Steel sheet, member, and methods for producing same

Abstract

Provided are: a steel sheet and member having excellent press formability and collision characteristics, and having tensile strength of 1180 MPa or more; and methods for producing the steel sheet and member.　This steel sheet has: a component composition containing prescribed amounts of C, Si, Mn, P, S, sol. Al, and N, with the balance being iron and inevitable impurities; and a steel structure in which the area ratio of polygonal ferrite is less than 10%, the area ratio of bainite is 10% to 60%, the area ratio of tempered martensite is 70% or less (excluding 0%), the area ratio of retained austenite is 5% to 20%, the total area ratio of the remaining structure is 5% or less (including 0%), the average amount of solute C in retained austenite adjacent to bainite or ferrite is 0.60 mass% or more and less than 1.20 mass%, and the number density of retained austenite adjacent to bainite or ferrite is 2.0 × 105 per 1 mm2 or more.

Description

Steel plates, components, and their manufacturing methods

[0001] The present invention relates to a steel plate, a member, and a method for manufacturing the same that are suitable for press-formed products having complex shapes used in automobile parts through a press-forming process and that can suppress fatigue fracture during assembly into an automobile.

[0002] Global CO ２ In response to tightening emission regulations, there is a growing demand for increased strength in automotive steel sheets to reduce vehicle weight, and the use of high-strength steel sheets of 980 MPa or higher for body and seat components is being promoted. Generally, increasing the strength of steel sheets reduces press formability, such as ductility, hole expandability, and bendability, making them more susceptible to cracking during press forming and reducing the degree of freedom in shaping, limiting their application to parts with simple shapes. Therefore, in order to apply high-strength steel sheets to parts with complex shapes, it is important to increase the strength of the steel sheets while maintaining or improving their press formability. Furthermore, increasing the strength of steel sheets is also required from the perspective of improving crashworthiness. Among these, in order to improve energy absorption properties, it is important not only to increase the strength of the steel sheets but also to improve their hole expandability and bendability in order to suppress fracture during crashes (e.g., Non-Patent Document 1).

[0003] In light of this background, TRIP (transformation-induced plasticity) steel, in which retained austenite (residual γ) is dispersed in the microstructure of the steel sheet, has been developed as a technology for improving the ductility of steel sheet. TRIP steel is manufactured using two heat treatment processes: austempering, which involves isothermal holding in the bainite transformation temperature range after soaking, and Q&P (Quenching & Partitioning) heat treatment, which involves cooling to a temperature range between the martensite transformation start temperature (Ms point) and the martensite transformation completion temperature (Mf point) during the cooling process, followed by reheating and holding to stabilize the retained austenite. In both of these heat treatment processes, a large amount of Si, which can suppress carbide precipitation, is added to form retained γ in the microstructure. For example, Patent Document 1 discloses that steel containing, by mass%, 0.04 to 0.12% C, 0.8 to 2.5% Si, and 0.5 to 2.0% Mn is subjected to soaking treatment followed by isothermal holding at 300 to 500°C for 10 to 900 seconds to form a volume fraction of retained austenite (residual γ) of 2 to 10%, resulting in a steel sheet with high ductility (TS×El≧21000 MPa·%). Furthermore, in Q&P, a portion of the structure is transformed into martensitic structure during the cooling process, and the martensitic structure is tempered by subsequent reheating, thereby reducing the hardness difference between different phases in the structure and improving not only ductility but also hole expandability. Patent Document 2 discloses a steel sheet having excellent hole expandability with a hole expansion ratio of 50% or more, in which a cold-rolled steel sheet containing 0.6 to 2.5% by mass of Si is held at a first soaking temperature of 750°C or higher, cooled to a cooling stop temperature in the temperature range of 150 to 350°C, and then reheated to a temperature range of 350 to 500°C, thereby achieving press formability with a volume fraction of 5 to 15% retained austenite, excellent collision characteristics with a TS of 980 MPa or higher, and ductility with an elongation of 17% or higher.

[0004] On the other hand, in the automotive manufacturing process, steel sheets blanked from coils are press-formed, then assembled to the vehicle body by spot welding or fastening, and then baked. This baked-finish process changes the yield strength and tensile strength of the steel sheets through work hardening (WH) due to the strain introduced during press forming, as well as bake hardening (BH) by holding the sheet at temperatures of 100–200°C for 10–30 minutes for drying. Thus, the material properties after BH determine the actual performance of automotive parts. In particular, to achieve high energy absorption during a collision in a vehicle that is driven repeatedly, it is important to suppress the formation of microvoids, which form when repeated strain is applied during driving and become crack initiation sites during a collision. To achieve this, improving fatigue strength after BH is necessary.

[0005] Patent No. 5515623 Patent No. 5821911 Patent No. 5589925

[0006] Proceedings of the Society of Automotive Engineers of Japan, Vol. 52, No. 1, (2021), P197

[0007] Among the above-mentioned press formability features, Patent Document 1 discloses ductility, and Patent Document 2 discloses TRIP steel with excellent ductility and hole expandability. However, neither discloses a technology for improving bendability, which is also important for crashworthiness. To ensure stable energy absorption characteristics even under complex deformation behavior during a crash, improving bendability is desirable. Furthermore, there is no disclosure of properties after bake-hardening, which is important after assembly into an automobile. Meanwhile, Patent Document 3 discloses a technology for achieving excellent bake-hardening properties even with low strain and stabilization of part material properties after bake-hardening by performing a two-stage soaking process on a steel containing, by mass, 0.05-0.35% C, 0.05-2.5% Si, and 0.6-3.0% Mn, followed by an isothermal holding process at 350-480°C and a subsequent cooling process. However, Patent Document 3 does not disclose a technology for improving hole expandability or bendability, which are important for both press formability and crashworthiness. Furthermore, Patent Documents 1, 2, and 3 do not disclose a technology for improving fatigue strength after bake-hardening.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a steel plate and a member having excellent press formability and crashworthiness and a tensile strength of 1180 MPa or more, and a method for manufacturing the same.

[0009] Here, the tensile strength (TS) is determined by a measurement method in accordance with JIS Z2241 (2011).

[0010] Furthermore, excellent press formability means that the total elongation (El) obtained in accordance with JIS Z2241 (2011) satisfies the ductility requirement of 14.0% or more at any TS level.

[0011] Furthermore, excellent press formability means that the hole expansion ratio (λ) (%) obtained by a hole expansion test conforming to the JFST1001 standard is 0.01 (= {(d - d ０ ) / d ０}×100) is 40% or more at any TS level.

[0012] Furthermore, excellent press formability refers to bendability in which the maximum bending radius (R / t) is 3.5 or less in the width direction of the steel sheet, the rolling direction, and in all bending directions in which the bending ridge line is directed at an angle of 45° to the rolling direction. R / t (R: limit bending radius (mm), t: sheet thickness (mm)) is measured in accordance with the V-block method in accordance with JIS Z2248 (1996).

[0013] Furthermore, excellent crashworthiness refers to a yield ratio (YR), which is the ratio (YS / TS) of yield strength (YS) to tensile strength (TS) obtained in accordance with JIS Z2241 (2011), of 0.80 or more.

[0014] Furthermore, excellent impact resistance is defined as a hole expansion ratio (λ) (= {(d - d ０ ) / d ０}×100) is 40% or more at any TS level.

[0015] Furthermore, excellent impact resistance refers to bendability in which the maximum bending radius (R / t) is 3.5 or less in the width direction of the steel sheet, the rolling direction, and in all bending directions in which the bending ridge line is directed at an angle of 45° to the rolling direction. R / t (R: limit bending radius (mm), t: sheet thickness (mm)) is measured in accordance with the V-block method in accordance with JIS Z2248 (1996).

[0016] Furthermore, excellent collision characteristics means that the ratio of the fatigue strength after BH treatment to the tensile strength (TS) obtained in accordance with JIS Z2241 (2011) (fatigue strength after BH treatment / TS) is 0.40 or more.

[0017] In order to solve the above problems, the present inventors have conducted extensive research into the steel components, heat treatment conditions, and microstructures that affect press formability and crashworthiness of various thin steel sheets having a tensile strength of 1180 MPa or more. As a result, they have found that the steel sheets contain, in mass%, C: 0.08 to 0.35%, Si: 0.50 to 2.00%, Mn: 1.5 to 3.5%, P: 0.050% or less, S: 0.01% or less, sol. The steel sheet has a component composition containing Al: 1.0% or less, N: 0.015% or less, and the balance being iron and unavoidable impurities, the area ratio of polygonal ferrite: less than 10%, the area ratio of bainite: 10% or more and 60% or less, the area ratio of tempered martensite: 70% or less (excluding 0%), the area ratio of retained austenite: 5% or more and 20% or less, the total area ratio of the retained structure: 5% or less (including 0%), the average amount of solute C in the retained austenite adjacent to the bainite or polygonal ferrite: 0.60 mass% or more and less than 1.20 mass%, and the number density of the retained austenite adjacent to the bainite or polygonal ferrite is 2.0 × 10 ５ pieces / mm ２ It has been found that by using the above steel structure, it is possible to obtain steel plates and members having a tensile strength of 1180 MPa or more, which are provided with excellent press formability and crashworthiness.

[0018] The present invention has been made based on the above findings, and its gist is as follows: [1] In mass %, C: 0.08 to 0.35%, Si: 0.50 to 2.00%, Mn: 1.5 to 3.5%, P: 0.050% or less, S: 0.01% or less, sol. A steel sheet having a component composition containing Al: 1.0% or less, N: 0.015% or less, with the balance being iron and unavoidable impurities, an area ratio of polygonal ferrite: less than 10%, an area ratio of bainite: 10% or more and 60% or less, an area ratio of tempered martensite: 70% or less (excluding 0%), an area ratio of retained austenite: 5% or more and 20% or less, a total area ratio of the retained structure: 5% or less (including 0%), an average amount of solute C in the retained austenite adjacent to the bainite or polygonal ferrite: 0.60 mass% or more and less than 1.20 mass%, and a number density of the retained austenite adjacent to the bainite or polygonal ferrite is 2.0 × 10 ５ pieces / mm ２ [2] The steel sheet according to [1] above, further comprising, as the chemical composition, one or more elements selected from, by mass%, Ti: 0.1% or less, B: 0.005% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, Co: 0.5% or less, Zr: 0.1% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.1% or less, Sb: 0.1% or less, and REM: 0.0050% or less. [3] A member made using the steel sheet according to [1] or [2] above. [4] A method for producing a steel sheet, comprising: hot rolling, pickling, and cold rolling a steel slab having the component composition according to [1] or [2]; and then annealing the resulting cold-rolled steel sheet; wherein the annealing comprises: ｃ３ Point -50℃ or higher A ｃ３ a soaking temperature T of 150°C or less for 30 to 500 seconds, and a soaking temperature M Ｓ Point +5℃ or more M Ｓ Cooling is performed at a first average cooling rate of 2 to 50°C / second in a temperature range from the point + 150°C to a first cooling stop temperature T1 of 500°C or less,Ｓ Point +5℃ or more M Ｓ a cooling step of cooling a steel slab at a temperature between +150°C and 500°C for a holding time t1 that satisfies the requirement of 10 seconds or more and ta seconds or less, to a second cooling stop temperature T2 at a second average cooling rate of 100°C / second or more to 50°C or less, and a reheating holding step of reheating from the second cooling stop temperature T2 to a reheating holding temperature of 50°C to 400°C at a heating rate of 50°C / second or less, and holding at the reheating holding temperature for a holding time t2 that satisfies the requirement of 10 seconds or more and tb seconds or less, where ta (seconds) and tb (seconds) are calculated by formulas (1) and (2), and in formulas (1) and (2), [Si] is the Si content (mass%) of the steel slab, and [sol. Al] is the sol. Al content (mass%) of the steel slab. ta=([Si]+[sol.Al])×336500 / (T1+273) (1) tb=([Si]+[sol.Al])×336500 / (T2+273)×(1-(t1 / ta)) (2) [5] A method for manufacturing a member, comprising a step of performing at least one of forming and joining on the steel plate according to [1] or [2] above to form a member.

[0019] According to the present invention, a steel plate and a member having a tensile strength of 1180 MPa or more, which have excellent press formability and crashworthiness, can be obtained. When the steel plate of the present invention is applied to a frame member of an automobile body, it is possible to manufacture difficult-to-form members having complex shapes by cold press working, which can greatly contribute to reducing the weight of the automobile body.

[0020] The present invention will be specifically described below, but the present invention is not limited to the following embodiments.

[0021] (Steel Plate) The steel plate of the present invention contains, in mass %, C: 0.08 to 0.35%, Si: 0.50 to 2.00%, Mn: 1.5 to 3.5%, P: 0.050% or less, S: 0.01% or less, sol. A component composition containing Al: 1.0% or less, N: 0.015% or less, and the balance being iron and unavoidable impurities, wherein the area ratio of polygonal ferrite is less than 10%, the area ratio of bainite is 10% or more and 60% or less, the area ratio of tempered martensite is 70% or less (excluding 0%), the area ratio of retained austenite is 5% or more and 20% or less, the total area ratio of the retained structure is 5% or less (including 0%), the average amount of solute C in the retained austenite adjacent to the bainite or polygonal ferrite is 0.60 mass% or more and less than 1.20 mass%, and the number density of the retained austenite adjacent to the bainite or polygonal ferrite is 2.0 × 10 ５ / mm ２ and a steel structure having at least one of the above.

[0022] The steel sheet of the present invention will be described below in the order of chemical composition and steel structure. First, the reasons for limiting the chemical composition of the present invention will be described. In the following description, all percentages indicating the steel composition are by mass % unless otherwise specified.

[0023] <C: 0.08-0.35%> C is included to ensure a predetermined strength through transformation strengthening and to secure a predetermined amount of retained austenite (residual γ) to improve ductility. A C content of less than 0.08% fails to ensure sufficient strength, the predetermined amount of retained γ, or both. On the other hand, exceeding the upper limit of the C content can result in a localized and excessive increase in the amount of solute C in the microstructure, potentially deteriorating hole expandability, bendability, and fatigue strength after BH treatment. Therefore, the C content must be 0.08-0.35%. The C content is preferably 0.10% or more, more preferably 0.12% or more. The C content is preferably 0.32% or less, more preferably 0.30% or less. From the perspective of ensuring fatigue strength and local deformability after BH treatment, a C content of 0.254% or less is even more preferable.

[0024] <Si: 0.50 to 2.00%> Si is added to strengthen ferrite, increasing strength and the yield ratio, and to suppress the formation of carbides in martensite and bainite, thereby ensuring a predetermined amount of retained γ and improving ductility. If the Si content is less than 0.50%, these effects cannot be fully achieved. On the other hand, if the Si content exceeds 2.00%, the effects saturate, and deformation resistance during hot rolling increases. Furthermore, if the Si content exceeds 2.00%, the desired hole expandability, bendability, and fatigue strength / TS after BH treatment cannot be achieved. Therefore, the Si content is set to 0.50 to 2.00%. The Si content is preferably 0.70% or more, more preferably 0.90% or more. The Si content is preferably 1.80% or less, more preferably 1.60% or less.

[0025] <Mn: 1.5 to 3.5%> Mn is added from the viewpoints of improving the hardenability of the steel sheet, promoting high strength through transformation strengthening, and improving ductility by promoting the formation of residual γ, which, like Si, suppresses the formation of carbides in bainite and contributes to ductility. To achieve these effects, the Mn content must be 1.5% or more. On the other hand, if the Mn content exceeds 3.5%, the bainite transformation is significantly delayed, making it impossible to ensure a predetermined amount of residual γ. For this reason, the Mn content is set to 1.5 to 3.5%. The Mn content is preferably 1.8% or more, more preferably 2.0% or more. The Mn content is preferably 3.2% or less, more preferably 3.0% or less.

[0026] <P: 0.050% or less> If the P content is high, it segregates at grain boundaries, resulting in deterioration of bendability. Furthermore, it deteriorates spot weldability. From this viewpoint, the P content is set to 0.050% or less. The P content is preferably 0.035% or less, and more preferably 0.020% or less. On the other hand, there is no particular restriction on the lower limit of P, but if the P content is significantly reduced, the steelmaking cost increases, so the P content is preferably set to 0.005% or more.

[0027] <S: 0.01% 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 bendability. It also deteriorates spot weldability. In order to reduce these negative effects, the S content is at least 0.01% or less, and preferably 0.0050% or less. Note that S may not be contained, but reducing it to less than 0.0001% requires a great deal of cost, so 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, and even more preferably 0.0010% or more.

[0028] <Sol. Al: 1.0% or Less> Al is contained for the purpose of deoxidation or obtaining residual γ. Although there is no particular lower limit for sol. Al, the sol. Al content is preferably 0.01% or more to ensure stable deoxidation. On the other hand, if the sol. Al content exceeds 1.0%, the amount of coarse Al-based inclusions increases significantly, deteriorating bendability. For this reason, the sol. Al content is set to 1.0% or less. The sol. Al content is preferably less than 1.0%, more preferably 0.80% or less, and even more preferably 0.06% or less.

[0029] <N: 0.015% or less> N is an element that forms nitrides such as BN, AlN, and TiN in steel and reduces bendability, so its content needs to be limited. Therefore, the N content is set to 0.015% or less. The N content is preferably less than 0.015%, more preferably 0.010% or less, and even more preferably 0.006% or less. Note that N may not be contained, but reducing the N content to less than 0.0001% requires significant costs, so the N content is preferably 0.0001% or more from the viewpoint of manufacturing costs. The N content is more preferably 0.0005% or more, and even more preferably 0.001% or more.

[0030] The steel sheet according to the present invention preferably has a composition containing the above-mentioned elemental elements as the basic components, with the balance being iron (Fe) and unavoidable impurities.

[0031] In addition to the above-mentioned components, the steel sheet of the present invention may contain one or more optional elements (selective elements) selected from the following: Ti: 0.1% or less, B: 0.005% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, Co: 0.5% or less, Zr: 0.1% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.1% or less, Sb: 0.1% or less, and REM: 0.0050% or less.

[0032] <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 and precipitating carbides to improve yield strength (YS). 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 even more preferably 0.010% or more. On the other hand, a Ti content exceeding 0.1% reduces the amount of solute C in the steel sheet, resulting in reduced TS and TS after bake-hardening. Furthermore, this increases the rolling load and precipitation strengthening, resulting in reduced ductility. Therefore, when Ti is contained, the Ti content is set to 0.1% or less. Preferably, the Ti content is 0.05% or less, more preferably 0.03% or less.

[0033] <B: 0.005% or less> B is an element that improves the hardenability of steel, and has the advantage of suppressing excessive ferrite formation and facilitating the formation of tempered martensite and / or bainite with a predetermined area ratio. Therefore, the B content is preferably 0.0005% or more. On the other hand, if the B content exceeds 0.005%, coarse BN is formed by bonding with solute N in the steel sheet, which deteriorates bendability. Therefore, when B is contained, the B content is set to 0.005% or less. The B content is preferably 0.003% or less.

[0034] <Cu: 1% or Less> Cu improves corrosion resistance in an automotive usage environment. Furthermore, Cu corrosion products coat the steel sheet surface, suppressing hydrogen penetration into the steel sheet. Cu is an element that is mixed in when scrap is utilized as a raw material. Allowing Cu to be incorporated allows recycled materials to be utilized as raw materials, thereby reducing manufacturing costs. From this perspective, Cu is preferably contained in an amount of 0.005% or more. Furthermore, from the perspective of improving delayed fracture resistance, Cu is more preferably contained in an amount of 0.05% or more. The Cu content is further preferably 0.10% or more. More preferably, the Cu content is 0.25% or more. On the other hand, excessive Cu content can cause surface defects and deteriorate bendability. Therefore, when Cu is contained, the Cu content is set to 1% or less. The Cu content is preferably 0.50% or less, more preferably 0.35% or less.

[0035] <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 desirable 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. On the other hand, if the Ni content is too high, scale generation in the heating furnace becomes non-uniform, which in turn causes surface defects and deteriorates bendability. It also leads to increased costs. Therefore, when Ni is contained, the Ni content is set to 1% or less. The Ni content is preferably 0.5% or less, and more preferably 0.3% or less.

[0036] <Cr: 1% or less> Cr can be added to improve the hardenability of steel and to suppress the formation of carbides in martensite and 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. The Cr content is preferably 0.10% or more, more preferably 0.20% or more, and even more preferably 0.30% or more. On the other hand, since an excessive Cr content deteriorates pitting corrosion resistance, when Cr is contained, the Cr content is set to 1% or less. The Cr content is preferably 0.8% or less, and even more preferably 0.6% or less.

[0037] <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 bainite. To achieve these effects, the Mo content is preferably 0.01% or more. The Mo content is more preferably 0.03% or more, and even more preferably 0.06% or more. The Mo content is more preferably 0.1% or more, and even more preferably 0.2% or more. On the other hand, Mo is an element that retards bainite transformation. If the Mo content exceeds 0.5%, the desired residual γ cannot be obtained, and ductility decreases. Therefore, when Mo is contained, the Mo content is set to 0.5% or less. The Mo content is preferably 0.4% or less, and even more preferably 0.35% or less.

[0038] <V: 0.5% or Less> V can be added to improve the hardenability of steel, inhibit the formation of Fe-based carbides in martensite and bainite, refine the structure to improve bendability, and precipitate carbides to improve yield strength (YS). To achieve these effects, the V content is preferably 0.003% or more. The V content is more preferably 0.005% or more, and even more preferably 0.010% or more. The V content is still more preferably 0.020% or more, and even more preferably 0.040% or more. On the other hand, since a large amount of V significantly deteriorates castability, when V is contained, the V content is set to 0.5% or less. Preferably, the V content is 0.3% or less, and more preferably 0.2% or less. The V content is preferably 0.2% or less, and even more preferably 0.1% or less.

[0039] <Nb: 0.1% or less> Nb can be added to refine the steel structure to improve bendability and precipitate carbides to increase yield strength (YS). To achieve these effects, the Nb content is preferably 0.010% or more. The Nb content is preferably 0.015% or more, and more preferably 0.020% or more. On the other hand, if a large amount of Nb is added, 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 added, the Nb content is set to 0.1% or less. Preferably, the Nb content is 0.08% or less, and more preferably 0.05% or less.

[0040] <Co: 0.5% or less> Co can be added to improve the hardenability of steel. To achieve this effect, the Co content is preferably 0.01% or more. The Co content is more preferably 0.05% or more, and even more preferably 0.1% or more. On the other hand, if a large amount of Co is added, coarse precipitates and inclusions increase, reducing the ultimate deformability of the steel sheet and reducing bendability and hole expandability. Therefore, when Co is added, the Co content is set to 0.5% or less. The Co content is preferably 0.4% or less, and more preferably 0.3% or less.

[0041] <Zr: 0.1% 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 a large amount of Zr is added, the amount of coarse precipitates such as ZrN and ZrS remaining 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 0.1% or less. The Zr content is preferably 0.050% or less, and more preferably 0.030% or less.

[0042] <Mg: 0.0050% or less> Mg fixes O as MgO and contributes to improving formability such as bendability. For this reason, the Mg content is preferably 0.0002% or more. The Mg content is more preferably 0.0010% or more, and more preferably 0.0015% or more. On the other hand, adding a large amount of Mg deteriorates the surface quality and bendability, so when Mg is contained, the Mg content is set to 0.0050% or less. Preferably, the Mg content is 0.0040% or less.

[0043] <Ca: 0.0050% or less> Ca fixes S as CaS and contributes to improving bendability. Therefore, the Ca content is preferably 0.0002% or more. The Ca content is more preferably 0.0005% or more, even more preferably 0.0010% or more, and even more preferably 0.0020% 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 0.0050% or less. Preferably, the Ca content is 0.0040% or less.

[0044] <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 content of C and B in the surface layer. This suppresses the formation of excessive ferrite in the surface layer of the steel sheet, increasing strength and improving fatigue properties. From this perspective, the Sn content is preferably 0.003% or more. The Sn content is more preferably 0.010% or more, and even more preferably 0.015% or more. The Sn content is preferably 0.020% or more, and more preferably 0.030% or more. On the other hand, if the Sn content exceeds 0.1%, castability deteriorates. Therefore, when Sn is contained, the Sn content is set to 0.1% or less. The Sn content is preferably 0.08% 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. This effect suppresses excessive ferrite formation in the surface layer of the steel sheet, increasing strength and improving fatigue properties. 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. More preferably, the Sb content is 0.008% or more, and even more preferably 0.010% or more. The Sb content is preferably 0.015% or more, and more preferably 0.020% or more. On the other hand, if the Sb content exceeds 0.1%, castability deteriorates. Therefore, when Sb is contained, the Sb content is set to 0.1% or less. The Sb content is preferably 0.080% or less, and more preferably 0.040% or less.

[0046] <REM: 0.0050% or less> REM is an element that spheroidizes the shape of sulfides, thereby suppressing the adverse effect of sulfides on stretch flangeability and improving bendability. To achieve these effects, the REM content is preferably 0.0005% or more. The REM content is more preferably 0.0010% or more, and even more preferably 0.0020% or more. On the other hand, if the REM content exceeds 0.0050%, the effect of improving bendability saturates. Therefore, when REM is contained, the REM content is set to 0.0050% or less. In the present invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoid elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content in the present invention refers to the total content of one or more elements selected from the above-mentioned REMs.

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

[0048] Next, the mechanical properties of the steel sheet of the present invention, which has excellent press formability and crashworthiness and has a tensile strength of 1180 MPa or more, will be described.

[0049] The steel sheet of the present invention has a tensile strength (TS) of 1180 MPa or more. There is no particular upper limit to the tensile strength, but from the viewpoint of compatibility with other properties, the tensile strength is preferably 1300 MPa or less.

[0050] The steel sheet of the present invention has excellent press formability, in terms of ductility, a total elongation (El) of 14.0% or more at any TS level. Also, in terms of hole expandability, the steel sheet of the present invention has a hole expansion ratio (λ) of 40% or more at any TS level. Furthermore, in terms of bendability, the steel sheet of the present invention has a maximum bending radius (R / t) of 3.5 or less in the width direction of the steel sheet, the rolling direction, and any bending direction in which the bending ridge line is at an angle of 45° to the rolling direction.

[0051] The steel sheet of the present invention has excellent impact properties, such as a yield ratio (YR) of 0.80 or more. Furthermore, among the excellent impact properties, the steel sheet has excellent bendability, such that the maximum bending radius (R / t) is 3.5 or less in the width direction of the steel sheet, the rolling direction, and in all bending directions in which the bending ridgeline is directed at an angle of 45° to the rolling direction. Furthermore, the steel sheet has excellent impact properties, such that the ratio of fatigue strength after BH treatment to TS (fatigue strength after BH treatment / TS) is 0.40 or more.

[0052] To evaluate the tensile properties (TS, El, YS), JIS No. 5 tensile test pieces are taken from the center position of the sheet width, and tensile tests (in accordance with JIS Z2241 (2011)) are performed with N=3. Each evaluation is based on the average value of the three points. Steel sheets with a tensile strength of 1180 MPa or more are considered high-strength steel sheets.

[0053] The hole expandability was evaluated by conducting a hole expansion test in accordance with the JFST1001 standard with N=3, and the obtained hole expansion ratio (λ) (= {(d - d ０ ) / d ０}×100).

[0054] To evaluate bendability, first, a bending test piece 30 mm wide and 100 mm long is taken from the width center of the steel plate so that the width direction, rolling direction, and 45° direction to the rolling direction of the steel plate are the bending ridge line directions. Next, using the taken bending test piece, a bending test (based on the V-block method of JIS Z2248 (1996)) is performed at a push-in speed of 100 mm / s and N = 3 tests at each bending radius, and the presence or absence of cracks is determined on the outside of the bent portion using a stereomicroscope. The maximum bending radius (R / t) refers to the maximum bending radius at which no cracks occur, and the width direction, rolling direction, and 45° direction to the rolling direction of the steel plate are the bending ridge line directions, and it is ensured to be 3.5 or less in all directions.

[0055] To evaluate the fatigue strength after BH treatment, the steel plate is first subjected to a 2% pre-strain in accordance with JIS Z2241 (2011), followed by a simulated baking treatment at 170°C for 20 minutes. The resulting steel plate is then subjected to fatigue strength evaluation using a plane bending fatigue test method for metal flat plates in accordance with JIS Z2275 (2019). Test specimens are processed to a minimum neck width of 20 mm and a curvature radius of 42.5 mm in accordance with JIS 1-20, and various bending stresses of 1 x 10 are applied at a stress ratio of -1 and a repetition rate of 25 Hz. ７ The bending stress at which no fracture occurs after repeated application of the stress is defined as the fatigue strength.

[0056] Next, the steel structure of the steel plate of the present invention will be described.

[0057] <Area ratio of polygonal ferrite: less than 10%> When the area ratio of polygonal ferrite increases, ductility improves, but the yield ratio decreases and hole expandability and bendability deteriorate. From this viewpoint, the area ratio of polygonal ferrite is limited to less than 10%. The area ratio of polygonal ferrite is preferably 8% or less, and more preferably 6% or less. There is no particular lower limit for the area ratio of polygonal ferrite, and the area ratio of polygonal ferrite may be 0%.

[0058] <Area Fraction of Bainite: 10% or More and 60% or Less> To obtain the desired ductility and bendability, the area fraction of bainite is set to 10% or more, and to obtain even higher strength, it is preferably set to 15% or more. On the other hand, if the area fraction of bainite exceeds 60%, it may not be possible to satisfy the tensile strength of 1,180 MPa or more and the yield ratio of 0.80 or more. For this reason, the area fraction of bainite is set to 60% or less. The area fraction of bainite is preferably 55% or less, and more preferably 50% or less.

[0059] <Area Fraction of Tempered Martensite: 70% or Less (Not Including 0%)> Tempered martensite is a structure necessary to obtain a predetermined strength and yield ratio. Therefore, the area fraction of tempered martensite is set to more than 0%. The area fraction of tempered martensite is preferably 15% or more, and more preferably 25% or more. On the other hand, if the area fraction of tempered martensite exceeds 70%, the predetermined retained γ cannot be obtained, and ductility cannot be ensured. Therefore, the area fraction of tempered martensite is set to 70% or less (not including 0%). The area fraction of tempered martensite is preferably 65% ​​or less, and more preferably 60% or less.

[0060] <Area Fraction of Retained Austenite (Residual γ): 5% or More and 20% or Less> If the area fraction of retained austenite is less than 5%, the desired ductility may not be ensured. From the viewpoint of ductility, the area fraction of retained austenite is set to 5% or more, preferably 7% or more. On the other hand, if the area fraction of retained austenite exceeds 20%, bendability and hole expandability deteriorate, making fracture more likely to occur during press forming. In addition, a large amount of fresh martensite may be formed when applying a 2% pre-strain during BH treatment, which may reduce fatigue strength. For this reason, the area fraction of retained austenite is set to 20% or less. The area fraction of retained austenite is preferably 18% or less, more preferably 16% or less.

[0061] <Average amount of dissolved C in the retained austenite adjacent to bainite or polygonal ferrite: 0.60 mass% or more and less than 1.20 mass%, and the number density of the retained austenite adjacent to bainite or polygonal ferrite: 2.0 × 10 ５ pieces / mm ２In TRIP steel, retained austenite (residual γ) forms adjacent to a bainite or martensite structure. As a result, retained γ may form adjacent to a polygonal ferrite structure. To ensure that retained γ remains stable even after 2% prestrain and ensures ductility while also ensuring bendability, hole expandability, and fatigue strength after BH treatment, it is necessary to increase the average amount of solute C in retained γ adjacent to bainite or polygonal ferrite, which are softer than martensite. Therefore, the average amount of solute C in retained γ adjacent to bainite or polygonal ferrite is set to 0.60 mass% or more. If the average amount of solute C falls below 0.60 mass%, at least one of bendability, hole expandability, and fatigue strength after BH treatment deteriorates. Therefore, the average amount of solute C in retained γ adjacent to bainite or polygonal ferrite is set to 0.60 mass% or more. It is preferably 0.70 mass% or more, and more preferably 0.80 mass% or more. On the other hand, if the average amount of solute C in the residual γ is too high, the residual γ remains until the later stage of deformation, so ductility and bendability are not improved, and the formation of extremely hard martensite may deteriorate hole expandability. For this reason, the average amount of solute C in the residual γ is limited to less than 1.20 mass%. The average amount of solute C in the residual γ is preferably less than 1.10 mass%, more preferably 1.05 mass% or less. Furthermore, even if the average amount of solute C in the residual γ adjacent to bainite or polygonal ferrite is 0.60 mass% or more, the number density may be 2.0 × 10 ５ pieces / mm ２ If the average amount of solute C is less than 0.60 mass% and less than 1.20 mass%, the number density of residual γ adjacent to bainite or polygonal ferrite is 2.0 × 10 ５ pieces / mm ２ The number density is preferably 2.5 × 10 ５ pieces / mm ２ More preferably, 3.0 × 10 ５ pieces / mm ２ Although there is no particular upper limit, the number density is preferably 8.0 × 10５ pieces / mm ２ More preferably, it is 6.0 × 10 ５ pieces / mm ２ The following is the result.

[0062] <Total area ratio of remaining structure: 5% or less (including 0%)> The steel structure preferably consists of a remaining structure other than the polygonal ferrite, bainite, tempered martensite, and retained γ described above. The total area ratio of the remaining structure is 5% or less. The total area ratio of the remaining structure may be 0%. Examples of the remaining structure include fresh martensite, unrecrystallized ferrite, carbide, and pearlite. These structures may be determined by SEM observation as described below. In particular, in the present invention, it is preferable that fresh martensite is not included because fresh martensite is a structural factor that adversely affects hole expandability, bendability, and fatigue properties.

[0063] Next, a method for measuring the steel structure will be described. The area ratios of polygonal ferrite, bainite, tempered martensite, and the remaining structure (fresh martensite, unrecrystallized ferrite, carbides, and pearlite) were measured by cutting a cross section of the plate parallel to the rolling direction, mirror-polishing it, and then etching it with 1 vol% nital. Ten fields of view of 40 μm × 30 μm were observed at 1 / 4 thickness using an SEM at 3000x magnification, and the photographed structure was quantified using image analysis. Polygonal ferrite refers to relatively equiaxed ferrite with almost no carbides inside. This is the region that appears the blackest under SEM. Bainite is a structure with the formation of carbides or retained austenite inside, which appears white under SEM. When it is difficult to distinguish between bainite and ferrite, the area ratios are calculated by classifying ferrite regions with an aspect ratio of ≦2.0 as polygonal ferrite and regions with an aspect ratio >2.0 as bainite. Here, 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 is cut across the longest in the direction perpendicular to the major axis a, with a / b being the aspect ratio. Tempered martensite is a region with lath-like substructure and carbide precipitation inside it when viewed under an SEM, while fresh martensite is a massive region that appears white under an SEM without visible substructure. Unrecrystallized ferrite is a structure that can be confirmed as ferrite with a black contrast containing deformed structure introduced by rolling, while carbides and pearlite are confirmed as white contrast. Carbides are distinguishable from pearlite because they have a particle diameter of 1 μm or less and have a lamellar (layer) structure.

[0064] The area fraction of retained austenite is determined by chemically polishing the steel sheet at a position 1 / 4 the thickness from the surface and then subjecting it to 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 retained austenite is randomly distributed in the steel sheet, the volume fraction of retained austenite determined by X-ray diffraction is equivalent to the area fraction. Therefore, in this specification, retained austenite is quantified as an area fraction.

[0065] The average amount of solute C in the retained γ adjacent to bainite or polygonal ferrite, and the number density of the retained γ adjacent to bainite or polygonal ferrite where the average amount of solute C is 0.60 mass% or more and less than 1.20 mass%, are measured by the following procedure. First, a cross section parallel to the rolling direction and perpendicular to the steel sheet surface (a thickness cross section parallel to the rolling direction) is cut out and mirror-polished. Then, using an EPMA (field emission electron probe microanalyzer) attached to an FE-SEM, a 3000 μm cross section is measured at a position 1 / 4 of the sheet thickness. ２The above measurement areas are measured to obtain an elemental map of C (C map). Next, in the same field of view, phase maps of the bcc phase and the fcc phase are obtained by SEM-EBSD, and the C map is matched to the residual γ adjacent to bainite or polygonal ferrite. Here, bainite, polygonal ferrite, and tempered martensite are all detected in the bcc phase, but bainite and tempered martensite can be distinguished by their IQ values ​​(image quality values). Although this is affected by the measurement conditions and the surface condition of the evaluation sample, tempered martensite generally has a structure with a high KAM value and a low IQ value, for example. In contrast, bainite or polygonal ferrite has a structure with a low KAM value compared to tempered martensite and a high IQ value compared to tempered martensite. By matching the obtained C map with the EBSD data, the residual γ adjacent to bainite or polygonal ferrite is analyzed, and the solute C at each data point in each residual γ grain is added up and divided by the number of data points for the residual γ grains. The average of these values ​​is taken as the average amount of solute C in the residual γ adjacent to bainite or polygonal ferrite. The number density of the residual γ adjacent to bainite or polygonal ferrite is calculated by dividing the number of residual γ adjacent to bainite or polygonal ferrite identified by the above method by the measured area. Here, "adjacent to bainite or polygonal ferrite" refers to the presence of some or all of the grain boundaries of the residual γ adjacent. It is necessary to ensure that there are at least 10 measurement data points for the amount of solute C in the residual γ grains. The amount of solute C in the residual γ varies depending on the case. Therefore, in such cases, the average amount of solute C can be calculated as the average value of the top 10% of data with the highest amount of solute C in the residual γ. In the phase map obtained by the SEM-EBSD method in the above measurement, retained austenite that does not contain a high-angle grain boundary of 15° or more inside is considered to be one retained austenite grain, and when the retained austenite contains a high-angle grain boundary, it is analyzed as two or three or more retained austenite grains separated by a high-angle grain boundary. In other words, retained austenite that does not have an orientation difference of 15° or more is considered to be one retained austenite.

[0066] (Method for Producing Steel Sheet) Next, a method for producing a steel sheet according to the present invention will be described.

[0067] The method for producing a steel sheet of the present invention is a method for producing a steel sheet, which comprises hot rolling, pickling and cold rolling a steel slab having the above-mentioned composition, and then annealing the obtained cold-rolled steel sheet, and the annealing is performed on the cold-rolled steel sheet. ｃ３ Point -50℃ or higher A ｃ３ a soaking temperature T of 150°C or less for 30 to 500 seconds, and a soaking temperature M Ｓ Point +5℃ or more M Ｓ Cooling is performed at a first average cooling rate of 2 to 50°C / second in a temperature range from the point + 150°C to a first cooling stop temperature T1 of 500°C or less, Ｓ Point +5℃ or more M Ｓ The method includes a cooling step of cooling the steel slab at a temperature between +150°C and 500°C for a holding time t1 that satisfies the requirement of 10 seconds or more and ta seconds or less, and then cooling the steel slab to a second cooling stop temperature T2 that is 50°C or less at a second average cooling rate of 100°C / second or more, and a reheating and holding step of reheating the steel slab from the second cooling stop temperature T2 to a reheating and holding temperature of 50°C or more and 400°C or less at a heating rate of 50°C / second or less, and holding the steel slab at the reheating and holding temperature for a holding time t2 that satisfies the requirement of 10 seconds or more and tb seconds or less, where ta (seconds) and tb (seconds) are calculated using formulas (1) and (2), respectively. In formulas (1) and (2), [Si] is the Si content (mass%) of the steel slab, and [sol. Al] is the sol. Al content (mass%) of the steel slab. ta=([Si]+[sol.Al])×336500 / (T1+273)...(1) tb=([Si]+[sol.Al])×336500 / (T2+273)×(1-(t1 / ta))...(2)

[0068] <Hot rolling> Methods for hot rolling a steel slab include a method of rolling the slab after heating, a method of directly rolling the slab after continuous casting without heating it, and a method of rolling the slab after continuous casting after subjecting it to a short-term heat treatment. Hot rolling may be carried out according to a conventional method, and for example, the slab heating temperature may be 1100°C or higher. The slab heating temperature may be 1300°C or lower. The soaking temperature may be 20 min or higher. The soaking temperature may be 300 min or lower. The finish rolling temperature may be A ｒ３ The temperature should be equal to or higher than the transformation point. ｒ３ The 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 fluctuation and stably ensuring high strength. Specifically, the coiling temperature is preferably set to 430°C or more. The coiling temperature is preferably set to 650°C or less. ｒ３ The transformation point can be calculated from the composition of the steel sheet and the following empirical formula (3): ｒ３ Point (°C) = 910 - 310 x [C] - 80 x [Mn] - 20 x [Cu] - 15 x [Cr] - 55 x [Ni] - 80 x [Mo] (3) (In the above formula, [M] is the content (mass%) of element M in the steel slab, and the value of an element that is not contained is zero (0).)

[0069] <Pickling> Pickling may be carried out in accordance with a conventional method.

[0070] <Cold Rolling> Cold rolling may be carried out according to a conventional method, with the rolling ratio (cumulative rolling ratio) being 30% or more. The rolling ratio (cumulative rolling ratio) may be 85% or less. The rolling ratio is preferably controlled 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).

[0071] <Annealing> A cold-rolled steel sheet manufactured according to a conventional method is annealed under the following conditions. The annealing equipment is not particularly limited, but from the viewpoints of productivity and ensuring the desired heating rate and cooling rate, it is preferable to carry out the annealing in a continuous annealing line (CAL).

[0072] [Soaking and holding process: A ｃ３ Point -50℃ or higher A ｃ３ The steel sheet obtained by the present invention is free from soft ferrite structure as much as possible, and retained γ is formed in the cooling process. This improves hole expandability and bendability while maintaining ductility. In order to achieve the above effect, the soaking temperature T is set to A ｃ３ On the other hand, if the soaking temperature T is excessively high, specifically, ｃ３ If the soaking temperature T exceeds the soaking point +150°C, the structure will become significantly coarse, the bainite transformation in the cooling process will be suppressed, and the desired retained γ will not be obtained. ｃ３ Point -50℃ or higher A ｃ３ The soaking temperature T is preferably A ｃ３ point -40°C or higher, more preferably A ｃ３ The soaking temperature T is preferably A ｃ３ point +100°C or less, and more preferably A ｃ３The soaking temperature is 50°C or less. Furthermore, if the soaking temperature T is maintained for less than 30 seconds, austenite formation at the soaking temperature is insufficient, resulting in an increase in polygonal ferrite, and the desired bainite and tempered martensite are not obtained. This can result in the desired strength while still maintaining the desired bendability and hole expandability, or the desired ductility can be impaired by insufficient retained austenite. On the other hand, if the soaking temperature T is maintained for more than 500 seconds, significant coarsening of the structure occurs, suppressing bainite transformation during the cooling process and potentially preventing the desired residual γ from being obtained. Therefore, the soaking temperature T is maintained for 30 to 500 seconds. The soaking temperature T is preferably maintained for 60 seconds or more, more preferably for 100 seconds or more. The soaking temperature T is preferably maintained for 400 seconds or less, more preferably for 300 seconds or less.

[0073] In addition, the above A ｃ３ is obtained from the empirical formula (4) below. ｃ３ It is sufficient to use the following. ｃ３ =910-203×([C]) １／２ −15.2 × [Ni] + 44.7 × [Si] + 104 × [V] + 31.5 × [Mo] + 13.1 × [W] Formula (4) In the above formula, [M] is the mass percentage of each element in the steel slab, and the value of an element that is not contained is zero (0).

[0074] [Cooling step (1): Soaking temperature T to M Ｓ Point +5℃ or more M Ｓ The temperature range from the point +150°C to a first cooling stop temperature T1 of 500°C or less is cooled at a first average cooling rate of 2 to 50°C / second, and M Ｓ Point +5℃ or more M Ｓ After holding at the soaking temperature T (after the soaking holding step), the temperature is changed from the soaking temperature T to M. Ｓ Point +5℃ or more M ＳThe temperature range from the soaking point +150°C or less to a first cooling stop temperature T1 of 500°C or less is cooled at a first average cooling rate of 2 to 50°C / second. If the first average cooling rate is less than 2°C / second, ferrite transformation during cooling proceeds excessively, making it impossible to suppress the formation of polygonal ferrite, so the first average cooling rate is set to 2°C / second or more. The first average cooling rate is preferably 5°C / second or more. On the other hand, if the first average cooling rate is too high, the plate shape deteriorates, so the first average cooling rate is set to 50°C / second or less. The first average cooling rate is preferably 40°C / second or less, and more preferably less than 30°C / second. Here, the first average cooling rate (°C / second) is "soaking temperature T (°C) - first cooling stop temperature T1 (°C) / cooling time (seconds) from soaking temperature T to first cooling stop temperature T1".

[0075] The above M Ｓ Point +5℃ or more M Ｓ At a temperature below +150°C and below 500°C (hereinafter also referred to as the residence temperature), bainite transformation occurs, thereby forming highly stable retained γ. Because bainite transforms with an incubation period, a holding time t1 (residence time t1) of 10 seconds or more at the residence temperature is required to sufficiently cause bainite transformation. On the other hand, if this residence time t1 is excessively long, the decomposition of the retained γ occurs, making it impossible to ensure the desired retained γ. The limit of this residence time t1 varies depending on the composition of the steel sheet, and after extensive investigation, it has been found that, for any composition, the decomposition of the retained γ does not occur and the desired retained γ can be obtained as long as the residence time is equal to or less than the residence time (ta seconds) calculated by the following formula (1). Therefore, the residence time t1 is set to 10 seconds or more and ta seconds or less. Note that the residence time t1 is determined by the M Ｓ Point +5℃ or more M Ｓ This is the time during which the material is held in a temperature range of 150°C or less and 500°C or less than the point above, and it is not necessary to hold the material at the same temperature.

[0076] ta=([Si]+[sol. Al])×336500 / (T1+273) (1) In formula (1), [Si] is the Si content (mass%) of the steel slab, and [sol. Al] is the sol. Al content (mass%) of the steel slab.

[0077] The above M Ｓis obtained from the empirical formula (5) below. Ｓ M Ｓ = 538 - 350 × [C] - 37.7 × [Mn] - 18.9 × [Ni] - 37.7 × [Cr] - 27 × [Mo] Formula (5) Here, [M] is the mass% of each element in the steel slab.

[0078] [Cooling step (2): Cooling to a second cooling stop temperature T2 of 50° C. or less at a second average cooling rate of 100° C. / second or more] Ｓ Point +5℃ or more M Ｓ The steel sheet is retained at a retention temperature of 150°C or lower and 500°C or lower, and then cooled to a temperature of 50°C or lower at a second average cooling rate of 100°C / s or higher. If the second cooling stop temperature T2 exceeds 50°C, the amount of untransformed austenite increases, resulting in an increase in fresh martensite formed during final cooling, deteriorating hole expandability and delayed fracture resistance. For this reason, the second cooling stop temperature T2 is set to 50°C or lower. Furthermore, if the second average cooling rate is lower than 100°C / s, non-uniform movement of solute C occurs during cooling due to bainite transformation and self-tempering of martensite, resulting in a decrease in the yield ratio. Therefore, the second average cooling rate is set to 100°C / s or higher. The second average cooling rate is preferably 120°C / s or higher. On the other hand, although there is no upper limit to the second average cooling rate, from the viewpoints of operability and prevention of meandering in a continuous annealing furnace, the second average cooling rate is preferably set to 2000°C / s or lower. The second average cooling rate is more preferably 1800°C / sec or less. Here, the second average cooling rate is "dwell end temperature (°C) - second cooling stop temperature (°C) / cooling time (seconds) from the dwell end temperature to the second cooling stop temperature." Note that the dwell end temperature is the same as the above-mentioned M Ｓ Point +5℃ or more M Ｓ The retention temperature is 150°C or lower and 500°C or lower (retention temperature) at the end of retention.

[0079] [Reheating and Holding Step: Reheating from the second cooling stop temperature T2 to a reheating and holding temperature of 50°C or higher and 400°C or lower at a heating rate of 50°C / second or lower, and holding at the reheating and holding temperature for a holding time t2 seconds that satisfies the requirement of 10 seconds or higher and tb seconds or lower] In holding at the reheating and holding temperature (reheating and holding), the steel sheet is held at a reheating and holding temperature of 50°C or higher and 400°C or lower for 10 seconds or higher and tb seconds or lower, from the viewpoint of adjusting the strength by tempering the formed martensite and improving the yield ratio and bendability. Here, tb (seconds) is calculated by the following formula (2), which takes into account the residence time (holding time t1) in the cooling step. If the reheating and holding temperature is lower than 50°C or the holding time t2 at the reheating and holding temperature is lower than 10 seconds, the tempering of the martensite will be insufficient, and the hardness difference in the steel sheet structure will increase, which may result in deterioration of one or more of the hole expandability, bendability, and delayed fracture resistance. On the other hand, if the reheating temperature exceeds 400°C or the holding time t2 at the reheating temperature exceeds tb seconds, decomposition of retained austenite occurs, making it impossible to ensure the desired ductility. Furthermore, if the holding time t2 at the reheating temperature exceeds tb seconds, excessive tempering of martensite occurs, making it impossible to ensure the desired strength. Furthermore, if the holding time t2 at the reheating temperature exceeds tb seconds, the decomposition reaction of retained austenite is promoted, making it impossible to obtain the desired area ratio of retained austenite and the desired ductility. Therefore, the holding time t2 at the reheating temperature is set to 10 seconds or more and tb seconds or less. The holding time t2 at the reheating temperature is preferably 50 seconds or more. Furthermore, the holding time t2 at the reheating temperature is preferably tb-50 seconds or less. The reheating temperature is 50°C or more and 400°C or less, preferably 60°C or more, and more preferably 100°C or more. The reheating temperature is preferably 350°C or less, more preferably 300°C or less. If the heating rate exceeds 50°C / sec, temperature unevenness in the width direction of the steel sheet increases, making it more likely that material variations in the width direction of the steel sheet will occur. Therefore, the heating rate is set to 50°C / sec or less. The heating rate is preferably 40°C / sec or less, more preferably 30°C / sec or less. The heating rate is preferably 5°C / sec or more, more preferably 10°C / sec or more.

[0080] tb=([Si]+[sol. Al])×336500 / (T2+273)×(1−(t1 / ta)) (2) In formula (2), [Si] is the Si content (mass%) of the steel slab, and [sol. Al] is the sol. Al content (mass%) of the steel slab.

[0081] Here, the temperature rise rate is "reheating holding temperature (°C) - second cooling stop temperature T2 (°C) / heating time (seconds) from the second cooling stop temperature to the reheating holding temperature".

[0082] [Thickness] The steel sheet of the present invention obtained as described above preferably has a thickness of 0.5 mm or more, and more preferably 3.0 mm or less.

[0083] (Member and manufacturing method of the member) Next, the member of the present invention and its manufacturing method will be described.

[0084] 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.

[0085] The steel sheet of the present invention has a tensile strength of 1180 MPa or more, a high yield ratio, excellent ductility, excellent hole expandability, excellent bendability, and excellent fatigue properties after BH treatment. Therefore, the member of the present invention also has a tensile strength of 1180 MPa or more, a high yield ratio, excellent ductility, excellent hole expandability, excellent bendability, and excellent fatigue properties after BH treatment. Furthermore, use of the member of the present invention enables weight reduction. Therefore, the member of the present invention can be suitably used, for example, in vehicle body frame parts.

[0086] 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.

[0087] A slab produced by continuous casting and having the chemical composition shown in Table 1 was heated to 1200°C with a soaking time of 200 min., a finish rolling temperature of 860°C or higher, and a coiling temperature of 550°C. After the hot rolling process, the slab was cold rolled at a rolling reduction (cumulative rolling reduction) of 50%. The resulting cold-rolled steel sheet having a thickness of 1.2 mm was treated under the annealing conditions shown in Table 2 to produce steel sheets according to the present invention and comparative examples. The measurement results are shown in Table 3.

[0088]

[0089]

[0090]

[0091] The steel structure was measured by the method described above, and the measurement results are shown in Table 3.

[0092] The obtained steel sheets were subjected to a tensile test using the method described above to obtain the tensile strength (TS), yield ratio (YR), and total elongation (El). A hole expansion test was also performed using the method described above to obtain the hole expansion ratio (λ). Bendability was also evaluated using the method described above, and the maximum values ​​of R / t in the width direction, rolling direction, and bending direction in which the ridgeline direction is at 45° to the rolling direction of the steel sheet are shown in Table 3. The R / t values ​​shown in Table 3 are the maximum values ​​of R / t in the width direction, rolling direction, and bending direction in which the ridgeline direction is at 45° to the rolling direction. The fatigue strength / TS after BH treatment was also obtained using the method described above.

[0093] Steel sheets with a tensile strength TS of 1180 MPa or more were judged to have excellent strength. Steel sheets with a total elongation El of 14.0% or more were judged to have excellent ductility. Steel sheets with a hole expansion ratio λ of 40% or more were judged to have excellent hole expandability. Steel sheets with a yield ratio YR of 0.80 or more were judged to have excellent yield ratio. Steel sheets with an R / t (maximum value of R / t) of 3.5 or less were judged to have excellent bendability. Steel sheets with a fatigue strength / TS after BH treatment of 0.40 or more were judged to have excellent fatigue properties after BH treatment.

[0094] The examples of the present invention shown in Tables 2 and 3 were excellent in strength, yield ratio, ductility, hole expandability, bendability and fatigue properties after BH treatment, whereas the comparative examples were inferior in any of these properties.

[0095] Furthermore, it was found that the components obtained by forming and joining the steel plates of the present invention have excellent strength, yield ratio, ductility, hole expandability, bendability and fatigue properties after BH treatment, similar to the steel plates of the present invention, because the steel plates of the present invention are excellent in strength, ductility, hole expandability, yield ratio, bendability and fatigue properties after BH treatment.

Claims

1. In mass %, C: 0.08 to 0.35%, Si: 0.50 to 2.00%, Mn: 1.5 to 3.5%, P: 0.050% or less, S: 0.01% or less, sol. A steel sheet having a component composition containing Al: 1.0% or less, N: 0.015% or less, with the balance being iron and unavoidable impurities, wherein the area ratio of polygonal ferrite is less than 10%, the area ratio of bainite is 10% or more and 60% or less, the area ratio of tempered martensite is 70% or less (excluding 0%), the area ratio of retained austenite is 5% or more and 20% or less, and the total area ratio of the retained structure is 5% or less (including 0%), the average amount of solute C in the retained austenite adjacent to the bainite or polygonal ferrite is 0.60 mass% or more and less than 1.20 mass%, and the number density of the retained austenite adjacent to the bainite or polygonal ferrite is 2.0 × 10 ５ pieces / mm ２ and a steel structure as described above.

2. The steel plate according to claim 1, wherein the chemical composition further contains, in mass %, one or more selected from the following: Ti: 0.1% or less, B: 0.005% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, Co: 0.5% or less, Zr: 0.1% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.1% or less, Sb: 0.1% or less, REM: 0.0050% or less.

3. A member made using the steel plate according to claim 1 or 2.

4. A method for producing a steel sheet, comprising the steps of hot rolling, pickling and cold rolling a steel slab having the chemical composition according to claim 1 or 2, and then annealing the resulting cold-rolled steel sheet, wherein the annealing comprises: ｃ３ Point -50℃ or higher A ｃ３ a soaking temperature T of 150°C or less for 30 to 500 seconds, and a soaking temperature M Ｓ Point +5℃ or more M Ｓ Cooling is performed at a first average cooling rate of 2 to 50°C / second in a temperature range from the point + 150°C to a first cooling stop temperature T1 of 500°C or less, Ｓ Point +5℃ or more M Ｓ a cooling step of cooling a steel slab at a temperature between +150°C and 500°C for a holding time t1 that satisfies the requirement of 10 seconds or more and ta seconds or less, to a second cooling stop temperature T2 at a second average cooling rate of 100°C / second or more to 50°C or less, and a reheating holding step of reheating from the second cooling stop temperature T2 to a reheating holding temperature of 50°C to 400°C at a heating rate of 50°C / second or less, and holding at the reheating holding temperature for a holding time t2 that satisfies the requirement of 10 seconds or more and tb seconds or less, where ta (seconds) and tb (seconds) are calculated by formulas (1) and (2), and in formulas (1) and (2), [Si] is the Si content (mass%) of the steel slab, and [sol. Al] is the sol. Al content (mass%) of the steel slab. ta=([Si]+[sol.Al])×336500 / (T1+273)...(1) tb=([Si]+[sol.Al])×336500 / (T2+273)×(1-(t1 / ta))...(2) 5. 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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