เหล็กกล้าแผ่นบางทนแรงสูง, ชิ้นส่วน, และวิธีการผลิตสิ่งเหล่านี้

Abstract

DEPCT68 การประดิษฐ์ปัจจุบันจัดให้มีเหล็กกล้าแผ่นบางทนแรงสูง(highstrengthsteelsheet)ที่มี ความแข็งแรงสูงและความแข็งแรงของส่วนประกอบ,สภาพดึงยืดได้,สภาพยืดขยายขอบรูได้, สภาพดัดขึ้นรูปได้,และสภาพขึ้นรูปอุ่นได้ที่ดีมากเหล็กกล้าแผ่นบางทนแรงสูงมีองค์ประกอบ ทางเคมีที่มีอยู่ด้วย(ในหน่วยเปอร์เซ็นต์โดยมวล)C:0.030เปอร์เซ็นต์ถึง0.500เปอร์เซ็นต์,Si:0.01 เปอร์เซ็นต์ถึง2.50เปอร์เซ็นต์,Mn:0.10เปอร์เซ็นต์ถึง5.00เปอร์เซ็นต์,P:0.100เปอร์เซ็นต์หรือ น้อยกว่านั้น,S:0.0200เปอร์เซ็นต์หรือน้อยกว่านั้น,Al:1.000เปอร์เซ็นต์หรือน้อยกว่านั้น, N:0.0100เปอร์เซ็นต์หรือน้อยกว่านั้น,และO:0.0100เปอร์เซ็นต์หรือน้อยกว่านั้นโดยมี ส่วนที่เหลือคือFeและสารเจือปนที่เลี่ยงไม่ได้ที่ตำแหน่ง1 / 4ของความหนาแผ่นโครงสร้างจุลภาค ของเหล็กกล้ามีลักษณะที่เศษส่วนพื้นที่คือมาร์เทนไซต์60เปอร์เซ็นต์หรือมากกว่านั้น,เฟร์ไรต์40 เปอร์เซ็นต์หรือน้อยกว่านั้น,และออสเทไนต์เหลือค้าง20เปอร์เซ็นต์หรือน้อยกว่านั้น,และค่าคงตัว แลตทิซaMของมาร์เทนไซต์สอดคล้องกับนิพจน์ต่อไปนี้:1.00005น้อยกว่าหรือเท่ากับaM / aRน้อยกว่าหรือเท่ากับ1.00500โดยที่aRคือ ค่าคงตัวแลตทิซของมาร์เทนไซต์ที่อุณหภูมิห้องหลังจากกรรมวิธีทางความร้อนของเหล็กกล้า แผ่นบางทนแรงสูงที่500องศาเซลเซียสเป็นเวลา30นาที;

Description

High-strength steel plates, components and their manufacturing methods

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

[0002] CO2 emissions from lighter vehicles ２ With the aim of reducing emissions and improving crashworthiness by reducing the weight of the vehicle body, efforts are underway to increase the strength of steel sheets for automobiles. New automobile-related regulations are also being introduced one after another.

[0003] For the purpose of increasing the strength of the vehicle body, there have been an increasing number of cases in which high-strength cold-rolled steel sheets having a tensile strength (TS) of 980 MPa or more are used for the main structural components that form the framework of the automobile cabin. Patent Document 1 describes a high-strength cold-rolled steel sheet having a tensile strength of 1310 MPa or more. Furthermore, in order to increase the application rate of high-strength steel sheets to automobiles, in addition to the application of hot stamping, attention has been focused on warm working, which involves forming at lower heating temperatures. Patent Document 2 describes a high-strength steel sheet for warm working that has excellent workability even at low heating temperatures of about 50 to 200°C.

[0004] International Publication No. 2019 / 181950 International Publication No. 2017 / 131053

[0005] However, there has not been a high-strength steel sheet that has excellent ductility, stretch-flangeability, and bendability at room temperature, as well as 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 take into consideration warm workability at all. Furthermore, the high-strength steel sheet of Patent Document 2 has excellent warm workability (total elongation), but the breakdown of uniform elongation and local elongation in warm work is unclear. Furthermore, Patent Document 2 does not consider stretch-flangeability or bendability at room temperature.

[0006] The present disclosure has been made in view of the above circumstances, and aims to provide a high-strength steel plate, a member, and methods for manufacturing the same, which have a tensile strength of 980 MPa or more and excellent part strength, ductility, stretch flangeability, bendability, and warm workability.

[0007] The present inventors have conducted extensive research to achieve the above-mentioned object, and have found that the above-mentioned object can be achieved by adopting the following configuration, thereby completing the present disclosure.

[0008] [1] A steel sheet having a composition containing, by mass%, C: 0.030% or more and 0.500% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.10% or more and 5.00% or less, 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, with the balance being Fe and unavoidable impurities, and having, at a 1 / 4 position in the sheet thickness, an area fraction of martensite of 60% or more, an area fraction of ferrite of 40% or less, and an area fraction of retained austenite of 20% or less, and a lattice constant a of the martensite of Ｍ A high-strength steel plate having a steel structure in which (nm) satisfies the following formula 1: Formula 1: 1.00005≦a Ｍ / a Ｒ ≦1.00500 In Formula 1, a Ｒ (nm) is the lattice constant of the martensite at room temperature after the high-strength steel plate is heat-treated at 500°C for 30 minutes.

[0009] [2] A high-strength steel plate according to [1], in which 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.

[0010] [3] The component composition further contains at least one element selected from the group consisting of, in mass %, 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, and is the high-strength steel sheet according to [1] or [2].

[0011] [4] The high-strength steel sheet according to any one of [1] to [3], having a plating layer on at least one side.

[0012] [5] A member made by using the high-strength steel sheet according to any one of [1] to [4].

[0013] [6] The member according to [5], which is for an automotive skeletal structure part or a reinforcing part.

[0014] [7] On a steel slab having the component composition according to [1] or [3], rough rolling is performed at an average strain rate of 1×10 －４ / s or more and 1×10 －１ / s or less with a total reduction ratio of 50% or more to obtain a rough-rolled sheet, the rough-rolled sheet is subjected to finish rolling to obtain a hot-rolled sheet, the hot-rolled sheet is subjected to cold rolling to obtain a cold-rolled sheet, the cold-rolled sheet is heated to a heating temperature of 800°C or more, and annealing is performed by cooling under the condition that the residence time t1 in the temperature range T1 of Ms point or more and 700°C or less is 1000 s or less. Then, the cold-rolled sheet is subjected to partial tempering by cooling from the Ms point to 50°C or less under the condition that the partial tempering parameter S (μm ２ ) satisfies the following formula 2. This is a method for manufacturing a high-strength steel sheet. In the above formulas 2 to 5, after the annealing, the time when the temperature of the cold-rolled sheet first reaches the Ms point is defined as t = 0 (s), and t = t Ｅ(s) is the time when the partial tempering is completed and the temperature of the cold-rolled sheet reaches 50°C. T (°C) is the average temperature of the cold-rolled sheet from time t-1 to t (s). T ｍｉｎ (°C) is the lowest temperature among the temperatures T from time 0 to t (s). Ms (°C) indicates the Ms point of the high-strength steel plate.

[0015] [8] The method for producing a high-strength steel plate according to [7], wherein in the partial tempering, cooling is stopped at a cooling stop temperature of room temperature or higher and Ms point or lower, then reheating is performed to a reheating temperature, and then cooling is performed to 50°C or lower.

[0016] [9] The method for producing a high-strength steel sheet according to [7] or [8], wherein the annealing further comprises plating at least one surface of the cold-rolled sheet.

[0017]

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

[0018] According to the present disclosure, it is possible to provide a high-strength steel plate, a member, and methods for manufacturing the same, which have a tensile strength of 980 MPa or more and excellent part strength, ductility, stretch flangeability, bendability, and warm workability.

[0019] Fig. 1 is a diagram showing an example of an electron diffraction pattern of martensite containing carbides. Fig. 2 is a schematic diagram of the thermal history of a partial tempering process in which continuous cooling is performed from the Ms point to 50°C, and a schematic diagram of f(t). Fig. 3 is a schematic diagram of the thermal history when reheating is performed in partial tempering, and a schematic diagram of f(t).

[0020] Hereinafter, an embodiment of the present invention will be described. However, the present invention is not limited to the following embodiment. A high-strength steel plate according to this embodiment has a chemical composition and a steel structure described below. Hereinafter, the "high-strength steel plate" may also be simply referred to as a "steel plate."

[0021] The high-strength steel sheet according to this embodiment has a tensile strength of 980 MPa or more, and is excellent in part strength, ductility, stretch flangeability, bendability, and warm workability.

[0022] High strength means that the tensile strength (TS) determined by the tensile test described below is 980 MPa or more.

[0023] "Excellent part strength" means that the yield ratio (YR) determined by the tensile test described below is 60% or more.

[0024] "Excellent ductility" means that the total elongation (T-El) determined by the tensile test described below is 6.0% or more.

[0025] The term "excellent stretch flangeability" means that the hole expansion ratio (λ) determined by the hole expansion test described below is 30% or more.

[0026] The term "excellent bendability" means that the limiting bend radius (R / t) determined in a bending test of a sample having a ground end face, which will be described later, is 5.0 or less.

[0027] Excellent warm workability means that the uniform elongation (U-El) is obtained by a tensile test at room temperature, which will be described later. ＲＴ ) and local elongation (L-El ＲＴ ) and the uniform elongation (U-El) obtained by a tensile test at 200 ° C. ２００ ) and local elongation (L-El ２００ ) in U-El ２００ / U-El ＲＴ is 1.10 or more and L-El ２００ / L-El ＲＴ is 1.30 or more.

[0028] During warm working, it is necessary to increase the uniform elongation of the steel sheet relative to room temperature from the viewpoint of strain dispersion ability, and to increase the local elongation of the steel sheet relative to room temperature to suppress fracture in the worked portion. In warm working, it is assumed that the mold and steel sheet are heated to a predetermined temperature before working, but the edge portions of the steel sheet are easily cooled, and it is assumed that working is essentially performed at room temperature. Therefore, in addition to excellent warm workability, steel sheets are required to have good ductility, stretch-flangeability, and bendability even at room temperature. Furthermore, high-strength steel sheets used for automotive reinforcement parts and frame structural parts are required to have excellent part strength (high 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 [%]) of the steel sheet. Until now, there have been no high-strength steel sheets that have good ductility, stretch-flangeability, and bendability at room temperature and excellent warm workability. In conventional steel sheets containing a large amount of retained austenite, very hard fresh martensite is generated by punching, and it is expected that stretch flangeability will be poor from the viewpoint of the generation, growth, and connection of voids. As will be described later, the inventors have intensively studied and come up with the idea of ​​appropriately adjusting the partial tempering parameter S. As a result, the lattice constant a of martensite at room temperature can be adjusted. Ｍ and the lattice constant a at room temperature after the high-strength steel plate having martensite is heat-treated at 500 ° C. for 30 minutes. Ｒ Ratio a Ｍ / a Ｒ can be set to an appropriate value. Ｍ / a Ｒ By setting the value of β to an appropriate value, it is possible to provide a high strength steel sheet having good ductility, stretch flangeability, bendability and warm workability at room temperature.

[0029] <Composition> First, the appropriate range of the composition of the high-strength steel sheet and the reasons for limiting it will be described. In the following description, "%" representing the content of the component elements of the steel sheet means "mass%" unless otherwise specified.

[0030] C: 0.030% or more and 0.500% or less Carbon (C) is one of the important basic components of a steel sheet, and particularly in the high-strength steel sheet according to this embodiment, it affects the area fraction of martensite and the lattice constant of martensite. If the C content is too low, the area fraction of martensite decreases, making it difficult to achieve a TS of 980 MPa or more. In addition, the lattice constant a of martensite Ｍ The C content of the steel sheet is reduced, resulting in a deterioration in warm workability. Furthermore, the area ratio of ferrite increases. For this reason, the C content of the steel sheet is set to 0.030% or more. The C content is preferably 0.050% or more, and more preferably 0.090% or more. On the other hand, if the C content is too high, the amount of retained austenite increases excessively, significantly increasing the hardness of martensite formed from the retained austenite during punching. As a result, crack propagation during hole expansion is promoted, the hole expansion ratio decreases, and stretch flangeability deteriorates. Furthermore, stress-induced transformation of the retained austenite reduces the YR and reduces part strength. For this reason, the C content is set to 0.500% or less. The C content is preferably 0.400% or less, and more preferably 0.350% or less.

[0031] <<Si: 0.01% or More and 2.50% or Less>> Si increases the strength of steel sheets by suppressing cementite precipitation in martensite and by solid-solution strengthening. To achieve this effect, the Si content is set to 0.01% or more. The Si content is preferably 0.05% or more, more preferably 0.10% or more. On the other hand, if the Si content is too high, carbide precipitation during bainite transformation is significantly suppressed, retained austenite increases excessively, and the hardness of martensite formed from the retained austenite during punching increases significantly. As a result, crack propagation during hole expansion is promoted, the hole expansion ratio decreases, and stretch flangeability deteriorates. Furthermore, stress-induced transformation of retained austenite reduces the YR and reduces part strength. For this reason, the Si content is set to 2.50% or less. The Si content is preferably 2.00% or less, more preferably 1.50% or less.

[0032] <<Mn: 0.10% or More and 5.00% or Less>> Mn is one of the important basic components of steel sheet. It affects the area ratio of martensite, particularly in the high-strength steel sheet according to this embodiment. If the Mn content is too low, the area ratio of martensite decreases, making it difficult to achieve a TS of 980 MPa or more. Furthermore, the area ratio of martensite decreases, making it difficult to obtain high warm workability. For this reason, the Mn content is set to 0.10% or more. The Mn content is preferably 0.90% or more, more preferably 1.80% or more. On the other hand, if the Mn content is too high, austenite is stabilized and retained austenite increases excessively. Furthermore, the hardness of martensite formed from retained austenite during punching increases significantly. As a result, crack propagation during hole expansion is promoted, the hole expansion ratio decreases, and stretch flangeability deteriorates. Furthermore, stress-induced transformation of retained austenite reduces YR and component strength. For this reason, the Mn content is set to 5.00% or less. The Mn content is preferably 4.20% or less, and more preferably 3.60% or less.

[0033] <P: 0.100% or less> P segregates at prior austenite grain boundaries and embrittles the grain boundaries, reducing the ultimate deformability of the steel sheet, thereby decreasing λ. It also reduces bendability. Therefore, the P content must be 0.100% or less. While there is no particular lower limit for the P content, since P is a solid-solution strengthening element and can increase the strength of the steel sheet, the P content is preferably 0.001% or more. The P content is preferably 0.070% or less.

[0034] <<S: 0.0200% or less>> S exists as sulfides and reduces the ultimate deformability of the steel sheet, thereby reducing λ. It also reduces bendability. Therefore, the S content must be 0.0200% or less. While there is no particular lower limit for the S content, due to constraints on production technology, the S content is preferably 0.0001% or more. The S content is preferably 0.0050% or less.

[0035] <<Al: 1.000% or less>> Al performs sufficient deoxidation and reduces inclusions in the steel. However, if the Al content is too high, a large amount of ferrite is generated, the hole expansion ratio decreases, and stretch flangeability decreases. Therefore, the Al content is set to 1.000% or less. The Al content is preferably 0.500% or less, and more preferably 0.100% or less. On the other hand, in order to perform stable deoxidation, the Al content is preferably 0.010% or more, more preferably 0.015% or more, and even more preferably 0.020% or more.

[0036] <N: 0.0100% or less> N exists as nitrides and reduces the ultimate deformability of the steel sheet, thereby reducing λ. It also reduces bendability. Therefore, the N content is set to 0.0100% or less. While there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably set to 0.0001% or more. The N content is preferably set to 0.0050% or less.

[0037] <O: 0.0100% or less> O exists as an oxide and reduces the ultimate deformability of the steel sheet, thereby reducing λ. It also reduces bendability. Therefore, the O content is set to 0.0100% or less. While there is no particular lower limit for the O content, due to constraints on production technology, the O content is preferably set to 0.0001% or more. The O content is preferably set to 0.0050% or less.

[0038] The high-strength steel sheet according to this embodiment has a composition containing the above-mentioned components, with the balance being Fe and unavoidable impurities. Examples of the unavoidable impurities include Zn, Pb, As, Ge, Sr, and Cs. The total content of these impurities is permitted to be 0.100% or less.

[0039] In addition to the above-mentioned chemical composition, the high-strength steel plate according to this embodiment further contains, in mass %, 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, and Cu: 1.00% or less. In addition, at least one element selected from the group consisting of 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 may be contained alone or in combination.

[0040] If Ti, Nb, and V are each 0.200% or less, large amounts of coarse precipitates and inclusions are not formed, and the ultimate deformability of the steel sheet is not reduced, so λ is not reduced. Furthermore, bendability is not reduced. Therefore, when Ti, Nb, or V is contained in the steel sheet, the Ti, Nb, and V contents are preferably 0.200% or less. While there are no particular lower limits for the Ti, Nb, and V contents, these elements form fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, thereby increasing the strength of the steel sheet. Therefore, the Ti, Nb, and V contents are more preferably 0.001% or more. The Ti, Nb, and V contents are more preferably 0.100% or less.

[0041] If Ta and W are each 0.10% or less, large amounts of coarse precipitates and inclusions are not generated, and the ultimate deformability of the steel sheet is not reduced, so λ does not decrease. Furthermore, bendability does not decrease. Therefore, the Ta and W contents are preferably 0.10% or less. While there are no particular lower limits for the Ta and W contents, the Ta and W contents are more preferably 0.01% or more because they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. The Ta and W contents are more preferably 0.08% or less.

[0042] If B is 0.0100% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet will not be reduced, so λ will not be reduced. Furthermore, bendability will not be reduced. Therefore, the B content is preferably 0.0100% or less. While there is no particular lower limit for the B content, since B is an element that segregates to austenite grain boundaries during annealing and improves hardenability, the B content is more preferably 0.0003% or more. The B content is more preferably 0.0080% or less.

[0043] If Cr, Mo, and Ni are each 1.00% or less, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so λ does not decrease. Furthermore, bendability does not decrease. Therefore, the Cr, Mo, and Ni contents are preferably each 1.00% or less. While there are no particular lower limits for the Cr, Mo, and Ni contents, since these elements improve hardenability, the Cr, Mo, and Ni contents are more preferably 0.01% or more. The Cr, Mo, and Ni contents are more preferably 0.80% or less.

[0044] If Co is 0.010% or less, the amount of coarse precipitates and inclusions does not increase, and the ultimate deformability of the steel sheet is not reduced, so λ does not decrease. Furthermore, bendability is not reduced. Therefore, the Co content is preferably 0.010% or less. While there is no particular lower limit for the Co content, since Co is an element that improves hardenability, the Co content is more preferably 0.001% or more. The Co content is more preferably 0.008% or less.

[0045] If Cu is 1.00% or less, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so λ does not decrease. Furthermore, bendability does not decrease. Therefore, the Cu content is preferably 1.00% or less. While there is no particular lower limit for the Cu content, since Cu is an element that improves hardenability, the Cu content is more preferably 0.01% or more. The Cu content is more preferably 0.80% or less.

[0046] If the Sn content is 0.200% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet will not be reduced, so λ will not be reduced. Furthermore, bendability will not be reduced. Therefore, the Sn content is preferably 0.200% or less. While there is no particular lower limit for the Sn content, since Sn is an element that improves hardenability, the Sn content is more preferably 0.001% or more. The Sn content is more preferably 0.100% or less.

[0047] If Sb is 0.200% or less, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so λ does not decrease. Furthermore, bendability does not decrease. Therefore, the Sb content is preferably 0.200% or less. While there is no particular lower limit for the Sb content, since Sb is an element that controls the softened surface thickness and enables strength adjustment, the Sb content is more preferably 0.001% or more. The Sb content is more preferably 0.100% or less.

[0048] If the Ca, Mg, and REM contents are each 0.0100% or less, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so λ does not decrease. Furthermore, bendability does not decrease. Therefore, the Ca, Mg, and REM contents are preferably each 0.0100% or less. While there are no particular lower limits for the Ca, Mg, and REM contents, since these elements spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, the Ca, Mg, and REM contents are preferably each 0.0005% or more. It is more preferable that the Ca, Mg, and REM contents are each 0.0050% or less.

[0049] If Zr and Te are each 0.100% or less, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so λ does not decrease. Furthermore, bendability does not decrease. Therefore, the Zr and Te contents are preferably 0.100% or less. While there are no particular lower limits for the Zr and Te contents, since these elements spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, the Zr and Te contents are more preferably 0.001% or more. The Zr and Te contents are more preferably 0.080% or less.

[0050] If Hf is 0.10% or less, the amount of coarse precipitates and inclusions does not increase, and the ultimate deformability of the steel sheet is not reduced, so λ does not decrease. Furthermore, bendability does not decrease. Therefore, the Hf content is preferably 0.10% or less. Although there is no particular lower limit for the Hf content, since Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel sheet, the Hf content is preferably 0.01% or more. The Hf content is more preferably 0.08% or less.

[0051] If Bi is 0.200% or less, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so λ does not decrease. Furthermore, bendability does not decrease. Therefore, the Bi content is preferably 0.200% or less. Although there is no particular lower limit for the Bi content, since Bi is an element that reduces segregation, the Bi content is preferably 0.001% or more. The Bi content is more preferably 0.100% or less.

[0052] In addition, when the content of each of the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi is less than the preferable lower limit, the effect of the present invention is not impaired, and therefore, these elements are treated as unavoidable impurities.

[0053] Next, the steel structure of the high-strength steel plate will be described.

[0054] <<Area Fraction of Martensite: 60% or More>> By including martensite, a TS of 980 MPa or more and excellent warm workability can be achieved. By increasing the area fraction of martensite, as described below, the ductility improvement mechanism caused by the microscopic interaction between dislocations contained in large amounts in martensite and properly controlled solute C can be expressed as elongation during macroscopic warm working. Therefore, the area fraction of martensite is set to 60% or more. The area fraction of martensite is preferably 70% or more, and more preferably 90% or more. There is no particular upper limit to the area fraction of martensite, and the above-described effects can be obtained even if the area fraction of martensite is 100%.

[0055] Martensite is a transformation phase that forms below the Ms point, regardless of whether tempering is performed. Martensite also includes lower bainite that forms below the Ms point. The martensite observation position is set at a quarter position in the thickness direction of the steel plate, as described below.

[0056] <<Ferrite Area Ratio: 40% or Less>> By setting the ferrite area ratio to 40% or less, the desired strength and stretch flangeability can be obtained. The effect can be obtained even if the ferrite area ratio is 0%. If the ferrite content is too high, a sufficient amount of martensite cannot be secured, and the desired TS cannot be obtained. Furthermore, the hardness difference between the structures increases, void generation and connection are promoted, the hole expansion ratio decreases, and the stretch flangeability deteriorates. Therefore, the ferrite area ratio is set to 40% or less. The ferrite area ratio is preferably 30% or less, and more preferably 20% or less.

[0057] Ferrite is soft BCC iron formed at temperatures higher than the Ms point, and includes allotriomorph ferrite, idiomorph ferrite, and upper bainite. The observation position for ferrite is set at a position corresponding to 1 / 4 of the sheet thickness of the steel sheet (a position corresponding to 1 / 4 of the sheet thickness in the depth direction from the surface of the steel sheet), as described below.

[0058] The method for measuring the area ratios of martensite and ferrite is as follows. First, a sample is cut out from the steel sheet so that the plate thickness cross section (L cross section at 1 / 4 of the plate thickness) parallel to the rolling direction serves as the observation surface. The observation surface of the sample is mirror-polished using diamond paste, then finish-polished using colloidal silica, and further etched using 1 volume % nital to reveal the structure. Next, the observation surface of the sample is observed at 3000x magnification using a scanning electron microscope (SEM) under an acceleration voltage of 10 kV, and SEM images of three fields of view are obtained. From the obtained SEM images, the area ratio of each structure is calculated using Adobe Photoshop (manufactured by Adobe Systems). Specifically, the value obtained by dividing the area of ​​each structure by the measured area is taken as the area ratio of each structure. The area ratio of each structure is calculated for three fields of view, and their average value is taken as the area ratio of each structure. In SEM images, ferrite appears as a gray, flat structure with no carbides, showing white contrast. In contrast, martensite has a hierarchical structure with fine internal irregularities. Therefore, martensite and ferrite can be distinguished from each other.

[0059] <<Area Fraction of Retained Austenite: 20% or Less>> By setting the area fraction of retained austenite to 20% or less, good component strength and stretch flangeability can be obtained. Therefore, the area fraction of retained austenite is set to 20% or less. The area fraction of retained austenite is preferably 15% or less. There is no particular lower limit for the area fraction of retained austenite, and even if it is 0%, the above-mentioned effects can be obtained.

[0060] The method for measuring the area fraction of retained austenite is as follows. First, a steel sheet is ground so that the 1 / 4 position of the sheet thickness becomes the measurement surface, and then the steel sheet is polished by an additional 0.1 mm by chemical polishing to obtain a sample. For the measurement surface of the sample, an X-ray diffractometer is used with a Co Kα radiation source to measure 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. The intensity ratio of the integrated reflection intensity of each plane of fcc iron to the integrated reflection intensity of each plane of bcc iron is calculated. The average value of the nine intensity ratios is taken as the volume fraction of retained austenite. The volume fraction of retained austenite is considered to be the area fraction of retained austenite.

[0061] <Lattice constant of martensite> The lattice constant of martensite is an extremely important structural factor in the present invention. The lattice constant a of martensite at room temperature is Ｍ When the a (nm) satisfies the formula 1, a high-strength steel sheet excellent in ductility, bendability, and warm workability at room temperature can be obtained. Formula 1: 1.00005≦a Ｍ / a Ｒ ≦1.00500 In Formula 1, a Ｒ (nm) is the lattice constant of martensite at room temperature after the high-strength steel plate is heat-treated at 500°C for 30 minutes.

[0062] The inventors have determined that 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 plate having martensite at 500 ° C. for 30 minutes. Ｒ Ratio a Ｍ / a Ｒ It has been discovered that by setting the lattice constant a of martensite to an appropriate value, it is possible to obtain a steel sheet with excellent ductility at room temperature and warm workability. It is presumed that by having an appropriate amount of solute C in martensite, which contains a large amount of dislocations responsible for plastic deformation, it is possible to improve both uniform elongation and local elongation during warm working through the interaction between the dislocations and solute C. Ｍis affected by not only C, an interstitial solid solution element, but also Si and Mn, which are substitutional solid solution elements. By heat treating a high-strength steel sheet at 500°C for 30 minutes, the C dissolved in martensite precipitates as cementite, and the concentration of dissolved C becomes very low. Therefore, the lattice constant a of martensite at room temperature Ｍ and the lattice constant a at room temperature after heat treatment of a high-strength steel plate having martensite at 500°C for 30 minutes. Ｒ Ratio a Ｍ / a Ｒ is a parameter that accurately represents the concentration of solute C in martensite. The concentration of solute C in martensite affects the ductility and warm workability at room temperature. Ｍ / a Ｒ By controlling the temperature so as to satisfy the formula 1, it is possible to obtain a high strength steel sheet having excellent ductility, bendability and warm workability at room temperature.

[0063] a Ｍ / a Ｒ If a is less than 1.00005, the concentration of solute C in martensite is low, and the interaction between dislocations in martensite and solute C during warm working is reduced, resulting in a deterioration in warm workability. Ｍ / a Ｒ If the lattice constant a of martensite exceeds 1.00500, the movement of dislocations at room temperature is significantly suppressed, and the ductility and bendability at room temperature are reduced. Ｍ is 1.00005≦a Ｍ / a Ｒ It is necessary to satisfy the following condition: a≦1.00500. Ｍ / a Ｒ is 1.00010 or more. Ｍ / a Ｒ is less than or equal to 1.00200.

[0064] The lattice constant of martensite is determined by X-ray diffraction. The high-strength steel plate is ground so that the measurement surface is at 1 / 4 of the plate thickness, and then further polished by 0.1 mm by chemical polishing to obtain a sample. For the measurement surface of the sample, an X-ray diffractometer is used with a Cu Kα radiation source to calculate the peak positions by fitting the peaks of the (110), (200), (211), (220), (310), and (222) planes of bcc iron using a pseudo-Voigt approximation. From the obtained peak positions, the lattice constant is calculated from Bragg's law, assuming that c / a is 1, and the average value is used to determine the lattice constant a of martensite. Ｍ The lattice constant a of martensite is also determined for the high-strength steel sheet after heat treatment at 500°C for 30 minutes, similarly to the high-strength steel sheet. Ｒ Ask for.

[0065] <<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>> 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, warm workability can be further improved. Carbides with a major axis of 200 nm or more present in martensite blocks are thermally stable from the standpoint of interface energy, and they tend to remove surrounding solute C during warm working, making it easier for the solute C to decrease locally. By reducing the number of martensite blocks containing carbides with a major axis of 200 nm or more, uniform elongation and local elongation 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 even more preferably 10% or less. The lower limit of the ratio of the number of martensite blocks containing carbides having a major axis of 200 nm or more to the number of martensite blocks containing carbides is not particularly limited, and may be 0%. The carbides contained in the martensite blocks are at least one type of carbide selected from cementite, epsilon carbide, eta carbide, and chi carbide.

[0066] 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 measured as follows. First, a steel sheet is ground so that the observation surface is at 1 / 4 of the sheet thickness, and then electropolished to prepare a sample. The observation surface of the prepared sample is observed using a transmission electron microscope (TEM) at an accelerating voltage of 200 kV. Since the dislocation density of martensite is significantly higher than that of ferrite and retained austenite, they can be distinguished from each other by observing the strain contrast in a TEM bright-field image. A martensite block is a hierarchical unit that constitutes martensite and is a group of laths with the same habit plane and the same crystal orientation. Therefore, in a TEM bright-field image, the diffraction contrast of each block within martensite is different, allowing it to be distinguished from other hierarchical structures such as packets and laths. An electron beam incident from the

[100] direction of the martensite block provides an electron diffraction pattern of the parent martensite. Adjacent martensite blocks have different crystal orientations across the block boundaries, and are therefore distinguishable from one another by the contrast differences in the bright-field images.

[0067] Figure 1 shows an example of an electron diffraction pattern of martensite containing carbides. When carbides are present in a single martensite block observed, an electron diffraction pattern of the carbides is obtained in addition to the electron diffraction pattern of the parent martensite (α), as shown in Figure 1. In Figure 1, the black circles indicate electron diffraction spots of the parent martensite when the electron beam is incident from the

[100] direction. Furthermore, the white circles indicate electron diffraction spots of the carbides. When such diffraction patterns of the parent martensite and carbides are obtained, the martensite block is considered to contain carbides. A dark-field image is obtained using the electron diffraction spots obtained from the carbides. In the dark-field image, metastable carbides exhibit white contrast.

[0068] The major axis of the obtained carbides is determined from the dark-field image. If the major axis of the largest carbide present in the martensite block is 200 nm or more, the martensite block is determined to contain carbides with a major axis of 200 nm or more. 50 martensite blocks are observed. The number of martensite blocks containing carbides with a major axis of 200 nm or more is divided by the number of martensite blocks containing carbides, and the result is multiplied by 100 to obtain the value ((number of martensite blocks containing carbides with a major axis of 200 nm or more) / (number of martensite blocks containing carbides) × 100). The obtained value is defined as 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.

[0069] <<Remaining Structure>> The structure of the steel sheet may have a structure (remaining structure) other than the above-mentioned martensite, ferrite, and retained austenite. However, the area ratio of the remaining structure is preferably 3% or less in terms of area ratio, so as not to impair the effects of the present invention. Examples of the remaining structure include pearlite, alloy carbonitrides precipitated in ferrite, and other structures known as the structure of steel sheet. Note that iron-based carbides present in martensite are not included in the remaining structure.

[0070] The area ratio of the remaining structure is calculated using Adobe Photoshop (manufactured by Adobe Systems) from the SEM image taken when measuring the area ratios of martensite and ferrite. Specifically, the value obtained by dividing the area of ​​the remaining structure by the measured area is taken as the area ratio of the remaining structure. The area ratios of the remaining structure are calculated for three fields of view, and their average value is taken as the area ratio of the remaining structure.

[0071] In the SEM image, pearlite appears as a lamellar structure consisting of cementite, which has a white contrast, and ferrite, which has a gray contrast, and alloy carbonitride appears as an angular structure, which has a dark contrast.

[0072] The thickness of the high-strength steel plate is not particularly limited, and may usually be 0.3 mm or more and 2.8 mm or less.

[0073] <Plated Layer> The high-strength steel sheet may have a plated layer on its surface. The plated layer is formed by a plating treatment described below. The plated layer is not particularly limited, and examples thereof include a hot-dip plated layer and an electroplated layer. The plated layer may be an alloyed plated layer (alloyed plated layer).

[0074] Examples of the plating layer include a zinc plating layer (Zn plating layer) and an Al plating layer. The plating layer is preferably a zinc plating layer. The zinc plating layer may contain elements such as Al and Mg.

[0075] The composition of the plating layer is not particularly limited and may be a general composition. For example, when the plating layer is a hot-dip galvanized layer or a galvannealed hot-dip galvanized layer, the plating layer generally contains 20 mass % or less of Fe, 0.001 to 1.0 mass % of Al, and further contains 0 mass % or more and 3.5 mass % or less of at least one element selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, with the balance being Zn and unavoidable impurities.

[0076] When the plating layer is a hot-dip galvanized layer, the coating weight of the plating layer per side is 20 g / m ２ It is preferable that the density is 80 g / m or more. ２ Furthermore, a galvannealed layer obtained by alloying a galvanized layer having such a coating amount is also preferred.

[0077] When the plating layer is a hot-dip galvanized layer, the Fe content in the plating layer is preferably less than 7 mass%. When the plating layer is a galvannealed hot-dip galvanized layer, the Fe content in the plating layer is preferably 7 mass% or more. Furthermore, when the plating layer is a galvannealed hot-dip galvanized layer, the Fe content in the plating layer is preferably 20 mass% or less, more preferably 15 mass% or less.

[0078] [Member] Next, a member according to one embodiment of the present invention will be described. The member is made using the high-strength steel sheet according to one embodiment of the present invention described above. The member is, for example, formed into a desired shape by forming or joining the high-strength steel sheet according to one embodiment of the present invention described above. The member according to one embodiment of the present invention is preferably a member for an automobile frame structural part or an automobile reinforcing part. Here, the high-strength steel sheet according to one embodiment of the present invention described above is a high-strength steel sheet having a tensile strength of 980 MPa or more and excellent in part strength, ductility, stretch flangeability, bendability, and warm workability. Therefore, the member according to one embodiment of the present invention can be suitably used in general as a member for an automobile frame structural part or an automobile reinforcing part.

[0079] [Method for manufacturing high-strength steel sheet] Next, a method for manufacturing a high-strength steel sheet will be described. First, a steel material having the above-described component composition is melted to manufacture a steel slab. The method for melting molten steel to become the steel slab (steel material) is not particularly limited, and known melting methods using a converter, electric furnace, or the like can be adopted. The steel slab is preferably manufactured by a continuous casting method to prevent macrosegregation, but can also be manufactured by other methods such as an ingot casting method or a thin slab casting method. The high-strength steel sheet according to this embodiment includes a cold-rolled steel sheet (cold-rolled sheet) manufactured by hot rolling, pickling, cold rolling, and annealing, and a high-strength steel sheet obtained by plating a cold-rolled steel sheet.

[0080] The steel slab is then hot-rolled to produce a hot-rolled sheet. In one example, the steel slab is cooled to room temperature, then reheated, and hot-rolled (rough rolling and finish rolling). The produced steel slab may be charged into a heating furnace as a hot strip without being cooled to room temperature, or may be briefly held at room temperature and then immediately rough-rolled.

[0081] <Rough Rolling> A rough rolled sheet is obtained by rough rolling a steel slab under the following conditions. The temperature at which the steel slab is heated (slab heating temperature) is preferably 1100°C or higher from the viewpoint of dissolving carbides and reducing the rolling load. On the other hand, in order to prevent an increase in scale loss, the slab heating temperature is preferably 1300°C or lower. The slab heating temperature is based on the surface temperature of the steel slab. Next, the steel slab heated to the slab heating temperature is subjected to rough rolling under the following conditions.

[0082] 《Average strain rate is 1×10 －４ / s or more 1×10 －１ / s or less, total reduction of 50% or more》 Carbon, an interstitial solid solution element in steel, interacts with Si and Mn, which are substitutional solid solution elements. In other words, solute atoms such as Si and Mn affect the solid solution C in martensite. By optimizing the average strain rate and total reduction during rough rolling, the lattice constant a of martensite in the final structure can be reduced. Ｍ The inventors consider the reason for this to be as follows: It is believed that solute atoms such as Si and Mn are distributed appropriately by high-speed diffusion through dislocations and grain boundaries of recrystallized grains during the plastic deformation and dynamic recrystallization process of austenite grains during rough rolling. By appropriately distributing solute atoms such as Si and Mn, the amount of dissolved C in martensite becomes uniform in the partial tempering treatment described later, and the lattice constant a of martensite in the final structure is reduced. Ｍ The inventors believe that the lattice constant a of martensite can be optimized. Ｍ By optimizing the above, it is possible to obtain a high strength steel sheet having excellent warm workability, as described above.

[0083] The average strain rate during rough rolling is calculated by multiplying the rolling rate ε(-) from the first mill to the final mill of rough rolling by the time t required from the start of rolling at the first mill to the completion of rolling at the final mill. Ｒ (ε / t Ｒ ) is defined as

[0084] The average strain rate during rough rolling is 1 × 10 －１If the ratio exceeds / s, the diffusion of solute atoms such as Si and Mn during the plastic deformation and dynamic recrystallization of austenite grains becomes insufficient, and the ratio of the lattice constants a Ｍ / a Ｒ On the other hand, when the average strain rate during rough rolling is 1 × 10 －４ When the ratio is less than / s, the recovery of dislocations in austenite grains is promoted, the driving force for recrystallization is reduced, and dynamic recrystallization is suppressed. This results in insufficient diffusion of solute atoms such as Si and Mn, and the ratio of the Ms lattice constants a Ｍ / a Ｒ Therefore, the average strain rate during rough rolling is 1×10 －４ / s or more 1×10 －１ The average strain rate during rough rolling is preferably 1×10 －３ The average strain rate during rough rolling is preferably 1×10 －２ / s or less.

[0085] If the total reduction rate of rough rolling is less than 50%, the diffusion of solute atoms such as Si and Mn during plastic deformation and dynamic recrystallization of austenite grains becomes insufficient, and the ratio of lattice constants a Ｍ / a Ｒ Therefore, the total reduction ratio of rough rolling is set to 50% or more. The total reduction ratio of rough rolling is preferably 60% or more. On the other hand, the total reduction ratio of rough rolling is preferably 90% or less.

[0086] The rolling end temperature of the rough rolling is not particularly limited, but is preferably 950°C or higher from the viewpoint of completing the recrystallization of austenite grains.

[0087] Next, the rough rolled sheet is subjected to finish rolling to obtain a hot rolled sheet. The hot rolled sheet is then appropriately coiled. When the slab heating temperature is set low, it is preferable to heat the rough rolled sheet using a bar heater or the like before finish rolling in order to prevent problems in the hot rolling. The temperature when performing finish rolling (finish rolling temperature) is preferably 700°C or higher. This reduces the rolling load. Furthermore, the reduction ratio in the unrecrystallized austenite state is reduced, suppressing the development of abnormal structures elongated in the rolling direction, and improving workability.

[0088] The finish rolling may be performed continuously by joining the rough rolled sheets together, or the rough rolled sheets may be temporarily wound up before the finish rolling is performed.

[0089] In order to reduce the rolling load, part or all of the finish rolling may be performed as lubricated rolling. Lubricated rolling is also preferred from the viewpoint of uniforming the shape and material properties of the steel sheet. The coefficient of friction during lubricated rolling is preferably 0.10 or more and 0.25 or less.

[0090] The coiling temperature after hot rolling is preferably 300°C or higher and 700°C or lower, from the viewpoint of improving the sheet passing properties during cold rolling and annealing, which will be described later.

[0091] Next, the hot-rolled sheet obtained by hot rolling is appropriately pickled. Pickling removes oxides from the surface of the hot-rolled sheet, and the final product, a high-strength steel sheet, has excellent chemical conversion treatability and coating layer quality. Pickling may be performed once or multiple times.

[0092] The hot-rolled sheet after pickling is optionally subjected to a softening heat treatment and then cold-rolled to obtain a cold-rolled sheet. The cold-rolling conditions are not particularly limited, but the cumulative reduction in cold rolling is preferably 20 to 75%. The number of rolling passes and the reduction in each pass are not particularly limited.

[0093] The cold-rolled sheet thus obtained is subjected to annealing as described below.

[0094] <Annealing> <Heating temperature: 800°C or higher> If the heating temperature is too low, the reverse transformation to austenite does not proceed sufficiently, the area ratio of martensite decreases, and the desired TS cannot be obtained. In addition, excellent warm workability cannot be obtained because there is little martensite that contributes to warm workability. For this reason, the heating temperature is set to 800°C or higher. Furthermore, the heating temperature is preferably 830°C or higher. There is no particular upper limit to the heating temperature, but from the viewpoint of operability, etc., the heating temperature is preferably 1000°C or lower. The heating temperature is based on the surface of the steel sheet.

[0095] The time for heating the cold-rolled sheet at the heating temperature (heating time) is not particularly limited, but if it is too short, the reverse transformation to austenite may not proceed sufficiently. Therefore, the heating time is preferably 30 seconds or more, more preferably 60 seconds or more. The upper limit of the heating time is not particularly limited, and for example, the heating time is preferably 6000 seconds or less, more preferably 3000 seconds or less. Here, "s" means seconds.

[0096] Cooling under conditions where the residence time t1 in the temperature range T1 from the Ms point to 700°C is 1000 seconds or less: The cold-rolled sheet cooled after heating 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, excessive ferrite transformation occurs, the area ratio of ferrite increases, and the desired TS and good stretch flangeability cannot be obtained. Therefore, the residence time t1 is set to 1000 seconds or less. The residence time t1 is preferably 800 seconds or less, more preferably 600 seconds or less. The lower limit of the residence time t1 is not particularly limited, but from the viewpoint of reducing the burden of capital investment, the residence time t1 is preferably 1 second or more, more preferably 5 seconds or more, and even more preferably 10 seconds or more. The temperature range T1 is based on the surface of the steel sheet.

[0097] The Ms point is calculated by the following formula (1): Ms = 499 - 317 [% C] - 11 [% Si] - 33 [Mn] - 17 [% Ni] - 28 [% Cr] - 11 [% Mo] (1) where [% M] indicates the M content (mass%) in the steel.

[0098] After the annealing, the cold-rolled sheet is subjected to a partial tempering step in which the sheet is cooled from the Ms point to 50° C. or less.

[0099] Partial tempering parameter S (μm ２ ) satisfies formula 2. In the above equations 2 to 5, the time when the temperature of the cold-rolled sheet first reaches the Ms point after the annealing is defined as t = 0 (s), and t = t Ｅ (s) is the time when the partial tempering process is completed and the temperature of the cold-rolled sheet reaches 50°C. T (°C) is the average temperature of the cold-rolled sheet from time t-1 to t (s). T ｍｉｎ(°C) is the lowest temperature among the temperatures T from time 0 to t (s). Ms (°C) refers to the Ms point of the high-strength steel plate. Note that the various temperatures in partial tempering are based on the surface of the steel plate.

[0100] After passing through the temperature region T1, the steel sheet reaches the Ms point and undergoes martensitic transformation. After the martensitic transformation, a tempering phenomenon occurs in which solute C precipitates as carbides, reducing the amount of solute C. Therefore, in the temperature region after the Ms point is reached for the first time, tempering of martensite always progresses, reducing the amount of solute C. Therefore, the inventors have discovered that it is necessary to consider the martensite fraction already formed at that temperature, the temperature, and the time throughout the entire region from the first time the Ms point is reached to 50°C, at which tempering can be ignored. Therefore, the inventors have defined a process of tempering that takes into account the martensite fraction already formed at that temperature as a partial tempering process.

[0101] The inventors have defined a partial tempering parameter S and have determined that by properly controlling the value of S, the ratio of the lattice constants a Ｍ / a Ｒ It has been found that the parameter S can be set within a predetermined range. The parameter S is a value obtained by integrating the tempering amount of already formed martensite per unit time, which is a combination of the volume fraction f(t) of martensite at the temperature T (°C) at time t (s) and the diffusion coefficient D(T) of C, over the thermal history from when the Ms point is first reached to 50°C, at which C diffusion can be ignored. After annealing, the time when the cold-rolled sheet first reaches the Ms point is defined as t = 0 (s), and Ｅ (s) is the time when partial tempering is completed and the temperature reaches 50°C. T (°C) is the average temperature of the cold-rolled sheet from time t-1 to t (s). T ｍｉｎ (°C) is the lowest temperature among the temperatures T from time 0 to t (s). Ms (°C) indicates the Ms point of the high-strength steel plate.

[0102] S (μm ２ If the ratio a) is too small, the partial tempering process will be insufficient, and the ratio a Ｍ / a Ｒ Therefore, the ductility and bendability at room temperature decrease. ２ ) is 0.01 or more.２ ) is preferably 0.03 or more, more preferably 0.10 or more. ２ If the ratio a) is too large, the partial tempering treatment becomes excessive, and the ratio a Ｍ / a Ｒ Therefore, S (μm ２ ) is 30 or less. S (μm ２ ) is preferably 15 or less, more preferably 5 or less.

[0103] 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 may have the temperature history shown in FIG. 2 or FIG.

[0104] FIG. 2 shows a schematic diagram of the thermal history of the partial tempering process in which continuous cooling is performed from the Ms point to 50°C or less, and a schematic diagram of f(t). In this example, the temperature history below the Ms point involves continuous cooling from the Ms point to 50°C or less. The cooling method is not particularly limited, but known cooling methods such as gas cooling can be used. The cooling rate when performing continuous cooling is not particularly limited, but can be, for example, in the range of 1°C / s to 100°C / s.

[0105] As shown in Figure 2, the time when the Ms point is reached for the first time after annealing is defined as t = 0 (s). When continuous cooling is performed as shown in Figure 2, t = t Ｅ (s) is the time when the temperature reaches 50°C for the first time after the annealing. t = t a When T ｍｉｎ (°C) is the lowest temperature among the temperatures T from time 0 to t (s), that is, t = t a This is the temperature at

[0106] <In the partial tempering step, cooling is stopped at a cooling stop temperature between room temperature and the Ms point, then reheating is performed to the reheating temperature, and then cooling to 50°C or less (preferred conditions)> In the partial tempering step, cooling may be stopped at any temperature between the Ms point and room temperature, then reheating is performed to the reheating temperature, and then cooling to 50°C or less. This reduces the hardness difference between the structures, and further improves bendability and stretch flangeability. Figure 3 shows a schematic diagram of the thermal history of the partial tempering step in which cooling is stopped at a temperature between the Ms point and room temperature, then heating is performed, and then cooling to 50°C or less. t = t ｂ In the example of FIG. ｍｉｎ (°C) is the cooling stop temperature. ｍｉｎ If the temperature (°C) falls below 50°C, the time when the steel plate reaches 50°C again after reheating is defined as t = t Ｅ (s) is defined as follows.

[0107] After annealing, the cold-rolled sheet is cooled to a cooling stop temperature between room temperature and the Ms point. The cooling stop temperature is not particularly limited, but is preferably 250°C or less in order to sufficiently generate martensite, promote partial tempering, and improve warm workability. The cooling stop temperature may be, for example, room temperature. The cooling method is also not particularly limited, and gas cooling, water cooling, or the like can be used. The cooling rate is not particularly limited, but can be, for example, in the range of 1°C / s to 100°C / s.

[0108] The cold-rolled sheet is then reheated to a reheating temperature. The heating method for reheating to the reheating temperature is not particularly limited, but for example, electromagnetic induction heating (IH) can be used.

[0109] The reheating temperature is preferably 500°C or less because it suppresses the recovery phenomenon of dislocations in martensite and provides favorable strength. The reheating temperature is preferably 130°C or more because it promotes the diffusion of C, promotes partial tempering in a short time, and provides favorable ductility and bendability. After reheating, the temperature may be held at a constant temperature until cooling.

[0110] Next, the cold-rolled sheet is cooled to 50°C or less. The cooling method is not particularly limited, and known methods such as water cooling or gas cooling can be used. The cooling rate is not particularly limited, and can be, for example, in the range of 1°C / s to 100°C / s. The cold-rolled sheet that has reached 50°C or less is cooled to room temperature by any cooling method. The cooling method is not particularly limited, and known methods such as gas cooling, air cooling, and water cooling can be used.

[0111] The partially tempered cold-rolled steel sheet is then cooled to room temperature, thereby obtaining the high-strength steel sheet (cold-rolled steel sheet) described above.

[0112] In the present production method, when a plating treatment described below is carried out, the resulting high-strength steel sheet is a plated steel sheet having a plating layer.

[0113] As long as the series of heat treatments in this manufacturing method satisfies the above-mentioned thermal history, other conditions are not particularly limited, and the equipment for carrying out the heat treatments is also not particularly limited.

[0114] <Plating Treatment> In the present manufacturing method, the cold-rolled steel sheet may be subjected to a plating treatment. Examples of plating treatments include hot-dip galvanizing (a treatment for forming a hot-dip galvanized layer) and galvannealing (a treatment for forming a hot-dip galvanized layer by performing an alloying treatment after hot-dip galvanizing). An electroplating layer may also be formed by electroplating.

[0115] When hot-dip galvanizing is performed, it is preferable to immerse the cold-rolled sheet in a galvanizing bath and then adjust the coating weight of the coating layer by gas wiping, etc. The bath temperature of the galvanizing bath is not particularly limited, but is preferably 440°C or higher and 500°C or lower.

[0116] The Al content of the zinc plating bath is preferably 0.10 mass % or more and 0.23 mass % or less.

[0117] The galvanizing treatment is preferably carried out while the steel sheet is held in the temperature range T1 of not less than the Ms point and not more than 700°C during the above-mentioned annealing.

[0118] The alloying temperature is preferably 470°C or higher to improve the Zn-Fe alloying rate and productivity. Furthermore, the alloying temperature is preferably 600°C or lower, more preferably 560°C or lower, to effectively prevent untransformed austenite from transforming into pearlite and improve TS. The alloying temperature is based on 530°C.

[0119] After the plating treatment, the plated steel sheet may be subjected to skin-pass rolling. The reduction ratio of the skin-pass rolling is preferably 0.05% or more from the viewpoint of increasing the yield strength. The upper limit of the reduction ratio is not particularly limited, but is preferably 1.50% or less from the viewpoint of productivity. The skin-pass rolling may be performed online or offline. Skin-pass rolling at the desired reduction ratio may be performed all at once, or may be performed in several steps.

[0120] From the viewpoint of productivity, the series of treatments such as the annealing and plating treatments described above are preferably carried out in a CAL (Continuous Annealing Line) or a CGL (Continuous Galvanizing Line).

[0121] The manufacturing conditions other than those mentioned above can be the same as those in the ordinary method.

[0122] [Method for manufacturing a member] A member can be manufactured by subjecting the high-strength steel plate described above to at least one of forming and joining. The forming and joining can be carried out by conventional methods.

[0123] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the examples described below.

[0124] <Production of Steel Sheet> Molten steel having the composition shown in Table 1 below (the balance consisting of Fe and unavoidable impurities) was melted in a converter and a steel slab was obtained by continuous casting.

[0125] The obtained steel slab was subjected to hot rolling to obtain a hot-rolled sheet. Specifically, the steel slab was heated to 1250 ° C and rough rolled at the average strain rate and total reduction shown in Table 2, then finish-rolled at a finish rolling temperature of 900 ° C, coiled at 500 ° C, and then cooled to room temperature to obtain a hot-rolled sheet. The obtained hot-rolled sheet was pickled, then softened at 500 ° C, and then cold-rolled at a rolling reduction of 50%. In this way, a cold-rolled sheet having a thickness of 1.6 mm was obtained.

[0126] The obtained cold-rolled steel sheet was subjected to annealing and partial tempering treatment under the conditions shown in Table 2 below to obtain a high-strength steel sheet (cold-rolled steel sheet) of the present invention. In the partial tempering treatment, when continuous cooling was performed from below the Ms point to below 50°C, "continuous" was recorded in the "Thermal history during partial tempering" column of Table 2. In addition, when cooling was stopped at a temperature from below the Ms point to room temperature, then reheating was performed, and then cooling to below 50°C, "reheating" was recorded in the "Thermal history during partial tempering" column of Table 2. The heating time at the heating temperature of the cold-rolled sheet was 200 s.

[0127] <<Plating Treatment>> Some cold-rolled sheets were subjected to hot-dip galvanizing treatment while retained in the temperature range T1 (Ms°C or higher and 700°C or lower) to form a coating layer (hot-dip galvanized layer) on both sides. That is, hot-dip galvanized steel sheets (GI) were obtained. For the hot-dip galvanizing treatment, a hot-dip galvanizing bath (bath temperature: 470°C) containing 0.20 mass% Al, with the balance being Zn and unavoidable impurities, was used. The coating weight of the hot-dip galvanized layer per side was 45 to 72 g / m ２ The composition of the formed hot-dip galvanized layer contained 0.1 to 1.0 mass % of Fe, 0.2 to 1.0 mass % of Al, and the balance being Fe and unavoidable impurities.

[0128] Another part of the cold-rolled sheet was subjected to a galvannealing treatment while the sheet was held in the temperature range T1 (Ms°C or higher and 700°C or lower) during annealing, to form a coating layer (galvannealed layer) on both sides. That is, a galvannealed steel sheet (GA) was obtained. For the galvannealing treatment, a hot-dip galvanizing bath (bath temperature: 470°C) containing 0.14 mass% Al, with the balance being Zn and unavoidable impurities, was used. The alloying treatment was carried out at 550°C. The coating weight of the galvannealed layer per side was 45 g / m ２ The composition of the formed alloyed hot-dip galvanized layer contained 7 to 15 mass % of Fe, 0.1 to 1.0 mass % of Al, and the balance being Fe and unavoidable impurities.

[0129] In the "Type of plating" column of Table 2 below, "GI" is entered when a hot-dip galvanized layer was formed, "GA" is entered when a galvannealed hot-dip galvanized layer was formed, and "CR" is entered when no plating layer was formed.

[0130] <Observation of Steel Structure> The area ratios of martensite, ferrite, and retained austenite, the lattice constant of martensite, the ratio of the number of martensite blocks containing carbides having 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 for the obtained steel sheets according to the methods described above. The results are shown in Table 3 below.

[0131] <Evaluation> The obtained steel sheets were subjected to the tests described below to evaluate various properties. The results are shown in Table 3 below.

[0132] Tensile Test The tensile test was carried out in accordance with JIS Z 2241:2021. Specifically, a JIS No. 5 test piece was taken from the obtained steel sheet so that the longitudinal direction was perpendicular to the rolling direction of the steel sheet. The taken test piece was used to perform a tensile test at a crosshead speed of 1.67 × 10 －１A tensile test was conducted under the condition of 100 mm / s to measure the yield strength (YS) [MPa], tensile strength (TS) [MPa], and total elongation (El) [%]. Furthermore, the yield ratio (YR) (= 100 × YS / TS) [%] was calculated. When the tensile strength (TS) was 980 MPa or more, it was judged to have high strength. When the yield ratio (YR) was 60% or more, it was judged to have excellent part strength. When the total elongation (El) was 6.0% or more, it was judged to have excellent ductility.

[0133] <Hole Expanding Test> The hole expanding test was performed in accordance with JIS Z 2256:2020. Specifically, the obtained steel plate was sheared to obtain a test piece measuring 100 mm x 100 mm. A hole with a diameter of 10 mm was punched into the obtained test piece with a clearance of 12.5%. Then, using a die with an inner diameter of 75 mm and a blank holding force of 9 tons (88.26 kN), a conical punch with an apex angle of 60° was pressed into the hole, and the hole diameter D at the crack initiation limit was measured. ｆ The initial hole diameter was measured as D [mm]. ０ [mm], the hole expansion ratio λ [%] was calculated from the following formula (6): Formula 6: λ = {(D ｆ -D ０ ) / D ０}×100 When the hole expanding ratio (λ) was 30% or more, it was determined that the stretch flangeability was excellent.

[0134] Bending Test The bending test was conducted in accordance with JIS Z 2248:2022. Specifically, strip-shaped test pieces 30 mm wide and 100 mm long were taken from the obtained steel sheets so that the axial direction of the bending test was parallel to the rolling direction of the steel sheets. The longitudinal end faces of the test pieces were ground end faces. Using the taken test pieces, a 90° V-bending test was conducted under conditions of an indentation load of 100 kN and a holding time of 5 seconds. Bending tests were conducted on five test pieces with an appropriate bending radius R. Next, the presence or absence of cracks at the ridgeline of the bend apex was confirmed. The presence or absence of cracks was confirmed by observing the ridgeline of the bend apex at 40x magnification using a digital microscope (RH-2000, manufactured by Hirox). The minimum bending radius R at which no cracks occurred in any of the five test pieces was determined, and the value (R / t) obtained by dividing this by the plate thickness t was used as the limit bending radius. When the limit bending radius (R / t) was 5.0 or less, it was determined that the bendability was excellent.

[0135] <Warm workability> Tensile tests were performed in accordance with JIS Z 2241:2021. Specifically, JIS No. 5 test pieces were taken from the obtained steel sheets so that the longitudinal direction was perpendicular to the rolling direction of the steel sheets. The taken test pieces were used for tensile tests at room temperature and 200°C at a crosshead speed of 1.67 × 10 －１ Tensile tests were carried out under conditions of 100 mm / s, and yield strength (YS) [MPa], tensile strength (TS) [MPa], uniform elongation (U-El) [%], and local elongation (L-El) [%] were measured. For the tensile test at 200°C, the specimen was held at 200°C for 10 minutes in a furnace attached to the tensile tester before starting the tensile test. The uniform elongation and local elongation determined by the tensile test at room temperature were measured as U-El, respectively. ＲＴ , L-El ＲＴ The uniform elongation and local elongation obtained by the tensile test at 200°C are defined as U-El ２００ , L-El ２００ U-El ２００ / U-El ＲＴ is 1.10 or more and L-El ２００ / L-El ＲＴWhen the value of the underlined parts in Tables 1 to 3 is 1.30 or more, the warm workability is judged to be excellent.

[0136]

[0137]

[0138]

[0139] As shown in Table 3, the inventive examples have a TS of 980 MPa or more and are excellent in part strength, ductility, stretch flangeability, bendability, and warm workability. On the other hand, the comparative examples are inferior in one or more of part strength, ductility, stretch flangeability, bendability, and warm workability.

[0140] Although the embodiments of the present invention have been described above, the present invention is not limited to the descriptions of the present embodiments, which form part of the disclosure of the present invention. In other words, other embodiments, examples, and operational techniques made by those skilled in the art based on the present embodiments are all included in the scope of the present invention. For example, in the series of heat treatments in the above-described manufacturing method, there are no particular limitations on the equipment used to perform the heat treatment on the steel sheet as long as the thermal history conditions are met.

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