EP4299769A1

EP4299769A1 - Steel sheet for hot stamping and hot stamped molded body

Info

Publication number: EP4299769A1
Application number: EP22807458.9A
Authority: EP
Inventors: Yuri TODA; Daisuke Maeda; Tamaki Suzuki; Ko SATAKE; Yuma Asada
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2021-05-13
Filing date: 2022-05-10
Publication date: 2024-01-03
Also published as: MX2023010466A; CN116997671A; WO2022239758A1; JP7549277B2; CN116997671B; KR102900041B1; EP4299769A4; KR20230146611A; JPWO2022239758A1; US20240183017A1

Abstract

This hot-stamping formed body has a predetermined chemical composition and has a microstructure in which an average grain size of prior austenite grains is 5 to 25 µm, and a standard deviation of grain sizes of the prior austenite grains is 0.1 to 2.0 µm. In addition, this steel sheet for hot stamping has a predetermined chemical composition and has a microstructure in which an average value of the pole densities of ferrite in the orientation group consisting of {100}<011> to {223}<110> is 10.0 or less, in entire ferrite, a number proportion of ferrite containing a carbide having an equivalent circle diameter of 0.2 µm or more in grains is 20% or more, and the steel sheet for hot stamping has a microstructure in which an area ratio of pearlite is 10% to 90% and an ratio of ferrite is 10% to 90%.

Description

[Technical Field of the Invention]

The present invention relates to a steel sheet for hot stamping and a hot-stamping formed body.
Priority is claimed on Japanese Patent Application No. 2021-081621, filed May 13, 2021 , the content of which is incorporated herein by reference.

[Background Art]

In the related art, from the viewpoint of global environmental problems and collision safety performance, thinning and high-strengthening of vehicle members have been required. In order to meet these demands, the number of vehicle members made of a high strength steel sheet as a material is increasing. In addition, as a forming method of a high strength steel sheet, a method called hot stamping is known. In the hot stamping, a high strength steel sheet is press-formed in a high temperature range of 700°C or higher and quenched inside or outside a press die. According to the hot stamping, since forming is performed in a high temperature range in which the strength of the steel sheet decreases, it is possible to suppress forming defects that occur in cold pressing. In addition, since a structure having martensite as a primary phase is obtained by quenching after forming, high strength can be obtained. For this reason, hot-stamping formed bodies having a tensile strength of about 1,500 MPa are widely used worldwide.
In order to obtain a higher effect of reducing the weight of a vehicle body from a vehicle member into which a high strength steel sheet is formed by hot stamping, it is necessary to obtain a member that has high strength and is also excellent in collision characteristics. In order to improve the collision characteristics of vehicle members, particularly, vehicle members are required to have excellent bendability.
Patent Document 1 discloses a steel sheet which is suitable for obtaining components such as gears by improving hardenability and material formability and in particular, performing forming by cold forging such as wall thickness increase or the like and a manufacturing method thereof.
The present inventors found that, in a vehicle member having an improved tensile strength, it is necessary to further improve the bendability in order to obtain a higher effect of reducing the weight of a vehicle body.

[Prior Art Document]

[Patent Document]

[Patent Document 1] PCT International Publication No. WO2016/190396

[Non-Patent Document]

Non-Patent Document 1: Acta Materialia, 58 (2010), 6393-6403

[Disclosure of the Invention]

[Problems to be Solved by the Invention]

The present invention has been made in view of the above-mentioned problem. An object of the present invention is to provide a hot-stamping formed body having high strength and excellent bendability, and a steel sheet for hot stamping capable of manufacturing this hot-stamping formed body.

[Means for Solving the Problem]

The gist of the present invention is as follows.

[1] A steel sheet for hot stamping according to an aspect of the present invention includes, as a chemical composition, by mass%:
- C: more than 0.40% and 0.70% or less;
- Si: 0.010% to 1.30%;
- Mn: 0.10% to 0.60%;
- P: 0.100% or less;
- S: 0.0100% or less;
- N: 0.0140% or less;
- O: 0.0200% or less;
- Al: 0.0010% to 0.500%;
- Cr: 0.010% to 0.80%;
- Nb: 0% to 0.100%;
- Ti: 0% to 0.100%;
- B: 0% to 0.0100%;
- Mo: 0% to 1.00%;
- Co: 0% to 2.00%;
- Ni: 0% or more and less than 3.00%;
- Cu: 0% to 1.00%;
- V: 0% to 1.00%;
- W: 0% to 1.000%;
- Ca: 0% to 0.010%;
- Mg: 0% to 1.000%;
- REM: 0% to 1.000%;
- Sb: 0% to 1.000%;
- Zr: 0% to 1.000%;
- Sn: 0% to 1.000%;
- As: 0% to 0.100%; and
- a remainder including Fe and impurities,
- wherein the steel sheet for hot stamping has a microstructure in which an average value of pole densities of ferrite in an orientation group consisting of {100}<011> to {223}<110> is 10.0 or less,
- in entire ferrite, a number proportion of ferrite containing a carbide having an equivalent circle diameter of 0.2 µm or more in grains is 20% or more, and
- an area ratio of pearlite is 10% to 90% and an area ratio of ferrite is 10% to 90%.
[2] The steel sheet for hot stamping according to [1], in which the steel sheet for hot stamping may contain, as the chemical composition, by mass%, one or more selected from the group consisting of:
- Nb: 0.001% to 0.100%;
- Ti: 0.010% to 0.100%;
- B: 0.0015% to 0.0100%;
- Mo: 0.05% to 1.00%;
- Co: 0.05% to 2.00%;
- Ni: 0.01% or more and less than 3.00%;
- Cu: 0.01% to 1.00%;
- V: 0.01% to 1.00%;
- W: 0.001% to 1.000%;
- Ca: 0.001% to 0.010%;
- Mg: 0.001% to 1.000%;
- REM: 0.001% to 1.000%;
- Sb: 0.005% to 1.000%;
- Zr: 0.001% to 1.000%;
- Sn: 0.001% to 1.000%; and
- As: 0.001% to 0.100%.
[3] A hot-stamping formed body according to another aspect of the present invention includes, as a chemical composition, by mass%:
- C: more than 0.40% and 0.70% or less;
- Si: 0.010% to 1.30%;
- Mn: 0.10% to 0.60%;
- P: 0.100% or less;
- S: 0.0100% or less;
- N: 0.0140% or less;
- O: 0.0200% or less;
- Al: 0.0010% to 0.500%;
- Cr: 0.010% to 0.80%;
- Nb: 0% to 0.100%;
- Ti: 0% to 0.100%;
- B: 0% to 0.0100%;
- Mo: 0% to 1.00%;
- Co: 0% to 2.00%;
- Ni: 0% or more and less than 3.00%;
- Cu: 0% to 1.00%;
- V: 0% to 1.00%;
- W: 0% to 1.000%;
- Ca: 0% to 0.010%;
- Mg: 0% to 1.000%;
- REM: 0% to 1.000%;
- Sb: 0% to 1.000%;
- Zr: 0% to 1.000%;
- Sn: 0% to 1.000%;
- As: 0% to 0.100%; and
- a remainder including Fe and impurities,
- in which the hot-stamping formed body has a microstructure in which an average grain size of prior austenite grains is 5 to 25 µm,
- a standard deviation of grain sizes of the prior austenite grains is 0.1 to 2.0 µm, and
- a tensile strength of the hot-stamping formed body is 2,200 MPa or more.
[4] The hot-stamping formed body according to [3], in which the hot-stamping formed body may contain, as the chemical composition, by mass%, one or more selected from the group consisting of:
- Nb: 0.001% to 0.100%;
- Ti: 0.010% to 0.100%;
- B: 0.0015% to 0.0100%;
- Mo: 0.05% to 1.00%;
- Co: 0.05% to 2.00%;
- Ni: 0.01% or more and less than 3.00%;
- Cu: 0.01% to 1.00%;
- V: 0.01% to 1.00%;
- W: 0.001% to 1.000%;
- Ca: 0.001% to 0.010%;
- Mg: 0.001% to 1.000%;
- REM: 0.001% to 1.000%;
- Sb: 0.005% to 1.000%;
- Zr: 0.001% to 1.000%;
- Sn: 0.001% to 1.000%; and
- As: 0.001% to 0.100%.
[5] In the hot-stamping formed body according to [3] or [4], an area ratio of the prior austenite grains having an average grain size of 0.5 to 3.0 µm may be 60% or less.

[Effects of the Invention]

According to the above-described aspects of the present invention, it is possible to provide a hot-stamping formed body having high strength and excellent bendability, and a steel sheet for hot stamping capable of manufacturing this hot-stamping formed body.

[Embodiments of the Invention]

The present inventors examined bendability of a hot-stamping formed body. As a result, the present inventors found that in a microstructure of the hot-stamping formed body, the bendability deteriorates when a large amount of fine prior austenite grains are present. In addition, the present inventors found that, in the microstructure of the hot-stamping formed body, when prior austenite grains are set to a desired size and unevenness in the size of the prior austenite grains is suppressed, that is, the prior austenite grains are grain-sized, the bendability of the hot-stamping formed body can be further improved.
Next, the present inventors examined a method for obtaining the above-described hot-stamping formed body. As a result, the present inventors found that when a Mn content in a chemical composition of a steel sheet for hot stamping is set to 0.60% or less, and in a microstructure, when pole densities of ferrite in an orientation group consisting of {100}<011> to {223}<110> are reduced and the number proportion of the ferrite containing a carbide in grains increase, the above-described hot-stamping formed body can be obtained.
Hereinafter, the steel sheet for hot stamping and the hot-stamping formed body according to the present embodiment made based on the above-described findings will be described. First, the reason why the chemical composition of the steel sheet for hot stamping according to the present embodiment is to be limited will be described.
A limited numerical range described using "to" to be described below includes a lower limit and an upper limit. Numerical values represented using "less than" or "more than" are not included in a numerical range. All percentages (%) related to the chemical composition mean mass%.
The steel sheet for hot stamping according to the present embodiment includes, as a chemical composition, by mass%, C: more than 0.40% and 0.70% or less, Si: 0.010% to 1.30%, Mn: 0.10% to 0.60%, P: 0.100% or less, S: 0.0100% or less, N: 0.0140% or less, O: 0.0200% or less, Al: 0.0010% to 0.500%, Cr: 0.010% to 0.80%, and a remainder including Fe and impurities. Each element will be described below.

C: more than 0.40% and 0.70% or less

C greatly contributes to improvement in the strength of the hot-stamping formed body. When the C content is 0.40% or less, it becomes difficult to obtain sufficient strength in the hot-stamping formed body. For this reason, the C content is set to more than 0.40%. The C content is preferably 0.42% or more, more preferably 0.45% or more, and still more preferably 0.47% or more.
Meanwhile, when the C content is more than 0.70%, coarse carbides are generated and the bendability of the hot-stamping formed body deteriorates. Therefore, the C content is set to 0.70% or less. The C content is preferably 0.65% or less and more preferably 0.60% or less.

Si: 0.010% to 1.30%

Si is an element that improves distortion capability of the hot-stamping formed body by suppressing the formation of an oxide which is combined with oxygen and becomes an origin of fracture. When the Si content is less than 0.010%, a coarse oxide is formed in the hot-stamping formed body, and desired bendability cannot be obtained. Therefore, the Si content is set to 0.010% or more. The Si content is preferably 0.05% or more and more preferably 0.10% or more.
Meanwhile, when the Si content is more than 1.30%, a coarse oxide is formed, and the bendability of the hot-stamping formed body deteriorates. For this reason, the Si content is set to 1.30% or less. The Si content is preferably less than 1.00% and more preferably 0.50% or less.

Mn: 0.10% to 0.60%

Mn stabilizes austenite and improves hardenability of a steel sheet. When the Mn content is less than 0.10%, sufficient hardenability cannot be obtained. For this reason, the Mn content is set to 0.10% or more. The Mn content is preferably 0.20% or more and more preferably 0.30% or more.
Meanwhile, when the Mn content is more than 0.60%, cracking attributed to Mn segregation is likely to occur unless the manufacturing method is appropriately controlled, and excellent bendability cannot be obtained in the hot-stamping formed body. For this reason, the Mn content is set to 0.60% or less. The Mn content is preferably 0.55% or less and more preferably 0.50% or less.

P: 0.100% or less

P segregates in the grain boundaries of the steel sheet and deteriorates the bendability of the hot-stamping formed body. Therefore, the lower P content is more preferable. In particular, when the P content is more than 0.100%, the workability of the steel sheet and the bendability of the hot-stamping formed body significantly deteriorate. For this reason, the P content is set to 0.100% or less. The P content is preferably 0.080% or less and more preferably 0.020% or less.
The lower limit of the P content is not particularly limited and may be 0%. However, when the P content is reduced to less than 0.0001%, the dephosphorization cost increases significantly, which is not preferable economically. For this reason, the P content may be set to 0.0001% or more.

S: 0.0100% or less

S forms coarse inclusions and deteriorates the bendability of the hot-stamping formed body. Accordingly, the lower S content is more preferable. In particular, when the S content is more than 0.0100%, the formability of the steel sheet and the bendability of the hot-stamping formed body significantly deteriorate. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0050% or less and more preferably 0.0010% or less.
The lower limit of the S content is not particularly limited and may be 0%. However, when the S content is reduced to less than 0.0001%, the desulfurization cost increases significantly, which is not preferable economically. For this reason, the S content may be set to 0.0001% or more.

N: 0.0140% or less

N forms a coarse nitride and deteriorates the bendability of the hot-stamping formed body. Therefore, the lower N content is more preferable. In particular, when the N content is more than 0.0140%, the formability of the steel sheet significantly deteriorates. Therefore, the N content is set to 0.0140% or less. The C content is preferably 0.0100% or less or 0.0070% or less and more preferably 0.0040% or less.
The lower limit of the N content is not particularly limited and may be 0%. However, when the N content is reduced to less than 0.0001%, the denitrification cost increases significantly, which is not preferable economically. For this reason, the N content may be set to 0.0001% or more.

O: 0.0200% or less

O forms a coarse oxide in steel and deteriorates the bendability of the hot-stamping formed body. Therefore, the lower O content is more preferable. In particular, when the O content is more than 0.0200%, the bendability of the hot-stamping formed body significantly deteriorates. Therefore, the O content is set to 0.0200% or less. The O content is preferably 0.0150% or less, more preferably 0.0100% or less, and still more preferably 0.0060% or less.
The lower limit of the O content is not particularly limited and may be 0%. However, when the O content is reduced to less than 0.0001%, the manufacturing cost increases significantly, which is not preferable economically. Therefore, the O content may be set to 0.0001% or more.

Al: 0.0010% to 0.500%"

Al is an element that improves the distortion capability by deoxidizing molten steel to suppress the formation of oxide which becomes the origin of fracture and improves the bendability of the hot-stamping formed body. In a case where the Al content is less than 0.0010%, deoxidation is not sufficiently performed and a coarse oxide is generated. As a result, the above-mentioned effects cannot be obtained. For this reason, the Al content is set to 0.0010% or more. The Al content is preferably 0.010% or more and more preferably 0.030% or more.
Meanwhile, when the Al content is more than 0.500%, a coarse oxide is formed in steel, and the bendability of the hot-stamping formed body deteriorates. Therefore, the Al content is set to 0.500% or less. The Al content is preferably 0.450% or less and more preferably 0.350% or less.

Cr: 0.010% to 0.80%

Cr increases the strength of the hot-stamping formed body by dissolving in prior austenite grains during heating at the time of hot stamping. When the Cr content is less than 0.010%, this effect cannot be obtained. Therefore, the Cr content is set to 0.010% or more. The Cr content is preferably 0.10% or more and more preferably 0.20% or more.
Meanwhile, when the Cr content is more than 0.80%, a coarse carbide is formed and the bendability of the hot-stamping formed body deteriorates. Therefore, the Cr content is set to 0.80% or less. The Cr content is preferably 0.60% or less and more preferably 0.40% or less.
The remainder of the chemical composition of the steel sheet for hot stamping according to the present embodiment may be Fe and impurities. An example of the impurities includes an element that is unavoidably incorporated from a steel raw material or scrap and/or during a steelmaking process and is allowed in a range in which properties of the hot-stamping formed body according to the present embodiment are not inhibited.
The steel sheet for hot stamping according to the present embodiment may contain the following elements as arbitrary elements instead of a part of Fe. The contents of the following arbitrary elements, which are obtained in a case where the following arbitrary elements are not contained, are 0%.

Nb: 0% to 0.100%

Nb forms carbonitride in steel to improve the strength of the hot-stamping formed body by precipitation hardening. In order to obtain this effect, the Nb content is preferably set to 0.001% or more.
Meanwhile, when the Nb content is more than 0.100%, a large amount of carbonitride is formed in steel, and the bendability of the hot-stamping formed body deteriorates. Therefore, the Nb content is set to 0.100% or less.

Ti: 0% to 0.100%

Similar to Nb, Ti forms carbonitride in steel to improve the strength of the hot-stamping formed body by precipitation hardening. In order to obtain the effects, a Ti content is preferably set to 0.010% or more.
Meanwhile, when the Ti content is more than 0.100%, a large amount of carbonitride is formed in steel, and the bendability of the hot-stamping formed body deteriorates. For this reason, the Ti content is set to 0.100% or less.

B: 0% to 0.0100%

B improves the hardenability of the steel and improves the strength of the hot-stamping formed body. In order to obtain the effects, the B content is preferably set to 0.0015% or more.
Meanwhile, when the B content is more than 0.0100%, a coarse carbide is generated and the bendability of the hot-stamping formed body deteriorates. Therefore, the B content is set to 0.0100% or less.

Mo: 0% to 1.00%

Mo improves the hardenability of the steel sheet and improves the strength of the hot-stamping formed body. In order to obtain the effects, the Mo content is preferably set to 0.05% or more.
Meanwhile, when the Mo content is more than 1.00%, a coarse carbide is generated and the bendability of the hot-stamping formed body deteriorates. Therefore, the Mo content is set to be 1.00% or less.

Co: 0% to 2.00%

Co improves the hardenability of the steel sheet and improves the strength of the hot-stamping formed body. In order to reliably exert the effects, it is preferable that the Co content is set to 0.05% or more.
Meanwhile, when the Co content is more than 2.00%, a coarse carbide is generated and the bendability of the hot-stamping formed body deteriorates. For this reason, the Co content is set to 2.00% or less.

Ni: 0% or more and less than 3.00%

Ni improves the hardenability of the steel sheet and improves the strength of the hot-stamping formed body. In order to obtain the effects, the Ni content is preferably set to 0.01% or more.
Meanwhile, when the Ni content is 3.00% or more, segregation is promoted and the bendability of the hot-stamping formed body deteriorates. Therefore, the Ni content is set to less than 3.00%.

Cu: 0% to 1.00%

Similar to Ni, Cu improves the hardenability of the steel sheet and improves the strength of the hot-stamping formed body. In order to obtain the effects, the Cu content is preferably set to 0.01% or more.
Meanwhile, when the Cu content is more than 1.00%, segregation is promoted and the bendability of the hot-stamping formed body deteriorates. Therefore, the Cu content is set to 1.00% or less.

V: 0% to 1.00%

V improves the hardenability of the steel sheet and improves the strength of the hot-stamping formed body. In order to obtain the effects, the V content is preferably set to 0.01% or more.
Meanwhile, when the V content is more than 1.00%, a large amount of carbonitride precipitates, and the bendability of the hot-stamping formed body deteriorates. Therefore, the V content is set to 1.00% or less.

W: 0% to 1.000%

W improves the hardenability of the steel sheet and improves the strength of the hot-stamping formed body. In order to obtain the effects, the W content is preferably set to 0.001 % or more.
Meanwhile, when the W content is more than 1.000%, segregation is promoted and the bendability of the hot-stamping formed body deteriorates. Therefore, the W content is set to 1.000% or less.

Ca: 0% to 0.010%

Ca improves the distortion capability by suppressing the formation of an oxide which becomes the origin of fracture and improve the bendability of the hot-stamping formed body. In order to obtain the effects, the Ca content is preferably set to 0.001% or more.
Meanwhile, when the Ca content is more than 0.010%, a coarse oxide is formed, and the bendability of the hot-stamping formed body deteriorates. Therefore, the Ca content is set to 0.010% or less.

Mg: 0% to 1.000%

Mg improves the distortion capability by suppressing the formation of an oxide which becomes the origin of fracture and improves the bendability of the hot-stamping formed body. In order to obtain the effects, the Mg content is preferably set to 0.001% or more.
Meanwhile, when the Mg content is more than 1.000%, a coarse oxide is generated, and the bendability of the hot-stamping formed body deteriorates. Therefore, the Mg content is set to 1.000% or less.

REM: 0% to 1.000%

REM improves the distortion capability by suppressing the formation of an oxide which becomes the origin of fracture and improves the bendability of the hot-stamping formed body. In order to obtain the effects, the REM content is preferably set to 0.001% or more.
Meanwhile, when the REM content is more than 1.000%, a coarse oxide is generated, and the bendability of the hot-stamping formed body deteriorates. Therefore, the REM content is set to 1.000% or less.
In the present embodiment, REM refers to a total of 17 elements that are composed of Sc, Y, and lanthanoid and the REM content refers to the total content of these elements.

Sb: 0% to 1.000%

Sb improves the distortion capability by suppressing the formation of an oxide which becomes the origin of fracture and improves the bendability of the hot-stamping formed body. In order to obtain the effects, the Sb content is preferably set to 0.005% or more.
Meanwhile, when the Sb content is more than 1.000%, a coarse oxide is generated, and the bendability of the hot-stamping formed body deteriorates. Therefore, the Sb content is set to 1.000% or less.

Zr: 0% to 1.000%

Zr improves the distortion capability by suppressing the formation of an oxide which becomes the origin of fracture and improves the bendability of the hot-stamping formed body. In order to obtain the effects, the Zr content is preferably set to 0.001% or more.
Meanwhile, when the Zr content is more than 1.000%, a coarse oxide is generated, and the bendability of the hot-stamping formed body deteriorates. Therefore, the Zr content is set to 1.000% or less.

Sn: 0% to 1.000%

Sn improves the distortion capability by suppressing the formation of an oxide which becomes the origin of fracture and improves the bendability of the hot-stamping formed body. In the case of reliably obtaining the effects, the Sn content is preferably set to 0.001% or more.
Meanwhile, since the above effects are saturated even when a large amount of Sn is contained, the Sn content is set to 1.000% or less.

As: 0% to 0.100%; and

As refines the prior austenite grains by lowering an austenite single-phase formation temperature and improve the bendability of the hot-stamping formed body. In the case of reliably obtaining the effects, the As content is preferably set to 0.001% or more.
Meanwhile, since the above effects are saturated even when a large amount of As is contained, the As content is set to 0.100% or less.
The above-mentioned chemical composition of the steel sheet for hot stamping may be measured by an ordinary analysis method. For example, the chemical composition of the steel sheet for hot stamping may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusionnondispersive infrared absorption method. In a case where a plating layer is provided on the surface of the steel sheet for hot stamping, the chemical composition may be analyzed after the plating layer is removed by mechanical grinding.
Next, the microstructure of the steel sheet for hot stamping according to the present embodiment will be described.
In the steel sheet for hot stamping according to the present embodiment has a microstructure in which an average value of pole densities of ferrite in an orientation group consisting of {100}<011> to {223}<110> is 10.0 or less, in entire ferrite, a number proportion of ferrite containing a carbide having an equivalent circle diameter of 0.2 µm or more in grains is 20% or more, and an area ratio of pearlite is 10% to 90% and an area ratio of ferrite is 10% to 90%. Hereinafter, each specification will be described.
In addition, in the present embodiment, it should be noted that, in a sheet thickness cross section parallel to a rolling direction, the microstructure is specified at a 1/4 depth position of the sheet thickness from the surface (in a region from a 1/8 depth of the sheet thickness from the surface to a 3/8 depth of the sheet thickness from the surface). The reason therefor is that the microstructure at this position indicates a typical microstructure of the steel sheet.

"Average value of pole densities of ferrite in orientation group consisting of {100}<011> to {223}<110> is 10.0 or less"

When the average value of the pole densities of ferrite in the orientation group consisting of {100}<011 > to {223 }<110> is more than 10.0, the average grain size of the prior austenite in the hot-stamping formed body cannot be controlled to a predetermined value, and a hot-stamping formed body having excellent bendability cannot be obtained. The average value of the pole densities of ferrite in the orientation group consisting of {100}<011> to {223}<110> is preferably 9.0 or less, more preferably 7.0 or less, still more preferably 6.0 or less, and even more preferably 5.0 or less. A lower limit of the pole density of ferrite in the orientation group consisting of {100}<011> to {223}<110> is not particularly limited and may be 0.1 or more.
In addition, in the orientation group consisting of {100}<011> to {223}<110>, crystal orientations of {100}<011>, {116}<110>, {114}<110>, {112}<110>, and {223}<110> are included.

Measurement method of pole density

The pole densities of ferrite in the orientation group consisting of t 100}<011> to {223} <110> can be obtained from an orientation distribution function (ODF) that displays a three-dimensional texture calculated by computing, using spherical harmonics, an orientation data measured by an electron backscattering diffraction (EBSD) method using a device in which a scanning electron microscope and an EBSD analyzer are combined and OlM Analysis (registered trademark) manufactured by TSL Solutions. A measurement region is set to the region from the 1/8 position of the sheet thickness from the surface to the 3/8 position of the sheet thickness from the surface so that the 1/4 depth position of the sheet thickness from the surface can be observed. Measurement pitches are set to 5 µm/step.
It should be noted that {hkl} indicates a crystal plane parallel to a rolled surface and <uvw> indicates a crystal direction parallel to a rolling direction. That is, {hkl } <uvw> indicates a crystal in which {hkl} is oriented in a sheet surface normal direction and <uvw> is oriented in the rolling direction.

"In entire ferrite, number proportion of ferrite containing carbide having equivalent circle diameter of 0.2 µm or more in grains is 20% or more"

When the number proportion of ferrite containing a carbide having the equivalent circle diameter of 0.2 µm or more in grains in entire ferrite is less than 20%, the prior austenite grains can be grain-sized in the hot-stamping formed body. As a result, it is not possible to obtain a hot-stamping formed body having excellent bendability. By setting the number proportion of the ferrite containing the carbide having the equivalent circle diameter of 0.2 µm or more in the grains in entire ferrite to 20% or more, the carbide in the grain function preferably as the origin of the prior austenite grains during the heating before the hot stamping. As a result, it is presumed that the prior austenite grains are uniformly dispersed and grain-sized in the microstructure of the hot-stamping formed body. In entire ferrite, the number proportion of the ferrite containing a carbide having the equivalent circle diameter of 0.2 µm or more in the grains is preferably 40% or more, more preferably 50% or more, and still more preferably 60% or more. An upper limit of the number proportion of ferrite containing a carbide having the equivalent circle diameter of 0.2 µm or more in the grains in entire ferrite is not particularly specified, but may be set to 90% or less.

Measurement method of number proportion of ferrite including carbide

A sample is collected from an arbitrary position (a position that avoids an end portion in a case where the sample cannot be collected at this position) away from an end surface of the steel sheet for hot stamping by a distance of 50 mm or more so that a sheet thickness cross section parallel to a rolling direction can be an observed section. Next, the observed section is finished by electropolishing. After that, the region from the 1/8 depth of the sheet thickness from the surface to the 3/8 depth of the sheet thickness from the surface is observed at 10 or more visual fields at a magnification of 20,000 times so that the 1/4 depth position of the sheet thickness from the surface can be observed. For the grains identified as ferrite by the measurement method of the microstructure described later, the equivalent circle diameter of each carbide is obtained from the area of each carbide observed in the grain of ferrite by image analysis. The number of ferrite grains including a carbide having the equivalent circle diameter of 0.2 µm or more in all grains of the observed ferrite is calculated. The obtained value is divided by the number of all grains of ferrite and multiplied by 100, thereby obtaining the number proportion of ferrite containing a carbide having an equivalent circle diameter of 0.2 µm or more in grains.
In the present embodiment, particles having the equivalent circle diameter of 0.2 to 30 µm are regarded as the carbide.

"10 to 90 area% of pearlite"
"10 to 90 area% of ferrite"

When the area ratio of ferrite is less than 10% and the area ratio of pearlite is more than 90%, the pearlite preferentially becomes the origin of the prior austenite in the hot stamping step, and it becomes impossible to obtain the grain size adjustment effect of the prior austenite grains. Therefore, the area ratio of ferrite is set to 10% or more, and the area ratio of pearlite is set to 90% or less. The area ratio of ferrite is preferably 20% or more and more preferably 40% or more. The area ratio of pearlite is preferably 80% or less and more preferably 60% or less.
Meanwhile, when the area ratio of ferrite is more than 90% and the area ratio of pearlite is less than 10%, carbon is excessively concentrated in pearlite, and the temperature at which carbon is transformed into austenite becomes low. As a result, in the hot stamping step, transformation starts at a low temperature, and the prior austenite grains are likely to coarsen, and it becomes impossible to obtain the grain size adjustment effect of the prior austenite grains. Therefore, the area ratio of ferrite is set to 90% or less and the area ratio of pearlite is set to 10% or more. The area ratio of ferrite is preferably 70% or less and more preferably 60% or less. The area ratio of pearlite is preferably 30% or more and more preferably 40% or more.
In the microstructure of the steel sheet for hot stamping according to the present embodiment, the remainder in microstructure is one or more of martensite, lower bainite, residual austenite, and tempered martensite. An area ratio of the remainder in microstructure may be set to 20% or less.

Measurement method of microstructure of steel sheet for hot stamping

A sample is cut out from an arbitrary position (a position that avoids an end portion in a case where the sample cannot be collected at this position) away from an end surface of the steel sheet for hot stamping by a distance of 50 mm or more so that a sheet thickness cross section parallel to a rolling direction can be observed. The size of the sample also depends on a measurement device, but is set to a size that can be observed by about 10 mm in the rolling direction.
The cross section of the sample is polished using silicon carbide paper having a grit of #600 to #1500, then, is finished as a mirror surface using liquid in which diamond powder having a grain size of 1 to 6 µm is dispersed in diluted solution of alcohol or the like or pure water and finish-polished by electrolytic polishing. Next, in a region that has a length of 50 µm and between the 1/8 depth of the sheet thickness from the surface and the 3/8 depth of the sheet thickness from the surface at an arbitrary position on the cross section of the sample in a longitudinal direction so that the 1/4 depth position of the sheet thickness from the surface can be observed, the structure is observed using a device including a thermal field emission type scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSD detector (DVC5-type detector manufactured by TSL Solutions). The scanning electron microscope used is equipped with a secondary electron detector. In a vacuum of 9.6 × 10^-5 Pa or less, the sample is irradiated with an electron beam at an acceleration voltage of 15 kV and an irradiation current level of 13, and a secondary electron image is photographed with the scanning electron microscope.
In the obtained photographed photograph, a region where cementite is precipitated in a lamellar shape in the grains is determined as pearlite. The area ratio of the pearlite is obtained by calculating the area ratio of the region determined to be pearlite. Lath-shaped grains are determined as lower bainite, martensite, and tempered martensite. Next, EBSD analysis is performed on the same visual field at an analysis speed of 200 to 300 points/sec using an EBSD analyzer. The area ratio of ferrite is calculated using the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. With this function, for grains having a body-centered structure, it is possible to calculate an orientation difference between adjacent measurement points and then obtain an average value of all measurement points in the grains. For the crystal orientation information obtained by the EBSD analysis, a region surrounded by grain boundaries having an average crystal orientation difference of 5° or more is defined as a grain, and a map is drawn by the "Grain Average Misorientation" function. In a region where regions determined to be pearlite, lower bainite, martensite, and tempered martensite are excluded from the map, a region where an average crystal orientation difference in grains is less than 5.0° is determined as ferrite. An area ratio of the region determined as ferrite is calculated, so that the area ratio of ferrite is obtained.
The steel sheet for hot stamping according to the present embodiment may have a plating layer formed on the surface for the purpose of improving corrosion resistance after hot stamping. The plating layer may be any of an electroplating layer and a hot-dip plating layer. The electroplating layer includes, for example, an electrogalvanized layer, an electrolytic Zn-Ni alloy plating layer, and the like. The hot-dip plating layer includes, for example, a hot-dip galvanized layer, a hot-dip galvannealed layer, a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, a hot-dip Zn-Al-Mg-Si alloy plating layer, and the like. An adhesion amount of a plating layer is not particularly limited and may be a general adhesion amount.
The sheet thickness of the steel sheet for hot stamping according to the present embodiment is not particularly limited, but is preferably 0.5 to 3.5 mm from the viewpoint of a reduction in the weight of the vehicle body or the like.
Next, a hot-stamping formed body according to the present embodiment that is obtained by hot-stamping the above-described steel sheet for hot stamping will be described. The hot-stamping formed body according to the present embodiment has the same chemical composition as the above-described steel sheet for hot stamping. A measurement method of the chemical composition may be the same as that for the steel sheet for hot stamping. In addition, in the hot-stamping formed body according to the present embodiment, the prior austenite grains are grain-sized in the microstructure. That is, the hot-stamping formed body according to the present embodiment has a microstructure in which the average grain size of the prior austenite grains is 5 to 25 µm and the standard deviation of the grain sizes of the prior austenite grains is 0.1 to 2.0 µm.
In addition, in the present embodiment, the microstructure is specified at the 1/4 depth position (the region from the 1/8 depth of the sheet thickness from the surface to the 3/8 depth of the sheet thickness from the surface) of the sheet thickness from the surface of the cross section perpendicular to the sheet surface. The reason therefor is that the microstructure at this position indicates a typical microstructure of the hot-stamping formed body. Hereinafter, the microstructure will be described.

"Average grain size of prior austenite grains is 5 to 25 µm"
"Standard deviation of grain size of prior austenite grains is 0.1 to 2.0 µm"

In the microstructure of the hot-stamping formed body, by setting the average grain size of the prior austenite grains to be 5 to 25 µm and setting the standard deviation of the grain sizes of the prior austenite grains to 0.1 to 2.0 µm, the bendability of the hot-stamping formed body can be improved. When the average grain size of the prior austenite grains or the standard deviation of the grain sizes of the prior austenite grains is outside the above range, it is not possible to obtain excellent bendability in the hot-stamping formed body.
The average grain size of the prior austenite grains is preferably 10 µm or more and more preferably 15 µm or more. The average grain size of the prior austenite grains is preferably 20 µm or less.
By setting the standard deviation of the grain sizes of the prior austenite grains to 2.0 µm or less, excellent bendability in the hot-stamping formed body can be obtained. Therefore, the standard deviation of the grain sizes of the prior austenite grains is set to 2.0 µm or less. The standard deviation is preferably 1.2 µm or less, more preferably 1.1 µm or less, and still more preferably 0.4 µm or less.
In an actual operation, since it is difficult to set the standard deviation of the grain sizes of the prior austenite grains to less than 0.1 µm, the substantial lower limit is set to 0.1 µm or more.
When the area ratio of the prior austenite grains having the average grain size of 0.5 to 3.0 µm is 60% or less, more excellent bendability can be obtained in the hot-stamping formed body. Therefore, the area ratio of the prior austenite grains having the average grain size of 0.5 to 3.0 µm may be set to 60% or less. The area ratio is more preferably 50% or less and still more preferably 40% or less.

Measurement method of average grain size and standard deviation of grain size of prior austenite grains

Next, the measurement method of the average grain size of the prior austenite grains will be described. A sample is cut out from an arbitrary position (a position that avoids an end portion in a case where the sample cannot be collected at this position) away from an end surface of the hot-stamping formed body by a distance of 50 mm or more so that a sheet thickness cross section parallel to a rolling direction can be observed. The size of the sample also depends on a measurement device, but is set to a size that can be observed by about 10 mm in the rolling direction. The cross section of the sample is polished using silicon carbide paper having a grit of #600 to #1500, then, is finished as a mirror surface using liquid in which diamond powder having a grain size of 1 to 6 µm is dispersed in diluted solution of alcohol or the like or pure water and finish-polished by electrolytic polishing.
Next, in a region from the 1/8 depth of the sheet thickness from the surface to the 3/8 depth of the sheet thickness from the surface at an arbitrary position of the sample cross section in the longitudinal direction so that the 1/4 depth position of the sheet thickness from the surface can be observed and in a region having 100 µm in the length and 100 µm in the sheet thickness direction, a sample is irradiated with an electron beam at an acceleration voltage of 15 kV and an irradiation current level of 13 in a vacuum of 9.6 × 10^-5 Pa or less using the device including a thermal field emission type scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSD detector (DVC5-type detector manufactured by TSL Solutions), and the EBSD analysis is performed at an analysis speed of 200 to 300 points/sec. Using the obtained crystal orientation information, the crystal orientation of the prior austenite grains is calculated from a crystal orientation relationship between the general prior austenite grains and grains having a body-centered structure after transformation, and the average grain size of the prior austenite grains is calculated using the calculated crystal orientation.
The method for calculating the crystal orientation of the prior austenite grains is not particularly limited, and for example, the calculation may be performed using the following method. First, the crystal orientation of the prior austenite grains is calculated by the method described in Non-Patent Document 1, and the crystal orientation of the prior austenite in each coordinate of the EBSD-measured region is specified. Next, a crystal orientation map of the prior austenite grain is created using the "Inverse Pole Figure" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. For one of the prior austenite grains included in the observed visual field, an average value of a shortest diameter and a longest diameter is calculated, and the average value is used as the grain size of the prior austenite grains. The above operation is performed on all the prior austenite grains except for the prior austenite grains which are not entirely included in the photographed visual fields, such as grains in an end portion of the photographed visual field, and the grain sizes of all the prior austenite grains in the photographed visual fields are obtained. The average grain size of the prior austenite grains in the photographed visual fields is obtained by calculating a value obtained by dividing the sum of the obtained grain sizes of the prior austenite grains by the total number of prior austenite grains of which grain sizes are measured. This operation is performed on all the photographed visual fields, and the average grain size of the prior austenite grains of all the photographed visual fields is calculated, thereby obtaining the average grain size of the prior austenite grains.
By calculating the standard deviation from the grain sizes of the prior austenite grains, the standard deviation of the grain sizes of the prior austenite grains is obtained. At this time, in order to eliminate the influence of locally generated fine grains or coarse grains, the standard deviation is calculated by excluding the minimum value and the maximum value of the prior austenite grain sizes.
By calculating a value obtained by dividing the area of the prior austenite grains having an average grain size of 0.5 to 3.0 µm by the area of the entire measurement visual field, the area ratio of the prior austenite grains having an average grain size of 0.5 to 3.0 µm is obtained.
The microstructure of the hot-stamping formed body is not particularly limited as long as desired strength and desired bendability can be obtained after hot stamping. However, the microstructure may include, for example, by area%, ferrite: 0% to 50%, bainite and martensite: 0% to 100%, pearlite: 0% to 30%, and residual austenite: 0% to 5%. The microstructure of the hot-stamping formed body may be measured by the following method.

Measurement method of microstructure of hot-stamping formed body

A sample is cut out from an arbitrary position (a position that avoids an end portion in a case where the sample cannot be collected at this position) away from an end surface of the hot-stamping formed body by a distance of 50 mm or more so that the cross section perpendicular to the sheet surface can be observed. The cross section of the sample is polished using silicon carbide paper having a grit of #600 to #1500, then, is finished as a mirror surface using liquid in which diamond powder having a grain size of 1 to 6 µm is dispersed in diluted solution of alcohol or the like or pure water and is performed on Nital etching. In a region that has a length of 100 µm and between the 1/8 depth of the sheet thickness from the surface and the 3/8 depth of the sheet thickness from the surface at an arbitrary position on the cross section of the sample in a longitudinal direction so that the 1/4 depth position of the sheet thickness from the surface can be observed, photographs having a plurality of visual fields are taken using a thermal field emission type scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.). Evenly spaced grids are drawn in the taken photographs, and structures at grid points are identified. The number of grid points corresponding to each structure is obtained and is divided by the total number of grid points, so that the area ratio of each structure is obtained. The area ratio can be more accurately obtained as the total number of grid points is larger. In the present embodiment, grid spacings are set to 2 µm × 2 µm and the total number of grid points is set to 1,500.
A region where cementite is precipitated in a lamellar shape in the grains is determined as pearlite. A region in which brightness is low and no sub-microstructure is observed is determined as ferrite. A region in which the brightness is high and the sub-microstructure is not exposed by etching is determined as "martensite or residual austenite". A region that does not correspond to any of the above-described microstructures is determined as bainite.
The area ratio of martensite is obtained by subtracting the area ratio of residual austenite obtained by EBSD analysis described later from the area ratio of martensite and residual austenite obtained from the taken photographs.
The area ratio of residual austenite is measured using an electron backscatter diffraction method (EBSD). In the analysis by EBSD, a sample collected at the same sample collection position as in the measurement using the above-described taken photograph is used, and the analysis is performed on the region between the 1/8 depth of the sheet thickness from the surface and the 3/8 depth of the sheet thickness from the surface. The sample is polished using silicon carbide paper having a grit of #600 to #1500, then, finished into a mirror surface using liquid in which diamond powder having a grain size of 1 to 6 µm is dispersed in diluted solution of alcohol or the like or pure water, and then finished by electrolytic polishing for the purpose of sufficiently removing strain in a cross section to be measured. In the electrolytic polishing, in order to remove mechanical polishing strain on the observed section, the sample may be polished a minimum of 20 µm and polished a maximum of 50 µm. The sample is preferably polished 30 µm or less in consideration of rollover at the end portion.
With regard to the measurement in EBSD, an acceleration voltage is set to 15 to 25 kV, the measurement is performed at intervals of at least 0.25 µm or less, and the crystal orientation information about each measurement point in a range of 150 µm or more in the sheet thickness direction and 250 µm or more in the rolling direction is obtained. In the obtained crystal structure, a measurement point at which a crystal structure is fcc is determined as residual austenite using "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. The ratio of measurement points determined as the residual austenite is obtained, thereby obtaining the area ratio of the residual austenite. Here, the larger the number of the measurement points, the more preferable, and thus it is preferable that the measurement intervals are narrow and the measurement range is wide. However, in a case where the measurement intervals are less than 0.01 µm, adjacent points interfere with the expansion width of an electron beam. For this reason, the measurement interval is set to 0.01 µm or more. In addition, the measurement range may be set to 200 µm in the sheet thickness direction and 400 µm in the sheet width direction at a maximum. An EBSD device including a thermal field emission type scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSD detector (DVC5-type detector manufactured by TSL Solutions) is used for measurement. In this case, a degree of vacuum in the device is set to 9.6 × 10^-5 Pa or less, the irradiation current level is set to 13, and the irradiation level of the electron beam is set to 62.
The hot-stamping formed body according to the present embodiment may have a plating layer formed on the surface for the purpose of improving corrosion resistance after the hot stamping or the like. The plating layer may be any of an electroplating layer and a hot-dip plating layer. The electroplating layer includes, for example, an electrogalvanized layer, an electrolytic Zn-Ni alloy plating layer, and the like. The hot-dip plating layer includes, for example, a hot-dip galvanized layer, a hot-dip galvannealed layer, a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, a hot-dip Zn-Al-Mg-Si alloy plating layer, and the like. An adhesion amount of a plating layer is not particularly limited and may be a general adhesion amount.
The sheet thickness of the hot-stamping formed body according to the present embodiment is not particularly limited. However, in terms of reducing the weight of a vehicle body or the like, it is preferable that the sheet thickness of the hot-stamping formed body according to the present embodiment is set to 0.5 to 3.5 mm.
The hot-stamping formed body according to the present embodiment has a tensile (maximum) strength of 2,200 MPa or more. The tensile strength is preferably 2,400 MPa or more, and more preferably 2,550 MPa or more. The tensile strength is obtained according to the test method described in JIS Z 2241 :2011 by producing a No. 5 test piece described in JIS Z 2241:2011 from a position as flat as possible in the hot-stamping formed body.
In addition, in the hot-stamping formed body according to the present embodiment, the maximum bending angle that is obtained by a bending test based on the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry is preferably 20° or more. The conditions in the bending test were as described below.

Dimensions of test piece: 60 mm (rolling direction) × 30 mm (direction parallel to sheet width direction)
Test piece sheet thickness: 1.6 mm
Bending ridge: direction parallel to sheet width direction
Test method: roll support and punch pressing
Roll diameter: ϕ 30 mm
Punch shape: tip end R=0.4 mm
Distance between rolls: 2.0 × sheet thickness (mm) + 0.5 mm
Pressing speed: 20 mm/min
Tester: SHIMADZU AUTOGRAPH 20 kN

Next, a manufacturing method of the steel sheet for hot stamping according to the present embodiment will be described.
In the manufacturing method of the steel sheet for hot stamping according to the present embodiment, the rolling reduction in the rolling one pass before the final pass of finish rolling in the hot rolling is set high in order to obtain the steel sheet for hot stamping having the above-described microstructure.
A steel piece (steel material) to be subjected to hot rolling may be a steel piece manufactured by an ordinary method, and may be, for example, a steel piece manufactured by a general method such as a continuous cast slab or a thin slab caster.
In the hot rolling, rough rolling and finish rolling are performed. In the finish rolling, the slab after the rough rolling is rolled by a plurality of finishing mills. In the present embodiment, the rolling one pass before the final pass of finish rolling is performed in a temperature range of 900°C to 1,050°C at a rolling reduction of 10% to 25%. After this rolling, the final pass is performed in a temperature range of 850°C or higher and lower than 1,000°C at a rolling reduction (final rolling reduction) of 6% or more. When a sheet thickness before the rolling one pass before the final pass is t0 and a sheet thickness after the rolling one pass before the final pass is t1, the rolling reduction in the rolling one pass before the final pass can be represented by {(T0 - t1)/t0} × 100 (%). When the sheet thickness before the final pass of the finish rolling is t1 and the sheet thickness after the final pass of the finish rolling is t2, the final rolling reduction can be represented by {(t1 - t2)/t1} × 100 (%).
By setting the rolling reduction in the rolling one pass before the final pass to 10% to 25%, dislocation in austenite is reduced, and by setting the rolling reduction (final rolling reduction) of the subsequent final pass to 6% or more, a small amount of dislocation can be introduced into the austenite grains. It is presumed that the dislocations introduced into the austenite grains function as the precipitation origins of carbides, and thus, as a result, a desired amount of ferrite containing the carbides can be formed in the grains. Since dislocations in austenite before the final rolling are combined with dislocations introduced in the final pass and disappear, it is presumed that unless the rolling reduction in the rolling one pass before the final pass is controlled within the above range, the precipitation origin of carbides decreases.
Usually, in the finish rolling, the rolling is performed by gradually reducing the rolling reduction in each pass. However, in the present embodiment, in the rolling one pass before the final pass of the finish rolling, the rolling is performed at the above-described rolling reduction with a rolling reduction higher than that of a pass (two passes before the final pass) before that. Accordingly, a desired microstructure can be obtained.
When the rolling reduction in the rolling one pass before the final pass is less than 10% or more than 25%, the recrystallization of austenite in the final pass is suppressed, and a desired texture cannot be obtained. The rolling reduction in the rolling one pass before the final pass is preferably 13% or more, more preferably 16% or more, and still more preferably 18% or more.
When the rolling temperature one pass before the final pass is lower than 900°C, the recrystallization of austenite in the final pass is suppressed, and a desired texture cannot be obtained. The rolling temperature one pass before the final pass is preferably 910°C or higher, and more preferably 930°C or higher.
Meanwhile, when the rolling temperature one pass before the final pass is higher than 1 ,050°C, the austenite grains become coarse and ferritic transformation is suppressed, so that a predetermined amount of ferrite cannot be obtained in the steel sheet for hot stamping. The rolling temperature one pass before the final pass is preferably 1,040°C or lower and more preferably 1,020°C or lower.
When the rolling reduction of the final pass (final rolling reduction) is less than 6%, the number of dislocations that are introduced decreases, and the number proportion of the ferrite containing the carbide having the equivalent circle diameter of 0.2 µm or more in the grains cannot be controlled to a predetermined amount. The final rolling reduction is preferably 8% or more, more preferably 10% or more, and still more preferably 12% or more. The upper limit of the final rolling reduction is not particularly specified and may be set to less than 40%.
When the rolling temperature of the final pass is lower than 850°C, austenite grains are excessively refined, ferritic transformation is excessively promoted, and it is not possible to obtain a predetermined amount of pearlite in the hot stamping steel sheet. The rolling temperature of the final pass is preferably 860°C or higher and more preferably 870°C or higher.
Meanwhile, when the rolling temperature of the final pass is 1,000°C or higher, the austenite grains become coarse and ferritic transformation is suppressed, so that it is not possible to obtain a predetermined amount of ferrite in the hot stamping steel sheet. The rolling temperature of the final pass is preferably 980°C or lower and more preferably 960°C or lower.
The heating temperature and holding time of the steel piece before hot rolling are not particularly limited, but it is preferable that the steel piece is held in a temperature range of 1200°C or higher for 20 minutes or longer.
After the finish rolling, the steel sheet is preferably coiled in the temperature range of 400°C to 750°C. When the coiling temperature is lower than 400°C, the area ratio of pearlite is more than 90% and the area ratio of ferrite is less than 10% in the steel sheet for hot stamping. The coiling temperature is preferably 450°C or higher and more preferably 530°C or higher.
Meanwhile, when the coiling temperature is higher than 750°C, the area ratio of pearlite is less than 10% and the area ratio of ferrite is more than 90% in the steel sheet for hot stamping. The coiling temperature is preferably 700°C or lower and more preferably 660°C or lower.
After the coiling, cold rolling may be performed as necessary. In addition, the above-mentioned plating may be formed after finish rolling or after cold rolling. Pickling may be performed between the hot rolling and the cold rolling. In the cold rolling, a normal cumulative rolling reduction, for example, 30% to 90% may be set. In addition, temper rolling may be performed under normal conditions. In addition, for the purpose of softening the hot-rolled steel sheet, hot-rolled sheet annealing in which the hot-rolled steel sheet is heated to a temperature range of 730°C or lower may be performed.
The steel sheet for hot stamping according to the present embodiment can be manufactured by the above method. Next, a manufacturing method of the hot-stamping formed body according to the present embodiment that can be manufactured using the above-described steel sheet for hot stamping will be described. The manufacturing method of the hot-stamping formed body according to the present embodiment is not particularly limited, and for example, the following manufacturing method may be used.
First, the above-mentioned steel sheet for hot stamping is heated in a temperature range of 800°C or higher. When the heating temperature is lower than 800°C, there are cases where coarse carbides that are being heated remain and the bendability of the hot-stamping formed body decreases. The heating temperature is preferably 820°C or higher and more preferably 860°C or higher.
The upper limit of the heating temperature is not particularly limited. However, when the heating temperature is too high, decarburization is promoted in the surface layer of the steel sheet, and the strength of the hot-stamping formed body decreases. Therefore, the heating temperature is preferably 1,000°C or lower, more preferably 960°C or lower, and even more preferably 930°C or lower.
The holding time at the heating temperature is preferably 1.0 to 10.0 minutes. When the holding time is shorter than 1.0 minutes, there are cases where coarse carbides remain and the bendability of the hot-stamping formed body decreases. Meanwhile, when the holding time is more than 10.0 minutes, decarburization is promoted in the surface layer of the steel sheet, and the strength of the hot-stamping formed body may decrease.
In addition, the average heating rate up to the heating temperature is preferably set to 1.0 °C/s or faster. When the average heating rate is slower than 1.0 °C/s, decarburization is promoted in the surface layer of the steel sheet, and the strength of the hot-stamping formed body decreases. Although the upper limit of the average heating rate is not particularly determined, since it is difficult to set the upper limit to faster than 1,000 °C/s in actual operation, the actual upper limit is 1,000 °Cls or slower.
Hot stamping is performed after the heating and the holding described above. After the hot stamping, it is preferable to perform cooling to a temperature range of, for example, 300°C or lower at an average cooling rate of 10 °C/s or faster. When the average cooling rate is slower than 10 °C/s, the strength may be insufficient. Although the upper limit of the average heating rate is not particularly determined, since it is difficult to set the upper limit to faster than 1,000 °C/s in actual operation, the actual upper limit is 1,000 °C/s or slower.
In the heating during hot stamping, it is not preferable to perform preheating, that is, to perform two-stage heating. The segregation region of carbon in the grain boundaries created in the stage of the steel sheet for hot stamping is eliminated, it is not possible to uniformly disperse and form the prior austenite grains, and as a result, the standard deviation of the prior austenite grains cannot be controlled within a desired range.
The hot-stamping formed body according to the present embodiment can be obtained by the preferable manufacturing method described above. After the hot stamping, a tempering treatment may be performed at 150°C to 600°C. In addition, a part of the hot-stamping formed body may be tempered by laser irradiation or the like to partially provide a softened region. Weldability improves in the softened region. For example, when spot welding is performed after softening the end portion of the hot-stamping formed body, it is possible to reduce a difference in strength between the softened end portion and the spot-welding portion of the end portion, and thus, the fracture from the interface between the end portion and the spot-welding portion can be suppressed. In addition, for example, in a case where the hot-stamping formed body is applied to a high strength member of an automobile, it is possible to control a fracture or deformation mode of the high strength member in the time of a collision by providing a softened region in a part of the high strength member.

[Example]

Next, examples of the present invention will be described. Conditions in the examples are one condition example that is employed to confirm the feasibility and effects of the present invention, and the present invention is not limited to this condition example. The present invention may employ various conditions to achieve the object of the present invention without departing from the scope of the present invention.
A steel piece manufactured by casting molten steel having a chemical composition shown in Tables 1A to 1D was heated, held in a temperature range of 1,200°C or higher for 20 minutes or longer, and then subjected to hot rolling and coiling under conditions shown in Tables 2A to 2G, and subjected to cold rolling, hot-rolled sheet annealing, pickling, and plating as necessary. As a result, steel sheets for hot stamping shown in Tables 2A to 2G were obtained. In the finish rolling, except for No. 195 marked with "*", in the rolling one pass before the final pass, the rolling was performed with a higher rolling reduction than the pass (two passes before the final pass) before that.
In addition, Steel sheet No. 149 was subjected to the hot-rolled sheet annealing of heating and holding in a temperature range of 730°C or lower.
The cold rolling was not performed on Steel sheet No. 150.
An electrogalvanized layer was formed on the surface of Steel sheet No. 151.
An electrolytic Zn-Ni alloy plating layer was formed on the surface of Steel sheet No. 152.
A hot-dip galvanized layer was formed on the surface of Steel sheet No. 153.
A hot-dip galvannealed layer was formed on the surface of Steel sheet No. 154.
A hot-dip aluminum plating layer was formed on the surface of Steel sheet No. 155.
A hot-dip Zn-Al alloy plating layer was formed on the surface of Steel sheet No. 156.
A hot-dip Zn-Al-Mg alloy plating layer was formed on the surface of Steel sheet No. 157.
A hot-dip Zn-Al-Mg-Si alloy plating layer was formed on the surface of Steel sheet No. 158.
For Steel sheet No. 195, in the finish rolling, the rolling was performed by gradually reducing the rolling reduction for each pass.
In Tables 2A to 2G, a "pole density" indicates the "average value of the pole densities of ferrite in the orientation group consisting of {100}<011> to {223 }<110>, a "number proportion of ferrite including carbide" indicates the "number proportion of ferrite including a carbide having an equivalent circle diameter of 0.2 µm or more in grains in entire ferrite".
The obtained steel sheets for hot stamping were subjected to hot stamping under the conditions shown in Tables 3A to 3G to obtain hot-stamping formed bodies shown in Tables 3A to 3G.
For Manufacturing No. 186, a tempering treatment was performed at 150°C to 600°C after hot stamping.
For Manufacturing No. 187, a partially softened region was formed by irradiating a portion of the hot-stamping formed body with a laser and tempering the portion.
After Manufacturing No. 188 was heated to a heating temperature shown in Table 3G, Manufacturing No. 188 was cooled to a temperature range of 250°C or lower. Thereafter, Manufacturing No. 188 was heated to 900°C and hot-stamped, and then cooled at the average cooling rate in Table 3G.
In the examples of the present invention shown in Tables 3A to 3G, the microstructures included, by area%, ferrite: 0% to 50%, bainite and martensite: 0% to 100%, pearlite: 0% to 30%, and residual austenite: 0% to 5%.

In addition, a method for measuring the microstructure of the steel sheet for hot stamping and a method for measuring the microstructure and mechanical properties of the hot-stamping formed body were as described above. In a case where the tensile strength of the hot-stamping formed body was 2,200 MPa or more, the hot-stamping formed body was determined to be acceptable for having high strength, and, in a case where the tensile strength of the hot-stamping formed body was less than 2,200 MPa, the hot-stamping formed body was determined to be unacceptable for not having high strength. In addition, in a case where the maximum bending angle was 20° or more, it was determined to be acceptable for having excellent bendability, and, in a case where the maximum bending angle was less than 20°, it was determined to be unacceptable for not having excellent bendability.

[Table 2A]

Steel sheet No.	Steel No.	Finish rolling				Coiling	Steel sheet for hot stamping				Remark
Steel sheet No.	Steel No.	Rolling temperature of rolling one pass before final pass (°C)	Rolling reduction of rolling one pass before final pass (%)	Rolling temperature of final pass (°C)	Final rolling reduction (%)	Coiling temperature (°C)	Pole density (-)	Number proportion of ferrite containing carbide (%)	Ferrite (area%)	Pearlite (area%)	Remark
1	1	971	19	927	18	660	3.4	76	49	51	Present invention example
2	2	975	19	919	17	607	23	74	42	58	Present invention example
3	3	996	19	940	16	586	3.0	84	53	47	Present invention example
4	4	950	23	904	16	577	3.1	61	60	40	Present invention example
5	5	964	20	926	19	552	6.6	40	29	71	Comparative example
6	6	995	23	921	15	638	6.1	44	32	68	Present invention example
7	7	978	21	911	18	554	7.0	48	29	71	Present invention example
8	8	983	22	908	16	569	6.9	45	23	77	Present invention example
9	9	986	22	933	17	565	6.1	44	36	64	Present invention example
10	10	975	19	909	15	638	6.5	40	32	68	Present invention example
11	11	994	21	931	18	570	7.0	47	27	73	Present invention example
12	12	957	19	920	18	636	6.5	44	24	76	Present invention example
13	11	961	21	926	16	587	6.2	42	37	63	Comparative example
14	14	994	21	901	18	639	6.4	47	22	78	Comparative example
15	15	961	20	925	15	626	6.7	42	24	76	Present invention example
16	16	950	21	911	18	615	67	46	22	78	Present invention example
17	17	970	21	929	17	627	6.1	48	36	64	Present invention example
18	18	952	19	902	18	545	7.0	45	23	77	Present invention example
19	19	958	20	939	15	640	7.0	43	34	66	Present invention example
20	20	975	21	900	15	552	6.5	48	26	74	Present invention example
21	21	984	23	929	19	586	6.9	48	35	65	Comparative example
22	22	962	19	923	16	637	6.5	43	23	77	Comparative example
23	23	957	21	926	17	614	62	49	22	78	Present invention example
24	24	961	23	938	15	579	6.4	40	25	75	Present invention example
25	25	996	22	902	17	542	6.9	48	30	70	Present invention example
26	26	995	23	920	15	633	6.8	41	36	64	Present invention example
27	27	951	23	925	16	592	6.4	49	24	76	Present invention example
28	28	997	23	904	15	590	6.2	46	22	78	Present invention example
29	29	969	21	903	19	554	6.2	49	23	77	Comparative example
30	30	962	22	939	17	594	6.7	40	22	78	Comparative example

Underscores indicate scope outside present invention and indicate that manufacturing conditions are not preferable.

[Table 2B]

Steel sheet No.	Steel No.	Finish rolling				Coiling	Steel sheet for hot stamping				Remark
Steel sheet No.	Steel No.	Rolling temperature of rolling one pass before final pass (°C)	Rolling reduction of rolling one pass before final pass (%)	Rolling temperature of final pass (°C)	Final rolling reduction (%)	Coiling temperature (°C)	Pole density (-)	Number proportion of ferrite containing carbide (%)	Ferrite (area%)	Pearlite (area%)	Remark
31	31	969	22	932	15	608	6.7	44	38	62	Present invention example
32	32	978	21	919	18	605	6.3	49	36	64	Present invention example
33	33	1000	23	941	19	595	6.1	45	39	61	Present invention example
34	34	971	22	944	17	541	6.1	42	21	79	Comparative example
35	35	987	21	931	15	580	6.6	40	21	79	Present invention example
36	36	967	19	919	17	653	6.4	47	28	72	Present invention example
37	37	975	20	903	18	556	6.5	40	21	79	Present invention example
38	38	982	20	932	15	561	7.0	43	38	62	Comparative example
39	39	955	19	914	17	580	6.8	41	32	68	Present invention example
40	40	996	20	932	19	587	6.6	46	25	75	Present invention example
41	41	962	19	929	18	545	6.9	40	39	61	Present invention example
42	42	961	22	900	16	557	6.2	44	28	72	Comparative example
43	43	994	21	915	19	578	6.6	47	38	62	Present invention example
44	44	999	22	901	17	580	6.4	44	23	77	Present invention example
45	45	972	21	934	19	615	7.0	46	26	74	Present invention example
46	46	994	19	920	16	652	6.3	48	21	79	Comparative example
47	47	998	23	911	is	638	6.3	47	28	72	Present invention example
48	48	966	20	930	19	600	6.4	41	26	74	Present invention example
49	49	950	22	908	19	561	6.7	44	26	74	Present invention example
50	50	953	22	904	17	563	6.3	48	25	75	Present invention example
51	51	972	21	920	19	645	6.2	44	37	63	Present invention example
52	52	975	20	926	15	592	6.1	48	30	70	Present invention example
53	53	980	20	928	is	617	6.9	49	23	77	Comparative example
54	54	991	23	937	18	552	6.9	41	24	76	Comparative example
55	55	955	22	916	18	556	6.6	41	30	70	Present invention example
56	56	971	21	928	18	630	7.0	41	21	79	Present invention example
57	57	963	19	944	19	621	6.8	44	38	62	Present invention example
58	58	983	23	924	16	551	6.6	48	34	66	Present invention example
59	59	990	23	906	19	544	6.9	41	35	65	Present invention example
60	60	997	19	926	17	541	6.8	44	21	79	Present invention example

[Table 2C]

Steel sheet No.	Steel No.	Finish rolling				Coiling	Steel sheet for hot stamping				Remark
Steel sheet No.	Steel No.	Rolling temperature of rolling one pass before final pass (°C)	Rolling reduction of rolling one pass before final pass (%)	Rolling temperature of final pass (°C)	Final rolling reduction (%)	Coiling temperature (°C)	Pole density (-)	Number proportion of ferrite containing carbide (%)	Ferrite (area%)	Pearlite (area%)	Remark
61	61	976	22	912	15	558	6.5	41	32	68	Comparative example
62	62	981	22	929	18	596	6.8	46	29	71	Comparative example
63	63	951	20	934	19	655	6.8	42	31	69	Present invention example
64	64	973	22	929	16	583	6.2	43	27	73	Present invention example
65	65	953	23	914	15	586	6.7	47	22	78	Present invention example
66	66	987	23	910	15	653	6.8	48	31	69	Comparative example
67	67	981	19	921	19	573	6.2	44	38	62	Present invention example
68	68	970	19	937	16	660	6.1	48	36	64	Present invention example
69	69	976	20	925	16	562	6.5	46	25	75	Present invention example
70	70	968	22	924	19	545	6.2	47	34	66	Comparative example
71	71	969	22	910	19	621	7.0	41	25	75	Present invention example
72	72	970	23	945	17	568	6.6	49	34	66	Present invention example
73	73	955	22	903	18	548	6.1	41	23	77	Present invention example
74	74	990	20	945	15	625	7.0	48	38	62	Comparative example
75	75	964	19	903	19	649	6.8	46	35	65	Present invention example
76	76	987	23	919	19	589	63	40	28	72	Present invention example
77	77	957	23	903	18	587	6.7	44	34	66	Present invention example
78	78	979	23	909	16	651	6.1	44	22	78	Comparative example
79	79	950	21	916	18	604	6.1	49	27	73	Present invention example
80	80	994	20	937	15	634	6.7	40	33	67	Present invention example
81	81	983	22	942	19	551	63	43	36	64	Present invention example
82	82	982	19	930	17	572	6.2	46	29	71	Comparative example
83	83	965	19	907	16	644	6.7	48	36	64	Present invention example
84	84	979	23	935	17	578	7.0	49	31	69	Present invention example
85	85	998	20	907	18	597	6.8	47	28	72	Comparative example
86	86	994	19	922	15	584	6.8	40	34	66	Present invention example
87	87	992	23	910	18	568	6.2	49	37	63	Present invention example
88	88	987	19	929	17	577	6.7	41	21	79	Comparative example
89	89	967	19	926	16	605	7.0	45	30	70	Present invention example
90	90	961	19	901	18	567	6.6	42	28	72	Present invention example

[Table 2D]

Steel sheet No.	Steel No.	Finish rolling				Coiling	Steel sheet for hot stamping				Remark
Steel sheet No.	Steel No.	Rolling temperature of rolling one pass before final pass (°C)	Rolling reduction of rolling one pass before final pass (%)	Rolling temperature of final pass (°C)	Final rolling reduction (%)	Coiling temperature (°C)	Pole density (-)	Number proportion of ferrite containing carbide (%)	Ferrite (area%)	Pearlite (area%)	Remark
91	21	977	19	938	19	646	63	47	39	61	Comparative example
92	92	996	19	927	19	635	6.1	47	30	70	Present invention example
93	93	954	20	915	15	571	6.1	41	34	66	Present invention example
94	94	972	23	916	18	541	6.1	40	32	68	Comparative example
95	95	956	23	933	16	545	7.0	42	26	74	Present invention example
96	96	988	23	917	15	612	7.0	44	34	66	Present invention example
97	97	961	23	937	15	583	7.0	44	21	79	Comparative example
98	98	973	23	929	is	627	6.6	42	23	77	Present invention example
99	99	996	21	918	16	541	6.6	45	25	75	Present invention example
100	100	951	19	927	19	549	6.2	42	22	78	Comparative example
101	101	978	22	909	18	643	6.6	45	22	78	Present invention example
102	102	985	23	910	17	646	6.8	45	28	72	Present invention example
103	103	962	20	917	17	544	6.7	48	29	71	Comparative example
104	104	994	22	905	19	602	6.7	46	28	72	Present invention example
105	105	953	21	927	18	655	6.4	48	25	75	Present invention example
106	106	962	19	903	19	551	6.9	48	24	76	Comparative example
107	107	954	20	922	15	628	6.4	46	21	79	Present invention example
108	108	971	22	935	19	627	6.9	47	25	75	Present invention example
109	2	1061	22	908	17	643	3.5	42	5	95	Comparative example
110	2	1050	22	939	15	617	2.2	40	14	86	Present invention example
111	2	1029	23	910	19	568	2.9	43	23	77	Present invention example
112	2	1012	21	903	16	573	5.6	43	22	78	Present invention example
113	2	972	19	942	15	613	6.5	44	38	62	Present invention example
114	2	936	19	927	17	610	6.2	46	32	68	Present invention example
115	2	918	22	900	15	593	6.6	48	28	72	Present invention example
116	2	908	22	923	18	631	8.3	49	22	78	Present invention example
117	2	871	19	917	15	545	11.0	47	33	67	Comparative example
118	2	983	36	942	18	601	12.0	43	23	77	Comparative example
119	2	982	23	939	16	639	8.1	48	38	62	Present invention example
120	2	986	19	922	16	637	4.0	48	28	72	Present invention example

[Table 2E]

Steel sheet No.	Steel No.	Finish rolling				Coiling	Steel sheet for hot stamping				Remark
Steel sheet No.	Steel No.	Rolling temperature of rolling one pass before final pass (°C)	Rolling reduction of rolling one pass before final pass (%)	Rolling temperature of final pass (°C)	Final rolling reduction (%)	Coiling temperature (°C)	Pole density (-)	Number proportion of ferrite containing carbide (%)	Ferrite (area%)	Pearlite (area%)	Remark
121	2	973	17	931	16	551	5.6	43	29	71	Present invention example
122	2	978	14	906	17	543	7.0	41	23	77	Present invention example
123	2	997	11	911	16	626	8.2	49	22	78	Present invention example
124	2	958	6	921	15	627	13.0	45	22	78	Comparative example
125	2	1000	21	1016	16	571	6.7	47	8	92	Comparative example
126	2	986	23	989	17	654	6.4	45	20	80	Present invention example
127	2	966	23	972	15	546	6.8	40	39	61	Present invention example
128	2	956	21	879	19	602	6.4	48	30	70	Present invention example
129	2	962	22	904	18	545	6.1	47	51	49	Present invention example
130	2	993	20	951	19	624	6.5	48	65	35	Present invention example
131	2	978	23	863	18	625	6.9	43	69	31	Present invention example
132	2	950	21	854	16	647	6.5	41	88	12	Present invention example
133	2	952	23	842	16	559	6.7	40	95	5	Comparative example
134	2	972	19	900	18	635	7.0	89	29	71	Present invention example
135	2	953	23	925	16	626	6.6	83	32	68	Present invention example
136	2	981	21	904	12	598	7.0	67	28	72	Present invention example
137	2	950	19	937	10	636	6.3	51	26	74	Present invention example
138	2	967	23	900	8	606	6.2	43	23	77	Present invention example
139	2	951	20	913	6	658	6.2	23	30	70	Present invention example
140	2	972	19	928	4	605	6.8	5	25	75	Comparative example
141	2	982	21	918	17	775	6.6	44	3	97	Comparative example
142	2	991	20	939	17	742	7.0	49	16	84	Present invention example
143	2	966	23	901	18	679	6.2	47	37	63	Present invention example
144	2	979	23	903	18	639	6.5	49	47	53	Present invention example
145	2	951	23	931	15	569	6.6	46	56	44	Present invention example
146	2	980	21	924	15	470	6.8	41	68	32	Present invention example
147	2	989	21	936	15	406	6.7	49	81	19	Present invention example
148	2	969	19	913	15	366	7.0	47	21	9	Comparative example
149	2	970	23	907	16	590	6.5	42	21	79	Present invention example
150	2	987	20	933	19	573	6.6	49	30	70	Present invention example

[Table 2F]

Steel sheet No.	Steel No.	Finish rolling				Coiling	Steel sheet for hot stamping				Remark
Steel sheet No.	Steel No.	Rolling temperature of rolling one pass before final pass (°C)	Rolling reduction of rolling one pass before final pass (%)	Rolling temperature of final pass (°C)	Final rolling reduction (%)	Coiling temperature (°C)	Pole density (-)	Number proportion of ferrite containing carbide (%)	Ferrite (area%)	Pearlite (area%)	Remark
151	2	966	21	927	17	577	63	44	27	73	Present invention example
152	2	967	22	940	18	562	6.1	41	38	62	Present invention example
153	2	988	23	905	19	605	6.4	44	28	72	Present invention example
154	2	977	19	944	18	650	6.5	47	37	63	Present invention example
155	2	990	23	938	16	571	6.3	49	34	66	Present invention example
156	2	956	21	906	17	649	6.8	41	27	73	Present invention example
157	2	963	22	931	15	580	7.0	41	33	67	Present invention example
158	2	985	20	930	19	659	6.8	41	29	71	Present invention example
159	2	973	23	907	16	577	7.0	42	33	67	Present invention example
160	2	950	20	932	15	634	6.9	45	39	61	Present invention example
161	2	981	20	940	15	649	6.6	40	35	65	Present invention example
162	2	981	20	925	19	596	6.1	46	29	71	Present invention example
163	2	968	21	940	15	605	6.1	44	29	71	Present invention example
164	2	974	23	937	16	597	6.4	48	39	61	Present invention example
165	2	999	20	937	17	576	6.5	49	27	73	Present invention example
166	2	960	22	900	18	566	6.9	43	35	65	Present invention example
167	2	970	20	911	18	549	6.5	41	27	73	Present invention example
168	2	999	23	942	19	631	6.2	42	30	70	Present invention example
169	2	970	20	942	16	630	6.8	46	24	76	Present invention example
170	2	964	22	925	16	563	6.3	48	38	62	Present invention example
171	2	992	20	930	16	651	6.9	43	31	69	Present invention example
172	2	953	19	943	16	625	6.5	41	34	66	Present invention example
173	2	989	20	932	15	636	6.2	47	33	67	Present invention example
174	2	977	21	904	16	652	6.1	44	36	64	Present invention example
175	2	981	21	936	19	655	7.0	42	37	63	Present invention example
176	2	989	19	900	18	573	6.7	43	38	62	Present invention example
177	2	966	20	933	16	632	6.8	45	38	62	Present invention example
178	2	991	22	905	19	584	6.8	44	37	63	Present invention example
179	2	951	22	941	is	625	6.7	45	37	63	Present invention example
180	2	999	19	933	16	561	6.9	45	39	61	Present invention example

[Table 2G]

Steel sheet No.	Steel No.	Finish rolling				Coiling	Steel sheet for hot stamping				Remark
Steel sheet No.	Steel No.	Rolling temperature of rolling one pass before final pass (°C)	Rolling reduction of rolling one pass before final pass (%)	Rolling temperature of final pass (°C)	Final rolling reduction (%)	Coiling temperature (°C)	Pole density (-)	Number proportion of ferrite containing carbide (%)	Ferrite (area%)	Pearlite (area%)	Remark
181	2	986	19	902	15	606	7.0	46	24	76	Present invention example
182	2	996	20	907	19	614	6.6	42	39	61	Present invention example
183	2	976	19	904	18	595	6.3	45	38	62	Present invention example
184	2	999	20	909	19	632	6.5	49	23	77	Present invention example
185	2	962	21	939	17	641	7.0	44	38	62	Present invention example
186	2	954	20	938	19	648	6.3	47	34	66	Present invention example
187	2	960	21	906	16	592	6.4	41	29	71	Present invention example
188	2	978	21	932	18	548	6.2	47	32	68	Present invention example
189	109	989	22	925	16	651	7.0	44	36	64	Present invention example
190	110	966	21	933	18	573	6.6	46	32	68	Present invention example
191	111	985	19	926	19	606	6.3	41	37	63	Present invention example
192	112	981	20	930	18	620	6.9	50	26	74	Present invention example
193	113	971	22	931	19	629	6.8	47	24	76	Present invention example
194	114	951	19	915	17	581	6.7	44	31	69	Present invention example
195*	115	962	20	911	18	623	12.0	45	24	76	Comparative example

[Table 3A]

Manufacturing No.	Steel sheet No.	Steel No.	Hot stamping				Hot-stamping formed body					Remark
Manufacturing No.	Steel sheet No.	Steel No.	Heating rate (°C/s)	Heating temperature (°C)	Holding time (minutes)	Cooling rate (°C/s)	Average grain size of prior γ grains (µm)	Standard deviation of grain sizes of prior γ grains (µm)	Prior γ grains having average grain size of 0.5 to 3.0 µm (area%)	Tensile strength (MPa)	Maximum bending angle (°)	Remark
1	1	1	6	896	4.5	445	8	0.3	35	2550	36	Present invention example
2	2	2	5	897	4.4	238	5	0.3	39	2550	32	Present invention example
3	3	3	8	914	4.1	257	7	0.4	39	2583	48	Present invention example
4	4	4	6	891	4.4	489	7	0.3	35	2626	32	Present invention example
5	5	5	8	920	4.8	291	10	0.6	40	2167	46	Comparative example
6	6	6	9	916	4.0	347	10	0.5	41	2439	46	Present invention example
7	7	7	8	915	4.7	430	10	1.1	47	2572	37	Present invention example
8	8	8	7	891	4.0	192	7	0.5	44	2440	32	Present invention example
9	9	9	10	893	4.0	193	8	1.2	48	2569	35	Present invention example
10	10	10	10	915	3.6	368	5	0.9	45	2551	27	Present invention example
11	11	11	7	907	3.7	487	9	1.0	45	2578	25	Present invention example
12	12	12	7	901	5.0	351	5	0.8	41	2636	29	Present invention example
13	13	13	10	907	4.8	284	5	1.0	46	2615	18	Comparative example
14	14	14	8	893	3.9	251	7	1.2	49	2563	19	Comparative example
15	15	15	5	904	4.5	237	6	0.5	41	2597	23	Present invention example
16	16	16	9	899	4.8	212	8	0.8	47	2598	45	Present invention example
17	17	17	10	919	4.4	216	9	0.9	48	2577	43	Present invention example
18	18	18	9	899	4.2	163	10	0.7	41	2558	47	Present invention example
19	19	19	5	892	4.6	413	6	1.2	41	2580	34	Present invention example
20	20	20	9	895	4.7	366	9	0.5	47	2570	23	Present invention example
21	21	21	10	905	4.5	334	9	0.6	49	2583	19	Comparative example
22	22	22	5	912	4.5	392	8	0.9	42	2090	42	Comparative example
23	23	23	7	906	4.9	223	5	1.1	43	2466	45	Present invention example
24	24	24	7	891	4.6	430	10	0.6	42	2595	44	Present invention example
25	25	25	10	899	3.8	310	10	1.1	47	2573	43	Present invention example
26	26	26	6	911	3.6	486	7	1.2	47	2609	39	Present invention example
27	27	27	7	903	5.0	293	7	1.0	44	2621	34	Present invention example
28	28	28	7	904	4.2	358	5	0.7	48	2633	28	Present invention example
29	29	29	7	892	3.6	358	5	0.9	41	2625	14	Comparative example
30	30	30	7	890	4.1	178	8	0.6	42	2577	12	Comparative example

[Table 3B]

Manufacturing No.	Steel sheet No.	Steel No.	Hot stamping				Hot-stamping formed body					Remark
Manufacturing No.	Steel sheet No.	Steel No.	Heating rate (°C/s)	Heating temperature (°C)	Holding time (minutes)	Cooling rate (°C/s)	Average grain size of prior γ grains (µm)	Standard deviation of grain sizes of prior γ grains (µm)	Prior γ grains having average grain size of 0.5 to 3.0 µm (area%)	Tensile strength (MPa)	Maximum bending angle (°)	Remark
31	31	31	10	915	4.3	342	7	1.2	46	2589	25	Present invention example
32	32	32	9	905	3.8	481	6	1.1	49	2562	37	Present invention example
33	33	33	8	904	4.9	226	6	0.6	43	2565	48	Present invention example
34	34	34	5	918	4.4	242	5	0.8	49	2571	14	Comparative example
35	35	35	5	900	4.8	306	7	0.5	43	2568	25	Present invention example
36	36	36	9	908	4.5	194	8	0.8	45	2579	38	Present invention example
37	37	37	8	901	4.4	484	6	0.8	41	2571	43	Present invention example
38	38	38	7	906	4.1	244	9	0.9	48	2556	19	Comparative example
39	39	39	7	902	4.9	319	5	0.6	43	2582	24	Present invention example
40	40	40	8	899	3.9	467	9	0.9	40	2579	35	Present invention example
41	41	41	8	905	4.8	374	6	0.9	44	2578	45	Present invention example
42	42	42	8	892	5.0	344	10	1.2	47	2581	19	Comparative example
43	43	43	8	899	4.4	360	7	1.0	48	2586	30	Present invention example
44	44	44	7	911	4.5	422	6	0.9	41	2575	34	Present invention example
45	45	45	8	906	4.9	223	6	0.6	48	2558	48	Present invention example
46	46	46	10	892	4.5	456	5	1.1	48	2582	18	Comparative example
47	47	47	7	902	4.8	460	8	1.0	49	2560	30	Present invention example
48	48	48	6	901	3.6	407	8	1.2	42	2588	45	Present invention example
49	49	49	7	898	4.0	271	7	1.1	45	2560	47	Present invention example
50	50	50	5	911	4.3	456	7	1.0	41	2575	43	Present invention example
51	51	51	9	899	4.5	278	9	0.7	41	2582	39	Present invention example
52	52	52	10	898	5.0	210	8	1.0	41	2553	28	Present invention example
53	53	53	9	891	3.6	390	9	0.8	41	2567	18	Comparative example
54	54	54	9	906	4.6	491	10	1.1	48	2027	48	Comparative example
55	55	55	7	901	3.9	441	8	0.6	47	2409	44	Present invention example
56	56	56	5	915	4.9	338	8	0.9	42	2597	48	Present invention example
57	57	57	7	919	4.4	418	8	0.5	43	2560	46	Present invention example
58	58	58	6	907	4.0	429	10	1.0	47	2615	36	Present invention example
59	59	59	9	918	4.5	444	9	0.9	48	2605	39	Present invention example
60	60	60	9	905	3.6	426	10	0.9	47	2619	30	Present invention example

[Table 3C]

Manufacturing No.	Steel sheet No.	Steel No.	Hot stamping				Hot-stamping formed body					Remark
Manufacturing No.	Steel sheet No.	Steel No.	Heating rate (°C/s)	Heating temperature (°C)	Holding time (minutes)	Cooling rate (°C/s)	Average grain size of prior γ grains (µm)	Standard deviation of grain sizes of prior γ grains (µm)	Prior γ grains having average grain size of 0.5 to 3.0 µm (area%)	Tensile strength (MPa)	Maximum bending angle (°)	Remark
61	61	61	7	897	3.6	357	6	1.1	40	2640	15	Comparative example
62	62	62	10	918	4.3	321	7	0.6	46	2601	15	Comparative example
63	63	63	7	890	4.6	247	6	1.1	47	2636	22	Present invention example
64	64	64	5	917	4.9	357	7	0.9	41	2632	37	Present invention example
65	65	65	9	893	3.9	234	6	0.7	46	2415	38	Present invention example
66	66	66	10	901	4.8	263	10	1.1	48	2610	19	Comparative example
67	67	67	9	916	4.2	256	9	1.2	47	2643	27	Present invention example
68	68	68	10	910	4.4	271	7	0.5	46	2608	35	Present invention example
69	69	69	8	902	4.4	169	5	1.2	42	2466	36	Present invention example
70	70	20	5	904	4.8	412	7	1.2	47	2647	12	Comparative example
71	71	71	10	897	3.9	290	7	0.8	49	2617	21	Present invention example
72	72	72	6	918	4.8	384	6	0.9	41	2646	40	Present invention example
73	73	73	7	899	4.6	284	6	1.1	45	2521	32	Present invention example
74	74	74	5	906	4.7	342	7	0.5	46	2629	12	Comparative example
75	75	75	5	913	4.6	482	5	1.0	42	2612	26	Present invention example
76	76	76	5	918	4.7	230	5	0.7	48	2626	36	Present invention example
77	77	77	8	909	4.0	410	5	0.8	42	2635	36	Present invention example
78	78	78	8	890	5.0	492	10	0.5	43	2639	12	Comparative example
79	79	79	9	900	4.0	457	5	1.2	48	2622	24	Present invention example
80	80	80	10	894	3.6	305	7	0.5	47	2637	40	Present invention example
81	81	81	8	908	4.1	373	8	0.5	48	2614	39	Present invention example
82	82	82	5	918	4.2	437	10	0.7	41	2628	15	Comparative example
83	83	83	6	907	4.6	377	6	0.7	49	2605	24	Present invention example
84	84	84	5	918	4.5	283	9	0.5	47	2613	31	Present invention example
85	85	85	7	898	4.2	404	7	0.9	46	2643	16	Comparative example
86	86	86	5	891	3.8	271	10	1.2	41	2619	27	Present invention example
87	87	87	6	912	3.6	289	5	0.6	44	2631	40	Present invention example
88	88	88	10	916	3.9	396	8	0.7	40	2601	14	Comparative example
89	89	89	8	909	4.3	357	5	0.8	44	2647	25	Present invention example
90	90	90	8	895	3.5	335	8	0.5	46	2603	36	Present invention example

[Table 3D]

Manufacturing No.	Steel sheet No.	Steel No.	Hot stamping				Hot-stamping formed body					Remark
Manufacturing No.	Steel sheet No.	Steel No.	Heating rate (°C/s)	Heating temperature (°C)	Holding time (minutes)	Cooling rate (°C/s)	Average grain size of prior γ grains (µm)	Standard deviation of grain sizes of prior γ grains (µm)	Prior γ grains having average grain size of 0.5 to 3.0 µm (area%)	Tensile strength (MPa)	Maximum bending angle (°)	Remark
91	91	21	10	900	3.9	332	7	1.2	49	2603	14	Comparative example
92	92	92	7	892	4.8	302	9	0.5	49	2629	25	Present invention example
93	93	93	7	902	4.9	258	7	0.5	41	2607	38	Present invention example
94	94	94	6	893	4.0	357	8	1.0	44	2601	16	Comparative example
95	95	95	9	900	4.5	390	8	0.6	40	2613	29	Present invention example
96	96	96	6	893	5.0	319	8	1.2	45	2644	32	Present invention example
97	97	97	9	890	4.0	250	10	0.5	45	2608	11	Comparative example
98	98	98	9	908	4.3	343	6	1.0	42	2612	23	Present invention example
99	99	99	8	902	4.7	154	9	1.2	44	2606	40	Present invention example
100	100	100	7	898	3.8	159	5	1.0	45	2610	17	Comparative example
101	101	101	9	909	4.5	489	7	0.5	41	2605	30	Present invention example
102	102	102	10	902	3.8	435	6	0.6	44	2605	39	Present invention example
103	103	103	7	893	4.6	493	10	1.2	45	2648	12	Comparative example
104	104	104	10	913	4.2	349	9	0.5	45	2638	27	Present invention example
105	105	105	6	908	3.5	266	10	0.5	43	2636	34	Present invention example
106	106	106	5	908	4.6	241	10	0.9	49	2618	15	Comparative example
107	107	107	10	920	3.8	302	5	0.5	47	2602	30	Present invention example
108	108	108	10	893	3.7	242	8	0.8	48	2640	38	Present invention example
109	109	2	5	891	4.2	222	9	2.4	46	2595	17	Comparative example
110	110	2	7	890	4.8	231	8	0.6	45	2551	21	Present invention example
111	111	2	6	891	3.8	249	6	1.2	46	2572	31	Present invention example
112	112	2	10	897	4.8	436	18	0.8	42	2562	38	Present invention example
113	113	2	10	897	4.5	407	14	0.9	40	2556	32	Present invention example
114	114	2	10	898	4.2	387	13	1.1	44	2575	37	Present invention example
115	115	2	7	914	4.9	402	18	1.1	40	2597	31	Present invention example
116	116	2	8	897	5.0	360	22	0.6	47	2594	29	Present invention example
117	117	2	6	910	43	363	34	0.9	48	2588	16	Comparative example
118	118	2	9	919	3.6	371	3	0.9	46	2560	11	Comparative example
119	119	2	10	908	4.7	258	23	0.8	46	2558	32	Present invention example
120	120	2	8	894	4.6	424	7	1.0	45	2588	48	Present invention example

[Table 3E]

Manufacturing No.	Steel sheet No.	Steel No.	Hot stamping				Hot-stamping formed body					Remark
Manufacturing No.	Steel sheet No.	Steel No.	Heating rate (°C/s)	Heating temperature (°C)	Holding time (minutes)	Cooling rate (°C/s)	Average grain size of prior γ grains (µm)	Standard deviation of grain sizes of prior γ grains (µm)	Prior γ grains having average grain size of 0.5 to 3.0 µm (area%)	Tensile strength (MPa)	Maximum bending angle (°)	Remark
121	121	2	6	890	3.6	408	20	0.6	48	2586	34	Present invention example
122	122	2	6	920	4.1	494	15	0.5	44	2580	33	Present invention example
123	123	2	7	919	3.7	361	25	0.7	49	2556	30	Present invention example
124	124	2	6	909	4.5	387	3	0.9	44	2581	17	Comparative example
125	125	2	7	913	4.0	399	13	3.2	85	2560	16	Comparative example
126	126	2	6	900	3.8	162	20	13	57	2592	24	Present invention example
127	127	2	6	914	4.4	414	18	0.6	47	2576	40	Present invention example
128	128	2	10	911	3.9	181	17	0.9	48	2595	34	Present invention example
129	129	2	6	897	3.8	193	17	0.3	35	2596	42	Present invention example
130	130	2	7	903	4.4	217	12	0.7	43	2595	40	Present invention example
131	131	2	6	903	4.1	392	13	1.0	47	2591	36	Present invention example
132	132	2	10	894	4.4	265	12	1.5	55	2553	26	Present invention example
133	133	2	5	918	3.7	246	38	3.0	66	2564	12	Comparative example
134	134	2	9	908	4.1	384	6	0.4	32	2596	45	Present invention example
135	135	2	9	891	4.5	483	9	0.3	32	2579	47	Present invention example
136	136	2	9	892	3.5	251	10	0.4	31	2568	48	Present invention example
137	137	2	8	919	3.9	477	10	0.3	32	2567	48	Present invention example
138	138	2	9	913	4.4	446	9	0.8	41	2588	34	Present invention example
139	139	2	10	900	4.6	467	9	1.6	55	2554	27	Present invention example
140	140	2	8	914	3.5	233	10	24	71	2556	14	Comparative example
141	141	2	5	896	4.9	273	17	2.3	65	2587	13	Comparative example
142	142	2	9	898	3.8	298	18	1.6	56	2569	30	Present invention example
143	143	2	5	912	5.0	293	18	0.6	45	2571	38	Present invention example
144	144	2	9	890	3.8	383	15	0.3	39	2599	45	Present invention example
145	145	2	9	903	3.8	295	12	0.2	36	2582	45	Present invention example
146	146	2	5	901	4.8	347	19	0.9	42	2580	40	Present invention example
147	147	2	10	895	4.7	383	16	1.5	54	2578	30	Present invention example
148	148	2	6	904	5.0	498	11	3.2	73	2574	18	Comparative example
149	149	2	9	894	4.1	484	10	1.0	44	2564	37	Present invention example
150	150	2	10	896	3.5	361	6	0.6	42	2556	36	Present invention example

[Table 3F]

Manufacturing No.	Steel sheet No.	Steel No.	Hot stamping				Hot-stamping formed body					Remark
Manufacturing No.	Steel sheet No.	Steel No.	Heating rate (°C/s)	Heating temperature (°C)	Holding time (minutes)	Cooling rate (°C/s)	Average grain size of prior γ grains (µm)	Standard deviation of grain sizes of prior γ grains (µm)	Prior γ grains having average grain size of 0.5 to 3.0 µm (area%)	Tensile strength (MPa)	Maximum bending angle (°)	Remark
151	151	2	10	895	4.6	434	9	0.7	47	2563	32	Present invention example
152	152	2	10	920	4.6	247	9	0.5	43	2580	33	Present invention example
153	153	2	7	902	4.6	333	6	1.1	49	2584	36	Present invention example
154	154	2	6	898	4.1	244	8	0.8	41	2575	36	Present invention example
155	155	2	10	891	3.8	169	6	0.9	45	2569	36	Present invention example
156	156	2	8	900	4.3	354	7	0.7	48	2560	36	Present invention example
157	157	2	7	901	4.0	439	6	0.9	45	2573	33	Present invention example
158	158	2	10	906	3.5	331	5	1.2	41	2586	35	Present invention example
159	159	2	955	902	4.9	256	8	0.6	48	2636	32	Present invention example
160	160	2	498	893	3.6	324	8	1.1	45	2611	37	Present invention example
161	161	2	126	913	3.7	392	6	1.0	46	2588	36	Present invention example
162	162	2	47	917	3.5	327	9	0.7	45	2583	32	Present invention example
163	163	2	11	894	3.5	461	5	0.8	42	2560	33	Present invention example
164	164	2	3	909	4.3	314	6	0.5	44	2545	31	Present invention example
165	165	2	0.6	907	3.5	436	9	1.1	41	1976	35	Comparative example
166	166	2	7	1031	3.6	336	7	0.9	44	2187	31	Comparative example
167	167	2	5	994	3.9	160	7	0.6	48	2533	41	Present invention example
168	168	2	7	969	4.5	383	5	1.0	48	2585	49	Present invention example
169	169	2	5	931	4.9	347	6	1.2	41	2576	44	Present invention example
170	170	2	7	904	4.8	364	7	0.7	40	2577	42	Present invention example
171	171	2	6	873	3.5	484	7	1.1	42	2567	43	Present invention example
172	172	2	6	829	4.9	323	5	0.7	40	2554	40	Present invention example
173	173	2	10	811	4.8	439	7	0.8	44	2580	24	Present invention example
174	174	2	7	908	120	191	9	0.8	46	2188	46	Comparative example
175	175	2	9	897	8.1	406	5	0.9	41	2516	48	Present invention example
176	176	2	8	909	7.4	434	10	0.8	40	2565	44	Present invention example
177	177	2	7	904	6.1	469	10	1.1	40	2618	43	Present invention example
178	178	2	10	917	4.4	358	6	1.1	46	2639	46	Present invention example
179	179	2	8	901	2.6	151	10	0.5	46	2589	35	Present invention example
180	180	2	6	896	1.2	411	5	0.6	49	2574	30	Present invention example

[Table 3G]

Manufacturing No.	Steel sheet No.	Steel No.	Hot stamping				Hot-stamping formed body					Remark
Manufacturing No.	Steel sheet No.	Steel No.	Heating rate (°C/s)	Heating temperature (°C)	Holding time (minutes)	Cooling rate (°C/s)	Average grain size of prior γ grains (µm)	Standard deviation of grain sizes of prior γ grains (µm)	Prior γ grains having average grain size of 0.5 to 3.0 µm (area%)	Tensile strength (MPa)	Maximum bending angle (°)	Remark
181	181	2	5	918	4.0	985	9	1.0	49	2606	34	Present invention example
182	182	2	8	920	4.5	561	8	1.1	45	2616	34	Present invention example
183	183	2	8	903	4.2	117	9	0.7	40	2594	38	Present invention example
184	184	2	8	893	4.3	19	7	0.7	41	2483	33	Present invention example
185	185	2	5	895	4.9	3	10	0.6	46	2167	34	Comparative example
186	186	2	7	916	4.7	208	8	0.9	45	2563	31	Present invention example
187	187	2	7	910	3.6	151	9	0.5	43	2576	40	Present invention example
188	188	2	6	891	4.0	182	9	3.1	65	2581	18	Comparative example
189	189	109	7	904	4.9	368	6	1.7	71	2620	31	Present invention example
190	190	110	9	903	4.8	251	6	1.6	77	2609	32	Present invention example
191	191	111	10	909	3.5	366	8	1.9	78	2611	35	Present invention example
192	192	112	11	891	3.5	241	8	0.9	50	2611	39	Present invention example
193	193	113	10	895	3.7	239	7	0.8	48	2640	38	Present invention example
194	194	114	7	900	4.8	314	5	0.6	42	2588	25	Present invention example
195	195*	115	7	904	4.4	381	3	0.8	45	2579	17	Comparative example

From Table 3A to Table 3G, it is found that the hot-stamping formed bodies according to the examples of the present invention have high strength and excellent bendability. Meanwhile, it can be seen that in the hot-stamping formed bodies according to comparative examples, one of the properties deteriorated.

[Industrial Applicability]

Claims

A steel sheet for hot stamping consisting of, as a chemical composition, by mass%:
C: more than 0.40% and 0.70% or less;

Si: 0.010% to 1.30%;

Mn: 0.10% to 0.60%;

P: 0.100% or less;

S: 0.0100% or less;

N: 0.0140% or less;

0: 0.0200% or less;

Al: 0.0010% to 0.500%;

Cr: 0.010% to 0.80%;

Nb: 0% to 0.100%;

Ti: 0% to 0.100%;

B: 0% to 0.0100%;

Mo: 0% to 1.00%;

Co: 0% to 2.00%;

Ni: 0% or more and less than 3.00%;

Cu: 0% to 1.00%;

V: 0% to 1.00%;

W: 0% to 1.000%;

Ca: 0% to 0.010%;

Mg: 0% to 1.000%;

REM: 0% to 1.000%;

Sb: 0% to 1.000%;

Zr: 0% to 1.000%;

Sn: 0% to 1.000%;

As: 0% to 0.100%; and

a remainder of Fe and impurities,

wherein the steel sheet for hot stamping has a microstructure in which an average value of pole densities of ferrite in an orientation group consisting of {100}<011> to {223}<110> is 10.0 or less,

in entire ferrite, a number proportion of ferrite containing a carbide having an equivalent circle diameter of 0.2 µm or more in grains is 20% or more, and

an area ratio of pearlite is 10% to 90% and an area ratio of ferrite is 10% to 90%.
The steel sheet for hot stamping according to claim 1, wherein the steel sheet for hot stamping contains, as the chemical composition, by mass%, one or more selected from the group consisting of:
Nb: 0.001% to 0.100%,

Ti: 0.010% to 0.100%,

B: 0.0015% to 0.0100%,

Mo: 0.05% to 1.00%,

Co: 0.05% to 2.00%,

Ni: 0.01% or more and less than 3.00%,

Cu: 0.01% to 1.00%,

V: 0.01% to 1.00%,

W: 0.001% to 1.000%,

Ca: 0.001% to 0.010%,

Mg: 0.001% to 1.000%,

REM: 0.001% to 1.000%,

Sb: 0.005% to 1.000%,

Zr: 0.001% to 1.000%,

Sn: 0.001% to 1.000%, and

As: 0.001% to 0.100%.
A hot-stamping formed body consisting of, as a chemical composition, by mass%:
C: more than 0.40% and 0.70% or less;

Si: 0.010% to 1.30%;

Mn: 0.10% to 0.60%;

P: 0.100% or less;

S: 0.0100% or less;

N: 0.0140% or less;

O: 0.0200% or less;

Al: 0.0010% to 0.500%;

Cr: 0.010% to 0.80%;

Nb: 0% to 0.100%;

Ti: 0% to 0.100%;

B: 0% to 0.0100%;

Mo: 0% to 1.00%;

Co: 0% to 2.00%;

Ni: 0% or more and less than 3.00%;

Cu: 0% to 1.00%;

V: 0% to 1.00%;

W: 0% to 1.000%;

Ca: 0% to 0.010%;

Mg: 0% to 1.000%;

REM: 0% to 1.000%;

Sb: 0% to 1.000%;

Zr: 0% to 1.000%;

Sn: 0% to 1.000%;

As: 0% to 0.100%; and

a remainder of Fe and impurities,

wherein the hot-stamping formed body has a microstructure in which an average grain size of prior austenite grains is 5 to 25 µm,

a standard deviation of grain sizes of the prior austenite grains is 0.1 to 2.0 µm, and

a tensile strength of the hot-stamping formed body is 2,200 MPa or more.
The hot-stamping formed body according to claim 3, wherein the hot-stamping formed body contains, as the chemical composition, by mass%, one or more selected from the group consisting of:
Nb: 0.001% to 0.100%,

Ti: 0.010% to 0.100%,

B: 0.0015% to 0.0100%,

Mo: 0.05% to 1.00%,

Co: 0.05% to 2.00%,

Ni: 0.01% or more and less than 3.00%,

Cu: 0.01% to 1.00%,

V: 0.01% to 1.00%,

W: 0.001% to 1.000%,

Ca: 0.001% to 0.010%,

Mg: 0.001% to 1.000%,

REM: 0.001% to 1.000%,

Sb: 0.005% to 1.000%,

Zr: 0.001% to 1.000%,

Sn: 0.001% to 1.000%, and

As: 0.001% to 0.100%.
The hot-stamping formed body according to claim 3 or 4, wherein an area ratio of the prior austenite grains having an average grain size of 0.5 to 3.0 µm is 60% or less.