Steel sheet, member, and production methods for same

Abstract

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

Description

Steel plates, components, and their manufacturing methods

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

[0002] CO2 emissions from improved fuel efficiency due to thinner and lighter steel sheets used in automobile bodies ２ To achieve both reduced emissions and improved crashworthiness, efforts are underway to increase the strength of automotive steel sheets. Furthermore, new regulations are being introduced one after another. Therefore, in order to increase vehicle body strength, the use of high-strength steel sheets, particularly high-strength steel sheets with a tensile strength (hereinafter also referred to as TS) of 980 MPa or more, for major structural and reinforcing components assembled to the framework of an automobile cabin (hereinafter also referred to as automotive framework structural components, etc.), is increasing. Furthermore, high-strength steel sheets used for automotive framework structural components, etc., are required to have high component strength when press-formed. To increase component strength, for example, it is effective to increase the yield ratio (hereinafter also referred to as YR), which is the yield stress (hereinafter also referred to as YS) of a steel sheet divided by TS. This increases the impact absorption energy (hereinafter also referred to as impact absorption energy) during a vehicle collision. Furthermore, among automotive framework structural components, for example, crash boxes, have bent portions. Therefore, from the viewpoint of press formability, it is preferable to apply steel sheets with high bendability to such parts. Furthermore, from the viewpoint of the rust prevention performance of the vehicle body, steel sheets used as raw materials for automotive components are often zinc-plated. Therefore, there is a demand for the development of hot-dip galvanized steel sheets that not only have high strength but also excellent press formability and impact resistance.

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

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

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

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

[0007] Patent No. 5354135 Patent No. 5352793 Patent No. 6536294 Patent No. 5256689

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

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

[0010]

[0003] Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet in which ductility is improved by the formation of retained austenite inside the steel sheet and mechanical cutting properties are improved by the formation of a decarburized layer on the surface of the steel sheet, but does not take into consideration at all the improvements in bendability and fracture resistance in the event of a vehicle collision that are achieved by the formation of a soft surface layer (decarburized layer), and the press formability of the steel sheet ends. Patent Document 2 discloses a high-strength hot-dip galvanized steel sheet in which the main structure inside the steel sheet is soft ferrite and the amount of unrecrystallized ferrite is limited to a small amount, thereby improving ductility, and the delayed fracture resistance and its anisotropy are improved by the formation of a decarburized layer on the surface of the steel sheet, but does not take into consideration at all the improvements in bendability and fracture resistance in the event of a vehicle collision that are achieved by the formation of a soft surface layer (decarburized layer), and the press formability of the steel sheet ends. Patent Document 3 discloses a hot-dip galvanized steel sheet and a galvannealed steel sheet in which ductility is improved by the formation of M-A inside the steel sheet and bendability is improved by the formation of a soft layer (decarburized ferrite layer) on the surface of the steel sheet, but no consideration is given to the press formability of the steel sheet ends. Patent Document 4 discloses a high-strength hot-dip galvanized steel sheet in which both ductility, which is the press formability inside the steel sheet, and stretch flangeability, which is the press formability of the steel sheet ends, are improved, but no consideration is given to the improvement of bendability or fracture resistance in the event of a vehicle collision due to the formation of a soft surface layer (decarburized layer). From the above, it cannot be said that the steel sheets disclosed in Patent Documents 1 to 4 have a TS of 1180 MPa or more, a high YS, excellent press formability inside the steel sheet (bendability inside the steel sheet), and excellent press formability of the steel sheet ends (bendability of the steel sheet ends (shear cross section)).

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

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

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

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

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

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

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

[0018] The El in the notched tensile test after the heat treatment at 170°C for 20 minutes is an index showing the resistance to crack propagation in the vertical wall portion of the axial crushable member (impact energy absorbing member). Also, the El in the notched tensile test after the heat treatment at 170°C for 20 minutes after the introduction of the 2% nominal tensile strain is an index showing the resistance to crack propagation in the bending ridge portion of the axial crushable member (impact energy absorbing member).

[0019] The present inventors conducted extensive research to achieve the above-mentioned objectives and have made the following discoveries. (1) With a predetermined chemical composition, a TS of 1180 MPa or more can be ensured by setting the area fraction of ferrite to 55.0% or less (including 0.0%) and the total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite) to more than 40.0% and less than 100.0% in the structure at the 1 / 4 position of the sheet thickness of the base steel sheet. (2) With a predetermined chemical composition, a high YS can be ensured by setting the total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite) to more than 40.0% and less than 100.0%, the area fraction of retained austenite to less than 3.5%, and the area fraction of fresh martensite to 10.0% or less (including 0.0%) in the structure at the 1 / 4 position of the sheet thickness of the base steel sheet. (3) The steel sheet has a soft surface layer with a predetermined chemical composition, and 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, and the soft surface layer satisfies the following formula (1): 20≦X≦120−3800×[Sb]−1900×[Sn] (1) where X is the thickness of the soft surface layer (μm), and [Sb] and [Sn] are the Sb and Sn contents (mass%) in the steel, respectively. Furthermore, the soft surface layer has a structure in which the area fraction of ferrite is 60.0% or more and 100.0% or less, the value obtained by dividing the area fraction of fresh martensite by the total area fraction of bainitic ferrite, fresh martensite, and tempered martensite, among the structures other than ferrite, is 0.5 or less, and the area fraction of retained austenite is 3.0% or less. This makes it possible to achieve excellent press formability inside the steel sheet (bendability inside the steel sheet) and high crack propagation resistance. (4) With a predetermined chemical composition, by setting the area ratio of retained austenite to less than 3.5% and the area ratio of fresh martensite to 10.0% or less (including 0.0%) as the structure at the 1 / 4 position of the sheet thickness of the base steel sheet, it is possible to achieve excellent press formability at the steel sheet end (bendability of the steel sheet end (shear cross section)).

[0020] The present disclosure has been made based on the above findings. That is, the gist of the present disclosure is as follows. [1] A base steel sheet having a component composition containing, in mass%, C: 0.050% or more and 0.400% or less, Si: 0.02% or more and 3.00% or less, Mn: 1.50% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.005% or more and 2.000% or less, N: 0.0100% or less, Sb: 0.200% or less (inclusive), and Sn: 0.200% or less (inclusive), with the balance consisting of Fe and unavoidable impurities, and a soft surface layer having a Vickers hardness of 84% or less of the Vickers hardness at a 1 / 4 sheet thickness position from the surface of the base steel sheet, wherein the soft surface layer satisfies the following formula (1): the area ratio of retained austenite is 3.0% or less; and the structure in the soft surface layer has an area ratio of ferrite of 60.0% or more and 100.0% or less; when the area ratio of ferrite is less than 100.0%, a value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less; the area ratio of retained austenite is 3.0% or less; and the structure at a 1 / 4 position in the plate thickness of the base steel plate has an area ratio of ferrite of 55.0% or less (including 0.0%); the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is more than 40.0% and 100.0% or less; the area ratio of retained austenite: less than 3.5%; and the area ratio of fresh martensite: 10.0% or less (including 0.0%); and the tensile strength is 1180 MPa or more and less than 1470 MPa. 20≦X≦120−3800×[Sb]−1900×[Sn] (1) In formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.[2] The composition further includes, in mass%, Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, The steel sheet according to [1] above, containing at least one selected from As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less. [3] The steel sheet according to [1] or [2] above, wherein one or both sides of the base steel sheet have a plating layer, the plating layer being a hot-dip galvanized layer. [4] The steel sheet according to [1] or [2] above, wherein one or both sides of the base steel sheet have a plating layer, the plating layer being a galvannealed hot-dip galvanized layer. [5] A member made using the steel sheet according to any of [1] to [4] above. [6] A hot rolling process in which a steel slab having the component composition according to [1] or [2] is hot rolled under the condition of a finish rolling temperature of 820°C or higher; an annealing process in which the steel sheet after the hot rolling process is heated and annealed under the conditions of an annealing temperature of 750°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, further satisfying formulas (2) and (3); a cooling process in which the steel sheet after the annealing process is cooled to a cooling stop temperature of less than 100°C; a first holding process in which the steel sheet after the cooling process is reheated to a reheat holding temperature range of the cooling stop temperature or higher and 440°C or lower, and held for 10 seconds or longer; and a pressure of 2.0 kgf / mm is applied to the steel sheet after the first holding process in the reheat holding temperature range. ２and a second holding step of holding the steel sheet after the surface strain introducing step in the reheating holding temperature range for 10 seconds or more, or further comprising a cold rolling step of cold rolling the steel sheet after the hot rolling step and before the annealing step at a rolling reduction of 20% or more and 80% or less to obtain a cold-rolled steel sheet. 2400≦Y≦20000 ... formula (2) Y = [{(T - Ac1) × t1} / 2}] + {(T - Ac1) × t2} ... formula (3) Where, in formula (3), T is the annealing temperature (°C), t1 is the time (s) from 650°C to the annealing temperature T during the temperature rise in the annealing step, t2 is the annealing time (s), and Ac1 is Ac1 (°C). [7] A method for producing a steel sheet according to [6] above, comprising a hot-dip galvanizing step of, after the annealing step, subjecting the steel sheet to a hot-dip galvanizing treatment to form a hot-dip galvanized layer. [8] A method for producing a steel sheet according to [6] above, comprising a hot-dip galvanizing step of, after the annealing step, subjecting the steel sheet to a hot-dip galvanizing treatment to form a hot-dip galvanized layer. [9] A method for producing a member, comprising a step of subjecting the steel sheet according to any one of [1] to [4] above to at least one of forming and joining to form a member.

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

[0022] 2(a) is a schematic diagram of a sample after 90-degree V bending, and FIG. 2(b) is a view of the sample shown in FIG. 2(a) viewed in the Z direction (negative direction). FIG. 2(b) is a schematic diagram of a sample after 90-degree V bending. FIG. 2(b) is a schematic diagram of an end face crack caused by 90-degree V bending.

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

[0024] [1. Steel Plate] The steel plate of the present invention has a base steel plate having a chemical composition containing, in mass%, C: 0.050% or more and 0.400% or less, Si: 0.02% or more and 3.00% or less, Mn: 1.50% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.005% or more and 2.000% or less, N: 0.0100% or less, Sb: 0.200% or less (inclusive), and Sn: 0.200% or less (inclusive), with the balance consisting of Fe and unavoidable impurities, and has a surface soft layer having a Vickers hardness of 84% or less of the Vickers hardness at a ¼ position of the plate thickness from the surface of the base steel plate, and the surface soft layer satisfies the following formula (1): the area ratio of ferrite in the soft surface layer is 60.0% or more and 100.0% or less, and when the area ratio of ferrite is less than 100.0%, the value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less, and the area ratio of retained austenite is 3.0% or less; In the structure at 1 / 4 of the plate thickness position of the base steel plate, the area ratio of ferrite is 55.0% or less (including 0.0%), the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is more than 40.0% and 100.0% or less, the area ratio of retained austenite is less than 3.5%, and the area ratio of fresh martensite is 10.0% or less (including 0.0%), when a 90-degree V-bend test with a bending radius of 0.5 mm is performed, the length of a crack propagating from the end of the bend ridge in the ridge direction is 200 μm or less, when a notched tensile test is performed after heat treatment at 170°C for 20 minutes, El is 6.0% or more, when a notched tensile test is performed after heat treatment at 170°C for 20 minutes with a 2% nominal tensile strain introduced, El is 5.0% or more, and the tensile strength is 1180 MPa or more and less than 1470 MPa. 20≦X≦120−3800×[Sb]−1900×[Sn] (1) In formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.

[0025] 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 tempered martensite and bainitic ferrite, etc., to ensure a TS of 1180 MPa or more and a high YS. Here, if the C content is less than 0.050%, the area fraction of ferrite increases, making it difficult to achieve a TS of 1180 MPa or more. This also results in a decrease in YS. On the other hand, if the C content exceeds 0.400%, the area fraction of fresh martensite increases excessively, resulting in an excessively high TS and a decrease in El. Furthermore, fresh martensite becomes the starting point for void formation during a 90-degree V-bend test, making it impossible to achieve the desired bendability and bendability of the sheared edge of the steel sheet. Furthermore, the area fraction of retained austenite increases excessively, significantly increasing the fraction of fresh martensite formed by the deformation-induced transformation of retained austenite during shearing, thereby promoting subsequent void formation and crack propagation. As a result, it is more difficult to achieve the desired bendability of the sheared edge. Therefore, the C content is set to 0.050% or more and 0.400% or less. The C content is preferably 0.070% or more. The C content is more preferably 0.080% or more, and even more preferably 0.090% or more. The C content is preferably 0.300% or less. The C content is more preferably 0.280% or less, and even more preferably 0.250% or less.

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

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

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

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

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

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

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

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

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

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

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

[0038] Nb: 0.200% or less Nb increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. To achieve this effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more. On the other hand, if the Nb content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions become the origins of voids and cracks during V-bend tests, making it difficult to ensure the press formability of the steel sheet edge (bendability of the steel sheet edge (shear cross section)). Furthermore, the crack length propagating from the bend edge toward the ridgeline in a 90-degree V-bend test with a bending radius of 0.5 mm cannot be controlled to 200 μm or less. Therefore, when 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 increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. To achieve this effect, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more. 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 origin of voids and cracks during a V-bend test, making it difficult to ensure the press formability of the steel sheet edge (bendability of the steel sheet edge (shear cross section)). Furthermore, the crack length propagating from the bend edge toward the ridgeline in a 90-degree V-bend test with a bending radius of 0.5 mm cannot be controlled to 200 μm or less. Therefore, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.

[0040] V: 0.200% or less Like Nb and Ti, V forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To achieve this effect, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. The V content is further preferably 0.010% or more, and even more preferably 0.030% or more. On the other hand, if the V content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such cases, the coarse precipitates and inclusions become the origin of voids and cracks during a V-bend test, making it difficult to ensure the press formability of the steel sheet end (bendability of the steel sheet end (shear cross section)). Therefore, the crack length propagating from the bend ridge end toward the ridgeline in a 90-degree V-bend test with a bending radius of 0.5 mm cannot be controlled to 200 μm or less. Therefore, when V is contained, the V content is preferably 0.200% or less, and more preferably 0.060% or less.

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

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

[0043] Ni: 1.000% or less. Ni is an element that improves hardenability, and the addition of Ni results in the formation of a large amount of tempered martensite, thereby increasing TS and YS. To achieve this effect, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more. The Ni content is 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, reducing bendability in a V-bend test. This makes it difficult to ensure the press formability of the steel sheet edge (bendability of the steel sheet edge (shear cross section)). Furthermore, in a 90-degree V-bend test with a bending radius of 0.5 mm, the crack length propagating from the bend edge toward the ridgeline cannot be controlled to 200 μm or less. Therefore, when Ni is contained, the Ni content is preferably 1.000% or less. The Ni content is more preferably 0.800% or less. The Ni content is more preferably 0.600% or less, and even more preferably 0.400% or less.

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

[0045] Cu: 1.000% or less Cu is an element that improves hardenability, and the addition of Cu results in the formation of a large amount of tempered martensite, thereby increasing TS and YS. To achieve this effect, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.008% or more, and even more preferably 0.010% or more. The Cu content is more preferably 0.020% or more. The Cu content is further preferably 0.100% or more, and even more preferably 0.150% or more. On the other hand, if the Cu content exceeds 1.000%, the area ratio of fresh martensite may increase excessively. Furthermore, a large amount of coarse precipitates and inclusions may be formed. In such cases, the excessively formed fresh martensite and coarse precipitates and inclusions become the starting points for voids and cracks during a V-bend test, making it difficult to ensure the press formability of the steel sheet edge (bendability of the steel sheet edge (shear cross section)). This makes it impossible to control the length of cracks propagating from the edge of the bend in the ridge direction to 200 μm or less in a 90-degree V-bend test with a bending radius of 0.5 mm. Therefore, when Cu is added, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.

[0046] Ta: 0.100% or less Like Ti, Nb, and V, Ta increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. Additionally, Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb, Ta)(C, N). This suppresses coarsening of precipitates and stabilizes precipitation strengthening. This further improves TS and YS. To achieve this effect, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.002% or more, and even more preferably 0.004% or more. On the other hand, if the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions may be formed. In such cases, excessively coarse precipitates and inclusions become the starting points for voids and cracks during a V-bend test, making it difficult to ensure the press formability of the steel sheet edge (bendability of the steel sheet edge (shear cross section)). This makes it impossible to control the length of cracks that propagate from the edge of the bend in the ridge direction to 200 μm or less in a 90-degree V-bend test with a bending radius of 0.5 mm. Therefore, when Ta is added, the Ta content is preferably 0.100% or less. The Ta content is more preferably 0.030% or less, and even more preferably 0.010% or less.

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

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

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

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

[0051] Zr: 0.1000% or less Like Zn and Co, Zr is an element that effectively spheroidizes the shape of inclusions and improves the hole expandability and bendability of steel sheets. To achieve this effect, the Zr content is preferably 0.0010% or more. On the other hand, if the Zr content exceeds 0.1000%, excessively coarse precipitates and inclusions may become the starting points for voids and cracks during V-bend tests, making it difficult to obtain the desired R / t. Therefore, when Zr is contained, the Zr content is preferably 0.1000% or less. The Zr content is more preferably 0.0300% or less, even more preferably 0.0150% or less, and even more preferably 0.0100% or less.

[0052] Ca: 0.0200% or less Ca exists as inclusions in steel. If the Ca content exceeds 0.0200%, a large amount of coarse inclusions may form. In such cases, excessively coarse precipitates and inclusions may become the starting point for voids and cracks during V-bend testing, preventing the desired R / t from being achieved. Therefore, when Ca is added, the Ca content is preferably 0.0200% or less. The Ca content is preferably 0.0020% or less. The Ca content is more preferably 0.0019% or less, and even more preferably 0.0018% or less. While the lower limit of the Ca content is not particularly limited, the Ca content is preferably 0.0005% or more. Due to production technology constraints, the Ca content is more preferably 0.0010% or more.

[0053] Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less and REM: 0.0200% or less Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are all effective elements for improving the hole expandability and bendability of steel sheet. In order to obtain such effects, it is preferable that the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are each 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 if the content of As exceeds 0.0500%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the excessively coarse precipitates and inclusions may become the starting points for voids and cracks during V-bend testing, making it difficult to obtain the desired R / t. Therefore, when at least one of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM is contained, the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM are preferably 0.0200% or less. Furthermore, when As is contained, the As content is preferably 0.0500% or less.

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

[0055] Steel Structure (Structure at 1 / 4 Plate Thickness Position of Base Steel Plate) Next, the structure of the steel plate according to one embodiment of the present invention excluding the surface soft layer will be described.

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

[0057] The total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): more than 40.0% and not more than 100.0%. Bainitic ferrite and tempered martensite have intermediate hardness between soft ferrite and hard fresh martensite, and are important phases for ensuring good bendability, shear edge bendability, and axial crush characteristics of steel sheets. 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 in improving TS. Therefore, the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): more than 40.0%. The total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is preferably 65.0% or more, and more preferably 80.0% or more. On the other hand, the upper limit of the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) may be 100.0%. The total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) may be 96.0% or less, or 92.0% or less. Bainitic ferrite is upper bainite with little carbide that is formed in a relatively high temperature range.

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

[0059] Area Fraction of Fresh Martensite: 10.0% or Less (Including 0.0%) In the present invention, if the area fraction of fresh martensite is excessively increased, fresh martensite may become the origin of void formation during a 90-degree V-bend test, which may prevent the desired R / t, bendability within the steel sheet, and bendability at the edge (shear cross section) of the steel sheet. Therefore, the area fraction of fresh martensite is set to 10.0% or less. The area fraction of fresh martensite is preferably set to 8.0% or less, and more preferably set to 6.0% or less. Note that the lower limit of the area fraction of fresh martensite is not particularly limited and may be 0.0%. The area fraction of fresh martensite may be set to 1.0% or more or 2.0% or more. Here, fresh martensite refers to as-quenched (untempered) martensite. Furthermore, fresh martensite also includes (isolated) island-like fresh martensite within ferrite grains.

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

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

[0062] Here, the area ratios of ferrite, bainitic ferrite, tempered martensite, and hard second phase (fresh martensite + retained austenite) are measured at a quarter-thickness position of the base steel sheet as follows. That is, a sample is cut out so that the thickness cross section (L cross section) parallel to the rolling direction of the base steel sheet serves as the observation surface. Next, the observation surface of the sample is polished with diamond paste, and then finish-polished using alumina. Next, the observation surface of the sample is etched with 1 vol. % nital to reveal the structure. Next, five fields of view are observed at a magnification of 3000x using an SEM, with the observation position being set at a quarter-thickness position of the steel sheet. The observation field is selected within a range of ±100 μm of ¼ of the steel sheet thickness, with each field of view being 38 μm × 30 μm. From the obtained structural images, Adobe Photoshop from Adobe Systems Inc. was used to calculate the area ratio of each constituent structure (ferrite, bainitic ferrite, tempered martensite, and hard second phase (fresh martensite + retained austenite)) divided by the measured area for five fields of view, and these values ​​were averaged to determine the area ratio of each structure. At the outermost surface, the galvanized layer was excluded, and the image was taken so as to include the internal oxide layer.

[0063] Ferrite: A region that is black and has a blocky shape. It contains almost no carbides. Isolated islands of fresh martensite and isolated islands of retained austenite within ferrite grains are not included in the area ratio of ferrite. Bainitic ferrite: A region that is black to dark gray and has a blocky or amorphous shape. It contains a relatively small number of carbides. Tempered martensite: A region that is gray and has an amorphous shape. It contains a relatively large number of carbides. Hard second phase (retained austenite + fresh martensite): A region that is white to light gray and has an amorphous shape. It does not contain carbides. Carbides: A region that is white and has a dotted or linear shape. It is contained in bainite, tempered bainite, and tempered martensite. Remaining structure: Examples include the above-mentioned pearlite and cementite, and the morphology of these is well known.

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

[0065] The area fraction of fresh martensite is determined by subtracting the area fraction of retained austenite from the area fraction of the hard second phase determined as described above: [Area fraction of fresh martensite (%)] = [Area fraction of hard second phase (%)] - [Area fraction of retained austenite (%)]

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

[0067] Soft Surface Layer The base steel sheet of the steel sheet according to one embodiment of the present invention preferably has a soft surface layer on the surface of the base steel sheet. The soft surface layer contributes to suppressing bending crack propagation during press forming and vehicle collisions, further improving bending fracture resistance. The soft surface layer refers to a decarburized layer, which 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 plate thickness. The Vickers hardness is measured based on JIS Z 2244-1 (2020) at a load of 10 gf.

[0068] Soft surface layer thickness (X): 20≦X≦120−3800×[Sb]−1900×[Sn] (1) In formula (1), X is the soft surface layer thickness (μm), and [Sb] and [Sn] are the Sb and Sn contents (mass%) in the steel, respectively. The soft surface layer in the present invention refers to a region in which the Vickers hardness is 84% ​​or less of the Vickers hardness at a position 1 / 4 of the sheet thickness from the surface of the base steel sheet. The soft surface layer thickness (X) must satisfy formula (1). If the soft surface layer thickness (X) is less than 20 μm, it will be impossible to achieve both high strength and excellent bendability as intended by the present invention. On the other hand, if the soft surface layer thickness (X) exceeds (120−3800×[Sb]−1900×[Sn]) μm, it will be impossible to achieve both high strength and excellent press formability as intended by the present invention. Therefore, the soft surface layer thickness (X) is specified to be 20 μm or more and (120-3800 × [Sb]-1900 × [Sn]) μm or less. In the present invention, as described above, Sb and Sn are added as needed to improve plating ability and chemical conversion treatability. However, when Sb and Sn are added, the surface segregation of these elements as described above reduces the allowable upper limit of the soft surface layer thickness (X) that affects bending cracking. For this reason, the upper limit of the soft surface layer that provides good bendability is (120-3800 × [Sb]-1900 × [Sn]) μm. The soft surface layer thickness is preferably 30 μm or more, more preferably 40 μm or more. The soft surface layer thickness is preferably 120 μm or less, more preferably 100 μm or less.

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

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

[0071] Area ratio of retained austenite: 3.0% or less 2.0 kgf / mm in the reheating holding temperature range after the first holding step ２In the surface strain introduction step in which the above tension is applied, untransformed austenite in the soft surface layer is transformed into hard martensite by stress-induced transformation, and in the subsequent second holding step in which the holding time is 10 seconds or more, the hard martensite is tempered to form tempered martensite, and by finally controlling the area fraction of retained austenite to 3.0% or less, the length of cracks propagating from the end of the bend ridgeline toward the ridgeline in a 90-degree V-bend test with a bend radius R of 0.5 mm can be reduced to 200 μm or less. If the area fraction of retained austenite exceeds 3.0%, hard martensite is formed by stress-induced transformation of the retained austenite during the V-bend test, and voids are generated and cracks propagate at the heterophase boundary with the hard martensite. As a result, it becomes difficult to ensure the press formability of the steel sheet end (bendability of the steel sheet end (shear cross section)), and the length of cracks propagating from the bend ridge end in the ridge direction in a 90-degree V-bend test with a bending radius of 0.5 mm cannot be controlled to 200 μm or less. Therefore, the area fraction of retained austenite is set to 3.0% or less. The area fraction of retained austenite is preferably set to 2.8% or less, and more preferably set to 2.5% or less. There is no particular restriction on the lower limit of the area fraction of retained austenite, but it is preferably set to 0.2% or more, and more preferably set to 0.5% or more.

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

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

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

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

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

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

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

[0079] The galvannealed layer is preferably composed of, for example, 20.0 mass% or less Fe and 0.001 mass% to 1.0 mass% or less Al. The galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% to 3.5 mass%. The Fe content of the galvannealed layer is more preferably 7.0 mass% or more, and even more preferably 8.0 mass% or more. The Fe content of the galvannealed layer is more preferably 15.0 mass% or less, and even more preferably 12.0 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 80g / m or more ２ It is preferable to do the following:

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

[0082] The thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably 0.5 mm or more, more preferably 0.6 mm or more, and even more preferably 0.8 mm or more. 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. Manufacturing Method of Steel Sheet] Next, a manufacturing method of a steel sheet according to one embodiment of the present invention will be described. The manufacturing method of a steel sheet of the present invention includes a hot rolling step of hot rolling a steel slab having the above-described chemical composition under a finish rolling temperature of 820°C or higher, an annealing step of heating the steel sheet after the hot rolling step and annealing it under conditions of an annealing temperature of 750°C to 900°C, an annealing time of 20 seconds or longer, and an atmosphere with a dew point of -10°C or higher, and further satisfying formulas (2) and (3), a cooling step of cooling the steel sheet after the annealing step to a cooling stop temperature of less than 100°C, a first holding step of reheating the steel sheet after the cooling step to a reheating holding temperature range of the cooling stop temperature to 440°C and holding for 10 seconds or longer, and a pressure of 2.0 kgf / mm ２ and a second holding step of holding the steel sheet after the surface strain introducing step in a reheating holding temperature range for 10 seconds or more, or further includes a cold rolling step of cold rolling the steel sheet after the hot rolling step but before the annealing step at a rolling reduction of 20% or more and 80% or less to obtain a cold-rolled steel sheet. 2400≦Y≦20000 ... formula (2) Y = [{(T - Ac1) × t1} / 2}] + {(T - Ac1) × t2} ... formula (3) Where, in formula (3), T is the annealing temperature (°C), t1 is the time (s) from 650°C to the annealing temperature T during the temperature rise in the annealing step, t2 is the annealing time (soaking time) (s), and Ac1 is Ac1 (°C). Unless otherwise specified, the above temperatures refer to the surface temperatures of the steel slab and steel plate.

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

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

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

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

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

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

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

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

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

[0093] Annealing temperature: 750°C or higher and 900°C or lower If the annealing temperature is lower than 750°C, the austenite generation rate during heating in the two-phase region of ferrite and austenite becomes insufficient. As a result, the area ratio of ferrite increases excessively after annealing, and the desired TS and YS cannot be obtained. On the other hand, if the annealing temperature exceeds 900°C, the ductility decreases. Therefore, the annealing temperature is set to 750°C or higher and 900°C or lower. The annealing temperature is preferably 880°C or lower. The annealing temperature is more preferably 870°C or lower. The annealing temperature is preferably 780°C or higher, more preferably 800°C or higher. The annealing temperature is the maximum temperature reached in the annealing process (soaking temperature).

[0094] Annealing time (soaking time): 20 seconds or more. If the annealing time is less than 20 seconds, the austenite generation rate during heating in the two-phase region of ferrite and austenite becomes insufficient. As a result, the area ratio of ferrite increases excessively after annealing, and TS and YS cannot be obtained. Therefore, the annealing time is set to 20 seconds or more. The annealing time is preferably 30 seconds or more, more preferably 50 seconds or more. Although the upper limit of the annealing time is not particularly limited, the annealing time is preferably 600 seconds or less, more preferably 250 seconds or less. The annealing time (soaking time) refers to the holding time in a temperature range of (annealing temperature -40°C) or more and the annealing temperature or less. In other words, the annealing time includes not only the holding time at the annealing temperature but also the residence time in a temperature range of (annealing temperature -40°C) or more and the annealing temperature or less during heating and cooling before and after reaching the annealing temperature. The number of annealing steps may be two or more, but one step is preferable from the viewpoint of energy efficiency.

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

[0096] 2400≦Y≦20000 ... Equation (2) Y = [{(T - Ac1) × t1} / 2}] + {(T - Ac1) × t2} ... Equation (3) Here, in equation (3), T: annealing temperature (°C), t1: time (s) from 650°C to the annealing temperature (soaking temperature) T during the temperature rise in the annealing process, t2: annealing time (s), and Ac1: Ac1 (°C). In the present invention, it is necessary to perform annealing under annealing conditions that satisfy equations (2) and (3). When Y in equation (3) is less than 2400, the soft surface layer defined in the present invention will be less than 20 μm. On the other hand, when Y is more than 20000, the soft surface layer defined in the present invention will be more than (120 - 3800 × [Sb] - 1900 × [Sn]) μm. Therefore, Y in formula (3) is set to be 2400 or more and 20000 or less. Y is preferably 9000 or more, more preferably 12000 or more. Y is preferably 19000 or less, more preferably 18000 or less. t1 is preferably 30 seconds or more. t1 is preferably 80 seconds or less.

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

[0098] (Cooling Step) Cooling to a Cooling Stop Temperature of Less than 100°C Average Cooling Rate: 10°C / s or More and 50°C / s or Less, Atmospheric Dew Point: -20°C or Less (Preferred Requirements) In the cooling step, the steel sheet after the annealing step is cooled to a cooling stop temperature of less than 100°C. At this time, the cooling start temperature can be 750°C or more and 900°C or less. The cooling stop temperature is preferably 80°C or less, more preferably 60°C or less. The cooling stop temperature is preferably 5°C or more, more preferably 15°C or more. The average cooling rate during this cooling step is preferably 10°C / s or more and 50°C / s or less. The steel structure specified in the present invention can be obtained in this cooling step. Furthermore, the atmospheric dew point during this cooling step is preferably -20°C or less. If the atmospheric dew point exceeds -20°C, the thickness of the soft surface layer of the steel sheet varies greatly in the in-plane direction, and the tensile strength specified in the present invention may not be obtained. Therefore, it is preferable to set the dew point of the atmosphere in this cooling step to −20° C. or lower. The average cooling rate (° C. / s) is obtained by dividing the difference between the cooling start temperature (° C.) and the cooling stop temperature (° C.) in the cooling step by the cooling time (s).

[0099] (First Holding Step (Reheating Holding Step)) In the first holding step (reheating holding step), the steel sheet is reheated to a reheating holding temperature range of not less than the cooling stop temperature but not more than 440°C, and held for 10 seconds or more. By reheating the steel sheet to a reheating holding temperature range of not less than the cooling stop temperature but not more than 440°C, and held for 10 seconds or more, it is possible to form bainitic ferrite and tempered martensite as defined in the present invention. The reheating holding temperature range is preferably not more than 420°C, and more preferably not more than 400°C. The holding time is preferably not less than 20 seconds, and more preferably not less than 30 seconds. The holding time is preferably not more than 100 seconds, and more preferably not more than 80 seconds.

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

[0101] (Second Holding Step) In the second holding step, the steel sheet after the surface strain introduction step is held in the reheating holding temperature range for 10 seconds or more. This allows the E1 to be controlled to be equal to or greater than the value specified in the present invention when a notched tensile test is performed after introducing a 2% nominal tensile strain and subjecting the steel sheet to a heat treatment at 170°C for 20 minutes. The holding time in the second holding step is preferably 15 seconds or more, more preferably 20 seconds or more. The holding time is preferably 60 seconds or less, more preferably 50 seconds or less.

[0102] (Plating Process (Hot-Dip Galvanizing Process, Galvannealed Hot-Dip Galvanizing Process)) In the present invention, after the annealing process, a galvanized steel sheet can be obtained by subjecting the steel sheet to a galvanizing process in the plating process. Examples of the galvanizing process include a hot-dip galvanizing process and an alloyed hot-dip galvanizing process. The galvanizing process may be carried out, for example, during the cooling process, during the first holding process, after the first holding process and before the surface layer strain introducing process, after the surface layer strain introducing process and before the second holding process, during the second holding process, or after the second holding process. The galvanizing process may be carried out, for example, after the annealing process and before the first holding process, during the cooling process in the cooling process.

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

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

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

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

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

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

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

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

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

[0112] Here, in the plating 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 plating process is also indicated as "GI" or "GA". In Table 2, in the case of GI steel sheets, no alloying treatment is performed, so the alloying temperature is indicated as -. In Table 2, steel sheets that are not treated in the plating process are indicated as "CR".

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

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

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

[0116] The structure of the soft surface layer was identified at a position halfway through the thickness of the soft surface layer by the same method as used to identify the steel structure of the base steel sheet.

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

[0118] TS (tensile strength) ◯ (pass): 1180 MPa or more and less than 1470 MPa × (fail): less than 1180 MPa or 1470 MPa or more

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

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

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

[0122] A 90-degree V-bend test was performed with a bending radius of 0.5 mm, and the length of the crack that propagated along the bend ridge formed other than the end of the bend ridge (press formability of the steel sheet end (bendability inside the steel sheet)) ○ (Pass): The length of the crack other than the V-bend end face was 200 μm or less × (Fail): The length of the crack other than the V-bend end face was more than 200 μm

[0123] A 90-degree V-bend test was performed with a bending radius of 0.5 mm, and the crack length propagating from the end of the bend ridge in the ridge direction (press formability of the steel sheet end (bendability of the steel sheet end (shear cross section))) ◯ (Pass): The V-bend end crack length is 200 μm or less × (Fail): The V-bend end crack length is more than 200 μm

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

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

[0126] (2) 90-degree V-bend test: A rectangular test piece measuring 100 mm C (C direction: the direction perpendicular to the rolling direction of the steel sheet) x 30 mm L (L direction: the direction along the rolling direction) was taken from the quarter-coil width position of the obtained steel sheet. The 100 mm long end surface was sheared, and the sheared state (without machining to remove burrs) was bent so that the burrs were on the outer periphery of the bend. The shear clearance was 15% and the rake angle was 0 degrees. V-bending was performed using an autograph manufactured by Shimadzu Corporation, with a punch bending radius of R = 0.5 mm, a punch bending angle of 90 degrees, a punch stroke speed of 30 mm / min, a pressing load of 10 tons, and a pressing time of 5 seconds, resulting in an L-direction bending (bending ridge length: 30 mmL).

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

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

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

[0130]

[0131]

[0132]

[0133]

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

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

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

Claims

1. A base steel plate having a component composition containing, in mass%, the following: C: 0.050% or more and 0.400% or less, Si: 0.02% or more and 3.00% or less, Mn: 1.50% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.005% or more and 2.000% or less, N: 0.0100% or less, Sb: 0.200% or less (inclusive), and Sn: 0.200% or less (inclusive) with the balance being Fe and unavoidable impurities; and a soft surface layer having a Vickers hardness of 84% or less of the Vickers hardness at a 1 / 4 position of the plate thickness from the surface of the base steel plate, wherein the soft surface layer satisfies the following formula (1), a structure in the soft surface layer, the area ratio of ferrite is 60.0% or more and 100.0% or less; when the area ratio of ferrite is less than 100.0%, a value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less; an area ratio of retained austenite is 3.0% or less; a structure at a 1 / 4 position of the sheet thickness of the base steel sheet, the area ratio of ferrite is 55.0% or less (including 0.0%); a total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is more than 40.0% and 100.0% or less; an area ratio of retained austenite: less than 3.5%; an area ratio of fresh martensite: 10.0% or less (including 0.0%); and a tensile strength of 1,180 MPa or more and less than 1,470 MPa. 20≦X≦120−3800×[Sb]−1900×[Sn] (1) In formula (1), X is the thickness (μm) of the surface soft layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.

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

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

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

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

6. A hot rolling process in which a steel slab having the chemical composition according to claim 1 or 2 is hot-rolled under the condition of a finish rolling temperature of 820°C or higher; an annealing process in which the steel sheet after the hot rolling process is heated and annealed under the conditions of an annealing temperature of 750°C to 900°C, an annealing time of 20 seconds or longer, and an atmosphere with a dew point of -10°C or higher, further satisfying the conditions of formulas (2) and (3); a cooling process in which the steel sheet after the annealing process is cooled to a cooling stop temperature of less than 100°C; a first holding process in which the steel sheet after the cooling process is reheated to a reheat holding temperature range of the cooling stop temperature to 440°C or lower and held for 10 seconds or longer; ２ and a second holding step of holding the steel sheet after the surface strain introducing step in the reheating holding temperature range for 10 seconds or more, or further comprising a cold rolling step of cold rolling the steel sheet after the hot rolling step and before the annealing step with a rolling reduction of 20% or more and 80% or less to obtain a cold rolled steel sheet. 2400≦Y≦20000 ... formula (2) Y = [{(T-Ac1)×t1} / 2}] + {(T-Ac1)×t2} ... formula (3) Where, in formula (3), T is the annealing temperature (°C), t1 is the time (s) from 650°C to the annealing temperature T during the temperature rise in the annealing step, t2 is the annealing time (s), and Ac1 is Ac1 (°C).

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

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

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

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