Steel sheet, member, and method for producing same

Abstract

Provided are: a steel plate which has a TS of 1180 MPa or more, and which has high YS, excellent press formability (ductility, hole expandability, and bendability), and fracture resistance characteristics during crushing (bending fracture characteristics and axial crushing characteristics); a member; and methods for producing the steel plate and the member. The base steel sheet has a prescribed component composition, and in the steel structure of the base steel sheet, bainitic ferrite, tempered martensite, retained austenite, and fresh martensite are set to prescribed ranges, the carbon concentration in the retained austenite and the carbide density in the tempered martensite are set to prescribed ranges, and the diffusible hydrogen content is 0.50 ppm by mass or less. A V-VDA bending test is performed to the maximum load point, the length of the crack in the L cross section is set to 400 [mu] m or less, and the amount of change in the grain diameter of the bainite in the plate thickness direction before and after processing is set to 5.0 or less in a predetermined region of the VDA bending.

Description

Technical Field

[0001] The present invention relates to a steel plate, a component using the steel plate as a raw material, and a method for producing the same. Background Art

[0002] In recent years, improving the fuel efficiency of automobiles has become an important issue from the perspective of protecting the global environment. Therefore, the trend of reducing the weight of automobile bodies by increasing the strength and reducing the thickness of steel sheets, which are raw materials for automobile parts, has become increasingly active.

[0003] In addition, social demands for improving the collision safety of automobiles are further increasing. Therefore, it is desired to develop a steel sheet that has not only high strength but also excellent impact resistance (hereinafter referred to as impact resistance) when a collision occurs while the automobile is running.

[0004] Regarding such steel sheets as raw materials for automobile components, for example, Patent Document 1 discloses a high-strength steel sheet having excellent stretch flange formability and collision resistance, characterized in that it contains, in mass %, 0.04 to 0.22% C, 1.0% or less Si, 3.0% or less Mn, 0.05% or less P, 0.01% or less S, 0.01 to 0.1% Al and 0.001 to 0.005% N, with the remainder being Fe and inevitable impurities, and is composed of a ferrite phase as a main phase and a martensite phase as a secondary phase, and the maximum grain size of the martensite phase is 2 μm or less, and its area ratio is 5% or more.

[0005] In addition, Patent Document 2 discloses a high-strength hot-dip galvanized steel sheet with excellent coating adhesion and formability, wherein the cold-rolled steel sheet is pre-plated with 0.2 g / m 2 Above and 2.0g / m 2A hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of a cold-rolled steel sheet obtained by reducing Ni to less than 7%, characterized in that the steel sheet has a hot-dip galvanized layer on the surface containing less than 7% Fe, and the balance consisting of Zn, Al and inevitable impurities, wherein the tensile strength is TS (MPa), the total elongation is EL (%), and the hole expansion ratio is λ (%), TS×EL is 18000 MPa·% or more, TS×λ is 35000 MPa·% or more, and the steel sheet has a tensile strength of 980 MPa or more, and the steel sheet contains, by mass%, C: 0.05% or more and 0.4% or less, Si: 0.01% or more and 3.0% or less, Mn: 0.1% or more and 3.0% or less, P: 0.04% or less, S: 0.05% or less, N: 0.01% or less, Al: 0.01% or more and 2.0% or less, Si+Al>0.5%, the balance being Fe and inevitable impurities, the microstructure containing 40% or more ferrite as a main phase by volume fraction, 8% or more retained austenite, two or more of martensite [3] among the three martensites [1][2][3] specified below, 1% or more bainite and 0-10% pearlite, wherein the three martensites [1][2][3] are respectively represented by volume fraction as martensite [1]: 0% or more and 50% or less, martensite [2]: 0% or more and less than 20%, martensite [3]: 1% or more and less than 30% (martensite [1]: C concentration (CM1) is less than 0.8%, and hardness Hv1 is Hv1 / (-982.1×CM1 2 +1676×CM1+189)≤0.60, martensite[2]: C concentration (CM2) is above 0.8%, hardness Hv2 is Hv2 / (-982.1×CM2 2 +1676×CM2+189)≤0.60, martensite[3]: C concentration (CM3) is above 0.8%, hardness Hv3 is Hv3 / (-982.1×CM3 2 +1676×CM3+189)≥0.80.

[0006] In addition, Patent Document 3 discloses a high-strength hot-dip galvanized steel sheet having, by mass%, C: 0.15% to 0.25%, Si: 0.50% to 2.5%, Mn: 2.3% to 4.0%, P: 0.100% to 0.100%, S: 0.02% to 0.02%, Al: 0.01% to 2.5%, the remainder being Fe and unavoidable impurities, and having tempered martensite by area ratio. phase: 30% or more and 73% or less, ferrite phase: 25% or more and 68% or less, retained austenite phase: 2% or more and 20% or less, other phases: 10% or less (including 0%), and having a martensite phase: 3% or less (including 0%), a bainitic ferrite phase: less than 5% (including 0%) as the other phases, the average grain size of the tempered martensite phase is 8 μm or less, and the C amount in the retained austenite phase is less than 0.7 mass % of the steel sheet structure.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent No. 3887235

[0010] Patent Document 2: Japanese Patent No. 5953693

[0011] Patent Document 3: Japanese Patent No. 6052472 Summary of the invention

[0012] Problems to be solved by the invention

[0013] However, currently, the impact energy absorbing members of automobiles, represented by the front side members and the rear side members, are still using steel plates with a tensile strength (hereinafter also referred to as TS) of 590 MPa class.

[0014] That is, in order to increase the absorbed energy during impact (hereinafter also referred to as impact absorbed energy), it is effective to increase the yield stress (hereinafter also referred to as YS). However, when the TS and YS of the steel plate are increased, the stamping formability, especially the ductility, hole expandability, and bendability, are generally reduced. Therefore, when it is assumed that such a steel plate with increased TS and YS is applied to the impact energy absorbing component of the above-mentioned automobile, not only will the stamping become difficult, but the component will also break in the axial crushing test of the simulated collision test. In other words, the actual impact absorbed energy will not be as high as expected based on the value of YS. Therefore, the current situation is that the above-mentioned impact energy absorbing component remains at the level of steel plates with a TS of 590MPa.

[0015] In fact, the steel plates disclosed in Patent Documents 1 to 3 cannot be said to have a TS of 1180 MPa or more and have high YS, excellent stamping formability (ductility, hole expandability and bendability) and fracture resistance during crushing (bending fracture characteristics and axial crushing characteristics).

[0016] The present invention has been developed in view of the above-mentioned situation, and its purpose is to provide a steel plate having a tensile strength TS of more than 1180 MPa, a high yield stress YS, excellent stamping formability (ductility, hole expandability and bendability) and fracture resistance during crushing (bending fracture characteristics and axial crushing characteristics), and a favorable manufacturing method thereof.

[0017] Another object of the present invention is to provide a member using the above-mentioned steel plate as a raw material and a method for producing the same.

[0018] The steel sheet mentioned here also includes galvanized steel sheet, and the galvanized steel sheet refers to hot-dip galvanized steel sheet (hereinafter also referred to as GI) or alloyed hot-dip galvanized steel sheet (hereinafter also referred to as GA).

[0019] Here, the tensile strength TS can be measured by a tensile test in accordance with JIS Z 2241 (2011).

[0020] In addition, having high yield stress YS, excellent stamping formability (ductility, hole expandability, and bendability), and fracture resistance during crushing (bending fracture characteristics and axial crushing characteristics) means satisfying the following conditions.

[0021] The high yield stress YS means that YS measured in a tensile test in accordance with JIS Z 2241 (2011) satisfies the following formula (A) or (B) based on TS measured in the tensile test.

[0022] (A) When 1180MPa≤TS<1320MPa, 750MPa≤YS

[0023] (B) When 1320MPa≤TS, 850MPa≤YS

[0024] In addition, excellent ductility means that the total elongation (E1) measured in a tensile test in accordance with JIS Z 2241 (2011) satisfies the following formula (A) or (B) based on TS measured by the tensile test.

[0025] (A) When 1180MPa≤TS<1320MPa, 12.0%≤El

[0026] (B) When 1320MPa≤TS, 10.0%≤El

[0027] In addition, excellent hole expandability means that the limiting hole expansion ratio (λ) measured in the hole expansion test in accordance with JIS Z 2256 (2020) is 30% or more.

[0028] Furthermore, excellent bendability means that the bending angle (α) at ​​the maximum load measured in a bending test in accordance with the VDA standard (VDA238-100) defined by the German Association of the Automotive Industry is 80° or more.

[0029] In addition, excellent bending fracture characteristics refer to the stroke (S) at the maximum load measured in the V-VDA bending test. Fmax ) is 26.0 mm or more.

[0030] In addition, excellent axial crushing characteristics refer to the fracture (appearance crack) after the axial crushing test. Figure 6-1 (b) The lower part has two curved ridges (see Figure 6-1 In the middle area Cx), there are no more than 3 locations in the area of ​​R=5.0mm and 200mm.

[0031] The above El, λ and α are characteristics indicating the ease of forming of the steel sheet during press forming. On the other hand, the V-VDA bending test is a test that simulates the deformation and fracture behavior of the bending ridge portion in a collision test. The stroke (S) at the maximum load measured in the V-VDA bending test is Fmax ) is a characteristic that indicates the ease with which a component can break.

[0032] Methods used to solve problems

[0033] In order to achieve the above object, the present inventors have conducted intensive research.

[0034] As a result, it was found that: the component composition of the base steel plate of the steel plate is appropriately adjusted, and the steel structure of the base steel plate of the steel plate has an area ratio of bainitic ferrite of 3.0% to 20.0%, an area ratio of tempered martensite (excluding retained austenite) of 40.0% to 90.0%, an area ratio of retained austenite of more than 3.0% to 15.0%, a carbon concentration in the retained austenite of 0.60 mass% to 1.30 mass%, an area ratio of fresh martensite of 10.0% or less (including 0.0%), and a density of carbides in the tempered martensite of 8.0 pieces / μm 2The following steel structure, the diffusible hydrogen amount of the base steel plate is 0.50 mass ppm or less, and, in a V-VDA bending test to the highest load point, in the L cross section in the overlapping area of ​​the V-bend ridge portion and the VDA bend ridge portion, the length of the crack is 400 μm or less, and, in an area formed from each position of a starting line starting from a bending vertex on the outer side of the VDA bend and existing to a position of 50 μm in the plate thickness direction to a position of 50 μm on both sides of the starting line in a perpendicular direction, with respect to the average grain size of bainitic ferrite in the plate thickness direction, the ratio of the average grain size before processing to the average grain size after processing (amount of change before and after processing: average grain size before processing (nm) / average grain size after processing (nm)) is set to 5.0 or less, thereby, a steel plate having a tensile strength TS of 1180 MPa or more, high YS, excellent stamping formability (ductility, hole expandability and bendability) and fracture resistance characteristics during crushing (bending fracture characteristics and axial crushing characteristics) can be obtained.

[0035] The present invention has been completed through further studies based on the above findings.

[0036] That is, the gist of the present invention is as follows.

[0037] [1] A steel plate having a base steel plate, wherein:

[0038] The base steel plate has a composition containing, in mass%, C: 0.050% or more and 0.400% or less, Si: greater than 0.75% and 3.00% or less, Mn: 2.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.010% or more and 2.000% or less, and N: 0.0100% or less, with the balance being Fe and inevitable impurities.

[0039] The base steel sheet has an area ratio of bainitic ferrite of 3.0% to 20.0%, an area ratio of tempered martensite of 40.0% to 90.0%, an area ratio of retained austenite of more than 3.0% to 15.0%, a carbon concentration in the retained austenite of 0.60 mass% to 1.30 mass%, an area ratio of fresh martensite of 10.0% or less, and a density of carbides in the tempered martensite of 8.0 pieces / μm 2 The following steel structures,

[0040] The diffusible hydrogen content of the base steel plate is 0.50 mass ppm or less,

[0041] Furthermore, V-VDA bending test was performed to the highest load point.

[0042] In the L section,

[0043] The length of the crack is less than 400 μm.

[0044] Furthermore, in the region formed from each position of the starting line which starts from the bending vertex on the outer side of the VDA bend and exists at a position of 50 μm in the plate thickness direction to each position of 50 μm on both sides in the perpendicular direction of the starting line,

[0045] Regarding the average grain size of bainitic ferrite in the plate thickness direction, the ratio of the average grain size before working to the average grain size after working is 5.0 or less.

[0046] The tensile strength of the steel plate is greater than 1180 MPa.

[0047] [2] The steel sheet according to [1] above, wherein the base steel sheet further comprises, in terms of mass%, Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, At least one of: less than 0.0200% Zn: less than 0.0200% Co: less than 0.0200% Zr: less than 0.1000% Ca: less than 0.0200% Se: less than 0.0200% Te: less than 0.0200% Ge: less than 0.0200% As: less than 0.0500% Sr: less than 0.0200% Cs: less than 0.0200% Hf: less than 0.0200% Pb: less than 0.0200% Bi: less than 0.0200% and REM: less than 0.0200%.

[0048] [3] The steel sheet according to [1] or [2] above, wherein a zinc plating layer is provided as an outermost layer on one or both surfaces of the steel sheet.

[0049] [4] The steel plate according to any one of [1] to [3] above, wherein

[0050] When the base steel plate has a surface layer of a region of 200 μm or less in the plate thickness direction from the surface of the base steel plate,

[0051] The surface layer has a surface soft layer having a Vickers hardness of 85% or less relative to the Vickers hardness at a position of 1 / 4 of the plate thickness,

[0052] When the nanohardness is measured at 300 points or more in a 50 μm×50 μm area of ​​the plate surface at each of the 1 / 4 position and the 1 / 2 position of the depth in the plate thickness direction of the surface soft layer from the surface of the base steel plate,

[0053] The ratio of the number of measurements of the plate surface having a nanohardness of 7.0 GPa or more at a position 1 / 4 of the depth in the plate thickness direction of the surface soft layer from the surface of the base steel plate is 0.10 or less relative to the total number of measurements at a position 1 / 4 of the depth in the plate thickness direction of the surface soft layer,

[0054] Furthermore, the standard deviation σ of the nanohardness of the plate surface at a position 1 / 4 of the depth in the plate thickness direction of the surface soft layer from the surface of the base steel plate is 1.8 GPa or less,

[0055] Furthermore, a standard deviation σ of the nanohardness of the plate surface at a position 1 / 2 of the depth in the plate thickness direction of the surface soft layer from the surface of the base steel plate is 2.2 GPa or less.

[0056] [5] The steel sheet according to any one of [1] to [4] above, wherein the steel sheet has a metal plating layer formed on the base steel sheet on one or both surfaces of the steel sheet.

[0057] [6] A member formed using the steel plate according to any one of [1] to [5] above.

[0058] [7] A method for manufacturing a steel plate, comprising:

[0059] A hot rolling step, wherein a steel slab having the component composition described in [1] or [2] is hot rolled to produce a hot-rolled steel sheet;

[0060] A pickling step, wherein the hot-rolled steel plate is pickled;

[0061] Annealing step, wherein the steel sheet after the pickling step is annealed at a temperature of: (Ac 1 +0.4×(Ac 3 -Ac 1 Annealing is performed under the conditions of ))°C or higher and 900°C or lower and annealing time: 20 seconds or longer;

[0062] A first cooling step, wherein the steel sheet after the annealing step is cooled to a first cooling stop temperature of 400° C. to 600° C.;

[0063] A second cooling step, wherein the steel sheet after the first cooling step is cooled to a second cooling stop temperature of 100°C to 300°C at an average cooling rate of 25.0°C / s or less, and during the cooling, a 2.0 kgf / mm2 pressure is applied to the steel sheet at least once in a temperature range of 300°C to 450°C. 2 and then applying a tension of more than 4 passes while bringing the steel plate into contact with a roll having a diameter of 500 mm or more and 1500 mm or less for 1 / 4 of the roll's revolution in each pass, and applying a treatment of more than 2 passes while bringing the steel plate into contact with a roll having a diameter of 500 mm or more and 1500 mm or less for 1 / 2 of the roll's revolution in each pass; and

[0064] A reheating process, wherein the steel plate after the second cooling process is heated to a tempering temperature in a temperature range higher than 300° C. and lower than 500° C. and maintained in the temperature range for a tempering time of 20 seconds to 900 seconds, and during the reheating process, a carbide control parameter CP represented by the following formula (1) is set to 10000 to 15000,

[0065] Alternatively, the method further comprises a cold rolling step of cold rolling the steel sheet after the pickling step and before the annealing step to obtain a cold-rolled steel sheet.

[0066] CP = (T + 273) × (k + 1.2 × logt) ... Formula (1)

[0067] Here, T is the tempering temperature (°C), k is a material constant that depends on the C content, and t is the tempering time (seconds).

[0068] k=-6×C M +17.8,

[0069] C M is the carbon content (mass %) in the martensite generated in the second cooling step.

[0070] [8] The method for manufacturing a steel sheet according to [7] above, further comprising a galvanizing step of galvanizing the steel sheet after the first cooling step and before the second cooling step to form a zinc plating layer on the steel sheet.

[0071] [9] The method for producing a steel sheet according to [7] or [8] above, wherein the annealing in the annealing step is performed in an atmosphere having a dew point of -30°C or higher.

[0072]

[10] The method for manufacturing a steel sheet according to any one of [7] to [9] above, comprising a metal plating step of plating metal on one or both surfaces of the steel sheet to form a metal plating layer after the pickling step and before the annealing step.

[0073]

[11] A method for manufacturing a component, comprising the step of subjecting the steel plate according to any one of [1] to [5] above to at least one of forming and joining to produce the component.

[0074] Effects of the Invention

[0075] According to the present invention, a steel plate can be obtained which has a tensile strength TS of 1180 MPa or more, a high yield stress YS, excellent stamping formability (ductility, hole expandability and bendability) and fracture resistance during crushing (bending fracture characteristics and axial crushing characteristics).

[0076] Furthermore, since a member made of the steel sheet of the present invention has high strength and excellent impact resistance, it can be used extremely advantageously as an impact energy absorbing member for automobiles and the like. BRIEF DESCRIPTION OF THE DRAWINGS

[0077] Figure 1 This is a tissue image obtained by SEM for explaining the identification of the tissue.

[0078] Figure 2-1 (a) is a diagram for explaining a V-bending process (primary bending process) in a V-VDA bending test of an example. Figure 2-1 (b) is a diagram for explaining VDA bending (secondary bending process) in the V-VDA bending test of the example.

[0079] Figure 2-2 (c) is a perspective view showing a test piece after V-bending (primary bending) in V-VDA. Figure 2-2 (d) is a perspective view of the test piece after VDA bending (secondary bending process) in V-VDA was performed.

[0080] Figure 2-3 (e) is a perspective view showing a test piece and an L-cross-sectional observation surface after VDA bending (secondary bending processing) in V-VDA is performed. Figure 2-3 (f) is a cross-sectional view showing a measurement position of the change in grain size of bainitic ferrite in the plate thickness direction before and after working in the L-cross-sectional observation plane of a test piece after VDA bending (secondary bending working) in V-VDA.

[0081] Figure 2-4 It is a schematic diagram for explaining the AB region.

[0082] Figure 3It is a schematic diagram of the stroke-load curve obtained when implementing the V-VDA test.

[0083] Figure 4 This is a structural image obtained by SEM for explaining the measurement of the length of a crack specified in the present invention (Example No. 36 of the present invention in the Examples).

[0084] Figure 5 (a) is a structural image obtained by SEM for explaining a method for measuring the grain size of bainitic ferrite before deformation by processing specified in the present invention (Inventive Example No. 35 in the Examples). Figure 5 (b) is a structural image obtained by SEM for explaining a method for measuring the grain size of bainitic ferrite after deformation caused by processing specified in the present invention (Inventive Example No. 35 in the Examples).

[0085] Figure 6-1 (a) is a front view of a test member produced for the shaft crushing test of the example, in which a hat-shaped member and a steel plate are spot-welded. Figure 6-1 (b) Yes Figure 6-1 (a) is a perspective view of the test component shown.

[0086] Figure 6-2 (c) is a schematic diagram for explaining the shaft crushing test of the example. DETAILED DESCRIPTION

[0087] The present invention will be described based on the following embodiments.

[0088] [1.Steel plate]

[0089] The steel sheet of the present invention is a steel sheet having a base steel sheet, wherein the base steel sheet contains, by mass%, C: 0.050% or more and 0.400% or less, Si: greater than 0.75% and 3.00% or less, Mn: 2.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.010% or more and 2.000% or less, and N: 0.0100% or less, and the balance is F. e and inevitable impurities, the base steel plate has an area ratio of bainitic ferrite of 3.0% or more and 20.0% or less, an area ratio of tempered martensite of 40.0% or more and 90.0% or less, an area ratio of retained austenite of more than 3.0% and 15.0% or less, a carbon concentration in the retained austenite of 0.60 mass % or more and 1.30 mass % or less, an area ratio of fresh martensite of 10.0% or less, and a density of carbides in the tempered martensite of 8.0 pieces / μm 2The following steel structure, the amount of diffusible hydrogen in the base steel plate is 0.50 mass ppm or less, and in a V-VDA bending test to the maximum load point, in the L cross section, the length of the crack is 400 μm or less, and in a region formed from each position of a starting line starting from a bending vertex on the outside of the bend of the VDA bend and existing at a position of 50 μm in the plate thickness direction to a position of 50 μm on both sides of the starting line in a perpendicular direction, with respect to the average grain size of bainitic ferrite in the plate thickness direction, the ratio of the average grain size before working to the average grain size after working is 5.0 or less, and the tensile strength is 1180 MPa or more.

[0090] The steel sheet may have a zinc coating as the outermost layer on one or both sides of the steel sheet. The steel sheet having the zinc coating may be a galvanized steel sheet.

[0091] Ingredients

[0092] First, the component composition of the base steel sheet of the steel sheet according to one embodiment of the present invention will be described. It should be noted that the unit in the component composition is "mass %", and hereinafter, "%" is used only unless otherwise specified.

[0093] C: 0.050% or more and 0.400% or less

[0094] C is an element that generates appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite, and retained austenite, and is effective for ensuring a tensile strength TS of 1180 MPa or more and a high YS. Here, when the C content is less than 0.050%, the area ratio of ferrite increases, making it difficult to make TS of 1180 MPa or more. In addition, it also leads to a decrease in YS.

[0095] On the other hand, when the C content exceeds 0.400%, the carbon concentration in the retained austenite increases excessively. Therefore, when the steel sheet is subjected to punching in the hole expansion test or V-bending in the V-VDA test, the hardness of the fresh martensite generated by the work-induced phase transformation increases significantly, which promotes the subsequent formation of voids and crack propagation, and the desired λ and S cannot be achieved. Fmax .

[0096] Therefore, the C content is set to be 0.050% or more and 0.400% or less. The C content is preferably 0.100% or more. In addition, the C content is preferably 0.300% or less.

[0097] Si: more than 0.75% and less than 3.00%

[0098] Si inhibits the formation of carbides during cooling and holding after annealing, and promotes the formation of retained austenite. That is, Si is an element that affects the volume fraction of retained austenite and the carbon concentration in the retained austenite. Here, when the Si content is 0.75% or less, the volume fraction of retained austenite decreases and the ductility decreases.

[0099] On the other hand, when the Si content is greater than 3.00%, the area ratio of ferrite increases excessively, making it difficult to achieve TS of 1180 MPa or more. In addition, it also leads to a decrease in YS. Moreover, the carbon concentration in austenite during annealing increases excessively, and the desired λ and S cannot be achieved. Fmax .

[0100] Therefore, the Si content is set to be greater than 0.75% and 3.00% or less. The Si content is preferably 2.00% or less.

[0101] Mn: 2.00% or more and less than 3.50%

[0102] Mn is an element that adjusts the area ratio of bainitic ferrite, tempered martensite, etc. Here, when the Mn content is less than 2.00%, the area ratio of ferrite increases excessively, making it difficult to make TS above 1180 MPa. In addition, it also leads to a decrease in YS.

[0103] On the other hand, when the Mn content is 3.50% or more, the martensitic transformation start temperature Ms (hereinafter also referred to as Ms point or Ms) decreases, and the martensite generated in the second cooling step decreases. As a result, the martensite generated during the final cooling increases, and the martensite generated at this time is not fully tempered, and the area ratio of hard fresh martensite increases. Fresh martensite becomes the starting point for void generation during the hole expansion test, VDA bending test, or V-VDA bending test. When the area ratio of fresh martensite exceeds 10%, the desired λ, α, and S cannot be achieved. Fmax .

[0104] Therefore, the Mn content is set to be 2.00% or more and less than 3.50%. The Mn content is preferably 2.50% or more. In addition, the Mn content is preferably 3.20% or less.

[0105] P: 0.001% or more and 0.100% or less

[0106] P is an element that has a solid solution strengthening effect and improves the TS and YS of the steel sheet. In order to obtain such an effect, the P content is set to 0.001% or more. On the other hand, when the P content exceeds 0.100%, P segregates at the original austenite grain boundary and embrittles the grain boundary. Therefore, after the steel sheet is subjected to blanking processing or V-bending processing in the V-VDA bending test, the amount of voids generated increases, and the desired λ and S cannot be achieved. Fmax .

[0107] Therefore, the P content is set to be 0.001% or more and 0.100% or less. The P content is preferably 0.030% or less.

[0108] S: 0.0001% or more and 0.0200% or less

[0109] S exists in the form of sulfides in steel. In particular, when the S content exceeds 0.0200%, the amount of voids generated after punching the steel sheet or after V-bending in the V-VDA bending test increases, and the desired λ and S cannot be achieved. Fmax .

[0110] Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0080% or less. It should be noted that, regarding the lower limit of the S content, due to limitations in production technology, the S content is set to 0.0001% or more.

[0111] Al: 0.010% or more and 2.000% or less

[0112] Al inhibits the formation of carbides during cooling and holding after annealing and promotes the formation of retained austenite. That is, Al is an element that affects the volume ratio of retained austenite and the carbon concentration in retained austenite. In order to obtain such an effect, the Al content is set to 0.010% or more.

[0113] On the other hand, when the Al content exceeds 2.000%, the area ratio of ferrite increases excessively, making it difficult to achieve TS of 1180 MPa or more. In addition, it also leads to a decrease in YS. Moreover, the C concentration in austenite during annealing increases excessively, and the desired λ and S cannot be achieved. Fmax .

[0114] Therefore, the Al content is set to be 0.010% or more and 2.000% or less. The Al content is preferably 0.015% or more. In addition, the Al content is preferably 1.000% or less.

[0115] N: 0.0100% or less

[0116] N exists in the form of nitrides in steel. In particular, when the N content exceeds 0.0100%, the amount of voids generated after punching the steel sheet or performing V-bending in the V-VDA bending test increases, and the desired λ and S cannot be achieved. Fmax .

[0117] Therefore, the N content is set to 0.0100% or less. In addition, the N content is preferably 0.0050% or less. It should be noted that there is no particular lower limit for the N content, but due to production technology limitations, the N content is preferably 0.0005% or more.

[0118] The basic component composition of the base steel sheet of the steel sheet according to one embodiment of the present invention has been described above. The base steel sheet of the steel sheet according to one embodiment of the present invention has a component composition containing the above-mentioned basic components, and the balance other than the above-mentioned basic components includes Fe (iron) and inevitable impurities. Here, the base steel sheet of the steel sheet according to one embodiment of the present invention preferably has a component composition containing the above-mentioned basic components, and the balance consists of Fe and inevitable impurities.

[0119] The base steel sheet of the steel sheet according to one embodiment of the present invention may contain at least one of the optional components shown below in addition to the above-mentioned basic components. It should be noted that the effect of the present invention can be obtained as long as the optional components shown below contain less than the upper limit amount shown below, so no lower limit is particularly set. It should be noted that when the optional elements shown below are contained in an amount less than the preferred lower limit value described below, the elements are contained as unavoidable impurities.

[0120] Selected from Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co At least one of: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less

[0121] Nb: 0.200% or less

[0122] Nb increases TS and YS by forming fine carbides, nitrides or carbonitrides during hot rolling and annealing. In order to obtain such an effect, it is preferred to set the Nb content to 0.001% or more. The Nb content is more preferably 0.005% or more. On the other hand, when the Nb content is greater than 0.200%, a large amount of coarse precipitates and inclusions are generated. In such a case, the coarse precipitates and inclusions become the starting point of cracks during the hole expansion test, the VDA bending test or the V-VDA bending test, and it may not be possible to achieve the desired λ, α and S. FmaxTherefore, when Nb is contained, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.

[0123] Ti: 0.200% or less

[0124] Ti, like Nb, increases TS and YS by forming fine carbides, nitrides or carbonitrides during hot rolling and annealing. In order to obtain such an effect, the Ti content is preferably set to 0.001% or more. The Ti content is more preferably 0.005% or more. On the other hand, when the Ti content is greater than 0.200%, a large amount of coarse precipitates and inclusions are generated. In such a case, the coarse precipitates and inclusions become the starting point of cracks during the hole expansion test, the VDA bending test or the V-VDA bending test, and it may not be possible to achieve the desired λ, α and S. Fmax Therefore, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.

[0125] V: 0.200% or less

[0126] V, like Nb and Ti, increases TS and YS by forming fine carbides, nitrides or carbonitrides during hot rolling and annealing. In order to obtain such an effect, it is preferred to set the V content to 0.001% or more. The V content is more preferably 0.005% or more. The V content is further preferably 0.010% or more, and further preferably 0.030% or more. On the other hand, when the V content is greater than 0.200%, a large amount of coarse precipitates and inclusions are generated. In such a case, the coarse precipitates and inclusions become the starting point of cracks during the hole expansion test, the VDA bending test or the V-VDA bending test, and it may not be possible to achieve the desired λ, α and S. Fmax Therefore, when V is contained, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.

[0127] B: 0.0100% or less

[0128] B is an element that improves hardenability by segregating at austenite grain boundaries. In addition, B is an element that suppresses the generation of ferrite and grain growth during cooling after annealing. In order to obtain such an effect, it is preferred that B content is set to more than 0.0001%. B content is more preferably more than 0.0002%.

[0129] The B content is more preferably 0.0005% or more, and still more preferably 0.0007% or more.

[0130] On the other hand, when the B content exceeds 0.0100%, cracks may be generated inside the steel sheet during hot rolling. In addition, after the steel sheet is subjected to blanking or V-bending in the V-VDA bending test, the amount of voids generated increases, and the desired λ and S may not be achieved. Fmax .

[0131] Therefore, when B is contained, the B content is preferably set to 0.0100% or less. The B content is more preferably set to 0.0050% or less.

[0132] Cr: 1.000% or less

[0133] Cr is an element that improves hardenability. Therefore, by adding Cr, a large amount of tempered martensite is generated, which can ensure a TS of more than 1180 MPa and a high YS. In order to obtain such an effect, the Cr content is preferably set to more than 0.0005%. In addition, the Cr content is more preferably more than 0.010%.

[0134] The Cr content is more preferably 0.030% or more, and still more preferably 0.050% or more.

[0135] On the other hand, when the Cr content exceeds 1.000%, the area ratio of hard fresh martensite increases excessively, and in the hole expansion test, VDA bending test or V-VDA bending test, the fresh martensite becomes the starting point of void formation, and the desired λ, α and S may not be achieved. Fmax Therefore, when Cr is contained, the Cr content is preferably set to 1.000% or less. In addition, the Cr content is more preferably set to 0.800% or less, and further preferably set to 0.700% or less.

[0136] Ni: 1.000% or less

[0137] Ni is an element that improves hardenability. Therefore, by adding Ni, tempered martensite is generated in large quantities, and TS and high YS of more than 1180MPa can be ensured. In order to obtain such an effect, it is preferred that the Ni content is set to more than 0.005%. The Ni content is more preferably more than 0.020%. The Ni content is further preferably more than 0.040%, and further more preferably more than 0.060%.

[0138] On the other hand, when the Ni content exceeds 1.000%, the area ratio of fresh martensite increases excessively, and in the hole expansion test, VDA bending test or V-VDA bending test, fresh martensite becomes the starting point of void generation, and the desired λ, α and S may not be achieved. Fmax Therefore, when Ni is contained, the Ni content is preferably set to 1.000% or less. The Ni content is more preferably set to 0.800% or less.

[0139] The Ni content is more preferably 0.600% or less, and still more preferably 0.400% or less.

[0140] Mo: 1.000% or less

[0141] Mo is an element that improves hardenability. Therefore, by adding Mo, a large amount of tempered martensite is generated, and a TS of more than 1180 MPa and a high YS can be ensured. In order to obtain such an effect, the Mo content is preferably set to more than 0.010%. The Mo content is more preferably more than 0.030%.

[0142] On the other hand, when the Mo content exceeds 1.000%, the area ratio of fresh martensite increases excessively, and in the hole expansion test, VDA bending test or V-VDA bending test, fresh martensite becomes the starting point of void generation, and the desired λ, α and S may not be achieved. Fmax Therefore, when Mo is contained, the Mo content is preferably set to 1.000% or less. The Mo content is more preferably 0.500% or less, further preferably 0.450% or less, and further more preferably 0.400% or less. The Mo content is more preferably 0.350% or less, and further more preferably 0.300% or less.

[0143] Sb: 0.200% or less

[0144] Sb is an element effective for suppressing the diffusion of C near the surface of the steel sheet during annealing and controlling the formation of a soft layer near the surface of the steel sheet. When the soft layer near the surface of the steel sheet increases excessively, it is difficult to make TS above 1180MPa. In addition, it also leads to a decrease in YS. Therefore, it is preferred that the Sb content is set to 0.002% or more. The Sb content is more preferably 0.005% or more.

[0145] On the other hand, when the Sb content exceeds 0.200%, a soft layer may not be formed near the surface of the steel sheet, resulting in a decrease in hole expandability and bendability. Therefore, when Sb is contained, the Sb content is preferably set to 0.200% or less. The Sb content is more preferably 0.020% or less.

[0146] Sn: 0.200% or less

[0147] Sn is an element that is effective for suppressing the diffusion of C near the surface of the steel sheet during annealing and controlling the formation of a soft layer near the surface of the steel sheet, similarly to Sb. When the soft layer near the surface of the steel sheet increases excessively, it is difficult to make TS above 1180MPa. In addition, it also leads to a decrease in YS. Therefore, it is preferred that the Sn content is set to 0.002% or more. The Sn content is more preferably 0.005% or more.

[0148] On the other hand, when the Sn content exceeds 0.200%, a soft layer may not be formed near the surface of the steel sheet, resulting in a decrease in hole expandability and bendability. Therefore, when Sn is contained, the Sn content is preferably set to 0.200% or less. The Sn content is more preferably 0.020% or less.

[0149] Cu: 1.000% or less

[0150] Cu is an element that improves hardenability. Therefore, by adding Cu, tempered martensite is generated in large quantities, and TS and high YS above 1180MPa can be ensured. In order to obtain such an effect, it is preferred that the Cu content is set to more than 0.005%. The Cu content is more preferably more than 0.008%, and further more preferably more than 0.010%. The Cu content is more preferably more than 0.020%.

[0151] On the other hand, when the Cu content exceeds 1.000%, the area ratio of fresh martensite increases excessively, and a large number of coarse precipitates and inclusions may be generated. In such a case, fresh martensite and coarse precipitates and inclusions become the starting points of voids and cracks during the hole expansion test, VDA bending test or V-VDA bending test, and the desired λ, α and S may not be achieved. Fmax Therefore, when Cu is contained, the Cu content is preferably set to 1.000% or less. The Cu content is more preferably set to 0.200% or less.

[0152] Ta: 0.100% or less

[0153] Ta, like Ti, Nb and V, increases TS and YS by forming fine carbides, nitrides or carbonitrides during hot rolling and annealing. Moreover, a portion of Ta is dissolved in Nb carbides and Nb carbonitrides to form composite precipitates such as (Nb, Ta) (C, N). Thus, the coarsening of precipitates is suppressed and precipitation strengthening is stabilized. Thus, TS and YS are further improved. In order to obtain such an effect, it is preferred that the Ta content is set to 0.001% or more. The Ta content is further preferably 0.002% or more, and further preferably 0.004% or more.

[0154] On the other hand, when the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions may become the starting point of cracks during the hole expansion test, VDA bending test or V-VDA bending test, and the desired λ, α and S may not be achieved. Fmax Therefore, when Ta is contained, the Ta content is preferably 0.100% or less.

[0155] The Ta content is more preferably 0.090% or less, and still more preferably 0.080% or less.

[0156] W: 0.500% or less

[0157] W is an element that improves hardenability. Therefore, by adding W, tempered martensite is generated in large quantities, and a TS of more than 1180 MPa and a high YS can be ensured. In order to obtain such an effect, it is preferred that the W content is set to more than 0.001%. The W content is more preferably more than 0.030%.

[0158] On the other hand, when the W content exceeds 0.500%, the area ratio of hard fresh martensite increases excessively, and in the hole expansion test, VDA bending test or V-VDA bending test, the fresh martensite becomes the starting point of void formation, and the desired λ, α and S may not be achieved. Fmax Therefore, when W is contained, the W content is preferably set to 0.500% or less. The W content is more preferably 0.450% or less, and further preferably 0.400% or less. The W content is further more preferably 0.300% or less.

[0159] Mg: 0.0200% or less

[0160] Mg is an element effective for spheroidizing the shape of inclusions such as sulfides and oxides and improving the hole expandability of the steel sheet. In order to obtain such an effect, the Mg content is preferably set to 0.0001% or more. The Mg content is more preferably 0.0005% or more, and further preferably 0.0010% or more.

[0161] On the other hand, when the Mg content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions may become the starting point of cracks during the hole expansion test, the VDA bending test, or the V-VDA bending test, and the desired λ, α, and S may not be achieved. Fmax Therefore, when Mg is contained, the Mg content is preferably set to 0.0200% or less. The Mg content is more preferably set to 0.0180% or less, and further preferably set to 0.0150% or less.

[0162] Zn: 0.0200% or less

[0163] Zn is an element effective for spheroidizing the shape of inclusions and improving the hole expandability of the steel plate. In order to obtain such an effect, the Zn content is preferably set to 0.0010% or more. The Zn content is more preferably 0.0020% or more, and further preferably 0.0030% or more.

[0164] On the other hand, when the Zn content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions become the starting point of cracks during the hole expansion test, VDA bending test or V-VDA bending test, and the desired λ, α and S may not be achieved. Fmax Therefore, when Zn is contained, the Zn content is preferably set to 0.0200% or less. The Zn content is more preferably set to 0.0180% or less, and further preferably set to 0.0150% or less.

[0165] Co: 0.0200% or less

[0166] Co is an element effective for spheroidizing the shape of inclusions and improving the hole expandability of the steel plate, similarly to Zn. In order to obtain such an effect, the Co content is preferably set to 0.0010% or more. The Co content is more preferably 0.0020% or more, and further preferably 0.0030% or more.

[0167] On the other hand, when the Co content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions become the starting point of cracks during the hole expansion test, the VDA bending test, or the V-VDA bending test, and the desired λ, α, and S may not be achieved. Fmax Therefore, when Co is contained, the Co content is preferably set to 0.0200% or less. The Co content is more preferably set to 0.0180% or less, and further preferably set to 0.0150% or less.

[0168] Zr: 0.1000% or less

[0169] Zr is an element effective for spheroidizing the shape of inclusions and improving the hole expandability of the steel sheet, similarly to Zn and Co. In order to obtain such an effect, the Zr content is preferably set to 0.0010% or more. On the other hand, when the Zr content is greater than 0.1000%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions become the starting point of cracks during the hole expansion test, the VDA bending test, or the V-VDA bending test, and the desired λ, α, and S may not be achieved. Fmax Therefore, when Zr is contained, the Zr content is preferably set to 0.1000% or less.

[0170] The Zr content is more preferably 0.0300% or less, and further preferably 0.0100% or less.

[0171] Ca: 0.0200% or less

[0172] Ca exists in the steel as inclusions. When the Ca content exceeds 0.0200%, a large number of coarse inclusions may be generated. In such cases, coarse precipitates and inclusions may become the starting point of cracks during the hole expansion test, VDA bending test, or V-VDA bending test, and the desired λ, α, and S may not be achieved. Fmax Therefore, when Ca is contained, the Ca content is preferably set to 0.0200% or less. The Ca content is preferably set to 0.0020% or less.

[0173] The Ca content is more preferably 0.0019% or less, and further preferably 0.0018% or less.

[0174] It should be noted that the lower limit of the Ca content is not particularly limited, but the Ca content is preferably 0.0005% or more. In addition, due to limitations in production technology, the Ca content is more preferably 0.0010% or more.

[0175] Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, REM: 0.0200% or less

[0176] Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are all elements effective for improving the hole expandability of the steel plate. In order to obtain such an effect, the content of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM is preferably set to 0.0001% or more.

[0177] On the other hand, when the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi and REM are respectively greater than 0.0200% and the content of As is greater than 0.0500%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions may become the starting point of cracks during the hole expansion test, VDA bending test or V-VDA bending test, and the desired λ, α and S may not be achieved. Fmax Therefore, in the case of containing at least one of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM, the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are preferably set to 0.0200% or less, respectively, and the content of As is preferably set to 0.0500% or less.

[0178] The Se content is more preferably 0.0005% or more, and further preferably 0.0008% or more. The Se content is more preferably 0.0180% or less, and further preferably 0.0150% or less.

[0179] The Te content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Te content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.

[0180] The Ge content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Ge content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.

[0181] The As content is more preferably 0.0010% or more, and even more preferably 0.0015% or more. The As content is more preferably 0.0400% or less, and even more preferably 0.0300% or less.

[0182] The Sr content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Sr content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.

[0183] The Cs content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Cs content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.

[0184] The Hf content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Hf content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.

[0185] The Pb content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Pb content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.

[0186] The Bi content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Bi content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.

[0187] The REM content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The REM content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.

[0188] It should be noted that the REM described in the present invention refers to scandium (Sc) with an atomic number of 21, yttrium (Y) with an atomic number of 39, and lanthanide elements from lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71. The REM concentration in the present invention refers to the total content of one or more elements selected from the above REM.

[0189] REM is not particularly limited, but Sc, Y, Ce, and La are preferred.

[0190] That is, the base steel sheet of the steel sheet according to one embodiment of the present invention contains, by mass%, C: 0.050% to 0.400%, Si: greater than 0.75% to 3.00% or less, Mn: 2.00% to 3.50%, P: 0.001% to 0.100% or less, S: 0.0001% to 0.0200% or less, Al: 0.010% to 2.000% or less, and N: 0.0100% or less, and optionally contains Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sb: 0.200% or less, and Sb: 0.200% or less. A component composition comprising at least one of n: 0.200% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less, and the balance being Fe and unavoidable impurities.

[0191] Steel structure

[0192] Next, the steel structure of the base steel plate of the steel plate according to one embodiment of the present invention will be described.

[0193] In the steel structure of the base steel sheet of the steel sheet according to one embodiment of the present invention, the area ratio of bainitic ferrite is 3.0% to 20.0%, the area ratio of tempered martensite (excluding retained austenite) is 40.0% to 90.0%, the volume ratio of retained austenite is greater than 3.0% to 15.0%, the carbon concentration in the retained austenite is 0.60 mass% to 1.30 mass%, the area ratio of fresh martensite is 10.0% to 10.0% (including 0.0%), and the density of carbides in the tempered martensite is 8.0 pieces / μm 2 Below, the amount of diffusible hydrogen in the base steel plate is 0.50 mass ppm or less, and in a V-VDA bending test to the maximum load point, the length of the crack in the L cross section is 400 μm or less, and in a region formed from each position of a starting line starting from a bending apex on the outer side of the bend in the VDA bending and existing at a position of 50 μm in the plate thickness direction to a position of 50 μm on both sides of the starting line in a perpendicular direction, with respect to the average grain size of bainitic ferrite in the plate thickness direction, the ratio of the average grain size before working to the average grain size after working is 5.0 or less.

[0194] The reasons for each limitation are described below.

[0195] Area ratio of bainitic ferrite: 3.0% or more and 20.0% or less

[0196] Bainitic ferrite has an intermediate hardness compared to soft ferrite, hard fresh martensite, etc., and is an important phase for ensuring good hole expansion, bendability, bending fracture characteristics, and axial crushing characteristics. In addition, bainitic ferrite is also a useful phase for effectively utilizing the diffusion of C from bainitic ferrite to untransformed austenite to obtain an appropriate amount of retained austenite area ratio and carbon concentration in the retained austenite. Therefore, the area ratio of bainitic ferrite is set to 3.0% or more.

[0197] The area ratio of bainitic ferrite is preferably 5.0% or more, more preferably 8.0% or more.

[0198] On the other hand, when the area ratio of bainitic ferrite increases excessively, the strength decreases, and the area ratio of retained austenite exceeds a predetermined amount. Therefore, the area ratio of bainitic ferrite is set to 20.0% or less.

[0199] The area ratio of bainitic ferrite is preferably 18.0% or less, more preferably 15.0% or less.

[0200] It should be noted that bainitic ferrite refers to upper bainite with a small amount of carbides generated in a relatively high temperature range.

[0201] Area ratio of tempered martensite (excluding retained austenite): 40.0% or more and 90.0% or less

[0202] Tempered martensite has an intermediate hardness compared to soft ferrite, hard fresh martensite, etc., and is an important phase for ensuring good hole expansion, bendability, bending fracture characteristics and axial crushing characteristics. In addition, tempered martensite is effective for improving TS. Therefore, the area ratio of tempered martensite is set to 40.0% or more. The area ratio of tempered martensite is preferably 60.0% or more. On the other hand, when the area ratio of tempered martensite increases excessively, ductility decreases. Therefore, the area ratio of tempered martensite is set to 90.0% or less.

[0203] The area ratio of tempered martensite is preferably 85.0% or less, more preferably 80.0% or less.

[0204] Area ratio of retained austenite: greater than 3.0% and less than 15.0%

[0205] From the viewpoint of obtaining good ductility, the area ratio of retained austenite is set to be greater than 3.0%. The area ratio of retained austenite is preferably 5.0% or more. On the other hand, when the area ratio of retained austenite is excessively increased, when the steel sheet is subjected to punching in a hole expansion test or V-bending in a V-VDA test, fresh martensite generated by the processing-induced phase transformation becomes the starting point of void generation, and the desired λ and S cannot be achieved. Fmax Therefore, the area ratio of retained austenite is set to 15.0% or less. The area ratio of retained austenite is preferably 12.0% or less, and more preferably 10.0% or less.

[0206] For example, by controlling the tension during the second cooling step in the manufacturing method described later, the area ratio of retained austenite can be suppressed to 15.0% or less. After the first cooling step (in the case of galvanizing, after galvanizing (after alloying as needed)), 2.0 kgf / mm 2 The steel plate is subjected to a tension of at least 4 passes while being in contact with a roll having a diameter of at least 500 mm and at most 1500 mm for 1 / 4 of a roll cycle in each pass, and the steel plate is subjected to a treatment of at least 2 passes while being in contact with a roll having a diameter of at least 500 mm and at most 1500 mm for 1 / 2 of a roll cycle in each pass, thereby causing unstable retained austenite to undergo a processing-induced phase transformation into fresh martensite, which is tempered during subsequent cooling and eventually becomes tempered martensite.

[0207] Carbon concentration in retained austenite: 0.60 mass % or more and 1.30 mass % or less

[0208] The carbon concentration in the retained austenite is an indicator that affects the stability of the phase transformation of the retained austenite into martensite during deformation. When the carbon concentration in the retained austenite is less than 0.60 mass%, the retained austenite is unstable, and a processing-induced martensitic phase transformation occurs after the stress is applied and before the plastic deformation, so the required elongation cannot be obtained. On the other hand, when the carbon concentration in the retained austenite is greater than 1.30 mass%, the hardness of the fresh martensite generated from the retained austenite when the steel plate is subjected to punching in the hole expansion test or V-bending in the V-VDA test increases significantly, the formation and connection of voids are promoted, and the desired λ and S cannot be achieved. Fmax Therefore, the carbon concentration in the retained austenite is set to be 0.60 mass % or more and 1.30 mass % or less. The carbon concentration in the retained austenite is preferably set to be 0.80 mass % or more. In addition, the carbon concentration in the retained austenite is preferably set to be 1.20 mass % or less.

[0209] Area ratio of fresh martensite: 10.0% or less (including 0.0%)

[0210] If the area ratio of fresh martensite increases excessively, fresh martensite becomes the starting point of void formation in the hole expansion test, VDA bending test or V-VDA bending test, and the desired λ, α and S cannot be achieved. Fmax . In addition, as the area ratio of fresh martensite increases, the amount of diffusible hydrogen in the steel sheet increases, and the hole expandability and bendability further decrease. From the viewpoint of ensuring good hole expandability and bendability, the area ratio of fresh martensite is set to 10.0% or less, preferably 5.0% or less. It should be noted that there is no particular restriction on the lower limit of the area ratio of fresh martensite, and it may be 0.0%.

[0211] It should be noted that fresh martensite refers to martensite in a quenched state (not tempered).

[0212] Density of carbides in tempered martensite (number density): 8.0 / μm 2 the following

[0213] In the present invention, the density of carbides in tempered martensite is greater than 8.0 carbides / μm 2 When the hole expansion test, VDA bending test or V-VDA bending test is performed, the number of voids caused by carbides increases, which promotes the occurrence and expansion of cracks, resulting in the failure to achieve the desired λ, α and S Fmax Therefore, the density of carbides in tempered martensite is set to 8.0 carbides / μm 2 The density of carbides in tempered martensite is preferably 7.0 carbides / μm 2 Below, more preferably 6.0 pieces / μm 2 the following.

[0214] The lower limit is not particularly limited, but the density of carbides in tempered martensite is preferably 1.0 carbide / μm. 2 More preferably, 2.0 pieces / μm 2 above.

[0215] It should be noted that the remaining structures other than the above mentioned ones include, for example, carbides such as ferrite, lower bainite, pearlite, and cementite. In order to ensure a TS of 1180 MPa or more and a high YS, the area ratio of pearlite is preferably set to 5.0% or less.

[0216] The type of the remaining tissue can be confirmed by, for example, observation using a SEM (Scanning Electron Microscope).

[0217] The area ratios of bainitic ferrite, tempered martensite, and hard second phase (retained austenite+fresh martensite) were measured as follows at a position of ¼ of the plate thickness of the base steel plate.

[0218] That is, the sample was cut out from the base steel plate so that the plate thickness section parallel to the rolling direction of the base steel plate became the observation surface. Then, the observation surface of the sample was mirror-polished using diamond paste. Then, the observation surface of the sample was finely polished using colloidal silica, and then etched using 3 volume % nitric acid ethanol solution to expose the structure.

[0219] Then, three fields of view of 25.6 μm×17.6 μm on the observation surface of the sample were photographed using a SEM (Scanning Electron Microscope) under conditions of an acceleration voltage of 15 kV and a magnification of 5000 times.

[0220] From the obtained structure image, bainitic ferrite, tempered martensite, hard second phase (retained austenite+fresh martensite) and the remaining structure were identified as follows.

[0221] Bainitic ferrite: It appears in areas ranging from black to dark gray, and has a blocky or amorphous shape. In addition, there are no or few iron-based carbides inside.

[0222] Tempered martensite: It is a gray area with an amorphous shape and contains a large amount of iron carbides.

[0223] Hard second phase (retained austenite + fresh martensite): presents a white to light gray area, and is amorphous. Also, there is no iron-based carbide contained inside. It should be noted that when the size is large, the color gradually darkens as it moves away from the interface with other tissues, and the inside appears dark gray.

[0224] Ferrite: A black area with a blocky shape. Also, it contains almost no iron-based carbides. However, in the case of iron-based carbides, the area of ​​ferrite is set to include the area of ​​iron-based carbides. The same is true for the above-mentioned bainitic ferrite and tempered martensite.

[0225] Cementite: White area in the form of dots and lines. It is contained in tempered martensite, bainitic ferrite and ferrite.

[0226] Lower bainite, pearlite, etc.: their morphologies and the like are well known.

[0227] Next, the area of ​​each phase identified in the structure image was calculated by the following method. A 20×20 grid was divided equally on an area of ​​23.1 μm×17.6 μm in actual length on the above SEM image with a magnification of 5000 times, and the area ratio of bainitic ferrite, tempered martensite and hard second phase was calculated by the point counting method of counting the number of points on each phase. The area ratio was set to the average of the three area ratios found in each SEM image with a magnification of 5000 times.

[0228] In addition, the area ratio of retained austenite was measured as follows.

[0229] That is, the base steel plate is mechanically ground along the plate thickness direction (depth direction) to 1 / 4 of the plate thickness, and then chemically polished with oxalic acid to form an observation surface. Next, the observation surface is observed using the X-ray diffraction method. Using MoKα rays as incident X-rays, the diffraction intensity of each surface of fcc iron (austenite) (200), (220) and (311) is calculated relative to the diffraction intensity of each surface of bcc iron (200), (211) and (220), and the volume ratio of retained austenite is calculated from the ratio of the diffraction intensity of each surface. In addition, it is considered that the retained austenite is three-dimensionally homogeneous, and the volume ratio of retained austenite is taken as the area ratio of retained austenite.

[0230] Regarding the distribution of carbon concentration in retained austenite, the lattice constant of retained austenite was determined using the diffraction peak of the (220) plane of fcc iron (austenite) measured by the above-mentioned X-ray diffraction method. Next, the carbon concentration in retained austenite was determined by the following formula.

[0231] Cγ=((A-(3.572+0.0012×[Mn%]-0.00157×[Si%]+0.0056×[Al%])) / 0.033

[0232] Here, A is the lattice constant of retained austenite, Cγ is the carbon concentration in retained austenite, and [Mn%], [Si%], and [Al%] are the contents (mass %) of Mn, Si, and Al in the steel sheet, respectively.

[0233] The area ratio of fresh martensite is determined by subtracting the area ratio of retained austenite from the area ratio of the hard second phase determined as described above.

[0234] [Area ratio of fresh martensite (%)] = [Area ratio of hard second phase (%)] - [Area ratio of retained austenite (%)]

[0235] The area ratio of the remaining structure is obtained by subtracting the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of the hard second phase obtained as described above from 100.0%.

[0236] [Area ratio of remaining structure (%)] = 100.0 - [Area ratio of bainitic ferrite (%)] - [Area ratio of tempered martensite (%)] - [Area ratio of hard second phase (%)]

[0237] The density of carbides in tempered martensite is measured as follows.

[0238] The tissue image obtained by SEM used in the above-mentioned tissue fraction determination is color-differentiated and extracted by hand-drawing to tempered martensite and carbide, and an image of only tempered martensite or carbide is made. Here, the carbide with a diameter (equivalent circle diameter) of more than 100nm is used as the object. Then, the area of ​​tempered martensite and the number of carbides in tempered martensite are obtained using open source ImageJ. The value obtained by dividing the number of carbides in tempered martensite by the area of ​​the tempered martensite is the density of carbides in tempered martensite, and 10 tempered martensites are randomly extracted in each SEM image and averaged, and the obtained value is used as the density of carbides in tempered martensite.

[0239] In addition, regarding one carbide, a granular region in which the outer periphery is surrounded by tempered martensite and is integrally formed without interruption in the SEM image is regarded as one carbide and measured.

[0240] Diffusible hydrogen content: 0.50 mass ppm or less

[0241] From the viewpoint of obtaining better hole expandability and bendability, the amount of diffusible hydrogen in the base steel plate is preferably set to 0.50 mass ppm or less. In addition, the amount of diffusible hydrogen in the base steel plate is more preferably set to 0.30 mass ppm or less. It should be noted that the lower limit of the amount of diffusible hydrogen in the base steel plate is not particularly specified and may be 0 mass ppm. In addition, due to limitations in production technology, the amount of diffusible hydrogen in the base steel plate is more preferably set to 0.01 mass ppm or more.

[0242] Here, the amount of diffusible hydrogen in the base steel sheet is measured as follows.

[0243] A test piece with a length of 30 mm and a width of 5 mm is cut from the steel plate. When a zinc coating is formed on the steel plate, the zinc coating is removed with an alkali. Next, the amount of hydrogen released from the test piece is determined by thermal desorption analysis. Specifically, the test piece is continuously heated from room temperature (-5 to 55°C) to 300°C at a heating rate of 200°C / hour, and then cooled to room temperature. At this time, the amount of hydrogen released from the test piece (cumulative hydrogen amount) is measured in the temperature range from room temperature to 210°C in the continuous heating. Then, the measured amount of hydrogen is divided by the mass of the test piece (the test piece after the zinc coating is removed and before continuous heating) and converted into a mass ppm unit to obtain a value as the diffusible hydrogen amount of the base steel plate. The determination of the diffusible hydrogen amount is preferably carried out after the manufacture of the steel plate is completed. The determination of the hydrogen amount is more preferably carried out within 1 week after the manufacture of the steel plate is completed.

[0244] In addition, the room temperature is set within the range of the local temperature change within one year in consideration of production in various countries around the world, and is usually preferably within the range of 10 to 50°C.

[0245] It should be noted that, for products (components) after the steel plates have been formed and joined, a test piece is cut out from the product placed in a normal use environment and the diffusible hydrogen content of the base steel plate portion is measured using the same points as above. If the value is 0.50 mass ppm or less, it can be considered that the diffusible hydrogen content of the base steel plate of the steel plate at the raw material stage before the forming and joining processes is also 0.50 mass ppm or less.

[0246] Surface soft layer

[0247] The base steel plate of the steel plate of one embodiment of the present invention preferably has a surface soft layer on the surface of the base steel plate. Since the surface soft layer helps to suppress the extension of bending cracks during stamping and vehicle body collision, the bending fracture resistance is further improved. It should be noted that the surface soft layer refers to a decarburized layer, which is a surface area with a Vickers hardness of less than 85% relative to the Vickers hardness of the cross section at the 1 / 4 position of the plate thickness.

[0248] Here, the surface soft layer is formed in an area 200 μm or less from the surface of the base steel plate in the plate thickness direction. Regarding the area where the surface soft layer is formed, it is preferably 150 μm or less, more preferably 120 μm or less from the surface of the base steel plate in the plate thickness direction. It should be noted that there is no particular lower limit for the thickness of the surface soft layer, but the thickness of the surface soft layer is preferably 8 μm or more, more preferably 11 μm or more. In addition, the surface soft layer is preferably 30 μm or more, more preferably 40 μm or more.

[0249] The 1 / 4 position of the plate thickness of the base steel plate for measuring the Vickers hardness is a non-surface soft layer (a layer that does not satisfy the hardness condition of the surface soft layer specified in the present invention).

[0250] The Vickers hardness was measured based on JIS Z 2244-1 (2020) with a load set to 10 gf.

[0251] Nanohardness of the soft surface layer

[0252] When the nanohardness of 300 or more points is measured in an area of ​​50 μm×50 μm on the plate surface at each of the position 1 / 4 of the depth in the plate thickness direction of the surface soft layer from the surface of the base steel plate and the position 1 / 2 of the depth in the plate thickness direction, the ratio of the number of measurements of the plate surface having a nanohardness of 7.0 GPa or more at the position 1 / 4 of the depth in the plate thickness direction of the surface soft layer from the surface of the base steel plate is 0.10 or less relative to the total number of measurements at the position 1 / 4 of the depth in the plate thickness direction of the surface soft layer

[0253] In the present invention, in order to obtain excellent bendability during stamping and excellent bending fracture characteristics during collision, it is preferred that when measuring the nanohardness of more than 300 points in the 50μm×50μm area of ​​the plate surface at each of the 1 / 4 position of the plate thickness direction depth of the surface soft layer and the 1 / 2 position of the plate thickness direction depth from the base steel plate surface, the ratio of the number of measurements of the nanohardness of the plate surface at the 1 / 4 position of the plate thickness direction depth of the surface soft layer from the base steel plate surface is 7.0GPa or more is less than 0.10 relative to the total number of measurements at the 1 / 4 position of the plate thickness direction depth of the surface soft layer. When the ratio of nanohardness of 7.0GPa or more is less than 0.10, it means that the ratio of hard structures (martensite, etc.), inclusions, etc. is small, which can further inhibit the generation, connection, and expansion of cracks of hard structures (martensite, etc.) and inclusions during stamping and collision, and can obtain excellent R / t, α and S Fmax .

[0254] The standard deviation σ of the nanohardness of the plate surface at a position 1 / 4 of the depth in the plate thickness direction of the surface soft layer from the steel plate surface is 1.8 GPa or less, and the standard deviation σ of the nanohardness of the plate surface at a position 1 / 2 of the depth in the plate thickness direction of the surface soft layer from the steel plate surface is 2.2 GPa or less

[0255] In the present invention, in order to obtain excellent bendability during stamping and excellent bending fracture characteristics during collision, it is preferred that the standard deviation σ of the nanohardness of the plate surface at a position 1 / 4 of the depth in the plate thickness direction of the surface soft layer from the surface of the steel plate is 1.8 GPa or less, and the standard deviation σ of the nanohardness of the plate surface at a position 1 / 2 of the depth in the plate thickness direction of the surface soft layer from the surface of the steel plate is 2.2 GPa or less. When the standard deviation σ of the nanohardness of the plate surface at a position 1 / 4 of the depth in the plate thickness direction of the surface soft layer from the surface of the steel plate is 1.8 GPa or less, and the standard deviation σ of the nanohardness of the plate surface at a position 1 / 2 of the depth in the plate thickness direction of the surface soft layer from the surface of the steel plate is 2.2 GPa or less, it means that the difference in tissue hardness in the microscopic area is small. Therefore, due to the small difference in tissue hardness in the microscopic area, the generation, connection and expansion of cracks during stamping or collision can be further suppressed, and excellent R / t, α and S can be obtained. Fmax .

[0256] The preferred range of the standard deviation σ of the nanohardness of the plate surface at a position 1 / 4 of the plate thickness direction depth of the surface soft layer from the base steel plate surface is preferably 1.7 GPa or less. The standard deviation σ of the nanohardness of the plate surface at a position 1 / 4 of the plate thickness direction depth of the surface soft layer from the base steel plate surface is more preferably 1.3 GPa or less. The lower limit is not particularly limited, but the standard deviation σ of the nanohardness of the plate surface at a position 1 / 4 of the plate thickness direction depth of the surface soft layer from the base steel plate surface can be set to 0.5 GPa or more.

[0257] The more preferred range of the standard deviation σ of the nanohardness of the plate surface at 1 / 2 of the plate thickness direction depth of the surface soft layer from the base steel plate surface is 2.1 GPa or less. The standard deviation σ of the nanohardness of the plate surface at 1 / 2 of the plate thickness direction depth of the surface soft layer from the base steel plate surface is more preferably 1.7 GPa or less. The lower limit is not particularly limited, but the standard deviation σ of the nanohardness of the plate surface at 1 / 2 of the plate thickness direction depth of the surface soft layer from the base steel plate surface can be set to 0.6 GPa or more.

[0258] Here, the nanohardness of the plate surface at the 1 / 4 position and the 1 / 2 position in the depth in the plate thickness direction refers to the hardness measured by the following method.

[0259] In the case where a coating is formed, after the coating is peeled off, mechanical grinding is performed until the position of -5 μm from the position of 1 / 4 of the depth in the thickness direction of the surface soft layer from the surface of the base steel plate, polishing with diamond and alumina is performed until the position of 1 / 4 of the depth in the thickness direction of the surface soft layer from the surface of the base steel plate, and further colloidal silica grinding is performed. Here, the peeled coating is the zinc coating when the zinc coating is formed, the metal coating when the metal coating is formed, and the zinc coating and the metal coating when the zinc coating and the metal coating are formed.

[0260] The nanohardness was measured using Hysitron's Tribo-950 with a Berkovich-shaped diamond indenter under the conditions of a load of 500 μN, a measurement area of ​​50 μm×50 μm, and a dot interval of 2 μm.

[0261] Next, mechanical grinding, polishing with diamond and alumina, and colloidal silica grinding were performed until the surface soft layer reached 1 / 2 of the depth in the thickness direction. Nanohardness was measured using Hysitron's Tribo-950 with a Berkovich-shaped diamond indenter under the conditions of load: 500 μN, measurement area: 50 μm×50 μm, and dot interval: 2 μm.

[0262] Nanohardness was measured at 300 or more points at 1 / 4 of the depth in the plate thickness direction, and nanohardness was also measured at 300 or more points at 1 / 2 of the depth in the plate thickness direction.

[0263] For example, when the thickness of the surface soft layer is 100 μm, the 1 / 4 position is 25 μm from the surface of the surface soft layer, and the 1 / 2 position is 50 μm from the surface of the surface soft layer. The nanohardness of more than 300 points is measured at the 25 μm position, and the nanohardness of more than 300 points is also measured at the 50 μm position.

[0264] Metal plating (first layer)

[0265] In addition, the steel sheet of one embodiment of the present invention preferably has a metal plating layer (first plating layer, pre-plating layer) on one or both surfaces of the base steel sheet (it should be noted that the metal plating layer (first plating layer) does not include a hot-dip galvanizing layer or a zinc plating layer of an alloyed hot-dip galvanizing layer). The metal plating layer is preferably a metal electroplating layer, and the following description will be made by taking the metal electroplating layer as an example.

[0266] By forming a metal plating layer on the surface of the steel sheet, the metal plating layer on the outermost surface helps to suppress the occurrence of bending cracks during press forming and vehicle body collision, thereby further improving the bending fracture resistance.

[0267] In the present invention, by setting the dew point higher than -5°C, the thickness of the soft layer can be further increased, and the shaft crushing characteristics can be made very excellent. In this regard, in the present invention, by having a metal plating layer, even if the dew point is set to below -5°C and the thickness of the soft layer is small, the shaft crushing characteristics equivalent to those when the thickness of the soft layer is large can be obtained.

[0268] The metal type of the metal plating layer can be any one of Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Rt, Au, Hg, Ti, Pb, and Bi, and more preferably Fe. Hereinafter, the Fe-based plating layer will be described as an example, but the following conditions for Fe can also be adopted for other metal types.

[0269] The Fe-based electroplating layer adhesion amount is set to be greater than 0 g / m 2 , preferably set to 2.0g / m 2 There is no particular upper limit on the amount of Fe-based electroplating layer deposited on each side, but from a cost perspective, it is preferred that the amount of Fe-based electroplating layer deposited on each side be set to 60 g / m 2 The Fe-based electroplating layer is preferably set to a weight of 50 g / m 2 Below, more preferably set to 40g / m 2 Below, it is further preferably set to 30g / m 2 the following.

[0270] The adhesion amount of the Fe-based electroplated layer is measured as follows. A sample of 10×15 mm in size is cut from the Fe-based electroplated steel plate, embedded in a resin, and made into a cross-section embedded sample. Use a scanning electron microscope (SEM) with an acceleration voltage of 15 kV and a magnification of 2000 to 10000 times according to the thickness of the Fe-based plating layer to observe any three places of the cross section, and multiply the average value of the thickness of the three fields of view by the specific gravity of iron, thereby converting it into the adhesion amount of each single side of the Fe-based electroplated layer.

[0271] As Fe system electroplating layer, except pure Fe can be used, alloy plating layers such as Fe-B alloy, Fe-C alloy, Fe-P alloy, Fe-N alloy, Fe-O alloy, Fe-Ni alloy, Fe-Mn alloy, Fe-Mo alloy, Fe-W alloy can also be used.The composition of Fe system electroplating layer is not particularly limited, but preferably set to contain one or more elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co below 10 mass %, the remainder is made of Fe and inevitable impurities.By setting the amount of the element except Fe to total below 10 mass %, the reduction of electrolysis efficiency can be prevented, and Fe system electroplating layer can be formed at low cost.In the case of Fe-C alloy, the content of C is preferably set to below 0.08 mass %.

[0272] It is more preferable to have a surface soft layer under the Fe-based electroplating layer, thereby significantly improving the bending fracture resistance. In the case of having an Fe-based electroplating layer, the Vickers hardness distribution is measured from the interface between the Fe-based electroplating layer and the base steel sheet in the plate thickness direction by the above method to evaluate the depth of the surface soft layer in the plate thickness direction.

[0273] Perform V-VDA bending test to the highest load point, the length of the crack in the L section: less than 400μm

[0274] In the present invention, for a steel plate having a crack length greater than 400 μm, the formation and expansion of voids in the steel plate structure are fast, and the bending fracture resistance is reduced. Therefore, the crack length is set to 400 μm or less. The crack length is preferably 300 μm or less, and more preferably 200 μm or less. The lower limit is not particularly limited, but the value may be 0 μm.

[0275] V-VDA bending test was performed to the highest load point. In the L cross section, in the region formed from each position of the starting line starting from the bending vertex on the outside of the VDA bending and existing at a position of 50 μm in the plate thickness direction to each position of 50 μm on both sides of the starting line in the perpendicular direction, regarding the average grain size of bainitic ferrite in the plate thickness direction, the ratio of the average grain size before processing to the average grain size after processing (the amount of change in the average grain size before and after processing): 5.0 or less

[0276] Next, the amount of change in the grain size of the bainitic ferrite in the plate thickness direction before and after working will be described.

[0277] Figure 1 In the formula, the symbol BF represents bainitic ferrite, the symbol F represents ferrite, and the symbol TM represents tempered martensite. Figure 1In the graph, θ(TM) represents carbides in tempered martensite, H1 represents the hard second phase, and X1(BF) represents the island second phase in bainitic ferrite.

[0278] like Figure 1 As shown, the bainitic ferrite BF in the steel plate structure forms island-shaped residual austenite inside due to carbon distribution. When the bainitic ferrite BF is deformed by processing, voids are easily generated at the boundary between the bainitic ferrite BF and the hard fresh martensite generated in the processing-induced phase transformation of the island-shaped residual austenite. When the change in the average grain size of the bainitic ferrite BF in the plate thickness direction before and after processing is greater than 5.0, the bainitic ferrite BF is subjected to tensile stress in the rolling direction, and the increase in the number of voids promotes the occurrence and expansion of cracks, resulting in a decrease in the bending fracture resistance. Therefore, the change in the average grain size of the bainitic ferrite in the plate thickness direction before and after processing is set to less than 5.0. The above-mentioned change is preferably less than 4.8, and more preferably less than 4.5.

[0279] In addition, when the change in the average grain size of bainitic ferrite before and after processing is less than 0.2, in the bainitic ferrite subjected to compressive stress, it is also possible that voids are easily generated at the boundary of the hard fresh martensite generated by the island-shaped retained austenite located inside the bainitic ferrite during the processing-induced phase transformation, and the bending fracture resistance may be reduced. Therefore, the change in the average grain size of bainitic ferrite in the plate thickness direction before and after processing is preferably set to 0.2 or more. The above-mentioned change is preferably 0.3 or more, and more preferably 0.5 or more.

[0280] The above-mentioned V-VDA bending test is performed as follows.

[0281] A 60 mm x 65 mm test piece was cut from the obtained steel plate by shearing. Here, the 60 mm side is parallel to the rolling (L) direction. The test piece was prepared by bending 90° (primary bending) in the rolling (L) direction with a curvature radius / plate thickness of 4.2 and with the width (C) direction as the axis. In the 90° bending (primary bending), if Figure 2-1 As shown in (a), a punch B1 is pressed into a steel plate placed on a die A1 having a V-shaped groove to obtain a test piece T1. Figure 2-1 As shown in (b), the test piece T1 placed on the support roll A2 is subjected to orthogonal bending (secondary bending) by pressing the punch B2 into the test piece so that the bending direction is at right angles to the rolling direction. Figure 2-1 (a) and Figure 2-1 In (b), symbol D1 indicates the width (C) direction, and symbol D2 indicates the rolling (L) direction.

[0282] The schematic diagram of the stroke-load curve obtained when performing the V-VDA test is shown in Figure 3The V-VDA test will be carried out to the maximum load point P, and then when the load reaches 94.9-99.9% of the maximum load ( Figure 3 The sample without loading (reference symbol R) is used as the evaluation sample for the V-VDA bending test.

[0283] The test piece T1 after V-bending (primary bending) of the steel plate in the V-VDA bending test is shown in FIG. Figure 2-2 (c) In addition, the test piece T2 after VDA bending (secondary bending) is performed on the test piece T1 is shown in Figure 2-2 (d) in. Figure 2-2 The position indicated by the short dashed line on the test piece T2 of (d) is the V-bending ridgeline described above, and is the same as the position before VDA bending. Figure 2-2 In the present invention, the overlapping area of ​​the V-bend ridgeline portion and the VDA-bend ridgeline portion refers to Figure 2-2 (d) The VDA bending apex indicated by a is the center position of the short dashed line.

[0284] Specifically, the V-bend ridgeline portion refers to the region ranging from the V-bend corner (apex) extending in the width direction to both sides 5 mm in length. The region other than the V-bend ridgeline portion is the V-bend flat portion.

[0285] The VDA bending ridgeline portion refers to a region ranging from a VDA bending corner (apex) extending in the rolling direction where VDA bending is performed to 5 mm on both sides.

[0286] The positional relationship between the L cross section AL of the V-bend ridgeline and the VDA-bend ridgeline and the test piece T2 is Figure 2-3 The L section AL when the D2 direction is perpendicular to the paper surface and the D1 direction is parallel to the paper surface is shown in (e). Figure 2-3 (f)

[0287] The V-VDA bending test was performed to the highest load point, and the length of the crack in the L cross section (hereinafter also referred to as AL plane) in the overlapping region of the V-bending ridgeline portion and the VDA-bending ridgeline portion was determined as follows.

[0288] The sample was cut out from the base steel plate so that the AL plane of the steel plate after the V-VDA bending test to the highest load point became the observation surface. Then, the observation surface of the sample was mirror-polished using diamond paste. Then, the observation surface of the sample was finely polished using colloidal silica, and then etched using 3 volume % nitric acid ethanol solution to expose the structure.

[0289] Then, using a SEM (Scanning Electron Microscope), under the conditions of accelerating voltage: 15 kV and magnification: 200 times, a field of view of 25.6 μm×17.6 μm was photographed so that the symmetry axis of the AL plane was perpendicular to the part of the bending vertex of the observation surface of the sample to observe the entire appearance of the crack.

[0290] In the obtained crack image, the distance between the starting point and the end point of the crack in the vertical direction is taken as the length of the crack. An example of the image of the crack actually measured is shown in Figure 4 middle. Figure 4 In the figure, symbol D2 indicates the rolling (L) direction, symbol D4 indicates the plate thickness direction, and symbol L indicates the length of the crack.

[0291] Next, the V-VDA bending test was performed to the highest load point, and the area (0 to 50 μm from the steel sheet surface outside the VDA bend and centered at the bend vertex of the VDA bend) in the L section in the overlapping area of ​​the V-bend ridgeline and the VDA bend ridgeline was 50 μm to the left and right of the steel sheet surface outside the VDA bend. Figure 2-3 The method for measuring the change in the average grain size of bainitic ferrite in the plate thickness direction before and after processing in the AB region (hereinafter referred to as the AB region) represented by the dotted line in (f) is described.

[0292] Here, first, for the AB region, in reference Figure 2-4 . Figure 2-4 is a schematic diagram for explaining the AB region. Figure 2-4 As shown, the AB area refers to the bending vertex t from the outer side of the bending at VDA. 0 The starting line L is a starting point and exists at a position of 50 μm in the plate thickness direction. 0 Each position plays a role in starting line L 0 The area formed is located at a position of 50 μm on both sides in the vertical direction.

[0293] As a method for measuring the above-mentioned amount of change, first, using a sample in which the AL plane is used as the observation plane after the above-mentioned V-VDA bending test to the highest load point (hereinafter also referred to as a deformed sample) and a sample used in the measurement of the area ratio of the above-mentioned steel plate structure (hereinafter also referred to as a sample before deformation), 5 fields of view of 25.6μm×17.6μm are photographed for each sample using a SEM (Scanning Electron Microscope) under the conditions of an acceleration voltage of 15kV and a magnification of 3000 times. For the deformed sample, the AB region is photographed to observe the bainitic ferrite that has been deformed by processing (hereinafter also referred to as the deformed bainitic ferrite). For the sample before deformation, the position from the surface of the base steel plate to the plate thickness of 50μm is photographed to observe the undeformed bainitic ferrite (hereinafter also referred to as the bainitic ferrite before deformation).

[0294] From the obtained structure image, 10 bainitic ferrites after deformation and 10 bainitic ferrites before deformation were randomly extracted by hand drawing, and the longest part of each bainitic ferrite in the plate thickness direction was taken as the grain size.

[0295] The particle sizes in the plate thickness direction of the 10 deformed bainitic ferrites and the 10 undeformed bainitic ferrites obtained were averaged respectively, and the value obtained by dividing the average particle size in the plate thickness direction of the bainitic ferrite before deformation by the average particle size in the plate thickness direction of the bainitic ferrite after deformation (the ratio of the average particle size before processing to the average particle size after processing: average particle size before processing (nm) / average particle size after processing (nm)) was used as the change in the average particle size in the plate thickness direction of the bainitic ferrite before and after processing.

[0296] An example of an image of bainitic ferrite before deformation and bainitic ferrite after deformation is shown in FIG. Figure 5 middle. Figure 5 In the figure, symbol BF1 represents bainitic ferrite before deformation, and symbol BF2 represents bainitic ferrite after deformation.

[0297] In addition, regarding one bainitic ferrite, a granular region which is surrounded by other structures on the periphery and formed integrally without interruption in the SEM image is regarded as one and measured.

[0298] Next, the mechanical properties of the steel sheet according to one embodiment of the present invention will be described.

[0299] Tensile strength (TS): 1180MPa or more

[0300] The tensile strength TS of the steel sheet according to one embodiment of the present invention is 1180 MPa or more. The upper limit is not particularly limited, but the tensile strength TS is preferably less than 1470 MPa.

[0301] It should be noted that the yield stress (YS), total elongation (El), limit hole expansion ratio (λ), limit bending angle (α) in the VDA bending test, and stroke (S) at maximum load in the V-VDA bending test of the steel sheet according to one embodiment of the present invention are as follows: Fmax ) and whether there is shaft crush fracture, as described above.

[0302] In addition, the tensile strength (TS), yield stress (YS) and total elongation (El) are measured by a tensile test based on JIS Z 2241 (2011) described later in the examples. The limiting hole expansion ratio (λ) is measured by a hole expansion test based on JIS Z 2256 (2020) described later in the examples. The limiting bending angle (α) in the VDA bending test is measured by a VDA bending test based on VDA238-100 described later in the examples. The stroke (S) at the maximum load in the V-VDA bending test Fmax ) was measured by the V-VDA bending test described later in the examples. The presence or absence of axial crush fracture was measured by the axial crush test described later in the examples.

[0303] Zinc coating (second coating)

[0304] According to one embodiment of the present invention, a steel plate may have a zinc coating as the outermost layer formed on a base steel plate (on the surface of the base steel plate or on the surface of the metal coating when a metal coating is formed). The zinc coating may be provided on only one surface of the base steel plate or on both surfaces.

[0305] That is, the steel sheet of the present invention may have a base steel sheet and a second coating (zinc coating) formed on the base steel sheet, or may have a base steel sheet and a metal coating (a first coating (excluding the zinc coating, i.e., the second coating)) and a second coating (zinc coating) sequentially formed on the base steel sheet.

[0306] The steel sheet having the zinc coating may be a galvanized steel sheet.

[0307] It should be noted that the zinc plating layer mentioned here refers to a plating layer containing Zn as a main component (Zn content is 50.0% or more), and examples thereof include a hot-dip galvanized layer and an alloyed hot-dip galvanized layer.

[0308] Here, the hot-dip galvanized layer is preferably composed of, for example, Zn and 20.0% by mass or less of Fe, and 0.001% by mass or more and 1.0% by mass or less of Al. In addition, the hot-dip galvanized layer may optionally contain a total of 0.0% by mass or more and 3.5% by mass or less of one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM. In addition, the Fe content of the hot-dip galvanized layer is more preferably less than 7.0% by mass. It should be noted that the remainder other than the above elements is an inevitable impurity.

[0309] In addition, the alloyed hot-dip galvanneal layer is preferably composed of, for example, Zn and Fe of less than 20.0 mass %, and Al of more than 0.001 mass % and less than 1.0 mass %. In addition, in the alloyed hot-dip galvanneal layer, one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM, which are totaling more than 0.0 mass % and less than 3.5 mass %, may be optionally contained. The Fe content of the alloyed hot-dip galvanneal layer is more preferably more than 7.0 mass %, and more preferably more than 8.0 mass %. In addition, the Fe content of the alloyed hot-dip galvanneal layer is more preferably less than 15.0 mass %, and more preferably less than 12.0 mass %. It should be noted that the remainder other than the above-mentioned elements is an inevitable impurity.

[0310] The coating weight per single side of the zinc coating is not particularly limited, but is preferably set to 20 g / m 2 In addition, the coating weight per single side of the zinc coating is preferably set to 80 g / m 2 the following.

[0311] In addition, the coating adhesion amount of the zinc plating layer was measured as follows.

[0312] That is, a treatment solution was prepared by adding 0.6 g of a corrosion inhibitor for Fe (manufactured by Asahi Chemical Industry Co., Ltd., "IBIT 700BK" (registered trademark)) to 1 L of a 10% by mass hydrochloric acid aqueous solution. Next, the steel plate as the test material was immersed in the treatment solution to dissolve the zinc coating. Then, the mass reduction of the test material before and after dissolution was measured, and the value was divided by the surface area of ​​the base steel plate (the surface area of ​​the portion covered by the coating), thereby calculating the coating adhesion (g / m 2 ).

[0313] It should be noted that the thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably 0.5 mm or more, and more preferably 0.6 mm or more.

[0314] The plate thickness is more preferably greater than 0.8 mm. The plate thickness is more preferably 0.9 mm or more. The plate thickness is more preferably 1.0 mm or more. The plate thickness is more preferably 1.2 mm or more.

[0315] In addition, the thickness of the steel plate is preferably 3.5 mm or less, and more preferably 2.3 mm or less.

[0316] The width of the steel plate of the present invention is not particularly limited, but is preferably set to 500 mm or more, more preferably 750 mm or more. The width of the steel plate is preferably set to 1600 mm or less, more preferably 1450 mm or less.

[0317] [2. Method for manufacturing steel plate]

[0318] Next, a method for producing a steel sheet according to an embodiment of the present invention will be described.

[0319] The method for manufacturing a steel plate according to one embodiment of the present invention comprises: a hot rolling process, wherein a steel slab having the above-mentioned composition is hot-rolled to form a hot-rolled steel plate; a pickling process, wherein the hot-rolled steel plate is pickled; an annealing process, wherein the steel plate after the pickling process is annealed at an annealing temperature: (Ac 1 +0.4×(Ac 3 -Ac 1 ))℃ to 900℃ and annealing time: 20 seconds or more; a first cooling step, wherein the steel plate after the annealing step is cooled to a first cooling stop temperature of 400℃ to 600℃; a second cooling step, wherein the steel plate after the first cooling step is cooled to a second cooling stop temperature of 100℃ to 300℃ at an average cooling rate of 25.0℃ / second or less, and during the cooling, the steel plate is subjected to 2.0kgf / mm2 more than once in a temperature range of 300℃ to 450℃ or less. 2 The steel sheet is subjected to a tension of at least 1000 mm and a tension of at least 2000 mm, and then subjected to a treatment of at least 4 passes while contacting a roll having a diameter of at least 500 mm and at most 1500 mm for 1 / 4 of a roll circle per pass, and subjected to a treatment of at least 2 passes while contacting a roll having a diameter of at least 500 mm and at most 1500 mm for 1 / 2 of a roll circle per pass; and a reheating process, wherein the steel sheet after the second cooling process is subjected to a reheating process in which the steel sheet is heated to a tempering temperature in a temperature range of greater than 300°C and less than 500°C and the tempering time is maintained in the above temperature range for a tempering time of at least 20 seconds and less than 900 seconds, and during the reheating process, a carbide control parameter CP represented by the following formula (1) is set to a value of at least 10000 and at most 15000, or further comprising a cold rolling process in which the steel sheet after the pickling process and before the annealing process is cold rolled to obtain a cold-rolled steel sheet.

[0320] CP = (T + 273) × (k + 1.2 × logt) ... Formula (1)

[0321] Here, T: tempering temperature (°C), k: material constant depending on the C content, t: tempering time (seconds),

[0322] k=-6×C M +17.8,

[0323] C M : Carbon content (mass %) in martensite generated in the second cooling step.

[0324] It should be noted that the above-mentioned temperatures indicate the surface temperatures of the steel slab and the steel plate unless otherwise specified.

[0325] First, a steel billet having the above-mentioned composition is prepared. For example, a steel raw material is melted to form molten steel having the above-mentioned composition. The smelting method is not particularly limited, and known smelting methods such as converter smelting and electric furnace smelting can be used. Next, the obtained molten steel is solidified to form a steel billet. There is no particular limitation on the method for obtaining a steel billet from molten steel, and for example, a continuous casting method, an ingot casting method, or a thin slab casting method can be used. From the viewpoint of preventing macrosegregation, a continuous casting method is preferred.

[0326] [Hot rolling process]

[0327] Next, in the hot rolling step, the steel slab is hot rolled to produce a hot-rolled steel sheet.

[0328] Hot rolling can be carried out by applying an energy-saving process. Examples of energy-saving processes include direct rolling (a method in which the steel slab is directly loaded into a heating furnace in a warm state without cooling it to room temperature and hot rolled) and direct rolling (a method in which the steel slab is rolled immediately after being slightly kept hot).

[0329] There are no particular restrictions on hot rolling conditions, but the hot rolling can be carried out under the following conditions, for example.

[0330] That is, the billet is temporarily cooled to room temperature, and then rolled after reheating. From the viewpoint of melting of carbides and reduction of rolling load, the billet heating temperature (reheating temperature) is preferably set to 1100°C or above. In addition, in order to prevent the increase of oxide scale loss, the billet heating temperature is preferably set to 1300°C or below. It should be noted that the billet heating temperature is based on the temperature of the billet surface.

[0331] Next, the steel billet is rough-rolled according to a conventional method to form a rough-rolled plate (hereinafter also referred to as a thin slab). Next, the thin slab is finish-rolled to form a hot-rolled steel plate. It should be noted that, when the heating temperature of the steel billet is lowered, from the viewpoint of preventing failures during finish rolling, it is preferred to heat the thin slab using a rod heater or the like before finish rolling. In order to reduce the rolling load, the finish rolling temperature is preferably set to above 800°C. In addition, when the reduction rate of austenite in the unrecrystallized state becomes higher, it is possible that an abnormal structure extending in the rolling direction develops, thereby reducing the workability of the annealed sheet. In addition, by setting the finish rolling temperature to above 800°C, the steel structure at the hot-rolled steel plate stage and the steel structure of the final product are easily uniform. It should be noted that when the steel structure becomes uneven, there is a tendency for the bendability to decrease.

[0332] On the other hand, when the finishing temperature is higher than 950°C, the amount of oxide (scale) generated increases. As a result, the interface between the steel base and the oxide may become rough, and the surface quality of the steel plate after pickling and cold rolling may deteriorate. In addition, since the grains become coarse, it may also become a cause of reducing the strength and bendability of the steel plate. Therefore, the finishing temperature is preferably set to a range of 950°C or more. In summary, the finishing temperature is preferably set to a range of 800°C or more and 950°C or more.

[0333] After the finish rolling, the hot rolled steel sheet is coiled. The coiling temperature is preferably set to 450° C. or higher. Alternatively, the coiling temperature is preferably set to 750° C. or lower.

[0334] It should be noted that the thin slabs can also be joined to each other during hot rolling and continuously finished rolling. In addition, the thin slabs can also be temporarily coiled before finishing rolling. In addition, in order to reduce the rolling load during hot rolling, part or all of the finishing rolling can also be set to lubrication rolling. From the perspective of uniformity of the shape of the steel plate and uniformity of the material, it is also effective to carry out lubrication rolling. It should be noted that the friction coefficient during lubrication rolling is preferably set to a range of 0.10 or more and 0.25 or less.

[0335] In the hot rolling process (hot rolling step) including rough rolling and finish rolling, the steel billet is usually rough rolled to become a thin slab, and the steel billet is finish rolled to become a hot rolled steel plate. However, depending on the grinding capacity, etc., there is no problem as long as the specified size is reached.

[0336] [Pickling process]

[0337] The hot rolled steel sheet after the hot rolling process is pickled. The pickling can remove the oxides on the surface of the steel sheet, and can ensure good chemical conversion treatment and coating quality. It should be noted that the pickling can be performed only once or in multiple times. There is no particular restriction on the pickling conditions, as long as they are performed according to conventional methods.

[0338] [Cold rolling process]

[0339] Next, the hot-rolled steel sheet is subjected to cold rolling as necessary to obtain a cold-rolled steel sheet. The cold rolling is performed by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.

[0340] The cold rolling reduction ratio (cumulative reduction ratio) is not particularly limited, but is preferably set to 20% or more. In addition, the cold rolling reduction ratio is preferably set to 80% or less. When the cold rolling reduction ratio is less than 20%, the steel structure is likely to be coarsened and non-uniform in the annealing process, and the TS and bendability of the final product may be reduced. On the other hand, when the cold rolling reduction ratio is greater than 80%, the shape of the steel sheet is likely to be poor, and the amount of zinc coating adhesion may become non-uniform.

[0341] In addition, the cold-rolled steel sheet obtained after cold rolling may be optionally subjected to pickling.

[0342] [Metal plating (electrometal plating, first plating) step]

[0343] In one embodiment of the present invention, it may include a first plating process of plating metal on one or both sides of a steel plate after a hot rolling process (after a pickling process, and after a cold rolling process after the pickling process when cold rolling is performed) and before an annealing process to form a metal plating layer (first plating layer).

[0344] For example, the surface of the hot-rolled steel sheet or cold-rolled steel sheet obtained as above can be subjected to electroplating metal treatment to produce a pre-annealing electroplated metal steel sheet having a pre-annealing metal plating layer formed on at least one side. It should be noted that the metal plating mentioned here does not include zinc plating (second plating).

[0345] The electroplating metal treatment method is not particularly limited, but as described above, the metal plating layer formed on the base steel sheet is preferably a metal electroplating layer, and therefore, the electroplating metal treatment is preferably performed.

[0346] For example, the Fe-based electroplating bath may be a sulfuric acid bath, a hydrochloric acid bath, or a mixture of the two. In addition, the amount of metal plating layer attached before annealing can be adjusted according to the power-on time, etc. It should be noted that the metal-plated steel sheet before annealing means that the metal plating layer has not undergone an annealing process, and does not exclude the method of pre-annealing the hot-rolled steel sheet before the metal plating treatment, the pickling treatment plate after hot rolling, or the cold-rolled steel sheet.

[0347] Here, the metal type of the electroplating layer can be any one of Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Rt, Au, Hg, Ti, Pb, and Bi, and Fe is more preferred. Therefore, the manufacturing method of the Fe-based electroplating layer is described below, but the following conditions for the Fe-based electroplating layer can also be adopted for other metal-based electroplating layers.

[0348] The Fe ion content in the Fe-based electroplating bath before power on is expressed as Fe 2+ The Fe ion content in the Fe-based electroplating bath is expressed as Fe 2+ When it is 0.5 mol / L or more, a sufficient Fe deposition amount can be obtained. In order to obtain a sufficient Fe deposition amount, the Fe ion content in the Fe-based electroplating bath before the start of energization is preferably set to 2.0 mol / L or less.

[0349] In addition, Fe ions and at least one element selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co can be contained in the Fe-based electroplating bath. The total content of these elements in the Fe-based electroplating bath is preferably set in a manner that the total content of these elements in the Fe-based electroplating layer before annealing is less than 10% by mass. It should be noted that the metal element can be contained in the form of metal ions, and the non-metallic element can be contained as a part of boric acid, phosphoric acid, nitric acid, organic acid, etc. In addition, the iron sulfate plating solution can also contain conductive additives such as sodium sulfate and potassium sulfate, chelating agents, and pH buffers.

[0350] There are no special restrictions on other conditions of the Fe-based electroplating bath. Considering the constant temperature maintenance, the temperature of the Fe-based electroplating solution is preferably set to above 30°C, preferably below 85°C. There are no special restrictions on the pH of the Fe-based electroplating bath, but from the viewpoint of preventing the reduction of current efficiency caused by the generation of hydrogen, it is preferably set to above 1.0. In addition, considering the conductivity of the Fe-based electroplating bath, it is preferably below 3.0. Regarding the current density, from the viewpoint of productivity, it is preferably set to 10A / dm 2 From the viewpoint of easy control of the adhesion amount of the Fe-based electroplating layer, it is preferably set to 150 A / dm 2 The sheet passing speed is preferably set to 5 mpm or more from the viewpoint of productivity, and is preferably set to 150 mpm or less from the viewpoint of stably controlling the amount of adhesion.

[0351] It should be noted that as a treatment before the Fe-based electroplating treatment, degreasing treatment and water washing for cleaning the surface of the steel plate, and pickling treatment and water washing for activating the surface of the steel plate can be implemented. Following these pretreatments, the Fe-based electroplating treatment is implemented. The methods of degreasing treatment and water washing are not particularly limited, and the usual methods can be used. In the pickling treatment, various acids such as sulfuric acid, hydrochloric acid, nitric acid and mixtures thereof can be used. Among them, sulfuric acid, hydrochloric acid or a mixture thereof is preferred. The concentration of the acid is not particularly limited, but if the ability to remove the oxide film and the prevention of surface roughness (surface defects) caused by excessive pickling are considered, it is preferably about 1% by mass to about 20% by mass. In addition, the pickling treatment solution may contain a defoaming agent, a pickling accelerator, a pickling inhibitor, etc.

[0352] [Annealing process]

[0353] Next, in one embodiment of the present invention, after the pickling step (after the cold rolling step in the case of cold rolling, after the metal plating (first plating) step in the case of metal plating to form a metal plating layer (first plating layer), after the metal plating (first plating) step in the case of cold rolling and metal plating), the steel sheet obtained as above is annealed at a temperature of: (Ac 1 +0.4×(Ac 3 -Ac 1 The annealing is performed under the conditions of a temperature of not less than 900° C. and not more than 900° C. and an annealing time of not less than 20 seconds. It should be noted that the number of annealing may be two or more times, but from the viewpoint of energy efficiency, it is preferably one time.

[0354] Annealing temperature: (Ac 1 +0.4×(Ac 3 -Ac 1 ))℃ or above and below 900℃

[0355] Annealing temperature is lower than (Ac 1 +0.4×(Ac 3 -Ac 1 ))℃, the formation ratio of austenite in the two-phase region of ferrite and austenite during heating becomes insufficient. Therefore, the area ratio of ferrite after annealing increases excessively, and YS decreases. In addition, the C concentration in austenite increases excessively during annealing, and the desired λ and S cannot be achieved. Fmax In addition, it is difficult to make TS more than 1180 MPa.

[0356] On the other hand, when the annealing temperature is higher than 900℃, excessive grain growth of austenite occurs and M S The annealing temperature is set to (Ac 1+0.4×(Ac 3 -Ac 1 ))°C or more and 900°C or less. The annealing temperature is preferably 880°C or less. The annealing temperature is more preferably 870°C or less. In addition, the annealing temperature is preferably (Ac 1 +0.5×(Ac 3 -Ac 1 ))℃ or more, more preferably (Ac 1 +0.6×(Ac 3 -Ac 1 ))℃ or above.

[0357] It should be noted that the annealing temperature is the highest temperature reached in the annealing process.

[0358] Ac 1 Point (℃) and Ac 3 The temperature (°C) is calculated by the following formula:

[0359] Ac 1 Point (℃) = 727.0-32.7×[%C]+14.9×[%Si]+2.0×[%Mn]

[0360] Ac 3 Point (℃) = 912.0-230×[%C]+31.6×[%Si]-20.4×[%Mn]

[0361] Here, [%C]: C content (mass %), [%Si]: Si content (mass %), and [%Mn]: Mn content (mass %).

[0362] Annealing time: more than 20 seconds

[0363] When the annealing time is less than 20 seconds, the formation ratio of austenite in the heating of the two-phase region of ferrite and austenite becomes insufficient. Therefore, the area ratio of ferrite increases excessively after annealing, and YS decreases. In addition, the C concentration in austenite increases excessively during annealing, and the desired λ and S cannot be achieved. Fmax . In addition, it is difficult to make TS above 1180 MPa. Therefore, the annealing time is set to be 20 seconds or more. The annealing time is preferably 30 seconds or more, and more preferably 50 seconds or more. It should be noted that there is no particular upper limit to the annealing time, but the annealing time is preferably set to be 900 seconds or less, and more preferably 800 seconds or less. The annealing time is further preferably set to be 300 seconds or less, and further more preferably 220 seconds or less.

[0364] It should be noted that the annealing time refers to the holding time in the temperature range of (annealing temperature - 40 ° C) or more and below the annealing temperature. That is, the annealing time includes not only the holding time at the annealing temperature, but also the residence time in the temperature range of (annealing temperature - 40 ° C) or more and below the annealing temperature during heating and cooling before and after reaching the annealing temperature.

[0365] Dew point of annealing atmosphere in annealing process: -30℃ or above

[0366] In one embodiment of the present invention, the dew point of the atmosphere (annealing atmosphere) in the annealing process is preferably set to -30°C. By setting the dew point of the annealing atmosphere in the annealing process to -30°C or higher for annealing, the decarburization reaction can be promoted, and a deeper surface soft layer can be formed. The dew point of the annealing atmosphere in the annealing process is more preferably -25°C or higher, further preferably -15°C or higher, and most preferably higher than -5°C.

[0367] The upper limit of the dew point of the annealing atmosphere in the annealing step is not particularly specified, but in order to appropriately prevent oxidation of the Fe-based electroplating layer surface and improve the coating adhesion when the zinc plating layer is provided, the dew point of the annealing atmosphere in the annealing step is preferably set to 30° C. or less.

[0368] [First Cooling Step]

[0369] Next, the steel sheet annealed as described above is cooled to a first cooling stop temperature of 400° C. to 600° C. inclusive.

[0370] First cooling stop temperature: 400°C or higher and 600°C or lower

[0371] When the first cooling stop temperature is lower than 400°C, the area ratio of bainitic ferrite increases excessively, the volume ratio of retained austenite exceeds a predetermined amount, and the desired λ and S cannot be achieved. Fmax On the other hand, when the first cooling stop temperature is higher than 600°C, the area ratio of pearlite increases, so there is a case where the strength decreases. Therefore, the first cooling stop temperature is set to be 400°C or higher and 600°C or lower. The first cooling stop temperature is preferably 460°C or higher. In addition, the first cooling stop temperature is preferably set to be 550°C or lower.

[0372] [Maintaining step (preferred requirements)]

[0373] After the first cooling step, a holding step of holding the steel plate in a temperature range of 400° C. to 600° C. (hereinafter referred to as a holding temperature range) for less than 80 seconds may be performed as needed. Here, the holding temperature range may be set to the first cooling stop temperature.

[0374] Keeping time within temperature range: less than 80 seconds

[0375] In the holding step, bainitic ferrite is generated, and diffusion of C from the generated bainitic ferrite to untransformed austenite adjacent to the bainitic ferrite occurs. As a result, a predetermined amount of area ratio of retained austenite can be ensured.

[0376] Here, when the holding time in the holding temperature range is 80 seconds or more, the area ratio of bainitic ferrite may increase excessively, and YS may decrease. In addition, the diffusion of C from bainitic ferrite to untransformed austenite may occur excessively, and the area ratio of retained austenite may exceed 15.0%, and the desired λ and S may not be achieved. Fmax Therefore, the holding time within the temperature range is preferably set to less than 80 seconds. The holding time within the temperature range is more preferably less than 60 seconds. It should be noted that the holding time within the temperature range does not include the residence time within the temperature range after the galvanizing treatment is performed in the plating process.

[0377] [Galvanizing step (second plating step)]

[0378] The steel sheet may be subjected to galvanizing after the first cooling step (after the holding step in the case of the holding step). By performing the galvanizing, a galvanized steel sheet may be obtained. Examples of the galvanizing include hot-dip galvanizing and alloy galvanizing.

[0379] In the case of hot-dip galvanizing, it is preferred to immerse the steel sheet in a galvanizing bath at 440°C or higher and 500°C or lower, and then adjust the coating adhesion by gas wiping or the like. The hot-dip galvanizing bath is not particularly limited as long as the composition of the zinc coating can be obtained. For example, it is preferred to use a galvanizing bath having an Al content of 0.10% by mass or higher and the balance consisting of Zn and unavoidable impurities. The Al content is preferably 0.23% by mass or lower.

[0380] In the case of galvannealing, it is preferred that the galvanized steel sheet be heated to an alloying temperature of 450° C. or higher after the hot-dip galvanizing treatment as described above. The alloying temperature is preferably set to 600° C. or lower.

[0381] When the alloying temperature is lower than 450°C, the Zn-Fe alloying speed slows down, and alloying becomes difficult. On the other hand, when the alloying temperature is higher than 600°C, the untransformed austenite transforms to pearlite, making it difficult to make TS above 1180MPa, and the ductility decreases. It should be noted that the alloying temperature is more preferably above 470°C. In addition, the alloying temperature is more preferably below 570°C.

[0382] In addition, the coating weight of hot-dip galvanized steel sheet (GI) and alloyed hot-dip galvannealed steel sheet (GA) is preferably set to 20 g / m per single side. 2 In addition, the coating weight per single side of the zinc coating is preferably set to 80 g / m 2 It should be noted that the coating adhesion amount can be adjusted by gas wiping or the like.

[0383] [Second Cooling Step]

[0384] Next, the steel sheet after the first cooling step is cooled to a second cooling stop temperature of 100° C. to 300° C. at an average cooling rate of 25.0° C. / sec or less.

[0385] Second cooling stop temperature: 100°C or higher and 300°C or lower

[0386] The second cooling step is a necessary step for controlling the area ratio of tempered martensite and the area ratio of retained austenite generated in the reheating step as a subsequent step within a specified range. Here, when the second cooling stop temperature is lower than 100°C, almost all of the untransformed austenite present in the steel is transformed into martensite in the second cooling step. As a result, the area ratio of the final tempered martensite increases excessively, making it difficult to obtain retained austenite greater than 3.0%, and the ductility decreases. On the other hand, when the second cooling stop temperature is higher than 300°C, the area ratio of tempered martensite decreases, and the area ratio of fresh martensite increases. In addition, the amount of diffusible hydrogen in the steel plate sometimes increases. As a result, the desired λ and S cannot be achieved. Fmax In addition, the desired α may not be obtained. Therefore, the second cooling stop temperature is set to 100° C. or higher and 300° C. or lower. The second cooling stop temperature is preferably 120° C. or higher. In addition, the first cooling stop temperature is preferably 280° C. or lower.

[0387] Average cooling rate in the second cooling step: 25.0°C / sec or less

[0388] When the cooling rate of the second cooling step is higher than 25.0°C / sec, fine carbides are generated and the density of carbides in the tempered martensite exceeds a predetermined value. As a result, the desired λ and S cannot be achieved. Fmax In addition, the desired α may not be obtained. Therefore, the average cooling rate in the second cooling step is set to 25.0° C. / sec or less.

[0389] Here, the average cooling rate can be calculated by "(cooling start temperature (° C.)-second cooling stop temperature (° C.)" / cooling time (s)".

[0390] In the present invention, when the steel sheet is cooled to the second cooling stop temperature, a 2.0 kgf / mm2 pressure is applied at least once in a temperature range of 300°C to 450°C. 2 Then, the steel sheet to which the tension is applied is subjected to treatment of applying 4 or more passes while making the steel sheet contact a roll having a diameter of 500 mm or more and 1500 mm or less for 1 / 4 of the roll's revolution in each pass, and the steel sheet is subjected to treatment of applying 2 or more passes while making the steel sheet contact a roll having a diameter of 500 mm or more and 1500 mm or less for 1 / 2 of the roll's revolution in each pass.

[0391] As described above, by applying 2.0 kgf / mm 2 With the above tension and the specified number of passes, the retained austenite excessively generated in the steel sheet structure undergoes a work-induced transformation to martensite, and then to tempered martensite during subsequent cooling. As a result, the desired λ and S can be obtained. Fmax .

[0392] The number of passes applied to the steel sheet while contacting the roll for 1 / 4 of its rotation is preferably 5 or more, more preferably 6 or more.

[0393] The upper limit is not particularly limited, but the number of passes applied to the steel sheet while contacting the roll for 1 / 4 of the roll is preferably 12 passes or less, more preferably 10 passes or less.

[0394] The number of passes applied to the steel sheet while the contact roll performs 1 / 2 of the rotation is preferably 3 or more, more preferably 4 or more.

[0395] The upper limit is not particularly limited, but the number of passes applied to the steel sheet while the contact roll is 1 / 2 of the rotation is preferably 6 or less, more preferably 5 or less.

[0396] Here, the tension is calculated by dividing the total value of the load (kgf) of the load cells on the left and right sides of the roll by the cross-sectional area of ​​the steel plate (=plate thickness (mm) × plate width (mm)) (mm 2 It should be noted that the force sensor needs to be arranged parallel to the tension direction.

[0397] Here, the configuration position of the load cell is preferably set at a position 200 mm away from both ends of the roller. In addition, the roller body length of the roller used is preferably set to be more than 1500 mm. In addition, the roller body length of the roller used is preferably set to be less than 2500 mm.

[0398] In addition, the tension is preferably 2.2 kgf / mm 2 More preferably 2.4 kgf / mm 2 above.

[0399] In addition, the tension is preferably 15.0 kgf / mm 2 Below, more preferably 10.0kgf / mm 2 The tension is more preferably 7.0 kgf / mm 2 Below, more preferably 4.0 kgf / mm 2 the following.

[0400] In addition, applying tension once or more, for example, applying tension twice means first applying 2.0 kgf / mm 2 Then, the tension becomes less than 2.0kgf / mm 2 After that, apply 2.0kgf / mm for the second time 2 In addition, applying tension three times means that first, 2.0 kgf / mm is applied once. 2 Then, the tension becomes less than 2.0kgf / mm 2 After that, apply 2.0kgf / mm for the second time 2 Then, the tension becomes less than 2.0kgf / mm 2 After that, apply 2.0kgf / mm for the third time 2 More tension.

[0401] [Reheating process]

[0402] Next, the steel sheet is reheated to a temperature range of 300°C to 500°C (hereinafter also referred to as a reheating temperature range), and the steel sheet is maintained in the temperature range of 300°C to 500°C for 20 seconds to 900 seconds.

[0403] Thus, the martensite present in the steel is tempered at the end of the second cooling step. In addition, the C supersaturated solid solution in the martensite is diffused into the untransformed austenite, thereby generating austenite stable at room temperature, that is, retained austenite.

[0404] Reheating temperature (tempering temperature): above 300℃ and below 500℃

[0405] When the reheating temperature (tempering temperature) is 300°C or lower, the tempering of martensite present in the steel at the end of the second cooling step is not sufficiently performed, the fresh martensite increases excessively, and the coarsening of carbides in the tempered martensite is not sufficiently performed, and the density of carbides in the tempered martensite exceeds a prescribed amount. As a result, the desired λ, α and S cannot be achieved. Fmax Furthermore, the hydrogen contained in the base steel sheet is not sufficiently released to the outside, and the amount of diffusible hydrogen in the base steel sheet increases. As a result, the hole expandability and bendability are further reduced.

[0406] On the other hand, when the reheating temperature (tempering temperature) is higher than 500°C, the tempering of the martensite present in the steel at the end of the second cooling process is excessively performed, so it is difficult to make TS above 1180MPa. In addition, the untransformed austenite present in the steel at the end of the second cooling process is decomposed into carbides (pearlite), so the ductility is reduced. In addition, there are also cases where the external release of hydrogen contained in the base steel plate becomes insufficient and the amount of diffusible hydrogen in the base steel plate increases. Therefore, the hole expandability is reduced. Therefore, the reheating temperature is set to be higher than 300°C and lower than 500°C. The reheating temperature is the highest temperature reached in the reheating process. The reheating temperature is preferably above 340°C, and more preferably above 360°C. The reheating temperature is preferably below 460°C, and more preferably below 440°C.

[0407] Holding time in the reheating temperature range (tempering time): 20 seconds or more and 900 seconds or less

[0408] When the holding time (tempering time) in the reheating temperature range is less than 20 seconds, the tempering of the martensite present in the steel at the end of the second cooling step is not sufficiently performed, and the fresh martensite increases excessively. In addition, the coarsening of the carbides in the tempered martensite may not be sufficiently performed, and the density of the carbides in the tempered martensite may exceed the specified amount. As a result, the desired λ, α and S cannot be achieved. Fmax Furthermore, the hydrogen contained in the base steel sheet is not sufficiently released to the outside, and the amount of diffusible hydrogen in the base steel sheet increases. As a result, the hole expandability and bendability are further reduced.

[0409] On the other hand, when the holding time (tempering time) in the reheating temperature range is greater than 900 seconds, the tempering of the martensite present in the steel at the end of the second cooling process is excessively performed, so it is difficult to make TS more than 1180MPa. In addition, the untransformed austenite present in the steel at the end of the second cooling process is decomposed into carbides (pearlite), so the ductility is reduced. Therefore, the holding time in the reheating temperature range is set to more than 20 seconds and less than 900 seconds. The holding time is preferably more than 30 seconds, more preferably more than 40 seconds. The holding time is preferably less than 500 seconds, more preferably less than 100 seconds.

[0410] The holding time in the reheating temperature range includes, in addition to the holding time at the reheating temperature, the residence time in the temperature range during heating and cooling before and after reaching the reheating temperature.

[0411] Carbide control parameter CP during reheating: 10000 or more and 15000 or less

[0412] When the carbide control parameter CP during reheating is less than 10000, the tempering of martensite present in the steel at the end of the second cooling step is not sufficiently performed, the fresh martensite increases excessively, and the coarsening of carbides in the tempered martensite is not sufficiently performed, and the density of carbides in the tempered martensite exceeds the prescribed amount. As a result, the desired λ, α and S cannot be achieved. Fmax Furthermore, the hydrogen contained in the base steel sheet is not sufficiently released to the outside, and the amount of diffusible hydrogen in the base steel sheet increases. As a result, the hole expandability and bendability are further reduced.

[0413] On the other hand, when the carbide control parameter CP during reheating is greater than 15000, the tempering of the martensite present in the steel at the end of the second cooling process is excessively performed, so it is difficult to make TS 1180 MPa or more. In addition, the untransformed austenite present in the steel at the end of the second cooling process is decomposed into carbides (pearlite), so the ductility is reduced. Therefore, the carbide control parameter CP during reheating is set to be greater than 10000 and less than 15000.

[0414] The carbide control parameter CP during reheating is preferably 11000 or more, and more preferably 12000 or more. The carbide control parameter CP during reheating is preferably 14500 or less, and more preferably 14000 or less.

[0415] The carbide control parameters during reheating are calculated by the following formula:

[0416] CP = (T + 273) × (k + 1.2 × logt) ... Formula (1)

[0417] Here, CP: carbide control parameter, T: tempering temperature (°C), k: material constant depending on the C content, t: tempering time (seconds), and 1.2 in the term 1.2×logt is a correction factor (predetermined correction factor) taking into account the cooling time after the reheating process.

[0418] In addition, the material constant k is calculated by the following formula:

[0419] k=-6×C M +17.8

[0420] Here, C M : Carbon content (mass %) in martensite generated in the second cooling step.

[0421] In addition, the carbon amount in the martensite produced|generated in the 2nd cooling process can be measured as follows.

[0422] First, the carbon amount in each phase immediately before the second cooling step satisfies the following relationship.

[0423] V γ1×C γ1 +V F ×C F +V BF ×C BF =C T …Formula (2)

[0424] V γ1 =1-V F -V BF …Formula (3)

[0425] Here, V γ1 , C γ1 is the area ratio (%) of the untransformed austenite immediately before the second cooling step and the carbon concentration (mass %) in the untransformed austenite,

[0426] V F , C F is the area ratio (%) of ferrite immediately before the second cooling step and the carbon concentration (mass %) in the ferrite,

[0427] V BF , C BF are the area ratio (%) of bainitic ferrite and the carbon concentration (mass %) in the bainitic ferrite immediately before the second cooling step,

[0428] C T It is the carbon concentration (mass %) in the steel (immediately before the second cooling step).

[0429] In addition, the area ratio V of ferrite immediately before the second cooling step is F (%) and the area ratio V of bainitic ferrite BF (%) may be the same as the area ratio (%) of ferrite and the area ratio (%) of bainitic ferrite in the final structure (the steel structure of the steel sheet finally obtained). In addition, the carbon concentration C in ferrite F (mass %) and the carbon concentration C in bainitic ferrite BF (mass %) can be set to zero.

[0430] Therefore, formula (2) is:

[0431] C T =V γ1 ×C γ1 +V F ×0(zero)+V BF ×0(zero)=V γ1 ×C γ1 …Formula (2-2).

[0432] In addition, according to formula (2-2) and formula (3), C T =(1-VF -V BF )×C γ1 , so it can be set to C γ1 =C T / (1-V F -V BF ). In addition, the phase transformation from γ1 (untransformed austenite) to martensite in the second cooling step is a phase transformation not accompanied by the diffusion of C, so C M =C γ1 , that is, C M =C γ1 =C T / (1-V F -V BF ).

[0433] The cooling conditions after holding within the reheating temperature range are not particularly limited, as long as they are in accordance with conventional methods. As cooling methods, for example, gas jet cooling, spray cooling, roller cooling, water cooling, and air cooling can be applied. In addition, from the viewpoint of preventing surface oxidation, it is preferred to cool to below 50°C after holding within the reheating temperature range, and more preferably to about room temperature. The average cooling rate of cooling after holding within the reheating temperature range is, for example, preferably above 1°C / second and below 50°C / second.

[0434] In addition, the steel plate obtained as above may be further subjected to temper rolling. When the reduction rate of temper rolling is greater than 2.00%, the yield stress may increase, and the dimensional accuracy when the steel plate is formed into a component may decrease. Therefore, the reduction rate of temper rolling is preferably less than 2.00%. It should be noted that there is no particular restriction on the lower limit of the reduction rate of temper rolling, but from the perspective of productivity, it is preferably greater than 0.05%. In addition, temper rolling can be performed on a device that is continuous with the annealing device used to perform the above-mentioned processes (online), or on a device that is discontinuous with the annealing device used to perform the various processes (offline). In addition, the number of temper rolling passes may be 1 time or more than 2 times. It should be noted that as long as the same elongation as temper rolling can be imparted, rolling using a leveler or the like may also be performed.

[0435] There are no particular limitations on the conditions other than those described above, and conventional methods may be followed.

[0436] [3.Component]

[0437] Next, components according to one embodiment of the present invention will be described.

[0438] A component according to one embodiment of the present invention is a component (as a raw material) formed using the above-mentioned steel plate. For example, the component is manufactured by subjecting the steel plate as a raw material to at least one of a forming process and a joining process.

[0439] Here, the steel sheet has a TS of 1180 MPa or more and has high YS, excellent press formability (ductility, hole expandability, and bendability), and fracture resistance during crushing (bending fracture characteristics and axial crushing characteristics). Therefore, the component according to one embodiment of the present invention has high strength and excellent impact resistance. Therefore, the component according to one embodiment of the present invention is particularly suitable for use in impact energy absorbing components used in the automotive field.

[0440] [4. Method for manufacturing component]

[0441] Next, a method for manufacturing a member according to one embodiment of the present invention will be described.

[0442] A method for manufacturing a component according to an embodiment of the present invention includes the step of performing at least one of forming and joining on the steel plate (for example, the steel plate manufactured by the method for manufacturing a steel plate) to manufacture the component.

[0443] Here, there is no particular limitation on the forming method, and general processing methods such as stamping can be used. In addition, there is no particular limitation on the joining method, and general welding such as spot welding, laser welding, arc welding, riveting, and caulking can be used. It should be noted that there is no particular limitation on the forming conditions and joining conditions, and conventional methods can be used.

[0444] Example

[0445] A steel material having the component composition shown in Table 1 (the balance is Fe and inevitable impurities) was melted in a converter and formed into a steel slab by a continuous casting method. In Table 1, - indicates the content of the inevitable impurity level.

[0446] The calculated phase transition points Ac shown in Table 1 1 Point (℃) and Ac 3 The temperature (°C) was calculated by the following formula.

[0447] Ac 1 Point (℃) = 727.0-32.7×[%C]+14.9×[%Si]+2.0×[%Mn]

[0448] Ac 3 Point (℃) = 912.0-230×[%C]+31.6×[%Si]-20.4×[%Mn]

[0449] Here, [%C]: C content (mass %), [%Si]: Si content (mass %), and [%Mn]: Mn content (mass %).

[0450] The obtained steel slabs were heated to 1200°C, and after heating, the steel slabs were subjected to hot rolling including rough rolling and finishing rolling at a finishing rolling temperature of 900°C to produce hot-rolled steel sheets. Next, the obtained hot-rolled steel sheets No. 1 to No. 61, No. 64 to No. 78, No. 84 to No. 98, and No. 104 to No. 109 were pickled and cold rolled (reduction ratio: 50%) to produce cold-rolled steel sheets having the plate thicknesses shown in Tables 3, 6, and 9. In addition, the obtained hot-rolled steel sheets No. 62 to No. 63, No. 79 to No. 83, and No. 99 to No. 103 were pickled to produce hot-rolled steel sheets (white skin) having the plate thicknesses shown in Tables 3, 6, and 9.

[0451] Next, the obtained cold-rolled steel sheet or hot-rolled steel sheet (white skin) is subjected to an annealing process, a first cooling process, a holding process, a galvanizing process, a second cooling process and a reheating process under the conditions shown in Table 2. In addition, the first plating process (metal plating process), an annealing process, a first cooling process, a holding process, a second plating process (galvanizing process), a second cooling process and a reheating process are subjected to the conditions shown in Tables 5 and 8 to obtain a steel sheet (galvanized steel sheet).

[0452] It should be noted that the presence or absence of the first plating step (metal plating step) and the type of plating when the treatment in the metal plating step is performed for the steel sheets No. 64 to No. 109 are shown in Tables 5 and 8. The thickness of the surface soft layer, the amount of metal plating, and the hardness distribution of the surface soft layer of the steel sheets No. 64 to No. 109 are shown in Tables 6 and 9.

[0453] Here, hot-dip galvanizing or alloyed galvanizing is performed in the galvanizing process to obtain a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or an alloyed hot-dip galvanized steel sheet (hereinafter also referred to as GA). It should be noted that in Table 2, Table 5, and Table 8, the types of plating processes are also expressed as "GI" and "GA". In the case of the GI steel sheet in Table 2, Table 5, and Table 8, since no alloying treatment is performed, the alloying temperature is expressed as -. In addition, in Table 8, the case of obtaining a cold-rolled steel sheet without performing a galvanizing treatment using the galvanizing process is expressed as "CR", and the case of obtaining a hot-rolled steel sheet without performing a galvanizing treatment using the galvanizing process is expressed as "HR".

[0454] The galvanizing bath temperature was set to 470° C. in the case of producing either GI or GA.

[0455] Regarding the zinc coating adhesion, when manufacturing GI, it is set to 45 to 72 g / m per side. 2 When manufacturing GA, it is set to 45g / m per side. 2 .

[0456] It should be noted that the composition of the zinc coating of the galvanized steel sheet finally obtained is that GI contains Fe: 0.1-1.0 mass%, Al: 0.2-0.33 mass%, and the balance is Zn and inevitable impurities. In addition, GA contains Fe: 8.0-12.0 mass%, Al: 0.1-0.23 mass%, and the balance is Zn and inevitable impurities.

[0457] In addition, the zinc plating layer is formed on both surfaces of the base steel sheet.

[0458] In Table 2, Table 5, and Table 8, Pass 1 means that during the second cooling process, 2.0 kgf / mm2 is applied at least once in the temperature range of 300°C to 450°C. 2 After the above average tension, the number of passes is given while making the steel plate contact with the roll with a diameter of 500mm to 1500mm for 1 / 4 of the roll per pass, and the number of passes 2 is the number of passes given while making the steel plate contact with the roll with a diameter of 500mm to 1500mm for 1 / 2 of the roll per pass.

[0459] The obtained steel plate was used to identify the steel structure of the base steel plate and measure the amount of diffusible hydrogen according to the above-mentioned points. The measurement results are shown in Table 3, Table 6, and Table 9. In Table 3, Table 6, and Table 9, BF ​​is bainitic ferrite, TM is tempered martensite, RA is retained austenite, FM is fresh martensite, LB is lower bainite, P is pearlite, and θ is carbide. In addition, C M is the carbon content of the martensite generated in the second cooling step, C γ is the carbon content in the retained austenite, ρ C is the density of carbides in tempered martensite.

[0460] The method for measuring the surface soft layer is as follows. The thickness section (L section) of the steel plate parallel to the rolling direction is smoothed by wet grinding, and then based on JIS Z 2244-1 (2020), a Vickers hardness tester is used to measure at a load of 10 gf from a position 1 μm from the surface of the steel plate in the thickness direction to a position 100 μm in the thickness direction at 1 μm intervals. Then, the measurement is performed at 20 μm intervals until the center of the plate thickness. The area where the hardness is reduced to less than 85% compared to the hardness at the 1 / 4 position of the plate thickness is defined as a soft layer (surface soft layer), and the thickness in the thickness direction of the area is defined as the thickness of the soft layer.

[0461] In addition, a tensile test, a hole expansion test, a VDA bending test, a V-VDA bending test, and an axial crush test were performed according to the following points, and the tensile strength (TS), yield stress (YS), total elongation (El), limit hole expansion ratio (λ), limit bending angle in the VDA bending test (α), stroke at maximum load in the V-VDA bending test (S Fmax ) and whether the shaft is crushed or fractured.

[0462] ·TS

[0463] ○(Qualified): 1180MPa or more

[0464] ×(Unqualified): less than 1180MPa

[0465] ·YS

[0466] 〇(Qualified):

[0467] (A) When 1180MPa≤TS<1320MPa, 750MPa≤YS

[0468] (B) When 1320MPa≤TS, 850MPa≤YS

[0469] ×(Unqualified):

[0470] (A) When 1180MPa≤TS<1320MPa, 750MPa>YS

[0471] (B) When 1320MPa≤TS, 850MPa>YS

[0472] ·El

[0473] 〇(Qualified):

[0474] (A) When 1180MPa≤TS<1320MPa, 12.0%≤El

[0475] (B) When 1320MPa≤TS, 10.0%≤El

[0476] ×(Unqualified):

[0477] (A) When 1180MPa≤TS<1320MPa, 12.0%>El

[0478] (B) When 1320MPa≤TS, 10.0%>El

[0479] ·λ

[0480] 0(Qualified): 30% or more

[0481] ×(Unqualified): Less than 30%

[0482] α

[0483] 0(Qualified): 80° or above

[0484] ×(Unqualified): Less than 80°

[0485] ·S Fmax

[0486] 〇(Qualified): 26.0mm or more

[0487] ×(Unqualified): less than 26.0mm

[0488] Whether the shaft is crushed or broken

[0489] A (Pass): No cracks were observed in the sample after the shaft crushing test

[0490] B (pass): No more than 2 cracks were observed in the sample after the shaft crushing test

[0491] C (Pass): No more than 3 cracks were observed in the sample after the shaft crushing test

[0492] D (failure): Four or more cracks were observed in the sample after the axial crush test or the sample was broken after the axial crush test

[0493] (1) Tensile test

[0494] The tensile test was performed based on JIS Z 2241 (2011). That is, a JIS No. 5 test piece was cut from the obtained steel plate so that the length direction was at right angles to the rolling direction of the base steel plate. The tensile test was performed using the cut test piece under the condition of a crosshead speed of 10 mm / min to measure TS, YS and E1. The results are shown in Table 4, Table 7, and Table 10.

[0495] (2) Hole expansion test

[0496] The hole expansion test is carried out based on JIS Z 2256 (2020). That is, a test piece of 100 mm × 100 mm is cut from the obtained steel plate by shearing. For the test piece, the gap is set to 12.5% ​​and a hole with a diameter of 10 mm is punched out. Next, a die with an inner diameter of 75 mm is used to apply an anti-wrinkle force of 9 tons (88.26 kN) around the hole, and a conical punch with a vertex angle of 60° is pressed into the hole to measure the diameter of the hole of the test piece at the crack generation limit (when cracks occur). Then, the limiting hole expansion rate is calculated by the following formula: λ (%). It should be noted that λ is an indicator for evaluating stretch flangeability. The results are shown in Tables 4, 7, and 10.

[0497] λ(%)={(D f -D 0 ) / D 0}×100

[0498] Here,

[0499] D f : The diameter of the hole of the test piece when cracks occurred (mm),

[0500] D 0 : The initial diameter of the hole of the test piece (mm).

[0501] (3) VDA bending test

[0502] The VDA bending test is performed by a bending test in accordance with the VDA standard (VDA238-100) prescribed by the German Association of the Automotive Industry.

[0503] Specifically, a test piece of 70 mm×60 mm was cut out from the obtained steel plate by shearing, wherein the side of 60 mm was parallel to the rolling (L) direction.

[0504] The test piece was subjected to a VDA bending test under the following conditions.

[0505] Test method: Roller support, punch press

[0506] Roller diameter: φ30mm

[0507] Punch tip R: 0.4mm

[0508] Distance between rollers: (plate thickness × 2) + 0.5 mm

[0509] Stroke speed: 20mm / min

[0510] Bending direction: rolling right angle (C) direction

[0511] At this time, the angle of the outer side of the bending of the central part of the plate-shaped test piece when the load F applied from the bending fixture from above reaches the maximum is measured as the limit bending angle (°). The average value of the limit bending angle when the load is the maximum when the above-mentioned VDA bending test is performed three times is set to α (°). The results are shown in Table 4, Table 7, and Table 10.

[0512] (4) V-VDA bending test (V bending + orthogonal VDA bending test)

[0513] The V-VDA bend test was performed as follows.

[0514] A 60 mm x 65 mm test piece was cut from the obtained steel plate by shearing. Here, the 60 mm side is parallel to the rolling (L) direction. The test piece was prepared by bending 90° (primary bending) in the rolling (L) direction with a curvature radius / plate thickness of 4.2 and with the width (C) direction as the axis. In the 90° bending (primary bending), if Figure 2-1 As shown in (a), a punch B1 is pressed into a steel plate placed on a die A1 having a V-shaped groove to obtain a test piece T1. Figure 2-1 As shown in (b), the test piece T1 placed on the support roll A2 is subjected to orthogonal bending (secondary bending) by pressing the punch B2 into the test piece so that the bending direction is perpendicular to the rolling direction. Figure 2-1 (a) and Figure 2-1 In (b), symbol D1 indicates the width (C) direction, and symbol D2 indicates the rolling (L) direction.

[0515] The conditions of the V-bend in the V-VDA bend test (V-bend + orthogonal VDA bend test) are as follows.

[0516] Test method: Die support, punch press

[0517] Forming load: 10t

[0518] Test speed: 30mm / min

[0519] Hold time: 5s

[0520] Bending direction: rolling (L) direction

[0521] The conditions of VDA bending in the V-VDA bending test are as follows.

[0522] Test method: Roller support, punch press

[0523] Roller diameter: φ30mm

[0524] Punch tip R: 0.4mm

[0525] Distance between rollers: (plate thickness × 2) + 0.5 mm

[0526] Stroke speed: 20mm / min

[0527] Test piece size: 60mm×60mm

[0528] Bending direction: rolling right angle (C) direction

[0529] The stroke at the maximum load was obtained from the stroke-load curve obtained when the VDA bending test was performed. The average value of the stroke at the maximum load when the V-VDA bending test was performed three times was taken as S FmaxThe results are shown in Tables 4, 7 and 10.

[0530] (5) Shaft crush test

[0531] A 160 mm x 200 mm test piece was cut from the obtained steel plate by shearing. Here, the 160 mm side was parallel to the rolling (L) direction. A die with a punch shoulder radius of 5.0 mm and a die shoulder radius of 5.0 mm was used to perform forming (bending) to a depth of 40 mm to produce Figure 6-1 (a) and Figure 6-1 (b) The hat-shaped member 10 shown in FIG. 1 is a hat-shaped member 10. In addition, the steel plate used as the raw material of the hat-shaped member is cut into a size of 80 mm × 200 mm. Then, the cut steel plate 20 and the hat-shaped member 10 are spot welded to produce a hat-shaped member 10 as shown in FIG. Figure 6-1 (a) and Figure 6-1 (b) is a test member 30 shown. Figure 6-1 (a) is a front view of a test member 30 produced by spot welding a hat-shaped member 10 and a steel plate 20 . Figure 6-1 (b) is a perspective view of the test component 30. The position of the spot weld 40 is as follows: Figure 6-1 As shown in (b), the ends of the steel plates are arranged so that the interval between the ends and the welded portions is 10 mm and the interval between the welded portions is 45 mm. Figure 6-2 As shown in (c), the test member 30 and the bottom plate 50 are joined by TIG welding to produce a shaft crush test sample. Then, the impactor 60 is made to collide with the produced shaft crush test sample at a constant speed of 10 mm / min, and the shaft crush test sample is crushed by 70 mm. Figure 6-2 As shown in (c), the crushing direction D3 is set to be parallel to the longitudinal direction of the test member 30.

[0532] The crushed samples were evaluated according to the above-mentioned points. The results are shown in Tables 4, 7 and 10.

[0533] In the VDA bending test, V-VDA bending test and shaft crushing test of steel plates with a thickness greater than 1.2 mm, all tests were conducted using steel plates with a thickness of 1.2 mm, taking into account the influence of the plate thickness. Steel plates with a thickness greater than 1.2 mm were ground on one side to a thickness of 1.2 mm. Since the bending property of the steel plate surface may be affected by the grinding process, the ground surface was set to the inner side of the bend (the side in contact with the punch) in the VDA bending test, and the ground surface was set to the outer side of the bend (the side in contact with the die) in the V-VDA bending test, and then the ground surface was set to the inner side of the bend (the side in contact with the punch) in the VDA bending test.

[0534] On the other hand, in the VDA bending test, V-VDA bending test, and shaft crushing test of a steel plate having a plate thickness of 1.2 mm or less, the test was performed without performing a grinding process because the influence of the plate thickness was small.

[0535] In addition, the “ 1" and " 2” are the lengths of cracks formed in the L section of the V-bend ridge and the VDA bend ridge when the V-VDA bend test was performed to the highest load point, and the changes in the grain size of bainitic ferrite in the plate thickness direction before and after processing in an area 50 μm from the steel plate surface on the outside of the VDA bend and 50 μm to the left and right of the VDA bend apex (an area formed from each position of a starting line starting from the apex of the VDA bend outside the plate thickness direction and extending to 50 μm on both sides of the starting line in the perpendicular direction).

[0536] <Nanohardness measurement>

[0537] In order to obtain excellent bendability during stamping and excellent bending fracture characteristics during collision, it is more preferred to measure the nanohardness of more than 300 points in the 50μm×50μm area of ​​the plate surface at each position of 1 / 4 position in the plate thickness direction of the surface soft layer and 1 / 2 position in the plate thickness direction from the base surface layer, and the number of measurements of the plate surface with a nanohardness of 7.0GPa or more at the position of 1 / 4 position in the plate thickness direction of the surface soft layer from the base steel plate surface is less than 0.10 relative to the total number of measurements at the position of 1 / 4 position in the plate thickness direction. When the ratio of nanohardness of 7.0GPa or more is less than 0.10, it means that the ratio of hard structures (martensite, etc.), inclusions, etc. is small, so the generation, connection and expansion of cracks of hard structures (martensite, etc.) and inclusions during stamping and collision can be further suppressed, and excellent R / t and S Fmax .

[0538] In the case of galvanizing, after stripping the coating, mechanical grinding is performed to 1 / 4 position -5 μm from the base steel plate surface in the plate thickness direction depth of the surface soft layer, diamond and alumina polishing is performed to 1 / 4 position from the base steel plate surface in the plate thickness direction depth of the surface soft layer, and then colloidal silica grinding is performed. Hysitron's tribo-950 is used, and a berkovich-shaped diamond indenter is used to measure the nanohardness of a total of 512 points under the conditions of load: 500 μN, measurement area: 50 μm×50 μm, and dot interval: 2 μm.

[0539] The peeled-off plating layer refers to the zinc plating when the zinc plating layer is formed, refers to the metal plating when the metal plating layer is formed, and refers to the zinc plating and the metal plating when the zinc plating and the metal plating are formed.

[0540] Next, mechanical grinding, polishing with diamond and alumina, and colloidal silica grinding were performed until the depth of the surface soft layer in the thickness direction was 1 / 2. Hysitron's tribo-950 was used, and a berkovich-shaped diamond indenter was used to measure the nanohardness of a total of 512 points under the conditions of load: 500 μN, measurement area: 50 μm×50 μm, and dot interval: 2 μm.

[0541]

[0542]

[0543]

[0544]

[0545]

[0546]

[0547]

[0548]

[0549]

[0550]

[0551]

[0552]

[0553]

[0554]

[0555] In Tables 1 to 10, the underlined parts indicate that the contents are outside the appropriate range of the present invention.

[0556] As shown in Tables 4, 7, and 10, for the inventive examples, the tensile strength (TS), yield stress (YS), total elongation (El), limiting hole expansion ratio (λ), limiting bending angle in the VDA bending test (α), and stroke at maximum load in the V-VDA bending test (S Fmax) are all qualified and there is no breakage in the shaft crush test.

[0557] On the other hand, for the comparative examples, the tensile strength (TS), yield stress (YS), total elongation (El), limiting hole expansion ratio (λ), limiting bending angle in VDA bending test (α), stroke at maximum load in V-VDA bending test (S Fmax ) At least one of the presence or absence of fracture in the axial crush test is insufficient.

[0558] It should be noted that in Tables 5 to 10, although there are cases where the dew point is in the range of above -30°C and below -5°C, the soft layer thickness is less than 11μm, and the fracture (appearance crack) in the shaft crush test is judged as "B" or "C", even when the soft layer thickness is less than 11μm, in the case of a metal coating, the fracture (appearance crack) in the shaft crush test is judged as "A".

[0559] In addition, it can be seen that the tensile strength (TS), yield stress (YS), total elongation (E1), limiting hole expansion ratio (λ), limiting bending angle (α) in the VDA bending test, and stroke (S) at the maximum load in the V-VDA bending test of the member obtained by forming or joining the steel sheet of the example of the present invention are Fmax ) all have the excellent characteristics that are the characteristics of the present invention, and do not break in the axial crush test, and have the excellent characteristics that are the characteristics of the present invention.

[0560] Explanation of symbols

[0561] 10 Hat-shaped components

[0562] 20 galvanized steel sheet

[0563] 30Test components

[0564] 40 spot welding

[0565] 50 base plate

[0566] 60 Impactor

[0567] A1 Die

[0568] A2 support roller

[0569] B1 Punch

[0570] B2 Punch

[0571] D1 width (C) direction

[0572] D2 rolling (L) direction

[0573] D3 crush direction

[0574] D4 Plate thickness direction

[0575] T1 test piece

[0576] T2 test piece

[0577] P Highest load point

[0578] RStarting from the highest load point, the stroke increases until the load reaches 94.9% to 99.9% of the highest load.

[0579] The area of ​​the L section in the overlapping area of ​​the ABV bending ridgeline and the VDA bending ridgeline, which is 0 to 50 μm from the steel sheet surface outside the VDA bending and 50 μm around the bending apex of the VDA bending

[0580] L-section in the overlapping area of ​​the ALV curved ridgeline and the VDA curved ridgeline

[0581] F Ferrite

[0582] BF bainitic ferrite

[0583] Bainitic ferrite before BF1 deformation

[0584] Bainitic ferrite after deformation of BF2

[0585] TM tempered martensite

[0586] θ Carbide

[0587] H1 Hard second phase

[0588] X1 island second phase

Claims

1. A steel plate, which is a steel plate having a base steel plate, wherein: The base steel plate has a composition containing, in mass%, C: 0.050% or more and 0.400% or less, Si: greater than 0.75% and 3.00% or less, Mn: 2.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.010% or more and 2.000% or less, and N: 0.0100% or less, with the balance being Fe and inevitable impurities. The base steel sheet has an area ratio of bainitic ferrite of 3.0% to 20.0%, an area ratio of tempered martensite of 40.0% to 90.0%, an area ratio of retained austenite of more than 3.0% to 15.0%, a carbon concentration in the retained austenite of 0.60 mass% to 1.30 mass%, an area ratio of fresh martensite of 10.0% or less, and a density of carbides in the tempered martensite of 8.0 pieces / μm 2 The following steel structures, The diffusible hydrogen content of the base steel plate is 0.50 mass ppm or less, Furthermore, V-VDA bending test was performed to the highest load point. In the L section, The length of the crack is less than 400 μm. Furthermore, in the region formed from each position of the starting line which starts from the bending vertex on the outer side of the VDA bend and exists at a position of 50 μm in the plate thickness direction to each position of 50 μm on both sides in the perpendicular direction of the starting line, Regarding the average grain size of bainitic ferrite in the plate thickness direction, the ratio of the average grain size before working to the average grain size after working is 5.0 or less. The tensile strength of the steel plate is 1180 MPa or more.

2. The steel plate according to claim 1, wherein: The base steel plate further comprises, in terms of mass%, Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0 200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less and REM: 0.0200% or less.

3. The steel plate according to claim 1 or 2, wherein: The steel sheet has a zinc coating as an outermost layer on one or both sides thereof.

4. The steel plate according to any one of claims 1 to 3, wherein When the base steel plate has a surface layer of a region of 200 μm or less in the plate thickness direction from the surface of the base steel plate, The surface layer has a surface soft layer having a Vickers hardness of 85% or less relative to the Vickers hardness at a position of 1 / 4 of the plate thickness, When the nanohardness is measured at 300 points or more in a 50 μm×50 μm area of ​​the plate surface at each of the 1 / 4 position and the 1 / 2 position of the depth in the plate thickness direction of the surface soft layer from the surface of the base steel plate, The ratio of the number of measurements of the plate surface having a nanohardness of 7.0 GPa or more at a position 1 / 4 of the depth in the plate thickness direction of the surface soft layer from the surface of the base steel plate is 0.10 or less relative to the total number of measurements at a position 1 / 4 of the depth in the plate thickness direction of the surface soft layer, Furthermore, the standard deviation σ of the nanohardness of the plate surface at a position 1 / 4 of the depth in the plate thickness direction of the surface soft layer from the surface of the base steel plate is 1.8 GPa or less, Furthermore, a standard deviation σ of the nanohardness of the plate surface at a position 1 / 2 of the depth in the plate thickness direction of the surface soft layer from the surface of the base steel plate is 2.2 GPa or less.

5. The steel plate according to any one of claims 1 to 4, wherein The steel sheet has a metal plating layer formed on the base steel sheet on one side or both sides thereof. 6 . A member formed using the steel sheet according to claim 1 .

7. A method for manufacturing a steel plate, comprising: A hot rolling step, wherein a steel slab having the component composition described in claim 1 or 2 is hot rolled to produce a hot-rolled steel sheet; A pickling step, wherein the hot-rolled steel plate is pickled; An annealing step, wherein the steel sheet after the pickling step is annealed at an annealing temperature of (Ac1+0.4×(Ac3-Ac1))°C or higher and 900°C or lower and an annealing time of 20 seconds or longer; A first cooling step, wherein the steel sheet after the annealing step is cooled to a first cooling stop temperature of 400° C. to 600° C.; A second cooling step, wherein the steel sheet after the first cooling step is cooled to a second cooling stop temperature of 100°C to 300°C at an average cooling rate of 25.0°C / s or less, and during the cooling, a 2.0 kgf / mm2 pressure is applied to the steel sheet at least once in a temperature range of 300°C to 450°C. 2 and then applying a tension of more than 4 passes while bringing the steel plate into contact with a roll having a diameter of 500 mm or more and 1500 mm or less for 1 / 4 of the roll's revolution in each pass, and applying a treatment of more than 2 passes while bringing the steel plate into contact with a roll having a diameter of 500 mm or more and 1500 mm or less for 1 / 2 of the roll's revolution in each pass; and A reheating process, wherein the steel plate after the second cooling process is heated to a tempering temperature in a temperature range higher than 300° C. and lower than 500° C. and maintained in the temperature range for a tempering time of 20 seconds to 900 seconds, and during the reheating process, a carbide control parameter CP represented by the following formula (1) is set to 10000 to 15000, Alternatively, the method further comprises a cold rolling step of cold rolling the steel sheet after the pickling step and before the annealing step to obtain a cold-rolled steel sheet. CP = (T + 273) × (k + 1.2 × logt) ... Formula (1) Here, T is the tempering temperature (°C), k is a material constant that depends on the C content, and t is the tempering time (seconds). k=-6×C M +17.8, C M is the carbon content (mass %) in the martensite generated in the second cooling step.

8. The method for manufacturing a steel plate according to claim 7, wherein: The method includes a galvanizing step of performing galvanizing on the steel sheet after the first cooling step and before the second cooling step to form a zinc plated layer on the steel sheet.

9. The method for manufacturing a steel plate according to claim 7 or 8, wherein: The annealing in the annealing step is performed in an atmosphere with a dew point of -30°C or higher.

10. The method for producing a steel plate according to any one of claims 7 to 9, wherein: The method includes a metal plating step of plating one or both surfaces of the steel sheet to form a metal plating layer after the pickling step and before the annealing step. 11 . A method for producing a member, comprising the step of subjecting the steel sheet according to claim 1 to at least one of a forming process and a joining process to produce a member.

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