Steel plates, components, and methods for manufacturing them.

Abstract

To provide steel plates, components, and methods for manufacturing them, which have excellent press formability and delayed fracture resistance, and a tensile strength of 1180 MPa or higher. A steel sheet having a composition in mass%, containing C: 0.10-0.35%, Si: 0.50-2.00%, Mn: 1.5-3.5%, P: 0.050% or less, S: 0.01% or less, sol.Al: 1.0% or less, N: 0.015% or less, with the remainder being iron and unavoidable impurities; and a steel structure in which the area ratio of polygonal ferrite is less than 10%, the total area ratio of bainite and tempered martensite is 70% or more, the volume ratio of retained austenite is 5% to 20%, the total area ratio of the remaining structure is 5% or less (including 0%), and the number of retained austenite particles with an aspect ratio of 2.0 or more and an equivalent circle diameter of less than 1.0 μm is 90% or more of the total number of retained austenite particles.

Description

[Technical Field]

[0001] The present invention relates to steel plates, components, and methods for manufacturing the same, which are suitable for press-formed products having complex shapes used in automotive components through a press-forming process, and which can suppress fatigue failure during automotive assembly. [Background technology]

[0002] Against the backdrop of increasing global CO2 emission regulations, there is a growing demand for reducing vehicle weight by increasing the strength of automotive steel sheets, and the application of high-strength steel sheets of 1180 MPa or higher is progressing for body and seat components. Generally, increasing the strength of steel sheets reduces press formability such as ductility, hole expandability, and bendability, making them more prone to cracking during press forming and reducing the freedom of shape, thus 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 press formability. Furthermore, increasing the strength of steel sheets is also required from the viewpoint of improving collision characteristics. In order to improve energy absorption characteristics, it is important not only to increase the strength of the steel sheets but also to improve hole expandability and bendability from the viewpoint of suppressing fracture during collisions (see, for example, Non-Patent Document 1).

[0003] Against this backdrop, TRIP (Transformation-Induced Plasticity) steel, which disperses retained austenite (retained gamma) in the microstructure of steel sheets, has been developed as a technology to improve the ductility of steel sheets. The production of TRIP steel involves austempering, which involves soaking and then isothermal holding in the bainite transformation temperature range, followed by a cooling process where the steel is cooled to a temperature range between the martensitic transformation initiation temperature (Ms point) and the martensitic transformation completion temperature (Mf point), and then reheated and held to stabilize the retained austenite—a process known as Q&P. Q uenching & PA heat treatment process called artitioning (quenching and distribution of carbon from martensite to austenite) is used. In all of the above heat treatment processes, a large amount of Si is added to suppress carbide precipitation in order to form residual gamma in the microstructure. For example, Patent Document 1 discloses that a steel containing C: 0.04~0.12%, Si: 0.8~2.5%, and Mn: 0.5~2.0% by mass is subjected to a soaking heat holding treatment followed by isothermal holding at 300~500°C for 10~900 sec to form retained austenite (retained γ) with a volume fraction of 2~10%, resulting in a steel sheet with high ductility of TS × El ≥ 21000 MPa·%. Furthermore, Q&P describes how a portion of the structure undergoes martensitic transformation during the cooling process, and the martensitic structure is tempered by subsequent reheating and holding, thereby reducing the hardness difference between different phases in the structure and improving not only ductility but also hole-expanding properties. Patent Document 2 discloses a method for manufacturing a steel sheet containing 0.6 to 2.5 mass% Si, which 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. This method ensures that the steel sheet has excellent press formability and impact characteristics, with a total strength of 980 MPa or higher, and ductility with an elongation of 17% or higher, as well as excellent hole-expanding properties with a hole-expanding ratio of 50% or higher.

[0004] On the other hand, when cold-pressed high-strength steel sheets are applied to actual vehicles, residual stress introduced during body assembly can degrade the delayed fracture resistance, increasing concerns about delayed fracture. Delayed fracture is a phenomenon in which fracture occurs after a certain period of time when subjected to high stress and placed in a hydrogen-ingress environment. Since delayed fracture becomes more apparent as the strength of the steel sheet increases, improvement of delayed fracture resistance is required when using high-strength steel sheets as automotive components. Patent document 3 discloses a technology for improving delayed fracture resistance by finely controlling the prior austenite grain size and the carbide particle size in the martensite, which are microstructural factors of martensitic steel. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent No. 5515623 [Patent Document 2] Japanese Patent No. 5821911 [Patent Document 3] Japanese Patent No. 7226673 [Non-Patent Document]

[0006] [Non-Patent Document 1] Transactions of the Japan Society of Automotive Engineers, Vol. 52, No. 1, (2021), P197 [Summary of the Invention] [Problems to be Solved by the Invention]

[0007] Among the above press formabilities, although Patent Document 1 discloses a technique related to TRIP steel with excellent ductility, and Patent Document 2 discloses a technique related to TRIP steel with excellent ductility and hole expansion property, there is no disclosure of a technique for improving bendability, which is also important in collision characteristics. In order to ensure stable energy absorption characteristics even in the complex deformation behavior during a collision, it is desirable to improve bendability. Further, in Patent Document 3, since the target steel sheet is martensitic steel and its ductility is inferior, the press formability is insufficient.

[0008] The present invention has been made in view of the above circumstances, and an object thereof is to provide a steel sheet, a member, and a method for manufacturing the same, which have excellent press formability and stress corrosion cracking resistance and a tensile strength of 1180 MPa or more.

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

[0010] Further, excellent press formability means that, as ductility, the total elongation (El) obtained in accordance with JIS Z2241 (2011) satisfies 12.0% or more at any TS level.

[0011] Furthermore, excellent press formability refers to a hole expansion ratio (λ) (={(d-d0) / d0}×100) obtained by a hole expansion test in accordance with the JFST1001 standard, which is 40% or more at any TS level.

[0012] Furthermore, excellent press formability refers to a maximum bending radius (R / t) of 2.5 or less in any bending direction, including the width direction of the steel sheet, the rolling direction, and the direction at 45° to the rolling direction where the bending ridge is the direction of the bending edge. Note that R / t (R: limiting bending radius (mm), t: sheet thickness (mm)) is measured based on the V-block method in accordance with JIS Z2248 (1996).

[0013] Excellent delayed fracture resistance refers to determining that a material has superior delayed fracture resistance based on the following criteria (1) to (3). (1) From the manufactured steel plate, a strip-shaped test piece measuring 100 mm x 30 mm is taken by shearing, such that the longitudinal direction is perpendicular to the rolling direction. (2) The test piece is bent so that the burrs generated by the shearing process are on the outside of the bend, and the test piece is fixed with bolts while maintaining the shape of the test piece at the time of bending. The shearing conditions are clearance: 13% and rake angle: 1°, and the bending conditions are U-bending with a bending radius of 10 mm. After that, in order to maintain the shape of the test piece at the time of bending, the test piece is tightened by clamping it with a hydraulic jack or the like so that the distance between the flange ends is the same as when it was bent, and then bolted in that state. In order to fasten with bolts, the test piece is fixed by passing it through an elliptical hole that has been made in advance at a distance of 10 mm from the short side end. (3) The test specimens obtained in (2) are immersed for 100 hours in a solution prepared by mixing a 0.1% by mass ammonium thiocyanate solution and Macilvein buffer in a 1:1 mass ratio, and adjusting the pH to 8.0. If no fracture occurs, the specimens are judged to have excellent delayed fracture resistance. [Means for solving the problem]

[0014] To solve the above problems, the inventors diligently investigated the steel composition, heat treatment conditions, and microstructure affecting press formability and delayed fracture resistance of various thin steel sheets having a tensile strength of 1180 MPa or more. As a result, we found that by creating a steel structure in which, by mass%, C: 0.10~0.35%, Si: 0.50~2.00%, Mn: 1.5~3.5%, P: 0.050% or less, S: 0.01% or less, sol.Al: 1.0% or less, and N: 0.015% or less, with the remainder being iron and unavoidable impurities, and with a polygonal ferrite area ratio of less than 10%, a total area ratio of bainite and tempered martensite of 70% or more, a retained austenite area ratio of 5% to 20%, a total area ratio of the remaining structure of 5% or less (including 0%), an aspect ratio of 2.0 or more, and retained austenite with an equivalent circle diameter of less than 1.0 μm accounting for 90% or more of the total number of retained austenites, we were able to obtain steel sheets and components with excellent press formability and delayed fracture resistance and a tensile strength of 1180 MPa or more.

[0015] This invention is based on the above findings, and its gist is as follows. [1] In mass%, C: 0.10~0.35%, Si: 0.50~2.00%, Mn: 1.5~3.5%, P: 0.050% or less, S: 0.01% or less, sol.Al: 1.0% or less, N: Contains 0.015% or less, The composition consists of iron and unavoidable impurities, Area ratio of polygonal ferrite: less than 10%, The total area ratio of bainite and tempered martensite is 70% or more. Area percentage of retained austenite: 5% to 20%. The total area ratio of the remaining tissue is 5% or less (including 0%). A steel plate having a microstructure in which the aspect ratio is 2.0 or greater and the number of retained austenite particles with an equivalent circular diameter of less than 1.0 μm is 90% or greater of the total number of retained austenite particles. [2] The above component composition is further, in 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, REM: 0.0050% or less The steel plate described in [1] above, which contains one or more selected from among them. [3] A member made of the steel plate described in [1] or [2] above. [4] A method for manufacturing a steel sheet, in which a steel slab having the component composition described in [1] or [2] above is subjected to hot rolling, pickling and cold rolling, and the resulting cold-rolled steel sheet is annealed. The aforementioned annealing process With respect to the cold-rolled steel sheet, A c3 Point -50℃ or above A c3 A soaking and holding process in which the temperature T is maintained at a soaking temperature of +150°C or lower for 30 to 500 seconds, Soaking temperature T to M S Point -100℃ or above M S The temperature range from -5°C or below to a first cooling stop temperature T1 of 500°C or below is cooled to the first cooling stop temperature T1 with a first average cooling rate of 2 to 50°C / second. M S Point -100℃ or above M S After being kept at a temperature of -5°C or lower and 500°C or lower for a holding time t1 seconds that satisfies the conditions of 10 seconds or more and ta seconds or less, Second mean cooling rate: A cooling process that cools down to a second cooling stop temperature T2 of 50°C or lower at a rate of 100°C / second or more. Reheating is performed 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. A method for manufacturing a steel sheet, comprising a reheating and holding step of holding the sheet at the aforementioned reheating and holding temperature for a holding time t2 seconds that satisfies the conditions of 10 seconds or more and tb seconds or less. Here, ta (seconds) and tb (seconds) are calculated by equations (1) and (2), respectively, where [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 the step of forming and joining a steel plate as described in [1] or [2] above to make a member. [Effects of the Invention]

[0016] According to the present invention, steel plates and components can be obtained that have high strength with a tensile strength TS of 1180 MPa or higher, and possess excellent press formability and delayed fracture resistance. When the steel sheet of the present invention is applied to the structural members of an automobile body, complex-shaped, difficult-to-form members can be manufactured by cold pressing, thus greatly contributing to the weight reduction of the automobile body. [Modes for carrying out the invention]

[0017] The present invention will be described in detail below. However, the present invention is not limited to the following embodiments.

[0018] (steel plate) The steel sheet of the present invention has a composition in which, by mass%, C: 0.10~0.35%, Si: 0.50~2.00%, Mn: 1.5~3.5%, P: 0.050% or less, S: 0.01% or less, sol.Al: 1.0% or less, N: 0.015% or less, with the remainder being iron and unavoidable impurities; and a structure in which the area ratio of polygonal ferrite is less than 10%, the total area ratio of bainite and tempered martensite is 70% or more, 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 aspect ratio is 2.0 or more, and the number of retained austenite (hereinafter also referred to as retained γ) with an equivalent circle diameter of less than 1.0 μm is 90% or more of the total number of retained austenite (retained γ).

[0019] The steel sheet of the present invention will be described below, starting with its component composition and then its structural composition. First, the reasons for limiting the component composition of the present invention will be explained. In the following description, all percentages indicating the components of steel are mass percentages unless otherwise specified.

[0020] <C:0.10~0.35%> C is included to ensure a predetermined strength through transformation strengthening, and to improve ductility by ensuring a predetermined amount of retained austenite (retained gamma). If the C content is less than 0.10%, it is not possible to sufficiently ensure either the strength, the predetermined amount of retained gamma, or both. On the other hand, exceeding the upper limit of C content may prevent the desired microstructure from being obtained, potentially degrading not only hole-expanding properties but also flexural properties and delayed fracture resistance. Therefore, it is necessary to keep the C content below 0.35%. Therefore, the carbon content is set to 0.10 to 0.35%. The carbon content is preferably 0.12% or more, and more preferably 0.14% or more. Furthermore, the carbon content is preferably 0.32% or less, more preferably 0.30% or less, and even more preferably 0.254% or less from the viewpoint of delayed fracture resistance (including bendability).

[0021] <Si:0.50~2.00%> Si is contained from the viewpoints of strengthening ferrite to increase strength and suppressing carbide formation in martensite and bainite to ensure a predetermined amount of retained γ to improve ductility. If the Si content is less than 0.50%, these effects cannot be sufficiently ensured. On the other hand, when the Si content exceeds 2.00%, in addition to the saturation of its effects, the deformation resistance during hot rolling increases. Also, when the Si content exceeds 2.00%, the desired bendability and anti-fatigue fracture properties cannot be obtained. Therefore, the Si content is set to 0.50 - 2.00%. The Si content is preferably 0.70% or more, more preferably 0.90% or more. Also, the Si content is preferably 1.80% or less, more preferably 1.60% or less.

[0022] <Mn: 1.5 - 3.5%> Mn is contained from the viewpoints of improving the hardenability of the steel sheet and promoting high strength by transformation strengthening, and promoting the formation of retained γ that contributes to ductility by suppressing carbide formation in bainite as well as Si, thereby improving ductility. To obtain these effects, the Mn content needs to be 1.5% or more. On the other hand, when the Mn content exceeds 3.5%, the bainite transformation is significantly delayed and a predetermined amount of retained γ cannot be ensured. Therefore, the Mn content is set to 1.5 - 3.5%. The Mn content is preferably 1.8% or more, more preferably 2.0% or more. Also, the Mn content is preferably 3.2% or less, more preferably 3.0% or less.

[0023] <P: 0.050% or less> When the content of P is high, it segregates at grain boundaries, resulting in deteriorated bendability. Furthermore, it deteriorates the spot weldability. From this viewpoint, the P content is set to 0.050% or less. The P content is preferably 0.035% or less, more preferably 0.020% or less. On the other hand, there are no particular restrictions on the lower limit value of P. However, when the content of P is significantly reduced, the steelmaking cost increases. Therefore, it is preferable that the lower limit value is 0.005% or more.

[0024] <Less than 0.01% S has the effects of improving the scale peeling property during hot rolling and suppressing nitridation during annealing, but it is an element that has an adverse effect on bendability. Furthermore, it also deteriorates spot weldability. In order to reduce these adverse effects, at least the S content should be 0.01% or less, and preferably 0.0050% or less. Although it is not necessary to contain S, since it takes a great deal of cost to reduce it to less than 0.0001%, the S content is preferably 0.0001% or more from the viewpoint of manufacturing cost. The S content is more preferably 0.0005% or more, and still more preferably 0.0010% or more.

[0025] <sol.Al: 1.0% or less Al is contained for the purpose of deoxidation or obtaining residual γ. The lower limit of sol.Al is not particularly specified, but in order to perform deoxidation stably, the sol.Al content is preferably 0.01% or more. On the other hand, when the sol.Al content exceeds 1.0%, a large amount of Al-based coarse inclusions increases, and the bendability deteriorates. Therefore, the sol.Al content is 1.0% or less. The sol.Al content is preferably less than 1.0%, more preferably 0.80% or less, and still more preferably 0.06% or less.

[0026] <N: 0.015% or less N is an element that forms nitrides such as BN, AlN, and TiN in steel and reduces bendability, so it is necessary to limit its content. Therefore, the N content is 0.015% or less. The N content is preferably less than 0.015%, more preferably 0.010% or less, and still more preferably 0.006% or less. Although it is not necessary to contain N, since it takes a great deal of cost to reduce it to less than 0.0001%, the N content is preferably 0.0001% or more from the viewpoint of manufacturing cost. The N content is more preferably 0.0005% or more, and still more preferably 0.001% or more.

[0027] The component composition of the steel sheet in the present invention contains the above component elements as basic components, and the balance contains iron (Fe) and inevitable impurities. In addition, the component composition of the steel sheet in the present invention preferably has a component composition in which the balance consists of Fe and inevitable impurities.

[0028] In addition to the above components, the component composition of the steel sheet of the present invention can appropriately contain one or more selected from the following as optional elements (selected elements). 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

[0029] <Ti: 0.1% or less> Ti fixes N in the steel as TiN, has the effect of improving hot ductility and the effect of improving the hardenability of B. In addition, the effect of refining the structure and the effect of precipitating carbides to improve the yield strength (YS) can be obtained. In order to obtain these effects, the Ti content is preferably set to 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 more preferably 0.010% or more. On the other hand, when the Ti content exceeds 0.1%, the amount of solid solution C in the steel sheet decreases, and the TS and the TS after BH decrease. In addition, it causes a decrease in ductility due to an increase in rolling load and an increase in precipitation strengthening amount. Therefore, when Ti is contained, the Ti content is set to 0.1% or less. Preferably, the Ti content is 0.05% or less, and more preferably 0.03% or less.

[0030] <B: 0.005% or less> B is an element that improves the hardenability of steel, has the advantage of suppressing excessive ferrite formation and facilitating the generation of tempered martensite and / or bainite with a predetermined area ratio. Also, B improves the stress corrosion cracking resistance. Therefore, it is preferable to set the B content to 0.0005% or more. On the other hand, when the B content exceeds 0.005%, by combining with the dissolved N in the steel sheet, coarse BN is formed, which deteriorates the bendability. Therefore, when containing B, the B content should be 0.005% or less. The B content is preferably 0.003% or less.

[0031] <Cu: 1% or less> Cu improves the corrosion resistance in the usage environment of automobiles. Also, the corrosion product of Cu has the effect of covering the steel sheet surface and suppressing the intrusion of hydrogen into the steel sheet. Cu is an element that is mixed in when using scrap as a raw material. By allowing the mixing of Cu, recycled materials can be utilized as raw materials, and the manufacturing cost can be reduced. From such a perspective, it is preferable to contain 0.005% or more of Cu. Further, from the perspective of improving the stress corrosion cracking resistance, it is more preferable to contain 0.05% or more of Cu. The Cu content is more preferably 0.10% or more. More preferably, the Cu content is 0.25% or more. On the other hand, if the Cu content becomes too high, it will cause the occurrence of surface defects and deteriorate the bendability. Therefore, when containing Cu, the Cu content should be 1% or less. The Cu content is preferably 0.5% or less, and more preferably 0.3% or less.

[0032] <Ni: 1% or less> Ni is also an element that has the effect of improving the corrosion resistance, similar to Cu. Also, Ni has the effect of suppressing the occurrence of surface defects that tend to occur when containing Cu. Therefore, it is preferable to contain 0.01% or more of Ni. The Ni content is more preferably 0.04% or more, and even more preferably 0.06% or more. On the one hand, if the Ni content is too high, scale formation in the heating furnace becomes non-uniform, which instead causes surface defects and deteriorates the bending property. In addition, it also increases the cost. Therefore, when Ni is contained, the Ni content should be 1% or less. The Ni content is preferably 0.5% or less, and more preferably 0.3% or less.

[0033] <Cr: 1% or less> Cr can be contained due to its effect of improving the hardenability of steel and its effect of suppressing carbide formation in martensite and bainite. To obtain such effects, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.03% or more, and still more preferably 0.06% or more. The Cr content is preferably 0.10% or more, more preferably 0.20% or more, and still more preferably 0.30% or more. On the other hand, if Cr is contained in excess, the pitting corrosion resistance deteriorates. Therefore, when Cr is contained, the Cr content should be 1% or less. The Cr content is preferably 0.8% or less, and more preferably 0.6% or less.

[0034] <Mo: 0.5% or less> Mo can be contained due to its effect of improving the hardenability of steel and its effect of suppressing carbide formation in martensite and bainite. To obtain such effects, the Mo content is preferably 0.01% or more. The Mo content is more preferably 0.03% or more, and still more preferably 0.06% or more. More preferably, the Mo content is 0.1% or more, and even more preferably 0.2% or more. On the one hand, Mo is an element that delays bainite transformation. When the Mo content exceeds 0.5%, a predetermined retained γ cannot be obtained and the ductility decreases. Therefore, when Mo is contained, the Mo content should be 0.5% or less. The Mo content is preferably 0.4% or less, and more preferably 0.35% or less.

[0035] <V: 0.5% or less> V can be contained for the effects of improving the hardenability of steel, suppressing the formation of Fe-based carbides in martensite and bainite, improving the bendability by refining the structure, and improving the stress corrosion cracking resistance by precipitating carbides. To obtain 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. Even more preferably, the V content is 0.020% or more, and even more preferably 0.040% or more. On the other hand, when a large amount of V is contained, the castability deteriorates significantly. Therefore, when V is contained, the V content should be 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 more preferably 0.1% or less.

[0036] <Nb: 0.1% or less> Nb can be contained for the effects of improving the bendability by refining the steel structure and improving the stress corrosion cracking resistance by precipitating carbides. To obtain these effects, the Nb content is preferably at least 0.010%. The Nb content is preferably 0.015% or more, and more preferably 0.020% or more. On the other hand, when a large amount of Nb is contained, the precipitation strengthening becomes too strong and the ductility decreases. Also, it causes an increase in the rolling load and deterioration of the castability. Therefore, when Nb is contained, the Nb content should be 0.1% or less. Preferably, the Nb content is 0.08% or less, and more preferably 0.05% or less.

[0037] <Co: 0.5% or less> Co can be contained for the effect of improving the hardenability of steel. To obtain 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 one hand, if a large amount of Co is contained, it will increase coarse precipitates and inclusions, reduce the ultimate deformation ability of the steel plate, and reduce the bending property and hole expansion property. Therefore, when Co is contained, the Co content should be 0.5% or less. The Co content is preferably 0.4% or less, and more preferably 0.3% or less.

[0038] <Zr: 0.1% or less> Zr can be contained due to its effects of improving the hardenability of steel, suppressing carbide formation in bainite, refining the structure, and depositing carbides to improve the stress corrosion cracking resistance. In order to obtain such 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 one hand, if a large amount of Zr is contained, coarse precipitates such as ZrN and ZrS that remain undissolved during slab heating before hot rolling will increase, and the stress corrosion cracking resistance will deteriorate. Therefore, when Zr is contained, the Zr content should be less than or equal to 0.1%. The Zr content is preferably 0.050% or less, and more preferably 0.030% or less.

[0039] <Mg: 0.0050% or less> Mg fixes O as MgO and contributes to the improvement of formability such as bending property. Therefore, the Mg content is preferably 0.0002% or more. The Mg content is more preferably 0.0010% or more, and even more preferably 0.0015% or more. On the one hand, if a large amount of Mg is added, the surface quality and bending property will deteriorate. Therefore, when Mg is contained, the Mg content should be 0.0050% or less. Preferably, the Mg content is 0.0040% or less.

[0040] <Ca: 0.0050% or less> Ca fixes S as CaS and contributes to the improvement of bending property. 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 0.0030% or more. On the other hand, when a large amount of Ca is added, the surface quality and bendability deteriorate. Therefore, when Ca is contained, the Ca content should be 0.0050% or less. Preferably, the Ca content is 0.0040% or less.

[0041] <Sn: 0.1% or less> Sn suppresses oxidation and nitridation of the steel sheet surface layer portion, thereby suppressing a decrease in the content of C and B in the surface layer. As a result, excessive ferrite formation in the steel sheet surface layer portion is suppressed, the strength is increased, and the fatigue characteristics are improved. From such a viewpoint, 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, when the Sn content exceeds 0.1%, the castability deteriorates. Therefore, when Sn is contained, the Sn content should be 0.1% or less. The Sn content is preferably 0.080% or less, and more preferably 0.060% or less.

[0042] <Sb: 0.1% or less> Sb suppresses oxidation and nitridation of the steel sheet surface layer portion, thereby suppressing a decrease in the content of C and B in the surface layer. By this effect, excessive ferrite formation in the steel sheet surface layer portion is suppressed, the strength is increased, and the fatigue characteristics are improved. From such a viewpoint, 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, when the Sb content exceeds 0.1%, the castability deteriorates. Therefore, when Sb is contained, the Sb content should be 0.1% or less. The Sb content is more preferably 0.080% or less, and even more preferably 0.040% or less.

[0043] <REM: 0.0050% or less> REM is an element that improves bendability by spheroidizing the shape of sulfides, thereby suppressing the adverse effects of sulfides on stretch flange formability. To obtain these effects, it is preferable to have a REM content of 0.0005% or more. More preferably, the REM content is 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 flexibility saturates; therefore, if REM is included, the REM content should be 0.0050% or less. In this invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanide elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In this invention, REM content refers to the total content of one or more elements selected from the above-mentioned REMs.

[0044] If the above-mentioned optional components are present in amounts below the lower limit, the optional elements present in amounts below the lower limit do not impair the effects of the present invention. Therefore, if the above-mentioned optional elements are present in amounts below the lower limit, the above-mentioned optional elements are considered to be present as unavoidable impurities.

[0045] Next, the mechanical properties of the steel sheet targeted by this invention (cold-rolled steel sheet with excellent material stability) will be described.

[0046] The steel sheet of the present invention shall have a tensile strength (TS) of 1180 MPa or higher. The upper limit of the tensile strength is not particularly limited, but from the viewpoint of compatibility with other properties, it is preferable that the tensile strength be 1500 MPa or lower.

[0047] In the steel sheet of the present invention, as part of the press formability, a total elongation (El) of 12.0% or more is ensured at all TS levels for ductility. Furthermore, as part of the press formability, a hole expansion ratio (λ) of 40% or more is ensured at all TS levels for hole expansion. Furthermore, as part of the press formability, a bendability of 2.5 or less is ensured in the width direction of the steel sheet, the rolling direction, and in any bending direction where the direction at 45° to the rolling direction is the bending ridge direction.

[0048] Tensile properties are evaluated by taking a JIS No. 5 tensile test specimen from the center of the plate width and conducting a tensile test (according to JIS Z2241 (2011)) with N=3. Each evaluation is based on the average value of the three points. Steel plates with a tensile strength of 1180 MPa or higher are classified as high-strength steel plates.

[0049] The hole expansion performance is evaluated based on the average value of the hole expansion ratio (λ) (={(d-d0) / d0}×100) obtained from a hole expansion test conducted in accordance with the JFST1001 standard with N=3.

[0050] The bendability is evaluated by performing a V-bending test based on the V-block method in accordance with JIS Z 2248 (1996), and the resulting R / t (R: critical bending radius (mm), t: plate thickness (mm)) is used. Steel plates are judged to have excellent bendability if R / t is 2.5 or less in any bending direction, including the width direction, the rolling direction, and the direction at 45° to the rolling direction where the bending ridge is the direction. Specifically, the evaluation of bendability involves first taking a 30 mm wide, 100 mm long bending test specimen from the center of the steel plate's width, so that the bending ridges are in the width direction, the rolling direction, and the direction 45° to the rolling direction. Next, using the collected bending test specimen, a bending test (in accordance with the V-block method of JIS Z2248 (1996)) is performed at an indentation speed of 100 mm / second, with N=3 tests at each bending radius. The presence or absence of cracks is then determined on the outer side of the bend using a stereomicroscope. The maximum bending radius (R / t) refers to the maximum bending radius at which no cracks occur, and the bending ridges are in the width direction, the rolling direction, and the direction 45° to the rolling direction, ensuring that the R / t is 2.5 or less in all directions.

[0051] The steel sheet of the present invention is judged to have excellent delayed fracture resistance by the following evaluation. (1) First, a 100mm x 30mm strip-shaped test piece is taken from the manufactured steel plate by shearing, with the longitudinal direction perpendicular to the rolling direction. (2) The test piece is bent so that the burrs generated by the shearing process are on the outside of the bend, and the test piece is fixed with bolts while maintaining the shape of the test piece at the time of bending. The shearing conditions are clearance: 13% and rake angle: 1°, and the bending conditions are U-bending with a bending radius of 10 mm. After that, in order to maintain the shape of the test piece at the time of bending, the test piece is tightened by clamping it with a hydraulic jack or the like so that the distance between the flange ends is the same as when it was bent, and then bolted in that state. In order to fasten with bolts, the test piece is fixed by passing it through an elliptical hole that has been made in advance at a distance of 10 mm from the short side end. (3) The test specimens obtained in (2) were immersed for 100 hours in a solution prepared by mixing a 0.1% by mass ammonium thiocyanate solution and Macilvein buffer in a 1:1 mass ratio, and adjusting the pH to 8.0. If no fracture occurred, the specimen was judged to have excellent delayed fracture resistance.

[0052] Next, the structural composition of the steel sheet of the present invention will be described.

[0053] <Area ratio of polygonal ferrite: less than 10%> As the area ratio of polygonal ferrite increases, ductility improves, but the hardness difference within the steel sheet structure widens, degrading hole-expanding properties, bendability, and delayed fracture resistance. From this perspective, the area ratio of polygonal ferrite should be 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 to the area ratio of polygonal ferrite, and the area ratio of polygonal ferrite may be 0%.

[0054] <Total area ratio of bainite and tempered martensite: 70% or more> Bainite and tempered martensite are necessary structures to obtain the required strength. Furthermore, if the total area ratio of bainite and tempered martensite is less than 70%, retained austenite, which contributes to ductility, cannot be secured, and it may not be possible to obtain the desired ductility while securing a strength of 1180 MPa or more. In addition, if the total area ratio of bainite and tempered martensite is less than 70%, it promotes the formation of adjacent structures with soft polygonal ferrite, increasing the number of phase boundaries with large differences in phase hardness, which deteriorates bendability and hole-expanding properties, making it impossible to satisfy the desired bendability and hole-expanding properties. For this reason, the total area ratio of bainite and tempered martensite is 70% or more. Preferably, the total area ratio of bainite and tempered martensite is 75% or more. The total area ratio of bainite and tempered martensite is preferably 95% or less, more preferably 90% or less, and even more preferably 85% or less.

[0055] <Area ratio of retained austenite (retained gamma): 5% to 20%> If the area ratio of retained austenite falls below 5%, it may become impossible to achieve the desired ductility. From the viewpoint of ductility, the area ratio of retained austenite should be 5% or more, preferably 7% or more. On the other hand, if the area ratio of retained austenite exceeds 20%, the hole-expanding properties and delayed fracture resistance may deteriorate. For this reason, the area ratio of retained austenite should be 20% or less. Preferably, the area ratio of retained austenite is 18% or less, and more preferably 16% or less.

[0056] <Number of retained austenite particles with an aspect ratio of 2.0 or greater and an equivalent circular diameter of less than 1.0 μm: 90% or more of the total number of retained austenite particles> By controlling the shape and size of retained austenite (retained γ), it is possible to improve delayed fracture resistance while maintaining press formability. By setting the aspect ratio of retained γ to 2.0 or higher and the equivalent circle diameter to less than 1.0 μm, stress concentration during deformation can be mitigated, suppressing the formation of minute voids that serve as crack initiation points in delayed fracture phenomena. By setting the number of such retained γ to 90% or more of the total number of retained γ, delayed fracture resistance can be improved without degrading ductility, bendability, and hole-expandability. Therefore, in this invention, the number of residual γ molecules having an aspect ratio of 2.0 or more and an equivalent circle diameter of less than 1.0 μm is set to 90% or more of the total number of residual γ molecules (total number of residual γ molecules). This number ratio is preferably 95% or more, and may be 100%.

[0057] <Total area ratio of remaining tissue: 5% or less (including 0%)> Regarding the steel structure, it is preferable that the remaining structure consists of the above-mentioned structures (polygonal ferrite, bainite, tempered martensite, and residual γ). The total area ratio of the remaining structure should be 5% or less. The total area ratio of the remaining structure may be 0%. Examples of the remaining structure include fresh martensite, unrecrystallized ferrite, carbides, and pearlite. These structures can be determined by SEM observation as described later. In particular, in the present invention, it is preferable that the steel does not contain fresh martensite, as fresh martensite is a structural factor that adversely affects hole expansion properties, bendability, in-plane isotropy, and fatigue properties.

[0058] Next, we will explain the method for measuring the steel structure. To measure the area percentage of polygonal ferrite, bainite, tempered martensite, and the remaining microstructure (fresh martensite, unrecrystallized ferrite, carbides, pearlite), a section of the plate thickness parallel to the rolling direction is cut out, mirror-polished, and then etched with 1 vol% nital. At the 1 / 4 thickness position, 10 fields of view are observed with SEM at 3000x magnification, with a field of view of 40 μm × 30 μm, and the resulting microstructure images are quantified by image analysis. Polygonal ferrites are relatively equiaxed ferrites that contain very little carbide internally. They appear as the darkest region in SEM. Bainite is a microstructure characterized by the formation of carbides or retained austenite within the material, which appear white under scanning electron microscopy (SEM). When distinguishing between bainite and ferrite is difficult, regions of ferrite with an aspect ratio ≤ 2.0 are classified as polygonal ferrite, and regions with an aspect ratio > 2.0 are classified as bainite, and their area ratios are calculated accordingly. Here, the aspect ratio is determined by finding the longest major axis length a of the particle, defining the short axis length b as the longest cross-particle length perpendicular to the major axis, and then dividing a / b by this ratio. Tempered martensite is a region in SEM that shows a lath-like substructure and carbide precipitates inside, while fresh martensite is a massive region in SEM that appears white with no visible substructure inside. Unrecrystallized ferrite is a black-contrast ferrite containing deformation structures introduced by rolling, while carbides and pearlite are visible as white contrast structures. Carbides have a particle size of 1 μm or less, and pearlite can be distinguished by its lamellar (layered) structure.

[0059] The area fraction of retained austenite is determined by chemical polishing the steel sheet surface at a thickness of 1 / 4 of the way down 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), (220) planes of ferrite and the (200), (220), (311) planes of austenite. Here, since retained austenite is randomly distributed in the steel sheet, the volume fraction of retained austenite obtained by X-ray diffraction is equivalent to the area fraction. Therefore, in this invention, retained austenite is quantified as an area fraction.

[0060] The aspect ratio and equivalent diameter of retained austenite are determined by cutting a cross section parallel to the rolling direction and perpendicular to the steel sheet surface (a thickness cross section parallel to the rolling direction) and mirror-polishing it. Then, using the SEM-EBSD method, the value at the 1 / 4 thickness position is measured at 3000 μm. 2Measure the above measurement area. And it is obtained by analyzing from the data of the phase map of the obtained fcc phase. Here, the measurement conditions of the SEM-EBSD method may follow the conventional method, and a structure in which 5 or more data points identified as the fcc phase are continuous on the fcc phase map is defined as retained austenite. The aspect ratio is obtained by measuring the major axis length and the minor axis length of the retained austenite and calculating (major axis length / minor axis length). In the above measurement, in the phase map obtained by the SEM-EBSD method, retained austenite that does not contain a large-angle grain boundary of 15° or more inside is regarded as one retained austenite particle, and when the retained austenite contains a large-angle grain boundary, it is analyzed as two or three or more retained austenites separated by the large-angle grain boundary. That is, in the retained austenite, those without an orientation difference of 15° or more are regarded as one retained austenite. Also, the equivalent circle diameter is calculated from the major axis length where the particle length is the longest and the minor axis length where the particle length is the shortest, which are obtained in the same manner as above, (4 × major axis length × minor axis length ÷ π) 1 / 2 by calculating. In the above analysis, the number ratio of retained austenite with an aspect ratio of 2.0 or more and an equivalent circle diameter of less than 1.0 μm is calculated by dividing the number of retained austenite with an aspect ratio of 2.0 or more and an equivalent circle diameter of less than 1.0 μm by the number of all retained austenite analyzed.

[0061] (Manufacturing method of steel sheet) Next, the manufacturing method of the steel sheet of the present invention will be described.

[0062] The manufacturing method of the steel sheet of the present invention is a manufacturing method of a steel sheet in which a steel slab having the above-described component composition is subjected to hot rolling, pickling, and cold rolling, and then annealing is performed on the obtained cold-rolled steel sheet. The above annealing is performed on the above cold-rolled steel sheet at A c3 point - 50°C or more and A c3 soaking holding step of holding at a soaking temperature T of point + 150°C or less for 30 to 500 seconds, and from the above soaking temperature T to M S point - 100°C or more and M SThe temperature range from -5°C or below to the first cooling stop temperature T1 (below 500°C) is cooled to the first cooling stop temperature T1 with a first average cooling rate of 2 to 50°C / second. M S Point -100℃ or above M S After being kept at a temperature of -5°C or lower and 500°C or lower for a holding time t1 seconds that satisfies the conditions of 10 seconds or more and ta seconds or less, Second mean cooling rate: A cooling process that cools down to a second cooling stop temperature T2 of 50°C or lower at a rate of 100°C / second or more. From the above-mentioned second cooling stop temperature T2, reheating is performed at a heating rate of 50°C / second or less until the reheating and holding temperature is between 50°C and 400°C. The process includes a reheating and holding step in which the reheating and holding temperature is maintained at the above-mentioned reheating and holding temperature for a holding time t2 seconds that satisfies the conditions of 10 seconds or more and tb seconds or less. Here, ta (seconds) and tb (seconds) are calculated by equations (1) and (2), respectively, where [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)

[0063] <Hot rolling> There are several methods for hot rolling steel slabs, including methods that involve heating the slab before rolling, methods that involve directly rolling the slab after continuous casting without heating, and methods that involve briefly heating the slab after continuous casting before rolling. Hot rolling should be carried out according to standard procedures; for example, the slab heating temperature should be 1100°C or higher. Alternatively, the slab heating temperature should be 1300°C or lower. Also, the soaking temperature should be 20 min or higher. Also, the soaking temperature should be 300 min or lower. The finishing rolling temperature should be A r3 It should be above the transformation point. Also, the finishing rolling temperature is A r3The temperature should be below the transformation point + 200°C. The winding temperature should be above 400°C. The winding temperature should be below 720°C. It is preferable to control the winding temperature from the viewpoint of suppressing plate thickness fluctuations and ensuring high strength stably. Specifically, it is preferable that the winding temperature be above 430°C. It is also preferable that the winding temperature be below 650°C. Note A r3 The transformation point can be calculated from the composition of the steel plate and the following empirical formula (3). A r3 Point (℃)=910-310×[C]-80×[Mn]-20×[Cu]-15×[Cr]-55×[Ni]-80×[Mo]...Equation (3) (In the above formula, [M] represents the mass %) content of element M in the steel slab, and the value for elements that are not present is zero (0).)

[0064] <Pickling> Pickling should be carried out according to standard procedures.

[0065] <Cold rolling> Cold rolling should be carried out according to conventional methods, and the rolling ratio (cumulative rolling ratio) should be 30% or more. Furthermore, the rolling ratio (cumulative rolling ratio) should be 85% or less. It is preferable to control the rolling ratio from the viewpoint of stably ensuring high strength and minimizing anisotropy. Specifically, it is preferable to have a rolling ratio of 35% or more. Furthermore, it is preferable to have a rolling ratio of 85% or less. If the rolling load is high, softening annealing treatment can be performed at 450-730°C using a CAL (continuous annealing line) or BAF (box annealing furnace).

[0066] <Annealing> Cold-rolled steel sheets manufactured according to conventional methods are annealed under the following conditions. While the annealing equipment is not particularly limited, it is preferable to carry it out on a continuous annealing line (CAL) from the viewpoint of productivity and ensuring the desired heating and cooling rates.

[0067] [Soaking process: A c3 Point -50℃ or above A c3[Hold at a uniform heating temperature T of +150℃ or lower for 30-500 seconds] The steel sheet obtained in this invention minimizes the presence of soft ferrite structures, while simultaneously forming residual gamma during the cooling process. This improves hole-expandability and bendability while maintaining ductility. To obtain the above effect, the soaking temperature T is set to A c3 The temperature should be -50°C or higher. Furthermore, if the soaking temperature T is excessively high, specifically, A c3 When the temperature exceeds +150°C, significant coarsening of the microstructure occurs, suppressing bainite transformation during the cooling process, and preventing the desired residual γ from being obtained. Therefore, the soaking temperature T is A c3 Point -50℃ or above A c3 The temperature should be below +150°C. The soaking temperature T is preferably A c3 The temperature is above -40°C, and more preferably A c3 The temperature is -30°C or higher. The soaking temperature T is preferably A c3 The temperature is below +100℃, more preferably A c3 The temperature is below +50°C. Furthermore, if the holding time at the soaking temperature T (soaking time) is less than 30 seconds, austenite formation at the soaking temperature T may not occur sufficiently, resulting in an excess of polygonal ferrite. This may prevent the acquisition of the desired bainite and tempered martensite, leading to insufficient strength or insufficient retained austenite, thus failing to ensure the desired ductility. In addition, sufficient hole-expanding properties, bendability, and delayed fracture resistance may not be obtained. On the other hand, if the holding time at the soaking temperature (soaking time) exceeds 500 seconds, significant coarsening of the microstructure occurs, suppressing bainite transformation during the cooling process, and potentially preventing the desired residual gamma from being obtained. Therefore, the holding time at the above annealing temperature (soaking time) is set to 30 to 500 seconds. The holding time at the soaking temperature T (soaking time) is preferably 60 seconds or more, and more preferably 100 seconds or more. Furthermore, the holding time at the soaking temperature (soaking time) is preferably 400 seconds or less, and more preferably 300 seconds or less.

[0068] Note that A c3 A is obtained from the empirical formula (4) below. c3 You can use it. A c3 =910-203×([C]) 1 / 2 -15.2×[Ni]+44.7×[Si]+104×[V]+31.5×[Mo]+13.1×[W]...Formula (4) Here, in the above formula, [M] is the mass percentage of each element in the steel slab, and the value of elements that are not present is set to zero (0).

[0069] [Cooling process (1): Soaking temperature T to M S Point -100℃ or above M S The temperature range up to the first cooling stop temperature T1, which is below -5°C and below 500°C, is cooled with a first average cooling rate of 2 to 50°C / second, M S Point -100℃ or above M S [A holding time (residence time) of t1 seconds satisfies the conditions of being below -5℃ and below 500℃ for at least 10 seconds and less than or equal to ta seconds.] After holding at the soaking temperature T (after the soaking and holding process described above), from the soaking temperature T to M S Point -100℃ or above M S The temperature range up to the first cooling stop temperature T1, which is below -5°C and below 500°C, is cooled at a first mean cooling rate of 2 to 50°C / second. If the first mean cooling rate falls below 2°C / second, the ferrite transformation during cooling will proceed excessively, and the formation of polygonal ferrite cannot be suppressed; therefore, the first mean cooling rate should be 2°C / second or higher. Preferably, the first mean cooling rate is 5°C / second or higher. On the other hand, if the first average cooling rate becomes too high, the plate shape deteriorates, so it should be kept below 50°C / second. The first average cooling rate is preferably below 40°C / second, and more preferably below 30°C / second. Here, the first average cooling rate (°C / second) is "(Soaking temperature T (°C) - First cooling stop temperature T1 (°C)) / Cooling time from soaking temperature T to first cooling stop temperature T1 (seconds)".

[0070] The above M S Point -100℃ or above M SAt a first cooling stop temperature (retention temperature) T1 below -5°C and below 500°C, martensitic and bainite transformations are induced, and by isothermal holding at the said temperature, residual γ with an aspect ratio of 2.0 or more and an equivalent circle diameter of less than 1.0 μm is formed. S Below -100°C, the fraction of untransformed austenite decreases, making it impossible to obtain the desired residual gamma. On the other hand, the first cooling stop temperature T1 is M S Above -5°C and / or above 500°C, residual γ with an aspect ratio of 2.0 or greater and an equivalent circle diameter of less than 1.0 μm decreases. Therefore, the first cooling stop temperature T1 is M S Point -100℃ or above M S The temperature should be -5°C or lower and 500°C or lower. The first cooling stop temperature T1 is preferably M S The temperature is above -90°C, and more preferably M S The temperature is above -80°C. The first cooling stop temperature T1 is preferably M S The temperature is below -15°C. Furthermore, since Baynight undergoes metamorphosis after a incubation period, in order to allow Baynight to fully metamorphose, M S Point -100℃ or above M S A holding time t1 (residence time t1) of 10 seconds or more is required at a temperature of -5°C or below and 500°C or below (hereinafter also referred to as residence temperature). On the other hand, if this residence time t1 is excessively long, the decomposition of residual γ occurs, making it impossible to secure the predetermined residual γ. The limiting residence time t1 differs depending on the composition of the steel plate. After careful investigation, it was found that if the residence time is less than or equal to the residence time (ta seconds) calculated by the following equation (1), the decomposition of residual γ does not occur for any component, and the predetermined residual γ can be obtained. Therefore, the residence time t1 should be 10 seconds or more and less than or equal to ta seconds. Note that the residence time t1 is M S Point -100℃ or above M S This refers to the time spent in a temperature range of -5°C or below and 500°C or below, and it is not necessarily required to maintain the same temperature throughout.

[0071] ta=([Si]+[sol.Al])×336500 / (T1+273)...(1) In equation (1), [Si] is the Si content (mass%) of the steel slab, and [sol.Al] is the sol.Al content (mass%) of the steel slab.

[0072] The above M S M is obtained from the empirical formula (5) below. S You can use it. M S =538-350×[C]-37.7×[Mn]-18.9×[Ni]-37.7×[Cr]-27×[Mo]...Formula (5) Here, [M] is the mass percentage of each element in the steel slab.

[0073] [Cooling process (2): Second mean cooling rate: Cooling at 100°C / second or higher until the second cooling stop temperature T2 is 50°C or lower] The above M S Point -100℃ or above M S After the steel plate is left to stand at a residence temperature of -5°C or lower and 500°C or lower, it is cooled to a second cooling stop temperature T2 of 50°C or lower at a second mean cooling rate of 100°C / second or higher. If the second cooling stop temperature T2 exceeds 50°C, the amount of untransformed austenite increases, leading to an increase in fresh martensite formed during final cooling, and degrading the hole-expanding properties and delayed fracture resistance. Therefore, the second cooling stop temperature should be kept below 50°C. Furthermore, if the second mean cooling rate falls below 100°C / second, non-uniform movement of dissolved carbon occurs during cooling due to bainite transformation and self-tempering of martensite, resulting in the formation of a large amount of fresh martensite. This leads to a deterioration in hole-expanding properties, bendability, and delayed fracture resistance. Therefore, the second mean cooling rate should be 100°C / second or higher. Preferably, the second mean cooling rate should be 120°C / second or higher. On the other hand, while there is no upper limit to the second mean cooling rate, it is preferable that the second mean cooling rate be 2000°C / second or less from the viewpoint of operability and suppression of meandering in the continuous annealing furnace. More preferably, the second mean cooling rate is 1800°C / second or less. Here, the second mean cooling rate (°C / sec) is defined as "(end of residence temperature (°C) - second cooling stop temperature (°C)) / cooling time from end of residence temperature to second cooling stop temperature (seconds)". Note that the end of residence temperature is the same as M mentioned above. S Point -100℃ or above M S This refers to the temperature at the end of the period, which is below -5°C and below 500°C (retention temperature).

[0074] [Reheating and holding process: Reheating is performed from the second cooling stop temperature T2 to a reheating and holding temperature of 50°C / second or less, between 50°C and 400°C, and the temperature is held at the reheating and holding temperature for a holding time t2 seconds that satisfies the condition of 10 seconds or more and tb seconds or less.] During the reheating and holding period, the martensite is held at a reheating and holding temperature of 50°C to 400°C for 10 seconds to tb seconds, in order to adjust its strength and improve its flexibility through tempering. Here, tb (seconds) is calculated using the following equation (2), which takes into account the residence time (holding time t1) during the cooling process. If the reheating and holding temperature is less than 50°C, or if the holding time t2 at the reheating and holding temperature is less than 10 seconds, the tempering of the martensite will be insufficient, and the hardness difference in the steel sheet structure will increase, which may cause deterioration in one or more of the hole-expanding properties, bendability, and delayed fracture resistance properties. On the other hand, if the reheating and holding temperature exceeds 400°C, or if the holding time t2 at the reheating and holding temperature exceeds tb seconds, decomposition of retained austenite occurs, making it impossible to secure the desired ductility. In addition, excessive tempering of the martensite may occur, making it impossible to secure the desired strength. Therefore, the holding time t2 at the reheating holding temperature is set to 10 seconds or more and tb seconds or less. Preferably, the holding time t2 at the reheating holding temperature is 50 seconds or more. Also, preferably, the holding time t2 at the reheating holding temperature is tb-50 seconds or less. The reheating and holding temperature is 50°C to 400°C, preferably 60°C or higher, and more preferably 100°C or higher. The reheating and holding temperature is preferably 350°C or lower, and more preferably 300°C or lower. Furthermore, if the heating rate exceeds 50°C / second, temperature unevenness in the width direction of the steel plate increases, making material variation in the width direction of the steel plate more likely. Therefore, the heating rate should be 50°C / second or less. Preferably, the heating rate is 40°C / second or less, and more preferably 30°C / second or less. Also, preferably, the heating rate is 5°C / second or more, and more preferably 10°C / second or more.

[0075] tb=([Si]+[sol.Al])×336500 / (T2+273)×(1-(t1 / ta))...(2) In equation (2), [Si] represents the Si content (mass%) and [sol.Al] represents the sol.Al content (mass%).

[0076] Here, the heating rate is defined as "(reheating and holding temperature (°C) - second cooling stop temperature T2 (°C)) / heating time from the second cooling stop temperature to the reheating and holding temperature (seconds)".

[0077] [plate thickness] The steel sheet obtained in the present invention as described above preferably has a thickness of 0.5 mm or more. Furthermore, it is preferable that the thickness be 3.0 mm or less.

[0078] (Components and methods for manufacturing components) Next, the component of the present invention and its manufacturing method will be described.

[0079] The component of the present invention is obtained by subjecting a steel sheet of the present invention to at least one of forming and joining processes. Furthermore, the method for manufacturing the component of the present invention includes the step of subjecting a steel sheet of the present invention to at least one of forming and joining processes to form the component.

[0080] The steel sheet of the present invention has a tensile strength of 1180 MPa or more, excellent press formability, excellent ductility, hole-expandability, and bendability, and further excellent delayed fracture resistance. Therefore, the member of the present invention also has a tensile strength of 1180 MPa or more, excellent press formability, excellent ductility, hole-expandability, and bendability, and further excellent delayed fracture resistance. Furthermore, using the components of the present invention makes weight reduction possible. Therefore, the components of the present invention can be suitably used, for example, in vehicle body frame components.

[0081] Forming processes can utilize general processing methods such as press working without restriction. Joining processes can utilize general welding methods such as spot welding and arc welding, as well as riveting and crimping without restriction. [Examples]

[0082] A slab produced by continuous casting having the component composition shown in Table 1 was heated to 1200°C, with a soaking time of 200 mins., a finish rolling temperature of 860°C or higher, and a coiling temperature of 550°C. After this hot rolling process, it was cold-rolled to a rolling rate of 50% (cumulative rolling rate). The resulting cold-rolled steel sheet with a thickness of 1.2 mm was treated with the annealing conditions shown in Table 2 to produce the steel sheet of the present invention and the steel sheet of the comparative example. The measurement results are shown in Table 3.

[0083] [Table 1]

[0084] [Table 2]

[0085] [Table 3]

[0086] The steel structure was measured using the method described above. The measurement results are shown in Table 3.

[0087] Tensile tests were performed on the obtained steel plates using the method described above to obtain the tensile strength TS and total elongation El. Furthermore, a hole expansion test was conducted using the method described above, and the hole expansion ratio λ was obtained. Furthermore, the bending properties were evaluated using the method described above, and Table 3 shows the maximum R / t values ​​in the width direction, rolling direction, and bending direction where the bending ridge direction is 45° to the rolling direction of the steel plate. Note that the R / t values ​​shown in Table 3 are the maximum values ​​of R / t in the width direction, rolling direction, and bending direction where the bending ridge direction is 45° to the rolling direction. Furthermore, the delayed fracture tolerance characteristics were evaluated using the method described above.

[0088] Steel plates with a tensile strength (TS) of 1180 MPa or higher were judged to have superior strength. Steel sheets with a total elongation El of 12.0% or higher were judged to have excellent ductility. Steel plates with a hole expansion ratio λ of 40% or more were judged to have excellent hole expansion properties. Steel plates with an R / t (maximum value of R / t) of 2.5 or less were judged to have excellent bendability. Steel plates that meet the criteria for delayed fracture resistance (pass) were judged to have excellent delayed fracture resistance.

[0089] The examples of the present invention shown in Tables 2 and 3 exhibited superior strength, ductility, hole-expanding properties, bendability, and delayed fracture resistance, while the comparative examples were inferior in at least one of these properties.

[0090] Furthermore, it was found that members obtained by forming and joining using the steel plate of the present invention exhibited similar strength, ductility, hole-expandability, bendability, and delayed fracture resistance to the steel plate of the present invention, as the steel plate of the present invention exhibited similar strength, ductility, hole-expandability, bendability, and delayed fracture resistance to the steel plate of the present invention.

Claims

1. In mass percent, C: 0.10-0.35%, Si: 0.50-2.00%, Mn: 1.5-3.5%, P: 0.050% or less, S: 0.01% or less, Sol. Al: 1.0% or less. N: Contains 0.015% or less, The composition consists of iron and unavoidable impurities, Area ratio of polygonal ferrite: less than 10%, The total area ratio of bainite and tempered martensite is 70% or more. Area percentage of retained austenite: 5% to 20%. The total area ratio of the remaining tissue is 5% or less (including 0%). A steel plate having a microstructure in which the aspect ratio is 2.0 or greater and the number of retained austenite particles with an equivalent circular diameter of less than 1.0 μm is 90% or greater of the total number of retained austenite particles.

2. The aforementioned component composition is further, in 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, REM: 0.0050% or less The steel plate according to claim 1, which contains one or more selected from among them.

3. A member made using the steel plate described in claim 1 or 2.

4. A method for manufacturing a steel sheet, comprising hot rolling, pickling and cold rolling a steel slab having the component composition described in claim 1 or 2, and then annealing the resulting cold-rolled steel sheet. The aforementioned annealing process With respect to the cold-rolled steel sheet, A ｃ３ Point -50℃ or above A ｃ３ A soaking and holding process in which the temperature T is maintained at a soaking temperature of +150°C or lower for 30 to 500 seconds, The soaking temperature T to M Ｓ Point -100℃ or more M Ｓ The temperature range from -5°C or below to a first cooling stop temperature T1 of 500°C or below is cooled to the first cooling stop temperature T1 with a first average cooling rate of 2 to 50°C / second. M Ｓ Point -100℃ or more M Ｓ After being kept at a temperature of -5°C or lower and 500°C or lower for a holding time t1 seconds that satisfies the conditions of 10 seconds or more and ta seconds or less, Second mean cooling rate: A cooling process that cools the material at a rate of 100°C / second or higher down to a second cooling stop temperature T2 of 50°C or lower. Reheating is performed 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. The process includes a reheating and holding step in which the reheating and holding temperature is maintained for a holding time t2 seconds that satisfies the conditions of 10 seconds or more and tb seconds or less. Area ratio of polygonal ferrite: less than 10%, The total area ratio of bainite and tempered martensite is 70% or more. Area percentage of retained austenite: 5% to 20%. The total area ratio of the remaining tissue is 5% or less (including 0%). A method for manufacturing a steel sheet having a steel structure in which the aspect ratio is 2.0 or greater and the number of retained austenite particles with an equivalent circular diameter of less than 1.0 μm is 90% or greater of the total number of retained austenite particles. Here, ta (seconds) and tb (seconds) are calculated by equations (1) and (2), respectively, where [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 forming and joining a steel plate according to claim 1 or 2 to form a component.