STEEL SHEET, MEMBER AND METHODS FOR ITS MANUFACTURE

Abstract

A technique is provided for a steel sheet having a tensile strength (TS) of 780 MPa or more and less than 1180 MPa, a high y-saturation (YS), and high pressure formability in the inner portion of the steel sheet and in an extreme portion of the steel sheet. In the steel sheet, a base steel sheet has a predetermined chemical composition and has a soft surface layer with a Vickers hardness of 84% or less with respect to that of one-quarter of the thickness from the surface of the base steel sheet and satisfying the following formula (1). The soft surface layer has a microstructure within a specified range, the base steel sheet has a microstructure within a specified range at one-quarter of the thickness, and the steel sheet has a tensile strength of 780 MPa or more and less than 1180 MPa.20< X < 120 - 3800 x [Sb] - 1900 x [Sn] (1) In formula (1), X denotes the thickness of the surface soft layer (µm), and [Sb] and [Sn] denote the Sb content and Sn content of the steel (% by mass), respectively.

Description

Steel plates, components, and their manufacturing methods

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

[0002] From the perspective of protecting the global environment, improving the fuel efficiency of automobiles has become an important issue. Therefore, there has been a growing movement to reduce the weight of automobile bodies by increasing the strength and thinning of steel sheets, which are the raw materials for automobile parts.

[0003] In addition, social demands for improved automobile collision safety are becoming increasingly stronger. Therefore, there is a demand for the development of steel sheets that not only have high strength but also have excellent impact resistance properties in the event of a collision while the automobile is in motion (hereinafter simply referred to as impact resistance properties). In particular, from the viewpoint of corrosion prevention performance of the vehicle body, steel sheets that are used as raw materials for automobile components are often zinc-plated. Therefore, there is a demand for the development of zinc-plated steel sheets that not only have high strength but also have excellent impact resistance properties.

[0004] As an example of a steel sheet that can be used as a material for such automobile parts, Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet having a thickness of 0.6 to 5.0 mm and a plating layer on the surface of the steel sheet, in which the steel sheet structure contains, by volume fraction, 40 to 90% of a ferrite phase and 3 to 25% of a retained austenite phase, the retained austenite phase having a solute carbon content of 0.70 to 1.00%, an average particle size of 2.0 μm or less, an average distance between particles of 0.1 to 5.0 μm, a decarburized layer thickness in the steel sheet surface layer of 0.01 to 10.0 μm, an average particle size of oxides contained in the steel sheet surface layer of 30 to 120 nm, and an average density of 1.0 × 10 １２ pieces / m ２ The present invention discloses a high-strength hot-dip galvanized steel sheet that has the above characteristics and further has a work-hardening coefficient (n value) of 0.080 or more on average at the time of plastic deformation of 3 to 7%, and that has high ductility while ensuring high strength of 900 MPa or more in maximum tensile strength and excellent mechanical cutting properties.

[0005] Furthermore, Patent Document 2 discloses a high-strength hot-dip galvanized steel sheet excellent in delayed fracture resistance, characterized in that it has, by volume fraction, 40 to 90% of a ferrite phase and 5% or less of a retained austenite phase, the proportion of unrecrystallized ferrite in the entire ferrite phase being 50% or less, by volume fraction, a grain size ratio, which is the value obtained by dividing the average grain size of crystal grains of the ferrite phase in the rolling direction by the average grain size in the sheet width direction, of 0.75 to 1.33, a length ratio, which is the value obtained by dividing the average length of hard structures dispersed in an island shape in the rolling direction by the average length in the sheet width direction, of 0.75 to 1.33, and an average aspect ratio of inclusions being 5.0 or less.

[0006] Patent Document 3 also discloses a steel sheet having a steel sheet and a hot-dip galvanized layer, the steel sheet including a base material and a decarburized ferrite layer, the structure at a depth of 1 / 4 of the sheet thickness of the steel sheet containing 5.0 volume % or more of tempered martensite and 0.5 volume % or more but less than 7.0 volume % of retained austenite, the balance mainly consisting of 4 to 70 volume % of ferrite and bainite, a part or all of the tempered martensite and the retained austenite forming M-A, the decarburized ferrite layer containing 120% or more of ferrite with respect to the content of ferrite at a depth of 1 / 4 of the sheet thickness, an average ferrite grain size of 20 μm or less, a thickness of 5 μm or more and 200 μm or less, 1.0 volume % or more of tempered martensite and a number density of 0.01 grains / μm ２ Disclosed are a hot-dip galvanized steel sheet and a hot-dip galvannealed steel sheet having good elongation and bendability, characterized by the above-mentioned properties, and methods for producing the same.

[0007] Patent Document 4 discloses a high-strength hot-dip galvanized steel sheet having a component composition containing, by mass%, 0.05 to 0.3% C, 0.01 to 2.5% Si, 0.5 to 3.5% Mn, 0.003 to 0.100% P, 0.02% or less S, 0.010 to 1.5% Al, 0.007% or less N, with the balance being Fe and unavoidable impurities, and having a microstructure containing, by area ratio, 20 to 87% ferrite, 3 to 10% martensite and retained austenite in total, and 10 to 60% tempered martensite, which has a high TS-El balance, excellent stretch flangeability, and low YR and excellent workability, and a method for producing the same.

[0008] Patent No. 5354135 Patent No. 5352793 Patent No. 6536294 Patent No. 5256689

[0009] In recent years, the practical application of steel sheets having a tensile strength (hereinafter also referred to as TS) of 780 MPa or more has been increasing for impact energy absorbing members of automobiles, such as front side members and rear side members.

[0010] That is, improving the yield stress YS (hereinafter also referred to as YS) is effective in increasing the energy absorption during impact (hereinafter also referred to as impact absorption energy) so as to obtain excellent fracture resistance characteristics during a vehicle collision. However, increasing the TS and YS of a steel sheet generally reduces press formability, particularly properties such as ductility, hole expandability, and bendability. Therefore, when applying a steel sheet with such increased TS and YS to the above-mentioned automotive impact energy absorbing components, press forming becomes difficult, and variations during forming reduce yield. In particular, a decrease in press formability at the end of the steel sheet leads to the occurrence of end cracks in the actual component.

[0011]

[0004] Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet in which ductility is improved by the formation of retained austenite inside the steel sheet and mechanical cutting properties are improved by the formation of a decarburized layer on the surface of the steel sheet, but does not take into consideration at all the improvements in bendability and fracture resistance in the event of a vehicle collision that are achieved by the formation of a soft surface layer (decarburized layer), and the press formability of the steel sheet ends. Patent Document 2 discloses a high-strength hot-dip galvanized steel sheet in which ductility is improved by making the main structure inside the steel sheet soft ferrite and limiting the amount of unrecrystallized ferrite to a small amount, and delayed fracture resistance and its anisotropy are improved by the formation of a decarburized layer on the surface of the steel sheet, but does not take into consideration at all the improvements in bendability and fracture resistance in the event of a vehicle collision that are achieved by the formation of a soft surface layer (decarburized layer), and the press formability of the steel sheet ends. Patent Document 3 discloses a hot-dip galvanized steel sheet and a galvannealed steel sheet in which ductility is improved by the formation of M-A inside the steel sheet and bendability is improved by the formation of a soft layer (decarburized ferrite layer) on the surface of the steel sheet, but does not consider at all the press formability of the steel sheet ends. Patent Document 4 discloses a high-strength hot-dip galvanized steel sheet in which both ductility, which is the press formability inside the steel sheet, and stretch flangeability, which is the press formability of the steel sheet ends, are improved, but does not consider at all the improvement in bendability or the improvement in fracture resistance during a vehicle collision due to the formation of a soft surface layer (decarburized layer). From the above, it cannot be said that the steel sheets disclosed in Patent Documents 1 to 4 have a TS of 780 MPa or more, a high YS, excellent press formability inside the steel sheet (bendability and stretch formability of the steel sheet), excellent press formability of the steel sheet ends (bendability of the steel sheet ends (shear cross section)), and fracture resistance during a collision (bending fracture properties and axial crush properties).

[0012] The present invention has been developed in view of the above-mentioned current situation, 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, excellent press formability inside the steel sheet (bendability and stretch formability of the steel sheet), and excellent press formability at the steel sheet end (bendability of the steel sheet end (shear cross section)), and a manufacturing method thereof. Another aim of the present invention is to provide a member made from the above-mentioned steel sheet, and a manufacturing method thereof.

[0013] The steel sheet referred to here also includes a galvanized steel sheet, and the galvanized steel sheet is a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or a hot-dip galvannealed steel sheet (hereinafter also referred to as GA).

[0014] Here, the tensile strength TS is measured by a tensile test conforming to JIS Z 2241 (2011). Furthermore, having a high yield stress YS, excellent press formability inside the steel sheet (bendability and stretch formability of the steel sheet), and excellent press formability at the steel sheet edge (bendability of the steel sheet edge (shear cross section)) means that the following is satisfied. A high yield stress YS means that the YS measured by a tensile test conforming to JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on the TS measured by the tensile test. (A) When 780 MPa ≦ TS < 980 MPa, 550 MPa ≦ YS. (B) When 980 MPa ≦ TS < 1180 MPa, 700 MPa ≦ YS.

[0015] Furthermore, excellent bendability of a steel plate means that, in accordance with JIS Z 2248 (2022), a 90-degree V-bend test with a bending radius of 0.5 mm is performed, and the crack length (crack length other than at the V-bend end face) that propagates along the bend ridge formed other than at the bend ridge end is 200 μm or less; in a close-contact bending test, the spacer plate thickness at which cracks of 0.5 mm or more do not occur along the bend ridge is 3.0 mm or less; in a close-contact bending test with a 3.0 mm spacer, the crack depth that propagates in the plate thickness direction at the bend ridge subjected to compressive stress (close-contact bending internal crack depth) is 200 μm or less; and in a close-contact bending + orthogonal 90-degree V-bend test, when the bending radius at which cracks of 0.5 mm or more do not occur along the bend ridge is defined as the crack limit bending radius (handkerchief bending boundary bending radius), the crack limit bending radius is 5.0 mm or less. Detailed measurement methods for the 90-degree V-bend test with a bending radius of 0.5 mm, the close bending test, and the close bending + perpendicular 90-degree V-bend test are as described in the examples below.

[0016] Furthermore, excellent internal stretch formability of the steel sheet means excellent ductility, and refers to the total elongation (El) measured in a tensile test in accordance with JIS Z 2241 (2011) satisfying the following formula (A) or (B) depending on the TS measured in the tensile test: (A) 17.0%≦El when 780 MPa≦TS<980 MPa; (B) 11.0%≦El when 980 MPa≦TS<1180 MPa.

[0017] Furthermore, excellent bendability at the steel plate end (shear cross section) means that, in a 90-degree V-bend test with a bending radius of 0.5 mm according to JIS Z 2248 (2022), the length of the crack propagating from the end of the bent ridge in the ridge direction (V-bend end face crack length) is 200 μm or less.

[0018] The inventors of the present invention have conducted extensive research to achieve the above object. As a result, the composition of a base steel sheet of a steel sheet is appropriately adjusted, and the base steel sheet of the steel sheet has a soft surface layer having a Vickers hardness of 84% or less of the Vickers hardness at a quarter-thickness position of the sheet, and the soft surface layer satisfies the following formula (1), and the structure in the soft surface layer is: an area ratio of ferrite: 50.0% or more and 100.0% or less, and among the structures other than ferrite, the value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less, and the structure at a quarter-thickness position of the base steel sheet is: It has been found that a steel sheet can be obtained which has an area ratio of ferrite: 76.5% or less (including 0.0%), a total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): 20.0% or more and 90.0% or less, a volume ratio of retained austenite: 3.5% or more and 10.0% or less, an area ratio of fresh martensite: 10.0% or less (including 0.0%), a tensile strength of 780 MPa or more and less than 1180 MPa, a high yield stress YS, excellent press formability inside the steel sheet (bendability and stretch formability of the steel sheet), and excellent press formability at the steel sheet end (bendability of the steel sheet end (shear cross section)). 20≦X≦120−3800×[Sb]−1900×[Sn] (1) In formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.

[0019] The present invention was completed based on the above findings and further investigations.

[0020] That is, the gist and configuration of the present invention are as follows. [1] A base steel sheet having a component composition containing, in mass%, 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 (inclusive), and Sn: 0.200% or less (inclusive), with the balance consisting of Fe and unavoidable impurities, and a soft surface layer having a Vickers hardness of 84% or less of the Vickers hardness at a 1 / 4 sheet thickness position from the surface of the base steel sheet, wherein the soft surface layer satisfies the following formula (1): the area fraction of bainitic ferrite and tempered martensite (excluding retained austenite) is 20.0% or more and 90.0% or less; the area fraction of retained austenite is 3.5% or more and 10.0% or less; the area fraction of fresh martensite is 10.0% or less (including 0.0%); and the tensile strength is 780 MPa or more and less than 1180 MPa. 20≦X≦120−3800×[Sb]−1900×[Sn] (1) In formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.[2] The composition further includes, in mass%, Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, The steel sheet according to [1] above, containing at least one selected from 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] above, wherein one or both sides of the base steel sheet have a plating layer, the plating layer being a hot-dip galvanized layer. [4] The steel sheet according to [1] or [2] above, wherein one or both sides of the base steel sheet have a plating layer, the plating layer being a galvannealed hot-dip galvanized layer. [5] A member made using the steel sheet according to any of [1] to [4] above. [6] A hot rolling process in which a steel slab having the chemical composition according to [1] or [2] is hot rolled to form a hot rolled steel sheet; a pickling process in which the hot rolled steel sheet is pickled after the hot rolling process; a cold rolling process in which the steel sheet after the pickling process is cold rolled at a rolling reduction of 20% to 80%; an annealing process in which the steel sheet after the cold rolling process is heated and annealed at an annealing temperature of Ac1 (°C) to 900°C inclusive, an annealing time of 20 seconds or more, and in an atmosphere with a dew point of -10°C or higher, under conditions satisfying formulas (2) and (3); a cooling process in which the steel sheet after the annealing process is cooled to a cooling stop temperature of 100°C to 300°C inclusive; and a first holding process in which the steel sheet after the cooling process is reheated to a reheat holding temperature range of 370°C to 460°C inclusive, and held for 10 seconds or more. The steel sheet after the first holding step was subjected to a load of 2.0 kgf / mm in the reheating holding temperature range. ２and a second holding step of holding the steel sheet after the surface strain introducing step at 300°C or more and 460°C or less for 10 seconds or more. 2400≦Y≦20000 ... formula (2) Y=[{(T-Ac1)×t1} / 2}]+{(T-Ac1)×t2} ... (3) where, in formula (3), T: annealing temperature (°C), t1: time (s) from 650°C to the annealing temperature T during temperature rise in the annealing step, t2: annealing time (s), and Ac1: Ac1 (°C). [7] A method for producing a steel sheet according to item [6] above, comprising a hot-dip galvanizing step of, after the annealing step, subjecting the steel sheet to a hot-dip galvanizing treatment to form a hot-dip galvanized layer. [8] A method for producing a steel sheet according to the above [6], comprising a galvannealed hot-dip galvanizing step of forming a galvannealed layer on the steel sheet after the annealing step. [9] A method for producing a member, comprising a step of forming the steel sheet according to any one of the above [1] to [4] into a member.

[0021] According to the present invention, a steel sheet can be obtained which has a tensile strength TS of 780 MPa or more but less than 1180, a high yield stress YS, excellent press formability inside the steel sheet (bendability and stretch formability of the steel sheet), and excellent press formability at the steel sheet end (bendability of the steel sheet end (shear cross section)). Furthermore, members made from the steel sheet of the present invention have high strength and can be very advantageously applied to automobile impact energy absorbing members, etc.

[0022] FIG. 1 is an example of a structural image taken by SEM used for structural identification. FIG. 2(a) is a schematic diagram of a sample after 90-degree V bending, and FIG. 2(b) is a view of the sample shown in FIG. 2(a) viewed in the Z direction (negative direction). FIGS. 3-1(a) and (b) are schematic diagrams for explaining end surface cracking during 90-degree V bending. FIG. 3-2(a) is a schematic diagram for explaining a method for measuring the crack length of end surface cracks that occurred during 90-degree V bending, and FIG. 3-2(b) is a diagram showing an example of a profile waveform used for measuring the crack length. FIG. 4-1(a) is a diagram for explaining a method for measuring the spacer plate thickness that is the crack limit when performing a contact bending test, and FIG. 4-1(b) is a diagram for explaining a method for measuring the crack depth that propagates in the plate thickness direction at the bend ridgeline subjected to compressive stress during a contact bending test. Figure 4-2(c) is a diagram for explaining a method for cutting out an observation cross section for measuring the depth of cracks that progress in the thickness direction at the bend ridgeline that has been subjected to compressive stress when a close contact bending test is performed, and Figure 4-2(d) is a diagram for explaining a method for measuring the depth of cracks that progress in the thickness direction at the bend ridgeline that has been subjected to compressive stress in the above observation cross section.

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

[0024] [1. Steel Plate] The steel plate of the present invention has a base steel plate having a chemical composition containing, in mass%, 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 (inclusive), and Sn: 0.200% or less (inclusive), with the balance consisting of Fe and unavoidable impurities, and has a surface soft layer having a Vickers hardness of 84% or less of the Vickers hardness at a ¼ position of the plate thickness from the surface of the base steel plate, the surface soft layer satisfies the following formula (1), and the structure in the surface soft layer is the area fraction of ferrite is 50.0% or more and 100.0% or less, and when the area fraction of ferrite is less than 100.0%, the value obtained by dividing the area fraction of fresh martensite by the total area fraction of bainite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less, the structure at the 1 / 4 position in the plate thickness of the base steel plate is such that the area fraction of ferrite is 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, the area fraction of retained austenite is 3.5% or more and 10.0% or less, and the area fraction of fresh martensite is 10.0% or less (including 0.0%), The tensile strength is 780 MPa or more and less than 1180 MPa, and the steel sheet has a high yield stress YS, excellent press formability inside the steel sheet (bendability and stretch formability of the steel sheet), and excellent press formability at the steel sheet end (bendability of the steel sheet end (shear cross section)). 20≦X≦120−3800×[Sb]−1900×[Sn] (1) In formula (1), X is the thickness (μm) of the surface soft layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.

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

[0026] C: 0.050% or more and 0.400% or less. C is an effective element for generating appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite, and retained austenite to ensure a TS of 780 MPa or more but less than 1180 MPa and a high YS. Here, if the C content is less than 0.050%, the area fraction of ferrite increases, making it difficult to achieve a TS of 780 MPa or more. This also results in a decrease in YS. On the other hand, if the C content exceeds 0.400%, the area fraction of fresh martensite increases excessively, making it difficult to achieve a TS of less than 1180 MPa. Furthermore, fresh martensite becomes the starting point for void formation during 90-degree V-bend tests, close-contact bending tests, and close-contact bending + orthogonal 90-degree V-bend tests, making it impossible to achieve the desired bendability and shear edge bendability of the steel sheet. Furthermore, the area fraction of retained austenite and the amount of solute C in the retained austenite increase excessively. Furthermore, when subjected to shearing, the hardness of fresh martensite generated by the stress-induced transformation of retained austenite increases significantly, promoting the subsequent generation of voids and crack propagation, making it more difficult to achieve the desired bendability of the sheared edge. Therefore, the C content is set to 0.050% or more and 0.400% or less. The C content is preferably 0.100% or more. Furthermore, the C content is preferably 0.300% or less. The C content is more preferably 0.200% or less.

[0027] Si: 0.20% or more and 3.00% or less Si suppresses the formation of carbides during cooling after annealing and promotes the formation of retained austenite. That is, Si is an element that affects the area fraction of retained austenite. Here, if the Si content is less than 0.20%, the area fraction of retained austenite decreases, and ductility decreases. On the other hand, if the Si content exceeds 3.00%, the area fraction of ferrite increases excessively, resulting in an excessive increase in the C concentration in austenite during annealing, making it impossible to achieve the desired bendability at the sheared edge. Therefore, the Si content is set to 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. Furthermore, the Si content is preferably 0.50% or more.

[0028] Mn: 1.00% or more but less than 3.50% Mn is an element that adjusts the area ratio of bainitic ferrite, tempered martensite, etc. Here, if the Mn content is less than 1.00%, the area ratio of ferrite increases excessively, making it difficult to achieve a TS of 780 MPa or more. It also results in a decrease in YS. On the other hand, if the Mn content is 3.50% or more, the martensitic transformation start temperature Ms (hereinafter simply referred to as the Ms point or Ms) decreases, and the amount of martensite generated during the cooling process decreases. As a result, the amount of martensite generated during final cooling increases, and the martensite generated at that time is not sufficiently tempered, resulting in an increase in the area ratio of hard fresh martensite. Fresh martensite serves as the starting point for void generation during a 90-degree V-bend test, a close-contact bending test, and a close-contact bending + orthogonal 90-degree V-bend test. If the area ratio of fresh martensite exceeds 10.0%, the desired bendability and bendability of the sheared edge of the steel sheet cannot be achieved. Therefore, the Mn content is set to 1.00% or more and less than 3.50%, preferably 2.00% or more, and preferably 3.00% or less.

[0029] P: 0.001% or more and 0.100% or less P is an element that has a solid solution strengthening effect and increases the TS and YS of steel sheets. To achieve this effect, the P content is set to 0.001% or more. On the other hand, if the P content exceeds 0.100%, P segregates at prior austenite grain boundaries, embrittling the grain boundaries. As a result, after shearing the steel sheet, the amount of voids generated increases, and the desired bendability of the sheared edge cannot be achieved. Therefore, the P content is set to 0.001% or more and 0.100% or less. The P content is preferably 0.003% or more. The P content is preferably 0.030% or less. The P content is preferably 0.010% or less, more preferably 0.005% or less.

[0030] S: 0.0001% or more and 0.0200% or less S exists as sulfides in steel. In particular, if the S content exceeds 0.0200%, the amount of voids generated increases after shearing the steel sheet, making it impossible to achieve the desired bendability of the sheared edge. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0080% or less. The S content is more preferably 0.0050% or less. Furthermore, due to constraints in production technology, the S content is set to 0.0001% or more. The S content is preferably 0.0003% or more, more preferably 0.0005% or more.

[0031] Al: 0.005% or more and 2.000% or less. Al suppresses carbide formation during cooling after annealing and promotes the formation of retained austenite. That is, Al is an element that affects the area fraction of retained austenite. To achieve this effect, the Al content is set to 0.005% or more. On the other hand, if the Al content exceeds 2.000%, the ferrite area fraction increases excessively, making it difficult to achieve a TS of 780 MPa or more. This also results in a decrease in YS. In addition, the C concentration in austenite during annealing increases excessively, making it difficult to achieve the desired bendability at the sheared edge. Therefore, the Al content is set to 0.005% or more and 2.000% or less. The Al content is preferably 0.010% or more. Furthermore, the Al content is preferably 1.000% or less. The Al content is more preferably 0.100% or less, and even more preferably 0.050% or less.

[0032] N: 0.0100% or less N exists as nitrides in steel. In particular, if the N content exceeds 0.0100%, the amount of voids generated increases after shearing the steel sheet, making it impossible to achieve the desired bendability of the sheared edge. Therefore, the N content is set to 0.0100% or less. Furthermore, the N content is preferably 0.0050% or less. Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0005% or more. The N content is more preferably 0.0010% or more, and even more preferably 0.0020% or more.

[0033] Sb: 0.200% or less (including 0%) Sb is a useful element that segregates on the steel sheet surface during annealing to improve platability and chemical conversion treatability. Therefore, 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, and even more preferably 0.009% or more. On the other hand, if the Sb content exceeds 0.200%, the effect of improving platability and chemical conversion treatability saturates, and there is a risk of deterioration in the press formability (internal bendability) and crack propagation resistance of the steel sheet. Therefore, when Sb is contained, the Sb content is set to 0.200% or less. The Sb content is more preferably 0.020% or less. Even more preferably 0.018% or less. The Sb content is more preferably 0.016% or less, and even more preferably 0.014% or less.

[0034] Sn: 0.200% or less (including 0%) Like Sb, Sn is a useful element that segregates on the steel sheet surface during annealing to improve platability and chemical conversion treatability. Therefore, 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, and even more preferably 0.009% or more. On the other hand, if the Sn content exceeds 0.200%, the effect of improving platability and chemical conversion treatability saturates, and there is a risk of deterioration in the press formability (internal bendability) and crack propagation resistance of the steel sheet. Therefore, when Sn is contained, the Sn content must be 0.200% or less. The Sn content is more preferably 0.020% or less, and even more preferably 0.016% or less. The Sn content is more preferably 0.014% or less, and even more preferably 0.012% or less.

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

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

[0037] Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less , Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.100 At least one selected from the group consisting of 0% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less

[0038] Nb: 0.200% or less Nb forms fine carbides, nitrides, or carbonitrides during hot rolling and annealing, thereby increasing TS and YS. To achieve this 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, and even more preferably 0.020% or more. On the other hand, if the Nb content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become crack initiation points during a 90-degree V-bend test, a close bend test, and a close bend + orthogonal 90-degree V-bend test, and the desired bendability and bendability of the sheared edge of the steel sheet may not be achieved. Therefore, when Nb is contained, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.

[0039] Ti: 0.200% or less Like Nb, Ti increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. To achieve this 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. On the other hand, if the Ti content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become crack initiation points during a 90-degree V-bend test, a close bend test, and a close bend + orthogonal 90-degree V-bend test, and the desired bendability and bendability of the sheared edge of the steel sheet may not be achieved. Therefore, 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, and further preferably 0.030% or less.

[0040] V: 0.200% or less Like Nb and Ti, V forms fine carbides, nitrides, or carbonitrides during hot rolling and annealing, thereby increasing TS and YS. To achieve this effect, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. The V content is even more preferably 0.010% or more, and even more preferably 0.020% or more. On the other hand, if the V content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for cracks during a 90-degree V-bend test, a close-contact bending test, and a close-contact bending + orthogonal 90-degree V-bend test, potentially preventing the desired bendability and bendability of the sheared edge of the steel sheet. Therefore, when V is added, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.

[0041] B: 0.0100% or less B is an element that segregates at austenite grain boundaries to improve hardenability. Furthermore, B is an element that suppresses the formation and grain growth of ferrite during cooling after annealing. To achieve this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. The B content is further preferably 0.0005% or more, and even more preferably 0.0007% or more. On the other hand, if the B content exceeds 0.0100%, cracks may occur inside the steel sheet during hot rolling. Furthermore, after shearing the steel sheet, the amount of voids generated may increase, making it difficult to achieve the desired bendability of the sheared edge. Therefore, 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.

[0042] Cr: 1.000% or less. Cr is an element that enhances hardenability, so the addition of Cr produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. To achieve this effect, the Cr content is preferably 0.0005% or more. The Cr content is more preferably 0.010% or more. The Cr content is even more preferably 0.030% or more, and even more preferably 0.040% or more. On the other hand, if the Cr content exceeds 1.000%, the area fraction of hard fresh martensite increases excessively, and fresh martensite becomes the origin of void formation in the 90-degree V-bend test, the close-contact bending test, and the close-contact bending + orthogonal 90-degree V-bend test. As a result, the desired bendability and bendability of the sheared edge of the steel sheet may not be achieved. Therefore, when Cr is added, the Cr content is preferably 1.000% or less. The Cr content is more preferably 0.800% or less, and further preferably 0.700% or less.The Cr content is more preferably 0.100% or less, and further preferably 0.080% or less.

[0043] Ni: 1.000% or less. Ni is an element that improves hardenability, so the addition of Ni produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. To achieve this effect, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more. The Ni content is even more preferably 0.040% or more, and even more preferably 0.060% or more. On the other hand, if the Ni content exceeds 1.000%, the area fraction of fresh martensite increases excessively, and fresh martensite becomes the origin of void formation in the 90-degree V-bend test, the close bend test, and the close bend + orthogonal 90-degree V-bend test. As a result, the desired bendability and bendability of the sheared edge of the steel sheet may not be achieved. Therefore, 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 more preferably 0.600% or less, even more preferably 0.400% or less, and more preferably 0.200% or less.

[0044] Mo: 1.000% or less. Mo is an element that improves hardenability, so adding Mo produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. To achieve this effect, the Mo content is preferably 0.010% or more. The Mo content is more preferably 0.030% or more. On the other hand, if the Mo content exceeds 1.000%, the area fraction of fresh martensite increases excessively, and fresh martensite becomes the origin of void formation in the 90-degree V-bend test, the close bend test, and the close bend + orthogonal 90-degree V-bend test. As a result, the desired bendability and bendability of the sheared edge of the steel sheet may not be achieved. Therefore, when Mo is added, the Mo content is preferably 1.000% or less. The Mo content is more preferably 0.500% or less, even more preferably 0.450% or less, and even more preferably 0.400% or less. The Mo content is more preferably 0.350% or less, even more preferably 0.300% or less, more preferably 0.100% or less, and even more preferably 0.080% or less.

[0045] Cu: 1.000% or less Cu is an element that improves hardenability, so the addition of Cu produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. To achieve this effect, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.008% or more, and even more preferably 0.010% or more. The Cu content is more preferably 0.020% or more. The Cu content is further preferably 0.050% or more, and even more preferably 0.100% or more. On the other hand, if the Cu content exceeds 1.000%, the area fraction of fresh martensite increases excessively, and a large amount of coarse precipitates and inclusions may be produced. In such cases, fresh martensite and coarse precipitates and inclusions may become the origin of void formation during a 90-degree V-bend test, a close bending test, and a close bending + perpendicular 90-degree V-bend test, which may prevent the desired bendability of the steel sheet and bendability of the sheared edge surface from being achieved. Therefore, when Cu is contained, the Cu content is preferably 1.000% or less, and more preferably 0.200% or less.

[0046] Ta: 0.100% or less Like Ti, Nb, and V, Ta increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. Additionally, Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb, Ta)(C, N). This suppresses coarsening of precipitates and stabilizes precipitation strengthening. This further improves TS and YS. To achieve this effect, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.002% or more, and even more preferably 0.004% or more. On the other hand, if the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions may be formed. In such cases, coarse precipitates and inclusions become the starting points for void formation during a 90-degree V-bend test, a close bending test, and a close bending + perpendicular 90-degree V-bend test. As a result, the desired bendability of the steel sheet and bendability of the sheared edge may not be achieved. Therefore, when Ta is contained, the Ta content is preferably 0.100% or less. The Ta content is more preferably 0.090% or less, and even more preferably 0.080% or less. The Ta content is more preferably 0.050% or less, and even more preferably 0.020% or less.

[0047] W: 0.500% or less. W is an element that enhances hardenability. Adding W results in the formation of a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. To achieve this effect, the W content is preferably 0.001% or more. The W content is more preferably 0.020% or more. On the other hand, if the W content exceeds 0.500%, the area fraction of hard fresh martensite increases excessively, and fresh martensite becomes the origin of void formation in the 90-degree V-bend test, the close bend test, and the close bend + orthogonal 90-degree V-bend test. As a result, the desired bendability and bendability of the sheared edge of the steel sheet may not be achieved. Therefore, when W is added, the W content is preferably 0.500% or less. The W content is more preferably 0.450% or less, and even more preferably 0.400% or less. The W content is even more preferably 0.300% or less. The W content is more preferably 0.100% or less, and further preferably 0.050% or less.

[0048] Mg: 0.0200% or less. Mg is an element effective in spheroidizing inclusions such as sulfides and oxides and improving the bendability of the sheared edge. To achieve this effect, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more, and even more preferably 0.0010% or more. The Mg content is more preferably 0.0020% or more, and even more preferably 0.0030% or more. On the other hand, if the Mg content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for void formation during a 90-degree V-bend test, a close-contact bending test, and a close-contact bending + orthogonal 90-degree V-bend test. As a result, the desired bendability of the steel sheet and the bendability of the sheared edge may not be achieved. Therefore, when Mg is contained, the Mg content is preferably 0.0200% or less, more preferably 0.0180% or less, and even more preferably 0.0150% or less, more preferably 0.0100% or less, and even more preferably 0.0080% or less.

[0049] Zn: 0.0200% or less Zn is an element effective in spheroidizing inclusions and improving the bendability of the sheared edge. To achieve this effect, the Zn content is preferably 0.0010% or more. The Zn content is more preferably 0.0020% or more, and even more preferably 0.0030% or more. On the other hand, if the Zn content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for void formation during a 90-degree V-bend test, a close-contact bending test, and a close-contact bending + orthogonal 90-degree V-bend test. As a result, the desired bendability and bendability of the sheared edge may not be achieved. Therefore, when Zn is contained, the Zn content is preferably 0.0200% or less. The Zn content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Zn content is more preferably 0.0100% or less, and further preferably 0.0080% or less.

[0050] Co: 0.0200% or less Like Zn, Co is an effective element for spheroidizing inclusions and improving the bendability of the sheared edge. To achieve this effect, the Co content is preferably 0.0010% or more. The Co content is more preferably 0.0020% or more, and even more preferably 0.0030% or more. The Co content is more preferably 0.0050% or more. On the other hand, if the Co content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for void formation during a 90-degree V-bend test, a close-contact bending test, and a close-contact bending + orthogonal 90-degree V-bend test. As a result, the desired bendability of the steel sheet and the bendability of the sheared edge may not be achieved. Therefore, when Co is contained, the Co content is preferably 0.0200% or less. The Co content is more preferably 0.0180% or less, and further preferably 0.0150% or less.

[0051] Zr: 0.1000% or less Like Zn and Co, Zr is an effective element for spheroidizing inclusions and improving the bendability of the sheared edge. To achieve this effect, the Zr content is preferably 0.0010% or more. On the other hand, if the Zr content exceeds 0.1000%, a large amount of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting point for void formation during a 90-degree V-bend test, a close bend test, and a close bend + perpendicular 90-degree V-bend test. As a result, the desired bendability and bendability of the sheared edge may not be achieved. Therefore, when Zr is contained, the Zr content is preferably 0.1000% or less. The Zr content is more preferably 0.0300% or less, and even more preferably 0.0100% or less. The Zr content is more preferably 0.0050% or less.

[0052] Ca: 0.0200% or less Ca exists as inclusions in steel. Here, if the Ca content exceeds 0.0200%, a large amount of coarse inclusions may be formed. In such cases, the coarse precipitates and inclusions become the starting points for void formation during a 90-degree V-bend test, a close bending test, and a close bending + perpendicular 90-degree V-bend test. As a result, the desired bendability and bendability of the sheared edge of the steel sheet may not be achieved. Therefore, when Ca is contained, the Ca content is preferably 0.0200% or less. The Ca content is preferably 0.0020% or less. While the lower limit of the Ca content is not particularly limited, a Ca content of 0.0005% or more is preferred. Furthermore, due to production technology constraints, a Ca content of 0.0010% or more is more preferred.

[0053] Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, REM: 0.0200% or less. Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are all effective elements for improving the bendability of the sheared edge. To achieve this effect, the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are each preferably 0.0001% or more. On the other hand, when the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM exceed 0.0200% each, and / or when the content of As exceeds 0.0500%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for void formation during a 90-degree V-bend test, a close bending test, and a close bending + orthogonal 90-degree V-bend test, which may prevent the desired bendability and bendability of the sheared edge of the steel sheet from being achieved. Therefore, when at least one of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM is contained, the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM are preferably each 0.0200% or less. When As is contained, the As content is preferably 0.0500% or less.

[0054] The Se content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Se content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Se content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Te content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Te content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Te content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Ge content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Ge content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Ge content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The As content is more preferably 0.0010% or more, and even more preferably 0.0015% or more. The As content is more preferably 0.0100% or more, and even more preferably 0.0150% or more. The As content is more preferably 0.0400% or less, and even more preferably 0.0300% or less. The Sr content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Sr content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Sr content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Cs content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Cs content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Cs content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Hf content is more preferably 0.0005% or more, and even more preferably 0.0008% or more.The Hf content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Hf content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Pb content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Pb content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Pb content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Bi content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Bi content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Bi content is more preferably 0.0100% or less, and even more preferably 0.0050% or less. The REM content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The REM content is more preferably 0.0010% or more, and even more preferably 0.0030% or more. The REM content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The REM content is more preferably 0.0100% or less. In the present invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM concentration in the present invention refers to the total content of one or more elements selected from the above-mentioned REMs. The REM is not particularly limited, but is preferably at least one of Sc, Y, Ce, and La.

[0055] That is, the base steel sheet of the steel sheet of the present invention contains, in mass %, 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 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. and at least one selected from 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 balance being Fe and unavoidable impurities.

[0056] Steel structure (structure at 1 / 4 position in plate thickness of base steel plate) Next, the steel structure of a steel plate according to one embodiment of the present invention will be described. As the structure at 1 / 4 position in plate thickness of the base steel plate, the area fraction of ferrite is 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, the area fraction of retained austenite is 3.5% or more and 10.0% or less, and the area fraction of fresh martensite is 10.0% or less (including 0.0%). The reasons for each limitation will be explained below.

[0057] Ferrite area fraction: 76.5% or less (including 0.0%) Soft ferrite is a phase that improves ductility. However, if the ferrite area fraction increases excessively, it becomes difficult to achieve a TS of 780 MPa or more. This also leads to a decrease in YS. In addition, the C concentration in austenite during annealing increases excessively, making it impossible to achieve the desired bendability of the sheared edge. Therefore, the ferrite area fraction is set to 76.5% or less. The ferrite area fraction is preferably 60.0% or less. The lower limit of the ferrite area fraction is not particularly limited and may be 0.0%. The ferrite area fraction may be 5.0% or more, or 10.0% or more.

[0058] Total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): 20.0% or more and 90.0% or less. Bainitic ferrite and tempered martensite have intermediate hardness between soft ferrite and hard fresh martensite, and are important phases for ensuring good bendability of steel sheets and bendability of sheared edges. Bainitic ferrite is also useful for obtaining an appropriate amount of retained austenite by utilizing the diffusion of C from bainitic ferrite to untransformed austenite. Tempered martensite is effective for improving TS. Therefore, the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is set to 20.0% or more, preferably 30.0% or more. On the other hand, if the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) increases excessively, ductility decreases. Therefore, the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is set to 90.0% or less. The total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is preferably 87.0% or less. It is more preferable that the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is 80.0% or less. Bainitic ferrite is upper bainite with little carbide that is formed in a relatively high temperature range.

[0059] Area fraction of retained austenite: 3.5% or more and 10.0% or less From the viewpoint of obtaining good ductility, the area fraction of retained austenite is set to 3.5% or more. The area fraction of retained austenite is preferably more than 3.5%. On the other hand, if the volume fraction of retained austenite increases excessively, fresh martensite generated by stress-induced transformation during shearing becomes the starting point for void generation, making it impossible to achieve the desired bendability of the sheared edge. Therefore, the area fraction of retained austenite is set to 10.0% or less. The area fraction of retained austenite is preferably 9.0% or less, more preferably 8.0% or less.

[0060] Area fraction of fresh martensite: 10.0% or less (including 0.0%). If the area fraction of fresh martensite is excessively increased, fresh martensite may become the origin of void generation in a 90-degree V-bend test, a close bending test, and a close bending + perpendicular 90-degree V-bend test, and the desired bendability of the steel sheet and the bendability of the sheared edge may not be achieved. From the viewpoint of ensuring good bendability of the steel sheet and the bendability of the sheared edge, the area fraction of fresh martensite is 10.0% or less, preferably 5.0% or less. The lower limit of the area fraction of fresh martensite is not particularly limited and may be 0.0%. Note that fresh martensite refers to as-quenched (untempered) martensite.

[0061] The area ratio of the remaining structure other than the above is preferably 10.0% or less. The area ratio of the remaining structure is more preferably 7.0% or less, and even more preferably 5.0% or less. The area ratio of the remaining structure may be 0.0%. The remaining structure is not particularly limited, and examples thereof include carbides such as lower bainite, pearlite, and cementite. The type of the remaining structure can be confirmed, for example, by observation using a scanning electron microscope (SEM).

[0062] Soft Surface Layer The base steel sheet of the steel sheet according to one embodiment of the present invention preferably has a soft surface layer on the surface of the base steel sheet. The soft surface layer contributes to suppressing bending crack propagation during press forming, further improving the bendability of the steel sheet. The soft surface layer refers to a decarburized layer, and is a surface region having a Vickers hardness of 84% or less of the Vickers hardness of the cross section at 1 / 4 of the sheet thickness. In order to obtain the effect of improving the bendability of the steel sheet, the thickness of the soft surface layer is 20 μm or more. The thickness of the soft surface layer is 120 μm or less. The Vickers hardness is measured based on JIS Z 2244-1 (2020) at a load of 10 gf.

[0063] Soft surface layer thickness (X): 20≦X≦120−3800×[Sb]−1900×[Sn] (1) In formula (1), X is the soft surface layer thickness (μm), and [Sb] and [Sn] are the Sb and Sn contents (mass%) in the steel, respectively. The soft surface layer in the present invention refers to a region in which the Vickers hardness is 84% ​​or less of the Vickers hardness at a position 1 / 4 of the sheet thickness from the surface of the base steel sheet. The soft surface layer thickness (X) must satisfy formula (1). If the soft surface layer thickness (X) is less than 20 μm, the desired bendability intended in the present invention may not be obtained. On the other hand, if the soft surface layer thickness (X) exceeds (120−3800×[Sb]−1900×[Sn]) μm, it is not possible to achieve both the high strength and excellent press formability intended in the present invention. Therefore, the soft surface layer thickness (X) is specified to be 20 μm or more and (120-3800 × [Sb]-1900 × [Sn]) μm or less. In the present invention, as described above, Sb and Sn are added as needed to improve plating ability and chemical conversion treatability. However, when Sb and Sn are added, the surface segregation of these elements as described above reduces the allowable upper limit of the soft surface layer thickness (X) that affects bending cracking. For this reason, the upper limit of the soft surface layer that provides good bendability is (120-3800 × [Sb]-1900 × [Sn]) μm. The soft surface layer thickness is preferably 25 μm or more, more preferably 30 μm or more. The soft surface layer thickness is preferably 100 μm or less, more preferably 90 μm or less.

[0064] Steel structure in the soft surface layer Ferrite area ratio: 50.0% or more and 100.0% or less When subjected to bending, the surface layer is deformed more than the interior. Therefore, voids are more likely to form in the surface layer. In the present invention, by controlling the amount of ferrite in the soft surface layer to 50.0% or more, voids that serve as crack initiation points are less likely to form in the surface layer, and crack propagation is suppressed. The ferrite area ratio in the soft surface layer is preferably 60.0% or more. The ferrite area ratio may be 100.0%. The ferrite area ratio may be 99.9% or less, 95.0% or less, or 90.0% or less.

[0065] Value obtained by dividing the area fraction of fresh martensite by the total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite): 0.5 or less. If the area fraction of fresh martensite in the soft surface layer is excessively increased, fresh martensite becomes the origin of void generation in a 90-degree V-bend test, a contact bending test, and a contact bending + orthogonal 90-degree V-bend test, making it impossible to achieve the desired bendability of the steel sheet. From the viewpoint of ensuring good bendability of the steel sheet and bendability of the sheared edge, when the area fraction of ferrite is less than 100.0%, the value obtained by dividing the area fraction of martensite in the soft surface layer by the area fraction of hard phases other than ferrite is set to 0.5 or less. Here, the hard phases other than ferrite refer to bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite). Note that the lower limit of the value obtained by dividing the area fraction of martensite in the soft surface layer by the area fraction of hard phases other than ferrite is not particularly limited and may be 0.00.

[0066] For example, by controlling the tension during the surface strain introduction step in the manufacturing method described later, the value obtained by dividing the area ratio of fresh martensite in the surface soft layer by the area ratio of hard phases other than ferrite can be suppressed to 0.5 or less. ２ By applying the above tension one or more times, untransformed austenite undergoes a deformation-induced transformation to become fresh martensite, which is then tempered during the second holding step, finally becoming tempered martensite.

[0067] Here, the area ratios of ferrite, bainitic ferrite, tempered martensite, and hard phase (hard second phase (retained austenite + fresh martensite)) in the base steel sheet at a position 1 / 4 of the sheet thickness and in the soft surface layer are measured as follows. The structure of the soft surface layer is measured at a position 1 / 2 of the thickness of the soft surface layer. A sample is cut out from the base steel sheet so that the thickness cross section (L cross section) parallel to the rolling direction of the base steel sheet serves as the observation surface. Next, the observation surface of the sample is mirror-polished using diamond paste. Next, the observation surface of the sample is finish-polished using colloidal silica, and then etched with 3 vol. % nital to reveal the structure. Then, using a scanning electron microscope (SEM) at an acceleration voltage of 15 kV and a magnification of 5000x, three 25.6 μm × 17.6 μm fields of view were photographed at the outermost layer of the specimen (at a position half the thickness of the soft surface layer) and within a range of ±100 μm at a position one-quarter of the plate thickness. The outermost layer was photographed to exclude the galvanized layer and include the internal oxide layer. Ferrite, bainitic ferrite, tempered martensite, and other hard phases (hard second phase (retained austenite + fresh martensite)) were identified from the obtained structural image (see FIG. 1 ) as follows:

[0068] Ferrite: A region that is black and has a blocky shape. It contains almost no iron-based carbides. However, if iron-based carbides are contained, the area of ​​the ferrite includes the area of ​​the iron-based carbides. The same applies to bainitic ferrite and tempered martensite, which will be described later. Bainitic ferrite: A region that is black to dark gray and has a blocky or amorphous shape. It contains no iron-based carbides or a relatively small number of iron-based carbides. Tempered martensite: A region that is gray and has an amorphous shape. It contains a relatively large number of iron-based carbides. Hard second phase (retained austenite + fresh martensite): A region that is white to light gray and has an amorphous shape. It does not contain iron-based carbides. When the size is relatively large, the color gradually darkens as it moves away from the interface with other structures, and the interior may be dark gray. Carbides: White regions with a dotted or linear shape. They are contained within tempered martensite, bainitic ferrite, and ferrite. Remaining structure: Examples include the above-mentioned lower bainite, pearlite, and internal oxides, and the morphology of these is well known.

[0069] Next, the area of ​​each phase identified in the structural image is calculated using the following method. An equally spaced 20 × 20 grid is placed in an area of ​​25.6 μm × 19.2 μm in actual length on the SEM image at 5000x magnification, and the area fractions of ferrite, bainitic ferrite, tempered martensite, and other hard phases (hard second phases) are investigated using the point counting method, which counts the number of points on each phase. The area fraction is calculated as the average of three area fractions determined on separate SEM images at 5000x magnification.

[0070] The area fraction of retained austenite is measured as follows. The base steel sheet is mechanically ground in the thickness direction (depth direction) to a position one-quarter of the sheet thickness, and then chemically polished with oxalic acid to obtain an observation surface. The observation surface is then observed by X-ray diffraction. MoKα rays are used as the incident X-rays, and the ratios of the diffraction intensities of the (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensities of the (200), (211), and (220) planes of bcc iron are calculated, and the volume fraction of retained austenite is calculated from the ratio of the diffraction intensities of each plane. The retained austenite is then considered to be three-dimensionally homogeneous, and the volume fraction of retained austenite is taken as the area fraction of retained austenite.

[0071] 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: [Area fraction of fresh martensite (%)] = [Area fraction of hard second phase (%)] - [Area fraction of retained austenite (%)]

[0072] The area ratio of the remaining structure is determined by subtracting the area ratio of ferrite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of other hard phases (hard second phases) determined as described above from 100.0%: [Area ratio of remaining structure (%)] = 100.0 - [Area ratio of ferrite (%)] - [Area ratio of bainitic ferrite (%)] - [Area ratio of tempered martensite (%)] - [Area ratio of hard second phases (%)]

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

[0074] Tensile strength (TS): 780 MPa or more and less than 1180 MPa The tensile strength TS of a steel plate according to one embodiment of the present invention is 780 MPa or more and less than 1180 MPa. The predetermined yield stress (YS), yield ratio (YR), stretch formability (total elongation (El)) of the steel plate internally, bendability of the steel plate, and bendability of the sheared edge surface of the steel plate according to one embodiment of the present invention are as described above. It is preferable that the ratio YR (yield ratio) of the yield stress YS to the tensile strength TS satisfies 0.70≦YR.

[0075] The tensile strength (TS), yield ratio (YR), yield stress (YS), and total elongation (El) are measured by a tensile test in accordance with JIS Z 2241 (2011), which will be described later in the Examples. The bendability of the steel sheet is measured by a close bending test and a close bending + perpendicular 90-degree V-bend test, which will be described later in the Examples. The bendability of the sheared edge is measured by a 90-degree V-bend test, which will be described later in the Examples.

[0076] Plated Layer (Hot-Dip Galvanized Layer, Galvannealed Hot-Dip Galvanized Layer) A steel sheet according to one embodiment of the present invention may have a plated layer formed on the base steel sheet (on the surface of the base steel sheet), and this plated layer may be provided on only one surface of the base steel sheet, or on both surfaces.

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

[0078] Here, the hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0 mass% or less Fe, and 0.001 mass% to 1.0 mass% Al. The hot-dip galvanized layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% to 3.5 mass%. The Fe content of the hot-dip galvanized layer is more preferably less than 7.0 mass%. The remainder other than the above elements is unavoidable impurities.

[0079] The galvannealed layer is preferably composed of, for example, 20% by mass or less of Fe and 0.001% by mass or more and 1.0% by mass or less of Al. The galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0% by mass or more and 3.5% by mass or less. The Fe content of the galvannealed layer is more preferably 7.0% by mass or more, and even more preferably 8.0% by mass or more. The Fe content of the galvannealed layer is more preferably 15.0% by mass or less, and even more preferably 12.0% by mass or less. The remainder other than the above elements is unavoidable impurities.

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

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

[0082] The thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably 0.5 mm or more, more preferably 0.6 mm or more, and even more preferably 0.8 mm or more. Furthermore, the thickness of the steel plate is preferably 2.3 mm or less, more preferably 1.6 mm or less, and even more preferably 1.2 mm or less.

[0083] 2. Method for Manufacturing Steel Sheet Next, a method for manufacturing a steel sheet according to one embodiment of the present invention will be described.

[0084] A method for producing a steel sheet according to one embodiment of the present invention includes a hot rolling step of hot rolling a steel slab having the above-described composition to obtain 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 with a rolling reduction of 20% or more and 80% or less, and a step of raising the temperature of the steel sheet after the cold rolling step and annealing it at an annealing temperature Ac. １ an annealing step of annealing the steel sheet at a temperature of 100°C or higher and 900°C or lower, an annealing time of 20 seconds or longer, and in an atmosphere with a dew point of -10°C or higher, under conditions satisfying formulas (2) and (3); a cooling step of cooling the steel sheet after the annealing step to a cooling stop temperature of 100°C or higher and 300°C or lower; a first holding step of reheating the steel sheet after the cooling step to a reheating holding temperature range of 370°C or higher and 460°C or lower and holding the temperature for 10 seconds or longer; and a pressure of 2.0 kgf / mm ２ and a second holding step of holding the steel sheet after the surface strain introducing step at 300°C or higher and 460°C or lower for 10 seconds or longer. 2400≦Y≦20000 ... formula (2) Y=[{(T−Ac1)×t1} / 2}]+{(T−Ac1)×t2} ... (3) Where, in formula (3), T is the annealing temperature (°C), t1 is the time (s) from 650°C to the annealing temperature T during the temperature rise in the annealing step, t2 is the annealing time (s), and Ac1 is Ac1 (°C). Ac1 (°C): 727.0 - 32.7 × [%C] + 14.9 × [%Si] + 2.0 × [%Mn], where [%C] is the C content of the steel plate (steel slab), [%Si] is the Si content of the steel plate (steel slab), and [%Mn] is the Mn content of the steel plate (steel slab). Note that the above temperatures refer to the surface temperatures of the steel slab and steel plate unless otherwise specified.

[0085] First, a steel slab having the above-described composition is prepared. For example, a steel material is melted to obtain molten steel having the above-described composition. The melting method is not particularly limited, and known melting methods such as converter melting and electric furnace melting can be used. Next, the obtained molten steel is solidified to obtain a steel slab. The method for obtaining a steel slab from molten steel is not particularly limited, and for example, continuous casting, ingot casting, thin slab casting, etc. can be used. From the viewpoint of preventing macrosegregation, continuous casting is preferred.

[0086] [Hot Rolling Process] Next, the steel slab is hot rolled to produce a hot-rolled steel sheet. Hot rolling may be performed using an energy-saving process. Examples of the energy-saving process include direct rolling (a method in which the steel slab is not cooled to room temperature, but is charged into a heating furnace as a hot slab and hot-rolled) and direct rolling (a method in which the steel slab is briefly kept at a constant temperature and then immediately rolled).

[0087] The hot rolling conditions are not particularly limited, and can be, for example, the following conditions. That is, the steel slab is once cooled to room temperature, and then reheated and rolled. The slab heating temperature (reheating temperature) is preferably 1100°C or higher from the viewpoint of dissolving carbides and reducing the rolling load. Furthermore, in order to prevent an increase in scale loss, the slab heating temperature is preferably 1300°C or lower. The slab heating temperature is based on the temperature of the steel slab surface.

[0088] Next, the steel slab is subjected to rough rolling according to a conventional method to obtain a rough-rolled plate (hereinafter also referred to as a sheet bar). The sheet bar is then subjected to finish rolling to obtain a hot-rolled steel plate. When the slab heating temperature is low, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling in order to prevent problems during finish rolling. The finish rolling temperature is preferably 800°C or higher to reduce the rolling load. Furthermore, if the reduction rate in the unrecrystallized austenite state becomes high, an abnormal structure elongated in the rolling direction may develop, which may reduce the workability of the annealed sheet. Furthermore, by setting the finish rolling temperature to 800°C or higher, the steel structure at the hot-rolled steel plate stage, and ultimately the steel structure of the final product, tends to become uniform. A non-uniform steel structure tends to reduce bendability. On the other hand, if the finish rolling temperature exceeds 950°C, the amount of oxide (scale) generated increases. As a result, the interface between the base steel and the oxide may become rough, which may deteriorate the surface quality of the steel sheet after pickling and cold rolling. Furthermore, the crystal grains may become coarse, which may cause a decrease in the strength and bendability of the steel sheet. For these reasons, the finish rolling temperature is preferably set to a range of 800°C or higher. Furthermore, the finish rolling temperature is preferably set to a range of 950°C or lower.

[0089] After finish rolling, the hot-rolled steel sheet is coiled. The coiling temperature is preferably 450°C or higher, and 750°C or lower.

[0090] Note that the sheet bars may be joined together during hot rolling and continuous finish rolling may be performed. The sheet bars may also be wound up before finish rolling. Furthermore, in order to reduce the rolling load during hot rolling, some or all of the finish rolling may be performed as lubricated rolling. Performing lubricated rolling is also effective from the viewpoint of uniforming the shape and material properties of the steel sheet. Note that the friction coefficient during lubricated rolling is preferably in the range of 0.10 to 0.25. In a hot rolling process (hot rolling process) including rough rolling and finish rolling, a steel slab is generally made into a sheet bar by rough rolling and then made into a hot-rolled steel sheet by finish rolling. However, depending on the mill capacity, etc., such classification is not essential as long as the desired size is achieved.

[0091] [Pickling Process] After the hot rolling process, the hot-rolled steel sheet is pickled. Pickling can remove oxides from the steel sheet surface, ensuring good chemical conversion treatability and plating quality. Pickling may be performed once or multiple times. The pickling conditions are not particularly limited, and may be performed according to conventional methods.

[0092] [Cold Rolling Process] Cold rolling is performed by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling. The reduction ratio (cumulative reduction ratio) of the cold rolling is not particularly limited, but is set to 20% or more and 80% or less. If the reduction ratio of the cold rolling is less than 20%, the steel structure is likely to become coarse and non-uniform during the annealing process, which may result in a decrease in TS and bendability in the final product. On the other hand, if the reduction ratio of the cold rolling exceeds 80%, the steel sheet is likely to have a defective shape and the coating weight may become non-uniform. Optionally, the cold-rolled steel sheet obtained after cold rolling may be pickled.

[0093] [Annealing Step] Next, in one embodiment of the present invention, after the cold rolling step, the steel sheet obtained as described above is heated and annealed in an atmosphere having an annealing temperature of Ac1 (°C) or higher and 900°C or lower, an annealing time of 20 seconds or longer, and a dew point (annealing dew point) of -10°C or higher. Note that the number of annealing steps may be two or more, but one step is preferred from the viewpoint of energy efficiency.

[0094] Annealing temperature: Ac1 (°C) or higher and 900°C or lower. If the annealing temperature is lower than the Ac1 point (°C), the austenite generation rate during heating in the two-phase region of ferrite and austenite becomes insufficient. As a result, the area fraction of ferrite increases excessively after annealing, resulting in a decrease in TS and YS. On the other hand, if the annealing temperature exceeds 900°C, excessive austenite grain growth occurs, the Ms point increases, and a large amount of tempered martensite containing carbides is generated. This makes it difficult to obtain an area fraction of 3.5% or more of retained austenite, resulting in a decrease in ductility. Therefore, the annealing temperature is set to the Ac1 point (°C) or higher and 900°C or lower. The annealing temperature is preferably 880°C or lower. The annealing temperature is the maximum temperature (soaking temperature) reached in the annealing process.

[0095] Ac1 (°C) is calculated by the following formula: Ac1 (°C) = 727.0 - 32.7 × [%C] + 14.9 × [%Si] + 2.0 × [%Mn], where [%C] is the C content of the steel plate (steel slab), [%Si] is the Si content of the steel plate (steel slab), and [%Mn] is the Mn content of the steel plate (steel slab).

[0096] Annealing time (soaking time): 20 seconds or more. If the annealing time is less than 20 seconds, the austenite generation rate during heating in the two-phase region of ferrite and austenite becomes insufficient. As a result, the area ratio of ferrite increases excessively after annealing, resulting in a decrease in TS and YS. In addition, the C concentration in austenite increases excessively during annealing, making it impossible to achieve the desired bendability of the sheared edge. Furthermore, a soft surface layer thickness of 20 μm or more cannot be formed during annealing, making it impossible to achieve the desired bendability of the steel sheet. Therefore, the annealing time is set to 20 seconds or more. The annealing time is preferably 40 seconds or more. The annealing time (soaking time) refers to the holding time in a temperature range from (annealing temperature -40°C) to (annealing temperature). In other words, the annealing time includes not only the holding time at the annealing temperature, but also the residence time in a temperature range from (annealing temperature -40°C) to (annealing temperature) during heating and cooling before and after reaching the annealing temperature.

[0097] 2400≦Y≦20000 ... Formula (2) Y = [{(T - Ac1) × t1} / 2}] + {(T - Ac1) × t2} ... (3) Here, in formula (3), T: annealing temperature (°C), t1: time (s) from 650 °C to the annealing temperature T during the temperature rise in the annealing process, t2: annealing time (s), and Ac1: Ac1 (°C). In the present invention, it is necessary to perform annealing under annealing conditions that satisfy formulas (2) and (3). When Y in formula (3) is less than 2400, the soft surface layer defined in the present invention will be less than 20 μm. On the other hand, when Y is more than 20000, the soft surface layer defined in the present invention will be more than (120 - 3800 × [Sb] - 1900 × [Sn]) μm. Therefore, Y in formula (3) is set to be equal to or greater than 2400 and equal to or less than 20000. t1 is preferably equal to or greater than 30 seconds. Also, t1 is preferably equal to or less than 80 seconds.

[0098] Dew point of the atmosphere (annealing atmosphere) in the annealing step (annealing dew point): -10°C or higher In one embodiment of the present invention, the dew point of the atmosphere (annealing atmosphere) in the annealing step is preferably -10°C or higher. By performing annealing with the dew point of the annealing atmosphere in the annealing step set to -10°C or higher, the decarburization reaction is promoted and a deeper soft surface layer can be formed. The dew point of the annealing atmosphere in the annealing step is preferably -5°C or higher, more preferably 0°C or higher, and even more preferably +10°C or higher. There is no particular upper limit for the dew point of the annealing atmosphere in the annealing step, but due to constraints in production technology, the dew point of the annealing atmosphere in the annealing step is preferably 30°C or lower.

[0099] [Isothermal Holding Step (Preferred Requirement)] After the annealing step, if necessary, during cooling from the annealing temperature, an isothermal holding step may be performed at 400°C or higher and 600°C or lower (hereinafter also referred to as an isothermal holding temperature range) for less than 80 seconds in order to promote bainite transformation. In the isothermal holding step, bainitic ferrite is formed, and C diffuses from the formed bainitic ferrite to untransformed austenite adjacent to the bainitic ferrite. As a result, a predetermined area fraction of retained austenite is secured, and elongation is improved.

[0100] Isothermal holding temperature range: 400°C or higher and 600°C or lower If the isothermal holding temperature is lower than 400°C, lower bainite and martensite containing a large amount of carbide are generated, which suppresses the diffusion of C into the untransformed austenite, and there is a risk that the area ratio of the predetermined amount of retained austenite cannot be secured. On the other hand, if the isothermal holding temperature exceeds 600°C, the untransformed austenite transforms to pearlite, which may result in the failure to secure TS and ductility. Therefore, the isothermal holding temperature is preferably set to 400°C or higher and 600°C or lower.

[0101] Holding time in isothermal holding temperature range: less than 80 seconds If the holding time in the isothermal holding temperature range is 80 seconds or more, the area ratio of bainitic ferrite will increase excessively, and the C concentration in the untransformed austenite will increase excessively, which may make it difficult to achieve the desired bendability of the sheared edge. Therefore, it is preferable that the holding time in the isothermal holding temperature range be less than 80 seconds.

[0102] [Cooling step (first cooling step)] Next, 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 of the atmosphere is preferably −20° C. or less.

[0103] Cooling stop temperature: 100°C or higher and 300°C or lower; Average cooling rate: 10°C / s or higher and 50°C / s or lower; Atmospheric dew point: -20°C or lower (preferred requirements). In the cooling process, the steel sheet after the annealing process is cooled to a cooling stop temperature of 100°C or higher and 300°C or lower. The cooling start temperature can be Ac1 (°C) or higher and 900°C or lower, and if an isothermal holding process is performed, the cooling start temperature can be 400°C or higher and 600°C or lower. The cooling process is necessary to control the area fraction of tempered martensite and the area fraction of retained austenite generated in the subsequent first holding process (reheating holding process) within predetermined ranges. Here, if the cooling stop temperature is lower than 100°C, almost all of the untransformed austenite present in the steel will be transformed into martensite during the cooling process. This ultimately results in an excessive increase in the area fraction of tempered martensite, making it difficult to obtain an area fraction of retained austenite of 3.5% or higher, and reducing ductility. On the other hand, if the cooling stop temperature exceeds 300°C, the area fraction of tempered martensite decreases and the area fraction of fresh martensite increases. As a result, fresh martensite becomes the origin of void formation in the 90° V-bend test, the close bending test, and the close bending + perpendicular 90° V-bend test, making it impossible to achieve the desired bendability and bendability of the sheared edge of the steel sheet. Therefore, the cooling stop temperature is set to 100°C or higher and 300°C or lower. The cooling stop temperature is preferably 120°C or higher. The cooling stop temperature is also preferably 280°C or lower. The average cooling rate during this cooling process is preferably 10°C / s or higher. The average cooling rate during this cooling process is also preferably 50°C / s or lower. The metallic phase defined in the present invention can be obtained through this cooling process. Here, if the average cooling rate is less than 10°C / s, the amount of untransformed austenite that is completely transformed into martensite increases during the cooling process, making it difficult to finally obtain an area fraction of 3.5% or more of retained austenite, which may result in reduced ductility. On the other hand, if the average cooling rate exceeds 50°C / s, self-relaxation during martensitic transformation is suppressed, and the sheet shape may deteriorate. In addition, it is preferable that the dew point of the atmosphere in this cooling step is -20°C or less.If the dew point of the atmosphere exceeds -20°C, the thickness of the soft surface layer in the in-plane direction of the steel sheet will vary greatly, and the tensile strength specified in the present invention may not be obtained. For these reasons, it is preferable that the dew point of the atmosphere in this cooling step be -20°C or less. The average cooling rate (°C / s) is obtained by dividing the difference between the cooling start temperature (°C) and the cooling stop temperature (°C) in the cooling step by the cooling time (s).

[0104] [First Holding Step (First Reheating Holding Step)] Next, in the first holding step (first reheating holding step), the steel sheet is reheated to a temperature range of 370°C or more and 460°C or less (also referred to as a reheating holding temperature range, but hereinafter also referred to as a first reheating holding temperature range to distinguish it from the reheating holding temperature range of the second holding step), and held for 10 seconds or more.

[0105] Reheating temperature (first reheating temperature): 370°C or higher and 460°C or lower. In the reheating process, carbon is enriched in the austenite remaining after the cooling process, thereby reducing the area fraction of fresh martensite in the final structure while ensuring a predetermined amount of retained austenite. If the reheating temperature (first reheating temperature) is lower than 370°C, carbon enrichment in the austenite remaining after the cooling process is insufficient, making it difficult to obtain retained austenite at an area fraction of 3.5% or higher, resulting in reduced ductility. On the other hand, if the reheating temperature (first reheating temperature) exceeds 460°C, carbon is excessively enriched in the untransformed austenite, and the untransformed austenite in the surface layer does not undergo strain-induced transformation in the surface strain introduction process described below, becoming retained austenite or fresh martensite. Furthermore, in the soft surface layer, the value obtained by dividing the area fraction of fresh martensite by the total area fraction of bainitic ferrite, fresh martensite, and tempered martensite exceeds 0.5. Therefore, the reheating holding temperature (first reheating holding temperature) is set to 370°C or higher and 460°C or lower.

[0106] Holding time in the reheating holding temperature range (first reheating holding temperature range): 10 seconds or more If the holding time in the reheating holding temperature range is less than 10 seconds, the concentration of C in the austenite remaining after the cooling step will be insufficient, making it difficult to obtain retained austenite with an area fraction of 3.5% or more, which may result in a decrease in ductility. Therefore, the holding time in the first reheating holding temperature range is set to 10 seconds or more.

[0107] [Surface Strain Inducing Step] In the surface strain introducing step, a pressure of 2.0 kgf / mm was applied between the first holding step (reheating holding step) and the second holding step. ２ By applying a tension of 2.0 kgf / mm or more, strain is introduced into the surface layer. ２ By applying the above tension one or more times, the untransformed austenite in the surface structure of the steel sheet undergoes a deformation-induced transformation to martensite, which then transforms into tempered martensite in the subsequent second holding step. As a result, the desired bendability of the steel sheet is obtained. Here, the tension is calculated by multiplying the total load (kgf) of the load cells on the left and right sides of the rolls through which the steel sheet passes while in contact with the roll by the cross-sectional area of ​​the steel sheet (= sheet thickness (mm) × sheet width (mm)) (mm ２ ) and the tension is obtained by dividing the load cell by the tension direction. The load cell must be placed parallel to the tension direction. The load cell is preferably placed 200 mm from both ends of the roll. The length of the roll body is preferably 1500 to 2500 mm. The tension is preferably 2.2 kgf / mm ２ More preferably, 2.4 kgf / mm ２ The tension is preferably 15.0 kgf / mm or more. ２ More preferably, it is 10.0 kgf / mm or less. ２ Here, the unit of tension is 1 kgf / mm ２ to 9.8 N / mm ２ As kgf / mm ２ to N / mm ２ can be converted into

[0108] [Second Holding Step] Next, in the second holding step, the steel sheet is held at 300° C. or higher and 460° C. or lower for 10 seconds or longer. Note that the holding here also includes cooling (slow cooling) within the range of 300° C. or higher and 460° C. or lower for 10 seconds or longer.

[0109] Second holding temperature (reheating holding temperature range (second reheating holding temperature range)): 300°C or higher and 460°C or lower In the second holding step, the martensite formed in the surface layer in the surface strain introduction step is tempered. As a result, the value obtained by dividing the area ratio of fresh martensite in the surface layer by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or lower, and the desired bendability of the steel sheet is obtained. If the second holding temperature is lower than 300°C, the martensite formed in the surface layer in the surface strain introduction step is not tempered, and the value obtained by dividing the area ratio of fresh martensite in the surface layer by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite exceeds 0.5. On the other hand, if the second holding temperature exceeds 460°C, the retained austenite inside the steel sheet decomposes, and the desired El is not obtained. Therefore, the second holding temperature (second reheating holding temperature range) is set to 300°C or higher and 460°C or lower.

[0110] Holding time at second holding temperature (second reheating holding temperature range): 10 seconds or more If the holding time at the second holding temperature (reheating holding temperature range: 300°C or more and 460°C or less) is less than 10 seconds, the martensite formed in the surface layer in the surface strain introduction step will not be sufficiently tempered, and the value obtained by dividing the area ratio of fresh martensite in the surface layer by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite will exceed 0.5. Therefore, the holding time in the reheating holding temperature range is set to 10 seconds or more.

[0111] [Plating process (hot-dip galvanizing process, galvannealed hot-dip galvanizing process)] Next, in the plating process, the steel sheet is subjected to a galvanizing process to produce a galvanized steel sheet. Examples of galvanizing processes include a galvanizing process and a galvannealed hot-dip galvanizing process. The galvanizing process in the plating process is performed after the annealing process. The galvanizing process may be performed, for example, during the cooling process, during the first holding process, after the first holding process and before the surface layer strain introducing process, after the surface layer strain introducing process and before the second holding process, during the second holding process, or after the second holding process.

[0112] In the case of hot-dip galvanizing treatment, it is preferable to immerse the steel sheet in a zinc plating bath (hot-dip galvanizing bath) at 440° C. or more and 500° C. or less, and then adjust the coating weight by gas wiping, etc. The hot-dip galvanizing bath is not particularly limited as long as it provides the above-mentioned composition of the zinc plating layer, but it is preferable to use, for example, a plating bath having a composition in which the Al content is 0.10 mass % or more and 0.23 mass % or less, with the balance consisting of Zn and unavoidable impurities.

[0113] In the case of alloying hot-dip galvanizing treatment, it is preferable to perform the hot-dip galvanizing treatment as described above, and then heat the hot-dip galvanized steel sheet to an alloying temperature of 450°C or higher and 600°C or lower to perform the alloying treatment. If the alloying temperature is lower than 450°C, the Zn-Fe alloying rate will be slow, and alloying may become difficult. On the other hand, if the alloying temperature exceeds 600°C, untransformed austenite will transform to pearlite, and ductility will decrease. The alloying temperature is more preferably 480°C or higher. The alloying temperature is more preferably 550°C or lower.

[0114] The coating weight of both the hot-dip galvanized steel sheet (GI) and the galvannealed steel sheet (GA) was 20 g / m per side. ２ In addition, the coating weight of both the hot-dip galvanized steel sheet (GI) and the galvannealed steel sheet (GA) is preferably 80 g / m per side. ２ It is preferable that the plating thickness is set to the following: The plating thickness can be adjusted by gas wiping or the like.

[0115] [Second Cooling Step (Preferable Condition)] Next, the steel sheet is preferably cooled to a second cooling stop temperature of 50°C or lower.

[0116] Second cooling stop temperature: 50°C or less The cooling conditions for the final cooling step are not particularly limited and may be conventional. Examples of cooling methods that can be used include gas jet cooling, mist cooling, roll cooling, water cooling, and air cooling. From the viewpoint of preventing surface oxidation, cooling to 50°C or less is preferred, and cooling to room temperature is more preferred. The average cooling rate is preferably, for example, 1°C / sec or more and 50°C / sec or less.

[0117] The steel sheet obtained as described above may further be subjected to temper rolling. If the temper rolling reduction exceeds 2.00%, the yield stress increases, which may result in a decrease in dimensional accuracy when the steel sheet is formed into a component. Therefore, the temper rolling reduction is preferably 2.00% or less. The lower limit of the temper rolling reduction is not particularly limited, but it is preferably 0.05% or more from the viewpoint of productivity. Furthermore, temper rolling may be performed on an apparatus continuous with the annealing apparatuses used for the above-mentioned processes (online), or may be performed on an apparatus discontinuous with the annealing apparatuses used for the above-mentioned processes (offline). Furthermore, the number of times temper rolling may be one, two, or more. Furthermore, rolling using a leveler or the like may be used as long as it can impart an elongation rate equivalent to that of temper rolling.

[0118] Conditions other than those described above are not particularly limited and may be performed in the usual manner. From the viewpoint of productivity, it is preferable that the series of treatments such as the annealing, hot-dip galvanizing, and alloying treatment of galvanizing described above be performed in a CGL (Continuous Galvanizing Line), which is a hot-dip galvanizing line. After hot-dip galvanizing, wiping can be performed to adjust the coating weight of the coating. Note that conditions for plating and the like other than those described above may be in the usual manner for hot-dip galvanizing.

[0119] [3. Member] Next, a member according to one embodiment of the present invention will be described. The member according to one embodiment of the present invention is a member made using (using as a raw material) the above-mentioned steel plate. For example, the raw material steel plate is subjected to at least one of forming and joining to form a member. Here, the above-mentioned steel plate has a TS of 780 MPa or more and less than 1180 MPa, and also has a high YS, excellent press formability within the steel plate (bendability and stretch formability of the steel plate), and excellent press formability at the steel plate end (bendability of the steel plate end (shear cross section)). Therefore, the member according to one embodiment of the present invention has high strength and excellent press formability. Therefore, the member according to one embodiment of the present invention is particularly preferably applied to impact energy absorbing members used in the automotive field.

[0120] [4. Manufacturing Method of Member] Next, a manufacturing method of a member according to one embodiment of the present invention will be described. The manufacturing method of a member according to one embodiment of the present invention includes a step of subjecting the above-mentioned steel plate (for example, a steel plate manufactured by the above-mentioned manufacturing method of steel plate) to at least one of forming and joining to form a member. Here, the forming method is not particularly limited, and for example, a general processing method such as press working can be used. Furthermore, the joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, rivet joining, caulking joining, etc. can be used. Note that the forming conditions and joining conditions are not particularly limited, and may be in accordance with conventional methods.

[0121] A steel material having the chemical composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and formed into a steel slab by a continuous casting method. In Table 1, "-" indicates the content of unavoidable impurities, which is treated as 0 (zero). The calculated transformation temperature Ac1 point (°C) shown in Table 1 was calculated using the following formula: Ac1 point (°C) = 727.0 - 32.7 × [%C] + 14.9 × [%Si] + 2.0 × [%Mn], where [%C] is the C content of the steel plate (steel slab), [%Si] is the Si content of the steel plate (steel slab), and [%Mn] is the Mn content of the steel plate (steel slab).

[0122] The obtained steel slab was heated to 1200°C, and after heating, the steel slab was subjected to hot rolling consisting of rough rolling and finish rolling at a finish rolling temperature of 900°C to obtain a hot-rolled steel sheet. Next, the obtained hot-rolled steel sheet was subjected to pickling and cold rolling (reduction rate: 50%) to obtain a cold-rolled steel sheet having a thickness shown in Table 3. Next, the obtained cold-rolled steel sheet was subjected to treatments in an annealing step, an isothermal holding step, a cooling step, a first holding step (reheating holding step), a surface strain introduction step, and a second holding step under the conditions shown in Table 2, and was also subjected to treatment in a plating step (hot-dip galvanizing step or alloyed hot-dip galvanizing step) as necessary to obtain a steel sheet. Note that the plating step was performed in No. 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, the cooling was performed after the first holding step and before the surface strain introduction step, and 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, the cooling was performed during the cooling step.

[0123] Here, in the coating process, a hot-dip galvanizing treatment or a galvannealed hot-dip galvanizing treatment was performed to obtain a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or a galvannealed hot-dip galvanized steel sheet (hereinafter also referred to as GA). In Table 2, the type of coating process is also indicated as "GI" or "GA". Furthermore, steel sheets that have not been subjected to either a hot-dip galvanizing treatment or a galvannealed hot-dip galvanizing treatment are indicated as "CR". In Table 2, in the case of CR steel sheets or GI steel sheets, the alloying temperature is indicated as - because no alloying treatment is performed.

[0124] The galvanizing bath temperature was 470°C for both GI and GA production. The galvanizing coating weight was 45 to 72 g / m per side for GI production. ２ When manufacturing GA, the thickness is 45 g / m per side. ２The composition of the plating layer (galvanized layer) of the finally obtained steel sheet was as follows: for GI, it contained 0.1 to 1.0 mass% Fe, 0.2 to 0.33 mass% Al, and the balance being Zn and unavoidable impurities. For GA, it contained 8.0 to 12.0 mass% Fe, 0.1 to 0.23 mass% Al, and the balance being Zn and unavoidable impurities. In all cases, the plating layer (galvanized layer) was formed on both sides of the base steel sheet.

[0125] The steel structure of the obtained steel sheet was identified in the same manner as described above. The measurement results are shown in Table 3. As shown in Figure 1, F represents ferrite, BF represents bainitic ferrite, TM represents tempered martensite, RA represents retained austenite, and FM represents fresh martensite. In Table 3, LB represents lower bainite, and θ represents carbide.

[0126] The soft surface layer was measured as follows: After smoothing the thickness cross section (L cross section) of the steel sheet parallel to the rolling direction by wet polishing, the hardness was measured using a Vickers hardness tester under a load of 10 gf (9.8 × 10 －２ Measurements were made at 1 μm intervals from a position 1 μm from the steel plate surface in the thickness direction to a position 100 μm in the thickness direction using a 100 μm (N) pressure. Measurements were then made at 20 μm intervals up to the center of the plate thickness. The region where the Vickers hardness was reduced to 84% or less compared to the hardness at the 1 / 4 position in the plate thickness was defined as the soft layer (surface soft layer), and the thickness of this region in the thickness direction was defined as the thickness of the soft layer.

[0127] The structure of the soft surface layer was identified at a position halfway through the thickness of the soft surface layer by the same method as used to identify the steel structure of the base steel sheet.

[0128] Further, a tensile test, a 90-degree V-bend test, a contact bending test, and a contact bending + perpendicular 90-degree V-bend test were carried out in the following manner, and the tensile strength (TS), yield stress (YS), yield ratio (YR), total elongation (El), bendability of the steel sheet, and bendability of the sheared edge were evaluated in accordance with the following criteria.

[0129] TS (tensile strength) ◯ (pass): 780 MPa or more and less than 1180 MPa × (fail): less than 780 MPa or 1180 MPa or more

[0130] YS (Yield Stress) ◯ (Pass): (A) When 780MPa≦TS<980MPa, 550MPa≦YS (B) When 980MPa≦TS<1180MPa, 700MPa≦YS × (Fail): (A) When 780MPa≦TS<980MPa, 550MPa>YS (B) When 980MPa≦TS<1180MPa, 700MPa>YS

[0131] El (stretch formability inside the steel sheet) ◯ (Pass): (A) When 780MPa≦TS<980MPa, 17.0%≦El (B) When 980MPa≦TS<1180MPa, 11.0%≦El × (Fail): (A) When 780MPa≦TS<980MPa, 17.0%>El (B) When 980MPa≦TS<1180MPa, 11.0%>El

[0132] - A 90-degree V-bend test was performed with a bending radius of 0.5 mm, and the length of the crack that propagates along the bend ridge formed other than the end of the bend ridge (crack length other than the V-bend end face) (bendability of steel plate) ○ (Pass): The crack length other than the V-bend end face is 200 μm or less × (Fail): The crack length other than the V-bend end face is more than 200 μm

[0133] A 90-degree V-bend test was performed with a bending radius of 0.5 mm, and the crack length that propagated from the end of the bend ridge in the ridge direction (V-bend end crack length) (bendability of the steel plate end (shear cross section)) ○ (Pass): V-bend end crack length is 200 μm or less × (Fail): V-bend end crack length is more than 200 μm

[0134] ・A close contact bending test was performed to determine the spacer thickness limit at which cracks of 0.5 mm or more did not occur along the bending ridgeline (close contact bending boundary spacer thickness) (bendability of steel plate). 〇 (Pass): Close contact bending boundary spacer thickness is 3.0 mm or less × (Fail): Close contact bending boundary spacer thickness is more than 3.0 mm

[0135] A close contact bending test was performed with a 3.0 mm spacer, and the crack depth (close contact bending internal crack depth) that progressed in the plate thickness direction at the bending ridge line subjected to compressive stress (bendability of steel plate) ○ (Pass): Close contact bending internal crack depth is 200 μm or less × (Fail): Close contact bending internal crack depth is more than 200 μm

[0136] - Conduct a close bending + perpendicular 90 degree V bending test, and determine the bending radius at which cracks of 0.5 mm or more do not occur along the bending ridge (handkerchief bending boundary bending radius) (bendability of steel plate) ○ (Pass): Handkerchief bending boundary bending radius is 5.0 mm or less × (Fail): Handkerchief bending boundary bending radius is more than 5.0 mm

[0137] (1) Tensile Test The tensile test was conducted in accordance with JIS Z 2241 (2011). That is, a JIS No. 5 test piece was taken from the obtained steel sheet at a quarter position of the coil width so that the longitudinal direction was perpendicular to the rolling direction of the base steel sheet. Using the taken test piece, a tensile test was conducted at a crosshead speed of 10 mm / min, and TS, YS, YR, and El were measured. The results are shown in Table 4.

[0138] (2) 90° V-bending test: A 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) strip test piece was taken from the 1 / 4 position of the coil width of the obtained steel sheet. The 100 mm long end face was sheared, and the sheared state (without machining to remove burrs) was bent so that the burrs were on the outer periphery of the bend. The shear clearance was 15% and the rake angle was 0 degrees. V-bending was performed using an autograph manufactured by Shimadzu Corporation, with a punch bending radius of R = 0.5 mm, a punch bending angle of 90 degrees, a punch stroke speed of 30 mm / min, a pressing load of 10 tons, and a pressing time of 5 seconds, resulting in an L-direction bending (bending ridge length: 30 mmL).

[0139] An example of a sample after the 90-degree V-bend test with a bending radius of 0.5 mm is shown in Figure 2. Figure 2(b) is an overhead view of the sample viewed from the Z direction shown in Figure 2(a). When the bend ridgeline is defined as a section extending from the bend apex along the steel sheet surface and extending 5 mm in the C direction (2.5 mm on both sides from the bend apex), the bend ridgeline end is defined as a section (region o) extending 5 mm in the L direction from the very end of the bend ridgeline. The crack length Y1 propagating from the bend ridgeline end in the ridgeline direction (L direction) and the crack length Y2 propagating in the L direction along the bend ridgeline formed other than the bend ridgeline end are each measured using the following methods.

[0140] After the 90-degree V-bend test with a bending radius of 0.5 mm, the crack length propagating from the end of the bend ridge toward the ridge was measured as follows. Figure 3-1(a) shows the crack at the end of the bend ridge of a sample after the V-bend test. When measuring the length of a crack at the center of the bend ridge, it is common to observe the plate surface (plane b) from the Z direction. Because the sample after the actual V-bend test has a saddle shape as shown in Figure 3-1(b), plane b is significantly deformed, reducing the accuracy of the crack length measurement and potentially preventing accurate evaluation of the bendability of the sheared edge. In the present invention, accurate measurements can be achieved by using the following measurement method. The symbol y in Figure 3-1(a) corresponds to symbol Y1 (crack length Y1) in Figure 2(b). The sheared surface a of the bent sample after the 90-degree V-bend test with a bending radius of 0.5 mm was placed facing up, and the end of the bend ridge was photographed at 40x magnification using a one-shot 3D shape measuring instrument (Keyence Corporation, VR6000 series or newer models). The obtained height data was analyzed using the analysis software attached to the one-shot 3D shape measuring instrument. As shown in Figure 3-2(a), an arc-shaped measurement line i was drawn as close as possible to the outside of the bent portion subjected to tensile stress, in line with the bend ridgeline. An example of the obtained profile waveform j is shown in Figure 3-2(b). The length of each crack (y1 + y2) / 2 was determined using the measurement tool in the software, and the length of the longest crack was taken as the crack length propagating from the end of the bend ridgeline toward the ridgeline after a 90-degree V-bend test with a bending radius of 0.5 mm.

[0141] After the 90° V-bend test with a bending radius of 0.5 mm, the length of cracks propagating along the bend ridge formed at other than the end of the bend ridge was measured by visual observation at 25x magnification using a stereomicroscope.

[0142] (3) Contact Bending Test A 60 mmC x 30 mmL test piece was taken from the 1 / 4 coil width position of the obtained steel plate. After finishing both end faces of the 60 mm length by grinding, a primary bending process (U-bending) was performed to prepare a test piece for contact bending. The U-bending was performed using a hydraulic bending tester with a punch bending radius of R = 5.0 mm, a stroke speed of 10 mm / s, and C-direction bending (bending ridge length: 30 mmL), so that no cracks occurred in any of the test materials. Next, contact bending was performed on the test piece after U-bending. The contact bending was performed using a hydraulic bending tester. As shown in Figure 4-1(a), a spacer q (plate thickness Z1) was inserted as necessary, and the stroke speed was 10 mm / min, the pressing load was 10 ton, and the pressing time was 3 seconds, so that the bending ridge of the test piece after U-bending was perpendicular to the pressing direction. In samples subjected to a tight bending test, if the area of ​​width Z2 (Z2 = (2t + Z1) × π / 2, where t is the sample thickness) from the bend apex along the steel plate surface on both sides in the circumferential direction (C direction) is defined as the outside of the bend ridgeline, the minimum spacer plate thickness at which the L-direction length Y3 of a crack extending in the L direction outside the bend ridgeline is less than 0.5 mm (no cracks of 0.5 mm or more occur) was defined as the critical spacer plate thickness for cracking. The crack length outside the bend ridgeline was measured by visual observation using a stereomicroscope at 25x magnification.

[0143] In addition, after a 3.0 mm spacer contact bending test (a 3.0 mm spacer thickness test), as shown in FIG. 4-1(b), a region Z3 (9 mm) from the bend apex along the steel sheet surface on both sides of the circumferential direction (C direction) on the side subjected to compressive stress (the inner surface of the sample) (see region s) was measured as the inside of the bend ridge. The crack depth propagating in the thickness direction on the inside of the bend ridge was measured as follows. As shown in FIG. 4-2(c), a specimen was cut from the sample after the contact bending so that the cross section k at 1 / 2 the L direction was the observation surface. Next, the observation surface of the specimen was mirror-polished using diamond paste. Then, using a SEM (Scanning Electron Microscope), under conditions of an acceleration voltage of 15 kV and a magnification of 50x, a field of view of 2560.0 μm × 1920.0 μm (m in FIG. 4-2 (d)) was photographed at the position m in FIG. 4-2 (d), which is the bending apex of the observation surface of the sample, and the entire crack was observed. In the obtained crack image, the distance X between the start and end points of the crack was taken as the crack depth. This X was evaluated as the crack depth that propagated in the plate thickness direction at the bending ridge subjected to compressive stress, by performing a close contact bending test with a 3.0 mm spacer.

[0144] (4) Close Bending + Perpendicular 90-Degree V-Bend Test Using the sample after the close bending test with a 3.0 mm spacer, a V-bend test in the L direction (perpendicular 90-degree V-bend test) was performed. When the r region (Z2 region in the figure) in Figure 4-1(a) after V-bending is the bend ridgeline after handkerchief bending, the bend radius at which cracks extending in the L direction at the bend ridgeline do not occur and which are 0.5 mm or longer in the L direction was defined as the crack limit bend radius (handkerchief bend boundary bend radius). The crack length of the bend ridgeline was measured by visual observation using a stereomicroscope at 25x magnification.

[0145]

[0146]

[0147]

[0148]

[0149] In Tables 1 to 4, underlined values ​​indicate values ​​outside the appropriate ranges of the present invention. As shown in Table 4, all of the inventive examples passed the tensile strength (TS), yield stress (YS), and total elongation (El), and the crack length other than at the V-bend end face, the crack length at the V-bend end face, the thickness of the spacer for tight bending, the crack depth inside the tight bending, and the bending radius at the boundary between the handkerchief and the bend were all within the specified ranges. On the other hand, in the comparative examples, at least one of the tensile strength (TS), yield stress (YS), total elongation (El), the crack length other than at the V-bend end face, the crack length at the V-bend end face, the thickness of the spacer for tight bending, the crack depth inside the tight bending, and the bending radius at the boundary between the handkerchief and the bend was insufficient.

[0150] F Ferrite FM Fresh martensite RA Retained austenite BF Bainitic ferrite TM Tempered martensite

Claims

1. A base steel plate having a component composition containing, in mass%, the following: 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 (inclusive), and Sn: 0.200% or less (inclusive), with the balance being Fe and unavoidable impurities; and a soft surface layer having a Vickers hardness of 84% or less of the Vickers hardness at a 1 / 4 sheet thickness position from the surface of the base steel plate, wherein the soft surface layer satisfies the following formula (1), a structure in the soft surface layer, the area ratio of ferrite is 50.0% or more and 100.0% or less; and when the area ratio of ferrite is less than 100.0%, a value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less; a structure at 1 / 4 of the sheet thickness of the base steel sheet, the area ratio of ferrite is 76.5% or less (including 0.0%), the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is 20.0% or more and 90.0% or less, the area ratio of retained austenite is 3.5% or more and 10.0% or less, and the area ratio of fresh martensite is 10.0% or less (including 0.0%); and a tensile strength is 780 MPa or more and less than 1,180 MPa. 20≦X≦120−3800×[Sb]−1900×[Sn] (1) In formula (1), X is the thickness (μm) of the surface soft layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.

2. The composition further includes, in mass%, Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, The steel plate according to claim 1, containing at least one selected from the group consisting of 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 one or both sides of the base steel sheet have a plating layer, the plating layer being a hot-dip galvanized layer.

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

5. A member made using the steel plate according to any one of claims 1 to 4.

6. A hot rolling process in which a steel slab having the composition according to claim 1 or 2 is hot-rolled to obtain a hot-rolled steel sheet; a pickling process in which the hot-rolled steel sheet is pickled after the hot rolling process; a cold rolling process in which the steel sheet after the pickling process is cold-rolled with a rolling reduction of 20% to 80%; an annealing process in which the steel sheet after the cold rolling process is heated and annealed under conditions satisfying formulas (2) and (3) at an annealing temperature of Ac1 (°C) to 900°C, an annealing time of 20 seconds or more, and an atmosphere with a dew point of -10°C or more; a cooling process in which the steel sheet after the annealing process is cooled to a cooling stop temperature of 100°C to 300°C; and a first holding process in which the steel sheet after the cooling process is reheated to a reheat holding temperature range of 370°C to 460°C and held for 10 seconds or more. The steel sheet after the first holding step is subjected to a strength of 2.0 kgf / mm in the reheating holding temperature range. ２ and a second holding step of holding the steel sheet after the surface strain introducing step at 300°C or more and 460°C or less for 10 seconds or more. 2400≦Y≦20000 ... (2) Y = [{(T-Ac1)×t1} / 2}] + {(T-Ac1)×t2} ... (3) In formula (3), T is annealing temperature (°C), t1 is time (s) from 650°C to annealing temperature T during temperature increase in the annealing step, t2 is annealing time (s), and Ac1 is Ac1 (°C).

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

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

9. A method for manufacturing a component, comprising the step of subjecting the steel plate according to any one of claims 1 to 4 to at least one of forming and joining to form the component.