WO2022059321A1 - Steel sheet for hot stamping, and hot stamp molded body - Google Patents

Abstract

This steel sheet for hot stamping has the desired chemical composition, and in the metal structure thereof, the pole density in the [112]<110> orientation of a central section in the sheet thickness direction is more than 3.0, ferrite constitutes 5-95% in terms of area ratio, and the number ratio of ferrite grains in which a hard phase is included in the ferrite grain to the total number of ferrite grains is 30% or more.

Description

Steel plate for hot stamping and hot stamp molded body

The present invention relates to a steel sheet for hot stamping and a hot stamp molded body.
This application claims priority based on Japanese Patent Application No. 2020-15656 filed in Japan on September 17, 2020, the contents of which are incorporated herein by reference.

In recent years, high-strength steel plates have been applied to automobile body parts due to the demand for weight reduction and collision safety improvement. Since the vehicle body parts are molded by press molding, improvement of press moldability, particularly improvement of shape freezing property, is an issue. Therefore, the hot stamping method is attracting attention as a method for manufacturing high-strength vehicle body parts having excellent shape accuracy.

Also, in recent years, a technique for applying a tailored blank to the hot stamping method has been studied. A tailored blank is a single steel plate made by joining a plurality of steel plates having different plate thicknesses, chemical compositions, metal structures, etc. by welding. In a tailored blank, the properties in a single joined steel sheet can be partially changed. For example, by giving a certain part a high strength, deformation in that part can be suppressed, and by giving another part a low strength, the part can be deformed and an impact can be absorbed. The low-strength portion is required to have excellent ductility so as to suppress breakage during deformation.

As a technique for applying a tailored blank to the hot stamping method, there is a technique for using a tailored blank in which a steel plate having low strength after hot stamping and a steel plate having high strength after hot stamping are joined by welding. As the steel sheet having high strength after hot stamping, for example, a steel sheet as disclosed in Patent Document 1 can be used. As the steel sheet having low strength after hot stamping, the chemical composition of the steel may be adjusted so as to have low strength after cooling the mold in the hot stamping.

Low carbon steel is one of the steel types applied to tailored blanks. Since low carbon steel has a low carbon content, it has the characteristic that it is difficult to increase its strength even if it is rapidly cooled after heating. Patent Document 2 discloses that ultra-low carbon steel is used as a low-strength material in the hot stamping method. Patent Document 2 discloses a technique for improving local deformability by heating a steel sheet to a temperature of ₃ points or more and then hot stamping it to form a metal structure having bainite and bainitic ferrite as main phases. ing. Patent Document 2 discloses that this technique makes it difficult for fracture to occur when a vehicle body part is deformed in a bending mode at the time of a collision, and is excellent in impact absorption ability due to plastic deformation.

In recent years, as a high-strength material having high collision performance, a hot stamp molded body having a tensile strength of less than 1500 MPa has been attracting attention. Such a hot stamped body is required to have a desired strength and a higher ductility after hot stamping so as to be able to sufficiently suppress fracture during deformation.

Japanese Patent Laid-Open No. 2004-197213 International Publication No. 2012/157581

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot stamped molded product having high strength and excellent ductility, and a steel plate for hot stamping capable of producing this hot stamped molded product.

The present inventors have studied a method for improving the ductility of a hot stamp molded product. As a result, it was found that the ductility of the hot stamped body can be improved by increasing the area ratio of the hard phase having a high dislocation density existing on martensite in the metal structure of the hot stamped body.

Further, the present inventors have determined that the hot stamped product can be obtained by preferably controlling the chemical composition of the hot stamping steel sheet and increasing the ratio of the number of ferrites containing a hard phase in the ferrite grains. I found out.

The present invention has been obtained based on the above findings, and the gist of the present invention is as follows.
(1) The steel sheet for hot stamping according to one aspect of the present invention has a chemical composition of% by mass.
C: 0.060 to 0.200%,
Si: 0.010 to 1.000%,
Mn: 1.20 to 3.00%,
Al: 0.010 to 0.500%,
P: 0.100% or less,
S: 0.0100% or less,
N: 0.0100% or less,
Nb: 0% or more, less than 0.020%,
Ti: 0 to 0.100%,
Cr: 0 to 0.50%,
B: 0 to 0.0100%,
Mo: 0 to 1.00%,
Co: 0 to 2.00%,
Ni: 0 to 0.50%,
V: 0 to 0.10%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%, and REM: 0 to 0.0100%,
The rest consists of Fe and impurities
In the metallographic structure
The extreme density of the {112} <110> orientation at the center of the plate thickness is over 3.0.
Area rule, ferrite is 5 to 95%,
Of all the ferrites, the number ratio of the ferrites containing a hard phase in the ferrite grains is 30% or more.
(2) The steel sheet for hot stamping according to (1) above has a chemical composition of% by mass.
Nb: 0.001% or more, less than 0.020%,
Ti: 0.010 to 0.100%,
Cr: 0.05 to 0.50%,
B: 0.0001 to 0.0100%,
Mo: 0.01-1.00%,
Co: 0.01-2.00%,
Ni: 0.01-0.50%,
V: 0.01-0.10%,
Ca: 0.0005-0.0100%,
Mg: 0.0005 to 0.0100%, and REM: 0.0005 to 0.0100%
It may contain one or more of the group consisting of.
(3) The hot stamp molded product according to another aspect of the present invention has a chemical composition of% by mass.
C: 0.060 to 0.200%,
Si: 0.010 to 1.00%,
Mn: 1.20 to 3.00%,
Al: 0.010 to 0.500%,
P: 0.100% or less,
S: 0.0100% or less,
N: 0.0100% or less,
Nb: 0% or more, less than 0.020%,
Ti: 0 to 0.100%,
Cr: 0 to 0.50%,
B: 0 to 0.0100%,
Mo: 0 to 1.00%,
Co: 0 to 2.00%,
Ni: 0 to 0.50%,
V: 0 to 0.10%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%, and REM: 0 to 0.0100%,
The rest consists of Fe and impurities
In the metallographic structure
In terms of area ratio, martensite is 80% or more,
The area ratio of the hard phase existing on the martensite and having a GAIQ value of 26000 or less is 1.0% or more.
(4) The hot stamp molded product according to (3) above has a chemical composition of% by mass.
Nb: 0.001% or more, less than 0.020%,
Ti: 0.010 to 0.100%,
Cr: 0.05 to 0.50%,
B: 0.0001 to 0.0100%,
Mo: 0.01-1.00%,
Co: 0.01-2.00%,
Ni: 0.01-0.50%,
V: 0.01-0.10%,
Ca: 0.0005-0.0100%,
Mg: 0.0005 to 0.0100%, and REM: 0.0005 to 0.0100%
It may contain one or more of the group consisting of.

According to the above aspect according to the present invention, it is possible to provide a hot stamped molded product having high strength and excellent ductility, and a steel plate for hot stamping capable of producing this hot stamped molded product.

Hereinafter, the steel plate for hot stamping and the hot stamp molded body according to the present embodiment will be described in detail. First, the reason for limiting the chemical composition of the hot stamping steel sheet according to the present embodiment will be described. In addition, the lower limit value and the upper limit value are included in the numerical limitation range described with "~" in between. Numerical values indicated as "less than" and "greater than" do not include the value in the numerical range. In addition,% of the chemical composition means mass%.

The hot stamped body according to the present embodiment has a chemical composition of% by mass, C: 0.060 to 0.200%, Si: 0.010 to 1.000%, Mn: 1.20 to 3.00. %, Al: 0.010 to 0.500%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, and the balance: Fe and impurities. Hereinafter, each element will be described.

C: 0.060 to 0.200%
C is an element that greatly affects the strength and ductility of the hot stamped product. If the C content is too low, the martensitic transformation is not promoted, the strength of the hot stamped compact is lowered, and fracture is likely to occur due to insufficient strength. Therefore, the C content is set to 0.060% or more. Preferably, it is 0.080% or more, 0.100% or more, or 0.120% or more.
On the other hand, if the C content is too high, the hardness of the martensite matrix becomes too high, and the ductility of the hot stamped product decreases. Therefore, the C content is set to 0.200% or less. Preferably, it is 0.170% or less or 0.150% or less.

Si: 0.010 to 1.000%
Si is an element having a solid solution strengthening ability and is an element necessary for obtaining the strength of a hot stamp molded product. If the Si content is too low, the desired strength cannot be obtained in the hot stamped body. Therefore, the Si content is 0.010% or more. Preferably, it is 0.100% or more, 0.300% or more, or 0.500% or more.
On the other hand, if the Si content is too high, the ferrite transformation proceeds excessively, and it becomes impossible to obtain a desired amount of martensite in the hot stamp molded product. Therefore, the Si content is 1.000% or less. It is preferably 0.900% or less or 0.800% or less.

Mn: 1.20 to 3.00%
Mn is an element having a solid solution strengthening ability, and is contained in order to obtain the strength of the hot stamp molded product. If the Mn content is too low, the ferrite transformation proceeds too much and martensite is difficult to be generated, and the desired strength cannot be obtained in the hot stamped body. Therefore, the Mn content is 1.20% or more. It is preferably 1.40% or more or 1.60% or more.
On the other hand, if the Mn content is too high, the hardenability of the steel becomes high, and the formation of ferrite in air cooling after heating during hot stamping is suppressed, so that the ductility of the hot stamped product is lowered. Therefore, the Mn content is set to 3.00% or less. It is preferably 2.80% or less or 2.60% or less.

Al: 0.010 to 0.500%
Al is an important element for promoting ferrite transformation. If the Al content is too low, the ferrite transformation is less likely to proceed, and a desired amount of ferrite cannot be obtained in the hot stamp molded product. Therefore, the Al content is 0.010% or more. Preferably, it is 0.020% or more or 0.030% or more.
On the other hand, if the Al content is too high, the transformation to ferrite proceeds excessively, and a desired amount of martensite cannot be obtained in the hot stamped body. Therefore, the Al content is set to 0.500% or less. Preferably, it is 0.450% or less or 0.400% or less.

P: 0.100% or less P is an element that has a solid solution strengthening ability and is effective for obtaining a desired strength in a hot stamped article. However, if the P content is too high, the ductility of the hot stamped body deteriorates. Therefore, the P content is set to 0.100% or less. Preferably, it is 0.080% or less, 0.060% or less, or 0.050% or less.
The lower limit of the P content is not particularly specified, but from the viewpoint of ensuring the strength by P, the P content may be 0.001% or more or 0.005% or more.

S: 0.0100% or less S is an element contained in steel as an impurity and embrittles the steel. Therefore, the smaller the S content, the more preferable. The S content shall be 0.0100% or less. Preferably, it is 0.0080% or less, 0.0060% or less, or 0.0040% or less.
The lower limit of the S content is not particularly specified, but the S content may be 0.0005% or more or 0.0010% or more because the cost in the desulfurization step increases if the S content is excessively reduced.

N: 0.0100% or less N is an impurity element, which is an element that forms a nitride in steel and deteriorates the ductility of the hot stamped body. If the N content is too high, the nitride in the steel becomes coarse and the ductility of the hot stamped body deteriorates. Therefore, the N content is 0.0100% or less. Preferably, it is 0.0080% or less or 0.0060% or less.
The lower limit of the N content is not particularly specified, but the N content may be 0.0010% or more because the cost in the steelmaking process increases if the N content is excessively reduced.

The steel sheet for hot stamping according to the present embodiment may contain the above elements, and the balance may consist of Fe and impurities. Examples of impurities include those that are inevitably mixed from the steel raw material or scrap and / or in the steelmaking process, or elements that are allowed within a range that does not impair the characteristics of the hot stamped product according to the present embodiment.

The hot stamping steel sheet according to the present embodiment may contain an optional element shown below in place of a part of Fe in order to improve various properties. Since it is not necessary to intentionally contain these arbitrary elements in the steel in order to reduce the alloy cost, the lower limit of the content of these optional elements is 0%.

Nb: 0.001% or more and less than 0.020% Nb is an element that suppresses the grain growth of austenite, atomizes the austenite grains, and promotes the transformation to ferrite. In order to surely obtain this effect, the Nb content is preferably 0.001% or more.
On the other hand, if the Nb content is too high, the above effects are saturated and the cost increases. Therefore, the Nb content is set to less than 0.020%.

Ti: 0.010 to 0.100%
Ti is an element that suppresses the grain growth of austenite, atomizes the austenite grains, and promotes the transformation to ferrite. In order to surely obtain this effect, the Ti content is preferably 0.010% or more.
On the other hand, if the Ti content is too high, coarse Ti sulfides, Ti nitrides and Ti oxides are formed, and the formability of the steel sheet is deteriorated. Therefore, the Ti content is set to 0.100% or less.

Cr: 0.05 to 0.50%
Cr is also an effective element for enhancing the hardenability of steel, promoting the formation of martensite, and increasing the strength of the hot stamped compact. In order to surely obtain this effect, the Cr content is preferably 0.05% or more.
On the other hand, if the Cr content is too high, a large amount of coarse Cr carbides that can be the starting point of fracture are formed. Therefore, the Cr content is set to 0.50% or less.

B: 0.0001 to 0.0100%
B is an element that segregates into the old austenite grain boundaries, has the effect of suppressing ferrite transformation, and contributes to the improvement of the strength of the hot stamped compact. In order to surely obtain this effect, the B content is preferably 0.0001% or more.
On the other hand, if the B content is too high, the ductility of the hot stamp molded product is lowered. Therefore, the B content is 0.0100% or less.

Mo: 0.01-1.00%
Mo forms carbides in the steel and improves the strength of the hot stamped body by strengthening precipitation. In order to surely obtain this effect, the Mo content is preferably 0.01% or more.
On the other hand, if the Mo content is too high, the ductility of the hot stamped product will decrease. Therefore, the Mo content is set to 1.00% or less.

Co: 0.01-2.00%
Co improves the strength of the hot stamped body by strengthening the solid solution. In order to surely obtain this effect, the Co content is preferably 0.01% or more.
On the other hand, if the Co content is too high, the effect of the above action is saturated and the cost increases. Therefore, the Co content is 2.00% or less.

Ni: 0.01-0.50%
Ni improves the strength of the hot stamped body. In order to surely obtain this effect, the Ni content is preferably 0.01% or more.
On the other hand, if the Ni content is too high, the castability may deteriorate. Therefore, the Ni content is set to 0.50% or less.

V: 0.01-0.10%
V improves the strength of the hot stamped compact by fortifying with the precipitate and suppressing the grain growth of austenite to make the austenite granules finer. In order to surely obtain this effect, the V content is preferably 0.01% or more.
On the other hand, if the V content is too high, a large amount of carbonitride is deposited and the formability of the steel sheet is deteriorated. Therefore, the V content is set to 0.10% or less.

Ca: 0.0005-0.0100%
Ca is an element having an action of deoxidizing molten steel to make the steel sound (suppressing the occurrence of defects such as blow holes in the steel). In order to surely obtain this effect, the Ca content is preferably 0.0005% or more.
On the other hand, if the Ca content is too high, the above effect is saturated, so the Ca content is preferably 0.0100% or less.

Mg: 0.0005-0.0100%
Mg is an element having an action of deoxidizing molten steel to make the steel sound. In order to surely obtain this effect, the Mg content is preferably 0.0005% or more.
On the other hand, if the Mg content is too high, the above effect is saturated and causes an increase in cost. Therefore, the Mg content is preferably 0.0100% or less.

REM: 0.0005-0.0100%
REM is an element having an action of deoxidizing molten steel to make the steel sound. In order to surely obtain this effect, the REM content is preferably 0.0005% or more.
On the other hand, if the REM content is too high, the above effect is saturated, so the REM content is preferably 0.0100% or less.
In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y and lanthanoids. In this embodiment, the REM content refers to the total content of these elements.

The above-mentioned chemical composition may be measured by a general analysis method. For example, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy) may be used for measurement. In addition, C and S may be measured by using the combustion-infrared absorption method, and N may be measured by using the inert gas melting-thermal conductivity method. When the steel plate for hot stamping or the hot stamped body has a plating layer on the surface, the plating layer on the surface may be removed by mechanical grinding, and then the chemical composition may be analyzed.

Next, the metal structure of the hot stamping steel sheet according to the present embodiment will be described.
The steel sheet for hot stamping according to the present embodiment has a metal structure having a polar density of more than 3.0 in the {112} <110> direction at the center of the plate thickness and an area ratio of 5 to 95%. Of all the ferrites, the number ratio of the ferrites containing a hard phase in the ferrite grains is 30% or more. Hereinafter, each regulation will be described in detail.
In addition, in this embodiment, the area ratio of the ferrite and the ferrite Specify the number ratio.

Extreme density of {112} <110> orientation at the center of the plate thickness: 3.0 or more It is desirable in the hot stamp molded body that the extreme density of the {112} <110> orientation at the center of the plate thickness is 3.0 or less. Cannot obtain the metallographic structure of. Therefore, the extreme density in the {112} <110> direction at the center of the plate thickness is set to more than 3.0. It is preferably 3.5 or more or 4.0 or more. The upper limit is not particularly limited, but may be 10.0 or less.
In the present embodiment, the central portion of the plate thickness means a region from the surface to a depth of 1/4 of the plate thickness to a region from the surface to a depth of 3/4 of the plate thickness.

The extreme density of the {112} <110> orientation at the center of the plate thickness is obtained by the following method.
For the measurement, a device combining a scanning electron microscope and an EBSD analysis device and OIM Analysis (registered trademark) manufactured by TSL Co., Ltd. are used. From the Crystal Orientation Distribution Function (ODF) that displays a three-dimensional aggregate structure calculated and calculated using the azimuth data measured by the EBSD (Electron Back Scattering Diffraction) method and the spherical harmonics, {112} <110> Obtain the extreme density of the orientation. The measurement range is from the surface to the depth of 1/4 of the plate thickness to the region of the surface to the depth of 3/4 of the plate thickness. The measurement pitch is 5 μm / step.

Note that {hkl} represents a crystal plane parallel to the rolling plane, and <uvw> represents a crystal plane parallel to the rolling direction. That is, {hkl} <uvw> indicates a crystal in which {hkl} is oriented in the plate normal direction and <uvw> is oriented in the rolling direction.

Area ratio of ferrite: 5 to 95%
If the area ratio of the ferrite is less than 5%, the desired metal structure cannot be obtained in the hot stamped body, and as a result, the desired ductility cannot be obtained. Therefore, the area ratio of ferrite is set to 5% or more. It is preferably 30% or more, 40% or more, 50% or more or 60% or more.
If the area ratio of the ferrite is more than 95%, the desired metal structure cannot be obtained in the hot stamped body. Therefore, the area ratio of ferrite is set to 95% or less. Preferably, it is 70% or less, 60% or less, 50% or less or 40% or less.

Remaining structure The remnant structure other than ferrite is a hard phase consisting of one or more of martensite, bainite and pearlite. The area ratio of the hard phase is preferably 5% or more in total. It is preferably 10% or more. The upper limit of the area ratio of the hard phase is not particularly limited, but may be 95% or less, 90% or less, 80% or less, or 70% or less in total.

Method for measuring the area ratio of the metallographic structure A sample is taken from a position 10 mm or more away from the end face of the hot stamping steel plate so that the thick cross section perpendicular to the surface becomes the observation surface. After polishing the observation surface, it corrodes with nital, and using an optical microscope and a scanning electron microscope (SEM), the plate thickness is 1/4 position from the surface (1/8 depth from the surface to the plate thickness from the surface). Observe at least 3 regions of 30 μm × 30 μm in the region of 3/8 depth). By performing image analysis on the microstructure photograph obtained by this microstructure observation, the area ratios of ferrite, pearlite and bainite are obtained. Then, after the repeller corrodes to the same observation position, the tissue is observed using an optical microscope and a scanning electron microscope, and the obtained tissue photograph is image-analyzed to obtain the area ratio of martensite. Is calculated.

In the above-mentioned tissue observation, each tissue is identified by the following method.
Martensite is a structure with a high dislocation density and substructures such as blocks and packets in the grain, so it can be distinguished from other metal structures by electron channeling contrast images using a scanning electron microscope. It is possible.
It is a collection of lath-shaped crystal grains, and has a structure that is not martensite among structures that do not contain Fe-based carbides with a major axis of 20 nm or more inside the structure, and Fe-based carbides that contain Fe-based carbides with a major axis of 20 nm or more inside the structure. A structure in which the carbide has a single variant, that is, an Fe carbide extending in the same direction, is considered bainite. Here, the Fe-based carbide elongated in the same direction means that the difference in the elongation direction of the Fe-based carbide is within 5 °.

A structure that is a lumpy crystal grain and does not contain a substructure such as a lath inside the structure is regarded as ferrite.
A structure in which plate-shaped ferrite and Fe-based carbide are layered is regarded as pearlite.

Ratio of the number of ferrites containing a hard phase in the ferrite grains: 30% or more When the ratio of the number of ferrites containing a hard phase in the ferrite grains is less than 30% of all ferrites, the metal structure of the hot stamped body will be affected. The ratio of the number of ferrite grains containing the hard phase becomes low, and as a result, excellent ductility cannot be obtained. Therefore, the number ratio of ferrites containing a hard phase in the ferrite grains is set to 30% or more. It is preferably 40% or more, 50% or more, or 60% or more.
The upper limit of the number ratio of ferrites containing a hard phase in the ferrite grains is not particularly limited, but may be 100% or less or 95% or less.
The hard phase referred to here is the above-mentioned residual structure, and refers to one or more of martensite, bainite and pearlite.

Method for measuring the number ratio of ferrites containing a hard phase in ferrite grains Using the microstructure photograph used for measuring the area ratio of the metal structure described above, the total number of ferrites and the hard phase (martensite) inside the ferrite grains were used. , Bainite and pearlite) and count the number of ferrites. By calculating the number of ferrites containing a hard phase inside the ferrite grains with respect to the total number of ferrites, the ratio of the number of ferrites containing a hard phase inside the ferrite grains ((the number of ferrites containing a hard phase inside the ferrite grains). / Number of all ferrites) × 100) is obtained.

The steel sheet for hot stamping according to this embodiment may have a plating layer on one side or both sides. Having a plating layer on the surface is preferable because the corrosion resistance of the hot stamped molded product after hot stamping is improved.
Examples of the plating to be applied include aluminum plating, aluminum-zinc plating, aluminum-silicon plating, hot-dip galvanizing, electrozinc plating, and alloyed hot-dip galvanizing.

The thickness of the steel plate for hot stamping is not particularly limited, but it is preferably 0.5 to 3.5 mm from the viewpoint of reducing the weight of the vehicle body.

Next, the hot stamping compact according to the present embodiment, which is obtained by hot stamping the above-mentioned hot stamping steel sheet, will be described. Since the chemical composition of the hot stamping compact according to the present embodiment can be regarded as the same as the chemical composition of the above-mentioned steel sheet for hot stamping, the description of the chemical composition will be omitted.

In the hot stamped body according to the present embodiment, the area ratio of the hard phase having martensite of 80% or more and the GAIQ value existing on the martensite of 26000 or less in the metal structure is 1. It is 0% or more. Hereinafter, each regulation will be described.
In this embodiment, the area ratio of the martensite and the hardness at the position of 1/4 of the plate thickness from the surface (the region from the depth of 1/8 of the plate thickness to the depth of 3/8 of the plate thickness from the surface). Specifies the area ratio of the phase.

Area ratio of martensite: 80% or more If the area ratio of martensite is less than 80%, the desired strength cannot be obtained in the hot stamp molded product. Therefore, the area ratio of martensite is 80% or more. It is preferably 85% or more or 90% or more. The upper limit of the area ratio of martensite is not particularly limited, but may be 100% or less or 95% or less.

Remaining structure The remaining structure other than martensite is one or two of ferrite, bainite and pearlite. If the area ratio of ferrite is less than 1%, excellent ductility may not be obtained. Therefore, the area ratio of ferrite may be 1% or more. More preferably, it is 2% or more.
The total area ratio of bainite and pearlite may be 15% or less or 10% or less.

The area ratio of the hard phase existing on martensite with a GAIQ value of 26000 or less is 1.0% or more. The higher the GAIQ value, the lower the dislocation density, and the lower the GAIQ value, the higher the dislocation density. show. Therefore, the GAIQ value is a parameter that can reflect the dislocation density of the crystal grains. By increasing the area ratio of the hard phase having a GAIQ value of 26000 or less, that is, the hard phase having a high dislocation density, which is present on martensite, the ductility of the hot stamped compact can be improved.

If the area ratio of the hard phase present on martensite having a GAIQ value of 26000 or less is less than 1.0%, excellent ductility cannot be obtained. Therefore, the area ratio of the hard phase existing on martensite and having a GAIQ value of 26000 or less is 1.0% or more. It is preferably 1.2% or more, 1.5% or more, 2.0% or more, 2.5% or more, or 3.0% or more.
The upper limit of the area ratio of the hard phase existing on martensite having a GAIQ value of 26000 or less is not particularly limited, but may be 10.0% or less or 7.0% or less.

The hard phase having a GAIQ value of 26000 or less includes martensite and bainite. In the present embodiment, either one or both of martensite and bainite may be contained as a hard phase having a GAIQ value of 26000 or less. Further, ferrite grains, bainite grains, etc. are present on martensite. It means that it exists outside the inside of pearlite grains, in other words, it exists at the lath boundary of martensite, between laths, inside the lath, the block boundary and the packet boundary, and the bainite grain boundary.

Method for measuring the area ratio of the metal structure and the area ratio of the hard phase having a GAIQ value of 26000 or less existing on martensite From a position 10 mm or more away from the end face of the hot stamped body (or a position avoiding the end). Take a sample so that the cross section of the plate thickness perpendicular to the surface is the observation surface. After polishing the observation surface, it corrodes with nital, and using an optical microscope and a scanning electron microscope (SEM), the plate thickness is 1/4 position from the surface (1/8 depth from the surface to the plate thickness from the surface). Observe at least 3 regions of 30 μm × 30 μm in the region of 3/8 depth). By performing image analysis on the tissue photograph obtained by this tissue observation, the area ratios of pearlite and bainite are obtained. Then, after the repeller corrodes to the same observation position, the tissue is observed using an optical microscope and a scanning electron microscope, and the obtained tissue photograph is image-analyzed to obtain the area ratio of martensite. Is calculated.
In the structure observation, each structure is identified by the same method as for the hot stamping steel sheet.

Next, cut out a sample so that the plate thickness cross section can be observed from a position 10 mm or more away from the end face of the hot stamp molded body (or a position avoiding the end). After polishing the plate thickness section of this sample with # 600 to # 1500 silicon carbide paper, a mirror surface is used with a diluted solution such as alcohol or a liquid in which diamond powder having a particle size of 1 to 6 μm is dispersed in pure water. Finish to. Next, the strain introduced into the surface layer of the sample is removed by polishing at room temperature with colloidal silica containing no alkaline solution for 8 minutes.

At an arbitrary position in the longitudinal direction of the plate thickness cross section of the sample, a region having 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 at a measurement interval of 0.1 μm. Crystal orientation information is obtained by electron backscatter diffraction method. For the measurement, an EBSD device composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the degree of vacuum in the EBSD device is 9.6 × 10 ^-5 Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the irradiation level of the electron beam is 62.

For the obtained crystal orientation information, use the software "OIM Data Collection" function attached to the EBSD device and the "Grain Average Simulation" function installed in "OIM Analysis (registered trademark)" to use the Rain Average Image Qual. GAIQ map) is obtained. In the obtained GAIQ map, a region surrounded by grain boundaries having a crystal orientation difference of 5 ° or more is defined as a crystal grain. A region having an average GAIQ value of 42000 or more in a unit crystal grain is regarded as ferrite, and the area ratio thereof is calculated to obtain the area ratio of ferrite.

Further, in the obtained GAIQ map, the area ratio of the hard phase existing on martensite and having a GAIQ value of 26000 or less is measured. As a result, the area ratio of the hard phase existing on martensite and having a GAIQ value of 26000 or less is obtained. In addition, martensite shall be identified by the above-mentioned method.

The hot stamp molded product according to the present embodiment may have a plating layer on one side or both sides. Having a plating layer on the surface is preferable because the corrosion resistance of the hot stamped molded product is improved.
Examples of the plating to be applied include aluminum plating, aluminum-zinc plating, aluminum-silicon plating, hot-dip galvanizing, electrozinc plating, and alloyed hot-dip galvanizing.

The plate thickness of the hot stamp molded product is not particularly limited, but is preferably 0.5 to 3.5 mm from the viewpoint of reducing the weight of the vehicle body.

The tensile (maximum) strength of the hot stamp molded product according to this embodiment may be 980 to 1400 MPa. Further, the total elongation of the hot stamp molded product according to the present embodiment may be 7.0% or more. Further, in the hot stamp molded product according to the present embodiment, the product (TS × El) of the tensile strength and the total elongation may be 12000 MPa ·% or more.
Tensile strength and total elongation are obtained by taking a JIS No. 5 test piece from a hot stamped body and performing a tensile test in accordance with JIS Z 2241: 2011.

Next, a preferable manufacturing method of the hot stamping steel sheet according to the present embodiment will be described. A preferred method for manufacturing a steel sheet for hot stamping according to the present embodiment includes the following steps.
A slab is obtained by setting the casting speed to 0.80 m / min or more.
A hot-rolled steel sheet is obtained by hot rolling with the winding temperature in the temperature range of 500 to 700 ° C.
After obtaining a cold-rolled steel sheet by cold rolling, the cold-rolled steel sheet is heated and held in a temperature range of 750 to Ac ₃ points (first holding), and then the average cooling rate in the temperature range of 600 to 700 ° C. Cool to 15 ° C./s or less. Then, it is rapidly cooled to a temperature range of 300 to 500 ° C. and held in this temperature range (second holding). After that, it is rapidly cooled to a temperature range of 100 ° C. or lower.
The term "quenching" as used herein means cooling having an average cooling rate of more than 15 ° C./s.
Hereinafter, each step will be described.

Casting speed: 0.80 m / min or more By manufacturing the slab at a casting speed of 0.80 m / min or more, Mn segregation in steel can be promoted. The casting speed may be 3.00 m / min or less from the viewpoint of suppressing slab cracking.

Winding temperature: 500-700 ° C
By performing hot rolling with the winding temperature in the temperature range of 500 to 700 ° C., Mn can be concentrated in the carbide. The other conditions for hot rolling are not particularly limited and may be general conditions. The conditions for cold rolling may also be general, and the cumulative rolling reduction may be 30 to 70%.

After the first holding, cool so that the average cooling rate is 15 ° C / s or less. After cold rolling, the cold-rolled steel sheet is heated and held in the two-phase region, that is, in the temperature range of 750 to Ac ₃ points (first holding). ), Then cooling is performed so that the average cooling rate in the temperature range of 600 to 700 ° C. is 15 ° C./s or less, so that a hard phase in which Mn is concentrated can remain inside the ferrite grains. By holding in the above temperature range, untransformed austenite in which Mn is not concentrated is transformed into ferrite, but untransformed austenite in which Mn is enriched has a lowered transformation point, so that untransformed austenite is used as untransformed austenite without ferrite transformation. Remains.

The holding time in the first holding may be 10 to 300 seconds. Further, in the present embodiment, the average cooling rate is a value obtained by dividing the temperature difference between the surface temperature at the start of cooling and the surface temperature at the stop of cooling by the time difference from the start of cooling to the stop of cooling.
Further, Ac ₃ points can be obtained by the following formula.

Ac ₃ (° C) = 910-203 x C ^0.5 +66 x Si-25 x Mn + 700 x P-11 x Cr + 109 x Al + 400 x Ti-15.2 x Ni + 104 x V + 31.5 x Mo
The element symbol in the above formula indicates the content of each element in mass%, and when the element is not contained, 0 is substituted.

After quenching, it is held for the second time, further cooled so that the average cooling rate in the temperature range of 600 to 700 ° C is 15 ° C / s or less, and then rapidly cooled to the temperature range of 300 to 500 ° C. Hold (hold for the second time) and then quench further. As a result, the carbide remaining in the ferrite grains can be transformed into a hard phase. As a result, the number ratio of ferrites containing a hard phase in the ferrite grains can be increased.
The holding time in the second holding may be 10 to 600 seconds.

By the manufacturing method described above, the steel sheet for hot stamping according to the present embodiment can be stably manufactured. In addition to the above-mentioned manufacturing method, a step of forming a plating layer on one side or both sides of the hot stamping steel sheet may be provided.

Next, a preferable manufacturing method of the hot stamp molded product according to the present embodiment will be described. The method for manufacturing a hot stamp molded product according to the present embodiment includes the following steps.
The steel sheet for hot stamping is heated to a temperature range of ₃ points or more and held.
Cool to a temperature range of 100 ° C. or lower so that the average cooling rate is 30 ° C./s or higher.
Hereinafter, each step will be described.

Heating temperature and holding temperature: Ac ₃ points or more By heating and holding the above-mentioned hot stamping steel sheet in a temperature range of Ac ₃ points or more, austenite can be sufficiently formed. The holding time in the temperature range of Ac ₃ points or more is not particularly limited, but may be, for example, 10 to 300 seconds. Ac After holding in a temperature range of ₃ points or more, hot stamp.

Average cooling rate up to a temperature range of 100 ° C. or lower: 30 ° C./s or more By cooling so that the average cooling rate up to a temperature range of 100 ° C. or lower is 30 ° C./s or more, a desired amount of hard phase is obtained. be able to. As a result, it is possible to increase the area ratio of the hard phase existing on martensite and having a GAIQ value of 26000 or less. Cooling to a temperature range of 100 ° C. or lower may be performed by contact with a mold.

By the method described above, the hot stamp molded product according to the present embodiment can be obtained. Since the steel sheet for hot stamping according to the present embodiment has relatively low strength, it is joined to a steel sheet having high strength after hot stamping to form a tailored blank, which is then hot stamped and formed into a vehicle body part. Since this vehicle body part is manufactured by hot-stamping a tailored blank made of a low-strength material and a high-strength material, it has a low-strength portion and a high-strength portion.

Various welding methods such as laser welding, seam welding, arc welding, and plasma welding can be considered when manufacturing a tailored blank, but the welding method is not particularly limited. Further, the high-strength material (steel plate having high strength after hot stamping) used together with the low-strength material (steel plate for hot stamping according to the present embodiment) is not particularly limited. These may be selected appropriately for each vehicle body part to be manufactured.

There is no problem even if the steel plate for hot stamping according to the present embodiment is not applied to the tailored blank and the vehicle body parts and the like are manufactured using only the steel plate. It is not a problem to make a blank by joining steel plates such as patchwork by spot welding and stacking them, and hot stamping the blank.

Next, an example of the present invention will be described. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is based on this one condition example. Not limited. The present invention can adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.

Using the slabs having the chemical compositions shown in Tables 1A and 1B, the steel sheets for hot stamping shown in Tables 2A to 2C were manufactured under the conditions shown in Tables 2A to 2C. Next, the hot stamp molded products shown in Tables 3A to 3C were obtained under the conditions shown in Tables 3A to 3C.

The slab was manufactured at the casting speeds shown in Tables 2A to 2C. In cold rolling after winding, the cumulative rolling reduction was set to 30 to 70%. The holding time in the first holding was 10 to 300 seconds, and the holding time in the second holding was 10 to 600 seconds. Further, after cooling so that the average cooling rate in the temperature range of 600 to 700 ° C. became the average cooling rate shown in Tables 2A to 2C, the mixture was rapidly cooled to the second holding temperature. After the second holding, it was rapidly cooled to a temperature range of 100 ° C. or lower.
Further, in the heating at the time of hot stamping, the holding time was set to 10 to 300 seconds.

By the above method, the metallographic structure of the hot stamped steel sheet, the metallic structure of the hot stamped body, and the mechanical properties (tensile strength and total elongation) were measured.
Examples having a tensile strength of 980 to 1400 MPa were judged to be acceptable because they had high strength. On the other hand, an example in which the tensile strength was less than 980 MPa or more than 1400 MPa was judged to be unacceptable.
Further, an example in which the total elongation was 7.0% or more and the product of the tensile strength and the total elongation (TS × El) was 12000 MPa ·% or more was judged to be acceptable because of its excellent ductility. On the other hand, the cases where the total elongation was less than 7.0% and the cases where the product of the tensile strength and the total elongation (TS × El) was less than 12000 MPa ·% were judged to be inferior in ductility and rejected.

According to Tables 1A to 3C, it can be seen that the hot stamp molded product according to the example of the present invention has high strength and excellent ductility.
On the other hand, it can be seen that the hot stamped product according to the comparative example does not have high strength and / or excellent ductility.

According to the above aspect according to the present invention, it is possible to provide a hot stamped molded product having high strength and excellent ductility, and a steel plate for hot stamping capable of producing this hot stamped molded product.

Claims (4)

1. The chemical composition is by mass%,
   C: 0.060 to 0.200%,
   Si: 0.010 to 1.000%,
   Mn: 1.20 to 3.00%,
   Al: 0.010 to 0.500%,
   P: 0.100% or less,
   S: 0.0100% or less,
   N: 0.0100% or less,
   Nb: 0% or more, less than 0.020%,
   Ti: 0 to 0.100%,
   Cr: 0 to 0.50%,
   B: 0 to 0.0100%,
   Mo: 0 to 1.00%,
   Co: 0 to 2.00%,
   Ni: 0 to 0.50%,
   V: 0 to 0.10%,
   Ca: 0-0.0100%,
   Mg: 0 to 0.0100%, and REM: 0 to 0.0100%,
   The rest consists of Fe and impurities
   In the metallographic structure
   The extreme density of the {112} <110> orientation at the center of the plate thickness is over 3.0.
   Area rule, ferrite is 5 to 95%,
   A steel sheet for hot stamping, wherein the number ratio of the ferrites containing a hard phase in the ferrite grains is 30% or more among all the ferrites.
2. The chemical composition is by mass%.
   Nb: 0.001% or more, less than 0.020%,
   Ti: 0.010 to 0.100%,
   Cr: 0.05 to 0.50%,
   B: 0.0001 to 0.0100%,
   Mo: 0.01-1.00%,
   Co: 0.01-2.00%,
   Ni: 0.01-0.50%,
   V: 0.01-0.10%,
   Ca: 0.0005-0.0100%,
   Mg: 0.0005 to 0.0100%, and REM: 0.0005 to 0.0100%
   The steel sheet for hot stamping according to claim 1, wherein the steel sheet for hot stamping comprises one or more of the group consisting of two or more.
3. The chemical composition is by mass%,
   C: 0.060 to 0.200%,
   Si: 0.010 to 1.000%,
   Mn: 1.20 to 3.00%,
   Al: 0.010 to 0.500%,
   P: 0.100% or less,
   S: 0.0100% or less,
   N: 0.0100% or less,
   Nb: 0% or more, less than 0.020%,
   Ti: 0 to 0.100%,
   Cr: 0 to 0.50%,
   B: 0 to 0.0100%,
   Mo: 0 to 1.00%,
   Co: 0 to 2.00%,
   Ni: 0 to 0.50%,
   V: 0 to 0.10%,
   Ca: 0-0.0100%,
   Mg: 0 to 0.0100%, and REM: 0 to 0.0100%,
   The rest consists of Fe and impurities
   In the metallographic structure
   In terms of area ratio, martensite is 80% or more,
   A hot stamped body having a GAIQ value of 26000 or less and an area ratio of a hard phase existing on martensite of 1.0% or more.
4. The chemical composition is by mass%.
   Nb: 0.001% or more, less than 0.020%,
   Ti: 0.010 to 0.100%,
   Cr: 0.05 to 0.50%,
   B: 0.0001 to 0.0100%,
   Mo: 0.01-1.00%,
   Co: 0.01-2.00%,
   Ni: 0.01-0.50%,
   V: 0.01-0.10%,
   Ca: 0.0005-0.0100%,
   Mg: 0.0005 to 0.0100%, and REM: 0.0005 to 0.0100%
   The hot stamp molded product according to claim 3, wherein the hot stamp molded product contains one or more of the group consisting of two or more.