[0001]The present invention relates to a steel sheet for hot stamping. Priority is
claimed on Japanese Patent Application No. 2019-101986, filed May 31, 2019, the
content of which is incorporated herein by reference.
[Background Art]
10 [0002]
In recent years, there has been a demand for the weight reduction of automotive
bodies from the viewpoint of environmental protection and resource saving, and the
application of high-strength steel sheets to automotive parts has been accelerating.
Automotive parts are manufactured by press forming, and an increase in the strength of
15 steel sheets does not only increase forming loads but also degrades formability, which
creates a problem with the formability of high-strength steel sheets into parts with a
complicated shape. In order to solve such a problem, the application of hot stamping
techniques, in which a steel sheet is heated to a high temperature in an austenite region
where the steel sheet softens and then formed by pressing, is underway. Hot stamping is
20 drawing attention as a technique in which a quenching treatment is carried out in a die at
the same time as pressing, thereby satisfying both formability into automotive parts and
the securement of the strength of automotive parts.
[0003]
However, in hot-stamped articles of the related art manufactured by hot
25 stamping, a hard structure (mainly martensite) is formed throughout the entire region in
1

the sheet thickness direction, and thus the deformability is poor. In order to obtain
superior collision characteristics in automotive parts, there is a need to enhance the
impact energy absorption capability. When deformation modes at the time of collision
are taken into account, there is a need to enhance deformability, particularly, bendability.
5 In addition, since an increase in the dislocation density of steel sheets increases hydrogen
embrittlement susceptibility, and a small amount of hydrogen may cause hydrogen embrittlement cracking, in the hot-stamped articles of the related art, there are cases where improvement in hydrogen embrittlement resistance is considered as a significant objective. That is, hot-stamped articles that are applied to automotive parts (steel sheets
10 for hot stamping that have been hot-stamped) are desirably excellent in terms of at least
one of bendability and hydrogen embrittlement resistance. [0004]
Patent Document 1 discloses a technique in which the cooling rate from finishing rolling through coiling in a hot rolling process is controlled to control the
15 crystal orientation difference in bainite to become 5° to 14°, thereby improving
deformability such as stretch-flangeability. [0005]
Patent Document 2 discloses a technique in which manufacturing conditions for finishing rolling through coiling in a hot rolling process are controlled to control the
20 strength of a specific crystal orientation group out of ferrite crystal grains, thereby
improving local deformability. [0006]
Patent Document 3 discloses a technique in which a heat treatment is carried out on a steel sheet for hot stamping to form ferrite in the surface layer, thereby reducing the
25 number of pores that are generated in the interface between ZnO and the steel sheet or
2

the interface between ZnO and a Zn-based plating layer during heating before hot
pressing and improving perforation corrosion resistance or the like.
[0007]
However, in the above-described techniques, there are cases where sufficient
5 strength and sufficient bendability or hydrogen embrittlement resistance cannot be
obtained.
[Citation List]
[Patent Document]
[0008]
10 [Patent Document 1]
PCT International Publication No. WO 2016/132545 [Patent Document 2]
Japanese Unexamined Patent Application, First Publication No. 2012-172203
[Patent Document 3]
15 Japanese Patent No. 5861766
[Non-Patent Document] [0009] [Non-Patent Document 1]
T. Ungar and three coauthors, Journal of Applied Crystallography (1999),
20 Volume 32 (pp. 992 to 1002)
[Summary of the Invention]
[Problems to be Solved by the Invention] [0010]
The present invention has been made in consideration of the problem of the
25 related art, and an objective of the present invention is to provide a steel sheet for hot
3

stamping enabling the obtainment of a hot-stamped article having an excellent strength
and being also excellent in terms of bendability or hydrogen embrittlement resistance.
[Means for Solving the Problem]
[0011]
5 The present inventors carried out intensive studies regarding a method for
solving the above-described problem and consequently obtained the following findings. [0012]
The present inventors carried out studies regarding the bendability of hot-
stamped articles. As a result, the present inventors found that, when the metallographic
10 structure of a surface layer region, which is a region from a surface to a depth of 50 μm,
of a base steel sheet that configures a hot-stamped article includes 80.0% or more of
martensite and 8.0% or more of residual austenite in terms of area percentage and the
concentration of Ni in the surface layer region is 8 mass% or more, the bendability of the
hot-stamped article improves.
15 [0013]
In addition, the present inventors carried out studies regarding the hydrogen
embrittlement resistance of hot-stamped articles. As a result, the present inventors
found that, when the metallographic structure of the surface layer region of the base steel
sheet that configures the hot-stamped article includes 90.0% or more of martensite in
20 terms of area percentage and the concentration of Ni in prior austenite grain boundaries
in the surface layer region is 5.5 mass% or more, the hydrogen embrittlement resistance of the hot-stamped article improves. [0014]
Furthermore, the present inventors found that, in order to obtain the above-
25 described metallographic structure in the surface layer region of the base steel sheet that
4

configures the hot-stamped article, in the surface layer region in the steel sheet for hot
stamping before hot stamping, it is necessary to set the average dislocation density to 4 ×
1015 m/m3 or more, to set the proportion of the crystal grains of one or more kinds of
unauto-tempered martensite and lower bainite to 15.0% or more in terms of area
5 percentage, and to carry out hot stamping under a predetermined condition.
[0015]
The present invention has been made by further progressing studies based on the above-described findings, and the gist thereof is as described below.
[1] A steel sheet for hot stamping according to one aspect of the present
10 invention having a base steel sheet containing, as chemical components, by mass%,
C: 0.15% or more and less than 0.70%,
Si: 0.005% or more and 0.250% or less,
Mn: 0.30% or more and 3.00% or less,
sol. Al: 0.0002% or more and 0.500% or less,
15 P: 0.100% or less,
S: 0.1000% or less, N: 0.0100% or less,
Nb: 0% or more and 0.150% or less,
Ti: 0% or more and 0.150% or less,
20 Mo: 0% or more and 1.000% or less,
Cr: 0% or more and 1.000% or less,
B: 0% or more and 0.0100% or less,
Ca: 0% or more and 0.0100% or less, and
REM: 0% or more and 0.30% or less
25 with a remainder including Fe and impurities; and
5

a plating layer on a surface of the base steel sheet, the plating layer being
attached to a single surface in an amount of 10 g/m2 or more and 90 g/m2 or less and
having a Ni content of 10 mass% or more and 25 mass% or less with a remainder
including Zn and impurities,
5 in which a metallographic structure of a surface layer region that is a region
from the surface of the base steel sheet to a depth of 50 μm includes 15.0% or more of crystal grains of one or two kinds of unauto-tempered martensite and lower bainite in terms of area percentage, and
an average dislocation density of the surface layer region is 4 × 1015 m/m3 or
10 more.
[2] The steel sheet for hot stamping according to [1] may further contain, in the base steel sheet, as chemical components, by mass%, one or more selected from the group consisting of
Nb: 0.010% or more and 0.150% or less,
15 Ti: 0.010% or more and 0.150% or less,
Mo: 0.005% or more and 1.000% or less,
Cr: 0.005% or more and 1.000% or less,
B: 0.0005% or more and 0.0100% or less,
Ca: 0.0005% or more and 0.0100% or less, and
20 REM: 0.0005% or more and 0.30% or less.
[Effects of the Invention] [0016]
According to the above-described aspect of the present invention, it is possible
to provide a steel sheet for hot stamping enabling the obtainment of a hot-stamped article
25 being excellent in terms of strength and bendability or hydrogen embrittlement
6

resistance.
[Brief Description of Drawings]
[0017]
Fig. 1 is a view showing a test piece that is used for the measurement of the
5 concentration of Ni in prior austenite grain boundaries.
Fig. 2 is a view showing a test piece used for the evaluation of the hydrogen embrittlement resistance of examples. [Embodiment for implementing the Invention] [0018]
10 In a steel sheet for hot stamping according to the present embodiment, in a
surface layer region that is a region from a surface of a base steel sheet to a depth of 50 μm, 15.0% or more of the crystal grains of one or two kinds of unauto-tempered martensite and lower bainite are included in terms of area percentage, and the average dislocation density is 4 × 1015 m/m3. Such characteristics make it possible to obtain a
15 desired metallographic structure in the surface layer region of the base steel sheet that
configures a hot-stamped article, in other words, the base steel sheet after hot stamping in a case where the steel sheet for hot stamping has been hot-stamped under a predetermined condition and to obtain a hot-stamped article being excellent in terms of strength and bendability or hydrogen embrittlement resistance. In the present
20 embodiment, the expression “having an excellent strength” refers to the fact that the
(maximum) tensile strength is 1500 MPa or higher. [0019]
In a hot-stamped article having an excellent strength and excellent bendability manufactured using the steel sheet for hot stamping according to the present embodiment
25 (hereinafter, referred to as the first application example in some cases), the
7

metallographic structure of the surface layer region of the steel sheet that configures the
hot-stamped article includes 80.0% or more of martensite and 8.0% or more of residual
austenite in terms of area percentage, and the concentration of Ni in the surface layer
region is 8 mass% or more.
5 [0020]
In a hot-stamped article having an excellent strength and excellent hydrogen embrittlement resistance manufactured using the steel sheet for hot stamping according to the present embodiment (hereinafter, referred to as the second application example in some cases), the metallographic structure of the surface layer region of the steel sheet
10 that configures the hot-stamped article includes 90.0% or more of martensite in terms of
area percentage, and the concentration of Ni in prior austenite grain boundaries in the surface layer region is 5.5 mass% or more. [0021]
As a result of intensive studies, the present inventors found that a steel sheet for
15 hot stamping and a hot-stamped article each having the above-described metallographic
structure can be obtained by the following method. [0022]
As a first stage, in a hot rolling process, cooling is initiated within five seconds from the end of finishing rolling in a manner that the average cooling rate on the surface
20 of a base steel sheet reaches 80 °C/s or faster, and the base steel sheet is cooled to a
temperature region of lower than 500°C, and the hot-rolled steel sheet is coiled. Even the hot-rolled coil that has been coiled is continuously cooled with water to room temperature (approximately 40°C or lower). As described above, compared with the related art, the average cooling rate is set to be fast and the coiling temperature is set to
25 be low, whereby it is possible to suppress the generation of a carbide, ferritic
8

transformation, and bainitic transformation. This makes it possible in the
metallographic structure of the surface layer region in the steel sheet for hot stamping to
set the proportion of the crystal grains of one or two kinds of unauto-tempered martensite
and lower bainite to 15.0% or more in terms of area percentage and to set the average
5 dislocation density of the surface layer region to 4 × 1015 m/m3 or more.
[0023]
As a second stage, a Zn-based plating layer containing 10 to 25 mass% of Ni is formed on the surface of the base steel sheet such that the amount attached to the single surface reaches 10 to 90 g/m2, thereby producing a steel sheet for hot stamping.
10 [0024]
As a third stage, the average heating rate of heating before hot stamping is controlled, thereby diffusing Ni in the plating layer disposed on the surface of the base steel sheet into the surface layer region of the base steel sheet. [0025]
15 Usually, in hot-rolled steel sheets having a high dislocation density that contain
0.15 mass% or more of C, have a metallographic structure including martensite, and are not tempered, ductility, toughness, and hydrogen embrittlement resistance deteriorate. Additionally, in the case of carrying out cold rolling after coiling, since the above-described hot-rolled steel sheets do not have excellent ductility, cracking is likely to
20 occur. Therefore, it is usual that the above-described hot-rolled steel sheets are
tempered after hot rolling and before post processes. In order to improve the bendability and hydrogen embrittlement resistance of hot-rolled steel sheets, it is important to improve the ductility of the surface layer region, and thus there are also cases where a treatment for softening the surface layer region (for example, a surface
25 layer decarburization treatment) is carried out on the above-described steel sheets.
9

[0026]
In addition, usually, when steel sheets containing 0.15 mass% or more of C are
hot-stamped, there are cases where the hot-stamped articles are not excellent in terms of
bendability or hydrogen embrittlement resistance.
5 [0027]
However, in the present embodiment, the metallographic structure of the surface layer region in the steel sheet for hot stamping is put into a preferred state, and Ni in the plating layer disposed on the surface of the base steel sheet is diffused into the surface layer region of the base steel sheet by heating before hot stamping, whereby it is possible
10 to improve the bendability or hydrogen embrittlement resistance of the hot-stamped
article even without tempering after hot stamping. [0028]
The metallographic structure of the surface layer region in the steel sheet for hot stamping according to the present embodiment includes 15.0% or more of the crystal
15 grains of one or two kinds of unauto-tempered martensite and lower bainite in terms of
area percentage. Inside the crystal grains of unauto-tempered martensite and lower bainite, the dislocation density is high, and these crystal grains have a small crystal grain size. Therefore, in the steel sheet for hot stamping according to the present embodiment, heating before hot stamping makes it easy for Ni contained in the plating
20 layer to diffuse into the surface layer region through crystal grain boundaries and
dislocations in the surface layer region of the base steel sheet as passages. Since Ni is an austenite-stabilizing element, when Ni in the plating layer diffuses into the surface layer region of the base steel sheet to increase the concentration of Ni in the surface layer region of the base steel sheet, residual austenite is likely to remain in the surface layer
25 region of the base steel sheet that configures the hot-stamped article. The bendability of
10

the hot-stamped article can be improved by causing a predetermined amount of residual
austenite to remain in the surface layer region of the base steel sheet that configures the
hot-stamped article using not only C but also Ni. The present inventors found that, in
order to diffuse Ni into the surface layer region of the base steel sheet to cause a
5 predetermined amount of residual austenite to remain in the surface layer region of the
base steel sheet that configures the hot-stamped article, there is a need to set the average heating rate of heating before hot stamping to slower than 100 °C/s. When the average heating rate of heating before hot stamping is set to slower than 100 °C/s, Ni diffuses through not only crystal grain boundaries but also dislocations in the surface layer region
10 of the base steel sheet as passages, which makes it possible to uniformly diffuse Ni into
the surface layer region. [0029]
In addition, in prior austenite grain boundaries of unauto-tempered martensite and lower bainite, since the number of grain boundary segregation elements such as C or
15 precipitates is small, Ni easily diffuses. Therefore, in the case of setting the average
heating rate of heating before hot stamping to be fast, it is possible to preferentially diffuse Ni into the prior austenite grain boundaries. The present inventors found that, when the average heating rate before hot stamping is set to 100 °C/s or faster and slower than 200 °C/s, and Ni is preferentially diffused into the prior austenite grain boundaries
20 in the surface layer region of the base steel sheet, these prior austenite grain boundaries
serve as an obstacle to hydrogen intrusion, and it is possible to improve the hydrogen embrittlement resistance of the hot-stamped article. [0030]
Hereinafter, the steel sheet for hot stamping according to the present
25 embodiment and a method for manufacturing the same will be described in detail. First,
11

reasons for limiting the chemical composition of the base steel sheet that configures the steel sheet for hot stamping according to the present embodiment will be described.
Numerical limitation ranges to be described below include the lower limit value
and the upper limit value in the range. Numerical values with an expression of “less
5 than” or “more than” do not include the numerical value in the numerical range.
Regarding the chemical composition, “%” indicates “mass%” in all cases. [0031]
The base steel sheet that configures the steel sheet for hot stamping according to the present embodiment contains, as chemical components, by mass%, C: 0.15% or more
10 and less than 0.70%, Si: 0.005% or more and 0.250% or less, Mn: 0.30% or more and
3.00% or less, sol. Al: 0.0002% or more and 0.500% or less, P: 0.100% or less, S: 0.1000% or less, N: 0.0100% or less with a remainder including Fe and impurities. [0032]
“C: 0.15% or more and less than 0.70%”
15 C is an important element for obtaining a tensile strength of 1500 MPa or higher
in the hot-stamped article. When the C content is less than 0.15%, martensite becomes soft, and it is difficult to obtain a tensile strength of 1500 MPa or higher. In addition, when the C content is less than 0.15%, the area rate of unauto-tempered martensite and lower bainite decreases. Therefore, the C content is set to 0.15% or more. The C
20 content is preferably 0.20% or more and more preferably 0.30% or more. On the other
hand, when the C content is 0.70% or more, a coarse carbide is generated, breakage is likely to occur, and the bendability and hydrogen embrittlement resistance of the hot-stamped article deteriorate. Therefore, the C content is set to less than 0.70%. The C content is preferably 0.50% or less and more preferably 0.45% or less.
25 [0033]
12

“Si: 0.005% or more and 0.250% or less”
Si is an element that is contained to secure hardenability. When the Si content
is less than 0.005%, the above-described effect cannot be obtained, and, in the steel sheet
for hot stamping, there are cases where the dislocation density decreases and cases where
5 unauto-tempered martensite and lower bainite cannot be obtained, which makes it
impossible to obtain a desired metallographic structure in the hot-stamped article.
Therefore, the Si content is set to 0.005% or more. Even when more than 0.250% of Si
is contained, the above-described effect is saturated, and thus the Si content is set to
0.250% or less. The Si content is preferably 0.210% or less.
10 [0034]
“Mn: 0.30% or more and 3.00% or less”
Mn is an element that contributes to improvement in the strength of the hot-
stamped article by solid solution strengthening. When the Mn content is less than
0.30%, the solid solution strengthening capability is poor, martensite becomes soft, and it
15 is difficult to obtain a tensile strength of 1500 MPa or higher in the hot-stamped article.
Therefore, the Mn content is set to 0.30% or more. The Mn content is preferably 0.50%
or more or 0.70% or more. On the other hand, when the Mn content is set to more than
3.00%, a coarse inclusion is generated in steel, breakage is likely to occur, and the
bendability and hydrogen embrittlement resistance of the hot-stamped article deteriorate.
20 Therefore, the lower limit is set to 3.00%. The Mn content is preferably 2.50% or less
or 2.00% or less. [0035]
“sol. Al (acid-soluble Al): 0.0002% or more and 0.500% or less”
Al is an element having an action of deoxidizing molten steel to make the steel
25 sound (suppressing the generation of a defect such as a blowhole in steel). When the
13

sol. Al content is less than 0.0002%, since molten steel is not sufficiently deoxidized, and
the above-described effect cannot be obtained, the sol. Al content is set to 0.0002% or
more. The sol. Al content is preferably 0.0010% or more or 0.0020% or more. On the
other hand, when the sol. Al content exceeds 0.500%, a coarse oxide is generated in steel,
5 and the bendability and hydrogen embrittlement resistance of the hot-stamped article
deteriorate. Therefore, the sol. Al content is set to 0.500% or less. The sol. Al content
is preferably 0.400% or less or 0.300% or less.
[0036]
“P: 0.100% or less”
10 P is an element that is segregated in grain boundaries and degrades the strength
of the grain boundaries. When the P content exceeds 0.100%, the strength of grain boundaries significantly decreases, and the bendability and hydrogen embrittlement resistance of the hot-stamped article deteriorate. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.050% or less. The lower limit of the P
15 content is not particularly limited. However, when the P content is decreased to lower
than 0.0001%, the dephosphorization cost increases significantly, which is not preferable economically, and thus the lower limit of the P content may be set to 0.0001% in actual operation. [0037]
20 “S: 0.1000% or less”
S is an element that forms an inclusion in steel. When the S content exceeds 0.1000%, a large amount of an inclusion is generated in steel, and the bendability and hydrogen embrittlement resistance of the hot-stamped article deteriorate. Therefore, the S content is set to 0.1000% or less. The S content is preferably 0.0050% or less. The
25 lower limit of the S content is not particularly limited. However, when the S content is
14

decreased to lower than 0.00015%, the desulfurization cost increases significantly, which
is not preferable economically, and thus the lower limit of the S content may be set to
0.00015% in actual operation.
[0038]
5 “N: 0.0100% or less”
N is an impurity element and an element that forms a nitride in steel to degrade the toughness and hydrogen embrittlement resistance of the hot-stamped article. When the N content exceeds 0.0100%, a coarse nitride is generated in steel to significantly degrade the bendability and hydrogen embrittlement resistance of the hot-stamped article.
10 Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0075%
or less. The lower limit of the N content is not particularly limited. However, when the N content is decreased to lower than 0.0001%, the denitrification cost increases significantly, which is not preferable economically, and thus the lower limit of the N content may be set to 0.0001% in actual operation.
15 [0039]
The remainder of the chemical composition of the base steel sheet that configures the steel sheet for hot stamping according to the present embodiment is Fe and impurities. As the impurities, exemplified is an element that is inevitably contained by accident from a steel raw material or a scrap and/or in a steel making process and is
20 permitted to an extent that the properties of hot-stamped articles, which are the steel sheet
for hot stamping according to the present embodiment that has been hot-stamped, are not impaired. [0040]
In addition, the base steel sheet that configures the steel sheet for hot stamping
25 according to the present embodiment substantially contains no Ni, and the Ni content is
15

less than 0.005%. Since Ni is an expensive element, in the present embodiment, it is
possible to suppress the cost at a low level compared with a case where the Ni content is
set to 0.005% or more by intentionally including Ni.
[0041]
5 The base steel sheet that configures the steel sheet for hot stamping according to
the present embodiment may contain the following elements as arbitrary elements. In a case where the following arbitrary elements are not contained, the content thereof is 0%. [0042]
“Nb: 0% or more and 0.150% or less”
10 Nb is an element that contributes to improvement in the strength of the hot-
stamped article by solid solution strengthening and thus may be contained as necessary.
In a case where Nb is contained, the Nb content is preferably set to 0.010% or more in
order to reliably exhibit the above-described effect. The Nb content is more preferably
0.035% or more. On the other hand, even when more than 0.150% of Nb is contained,
15 the above-described effect is saturated, and thus the Nb content is preferably set to
0.150% or less. The Nb content is more preferably 0.120% or less. [0043]
“Ti: 0% or more and 0.150% or less”
Ti is an element that contributes to improvement in the strength of the hot-
20 stamped article by solid solution strengthening and thus may be contained as necessary.
In a case where Ti is contained, the Ti content is preferably set to 0.010% or more in
order to reliably exhibit the above-described effect. The Ti content is preferably
0.020% or more. On the other hand, even when more than 0.150% of Ti is contained,
the above-described effect is saturated, and thus the Ti content is preferably set to
25 0.150% or less. The Ti content is more preferably 0.120% or less.
16

[0044]
“Mo: 0% or more and 1.000% or less”
Mo is an element that contributes to improvement in the strength of the hot-
stamped article by solid solution strengthening and thus may be contained as necessary.
5 In a case where Mo is contained, the Mo content is preferably set to 0.005% or more in
order to reliably exhibit the above-described effect. The Mo content is more preferably
0.010% or more. On the other hand, even when more than 1.000% of Mo is contained,
the above-described effect is saturated, and thus the Mo content is preferably set to
1.000% or less. The Mo content is more preferably 0.800% or less.
10 [0045]
“Cr: 0% or more and 1.000% or less”
Cr is an element that contributes to improvement in the strength of the hot-
stamped article by solid solution strengthening and thus may be contained as necessary.
In a case where Cr is contained, the Cr content is preferably set to 0.005% or more in
15 order to reliably exhibit the above-described effect. The Cr content is more preferably
0.100% or more. On the other hand, even when more than 1.000% of Cr is contained,
the above-described effect is saturated, and thus the Cr content is preferably set to
1.000% or less. The Cr content is more preferably 0.800% or less.
[0046]
20 “B: 0% or more and 0.0100% or less”
B is an element that is segregated in grain boundaries to improve the strength of
the grain boundaries and is thus contained as necessary. In a case where B is contained,
the B content is preferably set to 0.0005% or more in order to reliably exhibit the above-
described effect. The B content is preferably 0.0010% or more. On the other hand,
25 even when more than 0.0100% of B is contained, the above-described effect is saturated,
17

and thus the B content is preferably set to 0.0100% or less. The B content is more
preferably 0.0075% or less.
[0047]
“Ca: 0% or more and 0.0100% or less”
5 Ca is an element having an action of deoxidizing molten steel to make steel
sound. In order to reliably exhibit this action, the Ca content is preferably set to
0.0005% or more. On the other hand, even when more than 0.0100% of Ca is
contained, the above-described effect is saturated, and thus the Ca content is preferably
set to 0.0100% or less.
10 [0048]
“REM: 0% or more and 0.30% or less”
REM is an element having an action of deoxidizing molten steel to make steel
sound. In order to reliably exhibit this action, the REM content is preferably set to
0.0005% or more. On the other hand, even when more than 0.30% of REM is
15 contained, the above-described effect is saturated, and thus the REM content is
preferably set to 0.30% or less.
In the present embodiment, REM refers to a total of 17 elements consisting of
Sc, Y, and lanthanoids, and the REM content refers to the total amount of these elements.
[0049]
20 The above-described chemical composition of the steel sheet for hot stamping
may be measured by an ordinary analysis method. For example, the chemical
composition may be measured using inductively coupled plasma-atomic emission
spectrometry (ICP-AES). C and S may be measured using an infrared absorption
method after combustion, and N may be measured using an inert gas melting-thermal
25 conductivity method. The chemical composition is preferably analyzed after the plating
18

layer on the surface is removed by machining. [0050]
Next, the metallographic structure of the steel sheet that configures the steel
sheet for hot stamping according to the present embodiment will be described.
5 [0051]

“In metallographic structure of surface layer region that is region from surface
of base steel sheet to depth of 50 μm, 15.0% or more of crystal grains of one or two kinds
of unauto-tempered martensite and lower bainite in terms of area percentage are included
10 and average dislocation density of surface layer region is 4 × 1015 m/m3 or more”
In the metallographic structure of the surface layer region that is a region from
the surface of the base steel sheet to a depth of 50 μm, the proportion of the crystal grains
of one or two kinds of unauto-tempered martensite and lower bainite is set to 15.0% or
more in terms of area percentage, and the average dislocation density of the surface layer
15 region is set to 4 × 1015 m/m3 or more, whereby it is possible to diffuse Ni in the plating
layer into the surface layer region of the steel sheet by heating before hot stamping. The
upper limit of the average dislocation density of the surface layer region is not
particularly limited and the average dislocation density of the surface layer region may
be, for example, 5 × 1017 m/m3 or less or 1 × 1018 m/m3 or less.
20 [0052]
For example, in a case where the average heating rate of heating before hot
stamping is controlled to slower than 100 °C/s, Ni uniformly diffuses into the entire
surface layer region, and it is possible to set the proportion of residual austenite in the
surface layer region of the base steel sheet that configures the hot-stamped article to 8.0%
25 or more in terms of area percentage. This makes it possible to improve the bendability
19

of the hot-stamped article. [0053]
In the case of controlling the average heating rate of heating before hot stamping
to 100 °C/s or faster and slower than 200 °C/s, Ni in the plating layer preferentially
5 diffuses into the prior austenite grain boundaries in the surface layer region of the base
steel sheet that configures the hot-stamped article. The prior austenite grain boundaries into which Ni has diffused serve as an obstacle to hydrogen intrusion, which makes it possible to improve the hydrogen embrittlement resistance of the hot-stamped article. [0054]
10 In order to obtain the above-described effect, the proportion of the crystal grains
of one or two kinds of unauto-tempered martensite and lower bainite in the surface layer region is set to 15.0% or more in terms of area percentage. The proportion of the crystal grains thereof is preferably 20.0% or more in terms of area percentage. From the viewpoint of suppressing the occurrence of cracking at the time of cold rolling in post
15 processes, the proportion of the crystal grains thereof may be set to 30.0% or more in
terms of area percentage. The upper limit of the proportion of the crystal grains of one or two kinds of unauto-tempered martensite and lower bainite in the metallographic structure of the surface layer region is not particularly limited. The proportion of the crystal grains of one or two kinds of unauto-tempered martensite and lower bainite in the
20 metallographic structure of the surface layer region may be, for example, 50% or less or
90% or less in terms of area percentage. In addition, as a remaining structure other than the unauto-tempered martensite and the lower bainite, the metallographic structure in the surface layer region may include one or more of ferrite, upper bainite, residual austenite, and martensite that has been auto-tempered.
25 [0055]
20

The metallographic structure of the central portion of the base steel sheet is not
particularly limited, but is normally one or more of ferrite, upper bainite, lower bainite,
martensite, residual austenite, an iron carbide, and an alloy carbide. Here, the central
portion of the base steel sheet refers to a portion ranging from a position 0.2 mm apart
5 from one surface of the base steel sheet in the sheet thickness central direction to a
position 0.2 mm apart from the other surface of the base steel sheet in the sheet thickness
central direction.
[0056]
“Measurement of area fraction of crystal grains of unauto-tempered martensite
10 and lower bainite”
A method for measuring the area fraction of the crystal grains of the unauto-
tempered martensite and the lower bainite in the surface layer region of the base steel
sheet that configures the steel sheet for hot stamping according to the present
embodiment will be described.
15 [0057]
First, a sample is cut out from an arbitrary position 50 mm or more apart from
the end face of the steel sheet for hot stamping such that a rolling-direction cross section
(sheet thickness section) perpendicular to the surface can be observed. While also
depending on a measuring instrument, the size of the sample is set to a size large enough
20 to observe approximately 10 mm in the rolling direction. A measurement surface of the
sample that corresponds to the above-described rolling-direction cross section is polished
using silicon carbide paper #600 to #1500, and then the measurement surface is mirror-
finished using a liquid in which diamond powder having particle diameters of 1 to 6 μm
is dispersed in a diluted solution such as alcohol or pure water. Next, the measurement
25 surface is polished for eight minutes at room temperature using colloidal silica containing
21

no alkaline solution to remove strain present on the surface layer of the sample. After
that, the measurement surface is sputtered with argon ion beams using a cross section
polisher manufactured by JEOL Ltd. At this time, the argon ion beams are radiated to
the measurement surface from all directions using a specimen rotary holder manufactured
5 by JEOL Ltd. for the purpose of suppressing the generation of streaky unevenness on the
measurement surface. [0058]
At an arbitrary position of the measurement surface in the rolling direction, a 50 μm-long region from the interface between the plating layer and the surface of the base
10 steel sheet to a depth of 50 μm is measured by the electron backscatter diffraction method
at measurement intervals of 0.1 μm, thereby obtaining crystal orientation information. For the measurement, an instrument including a thermal field emission-type scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSD detector (DVC 5-type detector manufactured by TSL) is used. At this time, the degree of
15 vacuum in the instrument is set to 9.6 × 10-5 Pa or less, the accelerating voltage is set to
15 k V, the irradiation current level is set to 13, and the irradiation time of the electron beam is set to 0.5 seconds/point. The obtained crystal orientation information is analyzed using a “Grain Average Image Quality” function provided to software “OIM Analysis (registered trademark)” accompanied by the EBSD analyzer. With this
20 function, it is possible to digitalize the definition of the crystal orientation information as
an IQ value and to discriminate a structure that is not auto-tempered. The unauto-tempered martensite and the lower bainite have poor crystallinity and thus have a small IQ value. A region showing an IQ value calculated to be 60000 or less with the “Grain Average Image Quality” function is defined as unauto-tempered martensite and lower
25 bainite, and the area fraction thereof is calculated. The area percentage of the crystal
22

grains of the unauto-tempered martensite and the lower bainite in the surface layer region
is obtained by the above-described method.
[0059]
“Measurement of average dislocation density”
5 Next, a method for measuring the average dislocation density in the surface
layer region will be described. The average dislocation density can be measured by the X-ray diffraction method or transmission electron microscopic observation and is measured using the X-ray diffraction method in the present embodiment. [0060]
10 First, a sample is cut out from an arbitrary position 50 mm or more apart from
the end face of the base steel sheet. While also depending on a measuring instrument, the size of the sample is set to a size of approximately 20 mm × 20 mm. A thickness of 25 μm is reduced from each of the front surface and the rear surface of the sample using a mixed solution of distilled water (48%), a hydrogen peroxide solution (48%), and
15 hydrofluoric acid (4%), thereby reducing the thickness by a total of 50 μm. Therefore,
regions 25 μm deep from the surfaces of the sample before the thickness reduction are exposed. X-ray diffraction measurement is carried out on these exposed surfaces to specify a plurality of diffraction peaks of body-centered cubic lattices. The average dislocation density is analyzed from the half widths of these diffraction peaks, thereby
20 obtaining the average dislocation density of the surface layer region. Regarding the
analysis method, a modified Williamson-Hall method described in Non-Patent Document
1 is used.
[0061]
“Plating layer being attached to single surface in amount of 10 g/m2 or more and
25 90 g/m2 or less and having Ni content of 10 mass% or more and 25 mass% or less with
23

remainder including Zn and impurities”
The steel sheet for hot stamping according to the present embodiment has a
plating layer on the surface of the base steel sheet that configures the steel sheet for hot
stamping, the plating layer being attached to a single surface in an amount of 10 g/m2 or
5 more and 90 g/m2 or less and having a Ni content of 10 mass% or more and 25 mass% or
less with a remainder including Zn and impurities. This makes it possible to diffuse Ni into the surface layer region during heating before hot stamping. [0062]
When the amount of the plating layer attached to a single surface is less than 10
10 g/m2 or the Ni content in the plating layer is less than 10 mass%, the amount of Ni that
concentrates in the surface layer region of the base steel sheet becomes small, and it is not possible to obtain a desired metallographic structure in the surface layer region after hot stamping. On the other hand, in a case where the amount attached to a single surface exceeds 90 g/m2 or the Ni content in the plating layer exceeds 25 mass%, Ni
15 concentrates excessively in the interface between the plating layer and the base steel
sheet, the adhesion between the plating layer and the base steel sheet deteriorates, it becomes difficult for Ni in the plating layer to diffuse into the surface layer region of the base steel sheet, and it is not possible to obtain a desired metallographic structure in the hot-stamped article after hot stamping.
20 The amount of the plating layer attached to a single surface is preferably 30 g/m2
or more and more preferably 40 g/m2 or more. In addition, the amount of the Ni plating layer attached to a single surface is preferably 80 g/m2 or less and more preferably 60 g/m2 or less. [0063]
25 The amount of a plate attached in the steel sheet for hot stamping and the Ni
24

content in the plating layer are measured by the following methods.
The amount of the plate attached is measured by collecting a test piece from an
arbitrary position in the steel sheet for hot stamping according to the testing method
described in JIS H 0401: 2013. Regarding the Ni content in the plating layer, a test
5 piece is collected from an arbitrary position in the steel sheet for hot stamping according
to the testing method described in JIS K 0150: 2005, and the Ni content at a 1/2 position
of the total thickness of the plating layer is measured, thereby obtaining the Ni content in
the plating layer in the steel sheet for hot stamping.
[0064]
10 The sheet thickness of the steel sheet for hot stamping according to the present
embodiment is not particularly limited, but is preferably set to 0.5 to 3.5 mm from the viewpoint of the weight reduction of automotive bodies. [0065]
Next, a hot-stamped article, which is the steel sheet for hot stamping according
15 to the present embodiment that has been hot-stamped, will be described. In the
following description, a hot-stamped article that is manufactured using the steel sheet for
hot stamping according to the present embodiment and is excellent in terms of strength
and bendability will be referred to as the first application example, and a hot-stamped
article that is manufactured using the steel sheet for hot stamping according to the present
20 embodiment and is excellent in terms of strength and hydrogen embrittlement resistance
will be referred to as the second application example. [0066]

“First application example”
25 “Metallographic structure of surface layer region that is region from surface of
25

base steel sheet to depth of 50 μm includes 80.0% or more of martensite and 8.0% or
more of residual austenite in terms of area percentage”
The metallographic structure of the surface layer region that is a region from the
surface of the base steel sheet that configures the hot-stamped article to a depth of 50 μm
5 includes 80.0% or more of martensite and 8.0% or more of residual austenite in terms of
area percentage, whereby it is possible to obtain an excellent strength and excellent
bendability. As a remaining structure other than the martensite and the residual
austenite in the surface layer region, one or more of ferrite, upper bainite, and lower
bainite may be included.
10 [0067]
In a case where the proportion of the martensite is less than 80.0% in terms of
area percentage in the metallographic structure of the surface layer region, it is not
possible to obtain a desired strength in the hot-stamped article, and it is not possible to
apply the hot-stamped article to automotive parts or the like. The proportion of the
15 martensite is preferably 85.0% or more in terms of area percentage. The proportion of
the martensite may be set to 92.0% or less in terms of area percentage.
[0068]
In addition, when the proportion of the residual austenite is less than 8.0% in
terms of area percentage, the bendability of the hot-stamped article deteriorates. The
20 proportion of the residual austenite is preferably 10.0% or more in terms of area
percentage. There is no need to particularly limit the upper limit; however, in a case
where it is attempted to obtain a higher yield strength, the proportion of the residual
austenite may be set to 15.0% or less in terms of area percentage.
[0069]
25 Next, a method for measuring the metallographic structure of the surface layer
26

region will be described.
First, samples are cut out from an arbitrary position 50 mm or more apart from
the end face of the hot-stamped article such that a rolling-direction cross section (sheet
thickness cross section) perpendicular to the surface can be observed. While also
5 depending on a measuring instrument, the size of the sample is set to a size large enough
to observe approximately 10 mm in the rolling direction. The area fractions of the residual austenite and the martensite are measured using the samples collected by the above-described method. [0070]
10 “Measurement of area fraction of residual austenite”
A measurement surface of the sample that corresponds to the above-described rolling-direction cross section is polished using silicon carbide paper #600 to #1500, and then the measurement surface is mirror-finished using a liquid in which diamond powder having particle diameters of 1 to 6 μm is dispersed in a diluted solution such as alcohol or
15 pure water. Next, the measurement surface is polished for eight minutes at room
temperature using colloidal silica containing no alkaline solution to remove strain present on the surface layer of the sample. At an arbitrary position of the measurement surface of the sample in the rolling direction, a 50 μm-long region from the surface of the base steel sheet to a depth of 50 μm is measured by the electron backscatter diffraction method
20 at measurement intervals of 0.1 μm, thereby obtaining crystal orientation information.
For the measurement, an instrument including a thermal field emission-type scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSD detector (DVC 5-type detector manufactured by TSL) is used. At this time, the degree of vacuum in the instrument is set to 9.6 × 10-5 Pa or less, the accelerating voltage is set to
25 15 k V, the irradiation current level is set to 13, and the irradiation time of the electron
27

beam is set to 0.01 seconds/point. The area fraction of the residual austenite is
calculated from the obtained crystal orientation information using a “Phase Map”
function provided to software “OIM Analysis (registered trademark)” accompanied by
the EBSD analyzer, thereby obtaining the area fraction of the residual austenite in the
5 surface layer region. A structure having an fcc structure as the crystal structure is
determined to be the residual austenite. [0071]
“Measurement of area fraction of martensite”
A measurement surface of the sample (a sample different from the sample used
10 for the measurement of the area fraction of the residual austenite) is polished using
silicon carbide paper #600 to #1500, then, the measurement surface is mirror-finished using a liquid in which diamond powder having particle diameters of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water, and Nital-etched. Next, a region within 50 μm from the end portion of the observation surface on a side toward the
15 surface of the base steel sheet is observed as an observation visual field using a thermal
field emission-type scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.). The area fraction of martensite can be obtained as the total of the area fractions of tempered martensite and fresh martensite. The tempered martensite is an aggregate of lath crystal grains and is differentiated as a structure in which there are two or more
20 extension directions of an iron carbide. The fresh martensite is not sufficiently etched
by Nital etching and thus can be differentiated from other structures that are etched. Here, since the residual austenite is also, similar to the fresh martensite, not sufficiently etched, the area percentage of the fresh martensite is obtained from the difference between the area fraction of the structures that are not etched by Nital etching and the
25 area fraction of the residual austenite calculated above. The area fraction of the
28

martensite in the surface layer region is obtained by calculating the total of the area
percentages of the tempered martensite and the fresh martensite obtained by the above-
described method.
[0072]
5 “Concentration of Ni in surface layer region is 8 mass% or more”
The concentration of Ni in the surface layer region of the base steel sheet that
configures the hot-stamped article is 8 mass%. When the concentration of Ni in the
surface layer region is set to 8 mass% or more, the residual austenite in the surface layer
region is stabilized, and it is possible to increase the amount of the residual austenite in
10 the hot-stamped article. As a result, it is possible to improve the bendability of the hot-
stamped article. The concentration of Ni in the surface layer region is preferably 10
mass% or more and more preferably 12 mass% or more. In addition, the upper limit of
the concentration of Ni in the surface layer region is not particularly limited, and the
concentration of Ni may be, for example, 15 mass% or less or may be 20 mass% or less.
15 [0073]
“Method for measuring concentration of Ni in surface layer region” A method for measuring the concentration of Ni in the surface layer region will be described.
First, a sample is cut out from an arbitrary position 50 mm or more apart from
20 the end face of the hot-stamped article. While also depending on a measuring
instrument, the size of the sample is set to a size of approximately 20 mm × 20 mm.
Regarding the measurement of the concentration of Ni, an analysis is carried out in the
depth direction along the sheet thickness direction from the surface of the base steel sheet
by glow discharge optical emission spectrometry at 10 points on the surface of the
25 sample, the concentrations of Ni at positions 25 μm deep from the surface of the base
29

steel sheet in the sheet thickness direction are obtained, and the average value of the 10
points is calculated. The obtained average value is defined as the concentration of Ni in
the surface layer region.
[0074]
5 “Second application example”
“Metallographic structure of surface layer region that is region from surface of base steel sheet to depth of 50 μm includes 90.0% or more of martensite in terms of area percentage”
When the proportion of martensite in the metallographic structure of the surface
10 layer region that is a region from the surface of the base steel sheet to a depth of 50 μm is
90.0% or more, it is possible to obtain an excellent strength and excellent hydrogen embrittlement resistance in the hot-stamped article. The proportion of martensite is preferably as high as possible.
As a remaining structure other than the martensite in the surface layer region,
15 one or more of ferrite, upper bainite, lower bainite, and residual austenite may be
included.
The metallographic structure of the surface layer region that is a region from the
surface of the steel sheet to a depth of 50 μm is measured by the above-described
method.
20 [0075]
“Concentration of Ni in prior austenite grain boundaries in surface layer region is 5.5 mass% or more”
When the concentration of Ni in the prior austenite grain boundaries in the
surface layer region is 5.5 mass% or more, it is possible to obtain an excellent strength
25 and excellent hydrogen embrittlement resistance in the hot-stamped article. The
30

concentration of Ni is preferably 7.0 mass% or more. The concentration of Ni is
preferably as high as possible, but it is difficult to set to the concentration of Ni to 12.0
mass% or more in normal actual operation, and thus the substantial upper limit of the
concentration of Ni is 12.0 mass%.
5 [0076]
“Method for measuring concentration of Ni in prior austenite grain boundaries in surface layer region”
A method for measuring the concentration of Ni in the prior austenite grain boundaries in the surface layer region will be described.
10 A test piece having dimensions shown in Fig. 1 is produced from the central
portion of the hot-stamped article after a heat treatment. In a cut at the central portion of the test piece, the bonding portion at the bottom of the cut is controlled from 100 μm to 200 μm by inserting a wire cutter. Next, the test piece is immersed in a 40% ammonium thiocyanate solution for 24 to 48 hours. After the end of the immersion,
15 galvanization is carried out on the front and rear surfaces of the test piece within 0.5
hours. After the galvanization, an Auger electron emission spectroscopic analysis is carried out within 1.5 hours. The kind of a device for carrying out the Auger electron emission spectroscopic analysis is not particularly limited. The test piece is set in the analyzing device, and the prior austenite grain boundaries are exposed by breaking the
20 test piece from the cut portion in a vacuum of 9.6 × 10-5 Pa or less. An electron beam is
radiated to the prior austenite grain boundaries exposed in a 50 μm region in the surface layer in the sheet thickness direction at an accelerating voltage of 1 to 30 k V, and the concentration (mass%) of Ni in the grain boundaries is measured. The concentration of Ni is measured at 10 or more prior austenite grain boundaries. The measurement is
25 completed within 30 minutes from the breakage in order to prevent the contamination of
31

the grain boundaries. The average value of the obtained concentrations (mass%) of Ni
is calculated, thereby obtaining the concentration of Ni in the prior austenite grain
boundaries in the surface layer region.
[0077]
5 “Plating layer being attached to single surface in amount of 10 g/m2 or more and
90 g/m2 or less and having Ni content of 10 mass% or more and 25 mass% or less with remainder including Zn and impurities”
The hot-stamped articles of the first application example and the second
application example each have a plating layer on the surface of the base steel sheet that
10 configures the hot-stamped article, the plating layer being attached to a single surface in
an amount of 10 g/m2 or more and 90 g/m2 or less and having a Ni content of 10 mass% or more and 25 mass% or less with a remainder including Zn and impurities. [0078]
When the amount of the plating layer attached to a single surface of the base
15 steel sheet is less than 10 g/m2 or the Ni content in the plating layer is less than 10
mass%, the amount of Ni that concentrates in the surface layer region of the base steel
sheet becomes small, and it is not possible to obtain a desired metallographic structure in
the surface layer region after hot stamping. On the other hand, in a case where the
amount of the plating layer attached to a single surface of the base steel sheet exceeds 90
20 g/m2 or the Ni content in the plating layer exceeds 25 mass%, Ni concentrates
excessively in the interface between the plating layer and the base steel sheet, the
adhesion between the plating layer and the base steel sheet deteriorates, it becomes
difficult for Ni in the plating layer to diffuse into the surface layer region of the base steel
sheet, and it is not possible to obtain a desired metallographic structure in the hot-
25 stamped article.
32

The amount of the plating layer attached to a single surface of the base steel
sheet is preferably 30 g/m2 or more and more preferably 40 g/m2 or more. In addition,
the amount of the Ni plating layer attached to a single surface of the base steel sheet is
preferably 80 g/m2 or less and more preferably 60 g/m2 or less.
5 [0079]
The amount of the plate attached in the hot-stamped article and the Ni content in the plating layer are measured by the following methods.
The amount of the plate attached is measured by collecting a test piece from an
arbitrary position in the hot-stamped article according to the testing method described in
10 JIS H 0401: 2013. Regarding the Ni content in the plating layer, a test piece is collected
from an arbitrary position in the hot-stamped article according to the testing method
described in JIS K 0150: 2005, and the Ni content at a 1/2 position of the total thickness
of the plating layer is measured, thereby obtaining the Ni content in the plating layer in
the hot-stamped article.
15 [0080]
Next, preferred methods for manufacturing the steel sheet for hot stamping
according to the present embodiment and the hot-stamped article for which the steel sheet
for hot stamping according to the present embodiment is used will be described.
[0081]
20
A steel piece (steel) that is to be subjected to hot rolling may be, for example, a
steel piece manufactured by a usual method such as a continuously cast slag or a thin slab
caster as long as the steel piece is manufactured by a normal method. Rough rolling
may also be carried out by a usual method and is not particularly limited.
25 [0082]
33

“Finishing rolling”
In the final rolling (final pass) of finishing rolling, it is necessary to carry out the
finishing rolling in a temperature range of the A3 point or higher at a rolling reduction of
smaller than 20%. When the finishing rolling is carried out at a temperature of lower
5 than the A3 point or the rolling reduction is 20% or larger in the final rolling of the
finishing rolling, ferrite is formed in the surface layer region, and it is not possible to set
the proportion of the crystal grains of one or two kinds of the martensite and the lower
bainite that are not auto-tempered to 15.0% or more in terms of area percentage. The A3
point is represented by Formula (1).
10 [0083]
A3 point = 850 + 10 × (C + N) × Mn + 350 × Nb + 250 × Ti + 40 × B + 10 × Cr
+ 100 × Mo ∙∙∙ (1)
In Formula (1), C, N, Mn, Nb, Ti, B, Cr, and Mo indicate the amounts (mass%)
of the individual elements.
15 [0084]
“Cooling”
After the end of the finishing rolling, cooling at an average cooling rate of 80
°C/s or faster is initiated within five seconds, and the base steel sheet is cooled to a
temperature range of lower than 500°C and coiled. In addition, even after coiled, the
20 base steel sheet is continuously cooled with water to room temperature. In a case where
the cooling initiation time exceeds five seconds, a case where the average cooling rate is
slower than 80 °C/s, or a case where the coiling initiation temperature is higher than
500°C, ferrite, pearlite, and upper bainite are likely to be formed, and it is impossible to
set the proportion of the crystal grains of one or two kinds of the martensite and the lower
25 bainite that are not auto-tempered to 15.0% or more in terms of area percentage in the
34

surface layer region. The average cooling rate at this time is calculated from a change
in the temperature of the surface of the steel sheet and indicates an average cooling rate
while the temperature reaches the coiling initiation temperature from the finishing rolling
temperature.
5 [0085]
“Plating”
On the hot-rolled steel sheet as it is or after cold rolling, a plating layer being attached to a single surface in an amount of 10 g/m2 or more and 90 g/m2 or less and having a Ni content of 10 mass% or more and 25 mass% or less with a remainder
10 including Zn and impurities is formed to obtain the steel sheet for hot stamping. In a
case where cold rolling is carried out before plating, the rolling reduction in the cold rolling is not particularly limited, but is preferably set to 40% to 60% from the viewpoint of the shape stability of the steel sheet. The manufacturing of the steel sheet for hot stamping may additionally include a well-known manufacturing method such as pickling
15 or tempered rolling before the plating. However, when tempering is carried out at a
temperature of the Ms point - 15°C or higher, it is not possible to set the proportion of the crystal grains of one or two kinds of the unauto-tempered martensite and the lower bainite to 15.0% or more in terms of area percentage and to set the average dislocation density to 4 × 1015 m/m3 or more in the surface layer region, and consequently, it is not
20 possible to obtain a hot-stamped article having a desired metallographic structure.
Therefore, in a case where there is a need to carry out tempering before the plating for the reason of a high C content or the like, the tempering is carried out at a temperature of lower than the Ms point - 15°C. The Ms point is represented by Formula (2). [0086]
25 Ms = 493 - 300 × C - 33.3 × Mn - 11.1 × Si - 22.2 × Cr - 16.7 × Ni - 11.1 × Mo
35

∙∙∙ (2)
In Formula (2), C, Mn, Si, Cr, Ni, and Mo indicate the amounts (mass%) of the
individual elements.
[0087]
5 Next, the method for manufacturing the hot-stamped article for which the steel
sheet for hot stamping according to the present embodiment is used will be described. [0088]

The hot-stamped article is manufactured by heating the steel sheet for hot
10 stamping obtained as described above in a temperature range of 500°C or higher and the
A3 point or lower under a condition 1 for the first application example (at an average
heating rate of slower than 100 °C/s) or under a condition 2 for the second application
example (at an average heating rate of 100 °C/s or faster and slower than 200 °C/s), then,
holding the steel sheet for hot stamping at a temperature of the A3 point or higher and the
15 A3 point + 150°C or lower, hot-stamping the steel sheet for hot stamping such that the
elapsed time from the initiation of the heating to the initiation of forming reaches a predetermined time or shorter, and cooling the steel sheet for hot stamping to room temperature.
In addition, in order to adjust the strength of the hot-stamped article, a softened
20 region may be formed by tempering a partial region or the entire region of the hot-
stamped article at a temperature of lower than the Ms point - 15°C. [0089]
In the first application example, the steel sheet for hot stamping is heated in a
temperature range of 500°C or higher and the A3 point or lower under the condition 1 (at
25 an average heating rate of slower than 100 °C/s), and the elapsed time from the initiation
36

of the heating to the initiation of the forming is set to 240 to 480 seconds, whereby it is
possible to obtain a desired metallographic structure in the surface layer region. This
makes it possible to improve the bendability of the hot-stamped article. The average
heating rate under the condition 1 is preferably slower than 80 °C/s. The lower limit of
5 the average heating rate under the condition 1 is not particularly limited; however, in
actual operation, the average heating rate set to slower than 0.01 °C/s causes an increase in the manufacturing costs. Therefore, the average heating rate under the condition 1 may be set to 0.01 °C/s or faster. The elapsed time from the initiation of the heating to the initiation of the forming is preferably set to 280 seconds or longer and 320 seconds or
10 shorter.
[0090]
In the second application example, the steel sheet for hot stamping is heated in a temperature range of 500°C or higher and the A3 point or lower under the condition 2 (at an average heating rate of 100 °C/s or faster and slower than 200 °C/s), and the elapsed
15 time from the initiation of the heating to the initiation of the forming is set to be shorter
than 240 seconds, whereby it is possible to obtain a desired metallographic structure in the surface layer region. This makes it possible to obtain excellent hydrogen embrittlement resistance in the hot-stamped article. The average heating rate under the condition 2 is preferably 120 °C/s or faster. The average heating rate under the
20 condition 2 is set to slower than 200 °C/s since transformation into austenite is
accelerated while a carbide that is contained in the steel sheet for hot stamping is not completely dissolved, and the deterioration of the hydrogen embrittlement resistance of the hot-stamped article is caused. The average heating rate under the condition 2 is preferably slower than 180 °C/s. The elapsed time from the initiation of the heating to
25 the initiation of the forming is preferably set to 120 seconds or longer and 180 seconds or
37

shorter.
[0091]
The holding temperature during the hot stamping is preferably set to the A3 point
+ 10°C or higher and the A3 point + 150°C or lower. In addition, the average cooling
5 rate after the hot stamping is preferably set to 10 °C/s or faster.
[Examples]
[0092]
Next, examples of the present invention will be described. Conditions in the
examples are examples of conditions adopted to confirm the feasibility and effect of the
10 present invention, and the present invention is not limited to these examples of
conditions. The present invention is capable of adopting a variety of conditions as long
as the objective of the present invention is achieved without departing from the gist of the
present invention.
[0093]
15 Steel pieces manufactured by casting molten steel having a chemical
composition shown in Tables 1 and 2 were hot-rolled, cold-rolled, and plated under
conditions shown in Tables 3 and 4, thereby obtaining steel sheets for hot stamping
shown in Tables 3 and 4. The obtained steel sheets for hot stamping were hot-stamped
by carrying out a heat treatment shown in Tables 5 to 8, thereby obtaining hot-stamped
20 articles shown in Tables 5 to 8. A partial softened region was formed by irradiating a
part of the hot-stamped article with a laser, cooling the irradiated portion to lower than
the Ms - 15°C, and tempering the irradiated portion.
Underlined values in the tables indicate that the values are outside the scope of
the present invention, the preferred manufacturing conditions are not satisfied, or
25 property values are not preferable.
38

[0094]
[Table 1]

Steel No. Chemical composition (mass%) with remainder of Fe and impurities A3 (°C) Note

C Si Mn P S sol. Al N Nb Ti Mo Cr B Ca REM

1 0.16 0.250 1.10 0.006 0.0020 0.030 0.0026 0.130 865 Invention steel
2 0.44 0.250 1.80 0.010 0.0090 0.400 0.0040 0.03 858 Invention steel
3 0.23 0.250 1.20 0.010 0.0100 0.030 0.0050 0.020 0.200 860 Invention steel
4 0.08 0.220 0.81 0.008 0.0009 0.044 0.0026 851 Comparative steel
5 0.16 0.150 0.71 0.011 0.0006 0.043 0.0037 851 Invention steel
6 0.31 0.250 0.80 0.015 0.0011 0.041 0.0039 853 Invention steel
7 0.36 0.180 0.81 0.005 0.0005 0.045 0.0037 853 Invention steel
8 0.44 0.250 0.71 0.015 0.0007 0.034 0.0042 853 Invention steel
9 0.67 0.190 0.71 0.014 0.0003 0.037 0.0035 855 Invention steel
10 0.78 0.250 0.90 0.014 0.0011 0.031 0.0026 857 Comparative steel
11 0.36 0.002 0.86 0.005 0.0003 0.041 0.0032 853 Comparative steel
12 0.38 0.007 0.83 0.005 0.0011 0.050 0.0030 853 Invention steel
13 0.37 0.210 0.72 0.011 0.0007 0.030 0.0041 853 Invention steel
14 0.37 0.240 0.90 0.015 0.0007 0.047 0.0037 853 Invention steel
15 0.37 0.150 0.15 0.005 0.0003 0.035 0.0030 851 Comparative +steel
16 0.44 0.170 0.44 0.007 0.0005 0.049 0.0029 852 Invention steel
17 0.36 0.240 0.82 0.010 0.0011 0.035 0.0038 853 Invention steel
18 0.37 0.180 1.29 0.007 0.0010 0.030 0.0028 855 Invention steel
19 0.37 0.150 1.99 0.009 0.0005 0.035 0.0042 857 Invention steel
20 0.38 0.170 2.89 0.007 0.0005 0.046 0.0037 861 Invention steel
21 0.38 0.150 3.15 0.012 0.0009 0.036 0.0042 862 Comparative steel
22 0.38 0.240 0.82 0.0004 0.0007 0.045 0.0026 853 Invention steel
39

23 0.36 0.160 0.90 0.009
24 0.36 0.150 0.77 0.094
25 0.37 0.190 0.84 0.123
26 0.36 0.200 0.75 0.009
27 0.37 0.150 0.81 0.013
28 0.37 0.190 0.89 0.008
29 0.36 0.230 0.80 0.007
30 0.36 0.190 0.72 0.006
[0095]
0.0006 0.030 0.0038 853 Invention steel
0.0010 0.043 0.0033 853 Invention steel
0.0010 0.033 0.0032 853 Comparative steel
0.0002 0.047 0.0045 853 Invention steel
0.0003 0.031 0.0029 853 Invention steel
0.0022 0.044 0.0032 853 Invention steel
0.0900 0.049 0.0030 853 Invention steel
0.1334 0.045 0.0025 853 Comparative steel

[Table 2]

Steel No. Chemical composition (mass%) with remainder of Fe and impurities A3 (°C) Note

C Si Mn P S sol. Al N Nb Ti Mo Cr B Ca REM

31 0.38 0.230 0.79 0.013 0.0008 0.0001 0.0027 853 Comparative steel
32 0.38 0.160 0.85 0.010 0.0009 0.0003 0.0033 853 Invention steel
33 0.35 0.200 0.72 0.014 0.0007 0.0029 0.0042 853 Invention steel
34 0.37 0.160 0.73 0.006 0.0006 0.031 0.0026 853 Invention steel
35 0.35 0.240 0.83 0.009 0.0008 0.494 0.0034 853 Invention steel
36 0.37 0.240 0.84 0.011 0.0007 0.581 0.0040 853 Comparative steel
37 0.37 0.220 0.89 0.007 0.0007 0.035 0.0001 853 Invention steel
38 0.38 0.150 0.89 0.009 0.0008 0.038 0.0073 853 Invention steel
39 0.38 0.190 0.71 0.007 0.0007 0.039 0.0090 853 Invention steel
40 0.36 0.210 0.73 0.008 0.0003 0.035 0.0160 853 Comparative steel
41 0.37 0.230 0.87 0.009 0.0006 0.031 0.0025 0.012 857 Invention steel
42 0.36 0.170 0.70 0.009 0.0009 0.046 0.0030 0.032 864 Invention steel
43 0.37 0.220 0.73 0.008 0.0004 0.033 0.0038 0.120 895 Invention steel
44 0.37 0.230 0.90 0.009 0.0011 0.044 0.0044 0.013 857 Invention steel
40

45 0.35 0.170 0.89 0.011 0.0007 0.043 0.0028 0.036 862 Invention steel
46 0.36 0.170 0.88 0.007 0.0004 0.031 0.0033 0.140 888 Invention steel
47 0.36 0.210 0.80 0.005 0.0003 0.037 0.0035 0.006 854 Invention steel
48 0.37 0.200 0.78 0.009 0.0010 0.031 0.0026 0.012 854 Invention steel
49 0.38 0.160 0.82 0.015 0.0009 0.031 0.0041 0.980 951 Invention steel
50 0.36 0.230 0.77 0.011 0.0008 0.043 0.0038 0.006 853 Invention steel
51 0.35 0.160 0.70 0.005 0.0006 0.047 0.0026 0.009 853 Invention steel
52 0.37 0.250 0.83 0.006 0.0010 0.033 0.0039 0.960 863 Invention steel
53 0.37 0.150 0.70 0.015 0.0008 0.031 0.0044 0.0006 853 Invention steel
54 0.36 0.230 0.86 0.005 0.0003 0.050 0.0044 0.0011 853 Invention steel
55 0.36 0.160 0.74 0.015 0.0006 0.034 0.0044 0.0090 853 Invention steel
56 0.36 0.160 0.78 0.015 0.0006 0.037 0.0039 0.0080 853 Invention steel
57 0.36 0.190 0.80 0.010 0.0006 0.034 0.0027 0.28 853 Invention steel
58 0.14 0.110 0.84 0.011 0.0008 0.036 0.0031 851 Comparative steel
59 0.20 0.210 1.32 0.012 0.0007 0.028 0.0027 853 Invention steel
[0096]
[Table 3]

Steel No. Steel sheet No. Finishing rolling Cooling Coiling Cold rolling Heat
treatment
before
plating Steel sheet for hot stamping Note

Final
rolling
reduction
of
finishing
rolling (%) Finishing
rolling
temperature
(°C) Time
until
initiation
of
cooling
(s) Cooling
rate
(°C/s) Coiling
initiation
temperature
(°C) Rolling
reduction
(%) Heating
temperature
(°C) Amount
of plate
attached
(g/m2) Ni content
in
plating
layer
(mass
%) Average dislocation
density (1014 m/m3) Unauto-tempered martensite and lower bainite (area%) Sheet
thickness
(mm)

1 1 6 910 3 38 571 41 N/A 39 15 1 4.9 1.6 Comparative steel
2 2 5 921 2 32 596 40 N/A 55 12 29 2.1 1.6 Comparative steel
41

3 3 5 934 3 51 397 50 773 41 12 21 6.3 1.6 Comparative steel
4 4 9 954 1 126 372 53 N/A 56 15 3 9.8 1.6 Comparative steel
5 5 6 884 4 115 369 58 N/A 50 17 41 18.4 2.0 Invention steel
6 6 8 917 2 133 345 45 N/A 41 17 96 21.2 1.9 Invention steel
7 7 10 901 1 123 330 44 N/A 54 25 187 21.9 1.5 Invention steel
8 8 10 897 4 103 266 49 N/A 57 16 345 18.5 1.7 Invention steel
9 9 6 924 1 115 261 53 N/A 40 17 948 21.3 1.6 Invention steel
10 10 10 898 1 115 316 43 N/A 53 18 1120 24.7 1.3 Comparative steel
11 11 8 905 1 142 292 55 N/A 48 18 155 18.1 1.3 Comparative steel
12 12 5 887 4 118 353 60 N/A 58 15 230 19.9 1.6 Invention steel
13 13 5 926 3 146 390 47 N/A 48 15 223 22.3 1.6 Invention steel
14 14 9 890 3 134 283 55 N/A 46 15 219 17.6 1.8 Invention steel
15 15 8 933 1 123 329 53 N/A 58 18 11 3.7 1.6 Comparative steel
16 16 6 885 4 115 329 52 N/A 51 17 372 22.0 1.3 Invention steel
17 17 9 900 3 133 379 45 N/A 43 18 157 22.7 1.2 Invention steel
18 18 7 876 4 134 250 42 N/A 52 17 184 16.6 1.7 Invention steel
19 19 10 942 1 105 315 59 N/A 50 17 229 20.3 1.9 Invention steel
20 20 10 960 3 125 273 49 N/A 45 17 217 22.4 2.0 Invention steel
21 21 5 895 2 147 343 41 N/A 45 17 192 24.4 1.6 Comparative steel
22 22 8 931 3 123 354 43 N/A 60 17 178 20.3 1.8 Invention steel
23 23 10 961 1 138 272 48 N/A 47 15 159 18.4 2.0 Invention steel
24 24 10 955 2 124 330 45 N/A 60 15 189 18.9 1.2 Invention steel
42

25 25 5 901 1 104 310 50 N/A 58 15 237 20.9 1.6 Comparative steel
26 26 5 892 4 134 329 52 N/A 60 16 206 21.2 1.2 Invention steel
27 27 10 938 3 104 251 41 N/A 52 17 164 19.2 1.5 Invention steel
28 28 7 953 1 149 372 60 N/A 50 18 172 23.4 1.8 Invention steel
29 29 10 943 1 126 382 41 N/A 53 17 201 21.5 1.6 Invention steel
30 30 7 948 1 148 260 44 N/A 51 16 237 16.9 1.2 Comparative steel
31 31 5 895 1 115 328 52 N/A 46 16 152 18.5 1.8 Comparative steel
32 32 6 918 1 126 341 52 N/A 40 15 214 22.4 1.2 Invention steel
33 33 6 885 4 106 314 47 N/A 43 18 181 18.0 1.5 Invention steel
34 34 5 883 3 149 266 45 N/A 46 15 156 19.9 1.8 Invention steel
35 35 6 947 1 131 387 57 N/A 51 15 151 22.3 1.9 Invention steel
36 36 8 877 3 109 291 47 N/A 47 15 206 16.1 1.6 Comparative steel
37 37 9 947 2 118 365 45 N/A 52 16 171 21.4 1.7 Invention steel
38 38 6 924 4 145 393 45 N/A 46 15 212 23.1 1.8 Invention steel
39 39 9 938 3 143 354 51 N/A 60 16 173 22.2 1.2 Invention steel
40 40 5 943 1 144 395 40 N/A 60 16 220 23.7 1.8 Comparative steel
[0097]
[Table 4]

Steel No. Steel sheet No. Finishing rolling Cooling Coiling Cold rolling Heat
treatment
before
plating Steel sheet for hot stamping Note

Final
rolling
reduction Finishing
rolling
temperature Time
until
initiation Cooling
rate
(°C/s) Coiling
initiation
temperature Rolling
reduction
(%) Heating
temperature
(°C) Amount of plate attached Ni
content
in Average
dislocation
density Unauto-tempered martensite and Sheet
thickness
(mm)

43

of
finishing
rolling (%) (°C) of
cooling
(s) (°C) (g/m2) plating
layer
(mass
%) (1014 m/m3) lower bainite (area%)
41 41 9 921 2 114 394 58 N/A 45 16 169 23.1 2.0 Invention steel
42 42 9 898 1 106 330 48 N/A 58 15 229 18.9 1.8 Invention steel
43 43 10 952 4 131 376 55 N/A 59 15 233 23.2 1.6 Invention steel
44 44 5 902 4 119 291 52 N/A 45 16 219 19.7 1.2 Invention steel
45 45 6 929 4 131 342 42 N/A 42 18 223 22.4 1.5 Invention steel
46 46 10 929 3 117 316 54 N/A 58 17 219 21.5 2.0 Invention steel
47 47 9 896 4 149 269 55 N/A 42 17 226 17.5 1.5 Invention steel
48 48 10 928 2 110 331 46 N/A 48 17 223 20.5 1.8 Invention steel
49 49 9 987 2 102 386 58 N/A 58 17 176 23.4 1.3 Invention steel
50 50 5 953 2 109 343 43 N/A 42 17 198 19.7 1.6 Invention steel
51 51 9 928 4 101 316 46 N/A 51 15 216 19.2 1.6 Invention steel
52 52 10 939 4 106 338 56 N/A 60 16 197 21.5 1.6 Invention steel
53 53 5 936 4 144 276 57 N/A 49 17 239 19.9 1.9 Invention steel
54 54 10 876 1 104 315 51 N/A 40 18 211 18.3 1.4 Invention steel
55 55 9 944 2 108 338 40 N/A 54 16 209 21.6 1.7 Invention steel
56 56 9 883 3 138 344 50 N/A 44 15 181 22.6 1.7 Invention steel
57 57 9 916 4 127 263 47 N/A 46 16 166 18.9 1.5 Invention steel
7 58 7 887 7 137 255 40 N/A 54 25 21 8.9 1.9 Comparative steel
7 59 10 890 4 72 260 47 N/A 54 25 34 3.9 1.3 Comparative steel
7 60 9 900 2 88 379 54 N/A 54 25 213 19.7 1.8 Invention steel
44

7 61 6 946 2 176 325 47 N/A 54 25 216 21.3 1.4 Invention steel
7 62 8 963 2 113 536 56 N/A 54 25 28 1.8 1.8 Comparative steel
7 63 9 905 3 134 308 51 N/A 54 25 238 25.1 1.3 Invention steel
7 64 6 911 3 118 220 59 N/A 54 25 174 28.6 1.9 Invention steel
7 65 5 908 3 139 94 45 N/A 54 25 204 39.4 1.4 Invention steel
7 66 8 881 4 137 42 56 N/A 54 25 206 46.8 1.6 Invention steel
7 67 10 874 2 108 365 43 320 54 25 94 15.3 1.5 Invention steel
7 68 6 898 2 119 386 0 N/A 54 25 203 21.5 2.8 Invention steel
7 69 10 901 1 123 330 44 N/A 54 8 187 21.9 1.5 Comparative steel
7 70 10 901 1 123 330 44 N/A 54 11 187 21.9 1.5 Invention steel
2 71 40 901 5 82 418 51 N/A 47 15 2 3.1 1.4 Comparative steel
9 72 9 932 2 58 403 53 N/A 41 16 721 12.6 1.4 Comparative steel
58 73 8 946 3 87 486 49 N/A 55 14 29 44.4 1.6 Comparative steel
59 74 9 957 3 102 283 50 N/A 45 17 71 19.7 1.6 Invention steel
[0098]
[Table 5]

Heat treatment process during hot stamping Mechanical Surface layer region
properties
Steel No. Steel sheet No. Manufacture No. Heating rate (°C/s) Holding
temperature
(°C) Elapsed time from initiation of heating
to
initiation
of forming
(s) Tempering
temperature
(°C) Partial softened region Amount of plate attached (g/m2) Ni content in plating
layer (mass%) Concentration of Ni (mass%) Martensite (area%) Residual γ (area%) Tensile strength (MPa) Bending
angle
(°) Note
1 1 A1 6 901 296 N/A No 39 15 3 96.7 2.1 1503 48 Comparative steel
45

2 2 A2 48 889 303 N/A No 55 12 1 96.4 1.1 2403 33 Comparative steel
3 3 A3 23 906 305 N/A No 41 12 3 93.4 2.0 1618 45 Comparative steel
4 4 A4 4 903 319 N/A No 56 15 2 87.4 2.3 1001 - Comparative steel
5 5 A5 1 877 306 N/A No 50 17 13 85.5 12.4 1555 71 Invention steel
6 6 A6 4 909 284 N/A No 41 17 13 82.9 12.8 1867 68 Invention steel
7 7 A7 2 873 287 N/A No 54 25 18 86.5 13.2 2025 59 Invention steel
8 8 A8 13 867 297 N/A No 57 16 10 89.9 9.9 2520 51 Invention steel
9 9 A9 8 896 295 247 No 40 17 10 89.4 10.1 1716 65 Invention steel
10 10 A10 2 935 302 N/A No 53 18 11 85.3 10.8 1298 - Comparative steel
11 11 A11 15 894 318 N/A No 48 18 14 86.5 8.3 2208 41 Comparative steel
12 12 A12 10 946 292 N/A No 58 15 11 90.1 8.9 2022 52 Invention steel
13 13 A13 2 886 284 N/A No 48 15 11 84.9 10.3 1936 61 Invention steel
14 14 A14 12 904 313 N/A No 46 15 11 86.8 10.9 2134 66 Invention steel
15 15 A15 53 862 301 N/A No 58 18 3 60.9 4.5 1361 - Comparative steel
16 16 A16 68 902 306 N/A No 51 17 10 87.7 9.9 2544 50 Invention steel
17 17 A17 19 949 306 N/A No 43 18 8 87.5 8.6 2008 63 Invention steel
18 18 A18 9 881 307 N/A No 52 17 9 88.3 8.6 2163 57 Invention steel
19 19 A19 74 939 315 N/A No 50 17 14 82.9 13.0 2018 52 Invention steel
20 20 A20 23 911 316 N/A No 45 17 9 90.3 8.9 2101 51 Invention steel
21 21 A21 37 911 306 N/A No 45 17 15 81.7 14.9 2208 46 Comparative steel
22 22 A22 51 917 313 N/A No 60 17 15 81.5 15.3 1935 65 Invention steel
23 23 A23 33 898 285 N/A No 47 15 13 84.7 13.3 2111 61 Invention steel
24 24 A24 39 888 292 N/A No 60 15 12 85.4 11.6 1967 55 Invention steel
25 25 A25 34 870 305 N/A No 58 15 9 90.6 9.1 2043 42 Comparative steel
26 26 A26 31 932 310 N/A No 60 16 13 84.3 13.3 2204 64 Invention steel
27 27 A27 72 918 286 N/A No 52 17 14 85.8 13.1 2062 65 Invention steel
46

28 28 A28 19 940 296 N/A No 50 18 10 87.3 10.6 2069 64 Invention steel
29 29 A29 15 864 284 N/A No 53 17 15 80.6 15.3 2068 58 Invention steel
30 30 A30 30 942 312 N/A No 51 16 16 82.2 15.1 2083 48 Comparative steel
31 31 A31 33 941 302 N/A No 46 16 9 86.6 9.1 2170 38 Comparative steel
32 32 A32 42 893 314 N/A No 40 15 13. 84.4 13.7 1981 57 Invention steel
33 33 A33 27 915 288 N/A No 43 18 14 84.0 14.3 2053 60 Invention steel
34 34 A34 17 889 287 N/A No 46 15 14 81.8 13.4 2040 62 Invention steel
35 35 A35 49 881 304 N/A No 51 15 14 83.3 13.6 1985 56 Invention steel
36 36 A36 23 888 282 N/A No 47 15 16 81.9 16.4 2045 46 Comparative steel
37 37 A37 28 946 305 N/A No 52 16 9 86.6 9.2 2027 61 Invention steel
38 38 A38 13 921 295 N/A No 46 15 14 82.8 13.2 2096 55 Invention steel
39 39 A39 60 915 302 N/A No 60 16 12 83.7 11.9 2007 51 Invention steel
40 40 A40 65 925 305 N/A No 60 16 13 85.0 12.7 1988 45 Comparative steel
[0099]
[Table 6]

Heat treatment process during hot stamping Ni
content in
plating
layer
(mass%) Mechanical Surface layer region
properties
Steel No. Steel sheet No. Manufacture No. Heating rate (°C/s) Holding
temperature
(°C) Elapsed time
from
initiation of
heating to initiation of forming (s) Tempering
temperature
(°C) Partial softened region Amount of plate attached (g/m2)
Concentration of Ni (mass%) Martensite (area%) Residual γ (area%) Tensile strength (MPa) Bending angle (°) Note
41 41 A41 40 912 319 N/A No 45 16 13 84.3 12.8 2207 65 Invention steel
42 42 A42 65 877 299 N/A No 58 15 10 86.9 10.7 2171 65 Invention steel
43 43 A43 70 984 302 N/A No 59 15 10 86.8 10.1 2124 63 Invention steel
44 44 A44 76 898 289 N/A No 45 16 9 89.0 8.6 2250 58 Invention steel
45 45 A45 55 944 319 N/A No 42 18 13 87.3 12.4 2044 62 Invention
47

steel
46 46 A46 39 918 307 N/A No 58 17 15 81.6 16.0 2272 61 Invention steel
47 47 A47 57 885 320 N/A No 42 17 9 91.2 8.8 2217 61 Invention steel
48 48 A48 33 867 307 N/A No 48 17 9 90.5 8.8 2225 58 Invention steel
49 49 A49 49 1005 302 N/A No 58 17 13 85.7 12.7 2265 61 Invention steel
50 50 A50 52 879 285 N/A No 42 17 13 82.2 13.1 2143 64 Invention steel
51 51 A51 47 871 305 N/A No 51 15 16 81.6 15.0 2247 64 Invention steel
52 52 A52 70 903 311 N/A No 60 16 12 84.1 11.5 2088 61 Invention steel
53 53 A53 69 925 299 N/A No 49 17 9 88.8 9.4 2109 60 Invention steel
54 54 A54 59 923 305 N/A No 40 18 14 83.6 13.7 1999 61 Invention steel
55 55 A55 63 898 304 N/A No 54 16 16 83.4 14.7 2045 63 Invention steel
56 56 A56 57 953 307 N/A No 44 15 9 90.4 8.5 2029 64 Invention steel
57 57 A57 61 939 294 N/A No 46 16 10 88.3 10.3 2102 60 Invention steel
7 58 A58 52 933 307 N/A No 54 25 4 84.3 2.9 2132 41 Comparati ve steel
7 59 A59 6 878 302 N/A No 54 25 5 85.5 4.1 2112 43 Comparati ve steel
7 60 A60 67 890 280 N/A No 54 25 8 86.4 8.9 1989 51 Invention steel
7 61 A61 44 942 320 N/A No 54 25 18 88.0 10.1 2132 71 Invention steel
7 62 A62 75 878 301 N/A No 54 25 2 84.5 3.8 2112 47 Comparati ve steel
7 63 A63 7 865 318 N/A No 54 25 18 84.8 15.1 1968 58 Invention steel
7 64 A64 38 887 309 N/A No 54 25 20 82.1 15.8 1989 61 Invention steel
7 65 A65 31 882 299 N/A No 54 25 22 82.9 16.1 2009 69 Invention steel
7 66 A66 5 909 308 N/A No 54 25 23 84.7 15.1 2030 72 Invention steel
7 67 A67 21 917 284 N/A No 54 25 8 90.3 8.1 2132 56 Invention
48

steel
7 68 A68 42 950 290 N/A No 54 25 13 86.7 12.1 2009 58 Invention steel
7 69 A69 75 914 319 N/A No 54 8 7 90.0 7.5 1948 46 Comparati ve steel
7 70 A70 70 943 313 N/A No 54 11 8 88.1 8.2 2037 52 Invention steel
7 7 A71 92 937 294 N/A No 54 25 12 89.3 10.1 2018 53 Invention steel
7 7 A72 105 935 306 N/A No 54 25 4 94.7 2.8 2079 48 Comparati ve steel
7 7 A73 30 779 314 N/A No 54 25 5 76.4 0.8 1323 - Comparati ve steel
7 7 A74 40 1040 303 N/A No 54 25 14 82.9 14.1 2110 34 Comparati ve steel
7 7 A75 87 891 227 N/A No 54 25 6 90.3 6.2 2171 37 Comparati ve steel
7 7 A76 26 947 523 N/A No 54 25 14 66.6 1.1 1599 38 Comparati ve steel
7 7 A77 10 891 304 N/A Yes 54 25 13 83.8 13.1 2129 61 Invention steel
2 71 A78 16 902 300 N/A No 47 15 1 71.1 3.3 2289 48 Comparati ve steel
9 72 A79 77 907 289 N/A No 41 16 6 94.1 3.8 2601 29 Comparati ve steel
58 73 A80 6 918 280 N/A No 55 14 8 91.7 8.1 1458 - Comparati ve steel
59 74 A81 3 922 291 N/A No 45 17 9 88.2 9.9 1521 69 Invention steel
[0100]
[Table 7]
Heat treatment process during hot stamping Surface layer region Mechanical properties

Steel No. Steel sheet No. Manufacture No. Heating
rate
(°C/s) Holding
temperature
(°C) Elapsed time
from initiation of
heating to
initiation of
forming (s) Tempering
temperature
(°C) Partial softened region Amount of plate attached (g/m2) Ni content in plating
layer (mass%) Martensite (area%) Concentration of
Ni in prior γ grain
boundaries
(mass%) Tensile strength (MPa) Evaluation of hydrogen
embrittlement resistance Note
1 1 B1 130 879 173 N/A No 39 15 97.9 1.7 1601 NG Comparative steel
2 2 B2 139 900 176 N/A No 55 12 97.8 1.0 2505 NG Comparative steel
49

3 3 B3 145 906 150 N/A No 41 12 96.1 2.1 1664 NG Comparative steel
4 4 B4 145 948 160 N/A No 56 15 95.3 6.0 1081 - Comparative steel
5 5 B5 165 871 132 N/A No 50 17 91.3 7.1 1632 OK Invention steel
6 6 B6 142 915 160 N/A No 41 17 97.6 6.5 1990 OK Invention steel
7 7 B7 130 928 154 N/A No 54 25 90.2 9.7 2136 OK Invention steel
8 8 B8 143 929 161 N/A No 57 16 96.3 6.5 2575 OK Invention steel
9 9 B9 146 868 135 247 No 40 17 95.3 6.7 1866 OK Invention steel
10 10 B10 147 894 152 N/A No 53 18 93.0 7.3 1445 - Comparative steel
11 11 B11 139 950 148 N/A No 48 18 90.1 6.8 2260 NG Comparative steel
12 12 B12 129 952 167 N/A No 58 15 94.0 5.9 2011 OK Invention steel
13 13 B13 180 923 123 N/A No 48 15 98.6 5.8 2024 OK Invention steel
14 14 B14 154 904 152 N/A No 46 15 92.4 5.9 2271 OK Invention steel
15 15 B15 174 929 140 N/A No 58 18 90.0 7.6 1363 - Comparative steel
16 16 B16 157 945 129 N/A No 51 17 91.1 6.5 2727 OK Invention steel
17 17 B17 180 947 148 N/A No 43 18 93.3 7.3 2078 OK Invention steel
18 18 B18 128 916 157 N/A No 52 17 90.6 7.1 2280 OK Invention steel
19 19 B19 166 914 131 N/A No 50 17 92.7 6.5 2028 OK Invention steel
20 20 B20 133 897 165 N/A No 45 17 94.3 7.0 2173 OK Invention steel
21 21 B21 160 873 166 N/A No 45 17 96.2 7.1 2459 NG Comparative steel
22 22 B22 120 943 149 N/A No 60 17 96.8 6.5 1927 OK Invention steel
23 23 B23 151 944 143 N/A No 47 15 92.9 5.9 2099 OK Invention steel
24 24 B24 136 942 178 N/A No 60 15 92.3 6.0 2055 OK Invention steel
50

25 25 B25 168 900 143 N/A No 58 15 99.0 5.9 2134 NG Comparative steel
26 26 B26 124 912 126 N/A No 60 16 95.8 6.5 2300 OK Invention steel
27 27 B27 149 931 130 N/A No 52 17 95.1 6.7 2257 OK Invention steel
28 28 B28 139 931 145 N/A No 50 18 98.0 7.1 2141 OK Invention steel
29 29 B29 128 950 137 N/A No 53 17 97.4 7.1 2201 OK Invention steel
30 30 B30 143 924 130 N/A No 51 16 92.9 6.3 2217 NG Comparative steel
31 31 B31 127 878 132 N/A No 46 16 94.6 6.7 2287 NG Comparative steel
32 32 B32 159 944 153 N/A No 40 15 97.1 5.7 2210 OK Invention steel
33 33 B33 157 871 124 N/A No 43 18 91.7 7.5 2083 OK Invention steel
34 34 B34 142 886 131 N/A No 46 15 93.5 6.2 2192 OK Invention steel
35 35 B35 125 924 123 N/A No 51 15 92.9 5.9 1995 OK Invention steel
36 36 B36 138 912 163 N/A No 47 15 96.0 6.0 2280 NG Comparative steel
37 37 B37 142 920 146 N/A No 52 16 98.1 6.1 2118 OK Invention steel
38 38 B38 126 913 168 N/A No 46 15 98.0 6.2 2084 OK Invention steel
39 39 B39 163 932 169 N/A No 60 16 98.1 6.2 2137 OK Invention steel
40 40 B40 124 914 140 N/A No 60 16 95.0 6.5 2157 NG Comparative steel
[0101]
[Table 8]

Tempering
temperature
(°C)
Tensile strength (MPa)
Martensite (area%)
N/A
92.6
2369

Steel No. Steel sheet No.
41 41

Manufactur e No.
B41

Heating
rate
(°C/s)
172

Heat treatment process during hot stamping
Elapsed time
Holding
temperature
(°C)
from initiation of
heating to
initiation of
forming (s)
869
141

Partial softened region
No

Amount of plate attached (g/m2)
45

Ni content in plating
layer (mass%)
16

Surface layer region
Concentration of
Ni in prior γ
grain boundaries
(mass%)
6.5

Mechanical properties
Evaluation of hydrogen
embrittlement resistance
OK

Note
Invention

51

steel
42 42 B42 132 895 127 N/A No 58 15 95.7 5.8 2396 OK Invention steel
43 43 B43 162 962 130 N/A No 59 15 95.5 6.2 2133 OK Invention steel
44 44 B44 164 941 158 N/A No 45 16 97.3 6.4 2325 OK Invention steel
45 45 B45 168 937 156 N/A No 42 18 96.3 7.5 2115 OK Invention steel
46 46 B46 144 932 134 N/A No 58 17 95.7 7.1 2461 OK Invention steel
47 47 B47 177 928 132 N/A No 42 17 97.7 6.5 2402 OK Invention steel
48 48 B48 120 946 174 N/A No 48 17 90.4 7.1 2433 OK Invention steel
49 49 B49 153 1016 142 N/A No 58 17 92.6 7.1 2386 OK Invention steel
50 50 B50 136 913 120 N/A No 42 17 90.0 7.1 2280 OK Invention steel
51 51 B51 164 930 152 N/A No 51 15 94.9 5.9 2502 OK Invention steel
52 52 B52 158 945 141 N/A No 60 16 97.8 6.1 2202 OK Invention steel
53 53 B53 158 864 134 N/A No 49 17 94.5 6.8 2097 OK Invention steel
54 54 B54 158 944 168 N/A No 40 18 98.8 6.9 2129 OK Invention steel
55 55 B55 174 913 146 N/A No 54 16 96.1 6.6 2178 OK Invention steel
56 56 B56 151 907 121 N/A No 44 15 91.5 6.1 2038 OK Invention steel
57 57 B57 167 872 147 N/A No 46 16 90.5 6.1 2300 OK Invention steel
7 58 B58 178 886 151 N/A No 54 25 95.3 2.8 2205 NG Comparative steel
7 59 B59 151 888 124 N/A No 54 25 95.3 3.3 2142 NG Comparative steel
7 60 B60 132 936 173 N/A No 54 25 97.5 6.1 2019 OK Invention steel
7 61 B61 149 897 125 N/A No 54 25 98.1 9.4 2226 OK Invention steel
7 62 B62 128 900 141 N/A No 54 25 96.2 3.1 2098 NG Comparative steel
7 63 B63 124 905 136 N/A No 54 25 93.0 9.4 2116 OK Invention
52

steel
7 64 B64 176 907 150 N/A No 54 25 94.6 9.7 2058 OK Invention steel
7 65 B65 175 863 161 N/A No 54 25 95.5 11.6 1999 OK Invention steel
7 66 B66 125 934 165 N/A No 54 25 94.2 11.4 2181 OK Invention steel
7 67 B67 155 889 136 N/A No 54 25 90.9 8.9 2333 OK Invention steel
7 68 B68 131 927 135 N/A No 54 25 90.2 9.1 2039 OK Invention steel
7 69 B69 171 881 147 N/A No 54 8 97.2 3.2 2017 NG Comparative steel
7 70 B70 144 930 172 N/A No 54 11 97.3 5.6 2026 OK Invention steel
7 7 B71 92 926 136 N/A No 54 25 98.9 5.1 2110 NG Comparative steel
7 7 B72 108 952 124 N/A No 54 25 93.0 5.8 2080 OK Invention steel
7 7 B73 190 894 146 N/A No 54 25 92.5 9.5 2182 OK Invention steel
7 7 B74 155 796 147 N/A No 54 25 79.2 1.8 1276 - Comparative steel
7 7 B75 152 1059 150 N/A No 54 25 97.7 9.9 2059 NG Comparative steel
7 7 B76 154 944 144 N/A No 54 25 91.3 5.7 2121 OK Invention steel
7 7 B77 122 905 249 N/A No 54 25 95.6 5.1 2101 NG Comparative steel
7 7 B78 142 946 154 N/A Yes 54 25 92.9 9.1 2077 OK Invention steel
2 71 B79 188 897 174 N/A No 47 15 91.3 0.8 2427 NG Comparative steel
9 72 B80 161 908 126 N/A No 41 16 93.1 4.2 2547 NG Comparative steel
58 73 B81 121 896 123 N/A No 55 14 92.8 8.1 1489 - Comparative steel
59 74 B82 148 908 131 N/A No 45 17 94.1 6.9 1579 OK Invention steel
53

[0102]
The metallographic structures, the average dislocation densities, and the
concentrations (amounts) of Ni of the steel sheets for hot stamping and the hot-stamped
articles were measured by the above-described measurement methods. In addition, the
5 mechanical properties of the hot-stamped articles were evaluated by the following
methods. [0103]
“Tensile strength”
Regarding the tensile strength of the hot-stamped article, a No. 5 test piece
10 described in JIS Z 2201: 2011 was produced from an arbitrary position in the hot-
stamped article, and the tensile strength was obtained according to the testing method
described in JIS Z 2241: 2011. In a case where the tensile strength was lower than 1500
MPa, the tensile strength was evaluated as failure, and a test described below was not
carried out.
15 [0104]
“Bendability”
The bendability of the hot-stamped article was evaluated by the following
method based on the VDA standard (VDA238-100) specified by Verband der
Automobilindustrie. In the present examples, displacement under the maximum load
20 that was obtained in a bending test was converted to an angle based on VDA, thereby
obtaining the maximum bending angle (°).
Test piece dimensions: 60 mm (rolling direction) × 60 mm (direction parallel to
sheet width direction) or 30 mm (rolling direction) × 60 mm (direction parallel to sheet
width direction)
25 Test piece sheet thickness: 1.0 mm (the front and rear surfaces were polished the
54

same amount, respectively)
Bending ridge: Direction parallel to sheet width direction
Testing method: Supported by rolls and pressed by a punch
Roll diameter: ϕ30 mm
5 Punch shape: Tip R = 0.4 mm
Distance between rolls: 2.0 × sheet thickness (mm) + 0.5 mm Pressing rate: 20 mm/min
Tester: SHIMADZU AUTOGRAPH 20 kN
[0105]
10 In a case where the maximum bending angle obtained by the above-described
method was 50° or more, the bendability was considered to be excellent and determined
to be pass. However, in a case where the maximum bending angle was less than 50°,
the bendability was determined to be failure.
[0106]
15 “Hydrogen embrittlement resistance”
The hydrogen embrittlement resistance of the hot-stamped article was evaluated
by the following method. Fig. 2 shows the shape of a test piece used for the evaluation
of the hydrogen embrittlement resistance. A nominal stress of 1100 MPa, which was
calculated by dividing the applied load by the cross-sectional area of the bottom of the
20 cut, was imparted to a test piece shown in Fig. 2 imparted with V notches, then, a
constant load test in which electrolytic hydrogen charging was carried out on a 3 mass%
NaCl aqueous solution at a current density of 0.1 mA/cm2 for 48 hours was carried out at
room temperature, and the hydrogen embrittlement resistance was determined depending
on the presence or absence of breakage. In the tables, cases where no breakage
25 occurred were indicated as pass (OK), and cases where breakage occurred were indicated
55

as failure (NG). R10 shown in Fig. 2 indicates that the curvature radius is 10 mm. [0107]
In Tables 5 and 6, in cases where the tensile strength was 1500 MPa or higher
and the bendability was pass (50° or more), the strength and the bendability were
5 considered to be excellent, and the steels were determined to be invention steel. In
cases where any one of the two performances described above was not satisfied, the steels were determined to be comparative steel. [0108]
From Tables 5 and 6, it is found that hot-stamped articles in which the chemical
10 composition, plate composition, and metallographic structure of the steel sheet for hot
stamping were within the scope of the present invention and which were hot-stamped under the preferred conditions have an excellent strength and excellent bendability.
On the other hand, it is found that hot-stamped articles in which any one or more
of the chemical composition and the metallographic structure of the steel sheet for hot
15 stamping was outside the scope of the present invention or which were hot-stamped
under a non-preferred condition are poor in terms of one or more of strength and bendability. [0109]
In Tables 7 and 8, in cases where the tensile strength was 1500 MPa or higher
20 and the hydrogen embrittlement resistance was pass (OK), the strength and the hydrogen
embrittlement resistance were considered to be excellent, and the steels were determined
to be invention steel. In cases where any one of the two performances described above
was not satisfied, the steels were determined to be comparative steel.
[0110]
25 From Tables 7 and 8, it is found that hot-stamped articles in which the chemical
56

composition, plate composition, and metallographic structure of the steel sheet for hot
stamping were within the scope of the present invention and which were hot-stamped
under the preferred conditions have an excellent strength and excellent hydrogen
embrittlement resistance.
5 On the other hand, it is found that hot-stamped articles in which any one or more
of the chemical composition and the metallographic structure of the steel sheet for hot stamping was outside the scope of the present invention or which were hot-stamped under a non-preferred condition are poor in terms of one or more of strength and hydrogen embrittlement resistance.
10 [Industrial Applicability]
[0111]
According to the present invention, it is possible to provide a steel sheet for hot stamping enabling the obtainment of a hot-stamped article being excellent in terms of strength and bendability or hydrogen embrittlement resistance after hot stamping.

WE CLAIMS

A steel sheet for hot stamping comprising:
a base steel sheet containing, as chemical components, by mass%,
5 C: 0.15% or more and less than 0.70%,
Si: 0.005% or more and 0.250% or less,
Mn: 0.30% or more and 3.00% or less,
sol. Al: 0.0002% or more and 0.500% or less,
P: 0.100% or less,
10 S: 0.1000% or less,
N: 0.0100% or less,
Nb: 0% or more and 0.150% or less,
Ti: 0% or more and 0.150% or less,
Mo: 0% or more and 1.000% or less,
15 Cr: 0% or more and 1.000% or less,
B: 0% or more and 0.0100% or less, Ca: 0% or more and 0.0100% or less, and REM: 0% or more and 0.30% or less
with a remainder including Fe and impurities; and
20 a plating layer on a surface of the base steel sheet, the plating layer being
attached to a single surface in an amount of 10 g/m2 or more and 90 g/m2 or less and having a Ni content of 10 mass% or more and 25 mass% or less with a remainder including Zn and impurities,
wherein a metallographic structure of a surface layer region that is a region from
25 the surface of the base steel sheet to a depth of 50 μm includes 15.0% or more of crystal
58

grains of one or two kinds of unauto-tempered martensite and lower bainite in terms of area percentage, and
an average dislocation density of the surface layer region is 4 × 1015 m/m3 or more. 5
2. The steel sheet for hot stamping according to Claim 1,
wherein the base steel sheet contains, as the chemical components, by mass%, one or more selected from the group consisting of
Nb: 0.010% or more and 0.150% or less,
10 Ti: 0.010% or more and 0.150% or less,
Mo: 0.005% or more and 1.000% or less,
Cr: 0.005% or more and 1.000% or less,
B: 0.0005% or more and 0.0100% or less,
Ca: 0.0005% or more and 0.0100% or less, and
15 REM: 0.0005% or more and 0.30% or less.