WO2022239758A1 - Steel sheet for hot stamping and hot stamped molded body - Google Patents

Abstract

This hot stamped molded body has a metallographic structure having a prescribed chemical composition, the average grain size of old austenite grains being 5-25 μm, and the standard deviation of the grain size of the old austenite grains being 0.1-2.0 μm. Additionally, this steel sheet for hot stamping has a metallographic structure having a prescribed chemical composition, the average pole density of an orientation group composed of ferrite /{100/}<011>-/{223/}<110> being 10.0 or less, the numeric ratio of ferrites that include carbides having an equivalent circle diameter of 0.2 μm or greater in crystal grains among all ferrites being 20% or greater, and perlite and ferrite constituting 10-90% and 10-90% by area ratio, respectively.

Description

Steel plate for hot stamping and hot stamped compact

TECHNICAL FIELD The present invention relates to a steel sheet for hot stamping and a hot stamped product.
This application claims priority based on Japanese Patent Application No. 2021-081621 filed in Japan on May 13, 2021, the contents of which are incorporated herein.

Conventionally, from the perspective of global environmental issues and crash safety performance, there has been a demand for thinner and stronger automotive parts. In order to meet these demands, automobile parts made from high-strength steel sheets are increasing. A method called hot stamping is also known as a method for forming high-strength steel sheets. In hot stamping, a high-strength steel sheet is press-formed in a high temperature range of 700° C. or higher, and quenched inside or outside the press die. According to hot stamping, forming is performed in a high temperature range where the strength of the steel sheet decreases, so forming defects such as those that occur in cold pressing can be suppressed. In addition, since a structure having martensite as the main phase is obtained by quenching after molding, high strength can be obtained. Therefore, hot-stamped products having a tensile strength of about 1500 MPa are widely used worldwide.

　In order to achieve a higher effect of reducing the weight of automobile bodies, it is necessary to obtain high-strength automobile parts that are formed by hot stamping high-strength steel sheets and that also have excellent crash characteristics. In order to improve the collision characteristics of automobile members, in particular, automobile members are required to have excellent bendability.

Patent Document 1 discloses a steel sheet that improves hardenability and material formability and is particularly suitable for forming parts such as gears by cold forging such as thickening, and a method for manufacturing the same.

The inventors of the present invention have found that it is necessary to further improve the bendability of automobile components with improved tensile strength in order to obtain a higher effect of reducing the weight of the vehicle body.

WO2016/190396

The present invention has been made in view of the above problems. An object of the present invention is to provide a hot stamped article having high strength and excellent bendability, and a steel sheet for hot stamping from which the hot stamped article can be produced.

The gist of the present invention is as follows.
[1] A steel sheet for hot stamping according to one aspect of the present invention has a chemical composition, in mass%,
C: more than 0.40%, 0.70% or less,
Si: 0.010 to 1.30%,
Mn: 0.10-0.60%,
P: 0.100% or less,
S: 0.0100% or less,
N: 0.0140% or less,
O: 0.0200% or less,
Al: 0.0010 to 0.500%,
Cr: 0.010 to 0.80%,
Nb: 0 to 0.100%,
Ti: 0 to 0.100%,
B: 0 to 0.0100%,
Mo: 0 to 1.00%,
Co: 0 to 2.00%,
Ni: 0% or more and less than 3.00%,
Cu: 0 to 1.00%,
V: 0 to 1.00%,
W: 0 to 1.000%,
Ca: 0-0.010%,
Mg: 0-1.000%,
REM: 0 to 1.000%,
Sb: 0 to 1.000%,
Zr: 0 to 1.000%,
Sn: 0-1.000% and As: 0-0.100%
and the balance consists of Fe and impurities,
The average value of the pole density of the ferrite orientation group consisting of {100} <011> to {223} <110> is 10.0 or less,
Among all ferrite, the number ratio of the ferrite containing carbide having an equivalent circle diameter of 0.2 μm or more in the crystal grain is 20% or more,
It has a metal structure with an area ratio of 10 to 90% pearlite and 10 to 90% ferrite.
[2] The steel sheet for hot stamping according to [1] above, wherein the chemical composition is, in mass%,
Nb: 0.001 to 0.100%,
Ti: 0.010 to 0.100%,
B: 0.0015 to 0.0100%,
Mo: 0.05-1.00%,
Co: 0.05 to 2.00%,
Ni: 0.01% or more and less than 3.00%,
Cu: 0.01 to 1.00%,
V: 0.01 to 1.00%,
W: 0.001 to 1.000%,
Ca: 0.001-0.010%,
Mg: 0.001-1.000%,
REM: 0.001 to 1.000%,
Sb: 0.005 to 1.000%,
Zr: 0.001 to 1.000%,
Sn: 0.001-1.000% and As: 0.001-0.100%
It may contain one or more selected from the group consisting of.
[3] A hot stamped article according to another aspect of the present invention has a chemical composition, in mass %,
C: more than 0.40%, 0.70% or less,
Si: 0.010 to 1.30%,
Mn: 0.10-0.60%,
P: 0.100% or less,
S: 0.0100% or less,
N: 0.0140% or less,
O: 0.0200% or less,
Al: 0.0010 to 0.500%,
Cr: 0.010 to 0.80%,
Nb: 0 to 0.100%,
Ti: 0 to 0.100%,
B: 0 to 0.0100%,
Mo: 0 to 1.00%,
Co: 0 to 2.00%,
Ni: 0% or more and less than 3.00%,
Cu: 0 to 1.00%,
V: 0 to 1.00%,
W: 0 to 1.000%,
Ca: 0-0.010%,
Mg: 0-1.000%,
REM: 0 to 1.000%,
Sb: 0 to 1.000%,
Zr: 0 to 1.000%,
Sn: 0-1.000% and As: 0-0.100%
and the balance consists of Fe and impurities,
Having a metal structure in which the average grain size of the prior austenite grains is 5 to 25 μm and the standard deviation of the grain size of the prior austenite grains is 0.1 to 2.0 μm,
Tensile strength is 2200 MPa or more.
[4] The hot stamped article according to [3] above, wherein the chemical composition is, in mass%,
Nb: 0.001 to 0.100%,
Ti: 0.010 to 0.100%,
B: 0.0015 to 0.0100%,
Mo: 0.05-1.00%,
Co: 0.05 to 2.00%,
Ni: 0.01% or more and less than 3.00%,
Cu: 0.01 to 1.00%,
V: 0.01 to 1.00%,
W: 0.001 to 1.000%,
Ca: 0.001-0.010%,
Mg: 0.001-1.000%,
REM: 0.001 to 1.000%,
Sb: 0.005 to 1.000%,
Zr: 0.001 to 1.000%,
Sn: 0.001-1.000% and As: 0.001-0.100%
It may contain one or more selected from the group consisting of.
[5] In the hot stamped product described in [3] or [4] above, the area ratio of the prior austenite grains having an average grain size of 0.5 to 3.0 μm may be 60% or less.

According to the above aspect of the present invention, it is possible to provide a hot-stamped article having high strength and excellent bendability, and a hot-stamping steel sheet from which this hot-stamped article can be produced.

The inventors investigated the bendability of the hot stamped body. As a result, the present inventors have found that bendability deteriorates when a large amount of fine prior austenite grains are present in the metallographic structure of the hot stamped product. In addition, the present inventors made the prior austenite grains a desired size and suppressed the variation in the size of the prior austenite grains in the metal structure of the hot stamped product, that is, by regulating the prior austenite grains, It has been found that the bendability of the hot-stamped product can be further improved.

Next, the present inventors studied a method for obtaining the above hot-stamped molded product. As a result, the present inventors set the Mn content to 0.60% or less in the chemical composition of the steel sheet for hot stamping, and in the metal structure, the ferrite orientation consisting of {100} <011> to {223} <110> It has been found that the above-described hot-stamped compact can be obtained by reducing the pole density of the groups and increasing the number ratio of ferrite containing carbides in the crystal grains.

A hot-stamping steel sheet and a hot-stamping compact according to the present embodiment based on the above findings will be described below. First, reasons for limiting the chemical composition of the steel sheet for hot stamping 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 below between "-". Numerical values indicated as "less than" and "greater than" do not include the value within the numerical range. All percentages in the chemical composition are percentages by weight.

The steel sheet for hot stamping according to the present embodiment has a chemical composition in mass% of C: more than 0.40% and 0.70% or less, Si: 0.010 to 1.30%, Mn: 0.10 to 0.60%, P: 0.100% or less, S: 0.0100% or less, N: 0.0140% or less, O: 0.0200% or less, Al: 0.0010 to 0.500%, Cr: 0.010-0.80% and the balance consists of Fe and impurities. Each element will be described below.

C: more than 0.40%, 0.70% or less C greatly contributes to improving the strength of the hot-stamped product. If the C content is 0.40% or less, it becomes difficult to obtain sufficient strength in the hot-stamped product. Therefore, the C content should be more than 0.40%. It is preferably 0.42% or more, more preferably 0.45% or more, and still more preferably 0.47% or more.
On the other hand, if the C content exceeds 0.70%, coarse carbides are formed, degrading the bendability of the hot-stamped product. Therefore, the C content should be 0.70% or less. It is preferably 0.65% or less, more preferably 0.60% or less.

Si: 0.010-1.30%
Si is an element that improves the deformability of hot-stamped products by bonding with oxygen and suppressing the formation of oxides that act as fracture starting points. If the Si content is less than 0.010%, coarse oxides are formed in the hot-stamped product, making it impossible to obtain the desired bendability. Therefore, the Si content is set to 0.010% or more. It is preferably 0.05% or more, more preferably 0.10% or more.
On the other hand, if the Si content exceeds 1.30%, coarse oxides are formed, degrading the bendability of the hot stamped product. Therefore, the Si content should be 1.30% or less. It is preferably less than 1.00%, more preferably 0.50% or less.

Mn: 0.10-0.60%
Mn stabilizes austenite and improves the hardenability of the steel sheet. If the Mn content is less than 0.10%, sufficient hardenability cannot be obtained. Therefore, the Mn content is set to 0.10% or more. It is preferably 0.20% or more, more preferably 0.30% or more.
On the other hand, if the Mn content exceeds 0.60%, cracking due to Mn segregation tends to occur unless the manufacturing method is appropriately controlled, and excellent bendability cannot be obtained in the hot stamped product. Therefore, the Mn content is set to 0.60% or less. It is preferably 0.55% or less, more preferably 0.50% or less.

P: 0.100% or less P segregates at the grain boundary of the steel sheet and deteriorates the bendability of the hot-stamped product. Therefore, the lower the P content, the better. In particular, when the P content exceeds 0.100%, the workability of the steel sheet and the bendability of the hot-stamped product are significantly deteriorated. Therefore, the P content is set to 0.100% or less. It is preferably 0.080% or less, more preferably 0.020% or less.
Although the lower limit of the P content is not particularly limited, it may be 0%. However, if the P content is reduced to less than 0.0001%, the cost for removing P will increase significantly, which is economically undesirable. Therefore, the P content may be 0.0001% or more.

S: 0.0100% or less S forms coarse inclusions and deteriorates the bendability of the hot stamped product. Therefore, the lower the S content, the better. In particular, when the S content exceeds 0.0100%, the formability of the steel sheet and the bendability of the hot-stamped product are significantly deteriorated. Therefore, the S content should be 0.0100% or less. It is preferably 0.0050% or less, more preferably 0.0010% or less.
Although the lower limit of the S content is not particularly limited, it may be 0%. However, if the S content is reduced to less than 0.0001%, the deS cost will increase significantly, which is economically undesirable. Therefore, the S content may be 0.0001% or more.

N: 0.0140% or less N forms coarse nitrides and deteriorates the bendability of the hot-stamped product. Therefore, the lower the N content, the better. In particular, when the N content exceeds 0.0140%, the formability of the steel sheet is remarkably deteriorated. Therefore, the N content is made 0.0140% or less. It is preferably 0.0100% or less or 0.0070% or less, more preferably 0.0040% or less.
Although the lower limit of the N content is not particularly limited, it may be 0%. However, if the N content is reduced to less than 0.0001%, the N removal cost will increase significantly, which is economically unfavorable. Therefore, the N content may be 0.0001% or more.

O: 0.0200% or less O forms coarse oxides in the steel and deteriorates the bendability of the hot stamped product. Therefore, the lower the O content, the better. In particular, when the O content exceeds 0.0200%, the bendability of the hot-stamped product is significantly deteriorated. Therefore, the O content is set to 0.0200% or less. It is preferably 0.0150% or less, more preferably 0.0100% or less, and more preferably 0.0060% or less.
Although the lower limit of the O content is not particularly limited, it may be 0%. However, if the O content is reduced to less than 0.0001%, the manufacturing cost will increase significantly, which is economically undesirable. Therefore, the O content may be 0.0001% or more.

Al: 0.0010-0.500%
Al is an element that deoxidizes molten steel and suppresses the formation of oxides that serve as starting points for fracture, thereby improving deformability and enhancing the bendability of hot stamped products. If the Al content is less than 0.0010%, deoxidation is not sufficiently performed, and coarse oxides are formed, making it impossible to obtain the above effects. Therefore, the Al content is set to 0.0010% or more. It is preferably 0.010% or more, more preferably 0.030% or more.
On the other hand, if the Al content exceeds 0.500%, coarse oxides are formed in the steel, and the bendability of the hot-stamped product is lowered. Therefore, the Al content is set to 0.500% or less. It is preferably 0.450% or less, more preferably 0.350% or less.

Cr: 0.010-0.80%
Cr dissolves in the prior austenite grains during heating during hot stamping, thereby increasing the strength of the hot stamped compact. If the Cr content is less than 0.010%, this effect cannot be obtained. Therefore, the Cr content is set to 0.010% or more. It is preferably 0.10% or more, more preferably 0.20% or more.
On the other hand, when the Cr content exceeds 0.80%, coarse carbides are formed, which deteriorates the bendability of the hot-stamped product. Therefore, the Cr content is set to 0.80% or less. It is preferably 0.60% or less, more preferably 0.40% or less.

The rest of the chemical composition of the steel sheet for hot stamping according to the present embodiment may be Fe and impurities. Examples of impurities include elements that are unavoidably mixed from steel raw materials or scraps and/or during the steelmaking process, or elements that are allowed within a range that does not impair the properties of the hot stamped body according to the present embodiment.

The steel sheet for hot stamping according to the present embodiment may contain the following elements as arbitrary elements instead of part of Fe. The content is 0% when the following optional elements are not contained.

Nb: 0-0.100%
Nb forms carbonitrides in steel and improves the strength of hot stamped bodies through precipitation strengthening. In order to obtain this effect, the Nb content is preferably 0.001% or more.
On the other hand, if the Nb content exceeds 0.100%, a large amount of carbonitrides are formed in the steel, and the bendability of the hot-stamped product is lowered. Therefore, the Nb content is set to 0.100% or less.

Ti: 0-0.100%
Ti, like Nb, forms carbonitrides in steel and improves the strength of hot-stamped products by precipitation strengthening. In order to obtain this effect, the Ti content is preferably 0.010% or more.
On the other hand, when the Ti content exceeds 0.100%, a large amount of carbonitrides are formed in the steel, and the bendability of the hot stamped product is lowered. Therefore, the Ti content is set to 0.100% or less.

B: 0 to 0.0100%
B improves the hardenability of steel and improves the strength of hot-stamped products. In order to obtain this effect, the B content is preferably 0.0015% or more.
On the other hand, when the B content is more than 0.0100%, coarse carbides are formed and the bendability of the hot stamped product is deteriorated. Therefore, the B content is set to 0.0100% or less.

Mo: 0-1.00%
Mo improves the hardenability of the steel sheet and improves the strength of the hot-stamped product. In order to obtain this effect, it is preferable to set the Mo content to 0.05% or more.
On the other hand, when the Mo content is more than 1.00%, coarse carbides are formed, degrading the bendability of the hot-stamped product. Therefore, Mo content shall be 1.00% or less.

Co: 0-2.00%
Co improves the hardenability of the steel sheet and improves the strength of the hot-stamped product. In order to ensure this effect, the Co content is preferably 0.05% or more.
On the other hand, if the Co content exceeds 2.00%, coarse carbides are formed, degrading the bendability of the hot stamped product. Therefore, the Co content is set to 2.00% or less.

Ni: 0% or more and less than 3.00% Ni improves the hardenability of the steel sheet and improves the strength of the hot-stamped product. In order to obtain this effect, the Ni content is preferably 0.01% or more.
On the other hand, when the Ni content is 3.00% or more, the segregation is promoted and the bendability of the hot stamped product is deteriorated. Therefore, the Ni content is set to less than 3.00%.

Cu: 0-1.00%
Cu, like Ni, improves the hardenability of the steel sheet and improves the strength of the hot-stamped product. In order to obtain this effect, the Cu content is preferably 0.01% or more.
On the other hand, when the Cu content exceeds 1.00%, segregation is promoted and the bendability of the hot stamped product is deteriorated. Therefore, the Cu content is set to 1.00% or less.

V: 0-1.00%
V improves the hardenability of the steel sheet and improves the strength of the hot-stamped product. In order to obtain this effect, the V content is preferably 0.01% or more.
On the other hand, if the V content exceeds 1.00%, a large amount of carbonitrides precipitate, and the bendability of the hot-stamped body deteriorates. Therefore, the V content is set to 1.00% or less.

W: 0-1.000%
W improves the hardenability of the steel sheet and improves the strength of the hot-stamped product. In order to obtain this effect, the W content is preferably 0.001% or more.
On the other hand, when the W content exceeds 1.000%, the segregation is promoted and the bendability of the hot stamped product is deteriorated. Therefore, the W content is set to 1.000% or less.

Ca: 0-0.010%
Ca suppresses the formation of oxides, which act as starting points for fracture, thereby improving deformability and enhancing the bendability of the hot-stamped product. In order to obtain this effect, the Ca content is preferably 0.001% or more.
On the other hand, when the Ca content exceeds 0.010%, coarse oxides are formed, which deteriorates the bendability of the hot stamped product. Therefore, the Ca content is set to 0.010% or less.

Mg: 0-1.000%
Mg improves the deformability by suppressing the formation of oxides that act as starting points for fracture, and increases the bendability of the hot-stamped product. In order to obtain this effect, the Mg content is preferably 0.001% or more.
On the other hand, if the Mg content is more than 1.000%, coarse oxides are formed, degrading the bendability of the hot-stamped product. Therefore, the Mg content is set to 1.000% or less.

REM: 0-1.000%
REM improves deformability by suppressing the formation of oxides, which act as starting points for fracture, and enhances the bendability of hot-stamped products. To obtain this effect, the REM content is preferably 0.001% or more.
On the other hand, when the REM content is more than 1.000%, coarse oxides are formed, which deteriorates the bendability of the hot stamped product. Therefore, the REM content is set to 1.000% or less.
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 to 1.000%
Sb improves the deformability by suppressing the formation of oxides, which act as starting points for fracture, and increases the bendability of the hot stamped product. In order to obtain this effect, the Sb content is preferably 0.005% or more.
On the other hand, if the Sb content is more than 1.000%, coarse oxides are formed, degrading the bendability of the hot-stamped product. Therefore, the Sb content is set to 1.000% or less.

Zr: 0 to 1.000%
Zr improves the deformability by suppressing the formation of oxides that act as starting points for fracture, thereby enhancing the bendability of the hot stamped product. In order to obtain this effect, the Zr content is preferably 0.001% or more.
On the other hand, if the Zr content exceeds 1.000%, coarse oxides are formed, degrading the bendability of the hot stamped product. Therefore, the Zr content is set to 1.000% or less.

Sn: 0-1.000%
Sn improves the deformability by suppressing the formation of oxides, which act as starting points for fracture, and increases the bendability of the hot stamped product. In order to reliably obtain this effect, the Sn content is preferably 0.001% or more.
On the other hand, the above effect is saturated even if it is contained in a large amount, so the Sn content is made 1.000% or less.

As: 0-0.100%
As lowers the austenite single phase temperature, refines the prior austenite grains, and increases the bendability of the hot-stamped product. In order to reliably obtain this effect, the As content is preferably 0.001% or more.
On the other hand, even if the content of As is large, the above effect is saturated, so the content of As is set to 0.100% or less.

The chemical composition of the hot stamping steel sheet mentioned above can be measured by a general analysis method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Incidentally, C and S can be measured using a combustion-infrared absorption method, N can be measured using an inert gas fusion-thermal conductivity method, and O can be measured using an inert gas fusion-nondispersive infrared absorption method. When the steel sheet for hot stamping has a coating layer on its surface, the coating layer may be removed by mechanical grinding, and then the chemical composition may be analyzed.

Next, the metal structure of the steel sheet for hot stamping according to this embodiment will be described.
In the steel sheet for hot stamping according to the present embodiment, the average value of the pole density of the orientation group consisting of {100}<011> to {223}<110> of ferrite is 10.0 or less, and among all ferrites, the crystal A metal in which the number ratio of ferrite containing carbides having an equivalent circle diameter of 0.2 μm or more in grains is 20% or more, and the area ratio is 10 to 90% pearlite and 10 to 90% ferrite. have an organization; Each rule will be explained below.
In this embodiment, in the plate thickness cross section parallel to the rolling direction, the depth position of 1/4 of the plate thickness from the surface (1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface area) defines the metallographic structure. The reason is that the metallographic structure at this position shows the typical metallographic structure of the steel plate.

"The average value of the pole density of the orientation group consisting of {100} <011> to {223} <110> of ferrite is 10.0 or less"
When the average value of the pole density of the ferrite orientation group consisting of {100} <011> to {223} <110> is more than 10.0, the average grain size of the prior austenite in the hot stamped compact is set to a predetermined value. It cannot be controlled and a hot-stamped product with excellent bendability cannot be obtained. The average value of the pole density of the ferrite orientation group consisting of {100} <011> to {223} <110> is preferably 9.0 or less, more preferably 7.0 or less, and further preferably 6.0 or less, 5.0 or less is even more preferable. The lower limit of the pole density of the ferrite orientation group consisting of {100}<011> to {223}<110> is not particularly limited, but may be 0.1 or more.

The orientation group consisting of {100}<011> to {223}<110> includes {100}<011>, {116}<110>, {114}<110>, {112}<110>, The {223} <110> crystallographic orientation is included.

Measurement method of the pole density The pole density of the orientation group consisting of {100} <011> to {223} <110> of ferrite is measured by a device combining a scanning electron microscope and an EBSD analysis device and OIM Analysis (registered A crystal orientation distribution function (ODF: Orientation Distribution Function) that displays a three-dimensional texture calculated by calculating orientation data measured by an EBSD (Electron Back Scattering Diffraction) method using a spherical harmonic function using a trademark). can be obtained from The measurement area is from 1/8th of the thickness to 3/8th of the thickness from the surface so that the depth of 1/4th of the thickness from the surface can be observed. A measurement pitch is 5 μm/step.

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

"Among all ferrites, the ratio of the number of ferrites containing carbides having an equivalent circle diameter of 0.2 μm or more in crystal grains is 20% or more"
When the number ratio of ferrite containing carbide having an equivalent circle diameter of 0.2 μm or more in the crystal grains is less than 20% among all ferrites, the prior austenite grains can be regulated in the hot stamped compact. As a result, it is impossible to obtain a hot-stamped article having excellent bendability. By setting the number ratio of ferrite containing carbides having an equivalent circle diameter of 0.2 μm or more in the crystal grains to 20% or more among all ferrites, the carbides in the crystal grains are converted to former austenite when heated before hot stamping. It functions favorably as a starting point for grains. As a result, it is presumed that the prior austenite grains are uniformly dispersed in the metallographic structure of the hot-stamped compact and the grains are regulated. Among all ferrites, the number ratio of ferrite containing carbide having an equivalent circle diameter of 0.2 μm or more in crystal grains is preferably 40% or more, preferably 50% or more, and more preferably 60% or more. Although the upper limit of the number ratio of ferrite containing carbides having an equivalent circle diameter of 0.2 μm or more in crystal grains in all ferrite is not particularly defined, it may be 90% or less.

Method for measuring the number ratio of ferrite containing carbide A plate parallel to the rolling direction is measured from an arbitrary position 50 mm or more away from the end face of the hot stamping steel plate (if the sample cannot be taken from this position, avoid the end). Samples are collected so that the thick cross-section is the observation surface. The viewing surface is then finished by electropolishing. After that, observe 10 or more fields of view at a magnification of 20,000 times for the area from 1/8 the thickness to 3/8 the thickness from the surface so that the 1/4 depth position from the surface can be observed. do. For crystal grains identified as ferrite by the metal structure measurement method described below, the equivalent circle diameter of each carbide is determined from the area of each carbide observed in the ferrite crystal grains by image analysis. Among all observed ferrite crystal grains, the number of ferrite crystal grains containing carbide having an equivalent circle diameter of 0.2 μm or more is calculated. The obtained value is divided by the total number of ferrite crystal grains and multiplied by 100 to obtain the number ratio of ferrite containing carbide having an equivalent circle diameter of 0.2 μm or more in the crystal grains.
In this embodiment, particles having an equivalent circle diameter of 0.2 to 30 μm are regarded as carbides.

"Perlite is 10 to 90 area %"
"Ferrite is 10 to 90 area %"
When the area ratio of ferrite is less than 10% and the area ratio of pearlite is more than 90%, pearlite preferentially serves as a starting point for prior austenite in the hot stamping process, and an effect of regulating prior austenite grains can be obtained. Gone. Therefore, the ferrite area ratio is set to 10% or more, and the pearlite area ratio is set to 90% or less. The area ratio of ferrite is preferably 20% or more, more preferably 40% or more. The area ratio of pearlite is preferably 80% or less, more preferably 60% or less.
On the other hand, when the area ratio of ferrite is more than 90% and the area ratio of pearlite is less than 10%, the carbon in the pearlite becomes too concentrated and the temperature at which the pearlite transforms into austenite becomes low. As a result, the transformation starts at a low temperature in the hot stamping process, and the prior austenite grains tend to coarsen, making it impossible to obtain the effect of regulating the prior austenite grains. Therefore, the area ratio of ferrite is set to 90% or less, and the area ratio of pearlite is set to 10% or more. The area ratio of ferrite is preferably 70% or less, more preferably 60% or less. The area ratio of pearlite is preferably 30% or more, more preferably 40% or more.

In the metal structure of the steel sheet for hot stamping according to the present embodiment, the residual structure is one or more of martensite, lower bainite, retained austenite and tempered martensite. The area ratio of the residual tissue may be 20% or less.

Method for measuring metallographic structure of hot stamping steel plate A plate parallel to the rolling direction from an arbitrary position 50 mm or more away from the end face of the hot stamping steel plate (if the sample cannot be taken from this position, avoid the end) A sample is cut so that a thick section can be observed. Although the size of the sample depends on the measuring device, it should be a size that allows observation of about 10 mm in the rolling direction.

After polishing the cross section of the above sample using #600 to #1500 silicon carbide paper, a mirror finish is achieved using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water. , finish polishing by electropolishing. Then, at any position in the longitudinal direction of the sample cross section, so that the 1/4 depth position of the plate thickness can be observed from the surface, the length is 50 μm, and the thickness is 1/8 depth from the surface to the plate thickness from the surface. Tissues are observed in the 3/8 depth region using an apparatus consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector). The scanning electron microscope used shall be equipped with a two-electron detector. In a vacuum of 9.6×10 ⁻⁵ Pa or less, the sample is irradiated with an electron beam at an acceleration voltage of 15 kV and an irradiation current level of 13, and a secondary electron image is taken with a scanning electron microscope.

In the photograph obtained, the area where cementite is precipitated in lamellar form inside the grain is judged to be pearlite. By calculating the area ratio of the region determined to be pearlite, the area ratio of pearlite is obtained. Lath-like grains are determined as lower bainite, martensite and tempered martensite. Then, the same field of view is subjected to EBSD analysis using an EBSD analysis device at an analysis speed of 200 to 300 points/second. The area ratio of ferrite is calculated using the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. With this function, after calculating the misorientation between adjacent measurement points for crystal grains having a body-centered structure, it is possible to obtain an average value for all measurement points within the crystal grain. With respect to the crystal orientation information obtained by EBSD analysis, a region surrounded by grain boundaries with an average crystal orientation difference of 5° or more is defined as a crystal grain, and a map is drawn using the "Grain Average Misorientation" function. In the regions excluding the regions determined to be pearlite, lower bainite, martensite, and tempered martensite from the map, the regions having an average crystal orientation difference of less than 5.0° within the crystal grains are determined to be ferrite. By calculating the area ratio of the region determined to be ferrite, the area ratio of ferrite is obtained.

The steel sheet for hot stamping according to the present embodiment may have a plating layer formed on its surface for the purpose of improving corrosion resistance after hot stamping. The plating layer may be either an electroplating layer or a hot dipping layer. The electroplated layer includes, for example, an electrogalvanized layer, an electroplated Zn—Ni alloy layer, and the like. The hot-dip plating layer is, for example, a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, a hot-dip aluminum plating layer, a hot-dip Zn--Al alloy plating layer, a hot-dip Zn--Al--Mg alloy-plating layer, or a hot-dip Zn--Al--Mg--Si. Including alloy plating layer, etc. The coating amount of the plating layer is not particularly limited, and a general coating amount may be used.

Although the thickness of the hot stamping steel sheet according to the present embodiment is not particularly limited, it is preferably 0.5 to 3.5 mm from the viewpoint of reducing the weight of the vehicle body.

Next, a hot-stamped product according to the present embodiment, which is obtained by hot-stamping the steel plate for hot-stamping, will be described. The hot-stamped product according to this embodiment has the same chemical composition as that of the hot-stamping steel plate described above. The method for measuring the chemical composition may be the same method as for the steel plate for hot stamping. In addition, in the metal structure of the hot-stamped compact according to the present embodiment, the prior austenite grains are regulated. That is, the hot stamped body according to the present embodiment has a metal structure in which the average grain size of the prior austenite grains is 5 to 25 μm and the standard deviation of the grain size of the prior austenite grains is 0.1 to 2.0 μm. have.
In the present embodiment, in a cross section perpendicular to the plate surface, a depth position of 1/4 of the plate thickness from the surface (1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface) ) specifies the metal structure in The reason is that the metallographic structure at this position exhibits the typical metallographic structure of hot stamped compacts. The metal structure will be described below.

"The average grain size of prior austenite grains is 5 to 25 μm"
"The standard deviation of the grain size of prior austenite grains is 0.1 to 2.0 μm"
In the metal structure of the hot stamped body, the average grain size of the prior austenite grains is 5 to 25 μm, and the standard deviation of the grain size of the prior austenite grains is 0.1 to 2.0 μm. Flexibility can be improved. If the average grain size of the prior austenite grains or the standard deviation of the grain size of the prior austenite grains is outside the above range, the hot-stamped product cannot have excellent bendability.

The average grain size of the prior austenite grains is preferably 10 μm or more, more preferably 15 μm or more. Moreover, the average grain size of the prior austenite grains is preferably 20 μm or less.
By setting the standard deviation of the grain size of the prior austenite grains to 2.0 μm or less, excellent bendability can be obtained in the hot-stamped product. Therefore, the standard deviation of the grain size of prior austenite grains is set to 2.0 μm or less. It is more preferably 1.2 μm or less, still more preferably 1.1 μm or less, and still more preferably 0.4 μm or less.
In actual operation, it is difficult to make the standard deviation of the grain size of prior austenite grains less than 0.1 μm, so the practical lower limit is 0.1 μm or more.

If the area ratio of the prior austenite grains with an average grain size of 0.5 to 3.0 μm is 60% or less, the hot stamped product can have better bendability. Therefore, the area ratio of prior austenite grains having an average grain size of 0.5 to 3.0 μm may be 60% or less. It is more preferably 50% or less, and still more preferably 40% or less.

Method for Measuring Average Grain Size of Prior Austenite Grains and Standard Deviation of Grain Size Next, a method for measuring the average grain size of prior austenite grains will be described. Cut out a sample from an arbitrary position 50 mm or more away from the end face of the hot stamped product (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 a size that allows observation of about 10 mm in the rolling direction. After polishing the cross section of the above sample using #600 to #1500 silicon carbide paper, a mirror finish is achieved using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water. , finish polishing is performed using electropolishing.

Then, at any position in the longitudinal direction of the sample cross section, so that the 1/4 depth position of the plate thickness can be observed from the surface, 1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface A device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) for a region of 100 μm in length and 100 μm in the thickness direction in the area of the thickness. EBSD analysis is performed at an analysis speed of 200 to 300 points/second by irradiating the sample with an electron beam at an acceleration voltage of 15 kV and an irradiation current level of 13 in a vacuum of 9.6×10 ⁻⁵ Pa or less. Using the obtained crystal orientation information, the crystal orientation of the prior austenite grains was calculated from the crystal orientation relationship between the general prior austenite grains and the crystal grains having a body-centered structure after transformation. Calculate the average grain size of the grains.

The method for calculating the crystal orientation of the prior austenite grains is not particularly limited, but for example, the following method may be used. First, the crystal orientation of the prior austenite grains is calculated by the method described in Non-Patent Document 1, and the crystal orientation of the prior austenite at each coordinate of the EBSD-measured region is specified. Next, using the "Inverse Pole Figure" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device, a crystal orientation map of prior austenite grains is created. An average value of the shortest diameter and the longest diameter is calculated for one of the prior austenite grains included in the observation field, and the average value is taken as the grain size of the prior austenite grain. Perform the above operation for all prior austenite grains, except for prior austenite grains whose entire crystal grains are not included in the imaging field such as the edge of the imaging field, and obtain the grain size of all prior austenite grains in the imaging field. . The average grain size of the prior austenite grains in the field of view is obtained by dividing the sum of the grain sizes of the obtained prior austenite grains by the total number of the prior austenite grains whose grain sizes are measured. By performing this operation for every field of view taken and calculating the average grain size of the prior austenite grains in all the fields of view taken, the average grain size of the prior austenite grains is obtained.

By calculating the standard deviation from the grain size of the prior austenite grains, the standard deviation of the grain size of the prior austenite grains is obtained. At this time, in order to eliminate the influence of locally generated fine grains and coarse grains, the standard deviation is calculated by excluding the minimum and maximum values of the prior austenite grain size.
By calculating the value obtained by dividing the area of the prior austenite grains with an average grain size of 0.5 to 3.0 μm by the area of the entire measurement field, the prior austenite grains with an average grain size of 0.5 to 3.0 μm to obtain the area ratio of

The metal structure of the hot stamped product is not particularly limited as long as the desired strength and bendability can be obtained after hot stamping. , pearlite: 0-30%, and retained austenite: 0-5%. The metallographic structure of the hot-stamped product may be measured by the following method.

Measurement method of metallographic structure of hot stamped product Cut out the sample so that can be observed. After polishing the cross-section of this sample using #600 to #1500 silicon carbide paper, it is finished to a mirror surface using a liquid in which diamond powder with a grain size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water. , Nital etching. A length of 100 μm, a depth of 1/8 of the plate thickness from the surface to 3/ of the plate thickness from the surface, at any position in the longitudinal direction of the sample cross section so that the position of 1/4 of the plate thickness from the surface can be observed. Multiple fields of view are photographed using a thermal field emission scanning electron microscope (JEOL JSM-7001F) in an 8-depth region. An equidistant grid is drawn on the photograph to identify the tissue at the grid points. The area ratio of each tissue is obtained by calculating the number of grid points corresponding to each tissue and dividing it by the total number of grid points. The larger the total number of grid points, the more accurately the area ratio can be calculated. In this embodiment, the grid spacing is 2 μm×2 μm, and the total number of grid points is 1,500.

A region in which cementite is precipitated in a lamellar shape within grains is determined to be pearlite. A region with low brightness and no substructure is judged to be ferrite. Regions with high brightness and in which the substructure is not revealed by etching are judged to be martensite and retained austenite. A region that does not correspond to any of the above is determined to be bainite.
The area ratio of martensite is obtained by subtracting the area ratio of retained austenite obtained by EBSD analysis, which will be described later, from the area ratio of martensite and retained austenite obtained from the above photograph.

The area ratio of retained austenite is measured by backscattered electron diffraction (EBSD). Analysis by EBSD uses a sample collected at the same sample collection position as the measurement using the photographed photograph above, and the area from 1/8 depth of the plate thickness to 3/8 depth of the plate thickness from the surface about After polishing the sample using #600 to #1500 silicon carbide paper, it is mirror-finished using a liquid in which diamond powder with a grain size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water. , shall be finished by electropolishing for the purpose of sufficiently removing the distortion of the cross section to be measured. In the electropolishing, in order to remove the mechanical polishing distortion of the viewing surface, it is sufficient to polish the observation surface by a minimum of 20 μm and a maximum of 50 μm. 30 μm or less is preferable in consideration of sagging at the edge.

Measurement with EBSD is performed at an acceleration voltage of 15 to 25 kV, at intervals of at least 0.25 μm or less, and in the range of 150 μm or more in the plate thickness direction and 250 μm or more in the rolling direction Obtain crystal orientation information at each measurement point. . Among the obtained crystal structures, those with a crystal structure of fcc are determined to be retained austenite using the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. The area ratio of retained austenite is obtained by calculating the ratio of measurement points determined to be retained austenite. Here, since it is preferable that the number of measurement points is as large as possible, the measurement interval should be narrow and the measurement range should be wide. However, when the measurement interval is less than 0.01 μm, adjacent points interfere with the spread width of the electron beam. Therefore, the measurement interval should be 0.01 μm or more. Also, the maximum measurement range is 200 μm in the sheet thickness direction and 400 μm in the sheet width direction. For the measurement, an EBSD apparatus 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 apparatus is 9.6×10 ⁻⁵ Pa or less, the irradiation current level is 13, and the electron beam irradiation level is 62.

A plating layer may be formed on the surface of the hot-stamped body according to the present embodiment for the purpose of improving corrosion resistance after hot-stamping. The plating layer may be either an electroplating layer or a hot dipping layer. The electroplated layer includes, for example, an electrogalvanized layer, an electroplated Zn—Ni alloy layer, and the like. The hot-dip plating layer is, for example, a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, a hot-dip aluminum plating layer, a hot-dip Zn--Al alloy plating layer, a hot-dip Zn--Al--Mg alloy-plating layer, or a hot-dip Zn--Al--Mg--Si. Including alloy plating layer, etc. The coating amount of the plating layer is not particularly limited, and a general coating amount may be used.

Although the plate thickness of the hot-stamped body according to the present embodiment is not particularly limited, it is preferably 0.5 to 3.5 mm from the viewpoint of reducing the weight of the vehicle body.

The hot stamped product according to this embodiment has a tensile (maximum) strength of 2200 MPa or more. It is preferably 2400 MPa or more, more preferably 2550 MPa or more. Tensile strength is determined according to the test method described in JIS Z 2241:2011 by preparing a No. 5 test piece described in JIS Z 2241:2011 from a position as flat as possible on the hot-stamped molded body.

In addition, the hot stamped product according to the present embodiment preferably has a maximum bending angle of 20° or more obtained by a bending test based on the VDA standard (VDA238-100) specified by the German Automobile Manufacturers Association. The conditions for the bending test are as follows.

Test piece size: 60 mm (rolling direction) x 30 mm (direction parallel to plate width direction)
Test piece plate thickness: 1.6 mm
Bending ridgeline: direction parallel to sheet width direction Test method: roll support, punch pushing Roll diameter: φ30 mm
Punch shape: tip R = 0.4 mm
Distance between rolls: 2.0 x plate thickness (mm) + 0.5 mm
Pushing speed: 20mm/min
Testing machine: SHIMADZU AUTOGRAPH 20kN

Next, a method for manufacturing a steel sheet for hot stamping according to this embodiment will be described.
In the method for manufacturing a steel sheet for hot stamping according to the present embodiment, in order to obtain a steel sheet for hot stamping having the above-described metal structure, the reduction ratio of the rolling one pass before the final pass of the finish rolling in hot rolling is set high. do.

The steel slab (steel material) to be hot rolled may be a steel slab manufactured by a conventional method, for example, a steel slab manufactured by a general method such as continuous casting slabs or thin slab casters.

Hot rolling includes rough rolling and finish rolling. In finish rolling, the slab after rough rolling is rolled by a plurality of finish rolling mills. In this embodiment, rolling one pass before the final pass of finish rolling is performed in a temperature range of 900 to 1050° C. with a rolling reduction of 10 to 25%. After this rolling, a final pass is performed in a temperature range of 850° C. or more and less than 1000° C. at a rolling reduction (final rolling reduction) of 6% or more. The reduction ratio of the rolling _one pass before the final pass is given by _{ (t ₀ −t ₁ )/t ₀ }×100(%). The final rolling reduction is {(t ₁ −t ₂ )/ _{t 1} }× ₁₀₀ ( %).

By setting the reduction ratio of rolling one pass before the final pass to 10 to 25%, dislocations in the austenite are reduced, and the reduction ratio of the subsequent final pass (final reduction ratio) is set to 6% or more. , a small amount of dislocations can be introduced into the austenite grains. Since the dislocations introduced into the austenite grains function as starting points for the precipitation of carbides, it is presumed that as a result, a desired amount of ferrite containing carbides can be formed in the crystal grains. The dislocations in the austenite before the final rolling disappear together with the dislocations introduced in the final pass. Therefore, unless the rolling reduction in the rolling one pass before the final pass is controlled within the above range, the number of carbide precipitation starting points decreases. It is presumed that
Normally, in finish rolling, rolling is performed by gradually decreasing the rolling reduction for each pass. However, in this embodiment, in the rolling one pass before the final pass of the finish rolling, the rolling reduction is set higher than that of the previous pass (two passes before the final pass), and the rolling is performed at the above rolling reduction. Thereby, a desired metal structure can be obtained.

If the reduction in rolling one pass before the final pass is less than 10% or more than 25%, recrystallization of austenite in the final pass is suppressed and the desired texture cannot be obtained. The reduction in rolling one pass before the final pass is preferably 13% or more, more preferably 16% or more, and even more preferably 18% or more.

If the rolling temperature one pass before the final pass is less than 900° C., recrystallization of austenite in the final pass is suppressed and a desired texture cannot be obtained. The rolling temperature one pass before the final pass is preferably 910° C. or higher, more preferably 930° C. or higher.
On the other hand, if the rolling temperature one pass before the final pass exceeds 1050° C., the austenite grains are coarsened and ferrite transformation is suppressed, and a predetermined amount of ferrite cannot be obtained in the hot stamping steel sheet. The rolling temperature one pass before the final pass is preferably 1040° C. or lower, more preferably 1020° C. or lower.

When the rolling reduction of the final pass (final rolling reduction) is less than 6%, the number of dislocations introduced is reduced, and the number ratio of ferrite containing carbide having an equivalent circle diameter of 0.2 μm or more in the crystal grains is reduced by a predetermined amount. You can't control it. The final rolling reduction is preferably 8% or more, more preferably 10% or more, and even more preferably 12% or more. Although the upper limit of the final rolling reduction is not particularly specified, it may be less than 40%.

If the rolling temperature of the final pass is less than 850°C, the austenite grains become too fine and the ferrite transformation is excessively promoted, so that the desired amount of pearlite cannot be obtained in the hot stamped steel sheet. The rolling temperature in the final pass is preferably 860°C or higher, more preferably 870°C or higher.
On the other hand, if the rolling temperature of the final pass is 1000° C. or higher, the austenite grains are coarsened and the ferrite transformation is suppressed, and a predetermined amount of ferrite cannot be obtained in the hot stamped steel sheet. The rolling temperature in the final pass is preferably 980° C. or lower, more preferably 960° C. or lower.

The heating temperature and holding time of the billet before hot rolling are not particularly limited, but it is preferable to hold the billet in a temperature range of 1200°C or higher for 20 minutes or longer.

After finishing rolling, it is preferable to wind the steel sheet in a temperature range of 400 to 750°C. If the coiling temperature is less than 400° C., the steel sheet for hot stamping has a pearlite area ratio of more than 90% and a ferrite area ratio of less than 10%. The winding temperature is preferably 450°C or higher, more preferably 530°C or higher.
On the other hand, when the coiling temperature is higher than 750° C., the area ratio of pearlite is less than 10% and the area ratio of ferrite is more than 90% in the steel sheet for hot stamping. The winding temperature is preferably 700°C or lower, more preferably 660°C or lower.

After winding, cold rolling may be performed as necessary. Also, the plating described above may be formed after finishing rolling or after cold rolling. Furthermore, pickling may be performed between hot rolling and cold rolling. In cold rolling, a normal cumulative reduction rate, eg, 30 to 90%, may be used. Further, temper rolling may be performed under normal conditions. Moreover, for the purpose of softening the hot-rolled steel sheet, hot-rolled steel sheet annealing may be performed by heating the hot-rolled steel sheet to a temperature range of 730° C. or lower.

By the method described above, the steel sheet for hot stamping according to the present embodiment can be manufactured. Next, a method for manufacturing a hot-stamped body according to the present embodiment, which can be manufactured using the above-described hot-stamping steel sheet, will be described. The method for manufacturing the hot stamped body according to this embodiment is not particularly limited, but for example, the following manufacturing method may be used.

First, the steel plate for hot stamping described above is heated to a temperature range of 800°C or higher. If the heating temperature is less than 800° C., coarse carbides remain during heating, and the bendability of the hot-stamped product may deteriorate. The heating temperature is preferably 820° C. or higher, more preferably 860° C. or higher.

Although the upper limit of the heating temperature is not particularly limited, if the heating temperature is too high, decarburization is promoted in the surface layer of the steel sheet, and the strength of the hot stamped body is lowered. Therefore, the heating temperature is preferably 1000° C. or lower, more preferably 960° C. or lower, and even more preferably 930° C. or lower.

Also, the holding time at the above heating temperature is preferably 1.0 to 10.0 minutes. If the holding time is less than 1.0 minute, coarse carbides may remain and the bendability of the hot stamped product may deteriorate. On the other hand, when the holding time is more than 10.0 minutes, decarburization is promoted in the surface layer of the steel sheet, and the strength of the hot-stamped product may decrease.

Also, the average heating rate up to the above heating temperature is preferably 1.0°C/s or more. When the average heating rate is less than 1.0° C./s, decarburization is promoted in the surface layer of the steel sheet, and the strength of the hot-stamped product is lowered. Although the upper limit is not particularly defined, since it is difficult to exceed 1000° C./s in actual operation, 1000° C./s or less is the substantial upper limit.

After heating and holding as described above, hot stamping is performed. After hot stamping, for example, it is preferable to cool down to a temperature range of 300° C. or less at an average cooling rate of 10° C./s or more. If the average cooling rate is less than 10°C/s, the strength may be insufficient. Although the upper limit is not particularly defined, since it is difficult to exceed 1000° C./s in actual operation, 1000° C./s or less is the substantial upper limit.
It should be noted that preheating, that is, heating in two stages is not preferable in the heating at the time of hot stamping. The carbon segregation region in the grain boundary created at the stage of the hot stamping steel plate is eliminated, and the prior austenite grains cannot be uniformly dispersed and generated, and as a result, the standard deviation of the prior austenite grains is within the desired range. This is because it cannot be fully controlled.

The hot-stamped product according to the present embodiment can be obtained by the preferred manufacturing method described above. A tempering treatment may be performed at 150 to 600° C. after hot stamp molding. Also, a part of the hot-stamped body may be tempered by laser irradiation or the like to provide a partially softened region. Weldability improves in the softened region. For example, if spot welding is performed after softening the end of the hot stamped body, the difference in strength between the softened end and the spot-welded part of the end can be reduced. Destruction can be suppressed. Further, for example, when applying a hot-stamped body to a high-strength member of an automobile, by providing a softening region in a part of the high-strength member, it is possible to control the destruction and deformation mode of the high-strength member at the time of collision. can.

Next, examples of the present invention will be described. The conditions in the examples are one example of conditions adopted for confirming the feasibility and effect of the present invention, and the present invention is based on this one example of conditions. It is not limited. Various conditions can be adopted in the present invention as long as the objects of the present invention are achieved without departing from the gist of the present invention.

A billet produced by casting molten steel having the chemical composition shown in Tables 1A to 1D is heated and held in a temperature range of 1200 ° C. or higher for 20 minutes or more, and then hot rolled under the conditions shown in Tables 2A to 2G. and coiled, and subjected to cold rolling, hot-rolled sheet annealing, pickling and plating as required. As a result, steel sheets for hot stamping shown in Tables 2A to 2G were obtained. In the finish rolling, No. marked with "*" was used. Except for No. 195, in the rolling one pass before the final pass, rolling was performed with a higher rolling reduction than the previous pass (two passes before the final pass).

In addition, steel plate No. In No. 149, hot-rolled sheet annealing was performed by heating and holding in a temperature range of 730°C or less.
Steel plate no. 150 was not cold rolled.
Steel plate no. 151 formed an electrogalvanized layer on the surface.
Steel plate no. 152 formed an electric Zn-Ni alloy plating layer on the surface.
Steel plate no. 153 formed a hot-dip galvanized layer on the surface.
Steel plate no. 154 formed an alloyed hot-dip galvanized layer on the surface.
Steel plate no. 155 formed a hot-dip aluminum plating layer on the surface.
Steel plate no. 156 formed a hot-dip Zn-Al alloy plating layer on the surface.
Steel plate no. 157 formed a hot-dip Zn-Al-Mg alloy plating layer on the surface.
Steel plate no. 158 formed a hot-dip Zn-Al-Mg-Si alloy plating layer on the surface.
Steel plate no. In No. 195, rolling was performed by gradually lowering the rolling reduction for each pass in the finish rolling.

In addition, "polar density" in Tables 2A to 2G indicates "average value of extreme density of ferrite orientation group consisting of {100} <011> to {223} <110>", and "ferrite containing carbides" The number ratio” indicates “the number ratio of ferrite containing carbide having an equivalent circle diameter of 0.2 μm or more in crystal grains among all ferrites”.

The obtained steel sheets for hot stamping were subjected to hot stamping under the conditions shown in Tables 3A to 3G to obtain hot stamped bodies shown in Tables 3A to 3G.
Manufacturing No. 186 was tempered at 150 to 600°C after hot stamping.
Manufacturing No. In No. 187, a partially softened region was formed by irradiating and tempering a part of the hot-stamped body.
Manufacturing No. 188 was heated to the heating temperature shown in Table 3G, cooled to a temperature range of 250° C. or lower, then heated to 900° C. and then hot stamped, thereby cooling at the average cooling rate shown in Table 3G.

In addition, in the examples of the present invention in Tables 3A to 3G, the metal structure is, in terms of area%, ferrite: 0 to 50%, bainite and martensite: 0 to 100%, pearlite: 0 to 30%, and retained austenite: It consisted of 0-5%.

In addition, the method for measuring the metal structure of the hot stamping steel plate and the method for measuring the metal structure and mechanical properties of the hot stamped product were as described above. When the tensile strength of the hot stamped product was 2200 MPa or more, it was judged to have high strength and was judged to be acceptable. In addition, if the maximum bending angle is 20 ° or more, it is judged to have excellent bendability and is judged to be acceptable, and if the maximum bending angle is less than 20 °, it is judged to be unacceptable because it does not have excellent bendability. did.

From Tables 3A to 3G, it can be seen that the hot stamped bodies according to the examples of the present invention have high strength and excellent bendability. On the other hand, it can be seen that one of the properties of the hot-stamped article according to the comparative example deteriorated.

According to the above aspect of the present invention, it is possible to provide a hot-stamped article having high strength and excellent bendability, and a hot-stamping steel sheet from which this hot-stamped article can be produced.

Claims (5)

1. The chemical composition, in mass %,
   C: more than 0.40%, 0.70% or less,
   Si: 0.010 to 1.30%,
   Mn: 0.10-0.60%,
   P: 0.100% or less,
   S: 0.0100% or less,
   N: 0.0140% or less,
   O: 0.0200% or less,
   Al: 0.0010 to 0.500%,
   Cr: 0.010 to 0.80%,
   Nb: 0 to 0.100%,
   Ti: 0 to 0.100%,
   B: 0 to 0.0100%,
   Mo: 0 to 1.00%,
   Co: 0 to 2.00%,
   Ni: 0% or more and less than 3.00%,
   Cu: 0 to 1.00%,
   V: 0 to 1.00%,
   W: 0 to 1.000%,
   Ca: 0-0.010%,
   Mg: 0-1.000%,
   REM: 0 to 1.000%,
   Sb: 0 to 1.000%,
   Zr: 0 to 1.000%,
   Sn: 0-1.000% and As: 0-0.100%
   and the balance consists of Fe and impurities,
   The average value of the pole density of the ferrite orientation group consisting of {100} <011> to {223} <110> is 10.0 or less,
   Among all ferrite, the number ratio of the ferrite containing carbide having an equivalent circle diameter of 0.2 μm or more in the crystal grain is 20% or more,
   A steel sheet for hot stamping, characterized by having a metallographic structure in which pearlite is 10 to 90% and ferrite is 10 to 90% in terms of area ratio.
2. The chemical composition, in mass %,
   Nb: 0.001 to 0.100%,
   Ti: 0.010 to 0.100%,
   B: 0.0015 to 0.0100%,
   Mo: 0.05-1.00%,
   Co: 0.05 to 2.00%,
   Ni: 0.01% or more and less than 3.00%,
   Cu: 0.01 to 1.00%,
   V: 0.01 to 1.00%,
   W: 0.001 to 1.000%,
   Ca: 0.001-0.010%,
   Mg: 0.001-1.000%,
   REM: 0.001 to 1.000%,
   Sb: 0.005 to 1.000%,
   Zr: 0.001 to 1.000%,
   Sn: 0.001-1.000% and As: 0.001-0.100%
   The steel sheet for hot stamping according to claim 1, characterized by containing one or more selected from the group consisting of:
3. The chemical composition, in mass %,
   C: more than 0.40%, 0.70% or less,
   Si: 0.010 to 1.30%,
   Mn: 0.10-0.60%,
   P: 0.100% or less,
   S: 0.0100% or less,
   N: 0.0140% or less,
   O: 0.0200% or less,
   Al: 0.0010 to 0.500%,
   Cr: 0.010 to 0.80%,
   Nb: 0 to 0.100%,
   Ti: 0 to 0.100%,
   B: 0 to 0.0100%,
   Mo: 0 to 1.00%,
   Co: 0 to 2.00%,
   Ni: 0% or more and less than 3.00%,
   Cu: 0 to 1.00%,
   V: 0 to 1.00%,
   W: 0 to 1.000%,
   Ca: 0-0.010%,
   Mg: 0-1.000%,
   REM: 0 to 1.000%,
   Sb: 0 to 1.000%,
   Zr: 0 to 1.000%,
   Sn: 0-1.000% and As: 0-0.100%
   and the balance consists of Fe and impurities,
   Having a metal structure in which the average grain size of the prior austenite grains is 5 to 25 μm and the standard deviation of the grain size of the prior austenite grains is 0.1 to 2.0 μm,
   A hot-stamped article characterized by having a tensile strength of 2200 MPa or more.
4. The chemical composition, in mass %,
   Nb: 0.001 to 0.100%,
   Ti: 0.010 to 0.100%,
   B: 0.0015 to 0.0100%,
   Mo: 0.05-1.00%,
   Co: 0.05 to 2.00%,
   Ni: 0.01% or more and less than 3.00%,
   Cu: 0.01 to 1.00%,
   V: 0.01 to 1.00%,
   W: 0.001 to 1.000%,
   Ca: 0.001-0.010%,
   Mg: 0.001-1.000%,
   REM: 0.001 to 1.000%,
   Sb: 0.005 to 1.000%,
   Zr: 0.001 to 1.000%,
   Sn: 0.001-1.000% and As: 0.001-0.100%
   4. The hot-stamped article according to claim 3, comprising one or more selected from the group consisting of:
5. The hot stamped product according to claim 3 or 4, characterized in that the area ratio of the prior austenite grains having an average grain size of 0.5 to 3.0 µm is 60% or less.