Steel plates for hot stamping and hot stamped molded products

Abstract

This steel sheet for hot stamping comprises a steel sheet, wherein, when decarburization indexes of the steel sheet at five positions are measured, DIAVE, which is the average of the decarburization indexes at the five positions, DIMAX, which is the maximum value of the decarburization indexes at the five positions, and DIMIN, which is the minimum value of the decarburization indexes at the five positions, satisfy expressions (i) and (ii). This steel sheet for stamping is used to obtain a hot-stamp molded body.　Expression (i): 0.10 ≤ DIAVE ≤ 0.35 Expression (ii): DIMAX - DIMIN ≤ 0.15

Description

[Technical Field]

[0001] This disclosure relates to steel sheets for hot stamping and hot stamped molded articles. This application claims priority based on Japanese Patent Application No. 2024-045334, filed in Japan on March 21, 2024, and the contents of that application are incorporated herein by reference. [Background technology]

[0002] In recent years, there has been a demand for lighter automobile bodies from the perspective of environmental protection and resource conservation, and high-strength steel sheets are being applied to automobile components. When high-strength steel sheets are applied, the desired strength can be provided to the automobile body while reducing the weight of the vehicle body by making the steel sheet thickness thinner. Automobile components are manufactured by press forming steel sheets, but as the strength of the steel sheet increases, not only does the forming load increase, but the formability decreases, making it more prone to cracking and wrinkling. In addition, when high-strength steel sheets are press formed, the shape of the component changes significantly due to springback when the component is removed from the mold, making it difficult to ensure the dimensional accuracy of the component. Thus, it is not easy to manufacture high-strength automobile components by press forming.

[0003] To address the above-mentioned challenges, a technique has been proposed, for example, as disclosed in Patent Document 1, in which a heated steel sheet is press-formed using a low-temperature press die. This technique is called hot stamping or hot pressing, and because it press-forms a steel sheet that has been heated to a high temperature and is in a soft state, it is possible to manufacture components with complex shapes with high dimensional accuracy. Furthermore, because the steel sheet is rapidly cooled by contact with the die, it is possible to significantly increase its strength simultaneously with press-forming through quenching. Patent Document 1 discloses that a molded body with a tensile strength of 1400 MPa or more can be obtained by hot-stamping a steel sheet with a tensile strength of 500 to 600 MPa.

[0004] The strength of the hot stamping formed body can be further increased by increasing the C content of the steel sheet. However, when the C content of the steel sheet is increased, the deformability decreases as the strength of the hot stamping formed body increases, and when the hot stamping formed body is deformed during a collision, cracks are likely to occur at the initial stage of deformation. Thus, it is not easy to manufacture a high-strength hot stamping formed body with high deformability. In particular, when the tensile strength of the hot stamping formed body is 1800 MPa or more, it becomes difficult to achieve both strength and deformability.

[0005] As a technique for manufacturing a hot stamping formed body with excellent deformability, Patent Document 2 discloses a high-strength press part having a strength with a tensile strength of 1300 MPa or more and having high shock absorbency, and a method for manufacturing the same.

[0006] Patent Document 3 discloses an automotive hot stamping member having a tensile strength of 1100 MPa or more and having improved bendability of the member from the viewpoint of shock energy absorption, and a method for manufacturing the same.

[0007] <00​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​However, our investigations have shown that it is not easy to uniformly form a decarburized layer on a steel sheet for hot stamping, and that differences in the amount of decarburization tend to occur depending on the location within the steel sheet. Furthermore, we have found that when a hot-stamped molded body is manufactured using a steel sheet with an uneven amount of decarburization, the carbon concentration in the surface region of the hot-stamped molded body fluctuates, the deformability of the hot-stamped molded body is partially reduced, and the impact resistance characteristics may not be sufficiently improved.

[0010] This disclosure is made in view of the above circumstances and aims to provide a hot stamping steel sheet suitable as a material for hot stamped molded articles having high strength and excellent deformability, as well as a hot stamped molded article having high strength and excellent deformability. Furthermore, "excellent deformability" means that the deformability is highly uniform and that the deformability is high regardless of the position within the hot-stamped molded body. [Means for solving the problem]

[0011] The gist of this disclosure is as follows: [1] A steel plate for hot stamping comprising a steel plate, The chemical composition of the steel plate is, in mass%, C: more than 0.310%, less than 0.700%, Si: Less than 2.00% Mn: 0.01~3.00%, P: 0.200% or less, S: 0.0200% or less, sol.Al: 0.001~1.000%, N: 0.0200% or less, O: 0.0200% or less, B: 0.0002~0.0200%, Cr: 0~2.00%, Mo: 0~2.00%, W: 0~2.00%, Cu: 0~2.00%, Ni: 0~2.00%, Ti: 0~0.200%, Nb: 0~0.200%, V: 0~0.200%, Zr: 0~0.200%, Ca: 0~0.1000%, Mg: 0~0.1000%, REM: 0~0.1000%, Sn: 0~0.200%, As: 0~0.100%, and Contains Bi: 0~0.0500%, The remainder consists of Fe and impurities. When measuring the decarburization index at five locations: 50 mm from the end face in the width direction of the steel plate, 1 / 4 of the way across in the width direction, the center of the width direction, 3 / 4 of the way across in the width direction, and 50 mm from the end face opposite to the end face, The average value of the decarburization index at the five locations is the DI. AVE and, The maximum value of the decarburization index at the five locations is the DI. MAX and, The minimum value of the decarburization index at the five locations is the DI. MIN A steel sheet for hot stamping, characterized in that it satisfies the following formulas (i) and (ii). 0.10 ≤ DI AVE ≤0.35 …(i) DI MAX -DI MIN ≤0.15 …(ii) [2] The chemical composition of the steel plate is, in mass%, Cr: 0.01~2.00%, Mo: 0.01~2.00%, W: 0.01~2.00%, Cu: 0.01~2.00%, Ni: 0.01~2.00%, Ti: 0.001~0.200%, Nb: 0.001~0.200%, V: 0.001~0.200%, Zr: 0.001~0.200%, Ca: 0.0001~0.1000%, Mg: 0.0001~0.1000%, REM: 0.0001~0.1000%, Sn: 0.001~0.200%, As: 0.001~0.100%, and The hot stamping steel sheet according to [1] above, characterized by containing one or more elements from the group consisting of Bi: 0.0001 to 0.0500%. [3] A hot-stamped molded body comprising a steel plate, All or part of the aforementioned steel plate has a chemical composition of, by mass%, C: more than 0.310%, less than 0.700%, Si: Less than 2.00% Mn: 0.01~3.00%, P: 0.200% or less, S: 0.0200% or less, sol.Al: 0.001~1.000%, N: 0.0200% or less, O: 0.0200% or less, B: 0.0002~0.0200%, Cr: 0~2.00%, Mo: 0~2.00%, W: 0~2.00%, Cu: 0~2.00%, Ni: 0~2.00%, Ti: 0~0.200%, Nb: 0~0.200%, V: 0~0.200%, Zr: 0~0.200%, Ca: 0~0.1000%, Mg: 0~0.1000%, REM: 0~0.1000%, Sn: 0~0.200%, As: 0~0.100%, and Contains Bi: 0~0.0500%, The remainder consists of Fe and impurities. In the surface layer region, which is the region from a depth of 20 μm to a depth of 40 μm from the surface of the steel sheet, when measuring CS, which is the average value of the C concentration in mass% in the surface layer region of the steel sheet, at each of five locations that satisfy the following conditions A to D: CS AVE which is the average value of the CS at the five locations, CS MAX which is the maximum value of the CS at the five locations, CS MIN which is the minimum value of the CS at the five locations, satisfy the following formulas (iii) and (iv): A hot stamping formed body characterized in that the tensile strength is 1800 MPa or more. CS AVE ≦0.80×C1…(iii) CS MAX -CS MIN ≦0.100 mass% …(iv) Condition A: Each of the five locations is more than 25 mm away from the end face. Condition B: The five locations are more than 100 mm apart from each other. Condition C: Among the five locations, at least two locations are at a distance of 25 to 75 mm from the end face. Condition D: The distance between the two most distant locations among the locations at a distance of 25 to 75 mm from the end face is 400 mm or more. However, C1 in the above formula (iii) is the C content in mass% in the chemical composition of the steel sheet. [4] The chemical composition of the steel sheet is, in mass%, Cr: 0.01 to 2.00%, Mo: 0.01 to 2.00%, W: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Ti: 0.001 to 0.200%, Nb: 0.001 to 0.200%, V: 0.001 to 0.200%, Zr: 0.001 to 0.200%, Ca: 0.0001 to 0.1000%, Mg: 0.0001~0.1000%, REM: 0.0001~0.1000%, Sn: 0.001~0.200%, As: 0.001~0.100%, and Bi: 0.0001~0.0500% The hot stamped molded article according to [3] above, characterized in that it contains one or more from the group consisting of the above. [Effects of the Invention]

[0012] According to the above embodiments of this disclosure, it is possible to provide a hot stamping steel sheet suitable as a material for hot stamping molded articles having high strength and excellent deformability, as well as a hot stamping molded article having high strength and excellent deformability. The hot-stamped molded article according to the above embodiment has excellent deformability and does not crack in the initial stages of deformation during a collision, making it suitable for use in automotive components such as pillars, bumpers, and door beams. [Brief explanation of the drawing]

[0013] [Figure 1] This diagram illustrates the five locations in the hat component that satisfy conditions A to D. [Figure 2] This figure shows the hat component manufactured in the example. [Modes for carrying out the invention]

[0014] The inventors of this invention investigated methods to improve the deformability of hot-stamped molded articles having a tensile strength of 1800 MPa or more, and obtained the following findings.

[0015] (A) When hot stamping is performed using a hot stamping steel sheet equipped with a decarburized layer, a soft layer with a low C concentration is formed in the surface region of the hot stamped molded body, improving the deformability of the hot stamped molded body.

[0016] (B) In order to decarburize the surface layer of hot-stamping steel sheets, it is effective to anneal them in an atmosphere with a higher dew point than usual (hereinafter sometimes referred to as high-dew-point annealing). However, when high-dew-point annealing is performed, the amount of decarburization tends to vary depending on the location on the steel sheet. The variation in the amount of decarburization is often particularly large in the width direction of the steel sheet. The amount of decarburization is often high near the center of the steel sheet in the width direction and decreases towards the edges.

[0017] (C) When hot stamping is performed using hot stamping steel sheets with large variations in decarburization, the carbon concentration in the surface region after hot stamping becomes low in areas with high decarburization and high in areas with low decarburization. As a result, variations in carbon concentration occur depending on the location in the surface region of the hot stamped molded product. Furthermore, in the surface region of a hot-stamped molded body, areas with high carbon concentration reduce its flexibility. Therefore, if there are large fluctuations in the decarburization rate of the hot-stamped steel sheet used as the raw material, the deformability of the hot-stamped molded body may be partially reduced.

[0018] (D) Variations in the amount of decarburization in hot stamping steel sheets can be effectively suppressed by employing a process that satisfies the following conditions in the manufacturing of hot stamping steel sheets. (a) A reheating treatment is performed to reheat the end faces in the width direction of the hot-rolled and coiled steel sheet. (b) When annealing cold-rolled steel sheets, the dew point of the atmospheric gas in the furnace is set to the intermediate dew point.

[0019] The reason why the above process can suppress fluctuations in the amount of decarburization in hot-stamping steel sheets is not clear, but the inventors speculate as follows. After hot rolling and winding into a coil, areas within the coil that remain in a high-temperature region for a long time (near the center in the width direction of the sheet) tend to decarburize more easily during high-dew-point annealing. The winding temperature near the edge of the steel plate is lower than that near the center in the width direction of the plate, and the cooling rate after winding is also faster. However, by actively reheating the plate, the time spent in the high-temperature range is extended, and the difference between the edge and the center of the plate width is reduced. When performing high-dew-point annealing on cold-rolled steel sheets, slightly lowering the dew point (performing medium-dew-point annealing) results in little change in the decarburization rate in areas with long residence times in the high-temperature range, but decarburization is promoted in areas with short residence times in the high-temperature range.

[0020] Based on the findings in (A) to (D) above, the inventors have discovered that in the manufacturing process of hot-stamping steel sheets, fluctuations in the amount of decarburization in the hot-stamping steel sheets can be suppressed by reheating the end face portion of the hot-rolled and coiled steel sheet, and by performing annealing after cold rolling in an atmosphere with a medium dew point. Furthermore, they have found that by using a hot-stamping steel sheet with small fluctuations in the amount of decarburization as the material for hot stamping, fluctuations in the C concentration in the surface region of the hot-stamped molded body can be suppressed, resulting in a hot-stamped molded body with a tensile strength of 1800 MPa or more, excellent uniformity of deformability, and high deformability regardless of the position within the hot-stamped molded body.

[0021] The hot stamping steel sheet according to this embodiment will be described in detail below. First, the reason for limiting the chemical composition of the hot stamping steel sheet according to this embodiment will be explained.

[0022] The steel sheet constituting the hot stamping steel sheet according to this embodiment has the following chemical composition. Note that the numerical ranges indicated by "~" below include both the lower and upper limits. Numbers indicated as "less than" or "greater than" do not include the numerical range. All percentages for chemical composition represent mass percentages.

[0023] The steel sheet constituting the hot stamping steel sheet according to this embodiment has a chemical composition in mass%, of C: greater than 0.310% and 0.700% or less, Si: less than 2.00%, Mn: 0.01 to 3.00%, P: 0.200% or less, S: 0.0200% or less, sol.Al: 0.001 to 1.000%, N: 0.0200% or less, O: 0.0200% or less, B: 0.0002 to 0.0200%, and the remainder: Fe and impurities. The following describes each element.

[0024] C: More than 0.310%, less than 0.700% Carbon (C) is an element that improves the tensile strength of the steel sheet after hot stamping (the steel sheet that constitutes the hot-stamped molded body). If the C content is 0.310% or less, the tensile strength of the steel sheet after hot stamping will be less than 1800 MPa, resulting in insufficient strength for the hot-stamped molded body. Therefore, the C content should be greater than 0.310%. Preferably, the C content is 0.320% or more, 0.340% or more, 0.380% or more, 0.420% or more, or 0.450% or more. On the other hand, if the carbon content exceeds 0.700%, the strength of the hot-stamped molded article becomes too high, and excellent deformability cannot be obtained. Therefore, the carbon content should be 0.700% or less. Preferably, the carbon content is 0.650% or less, 0.600% or less, 0.550% or less, or 0.500% or less.

[0025] Si: Less than 2.00% Si is an element that embrittles steel. The adverse effect is particularly large when the Si content is 2.00% or higher. Therefore, the Si content should be less than 2.00%. Preferably, the Si content is 1.50% or less, 1.00% or less, 0.75% or less, 0.50% or less, or 0.20% or less. There is no particular lower limit to the Si content, but it may be 0%. However, excessively reducing the Si content will increase steelmaking costs, so it is preferable that the Si content be 0.001% or higher. Furthermore, since Si has the effect of improving the hardenability of steel, it may be actively included. From the viewpoint of improving hardenability, it is preferable that the Si content be 0.05% or higher, 0.10% or higher, 0.15% or higher, or 0.20% or higher.

[0026] Mn: 0.01~3.00% Mn is an element that combines with S to form MnS, which suppresses the harmful effects of S. If the Mn content is less than 0.01%, the above effect cannot be obtained. Therefore, the Mn content should be 0.01% or more. In addition, Mn is an element that improves the hardenability of steel, and is an effective element for ensuring the strength of the hot-stamped molded product by forming a martensite-based metallic structure in the inner layer region of the steel sheet after hot stamping. From the viewpoint of ensuring strength, the Mn content is preferably 0.50% or more, 0.75% or more, 1.00% or more, or 1.25% or more. On the other hand, if the Mn content exceeds 3.00%, excellent deformability cannot be obtained in the hot-stamped molded article. Therefore, the Mn content should be 3.00% or less. Preferably, the Mn content is 2.50% or less, 2.00% or less, or 1.50% or less.

[0027] P:0.200% or less P is an element that embrittles steel. When the P content exceeds 0.200%, the adverse effects become particularly large, and the weldability also deteriorates significantly. Therefore, the P content should be 0.200% or less. Preferably, the P content is 0.100% or less, 0.050% or less, or 0.020% or less. The phosphorus (P) content may be 0%. However, reducing the P content to less than 0.001% significantly increases the cost of P removal, which is economically undesirable, so it may be set to 0.001% or higher.

[0028] S: 0.0200% or less S is an element that embrittles steel. The adverse effects become particularly large when the S content exceeds 0.0200%. Therefore, the S content should be 0.0200% or less. Preferably, the S content is 0.0050% or less, 0.0020% or less, or 0.0010% or less. The sulfur content may be 0%. However, reducing the sulfur content to less than 0.0001% would significantly increase the cost of desulfurization, which is economically undesirable, so it may be set to 0.0001% or higher.

[0029] sol.Al:0.001~1.000% Al is an element that has the effect of deoxidizing molten steel. If the sol.Al content (acid-soluble Al content) is less than 0.001%, deoxidation will be insufficient. Therefore, the sol.Al content should be 0.001% or more. Preferably, the sol.Al content is 0.005% or more, 0.010% or more, or 0.020% or more. On the other hand, if the sol.Al content is too high, the transformation point rises, making it difficult to heat the steel sheet to a temperature above the Ac3 point during the hot stamping heating process. In addition, the strength and deformability of the hot stamped molded product decrease. For this reason, the sol.Al content should be 1.000% or less. Preferably, the sol.Al content is 0.500% or less, 0.100% or less, 0.060% or less, or 0.040% or less.

[0030] N: 0.0200% or less N is an element that forms nitrides during the continuous casting of steel. Since these nitrides reduce the deformability of the steel sheet after hot stamping, a low N content is preferable. The adverse effects become particularly large when the N content exceeds 0.0200%. Therefore, the N content should be 0.0200% or less. Preferably, the N content is 0.0100% or less, 0.0080% or less, or 0.0050% or less. The N content may be 0%. However, excessively reducing the N content would significantly increase the cost of removing N, which is economically undesirable, so it may be set to 0.0005% or more, or 0.0010% or more.

[0031] O: 0.0200% or less O is an element that forms oxide inclusions. If the O content exceeds 0.0200%, a large amount of coarse oxide inclusions are formed in the steel. This deteriorates the deformability of the hot-stamped molded product. Therefore, the O content should be 0.0200% or less. Preferably, the O content is 0.0150% or less, 0.0100% or less, 0.0060% or less, or 0.0040% or less. The oxygen content may be 0%. However, excessively reducing the oxygen content would significantly increase steelmaking costs and is therefore economically undesirable; therefore, the oxygen content may be 0.0005% or more, or 0.0010% or more.

[0032] B: 0.0002~0.0200% B is an element that improves the hardenability of steel and is effective in ensuring the strength of hot-stamped molded articles by forming a martensite-based metallic structure in the inner layer region of the steel sheet after hot stamping. If the B content is less than 0.0002%, the desired strength cannot be obtained in the hot-stamped molded article. In addition, the deformability of the hot-stamped molded article deteriorates. Therefore, the B content should be 0.0002% or more. Preferably, the B content is 0.0005% or more, 0.0010% or more, 0.0015% or more, or 0.0020% or more. On the other hand, if the B content exceeds 0.0200%, carbonites are formed in the hot-stamped molded body, impairing the hardenability-improving effect of B. Therefore, the B content should be 0.0200% or less. Preferably, the B content is 0.0080% or less, 0.0050% or less, 0.0040% or less, or 0.0030% or less.

[0033] The remainder of the chemical composition of the steel sheet constituting the hot stamping steel sheet according to this embodiment consists of Fe and impurities. Examples of impurities include elements that are inevitably mixed in from steel raw materials or scrap and / or during the steelmaking process, and are acceptable within a range that does not impair the properties of the hot stamped molded product according to this embodiment.

[0034] The steel sheet constituting the hot stamping steel sheet according to this embodiment may contain the following elements as optional elements in place of a portion of Fe. If the following optional elements are not included, the content is 0%.

[0035] Cr: 0.01~2.00% Cr is an element that increases the strength of hot-stamped molded articles by improving the hardenability of steel. To reliably obtain this effect, the Cr content is preferably 0.01% or more. More preferably, the Cr content is 0.05% or more, 0.10% or more, or 0.20% or more. On the other hand, if the Cr content exceeds 2.00%, the deformability of the hot-stamped molded article decreases. Therefore, the Cr content should be 2.00% or less. Preferably, the Cr content is 1.50% or less, 1.00% or less, 0.50% or less, 0.40% or less, or 0.30% or less.

[0036] Mo: 0.01~2.00% Mo is an element that enhances the strength of hot-stamped molded articles by improving the hardenability of steel. To reliably obtain this effect, the Mo content is preferably 0.01% or more. More preferably, the Mo content is 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, if the Mo content exceeds 2.00%, the deformability of the hot-stamped molded product decreases. Therefore, the Mo content should be 2.00% or less. Preferably, the Mo content is 1.00% or less, 0.50% or less, 0.40% or less, or 0.30% or less.

[0037] W: 0.01~2.00% W is an element that enhances the strength of hot-stamped molded products by improving the hardenability of steel. To reliably obtain this effect, the W content is preferably 0.01% or more. More preferably, the W content is 0.05% or more, or 0.10% or more. On the other hand, if the W content exceeds 2.00%, the deformability of the hot-stamped molded product decreases. Therefore, the W content should be 2.00% or less. Preferably, the W content is 0.50% or less, 0.40% or less, or 0.30% or less.

[0038] Cu: 0.01~2.00% Cu is an element that enhances the strength of hot-stamped molded products by improving the hardenability of steel. To reliably obtain these effects, the Cu content is preferably 0.01% or more. More preferably, the Cu content is 0.10% or more. On the other hand, if the Cu content exceeds 2.00%, the deformability of the hot-stamped molded product decreases. Therefore, the Cu content should be 2.00% or less. Preferably, the Cu content is 1.00% or less, 0.75% or less, or 0.50% or less.

[0039] Ni: 0.01~2.00% Ni is an element that enhances the strength of hot-stamped molded products by improving the hardenability of steel. To reliably obtain this effect, the Ni content is preferably 0.01% or more. More preferably, the Ni content is 0.10% or more. On the other hand, if the Ni content exceeds 2.00%, the deformability of the hot-stamped molded article decreases. Therefore, the Ni content should be 2.00% or less. Preferably, the Ni content is 1.00% or less, 0.75% or less, or 0.50% or less.

[0040] Ti: 0.001~0.200% Ti is an element that forms carbonitrides in steel, thereby increasing the strength of hot-stamped molded products through precipitation strengthening. Furthermore, Ti improves the deformability of hot-stamped molded products by refining the metallic structure. To reliably obtain these effects, a Ti content of 0.001% or more is preferable. More preferably, the Ti content is 0.005% or more, or 0.010% or more. On the other hand, if the Ti content exceeds 0.200%, a large amount of coarse carbonitrides are formed in the steel, reducing the deformability of the hot-stamped molded product. Therefore, the Ti content should be 0.200% or less. Preferably, the Ti content is 0.100% or less, 0.050% or less, or 0.030% or less.

[0041] Nb: 0.001~0.200% Nb is an element that forms carbonitrides in steel, thereby increasing the strength of hot-stamped molded products through precipitation strengthening. Furthermore, Nb improves the deformability of hot-stamped molded products by refining the metal structure. To reliably obtain these effects, the Nb content is preferably 0.001% or higher. More preferably, the Nb content is 0.005% or higher, or 0.010% or higher. On the other hand, if the Nb content exceeds 0.200%, a large amount of coarse carbonitrides are formed in the steel, reducing the deformability of the hot-stamped molded product. Therefore, the Nb content should be 0.200% or less. Preferably, the Nb content is 0.100% or less, 0.050% or less, less than 0.040%, or 0.030% or less.

[0042] V: 0.001~0.200% V is an element that forms carbonitrides in steel, thereby increasing the strength of hot-stamped molded articles through precipitation strengthening. Furthermore, V improves the deformability of hot-stamped molded articles through refinement of the metal structure. To reliably obtain these effects, the V content is preferably 0.001% or higher. More preferably, the V content is 0.005% or higher, or 0.010% or higher. On the other hand, if the V content exceeds 0.200%, a large amount of coarse carbonitrides are formed in the steel, reducing the deformability of the hot-stamped molded product. Therefore, the V content should be 0.200% or less. Preferably, the V content is 0.100% or less or 0.050% or less.

[0043] Zr: 0.001~0.200% Zr is an element that forms carbonitrides in steel, thereby increasing the strength of hot-stamped molded products through precipitation strengthening. Furthermore, Zr improves the deformability of hot-stamped molded products by refining the metal structure. To reliably obtain these effects, a Zr content of 0.001% or more is preferable. More preferably, the Zr content is 0.005% or more, or 0.010% or more. On the other hand, if the Zr content exceeds 0.200%, a large amount of coarse carbonitrides are formed in the steel, reducing the deformability of the hot-stamped molded product. Therefore, the Zr content should be 0.200% or less. Preferably, the Zr content is 0.100% or less, 0.050% or less, or 0.025% or less.

[0044] Ca: 0.0001~0.1000% Ca is an element that improves the deformability of steel sheets after hot stamping by adjusting the shape of inclusions. To reliably obtain this effect, it is preferable that the Ca content be 0.0001% or more. On the other hand, even if a large amount of Ca is included, the above effects will saturate, and furthermore, excessive costs will be incurred, so the Ca content should be 0.1000% or less. Preferably, the Ca content is less than 0.0100%, less than 0.0050%, or less than 0.0020%.

[0045] Mg: 0.0001~0.1000% Mg is an element that improves the deformability of steel sheets after hot stamping by adjusting the shape of inclusions. To reliably obtain these effects, it is preferable that the Mg content be 0.0001% or higher. On the other hand, even if a large amount of Mg is included, the above effects will saturate, and furthermore, excessive costs will be incurred, so the Mg content should be 0.1000% or less. Preferably, the Mg content is less than 0.0100%, less than 0.0050%, or less than 0.0020%.

[0046] REM: 0.0001~0.1000% REM is an element that improves the deformability of steel sheets after hot stamping by adjusting the shape of inclusions. To reliably obtain this effect, it is preferable that the REM content be 0.0001% or higher. On the other hand, even if a large amount of REM is included, the above effects will saturate, and furthermore, excessive costs will be incurred, so the REM content should be 0.1000% or less. Preferably, the REM content is less than 0.0100%, less than 0.0050%, or less than 0.0020%. In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these elements.

[0047] Sn: 0.001~0.200% Sn is an element that improves the corrosion resistance of hot-stamped molded articles. To reliably obtain this effect, the Sn content is preferably 0.001% or more. More preferably, the Sn content is 0.005% or more, 0.015% or more, or 0.030% or more. On the other hand, even if a large amount of Sn is included, the above effect will saturate, and furthermore, excessive costs will be incurred, so the Sn content should be 0.200% or less. Preferably, the Sn content is 0.150% or less, 0.100% or less, or 0.050% or less.

[0048] As: 0.001~0.100% As is an element that enhances the strength of hot-stamped molded products. To reliably obtain this effect, it is preferable that the As content be 0.001% or more. On the other hand, even if a large amount of As is included, the above effects will saturate, and furthermore, excessive costs will be incurred, so the As content should be 0.100% or less.

[0049] Bi: 0.0001~0.0500% Bi is an element that enhances the deformability of hot-stamped molded products by refining the solidification structure. To reliably obtain this effect, it is preferable that the Bi content be 0.0001% or higher. On the other hand, even if a large amount of Bi is included, the above effect will saturate, and furthermore, excessive costs will be incurred, so the Bi content should be 0.0500% or less. Preferably, the Bi content is 0.0100% or less or 0.0050% or less.

[0050] The chemical composition of the steel sheet constituting the hot stamping sheet described above can be determined by taking a test piece from the sheet and measuring the average elemental content over the entire sheet thickness using a general analytical method. For example, it can be measured using inductively coupled plasma atomic emission spectrometry or inductively coupled plasma mass spectrometry. C and S can be measured using combustion-infrared absorption spectrometry, while O and N can be measured using inert gas melting-infrared absorption spectrometry or inert gas melting-thermal conductivity spectrometry. If the steel sheet used for hot stamping has a coating or plating layer on its surface, these should be removed before measuring the chemical composition.

[0051] In this embodiment, when the decarburization index is measured at five locations in the steel sheet for hot stamping: 50 mm from the end face in the width direction of the steel sheet, 1 / 4 of the way along the width direction, the center of the width direction, 3 / 4 of the way along the width direction, and 50 mm from the end face opposite to the end face, the average value of the decarburization index at these five locations is called the DI (Dielectric Strength Index). AVE And the maximum value of the five decarburization indices mentioned above, which is DI. MAX And the minimum value of the five decarburization indices, which is DI. MIN The following equations (i) and (ii) are satisfied. 0.10 ≤ DI AVE ≤0.35…(i) DI MAX -DI MIN ≤0.15 …(ii)

[0052] The 1 / 4 position in the width direction of a steel plate is the position where the plate is 1 / 4 of its width away from the end face in the width direction, and can be expressed as the w / 4 position when the length of the steel plate in the width direction is w. Similarly, the 3 / 4 position in the width direction of a steel plate is the position where the plate is 3 / 4 of its width away from the end face in the width direction, and can be expressed as the w × 3 / 4 position when the length of the steel plate in the width direction is w.

[0053] The DI is the average value of the decarburization indicators at the five locations mentioned above. AVE If DI is less than 0.10, the C concentration in the surface region of the hot-stamped molded product cannot be favorably controlled. AVE The DI should be 0.10 or higher. AVE Preferably, it is 0.12 or higher, more preferably 0.15 or higher or 0.20 or higher. On the other hand, DI AVE To excessively increase DI, a long annealing process is required in the manufacturing process of hot-stamping steel sheets, as described later, which impairs the productivity of the steel sheets. AVE If the DI is greater than 0.35, the strength of the hot-stamped molded product decreases.AVE The DI should be 0.35 or less. AVE Preferably, it is 0.30 or less or 0.25 or less.

[0054] The maximum value of the decarburization index at the five locations mentioned above is the DI. MAX And the minimum value of the decarburization index at the five locations mentioned above is DI. MIN If the difference exceeds the right-hand side of equation (ii) above, i.e., greater than 0.15, then the C concentration in the surface region of the hot-stamped molded product cannot be favorably controlled. Therefore, DI MAX -DI MIN The DI should be 0.15 or less. MAX -DI MIN Preferably, it is 0.12 or less, and more preferably 0.10 or less. DI MAX -DI MIN There is no particular lower limit, but it may be 0.01 or higher, or 0.02 or higher.

[0055] The decarburization indices at five locations on the steel sheet constituting the hot-stamping steel sheet—50 mm from the end face in the width direction of the sheet, 1 / 4 of the way along the sheet, the center of the sheet, 3 / 4 of the way along the sheet, and 50 mm from the end face opposite to the aforementioned end face—are measured by the following method.

[0056] Test specimens are taken from the steel sheets that make up the hot-stamping steel sheets, and the distribution of carbon (C) and iron (Fe) concentrations in the thickness direction at the five locations mentioned above is measured using glow discharge optical emission spectrometry (GDS analysis). The measurement range is set to a depth of 120 μm or more from the surface of the steel sheet, and the measurement interval is set to 0.10 μm or less.

[0057] If the steel sheet used for hot stamping has a plating layer or coating on its surface, the plating layer or coating should be partially or completely removed before GDS measurement, so that measurements can be taken up to a depth of 120 μm from the surface of the steel sheet. In this embodiment, a region where the Fe concentration is 95% by mass or higher in the GDS measurement is determined to be a steel plate, and the depth position of the measurement point where the Fe concentration first reaches 95% by mass or higher from the start of the measurement is defined as the surface of the steel plate.

[0058] The average value of C concentration in the region from the surface of the steel plate to a depth of 120 μm from the surface of the steel plate (C * ) calculate C * The decarburization index (DI) is calculated using the following formula (v) from the C content (C0) in mass percent in the chemical composition of the hot-stamping steel sheet. DI={(C0-C * ) / C0}×0.6 …(v)

[0059] The steel sheet constituting the hot-stamping steel sheet according to this embodiment is not particularly limited as long as the desired strength and deformability can be obtained after hot stamping, but it may have the following microstructure. For example, it may consist of ferrite: 5-90%, bainite and martensite: 0-50%, pearlite: 10-95%, and retained austenite: 0-10% by area ratio. In addition, it may contain precipitates and inclusions such as iron carbides, alloy carbides, and intermetallic compounds.

[0060] The microstructure of the steel sheet used for hot stamping is measured by the following method. The microstructure of the steel sheet is measured from the surface to a depth of 1 / 4 of the sheet thickness (the region from the surface to a depth of 1 / 8 of the sheet thickness to a depth of 3 / 8 of the sheet thickness).

[0061] A test specimen is taken from a hot-stamping steel sheet, and after buffing the cross-section of the steel sheet, the microstructure is observed in the above-mentioned area. Specifically, after nital etching or electrolytic polishing of the polished surface, micrographs are taken using an optical microscope and a scanning electron microscope (SEM), and the area percentages of ferrite, pearlite, bainite, tempered martensite, precipitates, and inclusions are obtained by performing image analysis on the obtained micrographs based on differences in brightness or the morphology of iron carbides present in the phase. Subsequently, after repeller etching is performed on the same observation position, micrographs are taken using an optical microscope or a scanning electron microscope (SEM), and the total area percentage of "retained austenite and fresh martensite" is calculated by performing image analysis on the obtained micrographs.

[0062] Furthermore, for the same observation positions, after electropolishing the plate thickness cross-section, the area fraction of retained austenite is measured using a scanning electron microscope (SEM) equipped with an electron beam backscattering pattern analyzer (EBSP). The area fraction of retained austenite is obtained by calculating the area fraction of the region with an fcc crystal structure from the crystal orientation information obtained by the EBSP analysis. The area ratio of fresh martensite is obtained by subtracting the area ratio of retained austenite from the sum of the area ratios of "retained austenite and fresh martensite" mentioned above. In EBSP analysis, the measurement interval was 0.1 μm, and the vacuum level inside the instrument was 9.6 × 10⁻⁶. -5 The parameters are set to Pa or less, with an acceleration voltage of 25kV and an irradiation current level of 16. To display the EBSP map, version 7 or later of OIM Analysis® from EDAX / TSL solution is used with the obtained crystal orientation information.

[0063] Based on these results, the area percentages of ferrite, pearlite, bainite, martensite (tempered martensite and fresh martensite), retained austenite, precipitates, and inclusions are obtained.

[0064] In microstructural observation, tempered martensite can be distinguished from fresh martensite by the presence of iron carbides within it. Furthermore, tempered martensite can be distinguished from bainite by the fact that the iron carbides present within it are elongated in multiple directions, not just a single direction. Note that elongation in a single direction means that the difference in the direction of elongation is within 5°.

[0065] plate thickness The thickness of the hot-stamping steel plate according to this embodiment (or the thickness of the steel plate if the hot-stamping steel plate consists only of steel plate) is not particularly limited, but from the viewpoint of reducing vehicle weight, it is preferable to set it to 2.5 mm or less, 2.0 mm or less, 1.8 mm or less, or 1.6 mm or less. On the other hand, from the viewpoint of ensuring sufficient impact absorption, the plate thickness is preferably 0.4 mm or more, 0.6 mm or more, 0.8 mm or more, or 1.0 mm or more.

[0066] The hot-stamping steel sheet according to this embodiment may have a plating layer on its surface. The plating layer may be formed on both sides of the steel sheet or on one side. Having a plating layer on the surface of the steel sheet can improve corrosion resistance after hot stamping. Examples of plating layers include hot-dip aluminum plating, hot-dip aluminum-zinc alloy plating, hot-dip aluminum-silicon alloy plating, hot-dip galvanizing, electro-galvanizing, alloyed hot-dip galvanizing, hot-dip zinc-aluminum alloy plating, and electro-zinc-nickel alloy plating.

[0067] Next, we will describe the hot-stamped molded body according to this embodiment, which is obtained by hot-stamping the aforementioned hot-stamping steel sheet as the material.

[0068] In this embodiment, the hot-stamped molded body is composed of a steel sheet having all or part of the above-described chemical composition. Since the chemical composition is the same as that of the steel sheet that makes up the hot-stamping steel sheet described above, a detailed explanation is omitted.

[0069] If a hot-stamped molded article comprises a portion having a tensile strength of 1800 MPa or more and a portion having a tensile strength of less than 1800 MPa, it is sufficient that at least the portion with a tensile strength of 1800 MPa or more has the chemical composition described above. In order to analyze the chemical composition of the portion with a tensile strength of 1800 MPa or more, a tensile test described later should be performed, and a test specimen for chemical composition analysis should be taken from the tensile test specimen that obtained a tensile strength of 1800 MPa or more, or from a portion adjacent to the portion from which the tensile test specimen was taken.

[0070] In the hot-stamped molded body according to this embodiment, the steel sheet constituting the hot-stamped molded body is measured at five locations in the surface region, which is the area from a depth of 20 μm to a depth of 40 μm from the surface of the steel sheet, satisfying the following conditions A to D. The average value of the C concentration in the surface region of the steel sheet, which is the C concentration in mass%, is measured at each of the five locations. AVE And the CS which is the maximum value of the five locations mentioned above. MAX And the minimum value of the CS at the five locations mentioned above is the CS MIN The following equations (iii) and (iv) are satisfied. CS AVE ≤0.80 × C1…(iii) CS MAX -CS MIN ≦0.100% by mass…(iv) Condition A: Each of the five locations is at least 25 mm away from the end face. Condition B: The five locations are separated by at least 100 mm from each other. Condition C: Of the five locations mentioned above, at least two are between 25 and 75 mm from the end face. Condition D: Of the locations where the distance to the end face is between 25 and 75 mm, the distance between the two furthest locations is 400 mm or more. However, C1 in formula (iii) above is the C content in mass percent in the chemical composition of the steel sheet constituting the hot-stamped molded body.

[0071] The average value of CS at the five locations mentioned above is CS AVEHowever, if the right-hand side of equation (iii) above is exceeded, i.e., if it exceeds 0.80 × C1 mass%, the C concentration in the surface region of the steel sheet constituting the hot-stamped molded body is too high, and the desired deformability cannot be obtained in the hot-stamped molded body. Therefore, CS AVE The amount of C should be 0.80 × 1 mass% or less. AVE Preferably, the amount is 0.75 × C1 mass% or less, 0.70 × C1 mass% or less, or 0.65 × C1 mass% or less. CS AVE While there is no particular lower limit, lowering it excessively would require long annealing times in the manufacturing process of the hot-stamping steel sheet described later, which would impair the productivity of the steel sheet. Furthermore, it would necessitate shortening the heating time in the heating process for the hot-stamping steel sheet described later, making the production of hot-stamped molded products difficult. Therefore, CS AVE The lower limit is preferably 0.020% by mass or more, 0.050% by mass or more, or 0.100% by mass or more.

[0072] The maximum value of CS at the five locations mentioned above is the CS MAX And the minimum value of the CS at the five locations mentioned above is CS MIN The fact that the difference exceeds the right-hand side of equation (iv) above, i.e., exceeds 0.100 mass%, indicates that there is a large fluctuation in C concentration in the surface region of the steel sheet constituting the hot-stamped molded body. As a result, the desired deformability cannot be obtained in the hot-stamped molded body. Therefore, CS MAX -CS MIN The amount shall be 0.100% by mass or less. CS MAX -CS MIN Preferably, it is 0.080% by mass or less, and more preferably 0.050% by mass or less. CS MAX -CS MIN The lower limit is not particularly limited, but it may be set to 0.010% by mass or more, or to 0.020% by mass or more.

[0073] Here, conditions A through D will be explained in detail using diagrams. Examples of hot-stamped molded bodies include the one shown in Figure 2. Figure 2 shows a hat member created in the embodiment described later. Here, we will explain five locations that satisfy conditions A to D when the hot-stamped molded body is a hat member like the one in Figure 2.

[0074] Figure 1 illustrates five locations in the hat-shaped component that satisfy conditions A through D. The units of measurement in the figure are in millimeters (mm). Points 1 through 5 in the figure are examples of five locations that satisfy conditions A through D. In Figure 1, points 1 to 5 are each at least 25 mm away from the end face. That is, condition A is satisfied. Here, the end face refers to the surface where the cross-section of the steel plate constituting the hot-stamped molded body is exposed (including cases where the cross-section is painted). Furthermore, the distance from the measurement point to the end face refers to the shortest distance along the surface of the member from the measurement point to the end face. Points 1 through 5 are separated by more than 100 mm from each other. That is, condition B is satisfied. Here, the distance between measurement points refers to the shortest distance along the surface of the material between the measurement points. Points 1, 2, 4, and 5 are within a distance of 25 to 75 mm from the end face. That is, condition C is satisfied. Of points 1, 2, 4, and 5, point 1 and point 5 are the furthest apart, with a distance of 900 mm. This satisfies condition D. As described above, five locations were selected, and the average C concentration (CS) in the surface region was measured at those locations.

[0075] The five CS locations that satisfy conditions A to D are measured using the following method. Test specimens are taken from the hot-stamped molded body, and the distribution of carbon (C) and iron (Fe) concentrations in the thickness direction at the five locations mentioned above is measured by glow discharge optical emission spectrometry (GDS analysis). The measurement range is set to a depth of 40 μm or more from the surface of the steel plate, and the measurement interval is 0.10 μm or less.

[0076] Furthermore, if the hot-stamped molded body has a plating layer or coating on its surface, the plating layer or coating should be partially or completely removed before GDS measurement, so that measurements can be taken up to a depth of 40 μm from the surface of the steel plate. In this embodiment, the region where the Fe concentration is 95% by mass or higher in the GDS measurement is determined to be a steel plate, and the depth position of the measurement point where the Fe concentration first reaches 95% by mass or higher from the start of the measurement is defined as the surface of the steel plate.

[0077] From the GDS measurement results, the average C concentration (CS) in the surface region, which is the area from a depth of 20 μm to a depth of 40 μm from the surface of the steel plate, is determined. Using the CS obtained at five locations, the average of the CS at the five locations is calculated. AVE And the CS is the maximum value of the five locations mentioned above. MAX And the minimum value of the CS at the five locations mentioned above is CS MIN To obtain. If a hot-stamped molded article comprises a portion having a tensile strength of 1800 MPa or more and a portion having a tensile strength of less than 1800 MPa, then in at least the portion having a tensile strength of 1800 MPa or more, the above-mentioned CS AVE , CS MAX and CS MIN It is sufficient that the above equations (iii) and (iv) are satisfied. The test specimens to be used for GDS measurement should be taken from the portion adjacent to the portion from which a tensile test specimen was taken that yielded a tensile strength of 1800 MPa or more after the tensile test described later.

[0078] The microstructure of the steel sheet constituting the hot-stamped molded body is not particularly limited as long as the desired strength and deformability can be obtained, but it is preferable to have the following microstructure.

[0079] Since martensite is an effective structure for increasing the tensile strength of steel sheets after hot stamping, it is preferable that the area ratio of martensite in the region from a depth of 200 μm from the surface of the steel sheet constituting the hot-stamped molded body to the center of the sheet thickness (hereinafter sometimes referred to as the inner layer region) exceeds 90.0%. If the area ratio of martensite in the inner layer region is 90.0% or less, the tensile strength of the hot-stamped molded body may be less than 1800 MPa, resulting in insufficient strength. For this reason, it is preferable to have an area ratio of martensite in the inner layer region exceed 90.0%. More preferably, the area ratio of martensite in the inner layer region is greater than 91.0%, greater than 93.0%, or greater than 95.0%.

[0080] While there is no need to specifically define an upper limit on the area ratio of martensite in the inner layer region, significantly increasing the area ratio of martensite would require excessively high heating temperatures or excessively high cooling rates for the hot stamping steel sheet in the hot stamping process described later, which would greatly impair the productivity of the hot stamped molded product. Therefore, it is preferable to set the area ratio of martensite in the inner layer region to 99.0% or less, or 98.0% or less.

[0081] In this embodiment, the martensite includes not only fresh martensite that has not been tempered, but also tempered martensite that has been tempered and contains iron carbides internally.

[0082] The remaining metallic structure in the inner layer may contain ferrite, pearlite, bainite, or retained austenite, and may also contain precipitates such as cementite or inclusions such as oxides present on their own. Since it is not necessary to contain ferrite, pearlite, bainite, retained austenite, precipitates, and inclusions, the lower limit of the area percentage of ferrite, pearlite, bainite, retained austenite, precipitates, and inclusions is 0% for all of them.

[0083] Retained austenite has the effect of improving the deformability of steel sheets after hot stamping. To obtain this effect, it is preferable that the area ratio of retained austenite in the inner layer region be 0.5% or more, 1.0% or more, or 2.0% or more. On the other hand, excessively increasing the area ratio of retained austenite requires austempering at high temperatures after hot stamping, which significantly reduces the productivity of hot-stamped molded products. Furthermore, excessive retained austenite content may degrade the deformability of hot-stamped molded products. For this reason, it is preferable to keep the area ratio of retained austenite in the inner layer region below 9.0%, below 7.0%, below 5.0%, or below 4.0%.

[0084] Furthermore, if the hot-stamped molded article comprises a portion having a tensile strength of 1800 MPa or more and a portion having a tensile strength of less than 1800 MPa, it is sufficient that at least the portion with a tensile strength of 1800 MPa or more has the above-described metallic structure.

[0085] The area ratio of the metal structure of the steel sheet constituting the hot-stamped molded body is measured using the same method as described above for the hot-stamping steel sheet. Test specimens for microstructure observation should be taken from the area where a tensile strength of 1800 MPa or more was obtained through the tensile test described later, and from an adjacent area.

[0086] plate thickness The thickness of the hot-stamped molded body according to this embodiment (or the thickness of the steel plate if the hot-stamped molded body consists only of a steel plate) is not particularly limited, but from the viewpoint of reducing vehicle weight, it is preferable to have a thickness of 2.5 mm or less, 2.0 mm or less, 1.8 mm or less, or 1.6 mm or less. On the other hand, from the viewpoint of ensuring sufficient impact absorption, the plate thickness is preferably 0.4 mm or more, 0.6 mm or more, 0.8 mm or more, or 1.0 mm or more.

[0087] Tensile strength In this embodiment, all or part of the hot-stamped molded article has a tensile strength of 1800 MPa or more. To achieve this, it is necessary that all or part of the steel plate constituting the hot-stamped molded article in this embodiment has a tensile strength of 1800 MPa or more. If the tensile strength of at least part of the hot-stamped molded article is not 1800 MPa or more, it will not be possible to secure the deformation load when the hot-stamped molded article deforms. Therefore, the tensile strength of all or part of the hot-stamped molded article is set to 1800 MPa or more. Preferably, all or part of the hot-stamped molded article has a tensile strength of 1900 MPa or more, 2000 MPa or more, 2100 MPa or more, 2300 MPa or more, or 2500 MPa or more. On the other hand, excessively increasing the strength of a hot-stamped molded article can lead to a decrease in its deformability; therefore, it is preferable that the tensile strength of the hot-stamped molded article be less than 3000 MPa or less than 2800 MPa.

[0088] The hot-stamped molded body according to this embodiment may have a total tensile strength of 1800 MPa or more, but it may also have a mixture of parts with a tensile strength of 1800 MPa or more and parts with a tensile strength of less than 1800 MPa. By providing parts with different strengths, it is possible to control the deformation state of the hot-stamped molded body during impact. A hot-stamped molded body having parts with different strengths can be manufactured by a method of joining two or more steel plates with different chemical compositions and then hot-stamping them, a method of partially changing the heating temperature of the steel plates or the cooling rate after hot-stamping in the hot-stamping process, and a method of partially reheating the hot-stamped molded body.

[0089] When a hot-stamped molded body comprises a portion having a tensile strength of 1800 MPa or more and a portion having a tensile strength of less than 1800 MPa, it becomes difficult to ensure deformability in the portion having a tensile strength of 1800 MPa or more, and cracks often occur during impact. However, in the hot-stamped molded body according to this embodiment, the deformability of the hot-stamped molded body can be improved in the portion having a tensile strength of 1800 MPa or more by controlling the chemical composition of the steel sheet constituting the hot-stamped molded body and the carbon concentration distribution in the surface region, as described above. In this embodiment, "part" refers to the minimum amount necessary to ensure the properties of the hot-stamped molded article. For example, it is sufficient if 5% or more, 10% or more, 15% or more, 20% or more, 25% or more, or 30% or more of the steel sheet in the hot-stamped molded article has the above-mentioned chemical composition, C concentration distribution, and tensile strength.

[0090] The tensile strength of a hot-stamped molded body is obtained by taking a strip-shaped piece from the hot-stamped molded body, processing it into a tensile test specimen without surface grinding of the steel plate, and performing a tensile test. Specifically, it is preferable to take a plate-shaped test specimen of type 13B in accordance with JIS Z 2241:2022 and perform a tensile test at a tensile speed of 10 mm / min. If it is not possible to take a type 13B test specimen, a tensile test specimen with a parallel section of any width may be taken, and a tensile test may be performed at a tensile speed of 10 mm / min, and the tensile strength may be determined from the maximum test force and the original cross-sectional area of ​​the parallel section. If high-strength and low-strength areas are present within the hot-stamped molded body, tensile test specimens should be taken from the high-strength areas. Furthermore, if the tensile strength cannot be measured by the method described above, the Vickers hardness may be measured and converted to tensile strength. The Vickers hardness is measured by the following method. A test piece is cut from the hot-stamped molded body so that a cross-section perpendicular to the surface (thickness cross-section) can be observed. After buffing the thickness cross-section of the steel plate, a Vickers hardness test is performed in accordance with JIS Z 2244-1:2024. The Vickers hardness test is performed using a micro-Vickers hardness tester at a depth of 1 / 4 of the plate thickness from the surface (the region from 1 / 8 of the plate thickness to 3 / 8 of the plate thickness from the surface), with a load of 9.8 N and a load holding time of 10 seconds. Five arbitrary points are used as measurement points at a depth of 1 / 4 of the plate thickness. The interval between each measurement point is at least three times the size of the indentation. The Vickers hardness is measured at these five measurement points, and the average value is calculated to obtain the Vickers hardness. The obtained Vickers hardness is converted to tensile strength by multiplying it by 3.1.

[0091] The hot-stamped molded article according to this embodiment may have a plating layer on its surface. Having a plating layer on the surface can improve corrosion resistance after hot stamping. The plating layer may be formed on both sides of the hot-stamped molded article or on one side. Examples of the plating layer include an aluminum-based plating layer formed by hot-stamping a hot-stamping steel sheet equipped with a molten aluminum plating layer, a molten aluminum-zinc alloy plating layer, a molten aluminum-silicon alloy plating layer, etc., and a zinc-based plating layer formed by hot-stamping a hot-stamping steel sheet equipped with a molten zinc plating layer, an electroplated zinc layer, an alloyed molten zinc plating layer, a molten zinc-aluminum alloy plating layer, an electroplated zinc-nickel alloy plating layer, etc.

[0092] Next, a method for manufacturing hot-stamping steel sheets according to this embodiment will be described.

[0093] The steel sheet for hot stamping is manufactured by a manufacturing method that includes a hot rolling step, in which a hot-rolled steel sheet is obtained by hot-rolling and winding a slab having the above-mentioned chemical composition, followed by a reheating step, a cold rolling step, in which the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet, and an annealing step, in which the cold-rolled steel sheet is annealed to obtain an annealed steel sheet.

[0094] The method for manufacturing slabs used in the manufacturing method of hot-stamping steel sheets is not particularly limited. Steel having the above-mentioned chemical composition is melted by known means and then formed into ingots by continuous casting, or into billets by a method such as bloc rolling after being formed into ingots by any casting method. In the continuous casting process, it is preferable to generate additional external flow in the molten steel, such as electromagnetic stirring, within the mold in order to suppress the occurrence of surface defects caused by inclusions. The ingots or billets may be cooled and then reheated before being subjected to hot rolling, or the ingots or billets in a high-temperature state after continuous casting or bloc rolling may be subjected to hot rolling as they are, or after being kept warm or after auxiliary heating. Such ingots and billets are collectively referred to as "slabs" as materials for hot rolling.

[0095] The heating temperature of the slab to be subjected to hot rolling is preferably less than 1250°C, and more preferably less than 1200°C, in order to prevent coarsening of the austenite. Since rolling becomes difficult if the slab heating temperature is too low, the slab heating temperature may be 1050°C or higher.

[0096] A hot-rolled steel sheet is obtained by hot-rolling a heated slab. Hot-rolling is preferably completed in a temperature range above the Ar3 point in order to refine the microstructure of the hot-rolled steel sheet by transforming the austenite after the rolling is complete.

[0097] When winding hot-rolled steel sheets after hot rolling, the winding temperature is preferably 450°C or higher and less than 600°C. If the winding temperature is less than 450°C or more than 600°C, even if the reheating treatment described later is performed on the hot-rolled steel sheets wound into a coil, the temperature of the end faces of the steel sheets cannot be sufficiently raised, and it may not be possible to control the decarburization index favorably in the steel sheets for hot stamping. As a result, it may not be possible to obtain the desired deformability in the hot-stamped molded articles. The winding temperature is the average temperature in the width direction of the hot-rolled steel sheet being wound, and this average temperature can be measured with a radiation thermometer.

[0098] Hot-rolled steel sheets (hot-rolled coils), which have been hot-rolled and wound into a coil, are subjected to a heat-retention treatment by covering them with a heat-retaining cover. This causes the temperature of the end faces of the hot-rolled steel sheets to rise due to transformation heating and heat transfer from the center of the coil. It is preferable that the temperature increase of the end faces of the hot-rolled steel sheets due to the heat-retention treatment be 50°C or more. By increasing the temperature of the end faces of the hot-rolled steel sheets to 50°C or more, the difference between the amount of decarburization at the end faces in the width direction of the sheet and the amount of decarburization near the center in the width direction can be reduced in hot-stamping steel sheets. The heat-retention treatment time (time from covering to removing the cover) is preferably 4.0 to 6.0 hours. If the heat-retention time is less than 4.0 hours, the temperature increase may be insufficient. On the other hand, if the heat-retention time exceeds 6.0 hours, thermally stable iron carbides may be formed, which may deteriorate the deformability of the hot-stamped molded product. Furthermore, in the reheat treatment, it is preferable that the residence time of the end face of the hot-rolled steel sheet in the temperature range of 600°C or higher be less than 5.0 hours. If the residence time in the temperature range of 600°C or higher is too long during the reheat treatment, the amount of decarburization will be excessive, and DI AVE The price may become higher in some cases.

[0099] The amount of temperature increase at the end surface of the hot-rolled steel sheet due to the reheating treatment is the difference in average temperature at the end surface of the hot-rolled steel sheet before covering and after removing the cover. The average temperature at the end surface of the hot-rolled steel sheet can be determined by measuring the temperature of the entire side surface of the hot-rolled coil using a thermal viewer.

[0100] After removing the cover, the coil is unwound, pickled if necessary, and then cold-rolled according to conventional methods to obtain a cold-rolled steel sheet. In the cold-rolling process, it is preferable to have a cumulative reduction ratio of 40% or more. If the cumulative reduction ratio is less than 40%, the metal structure of the hot-stamping steel sheet may become coarse. If the metal structure of the hot-stamping steel sheet is coarse, the metal structure of the hot-stamped molded body will become coarse after hot stamping, causing a decrease in the deformability of the hot-stamped molded body. On the other hand, excessively increasing the cumulative reduction ratio increases the load on the rolling equipment and causes a decrease in productivity, so it is preferable to keep the cumulative reduction ratio below 70%. After cold rolling, treatments such as degreasing may be performed according to conventional methods.

[0101] After cold rolling, annealed steel sheets are obtained by annealing. In the annealing process, it is preferable to set the dew point of the atmospheric gas in the furnace to -30.0°C or higher and less than -10.0°C. The atmosphere is preferably a nitrogen-hydrogen atmosphere containing, for example, 1% or more and less than 4% by volume of hydrogen.

[0102] In this embodiment, by annealing a cold-rolled steel sheet that has undergone reheat treatment and cold rolling in an atmosphere with a dew point of -30.0°C or higher and less than -10.0°C, the difference between the amount of decarburization at the end face in the width direction of the annealed steel sheet (steel sheet for hot stamping) and the amount of decarburization near the center in the width direction of the sheet can be reduced. The annealing temperature can be determined by measuring the temperature at the center of the width direction of the cold-rolled steel sheet being annealed using a radiation thermometer.

[0103] It is preferable that the soaking temperature during annealing exceeds 700°C. If the soaking temperature is below 700°C, it may not be possible to control the decarburization index favorably in the hot-stamping steel sheet. As a result, it may not be possible to obtain the desired deformability in the hot-stamped molded product. On the other hand, if the heating rate is too slow, the soaking temperature is too high, or the soaking time is too long, grain growth can cause the metal structure of the annealed steel sheet to coarseen, which can reduce the deformability of the hot-stamped molded body. Therefore, it is preferable to set the average heating rate to the soaking temperature to 1°C / second or higher, the soaking temperature to 800°C or lower, and the soaking time (holding time at the soaking temperature) to less than 600 seconds. Furthermore, it is preferable to set the residence time in the temperature range of 700°C or higher and below (Ac3 point - 30°C) to more than 420 seconds and less than 600 seconds.

[0104] When the residence time in the temperature range of 700°C or higher and below (Ac3 point - 30°C) is 420 seconds or less, it may not be possible to favorably control the decarburization index in hot-stamped steel sheets. As a result, it may not be possible to obtain the desired deformability in hot-stamped molded articles. On the other hand, if the residence time in the above temperature range is 600 seconds or longer, excessive decarburization may occur in the hot stamping steel sheet, resulting in insufficient strength of the hot stamped molded product after hot stamping.

[0105] The annealed steel sheet produced by the method described above may be plated according to a conventional method to produce a plated steel sheet. The annealed steel sheet or plated steel sheet obtained in this way may be temper-rolled according to a conventional method.

[0106] The Ac3 point can be determined from the change in thermal expansion when a cold-rolled steel sheet is heated at a heating rate of 8°C / second.

[0107] The hot-stamped molded article according to this embodiment can be obtained by a manufacturing method that includes a heating step of heating a hot-stamping steel sheet (annealed steel sheet or plated steel sheet) manufactured by the method described above, and a hot-stamping step of performing hot stamping on the heated hot-stamping steel sheet. In order to stably obtain the hot-stamped molded article according to this embodiment, it is preferable to perform hot stamping by the following method.

[0108] In the heating process, the hot stamping steel sheet described above is heated prior to the hot stamping process. In the heating process, in order to form a martensite-based metal structure in the inner layer region of the hot stamped molded body and obtain the desired strength, it is preferable to set the heating temperature to over 900°C and above the Ac3 point, and the holding time at the heating temperature to over 60 seconds. The Ac3 point in the heating process may be the same value as the Ac3 point of the cold-rolled steel sheet determined by the method described above.

[0109] On the other hand, if the heating temperature is too high or the holding time at the heating temperature is too long, the metal structure of the hot-stamped molded product may become coarse, reducing its deformability and strength. Therefore, it is preferable to set the heating temperature below 1050°C and the holding time below 600 seconds.

[0110] In the hot stamping process, it is preferable to remove the heated hot stamping steel sheet from the heating furnace, allow it to cool in the atmosphere, and then start hot stamping at a temperature above 750°C. If the starting temperature for hot stamping is below 750°C, excessive ferrite may be generated in the metal structure of the hot stamped molded body, which may reduce the strength of the hot stamped molded body. After forming by hot stamping, the hot stamped molded body is cooled while being held in the mold, and / or the hot stamped molded body is removed from the mold and cooled by any method.

[0111] If the cooling rate is too low, the area ratio of martensite in the metal structure of the hot-stamped molded product may be insufficient, which can reduce the strength of the hot-stamped molded product. Therefore, it is preferable that the average cooling rate from the hot-stamping start temperature to 400°C be 30°C / second or higher, 60°C / second or higher, or 90°C / second or higher. Also, if the cooling stop temperature is too high, the area ratio of martensite in the metal structure of the hot-stamped molded product may be insufficient, which can reduce the strength of the hot-stamped molded product. Therefore, it is preferable that the cooling stop temperature achieved by the above cooling method be less than 90°C.

[0112] A hot-stamped molded article according to this embodiment is obtained by the method described above. It is preferable to perform a reheating treatment after hot-stamping. The deformability of the hot-stamped molded article is improved by the reheating treatment. It is preferable to set the heating temperature to 130°C or higher and the holding time at the heating temperature to 10 minutes or higher. On the other hand, if the heating temperature is too high or the holding time at the heating temperature is too long, the strength of the hot-stamped molded article may decrease. For this reason, it is preferable to set the heating temperature to less than 170°C and the holding time to less than 20 minutes. In addition, the hot-stamped molded article may be subjected to blasting, painting, and baking treatments. [Examples]

[0113] Next, embodiments of the present disclosure will be described. The conditions in the embodiments are examples of conditions adopted to confirm the feasibility and effectiveness of the present disclosure, and the present disclosure is not limited to these examples of conditions. The present disclosure may adopt various conditions insofar as they do not depart from the gist of the present disclosure and achieve the objectives of the present disclosure.

[0114] Steel having the chemical compositions shown in Tables 1A and 1B was melted and cast to obtain steel ingots. The obtained steel ingots were heated to 1200°C, hot-rolled in a temperature range of 920°C or higher, cooled, and wound into coils at a winding temperature of 410-630°C. The obtained hot-rolled coils were covered with heat-retaining covers and held for 3.4-6.0 hours for reheating treatment. Reheating treatment was omitted for some hot-rolled coils. After removing the covers, the hot-rolled coils were allowed to cool to obtain hot-rolled steel sheets with a thickness of 2.6 mm. After pickling the hot-rolled steel sheets, cold-rolling was performed to obtain cold-rolled steel sheets with a thickness of 1.4 mm. The cumulative reduction ratio during cold rolling was 46%.

[0115] Note that blank spaces in Tables 1A and 1B indicate that the content of the element in question was below the detection limit. The Ac3 points in Tables 2A and 2B were determined from the change in thermal expansion when cold-rolled steel sheets of each steel were heated at 8°C / second.

[0116] The obtained cold-rolled steel sheets were continuously annealed under the annealing conditions shown in Tables 2A and 2B. The atmosphere during soaking in the annealing furnace was a nitrogen-hydrogen atmosphere containing 2 volume% hydrogen, and the dew point was as described in Tables 2A and 2B. After soaking, the sheets were cooled to room temperature to obtain annealed steel sheets (for hot stamping) with a width of 1000 mm. In No. 4, the residence time of the end face of the hot-rolled steel sheet in the temperature range of 600°C or higher during the reheating treatment was less than 3.0 hours. Note that the "residence time" in Tables 2A and 2B refers to the residence time in the temperature range of 700°C or higher and below (Ac3 point - 30°C).

[0117] Furthermore, when the metal structure of the hot-stamping steel sheet according to the present invention example was observed using the method described above, the metal structure from the surface of the steel sheet to a depth of 1 / 4 of the sheet thickness consisted of, by area percentage, ferrite: 5-90%, bainite and martensite: 0-50%, pearlite: 10-95%, and retained austenite: 0-10%.

[0118] The obtained hot-stamping steel sheet was divided longitudinally (rolling direction), and test pieces for GDS measurement were taken from five locations in the width direction at the center of the longitudinal direction. Specifically, these locations were 50 mm from the end face in the width direction of the steel sheet, 1 / 4 of the way through the width direction, the center of the width direction, 3 / 4 of the way through the width direction, and 50 mm from the end face opposite to the aforementioned end face. The surface of the test piece was used as the measurement starting surface, and GDS measurements were performed from the measurement starting surface to a depth of 130 μm in the thickness direction using the method described above, and the decarburization index at the five locations was determined. There were 1800 measurement points from the start of measurement to a depth of 130 μm. From the measurement results at the five locations, the average value of the decarburization index at the five locations, called DI, was calculated. AVE And the DI, which is the maximum value of the five decarburization indices. MAX And the DI, which is the minimum value of the five decarburization indices. MIN And, we obtained. The results obtained are shown in Tables 3A and 3B.

[0119] Next, a hot-stamping base plate measuring 960 mm in width and 240 mm in length was taken from the longitudinal center of the hot-stamping steel sheet, and a hat-shaped member (hot-stamped molded body) as shown in Figure 2 was obtained by hot stamping. In the hot-stamping process, the hot-stamping base plate was heated under the conditions shown in Tables 3A and 3B. After that, the hot-stamping base plate was removed from the heating furnace and allowed to cool, then placed in a mold equipped with a cooling device, and hat forming (hot stamping) was performed with a forming start temperature of 770°C or higher. Subsequently, the average cooling rate from the forming start temperature to 400°C was set to 50°C / second or higher, and the material was cooled in the mold to a cooling stop temperature of 80°C or lower. In addition, the oxide scale (iron oxide) formed on the surface of the hat member was removed by shot blasting. Subsequently, the hat member was placed in an electric heating furnace and subjected to a reheating treatment in which it was heated to 160°C and held for 10 minutes.

[0120] A test specimen was taken from the longitudinal center of the top plate of the obtained hat member, and its chemical composition was measured and its microstructure observed using the method described above. Tables 3A and 3B show the carbon content obtained from measurements of the hat member. The content of elements other than carbon was the same as that shown in Tables 1A and 1B. In the example of the present invention, the metallic structure of the hot-stamped molded body showed that the area ratio of martensite in the inner layer region of the steel plate at the longitudinal center of the top plate of the hat member was 92.0% or more, and the total area ratio of structures other than martensite was 8.0% or less.

[0121] Furthermore, a plate-shaped test specimen of type 13B was taken from the longitudinal center of the top plate of the hat member, along the longitudinal direction of the hat member, in accordance with JIS Z 2241:2022, and the tensile strength was determined by performing a tensile test at a tensile speed of 10 mm / min.

[0122] If the obtained tensile strength was 1800 MPa or higher, it was judged to have high strength and was deemed acceptable. On the other hand, if the obtained tensile strength was less than 1800 MPa, it was judged to have insufficient strength and was deemed unacceptable. Note that the hat-shaped member shown in Figure 2 exhibits the lowest tensile strength at the longitudinal center of the top plate.

[0123] Furthermore, as shown in Figure 1, GDS measurement test pieces were taken from three locations on the top plate and two locations on the vertical wall of the hat member. The surface of the test piece was used as the measurement starting surface, and GDS measurement was performed from the measurement starting surface to a depth of 50 μm in the thickness direction using the method described above. The average value of the CS at the five locations in the surface region, which is the area from a depth of 20 μm to a depth of 40 μm from the surface of the steel plate, was then measured. AVE And the CS is the maximum value of the five locations mentioned above. MAX And the minimum value of the CS at the five locations mentioned above is CS MIN The results were obtained. The measurement points were set at 720 locations, from the surface of the steel plate to a depth of 50 μm.

[0124] Furthermore, 30mm square VDA bending test specimens were taken from three locations on the top plate and two locations on the vertical wall of the hat-shaped member, adjacent to the GDS measurement specimens, and bending tests were performed in accordance with the German Automotive Industry Association standard VDA 238-100. The specimens were bent so that the direction of the bending ridge was perpendicular to the rolling direction of the steel plate, and the bending angle (VDA bending angle) was determined when the bending load decreased by 30N from the maximum point. If cracking occurred before the bending load reached the maximum point (premature fracture occurred), the bending angle at the time of cracking was determined and used as the VDA bending angle.

[0125] If the tensile strength of the steel sheets constituting the hot-stamped molded body is less than 2300 MPa, and the average value of the VDA bending angles at five locations is 60.0° or higher, and the minimum value of the VDA bending angles at five locations is 55.0° or higher, the hot-stamped molded body is judged to be acceptable as having high uniformity of deformability and excellent deformability. Furthermore, if the tensile strength is 2300 MPa or higher, and the average value of the VDA bending angle at five locations is 40.0° or higher, and the minimum value of the VDA bending angle at five locations is 35.0° or higher, the hot-stamped molded article is judged to be acceptable as having high uniformity of deformability and excellent deformability. If these conditions were not met, the hot-stamped molded product was deemed unacceptable as it lacked sufficient deformability.

[0126] [Table 1A]

[0127] [Table 1B]

[0128] [Table 2A]

[0129] [Table 2B]

[0130] [Table 3A]

[0131] [Table 3B]

[0132] As can be seen from Tables 3A and 3B, the hot-stamped molded articles according to the present invention example had high strength and excellent deformability. On the other hand, the hot-stamped molded articles according to the comparative example were inferior in one or more of the above characteristics. [Industrial applicability]

[0133] According to the above embodiments of this disclosure, it is possible to provide a hot stamping steel sheet suitable as a material for hot stamping molded articles having high strength and excellent deformability, as well as a hot stamping molded article having high strength and excellent deformability.

Claims

1. A steel plate for hot stamping, comprising a steel plate, The chemical composition of the steel plate is, in mass%, C: more than 0.310%, less than 0.700%, Si: Less than 2.00% Mn: 0.01 to 3.00%, P: 0.200% or less, S: 0.0200% or less, Sol. Al: 0.001–1.000%, N: 0.0200% or less, O: 0.0200% or less, B: 0.0002 to 0.0200%, Cr: 0-2.00%, Mo: 0-2.00%, W: 0-2.00%, Cu: 0-2.00%, Ni: 0-2.00%, Ti: 0-0.200%, Nb: 0 to 0.200%, V: 0 to 0.200%, Zr: 0-0.200%, Ca: 0-0.1000%, Mg: 0 to 0.1000%, REM: 0-0.1000%, Sn: 0-0.200%, As: 0-0.100%, and Bi: Contains 0 to 0.0500%, The remainder consists of Fe and impurities. When measuring the decarburization index at five locations: 50 mm from the end face in the width direction of the steel plate, 1 / 4 of the way across in the width direction, the center of the width direction, 3 / 4 of the way across in the width direction, and 50 mm from the end face opposite to the end face, The average value of the decarburization index at the five locations is DI ＡＶＥ and, The maximum value of the decarburization index at the five locations is DI ＭＡＸ and, The minimum value of the decarburization index at the five locations is DI ＭＩＮ A steel sheet for hot stamping, characterized in that it satisfies the following formulas (i) and (ii). 0.10≦DI ＡＶＥ ≦0.35 …(i) DI ＭＡＸ -DI ＭＩＮ ≦0.15 …(ii)

2. The chemical composition of the steel plate is, in mass%, Cr: 0.01-2.00%, Mo: 0.01-2.00%, W: 0.01-2.00%, Cu: 0.01-2.00%, Ni: 0.01-2.00%, Ti: 0.001 to 0.200%, Nb: 0.001-0.200%, V: 0.001-0.200%, Zr: 0.001 to 0.200%, Ca: 0.0001-0.1000%, Mg: 0.0001 to 0.1000%, REM: 0.0001-0.1000%, Sn: 0.001-0.200%, As: 0.001 to 0.100%, and The hot stamping steel sheet according to claim 1, characterized in that it contains one or more elements from the group consisting of Bi: 0.0001 to 0.0500%.

3. A hot-stamped molded body comprising a steel plate, All or part of the aforementioned steel plate has a chemical composition of, by mass%, C: more than 0.310%, less than 0.700%, Si: Less than 2.00% Mn: 0.01 to 3.00%, P: 0.200% or less, S: 0.0200% or less, Sol. Al: 0.001–1.000%, N: 0.0200% or less, O: 0.0200% or less, B: 0.0002 to 0.0200%, Cr: 0-2.00%, Mo: 0-2.00%, W: 0-2.00%, Cu: 0-2.00%, Ni: 0-2.00%, Ti: 0-0.200%, Nb: 0 to 0.200%, V: 0 to 0.200%, Zr: 0-0.200%, Ca: 0-0.1000%, Mg: 0 to 0.1000%, REM: 0-0.1000%, Sn: 0-0.200%, As: 0-0.100%, and Bi: Contains 0 to 0.0500%, The remainder consists of Fe and impurities. In the surface region of the steel plate, which is the area from a depth of 20 μm to a depth of 40 μm from the surface, when the average value of the C concentration in mass percent in the surface region of the steel plate, CS, is measured at each of the five locations that satisfy the following conditions A to D, The average value of the CS at the five locations is CS ＡＶＥ and, The maximum value of the CS at the five locations mentioned above is CS ＭＡＸ and, CS, which is the minimum value of the five CSs ＭＩＮ satisfies the following formulas (iii) and (iv), A hot-stamped molded article characterized by having a tensile strength of 1800 MPa or more. CS ＡＶＥ ≦0.80×C １ …(iii) CS ＭＡＸ -CS ＭＩＮ ≤0.100% by mass … (iv) Condition A: Each of the five locations is at least 25 mm away from the end face. Condition B: The five locations are separated by at least 100 mm from each other. Condition C: Of the five locations mentioned above, at least two are between 25 and 75 mm from the end face. Condition D: Of the locations where the distance to the end face is between 25 and 75 mm, the distance between the two furthest locations is 400 mm or more. However, C in the above equation (iii) １ This is the C content in mass percent of the chemical composition of the steel plate.

4. The chemical composition of the steel plate is, in mass%, Cr: 0.01-2.00%, Mo: 0.01-2.00%, W: 0.01-2.00%, Cu: 0.01-2.00%, Ni: 0.01-2.00%, Ti: 0.001 to 0.200%, Nb: 0.001-0.200%, V: 0.001-0.200%, Zr: 0.001 to 0.200%, Ca: 0.0001-0.1000%, Mg: 0.0001 to 0.1000%, REM: 0.0001-0.1000%, Sn: 0.001-0.200%, As: 0.001 to 0.100%, and The hot-stamped molded article according to claim 3, characterized in that it contains one or more substances from the group consisting of Bi: 0.0001 to 0.0500%.

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