STEEL SHEET FOR HOT STAMPING AND HOT-STAMPING FORMED BODY

- NIPPON STEEL CORPORATION

This steel sheet for hot stamping has a predetermined chemical composition and has a microstructure in which Sα+SGB, which is a total of an area ratio Sα of ferrite and an area ratio SGB of a granular bainite, is 10% or more and less than 50% and SGB/Sα, which is a ratio between the area ratio SGB of the granular bainite and the area ratio Sα of the ferrite, is 0.30 to 0.70. In addition, a hot-stamping formed body manufactured using this steel sheet for hot stamping has a predetermined chemical composition and 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, and a tensile strength of the hot-stamping formed body is 2,200 MPa or more.

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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. 2021-081620, 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 hot-stamping formed body having a tensile strength of 1,900 MPa or more and capable of suppressing low-stress fracture 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. WO2018/134874

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: more than 0.60% and 3.00% or less;
    • P: 0.100% or less;
    • S: 0.0100% or less;
    • N: 0.0130% 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 steel sheet for hot stamping has a microstructure in which Sα+SGB, which is a total of an area ratio Sα of ferrite and an area ratio SGB of a granular bainite, is 10% or more and less than 50%, and
    • SGB/Sα, which is a ratio between the area ratio SGB of the granular bainite and the area ratio Sα of the ferrite, is 0.30 to 0.70.

[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: more than 0.60% and 3.00% or less;
    • P: 0.100% or less;
    • S: 0.0100% or less;
    • N: 0.0130% 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 the above-described hot-stamping formed body can be obtained by controlling an area ratio of ferrite and an area ratio of granular bainite so as to form a desired amount of ferrite and granular bainite in a microstructure of a steel sheet for hot stamping and to have a desired relationship.

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: more than 0.60% and 3.00% or less, P: 0.100% or less, S: 0.0100% or less, N: 0.0130% 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: more than 0.60% and 3.00% or less

Mn stabilizes austenite and improves the hardenability of the steel sheet. When the Mn content is 0.60% or less, sufficient hardenability cannot be obtained. Therefore, the Mn content is set to more than 0.60%. The Mn content is preferably 0.80% or more and more preferably 1.20% or more.

Meanwhile, when the Mn content is more than 3.00%, coarse inclusions are generated and the bendability of the hot-stamping formed body deteriorates. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.20% or less and more preferably 1.80% 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: 0.0130% 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 0.0130%, the formability of the steel sheet significantly deteriorates. Therefore, the N content is set to 0.0130% 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 0.0100% or less and 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%, coarse carbides are generated 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 reliably 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 fusion-nondispersive 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.

The steel sheet for hot stamping according to the present embodiment has a microstructure in which Sα+SGB, which is a total of an area ratio Sα of ferrite and an area ratio SGB of granular bainite, is 10% or more and less than 50%, and SGB/Sα which is a ratio between the area ratio SGB of the granular bainite and the area ratio Sα of the ferrite is 0.30 to 0.70. 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 ¼ depth position of the sheet thickness from the surface (in a region from a ⅛ depth of the sheet thickness from the surface to a ⅜ 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.

“Sα+SGB, which is total of area ratio Sα of ferrite and area ratio SGB of a granular bainite, is 10% or more and less than 50%”

When Sα+SGB, which is the total of the area ratio Sα of the ferrite and the area ratio SGB of the granular bainite, is less than 10%, the prior austenite grains cannot be grain-sized in the hot-stamping formed body, and as a result, it is not possible to obtain a hot-stamping formed body having excellent bendability. Since the solid solubility limits of carbon in ferrite and granular bainite are low, by setting Sα+SGB to 10% or more and setting SGB/Sα to be described below within a desired range, carbon diffuses into ferrite grain boundaries, and a segregation region of carbon is formed at ferrite grain boundaries. During hot stamping, the segregation region of carbon becomes the origin of the prior austenite grains, so that the prior austenite grains are uniformly dispersed and formed. As a result, it is presumed that prior austenite grains can be grain-sized in the hot-stamping formed body. Sα+SGB is preferably 20% or more and more preferably 30% or more.

Meanwhile, when Sα+SGB is 50% or more, segregation of carbon into ferrite grain boundaries is excessively promoted, the generation density of carbides at ferrite grain boundaries increases, and the prior austenite grains cannot be uniformly dispersed and generated after the hot stamping. Sα+SGB is preferably 40% or less.

“SGB/Sα which is a ratio between area ratio SGB of granular bainite and area ratio Sα of ferrite is 0.30 to 0.70”

SGB/Sα is set to 0.30 to 0.70. Since ferrite does not include subgrain boundaries, carbon is less likely to be segregated in the grains than granular bainite. Therefore, by controlling the area ratio of the ferrite and granular bainite to the above range, the amount of segregation of carbon at ferrite grain boundaries can increase. The subgrain boundaries contained in the grains of granular bainite can serve as the segregation origins of carbon and thus function as the origins of prior austenite during hot stamping heating. Accordingly, the average grain size of the prior austenite grains in the hot-stamping formed body can be controlled to 25 μm or less. SGB/Sα is preferably 0.40% or more.

Meanwhile, when SGB/Sα is more than 0.70, the segregation of carbon to subgrain boundaries is excessively promoted, and the distance between the austenite grains becomes short during hot stamping heating. Therefore, the average grain size of the prior austenite grains cannot be controlled to 5 μm or more. Therefore, SGB/Sα is set to 0.70 or less. SGB/Sα is preferably 0.50 or less.

In the microstructure of the steel sheet for hot stamping according to the present embodiment, the remainder in microstructure is one or more of pearlite, martensite, lower bainite, residual austenite, and tempered martensite. The area ratio of the remainder in the microstructure may be set to more than 50% and 90% or less in consideration of the relationship with Sα+SGB.

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 100 μm and between the ⅛ depth of the sheet thickness from the surface and the ⅜ 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 ¼ 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. 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 Sα of the ferrite and the area ratio SGB of the granular bainite are 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 0.4° is determined as ferrite, and a region where the average crystal orientation difference in grains is 0.4° or more and 3.0° or less is determined as granular bainite. An area ratio of the region determined as ferrite is calculated, so that the area ratio of ferrite is obtained. The area ratio of the granular bainite is obtained by calculating the area ratio of the region determined to be the granular bainite.

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 ¼ depth position (the region from the ⅛ depth of the sheet thickness from the surface to the ⅜ 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 ⅛ depth of the sheet thickness from the surface to the ⅜ depth of the sheet thickness from the surface at an arbitrary position of the sample cross section in the longitudinal direction so that the ¼ 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 50 μm and between the ⅛ depth of the sheet thickness from the surface and the ⅜ 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 ¼ 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 1500.

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 ⅛ depth of the sheet thickness from the surface and the ⅜ 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 maximum bending angle is more preferably 30° or more or 40° 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: φ30 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, in order to obtain the steel sheet for hot stamping having the above-described microstructure, the final rolling reduction of the finish rolling in the hot rolling is preferably set to 40% to 80%. Normally, the final rolling reduction of the finish rolling is less than 10%, but in the present embodiment, it is preferable to set the final rolling reduction to be higher than a normal final rolling reduction.

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 addition, in a casting step, the steel piece after solidification may be rolled at a rolling reduction of 30% to 70% in a temperature range in which a center temperature of a slab is 1,200° C. or higher and equal to or lower than a solidus temperature. As a result, the segregation of Mn is relaxed, which makes it possible to improve the bendability of the hot-stamping formed body. The solidus temperature can be obtained from Expression (1).


Solidus temperature (° C.)=1536−(415.5×% C+12.3×% Si+6.8×% Mn+124.5×% P+183.9×% S+4.3×% Ni+1.4×% Cr+4.1×% Al)  (1)

In Expression (1), % C, % Si, % Mn, % P, % S, % Ni, % Cr, and % Al mean the content (mass %) of each element.

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 finish rolling is preferably performed so that the rolling reduction (final rolling reduction) in the final pass of the finish rolling becomes 40% or more. When the sheet thickness before the final pass of the finish rolling is to and the sheet thickness after the final pass of the finish rolling is ti, the final rolling reduction can be represented by {(t0−t1)/t0}×100 (%).

By setting the final rolling reduction of the finish rolling to 40% to 80%, the prior austenite grains are refined, and the origins of ferrite and granular bainite increase. As a result, in the microstructure of the steel sheet for hot stamping, Sα+SGB and SGB/Sα can be set within desired ranges. When the final rolling reduction of the finish rolling is less than 40%, in the microstructure of the steel sheet for hot stamping, Sα+SGB and SGB/Sα cannot be set within desired ranges. Therefore, the final rolling reduction of the finish rolling is preferably set to 40% or more. The final rolling reduction of the finish rolling is preferably 50% or more. Meanwhile, when the final rolling reduction of the finish rolling is more than 80%, SGB/Sα cannot be controlled to 0.70 or less. Therefore, the final rolling reduction of the finish rolling is preferably set to 80% or less. The final rolling reduction is more preferably less than 70%.

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 1,200° 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 higher than 750° C., ferritic transformation is excessively promoted, and SGB+Sα becomes 50% or more and SGB/Sα becomes less than 0.30. The coiling temperature is preferably 700° C. or lower and more preferably 660° C. or lower.

In addition, the coiling temperature is preferably 400° C. or higher. When the coiling temperature is lower than 400° C., the formation of granular bainite is suppressed, and SGB/Sα becomes less than 0.30. The coiling temperature is preferably 450° C. or higher and more preferably 530° C. or higher.

In addition, after the finish rolling (after the completion of hot rolling), the cooling is preferably performed after 2.5 seconds or longer elapses. The cooling mentioned here is cooling that does not include air cooling and has an average cooling rate of 50 to 200° C./s. When the time from the finish rolling to the start of cooling is shorter than 2.5 seconds, a desired amount of Sα+SGB may not be obtained.

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° C./s 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 and lower than 1,350° C. for 20 minutes or longer, and then subjected to hot rolling, cooling, and coiling under conditions shown in Tables 2A to 2F, and subjected to cold rolling, hot-rolled sheet annealing, pickling, and plating as necessary. Therefore, steel sheets for hot stamping shown in Table 2A to Table 2F were obtained. The average cooling rate of cooling after the finish rolling to coiling was set to 50 to 200° C./s. In addition, cooling was performed at the above-described average cooling rate after a lapse of 2.5 seconds or longer after the finish rolling. Note that, for Steel sheet No. 172 marked with “*”, after the finish rolling, cooling was performed after 2.0 seconds elapsed.

In addition, for Steel sheet No. 107, in the casting step, the steel piece after solidification was rolled with a rolling reduction of 30% to 70% in a temperature range in which the center temperature of a slab was the solidus temperature or lower.

For Steel sheet No. 108, the heating temperature before the hot rolling was set to 1,350° C.

Steel sheet No. 125 was subjected to hot-rolled sheet annealing of heating and holding in a temperature range of 730° C. or lower.

For Steel sheet No. 126, the cold rolling was not performed.

An electrogalvanized layer was formed on the surface of Steel sheet No. 127.

An electrolytic Zn—Ni alloy plating layer was formed on the surface of Steel sheet No. 128.

A hot-dip galvanized layer was formed on the surface of Steel sheet No. 129.

A hot-dip galvannealed layer was formed on the surface of Steel sheet No. 130.

A hot-dip aluminum plating layer was formed on the surface of Steel sheet No. 131.

A hot-dip Zn—Al alloy plating layer was formed on the surface of Steel sheet No. 132.

A hot-dip Zn—Al—Mg alloy plating layer was formed on the surface of Steel sheet No. 133.

A hot-dip Zn—Al—Mg—Si alloy plating layer was formed on the surface of Steel sheet No. 134.

The obtained steel sheets for hot stamping were subjected to hot stamping under the conditions shown in Tables 3A to 3F to obtain hot-stamping formed bodies shown in Tables 3A to 3F.

For Manufacturing No. 161, a tempering treatment was performed at 150° C. to 600° C. after hot stamping.

For Manufacturing No. 162, 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. 163 was heated to a heating temperature shown in Table 3F, Manufacturing No. 163 was cooled to a temperature range of 250° C. or lower. Thereafter, Manufacturing No. 163 was heated to 900° C. and hot-stamped, and then cooled at the average cooling rate in Table 3D.

In the examples of the present invention shown in Tables 2A to 2F, the remainder in the microstructure was one or more of pearlite, martensite, lower bainite, residual austenite, and tempered martensite, and the total area ratio of these was more than 50% and 90% or less. In addition, in the examples of the present invention shown in Tables 3A to 3F, 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 1A Steel Chemical composition (mass %) remainder Fe and impurity No. C Si Mn P S N O Al Cr Nb Ti B 1 0.46 0.43 1.27 0.007 0.0003 0.0029 0.0010 0.030 0.27 0.019 0.028 0.0021 2 0.44 0.43 1.27 0.007 0.0003 0.0029 0.0010 0.030 0.27 0.019 0.028 0.0021 3 0.46 0.43 2.00 0.007 0.0003 0.0029 0.0010 0.030 0.27 0.019 0.028 0.0021 4 0.46 0.43 0.80 0.007 0.0003 0.0029 0.0010 0.030 0.27 0.019 0.028 0.0021 5 0.36 0.36 1.35 0.009 0.0005 0.0033 0.0008 0.044 0.18 0.015 0.026 0.0023 6 0.41 0.33 1.17 0.012 0.0014 0.0026 0.0014 0.035 0.29 0.022 0.033 0.0025 7 0.43 0.30 1.39 0.011 0.0019 0.0021 0.0010 0.038 0.30 0.017 0.032 0.0021 8 0.46 0.25 1.40 0.008 0.0006 0.0023 0.0009 0.043 0.22 9 0.48 0.29 1.19 0.012 0.0006 0.0031 0.0011 0.035 0.20 0.024 0.027 0.0018 10 0.55 0.31 1.12 0.008 0.0015 0.0030 0.0014 0.033 0.27 0.021 0.033 0.0020 11 0.62 0.27 1.40 0.010 0.0020 0.0021 0.0012 0.038 0.19 0.019 0.039 0.0019 12 0.67 0.39 1.22 0.012 0.0006 0.0035 0.0011 0.030 0.25 0.025 0.037 0.0027 13 0.73 0.28 1.36 0.008 0.0018 0.0016 0.0010 0.045 0.22 0.027 0.028 0.0025 14 0.44  0.006 1.16 0.009 0.0017 0.0015 0.0008 0.025 0.21 0.028 0.028 0.0023 15 0.44 0.02 1.28 0.011 0.0020 0.0029 0.0010 0.043 0.23 0.023 0.022 0.0024 16 0.44 0.07 1.39 0.011 0.0004 0.0019 0.0008 0.041 0.26 0.028 0.038 0.0028 17 0.44 0.20 1.42 0.010 0.0010 0.0021 0.0011 0.045 0.21 0.016 0.029 0.0023 18 0.46 0.40 1.38 0.008 0.0021 0.0028 0.0011 0.029 0.18 0.029 0.035 0.0019 19 0.44 0.90 1.33 0.008 0.0018 0.0032 0.0012 0.032 0.29 0.020 0.036 0.0021 20 0.44 1.10 1.25 0.012 0.0016 0.0033 0.0009 0.028 0.25 0.027 0.035 0.0026 21 0.46 1.40 1.32 0.009 0.0003 0.0016 0.0008 0.035 0.25 0.027 0.037 0.0023 22 0.45 0.22 0.40 0.009 0.0020 0.0030 0.0012 0.035 0.25 0.019 0.040 0.0018 23 0.44 0.26 0.65 0.009 0.0017 0.0032 0.0010 0.035 0.29 0.025 0.022 0.0022 24 0.44 0.35 1.00 0.010 0.0012 0.0029 0.0012 0.036 0.20 0.020 0.020 0.0024 25 0.45 0.29 1.30 0.012 0.0020 0.0033 0.0015 0.026 0.18 0.026 0.028 0.0020 26 0.44 0.40 1.70 0.012 0.0016 0.0016 0.0011 0.039 0.19 0.020 0.034 0.0023 27 0.45 0.26 2.00 0.008 0.0021 0.0019 0.0009 0.043 0.24 0.030 0.021 0.0019 28 0.45 0.23 2.70 0.009 0.0017 0.0019 0.0013 0.027 0.19 0.016 0.036 0.0025 Steel Chemical composition (mass %) remainder Fe and impurity No. Mo Co Ni Cu V W Ca Mg REM Sb Zr Sn As Remark 1 Present invention steel 2 Present invention steel 3 Present invention steel 4 Present invention steel 5 Comparative steel 6 Present invention steel 7 Present invention steel 8 Present invention steel 9 Present invention steel 10 Present invention steel 11 Present invention steel 12 Present invention steel 13 Comparative steel 14 Comparative steel 15 Present invention steel 16 Present invention steel 17 Present invention steel 18 Present invention steel 19 Present invention steel 20 Present invention steel 21 Comparative steel 22 Comparative steel 23 Present invention steel 24 Present invention steel 25 Present invention steel 26 Present invention steel 27 Present invention steel 28 Present invention steel Underscores indicate scope outside present invention.

TABLE 1B Steel Chemical composition (mass %) remainder Fe and impurity No. C Si Mn P S N O Al Cr Nb Ti B Mo 29 0.46 0.40 3.20 0.011 0.0004 0.0019 0.0013 0.035 0.22 0.021 0.024 0.0028 30 0.46 0.24 1.30 0.150 0.0022 0.0030 0.0014 0.034 0.29 0.021 0.028 0.0024 31 0.46 0.35 1.32 0.090 0.0020 0.0028 0.0008 0.036 0.20 0.016 0.032 0.0028 32 0.44 0.32 1.35 0.050 0.0020 0.0018 0.0014 0.029 0.21 0.025 0.024 0.0028 33 0.44 0.29 1.26 0.010 0.0021 0.0016 0.0015 0.028 0.21 0.021 0.029 0.0021 34 0.47 0.38 1.44 0.009 0.0150 0.0016 0.0010 0.035 0.18 0.028 0.028 0.0024 35 0.45 0.29 1.13 0.011 0.0080 0.0018 0.0011 0.025 0.23 0.028 0.027 0.0019 36 0.47 0.32 1.41 0.008 0.0030 0.0021 0.0009 0.032 0.22 0.022 0.020 0.0028 37 0.47 0.31 1.31 0.008 0.0008 0.0030 0.0012 0.033 0.25 0.025 0.022 0.0021 38 0.44 0.40 1.24 0.010 0.0014 0.0140 0.0013 0.045 0.26 0.030 0.021 0.0024 39 0.47 0.25 1.22 0.011 0.0011 0.0080 0.0008 0.034 0.25 0.030 0.021 0.0028 40 0.45 0.40 1.43 0.010 0.0003 0.0050 0.0013 0.045 0.29 0.028 0.027 0.0018 41 0.45 0.31 1.16 0.008 0.0011 0.0030 0.0010 0.031 0.21 0.029 0.028 0.0019 42 0.47 0.23 1.19 0.010 0.0008 0.0030 0.0250 0.040 0.18 0.026 0.036 0.0023 43 0.46 0.29 1.29 0.008 0.0009 0.0019 0.0150 0.037 0.18 0.026 0.035 0.0026 44 0.45 0.21 1.25 0.008 0.0017 0.0033 0.0080 0.041 0.29 0.027 0.022 0.0022 45 0.46 0.26 1.12 0.010 0.0016 0.0025 0.0030 0.041 0.26 0.020 0.036 0.0021 46 0.44 0.26 1.25 0.012 0.0008 0.0028 0.0015  0.0008 0.22 0.020 0.020 0.0019 47 0.44 0.27 1.28 0.011 0.0008 0.0022 0.0011 0.005 0.29 0.018 0.032 0.0027 48 0.47 0.32 1.14 0.012 0.0016 0.0028 0.0010 0.020 0.30 0.023 0.030 0.0023 49 0.45 0.20 1.18 0.010 0.0004 0.0015 0.0014 0.040 0.20 0.018 0.030 0.0027 50 0.44 0.35 1.37 0.012 0.0019 0.0034 0.0013 0.100 0.26 0.015 0.039 0.0025 51 0.47 0.37 1.20 0.009 0.0012 0.0024 0.0013 0.250 0.29 0.017 0.036 0.0020 52 0.47 0.32 1.32 0.010 0.0021 0.0020 0.0008 0.390 0.24 0.025 0.028 0.0019 53 0.44 0.28 1.14 0.012 0.0015 0.0019 0.0010 0.510 0.29 0.030 0.031 0.0028 54 0.45 0.35 1.19 0.010 0.0016 0.0027 0.0011 0.030  0.008 0.023 0.034 0.0018 55 0.45 0.31 1.35 0.008 0.0007 0.0032 0.0008 0.035  0.012 0.016 0.039 0.0027 56 0.46 0.26 1.22 0.010 0.0022 0.0019 0.0014 0.039 0.18 0.023 0.030 0.0019 Steel Chemical composition (mass %) remainder Fe and impurity No. Co Ni Cu V W Ca Mg REM Zr Sn As Remark 29 Comparative steel 30 Comparative steel 31 Present invention steel 32 Present invention steel 33 Present invention steel 34 Comparative steel 35 Present invention steel 36 Present invention steel 37 Present invention steel 38 Comparative steel 39 Present invention steel 40 Present invention steel 41 Present invention steel 42 Comparative steel 43 Present invention steel 44 Present invention steel 45 Present invention steel 46 Comparative steel 47 Present invention steel 48 Present invention steel 49 Present invention steel 50 Present invention steel 51 Present invention steel 52 Present invention steel 53 Comparative steel 54 Comparative steel 55 Present invention steel 56 Present invention steel Underscores indicate scope outside present invention.

TABLE 1C Steel Chemical composition (mass %) remainder Fe and impurity No. C Si Mn P S N O Al Cr Nb Ti B Mo 57 0.46 0.20 1.36 0.010 0.0003 0.0020 0.0009 0.030 0.27 0.026 0.040 0.0020 58 0.46 0.25 1.35 0.011 0.0008 0.0026 0.0009 0.044 0.34 0.027 0.026 0.0025 59 0.47 0.29 1.36 0.008 0.0021 0.0031 0.0014 0.038 0.55 0.019 0.027 0.0027 60 0.45 0.29 1.13 0.012 0.0019 0.0034 0.0014 0.044 0.67 0.022 0.033 0.0020 61 0.47 0.28 1.12 0.009 0.0018 0.0029 0.0008 0.028 0.85 0.021 0.038 0.0026 62 0.44 0.33 1.41 0.008 0.0017 0.0017 0.0010 0.043 0.28 0.130 0.028 0.0025 63 0.45 0.22 1.12 0.011 0.0015 0.0031 0.0013 0.036 0.29 0.080 0.036 0.0022 64 0.47 0.39 1.19 0.008 0.0008 0.0022 0.0010 0.028 0.21 0.019 0.039 0.0020 65 0.46 0.36 1.32 0.012 0.0015 0.0015 0.0009 0.034 0.24 0.028 0.0026 66 0.44 0.34 1.39 0.010 0.0022 0.0026 0.0009 0.030 0.19 0.015 0.150 0.0020 67 0.47 0.34 1.25 0.012 0.0007 0.0022 0.0015 0.036 0.25 0.024 0.090 0.0020 68 0.46 0.37 1.12 10.011 0.0021 0.0022 0.0008 0.029 0.27 0.027 0.028 0.0026 69 0.46 0.21 1.32 0.010 0.0005 0.0033 0.0011 0.033 0.28 0.025 0.0025 70 0.46 0.36 1.39 0.010 0.0012 0.0032 0.0013 0.027 0.27 0.015 0.020 0.0170 71 0.44 0.30 1.17 0.011 0.0017 0.0034 0.0009 0.039 0.22 0.015 0.037 0.0090 72 0.47 0.35 1.40 0.010 0.0014 0.0026 0.0015 0.026 0.18 0.018 0.024 0.0020 73 0.45 0.20 1.16 0.011 0.0015 0.0034 0.0008 0.041 0.28 0.021 0.038 74 0.45 0.28 1.33 0.012 0.0009 0.0035 0.0013 0.032 0.27 0.022 0.038 0.0019 1.20 75 0.45 0.29 1.41 0.010 0.0007 0.0024 0.0015 0.041 0.28 0.018 0.037 0.0022 0.88 76 0.45 0.39 1.25 0.008 0.0014 0.0032 0.0015 0.033 0.26 0.015 0.039 0.0021 0.10 77 0.47 0.38 1.23 0.012 0.0017 0.0028 0.0009 0.040 0.26 0.018 0.032 0.0027 78 0.46 0.38 1.31 0.009 0.0022 0.0028 0.0012 0.030 0.26 0.028 0.023 0.0024 79 0.45 0.23 1.36 0.008 0.0022 0.0020 0.0011 0.042 0.18 0.026 0.040 0.0025 80 0.46 0.37 1.43 0.009 0.0006 0.0029 0.0010 0.038 0.22 0.026 0.025 0.0026 81 0.46 0.40 1.43 0.008 0.0014 0.0017 0.0012 0.042 0.20 0.024 0.025 0.0027 82 0.47 0.36 1.20 0.010 0.0005 0.0026 0.0012 0.037 0.24 0.029 0.028 0.0028 83 0.46 0.20 1.26 0.012 0.0004 0.0015 0.0012 0.034 0.22 0.026 0.037 0.0026 Steel Chemical composition (mass %) remainder Fe and impurity No. Co Ni Cu V W Ca Mg REM Sb Zr Sn As Remark 57 Present invention steel 58 Present invention steel 59 Present invention steel 60 Present invention steel 61 Comparative steel 62 Comparative steel 63 Present invention steel 64 Present invention steel 65 Present invention steel 66 Comparative steel 67 Present invention steel 68 Present invention steel 69 Present invention steel 70 Comparative steel 71 Present invention steel 72 Present invention steel 73 Present invention steel 74 Comparative steel 75 Present invention steel 76 Present invention steel 77 2.20 Comparative steel 78 1.87 Present invention steel 79 0.20 Present invention steel 80 3.20 Comparative steel 81 2.77 Present invention steel 82 0.20 Present invention steel 83 1.30 Comparative steel Underscores indicate scope outside present invention.

TABLE 1D Steel Chemical composition (mass %) remainder Fe and impurity No. C Si Mn P S N O Al Cr Nb Ti B Mo Co Ni Cu V W Ca Mg REM Sb Zr Sn As Remark 84 0.47 0.25 1.21 0.009 0.0018 0.0027 0.0013 0.025 0.28 0.020 0.022 0.0020 0.85 Present invention steel 85 0.45 0.39 1.42 0.010 0.0004 0.0033 0.0011 0.028 0.23 0.028 0.036 0.0018 0.10 Present invention steel 86 0.45 0.36 1.11 0.012 0.0005 0.0019 0.0011 0.029 0.20 0.030 0.032 0.0025 1.10 Comparative steel 87 0.44 0.37 1.42 0.009 0.0012 0.0029 0.0010 0.035 0.23 0.020 0.026 0.0028 0.79 Present invention steel 88 0.46 0.33 1.16 0.009 0.0021 0.0021 0.0014 0.039 0.22 0.015 0.027 0.0023 0.05 Present invention steel 89 0.46 0.24 1.12 0.012 0.0018 0.0024 0.0011 0.043 0.20 0.028 0.021 0.0027 1.300 Comparative steel 90 0.45 0.26 1.20 0.008 0.0014 0.0018 0.0008 0.039 0.20 0.022 0.028 0.0026 0.910 Present invention steel 91 0.45 0.20 1.18 0.010 0.0010 0.0023 0.0013 0.040 0.26 0.015 0.037 0.0027 0.006 Present invention steel 92 0.46 0.34 1.12 0.011 0.0004 0.0029 0.0010 0.044 0.28 0.030 0.039 0.0025 0.016 Comparative steel 93 0.44 0.35 1.36 0.008 0.0003 0.0034 0.0012 0.039 0.30 0.022 0.035 0.0022 0.001 Present invention steel 94 0.46 0.38 1.35 0.011 0.0008 0.0027 0.0008 0.026 0.29 0.015 0.034 0.0025 0.003 Present invention steel 95 0.44 0.29 1.45 0.011 0.0017 0.0025 0.0008 0.034 0.28 0.016 0.031 0.0019 1.300 Comparative steel 96 0.45 0.32 1.34 0.009 0.0022 0.0033 0.0013 0.031 0.25 0.025 0.035 0.0024 0.950 Present invention steel 97 0.47 0.34 1.43 0.011 0.0017 0.0030 0.0009 0.032 0.18 0.025 0.027 0.0025 0.005 Present invention steel 98 0.44 0.24 1.32 0.008 0.0019 0.0030 0.0012 0.039 0.22 0.028 0.020 0.0026 1.300 Comparative steel 99 0.47 0.25 1.38 0.012 0.0011 0.0024 0.0012 0.036 0.24 0.015 0.022 0.0018 0.860 Present invention steel 100 0.44 0.22 1.20 0.008 0.0009 0.0023 0.0015 0.029 0.30 0.028 0.039 0.0025 0.003 Present invention steel 101 0.45 0.26 1.12 0.008 0.0013 0.0033 0.0010 0.027 0.22 0.023 0.036 0.0021 1.300 Comparative steel 102 0.44 0.40 1.32 0.009 0.0006 0.0035 0.0008 0.043 0.21 0.028 0.037 0.0021 0.920 Present invention steel 103 0.47 0.32 1.44 0.011 0.0010 0.0016 0.0008 0.032 0.21 0.030 0.024 0.0018 0.010 Present invention steel 104 0.44 0.24 1.35 0.010 0.0022 0.0015 0.0011 0.026 0.29 0.021 0.034 0.0018 1.200 Comparative steel 105 0.47 0.29 1.45 0.009 0.0015 0.0018 0.0015 0.043 0.22 0.021 0.023 0.0020 0.890 Present invention steel 106 0.47 0.38 1.21 0.009 0.0012 0.0028 0.0015 0.039 0.19 0.015 0.029 0.0022 0.020 Present invention steel 107 0.46 0.43 2.90 0.007 0.0003 0.0029 0.0010 0.030 0.27 0.019 0.028 0.0021 Present invention steel 108 0.44 0.43 2.92 0.007 0.0003 0.0029 0.0010 0.030 0.27 0.019 0.028 0.0021 Present invention steel 109 0.45 0.43 2.91 0.007 0.0003 0.0029 0.0010 0.030 0.27 0.019 0.028 0.0021 Present invention steel 110 0.47 0.24 1.21 0.011 0.0011 0.0081 0.0008 0.034 0.25 0.030 0.021 0.0028 Present invention steel 111 0.46 0.32 1.42 0.010 0.0010 0.0017 0.0008 0.032 0.21 0.028 0.024 0.0017 0.100 Present invention steel 112 0.46 0.39 1.21 0.009 0.0012 0.0028 0.0015 0.039 0.19 0.015 0.029 0.0022 0.011 Present invention steel Underscores indicate scope outside present invention.

TABLE 2A Steel Finish rolling Coiling sheet Steel Final rolling Coiling Steel sheet for hot stamping No. No. reduction (%) temperature (° C.) Sa (arca %) SGB (area %) Sa + SGB (area %) SGB/Sa Remark 1  1 43 631 13 9 22 0.43 Present invention example 2  2 45 637 19 15 34 0.44 Present invention example 3  3 54 593 12 11 23 0.48 Present invention example 4  4 55 569 17 16 33 0.47 Present invention example 5  5 45 551 10 4 14 0.32 Comparative example 6  6 42 585 6 4 10 0.36 Present invention example 7  7 45 565 9 5 14 0.33 Present invention example 8  8 54 630 10 6 16 0.37 Present invention example 9  9 46 659 11 6 17 0.35 Present invention example 10 10 45 570 12 6 18 0.33 Present invention example 11 11 46 565 10 5 15 0.36 Present invention example 12 12 45 540 8 4 12 0.33 Present invention example 13 13 49 608 10 4 14 0.32 Comparative example 14 14 47 597 11 7 18 0.37 Comparative example 15 15 50 590 7 4 11 0.36 Present invention example 16 16 53 591 6 4 10 0.38 Present invention example 17 17 54 617 8 5 13 0.39 Present invention example 18 18 42 556 12 7 19 0.38 Present invention example 19 19 55 570 10 6 16 0.35 Present invention example 20 20 49 597 9 5 14 0.37 Present invention example 21 21 55 599 12 6 18 0.34 Comparative example 22 22 46 549 8 5 13 0.36 Comparative example 23 23 56 544 13 7 20 0.37 Present invention example 24 24 50 581 10 6 16 0.38 Present invention example 25 25 44 646 8 4 12 0.32 Present invention example 26 26 53 622 12 6 18 0.31 Present invention example 27 27 51 545 11 7 18 0.38 Present invention example 28 28 45 623 9 5 14 0.33 Present invention example 29 29 52 542 10 6 16 0.36 Comparative example 30 30 56 548 8 4 12 0.31 Comparative example Underscores indicate scope outside present invention and indicate that manufacturing conditions are not preferable.

TABLE 2B Steel Finish rolling Coiling sheet Steel Final rolling Coiling Steel sheet for hot stamping No. No. reduction (%) temperature (° C.) Sa (arca %) SGB (area %) Sa + SGB (area %) SGB/Sa Remark 31 31 44 565 7 4 11 0.35 Present invention example 32 32 52 647 11 6 17 0.38 Present invention example 33 33 46 604 7 4 11 0.35 Present invention example 34 34 43 614 7 4 11 0.34 Comparative example 35 35 56 582 7 4 11 0.34 Present invention example 36 36 47 567 7 3 10 0.32 Present invention example 37 37 42 586 8 5 13 0.38 Present invention example 38 38 44 637 11 6 17 0.37 Comparative example 39 39 46 632 7 5 12 0.39 Present invention example 40 40 53 628 11 6 17 0.33 Present invention example 41 41 54 615 10 4 14 0.32 Present invention example 42 42 51 566 7 3 10 0.33 Comparative example 43 43 55 544 7 3 10 0.33 Present invention example 44 44 44 656 12 7 19 0.39 Present invention example 45 45 55 555 8 4 12 0.35 Present invention example 46 46 47 652 7 4 11 0.34 Comparative example 47 47 42 541 13 7 20 0.34 Present invention example 48 48 44 627 12 8 20 0.38 Present invention example 49 49 46 594 8 4 12 0.37 Present invention example 50 50 43 564 10 5 15 0.31 Present invention example 51 51 54 617 12 8 20 0.38 Present invention example 52 52 49 554 12 6 18 0.31 Present invention example 53 53 52 563 11 6 17 0.34 Comparative example 54 54 54 654 13 6 19 0.32 Comparative example 55 55 53 656 7 4 11 0.36 Present invention example 56 56 48 595 8 5 13 0.35 Present invention example 57 57 52 621 7 4 10 0.35 Present invention example 58 58 47 621 11 6 17 0.34 Present invention example 59 59 54 593 9 6 15 0.37 Present invention example 60 60 49 654 9 5 14 0.33 Present invention example Underscores indicate scope outside present invention and indicate that manufacturing conditions are not preferable.

TABLE 2C Steel Finish rolling Coiling sheet Steel Final rolling Coiling Steel sheet for hot stamping No. No. reduction (%) temperature (° C.) Sa (arca %) SGB (area %) Sa + SGB (area %) SGB/Sa Remark 61 61 44 565 12 5 17 0.31 Comparative example 62 62 43 554 13 7 20 0.37 Comparative example 63 63 56 615 9 5 14 0.38 Present invention example 64 64 42 638 12 6 18 0.35 Present invention example 65 65 49 606 13 6 19 0.32 Present invention example 66 66 42 545 9 6 15 0.39 Comparative example 67 67 56 596 11 6 17 0.35 Present invention example 68 68 45 587 10 5 15 0.31 Present invention example 69 69 42 583 12 6 18 0.33 Present invention example 70 70 47 580 8 3 11 0.31 Comparative example 71 71 42 605 12 7 19 0.39 Present invention example 72 72 49 616 8 4 12 0.31 Present invention example 73 73 43 541 8 4 12 0.36 Present invention example 74 74 53 595 8 4 12 0.32 Comparative example 75 75 45 642 12 6 18 0.32 Present invention example 76 76 48 594 7 4 11 0.34 Present invention example 77 77 53 568 13 7 20 0.37 Comparative example 78 78 52 540 12 8 20 0.38 Present invention example 79 79 53 540 11 6 17 0.34 Present invention example 80 80 42 635 10 6 16 0.36 Comparative example 81 81 52 643 10 6 16 0.37 Present invention example 82 82 49 561 11 7 18 0.39 Present invention example 83 83 47 585 9 6 15 0.37 Comparative example 84 84 42 570 7 4 11 0.35 Present invention example 85 85 54 652 7 4 11 0.35 Present invention example 86 86 44 584 8 3 11 0.31 Comparative example 87 87 43 560 10 6 16 0.39 Present invention example 88 88 54 631 8 4 12 0.32 Present invention example 89 89 44 651 8 3 11 0.31 Comparative example 90 90 50 645 7 5 12 0.39 Present invention example Underscores indicate scope outside present invention and indicate that manufacturing conditions are not preferable.

TABLE 2D Steel Finish rolling Coiling sheet Steel Final rolling Coiling Steel sheet for hot stamping No. No. reduction (%) temperature (° C.) Sa (arca %) SGB (area %) Sa + SGB (area %) SGB/Sa Remark 91 91  55 624 8 4 12 0.34 Present invention example 92 92  52 598 8 5 13 0.37 Comparative example 93 93  50 648 8 4 12 0.37 Present invention example 94 94  49 555 8 5 13 0.38 Present invention example 95 95  51 557 12 8 20 0.39 Comparative example 96 96  48 578 12 7 19 0.35 Present invention example 97 97  44 611 10 6 16 0.35 Present invention example 98 98  51 570 13 7 20 0.35 Comparative example 99 99  52 556 7 3 10 0.33 Present invention example 100 100  47 545 12 5 17 0.32 Present invention example 101 101  50 560 12 6 18 0.35 Comparative example 102 102  50 579 10 5 15 0.32 Present invention example 103 103  49 594 7 4 10 0.35 Present invention example 104 104  51 619 11 5 16 0.31 Comparative example 105 105  56 631 10 5 15 0.36 Present invention example 106 106  55 628 10 6 16 0.38 Present invention example 107 1 43 589 7 4 10 0.35 Present invention example 108 1 45 614 12 7 19 0.39 Present invention example 109 1 84 550 3 40 43 0.94 Comparative example 110 1 75 629 19 27 46 0.59 Present invention example 111 1 62 638 21 23 44 0.53 Present invention example 112 1 58 640 19 15 34 0.43 Present invention example 113 1 51 549 12 9 21 0.41 Present invention example 114 1 46 586 12 8 20 0.39 Present invention example 115 1 42 659 8 4 12 0.32 Present invention example 116 1 35 640 6 2  8 0.21 Comparative example 117 1 46 770 43 17 60 0.29 Comparative example 118 1 50 741 31 16 47 0.35 Present invention example 119 1 54 678 30 15 45 0.33 Present invention example 120 1 43 641 13 13 26 0.49 Present invention example Underscores indicate scope outside present invention and indicate that manufacturing conditions are not preferable.

TABLE 2E Steel Finish rolling Coiling sheet Steel Final rolling Coiling Steel sheet for hot stamping No. No. reduction (%) temperature (° C.) Sa (arca %) SGB (area %) Sa + SGB (area %) SGB/Sa Remark 121 1 46 556 18 13 31 0.43 Present invention example 122 1 46 473 4 7 11 0.66 Present invention example 123 1 52 411 4 6 10 0.58 Present invention example 124 1 44 376 14 3 17 0.20 Comparative example 125 1 50 624 11 6 17 0.33 Present invention example 126 1 55 550 8 5 13 0.36 Present invention example 127 1 45 594 8 5 13 0.35 Present invention example 128 1 46 564 8 4 12 0.35 Present invention example 129 1 48 586 8 5 13 0.38 Present invention example 130 1 51 649 12 7 19 0.38 Present invention example 131 1 43 631 12 5 17 0.32 Present invention example 132 1 43 630 13 6 19 0.33 Present invention example 133 1 53 558 7 4 11 0.33 Present invention example 134 1 54 547 12 8 20 0.39 Present invention example 135 1 50 597 10 5 15 0.35 Present invention example 136 1 44 577 7 3 10 0.33 Present invention example 137 1 42 600 7 3 10 0.33 Present invention example 138 1 48 549 9 5 14 0.34 Present invention example 139 1 52 607 11 6 17 0.37 Present invention example 140 1 47 656 6 4 10 0.37 Present invention example 141 1 44 625 9 4 13 0.32 Present invention example 142 1 43 590 8 4 12 0.35 Present invention example 143 1 45 646 8 4 12 0.31 Present invention example 144 1 45 633 8 5 13 0.39 Present invention example 145 1 51 565 8 4 12 0.36 Present invention example 146 1 42 650 11 6 17 0.38 Present invention example 147 1 50 621 7 4 11 0.32 Present invention example 148 1 47 644 12 5 17 0.31 Present invention example 149 1 52 638 12 7 19 0.36 Present invention example 150 1 49 637 11 6 17 0.33 Present invention example Underscores indicate scope outside present invention and indicate that manufacturing conditions are not preferable.

TABLE 2F Steel Finish rolling Coiling sheet Steel Final rolling Coiling Steel sheet for hot stamping No. No. reduction (%) temperature (° C.) Sa (arca %) SGB (area %) Sa + SGB (area %) SGB/Sa Remark 151 1 56 626 10 5 15 0.34 Present invention example 152 1 50 622 11 6 17 0.35 Present invention example 153 1 42 652 7 3 10 0.34 Present invention example 154 1 53 542 9 4 13 0.31 Present invention example 155 1 48 628 13 6 19 0.31 Present invention example 156 1 42 635 13 7 20 0.33 Present invention example 157 1 44 659 8 5 13 0.37 Present invention example 158 1 55 659 6 4 10 0.38 Present invention example 159 1 45 575 6 4 10 0.37 Present invention example 160 1 49 627 12 6 18 0.36 Present invention example 161 1 52 553 11 6 17 0.35 Present invention example 162 1 51 596 9 5 14 0.34 Present invention example 163 1 53 631 10 5 15 0.31 Present invention example 164 1 44 622 13 7 20 0.35 Present invention example 165 107 48 652 13 7 20 0.35 Present invention example 166 108 56 621 9 4 13 0.31 Present invention example 167 109 52 633 12 7 19 0.36 Present invention example 168 110 44 627 7 4 11 0.55 Present invention example 169 111 44 590 8 4 12 0.44 Present invention example 170 112 51 630 9 5 14 0.56 Present invention example 171 1 22 636 7 3  6 0.18 Comparative example  172* 1 24 640 8 2 10 0.21 Comparative example Underscores indicate scope outside present invention and indicate that manufacturing conditions are not preferable.

TABLE 3A Hot-stamping formed body Average Standard Prior γ Hot stamping grain deviation of grains having Maxi- Average Heating Average size of grain sizes average grain mum Manufac- Steel heating temper- Holding cooling prior γ of prior γ size of Tensile bending turing sheet Steel rate ature time rate grains grains 0.5 to 3.0 μm strength angle No. No. No. (° C./s) (° C.) (minutes) (° C./s) (μm) (μm) (area %) (MPa) (°) Remark 1  1  1 7 892 4.5 258 5 0.4 39 2558 31 Present invention example 2  2  2 5 906 4.6 195 6 0.3 35 2572 39 Present invention example 3  3  3 5 894 3.7 188 5 0.2 32 2561 47 Present invention example 4  4  4 8 898 3.9 267 7 0.3 31 2630 34 Present invention example 5  5  5 6 917 4.6 349 24 1.4 55 2149 44 Comparative example 6  6  6 8 908 3.9 367 22 1.4 56 2402 41 Present invention example 7  7  7 6 915 4.1 250 24 1.4 56 2569 37 Present invention example 8  8  8 10 910 3.5 474 25 1.5 51 2481 34 Present invention example 9  9  9 9 895 3.8 217 24 1.8 54 2598 35 Present invention example 10 10 10 5 920 4.5 280 25 1.8 59 2583 30 Present invention example 11 11 11 10 894 4.4 465 23 1.3 58 2556 21 Present invention example 12 12 12 8 913 4.3 175 25 1.6 50 2640 25 Present invention example 13 13 13 5 890 4.1 456 22 1.5 59 2630 16 Comparative example 14 14 14 7 917 3.7 290 21 1.5 55 2592 12 Comparative example 15 15 15 7 913 4.3 398 23 1.4 51 2586 22 Present invention example 16 16 16 6 898 5.0 481 22 1.4 57 2567 45 Present invention example 17 17 17 9 905 4.9 241 21 1.8 57 2589 46 Present invention example 18 18 18 7 916 4.8 442 23 1.3 57 2555 44 Present invention example 19 19 19 10 904 4.4 311 21 1.4 56 2586 32 Present invention example 20 20 20 7 891 3.7 335 21 1.7 58 2561 23 Present invention example 21 21 21 5 917 4.2 203 22 1.4 59 2565 17 Comparative example 22 22 22 9 902 4.9 389 22 1.6 55 2005 47 Comparative example 23 23 23 7 895 4.7 163 24 1.8 54 2467 43 Present invention example 24 24 24 5 918 4.9 217 25 1.5 57 2564 46 Present invention example 25 25 25 5 912 3.7 266 24 1.7 58 2598 47 Present invention example 26 26 26 8 907 4.2 383 24 1.7 55 2602 33 Present invention example 27 27 27 5 892 3.6 463 21 1.3 54 2642 39 Present invention example 28 28 28 10 896 4.7 259 24 1.5 55 2649 29 Present invention example 29 29 29 5 909 4.4 395 23 1.9 52 2625 15 Comparative example Underscores indicate scope outside present invention and indicate that manufacturing conditions are not preferable.

TABLE 3B Hot-stamping formed body Average Standard Prior γ Hot stamping grain deviation of grains having Maxi- Average Heating Average size of grain sizes average grain mum Manufac- Steel heating temper- Holding cooling prior γ of prior γ size of Tensile bending turing sheet Steel rate ature time rate grains grains 0.5 to 3.0 μm strength angle No. No. No. (° C./s) (° C.) (minutes) (° C./s) (μm) (μm) (area %) (MPa) (°) Remark 30 30 30 9 899 4.3 405 23 1.5 53 2556 16 Comparative example 31 31 31 6 892 4.6 190 22 1.5 54 2598 23 Present invention example 32 32 32 5 915 3.7 340 21 1.4 59 2590 40 Present invention example 33 33 33 10 890 4.5 402 21 1.7 57 2553 45 Present invention example 34 34 34 7 897 3.8 471 21 1.8 54 2590 16 Comparative example 35 35 35 8 902 5.0 351 21 1.8 59 2596 27 Present invention example 36 36 36 5 895 3.8 331 25 1.8 56 2561 37 Present invention example 37 37 37 7 890 3.5 299 25 1.5 53 2581 50 Present invention example 38 38 38 7 911 3.5 414 23 1.9 53 2586 12 Comparative example 39 39 39 7 920 3.6 477 21 1.3 51 2587 27 Present invention example 40 40 40 7 906 5.0 321 21 1.4 51 2566 37 Present invention example 41 41 41 8 913 3.9 251 22 1.9 54 2578 50 Present invention example 42 42 42 10 894 3.8 395 21 1.6 53 2594 14 Comparative example 43 43 43 7 914 3.6 394 25 1.9 52 2573 28 Present invention example 44 44 44 10 899 4.9 340 22 1.4 54 2584 38 Present invention example 45 45 45 7 896 3.5 296 24 1.4 50 2597 47 Present invention example 46 46 46 7 896 4.5 350 22 1.7 58 2557 15 Comparative example 47 47 47 8 895 4.5 227 24 1.5 53 2553 24 Present invention example 48 48 48 8 901 4.1 187 22 1.8 56 2555 46 Present invention example 49 49 49 10 890 4.3 279 25 1.6 58 2571 42 Present invention example 50 50 50 7 900 3.9 399 22 1.7 59 2593 47 Present invention example 51 51 51 9 909 4.0 286 22 1.6 54 2590 34 Present invention example 52 52 52 5 894 3.7 340 24 1.7 57 2579 30 Present invention example 53 53 53 5 903 4.6 448 25 1.3 58 2569 18 Comparative example 54 54 54 6 900 4.0 437 25 1.4 52 2188 47 Comparative example 55 55 55 5 901 4.5 429 22 1.3 56 2523 47 Present invention example 56 56 56 6 897 4.6 472 21 1.4 57 2571 42 Present invention example 57 57 57 7 897 4.5 418 24 1.8 52 2599 48 Present invention example 58 58 58 8 920 4.1 321 23 1.8 53 2609 37 Present invention example Underscores indicate scope outside present invention and indicate that manufacturing conditions are not preferable.

TABLE 3C Hot-stamping formed body Average Standard Prior γ Hot stamping grain deviation of grains having Maxi- Average Heating Average size of grain sizes average grain mum Manufac- Steel heating temper- Holding cooling prior γ of prior γ size of Tensile bending turing sheet Steel rate ature time rate grains grains 0.5 to 3.0 μm strength angle No. No. No. (° C./s) (° C.) (minutes) (° C./s) (μm) (μm) (area %) (MPa) (°) Remark 59 59 59 5 908 4.4 285 25 1.6 52 2620 32 Present invention example 60 60 60 5 894 4.0 418 23 1.3 59 2630 25 Present invention example 61 61 61 5 913 3.5 412 21 1.3 59 2629 12 Comparative example 62 62 62 9 894 4.4 325 21 1.7 58 2602 17 Comparative example 63 63 63 5 893 4.8 424 21 1.6 50 2609 27 Present invention example 64 64 64 8 890 4.6 196 24 1.8 58 2638 35 Present invention example 65 65 65 5 890 5.0 388 25 1.4 51 2490 40 Present invention example 66 66 66 7 907 4.8 371 24 1.5 54 2603 13 Comparative example 67 67 67 8 897 4.6 486 25 1.8 53 2649 21 Present invention example 68 68 68 5 895 4.6 255 23 1.5 53 2634 36 Present invention example 69 69 69 9 902 3.7 429 21 1.7 51 2410 32 Present invention example 70 70 70 5 913 4.4 199 23 1.6 52 2615 15 Comparative example 71 71 71 7 907 3.7 268 24 1.7 59 2602 24 Present invention example 72 72 72 5 914 4.6 395 23 1.8 53 2639 40 Present invention example 73 73 73 6 890 3.7 324 21 1.7 50 2441 32 Present invention example 74 74 74 9 901 3.7 402 25 1.5 55 2620 13 Comparative example 75 75 75 9 915 4.7 307 25 1.6 59 2623 23 Present invention example 76 76 76 8 904 4.3 237 21 1.3 50 2636 39 Present invention example 77 77 77 10 897 3.5 213 22 1.4 55 2627 12 Comparative example 78 78 78 7 906 3.8 466 22 1.8 50 2602 23 Present invention example 79 79 79 5 901 4.6 352 24 1.5 54 2630 34 Present invention example 80 80 80 10 901 4.5 247 23 1.9 51 2619 16 Comparative example 81 81 81 5 899 3.5 304 25 1.8 52 2618 23 Present invention example 82 82 82 5 919 4.5 277 22 1.4 50 2606 38 Present invention example 83 83 83 10 898 4.8 190 24 1.8 58 2618 19 Comparative example 84 84 84 8 898 4.9 478 25 1.3 51 2627 30 Present invention example 85 85 85 8 897 5.0 460 25 1.9 55 2620 39 Present invention example 86 86 86 5 891 4.1 325 25 1.8 54 2616 15 Comparative example 87 87 87 7 916 3.9 309 22 1.4 56 2642 29 Present invention example Underscores indicate scope outside present invention and indicate that manufacturing conditions are not preferable.

TABLE 3D Hot-stamping formed body Average Standard Prior γ Hot stamping grain deviation of grains having Maxi- Average Heating Average size of grain sizes average grain mum Manufac- Steel heating temper- Holding cooling prior γ of prior γ size of Tensile bending turing sheet Steel rate ature time rate grains grains 0.5 to 3.0 μm strength angle No. No. No. (° C./s) (° C.) (minutes) (° C./s) (μm) (μm) (area %) (MPa) (°) Remark 88  88 88 9 890 4.2 288 25 1.8 51 2623 34 Present invention example 89  89 89 5 912 3.6 345 22 1.9 55 2604 15 Comparative example 90  90 90 9 898 3.6 296 22 1.5 55 2613 30 Present invention example 91  91 91 9 905 4.1 434 25 1.4 58 2631 38 Present invention example 92  92 92 8 915 3.9 307 21 1.6 58 2636 13 Comparative example 93  93 93 10 917 4.4 288 22 1.7 51 2613 25 Present invention example 94  94 94 5 914 5.0 358 24 1.3 53 2631 40 Present invention example 95  95 95 7 894 4.8 228 24 1.8 52 2629 18 Comparative example 96  96 96 9 915 4.0 426 22 1.8 54 2633 28 Present invention example 97  97 97 8 905 3.5 300 24 1.6 59 2628 35 Present invention example 98  98 98 7 891 5.0 285 25 1.5 59 2642 17 Comparative example 99  99 99 7 903 3.5 310 25 1.3 52 2604 21 Present invention example 100 100 100  9 920 3.6 362 25 1.9 52 2630 33 Present invention example 101 101 101  5 904 4.8 355 24 1.6 57 2634 17 Comparative example 102 102 102  6 920 4.8 261 22 1.3 56 2643 27 Present invention example 103 103 103  8 895 4.9 480 25 1.7 56 2612 31 Present invention example 104 104 104  6 901 3.6 457 25 1.3 53 2630 15 Comparative example 105 105 105  8 908 4.7 210 23 1.3 54 2607 26 Present invention example 106 106 106  9 893 4.3 418 24 1.4 54 2650 37 Present invention example 107 107  1 9 901 4.0 291 21 1.7 57 2561 49 Present invention example 108 108  1 10 914 4.4 298 22 1.5 59 2582 42 Present invention example 109 109  1 10 919 4.9 287  2 0.5 44 2578 13 Comparative example 110 110  1 6 904 5.0 468 17 0.8 44 2582 40 Present invention example 111 111  1 7 909 4.1 355 19 0.6 49 2571 50 Present invention example 112 112  1 8 915 3.8 408  5 0.3 33 2558 48 Present invention example 113 113  1 7 915 4.8 323  5 0.2 34 2554 49 Present invention example 114 114  1 8 897 3.9 471 25 1.8 54 2553 37 Present invention example 115 115  1 6 899 4.4 249 23 1.7 51 2550 26 Present invention example 116 116  1 9 913 3.8 446 36 2.0 85 2571 14 Comparative example Underscores indicate scope outside present invention and indicate that manufacturing conditions are not preferable.

TABLE 3E Hot-stamping formed body Average Standard Prior γ Hot stamping grain deviation of grains having Maxi- Average Heating Average size of grain sizes average grain mum Manufac- Steel heating temper- Holding cooling prior γ of prior γ size of Tensile bending turing sheet Steel rate ature time rate grains grains 0.5 to 3.0 μm strength angle No. No. No. (° C./s) (° C.) (minutes) (° C./s) (μm) (μm) (area %) (MPa) (°) Remark 117 117 1 6 894 4.7 402 35 2.3 61 2568 12 Comparative example 118 118 1 6 904 4.9 280 23 0.5 41 2558 27 Present invention example 119 119 1 6 893 4.8 194 21 0.9 48 2554 31 Present invention example 120 120 1 10  895 3.9 217  7 0.2 33 2575 44 Present invention example 121 121 1 6 897 4.5 172  5 0.4 38 2585 45 Present invention example 122 122 1 6 905 4.1 164 12 1.8 52 2586 32 Present invention example 123 123 1 9 898 3.5 302 17 1.3 59 2566 24 Present invention example 124 124 1 5 907 4.3 494 29 1.5 57 2586 14 Comparative example 125 125 1 8 912 4.8 318 24 1.9 55 2582 39 Present invention example 126 126 1 6 909 4.8 478 25 1.3 56 2571 35 Present invention example 127 127 1 5 919 3.9 434 21 1.7 58 2566 40 Present invention example 128 128 1 8 903 3.5 344 21 1.9 52 2591 39 Present invention example 129 129 1 6 911 4.5 302 25 1.5 52 2593 34 Present invention example 130 130 1 6 901 4.8 168 24 1.5 57 2551 33 Present invention example 131 131 1 8 890 3.9 491 24 1.4 56 2561 35 Present invention example 132 132 1 5 897 5.0 208 23 1.9 55 2569 36 Present invention example 133 133 1 6 904 4.9 404 23 1.3 53 2579 34 Present invention example 134 134 1 10  915 4.3 210 25 1.6 50 2585 33 Present invention example 135 135 1 950  911 3.8 176 22 1.3 53 2611 31 Present invention example 136 136 1 500  910 3.9 311 23 1.9 58 2640 35 Present invention example 137 137 1 120  904 4.7 324 25 1.5 57 2560 36 Present invention example 138 138 1 50  919 4.7 298 24 1.5 56 2558 32 Present invention example 139 139 1 10  904 5.0 261 21 1.4 50 2596 36 Present invention example 140 140 1 4 903 4.2 203 25 1.4 58 2429 32 Present invention example 141 141 1   0.5 905 3.5 472 22 1.6 51 2099 39 Comparative example 142 142 1 5 1022  4.2 354 23 1.6 55 2009 38 Comparative example 143 143 1 5 997 4.8 289 25 1.7 54 2439 44 Present invention example 144 144 1 9 971 4.6 456 25 1.9 53 2578 49 Present invention example Underscores indicate scope outside present invention and indicate that manufacturing conditions are not preferable.

TABLE 3F Hot-stamping formed body Average Standard Prior γ Hot stamping grain deviation of grains having Maxi- Average Heating Average size of grain sizes average grain mum Manufac- Steel heating temper- Holding cooling prior γ of prior γ size of Tensile bending turing sheet Steel rate ature time rate grains grains 0.5 to 3.0 μm strength angle No. No. No. (° C./s) (° C.) (minutes) (° C./s) (μm) (μm) (area %) (MPa) (°) Remark 145 145 1 6 938 4.9 402 23 1.6 57 2567 41 Present invention example 146 146 1 6 905 4.6 383 23 1.8 53 2566 42 Present invention example 147 147 1 8 870 4.3 229 24 1.3 52 2594 50 Present invention example 148 148 1 10 831 4.9 188 24 1.4 55 2586 33 Present invention example 149 149 1 7 810 4.9 386 23 1.5 55 2560 30 Present invention example 150 150 1 5 902 11.0  405 22 1.8 50 2038 47 Comparative example 151 151 1 9 919 8.5 325 22 1.3 56 2407 48 Present invention example 152 152 1 10 896 7.8 290 25 1.5 57 2564 48 Present invention example 153 153 1 9 913 6.2 443 24 1.3 55 2642 46 Present invention example 154 154 1 10 919 4.1 321 25 1.8 57 2649 47 Present invention example 155 155 1 8 894 2.3 312 23 1.4 55 2555 35 Present invention example 156 156 1 6 89 1.5 276 25 1.6 51 2551 30 Present invention example 157 157 1 7 897 3.6 980 23 1.9 55 2638 36 Present invention example 158 158 1 7 914 4.4 560 24 1.9 52 2608 39 Present invention example 159 159 1 5 906 4.2 120 24 1.9 59 2552 39 Present invention example 160 160 1 8 910 3.8  18 23 1.8 59 2471 39 Present invention example 161 161 1 7 919 4.1  4 24 1.9 52 2188 39 Comparative example 162 162 1 10 895 4.0 330 22 1.7 50 2576 34 Present invention example 163 163 1 7 902 4.4 344 25 1.6 57 2580 31 Present invention example 164 164 1 5 895 4.1 371 24 3.3 71 2559 18 Comparative example 165 165 107 6 870 4.2 402 25 1.8 71 2610 23 Present invention example 166 166 108 7 919 4.9 345 23 1.7 77 2601 25 Present invention example 167 167 109 5 891 7.8 321 25 1.8 81 2598 24 Present invention example 168 168 110 7 921 3.7 460 22 1.2 55 2577 26 Present invention example 169 169 111 8 900 4.9 477 25 1.1 56 2613 30 Present invention example 170 170 112 9 899 4.1 420 24 1.3 51 2644 34 Present invention example 171 171 1 10 907 3.6 437 33 1.9 77 2566 13 Comparative example 172  172* 1 9 900 3.8 444 34 1.9 80 2566 14 Comparative example Underscores indicate scope outside present invention and indicate that manufacturing conditions are not preferable.

From Table 3A to Table 3F, 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

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.

Claims

1. 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: more than 0.60% and 3.00% or less;
P: 0.100% or less;
S: 0.0100% or less;
N: 0.0130% 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 steel sheet for hot stamping has a microstructure in which Sα+SGB, which is a total of an area ratio Sα of ferrite and an area ratio SGB of a granular bainite, is 10% or more and less than 50%, and
SGB/Sα, which is a ratio between the area ratio SGB of the granular bainite and the area ratio Sα of the ferrite, is 0.30 to 0.70.

2. 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 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 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: more than 0.60% and 3.00% or less;
P: 0.100% or less;
S: 0.0100% or less;
N: 0.0130% 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.

4. 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 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. The hot-stamping formed body according to claim 3, 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.

6. The hot-stamping formed body according to claim 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.

7. A steel sheet for hot stamping comprising, as a chemical composition, by mass %:

C: more than 0.40% and 0.70% or less;
Si: 0.010% to 1.30%;
Mn: more than 0.60% and 3.00% or less;
P: 0.100% or less;
S: 0.0100% or less;
N: 0.0130% 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 steel sheet for hot stamping has a microstructure in which Sα+SGB, which is a total of an area ratio Sα of ferrite and an area ratio SGB of a granular bainite, is 10% or more and less than 50%, and
SGB/Sα, which is a ratio between the area ratio SGB of the granular bainite and the area ratio Sα of the ferrite, is 0.30 to 0.70.
Patent History
Publication number: 20240183015
Type: Application
Filed: May 9, 2022
Publication Date: Jun 6, 2024
Applicant: NIPPON STEEL CORPORATION (Tokyo)
Inventors: Yuri TODA (Tokyo), Daisuke MAEDA (Tokyo), Tamaki SUZUKI (Tokyo), Yuma ASADA (Tokyo), Takuya TAKAYAMA (Tokyo)
Application Number: 18/279,370
Classifications
International Classification: C22C 38/38 (20060101); C21D 9/46 (20060101); C22C 38/02 (20060101); C22C 38/06 (20060101);