JP7350057B2 - hot stamp molded body - Google Patents

Description

TECHNICAL FIELD The present invention relates to a hot-stamped molded product suitably used for structural members and reinforcing members of automobiles and structures that require strength.
This application claims priority based on Japanese Patent Application No. 2019-057145 filed in Japan on March 25, 2019, the contents of which are incorporated herein.

In recent years, there has been a demand for lighter automobile bodies from the viewpoint of environmental protection and resource conservation. Therefore, the application of high-strength steel sheets to automobile parts is accelerating. However, as the strength of the steel sheet increases, the formability deteriorates, so the formability of high-strength steel sheets into members with complex shapes becomes an issue.
In order to solve these problems, hot stamping, in which a steel plate is heated to a high temperature in the austenite region and then press-formed, is being applied. Hot stamping involves quenching in the mold at the same time as pressing, so it is possible to obtain strength that corresponds to the amount of C in the steel sheet, and is attracting attention as a technology that can both form automobile parts and ensure strength. ing.

Conventional hot-stamped molded bodies manufactured by press quenching during hot-stamping have poor deformability because the entire region in the thickness direction is formed of a hard structure (mainly martensite). In order to obtain even better collision resistance properties in automobile parts, it is necessary to increase the ability to absorb impact energy, and when considering the deformation mode during a collision, it is necessary to increase the bendability in particular among the deformability.

In Patent Document 1, by controlling the Mn content or the total content of Mn and at least one of Cr, Mo, Cu, and Ni, the crystal grain size of martensite is made finer, resulting in high strength. However, techniques for improving collision resistance have been disclosed.
Patent Document 2 discloses a technique in which the average crystal grain size of prior austenite grains is made finer by selecting alloying elements to improve collision resistance.

Japanese Patent Application Publication No. 2010-070806 Japanese Patent Application Publication No. 2014-015638

Acta Materialia, 58 (2010), 6393-6403

In view of the problems of the prior art, an object of the present invention is to provide a hot-stamped molded product that has high hardness, which is a characteristic desired for general hot-stamped molded products, and has excellent bendability. .

The inventors of the present invention have intensively studied methods for solving the above problems.
In order to improve the bendability of a hot-stamped molded product mainly composed of martensite, the deformability can be increased by promoting the movement of dislocations in martensite. Since martensite has fine grains, it has a large grain boundary area, and dislocations that have moved within the grains are blocked by the grain boundaries, so martensite is characterized by low deformability. Therefore, the present inventors investigated a method for promoting the movement of dislocations even if the grain boundary area is large. As a result, the present inventors found that among the four types of grain boundaries contained in martensite, by increasing the proportion of grain boundaries with the lowest angle, it was possible to facilitate the movement of dislocations between grains. It has been found that the bendability of the hot-stamped molded product is improved. Specifically, the present inventors found that among grain boundaries with an average crystal orientation difference of 5 degrees or more in crystal grains having a body-centered structure such as martensite, the rotation angle is 57 degrees with the <011> direction as the rotation axis. The length of the grain boundary where the rotation angle is ~63°, the length of the grain boundary where the rotation angle is between 49° and 56°, the length of the grain boundary where the rotation angle is between 4° and 12°, and the length of the grain boundary where the rotation angle is between 64° and 72°. By controlling the ratio of the length of the grain boundary where the rotation angle is 4° to 12° to 15% or more of the total length of the grain boundary where the rotation angle is 4° to 12°, the bending of the hot stamped body It was found that sexual performance was improved.

Therefore, the present inventors investigated a method of increasing the ratio of the length of grain boundaries where the rotation angle is 4° to 12° with the <011> direction as the rotation axis. As a result, the present inventors found that by containing 3.0% by mass or more of Ni in a steel sheet and controlling the average crystal grain size of austenite before martensitic transformation to 10 μm or less, the body center of martensite, etc. Among the grain boundaries in which the average crystal orientation difference in crystal grains with a structure is 5° or more, the length of the grain boundary whose rotation angle is 57° to 63° with the <011> direction as the rotation axis, and the length of the grain boundary where the rotation angle is 49° For the total length of the grain boundary with a rotation angle of ~56°, the length of the grain boundary with a rotation angle of 4° to 12°, and the length of the grain boundary with a rotation angle of 64° to 72°. It has been found that the ratio of grain boundary length at which the rotation angle is 4° to 12° can be controlled to 15% or more.

Ni has the effect of improving the deformability of austenite by forming a solid solution in austenite. When austenite transforms into martensite, stress associated with the transformation is applied to austenite due to a change in crystal structure, so grain boundaries are generated that are advantageous for relieving this stress. The larger the rotation angle of a grain boundary, the greater the stress relaxation effect, so normally the grain boundary with the highest rotation angle, which is 64° to 72° with the <011> direction as the rotation axis, is preferential. to be generated. The present inventors have discovered that by incorporating Ni to improve the deformability of austenite, it is possible to alleviate the stress that accompanies martensitic transformation. As a result, it has been found that it is possible to increase the number of grain boundaries having a rotation angle of 4° to 12° with the <011> direction as the rotation axis, which was difficult to generate using conventional techniques. In addition, although the details will be described later, the present inventors have succeeded in reducing the average crystallization of prior austenite grains by increasing the generation rate of subgrain boundaries in a steel sheet for hot stamping and by rapidly heating in the heating process during hot stamping. It has been found that the particle size can be controlled to 10 μm or less. As a result, the present inventors found that since the grain boundary area of austenite increases, deformation (slippage) between crystal grains becomes easier, and the effect of relieving stress generated due to martensitic transformation can be enhanced. I found out.

The present inventors investigated a method for refining prior austenite grains. By making the steel sheet contain 3.0% by mass or more of Ni, dislocations in austenite can easily move, so austenite grains tend to become coarser. Therefore, in order to obtain fine prior austenite grains in Ni-containing steel, it is effective to delay the start of transformation by increasing the transformation temperature to austenite. In order to increase the transformation temperature to austenite, it is effective to utilize grain boundaries with a low carbon concentration as sites for reverse transformation of austenite.

The inventors have studied a method for obtaining fine prior austenite grains by generating grain boundaries with a low carbon concentration in a hot stamping steel sheet. As a result, the present inventors found that it is possible to generate a metal structure called granular bainite by hot rolling a steel sheet containing Ni under predetermined conditions, and that this granular bainite has It was found that large-angle grain boundaries and sub-grain boundaries are included, and that carbon is concentrated at a high concentration in the large-angle grain boundaries, and that carbon segregation is suppressed at the sub-grain boundaries.

Granular bainite, which includes high-angle grain boundaries and subgrain boundaries, is a metal structure that is generated through the following two steps. In the first stage, the transformation of austenite to bainitic ferrite occurs. In the second stage, grain boundaries between bainitic ferrites recover and become sub-grain boundaries, resulting in granular bainite.

As a first step, in the hot rolling step, hot rolling is completed at a temperature of 800° C. or higher, and coiling is performed at a temperature range of 500° C. or higher and 770° C. or lower. In order to obtain a desired amount of granular bainite, it is important to control the recrystallization rate of austenite before transformation, that is, the dislocation density. If recrystallization of austenite is promoted too much, the dislocation density in austenite decreases, making it impossible to obtain a desired amount of granular bainite. On the other hand, even if recrystallization is insufficient, the dislocation density in austenite increases too much and transformation to granular bainite no longer occurs. As a result of intensive study by the present inventors, the present inventors found that if the hot rolling end temperature is 800°C or higher, recrystallization of austenite is moderately promoted, and as a result, transformation to granular bainite is likely to occur. We found that the dislocation density can be controlled. Next, the 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 during winding 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, the grain boundaries between bainitic ferrites are recovered and subgrain boundaries are formed, resulting in granular bainite. Obtainable. Thereby, granular bainite can be generated in the hot rolling process. On the other hand, if the average cooling rate in the above temperature range exceeds 50° C./s, grain boundaries will recover and sub-grain boundaries cannot be formed. During cooling during winding, initial bainitic ferrite has grain boundaries with an average crystal orientation difference of 5° or more, but the average cooling rate is slow with an average cooling rate of 50°C/s or less in the temperature range where Fe can diffuse. By performing cooling, dislocation recovery occurs near the grain boundaries of bainitic ferrite, and sub-grain boundaries with an average crystal orientation difference of 0.4° or more and 3.0° or less are generated.

When dislocation recovery occurs, C in the steel diffuses from the sub-grain boundaries to the surrounding high-angle grain boundaries, so it is possible to reduce the segregation of carbon at the sub-grain boundaries. By containing 3.0% by mass or more of Ni, dislocation mobility increases, dislocation recovery is promoted, and the amount of sub-grain boundaries generated increases, so that the average crystal orientation difference is 0.4° or more. , the average crystal orientation difference is 0.4° or more and 3.0° or less relative to the sum of the length of grain boundaries of 3.0° or less and the length of grain boundaries with an average crystal orientation difference of more than 3.0° The length ratio of the grain boundaries becomes 60% or more, and the number of austenite reverse transformation sites can be increased during hot stamp heating, contributing to the refinement of prior austenite grains.

The present invention was completed based on the above findings, and the gist thereof is as follows.
[1] The hot stamp molded article according to one embodiment of the present invention has a chemical composition in 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, 0.283 % or less,
Ni: 3.0% or more, 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, 1.0000% or less,
Cu: 0% or more, 1.0000% or less,
Sn: 0% or more, 1.000% or less,
W: 0% or more, 1.000% or less,
Contains Ca: 0% or more and 0.010% or less, and REM: 0% or more and 0.300% or less,
The remainder consists of Fe and impurities,
The average crystal grain size of the prior austenite grains is 10 μm or less,
Among grain boundaries with an average crystal orientation difference of 5° or more in crystal grains with a body-centered structure, the length of the grain boundary where the rotation angle is 57° to 63° with the <011> direction as the rotation axis, and the rotation angle The total length of the grain boundary whose rotation angle is 49° to 56°, the length of the grain boundary whose rotation angle is 4° to 12°, and the length of the grain boundary whose rotation angle is 64° to 72°. On the other hand, it is characterized in that the ratio of the length of grain boundaries where the rotation angle is 4° to 12° is 15% or more.
[2] The hot-stamped molded article according to [1] above 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, 1.0000% or less,
Cu: 0.0010% or more, 1.0000% 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-stamped molded article according to [1] or [2] above may have a plating layer on its surface.
[4] The hot-stamped molded article according to any one of [1] to [3] above may have a softened region in part.

According to the above aspect of the present invention, it is possible to provide a hot-stamped molded article that has high hardness and excellent bendability.

Hereinafter, preferred embodiments of the present invention will be described in detail. However, the present invention is not limited to only the configuration disclosed in this embodiment, and various changes can be made without departing from the spirit of the present invention. The numerically limited range described below includes the lower limit value and the upper limit value. Numerical values indicated as “more than” or “less than” do not include the value within the numerical range. All percentages regarding chemical composition indicate mass %. First, the reason for limiting the chemical composition of the hot-stamped molded article according to this embodiment will be explained.

The hot-stamped molded article according to the present embodiment has a chemical composition, in mass %, of C: 0.15% or more and less than 0.70%, Si: 0.010% or more and less than 0.50%, and Mn: 0. .010% or more, less than 3.00%, sol. Al: 0.0002% or more, 3.000% or less, Ni: 3.0% or more, less than 15.0%, P: 0.100% or less, S: 0.1000% or less, N: 0.0100% The following and the remainder: Contains Fe and impurities. Each element will be explained in detail below.

"C: 0.15% or more, less than 0.70%"
C is an important element in order to obtain the hardness desired for automobile parts and the like in hot-stamped molded products. If the C content is less than 0.15%, 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, if the C content is 0.70% or more, coarse carbides will form in the steel and the toughness of the hot stamped product will decrease, so the C content should be 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 molded product and contributes to improving its toughness. If the Si content is less than 0.010%, the deformability is poor and the toughness of the hot stamped product deteriorates. Therefore, the Si content is set to 0.010% or more. Since the above effect is saturated even if the Si content is 0.50% or more, the Si content is made less than 0.50%.

"Mn: 0.010% or more, less than 3.00%"
Mn is an element that contributes to improving the hardness of the hot stamp molded product through solid solution strengthening. If the Mn content is less than 0.010%, the solid solution strengthening ability is poor and the martensite becomes soft, making it difficult to obtain the hardness desired for automobile parts 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, if the Mn content is 3.00% or more, the martensite becomes brittle and the toughness of the hot-stamped product is impaired, so the Mn content is made less than 3.00%.

"sol.Al: 0.0002% or more, 3.000% or less"
Al is an element that has the effect of deoxidizing molten steel and making the steel sound (suppressing the occurrence of defects such as blowholes in the steel). sol. If the Al content is less than 0.0002%, deoxidation is not sufficient, so sol. Al content shall be 0.0002% or more. sol. Al content is preferably 0.001% or more. On the other hand, sol. If the Al content exceeds 3.000%, coarse oxides will be generated and the toughness of the hot stamped product will be impaired. Al content shall be 3.000% or less.
Note that in this embodiment, sol. Al means acid-soluble Al, and indicates solid solution Al that exists in steel in a solid solution state.

"Ni: 3.0% or more, less than 15.0%"
Ni is an element that has the effect of refining prior austenite grains, and is also an element necessary to obtain a desired amount of granular bainite in the hot rolling process. If the Ni content is less than 3.0%, the above effects cannot be obtained, so the Ni content is set to 3.0% or more. In order to further increase the ability to absorb impact energy at low temperatures, the Ni content is preferably 5.0% or more. On the other hand, if the Ni content is 15.0% or more, the martensite becomes brittle and the toughness of the hot-stamped product is impaired, so the Ni content is made 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 grain boundaries and reduces the strength of the grain boundaries. If the P content exceeds 0.100%, the strength of the grain boundaries will decrease significantly and the bendability of the hot stamped product will decrease, so the P content should be 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 will increase significantly and it is economically unfavorable, so 0.0001% is the practical lower limit for practical steel sheets. be.

"S: 0.1000% or less"
S is an element that forms inclusions in steel. If the S content exceeds 0.1000%, inclusions will form in the steel and the bendability of the hot stamped product will decrease, so the S content should be 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 S removal cost will increase significantly and it is economically unfavorable, so 0.0015% is the practical lower limit for practical steel sheets. be.

"N: 0.0100% or less"
N is an impurity element that forms nitrides and reduces the bendability of the hot-stamped body. If the N content exceeds 0.0100%, coarse nitrides will form in the steel and the bendability of the hot-stamped product will significantly decrease, so the N content should be 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 de-N cost will increase significantly and it is economically unfavorable, so 0.0001% is the practical lower limit for practical steel plates. be.

The remainder of the chemical composition of the hot-stamped molded body according to this embodiment is Fe and impurities. Examples of impurities include elements that are unavoidably mixed in from steel raw materials or scraps and/or during the steel manufacturing process and are allowed within a range that does not impede the properties of the hot stamped molded product according to the present embodiment.
The hot-stamped molded article according to this embodiment may contain the following elements as optional elements. When the following arbitrary elements are not included, 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 through solid solution strengthening, it may be included 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 preferably contained at 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"
Ti is an element that contributes to improving the hardness of the hot-stamped molded product through solid solution strengthening, so it may be included if necessary. When Ti is contained, if it is less than 0.010%, a sufficient effect cannot be obtained by containing Ti, so it is preferable that the Ti content is 0.010% or more. The Ti content is more preferably 0.020% or more. On the other hand, since the above effect is saturated even if the Ti content exceeds 0.150%, 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"
Mo is an element that contributes to improving the hardness of the hot-stamped molded product through solid solution strengthening, so it may be included if necessary. When Mo is contained, it is preferable that the Mo content is 0.005% or more, since sufficient effects cannot be obtained by containing Mo if it is less than 0.005%. Mo content is more preferably 0.010% or more. On the other hand, since the above effect is saturated even if the Mo content exceeds 1.000%, the Mo content is set to 1.000% or less. Mo content is preferably 0.800% or less.

"Cr: 0% or more, 1.000% or less"
Cr is an element that contributes to improving the hardness of the hot-stamped molded product through solid solution strengthening, so it may be included if necessary. When Cr is contained, if the Cr content is less than 0.005%, sufficient effects cannot be obtained by containing Cr, so the Cr content is preferably 0.005% or more. The Cr content is more preferably 0.010% or more. On the other hand, since the above effect is saturated even if the Cr content exceeds 1.000%, 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 grain boundaries and improves the strength of the grain boundaries, it may be included as necessary. When B is contained, if the B content is less than 0.0005%, sufficient effects cannot be obtained by containing B, 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, 1.0000% or less"
Since V is an element that contributes to improving the hardness of the hot stamp molded product through solid solution strengthening, it may be included if necessary. When V is contained, if the V content is less than 0.0005%, sufficient effects cannot be obtained by containing V, 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 1.0000%, 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, 1.0000% or less"
Cu is an element that contributes to improving the hardness of the hot-stamped molded product through solid solution strengthening, so it may be included if necessary. When Cu is contained, if the Cu content is less than 0.0010%, a sufficient effect due to the Cu content cannot be obtained, so the Cu content is preferably 0.0010% or more. The Cu content is more preferably 0.0100% or more. On the other hand, since the above effect is saturated even if the Cu content exceeds 1.0000%, 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 that deoxidizes molten steel and makes the steel sound, it may be contained up to 1.000%. In order to reliably exhibit the above effects, the Sn content is preferably 0.001% or more.

"W: 0% or more, 1.000% or less"
Since W is an element that has the effect of deoxidizing molten steel and making the steel sound, it may be contained up to 1.000%. In order to reliably exhibit the above effects, the W content is preferably 0.001% or more.

"Ca: 0% or more, 0.010% or less"
Since Ca is an element that has the effect of deoxidizing molten steel and making the steel sound, it may be contained with an upper limit of 0.010%. In order to reliably exhibit the above effects, the Ca content is preferably 0.001% or more.

"REM: 0% or more, 0.300% or less"
Since REM is an element that has the effect of deoxidizing molten steel and making the steel sound, it may be contained up to 0.300%. In order to reliably exhibit the above effects, 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 as a misch metal, but in addition to La and Ce, lanthanide series elements may be contained in combination. Even when a lanthanide series element is contained in combination in addition to La and Ce, the hot-stamped molded article according to the present embodiment can exhibit its effects. Moreover, even if a metal REM such as metal La or Ce is contained, the hot-stamped molded article according to the present embodiment can exhibit its effects.

The chemical composition of the hot-stamped molded article described above may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Note that C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas melting-thermal conductivity method. sol. Al may be measured by ICP-AES using a solution obtained by thermally decomposing a sample with an acid. When the hot-stamped molded article has a plating layer on its surface, the chemical composition may be analyzed after removing the plating layer on the surface by mechanical grinding.

Next, a description will be given of the metallographic structure of the hot-stamped molded product according to the present embodiment and the hot-stamped steel plate applied thereto. First, the metallographic structure of the hot stamping steel plate applied to the hot stamping molded product according to the present embodiment will be explained.

[Steel plate for hot stamping]
“Sub-grains (granular bainite) with an average crystal orientation difference of 0.4° or more and 3.0° or less are placed inside grains surrounded by grain boundaries with an average crystal orientation difference of 5° or more. Contains 10% or more
The steel sheet for hot stamping is made of granular bainite (existing inside grains surrounded by grain boundaries with an average crystal orientation difference of 5° or more, with an average crystal orientation difference of 0.4° or more and 3.0° or less). It is necessary to contain 10% or more of the area ratio of certain subcrystalline grains. The granular bainite produced in the hot rolling process is transformed into austenite through a predetermined heat treatment process (cold rolling if necessary), and finally the desired metal structure can be obtained in the hot stamped compact. can. As a result of intensive research by the present inventors, it has been found that if the area ratio of granular bainite is less than 10%, a desired metal structure cannot be obtained in the hot-stamped molded product. Therefore, in the hot stamping steel plate applied to the hot stamping molded product according to the present embodiment, the area ratio of granular bainite is set to 10% or more. Preferably, the area ratio is 15% or more, 20% or more, 25% or more, or 30% or more. Although the upper limit is not particularly limited, the area ratio of granular bainite may be less than 95%.

The remainder of the metal structure is not particularly limited, but is usually one or more of ferrite, upper bainite, lower bainite, martensite, tempered martensite, retained austenite, iron-based carbides, and alloy carbides. The hot-stamping steel plate applied to the hot-stamping molded product 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 explained.
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 (plate thickness cross section) can be observed. The size of the sample is such that it can be observed about 10 mm in the rolling direction, although it depends on the measuring device. Regarding the cut sample, EBSD analysis is performed at a half-thickness position at a measurement interval of 0.2 μm to obtain crystal orientation information. Here, the EBSD analysis uses a device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) at a rate of 200 to 300 points/second. Perform at analysis speed.

The area ratio of granular bainite can be determined using, for example, "Grain Average" installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device.
By using the "Misorientation" function, calculation can be easily performed. With this function, for a crystal grain with a body-centered structure, after calculating the orientation difference between adjacent measurement points, it is possible to obtain an average value for all measurement points within the crystal grain. Based on the obtained crystal orientation information, a region surrounded by grain boundaries with an average crystal orientation difference of 5° or more is defined as a grain, and the "Grain Average Misorientation" function calculates the average crystal orientation difference within the grain. The area ratio of granular bainite can be obtained by calculating the area ratio of a region (sub-grain boundary) having an angle of 0.4° or more and 3.0° or less.

"The average length of grain boundaries with an average crystal orientation difference of 0.4° or more and 3.0° or less and the length of grain boundaries with an average crystal orientation difference of more than 3.0°," The ratio of the length of the grain boundary 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-stamped molded body according to the present embodiment, the length of grain boundaries with an average crystal orientation difference of 0.4° or more and 3.0° or less and the average crystal orientation difference of 3. The ratio of grain boundary lengths having an average crystal orientation difference of 0.4° or more and 3.0° or less to the total length of grain boundaries exceeding 0° is 60% or more. Average If the ratio of grain boundary lengths with a crystal orientation difference of 0.4° or more and 3.0° or less is less than 60%, a desired metal structure cannot be obtained in the hot-stamped compact. The length ratio 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, with respect to 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°, A method for measuring the ratio of grain boundary lengths with 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 surface of a steel plate for hot stamping so that a cross section perpendicular to the surface (plate thickness cross section) can be observed. The size of the sample is such that it can be observed about 10 mm in the rolling direction, although it depends on the measuring device. After polishing the cross section of the cut sample using #600 to #1500 silicon carbide paper, it was polished to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 μm was dispersed in a dilute solution such as alcohol and pure water. Finish. Next, the sample is polished for 8 minutes using colloidal silica without an alkaline solution at room temperature to remove the strain introduced into the surface layer of the sample. At any position in the longitudinal direction of the sample cross section, a region with a length of 50 μm and a depth of 50 μm from the surface of the steel plate is measured by electron backscatter diffraction at measurement intervals of 0.1 μm to obtain crystal orientation information. For the measurement, an apparatus consisting 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 electron beam irradiation time is 0.01 seconds/point. The obtained crystal orientation information is analyzed using the "Image Quality" function installed in the software "OIM Analysis (registered trademark)" included with the EBSD analyzer to determine whether the average crystal orientation difference is 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, relative to the total length of the following grain boundaries and the length of grain boundaries with an average crystal orientation difference of more than 3.0° Calculate the length ratio of. With this function, the total length of grain boundaries with any rotation angle can be calculated for grain boundaries of crystal grains with a body-centered structure. For all grains included in the measurement area, calculate the total length of these grain boundaries, and calculate the length of grain boundaries and average crystal orientation where the average crystal orientation difference is 0.4° or more and 3.0° or less. The ratio of the length of grain boundaries with an average crystal orientation difference of 0.4° or more and 3.0° or less to the total length of grain boundaries with a difference of more than 3.0° is calculated.

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

Next, the metallographic structure of the hot-stamped molded body according to this embodiment will be explained.

"Average grain size of prior austenite grains: 10 μm or less"
If the average crystal grain size of austenite before martensitic transformation is 10 μm or less, after martensitic transformation, among grain boundaries where the average crystal orientation difference in crystal grains with a body-centered structure such as martensite is 5° or more, The grain boundary length has a rotation angle of 57° to 63° with the <011> direction as the rotation axis, the grain boundary length has a rotation angle of 49° to 56°, and the rotation angle has a rotation angle of 4° to 12°. The ratio of the length of the grain boundary where the rotation angle is 4° to 12° is 15% of the total length of the grain boundary and the length of the grain boundary where the rotation angle is 64° to 72°. It can be controlled more than that. As a result, the bendability of the hot stamp molded product can be improved. Therefore, in the hot-stamped molded article according to the present embodiment, the average crystal grain size of the prior austenite grains is set to 10 μm or less. The average grain size of the prior austenite grains is preferably 8 μm or less. Although the lower limit is not particularly limited, since the average crystal grain size of prior austenite grains that can be realized in normal actual operations is 2 μm or more, the lower limit of the average crystal grain size of prior austenite grains may be 2 μm.

"Among the grain boundaries in which the average crystal orientation difference in grains with a body-centered structure is 5 degrees or more, the length and rotation angle of the grain boundaries where the rotation angle is 57 degrees to 63 degrees with the <011> direction as the rotation axis. The total length of the grain boundary where the rotation angle is 49° to 56°, the length of the grain boundary where the rotation angle is 4° to 12°, and the length of the grain boundary where the rotation angle is 64° to 72°. However, the ratio of grain boundary lengths with a rotation angle of 4° to 12° is 15% or more.
Among grain boundaries with an average crystal orientation difference of 5° or more in crystal grains with a body-centered structure, the length of the grain boundary where the rotation angle is 57° to 63° with the <011> direction as the rotation axis, and the rotation angle The total length of the grain boundary with a rotation angle of 49° to 56°, the length of the grain boundary with a rotation angle of 4° to 12°, and the length of the grain boundary with a rotation angle of 64° to 72°. On the other hand, if the ratio of the length of the grain boundary where the rotation angle is 4° to 12° is less than 15%, the bendability of the hot stamped product cannot be improved. Therefore, in the hot-stamped compact according to the present embodiment, among the grain boundaries in which the average crystal orientation difference in crystal grains having a body-centered structure is 5 degrees or more, the rotation angle is 57 degrees to 57 degrees with the <011> direction as the rotation axis. The length of the grain boundary where the rotation angle is 63°, the length of the grain boundary where the rotation angle is 49° to 56°, the length of the grain boundary where the rotation angle is 4° to 12°, and the length of the grain boundary where the rotation angle is 64° to 72°. The ratio of the grain boundary length where the rotation angle is 4° to 12° is 15% or more with respect to the total length of the grain boundary where the rotation angle is 4° to 12°. The ratio of the grain boundary length at which the rotation angle is 4° to 12° is preferably 20% or more. The upper limit of the ratio of the grain boundary length at which the rotation angle is 4° to 12° may be set to 50% from the viewpoint of ensuring strength by strengthening the grain boundaries.

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

Next, a method for measuring the average crystal grain size of prior austenite grains will be explained. Cut a sample from a position 50 mm or more away from the end surface of the hot-stamped molded product (if it is not possible to sample from this position, any position other than the end) so that a cross section perpendicular to the surface (plate thickness cross section) can be observed. . The size of the sample is such that it can be observed about 10 mm in the rolling direction, although it depends on the measuring device. Regarding the cut sample, EBSD analysis is performed at a half-thickness position at a measurement interval of 0.1 μm to obtain crystal orientation information. Here, the EBSD analysis uses a device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL), and the analysis speed is 200 to 300 points/second. It will be carried out. Using the obtained crystal orientation information, the crystal orientation of the prior austenite grains is calculated from the crystal orientation relationship between the general prior austenite grains and the crystal grains with a body-centered structure after transformation, and the average crystal orientation of the prior austenite grains is calculated. All you have to do is calculate the particle size. The method for calculating the crystal orientation of prior austenite grains is not particularly limited, but for example, a crystal orientation map of prior austenite grains is created by the method described in Non-Patent Document 1, and from the created crystal orientation map, prior austenite is calculated by an intercept method. What is necessary is to calculate the average crystal grain size of the grains.

Among grain boundaries with an average crystal orientation difference of 5° or more in crystal grains with a body-centered structure, the length of the grain boundary where the rotation angle is 57° to 63° with the <011> direction as the rotation axis, and the rotation angle The total length of the grain boundary with a rotation angle of 49° to 56°, the length of the grain boundary with a rotation angle of 4° to 12°, and the length of the grain boundary with a rotation angle of 64° to 72°. On the other hand, the ratio of grain boundary lengths at which the rotation angle is 4° to 12° is obtained by the following method.
Cut a sample from a position 50 mm or more away from the end surface of the hot-stamped molded product (if it is not possible to sample from this position, any position other than the end) so that a cross section perpendicular to the surface (plate thickness cross section) can be observed. . The length of the sample is such that it can be observed by about 10 mm in the rolling direction, although it depends on the measuring device. Regarding the cut sample, EBSD analysis is performed at a half-thickness position at a measurement interval of 0.1 μm to obtain crystal orientation information. Here, the EBSD analysis uses a device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL), and the analysis speed is 200 to 300 points/second. It will be carried out.
Next, based on the obtained crystal orientation information, among the grain boundaries in which the average crystal orientation difference in crystal grains with a body-centered structure is 5 degrees or more, the rotation angle is 57 degrees or more with the <011> direction as the rotation axis. The length of the grain boundary where the rotation angle is 63°, the length of the grain boundary where the rotation angle is 49° to 56°, the length of the grain boundary where the rotation angle is 4° to 12°, and the length of the grain boundary where the rotation angle is 64° to 72°. Calculate the length of the grain boundary where the rotation angle is from 4° to 12° to the sum of the lengths of each grain boundary. As a result, among the grain boundaries in which the average crystal orientation difference in grains with a body-centered structure is 5° or more, the length of the grain boundary where the rotation angle is 57° to 63° with the <011> direction as the rotation axis, The total length of the grain boundary where the rotation angle is 49° to 56°, the length of the grain boundary where the rotation angle is 4° to 12°, and the length of the grain boundary where the rotation angle is 64° to 72°. On the other hand, the ratio of grain boundary lengths at which the rotation angle is 4° to 12° is obtained.

The above grain boundary length can be easily calculated by using the "Inverse Pole Figure Map" and "Axis Angle" functions installed in the software "OIM Analysis (registered trademark)" included with the EBSD analyzer, for example. It is possible to do so. With these functions, the total length of the grain boundaries of crystal grains having a body-centered structure can be calculated by specifying a specific rotation angle with an arbitrary direction as the rotation axis. The above analysis is performed on all the crystal grains included in the measurement region, and the lengths of the four types of grain boundaries described above are calculated using the <011> direction as the rotation axis.

"Plating layer"
In this embodiment, a plating layer may be formed on the surface of the hot-stamped molded body for the purpose of improving corrosion resistance. 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 electrolytic Zn--Ni alloy plating layer, and the like. Examples of the hot-dip plating layer include a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, and a hot-dip Zn-Al-Mg-Si. Including alloy plating layer etc. The amount of the plating layer to be deposited is not particularly limited and may be a general amount to be deposited.

"Softening area"
The hot-stamped molded article according to this embodiment may have a softened region formed in a part thereof. Weldability improves in the softened region. For example, if spot welding is performed after softening the edges of a hot stamped product, the difference in strength between the softened edges and the spot welded portion of the edges can be reduced, so that the strength difference from the interface between the two can be reduced. Destruction can be suppressed. For example, when applying a hot stamp molded body to a high-strength member of an automobile, by providing a softened region in a part of the high-strength member, it is possible to control the destruction and deformation mode of the high-strength member in the event of a collision. can. In order to form the softened region, for example, after forming a hot-stamping steel plate into a hot-stamped body, the strength of a part of the hot-stamped body may be reduced by laser irradiation. Note that laser irradiation is an example of heat treatment as a softening means, and the softening means is not particularly limited. As another means, the softened region may be formed, for example, by tempering a portion of the hot stamped body.

Next, a preferred manufacturing method for obtaining the hot stamp molded article according to the present embodiment will be described.
In the method for manufacturing a hot stamping steel plate applied to the hot stamping molded body according to the present embodiment, a steel billet having the above-mentioned chemical composition is subjected to hot rolling, the hot rolling is finished at a temperature of 800°C or higher, and 500° C. It is preferable that the hot-rolled steel sheet is wound at a temperature of not lower than 770°C, and that the average cooling rate of the hot rolled steel sheet during winding is 50°C/s or less in the temperature range from 650°C to 400°C.

The steel billet (steel material) to be subjected to hot rolling may be a steel billet manufactured by a conventional method, for example, a steel billet manufactured by a conventional method such as a continuous casting slab or a thin slab caster.

"Hot rolling end temperature is 800℃ 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 recrystallization of austenite is promoted too much, the dislocation density in austenite decreases, making it impossible to obtain a desired amount of granular bainite. On the other hand, even if recrystallization is insufficient, the dislocation density in austenite increases too much and transformation to granular bainite no longer occurs. As a result of intensive study by the present inventors, the present inventors found that if the hot rolling end temperature is 800°C or higher, recrystallization of austenite will proceed appropriately, and as a result, dislocations that are likely to transform into granular bainite will 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. Preferably it is 820°C or higher. Further, in steel having the chemical composition defined in this embodiment, it is unlikely that recrystallization will be excessively promoted, so the upper limit of the hot rolling end temperature is not particularly defined, but is usually 1050°C.

"The average cooling rate of the hot-rolled steel sheet during coiling is 50°C/s or less in the temperature range from 650°C to 400°C."
It is preferable that winding is started at 500°C or higher and 770°C or lower, and that the average cooling rate of the hot rolled steel sheet during winding is controlled to 50°C/s or lower in the temperature range from 650°C to 400°C. If winding is started at a temperature higher than 770°C, transformation from austenite to bainitic ferrite may not occur, so the winding temperature is preferably 770°C or lower. When the winding temperature is 500°C, granular bainite may not be generated. Therefore, the winding temperature is preferably 500°C or higher.

It is preferable to cool the hot rolled steel sheet during winding 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 from 650°C to 400°C, the grain boundaries between bainitic ferrites are recovered due to the effect of Ni content, forming sub-grain boundaries, and the desired amount of granular bainite is produced. can be obtained. That is, a desired amount of granular bainite can be generated in the hot rolling process. On the other hand, if the average cooling rate in the above temperature range exceeds 50° C./s, grain boundaries between bainitic ferrites may recover and sub-grain boundaries may not be formed. Therefore, the average cooling rate in the above temperature range is preferably 50° C./s or less. In order to promote the formation of sub-grain boundaries, it is preferable that the cooling rate be slow, so 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 in normal actual operation, the lower limit is 0.1° C./s. Note that the average cooling rate during winding is calculated by measuring the temperature at the longitudinal center of the hot-rolled coil during winding using an infrared radiation thermometer for high temperature measurement.

The hot-rolled steel sheet rolled up in the hot rolling process may be rewound, pickled, and further cold rolled. By removing oxides on the surface of a hot rolled steel sheet by pickling and subjecting it to cold rolling, it is possible to improve tensile strength, chemical conversion treatment properties, plating properties, etc. Note that the pickling may be carried out once or in multiple steps. The cold rolling may be carried out at a normal cumulative reduction rate, for example, 30 to 90%, but is not limited to this cumulative reduction rate. Hot-rolled steel sheets and cold-rolled steel sheets include hot-rolled and cold-rolled steel sheets, as well as hot-rolled steel sheets or cold-rolled steel sheets that have been recrystallized and annealed under normal conditions. This also includes steel sheets that have been temper rolled.

When plating is applied to the surface, the conditions for plating are not particularly limited, and normal conditions may be used. If necessary, plating may be applied to a hot rolled steel plate, a cold rolled steel plate, or a steel plate obtained by recrystallization annealing and/or temper rolling to a cold rolled steel plate under normal plating conditions. For example, examples of plating include electroplating and hot-dip plating, examples of electroplating include electrogalvanizing, electrolytic Zn-Ni alloy plating, and examples of hot-dip plating include hot-dip galvanizing, alloyed hot-dip galvanizing, hot-dip aluminum plating, Examples 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 plate to be applied to the hot stamping molded article according to the present embodiment is obtained.

“After heating to a holding temperature of 800°C or more at an average heating rate of 100°C/s or more and less than 200°C/s, hold it and hot stamp it so that the elapsed time from the start of heating to molding is 240 seconds or less. and cool it down to a temperature range of 400 degrees Celsius or less.”
In the method for producing a hot-stamped molded article according to the present embodiment, the above-described steel plate for hot stamping is heated to a temperature of 800° C. or higher at an average heating rate of 100° C./s or more and less than 200° C./s, and then held; After hot stamping is performed such that the elapsed time from the start of heating to molding is 240 seconds or less, it is preferable to 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 process to austenite can be sufficiently promoted, and the grain boundaries of martensite can be controlled in 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 falls within a predetermined range. The hot-stamped molded product after hot-stamping is preferably cooled in a mold to a temperature range of 400° C. or lower. When cooling is stopped in a temperature range of 400° C. or lower, the grain boundaries of martensite can be controlled to a preferable form. By setting the average heating rate to 800°C or higher to 100°C/s or more and less than 200°C/s, and the elapsed time from the start of heating to forming to 240 seconds or less, the average crystal grain size of prior austenite grains is 10 μm or less. As a result, among the grain boundaries in which the average crystal orientation difference in crystal grains with a body-centered structure is 5 degrees or more, the rotation angle is 57 degrees to 63 degrees with the <011> direction as the rotation axis. The length of the grain boundary, the length of the grain boundary where the rotation angle is 49° to 56°, the length of the grain boundary where the rotation angle is 4° to 12°, and the grain boundary where the rotation angle is 64° to 72° The ratio of the length of the grain boundary where the rotation angle is 4° to 12° can be 15% or more with respect to the total length of the grain boundary. Therefore, it is preferable that the average heating rate to 800° C. or higher is 100° C./s or more and less than 200° C./s, and the elapsed time from the start of heating to molding is 240 seconds or less.

"Tempering at 150℃ or higher and 650℃ or lower"
For the purpose of adjusting the strength and improving the ductile-brittle transition temperature and low-temperature toughness, the hot-stamped molded product cooled to room temperature may be subjected to a tempering treatment at a temperature in the range of 150° C. to 650° C. In this case, for example, only a part of the hot stamp molded body may be tempered. Thereby, 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 depending on the region of the hot-stamped molded product. For example, when applying hot-stamped bodies to high-strength parts of automobiles, by tempering and softening only a part of the high-strength parts, it is possible to control the destruction and deformation mode of the high-strength parts in the event of a collision. can.

Next, examples of the present invention will be described. Note that the conditions in the Examples are examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as the purpose of the present invention is achieved without departing from the gist of the present invention.

A steel billet produced by casting molten steel having the chemical composition shown in Tables 1 to 3 is hot rolled under the conditions shown in Tables 4 to 6, and if necessary, cold rolled and/or plated to obtain the Steel plates for hot stamping shown in Nos. 4 to 6 were obtained. In addition, the steel plate for hot stamping is heat-treated and hot-stamped under the conditions shown in Tables 7 to 9, and if necessary, heated to the temperature shown in Table 9 to be tempered, or a part of the hot-stamped molded product is A softened region was formed by laser irradiation and tempering to obtain hot stamped molded bodies shown in Tables 7 to 9. In addition, "Al" in Tables 6 and 9 indicates hot-dip aluminum plating, "GA" indicates alloyed hot-dip galvanizing, and "electrozinc" indicates electrolytic galvanizing. The cooling rate during winding during hot rolling was calculated by measuring the temperature at the longitudinal center of the hot rolled coil during winding using an infrared radiation thermometer for high temperature measurement.

For hot stamping steel sheets, granular bainite (granular bainite with an average crystal orientation difference of 0.4° or more and 3.0° or less inside grains surrounded by grain boundaries with an average crystal orientation difference of 5° or more) is used. Total area ratio of crystal grains), length of grain boundaries with an average crystal orientation difference of 0.4° or more and 3.0° or less, and length of grain boundaries with an average crystal orientation difference of more than 3.0° The ratio of the length of grain boundaries with an average crystal orientation difference of 0.4° or more and 3.0° or less to the length of was obtained by the method described above.

For hot-stamped compacts, among the grain boundaries where the average crystal grain size of prior austenite grains and the average crystal orientation difference in crystal grains with a body-centered structure are 5° or more, the rotation angle is 57 with the <011> direction as the rotation axis. The length of the grain boundary where the rotation angle is between 49° and 56°, the length of the grain boundary where the rotation angle is between 4° and 12°, and the length of the grain boundary where the rotation angle is between 64° and 63°. The ratio of the grain boundary length at which the rotation angle is 4° to 12° to the total length including the grain boundary length at 72° was obtained by the method described above.

"Vickers hardness"
The Vickers hardness of the hot-stamped 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 surface of the hot stamp molded body. The sample was sized to allow observation of 10 mm in the rolling direction, although it depends on the measuring device. After polishing the cross section of the sample using #600 to #1500 silicon carbide paper, it was finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 μm was dispersed in a diluted solution such as alcohol and pure water. . Using a micro Vickers hardness tester at the 1/4 position of the plate thickness, the mirror-finished cross section was tested for hardness in the direction parallel to the plate surface (rolling direction) under a load of 1 kgf at intervals of three times the indentations or more. We measured the A total of 20 measurements were taken, and the average value was taken as the Vickers hardness of the hot stamped molded product. A case where the Vickers hardness was 450 Hv or more was judged to be excellent in hardness and was determined to pass, and a case where the Vickers hardness was less than 450 Hv was judged to be poor in hardness and judged to be a failure.

"Bendability"
The bendability of the hot stamp molded product was evaluated by the following method based on the VDA standard (VDA238-100) defined by the German Automobile Industry Association. In this example, the displacement at the maximum load obtained in the bending test was converted into an angle based on the VDA standard, and the maximum bending angle (°) was determined.
Test piece size: 60 mm (rolling direction) x 60 mm (parallel to the plate width direction), or 30 mm (rolling direction) x 60 mm (parallel to the plate width direction)
Test piece thickness: 1.0mm (ground the same amount on both sides)
Bending ridgeline: parallel to the sheet width direction Test method: roll support, punch pushing Roll diameter: φ30mm
Punch shape: Tip R = 0.4mm
Distance between rolls: 2.0 x plate thickness (mm) + 0.5 mm
Pushing speed: 20mm/min
Test machine: SHIMADZU AUTOGRAPH 20kN

When the product of the Vickers hardness obtained by the above method and the maximum bending angle is 25,000 Hv・° or more, it is judged as having excellent bendability and is judged as passing, and when it is less than 25,000 Hv・°, it is judged as being poor on bendability and is judged to be acceptable. It was judged as passing.

Tables 7 to 9 show the metallographic structures and mechanical properties of the hot stamped bodies. Note that Tables 7 to 9 show that hot-stamped molded bodies having chemical compositions and metal structures within the range of the present invention have high hardness and excellent bendability.
On the other hand, it can be seen that hot-stamped molded bodies in which one or more of the chemical composition and metal structure are outside the scope of the present invention are inferior in one or more of Vickers hardness and bendability.

According to the above aspect of the present invention, it is possible to provide a hot-stamped molded article that has high hardness and excellent bendability.

Claims (4)

The chemical composition is in 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, 0.283 % or less,
Ni: 3.0% or more, 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, 1.0000% or less,
Cu: 0% or more, 1.0000% or less,
Sn: 0% or more, 1.000% or less,
W: 0% or more, 1.000% or less,
Contains Ca: 0% or more and 0.010% or less, and REM: 0% or more and 0.300% or less,
The remainder consists of Fe and impurities,
The average crystal grain size of the prior austenite grains is 10 μm or less,
Among grain boundaries with an average crystal orientation difference of 5° or more in crystal grains with a body-centered structure, the length of the grain boundary where the rotation angle is 57° to 63° with the <011> direction as the rotation axis, and the rotation angle The total length of the grain boundary with a rotation angle of 49° to 56°, the length of the grain boundary with a rotation angle of 4° to 12°, and the length of the grain boundary with a rotation angle of 64° to 72°. On the other hand, the hot-stamped molded article is characterized in that the proportion of the length of grain boundaries where the rotation angle is 4° to 12° is 15% or more. The chemical composition is 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, 1.0000% or less,
Cu: 0.0010% or more, 1.0000% or less,
Sn: 0.001% or more, 1.000% or less,
W: 0.001% or more, 1.000% or less,
A claim 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. 1. The hot-stamped molded article according to 1. The hot-stamped molded article according to claim 1 or 2, comprising a plating layer on the surface. The hot-stamped molded article according to any one of claims 1 to 3, characterized in that it has a softened region in part.