[Technical Field]
5 [0001]
The present invention relates to a steel sheet for hot stamping. Priority is
claimed on Japanese Patent Application No. 2020-084584, filed May 13, 2020, 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
vehicle bodies from the viewpoint of environmental protection and resource saving, and
the application of high strength steel sheets to automotive members has been
accelerating. Automotive members are manufactured by press forming, and an increase
15 in the strength of 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 members 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
20 pressing, is underway. Hot stamping is 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 members and the securement of the strength
of automotive members.
[0003]
25 In a case where hot stamping is carried out on a steel sheet that is a bare material
1
on which plating or the like has not been carried out, there is a need to carry out hot
stamping in a non-oxidative atmosphere in order to suppress the formation of scale
during heating and the decarburization of the surface layer. However, even when hot
stamping is carried out in a non-oxidative atmosphere, the steel sheet is in the
5 atmospheric atmosphere when the steel sheet is conveyed from a heating furnace to a
pressing machine, thus, a scale is formed on the surface of the hot-stamped steel sheet.
The scale on the surface of the steel sheet is poorly adhesive and easily exfoliates, which
creates a concern of an adverse influence on other steps. Therefore, there is a need to
remove the scale by shot blasting or the like. Shot blasting has a problem of affecting
10 the shapes of steel sheets. In addition, there is a problem in that the productivity of a
hot stamping step deteriorates due to a scale removal step.
[0004]
In order to improve the adhesion of scale on the surface of a steel sheet, there is
a method in which plating is formed on the surface of the steel sheet. When plating is
15 formed, since scale that is formed on the surface of a steel by hot stamping has favorable
adhesion, a step of removing scale becomes unnecessary. Therefore, the productivity of
the hot stamping step is improved.
[0005]
As a method for forming plating on the surface of a steel sheet, a method in
20 which Zn plating orAl plating is formed is conceivable; however, in a case where Zn
plating is used, there is a problem with liquid metal embrittlement (hereinafter, referred
to as LME). LME refers to a phenomenon in which, when tensile strength is imparted
with a liquid metal in contact with the surface of a solid metal, the solid metal that
intrinsically exhibits ductility embrittles. Zn has a low melting point, molten Zn
25 intrudes along prior austenite grain boundaries of Fe during hot stamping, and micro-
2
cracks are initiated in steel sheets.
[0006]
In a case where Al plating is provided on a steel sheet, the above-described LME
problem is not caused, but a reaction is caused between Al and water on the surface of
5 the Al plating during hot stamping, and hydrogen is generated. Therefore, there is a
problem in that the amount of intruding hydrogen into the steel sheet is large. When the
amount of hydrogen intruding into the steel sheet is large, stress that is loaded after hot
stamping leads to cracking of the steel sheet (hydrogen embrittlement).
10
[0007]
Patent Document 1 discloses a technique for suppressing the intrusion of
hydrogen into steel at a high temperature by enriching nickel in the surface layer region
of a steel sheet.
[0008]
Patent Document 2 discloses a technique for suppressing the intrusion of
15 hydrogen into steel by coating a steel sheet with a barrier pre-coat containing nickel and
chromium and having a weight ratio Ni/Cr of 1.5 to 9.
[0009]
However, in the method of Patent Document 1, there has been a case where it is
not possible to sufficiently suppress the intrusion of hydrogen that is generated in the
20 case of providing Al plating. In addition, in the method of Patent Document 2, there has
been a case where it is not possible to sufficiently suppress the intrusion of hydrogen into
a steel sheet in an environment where the dew point is not controlled (for example, in a
high-dew point environment such as 30°C).
[Citation List]
25 [Patent Document]
3
[0010]
[Patent Document 1] PCT International Publication No. WO 2016/016707
[Patent Document 2] PCT International Publication No. WO 20171187255
[Non-Patent Document]
5 [0011]
[Non-Patent Document 1] T. Ungar and three coauthors, Journal of Applied
Crystallography (1999), Volume 32 (PP. 992 to 1002)
[Summary of the Invention]
[Problems to be Solved by the Invention]
10 [0012]
The present invention is an invention made in consideration of the abovedescribed
problem, and an objective of the present invention is to provide a steel sheet
for hot stamping having excellent hydrogen embrittlement resistance by suppressing the
intrusion of hydrogen into the steel sheet even in a high-dew point environment even in
15 the case of hot-stamping the steel sheet provided withAl plating.
[Means for Solving the Problem]
[0013]
As a result of intensive studies, the present inventors found that, when a steel
sheet for hot stamping including an Al-Si alloy plating layer includes a Ni plating layer
20 having a desired average layer thickness (thickness) and containing a desired amount of
Ni, and anAl oxide coating on the Al-Si alloy plating layer is limited to a predetermined
film thickness (thickness) or less, it is possible to sufficiently suppress the amount of
hydrogen intruding into the steel sheet for hot stamping even when hot stamping is
carried out in an environment where the dew point is not controlled.
25 [0014]
4
5
10
15
20
25
The present invention has been made by further progressing studies based on the
above-described finding, and the gist thereof is as described below.
(1) A steel sheet for hot stamping according to one aspect of the present
invention including:
a base material,
an Al-Si alloy plating layer in which anAl content is 75 mass% or more, a Si
content is 3 mass% or more and a total of the Al content and the Si content is 95 mass%
or more,
an Al oxide coating having a thickness of 0 to 20 nm, and
aNi plating layer having aNi content of more than 90 mass%
in this order,
in which the base material has a chemical composition of, by mass%,
C: 0.01% or more and less than 0.70%,
Si: 0.001% to 1.000%,
Mn: 0.40% to 3.00%,
sol. Al: 0.0002% to 0.5000%,
P: 0.100% or less,
S: 0.1000% or less,
N: 0.0100% or less,
Cu: 0% to 1.00%,
Ni: 0% to 1.00%,
Nb: 0% to 0.150%,
V: 0% to 1.000%,
Ti: 0% to 0.150%,
Mo: 0% to 1.000%,
5
5
Cr: 0% to 1.000%,
B: 0% to 0.0100%,
Ca: 0% to 0.010%,
REM: 0% to 0.300%, and
a remainder: Fe and an impurity,
the Al-Si alloy plating layer has a thickness of 7 to 148 ~m, and
the Ni plating layer has a thickness of more than 200 nm and 2500 nm or less.
(2) The steel sheet for hot stamping according to ( 1 ), in which the Ni plating
layer may be provided in direct contact with the Al-Si alloy plating layer as an upper
10 layer of the Al-Si alloy plating layer.
(3) The steel sheet for hot stamping according to (1), in which the Al oxide
coating may have a thickness of 2 to 20 nm.
(4) The steel sheet for hot stamping according to any one of (1) to (3), in which
the chemical composition of the base material may contain, by mass%, one or two or
15 more selected from the group consisting of
20
25
Cu: 0.005% to 1.000%,
Ni: 0.005% to 1.000%,
Nb: 0.010% to 0.150%,
V: 0.005% to 1.000%,
Ti: 0.010% to 0.150%,
Mo: 0.005% to 1.000%,
Cr: 0.050% to 1.000%,
B: 0.0005% to 0.0100%,
Ca: 0.001% to 0.010%, and
REM: 0.001% to 0.300%.
6
(5) The steel sheet for hot stamping according to any one of (1) to (4), in which
a dislocation density at a depth of 100 ~m from a surface of the base material may be 5 x
1013 m/m3 or more.
[Effects of the Invention]
5 [0015]
According to the above-described aspect of the present invention, it is possible
to provide a steel sheet for hot stamping having excellent hydrogen embrittlement
resistance by suppressing the intrusion of hydrogen into the steel sheet during hot
stamping in a high-dew point environment even when the steel sheet for hot stamping has
10 been provided withAl plating.
[Brief Description of Drawings]
15
20
[0016]
Fig. 1 is a schematic cross-sectional view of a steel sheet for hot stamping
according to an embodiment of the present invention.
Fig. 2 is a schematic cross-sectional view of a steel sheet for hot stamping
according to another embodiment of the present invention
[Embodiment(s) for implementing the Invention]
[0017]

As a result of intensive studies, the present inventors found that, when a steel
sheet having Al plating formed thereon is hot-stamped in an environment where the dew
point is not controlled, Al on the surface of the Al plating and water in the atmosphere
react with each other, whereby a large amount of hydrogen is generated and a large
amount of hydrogen intrudes into the steel sheet.
25 [0018]
7
As a result of additional intensive studies, the present inventors obtained the
following findings.
(A) When aNi plating layer where the Ni content is more than 90 mass% is
used, it is possible to suppress the intrusion of hydrogen into a steel sheet during hot
5 stamping at a high dew point.
(B) When the layer thickness (thickness) of the Ni plating layer is more than 200
nm, the reaction with water in the atmosphere is sufficient! y suppressed, and the amount
of hydrogen intruding into the steel sheet can be reduced.
(C) When the film thickness (thickness) of anAl oxide coating on an Al-Si alloy
10 plating layer is reduced, it is possible to reduce the area of a defect region in the Ni
plating layer where no Ni plating layer is formed, and consequently, Al on the surface of
the Al-Si alloy plating layer that comes into contact with the atmosphere can be reduced.
(D) In a case where a Ni plating layer is formed on anAl plating by electro
plating or the like, the adhesion of the Ni plating layer is not sufficient as a steel sheet for
15 hot stamping; however, when the thickness of the Al oxide coating is set to 0 to 20 nm,
sufficient adhesion of the Ni plating layer can be obtained such that the steel sheet can be
used as a steel sheet for hot stamping.
[0019]
In a steel sheet for hot stamping according to the present embodiment, the
20 configuration of the steel sheet for hot stamping was determined based on the abovedescribed
findings. In the steel sheet for hot stamping according to the present
embodiment, an intended effect of the present invention can be obtained due to the
synergistic effects of individual plating configurations. As shown in Fig. 1, a steel sheet
for hot stamping 10 includes a steel sheet (base material) 1, anAl-Si alloy plating layer 2,
25 anAl oxide coating 3 and a Ni plating layer 4. In a case where there is no Al oxide
8
coating 3, as shown in Fig. 2, a steel sheet for hot stamping 1 OA includes the base
material 1, the Al-Si alloy plating layer 2 and the Ni plating layer 4. Hereinafter, each
configuration will be described. In the present specification, numerical ranges
expressed using "to" include numerical values before and after "to" as the lower limit
5 value and the upper limit value. Numerical values expressed with "more than" and
"less than" are not included in numerical ranges. Regarding chemical compositions,
10
"%"indicates "mass%" in all cases.
[0020]
(Steel sheet)
A steel sheet (base material) that serves as the base material 1 of the steel sheet
for hot stamping 10 according to the present embodiment contains, as a chemical
composition, by mass%, C: 0.01% or more and less than 0.70%, Si: 0.001% to 1.000%,
Mn: 0.40% to 3.00%, sol. Al: 0.0002% to 0.5000%, P: 0.100% or less, S: 0.1000% or
less, N: 0.0100% or less and a remainder being Fe and an impurity.
15 [0021]
"C: 0.01% or more and less than 0.70%"
C is an important element for securing hardenability. When the C content of
the base material is less than 0.01 %, it becomes difficult to obtain sufficient
hardenability, and the tensile strength decreases. Therefore, the C content of the base
20 material is preferably set to 0.01% or more. In a case where the C content is 0.25% or
more, a tensile strength of 1600 MPa or more can be obtained, which is preferable. The
C content is more preferably 0.28% 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
hydrogen embrittlement resistance of a hot-stamping formed body deteriorates.
25 Therefore, the C content is set to less than 0.70%. The C content is preferably 0.36% or
9
less.
[0022]
"Si: 0.001% to 1.000%"
Si is an element that is contained to secure hardenability. When the Si content
5 is less than 0.001%, the above-described effect cannot be obtained. Therefore, the Si
content is set to 0.001% or more. A more preferable Si content is 0.005% or more. A
still more preferable Si content is 0.100% or more. In a case where Cu is contained, the
Si content is preferably set to 0.350% or more in order to suppress the hot embrittlement
of Cu. When more than 1.000% of Si is contained, the austenite transformation
10 temperature (Ac3 or the like) becomes extremely high, and there is a case where the cost
necessary for heating for hot stamping increases or ferrite remains during the heating for
hot stamping to decrease the tensile strength of a hot-stamping formed body. Therefore,
the Si content is set to 1.000% or less. The Si content is preferably 0.8000% or less.
In a case where Cu is contained, since the temperature of the austenite transformation
15 temperature becomes high, the Si content is preferably 0.600% or less. The Si content
may be 0.400% or less or 0.250% or less.
[0023]
"Mn: 0.40% to 3.00%"
Mn is an element that contributes to improvement in the tensile strength of a
20 hot-stamping formed body by solid solution strengthening. When the Mn content is less
than 0.40%, there is a case where the hot-stamping formed body breaks due to hydrogen
embrittlement. Therefore, the Mn content is set to 0.40% or more. The Mn content is
preferably 0.80% 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
10
5
additionally, the hydrogen embrittlement resistance deteriorates, and thus the Mn content
is set to 3.00% or less. The Mn content is preferably 2.00% or less.
[0024]
"sol. Al: 0.0002% to 0.5000%"
Al is an element having an action of deoxidizing molten steel to improve the
quality of the steel (suppressing the generation of a defect such as a blowhole in steel).
When the sol. Al content is less than 0.0002%, deoxidation does not sufficiently occur,
the above-described effect cannot be obtained, and additionally, there is a case where the
hydrogen embrittlement of the hot-stamping formed body occurs. Therefore, the sol. Al
10 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 0.5000%, a
coarse oxide is generated in steel, and there is a case where the hydrogen embrittlement
of the hot-stamping formed body occurs. Therefore, the sol. Al content is set to
0.5000% or less. The sol. Al content is preferably 0.4000% or less or 0.3000% or less.
15 sol. Al means acid -soluble Al and refers to the total amount of the solid solution of Al
that is present in steel in a solid solution state and Al that is present in steel as an acidsoluble
precipitate such as AlN.
20
[0025]
"P: 0.100% or less"
Pis 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 the grain
boundaries significantly decreases, and there is a case where the hydrogen embrittlement
of the hot-stamping formed body occurs. Therefore, the P content is set to 0.100% or
less. The P content is preferably 0.050% or less. A more preferable P content is
25 0.010% or less. The lower limit of the P content is not particularly limited; however,
11
5
when the lower limit is decreased to lower than 0.0005%, the dephosphorization cost
increases significantly, which is not preferable economically, and thus the lower limit
may be set to 0.0005% in actual operation.
[0026]
"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, the hydrogen
embrittlement resistance of the hot-stamping formed body deteriorates, and there is a
case where the hydrogen embrittlement of the hot-stamping formed body occurs.
10 Therefore, the S content is set to 0.1000% or less. The S content is preferably 0.0050%
or less. The lower limit of the S content is not particularly limited; however, when the
lower limit is decreased to lower than 0.00015%, the desulfurization cost increases
significantly, which is not preferable economically, and thus the lower limit may be set to
0.00015% in actual operation.
15 [0027]
"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-stamping formed body.
When theN content exceeds 0.0100%, a coarse nitride is generated in steel, and there is a
20 case where the hydrogen embrittlement of the hot-stamping formed body occurs.
Therefore, theN content is set to 0.0100% or less. TheN content is preferably 0.0050%
or less. The lower limit of theN content is not particularly limited; however, when the
lower limit is decreased to lower than 0.0001%, the denitrification cost increases
significantly, which is not preferable economically, and thus the lower limit may be set to
25 0.0001% in actual operation.
12
[0028]
The steel sheet (base material) that configures the steel sheet for hot stamping 10
according to the present embodiment may contain, instead of some of Fe, one or two or
more selected from the group consisting of Cu, Ni, Nb, V, Ti, Mo, Cr, B, Ca and REM as
5 an arbitrary element. In a case where the following arbitrary element is not contained,
the content thereof is 0%.
[0029]
"Cu: 0% to 1.00%"
Cu has an action of diffusing up to a plating layer of a hot stamping member
10 during hot stamping to reduce hydrogen that intrudes during heating in the manufacturing
of the hot stamping member. Therefore, Cu may be contained as necessary. In
addition, Cu is an effective element for enhancing the hardenability of steel to stably
secure the tensile strength of the quenched hot-stamping formed body. In a case where
Cu is contained, the Cu content is preferably set to 0.005% or more in order to reliably
15 exhibit the above-described effect. The Cu content is more preferably 0.150% or more.
20
On the other hand, even when more than 1.00% of Cu is contained, the above-described
effect is saturated, and thus the Cu content is preferably set to 1.00% or less. The Cu
content is more preferably 0.350% or less.
[0030]
"Ni: 0% to 1.00%"
Ni is an important element to suppress hot embrittlement caused by Cu during
the manufacturing of the steel sheet and secure stable production, and thus Ni may be
contained. When the Ni content is less than 0.005%, there is a case where the abovedescribed
effects cannot be sufficient! y obtained. Therefore, the Ni content is
25 preferably 0.005% or more. The Ni content is preferably 0.05% or more. On the other
13
5
hand, when the Ni content exceeds 1.00%, the limit hydrogen amount of the steel sheet
for hot stamping decreases. Therefore, the Ni content is set to 1.00% or less. The Ni
content is preferably 0.60% or less.
[0031]
"Nb: 0% to 0.150%"
Nb is an element that contributes to improvement in the tensile strength of the
hot-stamping formed body 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
10 preferably 0.030% or more. On the other hand, even when more than 0.150% ofNb is
contained, 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 0.100% or less.
15
[0032]
"V: 0% to 1.000%"
Vis an element that forms a fine carbide and improves the limit hydrogen
amount of steel by a refining effect or hydrogen trapping effect thereof. Therefore, V
may be contained. In order to obtain the above-described effects, 0.005% or more of V
is preferably contained, and 0.05% or more of Vis more preferably contained.
However, when the V content exceeds 1.000%, the above-described effects are saturated,
20 and the economic efficiency decreases. Therefore, in the case of being contained, the V
content is set to 1.000% or less.
[0033]
"Ti: 0% to 0.150%"
Ti is an element that contributes to improvement in the tensile strength of the
25 hot-stamping formed body by solid solution strengthening and thus may be contained as
14
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% ofTi is
contained, the above-described effect is saturated, and thus the Ti content is preferably
5 set to 0.150% or less. The Ti content is more preferably 0.120% or less.
[0034]
"Mo: 0% to 1.000%"
Mo is an element that contributes to improvement in the tensile strength of the
hot-stamping formed body by solid solution strengthening and thus may be contained as
10 necessary. 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% ofMo 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.
15 [0035]
"Cr: 0% to 1.000%"
Cr is an element that contributes to improvement in the tensile strength of the
hot-stamping formed body 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 0.050% or
20 more in 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.
[0036]
25 "B: 0% to 0.0100%"
15
B is an element that is segregated in grain boundaries to improve the strength of
the grain boundaries and thus may be 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
5 other hand, even when more than 0.0100% ofB is contained, the above-described effect
is saturated, and thus the B content is preferably set to 0.0100% or less. The B content
is more preferably 0.0075% or less.
10
[0037]
"Ca: 0% to 0.010%"
Ca is an element having an action of deoxidizing molten steel to improve the
quality of the steel. In order to reliably exhibit this action, theCa content is preferably
set to 0.001% or more. On the other hand, even when more than 0.010% of Ca is
contained, the above-described effect is saturated, and thus the Ca content is preferably
set to 0.010% or less.
15 [0038]
"REM: 0% to 0.300%"
REM is an element having an action of deoxidizing molten steel to improve the
quality of the steel. In order to reliably exhibit this action, the REM content is
preferably set to 0.001% or more. On the other hand, even when more than 0.300% of
20 REM is contained, the above-described effect is saturated, and thus the REM content is
preferably set to 0.300% or less.
25
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.
[0039]
"Remainder being Fe and impurity"
16
The remainder of the chemical composition of the base material 1 that
configures the steel sheet for hot stamping 10 according to the present embodiment is Fe
and an impurity. As the impurity, exemplified is an element that is inevitably
incorporated from a steel raw material or a scrap and/or in a steelmaking process or
5 intentionally added and is permitted to an extent that the properties of hot-stamping
formed bodies, which are the steel sheet for hot stamping 10 according to the present
embodiment that have been hot -stamped, are not impaired.
[0040]
The above-described chemical composition of the base material 1 may be
10 measured by an ordinary analytical method. For example, the chemical composition
may be measured using inductively coupled plasma-atomic emission spectrometry (ICPAES).
C and S may be measured using an infrared absorption method after combustion,
and N may be measured using an inert gas melting-thermal conductivity method. The
chemical composition needs to be analyzed after the plating layer on the surface is
15 removed by machining. sol. Al may be measured by ICP-AES using a filtrate obtained
by hydrolyzing a specimen with an acid.
[0041]
"Metallo graphic structure"
Next, the metallographic structure of the base material I that configures the steel
20 sheet for hot stamping 10 according to the present embodiment will be described. In the
metallographic structure of the base material 1 of the steel sheet for hot stamping 10, the
area ratio of ferrite is preferably 20% or more in terms of the area ratio in a cross section.
A more preferable area ratio of ferrite is 30% or more. The area ratio of ferrite is
preferably 80% or less. A more preferable area ratio of ferrite is 70% or less. The area
25 ratio of pearlite is preferably 20% or more in terms of the area ratio in a cross section.
17
5
The area fraction of pearlite is preferably 80% or less. A more preferable area ratio of
pearlite is 70% or less. The remainder may be bainite, martensite or residual austenite.
The area ratio of the remainder in microstructure may be less than 5%.
[0042]
(Measurement method of area ratios of ferrite and pearlite)
The area ratios of ferrite and pearlite are measured by the following method. A
cross section parallel to a rolling direction at the central position in the sheet width
direction is finished into a mirror-like surface and polished for eight minutes using
colloidal silica containing no alkaline solution at room temperature to remove strain
10 introduced into the surface layer of a sample. At an arbitrary position in the
longitudinal direction of the sample cross section, a region from a 1/8 depth of the sheet
thickness from the surface to a 3/8 depth of the sheet thickness from the surface, which is
50 ~m in length, is measured at measurement intervals of 0.1 ~m by an electron
backscatter diffraction method such that a 1/4 depth of the sheet thickness from the
15 surface can be analyzed to obtain crystal orientation information. For the measurement,
an instrument composed of a thermal field emission-type scanning electron microscope
(JSM 7001F manufactured by JEOL Ltd.) and an EBSP detector (DVC 5-type detector
manufactured by TSL) is used. In this case, the degree of vacuum in the instrument is
set to 9.6 x 10-5 Pa or less, the accelerating voltage is set to 15 kV, the irradiation current
20 level is set to 13, and the irradiation level of an electron beam is set to 62. Furthermore,
a reflected electron image is captured at the same visual field.
First, crystal grains where ferrite and cementite are precipitated in layers are
specified from the reflected electron image, and the area ratio of the crystal grains is
calculated, thereby obtaining the area ratio of pearlite. After that, for crystal grains
25 except the crystal grains determined as pearlite, from the obtained crystal orientation
18
information, regions where the grain average misorientation value is 1.0° or less are
determined as ferrite using a "Grain Average Misorientation" function mounted in
software "OIM Analysis (registered trademark)" included in the EBSP analyzer. The
area ratio of the regions determined as ferrite is obtained, thereby obtaining the area ratio
5 of ferrite.
[0043]
(Determination method of area ratio of remainder in microstructure)
The area ratio of the remainder in the present embodiment is a value obtained by
subtracting the area ratios of ferrite and pearlite from 100%.
10 [0044]
"Dislocation density at depth of 100 ~m from surface being 5 x 1013 m/m3 or
more"
The dislocation density of the base material 1 that configures the steel sheet for
hot stamping 10 according to the present embodiment will be described. The
15 dislocation density at a depth of 100 ~m from the surface of the base material 1 that
configures the steel sheet for hot stamping 10 according to the present embodiment is
preferably 5 x 1013 m/m3 or more. A more preferable dislocation density is 50 x 1013
m/m3 or more. When the dislocation density at 100 ~m from the surface of the base
material 1 is 5 x 1013 m/m3 or more, it becomes easy for Al in the Al-Si alloy plating
20 layer 2 to migrate toward the base material 1. Therefore, it is possible to suppress Al in
the Al-Si alloy plating layer 2 migrating up to the outermost surface of the Ni plating
layer 4 of the steel sheet for hot stamping 10 by heating during hot stamping. The
dislocation density is preferably 1000 x 1013 m/m3 or less. A more preferable
dislocation density is 150 x 1013 m/m3 or less.
25 [0045]
19
"Measurement of dislocation density"
Next, a measurement method of the dislocation density at a depth of 100 ~m
from the surface of the base material 1 will be described. The dislocation density can be
measured by the X-ray diffraction method or transmission electron microscopic
5 observation, but is measured using the X-ray diffraction method in the present
embodiment.
[0046]
First, a sample is cut out from an arbitrary position 50 mm or more apart from
an end face of the base material 1 that is used in the steel sheet for hot stamping 10.
10 While also depending on a measuring instrument, the size of the sample is set to a size of
approximately 20 mm x 20 mm. The thickness of the sample is reduced by 200 ~m
using a solution mixture of 48 mass% of diluted water, 48 mass% of hydrogen peroxide
water and 4 mass% of hydrofluoric acid. At this time, the front surface and the rear
surface of the sample are each reduced by 100 ~m, and 100 ~m regions are exposed from
15 the surfaces of the sample to be depressurized. X-ray diffraction measurement is
carried out on these exposed surfaces, and a plurality of diffraction peaks of bodycentered
cubic lattices is specified. The dislocation densities are analyzed from the
half-value widths of these diffraction peaks, thereby obtaining the dislocation density at a
depth of 100 ~m from the surface. As an analysis method, a modified Williamson-Hall
20 method described in Non-Patent Document 1 is used. In the case of measuring the
dislocation density of the steel sheet for hot stamping 10 including the Al-Si alloy plating
layer 2 and the Ni plating layer 4, the dislocation density is measured after the Al-Si alloy
plating layer 2 and the Ni plating layer 4 are removed. As a method for removing the
Al-Si alloy plating layer 2 and the Ni plating layer 4, for example, a method in which the
25 steel sheet for hot stamping 10 is immersed in a NaOH aqueous solution is an exemplary
20
example.
[0047]
The sheet thickness of the base material 1 of the steel sheet for hot stamping 10
according to the present embodiment is not particularly limited, but is preferably 0.4 mm
5 or more from the viewpoint of the weight reduction of vehicle bodies. A more
preferable sheet thickness of the base material 1 is 0.8 mm or more, 1.0 mm or more or
1.2 mm or more. The sheet thickness of the base material 1 is preferably set to 6.0 mm
or less. A more preferable sheet thickness of the base material 1 is 5.0 mm or less, 4.0
mm or less, 3.2 mm or less or 2.8 mm or less.
10 [0048]
(Al-Si alloy plating layer)
The Al-Si alloy plating layer 2 of the steel sheet for hot stamping 10 according
to the present embodiment is provided as an upper layer of the base material 1. The AlSi
alloy plating layer 2 is plating containing Aland Si as main components. Here, the
15 expression "containing Aland Si as main component" means that at least the Al content
is 75 mass% or more, the Si content is 3 mass% or more and the total of the Al content
and the Si content is 95 mass% or more. The Al content in the Al-Si alloy plating layer
2 is preferably 80 mass% or more. The Al content in the Al-Si alloy plating layer is
preferably 95 mass% or less. When the Al content in the Al-Si alloy plating layer 2 is in
20 this range, scale having favorable adhesion is formed on the surface of the steel sheet
during hot stamping.
[0049]
The Si content in the Al-Si alloy plating layer 2 is preferably 3 mass% or more.
The Si content in the Al-Si alloy plating layer 2 is more preferably 6 mass% or more.
25 The Si content in the Al-Si alloy plating layer 2 is preferably 20 mass% or less. The Si
21
content is more preferably 12 mass% or less. When the Si content in the Al-Si alloy
plating layer 2 is 3 mass% or more, alloying of Fe and Al can be suppressed. In
addition, when the Si content in the Al-Si alloy plating layer 2 is 20 mass% or less, it is
possible to suppress an increase in the melting point of the Al-Si alloy plating layer 2 and
5 to decrease the temperature of a hot-dip plating bath. Therefore, when the Si content in
the Al-Si alloy plating layer 2 is 20 mass% or less, it is possible to reduce the production
cost. The total of the Al content and the Si content may be 97 mass% or more, 98
mass% or more or 99 mass% or more. The remainder in the Al-Si alloy plating layer 2
is Fe and an impurity. As the impurity, a component that is inevitably incorporated
10 during the manufacturing of the Al-Si alloy plating layer 2, a component in the base
material 1 or the like is an exemplary example.
[0050]
The average layer thickness (thickness) of the Al-Si alloy plating layer 2 of the
steel sheet for hot stamping 10 according to the present embodiment is 7 ~m or more.
15 This is because, when the thickness of the Al-Si alloy plating layer 2 is less than 7 ~m,
there is a case where it is not possible to form scale having favorable adhesion during hot
stamping. A more preferable thickness of the Al-Si alloy plating layer 2 is 12 ~m or
more, 15 ~m or more, 18 ~m or more or 22 ~m or more. The thickness of the Al-Si
alloy plating layer 2 is 148 ~m or less. This is because, when the thickness of the Al-Si
20 alloy plating layer 2 is more than 148 ~m, not only is the above-described effect
saturated, but the cost also increases. A more preferable thickness of the Al-Si alloy
plating layer 2 is 100 ~m or less, 60 ~m or less, 45 ~m or less or 37 ~m or less.
[0051]
The thickness of the Al-Si alloy plating layer 2 is measured as described below.
25 The steel sheet for hot stamping 10 is cut in the sheet thickness direction, and then the
22
cross section of the steel sheet for hot stamping 10 is polished. On the polished cross
section of the steel sheet for hot stamping 10, a region from the surface of the steel sheet
for hot stamping 10 to the base material I is linearly analyzed by a ZAF method with an
electron probe microanalyzer (FE-EPMA), and, among detected components, the Al
5 concentration (content) and the Si concentration (content) are measured. As the
measurement conditions, the accelerating voltage needs to set to 15 kV, the beam
diameter needs to be set to approximately 100 nm, the irradiation time per point needs to
be set to 1000 ms, and the measurement pitches need to be set to 60 nm. A region
where the Al content is 75 mass% or more, the Si content is 3 mass% or more and the
10 total of the Al content and the Si content is 95 mass% or more is determined as the Al-Si
alloy plating layer 2. The thickness of the Al-Si alloy plating layer 2 is the length of the
above-described region in the sheet thickness direction. The thicknesses of layer of the
Al-Si alloy plating layer 2 are measured at five positions at 5 ~m intervals, and the
arithmetic average of the obtained values is regarded as the thickness of the Al-Si alloy
15 plating layer 2.
[0052]
Regarding the Al content and the Si content in the Al-Si alloy plating layer 2,
according to a testing method described in JIS K 0150 (2005), a test piece is collected,
the Al content and the Si content are measured at a 1/2 position of the thickness of the Al-
20 Si alloy plating layer 2, whereby the Al content and the Si content in the Al-Si alloy
plating layer 2 in the steel sheet for hot stamping 10 can be obtained.
[0053]
(Al oxide coating)
The Al oxide coating 3 of the steel sheet for hot stamping 10 according to the
25 present embodiment is provided in contact with the Al-Si alloy plating layer 2 as an
23
upper layer of the Al-Si alloy plating layer 2. The Al oxide coating is defined as a
region where the 0 content is 20 atomic% or more.
In a case where the thickness of the Al oxide coating 3 of the steel sheet for hot
stamping 10 according to the present embodiment is more than 20 nm, there is a
5 possibility that the adhesion to the Ni plating layer 4 that is provided over the Al-Si alloy
plating layer 2 may deteriorate and upper layer plating may exfoliate during handling
such as hot stamping forming. This plating peeling does not create any problems in
carrying out hot stamping, but degrades the hydrogen embrittlement resistance. In
addition, in a case where the thickness of the Al oxide coating 3 is more than 20 nm, the
10 coverage of the Ni plating layer 4 that is provided as an upper layer of the Al oxide
coating 3 becomes less than 90%. Therefore, the thickness of the Al oxide coating 3 is
0 to 20 nm or less. The thickness of the Al oxide coating 3 is more preferably 10 nm or
less. The thickness of the Al oxide coating 3 may be 2 nm or more. Since the Al oxide
coating 3 may not be provided, the lower limit of the Al oxide coating 3 is 0 nm. In that
15 case, the Ni plating layer 4 is formed so as to come into contact with the Al-Si alloy
plating layer 2.
[0054]
The thickness of the Al oxide coating 3 is evaluated by alternately repeating Ar
sputtering and X-ray photoelectron spectroscopy (XPS) measurement. Specifically, the
20 steel sheet for hot stamping 10 is sputtered by Ar sputtering (accelerating voltage: 20 kV,
sputtering rate: 1.0 nm/min), and then XPS measurement is carried out. The Ar
sputtering and the XPS measurement are alternate! y carried out, and these measurements
are repeated until a peak with a bonding energy of the 2p orbit of Al oxidized in the XPS
measurement of73.8 eV to 74.5 eV appears and then disappears. The thickness of the
25 Al oxide coating 3 is calculated from the sputtering time and the sputtering rate from a
24
position where the 0 content reaches 20 atomic% or more for the first time after the start
of the sputtering to a position where the 0 content reaches less than 20 atomic%. The
sputtering rate is obtained in terms of Si02. The thickness of the Al oxide coating 3 is
the arithmetic average value of two measurement sites.
5 [0055]
(Ni plating layer)
The Ni plating layer 4 of the steel sheet for hot stamping 10 according to the
present embodiment is provided in contact with the Al oxide coating 3 as an upper layer
of the Al oxide coating 3. In a case where the Al oxide coating 3 is not provided, the Ni
10 plating layer 4 is provided in contact with the Al-Si alloy plating layer 2 as an upper layer
of the Al-Si alloy plating layer 2. Ni is not easily oxidized and does not easily generate
hydrogen due to the suppression of oxidation by water at a high temperature.
Furthermore, even when hydrogen is generated and adsorbed to the surface, a Tafel
reaction where hydrogen atoms bond to each other, become hydrogen gas, and are
15 desorbed is accelerated, and thus Ni has an effect on suppressing the intrusion of
hydrogen into the steel sheet. Therefore, when the Ni plating layer 4 is formed, it is
possible to suppress the amount of hydrogen intruding into the steel sheet for hot
stamping 10 during hot stamping.
20
[0056]
The average layer thickness (thickness) of the Ni plating layer 4 according to the
present embodiment is more than 200 nm. A more preferable thickness of the Ni
plating layer 4 is 280 nm or more, 350 nm or more, 450 nm or more, 560 nm or more or
650 nm or more. When the thickness of the Ni plating layer 4 is 200 nm or less, it is not
possible to sufficiently suppress the intrusion of hydrogen into the base material 1 during
25 hot stamping. In addition, the thickness of the Ni plating layer 4 is 2500 nm or less. A
25
more preferable thickness of the Ni plating layer 4 is 1500 nm or less, 1200 nm or less or
1000 nm or less. When the thickness of the Ni plating layer 4 is more than 2500 nm,
the effect on suppressing the amount of hydrogen intruding into the base material I is
saturated, and the cost increases.
5 [0057]
When the Ni content in the Ni plating layer 4 is 90 mass% or less, there is a case
where the effect on suppressing the amount of hydrogen intruding into the steel sheet for
hot stamping 10 cannot be obtained. Therefore, the Ni content in the Ni plating layer 4
is more than 90 mass%. A more preferable Ni content is 92 mass% or more. A more
10 preferable Ni content is 93 mass% or more or 94 mass%. A still more preferable Ni
content is 96 mass% or more, 98 mass% or more or 99 mass% or more. The chemical
composition of the remainder of the Ni plating layer (excluding Ni) is not particularly
limited. Cr may be contained in the Ni plating layer, and the Ni/Cr ratio is preferably
larger than 9, and this ratio is more preferably 15 or more or 30 or more. More
15 preferably, the Cr content in the Ni plating layer is preferably 6.0 mass% or less and
more preferably 4.0 mass% or less or 3.0 mass% or less. The Cr content in the Ni
plating layer 3 is still more preferably 2.0 mass% or less. When the Cr content is
reduced, it is possible to reduce the amount of hydrogen intruding into the steel sheet.
[0058]
20 The coverage of the Ni plating layer 4 on the Al oxide coating 3 (in a case where
the Al oxide coating 3 is not provided, the coverage of the Ni plating layer 4 on the Al-Si
alloy plating layer 2) is preferably 90% or more. The coverage of the Ni plating layer 4
is more preferably 95% or more. When the coverage of the Ni plating layer 4 is less
than 90%, it is not possible to sufficiently suppress a reaction between water vapor and
25 Al on the surface of the Al-Si plating layer 2 during hot stamping. The coverage of the
26
Ni plating layer 4 may be 100% or less or may be 99% or less.
[0059]
The coverage of the Ni plating layer is evaluated by XPS measurement.
Specifically, the XPS measurement is carried out by scanning the steel sheet for hot
5 stamping 10 in the entire energy range using Quantum 2000 manufactured by ULVACPHI,
Inc. and Al Ka rays as a radiation source under conditions of an output of 15 kV, 25
W, a spot size of 100 ~m and the number of times of scanning of 10 times, analysis is
carried out using analysis software MultiPak V. 8.0 manufactured by ULVAC-PHI, Inc.,
and the Ni content (atomic%), the Al content (atomic%) and the amounts of other
10 components (atomic%) in the detected metal components are obtained. The obtained
content (atomic%) is converted to the content (mass%), whereby the Ni content (mass%)
and the Al content (mass%) can be obtained. Next, the proportion(%) of the Ni content
in the total of the Ni content and the Al content is calculated. The obtained proportion
is regarded as the coverage (%) of the Ni plating.
15 [0060]
The thickness of the Ni plating layer 4 is measured by alternate! y repeating Ar
sputtering etching and X-ray photoelectron spectroscopy (XPS) measurement.
Specifically, the steel sheet for hot stamping 10 is sputtering-etched by Ar sputtering
(accelerating voltage: 20 kV, sputtering rate: 1.0 nm/min), and then XPS measurement is
20 carried out. The Ar sputtering etching and the XPS measurement are alternately carried
out, and these measurements are repeated until a peak with a bonding energy of the 2p
orbit of Ni in the XPS measurement of 852.5 e V to 852.9 e V appears and then
disappears. The layer thickness of the Ni plating layer 4 is calculated from the
sputtering etching time and the sputtering etching rate while the peak in the above-
25 described range from a position where the Ni content reaches 10 atomic% or more for the
27
first time after the start of the sputtering to a position where the Ni content reaches less
than 10 atomic% appears and then disappears. The sputtering etching rate is obtained in
terms of Si02. The thickness of the Ni plating layer 4 is the arithmetic average value of
two measurement sites.
5 [0061]
10
Regarding the Ni content in the Ni plating layer 4, the Ni concentration at the
central position in the sheet thickness direction of the Ni plating layer 4 that is obtained
in the measurement of the thickness of the Ni plating layer is regarded as the Ni content.
[0062]
(Thickness)
The thickness of the steel sheet for hot stamping 10 is not particularly limited
and may be, for example, 0.4 mm or more. A more preferable thickness of the steel
sheet is 0.8 mm or more, 1.0 mm or more or 1.2 mm or more. The thickness of steel for
hot stamping may be 6.0 mm or less. A more preferable thickness of the steel sheet is
15 5.0 mm or less, 4.0 mm or less, 3.2 mm or less or 2.8 mm or less.
[0063]

Next, a preferable manufacturing method of the steel sheet for hot stamping 10
will be described. A slab that is to be subjected to hot rolling may be a slab
20 manufactured by a normal method and may be, for example, a slab manufactured by an
ordinary method such as a continuous cast slab or a thin slab caster. Hot rolling may
also be carried out by an ordinary method and is not particular! y limited.
[0064]
"Cooling start temperature"
25 The start temperature of cooling after hot rolling (cooling start temperature) is
28
preferably the Ac3 point to 1400°C. When cooling starts in this range, it is possible to
obtain a dislocation density of 5 x 1013 m/m3 or more at a depth of 100 ~m from the
surface of the base material 1 of the steel sheet for hot stamping 10. A more preferable
cooling start temperature is 1 ooooc to 1150°C. The Ac3 point (°C) is represented by the
5 following formula (1).
Ac3 = 912- 230.5 x C + 31.6 x Si- 20.4 x Mn- 14.8 x Cr- 18.1 x Ni + 16.8 x
Mo-39.8xCu .. (l)
Element symbols in the formula indicate the amounts by mass% of the
corresponding elements, and zero is assigned in a case where an element is not contained.
10

[CLAIMS]
1. A steel sheet for hot stamping comprising:
a base material;
an Al-Si alloy plating layer in which anAl content is 75 mass% or more, a Si
5 content is 3 mass% or more and a total of the Al content and the Si content is 95 mass%
or more;
10
15
20
25
an Al oxide coating having a thickness of 0 to 20 nm; and
aNi plating layer having aNi content of more than 90 mass%
in this order,
wherein the base material has a chemical composition of, by mass%,
C: 0.01% or more and less than 0.70%;
Si: 0.001% to 1.000%;
Mn: 0.40% to 3.00%;
sol. Al: 0.0002% to 0.5000%;
P: 0.100% or less;
S: 0.1000% or less;
N: 0.0100% or less;
Cu: 0% to 1.00%;
Ni: 0% to 1.00%;
Nb: 0% to 0.150%;
V: 0% to 1.000%;
Ti: 0% to 0.150%;
Mo: 0% to 1.000%;
Cr: 0% to 1.000%;
B: 0% to 0.0100%;
57
5
10
15
20
25
Ca: 0% to 0.010%;
REM: 0% to 0.300%; and
a remainder: Fe and an impurity,
the Al-Si alloy plating layer has a thickness of 7 to 148 ~m, and
the Ni plating layer has a thickness of more than 200 nm and 2500 nm or less.
2. The steel sheet for hot stamping according to claim 1,
wherein the Ni plating layer is provided in direct contact with the Al-Si alloy
plating layer as an upper layer of the Al-Si alloy plating layer.
3. The steel sheet for hot stamping according to claim 1,
wherein the Al oxide coating has a thickness of 2 to 20 nm.
4. The steel sheet for hot stamping according to any one of claims 1 to 3,
wherein the chemical composition of the base material contains, by mass%, one
or two or more selected from the group consisting of:
Cu: 0.005% to 1.000%;
Ni: 0.005% to 1.000%;
Nb: 0.010% to 0.150%;
V: 0.005% to 1.000%;
Ti: 0.010% to 0.150%;
Mo: 0.005% to 1.000%;
Cr: 0.050% to 1.000%;
B: 0.0005% to 0.0100%;
Ca: 0.001% to 0.01 0%; and
58
REM: 0.001% to 0.300%.
5. The steel sheet for hot stamping according to any one of claims 1 to 4,
wherein a dislocation density at a depth of 100 ~m from a surface of the base
5 material is 5 x 1013 m/m3 or more.