Steel plates, components, and their manufacturing methods

Abstract

This steel sheet is set to have a TS of not less than 1180 MPa but less than 1470 MPa, and to have high YS, excellent ductility, excellent press formability of the inside of the steel sheet and the end part of the steel sheet, and high crack propagation resistance.　The steel sheet is composed of a base steel sheet as a predetermined component composition, has Vickers hardness of 84% or less at the 1 / 4 position in the plate thickness from the surface of the base steel sheet, and has a surface soft layer that satisfies the following formula (1). The structure in the surface soft layer is set in a specific range, and the structure at the 1 / 4 position in the plate thickness of the base steel sheet is set in a specific range. The crack length progressing from a bent ridge line end part in the ridge line direction as a result of a 90-degree V bending test performed at a bending radius of 0.5 mm is 200 μm or less. The steel sheet exhibits EI of 6.0% or more as a result of a notching tension test performed after subjection to heat treatment at 170°C for 20 minutes, and exhibits El of 5.0% or more as a result of the notching tension test performed after introduction of the nominal tensile strain of 2% and subjection to the heat treatment at 170°C for 20 minutes. (1): 20≤X≤120-3800×[Sb]-1900×[Sn]

Description

[Technical Field]

[0001] The present invention relates to a steel plate, a member made from the steel plate, and a method for manufacturing the same. [Background technology]

[0002] Strengthening of automotive steel sheets is being promoted to achieve both improved fuel economy and CO2 emissions through the reduction of thickness and weight of steel sheets used in automobile bodies, while also improving crashworthiness. Furthermore, new regulations are being introduced one after another. Therefore, in order to increase vehicle body strength, the application of high-strength steel sheets, particularly high-strength steel sheets with a tensile strength (TS) of 980 MPa or more, is increasing for the main structural and reinforcing members (hereinafter referred to as automotive skeleton structural members) assembled to the framework of the automobile cabin. Furthermore, high-strength steel sheets used in automotive skeleton structural members are required to have high component strength when press-formed. To increase component strength, for example, increasing the yield ratio (YR), which is calculated by dividing the yield stress (YS) of the steel sheet by the TS, is effective. This increases the impact absorption energy (hereinafter referred to as impact absorption energy) during a vehicle collision. Furthermore, among automotive skeleton structural members, for example, crash boxes have bent sections. Therefore, from the viewpoint of press formability, it is preferable to apply steel sheets with high bendability to such parts. Furthermore, from the viewpoint of the rust prevention performance of the vehicle body, steel sheets used as raw materials for automotive components are often zinc-plated. Therefore, there is a demand for the development of hot-dip galvanized steel sheets that not only have high strength but also excellent press formability and impact resistance.

[0003] As an example of a steel sheet that can be used as a material for such automobile parts, Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet having a thickness of 0.6 to 5.0 mm and a plating layer on the surface of the steel sheet, in which the steel sheet structure contains a ferrite phase with a volume fraction of 40 to 90% and a retained austenite phase with a volume fraction of 3 to 25%, and the retained austenite phase has a solute carbon content of 0.70 to 1.00%, an average particle size of 2.0 μm or less, an average distance between particles of 0.1 to 5.0 μm, a decarburized layer in the surface layer of the steel sheet with a thickness of 0.01 to 10.0 μm, an average particle size of oxides contained in the surface layer of the steel sheet of 30 to 120 nm, and an average density of 1.0 × 10 12 pieces / m 2 The present invention discloses a high-strength hot-dip galvanized steel sheet that has the above characteristics and further has a work hardening coefficient (n value) of 0.080 or more on average at 3 to 7% plastic deformation, and that has high ductility while maintaining a high strength of 900 MPa or more in maximum tensile strength and excellent mechanical cutting properties.

[0004] Patent Document 2 discloses a high-strength hot-dip galvanized steel sheet with excellent delayed fracture resistance, characterized in that it has, by volume fraction, 40 to 90% of a ferrite phase and 5% or less of a retained austenite phase, the proportion of unrecrystallized ferrite in the entire ferrite phase being 50% or less, by volume fraction, a grain size ratio, which is the value obtained by dividing the average grain size of crystal grains of the ferrite phase in the rolling direction by the average grain size in the sheet width direction, of 0.75 to 1.33, a length ratio, which is the value obtained by dividing the average length of hard structures dispersed in an island shape in the rolling direction by the average length in the sheet width direction, of 0.75 to 1.33, and an average aspect ratio of inclusions being 5.0 or less.

[0005] Patent Document 3 also describes a steel sheet having a steel sheet and a hot-dip galvanized layer, the steel sheet including a base material and a decarburized ferrite layer, the structure at a depth of 1 / 4 of the steel sheet thickness containing 5.0 volume % or more of tempered martensite and 0.5 volume % or more but less than 7.0 volume % of retained austenite, the balance mainly consisting of 4 to 70 volume % of ferrite and bainite, a part or all of the tempered martensite and the retained austenite forming MA, the decarburized ferrite layer containing 120% or more of ferrite with respect to the content of ferrite at a depth of 1 / 4 of the sheet thickness, an average ferrite grain size of 20 μm or less, a thickness of 5 μm or more and 200 μm or less, 1.0 volume % or more of tempered martensite and a number density of 0.01 grains / μm 2 Disclosed are a hot-dip galvanized steel sheet and a hot-dip galvannealed steel sheet having good elongation and bendability, characterized by the above-mentioned properties, and methods for producing the same.

[0006] Patent Document 4 also discloses a high-strength hot-dip galvanized steel sheet having a chemical composition containing, by mass%, 0.05 to 0.3% C, 0.01 to 2.5% Si, 0.5 to 3.5% Mn, 0.003 to 0.100% P, 0.02% or less S, 0.010 to 1.5% Al, 0.007% or less N, with the balance being Fe and unavoidable impurities, and having a microstructure containing, by area ratio, 20 to 87% ferrite, 3 to 10% martensite and retained austenite in total, and 10 to 60% tempered martensite, which has a high TS-El balance, excellent stretch-flange formability, and low YR and a method for producing the same. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 5354135 [Patent Document 2] Patent No. 5352793 [Patent Document 3] Patent No. 6536294 [Patent Document 4] Patent No. 5256689 Summary of the Invention [Problem to be solved by the invention]

[0008] In automotive frame components such as center pillars, the use of steel sheets with a tensile strength (TS) of over 980 MPa is increasing, but in automotive impact energy absorbing components such as front side members and rear side members, the current situation is that steel sheets with a TS of 590 to 780 MPa are still used.

[0009] That is, improving the yield stress (YS) is effective in increasing the energy absorption during a collision (hereinafter also referred to as impact absorption energy). However, increasing the YS of a steel sheet generally reduces press formability, particularly properties such as ductility, hole expandability, and bendability. Therefore, when applying a steel sheet with increased TS and YS to the aforementioned automotive impact energy absorption components, not only does press forming become difficult, but the component cracks in an axial crushing test simulating a crash test. In other words, the actual impact absorption energy is not as high as expected from the YS value. For this reason, the current situation is that the impact energy absorption components are limited to steel sheets with a TS of 590 to 980 MPa.

[0010] Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet in which ductility is improved by the generation of retained austenite inside the steel sheet and mechanical cutting properties are improved by the formation of a decarburized layer on the surface of the steel sheet. However, no consideration is given at all to the improvement in bendability and fracture resistance in the event of a vehicle collision that are achieved by the formation of a soft surface layer (decarburized layer), and the press formability of the steel sheet end portions. Patent Document 2 discloses a high-strength hot-dip galvanized steel sheet in which the main structure inside the steel sheet is soft ferrite and the amount of unrecrystallized ferrite is limited to a small amount, thereby improving ductility, and the delayed fracture resistance and its anisotropy are improved by forming a decarburized layer on the surface of the steel sheet. However, no consideration is given at all to the improvement in bendability and fracture resistance in the event of a vehicle collision, which are achieved by forming a soft surface layer (decarburized layer), and the press formability of the steel sheet end portion. Patent Document 3 discloses a hot-dip galvanized steel sheet and a galvannealed steel sheet in which ductility is improved by the generation of MA inside the steel sheet and bendability is improved by the formation of a soft layer (decarburized ferrite layer) on the surface of the steel sheet, but does not take into consideration at all the press formability of the steel sheet end portions. Patent Document 4 discloses a high-strength hot-dip galvanized steel sheet that has improved ductility, which is the press formability inside the steel sheet, and stretch flangeability, which is the press formability at the ends of the steel sheet. However, no consideration is given at all to improving bendability by forming a soft surface layer (decarburized layer) or improving fracture resistance during a vehicle collision. From the above, it cannot be said that the steel sheets disclosed in Patent Documents 1 to 4 have a TS of 1180 MPa or more, a high YS, excellent press formability inside the steel sheet (bendability inside the steel sheet), and excellent press formability at the steel sheet end (bendability at the steel sheet end (shear cross section)).

[0011] The present invention has been developed in view of the above-mentioned current situation, and aims to provide a steel plate, a component, and a method for manufacturing the same, which have a tensile strength (TS) of 1180 MPa or more and less than 1470 MPa, a high yield stress (YS), excellent ductility, excellent press formability of the interior of the steel plate (bendability of the interior of the steel plate), excellent press formability of the steel plate edge (bendability of the steel plate edge (shear cross section)), and high crack propagation resistance.

[0012] The steel sheet referred to here also includes a galvanized steel sheet, and the galvanized steel sheet is a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or a galvannealed hot-dip galvanized steel sheet (hereinafter also referred to as GA).

[0013] Here, the tensile strength (TS) is measured by a tensile test in accordance with JIS Z 2241 (2011). Furthermore, having a high yield stress (YS) means satisfying the following. A high YS means that the YS measured in a tensile test conforming to JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on the TS measured in the tensile test. (A) When 1180MPa≦TS<1320MPa, 820MPa≦YS (B) When 1320MPa≦TS<1470MPa, 920MPa≦YS

[0014] Having excellent ductility means that the El measured in a tensile test conforming to JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on the TS measured in the tensile test. (A) When 1180MPa≦TS<1320MPa, 8.0%≦El (B) When 1320MPa≦TS<1470MPa, 7.0%≦El

[0015] Furthermore, excellent press formability (bendability) within the steel sheet means that R (limiting bending radius) / t (sheet thickness), measured in a 90-degree V-bend test conforming to JIS Z 2248 (2022), satisfies the following formula (A) or (B) depending on the TS. (A) When 1180MPa≦TS<1320MPa, 3.0≧R / t (B) When 1320MPa≦TS<1470MPa, 4.0≧R / t Furthermore, excellent press formability within the steel sheet (bendability within the steel sheet) means that, in a 90-degree V-bend test with a bending radius R of 0.5 mm in accordance with JIS Z 2248 (2022), the length of the cracks that propagate along the bending ridge formed other than at the end of the bending ridge is 200 μm or less (the length of the cracks other than at the V-bend end face is 200 μm or less).

[0016] Furthermore, excellent press formability of the steel plate end (bendability of the steel plate end (shear cross section)) means that, in a 90-degree V-bend test in accordance with JIS Z 2248 (2022) with a bending radius R of 0.5 mm, the crack length propagating from the end of the bend ridge in the ridge direction is 200 μm or less (the crack length at the V-bend end face is 200 μm or less).

[0017] Furthermore, having high crack propagation resistance means that formula (A) or (B) is satisfied in the following test (i), and formula (C) or (D) is satisfied in the following test (ii). (i) Notched tensile test after heat treatment at 170°C for 20 minutes (A) When 1180MPa≦TS<1320MPa, notch El≧6.5% (B) When 1320MPa≦TS<1470MPa, notch El≧6.0% (ii) Notched tensile test after introducing a 2% nominal tensile strain and heat treatment at 170°C for 20 minutes (C) When 1180MPa≦TS<1320MPa, notch El≧5.5% (D) When 1320MPa≦TS<1470MPa, notch El≧5.0%

[0018] The El value of the notched tensile test after the above-mentioned heat treatment at 170°C for 20 minutes is an index showing the resistance to crack propagation in the vertical wall of the axial crushing component (impact energy absorbing component).Furthermore, the El value of the notched tensile test after the above-mentioned 2% nominal tensile strain is introduced and the above-mentioned heat treatment at 170°C for 20 minutes is an index showing the resistance to crack propagation in the bending ridge of the axial crushing component (impact energy absorbing component). [Means for solving the problem]

[0019] The present inventors have conducted extensive research to achieve the above-mentioned object, and as a result have obtained the following findings. (1) With a specified chemical composition, a TS of 1180 MPa or more can be achieved by setting the area ratio of ferrite to 55.0% or less (including 0.0%) and the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) to more than 40.0% and less than 100.0% in the structure at the 1 / 4 position of the plate thickness of the base steel plate. (2) With a specified chemical composition, a high YS can be achieved by setting the total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite) at the 1 / 4 position in the thickness direction of the base steel plate to more than 40.0% and not more than 100.0%, the area fraction of retained austenite to less than 3.5%, and the area fraction of fresh martensite to 10.0% or less (including 0.0%). (3) The steel sheet has a specified chemical composition and a soft surface layer whose Vickers hardness is 84% ​​or less of the Vickers hardness at a position 1 / 4 of the plate thickness from the surface of the base steel sheet, and the soft surface layer satisfies the following formula (1). 20≦X≦120-3800×[Sb]-1900×[Sn] ···(1) In the formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively. Furthermore, the structure in the soft surface layer has an area ratio of ferrite of 60.0% to 100.0%, and among structures other than ferrite, the area ratio of fresh martensite divided by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite is set to 0.5 or less, and the area ratio of retained austenite is set to 3.0% or less. This enables excellent press formability (bendability within the steel sheet) and high crack propagation resistance within the steel sheet to be achieved. (4) With a specified chemical composition, by setting the area ratio of retained austenite to less than 3.5% and the area ratio of fresh martensite to 10.0% or less (including 0.0%) in the structure at the 1 / 4 position of the plate thickness of the base steel plate, excellent press formability of the steel plate end (bendability of the steel plate end (shear cross section)) can be achieved.

[0020] The present disclosure has been made based on the above findings. That is, the gist of the present disclosure is as follows. [1] In mass%, C: 0.050% or more and 0.400% or less, Si: 0.02% or more and 3.00% or less, Mn: 1.50% 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: 0.005% or more and 2.000% or less, N: 0.0100% or less, Sb: 0.200% or less (including 0%), and Sn: 0.200% or less (including 0%) and the balance being Fe and unavoidable impurities, The steel sheet has a soft surface layer having a Vickers hardness of 84% or less of the Vickers hardness at a position 1 / 4 of the sheet thickness from the surface of the base steel sheet, The surface soft layer satisfies the following formula (1): The structure in the superficial soft layer is The area ratio of ferrite is 60.0% or more and 100.0% or less, When the area fraction of ferrite is less than 100.0%, the value obtained by dividing the area fraction of fresh martensite by the total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less, The area fraction of retained austenite is 3.0% or less, The structure at 1 / 4 of the thickness of the base steel sheet is The area ratio of ferrite is 55.0% or less (including 0.0%), The total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is more than 40.0% and not more than 100.0%; The area ratio of retained austenite is less than 3.5%; Area ratio of fresh martensite: 10.0% or less (including 0.0%), A steel plate having a tensile strength of 1180 MPa or more but less than 1470 MPa. 20≦X≦120-3800×[Sb]-1900×[Sn] ···(1) In the formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively. [2] The component composition further includes, in 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 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 The steel sheet according to [1] above, containing at least one selected from the following: [3] The steel sheet according to [1] or [2], wherein one or both sides of the base steel sheet have a plating layer, and the plating layer is a hot-dip galvanized layer. [4] The steel sheet according to [1] or [2], wherein the base steel sheet has a plating layer on one or both sides, and the plating layer is a galvannealed layer. [5] A member made using the steel plate according to any one of [1] to [4] above. [6] A hot rolling process in which a steel slab having the composition described in [1] or [2] is hot rolled at a finish rolling temperature of 820 ° C or higher; an annealing step of heating the steel sheet after the hot rolling step and annealing it under conditions of an annealing temperature of 750°C or more and 900°C or less, an annealing time of 20 seconds or more, and a dew point of -10°C or more in an atmosphere, further satisfying formula (2) and formula (3); a cooling step of cooling the steel sheet after the annealing step to a cooling stop temperature of less than 100°C; a first holding step of reheating the steel sheet after the cooling step to a reheating holding temperature range of not less than the cooling stop temperature and not more than 440°C and holding the temperature for not less than 10 seconds; The steel sheet after the first holding step is subjected to a stress of 2.0 kgf / mm in the reheating holding temperature range. 2 a surface strain introduction step of applying the above tension; A second holding step of holding the steel sheet after the surface layer strain introduction step in the reheating holding temperature range for 10 seconds or more, or further comprising: A method for producing a steel sheet, comprising: a cold rolling step of cold rolling the steel sheet after the hot rolling step and before the annealing step at a rolling reduction of 20% or more and 80% or less to obtain a cold-rolled steel sheet. 2400≦Y≦20000...Formula (2) Y=[{(T-Ac1)×t1} / 2}]+{(T-Ac1)×t2}...Equation (3) In the formula (3), T is the annealing temperature (°C), t1 is the time (s) from 650°C to the annealing temperature T during the temperature rise in the annealing process, t2 is the annealing time (s), and Ac1 is Ac1 (°C). [7] The method for producing a steel sheet according to [6], further comprising a hot-dip galvanizing step of subjecting the steel sheet to a hot-dip galvanizing treatment after the annealing step to form a hot-dip galvanized layer. [8] The method for producing a steel sheet according to [6], further comprising a galvannealed hot-dip galvanizing step of subjecting the steel sheet to a galvannealed hot-dip galvanizing treatment after the annealing step to form a galvannealed hot-dip galvanized layer. [9] A method for manufacturing a component, comprising the step of subjecting the steel plate according to any one of [1] to [4] above to at least one of forming and joining to form the component. [Effects of the Invention]

[0021] According to the present invention, a steel plate and a member can be obtained which have a tensile strength (TS) of 1180 MPa or more and less than 1470 MPa, a high yield stress (YS), excellent ductility, excellent press formability inside the steel plate (bendability inside the steel plate), excellent press formability at the end of the steel plate (bendability at the end (shear cross section) of the steel plate), and high crack propagation resistance. Furthermore, members made from the steel plate of the present invention have high strength, excellent press formability, and crack propagation resistance, and therefore can be extremely advantageously applied to automotive frame members, impact energy absorbing members, and the like. [Brief explanation of the drawings]

[0022] [Figure 1] This is an example of a SEM tissue image used for tissue identification. [Figure 2] FIG. 2(a) is a schematic diagram of the sample after being bent at a 90-degree V angle, and FIG. 2(b) is a diagram of the sample shown in FIG. 2(a) viewed in the Z direction (negative direction). [Figure 3-1] FIG. 1 is a schematic diagram of a sample after being bent in a 90-degree V-shape. [Figure 3-2] This is a schematic diagram of end cracking during 90-degree V-bending. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention will be described based on the following embodiments.

[0024] [1. Steel plate] The steel sheet of the present invention has a base steel sheet having a chemical composition containing, in mass%, C: 0.050% or more and 0.400% or less, Si: 0.02% or more and 3.00% or less, Mn: 1.50% 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: 0.005% or more and 2.000% or less, N: 0.0100% or less, Sb: 0.200% or less (inclusive of 0%), and Sn: 0.200% or less (inclusive of 0%), with the balance being Fe and unavoidable impurities; The steel sheet has a soft surface layer having a Vickers hardness of 84% or less of the Vickers hardness at a position 1 / 4 of the sheet thickness from the surface of the base steel sheet, and the soft surface layer satisfies the following formula (1): As a structure in the soft surface layer, the area ratio of ferrite is 60.0% or more and 100.0% or less, and when the area ratio of ferrite is less than 100.0%, the value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less, and the area ratio of retained austenite is 3.0% or less, As a structure at 1 / 4 of the plate thickness position of the base steel plate, the area ratio of ferrite is 55.0% or less (including 0.0%), the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is more than 40.0% and 100.0% or less, the area ratio of retained austenite is less than 3.5%, and the area ratio of fresh martensite is 10.0% or less (including 0.0%), and when a 90-degree V-bend test with a bending radius of 0.5 mm is performed, the length of the crack propagating from the end of the bent ridge in the ridge direction is 200 μm or less, When a notched tensile test is conducted after heat treatment at 170°C for 20 minutes, El is 6.0% or more, and when a notched tensile test is conducted after introducing a 2% nominal tensile strain and heat treatment at 170°C for 20 minutes, El is 5.0% or more, and the tensile strength is 1180 MPa or more and less than 1470 MPa. 20≦X≦120-3800×[Sb]-1900×[Sn] ···(1) In the formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.

[0025] Ingredient composition First, the chemical composition of the base steel sheet of the steel sheet according to one embodiment of the present invention will be described. Note that the units for the chemical compositions are all "mass %", but hereinafter, unless otherwise specified, they will be simply expressed as "%".

[0026] C: 0.050% or more and 0.400% or less C is an effective element for ensuring a TS of 1180 MPa or more and a high YS by generating an appropriate amount of tempered martensite and bainitic ferrite. Here, if the C content is less than 0.050%, the area ratio of ferrite increases, making it difficult to achieve a TS of 1180 MPa or more. It also leads to a decrease in YS. On the other hand, if the C content exceeds 0.400%, the area fraction of fresh martensite increases excessively, resulting in an excessively high TS and a decrease in El. Furthermore, fresh martensite becomes the starting point for void formation during a 90-degree V-bend test, making it difficult to achieve the desired bendability of the steel sheet and the bendability of the sheared edge. Furthermore, the area fraction of retained austenite increases excessively, significantly increasing the fraction of fresh martensite formed by the deformation-induced transformation of retained austenite during shearing, accelerating the subsequent formation of voids and crack propagation. As a result, it becomes more difficult to achieve the desired bendability of the sheared edge. Therefore, the C content is set to 0.050% or more and 0.400% or less. The C content is preferably 0.070% or more. The C content is more preferably 0.080% or more, and even more preferably 0.090% or more. The C content is preferably 0.300% or less. The C content is more preferably 0.280% or less, and further preferably 0.250% or less.

[0027] Si: 0.02% or more and 3.00% or less Si is an element that suppresses excessive softening of tempered martensite. If the Si content is less than 0.02%, the tempered martensite will be excessively softened, making it difficult to ensure a TS of 1180 MPa or more. On the other hand, if the Si content exceeds 3.00%, the C concentration in austenite during annealing increases excessively due to an excessive increase in the area fraction of ferrite, and the desired bendability of the sheared edge cannot be achieved. Therefore, the Si content is set to 0.02% or more and 3.00% or less. The Si content is preferably 0.10% or more. The Si content is more preferably 0.20% or more, and even more preferably 0.30% or more. Furthermore, the Si content is preferably 1.80% or less. The Si content is more preferably 1.70% or less, and even more preferably 1.60% or less.

[0028] Mn: 1.50% or more and less than 3.50% Mn is an element that adjusts the area ratio of bainitic ferrite and tempered martensite. If the Mn content is less than 1.50%, the area ratio of ferrite increases, which may make it difficult to achieve a TS of 1180 MPa or more. This also leads to a decrease in YS. On the other hand, if the Mn content is 3.50% or more, the martensitic transformation start temperature Ms (hereinafter also referred to simply as the Ms point or Ms) decreases, and the amount of martensite formed during the cooling process decreases. As a result, the area ratio of fresh martensite ultimately increases, making it difficult to ensure the press formability of the steel sheet edge (bendability of the steel sheet edge (shear cross section)). In addition, in a 90-degree V-bend test with a bending radius of 0.5 mm, the crack length propagating from the bend edge toward the ridgeline may not be controlled to 200 μm or less. Therefore, the Mn content is set to 1.50% or more and less than 3.50%. The Mn content is preferably 2.00% or more, more preferably 2.20% or more. Furthermore, the Mn content is preferably 3.20% or less. The Mn content is more preferably 3.10% or less, and even more preferably 3.00% or less.

[0029] P: 0.001% or more and 0.100% or less P is an element that has a solid solution strengthening effect and increases the TS and YS of the steel sheet. To obtain this effect, the P content is set to 0.001% or more. On the other hand, if the P content exceeds 0.100%, P segregates at the prior austenite grain boundaries, embrittling the grain boundaries. Therefore, during a V-bend test, voids are generated and cracks propagate along the prior austenite grain boundaries, making it difficult to ensure press formability at the steel sheet edge (bendability at the steel sheet edge (shear cross section)). Furthermore, in a 90-degree V-bend test with a bending radius of 0.5 mm, the crack length propagating from the bend edge toward the ridgeline cannot be controlled to 200 μm or less. Therefore, the P content is set to 0.001% or more and 0.100% or less. The P content is preferably 0.002% or more, more preferably 0.004% or more. The P content is preferably 0.030% or less. The P content is preferably 0.025% or less, more preferably 0.020% or less.

[0030] S: 0.0001% or more and 0.0200% or less S exists in steel as sulfides. In particular, if the S content exceeds 0.0200%, voids will form and cracks will propagate from the sulfides during a V-bend test, making it difficult to ensure press formability at the steel sheet edge (bendability at the steel sheet edge (shear cross section)). Furthermore, in a 90-degree V-bend test with a bending radius of 0.5 mm, the length of cracks propagating from the edge of the bend ridge toward the ridge cannot be controlled to 200 μm or less. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0080% or less. The S content is more preferably 0.0050% or less, and even more preferably 0.0030% or less. On the other hand, due to constraints on production technology, the S content is set to 0.0001% or more, preferably 0.0003% or more, and more preferably 0.0005% or more.

[0031] Al: 0.005% or more and 2.000% or less Al promotes ferrite transformation during annealing and in the cooling process after annealing. That is, Al is an element that affects the area ratio of ferrite. Here, if the Al content is less than 0.005%, the area ratio of ferrite decreases, and ductility decreases. On the other hand, if the Al content exceeds 2.000%, the area ratio of ferrite increases excessively, making it difficult to achieve a TS of 1180 MPa or more. It also leads to a decrease in YS. Therefore, the Al content is set to 0.005% or more and 2.000% or less. The Al content is preferably 0.010% or more. The Al content is more preferably 0.020% or more, and even more preferably 0.030% or more. The Al content is preferably 1.000% or less, more preferably 0.800% or less, and further preferably 0.500% or less.

[0032] N: 0.0100% or less N exists in steel as nitrides. In particular, if the N content exceeds 0.0100%, voids will form and cracks will propagate from the nitrides during a V-bend test, making it difficult to ensure press formability at the steel sheet edge (bendability at the steel sheet edge (shear cross section)). Furthermore, in a 90-degree V-bend test with a bending radius of 0.5 mm, the length of cracks propagating from the edge of the bend ridge toward the ridge cannot be controlled to 200 μm or less. Therefore, the N content is set to 0.0100% or less. Furthermore, the N content is preferably 0.0050% or less. The N content is more preferably 0.0045% or less, and even more preferably 0.0040% or less. Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0015% or more.

[0033] Sb: 0.200% or less (including 0%) Sb is a useful element that segregates on the steel sheet surface during annealing to improve plating and chemical conversion treatment properties. Therefore, the Sb content may be 0%, but is preferably 0.002% or more. The Sb content is more preferably 0.005% or more. The Sb content is more preferably 0.007% or more, and even more preferably 0.008% or more. On the other hand, if the Sb content exceeds 0.200%, the effect of improving platability and chemical conversion treatability will saturate, and there is a risk of deterioration in the press formability (bendability within the steel sheet) and crack propagation resistance within the steel sheet. Therefore, when Sb is contained, the Sb content is set to 0.200% or less. The Sb content is more preferably 0.020% or less. The Sb content is further preferably 0.015% or less. The Sb content is even more preferably 0.012% or less, and even more preferably 0.011% or less.

[0034] Sn: 0.200% or less (including 0%) Like Sb, Sn is a useful element that segregates on the steel sheet surface during annealing to improve plating and chemical conversion treatment properties. Therefore, the Sn content may be 0%, but is preferably 0.002% or more. The Sn content is more preferably 0.003% or more. On the other hand, if the Sn content exceeds 0.200%, the effects of improving platability and chemical conversion treatability will saturate, and there is a risk of deterioration in the press formability (bendability within the steel sheet) and crack propagation resistance within the steel sheet. Therefore, when Sn is contained, the Sn content must be 0.200% or less. The Sn content is preferably 0.008% or less, and more preferably 0.004% or less.

[0035] The basic chemical composition of the base steel sheet of a steel sheet according to one embodiment of the present invention has been described above, but the base steel sheet of a steel sheet according to one embodiment of the present invention has a chemical composition containing the above basic chemical components, with the balance other than the above basic chemical components including Fe (iron) and unavoidable impurities. Here, it is preferable that the base steel sheet of a steel sheet according to one embodiment of the present invention has a chemical composition containing the above basic chemical components, with the balance consisting of Fe and unavoidable impurities.

[0036] In addition to the basic components described above, the base steel sheet of a steel sheet according to one embodiment of the present invention may contain at least one selected from the optional components shown below. Note that the effects of the present invention can be obtained so long as the optional components shown below are contained in amounts not exceeding the upper limit amounts shown below, so no lower limit is particularly set. Note that when the optional elements listed below are contained in amounts less than the preferred lower limit values ​​described below, the elements are considered to be included as inevitable impurities.

[0037] 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 , 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: 0.100 At least one selected from the group consisting of 0% 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

[0038] Nb: 0.200% or less Nb forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To achieve this effect, the Nb content is preferably 0.001% or more, and more preferably 0.005% or more. On the other hand, if the Nb content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions become the starting points for voids and cracks during a V-bend test, making it difficult to ensure the press formability of the steel sheet edge (bendability of the steel sheet edge (shear cross section)). In addition, in a 90-degree V-bend test with a bending radius of 0.5 mm, it is not possible to control the length of cracks that propagate from the edge of the bend ridge toward the ridge to 200 μm or less. Therefore, when Nb is added, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.

[0039] Ti:0.200% or less Like Nb, Ti forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To achieve this effect, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more. On the other hand, if the Ti content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions become the starting points for voids and cracks during V-bending tests, making it difficult to ensure the press formability of the steel sheet edge (bendability of the steel sheet edge (shear cross section)). In addition, in a 90-degree V-bending test with a bending radius of 0.5 mm, it is not possible to control the length of cracks that propagate from the edge of the bend ridge toward the ridge to 200 μm or less. Therefore, when Ti is added, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.

[0040] V:0.200% or less Like Nb and Ti, V forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To achieve this effect, the V content is preferably 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 even more preferably 0.030% or more. On the other hand, if the V content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions become the starting points for voids and cracks during a V-bend test, making it difficult to ensure the press formability of the steel sheet edge (bendability of the steel sheet edge (shear cross section)). In addition, in a 90-degree V-bend test with a bending radius of 0.5 mm, the length of cracks propagating from the edge of the bend ridge toward the ridge cannot be controlled to 200 μm or less. Therefore, when V is added, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.

[0041] B: 0.0100% or less B is an element that segregates at austenite grain boundaries to improve hardenability. B is also an element that controls the formation and grain growth of ferrite during cooling after annealing. To achieve this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. The B content is further preferably 0.0005% or more, and even more preferably 0.0007% or more. On the other hand, if the B content exceeds 0.0100%, cracks may occur inside the steel sheet during hot rolling. Furthermore, since the internal cracks become the starting points for cracks during a V-bend test, it becomes difficult to ensure the press formability of the steel sheet edge (bendability of the steel sheet edge (shear cross section)). In addition, in a 90-degree V-bend test with a bending radius of 0.5 mm, the length of the cracks that propagate from the edge of the bend ridge toward the ridge cannot be controlled to 200 μm or less. Therefore, when B is contained, the B content is preferably 0.0100% or less. The B content is more preferably 0.0050% or less.

[0042] Cr:1.000% or less Cr is an element that improves hardenability, and the addition of Cr produces an appropriate amount of tempered martensite, thereby increasing TS and YS. To achieve this effect, the Cr content is preferably 0.0005% or more. The Cr content is more preferably 0.100% or more, and even more preferably 0.150% or more. On the other hand, if the Cr content exceeds 1.000%, the area fraction of fresh martensite increases, bendability in a V-bend test decreases, making it difficult to ensure press formability at the steel sheet edge (bendability at the steel sheet edge (shear cross section)). Furthermore, in a 90-degree V-bend test with a bending radius of 0.5 mm, the length of cracks propagating from the edge of the bend ridge toward the ridge cannot be controlled to 200 μm or less. Therefore, when Cr is added, the Cr content is preferably 1.000% or less. The Cr content is more preferably 0.800% or less, and even more preferably 0.700% or less.

[0043] Ni: 1.000% or less Ni is an element that improves hardenability, and the addition of Ni results in the formation of a large amount of tempered martensite, thereby increasing TS and YS. To achieve this effect, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more. The Ni content is further preferably 0.040% or more, and even more preferably 0.060% or more. On the other hand, if the Ni content exceeds 1.000%, the area fraction of fresh martensite increases, reducing bendability in a V-bend test, making it difficult to ensure press formability at the steel sheet edge (bendability at the steel sheet edge (shear cross section)). Furthermore, in a 90-degree V-bend test with a bending radius of 0.5 mm, the length of cracks propagating from the edge of the bend ridge toward the ridge cannot be controlled to 200 μm or less. Therefore, when Ni is added, the Ni content is preferably 1.000% or less. The Ni content is more preferably 0.800% or less. The Ni content is even more preferably 0.600% or less, and even more preferably 0.400% or less.

[0044] Mo: 1.000% or less Mo is an element that improves hardenability, and the addition of Mo increases the formation of a large amount of tempered martensite, thereby increasing TS and YS. To obtain this effect, the Mo content is preferably 0.100% or more, and more preferably 0.150% or more. On the other hand, if the Mo content exceeds 1.000%, the area ratio of fresh martensite increases, bendability in a V-bend test decreases, and it becomes difficult to ensure the press formability of the steel sheet edge (bendability of the steel sheet edge (shear cross section)). Furthermore, in a 90-degree V-bend test with a bending radius of 0.5 mm, the crack length propagating from the bend edge toward the ridgeline cannot be controlled to 200 μm or less. Therefore, when Mo is added, the Mo content is preferably 1.000% or less. The Mo content is more preferably 0.500% or less, even more preferably 0.450% or less, and even more preferably 0.400% or less. The Mo content is more preferably 0.350% or less, and even more preferably 0.300% or less.

[0045] Cu:1.000% or less Cu is an element that improves hardenability, and the addition of Cu results in the formation of a large amount of tempered martensite, thereby increasing TS and YS. To achieve this effect, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.008% or more, and even more preferably 0.010% or more. The Cu content is more preferably 0.020% or more. The Cu content is even more preferably 0.100% or more, and even more preferably 0.150% or more. On the other hand, if the Cu content exceeds 1.000%, the area fraction of fresh martensite may increase excessively. Furthermore, large amounts of coarse precipitates and inclusions may form. In such cases, the excessively formed fresh martensite and coarse precipitates and inclusions become the starting points for voids and cracks during V-bending tests, making it difficult to ensure the press formability of the steel sheet edge (bendability of the steel sheet edge (shear cross section)). Furthermore, in a 90-degree V-bending test with a bending radius of 0.5 mm, the crack length propagating from the edge of the bend ridge toward the ridge cannot be controlled to 200 μm or less. Therefore, when Cu is added, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.

[0046] Ta:0.100% or less Ta, like Ti, Nb, and V, increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. Additionally, Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb,Ta)(C,N). This inhibits coarsening of precipitates and stabilizes precipitation strengthening. This further improves TS and YS. To achieve these effects, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.002% or more, and even more preferably 0.004% or more. On the other hand, if the Ta content exceeds 0.100%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the excessively coarse precipitates and inclusions become the starting points for voids and cracks during a V-bend test, making it difficult to ensure the press formability of the steel sheet edge (bendability of the steel sheet edge (shear cross section)). Furthermore, in a 90-degree V-bend test with a bending radius of 0.5 mm, the length of cracks propagating from the edge of the bend ridge toward the ridge cannot be controlled to 200 μm or less. Therefore, when Ta is added, the Ta content is preferably 0.100% or less. The Ta content is more preferably 0.030% or less, and even more preferably 0.010% or less.

[0047] W: 0.500% or less W is an element that improves hardenability, and the addition of W increases the formation of a large amount of tempered martensite, thereby increasing TS and YS. To achieve this effect, the W content is preferably 0.001% or more. The W content is more preferably 0.010% or more, and even more preferably 0.030% or more. On the other hand, if the W content exceeds 0.500%, the area ratio of fresh martensite increases, bendability in a V-bend test decreases, and it becomes difficult to ensure the press formability of the steel sheet edge (bendability of the steel sheet edge (shear cross section)). In addition, the crack length propagating from the bend edge toward the ridgeline in a 90-degree V-bend test with a bending radius of 0.5 mm cannot be controlled to 200 μm or less. Therefore, when W is contained, the W content is preferably 0.500% or less. The W content is more preferably 0.450% or less, and even more preferably 0.400% or less. The W content is even more preferably 0.300% or less.

[0048] Mg: 0.0200% or less Mg is an element that is effective in spheroidizing inclusions such as sulfides and oxides and improving the hole expandability and bendability of steel sheets. To achieve this effect, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0010% or more, and even more preferably 0.0030% or more. On the other hand, if the Mg content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the excessively coarse precipitates and inclusions become the starting points for voids and cracks during a V-bend test, making it difficult to ensure the press formability of the steel sheet edge (bendability of the steel sheet edge (shear cross section)). Furthermore, in a 90-degree V-bend test with a bending radius of 0.5 mm, the length of cracks propagating from the edge of the bend ridge toward the ridge cannot be controlled to 200 μm or less. Therefore, when Mg is added, the Mg content is preferably 0.0200% or less. The Mg content is more preferably 0.0150% or less, and even more preferably 0.0100% or less.

[0049] Zn: 0.0200% or less Zn is an element that effectively spheroidizes the shape of inclusions and improves the hole expandability and bendability of steel sheets. To achieve this effect, the Zn content is preferably 0.0010% or more. The Zn content is more preferably 0.0020% or more, and even more preferably 0.0030% or more. On the other hand, if the Zn content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the excessively coarse precipitates and inclusions become the starting points for voids and cracks during V-bending tests, making it difficult to ensure the press formability of the steel sheet edge (bendability of the steel sheet edge (shear cross section)). Furthermore, in a 90-degree V-bending test with a bending radius of 0.5 mm, the length of cracks propagating from the edge of the bend ridge toward the ridge cannot be controlled to 200 μm or less. Therefore, when Zn is added, the Zn content is preferably 0.0200% or less. The Zn content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.

[0050] Co:0.0200% or less Like Zn, Co is an element that effectively spheroidizes inclusions and improves the hole expandability and bendability of steel sheets. To achieve this effect, the Co content is preferably 0.0010% or more. The Co content is more preferably 0.0020% or more, and even more preferably 0.0030% or more. On the other hand, if the Co content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the excessively coarse precipitates and inclusions may become the starting points for voids and cracks during a V-bend test, making it difficult to obtain the desired R / t. Therefore, when Co is added, the Co content is preferably 0.0200% or less. The Co content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.

[0051] Zr: 0.1000% or less Like Zn and Co, Zr is an element that is effective in making inclusions spheroidal and improving the hole expandability and bendability of steel sheets. To obtain such effects, the Zr content is preferably 0.0010% or more. On the other hand, if the Zr content exceeds 0.1000%, excessively coarse precipitates and inclusions may become the starting points for voids and cracks during V-bend testing, making it difficult to obtain the desired R / t. Therefore, when Zr is added, the Zr content is preferably 0.1000% or less. The Zr content is more preferably 0.0300% or less, even more preferably 0.0150% or less, and even more preferably 0.0100% or less.

[0052] Ca:0.0200% or less Ca exists as inclusions in steel. If the Ca content exceeds 0.0200%, a large amount of coarse inclusions may form. In such cases, excessively coarse precipitates and inclusions may become the starting point for voids and cracks during V-bend testing, preventing the desired R / t ratio from being achieved. Therefore, when Ca is added, the Ca content is preferably 0.0200% or less. The Ca content is preferably 0.0020% or less. The Ca content is more preferably 0.0019% or less, and even more preferably 0.0018% or less. While there is no particular lower limit for the Ca content, the Ca content is preferably 0.0005% or more. Due to production technology constraints, the Ca content is more preferably 0.0010% or more.

[0053] 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 Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are all elements effective for improving the hole expandability and bendability of steel sheets. To obtain these effects, the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are each preferably 0.0001% or more. On the other hand, if the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM exceed 0.0200% each, and / or the content of As exceeds 0.0500%, large amounts of coarse precipitates and inclusions may form. In such cases, the excessively coarse precipitates and inclusions may become the initiation points for voids and cracks during V-bend testing, preventing the desired R / t ratio from being achieved. Therefore, when at least one of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM is contained, the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM are preferably 0.0200% or less. Furthermore, when As is contained, the As content is preferably 0.0500% or less.

[0054] The Se content is preferably 0.0010% or more, more preferably 0.0050% or more, more preferably 0.0180% or less, and further preferably 0.0150% or less. The Te content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Te content is even more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Te content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Ge content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Ge content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Ge content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The As content is more preferably 0.0010% or more, and even more preferably 0.0015% or more. The As content is even more preferably 0.0050% or more. The As content is more preferably 0.0400% or less, and even more preferably 0.0300% or less. The Sr content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Sr content is even more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Sr content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Cs content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Cs content is even more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Cs content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Hf content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Hf content is even more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Hf content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Pb content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Pb content is even more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Pb content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Bi content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Bi content is even more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Bi content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Bi content is even more preferably 0.0100% or less. The REM content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The REM content is more preferably 0.0010% or more, and even more preferably 0.0020% or more. The REM content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The REM content is even more preferably 0.0100% or less. In the present invention, REM refers to scandium (Sc), which has atomic number 21, yttrium (Y), which has atomic number 39, and the lanthanoids ranging from lanthanum (La), which has atomic number 57, to lutetium (Lu), which has atomic number 71. The REM concentration in the present invention refers to the total content of one or more elements selected from the above-mentioned REM. The REM is not particularly limited, but is preferably at least one of Sc, Y, Ce and La.

[0055] Steel structure (structure at 1 / 4 of the thickness of the base steel plate) Next, the structure of the steel sheet according to one embodiment of the present invention excluding the surface soft layer will be described.

[0056] Ferrite area ratio: 55.0% or less (including 0.0%) Soft ferrite is a phase that improves ductility. However, if the area fraction of ferrite increases excessively, it becomes difficult to achieve a TS of 1180 MPa or more. It also leads to a decrease in YS. Therefore, the area fraction of ferrite is set to 55.0% or less. The area fraction of ferrite is preferably set to 45.0% or less, and more preferably set to 30.0% or less. The lower limit of the area ratio of ferrite is not particularly limited and may be 0.0%. The area ratio of ferrite may be 1.0% or more, or 2.0% or more.

[0057] Total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): Over 40.0% and up to 100.0% Bainitic ferrite and tempered martensite have intermediate hardness between soft ferrite and hard fresh martensite, and are important phases for ensuring good bendability, shear edge bendability, and axial crush resistance of steel sheets. Bainitic ferrite is also useful for obtaining an appropriate amount of retained austenite by utilizing the diffusion of C from bainitic ferrite to untransformed austenite. Tempered martensite is effective for improving TS. Therefore, the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is set to more than 40.0%. The total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is preferably 65.0% or more, and more preferably 80.0% or more. On the other hand, the upper limit of the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) may be 100.0%. The total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) may be 96.0% or less, or 92.0% or less. Bainitic ferrite is upper bainite with little carbide that is produced in a relatively high temperature range.

[0058] Area ratio of retained austenite: Less than 3.5% (including 0.0%) In the present invention, if the area fraction of retained austenite is excessively increased, hard martensite is formed by the deformation-induced transformation of the retained austenite during a V-bend test. This generates voids and cracks at the heterophase boundary with the hard martensite, making it difficult to ensure press formability at the steel sheet edge (bendability at the steel sheet edge (shear cross section)). This may prevent the crack length propagating from the bend edge toward the ridgeline to 200 μm or less in a 90-degree V-bend test with a bending radius of 0.5 mm. Therefore, the area fraction of retained austenite is set to less than 3.5%. The area fraction of retained austenite is preferably 3.0% or less, more preferably 2.5% or less, and even more preferably 2.0% or less. The lower limit of the area ratio of retained austenite is not particularly limited, and the area ratio of retained austenite may be 0.0%, preferably 0.1% or more, and more preferably 0.2% or more. The retained austenite here also includes (isolated) island-like retained austenite within ferrite grains.

[0059] Area ratio of fresh martensite: 10.0% or less (including 0.0%) In the present invention, if the area fraction of fresh martensite is excessively increased, the fresh martensite becomes the starting point for void generation during a 90-degree V-bend test, and the desired R / t, bendability inside the steel sheet, and bendability at the edge (shear cross section) of the steel sheet may not be obtained. Therefore, the area fraction of fresh martensite is set to 10.0% or less. The area fraction of fresh martensite is preferably set to 8.0% or less, and more preferably to 6.0% or less. The lower limit of the area fraction of fresh martensite is not particularly limited and may be 0.0%. The area fraction of fresh martensite may be 1.0% or more, or 2.0% or more. The term "fresh martensite" used here refers to as-quenched (untempered) martensite, and also includes (isolated) fresh martensite islands within ferrite grains.

[0060] The area ratio of the remaining structure other than the above is preferably 10.0% or less. The area ratio of the remaining structure is more preferably 7.0% or less, and even more preferably 5.0% or less. The area ratio of the remaining structure may also be 0.0%.

[0061] The remaining structure is not particularly limited, and examples thereof include carbides such as pearlite and cementite. The type of the remaining structure can be confirmed, for example, by observation using a SEM (Scanning Electron Microscope).

[0062] Here, the area ratios of ferrite, bainitic ferrite, tempered martensite, and hard second phase (fresh martensite + retained austenite) are measured at a 1 / 4 position in the sheet thickness direction of the base steel sheet as follows. Specifically, a sample is cut out so that the observation surface is the thickness cross section (L cross section) parallel to the rolling direction of the base steel sheet. The observation surface of the sample is then polished with diamond paste, and then finish polished with alumina. The observation surface of the sample is then etched with 1 vol.% nital to reveal the structure. Next, the observation position is set at 1 / 4 of the steel plate thickness, and five fields of view are observed using an SEM at a magnification of 3000x. The fields of view to be observed are selected within a range of 1 / 4 of the steel plate thickness ±100 μm, with each field of view being 38 μm × 30 μm. From the obtained structural images, Adobe Photoshop from Adobe Systems Inc. is used to calculate the area ratios for each of the five fields of view by dividing the area of ​​each constituent structure (ferrite, bainitic ferrite, tempered martensite, and hard second phase (fresh martensite + retained austenite)) by the measured area, and these values ​​are averaged to determine the area ratio of each structure. At the outermost layer position, the galvanized layer is excluded, and the image is taken to include the internal oxide layer.

[0063] Ferrite: A black region with a blocky shape. It contains almost no carbides. Also, isolated fresh martensite islands and isolated retained austenite islands within the ferrite grains are not included in the ferrite area ratio. Bainitic ferrite: This region is black to dark gray in color and has a massive or amorphous shape. It also contains a relatively small number of carbides. Tempered martensite: This is a gray area with an amorphous shape. It also contains a relatively large number of carbides. Hard second phase (retained austenite + fresh martensite): This is a white to light gray region with an amorphous morphology and does not contain carbides. Carbides: These are white areas that are dotted or linear in shape and are included in bainite, tempered bainite, and tempered martensite. Remaining structure: The above-mentioned pearlite and cementite can be mentioned, and the forms thereof are as known.

[0064] The area ratio of retained austenite is measured as follows. The base steel sheet is mechanically ground in the thickness direction (depth direction) to a position one-quarter of the thickness, and then chemically polished with oxalic acid to obtain the observation surface. The observation surface is then observed using X-ray diffraction. MoKα radiation is used as the incident X-ray, and the ratio of the diffraction intensity of the (200), (211), and (220) planes of fcc iron (austenite) to the diffraction intensity of the (200), (220), and (311) planes of bcc iron is calculated. The volume fraction of retained austenite is then calculated from the ratio of the diffraction intensities of each plane. The retained austenite is then considered to be three-dimensionally homogeneous, and the volume fraction of retained austenite is taken as the area fraction of retained austenite.

[0065] 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. [Area fraction of fresh martensite (%)] = [Area fraction of hard second phase (%)] - [Area fraction of retained austenite (%)]

[0066] The area ratio of the remaining structure is determined by subtracting the area ratio of ferrite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of the hard second phase determined as described above from 100.0%. [Area fraction of remaining structure (%)] = 100.0 - [Area fraction of ferrite (%)] - [Area fraction of bainitic ferrite (%)] - [Area fraction of tempered martensite (%)] - [Area fraction of hard second phase (%)]

[0067] surface soft layer The base steel sheet of the steel sheet according to one embodiment of the present invention preferably has a soft surface layer on the surface of the base steel sheet. The soft surface layer contributes to suppressing the propagation of bending cracks during press forming and vehicle collisions, thereby further improving bending fracture resistance. The soft surface layer refers to a decarburized layer, and is a surface region having a Vickers hardness of 84% or less of the Vickers hardness of the cross section at 1 / 4 of the sheet thickness. Vickers hardness is measured based on JIS Z 2244-1 (2020) at a load of 10 gf.

[0068] Thickness of the soft surface layer (X): 20≦X≦120 − 3800×[Sb] − 1900×[Sn] (1) In the formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively. The soft surface layer in the present invention refers to a region where the Vickers hardness is 84% ​​or less of the Vickers hardness at a position 1 / 4 of the sheet thickness from the surface of the base steel sheet. The soft surface layer thickness (X) must satisfy the formula (1). If the surface soft layer thickness (X) is less than 20 μm, it is not possible to achieve both high strength and excellent bendability as intended by the present invention. On the other hand, if the surface soft layer thickness (X) exceeds (120-3800×[Sb]-1900×[Sn]) μm, it is not possible to achieve both high strength and excellent press formability as intended by the present invention. Therefore, the thickness (X) of the surface soft layer is specified to be 20 μm or more and (120−3800×[Sb]−1900×[Sn]) μm or less. In the present invention, as described above, Sb and Sn are added as needed to improve plating and chemical conversion properties, but when Sb and Sn are added, the surface segregation of these elements described above reduces the allowable upper limit of the soft surface layer thickness (X) that affects bending cracks. For this reason, the upper limit of the soft surface layer that provides good bendability is (120 - 3800 × [Sb] - 1900 × [Sn]) μm. The thickness of the surface soft layer is preferably 30 μm or more, and more preferably 40 μm or more. The thickness of the surface soft layer is preferably 120 μm or less, and more preferably 100 μm or less.

[0069] Tissues in the superficial soft layer Ferrite area ratio: 60.0% to 100.0% When subjected to bending, the surface layer is deformed more than the interior. Therefore, voids are likely to form in the surface layer. In the present invention, by controlling the area ratio of ferrite in the soft surface layer to 60.0% or more, voids that serve as crack initiation points are less likely to form in the surface layer, and crack propagation is suppressed. Therefore, the area ratio of ferrite is set to 60.0% or more. The area ratio of ferrite is preferably set to 80.0% or more, and more preferably to 90.0% or more. The area ratio of ferrite may be 100.0%, or less than 100.0%, or may be 98.0% or less, or 96.0% or less.

[0070] The area ratio of fresh martensite divided by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite): 0.5 or less If the area fraction of fresh martensite in the soft surface layer is excessively increased, the fresh martensite becomes the starting point for void formation in a 90-degree V-bend test, making it impossible to achieve the desired internal bendability of the steel sheet. From the viewpoint of ensuring good internal bendability of the steel sheet and bendability of the sheared edge, when the area fraction of ferrite in the soft surface layer is less than 100.0%, the value obtained by dividing the area fraction of fresh martensite by the total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is set to 0.5 or less. This value may be set to 0.4 or less, or 0.35 or less. The lower limit of the value obtained by dividing the area ratio of fresh martensite in the soft surface layer by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is not particularly limited and may be 0.0. This value may be 0.1 or more, or 0.15 or more.

[0071] Area ratio of retained austenite: 3.0% or less 2.0 kgf / mm in the reheating temperature range after the first holding process 2 In the surface strain introduction process, in which the above tension is applied, the untransformed austenite in the soft surface layer is transformed into hard martensite through deformation-induced transformation, and in the subsequent second holding process, in which the material is held for 10 seconds or more, the hard martensite is tempered to produce tempered martensite, and the area fraction of retained austenite is finally controlled to 3.0% or less.This makes it possible to reduce the length of cracks that propagate from the end of the bend ridge toward the ridge in a 90-degree V-bend test with a bending radius R of 0.5 mm to 200 μm or less. If the area fraction of retained austenite exceeds 3.0%, hard martensite is formed by the stress-induced transformation of the retained austenite during a V-bend test, and voids are generated and cracks propagate at the heterophase boundary with the hard martensite. As a result, it becomes difficult to ensure the press formability of the steel sheet edge (bendability of the steel sheet edge (shear cross section)). Furthermore, in a 90-degree V-bend test with a bending radius of 0.5 mm, the crack length propagating from the edge of the bend toward the ridge cannot be controlled to 200 μm or less. Therefore, the area fraction of retained austenite is set to 3.0% or less. The area fraction of retained austenite is preferably set to 2.8% or less, and more preferably to 2.5% or less. The lower limit of the area ratio of retained austenite is not particularly limited, but is preferably 0.2% or more, and more preferably 0.5% or more.

[0072] To measure the structure of the soft surface layer, if a zinc-plated layer is formed on the steel sheet, first peel off the zinc-plated layer and measure the structure at a position half the thickness of the soft surface layer in the same way as at a position 1 / 4 of the sheet thickness of the base steel sheet.

[0073] Next, the mechanical properties of the steel plate according to one embodiment of the present invention will be described.

[0074] Tensile strength (TS): 1180 MPa or more, less than 1470 MPa The tensile strength (TS) of the steel plate according to one embodiment of the present invention is 1180 MPa or more and less than 1470 MPa. The yield stress (YS) and total elongation (El) of the steel sheet according to one embodiment of the present invention are as described above.

[0075] The tensile strength (TS), yield stress (YS) and total elongation (El) are measured by a tensile test in accordance with JIS Z 2241 (2011) which will be described later in the examples. The ratio YR (yield ratio) of the yield stress (YS) to the tensile strength (TS) preferably satisfies 0.70≦YR. In addition, in a steel sheet according to one embodiment of the present invention, R (critical bending radius) / t (sheet thickness), the crack length propagating from the bend ridge end toward the ridge, El when a notched tensile test was conducted after heat treatment at 170°C for 20 minutes, and El when a notched tensile test was conducted after heat treatment at 170°C for 20 minutes with a 2% nominal tensile strain introduced are as described above. These are measured by the methods described later in the examples.

[0076] Plated layer (hot-dip galvanized layer, alloyed hot-dip galvanized layer) A steel sheet according to one embodiment of the present invention may have a plating layer formed on the base steel sheet (on the surface of the base steel sheet), and this plating layer may be provided on only one surface of the base steel sheet, or on both surfaces.

[0077] The plating layer (galvanized layer) referred to here refers to a plating layer containing Zn as the main component (Zn content of 50.0% or more), and examples thereof include a hot-dip galvanized layer and a hot-dip galvannealed layer.

[0078] Here, the hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0 mass% or less of Fe, and 0.001 mass% to 1.0 mass% of Al. The hot-dip galvanized layer may optionally contain 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 a total amount of 0.0 mass% to 3.5 mass%. The Fe content of the hot-dip galvanized layer is more preferably less than 7.0 mass%. The remainder other than the above elements is unavoidable impurities.

[0079] The galvannealed layer is preferably composed of, for example, 20.0% by mass or less of Fe and 0.001% by mass to 1.0% by mass of Al. The galvannealed layer may optionally contain 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 a total amount of 0.0% by mass to 3.5% by mass. The Fe content of the galvannealed layer is more preferably 7.0% by mass or more, and even more preferably 8.0% by mass or more. The Fe content of the galvannealed layer is more preferably 15.0% by mass or less, and even more preferably 12.0% by mass or less. The remainder other than the above elements is unavoidable impurities.

[0080] In addition, the coating weight of the plating layer (zinc plating layer) per side is not particularly limited, but is preferably 20 g / m 2 More than 80g / m 2 It is preferable to do the following:

[0081] The coating weight of the plating layer (zinc plating layer) is measured as follows. That is, a treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe ("Ivit 700BK" (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass % aqueous solution of hydrochloric acid. Next, a steel sheet (galvanized steel sheet) to be used as a test material is immersed in the treatment solution to dissolve the plating layer (galvanized layer). The mass loss of the test material before and after dissolution is measured, and this value is divided by the surface area of ​​the base steel sheet (the surface area of ​​the part that was covered with the plating) to determine the plating coverage (g / m 2 ) is calculated.

[0082] 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, more preferably 0.6 mm or more, and even more preferably 0.8 mm or more. The thickness of the steel plate is preferably 2.3 mm or less, more preferably 1.6 mm or less, and even more preferably 1.2 mm or less.

[0083] [2. Steel plate manufacturing method] Next, a method for manufacturing a steel sheet according to one embodiment of the present invention will be described. The method for producing a steel sheet of the present invention includes a hot rolling step of hot rolling a steel slab having the above-described chemical composition under a finish rolling temperature of 820°C or higher, and an annealing step of heating the steel sheet after the hot rolling step and annealing it under conditions of an annealing temperature of 750°C or higher and 900°C or lower, an annealing time of 20 seconds or longer, and an atmosphere with a dew point of -10°C or higher, and further satisfying formulas (2) and (3). a cooling step of cooling the steel sheet after the annealing step to a cooling stop temperature of less than 100°C; a first holding step of reheating the steel sheet after the cooling step to a reheating holding temperature range of not less than the cooling stop temperature and not more than 440°C and holding the temperature for not less than 10 seconds; The steel sheet after the first holding step is subjected to a pressure of 2.0 kgf / mm in the reheating holding temperature range. 2 a surface strain introduction step of applying the above tension; and a second holding step of holding the steel sheet after the surface strain introduction step in a reheating holding temperature range for 10 seconds or more, or further including a cold rolling step of cold rolling the steel sheet after the hot rolling step and before the annealing step at a rolling reduction of 20% or more and 80% or less to obtain a cold-rolled steel sheet. 2400≦Y≦20000...Formula (2) Y=[{(T-Ac1)×t1} / 2}]+{(T-Ac1)×t2}...Equation (3) In the formula (3), T is the annealing temperature (°C), t1 is the time (s) from 650°C to the annealing temperature T during the temperature rise in the annealing process, t2 is the annealing time (soaking time) (s), and Ac1 is Ac1 (°C). Unless otherwise specified, the above temperatures refer to the surface temperatures of the steel slab and steel plate.

[0084] In the present invention, the method for melting the steel material (steel slab) is not particularly limited, and any of the known melting methods, such as converters and electric furnaces, is suitable. Furthermore, to prevent macrosegregation, it is preferable to produce the steel slab by continuous casting, but it is also possible to produce it by ingot casting, thin slab casting, or other methods. Furthermore, in addition to the conventional method in which the steel slab is cooled to room temperature after production and then reheated, energy-saving processes such as direct rolling, in which the slab is charged into a heating furnace as a hot slab without being cooled to room temperature, or is rolled immediately after a short period of heat retention, can also be applied without any problems.

[0085] (Hot rolling process) When heating a slab, the slab heating temperature is preferably 1100°C or higher from the viewpoint of dissolving carbides and reducing the rolling load. Furthermore, in order to prevent an increase in scale loss, the slab heating temperature is preferably 1300°C or lower. The slab heating temperature is the temperature of the slab surface. Furthermore, the slab is made into a sheet bar by rough rolling under normal conditions. However, if the heating temperature is lower, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling in order to prevent problems during hot rolling.

[0086] Finishing rolling temperature: 820°C or higher Finish rolling increases the rolling load and the reduction ratio in the unrecrystallized austenite state, resulting in the development of an abnormal structure elongated in the rolling direction, which reduces the ductility, hole expandability, and bendability of the final material. For this reason, the finish rolling temperature is set to 820°C or higher. The finish rolling temperature is preferably 830°C or higher, and more preferably 850°C or higher. The finish rolling temperature is also preferably 1080°C or lower, and more preferably 1050°C or lower.

[0087] Furthermore, the coiling temperature after hot rolling is not particularly limited, but it is necessary to consider the possibility of reducing the ductility, hole expandability, and bendability of the final material. Therefore, the coiling temperature after hot rolling is preferably 300°C or higher. Furthermore, the coiling temperature after hot rolling is preferably 700°C or lower.

[0088] During hot rolling, the rough-rolled sheets may be joined together and continuously finished. The rough-rolled sheets may also be temporarily wound. To reduce the rolling load during hot rolling, some or all of the finish rolling may be performed as lubricated rolling. Lubricated rolling is also effective from the viewpoint of uniformity of the steel sheet shape and material properties. The coefficient of friction during lubricated rolling is preferably in the range of 0.10 to 0.25.

[0089] (pickling process) The hot-rolled steel sheet produced as described above may be subjected to pickling. Pickling can remove oxides from the steel sheet surface, and therefore can be performed to ensure good chemical conversion treatability and plating quality in the final high-strength steel sheet product. Furthermore, pickling may be performed once or multiple times.

[0090] (Cold rolling process) The hot-rolled pickled sheet or hot-rolled steel sheet obtained as described above is subjected to cold rolling as necessary. When cold rolling is performed, the pickled sheet may be subjected to cold rolling as it is after hot rolling, or may be subjected to heat treatment and then cold rolling. Optionally, the cold-rolled steel sheet obtained after cold rolling may be subjected to pickling. The cold rolling is carried out by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.

[0091] If necessary, cold rolling reduction: 20% to 80% When cold rolling is performed, the reduction ratio (cumulative reduction ratio) of the cold rolling is not particularly limited, but is preferably 20% or more and 80% or less. If the reduction ratio of the cold rolling is less than 20%, the steel structure is likely to become coarse and non-uniform during the annealing process, which may result in a decrease in TS and bendability in the final product. On the other hand, if the reduction ratio of the cold rolling exceeds 80%, the steel sheet is likely to have a defective shape and the amount of zinc coating may become non-uniform.

[0092] (Annealing process) In one embodiment of the present invention, the steel sheet after the hot rolling process (after the cold rolling process if cold rolling is performed) is heated and annealed under the conditions of an annealing temperature of 750°C or more and 900°C or less, an annealing time of 20 seconds or more, and an atmosphere with a dew point of -10°C or more, and further under conditions that satisfy formulas (2) and (3). 2400≦Y≦20000...Formula (2) Y=[{(T-Ac1)×t1} / 2}]+{(T-Ac1)×t2}...Equation (3) In the formula (3), T is the annealing temperature (°C), t1 is the time (s) from 650°C to the annealing temperature T during the temperature rise in the annealing process, t2 is the annealing time (s), and Ac1 is Ac1 (°C).

[0093] Annealing temperature: 750℃ to 900℃ If the annealing temperature is less than 750°C, the proportion of austenite generated during heating in the ferrite-austenite two-phase region will be insufficient, resulting in an excessive increase in the area fraction of ferrite after annealing, making it impossible to obtain the desired TS and YS. On the other hand, if the annealing temperature exceeds 900°C, the ductility decreases. Therefore, the annealing temperature is set to 750°C or higher and 900°C or lower. The annealing temperature is preferably 880°C or lower. The annealing temperature is more preferably 870°C or lower. The annealing temperature is preferably 780°C or higher, more preferably 800°C or higher. The annealing temperature is the maximum temperature (soaking temperature) reached in the annealing process.

[0094] Annealing time (soaking time): 20 seconds or more If the annealing time is less than 20 seconds, the austenite generation rate during heating in the two-phase region of ferrite and austenite becomes insufficient. As a result, the area ratio of ferrite increases excessively after annealing, and TS and YS cannot be obtained. Therefore, the annealing time is set to 20 seconds or more. The annealing time is preferably 30 seconds or more, more preferably 50 seconds or more. Although there is no particular upper limit for the annealing time, the annealing time is preferably 600 seconds or less, more preferably 250 seconds or less. The annealing time (soaking time) refers to the holding time in the temperature range of (annealing temperature -40°C) or more and (inclusive) of the annealing temperature. In other words, 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 (inclusive) of the annealing temperature during heating and cooling before and after reaching the annealing temperature. The number of annealing steps may be two or more, but one step is preferred from the viewpoint of energy efficiency.

[0095] Dew point of the annealing process atmosphere (annealing atmosphere): -10°C or higher In one embodiment of the present invention, the dew point of the atmosphere in the annealing step (annealing atmosphere) is preferably −10° C. or higher. By performing annealing in the annealing step with a dew point of −10° C. or higher, the decarburization reaction is promoted and a deeper soft surface layer can be formed. The dew point of the annealing atmosphere in the annealing step is preferably −5° C. or higher, more preferably 0° C. or higher, and even more preferably +5° C. or higher. There is no particular upper limit for the dew point of the annealing atmosphere in the annealing step, but in order to suitably prevent oxidation of the surface of the Fe-based electroplated layer and improve plating adhesion when a zinc plating layer is formed, the dew point of the annealing atmosphere in the annealing step is preferably 30° C. or lower. The dew point of the annealing atmosphere in the annealing step is more preferably 25° C. or lower, and even more preferably 20° C. or lower.

[0096] 2400≦Y≦20000...Formula (2) Y=[{(T-Ac1)×t1} / 2}]+{(T-Ac1)×t2}...Equation (3) In the formula (3), T is the annealing temperature (°C), t1 is the time (s) from 650°C to the annealing temperature (soaking temperature) T during the temperature rise in the annealing process, t2 is the annealing time (s), and Ac1 is Ac1 (°C). In the present invention, it is necessary to perform annealing under conditions that satisfy formulas (2) and (3). When Y in formula (3) is less than 2400, the soft surface layer defined in the present invention will be less than 20 μm. On the other hand, when Y exceeds 20,000, the soft surface layer defined in the present invention will be more than (120 - 3,800 × [Sb] - 1,900 × [Sn]) μm. Therefore, Y in formula (3) is set to be 2,400 or more and 20,000 or less. Y is preferably 9,000 or more, more preferably 12,000 or more. Also, Y is preferably 19,000 or less, and more preferably 18,000 or less. t1 is preferably 30 seconds or more, and t1 is preferably 80 seconds or less.

[0097] Ac1 (℃) is calculated using the following formula: Ac1(℃)=727.0-32.7×[%C]+14.9×[%Si]+2.0×[%Mn] Here, [%C] is the C content of the steel plate (steel slab), [%Si] is the Si content of the steel plate (steel slab), and [%Mn] is the Mn content of the steel plate (steel slab).

[0098] (cooling process) Cool to a cooling stop temperature of less than 100°C Average cooling rate: 10°C / s or more, 50°C / s or less, atmospheric dew point: -20°C or less (preferred requirements) In the cooling step, the steel sheet after the annealing step is cooled to a cooling stop temperature of less than 100° C. At this time, the cooling start temperature can be set to 750° C. or higher and 900° C. or lower. The cooling stop temperature is preferably 80°C or lower, more preferably 60°C or lower. The cooling stop temperature is preferably 5°C or higher, more preferably 15°C or higher. The average cooling rate during this cooling step is preferably 10° C. / s or more and 50° C. / s or less. This cooling step makes it possible to obtain the steel structure specified in the present invention. Furthermore, it is preferable that the dew point of the atmosphere in this cooling step be -20°C or lower. If the dew point of the atmosphere exceeds -20°C, the thickness of the soft surface layer in the in-plane direction of the steel sheet will vary greatly, and the tensile strength specified in the present invention may not be obtained. Therefore, it is preferable that the dew point of the atmosphere in this cooling step be -20°C or lower. The average cooling rate (°C / s) is obtained by dividing the difference between the cooling start temperature (°C) and the cooling stop temperature (°C) in the cooling step by the cooling time (s).

[0099] (First holding process (reheating holding process)) In the first holding step (reheating holding step), the steel sheet is reheated to a reheating holding temperature range of not less than the cooling stop temperature and not more than 440°C, and held for not less than 10 seconds. The bainitic ferrite and tempered martensite defined in the present invention can be formed by reheating the steel sheet to a reheating temperature range of from the cooling stop temperature to 440°C and holding the temperature for 10 seconds or more. The reheating temperature range is preferably 420°C or lower, and more preferably 400°C or lower. The retention time is preferably 20 seconds or more, more preferably 30 seconds or more. The retention time is preferably 100 seconds or less, more preferably 80 seconds or less.

[0100] (Surface strain introduction process) In the surface strain introduction process, the steel sheet after the first holding process is subjected to a strain of 2.0 kgf / mm in the reheating holding temperature range. 2 This allows the El value to be controlled to be equal to or greater than the value specified in the present invention when a notched tensile test is carried out after heat treatment at 170°C for 20 minutes. The tension is calculated by multiplying the total load (kgf) of the load cells on the left and right sides of the roll through which the steel plate passes while in contact with the roll by the cross-sectional area of ​​the steel plate (= plate thickness (mm) × plate width (mm)) (mm 2 ) The load cell must be placed parallel to the tension direction. Here, the load cells are preferably positioned 200 mm from both ends of the roll, and the body length of the roll used is preferably 1500 to 2500 mm. The tension is preferably 2.2 kgf / mm 2 More preferably, it is 2.4 kgf / mm 2 More preferably, this tension is 10.0 kgf / mm 2 More preferably, it is 4.0 kgf / mm or less. 2 Here, the unit of tension is 1 kgf / mm 2 9.8N / mm 2 As kgf / mm 2 to N / mm 2 can be converted into

[0101] (Second holding process) In the second holding step, the steel sheet after the surface strain introduction step is held in the reheating holding temperature range for 10 seconds or more. This allows the El value when a notched tensile test is performed after introducing a 2% nominal tensile strain and heat treating at 170°C for 20 minutes to be controlled to be equal to or greater than the value specified in the present invention. The holding time in the second holding step is preferably 15 seconds or more, more preferably 20 seconds or more. The holding time is preferably 60 seconds or less, more preferably 50 seconds or less.

[0102] (Plating process (hot-dip galvanizing process, galvannealed hot-dip galvanizing process)) In the present invention, after the annealing step, the steel sheet is subjected to a zinc plating treatment in the plating step, thereby obtaining a zinc-plated steel sheet. Examples of the galvanizing treatment include hot-dip galvanizing treatment and hot-dip galvannealing treatment. The zinc plating treatment may be carried out, for example, during the cooling step, during the first holding step, after the first holding step and before the surface strain introduction step, after the surface strain introduction step and before the second holding step, during the second holding step, or after the second holding step. The galvanizing treatment may be carried out, for example, after the annealing step and before the first holding step, during the cooling treatment in the cooling step.

[0103] In the case of hot-dip galvanizing, it is preferable to immerse the steel sheet in a galvanizing bath (hot-dip galvanizing bath) at a temperature of 440° C. or higher and 500° C. or lower, and then adjust the coating weight by gas wiping, etc. The hot-dip galvanizing bath is not particularly limited as long as it provides the above-mentioned composition of the galvanized layer, but it is preferable to use, for example, a plating bath having an Al content of 0.10 mass % or higher and 0.23 mass % or lower, with the balance consisting of Zn and unavoidable impurities.

[0104] In the case of alloying hot-dip galvanizing treatment, after hot-dip galvanizing treatment is performed as described above, the hot-dip galvanized steel sheet is preferably subjected to alloying treatment by heating to an alloying temperature of 450°C or higher and 600°C or lower. If the alloying temperature is lower than 450°C, the Zn-Fe alloying rate will be slow, making alloying difficult. On the other hand, if the alloying temperature exceeds 600°C, untransformed austenite will transform into pearlite, making it difficult to achieve a TS of 1180 MPa or higher. The alloying temperature is more preferably 500°C or higher, and even more preferably 510°C or higher. The alloying temperature is more preferably 570°C or lower.

[0105] In addition, the coating weight of both hot-dip galvanized steel sheet (GI) and galvannealed steel sheet (GA) is 20 to 80 g / m per side. 2 The plating weight can be adjusted by gas wiping or the like.

[0106] The steel sheet obtained as described above may further be subjected to temper rolling. If the temper rolling reduction exceeds 2.00%, the yield stress increases, which may result in a decrease in dimensional accuracy when the steel sheet is formed into a component. Therefore, the temper rolling reduction is preferably 2.00% or less. The lower limit of the temper rolling reduction is not particularly limited, but it is preferably 0.05% or more from the viewpoint of productivity. Temper rolling may be performed on an apparatus continuous with the annealing apparatuses used for the above-mentioned steps (online), or may be performed on an apparatus discontinuous with the annealing apparatuses used for the above-mentioned steps (offline). Temper rolling may be performed once or twice or more times. As long as an elongation rate equivalent to that of temper rolling can be imparted, rolling using a leveler or the like may be performed.

[0107] Other manufacturing method conditions are not particularly limited, but from the viewpoint of productivity, it is preferable that the above-mentioned series of processes such as annealing, hot-dip galvanizing, and galvanizing alloying be carried out in a continuous galvanizing line (CGL). After hot-dip galvanizing, wiping can be performed to adjust the coating weight. Note that plating conditions other than those described above can be based on conventional hot-dip galvanizing methods.

[0108] [3. Materials] Next, a member according to one embodiment of the present invention will be described. A member according to one embodiment of the present invention is a member made using (using as a raw material) the above-mentioned steel plate. For example, the raw material steel plate is subjected to at least one of forming and joining to form the member. Here, the above steel plate has a tensile strength (TS) of 1180 MPa or more and less than 1470 MPa, and also has a high yield stress (YS), excellent ductility, excellent press formability within the steel plate (bendability within the steel plate), excellent press formability at the steel plate edges (bendability at the steel plate edges (shear cross section)), and high crack propagation resistance. Therefore, a member according to one embodiment of the present invention has a tensile strength (TS) of 1180 MPa or more and less than 1470 MPa, and also has a high yield stress (YS), excellent ductility, excellent press formability within the steel plate (bendability within the steel plate), excellent press formability at the steel plate edges (bendability at the steel plate edges (shear cross section)), and high crack propagation resistance. Therefore, a member according to one embodiment of the present invention is particularly preferably applied to impact energy absorbing members used in the automotive field.

[0109] [4. Manufacturing methods for components] Next, a method for manufacturing a member according to one embodiment of the present invention will be described. A method for manufacturing a component according to one embodiment of the present invention includes a step of subjecting the above-mentioned steel plate (steel plate manufactured by the above-mentioned steel plate manufacturing method) to at least one of forming processing and joining processing to form a component. Here, the molding method is not particularly limited, and for example, a general processing method such as press working can be used. The joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, rivet joining, caulking joining, etc. The molding conditions and joining conditions are not particularly limited, and may be in accordance with ordinary methods. [Example]

[0110] Steel materials having the chemical composition shown in Table 1 (the balance being Fe and unavoidable impurities) were melted in a converter and then cast into steel slabs by continuous casting. In Table 1, - indicates the content of unavoidable impurities, which is treated as 0 (zero).

[0111] The obtained steel slabs were heated to 1200°C, and after heating, the steel slabs were subjected to rough rolling and hot rolling (finish rolling temperature: 900°C) to obtain hot-rolled steel sheets. Next, No. 1 to No. 70 of the obtained hot-rolled steel sheets were subjected to pickling and cold rolling to obtain cold-rolled steel sheets with the thicknesses shown in Table 3. The obtained cold-rolled steel sheet was then subjected to the annealing process, cooling process, plating process (hot-dip galvanizing process or alloyed hot-dip galvanizing process) during the cooling process, reheating and holding process (first holding process), surface strain introduction process, and second holding process under the conditions shown in Table 2 to obtain the steel sheet. In the cooling step, the average cooling rate was 10°C / s and the dew point of the atmosphere was -30°C.

[0112] Here, in the coating process, a hot-dip galvanizing treatment or a galvannealed hot-dip galvanizing treatment was performed to obtain a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or a galvannealed hot-dip galvanized steel sheet (hereinafter also referred to as GA). In Table 2, the type of coating process is also indicated as "GI" or "GA." In Table 2, since no alloying treatment is performed in the case of GI steel sheets, the alloying temperature is indicated as "-." In addition, in Table 2, steel sheets that are not treated in the coating process are indicated as "CR."

[0113] The galvanizing bath temperature was set to 470°C for both GI and GA production. When manufacturing GI, the amount of zinc plating applied is 45 to 72 g / m per side. 2 When manufacturing GA, the thickness is 45 g / m per side. 2 It was decided. The composition of the coating layer (galvanized layer) of the finally obtained steel sheet was as follows: GI: 0.1-1.0 mass% Fe, 0.2-0.33 mass% Al, and the balance Zn and unavoidable impurities; GA: 8.0-12.0 mass% Fe, 0.1-0.23 mass% Al, and the balance Zn and unavoidable impurities. In addition, the zinc plating layers were formed on both sides of the base steel sheet in all cases.

[0114] The steel structure of the obtained steel sheet was identified using the method described above. The measurement results are shown in Table 3. As shown in Figure 1, F represents ferrite, M represents fresh martensite, RA represents retained austenite, BF represents bainitic ferrite, and TM represents tempered martensite. In Table 3, θ represents carbide.

[0115] The soft surface layer was measured as follows: After smoothing the thickness cross section (L cross section) of the steel sheet parallel to the rolling direction by wet polishing, a Vickers hardness tester was used to measure the thickness of the soft surface layer under a load of 10 gf (9.8 × 10 -2 Measurements were taken at 1 μm intervals from a position 1 μm from the steel plate surface in the thickness direction to a position 100 μm in the thickness direction using a 100 μm (N) test. Measurements were then taken at 20 μm intervals up to the center of the plate thickness. The region where the Vickers hardness was reduced to 84% or less compared to the Vickers hardness at 1 / 4 of the plate thickness was defined as the soft layer (surface soft layer), and the thickness of this region in the thickness direction was defined as the soft layer thickness.

[0116] The structure of the soft surface layer was identified at a position halfway through the thickness of the soft surface layer using the same method as for identifying the steel structure of the base steel sheet.

[0117] In Tables 1 to 4, the underlined parts indicate values ​​outside the appropriate range of the present invention. In addition, tensile tests and 90-degree V-bend tests were conducted according to the following procedures, and the tensile strength (TS), yield stress (YS), yield ratio (YR), total elongation (El), R / t in the V-bend test, crack length propagating from the end of the bent ridge in the ridge direction, and notch El were evaluated according to the following criteria.

[0118] ·TS (tensile strength) 〇 (Pass): 1180MPa or more and less than 1470MPa × (Fail): Less than 1180 MPa or more than 1470 MPa

[0119] YS (yield stress) 〇(Pass): (A) When 1180MPa≦TS<1320MPa, 820MPa≦YS (B) When 1320MPa≦TS<1470MPa, 920MPa≦YS ×(Fail): (A) When 1180MPa≦TS<1320MPa, 820MPa>YS (B) When 1320MPa≦TS<1470MPa, 920MPa>YS

[0120] El (ductility) 〇(Pass): (A) When 1180MPa≦TS<1320MPa, 8.0%≦El (B) When 1320MPa≦TS<1470MPa, 7.0%≦El ×(Fail): (A) When 1180MPa≦TS<1320MPa, 8.0%>El (B) When 1320MPa≦TS<1470MPa, 7.0%>El

[0121] R / t (press formability inside the steel plate (bendability inside the steel plate)) 〇(Pass): (A) When 1180MPa≦TS<1320MPa, 3.0≧R / t (B) When 1320MPa≦TS<1470MPa, 4.0≧R / t ×(Fail): (A) 3.0 when 1180MPa≦TS<1320MPa <R / t (B) 4.0 when 1320MPa≦TS<1470MPa <R / t

[0122] - A 90-degree V-bend test with a bending radius of 0.5 mm was conducted to measure the length of the crack that develops along the bend ridge formed other than the end of the bend ridge (press formability of the steel plate end (bendability inside the steel plate)) 〇 (Pass): Crack length other than V-bend edge is 200 μm or less × (Fail): Crack length other than the V-bend edge is more than 200 μm

[0123] - A 90-degree V-bend test was conducted with a bending radius of 0.5 mm, and the crack length that propagated from the edge of the bend ridge toward the ridge (press formability of the steel plate edge (bendability of the steel plate edge (shear cross section))) 〇 (Pass): V-bend end crack length is 200 μm or less × (Fail): V-bend end crack length exceeds 200 μm

[0124] Notch El (crack propagation resistance) (i) Notched tensile test after heat treatment at 170°C for 20 minutes 〇(Pass): (A) When 1180MPa≦TS<1320MPa, notch El≧6.5% (B) When 1320MPa≦TS<1470MPa, notch El≧6.0% ×(Fail): (A) When 1180MPa≦TS<1320MPa, notch El<6.5% (B) When 1320MPa≦TS<1470MPa, notch El<6.0% (ii) Notched tensile test after introducing a 2% nominal tensile strain and heat treatment at 170°C for 20 minutes 〇(Pass): (C) When 1180MPa≦TS<1320MPa, notch El≧5.5% (D) When 1320MPa≦TS<1470MPa, notch El≧5.0% ×(Fail): (C) When 1180MPa≦TS<1320MPa, notch El<5.5% (D) When 1320MPa≦TS<1470MPa, notch El<5.0%

[0125] (1) Tensile test Tensile tests were conducted in accordance with JIS Z 2241 (2011). Specifically, JIS No. 5 test pieces were taken from the obtained steel sheets so that the longitudinal direction was perpendicular to the rolling direction of the base steel sheet. Using the taken test pieces, tensile tests were conducted at a crosshead speed of 10 mm / min, and TS, YS, YR, and El were measured. The results are shown in Table 3.

[0126] (2) 90-degree V-bend test A 100 mm C (C direction: perpendicular to the rolling direction of the steel sheet) x 30 mm L (L direction: along the rolling direction) strip specimen was cut from the resulting steel sheet at a position 1 / 4 of the coil width. The 100 mm long end was sheared, and the specimen was bent in the as-sheared state (without machining to remove burrs) so that the burrs were on the outer periphery of the bend. The shear clearance was 15% and the rake angle was 0°. V-bending was performed in the L direction (bending ridge length: 30 mmL) using a Shimadzu Autograph with a punch radius of 0.5 mm, a punch bend angle of 90°, a punch stroke speed of 30 mm / min, a pressing load of 10 ton, and a pressing time of 5 seconds.

[0127] An example of a sample after the 90-degree V-bend test with a bending radius of 0.5 mm is shown in Figure 2. Figure 2(b) is an overhead view of the sample viewed from the Z direction shown in Figure 2(a). If the bend ridgeline is defined as the area extending from the bend apex along the steel sheet surface to a total width of 5 mm in the C direction (2.5 mm on both sides from the bend apex), then the area (area o) extending 5 mm in the L direction from the very edge of the bend ridgeline is defined as the bend ridgeline edge. The crack length Y1 that propagates from the bend ridgeline edge in the ridgeline direction (L direction) and the crack length Y2 that propagates in the L direction along the bend ridgeline formed other than the bend ridgeline edge are measured using the following methods.

[0128] After the above 90-degree V-bend test with a bending radius of 0.5 mm, the length of the crack that propagates from the end of the bent ridge in the ridge direction is measured as follows. Figure 3-1(a) shows the crack at the end of the bending ridge of a sample after a V-bend test. When measuring the length of a crack at the center of the bending ridge, it is common to observe the plate surface (plane b) from the Z direction. Because the sample after an actual V-bend test has a saddle shape as shown in Figure 3-1(b), plane b is significantly deformed, reducing the accuracy of measuring the crack length and potentially making it impossible to accurately evaluate the bendability of the sheared edge. In the present invention, the following measurement method can be used to achieve accurate measurements. After a 90° V-bend test with a 0.5 mm radius, the shear plane (a) of the bent sample was placed facing up, and the edge of the bend ridge was photographed at 40x magnification using a one-shot 3D shape measuring instrument (Keyence, VR6000 series or newer models). The resulting height data was analyzed using the analysis software provided with the one-shot 3D shape measuring instrument. As shown in Figure 3-2(a), a circular arc-shaped measurement line (i) was drawn as close as possible to the outside of the bend, which was subjected to tensile stress, in line with the bend ridge. An example of the resulting profile waveform (j) is shown in Figure 3-2(b). The software's measurement tool was used to determine the length of each crack (y1 + y2) / 2. The length of the longest crack was taken as the crack length propagating from the edge of the bend ridge toward the ridge after the 90° V-bend test with a 0.5 mm radius.

[0129] (3) Notched tensile test The obtained steel sheets were subjected to a heat treatment at 170°C for 20 minutes. Then, JIS No. 5 tensile test specimens were prepared so that the longitudinal direction of the test specimen was aligned perpendicular to the rolling direction, and notches (V notches: depth 2 mm, V interior angle 60 degrees) were made at both width ends at the center position of the longitudinal parallel part of the test specimen. The test specimen had a gauge length of 15 mm, and a notched tensile test was performed in accordance with JIS Z 2241 (2011) to measure the total elongation (El). The steel sheets were also subjected to a 2% nominal tensile strain and heat treatment at 170°C for 20 minutes. JIS No. 5 tensile test specimens were then prepared so that the longitudinal direction of the test specimen was aligned perpendicular to the rolling direction. Notches (V-notches: depth 2 mm, V-angle 60 degrees) were made at both ends of the width at the center of the longitudinal parallel portion of the test specimen. The gauge length of the test specimen was set to 15 mm, and a notched tensile test was performed in accordance with JIS Z 2241 (2011) to measure the total elongation (El).

[0130] [Table 1]

[0131] [Table 2]

[0132] [Table 3]

[0133] [Table 4]

[0134] As shown in Table 4, all of the inventive examples passed the test in terms of tensile strength (TS), yield stress (YS), total elongation (El), R / t in the V-bend test, crack length propagating from the end of the bend ridge toward the ridge, and notch El. On the other hand, in the comparative examples, at least one of the following was insufficient: tensile strength (TS), yield stress (YS), total elongation (El), R / t in the V-bend test, crack length propagating from the end of the bend ridge toward the ridge, and notch El.

[0135] Furthermore, it was found that the components obtained by forming or joining using the steel plates of the present invention had the excellent properties characteristic of the present invention in terms of tensile strength (TS), yield stress (YS), total elongation (El), R / t in a V-bend test, crack length propagating from the end of the bend ridge in the ridge direction, and notch El. [Explanation of symbols]

[0136] F ferrite M Fresh martensite RA Retained austenite BF Bainitic ferrite TM Tempered Martensite

Claims

1. In mass%, C: 0.050% or more and 0.400% or less, Si: 0.02% or more and 3.00% or less, Mn: 1.50% 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: 0.005% or more and 2.000% or less, N: 0.0100% or less, Sb: 0.200% or less (including 0%), and Sn: 0.200% or less (including 0%) and the balance being Fe and unavoidable impurities, The steel sheet has a soft surface layer having a Vickers hardness of 84% or less of the Vickers hardness at a position 1 / 4 of the sheet thickness from the surface of the base steel sheet, The surface soft layer satisfies the following formula (1): The structure in the superficial soft layer is The area ratio of ferrite is 60.0% or more and 100.0% or less, When the area ratio of ferrite is less than 100.0%, the value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less; The area fraction of retained austenite is 3.0% or less, The structure at a 1 / 4 position of the plate thickness of the base steel plate is The area ratio of ferrite is 55.0% or less (including 0.0%), the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is more than 40.0% and 100.0% or less; Area ratio of retained austenite: less than 3.5%; Area ratio of fresh martensite: 10.0% or less (including 0.0%), A steel plate having a tensile strength of 1180 MPa or more and less than 1470 MPa. 20≦X≦120-3800×[Sb]-1900×[Sn]...(1) In the formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.

2. The component composition further includes, in 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, 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 The steel sheet according to claim 1, comprising at least one selected from the following:

3. The steel sheet according to claim 1 or 2, wherein one or both surfaces of the base steel sheet have a plating layer, the plating layer being a hot-dip galvanized layer.

4. The steel sheet according to claim 1 or 2, wherein one or both surfaces of the base steel sheet have a plating layer, the plating layer being a galvannealed layer.

5. A member made using the steel sheet according to claim 1 or 2.

6. A member made using the steel sheet according to claim 3.

7. A member made using the steel sheet according to claim 4.

8. A hot rolling process in which a steel slab having the component composition according to claim 1 or 2 is hot rolled at a finish rolling temperature of 820°C or higher; an annealing step of heating the steel sheet after the hot rolling step and annealing it under conditions of an annealing temperature of 750°C or more and 900°C or less, an annealing time of 20 seconds or more, and a dew point of -10°C or more in an atmosphere, further satisfying formula (2) and formula (3); a cooling step of cooling the steel sheet after the annealing step to a cooling stop temperature of less than 100°C; a first holding step of reheating the steel sheet after the cooling step to a reheating holding temperature range of not less than the cooling stop temperature and not more than 440°C and holding the temperature for not less than 10 seconds; The steel sheet after the first holding step is subjected to a pressure of 2.0 kgf / mm in the reheating holding temperature range. ２ a surface strain introduction step of applying the above tension; A second holding step of holding the steel sheet after the surface layer strain introduction step in the reheating holding temperature range for 10 seconds or more, or further comprising: The method includes a cold rolling step in which the steel sheet after the hot rolling step and before the annealing step is subjected to cold rolling at a rolling reduction rate of 20% or more and 80% or less to obtain a cold-rolled steel sheet. a base steel sheet having the above-described composition; and a surface soft layer having a Vickers hardness of 84% or less of the Vickers hardness at a position 1 / 4 of the sheet thickness from the surface of the base steel sheet, The surface soft layer satisfies the following formula (1): The structure in the superficial soft layer is The area ratio of ferrite is 60.0% or more and 100.0% or less, When the area ratio of ferrite is less than 100.0%, the value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less; The area fraction of retained austenite is 3.0% or less, The structure at a 1 / 4 position of the plate thickness of the base steel plate is The area ratio of ferrite is 55.0% or less (including 0.0%), the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is more than 40.0% and 100.0% or less; Area ratio of retained austenite: less than 3.5%; Area ratio of fresh martensite: 10.0% or less (including 0.0%), A method for producing a steel plate having a tensile strength of 1180 MPa or more and less than 1470 MPa. 20≦X≦120-3800×[Sb]-1900×[Sn] ...Formula (1) In the formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively. 2400≦Y≦20000...Formula (2) Y=[{(T-Ac1)×t1} / 2}]+{(T-Ac1)×t2} ...Formula (3) In the formula (3), T is the annealing temperature (°C), t1 is the time (s) from 650°C to the annealing temperature T during the temperature rise in the annealing step, t2 is the annealing time (s), and Ac1 is Ac1 (°C).

9. The method for producing a steel sheet according to claim 8, further comprising a hot-dip galvanizing step of performing hot-dip galvanizing on the steel sheet after the annealing step to form a hot-dip galvanized layer.

10. The method for producing a steel sheet according to claim 8, further comprising, after the annealing step, subjecting the steel sheet to a galvannealed hot-dip coating treatment to form a galvannealed layer.

11. A method for manufacturing a component, comprising the step of subjecting the steel plate according to claim 1 or 2 to at least one of forming and joining to form the component.

12. A method for manufacturing a component, comprising the step of subjecting the steel plate according to claim 3 to at least one of forming and joining to form the component.

13. A method for manufacturing a component, comprising the step of subjecting the steel plate according to claim 4 to at least one of forming and joining to form a component.

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