High-strength steel sheet, member, and method for producing same

Abstract

Provided is a high-strength steel sheet having high strength and excellent member strength, ductility, stretch flangeability, bendability, and warm workability. This high-strength steel sheet has a component composition containing, in mass%, 0.030% to 0.500% of C, 0.01% to 2.50% of Si, 0.10% to 5.00% of Mn, 0.100% or less of P, 0.0200% or less of S, 1.000% or less of Al, 0.0100% or less of N, and 0.0100% or less of O, with the remainder being Fe and unavoidable impurities, and also has a steel structure. And a steel structure in which the area ratio of martensite is 60% or more, the area ratio of ferrite is 40% or less, the area ratio of retained austenite is 20% or less, and the lattice constant aM of the martensite satisfies the following formula at a position of 1 / 4 of the sheet thickness. 1.00005 < = aM / aR < = 1.00500, where aR is the lattice constant of martensite at room temperature after the high-strength steel sheet is heat-treated at 500 DEG C for 30 minutes.

Description

Technical Field

[0001] The present disclosure relates to high-strength steel plates, components, and methods of manufacturing the same. Background Art

[0002] In order to reduce CO2 by reducing vehicle weight 2 The strength of automobile steel sheets is being increased to meet the needs of reducing vehicle emissions and improving collision resistance through lightweighting. New laws and regulations related to automobiles are also being introduced.

[0003] In order to improve the strength of the vehicle body, there are increasing cases of applying high-strength cold-rolled steel sheets with a tensile strength (TS) of 980 MPa or more to the main structural components that form the skeleton of the vehicle cab. Patent document 1 describes a high-strength cold-rolled steel sheet with a tensile strength of 1310 MPa or more. In addition, in order to increase the application ratio of high-strength steel sheets in automobiles, in addition to applying hot stamping, warm processing that is formed at a lower heating temperature is also considered. Patent document 2 describes a high-strength steel sheet for warm processing that has excellent processability even at low heating temperatures of about 50 to 200°C.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: International Publication No. 2019 / 181950

[0007] Patent Document 2: International Publication No. 2017 / 131053 Summary of the invention

[0008] However, there is no high-strength steel sheet that has excellent ductility, stretch flangeability and bendability at room temperature and excellent warm workability. For example, the high-strength steel sheet of Patent Document 1 has excellent bendability and ductility at room temperature, but does not consider warm workability at all. In addition, the high-strength steel sheet of Patent Document 2 has excellent warm workability (total elongation), but the details of uniform elongation and local elongation at warm temperature are unclear. In addition, in Patent Document 2, stretch flangeability and bendability at room temperature are not considered.

[0009] The present disclosure has been made in view of the above circumstances, and an object of the present disclosure is to provide a high-strength steel sheet and member having a tensile strength of 980 MPa or more and excellent in component strength, ductility, stretch flange formability, bendability, and warm workability, and a method for producing the same.

[0010] The present inventors have conducted intensive studies to achieve the above-mentioned problems, and as a result, have found that the above-mentioned object can be achieved by adopting the following configuration, thereby completing the present disclosure. That is, the gist of the present disclosure is as follows.

[0011] [1] A high-strength steel plate having the following composition and the following steel structure, the composition containing C: 0.030% to 0.500%, Si: 0.01% to 2.50%, Mn: 0.10% to 5.00%, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less by mass%, with the balance being composed of Fe and inevitable impurities;

[0012] In the steel structure at the 1 / 4 position of the plate thickness, the area ratio of martensite is 60% or more, the area ratio of ferrite is 40% or less, and the area ratio of retained austenite is 20% or less. The lattice constant a M (nm) of the above martensite satisfies the following formula 1.

[0013] Formula 1: 1.00005 ≤ a M / a R ≤ 1.00500

[0014] In Formula 1, a R (nm) is the lattice constant of the above martensite at room temperature after heat-treating the above high-strength steel plate at 500 °C for 30 minutes.

[0015] [2] The high-strength steel plate according to [1], wherein the number of martensite blocks containing carbides with a major axis of 200 nm or more is 50% or less relative to the number of martensite blocks containing carbides.

[0016] [3] The high-strength steel plate according to [1] or [2], wherein the composition further contains at least one element selected from Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less by mass%.

[0017] [4] The high-strength steel plate according to any one of [1] to [3], having a coating on at least one surface.

[0018] [5] A component made of the high-strength steel plate according to any one of [1] to [4].

[0019] [6] The member according to [5], wherein it is a skeleton structure member for an automobile or a strengthening member.

[0020] [7] A method for manufacturing a high-strength steel plate, wherein a steel slab having the composition described in [1] or [3] is subjected to rough rolling at an average strain rate of 1×10 －4 / s to 1×10 －1 / s and a total reduction ratio of 50% or more to obtain a rough-rolled plate, and the rough-rolled plate is subjected to finish rolling to obtain a hot-rolled plate.

[0021] The hot-rolled plate is subjected to cold rolling to obtain a cold-rolled plate.

[0022] Annealing is carried out as follows: the cold-rolled plate is heated to a heating temperature of 800°C or higher and cooled under the condition that the residence time t1 in the temperature region T1 from the Ms point to 700°C is 1000 s or less.

[0023] Then, partial tempering is carried out as follows: the cold-rolled plate is cooled from the Ms point to 50°C or lower under the condition that the partial tempering parameter S (μm 2 ) satisfies the following formula 2.

[0024] Formula 2: 0.01 ≤ S ≤ 30

[0025] Formula 3:

[0026] Formula 4: f(t) = 1 - exp(-1.1×10 -2 (Ms - T min ))

[0027] Formula 5:

[0028] In the above formulas 2 to 5,

[0029] The time when the temperature of the cold-rolled plate first reaches the Ms point after the above annealing is defined as t = 0 (s), and t = t E (s) is the time when the above partial tempering is completed and the temperature of the cold-rolled plate reaches 50°C.

[0030] T (°C) is the average temperature of the cold-rolled plate at time t - 1 to t (s).

[0031] T min (°C) is the lowest temperature in the temperature T from time 0 to t (s).

[0032] Ms (°C) refers to the Ms point of the above high-strength steel plate.

[0033] [8] The manufacturing method of the high-strength steel plate according to [7], wherein, in the above partial tempering, cooling is stopped at a cooling stop temperature above room temperature and below the Ms point, then reheating is performed to a reheating temperature, and then cooling is performed to 50°C or lower.

[0034] [9] The manufacturing method of the high-strength steel plate according to [7] or [8], wherein, in the above annealing, a plating treatment is further performed on at least one surface of the above cold-rolled plate.

[0035]

[10] A manufacturing method of a member, which is formed by performing at least one of forming processing or joining processing on the high-strength steel plate according to any one of [1] to [4] to form a member.

[0036] According to the present disclosure, it is possible to provide a high-strength steel plate, a member, and their manufacturing methods having a tensile strength of 980 MPa or more and excellent component strength, ductility, stretch flangeability, bendability, and warm workability. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a diagram showing an example of an electron diffraction pattern of martensite in which carbides are present.

[0038] Figure 2 It is a schematic diagram of the thermal history of the partial tempering process of continuously cooling from the Ms point to 50°C and a schematic diagram of f(t).

[0039] Figure 3 It is a schematic diagram of the thermal history and a schematic diagram of f(t) during reheating in partial tempering. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0040] Hereinafter, embodiments of the present invention will be described. It should be noted that the present invention is not limited to the following embodiments. The high-strength steel plate of the present embodiment has the composition and steel structure described below. Hereinafter, the "high-strength steel plate" will also be simply referred to as "steel plate".

[0041] The tensile strength of the high-strength steel plate of the present embodiment is 980 MPa or more, and the component strength, ductility, stretch flangeability, bendability, and warm workability are excellent.

[0042] High strength means that the tensile strength (TS) obtained by the tensile test described below is 980 MPa or more.

[0043] Excellent component strength means that the yield ratio (YR) obtained by the tensile test described below is 60% or more.

[0044] Excellent ductility means that the total elongation rate (T-El) obtained by the tensile test described below is 6.0% or more.

[0045] Excellent stretch flange property means that the hole expansion ratio (λ) obtained by the following reaming test is 30% or more.

[0046] Excellent bendability means that the limiting bend radius (R / t) obtained by the bend test of the sample with a ground end face described below is 5.0 or less.

[0047] Excellent warm workability means that in the uniform elongation (U-El RT ) and local elongation (L-El RT ) obtained by the following tensile test at room temperature and the uniform elongation (U-El 200 ) and local elongation (L-El 200 ) obtained by the tensile test at 200°C, U-El 200 / U-El RT is 1.10 or more and L-El 200 / L-El RT is 1.30 or more.

[0048] During warm working, from the viewpoint of strain dispersion ability, it is required that the uniform elongation of the steel sheet is higher than that at room temperature, and in order to suppress the fracture of the processed part, it is required that the local elongation of the steel sheet is higher than that at room temperature. In warm working, it is assumed that the die and the steel sheet are heated to a specified temperature and processed, but the end face part of the steel sheet is easily cooled and is substantially processed at room temperature. Therefore, in addition to excellent warm workability, the steel sheet is also required to have good ductility, stretch flange property, and bendability at room temperature. In addition, high-strength steel sheets used for automotive reinforcement parts and frame structure parts are required to have excellent part strength (large impact absorption energy during collision). This can be achieved by increasing the yield strength (YS) of the steel sheet or increasing the yield ratio (YR = yield strength YS / tensile strength TS × 100 [%]). Conventionally, there has been no high-strength steel sheet with good ductility, stretch flange property, and bendability at room temperature and excellent workability at warm temperature. In the steel sheets containing a large amount of retained austenite phase used in the past, very hard fresh martensite is generated by blanking processing, and low stretch flange property can be predicted from the viewpoints of the generation, growth, and connection of voids. As described later, as a result of in-depth research by the present inventors, it was conceived to appropriately adjust the partial tempering parameter S. Thereby, it is possible to make the lattice constant a of martensite at room temperature M and the lattice constant a at room temperature after heat-treating the above high-strength steel sheet having martensite at 500°C for 30 minutes R The ratio a M / a R be an appropriate value. Moreover, by making a M / a Ris an appropriate value, capable of providing a high-strength steel sheet with good ductility, stretch flangeability, and bendability at room temperature and excellent workability at elevated temperatures.

[0049] 〈Composition〉

[0050] First, the appropriate range of the composition of the high-strength steel sheet and the reasons for its limitation will be described. It should be noted that in the following description, "%" indicating the content of the component elements of the steel sheet refers to "mass%" unless otherwise specified.

[0051] 《C: 0.030% to 0.500%》

[0052] C is one of the important basic components of the steel sheet. Especially in the high-strength steel sheet of this embodiment, it affects the area ratio of martensite and the lattice constant of martensite.

[0053] If the C content is too low, the area ratio of martensite decreases, and it is difficult to achieve a TS of 980 MPa or more. In addition, the lattice constant a of martensite M decreases, and the warm workability decreases. In addition, the area ratio of ferrite increases. Therefore, the C content of the steel sheet is 0.030% or more. The C content is preferably 0.050% or more, and more preferably 0.090% or more.

[0054] On the other hand, if the C content is too high, the retained austenite increases excessively, and the hardness of the martensite formed from the retained austenite during blanking increases significantly. As a result, the crack propagation during reaming is promoted, the reaming rate decreases, and the stretch flangeability decreases. In addition, stress-induced phase transformation occurs in the retained austenite, resulting in a decrease in YR and a decrease in component strength. Therefore, the C content is 0.500% or less. The C content is preferably 0.400% or less, and more preferably 0.350% or less.

[0055] 《Si: 0.01% to 2.50%》

[0056] Si improves the strength of the steel sheet by suppressing the precipitation of cementite in martensite and solid solution strengthening. To obtain this effect, the Si content is 0.01% or more. The Si content is preferably 0.05% or more, and more preferably 0.10% or more.

[0057] On the other hand, if the Si content is too high, the precipitation of carbides during bainite transformation is significantly suppressed, the retained austenite increases excessively, and the hardness of the martensite formed from the retained austenite during blanking increases significantly. As a result, the crack propagation during reaming is promoted, the reaming rate decreases, and the stretch flangeability decreases. In addition, stress-induced phase transformation occurs in the retained austenite, resulting in a decrease in YR and a decrease in component strength. Therefore, the Si content is 2.50% or less. The Si content is preferably 2.00% or less, and more preferably 1.50% or less.

[0058] 《Mn: 0.10% - 5.00%》

[0059] Mn is one of the important basic components of the steel plate. Especially in the high-strength steel plate of this embodiment, it affects the area ratio of martensite.

[0060] If the Mn content is too low, the area ratio of martensite decreases, and it is difficult to achieve a TS of 980 MPa or more. In addition, with the decrease in the area ratio of martensite, it is difficult to obtain high hot workability. Therefore, the Mn content is 0.10% or more. The Mn content is preferably 0.90% or more, and more preferably 1.80% or more.

[0061] On the other hand, if the Mn content is too high, austenite is stabilized and the retained austenite increases excessively. Moreover, the hardness of the martensite generated from the retained austenite during blanking increases significantly. As a result, the crack propagation during reaming is promoted, the reaming rate decreases, and the stretch flangeability decreases. In addition, stress-induced phase transformation occurs in the retained austenite, resulting in a decrease in YR and a decrease in the strength of the component. Therefore, the Mn content is 5.00% or less. The Mn content is preferably 4.20% or less, and more preferably 3.60% or less.

[0062] 《P: 0.100% or less》

[0063] P segregates at the prior austenite grain boundaries, embrittling the grain boundaries. Therefore, the ultimate deformation ability of the steel plate decreases, and thus λ decreases. In addition, the bendability also decreases. Therefore, the P content needs to be 0.100% or less. It should be noted that there is no special regulation for the lower limit of the P content. However, since P is a solid solution strengthening element and can improve the strength of the steel plate, the P content is preferably 0.001% or more. The P content is preferably 0.070% or less.

[0064] 《S: 0.0200% or less》

[0065] S exists in the form of sulfides, reducing the ultimate deformation ability of the steel plate, so λ decreases. In addition, the bendability also decreases. Therefore, the S content needs to be 0.0200% or less. It should be noted that there is no special regulation for the lower limit of the S content. However, due to production technology constraints, the S content is preferably 0.0001% or more. The S content is preferably 0.0050% or less.

[0066] 《Al: 1.000% or less》

[0067] Al performs sufficient deoxidation, reducing inclusions in the steel.

[0068] However, if the Al content is too high, a large amount of ferrite is generated, the reaming rate decreases, and the stretch flangeability decreases. Therefore, the Al content is 1.000% or less. The Al content is preferably 0.500% or less, and more preferably 0.100% or less.

[0069] On the other hand, in order to stably perform deoxidation, the Al content is preferably 0.010% or more, more preferably 0.015% or more, and still more preferably 0.020% or more.

[0070] 《N: 0.0100% or less》

[0071] N exists in the form of nitrides, reducing the ultimate deformation ability of the steel sheet, so λ decreases. In addition, the bendability also decreases. Therefore, the N content is 0.0100% or less. It should be noted that the lower limit of the N content is not particularly specified, but due to production technical constraints, the N content is preferably 0.0001% or more. The N content is preferably 0.0050% or less.

[0072] 《O: 0.0100% or less》

[0073] O exists in the form of oxides, reducing the ultimate deformation ability of the steel sheet, so λ decreases. In addition, the bendability also decreases. Therefore, the O content is 0.0100% or less. It should be noted that the lower limit of the O content is not particularly specified, but due to production technical constraints, the O content is preferably 0.0001% or more. The O content is preferably 0.0050% or less.

[0074] The high-strength steel sheet of the present embodiment has a composition containing the above components and the remaining part is composed of Fe and inevitable impurities. Here, as inevitable impurities, Zn, Pb, As, Ge, Sr, and Cs can be cited. The total content of these impurities is allowed to be 0.100% or less.

[0075] In addition to the above composition, the high-strength steel sheet of the present embodiment may further contain, by mass%, at least one element selected from Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, either singly or in combination.

[0076] If Ti, Nb, and V are each 0.200% or less, a large amount of coarse precipitates and inclusions will not be generated, and the ultimate deformation ability of the steel sheet will not be reduced. Therefore, λ will not be reduced. In addition, the bendability will not be reduced. Therefore, when the steel sheet contains Ti, Nb, or V, the contents of Ti, Nb, and V are each preferably 0.200% or less. It should be noted that the lower limit of the contents of Ti, Nb, and V is not particularly specified, but the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, the contents of Ti, Nb, and V are each more preferably 0.001% or more. The contents of Ti, Nb, and V are each more preferably 0.100% or less.

[0077] If Ta and W are each 0.10% or less, a large amount of coarse precipitates and inclusions will not be generated, and the ultimate deformation ability of the steel sheet will not be reduced. Therefore, λ will not be reduced. In addition, the bendability will not be reduced. Therefore, the contents of Ta and W are each preferably 0.10% or less. It should be noted that the lower limit of the contents of Ta and W is not particularly specified, but the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, the contents of Ta and W are each more preferably 0.01% or more. The contents of Ta and W are each more preferably 0.08% or less.

[0078] If B is 0.0100% or less, cracks will not occur inside the steel sheet during casting or hot rolling, and the ultimate deformation ability of the steel sheet will not be reduced. Therefore, λ will not be reduced. In addition, the bendability will not be reduced. Therefore, the content of B is preferably 0.0100% or less. It should be noted that the lower limit of the content of B is not particularly specified, but since it is an element that segregates at the austenite grain boundaries and improves hardenability during annealing, the content of B is more preferably 0.0003% or more. The content of B is more preferably 0.0080% or less.

[0079] If Cr, Mo, and Ni are each 1.00% or less, the coarse precipitates and inclusions will not increase, and the ultimate deformation ability of the steel sheet will not be reduced. Therefore, λ will not be reduced. In addition, the bendability will not be reduced. Therefore, the contents of Cr, Mo, and Ni are each preferably 1.00% or less. It should be noted that the lower limit of the contents of Cr, Mo, and Ni is not particularly specified, but since they are elements that improve hardenability, the contents of Cr, Mo, and Ni are each more preferably 0.01% or more. The contents of Cr, Mo, and Ni are each more preferably 0.80% or less.

[0080] If Co is 0.010% or less, coarse precipitates and inclusions will not increase, and the ultimate deformation ability of the steel plate will not decrease. Therefore, λ will not decrease. In addition, the bendability will not decrease. Therefore, the content of Co is preferably 0.010% or less. It should be noted that there is no special regulation for the lower limit of the content of Co. However, since Co is an element that improves hardenability, the content of Co is more preferably 0.001% or more. The content of Co is more preferably 0.008% or less.

[0081] If Cu is 1.00% or less, coarse precipitates and inclusions will not increase, and the ultimate deformation ability of the steel plate will not decrease. Therefore, λ will not decrease. In addition, the bendability will not decrease. Therefore, the content of Cu is preferably 1.00% or less. It should be noted that there is no special regulation for the lower limit of the content of Cu. However, since Cu is an element that improves hardenability, the content of Cu is more preferably 0.01% or more. The content of Cu is more preferably 0.80% or less.

[0082] If Sn is 0.200% or less, cracks will not occur inside the steel plate during casting or hot rolling, and the ultimate deformation ability of the steel plate will not decrease. Therefore, λ will not decrease. In addition, the bendability will not decrease. Therefore, the content of Sn is preferably 0.200% or less. It should be noted that there is no special regulation for the lower limit of the content of Sn. However, since Sn is an element that improves hardenability, the content of Sn is more preferably 0.001% or more. The content of Sn is more preferably 0.100% or less.

[0083] If Sb is 0.200% or less, coarse precipitates and inclusions will not increase, and the ultimate deformation ability of the steel plate will not decrease. Therefore, λ will not decrease. In addition, the bendability will not decrease. Therefore, the content of Sb is preferably 0.200% or less. It should be noted that there is no special regulation for the lower limit of the content of Sb. However, since Sb is an element that controls the surface softening thickness and can adjust the strength, the content of Sb is more preferably 0.001% or more. The content of Sb is more preferably 0.100% or less.

[0084] If Ca, Mg, and REM are each 0.0100% or less, coarse precipitates and inclusions will not increase, and the ultimate deformation ability of the steel plate will not decrease. Therefore, λ will not decrease. In addition, the bendability will not decrease. Therefore, the contents of Ca, Mg, and REM are preferably 0.0100% or less, respectively. It should be noted that there is no special regulation for the lower limit of the contents of Ca, Mg, and REM. However, since Ca, Mg, and REM are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformation ability of the steel plate, the contents of Ca, Mg, and REM are preferably 0.0005% or more, respectively. The contents of Ca, Mg, and REM are more preferably 0.0050% or less, respectively.

[0085] If Zr and Te are each 0.100% or less, large precipitates and inclusions will not increase, and the ultimate deformation ability of the steel sheet will not be reduced. Therefore, λ will not be reduced. Additionally, the bendability will not be reduced. Therefore, the contents of Zr and Te are each preferably 0.100% or less. It should be noted that the lower limit of the contents of Zr and Te is not particularly specified, but since they are elements that spheroidize the shapes of nitrides and sulfides and improve the ultimate deformation ability of the steel sheet, the contents of Zr and Te are each more preferably 0.001% or more. The contents of Zr and Te are each more preferably 0.080% or less.

[0086] If Hf is 0.10% or less, large precipitates and inclusions will not increase, and the ultimate deformation ability of the steel sheet will not be reduced. Therefore, λ will not be reduced. Additionally, the bendability will not be reduced. Therefore, the content of Hf is preferably 0.10% or less. It should be noted that the lower limit of the content of Hf is not particularly specified, but since it is an element that spheroidizes the shapes of nitrides and sulfides and improves the ultimate deformation ability of the steel sheet, the content of Hf is preferably 0.01% or more. The Hf content is more preferably 0.08% or less.

[0087] If Bi is 0.200% or less, large precipitates and inclusions will not increase, and the ultimate deformation ability of the steel sheet will not be reduced. Therefore, λ will not be reduced. Additionally, the bendability will not be reduced. Therefore, the content of Bi is preferably 0.200% or less. It should be noted that the lower limit of the content of Bi is not particularly specified, but since it is an element that reduces segregation, the content of Bi is preferably 0.001% or more. The Bi content is more preferably 0.100% or less.

[0088] It should be noted that for the above Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi, when each content is less than the preferred lower limit value, the effects of the present invention will not be impaired, and thus they are treated as inevitable impurities.

[0089] Next, the steel structure of the high-strength steel sheet will be described.

[0090] 《Area ratio of martensite: 60% or more》

[0091] By containing martensite, a TS of 980 MPa or more and excellent warm workability can be achieved. This is because, as described later, by increasing the area ratio of martensite, the ductility improvement mechanism generated by the microscopic interaction between a large amount of dislocations contained in martensite and appropriately controlled solid-solution C can be manifested as the elongation rate in macroscopic warm working. Therefore, the area ratio of martensite is 60% or more. The area ratio of martensite is preferably 70% or more, and more preferably 90% or more.

[0092] The upper limit of the area ratio of martensite is not particularly limited, and the above effects can be obtained even when it is 100%.

[0093] Martensite is a phase transformation product formed below the Ms point and is independent of tempering. In addition, martensite includes lower bainite formed below the Ms point.

[0094] As described later, the observation position of martensite is at the 1 / 4 position of the plate thickness of the steel plate.

[0095] 《Area ratio of ferrite: 40% or less》

[0096] By making the area ratio of ferrite 40% or less, the desired strength and stretch flange formability can be obtained. It should be noted that the effects can be obtained even when the area ratio of ferrite is 0%. If there is too much ferrite, a sufficient amount of martensite cannot be ensured, and the desired TS cannot be obtained. In addition, the hardness difference between the tissues becomes larger, promoting the generation and connection of voids, reducing the hole expansion ratio, and deteriorating the stretch flange formability. Therefore, the area ratio of ferrite is 40% or less. The area ratio of ferrite is preferably 30% or less, and more preferably 20% or less.

[0097] Ferrite is a soft BCC iron formed at a temperature above the Ms point and includes allotriomorphic ferrite, idiomorphic ferrite, and upper bainite.

[0098] As described later, the observation position of ferrite is at the 1 / 4 position of the plate thickness of the steel plate (the position corresponding to 1 / 4 of the plate thickness in the depth direction from the steel plate surface).

[0099] The method for measuring the area ratios of martensite and ferrite is as follows.

[0100] First, a sample is cut out from the steel plate in such a way that the cross-section parallel to the rolling direction (the L-section at the 1 / 4 position of the plate thickness) is the observation surface. The observation surface of the sample is mirror-polished with diamond paste, then finally polished with colloidal silica, and further etched with 1% by volume nitric acid alcohol to expose the tissue.

[0101] Next, under the condition of an acceleration voltage of 10 kV, the observation surface of the sample is observed at a magnification of 3000 times using a scanning electron microscope (SEM) to obtain SEM images of 3 fields of view.

[0102] The area ratio of each structure was calculated from the obtained SEM images using Adobe Photoshop (manufactured by Adobe Systems Incorporated). Specifically, the value obtained by dividing the area of each structure by the measurement area was used as the area ratio of each structure. The area ratios of each structure in three fields of view were calculated, and their average value was used as the area ratio of each structure.

[0103] In the SEM image, ferrite is a flat tissue area that appears gray and does not contain carbides showing white contrast. In contrast, martensite is a tissue having a hierarchical structure with fine irregularities inside. Therefore, martensite and ferrite can be distinguished from each other.

[0104] 《Area ratio of retained austenite: 20% or less》

[0105] By making the area ratio of retained austenite 20% or less, good component strength and stretch flangeability can be obtained. Therefore, the area ratio of retained austenite is 20% or less. The area ratio of retained austenite is preferably 15% or less.

[0106] The lower limit of the area ratio of retained austenite is not particularly limited, and the above effects can be obtained even when it is 0%.

[0107] The method for measuring the area ratio of retained austenite is as follows.

[0108] First, the steel sheet was ground so that the measurement surface was at the 1 / 4 position of its plate thickness, and then further ground by 0.1 mm by chemical polishing to obtain a sample. For the measurement surface of the sample, the integrated reflection intensities of the (200), (220), and (311) planes of fcc iron (austenite) and the (200), (211), and (220) planes of bcc iron were measured using an X-ray diffractometer with a Co Kα ray source. The intensity ratios of the integrated reflection intensities of each plane of fcc iron to those of each plane of bcc iron were obtained. The average value of the nine intensity ratios was used as the volume ratio of retained austenite. The volume ratio of retained austenite was regarded as the area ratio of retained austenite.

[0109] 《Lattice constant of martensite》

[0110] The lattice constant of martensite is an extremely important tissue factor in the present invention. By the lattice constant a M (nm) of martensite at room temperature satisfies Formula 1, a high-strength steel sheet excellent in ductility, bendability, and warm workability at room temperature can be obtained.

[0111] Formula 1: 1.00005 ≤ a M / a R ≤ 1.00500

[0112] In Formula 1, a R(nm) is the lattice constant of martensite at room temperature after heat-treating the above high-strength steel sheet at 500 °C for 30 minutes.

[0113] The inventors found that by making the lattice constant a of martensite at room temperature M and the lattice constant a at room temperature after heat-treating the high-strength steel sheet having the above martensite at 500 °C for 30 minutes R The ratio a M / a R is an appropriate value, a steel sheet excellent in ductility and warm workability at room temperature can be obtained. It is presumed that due to the presence of an appropriate amount of dissolved C in martensite containing a large amount of dislocations responsible for plastic deformation, both the uniform elongation rate and the local elongation rate during warm working can be increased by the interaction between dislocations and dissolved C. The lattice constant a of martensite M is affected not only by the interstitial solid solution element C but also by the substitutional solid solution elements Si and Mn. By heat-treating the high-strength steel sheet at 500 °C for 30 minutes, the C dissolved in martensite precipitates in the form of cementite, and the dissolved C concentration becomes very low. Therefore, the lattice constant a of martensite at room temperature M and the lattice constant a at room temperature after heat-treating the high-strength steel sheet having martensite at 500 °C for 30 minutes R The ratio a M / a R is a parameter that accurately represents the dissolved C concentration in martensite. The dissolved C concentration in martensite affects the ductility and warm workability at room temperature. Therefore, by controlling to make a M / a R satisfy Formula 1, a high-strength steel sheet excellent in ductility, bendability, and warm workability at room temperature can be obtained.

[0114] If a M / a R is less than 1.00005, the dissolved C concentration in martensite is low, the interaction between dislocations and dissolved C in martensite during warm working becomes small, and the warm workability decreases. On the other hand, if a M / a R exceeds 1.00500, the movement of dislocations at room temperature is significantly suppressed, and the ductility and bendability at room temperature decrease. Therefore, the lattice constant a of martensite M needs to satisfy 1.00005 ≤ a M / a R ≤ 1.00500. Preferably, a M / a R is 1.00010 or more. In addition, preferably, a M / a R is 1.00200 or less.

[0115] The lattice constant of martensite was determined by X-ray diffraction method. The above high-strength steel plate was ground in such a way that the measurement surface was at the position of 1 / 4 of its plate thickness, and then further ground by chemical polishing for 0.1 mm to obtain a sample.

[0116] For the measurement surface of the sample, by using an X-ray diffractometer and a Cu Kα ray source, the peaks of the (110), (200), (211), (220), (310), and (222) planes of bcc iron were pseudo-Voigt approximated and fitted to calculate the peak positions. From the obtained peak positions, assuming c / a = 1, the lattice constant was calculated according to Bragg's law, and its average value was taken as the lattice constant a of martensite. M 。

[0117] For the steel plate obtained by heat-treating the high-strength steel plate at 500 °C for 30 minutes, the lattice constant a of martensite was also determined in the same manner as the above high-strength steel plate. R 。

[0118] 《Ratio of the number of martensite blocks containing carbides with a major axis of 200 nm or more to the number of martensite blocks containing carbides: 50% or less》

[0119] By reducing the ratio of the number of martensite blocks containing carbides with a major axis of 200 nm or more to the number of martensite blocks containing carbides, the warm workability can be further improved. The carbides with a major axis of 200 nm or more present in the martensite blocks are thermally stable from the perspective of interfacial energy and are likely to take away the surrounding solid-solved C during warm working, resulting in a local decrease in solid-solved C. By reducing the number of martensite containing carbides with a major axis of 200 nm or more, the uniform elongation rate and local elongation rate during warm working can be further improved. Therefore, the ratio of the number of martensite blocks containing carbides with a major axis of 200 nm or more to the number of martensite blocks containing carbides is preferably 50% or less, more preferably 30% or less, and still more preferably 10% or less. The lower limit of the ratio of the number of martensite blocks containing carbides with a major axis of 200 nm or more to the number of martensite blocks containing carbides is not particularly limited and can be 0%.

[0120] The carbides contained in the martensite blocks are at least one carbide selected from cementite, ε-carbide, η-carbide, and χ-carbide.

[0121] The method for measuring the ratio of the number of martensite blocks containing carbides with a major axis of 200 nm or more to the number of martensite blocks containing carbides is as follows.

[0122] First, grind the steel plate with the observation surface at 1 / 4 of its plate thickness, and then perform electrolytic polishing to produce a sample. Observe the observation surface of the produced sample using a Transmission Electron Microscope (TEM) under the condition of an acceleration voltage of 200 kV. The dislocation density of martensite is significantly higher than that of ferrite and retained austenite. Therefore, they are distinguished from each other by observing the strain contrast in the bright-field image of the TEM. Martensite packets are the hierarchical units that make up martensite and are a collection of laths with the same crystal habit plane and the same crystal orientation. Therefore, in the bright-field image of the TEM, the diffraction contrast of each packet within the martensite is different, so it can be distinguished from other hierarchical structures such as packets and laths. When the electron beam is incident from the

[100] direction of the martensite packet, an electron diffraction pattern of the parent-phase martensite can be obtained. The crystal orientations of adjacent martensite packets are different through the packet boundary, so the contrast in the bright-field image is different, and thus they are distinguishable from each other.

[0123] Figure 1 is an example of the electron diffraction pattern of martensite with carbide present.

[0124] When carbide is present in the observed single martensite packet, as Figure 1 shown, in addition to the electron diffraction pattern of the parent-phase martensite (α), an electron diffraction pattern of the carbide is also obtained. Figure 1 In, the black circles represent the electron diffraction spots of the parent-phase martensite when the electron beam is incident from the

[100] direction. In addition, the white circles represent the electron diffraction spots of the carbide. The case where such electron diffraction patterns of the parent-phase martensite and the carbide are obtained is regarded as a martensite packet containing carbide. Using the electron diffraction spots obtained from the carbide, a dark-field image is obtained. In the dark-field image, the metastable carbide shows white contrast.

[0125] Find the major axis length based on the obtained dark-field image of the carbide. When the major axis length of the largest carbide present within the martensite packet is 200 nm or more, it is regarded as a martensite packet containing carbide with a major axis length of 200 nm or more. Observe 50 martensite packets. Find the value obtained by dividing the number of martensite packets containing carbide with a major axis length of 200 nm or more by the number of martensite packets with carbide present and then multiplying by 100 ((the number of martensite packets containing carbide with a major axis length of 200 nm or more) / (the number of martensite packets with carbide present)×100). Take the obtained value as the ratio [%] of the number of martensite packets containing carbide with a major axis length of 200 nm or more to the number of martensite packets with carbide present.

[0126] 《Remaining Structure》

[0127] The structure of the steel sheet may have a structure (remaining structure) other than the above-mentioned martensite, ferrite, and retained austenite. However, for the reason of not impairing the effects of the present invention, the area ratio of the remaining structure is preferably 3% or less in terms of area ratio. Examples of the remaining structure include pearlite, alloy carbonitrides precipitated in ferrite, and structures known as structures of other steel sheets. It should be noted that the iron-based carbides present in martensite are not included in the remaining structure.

[0128] For the remaining structure, based on the SEM image taken when measuring the area ratios of martensite and ferrite, the area ratio of the remaining structure is calculated using Adobe Photoshop (manufactured by Adobe Systems Incorporated). Specifically, the value obtained by dividing the area of the remaining structure by the measurement area is used as the area ratio of the remaining structure. The area ratios of the remaining structure in three fields of view are calculated, and their average value is used as the area ratio of the remaining structure.

[0129] In the SEM image, pearlite presents a layered structure composed of cementite showing white contrast and ferrite showing gray contrast, and the alloy carbonitrides are angular structures showing dark contrast.

[0130] The thickness of the high-strength steel sheet is not particularly limited, and generally can be 0.3 mm or more and can be 2.8 mm or less.

[0131] 〈Coating〉

[0132] The high-strength steel sheet may have a coating on its surface. The coating is formed by the coating treatment described later. The coating is not particularly limited, and examples thereof include hot-dip coatings and electroplated coatings. The coating may also be an alloyed coating (alloyed coating).

[0133] Examples of the coating include a galvanized coating (Zn coating), an Al coating, etc. As the coating, a galvanized coating is preferred. The galvanized coating may contain elements such as Al and Mg.

[0134] The composition of the coating is not particularly limited and may be a general composition.

[0135] For example, when the coating is a hot-dip galvanized coating or an alloyed hot-dip galvanized coating, the following compositions are generally cited: containing Fe: 20% by mass or less, Al: 0.001 to 1.0% by mass, and further containing at least one selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total of 0% to 3.5% by mass, and the balance being composed of Zn and unavoidable impurities.

[0136] When the coating is a hot-dip galvanized coating, the adhesion amount on each side of the coating is preferably 20 g / m 2More preferably, it is 80 g / m² or less. In addition, an alloyed hot-dip galvanized layer obtained by alloying a hot-dip galvanized layer with such an adhesion amount is also preferred. 2

[0137] When the coating layer is a hot-dip galvanized layer, the Fe content in the coating layer is preferably less than 7% by mass. When the coating layer is an alloyed hot-dip galvanized layer, the Fe content in the coating layer is preferably 7% by mass or more. In addition, when the coating layer is an alloyed hot-dip galvanized layer, the Fe content in the coating layer is preferably 20% by mass or less, more preferably 15% by mass or less.

[0138] [Component]

[0139] Next, a component according to an embodiment of the present invention will be described. The component is made of the high-strength steel sheet according to an embodiment of the present invention described above. The component is, for example, formed into a target shape by forming processing or joining processing or the like of the high-strength steel sheet according to an embodiment of the present invention described above. The component according to an embodiment of the present invention is preferably a component for a skeletal structure part of an automobile or a reinforcing part of an automobile. Here, the high-strength steel sheet according to an embodiment of the present invention is a high-strength steel sheet having a tensile strength of 980 MPa or more and excellent in component strength, ductility, stretch flangeability, bendability, and warm workability. Therefore, the component according to an embodiment of the present invention can be particularly applied to all components for a skeletal structure part of an automobile or a reinforcing part of an automobile.

[0140] [Manufacturing method of high-strength steel sheet]

[0141] Next, a manufacturing method of the high-strength steel sheet will be described.

[0142] First, a steel billet is manufactured by melting a steel billet material having the above composition. The method of melting the molten steel into a steel billet (steel billet material) is not particularly limited, and a known melting method using a converter, an electric furnace, or the like can be adopted. In order to prevent macrosegregation, the steel billet is preferably manufactured by a continuous casting method, but can also be manufactured by other methods such as an ingot casting method and a thin slab casting method. The high-strength steel sheet of the present embodiment includes a cold-rolled steel sheet (cold-rolled sheet) manufactured by performing hot rolling, pickling, cold rolling, and annealing and a high-strength steel sheet having a coating applied to the cold-rolled steel sheet.

[0143] Next, the steel billet is hot-rolled to form a hot-rolled sheet. In one example, the steel billet is temporarily cooled to room temperature and then reheated and hot-rolled (rough rolling and finish rolling). It should be noted that the manufactured steel billet may be charged into a heating furnace in a warm sheet state without being cooled to room temperature, or rough rolling may be immediately performed after slightly maintaining heat.

[0144] 〈Rough rolling〉

[0145] By rough rolling the steel billet under the following conditions, a rough-rolled sheet is obtained.

[0146] From the viewpoints of dissolving carbides and reducing the rolling load, the temperature for heating the steel billet (slab heating temperature) is preferably 1100°C or higher. On the other hand, in order to prevent an increase in the loss of scale, the slab heating temperature is preferably 1300°C or lower. The slab heating temperature is based on the surface temperature of the steel billet. Next, the steel billet heated to the slab heating temperature is rough-rolled under the following conditions.

[0147] 《The average strain rate is 1×10 －4 / s to 1×10 －1 / s and the total reduction ratio is 50% or more》

[0148] The interstitial solid solution element C in the steel interacts with the substitutional solid solution elements Si and Mn. That is, solute atoms such as Si and Mn affect the solid solution C in martensite. By optimizing the average strain rate and the total reduction ratio in rough rolling, the lattice constant a of martensite in the final structure can be optimized. M For the reason, the inventors think as follows. It is considered that during the plastic deformation and dynamic recrystallization of austenite grains in rough rolling, solute atoms such as Si and Mn diffuse rapidly through dislocations and grain boundaries of recrystallized grains, and thus solute atoms such as Si and Mn are appropriately distributed. The inventors think that by appropriately distributing solute atoms such as Si and Mn, the amount of solid solution C in martensite becomes uniform during the partial tempering treatment described later, and thus the lattice constant a of martensite in the final structure can be optimized. M By optimizing the lattice constant a of martensite M , as described above, a high-strength steel sheet with excellent warm workability can be obtained.

[0149] The average strain rate in rough rolling is defined as the rolling rate ε(-) of rough rolling from the first rolling mill to the last rolling mill divided by the time t R (s) required for the rolling start of the first rolling mill to the rolling completion of the last rolling mill in rough rolling, and the obtained value (ε / t R ).

[0150] When the average strain rate in rough rolling exceeds 1×10 －1 / s, the diffusion of solute atoms such as Si and Mn in the plastic deformation and dynamic recrystallization of austenite grains becomes insufficient, and the lattice constant ratio a M / a R becomes lower, and the warm workability decreases. On the other hand, when the average strain rate in rough rolling is less than 1×10 －4 / s, the recovery of dislocations in austenite grains is promoted, the driving force for recrystallization decreases, and dynamic recrystallization is suppressed. As a result, the diffusion of solute atoms such as Si and Mn becomes insufficient, and the Ms lattice constant ratio a M / a Rdecreases, and the warm workability deteriorates. Therefore, the average strain rate in rough rolling is 1×10 －4 / s to 1×10 －1 / s. The average strain rate in rough rolling is preferably 1×10 －3 / s or more. The average strain rate in rough rolling is preferably 1×10 －2 / s or less.

[0151] When the total reduction ratio in rough rolling is less than 50%, the diffusion of solute atoms such as Si and Mn in the plastic deformation and dynamic recrystallization of austenite grains becomes insufficient, and the lattice constant ratio a M / a R decreases, and the warm workability deteriorates. Therefore, the total reduction ratio in rough rolling is 50% or more. The total reduction ratio in rough rolling is preferably 60% or more. On the other hand, the total reduction ratio in rough rolling is preferably 90% or less.

[0152] The finishing temperature of rough rolling is not particularly limited, but from the viewpoint of completing the recrystallization of austenite grains, it is preferably 950°C or higher.

[0153] Next, the rough rolled plate is subjected to finish rolling to produce a hot rolled plate. The hot rolled plate is appropriately coiled. When reducing the slab heating temperature, from the viewpoint of preventing failures during hot rolling, it is preferable to heat the rough rolled plate using a bar heater or the like before finish rolling. The temperature during finish rolling (finish rolling temperature) is preferably 700°C or higher. Thereby, the rolling load is reduced. In addition, the reduction ratio in the unrecrystallized state of austenite is reduced, the development of abnormal structures elongated in the rolling direction is suppressed, and the workability is excellent.

[0154] Finish rolling can be continuously performed by joining rough rolled plates to each other. The rough rolled plates can be temporarily coiled before performing finish rolling.

[0155] In order to reduce the rolling load, part or all of the finish rolling can be performed as lubricated rolling. From the viewpoint of making the shape and material quality of the steel plate uniform, lubricated rolling is also preferable. The friction coefficient during lubricated rolling is preferably 0.10 or more, and preferably 0.25 or less.

[0156] From the viewpoint of improving the through-feedability during subsequent cold rolling and annealing, the coiling temperature after hot rolling is preferably 300°C or higher, and preferably 700°C or lower.

[0157] Next, the hot rolled plate obtained by hot rolling is appropriately pickled. By pickling, the oxides on the surface of the hot rolled plate are removed, and in the high-strength steel plate as the final product, the chemical conversion treatability, the quality of the coating, etc. are excellent. Pickling can be performed once or in multiple times.

[0158] After arbitrarily performing softening heat treatment on the pickled hot-rolled sheet, cold rolling is carried out. Thus, a cold-rolled sheet is obtained. The conditions for cold rolling are not particularly limited, but the cumulative reduction ratio of cold rolling is preferably 20 to 75%. The number of rolling passes and the reduction ratio of each pass are not particularly limited.

[0159] The cold-rolled sheet thus obtained is subjected to annealing as described below.

[0160] 〈Annealing〉

[0161] 《Heating temperature: 800 °C or higher》

[0162] If the heating temperature is too low, the reverse transformation to austenite cannot proceed sufficiently, the area ratio of martensite becomes low, and the desired TS cannot be obtained. In addition, there is little martensite contributing to warm workability, so excellent warm workability cannot be obtained. Therefore, the heating temperature is 800 °C or higher. In addition, the heating temperature is preferably 830 °C or higher. The upper limit of the heating temperature is not particularly limited, but from the viewpoints of operability, etc., the heating temperature is preferably 1000 °C or lower. It should be noted that the heating temperature is based on the steel plate surface.

[0163] The time for heating the cold-rolled sheet at the heating temperature (heating time) is not particularly limited, but if it is too short, there is a risk that the reverse transformation to austenite cannot proceed sufficiently. Therefore, the heating time is preferably 30 s or longer, more preferably 60 s or longer. The upper limit of the heating time is not particularly limited. For example, the heating time is preferably 6000 s or shorter, more preferably 3000 s or shorter. It should be noted that "s" represents seconds.

[0164] 《Cooling under the condition that the residence time t1 in the temperature range T1 from the Ms point to 700 °C is 1000 s or shorter》

[0165] The cold-rolled sheet after heating and cooling passes through the temperature range T1 from the Ms point to 700 °C. If the residence time (residence time t1) of the cold-rolled sheet in the temperature range T1 is too long, the ferrite phase transformation occurs excessively, the area ratio of ferrite becomes high, and the desired TS and good stretch flangeability cannot be obtained. Therefore, the residence time t1 is 1000 s or shorter. The residence time t1 is preferably 800 s or shorter, more preferably 600 s or shorter. The lower limit of the residence time t1 is not particularly limited, but from the viewpoint of reducing the equipment investment burden, the residence time t1 is preferably 1 s or longer, more preferably 5 s or longer, and further preferably 10 s or longer. It should be noted that the temperature range T1 is based on the steel plate surface.

[0166] It should be noted that the Ms point is calculated by the following formula (1).

[0167] Ms = 499 - 317[%C] - 11[%Si] - 33[Mn] - 17[%Ni] - 28[%Cr] - 11[%Mo] ··· (1)

[0168] Among them, [%M] represents the content of M in steel (mass %).

[0169] After the above annealing, a partial tempering process of cooling the cold-rolled sheet from the Ms point to below 50 °C is carried out.

[0170] 《Partial tempering parameter S (μm 2 ) Satisfying the partial tempering treatment of Equation 2》

[0171] Equation 2:

[0172] 0.01 ≤ S ≤ 30

[0173] Equation 3:

[0174]

[0175] Equation 4:

[0176] f(t) = 1 - exp(-1.1×10 -2 (Ms - T min ))

[0177] Equation 5:

[0178]

[0179] In the above Equations 2 to 5,

[0180] The time when the temperature of the cold-rolled sheet after the above annealing first reaches the Ms point is defined as t = 0 (s), and t = t E (s) is the time when the partial tempering treatment is completed and the temperature of the cold-rolled sheet reaches 50 °C.

[0181] T (°C) is the average temperature of the cold-rolled sheet at time t - 1 to t (s).

[0182] T min (°C) is the lowest temperature in the temperature T from time 0 to t (s).

[0183] Ms (°C) refers to the Ms point of the above high-strength steel plate.

[0184] It should be noted that various temperatures in the partial tempering are based on the surface of the steel plate.

[0185] The steel plate passing through the temperature region T1 reaches the Ms point and undergoes martensitic transformation. After the martensitic transformation, tempering occurs in which the dissolved C precipitates in the form of carbides, and the dissolved C decreases. Therefore, in the temperature region after first reaching below the Ms point, tempering of the martensite always proceeds and the dissolved C decreases. Therefore, the present inventors found that in the entire region from the first reaching of the Ms point to 50°C where tempering can be neglected, it is necessary to consider the martensite fraction, temperature, and time that have already been generated at that temperature. Therefore, the process of tempering while considering the martensite fraction that has already been generated at that temperature is defined as the partial tempering process.

[0186] The present inventors found that by defining the partial tempering parameter S and appropriately controlling the value of S, the lattice constant ratio a M / a R can be within a specified value range. The parameter S is a value obtained by integrating the tempering amount of the martensite that has already been generated per unit time, which is a combination of the volume fraction f(t) of the martensite at the temperature T (°C) at time t (s) and the diffusion coefficient D(T) of C, over the thermal history from the first reaching of the Ms point to 50°C where C diffusion can be neglected. The time when the cold-rolled plate first reaches the Ms point after annealing is defined as t = 0 (s), and t = t E (s) is the time when the partial tempering is completed and 50°C is reached. T (°C) is the average temperature of the cold-rolled plate at time t - 1 to t (s). T min (°C) is the lowest temperature in the temperature T from time 0 to t (s). Ms (°C) refers to the Ms point of the above high-strength steel plate.

[0187] If S (μm 2 ) is too small, the partial tempering treatment becomes insufficient, and the lattice constant ratio a M / a R becomes high, and the ductility and bendability at room temperature decrease. Therefore, S (μm 2 ) is 0.01 or more. S (μm 2 ) is preferably 0.03 or more, more preferably 0.10 or more. If S (μm 2 ) is too large, the partial tempering treatment becomes excessive, and the lattice constant ratio a M / a R becomes low, and the warm workability decreases. Therefore, S (μm 2 ) is 30 or less. S (μm 2 ) is preferably 15 or less, more preferably 5 or less.

[0188] The temperature history in the partial tempering treatment is not particularly limited as long as the partial tempering parameter S is within the above range. In one example, the partial tempering treatment can be Figure 2 or Figure 3 the temperature history shown.

[0189] Figure 2 The figure shows a schematic diagram of the thermal history of the partial tempering process in which continuous cooling is carried out from the Ms point to below 50°C and a schematic diagram of f(t). In this example, the temperature history below the Ms point is continuously cooled from the Ms point to below 50°C. The cooling method is not particularly limited, and known cooling methods such as gas cooling can be used. The cooling rate during continuous cooling is not particularly limited, and for example, it can be in the range of 1°C / s to 100°C / s.

[0190] As Figure 2 shown, the time when reaching the Ms point for the first time after annealing is defined as t = 0 (s). When continuous cooling is carried out as Figure 2 shown, t = t E (s) is the time when reaching 50°C for the first time after the above annealing. When t = t a , T min (°C) is the lowest temperature among the temperatures T from time 0 to t (s), that is, the temperature when t = t a .

[0191] <During the partial tempering process, cooling is stopped at a cooling stop temperature above room temperature and below the Ms point, then reheating is carried out to the reheating temperature, and then cooling is carried out to below 50°C (preferred condition)>

[0192] In the partial tempering process, cooling can be stopped at any temperature from below the Ms point to room temperature, then reheating is carried out to the reheating temperature, and then cooling is carried out to below 50°C. Thereby, the hardness difference between the structures becomes smaller, and the bendability and stretch flangeability can be further improved. Figure 3 The figure shows a schematic diagram of the thermal history of the partial tempering process in which cooling is stopped at a temperature from below the Ms point to room temperature during the partial tempering process, then heating is carried out, and then cooling is carried out to below 50°C. When t = t b , in the example of Figure 3 , T min (°C) is the cooling stop temperature. It should be noted that when T min (°C) is lower than 50°C, the time when the steel plate reaches 50°C again after reheating is defined as t = t E (s).

[0193] After annealing, the cold-rolled plate is cooled to a cooling stop temperature above room temperature and below the Ms point. The cooling stop temperature is not particularly limited, and in order to sufficiently generate martensite, promote partial tempering, and make the warm workability appropriate, it is preferably 250°C or lower. The cooling stop temperature can be, for example, room temperature. The cooling method is also not particularly limited, and gas cooling or water cooling can be used. The cooling rate is not particularly limited, and for example, it can be in the range of 1°C / s to 100°C / s.

[0194] Next, the cold-rolled sheet is reheated to the reheating temperature. There is no particular limitation on the heating method when reheating to the reheating temperature. For example, induction heating (IH) can be used.

[0195] For the reason of suppressing the recovery phenomenon of dislocations in martensite and making the strength appropriate, the reheating temperature is preferably 500°C or lower. For the reason of promoting the diffusion of C, promoting partial tempering in a short time, and making the ductility and bendability appropriate, the reheating temperature is preferably 130°C or higher. After reheating, it can be held at a constant temperature until cooling.

[0196] Next, the cold-rolled sheet is cooled to 50°C or lower. There is no particular limitation on the cooling method. For example, well-known methods such as water cooling or gas cooling can be used. The cooling rate is not particularly limited. For example, it can be in the range of 1°C / s to 100°C / s. The cold-rolled sheet that reaches 50°C or lower is cooled to room temperature by any cooling method. There is no particular limitation on the cooling method, and well-known methods such as gas cooling, air cooling, and water cooling can be used.

[0197] Then, the cold-rolled sheet that has undergone partial tempering treatment is cooled to room temperature. In this way, the above high-strength steel sheet (cold-rolled steel sheet) is obtained.

[0198] In this manufacturing method, in the case of performing the plating treatment described later, the obtained high-strength steel sheet is a plated steel sheet having a plating layer.

[0199] For a series of heat treatments in this manufacturing method, as long as the above heat history is satisfied, other conditions are not particularly limited, and the equipment for performing the heat treatment, etc. are also not particularly limited.

[0200] 〈Plating Treatment〉

[0201] In this manufacturing method, a plating treatment can be performed on the cold-rolled sheet.

[0202] As the plating treatment, for example, hot-dip galvanizing treatment (treatment for forming a hot-dip galvanized layer), alloying hot-dip galvanizing treatment (treatment for forming an alloyed hot-dip galvanized layer by performing an alloying treatment after the hot-dip galvanizing treatment), etc. can be cited. An electroplated layer can also be formed by electroplating treatment.

[0203] When performing the hot-dip galvanizing treatment, it is preferable to immerse the cold-rolled sheet in a galvanizing bath, and then adjust the coating amount by gas wiping or the like. The bath temperature of the galvanizing bath is not particularly limited, and is preferably 440°C or higher, and further preferably 500°C or lower.

[0204] The Al content of the galvanizing bath is preferably 0.10 mass% or higher, and further preferably 0.23 mass% or lower.

[0205] The galvanizing treatment is preferably carried out while maintaining the temperature in the temperature range T1 from the Ms point to 700 °C in the above annealing.

[0206] In order to make the Zn-Fe alloying rate more appropriate and the productivity more appropriate, the temperature of the alloying treatment is preferably 470 °C or higher. In addition, in order to appropriately prevent the untransformed austenite from transforming into pearlite and make the TS more appropriate, the temperature of the alloying treatment is preferably 600 °C or lower, more preferably 560 °C or lower. The temperature of the alloying treatment is based on 530 °C.

[0207] After the plating treatment, skin rolling can be performed on the plated steel sheet. From the viewpoint of improving the yield strength, the reduction ratio of the skin rolling is preferably 0.05% or more. The upper limit of the reduction ratio is not particularly limited, but from the viewpoint of productivity, it is preferably 1.50% or less. The skin rolling can be carried out online or offline. The skin rolling with the target reduction ratio can be carried out once or in multiple times.

[0208] From the viewpoint of productivity, the above series of treatments such as annealing and plating treatment are preferably carried out in a CAL (Continuous Annealing Line) or a CGL (Continuous Galvanizing Line).

[0209] It should be noted that the manufacturing conditions other than the above conditions can be adopted by conventional methods.

[0210] [Manufacturing method of components]

[0211] At least one of the above-mentioned forming process or joining process can be applied to the high-strength steel sheet to manufacture components. The forming process and the joining process can be carried out by conventional methods.

[0212] Examples

[0213] Examples of the present invention are given below for specific illustration. However, the present invention is not limited to the examples described below.

[0214] 〈Manufacture of steel sheet〉

[0215] The molten steel having the composition shown in Table 1 below (the remaining part is composed of Fe and inevitable impurities) is melted in a converter and a steel slab is obtained by continuous casting.

[0216] The obtained steel billet is hot-rolled to obtain a hot-rolled sheet. Specifically, the steel billet is heated to 1250 °C, rough-rolled at the average strain rate and total reduction ratio shown in Table 2, then finish-rolled at a finish-rolling temperature of 900 °C, then coiled under the condition of 500 °C, and then cooled to room temperature to obtain a hot-rolled sheet. After pickling the obtained hot-rolled sheet, softening heat treatment is carried out under the condition of 500 °C, and then cold rolling is carried out under the condition of a rolling rate of 50%. In this way, a cold-rolled sheet with a thickness of 1.6 mm is obtained.

[0217] The obtained cold-rolled steel sheet is subjected to annealing and partial tempering treatment under the conditions shown in Table 2 below to obtain the high-strength steel sheet (cold-rolled steel sheet) of the present invention. In the partial tempering treatment, when continuously cooling from below the Ms point to below 50 °C, it is recorded as "continuous" in the column of "thermal history during partial tempering" in Table 2. In addition, in the partial tempering treatment, when cooling is stopped at a temperature from below the Ms point to room temperature, then reheated, and then cooled to below 50 °C, it is recorded as "reheating" in the column of "thermal history during partial tempering" in Table 2. The heating time at the heating temperature of the cold-rolled sheet is 200 s.

[0218] 《Coating Treatment》

[0219] A part of the cold-rolled sheet is subjected to hot-dip galvanizing treatment during residence in the temperature range T1 (Ms °C to 700 °C) to form coatings (hot-dip galvanized layers) on both sides. That is, a hot-dip galvanized steel sheet (GI) is obtained. In the hot-dip galvanizing treatment, a hot-dip galvanizing bath containing 0.20% by mass of Al and the balance consisting of Zn and inevitable impurities (bath temperature: 470 °C) is used. The coating amount on each side of the hot-dip galvanized layer is about 45 - 72 g / m 2 or so. The composition of the formed hot-dip galvanized layer is a composition containing 0.1 - 1.0% by mass of Fe and 0.2 - 1.0% by mass of Al and the balance consisting of Fe and inevitable impurities.

[0220] Another part of the cold-rolled sheet is subjected to alloying hot-dip galvanizing treatment during residence in the temperature range T1 (Ms °C to 700 °C) during annealing to form coatings (alloying hot-dip galvanized layers) on both sides. That is, an alloying hot-dip galvanized steel sheet (GA) is obtained. In the hot-dip galvanizing treatment, a hot-dip galvanizing bath containing 0.14% by mass of Al and the balance consisting of Zn and inevitable impurities (bath temperature: 470 °C) is used. The alloying treatment is at 550 °C. The coating amount on each side of the alloying hot-dip galvanized layer is about 45 g / m 2 or so. The composition of the formed alloying hot-dip galvanized layer is a composition containing 7 - 15% by mass of Fe and 0.1 - 1.0% by mass of Al and the balance consisting of Fe and inevitable impurities.

[0221] In the "Coating type" column of Table 2 below, the case where a hot-dip galvanized layer is formed is denoted as "GI", the case where an alloyed hot-dip galvanized layer is formed is denoted as "GA", and the case where no coating is formed is denoted as "CR".

[0222] 〈Observation of steel structure〉

[0223] For the obtained steel sheet, the area ratios of martensite, ferrite, and retained austenite, the lattice constant of martensite, the ratio of the number of martensite blocks containing carbides with a major axis of 200 nm or more to the number of martensite blocks containing carbides, and the area ratio of the remaining structure were measured by the above method. The results are shown in Table 3 below.

[0224] 〈Evaluation〉

[0225] The obtained steel sheet was subjected to the tests described below, and various properties were evaluated. The results are shown in Table 3 below.

[0226] 《Tensile test》

[0227] The tensile test was carried out in accordance with JIS Z 2241:2021.

[0228] Specifically, a JIS No. 5 test piece was taken from the obtained steel sheet with the long side direction perpendicular to the rolling direction of the steel sheet. Using the taken test piece, a tensile test was carried out under the condition that the crosshead speed was 1.67×10 －1 mm / s, and the yield strength (YS) [MPa], tensile strength (TS) [MPa], and total elongation (El) [%] were measured. Furthermore, the yield ratio (YR) (=100×YS / TS) [%] was calculated. When the tensile strength (TS) is 980 MPa or more, it is judged as high strength. When the yield ratio (YR) is 60% or more, it is judged that the component strength is excellent. When the total elongation (El) is 6.0% or more, it is judged that the ductility is excellent.

[0229] 《Hole expansion test》

[0230] The hole expansion test was carried out in accordance with JIS Z 2256:2020.

[0231] Specifically, the obtained steel sheet was sheared to take a test piece with a size of 100 mm×100 mm. A hole with a diameter of 10 mm was punched out on the taken test piece with a clearance of 12.5%. Then, using a die head with an inner diameter of 75 mm, a conical punch with a vertex angle of 60° was pressed into the hole in a state where the wrinkling force was 9 ton (88.26 kN), and the hole diameter D f at the limit of crack generation was measured [mm]. Let the initial hole diameter be D 0 [mm], and the hole expansion rate λ [%] was calculated according to the following formula (6).

[0232] Formula 6: λ = {(D f − D 0 ) / D 0} × 100

[0233] When the hole expansion ratio (λ) is 30% or more, it is judged that the stretch flange property is excellent.

[0234] "Bending Test"

[0235] The bending test is carried out in accordance with JIS Z 2248:2022. Specifically, a strip-shaped test piece with a width of 30 mm and a length of 100 mm is taken from the obtained steel plate in such a way that the axial direction of the bending test is parallel to the rolling direction of the steel plate. It should be noted that the end face in the long side direction of the test piece is taken as the ground end face. Using the taken test piece, a 90° V-bending test is carried out under the conditions of a pushing load of 100 kN and a pushing holding time of 5 seconds.

[0236] The bending test is carried out on 5 test pieces with an appropriate bending radius R. Then, it is confirmed whether cracks occur in the ridge line part of the bending vertex.

[0237] Whether cracks occur is confirmed by observing the ridge line part of the bending vertex at a magnification of 40 times using a digital microscope (RH-2000, manufactured by Hirox Co., Ltd.).

[0238] Find the minimum bending radius R at which no cracks occur in all 5 test pieces, divide it by the plate thickness t, and use the obtained value (R / t) as the limiting bending radius. When the limiting bending radius (R / t) is 5.0 or less, it is judged that the bending property is excellent.

[0239] "Warm Workability"

[0240] The tensile test is carried out in accordance with JIS Z 2241:2021.

[0241] Specifically, a JIS No. 5 test piece is taken from the obtained steel plate in such a way that the long side direction is perpendicular to the rolling direction of the steel plate. Using the taken test piece, a tensile test is carried out at room temperature and 200 °C under the condition that the crosshead speed is 1.67×10 －1 mm / s, and the yield strength (YS) [MPa], tensile strength (TS) [MPa], uniform elongation (U-El) [%], and local elongation (L-El) [%] are measured. During the tensile test at 200 °C, it is held at 200 °C for 10 minutes in the furnace equipped in the tensile testing machine, and then the tensile test is started. Let the uniform elongation and local elongation obtained from the tensile test at room temperature be U-El RT , L-El RTThe uniform elongation and local elongation obtained from the tensile test at 200 °C are designated as U-El 200 and L-El 200 When U-El 200 / U-El RT is 1.10 or more and L-El 200 / L-El RT is 1.30 or more, it is judged that the warm workability is excellent. The underlines in Tables 1 to 3 below indicate outside the appropriate range of the present invention.

[0242]

[0243] [Table 2]

[0244] Table 2

[0245]

[0246] Underlined part: Indicates outside the scope of the present invention.

[0247]

[0248] Underlined part: Indicates outside the scope of the present invention.

[0249] As shown in Table 3, in the examples of the present invention, TS is 980 MPa or more, and the component strength, ductility, stretch flangeability, bendability, and warm workability are excellent. On the other hand, in the comparative examples, one or more of the component strength, ductility, stretch flangeability, bendability, and warm workability are poor.

[0250] As described above, the embodiments of the present invention have been described, but the present invention is not limited to the description that constitutes a part of the disclosure of the present invention of this embodiment. That is, all other embodiments, examples, and application techniques and the like made by those skilled in the art based on this embodiment are included in the scope of the present invention. For example, in a series of heat treatments in the above manufacturing method, as long as the heat history conditions are satisfied, there are no particular limitations on the equipment for heat-treating the steel sheet.

Claims

1. A high-strength steel plate having the following composition and the following steel structure, wherein the composition contains, by mass%, C: 0.030% to 0.500%, Si: 0.01% to 2.50%, Mn: 0.10% to 5.00%, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, and the balance is composed of Fe and inevitable impurities; At the position of 1 / 4 of the plate thickness of the steel structure, the area ratio of martensite is 60% or more, the area ratio of ferrite is 40% or less, and the area ratio of retained austenite is 20% or less. The lattice constant a of the martensite M satisfies the following formula 1; Formula 1: 1.00005 ≤ a M / a R ≤ 1.00500 In Formula 1, a R is the lattice constant of the martensite at room temperature after heat-treating the high-strength steel sheet at 500 °C for 30 minutes, a M and a R are in the unit of nm.

2. The high-strength steel plate according to claim 1, wherein, the number of martensite blocks containing carbides with a major axis of 200 nm or more is 50% or less relative to the number of martensite blocks containing carbides.

3. The high-strength steel plate according to claim 1 or 2, wherein, the composition further contains, by mass%, at least one element selected from Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less.

4. The high-strength steel plate according to any one of claims 1 to 3, wherein, it has a plating layer on at least one surface.

5. A member made of the high-strength steel plate according to any one of claims 1 to 4.

6. The member according to claim 5, wherein, it is used for a skeleton structure member of an automobile or for a strengthening member.

7. A method for manufacturing a high-strength steel plate, which comprises rough rolling a steel billet having the composition described in claim 1 or 3 at an average strain rate of 1×10 －4 / s to 1×10 －1 / s and a total reduction ratio of 50% or more to obtain a rough-rolled plate, and then finish rolling the rough-rolled plate to obtain a hot-rolled plate. The hot-rolled plate is cold-rolled to produce a cold-rolled plate, and the following annealing is carried out: the cold-rolled plate is heated to a heating temperature of 800°C or higher, and cooled under the condition that the residence time t1 in the temperature range T1 from the Ms point to 700°C is 1000 s or less, then, the following partial tempering is carried out: the cold-rolled plate is cooled from the Ms point to 50°C or lower under the condition that the partial tempering parameter S satisfies the following formula 2, Formula 2: 0.01 ≤ S ≤ 30 Formula 3: Equation 4: f(t) = 1 - exp(-1.1×10 -2 (Ms - T min )) Formula 5: In the above formulas 2 to 5, Define the time when the temperature of the cold-rolled plate first reaches the Ms point after annealing as t = 0, and t = t E is the time when the partial tempering is completed and the temperature of the cold-rolled plate reaches 50 °C, T is the average temperature of the cold-rolled plate at time t - 1 to t, T min is the lowest temperature among the temperatures T from time 0 to t. Ms refers to the Ms point of the high-strength steel plate, The unit of S is μm 2 , the units of t and t E are s, and the units of T, T min and Ms are °C.

8. The manufacturing method of the high-strength steel plate according to claim 7, wherein, in the partial tempering, the cooling is stopped at a cooling stop temperature above room temperature and below the Ms point, then reheated to a reheating temperature, and then cooled to 50°C or lower.

9. The manufacturing method of the high-strength steel plate according to claim 7 or 8, wherein, in the annealing, a plating treatment is further carried out on at least one surface of the cold-rolled plate.

10. A method for manufacturing a component, comprising performing at least one of forming or joining on the high-strength steel sheet according to any one of claims 1 to 4 to manufacture the component.

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