WO2025084167A1 - Hot-stamp molded body - Google Patents

Abstract

This hot-stamp molded body has a desired chemical composition, has a tensile strength of 1800 MPa or more, has an average value of residual stress on a steel sheet surface of −250 MPa or less, and has an Hvs/Hvi ratio of 0.90 or less, Hvs/Hvi being the ratio of the average hardness Hvs in a surface layer region that is a region 20-50 μm deep from the surface and the average hardness Hvi at a depth of 1/4 of the plate thickness from the surface.

Description

Hot stamped compact

The present disclosure relates to a hot stamped body.
This application claims priority based on Japanese Patent Application No. 2023-178113, filed on October 16, 2023, the contents of which are incorporated herein by reference.

In recent years, there has been a demand for lighter automobile bodies from the standpoint of environmental protection and resource conservation, and high-strength steel sheets are being used in automobile components. When high-strength steel sheets are used, the thickness of the steel sheets can be reduced to reduce the weight of the automobile body while still providing the desired strength to the automobile body. Automobile components are manufactured by press-forming steel sheets, but as the strength of the steel sheets increases, not only does the forming load increase, but formability decreases, making the components more susceptible to cracks and wrinkles. 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 die, making it difficult to ensure the dimensional accuracy of the component. As such, it is not easy to manufacture high-strength automobile body components by press forming.

In order to solve the above problems, a technique has been proposed in the past, as disclosed in Patent Document 1, for example, in which heated steel plate is press-formed using a low-temperature press die. This technique is called hot stamping or hot pressing, and since steel plate that has been heated to a high temperature and is in a soft state is press-formed, components with complex shapes can be manufactured with high dimensional accuracy. In addition, since the steel plate is rapidly cooled by contact with the die, it is possible to significantly increase the strength by quenching at the same time as the press forming. Patent Document 1 discloses that by hot stamping a steel plate with a tensile strength of 500 to 600 MPa, a component with a tensile strength of 1400 MPa or more can be obtained.

The strength of the hot stamped body can be further increased by increasing the C content of the steel plate. However, increasing the C content of the steel plate increases the sensitivity to hydrogen embrittlement, making it more susceptible to hydrogen embrittlement cracking. In addition, the deformability of the steel plate that constitutes the hot stamped body decreases, making it more susceptible to cracking during collision. As such, it is not easy to manufacture a high-strength hot stamped body that has excellent hydrogen embrittlement resistance and deformability. In particular, in hot stamped bodies that have parts with a tensile strength of 1,800 MPa or more, cracks are likely to occur in those parts, making it more difficult to ensure hydrogen embrittlement resistance and deformability.

Patent Document 2 discloses a technology relating to a hot stamped body that has improved resistance to hydrogen embrittlement by making precipitates containing one or more of Nb, Ti, V, Cr, Mo, and Mg function as hydrogen trapping sites.

Patent Document 3 discloses a technology for producing hot stamped bodies with improved toughness and hydrogen embrittlement resistance by refining prior austenite grains.

Patent Document 4 discloses a hot-pressed member with high strength and excellent hydrogen embrittlement resistance, and a method for manufacturing the same. The method disclosed in Patent Document 4 improves the hydrogen embrittlement resistance of the member by refining prior austenite grains, making Nb-based precipitates into hydrogen trapping sites, and suppressing variation in hardness of the steel plate surface.

Japanese Patent Application Publication No. 2002-102980 Japanese Patent Application Publication No. 2005-97725 Japanese Patent Application Publication No. 2010-150612 International Publication No. 2019/003445

The technologies disclosed in Patent Documents 2 and 3 are excellent in that they can produce hot stamped bodies that have excellent hydrogen embrittlement resistance and a tensile strength of 950 MPa or more. However, according to the inventors' investigations, Patent Documents 2 and 3 have room for improvement in order to achieve a higher level of both high strength and hydrogen embrittlement resistance.

Patent Document 4 describes how a hot-pressed part with excellent hydrogen embrittlement resistance and a tensile strength of 1780 MPa or more can be obtained, but does not take into consideration the reduction in deformability, and it is believed that the collision performance will be insufficient for use as an automotive part.

As described above, it has been difficult in the prior art to manufacture, by hot stamping, a formed body having a portion with excellent hydrogen embrittlement resistance and deformability and a tensile strength of 1800 MPa or more.
The present disclosure has been made in view of the above-mentioned circumstances, and has an object to provide a hot stamped steel having high strength, as well as excellent hydrogen embrittlement resistance and deformability.

The gist of the present disclosure is as follows.
[1] A hot stamped product having a steel plate,
The whole or a part of the steel sheet has a chemical composition, 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: 0-3.00%,
Ti: 0-0.200%,
Nb: 0 to 0.200%,
V: 0-2.000%,
Zr: 0-0.200%,
Ca: 0-0.1000%,
Mg: 0 to 0.1000%,
REM: 0-0.1000%,
Sn: 0-0.200%,
As: 0 to 0.100%,
Bi: 0 to 0.0500%, and the balance: Fe and impurities;
The tensile strength is 1800 MPa or more,
The average residual stress on the surface of the steel plate is −250 MPa or less,
A hot stamped product characterized in that Hvs/Hvi, which is a ratio of an average Vickers hardness Hvs in a surface layer region that is a region from a depth of 20 μm to a depth of 50 μm from the surface, to an average Vickers hardness Hvi at a depth of 1/4 of the plate thickness from the surface, is 0.90 or less.
[2] The chemical composition 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 to 3.00%,
Ti: 0.001 to 0.200%,
Nb: 0.001-0.200%,
V: 0.001-2.000%,
Zr: 0.001 to 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 to 0.100%, and Bi: 0.0001 to 0.0500%
The hot stamped product according to the above [1], characterized in that it contains one or more of the group consisting of

According to the above-described aspects of the present disclosure, it is possible to provide a hot stamped steel having high strength, as well as excellent hydrogen embrittlement resistance and deformability.
The hot stamped steel according to the above embodiment is resistant to hydrogen embrittlement cracking, has excellent deformability, and is resistant to cracking during a collision, and therefore can be suitably applied to automobile components such as pillars and bumpers.

FIG. 4 is a diagram showing a hat member manufactured in an embodiment.

The inventors have investigated methods for improving the hydrogen embrittlement resistance and deformability of hot stamped bodies with a tensile strength of 1800 MPa or more, and have come to the following findings.

(A) In a hot stamped steel, by making the Vickers hardness of a region (surface layer region) from a depth of 20 μm to a depth of 50 μm from the surface of a steel plate constituting the hot stamped steel lower than the Vickers hardness of an inner layer portion of the steel plate (a depth of ¼ of the plate thickness from the surface), it is possible to improve the hydrogen embrittlement resistance and deformability of the hot stamped steel.
(B) Although the reason for this is not clear, it is presumed that this is because cracks due to hydrogen embrittlement and cracks due to reduced deformability are likely to originate from the surface layer of the steel plate that constitutes the hot stamped body, and by softening the surface layer, cracks in the surface layer are suppressed.
(C) By subjecting the hot stamped product to a blast treatment, the hydrogen embrittlement resistance of the hot stamped product can be improved.
(D) The reason for this is not clear, but it is assumed that this is because the blast treatment imparts compressive residual stress to the surface of the steel plate constituting the hot stamped body, thereby suppressing cracking in the surface layer.
(E) By subjecting the hot stamped product after the blasting treatment to a reheating treatment, the hydrogen embrittlement resistance and deformability of the hot stamped product can be improved.
(F) The reason for this is not clear, but it is assumed that this is because blasting treatment tends to increase the hardness of the surface region, but the reheating treatment keeps the surface soft.

Based on the findings of (A) to (F) above, the inventors discovered that by performing a blasting treatment and a reheating treatment after hot stamping to make the surface layer of the hot stamped body softer than the inner layer and to reduce the residual stress on the surface of the hot stamped body, it is possible to obtain a hot stamped body having a tensile strength of 1800 MPa or more and excellent hydrogen embrittlement resistance and deformability.

The hot stamped body according to this embodiment will be described in detail below. First, the reasons for limiting the chemical composition of the steel plate that constitutes the hot stamped body according to this embodiment will be explained.

All or a part of the steel sheet included in the hot stamped steel according to the present embodiment has the following chemical composition. When the hot stamped steel is composed of only a steel sheet, it can be said that all or a part of the hot stamped steel has the chemical composition shown below.
In the following, the numerical ranges described with "to" include the lower and upper limits. Numerical values indicated as "less than" and "more than" are not included in the numerical range. All percentages for chemical compositions are mass %.

If the hot stamped body has a portion with a tensile strength of 1800 MPa or more and a portion with a tensile strength of less than 1800 MPa, at least the portion with a tensile strength of 1800 MPa or more should have the following chemical composition.

The chemical composition of all or a part of the steel plate provided with the hot stamp formed product according to this embodiment includes, in mass%, C: more 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 balance: Fe and impurities.
Each element will be described below.

C: more than 0.310% and 0.700% or less C is an element that improves the tensile strength of the steel sheet after hot stamping (steel sheet constituting the hot stamped body). If the C content is 0.310% or less, the tensile strength of the steel sheet after hot stamping is less than 1800 MPa, and the strength of the hot stamped body is insufficient. Therefore, the C content is made to be more than 0.310%. The C content is preferably 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 C content exceeds 0.700%, the strength of the hot stamped steel becomes too high, and excellent hydrogen embrittlement resistance and deformability cannot be obtained. Therefore, the C content is set to 0.700% or less. Preferably, the C content is 0.650% or less, 0.600% or less, 0.550% or less, or 0.500% or less.

Si: less than 2.00% Si is an element that embrittles steel. If the Si content is 2.00% or more, the adverse effects become particularly large. Therefore, the Si content is set to less than 2.00%. The Si content is preferably less than 1.00%, less than 0.75%, less than 0.50%, or less than 0.20%.
The lower limit of the Si content is not particularly limited, but may be 0%. Since excessively decreasing the Si content increases the steelmaking cost, the Si content is preferably 0.001% or more. In addition, since Si has the effect of increasing the hardenability of steel, it may be actively contained. From the viewpoint of improving the hardenability, the Si content is preferably 0.05% or more, 0.10% or more, or 0.15% or more.

Mn: 0.01-3.00%
Mn is an element that combines with S to form MnS and suppresses the adverse effects of S. If the Mn content is less than 0.01%, the above effect cannot be obtained. Therefore, the Mn content is set to 0.01% or more. The Mn content is preferably 0.10% or more or 0.20% or more. In addition, Mn is an element that improves the hardenability of steel, and is an element that is effective for forming a metal structure mainly composed of martensite in the inner layer part of the steel sheet after hot stamping and ensuring the strength of the hot stamped body. From the viewpoint of ensuring strength, the Mn content is preferably 0.20% or more or 0.30% or more. In addition, Mn has the effect of making it easy to peel off the scale generated during hot stamping, and is an element that is effective for imparting compressive residual stress to the hot stamped body by blasting. Therefore, the Mn content is preferably 0.50% or more. The Mn content is more preferably 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 hydrogen embrittlement resistance and deformability cannot be obtained in the hot stamped steel. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.50% or less, 2.00% or less, or 1.50% or less.

P: 0.200% or less P is an element that embrittles steel. If the P content exceeds 0.200%, the adverse effects become particularly large, and weldability also deteriorates significantly. Therefore, the P content is set to 0.200% or less. The P content is preferably less than 0.100%, less than 0.050%, or less than 0.020%.
The P content may be 0%. However, if the P content is reduced to less than 0.001%, the dephosphorization cost increases significantly, which is economically undesirable. Therefore, the P content may be set to 0.001% or more or 0.005% or more.

S: 0.0200% or less S is an element that embrittles steel. If the S content exceeds 0.0200%, the adverse effects become particularly large. Therefore, the S content is set to 0.0200% or less. The S content is preferably less than 0.0050%, less than 0.0020%, or less than 0.0010%.
The S content may be 0%. However, if the S content is reduced to less than 0.0001%, the desulfurization cost increases significantly, which is economically undesirable. Therefore, the S content may be set to 0.0001% or more or 0.0002% or more.

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 becomes insufficient. Therefore, the sol. Al content is set to 0.001% or more. The sol. Al content is preferably 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 exceeding the Ac ₃ point in the heating process of hot stamping. Also, the strength of the hot stamped body is insufficient. Therefore, the sol. Al content is set to 1.000% or less. The sol. Al content is preferably less than 0.500%, less than 0.100%, less than 0.060%, or less than 0.040%.

N: 0.0200% or less N is an element that forms nitrides during continuous casting of steel. Since these nitrides deteriorate the deformability of the hot stamped body, it is preferable that the N content is low. If the N content exceeds 0.0200%, the adverse effects become particularly large. Therefore, the N content is set to 0.0200% or less. The N content is preferably less than 0.0100%, less than 0.0080%, or less than 0.0050%.
The N content may be 0%, but excessive reduction in the N content significantly increases the denitrification cost, which is economically undesirable. Therefore, the N content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more.

O: 0.0200% or less O is an element that forms oxide-based inclusions. If the O content exceeds 0.0200%, a large amount of coarse oxide-based inclusions are formed in the steel. This deteriorates the deformability of the hot stamped body. Therefore, the O content is set to 0.0200% or less. The O content is preferably 0.0150% or less, 0.0100% or less, 0.0060% or less, or 0.0040% or less.
The O content may be 0%. However, since O is an element that combines with B to form B oxides and reduces the hardenability of steel, O is an element that is effective for softening the surface layer of the steel sheet after hot stamping. In order to reliably obtain this effect, the O content is preferably 0.0005% or more. The O content is more preferably 0.0010% or more, 0.0015% or more, or 0.0020% or more.

B: 0.0002-0.0200%
B is an element that improves the hardenability of steel, and is an effective element for forming a metal structure mainly composed of martensite in the inner layer of the steel sheet after hot stamping, and for ensuring the strength of the hot stamped body. If the B content is less than 0.0002%, the desired strength cannot be obtained in the hot stamped body. Therefore, the B content is set to 0.0002% or more. The B content is preferably 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%, boron carbohydrates are formed in the hot stamped steel, and the hardenability improving effect of B is impaired. Therefore, the B content is set to 0.0200% or less. The B content is preferably less than 0.0050%, less than 0.0040%, or less than 0.0030%.

The remainder of the chemical composition of the steel plate constituting the hot stamped body according to this embodiment may be 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 permissible to the extent that they do not impair the properties of the hot stamped body according to this embodiment.

The steel plate constituting the hot stamped body 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 contained, the content is 0%.

Cr:0.01~2.00%
Cr is an element that enhances the hardenability of steel, thereby enhancing the strength of the hot stamped body. In order to reliably obtain this effect, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.05% or more or 0.10% or more.
On the other hand, if the Cr content exceeds 2.00%, the deformability of the hot stamped steel deteriorates. Therefore, the Cr content is set to 2.00% or less. The Cr content is preferably less than 0.50%, less than 0.40%, or less than 0.30%.

Mo: 0.01~2.00%
Mo is an element that enhances the hardenability of steel, thereby enhancing the strength of the hot stamped body. To reliably obtain this effect, the Mo content is preferably 0.01% or more. The Mo content is more preferably 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 steel deteriorates. Therefore, the Mo content is set to 2.00% or less. The Mo content is preferably less than 0.50%, less than 0.40%, or less than 0.30%.

W: 0.01~2.00%
W is an element that enhances the hardenability of steel, thereby enhancing the strength of the hot stamped body. To reliably obtain this effect, the W content is preferably 0.01% or more. The W content is more preferably 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 steel deteriorates. Therefore, the W content is set to 2.00% or less. The W content is preferably less than 0.50%, less than 0.40%, or less than 0.30%.

Cu: 0.01-2.00%
Cu is an element that enhances the hardenability of steel, thereby enhancing the strength of the hot stamped body. In order to reliably obtain these effects, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.10% or more.
On the other hand, if the Cu content exceeds 2.00%, the deformability of the hot stamped steel deteriorates. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably less than 1.00% or less than 0.50%.

Ni: 0.01-3.00%
Ni is an element that increases the hardenability of steel, thereby increasing the strength of the hot stamped body. In order to reliably obtain this effect, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.10% or more.
On the other hand, if the Ni content exceeds 3.00%, the deformability of the hot stamped steel deteriorates. Therefore, the Ni content is set to 3.00% or less. The Ni content is preferably less than 2.00%, less than 1.00%, or less than 0.50%.

Ti: 0.001-0.200%
Ti is an element that forms carbonitrides in steel and enhances the strength of the hot stamped body through precipitation strengthening. Ti also improves the hydrogen embrittlement resistance and deformability of the hot stamped body through refinement of the metal structure. To reliably obtain these effects, the Ti content is preferably 0.001% or more. The Ti content is more preferably 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, and the deformability of the hot stamped body deteriorates. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably less than 0.050% or less than 0.030%.

Nb: 0.001-0.200%
Nb is an element that forms carbonitrides in steel and enhances the strength of hot stamped bodies by precipitation strengthening. Nb also improves the hydrogen embrittlement resistance and deformability of hot stamped bodies through refinement of the metal structure. To reliably obtain these effects, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more or 0.010% or more.
On the other hand, if the Nb content exceeds 0.200%, a large amount of coarse carbonitrides are generated in the steel, and the deformability of the hot stamped body deteriorates. Therefore, the Nb content is set to 0.200% or less. The Nb content is preferably less than 0.050%, less than 0.030%, or less than 0.020%.

V:0.001~2.000%
V is an element that forms carbonitrides in steel and enhances the strength of hot stamped bodies by precipitation strengthening. V also improves the hydrogen embrittlement resistance and deformability of hot stamped bodies through refinement of the metal structure. To reliably obtain these effects, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more or 0.010% or more.
On the other hand, if the V content exceeds 2.000%, a large amount of coarse carbonitrides are formed in the steel, and the deformability of the hot stamped steel deteriorates. Therefore, the V content is set to 2.000% or less. The V content is preferably 1.000% or less or 0.500% or less.

Zr: 0.001-0.200%
Zr is an element that forms carbonitrides in steel and enhances the strength of hot stamped bodies by precipitation strengthening. Zr also improves the hydrogen embrittlement resistance and deformability of hot stamped bodies through refinement of the metal structure. To reliably obtain these effects, the Zr content is preferably 0.001% or more. The Zr content is more preferably 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, and the deformability of the hot stamped body deteriorates. Therefore, the Zr content is set to 0.200% or less. The Zr content is preferably less than 0.100% or less than 0.050%.

Ca:0.0001~0.1000%
Ca is an element that improves the deformability of the hot stamped steel by adjusting the shape of inclusions. In order to reliably obtain this effect, the Ca content is preferably 0.0001% or more.
On the other hand, even if a large amount of Ca is contained, the above effects are saturated and, furthermore, excessive costs are generated, so the Ca content is set to 0.1000% or less, and preferably less than 0.0100%.

Mg: 0.0001-0.1000%
Mg is an element that improves the deformability of a hot stamped steel by adjusting the shape of inclusions. In order to reliably obtain these effects, the Mg content is preferably 0.0001% or more.
On the other hand, even if Mg is contained in a large amount, the above effects are saturated and, furthermore, excessive costs are generated, so the Mg content is set to 0.1000% or less, and preferably less than 0.0100%.

REM: 0.0001~0.1000%
REM is an element that improves the deformability of a hot stamped steel by adjusting the shape of inclusions. In order to reliably obtain this effect, the REM content is preferably 0.0001% or more.
On the other hand, even if REM is contained in a large amount, the above effects are saturated and, furthermore, excessive costs are generated, so the REM content is set to 0.1000% or less, and preferably less than 0.0100%.
In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y and lanthanoids, and the REM content refers to the total content of these elements.

Sn: 0.001-0.200%
Sn is an element that has the effect of improving the corrosion resistance of a hot stamped steel. In order to reliably obtain this effect, the Sn content is preferably 0.001% or more. The Sn content is more preferably 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 contained, the above effects are saturated and, furthermore, excessive costs are generated, so the Sn content is set to 0.200% or less, and preferably 0.150% or less or 0.100% or less.

As: 0.001-0.100%
As is an element that has the effect of increasing the strength of a hot stamped steel. In order to reliably obtain this effect, the As content is preferably 0.001% or more.
On the other hand, even if As is contained in a large amount, the above effects are saturated and, furthermore, excessive costs are generated, so the As content is set to 0.100% or less.

Bi:0.0001~0.0500%
Bi is an element that refines the solidification structure to thereby improve the hydrogen embrittlement resistance and deformability of the hot stamped steel. In order to reliably obtain this effect, the Bi content is preferably 0.0001% or more.
On the other hand, even if a large amount of Bi is contained, the above effects are saturated and, furthermore, excessive costs are generated, so the Bi content is set to 0.0500% or less, and preferably 0.0100% or less or 0.0050% or less.

The chemical composition of the steel sheet constituting the above-mentioned hot stamp formed body can be determined by taking a test piece from the steel sheet constituting the hot stamp formed body, removing the paint if the steel sheet is painted, and then measuring the average element content throughout the thickness of the sheet using a general analytical method. For example, inductively coupled plasma atomic emission spectrometry (ICP-AES) or inductively coupled plasma mass spectrometry can be used. C and S can be measured using the combustion-infrared absorption method, and O and N can be measured using the inert gas fusion-infrared absorption method or the inert gas fusion-thermal conductivity method. Sol. Al can be measured by ICP-AES using the filtrate obtained by thermally decomposing the sample with acid. If the steel sheet constituting the hot stamp formed body has a plating layer on its surface, the plating layer can be removed before measuring the chemical composition.

As described above, when the hot stamped body has 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 having a tensile strength of 1800 MPa or more has the above-mentioned chemical composition. In order to analyze the chemical composition of the portion having a tensile strength of 1800 MPa or more, a tensile test as described below is performed, and a test specimen for chemical composition analysis is taken from a tensile test specimen that has a tensile strength of 1800 MPa or more, or from a portion adjacent to the portion from which the tensile test specimen was taken.

Next, the residual stress and Vickers hardness of the steel sheet constituting the hot stamped steel according to this embodiment will be described.
In the hot stamped steel according to this embodiment, the average value of residual stress on the surface of the steel plate constituting the hot stamped steel is −250 MPa or less, and Hvs/Hvi, which is the ratio of the average Vickers hardness Hvs in a surface layer region that is a region from a depth of 20 μm to a depth of 50 μm from the surface, to the average Vickers hardness Hvi at a depth of 1/4 of the plate thickness from the surface, is 0.90 or less.

The region from a depth of 20 μm from the surface of the steel plate to a depth of 50 μm from the surface can be rephrased as a region having a starting point at a position 20 μm deep from the surface of the steel plate and an ending point at a position 50 μm deep from the surface.
In addition, the 1/4 depth of the plate thickness from the surface of the steel plate can be rephrased as a region from 1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface, or a region starting from a position that is 1/8 depth of the plate thickness from the surface and ending at a position that is 3/8 depth of the plate thickness from the surface.

In addition, when the hot stamped body has 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 having a tensile strength of 1800 MPa or more has the above-mentioned average residual stress and Vickers hardness.
Each provision will be explained below.

Average value of residual stress on the surface: -250 MPa or less If the average value of residual stress on the surface of the steel plate constituting the hot stamped body exceeds -250 MPa, hydrogen embrittlement resistance deteriorates. Therefore, the average value of residual stress on the surface is set to -250 MPa or less. The average value of residual stress on the surface is preferably -300 MPa or less, -400 MPa or less, or -500 MPa or less.
The lower limit of the average value of the residual stress on the surface is not particularly limited, but may be −700 MPa or more.
The surface refers to the surface of the steel sheet constituting the hot stamped body. When the hot stamped body has a plating layer on the surface, the surface refers to the interface between the plating layer and the steel sheet.

The residual stress in the surface of the steel sheet constituting the hot stamped steel is measured by the following method.
Residual stress is measured on a 2 mm x 2 mm area on the surface of the hot stamped body (the surface of the steel plate constituting the hot stamped body) using a Rigaku Automate (using a Cr tube) with a tube voltage of 40 kV, a tube current of 40 mA, and a collimator size of φ0.3 mm, using the 2θ sin ² ψ method. At this time, positive values are defined as tensile stress and negative values as compressive stress. Measurements are performed on at least five or more 2 mm x 2 mm areas, and the average value is calculated to obtain the average value of residual stress on the surface.
Although X-rays penetrate into the interior of the steel sheet, the value measured by the above-mentioned method is regarded as the residual stress on the surface.

After the residual stress is measured, a test piece is taken from the part including the measurement part and a tensile test described later is performed. If a tensile strength of 1800 MPa or more is obtained, the measured value is adopted. If a tensile strength of 1800 MPa or more is not obtained, the measured value is not adopted.
In addition, when a plating layer is provided on the surface of the steel sheet constituting the hot stamped body, the measurement is performed after electrolytically removing the plating layer.

Hvs/Hvi: 0.90 or less If Hvs/Hvi, which is the ratio of the average Vickers hardness Hvs in the surface layer region to the average Vickers hardness Hvi at 1/4 depth of the plate thickness from the surface, exceeds 0.90, the desired hydrogen embrittlement resistance and deformability cannot be obtained. By setting Hvs/Hvi to 0.90 or less, it is possible to improve the hydrogen embrittlement resistance and the deformability. Therefore, Hvs/Hvi is set to 0.90 or less. Hvs/Hvi is preferably 0.85 or less, 0.80 or less, or 0.75 or less.
The lower limit of Hvs/Hvi is not particularly limited, but may be 0.60.
The surface refers to the surface of the steel sheet constituting the hot stamped body. When the hot stamped body has a plating layer on the surface, the surface refers to the interface between the plating layer and the steel sheet.

The average Vickers hardness Hvs in the surface layer region is measured by the following method.
A test piece is cut out from the hot stamped body so that a cross section (sheet thickness cross section) perpendicular to the surface can be observed. After buffing the sheet thickness cross section of the steel sheet, a Vickers hardness test is performed in accordance with JIS Z 2244-1:2020. The Vickers hardness test is performed on the surface region, which is a region from 20 μm deep from the surface of the steel sheet to 50 μm deep from the surface, using a micro Vickers hardness tester with a load of 0.098 N and a load holding time of 10 seconds. The measurement points are 20 μm deep from the surface, 25 μm deep from the surface, 30 μm deep from the surface, 35 μm deep from the surface, 40 μm deep from the surface, 45 μm deep from the surface, and 50 μm deep from the surface, with three points at each depth position for a total of 21 points. The interval between each measurement point is three times or more the size of the indentation. The Vickers hardness is measured at these 21 measurement points, and the average value is calculated to obtain the average Vickers hardness Hvs of the surface layer region.

The average Vickers hardness Hvi at a depth of 1/4 of the plate thickness from the surface is measured by the following method.
The Vickers hardness is measured at 1/4 depth from the surface of the steel plate (the region from 1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface) in the same manner as the Vickers hardness in the surface layer region. The measurement points are 20 arbitrary points in the region from 1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface, and the interval between each measurement point is 3 times or more the size of the indentation. The Vickers hardness is measured at the 20 measurement points, and the average value is calculated to obtain the average Vickers hardness Hvi at 1/4 depth of the plate thickness.

The test specimens used to measure Vickers hardness are taken from a portion adjacent to the portion from which the tensile test specimens that have a tensile strength of 1,800 MPa or more were taken by conducting the tensile test described below.

The metal structure of the steel sheet constituting the hot stamped body according to this embodiment is not particularly limited as long as it can obtain the desired strength, hydrogen embrittlement resistance, and deformability, but it is preferable for the steel sheet to have the metal structure shown below.

All or a part of the steel plate constituting the hot stamped body according to this embodiment preferably has a metal structure containing the amount of martensite shown below. In the following description of the metal structure, "%" means "volume %". When the hot stamped body has 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 having a tensile strength of 1800 MPa or more has the metal structure shown below.

The metal structure at a depth of 1/4 of the plate thickness from the surface (also called the inner layer) preferably contains more than 90.0% martensite.

Since martensite is an effective structure for increasing the tensile strength of steel sheets after hot stamping, it is preferable that the volume fraction of martensite is greater than 90.0%. If the volume fraction of martensite is 90.0% or less, the tensile strength of the hot stamped body will be less than 1800 MPa, and the strength may be insufficient. The volume fraction of martensite is more preferably greater than 91.0%, greater than 93.0%, or greater than 95.0%.

There is no need to set an upper limit for the volume fraction of martensite. However, in order to significantly increase the volume fraction of martensite, it would be necessary to excessively increase the heating temperature of the hot stamping steel sheet or the cooling rate during the hot stamping process, which would significantly impair the productivity of the hot stamped body. Therefore, it is preferable that the volume fraction of martensite be 99.0% or less or 98.0% or less.

In this embodiment, martensite includes not only fresh martensite that has not been tempered, but also tempered martensite that has been tempered and contains iron carbides inside.

The remainder of the metal structure of the inner layer may contain ferrite, pearlite, bainite or retained austenite, and may further contain precipitates such as cementite or oxides present alone. Since it is not necessary for the metal structure to contain ferrite, pearlite, bainite, retained austenite or precipitates, the lower limit for the volume fraction of ferrite, pearlite, bainite, retained austenite and precipitates is all 0%.

The retained austenite has the effect of improving the ductility of the steel sheet after hot stamping. In order to obtain this effect, the volume fraction of the retained austenite is preferably 0.5% or more, 1.0% or more, or 2.0% or more.
On the other hand, in order to excessively increase the volume fraction of the retained austenite, it is necessary to perform austempering treatment at high temperature after hot stamping, which significantly reduces the productivity of the hot stamped body. In addition, if the retained austenite is contained in excess, the deformability of the hot stamped body may deteriorate. Therefore, it is preferable to set the volume fraction of the retained austenite to less than 9.0%, less than 7.0%, less than 5.0%, or less than 4.0%.

In this embodiment, the volume fraction of each tissue is measured by the following method.
A test piece is taken from the hot stamped body, and the cross section of the steel plate is buffed. Then, the structure is observed 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).

Specifically, the polished surface is subjected to nital etching or electrolytic polishing, and then a structural photograph is taken using an optical microscope and a scanning electron microscope (SEM). The obtained structural photograph is subjected to image analysis based on brightness differences or differences in the morphology of iron carbides present within the phase to obtain the area ratios of ferrite, pearlite, bainite, tempered martensite, and precipitates. After that, the same observation positions are subjected to Lepera etching, and then a structural photograph is taken using an optical microscope or a scanning electron microscope (SEM). The obtained structural photograph is subjected to image analysis to calculate the total area ratio of "retained austenite and fresh martensite."

In addition, after electrolytic polishing of the plate thickness cross section at the same observation position, the area ratio of the retained austenite is measured using a SEM equipped with an electron backscatter pattern analyzer (EBSP device). The area ratio of the retained austenite is obtained by calculating the area ratio of the region having the 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 the above-mentioned "retained austenite and fresh martensite."
In the EBSP analysis, an EBSP analysis device consisting of a thermal field emission scanning electron microscope (JSM-7200F manufactured by JEOL) and an EBSP detector (EDAX Velocity (registered trademark) ultra-high speed operation EBSP detector) is used. In the EBSP analysis, the measurement interval is 0.1 μm, the degree of vacuum in the device is 9.6×10 ⁻⁵ Pa or less, the acceleration voltage is 25 kV, and the irradiation current level is 16.
To display the EBSP map, version 7 or later of OIM Analysis (registered trademark) manufactured by EDAX/TSL solution is used for the obtained crystal orientation information.

Based on these results, the area fractions of ferrite, pearlite, bainite, martensite (tempered martensite and fresh martensite), retained austenite, and precipitates are obtained. The area fractions are then considered to be equal to the volume fractions, and the obtained area fractions are regarded as the volume fractions of each structure.

In structural observation, tempered martensite can be distinguished from fresh martensite in that iron carbides are present inside. Tempered martensite can also be distinguished from bainite in that the iron carbides present inside are elongated in multiple directions rather than in a single direction. Elongated in a single direction means that the difference in elongation direction is within 5°.

Plate Thickness The plate thickness of the hot stamped body according to this embodiment (the plate thickness of the steel plate when the hot stamped body consists only of a steel plate) is not particularly limited, but from the viewpoint of reducing the vehicle body weight, it is preferably 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 a sufficient amount of shock 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.

Tensile strength All or a part of the steel plate constituting the hot stamped body according to the present embodiment has a tensile strength of 1800 MPa or more. When the tensile strength of at least a part of the hot stamped body is 1800 MPa or more, it is possible to ensure the deformation load when the hot stamped body is deformed. As a result, it is possible to further improve the impact resistance characteristics of the hot stamped body. Therefore, the tensile strength of all or a part of the hot stamped body is set to 1800 MPa or more. Preferably, the tensile strength of all or a part of the hot stamped body is 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 the hot stamped steel may cause a decrease in hydrogen embrittlement resistance and deformability, so the tensile strength of the hot stamped steel is preferably less than 3000 MPa or less than 2800 MPa.

The hot stamped product according to this embodiment may have a tensile strength of 1800 MPa or more throughout (the entire hot stamped product), but the hot stamped product may 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 becomes possible to control the deformation state of the hot stamped product during collision. A hot stamped product having parts with different strengths can be manufactured by a method of joining two or more types of steel sheets with different chemical compositions and then hot stamping, a method of partially changing the heating temperature of the steel sheet or the cooling rate after hot stamping in the hot stamping process, and a method of partially reheating the hot stamped product.
When a hot stamped steel has 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 often difficult to ensure hydrogen embrittlement resistance and deformability in the portion having a tensile strength of 1800 MPa or more. However, in the hot stamped steel according to this embodiment, the hydrogen embrittlement resistance and deformability can be improved in the portion having a tensile strength of 1800 MPa or more by controlling the chemical composition, residual stress, and Vickers hardness of the steel sheet as described above.
In the present embodiment, the term "part" refers to the minimum amount necessary to ensure the properties of the hot stamped steel. For example, 5% or more, 10% or more, 15% or more, 20% or more, 25% or more, or 30% or more of the steel sheet by volume in the hot stamped steel may have the above-mentioned chemical composition, tensile strength, residual stress, and Vickers hardness.

The tensile strength of the hot stamped body is obtained by taking a small rectangular piece from the hot stamped body, processing it into a tensile test piece without surface grinding the steel plate, and performing a tensile test. Specifically, it is preferable to take a No. 13B plate test piece from the hot stamped body in accordance with JIS Z 2241:2022 and perform a tensile test at a tensile speed of 10 mm/min. If the size of the hot stamped body is small or the shape is complex, and it is not possible to take a No. 13B plate test piece, a small rectangular piece having a parallel part 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 obtained from the maximum test force and the original cross-sectional area of the parallel part.
When the hot stamped product contains a mixture of high strength and low strength parts, the tensile test specimen is taken from the high strength part.
In addition, when the tensile strength cannot be measured by the above-mentioned method, the Vickers hardness may be measured and converted into the tensile strength. The Vickers hardness is measured by the following method.
A test piece is cut out from the hot stamped body so that a cross section (thickness cross section) perpendicular to the surface can be observed. After the thickness cross section of the steel plate is buffed, a Vickers hardness test is performed in accordance with JIS Z 2244-1:2020. The Vickers hardness test is performed at a depth of 1/4 of the plate thickness from the surface (a region from 1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface) using a micro Vickers hardness tester with a load of 9.8 N and a load holding time of 10 seconds. The measurement points are any five points at a depth of 1/4 of the plate thickness. The interval between each measurement point is three times or more 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 multiplied by 3.1 to convert it into tensile strength.

The hot stamped body according to this embodiment may have a plating layer on its surface. By having a plating layer on its surface, corrosion resistance can be improved after hot stamping. Examples of the plating layer include a zinc-based plating layer or an aluminum-based plating layer. A hot stamped body having these plating layers can be obtained by hot stamping a zinc-based plated steel sheet or an aluminum-based plated steel sheet. The plating layer may be formed on both sides or on one side of the hot stamped body. The plating layer of the hot stamped body can be formed by hot stamping a plated steel sheet having a plating layer. However, it is preferable that the hot stamped body according to this embodiment does not have a plating layer on its surface. This is because if a plating layer is applied to the hot stamped body, the residual stress on the surface tends to be high, and it may not be possible to obtain the desired characteristics.

The following describes a method for producing a hot stamp steel sheet to obtain the hot stamped body according to this embodiment.

　Hot stamping steel sheet is manufactured by a manufacturing method including a hot rolling process in which a slab having the above-mentioned chemical composition is hot rolled to form a hot rolled steel sheet, a cold rolling process in which the hot rolled steel sheet is cold rolled to form a cold rolled steel sheet, and an annealing process in which the cold rolled steel sheet is annealed to form an annealed steel sheet.

The method of manufacturing the slab used in the manufacturing method of steel plate for hot stamping is not particularly limited. Steel having the above-mentioned chemical composition is melted by a known means and then made into a steel ingot by a continuous casting method, or made into a steel billet by a method of making a steel ingot by any casting method and then blooming it. In the continuous casting process, it is preferable to generate an external additional flow such as electromagnetic stirring in the molten steel in the mold in order to suppress the occurrence of surface defects caused by inclusions. The steel ingot or billet may be cooled once and then reheated for hot rolling, or the steel ingot in a high temperature state after continuous casting or the steel billet in a high temperature state after blooming may be used for hot rolling as it is, or after keeping it warm, or after additional heating. Such steel ingots and billets are collectively called "slabs" as materials for hot rolling.

The heating temperature of the slab used for hot rolling is preferably less than 1250°C, more preferably less than 1200°C, in order to prevent coarsening of austenite. If the slab heating temperature is too low, rolling becomes difficult, so the slab heating temperature may be 1050°C or higher.

The heated slab is subjected to hot rolling to obtain a hot rolled steel sheet. The hot rolling is preferably completed in a temperature range of Ar ₃ point or higher in order to refine the metal structure of the hot rolled steel sheet by transforming austenite after the completion of rolling.

When the hot-rolled steel sheet after hot rolling is coiled, the coiling temperature is preferably less than 550° C. If the coiling temperature is 550° C. or higher, thermally stable iron carbide is generated, which may deteriorate the deformability of the hot stamped steel.
On the other hand, if the coiling temperature is too low, the hot-rolled steel sheet becomes excessively hard, making it difficult to perform cold rolling. Therefore, the coiling temperature is preferably set to more than 500°C.

The hot-rolled and coiled hot-rolled steel sheet is pickled in a conventional manner, and then cold-rolled in a conventional manner to obtain a cold-rolled steel sheet. In the cold-rolling process, the cumulative reduction in cold rolling is preferably 40% or more. If the cumulative reduction 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 body becomes coarse after hot stamping, which causes a decrease in the deformability of the body.
On the other hand, an excessive increase in the cumulative reduction rate increases the load on the rolling equipment and causes a decrease in productivity, so the cumulative reduction rate is preferably less than 70%. After cold rolling, treatment such as degreasing may be performed according to a conventional method.

The cold-rolled steel sheet is annealed to become an annealed steel sheet. In the annealing process, the soaking temperature is preferably 700° C. or higher in order to refine the metal structure of the annealed steel sheet (steel sheet for hot stamping) by recrystallization. If the soaking temperature is less than 700° C., the Vickers hardness may not be preferably controlled in the surface layer region of the hot stamped steel. As a result, the desired hydrogen embrittlement resistance and deformability may not be obtained in the hot stamped steel.
On the other hand, if the heating rate is too slow or the soaking temperature is too high, the metal structure of the annealed steel sheet may become coarse due to grain growth, and the hydrogen embrittlement resistance of the hot stamped steel may deteriorate. Therefore, it is preferable to set the average heating rate to the soaking temperature to 1° C./s or more, and the soaking temperature to 800° C. or less.

It is also preferable that the dew point of the atmosphere in the annealing furnace is -20.0°C or more and less than 10.0°C, and the residence time in a temperature range of 700°C or more and less than (Ac ₃ point -30°C) is more than 360 seconds and less than 600 seconds. It is also preferable that the atmosphere in the annealing furnace is a nitrogen-hydrogen atmosphere containing 1 volume % or more and less than 4 volume % of hydrogen.
If the dew point is lower than -20.0°C or higher than 10.0°C, or if the residence time in a temperature range of 700°C or higher and lower than (Ac ₃ point - 30°C) is 360 seconds or shorter, the Vickers hardness may not be controlled in a favorable manner in the surface layer region of the hot stamped steel, and as a result, the hot stamped steel may not be able to obtain the desired hydrogen embrittlement resistance and deformability.
On the other hand, if the residence time in the above temperature range is 600 seconds or longer, excessive decarburization occurs in the steel sheet for hot stamping, and the strength of the hot stamped product after hot stamping may decrease.
The annealed steel sheet produced by the above-mentioned method may be plated in a conventional manner to obtain a plated steel sheet. The annealed steel sheet or plated steel sheet thus obtained may be temper rolled in a conventional manner.

The Ac ₃ point is the temperature at which ferrite disappears from the metal structure when the base steel sheet is heated, and can be determined from the change in thermal expansion when the cold-rolled steel sheet is heated at a heating rate of 8°C/sec.

The hot stamped steel according to this embodiment can be obtained by a manufacturing method including a heating step of heating a steel sheet for hot stamping (annealed steel sheet or plated steel sheet) manufactured by the above-mentioned method, a hot stamping step of hot stamping the heated steel sheet for hot stamping, a blasting step of blasting the hot stamped hot stamped steel, and a reheating step of reheating the blasted hot stamped steel.
In order to stably obtain the hot stamped steel according to the present embodiment, it is preferable to perform hot stamping by the following method.

In the heating step, the hot stamping steel sheet having the above-mentioned chemical composition is heated prior to the hot stamping step. In the heating step, it is preferable to use a gas combustion furnace using a combustible gas including propane gas, and heat the hot stamping steel sheet with an air ratio of more than 0.84. It is preferable that the heating temperature is more than 900°C and more than the Ac ₃ point, and the holding time at the heating temperature is more than 200 seconds.
The air ratio is the ratio (A/ _A0 ) of the amount of air actually charged (A) to the theoretical amount of air ( _A0 ). The _Ac3 point in the heating process may be set to the same value as the Ac3 _point of the cold-rolled steel sheet obtained by the above-mentioned method.

If the air ratio is 0.84 or less, if the heating temperature is 900°C or less, or if the holding time is 200 seconds or less, the adhesion of scale increases, and it may not be possible to preferably impart residual stress to the surface of the hot stamped body by the blasting treatment described below. Also, if the heating temperature is Ac ₃ point or less, the volume fraction of martensite in the metal structure of the inner layer of the hot stamped body may be insufficient, and the strength of the hot stamped body may decrease.
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 steel may become coarse, and the hydrogen embrittlement resistance and deformability of the hot stamped steel may deteriorate, and the strength may decrease. Therefore, the heating temperature is preferably less than 950° C., and the holding time is preferably less than 360 seconds.

In the hot stamping process, it is preferable to start hot stamping at a temperature above 750°C after removing the heated steel sheet for hot stamping from the heating furnace and allowing it to cool in the atmosphere. If the starting temperature for hot stamping is below 750°C, excessive ferrite may be formed in the metal structure of the inner layer of the hot stamped body, which may reduce the strength of the hot stamped body. After forming by hot stamping, the hot stamped body is cooled while held in the die, and/or the hot stamped body is removed from the die and cooled by any method.

If the cooling rate is too slow, the volume fraction of martensite in the metal structure of the inner layer of the hot stamped body may be insufficient, and the strength of the hot stamped body may decrease. Therefore, the average cooling rate from the hot stamping start temperature to 400° C. is preferably 30° C./s or more, 60° C./s or more, or 90° C./s or more.
Moreover, if the cooling stop temperature is too high, the volume fraction of martensite in the metal structure of the inner layer of the hot stamped body may be insufficient, and the strength of the hot stamped body may decrease. Therefore, the cooling stop temperature for the above cooling is preferably set to less than 90°C.

In the blasting process, it is preferable to blast the hot stamped body with a projection intensity of more than 0.26 mmN in terms of Almen Ark Height value. If the projection intensity is 0.26 mmN or less, residual stress may not be imparted favorably to the surface of the steel plate constituting the hot stamped body. It is preferable to adjust the Almen Ark Height value by using cast steel shot, cut wire shot or conditioned cut wire shot as specified in JIS B 2711:2013 as the projection material, using an air blasting device, and changing the projection speed, projection time and projection distance. The Almen Ark Height value is measured in accordance with JIS B 2711:2013 using an Almen strip N piece and an Almen gauge.

In the reheating process, the hot stamped body is reheated after the blasting process. The surface region of the hot stamped body is hardened by the blasting process, so if it is used as an automotive component as is, it may not be possible to obtain excellent deformability. By reheating under the desired conditions and softening the hardened surface region, the deformability of the hot stamped body can be improved.

If the heating temperature for the reheating treatment (reheating temperature) is less than 130°C, the surface region of the hot stamped body cannot be softened sufficiently, and the Hvs/Hvi of the hot stamped body may not be controlled favorably. On the other hand, if the reheating temperature is 200°C or higher, the inner layer of the steel plate softens and the strength of the hot stamped body is insufficient. Therefore, it is preferable to set the reheating temperature to 130°C or higher and less than 200°C. It is more preferable to set the reheating temperature to 150°C or higher or 160°C or higher. It is also more preferable to set the reheating temperature to less than 190°C or less than 180°C.

If the holding time at the reheating temperature is short, the surface region of the hot stamped steel cannot be sufficiently softened, and the Hvs/Hvi of the hot stamped steel cannot be controlled appropriately. Therefore, the holding time is preferably 10.0 minutes or more. The holding time is more preferably 15.0 minutes or more.
On the other hand, if the holding time is long, the strength of the hot stamped steel sheet is insufficient. Therefore, the holding time is preferably less than 30.0 minutes, and more preferably less than 25.0 minutes.

The above method is used to obtain the hot stamped body according to this embodiment.

Next, an embodiment of the present invention will be described. However, the conditions in the embodiment are merely an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

Molten steel was cast using a vacuum melting furnace to obtain steel material having the chemical composition shown in Tables 1A and 1B. The obtained steel material was heated to 1180°C and held for 60 minutes, and then hot-rolled 10 times at a temperature range of 900°C or higher to obtain a hot-rolled steel sheet with a thickness of 3.5 mm. After hot rolling, the hot-rolled steel sheet was cooled to 540°C with a water spray. The cooling end temperature was set as the coiling temperature, and the hot-rolled steel sheet was charged into an electric heating furnace held at this coiling temperature and held there for 60 minutes. The hot-rolled steel sheet was then cooled to room temperature at an average cooling rate of 20°C/hour to simulate slow cooling after coiling. The hot-rolled steel sheet after furnace cooling was pickled and then cold-rolled to obtain a cold-rolled steel sheet with a thickness of 1.4 mm. The cumulative reduction rate during cold rolling was 60%.

In addition, the blank cells in Tables 1A and 1B indicate that the content of the corresponding element was below the lower limit of measurement.
The Ac ₃ points in Table 1B were determined from the change in thermal expansion when the cold-rolled steel sheets in the table were heated at 8° C./sec.

The obtained cold-rolled steel sheet was annealed under the annealing conditions shown in Tables 2A and 2B using a continuous annealing simulator. It was heated at an average heating rate of 8°C/s to the soaking temperature shown in Tables 2A and 2B. The atmosphere in the annealing furnace was a nitrogen-hydrogen atmosphere containing 3% by volume of hydrogen, and the dew point was as shown in Tables 2A and 2B. After soaking, the steel sheet was cooled to room temperature to obtain an annealed steel sheet (steel sheet for hot stamping).

A hot stamp blank with a width of 240 mm and a length of 800 mm was taken from the obtained hot stamp steel plate, and hot stamping was performed to obtain a hat member (hot stamped body) with the shape shown in Figure 1. In the hot stamping process, the hot stamp blank was heated using a gas combustion furnace under the conditions shown in Tables 2A and 2B. Specifically, propane gas was used as the combustion gas, and the heating temperature, holding time, and air ratio were set to the conditions shown in Tables 2A and 2B. The hot stamp blank was then removed from the heating furnace and allowed to cool, and then sandwiched between a die equipped with a cooling device and hat-formed with a forming start temperature of 770°C or higher. Next, the average cooling rate from the forming start temperature to 400°C was set to 50°C/sec or higher, and the blank was cooled in the die to a cooling stop temperature of 80°C or lower.

Then, the hat component was blasted with cast steel shot having an average particle size of about 300 μm using a direct pressure air blasting machine under the conditions shown in Tables 2A and 2B. The hat component was also reheated under the conditions shown in Tables 2A and 2B.

At the punch bottom of the obtained hat member, the average value of the residual stress on the surface of the steel plate was determined by the above-mentioned method.
Also, test pieces were taken from the punch bottom of the hat member, and the chemical compositions were measured by the above-mentioned method.

Further, a tensile test piece No. 13B was taken from the punch bottom of the hat member along the longitudinal direction of the hat member in accordance with JIS Z 2241:2022, and a tensile test was carried out at a tensile speed of 10 mm/min to determine the tensile strength.
When the obtained tensile strength was 1800 MPa or more, it was judged to have high strength and to pass, whereas when the obtained tensile strength was less than 1800 MPa, it was judged to not have high strength and to fail.

In addition, test pieces were taken from the punch bottom of the hat member, and the average Vickers hardness Hvs in the surface region and the average Vickers hardness Hvi at a depth of 1/4 of the plate thickness from the surface were determined by the method described above.
In addition, a test piece for microstructure observation was taken from the punch bottom of the hat member, and after the cross section of the plate thickness of this test piece was polished, the metal structure was observed at a depth of 1/4 of the plate thickness from the surface by the method described above.

In addition, a 60 mm square test piece for bending tests was taken from the punch bottom of the hat component, and a bending test was conducted in accordance with the German Association of the Automotive Industry standard VDA 238-100:2017-04. The test piece was bent so that the ridge direction of the bend was perpendicular to the rolling direction of the hot stamping steel sheet, and the bending angle (VDA bending angle) was determined at the point when the bending load decreased by 60 N from the maximum point. If a crack occurred before the bending load reached the maximum point, the bending angle at the time when the crack occurred was determined and used as the VDA bending angle.

If the tensile strength of the steel plate constituting the hot stamped body was less than 2300 MPa, and the VDA bend angle was 60° or more, it was judged to have excellent deformability and pass the test. Also, if the tensile strength was 2300 MPa or more, and the VDA bend angle was 40° or more, it was judged to have excellent deformability and pass the test. If these conditions were not met, it was judged to have no excellent deformability and fail the test.

In addition, test pieces for hydrogen embrittlement testing, measuring 6 mm in width and 68 mm in length, were taken from the punch bottom of the hat component and subjected to a four-point bending hydrochloric acid immersion test. The test pieces were immersed in hydrochloric acid with a pH of 4 while various stresses were applied to them, and it was examined whether cracks occurred within 72 hours.

If the tensile strength of the steel plate constituting the hot stamped body was less than 2300 MPa, and no cracks occurred when a load stress of 1400 MPa was applied, the steel was judged to have excellent hydrogen embrittlement resistance and to have passed the test. If the tensile strength was 2300 MPa or more, and no cracks occurred when a load stress of 900 MPa was applied, the steel was judged to have excellent hydrogen embrittlement resistance and to have passed the test. If these conditions were not met, the steel was judged to have no excellent hydrogen embrittlement resistance and to have failed the test. Examples judged to have passed the test are marked "OK" in the table, and examples judged to have failed the test are marked "NG" in the table.

Tables 3A and 3B show the results of measuring the chemical composition of the hot stamped steel and the results of evaluating the mechanical properties of the hot stamped steel.
The contents of elements other than C in the hot stamped steel were omitted because they were the same as the contents of the elements shown in Table 1A.

As shown in Tables 3A and 3B, the hot stamped steel according to the example of the present invention has high strength, as well as excellent hydrogen embrittlement resistance and deformability. In addition, in the metal structure of the hot stamped steel according to the example of the present invention, the volume fraction of martensite in the inner layer portion was 92.0% or more, and the total volume fraction of structures other than martensite was 8.0% or less.
On the other hand, it is seen that the hot stamped steel according to the comparative example is inferior in one or more characteristics.

The above aspects of the present disclosure make it possible to provide a hot stamped body having high strength, as well as excellent resistance to hydrogen embrittlement and deformability.

Claims (2)

A hot stamped product comprising a steel plate,
The whole or a part of the steel sheet has a chemical composition, 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: 0-3.00%,
Ti: 0-0.200%,
Nb: 0 to 0.200%,
V: 0-2.000%,
Zr: 0-0.200%,
Ca: 0-0.1000%,
Mg: 0 to 0.1000%,
REM: 0-0.1000%,
Sn: 0-0.200%,
As: 0 to 0.100%,
Bi: 0 to 0.0500%, and the balance: Fe and impurities;
The tensile strength is 1800 MPa or more,
The average residual stress on the surface of the steel plate is −250 MPa or less,
A hot stamped product characterized in that Hvs/Hvi, which is a ratio of an average Vickers hardness Hvs in a surface layer region that is a region from a depth of 20 μm to a depth of 50 μm from the surface, to an average Vickers hardness Hvi at a depth of 1/4 of the plate thickness from the surface, is 0.90 or less. The chemical composition, 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 to 3.00%,
Ti: 0.001 to 0.200%,
Nb: 0.001-0.200%,
V: 0.001-2.000%,
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 to 0.100%, and Bi: 0.0001 to 0.0500%
The hot stamped product according to claim 1, characterized in that it contains one or more types selected from the group consisting of

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