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

Abstract

The hot-press molded body has a chemical composition containing, in mass%, 0.25% or more but less than 0.40% of C, 0.01-1.00% of Si, 1.00-2.50% of Mn, 0.01% or less of P, 0.03% or less of S, and 0.0010-1.0000% of Al, and when a position that is 1 / 4 of the thickness from the surface in the thickness direction is defined as a 1 / 4 depth position, the thickness of the hot-press molded body is 1 / 4 depth position, the thickness of the hot-press molded body is 1 / 4 depth position, and the thickness of the hot-press molded body is 1 / 4 depth position, and the thickness of the hot-press molded body is 1 / 4 depth position. And the number density of epsilon carbides having an equivalent circle diameter of 5 nm or more is 20 / [mu] m2 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. 2023-016206, 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 achieve both fuel efficiency and improved crash safety, driven by recent tightening of environmental regulations and crash safety standards. However, with increased strength, the stamping properties of steel sheets decrease, making it difficult to manufacture products with complex shapes.

[0004] Specifically, as the strength increases, the following problems arise: the ductility of the steel sheet decreases, and when it is processed into a complex shape, it breaks at the high processing position. In addition, as the strength of the steel sheet increases, the following problems also arise: rebound and wall warping occur due to residual stress after processing, and dimensional accuracy decreases. Therefore, it is not easy to stamp a steel sheet with high strength, especially a tensile strength of more than 780 MPa, into a product with a complex shape. It is easy to process high-strength steel sheets by roll forming rather than stamping, but its application is limited to parts with the same cross-section in 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 it. Therefore, the steel is soft during forming, offering excellent formability. This allows even high-strength steel sheets to be formed into complex shapes with high precision. Furthermore, hot stamping involves simultaneous quenching of the steel using the stamping die, resulting in a sufficiently strong steel product (hot stamped body).

[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 plate by hot stamping.

[0008] In recent years, countries around the world have set higher CO2 reduction targets, and automobile companies are promoting fuel consumption reduction after considering collision safety. Needless to say, for fuel vehicles, even in the rapidly developing electric vehicles, in order to protect not only passengers but also batteries from collisions and offset their weight increase, higher strength materials are required as their materials. For example, for hot stamped parts used in automobiles, etc., higher strength (more than 1.5 GPa) steel is required, which exceeds the strength of the aforementioned Patent Document 1 and the strength currently used as 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 reducing 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, the present inventors' research results revealed that the method of Patent Document 2, while capable of achieving a certain degree of toughness improvement, cannot necessarily fully meet 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 properties in hot stamped products having high strength (particularly, tensile strength exceeding 1500 MPa), but conventional technologies have not necessarily been able to meet this demand.

[0019] Therefore, in view of the above problems, an object of the present invention is 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 carbides 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 completed 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 has a chemical composition comprising the following components, in mass %, C: 0.25% or more and less than 0.40%, Si: 0.01 to 1.00%, Mn: 1.00 to 2.50%, P: 0.100% or less, S: 0.01000% or less, Al: 0.0010 to 1.0000%, N: 0.0150% or less, Nb: 0 to 0.100%, Ti: 0 to 0.100%, Cr: 0 to 0.50%, V: 0 to 0.50%, Mo: 0 to 0.50%, B: 0 to 0.0100%, Co: 0 to 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 remainder: Fe and impurities, when a position which is 1 / 4 of the thickness from the surface in the thickness direction is referred to as a 1 / 4 depth position, the number density of ε carbides having an equivalent circle diameter (also referred to as "equivalent circle diameter") of 5 nm or more at the 1 / 4 depth position is 20 pieces / μm 2 above.

[0025] [2] The hot stamped product according to [1], wherein, when a position 50 μm from the surface in the thickness direction is defined 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 a depth of 50 μm can be lower than the hardness at a depth of 1 / 4 by at least HV100 in Vickers hardness.

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

[0028] [5] The hot stamped body according to any one of [1] to [4], wherein the chemical composition may include one or more selected from the group consisting of the following components: 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%.

[0029] [6] The hot stamped body according to any one of [1] to [5], wherein the chemical composition may include one or more selected from the group consisting of the following components: in mass %, 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 sheet according to another embodiment of the present invention has a chemical composition comprising the following components: in mass %, C: 0.25% or more and less than 0.40%, Si: 0.01 to 1.00%, Mn: 1.00 to 2.50%, P: 0.100% or less, S: 0.01000% or less, Al: 0.0010 to 1.0000%, N: 0.0150% or less, Nb: 0 to 0.100%, Ti: 0 to 0.100%, Cr: 0 to 0.50%, V: 0 to 0.50%, Mo: 0 to 0.50%, B: 0 to 0.0100%, Co: 0 to 1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0 to 3.00%, O: 0 to 0.100%, Ca: 0 to 1.0 0%, 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 remainder: Fe and impurities, when a position 1 / 4 of the thickness from the surface in the plate thickness direction is defined as a 1 / 4 depth position, at the 1 / 4 depth position, the microstructure comprises, in terms of area ratio, ferrite: greater than 50% and less than 100%, pearlite: 0-40%, bainite, martensite, austenite: a total of 0% or more and less than 10%, and at the 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] According to the steel plate described in [7], when a position 50 μm away from the surface in the plate thickness direction is defined as a 50 μm depth position, the hardness at the 50 μm depth position can 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 includes one or more selected from the group consisting of the following components: in mass %, 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 comprises one or more selected from the group consisting of the following components: 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%.

[0034]

[11] Another embodiment of the present invention is a method for manufacturing a hot stamped formed body according to the present invention, which comprises the following steps: a hot stamping step, wherein the steel plate according to the present invention is heated to a maximum heating temperature of at least the higher of the Ac3 point and 800°C and not more than 950°C, maintained at the maximum heating temperature for 60 to 720 seconds, and then cooled to below 300°C at an average cooling rate of 10 to 500°C / second from the maximum heating temperature to 300°C; and a tempering step, wherein the hot stamped formed body is heated to a maximum heating temperature of at least the higher of the Ac3 point and 800°C and not more than 950°C, maintained at the maximum heating temperature for 60 to 720 seconds, and then cooled to below 300°C at an average cooling rate of 10 to 500°C / second from the maximum heating temperature to 300°C; The steel plate after the pressing process is tempered, 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, and then 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 is a method for manufacturing a steel plate according to [7], comprising the following steps:

[0036] The heating step is to heat the slab to 1150-1350°C, wherein the slab has a chemical composition comprising the following components: in mass %, C: 0.25% or more and less than 0.40%, Si: 0.01-1.00%, Mn: 1.00-2.50%, 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: 0-0.5 0%, 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 remainder: Fe and impurities;

[0037] a hot rolling step of hot rolling the slab after the heating step at a finishing rolling temperature of 800 to 950° C. to obtain a steel plate;

[0038] a cooling step, which is started within 5.0 seconds after the hot rolling step, and cools the steel plate after the hot rolling step to a temperature of 750° C. or less at an average cooling rate of 10 to 100° C. / second;

[0039] a coiling step of coiling the steel sheet after the cooling step at a coiling temperature exceeding 500° C. and not exceeding 750° C., with an average cooling rate from the coiling temperature to 500° C. exceeding 50° C. / hour; and

[0040] A cold rolling step is performed on the steel sheet after the coiling step by cold rolling at a thickness reduction rate of 10 to 60%.

[0041]

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

[12] , wherein, after the cold rolling step, a heat treatment step may be included, in which the steel plate 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.

[0042]

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

[13] , wherein, after the heat treatment step, a skin pass rolling step may be included, in which the steel plate is subjected to skin pass rolling with a reduction ratio of 0.05 to 2.0%.

[0043] Effects of the Invention

[0044] 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

[0045] Hereinafter, a hot stamped product according to an embodiment of the present invention (hot stamped product according to the present embodiment), a steel sheet according to an embodiment of the present invention (steel sheet according to the present embodiment), and methods for manufacturing these will be described.

[0046] <Hot Stamping Formed Body>

[0047] The hot stamped body of this embodiment has a predetermined chemical composition described below. At a 1 / 4 depth position, the number density of ε carbides with an equivalent circle diameter of 5 nm or more is 20 pieces / μm. 2 above.

[0048] In this embodiment, a position ¼ the thickness from the surface in the thickness direction is described as a ¼ depth position, and a position 50 μm from the surface in the thickness direction is described as a 50 μm depth position.

[0049] <Chemical Composition>

[0050] The chemical composition of the hot stamped product of this embodiment will be described. Unless otherwise specified, the percentages of the contents of the elements constituting the chemical composition are mass %.

[0051] When the hot stamped product of the present embodiment is formed of a base steel material and a coating formed on the surface thereof, the chemical composition of the hot stamped product of the present embodiment means the chemical composition of the base steel material.

[0052] C: 0.25% or more and less than 0.40%

[0053] 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 0.25%, it is difficult to ensure sufficient strength in the hot stamped product. Therefore, the C content is set to 0.25% or more. The C content is preferably set to 0.27% or more, and more preferably set to 0.28% or more.

[0054] On the other hand, if the C content is 0.40% or more, the strength of the hot stamped body increases, and the bendability and ductility may decrease. Therefore, the C content is set to less than 0.40%. The C content is preferably set to 0.35% or less.

[0055] Si: 0.01~1.00%

[0056] Si is an effective element for 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.

[0057] 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) increases significantly. As a result, the cost required for heat treatment sometimes increases, or ferrite remains during heating, resulting in a decrease in the strength of the hot stamped body. In addition, if the Si content exceeds 1.00%, the generation behavior of oxide scale in steel plate manufacturing sometimes changes, impairing 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.

[0058] Mn: 1.00~2.50%

[0059] Mn contributes to the strength of hot stamped parts through solid solution strengthening. Furthermore, Mn is a very effective element for improving the hardenability of steel and stably ensuring the strength of hot stamped parts. To achieve these effects, the Mn content is set to 1.00% or more.

[0060] On the other hand, if the Mn content exceeds 2.50%, coarse inclusions such as MnS are likely to form in the steel, which may reduce bendability and ductility. Therefore, the Mn content is set to 2.50% or less. The Mn content is preferably set to 2.30% or less.

[0061] P: 0.100% or less

[0062] P is an element that segregates at grain boundaries and reduces the strength of 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 0.050% or less or 0.035% or less. The lower limit of the P content does not need to be specifically limited, and its 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 be set to 0.0001% or more.

[0063] S: 0.01000% or less

[0064] 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. The lower limit of the S content does not need to be specifically limited, and its lower limit is 0%. However, when the S content is reduced to less than 0.00015%, the cost of desulfurization increases significantly, which is not economically preferred. In actual operation, the S content can be set to more than 0.00015% and more than 0.00020%.

[0065] Al: 0.0010~1.0000%

[0066] Al is an element that deoxidizes molten steel, improving its soundness (suppressing defects such as pores in the steel). If the Al content is less than 0.0010%, deoxidation will not proceed sufficiently. 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.

[0067] 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.

[0068] N: 0.0150% or less

[0069] Nitrogen is an element that forms nitrides in steel. These nitrides serve as the starting point of fracture, so 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.

[0070] The lower limit of the N content does not need to be specifically specified; 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 can be set to 0.0001% or higher, or 0.0004% or higher, or 0.0010% or higher.

[0071] The chemical composition of the hot stamped body of this embodiment may be composed of the aforementioned elements (essential elements), with the remainder consisting of Fe and impurities. Alternatively, to improve various properties, one or more of the following elements (optional elements) may be further included. Since the inclusion of optional elements is not essential, the lower limit of their content is 0%.

[0072] Nb: 0~0.100%

[0073] Nb is an element that increases the strength of hot stamped parts by solid solution strengthening and contributes to the refinement of prior austenite grains by forming carbonitrides. Therefore, Nb may also 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 more than 0.100% of Nb is contained, Nb-based carbonitrides are excessively generated, sometimes inhibiting the formation of ε carbides that contribute to improving the toughness of 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.

[0074] Ti: 0~0.100%

[0075] Ti is an element that forms fine carbides, carbonitrides, and the like in steel along with Nb. These fine carbides and carbonitrides suppress Cu hot embrittlement cracking during the hot rolling process through the resulting grain refinement, while also improving 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, described later. Therefore, Ti may be included.

[0076] 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.

[0077] On the other hand, if the Ti content exceeds 0.100%, coarse TiN is generated, and the toughness of the hot stamped body deteriorates. 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.

[0078] Cr: 0~0.50%

[0079] Cr is an effective element for improving the hardenability of steel and stably ensuring the strength of hot stamped parts. Therefore, it can also be contained. When achieving the above effects, the Cr content is preferably set to 0.03% or more, and more preferably set to 0.05% or more.

[0080] 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.

[0081] V: 0~0.50%

[0082] 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.

[0083] On the other hand, if the V content exceeds 0.50%, excessive V-based carbonitrides may be generated, sometimes 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.

[0084] Mo: 0~0.50%

[0085] Mo is an extremely effective element for improving the hardenability of steel and ensuring stable strength in hot-stamped parts. In particular, its inclusion in combination with boron produces a synergistic effect in improving hardenability. Therefore, it is acceptable to include this element. To achieve these effects, the Mo content is preferably set to 0.05% or more, and more preferably 0.10% or more.

[0086] On the other hand, 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.

[0087] B: 0~0.0100%

[0088] Boron (B) is an element that improves the hardenability of steel even in trace amounts. Furthermore, B strengthens grain boundaries by segregating at them. Therefore, it may be included. To achieve the aforementioned effects, the B content is preferably set to 0.0010% or more.

[0089] 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.

[0090] Co: 0-1.00%

[0091] 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.

[0092] 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 be set to 0.10% or less.

[0093] Ni: 0-1.00%

[0094] Ni is an effective element for improving the hardenability of steel and stably ensuring the strength of hot-stamped parts. Furthermore, Ni suppresses Cu hot embrittlement cracking during steel sheet production. Therefore, Ni may be included. To achieve these effects, the Ni content is preferably set to 0.10% or more, and more preferably 0.20% or more.

[0095] On the other hand, if the Ni content exceeds 1.00%, the above 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.

[0096] Cu: 0-1.00%

[0097] Cu is an effective element 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.

[0098] On the other hand, if the Cu content exceeds 1.00%, the above 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.

[0099] W: 0~3.00%

[0100] W is an effective element for improving the hardenability of steel and stably ensuring the strength of hot stamped products. In order to obtain the above effects, the W content is preferably set to 0.10% or more.

[0101] On the other hand, if the W content exceeds 3.00%, the above-mentioned effect is saturated and the cost increases. Therefore, the Cu content is set to 3.00% or less.

[0102] O: 0~0.100%

[0103] O is an element that, when present in large amounts in steel, forms coarse oxides that serve as starting points for fracture, degrading 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.

[0104] 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.

[0105] Ca: 0-1.00%

[0106] Ca is an element that deoxidizes molten steel and suppresses the formation of oxides that can cause 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.

[0107] On the other hand, even if a large amount 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.

[0108] Mg: 0-1.00%

[0109] Mg is an element that deoxidizes molten steel and improves its soundness. Therefore, it may be included. 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.

[0110] 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.

[0111] REM: 0~0.0050%

[0112] REM is an element that deoxidizes molten steel and suppresses the formation of oxides that can cause damage. Therefore, this element 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.

[0113] 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.

[0114] In the present embodiment, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoid elements, and the content of REM refers to the total content of these elements.

[0115] Sb: 0~0.020%

[0116] Sb is an element that deoxidizes molten steel, suppressing the formation of oxides that can cause fracture, thereby improving the deformability of hot stamped parts. Therefore, it may be included. To achieve the above-mentioned effects, the Sb content is preferably set to 0.001% or more, and more preferably set to 0.005% or more.

[0117] On the other hand, even if a large amount is contained, the above-mentioned effect is saturated, so the Sb content is made 0.020% or less, and preferably 0.015% or less.

[0118] Zr: 0~0.10%

[0119] Zr is an element that helps control inclusions, particularly by finely dispersing them, and thus improves the toughness of hot-stamped parts. Therefore, it may be included. To achieve these effects, the Zr content is preferably set to 0.01% or more, and more preferably 0.03% or more.

[0120] On the other hand, if Zr is contained in a large amount, the surface properties may be significantly deteriorated. Therefore, the Zr content is set to 0.10% or less. The Zr content is preferably 0.08% or less.

[0121] Sn: 0~0.10%

[0122] Sn is an element that improves hydrogen embrittlement resistance. Therefore, it may be contained. In order to obtain the above-mentioned effect, the Sn content is preferably set to 0.01% or more.

[0123] 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.

[0124] As: 0~0.10%

[0125] As is an element that contributes to improving hydrogen embrittlement resistance by lowering the austenite single-phase transformation temperature and refining the prior austenite grains. Therefore, this element may be contained. To achieve the above-mentioned effects, the As content is preferably set to 0.01% or more.

[0126] 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.

[0127] The remainder: Fe and impurities

[0128] In the chemical composition of the hot stamped product of this embodiment, the remainder other than the aforementioned elements, i.e., Fe and impurities, is present. Specifically, the chemical composition of the hot stamped product of this embodiment may include the essential elements, with the remainder consisting of Fe and impurities. Alternatively, the chemical composition may include the essential elements and one or more optional elements, with the remainder consisting of Fe and impurities.

[0129] Here, "impurities" refer to components that are introduced into steel sheets during industrial production due to various factors, such as raw materials such as ores and scrap, and during the manufacturing process. These impurities are permitted as long as they do not adversely affect the properties of the hot stamped steel sheets of this embodiment. Industrial production methods include blast furnace steelmaking and electric furnace steelmaking, and include the level of impurities introduced during production using either method.

[0130] The chemical composition of the hot stamped body can be determined by the following method.

[0131] Elemental analysis is performed using standard methods such as ICP-AES at a depth of 1 / 4 the thickness from the surface of the hot-stamped part (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.

[0132] [Microstructure]

[0133] 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 a hot stamped body.

[0134] (At the 1 / 4 depth position, the number density of ε carbides with an equivalent circle diameter of 5 nm or more is 20 / μm 2 above)

[0135] In order to obtain high tensile strength, high-strength hot stamped products need to contain a large amount of C and other alloying elements. However, generally speaking, as the strength increases, the toughness of the hot stamped products decreases, and the impact absorption property decreases.

[0136] The present inventors have conducted research on this issue and have found that impact absorption can be improved by allowing ε carbide to exist in a hot stamped body and controlling its size and number density.

[0137] Specifically, it is found that the number density of ε carbides with an equivalent circle diameter of 5 nm or more is 20 per μm. 2 When the value is greater than or equal to 0.5, the impact absorbency is improved.

[0138] Therefore, in the hot stamped body of this 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.

[0139] 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 is difficult to achieve. For example, ε carbides with a diameter of 5 to 50 nm are targeted.

[0140] In addition, even for ε carbides with an equivalent circle diameter of 5 nm or more, the number density is less than 20 per μm. 2 On the other hand, if the number density exceeds 200 / μm 2 , the interface between ε carbide and base material may become the starting point of cracks, so it can be set to 200 / μm 2 the following.

[0141] In the present embodiment, the ε carbide is a carbide having a value of FexC (x: approximately 2 to 3).

[0142] 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 a thin film sample using a field emission transmission electron microscope equipped with an energy dispersive X-ray spectrometer (JEM-2100F manufactured by JEOL Ltd.).

[0143] Specifically, after cutting a small piece of about 10 mm square from the hot stamped body, mechanically or chemically polish both sides to make a thin film TEM sample (thickness of about 60 μm, φ3 mm) at the original 1 / 4 depth position (as long as the distance from the surface in the thickness direction is within the range of 1 / 8 to 3 / 8 of the thickness, it is allowed). In the preparation of this sample, the coarse water-resistant sandpaper of about #120 is gradually polished with fine mesh water-resistant sandpaper, and finally polished with water-resistant sandpaper of about #600, and the sample is punched out with a disc punch. Then, double-sided jet electrolytic polishing is performed until a hole is formed in the center to make a TEM observation sample. The electrolytic polishing device uses TENUPOL-2 made by STRUERS, and the electrolytic polishing liquid is a mixture of 5% perchloric acid and 95% glacial acetic acid solution. The thin film TEM sample is finely processed at a voltage of 70 V. Thin film TEM observation is performed at an acceleration voltage of 200 kV. To reduce variation in each area, the sample was observed in at least five viewing fields, each approximately 100 to 300 nm square. The types of precipitates observed were identified using diffraction patterns and EDX analysis results. Regarding number density, to reduce variation due to viewing field, the number density in each viewing field was counted and the averaged value was used as a representative value.

[0144] (It is preferred that the hardness at a depth of 50 μm is smaller than the hardness at a depth of 1 / 4)

[0145] In the hot stamped body of 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.

[0146] If the hardness at a depth of 50 μm is smaller than that at a quarter depth, an effect can be obtained. 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.

[0147] 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.

[0148] The hardness at a depth of 50 μm can be reduced (lowered) by decarburization by annealing as described later, for example. The target is set at a depth of 50 μm because the hardness near the outermost surface has a large variation in measurement principle.

[0149] The hardness at the 1 / 4 depth position and the hardness at the 50 μm depth position were evaluated by Vickers hardness in accordance with JIS Z2244-1:2020.

[0150] During the measurement, the cross section of the polished test piece was measured at five points at each location under a load of 50 gf, and the average value of the three points excluding the maximum and minimum values ​​was taken as the measured value.

[0151] In the hot stamped body of 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.

[0152] Here, martensite includes so-called fresh martensite and tempered martensite. The area ratio of martensite can be measured in the same manner as in the later-described microstructure observation of the steel sheet.

[0153] [covered]

[0154] The hot stamped product of the present embodiment may be coated partially or entirely on its surface.

[0155] 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.

[0156] A coating mainly composed of an Fe-Al alloy refers to a coating containing a total of 70% by mass or more of Fe and Al, and a coating mainly composed of an Fe-Zn alloy refers to a coating containing a total of 70% by mass or more of Fe and Zn. A coating mainly composed of an Fe-Al alloy may contain, in addition to Fe and Al, 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, with the remainder being impurities. A coating mainly composed of an Fe-Zn alloy may contain, in addition to Fe and Zn, 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, with the remainder being impurities.

[0157] The coating provides corrosion resistance, thereby achieving an effect of improving hydrogen embrittlement resistance during automobile use.

[0158] The coating thickness is preferably 5 to 100 μm.

[0159] The chemical composition and thickness of the coating can be determined by observing a cross section with a scanning electron microscope.

[0160] Specifically, a measurement sample is cut 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, for example, 400 times the magnification, which is 40,000 μm in area. 2 The cut sample was mechanically polished and then mirror-finished. The coating thickness of 10 random viewing fields was measured and the average value was taken as the coating thickness.

[0161] When observing with 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 evenly spaced within the observation photograph, with a distance of 6.50 μm between the measurement points. Furthermore, five fields of view are observed according to the above procedure, and the average value is used as the coating thickness.

[0162] The chemical composition of the coating can be determined by performing point elemental analysis (beam diameter: 1 μm or less) using an electron probe microanalyzer (EPMA) within the same observation range as above. The Fe, Al, and Zn contents in the coating can be determined by performing a point elemental analysis (beam diameter: 1 μm or less). Analysis is performed at a total of 10 points within the coating within any 10 viewing fields, and the average value is used as the Fe, Al, and Zn contents in the coating. The same method is used to determine the Fe, Al, and Zn contents in the coating even when elements other than Fe, Al, and Zn are present.

[0163] The surface serving as a reference for the above-mentioned 1 / 4 depth position and 50 μm depth position is the surface of the hot stamped body, but when the hot stamped body has a coating, that is, when 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.

[0164] [Mechanical properties]

[0165] (Tensile Strength)

[0166] In consideration of the contribution to fuel efficiency improvement and collision safety improvement when applied to automobile parts, the hot stamped product of this embodiment preferably has a tensile strength of 1800 MPa or more.

[0167] There is no upper limit for the tensile strength. However, as the tensile strength increases, the impact absorbability may decrease. Therefore, the tensile strength may be set to less than 2200 MPa.

[0168] The tensile strength can be determined by taking a No. 5 test piece described in JIS Z2241:2011 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 Z2241:2011.

[0169] (Shock Absorption)

[0170] As described above, the hot stamped body of the present embodiment can achieve excellent impact absorption by controlling the chemical composition and the existence state of ε carbide.

[0171] As a target for impact absorption, the maximum bending angle (Bending angle at maximum force) under the VDA (German Association of the Automotive Industry) bend test (238-100) is 40° or greater, calculated 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.

[0172] <Steel Plate>

[0173] Next, the steel sheet of this embodiment will be described. By hot stamping and tempering the steel sheet of this embodiment, the hot stamped body of this embodiment can be obtained. Therefore, the steel sheet of this embodiment is suitable as a raw material (hot stamping steel sheet) for the hot stamped body of this embodiment.

[0174] [Chemical composition]

[0175] The chemical composition of the steel plate of this embodiment needs to be set in a manner to obtain the preferred characteristics of the hot stamped formed body obtained by hot stamping and tempering, but the chemical composition does not substantially change due to hot stamping and tempering, so the chemical composition of the steel plate of this embodiment can be the same as that of the hot stamped formed body of this embodiment.

[0176] [Microstructure]

[0177] The microstructure of the steel plate of this embodiment will be described with the position 1 / 4 of the plate thickness from the surface in the plate thickness direction being the 1 / 4 depth position, the range 50 μm from the surface in the plate thickness direction being the surface layer portion, and the position 50 μm from the surface in the plate thickness direction being the 50 μm depth position. The 1 / 4 depth position represents a representative microstructure of the steel plate.

[0178] (At the 1 / 4 depth position, the following are included: by area ratio: 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%)

[0179] In the steel sheet of 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.

[0180] Pearlite is a structure composed of ferrite and cementite arranged in layers. In other words, a high pearlite area ratio 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 will not be obtained. Therefore, the pearlite area ratio is set to less than 40%. The pearlite area ratio can be 0% or 5% or higher.

[0181] The presence of bainite, martensite, and austenite increases the strength of the steel sheet before hot stamping. This can result in rough cut edges or cracks during trimming before hot stamping. Therefore, the combined area ratio of bainite, martensite, and austenite is set to 0% or more and less than 10%.

[0182] 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.

[0183] The L-section of the steel plate (a cross section parallel to the rolling direction and the thickness direction) was mirror-polished and then etched with Nital. Ten fields of view of each sample were observed using an FE-SEM at a magnification of 3000x, and the area ratio of each phase at the 1 / 4 depth position was calculated.

[0184] At this time, the tissues were identified based on the following characteristics of each tissue in the FE-SEM image.

[0185] Ferrite consists of massive grains and does not contain 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 massive ferrite). Bainite and tempered martensite are structures composed of lath-shaped grains and carbides, but differ from each other in the following ways.

[0186] First, bainite is observed to be divided into upper bainite and lower bainite. Upper bainite is a collection of lath-shaped grains, and is a collection of laths containing carbides between the laths. Lower bainite is a collection of lath-shaped 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 means that the difference in the elongation direction of the iron-based carbide group is within 5°. The area ratio of bainite is determined by the sum of the area ratios of upper bainite and lower bainite. Tempered martensite is a collection of lath-shaped grains like lower bainite, and is a structure containing iron-based carbides inside, but the carbides select two or more variants, so it is a structure in which the elongation directions of the iron-based carbides are two or more.

[0187] In this way, ferrite, pearlite, tempered martensite, and bainite can be characterized and identified separately by FE-SEM.

[0188] On the other hand, untempered fresh martensite and retained austenite are not fully corroded during nital etching. Therefore, in the observation performed 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 determined. 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 has been 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 used as the area ratio of retained austenite. The measurement is performed three times for each sample, and the average value obtained is used 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.

[0189] The above-mentioned area ratio can also be obtained in the hot stamped body in the same manner.

[0190] (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 / 10 μm 2 )

[0191] In the hot stamped part of the present embodiment described above, ε carbide is precipitated by hot stamping and tempering under specified conditions, thereby obtaining a specified ε carbide. However, if carbides exist in the raw steel sheet and these carbides do not dissolve during the hot stamping heating, the specified ε carbide cannot be obtained even after hot stamping and tempering.

[0192] For example, coarse carbides generated in the steel sheet do not dissolve during the heating process of hot stamping and tend to dissolve and remain. Therefore, in the steel sheet of 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 ).

[0193] 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.

[0194] The number density of carbides may not exist and may be 0.0 per 10 μm. 2 However, considering that the formation of ε carbides after hot stamping is not significantly hindered, the number can be 30 / 10000μm. 2 (0.03 pieces / 10μm 2 )above.

[0195] The upper limit of the equivalent circle diameter of the target carbide is not limited, but it can be 2.0 μm or less to avoid excessive influence caused by the variation of the hardenability during hot stamping. In other words, carbides with an equivalent circle diameter of 0.2 to 2.0 μm can also be targeted.

[0196] 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.

[0197] After mirror polishing the L-section of the steel plate, it was etched with nitric alcohol. The 1 / 4 depth position of the sample was observed using a scanning microscope. In the 50μm square area at this depth position, the observed precipitates were analyzed by EDX for composition. If they contained one or more of Nb, Ti, Fe, Mo, W and Cr and C, they were judged to be the target carbides. Regarding the number density, in order to reduce the deviation caused by the field of view, at least 5 fields of view were observed in the 50μm square area, and the number density of the above-mentioned carbides with an equivalent circle diameter of 0.2μm or more was counted, and the average was taken as the representative value of the number density.

[0198] (It is preferred that the hardness at a depth of 50 μm is smaller than the hardness at a depth of 1 / 4)

[0199] In the steel sheet as a raw material, by making the hardness at a depth of 50 μm smaller than the hardness at a quarter depth, the hardness at a depth of 50 μm in the hot stamped body can be made smaller than the hardness at a quarter depth. Therefore, for the steel sheet of this embodiment, it is preferred that the hardness at a depth of 50 μm is smaller than the hardness at a quarter depth.

[0200] [Plate thickness]

[0201] The plate thickness of the steel plate of the present embodiment is not limited, but is preferably 1.0 to 3.5 mm, assuming use as a steel plate for automobiles.

[0202] [covered]

[0203] The steel plate of this embodiment may have a coating on a portion of the surface. The coating may be a coating mainly composed of Al (Al-based coating) or a coating mainly composed of Zn (Zn-based coating). The coating is also called a film or a plating. A coating mainly composed of Al refers to a coating containing more than 70% by mass of Al, and a coating mainly composed of Zn refers to a coating containing more than 70% by mass of Zn. In addition to Al, the coating mainly composed of Al 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, and REM, with the remainder being impurities. In addition to Zn, the Zn-based coating 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, with the remainder being impurities.

[0204] 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.

[0205] <Manufacturing method>

[0206] The steel sheet and the hot stamped body of the present embodiment can achieve the effects as long as they have the above-mentioned characteristics regardless of the manufacturing method, but can be preferably manufactured according to the manufacturing method described below.

[0207] [Method for manufacturing steel sheet]

[0208] The steel sheet of the present embodiment can be obtained by a production method including the following steps.

[0209] (I) a heating process, in which a slab having a predetermined chemical composition is heated to 1150-1350° C.;

[0210] (II) a hot rolling step of hot rolling the slab after the heating step at a finishing rolling temperature of 800 to 950° C. to obtain a steel plate;

[0211] (III) a cooling step, which is started within 5 seconds after the hot rolling step, and cools the steel plate after the hot rolling step to a temperature of 750° C. or less at an average cooling rate of 10 to 100° C. / second;

[0212] (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., with an average cooling rate from the coiling temperature to 500° C. exceeding 50° C. / hour; and

[0213] (V) a cold rolling step of cold-rolling the steel sheet after the coiling step at a thickness reduction rate of 10 to 60%.

[0214] Furthermore, the method for manufacturing a steel plate according to the present embodiment may further include one or more of the following steps.

[0215] (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.

[0216] (VII) a skin pass rolling step, in which, after the heat treatment step, the steel sheet is subjected to skin pass rolling at a reduction ratio of 0.05 to 2.0%.

[0217] (VIII) A coating step of forming a coating on the surface of the steel sheet.

[0218] Preferred conditions for each step are described below. For undescribed conditions or steps, known conditions can be applied to the steps.

[0219] (Heating process)

[0220] The heating process is to heat the slab before hot rolling. The heating temperature is 1150-1350℃.

[0221] When the heating temperature is lower than 1150°C, carbides formed during casting do not melt, and coarse carbides remain even after the hot rolling process.

[0222] 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.

[0223] The chemical composition of the slab subjected to the heating step may be the same as the chemical composition of the steel plate to be obtained.

[0224] (Hot rolling process)

[0225] In the hot rolling step, the slab after the heating step is hot rolled at a finishing rolling temperature of 800 to 950° C. to obtain a steel plate.

[0226] 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 that are flat in the rolling direction remain, potentially causing anisotropy in the properties of the steel sheet. On the other hand, if the finishing rolling temperature exceeds 950°C, the grains of the steel sheet will coarsen.

[0227] (Cooling process)

[0228] After finish rolling, if the steel sheet is held 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 at an average cooling rate of 10 to 100°C / second to a cooling stop temperature of 750°C or less. Furthermore, this cooling process begins within 5.0 seconds of the completion of the hot rolling process.

[0229] 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 end 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.

[0230] 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 uniformly cool the steel sheet, and a defective sheet shape may occur.

[0231] (Coiling process)

[0232] The coiling step is to coil the steel sheet after the cooling step 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.

[0233] If the coiling temperature is 500°C or lower, hard phases such as bainite and martensite are formed, sometimes 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 concentrate in cementite present at the ferrite grain boundaries and in pearlite, sometimes remaining as undissolved carbides during annealing or hot stamping in subsequent steps.

[0234] On the other hand, coiling slows the cooling rate. If the average cooling rate to 500°C is slow, the internal oxide layer develops, increasing the load of the pickling process, or coarse carbides are generated during cooling. Therefore, the average cooling rate from the coiling temperature to 500°C is set to exceed 50°C / hour.

[0235] (Cold rolling process)

[0236] The cold rolling step is to cold-roll the steel sheet after the coiling step at a thickness reduction rate (reduction rate) of 10 to 60% to adjust the sheet thickness to a predetermined thickness.

[0237] (Heat treatment process)

[0238] The heat treatment step is to heat the cold-rolled steel sheet to an annealing temperature of 700 to 920° C., and maintain the annealing temperature for 120 to 500 seconds in an atmosphere with an oxygen potential of −1.50 or higher.

[0239] The heat treatment step is not essential, but is preferable because it reduces C (decarburization) in the surface layer of the steel plate and softens the surface layer of the steel plate by performing the heat treatment under the above-mentioned conditions.

[0240] 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.

[0241] On the other hand, if the annealing temperature exceeds 920° C., the crystal grains will coarsen.

[0242] On the other hand, if the holding time exceeds 500 seconds, productivity deteriorates, which causes an increase in material costs.

[0243] The upper limit of the oxygen potential is not limited. If the oxygen potential is too high, Fe and other alloy elements in the outermost layer will be oxidized, leaving an oxide scale pattern on the surface. Therefore, it is preferably set to -0.50 or less.

[0244] Oxygen potential refers to the partial molar Gibbs free energy of oxygen expressed as a function of the oxygen partial pressure in the gas phase and temperature.

[0245] <Coating process>

[0246] The surface may be coated as required. The coating method is not particularly limited and may include electroplating, vacuum evaporation, cladding, and spraying, among others, including hot-dip coating. Hot-dip coating is the most popular method in industry.

[0247] Examples of the coating include an Al-based coating containing Al and a Zn-based coating containing Zn.

[0248] When forming an Al-based coating by hot-dip plating, Fe is often mixed into the plating bath as an impurity in addition to Al. Furthermore, as long as the plating bath contains 70% by mass or more of Al, 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, and misch metals in addition to the above elements.

[0249] 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, 650-750°C for Al-based coating and 420-500°C for Zn-based coating) after annealing and then hot dip coated without cooling to room temperature.

[0250] There are no particular restrictions on the pre-treatment and post-treatment of the coating, and pre-coating, solvent coating, alloying treatment, temper rolling, etc. can be performed. As an alloying treatment, for example, annealing at 450-800°C can be performed. In addition, as a post-treatment, temper rolling is useful for shape adjustment, etc., and can be performed with a reduction of, for example, 0.1-0.5%.

[0251] (Skin-pass rolling process)

[0252] 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.

[0253] 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.

[0254] In order to obtain this effect, the reduction ratio of skin pass rolling is preferably set to 0.05% or more, and more preferably 0.1% or more.

[0255] On the other hand, if the skin-pass rolling reduction exceeds 2.0%, the load on the skin-pass rolling process increases, which causes an increase in material cost. Therefore, when skin-pass rolling is performed, the reduction is preferably set to 2.0% or less.

[0256] [Method for producing hot stamped body]

[0257] 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.

[0258] (i) a hot stamping step comprising heating the steel sheet of 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 temperature 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 at an average cooling rate of 10 to 500°C / second from the maximum heating temperature to 300°C; and

[0259] (ii) a tempering step of tempering the steel sheet after the hot stamping step.

[0260] Each process is described.

[0261] (Hot stamping process)

[0262] In the hot stamping process, the steel plate of 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 held at the maximum heating temperature for 60 to 720 seconds, the steel plate is cooled to below 300°C at an average cooling rate of 10 to 500°C / second from the maximum heating temperature to 300°C.

[0263] This step is to dissolve carbides present in the steel sheet and thereby increase the strength.

[0264] If the maximum heating temperature is lower than the Ac3 point, or lower than 800° C., or the holding time is lower than 60 seconds, carbides are not sufficiently dissolved, or austenite transformation becomes insufficient, and sufficient strength cannot be obtained after the hot stamping step.

[0265] 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 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.

[0266] Furthermore, when cooling from the maximum heating temperature to 300°C or lower, if the average cooling rate to 300°C is less than 10°C / s, sufficient quenching cannot be performed, and sufficient tensile strength cannot be obtained in the hot stamped body.

[0267] On the other hand, if the average cooling rate to 300° C. exceeds 500° C. / s, the cooling rate at each location will vary greatly, causing distortion in the shape of the molded article.

[0268] The Ac3 point can be determined from the point of change in thermal expansion coefficient when the heating rate is set to 5°C / second by, for example, a plate Formastor test.

[0269] (Tempering process)

[0270] In the tempering step, the steel sheet after the hot stamping step is tempered at a temperature of 80 to 300°C.

[0271] When the cooling stop temperature in the hot stamping process is 80-300°C, the tempering can be maintained as it is, or the steel sheet can be temporarily cooled to a temperature below 80°C and then heated to 80-300°C again and maintained at this temperature. Alternatively, after the hot stamping process, the steel sheet can be maintained at 80-300°C, temporarily cooled to a temperature below 80°C, and then heated to 80-300°C again and maintained at this temperature.

[0272] 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.

[0273] To fully precipitate ε carbides, the holding temperature at 80-300°C is set to 6 seconds or longer, regardless of whether or not 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.

[0274] Example

[0275] Slabs having the chemical compositions listed in Table 1-1 and Table 1-2 were prepared.

[0276] 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.

[0277] For the hot-rolled steel sheet, after cold rolling at the reduction rate of Table 2-2, except for a part, heat treatment is performed under the conditions of Table 2-2. In addition, for some examples, the coating (hot-dip galvanized layer or hot-dip aluminum layer) is formed by hot dipping. 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 aluminum layer. In addition, for some examples, skin-pass rolling is performed. "-" in Table 2-2 means not implemented.

[0278] Thus, steel plates Nos. 1 to 27 and 101 to 114 were obtained.

[0279] 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 with an equivalent circle diameter of 0.2 μm or more at the 1 / 4 depth position were measured according to the above-mentioned procedures.

[0280] Furthermore, the Vickers hardness at a depth of 50 μm and a depth of 1 / 4 was measured according to the above-mentioned procedure.

[0281] The results are shown in Table 2-3.

[0282] Table 1-1

[0283]

[0284] Table 1-2

[0285]

[0286] Table 2-1

[0287]

[0288] Table 2-2

[0289]

[0290] Table 2-3

[0291]

[0292] Next, the resulting steel sheets were 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), with all other conditions being the same. "-" in Table 3-1 indicates that this was not performed.

[0293] 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.

[0294] Furthermore, the Vickers hardness at a depth of 50 μm and a depth of 1 / 4 was measured according to the above-mentioned procedure.

[0295] The results are shown in Table 3-2.

[0296] Furthermore, the limit bending angle was determined as an index of the tensile strength (TS) and impact absorbability of the obtained hot stamped product.

[0297] (Tensile Strength)

[0298] The tensile strength was determined by taking a No. 5 test piece described in JIS Z2241:2011 from a position as flat as possible on the hot stamped part and subjecting the test piece to a tensile test according to the test method described in JIS Z2241:2011. The measurement was performed twice on each steel sheet, and the average value was used as the measured value.

[0299] If the tensile strength is 1800 MPa or more, it is determined that a preferable strength is obtained.

[0300] (Ultimate bending angle)

[0301] Regarding the limit bending angle, a test piece 60 mm wide by 30 mm long (length parallel to the rolling direction) was cut from a flatter position of the hot stamped part. Using this test piece, the steel sheet was bent along VDA 238-100 while applying a load between a pair of rollers with a punch. The bending angle of the steel sheet at which the punch's reaction force reached its maximum was defined as the limit bending angle of the steel sheet. The measurement was performed twice for each steel sheet, and the average value was used as the measured value.

[0302] If the limit bending angle is 40° or more when converted to a plate thickness of 2.0 mm, the plate is judged to have excellent impact absorption properties.

[0303] Table 3-1

[0304]

[0305] Table 3-2

[0306]

[0307] As can be seen from Tables 1-1 to 3-2, the chemical composition and number density of ε carbides in the hot stamped parts (formed parts No. H1 to H27) as inventive examples are within the range of the present invention, and are hot stamped parts having both high strength and excellent impact absorption.

[0308] In contrast, in the hot stamped parts (formed parts No. H101 to H107, H109, ​​H111 to H113) used as comparative examples, the steel plate as the raw material is not preferred. Even if the hot stamped parts are made, the chemical composition or the number density of ε carbides are outside the scope of the present invention, so the tensile strength or impact absorption (limit bending angle) is poor.

[0309] No. H108 and H114 were suitable steel sheets as raw materials, but 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).

[0310] In No. H110, since the steel sheet had a microstructure containing a large amount of martensite, cracks occurred at the end portion during the trimming step for hot stamping, and therefore hot stamping was not performed.

[0311] Industrial applicability

[0312] 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.

[0313] This hot stamped body meets the requirements of recent years for automobile steel sheets, which require both high strength and impact absorption, and can reduce automobile fuel consumption and improve collision safety.

Claims

1. A hot stamped body having a chemical composition comprising the following components: C: 0.25% or more and less than 0.40%, Si: 0.01-1.00%, Mn: 1.00~2.50%, 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 1 / 4 of the thickness from the surface in the thickness direction is defined as the 1 / 4 depth position, the number density of ε carbides with an equivalent circle diameter of 5 nm or more at the 1 / 4 depth position is 20 pieces / μm. 2 above. 2 . The hot stamped body according to claim 1 , wherein, when a position 50 μm from the surface in the thickness direction is defined as a 50 μm depth position, the hardness at the 50 μm depth position is smaller than the hardness at the ¼ depth position.

3. The hot stamped body according to claim 2, wherein: The hardness at the 50 μm depth is smaller than the hardness at the 1 / 4 depth 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 one or more selected from the group consisting of the following components: In mass %, Nb: 0.010~0.100%, Ti: 0.010~0.100%, Cr: 0.03-0.50%, and V：0.01～0.50％。 5. The hot stamped body according to any one of claims 1 to 3, wherein The chemical composition comprises one or more selected from the group consisting of the following components: 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％。 6. The hot stamped body according to any one of claims 1 to 3, wherein The chemical composition comprises one or more selected from the group consisting of the following components: 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%.

7. A steel plate having a chemical composition comprising the following components: In mass %, C: 0.25% or more and less than 0.40%, Si: 0.01-1.00%, Mn: 1.00~2.50%, 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 a position 1 / 4 of the plate thickness from the surface in the plate thickness direction is defined as a 1 / 4 depth position, the microstructure at the 1 / 4 depth position comprises, in terms of area ratio, ferrite: greater than 50% and less than 100%, pearlite: 0 to 40%, and bainite, martensite, and austenite: a total of 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 . The steel plate according to claim 7 , wherein, when a position 50 μm from the surface in the plate thickness direction is defined as a 50 μm depth position, the hardness at the 50 μm depth position is smaller than the hardness at the ¼ depth position.

9. The steel plate according to claim 7 or 8, wherein: The chemical composition comprises one or more selected from the group consisting of the following components: In mass %, Nb: 0.010~0.100%, Ti: 0.010~0.100%, Cr: 0.03-0.50%, and V：0.01～0.50％。 10. The steel plate according to claim 7 or 8, wherein: The chemical composition comprises one or more selected from the group consisting of the following components: 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％。 11. A method for producing a hot stamped part, comprising the following steps: a hot stamping step comprising 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, maintaining the temperature at the maximum heating temperature for 60 to 720 seconds, and then cooling the steel sheet to a temperature not more than 300°C at an average cooling rate from the maximum heating temperature to 300°C of 10 to 500°C / second; and A tempering process is performed to temper the steel plate after the hot stamping process. in, In the tempering process, The steel plate is kept at 80-300°C for more than 6.0 seconds, or The steel plate is cooled to below 80°C at an average cooling rate of 20 to 500°C / second, and then reheated and maintained at 80 to 300°C for 6.0 seconds or more, 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 heating process is to heat the slab to 1150-1350°C, wherein: The slab has a chemical composition containing the following components: by mass%, C: 0.25% or more and less than 0.40%, Si: 0.01 - 1.00%, Mn: 1.00 - 2.50%, 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 balance: Fe and impurities; A hot rolling process, which hot rolls the slab after the heating process at a finishing rolling temperature of 800 - 950°C to obtain a steel plate; A cooling process, which starts within 5.0 seconds after the end of the hot rolling process and cools the steel plate after the hot rolling process at an average cooling rate of 10 - 100°C / second to below 750°C; A coiling process, which coils the steel plate after the cooling process at a coiling temperature above 500°C and below 750°C, and makes the average cooling rate from the coiling temperature to 500°C exceed 50°C / hour; and A cold rolling process, which cold rolls the steel plate after the coiling process at a thickness reduction rate of 10 - 60%.

13. The method for manufacturing a steel plate according to claim 12, wherein After the cold rolling process, there is a heat treatment process, which heats the steel plate to an annealing temperature of 700 - 920°C and holds it at the annealing temperature for 120 - 500 seconds in an atmosphere with an oxygen potential of -1.50 or more.

14. The method for manufacturing a steel plate according to claim 13, wherein After the heat treatment process, there is a skin pass rolling process, which performs skin pass rolling on the steel plate with a reduction rate of 0.05 - 2.0%.

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