WO2009017256A1

WO2009017256A1 - High-strength steel sheet

Info

Publication number: WO2009017256A1
Application number: PCT/JP2008/064175
Authority: WO
Inventors: Koichi Nakagawa; Takeshi Yokota; Nobuyuki Nakamura; Kazuhiro Seto; Satoshi Kinoshiro; Katsumi Yamada
Original assignee: Jfe Steel Corporation
Priority date: 2007-07-31
Filing date: 2008-07-31
Publication date: 2009-02-05
Also published as: TW200912015A; EP2177640B1; CN101772584A; MX2010001110A; US20100196189A1; JP2009052139A; CA2693489C; CA2693489A1; EP2177640A4; KR20100029138A; EP2177640A1; JP5326403B2; TWI390050B; CN101772584B

Abstract

The invention provides a high-strength steel sheet excellent in stretch-flange characteristics after working and in corrosion resistance after coating. A steel sheet which has a composition containing by mass C: 0.02 to 0.20%, Si: 0.3% or below, Mn: 0.5 to 2.5%, P: 0.06% or below, S: 0.01% or below, Al: 0.1% or below, Ti: 0.05 to 0.25%, and V: 0.05 to 0.25% with the balance consisting of Fe and unavoidable impurities and a substantially ferrite single-phase structure wherein the contents of Ti, V, and solid-soluted V in precipitates of less than 20nm in size are 200 to 1750 mass ppm, 150 to 1750 mass ppm, and 200 to less than 1750 mass ppm respectively.

Description

Specification High Strength Steel Sheet Technical Field

The present invention relates to a high-strength steel sheet excellent in stretch flange characteristics after processing and corrosion resistance after coating. Background art

Parts such as automobile undercarriage and truck frames are required to have formability (mainly stretch and stretch flange characteristics), and steels with a tensile strength of 590 MPa have been used in the past. However, in recent years, from the viewpoint of reducing the environmental impact of automobiles and improving impact properties, the strength of automobile steel sheets has been increased, and the use of steel with a tensile strength of 780 MPa class has begun to be considered.

In general, steel materials have lower workability as the strength increases. For this reason, research has been conducted on steel sheets having high strength and high workability. Examples of techniques for improving stretch and stretch flange characteristics include Patent Documents 1 to 6.

Patent Document 1 has a tensile strength of 590 MPa or more, characterized in that it is substantially a ferrite single-phase structure and carbides containing Ti and Mo having an average particle size of lOnm are dispersed and precipitated. A technique relating to a high-tensile steel plate excellent in workability is disclosed.

In Patent Document 2, by mass, C: 0.08 to 0.20%, S i: 0.001% or more and less than 0.2%, Mn: more than 1.0¾ 3.0% or less, A1: 0.001 to 0.5%, V: more than 0.1% 0.5 % Or less, T i: 0.05% or more and less than 0.2¾ and Nb: 0.005¾ to 0.5%, and

(Equation 1) (T i / 48 + N b / 93) XC / 12 ≤ 4.5X10 one ^5,

(Equation 2) 0.5 ≤ (V / 51 + T i / 48 + N b / 93) / (C / 12) ≤ 1.5,

(Equation 3) V + T i X2 + X1.4 + CX2 + Mn X0.1 ≥ 0.80

3 and consists of the balance Fe and unavoidable impurities. A technology relating to a high-strength hot-rolled steel sheet having a steel structure containing 70 volume% or more of ferrite having a degree of 250 Hv or more and having a strength of 880 MPa or more and a yield ratio of 0.80 or more is disclosed. In Patent Document 3, by mass, C: 0.05 to 0.2%, S i: 0.001% to 3.0%, M n: 0.5 to 3.0, P: 0.001 to 0.2%, A 1: 0.001 to 3%, V: 0.1 over 1¾ to 1.5¾, the balance is Fe and impurities, and the structure has an average particle size of 1 to 5 m A technology related to hot-rolled steel sheets characterized by the presence of V carbonitrides with ferrite as the main phase and an average grain size of 50 nm or less in the ferrite grains is disclosed.

Patent Document 4 discloses a high-strength thin steel sheet having excellent thermal stability obtained by precipitating carbides in a steel structure. This thin steel sheet has a Na C 1 type crystal structure represented by MC when the carbide is M as a metal element, and the metal element M is composed of two or more kinds of metals. It is characterized by the superlattice structure in which the metals of the above are regularly arranged in the crystal lattice.

Patent Document 5 discloses the following hot-rolled steel sheet. Ingredient composition is% by mass, C: 0.0002 to 0.25%, Si: 0.003 to 3.0%, Mn: 0.003 to 3.0%, and A1: 0.002 to 2.0%, the remainder from Fe and inevitable impurities In the impurities, P is 0.15% or less, S is 0.05% or less, and N is 0.01% or less. In terms of area ratio, 70% or more of the metal structure is the ferrite phase, the average crystal grain size is 20 μπι or less, and the aspect ratio is 3 or less. Furthermore, 70% or more of ferrite grain boundaries are composed of large-angle grain boundaries, and among the ferrite phases formed at large-angle grain boundaries, the area ratio of precipitates with a maximum diameter of 30 μιη or less and a minimum diameter of 5 nm or more Is less than 2% of the metallographic structure. Furthermore, the average grain size of the second phase, which has the largest area ratio among the remaining phases excluding the ferrite phase and precipitates, is 20 / zm or less, and the large-angle grain boundary of the ferrite phase is between the nearest second phases. Exists.

Patent Document 6 includes, in mass%, ○: 0.01 to 0.1%, 8≤0.03%, 1 ^ ≤0.005%, 1 ¹ 1: 0.05 to 0.5%, and Ti-48 / 12C-48 / 14N -48 / 32S≥0% of Ti is contained, the balance is Fe and inevitable impurities, and {100} <011> to {223} of the plate surface at least 1/2 the plate thickness The average X-ray random intensity ratio of the 110> orientation group is 3 or more, and the X-ray random intensity ratio of the three directions of {554} <225>, {111} <112> and {111} <110> Average value is 3.5 or less A squeezable burring property with excellent shape freezing characteristics, characterized in that a composition having a lubricating effect is applied to a copper plate having an arithmetic average roughness Ra of 1 to 3.5 on the surface of at least one copper plate. The strength thin steel sheet is disclosed.

Patent Document 1: Registered Patent No. 3591502

Patent Document 2: JP-A-2006-161 112

Patent Document 3: Japanese Patent Application Laid-Open No. 2004-143518

Patent Document 4: Japanese Patent Laid-Open No. 2003-321740

Patent Document 5: Japanese Patent Laid-Open No. 2003-293083

Patent Document 6: Japanese Patent Laid-Open No. 2003-160836 Disclosure of Invention

However, the above-described conventional technology has the following problems.

Since Patent Documents 1 and 4 contain Mo, there is a problem in that the cost of raw materials for Mo has increased significantly in recent years, resulting in a significant increase in cost.

Furthermore, with the globalization of the automotive industry, copper plates used in automobiles are used in severe corrosive environments, and higher post-coating corrosion resistance is required for steel sheets. On the other hand, the addition of Mo hinders the formation or growth of chemical crystals, so that the corrosion resistance after coating of the steel sheet is lowered and the above requirement cannot be met. That is, the steels described in Patent Document 1 and Patent Document 4 cannot provide post-coating corrosion resistance that satisfies the demands of the automobile industry in recent years.

On the other hand, due to recent advances in press technology, processing processes such as drawing (drawing and stretching) — trimming (hole punching) → retriking (hole expanding) have begun to be adopted. The stretch flange part of the copper plate that is to be stretched requires draw flange characteristics after draw trimming, that is, after processing. However, in Patent Documents 2, 3, and 4, when trying to obtain a TS of 780 MPa or more, sufficient stretch flange characteristics after processing cannot always be obtained. Nb added to Patent Document 3 has a high function of suppressing recrystallization of austenite after hot rolling. Therefore, non-recrystallized grains remain on the steel sheet, and workability is reduced. There's a problem.. There is also a problem of increasing the rolling load during hot rolling.

According to Patent Document 5, a ferrite single-phase steel sheet having a tensile strength TS of up to 422 MPa (for example, Table 6 in Examples, Test Nos. 1 to 5 and Table 8 in Examples, Test No. 45), and a tensile strength TS of 780 MPa or more. A composite structure copper plate composed of a ferrite phase and a second phase (for example, Example Table 6, Test Nos. 33 to 36 and Example Table 8, Test No. 49) is disclosed. These steel sheets disclosed in Patent Document 5 mainly utilize solid solution strengthening by Si or Mn and transformation structure strengthening using a hard second phase. Therefore, the copper sheet is cooled to a temperature range of 600 to 800 ° C at an average cooling rate of 30 ° C / s or more within 2 seconds after finishing rolling, and then air-cooled for 3 to 15 seconds, and then further averaged. It must be water-cooled and scraped off at a cooling rate of 30¾ / sec or more. This promotes two-phase separation during ferrite transformation, and the steel sheet has a composite structure consisting of a ferrite phase and a second phase. Further, the finish rolling temperature is set to a temperature range (from Ae3 point +100) to Ae3 point, which is lower than a temperature range considered suitable for manufacturing the present invention described later. For example, in the case of a composite steel sheet having a tensile strength T S of 780 MPa or more (Table 6 in the example, test number 33 Kano et al. 36), the finish rolling temperature was 871 to ˜800. When the finish rolling temperature is low, the solid solubility limit of carbide-forming elements such as Ti in the austenite phase is lowered, and precipitation sites are introduced by processing by rolling, so precipitates of 20 nm or more are generated. This phenomenon is called strain-induced precipitation. In the steel sheet and the manufacturing method in Patent Document 5, strain-induced precipitation occurs, so that the amount of precipitates having a size of 20 nm or more increases.

Furthermore, Patent Document 5 discloses a technique that realizes the production of a ferrite single-phase structure by significantly reducing the C content of the steel composition and reducing the content of Mn, which is an austenite stabilizing element. (Reference: Table 2 in Examples, Steel numbers AA to AE). However, in this case, the content of Mn, which is also a solid solution strengthening element, decreases, so that the solid solution strengthening amount decreases. In addition, the decrease in the C content reduces the precipitation amount of carbides such as Ti and Nb, which are effective for precipitation strengthening, and the precipitation strengthening amount also decreases. As a result, even if the solid solution strengthening amount and precipitation strengthening amount are combined, the strength of 780 MPa or more cannot be obtained in the case of a ferritic single phase steel sheet (see: Table 6 of the example, test number 1). To 5 and Table 8 of Examples, Test No. 45). For the reasons described above, the technique of Patent Document 5 cannot produce a steel sheet having a structure that is substantially a ferrite single phase, has a tensile strength of 780 MPa or more, and has other characteristics.

Patent Document 6 discloses a copper plate (for example, steel symbols A-4, A-8, A-10, C, E, H in Example Table 2) having a tensile strength σ _以上 of 780 MPa or more. The YR (unit%, where YR is σ γ / σ _Β Χ 100) of these steel sheets is as low as 69% to 74%, so these copper plates have a hard second phase such as martensite phase. It is guessed that it contains.

From this, the design philosophy of steel sheets of 780 MPa or more in Patent Document 6 mainly uses solid solution strengthening by Si or Mn and transformation structure strengthening using a hard second phase, as in Patent Document 5. It is thought to do. Therefore, as in Patent Document 5, rolling with a total compression ratio of 25% or more is performed at a finish rolling temperature (Ar3 point +100 or less) lower than the temperature range considered suitable for manufacturing the present invention described later. Need to do. For example, according to the example of Patent Document 6, the finish rolling temperature of a steel sheet having a tensile strength σ B of 780 MPa or more was 800 ° C to 890 ° C. In the steel sheet and the manufacturing method in Patent Document 6, as in Patent Document 5, strain-induced precipitation occurs and the amount of precipitates having a size of 20 nm or more increases, resulting in the structure of the present invention. It is not possible to produce a steel sheet that is essentially a ferrite single phase with a tensile strength of 780 MPa or more and other properties.

In view of such circumstances, an object of the present invention is to provide a high-strength steel sheet having excellent stretch flange characteristics after processing and excellent corrosion resistance after coating.

The present inventors have studied to obtain a high-strength hot-rolled copper sheet having excellent stretch flange characteristics during processing and corrosion resistance after coating and having a tensile strength of 780 MPa or more, and obtained the following knowledge. i) In order to obtain a steel sheet with high strength and excellent corrosion resistance after painting, it is necessary to refine the precipitates (less than 20 mn in size) and increase the proportion of fine precipitates (less than 20 nm in size). In order to keep the precipitate fine, the precipitate contains Ti i Mo or Ti i-V. From the viewpoint of improving the corrosion resistance after coating, the precipitate is Ti. Combined analysis of V is useful.

ii) Solid solution of V is important for improving stretch flangeability after processing. There is a solid solution amount. 'The present invention has been made on the basis of the above findings, and the gist thereof is as follows.

[1] In mass%, C: 0.02% to 0.20%, Si: 0.3% or less, Mn: 0.5% to 2.5%, P: 0.06% or less, S: 0.01% or less, A1: 0.1¾ or less, Ti : 0.05% or more and 0.25% or less, V: 0.05% or more and 0.25% or less, with the remaining component composition consisting of Fe and inevitable impurities, and substantially a ferrite single-phase structure, In the precipitate with a size of less than 20 nm, Ti is 200 mass ppm or more and 1750 mass ppm or less, V force S is 150 mass ppm or more and 1750 mass ppm or less, and solid solution V is 200 mass ppm or more and less than 1750 mass ppm. A high-strength copper plate characterized by having a certain yarn and weave.

[2] In the above [1], any one of mass%, Cr: 0. ^) 1% to 0.5%, W: 0.005% to 0.2%, Zr: 0.0005% to 0.05% 1 A high-strength steel sheet characterized by containing two or more seeds.

[3] The high-strength steel sheet according to [1] or [2], wherein the tensile strength TS is 780 MPa or more.

[4] The high-strength steel sheet according to [1] or [2], wherein the maximum peel width on one side after the tape peel test in the salt warm water immersion test is 3.0 mm or less.

[5] The high-strength steel sheet according to [3], wherein the one-side maximum peel width after the tape peel test in the salt warm water immersion test is 3.0 or less.

[6] A high-strength copper sheet according to [1] or [2], wherein the stretch flange characteristic λ ₁₀ after rolling at an elongation rate of 10% is 60% or more.

[7] In the above [3], the stretch flange characteristics after rolling at an elongation rate of 10%. A high-strength steel sheet characterized by having a content of 60% or more.

In this specification, “%” and “ppm” indicating the components of steel are mass% and mass ppm, respectively. The high-strength steel plate in the present invention is a steel plate having a tensile strength (hereinafter sometimes referred to as TS) of 780 MPa or more, a hot-rolled steel plate, and further, for example, a surface such as a staking treatment. Surface treated steel sheets that have been treated are also targeted.

Furthermore, target properties of the present invention, stretch flangeability ₁₀ after rolling in elongation percentage 10%) Is 60% or more, and the maximum peel width on one side after the tape peel test in the salt warm water immersion test (SDT) described later is 3.0 mm or less. The invention's effect

According to the present invention, a high-strength hot-rolled steel sheet having excellent stretch flange characteristics after processing and corrosion resistance after coating and having a TS of 780 MPa or more can be obtained. Furthermore, in the present invention, the above effect can be obtained without adding Mo, so that the cost can be reduced.

For example, by using the high-strength hot-rolled steel sheet of the present invention for an automobile undercarriage member or a truck frame, the thickness can be reduced, the environmental load of the automobile is reduced, and the impact characteristics are greatly improved. It is expected. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail.

(1) First, the reasons for limiting the chemical component (component composition) of copper in the present invention will be described. C: 0.02% to 0.20%

C is an element that contributes to strengthening of the steel sheet by forming carbides with Ti and V and precipitating in the ferrite. In order to make TS more than 780MPa, C amount needs to be more than 0 · 02¾. On the other hand, if the C content exceeds 0.20%, the stretch flange characteristics after processing deteriorate due to coarsening of precipitates and formation of the second phase structure. Based on the above, the C content is 0.02% to 0.20%, preferably 0.03% to 0.15%.

S i: 0.3% or less

Si is an element that contributes to solid solution strengthening, but if added over 0.3%, cementite is generated at the grain boundaries, and the stretched flange characteristics after processing deteriorate. Therefore, the Si amount is 0.3% or less. Preferably, it is 0.001% or more and 0.2% or less.

Mn: 0.5% or more and 2.5% or less

Mn is an element that contributes to solid solution strengthening. However, if the amount is less than 0.5%, TS of 780 MPa or more cannot be obtained. On the other hand, if the Mn content exceeds 2.5%, weldability is reduced. Reduce significantly. From the above, the Mn content is 0.5 · 2.5% to 2.5%, preferably 0.6¾ to 2.0%.

Ρ: 0.06% or less

Ρ segregates at the prior austenite grain boundaries, leading to low temperature toughness degradation and workability degradation. Therefore, it is preferable to reduce the soot amount as much as possible, and it should be 0.06% or less. Preferably, the content is 0.001% or more and 0.055% or less.

S: 0.01% or less

When S segregates at the prior austenite grain boundaries or precipitates in large amounts as MnS, it lowers the low temperature toughness. In addition, the stretch flangeability is significantly reduced regardless of the presence or absence of processing. Therefore, it is preferable to reduce the amount of S as much as possible, and it should be 0.01% or less. Preferably, the content is 0.0001% or more and 0.005% or less.

A 1: 0.1% or less

A 1 is added as a steel deoxidizer and is an effective element for improving the cleanliness of steel. In order to acquire this effect, it is preferable to make it contain 0.001% or more. However, if it exceeds 0.1 ¾, a large amount of inclusions will be generated, causing copper plate wrinkles. Therefore, the A 1 amount is 0.1¾ or less. Preferably, it is 0.01% or more and 0.04% or less.

T i: 0.05% or more and 0.25 or less

Ti is an extremely important element for precipitation strengthening of fluorite, and is an important requirement for obtaining the effects of the present invention. If the Ti amount is less than 0.05%, it is difficult to secure the required strength. On the other hand, if it exceeds 0.25%, the effect is saturated and only the cost is increased. Therefore, the Ti amount is set to 0.05 to 0.25%, preferably 0.008 to 0.20%.

V: 0.05% or more 0.25% or less

V is an element that contributes to improving the strength as precipitation strengthening or solid solution strengthening, and along with Ti, is an important requirement for obtaining the effects of the present invention. By adding an appropriate amount together with Ti, there is a tendency to precipitate as fine Ti-V carbides with a particle size of less than 20nm (hereinafter also referred to as "size"), and like Mo There is no reduction in corrosion resistance after painting. If the amount of V is less than 0.05%, the effect of addition is poor. On the other hand, V amount exceeds 0.25% Then, the effect is saturated and only the cost is increased. Therefore, the V amount is 0.05¾ or more and 0.25% or less, preferably 0.06% or more and 0.20% or less.

With the above essential additive elements, the steel according to the present invention can achieve the desired properties. In addition to the above essential additive elements, the Cr is further 0.01% or more and 0.5% or less, and W: 0.005% or more for the following reasons. Any one of 0.2% or less, Zr: 0.0005% or more and 0.05% or less may be added.

C r: 0.01% or more and 0.5% or less, W: 0.005% or more and 0.2% or less, Z r: 0.0005% or more and 0.05% or less C, W, and Zr, like V, form precipitates or dissolve It has the function of strengthening the light in the state. If the amount of Cr is less than 0.01%, the amount of W is less than 0.005%, or the amount of Zr is less than 0.0005%, it will hardly contribute to increasing the strength. On the other hand, if the Cr content exceeds 0.5%, the W content exceeds 0.2%, or the Zr content exceeds 0.05%, the workability deteriorates. Therefore, when one or more of Cr, W, Zr is added, the amount added is Cr: 0.01% to 0.5%, W: 0.005% to 0.2%, Zr: 0.0005 % To 0.05%. Preferably, C r is 0.03% or more and 0.3% or less, W is 0.01% or more and 0.18¾ or less, and Z r is 0.001% or more and 0.04% or less.

The remainder other than the above consists of 'Fe and inevitable impurities. As an unavoidable impurity, for example, O forms nonmetallic inclusions and adversely affects quality, so it is desirable to reduce it to 0.003% or less. In the present invention, Cu, Ni, Sn, and Sb may be contained in a range of 0.1% or less as trace elements that do not impair the effects of the present invention.

(2) Next, the structure of the high strength steel sheet of the present invention will be described.

Substantial ferrite single phase structure

In order to improve the stretch flangeability after processing with TS of 780 MPa or more, ferrite with a low dislocation density is effective, and it is effective to have a single-phase structure. In particular, by using a ferrite single-phase structure rich in ductility, the effect of improving stretch flangeability after processing becomes significant. However, it is not always necessary to have a ferrite single-phase structure. If the ferrite single-phase structure is substantially obtained, the above effect can be sufficiently obtained. Here, substantially the ferrite single-phase structure means that a small amount of other phases or precipitates are allowed in addition to the carbide of the present invention. Preferably, the volume ratio of ferrite is 95% or more. In addition, if the volume ratio is in the range of up to 5%, the characteristics of the present invention are not affected even if cementite, pearlite, and vein structures are included.

As for the volume fraction of ferrite, the microstructure of the plate thickness cross section parallel to the rolling direction was revealed in 3¾ night, and the 1/4 position of the plate thickness was observed at 1500 times using a scanning electron microscope (SEM). For example, it is obtained by measuring the ferrite area ratio using “particle analysis II” image processing software manufactured by Sumitomo Metal Technology.

In the ferrite single-phase structure, Ti contained in precipitates with a size of less than 20 nm is 200 ppm or more and 1750 ppm or less, and V is 150 ppm or more and 1750 ppm or less.

In the high-strength copper plate of the present invention, precipitates containing Ti and / or V are mainly precipitated as carbides in the ferrite. This is thought to be because the solid solubility limit of C in ferrite is small, and supersaturated C tends to precipitate as carbonized in the ferrite. Such precipitates harden (strengthen) the soft fulite, and a TS of 780 MPa or more can be obtained. In addition, YS increases and YR (= YS / YR) of 83% or more is obtained.

In order to obtain a high-strength steel sheet, as described above, it is important to refine the precipitates (size less than 20 nm) and increase the proportion of the fine precipitates (size less than 20ηπι). If the size of the precipitate is 20 nm or more, the effect of suppressing the movement of dislocations is small, and the ferrite may not be sufficiently hardened, so the strength may decrease.

Furthermore, as a result of examination, it became clear that the fine precipitate size is important for post-coating corrosion resistance. In conventional Ti-based (Ti single addition) H S L A steel, precipitates tend to become coarser as the amount of Ti increases. For this reason, with such steel sheets, the corrosion resistance after coating tended to decrease as the strength decreased. The reason for the decrease in post-coating corrosion resistance associated with the coarsening of the precipitates is not clear, but it is thought that the coarse precipitates inhibit the formation or growth of chemical crystals.

From the above, the size of the precipitate is preferably less than 20 nm. This fine precipitate of less than 20 nm is achieved by adding both T i and V. V is mainly T i and mixed coal Forms a compound. Although the reason is not clear, it has been found that these precipitates exist stably and finely at a high temperature for a long time within the temperature range of the present invention.

Furthermore, it is important to control the amount of Ti and V contained in precipitates with a size of less than 20 nm. If the amount of Ti contained in precipitates of less than 20 nm is less than 200 ppm and the amount of V is less than 150 ppm, the number density of the precipitates is reduced and the interval between the precipitates is widened, thereby suppressing the movement of dislocations. It turned out that the effect to do becomes small. For this reason, ferrite cannot be hardened sufficiently, and a strength of TS of 780 MPa or more cannot be obtained. In addition, when the amount of Ti contained in the precipitate of less than 20 nm is 200 _P pm or more and the amount of V contained in the precipitate of less than 20 nm is less than 150 ppm, the precipitate tends to be coarsened. A strength of 780MPa or more may not be obtained. In addition, when the amount of Ti contained in the precipitate of less than 20 nm is less than 200 ppm and the amount of V contained in the precipitate of less than 20 nm is 150 ppm or more, the precipitation efficiency of V deteriorates, so TS is 780 MPa. The above strength may not be obtained. On the other hand, if the amount of Ti contained in precipitates of less than 20 nm exceeds 1750 ppm or if the amount of V exceeds 1750 ppm, the corrosion resistance after coating decreases and the target characteristics cannot be obtained. This is probably because a large amount of fine precipitates inhibits the formation or growth of chemical crystals on the surface of the plate. Therefore, both the amount of precipitation Ti and the amount of precipitation V contained in precipitates with a size of less than 20 nm must be satisfied.

Furthermore, when the ratio of Ti and V contained in precipitates with a size of less than 20 nm is 0.4≤ (Ti / 48) / (V / 51) ≤2.5, a TS of 785 MPa or more is obtained, which is a more favorable state. I found out that The reason is not clear, but it is thought that the thermal stability has been improved by optimizing the composition ratio of Ti and V.

Based on the above, the amount of Ti contained in precipitates with a size of less than 20 nm is 200 ppm or more and 1750 ppm or less, and the amount of V is 150 ppm or more and 1750 ppm or less. Furthermore, the ratio of Ti content to V content in precipitates with a size of less than 20 nm is preferably 0.4≤ (Ti / 48) / (V / 51) ≤2.5.

Note that precipitates and Z or inclusions may be collectively referred to as precipitates.

Note that the above Ti amount and V amount can be controlled by the tapping temperature. The cutting temperature at this time is preferably 500 ° C or higher and 700 ° C or lower. When the scraping temperature exceeds 700, The coarsening occurs, and the precipitation amounts of Ti and V contained in the precipitates of less than 20 nm are less than 200 ppm and less than 150 ppm, respectively, and TS of 780 MPa or more cannot be obtained. In addition, even when the cutting temperature is less than 500 ° C, the amounts of Ti and V contained in the precipitates of less than 20 nm are less than 200 ppm and 150 ppm, respectively. This is thought to be due to insufficient diffusion of Ti and V due to the low trapping temperature.

The amount of Ti and V contained in precipitates with a size of less than 20 nm can be confirmed by the following method.

After the sample is electrolyzed in a predetermined amount in the electrolytic solution, the sample piece is taken out of the electrolytic solution and immersed in a solution having dispersibility. Next, the precipitate contained in this solution is filtered using a filter having a pore diameter of 20 nm. Precipitates that have passed through the filter with a pore size of 20 nm together with the filtrate are less than 20 nm in size. Next, the filtrate after filtration is appropriately selected from inductively coupled plasma (ICP) emission spectroscopy, ICP mass spectrometry, atomic absorption spectrometry, etc., and contained in precipitates with a size of less than 20 nm. Obtain the amount of Ti and the amount of V.

Structure with a solid solution V content of 200ppm or more and less than 1750ppm

In the present invention, solute V is the most important requirement. V solid solution is important for improving the stretch flange characteristics after processing. If the solid solution V is less than 200 ppm, the effect is poor. In order to obtain the above effect, the solid solution V amount needs to be 200 ppm or more. On the other hand, when the amount of solute V is 1750ppm or more, the effect is saturated, so the upper limit was set.

Based on the above, the amount of solute V should be 200 _PP m or more and less than 1750 ppm. The steel of the present invention also has a slight decrease in workability as the strength increases. However, when the Ti content in the precipitates with a size of less than 20ηπι is 1750ppm or less and the V content is 1750ppm or less, it is a solid solution. By setting the V amount to 200 ppm or more, the target stretch flange characteristics after processing are sufficiently secured.

Note that less than solid solution V amount is 200ppm or 1750p _P m, for example, it can be confirmed by the following method.

Electrolyze a sample in a non-aqueous solvent electrolyte solution, and then use the electrolyte solution as the analysis solution for elemental analysis. Analytical methods include inductively coupled plasma (ICP) emission spectroscopy, ICP mass spectrometry, or atomic absorption spectrometry. (3) Next, a method for producing a high-strength steel sheet according to the present invention will be described.

The high-strength steel sheet of the present invention is, for example, hot-rolled by heating a steel slab adjusted to the above chemical composition range to 1150 ° C or more and 1350 ° C or less and then setting the finish rolling temperature to 850 or more and 1100 ° C or less. And then rolled up from 500 ° C to 700 ° C. These preferred conditions are described in detail below.

Steel slab heating temperature: 1150 "^ or more and 1350¾ or less

Most carbide-forming elements such as Ti and V exist as precipitates in steel slabs. In order to precipitate as desired in the ferrite structure after hot rolling, it is necessary to dissolve the precipitates that have precipitated as carbides before hot rolling. For this purpose, heating at 1150 ° C or higher is necessary.

If it is less than 1150 ° C, carbides with a size of 20 nm or more that do not contribute to precipitation strengthening and post-coating corrosion resistance remain, so that it is necessary to produce fine precipitates with a size of less than 20 nm necessary for obtaining the effects of the present invention. The amount of Ti and V involved is reduced, and the amount of precipitates with a size of less than 20 nm cannot be obtained as planned when cutting is described later. Furthermore, in the steel sheet manufacturing method of the present invention, carbides containing Ti and V remain dissolved during slab heating and finish rolling, and are precipitated as fine carbides containing Ti and V at the time of cutting after finish rolling. Is the most desirable form. Therefore, the heating temperature is more preferably 1170 or higher as the temperature at which the carbide is almost completely dissolved.

On the other hand, if it is heated above 1350 ° C, the crystal grain size becomes too coarse and the elongation flange characteristics and elongation characteristics after processing deteriorate. Further, considering such heat treatment conditions, if the temperature is 1300 ° C. or less, the coarsening of the crystal grain size can be almost completely prevented.

Therefore, the slab heating temperature is preferably 1150 ° C or higher and 1350 ° C or lower. More preferably, it is 1170 ° C or higher and 1300 ° C or lower.

Finishing rolling temperature in hot rolling: 850 ^ or more 1100T: or less

In order to obtain the Ti amount and the V amount contained in the precipitates having a size of less than 20 nm in the present invention, it is important to control the finish rolling temperature. The processed steel slab is preferably hot-rolled at a finish rolling temperature of 850 ° C to 1100 ° C, which is the end temperature of hot rolling. Finish rolling temperature If it is less than 850 ^, it will be rolled in the ferrite + austenite region, resulting in an expanded ferrite structure, which may degrade the stretch flange characteristics and elongation characteristics after processing. In addition, even if the precipitate precipitated as carbide before rolling at a steel slab heating temperature of 1150 ° C or higher is dissolved, if the final rolling temperature is less than 850 ¾, strain-induced precipitation may occur. , Carbides containing Ti and V are precipitated. Therefore, the amount of Ti and the amount of V related to the formation of fine precipitates with a size of less than 20 nm necessary for the effect of the present invention are reduced, and the amount of precipitates with a size of less than 20 nm at the time of cutting described later. Cannot be achieved as intended. In other words, it is important that the carbides containing Ti and V, which are dissolved during the slab heating described above, proceed to the next cutting process while still being dissolved in this finish rolling. Therefore, in order to keep the carbide dissolved, the finish rolling temperature is more preferably 935 or more. On the other hand, when the finish rolling temperature exceeds 1100, the ferrite grains become coarse, and a TS of 780 MPa may not be obtained. In order to prevent coarsening of the ferrite grains, the temperature is more preferably 990 ° C or lower.

Therefore, the finish rolling temperature is preferably 850 or more and 1100 or less. More preferably, it is 990 or more and the following.

Sampling temperature: 500 to 700

In order to obtain the amount of Ti and the amount of V contained in the precipitate having a size of less than 20 nm in the present invention, it is important to control the cutting temperature. As described above, in this cutting process, a large number of precipitation sites are formed, carbides are precipitated from this precipitation site, and the grain growth of the carbides is suppressed so as not to exceed 20 nm in size. This is because it is a desirable form of production. In order to obtain a structure of a ferrite single phase structure substantially and obtain the characteristics of the present invention, the cutting temperature is preferably 500 or more and 700 or less.

In the present invention, if the cutting temperature is less than 500, the amount of carbides containing Ti and / or V becomes insufficient, which may lead to a decrease in strength. In addition, a bainite phase may be generated, and a ferrite single phase structure may not be obtained.

In order to form a large number of precipitate sizes and to generate carbides from the parenthesis precipitation sites, the temperature is preferably higher, and more preferably 550 or more. On the other hand, if the cutting temperature exceeds 700 ° C, the precipitated carbides become coarse, which may lead to a decrease in strength. In addition, a pearlite phase is likely to be generated, and the stretch flangeability after processing may be reduced. When the temperature is 650 ° C. or lower, it is more preferable because the coarsely precipitated carbide can be prevented from coarsening.

Accordingly, the scraping temperature is preferably 500 ° C. or more and 700 or less, and more preferably 550 to 650 ° C. or less.

The steel sheet of the present invention includes those having a surface subjected to surface treatment or surface coating treatment. In particular, the steel sheet of the present invention can be suitably applied to a steel sheet having a hot-dip zinc-based steel sheet formed thereon. That is, since the copper plate of the present invention has good workability, good workability can be maintained even when a molten zinc-based plating film is formed. Here, the molten zinc-based plating is a molten solder mainly composed of zinc and zinc (that is, containing about 90% or more). In addition to zinc, alloy elements such as A 1 Cr are used. In addition, the alloy may be subjected to an alloying treatment after plating, even with the hot-dip zinc plating.

The method for melting steel is not particularly limited, and all known melting methods can be applied. For example, as a melting method, a method of melting in a converter, electric furnace or the like and performing secondary scouring in a vacuum degassing furnace is suitable. The forging method is preferably continuous forging from the viewpoint of productivity and quality. Further, the effect of the present invention is not affected even if the direct feed rolling in which the hot rolling is performed as it is immediately after the forging or after the heating for the purpose of supplementary heat is performed. Furthermore, the hot rolled material may be heated after rough rolling and before finish rolling, or even if continuous hot rolling is performed by joining the rolled material after rough rolling, and further, the heating material of the rolled material may be heated. Even if continuous rolling is performed simultaneously, the effect of the present invention is not impaired. Example

Example 1

Steels with the composition shown in Table 1 were melted in a converter and made into steel slabs by continuous forging. Next, these steel slabs were heated, hot-rolled and scraped under the conditions shown in Table 2 to produce hot rolled copper sheets having a thickness of 2.0. Ingredient composition (mass%)

Steel grade Remarks

C Si Mn P S Al Ti V

A 0.040 0.01 1.45 0.01 0.0015 0.03 0.105 0.120 Compatible steel

B 0.120 0.02 1.20 0.02 0.0008 0.03 0.240 0.100 Compatible steel

C 0.100 0.02 1.20 0.01 0.0080 0.03 0.1 10 0.245 Compatible steel

D 0.150 0.02 1.40 0.03 0.0020 0.03 0.230 0.224 Compatible steel

E 0.050 0.01 2.02 0.01 0.0020 0.03 0.120 0.120 Compatible steel

F 0.050 0.01 0.65 0.01 0.0015 0.03 0.1 10 0.136 Applicable steel

G 0.045 0.02 1.34 0.02 0.0007 0.02 0.060 0.1 10 Compatible steel

H 0.050 0.02 1.30 0.01 0.0008 0.02 0.1 10 0.052 Compatible steel

I 0.030 0.01 1.32 0.01 0.0007 0.02 0.080 0.070 Compatible steel

J 0.040 0.01 1.40 0.02 0.0015 0.03 0.126 0.152 Applicable steel

K 0.250 0.01 1.20 0.02 0.0020 0.03 0.120 0.130 Not compliant 0.001 0.01 1.19 0.02 0.0020 0.03 0.120 0.130 Not compliant

M 0.080 0.50 1.30 0.01 0.0012 0.03 0.070 0.070 Nonconforming

N 0.050 0.01 0.35 0.02 0.0015 0.03 0.080 0.080 Nonconforming

0 0.050 0.01 3.00 0.02 0.0014 0.03 0.080 0.080 Nonconforming

P 0.150 0.01 1.60 0.02 0.0015 0.03 0.040 0.120 Nonconforming

Q 0.160 0.01 1.60 0.02 0.0016 0.02 0.070 0.032 Nonconforming

R 0.152 0.01 1.62 0.02 0.0015 0.03 0.280 0.120 Nonconforming

S 0.161 0.01 1.61 0.02 0.0014 0.03 0.150 0.300 Nonconformity

X 0.090 0.06 1.35 0.04 0.0014 0.05 0.150 0.160 Suitable steel

The microstructure of the obtained hot-rolled steel sheet was analyzed by the method described below, and the amount of Ti and V in the precipitates of less than 20 nm were determined. Also, tensile strength: T S, stretch flange characteristics after processing: Corrosion resistance after painting: S D T One side maximum peel width was determined and evaluated. ,

Analysis of Miku mouth tissue

The hot-rolled steel sheet obtained above was cut to an appropriate size, and about 0.2 g of current was applied in 10% AA electrolyte (10 vol% acetylacetone-lmass% tetramethylammonium chloride-methanol). Constant current electrolysis was performed at a density of 20 mA / cm ² .

Measurement of the amount of Ti and V contained in precipitates with a size of less than 20 nm

After electrolysis, remove the sample piece with deposits on the surface from the electrolyte and immerse it in an aqueous solution of sodium hexametaphosphate (500 mg / l) (hereinafter referred to as SHMP aqueous solution). By applying vibration, the precipitate was peeled from the sample piece and extracted into an aqueous SHMP solution. Next, the SHMP aqueous solution containing the precipitate is filtered using a filter with a pore size of 20ηπι, and the filtrate after filtration is analyzed using an ICP emission spectroscopic analyzer. The absolute amounts of T i and V in the filtrate are analyzed. Was measured. Next, the absolute amounts of T i and V were divided by the electrolysis weight to obtain the amounts of Ti and V contained in precipitates having a size of less than 20 nm. The electrolytic weight was determined by measuring the weight of the sample after the deposit was peeled off and subtracting it from the sample weight before the electrolysis.

Measurement of the amount of solid solution V ·

The electrolytic solution after electrolysis was used as an analysis solution, and the concentration of Fe in the solution as a reference element and V was measured using ICP mass spectrometry. Based on the obtained concentration, the concentration ratio of V to Fe was calculated, and the amount of V in the solid solution state was obtained by multiplying the content of Fe in the sample. The content of Fe in the sample can be determined by subtracting the total composition value other than Fe from 100%.

T S

A tensile test was carried out by a method based on JIS Z 2241 using a JIS5 test piece with the rolling direction as the tensile direction, and TS was obtained.

Stretch flange characteristics after processing:

After rolling at an extension ratio 10%, subjected to hole expansion test in accordance with Tetsuren standards JFST 1001, it was determined lambda _10.

Corrosion resistance after painting: SD Chemical conversion treatment was performed using Nippon Paint Co., Ltd. degreasing agent: Surf Cleaner ECO 90, surface conditioner; Surf Fine 5 N—10, chemical conversion treatment agent: Surfdyne SD 2800 The temperature and concentration conditions were worse than the standard conditions. As an example of standard conditions, the degreasing process has a concentration of 16 g / l, a processing temperature of 42 to 44, a processing time of 120 s, spray degreasing, and the surface conditioning process has a total alkalinity of 1.5 to 2.5 points, free acidity 0.7 ~ 0.9 points, accelerator concentration 2.8 ~ 3.5 points, treatment temperature 44 "ϊ :, treatment time 120s. As a bad condition, the treatment temperature in the chemical conversion treatment process is 38 ° C After that, electrodeposition coating was performed using Nippon Paint's electrodeposition coating agent V-50, and the amount of chemical conversion coating was 2 to ^2.5 g / m ² . The coating was aimed at a film thickness of 25 / m.

The corrosion resistance after painting was evaluated by a salt warm water immersion test (S D T). For samples subjected to chemical conversion treatment and electrodeposition coating, apply a cross-cut kite with a force cutter and immerse in salt warm water (5% NaCl: 55 ° C) for 10 days, then rinse with water and dry. The tape was peeled off, and the maximum peel width on the left and right sides of the cut collar was measured. If the maximum peel width on one side is 3.0 mm or less, the corrosion resistance after painting is good.

The results obtained above are shown in Table 2 together with the manufacturing conditions.

Table 2

From Table 2, TS is 780 MPa or more in the present invention example. Is 60% or more, and the maximum peel width on one side of the SDT is 3.0 mm or less, which indicates that it is a hot-rolled copper sheet with excellent stretch flange characteristics after processing and excellent corrosion resistance after painting.

On the other hand, the comparative example is inferior in one or more of TS (strength), λ ₁₀ (stretch flange after processing), and SDT one-side maximum peel width (corrosion resistance after painting).

Example 2

Steel with the composition shown in Table 3 was melted in a converter and steel slabs were formed by continuous forging. Next, these steel slabs were heated, hot-rolled and wound under the conditions shown in Table 4 to produce hot-rolled steel sheets having a thickness of 2.0 mm. Table 3

The obtained hot-rolled copper sheet was analyzed for microtexture in the same manner as in Example 1, and the amounts of Ti and V contained in precipitates of less than 20 nm and the amount of solute V were determined. Also, tensile strength: TS, stretch flange characteristics after processing: Corrosion resistance after painting: The maximum peel width on one side of SDT was determined and evaluated.

Table 4 shows the results obtained as described above.

Table 4

Slab Finish Winding TS After pre-deformation Elongation after processing 20nm soon 20nm solid solution V amount Maximum on one side

No Steel type Heating temperature Rolling temperature / Elongation of dishness Flange characteristics: λ ₁₀ Precipitation Ti amount Precipitation V amount Peel width

(.C) (° C) (° C) (MPa) (%) (%) (mass ppm; (mass ppm; mass ppm) (mm)

25 T 1 250 921 625 832 1 7 99 750 815 250 2.5

26 U 1250 918 620 830 1 8 90 753 760 252 2.2

27 V 1250 920 "621 829 1 7 93 753 770 250 2.0

28 W 1250 921 620 842 1 8 98 760 823 251 2.6

35 T 1250 940 600 835 18 92 780 820 240 2.2

36 T 1270 960 630 840 17 93 782 823 244 2.1

37 1300 980 620 837 18 95 788 830 245 2.3

From Table 4, in the present invention, TS is 780 MPa or more; Is 60% or more, and the maximum peel width on one side of the SDT is 3.0 or less. It can be seen that this is a hot-rolled steel sheet with excellent stretch flange characteristics after processing and corrosion resistance after painting.

Furthermore, compared to steel plate No. l (Table 2), it can be seen that steel plate No25 28 35 37 to which CrW or Zr is added has improved TS. , Industrial applicability

The steel sheet of the present invention has high strength, and has excellent stretch flange characteristics after processing and corrosion resistance after painting. For example, parts that require stretch stretch flange characteristics such as automobile and truck frames. As best.

Claims

The scope of the claims

1. massy. C: 0.02% or more, 0.20% or less, Si: 0.3% or less, Mn: 0.5% or more, 2.5% or less, P: 0.06% or less, S: 0.013 or less A1: 0.1% or less, Ti: 0.05% or more and 0.25% or less, V: 0.05% or more and 0.25% or less, with the balance consisting of Fe and inevitable impurities ,

It is essentially a ferrite single-phase structure. In the pre-fc ferrite single-phase structure, Ti contained in precipitates with a size of less than 20mn is 200 mass ppm to 1750 mass ppm, V force S 150 mass p pm to 1750 mass A high-strength steel sheet characterized by having a structure of not more than ppm and having a solid solution V content of 200 mass ppm or more and less than 1750 mass ppm.

2. In mass%, Cr: 0.01% or more and 0.5% or less, W: 0.005% or more, 0.2% or less, Zr: any of 0.005% or more, 0.05% or less 1 The high-strength steel sheet according to claim 1, comprising seeds or two or more kinds.

3. The high-strength steel sheet according to claim 1 or 2, wherein the tensile strength T S is 780 MPa or more.

4. The high-strength steel sheet according to claim 1 or 2, wherein the maximum peel width on one side after the tape peel test in the salt warm water immersion test is 3.0 mm or less.

5. The high-strength steel sheet according to claim 3, wherein the one-side maximum peel width after the tape peel test in the salt warm water immersion test is 3.0 mm or less.

6. Steel plate according to claim 1 or 2, stretch flangeability tut _ten less than 60% ^ characterized after rolling with stretching rate of 10%.

7. Stretch flange characteristics after rolling at 10% elongation. 4. The high-strength steel plate according to claim 3, wherein is 60% or more.