Steel sheet, member, and production methods for same

Abstract

To provide a steel sheet having a TS of 1180 MPa or more and less than 1470 MPa, high YS, high ductility, high press formability in the inner portion of the steel sheet and at the end portion of the steel sheet, and high crack propagation resistance. The steel sheet has a base steel sheet having a predetermined chemical composition and has a surface soft layer having a Vickers hardness of 84% or less relative to that at the quarter thickness from the surface of the base steel sheet and satisfying the following formula (1), wherein the surface soft layer has a microstructure in a specific range, the base steel sheet has a microstructure in a specific range at the quarter thickness, the length of a crack that propagates from a bending ridge line end portion in a ridge line direction is 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm, El is 6.0% or more in a notch tensile test after a heat treatment at 170°C for 20 minutes, and El is 5.0% or more in a notch tensile test after a nominal tensile strain of 2% is introduced and after a heat treatment at 170°C for 20 minutes is performed. 20≤X≤120−3800×Sb−1900×Sn

Description

Technical Field

[0001] The present invention relates to a steel sheet, a member made of the steel sheet, and methods for producing the steel sheet and the member.Background Art

[0002] For the purpose of achieving both a reduction in CO 2 emissions due to an improvement of fuel efficiency by reducing the thickness and weight of steel sheets used for automobile bodies and an improvement in crashworthiness, an increase in the strength of steel sheets for automobiles has been promoted. Furthermore, new laws and regulations have been introduced one after another. Thus, for the purpose of increasing the strength of a vehicle body, high-strength steel sheets, particularly high-strength steel sheets having a tensile strength (hereinafter also referred to simply as TS) of 980 MPa or more, have been increasingly applied to main structural members and reinforcing members to be assembled to frames of automobile cabins (hereinafter also referred to as automobile frame structural members and the like). Furthermore, a high-strength steel sheet used for an automobile frame structural member or the like is required to have high member strength at the time of press forming. To increase the strength of a part, for example, it is effective to increase the yield ratio (hereinafter also referred to simply as YR) obtained by dividing the yield stress (hereinafter also referred to simply as YS) of a steel sheet by TS. This increases impact absorbed energy in case of a vehicle collision (hereinafter also referred to simply as impact absorbed energy). Furthermore, among automobile frame structural members, for example, a crash box has a bent portion. Thus, from the perspective of press formability, it is preferable to apply a steel sheet having high bendability to such a part. Furthermore, from the perspective of rust prevention performance of a vehicle body, a steel sheet as a material of an automotive member is often galvanized. Thus, it is desired to develop a hot-dip galvanized steel sheet having high press formability and crashworthiness in addition to high strength.

[0003] As such a steel sheet serving as a material for an automotive member, for example, Patent Literature 1 discloses a high-strength hot-dip galvanized steel sheet having a thickness in the range of 0.6 to 5.0 mm and having a coated layer on its surface, characterized in that the steel sheet has a microstructure including, in terms of volume fraction, 40% to 90% of a ferrite phase and 3% to 25% of a retained austenite phase, the retained austenite phase has a solute carbon content in the range of 0.70% to 1.00%, an average particle size of 2.0 µm or less, and an average interparticle distance in the range of 0.1 to 5.0 µm, a decarburized layer in a surface layer portion of the steel sheet has a thickness in the range of 0.01 to 10.0 µm, the surface layer portion of the steel sheet contains an oxide having an average particle size in the range of 30 to 120 nm and an average density of 1.0 x 10 12< particles / m 2< or more, and the steel sheet has an average work-hardening coefficient (n-value) of 0.080 or more during 3% to 7% plastic deformation, thereby exhibiting high ductility while maintaining a maximum tensile strength as high as 900 MPa or more and good mechanical cutting characteristics.

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

[0005] Patent Literature 3 discloses a hot-dip galvanized steel sheet and a hot-dip galvannealed steel sheet each having high ductility and bendability, characterized in that they include a steel sheet and a hot-dip galvanized layer, the steel sheet includes a base material and a decarburized ferrite layer, the microstructure at the quarter thickness of the steel sheet contains 5.0% or more by volume of tempered martensite and 0.5% or more by volume and less than 7.0% by volume of retained austenite, the remainder being composed mainly of 4% to 70% by volume of ferrite and bainite, part or all of the tempered martensite and the retained austenite form M-A, the decarburized ferrite layer contains 120% or more ferrite relative to the ferrite content at the quarter thickness, has an average ferrite grain size of 20 µm or less and a thickness of 5 µm or more and 200 µm or less, and contains 1.0% or more by volume of tempered martensite having a number density of 0.01 particles / µm 2< or more, and methods for producing the hot-dip galvanized steel sheet and the hot-dip galvannealed steel sheet.

[0006] Patent Literature 4 discloses a high-strength hot-dip galvanized steel sheet having a high TS-El balance, high stretch flangeability, a low YR, and high workability, characterized in that the steel sheet has a chemical composition containing, on a mass percent basis, C: 0.05% to 0.3%, Si: 0.01% to 2.5%, Mn: 0.5% to 3.5%, P: 0.003% to 0.100%, S: 0.02% or less, Al: 0.010% to 1.5%, and N: 0.007% or less, with the remainder being composed of Fe and incidental impurities, and has a microstructure containing, in terms of area fraction, 20% to 87% ferrite, a total of 3% to 10% martensite and retained austenite, and 10% to 60% tempered martensite, and a method for producing the high-strength hot-dip galvanized steel sheet.Citation ListPatent Literature

[0007] PTL 1: Japanese Patent No. 5354135 PTL 2: Japanese Patent No. 5352793 PTL 3: Japanese Patent No. 6536294 PTL 4: Japanese Patent No. 5256689 Summary of InventionTechnical Problem

[0008] While the application of a steel sheet having a tensile strength TS (hereinafter also referred to as TS) of more than 980 MPa has advanced for a structural member of an automobile exemplified by a center pillar, the use of a steel sheet for an impact energy absorbing member of an automobile exemplified by a front side member or a rear side member remains limited to a steel sheet having a TS in the range of 590 to 780 MPa.

[0009] Thus, to increase absorbed energy in case of a collision (hereinafter also referred to as impact absorbed energy), it is effective to improve yield stress YS (hereinafter also referred to as YS). However, an increase in the YS of a steel sheet generally decreases press formability, particularly, ductility, flangeability, bendability, and other characteristics. Thus, when it is assumed that such a steel sheet having increased TS and YS is applied to the above-described impact energy absorbing member of an automobile, not only press forming becomes difficult, but also the member cracks in an axial crushing test simulating a collision test. In other words, the actual impact absorbed energy is not as high as expected from the value of YS. Thus, the impact energy absorbing member is currently limited to a steel sheet having a TS in the range of 590 to 980 MPa.

[0010] Although Patent Literature 1 discloses a high-strength hot-dip galvanized steel sheet in which retained austenite is formed in the inner portion of the steel sheet to improve ductility and a decarburized layer is formed in the surface layer of the steel sheet to improve mechanical cutting characteristics, no consideration has been given to the improvement of bendability and the improvement of fracture resistance characteristics in case of a collision of an automotive body due to the formation of a surface soft layer (decarburized layer) and the press formability at the end portion of the steel sheet.

[0011] Although Patent Literature 2 discloses a high-strength hot-dip galvanized steel sheet in which ductility is improved by forming soft ferrite as a main microstructure in the inner portion of the steel sheet and limiting the amount of unrecrystallized ferrite to a small amount, and delayed fracture resistance and anisotropy thereof are improved by forming a decarburized layer in a surface layer of the steel sheet, but no consideration has been given to improvement of bendability and improvement of fracture resistance characteristics in case of a collision of an automotive body by forming a surface soft layer (decarburized layer), and press formability at an end portion of the steel sheet.

[0012] Although Patent Literature 3 discloses a hot-dip galvanized steel sheet and a hot-dip galvannealed steel sheet in which ductility is improved by forming M-A in the inner portion of the steel sheet and bendability is improved by forming a soft layer (decarburized ferrite layer) in a surface layer of the steel sheet, but no consideration has been given to press formability at an end portion of the steel sheet.

[0013] Although Patent Literature 4 discloses a high-strength hot-dip galvanized steel sheet in which both ductility, which is the press formability in the inner portion of the steel sheet, and stretch flangeability, which is the press formability at an end portion of the steel sheet, are improved, but no consideration has been given to the improvement of bendability by forming a surface soft layer (decarburized layer) or the improvement of fracture resistance characteristics in case of a collision of an automotive body.

[0014] As described above, it cannot be said that the steel sheets disclosed in Patent Literature 1 to Patent Literature 4 each have a TS of 1180 MPa or more, a high YS, high press formability in the inner portion of the steel sheet (bendability in the inner portion of the steel sheet), and high press formability at an end portion of the steel sheet (bendability at the end portion (sheared cross section) of the steel sheet).

[0015] The present invention has been made in view of the above circumstances and aims to provide a steel sheet and a member each having a tensile strength (TS) of 1180 MPa or more and less than 1470 MPa, a high yield stress (YS), high ductility, high press formability in the inner portion of the steel sheet (bendability in the inner portion of the steel sheet), high press formability at an end portion of the steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and high crack propagation resistance, and methods for producing the steel sheet and the member.

[0016] The term "steel sheet", as used herein, 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 hot-dip galvannealed steel sheet (hereinafter also referred to as GA).

[0017] The tensile strength (TS) is measured in a tensile test according to JIS Z 2241 (2011).

[0018] Furthermore, having a high yield stress (YS) means satisfying the following.

[0019] The phrase "high YS" means that YS measured in the tensile test according to JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on TS measured in the tensile test. (A) For 1180 MPa ≤ TS < 1320 MPa, 820 MPa ≤ YS (B) For 1320 MPa ≤ TS < 1470 MPa, 920 MPa ≤ YS

[0020] The phrase "high ductility" means that El measured in the tensile test according to JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on TS measured in the tensile test. (A) For 1180 MPa ≤ TS < 1320 MPa, 8.0% ≤ El (B) For 1320 MPa ≤ TS < 1470 MPa, 7.0% ≤ El

[0021] The phrase "high press formability in the inner portion of a steel sheet (bendability in the inner portion of the steel sheet)" means that R (critical bending radius) / t (sheet thickness) measured in a 90-degree V-bending test according to JIS Z 2248 (2022) satisfies the following formula (A) or (B) depending on TS. (A) For 1180 MPa ≤ TS < 1320 MPa, 3.0 ≥ R / t (B) For 1320 MPa ≤ TS < 1470 MPa, 4.0 ≥ R / t

[0022] The phrase "high press formability in the inner portion of a steel sheet (bendability in the inner portion of the steel sheet)" means that when a 90-degree V-bending test with a bending radius R of 0.5 mm is performed in accordance with JIS Z 2248 (2022), the length of a crack that propagates along a bending ridge line formed in a portion other than a bending ridge line end portion is 200 µm or less (the crack length in the portion other than the V-bent end surface is 200 µm or less).

[0023] The phrase "high press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet)" means that when a 90-degree V-bending test with a bending radius R of 0.5 mm is performed in accordance with JIS Z 2248 (2022), the length of a crack that propagates from a bending ridge line end portion in a ridge line direction is 200 µm or less (the V-bending edge crack length is 200 µm or less).

[0024] Furthermore, having high crack propagation resistance refers to satisfying the formula (A) or (B) in the following test (i) and satisfying the formula (C) or (D) in the following test (ii). (i) A notch tensile test after heat treatment at 170°C for 20 minutes (A) For 1180 MPa ≤ TS < 1320 MPa, notch El ≥ 6.5% (B) For 1320 MPa ≤ TS < 1470 MPa, notch El ≥ 6.0% (ii) A notch tensile test after introducing a nominal tensile strain of 2% and performing heat treatment at 170°C for 20 minutes (C) For 1180 MPa ≤ TS < 1320 MPa, notch El ≥ 5.5% (D) For 1320 MPa ≤ TS < 1470 MPa, notch El ≥ 5.0%

[0025] El in the notch tensile test after heat treatment at 170°C for 20 minutes is a measure of the difficulty of crack propagation in a vertical wall portion of an axial crushing member (impact energy absorbing member). Furthermore, El in the notch tensile test after introducing a nominal tensile strain of 2% and performing heat treatment at 170°C for 20 minutes is a measure of the difficulty of crack propagation in a bending ridge line portion of an axial crushing member (impact energy absorbing member).Solution to Problem

[0026] As a result of extensive studies to achieve the above objects, the present inventors have obtained the following findings. (1) A TS of 1180 MPa or more can be achieved by having a predetermined chemical composition, as microstructures at a quarter thickness of a base steel sheet, setting the area fraction of ferrite to 55.0% or less (including 0.0%) and the total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite) to more than 40.0% and 100.0% or less. (2) A high YS can be achieved by having a predetermined chemical composition, as microstructures at a quarter thickness of a base steel sheet, setting the total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite) to more than 40.0% and 100.0% or less, 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) With a predetermined chemical composition, a surface soft layer having a Vickers hardness of 84% or less relative to the Vickers hardness at a quarter thickness from a surface of a base steel sheet is included, and the surface soft layer satisfies the following formula (1). 20 ≤ X ≤ 120 − 3800 × Sb − 1900 × Sn

[0027] In the formula (1), X denotes the thickness of the surface soft layer (µm), and [Sb] and [Sn] denote the Sb content and the Sn content of steel (% by mass), respectively.

[0028] Furthermore, as microstructures in the surface soft layer, the area fraction of ferrite is set to 60.0% or more and 100.0% or less, and among microstructures other than ferrite, the area fraction of fresh martensite divided by the total area fraction of bainitic ferrite, fresh martensite, and tempered martensite is set to 0.5 or less, and the area fraction of retained austenite is set to 3.0% or less. This can achieve high press formability in the inner portion of a steel sheet (bendability in the inner portion of the steel sheet) and high crack propagation resistance.

[0029] (4) High press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet) can be achieved by having a predetermined chemical composition and, as microstructures at a quarter thickness of the base steel sheet, setting 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%).

[0030] The present disclosure has been made based on these findings. That is, the gist of the present disclosure is as described below. [1] A steel sheet having a base steel sheet with a chemical composition, on a mass percent basis, 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%), with the remainder being composed of Fe and incidental impurities, wherein the steel sheet has a surface soft layer having a Vickers hardness of 84% or less relative to a Vickers hardness at a quarter thickness from a surface of the base steel sheet, the surface soft layer satisfies the following formula (1), as microstructures in the surface soft layer, ferrite has an area fraction of 60.0% or more and 100.0% or less, when ferrite has an area fraction of less than 100.0%, an area fraction of fresh martensite divided by a total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less, retained austenite has an area fraction of 3.0% or less, and as microstructures at a quarter thickness of the base steel sheet, ferrite has an area fraction of 55.0% or less (including 0.0%), bainitic ferrite and tempered martensite (excluding retained austenite) have a total area fraction of more than 40.0% and 100.0% or less, retained austenite has an area fraction of less than 3.5%, and fresh martensite has an area fraction of 10.0% or less (including 0.0%), and the steel sheet has a tensile strength of 1180 MPa or more and less than 1470 MPa, 20 ≤ X ≤ 120 − 3800 × Sb − 1900 × Sn wherein, in the formula (1), X denotes the thickness of the surface soft layer (µm), and [Sb] and [Sn] denote an Sb content and a Sn content of steel (% by mass), respectively. [2] The steel sheet according to [1], wherein the chemical composition further contains, on a mass percent basis, at least one selected from Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, 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. [3] The steel sheet according to [1] or [2], wherein the steel sheet has a coated layer on one or both surfaces of the base steel sheet, and the coated layer is a hot-dip galvanized layer. [4] The steel sheet according to [1] or [2], wherein the steel sheet has a coated layer on one or both surfaces of the base steel sheet, and the coated layer is a hot-dip galvannealed layer. [5] A member including the steel sheet according to any one of [1] to [4]. [6] A method for producing a steel sheet, including: a hot rolling step of hot-rolling a steel slab having the chemical composition according to [1] or [2] at a finish rolling temperature of 820°C or more; an annealing step of heating the steel sheet after the hot rolling step and annealing the steel sheet at an annealing temperature of 750°C or more and 900°C or less for an annealing time of 20 seconds or more in an atmosphere with a dew-point temperature of -10°C or more under conditions satisfying the formulae (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 and holding temperature range of the cooling stop temperature or more and 440°C or less and holding the steel sheet for 10 seconds or more; a surface layer strain introduction step of applying a tension of 2.0 kgf / mm 2< or more to the steel sheet after the first holding step in the reheating and holding temperature range; and a second holding step of holding the steel sheet after the surface layer strain introduction step in the reheating and holding temperature range for 10 seconds or more, or further a cold rolling step of cold-rolling the steel sheet after the hot rolling step and before the annealing step at a rolling reduction ratio of 20% or more and 80% or less to form a cold-rolled steel sheet, 2400 ≤ Y ≤ 20000 Y = T − Ac 1 × t 1 / 2 + T − Ac 1 × t 2 wherein, in the formula (3), T denotes the annealing temperature (°C), t1 denotes the time (s) from 650°C to the annealing temperature T during the heating in the annealing step, t2 denotes the annealing time (s), and Ac1 denotes Ac1 (°C). [7] The method for producing a steel sheet according to [6], including, after the annealing step, a hot-dip galvanizing step of applying a hot-dip galvanizing treatment to the steel sheet to form a hot-dip galvanized layer. [8] The method for producing a steel sheet according to [6], including, after the annealing step, a hot-dip galvannealing step of applying a hot-dip galvannealing treatment to the steel sheet to form a hot-dip galvannealed layer. [9] A method for producing a member, including a step of subjecting the steel sheet according to any one of [1] to [4] to at least one of forming or joining to produce the member. Advantageous Effects of Invention

[0031] The present invention can provide a steel sheet and a member each having a tensile strength (TS) of 1180 MPa or more and less than 1470 MPa, a high yield stress (YS), high ductility, high press formability in the inner portion of the steel sheet (bendability in the inner portion of the steel sheet), high press formability at an end portion of the steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and high crack propagation resistance.

[0032] Furthermore, a member containing a steel sheet according to the present invention as a material has high strength, high press formability, and crack propagation resistance, and therefore can be very advantageously applied to a structural member, an impact energy absorbing member, and the like of an automobile.Brief Description of Drawings

[0033] [Fig. 1] Fig. 1 is an example of a microstructure image by SEM used for identification of the microstructure. [Fig. 2] Fig. 2(a) is a schematic view of a sample after 90-degree V-bending, and Fig. 2(b) is a view of the sample illustrated in Fig. 2(a) as viewed in the Z direction (negative direction). [Fig. 3-1] Fig. 3-1 is a schematic view of a sample after 90-degree V-bending. [Fig. 3-2] Fig. 3-2 is a schematic view of edge cracking in 90-degree V-bending. Description of Embodiments

[0034] The present invention will be described based on the following embodiments.[1. Steel Sheet]

[0035] A steel sheet according to the present invention has a base steel sheet with a chemical composition containing, on a mass percent basis, 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%), with the remainder being composed of Fe and incidental impurities, wherein the steel sheet has a surface soft layer having a Vickers hardness of 84% or less relative to a Vickers hardness at a quarter thickness from a surface of the base steel sheet, the surface soft layer satisfies the following formula (1), as microstructures in the surface soft layer, ferrite has an area fraction of 60.0% or more and 100.0% or less, when ferrite has an area fraction of less than 100.0%, an area fraction of fresh martensite divided by a total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less, retained austenite has an area fraction of 3.0% or less, and as microstructures at a quarter thickness of the base steel sheet, ferrite has an area fraction of 55.0% or less (including 0.0%), bainitic ferrite and tempered martensite (excluding retained austenite) have a total area fraction of more than 40.0% and 100.0% or less, retained austenite has an area fraction of less than 3.5%, fresh martensite has an area fraction of 10.0% or less (including 0.0%), the length of a crack that propagates from a bending ridge line end portion in a ridge line direction is 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm, El is 6.0% or more in a notch tensile test after heat treatment at 170°C for 20 minutes, El is 5.0% or more in a notch tensile test after a nominal tensile strain of 2% is introduced and after a heat treatment at 170°C for 20 minutes is performed, and the steel sheet has a tensile strength of 1180 MPa or more and less than 1470 MPa, 20 ≤ X ≤ 120 − 3800 × Sb − 1900 × Sn wherein, in the formula (1), X denotes the thickness of the surface soft layer (µm), and [Sb] and [Sn] denote the Sb content and the Sn content of steel (% by mass), respectively. Chemical Composition

[0036] First, the chemical composition of a base steel sheet of a steel sheet according to an embodiment of the present invention will be described. Although the unit in the chemical composition is "% by mass" in all cases, the unit is hereinafter simply indicated by "%" unless otherwise specified.C: 0.050% or more and 0.400% or less

[0037] C is an element that is effective in producing appropriate amounts of tempered martensite, bainitic ferrite, and the like to ensure a TS of 1180 MPa or more and a high YS. A C content of less than 0.050% results in an increase in the area fraction of ferrite and makes it difficult to achieve a TS of 1180 MPa or more. This also results in a decrease in YS.

[0038] On the other hand, a C content of more than 0.400% results in an excessive increase in the area fraction of fresh martensite, an excessive increase in TS, and a decrease in El. Furthermore, fresh martensite acts as a starting point of void formation in a 90-degree V-bending test and makes it impossible to achieve desired bendability of a steel sheet and desired bendability of a sheared end face. Furthermore, the area fraction of retained austenite increases excessively, the fraction of fresh martensite formed by deformation-induced transformation of the retained austenite when receiving shearing increases greatly, and the subsequent void formation and crack propagation are promoted. This makes it more difficult to achieve desired bendability of the sheared end face.

[0039] Thus, the C content is 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, 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, even more preferably 0.250% or less.Si: 0.02% or more and 3.00% or less

[0040] Si is an element that suppresses excessive softening of tempered martensite. A Si content of less than 0.02% results in excessive softening of tempered martensite and makes it difficult to achieve a TS of 1180 MPa or more.

[0041] On the other hand, a Si content of more than 3.00% results in, due to an excessive increase in the area fraction of ferrite, an excessive increase in the C concentration of austenite during annealing and makes it impossible to achieve desired bendability of a sheared end face. Thus, the Si content is 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, even more preferably 0.30% or more. The Si content is preferably 1.80% or less. The Si content is more preferably 1.70% or less, even more preferably 1.60% or less.Mn: 1.50% or more and less than 3.50%

[0042] Mn is an element that adjusts the area fractions of bainitic ferrite and tempered martensite. A Mn content of less than 1.50% may result in an increase in the area fraction of ferrite and make it difficult to achieve a TS of 1180 MPa or more. This also results in a decrease in YS.

[0043] On the other hand, a Mn content of 3.50% or more results in a decrease in martensite start temperature Ms (hereinafter also referred to simply as an Ms temperature or Ms) and a decrease in martensite formed in a cooling step. In the end, this may increase the area fraction of fresh martensite, make it difficult to ensure press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and make it impossible to control the length of a crack that propagates from a bending ridge line end portion in a ridge line direction to 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm. Thus, the Mn content is 1.50% or more and less than 3.50%. The Mn content is preferably 2.00% or more, more preferably 2.20% or more. The Mn content is preferably 3.20% or less. The Mn content is more preferably 3.10% or less, even more preferably 3.00% or less.P: 0.001% or more and 0.100% or less

[0044] P is an element that has a solid-solution strengthening effect and increases TS and YS of a steel sheet. To produce such an effect, the P content is 0.001% or more.

[0045] On the other hand, a P content of more than 0.100% results in segregation of P at a prior-austenite grain boundary and embrittlement of the grain boundary. This results in void formation and crack propagation along the prior-austenite grain boundary in a V-bending test, makes it difficult to ensure press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and makes it impossible to control the length of a crack that propagates from a bending ridge line end portion in a ridge line direction to 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm. Thus, the P content is 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.S: 0.0001% or more and 0.0200% or less

[0046] S is present as a sulfide in steel. In particular, when the S content is more than 0.0200%, void formation and crack propagation occur from the sulfide in a V-bending test and make it difficult to ensure press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and the length of a crack that propagates from a bending ridge line end portion in a ridge line direction cannot be controlled to 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm.

[0047] Thus, the S content is 0.0200% or less. The S content is preferably 0.0080% or less. The S content is more preferably 0.0050% or less, even more preferably 0.0030% or less.

[0048] On the other hand, due to constraints on production technology, the S content is 0.0001% or more. The S content is preferably 0.0003% or more, more preferably 0.0005% or more.Al: 0.005% or more and 2.000% or less

[0049] Al promotes ferrite transformation during annealing and in a cooling process after annealing. That is, Al is an element that affects the area fraction of ferrite. An Al content of less than 0.005% results in a decrease in the area fraction of ferrite and a decrease in ductility.

[0050] On the other hand, an Al content of more than 2.000% results in an excessive increase in the area fraction of ferrite and makes it difficult to achieve a TS of 1180 MPa or more. This also results in a decrease in YS. Thus, the Al content is 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, even more preferably 0.030% or more.

[0051] The Al content is preferably 1.000% or less. The Al content is more preferably 0.800% or less, even more preferably 0.500% or less.N: 0.0100% or less

[0052] N is present as a nitride in steel. In particular, when the N content is more than 0.0100%, void formation and crack propagation occur from the nitride in a V-bending test and make it difficult to ensure press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and the length of a crack that propagates from a bending ridge line end portion in a ridge line direction cannot be controlled to 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm. Thus, the N content is 0.0100% or less. The N content is preferably 0.0050% or less. The N content is more preferably 0.0045% or less, even more preferably 0.0040% or less.

[0053] The N content may have any lower limit but is preferably 0.0005% or more due to constraints on production technology. The N content is more preferably 0.0010% or more, even more preferably 0.0015% or more.Sb: 0.200% or less (including 0%)

[0054] Sb is a useful element that can segregate on the steel sheet surface during annealing and improve coatability and chemical convertibility. Thus, 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, even more preferably 0.008% or more.

[0055] On the other hand, an Sb content of more than 0.200% may result in saturation of the effects of improving coatability and chemical convertibility and a decrease in press formability in the inner portion of a steel sheet (bendability in the inner portion of the steel sheet) and crack propagation resistance. Thus, when Sb is contained, the Sb content is 0.200% or less. The Sb content is more preferably 0.020% or less. The Sb content is even more preferably 0.015% or less. The Sb content is even further more preferably 0.012% or less, even further more preferably 0.011% or less.Sn: 0.200% or less (including 0%)

[0056] Like Sb, Sn is a useful element that can segregate on the steel sheet surface during annealing and improve coatability and chemical convertibility. Thus, the Sn content may be 0% but is preferably 0.002% or more. The Sn content is more preferably 0.003% or more.

[0057] On the other hand, a Sn content of more than 0.200% may result in saturation of the effects of improving coatability and chemical convertibility and a decrease in press formability in the inner portion of a steel sheet (bendability in the inner portion of the steel sheet) and crack propagation resistance. Thus, when Sn is contained, the Sn content should be 0.200% or less. The Sn content is preferably 0.008% or less, more preferably 0.004% or less.

[0058] While a base chemical composition of a base steel sheet of a steel sheet according to an embodiment of the present invention has been described above, a base steel sheet of a steel sheet according to an embodiment of the present invention has a chemical composition that contains the base components and the remainder other than the base components including Fe (iron) and incidental impurities. A base steel sheet of a steel sheet according to an embodiment of the present invention preferably has a chemical composition that contains the base components and the remainder composed of Fe and incidental impurities.

[0059] A base steel sheet of a steel sheet according to an embodiment of the present invention may contain at least one selected from the following optional components in addition to the base components. Provided that the following optional components are contained in an amount equal to or lower than the upper limit amount described below, the advantages of the present invention can be achieved, and the lower limits are not particularly defined. The following optional elements, when contained below the appropriate lower limits described later, are contained as incidental impurities.

[0060] At least one selected from Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, 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 lessNb: 0.200% or less

[0061] Nb increases TS and YS by forming fine carbide, nitride, or carbonitride during hot rolling or annealing. To produce such an effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more.

[0062] On the other hand, a Nb content of more than 0.200% may result in a large number of coarse precipitates or inclusions. In such a case, a coarse precipitate or inclusion acts as a starting point of a void or a crack in a V-bending test and makes it difficult to ensure press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and the length of a crack that propagates from a bending ridge line end portion in a ridge line direction cannot be controlled to 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm. Thus, when Nb is contained, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.Ti: 0.200% or less

[0063] Like Nb, Ti increases TS and YS by forming fine carbide, nitride, or carbonitride during hot rolling or annealing. To produce such an effect, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more.

[0064] On the other hand, a Ti content of more than 0.200% may result in a large number of coarse precipitates or inclusions. In such a case, a coarse precipitate or inclusion acts as a starting point of a void or a crack in a V-bending test and makes it difficult to ensure press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and the length of a crack that propagates from a bending ridge line end portion in a ridge line direction cannot be controlled to 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm. Thus, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.V: 0.200% or less

[0065] Like Nb or Ti, V increases TS and YS by forming fine carbide, nitride, or carbonitride during hot rolling or annealing. To produce such an effect, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. The V content is even more preferably 0.010% or more, even further more preferably 0.030% or more.

[0066] On the other hand, a V content of more than 0.200% may result in a large number of coarse precipitates or inclusions. In such a case, a coarse precipitate or inclusion acts as a starting point of a void or a crack in a V-bending test and makes it difficult to ensure press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and the length of a crack that propagates from a bending ridge line end portion in a ridge line direction cannot be controlled to 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm. Thus, when V is contained, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.B: 0.0100% or less

[0067] B is an element that segregates at an austenite grain boundary and thereby increases hardenability. B is also an element that controls the formation and grain growth of ferrite during cooling after annealing. To produce such an effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. The B content is even more preferably 0.0005% or more, even further more preferably 0.0007% or more.

[0068] On the other hand, a B content of more than 0.0100% may result in cracking in the inner portion of a steel sheet during hot rolling. The internal crack acts as a starting point of cracking in a V-bending test and makes it difficult to ensure press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and the length of a crack that propagates from a bending ridge line end portion in a ridge line direction cannot be controlled to 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm. Thus, when B is contained, the B content is preferably 0.0100% or less. The B content is more preferably 0.0050% or less.Cr: 1.000% or less

[0069] Cr is an element that increases hardenability, and the addition of Cr forms an appropriate amount of tempered martensite and increases TS and YS. To produce such an effect, the Cr content is preferably 0.0005% or more. The Cr content is more preferably 0.100% or more, even more preferably 0.150% or more.

[0070] On the other hand, a Cr content of more than 1.000% results in an increase in the area fraction of fresh martensite and a decrease in bendability in a V-bending test and makes it difficult to ensure press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and the length of a crack that propagates from a bending ridge line end portion in a ridge line direction cannot be controlled to 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm. Thus, when Cr is contained, the Cr content is preferably 1.000% or less. The Cr content is more preferably 0.800% or less, even more preferably 0.700% or less.Ni: 1.000% or less

[0071] Ni is an element that increases hardenability, and the addition of Ni forms a large amount of tempered martensite and increases TS and YS. To produce such an effect, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more. The Ni content is even more preferably 0.040% or more, even further more preferably 0.060% or more.

[0072] On the other hand, a Ni content of more than 1.000% results in an increase in the area fraction of fresh martensite and a decrease in bendability in a V-bending test and makes it difficult to ensure press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and the length of a crack that propagates from a bending ridge line end portion in a ridge line direction cannot be controlled to 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm. Thus, when Ni is contained, 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, even further more preferably 0.400% or less.Mo: 1.000% or less

[0073] Mo is an element that increases hardenability, and the addition of Mo forms a large amount of tempered martensite and increases TS and YS. To produce such an effect, the Mo content is preferably 0.100% or more, more preferably 0.150% or more.

[0074] On the other hand, a Mo content of more than 1.000% results in an increase in the area fraction of fresh martensite and a decrease in bendability in a V-bending test and makes it difficult to ensure press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and the length of a crack that propagates from a bending ridge line end portion in a ridge line direction cannot be controlled to 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm. Thus, when Mo is contained, 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, even further more preferably 0.400% or less. The Mo content is more preferably 0.350% or less, even further more preferably 0.300% or less.Cu: 1.000% or less

[0075] Cu is an element that increases hardenability, and the addition of Cu forms a large amount of tempered martensite and increases TS and YS. To produce such an effect, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.008% or more, 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, even further more preferably 0.150% or more.

[0076] On the other hand, a Cu content of more than 1.000% may result in an excessive increase in the area fraction of fresh martensite. Furthermore, a large number of coarse precipitates or inclusions may be formed. In such a case, excessively formed fresh martensite or a coarse precipitate or inclusion acts as a starting point of a void or a crack in a V-bending test and makes it difficult to ensure press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and the length of a crack that propagates from a bending ridge line end portion in a ridge line direction cannot be controlled to 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm. Thus, when Cu is contained, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.Ta: 0.100% or less

[0077] Like Ti, Nb, and V, Ta increases TS and YS by forming fine carbide, nitride, or carbonitride during hot rolling or annealing. Furthermore, Ta partially dissolves in Nb carbide or Nb carbonitride and forms a complex precipitate, such as (Nb, Ta) (C, N). This suppresses coarsening of the precipitate and stabilizes precipitation strengthening. This further improves TS and YS. To produce such an effect, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.002% or more, even more preferably 0.004% or more.

[0078] On the other hand, a Ta content of more than 0.100% may result in a large number of coarse precipitates or inclusions. In such a case, an excessively coarse precipitate or inclusion acts as a starting point of a void or a crack in a V-bending test and makes it difficult to ensure press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and the length of a crack that propagates from a bending ridge line end portion in a ridge line direction cannot be controlled to 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm. Thus, when Ta is contained, the Ta content is preferably 0.100% or less. The Ta content is more preferably 0.030% or less, even more preferably 0.010% or less.W: 0.500% or less

[0079] W is an element that increases hardenability, and the addition of W forms a large amount of tempered martensite and increases TS and YS. To produce such an effect, the W content is preferably 0.001% or more. The W content is even more preferably 0.010% or more, even further more preferably 0.030% or more.

[0080] On the other hand, a W content of more than 0.500% results in an increase in the area fraction of fresh martensite and a decrease in bendability in a V-bending test and makes it difficult to ensure press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and the length of a crack that propagates from a bending ridge line end portion in a ridge line direction cannot be controlled to 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm. Thus, when W is contained, the W content is preferably 0.500% or less. The W content is more preferably 0.450% or less, even more preferably 0.400% or less. The W content is even further more preferably 0.300% or less.Mg: 0.0200% or less

[0081] Mg is an element that is effective in spheroidizing the shape of an inclusion of sulfide, oxide, or the like and improving the flangeability and bendability of a steel sheet. To produce such an effect, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0010% or more, even more preferably 0.0030% or more.

[0082] On the other hand, a Mg content of more than 0.0200% may result in a large number of coarse precipitates or inclusions. In such a case, an excessively coarse precipitate or inclusion acts as a starting point of a void or a crack in a V-bending test and makes it difficult to ensure press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and the length of a crack that propagates from a bending ridge line end portion in a ridge line direction cannot be controlled to 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm. Thus, when Mg is contained, the Mg content is preferably 0.0200% or less. The Mg content is more preferably 0.0150% or less, even more preferably 0.0100% or less.Zn: 0.0200% or less

[0083] Zn is an element that is effective in spheroidizing the shape of an inclusion and improving the flangeability and bendability of a steel sheet. To produce such an effect, the Zn content is preferably 0.0010% or more. The Zn content is more preferably 0.0020% or more, even more preferably 0.0030% or more.

[0084] On the other hand, a Zn content of more than 0.0200% may result in a large number of coarse precipitates or inclusions. In such a case, an excessively coarse precipitate or inclusion acts as a starting point of a void or a crack in a V-bending test and makes it difficult to ensure press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and the length of a crack that propagates from a bending ridge line end portion in a ridge line direction cannot be controlled to 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm. Thus, when Zn is contained, the Zn content is preferably 0.0200% or less. The Zn content is more preferably 0.0180% or less, even more preferably 0.0150% or less.Co: 0.0200% or less

[0085] Like Zn, Co is an element that is effective in spheroidizing the shape of an inclusion and improving the flangeability and bendability of a steel sheet. To produce such an effect, the Co content is preferably 0.0010% or more. The Co content is more preferably 0.0020% or more, even more preferably 0.0030% or more.

[0086] On the other hand, a Co content of more than 0.0200% may result in a large number of coarse precipitates or inclusions. In such a case, an excessively coarse precipitate or inclusion acts as a starting point of a void or a crack in a V-bending test, and desired R / t may not be achieved. Thus, when Co is contained, the Co content is preferably 0.0200% or less. The Co content is more preferably 0.0180% or less, even more preferably 0.0150% or less.Zr: 0.1000% or less

[0087] Like Zn and Co, Zr is an element that is effective in spheroidizing the shape of an inclusion and improving the flangeability and bendability of a steel sheet. To produce such an effect, the Zr content is preferably 0.0010% or more.

[0088] On the other hand, in a case where the Zr content is more than 0.1000%, an excessively coarse precipitate or inclusion acts as a starting point of a void or a crack in a V-bending test, and desired R / t may not be achieved. Thus, when Zr is contained, 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, even further more preferably 0.0100% or less.Ca: 0.0200% or less

[0089] Ca is present as an inclusion in steel. A Ca content of more than 0.0200% results in a large number of coarse inclusions. In such a case, an excessively coarse precipitate or inclusion acts as a starting point of a void or a crack in a V-bending test, and desired R / t may not be achieved. Thus, when Ca is contained, 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, even more preferably 0.0018% or less. The Ca content may have any lower limit but is preferably 0.0005% or more. Due to constraints on production technology, the Ca content is more preferably 0.0010% or more.

[0090] 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.

[0091] Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are elements that are effective in improving the flangeability and bendability of a steel sheet. To produce such an effect, each of the Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM contents is preferably 0.0001% or more.

[0092] On the other hand, Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM contents each exceeding 0.0200% and / or an As content of more than 0.0500% may result in a large number of coarse precipitates or inclusions. In such a case, an excessively coarse precipitate or inclusion acts as a starting point of a void or a crack in a V-bending test, and desired R / t may not be achieved. Thus, when at least one of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM is contained, each of the Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM contents is preferably 0.0200% or less. Furthermore, when As is contained, the As content is preferably 0.0500% or less.

[0093] The Se content is preferably 0.0010% or more, more preferably 0.0050% or more. The Se content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0094] The Te content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Te content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. The Te content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0095] The Ge content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Ge content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. The Ge content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0096] The As content is more preferably 0.0010% or more, even more preferably 0.0015% or more. The As content is even further more preferably 0.0050% or more. The As content is more preferably 0.0400% or less, even more preferably 0.0300% or less.

[0097] The Sr content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Sr content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. The Sr content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0098] The Cs content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Cs content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. The Cs content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0099] The Hf content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Hf content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. The Hf content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0100] The Pb content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Pb content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. The Pb content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0101] The Bi content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Bi content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. The Bi content is more preferably 0.0180% or less, even more preferably 0.0150% or less. The Bi content is even further more preferably 0.0100% or less.

[0102] The REM content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The REM content is more preferably 0.0010% or more, even more preferably 0.0020% or more. The REM content is more preferably 0.0180% or less, even more preferably 0.0150% or less. The REM content is even further more preferably 0.0100% or less.

[0103] The term "REM", as used herein, refers to scandium (Sc) with an atomic number of 21, yttrium (Y) with an atomic number of 39, and lanthanoids from lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71. The term "REM concentration", as used herein, refers to the total content of one or two or more elements selected from the REM.

[0104] The REM is preferably, but not limited to, at least one of Sc, Y, Ce, and La.Steel Microstructure (Microstructure at Quarter Thickness of Base Steel Sheet)

[0105] Next, the microstructure excluding a surface soft layer of a steel sheet according to an embodiment of the present invention will be described.Area fraction of ferrite: 55.0% or less (including 0.0%)

[0106] Soft ferrite is a phase that improves ductility. However, when the area fraction of ferrite is excessively increased, it is difficult to achieve a TS of 1180 MPa or more. This also results in a decrease in YS. Thus, ferrite has an area fraction of 55.0% or less. Ferrite preferably has an area fraction of 45.0% or less, more preferably 30.0% or less.

[0107] The lower limit of the area fraction of ferrite may be, but is not limited to, 0.0%. Ferrite may have an area fraction of 1.0% or more or 2.0% or more.

[0108] Total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite): more than 40.0% and 100.0% or less

[0109] Bainitic ferrite and tempered martensite have an intermediate hardness between soft ferrite and hard fresh martensite and the like and are important phases to ensure high bendability of a steel sheet, bendability of a sheared end face, and axial crushing characteristics. Bainitic ferrite is also a phase useful for obtaining an appropriate amount of retained austenite by utilizing the diffusion of C from the bainitic ferrite to non-transformed austenite. Tempered martensite is effective in improving TS. Thus, the total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite) is more than 40.0%. The total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite) is preferably 65.0% or more, more preferably 80.0% or more.

[0110] On the other hand, the upper limit of the total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite) may be 100.0%. Bainitic ferrite and tempered martensite (excluding retained austenite) may have a total area fraction of 96.0% or less or 92.0% or less.

[0111] The term "bainitic ferrite" refers to upper bainite that is formed in a relatively high temperature region and contains a small amount of carbide.Area fraction of retained austenite: less than 3.5% (including 0.0%)

[0112] In the present invention, when the area fraction of retained austenite is excessively increased, hard martensite may be formed by deformation-induced transformation of retained austenite in a V-bending test, void formation and crack propagation may occur at a heterophase boundary with the hard martensite and make it difficult to ensure press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and the length of a crack that propagates from a bending ridge line end portion in a ridge line direction cannot be controlled to 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm. Thus, the area fraction of retained austenite is less than 3.5%. Retained austenite preferably has an area fraction of 3.0% or less, more preferably 2.5% or less, even more preferably 2.0% or less.

[0113] The area fraction of retained austenite may have any lower limit and may be 0.0%, preferably 0.1% or more, more preferably 0.2% or more.

[0114] The term "retained austenite", as used herein, also includes island-like retained austenite (isolated) in ferrite grains.Area fraction of fresh martensite: 10.0% or less (including 0.0%)

[0115] In the present invention, when the area fraction of fresh martensite is excessively increased, fresh martensite may act as a starting point of void formation in a 90-degree V-bending test, and desired R / t, bendability in the inner portion of a steel sheet, and bendability at an end portion (sheared cross section) of the steel sheet may not be achieved. Thus, fresh martensite has an area fraction of 10.0% or less. Fresh martensite preferably has an area fraction of 8.0% or less, more preferably 6.0% or less.

[0116] The lower limit of the area fraction of fresh martensite may be, but is not limited to, 0.0%. Fresh martensite may have an area fraction of 1.0% or more or 2.0% or more.

[0117] The term "fresh martensite", as used herein, refers to as-quenched (untempered) martensite. The term "fresh martensite", as used herein, also includes Martensite-Austenite constituent (isolated) in ferrite grains.

[0118] The area fraction of the remaining microstructure other than the above is preferably 10.0% or less. The area fraction of the remaining microstructure is more preferably 7.0% or less, even more preferably 5.0% or less. The area fraction of the remaining microstructure may be 0.0%.

[0119] The remaining microstructure is, for example, but not limited to, pearlite or carbides, such as cementite and the like. The type of the remaining microstructure can be determined, for example, by scanning electron microscope (SEM) observation.

[0120] The area fractions of ferrite, bainitic ferrite, tempered martensite, and a hard second phase (fresh martensite + retained austenite) are measured at the quarter thickness of a base steel sheet as described below.

[0121] That is, a sample is cut out to form a cross section in the thickness direction (an L cross section) parallel to the rolling direction of the base steel sheet as an observation surface. The observation surface of the sample is then polished with a diamond paste and is then subjected to final polishing using alumina. The observation surface of the sample is then etched with 1% by volume nital to expose the microstructure.

[0122] Five visual fields are then observed at the quarter thickness of the steel sheet as observation positions using SEM at a magnification of 3000 times. The visual fields to be observed are selected in the range of 1 / 4 of the thickness of the steel sheet ± 100 µm, and one visual field is 38 µm x 30 µm. From the microstructure image thus obtained, the area fraction obtained by dividing the area of each constituent microstructure (ferrite, bainitic ferrite, tempered martensite, and a hard second phase (fresh martensite + retained austenite)) by the measurement area is calculated for five visual fields using Adobe Photoshop of Adobe Systems, and the values are averaged to obtain the area fraction of each microstructure. At the outermost surface layer position, a photograph is taken so as to exclude a galvanized layer and include an internal oxidation layer.

[0123] Ferrite: a region with a black color and with a massive form. Almost no carbides are included. Martensite-Austenite constituent and island-like retained austenite isolated in ferrite grains are not included in the area fraction of ferrite.

[0124] Bainitic ferrite: a region with a black to dark gray color and with a massive form, an indefinite form, or the like. It also includes a relatively small number of carbides.

[0125] Tempered martensite: a region with a gray color and with an indefinite form. A relatively large number of carbides are included.

[0126] Hard second phase (retained austenite + fresh martensite): a region with a white to light gray color and with an indefinite form. No carbides are included.

[0127] Carbide: a region with a white color and with a dot-like or linear form. It is included in bainite, tempered bainite, and tempered martensite.

[0128] Remaining microstructure: pearlite, cementite, and the like, as described above, and the forms and the like thereof are known.

[0129] The area fraction of retained austenite is measured as described below.

[0130] A base steel sheet is mechanically ground to the quarter thickness in the thickness direction (depth direction) and is then chemically polished with oxalic acid to form an observation surface. The observation surface is then observed by X-ray diffractometry. A MoKα ray is used as an incident X-ray, the ratio of the diffraction intensity of each of the (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensity of each of the

[0131] (200), (211), and (220) planes of bcc iron is determined to calculate the volume fraction of retained austenite from the ratio of the diffraction intensity of each plane. Assuming that the retained austenite is three-dimensionally homogeneous, the volume fraction of the retained austenite is defined as the area fraction of the retained austenite.

[0132] The area fraction of fresh martensite is determined by subtracting the area fraction of retained austenite from the area fraction of the hard second phase determined as described above.

[0133] The area fraction of the remaining microstructure is calculated by subtracting the area fraction of ferrite, the area fraction of bainitic ferrite, the area fraction of tempered martensite, and the area fraction of the hard second phase determined as described above from 100.0%. Surface Soft Layer

[0134] A base steel sheet of a steel sheet according to an embodiment of the present invention preferably has a surface soft layer in the surface of the base steel sheet. The surface soft layer contributes to suppression of bending crack propagation during press forming and in case of a collision of an automotive body and therefore further improves bending fracture resistance. The term "surface soft layer" means a decarburized layer and refers to a surface layer region having a Vickers hardness of 84% or less relative to the Vickers hardness of a cross section at the quarter thickness.

[0135] The Vickers hardness is measured under a load of 10 gf in accordance with JIS Z 2244-1 (2020).

[0136] In the formula (1), X denotes the thickness of the surface soft layer (µm), and [Sb] and [Sn] denote the Sb content and the Sn content of steel (% by mass), respectively.

[0137] The term "surface soft layer", as used herein, refers to a region having a Vickers hardness of 84% or less relative to the Vickers hardness at the quarter thickness from the surface of a base steel sheet. The surface soft layer thickness (X) needs to satisfy the formula (1).

[0138] A surface soft layer thickness (X) of less than 20 µm makes it impossible to achieve the high strength and high bendability intended in the present invention at the same time.

[0139] On the other hand, a surface soft layer thickness (X) exceeding (120 - 3800 x [Sb] - 1900 x [Sn]) µm makes it impossible to achieve the high strength and high press formability intended in the present invention at the same time.

[0140] Thus, the surface soft layer thickness (X) is set to be 20 µm or more and (120 - 3800 x [Sb] - 1900 x [Sn]) µm or less.

[0141] In the present invention, as described above, Sb and Sn are added as required to improve the coatability and chemical convertibility, but due to the surface segregation of these elements, the addition of Sb and Sn reduces the allowable upper limit of the surface soft layer thickness (X) affecting the bending cracks. For this reason, the upper limit of the surface soft layer for high bendability is (120 - 3800 x [Sb] - 1900 x [Sn]) µm.

[0142] The surface soft layer thickness is preferably 30 µm or more, more preferably 40 µm or more.

[0143] The surface soft layer thickness is preferably 120 µm or less, more preferably 100 µm or less.Microstructure in Surface Soft LayerArea ratio of ferrite: 60.0% or more and 100.0% or less

[0144] When subjected to bending, the surface layer is deformed more greatly than the inner portion. Thus, a void is likely to be formed in the surface layer. In the present invention, by controlling the area fraction of ferrite in the surface soft layer to 60.0% or more, a void that acts as a starting point of cracking is less likely to be formed in the surface layer, and the propagation of cracking is suppressed. Thus, the area fraction of ferrite is 60.0% or more. The area fraction of ferrite is preferably 80.0% or more, more preferably 90.0% or more. The area fraction of ferrite may be 100.0%. The area fraction of ferrite may be less than 100.0%. The area fraction of ferrite may be 98.0% or less or 96.0% or less.

[0145] Area fraction of fresh martensite divided by total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite): 0.5 or less

[0146] When the area fraction of fresh martensite in a surface soft layer is excessively increased, fresh martensite acts as a starting point of void formation in a 90-degree V-bending test, and the inner portion of a steel sheet cannot have desired bendability. From the perspective of ensuring high bendability in the inner portion of a base steel sheet and the bendability of a sheared end face, when ferrite has an area fraction of less than 100.0% in a surface soft layer, the area fraction of fresh martensite divided by the total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less. This value may be 0.4 or less or 0.35 or less.

[0147] The lower limit of the area fraction of fresh martensite divided by the total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) in a surface soft layer may be, but is not limited to, 0.0. This value may be 0.1 or more or 0.15 or more.Area fraction of retained austenite: 3.0% or less

[0148] In a surface layer strain introduction step of applying a tension of 2.0 kgf / mm 2< or more in a reheating and holding temperature range after the first holding step, non-transformed austenite in a surface soft layer is transformed into hard martensite by deformation-induced transformation, and in the subsequent second holding step of holding for 10 seconds or more, the hard martensite is tempered, tempered martensite is formed, and finally the area fraction of retained austenite is controlled to 3.0% or less, whereby the length of a crack that propagates from a bending ridge line end portion in a ridge line direction in a 90-degree V-bending test with a bending radius R of 0.5 mm can be set to 200 µm or less. When retained austenite has an area fraction of more than 3.0%, hard martensite is formed by deformation-induced transformation of retained austenite in a V-bending test, and void formation and crack propagation occur at a heterophase boundary with the hard martensite. This makes it difficult to ensure press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and the length of a crack that propagates from a bending ridge line end portion in a ridge line direction cannot be controlled to 200 µm or less in a 90-degree V-bending test with a bending radius of 0.5 mm. Thus, the area fraction of retained austenite is 3.0% or less. The area fraction of retained austenite is preferably 2.8% or less, more preferably 2.5% or less. The lower limit of the area fraction of retained austenite is preferably, but not limited to, 0.2% or more, more preferably 0.5% or more.

[0149] In the measurement of the microstructure in a surface soft layer, when a galvanized layer is formed on a steel sheet, the galvanized layer is first removed, and the microstructure is measured at the half thickness of the surface soft layer in the same manner as at the quarter thickness of a base steel sheet.

[0150] Next, mechanical characteristics of a steel sheet according to an embodiment of the present invention will be described. Tensile strength (TS): 1180 MPa or more and less than 1470 MPa A steel sheet according to an embodiment of the present invention has a tensile strength (TS) of 1180 MPa or more and less than 1470 MPa.

[0151] The yield stress (YS) and the total elongation (El) of a steel sheet according to an embodiment of the present invention are as described above.

[0152] The tensile strength (TS), the yield stress (YS), and the total elongation (El) are measured in the tensile test according to JIS Z 2241 (2011) described later in Examples.

[0153] The ratio YR (yield ratio) of yield stress (YS) to tensile strength (TS) preferably satisfies 0.70 ≤ YR.

[0154] Furthermore, in a steel sheet according to an embodiment of the present invention, R (critical bending radius) / t (sheet thickness), the length of a crack that propagates from a bending ridge line end portion in a ridge line direction, El in a notch tensile test after heat treatment at 170°C for 20 minutes, and El in a notch tensile test after introducing a nominal tensile strain of 2% and performing heat treatment at 170°C for 20 minutes are as described above. These are measured by the methods described later in Examples.Coated Layer (Hot-Dip Galvanized Layer, Hot-Dip Galvannealed Layer)

[0155] A steel sheet according to an embodiment of the present invention may have a coated layer on a base steel sheet (on the surface of the base steel sheet), and the coated layer may be provided on only one surface of the base steel sheet or may be provided on both surfaces thereof.

[0156] The term "coated layer (galvanized layer)", as used herein, refers to a coated layer containing Zn as a main component (Zn content: 50.0% or more), for example, a hot-dip galvanized layer or a hot-dip galvannealed layer.

[0157] The hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0% or less by mass Fe, and 0.001% or more by mass and 1.0% or less by mass Al. The hot-dip galvanized layer may optionally contain one or two 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% or more by mass and 3.5% or less by mass. The Fe content of the hot-dip galvanized layer is more preferably less than 7.0% by mass. The remainder other than the above elements is incidental impurities.

[0158] The hot-dip galvannealed layer is preferably composed of, for example, 20.0% or less by mass of Fe and 0.001% or more by mass and 1.0% or less by mass of Al. The hot-dip galvannealed layer may optionally contain one or two 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% or more by mass and 3.5% or less by mass. The Fe content of the hot-dip galvannealed layer is more preferably 7.0% or more by mass, even more preferably 8.0% or more by mass. The Fe content of the hot-dip galvannealed layer is more preferably 15.0% or less by mass, even more preferably 12.0% or less by mass. The remainder other than the above elements is incidental impurities.

[0159] Furthermore, the coating weight per side of a coated layer (galvanized layer) is preferably, but not limited to, 20 g / m 2< or more and 80 g / m 2< or less.

[0160] The coating weight of a coated layer (galvanized layer) is measured as described below.

[0161] That is, a treatment liquid is prepared by adding 0.6 g of a corrosion inhibitor for Fe ("Ibit 700BK" (registered trademark) manufactured by Asahi Chemical Co., Ltd.) to 1 L of 10% by mass aqueous hydrochloric acid. A steel sheet (galvanized steel sheet) as a specimen is then immersed in the treatment liquid to dissolve a coated layer (galvanized layer). The mass loss of the specimen due to the dissolution is then measured and is divided by the surface area of a base steel sheet (the surface area of a coated portion) to calculate the coating weight (g / m 2< ).

[0162] The thickness of a steel sheet according to an embodiment of the present invention is preferably, but not limited to, 0.5 mm or more, more preferably 0.6 mm or more, even more preferably 0.8 mm or more. The steel sheet preferably has a thickness of 2.3 mm or less, more preferably 1.6 mm or less, even more preferably 1.2 mm or less.[2. Method for Producing Steel Sheet]

[0163] Next, a method for producing a steel sheet according to an embodiment of the present invention will be described.

[0164] A method for producing a steel sheet according to the present invention includes a hot rolling step of hot-rolling a steel slab having the chemical composition described above at a finish rolling temperature of 820°C or more, an annealing step of heating the steel sheet after the hot rolling step and annealing the steel sheet at an annealing temperature of 750°C or more and 900°C or less for an annealing time of 20 seconds or more in an atmosphere with a dew-point temperature of -10°C or more under conditions satisfying the formulae (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 and holding temperature range of the cooling stop temperature or more and 440°C or less and holding the steel sheet for 10 seconds or more, a surface layer strain introduction step of applying a tension of 2.0 kgf / mm 2< or more to the steel sheet after the first holding step in the reheating and holding temperature range, and a second holding step of holding the steel sheet after the surface layer strain introduction step in the reheating and holding temperature range for 10 seconds or more, or further a cold rolling step of cold-rolling the steel sheet after the hot rolling step and before the annealing step at a rolling reduction ratio of 20% or more and 80% or less to produce a cold-rolled steel sheet. 2400 ≤ Y ≤ 20000 Y = T − Ac 1 × t 1 / 2 + T − Ac 1 × t 2

[0165] In the formula (3), T denotes the annealing temperature (°C), t1 denotes the time (s) from 650°C to the annealing temperature T during the heating in the annealing step, t2 denotes the annealing time (soaking time) (s), and Ac1 denotes Ac1 (°C).

[0166] Unless otherwise specified, the temperatures described above mean the surface temperatures of a steel slab and a steel sheet.

[0167] In the present invention, a steel material (steel slab) may be produced by any melting method and may be produced by any known melting method using a converter, an electric arc furnace, or the like. A steel slab (slab) is preferably produced by a continuous casting method to prevent macrosegregation but may also be produced by an ingot casting method, a thin slab casting method, or the like. Furthermore, in addition to a known method of producing a steel slab, then temporarily cooling the steel slab to room temperature, and then heating the steel slab again, it is also possible without problems to apply an energy-saving process, such as hot direct rolling or hot direct rolling, in which a hot piece is charged into a furnace as it is without being cooled to room temperature or is subjected to slight heat retention and is then immediately rolled.(Hot Rolling Step)

[0168] When a slab is heated, from the perspective of dissolution of carbide and reduction of rolling force, the slab heating temperature is preferably 1100°C or more. Furthermore, to prevent an increase in scale loss, the slab heating temperature is preferably 1300°C or less. The slab heating temperature is the temperature of the slab surface. Furthermore, although a slab is formed into a sheet bar by rough rolling under normal conditions, when the heating temperature is lowered, from the perspective of preventing troubles during hot rolling, the sheet bar is preferably heated with a bar heater or the like before finish rolling.Finish rolling temperature: 820°C or more

[0169] In the finish rolling, the rolling load increases and the rolling reduction ratio in the unrecrystallized state of austenite increases, and as a result of the development of an abnormal microstructure elongated in the rolling direction, the ductility, flangeability, and bendability of the final material are deteriorated. Thus, the finish rolling temperature is 820°C or more. The finish rolling temperature is preferably 830°C or more, more preferably 850°C or more. The finish rolling temperature is preferably 1080°C or less, more preferably 1050°C or less.

[0170] Furthermore, although the coiling temperature after hot rolling is not particularly limited, it is necessary to consider a case where the ductility, flangeability, and bendability of the final material are degraded. Thus, the coiling temperature after hot rolling is preferably 300°C or more. The coiling temperature after hot rolling is preferably 700°C or less.

[0171] During hot rolling, rough-rolled sheets may be joined and continuously subjected to finish rolling. A rough-rolled sheet may be once coiled. Furthermore, to reduce the rolling force during hot rolling, part or all of the finish rolling may be lubrication rolling. The lubrication rolling is also effective in making the shape and the material quality of a steel sheet uniform. The friction coefficient during the lubrication rolling is preferably 0.10 or more and 0.25 or less.(Pickling Step)

[0172] A hot-rolled steel sheet thus produced may be pickled. Pickling can remove oxides on the steel sheet surface and can therefore be performed to ensure high chemical convertibility and coating quality of a high-strength steel sheet as the final product. Furthermore, pickling may be performed once or may be divided into a plurality of times.(Cold Rolling Step)

[0173] A pickled sheet after hot rolling or a hot-rolled steel sheet produced as described above is cold-rolled as required. In cold rolling, after hot rolling, a pickled sheet may be cold-rolled as it is or may be cold-rolled after heat treatment. Optionally, a cold-rolled steel sheet after cold rolling may be pickled.

[0174] The cold rolling is performed, for example, by multipass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.Rolling reduction ratio of cold rolling as required: 20% or more and 80% or less

[0175] In cold rolling, the rolling reduction ratio (cumulative rolling reduction ratio) of the cold rolling is preferably, but not limited to, 20% or more and 80% or less. When the rolling reduction ratio of the cold rolling is less than 20%, coarsening and non-uniformity of the steel microstructure are likely to occur in the annealing step, and the final product may have a decrease in TS and bendability. On the other hand, a rolling reduction ratio of more than 80% in the cold rolling tends to result in a steel sheet with a poor shape and may result in an uneven galvanized coating weight.(Annealing Step)

[0176] In an embodiment of the present invention, a steel sheet after the hot rolling step (after the cold rolling step in the case of performing cold rolling) is heated and annealed at an annealing temperature of 750°C or more and 900°C or less for an annealing time of 20 seconds or more in an atmosphere with a dew-point temperature of -10°C or more under conditions satisfying the formulae (2) and (3). 2400 ≤ Y ≤ 20000 Y = T − Ac 1 × t 1 / 2 + T − Ac 1 × t 2

[0177] In the formula (3), T denotes the annealing temperature (°C), t1 denotes the time (s) from 650°C to the annealing temperature T during the heating in the annealing step, t2 denotes the annealing time (s), and Ac1 denotes Ac1 (°C).Annealing temperature: 750°C or more and 900°C or less

[0178] An annealing temperature of less than 750°C results in an insufficient formation ratio of austenite during heating in a two-phase region of ferrite and austenite. Thus, the area fraction of ferrite increases excessively after annealing, and desired TS and YS cannot be achieved.

[0179] On the other hand, an annealing temperature of more than 900°C results in a decrease in ductility.

[0180] Thus, the annealing temperature is 750°C or more and 900°C or less. The annealing temperature is preferably 880°C or less. The annealing temperature is more preferably 870°C or less. The annealing temperature is preferably 780°C or more, more preferably 800°C or more.

[0181] The annealing temperature is the highest temperature (soaking temperature) reached in the annealing step.Annealing time (soaking time): 20 seconds or more

[0182] An annealing time of less than 20 seconds results in an insufficient proportion of austenite formed during heating in a two-phase region of ferrite and austenite. Thus, the area fraction of ferrite increases excessively after annealing, and TS and YS cannot be achieved. Thus, the annealing time is 20 seconds or more. The annealing time is preferably 30 seconds or more, more preferably 50 seconds or more. The annealing time may have any upper limit but is preferably 600 seconds or less, more preferably 250 seconds or less.

[0183] The annealing time (soaking time) is a holding time in the temperature range of (annealing temperature - 40°C) or more and the annealing temperature or less. That is, the annealing time includes, in addition to the holding time at the annealing temperature, the residence time in the temperature range of (annealing temperature - 40°C) or more and the annealing temperature or less in heating and cooling before and after reaching the annealing temperature.

[0184] The annealing may be performed twice or more but is preferably performed once from the perspective of energy efficiency.

[0185] Dew-point temperature of atmosphere of annealing step (annealing atmosphere): -10°C or more

[0186] In an embodiment of the present invention, the dew-point temperature of the atmosphere in the annealing step (annealing atmosphere) is preferably -10°C or more. Annealing at a dew-point temperature of -10°C or more in the annealing atmosphere in the annealing step can promote a decarburization reaction and more deeply form a surface soft layer. The dew-point temperature of the annealing atmosphere in the annealing step is preferably -5°C or more, more preferably 0°C or more, even more preferably +5°C or more. Although the dew-point temperature of the annealing atmosphere in the annealing step may have any upper limit, to suitably prevent the oxidation of the surface of an Fe-based electroplated layer and to improve the adhesiveness of coating when a galvanized layer is provided, the dew-point temperature of the annealing atmosphere in the annealing step is preferably 30°C or less. The dew-point temperature of the annealing atmosphere in the annealing step is more preferably 25°C or less, even more preferably 20°C or less. 2400 ≤ Y ≤ 20000 Y = T − Ac 1 × t 1 / 2 + T − Ac 1 × t 2

[0187] In the formula (3), T denotes the annealing temperature (°C), t1 denotes the time (s) from 650°C to the annealing temperature (soaking temperature) T during the heating in the annealing step, t2 denotes the annealing time (s), and Ac1 denotes Ac1 (°C).

[0188] In the present invention, annealing should be performed under conditions that satisfy the formulae (2) and (3). Y of less than 2400 in the formula (3) results in less than 20 µm of a surface soft layer defined in the present invention. On the other hand, Y of more than 20000 results in a surface soft layer defined in the present invention exceeding (120 - 3800 x [Sb] - 1900 x [Sn]) µm. Thus, Y in the formula (3) is 2400 or more and 20000 or less.

[0189] Y is preferably 9000 or more, more preferably 12000 or more.

[0190] Y is preferably 19000 or less, more preferably 18000 or less.

[0191] t1 is preferably 30 s or more. t1 is preferably 80 s or less.

[0192] Ac1 (°C) is calculated by the following formula: wherein [%C] denotes the C content of the steel sheet (steel slab), [%Si] denotes the Si content of the steel sheet (steel slab), and [%Mn] denotes the Mn content of the steel sheet (steel slab).(Cooling Step)

[0193] Cooling to cooling stop temperature of less than 100°C Average cooling rate: 10°C / s or more and 50°C / s or less, dew-point temperature of atmosphere: -20°C or less(preferred requirement)

[0194] In the cooling step, a 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 750°C or more and 900°C or less.

[0195] The cooling stop temperature is preferably 80°C or less, more preferably 60°C or less.

[0196] The cooling stop temperature is preferably 5°C or more, more preferably 15°C or more.

[0197] The average cooling rate in the cooling step is preferably 10°C / s or more and 50°C / s or less. In this cooling step, a steel microstructure defined in the present invention can be formed.

[0198] The dew-point temperature of the atmosphere in the cooling step is preferably -20°C or less. An atmosphere with a dew-point temperature of more than -20°C may result in a large variation in the thickness of a surface soft layer in the in-plane direction of the steel sheet, making it impossible to achieve the tensile strength defined in the present invention. Thus, the dew-point temperature of the atmosphere in the cooling step is preferably -20°C or less.

[0199] The average cooling rate (°C / s) is calculated 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).(First Holding Step (Reheating and Holding Step))

[0200] In the first holding step (reheating and holding step), a steel sheet is reheated to a reheating and holding temperature range of the cooling stop temperature or more and 440°C or less and is held for 10 seconds or more.

[0201] Reheating the steel sheet to the reheating and holding temperature range of the cooling stop temperature or more and 440°C or less and holding the steel sheet for 10 seconds or more can form bainitic ferrite and tempered martensite defined in the present invention. The reheating and holding temperature range is preferably 420°C or less, more preferably 400°C or less.

[0202] The holding time is preferably 20 seconds or more, more preferably 30 seconds or more.

[0203] The holding time is preferably 100 seconds or less, more preferably 80 seconds or less.(Surface Layer Strain Introduction Step)

[0204] In the surface layer strain introduction step, a tension of 2.0 kgf / mm 2< or more is applied to a steel sheet after the first holding step in the reheating and holding temperature range. Thus, El in a notch tensile test after heat treatment at 170°C for 20 minutes can be controlled to be equal to or higher than the value defined in the present invention.

[0205] The tension is calculated by dividing the total value of the loads (kgf) of load cells at the left and right of a roll through which the steel sheet passes while contacting the roll by the cross-sectional area of the steel sheet (= sheet thickness (mm) x sheet width (mm)) (mm 2< ). The load cells should be arranged parallel to the direction of the tension.

[0206] The load cells are preferably disposed at a position of 200 mm from both ends of the roll. The length of the roll to be used preferably ranges from 1500 to 2500 mm.

[0207] The tension is preferably 2.2 kgf / mm 2< or more, more preferably 2.4 kgf / mm 2< or more. The tension is more preferably 10.0 kgf / mm 2< or less, more preferably 4.0 kgf / mm 2< or less. The unit of tension can be converted from kgf / mm 2< to N / mm 2< by setting 1 kgf / mm 2< to 9.8 N / mm 2< .(Second Holding Step)

[0208] In the second holding step, a steel sheet after the surface layer strain introduction step is held in the reheating and holding temperature range for 10 seconds or more. Thus, El in a notch tensile test after introducing a nominal tensile strain of 2% and performing a heat treatment at 170°C for 20 minutes can be controlled to be equal to or higher than the value defined 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.(Coating Step (Hot-Dip Galvanizing Step, Hot-Dip Galvannealing Step))

[0209] In the present invention, in a coating step after the annealing step, a steel sheet can be subjected to a galvanizing treatment to produce a galvanized steel sheet.

[0210] The galvanizing treatment is, for example, a hot-dip galvanizing treatment or a hot-dip galvannealing treatment.

[0211] The galvanizing treatment may be performed, for example, during the cooling step, during the first holding step, after the first holding step and before the surface layer strain introduction step, after the surface layer strain introduction step and before the second holding step, during the second holding step, or after the second holding step.

[0212] The galvanizing treatment may be performed, for example, during the cooling treatment in the cooling step after the annealing step and before the first holding step.

[0213] In the hot-dip galvanizing treatment, preferably, a steel sheet is immersed in a galvanizing bath (hot-dip galvanizing bath) at 440°C or more and 500°C or less, and the coating weight is then adjusted by gas wiping or the like. The hot-dip galvanizing bath is not particularly limited, provided that the galvanized layer has the composition described above, and is preferably, for example, a coating bath having a composition with an Al content of 0.10% or more by mass and 0.23% or less by mass and with the remainder being composed of Zn and incidental impurities.

[0214] In the hot-dip galvannealing treatment, after the hot-dip galvanizing treatment performed in the manner described above, the hot-dip galvanized steel sheet is preferably heated to an alloying temperature of 450°C or more and 600°C or less to perform an alloying treatment.

[0215] An alloying temperature of less than 450°C may result in a low Zn-Fe alloying speed and make alloying difficult. On the other hand, an alloying temperature of more than 600°C results in transformation of non-transformed austenite into pearlite and makes it difficult to achieve a TS of 1180 MPa or more. The alloying temperature is more preferably 500°C or more, even more preferably 510°C or more. The alloying temperature is more preferably 570°C or less.

[0216] The coating weight of each of the hot-dip galvanized steel sheet (GI) and the hot-dip galvannealed steel sheet (GA) preferably ranges from 20 to 80 g / m 2< per side. The coating weight can be adjusted by gas wiping or the like.

[0217] Furthermore, the steel sheet thus produced may be further subjected to temper rolling.

[0218] A rolling reduction ratio of more than 2.00% in the temper rolling may result in an increase in yield stress and a decrease in dimensional accuracy when the steel sheet is formed into a member. Thus, the rolling reduction ratio of the temper rolling is preferably 2.00% or less. The lower limit of the rolling reduction ratio in the temper rolling is preferably, but not limited to, 0.05% or more from the perspective of productivity. The temper rolling may be performed with an apparatus coupled to an annealing apparatus for each step (on-line) or with an apparatus separated from the annealing apparatus for each step (offline). The number of times of temper rolling may be one or two or more. Provided that the elongation percentage can be similar to that of temper rolling, rolling with a leveler or the like may be used.

[0219] Although the other conditions of the production method are not particularly limited, from the perspective of productivity, a series of treatments, such as the annealing, hot-dip galvanizing, and alloying treatment of galvanization, are preferably performed in a continuous galvanizing line (CGL), which is a hot-dip galvanizing line. After the hot-dip galvanizing, wiping can be performed to adjust the coating weight. The conditions of coating and the like other than the above-described conditions can be the same as in a usual hot-dip galvanizing method.[3. Member]

[0220] Next, a member according to an embodiment of the present invention will be described.

[0221] A member according to an embodiment of the present invention is a member produced by using the steel sheet described above (as a material). For example, the steel sheet as a material is subjected to at least one of forming or joining to produce the member.

[0222] The steel sheet has a TS of 1180 MPa or more and less than 1470 MPa, a high yield stress (YS), high ductility, high press formability in the inner portion of the steel sheet (bendability in the inner portion of the steel sheet), high press formability at an end portion of the steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and high crack propagation resistance. Thus, a member according to an embodiment of the present invention has a tensile strength (TS) of 1180 MPa or more and less than 1470 MPa, a high yield stress (YS), high ductility, high press formability in the inner portion of the steel sheet (bendability in the inner portion of the steel sheet), high press formability at an end portion of the steel sheet (bendability at the end portion (sheared cross section) of the steel sheet), and high crack propagation resistance. Thus, a member according to an embodiment of the present invention is particularly preferred for application to an impact energy absorbing member used in the automotive field.[4. Method for Producing Member]

[0223] Next, a method for producing a member according to an embodiment of the present invention will be described.

[0224] A method for producing a member according to an embodiment of the present invention includes a step of subjecting the steel sheet (a steel sheet produced by the method for producing a steel sheet) to at least one of forming or joining to produce the member.

[0225] The forming method may be, but is not limited to, for example, a typical processing method, such as press forming. Furthermore, the joining method may be, but is not limited to, for example, typical welding, such as spot welding, laser welding, or arc welding, riveting, caulking, or the like. The forming conditions and the joining conditions are not particularly limited and may be in accordance with a usual method.EXAMPLES

[0226] A steel material having a chemical composition shown in Table 1 (the remainder was composed of Fe and incidental impurities) was melted in a converter and was formed into a steel slab by a continuous casting method. In Table 1, "-" indicates the content at an incidental impurity level and is treated as 0 (zero).

[0227] The steel slab was heated to 1200°C and, after the heating, was subjected to rough rolling and hot rolling (finish rolling temperature: 900°C) to produce a hot-rolled steel sheet. Hot-rolled steel sheet Nos. 1 to 70 thus produced were then subjected to pickling and cold rolling to form cold-rolled steel sheets having the thickness shown in Table 3.

[0228] The cold-rolled steel sheets were subjected to treatments in an annealing step, a cooling step, a coating step (hot-dip galvanizing step or hot-dip galvannealing step) in the middle of the cooling step, a reheating and holding step (first holding step), a surface layer strain introduction treatment, and a second holding step under the conditions shown in Table 2 to produce steel sheets.

[0229] In the cooling step, the average cooling rate was 10°C / s, and the dew-point temperature of the atmosphere was -30°C.

[0230] In the coating step, a hot-dip galvanizing treatment or a hot-dip galvannealing treatment was performed to produce a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or a hot-dip galvannealed steel sheet (hereinafter also referred to as GA). In Table 2, the types of coating steps are also indicated as "GI" and "GA". In the GI steel sheets in Table 2, no alloying treatment is performed, and the alloying temperature is indicated by "-". In Table 2, steel sheets not subjected to the treatment in the coating step are indicated by "CR".

[0231] The galvanizing bath temperature was 470°C in the production of GI and GA.

[0232] The galvanizing coating weight ranged from 45 to 72 g / m 2< per side to produce GI and was 45 g / m 2< per side to produce GA.

[0233] The composition of the coated layer (galvanized layer) of the final steel sheet in GI contained Fe: 0.1% to 1.0% by mass and Al: 0.2% to 0.33% by mass, and the remainder was composed of Zn and incidental impurities. GA contained Fe: 8.0% to 12.0% by mass and Al: 0.1% to 0.23% by mass, and the remainder was composed of Zn and incidental impurities.

[0234] In each case, the galvanized layer was formed on both surfaces of the base steel sheet.

[0235] In the steel sheets thus produced, the steel microstructure of the base steel sheet was identified in the above-described manner. Table 3 shows the measurement results. As shown in Fig. 1, F denotes ferrite, M denotes fresh martensite, RA denotes retained austenite, BF denotes bainitic ferrite, and TM denotes tempered martensite. In Table 3, θ denotes a carbide.

[0236] Measurement is performed on a surface soft layer as described below. A cross section in the thickness direction (L cross section) parallel to the rolled direction of a steel sheet was smoothed by wet grinding, and measurement was then performed with a Vickers hardness tester at a load of 10 gf (9.8 x 10 -2< N) at intervals of 1 µm from a position of 1 µm in the thickness direction from the surface of the steel sheet to a position of 100 µm in the thickness direction.

[0237] The measurement was then performed at intervals of 20 µm up to the center of the sheet thickness. A region where the Vickers hardness decreased to 84% or less relative to the Vickers hardness at the quarter thickness is defined as a soft layer (surface soft layer), and the thickness of the region in the sheet thickness direction is defined as the thickness of the soft layer.

[0238] The microstructure of the surface soft layer was identified in the same manner as in the identification of the steel microstructure of the base steel sheet at the half thickness of the surface soft layer.

[0239] In Tables 1 to 4, the underlined portions indicate values outside the appropriate ranges of the present invention.

[0240] Furthermore, a tensile test and a 90-degree V-bending test were performed in the following manner, and the tensile strength (TS), the yield stress (YS), the yield ratio (YR), the total elongation (El), R / t in the V-bending test, the length of a crack that propagates from a bending ridge line end portion in a ridge line direction, and the notch El were evaluated according to the following criteria.- Tensile strength (TS)

[0241] O (pass): 1180 MPa or more and less than 1470 MPa X (fail): less than 1180 MPa or 1470 MPa or more - Yield stress (YS)

[0242] O (pass): (A) For 1180 MPa ≤ TS < 1320 MPa, 820 MPa ≤ YS (B) For 1320 MPa ≤ TS < 1470 MPa, 920 MPa ≤ YS X (fail): (A) For 1180 MPa ≤ TS < 1320 MPa, 820 MPa > YS (B) For 1320 MPa ≤ TS < 1470 MPa, 920 MPa > YS - Ductility (El)

[0243] O (pass): (A) For 1180 MPa ≤ TS < 1320 MPa, 8.0% ≤ El (B) For 1320 MPa ≤ TS < 1470 MPa, 7.0% ≤ El X (fail): (A) For 1180 MPa ≤ TS < 1320 MPa, 8.0% > El (B) For 1320 MPa ≤ TS < 1470 MPa, 7.0% > El

[0244] R / t (press formability in the inner portion of a steel sheet (bendability in the inner portion of the steel sheet)) O (pass): (A) For 1180 MPa ≤ TS < 1320 MPa, 3.0 ≥ R / t (B) For 1320 MPa ≤ TS < 1470 MPa, 4.0 ≥ R / t X (fail): (A) For 1180 MPa ≤ TS < 1320 MPa, 3.0<R / t (B) For 1320 MPa ≤ TS < 1470 MPa, 4.0<R / t The length of a crack that propagates along a bending ridge line formed in a portion other than a bending ridge line end portion in a 90-degree V-bending test with a bending radius of 0.5 mm (the press formability at an end portion of a steel sheet (bendability in the inner portion of the steel sheet)) O (pass): the crack length in the portion other than the V-bent end surface is 200 µm or less X (fail): the crack length in the portion other than the V-bent end surface is more than 200 µm The length of a crack that propagates from a bending ridge line end portion in a ridge line direction in a 90-degree V-bending test with a bending radius of 0.5 mm (the press formability at an end portion of a steel sheet (bendability at the end portion (sheared cross section) of the steel sheet)) O (pass): the V-bending edge crack length is 200 µm or less X (fail): the V-bending edge crack length is more than 200 µm Notch El (crack propagation resistance) (i) A notch tensile test after heat treatment at 170°C for 20 minutes O (pass): (A) For 1180 MPa ≤ TS < 1320 MPa, notch El ≥ 6.5% (B) For 1320 MPa ≤ TS < 1470 MPa, notch El ≥ 6.0% X (fail): (A) For 1180 MPa ≤ TS < 1320 MPa, notch El < 6.5% (B) For 1320 MPa ≤ TS < 1470 MPa, notch El < 6.0% (ii) A notch tensile test after introducing a nominal tensile strain of 2% and performing heat treatment at 170°C for 20 minutes O (pass): (C) For 1180 MPa ≤ TS < 1320 MPa, notch El ≥ 5.5% (D) For 1320 MPa ≤ TS < 1470 MPa, notch El ≥ 5.0% X (fail): (C) For 1180 MPa ≤ TS < 1320 MPa, notch El < 5.5% (D) For 1320 MPa ≤ TS < 1470 MPa, notch El < 5.0% (1) Tensile Test

[0245] The tensile test was performed in accordance with JIS Z 2241 (2011). More specifically, a JIS No. 5 specimen was taken from a steel sheet thus produced such that the longitudinal direction was perpendicular to the rolling direction of the base steel sheet. A tensile test was performed on the test specimen at a crosshead speed of 10 mm / min to measure TS, YS, YR, and El. Table 3 shows the results.(2) 90-Degree V-Bending Test

[0246] A strip specimen of 100 mm C (C direction: the direction perpendicular to the rolling direction of the steel sheet) x 30 mm L (L direction: the direction along the rolling direction) was taken from a steel sheet at a position corresponding to 1 / 4 of the coil width. An end surface 100 mm in length was cut by shearing, and bending was performed such that a burr was on the bending outer peripheral side in the state of the shearing (without machining of removing the burr). In the shearing, the clearance was 15%, and the rake angle was 0 degrees. In V-bending, L direction bending (bending ridge line length: 30 mm L) was performed using Autograph manufactured by Shimadzu Corporation under the conditions of a punch bending radius R of 0.5 mm, a punch bending angle of 90 degrees, a punch stroke speed of 30 mm / min, a pressing load of 10 ton, and a pressing time of 5 seconds.

[0247] An example of a sample after the 90-degree V-bending test with a bending radius of 0.5 mm is illustrated in Fig. 2. Fig. 2(b) is a bird's-eye view of the sample viewed from the Z direction illustrated in Fig. 2(a). When a portion having a full width of 5 mm from the bending apex in the C direction along the surface of the steel sheet (2.5 mm on both sides from the bending apex) was defined as a bending ridge line, a portion having a width of 5 mm (region o) from an endmost portion of the bending ridge line in the L direction was defined as a bending ridge line end portion. The length Y1 of a crack that propagates from the bending ridge line end portion in the ridge line direction (L direction) and the length Y2 of a crack that propagates in the L direction along the bending ridge line formed in a portion other than the bending ridge line end portion are measured by the following methods.

[0248] After the 90-degree V-bending test with a bending radius of 0.5 mm, the length of a crack that propagates from the bending ridge line end portion in the ridge line direction is measured as described below.

[0249] Fig. 3-1(a) illustrates cracking at a bending ridge line end portion in a sample after the V-bending test. To measure the crack length at the bending ridge line central portion, the sheet surface (b surface) is generally observed in the Z direction. An actual sample after the V-bending test has a saddle shape as illustrated in Fig. 3-1(b), the b surface is greatly deformed, the accuracy of measurement of the crack length is lowered, and the bendability of a sheared end face may not be accurately evaluated. In the present invention, the measurement can be performed with high accuracy by the following measurement method.

[0250] The shear surface a of a bent sample after the 90-degree V-bending test with a bending radius of 0.5 mm was placed on the upper side, and the bending ridge line end portion was photographed with a one-shot 3D shape measuring instrument (manufactured by Keyence Corporation, VR6000 series or newer model) at a magnification of 40 times. Height data was analyzed using analysis software attached to the one-shot 3D shape measuring instrument. As illustrated in Fig. 3-2(a), an arc measurement line i was drawn along the bending ridge line at a position as close as possible to the outside of the bend subjected to the tensile stress. An example of a profile waveform j is illustrated in Fig. 3-2(b), the length (y1 + y2) / 2 of each crack is determined using a measurement tool of the software, and the length of the longest crack is defined as the length of a crack that propagates from the bending ridge line end portion in the ridge line direction after the 90-degree V-bending test with a bending radius of 0.5 mm.(3) Notch Tensile Test

[0251] The steel sheet was subjected to a heat treatment at 170°C for 20 minutes. A JIS No. 5 test piece for tensile test was prepared such that the longitudinal direction of the test piece was aligned with the direction perpendicular to the rolling direction, a notch (V notch: depth 2 mm, V internal angle 60 degrees) was formed at both width end portions at the middle position of the longitudinal parallel portion of the test piece, and a notch tensile test was performed by a tensile test according to JIS Z 2241 (2011) with the gauge length of the test piece being 15 mm to measure the total elongation (El).

[0252] Furthermore, a nominal tensile strain of 2% was introduced into the steel sheet, and a heat treatment was performed at 170°C for 20 minutes. A JIS No. 5 test piece for tensile test was prepared such that the longitudinal direction of the test piece was aligned with the direction perpendicular to the rolling direction, a notch (V notch: depth 2 mm, V internal angle 60 degrees) was formed at both width end portions at the middle position of the longitudinal parallel portion of the test piece, and a notch tensile test was performed by a tensile test according to JIS Z 2241 (2011) with the gauge length of the test piece being 15 mm to measure the total elongation (El). [Table 1]Steel gradeChemical composition (% by mass)Ac1 (°C)NotesCSiMnPSAlNSbSnOthersA0.1250.552.880.0100.00120.0300.00320.0100--737Inventive steelB0.1190.352.960.0080.00100.0320.0027---734Inventive steelC0.1410.663.210.0090.00090.0380.00380.0090--739Inventive steelD0.1330.602.890.0090.00110.5000.00320.0080--737Inventive steelE 0.020 0.502.820.0100.00150.0280.00490.0090--739Comparative steelF 0.440 0.452.800.0130.00120.0320.00290.0040--725Comparative steelG 0.1283.20 2.750.0120.00180.0350.00320.0060--776Comparative steelH 0.1250.541.20 0.0140.00140.0270.00420.0060--733Comparative steelI 0.1230.483.80 0.0110.00080.0290.00380.0030--738Comparative steelJ0.1210.622.700.0120.00120.0410.00380.0040-Nb: 0.033738Inventive steelK0.1180.572.720.0100.00080.0320.00450.0010-Ti: 0.040737Inventive steelL0.1150.632.650.0090.00090.0350.00300.0100-Ti: 0.028, B: 0.0022738Inventive steelM0.1240.592.620.0100.00120.0460.00330.0050-Nb: 0.018, Ti: 0.022, B: 0.0014737Inventive steelN0.1210.452.510.0110.00120.0290.00320.00300.0010Nb: 0.035, Ti: 0.015, B: 0.0012, Cr: 0.580735Inventive steelO0.1370.693.120.0090.00080.0320.00350.0010-Nb: 0.012, Ti: 0.023, B: 0.0015739Inventive steelP0.1650.572.520.0110.00090.0390.00220.0080-Nb: 0.025, Ti: 0.025, B: 0.0010, Cr: 0.520735Inventive steelQ0.0750.612.550.0140.00150.0340.00510.0010-Nb: 0.035, Ti: 0.020, B: 0.0015, Cr: 0.440739Inventive steelR0.1150.732.530.0120.00250.0350.00330.00400.0020Nb: 0.040, Ti: 0.010, B: 0.0014, Cr: 0.680739Inventive steelS0.1190.072.500.0150.00380.0540.00380.0070-Nb: 0.015, Ti: 0.015, B: 0.0020, Cr: 0.350729Inventive steelT0.1210.433.350.0180.00200.0420.00290.0010-Nb: 0.025, Ti: 0.015, B: 0.0012, Cr: 0.550736Inventive steelU0.1160.582.150.0120.00140.0310.00260.0090-Nb: 0.020, Ti: 0.020, B: 0.0015, Cr: 0.600736Inventive steelV0.1250.602.630.0100.00120.0320.00350.01100.0010V: 0.055737Inventive steelW0.1300.552.780.0090.00120.0450.00320.0060-Cu: 0.180737Inventive steelX0.1130.502.720.0100.00100.0380.00190.0010-Cr: 0.590736Inventive steelY0.1090.182.950.0080.00090.0520.00260.00900.0030Ni: 0.150732Inventive steelZ0.1210.452.760.0090.00230.0410.0037--Mo: 0.200735Inventive steelAA0.1180.322.720.0110.00120.0350.00220.0030-Nb: 0.032, Ta: 0.007733Inventive steelAB0.0980.512.980.0100.00050.0520.00250.0005-Ta: 0.008737Inventive steelAC0.1550.482.480.0150.00290.0490.00440.0110-W: 0.090734Inventive steelAD0.1220.562.810.0040.00180.0300.00530.0005-Mg: 0.0050737Inventive steelAE0.1170.622.720.0090.00200.0320.00340.0030-Zn: 0.0060738Inventive steelAF0.1390.472.320.0110.00150.0380.00320.0005-Co: 0.0080734Inventive steelAG0.1100.532.730.0160.00090.0250.00270.0030-Zr: 0.0030737Inventive steelAH0.1000.582.820.0120.00160.0420.00380.0010-Ca: 0.0018738Inventive steelAI0.1170.492.780.0450.00090.0280.00320.0005-Se: 0.0075736Inventive steelAJ0.1120.532.730.0180.00780.0370.00650.01100.0030Te: 0.0140737Inventive steelAK0.1270.282.680.0270.00070.0320.00270.0020-Ge: 0.0060732Inventive steelAL0.1180.452.710.0160.00320.0390.0071--As: 0.0210735Inventive steelAM0.0910.603.180.0090.00170.0340.00350.0005-Sr: 0.0070739Inventive steelAN0.1220.522.690.0070.00210.0190.00390.00800.0010Cs: 0.0100736Inventive steelAO0.0990.413.290.0100.00360.0300.00310.01000.0020Hf: 0.0070736Inventive steelAP0.1150.472.750.0210.00190.0310.0033--Pb: 0.0100736Inventive steelAQ0.1170.502.720.0130.00110.0420.00350.0005-Bi: 0.0050736Inventive steelAR0.1160.482.770.0110.00150.0350.00420.0010-REM: 0.0040736Inventive steelAS0.1000.342.190.0130.00120.0340.00310.01000.0010Nb: 0.180, Ti: 0.180, V: 0.180, B: 0.0075, Cr: 0.950, Ni: 0.960, Mo: 0.950, Cu: 0.900, Ta: 0.095, W: 0.450, Mg: 0.0170, Zn: 0.0180, Co: 0.0180, Zr: 0.0930, Ca: 0.0180, Se: 0.0190, Te: 0.0185, Ge: 0.0190, As: 0.0400, Sr: 0.0180, Cs: 0.0185, Hf: 0.0185, Pb: 0.0190, Bi: 0.0190, REM: 0.0190733Inventive steelAT0.3500.891.500.0160.00170.0210.00410.0110--732Inventive steelAU0.0600.753.020.0200.00060.0110.00360.0130--742Inventive steelAV0.0892.682.740.0180.00120.0340.00450.0110--770Inventive steelAW0.2200.092.590.0050.00150.0120.00380.0140--726Inventive steelAX0.0810.723.380.0180.00220.0310.00270.0140--742Inventive steelAY0.2930.651.610.0170.00050.0100.00260.0130--730Inventive steelAZ0.1250.822.600.0800.00140.0360.00280.0060--740Inventive steelBA0.1230.692.740.0020.00170.0250.00370.0040--739Inventive steelBB0.1300.722.650.0060.01800.0190.00420.0090--739Inventive steelBC0.1310.772.560.0130.00080.0370.00250.0050--739Inventive steelThe remainder other than those described above is composed of Fe and incidental impurities. [Table 2] No.Steel gradeCold rolling stepAnnealing stepCooling stepFirst holding stepSurface layer strain introduction stepSecond holding stepCoating stepNotesRolling reduction ratio (%)Annealing temperature T (°C)Heating t1 (s)Annealing (soaking) t2 (s)Annealing dew-point temperature (°C)Formula (3) Y (-)Cooling stop temperature-(°C)Holding temperature (°C)Holding time (s)Average tension (kgf / mm 2< )Holding temperature (°C)Holding time (s)TypeAlloying temperature (°C)1A548507210051538644350503.034010GA500Example2B588004590-10887737380302.736010CRExample3C6085034100101670416270203.226040GA520Example4D56830777010972631310602.831060GA510Example5E 528807680101686832380602.736010GA490Comparative Example6F 4878080905743636290504.029010GA510Comparative Example7G 44890359001539134340703.836010GI8H 49880457051539736410503.139060GA510Comparative Example9I 4482059700863832280402.928040GA520Comparative Example10A62700 6516015-8848 36370603.236010GIComparative Example11A46910 538001843939360702.336010GA520Comparative Example Comparative Example12A698605710 01847 23360503.834010GA510Comparative Example13A5885038100-18 1697013350202.534040GA530Comparative Example14A42820515 201870 43350502.235040GA490Comparative Example15A5181580220-1025784 48330702.032050GA510Comparative Example16A548406190013923180 350403.735020GA480Comparative Example17A4884065100201547048490 702.8510 10GA520Comparative Example18A578505180-5135762235012.535010GA510Comparative Example19A64850678051357640350601.034010GA480Comparative Example20A698408080101237617360503.1450 10GIComparative Example21A638505490-51527330360903.33602 GA520Comparative Example22J4683048140101938723360504.034010GA520Example23K668507780151355136390403.639010GA510Example24L698507170-51176853340304.034010GA520Example25M41800567010661760370602.835060GIExample26N70850517051209937330403.933010GA510Example27O50810568020851536350303.537010GIExample28P70830768001138417330703.933010GIExample29Q40840639051367118340503.433010GA510Example30R518406470201058656360403.136020GIExample31S698606990101766532330903.732010GA520Example32T42830499051267033380603.440040GIExample33U5783041120-516893553301003.233010GA510Example34V51850787001185347330402.532040GA520Example35W638305280201122059370602.537050GIExample36X63840439001401415330503.831050GA510Example37Y4585063110-101946724360703.235010GA510Example38Z708505590-51548959350702.635010GA490Example39AA5983064100-51449830290503.328030GA510Example40AB458306570-5972843360504.038010GA480Example41AC4384041115151827753350202.035020GA510Example42AD68850637001186828300202.530020GIExample43AE60840729051379058340303.233030GA520Example44AF458204980201030811330603.833010GIExample45AG628505790101528748360603.734010GA520Example46AH588507011051847840420603.842020CRExample47AI60840647001091635400402.938010GA500Example48AJ538205310051249634330303.335040GA490Example49AK508307680201171542380403.438010GA500Example50AL488507390101548926350403.934010GA510Example51AM628106512001272228360303.838020CRExample52AN548205410001257926310602.129010GA510Example53AO508406490-51397957330502.232010GA500Example54AP41850388001371123350902.933010GA510Example55AO61830777020986324330703.631010GA500Example56AR418506111051882743330403.735040GIExample57AS438305990201307147320302.831040GA500Example58A568606880-11 1477617380202.738010GA530Comparative Example59A6284055120-51856438380502.537010GA510Example60A638405212051856416320603.131020GA500Example61AT54790731000872835380403.237020GA530Example62AU628006312001039432360303.538020GA520Example63AV548605610051357526320603.130015GA560Example64AW628506210051855129330503.232015GA510Example65AX58800409010785230360902.934020GA520Example66AY61800728010836131350803.433020GA510Example67AZ408506010001645045330402.735035GA530Example68BA458505811601835847340303.033035GA520Example69BB568406890-51366527390403.338025GA520Example70BC6084060110-51661428390502.837025GA520Example [Table 3] No.Steel gradeThickness t (mm)Base steel sheet steel microstructure (t / 4)Surface soft layerNotesArea fraction of each phase (*1)Remainder microstructureFormula (1) right side (*2)XF (*1)M / (BF+M+TM) (*1)RA (*1)FMRABFTMBF+TMArea fractionHard phaseArea fraction(%)(%)(%)(%)(%)(%)(µm)(µm)(%)(-)(%)1A1.238291.135.455.691.0θ825192.80.10.1Example2B1.23.13.51.442.249.291.4θ1204493.30.20.2Example3C1.20.04.52.37.882.089.8θ863483.80.30.1Example4D1.23.03.81.232.259.291.4θ904283.80.10.3Example5E 1.280.4 14.8 0.61.20.01.2 θ865093.10.30.2Comparative Example6F 1.29.336.3 15.8 0.332.032.3 θ1055580.90.15.2 Comparative Example7G 1.279.1 11.7 5.5 0.00.00.0 θ975781.80.11.3Comparative Example8H 1.282.2 10.9 2.01.40.01.4 θ975383.60.30.2Comparative Example9I 1.21.421.7 11.8 0.058.858.8θ1095871.50.24.3 Comparative Example10A1.288.2 8.61.20.00.00.0 θ823089.30.20.2Comparative Example11A1.20.05.31.96.679.486.0θ823271.50.21.3Comparative Example12A1.282.9 11.1 2.80.00.00.0 θ8212 88.20.25.4 Comparative Example13A1.217.70.31.217.755.272.9θ8215 45.6 0.41.1Comparative Example14A1.283.2 2.39.5 1.30.01.3 θ823296.60.12.2Comparative Example15A1.220.44.22.30.069.469.4θ82112 99.10.40.4Comparative Example16A1.221.44.37.0 7.659.667.2θ823581.50.20.3Comparative Example17A1.221.81.50.614.955.270.1θ823096.30.7 0.3Comparative Example18A1.219.339.1 1.45.828.234.0 θ823185.30.20.6Comparative Example19A1.215.81.66.1 19.650.970.5θ824178.60.6 0.3Comparative Example20A1.257.1 0.00.510.523.133.6 θ823585.60.30.4Comparative Example21A1.216.81.31.014.462.376.7θ823256.5 0.7 0.3Comparative Example22J1.20.45.92.619.671.290.8θ1058185.70.30.1Example23K1.24.87.91.830.451.982.3θ1162491.40.40.5Example24L1.218.22.30.328.549.377.8θ822994.50.30.3Example25M1.28.58.92.730.146.276.3θ1016586.90.20.4Example26N1.22.74.90.242.249.291.4θ1077480.80.30.1Example27O1.25.78.62.828.753.181.8θ1168093.50.40.4Example28P1.218.73.41.039.736.275.9θ904880.00.10.3Example29Q1.22.96.32.137.849.787.5θ1165679.20.30.5Example30R1.25.96.00.931.254.986.1θ1015294.40.20.1Example31S1.25.85.01.648.238.887.0θ937486.90.30.3Example32T1.26.56.21.724.859.884.6θ1166079.20.40.2Example33U1.26.33.81.136.251.787.9θ866289.90.20.2Example34V1.24.34.21.424.664.989.5θ765290.30.40.2Example35W1.23.23.81.242.649.091.6θ978396.80.10.5Example36X1.24.33.12.149.539.889.3θ1163288.50.20.5Example37Y1.26.37.12.133.350.984.2θ807388.70.20.2Example38Z1.25.53.31.638.550.889.3θ1208199.40.10.3Example39AA1.24.14.31.237.152.189.2θ1094591.20.10.4Example40AB1.23.23.01.141.649.791.3θ1186095.40.20.4Example41AC1.27.05.31.428.656.785.3θ782291.60.20.2Example42AD1.22.73.90.426.266.692.8θ1188380.80.10.5Example43AE1.23.23.81.242.649.091.6θ1097279.30.30.2Example44AF1.25.96.00.931.254.986.1θ1184393.30.20.5Example45AG1.22.72.91.646.345.792.0θ1092287.00.20.3Example46AH1.23.41.41.240.153.793.8θ1162579.20.10.2Example47AI1.26.71.00.940.151.091.1θ1182791.60.40.1Example48AJ1.21.90.91.731.962.894.7θ735089.00.10.2Example49AK1.21.44.42.124.867.191.9θ1126594.90.20.5Example50AL1.22.61.91.340.353.693.9θ1202591.20.10.2Example51AM1.23.15.11.443.746.390.0θ1187895.40.20.4Example52AN1.21.94.20.637.155.092.1θ883591.60.20.3Example53AO1.24.92.40.927.164.591.6θ783290.80.10.1Example54AP1.21.24.91.534.757.592.2θ1205179.30.30.5Example55AQ1.24.71.40.335.058.593.5θ1185179.80.30.4Example56AR1.25.96.00.931.254.986.1θ1164389.30.30.4Example57AS1.28.58.92.730.146.276.3θ805593.10.20.5Example58A1.23.15.11.443.746.390.0θ780 20.8 0.20.3Comparative Example59A1.21.94.20.637.155.092.1θ673588.00.30.1Example60A1.24.92.40.927.164.591.6θ675395.00.20.2Example61AT1.22.51.92.130.359.289.5θ784591.20.10.8Example62AU1.23.26.11.849.735.385.0θ714895.40.20.9Example63AV1.21.94.23.241.148.990.0θ783591.60.11.5Example64AW1.24.82.81.124.166.290.3θ673290.80.10.5Example65AX1.23.87.21.548.236.284.4θ674089.30.20.6Example66AY1.22.71.61.928.958.587.4θ714690.80.30.4Example67AZ1.22.72.91.235.253.688.8θ974389.30.20.7Example68BA1.22.93.21.133.152.885.9θ1055593.10.10.5Example69BB1.23.83.41.532.455.988.3θ865093.10.20.3Example70BC1.23.63.51.433.554.688.1θ1014588.00.20.2Example (*1) F: ferrite, M: fresh martensite, RA: retained austenite, BF: bainitic ferrite, TM: tempered martensite, θ: carbide (*2) Formula (1): 20 ≤ X ≤ 120 - 3800 x [Sb] - 1900 x [Sn] ([Sb]: Sb content of steel (% by mass), [Sn]: Sn content of steel (% by mass)) [Table 4] No.Steel gradeYS (MPa)TS (MPa)YR (-)EI (%)R / t (-)V-bending other than end face the crack length (µm)V-bending end face the crack length (µm)170°C, after 20 minutes notch El (%)Apply 2% strain 170°C, after 20 minutes notch El (%)Notes1A86412370.709.80.50906.86.1Example2B89212430.7210.91.00607.16.2Example3C95313410.718.21.001206.85.6Example4D86911910.7310.10.501207.56.6Example5E 404 620 0.6526.10.50407.16.0Comparative Example6F 12421498 0.8310.55.5 201 220 3.4 4.9 Comparative Example7G 589 856 0.6930.31.0139250 8.46.1Comparative Example8H 352 552 0.6428.50.501207.25.7Comparative Example9I 90412240.7411.84.5 203 2004.4 4.5 Comparative Example10A470 686 0.6826.00.5055665.8Comparative Example11A10791109 0.977.5 2.586907.06.1Comparative Example12A506 712 0.7126.41.001407.56.7Comparative Example13A9561032 0.9314.24.5 212 1104.0 4.6 Comparative Example14A503 735 0.6826.40.501208.360Comparative Example15A9381019 0.9210.21.51221136.97.0Comparative Example16A9311014 0.9210.11.51181236.55.6Comparative Example17A10321089 0.9516.14.5 201 1244.0 4.9 Comparative Example18A103013900.747.32.597225 7.46.3Comparative Example19A10771122 0.9615.64.5 221 1145.5 4.0 Comparative Example20A705 771 0.9114.31.001076.66.6Comparative Example21A10651111 0.9612.25.0 212 1065.3 3.7 Comparative Example22J91511840.779.50.001176.76.1Example23K91912430.7410.52.001246.76.7Example24L104812470.849.62.0981146.66.0Example25M95012410.779.50.501197.95.6Example26N88612600.7011.80.501148.17.1Example27O108912330.889.90.501277.86.2Example28P108712510.8710.31.01211228.46.9Example29Q87611880.7411.11.001137.25.6Example30R98112600.7812.31.001238.16.2Example31S96112030.8011.80.001158.25.8Example32T90612040.7510.01.001158.06.5Example33U90211950.759.61.01131216.57.3Example34V82912420.6711.61.51191218.06.5Example35W93412680.749.50.001247.36.1Example36X86212730.689.72.01311167.45.9Example37Y93712780.7310.60.01201207.36.6Example38Z99812790.789.00.01041237.17.4Example39AA95312360.7711.21.51211196.66.5Example40AB86312760.6812.41.0991187.46.3Example41AC94112550.7512.42.001168.06.9Example42AD95611970.8011.60.01261178.56.8Example43AE84112650.668.70.001248.37.1Example44AF90612040.7510.01.001148.26.4Example45AG90211950.759.62.001147.37.0Example46AH82912420.6711.62.01081118.27.2Example47AI101113410.758.22.01061086.96.9Example48AJ98213620.728.61.01051077.76.2Example49AK85811850.729.20.501067.37.0Example50AL90512240.749.02.001148.16.4Example51AM91511840.779.50.501146.76.3Example52AN96913750.708.21.01261188.07.4Example53AO89411890.759.41.001347.16.2Example54AP92812110.7711.00.501148.26.7Example55AQ83312440.679.90.501148.26.9Example56AR86212200.7110.61.01081118.27.2Example57AS92912030.7716.30.51051186.56.0Example58A85611880.7211.14.0 204 1214.4 3.4 Comparative Example59A98112600.7812.31.001196.66.1Example60A96112030.8011.80.501167.26.6Example61AT82812240.6813.11.00416.55.6Example62AU98511840.839.51.01121307.26.3Example63AV96912630.7713.51.01031426.55.5Example64AW92311890.789.11.00937.16.2Example65AX98211890.839.21.0981067.36.5Example66AY82212140.6812.91.00526.55.5Example67AZ87712300.7110.80.50687.06.1Example68BA86912210.7111.40.50726.86.0Example69BB85611920.7210.20.50736.66.0Example70BC87211980.7310.50.50696.96.1Example

[0253] As shown in Table 4, in all of the examples, all of the tensile strength (TS), the yield stress (YS), the total elongation (El), R / t in the V-bending test, the length of a crack that propagates from a bending ridge line end portion in a ridge line direction, and the notch El were acceptable.

[0254] In contrast, in the comparative examples, at least one of the tensile strength (TS), the yield stress (YS), the total elongation (El), R / t in the V-bending test, the length of a crack that propagates from a bending ridge line end portion in a ridge line direction, and the notch El was not sufficient.

[0255] Furthermore, it was found that all of the tensile strength (TS), the yield stress (YS), the total elongation (El), R / t in the V-bending test, the length of a crack that propagates from a bending ridge line end portion in a ridge line direction, and the notch El of a member produced by forming or joining the steel sheet of the example of the present invention had good characteristics characterized in the present invention and the member had good characteristics characterized in the present invention. Reference Signs List

[0256] Fferrite Mfresh martensite RAretained austenite BFbainitic ferrite TMtempered martensite

Claims

1. A steel sheet having a base steel sheet with a chemical composition comprising: on a mass percent basis, 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%), with the remainder being composed of Fe and incidental impurities, wherein the steel sheet has a surface soft layer having a Vickers hardness of 84% or less relative to a Vickers hardness at a quarter thickness from a surface of the base steel sheet, the surface soft layer satisfies the following formula (1), as microstructures in the surface soft layer, ferrite has an area fraction of 60.0% or more and 100.0% or less, when ferrite has an area fraction of less than 100.0%, an area fraction of fresh martensite divided by a total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less, retained austenite has an area fraction of 3.0% or less, and as microstructures at a quarter thickness of the base steel sheet, ferrite has an area fraction of 55.0% or less (including 0.0%), bainitic ferrite and tempered martensite (excluding retained austenite) have a total area fraction of more than 40.0% and 100.0% or less, retained austenite has an area fraction of less than 3.5%, and fresh martensite has an area fraction of 10.0% or less (including 0.0%), and the steel sheet has a tensile strength of 1180 MPa or more and less than 1470 MPa, 20 ≤ X ≤ 120 − 3800 × Sb − 1900 × Sn wherein, in the formula (1), X denotes the thickness of the surface soft layer (µm), and [Sb] and [Sn] denote an Sb content and a Sn content of steel (% by mass), respectively.

2. The steel sheet according to claim 1, wherein the chemical composition further comprising, on a mass percent basis, at least one selected from Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, 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.

3. The steel sheet according to claim 1 or 2, wherein the steel sheet has a coated layer on one or both surfaces of the base steel sheet, and the coated layer is a hot-dip galvanized layer.

4. The steel sheet according to claim 1 or 2, wherein the steel sheet has a coated layer on one or both surfaces of the base steel sheet, and the coated layer is a hot-dip galvannealed layer.

5. A member comprising the steel sheet according to any one of claims 1 to 4.

6. A method for producing a steel sheet, comprising: a hot rolling step of hot-rolling a steel slab having the chemical composition according to claim 1 or 2 at a finish rolling temperature of 820°C or more; an annealing step of heating the steel sheet after the hot rolling step and annealing the steel sheet at an annealing temperature of 750°C or more and 900°C or less for an annealing time of 20 seconds or more in an atmosphere with a dew-point temperature of -10°C or more under conditions satisfying the formulae (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 and holding temperature range of the cooling stop temperature or more and 440°C or less and holding the steel sheet for 10 seconds or more; a surface layer strain introduction step of applying a tension of 2.0 kgf / mm2 or more to the steel sheet after the first holding step in the reheating and holding temperature range; and a second holding step of holding the steel sheet after the surface layer strain introduction step in the reheating and holding temperature range for 10 seconds or more, or further a cold rolling step of cold-rolling the steel sheet after the hot rolling step and before the annealing step at a rolling reduction ratio of 20% or more and 80% or less to form a cold-rolled steel sheet, 2400 ≤ Y ≤ 20000 Y = T − Ac 1 × t 1 / 2 + T − Ac 1 × t 2 wherein, in the formula (3), T denotes the annealing temperature (°C), t1 denotes the time (s) from 650°C to the annealing temperature T during the heating in the annealing step, t2 denotes the annealing time (s), and Ac1 denotes Ac1 (°C).

7. The method for producing a steel sheet according to claim 6, comprising, after the annealing step, a hot-dip galvanizing step of applying a hot-dip galvanizing treatment to the steel sheet to form a hot-dip galvanized layer.

8. The method for producing a steel sheet according to claim 6, comprising, after the annealing step, a hot-dip galvannealing step of applying a hot-dip galvannealing treatment to the steel sheet to form a hot-dip galvannealed layer.

9. A method for producing a member, comprising a step of subjecting the steel sheet according to any one of Claims 1 to 4 to at least one of forming or joining to produce the member.

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