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WO2024185819A1 - Steel sheet and outer sheet member - Google Patents

Steel sheet and outer sheet member Download PDF

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Publication number
WO2024185819A1
WO2024185819A1 PCT/JP2024/008586 JP2024008586W WO2024185819A1 WO 2024185819 A1 WO2024185819 A1 WO 2024185819A1 JP 2024008586 W JP2024008586 W JP 2024008586W WO 2024185819 A1 WO2024185819 A1 WO 2024185819A1
Authority
WO
WIPO (PCT)
Prior art keywords
less
martensite
steel sheet
content
carbonitrides
Prior art date
Application number
PCT/JP2024/008586
Other languages
French (fr)
Japanese (ja)
Inventor
諭 弘中
真衣 永野
克哉 中野
Original Assignee
日本製鉄株式会社
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
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Application filed by 日本製鉄株式会社 filed Critical 日本製鉄株式会社
Publication of WO2024185819A1 publication Critical patent/WO2024185819A1/en

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention relates to steel plates, and more specifically to steel plates and exterior panel members with excellent appearance, primarily used for example as exterior panel members for automobiles.
  • Patent Document 1 describes a steel sheet for hot-dip galvanizing, which contains, by mass%, C: 0.02-0.3%, Si: 0.1-2.0%, Mn: less than 1.0%, Cr: more than 1.0-3.0%, P: 0.02% or less, S: 0.02% or less, Al: 0.014% or less, and N: 0.001-0.008%, and satisfies 2.5 ⁇ 1.5Mn%+Cr%, 4.1-2.3Mn%-1.2Cr% ⁇ Si%, with the balance being Fe and unavoidable impurities.
  • Patent Document 1 also teaches that by optimizing the amounts of Mn, Cr, and Si added, it is possible to achieve both the workability of a steel sheet for hot-dip galvanizing with a tensile strength of 390 MPa or more and an appearance after processing that allows it to be used as an automotive exterior panel. Furthermore, Patent Document 1 teaches that by setting the area ratio of the main phase, ferrite, to 70% or more and the area ratio of the hard second phase, including martensite, to 30% or less, it is possible to keep the strength, yield strength, yield ratio, and strength-ductility balance all within a good range.
  • the present invention describes a cold-rolled steel sheet having a composition that contains Ti* in a range that satisfies 0 ⁇ Ti* ⁇ 0.02 and further satisfies (Sb%) ⁇ (Cu%)/5, with the balance being Fe and unavoidable impurities, and that the content (mass%) of Ti element contained in precipitates less than 20 nm in size in the plate thickness surface layer portion up to 10 ⁇ m from each surface on both sides of the steel sheet is 9% or less of the total Ti content (mass%) in the steel sheet.
  • Patent Document 2 also teaches that by setting the content (mass%) of Ti element contained in precipitates less than 20 nm in size in the plate thickness surface layer portion up to 10 ⁇ m from each surface on both sides of the steel sheet to 9% or less of the total Ti content (mass%) in the steel sheet, it is possible to avoid the occurrence of appearance unevenness caused by such fine Ti-based precipitates, and to obtain a cold-rolled steel sheet with excellent surface properties, and further that the cold-rolled steel sheet can be suitably used for parts that require excellent surface quality after forming, mainly for the outer panels of automobiles.
  • thin and wide steel materials are relatively often required for automotive exterior plate applications such as those described in Patent Documents 1 and 2, but such thin and wide steel materials have a problem in that they are prone to bending, called heat buckling, during the manufacturing process, for example, when passing through a continuous annealing processing line (CAPL). If operation is continued with heat buckling occurring, it may lead to plate breakage, and if plate breakage occurs, it becomes necessary to stop the manufacturing line and perform recovery work, which can cause significant damage.
  • CTL continuous annealing processing line
  • the present invention aims to provide a steel sheet with a novel structure that can suppress the occurrence of heat buckling during the manufacturing process and achieve both strength and good appearance after forming.
  • the inventors conducted research focusing on both the chemical composition and metal structure of the steel sheet.
  • the inventors discovered that by optimizing the chemical composition of the steel sheet and appropriately controlling the form and amount of Nb carbonitride, the high-temperature strength of the steel sheet can be improved, thereby suppressing the occurrence of heat buckling during the manufacturing process, and further discovered that by uniformly dispersing martensite contained in a predetermined ratio in the metal structure in both the micro- and macro-regions in the metal structure, the desired high strength can be achieved based on such a hard structure, and even when strain is applied by press forming or the like, the generation of minute irregularities on the steel sheet surface can be significantly suppressed, thus completing the present invention.
  • the present invention which has achieved the above object is as follows. (1) In mass%, C: 0.030-0.100%, Mn: 0.70-3.00%, Si: 0.005-1.500%, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0010-0.0150%, O: 0.0100% or less, Nb: 0.005-0.200%, Cr: 0-1.00%, Mo: 0 to 0.80%, B: 0 to 0.0100%, Ti: 0-0.200%, V: 0 to 0.500%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0 to 1.00%, Ta: 0 to 0.10%, Co: 0-3.00%, Sn: 0-1.00%, Sb: 0 to 0.200%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0-0.0100%, Bi: 0 to 0.0500%, As
  • the Nb content in all Nb carbonitrides is 0.004% or more, and the Nb content in Nb carbonitrides having a particle size of 20 nm or more is 60% or more of the Nb content in all Nb carbonitrides,
  • the average grain spacing of the martensite is 2.5 ⁇ m or less,
  • the chemical composition is, in mass%, Cr: 0.001-1.00%, Mo: 0.001-0.80%, B: 0.0001 to 0.0100%, Ti: 0.001 to 0.200%, V: 0.001-0.500%, Ni: 0.001 to 1.00%, Cu: 0.001 to 1.00%, W: 0.001-1.00%, Ta: 0.001 to 0.10%, Co: 0.001 to 3.00%, Sn: 0.001 to 1.00%, Sb: 0.001-0.200%, Ca: 0.0001-0.0100%, Mg: 0.0001-0.0100%, Zr: 0.0001 to 0.0100%, REM: 0.0001-0.0100%, Bi: 0.0001 to 0.0500%, and As: 0.001 to 0.10%
  • the steel sheet according to the above (1) characterized in that it contains at least one of the following: (3)
  • the present invention provides a steel sheet that can suppress the occurrence of heat buckling during the manufacturing process and can achieve both strength and good appearance after forming.
  • the steel plate according to the embodiment of the present invention has, in mass%, C: 0.030-0.100%, Mn: 0.70-3.00%, Si: 0.005-1.500%, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0010-0.0150%, O: 0.0100% or less, Nb: 0.005-0.200%, Cr: 0-1.00%, Mo: 0 to 0.80%, B: 0 to 0.0100%, Ti: 0-0.200%, V: 0 to 0.500%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0 to 1.00%, Ta: 0 to 0.10%, Co: 0-3.00%, Sn: 0 to 1.00%, Sb: 0 to 0.200%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0-0.0100%, Bi: 0
  • the Nb content in all Nb carbonitrides is 0.004% or more, and the Nb content in Nb carbonitrides having a particle size of 20 nm or more is 60% or more of the Nb content in all Nb carbonitrides,
  • the average grain spacing of the martensite is 2.5 ⁇ m or less,
  • the steel sheet is characterized by having a metal structure in which the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is 1.5% or less.
  • DP steel which is a mixture of soft structure made of ferrite and hard structure made of martensite
  • uneven deformation is likely to occur during processing such as press forming, in which the soft structure and its surroundings are preferentially deformed, and fine irregularities are generated on the surface of the steel sheet after forming, which can cause appearance defects called ghost lines.
  • the soft structure made of ferrite deforms greatly and is recessed on the surface of the steel sheet.
  • the hard structure made of martensite is small in deformation. Therefore, compared to the soft structure, the hard structure does not recess on the surface of the steel sheet, but rises to be convex.
  • the deformation amount varies especially in the width direction of the steel plate (the direction perpendicular to the rolling direction and the plate thickness direction), and ghost lines are generated in a band shape (striped shape).
  • elements such as Mn may be added in relatively large amounts to improve the hardenability of the steel plate.
  • Mn is an element that is likely to segregate in a streaky manner in the steel plate. More specifically, Mn-enriched regions such as central segregation and microsegregation are formed during casting, and the enriched regions are elongated in the rolling direction by hot rolling or cold rolling, so that Mn segregates in a streaky manner.
  • a CAPL generally has a hearth roll with a crown that has a convex shape in the center. Therefore, when a steel sheet passes through a CAPL, compressive stress is applied to the steel sheet in the width direction center due to the convex crown of the hearth roll. On the other hand, since continuous annealing is performed at a relatively high temperature, the yield stress of the steel sheet decreases as the sheet temperature increases.
  • the steel sheet may not be able to fully resist the compressive stress due to the decrease in yield stress at a relatively high temperature, and in such a case, a phenomenon called heat buckling occurs in which the sheet breaks and wrinkles occur.
  • heat buckling occurs in which the sheet breaks and wrinkles occur.
  • the present inventors first conducted research into improving the high-temperature strength of a steel sheet from the viewpoints of both the chemical composition and metal structure of the steel sheet in order to suppress or reduce the occurrence of such heat buckling.
  • the present inventors found that, from the viewpoint of the chemical composition of the steel sheet, the high-temperature strength of the steel sheet can be improved by controlling the index A represented by the following formula 1 to 0.50% or more, and thus the occurrence of heat buckling can be suppressed or reduced.
  • the index A it is believed that by controlling the index A to 0.50% or more, not only can the high-temperature strength of the steel sheet be improved, but also the Ac3 point of the steel sheet can be lowered.
  • the steel sheet in the first heat treatment process corresponding to the CAPL after cold rolling, the steel sheet needs to be heated to a temperature higher than the Ac3 point at which the steel sheet becomes austenite single phase, more specifically, to Ac3+10°C or higher.
  • the heating temperature in the first heat treatment process can be lowered, and in connection with this, it is possible to suppress the decrease in the yield stress of the steel sheet due to the increase in the sheet temperature during heating. Therefore, by controlling the index A to 0.50% or more, it is possible to significantly improve the resistance of the steel sheet to the above-mentioned compressive stress that causes heat buckling, based on the active improvement of the high-temperature strength of the steel sheet itself and the suppression of the decrease in yield stress due to the decrease in the heating temperature in the first heat treatment process.
  • the inventors have found that, from the viewpoint of the metal structure of the steel sheet, the high-temperature strength of the steel sheet can be improved by controlling the form and amount of Nb carbonitrides so that a relatively large amount of Nb carbonitrides having an appropriate size are present in the steel sheet, more specifically, by controlling the form and amount of Nb carbonitrides so that the Nb content in all Nb carbonitrides is 0.004% or more and the Nb content in Nb carbonitrides having a particle size of 20 nm or more is 60% or more of the Nb content in all Nb carbonitrides.
  • Nb carbonitrides By controlling the form and amount of Nb carbonitrides within such a range, it is possible to cause a sufficient amount of Nb carbonitrides having a particle size of 20 nm or more, which is effective in improving high-temperature strength, to be present in the steel. Therefore, in combination with the control of the chemical composition by the index A described above, the high-temperature strength of the steel sheet can be significantly improved, and as a result, it becomes possible to significantly suppress or reduce the occurrence of heat buckling during the manufacturing process of the steel sheet.
  • the inventors have investigated means for achieving the desired high strength by optimizing the ratio of ferrite, which is a soft structure, and martensite, which is a hard structure, in the metal structure, while further improving the appearance after forming.
  • the inventors have focused on the distribution state of martensite, which is a hard structure in the metal structure, and more specifically, have investigated controlling the distribution of martensite from a viewpoint different from that of reducing Mn segregation.
  • the inventors have found that by forming the metal structure in the steel sheet before final annealing with a structure mainly composed of bainite and/or martensite, and then final annealing the steel sheet having such a metal structure under specified conditions, it is possible to uniformly disperse martensite in both the micro-region and the macro-region in the finally obtained metal structure, without necessarily depending on the presence or absence or the degree of Mn segregation.
  • the inventors have found that by subjecting a steel sheet having a metal structure consisting of bainite and/or martensite to final annealing under predetermined conditions, the average grain spacing of martensite can be controlled to 2.5 ⁇ m or less in the micro region, and the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction can be controlled to 1.5% or less in the macro region.
  • the average grain spacing of martensite can be densely and uniformly dispersed in the micro region.
  • the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is 1.5% or less.
  • the variation of the hard structure in the macro region can be significantly reduced.
  • a metal structure in which martensite, which is a hard structure, is finely and uniformly dispersed throughout the steel sheet can be formed.
  • the deformation amount of the steel sheet can be made more uniform, especially in the width direction, even during forming such as press forming, and it is possible to achieve an excellent post-forming appearance in which appearance defects such as ghost lines are significantly suppressed.
  • the average particle spacing of martensite is controlled to 2.5 ⁇ m or less
  • the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to 1.5% or less.
  • the martensite structure has substructures such as packets, blocks, and laths in the prior austenite grains, and therefore has many different interfaces inside compared to structures such as ferrite. Bainite is also a structure that has many different interfaces inside, similar to the case of martensite.
  • the steel plate according to the embodiment of the present invention ensures good formability by controlling the area ratio of ferrite, a soft structure, to 75-95%, while controlling the area ratio of martensite, a hard structure, to 5-25% and further controlling the chemical composition of the steel plate within a specified range to ensure high strength with a tensile strength of 540 MPa or more.
  • C is an element that secures a certain amount of martensite and improves the strength of the steel sheet.
  • C is also an austenite stabilizing element and is effective in lowering the Ac3 point.
  • the C content is set to 0.030% or more.
  • the C content may be 0.040% or more or 0.050% or more.
  • the C content is set to 0.100% or less.
  • the C content may be 0.090% or less, 0.080% or less, 0.070% or less, or 0.060% or less.
  • Mn is an element that improves hardenability and contributes to improving the strength of the steel sheet. Mn is also an austenite stabilizing element and is effective in lowering the Ac3 point. In order to fully obtain these effects, In addition, the Mn content is 0.70% or more. The Mn content may be 0.80% or more, 1.00% or more, 1.20% or more, or 1.50% or more. In a preferred method for producing the steel sheet, the metal structure of the steel sheet before final annealing is changed to bainite and/or annealed steel in order to uniformly disperse martensite in both the micro- and macro-regions in the final metal structure.
  • Mn hardenability by adding Mn
  • Mn is contained in an excessive amount, Mn
  • the effect of segregation cannot be sufficiently counteracted, and the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction may not be controlled within a desired range. Not more than 3.00%.
  • the Mn content may be 2.80% or less, 2.50% or less, 2.20% or less, or 2.00% or less.
  • Silicon is an element that improves the strength of steel sheet by solid solution strengthening. In order to fully obtain such an effect, the silicon content is set to 0.005% or more. The silicon content is set to 0.010% or more. The Si content may be 0.100% or more, 0.200% or more, 0.300% or more, or 0.400% or more. On the other hand, if the Si content is excessive, it becomes difficult to remove scale formed during hot rolling, This may cause deterioration of the appearance. Therefore, the Si content is set to 1.500% or less. In addition, since Si is a ferrite stabilizing element, reducing the Si content can lower the Ac3 point. Therefore, the Si content may be 1.200% or less, 1.000% or less, 0.900% or less, 0.800% or less, 0.700% or less, or 0.600% or less. .
  • P is an impurity element that embrittles welds and deteriorates plating properties. For this reason, the P content is set to 0.100% or less.
  • S is an impurity element that impairs weldability and also impairs manufacturability during casting and hot rolling. For this reason, the S content is set to 0.0200% or less.
  • the lower the S content the more preferable it is.
  • the lower limit is not particularly limited, and the upper limit is 0.0150% or less, 0.0120% or less, 0.0100% or less, 0.0060% or less, or 0.0030% or less.
  • the S content is set to 0.0001%. or more, or 0.0002% or more, or 0.0005% or more.
  • Al is an element that functions as a deoxidizer and is effective in increasing the strength of steel.
  • the Al content may be 0%, but in order to fully obtain these effects, The Al content is preferably 0.001% or more.
  • the Al content may be 0.005% or more, 0.010% or more, 0.025% or more, or 0.050% or more. If Al is contained in excess, coarse oxides may form, which may reduce toughness. Therefore, the Al content is set to 1.000% or less.
  • Al is a ferrite stabilizing element, Al By reducing the Al content, the Ac3 point can be lowered. Therefore, the Al content may be 0.800% or less, 0.600% or less, or 0.300% or less.
  • N is an element effective in improving the high-temperature strength of a steel sheet by forming carbonitrides with Nb.
  • the N content is set to 0.0010% or more.
  • the N content may be 0.0015% or more, 0.0020% or more, 0.0025% or more, or 0.0030% or more.
  • the N content is set to 0.0150% or less.
  • the N content is set to 0.0120% or less, 0.0100% or less, 0.0080% or less, or 0.0060% or less. This is also fine.
  • O is an element that causes blowholes during welding. Therefore, the O content is set to 0.0100% or less.
  • the O content is set to 0.0080% or less, 0.0050% or less, 0.
  • O is set to less than 0.0001%. If the O content is reduced, the production cost increases significantly, which is economically disadvantageous. Therefore, the O content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
  • Nb is an element effective in increasing the index A and improving the high-temperature strength of the steel sheet.
  • Nb content is set to 0.005% or more.
  • the Nb content is set to 0.010% or more, and 0.015% or more.
  • the Nb content is set to 0.200% or less.
  • the Nb content is set to 0.150% or less, 0.100% or less, 0 It may be 0.080% or less or 0.060% or less.
  • the steel plate may contain at least one of the following optional elements in place of a portion of the remaining Fe, if necessary, for the purpose of improving the properties.
  • the steel sheet may contain at least one of Cr: 0-1.00%, Mo: 0-0.80%, B: 0-0.0100%, Ti: 0-0.200%, V: 0-0.500%, Ni: 0-1.00%, Cu: 0-1.00%, W: 0-1.00%, Ta: 0-0.10%, Co: 0-3.00%, Sn: 0-1.00%, Sb: 0-0.200%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, REM: 0-0.0100%, Bi: 0-0.0500%, and As: 0-0.10%.
  • These optional elements will be described in detail below.
  • Cr 0-1.00%
  • Cr is an element that improves hardenability and contributes to improving the strength of the steel sheet, similar to Mn.
  • the Cr content may be 0%, but in order to obtain the above effect, the Cr content should be 0.001% or more.
  • the Cr content may be 0.01% or more, 0.10% or more, or 0.20% or more.
  • the Cr content is preferably 1.00% or less, and may be 0.80% or less, 0.60% or less, or 0.40% or less.
  • Mo is an element that contributes to improving the high-temperature strength of steel sheets. This effect can be obtained even with a small amount of Mo.
  • the Mo content may be 0%, but in order to obtain the above effect, Preferably, the Mo content is 0.001% or more.
  • the Mo content may be 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more. However, if Mo is contained excessively, hot workability may deteriorate, and productivity may decrease. Therefore, the Mo content is preferably 0.80% or less.
  • the Mo content is preferably 0.60% or less. % or less, 0.50% or less, 0.40% or less, or 0.20% or less.
  • B is an element that suppresses the formation of ferrite and pearlite during the cooling process from austenite and promotes the formation of martensite. B is also an element that is beneficial for increasing the strength of steel. These effects are only seen in small amounts.
  • the B content may be 0%, but in order to obtain the above effects, the B content is preferably 0.0001% or more. The B content is preferably 0.0005% or more.
  • the B content is preferably 0.0100% or less.
  • the B content may be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.
  • Ti is an element effective in controlling the morphology of carbides. Ti can promote an increase in the strength of ferrite.
  • the Ti content may be 0%, but in order to obtain these effects, the Ti content must be less than 0.
  • the Ti content is preferably 0.001% or more.
  • the Ti content may be 0.002% or more, 0.010% or more, 0.020% or more, or 0.040% or more.
  • an excessive Ti content However, the effect of Ti is saturated and there is a risk of an increase in manufacturing costs. Therefore, the Ti content is preferably 0.200% or less, more preferably 0.100% or less, 0.080% or less, or 0.050% or less. It may be the following.
  • V is an element effective in controlling the morphology of carbides, and is also an element effective in refining the structure and improving the toughness of the steel plate.
  • the V content may be 0%, but if the above effects are not to be obtained, To obtain this, the V content is preferably 0.001% or more.
  • the V content may be 0.005% or more, 0.010% or more, or 0.050% or more. If V is contained in an excessive amount, a large amount of precipitates may be formed, which may reduce the toughness. Therefore, the V content is preferably 0.500% or less.
  • the V content is preferably 0.400% or less. It may be 0.200% or less or 0.100% or less.
  • Ni is an element effective in improving the strength of a steel sheet.
  • the Ni content may be 0%, but in order to obtain the above effect, the Ni content is preferably 0.001% or more.
  • the Ni content may be 0.01% or more, or 0.05% or more.
  • the Ni content is set to 1.00 % or less.
  • the Ni content may be 0.80% or less, 0.40% or less, or 0.20% or less.
  • Cu is an element that contributes to improving the strength of the steel sheet. This effect can be obtained even with a small amount of Cu.
  • the Cu content may be 0%, but in order to obtain the above effect, the Cu content must be 0%.
  • the Cu content is preferably 0.001% or more.
  • the Cu content may be 0.01% or more or 0.05% or more.
  • excessive Cu content may cause red shortness and deteriorate the hardness during hot rolling. Therefore, the Cu content is preferably 1.00% or less.
  • the Cu content is preferably 0.80% or less, 0.60% or less, 0.30% or less, or It may be 0.20% or less.
  • W is an element effective in controlling the morphology of carbides and improving the strength of steel sheets.
  • the W content may be 0%, but in order to obtain these effects, the W content must be 0.001% or more.
  • the W content may be 0.01% or more, or 0.05% or more.
  • the W content is set to 1.
  • the W content may be 0.80% or less, 0.40% or less, or 0.20% or less.
  • Ta is an element that is effective in controlling the morphology of carbides and improving the strength of steel sheets.
  • the Ta content may be 0%, but in order to obtain these effects, the Ta content should be 0.001%.
  • the Ta content may be 0.01% or more, or 0.03% or more.
  • the Ta content is preferably 0.10% or less.
  • the Ta content is preferably 0.08% or less, 0.06% or less, or 0.04% or less. It's fine if there is.
  • Co is an element that is effective in improving the strength of steel sheets.
  • the Co content may be 0%, but in order to obtain the above effect, the Co content must be 0.001% or more.
  • the Co content may be 0.01% or more, 0.05% or more, or 0.10% or more.
  • the Co content is preferably 3.00% or less.
  • the Co content is preferably 2.00% or less, 1.00% or less, 0.50% or less, or 0.20% or less. % or less.
  • Sn is an element that may be contained in a steel sheet when scrap is used as the raw material for the steel sheet. In addition, Sn may cause embrittlement of ferrite. Therefore, the smaller the Sn content, the better.
  • the Sn content may be 0.10% or less, 0.040% or less, or 0.02% or less.
  • the Sn content may be 0%, but Sn Reducing the Sn content to less than 0.001% leads to an excessive increase in refining costs. Therefore, the Sn content is set to 0.001% or more, 0.005% or more, or 0.01% or more. good.
  • Sb is an element that can be contained in a steel sheet when scrap is used as the raw material for the steel sheet.
  • Sb may strongly segregate at grain boundaries and cause embrittlement of the grain boundaries. Therefore, the smaller the Sb content, the better, and it is preferably 0.200% or less.
  • the Sb content may be 0.100% or less, 0.040% or less, or 0.020% or less.
  • the Sb content may be 0%, but reducing the Sb content to less than 0.001% will lead to an excessive increase in refining costs. % or more, or 0.010% or more.
  • Ca, Mg, Zr and REM are elements that contribute to improving the formability of the steel sheet.
  • the Ca, Mg, Zr and REM contents may be 0%, but in order to obtain such effects,
  • the contents of Ca, Mg, Zr and REM are each preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. If these elements are contained in excess, the ductility of the steel sheet may decrease.
  • the Ca, Mg, Zr and REM contents are each preferably 0.0100% or less, 0.0080% or less, and 0. It may be 0.0060% or less, 0.0040% or less, or 0.0020% or less.
  • REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.
  • REM is a general term for the 17 elements, and the REM content is the total content of these elements.
  • Bi is an element that has the effect of improving formability by refining the solidification structure.
  • the Bi content may be 0%, but in order to obtain such an effect, the Bi content should be 0.0001%.
  • the Bi content is preferably 0.0500% or less, more preferably 0.0400% or less, 0.0200% or less, 0.0100% or less, or 0.0050% or less. % or less.
  • As is an element that can be contained in a steel sheet when scrap is used as the raw material for the steel sheet.
  • As is an element that strongly segregates at grain boundaries, and the lower the As content, the better.
  • the As content is preferably 0.10% or less, and may be 0.04% or less or 0.02% or less.
  • the As content may be 0%, but the As content Reducing As to less than 0.001% leads to an excessive increase in refining costs, so the As content may be 0.001% or more, 0.005% or more, or 0.01% or more.
  • the remainder excluding the above elements consists of Fe and impurities.
  • Impurities are elements that are mixed in from the steel raw materials and/or during the steelmaking process, and whose presence is permitted to the extent that they do not impair the properties of the steel plate according to the embodiment of the present invention.
  • index A 0.50% or more
  • the chemical composition of the steel sheet according to the embodiment of the present invention requires that the index A represented by the following formula 1 is 0.50% or more.
  • A [C]-0.1[Si]+0.3([Mn]-0.5)-0.3[Al]+0.1[Cr]+0.6[Mo]-[Ti]+15[Nb ] ...
  • [C], [Si], [Mn], [Al], [Cr], [Mo], [Ti] and [Nb] are the contents [mass %] of each element, When no Cr content is contained, the value is 0%.
  • the primary alloying step which will be described in detail later in relation to the manufacturing method of the steel plate, is performed. It is effective to lower the heating temperature required in the heat treatment process (i.e., Ac3+10°C or higher). By lowering the heating temperature required in the first heat treatment process, the yield of the steel sheet caused by the increase in sheet temperature during heating can be reduced.
  • the inventors set the index A defined by the contents of these elements together with a coefficient considering the degree of influence, that is, the index A expressed by the above formula 1, to 0.50% or more.
  • the resistance of the steel sheet to the compressive stress is improved based on the positive improvement of the high-temperature strength of the steel sheet itself and the suppression of the decrease in the yield stress caused by the decrease in the heating temperature in the first heat treatment process.
  • the larger the index A the more preferable it is.
  • the upper limit of the index A is not particularly limited, but the index A may be, for example, 0.60% or more, 0.65% or more, 0.70% or more, 0.75% or more, or 0.80% or more. , 2.00% or less, 1.80% or less, 1.50% or less, 1.30% or less, or 1.10% or less.
  • the chemical composition of the steel plate according to the embodiment of the present invention may be measured by a general analytical method.
  • the chemical composition of the steel plate may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES).
  • C and S may be measured using the combustion-infrared absorption method
  • N may be measured using the inert gas fusion-thermal conductivity method
  • O may be measured using the inert gas fusion-non-dispersive infrared absorption method.
  • ferrite Since ferrite is a soft structure, it is easily deformed and contributes to improving elongation. When the area ratio of ferrite is 75% or more, sufficient formability can be obtained. From the viewpoint of improving formability, the higher the area ratio of ferrite, the more preferable it is, and it may be, for example, 78% or more, 80% or more, 82% or more, or 85% or more. On the other hand, if ferrite is contained excessively, the desired strength may not be achieved in the steel plate. Therefore, the area ratio of ferrite is 95% or less. The area ratio of ferrite may be 93% or less, 90% or less, or 87% or less.
  • Martensite is a structure with high dislocation density and hardness, and therefore contributes to improving tensile strength.
  • the area ratio of martensite By setting the area ratio of martensite to 5% or more, it is possible to ensure a tensile strength of, for example, 540 MPa or more. From the viewpoint of improving strength, the higher the area ratio of martensite, the more preferable it is, and it may be, for example, 7% or more, 10% or more, or 13% or more.
  • the area ratio of martensite is 25% or less, it is possible to ensure formability and appearance.
  • the area ratio of martensite may be 22% or less, 20% or less, 18% or less, or 15% or less.
  • "martensite” includes not only as-quenched martensite (so-called fresh martensite) but also tempered martensite.
  • the remaining structure other than ferrite and martensite may be 0% in area ratio, but when the remaining structure exists, the remaining structure is at least one of bainite, pearlite, and retained austenite.
  • the area ratio of the remaining structure i.e., at least one of bainite, pearlite, and retained austenite, may be 10% or less in total, for example, 8% or less, 6% or less, 4% or less, or 2% or less.
  • the area ratio of the remaining structure 0% in order to make the area ratio of the remaining structure 0%, a high level of control is required in the manufacturing process of the steel plate, which may lead to a decrease in yield. Therefore, the area ratio of the remaining structure may be 0.5% or more, or 1% or more.
  • Identification of the metal structure and calculation of the area ratio are performed by FE-SEM (field emission scanning electron microscope) and optical microscope after corrosion using Nital reagent (3% nitric acid ethanol solution) and X-ray diffraction method.
  • the structure observation by FE-SEM and optical microscope is performed at a magnification of 500 to 50,000 times for a 100 ⁇ m ⁇ 100 ⁇ m area in the steel plate cross section in the direction perpendicular to the plate surface.
  • three measurement points are set, and the area ratio is determined by calculating the average value of the measured values.
  • the length in the plate thickness direction is reduced while securing a measurement area of 10,000 ⁇ m 2.
  • a measurement area of 20 ⁇ m in the plate thickness direction and 500 ⁇ m in the direction perpendicular to the plate thickness direction may be observed.
  • the measurement length in the plate thickness direction is 10 ⁇ m or more, preferably 50 ⁇ m or more. The same applies to the "100 ⁇ m ⁇ 100 ⁇ m area" in the following description.
  • plate thickness x/y position (where x and y are natural numbers satisfying x ⁇ y) refers to a position moved in the plate thickness direction from the surface (plate surface) of the steel plate in the plate thickness direction toward the center of the steel plate by a distance (depth) of x/y of the plate thickness t.
  • depth a distance of x/y of the plate thickness t.
  • plate thickness 1/8 position refers to a position that is 0.25 mm deep in the plate thickness direction from the surface of the steel plate.
  • the surface of the steel plate refers to the interface between the steel plate and the coating
  • plate thickness t refers to the thickness of the steel plate (base material) excluding the coating.
  • the area ratios of ferrite and martensite are determined by the following procedure. First, the observation surface of the sample is etched with a Nital reagent (a 3% nitric acid in ethanol solution), and then a 100 ⁇ m x 100 ⁇ m area within the range of 1/8 to 3/8 of the plate thickness, centered at the 1/4 position, is observed with an FE-SEM (e.g., JEOL's JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 500 to 2000 times). In Nital corrosion, martensite and retained austenite are not corroded, so the area ratio of the uncorroded area corresponds to the total area ratio of martensite and retained austenite.
  • a Nital reagent a 3% nitric acid in ethanol solution
  • the metal structure is binarized according to differences in brightness using the image analysis software Image J (Ver. 1.54f), and the black parts of the image data are ferrite, and the uncorroded white parts are the total structure of martensite and retained austenite. Therefore, the area ratio of ferrite is calculated from the area ratio of the black area, while the area ratio of martensite is calculated by subtracting the area ratio of retained austenite measured by the X-ray diffraction method described later from the area ratio of this uncorroded area.
  • the area ratio of martensite calculated by this method also includes the area ratio of tempered martensite.
  • the area fraction of retained austenite is calculated by X-ray diffraction.
  • the specimen is removed from the plate surface to a depth of 1/4 in the plate thickness direction by mechanical polishing and chemical polishing. More specifically, the specimen is thinned to the vicinity of the observation position by mechanical polishing, and then thinned to the target position by chemical polishing (with hydrofluoric acid).
  • the structure fraction of retained austenite is calculated from the integrated intensity ratio of the diffraction peaks of (200) and (211) of the bcc phase and (200), (220), and (311) of the fcc phase obtained at the 1/4 plate thickness position using, for example, a Rigaku X-ray diffraction device (RINT2500, X-ray output 40 kV-200 mA). The general five-peak method is used for this calculation.
  • the calculated structure fraction of retained austenite is determined as the area fraction of retained austenite.
  • the residual structures of bainite, pearlite, and retained austenite may be present at a total ratio of 0 to 10%. That is, in the steel plate according to this embodiment, ferrite and martensite are the main metal structures, and the residual structures of bainite, pearlite, and retained austenite are metal structures that may be unavoidably generated during manufacturing. Therefore, there is essentially no positive technical significance in identifying the residual structures of bainite, pearlite, and retained austenite or measuring their area ratios. From the chemical composition and manufacturing method of the steel plate described in this specification, it is clear that the residual structure in the steel plate according to this embodiment is bainite, pearlite, retained austenite, or a composite thereof. The following methods can be used to identify and measure the residual structures of bainite and pearlite. The method for measuring the area ratio of retained austenite is as described above.
  • bainite and calculation of the area ratio are carried out as follows. First, the observation surface of the sample is corroded with Nital reagent, and then a 100 ⁇ m x 100 ⁇ m area within the range of 1/8 to 3/8 of the plate thickness, centered at 1/4 of the plate thickness, is observed with an FE-SEM (e.g., JEOL's JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 500 to 2000 times). From the position and arrangement of cementite contained within the structure in this observation area, bainite is identified as follows.
  • FE-SEM e.g., JEOL's JSM-7200F
  • Bainite is classified into upper bainite and lower bainite, and in upper bainite, cementite or retained austenite exists at the interface of lath-shaped bainitic ferrite.
  • upper bainite cementite exists inside lath-shaped bainitic ferrite, there is one type of crystal orientation relationship between bainitic ferrite and cementite, and the cementite has the same variant.
  • upper bainite and lower bainite can be identified. In the present invention, these are collectively referred to as bainite, and the area ratio of the identified bainite is calculated based on image analysis.
  • Pearlite is identified and its area ratio calculated using the following procedure. First, the observation surface of the sample is corroded with Nital reagent, and then the area from 1/8 to 3/8 of the plate thickness, centered at 1/4 of the plate thickness, is observed using an SEM (e.g., JEOL's JSM-7200F, measured at an acceleration voltage of 15 kV and magnification of 500 to 2000 times). Areas in which lamellar cementite is observed in the SEM observation image are identified as pearlite, and the area ratio of this area is calculated based on image analysis.
  • SEM e.g., JEOL's JSM-7200F
  • the amount of Nb in all Nb carbonitrides is 0.004% or more, and the amount of Nb in Nb carbonitrides having a particle size of 20 nm or more is 60% or more of the amount of Nb in all Nb carbonitrides]
  • the steel plate according to the embodiment of the present invention contains Nb carbonitrides in the metal structure, the Nb content in all Nb carbonitrides is 0.004% or more, and the Nb content in Nb carbonitrides having a particle size of 20 nm or more is controlled to 60% or more of the Nb content in all Nb carbonitrides.
  • Nb carbonitrides include not only NbCN but also NbC and NbN, and further include NbCN, NbC and NbN in which a part of Nb is replaced by one or more other elements such as Ti.
  • Nb carbonitrides By controlling the form and amount of Nb carbonitrides within the above range, it is possible to cause a sufficient amount of Nb carbonitrides having a particle size of 20 nm or more, which is effective for improving high-temperature strength, to be present in the steel, and therefore, in combination with the control of the chemical composition by the index A described above, the high-temperature strength of the steel plate can be significantly improved. As a result, it is possible to reliably suppress or reduce the occurrence of heat buckling during the manufacturing process of the steel plate.
  • the ratio of the Nb amount in all Nb carbonitrides and the Nb amount in Nb carbonitrides having a particle size of 20 nm or more to the total Nb carbonitrides is preferably as large as possible.
  • the Nb amount in all Nb carbonitrides may be 0.006% or more, 0.008% or more, 0.010% or more, or 0.012% or more.
  • the upper limit is not particularly limited, but for example, the Nb amount in all Nb carbonitrides may be 0.100% or less, 0.060% or less, 0.040% or less, or 0.030% or less.
  • the Nb amount in Nb carbonitrides having a particle size of 20 nm or more may be 62% or more, 65% or more, 68% or more, or 70% or more of the Nb amount in all Nb carbonitrides.
  • the upper limit is not particularly limited, but for example, the Nb amount in Nb carbonitrides having a particle size of 20 nm or more may be 95% or less, 90% or less, or 85% or less of the Nb amount in all Nb carbonitrides. If the particle size of the Nb carbonitride is 20 nm or more, the effect of improving high-temperature strength can be obtained, and if the particle size is too large, the effect is not significantly reduced. Therefore, the upper limit of the particle size of the Nb carbonitride is not particularly limited, but the particle size of the Nb carbonitride may be, for example, 1000 nm or less, i.e., 1 ⁇ m or less.
  • a test piece is taken from the 1/2 position of the plate thickness, and the taken test piece is electrolyzed at a constant current in an electrolytic solution (10% by volume acetylacetone-1% by mass tetramethylammonium chloride-methanol), and the precipitates attached to the test piece after electrolysis are dispersed in an aqueous sodium hexametaphosphate solution, and then filtered and collected with a porous filter having a pore size of 0.02 ⁇ m ⁇ (20 nm ⁇ ).
  • the amount of Nb contained in the precipitates on the filter is measured by ICP emission spectroscopy, and the content of Nb in the steel precipitated as Nb precipitates having a particle size of 20 nm or more collected on the filter is obtained.
  • the amount of Nb in the filtrate having a particle size of less than 20 nm contained in the filtrate that has passed through the filter is measured by ICP emission spectroscopy.
  • the total mass of Nb precipitated as Nb carbonitrides is calculated by adding together the amount of Nb in Nb precipitates having a particle size of 20 nm or more and the amount of Nb in Nb precipitates having a particle size of less than 20 nm, and the obtained value is determined as the amount of Nb in all Nb carbonitrides.
  • the amount of Nb precipitated as Nb carbonitrides having a particle size of 20 nm or more is used to calculate the ratio of the amount of Nb precipitated as Nb carbonitrides to the total mass of Nb, and the calculated value is determined as the ratio of the amount of Nb in Nb carbonitrides having a particle size of 20 nm or more to the amount of Nb in all Nb carbonitrides.
  • the average particle spacing of the martensite which is a hard structure
  • the average particle spacing of the martensite is an index that indicates the uniformity of the hard structure distribution in the micro region. The smaller the average particle spacing of the martensite, the more densely and uniformly the hard structure is dispersed, and therefore the higher the uniformity. The more uniform the deformation amount of the steel sheet during press forming is, particularly in the width direction of the steel sheet, the better the appearance of the steel sheet after press forming.
  • the deformation amount of the steel sheet is strongly affected by the distribution state of the hard structure, in order to make the deformation amount of the steel sheet uniform in the width direction of the steel sheet, it is necessary to make the distribution of the hard structure in the metal structure uniform.
  • the deformation amount of the steel sheet can be made more uniform in the width direction even during forming such as press forming, and as a result, a good post-forming appearance can be achieved.
  • the average grain spacing of martensite is preferably 2.4 ⁇ m or less, more preferably 2.2 ⁇ m or less, and most preferably 2.0 ⁇ m or less or 1.8 ⁇ m or less. Although there is no particular lower limit, for example, the average grain spacing of martensite may be 0.5 ⁇ m or more, 0.8 ⁇ m or more, or 1.0 ⁇ m or more.
  • the average grain spacing of martensite is determined as follows. First, a sample having a steel sheet cross section perpendicular to the sheet surface is taken, and the cross section is used as the observation surface. A region of 100 ⁇ m ⁇ 100 ⁇ m within the range of 1/8 to 3/8 of the sheet thickness centered at 1/4 of the sheet thickness is used as the observation region of this observation surface, and martensite is identified using FE-SEM (for example, JEOL JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 1000 to 5000 times). Specifically, the image analysis software Image J (Ver.
  • the black part of the image data is ferrite, and the uncorroded white part is the total structure of martensite and retained austenite.
  • the white structure can be regarded as martensite.
  • the distance between the centers (centers of gravity) of all adjacent martensite grains among the identified martensite grains is calculated as the particle spacing based on image analysis, and the average of the calculated particle spacings is obtained. This operation is performed in the other two observation regions, and the average of the three values obtained is determined as the average particle spacing of martensite (strictly speaking, particles including martensite and/or retained austenite).
  • Standard deviation in area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is 1.5% or less
  • the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is controlled to 1.5% or less.
  • the standard deviation is an index representing the uniformity of the hard structure in the macro region. The appearance, which is an issue during press forming, depends on the minute irregularities on the steel sheet surface caused by the difference in the amount of deformation in the width direction of the steel sheet.
  • the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is preferably 1.4% or less, more preferably 1.2% or less, and most preferably 1.0% or less.
  • the lower limit is not particularly limited, but the standard deviation may be, for example, 0.1% or more, 0.3% or more, or 0.5% or more.
  • the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is determined as follows. First, a metal structure image of a steel plate cross section in a region of 50 mm in the direction perpendicular to the rolling direction and the plate thickness direction is obtained. In the case of an image of 10 mm or smaller, multiple images may be obtained and joined to make 50 mm.
  • the cross section is observed at 0°, 45°, 90°, and 135° to an arbitrary direction, and the cross section with the highest aspect ratio of the precipitates among them is determined as the cross section parallel to the rolling direction, and the direction perpendicular to the plate thickness direction is determined as the direction perpendicular to the rolling direction and the plate thickness direction.
  • the obtained image is divided into 100 ⁇ m (0.1 mm) in the direction perpendicular to the rolling direction and the plate thickness direction, and the area ratio of martensite in the entire plate thickness is calculated for each divided range. Based on the martensite area ratio calculated from each of the total 500 divided images, the standard deviation in the area ratio of martensite is calculated.
  • the area ratio of martensite in each divided region is calculated according to the procedure described in the section "Identification of metal structure and calculation of area ratio".
  • the measurement results of the steel sheet cross section in a region of 50 mm in the direction perpendicular to the rolling direction and the sheet thickness direction may be used instead of the measurement results of each divided region.
  • the average grain size of ferrite in the metal structure is 3.0 to 25.0 ⁇ m.
  • the average grain size of ferrite may be 5.0 ⁇ m or more, 7.0 ⁇ m or more, 8.0 ⁇ m or more, 9.0 ⁇ m or more, or 10.0 ⁇ m or more.
  • the average grain size of ferrite may be 22.0 ⁇ m or less, 20.0 ⁇ m or less, 16.0 ⁇ m or less, 14.0 ⁇ m or less, or 12.0 ⁇ m or less.
  • the average grain size of ferrite in steel plate is determined as follows. First, a sample having a steel plate cross section perpendicular to the plate surface is taken, and the cross section is used as the observation surface. A 100 ⁇ m x 100 ⁇ m region within the range of 1/8 to 3/8 plate thickness positions, centered at the 1/4 plate thickness position, is used as the observation region on this observation surface, and martensite is identified using an FE-SEM (e.g., JEOL's JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 500 to 2000 times). Specifically, the metal structure is binarized based on the difference in brightness using image analysis software Image J (Ver. 1.54f), and ferrite is identified.
  • FE-SEM e.g., JEOL's JSM-7200F
  • the black parts of the image data are ferrite, and the uncorroded white parts are the combined structure of martensite and retained austenite.
  • the circle equivalent diameter of all identified ferrite is calculated. This operation is repeated in the other two observation areas, and the arithmetic average of the circular equivalent diameters of all the ferrite particles obtained in the three observation areas is calculated, and the obtained value is determined as the average grain size of the ferrite.
  • the average grain size of martensite in the metal structure is 1.0 to 5.0 ⁇ m.
  • the average grain size of martensite may be 1.2 ⁇ m or more, 1.5 ⁇ m or more, 1.7 ⁇ m or more, or 2.0 ⁇ m or more.
  • the average grain size of martensite may be 4.7 ⁇ m or less, 4.5 ⁇ m or less, 4.2 ⁇ m or less, 4.0 ⁇ m or less, 3.8 ⁇ m or less, 3.6 ⁇ m or less, or 3.4 ⁇ m or less.
  • the average crystal grain size of martensite is determined as follows. First, a sample having a steel plate cross section perpendicular to the plate surface is taken, and the cross section is used as the observation surface. A 100 ⁇ m x 100 ⁇ m region within the range of 1/8 to 3/8 plate thickness positions centered at the 1/4 plate thickness position is used as the observation region on this observation surface, and martensite is identified using an FE-SEM (for example, JEOL's JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 500 to 2000 times). Specifically, the image analysis software Image J (Ver. 1.54f) is used to binarize the metal structure based on the difference in brightness, and martensite is identified.
  • FE-SEM for example, JEOL's JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 500 to 2000 times.
  • the black parts of the image data are ferrite, and the uncorroded white parts are the combined structure of martensite and retained austenite.
  • the white structure is regarded as martensite.
  • the circle equivalent diameter of all identified martensite is calculated. This operation is repeated in the other two observation regions, and the circle equivalent diameters of all the martensite particles obtained in the three observation regions are arithmetically averaged, and the obtained value is determined as the average crystal grain size of the martensite (strictly speaking, particles containing martensite and/or retained austenite).
  • the average aspect ratio of martensite in the metal structure is 2.5 or more.
  • the average aspect ratio of martensite may be 2.6 or more, 2.8 or more, or 3.0 or more.
  • the upper limit is not particularly limited, but for example, the average aspect ratio of martensite may be 4.0 or less, 3.8 or less, or 3.6 or less.
  • the average aspect ratio of martensite is determined as follows. First, a sample having a steel plate cross section perpendicular to the plate surface is taken, and the cross section is used as the observation surface. A 100 ⁇ m x 100 ⁇ m area is taken from this observation surface within the range of 1/8 to 3/8 plate thickness positions, centered at the 1/4 plate thickness position, and martensite is identified using an FE-SEM (e.g., JEOL JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 1000 to 5000 times). Specifically, the image analysis software Image J (Ver. 1.54f) is used to binarize the metal structure based on the difference in brightness, and martensite is identified.
  • FE-SEM e.g., JEOL JSM-7200F
  • the black parts of the image data are ferrite, and the uncorroded white parts are the combined structure of martensite and retained austenite.
  • the white structure is considered to be martensite.
  • the aspect ratios of all martensite grains are calculated from the obtained image data using the image analysis software Image J (Ver. 1.54f).
  • the aspect ratios of the particles (crystal grains) on the image can be measured using a function built into the image analysis software Image J (Ver. 1.54f).
  • This operation is then performed in the other two observation regions, and the aspect ratios of all martensite grains obtained in the three observation regions are arithmetically averaged, and the obtained value is determined as the average aspect ratio of martensite (strictly speaking, particles containing martensite and/or retained austenite).
  • the steel plate according to the embodiment of the present invention has a plate thickness of, for example, 0.1 to 2.0 mm, but is not particularly limited thereto.
  • a steel plate having such a plate thickness is suitable for use as a material for exterior plate members such as doors and hoods as automobile members.
  • the plate thickness may be 0.2 mm or more, 0.3 mm or more, or 0.4 mm or more.
  • the plate thickness may be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, or 1.0 mm or less.
  • the plate thickness 0.2 mm or more it becomes easier to maintain the shape of the molded product flat, and an additional effect of improving dimensional accuracy and shape accuracy can be obtained.
  • the plate thickness 1.0 mm or less the weight reduction effect of the member becomes remarkable.
  • the plate thickness of the steel plate is measured by a micrometer.
  • the steel sheet according to the embodiment of the present invention is a cold-rolled steel sheet, but may further include a plating layer on the surface for the purpose of improving corrosion resistance or the like.
  • the plating layer may be either a hot-dip plating layer or an electroplating layer. That is, the steel sheet according to the embodiment of the present invention may be a cold-rolled steel sheet having a hot-dip plating layer or an electroplating layer on its surface.
  • the hot-dip plating layer includes, for example, a hot-dip galvanized layer (GI), a hot-dip galvannealed layer (GA), a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, a hot-dip Zn-Al-Mg-Si alloy plating layer, and the like.
  • the electroplating layer includes, for example, an electrogalvanized layer (EG), an electrogalvanized Zn-Ni alloy plating layer, and the like.
  • the plating layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer.
  • the coating weight of the plating layer is not particularly limited and may be a general coating weight.
  • a high tensile strength specifically a tensile strength of 540 MPa or more can be achieved.
  • the tensile strength is preferably 570 MPa or more, more preferably 600 MPa or more.
  • the upper limit is not particularly limited, but for example, the tensile strength may be 980 MPa or less, 900 MPa or less, 850 MPa or less, 830 MPa or less, or 800 MPa or less.
  • the tensile strength is measured by taking a tensile test piece No. 5 of JIS Z2241:2011 from the steel plate, with the test direction being perpendicular to the rolling direction and the plate thickness direction, and performing a tensile test in accordance with JIS Z2241:2011.
  • the steel plate according to the embodiment of the present invention can suppress the occurrence of heat buckling during the manufacturing process, and can achieve both high strength, for example a tensile strength of 540 MPa or more, and an excellent appearance after forming such as press working. For this reason, the steel plate according to the embodiment of the present invention is particularly useful for use in parts in technical fields where both of these properties are required.
  • an exterior plate member particularly an automobile exterior plate member, is provided that includes the steel plate according to the embodiment of the present invention. Examples of the exterior plate members of an automobile include roofs, hoods, fenders, doors, and the like, which require high designability.
  • exterior plate members particularly the exterior plate members of an automobile, only need to include the steel plate according to the embodiment of the present invention in at least a portion of these exterior plate members, and therefore at least a portion of these exterior plate members will satisfy the chemical composition and metal structure characteristics described above.
  • the characteristics of the metal structure do not change particularly before and after forming.
  • the method for producing a steel sheet according to an embodiment of the present invention includes: A hot rolling process comprising finish rolling a slab having the chemical composition described above in relation to the steel sheet, followed by coiling, the hot rolling process satisfying the following conditions (a) to (d): (a) The finish rolling entry temperature is 1000 to 1080°C; (b) The reduction ratio of the rolling pass two passes before the final pass is 30% or more; (c) a ratio of the rolling reduction of the rolling pass two passes before the final pass/the rolling reduction of the final pass is 1.5 to 2.5; and (d) a coiling temperature is 520 to 670° C.
  • a cold rolling process in which the obtained hot-rolled steel sheet is cold-rolled at a rolling reduction of 70% or more.
  • the method is characterized by including a first annealing step including heating the obtained cold-rolled steel sheet to a temperature of Ac3+10°C or higher, and a second annealing step including heating the cold-rolled steel sheet and holding it at a temperature of (Ac1+20) to 820°C for 10 to 500 seconds.
  • a first annealing step including heating the obtained cold-rolled steel sheet to a temperature of Ac3+10°C or higher
  • a second annealing step including heating the cold-rolled steel sheet and holding it at a temperature of (Ac1+20) to 820°C for 10 to 500 seconds.
  • a slab having the chemical composition described above in relation to the steel plate is subjected to hot rolling.
  • the slab to be used is preferably cast by a continuous casting method from the viewpoint of productivity, but may be manufactured by an ingot casting method or a thin slab casting method.
  • the slab is preferably heated to 1100 ° C or higher prior to hot rolling.
  • the upper limit of the heating temperature is not particularly limited, but from an economic viewpoint, the heating temperature is preferably 1300 ° C or lower.
  • the heated slab may be subjected to rough rolling before finish rolling as an option, for plate thickness adjustment, etc. Such rough rolling is not particularly limited as long as the desired sheet bar dimensions can be secured.
  • Finish rolling entry temperature 1000 to 1080° C.
  • the heated slab or the slab that has been rough-rolled as necessary is then subjected to finish rolling.
  • the finish rolling needs to be performed under conditions where the inlet temperature of the finish rolling is 1000 to 1080 ° C.
  • Nb carbonitrides can be appropriately precipitated in the hot rolling process. Therefore, due to the appropriate precipitation of such Nb carbonitrides, the high-temperature strength of the steel sheet can be sufficiently improved, and the occurrence of heat buckling can be significantly suppressed even by heat treatment at high temperatures in the subsequent primary annealing process or the like.
  • the inlet temperature of the finish rolling is higher than 1080 ° C, recrystallization is likely to occur, making it difficult to accumulate strain in the later stage of the finish rolling, and it becomes impossible to promote the precipitation of Nb carbonitrides having a grain size of 20 nm or more. As a result, it becomes impossible to achieve the desired ratio of the Nb amount in Nb carbonitrides having a grain size of 20 nm or more to the Nb amount in all Nb carbonitrides.
  • the finish rolling entry temperature is lower than 1000°C, although strain accumulates in the latter stages of finish rolling, precipitation of Nb carbonitrides itself becomes difficult to occur, and the grain size of the Nb carbonitrides also becomes small. As a result, it becomes impossible to achieve a desired ratio of the amount of Nb in Nb carbonitrides having a grain size of 20 nm or more to the amount of Nb in all Nb carbonitrides.
  • the finish rolling is performed using a tandem rolling mill consisting of a plurality of rolling stands, for example, five or more rolling stands.
  • it is important to control the reduction ratio in the latter stage of the finish rolling and more specifically, it is important to control the reduction ratio of the rolling pass two passes before the final pass to 30% or more, and to control the ratio of the reduction ratio of the rolling pass two passes before the final pass/the reduction ratio of the final pass to 1.5 to 2.5.
  • the reduction rate of the rolling pass two passes before the final pass is low and the ratio of the reduction rate of the rolling pass two passes before the final pass/the reduction rate of the final pass is less than 1.5, the accumulation of strain after the reduction in the rolling pass two passes before the final pass becomes insufficient, and the precipitation of Nb carbonitrides with a particle size of 20 nm or more becomes insufficient.
  • the reduction rate of the final pass is high and the ratio of the reduction rate of the rolling pass two passes before the final pass/the reduction rate of the final pass is less than 1.5, the accumulation of strain after the reduction in the final pass becomes significant, and the amount of fine Nb carbonitrides precipitated after finish rolling increases, that is, the ratio of Nb carbonitrides with a particle size of 20 nm or more decreases. Therefore, in either case, it becomes impossible to achieve the desired ratio of the amount of Nb in Nb carbonitrides having a particle size of 20 nm or more relative to the amount of Nb in all Nb carbonitrides.
  • the reduction rate of the rolling pass two passes before the final pass is high or the reduction rate of the final pass is low, and in this regard the ratio of the reduction rate of the rolling pass two passes before the final pass/the reduction rate of the final pass exceeds 2.5, the accumulation of strain after the reduction in the rolling pass two passes before the final pass becomes too large, making it easier for recrystallization to occur.
  • Nb carbonitrides with a grain size of 20 nm or more it becomes difficult for Nb carbonitrides with a grain size of 20 nm or more to precipitate, and it becomes impossible to achieve the desired ratio of the amount of Nb in Nb carbonitrides having a grain size of 20 nm or more to the amount of Nb in all Nb carbonitrides.
  • the finish-rolled material is coiled at a coiling temperature of 520 to 670° C.
  • the coiling temperature exceeds 670°C, the alloy may be added to the cementite in the metal structure.
  • the metal structure in the first annealing process becomes a structure mainly composed of bainite and/or martensite. Therefore, the desired dispersion state of martensite cannot be obtained even by the subsequent secondary annealing step.
  • the average grain spacing of martensite cannot be controlled to 2.5 ⁇ m or less, and/or the martensite grains in the direction perpendicular to the rolling direction and the sheet thickness direction are not uniform. In other words, it becomes impossible to obtain a metal structure in which martensite is uniformly dispersed in both the micro-region and the macro-region. In this case, the occurrence of ghost lines and the like cannot be sufficiently suppressed, and the appearance after molding is deteriorated.
  • the obtained hot-rolled steel sheet is subjected to an appropriate pickling treatment to remove scale, and then to a cold rolling process.
  • the hot-rolled steel sheet is cold-rolled so that the reduction is 70% or more.
  • the desired plate thickness can be secured, and the recrystallization during heating in the subsequent primary annealing process can be completed early to ensure the desired precipitation state of Nb carbonitrides. If the reduction is less than 70%, recrystallization is delayed during heating in the primary annealing process, and strain remains until a high temperature is reached.
  • the reduction in the cold rolling process is 90% or less. By setting the reduction rate to 90% or less, it is possible to prevent the rolling load from becoming excessively large, which makes the rolling difficult.
  • the number of rolling passes and the reduction rate for each pass are not particularly limited, and may be appropriately set so that the reduction rate of the entire cold rolling is within the above range.
  • the obtained cold-rolled steel sheet is heated to a temperature of Ac3+10°C or more in the next primary annealing step.
  • the Ac3 point (°C) is determined by cutting a small piece from the cold-rolled steel sheet and calculating the thermal expansion of the small piece during heating from room temperature to 1000°C at 10°C/s.
  • the metal structure in the steel sheet after cooling can be reliably composed of a structure mainly composed of bainite and/or martensite, for example, full bainite or full martensite.
  • the structure mainly composed of bainite and/or martensite refers to a structure containing at least one of bainite and martensite in a total area ratio of 90% or more
  • full bainite refers to a structure composed of 100% bainite in area ratio
  • full martensite refers to a structure composed of 100% martensite in area ratio.
  • the bainite and/or martensite structure has many different interfaces inside compared to structures such as ferrite.
  • the metal structure of the steel sheet before the secondary annealing process i.e., the final annealing process
  • a structure mainly composed of bainite and/or martensite it becomes possible to disperse and generate a very large number of carbides that can become nucleation sites of austenite on these interfaces in the stage of heating such a metal structure in the secondary annealing.
  • austenite is generated finely and uniformly throughout the steel sheet from the nucleation sites dispersed in such a large number, and then martensite is generated from the austenite, so that in the metal structure obtained after the secondary annealing, the average grain spacing of martensite is controlled to 2.5 ⁇ m or less, and the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to 1.5% or less.
  • the heating temperature in the first annealing step is less than Ac3+10°C, austenitization will be insufficient, and the metal structure in the steel sheet will not be composed mainly of bainite and/or martensite even after subsequent cooling, meaning that the total area ratio of bainite and martensite will not be 90% or more.
  • the heating temperature in the first annealing step be 1050°C or less.
  • the holding time at the above heating temperature is preferably 10 to 500 seconds.
  • austenite can be generated finely and uniformly from the carbides throughout the steel sheet while maintaining the state in which the carbides are dispersed on the interfaces.
  • austenite can be generated finely and uniformly from the carbides throughout the steel sheet while maintaining the state in which the carbides are dispersed on the interfaces.
  • martensite can be appropriately generated from the finely dispersed austenite, and as a result, the average particle spacing of martensite is controlled to 2.5 ⁇ m or less, and the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to 1.5% or less.
  • the desired metal structure as described above cannot be obtained.
  • the heating temperature in the secondary annealing process is less than Ac1+20°C or the holding time is less than 10 seconds, the desired metal structure as described above cannot be obtained.
  • the heating temperature exceeds 820°C, the area ratio of austenite becomes too high, and the area ratio of ferrite cannot be increased to 75% or more.
  • the holding time exceeds 500 seconds, the austenite grains become coarse, and the martensite grains obtained by subsequent cooling are also relatively coarse. In such a case, it becomes impossible to obtain a fine martensite structure in which the average grain spacing of martensite is controlled to 2.5 ⁇ m or less.
  • a plating treatment may be applied to the surface of the obtained cold-rolled steel sheet as necessary.
  • the plating treatment may be a treatment such as hot-dip plating, alloying hot-dip plating, or electroplating.
  • the steel sheet may be subjected to hot-dip galvanizing treatment as the plating treatment, or the alloying treatment may be performed after the hot-dip galvanizing treatment.
  • the specific conditions of the plating treatment and the alloying treatment are not particularly limited, and may be any appropriate conditions known to those skilled in the art.
  • the alloying temperature may be 450 to 600°C.
  • steel sheets according to the embodiments of the present invention were manufactured under various conditions, and the occurrence of heat buckling during the manufacturing process, as well as the tensile strength and post-forming appearance characteristics of the resulting steel sheets were investigated.
  • a slab having the chemical composition shown in Table 1 and a thickness of 200 to 300 mm was cast by continuous casting.
  • the remainder other than the components shown in Table 1 was Fe and impurities.
  • the obtained slab was heated to a temperature of 1100 to 1300°C, and then hot rolling was performed.
  • the hot rolling was performed by performing rough rolling and finish rolling. More specifically, the rough rolling conditions were the same in all examples and comparative examples, and the finish rolling was performed using a tandem rolling mill consisting of seven rolling stands.
  • the reduction ratio of the rolling pass two passes before the final pass in the finish rolling (F5 rolling pass) was 30%.
  • Condition I Finishing roll entry temperature 1000-1080°C
  • Condition II Ratio of rolling reduction of the rolling pass two passes before the final pass/rolling reduction of the final pass (F5/F7 rolling reduction ratio) 1.5-2.5
  • Condition III Coiling temperature 520-670°C.
  • the finishing roll entry temperature is 1050°C
  • the finishing roll entry temperature is 950°C (Comparative Example 9) or 1120°C (Comparative Example 19).
  • the F5/F7 rolling reduction ratio is 2.0, while in the example where Condition II is not met, the F5/F7 rolling reduction ratio is 1.2 (Comparative Example 2) or 3.0 (Comparative Example 20).
  • the winding temperature was 600°C, while in the examples that did not satisfy condition III, the winding temperature was 470°C (Comparative Example 10) or 700°C (Comparative Example 21).
  • the obtained hot-rolled steel sheet was pickled, followed by cold rolling, primary annealing (holding time of 100 seconds at a specified heating temperature, and an average cooling rate of 40°C/sec to 200°C after annealing), and secondary annealing (holding time of 100 seconds at a heating temperature of 770°C, and an average cooling rate of 15°C/sec to 500°C after annealing) to produce a cold-rolled steel sheet with a thickness of 0.4 mm.
  • the heating temperature of 770°C satisfies the requirement of Ac1+20°C or more for all of the invention examples and comparative examples.
  • Table 2 shows the cases where cold rolling condition IV (reduction rate of 70% or more) and primary annealing condition V (heating temperature Ac3+10°C or more) are met and where they are not met.
  • cold rolling was performed at a rolling reduction of 80%
  • cold rolling was performed at a rolling reduction of 60%
  • the primary annealing was performed by heating to Ac3+15°C or higher, i.e., 900°C.
  • the primary annealing was performed by heating to a temperature lower than Ac3 (Comparative Examples 4 and 20).
  • the surface of the obtained cold-rolled steel sheet was appropriately plated to form a hot-dip galvanized layer (GI), a galvannealed layer (GA) or an electrogalvanized layer (EG).
  • GI hot-dip galvanized layer
  • GA galvannealed layer
  • EG electrogalvanized layer
  • the alloying conditions were 550°C for 20 seconds.
  • the properties of the obtained steel sheets were measured and evaluated by the following methods.
  • the occurrence of heat buckling during the manufacturing process the occurrence of buckling was checked after the first annealing, and if no buckling occurred, it was judged that no heat buckling had occurred and was deemed to have passed (OK), and if buckling occurred, it was judged that heat buckling had occurred and was deemed to have failed (NG).
  • TS Tensile strength
  • the tensile strength (TS) was measured by taking a No. 5 tensile test piece of JIS Z2241:2011 from the steel plate, the longitudinal direction of which was perpendicular to the rolling direction and the plate thickness direction, and conducting a tensile test in accordance with JIS Z2241:2011.
  • the appearance after forming was evaluated based on the degree of ghost lines that appeared on the surface of the door outer after forming.
  • a formed part simulating the door outer a pressed part was used in which a steel plate blanked to 600 mm square was press-formed so that the radius of curvature R at the center was 1200 mm.
  • the surface after press forming was ground with a grindstone, and stripes that appeared on the surface at intervals of several mm were judged to be ghost lines, and were scored from 1 to 5 depending on the degree of occurrence of the stripes.
  • the evaluation was "4" or more, the appearance after forming was judged to be poor and the specimen was judged to be unsatisfactory.
  • the appearance after forming was evaluated using a pressed member simulating a door outer, but the evaluation object may be a formed member that can be estimated to have been given a strain of 2.5% by press forming, or a test piece taken from a steel plate to which a pre-strain of 2.5% has been similarly given may be evaluated, and similar evaluations can be made by such test methods.
  • a test piece taken from a steel plate a JIS No. 5 test piece having a longitudinal direction perpendicular to the rolling direction and the plate thickness direction and given a pre-strain of 2.5% may be evaluated.
  • the proportion of Nb carbonitrides with a particle size of 20 nm or more decreased, and heat buckling occurred in the first annealing process.
  • the heating temperature in the first annealing step was low, so austenitization was insufficient, and it is believed that the metal structure in the steel sheet could not be constituted by a structure mainly composed of bainite and/or martensite even after subsequent cooling.
  • the average particle spacing of martensite exceeded 2.5 ⁇ m, and the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction exceeded 1.5%, resulting in a poor appearance after forming.
  • Comparative Example 20 the F5/F7 reduction ratio in the finish rolling was high, so that the accumulation of strain after reduction in the F5 rolling pass was too large, and recrystallization was promoted. As a result, the proportion of Nb carbonitrides with a particle size of 20 nm or more was reduced, and heat buckling occurred in the first annealing process.
  • Comparative Example 20 as in Comparative Example 4, the heating temperature in the first annealing process was low, so that austenitization was insufficient, and it is considered that the metal structure in the steel sheet could not be composed of a structure mainly composed of bainite and/or martensite even after subsequent cooling.
  • Comparative Example 28 since the value of index A was low, the high-temperature strength of the steel sheet could not be sufficiently improved, and heat buckling occurred in the primary annealing step.
  • the C or Mn content was high, so the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction exceeded 1.5%, resulting in poor appearance after forming.
  • the C or Mn content was low, so sufficient strength was not obtained.
  • all of the steel sheets according to the examples of the invention have a prescribed chemical composition and metal structure, and by appropriately controlling the proportions of ferrite and martensite in the metal structure, a TS of 540 MPa or more is achieved, and the average particle spacing of martensite is controlled to 2.5 ⁇ m or less in the microscopic region, while in the macroscopic region the standard deviation in the area ratio of martensite in the directions perpendicular to the rolling direction and plate thickness direction is controlled to 1.5% or less.
  • the Nb content in all Nb carbonitrides was controlled to 0.004% or more, and the Nb content in Nb carbonitrides with a grain size of 20 nm or more was controlled to 60% or more of the Nb content in all of the Nb carbonitrides, and by combining this with the control of the chemical composition by index A, it was possible to significantly suppress the occurrence of heat buckling during the manufacturing process of the steel sheets.
  • all of the Nb carbonitrides with a grain size of 20 nm or more in the invention examples had a grain size of 1000 nm or less, i.e., 1 ⁇ m or less.

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Abstract

Provided is a steel sheet that is characterized by: having a prescribed chemical composition; comprising, in terms of area%, 75 to 95% of ferrite, 5 to 25% of martensite and a total of 0 to 10% of at least one of bainite, pearlite and retained austenite; and having a metal structure in which the amount of Nb in all Nb carbonitrides is 0.004% or more, the amount of Nb in all Nb carbonitrides having particle sizes of 20 nm or more is 60% or more of the amount of Nb in all Nb carbonitrides, the average interparticle distance in the martensite is 2.5 µm or less, and the standard deviation of the areal ratio of martensite in the rolling direction and a direction perpendicular to the sheet thickness direction is 1.5% or less.

Description

鋼板及び外板部材Steel plates and exterior panel components
　本発明は、鋼板に関し、より詳しくは、例えば自動車の外板部材等が主たる用途の外観性に優れた鋼板及び外板部材に関する。 The present invention relates to steel plates, and more specifically to steel plates and exterior panel members with excellent appearance, primarily used for example as exterior panel members for automobiles.
　自動車からの炭酸ガスの排出量を抑えるために、高強度鋼板を使用して、安全性を確保しながら自動車車体を軽量化する試みが進められている。このような自動車用鋼板の高強度化は、自動車骨格部品で顕著に進んでいるが、ドアやフードなどの外板部材では引張強さで３００ＭＰａ以下の強度クラスの鋼板が主に使用されており、高強度化が進んでいない。このような外板部材には、高い成形性や外観性が求められる。一般に、鋼板の強度を高めると、成形性や成形後の外観性は低下する。したがって、高強度鋼板において、強度と成形性及び外観性、とりわけ成形後の外観性とを両立させることは困難である。従来、これらの課題を解決するために、いくつかの手段が提案されている。 In order to reduce carbon dioxide emissions from automobiles, efforts are underway to use high-strength steel sheets to reduce the weight of automobile bodies while ensuring safety. While the increase in strength of automotive steel sheets has been remarkable for automobile frame parts, steel sheets with a tensile strength of 300 MPa or less are mainly used for exterior panel components such as doors and hoods, and there has been little progress in increasing strength. Such exterior panel components require high formability and appearance. In general, increasing the strength of steel sheets reduces their formability and appearance after forming. Therefore, it is difficult to achieve both strength and formability and appearance, especially appearance after forming, in high-strength steel sheets. Several means have been proposed to solve these problems.
　例えば、特許文献１では、質量％で、Ｃ：０．０２～０．３％、Ｓｉ：０．１～２．０％、Ｍｎ：１．０％未満、Ｃｒ：１．０超～３．０％、Ｐ：０．０２％以下、Ｓ：０．０２％以下、Ａｌ：０．０１４％以下、Ｎ：０．００１～０．００８％を含有し、且つ、２．５≦１．５Ｍｎ％＋Ｃｒ％、４．１－２．３Ｍｎ％－１．２Ｃｒ％≦Ｓｉ％を満足し、残部Ｆｅおよび不可避不純物からなることを特徴とする溶融亜鉛めっき用鋼板が記載されている。また、特許文献１では、Ｍｎ、Ｃｒ、Ｓｉの添加量を最適化することによって、引張強度が３９０ＭＰａ以上の溶融亜鉛めっき用鋼板の加工性と、自動車用外板として使用可能な加工後の外観を両立できることが教示されている。さらに、特許文献１では、主相であるフェライトの面積率を７０％以上とし、マルテンサイトを含む硬質第２相の面積率を３０％以下とすることで、強度、降伏強度、降伏比、強度－延性バランスの全てを良好な範囲とすることが可能になると教示されている。 For example, Patent Document 1 describes a steel sheet for hot-dip galvanizing, which contains, by mass%, C: 0.02-0.3%, Si: 0.1-2.0%, Mn: less than 1.0%, Cr: more than 1.0-3.0%, P: 0.02% or less, S: 0.02% or less, Al: 0.014% or less, and N: 0.001-0.008%, and satisfies 2.5≦1.5Mn%+Cr%, 4.1-2.3Mn%-1.2Cr%≦Si%, with the balance being Fe and unavoidable impurities. Patent Document 1 also teaches that by optimizing the amounts of Mn, Cr, and Si added, it is possible to achieve both the workability of a steel sheet for hot-dip galvanizing with a tensile strength of 390 MPa or more and an appearance after processing that allows it to be used as an automotive exterior panel. Furthermore, Patent Document 1 teaches that by setting the area ratio of the main phase, ferrite, to 70% or more and the area ratio of the hard second phase, including martensite, to 30% or less, it is possible to keep the strength, yield strength, yield ratio, and strength-ductility balance all within a good range.
　特許文献２では、ｍａｓｓ％で、Ｃ：０．０００５～０．０１％、Ｓｉ：０．２％以下、Ｍｎ：０．１～１．５％、Ｐ：０．０３％以下、Ｓ：０．００５～０．０３％、Ｔｉ：０．０２～０．１％、Ａｌ：０．０１～０．０５％、Ｎ：０．００５％以下、Ｓｂ：０．０３％以下、Ｃｕ：０．００５％超０．０３％以下であり、かつ、Ｔｉ＊＝（Ｔｉ％）－３．４×（Ｎ％）－１．５×（Ｓ％）－４×（Ｃ％）で示されるＴｉ＊を０＜Ｔｉ＊＜０．０２を満たす範囲で、さらに、（Ｓｂ％）≧（Ｃｕ％）／５を満たす範囲で含有し、残部はＦｅおよび不可避的不純物からなる成分組成を有し、鋼板両面において、各表面から１０μｍまでの板厚表層部における大きさ２０ｎｍ未満の析出物に含まれるＴｉ元素の含有量（ｍａｓｓ％）が、鋼板中の全Ｔｉ含有量（ｍａｓｓ％）の９％以下であることを特徴とする冷延鋼板が記載されている。また、特許文献２では、鋼板両面の各表面から１０μｍまでの板厚表層部における大きさ２０ｎｍ未満の析出物に含まれるＴｉ元素の含有量（ｍａｓｓ％）を鋼板中の全Ｔｉ含有量（ｍａｓｓ％）の９％以下とすることで、このような微細なＴｉ系析出物に起因する外観ムラの発生を回避し、表面性状に優れた冷延鋼板が得られること、さらには当該冷延鋼板が自動車の外板を中心に優れた成形後表面品質を必要とする部品に対して好適に使用できることが教示されている。 In Patent Document 2, the mass percentages are C: 0.0005-0.01%, Si: 0.2% or less, Mn: 0.1-1.5%, P: 0.03% or less, S: 0.005-0.03%, Ti: 0.02-0.1%, Al: 0.01-0.05%, N: 0.005% or less, Sb: 0.03% or less, Cu: over 0.005% and 0.03% or less, and Ti* = (Ti%) - 3.4 x (N%) - 1.5 x (S%) - 4 x (C%). The present invention describes a cold-rolled steel sheet having a composition that contains Ti* in a range that satisfies 0<Ti*<0.02 and further satisfies (Sb%)≧(Cu%)/5, with the balance being Fe and unavoidable impurities, and that the content (mass%) of Ti element contained in precipitates less than 20 nm in size in the plate thickness surface layer portion up to 10 μm from each surface on both sides of the steel sheet is 9% or less of the total Ti content (mass%) in the steel sheet. Patent Document 2 also teaches that by setting the content (mass%) of Ti element contained in precipitates less than 20 nm in size in the plate thickness surface layer portion up to 10 μm from each surface on both sides of the steel sheet to 9% or less of the total Ti content (mass%) in the steel sheet, it is possible to avoid the occurrence of appearance unevenness caused by such fine Ti-based precipitates, and to obtain a cold-rolled steel sheet with excellent surface properties, and further that the cold-rolled steel sheet can be suitably used for parts that require excellent surface quality after forming, mainly for the outer panels of automobiles.
特開２００９－２４９７３７号公報JP 2009-249737 A 国際公開第２０１１／１４２４７３号International Publication No. 2011/142473
　例えば、特許文献１に記載されるような軟質なフェライトと硬質なマルテンサイトを含む金属組織を有する複合組織鋼の場合には、プレス成形などの加工時に軟質なフェライト及びその周辺が優先的に変形する不均一変形が起こりやすい。このため、このような軟質組織と硬質組織から構成される複合組織鋼を利用した場合には、成形後の鋼板表面に微小な凹凸が生じることで、ゴーストラインと呼ばれる外観不良が発生することがある。また、特許文献１及び２に記載されるような自動車の外板用途では、薄手広幅の鋼材が求められる場合が比較的多いが、このような薄手広幅の鋼材は、製造過程において、例えば、連続焼鈍処理設備（ＣＡＰＬ：Ｃｏｎｔｉｎｕｏｕｓ　Ａｎｎｅａｌｉｎｇ　Ｐｒｏｃｅｓｓｉｎｇ　Ｌｉｎｅ）に通板される際に、ヒートバックルと呼ばれる腰折れが起こりやすいという問題がある。ヒートバックルが発生したまま操業を続けると、板破断などに至る場合があり、板破断などが発生してしまうと、製造ラインを停止して復旧作業を行う必要が生じ、その損害は甚大なものとなる。 For example, in the case of a composite steel having a metal structure containing soft ferrite and hard martensite as described in Patent Document 1, non-uniform deformation is likely to occur during processing such as press forming, in which the soft ferrite and its surroundings are preferentially deformed. For this reason, when using such a composite steel consisting of a soft structure and a hard structure, minute irregularities may occur on the surface of the steel sheet after forming, which may cause appearance defects called ghost lines. In addition, thin and wide steel materials are relatively often required for automotive exterior plate applications such as those described in Patent Documents 1 and 2, but such thin and wide steel materials have a problem in that they are prone to bending, called heat buckling, during the manufacturing process, for example, when passing through a continuous annealing processing line (CAPL). If operation is continued with heat buckling occurring, it may lead to plate breakage, and if plate breakage occurs, it becomes necessary to stop the manufacturing line and perform recovery work, which can cause significant damage.
　そこで、本発明は、新規な構成により、製造過程におけるヒートバックルの発生を抑制することができるとともに、強度と成形後の外観性とを両立することができる鋼板を提供することを目的とする。 The present invention aims to provide a steel sheet with a novel structure that can suppress the occurrence of heat buckling during the manufacturing process and achieve both strength and good appearance after forming.
　本発明者らは、上記目的を達成するために、鋼板の化学組成と金属組織の両方に着目して検討を行った。その結果、本発明者らは、鋼板の化学組成を適正化するとともに、Ｎｂ炭窒化物の形態及び量を適切に制御することで鋼板の高温強度を向上させ、それによって製造過程におけるヒートバックルの発生を抑制することができること、さらには金属組織において所定の割合で含まれるマルテンサイトを金属組織中のミクロな領域とマクロな領域の両方において均一に分散させることで、このような硬質組織に基づいて所望の高強度化を達成するとともに、プレス成形等によってひずみが付与された場合においても、鋼板表面における微小な凹凸の生成が顕著に抑制されることを見出し、本発明を完成させた。 In order to achieve the above object, the inventors conducted research focusing on both the chemical composition and metal structure of the steel sheet. As a result, the inventors discovered that by optimizing the chemical composition of the steel sheet and appropriately controlling the form and amount of Nb carbonitride, the high-temperature strength of the steel sheet can be improved, thereby suppressing the occurrence of heat buckling during the manufacturing process, and further discovered that by uniformly dispersing martensite contained in a predetermined ratio in the metal structure in both the micro- and macro-regions in the metal structure, the desired high strength can be achieved based on such a hard structure, and even when strain is applied by press forming or the like, the generation of minute irregularities on the steel sheet surface can be significantly suppressed, thus completing the present invention.
　上記目的を達成し得た本発明は下記のとおりである。
　（１）質量％で、
　Ｃ　：０．０３０～０．１００％、
　Ｍｎ：０．７０～３．００％、
　Ｓｉ：０．００５～１．５００％、
　Ｐ　：０．１００％以下、
　Ｓ　：０．０２００％以下、
　Ａｌ：１．０００％以下、
　Ｎ　：０．００１０～０．０１５０％、
　Ｏ　：０．０１００％以下、
　Ｎｂ：０．００５～０．２００％、
　Ｃｒ：０～１．００％、
　Ｍｏ：０～０．８０％、
　Ｂ　：０～０．０１００％、
　Ｔｉ：０～０．２００％、
　Ｖ　：０～０．５００％、
　Ｎｉ：０～１．００％、
　Ｃｕ：０～１．００％、
　Ｗ　：０～１．００％、
　Ｔａ：０～０．１０％、
　Ｃｏ：０～３．００％、
　Ｓｎ：０～１．００％、
　Ｓｂ：０～０．２００％、
　Ｃａ：０～０．０１００％、
　Ｍｇ：０～０．０１００％、
　Ｚｒ：０～０．０１００％、
　ＲＥＭ：０～０．０１００％、
　Ｂｉ：０～０．０５００％、
　Ａｓ：０～０．１０％、並びに
　残部：Ｆｅ及び不純物からなり、下記式１で表される指数Ａが０．５０％以上である化学組成を有し、
　面積％で、
　フェライト：７５～９５％、
　マルテンサイト：５～２５％、並びに
　残部組織：合計で０～１０％からなり、
　全Ｎｂ炭窒化物中のＮｂ量が０．００４％以上であり、かつ粒径が２０ｎｍ以上のＮｂ炭窒化物中のＮｂ量が前記全Ｎｂ炭窒化物中のＮｂ量の６０％以上であり、
　前記マルテンサイトの平均粒子間隔が２．５μｍ以下であり、
　圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差が１．５％以下である金属組織を有することを特徴とする、鋼板。
　Ａ＝［Ｃ］－０．１［Ｓｉ］＋０．３（［Ｍｎ］－０．５）－０．３［Ａｌ］＋０．１［Ｃｒ］＋０．６［Ｍｏ］－［Ｔｉ］＋１５［Ｎｂ］　　　・・・式１
　ここで、［Ｃ］、［Ｓｉ］、［Ｍｎ］、［Ａｌ］、［Ｃｒ］、［Ｍｏ］、［Ｔｉ］及び［Ｎｂ］は、各元素の含有量［質量％］であり、元素を含有しない場合は０％である。
　（２）前記化学組成が、質量％で、
　Ｃｒ：０．００１～１．００％、
　Ｍｏ：０．００１～０．８０％、
　Ｂ　：０．０００１～０．０１００％、
　Ｔｉ：０．００１～０．２００％、
　Ｖ　：０．００１～０．５００％、
　Ｎｉ：０．００１～１．００％、
　Ｃｕ：０．００１～１．００％、
　Ｗ　：０．００１～１．００％、
　Ｔａ：０．００１～０．１０％、
　Ｃｏ：０．００１～３．００％、
　Ｓｎ：０．００１～１．００％、
　Ｓｂ：０．００１～０．２００％、
　Ｃａ：０．０００１～０．０１００％、
　Ｍｇ：０．０００１～０．０１００％、
　Ｚｒ：０．０００１～０．０１００％、
　ＲＥＭ：０．０００１～０．０１００％、
　Ｂｉ：０．０００１～０．０５００％、及び
　Ａｓ：０．００１～０．１０％
のうち少なくとも１種を含むことを特徴とする、上記（１）に記載の鋼板。
　（３）前記フェライトの平均結晶粒径が３．０～２５．０μｍであり、前記マルテンサイトの平均結晶粒径が１．０～５．０μｍであり、前記マルテンサイトの平均アスペクト比が２．５以上であることを特徴とする、上記（１）又は（２）に記載の鋼板。
　（４）上記（１）～（３）のいずれか１項に記載の鋼板を含む外板部材。 The present invention which has achieved the above object is as follows.
(1) In mass%,
C: 0.030-0.100%,
Mn: 0.70-3.00%,
Si: 0.005-1.500%,
P: 0.100% or less,
S: 0.0200% or less,
Al: 1.000% or less,
N: 0.0010-0.0150%,
O: 0.0100% or less,
Nb: 0.005-0.200%,
Cr: 0-1.00%,
Mo: 0 to 0.80%,
B: 0 to 0.0100%,
Ti: 0-0.200%,
V: 0 to 0.500%,
Ni: 0 to 1.00%,
Cu: 0 to 1.00%,
W: 0 to 1.00%,
Ta: 0 to 0.10%,
Co: 0-3.00%,
Sn: 0-1.00%,
Sb: 0 to 0.200%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%,
REM: 0-0.0100%,
Bi: 0 to 0.0500%,
As: 0 to 0.10%, and the balance: Fe and impurities, and has a chemical composition in which index A represented by the following formula 1 is 0.50% or more,
In area %,
Ferrite: 75-95%,
Martensite: 5-25%; and Remainder: 0-10% in total.
The Nb content in all Nb carbonitrides is 0.004% or more, and the Nb content in Nb carbonitrides having a particle size of 20 nm or more is 60% or more of the Nb content in all Nb carbonitrides,
The average grain spacing of the martensite is 2.5 μm or less,
A steel sheet having a metal structure in which the standard deviation in the area ratio of martensite in a direction perpendicular to the rolling direction and the sheet thickness direction is 1.5% or less.
A=[C]-0.1[Si]+0.3([Mn]-0.5)-0.3[Al]+0.1[Cr]+0.6[Mo]-[Ti]+15[Nb]...Formula 1
Here, [C], [Si], [Mn], [Al], [Cr], [Mo], [Ti] and [Nb] are the contents [mass %] of each element, and are 0% when the element is not contained.
(2) The chemical composition is, in mass%,
Cr: 0.001-1.00%,
Mo: 0.001-0.80%,
B: 0.0001 to 0.0100%,
Ti: 0.001 to 0.200%,
V: 0.001-0.500%,
Ni: 0.001 to 1.00%,
Cu: 0.001 to 1.00%,
W: 0.001-1.00%,
Ta: 0.001 to 0.10%,
Co: 0.001 to 3.00%,
Sn: 0.001 to 1.00%,
Sb: 0.001-0.200%,
Ca: 0.0001-0.0100%,
Mg: 0.0001-0.0100%,
Zr: 0.0001 to 0.0100%,
REM: 0.0001-0.0100%,
Bi: 0.0001 to 0.0500%, and As: 0.001 to 0.10%
The steel sheet according to the above (1), characterized in that it contains at least one of the following:
(3) The steel sheet according to (1) or (2) above, characterized in that the ferrite has an average crystal grain size of 3.0 to 25.0 μm, the martensite has an average crystal grain size of 1.0 to 5.0 μm, and the martensite has an average aspect ratio of 2.5 or more.
(4) An exterior panel member comprising the steel plate according to any one of (1) to (3) above.
　本発明によれば、製造過程におけるヒートバックルの発生を抑制することができるとともに、強度と成形後の外観性とを両立することができる鋼板を提供することができる。 The present invention provides a steel sheet that can suppress the occurrence of heat buckling during the manufacturing process and can achieve both strength and good appearance after forming.
＜鋼板＞
　本発明の実施形態に係る鋼板は、質量％で、
　Ｃ　：０．０３０～０．１００％、
　Ｍｎ：０．７０～３．００％、
　Ｓｉ：０．００５～１．５００％、
　Ｐ　：０．１００％以下、
　Ｓ　：０．０２００％以下、
　Ａｌ：１．０００％以下、
　Ｎ　：０．００１０～０．０１５０％、
　Ｏ　：０．０１００％以下、
　Ｎｂ：０．００５～０．２００％、
　Ｃｒ：０～１．００％、
　Ｍｏ：０～０．８０％、
　Ｂ　：０～０．０１００％、
　Ｔｉ：０～０．２００％、
　Ｖ　：０～０．５００％、
　Ｎｉ：０～１．００％、
　Ｃｕ：０～１．００％、
　Ｗ　：０～１．００％、
　Ｔａ：０～０．１０％、
　Ｃｏ：０～３．００％、
　Ｓｎ：０～１．００％、
　Ｓｂ：０～０．２００％、
　Ｃａ：０～０．０１００％、
　Ｍｇ：０～０．０１００％、
　Ｚｒ：０～０．０１００％、
　ＲＥＭ：０～０．０１００％、
　Ｂｉ：０～０．０５００％、
　Ａｓ：０～０．１０％、並びに
　残部：Ｆｅ及び不純物からなり、下記式１で表される指数Ａが０．５０％以上である化学組成を有し、
　面積％で、
　フェライト：７５～９５％、
　マルテンサイト：５～２５％、並びに
　残部組織：合計で０～１０％からなり、
　全Ｎｂ炭窒化物中のＮｂ量が０．００４％以上であり、かつ粒径が２０ｎｍ以上のＮｂ炭窒化物中のＮｂ量が前記全Ｎｂ炭窒化物中のＮｂ量の６０％以上であり、
　前記マルテンサイトの平均粒子間隔が２．５μｍ以下であり、
　圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差が１．５％以下である金属組織を有することを特徴としている。
　Ａ＝［Ｃ］－０．１［Ｓｉ］＋０．３（［Ｍｎ］－０．５）－０．３［Ａｌ］＋０．１［Ｃｒ］＋０．６［Ｍｏ］－［Ｔｉ］＋１５［Ｎｂ］　　　・・・式１
　ここで、［Ｃ］、［Ｓｉ］、［Ｍｎ］、［Ａｌ］、［Ｃｒ］、［Ｍｏ］、［Ｔｉ］及び［Ｎｂ］は、各元素の含有量［質量％］であり、元素を含有しない場合は０％である。 <Steel Plate>
The steel plate according to the embodiment of the present invention has, in mass%,
C: 0.030-0.100%,
Mn: 0.70-3.00%,
Si: 0.005-1.500%,
P: 0.100% or less,
S: 0.0200% or less,
Al: 1.000% or less,
N: 0.0010-0.0150%,
O: 0.0100% or less,
Nb: 0.005-0.200%,
Cr: 0-1.00%,
Mo: 0 to 0.80%,
B: 0 to 0.0100%,
Ti: 0-0.200%,
V: 0 to 0.500%,
Ni: 0 to 1.00%,
Cu: 0 to 1.00%,
W: 0 to 1.00%,
Ta: 0 to 0.10%,
Co: 0-3.00%,
Sn: 0 to 1.00%,
Sb: 0 to 0.200%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%,
REM: 0-0.0100%,
Bi: 0 to 0.0500%,
As: 0 to 0.10%, and the balance: Fe and impurities, and has a chemical composition in which index A represented by the following formula 1 is 0.50% or more,
In area %,
Ferrite: 75-95%,
Martensite: 5-25%; and Remainder: 0-10% in total.
The Nb content in all Nb carbonitrides is 0.004% or more, and the Nb content in Nb carbonitrides having a particle size of 20 nm or more is 60% or more of the Nb content in all Nb carbonitrides,
The average grain spacing of the martensite is 2.5 μm or less,
The steel sheet is characterized by having a metal structure in which the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is 1.5% or less.
A=[C]-0.1[Si]+0.3([Mn]-0.5)-0.3[Al]+0.1[Cr]+0.6[Mo]-[Ti]+15[Nb]...Formula 1
Here, [C], [Si], [Mn], [Al], [Cr], [Mo], [Ti] and [Nb] are the contents [mass %] of each element, and are 0% when the element is not contained.
　近年、自動車の外板部材（ルーフ、フード、フェンダー、ドア等）についても軽量化のニーズが高まっており、それゆえ骨格部材の場合と同様に、これらの外板部材においても高強度化及び薄肉化の要求がある。一方で、これらの外板部材においては、プレス成形等の際に生じる面ひずみと呼ばれる面欠陥を回避する観点から、降伏強度が比較的低い複合組織鋼（ＤＰ鋼）が用いられる場合が多い。しかしながら、フェライトからなる軟質組織とマルテンサイトからなる硬質組織が混在するＤＰ鋼の場合、プレス成形などの加工時に軟質組織及びその周辺が優先的に変形する不均一変形が起こりやすく、成形後の鋼板表面に微小な凹凸が生じることで、ゴーストラインと呼ばれる外観不良が発生することがある。より詳しく説明すると、プレス成形などの加工時には、フェライトからなる軟質組織は変形量が大きく、鋼板の表面において凹む。一方で、マルテンサイトからなる硬質組織は変形量が小さい。それゆえ硬質組織は軟質組織と比較して、鋼板の表面において凹まず、凸となるように盛り上がる。その結果、特に鋼板の幅方向（圧延方向及び板厚方向に直交する方向）において変形量のばらつきが発生してゴーストラインがバンド状（縞状）に生じる。一方で、鋼板の高強度化に伴い、鋼板の焼入れ性を改善するためにＭｎ等の元素が比較的多く添加される場合がある。Ｍｎは鋼板中で筋状に偏析しやすい元素であり、より詳しくは鋳造時に中心偏析やミクロ偏析といったＭｎ濃化領域が形成され、熱間圧延や冷間圧延によって当該濃化領域が圧延方向に延ばされることでＭｎは筋状に偏析する。このため、このようなＭｎの偏析に起因して、鋼板中に焼入れ性が高い領域と低い領域が存在することとなる。その結果として、焼入れ後の鋼板の金属組織において縞状の硬質組織が比較的多く生成する。この場合には、ゴーストラインの発生が特に顕著となる。これに対し、仮に鋼板中のＭｎ偏析を十分に抑制することができれば、このような縞状の硬質組織の生成を低減して当該硬質組織を金属組織中により均一に分散させることが可能となる。この場合には、プレス成形等によってひずみが付与された場合においても、鋼板表面における微小な凹凸の生成を十分に低減することができ、ゴーストラインの発生を抑制することが可能になると考えられる。しかしながら、高強度化の要求に伴い、特に鋼板中のＭｎ添加量が多くなる場合には、実際のところ、Ｍｎ偏析を確実かつ十分に抑制することは非常に困難である。このため、強度と成形後の外観性とを両立することは一般にかなり難しい。 In recent years, there has been an increasing need to reduce the weight of exterior panel components (roofs, hoods, fenders, doors, etc.) of automobiles, and therefore, like the case of frame components, there is a demand for high strength and thinning of these exterior panel components. On the other hand, in order to avoid surface defects called surface distortions that occur during press forming, etc., dual phase steel (DP steel) with a relatively low yield strength is often used for these exterior panel components. However, in the case of DP steel, which is a mixture of soft structure made of ferrite and hard structure made of martensite, uneven deformation is likely to occur during processing such as press forming, in which the soft structure and its surroundings are preferentially deformed, and fine irregularities are generated on the surface of the steel sheet after forming, which can cause appearance defects called ghost lines. To explain in more detail, during processing such as press forming, the soft structure made of ferrite deforms greatly and is recessed on the surface of the steel sheet. On the other hand, the hard structure made of martensite is small in deformation. Therefore, compared to the soft structure, the hard structure does not recess on the surface of the steel sheet, but rises to be convex. As a result, the deformation amount varies especially in the width direction of the steel plate (the direction perpendicular to the rolling direction and the plate thickness direction), and ghost lines are generated in a band shape (striped shape). On the other hand, as the strength of the steel plate increases, elements such as Mn may be added in relatively large amounts to improve the hardenability of the steel plate. Mn is an element that is likely to segregate in a streaky manner in the steel plate. More specifically, Mn-enriched regions such as central segregation and microsegregation are formed during casting, and the enriched regions are elongated in the rolling direction by hot rolling or cold rolling, so that Mn segregates in a streaky manner. For this reason, due to such Mn segregation, there are regions in the steel plate with high and low hardenability. As a result, a relatively large amount of striped hard structure is generated in the metal structure of the steel plate after quenching. In this case, the occurrence of ghost lines is particularly noticeable. On the other hand, if the Mn segregation in the steel plate can be sufficiently suppressed, it is possible to reduce the generation of such striped hard structure and to disperse the hard structure more uniformly in the metal structure. In this case, even when strain is applied by press forming or the like, it is possible to sufficiently reduce the generation of minute irregularities on the steel sheet surface, and it is believed that it is possible to suppress the occurrence of ghost lines. However, in reality, when the amount of Mn added to the steel sheet increases in response to the demand for higher strength, it is very difficult to reliably and sufficiently suppress Mn segregation. For this reason, it is generally quite difficult to achieve both strength and good appearance after forming.
　また、先に述べたとおり、自動車の外板用途では、薄手広幅の鋼材が求められる場合が比較的多いが、このような薄手広幅の鋼材は、製造過程において、例えば、連続焼鈍処理設備（ＣＡＰＬ）に通板させる際に、ヒートバックルと呼ばれる腰折れが起こりやすいという問題がある。より詳しく説明すると、ＣＡＰＬにおいては、一般に中央部が凸形状となるクラウンを設けたハースロールが配設されている。このため、鋼板をＣＡＰＬに通板すると、ハースロールの凸クラウンに起因して鋼板に幅方向中央部への圧縮応力が加わることとなる。一方で、連続焼鈍は比較的高温下で行われるため、板温上昇に伴い、鋼板の降伏応力が低下する。したがって、特に薄手広幅の鋼板をＣＡＰＬにおいて連続焼鈍した場合には、比較的高温下での降伏応力の低下により、鋼板が上記圧縮応力に十分に抵抗できなくなることがあり、このような場合には、板が折れてしわが発生するヒートバックルと呼ばれる現象が生じてしまう。ヒートバックルが発生したまま操業を続けると、板破断などに至る場合があり、板破断などが発生してしまうと、製造ラインを停止して復旧作業を行う必要が生じ、その損害は甚大なものとなる。 As mentioned above, thin and wide steel materials are often required for automotive exterior panel applications. However, such thin and wide steel materials have a problem in that they are prone to bending, called heat buckling, during the manufacturing process, for example, when passing through a continuous annealing processing line (CAPL). To explain in more detail, a CAPL generally has a hearth roll with a crown that has a convex shape in the center. Therefore, when a steel sheet passes through a CAPL, compressive stress is applied to the steel sheet in the width direction center due to the convex crown of the hearth roll. On the other hand, since continuous annealing is performed at a relatively high temperature, the yield stress of the steel sheet decreases as the sheet temperature increases. Therefore, particularly when a thin and wide steel sheet is continuously annealed in a CAPL, the steel sheet may not be able to fully resist the compressive stress due to the decrease in yield stress at a relatively high temperature, and in such a case, a phenomenon called heat buckling occurs in which the sheet breaks and wrinkles occur. Continuing operations while heat buckling is occurring can lead to plate breakage, and if plate breakage occurs, the production line must be stopped and recovery work carried out, resulting in significant damage.
　そこで、まず、本発明者らは、このようなヒートバックルの発生を抑制又は低減すべく、鋼板の化学組成と金属組織の両方の観点から鋼板の高温強度を向上させることについて検討を行った。その結果、本発明者らは、鋼板の化学組成の観点からは、下記式１で表される指数Ａを０．５０％以上に制御することにより、鋼板の高温強度を向上させることができ、それによってヒートバックルの発生を抑制又は低減することができることを見出した。
　Ａ＝［Ｃ］－０．１［Ｓｉ］＋０．３（［Ｍｎ］－０．５）－０．３［Ａｌ］＋０．１［Ｃｒ］＋０．６［Ｍｏ］－［Ｔｉ］＋１５［Ｎｂ］　　　・・・式１
　ここで、［Ｃ］、［Ｓｉ］、［Ｍｎ］、［Ａｌ］、［Ｃｒ］、［Ｍｏ］、［Ｔｉ］及び［Ｎｂ］は、各元素の含有量［質量％］であり、元素を含有しない場合は０％である。 Therefore, the present inventors first conducted research into improving the high-temperature strength of a steel sheet from the viewpoints of both the chemical composition and metal structure of the steel sheet in order to suppress or reduce the occurrence of such heat buckling. As a result, the present inventors found that, from the viewpoint of the chemical composition of the steel sheet, the high-temperature strength of the steel sheet can be improved by controlling the index A represented by the following formula 1 to 0.50% or more, and thus the occurrence of heat buckling can be suppressed or reduced.
A=[C]-0.1[Si]+0.3([Mn]-0.5)-0.3[Al]+0.1[Cr]+0.6[Mo]-[Ti]+15[Nb]...Formula 1
Here, [C], [Si], [Mn], [Al], [Cr], [Mo], [Ti] and [Nb] are the contents [mass %] of each element, and are 0% when the element is not contained.
　何ら特定の理論に束縛されることを意図するものではないが、指数Ａを０．５０％以上に制御することで、鋼板の高温強度を向上させることができるだけでなく、鋼板のＡｃ３点をも低下させることができると考えられる。鋼板の製造方法に関連して後で詳しく説明されるように、本発明の実施形態では、冷間圧延後のＣＡＰＬに対応する１次熱処理工程において鋼板をオーステナイト単相となるＡｃ３点よりも高い温度、より具体的にはＡｃ３＋１０℃以上に加熱する必要がある。このため、Ａｃ３点を低下させることで１次熱処理工程の加熱温度を低くすることができ、これに関連して加熱時の板温上昇に伴う鋼板の降伏応力における低下を抑制することが可能となる。したがって、指数Ａを０．５０％以上に制御することにより、鋼板自体の高温強度の積極的な向上と、１次熱処理工程における加熱温度の低下に起因する降伏応力の低下抑制とに基づいて、ヒートバックルを生じさせる上記圧縮応力に対する鋼板の抵抗力を顕著に向上させることが可能となる。 Without intending to be bound by any particular theory, it is believed that by controlling the index A to 0.50% or more, not only can the high-temperature strength of the steel sheet be improved, but also the Ac3 point of the steel sheet can be lowered. As will be described in detail later in relation to the manufacturing method of the steel sheet, in the embodiment of the present invention, in the first heat treatment process corresponding to the CAPL after cold rolling, the steel sheet needs to be heated to a temperature higher than the Ac3 point at which the steel sheet becomes austenite single phase, more specifically, to Ac3+10°C or higher. Therefore, by lowering the Ac3 point, the heating temperature in the first heat treatment process can be lowered, and in connection with this, it is possible to suppress the decrease in the yield stress of the steel sheet due to the increase in the sheet temperature during heating. Therefore, by controlling the index A to 0.50% or more, it is possible to significantly improve the resistance of the steel sheet to the above-mentioned compressive stress that causes heat buckling, based on the active improvement of the high-temperature strength of the steel sheet itself and the suppression of the decrease in yield stress due to the decrease in the heating temperature in the first heat treatment process.
　さらに、本発明者らは、鋼板の金属組織の観点から、鋼板中に適度な大きさを有するＮｂ炭窒化物が比較的多く存在するように当該Ｎｂ炭窒化物の形態及び量を制御すること、より具体的には全Ｎｂ炭窒化物中のＮｂ量が０．００４％以上であり、かつ粒径が２０ｎｍ以上のＮｂ炭窒化物中のＮｂ量が全Ｎｂ炭窒化物中のＮｂ量の６０％以上となるようにＮｂ炭窒化物の形態及び量を制御することで、鋼板の高温強度を向上させることができることを見出した。Ｎｂ炭窒化物の形態及び量をこのような範囲内に制御することで、高温強度の向上に有効な粒径２０ｎｍ以上のＮｂ炭窒化物を十分な量において鋼中に存在させることができるので、上で説明した指数Ａによる化学組成の制御との組み合わせにより、鋼板の高温強度を顕著に向上させることができ、その結果として鋼板の製造過程におけるヒートバックルの発生を顕著に抑制又は低減することが可能となる。 Furthermore, the inventors have found that, from the viewpoint of the metal structure of the steel sheet, the high-temperature strength of the steel sheet can be improved by controlling the form and amount of Nb carbonitrides so that a relatively large amount of Nb carbonitrides having an appropriate size are present in the steel sheet, more specifically, by controlling the form and amount of Nb carbonitrides so that the Nb content in all Nb carbonitrides is 0.004% or more and the Nb content in Nb carbonitrides having a particle size of 20 nm or more is 60% or more of the Nb content in all Nb carbonitrides. By controlling the form and amount of Nb carbonitrides within such a range, it is possible to cause a sufficient amount of Nb carbonitrides having a particle size of 20 nm or more, which is effective in improving high-temperature strength, to be present in the steel. Therefore, in combination with the control of the chemical composition by the index A described above, the high-temperature strength of the steel sheet can be significantly improved, and as a result, it becomes possible to significantly suppress or reduce the occurrence of heat buckling during the manufacturing process of the steel sheet.
　次に、本発明者らは、強度と成形後の外観性との両立を図るべく、金属組織中の軟質組織であるフェライトと硬質組織であるマルテンサイトの割合を適正化することで所望の高強度化を実現する一方で、さらに成形後の外観性を改善する手段について検討を行った。具体的には、本発明者らは、金属組織中の硬質組織であるマルテンサイトの分布状態に着目し、より詳しくはマルテンサイトの分布をＭｎ偏析の低減とは異なる別の観点から制御することについて検討を行った。その結果として、鋼板の製造方法について後で詳しく説明されるように、本発明者らは、最終焼鈍前の鋼板中の金属組織をベイナイト及び／又はマルテンサイトを主体とする組織によって構成し、次いでこのような金属組織を有する鋼板を所定の条件下で最終焼鈍することにより、Ｍｎ偏析の有無や程度に必ずしも依存することなく、最終的に得られる金属組織中でマルテンサイトをミクロな領域とマクロな領域の両方において均一に分散させることができることを見出した。より具体的には、本発明者らは、ベイナイト及び／又はマルテンサイトからなる金属組織を有する鋼板を所定の条件下で最終焼鈍することにより、ミクロな領域ではマルテンサイトの平均粒子間隔を２．５μｍ以下に制御することができ、マクロな領域では圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差を１．５％以下に制御することができることを見出した。マルテンサイトの平均粒子間隔を２．５μｍ以下に制御することでミクロな領域において硬質組織を密にかつ均一に分散させることができる。圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差を１．５％以下に制御することで、マクロな領域における硬質組織のばらつきを顕著に低減することができる。これら両方の要件を満足することで、硬質組織であるマルテンサイトが鋼板全体において微細かつ均一に分散した金属組織を形成することができる。その結果として、本発明の実施形態に係る鋼板によれば、プレス成形等の成形時においても鋼板の変形量をとりわけ幅方向においてより均一にすることができ、ゴーストライン等の外観不良が顕著に抑制された優れた成形後外観を達成することが可能となる。例えば、ミクロな領域でのマルテンサイトの均一性が確保されていても、マクロな領域でのマルテンサイトの均一性が確保されていなければ、マルテンサイトが鋼板全体において微細かつ均一に分散した金属組織を形成することはできない。同様に、マクロな領域でのマルテンサイトの均一性が確保されていても、ミクロな領域でのマルテンサイトの均一性が確保されていなければ、局所的にはマルテンサイトが不均一に存在し得ることになるため、マルテンサイトが鋼板全体において微細かつ均一に分散した金属組織を形成することはできない。したがって、本発明の実施形態に係る鋼板において、ゴーストライン等の外観不良が顕著に抑制された優れた成形後外観を達成するためには、マルテンサイトの平均粒子間隔を２．５μｍ以下に制御することと、圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差を１．５％以下に制御することの両方の要件を満足することが必要となる。 Next, in order to achieve both strength and appearance after forming, the inventors have investigated means for achieving the desired high strength by optimizing the ratio of ferrite, which is a soft structure, and martensite, which is a hard structure, in the metal structure, while further improving the appearance after forming. Specifically, the inventors have focused on the distribution state of martensite, which is a hard structure in the metal structure, and more specifically, have investigated controlling the distribution of martensite from a viewpoint different from that of reducing Mn segregation. As a result, as will be described in detail later with respect to the manufacturing method of steel sheet, the inventors have found that by forming the metal structure in the steel sheet before final annealing with a structure mainly composed of bainite and/or martensite, and then final annealing the steel sheet having such a metal structure under specified conditions, it is possible to uniformly disperse martensite in both the micro-region and the macro-region in the finally obtained metal structure, without necessarily depending on the presence or absence or the degree of Mn segregation. More specifically, the inventors have found that by subjecting a steel sheet having a metal structure consisting of bainite and/or martensite to final annealing under predetermined conditions, the average grain spacing of martensite can be controlled to 2.5 μm or less in the micro region, and the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction can be controlled to 1.5% or less in the macro region. By controlling the average grain spacing of martensite to 2.5 μm or less, the hard structure can be densely and uniformly dispersed in the micro region. By controlling the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction to 1.5% or less, the variation of the hard structure in the macro region can be significantly reduced. By satisfying both of these requirements, a metal structure in which martensite, which is a hard structure, is finely and uniformly dispersed throughout the steel sheet can be formed. As a result, according to the steel sheet according to the embodiment of the present invention, the deformation amount of the steel sheet can be made more uniform, especially in the width direction, even during forming such as press forming, and it is possible to achieve an excellent post-forming appearance in which appearance defects such as ghost lines are significantly suppressed. For example, even if the uniformity of martensite is ensured in the microscopic region, if the uniformity of martensite is not ensured in the macroscopic region, it is not possible to form a metal structure in which martensite is finely and uniformly distributed throughout the entire steel sheet. Similarly, even if the uniformity of martensite is ensured in the macroscopic region, if the uniformity of martensite is not ensured in the microscopic region, martensite may exist unevenly locally, and therefore it is not possible to form a metal structure in which martensite is finely and uniformly distributed throughout the entire steel sheet. Therefore, in order to achieve an excellent post-forming appearance in which appearance defects such as ghost lines are significantly suppressed in the steel sheet according to the embodiment of the present invention, it is necessary to satisfy both requirements: the average particle spacing of martensite is controlled to 2.5 μm or less, and the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to 1.5% or less.
　何ら特定の理論に束縛されることを意図するものではないが、最終的に得られる鋼板の金属組織においてマルテンサイトを鋼板全体で微細かつ均一に分散させるためには、最終焼鈍における加熱時に多数のオーステナイト核生成サイトを高分散に形成しておくことが極めて重要であると考えられる。これに関連して、マルテンサイト組織は、旧オーステナイト粒の中にさらにパケット、ブロック、ラス等の下部組織を有しており、それゆえフェライト等の組織と比較して内部に多くの様々な界面を有している組織である。ベイナイトもマルテンサイトの場合と同様に内部に多くの様々な界面を有している組織である。したがって、最終焼鈍前の鋼板における金属組織をベイナイト及び／又はマルテンサイトによって構成することで、このような金属組織を最終焼鈍において加熱していく段階でこれらの界面上にオーステナイトの核生成サイトとなり得る炭化物を非常に多く分散して生成させることが可能となる。したがって、界面上に多くの炭化物を生成した後、さらに温度をフェライトとオーステナイトの２相域まで加熱することで、鋼板全体にオーステナイトを微細かつ均一に生成させることが可能になると考えられる。最後に、このような金属組織を有する鋼板を急冷することで、これらのオーステナイトからマルテンサイトが生成するため、最終的に得られる金属組織において、マルテンサイトの平均粒子間隔が２．５μｍ以下に制御されるとともに、圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差が１．５％以下に制御される。すなわちマルテンサイトがミクロな領域とマクロな領域の両方において均一に分散した金属組織を得ることができるものと考えられる。このような熱処理を施すことでＭｎ偏析の影響を打ち消すほどにマルテンサイトを鋼板全体にわたって微細かつ均一に分散させることが可能になると考えられる。従来、Ｍｎ偏析自体を低減するという観点から硬質組織の分布制御を検討するのが一般的と考えられることから、Ｍｎ偏析の有無や程度に必ずしも依存することなく、最終的に得られる金属組織中でマルテンサイトをミクロな領域とマクロな領域の両方において均一に分散させることができるという事実は極めて意外であり、また驚くべきことである。 Without intending to be bound by any particular theory, it is believed that in order to disperse martensite finely and uniformly throughout the entire steel sheet in the metal structure of the finally obtained steel sheet, it is extremely important to form a large number of austenite nucleation sites in a highly dispersed manner during heating in the final annealing. In this regard, the martensite structure has substructures such as packets, blocks, and laths in the prior austenite grains, and therefore has many different interfaces inside compared to structures such as ferrite. Bainite is also a structure that has many different interfaces inside, similar to the case of martensite. Therefore, by forming the metal structure of the steel sheet before the final annealing with bainite and/or martensite, it is possible to generate a very large number of dispersed carbides that can become austenite nucleation sites on these interfaces during the heating stage of such a metal structure in the final annealing. Therefore, after generating many carbides on the interfaces, it is believed that by further heating the temperature to the two-phase region of ferrite and austenite, it is possible to generate austenite finely and uniformly throughout the steel sheet. Finally, by quenching the steel sheet having such a metal structure, martensite is generated from the austenite, and in the finally obtained metal structure, the average particle spacing of martensite is controlled to 2.5 μm or less, and the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is controlled to 1.5% or less. In other words, it is believed that a metal structure in which martensite is uniformly dispersed in both micro- and macro-regions can be obtained. It is believed that by carrying out such heat treatment, it is possible to disperse martensite finely and uniformly throughout the steel sheet to such an extent that the effect of Mn segregation is countered. Conventionally, it has been considered common to consider distribution control of hard structures from the viewpoint of reducing Mn segregation itself, so the fact that martensite can be uniformly dispersed in both micro- and macro-regions in the finally obtained metal structure, regardless of the presence or absence or degree of Mn segregation, is extremely unexpected and surprising.
　本発明の実施形態に係る鋼板によれば、ヒートバックル及びゴーストラインの発生を抑制することに関連する上記知見に加えて、軟質組織であるフェライトの面積率を７５～９５％に制御することで良好な成形性を確保するとともに、硬質組織であるマルテンサイトの面積率を５～２５％に制御し、さらに鋼板の化学組成を所定の範囲内に制御することで引張強さが５４０ＭＰａ以上の高強度を確保することができる。その結果として、製造過程におけるヒートバックルの発生を抑制することができるとともに、強度と成形後の外観性との両立を高いレベルで実現することが可能となる。 In addition to the above findings related to suppressing the occurrence of heat buckling and ghost lines, the steel plate according to the embodiment of the present invention ensures good formability by controlling the area ratio of ferrite, a soft structure, to 75-95%, while controlling the area ratio of martensite, a hard structure, to 5-25% and further controlling the chemical composition of the steel plate within a specified range to ensure high strength with a tensile strength of 540 MPa or more. As a result, it is possible to suppress the occurrence of heat buckling during the manufacturing process and to achieve a high level of both strength and appearance after forming.
　以下、本発明の実施形態に係る鋼板についてより詳しく説明する。以下の説明において、各元素の含有量の単位である「％」は、特に断りがない限り「質量％」を意味するものである。また、本明細書において、数値範囲を示す「～」とは、特に断りがない場合、その前後に記載される数値を下限値および上限値として含む意味で使用される。 Below, the steel sheet according to the embodiment of the present invention will be described in more detail. In the following description, the unit of content of each element, "%", means "mass %" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values before and after it are included as the lower and upper limits, unless otherwise specified.
［Ｃ：０．０３０～０．１００％］
　Ｃは、所定量のマルテンサイトを確保し、鋼板の強度を向上させる元素である。また、Ｃは、オーステナイト安定化元素でもあり、Ａｃ３点を低下させるのに有効である。これらの効果を十分に得るために、Ｃ含有量は０．０３０％以上とする。Ｃ含有量は０．０４０％以上又は０．０５０％以上であってもよい。一方、Ｃを過度に含有すると、強度が高くなりすぎてしまい、伸び性が低下する場合があるか、又は圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差を所望の範囲内に制御できない場合がある。このため、Ｃ含有量は０．１００％以下とする。Ｃ含有量は０．０９０％以下、０．０８０％以下、０．０７０％以下又は０．０６０％以下であってもよい。 [C:0.030-0.100%]
C is an element that secures a certain amount of martensite and improves the strength of the steel sheet. C is also an austenite stabilizing element and is effective in lowering the Ac3 point. In order to obtain this, the C content is set to 0.030% or more. The C content may be 0.040% or more or 0.050% or more. On the other hand, if C is contained excessively, the strength becomes high and This may result in excessive deformation, which may reduce the elongation, or may make it impossible to control the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction within a desired range. The C content is set to 0.100% or less. The C content may be 0.090% or less, 0.080% or less, 0.070% or less, or 0.060% or less.
［Ｍｎ：０．７０～３．００％］
　Ｍｎは、焼入れ性を高め、鋼板強度の向上に寄与する元素である。また、Ｍｎは、オーステナイト安定化元素でもあり、Ａｃ３点を低下させるのに有効である。これらの効果を十分に得るために、Ｍｎ含有量は０．７０％以上とする。Ｍｎ含有量は０．８０％以上、１．００％以上、１．２０％以上又は１．５０％以上であってもよい。後で説明する鋼板の好ましい製造方法では、最終的に得られる金属組織中でマルテンサイトをミクロな領域とマクロな領域の両方において均一に分散させるために、最終焼鈍前の鋼板中の金属組織をベイナイト及び／又はマルテンサイトを主体とする組織によって構成する必要がある。このため、Ｍｎ添加による焼入れ性の向上は成形後の外観性を改善する上でも重要といえる。一方、Ｍｎを過度に含有すると、Ｍｎ偏析による影響を十分に打ち消すことができず、圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差を所望の範囲内に制御できなくなる場合がある。そのため、Ｍｎ含有量は３．００％以下とする。Ｍｎ含有量は２．８０％以下、２．５０％以下、２．２０％以下又は２．００％以下であってもよい。 [Mn: 0.70-3.00%]
Mn is an element that improves hardenability and contributes to improving the strength of the steel sheet. Mn is also an austenite stabilizing element and is effective in lowering the Ac3 point. In order to fully obtain these effects, In addition, the Mn content is 0.70% or more. The Mn content may be 0.80% or more, 1.00% or more, 1.20% or more, or 1.50% or more. In a preferred method for producing the steel sheet, the metal structure of the steel sheet before final annealing is changed to bainite and/or annealed steel in order to uniformly disperse martensite in both the micro- and macro-regions in the final metal structure. For this reason, the improvement of hardenability by adding Mn is important for improving the appearance after forming. On the other hand, if Mn is contained in an excessive amount, Mn The effect of segregation cannot be sufficiently counteracted, and the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction may not be controlled within a desired range. Not more than 3.00%. The Mn content may be 2.80% or less, 2.50% or less, 2.20% or less, or 2.00% or less.
［Ｓｉ：０．００５～１．５００％］
　Ｓｉは、固溶強化により鋼板の強度を向上させる元素である。このような効果を十分に得るために、Ｓｉ含有量は０．００５％以上とする。Ｓｉ含有量は０．０１０％以上、０．１００％以上、０．２００％以上、０．３００％以上又は０．４００％以上であってもよい。一方、Ｓｉを過度に含有すると、熱間圧延で生成したスケール除去が困難となり、外観性の劣化を招く場合がある。したがって、Ｓｉ含有量は１．５００％以下とする。また、Ｓｉはフェライト安定化元素であるため、Ｓｉ含有量を低減することでＡｃ３点を低下させることができる。このため、Ｓｉ含有量は１．２００％以下、１．０００％以下、０．９００％以下、０．８００％以下、０．７００％以下又は０．６００％以下であってもよい。 [Si: 0.005 to 1.500%]
Silicon is an element that improves the strength of steel sheet by solid solution strengthening. In order to fully obtain such an effect, the silicon content is set to 0.005% or more. The silicon content is set to 0.010% or more. The Si content may be 0.100% or more, 0.200% or more, 0.300% or more, or 0.400% or more. On the other hand, if the Si content is excessive, it becomes difficult to remove scale formed during hot rolling, This may cause deterioration of the appearance. Therefore, the Si content is set to 1.500% or less. In addition, since Si is a ferrite stabilizing element, reducing the Si content can lower the Ac3 point. Therefore, the Si content may be 1.200% or less, 1.000% or less, 0.900% or less, 0.800% or less, 0.700% or less, or 0.600% or less. .
［Ｐ：０．１００％以下］
　Ｐは、不純物元素であり、溶接部の脆化やめっき性を劣化させる元素である。このため、Ｐ含有量は０．１００％以下とする。Ｐ含有量は０．０６０％以下、０．０４０％以下、０．０２０％以下又は０．０１０％以下であってもよい。Ｐ含有量は少ないほど好ましく、下限は特に限定されず０％であってもよい。一方、実用鋼板でＰ含有量を０．０００１％未満に低減すると、製造コストが大幅に上昇し、経済的に不利になる。そのため、Ｐ含有量は０．０００１％以上、０．０００２％以上又は０．０００５％以上であってもよい。 [P: 0.100% or less]
P is an impurity element that embrittles welds and deteriorates plating properties. For this reason, the P content is set to 0.100% or less. The lower the P content, the more preferable it is, and the lower limit is not particularly limited and may be 0%. If the P content is reduced to less than 0.0001%, the production cost increases significantly, which is economically disadvantageous. Therefore, the P content is set to 0.0001% or more, 0.0002% or more, or 0.0005% or more. It's fine if there is.
［Ｓ：０．０２００％以下］
　Ｓは、不純物元素であり、溶接性を阻害し、また、鋳造時と熱延時の製造性を阻害する元素である。このため、Ｓ含有量は０．０２００％以下とする。Ｓ含有量は０．０１５０％以下、０．０１２０％以下、０．０１００％以下、０．００６０％以下又は０．００３０％以下であってもよい。Ｓ含有量は少ないほど好ましく、下限は特に限定されず０％であってもよい。一方、実用鋼板でＳ含有量を０．０００１％未満に低減すると、製造コストが大幅に上昇し、経済的に不利になる。そのため、Ｓ含有量は０．０００１％以上、０．０００２％以上又は０．０００５％以上であってもよい。 [S: 0.0200% or less]
S is an impurity element that impairs weldability and also impairs manufacturability during casting and hot rolling. For this reason, the S content is set to 0.0200% or less. The lower the S content, the more preferable it is. The lower limit is not particularly limited, and the upper limit is 0.0150% or less, 0.0120% or less, 0.0100% or less, 0.0060% or less, or 0.0030% or less. On the other hand, if the S content of a practical steel sheet is reduced to less than 0.0001%, the manufacturing cost increases significantly, which is economically disadvantageous. Therefore, the S content is set to 0.0001%. or more, or 0.0002% or more, or 0.0005% or more.
［Ａｌ：１．０００％以下］
　Ａｌは、脱酸剤として機能する元素であり、鋼の強度を高めるのに有効な元素である。Ａｌ含有量は０％であってもよいが、これらの効果を十分に得るためには、Ａｌ含有量は０．００１％以上であることが好ましい。Ａｌ含有量は０．００５％以上、０．０１０％以上、０．０２５％以上又は０．０５０％以上であってもよい。一方、Ａｌを過度に含有すると、粗大な酸化物が形成し、靭性を低下させる場合がある。したがって、Ａｌ含有量は１．０００％以下とする。また、Ａｌはフェライト安定化元素であるため、Ａｌ含有量を低減することでＡｃ３点を低下させることができる。このため、Ａｌ含有量は０．８００％以下、０．６００％以下又は０．３００％以下であってもよい。 [Al: 1.000% or less]
Al is an element that functions as a deoxidizer and is effective in increasing the strength of steel. The Al content may be 0%, but in order to fully obtain these effects, The Al content is preferably 0.001% or more. The Al content may be 0.005% or more, 0.010% or more, 0.025% or more, or 0.050% or more. If Al is contained in excess, coarse oxides may form, which may reduce toughness. Therefore, the Al content is set to 1.000% or less. In addition, since Al is a ferrite stabilizing element, Al By reducing the Al content, the Ac3 point can be lowered. Therefore, the Al content may be 0.800% or less, 0.600% or less, or 0.300% or less.
［Ｎ：０．００１０～０．０１５０％］
　Ｎは、Ｎｂと炭窒化物を形成して鋼板の高温強度を向上させるのに有効な元素である。このような効果を十分に得るために、Ｎ含有量は０．００１０％以上とする。Ｎ含有量は０．００１５％以上、０．００２０％以上、０．００２５％以上又は０．００３０％以上であってもよい。一方、Ｎを過度に含有すると、溶接時のブローホール発生の原因となる場合がある。このため、Ｎ含有量は０．０１５０％以下とする。Ｎ含有量は０．０１２０％以下、０．０１００％以下、０．００８０％以下又は０．００６０％以下であってもよい。 [N: 0.0010-0.0150%]
N is an element effective in improving the high-temperature strength of a steel sheet by forming carbonitrides with Nb. In order to fully obtain such an effect, the N content is set to 0.0010% or more. The N content may be 0.0015% or more, 0.0020% or more, 0.0025% or more, or 0.0030% or more. On the other hand, if N is contained excessively, it may cause blowholes during welding. Therefore, the N content is set to 0.0150% or less. The N content is set to 0.0120% or less, 0.0100% or less, 0.0080% or less, or 0.0060% or less. This is also fine.
［Ｏ：０．０１００％以下］
　Ｏは、溶接時のブローホールの発生原因となる元素である。このため、Ｏ含有量は０．０１００％以下とする。Ｏ含有量は０．００８０％以下、０．００５０％以下、０．００３０％以下又は０．００２０％以下であってもよい。Ｏ含有量は少ないほど好ましく、下限は特に限定されず０％であってもよい。一方、実用鋼板でＯを０．０００１％未満に低減すると、製造コストが大幅に上昇し、経済的に不利になる。そのため、Ｏ含有量は０．０００１％以上、０．０００２％以上又は０．０００５％以上であってもよい。 [O: 0.0100% or less]
O is an element that causes blowholes during welding. Therefore, the O content is set to 0.0100% or less. The O content is set to 0.0080% or less, 0.0050% or less, 0. The lower the O content, the more preferable it is, and the lower limit is not particularly limited and may be 0%. On the other hand, in practical steel sheets, O is set to less than 0.0001%. If the O content is reduced, the production cost increases significantly, which is economically disadvantageous. Therefore, the O content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
［Ｎｂ：０．００５～０．２００％］
　Ｎｂは、指数Ａを高めて鋼板の高温強度を向上させるのに有効な元素である。とりわけ、Ｎｂを添加して適度な大きさを有するＮｂ炭窒化物を比較的多く形成することで、鋼板の高温強度をさらに向上させることが可能である。これらの効果を十分に得るために、Ｎｂ含有量は０．００５％以上とする。Ｎｂ含有量は０．０１０％以上、０．０１５％以上、０．０２０％以上又は０．０４０％以上であってもよい。一方、Ｎｂを過度に含有しても効果が飽和し、製造コストの上昇を招く虞があるか、又は鋼中に粗大な炭窒化物が生成することで鋼板の靭性を低下させる場合がある。このため、Ｎｂ含有量は０．２００％以下とする。Ｎｂ含有量は０．１５０％以下、０．１００％以下、０．０８０％以下又は０．０６０％以下であってもよい。 [Nb: 0.005-0.200%]
Nb is an element effective in increasing the index A and improving the high-temperature strength of the steel sheet. In particular, by adding Nb to form a relatively large amount of Nb carbonitrides having a suitable size, In order to fully obtain these effects, the Nb content is set to 0.005% or more. The Nb content is set to 0.010% or more, and 0.015% or more. On the other hand, if Nb is excessively contained, the effect becomes saturated, and there is a risk of increasing the manufacturing cost, or of forming coarse grains in the steel. The formation of carbonitrides may reduce the toughness of the steel plate. For this reason, the Nb content is set to 0.200% or less. The Nb content is set to 0.150% or less, 0.100% or less, 0 It may be 0.080% or less or 0.060% or less.
　本発明の実施形態に係る鋼板の基本化学組成は上記のとおりである。さらに、当該鋼板は、必要に応じて特性向上を目的として、残部のＦｅの一部に代えて以下の任意選択元素のうち少なくとも１種を含有してもよい。例えば、鋼板は、Ｃｒ：０～１．００％、Ｍｏ：０～０．８０％、Ｂ：０～０．０１００％、Ｔｉ：０～０．２００％、Ｖ：０～０．５００％、Ｎｉ：０～１．００％、Ｃｕ：０～１．００％、Ｗ：０～１．００％、Ｔａ：０～０．１０％、Ｃｏ：０～３．００％、Ｓｎ：０～１．００％、Ｓｂ：０～０．２００％、Ｃａ：０～０．０１００％、Ｍｇ：０～０．０１００％、Ｚｒ：０～０．０１００％、ＲＥＭ：０～０．０１００％、Ｂｉ：０～０．０５００％及びＡｓ：０～０．１０％のうち少なくとも１種を含んでもよい。以下、これらの任意選択元素について詳しく説明する。 The basic chemical composition of the steel plate according to the embodiment of the present invention is as described above. Furthermore, the steel plate may contain at least one of the following optional elements in place of a portion of the remaining Fe, if necessary, for the purpose of improving the properties. For example, the steel sheet may contain at least one of Cr: 0-1.00%, Mo: 0-0.80%, B: 0-0.0100%, Ti: 0-0.200%, V: 0-0.500%, Ni: 0-1.00%, Cu: 0-1.00%, W: 0-1.00%, Ta: 0-0.10%, Co: 0-3.00%, Sn: 0-1.00%, Sb: 0-0.200%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, REM: 0-0.0100%, Bi: 0-0.0500%, and As: 0-0.10%. These optional elements will be described in detail below.
［Ｃｒ：０～１．００％］
　Ｃｒは、Ｍｎと同様に焼入れ性を高め、鋼板強度の向上に寄与する元素である。Ｃｒ含有量は０％でもよいが、上記効果を得るためには、Ｃｒ含有量は０．００１％以上であることが好ましい。Ｃｒ含有量は０．０１％以上、０．１０％以上又は０．２０％以上であってもよい。一方、Ｃｒを過度に含有しても効果が飽和し、製造コストの上昇を招く虞がある。したがって、Ｃｒ含有量は１．００％以下であることが好ましく、０．８０％以下、０．６０％以下又は０．４０％以下であってもよい。 [Cr: 0-1.00%]
Cr is an element that improves hardenability and contributes to improving the strength of the steel sheet, similar to Mn. The Cr content may be 0%, but in order to obtain the above effect, the Cr content should be 0.001% or more. The Cr content may be 0.01% or more, 0.10% or more, or 0.20% or more. On the other hand, even if Cr is excessively contained, the effect is saturated and the manufacturing cost is increased. Therefore, the Cr content is preferably 1.00% or less, and may be 0.80% or less, 0.60% or less, or 0.40% or less.
［Ｍｏ：０～０．８０％］
　Ｍｏは、Ｎｂと同様に鋼板の高温強度の向上に寄与する元素である。この効果は微量であっても得ることができる。Ｍｏ含有量は０％でもよいが、上記効果を得るためには、Ｍｏ含有量は０．００１％以上であることが好ましい。Ｍｏ含有量は０．０１％以上、０．０２％以上、０．０５％以上又は０．１０％以上であってもよい。一方、Ｍｏを過度に含有すると、熱間加工性が低下して生産性が低下する場合がある。このため、Ｍｏ含有量は０．８０％以下であることが好ましい。Ｍｏ含有量は０．６０％以下、０．５０％以下、０．４０％以下又は０．２０％以下であってもよい。 [Mo: 0-0.80%]
Mo, like Nb, is an element that contributes to improving the high-temperature strength of steel sheets. This effect can be obtained even with a small amount of Mo. The Mo content may be 0%, but in order to obtain the above effect, Preferably, the Mo content is 0.001% or more. The Mo content may be 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more. However, if Mo is contained excessively, hot workability may deteriorate, and productivity may decrease. Therefore, the Mo content is preferably 0.80% or less. The Mo content is preferably 0.60% or less. % or less, 0.50% or less, 0.40% or less, or 0.20% or less.
［Ｂ：０～０．０１００％］
　Ｂは、オーステナイトからの冷却過程においてフェライト及びパーライトの生成を抑え、マルテンサイトの生成を促す元素である。また、Ｂは、鋼の高強度化に有益な元素である。これらの効果は微量であっても得ることができる。Ｂ含有量は０％でもよいが、上記効果を得るためには、Ｂ含有量は０．０００１％以上であることが好ましい。Ｂ含有量は０．０００５％以上又は０．００１０％以上であってもよい。一方、Ｂを過度に含有すると、靭性及び／又は溶接性が低下する場合がある。このため、Ｂ含有量は０．０１００％以下であることが好ましい。Ｂ含有量は０．００８０％以下、０．００５０％以下、０．００３０％以下又は０．００２０％以下であってもよい。 [B: 0 to 0.0100%]
B is an element that suppresses the formation of ferrite and pearlite during the cooling process from austenite and promotes the formation of martensite. B is also an element that is beneficial for increasing the strength of steel. These effects are only seen in small amounts. The B content may be 0%, but in order to obtain the above effects, the B content is preferably 0.0001% or more. The B content is preferably 0.0005% or more. On the other hand, if B is contained excessively, the toughness and/or weldability may decrease. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.
［Ｔｉ：０～０．２００％］
　Ｔｉは、炭化物の形態制御に有効な元素である。Ｔｉによってフェライトの強度増加が促され得る。Ｔｉ含有量は０％でもよいが、これらの効果を得るためには、Ｔｉ含有量は０．００１％以上であることが好ましい。Ｔｉ含有量は０．００２％以上、０．０１０％以上、０．０２０％以上又は０．０４０％以上であってもよい。一方、Ｔｉを過度に含有しても効果が飽和し、製造コストの上昇を招く虞がある。したがって、Ｔｉ含有量は０．２００％以下であることが好ましく、０．１００％以下、０．０８０％以下又は０．０５０％以下であってもよい。 [Ti: 0-0.200%]
Ti is an element effective in controlling the morphology of carbides. Ti can promote an increase in the strength of ferrite. The Ti content may be 0%, but in order to obtain these effects, the Ti content must be less than 0. The Ti content is preferably 0.001% or more. The Ti content may be 0.002% or more, 0.010% or more, 0.020% or more, or 0.040% or more. On the other hand, an excessive Ti content However, the effect of Ti is saturated and there is a risk of an increase in manufacturing costs. Therefore, the Ti content is preferably 0.200% or less, more preferably 0.100% or less, 0.080% or less, or 0.050% or less. It may be the following.
［Ｖ：０～０．５００％］
　Ｖは、Ｔｉと同様に炭化物の形態制御に有効な元素であり、組織を微細化して鋼板の靭性の向上にも効果的な元素である。Ｖ含有量は０％でもよいが、上記効果を得るためには、Ｖ含有量は０．００１％以上であることが好ましい。Ｖ含有量は０．００５％以上、０．０１０％以上又は０．０５０％以上であってもよい。一方、Ｖを過度に含有すると、多量の析出物が生成して靭性を低下させる場合がある。このため、Ｖ含有量は０．５００％以下であることが好ましい。Ｖ含有量は０．４００％以下、０．２００％以下又は０．１００％以下であってもよい。 [V: 0-0.500%]
V, like Ti, is an element effective in controlling the morphology of carbides, and is also an element effective in refining the structure and improving the toughness of the steel plate. The V content may be 0%, but if the above effects are not to be obtained, To obtain this, the V content is preferably 0.001% or more. The V content may be 0.005% or more, 0.010% or more, or 0.050% or more. If V is contained in an excessive amount, a large amount of precipitates may be formed, which may reduce the toughness. Therefore, the V content is preferably 0.500% or less. The V content is preferably 0.400% or less. It may be 0.200% or less or 0.100% or less.
［Ｎｉ：０～１．００％］
　Ｎｉは、鋼板の強度の向上に有効な元素である。Ｎｉの含有量は０％でもよいが、上記効果を得るためには、Ｎｉ含有量は０．００１％以上であることが好ましい。Ｎｉ含有量は０．０１％以上又は０．０５％以上であってもよい。一方、Ｎｉを過度に含有すると、鋼板の溶接性が低下する場合がある。このため、Ｎｉ含有量は１．００％以下であることが好ましい。Ｎｉ含有量は０．８０％以下、０．４０％以下又は０．２０％以下であってもよい。 [Ni: 0-1.00%]
Ni is an element effective in improving the strength of a steel sheet. The Ni content may be 0%, but in order to obtain the above effect, the Ni content is preferably 0.001% or more. The Ni content may be 0.01% or more, or 0.05% or more. On the other hand, if the Ni content is excessive, the weldability of the steel sheet may be reduced. For this reason, the Ni content is set to 1.00 % or less. The Ni content may be 0.80% or less, 0.40% or less, or 0.20% or less.
［Ｃｕ：０～１．００％］
　Ｃｕは、鋼板の強度の向上に寄与する元素である。この効果は微量であっても得ることができる。Ｃｕ含有量は０％でもよいが、上記効果を得るためには、Ｃｕ含有量は０．００１％以上であることが好ましい。Ｃｕ含有量は０．０１％以上又は０．０５％以上であってもよい。一方、Ｃｕを過度に含有すると、赤熱脆性を招いて熱間圧延での生産性を低下させる虞がある。このため、Ｃｕ含有量は１．００％以下であることが好ましい。Ｃｕ含有量は０．８０％以下、０．６０％以下、０．３０％以下又は０．２０％以下であってもよい。 [Cu: 0-1.00%]
Cu is an element that contributes to improving the strength of the steel sheet. This effect can be obtained even with a small amount of Cu. The Cu content may be 0%, but in order to obtain the above effect, the Cu content must be 0%. The Cu content is preferably 0.001% or more. The Cu content may be 0.01% or more or 0.05% or more. On the other hand, excessive Cu content may cause red shortness and deteriorate the hardness during hot rolling. Therefore, the Cu content is preferably 1.00% or less. The Cu content is preferably 0.80% or less, 0.60% or less, 0.30% or less, or It may be 0.20% or less.
［Ｗ：０～１．００％］
　Ｗは、炭化物の形態制御と鋼板の強度向上に有効な元素である。Ｗ含有量は０％でもよいが、これらの効果を得るためには、Ｗ含有量は０．００１％以上であることが好ましい。Ｗ含有量は０．０１％以上又は０．０５％以上であってもよい。一方、Ｗを過度に含有すると、溶接性が低下する場合がある。このため、Ｗ含有量は１．００％以下であることが好ましい。Ｗ含有量は０．８０％以下、０．４０％以下又は０．２０％以下であってもよい。 [W: 0-1.00%]
W is an element effective in controlling the morphology of carbides and improving the strength of steel sheets. The W content may be 0%, but in order to obtain these effects, the W content must be 0.001% or more. The W content may be 0.01% or more, or 0.05% or more. On the other hand, if W is contained excessively, the weldability may be deteriorated. For this reason, the W content is set to 1. The W content may be 0.80% or less, 0.40% or less, or 0.20% or less.
［Ｔａ：０～０．１０％］
　Ｔａは、Ｗと同様に炭化物の形態制御と鋼板強度の向上に有効な元素である。Ｔａ含有量は０％でもよいが、これらの効果を得るためには、Ｔａ含有量は０．００１％以上であることが好ましい。Ｔａ含有量は０．０１％以上又は０．０３％以上であってもよい。一方、Ｔａを過度に含有しても効果が飽和し、必要以上に鋼板中に含有させることは製造コストの上昇を招く。このため、Ｔａ含有量は０．１０％以下であることが好ましい。Ｔａ含有量は０．０８％以下、０．０６％以下又は０．０４％以下であってもよい。 [Ta: 0 to 0.10%]
Ta, like W, is an element that is effective in controlling the morphology of carbides and improving the strength of steel sheets. The Ta content may be 0%, but in order to obtain these effects, the Ta content should be 0.001%. The Ta content may be 0.01% or more, or 0.03% or more. On the other hand, if Ta is excessively contained, the effect is saturated, and if Ta is contained in the steel sheet more than necessary, Increasing the Ta content leads to an increase in manufacturing costs. For this reason, the Ta content is preferably 0.10% or less. The Ta content is preferably 0.08% or less, 0.06% or less, or 0.04% or less. It's fine if there is.
［Ｃｏ：０～３．００％］
　Ｃｏは、Ｎｉと同様に鋼板の強度の向上に有効な元素である。Ｃｏ含有量は０％でもよいが、上記効果を得るためには、Ｃｏ含有量は０．００１％以上であることが好ましい。Ｃｏ含有量は０．０１％以上、０．０５％以上又は０．１０％以上であってもよい。一方、Ｃｏを過度に含有すると、熱間加工性が低下する場合があり、原料コストの増加にも繋がる。このため、Ｃｏ含有量は３．００％以下であることが好ましい。Ｃｏ含有量は２．００％以下、１．００％以下、０．５０％以下又は０．２０％以下であってもよい。 [Co: 0-3.00%]
Co, like Ni, is an element that is effective in improving the strength of steel sheets. The Co content may be 0%, but in order to obtain the above effect, the Co content must be 0.001% or more. The Co content may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, if Co is contained excessively, the hot workability may be deteriorated, and the raw material This leads to an increase in costs. For this reason, the Co content is preferably 3.00% or less. The Co content is preferably 2.00% or less, 1.00% or less, 0.50% or less, or 0.20% or less. % or less.
［Ｓｎ：０～１．００％］
　Ｓｎは、鋼板の原料としてスクラップを用いた場合に、鋼板に含有され得る元素である。また、Ｓｎはフェライトの脆化を引き起こす虞がある。このため、Ｓｎ含有量は少ないほど好ましく、１．００％以下であることが好ましい。Ｓｎ含有量は０．１０％以下、０．０４０％以下又は０．０２％以下であってもよい。Ｓｎ含有量は０％であってもよいが、Ｓｎ含有量を０．００１％未満に低減することは精錬コストの過度な増加を招く。このため、Ｓｎ含有量は０．００１％以上、０．００５％以上又は０．０１％以上であってもよい。 [Sn: 0-1.00%]
Sn is an element that may be contained in a steel sheet when scrap is used as the raw material for the steel sheet. In addition, Sn may cause embrittlement of ferrite. Therefore, the smaller the Sn content, the better. The Sn content may be 0.10% or less, 0.040% or less, or 0.02% or less. The Sn content may be 0%, but Sn Reducing the Sn content to less than 0.001% leads to an excessive increase in refining costs. Therefore, the Sn content is set to 0.001% or more, 0.005% or more, or 0.01% or more. good.
［Ｓｂ：０～０．２００％］
　Ｓｂは、Ｓｎと同様に、鋼板の原料としてスクラップを用いた場合に鋼板に含有され得る元素である。また、Ｓｂは粒界に強く偏析して粒界の脆化を招く虞がある。このため、Ｓｂ含有量は少ないほど好ましく、０．２００％以下であることが好ましい。Ｓｂ含有量は０．１００％以下、０．０４０％以下又は０．０２０％以下であってもよい。Ｓｂ含有量は０％であってもよいが、Ｓｂ含有量を０．００１％未満に低減することは精錬コストの過度な増加を招く。このため、Ｓｂ含有量は０．００１％以上、０．００５％以上又は０．０１０％以上であってもよい。 [Sb: 0 to 0.200%]
Like Sn, Sb is an element that can be contained in a steel sheet when scrap is used as the raw material for the steel sheet. In addition, Sb may strongly segregate at grain boundaries and cause embrittlement of the grain boundaries. Therefore, the smaller the Sb content, the better, and it is preferably 0.200% or less. The Sb content may be 0.100% or less, 0.040% or less, or 0.020% or less. The Sb content may be 0%, but reducing the Sb content to less than 0.001% will lead to an excessive increase in refining costs. % or more, or 0.010% or more.
［Ｃａ：０～０．０１００％］
［Ｍｇ：０～０．０１００％］
［Ｚｒ：０～０．０１００％］
［ＲＥＭ：０～０．０１００％］
　Ｃａ、Ｍｇ、Ｚｒ及びＲＥＭは、鋼板の成形性の向上に寄与する元素である。Ｃａ、Ｍｇ、Ｚｒ及びＲＥＭ含有量は０％であってもよいが、このような効果を得るためには、Ｃａ、Ｍｇ、Ｚｒ及びＲＥＭ含有量はそれぞれ０．０００１％以上であることが好ましく、０．０００５％以上、０．００１０％以上又は０．００１５％以上であってもよい。一方で、これらの元素を過度に含有すると、鋼板の延性が低下する場合がある。したがって、Ｃａ、Ｍｇ、Ｚｒ及びＲＥＭ含有量はそれぞれ０．０１００％以下であることが好ましく、０．００８０％以下、０．００６０％以下、０．００４０％以下又は０．００２０％以下であってもよい。本明細書におけるＲＥＭとは、原子番号２１番のスカンジウム（Ｓｃ）、原子番号３９番のイットリウム（Ｙ）及びランタノイドである原子番号５７番のランタン（Ｌａ）～原子番号７１番のルテチウム（Ｌｕ）の１７元素の総称であり、ＲＥＭ含有量はこれら元素の合計含有量である。 [Ca: 0-0.0100%]
[Mg: 0 to 0.0100%]
[Zr: 0 to 0.0100%]
[REM: 0 to 0.0100%]
Ca, Mg, Zr and REM are elements that contribute to improving the formability of the steel sheet. The Ca, Mg, Zr and REM contents may be 0%, but in order to obtain such effects, The contents of Ca, Mg, Zr and REM are each preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. If these elements are contained in excess, the ductility of the steel sheet may decrease. Therefore, the Ca, Mg, Zr and REM contents are each preferably 0.0100% or less, 0.0080% or less, and 0. It may be 0.0060% or less, 0.0040% or less, or 0.0020% or less. In this specification, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. REM is a general term for the 17 elements, and the REM content is the total content of these elements.
［Ｂｉ：０～０．０５００％］
　Ｂｉは、凝固組織を微細化することにより成形性を高める作用を有する元素である。Ｂｉ含有量は０％でもよいが、このような効果を得るためには、Ｂｉ含有量は０．０００１％以上であることが好ましく、０．０００５％以上、０．００１０％以上又は０．００３０％以上であってもよい。一方、Ｂｉを過度に含有しても効果が飽和し、必要以上に鋼板中に含有させることは製造コストの上昇を招く。したがって、Ｂｉ含有量は０．０５００％以下であることが好ましく、０．０４００％以下、０．０２００％以下、０．０１００％以下又は０．００５０％以下であってもよい。 [Bi: 0 to 0.0500%]
Bi is an element that has the effect of improving formability by refining the solidification structure. The Bi content may be 0%, but in order to obtain such an effect, the Bi content should be 0.0001%. On the other hand, even if an excessive amount of Bi is contained, the effect is saturated and the amount of Bi in the steel sheet is more than necessary. Therefore, the Bi content is preferably 0.0500% or less, more preferably 0.0400% or less, 0.0200% or less, 0.0100% or less, or 0.0050% or less. % or less.
［Ａｓ：０～０．１０％］
　Ａｓは、Ｓｎ及びＳｂと同様に、鋼板の原料としてスクラップを用いた場合に鋼板に含有され得る元素である。また、Ａｓは、粒界に強く偏析する元素であり、Ａｓ含有量は少ないほど好ましい。Ａｓ含有量は０．１０％以下であることが好ましく、０．０４％以下又は０．０２％以下であってもよい。Ａｓ含有量は０％であってもよいが、Ａｓ含有量を０．００１％未満に低減することは精錬コストの過度な増加を招く。このため、Ａｓ含有量は０．００１％以上、０．００５％以上又は０．０１％以上であってもよい。 [As: 0 to 0.10%]
As, like Sn and Sb, is an element that can be contained in a steel sheet when scrap is used as the raw material for the steel sheet. In addition, As is an element that strongly segregates at grain boundaries, and the lower the As content, the better. The As content is preferably 0.10% or less, and may be 0.04% or less or 0.02% or less. The As content may be 0%, but the As content Reducing As to less than 0.001% leads to an excessive increase in refining costs, so the As content may be 0.001% or more, 0.005% or more, or 0.01% or more.
　本発明の実施形態に係る鋼板において、上記元素を除く残部は、Ｆｅ及び不純物からなる。不純物は、鋼原料から及び／又は製鋼過程で混入し、本発明の実施形態に係る鋼板の特性を阻害しない範囲で存在が許容される元素である。 In the steel plate according to the embodiment of the present invention, the remainder excluding the above elements consists of Fe and impurities. Impurities are elements that are mixed in from the steel raw materials and/or during the steelmaking process, and whose presence is permitted to the extent that they do not impair the properties of the steel plate according to the embodiment of the present invention.
［指数Ａ：０．５０％以上］
　本発明の実施形態に係る鋼板の化学組成は、下記式１で表される指数Ａが０．５０％以上であることを必要とする。
　Ａ＝［Ｃ］－０．１［Ｓｉ］＋０．３（［Ｍｎ］－０．５）－０．３［Ａｌ］＋０．１［Ｃｒ］＋０．６［Ｍｏ］－［Ｔｉ］＋１５［Ｎｂ］　　　・・・式１
　ここで、［Ｃ］、［Ｓｉ］、［Ｍｎ］、［Ａｌ］、［Ｃｒ］、［Ｍｏ］、［Ｔｉ］及び［Ｎｂ］は、各元素の含有量［質量％］であり、元素を含有しない場合は０％である。先に説明したとおり、本発明の実施形態に係る鋼板では、製造過程におけるヒートバックルの発生を抑制する上で鋼板の高温強度を向上させることが極めて重要である。鋼板の高温強度を向上させるためには、鋼板の高温強度を向上させるＮｂ及びＭｏ等の元素を多く添加することに加えて、鋼板の製造方法に関連して後で詳しく説明される１次熱処理工程で要求される加熱温度（すなわちＡｃ３＋１０℃以上）を低くすることが有効である。１次熱処理工程で要求される加熱温度を低くすることで、加熱時の板温上昇に伴う鋼板の降伏応力における低下を抑制することが可能となり、ヒートバックルを生じさせる鋼板の幅方向中央部への圧縮応力に対する抵抗力を顕著に向上させることが可能となるからである。このような観点から、鋼板自体の高温強度向上とＡｃ３点の低下に関する鋼中の各元素の影響度を実験的に調べた。より具体的には、本発明者らは、当該影響度を考慮した係数とともにこれらの元素の含有量によって規定される指数Ａ、すなわち上記式１で表される指数Ａを０．５０％以上に制御することにより、鋼板自体の高温強度の積極的な向上と、１次熱処理工程における加熱温度の低下に起因する降伏応力の低下抑制とに基づいて、上記圧縮応力に対する鋼板の抵抗力を向上させることができ、それによって製造過程におけるヒートバックルの発生を顕著に抑制することができることを見出した。ヒートバックルの発生を抑制する観点からは、指数Ａは大きいほど好ましく、例えば０．５５％以上、０．６０％以上、０．６５％以上、０．７０％以上、０．７５％以上又は０．８０％以上であってもよい。指数Ａの上限は特に限定されないが、例えば、指数Ａは、２．００％以下、１．８０％以下、１．５０％以下、１．３０％以下又は１．１０％以下であってもよい。 [Index A: 0.50% or more]
The chemical composition of the steel sheet according to the embodiment of the present invention requires that the index A represented by the following formula 1 is 0.50% or more.
A=[C]-0.1[Si]+0.3([Mn]-0.5)-0.3[Al]+0.1[Cr]+0.6[Mo]-[Ti]+15[Nb ] ...Formula 1
Here, [C], [Si], [Mn], [Al], [Cr], [Mo], [Ti] and [Nb] are the contents [mass %] of each element, When no Cr content is contained, the value is 0%. As described above, in the steel sheet according to the embodiment of the present invention, it is extremely important to improve the high-temperature strength of the steel sheet in order to suppress the occurrence of heat buckling during the manufacturing process. In order to improve the high-temperature strength of the steel plate, in addition to adding a large amount of elements such as Nb and Mo, which improve the high-temperature strength of the steel plate, the primary alloying step, which will be described in detail later in relation to the manufacturing method of the steel plate, is performed. It is effective to lower the heating temperature required in the heat treatment process (i.e., Ac3+10°C or higher). By lowering the heating temperature required in the first heat treatment process, the yield of the steel sheet caused by the increase in sheet temperature during heating can be reduced. This is because it is possible to suppress the decrease in stress, and it is possible to significantly improve the resistance to the compressive stress applied to the widthwise center portion of the steel sheet, which causes heat buckling. The effect of each element in the steel on improving the high temperature strength of the steel itself and lowering the Ac3 point was experimentally investigated. More specifically, the inventors set the index A defined by the contents of these elements together with a coefficient considering the degree of influence, that is, the index A expressed by the above formula 1, to 0.50% or more. By controlling the temperature, the resistance of the steel sheet to the compressive stress is improved based on the positive improvement of the high-temperature strength of the steel sheet itself and the suppression of the decrease in the yield stress caused by the decrease in the heating temperature in the first heat treatment process. It has been found that the occurrence of heat buckling during the manufacturing process can be significantly suppressed. From the viewpoint of suppressing the occurrence of heat buckling, the larger the index A, the more preferable it is. For example, 0.55% or more, The upper limit of the index A is not particularly limited, but the index A may be, for example, 0.60% or more, 0.65% or more, 0.70% or more, 0.75% or more, or 0.80% or more. , 2.00% or less, 1.80% or less, 1.50% or less, 1.30% or less, or 1.10% or less.
　本発明の実施形態に係る鋼板の化学組成は、一般的な分析方法によって測定すればよい。例えば、当該鋼板の化学組成は、誘導結合プラズマ発光分光分析（ＩＣＰ－ＡＥＳ：Ｉｎｄｕｃｔｉｖｅｌｙ　Ｃｏｕｐｌｅｄ　Ｐｌａｓｍａ－Ａｔｏｍｉｃ　Ｅｍｉｓｓｉｏｎ　Ｓｐｅｃｔｒｏｍｅｔｒｙ）を用いて測定すればよい。Ｃ及びＳは燃焼－赤外線吸収法を用い、Ｎは不活性ガス融解－熱伝導度法を用い、Ｏは不活性ガス融解－非分散型赤外線吸収法を用いて測定すればよい。 The chemical composition of the steel plate according to the embodiment of the present invention may be measured by a general analytical method. For example, the chemical composition of the steel plate may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method.
［フェライト：７５～９５％］
　フェライトは、軟質な組織であるので変形し易く、伸びの向上に寄与する。フェライトの面積率が７５％以上であると、十分な成形性を得ることができる。成形性向上の観点からは、フェライトの面積率は高いほど好ましく、例えば７８％以上、８０％以上、８２％以上又は８５％以上であってもよい。一方で、フェライトを過度に含むと、鋼板において所望の強度を達成することができない場合がある。したがって、フェライトの面積率は９５％以下とする。フェライトの面積率は９３％以下、９０％以下又は８７％以下であってもよい。 [Ferrite: 75-95%]
Since ferrite is a soft structure, it is easily deformed and contributes to improving elongation. When the area ratio of ferrite is 75% or more, sufficient formability can be obtained. From the viewpoint of improving formability, the higher the area ratio of ferrite, the more preferable it is, and it may be, for example, 78% or more, 80% or more, 82% or more, or 85% or more. On the other hand, if ferrite is contained excessively, the desired strength may not be achieved in the steel plate. Therefore, the area ratio of ferrite is 95% or less. The area ratio of ferrite may be 93% or less, 90% or less, or 87% or less.
［マルテンサイト：５～２５％］
　マルテンサイトは、転位密度が高く硬質な組織であるので、引張強度の向上に寄与する組織である。マルテンサイトの面積率を５％以上とすることで、例えば５４０ＭＰａ以上の引張強度を確保することができる。強度向上の観点からは、マルテンサイトの面積率は高いほど好ましく、例えば７％以上、１０％以上又は１３％以上であってもよい。一方で、マルテンサイトの面積率が２５％以下であると、成形性と外観性を確保することができる。マルテンサイトの面積率は２２％以下、２０％以下、１８％以下又は１５％以下であってもよい。本発明において、「マルテンサイト」とは、焼入れままマルテンサイト（いわゆるフレッシュマルテンサイト）だけでなく、焼戻しマルテンサイトをも包含するものである。 [Martensite: 5 to 25%]
Martensite is a structure with high dislocation density and hardness, and therefore contributes to improving tensile strength. By setting the area ratio of martensite to 5% or more, it is possible to ensure a tensile strength of, for example, 540 MPa or more. From the viewpoint of improving strength, the higher the area ratio of martensite, the more preferable it is, and it may be, for example, 7% or more, 10% or more, or 13% or more. On the other hand, if the area ratio of martensite is 25% or less, it is possible to ensure formability and appearance. The area ratio of martensite may be 22% or less, 20% or less, 18% or less, or 15% or less. In the present invention, "martensite" includes not only as-quenched martensite (so-called fresh martensite) but also tempered martensite.
［残部組織：合計で０～１０％］
　フェライト及びマルテンサイト以外の残部組織は、面積率で０％であってもよいが、残部組織が存在する場合には、当該残部組織はベイナイト、パーライト、及び残留オーステナイトの少なくとも１種である。フェライト及びマルテンサイトに基づく上記の効果を確保する観点から、残部組織すなわちベイナイト、パーライト、及び残留オーステナイトの少なくとも１種の面積率は、合計で１０％以下とし、例えば８％以下、６％以下、４％以下又は２％以下であってもよい。一方で、残部組織の面積率を０％とするには、鋼板の製造過程において高度な制御を要するため、歩留まりの低下を招く場合がある。したがって、残部組織の面積率は０．５％以上又は１％以上であってもよい。 [Remainder: 0-10% in total]
The remaining structure other than ferrite and martensite may be 0% in area ratio, but when the remaining structure exists, the remaining structure is at least one of bainite, pearlite, and retained austenite. From the viewpoint of ensuring the above-mentioned effects based on ferrite and martensite, the area ratio of the remaining structure, i.e., at least one of bainite, pearlite, and retained austenite, may be 10% or less in total, for example, 8% or less, 6% or less, 4% or less, or 2% or less. On the other hand, in order to make the area ratio of the remaining structure 0%, a high level of control is required in the manufacturing process of the steel plate, which may lead to a decrease in yield. Therefore, the area ratio of the remaining structure may be 0.5% or more, or 1% or more.
［金属組織の同定及び面積率の算出］
　金属組織の同定及び面積率の算出は、ナイタール試薬（３％硝酸エタノール溶液）を用いた腐食後のＦＥ－ＳＥＭ（電界放射型走査型電子顕微鏡）及び光学顕微鏡並びにＸ線回折法により行われる。ＦＥ－ＳＥＭ及び光学顕微鏡による組織観察は、板面に垂直な方向の鋼板断面における１００μｍ×１００μｍの領域に対して５００～５００００倍の倍率で行われる。いずれの金属組織についても測定箇所を３か所とし、それらの測定値の平均値を算出することによって面積率を決定する。例えば、測定対象の鋼板の板厚が薄いために、板厚方向に１００μｍの測定領域を確保できない場合には、板厚方向の長さを減少させつつ、測定領域１００００μｍ2を確保することとする。例えば、板厚方向に２０μｍ、板厚方向と垂直な方向に５００μｍの測定領域を観察対象としてもよい。ただし、板厚方向に含まれる結晶粒の数が少なくなりすぎると測定精度が低下する場合があるため、板厚方向の測定長さは１０μｍ以上、好ましくは５０μｍ以上とする。以下の説明中の「１００μｍ×１００μｍの領域」についても同様である。 [Identification of metal structure and calculation of area ratio]
Identification of the metal structure and calculation of the area ratio are performed by FE-SEM (field emission scanning electron microscope) and optical microscope after corrosion using Nital reagent (3% nitric acid ethanol solution) and X-ray diffraction method. The structure observation by FE-SEM and optical microscope is performed at a magnification of 500 to 50,000 times for a 100 μm × 100 μm area in the steel plate cross section in the direction perpendicular to the plate surface. For each metal structure, three measurement points are set, and the area ratio is determined by calculating the average value of the measured values. For example, if the thickness of the steel plate to be measured is thin and a measurement area of 100 μm in the plate thickness direction cannot be secured, the length in the plate thickness direction is reduced while securing a measurement area of 10,000 μm 2. For example, a measurement area of 20 μm in the plate thickness direction and 500 μm in the direction perpendicular to the plate thickness direction may be observed. However, if the number of crystal grains contained in the plate thickness direction becomes too small, the measurement accuracy may decrease, so the measurement length in the plate thickness direction is 10 μm or more, preferably 50 μm or more. The same applies to the "100 μm×100 μm area" in the following description.
　本明細書において、「板厚ｘ／ｙ位置（ここで、ｘ及びｙは、ｘ＜ｙを満たす自然数とする。）」とは、鋼板の板厚方向における表面（板面）から、板厚方向に、板厚ｔのｘ／ｙの距離（深さ）だけ鋼板の中心部に向かって移動した位置を意味する。たとえば、鋼板の板厚ｔが２ｍｍであった場合に「板厚１／８位置」とは、鋼板の表面から板厚方向に０．２５ｍｍの深さとなる位置を意味する。なお、鋼板が表面にめっき層等の被膜を有する場合、「鋼板の表面」は、鋼板と当該被膜との界面を意味し、「板厚ｔ」は、当該被膜を除いた鋼板（母材）の板厚を意味するものとする。 In this specification, "plate thickness x/y position (where x and y are natural numbers satisfying x<y)" refers to a position moved in the plate thickness direction from the surface (plate surface) of the steel plate in the plate thickness direction toward the center of the steel plate by a distance (depth) of x/y of the plate thickness t. For example, if the plate thickness t of the steel plate is 2 mm, "plate thickness 1/8 position" refers to a position that is 0.25 mm deep in the plate thickness direction from the surface of the steel plate. Note that, if the steel plate has a coating such as a plating layer on its surface, "the surface of the steel plate" refers to the interface between the steel plate and the coating, and "plate thickness t" refers to the thickness of the steel plate (base material) excluding the coating.
　フェライトの面積率及びマルテンサイトの面積率は以下の手順で求める。まず、試料の観察面をナイタール試薬（３％硝酸エタノール溶液）でエッチングし、次いで板厚１／４位置を中心とする板厚１／８位置～３／８位置の範囲内で１００μｍ×１００μｍの領域をＦＥ－ＳＥＭ（例えばＪＥＯＬ社製　ＪＳＭ－７２００Ｆ、加速電圧１５ｋＶ、倍率５００～２０００倍にて測定）で観察する。ナイタール腐食では、マルテンサイト及び残留オーステナイトは腐食されないため、腐食されていない領域の面積率は、マルテンサイト及び残留オーステナイトの合計面積率に対応する。具体的には、画像解析ソフトウェアＩｍａｇｅ　Ｊ（Ｖｅｒ．１．５４ｆ）を用いて、金属組織を輝度の違いにより二値化し、画像データの黒色部分がフェライトであり、腐食されていない白色部分がマルテンサイトと残留オーステナイトの合計組織である。したがって、黒色部分の領域の面積率からフェライトの面積率を算出し、一方で、この腐食されていない領域の面積率から、後で説明するＸ線回折法により測定した残留オーステナイトの面積率を引算することでマルテンサイトの面積率を算出する。この方法で求めたマルテンサイト面積率には、焼戻しマルテンサイト面積率も含まれる。 The area ratios of ferrite and martensite are determined by the following procedure. First, the observation surface of the sample is etched with a Nital reagent (a 3% nitric acid in ethanol solution), and then a 100 μm x 100 μm area within the range of 1/8 to 3/8 of the plate thickness, centered at the 1/4 position, is observed with an FE-SEM (e.g., JEOL's JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 500 to 2000 times). In Nital corrosion, martensite and retained austenite are not corroded, so the area ratio of the uncorroded area corresponds to the total area ratio of martensite and retained austenite. Specifically, the metal structure is binarized according to differences in brightness using the image analysis software Image J (Ver. 1.54f), and the black parts of the image data are ferrite, and the uncorroded white parts are the total structure of martensite and retained austenite. Therefore, the area ratio of ferrite is calculated from the area ratio of the black area, while the area ratio of martensite is calculated by subtracting the area ratio of retained austenite measured by the X-ray diffraction method described later from the area ratio of this uncorroded area. The area ratio of martensite calculated by this method also includes the area ratio of tempered martensite.
　残留オーステナイトの面積率はＸ線回折法により算出される。まず、試料の板面から板厚方向に深さ１／４位置までを機械研磨及び化学研磨により除去する。より具体的には機械研磨で観察位置近傍まで薄くした後、化学研磨（フッ酸）で目標位置まで薄くする。次いで、例えば、リガク社製Ｘ線回折装置（ＲＩＮＴ２５００、Ｘ線出力４０ｋＶ－２００ｍＡ）により、板厚１／４位置において、ＭｏＫα線を用いて得られたｂｃｃ相の（２００）及び（２１１）並びにｆｃｃ相の（２００）、（２２０）及び（３１１）の回折ピークの積分強度比から、残留オーステナイトの組織分率を算出する。この算出方法として一般的な５ピーク法が利用される。算出された残留オーステナイトの組織分率を残留オーステナイトの面積率として決定する。 The area fraction of retained austenite is calculated by X-ray diffraction. First, the specimen is removed from the plate surface to a depth of 1/4 in the plate thickness direction by mechanical polishing and chemical polishing. More specifically, the specimen is thinned to the vicinity of the observation position by mechanical polishing, and then thinned to the target position by chemical polishing (with hydrofluoric acid). Next, the structure fraction of retained austenite is calculated from the integrated intensity ratio of the diffraction peaks of (200) and (211) of the bcc phase and (200), (220), and (311) of the fcc phase obtained at the 1/4 plate thickness position using, for example, a Rigaku X-ray diffraction device (RINT2500, X-ray output 40 kV-200 mA). The general five-peak method is used for this calculation. The calculated structure fraction of retained austenite is determined as the area fraction of retained austenite.
　残部組織としてのベイナイト、パーライト、残留オーステナイトは、上述の通り、合計で０～１０％の割合で含まれていてもよいものである。すなわち、本実施形態に係る鋼板においては、フェライト及びマルテンサイトが主要な金属組織であり、残部組織としてのベイナイト、パーライト、残留オーステナイトは、製造上、不可避的に生成される可能性がある金属組織である。そのため、本質的に、これら残部組織としてのベイナイト、パーライト、残留オーステナイトを同定したり、面積率を測定したりする積極的な技術的意義は存在しない。本明細書に記載の鋼板の化学組成及び製造方法から、本実施形態に係る鋼板における残部組織は、ベイナイト、パーライト、残留オーステナイト、又はそれらの複合物であることは明らかである。なお、残部組織としてのベイナイト、パーライトの同定、測定方法としては、以下のような方法を採用することができる。残留オーステナイトの面積率の測定方法は、上述の通りである。 As described above, the residual structures of bainite, pearlite, and retained austenite may be present at a total ratio of 0 to 10%. That is, in the steel plate according to this embodiment, ferrite and martensite are the main metal structures, and the residual structures of bainite, pearlite, and retained austenite are metal structures that may be unavoidably generated during manufacturing. Therefore, there is essentially no positive technical significance in identifying the residual structures of bainite, pearlite, and retained austenite or measuring their area ratios. From the chemical composition and manufacturing method of the steel plate described in this specification, it is clear that the residual structure in the steel plate according to this embodiment is bainite, pearlite, retained austenite, or a composite thereof. The following methods can be used to identify and measure the residual structures of bainite and pearlite. The method for measuring the area ratio of retained austenite is as described above.
　ベイナイトの同定及び面積率の算出は以下の手順で行う。まず、試料の観察面をナイタール試薬で腐食し、次いで板厚１／４を中心とする板厚１／８～３／８の範囲内で１００μｍ×１００μｍの領域をＦＥ－ＳＥＭ（例えばＪＥＯＬ社製　ＪＳＭ－７２００Ｆ、加速電圧１５ｋＶ、倍率５００～２０００倍にて測定）で観察する。この観察領域において組織内部に含まれるセメンタイトの位置及びセメンタイトの配列から、以下のようにしてベイナイトを同定する。ベイナイトは、上部ベイナイトと下部ベイナイトに分類され、上部ベイナイトは、ラス状のベイニティックフェライトの界面にセメンタイト又は残留オーステナイトが存在する。下部ベイナイトは、ラス状のベイニティックフェライトの内部にセメンタイトが存在し、ベイニティックフェライトとセメンタイトの結晶方位関係が１種類であり、セメンタイトが同一のバリアントを持つ。これらの特徴点に基づき、上部ベイナイトと下部ベイナイトをそれぞれ同定することができる。本発明においてはこれらを合わせてベイナイトと呼び、同定されたベイナイトの面積率を画像解析に基づいて算出する。 The identification of bainite and calculation of the area ratio are carried out as follows. First, the observation surface of the sample is corroded with Nital reagent, and then a 100 μm x 100 μm area within the range of 1/8 to 3/8 of the plate thickness, centered at 1/4 of the plate thickness, is observed with an FE-SEM (e.g., JEOL's JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 500 to 2000 times). From the position and arrangement of cementite contained within the structure in this observation area, bainite is identified as follows. Bainite is classified into upper bainite and lower bainite, and in upper bainite, cementite or retained austenite exists at the interface of lath-shaped bainitic ferrite. In lower bainite, cementite exists inside lath-shaped bainitic ferrite, there is one type of crystal orientation relationship between bainitic ferrite and cementite, and the cementite has the same variant. Based on these characteristic points, upper bainite and lower bainite can be identified. In the present invention, these are collectively referred to as bainite, and the area ratio of the identified bainite is calculated based on image analysis.
　パーライトの同定及び面積率の算出は以下の手順で行う。まず、試料の観察面をナイタール試薬で腐食し、次いで板厚１／４を中心とする板厚１／８～３／８の範囲をＳＥＭ（例えばＪＥＯＬ社製　ＪＳＭ－７２００Ｆ、加速電圧１５ｋＶ、倍率５００～２０００倍にて測定）で観察する。ＳＥＭの観察像においてラメラ状のセメンタイトが観察される領域をパーライトと同定し、この領域の面積率を画像解析に基づいて算出する。 　Pearlite is identified and its area ratio calculated using the following procedure. First, the observation surface of the sample is corroded with Nital reagent, and then the area from 1/8 to 3/8 of the plate thickness, centered at 1/4 of the plate thickness, is observed using an SEM (e.g., JEOL's JSM-7200F, measured at an acceleration voltage of 15 kV and magnification of 500 to 2000 times). Areas in which lamellar cementite is observed in the SEM observation image are identified as pearlite, and the area ratio of this area is calculated based on image analysis.
［全Ｎｂ炭窒化物中のＮｂ量が０．００４％以上であり、かつ粒径が２０ｎｍ以上のＮｂ炭窒化物中のＮｂ量が全Ｎｂ炭窒化物中のＮｂ量の６０％以上］
　本発明の実施形態に係る鋼板は、金属組織中にＮｂ炭窒化物を含み、全Ｎｂ炭窒化物中のＮｂ量は０．００４％以上であり、かつ粒径が２０ｎｍ以上のＮｂ炭窒化物中のＮｂ量は当該全Ｎｂ炭窒化物中のＮｂ量の６０％以上に制御される。本発明において、Ｎｂ炭窒化物とは、ＮｂＣＮだけでなく、ＮｂＣ及びＮｂＮも包含し、さらにはＮｂＣＮ、ＮｂＣ及びＮｂＮにおけるＮｂの一部がＴｉなどの他の１つ又は複数の元素に置換されたものも包含するものである。Ｎｂ炭窒化物の形態及び量を上記の範囲内に制御することで、高温強度の向上に有効な粒径２０ｎｍ以上のＮｂ炭窒化物を十分な量において鋼中に存在させることができるので、上で説明した指数Ａによる化学組成の制御との組み合わせにより、鋼板の高温強度を顕著に向上させることができる。その結果として、鋼板の製造過程におけるヒートバックルの発生を確実に抑制又は低減することが可能となる。鋼板の高温強度を向上させる観点からは、全Ｎｂ炭窒化物中のＮｂ量及びそれに対する粒径が２０ｎｍ以上のＮｂ炭窒化物中のＮｂ量の割合は大きいほど好ましく、例えば、全Ｎｂ炭窒化物中のＮｂ量は０．００６％以上、０．００８％以上、０．０１０％以上又は０．０１２％以上であってもよい。上限は特に限定されないが、例えば、全Ｎｂ炭窒化物中のＮｂ量は０．１００％以下、０．０６０％以下、０．０４０％以下又は０．０３０％以下であってもよい。同様に、粒径が２０ｎｍ以上のＮｂ炭窒化物中のＮｂ量は、全Ｎｂ炭窒化物中のＮｂ量の６２％以上、６５％以上、６８％以上又は７０％以上であってもよい。上限は特に限定されないが、例えば、粒径が２０ｍ以上のＮｂ炭窒化物中のＮｂ量は、全Ｎｂ炭窒化物中のＮｂ量の９５％以下、９０％以下又は８５％以下であってもよい。Ｎｂ炭窒化物の粒径は２０ｎｍ以上であれば高温強度向上の効果が得られ、粒径が大きすぎることでその効果が大きく低下することはない。したがって、Ｎｂ炭窒化物における粒径の上限は特に限定されないが、例えば、Ｎｂ炭窒化物の粒径は１０００ｎｍ以下すなわち１μｍ以下であってもよい。 [The amount of Nb in all Nb carbonitrides is 0.004% or more, and the amount of Nb in Nb carbonitrides having a particle size of 20 nm or more is 60% or more of the amount of Nb in all Nb carbonitrides]
The steel plate according to the embodiment of the present invention contains Nb carbonitrides in the metal structure, the Nb content in all Nb carbonitrides is 0.004% or more, and the Nb content in Nb carbonitrides having a particle size of 20 nm or more is controlled to 60% or more of the Nb content in all Nb carbonitrides. In the present invention, Nb carbonitrides include not only NbCN but also NbC and NbN, and further include NbCN, NbC and NbN in which a part of Nb is replaced by one or more other elements such as Ti. By controlling the form and amount of Nb carbonitrides within the above range, it is possible to cause a sufficient amount of Nb carbonitrides having a particle size of 20 nm or more, which is effective for improving high-temperature strength, to be present in the steel, and therefore, in combination with the control of the chemical composition by the index A described above, the high-temperature strength of the steel plate can be significantly improved. As a result, it is possible to reliably suppress or reduce the occurrence of heat buckling during the manufacturing process of the steel plate. From the viewpoint of improving the high-temperature strength of the steel sheet, the ratio of the Nb amount in all Nb carbonitrides and the Nb amount in Nb carbonitrides having a particle size of 20 nm or more to the total Nb carbonitrides is preferably as large as possible. For example, the Nb amount in all Nb carbonitrides may be 0.006% or more, 0.008% or more, 0.010% or more, or 0.012% or more. The upper limit is not particularly limited, but for example, the Nb amount in all Nb carbonitrides may be 0.100% or less, 0.060% or less, 0.040% or less, or 0.030% or less. Similarly, the Nb amount in Nb carbonitrides having a particle size of 20 nm or more may be 62% or more, 65% or more, 68% or more, or 70% or more of the Nb amount in all Nb carbonitrides. The upper limit is not particularly limited, but for example, the Nb amount in Nb carbonitrides having a particle size of 20 nm or more may be 95% or less, 90% or less, or 85% or less of the Nb amount in all Nb carbonitrides. If the particle size of the Nb carbonitride is 20 nm or more, the effect of improving high-temperature strength can be obtained, and if the particle size is too large, the effect is not significantly reduced. Therefore, the upper limit of the particle size of the Nb carbonitride is not particularly limited, but the particle size of the Nb carbonitride may be, for example, 1000 nm or less, i.e., 1 μm or less.
［全Ｎｂ炭窒化物中のＮｂ量及びそれに対する粒径が２０ｎｍ以上のＮｂ炭窒化物中のＮｂ量の割合の測定］
　全Ｎｂ炭窒化物中のＮｂ量及びそれに対する粒径が２０ｎｍ以上のＮｂ炭窒化物中のＮｂ量の割合は、以下のようにして決定される。まず、板厚１／２位置から試験片を採取し、採取した試験片を電解液（１０体積％アセチルアセトン－１質量％塩化テトラメチルアンモニウム・メタノール）中で、定電流電解し、電解後の試験片に付着した析出物をヘキサメタリン酸ナトリウム水溶液に分散させ、次いで孔径０．０２μｍφ（２０ｎｍφ）の多孔質フィルタでろ過回収する。次に、フィルタ上の析出物に含まれるＮｂ量をＩＣＰ発光分光分析法で測定し、フィルタに捕集された粒径が２０ｎｍ以上のＮｂ析出物として析出したＮｂの鋼中含有率を求める。また、フィルタを通過したろ液に含まれる粒径が２０ｎｍ未満の析出物は、ろ液中のＮｂ量をＩＣＰ発光分光分析法で測定する。これらの方法で求めた粒径が２０ｎｍ以上のＮｂ析出物のＮｂ量と２０ｎｍ未満のＮｂ析出物のＮｂ量を足し合わせることで、Ｎｂ炭窒化物として析出したＮｂ総質量を求め、得られた値を全Ｎｂ炭窒化物中のＮｂ量として決定する。また、粒径が２０ｎｍ以上のＮｂ炭窒化物として析出したＮｂ量を用いて、Ｎｂ炭窒化物として析出したＮｂ総質量に対する割合を算出し、算出された値を全Ｎｂ炭窒化物中のＮｂ量に対する粒径が２０ｎｍ以上のＮｂ炭窒化物中のＮｂ量の割合として決定する。 [Measurement of the amount of Nb in all Nb carbonitrides and the ratio of the amount of Nb in Nb carbonitrides having a particle size of 20 nm or more]
The amount of Nb in the total Nb carbonitrides and the ratio of the amount of Nb in the Nb carbonitrides having a particle size of 20 nm or more are determined as follows. First, a test piece is taken from the 1/2 position of the plate thickness, and the taken test piece is electrolyzed at a constant current in an electrolytic solution (10% by volume acetylacetone-1% by mass tetramethylammonium chloride-methanol), and the precipitates attached to the test piece after electrolysis are dispersed in an aqueous sodium hexametaphosphate solution, and then filtered and collected with a porous filter having a pore size of 0.02 μmφ (20 nmφ). Next, the amount of Nb contained in the precipitates on the filter is measured by ICP emission spectroscopy, and the content of Nb in the steel precipitated as Nb precipitates having a particle size of 20 nm or more collected on the filter is obtained. In addition, the amount of Nb in the filtrate having a particle size of less than 20 nm contained in the filtrate that has passed through the filter is measured by ICP emission spectroscopy. The total mass of Nb precipitated as Nb carbonitrides is calculated by adding together the amount of Nb in Nb precipitates having a particle size of 20 nm or more and the amount of Nb in Nb precipitates having a particle size of less than 20 nm, and the obtained value is determined as the amount of Nb in all Nb carbonitrides. In addition, the amount of Nb precipitated as Nb carbonitrides having a particle size of 20 nm or more is used to calculate the ratio of the amount of Nb precipitated as Nb carbonitrides to the total mass of Nb, and the calculated value is determined as the ratio of the amount of Nb in Nb carbonitrides having a particle size of 20 nm or more to the amount of Nb in all Nb carbonitrides.
［マルテンサイトの平均粒子間隔：２．５μｍ以下］
　本発明の実施形態においては、硬質組織であるマルテンサイトの平均粒子間隔は２．５μｍ以下に制御される。マルテンサイトの平均粒子間隔は、ミクロ領域における硬質組織分布の均一性を表す指標である。マルテンサイトの平均粒子間隔が小さいほど、硬質組織が密にかつ均一に分散していることを意味し、よって均一性が高いといえる。プレス成形後の鋼板の外観性は、プレス成形時の鋼板の変形量がとりわけ鋼板の幅方向において均一であるほど良好なものとなる。鋼板の変形量は、硬質組織の分布状態の影響を強く受けるため、鋼板の変形量を鋼板の幅方向で均一にするためには、金属組織中の硬質組織の分布を均一にする必要がある。後で説明するマルテンサイトの面積率における標準偏差の制御に加えて、マルテンサイトの平均粒子間隔を２．５μｍ以下に制御することで、プレス成形等の成形時においても鋼板の変形量を幅方向においてより均一にすることができ、結果として良好な成形後外観を達成することができる。マルテンサイトの平均粒子間隔は、好ましくは２．４μｍ以下、より好ましくは２．２μｍ以下、最も好ましくは２．０μｍ以下又は１．８μｍ以下である。下限は特に限定されないが、例えば、マルテンサイトの平均粒子間隔は０．５μｍ以上、０．８μｍ以上又は１．０μｍ以上であってもよい。 [Average grain spacing of martensite: 2.5 μm or less]
In an embodiment of the present invention, the average particle spacing of the martensite, which is a hard structure, is controlled to 2.5 μm or less. The average particle spacing of the martensite is an index that indicates the uniformity of the hard structure distribution in the micro region. The smaller the average particle spacing of the martensite, the more densely and uniformly the hard structure is dispersed, and therefore the higher the uniformity. The more uniform the deformation amount of the steel sheet during press forming is, particularly in the width direction of the steel sheet, the better the appearance of the steel sheet after press forming. Since the deformation amount of the steel sheet is strongly affected by the distribution state of the hard structure, in order to make the deformation amount of the steel sheet uniform in the width direction of the steel sheet, it is necessary to make the distribution of the hard structure in the metal structure uniform. In addition to controlling the standard deviation in the area ratio of martensite, which will be described later, by controlling the average particle spacing of the martensite to 2.5 μm or less, the deformation amount of the steel sheet can be made more uniform in the width direction even during forming such as press forming, and as a result, a good post-forming appearance can be achieved. The average grain spacing of martensite is preferably 2.4 μm or less, more preferably 2.2 μm or less, and most preferably 2.0 μm or less or 1.8 μm or less. Although there is no particular lower limit, for example, the average grain spacing of martensite may be 0.5 μm or more, 0.8 μm or more, or 1.0 μm or more.
［マルテンサイトの平均粒子間隔の測定］
　マルテンサイトの平均粒子間隔は、以下のようにして決定される。まず、板面に垂直な方向の鋼板断面を有する試料を採取し、当該断面を観察面とする。この観察面のうち板厚１／４を中心とする板厚１／８～３／８の範囲内で１００μｍ×１００μｍの領域を観察領域とし、ＦＥ－ＳＥＭ（例えばＪＥＯＬ社製　ＪＳＭ－７２００Ｆ、加速電圧１５ｋＶ、倍率１０００～５０００倍にて測定）を用いてマルテンサイトを同定する。具体的には、画像解析ソフトウェアＩｍａｇｅ　Ｊ（Ｖｅｒ．１．５４ｆ）を用いて、金属組織を輝度の違いにより二値化し、マルテンサイトを同定する。具体的には、ナイタール液を用いた場合は、画像データの黒色部分がフェライトであり、腐食されていない白色部分がマルテンサイトと残留オーステナイトの合計組織である。しかしながら、本発明の実施形態に係る鋼板では、白色組織をマルテンサイトとみなすことができる。次に、同定されたマルテンサイトのうち、全ての隣り合うマルテンサイト粒の中心（重心）間の距離を粒子間隔として画像解析に基づいて算出し、算出された粒子間隔の平均値を求める。この操作を他の２つの観察領域にて行い、得られた３つの値の平均値をマルテンサイト（厳密にはマルテンサイト及び／又は残留オーステナイトを含む粒子）の平均粒子間隔として決定する。 [Measurement of average particle spacing in martensite]
The average grain spacing of martensite is determined as follows. First, a sample having a steel sheet cross section perpendicular to the sheet surface is taken, and the cross section is used as the observation surface. A region of 100 μm×100 μm within the range of 1/8 to 3/8 of the sheet thickness centered at 1/4 of the sheet thickness is used as the observation region of this observation surface, and martensite is identified using FE-SEM (for example, JEOL JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 1000 to 5000 times). Specifically, the image analysis software Image J (Ver. 1.54f) is used to binarize the metal structure based on the difference in brightness, and martensite is identified. Specifically, when a nital solution is used, the black part of the image data is ferrite, and the uncorroded white part is the total structure of martensite and retained austenite. However, in the steel sheet according to the embodiment of the present invention, the white structure can be regarded as martensite. Next, the distance between the centers (centers of gravity) of all adjacent martensite grains among the identified martensite grains is calculated as the particle spacing based on image analysis, and the average of the calculated particle spacings is obtained. This operation is performed in the other two observation regions, and the average of the three values obtained is determined as the average particle spacing of martensite (strictly speaking, particles including martensite and/or retained austenite).
［圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差が１．５％以下］
　本発明の実施形態においては、圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差は１．５％以下に制御される。当該標準偏差は、マクロ領域における硬質組織の均一性を表す指標である。プレス成形時に課題となる外観性は、鋼板の幅方向における変形量の差に起因した鋼板表面の微小な凹凸に依存している。このため、圧延方向及び板厚方向に垂直な方向の板厚内に含まれる硬質組織の面積率におけるばらつきが大きいと、鋼板の幅方向における変形量に差が生じ、その結果として鋼板表面に微小な凹凸が生成することとなる。したがって、圧延方向及び板厚方向に垂直な方向すなわち鋼板の幅方向のマルテンサイトの面積率における標準偏差を低減することが有効である。より具体的には、先に述べたマルテンサイトの平均粒子間隔の制御に加えて、当該標準偏差を１．５％以下に制御することで、プレス成形等の成形時においても鋼板の幅方向における変形量のばらつきをより小さくすることができ、結果として良好な成形後外観を達成することができる。圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差は、好ましくは１．４％以下、より好ましくは１．２％以下、最も好ましくは１．０％以下である。下限は特に限定されないが、例えば、当該標準偏差は０．１％以上、０．３％以上又は０．５％以上であってもよい。 [Standard deviation in area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is 1.5% or less]
In an embodiment of the present invention, the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is controlled to 1.5% or less. The standard deviation is an index representing the uniformity of the hard structure in the macro region. The appearance, which is an issue during press forming, depends on the minute irregularities on the steel sheet surface caused by the difference in the amount of deformation in the width direction of the steel sheet. For this reason, if the variation in the area ratio of the hard structure contained in the plate thickness in the direction perpendicular to the rolling direction and the plate thickness direction is large, a difference in the amount of deformation in the width direction of the steel sheet occurs, and as a result, minute irregularities are generated on the steel sheet surface. Therefore, it is effective to reduce the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction, i.e., the width direction of the steel sheet. More specifically, in addition to the control of the average particle spacing of martensite described above, by controlling the standard deviation to 1.5% or less, the variation in the amount of deformation in the width direction of the steel sheet can be further reduced even during forming such as press forming, and as a result, a good post-forming appearance can be achieved. The standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is preferably 1.4% or less, more preferably 1.2% or less, and most preferably 1.0% or less. The lower limit is not particularly limited, but the standard deviation may be, for example, 0.1% or more, 0.3% or more, or 0.5% or more.
［圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差の測定］
　圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差は、以下のようにして決定される。まず、圧延方向及び板厚方向に対して垂直な方向に５０ｍｍの領域の鋼板断面における金属組織画像を取得する。１０ｍｍ又はそれよりも小さい画像の場合、複数枚の画像を取得し、それらをつなぎ合わせて５０ｍｍとしてもよい。圧延方向が不明な場合には、任意の方向に対して０°、４５°、９０°及び１３５°の向きにて断面を観察し、その中で析出物のアスペクト比が最も高かった断面を圧延方向に平行な断面とし、それと板厚方向に垂直な方向を圧延方向及び板厚方向に垂直な方向として特定する。次に、取得した画像を圧延方向及び板厚方向に対して垂直な方向に１００μｍ（０．１ｍｍ）毎に分割して、分割した各範囲で板厚全体におけるマルテンサイトの面積率を算出する。合計５００個の各分割画像から算出したマルテンサイト面積率に基づいて、マルテンサイトの面積率における標準偏差を算出する。各分割領域におけるマルテンサイトの面積率は、［金属組織の同定及び面積率の算出］の項で説明した手順に従って算出される。なお、残留オーステナイトの面積率については、便宜的に、各分割領域における測定結果の代わりに、上述の圧延方向及び板厚方向に対して垂直な方向に５０ｍｍの領域の鋼板断面における測定結果を流用してもよい。 [Measurement of standard deviation in area fraction of martensite in the direction perpendicular to the rolling direction and the thickness direction]
The standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is determined as follows. First, a metal structure image of a steel plate cross section in a region of 50 mm in the direction perpendicular to the rolling direction and the plate thickness direction is obtained. In the case of an image of 10 mm or smaller, multiple images may be obtained and joined to make 50 mm. When the rolling direction is unknown, the cross section is observed at 0°, 45°, 90°, and 135° to an arbitrary direction, and the cross section with the highest aspect ratio of the precipitates among them is determined as the cross section parallel to the rolling direction, and the direction perpendicular to the plate thickness direction is determined as the direction perpendicular to the rolling direction and the plate thickness direction. Next, the obtained image is divided into 100 μm (0.1 mm) in the direction perpendicular to the rolling direction and the plate thickness direction, and the area ratio of martensite in the entire plate thickness is calculated for each divided range. Based on the martensite area ratio calculated from each of the total 500 divided images, the standard deviation in the area ratio of martensite is calculated. The area ratio of martensite in each divided region is calculated according to the procedure described in the section "Identification of metal structure and calculation of area ratio". For the area ratio of retained austenite, the measurement results of the steel sheet cross section in a region of 50 mm in the direction perpendicular to the rolling direction and the sheet thickness direction may be used instead of the measurement results of each divided region.
［フェライトの平均結晶粒径：３．０～２５．０μｍ］
　本発明の好ましい実施形態によれば、金属組織中のフェライトの平均結晶粒径は３．０～２５．０μｍである。フェライトの平均結晶粒径をこのような微細な範囲内に制御することで、鋼板の外観、特には成形後の外観をさらに向上させることが可能となる。フェライトの平均結晶粒径は、５．０μｍ以上、７．０μｍ以上、８．０μｍ以上、９．０μｍ以上又は１０．０μｍ以上であってもよい。同様に、フェライトの平均結晶粒径は、２２．０μｍ以下、２０．０μｍ以下、１６．０μｍ以下、１４．０μｍ以下又は１２．０μｍ以下であってもよい。 [Average grain size of ferrite: 3.0 to 25.0 μm]
According to a preferred embodiment of the present invention, the average grain size of ferrite in the metal structure is 3.0 to 25.0 μm. By controlling the average grain size of ferrite within such a fine range, it is possible to further improve the appearance of the steel sheet, particularly the appearance after forming. The average grain size of ferrite may be 5.0 μm or more, 7.0 μm or more, 8.0 μm or more, 9.0 μm or more, or 10.0 μm or more. Similarly, the average grain size of ferrite may be 22.0 μm or less, 20.0 μm or less, 16.0 μm or less, 14.0 μm or less, or 12.0 μm or less.
　鋼板におけるフェライトの平均結晶粒径は、以下のようにして決定される。まず、板面に垂直な方向の鋼板断面を有する試料を採取し、当該断面を観察面とする。この観察面のうち板厚１／４位置を中心とする板厚１／８位置～３／８位置の範囲内で１００μｍ×１００μｍの領域を観察領域とし、ＦＥ－ＳＥＭ（例えばＪＥＯＬ社製　ＪＳＭ－７２００Ｆ、加速電圧１５ｋＶ、倍率５００～２０００倍にて測定）を用いてマルテンサイトを同定する。具体的には、画像解析ソフトウェアＩｍａｇｅ　Ｊ（Ｖｅｒ．１．５４ｆ）を用いて、金属組織を輝度の違いにより二値化し、フェライトを同定する。具体的には、ナイタール液を用いた場合は、画像データの黒色部分がフェライトであり、腐食されていない白色部分がマルテンサイトと残留オーステナイトの合計組織である。次に、同定された全てのフェライトの円相当直径を算出する。この操作を他の２つの観察領域にて行い、３つの観察領域で得られた全てのフェライトの円相当直径を算術平均し、得られた値をフェライトの平均結晶粒径として決定する。 The average grain size of ferrite in steel plate is determined as follows. First, a sample having a steel plate cross section perpendicular to the plate surface is taken, and the cross section is used as the observation surface. A 100 μm x 100 μm region within the range of 1/8 to 3/8 plate thickness positions, centered at the 1/4 plate thickness position, is used as the observation region on this observation surface, and martensite is identified using an FE-SEM (e.g., JEOL's JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 500 to 2000 times). Specifically, the metal structure is binarized based on the difference in brightness using image analysis software Image J (Ver. 1.54f), and ferrite is identified. Specifically, when nital solution is used, the black parts of the image data are ferrite, and the uncorroded white parts are the combined structure of martensite and retained austenite. Next, the circle equivalent diameter of all identified ferrite is calculated. This operation is repeated in the other two observation areas, and the arithmetic average of the circular equivalent diameters of all the ferrite particles obtained in the three observation areas is calculated, and the obtained value is determined as the average grain size of the ferrite.
［マルテンサイトの平均結晶粒径：１．０～５．０μｍ］
　本発明の好ましい実施形態によれば、金属組織中のマルテンサイトの平均結晶粒径は１．０～５．０μｍである。マルテンサイトの平均結晶粒径をこのような微細な範囲内に制御することで、鋼板の外観、特には成形後の外観をさらに向上させることが可能となる。マルテンサイトの平均結晶粒径は、１．２μｍ以上、１．５μｍ以上、１．７μｍ以上又は２．０μｍ以上であってもよい。同様に、マルテンサイトの平均結晶粒径は、４．７μｍ以下、４．５μｍ以下、４．２μｍ以下、４．０μｍ以下、３．８μｍ以下、３．６μｍ以下又は３．４μｍ以下であってもよい。 [Average grain size of martensite: 1.0 to 5.0 μm]
According to a preferred embodiment of the present invention, the average grain size of martensite in the metal structure is 1.0 to 5.0 μm. By controlling the average grain size of martensite within such a fine range, it is possible to further improve the appearance of the steel sheet, particularly the appearance after forming. The average grain size of martensite may be 1.2 μm or more, 1.5 μm or more, 1.7 μm or more, or 2.0 μm or more. Similarly, the average grain size of martensite may be 4.7 μm or less, 4.5 μm or less, 4.2 μm or less, 4.0 μm or less, 3.8 μm or less, 3.6 μm or less, or 3.4 μm or less.
　マルテンサイトの平均結晶粒径は、以下のようにして決定される。まず、板面に垂直な方向の鋼板断面を有する試料を採取し、当該断面を観察面とする。この観察面のうち板厚１／４位置を中心とする板厚１／８位置～３／８位置の範囲内で１００μｍ×１００μｍの領域を観察領域とし、ＦＥ－ＳＥＭ（例えばＪＥＯＬ社製　ＪＳＭ－７２００Ｆ、加速電圧１５ｋＶ、倍率５００～２０００倍にて測定）を用いてマルテンサイトを同定する。具体的には、画像解析ソフトウェアＩｍａｇｅ　Ｊ（Ｖｅｒ．１．５４ｆ）を用いて、金属組織を輝度の違いにより二値化し、マルテンサイトを同定する。具体的には、ナイタール液を用いた場合は、画像データの黒色部分がフェライトであり、腐食されていない白色部分がマルテンサイトと残留オーステナイトの合計組織である。本発明の実施形態に係る鋼板では、白色組織をマルテンサイトとみなす。次に、同定された全てのマルテンサイトの円相当直径を算出する。この操作を他の２つの観察領域にて行い、３つの観察領域で得られた全てのマルテンサイトの円相当直径を算術平均し、得られた値をマルテンサイト（厳密にはマルテンサイト及び／又は残留オーステナイトを含む粒子）の平均結晶粒径として決定する。 The average crystal grain size of martensite is determined as follows. First, a sample having a steel plate cross section perpendicular to the plate surface is taken, and the cross section is used as the observation surface. A 100 μm x 100 μm region within the range of 1/8 to 3/8 plate thickness positions centered at the 1/4 plate thickness position is used as the observation region on this observation surface, and martensite is identified using an FE-SEM (for example, JEOL's JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 500 to 2000 times). Specifically, the image analysis software Image J (Ver. 1.54f) is used to binarize the metal structure based on the difference in brightness, and martensite is identified. Specifically, when a nital solution is used, the black parts of the image data are ferrite, and the uncorroded white parts are the combined structure of martensite and retained austenite. In the steel plate according to the embodiment of the present invention, the white structure is regarded as martensite. Next, the circle equivalent diameter of all identified martensite is calculated. This operation is repeated in the other two observation regions, and the circle equivalent diameters of all the martensite particles obtained in the three observation regions are arithmetically averaged, and the obtained value is determined as the average crystal grain size of the martensite (strictly speaking, particles containing martensite and/or retained austenite).
［マルテンサイトの平均アスペクト比：２．５以上］
　本発明の好ましい実施形態によれば、金属組織中のマルテンサイトの平均アスペクト比は２．５以上である。マルテンサイトの平均アスペクト比を２．５以上に制御することで、より大きなひずみが付与された状態とすることができ、鋼板の強度を向上させることが可能となる。マルテンサイトの平均アスペクト比は、２．６以上、２．８以上又は３．０以上であってもよい。上限は特に限定されないが、例えば、マルテンサイトの平均アスペクト比は、４．０以下、３．８以下又は３．６以下であってもよい。 [Average aspect ratio of martensite: 2.5 or more]
According to a preferred embodiment of the present invention, the average aspect ratio of martensite in the metal structure is 2.5 or more. By controlling the average aspect ratio of martensite to 2.5 or more, a state in which a larger strain is imparted can be achieved, and the strength of the steel sheet can be improved. The average aspect ratio of martensite may be 2.6 or more, 2.8 or more, or 3.0 or more. The upper limit is not particularly limited, but for example, the average aspect ratio of martensite may be 4.0 or less, 3.8 or less, or 3.6 or less.
　マルテンサイトの平均アスペクト比は、以下のようにして決定される。まず、板面に垂直な方向の鋼板断面を有する試料を採取し、当該断面を観察面とする。この観察面のうち板厚１／４位置を中心とする板厚１／８位置～３／８位置の範囲内で１００μｍ×１００μｍの領域を観察領域とし、ＦＥ－ＳＥＭ（例えばＪＥＯＬ社製　ＪＳＭ－７２００Ｆ、加速電圧１５ｋＶ、倍率１０００～５０００倍にて測定）を用いてマルテンサイトを同定する。具体的には、画像解析ソフトウェアＩｍａｇｅ　Ｊ（Ｖｅｒ．１．５４ｆ）を用いて、金属組織を輝度の違いにより二値化し、マルテンサイトを同定する。具体的には、ナイタール液を用いた場合は、画像データの黒色部分がフェライトであり、腐食されていない白色部分がマルテンサイトと残留オーステナイトの合計組織である。本発明の実施形態に係る鋼板では、白色組織をマルテンサイトとみなす。得られた画像データにおいて、画像解析ソフトウェアＩｍａｇｅ　Ｊ（Ｖｅｒ．１．５４ｆ）を用いて、全てのマルテンサイト粒のアスペクト比を算出する。画像上の粒子（結晶粒）のアスペクト比は、画像解析ソフトウェアＩｍａｇｅ　Ｊ（Ｖｅｒ．１．５４ｆ）に搭載の機能により測定することができる。次いで、この操作を他の２つの観察領域にて行い、３つの観察領域で得られた全てのマルテンサイト粒のアスペクト比を算術平均し、得られた値をマルテンサイト（厳密にはマルテンサイト及び／又は残留オーステナイトを含む粒子）の平均アスペクト比として決定する。 The average aspect ratio of martensite is determined as follows. First, a sample having a steel plate cross section perpendicular to the plate surface is taken, and the cross section is used as the observation surface. A 100 μm x 100 μm area is taken from this observation surface within the range of 1/8 to 3/8 plate thickness positions, centered at the 1/4 plate thickness position, and martensite is identified using an FE-SEM (e.g., JEOL JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 1000 to 5000 times). Specifically, the image analysis software Image J (Ver. 1.54f) is used to binarize the metal structure based on the difference in brightness, and martensite is identified. Specifically, when a nital solution is used, the black parts of the image data are ferrite, and the uncorroded white parts are the combined structure of martensite and retained austenite. In the steel plate according to the embodiment of the present invention, the white structure is considered to be martensite. The aspect ratios of all martensite grains are calculated from the obtained image data using the image analysis software Image J (Ver. 1.54f). The aspect ratios of the particles (crystal grains) on the image can be measured using a function built into the image analysis software Image J (Ver. 1.54f). This operation is then performed in the other two observation regions, and the aspect ratios of all martensite grains obtained in the three observation regions are arithmetically averaged, and the obtained value is determined as the average aspect ratio of martensite (strictly speaking, particles containing martensite and/or retained austenite).
［板厚］
　本発明の実施形態に係る鋼板は、特に限定されないが、例えば０．１～２．０ｍｍの板厚を有する。このような板厚を有する鋼板は、自動車部材としてのドアやフード等の外板部材のための素材として用いる場合に好適である。板厚は０．２ｍｍ以上、０．３ｍｍ以上、０．４ｍｍ以上であってもよい。同様に、板厚は１．８ｍｍ以下、１．５ｍｍ以下、１．２ｍｍ以下又は１．０ｍｍ以下であってもよい。例えば、板厚を０．２ｍｍ以上とすることで、成形品形状を平坦に維持することが容易になり、寸法精度及び形状精度が向上するという追加の効果を得ることができる。一方、板厚を１．０ｍｍ以下とすることで部材の軽量化効果が顕著となる。鋼板の板厚はマイクロメータによって測定される。 [Thickness]
The steel plate according to the embodiment of the present invention has a plate thickness of, for example, 0.1 to 2.0 mm, but is not particularly limited thereto. A steel plate having such a plate thickness is suitable for use as a material for exterior plate members such as doors and hoods as automobile members. The plate thickness may be 0.2 mm or more, 0.3 mm or more, or 0.4 mm or more. Similarly, the plate thickness may be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, or 1.0 mm or less. For example, by making the plate thickness 0.2 mm or more, it becomes easier to maintain the shape of the molded product flat, and an additional effect of improving dimensional accuracy and shape accuracy can be obtained. On the other hand, by making the plate thickness 1.0 mm or less, the weight reduction effect of the member becomes remarkable. The plate thickness of the steel plate is measured by a micrometer.
［めっき］
　本発明の実施形態に係る鋼板は、冷間圧延鋼板であるが、耐食性の向上等を目的として、表面にめっき層をさらに含んでもよい。めっき層は、溶融めっき層及び電気めっき層のいずれでもよい。つまり、本発明の実施形態に係る鋼板は、その表面に溶融めっき層又は電気めっき層を有する冷間圧延鋼板であってもよい。溶融めっき層は、例えば、溶融亜鉛めっき層（ＧＩ）、合金化溶融亜鉛めっき層（ＧＡ）、溶融アルミニウムめっき層、溶融Ｚｎ－Ａｌ合金めっき層、溶融Ｚｎ－Ａｌ－Ｍｇ合金めっき層、溶融Ｚｎ－Ａｌ－Ｍｇ－Ｓｉ合金めっき層等を含む。電気めっき層は、例えば、電気亜鉛めっき層（ＥＧ）、電気Ｚｎ－Ｎｉ合金めっき層等を含む。好ましくは、めっき層は、溶融亜鉛めっき層、合金化溶融亜鉛めっき層、又は電気亜鉛めっき層である。めっき層の付着量は、特に制限されず一般的な付着量でよい。 [Plating]
The steel sheet according to the embodiment of the present invention is a cold-rolled steel sheet, but may further include a plating layer on the surface for the purpose of improving corrosion resistance or the like. The plating layer may be either a hot-dip plating layer or an electroplating layer. That is, the steel sheet according to the embodiment of the present invention may be a cold-rolled steel sheet having a hot-dip plating layer or an electroplating layer on its surface. The hot-dip plating layer includes, for example, a hot-dip galvanized layer (GI), a hot-dip galvannealed layer (GA), a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, a hot-dip Zn-Al-Mg-Si alloy plating layer, and the like. The electroplating layer includes, for example, an electrogalvanized layer (EG), an electrogalvanized Zn-Ni alloy plating layer, and the like. Preferably, the plating layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer. The coating weight of the plating layer is not particularly limited and may be a general coating weight.
［機械特性］
　上記の化学組成及び金属組織を有する鋼板によれば、高い引張強さ、具体的には５４０ＭＰａ以上の引張強さを達成することができる。引張強さは、好ましくは５７０ＭＰａ以上、より好ましくは６００ＭＰａ以上である。上限は特に限定されないが、例えば、引張強さは９８０ＭＰａ以下、９００ＭＰａ以下、８５０ＭＰａ以下、８３０ＭＰａ以下又は８００ＭＰａ以下であってもよい。引張強さを８５０ＭＰａ以下とすることで、鋼板をプレス加工する際の成形性を確保しやすいという利点がある。引張強さは、圧延方向及び板厚方向に直角な方向を試験方向とするＪＩＳ　Ｚ２２４１：２０１１の５号引張試験片を鋼板から採取し、ＪＩＳ　Ｚ２２４１：２０１１に準拠して引張試験を行うことで測定される。 [Mechanical properties]
According to the steel plate having the above chemical composition and metal structure, a high tensile strength, specifically a tensile strength of 540 MPa or more can be achieved. The tensile strength is preferably 570 MPa or more, more preferably 600 MPa or more. The upper limit is not particularly limited, but for example, the tensile strength may be 980 MPa or less, 900 MPa or less, 850 MPa or less, 830 MPa or less, or 800 MPa or less. By setting the tensile strength to 850 MPa or less, there is an advantage that it is easy to ensure formability when the steel plate is press-processed. The tensile strength is measured by taking a tensile test piece No. 5 of JIS Z2241:2011 from the steel plate, with the test direction being perpendicular to the rolling direction and the plate thickness direction, and performing a tensile test in accordance with JIS Z2241:2011.
　本発明の実施形態に係る鋼板は、製造過程におけるヒートバックルの発生を抑制することができるとともに、高強度、例えば５４０ＭＰａ以上の引張強さと、プレス加工等の成形後における優れた外観とを両立することができる。このため、本発明の実施形態に係る鋼板は、これらの特性の両立が求められる技術分野の部品などにおいて使用するのに特に有用である。好ましい実施形態においては、本発明の実施形態に係る鋼板を含む外板部材、特には自動車の外板部材が提供される。自動車の外板部材の一例としては、高い意匠性が求められるルーフ、フード、フェンダー及びドア等が挙げられる。これらの外板部材、特には自動車の外板部材は、これらの外板部材の少なくとも一部において本発明の実施形態に係る鋼板を含んでいればよく、それゆえこれらの外板部材の少なくとも一部において先に述べた化学組成及び金属組織の特徴を満たすものである。プレス成形等の成形において加工の程度が比較的低い鋼板の部位では、金属組織の特徴は成形前後において特に変化しない。 The steel plate according to the embodiment of the present invention can suppress the occurrence of heat buckling during the manufacturing process, and can achieve both high strength, for example a tensile strength of 540 MPa or more, and an excellent appearance after forming such as press working. For this reason, the steel plate according to the embodiment of the present invention is particularly useful for use in parts in technical fields where both of these properties are required. In a preferred embodiment, an exterior plate member, particularly an automobile exterior plate member, is provided that includes the steel plate according to the embodiment of the present invention. Examples of the exterior plate members of an automobile include roofs, hoods, fenders, doors, and the like, which require high designability. These exterior plate members, particularly the exterior plate members of an automobile, only need to include the steel plate according to the embodiment of the present invention in at least a portion of these exterior plate members, and therefore at least a portion of these exterior plate members will satisfy the chemical composition and metal structure characteristics described above. In the area of the steel plate that is relatively lightly processed in forming such as press forming, the characteristics of the metal structure do not change particularly before and after forming.
＜鋼板の製造方法＞
　次に、本発明の実施形態に係る鋼板の好ましい製造方法について説明する。以下の説明は、本発明の実施形態に係る鋼板を製造するための特徴的な方法の例示を意図するものであって、当該鋼板を以下に説明するような製造方法によって製造されるものに限定することを意図するものではない。 <Method of manufacturing steel sheet>
Next, a preferred method for manufacturing the steel plate according to the embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing the steel plate according to the embodiment of the present invention, and is not intended to limit the steel plate to one manufactured by the manufacturing method described below.
　本発明の実施形態に係る鋼板の製造方法は、
　鋼板に関連して上で説明した化学組成を有するスラブを仕上げ圧延し、次いで巻き取ることを含み、下記（ａ）～（ｄ）の条件を満足する熱間圧延工程、
　　（ａ）仕上げ圧延入側温度が１０００～１０８０℃であること、
　　（ｂ）最終パスから２つ前の圧延パスの圧下率が３０％以上であること、
　　（ｃ）最終パスから２つ前の圧延パスの圧下率／最終パスの圧下率の比が１．５～２．５であること、及び
　　（ｄ）巻取温度が５２０～６７０℃であること
　得られた熱間圧延鋼板を７０％以上の圧下率で冷間圧延する冷間圧延工程、
　得られた冷間圧延鋼板をＡｃ３＋１０℃以上の温度に加熱することを含む１次焼鈍工程、並びに
　前記冷間圧延鋼板を加熱して（Ａｃ１＋２０）～８２０℃の温度にて１０～５００秒間保持することを含む２次焼鈍工程
を含むことを特徴としている。以下、各工程について詳しく説明する。 The method for producing a steel sheet according to an embodiment of the present invention includes:
A hot rolling process comprising finish rolling a slab having the chemical composition described above in relation to the steel sheet, followed by coiling, the hot rolling process satisfying the following conditions (a) to (d):
(a) The finish rolling entry temperature is 1000 to 1080°C;
(b) The reduction ratio of the rolling pass two passes before the final pass is 30% or more;
(c) a ratio of the rolling reduction of the rolling pass two passes before the final pass/the rolling reduction of the final pass is 1.5 to 2.5; and (d) a coiling temperature is 520 to 670° C. A cold rolling process in which the obtained hot-rolled steel sheet is cold-rolled at a rolling reduction of 70% or more.
The method is characterized by including a first annealing step including heating the obtained cold-rolled steel sheet to a temperature of Ac3+10°C or higher, and a second annealing step including heating the cold-rolled steel sheet and holding it at a temperature of (Ac1+20) to 820°C for 10 to 500 seconds. Each step will be described in detail below.
［熱間圧延工程］
　まず、鋼板に関連して上で説明した化学組成を有するスラブが熱間圧延に供される。使用するスラブは、生産性の観点から連続鋳造法によって鋳造することが好ましいが、造塊法又は薄スラブ鋳造法によって製造してもよい。スラブは、熱間圧延に先立ち１１００℃以上に加熱することが好ましい。加熱温度を１１００℃以上とすることで、熱間圧延において圧延反力が過度に大きくならず、目的とする製品厚を得やすくすることができる。加熱温度の上限は特に限定されないが、経済上の観点から、加熱温度は１３００℃以下とすることが好ましい。また、加熱されたスラブに対し、板厚調整等のために、任意選択で仕上げ圧延の前に粗圧延を施してもよい。このような粗圧延は、所望のシートバー寸法が確保できればよく、その条件は特に限定されない。 [Hot rolling process]
First, a slab having the chemical composition described above in relation to the steel plate is subjected to hot rolling. The slab to be used is preferably cast by a continuous casting method from the viewpoint of productivity, but may be manufactured by an ingot casting method or a thin slab casting method. The slab is preferably heated to 1100 ° C or higher prior to hot rolling. By setting the heating temperature to 1100 ° C or higher, the rolling reaction force does not become excessively large in hot rolling, and the desired product thickness can be easily obtained. The upper limit of the heating temperature is not particularly limited, but from an economic viewpoint, the heating temperature is preferably 1300 ° C or lower. In addition, the heated slab may be subjected to rough rolling before finish rolling as an option, for plate thickness adjustment, etc. Such rough rolling is not particularly limited as long as the desired sheet bar dimensions can be secured.
［（ａ）仕上げ圧延入側温度：１０００～１０８０℃］
　加熱されたスラブ又はそれに加えて必要に応じて粗圧延されたスラブは、次に仕上げ圧延を施される。仕上げ圧延は、当該仕上げ圧延の入側温度が１０００～１０８０℃となる条件下で実施する必要がある。仕上げ圧延入側温度をこのような範囲内に制御することで、熱間圧延工程においてＮｂ炭窒化物を適切に析出させることができる。それゆえ、このようなＮｂ炭窒化物の適切な析出に起因して鋼板の高温強度を十分に向上させることができ、その後の１次焼鈍工程等における高温下での熱処理によってもヒートバックルの発生を顕著に抑制することが可能となる。仕上げ圧延入側温度が１０８０℃よりも高いと、再結晶が起こりやすくなって仕上げ圧延後段でのひずみが蓄積されにくくなり、２０ｎｍ以上の粒径を有するＮｂ炭窒化物の析出を促進させることができなくなる。その結果として、全Ｎｂ炭窒化物中のＮｂ量に対し、２０ｎｍ以上の粒径を有するＮｂ炭窒化物中のＮｂ量の所望の割合を達成することができなくなる。一方で、仕上げ圧延入側温度が１０００℃よりも低いと、仕上げ圧延後段でのひずみは蓄積されるものの、Ｎｂ炭窒化物の析出自体が起こりにくくなり、Ｎｂ炭窒化物の粒径も小さくなってしまう。その結果、同様に全Ｎｂ炭窒化物中のＮｂ量に対し、２０ｎｍ以上の粒径を有するＮｂ炭窒化物中のＮｂ量の所望の割合を達成することができなくなる。 [(a) Finish rolling entry temperature: 1000 to 1080° C.]
The heated slab or the slab that has been rough-rolled as necessary is then subjected to finish rolling. The finish rolling needs to be performed under conditions where the inlet temperature of the finish rolling is 1000 to 1080 ° C. By controlling the inlet temperature of the finish rolling within such a range, Nb carbonitrides can be appropriately precipitated in the hot rolling process. Therefore, due to the appropriate precipitation of such Nb carbonitrides, the high-temperature strength of the steel sheet can be sufficiently improved, and the occurrence of heat buckling can be significantly suppressed even by heat treatment at high temperatures in the subsequent primary annealing process or the like. If the inlet temperature of the finish rolling is higher than 1080 ° C, recrystallization is likely to occur, making it difficult to accumulate strain in the later stage of the finish rolling, and it becomes impossible to promote the precipitation of Nb carbonitrides having a grain size of 20 nm or more. As a result, it becomes impossible to achieve the desired ratio of the Nb amount in Nb carbonitrides having a grain size of 20 nm or more to the Nb amount in all Nb carbonitrides. On the other hand, if the finish rolling entry temperature is lower than 1000°C, although strain accumulates in the latter stages of finish rolling, precipitation of Nb carbonitrides itself becomes difficult to occur, and the grain size of the Nb carbonitrides also becomes small. As a result, it becomes impossible to achieve a desired ratio of the amount of Nb in Nb carbonitrides having a grain size of 20 nm or more to the amount of Nb in all Nb carbonitrides.
［（ｂ）最終パスから２つ前の圧延パスの圧下率：３０％以上］
［（ｃ）最終パスから２つ前の圧延パスの圧下率／最終パスの圧下率の比：１．５～２．５］
　本製造方法では、仕上げ圧延は、複数の圧延スタンド、例えば５基以上の圧延スタンドからなるタンデム圧延機を用いて行われる。本製造方法の仕上げ圧延においては、仕上げ圧延後段での圧下率制御が重要であり、より具体的には最終パスから２つ前の圧延パスの圧下率を３０％以上に制御するとともに、最終パスから２つ前の圧延パスの圧下率／最終パスの圧下率の比を１．５～２．５に制御することが重要である。仕上げ圧延後段での圧下率制御をこのような条件下で実施することで、仕上げ圧延の後段にてひずみが適切に蓄積されて、粒径が２０ｎｍ以上のＮｂ窒化物を所望の割合において析出させることができる。それゆえ、その後の１次焼鈍工程等における高温下での熱処理によってもヒートバックルの発生を顕著に抑制することが可能となる。これに対し、例えば、最終パスから２つ前の圧延パスの圧下率が低く、これに関連して最終パスから２つ前の圧延パスの圧下率／最終パスの圧下率の比が１．５未満となる場合には、最終パスから２つ前の圧延パスでの圧下後にひずみの蓄積が不十分となり、粒径が２０ｎｍ以上のＮｂ炭窒化物の析出が不十分なものとなる。一方、最終パスの圧下率が高く、これに関連して最終パスから２つ前の圧延パスの圧下率／最終パスの圧下率の比が１．５未満となる場合には、最終パスでの圧下後におけるひずみの蓄積が顕著となってしまい、仕上げ圧延後に析出する微細なＮｂ炭窒化物が増加し、すなわち粒径が２０ｎｍ以上のＮｂ炭窒化物の割合が少なくなってしまう。したがって、いずれの場合も、全Ｎｂ炭窒化物中のＮｂ量に対し、２０ｎｍ以上の粒径を有するＮｂ炭窒化物中のＮｂ量の所望の割合を達成することができなくなる。 [(b) Reduction ratio of the rolling pass two passes before the final pass: 30% or more]
[(c) Ratio of reduction rate of the rolling pass two passes before the final pass/reduction rate of the final pass: 1.5 to 2.5]
In this manufacturing method, the finish rolling is performed using a tandem rolling mill consisting of a plurality of rolling stands, for example, five or more rolling stands. In the finish rolling of this manufacturing method, it is important to control the reduction ratio in the latter stage of the finish rolling, and more specifically, it is important to control the reduction ratio of the rolling pass two passes before the final pass to 30% or more, and to control the ratio of the reduction ratio of the rolling pass two passes before the final pass/the reduction ratio of the final pass to 1.5 to 2.5. By performing the reduction ratio control in the latter stage of the finish rolling under such conditions, strain is appropriately accumulated in the latter stage of the finish rolling, and Nb nitrides having a grain size of 20 nm or more can be precipitated in a desired ratio. Therefore, it is possible to significantly suppress the occurrence of heat buckling even by heat treatment at high temperatures in the subsequent primary annealing process, etc. In contrast, for example, if the reduction rate of the rolling pass two passes before the final pass is low and the ratio of the reduction rate of the rolling pass two passes before the final pass/the reduction rate of the final pass is less than 1.5, the accumulation of strain after the reduction in the rolling pass two passes before the final pass becomes insufficient, and the precipitation of Nb carbonitrides with a particle size of 20 nm or more becomes insufficient. On the other hand, if the reduction rate of the final pass is high and the ratio of the reduction rate of the rolling pass two passes before the final pass/the reduction rate of the final pass is less than 1.5, the accumulation of strain after the reduction in the final pass becomes significant, and the amount of fine Nb carbonitrides precipitated after finish rolling increases, that is, the ratio of Nb carbonitrides with a particle size of 20 nm or more decreases. Therefore, in either case, it becomes impossible to achieve the desired ratio of the amount of Nb in Nb carbonitrides having a particle size of 20 nm or more relative to the amount of Nb in all Nb carbonitrides.
　一方で、例えば、最終パスから２つ前の圧延パスの圧下率が高いか又は最終パスの圧下率が低く、これに関連して最終パスから２つ前の圧延パスの圧下率／最終パスの圧下率の比が２．５超となる場合には、最終パスから２つ前の圧延パスでの圧下後にひずみの蓄積が大きくなりすぎてしまい、再結晶が起こりやすくなってしまう。その結果として、粒径が２０ｎｍ以上のＮｂ炭窒化物が析出しにくくなり、全Ｎｂ炭窒化物中のＮｂ量に対し、２０ｎｍ以上の粒径を有するＮｂ炭窒化物中のＮｂ量の所望の割合を達成することができなくなる。 On the other hand, for example, if the reduction rate of the rolling pass two passes before the final pass is high or the reduction rate of the final pass is low, and in this regard the ratio of the reduction rate of the rolling pass two passes before the final pass/the reduction rate of the final pass exceeds 2.5, the accumulation of strain after the reduction in the rolling pass two passes before the final pass becomes too large, making it easier for recrystallization to occur. As a result, it becomes difficult for Nb carbonitrides with a grain size of 20 nm or more to precipitate, and it becomes impossible to achieve the desired ratio of the amount of Nb in Nb carbonitrides having a grain size of 20 nm or more to the amount of Nb in all Nb carbonitrides.
［（ｄ）巻取温度：５２０～６７０℃］
　次に、仕上げ圧延された圧延材は、５２０～６７０℃の巻取温度で巻き取られる。巻取温度をこのような温度域に適切に制御することで、スケールの成長を抑制するとともに金属組織を微細かつ均一にすることができ、最終的に得られる金属組織においてマルテンサイトの所望の分散状態を得るのに重要である。巻取温度が６７０℃を超えると、金属組織中のセメンタイトに合金元素が濃化してしまい、その後の１次焼鈍工程における加熱時に未溶解炭化物が残存してしまうこととなる。その結果、１次焼鈍工程において金属組織をベイナイト及び／又はマルテンサイトを主体とする組織によって構成することができず、その後の２次焼鈍工程によってもマルテンサイトの所望の分散状態を得ることができなくなる。より具体的には、その後の２次焼鈍工程によっても、マルテンサイトの平均粒子間隔を２．５μｍ以下に制御することができないか、及び／又は圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差を１．５％以下に制御することができなくなり、すなわちマルテンサイトがミクロな領域とマクロな領域の両方において均一に分散した金属組織を得ることができなくなる。この場合には、ゴーストライン等の発生を十分に抑制することができなくなり、成形後の外観性が低下する。 [(d) Winding temperature: 520-670°C]
Next, the finish-rolled material is coiled at a coiling temperature of 520 to 670° C. By appropriately controlling the coiling temperature within this temperature range, the growth of scale is suppressed and the metal structure is improved. This is important for obtaining a desired distribution state of martensite in the final metal structure. If the coiling temperature exceeds 670°C, the alloy may be added to the cementite in the metal structure. As a result, the metal structure in the first annealing process becomes a structure mainly composed of bainite and/or martensite. Therefore, the desired dispersion state of martensite cannot be obtained even by the subsequent secondary annealing step. More specifically, even after the subsequent secondary annealing process, the average grain spacing of martensite cannot be controlled to 2.5 μm or less, and/or the martensite grains in the direction perpendicular to the rolling direction and the sheet thickness direction are not uniform. In other words, it becomes impossible to obtain a metal structure in which martensite is uniformly dispersed in both the micro-region and the macro-region. In this case, the occurrence of ghost lines and the like cannot be sufficiently suppressed, and the appearance after molding is deteriorated.
　一方で、巻取温度が５２０℃よりも低いと、固溶Ｎｂ量が多くなり、これに関連してＮｂ炭窒化物の析出が少なくなる。このため、全Ｎｂ炭窒化物中の所望のＮｂ量を達成することができなくなるか、及び／又は全Ｎｂ炭窒化物中のＮｂ量に対し、２０ｎｍ以上の粒径を有するＮｂ炭窒化物中のＮｂ量の所望の割合を達成することができなくなる。このような場合には、鋼板の高温強度を十分に向上させることができなくなるため、その後の１次焼鈍工程等の高温下での熱処理においてヒートバックルの発生を十分に抑制又は低減することができなくなる。 On the other hand, if the coiling temperature is lower than 520°C, the amount of dissolved Nb increases, and the precipitation of Nb carbonitrides decreases accordingly. As a result, it becomes impossible to achieve the desired amount of Nb in all Nb carbonitrides, and/or it becomes impossible to achieve the desired ratio of the amount of Nb in Nb carbonitrides having a grain size of 20 nm or more to the amount of Nb in all Nb carbonitrides. In such cases, it becomes impossible to sufficiently improve the high-temperature strength of the steel sheet, and therefore it becomes impossible to sufficiently suppress or reduce the occurrence of heat buckling in the subsequent heat treatment at high temperatures, such as the primary annealing process.
［冷間圧延工程］
　得られた熱間圧延鋼板は、スケールを除去するために適宜酸洗処理を施され、次いで冷間圧延工程に供される。冷間圧延工程では、圧下率が７０％以上となるように熱間圧延鋼板に対して冷間圧延が施される。圧下率をこのような範囲に制御することで、所望の板厚を確保するとともに、その後の１次焼鈍工程の加熱時における再結晶を早期に完了させてＮｂ炭窒化物の所望の析出状態を確実なものとすることができる。圧下率が７０％未満であると、１次焼鈍工程の加熱時に再結晶が遅延し、高温になるまでひずみが残存することとなる。その結果、１次焼鈍工程の加熱中に微細なＮｂ炭窒化物が多く析出してしまい、全Ｎｂ炭窒化物中のＮｂ量に対し、２０ｎｍ以上の粒径を有するＮｂ炭窒化物中のＮｂ量の所望の割合を達成することができなくなる。一方で、冷間圧延工程の圧下率は９０％以下とすることが好ましい。圧下率を９０％以下とすることにより、圧延荷重が過大になって圧延が困難となることを防ぐことができる。圧延パスの回数及びパス毎の圧下率は、特に限定されず、冷間圧延全体の圧下率が上記範囲となるように適宜設定すればよい。 [Cold rolling process]
The obtained hot-rolled steel sheet is subjected to an appropriate pickling treatment to remove scale, and then to a cold rolling process. In the cold rolling process, the hot-rolled steel sheet is cold-rolled so that the reduction is 70% or more. By controlling the reduction within such a range, the desired plate thickness can be secured, and the recrystallization during heating in the subsequent primary annealing process can be completed early to ensure the desired precipitation state of Nb carbonitrides. If the reduction is less than 70%, recrystallization is delayed during heating in the primary annealing process, and strain remains until a high temperature is reached. As a result, a large amount of fine Nb carbonitrides is precipitated during heating in the primary annealing process, and the desired ratio of the Nb amount in Nb carbonitrides having a particle size of 20 nm or more to the Nb amount in the total Nb carbonitrides cannot be achieved. On the other hand, it is preferable that the reduction in the cold rolling process is 90% or less. By setting the reduction rate to 90% or less, it is possible to prevent the rolling load from becoming excessively large, which makes the rolling difficult. The number of rolling passes and the reduction rate for each pass are not particularly limited, and may be appropriately set so that the reduction rate of the entire cold rolling is within the above range.
［１次焼鈍工程］
　得られた冷間圧延鋼板は、次の１次焼鈍工程においてＡｃ３＋１０℃以上の温度に加熱される。Ａｃ３点（℃）は、冷延鋼板から小片を切り出し、当該小片における室温から１０℃／秒で１０００℃への加熱中の熱膨張から求められる。冷間圧延鋼板をＡｃ３＋１０℃以上の温度に加熱することで、オーステナイト化を促進しその後適切に冷却すること、例えば２００℃までの温度域を平均冷却速度３０℃／秒以上で冷却することで、冷却後の鋼板中の金属組織を確実にベイナイト及び／又はマルテンサイトを主体とする組織、例えばフルベイナイト又はフルマルテンサイトによって構成することが可能となる。ここで、ベイナイト及び／又はマルテンサイトを主体とする組織とは、ベイナイト及びマルテンサイトの少なくとも１種を合計の面積率で９０％以上含む組織をいうものであり、フルベイナイトとは、面積率で１００％のベイナイトからなる組織をいうものであり、フルマルテンサイトとは、面積率で１００％のマルテンサイトからなる組織をいうものである。ベイナイト及び／又はマルテンサイト組織は、フェライト等の組織と比較して内部に多くの様々な界面を有している組織である。このため、２次焼鈍工程すなわち最終焼鈍工程前の鋼板における金属組織をベイナイト及び／又はマルテンサイトを主体とする組織によって構成することで、このような金属組織を２次焼鈍において加熱していく段階でこれらの界面上にオーステナイトの核生成サイトとなり得る炭化物を非常に多く分散して生成させることが可能となる。その結果として、このように多く分散された核生成サイトから鋼板全体にオーステナイトを微細かつ均一に生成させ、次いでこれらのオーステナイトからマルテンサイトを生成させることで、２次焼鈍後に得られる金属組織において、マルテンサイトの平均粒子間隔が２．５μｍ以下に制御されるとともに、圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差が１．５％以下に制御される。すなわちマルテンサイトがミクロな領域とマクロな領域の両方において均一に分散した金属組織を達成することが可能となる。 [Primary annealing process]
The obtained cold-rolled steel sheet is heated to a temperature of Ac3+10°C or more in the next primary annealing step. The Ac3 point (°C) is determined by cutting a small piece from the cold-rolled steel sheet and calculating the thermal expansion of the small piece during heating from room temperature to 1000°C at 10°C/s. By heating the cold-rolled steel sheet to a temperature of Ac3+10°C or more, austenitization is promoted and the sheet is then appropriately cooled, for example, by cooling the sheet to 200°C at an average cooling rate of 30°C/s or more, the metal structure in the steel sheet after cooling can be reliably composed of a structure mainly composed of bainite and/or martensite, for example, full bainite or full martensite. Here, the structure mainly composed of bainite and/or martensite refers to a structure containing at least one of bainite and martensite in a total area ratio of 90% or more, full bainite refers to a structure composed of 100% bainite in area ratio, and full martensite refers to a structure composed of 100% martensite in area ratio. The bainite and/or martensite structure has many different interfaces inside compared to structures such as ferrite. Therefore, by forming the metal structure of the steel sheet before the secondary annealing process, i.e., the final annealing process, with a structure mainly composed of bainite and/or martensite, it becomes possible to disperse and generate a very large number of carbides that can become nucleation sites of austenite on these interfaces in the stage of heating such a metal structure in the secondary annealing. As a result, austenite is generated finely and uniformly throughout the steel sheet from the nucleation sites dispersed in such a large number, and then martensite is generated from the austenite, so that in the metal structure obtained after the secondary annealing, the average grain spacing of martensite is controlled to 2.5 μm or less, and the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to 1.5% or less. In other words, it is possible to achieve a metal structure in which martensite is uniformly dispersed in both micro- and macro-regions.
　１次焼鈍工程における加熱温度がＡｃ３＋１０℃未満であると、オーステナイト化が不十分となり、その後の冷却によっても鋼板中の金属組織をベイナイト及び／又はマルテンサイトを主体とする組織によって構成することができなくなり、すなわちベイナイト及びマルテンサイトの面積率の合計を９０％以上にすることができなくなる。その結果として、最終的に得られる金属組織において、マルテンサイトの平均粒子間隔を２．５μｍ以下に制御することができないか、及び／又は圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差を１．５％以下に制御することができなくなる。この場合には、ゴーストライン等の発生を十分に抑制することができなくなり、成形後の外観性が低下する。一方で、より高温での加熱は生産性を低下させることから、１次焼鈍工程における加熱温度は１０５０℃以下とすることが好ましい。上記加熱温度での保持時間は１０～５００秒であることが好ましい。 If the heating temperature in the first annealing step is less than Ac3+10°C, austenitization will be insufficient, and the metal structure in the steel sheet will not be composed mainly of bainite and/or martensite even after subsequent cooling, meaning that the total area ratio of bainite and martensite will not be 90% or more. As a result, in the finally obtained metal structure, it will be impossible to control the average grain spacing of martensite to 2.5 μm or less, and/or it will be impossible to control the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction to 1.5% or less. In this case, it will be impossible to sufficiently suppress the occurrence of ghost lines, etc., and the appearance after forming will deteriorate. On the other hand, since heating at a higher temperature reduces productivity, it is preferable that the heating temperature in the first annealing step be 1050°C or less. The holding time at the above heating temperature is preferably 10 to 500 seconds.
［２次焼鈍工程（最終焼鈍工程）］
　１次焼鈍後の冷間圧延鋼板は、次の２次焼鈍工程において再び加熱され、（Ａｃ１＋２０）～８２０℃の温度にて１０～５００秒間保持される。ここで、Ａｃ１点（℃）は、Ａｃ３点の場合と同様に、冷延鋼板から小片を切り出し、当該小片における室温から１０℃／秒で１０００℃への加熱中の熱膨張から求められる。まず、１次冷却後の鋼板をＡｃ１～８２０℃の温度まで加熱していく段階で、金属組織中のベイナイト及び／又はマルテンサイトの内部に含まれる多くの界面上に炭化物を分散して生成させることができる。次に、フェライトとオーステナイトの２相域に対応するＡｃ１～８２０℃の温度にて１０～５００秒間保持することで、界面上に炭化物が分散された状態を維持しつつ、当該炭化物からオーステナイトを鋼板全体に微細かつ均一に生成させることができる。最後に、鋼板を適切に冷却すること、例えば５００℃までの温度域を平均冷却速度１０℃／秒以上で冷却することで、微細分散されたオーステナイトからマルテンサイトを適切に生成させることができ、その結果としてマルテンサイトの平均粒子間隔が２．５μｍ以下に制御されるとともに、圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差が１．５％以下に制御される。すなわちマルテンサイトがミクロな領域とマクロな領域の両方において均一に分散した金属組織を達成することが可能となる。 [Secondary annealing process (final annealing process)]
The cold-rolled steel sheet after the primary annealing is heated again in the next secondary annealing step and held at a temperature of (Ac1+20) to 820°C for 10 to 500 seconds. Here, the Ac1 point (°C) is obtained from the thermal expansion of a small piece cut from the cold-rolled steel sheet during heating from room temperature to 1000°C at 10°C/sec, as in the case of the Ac3 point. First, in the stage of heating the steel sheet after the primary cooling to a temperature of Ac1 to 820°C, carbides can be dispersed and generated on many interfaces contained inside the bainite and/or martensite in the metal structure. Next, by holding at a temperature of Ac1 to 820°C corresponding to the two-phase region of ferrite and austenite for 10 to 500 seconds, austenite can be generated finely and uniformly from the carbides throughout the steel sheet while maintaining the state in which the carbides are dispersed on the interfaces. Finally, by appropriately cooling the steel sheet, for example, by cooling in a temperature range up to 500°C at an average cooling rate of 10°C/sec or more, martensite can be appropriately generated from the finely dispersed austenite, and as a result, the average particle spacing of martensite is controlled to 2.5 μm or less, and the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to 1.5% or less. In other words, it is possible to achieve a metal structure in which martensite is uniformly dispersed in both micro- and macro-regions.
　２次焼鈍工程における加熱温度がＡｃ１＋２０℃未満であるか又は保持時間が１０秒未満であると、上記のような所望の金属組織を得ることができない。一方で、加熱温度が８２０℃超の場合には、オーステナイトの面積率が高くなりすぎて、フェライトの面積率を７５％以上にすることができない。さらに、高温に起因して界面上に炭化物が分散された状態を維持することができなくなり、最終的に得られる金属組織において、ミクロな領域とマクロな領域の両方でのマルテンサイトの均一分散を達成することができなくなる。また、保持時間が５００秒超であると、オーステナイト粒が粗大化してしまい、その後の冷却によって得られるマルテンサイト粒も比較的粗大なものとなる。このような場合には、マルテンサイトの平均粒子間隔が２．５μｍ以下に制御された微細なマルテンサイト組織を得ることができなくなる。 If the heating temperature in the secondary annealing process is less than Ac1+20°C or the holding time is less than 10 seconds, the desired metal structure as described above cannot be obtained. On the other hand, if the heating temperature exceeds 820°C, the area ratio of austenite becomes too high, and the area ratio of ferrite cannot be increased to 75% or more. Furthermore, due to the high temperature, it becomes impossible to maintain the state in which carbides are dispersed on the interface, and it becomes impossible to achieve uniform dispersion of martensite in both the micro- and macro-regions in the finally obtained metal structure. Furthermore, if the holding time exceeds 500 seconds, the austenite grains become coarse, and the martensite grains obtained by subsequent cooling are also relatively coarse. In such a case, it becomes impossible to obtain a fine martensite structure in which the average grain spacing of martensite is controlled to 2.5 μm or less.
［めっき工程］
　耐食性の向上等を目的として、必要に応じて、得られた冷間圧延鋼板の表面にめっき処理を施してもよい。めっき処理は、溶融めっき、合金化溶融めっき、電気めっき等の処理であってよい。例えば、めっき処理として鋼板に溶融亜鉛めっき処理を行ってもよく、溶融亜鉛めっき処理後に合金化処理を行ってもよい。めっき処理及び合金化処理の具体的な条件は特に限定されず、当業者に公知の任意の適切な条件であってよい。例えば、合金化温度は４５０～６００℃であってもよい。 [Plating process]
For the purpose of improving corrosion resistance, etc., a plating treatment may be applied to the surface of the obtained cold-rolled steel sheet as necessary. The plating treatment may be a treatment such as hot-dip plating, alloying hot-dip plating, or electroplating. For example, the steel sheet may be subjected to hot-dip galvanizing treatment as the plating treatment, or the alloying treatment may be performed after the hot-dip galvanizing treatment. The specific conditions of the plating treatment and the alloying treatment are not particularly limited, and may be any appropriate conditions known to those skilled in the art. For example, the alloying temperature may be 450 to 600°C.
　以下、実施例によって本発明をより詳細に説明するが、本発明はこれらの実施例に何ら限定されるものではない。 The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples in any way.
　以下の実施例では、本発明の実施形態に係る鋼板を種々の条件下で製造し、製造過程におけるヒートバックル発生の有無、並びに得られた鋼板の引張強さ及び成形後外観の特性について調べた。 In the following examples, steel sheets according to the embodiments of the present invention were manufactured under various conditions, and the occurrence of heat buckling during the manufacturing process, as well as the tensile strength and post-forming appearance characteristics of the resulting steel sheets were investigated.
　まず、連続鋳造法により表１に示す化学組成を有しかつ厚さが２００～３００ｍｍのスラブを鋳造した。表１に示す成分以外の残部はＦｅ及び不純物である。次に、得られたスラブを１１００～１３００℃の温度に加熱し、次いで熱間圧延を行った。熱間圧延は、粗圧延と仕上げ圧延を行うことにより実施した。より具体的には、粗圧延は全ての実施例及び比較例で同じ条件であり、仕上げ圧延は７基の圧延スタンドからなるタンデム圧延機を用いて実施した。仕上げ圧延における最終パスから２つ前の圧延パス（Ｆ５圧延パス）の圧下率は３０％であった。 First, a slab having the chemical composition shown in Table 1 and a thickness of 200 to 300 mm was cast by continuous casting. The remainder other than the components shown in Table 1 was Fe and impurities. Next, the obtained slab was heated to a temperature of 1100 to 1300°C, and then hot rolling was performed. The hot rolling was performed by performing rough rolling and finish rolling. More specifically, the rough rolling conditions were the same in all examples and comparative examples, and the finish rolling was performed using a tandem rolling mill consisting of seven rolling stands. The reduction ratio of the rolling pass two passes before the final pass in the finish rolling (F5 rolling pass) was 30%.
　各例の熱間圧延における条件Ｉ：仕上げ圧延入側温度１０００～１０８０℃、条件ＩＩ：最終パスから２つ前の圧延パスの圧下率／最終パスの圧下率の比（Ｆ５／Ｆ７圧下率比）１．５～２．５、及び条件ＩＩＩ：巻取温度５２０～６７０℃を満たす場合と満たさない場合を表２に示す。具体的には、条件Ｉを満たす例では、仕上げ圧延入側温度は１０５０℃であり、一方で条件Ｉを満たさない例では、仕上げ圧延入側温度は９５０℃（比較例９）又は１１２０℃（比較例１９）であった。また、条件ＩＩを満たす例では、Ｆ５／Ｆ７圧下率比は２．０であり、一方で条件ＩＩを満たさない例では、Ｆ５／Ｆ７圧下率比は１．２（比較例２）又は３．０（比較例２０）であった。また、条件ＩＩＩを満たす例では、巻取温度は６００℃であり、一方で条件ＩＩＩを満たさない例では、巻取温度は４７０℃（比較例１０）又は７００℃（比較例２１）であった。 Table 2 shows the cases where the following conditions are met and not met in the hot rolling of each example: Condition I: Finishing roll entry temperature 1000-1080°C, Condition II: Ratio of rolling reduction of the rolling pass two passes before the final pass/rolling reduction of the final pass (F5/F7 rolling reduction ratio) 1.5-2.5, and Condition III: Coiling temperature 520-670°C. Specifically, in the example where Condition I is met, the finishing roll entry temperature is 1050°C, while in the example where Condition I is not met, the finishing roll entry temperature is 950°C (Comparative Example 9) or 1120°C (Comparative Example 19). In addition, in the example where Condition II is met, the F5/F7 rolling reduction ratio is 2.0, while in the example where Condition II is not met, the F5/F7 rolling reduction ratio is 1.2 (Comparative Example 2) or 3.0 (Comparative Example 20). In addition, in the examples that satisfied condition III, the winding temperature was 600°C, while in the examples that did not satisfy condition III, the winding temperature was 470°C (Comparative Example 10) or 700°C (Comparative Example 21).
　次に、得られた熱間圧延鋼板を酸洗し、次いで冷間圧延、１次焼鈍（所定の加熱温度で保持時間１００秒、及び焼鈍後２００℃までの平均冷却速度４０℃／秒）、及び２次焼鈍（７７０℃の加熱温度で保持時間１００秒、及び焼鈍後５００℃までの平均冷却速度１５℃／秒）を実施して、板厚が０．４ｍｍの冷間圧延鋼板を製造した。７７０℃の加熱温度は、全ての発明例及び比較例についてＡｃ１＋２０℃以上の要件を満足するものである。冷間圧延の条件ＩＶ（圧下率７０％以上）と１次焼鈍の条件Ｖ（加熱温度Ａｃ３＋１０℃以上）を満たす場合と満たさない場合を表２に示す。具体的には、条件ＩＶを満たす例では、圧下率８０％で冷間圧延を行い、一方で条件ＩＶを満たさない例では、圧下率６０％（比較例３及び９）で冷間圧延を行った。また、条件Ｖを満たす例では、Ａｃ３＋１５℃以上すなわち９００℃に加熱して１次焼鈍を実施した。一方、条件Ｖを満たさない例では、Ａｃ３未満の温度（比較例４及び２０）に加熱して１次焼鈍を実施した。 Then, the obtained hot-rolled steel sheet was pickled, followed by cold rolling, primary annealing (holding time of 100 seconds at a specified heating temperature, and an average cooling rate of 40°C/sec to 200°C after annealing), and secondary annealing (holding time of 100 seconds at a heating temperature of 770°C, and an average cooling rate of 15°C/sec to 500°C after annealing) to produce a cold-rolled steel sheet with a thickness of 0.4 mm. The heating temperature of 770°C satisfies the requirement of Ac1+20°C or more for all of the invention examples and comparative examples. Table 2 shows the cases where cold rolling condition IV (reduction rate of 70% or more) and primary annealing condition V (heating temperature Ac3+10°C or more) are met and where they are not met. Specifically, in examples that satisfied condition IV, cold rolling was performed at a rolling reduction of 80%, while in examples that did not satisfy condition IV, cold rolling was performed at a rolling reduction of 60% (Comparative Examples 3 and 9). In addition, in examples that satisfied condition V, the primary annealing was performed by heating to Ac3+15°C or higher, i.e., 900°C. On the other hand, in examples that did not satisfy condition V, the primary annealing was performed by heating to a temperature lower than Ac3 (Comparative Examples 4 and 20).
　最後に、得られた冷間圧延鋼板の表面に適宜めっき処理を施し、溶融亜鉛めっき層（ＧＩ）、合金化溶融亜鉛めっき層（ＧＡ）又は電気亜鉛めっき層（ＥＧ）を形成した。合金化溶融亜鉛めっき層（ＧＡ）について、合金化条件は、５５０℃で２０秒とした。 Finally, the surface of the obtained cold-rolled steel sheet was appropriately plated to form a hot-dip galvanized layer (GI), a galvannealed layer (GA) or an electrogalvanized layer (EG). For the galvannealed layer (GA), the alloying conditions were 550°C for 20 seconds.
　得られた鋼板の特性は以下の方法によって測定及び評価した。また、製造過程におけるヒートバックル発生の有無については、１次焼鈍後にバックリングの発生有無を確認し、バックリングが発生していない場合をヒートバックルの発生なしと判定して合格（ＯＫ）とし、バックリングが発生している場合をヒートバックルの発生ありと判定して不合格（ＮＧ）とした。 The properties of the obtained steel sheets were measured and evaluated by the following methods. In addition, regarding the occurrence of heat buckling during the manufacturing process, the occurrence of buckling was checked after the first annealing, and if no buckling occurred, it was judged that no heat buckling had occurred and was deemed to have passed (OK), and if buckling occurred, it was judged that heat buckling had occurred and was deemed to have failed (NG).
［引張強さ（ＴＳ）］
　引張強さ（ＴＳ）は、圧延方向及び板厚方向に直角な方向を長手方向とするＪＩＳ　Ｚ２２４１：２０１１の５号引張試験片を鋼板から採取し、ＪＩＳ　Ｚ２２４１：２０１１に準拠して引張試験を行うことで測定した。 [Tensile strength (TS)]
The tensile strength (TS) was measured by taking a No. 5 tensile test piece of JIS Z2241:2011 from the steel plate, the longitudinal direction of which was perpendicular to the rolling direction and the plate thickness direction, and conducting a tensile test in accordance with JIS Z2241:2011.
［成形後外観］
　成形後外観は、成形後のドアアウタの表面に発生するゴーストラインの程度により評価した。ドアアウタを模した成形部材として、６００ｍｍ角にブランキングした鋼板を中央部の曲率半径Ｒが１２００ｍｍとなるようプレス成形したプレス部材を用いた。プレス成形後の表面を砥石掛けし、表面に生じた数ｍｍオーダー間隔の縞模様を、ゴーストラインと判断し、筋模様の発生程度によって１～５で評点付けした。１００ｍｍ×１００ｍｍの任意の領域を目視で確認し、筋模様が全く確認されなかった場合を「１」とし、筋模様の最大長さが２０ｍｍ以下の場合を「２」とし、筋模様の最大長さが２０ｍｍ超、５０ｍｍ以下の場合を「３」とし、筋模様の最大長さが５０ｍｍ超、７０ｍｍ以下の場合を「４」とし、筋模様の最大長さが７０ｍｍを超える場合を「５」とした。評価が「３」以下であった場合、成形後外観に優れるとして合格と判定した。一方、評価が「４」以上であった場合、成形後外観に劣るとして不合格と判定した。今回の試験では、ドアアウタを模擬したプレス部材にて成形後外観を評価したが、プレス成形により２．５％のひずみが付与されたものと推定可能な成形部材を評価対象としてもよく、鋼板から採取した試験片に対して同様に２．５％の予ひずみを付与したものを評価対象としてもよく、それらのような試験方法によっても同等の評価を行うことができる。鋼板から採取した試験片の場合、圧延方向及び板厚方向に直交する方向を長手方向とするＪＩＳ５号試験片に対し、２．５％の予ひずみを与えたものについて評価することができる。 [Appearance after molding]
The appearance after forming was evaluated based on the degree of ghost lines that appeared on the surface of the door outer after forming. As a formed part simulating the door outer, a pressed part was used in which a steel plate blanked to 600 mm square was press-formed so that the radius of curvature R at the center was 1200 mm. The surface after press forming was ground with a grindstone, and stripes that appeared on the surface at intervals of several mm were judged to be ghost lines, and were scored from 1 to 5 depending on the degree of occurrence of the stripes. An arbitrary area of 100 mm x 100 mm was visually confirmed, and a score of "1" was given if no stripes were observed, a score of "2" was given if the maximum length of the stripes was 20 mm or less, a score of "3" was given if the maximum length of the stripes was more than 20 mm and 50 mm or less, a score of "4" was given if the maximum length of the stripes was more than 50 mm and 70 mm or less, and a score of "5" was given if the maximum length of the stripes was more than 70 mm. If the score was "3" or less, the appearance after forming was judged to be excellent and the product was judged to be acceptable. On the other hand, when the evaluation was "4" or more, the appearance after forming was judged to be poor and the specimen was judged to be unsatisfactory. In this test, the appearance after forming was evaluated using a pressed member simulating a door outer, but the evaluation object may be a formed member that can be estimated to have been given a strain of 2.5% by press forming, or a test piece taken from a steel plate to which a pre-strain of 2.5% has been similarly given may be evaluated, and similar evaluations can be made by such test methods. In the case of a test piece taken from a steel plate, a JIS No. 5 test piece having a longitudinal direction perpendicular to the rolling direction and the plate thickness direction and given a pre-strain of 2.5% may be evaluated.
　ヒートバックルが発生しておらず、かつ引張強さが５４０ＭＰａ以上及び成形後外観の評価が３以下の場合を、製造過程におけるヒートバックルの発生を抑制することができるとともに、強度と成形後の外観性とを両立することができる鋼板として評価した。その結果を表２に示す。表２に示す金属組織において、残部組織は、ベイナイト、パーライト、及び残留オーステナイトの少なくとも１種であった。 Steel sheets that did not have heat buckling, had a tensile strength of 540 MPa or more, and had a post-forming appearance rating of 3 or less were evaluated as being capable of suppressing the occurrence of heat buckling during the manufacturing process and achieving both strength and good post-forming appearance. The results are shown in Table 2. In the metal structure shown in Table 2, the remaining structure was at least one of bainite, pearlite, and retained austenite.
　表１及び２を参照すると、比較例２では、仕上げ圧延におけるＦ５／Ｆ７圧下率比が低かったために、最終パス（Ｆ７）での圧下後におけるひずみの蓄積が顕著となってしまい、仕上げ圧延後に析出する微細なＮｂ炭窒化物が増加したものと考えられる。その結果として、粒径が２０ｎｍ以上のＮｂ炭窒化物の割合が少なくなってしまい、１次焼鈍工程においてヒートバックルが発生した。比較例３では、冷間圧延工程の圧下率が低かったために、その後の１次焼鈍工程の加熱時に再結晶が遅延し、高温になるまでひずみが残存したことに起因して、１次焼鈍工程の加熱中に微細なＮｂ炭窒化物が多く析出してしまったと考えられる。その結果として、粒径が２０ｎｍ以上のＮｂ炭窒化物の割合が少なくなってしまい、１次焼鈍工程においてヒートバックルが発生した。比較例４では、１次焼鈍工程の加熱温度が低かったために、オーステナイト化が不十分となり、その後の冷却によっても鋼板中の金属組織をベイナイト及び／又はマルテンサイトを主体とする組織によって構成することができなかったと考えられる。その結果として、２次焼鈍後に得られる金属組織において、マルテンサイトの平均粒子間隔が２．５μｍ超となり、また圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差が１．５％超となってしまい、成形後外観が低下した。比較例９では、仕上げ圧延入側温度が低かったために、Ｎｂ炭窒化物が十分に析出しなかった。その結果として、粒径が２０ｎｍ以上のＮｂ炭窒化物の割合が少なくなってしまい、１次焼鈍工程においてヒートバックルが発生した。比較例１０では、巻取温度が低かったために、固溶Ｎｂ量が多くなり、これに関連してＮｂ炭窒化物の析出が少なくなったものと考えられる。その結果として、全Ｎｂ炭窒化物中のＮｂ量が少なくなり、また粒径が２０ｎｍ以上のＮｂ炭窒化物の割合も少なくなってしまい、１次焼鈍工程においてヒートバックルが発生した。比較例１９では、仕上げ圧延入側温度が高かったために、再結晶が起こり、仕上げ圧延後段でのひずみの蓄積が不十分となり、粒径が２０ｎｍ以上のＮｂ炭窒化物の析出を促進させることができず、その割合が少なくなってしまったと考えられる。その結果として、１次焼鈍工程においてヒートバックルが発生した。 Referring to Tables 1 and 2, in Comparative Example 2, since the F5/F7 reduction ratio in the finish rolling was low, the accumulation of strain after reduction in the final pass (F7) became significant, and it is believed that the amount of fine Nb carbonitrides precipitated after the finish rolling increased. As a result, the proportion of Nb carbonitrides with a particle size of 20 nm or more decreased, and heat buckling occurred in the first annealing process. In Comparative Example 3, since the reduction ratio in the cold rolling process was low, recrystallization was delayed during heating in the subsequent first annealing process, and strain remained until the temperature reached a high level, which is believed to have caused a large amount of fine Nb carbonitrides to precipitate during heating in the first annealing process. As a result, the proportion of Nb carbonitrides with a particle size of 20 nm or more decreased, and heat buckling occurred in the first annealing process. In Comparative Example 4, the heating temperature in the first annealing step was low, so austenitization was insufficient, and it is believed that the metal structure in the steel sheet could not be constituted by a structure mainly composed of bainite and/or martensite even after subsequent cooling. As a result, in the metal structure obtained after the second annealing, the average particle spacing of martensite exceeded 2.5 μm, and the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction exceeded 1.5%, resulting in a poor appearance after forming. In Comparative Example 9, the finish rolling inlet temperature was low, so Nb carbonitrides were not precipitated sufficiently. As a result, the proportion of Nb carbonitrides with a particle size of 20 nm or more was reduced, and heat buckling occurred in the first annealing step. In Comparative Example 10, the coiling temperature was low, so the amount of solid-solubilized Nb increased, and it is believed that the precipitation of Nb carbonitrides was reduced in relation to this. As a result, the amount of Nb in the total Nb carbonitrides was reduced, and the proportion of Nb carbonitrides with a particle size of 20 nm or more was also reduced, causing heat buckling in the first annealing process. In Comparative Example 19, the temperature at the inlet side of the finish rolling was high, causing recrystallization, which resulted in insufficient accumulation of strain in the latter stages of the finish rolling, making it impossible to promote the precipitation of Nb carbonitrides with a particle size of 20 nm or more, resulting in a reduced proportion. As a result, heat buckling occurred in the first annealing process.
　比較例２０では、仕上げ圧延におけるＦ５／Ｆ７圧下率比が高かったために、Ｆ５圧延パスでの圧下後にひずみの蓄積が大きくなりすぎてしまい、再結晶が促進されてしまったものと考えられる。その結果として、粒径が２０ｎｍ以上のＮｂ炭窒化物の割合が少なくなってしまい、１次焼鈍工程においてヒートバックルが発生した。また、比較例２０では、比較例４の場合と同様に、１次焼鈍工程の加熱温度が低かったために、オーステナイト化が不十分となり、その後の冷却によっても鋼板中の金属組織をベイナイト及び／又はマルテンサイトを主体とする組織によって構成することができなかったと考えられる。その結果として、２次焼鈍後に得られる金属組織において、マルテンサイトの平均粒子間隔が２．５μｍ超となり、また圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差が１．５％超となってしまい、成形後外観が低下した。比較例２１では、巻取温度が高かったために、金属組織中のセメンタイトに合金元素が濃化してしまい、その後の１次焼鈍工程における加熱時に未溶解炭化物が残存してしまい、当該１次焼鈍工程において金属組織をベイナイト及び／又はマルテンサイトを主体とする組織によって構成することができなかったと考えられる。その結果として、２次焼鈍後に得られる金属組織において、マルテンサイトの平均粒子間隔が２．５μｍ超となり、また圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差が１．５％超となってしまい、成形後外観が低下した。比較例２７では、Ｎｂを添加しなかったために粒径が２０ｎｍ以上のＮｂ炭窒化物は析出せず、また指数Ａの値も低かった。その結果として、鋼板の高温強度を十分に向上させることができず、１次焼鈍工程においてヒートバックルが発生した。比較例２８では、指数Ａの値が低かったために、同様に鋼板の高温強度を十分に向上させることができず、１次焼鈍工程においてヒートバックルが発生した。比較例２９及び３０では、Ｃ又はＭｎ含有量が高かったために、圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差が１．５％超となってしまい、成形後外観が低下した。比較例３１及び３２では、Ｃ又はＭｎ含有量が低かったために、十分な強度が得られなかった。 In Comparative Example 20, the F5/F7 reduction ratio in the finish rolling was high, so that the accumulation of strain after reduction in the F5 rolling pass was too large, and recrystallization was promoted. As a result, the proportion of Nb carbonitrides with a particle size of 20 nm or more was reduced, and heat buckling occurred in the first annealing process. In Comparative Example 20, as in Comparative Example 4, the heating temperature in the first annealing process was low, so that austenitization was insufficient, and it is considered that the metal structure in the steel sheet could not be composed of a structure mainly composed of bainite and/or martensite even after subsequent cooling. As a result, in the metal structure obtained after the second annealing, the average particle spacing of martensite exceeded 2.5 μm, and the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction exceeded 1.5%, resulting in a deterioration in the appearance after forming. In Comparative Example 21, since the coiling temperature was high, the alloy elements were concentrated in the cementite in the metal structure, and undissolved carbides remained during heating in the subsequent primary annealing step, and it is considered that the metal structure could not be constituted by a structure mainly composed of bainite and/or martensite in the primary annealing step. As a result, in the metal structure obtained after the secondary annealing, the average particle spacing of martensite exceeded 2.5 μm, and the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction exceeded 1.5%, resulting in a poor appearance after forming. In Comparative Example 27, since Nb was not added, Nb carbonitrides having a particle size of 20 nm or more did not precipitate, and the value of index A was also low. As a result, the high-temperature strength of the steel sheet could not be sufficiently improved, and heat buckling occurred in the primary annealing step. In Comparative Example 28, since the value of index A was low, the high-temperature strength of the steel sheet could not be sufficiently improved, and heat buckling occurred in the primary annealing step. In Comparative Examples 29 and 30, the C or Mn content was high, so the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction exceeded 1.5%, resulting in poor appearance after forming. In Comparative Examples 31 and 32, the C or Mn content was low, so sufficient strength was not obtained.
　これとは対照的に、全ての発明例に係る鋼板において、所定の化学組成及び金属組織を有し、さらに金属組織中のフェライト及びマルテンサイトの割合を適切に制御することで５４０ＭＰａ以上のＴＳを達成するとともに、ミクロな領域ではマルテンサイトの平均粒子間隔を２．５μｍ以下に制御し、一方でマクロな領域では圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差を１．５％以下に制御することで、プレス成形によってひずみが付与された場合においても、鋼板表面における微小な凹凸の生成を抑制してゴーストラインの発生を顕著に抑制することができた。全ての発明例に係る２次焼鈍前の冷間圧延鋼板における金属組織を断面観察したところ、いずれも面積率で９０％以上のマルテンサイトによって構成されていた。また、全ての発明例に係る鋼板において、全Ｎｂ炭窒化物中のＮｂ量を０．００４％以上、かつ粒径が２０ｎｍ以上のＮｂ炭窒化物中のＮｂ量を当該全Ｎｂ炭窒化物中のＮｂ量の６０％以上に制御することで、指数Ａによる化学組成の制御との組み合わせにより、鋼板の製造過程におけるヒートバックルの発生を顕著に抑制することができた。また、発明例における粒径が２０ｎｍ以上のＮｂ炭窒化物は、全て粒径が１０００ｎｍ以下すなわち１μｍ以下であった。 In contrast, all of the steel sheets according to the examples of the invention have a prescribed chemical composition and metal structure, and by appropriately controlling the proportions of ferrite and martensite in the metal structure, a TS of 540 MPa or more is achieved, and the average particle spacing of martensite is controlled to 2.5 μm or less in the microscopic region, while in the macroscopic region the standard deviation in the area ratio of martensite in the directions perpendicular to the rolling direction and plate thickness direction is controlled to 1.5% or less. This makes it possible to suppress the generation of minute irregularities on the steel sheet surface and significantly suppress the occurrence of ghost lines, even when strain is imparted by press forming. When the metal structure of the cold-rolled steel sheets according to all of the examples of the invention before secondary annealing was observed in cross section, all were composed of martensite by area ratio of 90% or more. In addition, in all of the steel sheets according to the invention, the Nb content in all Nb carbonitrides was controlled to 0.004% or more, and the Nb content in Nb carbonitrides with a grain size of 20 nm or more was controlled to 60% or more of the Nb content in all of the Nb carbonitrides, and by combining this with the control of the chemical composition by index A, it was possible to significantly suppress the occurrence of heat buckling during the manufacturing process of the steel sheets. In addition, all of the Nb carbonitrides with a grain size of 20 nm or more in the invention examples had a grain size of 1000 nm or less, i.e., 1 μm or less.

Claims (4)

  1. 　質量％で、
    　Ｃ　：０．０３０～０．１００％、
    　Ｍｎ：０．７０～３．００％、
    　Ｓｉ：０．００５～１．５００％、
    　Ｐ　：０．１００％以下、
    　Ｓ　：０．０２００％以下、
    　Ａｌ：１．０００％以下、
    　Ｎ　：０．００１０～０．０１５０％、
    　Ｏ　：０．０１００％以下、
    　Ｎｂ：０．００５～０．２００％、
    　Ｃｒ：０～１．００％、
    　Ｍｏ：０～０．８０％、
    　Ｂ　：０～０．０１００％、
    　Ｔｉ：０～０．２００％、
    　Ｖ　：０～０．５００％、
    　Ｎｉ：０～１．００％、
    　Ｃｕ：０～１．００％、
    　Ｗ　：０～１．００％、
    　Ｔａ：０～０．１０％、
    　Ｃｏ：０～３．００％、
    　Ｓｎ：０～１．００％、
    　Ｓｂ：０～０．２００％、
    　Ｃａ：０～０．０１００％、
    　Ｍｇ：０～０．０１００％、
    　Ｚｒ：０～０．０１００％、
    　ＲＥＭ：０～０．０１００％、
    　Ｂｉ：０～０．０５００％、
    　Ａｓ：０～０．１０％、並びに
    　残部：Ｆｅ及び不純物からなり、下記式１で表される指数Ａが０．５０％以上である化学組成を有し、
    　面積％で、
    　フェライト：７５～９５％、
    　マルテンサイト：５～２５％、並びに
    　残部組織：合計で０～１０％からなり、
    　全Ｎｂ炭窒化物中のＮｂ量が０．００４％以上であり、かつ粒径が２０ｎｍ以上のＮｂ炭窒化物中のＮｂ量が前記全Ｎｂ炭窒化物中のＮｂ量の６０％以上であり、
    　前記マルテンサイトの平均粒子間隔が２．５μｍ以下であり、
    　圧延方向及び板厚方向に垂直な方向のマルテンサイトの面積率における標準偏差が１．５％以下である金属組織を有することを特徴とする、鋼板。
    　Ａ＝［Ｃ］－０．１［Ｓｉ］＋０．３（［Ｍｎ］－０．５）－０．３［Ａｌ］＋０．１［Ｃｒ］＋０．６［Ｍｏ］－［Ｔｉ］＋１５［Ｎｂ］　　　・・・式１
    　ここで、［Ｃ］、［Ｓｉ］、［Ｍｎ］、［Ａｌ］、［Ｃｒ］、［Ｍｏ］、［Ｔｉ］及び［Ｎｂ］は、各元素の含有量［質量％］であり、元素を含有しない場合は０％である。     In mass percent,
    C: 0.030-0.100%,
    Mn: 0.70-3.00%,
    Si: 0.005-1.500%,
    P: 0.100% or less,
    S: 0.0200% or less,
    Al: 1.000% or less,
    N: 0.0010-0.0150%,
    O: 0.0100% or less,
    Nb: 0.005-0.200%,
    Cr: 0-1.00%,
    Mo: 0 to 0.80%,
    B: 0 to 0.0100%,
    Ti: 0-0.200%,
    V: 0 to 0.500%,
    Ni: 0 to 1.00%,
    Cu: 0 to 1.00%,
    W: 0 to 1.00%,
    Ta: 0 to 0.10%,
    Co: 0-3.00%,
    Sn: 0 to 1.00%,
    Sb: 0 to 0.200%,
    Ca: 0-0.0100%,
    Mg: 0 to 0.0100%,
    Zr: 0 to 0.0100%,
    REM: 0-0.0100%,
    Bi: 0 to 0.0500%,
    As: 0 to 0.10%, and the balance: Fe and impurities, and has a chemical composition in which index A represented by the following formula 1 is 0.50% or more,
    In area %,
    Ferrite: 75-95%,
    Martensite: 5-25%; and Remainder: 0-10% in total.
    The Nb content in all Nb carbonitrides is 0.004% or more, and the Nb content in Nb carbonitrides having a particle size of 20 nm or more is 60% or more of the Nb content in all Nb carbonitrides,
    The average grain spacing of the martensite is 2.5 μm or less,
    A steel sheet having a metal structure in which the standard deviation in the area ratio of martensite in a direction perpendicular to the rolling direction and the sheet thickness direction is 1.5% or less.
    A=[C]-0.1[Si]+0.3([Mn]-0.5)-0.3[Al]+0.1[Cr]+0.6[Mo]-[Ti]+15[Nb]...Formula 1
    Here, [C], [Si], [Mn], [Al], [Cr], [Mo], [Ti] and [Nb] are the contents [mass %] of each element, and are 0% when the element is not contained.
  2. 　前記化学組成が、質量％で、
    　Ｃｒ：０．００１～１．００％、
    　Ｍｏ：０．００１～０．８０％、
    　Ｂ　：０．０００１～０．０１００％、
    　Ｔｉ：０．００１～０．２００％、
    　Ｖ　：０．００１～０．５００％、
    　Ｎｉ：０．００１～１．００％、
    　Ｃｕ：０．００１～１．００％、
    　Ｗ　：０．００１～１．００％、
    　Ｔａ：０．００１～０．１０％、
    　Ｃｏ：０．００１～３．００％、
    　Ｓｎ：０．００１～１．００％、
    　Ｓｂ：０．００１～０．２００％、
    　Ｃａ：０．０００１～０．０１００％、
    　Ｍｇ：０．０００１～０．０１００％、
    　Ｚｒ：０．０００１～０．０１００％、
    　ＲＥＭ：０．０００１～０．０１００％、
    　Ｂｉ：０．０００１～０．０５００％、及び
    　Ａｓ：０．００１～０．１０％
    のうち少なくとも１種を含むことを特徴とする、請求項１に記載の鋼板。     The chemical composition, in mass%,
    Cr: 0.001-1.00%,
    Mo: 0.001-0.80%,
    B: 0.0001 to 0.0100%,
    Ti: 0.001 to 0.200%,
    V: 0.001-0.500%,
    Ni: 0.001 to 1.00%,
    Cu: 0.001 to 1.00%,
    W: 0.001-1.00%,
    Ta: 0.001 to 0.10%,
    Co: 0.001 to 3.00%,
    Sn: 0.001 to 1.00%,
    Sb: 0.001-0.200%,
    Ca: 0.0001-0.0100%,
    Mg: 0.0001-0.0100%,
    Zr: 0.0001 to 0.0100%,
    REM: 0.0001-0.0100%,
    Bi: 0.0001 to 0.0500%, and As: 0.001 to 0.10%
    The steel sheet according to claim 1, characterized in that it contains at least one of the following:
  3. 　前記フェライトの平均結晶粒径が３．０～２５．０μｍであり、前記マルテンサイトの平均結晶粒径が１．０～５．０μｍであり、前記マルテンサイトの平均アスペクト比が２．５以上であることを特徴とする、請求項１又は２に記載の鋼板。 The steel sheet according to claim 1 or 2, characterized in that the average grain size of the ferrite is 3.0 to 25.0 μm, the average grain size of the martensite is 1.0 to 5.0 μm, and the average aspect ratio of the martensite is 2.5 or more.
  4. 　請求項１～３のいずれか１項に記載の鋼板を含む外板部材。 An exterior panel member comprising the steel plate according to any one of claims 1 to 3.
PCT/JP2024/008586 2023-03-06 2024-03-06 Steel sheet and outer sheet member WO2024185819A1 (en)

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Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2012172159A (en) * 2011-02-17 2012-09-10 Nippon Steel Corp High-strength cold-rolled steel sheet excellent in homogeneous deformability and local deformability
WO2012161241A1 (en) * 2011-05-25 2012-11-29 新日鐵住金株式会社 Cold-rolled steel sheet and method for producing same

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2012172159A (en) * 2011-02-17 2012-09-10 Nippon Steel Corp High-strength cold-rolled steel sheet excellent in homogeneous deformability and local deformability
WO2012161241A1 (en) * 2011-05-25 2012-11-29 新日鐵住金株式会社 Cold-rolled steel sheet and method for producing same

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