Steel sheet, component, and method for manufacturing the same

Abstract

This invention provides a steel plate with a tensile strength of 1310 MPa or higher, which achieves excellent compressibility in a steel primarily composed of martensite with excellent resistance to delayed fracture. The steel plate is characterized by having the following composition and microstructure, wherein the composition, by mass%, contains C: 0.12%–0.40%, Si: less than 1.5%, Mn: greater than 1.7% and less than 3.5%, P: less than 0.05%, S: less than 0.010%, sol.Al: less than 1.00%, N: less than 0.010%, Ti: 0.002%–0.080%, and B: 0.0002%–0.0050%, with the remainder being Fe and unavoidable impurities. The martensite in the microstructure has an area fraction of 85% or more relative to the overall microstructure, and the length L of the subplate boundaries in this martensite is... S Length L of the lath boundary B The ratio of L S / L B It satisfies the specified formula (1) and has a tensile strength of 1310 MPa or more.

Description

Technical Field

[0001] This invention relates to high-strength steel sheets for cold pressing forming, which are required for use in automobiles, home appliances, etc., and parts using the steel sheets, as well as methods for manufacturing them. Background Technology

[0002] In recent years, due to the increasing demand for lightweight automotive bodies, the application of high-strength steel sheets with a tensile strength (TS) of 1310 MPa or higher in vehicle body frame components has been gradually advancing. Furthermore, from the perspective of further weight reduction, research is gradually beginning on increasing strength to 1.8 GPa or higher. Previously, hot pressing based on thermal compression was actively studied for increasing strength, but recently, from a cost and productivity perspective, the application of cold pressing in high-strength steel is being re-examined.

[0003] Martensitic structures are easier to achieve high strength than softer structures like ferrite and bainite, making a martensitic structure the primary microstructure effective in high-strength steel plate design. However, compared to composite steels containing softer structures like ferrite and bainite, martensitic steels lack ductility. Therefore, martensitic steels are only suitable for relatively simple-shaped components such as car door beams and bumpers, which are primarily formed through bending processes.

[0004] On the other hand, composite-structure steels exhibit inferior resistance to delayed fracture compared to martensitic steels. Specifically, to achieve the same strength as martensitic steels in composite-structure steels, a phase with higher hardness is required. However, this hard phase, due to high stress concentration, becomes the initiation point for delayed fracture. Therefore, it is difficult to simultaneously achieve excellent resistance to delayed fracture and formability in high-strength steel plates.

[0005] Here, if the ductility of the martensitic structure itself, which gives it excellent resistance to delayed fracture, can be improved, it is possible to achieve both excellent resistance to delayed fracture and formability even without composite microstructure. One method to improve the ductility of martensitic structure is to increase the tempering temperature, but this method has a small effect on improving ductility and significantly deteriorates bending properties due to the formation of coarse carbides.

[0006] Patent Document 1 discloses a technology for a high-strength cold-rolled steel sheet with excellent bending properties, a yield strength of 1180 MPa or more, a tensile strength of 1470 MPa or more, and a yield strength of 1180 MPa or more. The technology is characterized by containing at least 95% martensite by area, and the combined area percentage of retained austenite and ferrite being less than 5% (including 0%). Furthermore, the average size of the carbides is less than 60 nm by equivalent circle diameter, and the number density of carbides with an equivalent circle diameter of 25 nm or more is 1 carbide / mm. 2 .

[0007] Patent document 2 discloses a technology for an ultra-high strength steel plate with excellent yield ratio and workability. The plate is characterized by having a microstructure consisting of more than 90% martensite and more than 0.5% retained austenite, with a local Mn concentration of more than 1% by area in regions where the Mn concentration reaches more than 1.2 times the Mn content of the whole steel plate, and a tensile strength of more than 1470 MPa, a yield ratio of more than 0.75, and a total elongation of more than 10%.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent No. 6017341

[0011] Patent Document 2: Japanese Patent Application Publication No. 2019-2078 Summary of the Invention

[0012] In recent years, by utilizing pressing technology, even steel sheets lacking ductility have gradually become capable of being processed into complex component shapes. One such manufacturing method is pre-forming technology, which involves partially forming the steel sheet before it reaches its final shape, rather than forming it in a single pressing operation. This disperses strain throughout the steel sheet, thereby controlling its fracture. In this method, the strain application is complex; for example, sometimes strain is applied along two axes in the next process after uniaxial stretching. In other words, sometimes the strain application directions in the first and second processes are orthogonal. The pressability of this manufacturing method is not necessarily related to the characteristic values ​​evaluated in uniaxial tensile tests, which are commonly used to assess formability.

[0013] In the technology described in Patent Document 1, excellent flexibility is achieved, thus providing sufficient ductility for bending deformation commonly used in component forming. However, for martensitic steel, the ductility is insufficient when processing components with more complex shapes.

[0014] In the technology described in Patent Document 2, although certain elongation properties can be obtained by containing retained austenite, the retained austenite transforms into hard martensite during processing in a certain direction. Since hard martensite is prone to become the starting point for deformation concentration, it may not be able to achieve sufficient formability in more complex pressing processes that involve multiple steps.

[0015] As mentioned above, it is difficult to achieve excellent compressibility in high-strength steel sheets with martensitic structure in existing technologies. Furthermore, components obtained by forming or welding the aforementioned steel sheets also require this excellent compressibility.

[0016] The present invention was made to solve this problem, and its object is to provide a steel plate with a tensile strength of 1310 MPa or more that can achieve excellent compressibility in steel with a martensitic structure that has excellent resistance to delayed fracture, as well as parts using the steel plate and methods for manufacturing the same.

[0017] To address the aforementioned issues, the inventors conducted repeated and in-depth research, resulting in the insights described below (i) to (v). As a fundamental consideration, in more complex pressing processes involving multiple steps, the deformation of soft structures such as retained austenite and ferrite tends to concentrate. Therefore, limiting their content and improving the strain dispersion of the dominant martensite structure is crucial.

[0018] i) Martensitic steels can sometimes develop complex internal stress fields due to thermal shrinkage and phase transformation expansion during the fabrication of martensitic structures.

[0019] ii) If such an internal stress field exists, when deformation is performed through processing, a specific area will begin to deform preferentially. As deformation progresses, multiple areas will begin to deform in stages, thereby dispersing the strain throughout the steel plate.

[0020] iii) It is difficult to directly observe this internal stress field, but since the crystal orientation of the lath, which is a substructure of martensite, is affected by the stress field during martensite formation, the magnitude of the stress field can be indirectly inferred from the crystal orientation information of the lath.

[0021] iv) The selective orientation of lath crystals is altered by controlling the cooling rate within a specific temperature range during the formation of martensite.

[0022] v) The crystal orientation of laths is greatly influenced by the Ms point, which is the starting temperature for martensite formation. A more uniform distribution of Mn concentration at the Ms point results in higher strain dispersion. This Mn concentration distribution is achieved by forming an appropriate hot-rolled microstructure.

[0023] This invention is based on the above insights, and its main points are as follows.

[0024] (1) A steel plate having the following composition and metal structure:

[0025] The above composition, by mass%, contains C: 0.12%–0.40%, Si: less than 1.5%, Mn: greater than 1.7% and less than 3.5%, P: less than 0.05%, S: less than 0.010%, sol.Al: less than 1.00%, N: less than 0.010%, Ti: 0.002%–0.080%, and B: 0.0002%–0.0050%, with the remainder being Fe and unavoidable impurities.

[0026] In the aforementioned metal microstructure, the martensite area fraction relative to the overall microstructure is over 85%, and the length L of the subplate boundary is... S Length L of the lath boundary B The ratio of L S / L B Satisfy the following equation (1),

[0027] Furthermore, the tensile strength is above 1310 MPa.

[0028] 0.06 / [C%] 0.8 ≦L S / L B ≤0.13 / [C%] 0.8 ···(1)

[0029] Wherein, [C%]: C content (mass %).

[0030] (2) The steel plate according to (1), wherein the above composition further contains, by mass %, one or more of the following: Cu: 0.01% to 1.00%, Ni: 0.01% to 1.00%, Mo: 0.005% to 0.350%, Cr: 0.005% to 0.350%, Zr: 0.005% to 0.350%, Ca: 0.0002% to 0.0050%, Nb: 0.002% to 0.060%, V: 0.005% to 0.500%, W: 0.005% to 0.200%, Sb: 0.001% to 0.100%, Sn: 0.001% to 0.100%, Mg: 0.0002% to 0.0100%, and REM: 0.0002% to 0.0100%.

[0031] (3) The steel plate according to (1) or (2) above, wherein the standard deviation of the concentration of Mn is less than 0.35%.

[0032] (4) The steel plate according to any one of (1) to (3) above, wherein a zinc coating is provided on the surface.

[0033] (5) A component formed by forming and welding at least one of the steel plates described in any one of (1) to (4) above.

[0034] (6) A method for manufacturing a steel plate, wherein a steel billet having the composition described in (1) or (2) above is hot-rolled to obtain a hot-rolled steel plate, the hot-rolled steel plate is then cold-rolled to obtain a cold-rolled steel plate, the cold-rolled steel plate is subjected to a homogenization heat treatment of Ac3 point or above for 240 seconds or more, and then a first cooling is performed: the temperature range from the cooling start temperature of 680°C or above to the Ms point is cooled at an average cooling rate of 10°C / s or above, followed by a second cooling: the temperature range from the Ms point to (Ms point - 50°C) is cooled at an average cooling rate of 100°C / s or above, and then a third cooling is performed: the temperature is cooled to below 50°C at an average cooling rate of 70°C / s or above.

[0035] (7) The steel plate manufacturing method according to (6) above, wherein after the above three coolings, reheating is performed and held in a temperature range of 150 to 300°C for 20 to 1500 seconds.

[0036] (8) The method for manufacturing the steel plate according to (6) or (7) above, wherein the refrigerant used in the two cooling processes is water, and the water flow density in the two cooling processes is 0.5 m³ / s. 3 / m 2 / min~10.0m 3 / m 2 / min.

[0037] (9) The method for manufacturing a steel plate according to any one of (6) to (8) above, wherein, in the hot rolling process, after rolling at a finishing temperature of 840°C or above, the plate is cooled to 640°C or below within 3 seconds, held at a temperature range of 600°C to 500°C for 5 seconds or more, and then wound at a temperature of 550°C or below.

[0038] (10) The method for manufacturing steel plate according to any one of (7) to (9) above, wherein plating treatment is performed after the above reheating.

[0039] (11) A method for manufacturing a component, wherein a steel plate manufactured by the steel plate manufacturing method described in any one of (6) to (10) above is subjected to at least one of forming and welding.

[0040] According to the present invention, a steel sheet with a tensile strength of 1310 MPa or higher can be provided, simultaneously achieving excellent resistance to delayed fracture and good press formability. This improved property promotes the widespread use of high-strength steel sheets in cold-pressing applications for parts with more complex shapes, contributing to increased part strength and weight reduction. Attached Figure Description

[0041] Figure 1 It represents the ratio L. S / LB A graph showing the relationship between the cup-shaped height and the cup-shaped height.

[0042] Figure 2 It is a graph showing the tensile strength versus cupping height in the embodiment. Detailed Implementation

[0043] The embodiments of the present invention will now be described. It should be noted that the present invention is not limited to the embodiments described below. First, the content of each component in the composition of the steel plate of the present invention will be described. Hereinafter, unless otherwise stated, "%" indicating the content of a component refers to "mass %".

[0044] C: 0.12%~0.40%

[0045] Carbon (C) is included to improve hardenability and obtain a specified martensite area ratio. It is also included from the viewpoint of improving the strength of martensite and ensuring a strength (TS) ≥ 1310 MPa. When the C content is less than 0.12%, it is difficult to consistently obtain the specified strength. Furthermore, from the viewpoint of obtaining a TS ≥ 1470 MPa, it is preferable to set the C content to 0.18% or more. If the C content exceeds 0.40%, the strength becomes too high while the toughness decreases, and the pressing formability deteriorates. Therefore, the C content is set to 0.12–0.40%, preferably 0.36% or less.

[0046] Si: below 1.5%

[0047] Si is added as a strengthening element for solid solution strengthening. While no lower limit is specified for the Si content, from the viewpoint of achieving the aforementioned effects, a Si content of 0.02% or more is preferred. Furthermore, a Si content of 0.1% or more is more preferable. On the other hand, if the Si content exceeds 1.5%, it leads to a decrease in toughness and deterioration in press formability. Additionally, if the Si content exceeds 1.5%, it results in a significant increase in rolling load during hot rolling. Therefore, the Si content is set to 1.5% or less, preferably 1.2% or less.

[0048] Mn: greater than 1.7% and less than 3.5%

[0049] Mn is included to improve the hardenability of steel and achieve a martensite area ratio within a specified range. Furthermore, it dissolves in martensite, increasing its strength. To ensure an industrially stable and specified martensite area ratio, the Mn content is greater than 1.7%. On the other hand, from the viewpoint of ensuring weld stability and avoiding deterioration in press formability due to the formation of coarse MnS, the upper limit of the Mn content is set at 3.5%. Preferably, it is 3.2% or less, more preferably 3.0% or less.

[0050] P: below 0.05%

[0051] Polymer (P) is an element that strengthens steel, but if its content is too high, toughness decreases, and the formability and weldability deteriorate. Therefore, the P content is set to 0.05% or less. From the above perspective, it is preferable to set the P content to 0.02% or less. It should be noted that the lower limit of the P content does not need to be particularly limited, but reducing it to less than 0.002% requires a significant amount of cost; from a cost perspective, it is preferable to be 0.002% or more.

[0052] S: below 0.010%

[0053] Since sulfur (S) degrades compressibility by forming coarse MnS, the S content needs to be set to 0.010% or less. From the above perspective, it is preferable to set the S content to 0.005% or less. More preferably, it is 0.002% or less. It should be noted that the lower limit of the S content does not need to be particularly limited, but reducing it to less than 0.0002% requires a significant amount of cost, so from a cost perspective, it is preferable to have a content of 0.0002% or more.

[0054] sol.Al: 1.00% or less

[0055] Al is included to ensure sufficient deoxidation and reduce inclusions in the steel. There is no specific lower limit for the sol.Al content, but for stable deoxidation, it is preferably set to 0.003% or more, more preferably 0.01% or more. On the other hand, if the sol.Al content exceeds 1.00%, a large number of coarse Al-based inclusions are generated, leading to deterioration in press formability. Therefore, the sol.Al content is set to 1.00% or less, preferably 0.80% or less.

[0056] N: below 0.010%

[0057] Because nitrogen forms large nitrides that degrade compression molding properties, its addition amount needs to be controlled. Therefore, the nitrogen content needs to be set to below 0.010%, preferably below 0.006%. There is no specified lower limit for the nitrogen content, but the currently industrially feasible lower limit is around 0.0005%, practically above 0.0005%.

[0058] Ti: 0.002%~0.080%

[0059] Ti is added to ensure the solid solution of B and stabilize hardenability by forming TiN before the formation of BN. From the viewpoint of achieving the above-mentioned effects, the Ti content needs to be 0.002% or more. The Ti content is preferably 0.005% or more. On the other hand, if excessive Ti is present, a large number of coarse inclusions such as TiN and TiC are formed, deteriorating the pressing formability. Therefore, the Ti content needs to be 0.080% or less. Preferably 0.060% or less, more preferably 0.055% or less.

[0060] B: 0.0002%~0.0050%

[0061] Boron (B) is an element that improves the hardenability of steel, and even with a low Mn content, it has the effect of generating martensite with a specified area ratio. To obtain this effect of B, it is preferable to set the B content to 0.0002% or more, and more preferably 0.0005% or more. On the other hand, if the B content exceeds 0.0050%, its effect saturates. Therefore, the B content is set to 0.0002% to 0.0050%. The B content is preferably 0.0040% or less, and more preferably 0.0030% or less.

[0062] The steel sheet of the present invention has a composition comprising the above-mentioned component group (C, Si, Mn, P, S, sol.Al, N, Ti, and B) as a basic component, with the remainder consisting of Fe (iron) and unavoidable impurities. In particular, the steel sheet of one embodiment of the present invention preferably has a composition comprising the above-mentioned components as a basic component, with the remainder consisting of Fe and unavoidable impurities. It should be noted that the unavoidable impurities are not limited and can include H, He, Li, Be, O (oxygen), F, Ne, Na, Cl, Ar, K, Co, Zn, Ga, Ge, As, Se, Br, Kr, Rb, Sr, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, I, Xe, Cs, Ba, La, Hf, Ta, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Po, At, Rn, Fr, Ra, Ac, Rf, Ha, Sg, Ns, Hs, Mt, etc.

[0063] In addition to the composition groups mentioned above, the composition of the steel plate may, as needed, contain one or more elements selected from any of the elements shown below.

[0064] Cu: 0.01%~1.00%

[0065] Cu improves corrosion resistance in automotive environments. Furthermore, the presence of Cu allows corrosion products to coat the steel surface, thus inhibiting hydrogen penetration. From this perspective, the Cu content is preferably 0.01% or more, and more preferably 0.05% or more from the viewpoint of improving resistance to delayed fracture. However, excessive Cu content can lead to surface defects, so it is preferable to set the Cu content to 1.00% or less. More preferably, it is 0.5% or less, and even more preferably 0.3% or less.

[0066] Ni: 0.01%~1.00%

[0067] Like Cu, Ni is an element that improves corrosion resistance. Furthermore, in the presence of Cu, Ni helps reduce easily formed surface defects. Therefore, from the above perspective, it is preferable that Ni contains 0.01% or more. However, if the Ni content is too high, the formation of oxide scale in the heating furnace becomes uneven, contributing to surface defects and significantly increasing costs. Therefore, it is preferable to set the Ni content to 1.00% or less. More preferably, it is 0.5% or less, and even more preferably, it is 0.3% or less.

[0068] Mo: 0.005%–0.350%

[0069] To improve the hardenability of steel and achieve a stable, guaranteed strength, Mo can be added. To obtain this effect, the Mo content is preferably 0.005% or more. However, if the Mo content exceeds 0.350%, the chemical conversion treatability deteriorates. Therefore, the Mo content is preferably 0.005% to 0.350%, and more preferably 0.20% or less.

[0070] Cr: 0.005%~0.350%

[0071] To improve the hardenability of steel, Cr can be added. Ideally, the Cr content should be 0.005% or more. However, if the Cr content exceeds 0.350%, the chemical conversion treatability deteriorates. Therefore, the Cr content is preferably 0.005 to 0.350%. Since chemical conversion treatability tends to deteriorate when the Cr content exceeds 0.20%, from the viewpoint of preventing this, the Cr content is more preferably 0.200% or less.

[0072] Zr: 0.005%~0.350%

[0073] Zr contributes to increased strength by refining the original γ grain size and the resulting refinement of the internal structure of martensite. From this perspective, the Zr content is preferably 0.005% or more. However, if a large amount of Zr is added, coarse precipitates of the Zr system increase, deteriorating the compressibility. Therefore, the Zr content is preferably 0.350% or less. More preferably, it is 0.20% or less, and even more preferably, it is 0.05% or less.

[0074] Ca: 0.0002%~0.0050%

[0075] Ca fixes S into CaS, improving compression molding properties. To achieve this effect, a content of 0.0002% or more is preferred. However, if a large amount of Ca is added, the surface quality deteriorates; therefore, the Ca content is preferably 0.0050% or less, more preferably 0.0030% or less.

[0076] Nb: 0.002%~0.060%

[0077] Nb contributes to increased strength by refining the original γ-particle size and consequently the internal structure of martensite. From this perspective, the Nb content is preferably 0.002% or more. However, if a large amount of Nb is added, coarse Nb precipitates will increase, leading to a deterioration in compressibility. Therefore, the Nb content is preferably 0.060% or less, more preferably 0.030% or less, and even more preferably 0.015% or less.

[0078] V: 0.005%~0.500%

[0079] To improve the hardenability of steel and increase its strength through martensite refinement, v can be added. To achieve this effect, a v content of 0.005% or more is preferred. However, if the v content exceeds 0.500%, castability deteriorates significantly. Therefore, the v content is preferably 0.005 to 0.500%, more preferably 0.200% or less, and even more preferably 0.100% or less.

[0080] W: 0.005%~0.200%

[0081] W contributes to high strength by forming fine W-based carbides and W-based carbonitrides. From this perspective, it is preferable that the W content is 0.005% or more. However, if the W content is high, the amount of coarse precipitates remaining undissolved during the slab heating process in the hot rolling process will increase, leading to a deterioration in press formability. Therefore, it is preferable to set the W content to 0.200% or less. More preferably, it is 0.100% or less, and even more preferably, it is 0.050% or less.

[0082] Sb: 0.001%~0.100%

[0083] Sb inhibits surface oxidation and nitriding, thereby suppressing the reduction of C and B. By suppressing the reduction of C and B, the formation of ferrite in the surface layer is suppressed, which contributes to high strength. From this point of view, the Sb content is preferably 0.001% or more. However, if the Sb content exceeds 0.100%, castability deteriorates, and Sb segregates at the original γ grain boundaries, resulting in deteriorated toughness and poor pressability. Therefore, the Sb content is preferably 0.100% or less. More preferably, it is 0.050% or less, and even more preferably, it is 0.015% or less.

[0084] Sn: 0.001%~0.100%

[0085] Sn inhibits surface oxidation and nitriding, thereby suppressing the reduction of C and B content in the surface layer. By suppressing the reduction of C and B, the formation of ferrite in the surface layer is suppressed, which contributes to the improvement of high strength and resistance to delayed fracture. From this point of view, the Sn content is preferably 0.001% or more. However, if the Sn content exceeds 0.100%, castability deteriorates, and Sn segregates at the original γ grain boundaries, resulting in deterioration of toughness and compressibility. Therefore, the Sn content is preferably 0.100% or less. More preferably, it is 0.050% or less, and even more preferably, it is 0.015% or less.

[0086] Mg: 0.0002%~0.0100%

[0087] Mg fixes O to MgO, improving compressibility. To achieve this effect, a content of 0.0002% or more is preferred. However, if a large amount of Mg is added, the surface quality and compressibility deteriorate; therefore, the Mg content is preferably 0.0100% or less. More preferably, it is 0.0050% or less, and even more preferably 0.0030% or less.

[0088] REM: 0.0002%~0.0100%

[0089] REM improves compressibility by refining inclusions and reducing the initiation point of fracture. Therefore, it is preferable to contain 0.0002% or more. However, if a large amount of REM is added, the inclusions become coarser, and the compressibility deteriorates. Therefore, the REM content is preferably 0.0100% or less. More preferably, it is 0.0050% or less, and even more preferably, it is 0.0030% or less.

[0090] It should be noted that for any of the above-mentioned elements, if the content is less than the lower limit value mentioned above, the element is considered an unavoidable impurity.

[0091] Next, the metal structure and tensile strength of the steel plate of the present invention will be described.

[0092] (Requirement 1 for metallic structure)

[0093] Martensite area relative to the total tissue area: over 85%

[0094] To achieve the specified strength, the martensite area ratio relative to the overall microstructure in the steel sheet of this invention needs to be 85% or more. The martensite area ratio can also be 100%. Examples of the remaining microstructure besides martensite include bainite, ferrite, and retained austenite; however, if their combined proportion exceeds 15%, i.e., if martensite is less than 85%, the increase in bainite, ferrite, and retained austenite as remaining microstructures makes it difficult to achieve the specified strength.

[0095] It should be noted that methods to ensure the specified strength while maintaining a martensite area ratio of less than 85% include, for example, lowering the tempering temperature. However, if the tempering temperature is too low, toughness decreases and pressability deteriorates. Furthermore, while increasing the carbon content can increase strength, it may worsen weldability and is not preferred. Therefore, to ensure excellent pressability and the specified strength, the martensite area ratio needs to be set to 85% or more. Here, martensite includes tempered martensite, self-tempered martensite generated during continuous cooling, and martensite that has not undergone tempering. The remaining portion can include bainite, ferrite, residual γ-rays, and inclusions such as carbides, sulfides, nitrides, and oxides. It should also be noted that the remaining portion can be excluded, and the martensite area ratio can be 100%.

[0096] (Requirement 2 for metallic structure)

[0097] In the steel plate of the present invention, the length L of the sub-plate strip boundary is... S Length L of the lath boundary B The ratio of L S / L B It satisfies the following equation (1).

[0098] 0.06 / [C%] 0.8 ≦L S / L B ≤0.13 / [C%] 0.8 ···(1)

[0099] Where [C%] represents the C content.

[0100] The substructure of martensite is a layered structure, named in order of size as packets, blocks, and laths. A packet is a group of regions within the structure that divides the original γ-grain into several areas, with the habit planes belonging to the same group. A block is a group of regions that divides the packets, with the crystal orientations being essentially the same group. Typically, lath-block boundaries form at large-angle grain boundaries with an orientation difference of 15 degrees or more. However, sometimes a relatively low angular orientation difference can be observed within a lath-block; this is called a sub-lamellar-block boundary. The inventors studied the correlation between the number of sub-lamellar-block boundaries and pressing tests in actual parts, finding that the more sub-lamellar-block boundaries there are, the smaller the reduction in sheet thickness in the actual part, even in complex pressing processes, and the potentially improved strain dispersion.

[0101] The mechanism is not yet fully understood, but it can be attributed to an internal stress field formed within the martensite. Each grain of martensite possesses a different yield strength and undergoes deformation in various regions. That is, it can be considered that during martensitic transformation, with the formation of numerous lath boundaries acting as large-angle grain boundaries, the strain decreases due to the martensitic transformation, resulting in a smaller internal stress field at the completion of the transformation. On the other hand, sub-lamellar boundaries are frequently observed in steels with relatively low carbon content. This is presumably because during the transformation expansion induced by the martensitic transformation, the deformation resistance of the surrounding austenite depends on the carbon content, indirectly affecting the crystal orientation selection of the laths.

[0102] Based on the above experimental results and speculations, the inventors conceived that C diffuses and enriches in the austenite region immediately after the formation of martensite, which may affect the choice of crystal orientation of lath blocks.

[0103] The inventors conducted more detailed experiments and found that: as the number of subplate boundaries, the length L of the subplate boundary... S Length L of the lath boundary B The ratio (hereinafter referred to as the ratio) L S / L B When used as an indicator, this ratio L S / L B It depends on the amount of carbon (C); by controlling the above ratio within a specified range according to the amount of carbon, the moldability can be improved; the above ratio is achieved through appropriate cooling conditions.

[0104] First, for steel plates with different carbon contents ranging from 0.10% to 0.46%, the L-section was finely ground with colloidal silica after grinding. Backscattered electron diffraction (EBSD) was used to analyze a 200 μm × 200 μm region at a position 1 / 4 thickness from the steel plate surface. The obtained crystal orientation data was analyzed using analysis software (OIMAnalysis Ver.7) manufactured by TSLSolutions Co., Ltd. The step size was set to 0.2 μm. On the EBSD-based crystal orientation map (crystal orientation data), ferrite, bainite, and martensite are difficult to distinguish due to their identical body-centered cubic (BCC) structure. Furthermore, in this invention, most of them have martensitic structures. Therefore, regions containing these structures with BCC crystal structures were selected as the target, and the orientation relationships of grain boundaries were quantified. Lath boundaries were defined as those with a crystal orientation difference of 15 degrees or more between adjacent steps, and sub-lamellar boundaries were defined as those with a difference of 3 degrees or more but less than 15 degrees. When drawing boundaries on the aforementioned analysis software, the length of each boundary is automatically measured, thereby determining the length L of the lath boundary. B The length L of the subplate boundary S In addition, for each steel plate, the formability was evaluated according to the method described in the embodiments below.

[0105] The measurement (ratio L) S / L B The results of the evaluation (cup formation height) are shown in Figure 1 As shown in the figure, it can be seen that in ratio L... S / L B The relationship between 0.06 / [C%] and C quantity. 0.8 In the above-mentioned areas, excellent formability can be obtained when the cupping height is 19.5 mm or more. This ratio L S / L B The higher the value, the more effective it is, but it also clearly indicates that the effect saturates within a certain range. That is, the ratio L S / L B Even if it rises to more than 0.13 / [C%] 0.8 The effect also saturates, therefore the actual upper limit is 0.13 / [C%]. 0.8 .

[0106] In order to make the ratio L S / L B The range of equation (1) can be achieved primarily through appropriate cooling conditions. Details of these cooling conditions will be described later. Furthermore, in the past, the cooling rate for martensitic formation was mainly focused on suppressing the formation of ferrite and bainite on the high-temperature side above the Ms point, as excessively increasing the cooling rate would increase equipment costs, and therefore, little research was conducted. From this perspective, the cooling rate for martensitic formation was mostly controlled by the average cooling rate from the high-temperature region of around 700°C, where ferrite does not form, to the temperature at which the martensitic transformation ends. However, in reality, the cooling rate decreases rapidly as the steel plate temperature decreases.

[0107] For example, in the technology described in Patent Document 1 above, only the average cooling rate is specified. Even in the examples, it is recorded that the rate exceeds 1000°C / s in all cases, and no attempt is found to precisely grasp and control the cooling rate in each temperature range during the cooling process. In addition, the reason for limiting the cooling rate is only the viewpoint of suppressing the formation of ferrite and bainite, and the viewpoint of suppressing the precipitation of coarse carbides after the formation of martensite, without considering the fact that the crystal orientation selection of laths as substructures is controlled.

[0108] The inventors have made a new discovery that, in order to control the crystal orientation selection of the above-mentioned lath blocks, it is necessary to control the cooling rate in a specific temperature range below the Ms point. To achieve this cooling rate, the conventional cooling method is insufficient, and the cooling conditions described later are required.

[0109] (Preferred requirements for metallic structure)

[0110] Standard deviation of Mn concentration: below 0.35%

[0111] Mn segregates during casting and tends to exhibit a strong banded distribution along the thickness direction after rolling. Because Mn significantly affects the Ms point, a banded Mn concentration distribution leads to anisotropic, banded distribution of internal stress due to martensitic transformation. From this perspective, a uniform Mn concentration distribution is desirable; specifically, the standard deviation of Mn concentration is preferably below 0.35%. Mn is known to be enriched in cementite, and the microstructure formed during hot rolling influences cementite formation as described later.

[0112] It should be noted that the standard deviation of the Mn concentration was calculated as follows. After mirror polishing of the L-section of the steel plate, an electron beam microanalyzer (EPMA) was used to analyze a 300 μm × 300 μm region corresponding to the 3 / 8 to 5 / 8 thickness of the steel plate. The accelerating voltage was set to 15 kV, the beam diameter to 1 μm, and the beam current to 2.5 × 10⁻⁶. －6 A. Calculate the standard deviation based on the obtained quantitative values ​​of Mn at 300 points × 300 points.

[0113] (Tensile strength (TS): ≥1310MPa)

[0114] Martensitic structures are commonly used in steel plates with tensile strengths of 1310 MPa and above. One feature of this invention is that it maintains good formability even at tensile strengths of 1310 MPa and above. Therefore, the steel plate of this invention has a tensile strength of 1310 MPa and above.

[0115] Furthermore, the steel sheet of the present invention may also have a coating on its surface. The type of coating is not particularly limited and may be any of a zinc (Zn) coating or a coating of metals other than Zn. Additionally, the coating may include components other than those primarily composed of Zn. For example, an electroplated zinc coating may be used for zinc coatings.

[0116] Next, the manufacturing method of the steel sheet of the present invention will be described. In this manufacturing method, a steel billet such as a slab having the above-described composition is hot-rolled to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. Next, a first cooling is performed, and the cold-rolled steel sheet is subjected to a homogenization heat treatment of Ac3 point or higher for 240 seconds or higher, and cooled at an average cooling rate of 10°C / s or higher in the temperature range from the cooling start temperature of 680°C or higher to the Ms point. Next, a second cooling is performed, and cooled at an average cooling rate of 100°C / s or higher from the Ms point to (Ms point - 50°C) in the temperature range. Then, a third cooling is performed, and cooled to below 50°C at an average cooling rate of 70°C / s or higher. The steel sheet of the present invention can be manufactured by this manufacturing method. In the present invention, the preparation of the steel billet, hot rolling, and cold rolling can be carried out according to conventional methods, but the key is to perform heat treatment (homogenization heat treatment, first cooling, second cooling, and third cooling) on ​​the cold-rolled steel sheet under specified conditions. It should be noted that hot rolling is preferably carried out according to the following conditions as needed.

[0117] (Hot-rolled)

[0118] In hot rolling, rolling, cooling, holding, and winding are preferably performed sequentially. From the viewpoint of preventing ferrite formation and increased plate thickness variation, the finishing rolling temperature is preferably 840°C or higher. After rolling (finishing), it is preferable to cool to below 640°C within 3 seconds and hold at a temperature range of 600°C to 500°C for at least 5 seconds. This is because holding at high temperatures results in the formation of coarse ferrite, with carbon enrichment in the untransformed region, easily leading to localized cementite formation. Holding at the specified temperature facilitates the formation of bainite and prevents excessive carbon enrichment. Furthermore, the winding process after holding is preferably performed at a temperature below 550°C. Winding at a temperature below 550°C suppresses the formation of pearlite containing coarse cementite. It should be noted that the upper limit of the finishing rolling temperature does not need to be specifically limited, but from the viewpoint of preventing large plate thickness variation due to coarse particles, 950°C is preferred.

[0119] (Heat treatment)

[0120] <Heat soaking: Ac3 or higher, 240 seconds or more>

[0121] In this invention, to obtain the specified martensite, the cold-rolled steel sheet (cold-rolled steel sheet) needs to undergo a homogenization heat treatment with a point at or above Ac3 and a duration of at least 240 seconds. When the homogenization temperature (annealing temperature) is lower than the Ac3 point or the homogenization time is less than 240 seconds, sufficient austenite is not generated during annealing, and the specified martensite area ratio cannot be ensured in the final product, resulting in a tensile strength of at least 1310 MPa. There is no particular upper limit to the annealing temperature and homogenization time. However, if the annealing temperature and homogenization time exceed a certain limit, the austenite grain size may become coarser, potentially deteriorating the toughness. Therefore, the preferred annealing temperature is below 1150°C, and the preferred homogenization time is below 900 seconds.

[0122] <1 Cooling>

[0123] To reduce bainite, ferrite, and residual γ, and to achieve a martensite area ratio of 85% or more, a cooling process is required after the aforementioned soaking treatment. This cooling is performed at an average rate of 10°C / s or higher over the temperature range from the high temperature (cooling start temperature) above 680°C to the Ms point. Firstly, if the cooling start temperature is below 680°C, a large amount of ferrite is formed. Furthermore, if the average cooling rate is less than 10°C / s, bainite is formed. It should be noted that the upper limit of the average cooling rate is not particularly limited, but from the viewpoint of avoiding increased manufacturing costs, 1500°C / s is preferred.

[0124] <2-stage cooling>

[0125] After the first cooling, a second cooling is required, with an average cooling rate of over 100℃ / s for the temperature range from Ms point to (Ms point - 50℃). This is because during the martensitic transformation, the diffusion and enrichment of carbon are suppressed, resulting in more sublamellar boundaries. Due to the reduced temperature difference between the steel plate and the refrigerant, coupled with the heat generated by the martensitic transformation, the cooling rate in the low-temperature range tends to be slow. However, previously, the importance of controlling the cooling rate in this temperature range was not well understood, and few attempts were made to measure or even control it. Moreover, microstructure design was managed through the average cooling rate from the quenching start temperature.

[0126] The inventors conducted cooling experiments using a sample with a thermocouple embedded in the center of a 2mm thick steel plate, employing water as the refrigerant. They investigated the relationship between cooling conditions and cooling rate in detail. Their results showed that to achieve the specified cooling rate, a 0.5m... 3 / m 2 Water cooling with a flow rate of 100 m / min or higher is effective. Here, we assume that the refrigerant is inexpensive water, but from the viewpoint of further obtaining cooling capacity, the refrigerant is not limited to water.

[0127] Furthermore, based on achieving the specified water flow density, the shape, configuration, and flow rate of the refrigerant injection nozzle can be appropriately modified. There is no specific upper limit to the water flow density; however, to avoid excessive increases in manufacturing costs, the water flow density for cooling water is set at 10 m³ / s. 3 / m 2 The cooling rate is below / min. It should be noted that the embodiments described later were implemented on an actual production line. While the cooling rate can be measured using a plate thermometer in a gas atmosphere, it is impossible to measure the plate temperature in water cooling. Therefore, the cooling rate in water cooling on the actual production line was calculated using heat transfer calculations based on the billet thickness, the plate temperature before water cooling, the plate flow rate, and the water flow density. The effectiveness of the heat transfer calculations was verified and confirmed by comparing the characteristics of the actual manufactured material with those of the steel plate in the aforementioned laboratory cooling experiment.

[0128] <3 Cooling Cycles>

[0129] After the two cooling cycles described above, a third cooling cycle is required, with an average cooling rate of 70°C / s or higher, to bring the temperature down to below 50°C. This helps to suppress softening caused by martensite self-tempering. If the average cooling rate is less than 70°C / s, it is difficult to achieve the required strength by tempering the martensite.

[0130] It should be noted that points Ac3 and Ms can be calculated using the following formulas respectively.

[0131] Ac3 point (°C) =

[0132] 910-203×[C%] 0.5 +44.7×[Si%]+31.5×[Mo%]-30×[Mn%]-11×[Cr%]+700×[P%]+400×[Al%]+400×[Ti%]

[0133] Ms point (°C) =

[0134] 561-474×[C%]-33×[Mn%]-17×[Cr%]-17×[Ni%]-21×[Mo%]

[0135] <Reheating (annealing)>

[0136] It is known that the toughness of martensite is improved by tempering. To ensure excellent compressibility, appropriate temperature control is preferred. Specifically, reheating is preferable, followed by three cooling quenchings to below 50°C, and then holding at a temperature range of 150–300°C for 20–1500 seconds. When the holding temperature is below 150°C or the holding time is less than 20 seconds, the tempering of martensite is insufficient, and compressibility may deteriorate. Furthermore, if the holding temperature is above 300°C, coarse cementite is formed, which may also deteriorate compressibility. Additionally, if the holding time exceeds 1500 seconds, not only will the tempering effect saturate, but manufacturing costs will also increase, and coarse carbides may be formed, further deteriorating compressibility.

[0137] From the perspective of stabilizing the shape accuracy of pressing and forming by adjusting surface roughness and flattening the plate shape, the steel plate obtained in this way can be subjected to surface smoothing and straightening processes.

[0138] In addition, the obtained steel sheet can be plated. By performing a plating process, a steel sheet with a coating such as a zinc layer on the surface can be obtained. There is no particular limitation on the type of plating process; it can be either melt plating or electroplating. Alternatively, an alloying plating process can be performed after melt plating. It should be noted that when performing a plating process, if the above-mentioned surface finishing is performed, surface finishing is performed after the plating process.

[0139] Next, the components of the present invention and their manufacturing method will be described.

[0140] The component of the present invention is formed by performing at least one of forming and welding on the steel plate of the present invention. Furthermore, the manufacturing method of the component of the present invention is a method of performing at least one of forming and welding on a steel plate manufactured by the steel plate manufacturing method of the present invention.

[0141] The steel sheet of the present invention has a tensile strength of 1310 MPa or higher and exhibits excellent press formability. Therefore, components obtained using the steel sheet of the present invention also possess high strength and superior press formability compared to conventional high-strength components. Furthermore, lightweighting can be achieved by using components of the present invention. Therefore, components of the present invention can be applied, for example, to vehicle body frame components.

[0142] As for forming, there are no particular limitations; conventional processing methods such as pressing can be used. Similarly, as for welding, there are no particular limitations; conventional welding methods such as spot welding and arc welding can be used.

[0143] Example

[0144] (Example 1)

[0145] Steel with the composition shown in Table 1 was melted, cast into slabs, and hot-rolled according to the conditions shown in Table 2. The resulting hot-rolled steel sheet was then pickled and cold-rolled to obtain a cold-rolled steel sheet. The obtained cold-rolled steel sheet was heat-treated according to the conditions shown in Table 2. Then, it was subjected to 0.1% quenching and tempering rolling to obtain a steel sheet. Furthermore, to confirm the effect of the microstructure differences formed by hot rolling on the uniformity of Mn concentration and press formability, the hot rolling conditions were changed as shown in Table 3; otherwise, two example steel sheets were manufactured under essentially the same conditions.

[0146]

[0147] [Table 2]

[0148]

[0149]

[0150] The obtained steel plates were subjected to quantitative analysis of their microstructure, followed by evaluation of their tensile properties and compressibility. The results are shown in Table 4.

[0151] Quantification of the microstructure was performed by etching the L-section (a section perpendicular to the rolling direction) of the steel plate with nitric acid and ethanol after grinding. At a position 1 / 4 of the plate thickness from the surface (hereinafter referred to as the 1 / 4 thickness position), four fields of view were observed using a scanning electron microscope (SEM) at 2000x magnification, and the microstructure images were analyzed. Here, martensite and bainite refer to the gray microstructure observed under SEM. On the other hand, ferrite is the region appearing as a black contrast under SEM. It should be noted that martensite and bainite contain trace amounts of carbides, nitrides, sulfides, and oxides; however, since these are difficult to exclude, the area ratio of the regions containing these is used as their area ratio.

[0152] The residual γ was determined using X-ray diffraction intensity analysis on the surface of a steel plate after chemically grinding and removing the top 200 μm layer with oxalic acid. The determination was performed using Mo-K... α The intensity of the diffraction peaks (200)α, (211)α, (220)α, (200)γ, (220)γ, and (311)γ measured by X-ray is calculated.

[0153] Martensite and bainite can be distinguished by observing the location and deformation of internal carbides under 10,000x magnification in SEM. Specifically, bainite forms carbides at the interfaces or within laths of a lath structure. Due to the crystalline orientation relationship between bainitic ferrite and cementite, the formed carbides extend in one direction. On the other hand, martensite forms carbides within laths, where the laths and carbides have two or more crystalline orientation relationships, resulting in carbides extending in multiple directions. Furthermore, the residual γ-rays generated by the high aspect ratio and C enrichment of bainite can be observed as a white contrast between the laths.

[0154] The length L of the subplate block boundary S The length L of the lath boundary B The following method was used for determination. The L-section of the steel plate was finely ground with colloidal silica after grinding, and backscattered electron diffraction (EBSD) was used to analyze a 200 μm × 200 μm region at a position 1 / 4 thickness from the steel plate surface. The obtained crystal orientation data was analyzed using analysis software (OIM Analysis Ver.7) manufactured by TSLSolutions Co., Ltd. The step size was set to 0.2 μm. On the EBSD-based crystal orientation map, ferrite, bainite, and martensite are difficult to distinguish due to their identical body-centered cubic (BCC) structure. Since most of the material in this invention has martensitic structure, the orientation relationship of grain boundaries was quantified in regions containing these structures with a BCC crystal structure. A lath boundary was defined as a crystal orientation difference of 15 degrees or more between adjacent steps, and a sub-lamellar boundary was defined as 3 degrees or more but less than 15 degrees. When drawing boundaries on the aforementioned analysis software, the length of each boundary (L of the lath boundary) was automatically measured. B The length L of the subplate boundary S ).

[0155] In addition, the standard deviation of Mn concentration was calculated as follows. After mirror polishing of the L-section of the steel plate, an electron beam microanalyzer (EPMA) was used to analyze a 300 μm × 300 μm region corresponding to the 3 / 8 to 5 / 8 thickness of the steel plate. The accelerating voltage was set to 15 kV, the beam diameter to 1 μm, and the beam current to 2.5 × 10⁻⁶. －6 A. Calculate the standard deviation based on the obtained quantitative values ​​of Mn at 300 points × 300 points.

[0156] In the tensile test, JIS No. 5 tensile test specimens are cut from the steel plate with the rolling right angle as the long side direction, and the tensile strength is evaluated by performing a tensile test (according to JIS Z2241). The tensile strength is considered acceptable if it is above 1310 MPa.

[0157] The press formability was evaluated using a cupping test, which confirmed its correlation with actual press formability evaluation tests using model components. It is known that this cupping property is related to indicators such as elongation characteristics and n-value in tensile tests. However, the martensitic steel used in this invention has low ductility. Even if superiority was not confirmed in the tensile test results, it can be inferred that superiority could be evaluated in more complex forming tests. A 210mm × 210mm plate was cut from the aforementioned steel sheet. A cupping test was performed on the punch. The clamping load was 100 tons, the feed rate was 30 mm / min, and R352L was applied as a lubricant. The maximum cupping height at crack initiation was evaluated using N=5, and the average value was taken as the cupping height. A cupping height of 19.5 mm or higher was considered acceptable.

[0158] [Table 4]

[0159]

[0160] As shown in Table 4, steel with optimized composition and heat treatment conditions can achieve tensile strength of over 1310 MPa and excellent compressibility.

[0161] here, Figure 2 The results are shown by organizing examples (inventive examples and comparative examples) for the above evaluation with tensile strength as the horizontal axis and cupping height as the vertical axis. For example... Figure 2 As shown, according to the inventive example of the present invention, both a tensile strength of 1310 MPa or more and a cupping height of 19.5 mm or more are simultaneously satisfied. In particular, when comparing the formability at the same strength, it can be seen that the formability of the inventive example is significantly improved. In addition, according to the comparison of No. 42 (Inventive Example) and No. 43 (Inventive Example), although they are both good results, the pressing formability can be further improved by attempting to optimize hot rolling and suppress Mn segregation.

[0162] (Example 2)

[0163] The galvanized steel sheet obtained by galvanizing Table 4 No. 1 (Example of the Invention) in Example 1 was pressed and formed to manufacture the first component of this invention. Furthermore, the galvanized steel sheet obtained by galvanizing Table 4 No. 1 (Example of the Invention) in Example 1 and the galvanized steel sheet obtained by galvanizing Table 4 No. 7 (Example of the Invention) in Example 1 were joined by spot welding to manufacture the second component of this invention. The cupping heights of the first and second components were measured to be 20.8 mm and 21.2 mm, respectively. That is, it can be seen that the press formability of both the first and second components is excellent.

[0164] Similarly, the steel sheet of Table 4 No. 1 (Example of the Invention) in Example 1 was press-formed to manufacture the third component of this invention. Furthermore, the steel sheet of Table 4 No. 1 (Example of the Invention) in Example 1 and the steel sheet of Table 4 No. 7 (Example of the Invention) in Example 1 were spot-welded together to manufacture the fourth component of this invention. The cupping heights of the third and fourth components were measured to be 21.3 mm and 21.5 mm, respectively. That is, it can be seen that the press-formability of both the third and fourth components is excellent.

Claims

1. A steel plate having the following composition and metallic structure, The composition, by mass%, contains C: 0.12%–0.40%, Si: less than 1.5%, Mn: greater than 1.7% and less than 3.5%, P: less than 0.05%, S: less than 0.010%, sol.Al: less than 1.00%, N: less than 0.010%, Ti: 0.002%–0.080%, and B: 0.0002%–0.0050%, with the remainder being Fe and unavoidable impurities. In the aforementioned metal microstructure, the martensite area fraction relative to the overall microstructure is over 85%, and the length L of the subplate boundary is... S Length L of the lath boundary B The ratio of L S / L B Satisfy the following equation (1), Furthermore, the tensile strength is above 1310 MPa. 0.06 / [C%] 0.8 ≦L S / L B ≦0.13 / [C%] 0.8 ···(1) in, [C%]: C content in mass % The steel plate is obtained by hot rolling a steel billet having the aforementioned composition to produce a hot-rolled steel plate, cold rolling the hot-rolled steel plate to produce a cold-rolled steel plate, subjecting the cold-rolled steel plate to a homogenization heat treatment of Ac3 point or higher for 240 seconds or higher, performing a first cooling process from a cooling start temperature of 680°C or higher to Ms point at an average cooling rate of 10°C / s or higher, followed by a second cooling process from Ms point to (Ms point - 50°C) at an average cooling rate of 100°C / s or higher and 380°C / s or lower, and finally performing a third cooling process to cool to 50°C or lower at an average cooling rate of 70°C / s or higher and 290°C / s or lower.

2. The steel plate according to claim 1, wherein, The composition, by mass%, further contains one or more of the following components: Cu: 0.01%–1.00%, Ni: 0.01%–1.00%, Mo: 0.005%–0.350%, Cr: 0.005%–0.350%, Zr: 0.005%–0.350%, Ca: 0.0002%–0.0050%, Nb: 0.002%–0.060%, V: 0.005%–0.500%, W: 0.005%–0.200%, Sb: 0.001%–0.100%, Sn: 0.001%–0.100%, Mg: 0.0002%–0.0100%, and REM: 0.0002%–0.0100%.

3. The steel plate according to claim 1 or 2, wherein, The standard deviation of the Mn concentration is less than 0.35%.

4. The steel plate according to any one of claims 1 to 3, wherein, It has a zinc-plated layer on the surface.

5. A component is formed by performing at least one of forming and welding on the steel plate according to any one of claims 1 to 4.

6. A method for manufacturing a steel plate, comprising hot rolling a steel billet having the composition described in claim 1 or 2 to produce a hot-rolled steel plate, cold rolling the hot-rolled steel plate to produce a cold-rolled steel plate, subjecting the cold-rolled steel plate to a homogenization heat treatment at point Ac3 or higher for 240 seconds or higher, performing a first cooling at an average cooling rate of 10°C / s or higher in the temperature range from a cooling start temperature of 680°C or higher to point Ms, performing a second cooling at an average cooling rate of 100°C / s or higher and 380°C / s or lower in the temperature range from point Ms to (Ms point - 50°C), and then performing a third cooling at an average cooling rate of 70°C / s or higher and 290°C / s or lower to 50°C or lower.

7. The method for manufacturing a steel plate according to claim 6, wherein, After the three cooling cycles, the product is reheated for 20 to 1500 seconds within a temperature range of 150 to 300°C.

8. The method for manufacturing a steel plate according to claim 6 or 7, wherein, The refrigerant used in the two cooling processes is water, and the water flow density in the two cooling processes is 0.5 m³ / s. 3 / m 2 / min~10.0m 3 / m 2 / min.

9. The method for manufacturing the steel plate according to any one of claims 6 to 8, wherein, In the hot rolling process, after rolling at a finishing temperature of 840°C or higher, the temperature is cooled to below 640°C within 3 seconds, held at a temperature range of 600°C to 500°C for more than 5 seconds, and then wound at a temperature of 550°C or lower.

10. The method for manufacturing a steel plate according to claim 7, wherein, The plating process is performed after the reheating.

11. A method for manufacturing a component, wherein a steel plate manufactured by the method for manufacturing a steel plate according to any one of claims 6 to 10 is subjected to at least one of forming and welding.

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