WO2023162614A1 - Hot stamped compact - Google Patents

Abstract

Provided is a hot stamped compact that has a prescribed chemical composition, and has a metal structure containing at least 90 vol% martensite, wherein the average grain diameter of the prior austenite grains is 3.0 µm or less, the standard deviation in the grain diameter distribution of the prior austenite grains is 1.5 µm or less, and the difference between the minimum and maximum values of the Vickers hardness distribution in the plate thickness direction is 35% or less of the average value of the Vickers hardness distribution.

Description

hot stamped body

The present invention relates to a hot-stamped molded product.

In recent years, the automobile industry has been demanding lighter vehicle bodies from the perspective of improving fuel efficiency. Increasing the strength of the steel plate used is one of the effective ways to achieve both weight reduction of the vehicle body and collision safety. On the other hand, increasing the strength of a steel sheet lowers its formability, so it is generally difficult to achieve both strength and formability in a steel sheet.

In this regard, Patent Document 1 has a predetermined chemical composition, Ceq defined by C + 1/24Si + 1/6Mn + 1/40Ni + 1/5Cr + 1/4Mo + 1/14V is 0.10 to 1.00, and the metal structure However, a steel material containing 95.0% or more by volume of martensite, having a prior austenite grain size of 5.0 μm or less, and having a number of packets within the prior austenite grains of 3.0 or less is described. ing. Further, Patent Document 1 teaches that it is possible to obtain a steel material having an ultrafine structure with an average grain size of 5.0 μm or less and having excellent strength, ductility and toughness.

Hot stamping (hot press) is known as a technique for press forming steel materials that have high strength and are therefore difficult to form, as described in Patent Document 1. Hot stamping is a hot forming technique in which the material to be formed is heated and then formed. In this technique, since the material is heated and then formed, the steel material is soft and has good formability during forming. Therefore, even high-strength steel can be formed into complex shapes with high accuracy. In addition, since quenching is performed at the same time as forming with a press die, the steel after forming must have sufficient strength. It has been known.

In this regard, in Patent Document 2, it has a predetermined component composition, the microstructure includes prior austenite with an average grain size of 3 μm or less, and at least lower bainite, martensite, and tempered martensite One type is included in an area ratio of 90% or more, and defined by Z = (mass% of one or two types of Nb and Mo at the grain boundary) / (mass% of one or two types of Nb and Mo during melting) A hot-stamped body is described in which the grain boundary solid solution ratio Z is 0.3 or more. Further, in Patent Document 2, the grain size of the prior austenite is set to 3 μm or less, and one or both of Nb and Mo are dissolved in the prior austenite grain boundary to increase the embrittlement strength of the grain boundary. More specifically, the hot stamped body having the above configuration has a tensile strength of 2000 MPa or more, and early breakage is suppressed. ing.

In Patent Document 3, it has a predetermined component composition, the microstructure contains at least one of lower bainite, martensite and tempered martensite in an area ratio of 90% or more, and Z = (1 of Nb and Mo at the grain boundary The grain boundary solid solution ratio Z defined by (% by mass of one or two of Nb and Mo at the time of melting) is 0.4 or more, and the above lower bainite and martensite Or, the X-ray random intensity ratio of {112} <111> of the crystal grains constituting the tempered martensite is 2.8 or more, and the total number density of cementite and epsilon carbide having a grain size of 50 nm or less is 1 × 10 A steel plate for hot stamping is described which has ¹⁶ pieces/cm ³ or more. Further, in Patent Document 3, in a steel plate for hot stamping, by controlling the X-ray random intensity ratio of {112}<111>, which is the crystal orientation of the crystal grains of lower bainite or martensite or tempered martensite, It is taught that due to the texture memory effect of martensite, a crystal orientation with a high crack growth suppression effect is generated in the hot stamped product, and excellent bending deformability is obtained in the hot stamped product. More specifically, It is disclosed that the hot stamped body having the above structure has a tensile strength of 2000 MPa or more and a maximum bending angle of 50° or more.

JP-A-2020-012173 WO2019/186928 WO2019/186927

Since plastic deformation occurs when an impact exceeds the yield strength, it is required to improve not only the tensile strength but also the yield strength of hot stamped products from the viewpoint of ensuring the collision safety of automobiles. ing. However, hydrogen embrittlement cracking (also referred to as delayed fracture) may pose a problem in hot-stamped compacts having such high strength. Hydrogen embrittlement cracking is a phenomenon in which a steel member, which is subjected to high stress during use, suddenly fractures due to hydrogen entering the steel from the environment. It is generally known that hydrogen embrittlement cracking is more likely to occur as the strength of the steel increases. On the other hand, in the automobile industry and the like, there is a demand for further weight reduction of steel materials, and in order to achieve such weight reduction, it becomes necessary to increase the strength of steel materials more than ever. Therefore, there is a strong need for a steel material, more specifically a hot-stamped product, which can solve the problem of hydrogen embrittlement even when the strength is increased to a level equal to or higher than that of the conventional steel.

Therefore, an object of the present invention is to provide a hot-stamped molded article that has high strength and is capable of suppressing hydrogen embrittlement through a novel configuration.

In order to achieve the above objectives, the present inventors focused on the metallographic structure of the hot-stamped product. As a result, the present inventors have found that the prior austenite grains in the hot-stamped compact are refined and regulated, and in association with this, the hardness variation in the metallographic structure of the hot-stamped compact is reduced, resulting in microscopic In addition, the amount of hydrogen captured per unit grain boundary area can be reduced by increasing the grain boundary area due to the refinement and regulating of prior austenite grains. , and the combination of such microscopic stress concentration suppression or reduction and the reduction of the amount of trapped hydrogen per unit grain boundary area allows the hot stamped compact to have high tensile strength and yield strength. Regardless, the inventors have found that the hydrogen embrittlement resistance can be remarkably improved, and completed the present invention.

The present invention that has achieved the above object is as follows.
(1) in mass %,
C: 0.40 to 0.70%,
Si: 0.01 to 1.30%,
Mn: 0.05-3.00%,
P: 0.100% or less,
S: 0.0100% or less,
N: 0.0200% or less,
O: 0.0200% or less,
Al: 0.001 to 1.000%,
Cr: 0.01 to 1.00%,
Nb: 0 to 0.200%,
Ti: 0 to 0.200%,
Mo: 0 to 1.00%,
B: 0 to 0.1000%,
Co: 0 to 4.00%,
Ni: 0 to 3.00%,
Cu: 0 to 3.00%,
V: 0 to 3.00%,
W: 0 to 1.00%,
Ca: 0 to 1.000%,
Mg: 0-1.000%,
REM: 0 to 1.000%,
Sb: 0 to 1.000%,
Zr: 0 to 1.000%,
Sn: 0 to 1.000%,
As: 0 to 0.100%, and the balance: having a chemical composition consisting of Fe and impurities,
90% or more of martensite is included in the volume fraction,
The average grain size of the prior austenite grains is 3.0 μm or less,
The standard deviation in the grain size distribution of the prior austenite grains is 1.5 μm or less,
A hot-stamped article having a metallographic structure in which the difference between the maximum value and the minimum value of the Vickers hardness distribution in the plate thickness direction is 35% or less of the average value of the Vickers hardness distribution.
(2) the chemical composition, in mass %,
Nb: 0.001 to 0.200%,
Ti: 0.001 to 0.200%,
Mo: 0.001 to 1.00%,
B: 0.0001 to 0.1000%,
Co: 0.001 to 4.00%,
Ni: 0.001 to 3.00%,
Cu: 0.001 to 3.00%,
V: 0.001 to 3.00%,
W: 0.001 to 1.00%,
Ca: 0.0001 to 1.000%,
Mg: 0.0001-1.000%,
REM: 0.0001 to 1.000%,
Sb: 0.001 to 1.000%,
Zr: 0.001 to 1.000%,
Sn: 0.001 to 1.000%, and As: 0.001 to 0.100%
The hot stamped article according to (1) above, which contains one or more selected from the group consisting of:

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

<Hot stamp molded product>
The hot stamped body according to the embodiment of the present invention is mass%,
C: 0.40 to 0.70%,
Si: 0.01 to 1.30%,
Mn: 0.05-3.00%,
P: 0.100% or less,
S: 0.0100% or less,
N: 0.0200% or less,
O: 0.0200% or less,
Al: 0.001 to 1.000%,
Cr: 0.01 to 1.00%,
Nb: 0 to 0.200%,
Ti: 0 to 0.200%,
Mo: 0 to 1.00%,
B: 0 to 0.1000%,
Co: 0 to 4.00%,
Ni: 0 to 3.00%,
Cu: 0 to 3.00%,
V: 0 to 3.00%,
W: 0 to 1.00%,
Ca: 0 to 1.000%,
Mg: 0-1.000%,
REM: 0 to 1.000%,
Sb: 0 to 1.000%,
Zr: 0 to 1.000%,
Sn: 0 to 1.000%,
As: 0 to 0.100%, and the balance: having a chemical composition consisting of Fe and impurities,
90% or more of martensite is included in the volume fraction,
The average grain size of the prior austenite grains is 3.0 μm or less,
The standard deviation in the grain size distribution of the prior austenite grains is 1.5 μm or less,
It is characterized by having a metallographic structure in which the difference between the maximum value and the minimum value of the Vickers hardness distribution in the plate thickness direction is 35% or less of the average value of the Vickers hardness distribution.

As mentioned earlier, it is known that hydrogen embrittlement cracking is more likely to occur as the strength of the steel increases. For example, in high-strength steel materials, the metal structure generally contains martensite in order to ensure high strength, and especially in hot stamped products with a tensile strength of 2000 MPa or more, the metal structure is controlled to have martensite as the main phase. in many cases. Such a martensite-based structure has a high dislocation density, while dislocations can become hydrogen trap sites. Therefore, there is a continuing need for solving the problem of hydrogen embrittlement in high-strength hot-stamped products containing martensite as the main phase. Therefore, the present inventors have found that in such a high-strength hot stamped body having martensite as the main phase, from the viewpoint of reducing or suppressing the region that can be the starting point of hydrogen embrittlement cracking, especially the hot stamping The study focused on the metal structure of the body. More specifically, the present inventors first found that if the prior-austenite grain size varies greatly in the metallographic structure of the hot-stamped compact, the hardness increases in a region with a smaller prior-austenite grain size. It was found that local high-hardness regions can be the origin of hydrogen embrittlement cracking. In response to this, the present inventors refined the prior austenite grains to reduce the average grain size to 3.0 μm or less, and controlled the standard deviation in the grain size distribution to 1.5 μm or less to regulate the grain size. Relatedly, by controlling the difference between the maximum value and the minimum value of the Vickers hardness distribution in the plate thickness direction to 35% or less of the average value of the Vickers hardness distribution, such a local increase in hardness is suppressed. It has been found that the microscopic stress concentration can be reliably suppressed or reduced by

Although it is not intended to be bound by any particular theory, it is believed that during hot stamping, the starting temperature of martensite transformation changes according to the grain size of the austenite grains. More specifically, austenite grains with a larger grain size have a higher starting temperature for martensite transformation than austenite grains with a smaller grain size, and auto-tempering occurs between the completion of transformation and cooling to room temperature. It is thought that the hardness becomes lower in order to advance. Austenite grains with a smaller grain size undergo a martensitic transformation at a lower temperature than larger grains, resulting in an increase in hardness. Therefore, in order to suppress or reduce such a local increase in hardness, it is important to reduce the variation in austenite grain size before martensite transformation. In other words, reducing the variability in the austenite grain size before martensitic transformation can reduce the variability in the prior austenite grain size after the martensitic transformation, resulting in a hardness increase in the metallographic structure of the hot stamped compact. It is considered possible to reduce the variation of For this reason, the difference between the maximum value and the minimum value of the Vickers hardness distribution in the metal structure of the hot stamped product is controlled to 35% or less of the average value of the Vickers hardness distribution to reduce the hardness variation. As a result, it is considered possible to remarkably suppress local increases in hardness due to differences in the timing of martensite transformation. If there is a locally high hardness region, it is highly likely that stress will concentrate on the interface of the prior austenite grains, which have a difference in hardness, and hydrogen embrittlement cracking will occur. It is very effective to suppress or reduce microscopic stress concentration and improve hydrogen embrittlement resistance.

Furthermore, in the hot stamped body according to the embodiment of the present invention, the prior austenite grains have an average grain size of 3.0 μm or less, and are therefore very fine, and the standard deviation in the grain size distribution of the prior austenite grains is is controlled to 1.5 μm or less, that is, prior austenite grains are highly regulated. The fine prior austenite grains are considered to undergo martensite transformation at a lower temperature as described above, and since they are highly grain-regulated, the finer and grain-regulated prior austenite grains A metal structure containing austenite grains is considered to undergo martensite transformation at a lower temperature than the entire metal structure. Therefore, a hot-stamped compact containing such a metal structure can significantly improve the hardness as a whole, so that extremely high strength, specifically, extremely high tensile strength and yield strength can be achieved. It becomes possible to In addition, in the metal structure containing such refined and regulated prior austenite grains, the grain boundary area is greatly increased, and as a result, the amount of hydrogen captured per unit grain boundary area is significantly reduced. can be made Hydrogen embrittlement cracking is caused, for example, by hydrogen entering the steel from the environment being trapped at grain boundaries under high stress. Therefore, by reducing the amount of hydrogen captured per unit grain boundary area, it is possible to greatly reduce the risk of hydrogen embrittlement cracking. Therefore, according to the hot stamped body according to the embodiment of the present invention, in addition to the effect of improving the hydrogen embrittlement resistance due to the reduction in hardness variation described above, the prior austenite grains are refined and the grains are regulated. The hydrogen embrittlement resistance of the hot-stamped product can be improved also from the viewpoint of the increase in the grain boundary area caused by the quenching.

The hot-stamped article according to the embodiment of the present invention will be described in more detail below. In the following description, the unit of content of each element, "%", means "% by mass" unless otherwise specified. In addition, in this specification, the term "to" indicating a numerical range is used to include the numerical values before and after it as a lower limit and an upper limit, unless otherwise specified.

[C: 0.40 to 0.70%]
C is an element that improves the strength of the hot stamped compact. If the C content is less than 0.40%, the desired strength cannot be obtained in the hot-stamped product. Therefore, the C content is made 0.40% or more. The C content is preferably more than 0.40%, 0.42% or more, 0.44% or more, or 0.45% or more.
On the other hand, if the C content exceeds 0.70%, the strength becomes too high, and excellent hydrogen embrittlement resistance may not be obtained. Therefore, the C content is made 0.70% or less. Preferably, the C content is 0.68% or less, 0.67% or less, 0.65% or less or 0.60% or less.

[Si: 0.01 to 1.30%]
Si is an element that improves the strength of hot-stamped products by solid-solution strengthening. If the Si content is less than 0.01%, the desired strength cannot be obtained. Therefore, the Si content is set to 0.01% or more. Si content is preferably 0.05% or more, 0.10% or more, 0.20% or more, 0.25% or more, 0.26% or more, 0.27% or more, 0.30% or more or 0 .40% or more.
On the other hand, if the Si content exceeds 1.30%, the amount of ferrite increases and a desired metal structure may not be obtained. Therefore, the Si content is set to 1.30% or less. The Si content is preferably 1.20% or less, 1.00% or less, 0.80% or less, 0.60% or less, or 0.50% or less.

[Mn: 0.05 to 3.00%]
Mn is an element that enhances the hardenability of steel and contributes to the improvement of strength. If the Mn content is less than 0.05%, such effects cannot be sufficiently obtained. Therefore, the Mn content is set to 0.05% or more. The Mn content is preferably 0.10% or more, 0.50% or more, 1.00% or more, 1.30% or more, or 1.50% or more.
On the other hand, if the Mn content exceeds 3.00%, Mn segregation becomes significant, and this may result in insufficient suppression of variations in the prior austenite grain size. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.80% or less, 2.50% or less, 2.30% or less or 2.00% or less.

[P: 0.100% or less]
P is an impurity element and segregates at grain boundaries to deteriorate hydrogen embrittlement resistance. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.070% or less, 0.050% or less, or 0.010% or less.
The lower limit of the P content is not particularly limited. Therefore, the P content may be 0.0001% or more.

[S: 0.0100% or less]
S is an impurity element and forms inclusions in steel. Since these inclusions degrade hydrogen embrittlement resistance, the S content should be 0.0100% or less. The S content is preferably 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.
The lower limit of the S content is not particularly limited, but if it is reduced to less than 0.0001%, the deS cost will increase significantly, which is economically unfavorable. Therefore, the S content may be 0.0001% or more.

[N: 0.0200% or less]
N is an impurity element and forms nitrides in steel. Since this nitride deteriorates the hydrogen embrittlement resistance, the N content is made 0.0200% or less. The N content is preferably 0.0180% or less, 0.0150% or less, 0.0100% or less, 0.0060% or less, or 0.0040% or less.
The lower limit of the N content is not particularly limited, but if it 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]
When O is contained in a large amount in steel, it forms coarse oxides and degrades hydrogen embrittlement resistance. Therefore, the O content is set to 0.0200% or less. The O content is preferably 0.0150% or less, 0.0100% or less, 0.0070% or less, or 0.0040% or less.
From the viewpoint of reducing refining costs, the O content may be 0.0001% or more. The O content may be 0.0005% or more in order to disperse a large number of fine oxides when deoxidizing molten steel.

[Al: 0.001 to 1.000%]
Al is an element that has the effect of deoxidizing molten steel and making the steel sound. If the Al content is less than 0.001%, deoxidation is not sufficiently performed, and coarse oxides are formed, degrading hydrogen embrittlement resistance. Therefore, the Al content is set to 0.001% or more. The Al content is preferably 0.003% or more, 0.005% or more, 0.010% or more, or 0.030% or more.
On the other hand, if the Al content exceeds 1.000%, coarse oxides are formed in the steel, and the hydrogen embrittlement resistance of the hot stamped product is lowered. Therefore, the Al content is set to 1.000% or less. The Al content is preferably 0.800% or less, 0.600% or less, 0.400% or less, 0.200% or less, or 0.100% or less.

[Cr: 0.01 to 1.00%]
Cr is an element that enhances the strength of the hot-stamped body by forming a solid solution in prior austenite grains during heating before hot-stamping. If the Cr content is less than 0.01%, the desired strength cannot be obtained. Therefore, the Cr content is set to 0.01% or more. The Cr content is preferably 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more.
On the other hand, when the Cr content exceeds 1.00%, coarse intermetallic compounds are formed in the hot-stamped body, degrading the hydrogen embrittlement resistance of the hot-stamped body. Therefore, the Cr content is set to 1.00% or less. The Cr content is preferably 0.80% or less, 0.60% or less, 0.50% or less or 0.40% or less.

The basic chemical composition of the hot stamped body according to the embodiment of the present invention is as described above. Furthermore, the hot-stamped compact may contain at least one of the following optional elements in place of part of the remaining Fe, if necessary. For example, the hot stamped body contains Nb: 0-0.200%, Ti: 0-0.200%, Mo: 0-1.00%, B: 0-0.1000%, Co: 0-4. 00%, Ni: 0 to 3.00%, Cu: 0 to 3.00%, V: 0 to 3.00%, and W: 0 to 1.00%. You may In addition, the hot stamped body may contain at least one selected from the group consisting of Ca: 0 to 1.000%, Mg: 0 to 1.000% and REM: 0 to 1.000%. . In addition, the hot stamped product may contain at least one selected from the group consisting of Sb: 0 to 1.000%, Zr: 0 to 1.000% and Sn: 0 to 1.000%. . Also, the hot stamped product may contain As: 0 to 0.100%. These optional elements are described in detail below.

[Nb: 0 to 0.200%]
Nb is an element that forms carbonitrides in steel and improves the strength of hot stamped products by precipitation strengthening. Although the Nb content may be 0.001% or more, the Nb content is preferably 0.010% or more or 0.020% or more in order to reliably obtain this effect.
On the other hand, even if the Nb content is large, the above effect is saturated, so the Nb content is preferably 0.200% or less. The Nb content may be 0.180% or less, 0.150% or less, 0.100% or less, 0.080% or less, or 0.060% or less.

[Ti: 0 to 0.200%]
Ti is an element that forms carbonitrides in steel and improves the strength of hot-stamped products by precipitation strengthening. Although the Ti content may be 0.001% or more, the Ti content is preferably 0.010% or more or 0.020% or more in order to reliably obtain this effect.
On the other hand, even if the Ti content is large, the above effect is saturated, so the Ti content is preferably 0.200% or less. The Ti content may be 0.180% or less, 0.150% or less, 0.100% or less, 0.080% or less, or 0.060% or less.

[Mo: 0 to 1.00%]
Mo is an element that improves the hardenability of steel. The Mo content may be 0.001% or more, but in order to reliably obtain this effect, the Mo content is preferably 0.005% or more or 0.01% or more.
On the other hand, even if it is contained in a large amount, the above effect is saturated, so the Mo content is preferably 1.00% or less. The Mo content may be 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.10% or less.

[B: 0 to 0.1000%]
B is an element that improves the hardenability of steel. Although the B content may be 0.0001% or more, the B content is preferably 0.0005% or more or 0.0010% or more in order to reliably obtain this effect.
On the other hand, even if it is contained in a large amount, the above effect is saturated, so the B content is preferably 0.1000% or less. The B content may be 0.0500% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0015% or less.

[Co: 0 to 4.00%]
Co is an element that improves the strength of hot-stamped products by solid-solution strengthening. Although the Co content may be 0.001% or more, the Co content is preferably 0.01% or more or 0.05% or more in order to reliably obtain this effect.
On the other hand, even if the Co content is large, the above effect is saturated, so the Co content is preferably 4.00% or less. The Co content may be 3.00% or less, 2.00% or less, 1.00% or less, 0.80% or less, or 0.60% or less.

[Ni: 0 to 3.00%]
Ni forms a solid solution in the austenite grains during heating in the hot stamp molding process, thereby increasing the strength of the hot stamp molded body. Although the Ni content may be 0.001% or more, the Ni content is preferably 0.01% or more in order to reliably obtain this effect.
On the other hand, even if the Ni content is large, the above effect is saturated, so the Ni content is preferably 3.00% or less. The Ni content may be 2.80% or less, 2.50% or less, 2.00% or less, 1.50% or less, 1.00% or less, or 0.80% or less.

[Cu: 0 to 3.00%]
Cu has the effect of increasing the strength of the hot stamped body by forming a solid solution in the austenite grains during heating in the hot stamping process. Although the Cu content may be 0.001% or more, the Cu content is preferably 0.01% or more or 0.05% or more in order to reliably obtain this effect.
On the other hand, even if the Cu content is large, the above effect is saturated, so the Cu content is preferably 3.00% or less. The Cu content may be 2.00% or less, 1.00% or less, 0.50% or less, 0.30% or less, or 0.10% or less.

[V: 0 to 3.00%]
V has the effect of forming carbonitrides in steel and improving the strength of hot-stamped products through precipitation strengthening. Although the V content may be 0.001% or more, the V content is preferably 0.01% or more or 0.05% or more in order to reliably obtain this effect.
On the other hand, even if the V content is large, the above effect is saturated, so the V content is preferably 3.00% or less. The V content may be 2.00% or less, 1.00% or less, 0.50% or less, 0.30% or less, or 0.10% or less.

[W: 0 to 1.00%]
W is an element that improves the hardenability of steel. Although the W content may be 0.001% or more, the W content is preferably 0.005% or more or 0.01% or more in order to reliably obtain this effect.
On the other hand, the W content is preferably 1.00% or less because the above effect is saturated even if the W content is large. The W content may be 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.10% or less.

[Ca: 0 to 1.000%]
Ca is an element that can control the morphology of sulfide. Although the Ca content may be 0.0001% or more, the Ca content is preferably 0.0005% or more or 0.001% or more in order to reliably obtain this effect.
On the other hand, the Ca content is preferably 1.000% or less because the above effect is saturated even if the Ca content is large. The Ca content may be 0.500% or less, 0.100% or less, 0.050% or less, 0.010% or less, 0.005% or less, or 0.002% or less.

[Mg: 0 to 1.000%]
Mg is an element that can control the morphology of sulfides. Although the Mg content may be 0.0001% or more, the Mg content is preferably 0.0005% or more or 0.001% or more in order to reliably obtain this effect.
On the other hand, even if the Mg content is large, the above effect is saturated, so the Mg content is preferably 1.000% or less. The Mg content may be 0.500% or less, 0.100% or less, 0.050% or less, 0.010% or less, 0.005% or less, or 0.002% or less.

[REM: 0 to 1.000%]
REM is an element that can control the morphology of sulfides. Although the REM content may be 0.0001% or more, the REM content is preferably 0.0005% or more or 0.001% or more in order to reliably obtain this effect.
On the other hand, since the above effect is saturated even if it is contained in a large amount, the REM content is preferably 1.000% or less. The REM content may be 0.500% or less, 0.100% or less, 0.050% or less, 0.010% or less, 0.005% or less, or 0.002% or less.
In the present embodiment, REM means scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanide (La) with atomic number 57 to lutetium with atomic number 71. (Lu) is a general term for the 17 elements, and the REM content is the total content of these elements.

[Sb: 0 to 1.000%]
Sb is an element that suppresses the formation of oxides. In order to reliably obtain this effect, the Sb content is preferably 0.001% or more.
On the other hand, even if the Sb content is large, the above effect is saturated, so the Sb content is preferably 1.000% or less. The Sb content may be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.050% or less.

[Zr: 0 to 1.000%]
Zr is an element that suppresses the formation of oxides. In order to reliably obtain this effect, the Zr content is preferably 0.001% or more.
On the other hand, since the above effect is saturated even if the Zr content is large, the Zr content is preferably 1.000% or less. The Zr content may be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.050% or less.

[Sn: 0 to 1.000%]
Sn is an element that suppresses the formation of oxides. In order to reliably obtain this effect, the Sn content is preferably 0.001% or more.
On the other hand, even if the content is large, the above effect is saturated, so the Sn content is preferably 1.000% or less. The Sn content may be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.050% or less.

[As: 0 to 0.100%]
As lowers the austenite single phase temperature, thereby contributing to the refinement of prior austenite grains. 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 preferably 0.100% or less. The As content may be 0.080% or less, 0.050% or less, 0.020% or less, 0.010% or less, or 0.005% or less.

In the hot stamped compact according to the embodiment of the present invention, the balance other than the above elements consists of Fe and impurities. Impurities are components and the like that are mixed due to various factors in the manufacturing process, including raw materials such as ores and scraps, when industrially manufacturing hot stamped products.

The chemical composition of the hot-stamped body described above may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Incidentally, C and S may be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-nondispersive infrared absorption method.
When the surface of the hot stamped body is provided with a plating layer, the chemical composition may be analyzed after removing the plating layer by mechanical grinding.

[Martensite: 90% or more]
The metal structure of the hot-stamped article according to the embodiment of the present invention contains 90% or more of martensite in volume fraction. Although the residual structure is not particularly limited, it may consist of at least one of bainite, ferrite, retained austenite and pearlite in an amount of 10% or less. Martensite is a very hard structure, so by including 90% or more by volume of martensite in a hot stamped product, high tensile strength and yield strength, specifically a tensile strength of 2200 MPa or more and a yield strength of 1800 MPa or more can be achieved. On the other hand, when the volume fraction of martensite is low and the ratio of soft structures such as ferrite is high, the variation in hardness in the plate thickness direction becomes remarkable, and the difference between the maximum and minimum values of the Vickers hardness distribution in the plate thickness direction ratio to the average value of Therefore, the larger the volume fraction of martensite, the better. For example, it may be 92% or more, 94% or more, 96% or more, or 98% or more. The upper limit of the volume fraction of martensite is not particularly limited and may be 100%.

[Identification of metal structure and calculation of volume fraction]
The identification of the metal structure and the calculation of the volume fraction in the hot-stamped product are performed as follows. First, a sample is taken so that a cross section parallel to the plate thickness direction of the hot stamped body serves as an observation surface. Next, the observation surface is mirror-polished, corroded with a nital corrosive solution, and then subjected to structural observation using a scanning electron microscope (SEM). A range of 300 μm×300 μm is photographed at a magnification of 1000, centering on the depth position of 1/4 of the plate thickness of the observation surface. The obtained microstructure photograph is subjected to black and white binarization processing, image analysis is performed, pearlite, bainite and ferrite are identified, and "steel-microscopic test method for grain size" specified in JIS G 0551: 2020. Calculate the total amount of those area ratios using the method based on Furthermore, the conversion from the area ratio of the structure to the volume ratio is performed by the line segment method. The line segment method is described, for example, by Robert T. et al. DeHoff, Frederik N.; Rhines co-ed. (Quantitative Microscopy, 1968). Since it is difficult to distinguish retained austenite from martensite by SEM, the volume fraction of retained austenite is measured by the X-ray diffraction method. Finally, the volume fraction of martensite is determined by subtracting the total volume fraction of pearlite, bainite, ferrite and retained austenite obtained by the above method from 100%.

[Average grain size of prior austenite grains: 3.0 μm or less]
In an embodiment of the present invention, the average grain size of prior austenite grains is 3.0 μm or less. Such refinement of the prior austenite grains, in combination with the grain size regulation of the prior austenite grains, which will be described later, contributes to the enhancement of the strength of the hot stamped compact, and also increases the grain boundary area to increase the unit grain boundary. By reducing the amount of hydrogen captured per area, the hydrogen embrittlement resistance of the hot stamped product can be improved. From the viewpoint of increasing the strength of the hot stamped product and improving the hydrogen embrittlement resistance, the smaller the average grain size of the prior austenite grains, the better. 0 μm or less. Although the lower limit is not particularly limited, the average grain size of the prior austenite grains may be, for example, 1.0 μm or more, 1.2 μm or more, or 1.5 μm or more.

[Standard deviation in grain size distribution of prior austenite grains: 1.5 μm or less]
In an embodiment of the present invention, the standard deviation in the grain size distribution of prior austenite grains is 1.5 μm or less. By suppressing the variation in the prior-austenite grain size by regulating the prior-austenite grains in this way, as explained in relation to the average grain size of the prior-austenite grains, the strength of the hot-stamped compact can be increased, and furthermore, the grain boundary Hydrogen embrittlement resistance can be improved due to the increase in area. In addition, according to the embodiment of the present invention, the standard deviation in the grain size distribution of the prior austenite grains is controlled to 1.5 μm or less to reduce the variation in the grain size of the prior austenite grains, thereby reducing the variation in hardness. As a result, it is possible to remarkably suppress a local increase in hardness in the hot-stamped article. By suppressing the local hardness increase in the hot stamped product, it is possible to reliably suppress or reduce the microscopic stress concentration, so that the hydrogen embrittlement resistance of the hot stamped product is further improved. becomes possible. From the viewpoint of improving these effects, it is preferable that the standard deviation in the grain size distribution of the prior austenite grains is as small as possible. . Although the lower limit is not particularly limited, the standard deviation may be, for example, 0.1 μm or more, 0.2 μm or more, or 0.4 μm or more.

[Method for Determining Standard Deviation in Average Grain Size and Grain Size Distribution of Prior Austenite Grains]
The average grain size of prior austenite grains and the standard deviation in the grain size distribution are determined as follows. First, 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 perpendicular to the surface 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 direction perpendicular to the plate thickness direction. After polishing the cross section of the above sample using silicon carbide paper of #600 to #1500, a diamond powder with a particle size of 1 to 6 μm is dispersed in a dilute solution such as alcohol or pure water to make a mirror finish. . Next, the observation surface is finished by electropolishing. An area of 50 μm in length and 50 μm in the plate thickness direction centered at the 1/4 depth position of the plate thickness at an arbitrary position in the longitudinal direction of the sample cross section is measured by the electron backscatter diffraction method at a measurement interval of 0.1 μm. to obtain crystal orientation information. For the measurement, an EBSD analyzer composed of a thermal field emission scanning electron microscope and an EBSD detector may be used. For example, an EBSD analyzer composed of JEOL JSM-7001F and TSL DVC5 type detector may be used. You can use it. At this time, the degree of vacuum in the EBSD analysis apparatus may be 9.6×10 ⁻⁵ Pa or less, the acceleration voltage may be 15 kV, and the irradiation current level may be 13. Using the obtained crystal orientation information, the crystal orientation of prior austenite grains is calculated from the crystal orientation relationship between general prior austenite grains and crystal grains having a body-centered structure after transformation. The following method is used to calculate the crystal orientation of prior austenite grains. First, a crystal orientation map of prior austenite grains is created by the method described in Acta Materialia, 58 (2010), 6393-6403. 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 end of the imaging field, and obtain the grain size of all prior austenite grains in the imaging field. . By calculating the average grain size and the standard deviation from the obtained grain sizes of all the prior austenite grains, the average grain size of the prior austenite grains and the standard deviation in the grain size distribution are determined.

[Difference between the maximum value and the minimum value of the Vickers hardness distribution in the plate thickness direction: 35% or less of the average value of the Vickers hardness distribution]
In an embodiment of the present invention, the difference between the maximum value and the minimum value of the Vickers hardness distribution in the plate thickness direction is 35% or less of the average value of the Vickers hardness distribution. By suppressing the variation in the Vickers hardness distribution within such a range, it is possible to remarkably suppress a local increase in hardness in the hot stamped product. As a result, microscopic stress concentration can be reliably suppressed or reduced, so that the hydrogen embrittlement resistance of the hot stamped product can be improved. From the viewpoint of improving the hydrogen embrittlement resistance of the hot stamped product, the smaller the difference between the maximum value and the minimum value of the Vickers hardness distribution, the better. Below, it may be 20% or less or 15% or less. Although the lower limit is not particularly limited, the difference between the maximum value and the minimum value of the Vickers hardness distribution may be, for example, 1% or more, 3% or more, or 5% or more of the average value of the Vickers hardness distribution.

As mentioned above, it is possible to reduce the variation in hardness by reducing the variation in the grain size of the prior austenite. However, in the embodiment of the present invention, the standard deviation in the grain size distribution of the prior austenite grains is measured and determined in the region centered on the same 1/4 plate thickness depth position, whereas the Vickers The difference between the maximum value and the minimum value of hardness distribution is determined based on Vickers hardness values measured at various depth positions in the plate thickness direction. Therefore, when the standard deviation in the grain size distribution of the prior austenite grains satisfies 3.0 μm or less, the difference between the maximum value and the minimum value of the Vickers hardness distribution in the plate thickness direction naturally becomes the average value of the Vickers hardness distribution. We cannot say that it will be less than 35%. In addition, the effect obtained by reducing the variation in prior austenite grain size does not exactly match the effect obtained by reducing the variation in hardness. For example, as explained above, the reduction of the variation in the prior austenite grain size (regular grain size) is due to the increase in the strength of the hot stamped compact and the increase in the grain boundary area in combination with the prior austenite grain size. While it contributes to the improvement of hydrogen embrittlement resistance, the reduction in hardness variation suppresses the local increase in hardness in the hot stamped product and suppresses the microscopic stress concentration. and thereby improve the hydrogen embrittlement resistance of the hot stamped compact. The object of the present invention, ``to provide a hot stamped body that is high in strength and capable of suppressing hydrogen embrittlement,'' is, in addition to the chemical composition of the hot stamped body, the average grain size and grain size distribution of the prior austenite grains. and the difference between the maximum and minimum values of the Vickers hardness distribution. It cannot be achieved only by defining the standard deviation in the particle size distribution.

[Method for determining the difference between the maximum value and the minimum value of the Vickers hardness distribution in the plate thickness direction]
The difference between the maximum value and the minimum value of the Vickers hardness distribution in the plate thickness direction is determined as follows. First, a sample is cut from an arbitrary position 50 mm or more away from the end face of the hot stamped body so that a cross section perpendicular to the surface (thickness cross section) can be observed. The size of the sample should be such that it can be observed by 10 mm in the direction perpendicular to the plate thickness direction, depending on the measurement equipment. After polishing the cross section of the sample using silicon carbide paper of #600 to #1500, a liquid of diamond powder with a grain size of 1 to 6 μm dispersed in diluted liquid such as alcohol and pure water is used to mirror finish. 25 to 25 μm in the plate thickness direction from a depth of 100 μm from the surface of the hot stamped product (when the plating layer is present on the surface, the surface of the hot stamped product excluding the plating layer) for the mirror-finished cross section Vickers hardness is measured at intervals of 30 μm using a micro Vickers hardness tester with a load of 25 gf, and Vickers hardness is obtained at a distance of more than half the plate thickness excluding 100 μm from the front and back surfaces of the hot stamped product. . It is necessary that the distance between the measurement points arranged in the plate thickness direction (the distance between the centers of the indentations) be three times or more the distance of the indentations. A distance of three times or more of the indentation means a distance of three times or more of the diagonal length of the rectangular opening of the indentation produced by the diamond indenter during Vickers hardness measurement. If the hot stamped product to be measured is thin and it is difficult to stamp linearly in the plate thickness direction while setting the distance between each measurement point to a distance of three times or more the indentation, the distance between each measurement point should be the distance of the indentation. It may be stamped zigzag in the plate thickness direction with a distance of three times or more. Finally, select the maximum and minimum values from all Vickers hardness measurements and calculate the difference between them, and calculate the difference between the calculated maximum and minimum values and all Vickers hardness measurements From the average value obtained by averaging, the ratio of the difference between the maximum value and the minimum value of the Vickers hardness distribution in the plate thickness direction to the average value is determined.

[Plating]
The hot-stamped article according to this embodiment may have a plating layer on its surface. Corrosion resistance can be improved by having a plating layer on the surface. As plating layers, aluminum plating layer, aluminum-zinc plating layer, aluminum-silicon plating layer, hot-dip galvanization layer, electrogalvanization layer, alloyed hot-dip galvanizing layer, zinc-nickel plating layer, aluminum-magnesium-zinc system A plating layer etc. are illustrated.

[Mechanical properties]
According to the hot-stamped article according to the embodiment of the present invention, excellent mechanical properties such as tensile strength of 2200 MPa or more can be achieved. The tensile strength is preferably 2300 MPa or higher, more preferably 2400 MPa or higher, most preferably 2500 MPa or higher. Although the upper limit is not particularly limited, for example, the tensile strength may be 3500 MPa or less, 3300 MPa or less, or 3000 MPa or less. Similarly, according to the hot-stamped article according to the embodiment of the present invention, a yield strength of 1800 MPa or more can be achieved. The yield strength is preferably 1900 MPa or higher, more preferably 2000 MPa or higher, most preferably 2100 MPa or higher. Although the upper limit is not particularly limited, the yield strength may be, for example, 3000 MPa or less, 2800 MPa or less, or 2500 MPa or less. The tensile strength and yield strength of the hot stamped product are measured by preparing a No. 5 test piece and performing a tensile test in accordance with JIS Z 2241:2011.

The hot stamped article according to the embodiment of the present invention has excellent hydrogen embrittlement resistance despite having a high tensile strength of 2200 MPa or more and a high yield strength of 1800 MPa or more, as described above. For example, it is very useful for use as a frame member for automobiles, bumpers, and other structural members and reinforcing members that require strength.

<Manufacturing method of hot stamp molded body>
Next, a preferred method for manufacturing a hot stamped body according to the embodiment of the present invention will be described. The following description is intended to be illustrative of a particular method for manufacturing hot stamped bodies according to embodiments of the present invention, wherein the hot stamped bodies are manufactured by a manufacturing method as described below. It is not intended to be limited to what is manufactured.

In the method for producing a hot stamped compact according to an embodiment of the present invention, the prior austenite grains are refined and regulated, and in association with this, tempered martensite is mainly used to reduce variations in hardness. It is characterized by cold working a hot-rolled steel sheet having a metallographic structure and then rapidly heating it during hot stamping. More specifically, the method for manufacturing a steel sheet for hot stamping according to an embodiment of the present invention includes:
hot rolling a slab having the chemical composition described above in relation to hot stamped bodies, then cooling at an average cooling rate of 50° C./s or more and coiling at a temperature below the M point (hot rolling process),
a step of tempering the obtained hot-rolled steel sheet in a temperature range of 400 to 600°C (tempering step);
A step of cold-rolling the tempered hot-rolled steel sheet at a rolling reduction of 30% or more (cold-rolling step), and heat-treating the obtained cold-rolled steel sheet from 600°C at an average heating rate of more than 100°C/sec. : A process of heating to A3 + 50 to A3 + 150 ° C. and holding for 30 seconds or less, then starting hot stamp molding until the temperature reaches 550 ° C. and cooling at an average cooling rate of 10 ° C./sec or more (hot stamp molding process)
is characterized by including

In order to obtain fine prior austenite grains in the metal structure of the final hot stamped compact, it is important to form a large number of austenite nucleation sites during heating in the hot stamping process. In this connection, in the martensitic structure, prior austenite grain boundaries can serve as austenite nucleation sites. It is known that their boundaries (interfaces) can also serve as austenite nucleation sites. For this reason, the martensite structure has many austenite nucleation sites compared to structures such as ferrite and pearlite, for example. Therefore, in the present manufacturing method, first, in the hot rolling step, the hot-rolled slab is cooled at a cooling rate equal to or higher than the critical cooling rate, more specifically, at an average cooling rate equal to or higher than 50°C/sec to obtain the Ms point. By winding at the following temperature, a hot-rolled steel sheet having a structure mainly composed of martensite is formed. Next, by heat-treating the obtained hot-rolled steel sheet in an appropriate temperature range in the tempering process, a large number of carbides are generated (precipitated ). The interface between many of the precipitated carbides and these substructures also become major austenite nucleation sites during heating in the hot stamping process. Therefore, by performing the hot stamping process after the tempering process, austenite is extracted from more austenite nucleation sites than when the hot stamping process is performed without the tempering process. It is possible to generate

Next, by cold working (cold rolling) the tempered hot-rolled steel sheet at a predetermined reduction rate, the metal structure can be refined and the interface of the substructure can be increased, resulting in austenite Nucleation sites can be further increased. For example, in the tempering process, the carbides precipitated at the interface of the substructure such as packets, blocks and laths in the same grain have a close orientation relationship, so if they are heated as they are in the hot stamping process, these carbides and The orientation relationship of the austenite that forms the interface with the underlying structure as a nucleation site can also be close. In such a case, the generated austenite grains are likely to be connected, making it difficult to keep the final metal structure fine. On the other hand, by cold rolling at a predetermined rolling reduction, the orientation relationship between carbides as described above is changed, and the interface between these carbides and the substructure is used as a nucleation site. can be randomized. As a result, the formed austenite is less likely to be connected, and therefore the metal structure finally obtained in the subsequent hot stamping process can be maintained not only in a fine but also in a regular grain state. Finally, a cold-rolled steel sheet having a metallographic structure mainly composed of cold-worked tempered martensite is rapidly heated at a predetermined average heating rate in a temperature zone where recrystallization progresses during heating for hot stamping. be. When recrystallization progresses during heating for hot stamping, the austenite nucleation sites formed in the previous steps are reduced or lost, so the final metal structure achieves the desired refinement and grain size regulation. I can't do it. Therefore, in the present production method, recrystallization is suppressed by passing through such a recrystallization progress temperature zone in a short time with a predetermined rapid heating, and the number of austenite nucleation sites caused by the progress of the recrystallization is reduced. Alternatively, by suppressing the disappearance, it is possible to refine the finally obtained metal structure and to make the grains uniform. According to the present manufacturing method, since the austenite grains refined during the manufacturing process do not connect or are difficult to connect, the prior austenite grains obtained after heating and cooling in the hot stamping process can be simply refined. Instead, it becomes possible to regulate the grain size, and as a result, it becomes possible to reduce the variation in the grain size of prior austenite and to reliably reduce the variation in hardness. Each step will be described in more detail below.

[Hot rolling process]
The hot rolling process first heats a slab having the chemical composition described above in connection with the hot stamped compact. The casting method of molten steel is not particularly limited, and it may be manufactured by a continuous casting method, an ingot casting method, or a thin slab casting method. Heating before hot rolling is not particularly limited, but the slab used contains relatively large amounts of alloying elements in order to obtain a high-strength steel sheet. Therefore, the heating temperature may be 1100° C. or higher for the purpose of heating the slab before subjecting it to hot rolling to dissolve the alloying elements in the slab. In addition, the heated slab may optionally be subjected to rough rolling before finish rolling for thickness adjustment and the like. Conditions for the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured. The heated slab, or optionally rough rolled slab, is then subjected to finish rolling. Finish rolling is not particularly limited, but is generally carried out under such conditions that the completion temperature of finish rolling is 650° C. or higher. This is because if the finish rolling completion temperature is too low, the rolling reaction force increases, making it difficult to stably obtain a desired plate thickness. The upper limit is not particularly limited, but generally the finish rolling completion temperature is 950° C. or less.

[Take-up]
Next, the finish-rolled hot-rolled steel sheet is cooled at an average cooling rate of 50° C./sec or more and coiled at a temperature of Ms point or less. By setting the average cooling rate to 50° C./second or more, preferably 100° C./second or more, and the coiling temperature to Ms point or less, the hot-rolled steel sheet after coiling forms a metal structure mainly composed of martensite. It is possible to reliably form a cold-rolled steel sheet having a metallographic structure mainly composed of tempered martensite in the subsequent tempering process. If the average cooling rate is less than 50° C./sec and/or the coiling temperature is higher than the Ms point, a metal structure mainly composed of martensite cannot be formed in the hot-rolled steel sheet after coiling, Even if the subsequent steps are appropriately performed, the desired hard structure cannot be obtained in the finally obtained hot stamped body and / or the prior austenite grains are refined and regulated, and the hardness is uneven. reduction cannot be achieved. From the viewpoint of refining the metal structure, it is preferable to set the average cooling rate to 100° C./sec or higher and the coiling temperature to the Mf point or lower. By setting the average cooling rate to 100° C./sec or higher and the winding temperature to the Mf point or lower, the fraction of martensitic structure obtained can be further increased. As described above, the martensite structure has multiple substructures, and many austenite nucleation sites can be created by these substructures and carbides that precipitate during the tempering process. Therefore, the martensite structure can be said to be a very effective structure for promoting refinement of the metal structure. On the other hand, however, if the fraction of the martensitic structure, which is a hard structure, is increased in the hot-rolled steel sheet, the rolling load of the rolling mill will significantly increase in the subsequent cold rolling process. Therefore, by combining low-temperature coiling below the Ms point, subsequent tempering and cold rolling, and rapid heating in the hot stamping process, both refinement and grain size regulation of the prior austenite grains should be achieved. The technical idea has never existed before, and was discovered by the present inventors for the first time this time. Here, the Ms point (° C.) and Mf point (° C.) can be approximately calculated based on Equations 1 and 2 below.
Ms = 550 - 361 x [C] - 39 x [Mn] - 35 x [V] - 20 x [Cr] - 17 x [Ni] - 10 x [Cu] - 5 x ([Mo] + [W] ) + 15 × [Co] + 30 × [Al] Formula 1
Mf = 410.5 - 407.3 x [C] - 7.3 x [Si] - 37.8 x [Mn] - 20.5 x [Cu] - 19.5 x [Ni] - 19.8 x [Cr]−4.5×[Mo]
・・・Formula 2
where [C], [Mn], [V], [Cr], [Ni], [Cu], [Mo], [W], [Co], [Al] and [Si] are hot stamping It is the content (% by mass) of each element in the body.

[Tempering process]
Next, the obtained hot-rolled steel sheet is tempered in a temperature range of 400 to 600°C in a tempering process. By tempering the hot-rolled steel sheet in the relevant temperature range, a large number of carbides can be precipitated at the prior austenite grain boundaries, packet/block boundaries, between laths, etc. in the martensite structure, and as a result, a large number of precipitated carbides. and many austenite nucleation sites can be generated at the interfaces of these substructures. If the tempering step is not performed or the tempering temperature is lower than 400°C, the carbides cannot be precipitated sufficiently, and therefore even if the subsequent steps are properly performed, the desired metallographic structure will not be obtained. Refinement and/or grain size regulation cannot be achieved. Preferably, the tempering temperature is 450°C or higher. On the other hand, if the tempering temperature is higher than 600° C., the precipitated carbides become coarse, and likewise the desired refinement and uniform grain size cannot be achieved in the metal structure. Preferably, the tempering temperature is 550°C or less. From the viewpoint of sufficiently precipitating carbides, the tempering time is preferably 1200 seconds or more. Although the upper limit is not particularly limited, for example, the tempering time may be 7200 seconds or less.

[Pickling process]
Optionally, pickling may be performed to remove oxide scale formed on the surface of the hot rolled steel sheet prior to the cold rolling process. The pickling may be carried out under conditions suitable for removing the oxide scale, and may be carried out once, or may be carried out in a plurality of steps to ensure the removal of the oxide scale. Implementation of such a pickling process is not necessarily limited to before the cold rolling process, and may be implemented, for example, after the cold rolling process.

[Cold rolling process]
The tempered hot-rolled steel sheet is cold-rolled at a rolling reduction of 30% or more. By applying cold rolling at a rolling reduction of 30% or more, as described above, the metal structure is refined to increase the interface of the substructure of martensite, and the orientation relationship of carbides precipitated in the tempering process is changed. Therefore, it is possible to reliably randomize the orientation relationship of the austenite that forms the interface between these carbides and the substructure as nucleation sites. Such an effect cannot be sufficiently obtained when the draft of cold rolling is low. Preferably, the rolling reduction of cold rolling is 40% or more. Although the upper limit is not particularly limited, the rolling reduction in cold rolling may be, for example, 80% or less or 70% or less from the viewpoint of reducing the rolling load of the rolling mill.

[Annealing process]
For example, after the cold rolling step, an optional annealing may be performed to adjust the metallographic structure and/or properties. Although not particularly limited, the heating temperature in the annealing step is, for example, 600°C from the viewpoint of avoiding the loss of the metal structure created in the previous steps, such as the loss of austenite nucleation sites due to the progress of recrystallization. It is preferable to:

[Plating process]
For the purpose of improving corrosion resistance, etc., the surface of the cold-rolled steel sheet may be plated. The plating process may be a process such as hot-dip plating, hot-dip alloying, electroplating, or the like. For example, the steel sheet may be subjected to hot-dip galvanizing treatment as the plating treatment, or alloying treatment may be performed after the hot-dip galvanizing treatment. As plating layers, aluminum plating layer, aluminum-zinc plating layer, aluminum-silicon plating layer, hot-dip galvanization layer, electrogalvanization layer, alloyed hot-dip galvanizing layer, zinc-nickel plating layer, aluminum-magnesium-zinc system A plating layer etc. are illustrated. Specific conditions for the plating treatment and alloying treatment are not particularly limited, and may be any appropriate conditions known to those skilled in the art. However, as in the case of the annealing process, from the viewpoint of avoiding the disappearance of austenite nucleation sites due to the progress of recrystallization, the plating process and alloying process should be performed at the lowest possible temperature and in the shortest possible time. is preferred.

[Temperature rolling process]
For the purpose of correcting the shape of the steel sheet and adjusting the surface roughness, for example, the steel sheet may be subjected to temper rolling after the cold rolling process, after the annealing process, or after the plating process. It is preferable that the rolling reduction of temper rolling is, for example, 1.0% or less.

[Hot stamp molding process]
Finally, in the hot stamping process, the obtained cold-rolled steel sheet is heated from 600° C. to a heat treatment temperature of A3+50 to A3+150° C. at an average heating rate of more than 100° C./sec and held at the heat treatment temperature for 30 seconds or less. Hot stamp molding is then started until the temperature reaches 550° C., and cooling is performed at an average cooling rate of 10° C./sec or more. By rapidly heating from 600 ° C. to the above heat treatment temperature at an average heating rate of more than 100 ° C./sec, it is possible to avoid the temperature range in which recrystallization progresses, so that the progress of recrystallization may cause the process up to that time. It is possible to suppress the reduction or disappearance of the austenite nucleation sites formed in . Therefore, due to the martensite transformation in the subsequent cooling process, the final hot stamped body contained refined and regulated prior austenite grains, and the variation in hardness in the plate thickness direction was significantly reduced. It becomes possible to obtain a metal structure. As the strength of steel materials increases, a relatively large amount of Mn is sometimes added to improve the hardenability of steel materials. Variation may not be suppressed appropriately. However, according to the present production method, by performing rapid heating at a high average heating rate of more than 100 ° C./sec, even when the Mn content is relatively high, the grain size and hardness of the prior austenite grains vary. can be reduced to obtain the desired metal structure. On the other hand, when the average heating rate is 100° C./s or less, or the heating start temperature at an average heating rate of more than 100° C./s is higher than 600° C., recrystallization proceeds and the number of austenite nucleation sites is reduced. Alternatively, it disappears, making it impossible to achieve the desired refinement and uniformity of grains in the metallographic structure. The average heating rate is preferably 120° C./s or higher from the viewpoint of achieving the desired fineness and particle size regulation. Also, when rapid heating is performed at a high average heating rate such as over 100° C./sec, the A3 point generally tends to increase. Therefore, in order to sufficiently austenitize the metal structure to obtain the desired hard structure and to achieve the desired refinement and grain size regulation, rapid heating is approximately required from the chemical composition of the hot stamped compact. It is necessary to heat to a temperature 50 to 150° C. higher than the A3 point (° C.) in Formula 3 below.
A3=850+10×([C]+[N])×[Mn]+350×[Nb]+250×[Ti]+40×[B]+10×[Cr]+100×[Mo] Formula 3
In the formula, [C], [N], [Mn], [Nb], [Ti], [B], [Cr] and [Mo] are the contents of each element in the hot stamped compact (% by mass) is.

If the heat treatment temperature is lower than A3 + 50°C, the austenitization will be insufficient, and the desired hard structure cannot be obtained and/or the desired refinement and grain size regulation cannot be achieved. On the other hand, when the heat treatment temperature exceeds A3 + 150 ° C., the austenite grains grow excessively, and similarly the desired refinement and grain size regulation cannot be achieved, and as a result, the desired thickness in the plate thickness direction Vickers hardness distribution cannot be obtained. Also, when the holding time at the heat treatment temperature exceeds 30 seconds, the austenite grains grow excessively, making it impossible to achieve the desired refinement and grain size regulation. Therefore, the holding time at the heat treatment temperature is preferably 10 seconds or less, more preferably 3 seconds or less. Although the lower limit is not particularly limited, the retention time may be 1 second or more.

The heating atmosphere is not particularly limited, and ordinary conditions may be used. For example, the atmosphere, a gas combustion atmosphere in which the ratio of air and fuel is controlled, and a nitrogen atmosphere may be used, and the dew point may be controlled in these gases. . Examples of the heating method include furnace heating using an electric furnace or gas furnace, flame heating, electrical heating, high-frequency heating, induction heating, and the like. After holding at the above heat treatment temperature, hot stamp molding is started until the temperature reaches 550°C. If hot stamping is started at a temperature lower than 550°C, bainite transformation and the like will proceed, and in such a case, it will be impossible to obtain the desired hard structure containing 90% by volume or more of martensite. . After hot stamping, cooling to a temperature range of 250° C. or less at an average cooling rate of 10° C./second or more is sufficient to obtain a desired hard structure containing 90% by volume or more of martensite. Further, after hot stamp molding, baking hardening treatment (BH treatment) after painting, for example, BH treatment at 170 to 200° C. for 20 to 30 minutes may be performed.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

In the following examples, hot-stamped articles according to embodiments of the present invention were produced under various conditions, and the tensile strength, yield strength, and resistance to hydrogen embrittlement of the resulting hot-stamped articles were examined. .

First, molten steel having the chemical composition shown in Table 1 was cast by a continuous casting method to produce a slab. The balance other than the components shown in Table 1 is Fe and impurities. These slabs were heated to a temperature of 1100° C. or higher and rough-rolled under predetermined conditions, then finish-rolled, cooled, coiled, tempered and cold-rolled under the conditions shown in Table 2. Next, the obtained cold-rolled steel sheet was heated at an average heating rate shown in Table 3 from 600°C to the heat treatment temperature shown in Table 3, and then hot stamping was started until the temperature reached 550°C. Then, it was cooled to a temperature range of 250° C. or lower at the average cooling rate shown in Table 3. For Comparative Examples 42, 44 and 46 and Invention Examples 43 and 45, the cold-rolled steel sheets obtained by cold rolling were heated at the average heating rate shown in Table 4 from the temperature T1 shown in Table 4 to the heat treatment temperature. After holding, hot stamp molding was started until the temperature reached 550°C, and the temperature was cooled to 250°C or less at the average cooling rate shown in Table 4. The heating atmosphere and heating method in the hot stamping process were gas combustion atmosphere (air-fuel ratio 0.85) and electrical heating. In addition, from the viewpoint of evaluating the performance under conditions closer to the actual product, the obtained hot stamped product was subjected to a heat treatment (without painting) at 170 ° C. for 20 minutes as a bake hardening treatment (BH treatment) after painting. went.

The properties of the obtained hot stamped product were measured and evaluated by the following methods.

[Tensile strength (TS) and yield strength (YS)]
The tensile strength (TS) and (YS) of the hot stamped body are determined by preparing a No. 5 test piece from an arbitrary position of the hot stamped body according to JIS Z 2241: 2011 and performing a tensile test. Obtained. The crosshead speed was set to 1 mm/min.

[Hydrogen embrittlement resistance]
Hydrogen embrittlement resistance of the hot stamped body was evaluated by a slow strain rate tensile test (SSRT) as follows. First, a test piece of 1.0t × 9.0W × 140L (mm) was prepared, and the test piece had a parallel part length of 25 mm, a parallel part diameter of 2.0 mm, and a notch depth on both sides of the center of the parallel part. A U-notch with a depth of 0.35 mm and a notch base radius of 0.1 mm was provided. This test piece was immersed in a 3% NaCl solution and charged with hydrogen using a galvanostat as a power supply while controlling the current density at the immersed portion of the surface of the test piece to 0.1 mA/cm ² . Next, the test piece charged with hydrogen was subjected to a low strain rate tensile test at a tensile speed of 0.0060 mm/min to investigate the load at break. When the breaking load in such a hydrogen environment was 600 MPa or more, it was evaluated as acceptable, and when the breaking load was less than 600 MPa, it was evaluated as unacceptable.

A hot-stamped product having a tensile strength (TS) of 2200 MPa or more, a yield strength (YS) of 1800 MPa or more, and passing the evaluation of hydrogen embrittlement resistance is regarded as a hot stamped body having high strength and capable of suppressing hydrogen embrittlement. evaluated. Table 3 shows the results. Table 3 shows the volume fraction of martensite, the average grain size of the prior austenite grains (prior γ grains), and the standard deviation of the grain size distribution in the area centered at the depth position of 1/4 of the sheet thickness of the hot stamped compact. ing. Table 3 also shows the ratio of the difference between the maximum value and the minimum value of the Vickers hardness distribution in the plate thickness direction to the average value (hardness definition). In the hot-stamped compacts shown in Table 3, residual structures other than martensite were bainite, ferrite, retained austenite and/or pearlite.

Referring to Tables 1 to 4, in Comparative Example 2, the average heating rate in the hot stamping process was slow, so recrystallization progressed and the number of austenite nucleation sites was reduced. As a result, it is not possible to achieve the desired refinement and uniformity in the metallographic structure, and in relation to this, it is difficult to obtain the desired average grain size of γ grains, the standard deviation of the grain size distribution, and the hardness definition. was not possible, and the hydrogen embrittlement resistance decreased. In Comparative Example 3, since the tempering process was not performed, it is considered that sufficient austenite nucleation sites due to precipitation of carbides could not be generated. As a result, the desired average grain size and hardness of the γ grains could not be obtained in the metal structure, and the TS and hydrogen embrittlement resistance decreased. In Comparative Example 4, since the cold rolling process was not performed, the orientation relationship of austenite generated as a nucleation site at the interface between the carbide and the substructure of martensite could not be randomized, and the austenite was connected. It is thought that it has become easier. As a result, the desired refinement and uniform grain size could not be achieved in the metallographic structure, resulting in a decrease in YS. In Comparative Example 6, since the heat treatment temperature in the hot stamping process was low, the austenitization was insufficient, and the desired hard structure and hardness could not be achieved, and the YS and hydrogen embrittlement resistance decreased. . In Comparative Example 7, the high coiling temperature presumably prevented the formation of a metallographic structure mainly composed of martensite in the hot-rolled steel sheet after coiling. As a result, the metal structure of the finally obtained hot-stamped compact could not achieve the desired refinement and uniformity of grains, resulting in a decrease in YS.

In Comparative Example 9, it is considered that the precipitated carbides became coarse due to the high tempering temperature. As a result, the desired refinement and uniform grain size could not be achieved in the metallographic structure, resulting in a decrease in YS. On the other hand, in Comparative Example 10, the tempering temperature was low, so it is considered that the carbide could not be sufficiently precipitated. As a result, the desired refinement and uniform grain size in the metal structure could not be achieved, and the hydrogen embrittlement resistance decreased. In Comparative Example 11, it is considered that the austenite grains grew excessively due to the long holding time in the hot stamping process. As a result, it was not possible to achieve the desired refinement and uniformity of grains in the metal structure, and the hydrogen embrittlement resistance deteriorated. In Comparative Example 12, since the average heating rate in the hot stamping process was slow and the holding time was long, recrystallization progressed and the number of austenite nucleation sites decreased, and the austenite generated also excessively grew grains. It is thought that As a result, it was not possible to achieve the desired refinement and uniformity of grains in the metal structure, and the hydrogen embrittlement resistance deteriorated. In Comparative Example 14, the heat treatment temperature in the hot stamping process was high, and thus the austenite grains grew excessively. As a result, it was not possible to achieve the desired refinement and uniformity of grains in the metal structure, and the hydrogen embrittlement resistance deteriorated. In Comparative Example 15, since the average cooling rate in the hot stamping process was slow, the volume fraction of martensite in the metal structure was greatly reduced. For this reason, depending on the method specified in this specification, the average particle size of γ grains, the standard deviation of the particle size distribution, and the hardness definition could not be appropriately measured, and YS and TS also greatly decreased. . In Comparative Examples 18-20, YS and TS decreased due to the low C content. In Comparative Example 22, the coiling temperature was high, so it is considered that the metal structure mainly composed of martensite could not be formed in the hot-rolled steel sheet after coiling. As a result, the desired hard structure and hardness regulation could not be achieved in the metal structure of the finally obtained hot stamped product, and YS and TS decreased. In Comparative Example 24, the heat treatment temperature in the hot stamping process was low, so the austenitization was insufficient, the desired hard structure and hardness regulation could not be achieved, and YS and TS decreased.

In Comparative Example 38, the average cooling rate after finish rolling in the hot rolling process was slow, so it is considered that the metal structure mainly composed of martensite could not be formed in the hot-rolled steel sheet after coiling. As a result, it was not possible to achieve the desired refinement and uniformity of grains in the metal structure, and the hydrogen embrittlement resistance deteriorated. In Comparative Example 39, the average heating rate in the hot stamping process was low, so recrystallization progressed and the austenite nucleation sites decreased or disappeared. As a result, it was not possible to achieve the desired refinement and uniformity of grains in the metal structure, and the hydrogen embrittlement resistance deteriorated. In Comparative Example 40, since the reduction ratio of cold rolling was low, the orientation relationship of austenite generated as a nucleation site at the interface between the carbide and the substructure of martensite could not be randomized, and the austenite was connected. It is thought that it has become easier. As a result, it was not possible to achieve the desired refinement and uniformity of grains in the metal structure, and the hydrogen embrittlement resistance deteriorated. In Comparative Examples 42, 44 and 46, the heating start temperature (speed change temperature T1 in Table 4) at an average heating rate of more than 100 ° C./sec in the hot stamping process was higher than 600 ° C., so recrystallization occurred. It is considered that the austenite nucleation sites have been reduced or disappeared as it progresses. As a result, it was not possible to achieve the desired refinement and uniformity of grains in the metal structure, and the hydrogen embrittlement resistance deteriorated.

In contrast, all the hot stamped bodies according to the invention examples have a predetermined chemical composition and a metal structure containing 90% by volume or more of martensite, and the prior austenite grains are refined to reduce the average grain size to 3.5. It is reduced to 0 μm or less and the standard deviation in the grain size distribution is controlled to 1.5 μm or less to regulate the grain size. Despite having a high tensile strength of 2200 MPa or more and a high yield strength of 1800 MPa or more due to the high C content and martensite volume fraction by controlling the thickness distribution to 35% or less of the average value , a high breaking load of 600 MPa or more could be achieved, and therefore excellent hydrogen embrittlement resistance could be achieved.

Claims (2)

1. in % by mass,
   C: 0.40 to 0.70%,
   Si: 0.01 to 1.30%,
   Mn: 0.05-3.00%,
   P: 0.100% or less,
   S: 0.0100% or less,
   N: 0.0200% or less,
   O: 0.0200% or less,
   Al: 0.001 to 1.000%,
   Cr: 0.01 to 1.00%,
   Nb: 0 to 0.200%,
   Ti: 0 to 0.200%,
   Mo: 0 to 1.00%,
   B: 0 to 0.1000%,
   Co: 0 to 4.00%,
   Ni: 0 to 3.00%,
   Cu: 0 to 3.00%,
   V: 0 to 3.00%,
   W: 0 to 1.00%,
   Ca: 0 to 1.000%,
   Mg: 0-1.000%,
   REM: 0 to 1.000%,
   Sb: 0 to 1.000%,
   Zr: 0 to 1.000%,
   Sn: 0 to 1.000%,
   As: 0 to 0.100%, and the balance: having a chemical composition consisting of Fe and impurities,
   90% or more of martensite is included in the volume fraction,
   The average grain size of the prior austenite grains is 3.0 μm or less,
   The standard deviation in the grain size distribution of the prior austenite grains is 1.5 μm or less,
   A hot-stamped article having a metallographic structure in which the difference between the maximum value and the minimum value of the Vickers hardness distribution in the plate thickness direction is 35% or less of the average value of the Vickers hardness distribution.
2. The chemical composition, in mass %,
   Nb: 0.001 to 0.200%,
   Ti: 0.001 to 0.200%,
   Mo: 0.001 to 1.00%,
   B: 0.0001 to 0.1000%,
   Co: 0.001 to 4.00%,
   Ni: 0.001 to 3.00%,
   Cu: 0.001 to 3.00%,
   V: 0.001 to 3.00%,
   W: 0.001 to 1.00%,
   Ca: 0.0001 to 1.000%,
   Mg: 0.0001-1.000%,
   REM: 0.0001 to 1.000%,
   Sb: 0.001 to 1.000%,
   Zr: 0.001 to 1.000%,
   Sn: 0.001 to 1.000%, and As: 0.001 to 0.100%
   The hot stamped article according to claim 1, comprising one or more selected from the group consisting of: