JPWO2020194995A1 - Hot stamp molded body - Google Patents

Abstract

This hot stamped body has a predetermined chemical composition, the average crystal grain size of the old austenite grains is more than 10 μm and 40 μm or less, and the length of the major axis and the length of the minor axis of the crystal lattice of the martensite crystal grains. The major axis length / minor axis length, which is the ratio to the value, is less than 1.020.

Description

The present invention relates to a hot stamp molded body that is suitably used for automobiles, structural members of structures, reinforcing members, and the like, which require strength.
The present application claims priority based on Japanese Patent Application No. 2019-057144 filed in Japan on March 25, 2019, the contents of which are incorporated herein by reference.

In recent years, there has been a demand for weight reduction of automobile bodies from the viewpoint of environmental protection and resource saving. Therefore, the application of high-strength steel sheets to automobile parts is accelerating. However, since the formability deteriorates as the strength of the steel sheet increases, the formability of a high-strength steel sheet into a member having a complicated shape becomes an issue.

In order to solve such a problem, the application of hot stamping in which a steel sheet is heated to a high temperature in the austenite region and then press-formed is being promoted. Since hot stamping is hardened in a die at the same time as press working, it is possible to obtain strength according to the amount of C in the steel sheet, and it is attracting attention as a technology that achieves both molding into automobile parts and ensuring strength. ing.

In the conventional hot stamping molded body manufactured by press quenching at the time of hot stamping, the entire area in the plate thickness direction is formed of a hard structure (mainly martensite). Therefore, early fracture, which is a phenomenon leading to fracture with a stress lower than the tensile strength estimated from the hardness of the hot stamped compact, is likely to occur. In order to obtain even better collision resistance in automobile members, it is necessary to suppress premature fracture.

In Patent Document 1, the crystal grain size of martensite is refined by controlling the Mn content or the total content of at least one of Cr, Mo, Cu, and Ni and Mn, and the strength is high. However, a technique for improving collision resistance is disclosed.
Patent Document 2 discloses a technique for improving collision resistance by finely reducing the average crystal grain size of old austenite grains by selecting an alloying element.

Japanese Patent Application Laid-Open No. 2010-070806 Japanese Patent Application Laid-Open No. 2014-015638

Acta Materialia, 58 (2010), 6393-6403

In view of the problems of the prior art, it is an object of the present invention to provide a hot stamped body having high hardness, which is a characteristic desired for a general hot stamped body, and excellent in early fracture resistance. And.

The present inventors have diligently studied a method for solving the above problems.
The reason why premature fracture is likely to occur in a hot stamped compact mainly composed of martensite is that internal stress is generated in martensite. When an external force is applied to this hot stamped body, it is easy to break from a region where the internal stress is high, so it is considered that the hot stamped body breaks at a stress lower than the tensile strength estimated from the hardness.

The reason why internal stress is generated in martensite is due to its crystal structure. When austenite is transformed into martensite, it changes from an fcc structure to a bct structure containing carbon in supersaturation. The ratio of the length of the major axis to the length of the minor axis of the bct structure affects the strength of the internal stress, and the equal the length of the major axis and the length of the minor axis, the smaller the internal stress. In general, as the C concentration in martensite increases, the ratio of the length of the major axis to the length of the minor axis of the crystal lattice of martensite crystal grains becomes significantly larger than 1.000, and the internal stress increases. Cause an increase.

As a result of diligent studies, the present inventors have made the steel plate contain Ni, and further controlled the average crystal grain size of austenite before martensitic transformation, so that even martensite having a high C concentration has a bct structure. It was clarified that the ratio between the length of the major axis and the length of the minor axis of the crystal lattice of martensite crystal grains can be reduced (it can be approached to 1.000). Specifically, the present inventors impose 3.0% by mass or more of Ni in the steel plate, and further control the heating rate during hot stamping to obtain the average crystal grain size of austenite before martensitic transformation. By controlling to more than 10 μm and 40 μm or less, the major axis length / minor axis length, which is the ratio between the major axis length and the minor axis length of the crystal lattice of martensite crystal grains, is controlled to less than 1.020. As a result, it was found that premature fracture of the hot stamped body after hot stamping can be suppressed.

The present inventors have made a steel sheet containing 3.0% by mass or more of Ni, and further controlled the metal structure of the hot stamping steel sheet and the heating rate during hot stamping to control the hot stamped steel sheet (the steel sheet after hot stamping (). We came up with the idea that the ratio (major axis length / minor axis length) between the length of the major axis and the length of the minor axis of the crystal lattice of martensite crystal grains in a hot stamped product) can be controlled. The major axis length / minor axis length of the crystal lattice of martensite crystal grains is determined by the alloying elements and the crystal grain size of austenite before the martensitic transformation. By containing 3.0% or more of Ni as an alloying element, a part of iron atoms contained in the crystal lattice of martensite crystal grains is replaced with Ni, and the major axis length / minor axis length is reduced. Contribute.

On the other hand, the Ni content makes it easy for some regions in the austenite grains to remain as untransformed austenite after hot stamping. Ni has an action of stabilizing austenite to facilitate the retention of untransformed austenite, and an action of accelerating the growth rate of austenite to coarsen the austenite.
If some regions within the austenite grain remain as untransformed austenite after hot stamping, the martensite adjacent to the untransformed austenite tends to expand its crystal lattice, resulting in the major axis length of martensite /. The length of the minor axis becomes large. Therefore, from the viewpoint of reducing the major axis length / minor axis length of martensite, it is desirable that untransformed austenite does not remain as much as possible. Since Ni is an element that stabilizes austenite, in the prior art, untransformed austenite remained when the steel sheet contained 3.0% by mass or more of Ni.

The present inventors have investigated a means capable of coarsening old austenite grains even with Ni-containing steel. As a result, in the heating process at the time of hot stamping, the present inventors use the large tilted grain boundaries having a high carbon concentration as the nucleation site of austenite, and lower the transformation temperature to austenite to achieve early martensitic transformation. Found that can be started. Furthermore, the present inventors controlled the heating rate to less than 100 ° C./s in the heating step during hot stamping to promote the grain growth of austenite, thereby increasing the average crystal grain size of the old austenite grains to more than 10 μm. It was found that it can be controlled to 40 μm or less. If the average crystal grain size of the old austenite grains is more than 10 μm and 40 μm or less, it is possible to suppress the remaining of untransformed austenite even if Ni is contained in an amount of 3.0% by mass or more, and martensite It is possible to reduce the major axis length / minor axis length of the crystal lattice of crystal grains.

Next, the present inventors examined a method for generating a large tilt angle grain boundary having a high carbon concentration in a hot stamping steel sheet. In order for the grain boundaries to function as reverse transformation sites of austenite, it is preferable that the orientation difference between the crystal grains sandwiching the grain boundaries is large. Therefore, it is important to generate large tilt angle grain boundaries in the hot stamping steel sheet. .. In addition, carbon has the effect of lowering the transformation temperature to austenite. Therefore, it is important to lower the transformation temperature to austenite by enriching carbon at the large-tilt grain boundaries and to start martensitic transformation at an early stage.

In order to generate large tilt angle grain boundaries, it is effective to use bainite and martensite as the main metal structures of the hot stamping steel sheet, but carbon diffusion is not sufficient in the temperature range where these metal structures can be obtained. No. In the prior art, the concentration of carbon concentrated at the large tilt angle grain boundaries was not sufficient, and it was difficult to obtain the effect of lowering the transformation temperature to austenite. Therefore, the present inventors reduced the transformation temperature to austenite by generating large tilt angle grain boundaries and promoting carbon concentration in the hot stamping steel sheet, and used the old austenite grains having a desired size. I examined how to get it. As a result, the present inventors can generate a metal structure called a granular bainite by hot rolling a steel sheet containing Ni under predetermined conditions, and the granular bainite has a metal structure. It was found that the large-tilt grain boundaries and the sub-grain boundaries are included, and that the high-concentration carbon is concentrated in the large-angle grain boundaries and that the segregation of carbon is suppressed in the sub-grain boundaries.

Granular bainite, which includes large-inclined grain boundaries and sub-grain boundaries, is a metallic structure formed through the following two steps. In the first stage, the transformation from austenite to bainitic ferrite occurs. In the second stage, the grain boundaries between the bainitic ferrites are restored to become subgrain boundaries and become granular bainite.

As a first step, in the hot rolling step, hot rolling is completed at a temperature of 800 ° C. or higher, and winding is performed in a temperature range of 500 ° C. or higher and 770 ° C. or lower. In order to obtain the desired amount of granular bainite, it is important to control the recrystallization rate of austenite before transformation, that is, the dislocation density. If the recrystallization of austenite is promoted too much, the dislocation density in austenite decreases, and a desired amount of granular bainite cannot be obtained. On the other hand, even if recrystallization is insufficient, the dislocation density in austenite increases too much, and transformation to granular bainite does not occur. As a result of diligent studies by the present inventors, when the hot rolling end temperature is 800 ° C. or higher, the recrystallization of austenite is appropriately promoted, and as a result, the transformation to granular bainite is likely to occur. It was found that the dislocation density can be controlled. Next, austenite is transformed into bainitic ferrite by winding in a temperature range of 500 ° C. or higher and 770 ° C. or lower.

As a second step, the average cooling rate of the hot-rolled steel sheet being wound in the temperature range from 650 ° C to 400 ° C is controlled to 50 ° C / s or less. By cooling at an average cooling rate of 50 ° C./s or less in the temperature range from 650 ° C to 400 ° C, grain boundaries between bainitic ferrite are restored and subgrain boundaries are formed, thereby forming granular bainite. Obtainable. This makes it possible to generate granular bainite in the hot rolling process. On the other hand, when the average cooling rate in the above temperature range exceeds 50 ° C./s, the grain boundaries are restored and subgrain boundaries cannot be formed. In cooling during winding, the initial vanitic ferrite has grain boundaries with an average crystal orientation difference of 5 ° or more, but the average cooling rate is slow at 50 ° C / s or less in the temperature range where Fe can diffuse. By cooling, dislocation recovery occurs in the vicinity of the grain boundaries of vanitic ferrite, and subgrain boundaries having an average crystal orientation difference of 0.4 ° or more and 3.0 ° or less are generated. When the dislocation recovery occurs, C in the steel diffuses from the subgrain boundaries to the surrounding large-inclined grain boundaries, so that carbon can be concentrated at high concentrations in the large-inclined grain boundaries. Further, when the dislocation recovery occurs, C in the steel diffuses from the subgrain boundaries to the surrounding large tilt angle grain boundaries, so that it is possible to reduce the segregation of carbon at the subgrain boundaries.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] The hot stamp molded article according to one aspect of the present invention has a chemical composition of C: 0.15% or more and less than 0.70% in mass%.
Si: 0.010% or more, less than 0.50%,
Mn: 0.010% or more, less than 3.00%,
sol. Al: 0.0002% or more, 3.000% or less,
Ni: 3.0% or more and less than 15.0%,
P: 0.100% or less,
S: 0.1000% or less,
N: 0.0100% or less,
Nb: 0% or more, 0.150% or less,
Ti: 0% or more, 0.150% or less,
Mo: 0% or more, 1.000% or less,
Cr: 0% or more, 1.000% or less,
B: 0% or more, 0.0100% or less,
V: 0% or more, 1.000% or less,
Cu: 0% or more, 1.000% or less,
Sn: 0% or more, 1.000% or less,
W: 0% or more, 1.000% or less,
Ca: 0% or more, 0.010% or less, and REM: 0% or more, 0.300% or less,
The rest consists of Fe and impurities
The average crystal grain size of the old austenite grains is more than 10 μm and 40 μm or less.
It is characterized in that the major axis length / minor axis length, which is the ratio of the length of the major axis to the length of the minor axis of the crystal lattice of martensite crystal grains, is less than 1.020.
[2] The hot stamp molded article according to the above [1] has the chemical composition in mass%.
Nb: 0.010% or more, 0.150% or less,
Ti: 0.010% or more, 0.150% or less,
Mo: 0.005% or more, 1.000% or less,
Cr: 0.005% or more, 1.000% or less,
B: 0.0005% or more, 0.0100% or less,
V: 0.0005% or more, 1.000% or less,
Cu: 0.0010% or more, 1.000% or less,
Sn: 0.001% or more, 1.000% or less,
W: 0.001% or more, 1.000% or less,
It may contain one or more of the group consisting of Ca: 0.001% or more and 0.010% or less, and REM: 0.001% or more and 0.300% or less.
[3] The hot stamp molded product according to the above [1] or [2] may be provided with a plating layer on the surface.
[4] The hot stamp molded product according to any one of the above [1] to [3] may have a softened region in a part thereof.

According to the above aspect according to the present invention, it is possible to provide a hot stamp molded product having high hardness and excellent early breaking resistance.

Hereinafter, preferred embodiments of the present invention will be described in detail. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the spirit of the present invention. The numerical limitation range described below includes the lower limit value and the upper limit value. Numerical values indicated as "super" and "less than" do not include the value in the numerical range. All% of the chemical composition indicate mass%. First, the reason for limiting the chemical composition of the hot stamped molded product according to the present embodiment will be described.

The hot stamped body according to the present embodiment has a chemical composition of% by mass, C: 0.15% or more, less than 0.70%, Si: 0.010% or more, less than 0.50%, Mn: 0. .010% or more, less than 3.00%, sol. Al: 0.0002% or more, 3.000% or less, Ni: 3.0% or more and less than 15.0%, P: 0.100% or less, S: 0.1000% or less, N: 0.0100% The following and the balance: Fe and impurities are included. Hereinafter, each element will be described in detail.

"C: 0.15% or more, less than 0.70%"
C is an important element for obtaining a hardness desired for an automobile member or the like in a hot stamped molded product. If the C content is less than 0.15%, the martensite is soft and it is difficult to obtain the desired hardness. Therefore, the C content is set to 0.15% or more. The C content is preferably 0.30% or more. On the other hand, when the C content is 0.70% or more, coarse carbides are generated in the steel and the toughness of the hot stamped compact is lowered, so the C content is set to less than 0.70%. The C content is preferably 0.50% or less.

"Si: 0.010% or more, less than 0.50%"
Si is an element that enhances the deformability of the hot stamped compact and contributes to the improvement of toughness. If the Si content is less than 0.010%, the deformability is poor and the toughness of the hot stamp molded product deteriorates. Therefore, the Si content is 0.010% or more. Even if the Si content is 0.50% or more, the above effect is saturated, so the Si content is set to less than 0.50%.

"Mn: 0.010% or more, less than 3.00%"
Mn is an element that contributes to the improvement of the hardness of the hot stamp molded product by strengthening the solid solution. If the Mn content is less than 0.010%, the solid solution strengthening ability is poor, martensite becomes soft, and it becomes difficult to obtain the desired hardness for automobile members and the like. Therefore, the Mn content is set to 0.010% or more. The Mn content is preferably 0.70% or more. On the other hand, when the Mn content is 3.00% or more, the martensite becomes brittle and the toughness of the hot stamp molded product is impaired, so the Mn content is set to less than 3.00%.

"Sol.Al: 0.0002% or more, 3.000% or less"
Al is an element having an action of deoxidizing molten steel to make the steel sound (suppressing the occurrence of defects such as blow holes in the steel). sol. If the Al content is less than 0.0002%, deoxidation is not sufficient. The Al content is 0.0002% or more. sol. The Al content is preferably 0.001% or more. On the other hand, sol. When the Al content is more than 3.000%, coarse oxides are generated and the toughness of the hot stamped compact is impaired. The Al content is 3.000% or less.
In this embodiment, sol. Al means an acid-soluble Al, and indicates a solid solution Al existing in the steel in a solid solution state.

"Ni: 3.0% or more and less than 15.0%"
Ni is an element having an effect of atomizing old austenite grains, and is also an element necessary for obtaining a desired amount of granular bainite in a hot rolling process. Since the above effect cannot be obtained when the Ni content is less than 3.0%, the Ni content is set to 3.0% or more. The Ni content is preferably 5.0% or more in order to further enhance the ability to absorb impact energy at low temperatures. On the other hand, if the Ni content is 15.0% or more, the martensite becomes brittle and the toughness of the hot stamped compact is impaired, so the Ni content is set to less than 15.0%. The Ni content is preferably less than 12.0%.

"P: 0.100% or less"
P is an element that segregates at the grain boundaries and reduces the strength of the grain boundaries. When the P content exceeds 0.100%, the strength of the grain boundaries is remarkably lowered, and the hydrogen embrittlement resistance property of the hot stamped compact is lowered. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.050% or less. The lower limit of the P content is not particularly limited, but if it is reduced to less than 0.0001%, the cost of removing P is significantly increased, which is economically unfavorable. be.

"S: 0.1000% or less"
S is an element that forms inclusions in the steel. If the S content exceeds 0.1000%, inclusions are formed in the steel and the hydrogen embrittlement resistance of the hot stamped compact is deteriorated. Therefore, the S content is set to 0.1000% or less. The S content is preferably 0.0050% or less. The lower limit of the S content is not particularly limited, but if it is reduced to less than 0.0015%, the cost of removing S is significantly increased, which is economically unfavorable. be.

"N: 0.0100% or less"
N is an impurity element, which is an element that forms a nitride and lowers the bendability of the hot stamped compact. If the N content exceeds 0.0100%, coarse nitrides are formed in the steel and the bendability of the hot stamped compact is significantly reduced. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0075% 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 and it is economically unfavorable. Therefore, 0.0001% is the practical lower limit on the practical steel sheet. be.

The rest of the chemical composition of the hot stamped article according to this embodiment is Fe and impurities. Examples of impurities include elements that are unavoidably mixed from steel raw materials or scrap and / or in the steelmaking process and are allowed as long as they do not impair the characteristics of the hot stamped compact according to the present embodiment.
The hot stamp molded article according to the present embodiment may contain the following elements as optional elements. When the following optional elements are not contained, the content is 0%.

"Nb: 0% or more, 0.150% or less"
Since Nb is an element that contributes to improving the hardness of the hot stamped molded product by strengthening the solid solution, it may be contained as necessary. When Nb is contained, if it is less than 0.010%, a sufficient effect cannot be obtained by containing Nb, so it is preferable to contain 0.010% or more. The Nb content is more preferably 0.035% or more. On the other hand, even if the Nb content exceeds 0.150%, the above effect is saturated, so the Nb content is set to 0.150% or less. The Nb content is preferably 0.120% or less.

"Ti: 0% or more, 0.150% or less"
Since Ti is an element that contributes to the improvement of the hardness of the hot stamped molded product by strengthening the solid solution, it may be contained if necessary. When Ti is contained, the Ti content is preferably 0.010% or more because a sufficient effect cannot be obtained by the Ti content if it is less than 0.010%. The Ti content is more preferably 0.020% or more. On the other hand, even if the Ti content exceeds 0.150%, the above effect is saturated, so the Ti content is set to 0.150% or less. The Ti content is preferably 0.120% or less.

"Mo: 0% or more, 1.000% or less"
Since Mo is an element that contributes to improving the hardness of the hot stamped molded product by strengthening the solid solution, it may be contained as necessary. When Mo is contained, the Mo content is preferably 0.005% or more because a sufficient effect cannot be obtained by the Mo content if it is less than 0.005%. The Mo content is more preferably 0.010% or more. On the other hand, even if the Mo content exceeds 1.000%, the above effect is saturated, so the Mo content is set to 1.000% or less. The Mo content is preferably 0.800% or less.

"Cr: 0% or more, 1.000% or less"
Since Cr is an element that contributes to improving the hardness of the hot stamped molded product by strengthening the solid solution, it may be contained if necessary. When Cr is contained, if the Cr content is less than 0.005%, a sufficient effect cannot be obtained by the Cr content, so the Cr content is preferably 0.005% or more. The Cr content is more preferably 0.010% or more. On the other hand, even if the Cr content exceeds 1.000%, the above effect is saturated, so the Cr content is set to 1.000% or less. The Cr content is preferably 0.800% or less.

"B: 0% or more, 0.0100% or less"
Since B is an element that segregates at the grain boundaries and improves the strength of the grain boundaries, it may be contained as necessary. When B is contained, if the B content is less than 0.0005%, a sufficient effect cannot be obtained by the B content, so the B content is preferably 0.0005% or more. The B content is more preferably 0.0010% or more. On the other hand, even if the B content exceeds 0.0100%, the above effect is saturated, so the B content is set to 0.0100% or less. The B content is preferably 0.0075% or less.

"V: 0% or more, 1.000% or less"
Since V is an element that contributes to improving the hardness of the hot stamped molded product by strengthening the solid solution, it may be contained as necessary. When V is contained, if the V content is less than 0.0005%, a sufficient effect cannot be obtained by the V content, so the V content is preferably 0.0005% or more. The V content is more preferably 0.0100% or more. On the other hand, even if the V content exceeds 1.000%, the above effect is saturated, so the V content is set to 1.0000% or less. The V content is preferably 0.8000% or less.

"Cu: 0% or more, 1.000% or less"
Since Cu is an element that contributes to the improvement of the hardness of the hot stamped molded product by strengthening the solid solution, it may be contained if necessary. When Cu is contained, if the Cu content is less than 0.0010%, a sufficient effect cannot be obtained by the Cu content, so the Cu content is preferably 0.0010% or more. The Cu content is more preferably 0.0100% or more. On the other hand, even if the Cu content exceeds 1.000%, the above effect is saturated, so the Cu content is set to 1.0000% or less. The Cu content is preferably 0.8000% or less.

"Sn: 0% or more, 1.000% or less"
Since Sn is an element having an action of deoxidizing molten steel to make the steel sound, it may be contained up to 1.000%. In order to surely exert the above effect, the Sn content is preferably 0.001% or more.

"W: 0% or more, 1.000% or less"
Since W is an element having an action of deoxidizing molten steel to make the steel sound, it may be contained up to 1.000%. In order to surely exert the above effect, the W content is preferably 0.001% or more.

"Ca: 0% or more, 0.010% or less"
Since Ca is an element having an action of deoxidizing molten steel to make the steel sound, it may be contained up to 0.010%. In order to surely exert the above effect, the Ca content is preferably 0.001% or more.

"REM: 0% or more, 0.300% or less"
Since REM is an element having an action of deoxidizing molten steel to make the steel sound, it may be contained up to 0.300%. In order to surely exert the above effect, the REM content is preferably 0.001% or more.
In this embodiment, REM is a general term for a total of 17 elements consisting of Sc, Y and lanthanoids, and the content of REM means the total amount of the above elements. REM is often contained by misch metal, but may contain elements of the lanthanoid series in combination with La and Ce. Even when a lanthanoid-series element is contained in a complex in addition to La and Ce, the hot stamp molded product according to the present embodiment can exert its effect. Further, even if a metal REM such as metal La or Ce is contained, the hot stamp molded product according to the present embodiment can exert its effect.

The chemical composition of the hot stamped molded article described above may be measured by a general analytical method. For example, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrum) may be used for measurement. In addition, C and S may be measured by using the combustion-infrared absorption method, and N may be measured by using the inert gas melting-thermal conductivity method. sol. Al may be measured by ICP-AES using a solution obtained by heat-decomposing the sample with an acid. When the hot stamped product has a plating layer on the surface, the plating layer on the surface may be removed by mechanical grinding, and then the chemical composition may be analyzed.

Next, the metal structure of the hot stamping molded body according to the present embodiment and the hot stamping steel plate applied thereto will be described. First, the metal structure of the hot stamping steel plate applied to the hot stamping compact according to the present embodiment will be described.

[Steel sheet for hot stamping]
"A sub-crystal grain (granular bainite) with an average crystal orientation difference of 0.4 ° or more and 3.0 ° or less is placed inside a crystal grain surrounded by grain boundaries with an average crystal orientation difference of 5 ° or more. Including 10% or more in rate "
The steel plate for hot stamping is granular bainite (when the average crystal orientation difference is 0.4 ° or more and 3.0 ° or less, which exists inside the crystal grains surrounded by the grain boundaries with the average crystal orientation difference of 5 ° or more). It is necessary to contain 10% or more of a certain subcrystal grain) in terms of area ratio. Granular bainite produced in the hot rolling step can be transformed into austenite through a predetermined heat treatment step (cold rolling if necessary) and finally obtain a desired metallographic structure in a hot stamped body. can. As a result of diligent research by the present inventors, it has been found that when the area ratio of granular bainite is less than 10%, a desired metal structure cannot be obtained in a hot stamped compact. Therefore, in the hot stamping steel sheet applied to the hot stamping compact according to the present embodiment, the area ratio of granular bainite is set to 10% or more. The area ratio is preferably 15% or more, 20% or more, 25% or more, and 30% or more. The upper limit is not particularly limited, but the area ratio of granular bainite may be less than 95%.

The remainder of the metallographic structure is not particularly limited, but is usually one or more of ferrite, upper bainite, lower bainite, martensite, tempered martensite, retained austenite, and iron-based carbides and alloy carbides. The hot stamping steel sheet applied to the hot stamping compact according to the present embodiment may contain more than 5% and 90% or less of these metal structures.

Next, a method for measuring the area ratio of granular bainite will be described.
A sample is cut out from a position 50 mm or more away from the end face of the hot stamping steel plate so that a cross section perpendicular to the surface (thickness cross section) can be observed. The size of the sample shall be such that it can be observed by about 10 mm in the rolling direction, although it depends on the measuring device. Crystal orientation information is obtained by EBSD analysis of the cut-out sample at the plate thickness 1/2 position at a measurement interval of 0.2 μm. Here, the EBSD analysis is performed at 200 to 300 points / sec using a device composed of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 type detector). Perform at analysis speed.

The area ratio of granular bainite can be easily calculated by using, for example, the "Grain Average Simulation" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. With this function, it is possible to calculate the directional difference between adjacent measurement points for a crystal grain having a body-centered structure, and then obtain an average value for all the measurement points in the crystal grain. With respect to the obtained crystal orientation information, a region surrounded by grain boundaries having an average crystal orientation difference of 5 ° or more is defined as a crystal grain, and "Grain Average" is defined.
By calculating the area ratio of the region (sub-crystal grain) where the average crystal orientation difference in the crystal grain is 0.4 ° or more and 3.0 ° or less by the "Missionation" function, the area ratio of granular bainite can be obtained. Can be done.

"The above average with respect to the total length of the grain boundaries having an average crystal orientation difference of 0.4 ° or more and 3.0 ° or less and the grain boundaries having an average crystal orientation difference of more than 3.0 °. The ratio of the length of the grain boundaries with a crystal orientation difference of 0.4 ° or more and 3.0 ° or less is 60% or more. "
In the hot stamping steel plate applied to the hot stamping compact according to the present embodiment, the grain boundary length and the average crystal orientation difference having an average crystal orientation difference of 0.4 ° or more and 3.0 ° or less are 3. The ratio of the grain boundary lengths having an average crystal orientation difference of 0.4 ° or more and 3.0 ° or less to the total lengths of grain boundaries exceeding 0 ° is 60% or more. Average for the total length of grain boundaries with an average crystal orientation difference of 0.4 ° or more and 3.0 ° or less and grain boundaries with an average crystal orientation difference of more than 3.0 ° If the ratio of the grain boundary lengths having a crystal orientation difference of 0.4 ° or more and 3.0 ° or less is less than 60%, the desired metal structure cannot be obtained in the hot stamped body. The ratio of the length of the grain boundaries is preferably 70% or more, or 80% or more. The upper limit is not particularly limited, but may be less than 95%.

Next, for the total length of the grain boundaries having an average crystal orientation difference of 0.4 ° or more and 3.0 ° or less and the grain boundaries having an average crystal orientation difference of more than 3.0 °. A method for measuring the ratio of grain boundary lengths having an average crystal orientation difference of 0.4 ° or more and 3.0 ° or less will be described.
First, a sample is cut out from a position 50 mm or more away from the end face of the hot stamping steel plate so that a cross section perpendicular to the surface (thickness cross section) can be observed. The size of the sample shall be such that it can be observed by about 10 mm in the rolling direction, although it depends on the measuring device. After polishing the cross section of the cut sample using silicon carbide paper of # 600 to # 1500, a diamond powder having a particle size of 1 to 6 μm is mirrored using a diluted solution such as alcohol and a liquid dispersed in pure water. Finish. Next, the strain introduced into the surface layer of the sample is removed by polishing at room temperature with colloidal silica containing no alkaline solution for 8 minutes. Crystal orientation information is obtained by measuring a region having a length of 50 μm and a depth of 50 μm from the surface of the steel sheet at an arbitrary position in the longitudinal direction of the sample cross section by an electron backscatter diffraction method at a measurement interval of 0.1 μm. For the measurement, a device composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the degree of vacuum in the apparatus is 9.6 × 10 ^-5 Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the irradiation time of the electron beam is 0.01 seconds / point. Using the "Image Quality" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer, the obtained crystal orientation information has an average crystal orientation difference of 0.4 ° or more and 3.0 °. Grain boundaries with an average crystal orientation difference of 0.4 ° or more and 3.0 ° or less with respect to the total length of the following grain boundaries and the lengths of grain boundaries with an average crystal orientation difference of more than 3.0 °. Calculate the ratio of the length of. With this function, it is possible to calculate the total length of grain boundaries having an arbitrary angle of rotation for the grain boundaries of crystal grains having a body-centered structure. For all the crystal grains included in the measurement area, the total length of these grain boundaries was calculated, and the grain boundary length and average crystal orientation with an average crystal orientation difference of 0.4 ° or more and 3.0 ° or less were calculated. The ratio of the grain boundary lengths having an average crystal orientation difference of 0.4 ° or more and 3.0 ° or less to the total length of the grain boundaries having a difference of more than 3.0 ° is calculated.

The thickness of the hot stamping steel plate applied to the hot stamping compact according to the present embodiment is not particularly limited, but is preferably 0.5 to 3.5 mm from the viewpoint of reducing the weight of the vehicle body.

Next, the metal structure of the hot stamping molded product according to the present embodiment will be described.

"The average crystal grain size of old austenite grains is more than 10 μm and 40 μm or less"
By setting the average crystal grain size of austenite before the martensitic transformation to more than 10 μm and 40 μm or less, the ratio of the major axis length to the minor axis length of the crystal lattice of the martensitic crystal grains after the martensitic transformation. A certain major axis length / minor axis length can be less than 1.020. As a result, premature breakage of the hot stamp molded product can be suppressed. Therefore, in the hot stamp molded product according to the present embodiment, the average crystal grain size of the old austenite grains is set to more than 10 μm and 40 μm or less. The average crystal grain size of the old austenite grains is preferably 15 μm or more, or 20 μm or more. The average crystal grain size of the old austenite grains is preferably 35 μm or less, or 30 μm or less.

"The long axis length / minor axis length, which is the ratio of the length of the major axis to the length of the minor axis of the crystal lattice of martensite crystal grains, is less than 1.020."
When the major axis length / minor axis length, which is the ratio of the major axis length to the minor axis length of the crystal lattice of martensite crystal grains, is 1.020 or more, premature fracture of the hot stamped compact is suppressed. Can not do it. Therefore, in the hot stamped body according to the present embodiment, the major axis length / minor axis length, which is the ratio between the length of the major axis and the length of the minor axis of the crystal lattice of martensite crystal grains, is less than 1.020. And. The major axis length / minor axis length is preferably 1.015 or less. From the viewpoint of ensuring strength by strengthening the solid solution of carbon, the lower limit may be set to 1.001.

The metal structure of the hot stamped body according to the present embodiment may be mainly composed of crystal grains having a body-core structure such as martensite, tempered martensite, upper bainite, and lower bainite. The body-centered structure is a general term for those whose crystal structure is a body-centered cubic structure, a body-centered cubic structure, or the like. The term "mainly" means that the crystal grains have an area ratio of 80% or more in the metal structure. The remaining structure is one or more of pearlite and ferrite of 20% or less.

Next, a method for measuring the average crystal grain size of the old austenite grains will be described. Cut out a sample from a position 50 mm or more away from the end face of the hot stamped body (if it cannot be collected from this position, any position except the end) so that a cross section perpendicular to the surface (thickness cross section) can be observed. .. The size of the sample shall be such that it can be observed by about 10 mm in the rolling direction, although it depends on the measuring device. Crystal orientation information is obtained by EBSD analysis of the cut-out sample at the plate thickness 1/2 position at a measurement interval of 0.1 μm. Here, the EBSD analysis uses an apparatus composed of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 type detector), and has an analysis speed of 200 to 300 points / sec. It will be carried out at. Using the obtained crystal orientation information, the crystal orientation of the former austenite grains is calculated from the crystal orientation relationship between the general old austenite grains and the crystal grains having a body-centered structure after transformation, and the average crystal of the former austenite grains is calculated. The particle size may be calculated. The method for calculating the crystal orientation of the former austenite grains is not particularly limited, but for example, a crystal orientation map of the former austenite grains is created by the method described in Non-Patent Document 1, and the former austenite is obtained from the created crystal orientation map by the section method. The average crystal grain size of the grains may be calculated.

The major axis length / minor axis length, which is the ratio of the length of the major axis to the length of the minor axis of the crystal lattice of martensite crystal grains, is obtained by the following method.
A sample is taken from a position 50 mm or more away from the end face of the hot stamped body (if it cannot be taken from this position, any position except the end), and the surface is face-cut to a depth of 1/4 in the plate thickness direction. Used for X-ray diffraction measurement. The X-ray diffractometer uses RINT2000 manufactured by Rigaku Co., Ltd., and Co Kα rays are used as the X-ray source. The measurement angle 2θ is 10 ° to 130 °, the measurement interval is 0.02 °, and the peak intensity profile at each 2θ is obtained. The obtained profile is profile-fitted by Rietveld analysis, and the ratio of the length of the major axis to the minor axis of the crystal lattice of martensite crystal grains is calculated to obtain the major axis length / minor axis length. ..

"Plating layer"
In the present embodiment, a plating layer may be formed on the surface of the hot stamp molded product for the purpose of improving corrosion resistance and the like. The plating layer may be either an electroplating layer or a hot-dip plating layer. The electroplating layer includes, for example, an electrogalvanizing layer, an electric Zn—Ni alloy plating layer, and the like. The hot-dip plating layer is, for example, a hot-dip zinc-plated layer, an alloyed hot-dip zinc-plated layer, a hot-dip aluminum plated layer, a hot-dip Zn-Al alloy plated layer, a hot-dip Zn-Al-Mg alloy plated layer, and a hot-dipped Zn-Al-Mg-Si. Includes alloy plating layer and the like. The amount of adhesion of the plating layer is not particularly limited and may be a general amount of adhesion.

"Softening area"
The hot stamp molded product according to the present embodiment may have a softened region partially formed. Weldability is improved in the softened region. For example, if spot welding is performed after softening the end portion of the hot stamp molded body, the strength difference between the softened end portion and the spot welded portion of the end portion can be reduced, so that the strength difference between the two ends can be reduced. Destruction can be suppressed. Further, for example, when a hot stamp molded body is applied to a high-strength member of an automobile, it is possible to control the destruction and deformation modes of the high-strength member at the time of a collision by providing a softened region in a part of the high-strength member. can. In order to form a softened region, for example, after forming a hot stamping steel sheet into a hot stamping compact, the strength of a part of the hot stamping compact may be reduced by laser irradiation. Laser irradiation is an example of heat treatment which is a softening means, and the softening means is not particularly limited. As another means, the softened region may be formed, for example, by baking a part of the hot stamped body.

Next, a suitable manufacturing method for obtaining the hot stamped molded product according to the present embodiment will be described.
In the method for producing a steel sheet for hot stamping applied to the hot stamped body according to the present embodiment, the steel pieces having the above-mentioned chemical composition are subjected to hot rolling, hot rolling is completed at a temperature of 800 ° C. or higher, and 500. It is preferable that the average cooling rate of the hot-rolled steel sheet being wound at a temperature of ° C. or higher and 770 ° C. or lower in the temperature range of 650 ° C. to 400 ° C. is 50 ° C./s or lower.

The steel piece (steel material) to be subjected to hot rolling may be a steel piece manufactured by a conventional method, and may be, for example, a steel piece manufactured by a general method such as a continuously cast slab or a thin slab caster.

"Set the hot rolling end temperature to 800 ° C or higher"
In order to obtain a desired amount of granular bainite, it is effective to control the recrystallization rate of austenite before transformation, that is, the dislocation density. If the recrystallization of austenite is promoted too much, the dislocation density in austenite decreases, and a desired amount of granular bainite cannot be obtained. On the other hand, even if recrystallization is insufficient, the dislocation density in austenite increases too much, and transformation to granular bainite does not occur. As a result of diligent studies by the present inventors, when the hot rolling end temperature is 800 ° C. or higher, the recrystallization of austenite proceeds moderately, and as a result, dislocations that are likely to be transformed into granular bainite are likely to occur. We found that the density can be controlled. If the hot rolling end temperature is less than 800 ° C., recrystallization of austenite does not occur and a desired amount of granular bainite may not be obtained. Therefore, the hot rolling end temperature is preferably 800 ° C. or higher. It is preferably 820 ° C. or higher. Further, since it is unlikely that recrystallization is over-promoted in the steel having the chemical composition specified in the present embodiment, the upper limit of the hot rolling end temperature is not particularly specified, but it is usually 1050 ° C.

"The average cooling rate of the hot-rolled steel sheet being wound at a temperature of 500 ° C. or higher and 770 ° C. or lower in the temperature range of 650 ° C. to 400 ° C. shall be 50 ° C./s or less."
It is preferable to start winding at 500 ° C. or higher and 770 ° C. or lower, and control the average cooling rate of the hot-rolled steel sheet being rolled up in the temperature range from 650 ° C. to 400 ° C. to 50 ° C./s or lower. When winding is started at a temperature higher than 770 ° C, the transformation from austenite to bainitic ferrite may not occur, so the winding temperature is preferably 770 ° C or lower. Since the formation of granular bainite may not occur when the winding temperature is 500 ° C., the winding temperature is set to 500 ° C. or higher. The winding temperature is preferably 560 ° C. or higher.

It is preferable to cool the hot-rolled steel sheet being wound in the temperature range from 650 ° C to 400 ° C at an average cooling rate of 50 ° C / s or less. By cooling at the above-mentioned average cooling rate in the temperature range of 650 ° C to 400 ° C, the grain boundaries between the bainitic ferrites are restored due to the effect of Ni content to form subgrain boundaries, and a desired amount of granular bainite is formed. Can be obtained. That is, in the hot rolling step, a desired amount of granular bainite can be produced. On the other hand, if the average cooling rate in the above temperature range exceeds 50 ° C./s, the grain boundaries between the bainitic ferrites may be restored and subgrain boundaries may not be formed. Therefore, the average cooling rate in the above temperature range is preferably 50 ° C./s or less. Since the slower the cooling rate is, the more preferable it is to promote the formation of subgrain boundaries, the average cooling rate in the above temperature range is preferably 30 ° C./s or less and 20 ° C./s or less. The lower limit of the average cooling rate in the above temperature range is not particularly limited, but the lower limit is 0.1 ° C./s in normal actual operation. The average cooling rate during winding is calculated by measuring the temperature at the center of the hot-rolled coil during winding in the longitudinal direction using an infrared radiation thermometer for high temperature measurement.

The hot-rolled steel sheet wound in the hot-rolling step may be unwound, pickled, and further cold-rolled. By removing the oxide on the surface of the hot-rolled steel sheet by pickling and subjecting it to cold rolling, it is possible to improve the tensile strength, the chemical conversion treatment property, the plating property, and the like. The pickling may be performed once or may be divided into a plurality of times. The cold rolling may be cold rolling performed at a normal cumulative reduction rate, for example, a cumulative reduction rate of 30 to 90%, but the cold rolling is not limited to this cumulative reduction rate. In addition to hot-rolled and cold-rolled steel sheets, hot-rolled steel sheets and cold-rolled steel sheets include hot-rolled steel sheets or cold-rolled steel sheets that have been recrystallized and annealed under normal conditions, and under normal conditions. It also includes steel sheets that have been temper-rolled in.

When plating is applied to the surface, the plating conditions are not particularly limited, and normal conditions may be used. A hot-rolled steel sheet, a cold-rolled steel sheet, or a steel sheet obtained by recrystallizing and / or tempering and rolling a cold-rolled steel sheet may be plated under normal plating conditions, if necessary. For example, examples of plating include electric plating and hot-dip plating, examples of electric plating include electric zinc plating and electric Zn—Ni alloy plating, and hot-dip plating includes hot-dip zinc plating, alloyed hot-dip zinc plating, and hot-dip aluminum plating. Examples thereof include hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating.

By the above manufacturing method, a hot stamping steel sheet to be applied to the hot stamping compact according to the present embodiment is obtained.

"After heating to a holding temperature of 800 ° C or higher at an average heating rate of less than 100 ° C / s, hold it, hot stamp it so that the elapsed time from the start of heating to molding is 300 seconds or less, and 400 ° C or lower. Cool down to the temperature range of
In the method for producing a hot stamped body according to the present embodiment, the above-mentioned hot stamping steel plate is heated to a temperature of 800 ° C. or higher at an average heating rate of less than 100 ° C./s and then held, and from the start of heating to molding. It is preferable to perform hot stamping so that the elapsed time is 300 seconds or less, and then cool to a temperature range of 400 ° C. or less. By setting the holding temperature to 800 ° C. or higher, the transformation from granular bainite generated in the hot rolling step to austenite can be sufficiently promoted, and the grain boundaries of martensite can be controlled to a preferable form. Therefore, the holding temperature is preferably 800 ° C. or higher. The holding time may be set so that the elapsed time from the start of heating to the start of molding is within a predetermined range. It is preferable that the molded product after hot stamping is cooled by a mold to a temperature range of 400 ° C. or lower. By stopping the cooling in the temperature range of 400 ° C. or lower, the grain boundaries of martensite can be controlled to a preferable form. When the average heating rate up to 800 ° C. or higher is less than 100 ° C./s and the elapsed time from the start of heating to molding is 300 seconds or less, the average crystal grain size of the old austenite grains should be controlled to more than 10 μm and 40 μm or less. As a result, the major axis length / minor axis length of martensite crystal grains can be reduced. Therefore, the average heating rate up to 800 ° C. or higher is set to less than 100 ° C./s, and the elapsed time from the start of heating to molding is set to 300 seconds or less. The lower limit of the average heating rate up to 800 ° C. or higher is 0.01 ° C./s in normal actual operation. It is preferable to heat the hot stamping steel sheet to a holding temperature of 860 ° C. or higher.

"Bake back at 150 ° C or higher and 650 ° C or lower"
For the purpose of adjusting the strength and improving the ductile brittle transition temperature and low temperature toughness, the hot stamped compact cooled to room temperature may be tempered in the range of 150 ° C to 650 ° C. In this case, for example, only a part of the hot stamp molded product may be tempered. As a result, a softened region can be formed in a part of the hot stamped molded product, and properties such as strength and toughness can be controlled according to the portion of the hot stamped molded product. For example, when the hot stamp molded body is applied to a high-strength member of an automobile, it is possible to control the fracture and deformation modes of the high-strength member at the time of collision by tempering and softening only a part of the high-strength member. can.

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

Steel pieces produced by casting molten steel having the chemical compositions shown in Tables 1 to 3 are hot-rolled under the conditions shown in Tables 4 to 6, and if necessary, cold-rolled and / or plated. The steel sheets for hot stamping shown in 4 to 6 were obtained. Further, the hot stamping steel sheet is heat-stamped under the conditions shown in Tables 7 to 9, and if necessary, it is heated to the temperature shown in Table 9 and baked, or the hot stamped body is formed. A softened region was formed by irradiating a part with a laser and baking it to obtain a hot stamped body shown in Tables 7-9. In Tables 6 and 9, "Al" indicates hot-dip aluminum plating, "GA" indicates alloyed hot-dip galvanization, and "electric zinc" indicates electric zinc plating. The cooling rate during winding in hot rolling was calculated by measuring the temperature at the center of the hot-rolled coil during winding in the longitudinal direction using an infrared radiation thermometer for high temperature measurement.

Regarding the steel plate for hot stamping, granular baynite (with an average crystal orientation difference of 0.4 ° or more and 3.0 ° or less inside the crystal grains surrounded by grain boundaries with an average crystal orientation difference of 5 ° or more). The total of the grain boundary lengths with an area ratio of (crystal grains) and an average crystal orientation difference of 0.4 ° or more and 3.0 ° or less and the grain boundaries with an average crystal orientation difference of more than 3.0 °. The ratio of the grain boundary lengths having an average crystal orientation difference of 0.4 ° or more and 3.0 ° or less with respect to the length of the grain boundaries was obtained by the above method.

For the hot stamped body, the major axis length / minor axis length, which is the ratio between the major axis length and the minor axis length of the average crystal grain size of the old austenite grains and the crystal lattice of the martensite crystal grains, is described above. Obtained by the method.

"Vickers hardness"
The Vickers hardness of the hot stamp molded product was obtained by the following method. First, a sample was cut out so that a cross section perpendicular to the surface (thickness cross section) could be observed from an arbitrary position 50 mm or more away from the end face of the hot stamp molded body. The size of the sample was set so that it could be observed by 10 mm in the rolling direction, although it depends on the measuring device. The cross section of the sample was polished using # 600 to # 1500 silicon carbide paper, and then mirror-finished using a diluted solution such as alcohol and a liquid in which diamond powder having a particle size of 1 to 6 μm was dispersed in pure water. .. Using a Micro Vickers hardness tester at a plate thickness of 1/4 of the mirror-finished cross section, it is hardened in the direction parallel to the plate surface (rolling direction) at a load of 1 kgf and at intervals of 3 times or more the indentation. Was measured. A total of 20 points were measured, and the average value was taken as the Vickers hardness of the hot stamped product. When the Vickers hardness was 450 Hv or more, it was judged to be excellent in hardness, and when it was less than 450 Hv, it was judged to be inferior in hardness, and it was judged to be unacceptable.

"Tensile strength"
The tensile strength of the hot stamped body was determined by preparing the No. 5 test piece described in JIS Z 2241: 2011 from an arbitrary position of the hot stamped body and following the test method described in JIS Z 2241: 2011. The obtained tensile strength was used in the evaluation of early fracture resistance described later.

"Evaluation method of early fracture resistance"
The early fracture resistance of the hot stamped body was evaluated as follows. A numerical value obtained by dividing the tensile strength of the hot stamped product obtained by the above method by the value obtained by multiplying the Vickers hardness obtained by the above method by 3.3 (tensile strength / (Vickers hardness × 3.3)). When is 0.60 or more, it is judged to be acceptable as having excellent early breaking resistance, and when it is less than 0.60, it is judged to be rejected. The value obtained by multiplying the Vickers hardness by 3.3 is the tensile strength estimated from the hardness, and if the measured value of the tensile strength is 0.60 times or more of the estimated tensile strength, the early fracture resistance is excellent. You can judge.

Tables 7-9 show the metallographic structure and mechanical properties of the hot stamped article. Looking at Tables 7 to 9, it can be seen that the hot stamped compact having a chemical composition and a metal structure within the scope of the present invention has high hardness and excellent early fracture resistance.
On the other hand, it can be seen that a hot stamped body in which any one or more of the chemical composition and the metal structure deviates from the present invention is inferior in one or more of Vickers hardness and early fracture resistance.

According to the above aspect according to the present invention, it is possible to provide a hot stamp molded product having high hardness and excellent early breaking resistance.

Claims (4)

The chemical composition is by mass%,
C: 0.15% or more, less than 0.70%,
Si: 0.010% or more, less than 0.50%,
Mn: 0.010% or more, less than 3.00%,
sol. Al: 0.0002% or more, 3.000% or less,
Ni: 3.0% or more and less than 15.0%,
P: 0.100% or less,
S: 0.1000% or less,
N: 0.0100% or less,
Nb: 0% or more, 0.150% or less,
Ti: 0% or more, 0.150% or less,
Mo: 0% or more, 1.000% or less,
Cr: 0% or more, 1.000% or less,
B: 0% or more, 0.0100% or less,
V: 0% or more, 1.000% or less,
Cu: 0% or more, 1.000% or less,
Sn: 0% or more, 1.000% or less,
W: 0% or more, 1.000% or less,
Ca: 0% or more, 0.010% or less, and REM: 0% or more, 0.300% or less,
The rest consists of Fe and impurities
The average crystal grain size of the old austenite grains is more than 10 μm and 40 μm or less.
A hot stamped body characterized in that the major axis length / minor axis length, which is the ratio of the length of the major axis to the length of the minor axis of the crystal lattice of martensite crystal grains, is less than 1.020. The chemical composition is by mass%.
Nb: 0.010% or more, 0.150% or less,
Ti: 0.010% or more, 0.150% or less,
Mo: 0.005% or more, 1.000% or less,
Cr: 0.005% or more, 1.000% or less,
B: 0.0005% or more, 0.0100% or less,
V: 0.0005% or more, 1.000% or less,
Cu: 0.0010% or more, 1.000% or less,
Sn: 0.001% or more, 1.000% or less,
W: 0.001% or more, 1.000% or less,
The claim is characterized by containing one or more of the group consisting of Ca: 0.001% or more and 0.010% or less, and REM: 0.001% or more and 0.300% or less. The hot stamp molded body according to 1. The hot stamp molded article according to claim 1 or 2, wherein the surface is provided with a plating layer. The hot stamp molded product according to any one of claims 1 to 3, which has a softened region in a part thereof.