Hot press molded body, steel sheet, and method for manufacturing

Abstract

The hot press molded body has a chemical composition containing, in mass%, 0.40-1.00% of C, 0.01-1.00% of Si, 0.01% or more but less than 1.00% of Mn, 0.100% or less of P, 0.01000% or less of S, and 0.0010-1.0000% of Al, and when a position that is 1 / 4 of the thickness in the thickness direction from the surface is set as a 1 / 4 depth position, the number density of epsilon carbides having an equivalent circle diameter of 5 nm or more at the 1 / 4 depth position is 20 / [mu] m2 or more, and the number density of the epsilon carbides having an equivalent circle diameter of 5 nm or more at the 1 / 4 depth position is 20 / [mu] m2 or more. The hot press molded body has a tensile strength of 2100 MPa or more.

Description

Technical Field

[0001] The present invention relates to a hot stamped body and a steel sheet, and methods for producing the same.

[0002] This application claims priority based on Japanese Patent Application No. 2023-016207 filed in Japan on February 6, 2023, the contents of which are incorporated herein by reference. Background Art

[0003] In the field of automotive steel sheets, the use of steel sheets with high tensile strength (high-strength steel sheets) is expanding to improve both fuel efficiency and crash safety, driven by recent tightening environmental regulations and crash safety standards. However, as higher strength increases, the press-formability of steel sheets decreases, making the manufacture of complex-shaped products increasingly difficult.

[0004] Specifically, as steel sheets increase in strength, their ductility decreases, leading to fractures in the highly processed areas when processed into complex shapes. Furthermore, as steel sheets increase in strength, residual stresses after processing cause springback and wall warping, leading to a decrease in dimensional accuracy. Therefore, press-forming high-strength steel sheets, particularly those with a tensile strength of 780 MPa or higher, into products with complex shapes is not easy. While roll forming, rather than press forming, can easily process high-strength steel sheets, its application is limited to components with a uniform cross-section along the longitudinal direction.

[0005] Therefore, in recent years, hot stamping has been adopted as a technique for press-forming difficult-to-form materials such as high-strength steel sheets, as disclosed in Patent Document 1. Hot stamping is a thermoforming technique in which a material to be formed is heated and then formed.

[0006] This technology heats the material before forming. Therefore, the steel is soft during forming, offering excellent formability. This allows even high-strength steel sheets to be precisely formed into complex shapes. Furthermore, hot stamping utilizes a press die for simultaneous quenching, resulting in a sufficiently strong steel product (hot stamped product).

[0007] For example, Patent Document 1 discloses that a tensile strength of 1400 MPa or more can be imparted to a steel member (hot stamped body) obtained by forming a steel sheet by hot stamping.

[0008] In recent years, countries around the world have set higher CO2 reduction targets, and automobile companies are promoting fuel efficiency reductions with collision safety concerns. Gasoline vehicles, of course, are also experiencing rapid development. To protect not only passengers but also batteries from collisions and offset the increased weight, higher-strength materials are required. For example, hot stamped parts used in automobiles and the like require higher-strength steel (exceeding 1.5 GPa) than that described in Patent Document 1 or currently used for hot stamped parts formed by hot stamping.

[0009] However, as the strength of a hot stamped body is increased, the toughness tends to decrease, and there is a concern that sufficient impact absorption may not be obtained.

[0010] To address such a problem, Patent Document 2 discloses a hot stamped body having a tensile strength of 2000 MPa or more.

[0011] Patent Document 2 discloses that a hot stamped body having excellent strength and toughness can be obtained by setting the average grain size of prior austenite grains to 5.0 μm or less and the average Mn concentration of grain boundaries of prior austenite grains to 1.0 mass % or less through secondary heat treatment.

[0012] However, as a result of research by the present inventors, it was found that the method of Patent Document 2 can achieve a certain degree of toughness improvement effect, but it cannot necessarily be said to be able to fully respond to the higher demands in recent years.

[0013] Prior art literature

[0014] Patent Literature

[0015] Patent Document 1: Japanese Patent Application Laid-Open No. 2002-102980

[0016] Patent Document 2: Japanese Patent No. 6966023 Summary of the Invention

[0017] Problems to be solved by the invention

[0018] As described above, in recent years, there has been a demand for improved impact absorption in high-strength (particularly tensile strength exceeding 1500 MPa or 2100 MPa) hot stamped products. However, conventional technologies have not necessarily been able to meet this demand.

[0019] Therefore, in view of the above problems, the present invention aims to provide a hot stamped body having both high strength and excellent impact absorption, a steel sheet suitable as a raw material thereof, and methods for producing the same.

[0020] Means for solving problems

[0021] The present inventors have studied methods for improving the impact absorption of high-strength hot stamped products and have found that the impact absorption is improved by precipitating ε carbide in the hot stamped products.

[0022] Furthermore, it was found that, in order to obtain a hot stamped body having such ε carbides, it is effective to reduce the number density of carbides of one or more of Nb, Ti, Fe, Mo, W, and Cr in the steel sheet (hot stamping steel sheet) serving as the raw material.

[0023] The present invention has been made based on the above findings. The gist of the present invention is as follows.

[0024] [1] A hot stamped product according to one embodiment of the present invention comprises, in mass%, C: 0.40-1.00%, Si: 0.01-1.00%, Mn: 0.01% or more and less than 1.00%, P: 0.100% or less, S: 0.01000% or less, Al: 0.0010-1.0000%, N: 0.0150% or less, Nb: 0-0.100%, Ti: 0-0.100%, Cr: 0-0.50%, V: 0-0.50%, Mo: 0-0.50%, B: 0-0.0100%, Co: 0-1.00%, Ni: The chemical composition of the steel sheet is as follows: 1.00% by weight of Ni: 0-1.00%, 0.5% by weight of Cu: 0-1.00%, 0.5% by weight of W: 0-3.00%, 0.10% by weight of O: 0-0.100%, 0.5% by weight of Ca: 0-1.00%, 0.5% by weight of Mg: 0-1.00%, 0.0050% by weight of REM: 0-0.020%, 0.020% by weight of Sb: 0-0.10%, 0.10% by weight of Zr: 0-0.10%, 0.10% by weight of Sn: 0-0.10%, 0.10% by weight of As: 0-0.10%, and the remainder: Fe and impurities; when a position which is 1 / 4 of the thickness in the thickness direction from the surface is defined as a 1 / 4 depth position, at the above 1 / 4 depth position, the number density of ε carbides having an equivalent circle diameter of 5 nm or more is 20 pieces / μm 2 As described above, the tensile strength of the hot stamped body is 2100 MPa or more.

[0025] [2] The hot stamped part according to [1], wherein when a position 50 μm away from the surface in the thickness direction is set as a 50 μm depth position, the hardness at the 50 μm depth position may be smaller than the hardness at the 1 / 4 depth position.

[0026] [3] The hot stamped product according to [2], wherein the hardness at the 50 μm depth position is smaller than the hardness at the 1 / 4 depth position by at least HV100 in Vickers hardness.

[0027] [4] The hot stamped product according to any one of [1] to [3], wherein the chemical composition may also contain, in mass %, one or more selected from the group consisting of Nb: 0.010-0.100%, Ti: 0.010-0.100%, Cr: 0.03-0.50%, and V: 0.01-0.50%.

[0028] [5] The hot stamped body according to any one of [1] to [4], wherein the chemical composition may also contain, in mass %, one or more selected from the group consisting of Mo: 0.05-0.50%, B: 0.0010-0.0100%, Co: 0.01-1.00%, Ni: 0.10-1.00%, Cu: 0.10-1.00%, and W: 0.10-3.00%.

[0029] [6] The hot stamped part according to any one of [1] to [5], wherein the chemical composition may also contain, in mass %, one or more selected from the group consisting of O: 0.001 to 0.100%, Ca: 0.01 to 1.00%, Mg: 0.01 to 1.00%, REM: 0.0001 to 0.0050%, Sb: 0.001 to 0.020%, Zr: 0.01 to 0.10%, Sn: 0.01 to 0.10%, and As: 0.01 to 0.10%.

[0030] [7] A steel plate according to another embodiment of the present invention comprises, in mass%, C: 0.40-1.00%, Si: 0.01-1.00%, Mn: 0.01% or more and less than 1.00%, P: 0.100% or less, S: 0.01000% or less, Al: 0.0010-1.0000%, N: 0.0150% or less, Nb: 0-0.100%, Ti: 0-0.100%, Cr: 0-0.50%, V: 0-0.50%, Mo: 0-0.50%, B: 0-0.0100%, Co: 0-1.00%, Ni: 0-1.00%, Cu: 0-1.00%, W: 0-3.00%, O: 0-0.100%, Ca: 0-1.00%, Mg: The chemical composition of the present invention is as follows: 0-1.00%, REM: 0-0.0050%, Sb: 0-0.020%, Zr: 0-0.10%, Sn: 0-0.10%, As: 0-0.10%, and the remainder: Fe and impurities; when a position that is 1 / 4 of the thickness in the thickness direction from the surface is set as a 1 / 4 depth position, at the above 1 / 4 depth position, the microstructure comprises, by area ratio, ferrite: more than 50% and less than 100%, pearlite: 0-40%, and bainite, martensite, and austenite: a total of 0% or more and less than 10%; at the above 1 / 4 depth position, the number density of one or more carbides of Nb, Ti, Fe, Mo, W, and Cr having an equivalent circle diameter of 0.2 μm or more is less than 5.0 pieces / 10 μm. 2 .

[0031] [8] The steel plate according to [7], wherein when a position 50 μm away from the surface in the plate thickness direction is set as a 50 μm depth position, the hardness at the 50 μm depth position may be smaller than the hardness at the 1 / 4 depth position.

[0032] [9] The steel plate according to [7] or [8], wherein the chemical composition may also contain, in mass %, one or more selected from the group consisting of Nb: 0.010-0.100%, Ti: 0.010-0.100%, Cr: 0.03-0.50%, and V: 0.01-0.50%.

[0033]

[10] The steel plate according to any one of [7] to [9], wherein the chemical composition may also contain, in mass %, one or more selected from the group consisting of Mo: 0.05-0.50%, B: 0.0010-0.0100%, Co: 0.01-1.00%, Ni: 0.10-1.00%, Cu: 0.10-1.00%, and W: 0.10-3.00%.

[0034]

[11] Another embodiment of the present invention relates to a method for manufacturing a hot stamped formed body. The method comprises the following steps: heating the steel plate described in [7] to a maximum heating temperature of at least the higher of the Ac3 point and 800°C and not more than 950°C, maintaining the temperature at the maximum heating temperature for 60 to 720 seconds, and then cooling the steel plate to a temperature below 300°C at an average cooling rate of 10 to 500°C / second from the maximum heating temperature to 300°C; and hot stamping the steel plate after the hot stamping step. The steel plate is subjected to a tempering process for tempering, wherein in the tempering process, the steel plate is held at 80-300°C for more than 6.0 seconds; or the steel plate is cooled to less than 80°C at an average cooling rate of 20-500°C / second, and then reheated and held at 80-300°C for more than 6.0 seconds; or the steel plate is held at 80-300°C for more than 6.0 seconds, cooled to less than 80°C at an average cooling rate of 20-500°C / second, and then reheated and held at 80-300°C for more than 6.0 seconds.

[0035]

[12] Another embodiment of the present invention relates to a method for manufacturing a steel plate according to [7], comprising the following steps: preparing a steel plate having, in mass %, C: 0.40-1.00%, Si: 0.01-1.00%, Mn: 0.01% or more and less than 1.00%, P: 0.100% or less, S: 0.01000% or less, Al: 0.0010-1.0000%, N: 0.0150% or less, N b: 0~0.100%, Ti: 0~0.100%, Cr: 0~0.50%, V: 0~0.50%, Mo: 0~0.50%, B: 0~0.0100%, Co: 0~1.00%, Ni: 0~1.00%, Cu: 0~1.00%, W: 0~3.00%, O: 0~0.100%, Ca: 0~1.00%, Mg: 0~1.00%, REM: 0~0.0050%, A heating step of heating a slab having a chemical composition of Sb: 0-0.020%, Zr: 0-0.10%, Sn: 0-0.10%, As: 0-0.10%, and the remainder: Fe and impurities to 1150-1350°C; a hot rolling step of hot-rolling the slab after the heating step to a finish rolling temperature of 800-950°C to obtain a steel plate; a cooling step of cooling the steel plate after the hot rolling step to 750°C or less at an average cooling rate of 10-100°C / second within 5.0 seconds from the completion of the hot rolling step; a coiling step of coiling the steel plate after the cooling step at a coiling temperature of more than 500°C and less than 750°C, and setting the average cooling rate from the coiling temperature to 500°C to more than 50°C / hour; and a cold rolling step of cold-rolling the steel plate after the coiling step at a plate thickness reduction rate of 10-60%.

[0036]

[13] The method for manufacturing a steel plate according to

[12] , wherein, after the above-mentioned cold rolling step, the following heat treatment step may also be included: the above-mentioned steel plate is heated to an annealing temperature of 700 to 920°C, and maintained at the above-mentioned annealing temperature for 120 to 500 seconds in an atmosphere with an oxygen potential of -1.50 or higher.

[0037]

[14] The method for manufacturing a steel plate according to

[13] , wherein, after the heat treatment step, a skin pass rolling step of performing skin pass rolling on the steel plate at a reduction ratio of 0.05 to 2.0% may be included.

[0038] Effects of the Invention

[0039] According to the above aspects of the present invention, it is possible to provide a hot stamped body having both high strength and excellent impact absorption, a steel sheet suitable as a raw material thereof, and methods for producing the same. DETAILED DESCRIPTION

[0040] Hereinafter, a hot stamped body according to an embodiment of the present invention (hot stamped body according to this embodiment), a steel sheet according to an embodiment of the present invention (steel sheet according to this embodiment), and methods for manufacturing these will be described.

[0041] <Hot Stamping Formed Body>

[0042] The hot stamped body according to this embodiment has a predetermined chemical composition described below, and the number density of ε carbides with an equivalent circle diameter of 5 nm or more at a 1 / 4 depth position is 20 pieces / μm. 2 above.

[0043] In this embodiment, a position that is 1 / 4 of the thickness from the surface in the thickness direction is set as a 1 / 4 depth position, and a position that is 50 μm from the surface in the thickness direction is set as a 50 μm depth position.

[0044] <Chemical Composition>

[0045] The chemical composition of the hot stamped product according to the present embodiment will be described. Unless otherwise specified, the percentages of the contents of the elements constituting the chemical composition are mass %.

[0046] The chemical composition of the hot stamped product according to the present embodiment refers to the chemical composition of the base steel material when the hot stamped product according to the present embodiment includes a base steel material and a coating formed on the surface thereof.

[0047] C: 0.40~1.00%

[0048] C is an element that improves the hardenability of steel and increases the strength of the hot stamped product obtained by hot stamping the steel sheet. If the C content is less than 0.40%, it becomes difficult to ensure sufficient strength in the hot stamped product. Therefore, the C content is set to 0.40% or more. The C content is preferably set to 0.45% or more.

[0049] On the other hand, if the C content exceeds 1.00%, there is a concern that the strength of the hot stamped product will increase, while the bendability and ductility will decrease. Therefore, the C content is set to 1.00% or less. The C content is preferably set to 0.80% or less.

[0050] Si: 0.01~1.00%

[0051] Si is an element effective in improving the hardenability of steel and stably ensuring the strength of hot stamped parts. To achieve these effects, the Si content is set to 0.01% or more, and preferably 0.10% or more.

[0052] On the other hand, if the Si content in the steel exceeds 1.00%, the heating temperature required for austenite transformation during heat treatment (quenching) becomes significantly higher. As a result, the cost required for heat treatment sometimes increases, or ferrite remains during heating and the strength of the hot stamped body decreases. In addition, if the Si content exceeds 1.00%, the generation behavior of oxide scale in steel plate manufacturing sometimes changes, which impairs the appearance of the product surface. Therefore, the Si content is set to 1.00% or less. The Si content is preferably set to 0.90% or less.

[0053] Mn: 0.01% or more and less than 1.00%

[0054] Mn contributes to the strength of hot stamped parts through solid solution strengthening. Furthermore, Mn is an element that is very effective in improving the hardenability of steel and stably ensuring the strength of hot stamped parts. To achieve these effects, the Mn content is set to 0.01% or more.

[0055] On the other hand, if the Mn content exceeds 1.00%, there is a concern that coarse steel inclusions such as MnS will be easily formed, which may reduce bendability and ductility. Therefore, the Mn content is set to less than 1.00%. The Mn content is preferably set to 0.90% or less.

[0056] P: 0.100% or less

[0057] P is an element that segregates at grain boundaries and reduces the strength of the grain boundaries. If the P content exceeds 0.100%, the strength of the grain boundaries is significantly reduced, and the toughness of the hot stamped body is reduced. Therefore, the P content is set to 0.100% or less. The P content is preferably less than 0.050% and less than 0.035%. There is no need to specifically limit the lower limit of the P content, and the lower limit is 0%. However, if the P content is reduced to less than 0.0001%, the cost of P removal increases significantly, which is not economically preferred. In actual operation, the P content can also be set to 0.0001% or more.

[0058] S: 0.01000% or less

[0059] S is an element that forms inclusions in steel. If the S content exceeds 0.01000%, a large amount of inclusions will be generated in the steel, and the toughness of the hot stamped body will decrease. Therefore, the S content is set to 0.01000% or less. The S content is preferably 0.00400% or less. There is no need to specifically limit the lower limit of the S content, and the lower limit is 0%. However, if the S content is reduced to less than 0.00015%, the cost of desulfurization will increase significantly, which is not economically preferred. In actual operation, the S content can also be set to more than 0.00015% or more than 0.00020%.

[0060] Al: 0.0010~1.0000%

[0061] Al is an element that deoxidizes molten steel and improves its soundness (suppressing defects such as bubbles in the steel). If the Al content is less than 0.0010%, deoxidation is insufficient. Therefore, the Al content is set to 0.0010% or more. The Al content is preferably 0.0100% or more, and more preferably 0.0200% or more.

[0062] On the other hand, if the Al content exceeds 1.0000%, coarse oxides and coarse nitrides will form in the steel, reducing the toughness of the hot stamped body. Therefore, the Al content is set to 1.0000% or less. The Al content is preferably 0.5000% or less, and more preferably 0.3000% or less.

[0063] N: 0.0150% or less

[0064] Nitrogen is an element that forms nitrides in steel. Since these nitrides become the starting point of fracture, the N content is set to 0.0150% or less. The N content is preferably 0.0100% or less, and more preferably 0.0050% or less.

[0065] There is no need to specify a lower limit for the N content; it is 0%. However, reducing the N content to less than 0.0001% significantly increases the cost of N removal, making it economically undesirable. Therefore, the N content may be set to 0.0001% or higher, 0.0004% or higher, or 0.0010% or higher.

[0066] The hot stamped body of this embodiment may also contain the aforementioned elements (basic elements) in its chemical composition, with the remainder consisting of Fe and impurities. Alternatively, to improve various properties, it may further contain one or more of the following elements (optional elements). The inclusion of optional elements is not necessary, so the lower limit of their content is 0%.

[0067] Nb: 0~0.100%

[0068] Nb is an element that increases the strength of hot stamped parts by solid solution strengthening and contributes to the refinement of old austenite grains by forming carbonitrides. Therefore, Nb may be contained as needed. When Nb is contained, in order to reliably exert the above-mentioned effects, the Nb content is preferably set to 0.010% or more. The Nb content is more preferably 0.035% or more. On the other hand, if Nb is contained in an amount exceeding 0.100%, Nb-based carbonitrides may be excessively generated, thereby suppressing the formation of ε carbides that contribute to the improvement of the toughness of the hot stamped parts. Therefore, the Nb content is preferably set to 0.100% or less. The Nb content is more preferably 0.080% or less.

[0069] Ti: 0~0.100%

[0070] Ti is an element that forms fine carbides, carbonitrides, and the like in steel together with Nb. These fine carbides and carbonitrides suppress Cu hot embrittlement cracking during the hot rolling process through the resulting grain refinement, and also improve the hydrogen embrittlement resistance of hot stamped parts. Furthermore, Ti preferentially combines with N in steel to form nitrides, suppressing the consumption of dissolved B due to BN precipitation and promoting the effect of B on hardenability, which will be described later. Therefore, Ti may also be contained.

[0071] In order to obtain the above-mentioned effects, the Ti content is preferably set to 0.010% or more, and more preferably set to 0.020% or more.

[0072] On the other hand, if the Ti content exceeds 0.100%, coarse TiN is generated, and the toughness of the hot stamped body is degraded. Therefore, the Ti content is set to 0.100% or less. The Ti content is preferably 0.080% or less, and more preferably 0.060% or less.

[0073] Cr: 0~0.50%

[0074] Cr is an element effective in improving the hardenability of steel and stably ensuring the strength of hot stamped parts. Therefore, it can also be contained. When the above-mentioned effects are achieved, the Cr content is preferably set to 0.03% or more, and more preferably set to 0.05% or more.

[0075] On the other hand, if the Cr content exceeds 0.50%, the above-mentioned effects are saturated and the cost increases. Therefore, the Cr content is set to 0.50% or less. The Cr content is preferably set to 0.30% or less.

[0076] V: 0~0.50%

[0077] V is an element that improves the strength of hot stamped parts by solid solution strengthening. Therefore, V may be contained. In order to reliably obtain the above-mentioned effect, the V content is preferably set to 0.01% or more.

[0078] On the other hand, if the V content exceeds 0.50%, excessive V-based carbonitrides may be generated, inhibiting the formation of ε carbides that contribute to improving the toughness of the hot stamped body. Therefore, the V content is set to 0.50% or less. The V content is preferably 0.40% or less.

[0079] Mo: 0~0.50%

[0080] Mo is a very effective element for improving the hardenability of steel and ensuring stable strength in hot stamped parts. In particular, its inclusion in combination with B produces a synergistic effect in improving hardenability. Therefore, Mo can be included. To achieve the above-mentioned effects, the Mo content is preferably set to 0.05% or more, and more preferably to 0.10% or more.

[0081] On the other hand, even if the Mo content exceeds 0.50%, not only will the above effects be saturated, but the alloy cost will also increase. Therefore, the Mo content is set to 0.50% or less. The Mo content is preferably 0.40% or less.

[0082] B: 0~0.0100%

[0083] B is an element that has the effect of improving the hardenability of steel even in a trace amount. In addition, B is an element that strengthens the grain boundaries by segregating at the grain boundaries. Therefore, it can also be contained. When obtaining the above-mentioned effect, the B content is preferably set to 0.0010% or more.

[0084] On the other hand, if the B content exceeds 0.0100%, a large amount of coarse compounds will precipitate, reducing the toughness of the hot stamped body. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less.

[0085] Co: 0-1.00%

[0086] Co is an element that increases the martensite start temperature (Ms point) and improves the toughness of hot stamped parts. Therefore, Co may be contained. To achieve the above effects, the Co content is preferably set to 0.01% or more.

[0087] On the other hand, Co is an expensive element. If the Co content exceeds 1.00%, the alloy cost increases. Therefore, the Co content is set to 1.00% or less. The Co content can also be set to 0.10% or less.

[0088] Ni: 0-1.00%

[0089] Nickel is an element effective for improving the hardenability of steel and ensuring stable strength in hot stamped parts. Furthermore, nickel suppresses hot embrittlement cracking of Cu during steel sheet production. Therefore, nickel may be present. To achieve the aforementioned effects, the nickel content is preferably set to 0.10% or more, and more preferably 0.20% or more.

[0090] On the other hand, if the Ni content exceeds 1.00%, the above-mentioned effects are saturated and the cost increases. Therefore, the Ni content is set to 1.00% or less. The Ni content is preferably set to 0.80% or less, and more preferably set to 0.50% or less.

[0091] Cu: 0-1.00%

[0092] Cu is an element effective for improving the hardenability of steel and stably ensuring the strength of hot stamped parts. Furthermore, Cu improves corrosion resistance in corrosive environments. Therefore, it may be included. To achieve the above-mentioned effects, the Cu content is preferably set to 0.10% or more. A Cu content of 0.20% or more is more preferred.

[0093] On the other hand, if the Cu content exceeds 1.00%, the above-mentioned effects are saturated and the cost increases. Therefore, the Cu content is set to 1.00% or less. The Cu content is preferably set to 0.90% or less.

[0094] W: 0~3.00%

[0095] W is an element effective in improving the hardenability of steel and stably ensuring the strength of hot stamped products. In order to obtain the above-mentioned effects, the W content is preferably set to 0.10% or more.

[0096] On the other hand, if the W content exceeds 3.00%, the above-mentioned effects are saturated and the cost increases. Therefore, the Cu content is set to 3.00% or less.

[0097] O: 0~0.100%

[0098] O is an element that, if contained in large quantities in steel, forms coarse oxides that serve as the starting point of fracture, deteriorating the toughness of hot stamped parts. Therefore, the O content is set to 0.100% or less. The O content is preferably set to 0.080% or less, 0.050% or less, or 0.030% or less.

[0099] The lower limit of the O content does not need to be particularly specified, and is 0%. However, in order to disperse a large amount of fine oxides during deoxidation of molten steel, the O content may be set to 0.001% or more, or 0.005% or more.

[0100] Ca: 0-1.00%

[0101] Ca is an element that deoxidizes the molten steel and suppresses the formation of oxides that can become the starting point of damage. Therefore, it may be contained. To achieve the above-mentioned effects, the Ca content is preferably set to 0.01% or more, and more preferably to 0.05% or more.

[0102] On the other hand, even if a large amount of Ca is contained, the above-mentioned effect is saturated, so the Ca content is set to 1.00% or less. The Ca content is preferably 0.40% or less, 0.20% or less, or 0.15% or less.

[0103] Mg: 0-1.00%

[0104] Mg is an element that deoxidizes molten steel and improves its soundness. Therefore, it may be contained. To achieve the aforementioned effects, the Mg content is preferably set to 0.01% or more. More preferably, the Mg content is 0.05% or more.

[0105] On the other hand, if the Mg content exceeds 1.00%, the oxide content in the steel increases, adversely affecting the toughness of the hot stamped steel. Therefore, the Mg content is set to 1.00% or less. The Mg content is preferably 0.40% or less, 0.20% or less, or 0.15% or less.

[0106] REM: 0~0.0050%

[0107] REM is an element that deoxidizes the molten steel and suppresses the formation of oxides that can become the starting point of fracture. Therefore, it may be contained. To achieve the above-mentioned effects, the REM content is preferably set to 0.0001% or more, and more preferably set to 0.0010% or more.

[0108] On the other hand, even if a large amount is contained, the above-mentioned effect is saturated, so the REM content is made 0.0050% or less. The REM content is preferably 0.0040% or less or 0.0020% or less.

[0109] In the present embodiment, REM refers to a total of 17 elements including Sc, Y, and lanthanoid elements, and the content of REM refers to the total content of these elements.

[0110] Sb: 0~0.020%

[0111] Sb is an element that improves the deformability of hot stamped parts by deoxidizing the molten steel and suppressing the formation of oxides that can cause damage. Therefore, it can be contained. To achieve the above effects, the Sb content is preferably set to 0.001% or more, and more preferably set to 0.005% or more.

[0112] On the other hand, even if a large amount of Sb is contained, the above-mentioned effect is saturated, so the Sb content is made 0.020% or less. The Sb content is preferably made 0.015% or less.

[0113] Zr: 0~0.10%

[0114] Zr contributes to inclusion control, particularly the fine dispersion of inclusions, and improves the toughness of hot stamped parts. Therefore, it may be present. To achieve these effects, the Zr content is preferably set to 0.01% or more, and more preferably 0.03% or more.

[0115] On the other hand, if Zr is contained in large amounts, the surface properties may be significantly deteriorated. Therefore, the Zr content is set to 0.10% or less, and preferably 0.08% or less.

[0116] Sn: 0~0.10%

[0117] Sn is an element that improves hydrogen embrittlement resistance. Therefore, it may be contained. In order to obtain the above-mentioned effects, the Sn content is preferably set to 0.01% or more.

[0118] On the other hand, if the Sn content exceeds 0.10%, the effect is saturated and the cost increases. Therefore, when Sn is contained, the Sn content is set to 0.10% or less.

[0119] As: 0~0.10%

[0120] As is an element that reduces the austenite single-phase transformation temperature, refines the prior austenite grains, and contributes to improving hydrogen embrittlement resistance. Therefore, it may be contained. To achieve the above-mentioned effects, the As content is preferably set to 0.01% or more.

[0121] On the other hand, even if a large amount of As is contained, the above-mentioned effect is saturated, so the As content is made 0.10% or less. The As content is preferably made 0.06% or less.

[0122] The rest: Fe and impurities

[0123] In the chemical composition of the hot stamped product according to this embodiment, the balance, other than the aforementioned elements, is Fe and impurities. Specifically, the hot stamped product according to this embodiment may have a chemical composition containing the essential elements, with the balance consisting of Fe and impurities. Alternatively, the chemical composition may contain the essential elements and further include one or more optional elements, with the balance consisting of Fe and impurities.

[0124] Here, "impurities" refer to components that are introduced during industrial steel plate production due to various factors, such as raw materials such as ores and scrap, and during the manufacturing process. These components are permitted within a range that does not adversely affect the properties of the hot stamped steel sheet according to this embodiment. Industrial production methods include blast furnace steelmaking and electric furnace steelmaking, and the level of impurities introduced during production using either method is also included.

[0125] The chemical composition of the hot stamped body can be determined by the following method.

[0126] The elemental analysis can be performed using standard methods such as ICP-AES, starting at a point 1 / 4 of the thickness from the surface of the hot stamped part in the thickness direction (a range of 1 / 8 to 3 / 8 of the thickness from the surface is acceptable). C and S, which are difficult to measure using ICP-AES, can be measured using combustion-infrared absorption, N using inert gas fusion-thermal conductivity, and O using inert gas fusion-non-dispersive infrared absorption.

[0127] [Microstructure]

[0128] In this embodiment, the microstructures at the 1 / 4 depth position and the 50 μm depth position are specified. The 1 / 4 depth position is a position showing a representative microstructure of the hot stamped body.

[0129] (At the 1 / 4 depth position, the number density of ε carbides with an equivalent circle diameter of 5nm or more is 20 / μm 2 above)

[0130] In order to obtain high tensile strength, high-strength hot stamped products must contain a large amount of C or other alloying elements. However, generally speaking, as the strength increases, the toughness of the hot stamped products decreases, and the impact absorption property decreases.

[0131] The present inventors have conducted research on this issue and have found that the impact absorbency can be improved by allowing ε carbides to exist in a hot stamped body and controlling their size and number density.

[0132] Specifically, it was found that the number density of ε carbides with an equivalent circle diameter of 5 nm or more was 20 per μm. 2 In the above case, the impact absorbency is improved.

[0133] Therefore, in the hot stamped body according to the present embodiment, the number density of ε carbides having an equivalent circle diameter of 5 nm or more is set to 20 pieces / μm at the 1 / 4 depth position. 2 above.

[0134] ε carbides with an equivalent circle diameter of 5 nm or greater are targeted because smaller ε carbides do not fully improve impact absorption. While there is no upper limit on the equivalent circle diameter of the targeted ε carbides, excessively large diameters are not preferred because sufficient number density becomes difficult to achieve. For example, ε carbides with a diameter of 5 to 50 nm are targeted.

[0135] In addition, even for ε carbides with an equivalent circle diameter of 5 nm or more, the number density is less than 20 / μm. 2On the other hand, if the number density exceeds 200 / μm 2 , then the interface between ε carbide and base material may become the starting point of cracks, so it can also be set to 200 pieces / μm 2 the following.

[0136] In the present embodiment, the ε carbide is a carbide having a value of FexC (x: approximately 2 to 3).

[0137] The equivalent circle diameter of ε carbides and the number density of ε carbides with an equivalent circle diameter of 5 nm or more can be determined by observing the thin film sample using a field emission transmission electron microscope (JEM-2100F manufactured by JEOL Ltd.) equipped with an energy dispersive X-ray spectrometer.

[0138] Specifically, after cutting out a small piece of about 10 mm square from the hot stamped body, both sides are mechanically or chemically polished to produce a thin film TEM sample (thickness of about 60 μm, φ3 mm) at the original 1 / 4 depth position (as long as it is within the range of 1 / 8 to 3 / 8 of the thickness from the surface in the thickness direction, it is allowed). In the preparation of this sample, starting from rough water-resistant abrasive paper of about #120, slowly polishing with fine mesh water-resistant abrasive paper, and finally polishing with water-resistant abrasive paper of about #600, the sample is punched out with a sample punch. After that, double-sided jet electrolytic polishing is performed until a hole is opened in the center, as a TEM observation sample. The electrolytic polishing device uses TENUPOL-2 made by STRUERS, and the electrolytic polishing liquid is set to a mixture of 5% perchloric acid and 95% glacial acetic acid solution, and the thin film TEM sample is finely processed at a voltage of 70 V. In the thin film TEM observation, observation is performed at an acceleration voltage of 200 kV. To reduce variations in the sample size per area, the sample was observed in at least five viewing fields, each with a square area of ​​approximately 100 to 300 nm. The type of precipitate observed was identified using diffraction patterns and EDX analysis results. Regarding number density, to reduce variations in the viewing field, the number density in each viewing field was counted and the average value was used as the representative number density value.

[0139] (It is preferred that the hardness at a depth of 50 μm is smaller than the hardness at a depth of 1 / 4)

[0140] In the hot stamped body according to this embodiment, the hardness at a depth of 50 μm is preferably smaller (lower) than the hardness at a depth of 1 / 4. By reducing the hardness (softening) near the surface, the impact absorption is further improved.

[0141] An effect can be obtained if the hardness at a depth of 50 μm is smaller than that at a quarter depth. However, to obtain a more pronounced effect, the hardness at a depth of 50 μm is preferably smaller than that at a quarter depth by at least HV100 in Vickers hardness.

[0142] The upper limit of the difference in Vickers hardness is not limited, but may be HV300 or less from the perspective of ensuring the strength of the entire molded body.

[0143] The hardness at a depth of 50 μm can be reduced (lowered) by, for example, decarburization by annealing as described later. The target is set at a depth of 50 μm because the hardness variation near the outermost surface increases based on the measurement principle.

[0144] The hardness at the 1 / 4 depth position and the hardness at the 50 μm depth position were evaluated using Vickers hardness in accordance with JIS Z 2244-1:2020.

[0145] During the measurement, the cross section of the polished test piece was measured at five points at each location with a load of 50 gf, and the average value of the three points excluding the maximum and minimum values ​​was set as the measured value.

[0146] In the hot stamped body according to the present embodiment, the constituent phases of the microstructure are not limited and may be controlled according to the target tensile strength. However, it is preferred that the area ratio of martensite is 95% or more.

[0147] Here, the martensite includes so-called fresh martensite and tempered martensite. The area ratio of martensite can be measured by the same method as the microstructure observation of the steel sheet described later.

[0148] [covered]

[0149] The hot stamped product according to the present embodiment may include a coating on a part or all of the surface.

[0150] The coating may be mainly composed of an Fe-Al alloy or a Fe-Zn alloy. The coating is also called a film, an alloyed plating layer, or an intermetallic compound layer.

[0151] A coating mainly composed of an Fe-Al alloy refers to a coating containing 70% by mass or more of Fe and Al, and a coating mainly composed of an Fe-Zn alloy refers to a coating containing 70% by mass or more of Fe and Zn. A coating mainly composed of an Fe-Al alloy may further contain Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and REM in addition to Fe and Al, with the remainder being impurities. A coating mainly composed of an Fe-Zn alloy may further contain Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM in addition to Fe and Zn, with the remainder being impurities.

[0152] The coating provides corrosion resistance, thereby achieving an effect of improving hydrogen embrittlement resistance during automobile use.

[0153] The coating thickness is preferably 5 to 100 μm.

[0154] The chemical composition and thickness of the coating can be determined by observing a cross section with a scanning electron microscope.

[0155] Specifically, a measurement sample is cut out from the 1 / 2 portion in the longitudinal direction (the position of 1 / 2 of the length in the longitudinal direction from the longitudinal end) and the 1 / 4 portion in the width direction (the position of 1 / 4 of the width in the width direction from the width end) of the hot stamped body and observed. The observation range of the microscope is set to, for example, 40000 μm in area at a magnification of 400 times. 2 The cut sample was mechanically polished and then mirror-finished. The coating thickness was then measured in 10 random viewing fields, and the average value was taken as the coating thickness.

[0156] When observed using a BSE image (or COMPO image), a clear contrast difference is observed between the coating and the base metal (steel plate substrate). Therefore, the coating thickness can be determined by measuring the thickness from the outermost surface to the point where the contrast changes. Measurements are made at 20 locations at equal intervals within the observation photograph, with the distance between measurement locations set to 6.50 μm. Furthermore, during the measurement, observations are made across five viewing fields according to the above-mentioned procedure, and the average value is used as the coating thickness.

[0157] The chemical composition of the coating can be determined by performing elemental analysis of the same observation area as above using an electron probe microanalyzer (EPMA) with a spot size of 1 μm or less. Analysis is performed at a total of 10 points in the coating over 10 random viewing fields, and the average value is used as the Fe, Al, and Zn content of the coating. Even when elements other than Fe, Al, and Zn are present, the same method can be used to determine the Fe, Al, and Zn content.

[0158] The surface serving as the basis for the above-mentioned 1 / 4 depth position and 50 μm depth position is the surface of the hot stamped body, but in the case where the hot stamped body has a coating, that is, the hot stamped body has a base steel material and a coating formed on the surface of the base steel material, the surface refers to the surface of the base steel material excluding the coating.

[0159] [Mechanical properties]

[0160] (tensile strength)

[0161] The tensile strength of the hot stamped body according to the present embodiment is 2100 MPa or more in consideration of its contribution to improvements in fuel efficiency and collision safety when applied to automobile parts.

[0162] There is no upper limit for the tensile strength. However, since an increase in the tensile strength may lead to a decrease in the impact absorbability, the tensile strength may be set to less than 2900 MPa.

[0163] The tensile strength can be determined by taking a No. 5 test piece described in JIS Z 2241:2011 from a position as flat as possible on the hot stamped body and subjecting the test piece to a tensile test according to the test method described in JIS Z 2241:2011.

[0164] (Shock Absorption)

[0165] In the hot stamped body according to the present embodiment, excellent impact absorption can be obtained by controlling the chemical composition and the existence state of ε carbides as described above.

[0166] As a target for impact absorption, the maximum bending angle (Bending angle at maximum force) in the VDA (German Association of the Automotive Industry) bend test 238-100 is set to 40° or greater, based on a sheet thickness of 2.0 mm. Sheet thickness conversion is based on "Bending angle correction regarding sheet thickness," Materials Science and Engineering 418 (2018) 012076.

[0167] <Steel Plate>

[0168] Next, the steel plate involved in this embodiment will be described. By hot stamping and tempering the steel plate involved in this embodiment, the hot stamped body involved in this embodiment can be obtained. Therefore, the steel plate involved in this embodiment is suitable as a raw material (steel plate for hot stamping) for the hot stamped body involved in this embodiment.

[0169] [Chemical composition]

[0170] The chemical composition of the steel plate involved in this embodiment needs to be set in a manner so that preferred properties can be obtained as a hot stamped part obtained by hot stamping and tempering. However, since the chemical composition does not substantially change by hot stamping and tempering, the chemical composition of the steel plate involved in this embodiment can be equivalent to that of the hot stamped part involved in this embodiment.

[0171] [Microstructure]

[0172] The microstructure of the steel plate according to this embodiment is described by defining the position 1 / 4 of the plate thickness from the surface in the plate thickness direction as the 1 / 4 depth position, the range from the surface to 50 μm in the plate thickness direction as the surface layer portion, and the position 50 μm from the surface in the plate thickness direction as the 50 μm depth position. The 1 / 4 depth position represents a representative microstructure of the steel plate.

[0173] (At the 1 / 4 depth position, the area ratio includes ferrite: more than 50% and less than 100%, pearlite: 0-40%, bainite, martensite (including fresh martensite and tempered martensite), and austenite: a total of 0% or more and less than 10%)

[0174] In the steel sheet according to this embodiment, the microstructure is mainly composed of ferrite (set to an area ratio of more than 50%) in consideration of workability. The area ratio of ferrite may be 100%, but pearlite, bainite, martensite, and austenite (retained austenite) may be included as structures other than ferrite.

[0175] Pearlite is a structure composed of ferrite and cementite arranged in lamellar layers. In other words, a high area fraction of pearlite means a high amount of carbon present as cementite. If the amount of carbon present as cementite is high, the cementite (pearlite) will not fully dissolve during hot stamping heating, and even with hot stamping and tempering, sufficient epsilon carbide cannot be obtained. Therefore, the area fraction of pearlite is set to less than 40%. The area fraction of pearlite can be 0%, but it can also be set to 5% or above.

[0176] The presence of bainite, martensite, and austenite increases the strength of the steel sheet before hot stamping. However, this may cause roughness or cracking of the cut end surface during trimming or other processes before hot stamping. Therefore, the combined area ratio of bainite, martensite, and austenite is set to 0% or more and less than 10%.

[0177] The area ratios of ferrite, pearlite, bainite, martensite, and retained austenite in the microstructure of the steel sheet can be determined by the following method using a field emission scanning microscope (FE-SEM) and X-ray diffraction measurement.

[0178] The L-section of the steel plate (a section parallel to the rolling direction and the plate thickness direction) was mirror-polished and then etched with Nital. The sample was observed using an FE-SEM at a magnification of 3000x for 10 fields, and the area ratio of each phase at the 1 / 4 depth position was calculated.

[0179] At this time, in the FE-SEM image, the tissues were identified based on the following characteristics of each tissue.

[0180] Ferrite is a massive crystal grain with no underlying structures such as laths. Pearlite is a structure composed of alternating layers of ferrite and cementite (the lamellar ferrite in pearlite is distinct from the massive ferrite described above and is not included in the area ratio of the massive ferrite). Bainite and tempered martensite are structures containing lath-shaped crystal grains and carbides, but differ from each other as follows.

[0181] First, bainite is classified into upper bainite and lower bainite and observed. Upper bainite is a collection of lamellar grains, which is a collection of laths containing carbides between the laths. Lower bainite is a collection of lamellar grains, which contains iron-based carbides with a long diameter of 5nm or more inside. Furthermore, the carbides belong to a single variant, that is, a group of iron-based carbides extending in the same direction. Here, the group of iron-based carbides extending in the same direction refers to carbides whose elongation directions of the iron-based carbides differ by less than 5°. The area ratio of bainite is determined by the sum of the area ratios of the upper bainite and the lower bainite. Tempered martensite is also a collection of lamellar grains like the lower bainite, and is a structure containing iron-based carbides inside. However, since the carbides select more than two variants, the elongation directions of the iron-based carbides are more than two.

[0182] Thus, ferrite, pearlite, tempered martensite, and bainite can be identified by confirming their characteristics using FE-SEM.

[0183] On the other hand, the untempered fresh martensite and retained austenite are not fully corroded in the nital etching, so in the observation using FE-SEM, they can be distinguished from other etched structures (tempered martensite, bainite, ferrite), but the difference between fresh martensite and retained austenite cannot be distinguished. Therefore, the area ratio of retained austenite is measured by X-ray diffraction. X-ray diffraction is measured on the surface of a 20 mm square test piece that is mechanically ground by 50 μm and then chemically ground. The integrated intensity of the diffraction peaks of the BCC phase and the FCC phase is measured by X-ray diffraction, and the ratio of the integrated intensity of the FCC phase in the sum of the total integrated intensity is set as the area ratio of retained austenite. The measurement is carried out 3 times for each sample, and the average obtained is set as the area ratio of retained austenite. The area ratio of fresh martensite was determined as the difference between the area ratio of the uncorroded region (fresh martensite or retained austenite) observed by FE-SEM and the area ratio of retained austenite measured by X-ray diffraction.

[0184] The above-mentioned area ratio can also be obtained in the hot stamped body by the same method.

[0185] (At the 1 / 4 depth position, the number density of one or more carbides of Nb, Ti, Fe, Mo, W and Cr with an equivalent circle diameter of 0.2 μm or more is less than 5.0 pieces / 10 μm 2 )

[0186] In the hot stamped part according to the present embodiment described above, ε carbide is precipitated by hot stamping and tempering under predetermined conditions, thereby obtaining a predetermined ε carbide. However, if carbides exist in the steel sheet serving as the raw material and the carbides do not dissolve during the heating for hot stamping, the predetermined ε carbide cannot be obtained even after hot stamping and tempering.

[0187] For example, coarse carbides generated in the steel sheet are not dissolved during the heating of hot stamping and tend to remain. Therefore, in the steel sheet involved in this embodiment, coarse carbides are reduced. More specifically, at the 1 / 4 depth position, the number density of one or more carbides of Nb, Ti, Fe, Mo, W, and Cr with an equivalent circle diameter of 0.2 μm or more is set to less than 5.0 per 10 μm. 2 (less than 0.50 / μm 2 ).

[0188] If the number of coarse carbides with an equivalent circle diameter of 0.2 μm or more is 5.0 per 10 μm 2 If the above amount is exceeded, more carbides will remain after being dissolved during heating for hot stamping.

[0189] The number density of carbides may not exist or may be 0.0 per 10 μm. 2 However, in order to not greatly hinder the formation of ε carbides after hot stamping, it can also be 30 pieces / 10000μm 2 (0.03 / 10μm 2 )above.

[0190] The upper limit of the equivalent circle diameter of the target carbide is not limited, but in order to avoid excessive influence on the unevenness of the hardenability due to different locations during hot stamping, it can be 2.0 μm or less. In other words, carbides with an equivalent circle diameter of 0.2 to 2.0 μm can be set as the target.

[0191] The number density of one or more carbides of Nb, Ti, Fe, Mo, W, and Cr having an equivalent circle diameter of 0.2 μm or more can be determined by the following method.

[0192] After the L-section of the steel plate is mirror-polished, it is etched with nitric alcohol. The 1 / 4 depth position of the sample is observed with a scanning microscope. In the area of ​​50 μm square at this depth position, the observed precipitates are analyzed by EDX for composition. In the case of containing one or more of Nb, Ti, Fe, Mo, W and Cr and C, it is judged to be the target carbide. In order to reduce the unevenness caused by the field of view, the area of ​​50 μm square is observed for at least 5 fields of view, and the number density of the above-mentioned carbides with an equivalent circle diameter of 0.2 μm or more is counted, and the average is set as the representative value of the number density.

[0193] (It is preferred that the hardness at a depth of 50 μm is smaller than the hardness at a depth of 1 / 4)

[0194] By making the hardness at a depth of 50 μm smaller than the hardness at a quarter depth in the steel sheet as the raw material, the hardness at a depth of 50 μm in the hot stamped part can be made smaller than the hardness at a quarter depth. Therefore, in the steel sheet according to this embodiment, the hardness at a depth of 50 μm is preferably smaller than the hardness at a quarter depth.

[0195] [Plate thickness]

[0196] The plate thickness of the steel plate according to the present embodiment is not limited, but is preferably 1.0 to 3.5 mm, assuming use as a steel plate for automobiles.

[0197] [covered]

[0198] The steel plate involved in this embodiment may also have a coating on a part of the surface. The coating may be a coating with Al as the main component (Al-based coating) or a coating with Zn as the main component (Zn-based coating). The coating is also called a film or a plating. A coating with Al as the main component refers to a coating containing 70% by mass or more of Al, and a coating with Zn as the main component refers to a coating containing 70% by mass or more of Zn. The coating with Al as the main component may further contain Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, REM in addition to Al, with the remainder being impurities. The coating with Zn as the main body may also contain Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM in addition to Zn, with the remainder being impurities.

[0199] The above-mentioned 1 / 4 depth position, 50μm depth position, and surface serving as a reference for the surface layer are the surfaces of the steel plates. However, when the steel plate has a coating, that is, when the steel plate has a base material and a coating formed on the surface of the base material, the surface refers to the surface of the base material excluding the coating.

[0200] <Manufacturing method>

[0201] The steel sheet and the hot stamped product according to the present embodiment can achieve the effects as long as they have the above-mentioned characteristics regardless of the manufacturing method. However, they can be preferably manufactured by the manufacturing method described below.

[0202] [Method for manufacturing steel sheet]

[0203] The steel sheet according to the present embodiment can be obtained by a production method including the following steps.

[0204] (I) a heating step of heating a slab having a predetermined chemical composition to 1150 to 1350° C.;

[0205] (II) hot rolling the slab after the heating step so that the finishing rolling temperature becomes 800 to 950° C. to obtain a steel plate;

[0206] (III) a cooling step of cooling the hot-rolled steel sheet to a temperature of 750° C. or lower at an average cooling rate of 10 to 100° C. / second, starting within 5 seconds after completion of the hot rolling step;

[0207] (IV) a coiling step of coiling the steel sheet after the cooling step at a coiling temperature exceeding 500° C. and not exceeding 750° C., and setting the average cooling rate from the coiling temperature to 500° C. to exceed 50° C. / hour;

[0208] (V) A cold rolling step of cold-rolling the steel sheet after the coiling step at a thickness reduction rate of 10 to 60%.

[0209] Furthermore, the method for manufacturing a steel plate according to the present embodiment may further include one or more of the following steps.

[0210] (VI) a heat treatment step of heating the steel sheet after the cold rolling step to an annealing temperature of 700 to 920° C. and maintaining the annealing temperature for 120 to 500 seconds in an atmosphere with an oxygen potential of −1.50 or higher;

[0211] (VII) after the heat treatment step, subjecting the steel sheet to a skin pass rolling step of skin pass rolling at a reduction ratio of 0.05 to 2.0%;

[0212] (VIII) A coating step of forming a coating on the surface of the steel sheet.

[0213] Preferred conditions for each step are described below. For conditions or steps not described, known conditions can be applied.

[0214] (Heating process)

[0215] In the heating process, the slab is heated before hot rolling. The heating temperature is set to 1150-1350°C.

[0216] When the heating temperature is lower than 1150° C., carbides formed during casting are not dissolved, and coarse carbides remain even after the hot rolling step.

[0217] On the other hand, from the viewpoint of suppressing scale loss and energy saving, the slab heating temperature is set to 1350° C. or lower.

[0218] The chemical composition of the slab to be subjected to the heating step may be set to be the same as the chemical composition of the steel plate to be obtained.

[0219] (Hot rolling process)

[0220] In the hot rolling step, the slab after the heating step is hot rolled so that the finishing rolling temperature becomes 800 to 950° C. to obtain a steel plate.

[0221] If the finishing rolling temperature (surface temperature at the exit of the final pass) is below 800°C, a large number of non-recrystallized regions flattened in the rolling direction may remain, causing anisotropy in the properties of the steel sheet. On the other hand, if the finishing rolling temperature exceeds 950°C, the grain size of the steel sheet will coarsen.

[0222] (Cooling process)

[0223] After finish rolling, if the steel sheet is kept at a temperature exceeding 750°C for a long period of time, coarse carbides will form. Therefore, in the cooling process, the hot-rolled steel sheet is cooled to a cooling stop temperature of 750°C or below at an average cooling rate of 10 to 100°C / second. Furthermore, this cooling process is initiated within 5.0 seconds of the completion of the hot rolling process.

[0224] If the average cooling rate to the cooling stop temperature of 750°C or lower is lower than 10°C / second, the time from the completion of the hot rolling process to the start of the cooling process exceeds 5.0 seconds, or the cooling stop temperature exceeds 750°C, a large amount of coarse carbides are generated.

[0225] On the other hand, if the average cooling rate to the cooling stop temperature of 750° C. or lower exceeds 100° C. / second, it becomes difficult to cool the steel sheet uniformly, and defects in the sheet shape may occur.

[0226] (Coiling process)

[0227] In the coiling process, the steel sheet after the cooling process is coiled at a temperature exceeding 500° C. and not exceeding 750° C. Furthermore, after coiling, the average cooling rate from the coiling temperature to 500° C. is set to exceed 50° C. / hour.

[0228] If the coiling temperature is 500°C or lower, hard phases such as bainite and martensite may form, making cold rolling impossible or increasing the load required for cold rolling. The coiling temperature is preferably 520°C or higher, more preferably 540°C or higher. On the other hand, if the coiling temperature exceeds 750°C, Cr and Mn may concentrate in cementite present in ferrite grain boundaries and pearlite, remaining as undissolved carbides during subsequent annealing and hot stamping.

[0229] On the other hand, coiling slows the cooling rate. If the average cooling rate to 500°C is slow, the internal oxide layer develops, the load of the pickling process increases, and coarse carbides are generated during cooling. Therefore, the average cooling rate from the coiling temperature to 500°C is set to more than 50°C / hour.

[0230] (Cold rolling process)

[0231] In the cold rolling process, the steel sheet after the coiling process is cold rolled at a thickness reduction rate (reduction rate) of 10 to 60% to adjust the sheet thickness to a predetermined thickness.

[0232] (Heat treatment process)

[0233] In the heat treatment step, the cold-rolled steel sheet is heated to an annealing temperature of 700 to 920° C. and maintained at the annealing temperature for 120 to 500 seconds in an atmosphere having an oxygen potential of −1.50 or higher.

[0234] The heat treatment step is not essential, but is preferred because the C content (decarburization) of the surface layer of the steel plate can be reduced and the surface layer of the steel plate can be softened by performing the heat treatment under the above-mentioned conditions.

[0235] When the annealing temperature is lower than 700°C, the oxygen potential is lower than -1.50, or the holding time is lower than 120 seconds, no sufficient effect can be obtained.

[0236] On the other hand, when the annealing temperature exceeds 920°C, the crystal grains become coarse.

[0237] Furthermore, if the holding time exceeds 500 seconds, productivity deteriorates, which causes an increase in material costs.

[0238] The upper limit of the oxygen potential is not limited. However, if the oxygen potential is too high, Fe and other alloy elements in the outermost layer will be oxidized, resulting in the formation of scale patterns on the surface. Therefore, it is preferably set to -0.50 or less.

[0239] Oxygen potential refers to the partial molar Gibbs free energy of oxygen expressed in terms of oxygen partial pressure in the gas phase and temperature.

[0240] <Coating process>

[0241] The coating can also be formed on the surface as required. There is no particular limitation on the coating method, and electroplating, vacuum evaporation, metal coating, spraying, etc. represented by hot-dip coating can be performed. Hot-dip coating is the most popular industrial method.

[0242] Examples of the coating include an Al-based coating containing Al and a Zn-based coating containing Zn.

[0243] When forming an Al-based coating by hot-dip plating, in addition to Al, Fe is often mixed into the plating bath as an impurity. Furthermore, in addition to the above elements, the plating bath may also contain Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, or a mischmetal alloy, as long as it contains 70% by mass or more of Al.

[0244] In the case of hot dip coating, the steel sheet after the heat treatment process can be cooled to room temperature and then heated again for coating, or it can be cooled to a temperature near the coating bath temperature (for example, 650 to 750°C in the case of Al-based coating and 420 to 500°C in the case of Zn-based coating) after annealing and then hot dip coated without temporarily cooling to room temperature.

[0245] There are no particular limitations on the pre- and post-treatments for coating, and pre-coating or solvent coating, alloying treatment, and temper rolling can be performed. For example, alloying treatment can include annealing at 450 to 800°C. Furthermore, as a post-treatment, temper rolling is useful for shape adjustment, and for example, a reduction of 0.1 to 0.5% can be performed.

[0246] (Skin-pass rolling process)

[0247] The method for producing a steel sheet according to the present embodiment may further include a skin pass rolling step of skin pass rolling the steel sheet after the heat treatment step or after the coating step.

[0248] Skin-pass rolling increases the diffusion rate of elements in the material, making it easier for carbides to dissolve during hot stamping. This allows for a higher density of ε carbides.

[0249] In order to obtain this effect, the reduction ratio in skin pass rolling is preferably set to 0.05% or more, more preferably 0.1% or more.

[0250] On the other hand, if the skin-pass rolling reduction exceeds 2.0%, the load on the skin-pass rolling process increases, causing an increase in material cost. Therefore, when skin-pass rolling is performed, the reduction is preferably set to 2.0% or less.

[0251] [Method for producing hot stamped body]

[0252] The method for producing the hot stamped body according to the present embodiment can be obtained by a production method including the following steps using the steel sheet according to the present embodiment described above.

[0253] (i) a hot stamping step of heating the steel sheet according to the present embodiment to a maximum heating temperature of at least the higher of the Ac3 point and 800°C and not more than 950°C, holding the steel sheet at the maximum heating temperature for 60 to 720 seconds, and then cooling the steel sheet to a temperature of not more than 300°C such that the average cooling rate from the maximum heating temperature to 300°C is 10 to 500°C / second;

[0254] (ii) A tempering step of tempering the steel sheet after the hot stamping step.

[0255] Each process is described.

[0256] (Hot stamping process)

[0257] In the hot stamping process, the steel plate involved in this embodiment is used as a raw material (steel plate for hot stamping), and the steel plate is heated to a maximum heating temperature of at least the higher of the Ac3 point (°C) and 800°C and not more than 950°C. After being maintained at the maximum heating temperature for 60 to 720 seconds, the steel plate is cooled to below 300°C so that the average cooling rate from the maximum heating temperature to 300°C becomes 10 to 500°C / second.

[0258] This process causes carbides present in the steel sheet to dissolve in solid form, thereby increasing the strength.

[0259] If the maximum heating temperature is lower than the Ac3 point or lower than 800° C., or the holding time is shorter than 60 seconds, carbides are not sufficiently dissolved or austenite transformation becomes insufficient, and sufficient strength cannot be obtained after the hot stamping step.

[0260] On the other hand, if the maximum heating temperature is too high or the holding time at the maximum heating temperature is too long, the crystal grains will coarsen, and the toughness and bendability of the formed body after the hot stamping process will become insufficient. Therefore, the maximum heating temperature is set to 950°C or less, and the holding time at the maximum heating temperature is set to 720 seconds or less.

[0261] Furthermore, in cooling from the maximum heating temperature to 300° C. or lower, if the average cooling rate to 300° C. is lower than 10° C. / s, sufficient quenching does not occur, and sufficient tensile strength cannot be obtained in the hot stamped body.

[0262] On the other hand, if the average cooling rate to 300° C. exceeds 500° C. / s, the variation in cooling rate among different locations becomes large, causing distortion in the shape of the molded article.

[0263] The Ac3 point can be determined from the change point of the thermal expansion coefficient when the heating rate is set to 5°C / second by, for example, a plate Formastor test.

[0264] (Tempering process)

[0265] In the tempering step, the steel sheet after the hot stamping step is tempered at a temperature of 80 to 300°C.

[0266] Tempering can be performed when the cooling stop temperature during the hot stamping process is 80-300°C. Alternatively, the steel sheet may be temporarily cooled to a temperature below 80°C and then heated to 80-300°C and held at this temperature. Furthermore, after the hot stamping process, the steel sheet may be held at 80-300°C, then temporarily cooled to a temperature below 80°C, then heated to 80-300°C and held at this temperature.

[0267] When the cooling stop temperature in the hot stamping process is lower than 80° C., the steel sheet may be heated again to 80 to 300° C. and maintained at this temperature.

[0268] In order to fully precipitate ε carbides, the holding temperature at 80-300°C is set to 6 seconds or longer, regardless of whether reheating is performed. There is no upper limit to the holding time, but holding longer than necessary reduces productivity, so the holding time can be set to 1800 seconds or less.

[0269] Example

[0270] Slabs having the chemical compositions listed in Table 1-1 and Table 1-2 were prepared.

[0271] The slab was heated under the conditions shown in Table 2-1, hot-rolled, cooled, and coiled to produce a 2.6 mm hot-rolled steel sheet.

[0272] After the hot-rolled steel sheet is cold-rolled at the reduction ratio shown in Table 2-2, it is heat-treated under the conditions shown in Table 2-2 except for a part. In addition, for some examples, a coating (hot-dip galvanized layer or hot-dip Al-plated layer) is formed by hot-dip plating. For the hot-dip galvanized layer, an alloyed hot-dip galvanized layer is made by alloying. In Table 2-2, the coating type GA is an alloyed hot-dip galvanized layer, and Al is a hot-dip Al-plated layer. In addition, for some examples, skin-pass rolling is performed. "-" in Table 2-2 means that it is not implemented.

[0273] Thus, steel plates Nos. 1 to 23 and 101 to 113 were obtained.

[0274] For the obtained steel plate, the microstructure fraction at the 1 / 4 depth position and the number density of one or more carbides of Nb, Ti, Fe, Mo, W and Cr having an equivalent circle diameter of 0.2 μm or more at the 1 / 4 depth position were measured according to the above-mentioned procedure.

[0275] Furthermore, the Vickers hardness at a depth of 50 μm and a depth of 1 / 4 was measured according to the above-mentioned procedure.

[0276] The results are shown in Table 2-3.

[0277] Table 1-1

[0278]

[0279] Table 1-2

[0280]

[0281] Table 2-1

[0282]

[0283] Table 2-2

[0284]

[0285] Table 2-3

[0286]

[0287] The resulting steel sheets were then hot stamped and tempered under the conditions listed in Table 3-1 to produce hot stamped parts. Part No. H1 in the table indicates that steel sheet No. 1 was used as the raw material (hot stamping steel sheet), and Part No. H2 indicates that steel sheet No. 2 was used as the raw material (hot stamping steel sheet). The same applies to the other parts. "-" in Table 3-1 indicates that no hot stamping was performed.

[0288] The obtained hot stamped body was measured according to the above-mentioned procedure for the area ratio of martensite in the microstructure at the 1 / 4 depth position and the number density of ε carbides having an equivalent circle diameter of 5 nm or more at the 1 / 4 depth position.

[0289] Furthermore, the Vickers hardness at a depth of 50 μm and a depth of 1 / 4 was measured according to the above-mentioned procedure.

[0290] The results are shown in Table 3-2.

[0291] Furthermore, the limit bending angle was determined as an index of the tensile strength (TS) and impact absorbability of the obtained hot stamped product.

[0292] (tensile strength)

[0293] The tensile strength was determined by taking a No. 5 test piece described in JIS Z 2241: 2011 from a position as flat as possible on the hot stamped body and subjecting the test piece to a tensile test according to the test method described in JIS Z 2241: 2011. The measurement was performed twice for each steel plate, and the average of the results was used as the measured value.

[0294] If the tensile strength is 2000 MPa or more, it is judged that a preferable strength is obtained.

[0295] (Ultimate bending angle)

[0296] The limit bending angle is determined by taking a 60mm wide x 30mm long (length parallel to the rolling direction) test piece from a flatter position of the hot stamped sheet. Using this test piece, the steel sheet is gradually bent between a pair of rolls while applying a load with a punch according to VDA 238-100. The bending angle at which the punch's reaction force reaches its maximum is defined as the limit bending angle of the steel sheet. The measurement is performed twice for each steel sheet, and the average of the results is used as the measured value.

[0297] If the limit bending angle is 40° or more when converted to a plate thickness of 2.0 mm, the impact absorbency is judged to be excellent.

[0298] Table 3-1

[0299]

[0300] Table 3-2

[0301]

[0302] As can be seen from Tables 1-1 to 3-2, the hot stamped bodies (formed bodies No. H1 to H23) serving as inventive examples have chemical compositions and ε carbide number densities within the ranges of the present invention, and are hot stamped bodies having both high strength and excellent impact absorption.

[0303] In contrast, for the hot stamped parts (formed parts No. H101 to H103, H105 to H109) used as comparative examples, the steel plate used as the raw material is not preferred, and the chemical composition or the number density of ε carbides as hot stamped parts are also outside the scope of the present invention, resulting in poor tensile strength and impact absorption (limit bending angle) (H101 broke early before reaching TS, etc. during tensile tests and bending tests).

[0304] Since H104 cracked during blanking before hot stamping, no further testing was performed.

[0305] Regarding No. H110, since the heating temperature in the heat treatment step was too high and the heating time was too long, sufficient quenching was not achieved, and sufficient tensile strength was not obtained as a hot stamped body.

[0306] Regarding Nos. H111 and H112, although the steel sheets used as the raw materials were suitable steel sheets, the hot stamping conditions were not suitable, and sufficient tensile strength was not obtained as the hot stamped bodies.

[0307] Regarding No. H113, although the steel sheet used as the raw material was a suitable steel sheet, the tempering after hot stamping was insufficient, and the number density of ε carbides was outside the range of the present invention, resulting in poor impact absorption (limit bending angle).

[0308] Industrial applicability

[0309] According to the present invention, it is possible to provide a hot stamped body having both high strength and excellent impact absorption, a steel sheet suitable as a raw material thereof, and methods for producing the same.

[0310] This hot stamped body can meet the recent demand for high strength and impact absorption properties required of automobile steel sheets, thereby improving the fuel efficiency and collision safety of automobiles.

Claims

1. A hot stamped body having the following chemical composition: comprising, by mass %, C：0.40～1.00％、 Si: 0.01-1.00%, Mn: 0.01% or more and less than 1.00%, P: 0.100% or less, S: 0.01000% or less, Al：0.0010～1.0000％、 N: 0.0150% or less, Nb: 0~0.100%, Ti: 0~0.100%, Cr：0～0.50％、 V：0～0.50％、 Mo: 0~0.50%, B：0～0.0100％、 Co: 0-1.00%, Ni: 0-1.00%, Cu: 0-1.00%, W：0～3.00％、 O：0～0.100％、 Ca: 0-1.00%, Mg: 0-1.00%, REM: 0~0.0050% Sb: 0~0.020%, Zr：0～0.10％、 Sn: 0~0.10%, As: 0~0.10%, and The rest: Fe and impurities, When the position that is 1 / 4 of the thickness in the thickness direction from the surface is set as the 1 / 4 depth position, At the 1 / 4 depth position, the number density of ε carbides with an equivalent circle diameter of 5 nm or more is 20 per μm. 2 above, The hot stamping formed body has a tensile strength of 2100 MPa or more.

2. The hot stamped body according to claim 1, wherein When a position 50 μm away from the surface in the thickness direction is set as a 50 μm depth position, The hardness at the 50 μm depth position is smaller than the hardness at the 1 / 4 depth position.

3. The hot stamped body according to claim 2, wherein: The hardness at the 50 μm depth position is smaller than the hardness at the 1 / 4 depth position by at least HV100 in Vickers hardness.

4. The hot stamped body according to any one of claims 1 to 3, wherein The chemical composition comprises, in mass %, Nb: 0.010~0.100%, Ti: 0.010~0.100%, Cr: 0.03-0.50%, and V：0.01～0.50％、 One or more of the group consisting of.

5. The hot stamped body according to any one of claims 1 to 3, wherein The chemical composition comprises, in mass %, Mo: 0.05~0.50%, B：0.0010～0.0100％、 Co: 0.01-1.00%, Ni: 0.10-1.00%, Cu: 0.10-1.00%, and W：0.10～3.00％、 One or more of the group consisting of.

6. The hot stamped body according to any one of claims 1 to 3, wherein The chemical composition comprises, in mass %, O：0.001～0.100％、 Ca: 0.01-1.00%, Mg: 0.01~1.00%, REM: 0.0001~0.0050% Sb: 0.001~0.020%, Zr：0.01～0.10％、 Sn: 0.01-0.10%, and As: 0.01~0.10%, One or more of the group consisting of.

7. A steel plate having the following chemical composition: comprising, by mass %, C：0.40～1.00％、 Si: 0.01-1.00%, Mn: 0.01% or more and less than 1.00%, P: 0.100% or less, S: 0.01000% or less, Al：0.0010～1.0000％、 N: 0.0150% or less, Nb: 0~0.100%, Ti: 0~0.100%, Cr：0～0.50％、 V：0～0.50％、 Mo: 0~0.50%, B：0～0.0100％、 Co: 0-1.00%, Ni: 0-1.00%, Cu: 0-1.00%, W：0～3.00％、 O：0～0.100％、 Ca: 0-1.00%, Mg: 0-1.00%, REM: 0~0.0050% Sb: 0~0.020%, Zr：0～0.10％、 Sn: 0~0.10%, As: 0~0.10%, and The rest: Fe and impurities, When the position that is 1 / 4 of the plate thickness in the plate thickness direction from the surface is set as the 1 / 4 depth position, At the 1 / 4 depth position, the microstructure comprises, in terms of area ratio: Ferrite: more than 50% and less than 100%, Pearlite: 0-40%, Bainite, martensite, austenite: the total is 0% or more and less than 10%, At the 1 / 4 depth position, the number density of one or more carbides of Nb, Ti, Fe, Mo, W and Cr with an equivalent circle diameter of 0.2 μm or more is less than 5.0 per 10 μm. 2 .

8. The steel plate according to claim 7, wherein: When a position 50 μm away from the surface in the plate thickness direction is set as a 50 μm depth position, The hardness at the 50 μm depth position is smaller than the hardness at the 1 / 4 depth position.

9. The steel plate according to claim 7 or 8, wherein: The chemical composition comprises, in mass %, Nb: 0.010~0.100%, Ti: 0.010~0.100%, Cr: 0.03-0.50%, and V：0.01～0.50％、 One or more of the group consisting of.

10. The steel plate according to claim 7 or 8, wherein: The chemical composition comprises, in mass %, Mo: 0.05~0.50%, B：0.0010～0.0100％、 Co: 0.01-1.00%, Ni: 0.10-1.00%, Cu: 0.10-1.00%, and W：0.10～3.00％、 One or more of the group consisting of.

11. A method for producing a hot stamped part, comprising the following steps: A hot stamping step of heating the steel sheet according to claim 7 to a maximum heating temperature of at least the higher of the Ac3 point and 800°C and not more than 950°C, holding the steel sheet at the maximum heating temperature for 60 to 720 seconds, and then cooling the steel sheet to 300°C or less so that the average cooling rate from the maximum heating temperature to 300°C is 10 to 500°C / second; and A tempering step of tempering the steel sheet after the hot stamping step, In the tempering process, Maintaining the steel plate at 80-300° C. for more than 6.0 seconds; Cooling the steel plate to below 80°C at an average cooling rate of 20-500°C / s, and then reheating the steel plate and maintaining the temperature at 80-300°C for more than 6.0 seconds; or The steel plate is held at 80 to 300° C. for more than 6.0 seconds, cooled to below 80° C. at an average cooling rate of 20 to 500° C. / second, and then reheated and held at 80 to 300° C. for more than 6.0 seconds.

12. A method for manufacturing a steel plate, comprising the following steps: The present invention relates to a composite material comprising, in mass%, C: 0.40-1.00%, Si: 0.01-1.00%, Mn: 0.01% or more and less than 1.00%, P: 0.100% or less, S: 0.01000% or less, Al: 0.0010-1.0000%, N: 0.0150% or less, Nb: 0-0.100%, Ti: 0-0.100%, Cr: 0-0.50%, V: 0-0.50%, Mo: 0-0.50%, B: 0-0.010% or less, and the like. a heating step of heating a slab having a chemical composition of 0%, Co: 0-1.00%, Ni: 0-1.00%, Cu: 0-1.00%, W: 0-3.00%, O: 0-0.100%, Ca: 0-1.00%, Mg: 0-1.00%, REM: 0-0.0050%, Sb: 0-0.020%, Zr: 0-0.10%, Sn: 0-0.10%, As: 0-0.10%, and the balance: Fe and impurities to 1150-1350° C.; a hot rolling step of hot rolling the slab after the heating step so that the finishing rolling temperature becomes 800 to 950° C. to obtain a steel plate; a cooling step of cooling the hot-rolled steel plate to a temperature of 750° C. or lower at an average cooling rate of 10 to 100° C. / second, starting within 5.0 seconds after completion of the hot rolling step; a step of coiling the steel sheet after the cooling step at a coiling temperature exceeding 500° C. and not exceeding 750° C., and setting an average cooling rate from the coiling temperature to 500° C. to exceed 50° C. / hour; and A cold rolling step is performed on the steel sheet after the coiling step at a thickness reduction rate of 10 to 60%.

13. The method for manufacturing a steel plate according to claim 12, wherein: After the cold rolling step, a heat treatment step is performed in which the steel sheet is heated to an annealing temperature of 700 to 920° C. and maintained at the annealing temperature for 120 to 500 seconds in an atmosphere with an oxygen potential of −1.50 or higher.

14. The method for manufacturing a steel plate according to claim 13, wherein: After the heat treatment step, a skin pass rolling step is performed on the steel sheet at a reduction ratio of 0.05 to 2.0%.

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