WO2023199635A1

WO2023199635A1 - Hot-stamp-formed article

Info

Publication number: WO2023199635A1
Application number: PCT/JP2023/007780
Authority: WO
Inventors: 真吾藤中; 純芳賀; 由梨戸田
Original assignee: 日本製鉄株式会社
Priority date: 2022-04-14
Filing date: 2023-03-02
Publication date: 2023-10-19
Also published as: MX2024009297A; KR20240132338A; CN118647743A; JPWO2023199635A1

Abstract

This hot-stamp-formed article has a specified chemical composition, in which the area ratio of bainite in a surface layer region is more than 10%, the maximum value of a pole density in a texture is 4.0 or less, and the de-B index is 0.05 or more.

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. 2022-067023 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. Automotive parts are manufactured by press forming, but as the strength of steel plates increases, not only does the forming load increase, but also formability decreases. Therefore, in high-strength steel sheets, formability into members with complex shapes becomes an issue.

In order to solve the above-mentioned problems, the application of hot stamping technology is progressing, in which press forming is performed after heating the steel plate to a high temperature in the austenite region where the steel plate becomes soft. Hot stamping is attracting attention as a technology that achieves both moldability into automobile parts and strength of automobile parts by performing quenching treatment in a mold at the same time as press working.

For example, Patent Document 1 discloses a high-yield ratio, high-strength electrogalvanized steel sheet in which the amount of diffusible hydrogen in the steel is 0.20 mass ppm or less and excellent bendability.

International Publication No. 2020/079925

In order to further reduce the weight of automobile bodies than before, it is effective to increase the strength of steel plates. In order to increase the strength of a steel sheet, it is effective to make the metal structure of the steel sheet a metal structure in which martensite with a high dislocation density is the main phase. However, in a metal structure whose main phase is martensite with a high dislocation density, a large amount of hydrogen penetrating from the outside is trapped, making hydrogen embrittlement cracking more likely to occur in automobile parts.

Hydrogen embrittlement cracking is a phenomenon in which a steel member under high stress during use breaks down due to hydrogen penetrating into the steel from the external environment. This phenomenon is also called delayed fracture because of the manner in which the fracture occurs. It is generally known that hydrogen embrittlement cracking of a steel plate occurs more easily as the tensile strength of the steel plate increases. This is thought to be because the higher the tensile strength of the steel plate, the greater the stress remaining in the steel plate after forming the part. This susceptibility to hydrogen embrittlement cracking (delayed fracture) is called hydrogen embrittlement resistance.

In Patent Document 1, bendability is considered, but hydrogen embrittlement resistance is not considered.

The present invention has been made in view of the above problems. An object of the present invention is to provide a hot-stamped molded article that has high strength and excellent hydrogen embrittlement resistance.

The gist of the invention is as follows.
(1) The hot stamp molded article according to one embodiment of the present invention has a chemical composition in mass %,
C: more than 0.40%, less than 0.70%,
Si: 0.010-3.000%,
Mn: 0.10% or more, less than 0.60%,
P: 0.100% or less,
S: 0.0100% or less,
N: 0.0200% or less,
O: 0.0200% or less,
Al: 0.0010-0.5000%,
Nb: 0.0010-0.1000%,
Ti: 0.010-0.200%,
Cr: 0.010-0.800%,
Mo: 0.0010-1.0000%,
B: 0.0005-0.0200%,
Co: 0-4.00%,
Ni: 0-3.00%,
Cu: 0-3.00%,
V: 0 to 3.00%,
W: 0-3.00%,
Ca: 0-1.0000%,
Mg: 0 to 1.0000%,
REM: 0-1.0000%,
Sb: 0 to 1.000%,
Sn: 0-1.000%,
Zr: 0 to 1.000%,
As: 0 to 0.100%, and
The remainder: Fe and impurities,
In a surface layer region that is a region from the surface of the hot stamp molded body to a depth of 1/25 of the plate thickness from the surface,
The area ratio of bainite is more than 10%,
The maximum value of the polar density of the texture is 4.0 or less,
The anti-B index is 0.05 or more.
(2) The hot-stamped molded article according to (1) above has the chemical composition in mass %,
Co: 0.01-4.00%,
Ni: 0.01 to 3.00%,
Cu: 0.01-3.00%,
V: 0.01 to 3.00%,
W: 0.01-3.00%,
Ca: 0.0001-1.0000%,
Mg: 0.0001 to 1.0000%,
REM: 0.0001-1.0000%,
Sb: 0.001 to 1.000%,
Sn: 0.001 to 1.000%,
Zr: 0.001 to 1.000%, and As: 0.001 to 0.100%
It may contain one or more selected from the group consisting of:

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

FIG. 3 is a diagram for explaining how to obtain a B-free index.

The present inventors have succeeded in improving the hydrogen embrittlement resistance of hot-stamped bodies by generating a desired amount of bainite in the surface layer region, creating a texture with a desired crystal orientation, and achieving a desired B removal index. It was found that the chemical properties can be improved.

The present inventors have found that in order to obtain a hot-stamped molded body having the above characteristics, it is particularly effective to perform annealing under desired conditions when manufacturing a steel plate to be subjected to hot-stamping.

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

Note that the numerically limited ranges described below with "~" in between include the lower limit and the upper limit. 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 %.

The hot-stamped molded article according to this embodiment has a chemical composition in mass %: C: more than 0.40% and 0.70% or less, Si: 0.010 to 3.000%, and Mn: 0.10%. or more, less than 0.60%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.0010 to 0.5000% , Nb: 0.0010-0.1000%, Ti: 0.010-0.200%, Cr: 0.010-0.800%, Mo: 0.0010-1.0000%, B: 0.0005 ~0.0200%, and the balance: Contains Fe and impurities.
Each element will be explained below.

C: More than 0.40% and 0.70% or less C is an element that improves the strength of the hot stamp molded product. If the C content is less than 0.40%, the desired strength cannot be obtained in the hot-stamped molded product. Therefore, the C content is set to exceed 0.40%. The C content is preferably 0.42% or more or 0.44% or more.
On the other hand, if the C content exceeds 0.70%, the amount of hydrogen trapped in martensite increases, making it impossible to obtain excellent hydrogen embrittlement resistance. Therefore, the C content is set to 0.70% or less. Preferably, the C content is 0.65% or less or 0.60% or less.

Si: 0.010-3.000%
Si is an element that improves the strength of the hot stamp molded product through solid solution strengthening. If the Si content is less than 0.010%, desired strength cannot be obtained. Therefore, the Si content is set to 0.010% or more. The Si content is preferably 0.050% or more, 0.100% or more, or 0.150% or more.
On the other hand, if the Si content exceeds 3.000%, the amount of ferrite increases and a desired metal structure cannot be obtained. Therefore, the Si content is set to 3.000% or less. The Si content is preferably 2.000% or less, 1.000% or less, or 0.600% or less.

Mn: 0.10% or more, less than 0.60% Mn is an element that improves the hardenability of steel and increases the strength of hot stamped compacts. In order to obtain the desired strength, the Mn content is set to 0.10% or more. The Mn content is preferably 0.20% or more or 0.25% or more.
On the other hand, if the Mn content is 0.60% or more, the desired texture cannot be obtained. Therefore, the Mn content is made less than 0.60%. Preferably, the Mn content is 0.55% or less, 0.50% or less or 0.45% or less.

P: 0.100% or less P is an impurity element and reduces grain boundary strength by segregating at grain boundaries. This deteriorates the hydrogen embrittlement resistance of the hot stamped body. When the P content exceeds 0.100%, the above effects become significant. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.050% or less or 0.010% or less.
The lower limit of the P content is not particularly limited, but may be 0%. However, reducing the P content to less than 0.0001% significantly increases the cost of removing P, which is economically unfavorable. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more.

S: 0.0100% or less S is an impurity element and forms inclusions in steel. These inclusions trap a large amount of hydrogen and form regions where the hydrogen concentration locally increases, thereby degrading the hydrogen embrittlement resistance of the hot-stamped compact. When the S content is more than 0.0100%, the above effects become significant. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, 0.0050% or less, or 0.0030% or less.
The lower limit of the S content is not particularly limited, but may be 0%. However, if the S content is reduced to less than 0.0001%, the cost for removing S will increase significantly, which is economically unfavorable. Therefore, the S content may be 0.0001% or more, 0.0002% or more, 0.0003% or more, or 0.0010% or more.

N: 0.0200% or less N is an impurity element and forms nitrides in steel. This nitride traps a large amount of hydrogen and forms regions where the hydrogen concentration locally increases, thereby degrading the hydrogen embrittlement resistance of the hot stamped body. When the N content exceeds 0.0200%, the above effects become significant. Therefore, the N content is set to 0.0200% or less. The N content is preferably 0.0150% or less, 0.0100% or less, 0.0060% or less, or 0.0040% or less.
The lower limit of the N content is not particularly limited, but may be 0%. However, reducing the N content to less than 0.0001% significantly increases the cost of removing N, which is economically unfavorable. Therefore, the N content may be 0.0001% or more or 0.0010% or more.

O: 0.0200% or less When O is contained in large amounts in steel, it forms coarse oxides. This oxide traps a large amount of hydrogen and forms regions where the hydrogen concentration locally increases, thereby deteriorating the hydrogen embrittlement resistance of the hot stamped product. When the O content exceeds 0.0200%, the above effects become significant. Therefore, the O content is set to 0.0200% or less. The O content is preferably 0.0100% or less, 0.0070% or less, or 0.0040% or less.
The O content may be 0%, but in order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be 0.0005% or more or 0.0010% or more.

Al: 0.0010-0.5000%
Al is an element that has the effect of deoxidizing molten steel and making the steel sound. When the Al content is less than 0.0010%, deoxidation is not performed sufficiently and coarse oxides are produced. This oxide traps a large amount of hydrogen and forms regions where the hydrogen concentration locally increases, thereby deteriorating the hydrogen embrittlement resistance of the hot stamped body. Therefore, the Al content is set to 0.0010% or more. The Al content is preferably 0.0050% or more, 0.0100% or more, or 0.0300% or more.
On the other hand, if the Al content exceeds 0.5000%, coarse oxides will be generated in the steel. This oxide traps a large amount of hydrogen and forms regions where the hydrogen concentration locally increases, thereby deteriorating the hydrogen embrittlement resistance of the hot stamped product. Therefore, the Al content is set to 0.5000% or less. The Al content is preferably 0.4000% or less, 0.3000% or less, 0.2000% or less, or 0.1000% or less.

Nb: 0.0010-0.1000%
Nb is an element that forms carbonitrides in steel and improves the strength of hot stamped products through precipitation strengthening. If the Nb content is less than 0.0010%, desired strength cannot be obtained. Therefore, the Nb content is set to 0.0010% or more. The Nb content is preferably 0.0050% or more, 0.0090% or more, or 0.0150% or more.
On the other hand, if the Nb content exceeds 0.1000%, a large amount of carbonitrides will be generated in the steel, deteriorating the hydrogen embrittlement resistance of the hot stamped body. Therefore, the Nb content is set to 0.1000% or less. The Nb content is preferably 0.0800% or less or 0.0600% or less.

Ti: 0.010-0.200%
Ti is an element that forms carbonitrides in steel and improves the strength of hot stamped products through precipitation strengthening. If the Ti content is less than 0.010%, desired strength cannot be obtained. Therefore, the Ti content is set to 0.010% or more. The Ti content is preferably 0.020% or more or 0.025% 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, resulting in sites where the hydrogen concentration locally increases, resulting in the durability of the hot-stamped compact. Hydrogen embrittlement properties deteriorate. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably 0.150% or less, 0.090% or less, 0.080% or less, 0.070% or less, 0.060% or less, or 0.050% or less.

Cr:0.010~0.800%
Cr is an element that increases the strength of the hot-stamped molded product by forming a solid solution in the prior austenite grains during heating before hot-stamping. If the Cr content is less than 0.010%, desired strength cannot be obtained. Therefore, the Cr content is set to 0.010% or more. The Cr content is preferably 0.100% or more, 0.150% or more, or 0.200% or more.
On the other hand, if the Cr content exceeds 0.800%, a desired texture cannot be obtained in the hot-stamped molded product, and the hydrogen embrittlement resistance deteriorates. Therefore, the Cr content is set to 0.800% or less. The Cr content is preferably 0.700% or less, 0.500% or less, or 0.400% or less.

Mo: 0.0010-1.0000%
Mo is an element that increases the strength of the hot-stamped molded product by forming a solid solution in the prior austenite grains during heating before hot-stamping. If the Mo content is less than 0.0010%, desired strength cannot be obtained. Therefore, the Mo content is set to 0.0010% or more. The Mo content is preferably 0.0100% or more, 0.0500% or more, or 0.1000% or more.
On the other hand, if the Mo content exceeds 1.0000%, a desired texture cannot be obtained in the hot-stamped molded product, and the hydrogen embrittlement resistance deteriorates. Therefore, the Mo content is set to 1.0000% or less. Mo content is preferably 0.8000% or less, 0.6000% or less, or 0.4000% or less.

B: 0.0005-0.0200%
B is an element that improves the hardenability of steel. If the B content is less than 0.0005%, desired strength cannot be obtained. Therefore, the B content is set to 0.0005% or more. The B content is preferably 0.0010% or more or 0.0015% or more.
On the other hand, if the B content exceeds 0.0200%, coarse intermetallic compounds will be formed in the hot stamped product. This intermetallic compound becomes a site where the hydrogen concentration locally increases, degrading the hydrogen embrittlement resistance of the hot stamped product. Therefore, the B content is set to 0.0200% or less. The B content is preferably 0.0150% or less, 0.0100% or less, 0.0080% or less, 0.0040% or less, or 0.0030% or less.

The remainder of the chemical composition of the hot-stamped molded body 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 hot stamp molded product may contain the following elements as optional elements. When the following arbitrary elements are not included, the content is 0%.

Co: 0-4.00%
Co is an element that improves the strength of the hot stamp molded product through solid solution strengthening. To ensure this effect, the Co content is preferably 0.01% or more. More preferably, the Co content is 0.05% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, the Co content is set to 4.00% or less. If necessary, the upper limit of the Co content may be set to 1.00%, 0.50%, 0.10%, 0.05% or 0.02%.

Ni: 0-3.00%
Ni has the effect of increasing the strength of the hot-stamped molded product by solidly dissolving in the prior austenite grains during heating before hot-stamping. To ensure this effect, the Ni content is preferably 0.01% or more.
On the other hand, since the above effect is saturated even if Ni is contained in a large amount, it is preferable that the Ni content is 3.00% or less. If necessary, the upper limit of the Ni content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05% or 0.02%.

Cu: 0-3.00%
Cu has the effect of increasing the strength of the hot-stamped molded product by solidly dissolving in the prior austenite grains during heating before hot-stamping. To ensure this effect, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.05% or more.
On the other hand, since the above effects are saturated even if Cu is contained in a large amount, the Cu content is preferably 3.00% or less. If necessary, the upper limit of the Cu content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05% or 0.02%.

V: 0-3.00%
V has the effect of forming carbonitrides in the steel and improving the strength of the hot stamped product through precipitation strengthening. To ensure this effect, the V content is preferably 0.01% or more. The V content is more preferably 0.05% or more.
On the other hand, if the V content exceeds 3.00%, a large amount of coarse carbonitrides will be generated in the steel. These carbonitrides become sites where the hydrogen concentration locally increases, degrading the hydrogen embrittlement resistance of the hot stamped compact. Therefore, the V content is set to 3.00% or less. If necessary, the upper limit of the V content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05% or 0.02%.

W: 0-3.00%
W has the effect of improving the strength of the hot stamp molded product. To ensure this effect, the W content is preferably 0.01% or more. The W content is preferably 0.05% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, the W content is set to 3.00% or less. If necessary, the upper limit of the W content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05% or 0.02%.

Ca: 0~1.0000%
Ca is an element that suppresses the formation of oxides that become the starting point of fracture, and contributes to improving the hydrogen embrittlement resistance of the hot stamped compact. To ensure this effect, the Ca content is preferably 0.0001% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, the Ca content is set to 1.0000% or less. If necessary, the upper limit of the Ca content may be set to 0.1000%, 0.0100%, 0.0050%, 0.0010%, 0.0005% or 0.0002%.

Mg: 0-1.0000%
Mg forms oxides and sulfides in molten steel, suppresses the formation of coarse MnS, and disperses many fine oxides, thereby refining the metal structure. This contributes to improving the hydrogen embrittlement resistance of the hot-stamped molded body. In order to reliably obtain these effects, it is preferable that the Mg content is 0.0001% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, the Mg content is set to 1.0000% or less. If necessary, the upper limit of the Mg content may be set to 0.1000%, 0.0100%, 0.0050%, 0.0010%, 0.0005% or 0.0002%.

REM: 0~1.0000%
REM suppresses the formation of coarse oxides that become sites with local increases in hydrogen concentration. This contributes to improving the hydrogen embrittlement resistance of the hot-stamped molded body. In order to reliably obtain this effect, it is preferable that the REM content is 0.0001% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, the REM content is set to 1.0000% or less. If necessary, the upper limit of the REM content may be set to 0.1000%, 0.0100%, 0.0050%, 0.0010%, 0.0005% or 0.0002%.
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.

Sb: 0-1.000%
Sb suppresses the formation of coarse oxides that become sites that are accompanied by a local increase in hydrogen concentration. This contributes to improving the hydrogen embrittlement resistance of the hot-stamped molded body. To ensure this effect, the Sb content is preferably 0.001% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, the Sb content is set to 1.000% or less. If necessary, the upper limit of the Sb content may be set to 0.100%, 0.050%, 0.020%, 0.010%, 0.005% or 0.002%.

Sn: 0-1.000%
Sn suppresses the formation of coarse oxides that become sites that cause a local increase in hydrogen concentration. This contributes to improving the hydrogen embrittlement resistance of the hot-stamped molded body. To ensure this effect, the Sn content is preferably 0.001% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, the Sn content is set to 1.000% or less. If necessary, the upper limit of the Sn content may be set to 0.100%, 0.050%, 0.020%, 0.010%, 0.005% or 0.002%.

Zr: 0-1.000%
Zr suppresses the formation of coarse oxides that become sites associated with local increases in hydrogen concentration. This contributes to improving the hydrogen embrittlement resistance of the hot-stamped molded body. To ensure this effect, the Zr content is preferably 0.001% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, the Zr content is set to 1.000% or less. If necessary, the upper limit of the Zr content may be set to 0.100%, 0.050%, 0.020%, 0.010%, 0.005% or 0.002%.

As: 0~0.100%
As reduces the austenite single-phase temperature, thereby refining the prior austenite grains. This contributes to improving the hydrogen embrittlement resistance of the hot-stamped molded body. To ensure this effect, the As content is preferably 0.001% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, the As content is set to 0.100% or less. If necessary, the upper limit of the As content may be set to 0.100%, 0.050%, 0.020%, 0.010%, 0.005% or 0.002%.

The chemical composition of the hot-stamped molded article described above may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Note that C and S may be measured using a combustion-infrared absorption method, N using an inert gas melting-thermal conductivity method, and O using an inert gas melting-non-dispersive infrared absorption method.
If the surface of the hot-stamped body is provided with a plating layer, paint film, etc., the chemical composition is analyzed after removing the plating layer, paint film, etc. by mechanical grinding.

Next, the metallographic structure of the hot-stamped molded body according to this embodiment will be explained.
The hot-stamped molded body according to the present embodiment has an area of bainite in a surface layer region that is a region from the surface of the hot-stamped molded body to a depth of 1/25 of the plate thickness (thickness of the hot-stamped molded body). ratio is more than 10%, the maximum value of the polar density of the texture is 4.0 or less, and the B removal index is 0.05 or more.

In this embodiment, the surface layer region refers to a region from the surface of the hot-stamped molded product to a depth of 1/25 of the plate thickness (thickness of the hot-stamped molded product) from the surface.
In addition, when the hot-stamped molded product has a plating layer, a paint film, etc. on the surface, the "surface" here refers to the interface between the plating layer and the base steel plate, and for convenience, the plating from the hot-stamped molded product is Exclude layers, paint films, etc. Specifically, when the surface of the hot-stamped molded product has a plating layer, a paint film, etc., as described below, for convenience, the area where the iron concentration is less than 90% by mass in GD-OES measurement, that is, the plating layers, paint films, etc. are excluded from the hot-stamped molded body, and the measurement point where the iron concentration is 90% by mass (that is, the interface between the base steel material and the plating layer, etc.) is regarded as the surface of the hot-stamped molded body. As mentioned above, the plating layer, paint film, etc. were excluded from the hot-stamped product, but the thickness of the plating layer, paint film, etc. is very small compared to the plate thickness (thickness) of the hot-stamped product, so it can be ignored. If possible (however, in the case of only a plating layer, the thickness of the plating layer is often very small and can be ignored in most cases), when measuring the plate thickness (thickness) of a hot stamp molded product, use a hot stamp. The plate thickness (thickness) of the molded body may be the plate thickness (thickness) including the plating layer, paint film, etc.

(Surface region) Area ratio of bainite: more than 10% By generating bainite in the surface region, the dislocation density in the surface region can be reduced. As a result, the intrusion of hydrogen from the external environment can be suppressed, and the hydrogen embrittlement resistance of the hot-stamped molded article can be improved. Furthermore, by generating bainite in the surface layer region, excessive softening of the surface layer can be suppressed, so that the hydrogen embrittlement resistance can be further enhanced while maintaining the load capacity of the member. If the area ratio of bainite in the surface layer region is less than 10%, the hydrogen embrittlement resistance of the hot-stamped molded product deteriorates. Therefore, the area ratio of bainite is set to exceed 10%. Preferably, it is 20% or more, 40% or more, or 60% or more.
The upper limit of the area ratio of bainite is not particularly limited, but may be 100%, 90%, or 80%.

In addition to bainite, the metal structure of the surface layer region includes 0 to 90% (0% or more, 90% or less) of martensite, and a total of 0 to 65% (0% or more, 65% or less) of ferrite and residual Austenite may be included. Note that martensite in this embodiment includes untempered martensite (fresh martensite) and tempered martensite.
The area ratio of the metal structure is calculated for the surface layer region (the region from the surface to a depth of 1/25 of the plate thickness) by the following method.

Cut out a sample so that the thickness section parallel to the rolling direction can be observed from an arbitrary position 50 mm or more away from the end surface of the hot stamped body (if the sample cannot be taken from this position, avoid the edge). Although the size of the sample depends on the measuring device, it should be large enough to allow observation of about 10 mm in the rolling direction.

After polishing the cross section of the sample above using #600 to #1500 silicon carbide paper, polish it to a mirror surface using a liquid made by dispersing diamond powder with a particle size of 1 to 6 μm in diluted liquid such as alcohol or pure water. . Next, the observation surface is finished by electrolytic polishing. At any position in the longitudinal direction of the sample cross section, a region from the surface of the hot-stamped molded body with a length of 50 μm to a depth of 1/25 of the plate thickness from the surface was measured using an electron backscatter diffraction method at a measurement interval of 0.1 μm. The crystal orientation information is obtained by measuring. For the measurement, an EBSD analysis device consisting of a thermal field emission scanning electron microscope and an EBSD detector may be used. For example, an EBSD analysis device consisting of a JEOL JSM-7001F and a TSL DVC5 type detector may be used. You can use At this time, the degree of vacuum in the EBSD analyzer may be 9.6×10 ⁻⁵ Pa or less, the acceleration voltage may be 15 kV, and the irradiation current level may be 13.

Using the obtained crystal orientation information and the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer, those whose crystal structure is fcc are determined to be retained austenite. By calculating the area ratio of this retained austenite, the area ratio of retained austenite is obtained. Next, in the region where the crystal structure is bcc, using the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" included with the EBSD analyzer, 5° grain boundaries are regarded as grain boundaries. Under these conditions, a region where "Grain Average Misorientation" is greater than 0.50° and less than 0.75° is extracted as bainite. The area ratio of bainite is obtained by calculating the area ratio of the extracted bainite.
Subsequently, a region where "Grain Average Misorientation" is 0.5° or less is extracted as ferrite. The area ratio of ferrite is obtained by calculating the area ratio of the extracted ferrite. The remaining area (area where "Grain Average Misorientation" exceeds 0.75°) is extracted as martensite, and the area rate of martensite is calculated by calculating the area rate of martensite.
In addition, in this embodiment, the rolling direction of the hot stamp molded body is determined by the following method.
First, a test piece is taken from an arbitrary position 50 mm or more away from the end of the hot-stamped molded body so that the cross-section of the plate thickness can be observed. After finishing the thickness section of the sampled test piece by mirror polishing, it is observed using an optical microscope at 100x, 200x, 500x, and 1000x magnification. Depending on the size of the inclusion, select an observation result with an appropriate magnification that allows the size of the inclusion to be measured. The observation range is a width of 500 μm or more and the full thickness of the plate, and areas with low brightness are determined to be inclusions. When observing, you may observe from multiple fields of view. Next, the same method as above is applied to the plane parallel to the plane rotated in 5° increments in the range of 0° to 180° with the thickness direction as the axis, using the thickness cross section initially observed by the above method as a reference. Observe the cross section according to the method. The average value of the lengths of the long axes of the plurality of inclusions in each cross section is calculated for each cross section. The cross section in which the average length of the long axes of the obtained inclusions is maximum is specified. A direction parallel to the longitudinal direction of the inclusion in the cross section is determined as the rolling direction.

(Surface region) Crystal orientation in the surface region: The maximum value of the polar density of the texture is 4.0 or less By controlling the texture in the surface region, it is possible to suppress the intrusion of hydrogen from the external environment into the surface region, and hot stamping The hydrogen embrittlement resistance of the molded body can be improved. If the maximum value of the polar density of the texture in the surface region exceeds 4.0, the hydrogen embrittlement resistance of the hot-stamped molded product deteriorates. Therefore, the maximum value of the polar density of the texture in the surface layer region is set to 4.0 or less. Preferably, it is 3.5 or less, 3.0 or less, or 2.5 or less.
The lower limit of the polar density of the texture in the surface layer region is not particularly limited, but may be set to 1.0 or 1.2.

The texture in the surface layer region is obtained for the surface layer region (the region from the surface to a depth of 1/25 of the plate thickness) by the following method.
A sample is cut out from an arbitrary position 50 mm or more away from the end surface of the hot-stamped compact (if the sample cannot be taken from this position, avoid the end) so that the thickness cross section parallel to the rolling direction can be observed. Although the size of the sample depends on the measuring device, it should be large enough to allow observation of about 10 mm in the rolling direction.

After polishing the cross section of the sample above using #600 to #1500 silicon carbide paper, polish it to a mirror surface using a liquid made by dispersing diamond powder with a particle size of 1 to 6 μm in diluted liquid such as alcohol or pure water. . Next, it is finished by electrolytic polishing. At any position in the longitudinal direction of the sample cross section, a region with a length of 1000 μm from the surface to a depth of 1/25 of the plate thickness is measured using electron backscatter diffraction at measurement intervals of 5.0 μm to determine the crystal orientation. get information. For measurement, an EBSD analysis device consisting of a thermal field emission scanning electron microscope and an EBSD detector may be used. For example, an EBSD analysis device consisting of a JEOL JSM-7001F and a TSL DVC5 type detector may be used. Just use it. At this time, the degree of vacuum in the EBSD analyzer may be 9.6×10 ⁻⁵ Pa or less, the acceleration voltage may be 15 kV, and the irradiation current level may be 13.

Using the "Texture" function installed in the software "OIM Analysis (registered trademark)" that comes with the EBSD analyzer, the obtained crystal orientation information is used to calculate a harmonic function (Harmonic Series) for crystal grains whose crystal structure is bcc. Intensity calculations are performed using (Expansion). At this time, the expansion order is 16, and the half width when applied to Gaussian distribution is 5°. Next, the "Texture Plot" function is used for the output file after the strength calculation to output a φ2=45° cross section in the crystal orientation distribution function (ODF). The maximum value of the polar density in the φ2=45° cross section is defined as the polar density of the texture in the surface layer region.

(Surface layer region) B removal index: 0.05 or more The B removal index is an index that quantitatively represents the amount of decrease in B concentration in the surface layer region. By reducing the B concentration in the surface layer region, the strength of the prior austenite before transformation is reduced, the deformability of the prior austenite grains is improved, and randomly oriented crystal grains are more likely to be generated in the surface layer region. If the B removal index in the surface layer region is less than 0.05, crystal grains with a desired texture cannot be obtained in the surface layer region. Therefore, the de-B index is set to 0.05 or more. Preferably, it is 0.20 or more, 0.30 or more, or 0.35 or more.
The upper limit of the B removal index is not particularly limited, but may be 1.00, 0.80, or 0.60.

The B removal index in the surface layer region is obtained by the following method.
Glow Discharge Optical Emission Spectrometry (GD-OES: Marcus type high frequency glow discharge optical emission spectrometer, GD-PROFILER-HR manufactured by Horiba, Ltd.) was used to determine the element concentration distribution in the thickness direction of the hot stamped compact. Measure. The measurement conditions are an analysis diameter of 4 mmφ, a sputtering rate of 4 μm/min, an argon pressure of 600 Pa, an RF output of 35 W, and a measurement interval of 0.02 μm or less. Measurements are performed for all elements contained in the hot stamped compact.

Note that when the hot-stamped molded body has a plating layer or the like on the surface, the "surface" here refers to the interface between the plating layer or the like and the base steel plate. If the surface has a plating layer, paint film, etc., mechanical polishing or Part or all of the plating layer, paint, etc. is removed by chemical polishing, and then subjected to GD-OES measurement. In the GD-OES measurement, the measurement point where the iron concentration is 90% by mass is regarded as the surface of the hot stamped body. In addition, in the following description, for convenience of explanation, the hot-stamped molded body may be referred to as a base material steel plate.

Next, the B concentration is measured from the surface of the hot-stamped molded article to a depth of at least 100 μm from the surface. After measuring the B concentration at a depth of 100 μm from the surface, the absolute value of the difference between the average B concentration in the 80 to 100 μm region and the maximum measured B concentration in the 80 to 100 μm region is 0.0006. % by mass or less, and the absolute value of the difference between the average value of the B concentration in the 80 to 100 μm region and the minimum value of the measured B concentration in the 80 to 100 μm region is 0.0006 mass % or less , the measurement of the B concentration in the depth direction is completed at a depth of 100 μm from the surface.
If the requirements for ending the measurement are not met, the measurement of the B concentration in the depth direction is continued. Then, each time a new B concentration measurement value is obtained in the depth direction, calculate the average value of the B concentration in an area of 20 μm from the deepest part to the surface side, and from the deepest part to the surface side. The absolute value of the difference between the average value of B concentration in a region of 20 μm from the deepest part to the maximum value of the measured value of B concentration in a region of 20 μm from the deepest part to the surface side is 0.0006% by mass or less, and , the absolute value of the difference between the average value of the B concentration in the region of 20 μm from the deepest part to the surface side and the minimum value of the measured value of B concentration in the region of 20 μm from the deepest part to the surface side is 0. If it is .0006% by mass or less, the measurement of the B concentration in the depth direction ends at that position. For example, if the measured value of the B concentration is obtained at a depth of 150 μm from the surface, the average value of the B concentration in the region 130 to 150 μm deep from the surface and the measured value of the B concentration in the region 130 to 150 μm deep from the surface The absolute value of the difference between the maximum value of If the absolute value of the difference from the minimum value is 0.0006% by mass or less, the measurement of the B concentration in the depth direction is terminated at a depth of 150 μm from the surface.
Even if the measurement of the B concentration in the depth direction cannot be completed because the requirements for completing the measurement described above are not met, the B concentration in the depth direction will be terminated as soon as the measurement of the B concentration at a depth of 200 μm from the surface is completed. Finish the measurement. Then, at the point when the measurement of the B concentration in the depth direction is completed, the area from the deepest part (the deepest position where the B concentration used for calculating the B removal index was obtained) to the region 20 μm from the deepest part to the surface side is The average value of the B concentration (hereinafter, the average value of the B concentration in this region will be referred to as the average B concentration at the deepest part of 20 μm) is used in the calculation of the B removal index below.
For convenience of measurement, for example, after measuring the B concentration from the surface to a depth of 200 μm, the above-mentioned termination condition for B concentration measurement in the depth direction is satisfied in a region 100 to 200 μm deep from the surface. The shallowest depth position may be found, and if that position is found, the B removal index may be calculated without using the B concentration measurement results at positions deeper than that depth position. For example, the B concentration may be measured from the surface to a depth of 200 μm, and in this case, in a region 100 μm or more from the surface, the shallowest depth position that satisfies the termination condition for the B concentration measurement in the depth direction. If there is, it is assumed that the measurement has ended at that depth position, and the B removal index is calculated.

The amount of decrease in B concentration per unit depth in the region from the deepest part to 20 μm from the deepest part to the surface side of the hot stamped compact (the B concentration at each measurement point was subtracted from the average B concentration at the deepest part of 20 μm) value) is calculated, and the integral value of the product of the unit depth and the amount of decrease in B concentration is determined to determine the area of the B-deficient region (area of region A in FIG. 1). However, if the value obtained by subtracting the B concentration at each measurement point from the average B concentration at the deepest 20 μm is negative, it is integrated as 0 (because of the de-B phenomenon near the surface, the average B concentration at the deepest 20 μm is In most cases, the B concentration at a point is small, and this integral value is positive.) Next, the product of the average B concentration at the deepest part of 20 μm and the length of 200 μm is calculated as a reference area (area of rectangular region B in FIG. 1). The value obtained by dividing the B deficient area (area of region A) by the reference area (area of region B) is defined as the B depletion index (area of region A/area of region B). Note that even in the case where the above-mentioned requirement of finishing the measurement up to 200 μm from the surface is satisfied, the reference area (area of region B) is calculated by assuming that the length by which the average B concentration at the deepest part of 20 μm is multiplied is 200 μm.

The metallographic structure of the region other than the surface region of the hot-stamped compact (for example, the region from the surface of the hot-stamped compact to a depth of 4/16 of the plate thickness to a depth of 5/16 of the plate thickness from the surface) is as follows: There are no particular limitations as long as the desired strength and hydrogen embrittlement resistance properties can be obtained, but for example, martensite and bainite with a total area percentage of 90 to 100% (90% or more, 100% or less), and 0 to 100% martensite and bainite It may consist of 10% (0% or more, 10% or less) of ferrite and retained austenite.
The metal structure in areas other than the surface layer area is measured by the following method. Note that the metallographic structure in a region other than the surface layer region is measured from a depth of 1/8 of the plate thickness from the surface to a depth of 3/8 of the plate thickness from the surface. The reason for this is that the metal structure in this region shows a typical metal structure of a hot-stamped molded product.

After polishing the cross section of the sample above using #600 to #1500 silicon carbide paper, polish it to a mirror surface using a liquid made by dispersing diamond powder with a particle size of 1 to 6 μm in diluted liquid such as alcohol or pure water. . Next, the observation surface is finished by electrolytic polishing. At any position in the longitudinal direction of the sample cross section, a region with a length of 50 μm and a depth of 1/8 of the plate thickness from the surface to a depth of 3/8 of the plate thickness from the surface is measured with an electron back at a measurement interval of 0.1 μm. Obtain crystal orientation information by measuring by scattering diffraction method. For measurement, an EBSD analysis device consisting of a thermal field emission scanning electron microscope and an EBSD detector may be used. For example, an EBSD analysis device consisting of a JEOL JSM-7001F and a TSL DVC5 type detector may be used. Just use it. At this time, the degree of vacuum in the EBSD analyzer may be 9.6×10 ⁻⁵ Pa or less, the acceleration voltage may be 15 kV, and the irradiation current level may be 13.

Using the obtained crystal orientation information and the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer, those whose crystal structure is fcc are determined to be retained austenite. By calculating the area ratio of this retained austenite, the area ratio of retained austenite is obtained. Next, regions having a bcc crystal structure are determined to be bainite, martensite, and ferrite. Regarding these regions, using the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" included with the EBSD analyzer, "Grain Average Misorientation" is performed under conditions where 5° grain boundaries are regarded as grain boundaries. A region where "Average Misorientation" is 0.5° or less is extracted as ferrite. The area ratio of ferrite is obtained by calculating the area ratio of the extracted ferrite.

Subsequently, the remaining region (the region where "Grain Average Misorientation" exceeds 0.5°) is defined as the total area ratio of martensite and bainite.

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. Plating layers include aluminum plating layer, aluminum-zinc plating layer, aluminum-silicon plating layer, hot-dip galvanizing layer, electrolytic galvanizing layer, alloyed hot-dip galvanizing layer, zinc-nickel plating layer, aluminum-magnesium-zinc system. Examples include a plating layer.

Next, a hot stamping steel plate for obtaining a hot stamping molded article according to the present embodiment will be explained.
The hot stamping steel plate has the above-mentioned chemical composition. The metal structure of the steel sheet for hot stamping is not particularly limited as long as the desired strength and hydrogen embrittlement resistance can be obtained after hot stamping, but for example, in terms of area ratio, ferrite: 5 to 90%, bainite and martensite: 0 ~100%, pearlite: 10~95%, and retained austenite: 0~5%. In addition, iron carbides, alloy carbides, intermetallic compounds, and inclusions may be included.

Moreover, the steel plate for hot stamping may have a plating layer on the surface. By having a plating layer on the surface, corrosion resistance can be improved after hot stamping. Plating layers include aluminum plating layer, aluminum-zinc plating layer, aluminum-silicon plating layer, hot-dip galvanizing layer, electrolytic galvanizing layer, alloyed hot-dip galvanizing layer, zinc-nickel plating layer, aluminum-magnesium-zinc system. Examples include a plating layer.

Method for manufacturing a steel plate for hot stamping Hereinafter, a method for manufacturing a steel plate for hot stamping to obtain a hot stamping molded body according to the present embodiment will be described. In order to obtain the hot-stamped molded body described above, it is particularly effective to control the annealing conditions in the method for manufacturing a hot-stamped steel plate.

Note that the casting method of molten steel, the conditions for heating before hot rolling, rough rolling, finish rolling, winding, and cold rolling are not particularly limited, and may be general conditions.
Further, for the purpose of softening the hot rolled steel sheet, the coil after winding may be subjected to a softening heat treatment. The method of softening heat treatment is not particularly limited, and general conditions may be used.

Annealing After cold rolling, it is preferable to perform annealing by heating in an oxidizing atmosphere for 15 seconds or more. Normally, it is preferable to perform annealing in a reducing atmosphere in order to suppress scale formation, but in this embodiment, scale formation on the steel plate surface is promoted by performing annealing in an oxidizing atmosphere. During heating during hot stamping, the scale formed on the surface of the steel sheet becomes an oxidation source, and C and B in the surface layer region are oxidized. Since the oxidized C and B separate from the surface layer of the steel sheet, the amounts of C and B are reduced in the surface layer region. As a result, the strength of the prior austenite grains decreases and becomes easily deformed, making it easier to generate randomly oriented crystal grains. Thereby, crystal grains having a desired texture can be generated in the surface layer region.

Note that the heating temperature during annealing may be in the temperature range of 730 to 900°C, and by staying in this heating temperature range for 15 seconds or more, scale formation can be promoted while suppressing scale peeling. . The time for annealing is preferably 100 seconds or more, more preferably 200 seconds or more, and even more preferably 300 seconds or more. On the other hand, annealing for more than 3,600 seconds is undesirable because the prior austenite grain size becomes coarser, the grain boundary diffusion rate of B decreases, B removal does not proceed, and the B removal index does not exceed 0.05. . Therefore, the annealing time is preferably 3600 seconds or less.
Note that, after annealing in an oxidizing atmosphere, the annealing process may be performed again in an oxidizing atmosphere or a non-oxidizing atmosphere unless a treatment for removing oxide scale (for example, pickling) is performed.

In this embodiment, the oxidizing atmosphere may be any heating atmosphere that generates oxide scale on the surface layer of the steel sheet, and may be a general condition. For example, in a gas combustion atmosphere, it is preferable to create an atmosphere in which the mixture ratio of air and fuel (air-fuel ratio) is controlled to 1.00 or more, and more preferably to be controlled to 1.10 or more. It is preferable to generate an oxide scale of 15 μm or more on the surface of the steel sheet by annealing in an oxidizing atmosphere.
It is preferable that the oxidized scale on the surface of the steel sheet remain in subsequent steps. That is, it is preferable to perform hot stamping, which will be described later, with the oxide scale remaining. Oxide scale is removed by shot blasting after hot stamping.
Further, even when a plating layer is formed on the surface of a hot stamping steel sheet, oxide scale remains at the interface between the base steel sheet and the plating layer. When a plating layer is formed, the oxide scale disappears after hot stamping due to an alloying reaction during heating before hot stamping.

Hot Stamping A hot-stamped molded body according to the present embodiment is obtained by hot-stamping the hot-stamping steel plate manufactured by the method described above. Although hot stamping conditions are not particularly limited, it is preferable, for example, to heat the steel plate for hot stamping to a temperature range of 800° C. to 1000° C. and hold it in this temperature range for 60 to 1200 seconds.

If the heating temperature is less than 800°C, austenitization will be insufficient, and the hot-stamped molded product may have deteriorated hydrogen embrittlement resistance or may not have the desired strength. On the other hand, if the heating temperature exceeds 1000° C., grains of prior austenite will grow excessively, and the hydrogen embrittlement resistance of the hot-stamped product may deteriorate or the desired strength may not be obtained. If the holding time is less than 60 seconds, austenitization will be insufficient, and the hot-stamped molded product may have deteriorated hydrogen embrittlement resistance or may not have the desired strength. If the holding time exceeds 1200 seconds, grains of prior austenite will grow excessively, and the hydrogen embrittlement resistance of the hot-stamped molded product may deteriorate or the desired strength may not be obtained.

Note that the heating atmosphere is not particularly limited and may be under normal conditions, such as the atmosphere, a gas combustion atmosphere with a controlled ratio of air and fuel, or a nitrogen atmosphere, and the dew point of these gases is controlled. Also good.
After holding in the relevant temperature range, hot stamping is performed. After hot stamping, cooling may be performed to a temperature range of 250°C or lower at an average cooling rate of 20°C/s or higher.

Examples of heating methods before hot stamping include heating in an electric furnace, gas furnace, etc., flame heating, electrical heating, high frequency heating, induction heating, and the like.

By the above method, a hot stamp molded article according to the present embodiment is obtained. Incidentally, in order to soften the material, a tempering treatment at 130 to 600° C. may be performed after hot stamp molding, or a baking hardening treatment may be performed after painting. Alternatively, a portion of the hot stamp molded body may be tempered by laser irradiation or the like to provide a partially softened region.

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.

Slabs manufactured by casting molten steel having the chemical compositions shown in Tables 1A to 2C are subjected to rough rolling, finish rolling, cold rolling and winding under general conditions after being held in a temperature range of 1200°C or higher for 20 minutes or more. I took it. Thereafter, annealing was performed under the conditions shown in Tables 3A to 3C. Note that the annealing was performed in an oxidizing atmosphere. In annealing in an oxidizing atmosphere, the mixture ratio of air and fuel (air-fuel ratio) was controlled to 1.05 in a gas combustion atmosphere.

The obtained hot stamping steel plate was heated to the temperature range listed in Tables 3A to 3C in a furnace continuously supplied with nitrogen gas, held in the temperature range, hot stamped, and then heated to a temperature of 250°C or less. Hot stamping was performed under conditions of cooling down to the temperature range at an average cooling rate of 20° C./s or more. As a result, hot-stamped molded bodies shown in Tables 4A to 4C were obtained.
However, some examples were subjected to re-annealing, plating, or heat treatment for softening as described in the table.

Note that the underline in the table indicates that it is outside the scope of the present invention, that it falls outside of the preferred manufacturing conditions, or that the characteristic value is unfavorable. Furthermore, in addition to bainite, the metal structure of the surface layer region of the hot-stamped molded article according to the example of the present invention contains martensite of 90% or more in terms of area %, and ferrite and retained austenite of 65% or less in total. was. Furthermore, the metal structure of the hot-stamped molded article according to the example of the present invention in a region other than the surface region consists of a total of 90% or more of martensite and bainite, and 10% or less of ferrite and retained austenite, in terms of area %. Ta.

The measurement of the metallographic structure, B removal index, and extreme density of the texture of the hot-stamped compact was performed by the method described above. In addition, the mechanical properties of the hot-stamped molded product were evaluated by the following method.

Tensile strength The tensile (maximum) strength TS of the hot-stamped molded product can be determined by preparing a No. 5 test piece from any position of the hot-stamped molded product in accordance with JIS Z 2241:2011 and performing a tensile test. Obtained. Note that the crosshead speed was 1 mm/min. A case where the tensile strength TS was 2200 MPa or more was judged to have high strength and was determined to pass, and a case where the tensile strength TS was less than 2200 MPa was judged to be failed as not to have high strength.

Hydrogen embrittlement resistance The hydrogen embrittlement resistance of the hot-stamped molded product was evaluated by the following method. A test piece with a length of 68 mm and a width of 6 mm is taken from any position of the hot stamp molded body, and the edges of the test piece are polished using #200 to #1500 silicon carbide paper, and the grain size is 1 to 1. A mirror finish was obtained using a liquid in which 6 μm diamond powder was dispersed in diluted liquid such as alcohol and pure water. Furthermore, the corners of the test pieces were chamfered using #200 to #1500 silicon carbide paper. A stress of 800 MPa or more was applied to the test piece, and the test piece was immersed in 1 liter of hydrochloric acid adjusted to pH 4 at room temperature for 48 hours, and the presence or absence of cracks was determined.

A case in which no cracking occurred even under a load stress of 800 MPa or more was judged as passing. In the table, there is no cracking at 800MPa, "Fair", no cracking at 900MPa, "Good", no cracking at 1000MPa, "Very Good", and no cracking at 1100MPa or higher, "Excellent". did. On the other hand, the case where there was cracking at a load stress of 800 MPa was determined to be a failure and was written as "Bad" in the table.

Looking at Tables 4A to 4C, it can be seen that the hot-stamped molded products of the examples of the present invention have high strength and excellent hydrogen embrittlement resistance. On the other hand, it can be seen that the hot-stamped molded article as a comparative example is inferior in one or more properties.

Claims

The chemical composition is in mass%,
C: more than 0.40%, less than 0.70%,
Si: 0.010-3.000%,
Mn: 0.10% or more, less than 0.60%,
P: 0.100% or less,
S: 0.0100% or less,
N: 0.0200% or less,
O: 0.0200% or less,
Al: 0.0010-0.5000%,
Nb: 0.0010-0.1000%,
Ti: 0.010-0.200%,
Cr: 0.010-0.800%,
Mo: 0.0010-1.0000%,
B: 0.0005-0.0200%,
Co: 0-4.00%,
Ni: 0-3.00%,
Cu: 0-3.00%,
V: 0 to 3.00%,
W: 0-3.00%,
Ca: 0-1.0000%,
Mg: 0 to 1.0000%,
REM: 0-1.0000%,
Sb: 0 to 1.000%,
Sn: 0-1.000%,
Zr: 0 to 1.000%,
As: 0 to 0.100%, and
The remainder: Fe and impurities,
In a surface layer region that is a region from the surface of the hot stamp molded body to a depth of 1/25 of the plate thickness from the surface,
The area ratio of bainite is more than 10%,
The maximum value of the polar density of the texture is 4.0 or less,
A hot-stamped molded article having a B removal index of 0.05 or more.
The chemical composition is in mass%,
Co: 0.01-4.00%,
Ni: 0.01 to 3.00%,
Cu: 0.01-3.00%,
V: 0.01 to 3.00%,
W: 0.01-3.00%,
Ca: 0.0001-1.0000%,
Mg: 0.0001 to 1.0000%,
REM: 0.0001-1.0000%,
Sb: 0.001 to 1.000%,
Sn: 0.001 to 1.000%,
Zr: 0.001 to 1.000%, and As: 0.001 to 0.100%
The hot-stamped molded article according to claim 1, characterized in that it contains one or more selected from the group consisting of: