Hot stamp molded body, steel sheet, and methods for manufacturing same

Abstract

This hot-stamping formed body has a chemical composition including, by mass%, C: 0.40% to 1.00%, Si: 0.01% to 1.00%, Mn: 0.01% or more and less than 1.00%, P: 0.100% or less, S: 0.01000% or less, and Al: 0.0010% to 1.0000%, in which, when a position at 1 / 4 of a thickness in a thickness direction from a surface is defined as a 1 / 4 depth position, a number density of ε-carbides having a circle equivalent diameter of 5 nm or more at the 1 / 4 depth position is 20 / µm2 or more, and a tensile strength of the hot-stamping formed body is 2,100 MPa or more.

Description

TECHNICAL FIELD

[0001] The present invention relates to a hot-stamping formed body, a steel sheet, and manufacturing methods of the same.

[0002] Priority is claimed on Japanese Patent Application No. 2023-016207, filed February 06, 2023, the content of which is incorporated herein by reference.BACKGROUND ART

[0003] In the field of steel sheets for a vehicle, against the background of tightening of recent environmental regulations and collision safety standards, the application of steel sheets having high tensile strength (high strength steel sheets) has expanded in order to improve both fuel efficiency and collision safety. However, press formability of a steel sheet decreases with high-strengthening, which makes it difficult to manufacture a product having a complex shape.

[0004] Specifically, ductility of the steel sheet decreases with the high-strengthening, and there is a problem in that the steel sheet is fractured at a highly processed portion in the case of being processed into a complex shape. In addition, with the high-strengthening of the steel sheet, residual stress after processing causes springback and wall warpage, which also causes a problem that dimensional accuracy is deteriorated. Therefore, it is not easy to perform press forming on a steel sheet having high strength, particularly a tensile strength of 780 MPa or more, into a product having a complex shape. Roll forming rather than press forming makes it easier to process a high strength steel sheet, but the application thereof is limited to components having a uniform cross section in a longitudinal direction thereof.

[0005] Therefore, in recent years, for example, as disclosed in Patent Document 1, hot stamping has been adopted as a technology of press-forming a material that is difficult to form, such as a high strength steel sheet. The hot stamping is a hot forming technology of heating a material to be subjected to forming and then forming the material.

[0006] In this technology, the material is formed after being heated. Therefore, the steel is soft at the time of forming and has good formability. Accordingly, even a high strength steel sheet can be accurately formed into a complex shape. Furthermore, in hot stamping, since quenching is performed simultaneously with forming by a press die, steel (hot-stamping formed body) after the forming has sufficient strength.

[0007] For example, according to Patent Document 1, it is disclosed that it is possible to impart a tensile strength of 1,400 MPa or more to a steel member (hot-stamping formed body) obtained by forming a steel sheet through hot stamping.

[0008] In recent years, countries around the world have set higher CO 2 reduction targets, and each vehicle manufacturer has progressed in reducing fuel consumption in consideration of collision safety. Not only gasoline vehicles but also rapidly developing electric vehicles require, in terms of materials, higher strength materials to protect not only passengers but also batteries from collision and to cancel out the amount of an increase in weight. For example, in a hot-stamping formed body used in vehicles and the like, there is a need for a higher strength (higher than 1.5 GPa) steel that exceeds the strength described above in Patent Document 1 or a strength generally used as a hot-stamping formed body currently formed by hot stamping.

[0009] However, as the strength of the hot-stamping formed body is increased, there is a tendency for toughness to decrease, and there is a concern that sufficient impact absorption properties cannot be obtained.

[0010] As a solution to such a problem, Patent Document 2 discloses a hot-stamping formed body having a tensile strength of 2,000 MPa or more.

[0011] Patent Document 2 discloses that by performing two heat treatments to set an average grain size of prior austenite grains to 5.0 µm or less and an average Mn concentration at grain boundaries of the prior austenite grains to 1.0 mass% or less, a hot-stamping formed body having excellent strength and toughness can be obtained.

[0012] However, as a result of the studies conducted by the present inventors, it has been found that the method of Patent Document 2 provides an improvement in toughness to some extent, but does not necessarily sufficiently meet recent higher demands.Citation ListPatent Documents

[0013] Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2002-102980 Patent Document 2: Japanese Patent No. 6966023 SUMMARY OF INVENTIONTechnical Problem

[0014] As described above, in recent years, high strength (particularly, a tensile strength of more than 1,500 MPa or 2,100 MPa or more) hot-stamping formed bodies have been required to have improved impact absorption properties, but technologies in the related art have not necessarily been able to fully meet this requirement.

[0015] Therefore, in view of the above problems, an object of the present invention is to provide a hot-stamping formed body having both high strength and excellent impact absorption properties, a steel sheet suitable as a material for the hot-stamping formed body, and manufacturing methods of the same.Solution to problem

[0016] The present inventors have studied a method for increasing the impact absorption properties in a high strength hot-stamping formed body. As a result, it has been found that the impact absorption properties are increased by precipitating ε-carbides in the hot-stamping formed body.

[0017] In addition, in a case of obtaining such a hot-stamping formed body having ε-carbides, it has been found that it is effective to reduce a number density of carbides of one or more of Nb, Ti, Fe, Mo, W, and Cr in a steel sheet (steel sheet for hot stamping) serving as a material for the hot-stamping formed body.

[0018] The present invention has been made in view of the above findings. The gist of the present invention is as follows. [1] A hot-stamping formed body according to an aspect of the present invention includes, as a chemical composition, by mass%: C: 0.40% to 1.00%; Si: 0.01% to 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% 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 1.00%; Mg: 0% to 1.00%; REM: 0% to 0.0050%; Sb: 0% to 0.020%; Zr: 0% to 0.10%; Sn: 0% to 0.10%; As: 0% to 0.10%; and a remainder: Fe and impurities, in which, when a position at 1 / 4 of a thickness in a thickness direction from a surface is defined as a 1 / 4 depth position, a number density of ε-carbides having a circle equivalent diameter of 5 nm or more at the 1 / 4 depth position is 20 / µm 2< or more, and a tensile strength of the hot-stamping formed body is 2,100 MPa or more. [2] In the hot-stamping formed body according to [1], when a position at 50 µm in the thickness direction from the surface is defined as a 50 µm depth position, a hardness at the 50 µm depth position may be smaller than a hardness at the 1 / 4 depth position. [3] In the hot-stamping formed body according to [2], the hardness at the 50 µm depth position may be smaller than the hardness at the 1 / 4 depth position by HV100 or more in Vickers hardness. [4] In the hot-stamping formed body according to any one of [1] to [3], the chemical composition may include, by mass%, one or more selected from the group consisting of 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%. [5] In the hot-stamping formed body according to any one of [1] to [4], the chemical composition may include, by mass%, one or more selected from the group consisting of Mo: 0.05% to 0.50%, B: 0.0010% to 0.0100%, Co: 0.01% to 1.00%, Ni: 0.10% to 1.00%, Cu: 0.10% to 1.00%, and W: 0.10% to 3.00%. [6] In the hot-stamping formed body according to any one of [1] to [5], the chemical composition may include, by 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%. [7] A steel sheet according to another aspect of the present invention includes, as a chemical composition, by mass%: C: 0.40% to 1.00%; Si: 0.01% to 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% 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 1.00%; Mg: 0% to 1.00%; REM: 0% to 0.0050%; Sb: 0% to 0.020%; Zr: 0% to 0.10%; Sn: 0% to 0.10%; As: 0% to 0.10%; and a remainder: Fe and impurities, in which, when a position at 1 / 4 of a sheet thickness in a sheet thickness direction from a surface is defined as a 1 / 4 depth position, a microstructure at the 1 / 4 depth position includes, by area ratio, ferrite: more than 50% and 100% or less, pearlite: 0% to 40%, and bainite, martensite, and austenite: 0% or more and less than 10% in total, and a number density of carbides of one or more of Nb, Ti, Fe, Mo, W, and Cr having a circle equivalent diameter of 0.2 µm or more at the 1 / 4 depth position is less than 5.0110 µm 2< . [8] In the steel sheet according to [7], when a position at 50 µm in the sheet thickness direction from the surface is defined as a 50 µm depth position, a hardness at the 50 µm depth position may be smaller than a hardness at the 1 / 4 depth position. [9] In the steel sheet according to [7] or [8], the chemical composition may include, by mass%, one or more selected from the group consisting of 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%.

[10] In the steel sheet according to any one of [7] to [9], the chemical composition may include, by mass%, one or more selected from the group consisting of Mo: 0.05% to 0.50%, B: 0.0010% to 0.0100%, Co: 0.01% to 1.00%, Ni: 0.10% to 1.00%, Cu: 0.10% to 1.00%, and W: 0.10% to 3.00%.

[11] A manufacturing method of the hot-stamping formed body according to an aspect of the present invention is a method of manufacturing the hot-stamping formed body according to [1], the method including: a hot stamping process of heating the steel sheet according to [7] to a highest heating temperature of higher of an Ac3 point and 800°C or higher and 950°C or lower, holding the steel sheet at the highest heating temperature for 60 to 720 seconds, and then cooling the steel sheet to 300°C or lower so that an average cooling rate from the highest heating temperature to 300°C is 10 to 500 °C / sec; and a tempering process of tempering the steel sheet after the hot stamping process, in which, in the tempering process, the steel sheet is held at 80°C to 300°C for 6.0 seconds or longer, the steel sheet is cooled to lower than 80°C at an average cooling rate of 20 to 500°C / sec, and then heated again and held at 80°C to 300°C for 6.0 seconds or longer, or the steel sheet is held at 80°C to 300°C for 6.0 seconds or longer, the steel sheet is cooled to lower than 80°C at an average cooling rate of 20 to 500°C / sec, and then heated again and held at 80°C to 300°C for 6.0 seconds or longer.

[12] A manufacturing method of the steel sheet according to another aspect of the present invention is a method of manufacturing the steel sheet according to [7], the method including: a heating process of heating a slab having a chemical composition including, by mass%, C: 0.40% to 1.00%; Si: 0.01% to 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% 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 1.00%; Mg: 0% to 1.00%; REM: 0% to 0.0050%; Sb: 0% to 0.020%; Zr: 0% to 0.10%; Sn: 0% to 0.10%; As: 0% to 0.10%; and a remainder: Fe and impurities, to 1,150°C to 1,350°C; a hot rolling process of hot-rolling the slab after the heating process so that a finish rolling temperature is 800°C to 950°C to obtain a steel sheet; a cooling process of cooling the steel sheet after the hot rolling process to 750°C or lower so that an average cooling rate is 10 to 100 °C / sec, the cooling process starting within 5.0 seconds from completion of the hot rolling process; a coiling process of coiling the steel sheet after the cooling process at a coiling temperature of higher than 500°C and 750°C or lower and setting an average cooling rate from the coiling temperature to 500°C to faster than 50 °C / hr; and a cold rolling process of cold-rolling the steel sheet after the coiling process at a sheet thickness reduction ratio of 10% to 60%.

[13] The manufacturing method of the steel sheet according to

[12] , may further include: a heat treatment process of heating the steel sheet after the cold rolling process to an annealing temperature of 700°C to 920°C, and holding the steel sheet at the annealing temperature for 120 to 500 seconds in an atmosphere having an oxygen potential of -1.50 or more.

[14] The manufacturing method of the steel sheet according to

[13] , may further include: a skin pass process of performing skin pass rolling on the steel sheet after the heat treatment process at a rolling reduction of 0.05% to 2.0%. Advantageous Effects of Invention

[0019] According to the above aspects of the present invention, it is possible to provide a hot-stamping formed body having both high strength and excellent impact absorption properties, a steel sheet suitable as a material for the hot-stamping formed body, and manufacturing methods of the same.DESCRIPTION OF EMBODIMENTS

[0020] Hereinafter, a hot-stamping formed body according to an embodiment of the present invention (a hot-stamping formed body according to the present embodiment), a steel sheet according to the embodiment of the present invention (a steel sheet according to the present embodiment), and manufacturing methods of the same will be described.<Hot-Stamping Formed Body>

[0021] The hot-stamping formed body according to the present embodiment has a predetermined chemical composition, which will be described later, in which a number density of ε-carbides having a circle equivalent diameter of 5 nm or more is 20 / µm 2< or more at a 1 / 4 depth position.

[0022] In the present embodiment, a position at 1 / 4 of a thickness in a thickness direction from a surface is referred to as a 1 / 4 depth position, and a position at 50 µm in the thickness direction from the surface is referred to as a 50 µm depth position in the description.<Chemical Composition>

[0023] The chemical composition of the hot-stamping formed body according to the present embodiment will be described. Unless otherwise specified, % of the amount of each element constituting the chemical composition is mass%.

[0024] The chemical composition of the hot-stamping formed body according to the present embodiment means a chemical composition of a base steel in a case where the hot-stamping formed body according to the present embodiment includes the base steel and a coating formed on a surface of the base steel.C: 0.40% to 1.00%

[0025] C is an element that enhances hardenability of steel and increases strength of a hot-stamping formed body obtained after a steel sheet is subjected to hot stamping. When a C content is less than 0.40%, it becomes difficult to secure sufficient strength in the hot-stamping formed body. Therefore, the C content is set to 0.40% or more. The C content is preferably set to 0.45% or more.

[0026] On the other hand, when the C content is more than 1.00%, the strength of the hot-stamping formed body increases, and there is a concern that bendability and ductility decrease. Therefore, the C content is set to 1.00% or less. The C content is preferably set to 0.80% or less.Si: 0.01% to 1.00%

[0027] Si is an effective element for enhancing the hardenability of the steel and stably securing the strength of the hot-stamping formed body. In order to obtain this effect, a Si content is set to 0.01% or more. The Si content is preferably 0.10% or more.

[0028] On the other hand, when the Si content in steel is more than 1.00%, a heating temperature required for austenitic transformation becomes significantly high during a heat treatment (quenching). Accordingly, there are cases where the cost required for the heat treatment increases or ferrite remains during heating, and the strength of the hot-stamping formed body decreases. In addition, when the Si content is more than 1.00%, there are cases where the behavior of scale generated during manufacture of the steel sheet changes, and external appearance of a surface of a product is impaired. Therefore, the Si content is set to 1.00% or less. The Si content is preferably set to 0.90% or less.Mn: 0.01% or More and Less Than 1.00%

[0029] Mn contributes to an improvement in the strength of the hot-stamping formed body by solid solution strengthening. In addition, Mn is a very effective element for enhancing the hardenability of the steel and stably securing the strength of the hot-stamping formed body. In order to obtain this effect, a Mn content is set to 0.01% or more.

[0030] On the other hand, when the Mn content is 1.00% or more, coarse steel inclusions such as MnS are likely to be generated, and there is a concern that the bendability and the ductility decrease. Therefore, the Mn content is set to less than 1.00%. The Mn content is preferably set to 0.90% or less.P: 0.100% or Less

[0031] P is an element that segregates to grain boundaries and reduces intergranular strength. When a P content is more than 0.100%, intergranular strength significantly decreases, and the toughness of the hot-stamping formed body decreases. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.050% or less, and 0.035% or less. A lower limit of the P content does not need to be particularly limited, and is 0%. However, when the P content is reduced to less than 0.0001%, a dephosphorization cost increases significantly, which is not preferable economically. In an actual operation, the P content may be set to 0.0001% or more.S: 0.01000% or Less

[0032] S is an element that forms inclusions in the steel. When a S content is more than 0.01000%, a large amount of inclusions are generated in the steel, and the toughness of the hot-stamping formed body decreases. Therefore, the S content is set to be 0.01000% or less. The S content is preferably 0.00400% or less. A lower limit of the S content does not need to be particularly limited, and is 0%. However, when the S content is reduced to less than 0.00015%, a desulfurization cost increases significantly, which is not preferable economically. In an actual operation, the S content may be set to 0.00015% or more, and 0.00020% or more.Al: 0.0010% to 1.0000%

[0033] Al is an element having an action of deoxidizing molten steel and achieving soundness of the steel (suppressing the occurrence of defects such as blowholes in the steel). When an Al content is less than 0.0010%, deoxidation is not sufficiently performed. 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.

[0034] On the other hand, when the Al content is more than 1.0000%, coarse oxides or coarse nitrides are generated in steel, and the toughness of the hot-stamping formed body decreases. Therefore, the Al content is set to be 1.0000% or less. The Al content is preferably 0.5000% or less, and more preferably 0.3000% or less.N: 0.0150% or Less

[0035] N is an element that forms nitrides in the steel. Since the nitrides serve as the origin of fracture, a 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.

[0036] A lower limit of the N content does not need to be particularly specified, and is 0%. However, when the N content is reduced to less than 0.0001%, a denitrification cost increases significantly, which is not preferable economically. Therefore, the N content may be set to 0.0001% or more, 0.0004% or more, or 0.0010% or more.

[0037] The hot-stamping formed body according to the present embodiment may have a chemical composition containing the above elements (base elements) and a remainder of Fe and impurities. On the other hand, for the purpose of improving various properties, one or more of the following elements (optional elements) may be further contained. Since the optional element does not necessarily have to be contained, a lower limit of an amount thereof is 0%.Nb: 0% to 0.100%

[0038] Nb is an element that increases the strength of the hot-stamping formed body by solid solution strengthening and forms carbonitrides, thereby contributing to grain refinement of prior austenite grains. Therefore, Nb may be contained as necessary. In a case where Nb is contained, in order to reliably exhibit the above effect, a Nb content is preferably set to 0.010% or more. The Nb content is more preferably 0.035% or more. On the other hand, when Nb is contained in an amount of more than 0.100%, there are cases where Nb-based carbonitrides are excessively generated and the generation of ε-carbides that contribute to the improvement in the toughness of the hot-stamping formed body is suppressed. Therefore, the Nb content is preferably set to 0.100% or less. The Nb content is more preferably 0.080% or less.Ti: 0% to 0.100%

[0039] Ti is an element that forms fine carbides, carbonitrides, and the like together with Nb in the steel, suppresses Cu hot embrittlement cracking in a hot rolling step by an grain refinement effect thereof, and has an action of improving hydrogen embrittlement resistance of the hot-stamping formed body. In addition, Ti is an element that also forms nitrides by being preferentially bonded to N in the steel, suppresses the consumption of solute B due to precipitation of BN, and promotes an effect of improving the hardenability by B, which will be described later. Therefore, Ti may be contained.

[0040] In a case of obtaining the above effect, a Ti content is preferably set to 0.010% or more. The Ti content is more preferably set to 0.020% or more.

[0041] On the other hand, when the Ti content is more than 0.100%, coarse TiN is formed, and the toughness of the hot-stamping formed body deteriorates. Therefore, a 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.Cr: 0% to 0.50%

[0042] Cr is an element effective for enhancing the hardenability of the steel and stably securing the strength of the hot-stamping formed body. Therefore, Cr may be contained. In a case of obtaining the above effect, a Cr content is set to preferably 0.03% or more, and more preferably 0.05% or more.

[0043] On the other hand, when the Cr content is more than 0.50%, the above effect is 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.V: 0% to 0.50%

[0044] V is an element that increases the strength of the hot-stamping formed body by solid solution strengthening. Therefore, V may be contained. In order to reliably obtain the effect, it is preferable that a V content is set to 0.01% or more.

[0045] On the other hand, when the V content is more than 0.50%, there are cases where V-based carbonitrides are excessively generated and the generation of ε-carbides that contribute to the improvement in the toughness of the hot-stamping formed body is suppressed. Therefore, the V content is set to 0.50% or less. The V content is preferably 0.40% or less.Mo: 0% to 0.50%

[0046] Mo is a very effective element for enhancing the hardenability of the steel and stably securing the strength of the hot-stamping formed body. In particular, a synergistic effect of improving the hardenability can be obtained by containing B in combination. Therefore, Mo may be contained. In a case of obtaining the above effect, a Mo content is set to preferably 0.05% or more, and more preferably 0.10% or more.

[0047] On the other hand, even when the Mo content is more than 0.50%, the above effect is only saturated, and an alloying cost increases. Therefore, the Mo content is set to 0.50% or less. The Mo content is preferably 0.40% or less.B: 0% to 0.0100%

[0048] B is an element having an action of enhancing the hardenability of the steel even in a small amount. In addition, B is an element that segregates to the grain boundaries and strengthens the grain boundaries. Therefore, B may be contained. In a case of obtaining the above effect, a B content is preferably set to 0.0010% or more.

[0049] On the other hand, when the B content is more than 0.0100%, a large amount of coarse compounds are precipitated, and the toughness of the hot-stamping formed body decreases. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less.Co: 0% to 1.00%

[0050] Co is an element having an action of raising a martensite start temperature (Ms point) and is an element that improves the toughness of the hot-stamping formed body. Therefore, Co may be contained. In a case of obtaining the above effect, a Co content is preferably set to 0.01% or more.

[0051] On the other hand, Co is an expensive element, and when the Co content is more than 1.00%, the alloy cost increases. Therefore, the Co content is set to 1.00% or less. The Co content may be set to 0.10% or less.Ni: 0% to 1.00%

[0052] Ni is an effective element for enhancing the hardenability of the steel and stably securing the strength of the hot-stamping formed body. In addition, Ni is an element having an action of suppressing Cu hot embrittlement cracking in the manufacturing of a steel sheet. Therefore, Ni may be contained. In a case of obtaining the above effect, a Ni content is set to preferably 0.10% or more, and more preferably 0.20% or more.

[0053] On the other hand, when the Ni content is more than 1.00%, the above effect is saturated and the cost increases. Therefore, the Ni content is set to 1.00% or less. The Ni content is set to preferably 0.80% or less, and more preferably 0.50% or less.Cu: 0% to 1.00%

[0054] Cu is an element effective for enhancing the hardenability of the steel and stably securing the strength of the hot-stamping formed body. In addition, Cu is an element that improves corrosion resistance in a corrosive environment. Therefore, Cu may be contained. In a case of obtaining the above effects, a Cu content is preferably set to 0.10% or more. The Cu content is more preferably 0.20% or more.

[0055] On the other hand, when the Cu content is more than 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.W: 0% to 3.00%

[0056] W is an element effective for enhancing the hardenability of the steel and stably securing the strength of the hot-stamping formed body. In a case of obtaining the above effect, a W content is preferably set to 0.10% or more.

[0057] On the other hand, when the W content is more than 3.00%, the above effect is saturated and the cost increases. Therefore, the W content is set to 3.00% or less.O: 0% to 0.100%

[0058] O is an element that forms coarse oxides that serve as the origin of fracture when contained in a large amount in steel and deteriorates the toughness of the hot-stamping formed body. Therefore, an 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.

[0059] A lower limit of the O content does not need to be particularly specified, and is 0%. However, the O content may be set to 0.001% or more or 0.005% or more in order to disperse a large number of fine oxides during deoxidation of molten steel.Ca: 0% to 1.00%

[0060] Ca is an element that deoxidizes molten steel and suppresses the generation of oxides that serve as the origin of fracture. Therefore, Ca may be contained. In a case of obtaining the above effect, a Ca content is set to preferably 0.01% or more, and more preferably 0.05% or more.

[0061] On the other hand, the above effect is saturated even when a large amount of Ca is contained. Therefore, the Ca content is set to 1.00% or less. The Ca content is preferably 0.40% or less, 0.20% or less, and 0.15% or less.Mg: 0% to 1.00%

[0062] Mg is an element having an action of deoxidizing the molten steel and achieving the soundness of the steel. Therefore, Mg may be contained. In a case of obtaining the above effect, a Mg content is preferably set to 0.01% or more. The Mg content is more preferably 0.05% or more.

[0063] On the other hand, when the Mg content is more than 1.00%, the amount of oxides in the steel increases, which adversely affects the toughness of the hot-stamping formed body. 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.REM: 0% to 0.0050%

[0064] REM is an element that deoxidizes the molten steel and suppresses the generation of oxides that serve as the origin of fracture. Therefore, REM may be contained. In a case of obtaining the above effect, a REM content is set to preferably 0.0001% or more, and more preferably 0.0010% or more.

[0065] On the other hand, the above effect is saturated even when a large amount of REM is contained. Therefore, the REM content is set to 0.0050% or less. The REM content is preferably 0.0040% or less or 0.0020% or less.

[0066] Further, in the present embodiment, the REM refers to a total of 17 elements including Sc, Y, and lanthanoids, and the REM content refers to the total amount of these elements.Sb: 0% to 0.020%

[0067] Sb is an element that deoxidizes the molten steel and suppress the generation of oxides that serve as the origin of fracture, thereby improving deformability of the hot-stamping formed body. Therefore, Sb may be contained. In a case of obtaining the above effect, a Sb content is set to preferably 0.001% or more, and more preferably 0.005% or more.

[0068] On the other hand, the above effect is saturated even when a large amount of Sb is contained. Therefore, the Sb content is set to 0.020% or less. The Sb content is preferably 0.015% or less.Zr: 0% to 0.10%

[0069] Zr is an element that contributes to inclusion control, particularly, fine dispersion of inclusions and that increases the toughness of the hot-stamping formed body. Therefore, Zr may be contained. In a case of obtaining the above effect, a Zr content is set to preferably 0.01% or more, and more preferably 0.03% or more.

[0070] On the other hand, when a large amount of Zr is contained, there are cases where deterioration of surface properties become noticeable. Therefore, the Zr content is set to 0.10% or less. The Zr content is preferably 0.08% or less.Sn: 0% to 0.10%

[0071] Sn is an element that improves the hydrogen embrittlement resistance. Therefore, Sn may be contained. In a case of obtaining the above effect, a Sn content is preferably set to 0.01% or more.

[0072] On the other hand, when the Sn content is more than 0.10%, the effect is saturated and the cost increases. Therefore, in a case where Sn is contained, the Sn content is set to 0.10% or less.As: 0% to 0.10%

[0073] As is an element that lowers an temperature to toransform to austenite single phase and refines prior austenite grains, thereby contributing to the improvement of the hydrogen embrittlement resistance. Therefore, As may be contained. In a case of obtaining the above effect, an As content is preferably set to 0.01% or more.

[0074] On the other hand, the above effect is saturated even when a large amount of As is contained. Therefore, the As content is set to 0.10% or less. The As content is preferably 0.06% or less.Remainder: Fe and Impurities

[0075] In the chemical composition of the hot-stamping formed body according to the present embodiment, elements other than the above-described elements, that is, the remainder is Fe and impurities. That is, the hot-stamping formed body according to the present embodiment may have a chemical composition including the base elements and a remainder of Fe and impurities, or may have a chemical composition including the base elements, one or more optional elements, and a remainder of Fe and impurities.

[0076] Here, the "impurities" mean elements that are incorporated due to various factors including raw materials such as ore and scrap and the manufacturing process when the steel sheet is industrially manufactured, and are acceptable in a range without adversely affecting the properties of the hot-stamping formed body according to the present embodiment. An industrial manufacturing method is a blast furnace steelmaking method or an electric furnace steelmaking method, and includes a level (impurity level) incorporated during manufacturing by any of the methods.

[0077] The chemical composition of the hot-stamping formed body can be obtained by the following method.

[0078] The chemical composition can be obtained by performing elemental analysis on the 1 / 4 depth position, which is the position at 1 / 4 of the thickness in the thickness direction from the surface of the hot-stamping formed body (a range of 1 / 8 to 3 / 8 of the thickness in the thickness direction from the surface is acceptable) using a general method such as ICP-AES. For elements, which are difficult to measure using ICP-AES, C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method.[Microstructure]

[0079] In the present embodiment, microstructures at the 1 / 4 depth position and the 50 µm depth position are specified. The 1 / 4 depth position is a position representing a typical microstructure of the hot-stamping formed body. (At 1 / 4 Depth Position, Number Density of ε-Carbides Having Circle Equivalent Diameter of 5 nm or More Is 20 / µm 2< or More)

[0080] In order to obtain a high tensile strength, a high strength hot-stamping formed body needs to contain a large amount of C and other alloying elements. However, in general, with an increase in strength, the toughness of the hot-stamping formed body decreases, and the impact absorption properties decrease.

[0081] Contrary to this, as a result of studies conducted by the present inventors, it has been found that the impact absorption properties are improved by controlling a size and number density of ε-carbides present in the hot-stamping formed body.

[0082] Specifically, it has been found that the impact absorption properties are improved in a case where the ε-carbides having a circle equivalent diameter of 5 nm or more are present in a number density 20 / µm 2< or more.

[0083] Therefore, in the hot-stamping formed body according to the present embodiment, the number density of the ε-carbides having a circle equivalent diameter of 5 nm or more at the 1 / 4 depth position is set to 20 / µm 2< or more.

[0084] The reason why the ε-carbides having a circle equivalent diameter of 5 nm or more are targeted is that the effect of improving the impact absorption properties cannot be sufficiently obtained by smaller ε-carbides. On the other hand, although an upper limit of the circle equivalent diameter of the ε-carbide to be targeted is not limited, excessively large ε-carbides are not preferable because it is difficult to obtain a sufficient number density. For example, ε-carbides having a size of 5 to 50 nm are targeted.

[0085] In addition, even in ε-carbides having a circle equivalent diameter of 5 nm or more, the effect is not sufficient when the number density is less than 20 / µm 2< . On the other hand, when the number density is more than 200 / µm 2< , there is a possibility that an interface between the ε-carbide and the base metal serves as the origin of cracks. Therefore, the number density may be set to 200 / µm 2< or less.

[0086] In the present embodiment, the ε-carbide is a carbide that becomes FexC (x: about 2 to 3).

[0087] The circle equivalent diameter of the ε-carbides or the number density of the ε-carbides having a circle equivalent diameter of 5 nm or more can be obtained by observing a thin film sample using a field emission transmission electron microscope (JEM-2100F manufactured by JEOL Ltd.) provided with an energy dispersive X-ray spectrometer.

[0088] Specifically, a small piece of about 10 mm square is cut out from the hot-stamping formed body, both surfaces are then mechanically or chemically polished, and a thin film TEM sample (thickness of about 60 µm, φ3 mm) at the original 1 / 4 depth position (a range of 1 / 8 to 3 / 8 of the thickness from the surface in the thickness direction is acceptable) is produced. In the production of the sample, the sample is polished starting with a coarse waterproof abrasive paper of about #120, then gradually with a finer waterproof abrasive paper, and finally with a waterproof abrasive paper of about #600, and then the sample is punched out with a specimen punch. Thereafter, double-sided jet electropolishing is performed until a hole is formed in the center to obtain a TEM observation sample. An electropolishing device (TENUPOL-2 manufactured by Struers) is used, an electropolishing solution is a liquid mixture of 5% perchloric acid and 95% glacial acetic acid, and the thin film TEM sample is finished at a voltage of 70 V. In the thin film TEM observation, the observation is performed at an acceleration voltage of 200 kV. In order to reduce variation across parts, at least five visual fields of the sample having a visual field of about 100 to 300 nm square are observed. Types of precipitates observed are identified using a diffraction pattern and an analysis result by EDX. Regarding the number density, in order to reduce variation due to the visual field, the number density in each visual field is counted, and an average thereof is regarded as a representative value of the number density.(Preferably, Hardness at 50 µm Depth Position Is Smaller Than Hardness at 1 / 4 Depth Position)

[0089] In the hot-stamping formed body according to the present embodiment, it is preferable that a hardness at the 50 µm depth position is smaller (lower) than a hardness at the 1 / 4 depth position. The impact absorption properties are further improved by reducing (softening) the hardness at a position close to the surface.

[0090] Although the effect is obtained when the hardness at the 50 µm depth position is smaller than the hardness at the 1 / 4 depth position, in a case of obtaining a more clear effect, the hardness at the 50 µm depth position is more preferably smaller than the hardness at the 1 / 4 depth position by HV100 or more in terms of Vickers hardness.

[0091] An upper limit of a difference in Vickers hardness is not limited, but may be HV300 or less in order to secure the strength of the entire formed body.

[0092] The hardness at the 50 µm depth position can be reduced (lowered) by, for example, decarburization annealing as will be described below. The 50 µm depth position is targeted because the hardness varies greatly in the vicinity of an outermost surface due to the measurement principle.

[0093] The hardness at the 1 / 4 depth position and the hardness at the 50 µm depth position are evaluated in terms of Vickers hardness according to JIS Z 2244-1:2020.

[0094] In the measurement, measurement is performed on a polished cross section of a test piece at five points for each part with a load of 50 gf, and an average value of three points excluding a maximum value and a minimum value is used as a measurement value.

[0095] In the hot-stamping formed body according to the present embodiment, phases constituting the microstructure are not limited, and may be controlled according to a target tensile strength. However, it is preferable that an area ratio of martensite is 95% or more.

[0096] Here, martensite includes so-called fresh martensite and tempered martensite. The area ratio of martensite can be measured in the same manner as in structure observation of the steel sheet, which will be described later.[Coating]

[0097] The hot-stamping formed body according to the present embodiment may be provided with a coating on a part or the entirety of the surface.

[0098] The coating may be a coating primarily containing an Fe-Al-based alloy or a coating primarily containing an Fe-Zn-based alloy. The coating is also referred to as a film, an alloyed plating layer, or an intermetallic compound layer.

[0099] The coating primarily containing an Fe-Al-based alloy is a coating containing 70 mass% or more of Fe and Al in total, and the coating primarily containing an Fe-Zn-based alloy is a coating containing 70 mass% or more of Fe and Zn in total. The coating primarily containing an Fe-Al-based alloy may further 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, and a remainder of impurities. The coating primarily containing an Fe-Zn-based alloy may further 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, and a of including impurities.

[0100] By including the coating, corrosion resistance is obtained, so that the effect of improving the hydrogen embrittlement resistance in use in a vehicle can be obtained.

[0101] A thickness of the coating is preferably 5 to 100 µm.

[0102] The chemical composition and the thickness of the coating can be obtained by observing a cross section with a scanning electron microscope.

[0103] Specifically, a measurement sample is cut out from a 1 / 2 portion in a longitudinal direction (a position at 1 / 2 of a length in the longitudinal direction from a longitudinal end portion) and a 1 / 4 width portion (a position at 1 / 4 of a width in a width direction from a width-directional end portion) of the hot-stamping formed body and is observed. An observation range of the microscope is set to, for example, a range of 40,000 µm 2< or more in terms of area at a magnification of 400-fold. The cut sample is mechanically polished and subsequently mirror-finished. Next, the thickness of the coating is measured in any 10 visual fields, and an average value thereof is used as the thickness of the coating.

[0104] Observation with a BSE image (or a COMPO image) confirms a clear difference in contrast between the coating and the base metal (steel sheet substrate). Therefore, the thickness of the coating can be measured by measuring a thickness from an outermost surface to a position where the contrast changes. Measurement is performed at 20 points at equal intervals in an observation photograph, and a distance between the measurement points is set to 6.50 µm. During the measurement, observation is performed in five visual fields in the above-described manner, and an average value thereof is used as the thickness of the coating.

[0105] In addition, as the chemical composition of the coating, the amounts of Fe, Al, and Zn contained in the coating can be obtained by performing spot elemental analysis (beam diameter: 1 µm or less) on the observation range described above using an electron probe micro-analyzer (EPMA). A total of 10 points are analyzed in the coating in any 10 visual fields, and average values thereof are used as the amounts of Fe, Al, and Zn contained in coating. Even in a case where an element other than Fe, Al, and Zn is contained, the amount thereof is obtained using the same method.

[0106] The surface serving as a reference of the 1 / 4 depth position and the 50 µm depth position described above is the surface of the hot-stamping formed body. However, in a case where the hot-stamping formed body includes the coating, that is, in a case where the hot-stamping formed body includes the base steel and the coating formed on the surface of the base steel, the surface means the surface of the base steel material excluding the coating.[Mechanical Properties](Tensile Strength)

[0107] The tensile strength of the hot-stamping formed body according to the present embodiment is 2,100 MPa or more in consideration of the contribution to the improvement in fuel efficiency and collision safety when the hot-stamping formed body is applied to vehicle components.

[0108] An upper limit of the tensile strength is not limited. However, there is a concern that the impact absorption properties decrease with an increase in the tensile strength. Therefore, the tensile strength may be set to less than 2,900 MPa.

[0109] The tensile strength can be obtained by collecting a No. 5 test piece according to JIS Z 2241:2011 from a position as flat as possible of the hot-stamping formed body and conducting a tensile test on the test piece according to a test method described in JIS Z 2241:2011.(Impact Absorption Properties)

[0110] In the hot-stamping formed body according to the present embodiment, excellent impact absorption properties can be obtained by controlling the chemical composition and the state of presence of ε-carbide as described above.

[0111] A target for the impact absorption properties is a bending angle at maximum force of 40° or more in terms of a sheet thickness of 2.0 mm based on the VDA (German Association of the Automotive Industry) bending test) 238-100. The sheet thickness conversion is performed based on "Bending angle correction regarding sheet thickness", Materials Science and Engineering 418 (2018) 012076.<Steel Sheet>

[0112] Next, the steel sheet according to the present embodiment will be described. By performing hot stamping and tempering on the steel sheet according to the present embodiment, the hot-stamping formed body according to the present embodiment described above can be obtained. Therefore, the steel sheet according to the present embodiment is suitable as a material (steel sheet for hot stamping) for the hot-stamping formed body according to the present embodiment.[Chemical Composition]

[0113] A chemical composition of the steel sheet according to the present embodiment needs to be set so that the hot-stamping formed body obtained by hot stamping and tempering has preferable properties. However, since the chemical composition does not substantially change by hot stamping and tempering, the chemical composition of the steel sheet according to the present embodiment may be the same as the hot-stamping formed body according to the present embodiment.[Microstructure]

[0114] Regarding a microstructure of the steel sheet according to the present embodiment, a position at 1 / 4 of a sheet thickness in a sheet thickness direction from a surface is defined as a 1 / 4 depth position, a range of 50 µm in the sheet thickness direction from the surface is defined as a surface layer portion, and a position at 50 µm in the sheet thickness direction from the surface is defined as a 50 µm depth position. The 1 / 4 depth position is a position representing a typical microstructure of the steel sheet.(At 1 / 4 Depth Position, in Terms of Area Ratio, Ferrite: More Than 50% and 100% or Less, Pearlite: 0% to 40%, Bainite, Martensite (Including Fresh Martensite and Tempered Martensite), and Austenite: 0% or More and Less Than 10% in Total)

[0115] In the steel sheet according to the present embodiment, ferrite is primarily contained (in an area ratio of more than 50%) in the microstructure in consideration of workability. The area ratio of ferrite may be 100%, but structures other than ferrite may include pearlite, bainite, martensite, and austenite (residual austenite).

[0116] Pearlite is a structure in which ferrite and cementite are arranged in a lamellar form. That is, a large area ratio of pearlite means that the amount of C present as cementite is large. When the amount of C present as cementite is large, the cementite (pearlite) is not sufficiently dissolved during heating for hot stamping, and even in a case where hot stamping and tempering are performed, a sufficient amount of ε-carbide cannot be obtained. Therefore, the area ratio of pearlite is set to less than 40%. The area ratio of pearlite may be 0%, but may be set to 5% or more.

[0117] When bainite, martensite, and austenite are present, the strength of the steel sheet before hot stamping is high. As a result, there are cases where a cut end surface becomes rough or cracks occur in a trimming step or the like before hot stamping. Therefore, the total area ratio of bainite, martensite, and austenite is set to 0% or more and less than 10%.

[0118] The area ratios of ferrite, pearlite, bainite, martensite, and residual austenite in the microstructure of the steel sheet can be obtained by the following method using an electron backscatter diffraction (EBSD) and X-ray diffraction measurement.

[0119] An L-section (cross section parallel to a rolling direction and the sheet thickness direction) of the steel sheet is mirror-polished and then etched with nital. The same sample is observed with FE-SEM at a magnification of 3,000-fold in ten visual fields, and the area ratio of each phase at the 1 / 4 depth position is calculated.

[0120] In this case, in an FE-SEM image, the structure is identified based on the following characteristics of each structure.

[0121] Ferrite contains block-like crystal grains and does not include a substructure such as laths inside. Pearlite is a structure in which ferrite and cementite are alternately layered (laminated ferrite in pearlite is distinguished from the above-described block-like ferrite and is not included in the area ratio of the block-like ferrite). Bainite and tempered martensite are structures formed of lath-shaped crystal grains and carbides, but have the following differences.

[0122] First, bainite is observed separately into upper bainite and lower bainite. Upper bainite is an aggregate of lath-shaped crystal grains and is an aggregate of laths including carbides between the laths. Lower bainite is an aggregate of lath-shaped crystal grains and contains iron-based carbides having a major axis of 5 nm or more therein, and the carbides belong to a single variant, that is, an iron-based carbide group elongated in the same direction. Here, the iron-based carbide group elongated in the same direction means a group in which a difference in 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. Similar to lower bainite, tempered martensite is an aggregate of lath-shaped crystal grains and has a structure containing iron-based carbides therein. However, since two or more variants of the carbides are selected, tempered martensite has a structure in which the elongation direction of the iron-based carbides is two or more.

[0123] In this way, ferrite, pearlite, tempered martensite, and bainite can be identified by confirming characteristics of each phase with FE-SEM.

[0124] On the other hand, fresh martensite that has not been tempered and residual austenite are not sufficiently corroded by nital etching and therefore can be distinguished from other etched structures (tempered martensite, bainite, and ferrite) in observation by FE-SEM, but the difference between fresh martensite and residual austenite cannot be determined. Therefore, the area ratio of residual austenite is measured by X-ray diffraction. In X-ray diffraction, a surface of a test piece cut into a 20 mm square is mechanically polished to 50 µm and then chemically polished, and the resulting surface is measured. Integrated intensities of diffraction peaks of BCC phases and FCC phases are measured by X-ray diffraction, and a ratio of the integrated intensity of the FCC phases in the sum of the integrated intensities is defined as the area ratio of the residual austenite. The measurement is performed three times for each sample, and an average value obtained is used as the area ratio of residual austenite. The area ratio of fresh martensite is obtained as a difference between an area ratio of an uncorroded region (either fresh martensite or residual austenite) observed with FE-SEM and the area ratio of residual austenite measured by X-ray diffraction.

[0125] The area ratio can be obtained in the same manner even in the hot-stamping formed body.(At 1 / 4 Depth Position, Number Density of Carbides of One or More of Nb, Ti, Fe, Mo, W, and Cr Having Circle Equivalent Diameter of 0.2 µm or More Is Less Than 5.0 / 10 µm 2< )

[0126] In the hot-stamping formed body according to the present embodiment described above, predetermined ε-carbides are obtained by precipitating ε-carbides through hot stamping and tempering under predetermined conditions. However, in a case where carbides are present in the steel sheet serving as a material and the carbides are not dissolved during heating for hot stamping, the predetermined ε-carbides cannot be obtained even after hot stamping and tempering.

[0127] For example, coarse carbides generated in the steel sheet are not dissolved during heating for hot stamping and are likely to remain undissolved. Therefore, in the steel sheet according to the present embodiment, the amount of coarse carbides is reduced. More specifically, at the 1 / 4 depth position, the number density of carbides of one or more of Nb, Ti, Fe, Mo, W, and Cr having a circle equivalent diameter of 0.2 µm or more is set to less than 5.0 / 10 µm 2< (less than 0.50 / µm 2< ).

[0128] When coarse carbides having a circle equivalent diameter of 0.2 µm or more are 5.0 / 10 µm 2< or more, a large number of carbides remain undissolved during heating for hot stamping.

[0129] The number density of carbides may not be present, and may be 0.0 / 10 µm 2< , but may be 30 / 10,000 µm 2< (0.03 / 10 µm 2< ) or more from a viewpoint of not significantly inhibiting the generation of ε-carbides after hot stamping.

[0130] An upper limit of the circle equivalent diameter of the target carbides is not limited, but may be 2.0 µm or less in order not to excessively affect variation in hardenability across parts during hot stamping. That is, carbides having a circle equivalent diameter of 0.2 to 2.0 µm may be targeted.

[0131] The number density of carbides of one or more of Nb, Ti, Fe, Mo, W, and Cr having a circle equivalent diameter of 0.2 µm or more can be obtained by the following method.

[0132] The L-section of the steel sheet is mirror-polished and then etched with nital. The same sample is observed at the 1 / 4 depth position with a scanning electron microscope. In a region of 50 µm square at the same depth position, composition analysis is performed on observed precipitates using EDX, and precipitates containing one or more of Nb, Ti, Fe, Mo, W, and Cr and C are determined to be the target carbides. Regarding the number density, in order to reduce variation across visual fields, the number density of the carbides having a circle equivalent diameter of 0.2 µm or more is counted by observing a region of 50 µm square in at least five visual fields, and an average thereof is used as a representative value of the number density.(Preferably, Hardness at 50 µm Depth Position Is Smaller Than Hardness at 1 / 4 Depth Position)

[0133] In a stage of the steel sheet which is a material, a hardness at the 50 µm depth position is set to be smaller than a hardness at the 1 / 4 depth position, so that the hardness at the 50 µm depth position in the hot-stamping formed body can be set to be smaller than the hardness at the 1 / 4 depth position. Therefore, in the steel sheet according to the present embodiment, it is preferable that the hardness at the 50 µm depth position is smaller than the hardness at the 1 / 4 depth position.[Sheet Thickness]

[0134] The sheet thickness of the steel sheet according to the present embodiment is not limited, but is preferably 1.0 to 3.5 mm in consideration of use as a steel sheet for a vehicle.[Coating]

[0135] A part of the surface of the steel sheet according to the present embodiment may have a coating. The coating may be a coating primarily containing Al (Al-based coating) or a coating primarily containing Zn (Zn-based coating). The coating is also referred to as a film or a plating layer. The coating primarily containing Al is a coating containing 70 mass% or more of Al, and the coating primarily containing Zn is a coating containing 70 mass% or more of Zn. The coating primarily containing Al may further contain, in addition to 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, and a remainder including impurities. The coating primarily containing Zn may further contain, in addition to 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, and a remainder including impurities.

[0136] Here, a surface that serves as a reference for the 1 / 4 depth position, the 50 µm depth position, and the surface layer portion is the surface of the steel sheet. However, in a case where the surface has a coating, that is, a case where the steel sheet has a base metal and a coating formed on a surface of the base metal, the surface means the surface of the base metal excluding the coating.<Manufacturing Method>

[0137] The steel sheet according to the present embodiment and the hot-stamping formed body according to the present embodiment can obtain the effects as long as the above-described characteristics are provided regardless of the manufacturing method. However, the steel sheet and the hot-stamping formed body can be preferably manufactured according to the manufacturing method described below.[Manufacturing Method of Steel Sheet]

[0138] The steel sheet according to the present embodiment can be obtained by a manufacturing method including the following steps: (I) a heating step of heating a slab having a predetermined chemical composition to 1,150°C to 1,350°C; (II) a hot rolling step of hot-rolling the slab after the heating step so that a finish rolling temperature is 800°C to 950°C to obtain a steel sheet; (III) a cooling step of cooling the steel sheet after the hot rolling step to 750°C or lower so that an average cooling rate is 10 to 100 °C / sec, the cooling step starting within 5 seconds from completion of the hot rolling step; (IV) a coiling step of coiling the steel sheet after the cooling step at a coiling temperature of higher than 500°C and 750°C or lower and setting an average cooling rate from the coiling temperature to 500°C to faster than 50 °C / hr; and (V) a cold rolling step of cold-rolling the steel sheet after the coiling step at a sheet thickness reduction ratio of 10% to 60%.

[0139] In addition, the manufacturing method of the steel sheet according to the present embodiment may further include one or more of the following steps: (VI) a heat treatment step of heating the steel sheet after the cold rolling step to an annealing temperature of 700°C to 920°C, and holding the steel sheet at the annealing temperature for 120 to 500 seconds in an atmosphere having an oxygen potential of -1.50 or more; (VII) a skin pass step of performing skin pass rolling on the steel sheet after the heat treatment step at a rolling reduction of 0.05% to 2.0%; and (VIII) a coating step of forming a coating on a surface of the steel sheet.

[0140] Preferable conditions of each step will be described. Known conditions can be applied to conditions or steps not described.(Heating Step)

[0141] In the heating step, the slab is heated before the hot rolling. The heating temperature is set to 1,150°C to 1,350°C.

[0142] When the heating temperature is lower than 1,150°C, the carbides formed during casting are not dissolved, and coarse carbides remain even after the hot rolling step.

[0143] On the other hand, from the viewpoint of suppressing scale loss and the viewpoint of energy saving, the slab heating temperature is set to 1,350°C or lower.

[0144] A chemical composition of the slab to be subjected to the heating step may be the same as the chemical composition of the steel sheet to be obtained.(Hot Rolling Step)

[0145] In the hot rolling step, the slab after the heating step is hot-rolled so that the finish rolling temperature is 800°C to 950°C to obtain a steel sheet.

[0146] When the finish rolling temperature (surface temperature on a final pass exit side) is lower than 800°C, there is a possibility that a large number of unrecrystallized regions flattened in the rolling direction remain, which may cause anisotropy in the properties of the steel sheet. On the other hand, when the finish rolling temperature is higher than 950°C, the crystal grains of the steel sheet become coarse.(Cooling Step)

[0147] After the finish rolling is completed, when the steel sheet is left at a temperature of higher than 750°C for a long period of time, coarse carbides are generated. Therefore, in the cooling step, the steel sheet after the hot rolling step is cooled to a cooling stop temperature of 750°C or lower so that the average cooling rate is 10 to 100 °C / sec. In addition, the cooling is started within 5.0 seconds from the completion of the hot rolling step.

[0148] In a case where the average cooling rate to the cooling stop temperature of 750°C or lower is slower than 10°C / sec, a time from the completion of the hot rolling step to the start of the cooling step is longer than 5.0 seconds, or the cooling stop temperature is higher than 750°C, a large amount of coarse carbides are generated.

[0149] On the other hand, in a case where the average cooling rate up to the cooling stop temperature of 750°C or lower is faster than 100 °C / sec, it is difficult to uniformly cool the steel sheet, and there are cases where defects in a sheet shape occur.(Coiling Step)

[0150] In the coiling step, the steel sheet after the cooling step is coiled at higher than 500°C and 750°C or lower. In addition, after the coiling, the average cooling rate from the coiling temperature to 500°C is set to be faster than 50 °C / hr.

[0151] When the coiling temperature is 500°C or lower, hard phases such as bainite or martensite may be generated, and cold rolling may not be possible or a load for cold rolling may increase. The coiling temperature is preferably 520°C or higher, and more preferably 540°C or higher. On the other hand, in a case where the coiling temperature is higher than 750°C, Cr and Mn may be concentrated in ferrite grain boundaries or in cementite present in pearlite and may remain as unsolved carbides during subsequent annealing or hot stamping.

[0152] On the other hand, when coiling is performed, the cooling rate decreases. When the average cooling rate up to 500°C is slow, an internal oxidation layer develops, a load for a pickling step increases, or coarse carbides are generated during cooling. Therefore, the average cooling rate from the coiling temperature to 500°C is set to be faster than 50 °C / hr.(Cold Rolling Step)

[0153] In the cold rolling step, the steel sheet after the coiling step is cold-rolled at a sheet thickness reduction ratio (rolling reduction) of 10% to 60% and adjusted to a predetermined sheet thickness.(Heat Treatment Step)

[0154] In the heat treatment step, the steel sheet after the cold rolling step is heated to an annealing temperature of 700°C to 920°C and held at the annealing temperature for 120 to 500 seconds in an atmosphere having an oxygen potential of -1.50 or more.

[0155] The heat treatment step is not essential, but it is preferable to perform heat treatment under the above conditions in order to reduce (decarburize) C in the surface layer portion of the steel sheet and soften the surface layer portion of the steel sheet.

[0156] When the annealing temperature is lower than 700°C, the oxygen potential is lower than -1.50, or the holding time is shorter than 120 seconds, a sufficient effect cannot be obtained.

[0157] On the other hand, when the annealing temperature is higher than 920°C, the crystal grains becomes coarse.

[0158] In addition, when the holding time is longer than 500 seconds, productivity deteriorates, which causes an increase in material cost.

[0159] An upper limit of the oxygen potential is not limited. However, when the oxygen potential is excessively high, Fe or other alloying elements in an outermost layer are oxidized, causing a scale pattern to form on the surface. Therefore, the oxygen potential is preferably set to -0.50 or less.

[0160] The oxygen potential is a partial molar Gibbs free energy of oxygen represented by an oxygen partial pressure in a vapor phase and a temperature.<Coating Step>

[0161] A coating may be formed on the surface as necessary. A method for forming the coating is not particularly limited, and a hot-dip plating method, an electroplating method, a vacuum vapor deposition method, a cladding method, a thermal spraying method, and the like can be used. The hot-dip plating method is the most popular in the industry.

[0162] Examples of the coating include an Al-based coating containing Al and a Zn-based coating containing Zn.

[0163] In a case where the Al-based coating is formed by hot-dip plating, in addition to Al, Fe is mixed in a plating bath as an impurity in many cases. Furthermore, in addition to the above elements, 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 mischmetal may be contained in the plating bath as long as 70 mass% or more of Al is contained.

[0164] In the case of performing hot-dip plating, the steel sheet after the heat treatment step may be cooled to room temperature and is then heated again to perform plating, or may be cooled to a temperature near a plating bath temperature (for example, 650°C to 750°C in the case of an Al-based plating and 420°C to 500°C in the case of a Zn-based coating) after the annealing, and then subjected to hot-dip plating without being cooled to room temperature.

[0165] Pretreatments and post-treatments of the coating are not particularly limited, and precoating, solvent coating, an alloying treatment, temper rolling, or the like can be performed. As the alloying treatment, for example, annealing at 450°C to 800°C can be performed. Furthermore, as a post-treatment, temper rolling is useful for shape adjustment and the like, and can achieve, for example, a rolling reduction of 0.1% to 0.5%.(Skin Pass Step)

[0166] The manufacturing method of the steel sheet according to the present embodiment may further include the skin pass step of performing skin pass rolling on the steel sheet after the heat treatment step or after the coating step.

[0167] By performing the skin pass rolling, a diffusion rate of the elements in the material increases, and the carbides are more likely to be dissolved during hot stamping. In this case, the number density of the ε-carbides can be further increased.

[0168] In a case of obtaining this effect, a rolling reduction of the skin pass rolling is preferably set to 0.05% or more. The rolling reduction of the skin pass rolling is more preferably 0.1% or more.

[0169] On the other hand, when the rolling reduction of the skin pass rolling is more than 2.0%, a load of the skin pass step is large, which is a factor of an increase in material cost. Therefore, in a case where the skin pass rolling is performed, the rolling reduction is preferably set to 2.0% or less.[Manufacturing Method of Hot-Stamping Formed Body]

[0170] The manufacturing method of the hot-stamping formed body according to the present embodiment can be obtained by a manufacturing method including the following steps using the steel sheet according to the present embodiment described above: (i) a hot stamping step of heating the steel sheet according to the present embodiment to a highest heating temperature of higher of an Ac3 point and 800°C or higher and 950°C or lower, holding the steel sheet at the highest heating temperature for 60 to 720 seconds, and then cooling the steel sheet to 300°C or lower so that an average cooling rate from the highest heating temperature to 300°C is 10 to 500 °C / sec; and (ii) a tempering step of tempering the steel sheet after the hot stamping step.

[0171] Each step will be described.(Hot Stamping Step)

[0172] In the hot stamping step, the steel sheet according to the present embodiment is used as a material (steel sheet for hot stamping), the steel sheet is heated to a highest heating temperature of the higher of an Ac3 point (°C) and 800°C or higher and 950°C or lower, and held at the highest heating temperature for 60 to 720 seconds, and then cooled to 300°C or lower so that an average cooling rate from the highest heating temperature to 300°C is 10 to 500°C / sec.

[0173] In this step, the carbides present in the steel sheet are dissolved and then the steel sheet is high-strengthened.

[0174] When the highest heating temperature is lower than the Ac3 point or lower than 800°C, or the holding time is shorter than 60 seconds, the carbides are not sufficiently dissolved or austenitic transformation is insufficient, and sufficient strength cannot be obtained after the hot stamping step.

[0175] On the other hand, in a case where the highest heating temperature is too high or the holding time at the highest heating temperature is too long, the crystal grains become coarse, and the toughness and bendability of the formed body after the hot stamping step become insufficient. Therefore, the highest heating temperature is set to 950°C or lower, and the holding time at the highest heating temperature is set to 720 seconds or shorter.

[0176] In addition, during the cooling from the highest heating temperature to 300°C or lower, when the average cooling rate up to 300°C is slower than 10 °C / sec, sufficient annealing is not achieved, and a sufficient tensile strength cannot be obtained in the hot-stamping formed body.

[0177] On the other hand, when the average cooling rate up to 300°C is faster than 500 °C / sec, the cooling rate varies greatly across the parts, which causes strain in the shape of the formed body.

[0178] The Ac3 point can be obtained from a change point of a thermal expansion coefficient when a temperature rising rate is set to 5 °C / sec by a Formaster test or the like.(Tempering Step)

[0179] In the tempering step, the steel sheet after the hot stamping step is tempered at a temperature of 80°C to 300°C.

[0180] In a case where the cooling stop temperature in the hot stamping step is 80°C to 300°C, the steel sheet may be held as it is for the tempering, or the steel sheet may be once cooled to a temperature of lower than 80°C, and then heated to 80°C to 300°C again and held at that temperature. Alternatively, after the hot stamping step, the steel sheet may be held at 80°C to 300°C, then once cooled to a temperature of lower than 80°C, and then heated to 80°C to 300°C again and held at that temperature.

[0181] In a case where the cooling stop temperature in the hot stamping step is lower than 80°C, the steel sheet may be heated to 80°C to 300°C again and held at the temperature.

[0182] The holding temperature at 80°C to 300°C is set to 6 seconds or longer in order to sufficiently precipitate ε-carbides, regardless of whether reheating is performed. An upper limit of the holding time is not limited. However, when the holding time is longer than necessary, the productivity decreases. Therefore, the holding time may be set to 1,800 seconds or shorter.Examples

[0183] Slabs having the chemical compositions shown in Tables 1-1 and 1-2 were prepared.

[0184] The slabs were heated, hot-rolled, cooled, and coiled under the conditions shown in Table 2-1 to manufacture hot-rolled steel sheets of 2.6 mm.

[0185] The hot-rolled steel sheets were cold-rolled at the rolling reductions shown in Table 2-2 and then subjected to a heat treatment under the conditions shown in Table 2-2 except for some of the hot-rolled steel sheets. In addition, in some examples, a coating (a hot-dip galvanized layer or a hot-dip Al plating layer) was formed by hot-dip plating. The hot-dip galvanized layer was alloyed to form a hot-dip galvannealed layer. In Table 2-2, the plating type GA is a hot-dip galvannealed layer, and Al is a hot-dip Al plating layer. In addition, skin pass rolling was performed on some of the examples. In Table 2-2, "-" indicates not applicable.

[0186] As a result, steel sheets Nos. 1 to 23 and Nos. 101 to 113 were obtained.

[0187] For the obtained steel sheets, the structure fractions of the microstructure at the 1 / 4 depth position and the number density of carbides of one or more of Nb, Ti, Fe, Mo, W, and Cr, the carbides having a circle equivalent diameter of 0.2 µm or more at the 1 / 4 depth position, were measured in the above-described manners.

[0188] In addition, the Vickers hardnesses at the 50 µm depth position and the 1 / 4 depth position were measured in the above-described manner.Results are shown in Table 2-3.

[0189] [Table 1-1]Steel sheet No.Chemical composition (unit mass%) remainder : Fe and impuritiesCSiMnPSAlNNbTi10.410.200.410.0090.005200.00500.0022--20.580.400.400.0330.005100.10000.0022--30.640.400.390.0150.005100.09000.00220.0450-40.800.520.610.0020.005300.10000.0023--50.580.800.350.0310.008200.02600.0031-0.02160.510.050.510.0310.005200.00500.0022--70.610.220.950.0310.005100.00100.0023--80.520.400.150.0300.005200.00100.0012--90.510.400.500.0330.005000.46000.0022-0.025100.520.400.400.0070.004000.02600.00300.02100.025110.580.400.500.0310.005200.12000.0019--120.580.610.500.0330.005000.10000.0020--130.580.620.500.0290.004400.09000.0022--140.580.660.500.0280.005300.08000.0023--150.580.610.500.0310.004800.11000.0022--160.570.610.500.0300.005100.08200.0023--170.580.660.500.0310.005100.12000.0015--180.590.610.500.0320.005400.09000.0022--190.600.600.900.0300.005600.11000.0022--200.580.400.500.0310.005200.12000.0019--210.480.220.410.0090.005200.04800.0022--220.480.220.410.0090.005200.04800.0022--230.480.610.410.0070.004900.00100.0022--1011.100.400.420.0320.005100.10000.0022--1020.380.220.420.0310.005100.12000.0022--1030.491.100.500.0310.005100.10000.0022--1040.540.601.110.0320.005100.10000.0022--1050.580.600.500.0290.004401.20000.0510--1060.810.600.500.0270.005400.10000.00220.12000.2001070.620.900.310.0310.005200.10000.0039--1080.620.900.310.0310.005200.10000.0039--1090.620.900.310.0310.005200.10000.0039--1100.620.900.310.0310.005200.10000.0039--1110.620.900.310.0310.005200.10000.0039--1120.620.900.310.0310.005200.10000.0039--1130.620.900.310.0310.005200.10000.0039-- [Table 1-2] Steel sheet No.Chemical composition (unit mass%) remainder : Fe and impuritiesCrVMoBCoNiCuOther elements1--------2--------3--------4--------50.10--0.0021----6--------7--------8--------90.200.050.100.00210.05---100.10-0.210.0024-0.080.19-11-------W: 0.2212-------O: 0.00813-------Ca: 0.10 - Mg: 0.1514-------La: 0.002215-------Sb: 0.01216-------Zr: 0.0517-------Sn: 0.0118-------As: 0.0719-----0.200.20-20--------21--------22--------23--------101--0.100.0021-0.110.12-102--------103--0.100.0010----104--0.220.0021----105---0.0020----106---0.0021----107---0.0021----108---0.0021----109---0.0021----110---0.0021----111---0.0021----112---0.0021----113---0.0021---- [Table 2-1] Steel sheet No.Manufacturing conditionsHot rolling stepCooling stepCoiling stepSlab heating temperature (°C)Finish rolling temperature (°C)Time from finish rolling to start of cooling (s)Average cooling rate up to cooling stop temperature of 750°C or lower (°C / s)Coiling temperature (°C)Average cooling rate from coiling temperature to 500°C (°C / hr)112509021.05062082212509011.25162278312509001.55058064412509051.05159266512509003.05260169612509021.55062075712509001.55061069812509051.55061368912508901.050582581012509001.050530551112509101.050552551212509001.050601701312509011.050600711412508921.050611721512509041.050530601612509051.050592701712509021.050581631812509041.050602621912509001.050600662012509201.050600622112509021.050620802212509001.050600712312509001.0506006710112509001.0506007010212508701.0505305610312509001.0505906010412509001.0505506210512509001.0506007010612509001.0516027310712509641.0407558010812509006.296057110912509001.0507558211012509001.0515903011112509001.0506007211212509001.0506006811312509001.05060071 [Table 2-2] Steel sheet No.Manufacturing conditionsCold rolling stepHeat treatment stepCoating stepSkin pass stepSheet thickness reduction ratio (%)Holding temperature (°C)Holding time (s)Oxygen potentialType of coatingThickness (µm)Rolling reduction (%)142820300-1.61---242820300-0.77GA150.1342842300-0.77Al200.1442822300-0.77--0.1542800300-0.77--0.1642800300-1.00---750800300-1.05--0.1841800300-1.70--0.5942820300-1.02--0.11042800300-0.77GA120.051142820300-0.80GA150.051242820300-0.77--0.11342790300-0.77--0.11442822300-0.77--0.11542820300-0.77--0.11642831300-0.78--0.11742820300-1.01--0.11842819300-0.79--0.11942821300-0.80--0.12042820300-0.80--0.12142-----2242862300-0.71--0.12342780300-0.55--0.0510142820300-1.25---10242780200-1.25--0.110342820300-0.77--0.110442820300-0.77--0.110542820300-0.77--0.110642811300-0.77---10742900300-0.99--0.110842800150-0.77--0.110942780150-0.77--0.111042950600-2.00--0.111142820300-0.77--0.111242820300-0.77--0.111342820300-0.77--0.1 [Table 2-3] Steel sheet No.Steel sheetSheet thickness (mm)Fraction (area ratio) of microstructure at 1 / 4 depth positionCarbideHardnessFerrite (%)Pearlite (%)Bainite (%)Martensite (%)Austenite (%)Total of bainite, martensite, and austenite (%)Number density of carbides having circle equivalent diameter of 0.2 µm or more at 1 / 4 depth position ( / 10 µm 2< )Hardness at 50 µm depth position (HV)Hardness at 1 / 4 depth position (HV)12.091900001.025225222.0782200002.213525532.0673300002.012026142.0702802023.010928252.0752020353.010424062.0683200003.528228272.0732403031.017123782.0871300003.112229591.4802000003.0181251102.0772030033.0129233112.0771850053.9123240122.0802000003.0110240132.0801820023.2119242142.0703000001.2109255152.0801550053.0121258162.0802000003.2112242172.0802000003.1172241182.0802000003.0111252192.0841204043.6121251202.0722800003.0123240212.0703000004.1242245222.0802000002.0112269232.0851500003.01332521012.0772000339.23213321022.0881200000.02442441032.076120012123.01022321042.070180120123.01222721052.0703000004.21232401062.08018200210.11222991072.073120015157.12512721082.08020000012.11212931092.0693100008.11182741102.0881200008.22662621112.0802000003.61202401122.0802000003.61202401132.0802000003.6120240

[0190] Next, hot stamping and tempering were performed on the obtained steel sheets under the conditions shown in Table 3-1 to obtain hot-stamping formed bodies. In the table, formed body No. H1 indicates that steel sheet No. 1 was used as the material (steel sheet for hot stamping), and No. H2 indicates that steel sheet No. 2 was used as the material (steel sheet for hot stamping). The same applies to the others. In Table 3-1, "-" indicates not applicable.

[0191] In the obtained hot-stamping formed bodies, the area ratio of martensite in the microstructure at the 1 / 4 depth position and the number density of ε-carbides having a circle equivalent diameter of 5 nm or more at the 1 / 4 depth position were measured in the above-described manners.

[0192] In addition, the Vickers hardnesses at the 50 µm depth position and the 1 / 4 depth position were measured in the above-described manner.

[0193] Results are shown in Table 3-2.

[0194] In addition, the bending angle at maximum force was obtained as an index of the tensile strength (TS) and the impact absorption properties of the obtained hot-stamping formed body.(Tensile Strength)

[0195] The tensile strength was obtained by collecting a No. 5 test piece according to JIS Z 2241:2011 from a position as flat as possible of the hot-stamping formed body and conducting a tensile test on the test piece according to a test method described in JIS Z 2241:2011. The measurement was performed twice for each steel sheet, and an average thereof was used as a measurement value.

[0196] It was determined that a preferable strength was obtained when the tensile strength was 2,000 MPa or more.(Bending Angle at Maximum Force)

[0197] Regarding the bending angle at maximum force, a bending angle of the steel sheet when a load was applied to the steel sheet between a pair of rolls with a punch while bending the steel sheet along VDA238-100 using a test piece having a width of 60 mm and a length of 30 mm (the length is in a direction parallel to the rolling direction), the test piece being collected from a position as flat as possible of the hot-stamping formed body, and the punch-back force was maximized, was defined as the bending angle at maximum force of the steel sheet. The measurement was performed twice for each steel sheet, and an average thereof was used as a measurement value.

[0198] In a case where the bending angle at maximum force was 40° or more in terms of a sheet thickness of 2.0 mm, excellent impact absorption properties were determined. [Table 3-1]Formed body No.Ac3 (°C)Manufacturing conditions of hot-stamping formed bodyHot stamping stepTempering stepHeating temperature (°C)Heating time (s)Average cooling rate up to 300 °C (°C / s)Holding time at 80°C or higher and 300°C or lower (s)Tempering temperature (°C)Tempering time (s)H1815900240709.9--H27839002407010.0--H3769902240707.01701200H4732900240701.81701200H5795901240401.61701200H67869002407010.0--H7759902240709.8--H8802900240408.0--H97969304807010.0--H107909002405010.01701200H117819002407010.017020H127879002407010.017020H137889002407010.017020H147899002407010.017020H157879002407010.017020H167909002407010.017020H177899002407010.017020H187859002407010.017020H197639202404010.017020H207819002407010.017020H218009002407010.0--H228009002407010.0--H238129102405010.017020H1016579002407010.017020H1028239002407010.017020H1038259002407010.017020H104Cracking occurred in processing before HSH1057879002407010.0--H106734880240705.0--H 1077919002407010.017020H1087829002407010.017020H1097829002407010.017020H1107578002407010.0--H111782770301204.217020H11278290024081.217020H1137828501201501.5-- [Table 3-2] Formed body No.Hot-stamping formed bodyCarbideFraction (area ratio) of microstructure at 1 / 4 depth positionHardnessPropertiesNumber density of ε-carbides having circle equivalent diameter of 5 nm or more at 1 / 4 depth position ( / µm 2< )Martensite (%)Hardness at 50 µm depth position (HV)Hardness at 1 / 4 depth position (HV)TS (MPa)Bending angle at maximum force (°)H15198662667220141H262100356778246646H375100320813258447H472100366900287143H542100299778245649H630100511724233842H759100411796257644H840100724750247641H975100420734237143H1075100314740234344H1175100292778246646H1275100301783248344H1375100333782248445H1451100312783248344H1575100305778246643H1675100315771244643H1775100451778250642H1875100321785248943H1940100388790250646H2075100292778246645H213199692704232441H2275100300714224745H238299320709224147H101010010011030FracturedFracturedH1022397641641201644H1037599261716226242H104Cracking occurred in processing before HSH1052899292773245032H1060100341912291033H107099653809266837H108099225814258531H109099226797252936H1105996580581189236H1114190224521171942H1125285191417137644H1131399250797252938

[0199] As can be seen from Tables 1-1 to 3-2, the hot-stamping formed bodies (formed bodies Nos. H1 to H23), which are invention examples, were hot-stamping formed bodies in which the chemical composition and the number density of ε-carbides were within the ranges of the present invention, and both high strength and excellent impact absorption properties were provided.

[0200] Contrary to this, in the hot-stamping formed bodies (formed bodies Nos. H101 to H103 and Nos. H105 to H109), which are comparative examples, the steel sheets used as the materials were not preferable, and the chemical composition or the number density of ε-carbides was outside the ranges of the present invention even as the hot-stamping formed bodies, so that the tensile strength or the impact absorption properties (bending angle at maximum force) were inferior (H101 fractured at an early stage before reaching TS or the like in the tensile test and the bending test).

[0201] In H104, cracks occurred during blank processing before hot stamping, and thus subsequent tests were not conducted.

[0202] In No. H110, the heating temperature in the heat treatment step was too high and the heating time was too long, so that sufficient hardening could not be achieved, and a sufficient tensile strength could not be obtained as the hot-stamping formed body.

[0203] In Nos. H111 and H112, the steel sheets used as the materials were preferable, but the hot stamping conditions were not preferable, and a sufficient tensile strength could not be obtained as the hot-stamping formed body.

[0204] In No. H113, the steel sheet used as the material was preferable, but the tempering after hot stamping was not sufficient, and the number density of ε-carbides was outside the range of the present invention, so that the impact absorption properties (bending angle at maximum force) were inferior.INDUSTRIAL APPLICABILITY

[0205] According to the present invention, it is possible to provide a hot-stamping formed body having both high strength and excellent impact absorption properties, a steel sheet suitable as a material for the hot-stamping formed body, and manufacturing methods of the same.

[0206] This hot-stamping formed body meets the recent demand for both high strength and impact absorption properties required for a steel sheet for a vehicle, and can improve the fuel efficiency and collision safety of vehicles.

Claims

1. A hot-stamping formed body comprising, as a chemical composition, by mass%: C: 0.40% to 1.00%; Si: 0.01% to 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% 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 1.00%; Mg: 0% to 1.00%; REM: 0% to 0.0050%; Sb: 0% to 0.020%; Zr: 0% to 0.10%; Sn: 0% to 0.10%; As: 0% to 0.10%; and a remainder: Fe and impurities, wherein, when a position at 1 / 4 of a thickness in a thickness direction from a surface is defined as a 1 / 4 depth position, a number density of ε-carbides having a circle equivalent diameter of 5 nm or more at the 1 / 4 depth position is 20 / µm2 or more, and a tensile strength of the hot-stamping formed body is 2,100 MPa or more.

2. The hot-stamping formed body according to Claim 1, wherein, when a position at 50 µm in the thickness direction from the surface is defined as a 50 µm depth position, a hardness at the 50 µm depth position is smaller than a hardness at the 1 / 4 depth position.

3. The hot-stamping formed 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 HV100 or more in Vickers hardness.

4. The hot-stamping formed body according to any one of Claims 1 to 3, wherein the chemical composition includes, by mass%, one or more selected from the group consisting of 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%.

5. The hot-stamping formed body according to any one of Claims 1 to 3, wherein the chemical composition includes, by mass%, one or more selected from the group consisting of Mo: 0.05% to 0.50%, B: 0.0010% to 0.0100%, Co: 0.01% to 1.00%, Ni: 0.10% to 1.00%, Cu: 0.10% to 1.00%, and W: 0.10% to 3.00%.

6. The hot-stamping formed body according to any one of Claims 1 to 3, wherein the chemical composition includes, by 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%.

7. A steel sheet comprising, as a chemical composition, by mass%: C: 0.40% to 1.00%; Si: 0.01% to 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% 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 1.00%; Mg: 0% to 1.00%; REM: 0% to 0.0050%; Sb: 0% to 0.020%; Zr: 0% to 0.10%; Sn: 0% to 0.10%; As: 0% to 0.10%; and a remainder: Fe and impurities, wherein, when a position at 1 / 4 of a sheet thickness in a sheet thickness direction from a surface is defined as a 1 / 4 depth position, a microstructure at the 1 / 4 depth position includes, by area ratio, ferrite: more than 50% and 100% or less, pearlite: 0% to 40%, and bainite, martensite, and austenite: 0% or more and less than 10% in total, and a number density of carbides of one or more of Nb, Ti, Fe, Mo, W, and Cr having a circle equivalent diameter of 0.2 µm or more at the 1 / 4 depth position is less than 5.0 / 10 µm2.

8. The steel sheet according to Claim 7, wherein, when a position at 50 µm in the sheet thickness direction from the surface is defined as a 50 µm depth position, a hardness at the 50 µm depth position is smaller than a hardness at the 1 / 4 depth position.

9. The steel sheet according to Claim 7 or 8, wherein the chemical composition includes, by mass%, one or more selected from the group consisting of 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%.

10. The steel sheet according to Claim 7 or 8, wherein the chemical composition includes, by mass%, one or more selected from the group consisting of Mo: 0.05% to 0.50%, B: 0.0010% to 0.0100%, Co: 0.01% to 1.00%, Ni: 0.10% to 1.00%, Cu: 0.10% to 1.00%, and W: 0.10% to 3.00%.

11. A manufacturing method of the hot-stamping formed body according to Claim 1, the manufacturing method comprising: a hot stamping process of heating the steel sheet according to claim 7 to a highest heating temperature of higher of an Ac3 point and 800°C or higher and 950°C or lower, holding the steel sheet at the highest heating temperature for 60 to 720 seconds, and then cooling the steel sheet to 300°C or lower so that an average cooling rate from the highest heating temperature to 300°C is 10 to 500 °C / sec; and a tempering process of tempering the steel sheet after the hot stamping process, wherein, in the tempering process, the steel sheet is held at 80°C to 300°C for 6.0 seconds or longer, the steel sheet is cooled to lower than 80°C at an average cooling rate of 20 to 500°C / sec, and then heated again and held at 80°C to 300°C for 6.0 seconds or longer, or the steel sheet is held at 80°C to 300°C for 6.0 seconds or longer, the steel sheet is cooled to lower than 80°C at an average cooling rate of 20 to 500°C / sec, and then heated again and held at 80°C to 300°C for 6.0 seconds or longer.

12. A manufacturing method of the steel sheet according to Claim 7, the manufacturing method comprising: a heating process of heating a slab having a chemical composition including, by mass%, C: 0.40% to 1.00%; Si: 0.01% to 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% 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 1.00%; Mg: 0% to 1.00%; REM: 0% to 0.0050%; Sb: 0% to 0.020%; Zr: 0% to 0.10%; Sn: 0% to 0.10%; As: 0% to 0.10%; and a remainder: Fe and impurities, to 1,150°C to 1,350°C; a hot rolling process of hot-rolling the slab after the heating process so that a finish rolling temperature is 800°C to 950°C to obtain a steel sheet; a cooling process of cooling the steel sheet after the hot rolling process to 750°C or lower so that an average cooling rate is 10 to 100 °C / sec, the cooling process starting within 5.0 seconds from completion of the hot rolling process; a coiling process of coiling the steel sheet after the cooling process at a coiling temperature of higher than 500°C and 750°C or lower and setting an average cooling rate from the coiling temperature to 500°C to faster than 50 °C / hr; and a cold rolling process of cold-rolling the steel sheet after the coiling process at a sheet thickness reduction ratio of 10% to 60%.

13. The manufacturing method of the steel sheet according to Claim 12, further comprising: a heat treatment process of heating the steel sheet after the cold rolling process to an annealing temperature of 700°C to 920°C, and holding the steel sheet at the annealing temperature for 120 to 500 seconds in an atmosphere having an oxygen potential of -1.50 or more.

14. The manufacturing method of the steel sheet according to Claim 13, further comprising: a skin pass process of performing skin pass rolling on the steel sheet after the heat treatment process at a rolling reduction of 0.05% to 2.0%.

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