[Technical Field]
1
5 [0001]
The present invention relates to a steel sheet for hot stamping. Priority is
claimed on Japanese Patent Application No. 2020-084585, 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
2
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 IME). 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 micro3
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 the surface layer region of a steel
sheet with nickel.
[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 methods of Patent Documents 1 and 2, in a hot-stamping
formed body after hot stamping, Fe diffuses up to the surface of the hot- stamping formed
20 body, which creates a problem in that the corrosion of the hot-stamping formed body
cannot be sufficiently suppressed.
[Citation List]
[Patent Document]
[0010]
25 [Patent Document 1]
4
PCT International Publication No. WO 2016/016707
[Patent Document 2]
PCT International Publication No. WO 2017/187255
[Summary of the Invention]
5 [Problems to be Solved by the Invention]
[0011]
The present invention has been made in consideration of the above-described
problem, and an objective of the present invention is to provide a steel sheet for hot
stamping from which a hot-stamping formed body having excellent corrosion resistance
10 can be manufactured.
[Means for Solving the Problem]
[0012]
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 aNi plating layer
15 having a desired average layer thickness (thickness) on the Al-Si alloy plating layer and
contains a desired amount of a solid solution of Nb in a steel sheet that is a substrate of
the steel sheet for hot stamping, it is possible to sufficiently suppress the corrosion of
hot-stamping formed bodies.
20
25
[0013]
The present invention has been made by further progres sing 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
5
10
15
20
25
5
content is 3 mass% or more and a total of the Al content and the Si content is 95 mass%
or more, and
aNi plating layer having aNi content of more than 90 mass%
in this order,
in which a chemical composition of the base material is, by mass%,
C: 0.01% or more and less than 0.70%,
Si: 0.005% to 1.000%,
Mn: 0.40% to 3.00%,
Nb: 0.010% to 0.200%,
a solid solution ofNb: 0.010% to 0.150%,
sol. Al: 0.00020% to 0.50000%,
P: 0.100% or less,
S: 0.1000% or less,
N: 0.0100% orless,
Cu: 0% to 1.00%,
Ni: 0% to 1.00%,
V: 0% to 1.00%,
Ti: 0% to 0.150%,
Mo: 0% to 1.000%,
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 )liD, and
5
10
6
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 chemical
composition of the base material may contain, by mass%, one or two or more selected
from the group consisting of
Cu: 0.005% to 1.00%,
Ni: 0.005% to 1.00%,
V: 0.01% to 1.00%,
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%.
[Effects of the Invention]
15 [0014]
According to the aspect of the present invention, it is possible to obtain a steel
sheet for hot stamping from which a hot-stamping formed body having excellent
corrosion resistance can be manufactured.
[Brief Description of Drawings]
20 [0015]
25
Fig. 1 is a schematic cross-sectional view of a steel sheet for hot stamping
according to an embodiment of the present invention.
[Embodiment(s) for implementing the Invention]
[0016]

5
10
7
As a result of intensive studies, the present inventors found that, in a case where
anAl-plated steel sheet is hot-stamped, Fe diffuses to the surface and the corrosion
resistance deteriorates.
[0017]
As a result of additional intensive studies, the present inventors obtained the
following findings.
(A) When the average layer thickness (thickness) of aNi plating layer is more
than 200 nm, it is possible to suppress the diffusion of Fe to the surface of a hot-stamping
formed body.
(B) When a sufficient amount of a solid solution of Nb is present in a steel sheet
that serves as a substrate of a steel sheet for hot stamping, the alloying rate (diffusion
rate) of Fe from the interface between the steel sheet and anAl plating layer toward the
surface of the hot-stamping formed body decreases, and it is possible to further suppress
the diffusion of Fe.
15 [0018]
In a steel sheet for hot stamping according to the present embodiment, the
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
20 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, an Al-Si alloy plating layer 2
and aNi plating layer 3. 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 value and the upper limit value. Numerical
25 values expressed with "more than" and "less than" are not included in numerical ranges.
8
Regarding chemical compositions, "%" indicates "mass%" in all cases.
[0019]
(Steel sheet (base material))
The chemical composition of the steel sheet (base material) that serves as the
5 steel sheet (base material) 1 of the steel sheet for hot stamping 10 according to the
present embodiment is, by mass%, C: 0.01% or more and less than 0.70%, Si: 0.005% to
1.000%, Mn: 0.40% to 3.00%, sol. Al: 0.00020% to 0.50000%, Nb: 0.010% to 0.200%, a
solid solution ofNb: 0.010% to 0.150%, 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%, V: 0% to 1.00%, Ti: 0% to 0.150%,
10 Mo: 0% to 1.000%, 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.
[0020]
"C: 0.01% or more and less than 0.70%"
C is an important element for securing hardenability. When the C content is
15 less than 0.01 %, it becomes difficult to obtain sufficient hardenability, and the tensile
strength decreases. Therefore, the C content may be 0.01% or more, 0.08% or more,
0.18% or more or 0.25% or more. On the other hand, when the C content is 0.70% or
more, a coarse carbide is formed, and brittle fracture is likely to occur. Therefore, the C
content is preferably set to less than 0.70%. The C content is preferably 0.38% or les s.
20 [0021]
"Si: 0.005% to 1.000%"
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. Therefore, the Si
content is set to 0.005% or more. A more preferable Si content is 0.100% or more. In
25 a case where Cu is contained, the Si content is preferably set to 0.350% or more in order
9
to suppress the hot embrittlement of Cu. When more than 1.000% of Si is contained,
the austenite transformation 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-
5 stamping formed body. Therefore, the Si content is set to 1.000% or less. The Si
content is preferably 0.800% or less. In a case where u is contained, since the
temperature of the austenite transformation temperature becomes high, the Si content is
preferably 0.600% or less. The Si content may be 0.400% or less or 250% or less.
10
[0022]
"Mn: 0.40% to 3.00%"
Mn is an element necessary to ensure a solid solution of Nb. When the Mn
content is less than 0.40%, it is not possible to suppress the precipitation of a Nb
carbonitride, and it is difficult to obtain a desired amount of a solid solution of Nb.
Therefore, the Mn content is set to 0.40% or more. The Mn content is preferably 0.80%
15 or more. On the other hand, when the Mn content is set to more than 3.00%, since a
coarse inclusion is generated in steel, and breakage is likely to occur, the Mn content is
set to 3.00% or less. The Mn content is preferably 2.00% or less.
20
[0023]
"soL Al: 0.00020% to 0.50000%"
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 0.00020%, since molten steel is not sufficiently
deoxidized, and the above-described effect cannot be obtained, the soL Al content is set
to 0.00020% or more. The soL Al content is preferably 0.00100% or more or 0.00200%
25 or more. On the other hand, when the soL Al content exceeds 0.50000%, a coarse oxide
10
is generated in steel, and the brittle fracture of the hot-stamping formed body is likely to
occur. Therefore, the soL Al content is set to 0.50000% or les s. The soL Al content is
preferably 0.40000% or less or 0.30000% or less. soL Al means acid-soluble Aland
refers to the total amount of the solid solution of Al that is present in steel in a solid
5 solution state and Al that is present in steel as an acid-soluble precipitate such as AlN.
[0024]
"Solid solution of Nb: 0.010% to 0.150%"
A solid solution of Nb decreases the diffusion rate of Fe. In a case where the
amount of a solid solution ofNb is less than 0.010%, the above-described effect cannot
10 be obtained. Therefore, the amount of the solid solution of Nb is 0.010% or more. A
more preferable amount of the solid solution of Nb is 0.030% or more. A still more
preferable amount of the solid solution of Nb is 0.050% or more. On the other hand,
even when more than 0.150% of the solid solution ofNb is contained, the abovedescribed
effect is saturated, and thus the amount of the solid solution of Nb is set to
15 0.150% or les s. The amount of the solid solution ofNb is more preferably 0. 100% or
less. The solid solution of Nb refers to Nb that is present in steel in a solid solution
state.
20
[0025]
"Nb: 0.010% to 0.200%"
In a case where the Nb content is less than 0.010%, it is not possible to set the
amount of the solid solution ofNb to 0.010% or more. Therefore, the Nb content is
0.010% or more. A more preferable Nb content is 0.030% or more. A still more
preferable Nb content is 0.050% or more. On the other hand, even when the Nb content
is more than 0.200%, Nb is not capable of forming a solid solution, and a carbide is
25 coarsened, and thus the Nb content is set to 0.200% or less. The Nb content is more
preferably 0.100% or less.
[0026]
"P: 0.100% or less"
11
Pis an element that is segregated in grain boundaries and degrades the strength
5 of the grain boundaries. When the P content exceeds 0.100%, the strength of grain
boundaries significantly decreases, and the brittle fracture of the hot-stamping formed
body is likely to occur. Therefore, the P content is preferably set to 0.100% or less.
The P content is preferably 0.050% or less. A more preferable P content is 0.010% or
less. The lower limit of the P content is not particularly limited; however, when the
10 lower limit is decreased to lower than 0.0005%, the dephosphorization cost increases
significantly, which is not preferable economically, and thus the lower limit of the P
content may be set to 0.0005% in actual operation.
15
[0027]
"S: 0.1000% or less"
S is an element that forms an inclusion in steel. When the S content of the base
material exceeds 0.1000%, a large amount of an inclusion is generated in steel, and the
brittle fracture of the hot-stamping formed body is likely to occur. Therefore, the S
content is preferably 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
20 lower limit is decreased to lower than 0.00015%, the desulfurization cost increases
25
significantly, which is not preferable economically, and thus the lower limit of the S
content may be set to 0.00015% in actual operation.
[0028]
"N: 0.0100% or less"
N is an impurity element and an element that forms a nitride in steel to degrade
12
the toughness and tensile strength of the hot-stamping formed body. When theN
content exceeds 0.0100%, a coarse nitride is generated in steel, and the brittle fracture of
the hot-stamping formed body is likely to occur. Therefore, theN content is set to
0.0100% or less. TheN content is preferably 0.0050% or less. The lower limit of the
5 N 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 0.0001% in actual operation.
[0029]
The steel sheet that configures the steel sheet for hot stamping 10 according to
10 the present embodiment may contain, instead of some of Fe, one or two or more selected
from the group consisting of Cu, Ni, V, Ti, Mo, Cr, B, Ca and REM as an arbitrary
element. In a case where the following arbitrary element is not contained, the content
thereof is 0%.
15
[0030]
"Cu: 0% to 1.00%"
Cu has an action of diffusing up to a plating layer of a hot stamping member
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
20 secure the 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 exhibit
the above-described effect. The Cu content is more preferably 0. 150% or more. 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
25 content is more preferably 0.350% or less.
13
[0031]
"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
5 contained. When the Ni content is less than 0.005%, there is a case where the abovedescribed
effects cannot be sufficiently obtained. Therefore, the Ni content is
preferably 0.005% or more. The Ni content is preferably 0.05% or more. On the other
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
10 content is preferably 0.60% or less.
[0032]
"V: 0% to 1.00%"
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
15 may be contained. In order to obtain the above-described effects, 0.01% or more of Vis
preferably contained, and 0.05% or more of Vis more preferably contained. However,
when the V content exceeds 1.00%, the above-described effects are saturated, and the
economic efficiency decreases. Therefore, in the case of containing V, the V content is
set to 1.00% or less.
20 [0033]
"Ti: 0% to 0.150%"
Ti is an element that contributes to improvement in the strength of the hotstamping
formed body 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
25 more in order to reliably exhibit the above-described effect. The Ti content is
5
14
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
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 strength of the hotstamping
formed body by solid solution strengthening and thus may be contained as
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
10 preferably 0.010% or more. On the other hand, even when more than 1.000% ofMo is
15
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.
[0035]
"Cr: 0% to 1.000%"
Cr is an element that contributes to improvement in the strength of the hotstamping
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
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% ofCr is
20 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]
"B: 0% to 0.0100%"
B is an element that is segregated in grain boundaries to improve the strength of
25 the grain boundaries and thus may be contained as necessary. In a case where B is
15
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, 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
5 is more preferably 0.0075% or less.
[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
10 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 theCa 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
REM is contained, the above-described effect is saturated, and thus the REM content is
preferably set to 0.300% or less. In the present embodiment, REM refers to a total of 17
20 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"
The remainder of the chemical composition of the base material 1 that
25 configures the steel sheet for hot stamping 10 according to the present embodiment is Fe
16
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
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
5 embodiment that have been hot-stamped, are not impaired.
[0040]
The above-described chemical composition of the base material 1 may be
measured by an ordinary analytical method. For example, the chemical composition
may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-
10 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 conductivity method. The
chemical composition needs to be analyzed after the plating layer on the surface is
removed by machining. soL Al may be measured by ICP-AES using a filtrate obtained
by hydrolyzing a specimen with an acid.
15 [0041]
The amount of the solid solution of Nb is measured by the following method.
An electrolytic extraction (electrolytic solution: 10 vol% ofacetylacetone-1 mass% of
tetraammonium chloride-methanol) residue is separated, then, only this residue is
dissolved by a sulfuric phosphoric acid white fume treatment, the solution is analyzed by
20 ICP-AES, and the amount ofNb precipitated (insoL Nb) is determined. insoL Nb is
subtracted from the total amount of Nb in the base material 1, thereby determining the
amount of a solid solution ofNb (soL Nb). At this time, insoL Nb is rounded down
after four decimal places (mass%). The total amount of Nb in the base material 1 is
determined according to JIS G 1258-4 (2007).
25 [0042]
17
"Metallographic structure"
Next, the metallographic structure of the base material 1 that configures the steel
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
5 area ratio of ferrite is preferably 10% or more in terms of the area ratio in a cross section.
A more preferable area ratio of ferrite is 20% or more. The area ratio of ferrite is
preferably 40% or less in terms of the area ratio in a cross section. A more preferable
area ratio of ferrite is 30% or less. The area ratio of pearlite is preferably 10% or more
in terms of the area ratio in a cross section. A more preferable area ratio of pearlite is
10 20% or more. The area fraction of pearlite is preferably 40% or less. A more
preferable area ratio of pearlite is 30% or less. The area ratio of bainite is preferably
20% or more in terms of the area ratio in a cross section. A more preferable area ratio
of bainite is 30% or more. 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 martensite
15 or residual austenite. The area ratio of the remainder in microstructure may be less than
5%.
[0043]
(Measurement method of area ratios of ferrite, pearlite and bainite)
The area ratios of ferrite and pearlite are measured by the following method. A
20 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
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 118 depth of the sheet
25 thickness from the surface to a 3/8 depth of the sheet thickness from the surface, which is
18
50 11m in length, is measured at measurement intervals of 0.1 11m by an electron
backscatter diffraction method such that a 1/4 depth of the sheet thickness from the
surface can be analyzed to obtain crystal orientation information. For the measurement,
an instrument composed of a thermal field emission-type scanning electron microscope
5 (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 w-s Pa or less, the accelerating voltage is set to 15 kV, the irradiation current
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.
10 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
except the crystal grains determined as pearlite, from the obtained crystal orientation
information, regions where the grain average misorientation value is 1.0° or less are
15 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
of ferrite.
Regions where the grain average misorientation value is more than 1.0° and 5.0°
20 or less are determined as bainite using the above-described function. The area ratio of
the regions determined as bainite is obtained, thereby obtaining the area ratio of bainite.
[0044]
(Determination method of area ratio of remainder)
The area ratio of the remainder in the present embodiment is a value obtained by
25 subtracting the total area of ferrite, pearlite and bainite from 100%.
19
[0045]
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
or more from the viewpoint of the weight reduction of vehicle bodies. The sheet
5 thickness of the base material I is more preferably 0.8 mm or more, 1.0 mm or more or
1.2 mm or more. The sheet thickness of the base material! is preferably set to 6.0 mm
or less. The sheet thickness of the base material I is more preferably 5.0 mm or les s,
4.0 mm or less, 3.2 mm or less or 2.8 mm or less.
10
[0046]
(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
expression "containing Al and Si as main component" means that at least the Al content
15 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 2 is
preferably 95 mass% or less. When the Al content in the Al-Si alloy plating layer 2 is in
this range, it is possible to prevent the formation of scale on the surface of the steel sheet
20 during hot stamping.
[0047]
The Si content in the Al-Si alloy plating layer 2 is 3 mass% or more. The Si
content in the Al-Si alloy plating layer 2 is more preferably 6 mass% or more. The Si
content in the Al-Si alloy plating layer 2 is 20 mass% or less. The Si content is more
25 preferably 12 mass% or less. When the Si content in the Al-Si alloy plating layer 2 is 3
20
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 obtain
favorable adhesion of scale. 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
5 alloy plating layer 2 is Fe and an impurity. As the impurity, a component that is
inevitably incorporated 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.
[0048]
The average layer thickness (thickness) oftheAl-Si alloy plating layer 2 of the
10 steel sheet for hot stamping 10 according to the present embodiment is 7 J.lm or more.
This is because, when the thickness of the Al-Si alloy plating layer 2 is less than 7 J.lm, it
is not possible to sufficiently suppress the formation of scale during hot stamping. A
more preferable thickness oftheAl-Si alloy plating layer 2 is 12 J.lm or more, 15 J.lm or
more, 18 J.lm or more or 22 J.lm or more. When the thickness of the Al-Si alloy plating
15 layer 2 is more than 148 J.lm, not only is the above-described effect saturated, but the cost
also increases, and thus the thickness oftheAl-Si alloy plating layer 2 is 148 J.lm or les s.
A more preferable thickness of the Al-Si alloy plating layer 2 is 100 J.lm or less, 60 J.lm or
less, 45 J.lm or less or 37 J.lm or less.
20
[0049]
The thickness of the Al-Si alloy plating layer 2 is measured as described below.
The steel sheet for hot stamping 10 is cut in the sheet thickness direction, and then the
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
25 electron probe microanalyzer (FE-EPMA), and, among detected components, the Al
21
concentration and the Si concentration 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 Si content is 3 mass%
5 or more and the 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 layer thickness of the Al-Si alloy
plating layer 2 is the length of the above-described region in the sheet thickness direction.
The layer thicknesses of the Al-Si alloy plating layer 2 are measured at five positions at 5
11m intervals, and the arithmetic average of the obtained values is regarded as the
10 thickness oftheAl-Si alloy plating layer 2.
[0050]
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 112 position of the total thickness of
15 the Al-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.
[0051]
(Ni plating layer)
The Ni plating layer 3 of the steel sheet for hot stamping 10 according to the
20 present embodiment is provided on theAl-Si alloy plating layer 2 as an upper layer of the
Al-Si alloy plating layer 2. Between the Al-Si alloy plating layer 2 and the Ni plating
layer 3, anAl oxide coating formed by the oxidation of the surface of the Al-Si alloy
plating layer may be present.
[0052]
25 The average layer thickness (thickness) of the Ni plating layer 3 according to the
22
present embodiment is more than 200 nm. A more preferable thickness of the Ni
plating layer 3 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 3 is 200 nm or less, it is not
possible to sufficiently suppress the diffusion of Fe to the surface of the hot-stamping
5 formed body. In addition, from the viewpoint of suppressing the diffusion of Fe, the
thickness of the Ni plating layer 3 is desirably as thick as possible. On the other hand,
from the viewpoint of the cost, the thickness of the Ni plating layer 3 is 2500 nm or less.
The thickness of the Ni plating layer 3 is preferably 1500 nm or less, 1200 nm or less or
1000 nm or less and more preferably 900 nm or less or 730 nm or less. When the
10 thickness of the Ni plating layer 3 is more than 2500 nm, the effect on suppressing the
diffusion of Fe is saturated.
[0053]
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
15 hot stamping 10 cannot be obtained. Therefore, the Ni content in the Ni plating layer 3
is more than 90 mass%. A preferable Ni content is 92 mass% or more. A more
preferable Ni content is 93 mass% or more or 94 mass%. A still more preferable Ni
content is 95 mass% or more, 98 mass% or more or 99 mass% or more. The remainder
of the Ni plating layer 3 is not particularly limited as long as the Ni content is more than
20 90 mass%. Cr may be contained in the Ni plating layer 3, and the Ni/Cr ratio is
preferably larger than 9, and this ratio is more preferably 15 or more or 30 or more. The
Cr content in the Ni plating layer 3 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.
25 [0054]
23
The thickness of the Ni plating layer 3 is measured by alternately 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
5 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 ofNi in the XPS measurement of 852.5 eV to 852.9 eV appears and then
disappears. The layer thickness of the Ni plating layer 3 is calculated from the
sputtering etching time and the sputtering etching rate from a position where the Ni
10 content reaches 10 atomic% or more for the first time after the start of the sputtering to a
position where the Ni content reaches less than 10 atomic%. The sputtering etching rate
is obtained in terms of Si02. The thickness of the Ni plating layer 3 is the arithmetic
15
20
average value of two measurement sites.
[0055]
Regarding the Ni content in the Ni plating layer 3, the Ni content at the central
position in the sheet thickness direction of the Ni plating layer 3 that is obtained in the
measurement of the thickness of the Ni plating layer is regarded as the Ni content of the
Ni plating layer.
[0056]
(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
25 5.0 mm or less, 4.0 mm or less, 3.2 mm or less or 2.8 mm or less.
24
[0057]
<.Manufacturing method of steel sheet for hot stamping>
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
5 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.
[0058]
In a case where the Nb content in the slab is less than 0.010%, it is not possible
to sufficiently obtain a solid solution of Nb even when all Nb in the slab turns into the
10 solid solution ofNb. Therefore, the Nb content in the slab is 0.010% or more. In a
case where the Nb content in the slab is more than 0.200%, a carbide is coarsened.
Therefore, the Nb content in the slab is 0.200% or less.
[0059]
The slab is heated to 1200°C or higher before hot rolling. In a case where the
15 heating temperature of the slab is lower than 1200°C, a Nb carbide in the slab is not
dissolved, and the amount of the solid solution of Nb decreases, which is not preferable.
Therefore, the heating temperature of the slab is set to 1200°C or higher.
[0060]
Hot rolling after the heating of the slab may be carried out by an ordinary
20 method and is not particularly limited.
[0061]
After the hot rolling, the steel sheet is cooled to a temperature range of room
temperature to 500°C and coiled. When the coiling start temperature is higher than
500°C, the amount of the solid solution of Nb in the steel sheet for hot stamping 10
25 decreases, which is not preferable. Here, room temperature refers to a temperature
25
region of 23°C to 28°C. The cooling rate up to 500°C after the rolling is preferably
30 °C/second or faster.
[0062]
The steel sheet is cooled to a temperature range of 500°C or lower and then
5 coiled. After the coiling, cold rolling may be further carried out as necessary. The
cumulative 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.
[0063]
Among the manufacturing steps of the steel sheet for hot stamping, in steps after
10 the coiling (from annealing before plating to coiling after plating), in a case where the
total of times taken for the temperature of the steel sheet to reach 500°C or higher
(holding time at 5oooc or higher) is longer than 180 seconds, the amount of the solid
solution of Nb in the steel sheet for hot stamping 10 becomes low. Therefore, the total
of the times taken for the temperature of the steel sheet to reach 500°C or higher (holding
15 time at 5oooc or higher) is set to 180 seconds or shorter. Hereinafter, manufacturing
conditions for setting the holding time at 5oooc or higher to 180 seconds or shorter will
be described as an example, but the present invention is not limited to the following
method.
20
[0064]
Before Al-Si alloy plating is provided on the steel sheet, annealing before plating
is carried out. Specifically, it is preferable that, after the coiling, the steel is heated up to
a temperature range of 780°C to 81 ooc at a temperature rising rate of 10 °C/second to
100 °C/second and retained in the temperature range for 90 seconds to 110 seconds. In
a case where the temperature rising rate is slower than 10 oc /second, there is a case
25 where the holding time at 5oooc or higher exceeds 180 seconds. Usual temperature
26
rising rates during the annealing before plating are 3 °C/second to 6 °C/second, and there
is a case where the holding time at 500°C or higher exceeds 180 seconds. Therefore,
the steel sheet is preferably heated rapidly by energization heating or the like in order to
set the temperature rising rate to 10 °C/second or faster.
5 [0065]
After the end of the retention, the steel sheet is rapidly cooled to a temperature
range of 660°C to 680°C at a cooling rate of 12 °C/second to 20 °C/second (cooling
before plating). In a case where the cooling rate is slower than 12 °C/second, there is a
case where the holding time at 5oooc or higher becomes longer than 180 seconds.
10 [0066]
"Al-Si alloy plating"
Al-Si alloy plating is provided on the steel sheet. A method for forming the AlSi
alloy plating layer 2 is not particularly limited, and a hot-dip plating method, an
electro plating method, a vacuum deposition method, a cladding method, a thermal
15 spraying method or the like can be used. The hot-dip plating method is particularly
preferable. In the case of the hot-dip plating method, hot-dip plating is carried out after
the cooling before plating.
[0067]
In a case where the Al-Si alloy plating layer 2 is formed by the hot-dip plating
20 method, the base material 1 is immersed in a plating bath where the components have
been adjusted such that at least the Si content reaches 3 mass% or more and the total of
the Al content and the Si content reaches 95 mass% or more, thereby obtaining the Al-Si
alloy-plated steel sheet. In this case, the Al-Si alloy plating layer 2 turns into a molten
Al-Si alloy plating layer. The temperature of the plating bath is preferably within a
25 temperature range of 66ooc to 680°C. After the hot-dip plating, the steel sheet is
27
preferably cooled to 500°C or lower at 15 °C/second to 40 °C/second (cooling after
plating). In a case where the cooling rate is slower than 15 °C/second, there is a case
where the holding time at 500°C or exceeds 180 seconds. Usually, the cooling rate after
the hot-dip plating is 8°C/second to 12 °C/second, the steel sheet is preferably cooled by
5 mist cooling or the like in order to set the cooling rate to 15 °C/second or faster.
[0068]
In addition, in a case where hot-dip plating is carried out, there is a case where
Fe is incorporated into the plating bath as an impurity other than Al or Si. In addition,
Ni, Mg, Ti, Zn, Sb, Sn, Cu, Co, In, Bi, Ca, mischmetal, and the like may be further
10 contained in the plating bath as long as the Si content reaches 3 mass% or more and the
total of the Al content and the Si content reaches 95 mass% or more.
[0069]
"Ni plating"
The steel sheet for hot stamping is obtained by providing Ni plating on the Al-Si
15 alloy-plated steel sheet to form aNi plating layer 3. The Ni plating layer 3 may be
formed by an electro plating method, a vacuum deposition method or the like. Before
the Ni plating is provided, an oxide film on the Al-Si alloy plating layer 2 may be
removed.
In a case where the Ni plating layer 3 is formed by electro plating, the Al-Si
20 alloy-plated steel sheet is immersed in a plating bath containing nickel sulfate, nickel
chloride and boric acid and the current density and the energization time are controlled as
appropriate using soluble Ni as an anode, whereby the Ni plating layer 3 can be formed
such that the thickness reaches more than 200 nm.
In a case where the plating original sheet is a cold-rolled steel sheet or the like,
25 temper rolling may be carried out at a cumulative rolling reduction of approximately
0.5% to 2% after the Ni plating).
[0070]

28
A hot-stamping formed body is obtained by hot-stamping the steel sheet for hot
5 stamping manufactured above. Hereinafter, conditions for the hot stamping will be
described as an example, but the hot stamping conditions are not limited to these
conditions.

[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%
10
15
20
25
or more; 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: 0.005% to 1.000%;
Mn: 0.40% to 3.00%;
Nb: 0.010% to 0.200%;
a solid solution ofNb: 0.010% to 0.150%;
sol. Al: 0.00020% to 0.50000%;
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%;
V: 0% to 1.00%;
Ti: 0% to 0.150%;
Mo: 0% to 1.000%;
Cr: 0% to 1.000%;
B: 0% to 0.0100%;
51
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 J.lm, and
5 the Ni plating layer has a thickness of more than 200 nm and 2500 nm or less.
10
15
2. The steel sheet for hot stamping according to claim 1,
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 1.00%;
Ni: 0.005% to 1.00%;
V: 0.01% to 1.00%;
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%.