Steel plates, components, and their manufacturing methods

Abstract

Provided is a steel sheet having a TS of 780 MPa or more and less than 1,180 MPa and having high YS and excellent press formability of the inside of the steel sheet and at an edge of the steel sheet.　This steel sheet has a base steel sheet having a predetermined composition, and a surface soft layer having a Vickers hardness of 84% or less with respect to the 1 / 4 position of the sheet thickness from the surface of the base steel sheet and satisfying formula (1), wherein the surface soft layer has a structure within a specific range, the structure at the 1 / 4 position of the sheet thickness of the base steel sheet is within a specific range, and the steel sheet has a tensile strength of 780 MPa or more and less than 1,180 MPa.　Formula (1): 20≤X≤120-3800×[Sb]-1900×[Sn], provided that in formula (1), X is the thickness (μm) of the surface soft layer; and [Sb] and [Sn] are the content (mass%) of Sb and Sn in the steel, respectively.

Description

[Technical Field]

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

[0002] 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 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 a ferrite phase with a volume fraction of 40 to 90% and a retained austenite phase with a volume fraction of 3 to 25%, the retained austenite phase has a solute carbon content of 0.70 to 1.00%, an average particle size of 2.0 μm or less, an average distance between particles of 0.1 to 5.0 μm, a decarburized layer 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 3 to 7% plastic deformation, and that has high ductility while maintaining a high strength of 900 MPa or more in maximum tensile strength and excellent mechanical cutting properties.

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

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

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

[0008] [Patent Document 1] Patent No. 5354135 [Patent Document 2] Patent No. 5352793 [Patent Document 3] Patent No. 6536294 [Patent Document 4] Patent No. 5256689 Summary of the Invention [Problem to be solved by the invention]

[0009] In recent years, steel sheets with a tensile strength (hereinafter also referred to as TS) of 780 MPa or more have been increasingly used in automobile impact energy absorbing components, 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 such steel sheets with increased TS and YS to the aforementioned automotive impact energy absorption components, press forming becomes difficult, and variations during forming result in reduced yields. In particular, reduced press formability at the steel sheet edges can lead to end-face cracking in the actual components.

[0011] Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet in which ductility is improved by the generation of retained austenite inside the steel sheet and mechanical cutting properties are improved by the formation of a decarburized layer on the surface of the steel sheet. However, no consideration is given at all to the improvement in bendability and fracture resistance in the event of a vehicle collision that are achieved by the formation of a soft surface layer (decarburized layer), and the press formability of the steel sheet end portions. Patent Document 2 discloses a high-strength hot-dip galvanized steel sheet in which the main structure inside the steel sheet is soft ferrite and the amount of unrecrystallized ferrite is limited to a small amount, thereby improving ductility, and the formation of a decarburized layer on the surface of the steel sheet to improve delayed fracture resistance and its anisotropy. However, no consideration is given at all to the improvement in bendability and fracture resistance in the event of a vehicle collision that are achieved by forming a soft surface layer (decarburized layer), and the press formability of the steel sheet end portions. Patent Document 3 discloses a hot-dip galvanized steel sheet and a galvannealed steel sheet in which ductility is improved by the generation of MA inside the steel sheet and bendability is improved by the formation of a soft layer (decarburized ferrite layer) on the surface of the steel sheet, but does not take into consideration at all the press formability of the steel sheet end portions. Patent Document 4 discloses a high-strength hot-dip galvanized steel sheet that has improved ductility, which is the press formability inside the steel sheet, and stretch flangeability, which is the press formability at the ends of the steel sheet. However, no consideration is given at all to improving bendability by forming a soft surface layer (decarburized layer) or improving fracture resistance during a vehicle collision. From the above, it cannot be said that the steel sheets disclosed in Patent Documents 1 to 4 have a TS of 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 at the steel sheet end (bendability of the steel sheet end (shear cross section)), and fracture resistance properties during 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 edge (bendability of the steel sheet edge (shear cross section)), and a method for manufacturing the same. Another object of the present invention is to provide a member made from the above steel plate and a method for manufacturing the same.

[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 galvannealed hot-dip galvanized steel sheet (hereinafter also referred to as GA).

[0014] Here, the tensile strength TS is measured by a tensile test in accordance with JIS Z 2241 (2011). In addition, 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 satisfying the following: A high yield stress YS means that the YS measured in a tensile test conforming to JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on the TS measured in the tensile test. (A) When 780MPa≦TS<980MPa, 550MPa≦YS (B) When 980MPa≦TS<1180MPa, 700MPa≦YS

[0015] Furthermore, the steel plate has excellent bendability when, in accordance with JIS Z 2248 (2022), a 90-degree V-bend test is conducted with a bending radius of 0.5 mm, and the length of the cracks that propagate along the bend ridges formed other than at the ends of the bend ridges (crack length other than at the V-bend end surface) is 200 μm or less. A close contact bending test was conducted, and the spacer plate thickness at the crack limit where cracks of 0.5 mm or more do not occur along the bending ridge is 3.0 mm or less. A close contact bending test was conducted with a 3.0 mm spacer, and the crack depth (close contact bending internal crack depth) that progressed in the thickness direction at the bending ridge line subjected to compressive stress was 200 μm or less, and When a close bending + perpendicular 90-degree V-bending test is performed and the bending radius at which no cracks of 0.5 mm or more occur along the bending ridge is defined as the crack limit bending radius (handkerchief bending boundary bending radius), this indicates that 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 a steel sheet means excellent ductility, and refers to the total elongation (El) measured in a tensile test conforming to JIS Z 2241 (2011) satisfying the following formula (A) or (B) depending on the TS measured in the tensile test. (A) When 780MPa≦TS<980MPa, 17.0%≦El (B) 980MPa≦TS<1180MPa, 11.0%≦El

[0017] Furthermore, excellent bendability at the steel plate end (shear cross section) means that, in accordance with JIS Z 2248 (2022), a 90-degree V-bend test with a bending radius of 0.5 mm is conducted, and the crack length that propagates from the end of the bend ridge toward the ridge (V-bend end face crack length) is 200 μm or less. [Means for solving the problem]

[0018] The present inventors have conducted extensive research to achieve the above object. As a result, the composition of the base steel sheet of the steel sheet is appropriately adjusted, and the base steel sheet of the steel sheet has a surface soft layer whose Vickers hardness is 84% ​​or less of the Vickers hardness at the 1 / 4 position of the sheet thickness, and the surface soft layer satisfies the following formula (1): The tissue in the superficial soft layer is Ferrite area ratio: 50.0% or more and 100.0% or less, Among the structures other than ferrite, the area ratio of fresh martensite divided by the total area ratio of bainite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less, The structure at 1 / 4 of the thickness of the base steel sheet is 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 volume 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%), It has been found that a steel sheet can be obtained that has a tensile strength of 780 MPa or more but 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 edge (bendability of the steel sheet edge (shear cross section)). 20≦X≦120-3800×[Sb]-1900×[Sn] ···(1) In the 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.

[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] 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 (including 0%), and Sn: 0.200% or less (including 0%) and the balance being Fe and unavoidable impurities, The steel sheet has a soft surface layer having a Vickers hardness of 84% or less of the Vickers hardness at a position 1 / 4 of the sheet thickness from the surface of the base steel sheet, The surface soft layer satisfies the following formula (1): The structure in the superficial soft layer is The area ratio of ferrite is 50.0% or more and 100.0% or less, When the area fraction of ferrite is less than 100.0%, the value obtained by dividing the area fraction of fresh martensite by the total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less, The structure at 1 / 4 of the thickness of the base steel sheet is 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 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%), A steel plate having a tensile strength of 780 MPa or more but less than 1180 MPa. 20≦X≦120-3800×[Sb]-1900×[Sn] ···(1) In the 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 component composition further includes, in mass%, Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co:0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less and REM: 0.0200% or less The steel sheet according to [1] above, containing at least one selected from the following: [3] The steel sheet according to [1] or [2], wherein one or both sides of the base steel sheet have a plating layer, and the plating layer is a hot-dip galvanized layer. [4] The steel sheet according to [1] or [2], wherein the base steel sheet has a plating layer on one or both sides, and the plating layer is a galvannealed layer. [5] A member made using the steel plate according to any one of [1] to [4] above. [6] A hot rolling process in which a steel slab having the composition described in [1] or [2] is hot rolled to form a hot-rolled steel sheet; After the hot rolling step, a pickling step of pickling the hot-rolled steel sheet; a cold rolling step of cold rolling the steel sheet after the pickling step at a rolling reduction of 20% or more and 80% or less; an annealing step of heating the steel sheet after the cold rolling step and annealing it under conditions that satisfy formulas (2) and (3) at 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 of -10°C or higher in an atmosphere; 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; The steel sheet after the first holding step is subjected to a stress of 2.0 kgf / mm in the reheating holding temperature range. 2 a surface strain introduction step of applying the above tension; A second holding step of holding the steel sheet after the surface strain introduction step at 300 ° C or higher and 460 ° C or lower for 10 seconds or more. Steel plate manufacturing method. 2400≦Y≦20000...Formula (2) Y=[{(T-Ac1)×t1} / 2}]+{(T-Ac1)×t2} ···(3) In the formula (3), T is the annealing temperature (°C), t1 is the time (s) from 650°C to the annealing temperature T during the temperature rise in the annealing process, t2 is the annealing time (s), and Ac1 is Ac1 (°C). [7] The method for producing a steel sheet according to [6], further comprising a hot-dip galvanizing step of subjecting the steel sheet to a hot-dip galvanizing treatment after the annealing step to form a hot-dip galvanized layer. [8] The method for producing a steel sheet according to [6], further comprising a galvannealed hot-dip galvanizing step of subjecting the steel sheet to a galvannealed hot-dip galvanizing treatment after the annealing step to form a galvannealed hot-dip galvanized layer. [9] A method for manufacturing a component, comprising the step of subjecting the steel plate according to any one of [1] to [4] above to at least one of forming and joining to form the component. [Effects of the Invention]

[0021] According to the present invention, a steel sheet can be obtained which has a tensile strength TS of 780 MPa or more and 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 plate of the present invention have high strength and can be used extremely advantageously as impact energy absorbing members for automobiles. [Brief explanation of the drawings]

[0022] [Figure 1] Figure 1 shows an example of a SEM tissue image used for tissue identification. [Figure 2] FIG. 2(a) is a schematic diagram of the sample after being bent at a 90-degree V angle, and FIG. 2(b) is a diagram of the sample shown in FIG. 2(a) viewed in the Z direction (negative direction). [Figure 3-1] 3-1(a) and (b) are schematic diagrams for explaining end surface cracking during 90-degree V-bending. [Figure 3-2] Figure 3-2(a) is a schematic diagram for explaining a method for measuring the crack length of an end crack that occurs during a 90-degree V-bend, and Figure 3-2(b) is a diagram showing an example of a profile waveform used to measure the crack length. [Figure 4-1] Figure 4-1(a) is a diagram to explain how to measure the spacer plate thickness at the crack limit when performing a close contact bending test, and Figure 4-1(b) is a diagram to explain how to measure the crack depth that progresses in the plate thickness direction at the bending ridge that is subjected to compressive stress when performing a close contact bending test. [Figure 4-2] 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 ridge line that has been subjected to compressive stress during a close contact bending test, 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 ridge line that has been subjected to compressive stress in the above observation cross section. DETAILED DESCRIPTION OF THE INVENTION

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

[0024] [1. Steel plate] The steel sheet of the present invention has a base steel sheet having a chemical composition containing, in mass%, C: 0.050% or more and 0.400% or less, Si: 0.20% 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 of 0%), and Sn: 0.200% or less (inclusive of 0%), with the balance being Fe and unavoidable impurities; It has a soft surface layer whose Vickers hardness is 84% ​​or less of the Vickers hardness at a position 1 / 4 of the plate thickness from the surface of the base steel plate, The soft surface layer satisfies the following formula (1): The tissue in the superficial soft layer is The area ratio of ferrite is 50.0% or more and 100.0% or less, When the area fraction of ferrite is less than 100.0%, the value obtained by dividing the area fraction of fresh martensite by the total area fraction of bainite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less, The structure at 1 / 4 of the thickness of the base steel sheet is 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 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%), The tensile strength is 780 MPa or more but less than 1180 MPa, and the steel has a high yield stress YS, excellent press formability within 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)). 20≦X≦120-3800×[Sb]-1900×[Sn] ···(1) In the 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] Ingredient composition First, the chemical composition of the base steel sheet of the steel sheet according to one embodiment of the present invention will be described. Note that the units for the chemical compositions are all "mass %", but hereinafter, unless otherwise specified, they will be simply expressed as "%".

[0026] C: 0.050% or more and 0.400% or less C is an effective element for generating appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite, and retained austenite to ensure a TS of 780 MPa or more and less than 1180 MPa and a high YS. 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 leads to a decrease in YS. On the other hand, if the C content exceeds 0.400%, the area fraction of fresh martensite increases excessively, making it difficult to achieve a tensile strength of less than 1180 MPa. Furthermore, fresh martensite becomes the starting point for void formation during 90° V-bend tests, close-contact bending tests, and close-contact bending + orthogonal 90° V-bend tests, making it difficult to achieve the desired bendability of the steel sheet and the bendability of the sheared edge. Furthermore, the area fraction of retained austenite and the amount of solute C in the retained austenite increase excessively. Furthermore, the hardness of fresh martensite formed by the deformation-induced transformation of retained austenite during shearing increases significantly, promoting the subsequent formation 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 ratio of ferrite increases excessively, and the C concentration in austenite during annealing increases excessively, 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, preferably 2.00% or less, and more preferably 1.50% or less. The Si content is preferably 0.50% or more.

[0028] Mn: 1.00% or more and 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 leads to a decrease in YS. On the other hand, if the Mn content is 3.50% or more, the martensitic transformation start temperature Ms (hereinafter also referred to simply as the Ms point or Ms) decreases, and the amount of martensite formed during the cooling process decreases. As a result, the amount of martensite formed during final cooling increases, and the martensite formed at that time is not sufficiently tempered, resulting in an increase in the area fraction of hard fresh martensite. Fresh martensite serves as the starting point for void formation during 90° V-bend tests, close-contact bending tests, and close-contact bending + orthogonal 90° V-bend tests. If the area fraction of fresh martensite exceeds 10.0%, the desired bendability of the steel sheet and the bendability of the sheared edge 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 the steel sheet. To obtain this effect, the P content is set to 0.001% or more. On the other hand, if the P content exceeds 0.100%, P segregates at the prior austenite grain boundaries and embrittles the grain boundaries, which increases the amount of voids generated after shearing the steel sheet, making it impossible to achieve the desired bendability at the sheared edge. Therefore, the P content is set to 0.001% or more and 0.100% or less, and preferably 0.003% or more. The P content is preferably 0.030% or less, more preferably 0.010% or less, and even more preferably 0.005% or less.

[0030] S: 0.0001% or more and 0.0200% or less S exists as sulfide 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, preferably 0.0080% or less, and more preferably 0.0050% or less. Furthermore, due to constraints on production technology, the S content is set to 0.0001% or more, preferably 0.0003% or more, and more preferably 0.0005% or more.

[0031] Al: 0.005% or more and 2.000% or less Al suppresses the formation of carbides during cooling after annealing and promotes the formation of retained austenite. In other words, Al is an element that affects the area ratio of retained austenite. To achieve these effects, the Al content is set to 0.005% or more. On the other hand, if the Al content exceeds 2.000%, the area fraction of ferrite increases excessively, making it difficult to achieve a TS of 780 MPa or more. This also leads to a decrease in YS. Additionally, the C concentration in austenite during annealing increases excessively, making it impossible 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, and preferably 0.010% or more. The Al content is preferably 1.000% or less, more preferably 0.100% or less, and further 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, and preferably to 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, 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 galvanizability and chemical conversion treatability will saturate, and there is a risk of deterioration in the press formability (bendability within the steel sheet) and crack propagation resistance within the steel sheet. Therefore, when Sb is contained, the Sb content is set to 0.200% or less. The Sb content is more preferably 0.020% or less. Further preferably, it is 0.018% or less. The Sb content is more preferably 0.016% or less, and further preferably, it is 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 will saturate, and there is a risk of deterioration in the press formability (bendability within the steel sheet) and crack propagation resistance within the steel sheet. Therefore, when Sn is contained, the Sn content must be 0.200% or less. The Sn content is 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 a steel sheet according to one embodiment of the present invention has been described above, but the base steel sheet of a steel sheet according to one embodiment of the present invention has a chemical composition containing the above basic chemical components, with the balance other than the above basic chemical components including Fe (iron) and unavoidable impurities. Here, it is preferable that the base steel sheet of a steel sheet according to one embodiment of the present invention has a chemical composition containing the above basic chemical components, with the balance consisting of Fe and unavoidable impurities.

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

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

[0038] Nb: 0.200% or less Nb forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To achieve this effect, the Nb content is preferably 0.001% or more. 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 the starting points for cracks during 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 shear edge may not be achieved. Therefore, when Nb is added, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.

[0039] Ti:0.200% or less Like Nb, Ti forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To achieve this effect, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more. 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 the starting points for cracks during 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. 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 even more 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 or annealing, thereby increasing TS and YS. To achieve this effect, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. The V content is further preferably 0.010% or more, and even more preferably 0.020% 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 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 V is added, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.

[0041] B: 0.0100% or less B is an element that segregates at austenite grain boundaries to improve hardenability. B is also an element that suppresses the formation of ferrite and grain growth 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. In addition, after the steel sheet is subjected to shearing, the amount of voids generated increases, and the desired bendability of the sheared edge may not be achieved. Therefore, when B is contained, the B content is preferably 0.0100% or less, more preferably 0.0050% or less, and more preferably 0.0020% or less.

[0042] Cr:1.000% or less Cr is an element that improves hardenability, and 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 further 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° V-bend test, the close bend test, and the close bend + orthogonal 90° 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 Cr is added, the Cr content is preferably 1.000% or less. The Cr content is more preferably 0.800% or less, and even more preferably 0.700% or less. The Cr content is more preferably 0.100% or less, and even more preferably 0.080% or less.

[0043] Ni: 1.000% or less Since Ni is an element that improves hardenability, 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 of the steel sheet and the bendability of the sheared edge may not be achieved. Therefore, when Ni is added, the Ni content is preferably 1.000% or less. The Ni content is more preferably 0.800% or less. The Ni content is 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, and the addition of 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 bending test, and the close bending + perpendicular 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 Mo is added, the Mo content is preferably 1.000% or less. The Mo content is more preferably 0.500% or less, even more preferably 0.450% or less, and even more preferably 0.400% or less. The Mo content is more preferably 0.350% or less, and even more preferably 0.300% or less. The Mo content is more preferably 0.100% or less, and even more preferably 0.080% or less.

[0045] Cu:1.000% or less Since Cu is an element that improves hardenability, 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 even more 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, which may result in the formation of large amounts of coarse precipitates and inclusions. In such cases, the fresh martensite and coarse precipitates and inclusions may become the starting points for void formation during 90-degree V-bend tests, close-contact bending tests, and close-contact bending + orthogonal 90-degree V-bend tests, which may prevent the desired bendability of the steel sheet and bendability of the sheared edge. Therefore, if Cu is added, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.

[0046] Ta:0.100% or less Ta, like Ti, Nb, and V, increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. Additionally, Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb,Ta)(C,N). This inhibits coarsening of precipitates and stabilizes precipitation strengthening. This further improves TS and YS. To achieve these effects, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.002% or more, and even more preferably 0.004% or more. On the other hand, if the Ta content exceeds 0.100%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the 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 + 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, if Ta is added, the Ta content is preferably 0.100% or less. The Ta content is more preferably 0.090% or less, even more preferably 0.080% or less, 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 improves hardenability, and the addition of W produces 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 bending test, and the close bending + perpendicular 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 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 even more preferably 0.050% or less.

[0048] Mg: 0.0200% or less Mg is an element that is effective in spheroidizing the shape of inclusions such as sulfides and oxides and improving the bendability of the sheared end face. 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 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 Mg is added, the Mg content is preferably 0.0200% or less. The Mg content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Mg content is more preferably 0.0100% or less, and even more preferably 0.0080% or less.

[0049] Zn: 0.0200% or less Zn is an element that effectively spheroidizes the shape of inclusions and improves the bendability of the sheared end surface. 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 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 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 even more preferably 0.0080% or less.

[0050] Co:0.0200% or less Like Zn, Co is an element that is effective in spheroidizing the shape of inclusions and improving the bendability of the sheared end face. 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 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 Co is added, the Co content is preferably 0.0200% or less. The Co content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.

[0051] Zr: 0.1000% or less Zr, like Zn and Co, is an element that is effective in making the shape of inclusions spheroidal and improving the bendability of the sheared edge. To obtain this effect, the Zr content is preferably 0.0010% or more. On the other hand, if the Zr content exceeds 0.1000%, 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 + 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 Zr is added, the Zr content is preferably 0.1000% or less. The Zr content is more preferably 0.0300% or less, further preferably 0.0100% or less, and more preferably 0.0050% or less.

[0052] Ca:0.0200% or less Ca exists as inclusions in steel. If the Ca content exceeds 0.0200%, a large amount of coarse inclusions may form. In such cases, the coarse precipitates and inclusions may become the starting point 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 shear edge bendability of the steel sheet may not be achieved. Therefore, if Ca is added, the Ca content is preferably 0.0200% or less. The Ca content is preferably 0.0020% or less. Although there is no particular lower limit for the Ca content, the Ca content is preferably 0.0005% or more. Furthermore, due to constraints on production technology, the Ca content is more preferably 0.0010% or more.

[0053] Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, REM: 0.0200% or less Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are all elements effective in 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, if the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM exceed 0.0200% each, and / or the content of As exceeds 0.0500%, large amounts of coarse precipitates and inclusions may form. In such cases, the coarse precipitates and inclusions may become the origin of void formation during 90° V-bend tests, close bend tests, and close bend + orthogonal 90° V-bend tests, 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 0.0200% or less. Furthermore, 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), which has atomic number 21, yttrium (Y), which has atomic number 39, and the lanthanoids ranging from lanthanum (La), which has atomic number 57, to lutetium (Lu), which has atomic number 71. The REM concentration in the present invention refers to the total content of one or more elements selected from the above-mentioned REM. The REM is not particularly limited, but is preferably at least one of Sc, Y, Ce and La.

[0055] 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 of the thickness of the base steel plate) Next, the steel structure of the steel plate according to one embodiment of the present invention will be described. In terms of the structure at the 1 / 4 position in the plate thickness of the base steel plate, 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%). The reasons for each limitation will be explained below.

[0057] Ferrite area ratio: 76.5% or less (including 0.0%) Soft ferrite is a phase that improves ductility. However, if the area fraction of ferrite increases excessively, it becomes difficult to achieve a TS of 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 at the sheared edge. Therefore, the area fraction of ferrite is set to 76.5% or less. The area fraction of ferrite is preferably 60.0% or less. The lower limit of the area ratio of ferrite is not particularly limited and may be 0.0%. The area ratio of ferrite may be 5.0% or more, or 10.0% or more.

[0058] Total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): 20.0% to 90.0% 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 a useful phase 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 produced in a relatively high temperature range.

[0059] Area ratio of retained austenite: 3.5% to 10.0% In order to obtain good ductility, the area fraction of retained austenite is set to 3.5% or more, and preferably exceeds 3.5%. On the other hand, if the volume fraction of retained austenite increases excessively, fresh martensite formed by stress-induced transformation during shearing becomes the starting point for void formation, 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, and more preferably 8.0% or less.

[0060] Area ratio of fresh martensite: 10.0% or less (including 0.0%) If the area fraction of fresh martensite increases excessively, the fresh martensite may become the origin of void generation in the 90-degree V-bend test, the close bending test, and the close bending + orthogonal 90-degree V-bend test, and the desired bendability of the steel plate and the bendability of the sheared edge may not be achieved. From the viewpoint of ensuring good bendability of the steel sheet and bendability of the sheared edge, the area ratio of fresh martensite is set to 10.0% or less, and preferably 5.0% or less. The lower limit of the area ratio of fresh martensite is not particularly limited, and may be 0.0%. Fresh martensite is martensite that has not been quenched (i.e., has not been tempered).

[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 also be 0.0%. The remaining structure is not particularly limited, and examples thereof include carbides such as lower bainite, pearlite, cementite, etc. The type of the remaining structure can be confirmed by observation using, for example, a scanning electron microscope (SEM).

[0062] surface soft layer The base steel sheet of the steel sheet according to one embodiment of the present invention preferably has a soft surface layer on the surface of the base steel sheet. The soft surface layer contributes to suppressing the propagation of bending cracks during press forming, 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 surface soft layer is set to 20 μm or more, and 120 μm or less. Vickers hardness is measured based on JIS Z 2244-1 (2020) at a load of 10 gf.

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

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

[0065] The area ratio of fresh martensite divided by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite): 0.5 or less If the area fraction of fresh martensite in the soft surface layer increases excessively, the fresh martensite becomes the origin of void generation in a 90-degree V-bend test, a close bending test, and a close 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 phase other than ferrite refers to bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite). The lower limit of the value obtained by dividing the area ratio of martensite in the soft surface layer by the area ratio of the hard phase 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 plate thickness cross section (L cross section) parallel to the rolling direction of the base steel sheet serves as the observation surface. The observation surface of the sample is then mirror-polished using diamond paste. The observation surface of the sample is then finish-polished using colloidal silica, and then etched with 3 vol.% nital to reveal the structure. Then, using a SEM (Scanning Electron Microscope), under conditions of 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 observation surface (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. At the outermost layer, the zinc plating layer was excluded and the internal oxide layer was included in the photograph. From the obtained structural image (see Figure 1), ferrite, bainitic ferrite, tempered martensite, and other hard phases (hard second phase (retained austenite + fresh martensite)) are identified as follows.

[0068] Ferrite: A black region that is lumpy. 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: This is a region that is black to dark gray in color and has a massive or amorphous shape. It contains no iron-based carbides or contains relatively small amounts of them. Tempered martensite: A gray region with an amorphous shape. It also contains a relatively large number of iron-based carbides. Hard second phase (retained austenite + fresh martensite): This is a region that is white to light gray in color and has an amorphous form. It does not contain iron-based carbides. If 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 areas that appear as dots or lines. They are included in tempered martensite, bainitic ferrite, and ferrite. Remaining structure: The above-mentioned lower bainite, pearlite, inner oxides, etc. may be mentioned, and the forms thereof are as known.

[0069] Next, the area of ​​each phase identified in the structural image is calculated using the following method. An equally spaced 20 x 20 grid is placed in an area of ​​25.6 μm x 19.2 μm in actual length on the 5000x magnification SEM image, and the area fraction of ferrite, bainitic ferrite, tempered martensite, and other hard phases (hard second phases) is 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 5000x magnification SEM images.

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

[0071] The area ratio of fresh martensite is determined by subtracting the area ratio of retained austenite from the area ratio of the hard second phase determined as described above. [Area fraction of fresh martensite (%)] = [Area fraction of hard second phase (%)] - [Area fraction of retained austenite (%)]

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

[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, less than 1180 MPa The tensile strength TS of the 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)) within the steel sheet, bendability of the steel sheet, and bendability of the sheared edge surface of the steel sheet according to one embodiment of the present invention are as described above. The ratio YR (yield ratio) of the yield stress YS to the tensile strength TS preferably 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, alloyed hot-dip galvanized layer) A steel sheet according to one embodiment of the present invention may have a plating layer formed on the base steel sheet (on the surface of the base steel sheet), and this plating layer may be provided on only one surface of the base steel sheet, or on both surfaces.

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

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

[0079] The galvannealed layer is preferably composed of, for example, 20% by mass or less of Fe and 0.001% by mass to 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 to 3.5% by mass. The Fe content of the galvannealed layer is more preferably 7.0% by mass or more, and even more preferably 8.0% by mass or more. The Fe content of the galvannealed layer is more preferably 15.0% by mass or less, and even more preferably 12.0% by mass or less. The remainder other than the above elements is unavoidable impurities.

[0080] In addition, the coating weight of the plating layer (zinc plating layer) per side is not particularly limited, but is preferably 20 g / m 2 The coating 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 solution of hydrochloric acid. Next, a steel sheet (galvanized steel sheet) to be used as a test material is immersed in the treatment solution to dissolve the plating layer (galvanized layer). The mass loss of the test material before and after dissolution is measured, and this value is divided by the surface area of ​​the base steel sheet (the surface area of ​​the part that was covered with the plating) to determine the plating coverage (g / m 2 ) is calculated.

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

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

[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-mentioned component composition to obtain a hot-rolled steel sheet; After the hot rolling step, a pickling step of pickling the hot-rolled steel sheet; a cold rolling step of cold rolling the steel sheet after the pickling step at a rolling reduction of 20% or more and 80% or less; an annealing step of heating the steel sheet after the cold rolling step, and annealing the steel sheet under conditions satisfying formulas (2) and (3) at an annealing temperature of Ac1 point (°C) or higher and 900°C or lower, an annealing time of 20 seconds or longer, and a dew point of -10°C or higher in an atmosphere; 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; The steel sheet after the first holding step is subjected to a reheating first holding temperature range of 2.0 kgf / mm 2 a surface strain introduction step of applying the above tension; and a second holding step of holding the steel sheet after the surface strain 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) In the formula (3), T is the annealing temperature (°C), t1 is the time (s) from 650°C to the annealing temperature T during the temperature rise in the annealing process, t2 is the annealing time (s), and Ac1 is Ac1 (°C). Ac1 (℃): 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). Unless otherwise specified, the above temperatures refer to the surface temperatures of the steel slab and steel plate.

[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] The steel slab is then hot rolled to form a hot rolled steel sheet. Hot rolling may be performed using an energy-saving process, such as direct rolling (a method in which a steel slab is charged into a heating furnace as a hot strip without being cooled to room temperature, and then hot rolled) or direct rolling (a method in which a steel slab is briefly kept at a constant temperature and then immediately rolled).

[0087] The hot rolling conditions are not particularly limited, and the hot rolling can be carried out under the following conditions, for example. That is, the steel slab is 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 sheet. 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 ratio 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 sheet stage and, ultimately, the steel structure of the final product, tends to be 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 at a coiling temperature of preferably 450°C or higher, and preferably 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 temporarily wound before finish rolling. Furthermore, in order to reduce the rolling load during hot rolling, part or all of the finish rolling may be performed as lubricated rolling. Performing lubricated rolling is also effective from the viewpoint of uniformity of the steel sheet shape and material properties. The friction coefficient during lubricated rolling is preferably in the range of 0.10 to 0.25. In the hot rolling process (hot rolling process), which includes rough rolling and finish rolling, steel slabs are generally made into sheet bars by rough rolling and then made into hot-rolled steel sheets by finish rolling. However, depending on the mill capacity, etc., such divisions do not matter as long as the specified 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. There are no particular restrictions on the pickling conditions, and conventional methods may be used.

[0092] [Cold rolling process] The cold rolling is carried out by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling. The reduction rate (cumulative reduction rate) of cold rolling is not particularly limited, but is set to 20% or more and 80% or less. If the reduction rate of 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 rate in cold rolling exceeds 80%, the steel sheet is more likely to have defective shape and the coating weight may become non-uniform. Furthermore, the cold-rolled steel sheet obtained after cold rolling may be optionally subjected to pickling.

[0093] [Annealing process] 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 (℃) or higher and 900℃ or lower If the annealing temperature is lower than the Ac1 point (°C), the rate of austenite formation during heating in the two-phase region of ferrite and austenite becomes insufficient, resulting in an excessive increase in the area fraction of ferrite after annealing, and 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, a large amount of tempered martensite containing carbides is formed, it becomes difficult to obtain an area fraction of retained austenite of 3.5% or more, and ductility decreases. Therefore, the annealing temperature is set to be equal to or higher than the Ac1 point (° C.) and equal to or lower than 900° C. The annealing temperature is preferably equal to or lower than 880° C. The annealing temperature is the maximum temperature (soaking temperature) reached in the annealing process.

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

[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, it is impossible to form a soft surface layer with a thickness of 20 μm or more 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 the temperature range of (annealing temperature -40°C) or more and (inclusive) of the annealing temperature. In other words, the annealing time includes not only the holding time at the annealing temperature but also the residence time in the temperature range of (annealing temperature -40°C) or more and (inclusive) of the annealing temperature during heating and cooling before and after reaching the annealing temperature.

[0097] 2400≦Y≦20000...Formula (2) Y=[{(T-Ac1)×t1} / 2}]+{(T-Ac1)×t2} ···(3) In the formula (3), T is the annealing temperature (°C), t1 is the time (s) from 650°C to the annealing temperature T during the temperature rise in the annealing process, t2 is the annealing time (s), and Ac1 is Ac1 (°C). In the present invention, it is necessary to perform annealing under annealing conditions that satisfy formulas (2) and (3). If 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, if Y exceeds 20,000, the soft surface layer defined in the present invention will be more than (120 - 3,800 × [Sb] - 1,900 × [Sn]) μm. Therefore, Y in formula (3) is set to be 2,400 or more and 20,000 or less. t1 is preferably 30 seconds or more, and t1 is preferably 80 seconds or less.

[0098] Dew point of the annealing process atmosphere (annealing atmosphere): -10°C or higher In one embodiment of the present invention, the dew point of the atmosphere in the annealing step (annealing atmosphere) is preferably -10°C or higher. By performing annealing in the annealing step with the dew point of the annealing atmosphere being -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 process (preferred requirements)] After the annealing step, during cooling from the annealing temperature, an isothermal holding step may be performed, if necessary, at 400°C or more and 600°C or less (hereinafter also referred to as an isothermal holding temperature range) for less than 80 seconds in order to promote bainite transformation. In the isothermal holding step, bainitic ferrite is formed, and C diffuses from the formed bainitic ferrite to the untransformed austenite adjacent to the bainitic ferrite, thereby ensuring a predetermined area ratio of retained austenite and improving elongation.

[0100] Isothermal holding temperature range: 400℃ or higher and 600℃ or lower If the isothermal holding temperature is less than 400°C, lower bainite and martensite containing a large amount of carbide are generated, which suppresses the diffusion of C into untransformed austenite, and there is a risk that the area ratio of the specified amount of retained austenite cannot be secured. On the other hand, if the isothermal holding temperature exceeds 600°C, untransformed austenite may transform into pearlite, making it difficult to ensure 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 region is 80 seconds or more, the area ratio of bainitic ferrite increases excessively, the C concentration in the untransformed austenite increases excessively, and there is a risk that the desired bendability of the sheared edge cannot be achieved. Therefore, it is preferable that the holding time in the isothermal holding temperature region is less than 80 seconds.

[0102] [Cooling process (first cooling process)] 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℃ or more and 300℃ or less Average cooling rate: 10°C / s or more, 50°C / s or less, atmospheric dew point: -20°C or less (preferred requirements) In the cooling step, the steel sheet after the annealing step is cooled to a cooling stop temperature of 100°C or higher and 300°C or lower. At this time, the cooling start temperature can be Ac1 (°C) or more and 900°C or less, and can be 400°C or more and 600°C or less when an isothermal holding step is performed. The cooling step is necessary to control the area fractions of tempered martensite and retained austenite formed in the subsequent first holding step (reheating and holding step) within predetermined ranges. If the cooling stop temperature is below 100°C, the untransformed austenite present in the steel during the cooling step will be almost entirely transformed into martensite. 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 more, and reducing ductility. On the other hand, if the cooling stop temperature exceeds 300°C, the area ratio of tempered martensite decreases and the area ratio of fresh martensite increases. As a result, fresh martensite becomes the origin of void generation in the 90° V-bend test, the close bending test, and the close bending + orthogonal 90° V-bend test, and the desired bendability of the steel sheet and the bendability of the sheared edge cannot be achieved. 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. Furthermore, the cooling stop temperature is preferably 280°C or lower. The average cooling rate during this cooling step is preferably 10°C / s or more. Furthermore, the average cooling rate during this cooling step is preferably 50°C / s or less. The metallic phase defined in the present invention can be obtained through this cooling step. Here, if the average cooling rate is less than 10°C / s, the amount of untransformed austenite that is completely transformed into martensite during the cooling step increases, making it difficult to ultimately obtain retained austenite in an area fraction of 3.5% or more, and ductility may decrease. On the other hand, if the average cooling rate exceeds 50°C / s, self-relaxation during martensite transformation is suppressed, which may result in a deterioration in the sheet shape. Furthermore, the dew point of the atmosphere in this cooling step is preferably -20°C or lower. If the dew point of the atmosphere exceeds -20°C, the thickness of the soft surface layer in the in-plane direction of the steel sheet will vary greatly, and the tensile strength specified in the present invention may not be obtained. For these reasons, the dew point of the atmosphere in this cooling step is preferably -20°C or lower. The average cooling rate (°C / s) is obtained by dividing the difference between the cooling start temperature (°C) and the cooling stop temperature (°C) in the cooling step by the cooling time (s).

[0104] [First holding process (first reheating holding process)] 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 the reheating holding temperature range, but hereinafter also referred to as the 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 holding temperature (first reheating holding temperature): 370℃ or higher and 460℃ or lower In the reheating and holding step, C is concentrated in the austenite remaining after the cooling step, thereby reducing the area ratio of fresh martensite in the final structure while ensuring a predetermined amount of area ratio of retained austenite. If the reheating temperature (first reheating temperature) is less than 370°C, the carbon concentration in the austenite remaining after the cooling step is insufficient, making it difficult to obtain retained austenite in an area ratio of 3.5% or more, and ductility is reduced. On the other hand, if the reheating temperature (first reheating temperature) exceeds 460°C, C will be excessively concentrated in the untransformed austenite, and the untransformed austenite in the surface layer will not undergo strain-induced transformation in the surface strain introduction process described below, but will instead become 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 will exceed 0.5. Therefore, the reheating temperature (first reheating temperature) is set to 370°C or higher and 460°C or lower.

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

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

[0108] [Second holding process] 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. The term "holding" as used herein also includes cooling (slow cooling) within a 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℃ or higher and 460℃ 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 area ratio of fresh martensite in the surface layer divided by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) becomes 0.5 or less, and the desired bendability of the steel sheet is obtained. If the second holding temperature is less than 300°C, the martensite formed in the surface layer during the surface strain introduction process 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 cannot be 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 the 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 to 460°C) is less than 10 seconds, the martensite formed in the surface layer during the surface strain introduction process will not be tempered sufficiently, 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 step, the steel sheet is subjected to a galvanizing treatment to obtain a galvanized steel sheet. Examples of the galvanizing treatment include a hot-dip galvanizing treatment and a galvannealing treatment. The galvanizing treatment in the plating step is performed after the annealing step. The galvanizing treatment may be performed, for example, during the cooling step, during the first holding step, after the first holding step and before the surface layer strain introducing step, after the surface layer strain introducing step and before the second holding step, during the second holding step, or after the second holding step.

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

[0113] In the case of alloying hot-dip galvanizing treatment, after hot-dip galvanizing treatment is performed as described above, the hot-dip galvanized steel sheet is preferably subjected to alloying treatment by heating to an alloying temperature of 450°C or higher and 600°C or lower. If the alloying temperature is less than 450°C, the Zn-Fe alloying rate will be slow, making alloying difficult in some cases. On the other hand, if the alloying temperature exceeds 600°C, untransformed austenite transforms to pearlite, resulting in a decrease in ductility. The alloying temperature is more preferably 480°C or higher. The alloying temperature is more preferably 550°C or lower.

[0114] In addition, the coating weight of both hot-dip galvanized steel sheet (GI) and galvannealed steel sheet (GA) is 20 g / m per side. 2 In addition, the coating weight of both hot-dip galvanized steel sheet (GI) and galvannealed steel sheet (GA) is preferably 80 g / m per side. 2It 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 process (preferable condition)] The steel sheet is then preferably cooled to a second cooling stop temperature of 50°C or less.

[0116] Second cooling stop temperature: 50℃ or less The cooling conditions for the final cooling step are not particularly limited and may be any conventional method, such as gas jet cooling, mist cooling, roll cooling, water cooling, and air cooling. From the viewpoint of preventing surface oxidation, the material is preferably cooled to 50° C. or less, more preferably to room temperature. 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. Temper rolling may be performed on an apparatus continuous with the annealing apparatuses used for the above-mentioned steps (online), or may be performed on an apparatus discontinuous with the annealing apparatuses used for the above-mentioned steps (offline). Temper rolling may be performed once or twice or more times. As long as an elongation rate equivalent to that of temper rolling can be imparted, rolling using a leveler or the like may be performed.

[0118] Conditions other than those mentioned above are not particularly limited and may be performed in accordance with conventional methods. From the viewpoint of productivity, it is preferable that the above-mentioned series of treatments, such as annealing, hot-dip galvanizing, and alloying treatment of galvanizing, 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 mentioned above may be in accordance with conventional hot-dip galvanizing methods.

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

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

[0121] Steel materials having the chemical composition shown in Table 1 (the balance being Fe and unavoidable impurities) were melted in a converter and then cast into steel slabs by continuous casting. In Table 1, - indicates the content of unavoidable impurities, which is treated as 0 (zero). The calculated transformation point Ac1 (℃) shown in Table 1 is calculated using the following formula: Ac1 point (℃)=727.0-32.7×[%C]+14.9×[%Si]+2.0×[%Mn] Here, [%C] is the C content of the steel plate (steel slab), [%Si] is the Si content of the steel plate (steel slab), and [%Mn] is the Mn content of the steel plate (steel slab).

[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 with the 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 treatments in a coating step (hot-dip galvanizing step or galvannealed hot-dip coating step) as necessary to obtain a steel sheet. The plating process was performed after the first holding process and before the surface strain introduction process for Nos. 1, 5 to 9, 11, 14 to 16, 21, 23 to 31, 33 to 36, 38, 40, 44, 45, 47, 51 to 56, 59, 61, 63, 64, 66, and 67, and during the cooling process for 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.

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

[0125] The steel microstructure of the obtained steel sheet was identified using the method described above. The measurement results are shown in Table 3. As shown in Figure 1, F represents ferrite, 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, a Vickers hardness tester was used to measure the thickness of the soft surface layer under a load of 10 gf (9.8 × 10 -2Measurements were taken at 1 μm intervals from a position 1 μm from the steel plate surface in the thickness direction to a position 100 μm in the thickness direction using a 100 μm (N) pressure. Measurements were then taken at 20 μm intervals up to the center of the plate thickness. The region where the Vickers hardness was reduced to 84% or less compared to the hardness at 1 / 4 of the plate thickness was defined as the soft layer (surface soft layer), and the thickness of this region in the thickness direction was defined as the soft layer thickness.

[0127] The structure of the soft surface layer was identified at a position halfway through the thickness of the soft surface layer using the same method as for identifying the steel structure of the base steel sheet.

[0128] In addition, tensile tests, 90-degree V-bend tests, contact bending tests, and contact bending + perpendicular 90-degree V-bend tests were conducted according to the following procedures, and the tensile strength (TS), yield stress (YS), yield ratio (YR), total elongation (El), bendability of the steel plate, and bendability of the sheared edge were evaluated according to the following criteria.

[0129] ·TS (tensile strength) 〇 (Pass): 780MPa or more and less than 1180MPa × (Fail): Less than 780 MPa or more than 1180 MPa

[0130] YS (yield stress) 〇(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 plate) 〇(Pass): (A) When 780MPa≦TS<980MPa, 17.0%≦El (B) 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 conducted with a bending radius of 0.5 mm, and the crack length that develops along the bend ridge formed other than at the end of the bend ridge (crack length other than at the V-bend end) (bendability of steel plate) 〇 (Pass): Crack length is 200 μm or less except for the V-bend edge × (Fail): Crack length exceeds 200 μm except for the V-bend end surface

[0133] - A 90-degree V-bend test was conducted with a bending radius of 0.5 mm, and the crack length that propagated from the edge of the bend ridge toward the ridge (V-bend edge crack length) (bendability of the steel plate edge (shear cross section)) 〇 (Pass): V-bend end crack length is 200 μm or less × (Fail): V-bend end crack length exceeds 200 μm

[0134] · Conduct a close contact bending test and determine the spacer thickness at which cracks of 0.5 mm or more do not occur along the bending ridge (close contact bending boundary spacer thickness) (bendability of steel plate) 〇 (Pass): Thickness of spacer at boundary of tight bending is 3.0 mm or less × (Fail): The thickness of the spacer at the boundary between the bent parts is more than 3.0 mm.

[0135] Conduct a close contact bending test with a 3.0 mm spacer to measure the depth of the crack that develops in the thickness direction at the bending ridge under compressive stress (close contact bending internal crack depth) (bendability of steel plate) ○ (Pass): Internal crack depth of tightly bent joint is 200 μm or less × (Fail): Internal crack depth exceeds 200 μm

[0136] · Conduct a close bending + perpendicular 90 degree V-bending test and determine the limit 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 sheets) 〇 (Pass): Handkerchief bending boundary bending radius is 5.0 mm or less × (Fail): Handkerchief bending boundary bending radius is over 5.0 mm

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

[0138] (2) 90-degree V-bend test A 100 mm C (C direction: perpendicular to the rolling direction of the steel sheet) x 30 mm L (L direction: along the rolling direction) strip specimen was cut from the resulting steel sheet at a position 1 / 4 of the coil width. The 100 mm long end was sheared and then bent in the as-sheared state (without machining to remove burrs) so that the burrs were on the outer periphery of the bend. The shearing clearance was 15% and the rake angle was 0°. V-bending was performed using a Shimadzu Autograph with a punch radius of 0.5 mm, a punch bend angle of 90°, a punch stroke speed of 30 mm / min, a pressing load of 10 ton, and a pressing time of 5 seconds to obtain an L-direction bend (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). If the bend ridgeline is defined as the area extending from the bend apex along the steel sheet surface to a total width of 5 mm in the C direction (2.5 mm on both sides from the bend apex), then the area (area o) extending 5 mm in the L direction from the very edge of the bend ridgeline is defined as the bend ridgeline edge. The crack length Y1 that propagates from the bend ridgeline edge in the ridgeline direction (L direction) and the crack length Y2 that propagates in the L direction along the bend ridgeline formed other than the bend ridgeline edge are measured using the following methods.

[0140] After the 90-degree V-bend test with a bending radius of 0.5 mm, the length of the crack propagating from the end of the bent ridge toward the ridge was measured as follows. Figure 3-1(a) shows the crack at the end of the bending ridge of a sample after a V-bend test. When measuring the length of a crack at the center of the bending ridge, it is common to observe the plate surface (plane b) from the Z direction. Since the sample after an actual V-bend test has a saddle shape as shown in Figure 3-1(b), plane b is significantly deformed, reducing the accuracy of measuring the crack length and potentially making it difficult to accurately evaluate 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). After a 90° V-bend test with a 0.5 mm radius, the shear plane (a) of the bent sample was placed facing up, and the edge of the bend ridge was photographed at 40x magnification using a one-shot 3D shape measuring instrument (Keyence, VR6000 series or newer models). The obtained height data was analyzed using the analysis software provided with the one-shot 3D shape measuring instrument. As shown in Figure 3-2(a), a circular arc measurement line (i) was drawn as close as possible to the outside of the bend, where tensile stress was applied, in line with the bend ridge. An example of the obtained profile waveform (j) is shown in Figure 3-2(b). The length of each crack (y1 + y2) / 2 was calculated using the software's measurement tool. The length of the longest crack was taken as the crack length propagating from the edge of the bend ridge toward the ridge after the 90° V-bend test with a 0.5 mm radius.

[0141] After the 90-degree V-bend test with a bending radius of 0.5 mm, the length of cracks that propagated along the bend ridge formed at the ends of the bend ridge was measured by visual observation at 25x magnification using a stereomicroscope.

[0142] (3) Adhesion bending test A 60mm x 30mm test piece was cut from the resulting steel sheet at a quarter-width position. Both ends of the 60mm length were ground, followed by a primary bending (U-bending) to prepare the test piece for close bending. U-bending was performed using a hydraulic bending tester with a punch radius of R = 5.0mm, a stroke speed of 10mm / s, and a C-direction bending (bending ridge length: 30mmL), which did not cause cracks in any of the test materials. Next, close bending was performed on the U-bended test piece. This close bending was performed using a hydraulic bending tester. As shown in Figure 4-1(a), a spacer q (plate thickness Z1) was inserted as needed, and the stroke speed was 10mm / 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 the close bending test, if the area from the bend apex along the steel plate surface to both sides in the circumferential direction (C direction) with a width Z2 (Z2 = (2t + Z1) × π / 2, where t is the sample thickness) is considered to be the outside of the bend ridge, the minimum spacer thickness at which the L-direction length Y3 of the crack extending in the L direction outside the bend ridge is less than 0.5 mm (no cracks of 0.5 mm or more will occur) was defined as the critical spacer thickness for cracking. The crack length outside the bend ridge was measured by visual observation using a stereomicroscope at 25x magnification.

[0143] Furthermore, after conducting a close contact bending test with a 3.0 mm spacer (a close contact bending test with a spacer plate thickness of 3.0 mm), as shown in Figure 4-1(b), if the area with width Z3 (9 mm) on both sides of the circumferential direction (C direction) from the bending apex along the steel plate surface on the side subjected to compressive stress (the inner surface of the sample) (see area s) is defined as the inside of the bending ridge, the depth of the crack propagating in the plate thickness direction on the inside of the bending ridge was measured as follows. As shown in Figure 4-2(c), a specimen was cut out from the sample after the above-mentioned close bending so that the cross section k at the 1 / 2 position in the L direction was the observation surface. Next, the observation surface of the sample was mirror-polished using diamond paste. Then, using a SEM (Scanning Electron Microscope), an accelerating voltage of 15 kV and a magnification of 50x was used to photograph a field of view of 2560.0 μm × 1920.0 μm (m in Figure 4-2(d)) at the position m in Figure 4-2(d), which is the bending apex of the observation surface of the sample, and the entire crack was observed. In the obtained image of the crack, the distance X between the start point and the end point of the crack was taken as the crack depth. This X A close contact bending test was conducted using a 3.0 mm spacer, and the crack depth was evaluated as the crack depth that propagated in the thickness direction at the bending ridge line subjected to compressive stress.

[0144] (4) Close bending + perpendicular 90 degree V bending test After the above-mentioned 3.0 mm spacer tight-fitting bending test, a V-bend test in the L direction (a 90-degree V-bend test) was conducted using the sample. If the r region (Z2 region in the figure) in Figure 4-1(a) after V-bending is taken as the bend ridgeline after handkerchief bending, the bending radius at which cracks extending in the L direction along the bend ridgeline do not occur and which are 0.5 mm or longer in the L direction was taken as the crack limit bending radius (handkerchief bending boundary bending radius). The crack length of the bend ridgeline was measured by visual observation using a stereomicroscope at 25x magnification.

[0145] [Table 1]

[0146] [Table 2]

[0147] [Table 3]

[0148] [Table 4]

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

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

Claims

1. In mass%, C: 0.050% or more and 0.400% or less, Si: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%; P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.005% or more and 2.000% or less, N: 0.0100% or less, Sb: 0.200% or less (including 0%), and Sn: 0.200% or less (including 0%) and the balance being Fe and unavoidable impurities, The steel sheet has a soft surface layer having a Vickers hardness of 84% or less of the Vickers hardness at a position 1 / 4 of the sheet thickness from the surface of the base steel sheet, The surface soft layer satisfies the following formula (1): The structure in the superficial soft layer is The area ratio of ferrite is 50.0% or more and 100.0% or less, When the area ratio of ferrite is less than 100.0%, the value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less; The structure at a 1 / 4 position of the plate thickness of the base steel plate is 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, The area fraction of fresh martensite is 10.0% or less (including 0.0%), A steel plate having a tensile strength of 780 MPa or more and less than 1180 MPa. 20≦X≦120-3800×[Sb]-1900×[Sn]...(1) In the formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.

2. The component composition further includes, in mass %, Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less; and REM: 0.0200% or less The steel sheet according to claim 1, comprising at least one selected from the following:

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

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

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

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

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

8. A hot rolling step of hot rolling a steel slab having the component composition according to claim 1 or 2 to obtain a hot-rolled steel sheet; After the hot rolling step, a pickling step of pickling the hot-rolled steel sheet; a cold rolling step of cold rolling the steel sheet after the pickling step at a rolling reduction of 20% or more and 80% or less; An annealing step in which the steel sheet after the cold rolling step is heated and annealed at an annealing temperature of Ac1 (°C) or higher and 900°C or lower, an annealing time of 20 seconds or longer, and an atmosphere with a dew point of -10°C or higher under conditions that satisfy formula (2) and formula (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; The steel sheet after the first holding step is subjected to a pressure of 2.0 kgf / mm in the reheating holding temperature range. ２ a surface strain introduction step of applying the above tension; A second holding step of holding the steel sheet after the surface strain introduction step at 300 ° C. or higher and 460 ° C. or lower for 10 seconds or more. a base steel sheet having the above-described composition; and a surface soft layer having a Vickers hardness of 84% or less of the Vickers hardness at a position 1 / 4 of the sheet thickness from the surface of the base steel sheet, The surface soft layer satisfies the following formula (1): The structure in the superficial soft layer is The area ratio of ferrite is 50.0% or more and 100.0% or less, When the area ratio of ferrite is less than 100.0%, the value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less; The structure at a 1 / 4 position of the plate thickness of the base steel plate is 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, The area fraction of fresh martensite is 10.0% or less (including 0.0%), A method for producing a steel plate having a tensile strength of 780 MPa or more and less than 1180 MPa. 20≦X≦120-3800×[Sb]-1900×[Sn]...(1) In the formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively. 2400≦Y≦20000 (2) Y=[{(T-Ac1)×t1} / 2}]+{(T-Ac1)×t2}...(3) Here, 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).

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

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

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

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

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

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