Steel sheet, member, and production methods thereof

Abstract

To provide a technique for a steel sheet having a TS of 780 MPa or more and less than 1180 MPa, a high YS, and high press formability in the inner portion of the steel sheet and at an end portion of the steel sheet. In the steel sheet, a base steel sheet has a predetermined chemical composition and has a surface soft layer having a Vickers hardness of 84% or less with respect to that at the quarter thickness from the surface of the base steel sheet and satisfying the following formula (1), 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, and the steel sheet has a tensile strength of 780 MPa or more and less than 1180 MPa. 20≤X≤120−3800×Sb−1900×Sn 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.

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] From the viewpoint of global environmental conservation, improvement of fuel efficiency in automobiles has been an important issue. Thus, there has been an active movement to reduce the weight of automobile bodies by increasing the strength and reducing the thickness of steel sheets used as materials for automotive body components.

[0003] Furthermore, a social demand for improvement of crash safety of automobiles is further increased. Thus, there is a demand for the development of a steel sheet having high strength and crashworthiness when a vehicle collides in the course of travel (hereinafter referred to simply as crashworthiness). In particular, 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 galvanized steel sheet having high crashworthiness in addition to high strength.

[0004] 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.

[0005] 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 in terms of volume fraction, the grain size ratio, which is 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 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.

[0006] 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.

[0007] 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

[0008] 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

[0009] In recent years, application of a steel sheet having a tensile strength (hereinafter also referred to as TS) of 780 MPa grade or more to an impact energy absorbing member of an automobile exemplified by a front side member or a rear side member has been put to practical use.

[0010] Thus, to increase absorbed energy at the time of impact (hereinafter also referred to as impact absorbed energy) so as to achieve good fracture resistance characteristics in case of a collision of an automotive body, it is effective to improve yield stress YS (hereinafter also referred to as YS). However, an increase in the TS and 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, the steel sheet is difficult to press and has a low yield due to variations at the time of forming. In particular, a decrease in press formability at an end portion of a steel sheet results in edge cracking of an actual member.

[0011] 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 a surface layer of the steel sheet to improve mechanical cutting characteristics, no consideration has been given to the improvement of bendability by forming a surface soft layer (decarburized layer), the improvement of fracture resistance characteristics in case of a collision of an automotive body, and the press formability at an end portion of the steel sheet.

[0012] 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, 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.

[0013] 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, no consideration has been given to press formability at an end portion of the steel sheet.

[0014] 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, 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.

[0015] 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 780 MPa or more, a high YS, high press formability in the inner portion of the steel sheet (bendability and stretch formability 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 fracture resistance characteristics (bending fracture characteristics and axial crushing characteristics) in case of a collision.

[0016] The present invention has been made in view of the above circumstances and aims to provide a steel sheet having a tensile strength TS of 780 MPa or more and less than 1180 MPa, a high yield stress YS, high press formability in the inner portion of the steel sheet (bendability and stretch formability 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), and a method for producing the steel sheet.

[0017] The present invention also aims to provide a member using the steel sheet as a material, and a method for producing the member.

[0018] 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).

[0019] The tensile strength TS is measured in a tensile test according to JIS Z 2241 (2011).

[0020] Having a high yield stress YS, high press formability in the inner portion of a steel sheet (bendability and stretch formability 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) means satisfying the following.

[0021] The phrase "high yield stress 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 780 MPa ≤ TS < 980 MPa, 550 MPa ≤ YS (B) For 980 MPa ≤ TS < 1180 MPa, 700 MPa ≤ YS

[0022] The term "high bendability of a steel sheet" means that, when a 90-degree V-bending test with a bending radius 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 (the crack length in a portion other than the V-bent end surface) is 200 µm or less; when a tight bending test is performed, the thickness of a spacer at a cracking limit at which cracking of 0.5 mm or more does not occur along the bending ridge line is 3.0 mm or less; when a tight bending test with a 3.0-mm spacer is performed, the depth of a crack that propagates in the thickness direction at a compressive-stressed bending ridge line portion (tight bending internal cracking depth) is 200 µm or less; and when a tight bending + orthogonal 90-degree V-bending test is performed, the bending radius at a cracking limit (handkerchief bending boundary bending radius), defined as a bending radius at which cracking of 0.5 mm or more does not occur along the bending ridge line, is 5.0 mm or less.

[0023] Detailed measurement methods of the 90-degree V-bending test with a bending radius of 0.5 mm, the tight bending test, and the tight bending + orthogonal 90-degree V-bending test are as described below in Examples.

[0024] The phrase "high stretch formability in the inner portion of a steel sheet" refers to high ductility and means that the total elongation (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 780 MPa ≤ TS < 980 MPa, 17.0% ≤ El (B) For 980 MPa ≤ TS < 1180 MPa, 11.0% ≤ El

[0025] The phrase "high bendability at an end portion (sheared cross section) of a steel sheet" means that, when a 90-degree V-bending test with a bending radius of 0.5 mm is performed in accordance with JIS Z 2248 (2022), the length of a crack (V-bending edge crack length) that propagates from a bending ridge line end portion in a ridge line direction is 200 µm or less.Solution to Problem

[0026] The present inventors have conducted extensive studies to achieve the above objects.

[0027] Consequently, it was found that a steel sheet having a high yield stress YS, high press formability in the inner portion of the steel sheet (bendability and stretch formability 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) can be provided, wherein the chemical composition of a base steel sheet of the steel sheet is appropriately adjusted, the base steel sheet of the steel sheet has a surface soft layer having a Vickers hardness of 84% or less relative to the Vickers hardness at the quarter thickness, the surface soft layer satisfies the following formula (1), as microstructures in the surface soft layer, ferrite has an area fraction of 50.0% 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 bainite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less, as microstructures at the quarter thickness of the base steel sheet, ferrite has an area fraction of 76.5% or less (including 0.0%), the total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite) is 20.0% or more and 90.0% or less, retained austenite has a volume fraction of 3.5% or more and 10.0% or less, and fresh martensite has an area fraction of 10.0% or less (including 0.0%), and the steel sheet has a tensile strength of 780 MPa or more and less than 1180 MPa. 20 ≤ X ≤ 120 − 3800 × Sb − 1900 × Sn

[0028] 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.

[0029] The present invention has been completed on the basis of the above findings and further studies.

[0030] The following is the gist of the present invention. [1] A steel sheet having 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: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.005% 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 50.0% or more and 100.0% or less, and 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, as microstructures at a quarter thickness of the base steel sheet, ferrite has an area fraction of 76.5% or less (including 0.0%), bainitic ferrite and tempered martensite (excluding retained austenite) have a total area fraction of 20.0% or more and 90.0% or less, retained austenite has an area fraction of 3.5% or more and 10.0% or less, and fresh martensite has an area fraction of 10.0% or less (including 0.0%), and the steel sheet has a tensile strength of 780 MPa or more and less than 1180 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 containing, 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] to form a hot-rolled steel sheet; a pickling step of pickling the hot-rolled steel sheet after the hot rolling step; a cold rolling step of cold-rolling the steel sheet after the pickling step at a rolling reduction ratio of 20% or more and 80% or less; an annealing step of heating the steel sheet after the cold rolling step and annealing the steel sheet at an annealing temperature Ac1 (°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 100°C or more and 300°C or less; a first holding step of reheating the steel sheet after the cooling step to a reheating and holding temperature range of 370°C or more and 460°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 at 300°C or more and 460°C or less for 10 seconds or more, 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 having a tensile strength TS of 780 MPa or more and less than 1180, a high yield stress YS, high press formability in the inner portion of the steel sheet (bendability and stretch formability 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).

[0032] Furthermore, a member including a steel sheet according to the present invention as a material has high strength and can be extremely advantageously applied to an impact energy absorbing member of an automobile or the like.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] Figs. 3-1(a) and (b) are schematic explanatory views of edge cracking in 90-degree V-bending. [Fig. 3-2] Fig. 3-2(a) is a schematic explanatory view of a method for measuring the crack length of edge cracking in 90-degree V-bending, and Fig. 3-2(b) is a view of an example of a profile waveform used for measurement of the crack length. [Fig. 4-1] Fig. 4-1(a) is an explanatory view of a method for measuring a spacer thickness at a cracking limit in a tight bending test, and Fig. 4-1(b) is an explanatory view of a method for measuring the depth of a crack that propagates in the thickness direction in a compressive-stressed bending ridge line portion in the tight bending test. [Fig. 4-2] Fig. 4-2(c) is an explanatory view of a method of cutting out an observation cross section for measuring the depth of a crack that propagates in the thickness direction in a compressive-stressed bending ridge line portion in a tight bending test, and Fig. 4-2(d) is an explanatory view of a method of measuring the depth of a crack that propagates in the thickness direction in a compressive-stressed bending ridge line portion in the observation cross section. 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: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.005% 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 50.0% or more and 100.0% or less, and when ferrite has an area fraction of less than 100.0%, an area fraction of fresh martensite divided by a total area fraction of bainite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less, as microstructures at a quarter thickness of the base steel sheet, ferrite has an area fraction of 76.5%% or less (including 0.0%), bainitic ferrite and tempered martensite (excluding retained austenite) have a total area fraction of 20.0% or more and 90.0% or less, retained austenite has an area fraction of 3.5% or more and 10.0% or less, and fresh martensite has an area fraction of 10.0% or less (including 0.0%), and the steel sheet has a tensile strength of 780 MPa or more and less than 1180 MPa, thereby having a high yield stress YS, high press formability in the inner portion of the steel sheet (bendability and stretch formability 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), 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 forming appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite, and retained austenite and ensuring a TS of 780 MPa or more and less than 1180 MPa 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 780 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 and makes it difficult to achieve a TS of less than 1180 MPa.

[0039] Furthermore, fresh martensite acts as a starting point of void formation in a 90-degree V-bending test, a tight bending test, or a tight bending + orthogonal 90-degree V-bending test, and it is therefore impossible to achieve desired bendability of a steel sheet and desired bendability of a sheared end face. This also excessively increases the area fraction of retained austenite and the solute C content of retained austenite. Furthermore, the hardness of fresh martensite formed by deformation-induced transformation of retained austenite when subjected to shearing increases greatly, subsequent void formation and crack propagation are promoted, and it is more difficult to achieve the desired bendability of the sheared end face.

[0040] Thus, the C content is 0.050% or more and 0.400% or less. The C content is preferably 0.100% or more. The C content is preferably 0.300% or less. The C content is more preferably 0.200% or less.Si: 0.20% or more and 3.00% or less

[0041] Si suppresses the formation of carbide and promotes the formation of retained austenite during cooling and holding after annealing. Thus, Si is an element that affects the area fraction of retained austenite. A Si content of less than 0.20% results in a decrease in the area fraction of retained austenite and a decrease in ductility.

[0042] 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.

[0043] Thus, the Si content is 0.20% or more and 3.00% or less. The Si content is preferably 2.00% or less. The Si content is more preferably 1.50% or less.

[0044] The Si content is preferably 0.50% or more.Mn: 1.00% or more and less than 3.50%

[0045] Mn is an element that adjusts the area fraction of bainitic ferrite, tempered martensite, or the like. A Mn content of less than 1.00% results in an excessive increase in the area fraction of ferrite and makes it difficult to achieve a TS of 780 MPa or more. This also results in a decrease in YS.

[0046] 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. This increases martensite formed during final cooling, does not sufficiently temper martensite formed at that time, and increases the area fraction of hard fresh martensite. Fresh martensite acts as a starting point of void formation in a 90-degree V-bending test, a tight bending test, and a tight bending + orthogonal 90-degree V-bending test, and an area fraction of fresh martensite exceeding 10.0% makes it impossible to achieve desired bendability of a steel sheet and desired bendability of a sheared end face.

[0047] Thus, the Mn content is 1.00% or more and less than 3.50%. The Mn content is preferably 2.00% or more. The Mn content is preferably 3.00% or less.P: 0.001% or more and 0.100% or less

[0048] 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.

[0049] 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. After a steel sheet is sheared, this increases the number of voids and makes it impossible to achieve desired bendability of a sheared end face.

[0050] Thus, the P content is 0.001% or more and 0.100% or less. The P content is preferably 0.003% or more.

[0051] The P content is preferably 0.030% or less. The P content is preferably 0.010% or less, more preferably 0.005% or less.S: 0.0001% or more and 0.0200% or less

[0052] S is present as a sulfide in steel. In particular, after a steel sheet is sheared, a S content of more than 0.0200% results in an increase in the number of voids and makes it impossible to achieve desired bendability of a sheared end face.

[0053] 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.

[0054] The S content is 0.0001% or more due to constraints on production technology. 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

[0055] Al suppresses the formation of carbide and promotes the formation of retained austenite during cooling and holding after annealing. Thus, Al is an element that affects the area fraction of retained austenite. To produce such effects, the Al content is 0.005% or more.

[0056] 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 780 MPa or more. This also results in a decrease in YS. This also excessively increases the C concentration of austenite during annealing and makes it impossible to achieve desired bendability of a sheared end face.

[0057] Thus, the Al content is 0.005% or more and 2.000% or less. The Al content is preferably 0.010% or more.

[0058] The Al content is preferably 1.000% or less. The Al content is more preferably 0.100% or less, even more preferably 0.050% or less.N: 0.0100% or less

[0059] N is present as a nitride in steel. In particular, after a steel sheet is sheared, a N content of more than 0.0100% results in an increase in the number of voids and makes it impossible to achieve desired bendability of a sheared end face.

[0060] Thus, the N content is 0.0100% or less. The N content is preferably 0.0050% or less.

[0061] 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 0.0020% or more.Sb: 0.200% or less (including 0%)

[0062] 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 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 0.009% or more.

[0063] 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 a 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, even more preferably 0.018% or less. The Sb content is more preferably 0.016% or less, even more preferably 0.014% or less.Sn: 0.200% or less (including 0%)

[0064] 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 is preferably 0.002% or more. The Sn content is more preferably 0.005% or more. The Sn content is more preferably 0.007% or more, even more preferably 0.009% or more.

[0065] 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 a steel sheet) and crack propagation resistance. Thus, when Sn is contained, the Sn content should be 0.200% or less. The Sn content is more preferably 0.020% or less, even more preferably 0.016% or less. The Sn content is more preferably 0.014% or less, even more preferably 0.012% or less.

[0066] 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.

[0067] 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.

[0068] 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

[0069] 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. The Nb content is more preferably 0.010% or more, even more preferably 0.020% or more.

[0070] 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 may act as a starting point of cracking in a 90-degree V-bending test, a tight bending test, and a tight bending + orthogonal 90-degree V-bending test and may make it impossible to achieve desired bendability of a steel sheet and desired bendability of a sheared end face. 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

[0071] 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. The Ti content is more preferably 0.010% or more.

[0072] 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 may act as a starting point of cracking in a 90-degree V-bending test, a tight bending test, and a tight bending + orthogonal 90-degree V-bending test and may make it impossible to achieve desired bendability of a steel sheet and desired bendability of a sheared end face. Thus, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less. The Ti content is more preferably 0.050% or less, even more preferably 0.030% or less.V: 0.200% or less

[0073] 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 0.020% or more.

[0074] 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 may act as a starting point of cracking in a 90-degree V-bending test, a tight bending test, and a tight bending + orthogonal 90-degree V-bending test and may make it impossible to achieve desired bendability of a steel sheet and desired bendability of a sheared end face. 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

[0075] B is an element that segregates at an austenite grain boundary and thereby increases hardenability. B is also an element that suppresses 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.

[0076] 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. Thus, after a steel sheet is sheared, the number of voids formed may increase, and a sheared end face may not have desired bendability.

[0077] Thus, when B is contained, the B content is preferably 0.0100% or less. The B content is more preferably 0.0050% or less. The B content is more preferably 0.0020% or less.Cr: 1.000% or less

[0078] Cr is an element that increases hardenability, and the addition of Cr can form a large amount of tempered martensite and ensure a TS of 780 MPa or more and high YS. To produce such an effect, the Cr content is preferably 0.0005% or more. The Cr content is more preferably 0.010% or more. Cr is even more preferably 0.030% or more, even further more preferably 0.040% or more.

[0079] On the other hand, a Cr content of more than 1.000% results in an excessive increase in the area fraction of hard fresh martensite, and fresh martensite acts as a starting point of void formation in a 90-degree V-bending test, a tight bending test, and a tight bending + orthogonal 90-degree V-bending test. This may make it impossible to achieve desired bendability of a steel sheet and desired bendability of a sheared end face. 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. The Cr content is more preferably 0.100% or less, even more preferably 0.080% or less.Ni: 1.000% or less

[0080] Ni is an element that increases hardenability, and the addition of Ni can form a large amount of tempered martensite and ensure a TS of 780 MPa or more and high 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.

[0081] On the other hand, a Ni content of more than 1.000% results in an excessive increase in the area fraction of fresh martensite, and fresh martensite acts as a starting point of void formation in a 90-degree V-bending test, a tight bending test, and a tight bending + orthogonal 90-degree V-bending test. This may make it impossible to achieve desired bendability of a steel sheet and desired bendability of a sheared end face. 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. The Ni content is more preferably 0.200% or less.Mo: 1.000% or less

[0082] Mo is an element that increases hardenability, and the addition of Mo can form a large amount of tempered martensite and ensure a TS of 780 MPa or more and high YS. To produce such an effect, the Mo content is preferably 0.010% or more. The Mo content is more preferably 0.030% or more.

[0083] On the other hand, a Mo content of more than 1.000% results in an excessive increase in the area fraction of fresh martensite, and fresh martensite acts as a starting point of void formation in a 90-degree V-bending test, a tight bending test, and a tight bending + orthogonal 90-degree V-bending test. This may make it impossible to achieve desired bendability of a steel sheet and desired bendability of a sheared end face. 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 more preferably 0.400% or less. The Mo content is more preferably 0.350% or less, even further more preferably 0.300% or less. The Mo content is more preferably 0.100% or less, even more preferably 0.080% or less.Cu: 1.000% or less

[0084] Cu is an element that increases hardenability, and the addition of Cu can form a large amount of tempered martensite and ensure a TS of 780 MPa or more and high YS. To produce such an effect, the Cu content is preferably 0.005% or more. The Cu content is even more preferably 0.008% or more, even further more preferably 0.010% or more. The Cu content is more preferably 0.020% or more. The Cu content is even more preferably 0.050% or more, even further more preferably 0.100% or more.

[0085] 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 and a large number of coarse precipitates or inclusions. In such a case, fresh martensite and a coarse precipitate or inclusion may act as a starting point of void formation in a 90-degree V-bending test, a tight bending test, and a tight bending + orthogonal 90-degree V-bending test and may make it impossible to achieve desired bendability of a steel sheet and desired bendability of a sheared end face. 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

[0086] 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.

[0087] 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, a coarse precipitate or inclusion acts as a starting point of void formation in a 90-degree V-bending test, a tight bending test, and a tight bending + orthogonal 90-degree V-bending test. This may make it impossible to achieve desired bendability of a steel sheet and desired bendability of a sheared end face. Thus, when Ta is contained, the Ta content is preferably 0.100% or less. The Ta content is even more preferably 0.090% or less, even further more preferably 0.080% or less. The Ta content is more preferably 0.050% or less, even more preferably 0.020% or less.W: 0.500% or less

[0088] W is an element that increases hardenability, and the addition of W can form a large amount of tempered martensite and ensure a TS of 780 MPa or more and high YS. To produce such an effect, the W content is preferably 0.001% or more. The W content is more preferably 0.020% or more.

[0089] On the other hand, a W content of more than 0.500% results in an excessive increase in the area fraction of hard fresh martensite, and fresh martensite acts as a starting point of void formation in a 90-degree V-bending test, a tight bending test, and a tight bending + orthogonal 90-degree V-bending test. This may make it impossible to achieve desired bendability of a steel sheet and desired bendability of a sheared end face. 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. The W content is more preferably 0.100% or less, even more preferably 0.050% or less.Mg: 0.0200% or less

[0090] Mg is an element that is effective in spheroidizing the shape of an inclusion, such as sulfide or oxide, and improving the bendability of a sheared end face. To produce such an effect, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more, even more preferably 0.0010% or more. The Mg content is more preferably 0.0020% or more, even more preferably 0.0030% or more.

[0091] 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, a coarse precipitate or inclusion acts as a starting point of void formation in a 90-degree V-bending test, a tight bending test, and a tight bending + orthogonal 90-degree V-bending test. This may make it impossible to achieve desired bendability of a steel sheet and desired bendability of a sheared end face. Thus, when Mg is contained, the Mg content is preferably 0.0200% or less. The Mg content is more preferably 0.0180% or less, even more preferably 0.0150% or less. The Mg content is more preferably 0.0100% or less, even more preferably 0.0080% or less.Zn: 0.0200% or less

[0092] Zn is an element that is effective in spheroidizing the shape of an inclusion and improving the bendability of a sheared end face. 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.

[0093] 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, a coarse precipitate or inclusion acts as a starting point of void formation in a 90-degree V-bending test, a tight bending test, and a tight bending + orthogonal 90-degree V-bending test. This may make it impossible to achieve desired bendability of a steel sheet and desired bendability of a sheared end face. 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. The Zn content is more preferably 0.0100% or less, even more preferably 0.0080% or less.Co: 0.0200% or less

[0094] Like Zn, Co is an element that is effective in spheroidizing the shape of an inclusion and improving the bendability of a sheared end face. 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. The Co content is more preferably 0.0050% or more.

[0095] 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, a coarse precipitate or inclusion acts as a starting point of void formation in a 90-degree V-bending test, a tight bending test, and a tight bending + orthogonal 90-degree V-bending test. This may make it impossible to achieve desired bendability of a steel sheet and desired bendability of a sheared end face. 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

[0096] Like Zn and Co, Zr is an element that is effective in spheroidizing the shape of an inclusion and improving the bendability of a sheared end face. To produce such an effect, the Zr content is 0.0010% or more.

[0097] On the other hand, a Zr content of more than 0.1000% 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 void formation in a 90-degree V-bending test, a tight bending test, and a tight bending + orthogonal 90-degree V-bending test. This may make it impossible to achieve desired bendability of a steel sheet and desired bendability of a sheared end face. 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 0.0100% or less. The Zr content is more preferably 0.0050% or less.Ca: 0.0200% or less

[0098] 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, a coarse precipitate or inclusion acts as a starting point of void formation in a 90-degree V-bending test, a tight bending test, and a tight bending + orthogonal 90-degree V-bending test. This may make it impossible to achieve desired bendability of a steel sheet and desired bendability of a sheared end face. 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 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.

[0099] 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

[0100] Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are elements effective in improving the bendability of a sheared end face. 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.

[0101] 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, a coarse precipitate or inclusion may act as a starting point of void formation in a 90-degree V-bending test, a tight bending test, and a tight bending + orthogonal 90-degree V-bending test and may make it impossible to achieve desired bendability of a steel sheet and desired bendability of a sheared end face. 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.

[0102] The Se content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Se content is more preferably 0.0010% or more, even more preferably 0.0050% or more. The Se content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0103] The Te content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Te content is more preferably 0.0010% or more, even more preferably 0.0050% or more. The Te content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0104] The Ge content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Ge content is more preferably 0.0010% or more, even more preferably 0.0050%% or more.

[0105] The Ge content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0106] The As content is more preferably 0.0010% or more, even more preferably 0.0015% or more. The As content is more preferably 0.0100% or more, even more preferably 0.0150% or more. The As content is more preferably 0.0400% or less, even more preferably 0.0300% or less.

[0107] The Sr content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Sr content is more preferably 0.0010% or more, even more preferably 0.0050% or more. The Sr content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0108] The Cs content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Cs content is more preferably 0.0010% or more, even more preferably 0.0050% or more. The Cs content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0109] The Hf content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Hf content is more preferably 0.0010% or more, even more preferably 0.0050% or more. The Hf content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0110] The Pb content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Pb content is more preferably 0.0010% or more, even more preferably 0.0050% or more. The Pb content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0111] The Bi content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Bi content is more preferably 0.0180% or less, even more preferably 0.0150% or less. The Bi content is more preferably 0.0100% or less, even more preferably 0.0050% or less.

[0112] 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 0.0030% or more. The REM content is more preferably 0.0180% or less, even more preferably 0.0150% or less. The REM content is more preferably 0.0100% or less.

[0113] 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.

[0114] The REM is preferably, but not limited to, at least one of Sc, Y, Ce, and La.

[0115] Thus, a base steel sheet of a steel sheet according to the present invention has a chemical composition containing, on a mass percent basis, C: 0.050% or more and 0.400% or less, Si: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, Sb: 0.200% or less (including 0%), and Sn: 0.200% or less (including 0%), and optionally 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, with the remainder being composed of Fe and incidental impurities.Steel Microstructure (Microstructure at Quarter Thickness of Base Steel Sheet)

[0116] Next, the steel microstructure of a steel sheet according to an embodiment of the present invention will be described.

[0117] As microstructures at the quarter thickness of the base steel sheet, ferrite has an area fraction of 76.5% or less (including 0.0%), bainitic ferrite and tempered martensite (excluding retained austenite) have a total area fraction of 20.0% or more and 90.0% or less, retained austenite has an area fraction of 3.5% or more and 10.0% or less, and fresh martensite has an area fraction of 10.0% or less (including 0.0%).

[0118] The reasons for these limitations will be described below.Area fraction of ferrite: 76.5% or less (including 0.0%)

[0119] Soft ferrite is a phase that improves ductility. However, the area fraction of ferrite increases excessively, and it is difficult to achieve a TS of 780 MPa or more. This also results in a decrease in YS. This also excessively increases the C concentration of austenite during annealing and makes it impossible to achieve desired bendability of a sheared end face. Thus, the area fraction of ferrite is 76.5% or less. The area fraction of ferrite is preferably 60.0% or less.

[0120] 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 5.0% or more or 10.0% or more.

[0121] Total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite): 20.0% or more and 90.0% or less

[0122] Bainitic ferrite and tempered martensite have an intermediate hardness between soft ferrite and hard fresh martensite and the like and are important phases to provide a steel sheet having high bendability and a sheared end face having high bendability. 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, bainitic ferrite and tempered martensite (excluding retained austenite) preferably have a total area fraction of 20.0% or more, more preferably 30.0% or more.

[0123] On the other hand, an excessive increase in the total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite) results in a decrease in ductility. Thus, the total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite) is 90.0% or less. The total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite) is preferably 87.0% or less. The total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite) is more preferably 80.0% or less.

[0124] 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: 3.5% or more and 10.0% or less

[0125] From the perspective of achieving high ductility, the area fraction of retained austenite is 3.5% or more. The area fraction of retained austenite is preferably more than 3.5%.

[0126] On the other hand, when the volume fraction of retained austenite is excessively increased, fresh martensite formed by deformation-induced transformation when shearing is performed acts as a starting point of void formation, and a sheared end face therefore cannot have desired bendability. Thus, the area fraction of retained austenite is 10.0% or less. The area fraction of retained austenite is preferably 9.0% or less, more preferably 8.0% or less.Area fraction of fresh martensite: 10.0% or less (including 0.0%)

[0127] 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, a tight bending test, and a tight bending + orthogonal 90-degree V-bending test and may make it impossible to achieve desired bendability of a steel sheet and desired bendability of a sheared end face.

[0128] From the perspective of providing a steel sheet having high bendability and a sheared end face having high bendability, the area fraction of fresh martensite is 10.0% or less, preferably 5.0% or less.

[0129] The lower limit of the area fraction of fresh martensite may be, but is not limited to, 0.0%.

[0130] The term "fresh martensite" refers to as-quenched (untempered) martensite.

[0131] 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%.

[0132] The remaining microstructure is, for example, but not limited to, lower bainite, 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.Surface Soft Layer

[0133] 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 therefore further improves the bendability of the steel sheet. 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.

[0134] To produce the effect of improving the bendability of the steel sheet, the surface soft layer has a thickness of 20 µm or more. The surface soft layer has a thickness of 120 µm or less.

[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 a 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 may make it impossible to achieve the desired bendability intended in the present invention.

[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. 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.

[0140] 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.

[0141] The surface soft layer thickness is preferably 25 µm or more, more preferably 30 µm or more.

[0142] The surface soft layer thickness is preferably 100 µm or less, more preferably 90 µm or less.Steel Microstructure in Surface Soft LayerArea ratio of ferrite: 50.0% or more and 100.0% or less

[0143] 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 amount of ferrite in the surface soft layer to 50.0% 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. The area fraction of ferrite in the surface soft layer is preferably 60.0% or more.

[0144] The area fraction of ferrite may be 100.0%. The area fraction of ferrite may be 99.9% or less, 95.0% or less, or 90.0% 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, a tight bending test, and a tight bending + orthogonal 90-degree V-bending test and makes it impossible to achieve desired bendability of a steel sheet. From the perspective of providing a steel sheet having high bendability and a sheared end face having high bendability, when ferrite has an area fraction of less than 100.0%, the area fraction of martensite divided by the area fraction of a hard phase other than ferrite in a surface soft layer is 0.5 or less.

[0147] The hard phase other than ferrite refers to (bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite).

[0148] The lower limit of the area fraction of martensite divided by the area fraction of the hard phase other than ferrite in a surface soft layer may be, but is not limited to, 0.00.

[0149] For example, tension in a surface layer strain introduction step in a production method described later can be controlled to suppress the area fraction of fresh martensite divided by the area fraction of a hard phase other than ferrite in a surface soft layer to 0.5 or less. After the first holding step, a tension of 2.0 kgf / mm 2< or more is applied one or more times to cause deformation-induced transformation of non-transformed austenite into fresh martensite and cause tempering in the subsequent second holding step, finally forming tempered martensite.

[0150] The area fractions of ferrite, bainitic ferrite, tempered martensite, and a hard phase (hard second phase (retained austenite + fresh martensite)) at the quarter thickness of a base steel sheet and a surface soft layer are measured as described below. The microstructure of a surface soft layer is measured at the half thickness of the surface soft layer.

[0151] A sample is cut out from a base steel sheet to form a cross section in the thickness direction (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 mirror-polished with a diamond paste. The observation surface of the sample is then subjected to final polishing with colloidal silica and is then etched with 3% by volume nital to expose the microstructure.

[0152] Then, using a scanning electron microscope (SEM) under conditions of an acceleration voltage of 15 kV and a magnification of 5000 times, three visual fields of 25.6 µm x 17.6 µm are photographed at the outermost surface layer position (at the half thickness of a surface soft layer) of the observation surface of the sample and at the quarter thickness ± 100 µm. At the outermost surface layer position, a photograph is taken so as to exclude a galvanized layer and include an internal oxidation layer.

[0153] From a microstructure image (see Fig. 1) thus taken, ferrite, bainitic ferrite, tempered martensite, and other hard phases (a hard second phase (retained austenite + fresh martensite)) are identified as described below.

[0154] Ferrite: a region with a black color and with a massive form. Almost no iron-based carbide is contained. When an iron-based carbide is contained, however, the area of ferrite includes the area of the iron-based carbide. The same applies to bainitic ferrite and tempered martensite described later.

[0155] Bainitic ferrite: a region with a black to dark gray color and with a massive form, an indefinite form, or the like. No or a relatively small amount of iron-based carbide is contained.

[0156] Tempered martensite: a region with a gray color and with an indefinite form.

[0157] A relatively large amount of iron-based carbide is contained.

[0158] Hard second phase (retained austenite + fresh martensite): a region with a white to light gray color and with an indefinite form. No iron-based carbide is contained. A relatively large one may have a gradually darker color with increasing distance from the interface with another microstructure and may have a dark gray interior.

[0159] Carbide: a region with a white color and with a dot-like or linear form. It is contained in tempered martensite, bainitic ferrite, and ferrite.

[0160] Remaining microstructure: the above-described lower bainite, pearlite, internal oxide, or the like with a known form and the like.

[0161] Next, the region of each phase identified in the microstructure image is subjected to calculation by the following method. The area fractions of ferrite, bainitic ferrite, tempered martensite, and another hard phase (a hard second phase) are examined by a point counting method in which 20 x 20 grids are placed at equal intervals on a region having an actual length of 25.6 µm x 19.2 µm on the 5000x SEM image, and the number of points on each phase is counted. Each area fraction is the average value of three area fractions determined from different 5000x SEM images.

[0162] The area fraction of retained austenite is measured as described below.

[0163] 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

[0164] (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.

[0165] 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.

[0166] 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 other hard phases (a hard second phase) determined as described above from 100.0%.

[0167] Next, mechanical characteristics of a steel sheet according to an embodiment of the present invention will be described.Tensile strength (TS): 780 MPa or more and less than 1180 MPa

[0168] A steel sheet according to an embodiment of the present invention has a tensile strength TS of 780 MPa or more and less than 1180 MPa.

[0169] The predetermined yield stress (YS) and yield ratio (YR) of a steel sheet according to an embodiment of the present invention, stretch formability (total elongation (El)) of the inner portion of the steel sheet, bendability of the steel sheet, and bendability of a sheared end face are as described above.

[0170] The ratio YR (yield ratio) of the yield stress YS to the tensile strength TS preferably satisfies 0.70 ≤ YR.

[0171] The tensile strength (TS), the yield ratio (YR), 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. The bendability of a steel sheet is measured in a tight bending test and a tight bending + orthogonal 90-degree V-bending test described later in Examples. The bendability of a sheared end face is measured in a 90-degree V-bending test described later in Examples.Coated Layer (Hot-Dip Galvanized Layer, Hot-Dip Galvannealed Layer)

[0172] 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.

[0173] 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.

[0174] The hot-dip galvanized layer is preferably composed of, for example, Zn, 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 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.

[0175] The hot-dip galvannealed layer is preferably composed of, for example, 20% 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% 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.

[0176] Furthermore, the coating weight per side of a coated layer (galvanized layer) is preferably, but not limited to, 20 g / m 2< or more. The coating weight per side of a coated layer (galvanized layer) is preferably 80 g / m 2< or less.

[0177] The coating weight of a coated layer (galvanized layer) is measured as described below.

[0178] 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< ).

[0179] 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]

[0180] Next, a method for producing a steel sheet according to an embodiment of the present invention will be described.

[0181] A method for producing a steel sheet according to an embodiment of the present invention includes: a hot rolling step of hot-rolling a steel slab having the chemical composition described above to form a hot-rolled steel sheet; a pickling step of pickling the hot-rolled steel sheet after the hot rolling step; a cold rolling step of cold-rolling the steel sheet after the pickling step at a rolling reduction ratio of 20% or more and 80% or less; an annealing step of heating the steel sheet after the cold rolling step and annealing the steel sheet at an annealing temperature Ac1 (°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 100°C or more and 300°C or less; a first holding step of reheating the steel sheet after the cooling step to a reheating and holding temperature range of 370°C or more and 460°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 first holding temperature range; and a second holding step of holding the steel sheet after the surface layer strain introduction step at 300°C or more and 460°C or less for 10 seconds or more. 2400 ≤ Y ≤ 20000 Y = T − Ac 1 × t 1 / 2 + T − Ac 1 × t 2

[0182] 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).

[0183] Ac1 (°C): 727.0 - 32.7 x [%C] + 14.9 x [%Si] + 2.0 x [%Mn], 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).

[0184] Unless otherwise specified, the temperatures described above mean the surface temperatures of a steel slab and a steel sheet.

[0185] First, a steel slab having the chemical composition described above is prepared. For example, a steel material is melted to produce a molten steel having the chemical composition described above. The melting method may be, but is not limited to, any known melting method using a converter, an electric arc furnace, or the like. The resulting molten steel is then solidified into a steel slab. A steel slab can be produced from molten steel by any method, for example, a continuous casting method, an ingot casting method, a thin slab casting method, or the like. From the perspective of preventing macrosegregation, a continuous casting method is preferred.[Hot Rolling Step]

[0186] The steel slab is then hot-rolled to form a hot-rolled steel sheet.

[0187] The hot rolling may be performed in an energy-saving process. The energy-saving process may be hot direct rolling (a method of charging a furnace with the steel slab as a hot piece not cooled to room temperature and hot-rolling the steel slab), hot direct rolling (a method of keeping the steel slab slightly warm and then immediately rolling the steel slab), or the like.

[0188] The hot rolling may be performed under any conditions, for example, under the following conditions.

[0189] That is, the steel slab is temporarily cooled to room temperature and is then reheated and rolled. The slab heating temperature (reheating temperature) is preferably 1100°C or more from the perspective of melting carbide and reducing rolling force. Furthermore, to prevent an increase in scale loss, the slab heating temperature is preferably 1300°C or less. The slab heating temperature is based on the temperature of the steel slab surface.

[0190] The steel slab is then rough-rolled in the usual manner to form a rough-rolled sheet (hereinafter also referred to as a sheet bar). The sheet bar is then finish-rolled to form a hot-rolled steel sheet. When the slab heating temperature is relatively low, it is preferable to heat the sheet bar using a bar heater or the like before the finish rolling from the perspective of preventing trouble during the finish rolling. The finish rolling temperature is preferably 800°C or more to reduce the rolling load. Furthermore, when the rolling reduction ratio of austenite in an unrecrystallized state is increased, an abnormal microstructure elongated in the rolling direction may be developed and impair the workability of an annealed sheet. Furthermore, at a finish rolling temperature of 800°C or more, not only the steel microstructure of the hot-rolled steel sheet but also the steel microstructure of the final product is likely to be uniform. A nonuniform steel microstructure tends to result in a decrease in bendability. On the other hand, at a finish rolling temperature of more than 950°C, the amount of oxide (scale) formed increases. This may roughen the interface between a steel substrate and the oxide and impair the surface quality of the steel sheet after pickling and cold rolling. This may also coarsen crystal grains and reduce the strength and bendability of the steel sheet.

[0191] Thus, the finish rolling temperature is preferably 800°C or more.

[0192] The finish rolling temperature is preferably 950°C or less.

[0193] After the finish rolling, the hot-rolled steel sheet is coiled. The coiling temperature is preferably 450°C or more. The coiling temperature is preferably 750°C or less.

[0194] Sheet bars may be joined together during hot rolling to continuously perform the finish rolling. The sheet bar may be temporarily coiled before the finish rolling. 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.

[0195] In the hot rolling step including rough rolling and finish rolling (hot rolling step), the steel slab is typically formed into a sheet bar by the rough rolling and then into a hot-rolled steel sheet by the finish rolling. Depending on the mill capacity or the like, however, such classification is not concerned, provided that a predetermined size is obtained.[Pickling Step]

[0196] After the hot rolling step, the hot-rolled steel sheet is pickled. The pickling can remove an oxide from the surface of the steel sheet and ensure high chemical convertibility and coating quality. The pickling may be performed once or multiple times. The pickling may be performed under any conditions and may be performed in the usual manner.[Cold Rolling Step]

[0197] The cold rolling is performed, for example, by multipass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.

[0198] The rolling reduction ratio (cumulative rolling reduction ratio) of the cold rolling is, 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.

[0199] On the other hand, a rolling reduction ratio of more than 80% in the cold rolling tends to result in a steel sheet having a poor shape and may result in an uneven coating weight.

[0200] Optionally, a cold-rolled steel sheet after cold rolling may be pickled.[Annealing Step]

[0201] In an embodiment of the present invention, after the cold rolling step, a steel sheet formed as described above is heated and annealed at an annealing temperature of Ac1 (°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 (annealing dew-point temperature) of -10°C or more. The annealing may be performed twice or more but is preferably performed once from the perspective of energy efficiency.Annealing temperature: Ac1 (°C) or more and 900°C or less

[0202] An annealing temperature below the Ac1 temperature (°C) results in an insufficient formation ratio of austenite during heating in a two-phase region of ferrite and austenite. This results in an excessive increase in the area fraction of ferrite after annealing and a decrease in TS and YS.

[0203] On the other hand, an annealing temperature of more than 900°C results in excessive grain growth of austenite, a higher MS temperature, and a large amount of tempered martensite containing carbide, makes it difficult to form 3.5% or more by area of retained austenite, and results in lower ductility.

[0204] Thus, the annealing temperature is the Ac1 temperature (°C) or more and 900°C or less. The annealing temperature is preferably 880°C or less. The annealing temperature is the highest temperature (soaking temperature) reached in the annealing step.Ac1 (°C) is calculated using the following formula: Ac1 (°C) = 727.0 - 32.7 x [%C] + 14.9 x [%Si] + 2.0 x [%Mn]

[0205] [%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).Annealing time (soaking time): 20 seconds or more

[0206] 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. This results in an excessive increase in the area fraction of ferrite after annealing and a decrease in TS and YS. This also excessively increases the C concentration of austenite during annealing and makes it impossible to achieve desired bendability of a sheared end face. Furthermore, a surface soft layer thickness of 20 µm or more cannot be formed during annealing, and a steel sheet cannot have desired bendability. Thus, the annealing time is 20 seconds or more. The annealing time is preferably 40 seconds or more.

[0207] 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. 2400 ≤ Y ≤ 20000 Y = T − Ac 1 × t 1 / 2 + T − Ac 1 × t 2

[0208] 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).

[0209] 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.

[0210] t1 is preferably 30 s or more. t1 is preferably 80 s or less.

[0211] Dew-point temperature of atmosphere in annealing step (annealing atmosphere) (annealing dew-point temperature): - 10°C or more

[0212] 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 +10°C or more. The dew-point temperature of the annealing atmosphere in the annealing step may have any upper limit and is preferably 30°C or less due to constraints on production technology.[Isothermal Holding Step (Preferred Requirement)]

[0213] After the annealing step, during cooling from the annealing temperature, when necessary, in an isothermal holding step, isothermal holding may be performed at 400°C or more and 600°C or less (hereinafter also referred to as an isothermal holding temperature range) for less than 80 seconds for the purpose of promoting bainite transformation.

[0214] In the isothermal holding step, bainitic ferrite is formed, and C diffuses from the formed bainitic ferrite to non-transformed austenite adjacent to the bainitic ferrite. This ensures a predetermined area fraction of retained austenite and improves elongation.Isothermal holding temperature range: 400°C or more and 600°C or less

[0215] An isothermal holding temperature of less than 400°C may result in the formation of lower bainite and martensite, which contain a large amount of carbide, and suppressed diffusion of C into non-transformed austenite, which may make it impossible to achieve a predetermined area fraction of retained austenite.

[0216] On the other hand, an isothermal holding temperature of more than 600°C may result in transformation of non-transformed austenite into pearlite, making it impossible to achieve TS and ductility. Thus, the isothermal holding temperature is preferably 400°C or more and 600°C or less.Holding time in isothermal holding temperature range: less than 80 seconds

[0217] A holding time of 80 seconds or more in the isothermal holding temperature range may result in an excessive increase in the area fraction of bainitic ferrite and an excessive increase in the C concentration of non-transformed austenite and make it impossible to achieve desired bendability of a sheared end face. Thus, the holding time in the isothermal holding temperature range is preferably less than 80 seconds.[Cooling Step (First Cooling Step)]

[0218] In the cooling step, the steel sheet after the annealing step is cooled to a cooling stop temperature of 100°C or more and 300°C or less. The average cooling rate is preferably 10°C / s or more and 50°C / s or less, and the dew-point temperature of the atmosphere is preferably -20°C or less.Cooling stop temperature: 100°C or more and 300°C or lessAverage cooling rate: 10°C / s or more and 50°C / s or less, dew-point temperature of atmosphere: -20°C or less (preferred requirement)

[0219] In the cooling step, the steel sheet after the annealing step is cooled to a cooling stop temperature of 100°C or more and 300°C or less.

[0220] At this time, the cooling start temperature can be Ac1 (°C) or more and 900°C or less, and when the isothermal holding step is performed, the cooling start temperature can be 400°C or more and 600°C or less.

[0221] The cooling step is a necessary step to control the area fraction of tempered martensite and the area fraction of retained austenite within predetermined ranges, which are formed in the subsequent first holding step (reheating and holding step). At a cooling stop temperature of less than 100°C, almost the entire amount of non-transformed austenite present in the steel is transformed into martensite in the cooling step. This finally results in an excessive increase in the area fraction of tempered martensite, makes it difficult to form 3.5% by area or more of retained austenite, and results in a decrease in ductility.

[0222] On the other hand, a cooling stop temperature of more than 300°C results in a decrease in the area fraction of tempered martensite and an increase in the area fraction of fresh martensite. Consequently, fresh martensite acts as a starting point of void formation in a 90-degree V-bending test, a tight bending test, and a tight bending + orthogonal 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. Thus, the cooling stop temperature is 100°C or more and 300°C or less. The cooling stop temperature is preferably 120°C or more. The cooling stop temperature is preferably 280°C or less.

[0223] The average cooling rate in the cooling step is preferably 10°C / s or more. The average cooling rate in the cooling step is preferably 50°C / s or less. In this cooling step, a metal phase defined in the present invention can be formed. Here, an average cooling rate of less than 10°C / s may result in an increase in the amount of non-transformed austenite that is entirely transformed into martensite in the cooling step, make it difficult to finally form 3.5% or more by area of retained austenite, and result in a decrease in ductility. On the other hand, an average cooling rate of more than 50°C / s may result in suppressed self-relaxation during martensite transformation and a deterioration in sheet shape.

[0224] 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.

[0225] 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 (First Reheating and Holding Step)]

[0226] Next, in the first holding step (first reheating and holding step), the steel sheet is reheated to the temperature range of 370°C or more and 460°C or less (also referred to as a reheating and holding temperature range and, to be distinguished from the reheating and holding temperature range in the second holding step, hereinafter also referred to as a first reheating and holding temperature range) and is held for 10 seconds or more.Reheating and holding temperature (first reheating and holding temperature): 370°C or more and 460°C or less

[0227] In the reheating and holding step, by concentrating C in austenite remaining after the cooling step, the area fraction of fresh martensite in the final microstructure is reduced while maintaining a predetermined area fraction of retained austenite.

[0228] A reheating and holding temperature (first reheating and holding temperature) of less than 370°C results in insufficient C concentration in austenite remaining after the cooling step, makes it difficult to form 3.5% or more by area of retained austenite, and results in a decrease in ductility.

[0229] On the other hand, when the reheating and holding temperature (first reheating and holding temperature) is more than 460°C, C is excessively concentrated in non-transformed austenite, and non-transformed austenite in a surface layer does not undergo deformation-induced transformation in the surface layer strain introduction step described later, resulting instead in retained austenite or fresh martensite. Furthermore, 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 exceeds 0.5. Thus, the reheating and holding temperature (first reheating and holding temperature) is 370°C or more and 460°C or less.

[0230] Holding time in reheating and holding temperature range (first reheating and holding temperature range): 10 seconds or more

[0231] A holding time in the reheating and holding temperature range below 10 seconds may result in insufficient concentration of C in austenite remaining after the cooling step, make it difficult to achieve 3.5% or more by area of retained austenite, and result in a decrease in ductility.

[0232] Thus, the holding time in the first reheating and holding temperature range is 10 seconds or more.[Surface Layer Strain Introduction Step]

[0233] In the surface layer strain introduction step, a tension of 2.0 kgf / mm 2< or more is applied between the first holding step (reheating and holding step) and the second holding step to introduce strain into the surface layer.

[0234] Applying a tension of 2.0 kgf / mm 2< or more one or more times causes deformation-induced transformation of non-transformed austenite in a surface layer microstructure of a steel sheet into martensite and then into tempered martensite in the subsequent second holding step. This can achieve desired bendability of a steel sheet.

[0235] 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 a 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.

[0236] 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.

[0237] The tension is preferably 2.2 kgf / mm 2< or more, more preferably 2.4 kgf / mm 2< or more. The tension is preferably 15.0 kgf / mm 2< or less, more preferably 10.0 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]

[0238] Next, in the second holding step, the steel sheet is held at 300°C or more and 460°C or less for 10 seconds or more.

[0239] The term "holding", as used herein, also includes cooling (gradual cooling) for 10 seconds or more in the range of 300°C or more and 460°C or less.Second holding temperature (reheating and holding temperature range (second reheating and holding temperature range)): 300°C or more and 460°C or less

[0240] In the second holding step, martensite formed in the surface layer in the surface layer strain introduction step is tempered. As a result, the area fraction of fresh martensite divided by the total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) in the surface layer becomes 0.5 or less, thus achieving the desired bendability of the steel sheet.

[0241] When the second holding temperature is less than 300°C, martensite formed in the surface layer in the surface layer strain introduction step is not tempered, and the area fraction of fresh martensite in the surface layer divided by the total area fraction of bainitic ferrite, fresh martensite, and tempered martensite exceeds 0.5.

[0242] On the other hand, a second holding temperature of more than 460°C results in decomposition of retained austenite in the inner portion of the steel sheet and undesired El.

[0243] Thus, the second holding temperature (second reheating and holding temperature range) is 300°C or more and 460°C or less.Holding time at second holding temperature (second reheating and holding temperature range): 10 seconds or more

[0244] When the holding time at the second holding temperature (reheating and holding temperature range: 300°C or more and 460°C or less) is less than 10 seconds, tempering of martensite formed in the surface layer in the surface layer strain introduction step is insufficient, and the area fraction of fresh martensite in the surface layer divided by the total area fraction of bainitic ferrite, fresh martensite, and tempered martensite exceeds 0.5. Thus, the holding time in the reheating and holding temperature range is 10 seconds or more.[Coating Step (Hot-Dip Galvanizing Step, Hot-Dip Galvannealing Step)]

[0245] Next, in a coating step, a steel sheet is subjected to a galvanizing treatment to form a galvanized steel sheet. The galvanizing treatment is, for example, a hot-dip galvanizing treatment or a galvannealing treatment.

[0246] The galvanizing treatment in the coating step is performed after the annealing step. It is considered that the galvanizing treatment is 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.

[0247] 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.

[0248] 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.

[0249] An alloying temperature of less than 450°C may result in a low Zn-Fe alloying speed and make alloying difficult.

[0250] On the other hand, an alloying temperature of more than 600°C results in transformation of non-transformed austenite into pearlite and a decrease in ductility. The alloying temperature is more preferably 480°C or more. The alloying temperature is more preferably 550°C or less.

[0251] The coating weight of each of the hot-dip galvanized steel sheet (GI) and a galvannealed steel sheet (GA) is preferably 20 g / m 2< or more per side. The coating weight of each of the hot-dip galvanized steel sheet (GI) and the galvannealed steel sheet (GA) is preferably 80 g / m 2< or less per side. The coating weight can be adjusted by gas wiping or the like.[Second Cooling Step (Preferred Requirement)]

[0252] The steel sheet is then preferably cooled to a second cooling stop temperature of 50°C or less.Second cooling stop temperature: 50°C or less

[0253] The cooling conditions in the final cooling step are not particularly limited and may be in accordance with a usual method. The cooling method is, for example, gas jet cooling, mist cooling, roll cooling, water cooling, natural cooling, or the like.

[0254] From the perspective of preventing surface oxidation, cooling to 50°C or less is preferred, and cooling to approximately room temperature is more preferred. The average cooling rate is preferably, for example, 1°C / s or more and 50°C / s or less.

[0255] Furthermore, the steel sheet thus produced may be further subjected to temper rolling.

[0256] 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 a 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.

[0257] Conditions other than those described above are not particularly limited and may be based on a usual method. 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]

[0258] Next, a member according to an embodiment of the present invention will be described.

[0259] 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.

[0260] The steel sheet has a TS of 780 MPa or more and less than 1180 MPa, a high YS, high press formability in the inner portion of the steel sheet (bendability and stretch formability 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). Thus, a member according to an embodiment of the present invention has high strength and high press formability. 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]

[0261] Next, a method for producing a member according to an embodiment of the present invention will be described.

[0262] A method for producing a member according to an embodiment of the present invention includes a step of subjecting the steel sheet (for example, a steel sheet produced by the method for producing a steel sheet) to at least one of forming or joining to produce the member.

[0263] 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

[0264] 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).

[0265] The calculated transformation temperature Ac1 temperature (°C) shown in Table 1 is calculated using the following formula:

[0266] 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).

[0267] The steel slab was heated to 1200°C and, after the heating, was subjected to hot rolling composed of rough rolling and finish rolling at a finish rolling temperature of 900°C to form a hot-rolled steel sheet. The hot-rolled steel sheet was then subjected to pickling and cold rolling (rolling reduction ratio: 50%) to form a cold-rolled steel sheet having the thickness shown in Table 3. The cold-rolled steel sheet was then subjected to treatments in the annealing step, the isothermal holding step, the cooling step, the first holding step (reheating and holding step), the surface layer strain introduction step, and the second holding step under the conditions shown in Table 2 and, when necessary, treatment in the coating step (hot-dip galvanizing step or hot-dip galvannealing step), to produce a steel sheet.

[0268] The coating step was performed after the first holding step and before the surface layer strain introduction step in Nos. 1, 5 to 9, 11, 14 to 16, 21, 23 to 31, 33 to 36, 38, 40, 44, 45, 47, 51 to 56, 59, 61, 63, 64, 66, and 67, and was performed in the middle of the cooling step in Nos. 3, 4, 10, 12, 13, 17 to 20, 22, 32, 37, 39, 41 to 43, 46, 48 to 50, 57, 58, 60, 62, 65, and 68.

[0269] 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". A steel sheet not subjected to the hot-dip galvanizing treatment or the hot-dip galvannealing treatment is indicated by "CR". In Table 2, for CR steel sheets or GI steel sheets, the alloying treatment is not performed, and the alloying temperature is indicated by "-".

[0270] The galvanizing bath temperature was 470°C in the production of GI and GA.

[0271] 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.

[0272] 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.

[0273] The coated layer (galvanized layer) was formed on both sides of the base steel sheet.

[0274] 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, BF denotes bainitic ferrite, TM denotes tempered martensite, RA denotes retained austenite, and FM denotes fresh martensite. In Table 3, LB denotes lower bainite, and θ denotes carbide.

[0275] 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.

[0276] 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 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.

[0277] 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.

[0278] Furthermore, a tensile test, a 90-degree V-bending test, a tight bending test, and a tight bending + orthogonal 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), the bendability of a steel sheet, and the bendability of a sheared end face were evaluated according to the following criteria.

[0279] - Tensile strength (TS) O (pass): 780 MPa or more and less than 1180 MPa X (fail): less than 780 MPa or 1180 MPa or more

[0280] - Yield stress (YS) O (pass): (A) For 780 MPa ≤ TS < 980 MPa, 550 MPa ≤ YS (B) For 980 MPa ≤ TS < 1180 MPa, 700 MPa ≤ YS X (fail): (A) For 780 MPa ≤ TS < 980 MPa, 550 MPa > YS (B) For 980 MPa ≤ TS < 1180 MPa, 700 MPa > YS

[0281] - Stretch formability in inner portion of steel sheet (El) O (pass): (A) For 780 MPa ≤ TS < 980 MPa, 17.0% ≤ El (B) For 980 MPa ≤ TS < 1180 MPa, 11.0% ≤ El X (fail): (A) For 780 MPa ≤ TS < 980 MPa, 17.0% > El (B) For 980 MPa ≤ TS < 1180 MPa, 11.0% > El - 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 crack length in a portion other than the V-bent end surface) (the bendability 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 V-bending edge crack length) (bendability at an 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 - The thickness of a spacer at a cracking limit at which cracking of 0.5 mm or more does not occur along a bending ridge line in a tight bending test (the tight bending boundary spacer thickness) (the bendability of the steel sheet) O (pass): the tight bending boundary spacer thickness is 3.0 mm or less X (fail): the tight bending boundary spacer thickness is more than 3.0 mm - The depth of a crack that propagates in the thickness direction in a compressive-stressed bending ridge line portion in a tight bending test with a 3.0-mm spacer (the tight bending internal cracking depth) (the bendability of the steel sheet) O (pass): the tight bending internal cracking depth is 200 µm or less X (fail): the tight bending internal cracking depth is more than 200 µm - The bending radius at a cracking limit at which cracking of 0.5 mm or more does not occur along a bending ridge line in a tight bending + orthogonal 90-degree V-bending test (handkerchief bending boundary bending radius) (the bendability of the steel sheet) O (pass): the handkerchief bending boundary bending radius is 5.0 mm or less X (fail): the handkerchief bending boundary bending radius is more than 5.0 mm (1) Tensile Test

[0282] 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 at a position corresponding to 1 / 4 of the coil width with the longitudinal direction of the test specimen being perpendicular to the rolling direction of a 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 4 shows the results.(2) 90-Degree V-bending Test

[0283] 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.

[0284] 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.

[0285] After the 90-degree V-bending test with a bending radius of 0.5 mm, the length of a crack that propagated from the bending ridge line end portion in the ridge line direction was measured as described below.

[0286] 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. A reference sign y in Fig. 3-1(a) corresponds to a reference sign Y1 (the length Y1 of a crack) in Fig. 2(b)

[0287] 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 as illustrated in Fig. 3-2(b), the length (y1 + y2) / 2 of each crack was determined using a measurement tool of the software, and the length of the longest crack was defined as the length of a crack that propagated 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.

[0288] After the 90-degree V-bending test with a bending radius of 0.5 mm, the length of a crack that propagated along the bending ridge line formed in a portion other than the bending ridge line end portion was measured by visual observation with a stereomicroscope at a magnification of 25 times.(3) Tight bending test

[0289] A test specimen of 60 mm C x 30 mm L was taken from the steel sheet at a position corresponding to 1 / 4 of the coil width. After both end faces having a length of 60 mm were finished by grinding, primary bending (U-bending) was performed to prepare a test specimen for tight bending. The U-bending was performed using a hydraulic bending tester under the conditions of a punch bending radius R of 5.0 mm, at which no cracking occurred in any specimen, a stroke speed of 10 mm / s, and C direction bending (bending ridge line length: 30 mm L). The test specimen after the U-bending was then subjected to tight bending. The tight bending was performed with a hydraulic bending tester. As illustrated in Fig. 4-1(a), a spacer q (thickness Z1) was interposed as necessary, and the test was performed at a stroke speed of 10 mm / min, a pressing load of 10 ton, and a pressing time of 3 seconds such that the bending ridge line of the test specimen after the U-bending was perpendicular to the pressing direction.

[0290] In a sample subjected to the tight bending test, when a portion having a width Z2 (Z2 = (2t + Z1) x n / 2, wherein t denotes the thickness of the sample) on both sides (C direction) in the circumferential direction along the surface of the steel sheet from the bending apex was defined as the outside of the bending ridge line, the minimum spacer thickness at which the length Y3 in the L direction of a crack formed by extension in the L direction on the outside of the bending ridge line was less than 0.5 mm (a crack of 0.5 mm or more does not occur) was defined as the spacer thickness at a cracking limit. The crack length outside the bending ridge line was measured by visual observation with a stereomicroscope at a magnification of 25 times.

[0291] Furthermore, after a tight bending test with a 3.0-mm spacer (a tight bending test using a spacer having a thickness of 3.0 mm), as illustrated in Fig. 4-1(b), when a compressive-stressed side (the inner surface side of the sample) had, on both sides in the circumferential direction (C direction), a portion having a width Z3 (9 mm) (see region s) along the steel sheet surface from the bending apex located inside the bending ridge line, the depth of a crack that propagated in the thickness direction on the inside of the bending ridge line was measured as follows.

[0292] As illustrated in Fig. 4-2(c), a specimen was cut out from the sample after the tight bending so that the cross section k at the half position in the L direction was an observation surface.

[0293] The observation surface of the specimen was then mirror-polished with a diamond paste. A visual field (m in Fig. 4-2(d)) of 2560.0 µm x 1920.0 µm was then photographed with a scanning electron microscope (SEM) under the conditions of an acceleration voltage of 15 kV and a magnification of 50 times at the position m in Fig. 4-2(d), which is the bending apex of the observation surface of the specimen, and the entire crack morphology was observed. In the image of the crack, the distance X between the starting point and the end point of the crack was defined as the depth of the crack. This X was evaluated as the depth of a crack that propagated in the thickness direction in the compressive-stressed bending ridge line portion in the tight bending test with a 3.0-mm spacer.(4) Tight Bending + Orthogonal 90-Degree V-Bending Test

[0294] A V-bending test in the L direction (orthogonal 90-degree V-bending test) was performed using a sample after the tight bending test with a 3.0-mm spacer. When an r region (Z2 region in the drawing) in Fig. 4-1(a) after the V-bending is a bending ridge line after handkerchief bending, a bending radius at a cracking limit at which a crack having a length of 0.5 mm or more in the L direction does not occur in cracking in the bending ridge line formed by extension in the L direction was defined as a bending radius at a cracking limit (handkerchief bending boundary bending radius). The crack length on the bending ridge line was measured by visual observation with a stereomicroscope at a magnification of 25 times. [Table 1]Steel gradeChemical composition (% by mass)Calculated transformation point (°C)NotesCSiMnPSAlNSbSnOthersAc1A0.1071.462.650.0120.00110.0320.00280.005--751Inventive steelB0.0580.962.290.0050.00190.0300.0050-0.005-744Inventive steelC0.3041.711.030.0110.00200.0320.00320.012--745Inventive steelD0.1700.442.660.0180.00090.0200.00400.006--733Inventive steelE0.1062.032.450.0170.00110.0230.0036---759Inventive steelF0.2750.821.880.0100.00080.0320.0029---734Inventive steelG0.2061.323.120.006,0.001210.0300.00440.005--746Inventive steelH 0.040 1.652.280.0110.00220.0190.00200.008--755Comparative steelI 0.446 1.102.810.0070.00070.0160.00490.0120.014-734Comparative steelJ 0.1040.12 2.290.0120.00180.0340.00280.009--730Comparative steelK 0.1753.20 2.350.0100.00150.0280.00450.007--774Comparative steelL 0.1541.530.83 0.0080.00050.0350.00480.006--746Comparative steelM 0.1631.043.55 0.0160.00200.0110.00360.002--744Comparative steelN0.1831.232.550.0080.00100.0130.00230.002-Ti: 0.017744Inventive steelO0.10910.852.710.0130.00120.0160.0027--Nb: 0.023742Inventive steelP0.1161.472.690.0250.00050.0180.0026--Ti: 0.021, B: 0.0015750Inventive steelQ0.0891.512.340.0160.00060.0190.00370.008-Ti: 0.015, Mo: 0.016, B: 0.0017751Inventive steelR0.1201.522.750.0150.00130.0190.00450.011-Ti: 0.019, Mo: 0.049, B: 0.0011751Inventive steelS0.1960.602.620.0080.00070.0200.0049--V: 0.028735Inventive steelT0.1440.882.660.0120.00180.0330.00390.006-Cu: 0.133741Inventive steelU0.1261.332.940.0130.00190.0320.00470.007-Cr: 0.047749Inventive steelV0.1551.102.600.0080.00070.0160.0040--Ni: 0.112744Inventive steelW0.1131.152.770.0190.00140.0170.00270.004-Mo: 0.042746Inventive steelX0.1291.042.990.0060.00130.0260.00500.007--744Inventive steelY0.1791.082.530.0050.00180.0150.00370.0060.014-742Inventive steelZ0.1371.562.510.0250.00100.0320.0025-0.007Ta: 0.009751Inventive steelAA0.1710.872.580.0180.00210.0120.00300.0070.016W: 0.027740Inventive steelAB0.1231.372.930.0160.00090.0150.0031--Mg: 0.0043749Inventive steelAC0.1071.292.730.0250.00200.0230.00270.002-Zn: 0.0055748Inventive steelAD0.1350.932.570.0140.00140.0340.0023--Co: 0.0092742Inventive steelAE0.1671.032.620.0180.00120.0120.00390.003-Zr: 0.0023742Inventive steelAF0.1311.062.830.0110.00180.0260.00390.008-Ca: 0.0018744Inventive steelAG0.1541.332.820.0140.00240.0250.0043--Se: 0.0088747Inventive steelAH0.1361.532.600.0130.00190.0320.0042--Te: 0.0120751Inventive steelAI0.1211.142.510.0150.00100.0350.0033--Ge: 0.0120745Inventive steelAJ0.1900.942.920.0100.00080.0280.0028--As: 0.0211741Inventive steelAK0.1561.572.730.0240.00190.0350.0026--Sr: 0.0090751Inventive steelAL0.1851.102.960.0080.00080.0120.0023-0.007Cs: 0.0150743Inventive steelAM0.1471.452.960.0080.00170.0320.00420.005-Hf: 0.0070750Inventive steelAN0.1191.412.510.0100.00240.0260.00490.006-Pb: 0.0110749Inventive steelAO0.1721.132.880.0210.00240.0300.00380.0100.013Bi: 0.0020744Inventive steelAP0.1042.750.0170.104 0.00130.0100.00430.004-Ti: 0.019, B: 0.0015747Inventive steelAQ0.1421.122.860.0110.00100.0390.00210.006-REM: 0.0050745Inventive steelAR0.3952.953.450.0950.01951.9550.00950.0180.016-765Inventive steelAS0.0540.231.060.0020.00010.0060.0001---731Inventive steelAT0.1842.770.184 0.0050.00190.0160.0035--Nb: 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: 0.0190751Inventive steelAU0.0510.251.100.0040.00050.0100.00090.0170.015-731Inventive steel- The remainder other than those described above is composed of Fe and incidental impurities. [Table 2] No.I Steel gradeAnnealing stepIsothermal holding stepCooling stepFirst holding stepsurface layer strain introduction stepSecond holding stepCoating stepNotesAnnealing temperature T (°C)Heating time t1 (s)Annealing time (s)Annealing dew-point temperature (°C)Formula (3) Y (-)Holding temperature (°C)Holding time (s)Cooling stop temperature (°C)Holding temperature (°C)Holding time (s)Average tension (kgf / mm2)Holding temperature (°C)Holding time (s)TypeAlloying temperature (°C)1A850187100-1019238--180410503.041010GA520Example2B80017990-510064--230420302.741010CR-Example3C850169100-101945140080110410203.240040GA520Example4D83010070-51161742090160420602.830060GA510Example5E87010070-513376--100380303.138010GA510Example6F8905190-1018033--200450502.845010GA520Example7G820201170-1019977--140460502.441010GA520Example8H 88014580-519074--230420602.740010GA510Comparative Example9I 78017990-58189--0 460504.033010GA510Comparative Example10J 82014580-101372049080210430403.343020GA520Comparative Example11K 89016290-519904--150390703.830010GI-Comparative Example12L 88010070-51605141050240460503.132060GA510Comparative Example13M 82010070-10910046070110440402.936040GA520Comparative Example14A750 0160-10-89 --180440603.244010GI-Comparative Example15A910 5490-1018649--190450702.334010GA520Comparative Example16A8601010 -51642 --170430503.831010GA510Comparative Example17A85036180-15 1969648080170450202.534040GA530Comparative Example18A860020-102200 42090180370502.230040GA520Comparative Example19A85062190-522000 46070180370702.037050GA510Comparative Example20A8401799010160604106050 440403.737020GA480Comparative Example21A86014580-516668--350 370602.635010GA510Comparative Example22P84014380-51355848090170150 203.346020GI-Comparative Example23P840205100-1018107--170500 702.845010GA520Comparative Example24P85014380-515082--1804201 2.530010GA510Comparative Example25P850143802015082--180370601.0 37010GA480Comparative Example26P86014380-1016606--170420502.9150 10GA520Comparative Example27P84014380-1013558--180430503.1500 10GI-Comparative Example28P85017790-1017783--180450903.34001GA520Comparative Example29N83084180-1018990--170410504.040010GA520Example30O85014580-516531--200460403.634010GA510Example31P8509970-1011892--170400304.034010GA520Example32Q8009570-55718400100170390602.839060GI-Example33R85010570-512107--170370403.937010GA510Example34S81014580-1011464--160440303.542010GI-Example35T83014580-513605--200380703.932010GI-Example36U84017990-516427--160420503.432010GA510Example37V84010070-51159343080200400403.140020GI-Example38W86016290-1019487--160370903.737010GA520Example39X83017990-101540645070200430603.439040GI-Example40Y83010070-510538--1503801003.234010GA510Example41Z85010070-101192248050150390402.539040GA520Example42AA83014580-51378748090190420602.541050GI-Example43AB84017990-51630649070200370503.837050GA510Example44AC85017990-518295--180400703.236010GA510Example45AD85010070-1013027--170380702.630010GA520Example46AE83014580-51339145090140410503.341030GA510Example47AF830100701010313--180430504.039010GA480Example48AG84084160-51871745070180450202.032020GA510Example49AH85010070-51195048050140440202.530020GI-Example50AI84017990-101706146080180400303.231030GA520Example51AJ82014580-1012095--160390603.830010GI-Example52AK85017990-1017833--180460603.744010GA520Example53AL820207100-515617--130410602.136010GA510Example54AM84017990-1016222--190390502.239010GA520Example55AN85014580-515370--180400902.940010GA510Example56AO83010070-1010337--150370703.637010GA520Example57AP85047160-101881747080180390403.739040GI-Example58AQ83017990-101531245070150400302.840040GA520Example59AR800604052454--160380402.031030GA480Example60AS8201059001271448070160410703.632060GI-Example61AT86095601011770--200380802.838010GA480Example62AS8402001001021844 420100150420803.142010GA480Comparative Example63AA7604090-102252 --180410602.939070GA480Comparative Example64AA7705050-102285 --150400403.937010GA500Comparative Example65AA7904020-102019 42040160380703.134060GA480Comparative Example66AA7905020-102271 --200420503.442010GI-Comparative Example67AA8001501001010582--1804202 2.842010GA490Comparative Example68AU8501901101024342 45080200410503.040020GA480Comparative Example [Table 3] No.Steel gradeThickness (mm)Base steel sheet steel microstructure (t / 4)Surface soft layerNotesArea fraction of each phaseRemainder microstructureFormula (1) right side (*2)XFFM / hard phase (*1)FBFTMBF+TMRAFMArea fraction(%)(%)(%)(%)(%)(%)(µm)(µm)(%)(-)1A1.216.913.760.774.44.908LB, θ1013280.00.3Example2B1.256.45.222.527.76.19.0LB, θ1112850.20.4Example3C1.221.16.557.764.26.64.2LB, θ743983.80.4Example4D1.210.51.671.973.53.68.2LB, θ973483.80.1Example5E1.223.012.751.764.47.04.9θ1203379.80.2Example6F1.216.310.165.475.54.72.0θ1203793.10.4Example7G1.220.89.659.869.44.21.0LB, θ1013885.10.2Example8H 1.280.4 1.20.01.2 2.4 14.8 θ903983.30.4Comparative Example9I 1.29.30.332.032.318.8 36.3 θ482680.90.1Comparative Example10J 1.20.018.774.793.4 0.3 1.5LB, θ862880.70.2Comparative Example11K 1.279.1 0.00.00.0 8.511.7 LB, θ934181.80.1Comparative Example12L 1.282.2 1.40.01.4 5.010.9 θ974383.60.3Comparative Example13M 1.21.40.058.858.814.8 21.7 LB, θ1122161.50.2Comparative Example14A1.288.2 0.00.00.0 0.0 8.6θ10118 89.30.5Comparative Example15A1.20.06.679.486.01.1 5.3LB, θ1013561.50.2Comparative Example16A1.222.93.056.059.04.79.1θ10112 88.20.2Comparative Example17A1.217.717.755.272.94.20.3LB, θ10111 45.6 0.4Comparative Example18A1.223.211.350.061.312.5 2.3θ10116 96.60.1Comparative Example19A1.220.40.069.469.43.74.2θ101129 99.10.5Comparative Example20A1.221.47.659.667.20.3 4.3LB, θ1016261.50.4Comparative Example21A1.219.818.726.745.43.830.2 θ1013790.20.1Comparative Example22P1.221.712.446.458.82.3 15.9 θ1203285.90.3Comparative Example23P1.221.814.955.270.14.41.5LB, θ1203296.30.9 Comparative Example24P1.220.35.863.269.03.66.1θ1204155.30.9 Comparative Example25P1.215.819.650.970.59.11.6LB, θ1207678.60.8 Comparative Example26P1.222.89.152.061.13.4 9.2LB, θ1203388.80.8 Comparative Example27P1.220.110.565.175.60.0 0.0θ1202785.60.3Comparative Example28P1.216.814.462.376.74.01.3θ1203856.50.9 Comparative Example29N1.224.05.162.467.55.32.5θ1123785.70.3Example30O1.29.618.763.582.25.00.8LB, θ1203791.40.4Example31P1.217.513.961.975.84.01.1θ1202554.50.3Example32Q1.246.28.431.640.06.03.7LB, θ902986.90.4Example33R1.217.217.858.175.93.50.0LB, θ783380.80.3Example34S1.222.011.856.868.66.10.6LB, θ1202293.50.4Example35T1.219.719.654.674.24.21.9-973360.00.1Example36U1.216.418.955.774.65.01.9LB, θ933679.20.3Example37v1.222.514.951.266.14.22.2LB, θ1202994.40.5Example38W1.212.918.459.477.83.92.0LB, θ1053586.90.3Example39X1.221.58.264.873.04.80.7-933579.20.4Example40Y1.219.117.857.375.13.81.7θ712859.90.2Example41Z1.223.76.065.571.53.61.0θ1073160.30.4Example42AA1.218.212.958.171.04.51.6LB, θ633296.80.5Example43AB1.221.38.660.869.43.64.0θ1203958.50.4Example44AC1.222.210.660.671.24.31.2θ1124658.70.4Example45AD1.221.111.158.169.25.10.3LB, θ1202950.80.5Example46AE1.217.312.661.373.94.20.3LB, θ1093551.20.1Example47AF1.219.416.656.873.43.81.8LB, θ904255.40.2Example48AG1.222.28.461.670.03.70.1LB, θ1204291.60.2Example49AH1.215.414.764.379.04.00.1θ1203560.80.1Example50AI1.220.36.465.872.23.81.0LB, θ1203479.30.3Example51AJ1.220.26.063.969.96.71.7θ1202393.30.2Example52AK1.214.419.856.276.04.01.7LB, θ1203887.00.2Example53AL1.216.413.262.275.44.50.4LB, θ1073359.20.1Example54AM1.221.213.753.467.16.73.9θ1012750.50.4Example55AN1.217.416.558.875.34.40.5LB, θ973659.00.4Example56AO1.221.26.765.772.45.10.2θ573094.90.2Example57AP1.217.517.056.973.93.82.0LB, θ1053581.10.3Example58AQ1.219.315.056.671.64.90.7LB, θ972695.50.4Example59AR1.221.56.952.759.69.28.1LB, θ212179.70.1Example60AS1.219.617.554.972.43.80.9LB, θ1206995.30.1Example61AT1.216.712.165.377.44.61.1θ1207286.80.2Example62AS1.239.47.743.451.16.01.7θ120128 98.30.1Comparative Example63AA1.212.910.869.880.64.61.0θ6318 51.50.3Comparative Example64AA1.221.414.256.270.45.20.2LB, θ6317 58.60.5Comparative Example65AA1.218.916.058.374.34.51.5θ6319 54.80.2Comparative Example66AA1.223.89.353.662.96.73.1LB, θ6318 70.60.1Comparative Example67AA1.219.910.759.670.32.8 5.6θ635095.90.4Comparative Example68AU1.236.68.948.657.54.11.5θ27100 99.00.1Comparative Example (*1) FM / hard phase: the area fraction of fresh martensite divided by the total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) (*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)YRTS (MPa)EI (%)V-bending other than end face the crack length (µm)V-bending end face the crack length (µm)Tight bending boundary spacer thickness (mm)Tight bending internal crack depth (µm)Handkerchief bending boundary bending radius (mm)Notes1A9620.93103614.501100.01301.5Example2B6810.8778221.30400.0660.0Example3C8180.72112816.201410.01781.0Example4D8690.8899112.401160.01790.5Example5E9880.90109816.701130.01792.0Example6F8370.81103714.301260.01200.0Example7G10220.87117114.601080.01490.5Example8H 404 0.65620 26.50560.01030.0Comparative Example9I 12420.831498 10.5 201 214 4.0 240 5.5 Comparative Example10J 8200.8398210.2 01110.01081.5Comparative Example11K 5890.6985630.3139248 0.5570.0Comparative Example12L 352 0.64552 28.501601.5364.0Comparative Example13M 9040.741224 11.8203 208 3.5 233 6.0 Comparative Example14A470 0.68686 26.00490.018600Comparative Example15A10790.9611199.7 861060.5543.0Comparative Example16A9060.89101216.4220 1434.0 269 6.0 Comparative Example17A9560.93103214.2212 1103.5 247 6.0 Comparative Example18A9380.92101911.2209 1123.5 336 5.5 Comparative Example19A503 0.68735 26.41221112.55500Comparative Example20A9310.92101410.1 1181110.5392.5Comparative Example21A7420.73102214.2205 211 4.0 231 5.5 Comparative Example22P10130.9510696.1 01080.018610Comparative Example23P10320.95108913.9207 1084.5 208 6.5 Comparative Example24P10300.95109016.1201 1084.5 201 5.5 Comparative Example25P10770.96112215.6221 1064.0 236 6.5 Comparative Example26P10270.95108517.7210 1084.5 253 6.0 Comparative Example27P7050.6610719.3 01070.01701.0Comparative Example28P10650.96111112.2212 1064.5 210 5.5 Comparative Example29N9250.83111120.401230.01150.5Example30O9050.88102915.701130.01870.0Example31P10420.94111413.6981081.5494.0Example32Q7040.8187119.001280.01950.0Example33R9510.90105617.301140.01111.5Example34S8410.82102113.601270.01350.0Example35T8670.84103616.01211220.5662.5Example36U10270.89115814.401130.01992.0Example37V8900.82108814.501230.01480.0Example38W9610.88108814.801150.01100.5Example39X9480.90105713.301150.01072.0Example40Y9190.84109815.11131212.5624.5Example41Z9350.82113614.11191210.5373.0Example42AA8850.83106914.401240.01170.0Example43AB9960.88113314.41311160.5633.0Example44AC9100.87105013.91201202.5505.0Example45AD8370.8499714.21041232.5684.5Example46AE9290.86107415.21211192.5535.0Example47AF9320.87106714.1991181.5603.5Example48AG9890.86114613.601160.01850.0Example49AH10060.87116214.01261170.5472.5Example50AI8560.85100413.701240.01922.0Example51AJ9860.89111113.801140.01460.0Example52AK10730.91117616.101140.01011.0Example53AL10480.90116413.61081110.5412.5Example54AM10450.90116614.21061182.5495.0Example55AN9290.84110316.31051180.5363.0Example56AO9890.91108414.201150.01220.0Example57AP8310.80104115.201140.01131.5Example58AQ9630.89107613.901140.01230.0Example59AR8370.83100514.2491221.0802.5Example60AS9290.85109915.201210.5561.5Example61AT9320.87106714.1431170.0401.0Example62AS540 0.74727 17.601160.0110.0Comparative Example63AA10060.88114914.0212 1172.01984.5Comparative Example64AA8560.83102713.71111223.5 1694.0Comparative Example65AA9860.91108213.81111122.5298 4.5Comparative Example66AA10730.92117316.11111112.51895.5 Comparative Example67AA10480.92113410.6 1081102.010630Comparative Example68AU544 0.71770 19.00560.0200.0Comparative Example

[0295] In Tables 1 to 4, the underlined portions indicate values outside the appropriate ranges of the present invention.

[0296] As shown in Table 4, in all of the examples, the tensile strength (TS), the yield stress (YS), and the total elongation (El) were acceptable, and the crack length in the portion other than the V-bent end surface, the crack length on the V-bent end surface, the tight bending spacer thickness, the tight bending internal cracking depth, and the handkerchief bending boundary bending radius were within the specified ranges.

[0297] In contrast, in the comparative examples, at least one of the tensile strength (TS), the yield stress (YS), the total elongation (El), the crack length in the portion other than the V-bent end surface, the crack length on the V-bent end surface, the tight bending spacer thickness, the tight bending internal cracking depth, and the handkerchief bending boundary bending radius was not sufficient.Reference Signs List

[0298] Fferrite FMfresh 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: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.005% 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 50.0% or more and 100.0% or less, and 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, as microstructures at a quarter thickness of the base steel sheet, ferrite has an area fraction of 76.5% or less (including 0.0%), bainitic ferrite and tempered martensite (excluding retained austenite) have a total area fraction of 20.0% or more and 90.0% or less, retained austenite has an area fraction of 3.5% or more and 10.0% or less, and fresh martensite has an area fraction of 10.0% or less (including 0.0%), and the steel sheet has a tensile strength of 780 MPa or more and less than 1180 MPa, 20 ≤ X ≤ 120 − 3800 × Sb − 1900 × Sn wherein, in the formula (1), X denotes a 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 to form a hot-rolled steel sheet; a pickling step of pickling the hot-rolled steel sheet after the hot rolling step; a cold rolling step of cold-rolling the steel sheet after the pickling step at a rolling reduction ratio of 20% or more and 80% or less; an annealing step of heating the steel sheet after the cold rolling step and annealing the steel sheet at an annealing temperature Ac1 (°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 100°C or more and 300°C or less; a first holding step of reheating the steel sheet after the cooling step to a reheating and holding temperature range of 370°C or more and 460°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 at 300°C or more and 460°C or less for 10 seconds or more, 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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