Steel sheet, member, and production methods for these

Abstract

The present invention provides a steel sheet having a tensile strength of 780 MPa or more, having high ductility and excellent stretch flange formability, and having excellent time-of-collision energy absorption characteristics, and also provides a member, and production methods for these.　The steel sheet has a composition containing specific quantities of C, Si, Mn, P, S, sol. Al, and N, and has a steel structure in which, in terms of area ratio, the total of ferrite and bainitic ferrite is 5-60%, tempered martensite is 20-80%, fresh martensite is 20% or less (including 0%), and retained austenite is 5-25%, and a structure comprising one or two or more among ferrite, tempered martensite, fresh martensite, bainite, and retained austenite is 90% or more (including 100%). Of the ferrite, high-Mn ferrite is 20-70%, inclusive, in terms of area ratio, and SC≥0.5 / SC≥0.3×100 is 20% or more.

Description

Steel plates, components, and their manufacturing methods

[0001] The present invention relates to steel sheets and members used in various applications such as automobiles and home appliances, and to methods for manufacturing the same.

[0002] In recent years, due to the increasing need for lighter automobile bodies, the application of high-strength steel sheets having a tensile strength of 780 MPa or more to automobile frame parts and seat parts has been increasing. However, when high-strength steel sheets having a tensile strength of 780 MPa or more are applied to automobile parts, press cracks tend to occur due to reduced ductility and reduced stretch flange formability. Therefore, these high-strength steel sheets are desired to have better formability than conventional steel sheets.

[0003] Furthermore, from the perspective of passenger safety, it is necessary to suppress deformation around the cabin during a collision, and therefore energy-absorbing components such as side members are required to absorb collision energy by deforming during a collision. However, high-strength steel plates with a tensile strength of 780 MPa or more have a problem in that, due to reduced axial crushing properties, fractures of the components are likely to occur at locations that have undergone primary processing by forming during a collision, making it difficult to stably exhibit collision energy absorption capacity.

[0004] In light of this background, TRIP steel, in which residual γ is dispersed in the microstructure of a steel sheet, has been developed as a technique for improving the ductility of the steel sheet. For example, Patent Document 1 discloses a steel sheet containing, in mass %, C: 0.15% to 0.30%, P: 0.040% to 0.0100%, S: 0.0100% to 0.0100%, N: 0.0100% to 0.0060%, one or two of Si and Al: 0.70% to 2.50% in total, one or two of Mn and Cr: 1.50% to 3.50% in total, Mo: 0% or more, 1.00% or less, Ni: 0% or more, 1.00% or less, Cu: 0% or more, 1.00% or less, Nb: 0% or more, 0.30% or less, Ti: 0% or more, 0.30% or less, V: 0% or more, 0.30% or less, B: 0% or more, 0.0050% or less, Ca: 0% or more, 0.0400% or less, Mg: 0% or more, 0.0400% or less, and REM: 0% or more, and 0.0400% or less, with the remainder consisting of Fe and impurities, and the steel sheet has, in terms of area ratio relative to the entire structure, one or two of ferrite and granular bainite: 10% or more and 50% or less in total, one or two of upper bainite and lower bainite: 10% or more and 50% or less in total, tempered martensite: more than 0% and 30% or less, retained austenite: 5% or more, and one or two or more of pearlite, cementite, and martensite: 0% or more and 10% or less in total, and by setting the area ratio of the ferrite to the total area ratio of the ferrite and the granular bainite to 25% or less, a steel sheet having a tensile strength of 980 MPa or more and excellent elongation and hole expandability can be obtained.

[0005] In Patent Document 2, the steel composition contains, in mass %, C: 0.07 to 0.20%, Si: 0.1 to 2.0%, Mn: 2.0 to 3.5%, P: 0.05% or less, S: 0.05% or less, and Sol. Al: 0.005 to 0.1%, with the balance being Fe and unavoidable impurities, and the steel structure has, in area ratios, ferrite: 60% or less, tempered martensite: 40% or more, and fresh martensite: 10% or less, and the void number density of the bent portion in a VDA bending test is 1500 voids / mm ２It is disclosed that by satisfying the following conditions, a high-strength hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more and excellent fracture resistance characteristics in the event of a collision can be obtained.

[0006] Patent Document 3 discloses a high-strength cold-rolled steel sheet containing, by mass%, C: 0.10 to 0.40%, Mn: 0.5 to 4.0%, Si: 0.005 to 2.5%, Al: 0.005 to 2.5%, and Cr: 1.0% or less, with the balance being iron and inevitable impurities. P: 0.05% or less, S: 0.02% or less, and N: 0.006% or less. The steel structure contains, in terms of area ratio, 2 to 30% retained austenite, martensite is limited to 20% or less, the average grain size of cementite is 0.01 μm or more and 1 μm or less, and the cementite contains 30% or more and 100% or less cementite having an aspect ratio of 1 or more and 3 or less. This discloses that a high-strength cold-rolled steel sheet having excellent strength, ductility, and hole expandability can be obtained.

[0007] Patent No. 6338038 Patent No. 6795122 Patent No. 4903915

[0008] The technology described in Patent Document 1 utilizes granular bainite to minimize the difference in hardness between different phases in a dual-phase steel sheet, thereby suppressing the deterioration of stretch flangeability associated with an increase in ferrite. This enables the production of steel sheets with both excellent ductility and stretch flangeability, but the area ratio of tempered martensite is small, at 30% or less, resulting in poor axial crushing properties. The technology described in Patent Document 2 reduces void density to enable the production of steel sheets with excellent axial crushing properties, but the absence of bainite, which has a hardness intermediate between ferrite and tempered martensite, results in a large difference in hardness between different phases and poor stretch flangeability. Patent Document 3 describes that controlling the shape of cementite can produce steel sheets with excellent ductility and stretch flangeability, but does not consider the axial crushing properties. During bending, cracks originate in the cementite, resulting in poor axial crushing properties.

[0009] As described above, the prior art was inferior in at least one of ductility, stretch flangeability, and energy absorption characteristics (axial crushing characteristics) during a collision.

[0010] The present invention has been made to solve these problems, and aims to provide a steel plate, a member, and a method for manufacturing the same, which have a tensile strength of 780 MPa or more, high ductility and excellent stretch flange formability, and further have excellent energy absorption properties during a collision.

[0011] Here, in the present invention, the tensile strength is measured by a tensile test in accordance with JIS Z 2241 (2011).

[0012] In the present invention, high ductility refers to the following, with respect to total elongation (T-El) measured in a tensile test in accordance with JIS Z 2241 (2011): (A) when TS is 780 MPa or more and less than 980 MPa, T-El is 18.0% or more; (B) when TS is 980 MPa or more and less than 1180 MPa, T-El is 16.0% or more; (C) when TS is 1180 MPa or more and less than 1320 MPa, T-El is 14.0% or more; and (D) when TS is 1320 MPa or more, T-El is 13.0% or more.

[0013] In the present invention, excellent stretch flangeability is defined as a property in which a sample having a size of 100 mm x 100 mm square is punched using a punching tool having a punch diameter of 10 mm and a die diameter of 10.3 mm (clearance 13%), and then the hole is expanded using a conical punch with an apex angle of 60 degrees so that the burrs generated during the formation of the punched hole are on the outside until a crack penetrating the plate thickness occurs. ０ : initial hole diameter (mm), d: hole diameter at crack occurrence (mm), hole expansion ratio λ (%) = {(d - d ０ ) / d ０}×100 is 30% or more.

[0014] In the present invention, "excellent energy absorption properties during a collision" refers to an energy absorption amount of 13,000 MPa·% or more, where the area up to the maximum stress in a nominal stress-nominal strain curve in a tensile test in accordance with JIS Z 2241 (2011) is regarded as the energy absorption amount during deformation.

[0015] The present inventors conducted extensive research into means for imparting high ductility, excellent stretch flangeability, and excellent energy absorption characteristics during a collision. As a result, they discovered that by creating a structure in which the total area fraction of ferrite, tempered martensite, fresh martensite, bainite, and retained austenite is 90% or more, high ductility and excellent stretch flangeability are ensured, and by appropriately controlling the phase fractions of each structure and adjusting the amount of solute Mn in ferrite over a wide appropriate range, excellent energy absorption characteristics during a collision can also be obtained. Furthermore, they clarified that these structures can be realized by annealing at a predetermined temperature and then cooling to a temperature range of −15°C from the annealing temperature at an average cooling rate CR1 of 0.01°C / s or more and 5°C / s or less, thereby promoting ferrite transformation without Mn diffusion and including both ferrite formed during annealing and cooling.

[0016] The present invention has been made based on the above findings, and specifically provides the following: [1] In mass%, C: 0.06% or more and 0.25% or less, Si: 0.4% or more and 2.5% or less, Mn: 1.5% or more and 3.5% or less, P: 0.10% or less, S: 0.010% or less, sol. and has a component composition containing Al: 1.0% or less, N: 0.015% or less, with the balance being Fe and unavoidable impurities, and has a steel structure in which, in terms of area percentages in the entire structure, the total of ferrite and bainitic ferrite is 5% or more and 60% or less, tempered martensite is 20% or more and 80% or less, fresh martensite is 20% or less (including 0%), and in terms of volume percentages, retained austenite is 5% or more and 25% or less, and has a steel structure in which the total area percentage of ferrite, tempered martensite, fresh martensite, bainite and retained austenite is 90% or more (including 100%), and when the ferrite exceeds 0%, in the ferrite, the proportion of ferrite having a solute Mn content of 2.0 mass% or more in the entire ferrite is 20% or more and 70% or less in terms of area percentage, and Ｃ≧０．３ The area S of the region where the C concentration is 0.5 mass% or more Ｃ≧０．５ The ratio of: (S Ｃ≧０．５ / S Ｃ≧０．３ ) × 100 is 20% or more. [2] The steel sheet according to [1] above, wherein the chemical composition further contains, in mass%, one or more selected from Ti: 0.1% or less, B: 0.01% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1.0% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, Zr: 0.2% or less, W: 0.2% or less, Ca: 0.0040% or less, Ce: 0.0040% or less, La: 0.0040% or less, Mg: 0.0030% or less, Sb: 0.1% or less, and Sn: 0.1% or less. [3] The steel sheet according to [1] above, wherein the steel structure further contains one or more internal carbides having a size of 10 μm or less. ２[4] The steel sheet according to any one of [1] to [3], which has a zinc-plated layer on its surface. [5] A member made using the steel sheet according to any one of [1] to [4]. [6] A steel slab having the chemical composition according to [1] or [2] is hot-rolled and cold-rolled, and then the resulting cold-rolled steel sheet is annealed, and the annealing comprises: a step of holding at an annealing temperature of 775°C or more and 830°C or less; a step of cooling at an average cooling rate CR1 of 0.01°C / s or more and 5°C / s or less in a temperature range from the annealing temperature to the annealing temperature -15°C; a step of cooling at an average cooling rate CR2 of 3°C / s or more in a temperature range from the annealing temperature -15°C to a cooling stop temperature of 200°C or more and 300°C or less; a step of heating at an average heating rate of 2°C / s or more in a temperature range from the cooling stop temperature to 380°C; and a step of retaining at an average cooling rate CR4 of 0.01 to 5°C / s for 20 seconds or more and 3000 seconds or less in a temperature range of 340°C or more and 590°C or less. and a step of cooling the steel sheet to a temperature of 50°C or less at an average cooling rate CR5 of 0.1°C / s or more.[7] A steel slab having the chemical composition described in [1] or [2] is hot-rolled and cold-rolled, and then the resulting cold-rolled steel sheet is annealed, and the annealing comprises: a step of holding at an annealing temperature of 775°C or more and 830°C or less; a step of cooling at an average cooling rate CR1 of 0.01°C / s or more and 5°C / s or less in a temperature range from the annealing temperature to the annealing temperature -15°C; a step of cooling at an average cooling rate CR2A of 3°C / s or more in a temperature range from the annealing temperature -15°C to 500°C; a step of holding at an average cooling rate CR3 of 10°C / s or less for 10 seconds or more and 60 seconds or less in a temperature range from 500°C to a martensitic transformation start temperature Ms or more and a residence stop temperature of 320°C or more; and a step of cooling at an average cooling rate CR2B of 3°C / s or more in a temperature range from the residence stop temperature to a cooling stop temperature of 200°C to 300°C. A method for manufacturing a steel sheet, comprising, in this order: heating the steel sheet at a temperature range from the cooling stop temperature to 380°C at an average heating rate of 2°C / s or more, retaining the steel sheet at a temperature range of 340°C to 590°C at an average cooling rate CR4 of 0.01 to 5°C / s for 20 seconds to 3,000 seconds, and cooling the steel sheet at an average cooling rate CR5 of 0.1°C / s to a temperature of 50°C or less. [8] A method for manufacturing a steel sheet according to [6] or [7] above, wherein the steel sheet is subjected to a hot-dip galvanizing treatment or a galvannealed hot-dip galvanizing treatment in the retaining step at an average cooling rate CR4 of 0.01 to 5°C / s. [9] A method for manufacturing a steel sheet according to [6] or [7] above, comprising a step of performing an electrogalvanizing treatment after the cooling step at an average cooling rate CR5 of 0.1°C / s or more.

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

[0017] According to the present invention, it is possible to obtain a steel sheet having high ductility and excellent stretch flangeability, and further having excellent energy absorption properties in a collision. Furthermore, according to the present invention, it is also possible to increase the strength. If the steel sheet of the present invention is applied to automobile parts, the weight of the automobile parts can be reduced, and improvement in fuel efficiency is expected.

[0018] Fig. 1 is an example of an SEM photograph of the steel structure of a steel sheet. Fig. 2 is a diagram for explaining a method for measuring the steel structure of a steel sheet of the present invention. Fig. 3 is a diagram for explaining a method for manufacturing a steel sheet of the present invention, where (a) is a diagram for explaining a manufacturing method without a retention treatment and (b) is a diagram for explaining a manufacturing method with a retention treatment.

[0019] The present invention will be specifically described below, but the present invention is not limited to the following embodiments.

[0020] <Steel Sheet> The steel sheet of the present invention contains, in mass %, C: 0.06% or more and 0.25% or less, Si: 0.4% or more and 2.5% or less, Mn: 1.5% or more and 3.5% or less, P: 0.10% or less, S: 0.010% or less, sol. The steel has a component composition containing Al: 1.0% or less, N: 0.015% or less, and the balance being Fe and unavoidable impurities, and has a steel structure in which, in terms of area percentages in the entire structure, the total of ferrite and bainitic ferrite is 5% or more and 60% or less, tempered martensite is 20% or more and 80% or less, fresh martensite is 20% or less (including 0%), and, in terms of volume percentages, retained austenite is 5% or more and 25% or less, and the total of ferrite, tempered martensite, fresh martensite, bainite, and retained austenite is 90% or more (including 100%), and when the ferrite content exceeds 0%, the proportion of ferrite in which the amount of solute Mn is 2.0% by mass or more in the entire ferrite is 20% or more and 70% or less, and the area S of a region in which the C concentration is 0.3% by mass or more Ｃ≧０．３ The area S of the region where the C concentration is 0.5 mass% or more Ｃ≧０．５ The ratio of: (S Ｃ≧０．５ / S Ｃ≧０．３ ) × 100 is 20% or more. The steel sheet of the present invention will be described below in the order of chemical composition and steel structure.

[0021] The steel sheet of the present invention contains the following components: In the following description, the unit of "%" for the content of each component means "mass %."

[0022] C: 0.06% or more and 0.25% or less. C is contained from the viewpoints of ensuring the area ratio of tempered martensite to ensure a predetermined strength, ensuring the area ratio (volume ratio) of retained austenite (residual γ) to improve ductility, and concentrating C in the retained γ to stabilize the retained γ to improve ductility. Since these effects cannot be fully ensured when the C content is less than 0.06%, the lower limit is set to 0.06%. The C content is preferably 0.09% or more, and more preferably 0.11% or more. On the other hand, when the C content exceeds 0.25%, the strength becomes excessively high and ductility decreases. In addition, the increase in blocky martensite may deteriorate stretch flangeability. Therefore, the upper limit of the C content is set to 0.25%. From the viewpoint of improving ductility, the C content is preferably 0.22% or less. From the viewpoint of further improving ductility, the C content is more preferably 0.20% or less.

[0023] Si: 0.4% or more and 2.5% or less. Si is contained from the viewpoint of strengthening ferrite to increase strength, suppressing carbide formation in martensite and bainite, improving the stability of residual γ, and improving ductility. From these viewpoints, the Si content is set to 0.4% or more. From the viewpoint of improving ductility, the Si content is preferably 0.6% or more. The Si content is more preferably 0.8% or more. On the other hand, if the Si content exceeds 2.5%, the temper softening resistance increases excessively, resulting in increased strength of massive fresh martensite, thereby reducing stretch flangeability. In addition, the rolling load during hot rolling becomes extremely high, making it difficult to manufacture thin plates. Furthermore, the chemical conversion treatability and toughness of welds may deteriorate. Therefore, the Si content is set to 2.5% or less. From the viewpoints of ensuring chemical conversion treatability and the toughness of the material and welds, the Si content is preferably less than 2.0%. From the viewpoint of ensuring toughness of welds, the Si content is preferably 1.8% or less, and even more preferably 1.5% or less.

[0024] Mn: 1.5% or more and 3.5% or less. Mn is an important element from the viewpoint of ensuring strength by ensuring a predetermined area fraction of tempered martensite and / or bainite, and from the viewpoint of stabilizing residual γ by lowering the Ms point of residual γ and improving ductility. Similarly to Si, Mn is an important element from the viewpoint of suppressing the formation of carbides in bainite to improve ductility, and from the viewpoint of increasing the volume fraction of residual γ to improve ductility. To achieve these effects, the Mn content is set to 1.5% or more. From the viewpoint of stabilizing residual γ and improving ductility, the Mn content is preferably 2.5% or more. The Mn content is more preferably 2.6% or more, and even more preferably 2.7% or more. On the other hand, if the Mn content exceeds 3.5%, the bainite transformation is significantly delayed, resulting in reduced ductility. Furthermore, if the Mn content exceeds 3.5%, it becomes difficult to suppress the formation of blocky coarse γ and blocky coarse martensite, and stretch flangeability also deteriorates. Furthermore, if the Mn content exceeds 3.5%, the hardenability increases excessively. Therefore, the amount of ferrite with a solute Mn content of 2.0 mass% or more, which is generated in the process of cooling the temperature range from the annealing temperature to the annealing temperature −15°C at an average cooling rate CR1 of 0.01°C / s or more and 5°C / s or less, becomes insufficient, and sufficient energy absorption characteristics during collision may not be obtained. Therefore, the Mn content is set to 3.5% or less. From the viewpoint of promoting ferrite transformation and bainite transformation to ensure high ductility and energy absorption characteristics during collision, the Mn content is preferably set to 3.2% or less. The Mn content is more preferably 3.1% or less.

[0025] P: 0.10% or less P is an element that strengthens steel, but a high content of P deteriorates spot weldability. Therefore, the P content is set to 0.10% or less. The P content is preferably set to 0.02% or less. From the viewpoint of improving spot weldability, the P content is more preferably set to 0.01% or less. Note that P may not be contained, but from the viewpoint of manufacturing costs, the P content is preferably 0.001% or more.

[0026] S: 0.010% or less S has the effect of improving scale peelability during hot rolling and suppressing nitriding during annealing, but is an element that has a negative effect on spot weldability, bendability, and hole expandability. To reduce these negative effects, the S content is set to 0.010% or less. In the present invention, the high contents of C, Si, and Mn tend to deteriorate spot weldability. Therefore, from the viewpoint of improving spot weldability, the S content is preferably set to 0.0020% or less, and more preferably less than 0.0010%. While S may not be contained, the S content is preferably 0.0001% or more from the viewpoint of manufacturing costs. The S content is more preferably 0.0005% or more.

[0027] Sol. Al: 1.0% or less. Al is contained for the purpose of deoxidation or as a substitute for Si to stabilize residual γ. Although there is no particular lower limit for sol. Al, the sol. Al content is preferably 0.005% or more to ensure stable deoxidation. The sol. Al content is more preferably 0.01% or more, even more preferably 0.02% or more, and even more preferably 0.03% or more. On the other hand, if the sol. Al content exceeds 1.0%, the strength of the material will be extremely reduced and will have a negative impact on chemical conversion treatability, so the sol. Al content is set to 1.0% or less. To obtain high strength, the sol. Al content is preferably less than 0.50%, more preferably 0.20% or less. The sol. Al content is more preferably 0.15% or less, and even more preferably 0.10% or less.

[0028] N: 0.015% or less N is an element that forms nitrides such as BN, AlN, and TiN in steel, and reduces the hot ductility of steel and the surface quality. Furthermore, in steels containing B, N has the adverse effect of eliminating the effect of B through the formation of BN. If the N content exceeds 0.015%, the surface quality deteriorates significantly. Therefore, the N content is set to 0.015% or less. The N content is preferably 0.010% or less. While N may not be contained, the N content is preferably 0.0001% or more from the viewpoint of manufacturing costs. The N content is more preferably 0.001% or more.

[0029] The balance other than the above is Fe and unavoidable impurities. The steel sheet of the present invention preferably has a component composition containing the above basic components with the balance being Fe and unavoidable impurities.

[0030] The steel sheet of the present invention may contain, in addition to the above-mentioned components, one or more of the following optional elements in its chemical composition: Ti: 0.1% or less, B: 0.01% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1.0% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, Zr: 0.2% or less, W: 0.2% or less, Ca: 0.0040% or less, Ce: 0.0040% or less, La: 0.0040% or less, Mg: 0.0030% or less, Sb: 0.1% or less, and Sn: 0.1% or less.

[0031] Ti: 0.1% or less Ti fixes N in steel as TiN, improving hot ductility and improving the hardenability of B. It also has the effect of refining the structure by precipitation of TiC. To achieve these effects, the Ti content is preferably 0.002% or more. From the viewpoint of sufficiently fixing N, the Ti content is more preferably 0.008% or more. The Ti content is further preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.1%, an increase in rolling load and a decrease in ductility due to an increase in the amount of precipitation strengthening are caused. Therefore, when Ti is contained, the Ti content is set to 0.1% or less. The Ti content is preferably 0.05% or less. To ensure high ductility, the Ti content is more preferably 0.03% or less.

[0032] B: 0.01% or less B is an element that improves the hardenability of steel and has the advantage of facilitating the formation of a predetermined area ratio of tempered martensite and / or bainite. Furthermore, residual solute B improves delayed fracture resistance. To obtain these effects of B, the B content is preferably 0.0002% or more. Furthermore, the B content is more preferably 0.0005% or more. The B content is even more preferably 0.0010% or more. On the other hand, if the B content exceeds 0.01%, not only does the effect saturate, but it also significantly reduces hot ductility and causes surface defects. Therefore, when B is contained, the B content is set to 0.01% or less. The B content is preferably 0.0050% or less. The B content is more preferably 0.0030% or less.

[0033] Cu: 1% or less Cu improves corrosion resistance in the automotive operating environment. Furthermore, the corrosion products of Cu coat the steel sheet surface, suppressing hydrogen penetration into the steel sheet. Cu is an element that is mixed in when scrap is used as a raw material. Allowing Cu to be mixed in allows recycled materials to be used as raw materials, thereby reducing manufacturing costs. From this perspective, a Cu content of 0.005% or more is preferable. Furthermore, from the perspective of improving delayed fracture resistance, a Cu content of 0.05% or more is more preferable. More preferably, a Cu content of 0.10% or more is preferable. However, since an excessive Cu content can cause surface defects, when Cu is contained, the Cu content is set to 1% or less. The Cu content is preferably 0.4% or less, more preferably 0.2% or less.

[0034] Ni: 1% or less Like Cu, Ni is an element that improves corrosion resistance. Ni also has the effect of suppressing the occurrence of surface defects, which tend to occur when Cu is contained. Therefore, it is preferable to contain 0.01% or more of Ni. The Ni content is more preferably 0.04% or more, and even more preferably 0.06% or more. However, if the Ni content is too high, scale formation in the heating furnace becomes non-uniform, which may actually cause surface defects. It also increases costs. Therefore, when Ni is contained, the Ni content is set to 1% or less. The Ni content is preferably 0.4% or less, and more preferably 0.2% or less.

[0035] Cr: 1.0% or less Cr can be added to improve the hardenability of steel and to suppress the formation of carbides in martensite and upper / lower bainite. To achieve these effects, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.03% or more, and even more preferably 0.06% or more. On the other hand, excessive Cr content deteriorates pitting corrosion resistance, so when Cr is added, the Cr content is set to 1.0% or less. The Cr content is preferably 0.8% or less, more preferably 0.4% or less. The Cr content is more preferably 0.2% or less, and even more preferably 0.1% or less.

[0036] Mo: 0.5% or less Mo can be added to improve the hardenability of steel and to suppress the formation of carbides in martensite and upper / lower bainite. To achieve these effects, the Mo content is preferably 0.01% or more, more preferably 0.03% or more, and even more preferably 0.06% or more. On the other hand, Mo significantly deteriorates the chemical conversion treatability of cold-rolled steel sheets, so when Mo is contained, the Mo content is set to 0.5% or less. From the viewpoint of improving chemical conversion treatability, the Mo content is preferably set to 0.15% or less.

[0037] V: 0.5% or less V can be added to improve the hardenability of steel, suppress the formation of carbides in martensite and upper / lower bainite, refine the structure, and precipitate carbides to improve delayed fracture resistance. To achieve these effects, the V content is preferably 0.003% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. However, since a large amount of V significantly deteriorates castability, when V is contained, the V content is set to 0.5% or less. The V content is preferably 0.3% or less, more preferably 0.1% or less, and even more preferably 0.05% or less.

[0038] Nb: 0.1% or less Nb can be added to refine the steel structure and increase its strength, promote bainite transformation through grain refinement, improve bendability, and enhance delayed fracture resistance. To achieve these effects, the Nb content is preferably 0.002% or more. The Nb content is more preferably 0.004% or more, even more preferably 0.010% or more, and even more preferably 0.020% or more. On the other hand, a large amount of Nb content leads to excessive precipitation strengthening and reduced ductility. It also increases the rolling load and deteriorates castability. Therefore, when Nb is added, the Nb content is set to 0.1% or less. The Nb content is preferably 0.05% or less, and even more preferably 0.03% or less.

[0039] Zr: 0.2% or less Zr can be added to improve the hardenability of steel, suppress the formation of carbides in bainite, refine the structure, and precipitate carbides to improve delayed fracture resistance. To achieve these effects, the Zr content is preferably 0.005% or more. The Zr content is more preferably 0.008% or more, and even more preferably 0.010% or more. On the other hand, if Zr is added in a large amount, the amount of coarse precipitates such as ZrN and ZrS that remain in an undissolved state during slab heating before hot rolling increases, deteriorating delayed fracture resistance. Therefore, when Zr is added, the Zr content is set to 0.2% or less. The Zr content is preferably 0.15% or less, more preferably 0.08% or less, and even more preferably 0.05% or less.

[0040] W: 0.2% or less W can be added to improve the hardenability of steel, suppress the formation of carbides in bainite, refine the structure, and precipitate carbides to improve delayed fracture resistance. To achieve these effects, the W content is preferably 0.005% or more. The W content is more preferably 0.008% or more, and even more preferably 0.010% or more. On the other hand, if a large amount of W is added, the amount of coarse precipitates such as WN and WS that remain in an undissolved state during slab heating before hot rolling increases, deteriorating delayed fracture resistance. Therefore, when W is added, the W content is set to 0.2% or less. The W content is preferably 0.15% or less, more preferably 0.08% or less, and even more preferably 0.05% or less.

[0041] Ca: 0.0040% or less Ca fixes S as CaS, contributing to improvements in bendability and delayed fracture resistance. Therefore, the Ca content is preferably 0.0002% or more. The Ca content is more preferably 0.0005% or more, and even more preferably 0.0010% or more. On the other hand, adding a large amount of Ca deteriorates surface quality and bendability, so when Ca is contained, the Ca content is set to 0.0040% or less. The Ca content is preferably 0.0035% or less, and more preferably 0.0020% or less.

[0042] Ce: 0.0040% or less Like Ca, Ce also fixes S and contributes to improving bendability and delayed fracture resistance. Therefore, the Ce content is preferably 0.0002% or more. The Ce content is more preferably 0.0004% or more, and even more preferably 0.0006% or more. On the other hand, adding a large amount of Ce deteriorates the surface quality and bendability, so when Ce is contained, the Ce content is set to 0.0040% or less. The Ce content is preferably 0.0035% or less, and more preferably 0.0020% or less.

[0043] La: 0.0040% or less Like Ca, La also fixes S and contributes to improving bendability and delayed fracture resistance. Therefore, the La content is preferably 0.0002% or more. The La content is more preferably 0.0004% or more, and even more preferably 0.0006% or more. On the other hand, adding a large amount of La deteriorates the surface quality and bendability, so when La is contained, the La content is set to 0.0040% or less. The La content is preferably 0.0035% or less, and more preferably 0.0020% or less.

[0044] Mg: 0.0030% or less Mg fixes O as MgO and contributes to improving delayed fracture resistance. Therefore, the Mg content is preferably 0.0002% or more. The Mg content is more preferably 0.0004% or more, and even more preferably 0.0006% or more. On the other hand, adding a large amount of Mg deteriorates surface quality and bendability, so when Mg is contained, the Mg content is set to 0.0030% or less. The Mg content is preferably 0.0025% or less, and even more preferably 0.0010% or less.

[0045] Sb: 0.1% or less Sb suppresses oxidation and nitriding of the surface layer of the steel sheet, thereby suppressing the resulting reduction in the C and B contents in the surface layer. Furthermore, by suppressing the reduction in the C and B contents, the formation of ferrite in the surface layer of the steel sheet is suppressed, resulting in increased strength and improved delayed fracture resistance. From this perspective, the Sb content is preferably 0.002% or more. The Sb content is more preferably 0.004% or more, and even more preferably 0.006% or more. On the other hand, if the Sb content exceeds 0.1%, castability deteriorates and the Sb segregates to prior γ grain boundaries, deteriorating the delayed fracture resistance of the sheared edge. Therefore, when Sb is contained, the Sb content is set to 0.1% or less. The Sb content is preferably 0.04% or less, more preferably 0.03% or less, and even more preferably 0.02% or less.

[0046] Sn: 0.1% or less Sn suppresses oxidation and nitriding of the surface layer of the steel sheet, thereby suppressing the resulting reduction in the C and B contents in the surface layer. Furthermore, by suppressing the reduction in the C and B contents, the formation of ferrite in the surface layer of the steel sheet is suppressed, increasing strength and improving delayed fracture resistance. From this perspective, the Sn content is preferably 0.002% or more. The Sn content is more preferably 0.004% or more, and even more preferably 0.006% or more. On the other hand, if the Sn content exceeds 0.1%, castability deteriorates. Furthermore, Sn segregates at prior γ grain boundaries, deteriorating the delayed fracture resistance of the sheared edge. Therefore, when Sn is contained, the Sn content is set to 0.1% or less. More preferably, it is 0.04% or less, and even more preferably, it is 0.03% or less.

[0047] When the optional components are contained in amounts less than the preferred lower limit, the optional elements contained in amounts less than the lower limit do not impair the effects of the present invention. Therefore, when the optional elements are contained in amounts less than the lower limit, the optional elements are considered to be contained as inevitable impurities.

[0048] Next, the steel structure of the steel plate of the present invention will be described.

[0049] Total area ratio of ferrite and bainitic ferrite: 5% to 60%. Ferrite formed during annealing or cooling, and bainitic ferrite in upper bainite formed during cooling, subsequent heating, or retention contribute to improved ductility. Additionally, it concentrates carbon in the surrounding untransformed austenite, contributing to stabilization of retained austenite. On the other hand, excessive ferrite or bainitic ferrite can cause a decrease in strength and a hardness difference with surrounding hard phases such as martensite. This can lead to cracks propagating from the interface with the hard phase during bending, resulting in a decrease in stretch flangeability and axial crush resistance. Therefore, the total area ratio of ferrite and bainitic ferrite is set to 5% to 60%. The total area ratio of ferrite and bainitic ferrite is preferably 10% or more, more preferably 15% or more. The total area ratio of ferrite and bainitic ferrite is preferably 55% or less, more preferably 50% or less. The bainitic ferrite mentioned above refers to the BCC phase portion obtained by excluding precipitates such as carbides from bainite, which will be described later.Ferrite content is preferably more than 0%.

[0050] Area fraction of tempered martensite: 20% or more and 80% or less To obtain predetermined strength and stretch flangeability, the area fraction of tempered martensite is set to 20% or more. The area fraction of tempered martensite is preferably 30% or more, more preferably 40% or more. On the other hand, if the area fraction of tempered martensite exceeds 80%, excessive strength is obtained, resulting in a decrease in ductility. Therefore, the area fraction of tempered martensite is set to 80% or less, preferably 70% or less, and more preferably 60% or less.

[0051] Area fraction of fresh martensite: 20% or less (including 0%) Since this leads to a decrease in at least one of ductility and stretch flangeability, the area fraction of fresh martensite is set to 20% or less, preferably 15% or less, and more preferably 10% or less. The fresh martensite may be 0%. The fresh martensite may be 5% or more, or 10% or more.

[0052] Volume fraction of retained austenite: 5% or more and 25% or less In order to ensure high ductility, the volume fraction of retained austenite (retained γ) relative to the entire steel structure is set to 5% or more. The volume fraction of retained γ is preferably 7% or more, more preferably 9% or more. This amount of retained γ includes the volume fraction of retained γ formed adjacent to bainite. A volume fraction of retained γ exceeding 25% leads to a decrease in strength, a decrease in stretch flangeability, and a deterioration in delayed fracture resistance. Therefore, the volume fraction of retained γ is set to 25% or less. The volume fraction of retained γ is preferably 20% or less, more preferably 18% or less. Furthermore, the percentage as a "volume fraction" of retained γ measured by the measurement method described below can also be considered as a percentage as an "area fraction."

[0053] Total area ratio of ferrite, tempered martensite, fresh martensite, bainite, and retained austenite: 90% or more (including 100%). In order to ensure the required strength, ductility, and stretch flangeability, the total area ratio of ferrite, tempered martensite, fresh martensite, bainite, and retained austenite is set to 90% or more. Of these, bainite contains the above-mentioned bainitic ferrite, and bainite has internal carbides of 10 μm or more. ２ Bainitic ferrite contains 20 or less particles per 10 μm. ２ More than 20 bainitic ferrite particles may be contained per one grain.

[0054] Area ratio of ferrite with a solute Mn content of 2.0 mass% or more (high Mn ferrite) to the total ferrite: 20% to 70%. When the ferrite content exceeds 0%, high work hardening is exhibited over a wide strain range, and excellent energy absorption characteristics are achieved. Therefore, the area ratio of ferrite with a solute Mn content of 2.0 mass% or more (high Mn ferrite) is set to 20% to 70%. The area ratio of high Mn ferrite is preferably 25% or more, more preferably 30% or more. The area ratio of high Mn ferrite is preferably 65% ​​or less, more preferably 60% or less. The area ratio of ferrite with a solute Mn content of less than 2.0 mass% (low Mn ferrite) to the total ferrite is 30% to 80%, preferably 75% or less, more preferably 70% or less. The area ratio of low Mn ferrite to the total ferrite is preferably 35% or more, more preferably 40% or more.

[0055] Area S of the region where the C concentration is 0.3 mass% or more Ｃ≧０．３ The area S of the region where the C concentration is 0.5 mass% or more Ｃ≧０．５ The ratio of: (S Ｃ≧０．５ / S Ｃ≧０．３ ) × 100: 20% or more In order to ensure high ductility, the area S of the region where the C concentration is 0.3 mass% or more Ｃ≧０．３ The area S of the region where the C concentration is 0.5 mass% or more Ｃ≧０．５ The ratio (S Ｃ≧０．５ / S Ｃ≧０．３ ) × 100 is set to 20% or more. The above ratio is preferably 25% or more, and more preferably 30% or more. There is no particular upper limit to the above ratio, but the above ratio is preferably 70% or less, and more preferably 60% or less.

[0056] Internal carbide is 10 μm ２ Area ratio of bainitic ferrite of 20 or less per 10000: 3% or more and 40% or less Bainite is preferably more than 0%, and when bainite is more than 0%, the bainite has an internal carbide of 10 μm or less. ２By containing 20 or less bainitic ferrite particles per 1000 μm, C is efficiently concentrated in the residual γ around the bainitic ferrite, and ductility is further improved. ２ The area ratio of bainitic ferrite particles with 20 or less particles per 10 ... ２ The area ratio of bainitic ferrite particles of 20 or less per 10 ... ２ More than 20 bainitic ferrite particles may be present per one grain. The bainitic ferrite refers to the BCC phase portion obtained by excluding precipitates such as carbides from the bainite described above.

[0057] Next, a method for measuring the steel structure of the steel sheet of the present invention will be described. The area ratios of ferrite, bainitic ferrite, tempered martensite, and fresh martensite are measured by cutting out a cross section of the sheet parallel to the rolling direction. The cut cross section is then mirror-polished, etched with 3 vol% nital, and observed at a quarter-thickness position using a SEM at 5000x magnification with a field of view of 30 μm × 40 μm, for a total of 10 fields of view. FIG. 1 shows an example of an enlarged SEM photograph of the steel structure of the steel sheet. As shown in FIG. 1, ferrite (see symbol F in FIG. 1) is a relatively equiaxed polygonal ferrite with almost no carbides inside. This is the region that appears blackest in the SEM. Bainitic ferrite (see symbol BF in FIG. 1) is a ferrite structure with carbides or residual γ inside, which appears white in the SEM. Fresh martensite and retained austenite (see symbols FM and RA in Figure 1) are massive regions that appear white under an SEM, with the substructure hidden within. The area fraction of fresh martensite can be determined by subtracting the volume fraction of retained γ, measured by the method described below, from the area fraction of the white massive regions.

[0058] In the present invention, when it is difficult to distinguish between ferrite and bainitic ferrite, the area ratio is calculated by classifying the polygonal ferrite region having an aspect ratio of ≦2.5 as ferrite and the region having an aspect ratio >2.5 as bainitic ferrite. Fig. 2 is a diagram for explaining a method for measuring the steel structure of the steel sheet of the present invention. As shown in Fig. 2, the aspect ratio is determined by determining the major axis length a at which the particle length is longest, and the minor axis length b at the particle length when it crosses the particle the longest in the direction perpendicular to that, and a / b is the aspect ratio.

[0059] Internal carbide is 10 μm ２ Regarding the area ratio of bainitic ferrite, which is 20 or less per 10 μm, the area ratio of each bainitic ferrite and the number of carbides therein are measured in an SEM photograph at 5000 times magnification (field of view: 30 μm×40 μm). ２ The area ratio of each bainitic ferrite grain in which the number of carbides is 20 or less when converted into the number per bainitic ferrite grain can be calculated by adding up the area ratio of each bainitic ferrite grain in the entire structure.

[0060] The total area ratio of ferrite, tempered martensite, fresh martensite, bainite, and retained austenite can be determined by subtracting the area ratio of the remaining structures other than the above structures from the entire steel structure. Here, the area ratio of carbides is very small, so it is included in the area ratio of the above structures (ferrite, tempered martensite, fresh martensite, bainite, and retained austenite). The remaining structures include precipitates other than pearlite and carbides, and their area ratios can be determined using a SEM.

[0061] The volume fraction of retained austenite (retained γ) is determined by chemically polishing the steel sheet at a position 1 / 4 of the thickness from the surface and then performing X-ray diffraction. A Co-Kα radiation source is used for the incident X-rays, and the volume fraction of retained austenite is calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite. Here, since the retained γ is randomly distributed, the volume fraction of retained γ determined by X-ray diffraction can be treated as the area fraction of retained γ in the steel structure.

[0062] The area ratio of ferrite (high Mn ferrite) with a solute Mn content of 2.0 mass% or more, and the area S of the region with a C concentration of 0.5 mass% or more Ｃ≧０．５ and the area S of the region where the C concentration is 0.3 mass% or more Ｃ≧０．３ The measurement was carried out at a quarter-thickness position of the plate thickness cross section parallel to the rolling direction using a JEOL field emission electron probe microanalyzer (FE-EPMA) JXA-8500F, with an acceleration voltage of 6 kV and a probe current of 7 × 10 －８ A, the beam diameter is minimized to measure the Mn concentration distribution and the C concentration distribution by mapping analysis. However, when measuring the C concentration distribution, in order to eliminate the influence of contamination, a background component is subtracted so that the average C concentration obtained by the analysis is equal to the carbon content of the base material (the C content of the steel sheet). In other words, if the average measured carbon content is greater than the carbon content of the base material, this increase is considered to be contamination, and the true C concentration at each position is determined by uniformly subtracting this increase from the analytical value at each position. Furthermore, ferrite and bainitic ferrite can be distinguished by the SEM observation described above. Therefore, by performing SEM observation on the region where the C concentration distribution mapping analysis was performed, the proportion of high-Mn ferrite and low-Mn ferrite within the ferrite can be determined.

[0063] The steel sheet of the present invention has a tensile strength (TS) of 780 MPa or more, more preferably 980 MPa or more. The upper limit of the tensile strength is preferably 1469 MPa or less, more preferably 1320 MPa or less, from the viewpoint of compatibility with other properties.

[0064] In the steel sheet of the present invention, the total elongation T-El is 18.0% or more when TS is 780 MPa or more but less than 980 MPa, 16.0% or more when TS is 980 MPa or more but less than 1180 MPa, 14.0% or more when TS is 1180 MPa or more but less than 1320 MPa, and 13.0% or more when TS is 1320 MPa or more, thereby significantly improving forming stability. The hole expansion ratio λ is 30% or more. The upper limit of λ is not particularly limited, but from the viewpoint of compatibility with other properties, λ is preferably 90% or less, more preferably 80% or less, at any strength level.

[0065] Furthermore, in the steel plate of the present invention, from the viewpoint of ensuring excellent energy absorption characteristics during a collision, the area up to the maximum stress in the nominal stress-nominal strain curve of a tensile test (amount of energy absorbed during deformation) is preferably 13,000 MPa % or more, more preferably 14,000 MPa % or more.

[0066] The steel sheet of the present invention may have a zinc-plated layer on one or both sides of the steel sheet surface. The plated layer may be either a hot-dip plated layer or an electroplated layer.

[0067] Next, a method for manufacturing a steel sheet according to the present invention will be described. Hereinafter, a case where a holding treatment under predetermined conditions is not performed in a subsequent cooling treatment after holding at the annealing temperature will be described as a first embodiment. Also, a case where a holding treatment under predetermined conditions is performed in a subsequent cooling treatment after holding at the annealing temperature will be described as a second embodiment.

[0068] The temperatures specified in each step in the present invention refer to the surface temperatures of the slab (steel slab) or steel plate, and can be measured using a radiation thermometer, etc. The average cooling rate (°C / s) is defined as "(cooling start temperature - cooling stop temperature) (°C) / cooling time (s)", and the average heating rate (°C / s) is defined as "(heating stop temperature - heating start temperature) (°C) / heating time (s)".

[0069] FIG. 3 is a diagram for explaining the steel sheet manufacturing method of the present invention, and particularly shows the change in the surface temperature of a slab (steel slab) or a steel sheet over time. Details of each step, including the change in temperature over time, will be described below. FIG. 3(a) shows the change in the surface temperature of a slab (steel slab) or a steel sheet over time in the steel sheet manufacturing method of the first embodiment (when no holding treatment is performed). FIG. 3(b) shows the change in the surface temperature of a slab (steel slab) or a steel sheet over time in the steel sheet manufacturing method of the second embodiment (when holding treatment is performed).

[0070] <First embodiment (without retention treatment)> In a manufacturing method of a first embodiment of a steel sheet of the present invention, as also shown in FIG. 3(a), a steel slab having the above-described chemical composition is hot-rolled and cold-rolled, and then the resulting cold-rolled steel sheet is annealed, and the annealing includes: a step of holding at an annealing temperature of 775°C or more and 830°C or less; a step of cooling at an average cooling rate CR1 of 0.01°C / s or more and 5°C / s or less in a temperature range from the annealing temperature to the annealing temperature -15°C; a step of cooling at an average cooling rate CR2 of 3°C / s or more in a temperature range from the annealing temperature -15°C to a cooling stop temperature of 200°C or more and 300°C or less; a step of heating at an average heating rate of 2°C / s or more in a temperature range from the cooling stop temperature to 380°C; and a step of retention at an average cooling rate CR4 of 0.01 to 5°C / s for 20 s or more and 3000 s or less in a temperature range of 340°C or more and 590°C or less. and a step of cooling to a temperature of 50°C or less at an average cooling rate CR5 of 0.1°C / s or more.

[0071] Hot rolling of steel slabs can be performed by heating the slab before rolling, directly rolling the slab after continuous casting without heating it, or by subjecting the slab after continuous casting to a short heat treatment before rolling. Hot rolling can be performed according to a conventional method, and for example, the slab heating temperature can be 1100°C or higher. The slab heating temperature can be 1300°C or lower. The soaking temperature can be 20 min or higher. The soaking temperature can be 300 min or lower. The finish rolling temperature can be Ar ３ The finishing temperature is set to be equal to or higher than the transformation point. ３The coiling temperature may be set to transformation point +200°C or less. The coiling temperature may be set to 400°C or more. The coiling temperature may be set to 720°C or less. The coiling temperature is preferably controlled from the viewpoint of suppressing thickness fluctuations and stably ensuring high strength. Specifically, the coiling temperature is preferably set to 430°C or more. The coiling temperature is preferably set to 630°C or less.

[0072] Cold Rolling In cold rolling, the rolling ratio (cumulative rolling ratio) may be 30% or more. The rolling ratio (cumulative rolling ratio) may be 85% or less. It is preferable to control the rolling ratio from the viewpoint of stably ensuring high strength and reducing anisotropy. Specifically, the rolling ratio is preferably 35% or more. The rolling ratio is preferably 85% or less. When the rolling load is high, softening annealing can be performed at 450 to 730°C in a CAL (continuous annealing line) or BAF (box annealing furnace).

[0073] Annealing After hot rolling and cold rolling the steel slab having the above-mentioned composition, annealing is performed under the following specified conditions. Although the annealing equipment is not particularly limited, it is preferable to perform the annealing in a continuous annealing line (CAL) or a continuous hot-dip galvanizing line (CGL) from the viewpoints of productivity and ensuring the desired heating and cooling rates.

[0074] Annealing temperature: maintained at 775°C or higher and 830°C or lower. To ensure a predetermined area ratio of ferrite, tempered martensite, bainite, and residual γ, the annealing temperature is set to 775°C or higher and 830°C or lower. In order to achieve an area ratio of 5% or higher and 60% or lower for the total of ferrite and bainitic ferrite, the annealing temperature is preferably adjusted to achieve ferrite + austenite two-phase annealing. The annealing temperature is preferably 780°C or higher. On the other hand, if the annealing temperature exceeds 830°C, the γ grain size becomes excessively large, the diffusion distance of C atoms required to obtain the desired area ratio of residual γ becomes longer, and the predetermined amount of ferrite cannot be obtained, resulting in a decrease in ductility. Therefore, the annealing temperature is set to 830°C or lower. The holding time at the annealing temperature is not particularly limited, but is preferably set to 10 seconds or higher, more preferably 30 seconds or higher, from the viewpoint of sufficiently promoting recrystallization and reverse transformation from the structure after cold rolling. In addition, from the viewpoint of avoiding excessive coarsening of the structure after recrystallization and reverse transformation, the holding time is preferably 600 seconds or less, and more preferably 500 seconds or less.

[0075] Cooling is performed at an average cooling rate CR1 of 0.01°C / s or more and 5°C / s or less in the temperature range from the annealing temperature to the annealing temperature -15°C. Ferrite formed during annealing can undergo Mn diffusion during formation, resulting in a low Mn concentration in the ferrite. However, by cooling at an average cooling rate CR1 of 0.01°C / s or more and 5°C / s or less in the temperature range from the annealing temperature to the annealing temperature -15°C, ferrite with a high Mn concentration can be obtained without Mn diffusion. As a result, ferrite with a wide Mn concentration distribution can be obtained, resulting in high work hardening capacity over a wider strain range during deformation and improved energy absorption characteristics. If the average cooling rate CR1 in the temperature range from the annealing temperature to the annealing temperature -15°C exceeds 5°C / s, a sufficient amount of ferrite with a high Mn concentration cannot be obtained. For this reason, the average cooling rate CR1 is set to 5°C / s or less, preferably 4°C / s or less. On the other hand, if the average cooling rate CR1 in the temperature range from the annealing temperature to the annealing temperature -15°C is less than 0.01°C / s, Mn diffusion progresses, and a sufficient amount of ferrite with a high Mn concentration cannot be obtained. For this reason, the average cooling rate CR1 is set to 0.01°C / s or more. Preferably, it is 0.1°C / s or more. Here, the average cooling rate CR1 (°C / s) is calculated by "15 (°C) / (cooling time (s) from the annealing temperature to the annealing temperature -15°C))".

[0076] Cooling is performed at an average cooling rate CR2 of 3°C / s or more in the temperature range from the annealing temperature of -15°C to a cooling stop temperature of 200°C to 300°C. After cooling at an average cooling rate CR1 of 0.01°C / s to 5°C / s in the temperature range from the annealing temperature to the annealing temperature of -15°C, cooling is performed at an average cooling rate CR2 of 3°C / s or more in the temperature range from 200°C to 300°C to a cooling stop temperature. This results in a predetermined amount of tempered martensite and residual γ in the final structure, improving strength, stretch flangeability, and energy absorption properties. If the average cooling rate CR2 is less than 3°C / s, excessive ferrite, bainite, and pearlite are formed during the cooling process, which reduces strength, stretch flangeability, and energy absorption properties. For this reason, the average cooling rate CR2 in the temperature range from the annealing temperature of -15°C to a cooling stop temperature of 200°C to 300°C is set to 3°C / s or more. The average cooling rate CR2 is preferably 5°C / s or more, more preferably 8°C / s or more. Furthermore, if the average cooling rate CR2 in this temperature range becomes too large, the sheet shape deteriorates, so the average cooling rate CR2 in this temperature range is preferably 100°C / s or less. The average cooling rate CR2 is more preferably 50°C / s or less. To ensure a predetermined amount of tempered martensite and retained austenite, the cooling stop temperature is 200°C or more. The cooling stop temperature is preferably 210°C or more, more preferably 220°C or more. If the cooling stop temperature exceeds 300°C, a large amount of massive untransformed austenite remains, the amount of fresh martensite at the time of final cooling increases, and stretch flangeability deteriorates. Therefore, the cooling stop temperature is set to 300°C or less. The cooling stop temperature is preferably 280°C or less. Here, the average cooling rate CR2 (°C / s) is calculated by "(annealing temperature (°C) - 15 (°C) - cooling stop temperature (°C)) / (cooling time (s) from annealing temperature - 15°C to cooling stop temperature)".

[0077] Heating the temperature range from the cooling stop temperature to 380°C at an average heating rate of 2°C / s or more. Heating the temperature range from the cooling stop temperature to 380°C in a short time can suppress carbide precipitation and ensure high ductility. Furthermore, when reheating to 380°C or higher, upper bainite is generated using the martensite or bainite generated by cooling as nuclei. If the average heating rate to 380°C is slow, these effects cannot be obtained. As a result, the amount of retained γ decreases and ductility deteriorates. For this reason, the average heating rate in the temperature range from the cooling stop temperature to 380°C is set to 2°C / s or more. From the viewpoint of suppressing carbide precipitation and generating upper bainite during reheating, the average heating rate is preferably 5°C / s or more, and more preferably 10°C / s or more. The upper limit of the average heating rate is not particularly limited, but is preferably 50°C / s or less, more preferably 30°C / s or less.

[0078] Here, the average heating rate (°C / s) is calculated by (380 (°C) (heating end temperature) - cooling stop temperature (°C)) / (heating time (s) from the cooling stop temperature to 380°C)). Note that the heating end temperature (380°C) here refers to the end temperature when calculating the average heating temperature, and after the above heating, further heating may be performed continuously before retention in the temperature range of 340°C or higher and 590°C or lower, as described below.

[0079] The temperature range is from 340°C to 590°C, with an average cooling rate of 0.01 to 5°C / s, and the temperature is held for from 20 to 3000 seconds. From the perspective of distributing carbon to the residual γ to stabilize it and improve ductility, and from the perspective of subdividing the regions distributed as untransformed γ in a blocky form through bainite transformation and improving λ, the steel is held (slowly cooled) for from 20 to 3000 seconds in the temperature range from 340°C to 590°C. Furthermore, to suppress the formation of blocky structures due to the distribution of excess carbon to the residual γ and improve λ through self-tempering of fresh martensite, this temperature range is slowly cooled at an average cooling rate of 0.01 to 5°C / s. If the average cooling rate CR4 is less than 0.01°C / s, excessive carbon is distributed to the residual γ, resulting in the formation of blocky structures and a decrease in λ. For this reason, the average cooling rate CR4 is set to 0.01°C / s or greater. On the other hand, if the average cooling rate CR4 exceeds 5°C / s, C distribution to the residual γ is suppressed, and a sufficient amount of C-enriched region cannot be obtained. In addition, fresh martensite is generated, resulting in deterioration of λ. Therefore, the average cooling rate CR4 is set to 5°C / s or less.

[0080] Here, the average cooling rate CR4 is calculated by "(cooling start temperature (°C) - cooling stop temperature (°C)) / (cooling time (s) from the cooling start temperature to the cooling stop temperature)". Here, the cooling start temperature and the cooling stop temperature are not particularly limited as long as they are in the range of 340°C to 590°C, but the cooling start temperature is preferably 360°C or higher. The cooling start temperature is preferably 580°C or lower. The cooling stop temperature is preferably 350°C or higher. The cooling stop temperature is preferably 450°C or lower. When the cooling start temperature is higher than 380°C, after the step of heating at an average heating rate of 2°C / s or higher, a separate heating is performed up to the cooling start temperature. When the cooling start temperature is lower than 380°C, after the step of heating at an average heating rate of 2°C / s or higher, a separate cooling is performed up to the cooling start temperature.

[0081] Note that holding (dwelling, slow cooling) in the temperature range of 340°C to 590°C may also serve as a hot-dip galvanizing treatment or a galvannealing treatment. That is, in the step of holding at the aforementioned average cooling rate CR4: 0.01 to 5°C / s, the steel sheet may be subjected to a hot-dip galvanizing treatment or a galvannealing treatment. When hot-dip galvanizing is performed, it is preferable to immerse the steel sheet in a galvanizing bath at 440°C to 500°C, perform the hot-dip galvanizing treatment, and then adjust the coating weight by gas wiping or the like. For hot-dip galvanizing, it is preferable to use a galvanizing bath containing 0.10% to 0.22% Al. Furthermore, as a galvannealing treatment, a galvannealing treatment can be performed after the hot-dip galvanizing treatment. When performing the galvannealing treatment, it is preferable to perform it in a temperature range of 470°C to 590°C. This step is a step of cooling (retention and slow cooling (slow cooling)), but as long as the above-mentioned temperature range, retention time range, and average cooling rate CR4 range are satisfied, hot-dip galvanizing treatment or galvanizing alloying treatment can be performed during this step. The hot-dip galvanizing treatment or galvanizing alloying treatment may be accompanied by a temperature rise.

[0082] Cooling is performed at an average cooling rate CR5 of 0.1°C / s or more to a temperature of 50°C or less. Thereafter, cooling is performed at an average cooling rate CR5 of 0.1°C / s or more to a temperature of 50°C or less from the viewpoint of preventing softening due to excessive tempering and a decrease in ductility due to carbide precipitation. Skin-pass rolling can be performed on the steel sheet from the viewpoint of stabilizing press formability, such as adjusting the surface roughness and flattening the sheet shape, and from the viewpoint of increasing the YS. The skin-pass elongation is preferably 0.1 to 0.5%. The sheet shape can also be flattened using a leveler. The average cooling rate CR5 to the temperature of 50°C or less is preferably 5°C / s or more. Furthermore, the average cooling rate CR5 is preferably 100°C / s or less.

[0083] Here, the average cooling rate CR5 is calculated by (340 (°C) (cooling start temperature) - cooling stop temperature (°C) of 50°C or less) / (cooling time (s) from cooling start temperature to cooling stop temperature).

[0084] From the viewpoint of improving stretch flangeability, it is also possible to perform a low-temperature heat treatment at 100 to 300°C for 30 seconds to 10 days after the above-mentioned annealing (heat treatment) or after skin-pass rolling. This treatment causes hydrogen that has entered the steel sheet during tempering or annealing, which occurred during final cooling or skin-pass rolling, to be released from the steel sheet. The low-temperature heat treatment can reduce hydrogen to less than 0.1 ppm.

[0085] It is also possible to apply electroplating. That is, the steel sheet may be subjected to electrogalvanizing treatment after the step of cooling at the aforementioned average cooling rate CR5: 0.1°C / s or more. After the electroplating, it is preferable to apply the above-mentioned low-temperature heat treatment from the viewpoint of reducing hydrogen in the steel.

[0086] <Second embodiment (with retention treatment)> In a manufacturing method of a second embodiment of a steel sheet of the present invention, as also shown in FIG. 3(b), a steel slab having the above-described chemical composition is hot-rolled and cold-rolled, and then the resulting cold-rolled steel sheet is annealed, and the annealing includes: a step of holding the annealing temperature at 775°C or more and 830°C or less; a step of cooling at an average cooling rate CR1 of 0.01°C / s or more and 5°C / s or less in a temperature range from the annealing temperature to the annealing temperature -15°C; a step of cooling at an average cooling rate CR2A of 3°C / s or more in a temperature range from the annealing temperature -15°C to 500°C; and a step of retaining the annealed steel sheet at an average cooling rate CR3 of 10°C / s or less for 10 seconds or more and 60 seconds or less in a temperature range from 500°C to a martensitic transformation start temperature Ms or more and a retention stop temperature of 320°C or more. The method includes, in this order, a step of cooling at an average cooling rate CR2B of 3°C / s or more in the temperature range from the residence stop temperature to a cooling stop temperature of 200°C or more and 300°C or less, a step of heating at an average heating rate of 2°C / s or more in the temperature range from the cooling stop temperature to 380°C, a step of retaining the material in the temperature range of 340°C or more and 590°C or less at an average cooling rate CR4 of 0.01 to 5°C / s for 20 seconds or more and 3000 seconds or less, and a step of cooling at an average cooling rate CR5 of 0.1°C / s or more to a temperature of 50°C or less.

[0087] In the second embodiment, hot rolling and cold rolling can be performed under the same conditions as in the first embodiment. Furthermore, in the second embodiment, the annealing temperature: the step of holding at 775°C or higher and 830°C or lower and the step of cooling at an average cooling rate CR1 of 0.01°C / s or higher and 5°C / s or lower can be performed under the same conditions as in the first embodiment. Furthermore, in the second embodiment, the step of cooling at an average cooling rate CR2 of 3°C / s or higher in the first embodiment is replaced by the step of cooling at an average cooling rate CR2A of 3°C / s or higher, the step of retaining at an average cooling rate CR3 of 10°C / s or lower for 10 seconds or higher and 60 seconds or lower, and the step of cooling at an average cooling rate CR2B of 3°C / s or higher. In the second embodiment, the process of heating at an average heating rate of 2°C / s or more, the process of retaining the steel sheet at an average cooling rate CR4 of 0.01 to 5°C / s for 20 seconds or more and 3,000 seconds or less, and the process of cooling at an average cooling rate CR5 of 0.1°C / s or more can be performed under the same conditions as in the first embodiment. In the second embodiment, other processes such as hot-dip galvanizing, skin-pass rolling, low-temperature heat treatment after annealing, and electroplating can also be performed under the same conditions as in the first embodiment. Hereinafter, in this embodiment, the process of cooling at an average cooling rate CR2A of 3°C / s or more, the process of retaining the steel sheet at an average cooling rate CR3 of 10°C / s or less for 10 seconds or more and 60 seconds or less, and the process of cooling at an average cooling rate CR2B of 3°C / s or more will be mainly described.

[0088] Annealing temperature: Cool at an average cooling rate of CR2A: 3°C / s or more in the temperature range from -15°C to 500°C. Temperature range from 500°C to the martensitic transformation start temperature Ms or higher and the dwell stop temperature of 320°C or higher: Cool at an average cooling rate of CR3: 10°C / s or less, dwell for 10 seconds to 60 seconds. Temperature range from the dwell stop temperature to the cooling stop temperature of 200°C to 300°C: Cool at an average cooling rate of CR2B: 3°C / s or more. During the cooling process from an annealing temperature of -15°C to a cooling stop temperature of 200°C to 300°C, a retention (slow cooling) treatment step is included in which the steel is retained for 10 seconds to 60 seconds at an average cooling rate of CR3: 10°C / s or less in the temperature range from 500°C to a martensitic transformation start temperature Ms or higher and a retention stop temperature of 320°C or higher. This allows the formation of bainitic ferrite with a low carbide density and the formation of high-carbon-concentration residual γ adjacent to the bainitic ferrite, resulting in a steel sheet with superior ductility. If the retention stop temperature is lower than Ms or lower than 320°C, martensite forms first, which can cause excessive bainite transformation due to the swing back phenomenon, resulting in reduced strength and stretch flangeability. On the other hand, if the temperature exceeds 500°C, the driving force for bainite transformation decreases, and the amount of bainitic ferrite with a low carbide density produced decreases. Therefore, when a retention treatment is performed during cooling, the retention temperature range is set to be equal to or higher than Ms and equal to or higher than 320°C and equal to or lower than 500°C. This temperature range is preferably equal to or higher than 380°C, and more preferably equal to or higher than 420°C. Furthermore, this temperature range is preferably equal to or lower than 480°C, and more preferably equal to or lower than 460°C.

[0089] The reason why the average cooling rate CR2A is set to 3°C / s or more and the average cooling rate CR2B is set to 3°C / s or more is the same as the reason why the average cooling rate CR2 is set to 3°C / s or more described in the first embodiment. Both of the average cooling rates CR2A and CR2B are preferably 5°C / s or more, and more preferably 8°C / s or more. Furthermore, if the average cooling rates CR2A and CR2B are both too large, the sheet shape will deteriorate, so the average cooling rates CR2A and CR2B are preferably set to 100°C / s or less. Both of the average cooling rates CR2A and CR2B are more preferably 50°C / s or less. Here, the average cooling rate CR2A (°C / s) is calculated by "(annealing temperature (°C) - 15 (°C) - 500 (°C)) / (annealing temperature - cooling time (s) from 15°C to 500°C))". The average cooling rate CR2B (°C / s) is obtained by dividing (retention stop temperature (°C) - cooling stop temperature (°C)) by (cooling time (s) from the retention stop temperature to the cooling stop temperature).

[0090] If the average cooling rate CR3 exceeds 10°C / s, the amount of bainite transformation decreases, and the above-mentioned ductility improvement effect becomes insufficient. Therefore, if a holding treatment is performed during cooling, the average cooling rate CR3 is set to 10°C / s or less. Furthermore, if the holding time is less than 10 s, the amount of bainite transformation decreases, and the above-mentioned ductility improvement effect becomes insufficient. On the other hand, if it exceeds 60 s, C concentration from bainite to massive untransformed γ progresses, resulting in an increase in the amount of residual massive structure. Therefore, if a holding treatment is performed during cooling, the holding time is set to 10 s or more and 60 s or less. From the viewpoint of ensuring bainitic ferrite and retained austenite and improving ductility, a holding time of 20 s or more is preferable. Furthermore, from the viewpoint of improving stretch flange formability by reducing the massive structure, a holding time of 50 s or less is preferable.

[0091] Here, the average cooling rate CR3 (°C / s) is calculated by (500 (°C) - residence stop temperature (°C)) / (cooling time (s) from 500°C to residence stop temperature).

[0092] The martensitic transformation start temperature Ms can be determined by using a cylindrical test piece (diameter 3 mm × height 10 mm) and measuring the change in height of the test piece when it is held at a predetermined annealing temperature in a Formaster testing machine and then quenched with helium gas.

[0093] The steel sheet of the present invention preferably has a thickness of 0.5 mm or more, and more preferably has a thickness of 2.0 mm or less.

[0094] <Component> Next, the component of the present invention and the method for producing the same will be described.

[0095] The member of the present invention is obtained by subjecting the steel plate of the present invention to at least one of forming and joining. Also, the method for manufacturing the member of the present invention includes a step of subjecting the steel plate of the present invention to at least one of forming and joining to form the member.

[0096] The steel sheet of the present invention has a tensile strength of 780 MPa or more, high ductility, excellent stretch flangeability, and excellent energy absorption properties during a collision. Therefore, members obtained using the steel sheet of the present invention also have high strength and, compared to conventional high-strength members, have high ductility, excellent stretch flangeability, and excellent energy absorption properties during a collision. Furthermore, the use of the member of the present invention enables weight reduction. Therefore, the member of the present invention can be suitably used, for example, for vehicle body frame parts. The member of the present invention also includes welded joints.

[0097] The forming process can be performed using a general processing method such as press working without any restrictions, and the joining process can be performed using general welding methods such as spot welding and arc welding, riveting, crimping, etc. without any restrictions.

[0098] Cold-rolled steel sheets having a thickness of 1.4 mm and having the chemical composition shown in Table 1 were treated under the annealing conditions shown in Table 2 to produce steel sheets of the present invention and comparative examples. Each cold-rolled steel sheet was obtained by hot-rolling (slab heating temperature: 1200°C, soaking time: 60 min, finish rolling temperature: 900°C, coiling temperature: 500°C) and cold-rolling (rolling reduction (cumulative rolling reduction): 50%) a steel slab having the chemical composition shown in Table 1.

[0099] In Table 2, the martensitic transformation start temperature Ms was determined by using a cylindrical test piece (diameter 3 mm × height 10 mm) and measuring the change in height of the test piece when it was held at a predetermined annealing temperature in a Formaster testing machine and then quenched with helium gas.

[0100]

[0101]

[0102] Some steel sheets (cold-rolled steel sheets: CR) were subjected to a hot-dip galvanizing process in a step of retaining the steel sheet in a temperature range of 340°C to 590°C at an average cooling rate of 0.01 to 5°C / s for 20 seconds to 3,000 seconds, thereby obtaining hot-dip galvanized steel sheets (GI). Here, the steel sheet was subjected to the hot-dip galvanizing process by immersing it in a galvanizing bath at a temperature of 440°C to 500°C, and then the coating weight was adjusted by gas wiping or the like. For the hot-dip galvanizing, a galvanizing bath containing 0.10% to 0.22% Al was used. Furthermore, some hot-dip galvanized steel sheets were subjected to an alloying process after the hot-dip galvanizing process, resulting in galvannealed steel sheets (GA). Here, the alloying process was performed in a temperature range of 460°C to 590°C. In addition, some of the steel sheets (cold-rolled steel sheets: CR) were subjected to a step of cooling at an average cooling rate of CR5: 0.1°C / s or more, and then electroplated to form electrogalvanized steel sheets (EG).

[0103] In addition, some steel sheets were annealed under the conditions of holding (these steel sheets have values ​​other than "-" in the columns for CR3, holding time, and holding stop temperature in Table 2). In these cases, CR2A and CR2B were both equal to CR2B, and CR2A and CR2B are collectively shown as CR2 in Table 2.

[0104] The steel structure was measured by the method described above. The measurement results are shown in Table 3.

[0105]

[0106] JIS No. 5 tensile test pieces and hole expansion test pieces were taken from the obtained steel sheets, and tensile tests (in accordance with JIS Z2241 (2011)) were conducted. TS and T-El are shown in Table 3. Those with a tensile strength of 780 MPa or more were judged to have excellent strength. Those with a total elongation T-El of 18.0% or more for TS less than 980 MPa, 16.0% or more for TS 980 MPa or more and less than 1180 MPa, 14.0% or more for TS 1180 MPa or more and less than 1320 MPa, and 13.0% or more for TS 1320 MPa or more were judged to have excellent ductility.

[0107] Furthermore, the area up to the maximum stress in the nominal stress-nominal strain curve of the tensile test was considered to be the amount of energy absorbed during deformation, and a value of 13,000 MPa·% or more was considered to have excellent energy absorption properties during a collision.

[0108] In addition, stretch flangeability was evaluated by a hole expansion test specimen taken from the steel sheet obtained after heat treatment and a hole expansion test in accordance with the provisions of the Japan Iron and Steel Federation standard JFST1001. That is, a 100 mm x 100 mm square sample was punched using a punching tool with a punch diameter of 10 mm and a die diameter of 10.3 mm (clearance 13%), and then using a conical punch with an apex angle of 60 degrees, the hole was expanded until a crack penetrating the plate thickness occurred, with the burrs generated during punching being on the outside. At this time, d ０ : initial hole diameter (mm), d: hole diameter at crack occurrence (mm), hole expansion ratio λ (%) = {(d - d ０ ) / d ０}×100, and the results are shown in Table 3. Steels having a λ of 30% or more were judged to have excellent stretch flangeability (hole expandability).

[0109] The examples of the present invention shown in Tables 2 and 3 are excellent in strength, ductility, stretch flangeability and energy absorption properties, whereas the comparative examples are inferior in any of these properties.

[0110] Furthermore, using the steel plate of the present invention, the components obtained by forming, the components obtained by joining, and the components obtained by further forming and joining were found to have high strength, high ductility, excellent stretch flange formability, and excellent energy absorption properties during a collision, similar to the steel plate of the present invention, because the steel plate of the present invention has high strength, high ductility, excellent stretch flange formability, and excellent energy absorption properties during a collision.

[0111] The present invention has extremely high ductility, excellent stretch flangeability, and excellent energy absorption properties, and can be suitably applied to press-molded parts that are used in automobiles, home appliances, etc. through press molding processes.

[0112] F Ferrite TM Tempered martensite BF Bainitic ferrite FM Fresh martensite RA Retained austenite

Claims

1. By mass percentage, C: 0.06% or more and 0.25% or less, Si: 0.4% or more and 2.5% or less, Mn: 1.5% or more and 3.5% or less, P: 0.10% or less, S: 0.010% or less, sol.Al: 1.0% or less, N: 0.015% or less, having a component composition consisting of the balance Fe and inevitable impurities, with the total area ratio in the entire structure, the total of ferrite and bainitic ferrite: 5% or more and 60% or less, tempered martensite: 20% or more and 80% or less, fresh martensite: 20% or less (including 0%), by volume ratio, retained austenite: 5% or more and 25% or less, having a steel structure with the total area ratio of ferrite, tempered martensite, fresh martensite, bainite, and retained austenite: 90% or more (including 100%). When the ferrite is more than 0%, in the ferrite, the ratio of the ferrite with a solid-solution Mn amount of 2.0% by mass or more to the total ferrite is 20% or more and 70% or less by area ratio, and the area S of the region where the C concentration is 0.3% by mass or more Ｃ≧０．３ to the area S of the region where the C concentration is 0.5% by mass or more Ｃ≧０．５ ratio: (S Ｃ≧０．５ / S Ｃ≧０．３ )×100 is 20% or more, a steel plate.

2. The steel sheet according to claim 1, further containing, by mass%, one or more selected from Ti: 0.1% or less, B: 0.01% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1.0% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, Zr: 0.2% or less, W: 0.2% or less, Ca: 0.0040% or less, Ce: 0.0040% or less, La: 0.0040% or less, Mg: 0.0030% or less, Sb: 0.1% or less, and Sn: 0.1% or less in the component composition.

3. In the steel structure, furthermore, the bainitic ferrite in which the internal carbide is 20 or less per 10 μm ２ is included in an area ratio of 3% or more and 40% or less, and the steel sheet according to claim 1 or 2.

4. The steel sheet according to any one of claims 1 to 3, having a zinc plating layer on the surface.

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

6. After hot rolling and cold rolling are performed on a steel slab having the component composition according to claim 1 or 2, annealing is performed on the obtained cold-rolled steel sheet. The annealing includes a step of holding at an annealing temperature of 775°C or higher and 830°C or lower, a step of cooling in a temperature range from the annealing temperature to the annealing temperature - 15°C at an average cooling rate CR1 of 0.01°C / s or higher and 5°C / s or lower, a step of cooling in a temperature range from the annealing temperature - 15°C to a cooling stop temperature of 200°C or higher and 300°C or lower at an average cooling rate CR2 of 3°C / s or higher, a step of heating in a temperature range from the cooling stop temperature to 380°C at an average heating rate of 2°C / s or higher, a step of retaining in a temperature range of 340°C or higher and 590°C or lower at an average cooling rate CR4 of 0.01 to 5°C / s for 20 s or longer and 3000 s or shorter, and a step of cooling to a temperature of 50°C or lower at an average cooling rate CR5 of 0.1°C / s or higher, in this order. A method for manufacturing a steel sheet.

7. After hot rolling and cold rolling are performed on a steel slab having the component composition according to claim 1 or 2, annealing is performed on the obtained cold-rolled steel sheet. The annealing includes: a step of holding at an annealing temperature of 775°C or higher and 830°C or lower; a step of cooling in a temperature range from the annealing temperature to the annealing temperature - 15°C at an average cooling rate CR1 of 0.01°C / s or higher and 5°C / s or lower; a step of cooling in a temperature range from the annealing temperature - 15°C to 500°C at an average cooling rate CR2A of 3°C / s or higher; a step of retaining in a temperature range from 500°C to a temperature equal to or higher than the martensite transformation start temperature Ms and equal to or higher than 320°C at an average cooling rate CR3 of 10°C / s or lower for 10 s or longer and 60 s or shorter; a step of cooling in a temperature range from the retention stop temperature to a cooling stop temperature of 200°C or higher and 300°C or lower at an average cooling rate CR2B of 3°C / s or higher; a step of heating in a temperature range from the cooling stop temperature to 380°C at an average heating rate of 2°C / s or higher; a step of retaining in a temperature range of 340°C or higher and 590°C or lower at an average cooling rate CR4 of 0.01 to 5°C / s for 20 s or longer and 3000 s or shorter; and a step of cooling to a temperature of 50°C or lower at an average cooling rate CR5 of 0.1°C / s or higher, in this order. A method for manufacturing a steel sheet.

8. The method for manufacturing a steel sheet according to claim 6 or 7, wherein in the step of retaining at an average cooling rate CR4 of 0.01 to 5°C / s, a hot-dip galvanizing treatment or an alloyed hot-dip galvanizing treatment is performed on the steel sheet.

9. The method for manufacturing a steel sheet according to claim 6 or 7, including a step of performing an electro-galvanizing treatment after the step of cooling at an average cooling rate CR5 of 0.1°C / s or higher.

10. A method for manufacturing a member, including a step of subjecting the steel sheet according to any one of claims 1 to 4 to at least one of forming and joining to form a member.

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