WO2023199776A1 - Hot stamp molded body - Google Patents

Abstract

The present invention provides a hot stamp molded body which is provided with a steel sheet, wherein: the steel sheet has a specific chemical composition; the average B concentration in a region ranging from the depth of 5 µm from the surface of the steel sheet to the depth of 25 µm from the surface is not more than 0.700 times the B concentration at the depth of 100 µm from the surface; and the average O concentration in a region ranging from the surface of the steel sheet to the depth of 0.5 µm from the surface is 4.000% by mass or less.

Description

hot stamp molded body

The present invention relates to a hot stamp molded article.
This application claims priority based on Japanese Patent Application No. 2022-067028 filed in Japan on April 14, 2022, the contents of which are incorporated herein.

In recent years, there has been a demand for lighter automobile bodies from the viewpoint of environmental protection and resource conservation, and high-strength steel plates are being applied to automobile parts. When high-strength steel plates are used, desired strength can be imparted to the vehicle body while reducing the thickness of the steel plate and reducing the weight of the vehicle body. Automotive parts are manufactured by press forming steel plates, but as the strength of the steel plates increases, not only does the forming load increase, but also the formability decreases, making cracks and wrinkles more likely to occur. Furthermore, when a high-strength steel plate is press-formed, the shape of the member changes significantly due to springback when the member is taken out of the mold, making it difficult to ensure the dimensional accuracy of the member. As described above, it is not easy to manufacture high-strength vehicle body members by press molding.

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

The strength of the hot-stamped compact can be further increased by increasing the C content of the steel plate. However, increasing the C content of a steel sheet increases its susceptibility to hydrogen embrittlement, making hydrogen embrittlement cracking more likely to occur. Moreover, the deformability of the steel plate constituting the hot-stamped body is reduced, and cracks are more likely to occur during a collision. As described above, it is not easy to produce a high-strength hot-stamped molded product with excellent hydrogen embrittlement resistance and deformability, especially when the tensile strength of the hot-stamped molded product exceeds 1900 MPa. It becomes more difficult to achieve both hydrogen embrittlement characteristics and deformability.

Patent Document 2 describes a hot stamp with improved hydrogen embrittlement resistance by making precipitates containing one or more of Nb, Ti, V, Cr, Mo, and Mg function as hydrogen trap sites. Techniques related to molded bodies are disclosed.

Patent Document 3 discloses a technique for producing a hot-stamped molded body with improved toughness and hydrogen embrittlement resistance by refining prior austenite grains.

Patent Document 4 discloses a hot press 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 members by refining prior austenite grains, using Nb-based precipitates as hydrogen trap sites, and suppressing variations in hardness on the steel plate surface. It has improved chemical properties.

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 techniques disclosed in Patent Document 2 and Patent Document 3 are excellent in that they are capable of producing hot-stamped molded articles having excellent hydrogen embrittlement resistance and a tensile strength of 950 MPa or more. However, Patent Documents 2 and 3 do not describe a hot-stamped molded article having a tensile strength of 1900 MPa or more. According to the studies of the present inventors, in Patent Documents 2 and 3, there is room for improvement in order to achieve both high strength and hydrogen embrittlement resistance at a higher level.

Patent Document 4 states that a hot pressed member with excellent hydrogen embrittlement resistance and a tensile strength of 1780 MPa or more can be obtained, but it does not take into account the reduction in deformability and is not suitable for use as an automobile member. It is thought that collision performance is insufficient.

The present invention was made in view of the above circumstances, and an object thereof is to provide a hot-stamped molded article having high tensile strength of 1900 MPa or more, as well as excellent hydrogen embrittlement resistance and deformability. do.

The gist of the present invention is as follows.

(1) A hot-stamped molded body according to one aspect of the present invention is a hot-stamped molded body that includes a steel plate, and all or part of the steel plate has a chemical composition in mass %,
C: more than 0.32%, less than 0.70%,
Si: less than 2.00%,
Mn: 0.01% or more, less than 1.00%,
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: 0.0005-0.0200%,
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%,
Contains As: 0 to 0.100% and Bi: 0 to 0.0500%,
The remainder consists of Fe and impurities,
The tensile strength is 1900 MPa or more,
The average B concentration in a region from 5 μm deep to 25 μm deep from the surface of the steel plate is 0.700 times or less of the B concentration at a position 100 μm deep from the surface,
The average O concentration in a region from the surface to a depth of 0.5 μm from the surface is 4.000% by mass or less.
(2) The hot-stamped molded article according to (1) above has 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: 0.01-2.00%,
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 Bi: 0.0001 to 0.0500%
It may contain one or more types from the group consisting of.

According to the above aspect of the present invention, it is possible to provide a hot-stamped molded article having a high tensile strength of 1900 MPa or more, as well as excellent hydrogen embrittlement resistance and deformability.
The hot-stamped molded article according to the above aspect is less likely to cause hydrogen embrittlement cracking, has excellent deformability, and is less likely to crack during a collision, and thus can be suitably applied to automobile parts such as pillars and bumpers.

The figure which shows the hat member manufactured in the Example.

The present inventors studied methods for improving the hydrogen embrittlement resistance and deformability of hot-stamped molded articles having a tensile strength of 1900 MPa or more, and as a result, obtained the following knowledge.

(A) In the hot-stamped compact, by lowering the average B concentration in the region from 5 μm deep to 25 μm deep from the surface of the steel plate constituting the hot-stamped compact, the hot-stamped compact can be resistant to hydrogen embrittlement. It is possible to improve the deformation properties and deformability.

(B) Although the reason for this is not clear, (a) cracks due to hydrogen embrittlement and cracks due to reduced deformability tend to occur starting from the surface layer of the steel plate that constitutes the hot stamped compact, (b) hot stamping In a region (surface layer region) from 5 μm deep to 25 μm deep from the surface of the steel plate constituting the compact, the hardenability of the steel sheet decreases due to a decrease in the average B concentration, and (c) decrease in hardenability. This is presumed to be because the surface layer becomes softer and cracking in the surface layer is suppressed.

(C) In the hot-stamped compact, by lowering the average O concentration in the region from the surface of the steel plate constituting the hot-stamped compact to a region 0.5 μm deep from the surface (near-surface region), the hot-stamped compact is Hydrogen embrittlement resistance can be improved.

(D) Although the reason for this is not clear, (d) if the scale generated in the manufacturing process of the hot-stamped molded product remains without being sufficiently removed, the average O concentration in the region near the surface increases; and (e) It is presumed that if scale remains, it becomes difficult for hydrogen that has entered the hot-stamped molded product to escape, making hydrogen embrittlement cracking more likely to occur.

(E) By lowering the Mn content in the chemical composition of the steel plate constituting the hot-stamped body, the hydrogen embrittlement resistance of the hot-stamped body can be improved.

(F) Although the reason for this is not clear, (f) the Mn concentration at the grain boundaries decreases and cracks originating from the grain boundaries are suppressed, and (g) the decrease in the Mn content causes hot stamping. It is presumed that this is because the average B concentration in the surface layer region of the steel plate constituting the compact is likely to decrease.

Based on the knowledge of (A) to (F) above, the present inventors controlled the B concentration and O concentration within a specific range in the surface layer region (including the surface layer region and near-surface region described below) of the hot stamp molded product. and by lowering the Mn content in the chemical composition of the hot-stamped molded body, a hot-stamped molded body with a tensile strength of 1900 MPa or more and excellent hydrogen embrittlement resistance and deformability can be obtained. I found out.

Hereinafter, the hot stamp molded article according to this embodiment will be described in detail. First, the reason for limiting the chemical composition of the steel plate constituting the hot-stamped molded body according to the present embodiment will be explained.

All or part of the steel plate included in the hot-stamped molded body according to the present embodiment has the following chemical composition. When the hot-stamped molded body is made of only a steel plate, it can be said that all or part of the hot-stamped molded body has the chemical composition shown below.
Note that the numerically limited range described below with "~" in between includes the lower limit value and the upper limit value. Numerical values indicated as "less than" or "greater than" do not include the value within the numerical range. All percentages regarding chemical composition indicate mass %.

When the hot stamped molded article includes a part having a tensile strength of 1900 MPa or more and a part having a tensile strength of less than 1900 MPa, at least the part having a tensile strength of 1900 MPa or more has the following chemical composition. All you have to do is do it.

All or part of the steel plate included in the hot-stamped molded body according to the present embodiment has a chemical composition, in mass %, of C: more than 0.32% and 0.70% or less, Si: less than 2.00%, and Mn. : 0.01% or more, less than 1.00%, 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: 0.0005 to 0.0200%, and the remainder: Fe and impurities.
Each element will be explained below.

C: More than 0.32%, 0.70% or less C is an element that improves the tensile strength of the steel plate after hot stamping (the steel plate that constitutes the hot stamped body). If the C content is 0.32% or less, the tensile strength of the steel plate after hot stamping will be less than 1900 MPa, resulting in insufficient strength of the hot stamped product. Therefore, the C content is set to more than 0.32%. The C content is preferably greater than 0.34%, greater than 0.38%, greater than 0.42%, or greater than 0.45%.
On the other hand, if the C content exceeds 0.70%, the strength of the hot-stamped molded product becomes too high, making it impossible to obtain excellent hydrogen embrittlement resistance and deformability. Therefore, the C content is set to 0.70% or less. Preferably, the C content is 0.65% or less, 0.60% or less, 0.55% or less or 0.50% or less.

Si: less than 2.00% Si is an element that may be contained as an impurity in steel and makes the steel brittle. When the Si content exceeds 2.00%, its adverse effects become particularly large. Therefore, the Si content is made 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 lowering the Si content causes an increase in steel manufacturing costs, the Si content is preferably 0.001% or more. Further, since Si has the effect of improving the hardenability of steel, it may be actively included. From the viewpoint of improving hardenability, the Si content is preferably 0.05% or more, 0.10% or more, or 0.15% or more.

Mn: 0.01% or more and less than 1.00% Mn is an element that combines with S to form MnS and has the effect of suppressing the harmful effects of S. If the Mn content is less than 0.01%, the above effects 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 forms a metal structure mainly composed of martensite inside the steel sheet after hot stamping, which is effective for ensuring the strength of the hot stamped product. It is an element. From the viewpoint of ensuring strength, the Mn content is preferably 0.20% or more or 0.30% or more.
On the other hand, if the Mn content is 1.00% or more, excellent hydrogen embrittlement resistance cannot be obtained in the hot-stamped molded product. Therefore, the Mn content is set to less than 1.00%. The Mn content is preferably less than 0.80%, less than 0.60% or less than 0.50%.

P: 0.200% or less P is an element that may be contained as an impurity in steel and makes the steel brittle. If the P content exceeds 0.200%, the adverse effects will be particularly large, and weldability will also deteriorate 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%, but if the P content is reduced to less than 0.001%, the cost of removing P will increase significantly, which is economically unfavorable. % or more.

S: 0.0200% or less S is an element that may be contained as an impurity in steel and makes the steel brittle. When 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%, but if the S content is reduced to less than 0.0001%, the S removal cost will increase significantly and it is economically unfavorable. % or more.

sol. Al: 0.001-1.000%
Al is an element that has the effect of deoxidizing molten steel. sol. If the Al content (acid-soluble Al content) is less than 0.001%, deoxidation will be insufficient. Therefore, sol. Al content shall be 0.001% or more. sol. The Al content is preferably 0.005% or more, 0.010% or more, or 0.020% or more.
On the other hand, sol. If the Al content is too high, the transformation point will rise, making it difficult to heat the steel plate to a temperature exceeding Ac ₃ in the hot stamping heating process. Moreover, the strength of the hot stamp molded product is insufficient. Therefore, sol. Al content shall be 1.000% or less. sol. The 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 may be contained as an impurity in steel and forms nitrides during continuous casting of steel. Since this nitride deteriorates the deformability of the hot-stamped molded body, it is preferable that the N content is low. If the N content exceeds 0.0200%, its adverse effects will be 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 if the N content is reduced excessively, the cost of removing N will increase significantly and it is economically unfavorable. It may be .0020% or more.

O: 0.0200% or less O is an element that may be contained as an impurity in steel and forms oxide inclusions. When 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 stamp molded 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.
O may be 0%, but O is an element that forms B oxide by combining with B and reduces the hardenability of steel, softening the surface layer of the steel plate after hot stamping. It is an effective element for 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: 0.0005-0.0200%
B is an element that improves the hardenability of steel, and is an effective element for forming a metal structure mainly composed of martensite inside the steel plate after hot stamping, and ensuring the strength of the hot stamped product. be. If the B content is less than 0.0005%, the desired strength cannot be obtained in the hot stamp molded product. Therefore, the B content is set to 0.0005% or more. The B content is preferably 0.0010% or more, 0.0015% or more, or 0.0020% or more.
On the other hand, if the B content exceeds 0.0200%, carborides are formed in the hot-stamped compact, and the hardenability improvement 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 molded body according to this embodiment may be Fe and impurities. Examples of impurities include elements that are unavoidably mixed in from steel raw materials or scraps and/or during the steel manufacturing process and are allowed within a range that does not impede the properties of the hot-stamped molded product according to the present embodiment.

The steel plate constituting the hot-stamped molded product according to the present embodiment may contain the following elements as optional elements in place of a part of Fe. When the following arbitrary elements are not included, the content is 0%.

Cr:0.01~2.00%
Cr is an element that increases the strength of the hot stamped body by increasing the hardenability of steel. To ensure 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 molded product 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 increases the strength of the hot stamped body by increasing the hardenability of steel. To ensure this effect, the Mo content is preferably 0.01% or more. 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 molded product 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 increases the strength of the hot stamped body by increasing the hardenability of the steel. To ensure 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 body 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 increases the strength of the hot stamped body by increasing the hardenability of steel. 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 body 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: 0.01-2.00%
Ni is an element that increases the strength of the hot stamped body by increasing the hardenability of steel. To ensure 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 2.00%, the deformability of the hot-stamped body deteriorates. Therefore, the Ni content is preferably 2.00% or less. The Ni content is preferably less than 1.00% or less than 0.50%.

Ti: 0.001-0.200%
Ti is an element that forms carbonitrides in steel and increases the strength of hot stamped products through precipitation strengthening. Furthermore, Ti is an element that improves the hydrogen embrittlement resistance and deformability of the hot-stamped molded product through refinement of the metal structure. To ensure 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 will be generated in the steel, and the deformability of the hot stamped body will deteriorate. 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 increases the strength of hot stamped products through precipitation strengthening. Furthermore, Nb is an element that improves the hydrogen embrittlement resistance and deformability of the hot-stamped compact through refinement of the metal structure. To ensure 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 will be generated in the steel, and the deformability of the hot-stamped compact will deteriorate. 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:0.001~0.200%
V is an element that forms carbonitrides in steel and increases the strength of the hot stamped product through precipitation strengthening. Further, V is an element that improves the hydrogen embrittlement resistance and deformability of the hot-stamped molded product through refinement of the metal structure. To ensure 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 0.200%, a large amount of coarse carbonitrides will be generated in the steel, and the deformability of the hot-stamped body will deteriorate. Therefore, the V content is set to 0.200% or less. The V content is preferably less than 0.100% or less than 0.050%.

Zr: 0.001-0.200%
Zr is an element that forms carbonitrides in steel and increases the strength of hot-stamped products through precipitation strengthening. Further, Zr is an element that improves the hydrogen embrittlement resistance and deformability of the hot-stamped molded product through refinement of the metal structure. To ensure 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 will be generated in the steel, and the deformability of the hot-stamped product will deteriorate. 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 body by adjusting the shape of inclusions. To ensure 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-mentioned effects are saturated, and furthermore, excessive costs occur, so the Ca content is set to 0.1000% or less. Ca content is preferably less than 0.0100%.

Mg: 0.0001-0.1000%
Mg is an element that improves the deformability of the hot stamped body by adjusting the shape of inclusions. To ensure these effects, the Mg content is preferably 0.0001% or more.
On the other hand, even if a large amount of Mg is contained, the above-mentioned effects are saturated, and furthermore, excessive costs occur, so the Mg content is set to 0.1000% or less. The Mg content is preferably less than 0.0100%.

REM: 0.0001~0.1000%
REM is an element that improves the deformability of a hot stamped body by adjusting the shape of inclusions. To ensure this effect, the REM content is preferably 0.0001% or more.
On the other hand, even if a large amount of REM is contained, the above-mentioned effects are saturated, and furthermore, excessive costs occur, so the REM content is set to 0.1000% or less. The REM content is preferably less than 0.0100%.
In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the content of REM 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 the hot stamp molded product. To ensure 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-mentioned effects are saturated and, furthermore, excessive costs occur, so the Sn content is set to 0.200% or less. Sn content is 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 the hot stamp molded product. To ensure this effect, the As content is preferably 0.001% or more.
On the other hand, even if a large amount of As is contained, the above-mentioned effects will be saturated and furthermore, excessive costs will occur, so the As content is set to 0.100% or less.

Bi:0.0001~0.0500%
Bi is an element that improves the hydrogen embrittlement resistance and deformability of the hot-stamped compact by making the solidified structure fine. To ensure 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-mentioned effects will be saturated and furthermore, excessive costs will occur, so the Bi content is set to 0.0500% or less. Bi content is preferably 0.0100% or less or 0.0050% or less.

The chemical composition of the steel plate constituting the hot-stamped body described above can be determined by taking a test piece from the steel plate constituting the hot-stamping body, removing the paint film if the steel plate has been painted, and then using a general method. The average element content throughout the plate thickness may be measured using an analytical method. For example, measurement may be performed using inductively coupled plasma optical emission spectrometry or inductively coupled plasma mass spectrometry. Note that C and S may be measured using a combustion-infrared absorption method, and O and N may be measured using an inert gas melting-infrared absorption method or an inert gas melting-thermal conductivity method. When a plating layer is provided on the surface of the steel plate constituting the hot-stamped compact, the chemical composition may be measured after removing the plating layer.

As described above, when the hot stamp molded article includes a portion having a tensile strength of 1900 MPa or more and a portion having a tensile strength of less than 1900 MPa, at least the portion having a tensile strength of 1900 MPa or more is as described above. It is sufficient that the chemical composition is as follows. In order to analyze the chemical composition of a part with a tensile strength of 1900 MPa or more, a tensile test described below was performed and a tensile test piece was collected from a tensile test piece that had a tensile strength of 1900 MPa or more. A test piece for chemical composition analysis may be taken from a part adjacent to the part.

Next, the B concentration distribution and O concentration distribution in the thickness direction of the steel plate constituting the hot-stamped molded body according to the present embodiment will be described.
In this embodiment, in the process of heating the hot stamping steel plate, by heating the hot stamping steel plate under specific conditions, the amount of B present in the surface layer region can be reduced, and the hydrogen resistance of the hot stamping molded body can be reduced. Embrittlement properties and deformability can be improved. Further, by performing blasting under specific conditions after hot stamping, the amount of O present in the region near the surface can be reduced, and the hydrogen embrittlement resistance of the hot stamped molded product can be improved.

In the hot-stamped compact according to the present embodiment, the average B concentration in a region from 5 μm deep to 25 μm deep from the surface of the steel plate constituting the hot-stamped compact is such that the B concentration at a position 100 μm deep from the surface is The average O concentration in a region from the surface to a depth of 0.5 μm from the surface is 4.000% by mass or less.
Note that the region from 5 μm deep to 25 μm deep from the surface of the steel plate can be referred to as a region whose starting point is 5 μm deep from the surface of the steel plate and ends at 25 μm deep from the surface. be able to. Furthermore, the region from the surface of the steel plate to a depth of 0.5 μm from the surface can be expressed as a region starting from the surface of the steel plate and ending at a position 0.5 μm deep from the surface.

When the hot-stamped molded product includes a portion having a tensile strength of 1900 MPa or more and a portion having a tensile strength of less than 1900 MPa, at least the portion having a tensile strength of 1900 MPa or more has the following B concentration distribution and It is sufficient as long as it has an O concentration distribution.
Each regulation will be explained below.

Average B concentration in a region from 5 μm deep to 25 μm deep from the surface: 0.700 times or less of the B concentration at a position 100 μm deep from the surface Region from 5 μm deep to 25 μm deep from the surface (hereinafter referred to as If the average B concentration in the surface layer region (sometimes referred to as the surface layer region) is more than 0.700 times the B concentration at a depth of 100 μm from the surface, the surface layer region will not become soft and the hot stamped molded product will not have the desired durability. Hydrogen embrittlement properties and deformability cannot be obtained. Therefore, the average B concentration in the surface layer region is set to be 0.700 times or less of the B concentration at a position 100 μm deep from the surface. The average B concentration in the surface layer region is preferably 0.700 times or less of the B concentration at a depth of 100 μm from the surface and 0.0015% by mass or less. More preferably, the average B concentration in the surface layer region is 0.500 times or less or 0.300 times or less of the B concentration at a depth of 100 μm from the surface. Further, the average B concentration in the surface layer region is more preferably 0.0010% by mass or less or 0.0006% by mass or less.
Although the lower limit is not particularly defined, the average B concentration in the surface layer region may be set to 0.0002% by mass or more, since if it is lowered too much, the above effect will not only be saturated, but also the strength of the hot-stamped molded article will be reduced.

Note that the surface refers to the surface of the steel plate that constitutes the hot stamp molded body. When the hot-stamped molded body has a plating layer on the surface, the surface refers to the interface between the plating layer and the steel plate.

Average O concentration in a region from the surface to a depth of 0.5 μm from the surface: 4.000% by mass or less Average O concentration in a region from the surface to a depth of 0.5 μm from the surface (hereinafter sometimes referred to as the near-surface region) If it exceeds 4.000% by mass, the desired hydrogen embrittlement resistance cannot be obtained in the hot-stamped molded article. Therefore, the average O concentration in the region near the surface is set to 4.000% by mass or less. The average O concentration in the near-surface region is preferably 3.500% by mass or less, 3.000% by mass or less, or 2.500% by mass or less.
Although the lower limit is not particularly defined, if the average O concentration in the near-surface region is excessively reduced, the above effects will not only be saturated, but also the productivity of the hot-stamped molded product will be greatly impaired. It may be more than 0.0100% by mass or more than 0.0200% by mass.

The average B concentration in the surface region, the average O concentration in the near-surface region, and the B concentration at a depth of 100 μm from the surface are measured by the following method.
A test piece is taken from the hot-stamped molded product, and after removing the paint film if the steel plate is coated, it is measured at a depth from the measurement starting surface using Glow Discharge Optical Emission Spectrometry (GDS analysis). The concentration (mass %) of each element is measured at a depth of 100 μm or more in the width direction (plate thickness direction). Note that the "measurement start surface" here is different from the "surface of the steel plate."

In the GDS analysis, the measurement pitch is adjusted so that there are 1200 to 1800 measurement points from the surface of the steel plate to a depth of 100 μm. In order to eliminate the influence of foreign substances such as oil adhering to the measurement starting surface, from the start of the measurement, set the depth at which the Fe concentration is at least 95% of the "Fe concentration at a depth of 100 μm from the measurement starting surface". Defined as the surface of the steel plate. Similarly, when the hot stamp molded body has a plating layer on the surface, the depth at which the Fe concentration initially becomes 95% or more of "the Fe concentration at a depth of 100 μm from the measurement starting surface" is set as the plating layer. It is defined as the interface with the steel plate, that is, the surface of the steel plate.

From the obtained measurement results, the average B concentration in the surface layer region is obtained by calculating the average value of the B concentration in the region from 5 μm depth to the steel sheet surface to 25 μm depth from the steel sheet surface. Further, by calculating the average value of the O concentration in a region from the surface of the steel plate to a depth of 0.5 μm from the surface of the steel plate, the average O concentration in the region near the surface is obtained. Further, by determining the B concentration at a position 100 μm deep from the surface of the steel plate, the B concentration at a position 100 μm deep from the surface is obtained. In addition, if there is no measured value of GDS analysis at a position 100 μm deep from the surface of the steel plate, the first measured value beyond the position 100 μm deep from the surface may be taken as the B concentration at a position 100 μm deep from the surface. .

It is preferable that the GDS analysis is performed on test pieces taken from three or more locations of the hot-stamped molded body, and the average value of the obtained results is taken as the B concentration and the O concentration. In addition, the test piece may be taken from a part adjacent to a part where a tensile test piece having a tensile strength of 1900 MPa or more was taken after conducting a tensile test to be described later.

The metal structure of the steel plate constituting the hot-stamped compact is not particularly limited as long as the desired strength, hydrogen embrittlement resistance, and deformability can be obtained, but it is preferable to have the metal structure shown below.

It is preferable that all or part of the steel plate constituting the hot-stamped molded body according to the present embodiment has a metal structure containing martensite in the amount shown below. In the following description regarding metallographic structure, "%" means "volume %". When the hot-stamped molded body has a part having a tensile strength of 1900 MPa or more and a part having a tensile strength of less than 1900 MPa, at least the part having a tensile strength of 1900 MPa or more has the following metal structure. All you have to do is do it.

Metal structure of the inner layer region The metal structure of the inner layer region (area from 100 μm depth to the center of the plate thickness (1/2 position of the plate thickness) from the surface of the steel plate constituting the hot-stamped compact) is more than 90.0% martensite. It is preferable to include.

Martensite is an effective structure for increasing the tensile strength of the steel plate after hot stamping, so martensite is a structure that is effective for increasing the tensile strength of the steel plate after hot stamping. It is preferable that the volume fraction of martensite is more than 90.0%. If the volume fraction of martensite in the inner layer region is 90.0% or less, the tensile strength of the hot-stamped molded product is less than 1900 MPa, which may result in insufficient strength. Therefore, it is preferable that the volume fraction of martensite in the inner layer region is greater than 90.0%. The volume fraction of martensite in the inner layer region is more preferably over 91.0%, over 93.0%, or over 95.0%.

There is no need to set a particular upper limit for the volume fraction of martensite in the inner layer region, but in order to greatly increase the volume fraction of martensite, it is necessary to excessively raise the heating temperature of the steel plate for hot stamping during the hot stamping process. , it is necessary to increase the cooling rate excessively, and the productivity of the hot-stamped molded product is greatly impaired. Therefore, the volume fraction of martensite in the inner layer region is preferably 99.0% or less or 98.0% or less.

In the present embodiment, martensite includes fresh martensite that has not been tempered, as well as tempered martensite that has been tempered and has iron carbides inside.

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

Retained austenite has the effect of improving the ductility of the steel plate after hot stamping. In order to obtain this effect, it is preferable that the volume fraction of retained austenite in the inner layer region is 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 retained austenite, it is necessary to perform an austempering treatment at a high temperature after hot stamping, which significantly reduces the productivity of the hot stamped molded product. Furthermore, if the residual austenite is contained excessively, the collision resistance properties of the hot-stamped molded article may be deteriorated. Therefore, the volume fraction of retained austenite in the inner layer region is preferably 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.
First, a test piece is taken from the hot-stamped compact, and after buffing the vertical cross section (thickness cross section) of the steel plate, a depth of 100 μm from the surface of the steel plate constituting the hot-stamped compact to the center of the plate thickness (plate thickness 1 Tissue observation is performed on the inner layer region (position 2).

Specifically, after subjecting the polished surface to nital corrosion or electrolytic polishing, a microstructure photograph is taken using an optical microscope and a scanning electron microscope (SEM), and the resulting microstructure photographs are analyzed based on brightness differences or presence within the phase. By performing image analysis based on the differences in the morphology of iron carbides, the area percentages of ferrite, pearlite, bainite, tempered martensite, and precipitates are obtained. After that, after repeller corrosion was performed on the same observation position, the structure was photographed using an optical microscope and a scanning electron microscope (SEM), and image analysis was performed on the obtained structure photograph. Calculate the total area ratio of "austenite and fresh martensite".

Further, after electrolytically polishing the longitudinal section at the same observation position, the area ratio of retained austenite is measured using a SEM equipped with an electron beam backscatter pattern analyzer (EBSP device). Note that the area ratio of retained austenite is obtained by calculating the area ratio of a region having 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 the above-mentioned "retained austenite and fresh martensite."

Based on these results, obtain the area percentages of ferrite, pearlite, bainite, martensite (tempered martensite and fresh martensite), retained austenite, and precipitates. Then, assuming that the area ratio is equal to the volume ratio, the obtained area ratio is regarded as the volume ratio of each tissue.

In microstructural observation, tempered martensite can be distinguished from fresh martensite by the presence of iron carbides inside. Furthermore, tempered martensite can be distinguished from bainite in that the iron carbide present inside extends not in a single direction but in multiple directions. Note that elongation 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 the present embodiment (the thickness of the steel plate when the hot-stamped body is made only of steel plates) is not particularly limited, but from the viewpoint of reducing the weight of the vehicle body, it is 2.5 mm or less, 2.5 mm or less, It is preferable to set it as 0 mm or less, 1.8 mm or less, or 1.6 mm or less.
On the other hand, from the viewpoint of ensuring the 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 part of the hot stamp molded article according to this embodiment has a tensile strength of 1900 MPa or more. For this purpose, it is necessary that the tensile strength of all or part of the steel plate constituting the hot-stamped molded product according to this embodiment is 1900 MPa or more. If the tensile strength of at least a portion of the hot-stamped molded product is not 1900 MPa or more, it becomes impossible to secure the deformation load when the hot-stamped molded product is deformed. As a result, the collision resistance properties of the hot-stamped molded product deteriorate. Therefore, the tensile strength of all or part of the hot stamp molded product is set to 1900 MPa or more. Preferably, the tensile strength of all or part of the hot-stamped molded product is 2000 MPa or more, 2100 MPa or more, 2300 MPa or more, or 2500 MPa or more.
On the other hand, since excessively increasing the strength of the hot-stamped molded product causes a decrease in hydrogen embrittlement resistance and deformability, the tensile strength of the hot-stamped molded product is preferably less than 3000 MPa or less than 2800 MPa.

The hot stamp molded product according to the present embodiment may have a tensile strength of 1900 MPa or more as a whole (the entire hot stamp molded product), but a portion of the hot stamp molded product having a tensile strength of 1900 MPa or more may have a tensile strength of 1900 MPa or more. There may be a mixture of portions that are less than the above. By providing portions with different strengths, it becomes possible to control the deformation state of the hot stamp molded body at the time of collision. A hot-stamped molded body having parts with different strengths can be produced by hot-stamping after joining two or more types of steel plates with different chemical compositions, or by adjusting the heating temperature of the steel plate or the cooling rate after hot-stamping in the hot-stamping process. It can be produced by a method of partially changing the shape, a method of partially reheating a hot stamp molded product, and the like.

The tensile strength of a hot-stamped compact is obtained by taking a small strip-shaped piece from the hot-stamping compact, 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-shaped test piece from the hot-stamped molded product in accordance with JIS Z 2241:2011 and conduct a tensile test at a tensile speed of 10 mm/min. If it is not possible to collect a No. 13B plate-shaped test piece because the size of the hot-stamped molded product is small or the shape is complicated, a small strip-shaped piece having a parallel part of an arbitrary width is collected, 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 portion.
In addition, when a hot-stamped molded article contains a high-strength part and a low-strength part coexisting, a tensile test piece is taken from the high-strength part.

The hot stamp molded article according to this embodiment may have a plating layer on the surface. By having a plating layer on the surface, corrosion resistance can be improved after hot stamping. Examples of the plating layer include a zinc-based plating layer and an aluminum-based plating layer. A hot-stamped molded body having these plating layers is obtained by hot-stamping using a zinc-based plated steel sheet or an aluminum-based plated steel sheet. The plating layer may be formed on both sides of the hot-stamped molded body, or may be formed on one side. The plating layer of the hot stamp molded body can be formed by hot stamping using a plated steel plate provided with the plating layer. However, the plating layer provided on the plated steel sheet prevents the formation of preferable B and O concentration distributions in the surface region and near-surface region of the hot-stamped compact during the process of manufacturing the hot-stamped compact, so it must be manufactured more strictly. It is necessary to control the method, and the productivity of hot-stamped molded products may be significantly reduced. Therefore, from the viewpoint of productivity, it is preferable that the hot stamp molded product does not have a plating layer on the surface.

Next, a hot stamping steel plate suitable for obtaining a hot stamping molded article according to the present embodiment will be described.
Since the change in chemical composition caused by hot stamping is negligibly small, the chemical composition of the hot stamping steel sheet may be the same as that of the hot stamping molded product described above. The chemical composition of the hot stamping steel plate may be determined by taking a test piece from the hot stamping steel plate and measuring it in the same manner as in the case of the hot stamping molded product.

The steel sheet for hot stamping has an average B concentration in a region from 5 μm deep to 25 μm deep from the surface (surface layer region) of the steel sheet, which is 0.850 times or less than the B concentration at a position 100 μm deep from the surface of the steel sheet. It is preferable that there be. If the average B concentration in the surface layer region is more than 0.850 times the B concentration at a depth of 100 μm from the surface of the steel sheet, even if the hot stamping conditions described below are applied, the B concentration in the surface layer region of the hot stamped compact is Concentration distribution cannot be controlled favorably. As a result, desired hydrogen embrittlement resistance and deformability cannot be obtained in the hot-stamped molded article. In addition, when the steel plate for hot stamping has a plating layer, the surface refers to the interface between the plating layer and the steel plate.

The B concentration distribution in the thickness direction of a hot stamping steel sheet can be determined by taking a test piece from the hot stamping steel sheet and performing a GDS analysis in the same manner as in the case of a hot stamping compact.

Hereinafter, a method for manufacturing a hot stamping steel plate for obtaining a hot stamping molded body according to the present embodiment will be described.

Steel plates for hot stamping are produced through a hot rolling process in which a slab having the above-mentioned chemical composition is hot-rolled to produce a hot-rolled steel plate, and a cold-rolled steel plate is obtained by cold-rolling the hot-rolled steel plate. It is manufactured by a manufacturing method including a cold rolling step and an annealing step of annealing the cold rolled steel sheet to obtain an annealed steel sheet.

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

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

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

When winding up the hot-rolled steel sheet after hot rolling, the winding temperature is preferably less than 550°C. If the winding temperature is 550° C. or higher, thermally stable iron carbides are generated, which may deteriorate the collision resistance properties of the hot-stamped molded product.
On the other hand, if the coiling temperature becomes too low, the hot rolled steel sheet will become excessively hard and it will be difficult to perform cold rolling, so the coiling temperature is preferably over 500°C.

The hot-rolled steel sheet that has been hot-rolled and wound up is pickled according to a conventional method, and then cold-rolled according to a conventional method to obtain a cold-rolled steel sheet. In the cold rolling process, it is preferable that the cumulative reduction rate in cold rolling is 40% or more. If the cumulative rolling reduction is less than 40%, the metal structure of the hot stamping steel sheet may become coarse. If the metal structure of the steel plate for hot stamping is coarse, the metal structure of the hot stamp molded body will become coarse after hot stamping, which will cause the collision resistance of the molded body to deteriorate.
On the other hand, excessively increasing the cumulative rolling reduction increases the load on the rolling equipment and causes a decrease in productivity, so the cumulative rolling reduction is preferably less than 70%. After cold rolling, treatments such as degreasing may be performed in a conventional manner.

A cold-rolled steel plate is annealed to become an annealed steel plate. In the annealing step, in order to refine the metal structure of the annealed steel plate (steel plate for hot stamping) by recrystallization, it is preferable that the soaking temperature be higher than 700°C. If the soaking temperature is 700° C. or lower, it may not be possible to preferably control the B concentration distribution in the surface layer region of the hot stamping steel sheet. As a result, it may not be possible to obtain the desired hydrogen embrittlement resistance and deformability in the hot-stamped molded article.
On the other hand, if the heating rate is too slow, the soaking temperature is too high, or the soaking time is too long, the metal structure of the annealed steel sheet will become coarse due to grain growth, and the hydrogen embrittlement resistance of the hot stamped product will deteriorate. performance may decrease. Therefore, it is preferable that the average heating rate up to the soaking temperature is 1°C/sec or more, the soaking temperature is preferably 800°C or less, and the soaking time (holding time at the soaking temperature) is less than 600 seconds. It is preferable that

In addition, the dew point of the atmosphere in the annealing furnace should be set to -20°C or higher and lower than 0°C, and the residence time in the temperature range of 700°C or higher and lower than (Ac ₃ points -30°C) should be set to more than 360 seconds and less than 600 seconds. It is preferable to do so. Further, the atmosphere in the annealing furnace is preferably a nitrogen-hydrogen atmosphere containing 1% by volume or more and less than 4% by volume of hydrogen.
If the dew point is less than -20°C or 0°C or more, or the residence time in the temperature range of 700°C or more and less than (Ac ₃ points -30°C) is 360 seconds or less, B in the surface area of the hot stamping steel plate In some cases, it may not be possible to control the concentration distribution favorably. As a result, it may not be possible to obtain the desired hydrogen embrittlement resistance and deformability in the hot-stamped molded article.
On the other hand, if the residence time in the above temperature range is 600 seconds or more, excessive decarburization may occur in the hot stamping steel plate, and the strength of the hot stamped molded product may be insufficient after hot stamping. The annealed steel sheet produced by the method described above may be plated according to a conventional method to obtain a plated steel sheet. The annealed steel sheet or plated steel sheet thus obtained may be subjected to temper rolling according to a conventional method.

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

The hot stamping molded article according to the present embodiment includes a heating process of heating a hot stamping steel plate (annealed steel plate or plated steel plate) produced by the method described above, and a hot stamping process of hot stamping the heated steel plate for hot stamping. It can be obtained by a manufacturing method including a stamping step and a blasting step of performing a blasting treatment on the hot-stamped hot-stamped molded body. In order to stably obtain the hot-stamped molded article according to this embodiment, hot-stamping is preferably performed by the following method.

In the heating step, prior to the hot stamping step, a hot stamping steel sheet having the above-described chemical composition and B concentration distribution in the thickness direction is heated. In the heating step, it is preferable to use a gas combustion furnace using a flammable gas containing propane gas to heat the hot stamping steel plate at an air ratio of 0.84 or less. It is preferable that the heating temperature is higher than 950° C. and higher than ₃ Ac points, and the holding time at the heating temperature is higher than 360 seconds.
Note that the air ratio is the ratio (A/A ₀ ) of the amount of air (A) actually introduced to the theoretical amount of air (A ₀ ). Moreover, the Ac ₃ points in the heating process means the Ac ₃ points of the inner layer region of the hot stamping steel sheet, and may be the same value as the Ac ₃ points of the cold rolled steel sheet determined by the above method.

If the air ratio exceeds 0.84, the heating temperature is 950°C or less, or the holding time is 360 seconds or less, the B concentration distribution in the surface region of the hot-stamped molded product cannot be preferably controlled. There is. Moreover, if the heating temperature is ₃ points or less of Ac, the volume fraction of martensite may be insufficient in the metal structure of the inner layer region of the hot-stamped molded product, and the strength of the hot-stamped molded product 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 molded product will become coarse, and the hydrogen embrittlement resistance and deformability of the hot-stamped molded product will decrease. Strength may decrease. Therefore, the heating temperature is preferably less than 1050°C, and the holding time is preferably less than 600 seconds.

In the hot stamping step, it is preferable to take out the heated steel plate for hot stamping from the heating furnace and allow it to cool in the atmosphere, and then start hot stamping in a temperature range of over 750°C. If the starting temperature of hot stamping is 750° C. or lower, ferrite may be excessively produced in the metal structure of the inner layer region of the hot stamped body, and the strength of the hot stamped body may decrease. After molding by hot stamping, the hot stamp molded product is cooled while being held in a mold, and/or the hot stamp molded product is taken out from the mold and cooled by an arbitrary method.

If the cooling rate is low, the volume fraction of martensite may be insufficient in the metal structure of the inner layer region of the hot-stamped body, 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/second or more, 60°C/second or more, or 90°C/second or more. Furthermore, if the cooling stop temperature is high, the volume fraction of martensite may be insufficient in the metal structure of the inner layer region of the hot-stamped body, and the strength of the hot-stamped body may decrease. Therefore, it is preferable that the cooling stop temperature by the above-mentioned cooling is less than 90°C.

In the blasting process, it is preferable that the hot-stamped hot-stamped molded body is blasted with a blasting intensity of more than 0.20 mmN in Almen arc height value. The hot-stamped molded article according to this embodiment has high scale adhesion due to its low Mn content, and when the projection strength is 0.20 mmN or less, the O concentration distribution is favorable in the region near the surface of the hot-stamped molded article. It may be out of control. The Almen arc height value is determined by using cast steel shot, cut wire shot, or conditioned cut wire shot specified in JIS B 2711:2013 as the projectile material, using a pneumatic blasting device, and determining the projection speed, projection time, and projection distance. It is preferable to change and adjust. The Almen arc height value is measured using an Almen strip N piece and an Almen gauge in accordance with JIS B 2711:2013.

By the above method, a hot stamp molded article according to the present embodiment is obtained. Note that after the hot stamp molding, reheating treatment may be performed as long as the strength of the hot stamp molded product is ensured. When performing reheating treatment, the heating temperature is preferably less than Ac ₃ -100°C. If the heating temperature of the reheating treatment is higher than (Ac ₃ points - 100°C), the surface layer region of the hot-stamped molded product will not be sufficiently softened, and the hydrogen embrittlement resistance and deformability of the hot-stamped molded product will decrease. There are cases where A portion of the hot stamp molded body may be reheated by laser irradiation or the like to provide a partially softened region. Further, the hot stamp molded body may be subjected to painting and baking treatments.

Next, an example of the present invention will be described. The conditions in the example are examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is based on this example of conditions. It is not limited. The present invention can adopt various conditions as long as the purpose of the present invention is achieved without departing from the gist of the present invention.

A steel material having the chemical composition shown in Table 1 was obtained by casting molten steel using a vacuum melting furnace. The obtained steel material was heated to 1200° C. and held for 60 minutes, and then hot rolled for 10 passes in a temperature range of 900° C. or higher to obtain a hot rolled steel plate with a thickness of 3.5 mm. After hot rolling, the hot rolled steel sheet was cooled to 540° C. with water spray. The cooling end temperature was defined as the coiling temperature, and the hot rolled steel sheet was charged into an electric heating furnace maintained at this coiling temperature and maintained for 60 minutes. Thereafter, the hot rolled steel sheet was furnace cooled to room temperature at an average cooling rate of 20° C./hour to simulate slow cooling after winding. After the hot-rolled steel sheet was cooled in the furnace, it 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%.

Note that "-" in Table 1 indicates that the content of the element was below the detection limit.
_{The three} Ac points in Table 1 were determined from the change in thermal expansion when the cold-rolled steel sheets in the table were heated at 8° C./second.

The obtained cold rolled steel sheets were annealed using a continuous annealing simulator under the annealing conditions shown in Table 2A and Table 2B. Note that heating was performed at an average heating rate of 8° C./sec to the soaking temperature shown in Table 2A and Table 2B. The atmosphere in the annealing furnace was a nitrogen-hydrogen atmosphere containing 3% by volume of hydrogen, and the dew points were as shown in Tables 2A and 2B. After soaking, an annealed steel plate (a steel plate for hot stamping) was obtained by cooling to room temperature.

Test pieces for GDS analysis were taken from three locations on the obtained steel plate for hot stamping, the surface of the test piece was used as the measurement starting surface, and GDS was performed from the measurement starting surface to a depth of 120 μm in the sheet thickness direction using the method described above. Analysis was carried out. As a result, the average B concentration in a region (surface layer region) from 5 μm deep to 25 μm deep from the surface of the hot stamping steel plate and the B concentration at a position 100 μm deep from the surface were obtained. The number of measurement points from the surface of the steel plate to a depth of 100 μm was 1500 points. The results obtained are shown in Table 2.

Next, a blank plate for hot stamping with a width of 240 mm and a length of 800 mm is taken from the obtained steel plate for hot stamping, and hot stamping is performed to obtain a hat member (hot stamp molded body) having the shape shown in Fig. 1. Ta. In the hot stamping step, the hot stamping blanks were heated under the conditions shown in Table 3 using a gas combustion furnace. Specifically, propane gas was used as the combustion gas, and the heating temperature, holding time, and air ratio were as shown in Table 3. Thereafter, the hot stamp blank was taken out of the heating furnace and allowed to cool, and then placed between molds equipped with a cooling device to perform hat molding at a molding start temperature of 770° C. or higher. Subsequently, the average cooling rate from the molding start temperature to 400°C was set to 50°C/sec or more, and the mold was cooled to a cooling stop temperature of 80°C or less.
Subsequently, cast steel shot having an average particle diameter of about 300 μm was projected onto the obtained hat member using a direct pressure air blasting machine, and blasting was performed under the conditions shown in Table 3.

A test piece was taken from the vertical wall of the obtained hat member, and its chemical composition was measured by the method described above.

In addition, a No. 13B tensile test piece was taken from the vertical wall of the hat member along the longitudinal direction of the hat member in accordance with JIS Z 2241:2011, and a tensile test was conducted at a tensile speed of 10 mm/min. The tensile strength was determined.
When the obtained tensile strength was 1900 MPa or more, it was determined that the product had high strength and passed. On the other hand, when the obtained tensile strength was less than 1900 MPa, it was determined that the product did not have high strength and was rejected.

In addition, test pieces for GDS analysis were taken from three locations on the vertical wall of the hat member, and using the surface of the test piece as the measurement starting surface, the method described above was carried out to a depth of 120 μm in the thickness direction from the measurement starting surface. GDS analysis was performed. As a result, the average B concentration in a region from 5 μm deep to 25 μm deep from the surface (surface layer region), the average O concentration in a region from the surface to 0.5 μm deep from the surface (near surface region), and The B concentration at a depth of 100 μm was determined. The number of measurement points from the surface of the steel plate to a depth of 100 μm was 1500 points.

In addition, a test piece for tissue observation was taken from the vertical wall of the hat member, and after polishing the vertical cross section of this test piece, the method described above was used to conduct a test piece at a depth of 100 μm from the surface to 1/2 the plate thickness (inner layer region). The metal structure was observed.

In addition, a 60 mm square test piece for bending test was taken from the bottom of the hat member, and a bending test was conducted in accordance with the German Automobile Industry Association standard VDA 238-100. The test piece was bent so that the bending ridge direction was perpendicular to the rolling direction of the steel plate for hot stamping, and the bending angle (VDA bending angle) at the time when the bending load decreased by 60 N from the highest point was determined. If a crack occurred before the bending load reached its maximum point, the bending angle at the time the crack occurred was determined and defined as the VDA bending angle.

When the tensile strength of the steel plate constituting the hot-stamped molded product was less than 2300 MPa, the case where the VDA bending angle was 60° or more was judged as having excellent deformability and passed. Moreover, when the tensile strength was 2300 MPa or more, and when the VDA bending angle was 40° or more, it was judged as having excellent deformability and passed. If these conditions were not met, it was determined that the product did not have excellent deformability and was rejected.

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

If the tensile strength of the steel plate constituting the hot-stamped compact was less than 2300 MPa, if no cracking occurred when the applied stress was 1400 MPa, it was judged as having excellent hydrogen embrittlement resistance and passed. . Further, when the tensile strength is 2300 MPa or more, and no cracking occurs when the applied stress is 900 MPa, it is judged as having excellent hydrogen embrittlement resistance and passed. If these conditions were not met, it was determined that the product did not have excellent hydrogen embrittlement resistance and was rejected. Examples that were determined to be passed were written as "OK" in the table, and examples that were judged to be rejected were written as "NG" in the table.

Tables 4A and 4B show the results of measuring the chemical composition of the hot-stamped molded product, the results of measuring the mechanical properties of the hot-stamped molded product, the results of measuring the B and O concentration distribution in the hot-stamped molded product, and the results of measuring the chemical composition of the hot-stamped molded product. The results of evaluating the deformability of and the results of evaluating the hydrogen embrittlement resistance of the hot-stamped molded body are shown.
Note that the contents of the elements other than C in the hot-stamped molded body were omitted because they were the same as the contents of the elements shown in Table 1.

The hot-stamped molded article according to the example of the present invention had a tensile strength of 1900 MPa or more, and had high strength. Furthermore, the average B concentration in the surface layer region was low, the average O concentration in the near-surface region was low, and the deformability and hydrogen embrittlement resistance were excellent. In addition, in the metal structure of the hot-stamped molded article according to the example of the present invention, the volume percentage of martensite is 91.0% or more in the inner layer region, and the total volume percentage of structures other than martensite is 9.0% or less. there were.

On the other hand, in the comparative examples (test numbers 1, 6, 19 and 20) in which the chemical composition of the hot-stamped molded product was outside the scope of the invention, the C content was too low, the B content was too low, or sol. Since the Al content was too high, the tensile strength of the hot-stamped molded product was less than 1900 MPa, resulting in poor strength.

In test number 17, the Mn content was too high, so the hydrogen embrittlement resistance of the hot-stamped molded product was poor.

In test number 18, the C content was too high, so the deformability and hydrogen embrittlement resistance of the hot-stamped molded product were poor. In addition, in the tensile test, early breakage occurred and the tensile strength could not be determined, and the breaking strength was less than 1900 MPa.

The chemical composition of the hot-stamped molded body was within the preferred range, but the comparative example test numbers 2, 4, 5, 8, 9, 11 to 13, 15, 16, 27, and 29 had manufacturing conditions outside the preferred range. , the average B concentration in the surface layer region or the average O concentration in the near-surface region was out of the invention range. Therefore, the deformability and/or hydrogen embrittlement resistance of the hot-stamped molded product was poor.

According to the above aspect of the present invention, it is possible to provide a hot-stamped molded article having high strength and excellent hydrogen embrittlement resistance and deformability.

Claims (2)

1. A hot stamped molded body comprising a steel plate,
   All or part of the steel plate has a chemical composition in mass%,
   C: more than 0.32%, less than 0.70%,
   Si: less than 2.00%,
   Mn: 0.01% or more, less than 1.00%,
   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: 0.0005-0.0200%,
   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%,
   Contains As: 0 to 0.100% and Bi: 0 to 0.0500%,
   The remainder consists of Fe and impurities,
   The tensile strength is 1900 MPa or more,
   The average B concentration in a region from 5 μm deep to 25 μm deep from the surface of the steel plate is 0.700 times or less of the B concentration at a position 100 μm deep from the surface,
   A hot-stamped molded article characterized in that an average O concentration in a region from the surface to a depth of 0.5 μm from the surface is 4.000% by mass 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: 0.01-2.00%,
   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: 0.0001-0.1000%,
   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 molded article according to claim 1, characterized in that it contains one or more of the group consisting of:

Images