WO2023199638A1 - Hot-stamp-formed article - Google Patents

Abstract

This hot-stamp-formed article has a specified chemical composition, in which the standard deviation of crystal grain diameters of prior austenite grains in an internal region is 5.0 μm or less, 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. 2022-067020 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 plate, there is a method of increasing the Mn content in order to improve the hardenability of the steel plate, but increasing the Mn content poses problems such as hydrogen embrittlement cracking and early fracture.

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.

Early fracture is a phenomenon in which fracture occurs at a stress lower than the tensile strength estimated from the hardness of the steel member. This susceptibility to early rupture is referred to as early rupture resistance.

In Patent Document 1, bendability is considered, but hydrogen embrittlement resistance and early rupture resistance are 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 having high strength and excellent hydrogen embrittlement resistance and early breakage 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: 0.010-3.00%,
Mn: 0.60-3.00%,
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 to 0.100%,
Ti: 0.010-0.200%,
Cr: 0.01-0.80%,
Mo: 0.0010-1.000%,
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: 0-1.000%,
Mg: 0-1.000%,
REM: 0-1.000%,
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 an internal region that is a region from a depth of 4/16 of the plate thickness from the surface to a depth of 5/16 of the plate thickness from the surface,
The standard deviation of the crystal grain size of prior austenite grains is 5.0 μm or less,
In a surface layer region that is a region from the surface to a depth of 1/25 of the plate thickness,
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: 0.001-1.000%,
Mg: 0.001-1.000%,
REM: 0.001-1.000%,
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 and early breakage resistance.

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

The present inventors have discovered that by reducing the standard deviation of the crystal grain size of prior austenite grains in the internal region, it is possible to improve the hydrogen embrittlement resistance and early rupture resistance of a hot stamped compact. In addition, the present inventors improved hydrogen embrittlement resistance by generating a desired amount of bainite in the surface layer region, creating a texture with a desired crystal orientation, and setting a desired B removal index. We discovered that further improvements can be made.

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

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 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 %.

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 3.00%, Mn: 0.60 to 3.00%, 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: 0.0010-0.100%, Ti: 0.010-0.200%, Cr: 0.01-0.80%, Mo: 0.0010-1.000%, B: 0.0005-0. 0200%, and the remainder: 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 toughness of martensite is too low and excellent early rupture resistance cannot be obtained. 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:0.010~3.00%
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 0.05% or more, 0.10% or more, or 0.15% or more.
On the other hand, if the Si content exceeds 3.00%, the amount of ferrite increases, making it impossible to obtain the desired metal structure. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.00% or less, 1.00% or less, or 0.70% or less.

Mn: 0.60-3.00%
Mn is an element that promotes the transformation from prior austenite to pearlite in the hot rolled steel sheet having the chemical composition according to the present embodiment, and contributes to controlling the prior austenite grain size distribution of the hot stamped compact. In order to keep the standard deviation of the crystal grain size of prior austenite grains within a desired range, the Mn content is set to 0.60% or more. The Mn content is preferably 0.70% or more or 1.00% or more.
On the other hand, if the Mn content exceeds 3.00%, the transformation from prior austenite to pearlite in the hot-rolled steel sheet having the chemical composition according to the present embodiment is excessively promoted, and prior austenite grains in the hot stamped body are It is not possible to set the standard deviation of the crystal grain size within the desired range. Therefore, the Mn content is set to 3.00% or less. Preferably, the Mn content is 2.50% or less or 2.30% or less.

P: 0.100% or less P is an impurity element, and when it segregates at grain boundaries, it becomes a starting point for fracture and deteriorates early rupture resistance, so 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. Since this inclusion becomes a starting point for fracture and deteriorates early fracture resistance, 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. Since this nitride becomes a starting point for fracture and deteriorates early fracture resistance, 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 a large amount in steel, it forms coarse oxides that become a starting point for fracture, and deteriorates the early breakage resistance of the hot-stamped molded product. Therefore, the O content is set to 0.0200% or less. The O content is preferably 0.00100% 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 generated, which deteriorates the early rupture resistance. 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 are generated in the steel, and the early breakage resistance of the hot-stamped body is reduced. 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: 0.0010-0.100%
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 0.005% or more, 0.009% or more, or 0.015% or more.
On the other hand, if the Nb content exceeds 0.100%, a large amount of carbonitrides will be generated in the steel, and the early breakage resistance of the hot-stamped body will deteriorate. Therefore, the Nb content is set to 0.100% or less. The Nb content is preferably 0.080% or less or 0.060% 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 carbonitrides will be generated in the steel, and the early breakage resistance of the hot stamped body will deteriorate. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably 0.150% or less, 0.100% or less, 0.080% or less, 0.060% or less, or 0.050% or less.

Cr:0.01~0.80%
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 0.01%, desired strength cannot be obtained. Therefore, the Cr content is set to 0.01% or more. The Cr content is preferably 0.10% or more, 0.15% or more, or 0.20% or more.
On the other hand, if the Cr content is more than 0.80%, coarse intermetallic compounds are formed in the hot-stamped molded product, and the early rupture resistance deteriorates. Therefore, the Cr content is set to 0.80% or less. The Cr content is preferably 0.70% or less, 0.50% or less, or 0.40% or less.

Mo: 0.0010-1.000%
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 0.010% or more, 0.050% or more, or 0.100% or more.
On the other hand, if the Mo content exceeds 1.000%, coarse intermetallic compounds are formed in the hot-stamped molded product, and the early rupture resistance deteriorates. Therefore, the Mo content is set to 1.000% or less. Mo content is preferably 0.800% or less, 0.600% or less, or 0.400% 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 are formed in the hot-stamped molded product, and the early rupture resistance deteriorates. 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, when the V content exceeds 3.00%, a large amount of carbonitrides are generated in the steel, and the early rupture resistance of the hot-stamped body deteriorates. 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: 0-1.000%
Ca is an element that suppresses the formation of oxides that become fracture starting points, and contributes to improving early fracture resistance. To ensure this effect, the Ca 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 Ca content is set to 1.000% or less. If necessary, the upper limit of the Ca content may be set to 0.100%, 0.010%, 0.005%, 0.001%, 0.0005% or 0.0002%.

Mg: 0-1.000%
Mg forms oxides and sulfides in molten steel, suppresses the formation of coarse MnS, disperses many fine oxides, refines the metal structure, and contributes to improving early fracture resistance. In order to reliably obtain these effects, it is preferable that the Mg content is 0.001% 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 1.000% or less. If necessary, the upper limit of the Mg content may be set to 0.100%, 0.010%, 0.005%, 0.001%, 0.0005% or 0.0002%.

REM: 0~1.000%
REM suppresses the formation of oxides that become the starting point of fracture, and contributes to improving early fracture resistance. To ensure this effect, the REM 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 REM content is set to 1.000% or less. If necessary, the upper limit of the REM content may be set to 0.100%, 0.010%, 0.005%, 0.001%, 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 oxides that become fracture starting points, and contributes to improving early fracture resistance. 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 oxides that become a starting point for fracture, and contributes to improving early fracture resistance. 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 oxides that become fracture starting points, and contributes to improving early fracture resistance. 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%
By lowering the austenite single-phase temperature, As makes prior austenite grains finer and contributes to improving early fracture resistance. 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 product according to the present embodiment has a depth from the surface of the hot-stamped molded product to a depth of 4/16 of the plate thickness (thickness of the hot-stamped molded product) to a depth of 5/16 of the plate thickness from the surface. In the inner region, which is a region, the standard deviation of the crystal grain size of prior austenite grains is 5.0 μm or less, and in the surface region, which is a region from the surface to a depth of 1/25 of the plate thickness, bainite The area ratio of 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 internal region refers to a region from a depth of 4/16 of the plate thickness from the surface of the hot stamp molded product to a depth of 5/16 of the plate thickness from the surface.
Further, the surface layer region refers to a region from the surface of the hot stamp molded product to a depth of 1/25 of the plate thickness 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.

(Internal area)
Standard deviation of the crystal grain size of prior austenite grains: 5.0 μm or less By reducing the variation in the crystal grain size of prior austenite grains in the internal region, that is, by reducing the standard deviation, suppress the increase in local residual stress. be able to. As a result, the hydrogen embrittlement resistance and early breakage resistance of the hot-stamped molded article can be improved. If the standard deviation of the crystal grain size of prior austenite grains is more than 5.0 μm, the hydrogen embrittlement resistance and early rupture resistance deteriorate. Therefore, the standard deviation of the crystal grain size of prior austenite grains is set to 5.0 μm or less. Preferably, it is 4.0 μm or less, 3.0 μm or less, or 2.5 μm or less.
The lower limit of the standard deviation of the crystal grain size of prior austenite grains does not need to be particularly limited, but may be 0.1 μm, 0.5 μm, 1.0 μm or 1.5 μm.

The standard deviation of the grain size of prior austenite grains is obtained 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, 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 4/16 of the plate thickness from the surface to a depth of 5/16 of the plate thickness from the surface is measured with an electron beam at a measurement interval of 0.1 μm. Obtain crystal orientation information by measuring by backscattering 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, the crystal orientation of prior austenite grains is calculated from the crystal orientation relationship between general prior austenite grains and crystal grains with a body-centered structure after transformation, and this is used to calculate prior austenite grains. After calculating the average crystal grain size of the grains, its standard deviation is calculated.

The method for calculating the crystal orientation of prior austenite grains is as follows. First, a crystal orientation map of prior austenite grains is created using the method described in Non-Patent Document 1. For one of the prior austenite grains included in the observation field, the average value of the shortest diameter and the longest diameter is calculated, and the average value is taken as the grain size of the prior austenite grain. Perform the above operation for all prior austenite grains, excluding prior austenite grains whose entire crystal grains are not included in the photographic field of view, such as at the edges of the photographic field of view, to determine the grain size of all prior austenite grains in the relevant photographic field of view. . By calculating the standard deviation from the grain size of all the obtained prior austenite grains, the standard deviation of the grain size of the prior austenite grains is obtained.
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 identified. A direction parallel to the longitudinal direction of the inclusion in the cross section is determined as the rolling direction.

The metal structure of the internal region is not particularly limited as long as the desired strength, hydrogen embrittlement resistance, and early fracture resistance can be obtained, but for example, the metal structure in the internal region is 90 to 100% in total (90% or more, 100% (below) martensite and bainite, and 0 to 10% (0% or more, 10% or less) of ferrite and retained austenite. Note that martensite in this embodiment includes untempered martensite (fresh martensite) and tempered martensite.
The metallographic structure of the hot-stamped compact is measured 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 with a length of 50 μm and a depth of 4/16 of the plate thickness from the surface of the hot-stamped molded body to a depth of 5/16 of the plate thickness from the surface is 0. Crystal orientation information is obtained by measuring with an electron backscatter diffraction method at a measurement interval of 1 μm. 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 area ratio of the remaining region (the area where "Grain Average Misorientation" exceeds 0.5°) is calculated, and this area ratio is taken as the total area ratio of martensite and bainite.

(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 improved 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 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.
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.

(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 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, hydrogen embrittlement resistance, and early fracture resistance can be obtained after hot stamping, but for example, in terms of area percentage, ferrite: 5 to 90%, bainite. and martensite: 0 to 100%, pearlite: 10 to 95%, and retained austenite: 0 to 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 finish rolling conditions and annealing conditions in the method for manufacturing a hot-stamped steel plate.

Finish Rolling In finish rolling, the rolling reduction ratio (final rolling ratio) of the final pass is preferably 20% or more. The final rolling reduction ratio here is {(t ₀ - _{t 1} )/t ₀ }×100, where the plate thickness before rolling in the final pass is t ₀ and the plate thickness after rolling in the final pass is t ₁ . It can be expressed as (%). By increasing the final rolling reduction rate, pearlite is uniformly dispersed in the hot rolled steel sheet after rolling. This pearlite becomes a reverse transformation site of prior austenite during heating during hot stamping. Therefore, when pearlite is uniformly dispersed, the standard deviation of the crystal grain size of prior austenite grains in the hot-stamped molded body becomes small. As a result, the early breakage resistance of the hot-stamped molded product can be improved. More preferably, it is 30% or more, 40% or more, or 45% or more.
When the Mn content is 0.60% or more, as in the chemical composition of the hot-stamped molded product according to the present embodiment, in order to preferably control the texture of the surface layer region of the hot-stamped molded product, it is necessary to perform finish rolling. It is important to increase the final rolling reduction as described above.

Note that the casting method of molten steel, the conditions for heating before hot rolling, rough rolling, winding, and cold rolling are not particularly limited, and may be general conditions. The winding temperature may be 750° C. or lower. By setting the coiling temperature to 750° C. or lower, it is possible to suppress ferrite from being arranged in a connected manner in the hot-rolled steel sheet after rolling, and pearlite is uniformly dispersed. This pearlite becomes a reverse transformation site of prior austenite during heating during hot stamping. Therefore, when pearlite is uniformly dispersed, the standard deviation of the crystal grain size of prior austenite grains in the hot-stamped molded body becomes small. As a result, the early breakage resistance of the hot-stamped molded product can be improved.
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.
The heating temperature during annealing may be in the 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. can. 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 0.80 or more, and more preferably to be controlled to exceed 1.00. 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.

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. The hot stamping conditions are not particularly limited, but 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 600 seconds. If the heating temperature is less than 800°C, austenitization will be insufficient, the desired prior austenite grain size distribution cannot be obtained, and the early rupture resistance may deteriorate. On the other hand, if the heating temperature exceeds 1000° C., the grains of prior austenite will grow excessively, making it impossible to obtain the desired prior austenite grain size distribution, and the early rupture resistance may deteriorate. If the holding time is less than 60 seconds, austenitization becomes insufficient, a desired prior austenite particle size distribution cannot be obtained, and the early rupture resistance may deteriorate. If the holding time exceeds 600 seconds, grains of prior austenite will grow excessively, making it impossible to obtain a desired prior austenite grain size distribution, and the early rupture resistance may deteriorate.

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. Note that 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 with chemical compositions shown in Tables 1A to 1T are held at a temperature of 1200°C or higher for 20 minutes or more, and then finish rolled, coiled, and rolled under the conditions shown in Tables 2A to 2H. Annealing was performed. Note that, except for some examples, annealing was performed in an oxidizing atmosphere. For examples not specifically mentioned in the notes column in the table, 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. For some examples, as described in the table, annealing was performed in a reducing atmosphere, and the coils after winding were subjected to softening heat treatment.

The obtained steel plate for hot stamping is heated to a temperature range higher than 800°C in a furnace continuously supplied with nitrogen gas (hot stamp heating), held in the temperature range, hot stamped, and then heated to 250°C. Hot stamping was performed under conditions of cooling at an average cooling rate of 20° C./s or more to the following temperature range. As a result, hot stamp molded bodies shown in Tables 3A to 3H were obtained. In addition, for the examples not specifically mentioned in the remarks column in the table, a gas combustion atmosphere was used in which the mixture ratio of air and fuel (air-fuel ratio) was controlled to 0.85.
However, for some examples, as described in the table, heating in a furnace adjusted to a different atmosphere, reannealing, plating, tempering, hot stamp heating, etc. were performed.

Note that the underline in the table indicates that it is outside the scope of the present invention, that it falls outside the preferred manufacturing conditions, or that the characteristic value is unfavorable.
The metallographic structure (including the standard deviation of the crystal grain size of austenite grains), deboronization index, and ultra-density of the texture of the hot-stamped compact were measured by the methods 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.
In addition, for examples in which the early rupture resistance described below failed, the Vickers hardness measured by the method for early rupture resistance evaluation described later was multiplied by 3.3 (=Vickers hardness x 3.3). ) was taken as the tensile 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 to be acceptable. 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.

Early rupture resistance properties The early rupture resistance properties are calculated by dividing the tensile strength of the hot-stamped molded product obtained by the above method by the value obtained by multiplying the Vickers hardness obtained by the following method by 3.3 (tensile strength/ (Vickers hardness x 3.3)). When this value was 0.60 or more, it was determined to be excellent in early breakage resistance and was determined to pass, and when this value was less than 0.60, it was determined to be rejected. The value obtained by multiplying the Vickers hardness by 3.3 is the tensile strength estimated from the hardness, and if the measured value of the tensile strength is 0.60 times or more of the estimated tensile strength, then the early rupture resistance is can be judged to be excellent.

The Vickers hardness used to evaluate early breakage resistance was obtained by the following method. First, a sample was cut out so that a cross section perpendicular to the surface (thickness cross section) could be observed from an arbitrary position 50 mm or more away from the end surface of the hot stamp molded body. The sample was sized to allow observation of 10 mm in the rolling direction, although it depends on the measuring device. After polishing the cross section of the sample using #600 to #1500 silicon carbide paper, it was finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 μm was dispersed in a diluted solution such as alcohol and pure water. . For a mirror-finished cross section, use a micro Vickers hardness tester at any position in the area from a depth of 4/16 of the plate thickness from the surface to a depth of 5/16 of the plate thickness from the surface, Hardness was measured in a direction parallel to the plate surface (rolling direction) under a load of 1 kgf at intervals of three times or more the indentations. The Vickers hardness was obtained by measuring a total of 20 points and calculating the average value.

Looking at Tables 3A to 3H, it can be seen that the hot-stamped molded articles according to the present invention have high strength and excellent hydrogen embrittlement resistance and early fracture 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.

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 and early breakage resistance.

Claims (2)

1. The chemical composition is in mass%,
   C: more than 0.40%, less than 0.70%,
   Si: 0.010-3.00%,
   Mn: 0.60-3.00%,
   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 to 0.100%,
   Ti: 0.010-0.200%,
   Cr: 0.01-0.80%,
   Mo: 0.0010-1.000%,
   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: 0-1.000%,
   Mg: 0-1.000%,
   REM: 0-1.000%,
   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 an internal region that is a region from a depth of 4/16 of the plate thickness from the surface of the hot stamp molded body to a depth of 5/16 of the plate thickness from the surface,
   The standard deviation of the crystal grain size of prior austenite grains is 5.0 μm or less,
   In a surface layer region that is a region from the surface to a depth of 1/25 of the plate thickness,
   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.
2. 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: 0.001-1.000%,
   Mg: 0.001-1.000%,
   REM: 0.001-1.000%,
   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:

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