Steel plates, components, and their manufacturing methods

Abstract

The present invention provides a steel plate and a member having a tensile strength of 780 MPa or more, high ductility and excellent stretch flange formability, and further having excellent energy absorption properties in the event of a collision, and a method for manufacturing the same. It has a chemical composition with specific contents of C, Si, Mn, P, S, sol.Al, and N, and in terms of area ratio, the total of ferrite and bainitic ferrite: 5 to 60%, tempered martensite: 20 to 80%, fresh martensite: 20% or less (including 0%), retained austenite: 5 to 25%, and has a steel structure consisting of one or more of ferrite, tempered martensite, fresh martensite, bainite, and retained austenite: 90% or more (including 100%), and of the ferrite, high Mn ferrite is 20% or more and 70% or less in terms of area ratio, and S C≧0.5 / S C≧0.3 x 100 is 20% or more.

Description

[Technical Field]

[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. [Background technology]

[0002] In recent years, the application of high-strength steel sheets with a tensile strength of 780 MPa or more to automotive frame components and seat components has been increasing due to the increasing need for lightweight automobile bodies. However, when high-strength steel sheets with a tensile strength of 780 MPa or more are used in automotive parts, press cracking is likely to occur due to reduced ductility and stretch flangeability. Therefore, these high-strength steel sheets are desired to have better formability than conventional steel sheets.

[0003] Furthermore, to ensure passenger safety, it is necessary to suppress deformation around the cabin during a collision, so 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 reduced axial crush resistance, which makes them prone to fracture during a collision, originating from areas that have undergone primary forming, making it difficult to stably absorb collision energy.

[0004] Against this background, TRIP steel, in which residual γ is dispersed in the microstructure of a steel sheet, has been developed as a technology for improving the ductility of steel sheets. For example, Patent Document 1 describes the following contents, 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%, Si and / or Al: 0.70% to 2.50% in total, Mn and / or 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 area ratio relative to the entire structure is as follows: 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, by 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, Sol.Al: 0.005 to 0.1%, and the balance is Fe and unavoidable impurities, and the steel structure has, by area ratio, ferrite: 60% or less, tempered martensite: 40% or more, and fresh martensite: 10% or less, and the void number density of the bent part 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 in the event of a collision can be obtained.

[0006] Patent Document 3 discloses that a high-strength cold-rolled steel sheet excellent in strength, ductility, and hole expandability can be obtained by 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%, Cr: 1.0% or less, with the balance being iron and unavoidable impurities, with P: 0.05% or less, S: 0.02% or less, and N: 0.006% or less, with the steel structure containing, by area fraction, 2 to 30% retained austenite and limiting martensite to 20% or less, with the average grain size of cementite being 0.01 μm or more and 1 μm or less, and the cementite containing 30% or more and 100% or less cementite having an aspect ratio of 1 to 3. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 6338038 [Patent Document 2] Patent No. 6795122 [Patent Document 3] Patent No. 4903915 Summary of the Invention [Problem to be solved by the invention]

[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 that accompanies an increase in ferrite. This makes it possible to manufacture a steel sheet that is excellent in both ductility and stretch flangeability, but the area fraction of tempered martensite is small, at 30% or less, so the axial crushing properties tend to be inferior. The technology described in Patent Document 2 makes it possible to manufacture steel sheets with excellent axial crushing properties by reducing the void density. However, since the technology does not contain a bainite phase, which has a hardness intermediate between that of ferrite and tempered martensite, there is a large difference in hardness between the different phases, resulting in poor stretch flangeability. Patent Document 3 states that controlling the shape of cementite makes it possible to obtain a steel sheet with excellent ductility and stretch flange formability, but does not take into consideration the axial crushing properties, and cracks are generated starting from the cementite during bending deformation, 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 and a member having a tensile strength of 780 MPa or more, high ductility and excellent stretch flange formability, and further having excellent energy absorption properties during a collision, as well as a method for manufacturing the same.

[0011] 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 means that the total elongation (T-El) measured in a tensile test according to JIS Z 2241 (2011) is: (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, (D) This means that when TS is 1320 MPa or more, T-El is 13.0% or more.

[0013] In the present invention, excellent stretch flangeability refers to a hole expansion ratio λ (%) = {(d - d) / d × 100} of 30% or more when a 100 mm × 100 mm square sample is punched using a punching tool with 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 burrs generated during punching face outward until a crack penetrates the plate thickness, where d is the initial hole diameter (mm) and d is the hole diameter (mm) at the time of crack initiation.

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

[0015] The present inventors have conducted extensive research into means for imparting high ductility, excellent stretch flangeability, and excellent energy absorption characteristics during a collision. As a result, they have discovered that a microstructure in which the total area fraction of ferrite, tempered martensite, fresh martensite, bainite, and retained austenite is 90% or more ensures high ductility and excellent stretch flangeability, while also achieving excellent energy absorption characteristics during a collision by appropriately controlling the phase fractions of each microstructure and adjusting the amount of solute Mn in ferrite over a wide range. They have also found that this microstructure can be achieved 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 of 0.01°C / s to 5°C / s, 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.Al: 1.0% or less, N: 0.015% or less and the balance being Fe and unavoidable impurities, The area ratio of the entire tissue is The sum 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%), The volume fraction of retained austenite is 5% or more and 25% or less. The steel structure has a total area ratio of ferrite, tempered martensite, fresh martensite, bainite, and retained austenite of 90% or more (including 100%), When the ferrite is more than 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 ratio, 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 Percentage of: (S C≧0.5 / S C≧0.3 ) × 100 is 20% or more. [2] The component composition further includes, in mass%, 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, Sn: 0.1% or less The steel sheet according to [1] above, containing one or more selected from the following: [3] In the steel structure, further, the internal carbides are 10 μm 2 The steel sheet according to [1] or [2], wherein the steel sheet contains 20 or less bainitic ferrite particles per square meter in an area ratio of 3% to 40%. [4] The steel sheet according to any one of [1] to [3] above, which has a zinc-plated layer on its surface. [5] A member made using the steel plate according to any one of [1] to [4] above. [6] A steel slab having the composition described in [1] or [2] is hot-rolled and cold-rolled, and then the obtained cold-rolled steel sheet is annealed; The annealing is Annealing temperature: A process of holding at 775 ° C or more and 830 ° C or less; Cooling the temperature range from the annealing temperature to the annealing temperature -15°C at an average cooling rate CR1: 0.01°C / s or more and 5°C / s or less; a step of cooling the steel sheet at an average cooling rate of CR2:3°C / s or more in a temperature range from an annealing temperature of -15°C to a cooling stop temperature of 200°C or more and 300°C or less; a step of heating the temperature range from the cooling stop temperature to 380°C at an average heating rate of 2°C / s or more; A step of retaining the temperature range of 340 ° C or more and 590 ° C or less at an average cooling rate CR4: 0.01 to 5 ° C / s for 20 seconds to 3000 seconds; a step of cooling to a temperature of 50°C or less at an average cooling rate of CR5: 0.1°C / s or more; A method for manufacturing a steel plate, comprising the steps of: [7] A steel slab having the composition described in [1] or [2] is hot-rolled and cold-rolled, and then the obtained cold-rolled steel sheet is annealed; The annealing is Annealing temperature: A process of holding at 775 ° C or more and 830 ° C or less; Cooling the temperature range from the annealing temperature to the annealing temperature -15°C at an average cooling rate CR1: 0.01°C / s or more and 5°C / s or less; A step of cooling the annealed steel sheet at an average cooling rate of CR2A: 3°C / s or more in a temperature range of -15°C to 500°C; a step of retaining the material in a temperature range from 500°C to a retention stop temperature of 320°C or higher and at an average cooling rate of CR3:10°C / s or less for 10 seconds or more and 60 seconds or less; cooling the 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: 3°C / s or higher; a step of heating the temperature range from the cooling stop temperature to 380°C at an average heating rate of 2°C / s or more; A step of retaining the temperature range of 340 ° C or more and 590 ° C or less at an average cooling rate CR4: 0.01 to 5 ° C / s for 20 seconds to 3000 seconds; a step of cooling to a temperature of 50°C or less at an average cooling rate of CR5: 0.1°C / s or more; A method for manufacturing a steel plate, comprising the steps of: [8] The method for producing a steel sheet according to [6] or [7], wherein the steel sheet is subjected to a hot-dip galvanizing treatment or a hot-dip galvannealing treatment in the step of retaining the steel sheet at an average cooling rate CR4 of 0.01 to 5°C / s. [9] The method for producing a steel sheet according to [6] or [7], further comprising the step of performing an electrolytic galvanizing treatment after the step of cooling at an average cooling rate CR5 of 0.1°C / s or more.

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

[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. [Brief explanation of the drawings]

[0018] [Figure 1] Figure 1 shows an example of an SEM photograph of the steel structure of a steel plate. [Figure 2] FIG. 2 is a diagram for explaining a method for measuring the steel structure of the steel plate of the present invention. [Figure 3] FIG. 3 is a diagram illustrating the manufacturing method of the steel sheet of the present invention, where (a) is a diagram illustrating the manufacturing method without retention treatment, and (b) is a diagram illustrating the manufacturing method with retention treatment. DETAILED DESCRIPTION OF THE INVENTION

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

[0020] <Steel plate> The steel sheet of the present invention has a composition containing, by mass%, C: 0.06% to 0.25%, Si: 0.4% to 2.5%, Mn: 1.5% to 3.5%, P: 0.10% or less, S: 0.010% or less, sol.Al: 1.0% or less, N: 0.015% or less, with the balance being Fe and unavoidable impurities, and the area ratio of ferrite and bainitic ferrite to the entire structure is 5% to 60%, tempered martensite is 20% to 80%, and fresh martensite is 1.0% to 1.5%. The steel structure has a volume fraction of ferrite, tempered martensite, fresh martensite, bainite, and retained austenite of 20% or less (including 0%), a volume fraction of retained austenite of 5% or more and 25% or less, and a total of ferrite, tempered martensite, fresh martensite, bainite, and retained austenite of 90% or more (including 100%), and when the ferrite content is more than 0%, the area fraction of ferrite in which the amount of solute Mn is 2.0% or more by mass is 20% or more and 70% or less by area, and the area S of the region in which the C concentration is 0.3% or more by mass C≧0.3 The area S of the region where the C concentration is 0.5 mass% or more C≧0.5 Percentage 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 added 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 (retained γ) to improve ductility, and concentrating C in the retained γ to stabilize the retained γ to improve ductility. Since these effects cannot be sufficiently ensured if 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, if the C content exceeds 0.25%, the strength becomes excessively high and the ductility decreases, and in addition, the amount of blocky martensite increases, which may deteriorate the stretch flangeability. For this reason, the upper limit of the C content is set to 0.25%. From the viewpoint of improving ductility, the C content is preferably set to 0.22% or less. From the viewpoint of further improving ductility, the C content is more preferably set to 0.20% or less.

[0023] Si: 0.4% or more and 2.5% or less Si is added from the viewpoints of strengthening ferrite to increase strength, suppressing the formation of carbides in martensite and bainite, improving the stability of retained 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, and more preferably 0.8% or more. On the other hand, if the Si content exceeds 2.5%, the stretch flangeability deteriorates due to the high strength of massive fresh martensite caused by an excessive increase in temper softening resistance. Furthermore, the rolling load during hot rolling becomes extremely high, making it difficult to manufacture thin plates. Furthermore, the chemical conversion treatability and the toughness of welds may deteriorate. For these reasons, the Si content is set to 2.5% or less. From the viewpoint of ensuring chemical conversion treatability and toughness of the material and welded joints, the Si content is preferably less than 2.0%. From the viewpoint of ensuring toughness of welded joints, the Si content is preferably 1.8% or less, and more preferably 1.5% or less.

[0024] Mn: 1.5% or more and 3.5% or less Mn is an important element from the viewpoints of ensuring strength by ensuring a predetermined area fraction of tempered martensite and / or bainite, and stabilizing retained γ by lowering the Ms point of retained γ, thereby improving ductility. Similarly to Si, Mn is also an important element from the viewpoints of suppressing the formation of carbides in bainite to improve ductility, and increasing the volume fraction of retained γ to improve ductility. To achieve these effects, the Mn content is set to 1.5% or more. From the viewpoint of stabilizing retained 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 a decrease in ductility. Also, if the Mn content exceeds 3.5%, it becomes difficult to suppress the formation of coarse γ and martensite, resulting in a decrease in stretch flangeability. Furthermore, if the Mn content exceeds 3.5%, the hardenability increases excessively, and therefore the amount of ferrite with a solute Mn content of 2.0 mass% or more, which is generated during the cooling process 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 minus 15°C, becomes insufficient, and sufficient energy absorption characteristics during a 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 properties during a collision, the Mn content is preferably set to 3.2% or less, and more preferably set to 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 peeling 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, so from the perspective of improving spot weldability, the S content is preferably set to 0.0020% or less, and more preferably less than 0.0010%. Although S may not be contained, the S content is preferably 0.0001% or more from the viewpoint of production costs, and more preferably 0.0005% or more.

[0027] sol.Al: 1.0% or less Al is contained for the purpose of deoxidation or to stabilize residual γ as a substitute for Si. 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 significantly reduced and chemical conversion treatability will be adversely affected, 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%, and more preferably 0.20% or less. The sol.Al content is further 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, reducing the hot ductility and surface quality of the steel. Furthermore, in steel containing B, N has the detrimental effect of eliminating the effects 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. Although N does not necessarily have to be contained, the N content is preferably 0.0001% or more from the viewpoint of production costs, and 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 composition of the steel sheet of the present invention may contain, in addition to the above-mentioned components, one or more of the following optional elements. 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% Below, 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, Sn: 0.1% or less

[0031] Ti: 0.1% or less Ti has the effect of fixing N in steel as TiN, improving hot ductility, and improving the hardenability of B. It also has the effect of refining the structure by precipitating TiC. To obtain 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%, the rolling load increases and the ductility decreases due to the increased amount of precipitation strengthening. 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 tempered martensite and / or bainite with a predetermined area ratio. 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 further 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 environment in which an automobile is used. Furthermore, the corrosion products of Cu coat the surface of the steel sheet, suppressing hydrogen penetration into the steel sheet. Cu is an element that is mixed in when scrap is used as a raw material. By allowing the inclusion of Cu, recycled materials can be used as raw materials, reducing manufacturing costs. From this perspective, it is preferable for Cu to be contained in an amount of 0.005% or more. Furthermore, from the perspective of improving delayed fracture resistance, it is more desirable for Cu to be contained in an amount of 0.05% or more. A Cu content of 0.10% or more is even more preferable. However, if the Cu content is too high, it will cause surface defects, so if Cu is contained, the Cu content is set to 1% or less, preferably 0.4% or less, and more preferably 0.2% or less.

[0034] Ni: 1% or less Ni, like Cu, is an element that improves corrosion resistance. Ni also has the effect of suppressing the occurrence of surface defects that tend to occur when Cu is contained. For this reason, it is preferable that Ni be contained in an amount of 0.01% or more. 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 uneven, which can actually cause surface defects. It also leads to increased costs. For this reason, if 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 if Cr is contained, 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 further 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, since Mo significantly deteriorates the phosphatability of cold-rolled steel sheets, when Mo is contained, the Mo content is set to 0.5% or less, and from the viewpoint of improving the phosphatability, 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, to suppress the formation of carbides in martensite and upper / lower bainite, to refine the structure, and to 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, 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 to increase 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, if Nb is contained in a large amount, precipitation strengthening becomes too strong and ductility decreases. It also leads to an increase in rolling load and deterioration of castability. Therefore, when Nb is contained, the Nb content is set to 0.1% or less. The Nb content is preferably 0.05% or less, and 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 contained in a large amount, the amount of coarse precipitates such as ZrN and ZrS remaining in an undissolved state during slab heating before hot rolling increases, deteriorating the delayed fracture resistance. Therefore, if Zr is contained, 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 remaining 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 the surface quality and bendability, so when Ca is contained, the Ca content is set to 0.0040% or less, 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, if a large amount of Ce is added, the surface quality and bendability deteriorate, so if Ce is contained, the Ce content is set to 0.0040% or less, 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, 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, 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 the surface quality and bendability, so if Mg is contained, the Mg content is set to 0.0030% or less, preferably 0.0025% or less, and more preferably 0.0010% or less.

[0045] Sb: 0.1% or less Sb suppresses oxidation and nitriding in 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 at the 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 in 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 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, if Sn is contained, the Sn content is set to 0.1% or less, more preferably 0.04% or less, and even more preferably 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 dwelling contribute to improved ductility. In addition, it concentrates carbon in the surrounding untransformed austenite, contributing to the stabilization of retained austenite. On the other hand, excessive ferrite or bainitic ferrite can reduce strength and create 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 reduced stretch flangeability and axial crush resistance. Therefore, the total area ratio of ferrite and bainitic ferrite is set to 5% or more and 60% or less. The total area ratio of ferrite and bainitic ferrite is preferably 10% or more, more preferably 15% or more, and is preferably 55% or less, more preferably 50% or less. The above-mentioned bainitic ferrite refers to the BCC phase portion remaining after removing precipitates such as carbides from bainite, which will be described later. Moreover, the ferrite content is preferably more than 0%.

[0050] Area ratio of tempered martensite: 20% to 80% To obtain a predetermined strength and stretch flangeability, the area ratio of tempered martensite is set to 20% or more, preferably 30% or more, and more preferably 40% or more. On the other hand, if the area ratio of tempered martensite exceeds 80%, ductility decreases due to excessively high strength, so the area ratio of tempered martensite is set to 80% or less, preferably 70% or less, and more preferably 60% or less.

[0051] Area ratio 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 ratio of fresh martensite is set to 20% or less, preferably 15% or less, and more preferably 10% or less. Also, the area ratio of fresh martensite may be 0%. The area ratio of fresh martensite may be 5% or more, or 10% or more.

[0052] Volume fraction of retained austenite: 5% to 25% 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, and 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, and 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%) To ensure the required strength, ductility, and stretch flangeability, the total area ratio of ferrite, tempered martensite, fresh martensite, bainite, and retained austenite is 90% or more. Of these, bainite contains the aforementioned bainitic ferrite, and bainite has internal carbides of 10 μm or less. 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 properties are obtained. Therefore, the content of ferrite with a solute Mn content of 2.0 mass% or more (high Mn ferrite) is set to 20% or more and 70% or less. The area ratio of high Mn ferrite is preferably 25% or more, more preferably 30% or more, and 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 entire ferrite is 30% to 80%, preferably 75% or less, and more preferably 70% or less. The area ratio of low Mn ferrite to the entire ferrite is preferably 35% or more, and more preferably 40% or more.

[0055] 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 Percentage of: (S C≧0.5 / S C≧0.3 )×100: 20% or more 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. The upper limit of the above ratio is not particularly limited, 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 particles of 20 or less per particle: 3% to 40% Bainite is preferably more than 0%, and when it is more than 0%, the carbides inside the bainite are 10 μm. 2By containing 20 or less bainitic ferrite particles per 1000 sq.m., C is efficiently concentrated in the residual γ around the bainitic ferrite, further improving ductility. To obtain this effect, the internal carbides must be 10 μm or less. 2 The area ratio of 20 or less bainitic ferrite particles per grain is preferably 3% or more, more preferably 5% or more, and further preferably 7% or more. On the other hand, to prevent a decrease in strength, the internal carbides are 10 μm 2 The area ratio of 20 or less bainitic ferrite particles per grain is preferably 40% or less, more preferably 30% or less, and further preferably 25% or less. In the present invention, the internal carbides are 10 μm 2 There may be more than 20 bainitic ferrite particles per grain. The above-mentioned bainitic ferrite refers to the BCC phase portion remaining after excluding precipitates such as carbides from the above-mentioned bainite.

[0057] Next, a method for measuring the steel structure of the steel plate of the present invention will be described. The area fractions of ferrite, bainitic ferrite, tempered martensite, and fresh martensite are measured by cutting a cross section parallel to the rolling direction. The cut cross section is then mirror-polished and etched with 3 vol% nital. The cross section is then observed at the 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. Figure 1 shows an example of a magnified SEM image of the steel plate's microstructure. As shown in Figure 1, ferrite (see symbol F in Figure 1) is a relatively equiaxed polygonal ferrite with almost no carbides inside. This is the region that appears darkest in the SEM image. Bainitic ferrite (see symbol BF in Figure 1) is a ferrite structure with carbides or residual γ inside, which appears white in the SEM image. Fresh martensite and retained austenite (see symbols FM and RA in Figure 1) are massive regions that appear white in the SEM image, with no visible substructure inside. The area fraction of fresh martensite can be determined by regarding the volume fraction of retained γ measured by the method described below as the area fraction and subtracting it from the area fraction of the white lumpy regions.

[0058] In the present invention, when it is difficult to distinguish between ferrite and bainitic ferrite, the area of ​​polygonal ferrite having an aspect ratio of ≦2.5 is classified as ferrite, and the area of ​​an aspect ratio >2.5 is classified as bainitic ferrite, and the area ratio is calculated. Fig. 2 is a diagram illustrating 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 which the particle length is longest in the direction perpendicular to the major axis length a, 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 1000μm, the area ratio of each bainitic ferrite and the number of carbides therein are measured in a 5000x SEM photograph (field of view 30μm x 40μm). 2 The area ratio of each bainitic ferrite grain in which the number of carbides is 20 or less when converted into the number of carbides 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 fraction of ferrite, tempered martensite, fresh martensite, bainite, and retained austenite can be determined by subtracting the area fraction of the remaining structures other than the above from the overall steel structure. Here, the area fraction of carbides is very small, so it is included in the area fraction of the above structures (ferrite, tempered martensite, fresh martensite, bainite, and retained austenite). Remaining structures include precipitates other than pearlite and carbides, and their area fractions can be determined using an 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 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 with a solute Mn content of 2.0 mass% or more (high Mn ferrite), and the area S of the region with a C concentration of 0.5 mass% or more C≧0.5 and the area S where the C concentration is 0.3 mass% or more C≧0.3 The measurement was carried out at a quarter-thickness position on the cross section of the plate 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 and the Mn concentration distribution and C concentration distribution are measured by mapping analysis. However, when measuring the C concentration distribution, in order to eliminate the effects of contamination, the background is subtracted so that the average C concentration obtained by analysis is equal to the carbon amount of the base material (C content of the steel plate). In other words, if the average measured carbon amount is greater than the carbon amount of the base material, this increase is considered to be contamination, and the true C concentration at each position is determined by subtracting this increase uniformly from the analysis value at each position. Furthermore, ferrite and bainitic ferrite can be distinguished by the SEM observation described above, and therefore, by performing SEM observation on the region where the C concentration distribution mapping analysis was performed, the proportions 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 forming stability is significantly improved by ensuring that the total elongation T-El is 18.0% or more when TS is 780 MPa or more and less than 980 MPa, 16.0% or more when TS is 980 MPa or more and less than 1180 MPa, 14.0% or more when TS is 1180 MPa or more and less than 1320 MPa, and 13.0% or more when TS is 1320 MPa or more. The hole expansion ratio λ is 30% or more. There is no particular upper limit for λ, 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 sheet of the present invention, from the viewpoint of ensuring excellent energy absorption properties during a collision, the area up to the maximum stress in the nominal stress-nominal strain curve in a tensile test (amount of energy absorbed during deformation) is preferably 13000 MPa·% or more, more preferably 14000 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 producing a steel sheet according to the present invention will be described. In the following, a case where the steel sheet is held at the annealing temperature and then cooled without holding under predetermined conditions will be described as a first embodiment, and a case where the steel sheet is held at the annealing temperature and then cooled with holding under predetermined conditions 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 steel sheet over time. Details of each step, including the change in temperature over time, will be explained below. FIG. 3(a) shows the change in the surface temperature of a slab (steel slab) or steel sheet over time in the steel sheet manufacturing method of the first embodiment (when retention treatment is not performed). FIG. 3(b) shows the change in the surface temperature of a slab (steel slab) or steel sheet over time in the steel sheet manufacturing method of the second embodiment (when retention treatment is performed).

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

[0071] hot rolling 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 rolling the slab after continuous casting after a short heat treatment. Hot rolling can be performed according to a conventional method, 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 the Ar3 transformation point or higher. The finish rolling temperature can be Ar3 transformation point + 200°C or lower. The coiling temperature can be 400°C or higher. The coiling temperature can be 720°C or lower. The coiling temperature is preferably controlled from the viewpoint of suppressing thickness fluctuations and stably ensuring high strength. Specifically, the coiling temperature is preferably 430°C or higher. The coiling temperature is preferably 630°C or lower.

[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. It is also preferable to set the rolling ratio to 85% or less. If 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 A steel slab having the above-described chemical composition is hot-rolled and cold-rolled, and then annealed under the following specified conditions. Although the annealing equipment is not particularly limited, it is preferable to carry out 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: Maintain between 775°C and 830°C To ensure the specified area ratios of ferrite, tempered martensite, bainite, and residual γ, the annealing temperature is set to 775°C or higher and 830°C or lower. To ensure that the total area ratio of ferrite and bainitic ferrite is 5% or higher and 60% or lower, 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 gamma grain size becomes excessively large, the diffusion distance of C atoms required to obtain the desired area ratio of retained gamma becomes long, and the required amount of ferrite is not obtained, resulting in a decrease in ductility. Therefore, the annealing temperature is set to 830°C or less. The holding time at the annealing temperature is not particularly limited, but is preferably 10 seconds or more, more preferably 30 seconds or more, from the viewpoint of sufficiently promoting recrystallization and reverse transformation from the structure after cold rolling. Moreover, from the viewpoint of avoiding excessive coarsening of the structure after recrystallization and reverse transformation, the holding time is preferably 600 seconds or less, more preferably 500 seconds or less.

[0075] Cooling is performed at an average cooling rate of CR1: 0.01°C / s to 5°C / s in the temperature range from the annealing temperature to the annealing temperature -15°C. The ferrite formed during annealing can be accompanied by diffusion of Mn during formation, resulting in a low Mn concentration in the ferrite, but by 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 -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, which provides high work hardening ability over a wider strain range during deformation and improves energy absorption characteristics. If the average cooling rate CR1 in the temperature range from the annealing temperature to the annealing temperature minus 15°C exceeds 5°C / s, a sufficient amount of ferrite with a high Mn concentration cannot be obtained. Therefore, 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 minus 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. Therefore, the average cooling rate CR1 is set to 0.01°C / s or more, and preferably 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] Annealing temperature -15℃ to the cooling stop temperature of 200℃ to 300℃, cooling at an average cooling rate of CR2:3℃ / s or more Cooling is performed at an average cooling rate CR1 of 0.01°C / s to 5°C / s in the temperature range from the annealing temperature to -15°C below the annealing temperature, followed by cooling at an average cooling rate CR2 of 3°C / s or higher 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 retained 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, resulting in reduced strength, stretch flangeability, and energy absorption properties. For this reason, the average cooling rate CR2 in the temperature range from -15°C below the annealing temperature to a cooling stop temperature of 200°C to 300°C is set to 3°C / s or higher. The average cooling rate CR2 is preferably 5°C / s or higher, more preferably 8°C / s or higher. Furthermore, if the average cooling rate CR2 in this temperature range becomes too high, the sheet shape will deteriorate, so the average cooling rate CR2 in this temperature range is preferably 100°C / s or less, and more preferably 50°C / s or less. To ensure the desired amount of tempered martensite and retained austenite, the cooling stop temperature is set to 200°C or higher. The cooling stop temperature is preferably 210°C or higher, and more preferably 220°C or higher. If the cooling stop temperature exceeds 300°C, a large amount of blocky 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 lower. The cooling stop temperature is preferably 280°C or lower. 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] Heat the temperature range from the cooling stop temperature above to 380°C at an average heating rate of 2°C / s or more By heating in the temperature range from the cooling stop temperature to 380°C in a short time, carbide precipitation can be suppressed and high ductility can be ensured. Furthermore, when reheating to 380°C or higher, upper bainite is formed using the martensite or bainite formed by cooling as nuclei. If the average heating rate up to 380°C is slow, these effects cannot be obtained. As a result, the amount of retained γ decreases, and ductility deteriorates. Therefore, the average heating rate in the temperature range from the cooling stop temperature to 380°C is set to 2°C / s or higher. From the viewpoint of suppressing carbide precipitation and forming upper bainite during reheating, the average heating rate is preferably 5°C / s or higher, and more preferably 10°C / s or higher. The upper limit of the average heating rate is not particularly limited, but is preferably 50°C / s or lower, more preferably 30°C / s or lower.

[0078] Here, the average heating rate (°C / s) is calculated by (380 (°C) (heating end temperature) - cooling stop temperature (°C)) / (heating time (s) from cooling stop temperature to 380°C). 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 340℃ to 590℃ with an average cooling rate of CR4: 0.01 to 5℃ / s, and the retention time is 20 to 3000 seconds. To stabilize the residual γ by distributing C to it and improve ductility, and to subdivide the regions distributed as untransformed γ in a blocky manner through bainite transformation and improve λ, the steel is allowed to dwell (slowly cooled) in the temperature range of 340°C to 590°C for 20 seconds to 3000 seconds. Furthermore, to suppress the formation of blocky structures due to the distribution of excess C to the residual γ and improve λ through self-tempering of fresh martensite, the steel is slowly cooled within this temperature range at an average cooling rate CR4 of 0.01 to 5°C / s. If the average cooling rate CR4 is less than 0.01°C / s, excess C will be 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, the distribution of C to the residual γ is suppressed, and a sufficient amount of C-enriched region cannot be obtained. In addition, fresh martensite is formed, resulting in a 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 cooling start temperature to 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. If the cooling start temperature is higher than 380°C, after the step of heating at an average heating rate of 2°C / s or more, a separate heating is carried out up to the cooling start temperature. If the cooling start temperature is lower than 380°C, after the step of heating at an average heating rate of 2°C / s or more, a separate cooling is carried out 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, the steel sheet may be subjected to a hot-dip galvanizing treatment or a galvannealing treatment in the step of holding at the aforementioned average cooling rate CR4 of 0.01 to 5°C / s. When hot-dip galvanizing is performed, it is preferable to immerse the steel sheet in a galvanizing bath at 440°C to 500°C to 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 the galvannealing treatment is performed, it is preferable to perform it in a temperature range of 470°C to 590°C. This step involves 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 involve a temperature rise.

[0082] Cool to a temperature of 50°C or less at an average cooling rate of CR5: 0.1°C / s or more Thereafter, cooling is carried out 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. The steel sheet can be subjected to skin-pass rolling from the viewpoint of stabilizing press formability, such as adjusting surface roughness and flattening the sheet shape, and from the viewpoint of increasing 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. The average cooling rate CR5 is also 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) below 50°C) / (cooling time (s) from cooling start temperature to cooling stop temperature).

[0084] To improve stretch flangeability, low-temperature heat treatment at 100 to 300°C for 30 seconds to 10 days can be performed after the above annealing (heat treatment) or after skin-pass rolling. This treatment causes the hydrogen that entered the steel sheet during annealing or tempering of martensite formed during final cooling or skin-pass rolling to be released from the steel sheet. 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 process)> In the manufacturing method of the second embodiment of the steel sheet of the present invention, as shown in FIG. 3(b), a steel slab having the above-mentioned composition is hot-rolled and cold-rolled, and then the obtained cold-rolled steel sheet is annealed. Annealing temperature: A process of holding at 775 ° C or more and 830 ° C or less; a step of cooling the steel sheet 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 the annealed steel sheet at an average cooling rate of CR2A: 3°C / s or more in a temperature range of -15°C to 500°C; a step of retaining the material in a temperature range from 500°C to a retention stop temperature of 320°C or higher and at an average cooling rate of CR3:10°C / s or less for 10 seconds or more and 60 seconds or less; a step of cooling the material at an average cooling rate of 3°C / s or more in a 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 the material in a temperature range from the cooling stop temperature to 380°C at an average heating rate of 2°C / s or more; A step of retaining the temperature range of 340 ° C or more and 590 ° C or less at an average cooling rate CR4: 0.01 to 5 ° C / s for 20 seconds to 3000 seconds; and cooling the resulting mixture to a temperature of 50°C or less at an average cooling rate of CR5:0.1°C / s or more.

[0087] In the second embodiment, the hot rolling and cold rolling can be performed under the same conditions as in the first embodiment. In the second embodiment, the annealing temperature: the process of holding at 775°C or more and 830°C or less and the process of cooling at an average cooling rate CR1: 0.01°C / s or more and 5°C / s or less can be performed under the same conditions as in the first embodiment. In the second embodiment, the process of cooling at an average cooling rate CR2 of 3°C / s or more in the first embodiment is replaced by a process of cooling at an average cooling rate CR2A of 3°C / s or more, a process of retaining the material at an average cooling rate CR3 of 10°C / s or less for 10 seconds to 60 seconds, and a process of cooling at an average cooling rate CR2B of 3°C / s or more. In the second embodiment, the process of heating at an average heating rate of 2°C / s or more, the process of retaining the material at an average cooling rate CR4 of 0.01 to 5°C / s for 20 seconds to 3000 seconds, and the process of cooling at an average cooling rate CR5 of 0.1°C / s or more can be carried out under the same conditions as in the first embodiment. In the second embodiment, other treatments 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. In the following, in this embodiment, we will mainly explain the process of cooling at an average cooling rate CR2A of 3°C / s or more, the process of retaining at an average cooling rate CR3 of 10°C / s or less for 10 seconds to 60 seconds, and the process of cooling at an average cooling rate CR2B of 3°C / s or more.

[0088] Annealing temperature: Cooling in the temperature range from -15°C to 500°C at an average cooling rate of CR2A: 3°C / s or more The temperature range is from 500°C to the martensitic transformation start temperature Ms or higher and the retention stop temperature is 320°C or higher, and the average cooling rate is CR3: 10°C / s or lower, with retention for 10 seconds to 60 seconds. Cooling is performed in the temperature range from the dwell stop temperature to the cooling stop temperature of 200°C to 300°C at an average cooling rate of CR2B: 3°C / s or more. During the process of cooling from the annealing temperature of -15°C to a cooling stop temperature of 200°C or higher and 300°C or lower, a retention (slow cooling) treatment step is included in which the temperature range from 500°C to a retention stop temperature of 320°C or higher and the temperature is retained at an average cooling rate of CR3:10°C / s or lower for 10 seconds or higher and 60 seconds or lower.This makes it possible to generate bainitic ferrite with a low carbide density and to generate residual γ with a high carbon concentration adjacent to the bainitic ferrite, resulting in a steel sheet with better ductility. If the retention stop temperature is less than Ms or less than 320°C, martensite forms first, and the swing back phenomenon causes excessive bainite transformation, which may result in a decrease in strength and stretch flangeability. On the other hand, if the temperature exceeds 500°C, the driving force for the bainite transformation decreases, and the amount of bainitic ferrite, which has a low carbide density, produced decreases. Therefore, when a retention treatment is performed during cooling, the retention temperature range is set to Ms or higher and 320°C or higher, but not higher than 500°C. This temperature range is preferably 380°C or higher, and more preferably 420°C or higher. Furthermore, this temperature range is preferably 480°C or lower, and more preferably 460°C or lower.

[0089] The reason why the average cooling rates CR2A and CR2B are set to 3°C / s or more and 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 as 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 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)) / (cooling time (s) from annealing temperature - 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 retention 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 seconds, the amount of bainite transformation decreases, resulting in insufficient ductility improvement. On the other hand, if it exceeds 60 seconds, carbon concentration from bainite to the blocky untransformed γ progresses, resulting in an increase in the amount of remaining blocky structure. Therefore, if holding treatment is performed during cooling, the holding time is set to 10 seconds or more and 60 seconds or less. From the viewpoint of securing bainitic ferrite and retained austenite and improving ductility, a holding time of 20 seconds or more is preferable. Furthermore, from the viewpoint of improving stretch flange formability by reducing blocky structure, a holding time of 50 seconds 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] <Components> Next, the member of the present invention and the method for manufacturing 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 have higher ductility, excellent stretch flangeability, and excellent energy absorption properties during a collision compared to conventional high-strength members. 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. [Example]

[0098] Cold rolled steel sheets having the chemical compositions shown in Table 1 and a thickness of 1.4 mm were treated under the annealing conditions shown in Table 2 to produce steel sheets according to the present invention and comparative steel sheets. 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 measuring the change in height of a cylindrical test piece (diameter 3 mm x height 10 mm) when it was held at a predetermined annealing temperature in a Formaster testing machine and then quenched with helium gas.

[0100] [Table 1]

[0101] [Table 2]

[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 at a temperature range of 340°C to 590°C at an average cooling rate of 0.01 to 5°C / s for 20 to 3,000 seconds to produce hot-dip galvanized steel sheets (GI). Here, the steel sheet was immersed in a galvanizing bath at a temperature of 440°C to 500°C to produce hot-dip galvanized steel sheets, 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, as a galvannealed hot-dip galvanizing process, to produce 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 cooling process at an average cooling rate of CR5:0.1°C / s or more, and then electroplated to produce 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] [Table 3]

[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 carried out. TS and T-El are shown in Table 3. A tensile strength of 780 MPa or more was judged to be excellent in strength. The total elongation T-El was judged to be excellent when it was 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.

[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 materials with a value of 13,000 MPa·% or more were considered to have excellent energy absorption properties during a collision.

[0108] Stretch flangeability was evaluated by hole expansion tests in accordance with the Japan Iron and Steel Federation standard JFST1001. Specifically, a 100 mm x 100 mm square sample was punched using a punch with a 10 mm punch diameter and a 10.3 mm die diameter (13% clearance). The hole was then expanded using a conical punch with a 60° apex angle, with the burrs formed during punching facing outward until a crack penetrated the plate thickness. The hole expansion ratio λ (%) was calculated as {(d - d) / d} × 100, where d is the initial hole diameter (mm) and d is the hole diameter at the time of crack initiation (mm). The results are shown in Table 3. Steels with a λ of 30% or greater were considered 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. [Industrial Applicability]

[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. [Explanation of symbols]

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

Claims

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. Al: 1.0% or less, N: 0.015% or less and the balance being Fe and unavoidable impurities, The area ratio of the entire tissue is The sum 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%), The volume fraction of retained austenite is 5% or more and 25% or less, The steel has a steel structure in which the total area ratio of ferrite, tempered martensite, fresh martensite, bainite, and retained austenite is 90% or more (including 100%), When the ferrite is more than 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 ratio, 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 is 20% or more.

2. The component composition further includes, in mass%, 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, Sn: 0.1% or less The steel sheet according to claim 1, comprising one or more selected from the following:

3. In the steel structure, further, the internal carbides are 10 μm ２ The steel sheet according to claim 1 or 2, wherein bainitic ferrite particles having 20 or less particles per grain are contained in an area ratio of 3% to 40%.

4. The steel sheet according to claim 1 or 2, which has a zinc-plated layer on its surface.

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

6. A steel slab having the chemical composition according to claim 1 or 2 is hot-rolled and cold-rolled, and then the resulting cold-rolled steel sheet is annealed; The annealing is Annealing temperature: A step of holding at 775°C or higher and 830°C or lower; Cooling the steel sheet 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 the steel sheet at an average cooling rate CR2 of 3°C / s or more in a temperature range from an annealing temperature of -15°C to a cooling stop temperature of 200°C or more and 300°C or less; a step of heating the temperature range from the cooling stop temperature to 380°C at an average heating rate of 2°C / s or more; A step of retaining the material in a temperature range of 340°C or higher and 590°C or lower at an average cooling rate CR4: 0.01 to 5°C / s for 20 seconds or higher and 3000 seconds or lower; Cooling the mixture to a temperature of 50°C or less at an average cooling rate CR5 of 0.1°C / s or more; in this order, The area ratio of the entire tissue is The sum 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%), The volume fraction of retained austenite is 5% or more and 25% or less, The steel has a steel structure in which the total area ratio of ferrite, tempered martensite, fresh martensite, bainite, and retained austenite is 90% or more (including 100%), When the ferrite is more than 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 ratio, A method for manufacturing a steel sheet, wherein a ratio of an area SC≧0.5 of a region having a C concentration of 0.5 mass% or more to an area SC≧0.3 of a region having a C concentration of 0.3 mass% or more: (SC≧0.5 / SC≧0.3)×100 is 20% or more.

7. A steel slab having the chemical composition according to claim 1 or 2 is hot-rolled and cold-rolled, and then the resulting cold-rolled steel sheet is annealed; The annealing is Annealing temperature: A step of holding at 775°C or higher and 830°C or lower; Cooling the steel sheet 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 the steel sheet at an average cooling rate of CR2A: 3°C / s or more in a temperature range of annealing temperature -15°C to 500°C; a step of retaining the steel sheet in a temperature range from 500°C to a retention stop temperature of not less than the martensitic transformation start temperature Ms and not less than 320°C at an average cooling rate CR3 of not more than 10°C / s for not less than 10 seconds and not more than 60 seconds; cooling the 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 the temperature range from the cooling stop temperature to 380°C at an average heating rate of 2°C / s or more; A step of retaining the material in a temperature range of 340°C or higher and 590°C or lower at an average cooling rate CR4: 0.01 to 5°C / s for 20 seconds or higher and 3000 seconds or lower; Cooling the mixture to a temperature of 50°C or less at an average cooling rate CR5 of 0.1°C / s or more; in this order, The area ratio of the entire tissue is The sum 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%), The volume fraction of retained austenite is 5% or more and 25% or less, The steel has a steel structure in which the total area ratio of ferrite, tempered martensite, fresh martensite, bainite, and retained austenite is 90% or more (including 100%), When the ferrite is more than 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 ratio, A method for manufacturing a steel sheet, wherein a ratio of an area SC≧0.5 of a region having a C concentration of 0.5 mass% or more to an area SC≧0.3 of a region having a C concentration of 0.3 mass% or more: (SC≧0.5 / SC≧0.3)×100 is 20% or more.

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

9. The method for producing a steel sheet according to claim 7, further comprising the step of performing an electrogalvanizing treatment after the step of cooling at an average cooling rate CR5 of 0.1°C / s or more.

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

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