WO2024095532A1 - Tôle d'acier laminée à chaud - Google Patents
Tôle d'acier laminée à chaud Download PDFInfo
- Publication number
- WO2024095532A1 WO2024095532A1 PCT/JP2023/024341 JP2023024341W WO2024095532A1 WO 2024095532 A1 WO2024095532 A1 WO 2024095532A1 JP 2023024341 W JP2023024341 W JP 2023024341W WO 2024095532 A1 WO2024095532 A1 WO 2024095532A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- less
- content
- rolling
- hot
- strength
- Prior art date
Links
- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 136
- 239000010959 steel Substances 0.000 title claims abstract description 136
- 239000013078 crystal Substances 0.000 claims abstract description 83
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 29
- 229910001563 bainite Inorganic materials 0.000 claims abstract description 27
- 229910000734 martensite Inorganic materials 0.000 claims abstract description 27
- 239000000203 mixture Substances 0.000 claims abstract description 22
- 239000000126 substance Substances 0.000 claims abstract description 22
- 229910052751 metal Inorganic materials 0.000 claims description 25
- 239000002184 metal Substances 0.000 claims description 25
- 239000012535 impurity Substances 0.000 claims description 5
- 229910052718 tin Inorganic materials 0.000 claims description 2
- 238000005096 rolling process Methods 0.000 description 125
- 238000001816 cooling Methods 0.000 description 47
- 230000000694 effects Effects 0.000 description 33
- 238000004519 manufacturing process Methods 0.000 description 33
- 230000002829 reductive effect Effects 0.000 description 33
- 239000002344 surface layer Substances 0.000 description 30
- 230000001276 controlling effect Effects 0.000 description 22
- 230000000052 comparative effect Effects 0.000 description 20
- 238000000034 method Methods 0.000 description 19
- 238000004458 analytical method Methods 0.000 description 15
- 238000005098 hot rolling Methods 0.000 description 14
- 229910001566 austenite Inorganic materials 0.000 description 13
- 229910052761 rare earth metal Inorganic materials 0.000 description 13
- 238000012360 testing method Methods 0.000 description 12
- 230000007423 decrease Effects 0.000 description 11
- 238000005259 measurement Methods 0.000 description 11
- 230000009466 transformation Effects 0.000 description 11
- 229920006395 saturated elastomer Polymers 0.000 description 9
- 230000001186 cumulative effect Effects 0.000 description 8
- 238000005728 strengthening Methods 0.000 description 8
- 238000010438 heat treatment Methods 0.000 description 7
- 230000000717 retained effect Effects 0.000 description 7
- 230000003247 decreasing effect Effects 0.000 description 6
- 150000001247 metal acetylides Chemical class 0.000 description 6
- 239000000047 product Substances 0.000 description 6
- 230000015572 biosynthetic process Effects 0.000 description 5
- 229910001567 cementite Inorganic materials 0.000 description 5
- 238000009661 fatigue test Methods 0.000 description 5
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 5
- 238000001556 precipitation Methods 0.000 description 5
- 238000009864 tensile test Methods 0.000 description 5
- 238000005260 corrosion Methods 0.000 description 4
- 230000007797 corrosion Effects 0.000 description 4
- 238000011156 evaluation Methods 0.000 description 4
- 230000003287 optical effect Effects 0.000 description 4
- 239000002244 precipitate Substances 0.000 description 4
- 239000006104 solid solution Substances 0.000 description 4
- 239000000243 solution Substances 0.000 description 4
- 238000002441 X-ray diffraction Methods 0.000 description 3
- 238000009825 accumulation Methods 0.000 description 3
- 229910052782 aluminium Inorganic materials 0.000 description 3
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 3
- 229910052785 arsenic Inorganic materials 0.000 description 3
- 229910052797 bismuth Inorganic materials 0.000 description 3
- 238000004364 calculation method Methods 0.000 description 3
- 229910052802 copper Inorganic materials 0.000 description 3
- 238000011161 development Methods 0.000 description 3
- 229910052735 hafnium Inorganic materials 0.000 description 3
- 230000000670 limiting effect Effects 0.000 description 3
- 229910052759 nickel Inorganic materials 0.000 description 3
- 238000004080 punching Methods 0.000 description 3
- 238000010998 test method Methods 0.000 description 3
- 238000010521 absorption reaction Methods 0.000 description 2
- 238000005266 casting Methods 0.000 description 2
- 238000009749 continuous casting Methods 0.000 description 2
- 230000008094 contradictory effect Effects 0.000 description 2
- 230000000875 corresponding effect Effects 0.000 description 2
- 238000010586 diagram Methods 0.000 description 2
- 238000010191 image analysis Methods 0.000 description 2
- 238000002354 inductively-coupled plasma atomic emission spectroscopy Methods 0.000 description 2
- 239000011261 inert gas Substances 0.000 description 2
- 229910052748 manganese Inorganic materials 0.000 description 2
- 238000010899 nucleation Methods 0.000 description 2
- 230000006911 nucleation Effects 0.000 description 2
- 229910001562 pearlite Inorganic materials 0.000 description 2
- 239000002994 raw material Substances 0.000 description 2
- 238000001953 recrystallisation Methods 0.000 description 2
- 238000007670 refining Methods 0.000 description 2
- 229910052765 Lutetium Inorganic materials 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- 239000003153 chemical reaction reagent Substances 0.000 description 1
- 230000002596 correlated effect Effects 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 239000006185 dispersion Substances 0.000 description 1
- 238000005315 distribution function Methods 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 229910052747 lanthanoid Inorganic materials 0.000 description 1
- 150000002602 lanthanoids Chemical class 0.000 description 1
- 229910052746 lanthanum Inorganic materials 0.000 description 1
- FZLIPJUXYLNCLC-UHFFFAOYSA-N lanthanum atom Chemical compound [La] FZLIPJUXYLNCLC-UHFFFAOYSA-N 0.000 description 1
- 239000010410 layer Substances 0.000 description 1
- OHSVLFRHMCKCQY-UHFFFAOYSA-N lutetium atom Chemical compound [Lu] OHSVLFRHMCKCQY-UHFFFAOYSA-N 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 238000004377 microelectronic Methods 0.000 description 1
- 229910052758 niobium Inorganic materials 0.000 description 1
- 150000004767 nitrides Chemical class 0.000 description 1
- 230000001376 precipitating effect Effects 0.000 description 1
- 238000011084 recovery Methods 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 229910052706 scandium Inorganic materials 0.000 description 1
- SIXSYDAISGFNSX-UHFFFAOYSA-N scandium atom Chemical compound [Sc] SIXSYDAISGFNSX-UHFFFAOYSA-N 0.000 description 1
- 229910001220 stainless steel Inorganic materials 0.000 description 1
- 239000010935 stainless steel Substances 0.000 description 1
- 229910052727 yttrium Inorganic materials 0.000 description 1
- VWQVUPCCIRVNHF-UHFFFAOYSA-N yttrium atom Chemical compound [Y] VWQVUPCCIRVNHF-UHFFFAOYSA-N 0.000 description 1
Images
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
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
-
- 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 hot-rolled steel sheets.
- Patent Document 1 describes a hot-rolled steel sheet having a predetermined chemical composition, a structure containing, in terms of area ratio, a total of 80-98% ferrite and bainite, and 2-10% martensite, and, when the boundaries in the structure where the misorientation is 15° or more are defined as grain boundaries, and the regions surrounded by the grain boundaries and having a circle equivalent diameter of 0.3 ⁇ m or more are defined as crystal grains, the proportion of the crystal grains where the misorientation within the grains is 5-14° is 10-60% in terms of area ratio.
- Patent Document 1 also teaches that by setting the proportion of the above crystal grains where the misorientation within the grains is 5-14° to an area ratio of 10-60%, it is possible to improve stretch flangeability and ductility while maintaining high strength, and further teaches that by controlling the total area ratio of ferrite and bainite in the structure and the area ratio of martensite within a predetermined range, it is possible to improve notch fatigue properties.
- High-strength steel plates are manufactured by hot rolling cast slabs, and it is known that the hot rolling can cause strength anisotropy between the strength in the rolling direction (L direction) and the strength in the width direction (C direction) perpendicular thereto.
- L direction strength in the rolling direction
- C direction width direction
- the strength anisotropy becomes large, it generally becomes a problem because the workability of the steel plate decreases. Therefore, in order to improve the workability of steel plates, there is a high demand for high-strength steel plates with reduced strength anisotropy in addition to the stretch flangeability, ductility, and notch fatigue properties described in Patent Document 1.
- the present invention aims to provide a hot-rolled steel sheet that, despite its high strength, has improved stretch flangeability, ductility, and notch fatigue properties, and has reduced strength anisotropy.
- the inventors conducted research with a particular focus on the metal structure of hot-rolled steel sheet.
- the inventors discovered that by configuring the metal structure of a hot-rolled steel sheet having a specified chemical composition to contain at least one of ferrite and bainite, and martensite in specific proportions, and further controlling the proportion of crystal grains within a specified range, it is possible to improve stretch flangeability, ductility, and notch fatigue properties, while in addition, by appropriately controlling the texture in the surface layer and center of the steel sheet, it is possible to reduce the anisotropy of strength, thus completing the present invention.
- the present invention which has achieved the above object, is as follows. (1) In mass%, C: 0.020 to 0.070%, Si: 0.010 to 2.000%, Mn: 0.60 to 2.00%, Ti: 0.015 to 0.200%, sol. Al: 0.010 to 1.000%, P: 0.100% or less, S: 0.030% or less, N: 0.0060% or less, O: 0.0100% or less, Nb: 0 to 0.050%, V: 0 to 0.300%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0 to 2.00%, Mo: 0 to 1.000%, B: 0 to 0.0100%, Sb: 0 to 1.00%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Hf: 0 to 0.0100%, REM: 0 to 0.1000%, Bi: 0 to 0.0100%, As: 0 to 0.0100%, Zr: 0 to 1.00%, Co: 0 to
- the chemical composition is, in mass%, Nb: 0.001 to 0.050%, V: 0.001 to 0.300%, Cr: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Mo: 0.001 to 1.000%, B: 0.0001 to 0.0100%, Sb: 0.01 to 1.00%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Hf: 0.0001 to 0.0100%, REM: 0.0001 to 0.1000%, Bi: 0.0001 to 0.0100%, As: 0.0001 to 0.0100%, Zr: 0.01 to 1.00%, Co: 0.01 to 1.00%, Zn: 0.01 to 1.00%, W: 0.01 to 1.00%, and Sn: 0.01 to 1.00%
- the hot-rolled steel sheet according to the above (1) characterized in that it contains at least one of the following:
- the present invention provides a hot-rolled steel sheet that has high strength, but also has improved stretch flangeability, ductility, and notch fatigue properties, and has reduced strength anisotropy.
- FIG. 2 is a diagram showing the shape of a saddle-shaped molded product used in a saddle-shaped stretch flange test method.
- FIG. 2 is a diagram showing the shape of a fatigue test specimen used to evaluate notch fatigue properties.
- the hot-rolled steel sheet according to the embodiment of the present invention has, in mass%, C: 0.020 to 0.070%, Si: 0.01 to 2.00%, Mn: 0.600 to 2.00%, Ti: 0.015 to 0.200%, sol.
- the chemical composition satisfies 0.100 ⁇ [Si]+[sol
- the metal structure of a hot-rolled steel sheet having a predetermined chemical composition is configured to contain at least one of ferrite and bainite and martensite in a specific ratio, more specifically, by configuring it to contain at least one of ferrite and bainite: 80 to 98% in total and martensite: 2 to 10% by area percentage, it is possible to improve strength, stretch flangeability, ductility, and notch fatigue properties in a well-balanced manner.
- a crystal grain with an orientation difference within the grain of 5 to 14° is effective in improving strength, stretch flangeability, and ductility. Therefore, by appropriately controlling the proportion of these crystal grains, more specifically by controlling it to within the range of 10 to 60% by area, it is possible to further improve the balance between strength, stretch flangeability, and ductility.
- the anisotropy of strength due to the anisotropic metal structure obtained by hot rolling during steel sheet manufacturing, the tensile strength tends to differ between the rolling direction (L direction) and the width direction (C direction) perpendicular thereto, and generally, the tensile strength of hot-rolled steel sheets tends to be lower in the L direction than in the C direction.
- the present inventors conducted a study, focusing particularly on the texture of hot-rolled steel sheets, in order to achieve both high strength steel sheets by reducing the anisotropy of strength in addition to improving the stretch flangeability, ductility, and notch fatigue properties.
- the inventors discovered that by controlling the average pole density of the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the region from the surface of the hot-rolled steel plate to the 1/6 position of the plate thickness to 2.50 or more, and controlling the average pole density of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011>, and ⁇ 332 ⁇ 113> orientations in the region from the 2/5 position to the 3/5 position of the plate thickness to 7.00 or less, it is possible to significantly reduce the anisotropy of the strength in the tensile strength of the hot-rolled steel plate in the L direction and C direction.
- the crystal orientation is different between the surface layer part of the plate thickness (i.e., the region from the surface of the hot-rolled steel plate to the 1/6 position of the plate thickness) which is directly affected by rolling, and the center part of the plate thickness (i.e., the region from the 2/5 position of the plate thickness to the 3/5 position of the plate thickness). More specifically, in the surface layer part of the plate thickness, textures of ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations are developed, and it is considered that the strength in the L direction is increased due to the development of such textures.
- the crystal orientation of the rolled sheet is usually expressed as ⁇ hkl ⁇ or (hkl) for the crystal orientation perpendicular to the rolling surface, and ⁇ uvw> or [uvw] for the crystal orientation parallel to the rolling direction.
- ⁇ hkl ⁇ and ⁇ uvw> are generic names for equivalent planes and orientations, and (hkl) and [uvw] refer to individual crystal planes.
- the hot-rolled steel sheet according to the embodiment of the present invention is mainly intended for body-centered cubic structures (bcc structures), so for example, (110), (-110), (1-10), (-1-10), (101), (-101), (10-1), (-10-1), (011), (0-11), (01-1) and (0-1-1) are equivalent and indistinguishable. In the embodiment of the present invention, these orientations are collectively expressed as ⁇ 110 ⁇ .
- the inventors have determined that the average value of the pole density of the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the plate thickness surface layer portion is increased to a predetermined value or more to increase the strength in the L direction, while the average value of the pole density of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> orientations in the plate thickness center portion is decreased to a predetermined value or less to decrease the strength in the C direction, more specifically, from the surface of the hot-rolled steel plate to a position 1/6 of the plate thickness.
- the anisotropy of strength in the tensile strength in the L direction and the C direction of the hot rolled steel sheet can be significantly reduced by controlling the average value of the pole density of the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the region to 2.50 or more and controlling the average value of the pole density of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> orientations in the region from the 2/5 position of the sheet thickness to the 3/5 position of the sheet thickness to 7.00 or less.
- the pole density refers to the ratio of the accumulation degree in a specific orientation of the test material to that of a standard sample that does not have accumulation in a specific orientation.
- the metal structure in order to improve the stretch flangeability, ductility and notch fatigue properties, is configured to contain at least one of ferrite and bainite and martensite in a specific ratio, and the area percentage of the specific crystal grains having an intragranular orientation difference of 5 to 14° is controlled to be within the range of 10 to 60%. Therefore, it is extremely difficult to control the pole density of a specific texture in the thickness surface layer and the pole density of a specific texture in the thickness center to within a desired range while maintaining the configuration of the metal structure controlled in this way.
- the rolling conditions in the hot rolling process appropriate, it is possible to realize a metal structure in which the average value of the pole density in the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the thickness surface layer is 2.50 or more and the average value of the pole density in the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> orientations in the thickness center is 7.00 or less while maintaining the configuration of the metal structure for improving stretch flangeability, ductility and notch fatigue properties.
- the hot-rolled steel sheet according to the embodiment of the present invention can reliably achieve both the contradictory properties of high strength and excellent workability, and is therefore particularly useful in the automotive field, where both properties are required to be achieved.
- C is an element effective in increasing the strength of steel plate.
- C forms carbides and/or carbonitrides with Ti and Nb in steel, and contributes to precipitation strengthening based on the formed precipitates and to refinement of the structure due to the pinning effect of the precipitates.
- the C content is set to 0.020% or more.
- the C content may be 0.022% or more, 0.025% or more, 0.028% or more, or 0.030% or more.
- the C content is set to 0.070% or less.
- the C content may be 0.065% or less, 0.060% or less, 0.055% or less, or 0.050% or less.
- Si is an element that is effective in increasing strength as a solid solution strengthening element.
- the Si content is set to 0.010% or more.
- the Si content may be 0.100% or more, more than 0.100%, 0.110% or more, 0.120% or more, 0.150% or more, 0.180% or more, 0.200% or more, 0.300% or more, 0.500% or more, 0.800% or more, or 1.000% or more.
- the Si content is set to 2.000% or less.
- the Si content may be 1.800% or less, 1.600% or less, 1.400% or less, or 1.200% or less.
- Mn is an element that is effective in increasing strength as a hardenability and solid solution strengthening element. In order to fully obtain these effects, the Mn content is set to 0.60% or more. The Mn content may be 0.70% or more, 0.80% or more, 0.90% or more, or 1.00% or more. On the other hand, if Mn is contained excessively, stretch flangeability may be reduced. Therefore, the Mn content is set to 2.00% or less. The Mn content may be 1.80% or less, 1.60% or less, 1.40% or less, or 1.20% or less.
- Ti is an element that precipitates finely in steel as carbide (TiC) and improves the strength of steel by precipitation strengthening. Ti also forms carbides to fix C and suppresses the formation of cementite, which is harmful to stretch flangeability. In order to fully obtain these effects, the Ti content is set to 0.015% or more. The Ti content may be 0.020% or more, 0.030% or more, 0.040% or more, or 0.050% or more. On the other hand, if Ti is contained excessively, the carbides may become coarse and the ductility may decrease. Therefore, the Ti content is set to 0.200% or less. The Ti content may be 0.180% or less, 0.170% or less, 0.150% or less, or 0.120% or less.
- sol. Al is an element that acts as a deoxidizer for molten steel. In order to fully obtain this effect, the sol. Al content is set to 0.010% or more. On the other hand, if the sol. Al content is excessive, coarse oxides are formed, which reduces toughness and ductility and causes fatigue during rolling. Therefore, the sol. Al content is set to 1.000% or less. The sol. Al content is set to 0.800% or less, 0.600% or less, or 0.400% or less.
- the term "solubilized aluminum" means acid-soluble aluminum, and indicates solute aluminum that is present in the steel in a solid solution state.
- the P content is set to 0.100% or less.
- the P content may be 0.050% or less, 0.030% or less, 0.020% or less, or 0.015% or less.
- the lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in costs. Therefore, the P content may be 0.001% or more, 0.003% or more, or 0.005% or more.
- the Si content is set to 0.030% or less.
- the S content may be 0.020% or less, 0.010% or less, or 0.005% or less.
- the lower limit of the S content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in costs. Therefore, the S content may be 0.001% or more, 0.002% or more, or 0.003% or more.
- N may form precipitates with Ti preferentially over C, and may reduce the amount of Ti that is effective for fixing C. Therefore, the N content is set to 0.0060% or less.
- the N content may be 0.0050% or less, 0.0040% or less, or 0.0030% or less.
- the lower limit of the N content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in costs. Therefore, the N content may be 0.0001% or more, or 0.0005% or more.
- O is an element that is mixed in during the manufacturing process. If O is contained excessively, coarse inclusions may be formed, which may reduce the toughness of the steel plate. Therefore, the O content is set to 0.0100% or less.
- the O content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less.
- the lower limit of the O content is not particularly limited and may be 0%, but reducing the O content to less than 0.0001% requires a long time for refining, which leads to a decrease in productivity. Therefore, the O content may be 0.0001% or more, or 0.0005% or more.
- the basic chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention is as described above. Furthermore, the hot-rolled steel sheet may contain at least one of the following optional elements in place of a portion of the remaining Fe, as necessary.
- Nb is an element that forms carbides, nitrides and/or carbonitrides in steel and contributes to refining the structure by the pinning effect, and thus to increasing the strength of the steel sheet.
- Nb is also an element that fixes C by forming carbides and/or carbonitrides, and suppresses the formation of cementite that is harmful to stretch flangeability.
- the Nb content may be 0%, but in order to obtain these effects, the Nb content is preferably 0.001% or more.
- the Nb content may be 0.005% or more, 0.010% or more, or 0.015% or more.
- the Nb content is set to 0.050% or less.
- the Nb content may be 0.040% or less, 0.030% or less, or 0.020% or less.
- V is an element that contributes to improving strength by precipitation strengthening, etc.
- the V content may be 0%, but in order to obtain such an effect, the V content is preferably 0.001% or more.
- the V content may be 0.010% or more, 0.030% or more, or 0.050% or more.
- the V content is preferably 0.300% or less.
- the V content may be 0.200% or less, 0.100% or less, or 0.080% or less.
- Cr is an element that enhances the hardenability of steel and contributes to improving strength.
- the Cr content may be 0%, but in order to obtain such an effect, the Cr content is preferably 0.01% or more.
- the Cr content may be 0.03% or more or 0.05% or more.
- the Cr content is preferably 2.00% or less.
- the Cr content may be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.
- Ni and Cu are elements that contribute to improving strength by precipitation strengthening or solid solution strengthening.
- the Ni and Cu contents may be 0%, but in order to obtain such effects, the contents of these elements are preferably 0.01% or more, and may be 0.03% or more or 0.05% or more. On the other hand, even if these elements are contained excessively, the effects are saturated and there is a risk of increasing manufacturing costs. Therefore, the Ni and Cu contents are preferably 2.00% or less, and may be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.
- Mo is an element that improves the hardenability of steel and contributes to improving strength.
- the Mo content may be 0%, but in order to obtain such an effect, the Mo content is preferably 0.001% or more.
- the Mo content may be 0.010% or more, 0.020% or more, or 0.050% or more.
- the Mo content is preferably 1.000% or less.
- the Mo content may be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.080% or less.
- [B: 0 to 0.0100%] B segregates at grain boundaries to increase grain boundary strength, thereby improving low-temperature toughness.
- the B content may be 0%, but in order to obtain such an effect, the B content is preferably 0.0001% or more.
- the B content may be 0.0002% or more, 0.0003% or more, or 0.0005% or more.
- the B content is preferably 0.0100% or less.
- the B content may be 0.0050% or less, 0.0030% or less, 0.0015% or less, or 0.0010% or less.
- Sb is an element effective in improving corrosion resistance.
- the Sb content may be 0%, but in order to obtain such an effect, the Sb content is preferably 0.01% or more.
- the Sb content may be 0.02% or more or 0.05% or more.
- excessive Sb content may cause a decrease in toughness. Therefore, the Sb content is preferably 1.00% or less.
- the Sb content may be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.
- Ca, Mg and Hf are elements capable of controlling the morphology of nonmetallic inclusions.
- the Ca, Mg and Hf contents may be 0%, but in order to obtain such effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
- the Ca, Mg and Hf contents are preferably 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.
- REM is an element capable of controlling the morphology of nonmetallic inclusions.
- the REM content may be 0%, but in order to obtain such an effect, the REM content is preferably 0.0001% or more.
- the REM content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
- the REM content is preferably 0.1000% or less.
- the REM content may be 0.0500% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.
- REM is a collective term for 17 elements: 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, and the REM content is the total content of these elements.
- Bi and As are elements effective in improving corrosion resistance.
- the Bi and As contents may be 0%, but in order to obtain such effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
- the Bi and As contents are preferably 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.
- Zr is an element capable of controlling the form of nonmetallic inclusions.
- the Zr content may be 0%, but in order to obtain such an effect, the Zr content is preferably 0.01% or more.
- the Zr content may be 0.05% or more or 0.10% or more.
- the Zr content is preferably 1.00% or less.
- the Zr content may be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.
- Co is an element that contributes to improving hardenability and/or heat resistance.
- the Co content may be 0%, but in order to obtain these effects, the Co content is preferably 0.01% or more.
- the Co content may be 0.05% or more or 0.10% or more.
- the Co content is preferably 1.00% or less.
- the Co content may be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.
- Zn is an element effective in controlling the shape of inclusions.
- the Zn content is preferably 0.01% or more.
- the Zn content may be 0.05% or more or 0.10% or more.
- the Zn content is preferably 1.00% or less.
- the Zn content may be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.
- W is an element that enhances the hardenability of steel and contributes to improving strength.
- the W content may be 0%, but in order to obtain such an effect, the W content is preferably 0.01% or more.
- the W content may be 0.05% or more or 0.10% or more.
- excessive W content may reduce weldability. Therefore, the W content is preferably 1.00% or less.
- the W content may be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.
- Sn is an element effective in improving corrosion resistance.
- the Sn content may be 0%, but in order to obtain such an effect, the Sn content is preferably 0.01% or more.
- the Sn content may be 0.02% or more or 0.05% or more.
- excessive Sn content may cause a decrease in toughness. Therefore, the Sn content is preferably 1.00% or less.
- the Sn content may be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.
- the remainder other than the above elements consists of Fe and impurities.
- Impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when industrially manufacturing hot-rolled steel sheets.
- [0.100 ⁇ [Si]+[sol. Al] ⁇ 2.500] The chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention must satisfy the following formula. 0.100 ⁇ [Si]+[sol. Al] ⁇ 2.500
- [Si] and [sol. Al] are the contents (mass%) of each element.
- the grains are surrounded by a boundary with an orientation difference of 15° or more and have a circle equivalent diameter of 0.3 ⁇ m or more.
- crystal grains having an intragranular misorientation of 5 to 14° are effective in improving strength and stretch flangeability.
- the ratio of the crystal grains is controlled within the range of 10 to 60% by area to improve the balance between strength and stretch flangeability.
- sol. Al is also an element effective in controlling the ratio of crystal grains having an intragranular misorientation of 5 to 14 degrees within the range of 10 to 60%. This is believed to be due to the fact that the temperature of the Ar3 point is increased by the inclusion of Si and sol. Al, and the transformation strain introduced into the grains is reduced.
- the chemical composition of the hot rolled steel sheet according to the embodiment of the present invention is such that the contents of each element are controlled within the ranges described above, while Si and sol. The total content of Si and sol.
- Al is controlled to be 0.100% or more, i.e., to satisfy [Si] + [sol. Al] ⁇ 0.100.
- the total content of Si and sol. Al is controlled to be 0.120%.
- the total content of Si and sol. Al is set to 2.500% or less, that is, [Si] + [sol. Al] ⁇ 2.500.
- the content may be 2.000% or less, 1.500% or less, 1.000% or less, or 0.000% or less.
- the chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention may be measured by a general analytical method.
- the chemical composition of the hot-rolled steel sheet 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.
- the metal structure of the hot-rolled steel sheet according to the embodiment of the present invention includes, in area %, at least one of ferrite and bainite: 80 to 98% in total, and martensite: 2 to 10%.
- the total area ratio of at least one of ferrite and bainite is 80% or more, and may be, for example, 82% or more, 85% or more, 88% or more, or 90% or more.
- the area ratio of martensite may be 10% or less, and may be, for example, 9% or less, 8% or less, 7% or less, or 6% or less.
- the total area ratio of at least one of ferrite and bainite is high or the area ratio of martensite is low, the balance between strength and notch fatigue properties may be reduced, and desired properties may not be obtained.
- the total area ratio of at least one of ferrite and bainite is 98% or less, and may be, for example, 96% or less, 94% or less, or 92% or less.
- the area ratio of martensite is 2% or more, and may be, for example, 3% or more, 4% or more, or 5% or more.
- the metal structure of the hot-rolled steel sheet may contain either ferrite or bainite, and preferably contains both ferrite and bainite. Therefore, the area ratio of either ferrite or bainite may be 0%, or may be, for example, 2% or more, 5% or more, 10% or more, 20% or more, 30% or more, or 40% or more, respectively. Similarly, the area ratio of ferrite and bainite may be, for example, 90% or less, 80% or less, 70% or less, 60% or less, or 50% or less, respectively. From the viewpoint of improving the ductility of the hot-rolled steel sheet, the area ratio of bainite is preferably 80% or less, and more preferably 70% or less.
- the remaining structure other than ferrite, bainite, and martensite may be 0% by area, but when the remaining structure is present, the remaining structure may be at least one of retained austenite and pearlite.
- the area ratio of the remaining structure is not particularly limited, but may be, for example, 1% or more, 2% or more, or 3% or more. From the viewpoint of further improving the stretch flangeability, the area ratio of the remaining structure is preferably, for example, 10% or less, and may be 8% or less, 6% or less, or 5% or less.
- Identification of the metal structure and calculation of the area ratio in the hot-rolled steel sheet are performed by optical microscope observation after corrosion using a Nital reagent or a Lepera solution and X-ray diffraction method.
- the structure observation by an optical microscope is performed on a plate thickness cross section parallel to the rolling direction and perpendicular to the plate surface. Specifically, first, a sample is taken from the hot-rolled steel sheet, and the observation surface of the sample is etched with Nital.
- image analysis is performed on a structure photograph obtained at a 1/4 depth position of the plate thickness in a field of view of 300 ⁇ m ⁇ 300 ⁇ m using an optical microscope, thereby calculating each area ratio of ferrite and pearlite, and the total area ratio of bainite and martensite.
- image analysis is performed on a structure photograph obtained at a 1/4 depth position of the plate thickness in a field of view of 300 ⁇ m ⁇ 300 ⁇ m using an optical microscope, thereby calculating the total area ratio of retained austenite and martensite.
- the volume ratio of retained austenite is calculated by X-ray diffraction measurement. Since the volume fraction of the retained austenite is equivalent to the area fraction, this is taken as the area fraction of the retained austenite.
- the area fraction of martensite is calculated by subtracting the obtained area fraction of the retained austenite from the total area fraction of the retained austenite and martensite calculated previously.
- the area fraction of bainite is calculated by subtracting the obtained area fraction of martensite from the total area fraction of bainite and martensite calculated previously.
- an increase in the dislocation density within the grain improves strength while decreasing workability.
- the strength can be improved without decreasing workability in crystal grains in which the orientation difference within the grain is controlled to 5 to 14°.
- crystal grains with an orientation difference within the grain of less than 5° are excellent in workability but difficult to increase in strength.
- crystal grains with an orientation difference within the grain of more than 14° do not necessarily contribute to improving stretch flangeability because the deformability is different within the crystal grain.
- the proportion of crystal grains having an intragranular misorientation of 5 to 14° by appropriately controlling the proportion of crystal grains having an intragranular misorientation of 5 to 14°, more specifically, by controlling it to within a range of 10 to 60% in terms of area%, it is possible to improve the stretch flangeability while achieving the desired steel sheet strength, and it is possible to further improve the balance between strength and stretch flangeability. If the proportion of crystal grains having an intragranular misorientation of 5 to 14° is small, the stretch flangeability may be reduced. Therefore, from the viewpoint of improving the stretch flangeability, the proportion of crystal grains having an intragranular misorientation of 5 to 14° may be 15% or more, 18% or more, or 20% or more.
- the proportion of crystal grains having an intragranular misorientation of 5 to 14° may be 55% or less, 50% or less, 45% or less, or 40% or less.
- the proportion of crystal grains with an intragranular orientation difference of 5 to 14° is measured by electron backscattered diffraction (EBSD). More specifically, first, a sample is taken from the steel sheet so that the plate thickness cross section parallel to the rolling direction and perpendicular to the plate surface is the observation surface. Next, at a depth position of 1/4 of the plate thickness from the surface of the steel sheet, an area of 200 ⁇ m in the rolling direction of the steel sheet and 100 ⁇ m in the normal direction to the rolling surface is analyzed by EBSD analysis at a measurement interval of 0.2 ⁇ m to obtain crystal orientation information.
- EBSD electron backscattered diffraction
- the EBSD analysis is performed at an analysis speed of 50 to 300 points/second using an apparatus consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL).
- JSM-7001F thermal field emission scanning electron microscope
- HTKARI detector HAI detector manufactured by TSL.
- regions with an orientation difference of 15° or more and a circle equivalent diameter of 0.3 ⁇ m or more are defined as crystal grains
- the average orientation difference within the crystal grains is calculated
- the ratio of crystal grains with an orientation difference of 5 to 14° within the grains is obtained.
- the crystal grains and the average orientation difference within the grains defined as above can be calculated using the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device.
- orientation difference within a grain refers to "Grain Orientation Spread (GOS)", which is the orientation dispersion within a crystal grain.
- GOS Grain Orientation Spread
- the value of the orientation difference within a grain is described in "Analysis of Misorientation in Plastic Deformation of Stainless Steel by EBSD Method and X-ray Diffraction Method", Hidehiko Kimura et al., Transactions of the Japan Society of Mechanical Engineers (Series A), Vol. 71, No. 712, 2005, p.
- the GOS is calculated as an average value of the misorientation between a reference crystal orientation and all measurement points within the same crystal grain.
- the reference crystal orientation is an average orientation of all measurement points within the same crystal grain.
- the GOS value can be calculated using the software "OIM Analysis (registered trademark) Version 7.0.1" that comes with the EBSD analyzer.
- the average value of the pole densities of the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the region from the surface of the hot-rolled steel plate to the 1/6 position of the plate thickness (i.e., the plate thickness surface layer portion) is controlled to 2.50 or more, and the average value of the pole densities of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> orientations in the region from the 2/5 position of the plate thickness to the 3/5 position of the plate thickness (i.e., the plate thickness center portion) is controlled to 7.00 or less.
- the average value of the pole density of the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the sheet thickness surface layer portion to 2.50 or more to increase the strength in the L direction
- the average value of the pole density of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> orientations in the sheet thickness center portion to 7.00 or less to reduce the strength in the C direction
- the difference in tensile strength between the L direction and the C direction of the obtained hot rolled steel sheet can be reduced, and as a result, the anisotropy of the strength in the tensile strength in the L direction and the C direction can be significantly reduced.
- the larger the average value of the pole density of the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the sheet thickness surface layer portion the more preferable it is, and it may be, for example, 2.80 or more, 3.00 or more, 3.20 or more, or 3.50 or more.
- the upper limit is not particularly limited, but for example, the average value of the pole density of the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the sheet thickness surface layer portion may be 5.00 or less, 4.80 or less, 4.70 or less, 4.50 or less, 4.20 or less, 4.00 or less, or 3.80 or less.
- the smaller the average value of the pole density of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011>, and ⁇ 332 ⁇ 113> orientations in the sheet thickness center portion the more preferable, and may be, for example, 6.80 or less, 6.50 or less, 6.20 or less, or 6.00 or less.
- the lower limit is not particularly limited, but for example, the average pole density of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011>, and ⁇ 332 ⁇ 113> orientations in the center part of the sheet thickness may be 3.50 or more, 4.00 or more, 4.20 or more, 4.40 or more, 4.50 or more, or 5.00 or more.
- a sample is taken from the steel sheet so that the sheet thickness cross section parallel to the rolling direction and perpendicular to the sheet surface becomes the observation surface, and EBSD analysis is performed at measurement intervals of 1 ⁇ m on a rectangular region of the steel sheet, which is 1000 ⁇ m in the rolling direction and 100 ⁇ m in the normal direction to the rolling surface and is centered at a depth position of 1/12 of the sheet thickness from the steel sheet surface, to obtain crystal orientation information of this rectangular region.
- the EBSD analysis is performed at an analysis speed of 50 to 300 points/second using an apparatus consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL).
- the ODF Orientation Distribution Function
- the Harmonic Series Expansion spherical harmonic function method
- the expansion order was set to 16.
- the calculation was performed taking into account symmetry (orthotropic).
- the hot rolled steel sheet according to the embodiment of the present invention generally has a thickness of 1.0 to 6.0 mm, although not particularly limited thereto.
- the thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and/or 5.0 mm or less, or 4.0 mm or less.
- the upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the hot-rolled steel sheet may be 1470 MPa or less, 1250 MPa or less, 1180 MPa or less, 1080 MPa or less, or 980 MPa or less.
- the tensile strength is measured by taking a JIS No. 5 test piece from a direction (C direction) in which the longitudinal direction of the test piece is parallel to the rolling perpendicular direction of the hot-rolled steel sheet, and performing a tensile test in accordance with JIS Z 2241:2011.
- the tensile strength thus obtained is also referred to herein as C-direction TS (TSC).
- Total elongation: El According to the hot-rolled steel sheet having the above chemical composition and metal structure, in addition to high tensile strength, the total elongation can be improved, and more specifically, a total elongation of 15.0% or more can be achieved.
- the total elongation is preferably 18.0% or more, more preferably 20.0% or more, and most preferably 22.0% or more.
- the upper limit is not particularly limited, but for example, the total elongation may be 40.0% or less or 35.0% or less.
- the total elongation is measured by taking a JIS No. 5 test piece from a direction (C direction) in which the longitudinal direction of the test piece is parallel to the rolling perpendicular direction of the hot-rolled steel sheet, and performing a tensile test in accordance with JIS Z 2241:2011.
- the method for producing a hot-rolled steel sheet according to an embodiment of the present invention includes: (A) a hot rolling process including heating a slab having the chemical composition described above in relation to the hot rolled steel sheet and then finish rolling the slab, and satisfying the following conditions (A1) to (A5); and (A1) the heating temperature of the slab is a solution temperature (SRTmin) °C or higher represented by the following formula 1 and 1260 °C or lower; (A2) The cumulative strain ( ⁇ eff.) in the last three stages of finish rolling, represented by the following formula 2, is 0.50 to 0.60; (A3) The end temperature of the finish rolling is Ar3+30°C or higher; (A4) In the finish rolling, two or more rolling passes having a shape ratio (X) represented by the following formula 3 of 2.3 or more are performed at 1100 ° C.
- X shape ratio
- the rolling temperature of the first three stages of the finish rolling is equal to or higher than the entry temperature of the finish rolling (FT0) - 50 ° C.
- SRTmin 7000 / ⁇ 2.75 - log ([Ti] x [C]) ⁇ - 273 ...Equation 1
- [Ti] and [C] are the contents (mass%) of each element in the steel.
- ⁇ eff. ⁇ i(t,T) ...
- Q 183200J
- t indicates the cumulative time (seconds) until just before cooling in the pass
- T indicates the rolling temperature (° C.) in the pass.
- a cooling step includes primarily cooling the finish-rolled steel sheet to a temperature range of 650 to 750°C at an average cooling rate of 10°C/s or more, holding the steel sheet in the temperature range for 3.0 to 10.0 seconds, and then secondary cooling to 100°C or less at an average cooling rate of 30°C/s or more. Each step will be described in detail below.
- the hot-rolled steel sheet according to the embodiment of the present invention contains Ti, and if the heating temperature of the slab is less than the solution temperature (SRTmin) ° C., Ti is not sufficiently dissolved. If Ti is not sufficiently dissolved during slab heating, it is difficult to improve the strength of the steel by precipitation strengthening by finely precipitating Ti as carbide (TiC) in the steel during the cooling process after the hot rolling process. In addition, it is difficult to fix C by forming carbide (TiC) and suppress the generation of cementite, which is harmful to stretch flangeability. On the other hand, if the heating temperature of the slab is more than 1260 ° C., the yield decreases due to scale-off.
- SiC solution temperature
- the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc.
- the conditions of the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.
- the cumulative strain in the latter three stages of finish rolling is less than 0.50, the dislocation density of the introduced austenite is insufficient, and the proportion of crystal grains with an intragranular misorientation of 5 to 14° is less than 10%.
- the cumulative strain in the latter three stages of finish rolling is more than 0.60, recrystallization of austenite occurs during hot rolling, and the accumulated dislocation density during transformation decreases. As a result, the proportion of crystal grains with an intragranular misorientation of 5 to 14° is similarly less than 10%.
- Q 183200J
- t indicates the cumulative time (seconds) until just before cooling in the corresponding pass
- T indicates the rolling temperature (° C.) in the corresponding pass.
- the end temperature of the finish rolling needs to be Ar3+30°C or higher. If the end temperature of the finish rolling is less than Ar3+30°C, when ferrite is generated in a part of the structure due to the variation of the components in the steel sheet and the rolling temperature, the ferrite may be processed. The processed ferrite may cause a decrease in ductility. In addition, if the end temperature of the finish rolling is less than Ar3+30°C, the ratio of crystal grains having an intragranular misorientation of 5 to 14° may exceed 60% and become excessively high.
- Ar3 (°C) is calculated based on the chemical composition of the hot-rolled steel sheet by the following formula 4.
- Ar3 901 - 325 x [C] + 33 x [Si] + 287 x [P] + 40 x [sol. Al] - 92 x ([Mn] + [Mo] + [Cu]) - 46 x ([Cr] + [Ni]) ...Equation 4
- [C], [Si], [P], [sol. Al], [Mn], [Mo], [Cu], [Cr] and [Ni] are the contents (mass%) of each element in the steel, and are 0 when the element is not contained.
- the inventors have found that by increasing the shear strain introduced into the plate thickness surface layer portion of the steel plate in finish rolling, the degree of accumulation in the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the plate thickness surface layer portion can be increased, thereby controlling the average value of the pole density of these orientations within a desired range. More specifically, by performing two or more rolling passes at 1100°C or less in the finish rolling such that the shape ratio (X) represented by the following formula 3 is 2.3 or more, it is possible to increase the average pole density of the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the sheet thickness surface layer portion to 2.50 or more.
- the shape ratio (X) means the roll contact arc length ( ⁇ (R(h 0 -h 1 ))) divided by the average plate thickness ((h 0 +h 1 )/2).
- a shape ratio (X) of 2.3 or more can be achieved, and the shear strain introduced into the plate thickness surface layer portion of the steel plate can be increased.
- the rolling temperature at that time is 1100 ° C or less, the recovery of the introduced shear strain can be suppressed.
- the average value of the pole density of the ⁇ 110 ⁇ ⁇ 111> and ⁇ 112 ⁇ ⁇ 111> orientations in the plate thickness surface layer portion can be reliably increased to 2.50 or more.
- the upper limit of the number of such rolling passes is not particularly limited, and for example, the number of rolling passes may be 5 passes or less.
- the rolling temperature is more than 1100 ° C, or X is 2.3 or more and the number of rolling passes at 1100 ° C or less is 1 pass or less, sufficient shear strain cannot be introduced into the plate thickness surface layer.
- the roll radius of the roll used in the rolling mill can be selected from a range in which X is 2.3 or more. Although not particularly limited, for example, the roll radius can be selected from a range of 150 to 400 mm.
- the rolling temperatures of the first three stages of the finish rolling i.e., the rolling temperature of the first stage of the finish rolling (FT1), the rolling temperature of the second stage of the finish rolling (FT2) and the rolling temperature of the third stage of the finish rolling (FT3), are controlled to be equal to or higher than the entry temperature of the finish rolling (FT0) -50°C, thereby reducing the shear strain introduced in the center of the plate thickness, thereby achieving an average pole density of 7.00 or less in the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> orientations.
- the shear strain introduced in the plate thickness center portion is smaller than that in the plate thickness surface layer portion that directly contacts the roll, and therefore, by controlling the rolling temperature of the first three stages of the finish rolling to a relatively high temperature as described above, it is possible to sufficiently reduce the introduced shear strain. From the viewpoint of further reducing the anisotropy of strength, it is preferable to control the rolling temperature of the first three stages of the finish rolling to FT0-45 ° C. or higher.
- the rolling temperature of even one of the first three stages of the rolling is less than FT0-50 ° C., the effect of reducing the shear strain is not sufficient, and the average value of the pole density of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> orientations in the plate thickness center portion cannot be reduced to 7.00 or less.
- the upper limit of the rolling temperature of the first three stages of the finish rolling is not particularly limited, but for example, the rolling temperature of the first three stages of the finish rolling may be 1100 ° C. or less or 1000 ° C. or less.
- one or more rolling passes under the condition (A4) and one or more rolling passes under the condition (A5) may overlap with each other.
- the finish-rolled steel sheet is subjected to two-stage cooling in the next cooling step. Specifically, the finish-rolled steel sheet is first cooled to a temperature range of 650 to 750 ° C. at an average cooling rate of 10 ° C./s or more, held in that temperature range for 3.0 to 10.0 seconds, and then secondarily cooled to 100 ° C. or less at an average cooling rate of 30 ° C./s or more.
- the cooling end temperature of the primary cooling is less than 650 ° C
- transformation due to para-equilibrium occurs at a temperature lower than the desired temperature range
- the proportion of crystal grains with an orientation difference of 5 to 14 ° in the grains is less than 10%.
- the holding time at 650 to 750 ° C is less than 3.0 seconds
- the proportion of crystal grains with an orientation difference of 5 to 14 ° in the grains is also less than 10%.
- the holding time at 650 to 750 ° C exceeds 10.0 seconds or the average cooling rate of the secondary cooling is less than 30 ° C / s, cementite that is harmful to stretch flangeability is likely to be generated.
- the cooling end temperature of the secondary cooling is more than 100 ° C, the area ratio of martensite is less than 2%.
- the average cooling rate of the primary and secondary cooling may be 200 ° C / s or less in consideration of the equipment capacity of the cooling equipment.
- Hot-rolled steel sheet manufactured by the above manufacturing method can have a metal structure that contains, by area percentage, at least one of ferrite and bainite: 80-98% in total, and martensite: 2-10%, and where, if boundaries with an orientation difference of 15° or more are defined as grain boundaries, and regions surrounded by such grain boundaries and having a circle equivalent diameter of 0.3 ⁇ m or more are defined as crystal grains, the proportion of crystal grains with an intragranular orientation difference of 5-14° is 10-60% by area percentage.
- a metal structure that contains, by area percentage, at least one of ferrite and bainite: 80-98% in total, and martensite: 2-10%, and where, if boundaries with an orientation difference of 15° or more are defined as grain boundaries, and regions surrounded by such grain boundaries and having a circle equivalent diameter of 0.3 ⁇ m or more are defined as crystal grains, the proportion of crystal grains with an intragranular orientation difference of 5-14° is 10-60% by area percentage.
- the average pole density of the ⁇ 110 ⁇ 111> and ⁇ 112 ⁇ 111> orientations in the region from the surface to the 1/6 position of the plate thickness is controlled to 2.50 or more, and the average pole density of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011>, and ⁇ 332 ⁇ 113> orientations in the region from the 2/5 position to the 3/5 position of the plate thickness is controlled to 7.00 or less, so that the anisotropy of strength in the tensile strength in the L direction and C direction of the hot rolled steel sheet can be significantly reduced. Therefore, according to the hot rolled steel sheet manufactured by the above manufacturing method, it is possible to reliably achieve the contradictory properties of high strength and excellent workability at the same time, and it is particularly useful in the automotive field where both properties are required.
- hot-rolled steel sheets according to the embodiments of the present invention were manufactured under various conditions, and the tensile strength, stretch flangeability, ductility, notch fatigue properties, and strength anisotropy of the obtained hot-rolled steel sheets were investigated.
- molten steel was cast by continuous casting to form slabs having various chemical compositions shown in Tables 1 and 2, and these slabs were heated under the conditions shown in Table 3, and then hot rolling was performed.
- 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 finish rolling was performed using a tandem rolling mill consisting of seven rolling stands.
- the entry temperature (F0) of the finish rolling, the rolling temperature of the first stage of the finish rolling (FT1), the rolling temperature of the second stage of the finish rolling (FT2), the rolling temperature of the third stage of the finish rolling (FT3), the end temperature of the finish rolling, and the accumulated strain ( ⁇ eff.) of the last three stages of the finish rolling were as shown in Table 2.
- finish rolling was performed using rolls having the roll radii shown in Table 3, and the number of rolling passes at 1100°C or less to achieve a shape ratio (X) of 2.3 or more was shown in Table 3.
- finish-rolled steel plate was subjected to primary and secondary cooling under the conditions shown in Table 3 to obtain a hot-rolled steel plate having the plate thickness shown in Table 2.
- the properties of the resulting hot-rolled steel sheets were measured and evaluated using the following methods.
- TSC tensile strength
- El total elongation
- the stretch flangeability was evaluated by a saddle-shaped stretch flange test method using a saddle-shaped molded product. Specifically, a molded product of a saddle-shaped shape simulating a stretch flange shape consisting of a straight part and a circular part as shown in FIG. 1 was pressed, and the stretch flangeability was evaluated by the limit forming height at that time.
- a saddle-shaped stretch flange test method a saddle-shaped molded product with a corner curvature radius R of 50 to 60 mm and an opening angle ⁇ of 120° is used to measure the limit forming height H (mm) when the clearance when punching the corner part is 11%.
- the clearance indicates the ratio of the gap between the punching die and the punch to the thickness of the test piece. Since the clearance is actually determined by the combination of the punching tool and the plate thickness, 11% means that the range of 10.5 to 11.5% is satisfied.
- the judgment of the limit forming height H was made by visually observing the presence or absence of cracks having a length of 1/3 or more of the plate thickness after forming, and the limit forming height at which no cracks existed was determined.
- the product (TSC ⁇ H) of the tensile strength TSC (MPa) and the limit forming height H (mm) was used as an index of stretch flangeability, and the stretch flangeability was evaluated as being improved when TSC ⁇ H ⁇ 19,500 MPa ⁇ mm.
- the hot-rolled steel sheet was evaluated as having high strength, yet improved stretch flangeability, ductility, and notch fatigue properties, and reduced strength anisotropy. The results are shown in Tables 4 and 5.
- the average value of the pole density of the ⁇ 100 ⁇ 011>, ⁇ 211 ⁇ 011>, and ⁇ 332 ⁇ 113> orientations exceeded 7.00, the strength in the C direction could not be reduced, and the anisotropy of the strength became prominent.
- the average cooling rate of the first cooling in the cooling step was low, so it is believed that transformation due to paraequilibrium occurred at a relatively high temperature.
- the proportion of crystal grains with an orientation difference of 5 to 14° in the grains was less than 10%, and the stretch flangeability was reduced.
- the cooling stop temperature of the first cooling was high, so it is believed that transformation due to paraequilibrium occurred at a relatively high temperature.
- the cooling stop temperature of the first cooling was low, so it is believed that transformation due to paraequilibrium occurred at a temperature lower than the desired temperature range.
- the proportion of crystal grains with an orientation difference of 5 to 14° in the grains was less than 10%, and the stretch flangeability was reduced.
- the holding time at 650 to 750 ° C in the first cooling was short, so the proportion of crystal grains with an orientation difference of 5 to 14° in the grains was less than 10%, and the stretch flangeability was reduced.
- the cooling stop temperature of the secondary cooling in the cooling process was high, so the area ratio of martensite was less than 2%. As a result, the TSC and notch fatigue properties were reduced.
- Comparative Examples 31 and 33 the C and Mn contents were high, respectively, and therefore the stretch flangeability was reduced.
- Comparative Examples 32 and 34 the C and Mn contents were low, respectively, and therefore sufficient strength could not be obtained.
- Comparative Example 35 the Al content was high, and therefore cracks occurred during rolling, and subsequent testing could not be performed.
- Comparative Example 36 the total content of Si and sol. Al was high, and therefore ferrite formation was promoted, and the TSC was reduced.
- Comparative Example 37 the total content of Si and sol. Al was low, and therefore the proportion of crystal grains with an intragranular misorientation of 5 to 14° was less than 10%, and therefore the stretch flangeability was reduced.
- Comparative Example 38 the Ti content was high, and therefore the carbide (TiC) became coarse, and therefore the ductility was reduced.
- Comparative Example 39 the Ti content was low, and therefore it is believed that the formation of cementite could not be sufficiently suppressed, and as a result, the stretch flangeability was reduced.
Landscapes
- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Mechanical Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Heat Treatment Of Sheet Steel (AREA)
Abstract
Est proposée une tôle d'acier laminée à chaud qui a une composition chimique prescrite et contient 80 à 98 % d'une ferrite et/ou d'une bainite au total et 2 à 10 % d'une martensite en termes de pourcentage surfacique et qui présente une structure métallique. Étant donné qu'une limite ayant une différence d'orientation de 15° ou plus est définie comme une limite de grain et qu'une région qui est entourée par la limite de grain a un diamètre de cercle équivalent de 0,3 µm ou plus est définie comme grain cristallin, la proportion de grains cristallins ayant une différence d'orientation dans le grain de 5 à 14° est de 10 à 60 % en termes de pourcentage surfacique ; la densité de pôle moyenne dans les orientations {110}<111> et {112}<111> dans une région allant de la surface à la position d'épaisseur de 1/6 de feuille est de 2,50 ou plus ; et la densité de pôle moyenne dans les orientations {100}<011>, {211}<011> et {332}<113> dans une région allant de la position d'épaisseur de 2/5 de feuille à la position d'épaisseur de feuille de 3/5 est de 7,00 ou moins.
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP2022176187 | 2022-11-02 | ||
JP2022-176187 | 2022-11-02 |
Publications (1)
Publication Number | Publication Date |
---|---|
WO2024095532A1 true WO2024095532A1 (fr) | 2024-05-10 |
Family
ID=90930107
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
PCT/JP2023/024341 WO2024095532A1 (fr) | 2022-11-02 | 2023-06-30 | Tôle d'acier laminée à chaud |
Country Status (1)
Country | Link |
---|---|
WO (1) | WO2024095532A1 (fr) |
Citations (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JP2005298956A (ja) * | 2004-04-16 | 2005-10-27 | Sumitomo Metal Ind Ltd | 熱延鋼板およびその製造方法 |
JP2009132988A (ja) * | 2007-04-19 | 2009-06-18 | Nippon Steel Corp | 低降伏比高ヤング率鋼板、溶融亜鉛メッキ鋼板、合金化溶融亜鉛メッキ鋼板、及び、鋼管、並びに、それらの製造方法 |
WO2016133222A1 (fr) * | 2015-02-20 | 2016-08-25 | 新日鐵住金株式会社 | Tôle d'acier laminée à chaud |
WO2020195605A1 (fr) * | 2019-03-26 | 2020-10-01 | 日本製鉄株式会社 | Tôle en acier ainsi que procédé de fabrication de celle-ci, et tôle en acier plaquée |
-
2023
- 2023-06-30 WO PCT/JP2023/024341 patent/WO2024095532A1/fr unknown
Patent Citations (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JP2005298956A (ja) * | 2004-04-16 | 2005-10-27 | Sumitomo Metal Ind Ltd | 熱延鋼板およびその製造方法 |
JP2009132988A (ja) * | 2007-04-19 | 2009-06-18 | Nippon Steel Corp | 低降伏比高ヤング率鋼板、溶融亜鉛メッキ鋼板、合金化溶融亜鉛メッキ鋼板、及び、鋼管、並びに、それらの製造方法 |
WO2016133222A1 (fr) * | 2015-02-20 | 2016-08-25 | 新日鐵住金株式会社 | Tôle d'acier laminée à chaud |
WO2020195605A1 (fr) * | 2019-03-26 | 2020-10-01 | 日本製鉄株式会社 | Tôle en acier ainsi que procédé de fabrication de celle-ci, et tôle en acier plaquée |
Similar Documents
Publication | Publication Date | Title |
---|---|---|
CN109563586B (zh) | 钢板及镀覆钢板 | |
CN103459645B (zh) | 局部变形能力优异的高强度热轧钢板及其制造方法 | |
CN103403208B (zh) | 热轧钢板及其制造方法 | |
CN103459647B (zh) | 热轧钢板及其制造方法 | |
CN104395490B (zh) | 高强度热轧钢板及其制造方法 | |
CN107250404B (zh) | 热轧钢板 | |
WO2015162932A1 (fr) | Tôle d'acier laminée à chaud pour ébauche laminée sur mesure, ébauche laminée sur mesure et leur procédé de fabrication | |
JP6358386B2 (ja) | 熱延鋼板 | |
WO2013065346A1 (fr) | Feuille d'acier laminée à chaud, de haute résistance, ayant d'excellentes caractéristiques de flexion et une excellente ténacité aux basses températures et son procédé de fabrication | |
US11578394B2 (en) | Nickel-containing steel for low temperature | |
US11198929B2 (en) | Hot rolled steel sheet and method for producing same | |
US20220282359A1 (en) | Nickel-containing steel for low temperature | |
WO2019082324A1 (fr) | Acier comprenant du nickel pour basse température | |
EP3936629A1 (fr) | Tôle d'acier laminée à chaud et procédé de production s'y rapportant | |
WO2023063010A1 (fr) | Tôle d'acier laminée à chaud | |
CN114829654B (zh) | 热轧钢板 | |
WO2024095532A1 (fr) | Tôle d'acier laminée à chaud | |
JP7440804B2 (ja) | 熱間圧延鋼板 | |
EP4074854A1 (fr) | Plaque d'acier laminée à chaud | |
JP2008013831A (ja) | 高ヤング率溶接構造用厚鋼板およびその製造方法 | |
JP7469706B2 (ja) | 高強度鋼板 | |
WO2024096073A1 (fr) | Bobine laminée à chaud | |
WO2024095534A1 (fr) | Tôle en acier laminé à chaud | |
WO2024095533A1 (fr) | Feuille d'acier laminée à chaud | |
WO2024135043A1 (fr) | Feuille d'acier laminée à chaud |
Legal Events
Date | Code | Title | Description |
---|---|---|---|
121 | Ep: the epo has been informed by wipo that ep was designated in this application |
Ref document number: 23885305 Country of ref document: EP Kind code of ref document: A1 |