[0001]The present invention relates to a hot-stamping formed body.
Priority is claimed on Japanese Patent Application No. 2019-052103, filed
March 20, 2019, the content of which is incorporated herein by reference.
[Background Art]
[0002]
In recent years, there has been a demand for a reduction in the weight of
vehicle body of a vehicle from the viewpoint of environmental protection and resource
saving, and a high strength steel sheet has been increasingly applied to a member for a
vehicle. The higher the strength of the steel sheet, the greater the load during press
forming on the member for a vehicle. In addition, when a high strength steel sheet is
used, formability into a member having a complex shape becomes a problem. In
order to solve such a problem, a hot stamping technique in which press forming is
performed after heating to the austenite region where the steel sheet softens has been
applied.
[0003]
Hot stamping has attracted attention as a technique that achieves both forming
into a member for a vehicle and securing strength by performing a hardening treatment
in a die simultaneously with press working. Hot stamping has been employed as a
working method for a deformation suppressing member and an impact absorbing
member of a vehicle. In particular, the deformation suppressing member is required
to be a member that is hardly deformed by a collision, and is required to be subjected
- 1 -
to high-strengthening.
[0004]
However, in general, the toughness decreases as the strength of the steel sheet
increases, so that cracks are likely to occur during the collision deformation. As a
result, there are cases where the proof stress and absorbed energy required for the
member for a vehicle cannot be obtained.
[0005]
Patent Document 1 proposes a technique in which spheroidizing annealing at
650 to Act + 20°C before hardening and tempering to spheroidize carbides and
undissolved carbides are reduced in amount during hardening and tempering heat
treatments, thereby improving toughness.
[0006]
Patent Document 2 proposes a hot-rolled steel sheet in which the total amount
of tempered martensite and lower bainite is set to 90% or more to provide a
homogeneous microstructure, thereby achieving both high strength and low
temperature toughness.
[0007]
Patent Document 3 proposes an ultrahigh-strength cold-rolled steel sheet
having a tempered martensite single phase as its microstructure and improved stretch
flangeability.
[0008]
Patent Document 4 proposes a method of manufacturing a formed body
capable of achieving both high strength and toughness by hardening performed twice.
In this manufacturing method, the microstructure of steel is formed into martensite
containing a large amount of fine carbides by a first hardening heat treatment (it is
- 2 -
described that the number density of the carbides is preferably 0.50 /)lm2 or more).
Thereafter, rapid heating is performed in a second hardening heat treatment to cause
the carbides to act as nucleation sites for reverse transformation to austenite, thereby
achieving the refinement of the microstructure.
[Prior Art Document]
[Patent Document]
[0009]
[Patent Document 1] Japanese Patent No. 5030280
[Patent Document 2] Japanese Patent No. 6132017
[Patent Document 3] Japanese Patent No. 5402191
[Patent Document 4] PCT International Publication No. W020181134874
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0010]
In the technique described in Patent Document 1, annealing is performed by
heating at lower than the Ac3 point for the purpose of spheroidizing carbides.
Therefore, Mn is not sufficiently diffused, and a portion having a high Mn
concentration is present in the annealed steel, and the toughness of the steel
deteriorates. In addition, coarse carbides are generated in the microstructure of the
steel due to the spheroidizing annealing. Since such carbides are likely to be a
fracture origin in a high strength steel of 2,000 MPa or more, there are cases where the
toughness of the steel significantly deteriorates.
[0011]
In the technique described in Patent Document 2, although the microstructure
is uniform as a whole, there are cases where Mn is segregated in prior austenite grains.
- 3 -
When the degree of segregation of Mn is reduced, the portion having a high Mn
concentration does not become the fracture origin, and a further improvement in
toughness can be expected. However, in Patent Document 2, the method has not been
clarified.
[0012]
In the technique described in Patent Document 3, although annealing is
performed at 900°C or lower in order not to coarsen the prior austenite grains, Mn is
not sufficiently diffused, and there are cases where Mn is segregated in the
microstructure. As described above, the portion having a locally high Mn
concentration tends to be a fracture origin in a high strength steel of 2,000 MPa or
more, so that there are cases where the toughness of the steel deteriorates. In addition,
in this technique, it is necessary to perform tempering at 250oc after the microstructure
is formed into martensite, which causes an increase in manufacturing cost due to an
increase in the number of processes.
[0013]
In the technique described in Patent Document 4, the steel in which carbides
are generated as much as possible during the first heat treatment is subjected to the
second heat treatment for reverse transformation to austenite using the carbides as the
nucleation site. Therefore, the amount of residual austenite is small during the first
heat treatment and the grain growth of austenite is likely to proceed during the second
heat treatment. Therefore, a method of further refining grains is required.
[0014]
The present invention has been made to solve the problems of the related art,
and an object thereof is to provide a hot-stamping formed body having excellent
strength and toughness.
- 4 -
[Means for Solving the Problem]
[0015]
As a result of intensive examinations on a method for solving the above
problems, the present inventors have obtained the following findings.
[0016]
In the related art, in order to secure a tensile strength of 2,000 MPa or more, it
is necessary to secure hardenability, and it has been considered that it is effective to
contain Mn. However, the containing of Mn promotes Mn segregation at the grain
boundaries, resulting in inferior toughness of the hot-stamping formed body.
Therefore, as a result of intensive studies, the present inventors found that a hotstamping
formed body having better toughness than in the related art can be obtained
even with a material containing Mn.
[0017]
The present inventors found that, as a microstructure of a hot-stamping
formed body, the occurrence of a crack can be suppressed by controlling the average
grain size of prior austenite grains to 5.0 )liD or less, and setting the average Mn
concentration at the grain boundaries of the prior austenite grains (hereinafter,
sometimes described as prior austenite grain boundaries) to 1.0 mass% or less. In
addition, as a result of intensive examinations by the present inventors, it was found
that the above-mentioned microstructure can be obtained by the following method.
[0018]
First, a pre-heat treatment (hereinafter, referred to as "first heat treatment") is
performed before a hot stamping step. The first heat treatment is a heat treatment
including a heating step of heating to a heating temperature T 1 of an Ac3 point to the
Ac3 point + 200°C, a holding step of holding at the heating temperature T1, and a
- 5 -
cooling step of cooling from the heating temperature T1 to a cooling stop temperature
of "250°C to 400°C" at an average cooling rate of 10 °C/s to 500 °C/s. The heating
step and the holding step of the first heat treatment have a role of re-dissolving coarse
carbides formed before the first heat treatment and a role of concentrating Mn at the
prior austenite grain boundaries. In addition, since the microstructure is controlled to
include martensite, tempered martensite, bainite, and tempered bainite by the cooling
step of the first heat treatment, a large amount of high angle grain boundaries are
formed in the prior austenite grains.
[0019]
Next, a thermo-mechanical treatment (hereinafter, referred to as "second heat
treatment") of a hot stamping step is performed. The second heat treatment is a heat
treatment including a heating step of performing rapid heating to a heating temperature
T2 of an Ac3' point to (Ac3' point+ 100°C) at an average heating rate of 10 °C/s to
500 oc/s, and a holding step of holding at the heating temperature T2 for longer than
10 seconds and 60 seconds or shorter. Here, the difference (T2 - cooling stop
temperature) between the cooling stop temperature during the first heat treatment and
the heating temperature T2 during the second heat treatment is lower than 600°C.
The steel after the holding step of the second heat treatment is subjected to hot
stamping and cooling.
The Ac3' point is a temperature obtained by an experiment. Details thereof
will be described later.
[0020]
In the heating step of the second heat treatment, diffusion of Mn from the
prior austenite grain boundaries to the high angle grain boundaries formed in the first
heat treatment occurs. Accordingly, Mn is concentrated in fine residual austenite
- 6 -
present at the high angle grain boundaries (between blocks). As Mn is concentrated
in the residual austenite, the stability of the residual austenite increases, and the Ac3
point decreases. The decreased Ac3 point is referred to as "Ac3' point" for
convemence.
[0021]
In a temperature range exceeding the Ac3' point, austenitizing proceeds.
Here, since austenitizing at this stage proceeds at a low temperature, the grain growth
of austenite is suppressed. In addition, since fine austenite is maintained, Mn
concentration from the prior austenite grain boundaries to the high angle grain
boundaries continues.
[0022]
The steel after the second heat treatment is subjected to hot stamping and
cooled to room temperature. Accordingly, a hot-stamping formed body is obtained.
By these steps, a fine grain structure in which the average grain size of the prior
austenite grains of the hot-stamping formed body is 5.0 11m or less can be achieved,
and the average Mn concentration at the grain boundaries of the prior austenite grains
can be reduced to 1.0 mass% or less. As a result, fracture (the occurrence of a crack)
at the time of a collision is suppressed due to a reduction in a high Mn concentration
region of the prior austenite grain boundaries, and the propagation of a crack is
suppressed due to fine prior austenite grain sizes. As a result, it becomes possible to
obtain a hot-stamping formed body having excellent toughness.
[0023]
The gist of the present invention made based on the above findings is as
follows .
[1] A hot-stamping formed body according to an aspect of the present
- 7 -
invention includes, as a chemical composition, by mass%:
C: 0.40% to 0.70%;
Si: 0.010% to 1.30%;
Mn: 0.40% to 3.00%;
sol. Al: 0.0010% to 0.500%;
Ti: 0.010% to 0.100%;
Cr: 0.010% to 0.80%;
B: 0.0005% to 0.0100%;
P: 0.100% or less;
S: 0.0100% or less;
N: 0.0100% or less;
Nb: 0% to 0.100%;
Mo: 0% to 1.00%;
V: 0% to 0.100%;
Ni: 0% to 0.50%;
REM: 0% to 0.0100%;
Mg: 0% to 0.0100%;
Ca: 0% to 0.0100%;
Co: 0% to 4.00%; and
a remainder consisting of Fe and impurities,
in which an average grain size of prior austenite grains in a microstructure is
5.0 11m or less, and
an average Mn concentration at grain boundaries of the prior austenite grains
is 1.0 mass% or less.
[2] The hot-stamping formed body according to [1] may include, as the
- 8 -
chemical composition, by mass%, one or two or more elements selected from:
Nb: 0.010% to 0.100%;
Mo: 0.01% to 1.00%;
V: 0.001% to 0.100%;
Ni: 0.001% to 0.50%;
REM: 0.0010% to 0.0100%;
Mg: 0.0010% to 0.0100%;
Ca: 0.0010% to 0.0100%; and
Co: 0.10% to 4.00%.
[3] The hot-stamping formed body according to [1] or [2] may further include:
a plating layer on a surface of the hot-stamping formed body.
[4] In the hot-stamping formed body according to any one of [1] to [3], a
portion of the hot-stamping formed body may have a softened region.
[Effects of the Invention]
[0024]
According to the present invention, it is possible to provide a hot-stamping
formed body having excellent strength and toughness.
[Brief Description of the Drawings]
[0025]
FIG. 1 is a diagram showing the shape of a test piece used for measuring the
average Mn concentration at the grain boundaries of prior austenite grains.
FIG. 2 is a diagram showing the relationship between T2- cooling stop
temperature and the average Mn concentration at the grain boundaries of the prior
austenite grains.
FIG. 3 is a diagram showing the relationship between T2 - cooling stop
- 9 -
temperature and the average grain size of the prior austenite grains.
FIG. 4 is a diagram showing the relationship between a retention time at a
heating temperature T2 and the average Mn concentration at the grain boundaries of
the prior austenite grains.
FIG. 5 is a diagram showing the relationship between s retention time at s
heating temperature T2 and the average grain size of the prior austenite grains.
[Embodiments ofthe Invention]
[0026]
Hereinafter, a hot-stamping formed body according to the present embodiment
and a method of manufacturing the same will be described in detail. However, the
present invention is not limited to the configuration disclosed in the present
embodiment, and various modifications can be made without departing from the gist of
the present invention.
[0027]

First, the reason for limiting the chemical composition of the hot-stamping
formed body according to the present embodiment will be described. Hereinafter,
all % regarding the chemical composition means mass%. Numerical values indicated
as "more than or equal to" or "less than or equal to" fall within the numerical range.
Numerical values indicated as "less than" or "more than" do not fall within the
numerical range.
[0028]
The hot-stamping formed body according to the present embodiment includes,
as a chemical composition, by mass%: C: 0.40% to 0.70%; Si: 0.010% to 1.30%; Mn:
0.40% to 3.00%; sol. Al: 0.0010% to 0.500%; Ti: 0.010% to 0.100%; Cr: 0.010% to
- 10 -
0.80%; B: 0.0005% to 0.0100%; P: 0.100% or less; S: 0.0100% or less; N: 0.0100% or
less; and a remainder consisting of Fe and impurities. Hereinafter, each element will
be described in detail.
[0029]
"C: 0.40% to 0.70%"
Cis an important element for obtaining a tensile strength of 2,000 MPa or
more in the hot-stamping formed body. When the C content is less than 0.40%,
martensite becomes soft and it is difficult to obtain a tensile strength of 2,000 MPa or
more. Therefore, the C content is set to 0.40% or more. The C content is preferably
0.43% or more, and 0.45% or more. On the other hand, when the C content exceeds
0.70%, coarse carbides are generated and fracture is likely to occur, resulting in a
decrease in the toughness of the hot-stamping formed body. For this reason, the C
content is set to 0.70% or les s. The C content is preferably 0.60% or less, and 0.55%
or less.
[0030]
"Si: 0.010% to 1.30%"
Si has an effect of suppressing the formation of coarse cementite, and is an
important element for securing the toughness of the hot-stamping formed body. In
addition, Si has resistance to temper softening, and has an action of suppressing a
decrease in strength due to self-tempering during hot stamping hardening. When the
Si content is less than 0.010%, the above effect cannot be obtained, and there are cases
where the toughness of the hot- stamping formed body deteriorates. Therefore, the Si
content is set to 0.010% or more. The Si content is preferably 0.02% or more, and
0.03% or more. On the other hand, in a case where Si is contained in an amount of
more than 1.30%, the stability of austenite decreases, and the diffusion of Mn to high
- 11 -
angle grain boundaries does not proceed sufficiently during a second heat treatment, so
that the toughness of the hot-stamping formed body deteriorates. Therefore, the Si
content is set to 1.30% or less. The Si content is preferably 1.20% or less, and 1.00%
or less.
[0031]
"Mn: 0.40% to 3.00%"
Mn is an element that contributes to an improvement in the strength of the
hot-stamping formed body by solid solution strengthening. When the Mn content is
less than 0.40%, the solid solution strengthening ability is poor and martensite
becomes soft, so that it is difficult to obtain a tensile strength of 2, 000 MPa or more in
the hot-stamping formed body. Therefore, the Mn content is set to 0.40% or more.
The Mn content is more preferably 0.50% or more, and 0.60% or more. On the other
hand, when the Mn content exceeds 3.00%, coarse inclusions are generated in the steel
and fracture is likely to occur, resulting in a decrease in the toughness of the hotstamping
formed body. Therefore, the Mn content is set to 3.00% or less. The Mn
content is preferably 2.50% or less, 2.00% or less, and 1.50% or less.
[0032]
"soL Al: 0.0010% to 0.500%"
Al is an element having an action of deoxidizing molten steel and achieving
soundness of the steel (suppressing the occurrence of defects such as blowholes in the
steel). When the soL Al content is less than 0.0010%, deoxidation does not
sufficiently proceed. Therefore, the soL Al content is set to 0.0010% or more. The
soL Al content is preferably 0.010% or more, and 0.020% or more. On the other hand,
when the soL Al content exceeds 0.500%, coarse oxides are generated in the steel, and
the toughness of the hot-stamping formed body decreases. Therefore, the soL Al
- 12 -
content is set to 0.500% or less. The sol. Al content is preferably 0.400% or less, and
0.350% or less.
In addition, sol. Al means acid-soluble Al, and indicates solute Al present in
the steel in a solid solution state.
[0033]
"Ti: 0.010% to 0.100%"
Ti is an element that forms carbonitrides and suppresses the grain growth of
austenite during hot-stamping heating (particularly during a second heat treatment).
When the Ti content is less than 0.010%, the above effect cannot be obtained, and prior
austenite grains become coarse, so that the toughness of the hot-stamping formed body
deteriorates. Therefore, the Ti content is set to 0.010% or more. The Ti content is
preferably 0.020% or more, and 0.025% or more. On the other hand, when Ti is
contained in an amount of more than 0.100%, coarse TiN is generated, so that the
toughness of the hot-stamping formed body deteriorates. Therefore, the Ti content is
set to 0.100% or less. The Ti content is preferably 0.080% or less, or 0.060% or less.
[0034]
"Cr: 0.010% to 0.80%"
Cr is an element forming carbides and is also an element that improves the
toughness of the hot-stamping formed body by refining the carbides. When the Cr
content is less than 0.010%, the above effect cannot be obtained. Therefore, the Cr
content is set to 0.010% or more. The Cr content is preferably 0.10% or more, and
0.15% or more. On the other hand, even ifCr is contained in an amount of more than
0.80%, the above effect is saturated. In addition, Cr fills Mg segregation sites of prior
austenite grain boundaries and inhibits the segregation of Mn to the prior austenite
grain boundaries during a first heat treatment. As a result, the amount of Mn in the
- 13 -
prior austenite grains increases, and there are cases where the toughness of the hotstamping
formed body deteriorates. Therefore, the Cr content is set to 0.80% or less.
The Cr content is preferably 0.60% or less, 0.50% or less, and 0.40% or less.
[0035]
"B: 0.0005% to 0.0100%"
B is an element that segregates to grain boundaries and enhances the
hardenability of the steeL When the B content is less than 0.0005%, the above effect
cannot be obtained, and there are cases where ferrite is formed. As a result, there are
cases where it is difficult to obtain a tensile strength of 2,000 MPa or more, and the
toughness of the hot-stamping formed body deteriorates. Therefore, the B content is
set to 0.0005% or more. The B content is preferably 0.0010% or more, 0.0015% or
more, and 0.0020% or more. On the other hand, since B is likely to segregate to the
prior austenite grain boundaries, when B is contained in an amount of more than
0.0100%, B inhibits the segregation ofMn to the prior austenite grain boundaries
during the first heat treatment. As a result, the amount of Mn in the prior austenite
grains increases, and there are cases where the toughness of the hot-stamping formed
body deteriorates. Therefore, the B content is set to 0.0100% or less. The B content
is preferably 0.0075% or less, and 0.0050% or less.
[0036]
"P: 0.100% or Less"
Pis an element that segregates to the grain boundaries and reduces
intergranular strength. When the P content exceeds 0.100%, the intergranular
strength significantly decreases, and the toughness of the hot-stamping formed body
decreases. Therefore, the P content is set to 0.100% or less. The P content is
preferably 0.050% or less, and 0.030% or less. The lower limit of the P content is not
- 14 -
particularly limited. However, when the P content is reduced to less than 0.0001%,
the dephosphorization cost is increased significantly, which is economically
unfavorable. In an actual operation, the P content may be set to 0.0001% or more.
[0037]
"S: 0.0100% or Less"
S is an element that forms inclusions in the steel. When the S content
exceeds 0.0100%, a large amount of inclusions are generated in the steel, and the
toughness of the hot-stamping formed body decreases. Therefore, the S content is set
to 0.0100% or less. The S content is preferably 0.0040% or less. The lower limit of
the S content is not particularly limited. However, when the S content is reduced to
less than 0.00015%, the desulfurization cost is increased significantly, which is
economically unfavorable. In an actual operation, the S content may be set to
0.00015% or more, and 0.0002% or more.
[0038]
"N: 0.0100% or Less"
N is an impurity element that forms nitrides in the steel and is an element that
deteriorates the toughness of the hot-stamping formed body. When theN content
exceeds 0.0100%, coarse nitrides are generated in the steel, and the toughness of the
hot-stamping formed body significantly decreases. Therefore, theN content is set to
0.0100% or less. TheN content is preferably 0.0075% or less, and 0.0050% or les s.
The lower limit of theN content is not particularly limited. However, when theN
content is reduced to less than 0.0001%, the denitrification cost is increased
significantly, which is economically unfavorable. In an actual operation, theN
content may be set to 0.0001% or more.
[0039]
- 15 -
The remainder of the chemical composition of the hot-stamping formed body
according to the present embodiment consists of Fe and impurities. The impurities
are elements unavoidably incorporated from steel raw materials or scrap, elements
unavoidably incorporated in a steelmaking process, and/or elements intentionally
added in a small amount, and examples thereof are elements that are allowed in a range
in which the characteristics of the hot-stamping formed body according to the present
embodiment are not inhibited.
[0040]
In the hot-stamping formed body according to the present embodiment, the
following optional elements may be contained instead of a portion of Fe. The lower
limit of the amounts of the optional elements in a case where the following optional
elements are not contained is 0%. Hereinafter, each optional element will be
described in detaiL
[0041]
"Nb: 0% to 0.100%"
Nb is an element that improves the strength of the hot-stamping formed body
by solid solution strengthening and forms carbonitrides, thereby contributing to grain
refinement of the prior austenite grains. Therefore, Nb 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 effect. The Nb content is more preferably 0.035%
or more. On the other hand, when Nb is contained in an amount of more than 0.100%,
carbonitrides are excessively generated, and there are cases where the toughness of the
hot- stamping formed body decreases. Therefore, the Nb content is preferably set to
0.100% or less. The Nb content is more preferably 0.080% or less.
[0042]
- 16 -
"Mo: 0% to 1.00%"
Moisan element that improves the strength of the hot-stamping formed body
by solid solution strengthening and increase the hardenability of the steel, thereby
suppressing the formation of ferrite that deteriorates the toughness. Therefore, Mo
may be contained are necessary. In a case where Mo is contained, the Mo content is
preferably set to 0.01% or more in order to reliably exhibit the above effect. The Mo
content is more preferably 0.02% or more. On the other hand, even if Mo is
contained in an amount of more than 1.00%, not only is the above effect saturated, but
also an increase in the alloy cost is incurred. Therefore, the Mo content is preferably
set to 1.00% or less. The Mo content is more preferably 0.80% or less.
[0043]
"V: 0% to 0.100%"
Vis an element that improves the strength of the hot-stamping formed body
by solid solution strengthening. In order to reliably obtain the effect, the V content is
preferably set to 0.001% or more. The V content is more preferably 0.050% or more.
On the other hand, when the V content exceeds 0.100%, carbonitrides are excessively
generated, and the toughness of the hot-stamping formed body decreases. Therefore,
the V content is preferably set to 0.100% or less. The V content is more preferably
0.090% or les s.
[0044]
"Ni: 0% to 0.50% "
Ni is an element that dissolves in austenite as a solid solution, has an action of
enhancing the hardenability of the steel, and improves the toughness of the hotstamping
formed body. In order to reliably obtain the above effect, the Ni content is
preferably set to 0.001% or more. The Ni content is more preferably 0.01% or more.
- 17 -
On the other hand, even ifNi is contained in an amount of more than 0.50%, the above
effect is saturated, and an increase in the alloy cost is incurred. Therefore, the Ni
content is preferably set to 0.50% or less. The Ni content is more preferably 0.40%
or less.
[0045]
"REM: 0% to 0.0100%"
REM is an element that has an action of deoxidizing molten steel and
achieving soundness of the steel, and is also an element that improves the toughness of
the hot-stamping formed body. Therefore, REM may be contained as necessary. In
order to reliably obtain the above effect, the REM content is preferably set to 0.0010%
or more. The REM content is more preferably 0.0020% or more. On the other hand,
even if REM is contained in an amount of more than 0.0100%, the above effect is
saturated, and an increase in the cost is incurred. Therefore, the REM content is
preferably set to 0.0100% or less. The REM content is more preferably 0.0080% or
less.
In the present embodiment, REM refers to a total of 17 elements including Sc,
Y, and lanthanoids. In the present embodiment, the REM content refers to the total
amount of these elements. Lanthanoids are added in the form of mischmetal in
industry.
[0046]
"Mg: 0% to 0.0100%"
Mg is an element having an action of deoxidizing molten steel and achieving
soundness of the steel, and improves the toughness of the hot-stamping formed body.
Therefore, Mg may be contained as necessary. In order to reliably obtain the above
effect, the Mg content is preferably set to 0.0010% or more. The Mg content is more
- 18 -
preferably 0.0020% or more. On the other hand, even ifMg is contained in an
amount of more than 0.0100%, the above effect is saturated, and an increase in the cost
is incurred. Therefore, the Mg content is preferably set to 0.0100% or less. The Mg
content is more preferably 0.0080% or less.
[0047]
"Ca: 0% to 0.0100%"
Ca is an element having an action of deoxidizing molten steel and achieving
soundness of the steel, and improves the toughness of the hot-stamping formed body.
Therefore, Ca may be contained as necessary. In order to reliably obtain the above
effect, theCa content is preferably set to 0.0010% or more. TheCa content is more
preferably 0.0020% or more. On the other hand, even if Ca is contained in an amount
of more than 0.0100%, the above effect is saturated, and an increase in the cost is
incurred. Therefore, theCa content is preferably set to 0.0100% or less. TheCa
content is more preferably 0.0080% or less.
[0048]
"Co: 0% to 4.00%"
Co is an element having an action of raising a martensite start temperature
(Ms point) and improves the toughness of the hot-stamping formed body. Therefore,
Co may be contained as necessary. In a case where Co is contained, the Co content is
preferably set to 0.10% or more in order to reliably exhibit the above effect. The Co
content is more preferably 0.20% or more. On the other hand, when the Co content
exceeds 4.00%, the hardenability of the steel decreases, and it becomes difficult to
obtain a tensile strength of 2,000 MPa or more. Therefore, the Co content is
preferably set to 4 .00% or less. The Co content is more preferably 3.00% or less.
[0049]
- 19 -
The chemical composition of the hot-stamping formed body described above
may be measured by a general analytical method. For example, the chemical
composition may be measured using inductively coupled plasma-atomic emission
spectrometry (ICP-AES). In addition, sol. Al may be measured by ICP-AES using a
filtrate obtained by heating and decomposing a sample with an acid. C and S may be
measured using a combustion-infrared absorption method, and N may be measured
using an inert gas fusion-thermal conductivity method.
[0050]

Next, the microstructure of the hot-stamping formed body according to the
present embodiment will be described. In the present embodiment, the microstructure
of the hot-stamping formed body means a microstructure in a region from a t/8
thickness depth from the surface to a 3t/8 thickness depth from the surface centered on
a t/4 thickness position (tis the sheet thickness) from the surface.
In the hot-stamping formed body according to the present embodiment, the
average grain size of the prior austenite grains in the microstructure is 5.0 )lm or less,
and the average Mn concentration at the grain boundaries of the prior austenite grains
is 1.0 mass% or less. Hereinafter, each regulation will be described.
[0051]
"Average Grain Size of Prior Austenite Grains Is 5.0 )lm or Less, and Average
Mn Concentration at Grain Boundaries of Prior Austenite Grains Is 1.0 mass% or
Less."
In order to obtain excellent toughness in a hot-stamping formed body, it is
preferable that the microstructure is finer. The present inventors found that in a high
strength hot-stamping formed body having a tensile strength of more than 2,000 MPa,
- 20 -
the toughness deteriorates when the average grain size of the prior austenite grains
exceeds 5.0 11m. Therefore, the average grain size of the prior austenite grains is set
to 5.0 11m or less. The average grain size of the prior austenite grains is more
preferably 4.5 11m or less, 4.0 11m or less, and 3.5 11m or less.
The average grain size of the prior austenite grains may be set to 1.0 11m or
more or 2.0 11m or more.
[0052]
In addition, the present inventors also found that in order to obtain excellent
toughness in a hot-stamping formed body, it is important to reduce the Mn
concentration at the grain boundaries of the prior austenite grains (prior austenite grain
boundaries). When a large amount of Mn is unevenly distributed at the prior
austenite grain boundaries, the ductile fracture limit is significantly deteriorated, and
Mn becomes a fracture origin at the time of a collision. As a result, the toughness of
the hot-stamping formed body deteriorates. When the average Mn concentration at
the prior austenite grain boundaries exceeds 1.0 mass%, the sensitivity to fracture is
increased and the toughness of the hot-stamping formed body significantly deteriorates.
Therefore, the average Mn concentration at the prior austenite grain boundaries is set
to 1.0 mass% or less. The average Mn concentration at the prior austenite grain
boundaries is preferably 0.8 mass% or less, 0.6 mass% or less, and 0.5 mass% or less.
The average Mn concentration at the prior austenite grain boundaries may be
set to 0.1 mass% or more, or 0.2 mass% or more.
[0053]
(Method of Measuring Average Grain Size of Prior Austenite Grains)
The average grain size of the prior austenite grains is measured by the
following method.
- 21 -
First, the hot-stamping formed body is subjected to a heat treatment at 540°C
for 24 hours. This promotes corrosion of the prior austenite grain boundaries. As
the heat treatment, furnace heating or energization heating may be performed, the
temperature rising rate is set to 0.1 to 100 oc/s, and the cooling rate is set to 0.1 to
150 °C/s. A sheet thickness cross section perpendicular to the sheet surface is cut out
from a center portion (a portion avoiding end portions) of the hot-stamping formed
body after the heat treatment. This sheet thickness cross section is polished using
#600 to #1500 silicon carbide paper and thereafter mirror-finished using a liquid
obtained by dispersing a diamond powder having a particle size of 1 to 6 11m in a
diluted solution such as alcohol or pure water. This sheet thickness cross section is
used as an observed section.
[0054]
Next, the observed section is immersed in a 3% to 4% sulfuric acid-alcohol
(or water) solution (% is volume%) for 1 minute to reveal the prior austenite grain
boundaries. The immersion work is performed in an exhaust treatment apparatus, and
the temperature of the work atmosphere is room temperature (1 ooc to 30°C, the same
applies hereinafter). The observed section that reveals the prior austenite grain
boundaries is washed with acetone or ethyl alcohol and dried. Thereafter, the
observed section is observed with a scanning electron microscope. The scanning
electron microscope used is equipped with a secondary electron detector.
[0055]
In a vacuum of 9.6 x w-s Pa or less, a sample is irradiated with an electron
beam at an acceleration voltage of 15 kV and an irradiation current level of 13, and a
secondary electron image of a region from a t/8 thickness depth from the surface to a
3t/8 thickness depth from the surface of the hot-stamping formed body is photographed.
- 22 -
The photographing magnification is set to 4,000-fold based on a screen of 386 mm in
width x 290 mm in length, and the number of photographed visual fields is set to 10 or
more visual fields.
[0056]
In the photographed secondary electron image, the prior austenite grain
boundaries are imaged as a bright contrast. The shortest diameter and the longest
diameter of each of the prior austenite grains included in the photographed visual field
are measured, and the average value thereof is calculated, thereby obtaining the grain
size of the observed prior austenite grains. In a case where the entirety of a prior
austenite grain is not included in the photographed visual field, such as in a case of an
end portion of the photographed visual field, the grain size of the prior austenite grain
is not measured. The grain sizes of all the prior austenite grains in all the
photographed visual fields are calculated, and the average value thereof is calculated,
thereby obtaining the average grain size of the prior austenite grains. The average
grain size of the prior austenite grains is a value obtained by dividing the sum of the
calculated grain sizes of the prior austenite grains by the total number of prior austenite
grains whose grain sizes have been measured.
[0057]
(Method of Measuring Average Mn Concentration at Grain Boundaries of
Prior Austenite Grains)
A method of measuring the average Mn concentration at the grain boundaries
of the prior austenite grains will be described.
A test piece having the dimensions shown in FIG. 1 is produced from the
center portion (a portion avoiding the end portion) of the hot-stamping formed body.
The front and rear surfaces of the test piece are removed by mechanical grinding in
- 23 -
equal amounts so that the sheet thickness (the test piece length in a direction
perpendicular to FIG 1) becomes 1.2 mm. A notch is provided in the center portion
of the test piece in the length direction (left -right direction in FIG 1 ). This notch is
formed by inserting a wire cutter having a thickness of 1 mm. In the width direction
of the test piece (up-down direction in FIG 1 ), the distance between the bottom of the
notch and a side surface where the notch is not provided is controlled to 100 to 200 Jlm.
[0058]
Next, the test piece is immersed in a 20%-ammonium thiocyanate solution(%
is volume%) for 24 to 48 hours. The front and rear surfaces of the test piece are
galvanized within 0.5 hours after the immersion is completed. After the galvanizing,
the test piece is subjected to Auger electron emission spectroscopy within 1.5 hours.
The kind of apparatus for performing the Auger electron emission spectroscopy is not
particularly limited. The test piece is set in an analyzer, and in a vacuum of 9.6 x w-s
Pa or less, and the test piece is fractured from the notch portion to expose the prior
austenite grain boundaries. The exposed prior austenite grain boundaries are
irradiated with an electron beam at an acceleration voltage of 1 to 30 kV, and the Mn
concentration (mass%) at the prior austenite grain boundaries is measured. The
measurement is performed for three or more prior austenite grains at 10 or more
positions at each prior austenite grain boundary. The measurement is completed
within 30 minutes after the fracture to prevent contamination of the prior austenite
grain boundaries. By calculating the average value of the obtained Mn concentrations
(mass%), the average Mn concentration at the prior austenite grain boundaries is
obtained.
[0059]
The microstructure of the hot-stamping formed body is not particularly
- 24 -
limited, but may include martensite (including fresh martensite and tempered
martensite), upper bainite, lower bainite, residual austenite, and iron carbides and/or
alloy carbides.
Preferably, the microstructure has martensite (including fresh martensite and
tempered martensite) as the primary phase (90% or more in area ratio) and the
remainder in the microstructure (upper bainite, lower bainite, residual austenite, and
iron carbides and/or alloy carbides) in an area ratio of 10% or less. The area ratio of
martensite is more preferably 95% or more, and even more preferably 100%. The
area ratio of the remainder in the microstructure is more preferably 5% or less, and
even more preferably 0%, in relation to the area ratio of martensite.
[0060]
(Method of Measuring Area Ratio of Martensite)
The area ratio of martensite is measured by the following method.
A sample is taken from a position 50 mm or more away from the end surface
of the hot-stamping formed body (or a position avoiding the end portion) so that the
sheet thickness cross section can be observed. After polishing the observed section,
nital etching is performed to clarify the contrast between carbides and grain boundaries.
Next, using a field-emission scanning electron microscope (FE-SEM) equipped with a
secondary electron detector, a secondary electron image of a region centered on a t/4
thickness position of the sample (a region from a 118 thickness depth from the surface
to a 3/8 thickness depth from the surface) is photographed at a photographing
magnification of 5,000-fold.
[0061]
In the photograph obtained by the above method, phases other than martensite
(ferrite, pearlite, upper bainite, lower bainite, residual austenite, and the like) and
- 25 -
martensite (fresh martensite and tempered martensite) are distinguished from each
other. Upper bainite, lower bainite, and tempered martensite can be distinguished by
the presence or absence of iron carbides in the lath-like grains and the stretching
direction of the iron carbides. Fresh martensite is not sufficiently etched by nital
etching and is therefore distinguishable from other etched structures. However, since
residual austenite is not sufficiently etched like martensite, the area ratio of fresh
martensite is obtained by obtaining the difference from the area ratio of residual
austenite obtained by a method described later.
[0062]
Upper bainite is a phase formed of aggregates oflath-like grains, and is
accompanied by precipitation of carbides between laths.
Lower bainite and tempered martensite are also phases formed of aggregates
oflath-like grains, but are phases containing carbides inside the laths. Lower bainite
and tempered martensite are distinguished from each other by the stretching direction
of carbides. The carbides of lower bainite have a single variant, have an angular
difference of sa or less between carbides present in a single grain, and thus have
substantially a single direction. On the other hand, the carbides of tempered
martensite have a plurality of variants, and the carbides present in a single grain are
stretched in a plurality of directions. By the difference, lower bainite and tempered
martensite are distinguished from each other.
[0063]
The area ratio of residual austenite is measured in the same region as the
observed region from which the photograph is obtained. The observed section is
polished using #600 to # 1500 silicon carbide paper and thereafter mirror-finished using
a liquid obtained by dispersing a diamond powder having a particle size of 1 to 6 )liD in
- 26 -
a diluted solution such as alcohol or pure water. Next, the observed section is
polished at room temperature using colloidal silica containing no alkaline solution for
8 minutes to remove strain introduced into the surface layer of the observed section.
The observed section is measured by an electron backscatter diffraction method at a
measurement interval of 0.1 J.lm to obtain crystal orientation information. For the
measurement, an apparatus including a thermal field-emission scanning electron
microscope (JSM-7001F manufactured by JEOLLtd.) and an EBSD detector (DVC5
type detector manufactured by TSL) is used. At this time, the degree of vacuum in
the apparatus is set to 9.6 x w-s Pa or less, the acceleration voltage is set to 15 kv, the
irradiation current level is set to 13, and the electron beam irradiation level is set to 62.
The area ratio of residual austenite, which is an fcc structure, is calculated from the
obtained crystal orientation information using the "Phase Map" function installed in
the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer,
thereby obtaining the area ratio of residual austenite.
[0064]
By distinguishing the structures from each other by the above-described
method, the area ratio of martensite (fresh martensite and tempered martensite) is
obtained.
The area ratio of the remainder in the microstructure is obtained by
subtracting the area ratio of martensite from 100%.
[0065]
"Number Density of Carbides Having Circle Equivalent Diameter of 0.20 J.lm
or More Is 0.5 IJ..1m2 or Less"
When the microstructure of the hot-stamping formed body contains a large
amount of coarse carbides, there are cases where the toughness of the hot-stamping
- 27 -
formed body deteriorates. Therefore, it is desirable that the amount of coarse carbide
is as small as possible. In the present embodiment, the number density of carbides
having a circle equivalent diameter of 0.20 11m or more is preferably 0.5 111m2 or less.
The number density thereof is more preferably 0.3 111m2 or less, and 0.2 111m2 or less.
Since it is preferable that the number density of carbides having a circle equivalent
diameter of 0.20 11m or more is smaller, the number density thereof may be set to 0
111m2.
[0066]
(Method of Measuring Number Density of Carbides)
A sample is taken so that the sheet thickness cross section of the hot -stamping
formed body becomes an observed section, and the observed section is fini shed by
electrolytic polishing. Thereafter, a region from a t/8 thickness depth from the
surface to a 3t/8 thickness depth from the surface is observed for 10 or more visual
fields at a magnification of 20,000-fold. The circle equivalent diameter of each
carbide is obtained from the observed area of each carbide by image analysis. By
calculating the number density of carbides having a circle equivalent diameter of 0.20
11m or more, the number density of carbides having a circle equivalent diameter of 0.20
11m or more is obtained.
In the present embodiment, particles having a major axis of 5 nm or more
present in the laths or in the form of laths in martensite are regarded as carbides.
[0067]
"Tensile Strength"
The hot-stamping formed body according to the present embodiment may
have a tensile (maximum) strength of 2,000 MPa or more. The tensile strength
thereof is more preferably 2,200 MPa or more. The upper limit thereof is not
- 28 -
particularly limited, but may be 2,600 MPa or less and 2,500 MPa or less.
[0068]
The tensile (maximum) strength is obtained according to the test method
described in JIS Z 2241:2011 by producing a No. 5 test piece described in JIS Z
2241:2011 from a position as flat as possible in the hot-stamping formed body.
[0069]
"Toughness"
The hot-stamping formed body according to the present embodiment may
have a value of 0.60 MPa/Hv or more, which is an index of early fracture properties,
and a hardness variation (~Hv) of 50 Hv or less. The value that is an index of the
early fracture properties is a value (tensile strength I (average hardness x 3.3)) obtained
by dividing the tensile strength (unit: MPa) by a value obtained by multiplying an
average hardness (unit: Hv) obtained by a method described later by 3.3. This value
is preferably 0.75 MPa/Hv or more and 0.80 MPa/Hv or more. The value obtained by
multiplying the average hardness by 3.3 is an estimated tensile strength which is
estimated from the hardness. When an actual measurement value of the tensile
strength is 0.60 MPa/Hv or more times the estimated tensile strength, early fracture
properties are excellent, so that excellent toughness can be determined.
[0070]
When the hardness variation (~Hv) is 50 Hv or less, a stress concentration is
less likely to occur in a case where deformation (stress) occurs from the outside in the
hot- stamping formed body having a tensile strength of 2,000 MPa or more, so that
excellent toughness can be determined. The hardness variation (~Hv) is preferably
40 Hv or less, 30 Hv or less, and 20 Hv or less.
[0071]
- 29 -
The average hardness used to calculate the index of early fracture properties is
measured by the following method.
A test piece is cut out from any position (a position avoiding the end portion)
of the hot-stamping formed body so that a sheet thickness cross section perpendicular
to the surface can be observed. The length of the test piece depends on the measuring
apparatus, but may be about 10 mm. The sheet thickness cross section of the test
piece is polished using #600 to #1500 silicon carbide paper and thereafter mirrorfinished
using a liquid obtained by dispersing a diamond powder having a particle size
of 1 to 6 J.lm in a diluted solution such as alcohol or pure water. This sheet thickness
cross section is used as a measurement surface. Using a Micro Vickers hardness
tester, Vickers hardnesses are measured at intervals of three or more times an
indentation under a load of 1 kgf at a t/4 thickness position (a region from a t/8
thickness depth from the surface to a 3t/8 thickness depth from the surface) of the
measurement surface. By measuring 20 points in total and calculating the average
value thereof, the average value (average hardness) of the Vickers hardnesses is
obtained.
The hardness variation (fl.Hv) is obtained by calculating the difference
between the maximum value and the minimum value of the Vickers hardnesses at the
20 points, which are obtained when the average hardness is obtained by the above
method.
[0072]
The hot- stamping formed body according to the present embodiment can be
obtained by a manufacturing method in which a steel sheet for hot stamping is
subjected to a first heat treatment and a second heat treatment. By performing the
first heat treatment, a large amount of high angle grain boundaries are formed in prior
- 30 -
austenite grains. During the second heat treatment, Mn is diffused from the prior
austenite grain boundaries to the high angle grain boundaries in the prior austenite
grams. As a result, the Mn concentration at the prior austenite grain boundaries can
be reduced in the microstructure of the hot-stamping formed body. That is, it is
preferable that a sufficient amount of high angle grain boundaries is formed in the steel
sheet for hot stamping (steel sheet after the first heat treatment and before the second
heat treatment), which is to be processed into the hot-stamping formed body according
to the present embodiment.
[0073]
In the steel sheet for hot stamping, which is to be processed into the hotstamping
formed body according to the present embodiment, it is preferable that the
proportion of the high angle grain boundaries at a t/4 thickness position (a region from
a t/8 thickness depth from the surface to a 3t/8 thickness depth from the surface) is
40% or more. However, even if the proportion of the high angle grain boundaries of
the steel sheet for hot stamping is less than 40%, the hot-stamping formed body
according to the present embodiment can be manufactured depending on the
manufacturing conditions after the first heat treatment. Therefore, the proportion of
the high angle grain boundaries of the steel sheet for hot stamping is not particularly
limited.
[0074]
(Method of Calculating Proportion of High angle grain boundaries)
A method of calculating the proportion of the high angle grain boundaries of
the steel sheet for hot stamping will be described.
A test piece is cut out from any position on the steel sheet for hot stamping so
that a cross section perpendicular to the surface (sheet thickness cross section) can be
- 31 -
observed. The length of the test piece depends on the measuring apparatus, but may
be about 10 mm. The cross section of the test piece is polished using #600 to #1500
silicon carbide paper and thereafter mirror-finished using a liquid obtained by
dispersing a diamond powder having a particle size of 1 to 6 Jlm in a diluted solution
such as alcohol or pure water. This sheet thickness cross section is used as an
observed section.
[0075]
Next, the observed section is polished at room temperature using colloidal
silica containing no alkaline solution for 8 minutes to remove strain introduced into the
surface layer of the test piece. At any position in the longitudinal direction of the
observed section, the t/4 thickness position of the steel sheet (a region from a t/8
thickness depth from the surface to a 3t/8 thickness depth from the surface) is
measured by an electron backscatter diffraction method at a measurement interval of
0.1 Jlm to obtain crystal orientation information. For the measurement, an apparatus
including a thermal field-emission scanning electron microscope (JSM-7001F
manufactured by JEOL Ltd.) and an EBSD detector (DVC5 type detector
manufactured by TSL) is used. At this time, the degree of vacuum in the apparatus is
set to 9.6 x 10-5 Pa or less, the acceleration voltage is set to 15 kv, the irradiation
current level is set to 13, and the electron beam irradiation time is set to 0.01 sec/point.
[0076]
The proportion of the lengths of grain boundaries in which the rotation angle
between adjacent crystal lattices 15° or more in the sum of the lengths of the grain
boundaries in which the rotation angle is 15° or more and the lengths of grain
boundaries in which rotation angle is less than 15° is calculated from the obtained
crystal orientation information using the "Image Quality" function installed in the
- 32 -
software "OIM Analysis (registered trademark)" attached to the EBSD analyzer.
With this function, regarding the grain boundaries of grains having a body-centered
cubic structure, the length of the sum of grain boundaries having any rotation angle can
be calculated. Regarding all the grains included in the measurement region, the
length of the sum of such grain boundaries is calculated, and the proportion of the
lengths of the grain boundaries in which the rotation angle is 15° or more is obtained.
This proportion is defined as the proportion of the high angle grain boundaries.
[0077]

Next, a preferred manufacturing method of the hot-stamping formed body
according to the present embodiment will be described. First, a method of
manufacturing the steel sheet for hot stamping applied to the hot-stamping formed
body according to the present embodiment will be described.
[0078]
(Method of Manufacturing Steel Sheet for Hot Stamping)
"Heating Step"
A steel piece (steel) to be subjected to hot rolling may be a steel piece
manufactured by an ordinary method, and may be, for example, a steel piece
manufactured by a general method such as a continuously cast slab or a thin slab caster.
It is preferable that the steel having the above-described chemical composition is
subjected to hot rolling to be heated in a temperature range of 1,1 00°C or higher in a
hot rolling step, and is held in this temperature range for 20 minutes or longer. In a
case where the heating temperature is lower than 1,1 oooc or the retention time is
shorter than 20 minutes, re-dissolving of coarse inclusions such as Ti does not proceed
and the coarse inclusions remain as fracture origins, so that there are cases where the
- 33 -
toughness of the hot-stamping formed body deteriorates. More preferably, the
heating temperature is 1 ,200°C or higher, and the retention time is 25 minutes or longer.
The heating temperature is preferably 1,400°C or lower, and the retention time is
preferably 120 minutes or shorter.
[0079]
"Finish Rolling Step"
Next, it is preferable to perform hot rolling so that the completion temperature
of finish rolling (finish rolling temperature) is in a temperature range of an An point or
higher. When the finish rolling is completed at a temperature lower than the An point,
there are cases where dual phase rolling is performed and the shape of the sheet during
the rolling deteriorates. Therefore, the finish rolling temperature is preferably set to
the An point or higher. More preferably, the finish rolling temperature is the An
point+ 1 ooc or higher. The finish rolling temperature is preferably set to the Ar3
point+ l00°C or lower.
[0080]
The Ar3 point is represented by Expression ( 1 ). Each element symbol in
Expression ( 1) indicates the amount (mass%) of the corresponding element. In a case
where the corresponding elements are not contained, 0 is substituted.
Ar3 point= 850 + 10 x (C + N) x Mn + 350 x Nb + 250 x Ti + 40 x B + 10 x
Cr + 100 x Mo ... Expression (1)
[0081]
"Coiling Step"
The steel sheet after the finish rolling is coiled into a coil shape in a
temperature range of 750°C or lower. When the coiling temperature exceeds 750°C,
a large amount of scale is generated, which makes it difficult to remove the scale in a
- 34 -
pickling step which is a subsequent step. Therefore, the coiling temperature is
preferably set to 750°C or lower. The coiling temperature is more preferably 600°C
or lower. In addition, the coiling temperature is preferably set to 400°C or higher.
A hot-rolled steel sheet is obtained by the above method.
[0082]
The hot-rolled steel sheet obtained by the above method may be subjected to a
re-heating treatment for the purpose of softening, as necessary. A cold-rolled steel
sheet may be obtained by cold-rolling the hot-rolled steel sheet, or a plated steel sheet
may be obtained by applying plating. In addition, continuous annealing may also be
performed.
[0083]
The cold rolling may be cold rolling performed at a normal cumulative rolling
reduction of, for example, 30% to 90%. The hot-rolled steel sheet may be subjected
to a hot stamping step without being subjected to the cold rolling.
[0084]
The hot-rolled steel sheet or the cold-rolled steel sheet may have a plating
layer on the surface. Various known hot-dip metal plating, electro plating, and the
like may be performed depending on the purpose such as suppressing the generation of
scale in the hot stamping step and improving the corrosion resistance of the hotstamping
formed body.
[0085]
Examples of the hot-dip metal plating include hot-dip galvanizing, hot-dip
galvannealing, hot-dip aluminum plating, and hot-dip aluminum-zinc plating. When
a hot-dip metal plating layer is full hard, there are cases where a crack occurs during
hot-stamping forming and the corrosion r esistance of the hot-stamping formed body
- 35 -
deteriorates. Therefore, the hot-dip metal plating is preferably hot-dip galvanizing or
hot-dip galvannealing in which the plating layer becomes soft.
[0086]
In a case where the hot-dip metal plating is hot-dip galvanizing or hot-dip
gal vannealing, the amount of plating adhered to the surface of the hot -rolled steel sheet
or cold-rolled steel sheet is preferably 3 to 800 g/m2 per surface. When the plating
adhesion amount is less than 3 g/m2 per surface, there are cases where the effect of
improving corrosion resistance cannot be reliably obtained. On the other hand, when
the plating adhesion amount exceeds 800 g/m2 per surface, there are cases where
defects such as blowholes easily occur during welding. From the viewpoint of
improving corrosion resistance and suppressing an increase in cost, it is more
preferable that the plating adhesion amount is 10 to 200 g/m2
.
[0087]
In order to suppress evaporation of the plating layer before hot-stamping
forming and improve the corrosion resistance of the hot-stamping formed body, it is
preferable that the plating is hot-dip galvannealing. As for the degree of alloying of
the hot-dip galvannealing, it is preferable that the Fe content in the plating layer is 3%
to 25%. When the Fe content in the plating layer is less than 3%, there are cases
where evaporation of the plating layer during hot-stamping forming cannot be
sufficiently suppressed. When the Fe content in the plating layer exceeds 25%, there
are cases where the powdering property of the hot-stamping formed body deteriorates.
[0088]
From the viewpoint of suppressing evaporation of the plating layer and
securing the powdering property, the Fe content in the plating layer is more preferably
7% to 18%. The surface of the hot-dip galvanized layer or the hot-dip galvannealed
- 36 -
layer may be further subjected to an organic or inorganic coating.
[0089]
(Method of Manufacturing Hot-Stamping Formed Body)
Using the steel sheet for hot stamping obtained by the above method, for
example, the hot-stamping formed body according to the present embodiment is
manufactured by the following manufacturing method. As described above, in the
present embodiment, two heat treatments are performed in order to obtain a desired
microstructure in the hot-stamping formed body.
[0090]
(First Heat Treatment) Heating Temperature Tl: Ac3 Point to Ac3 + 200ac
Regarding the hot-stamping formed body according to the present
embodiment, the steel sheet for hot stamping is subjected to the first heat treatment
before being subjected to the hot stamping step. In the first heat treatment, heating to
a heating temperature Tl of an Ac3 point to the Ac3 point + 200ac and holding at this
temperature Tl are performed. In the heating of this first heat treatment, Mn is
concentrated at the prior austenite grain boundaries. In a case where the heating
temperature Tl is lower than the Ac3 point, the concentration of Mn in the prior
austenite grain boundaries does not proceed sufficiently, and the Mn concentration
cannot be sufficiently reduced in the subsequent second heat treatment. Therefore,
the heating temperature Tl is set to the Ac3 point or higher. The heating temperature
Tl is preferably the Ac3 point+ 20°C or higher. On the other hand, in a case where
the heating temperature Tl exceeds the Ac3 point+ 200°C, there are cases where the
prior austenite grains become coarse and the average grain size of the prior austenite
grains cannot be set to 5.0 J.lm or less. Therefore, the heating temperature Tl is set to
Ac3 + 200ac or lower. The average heating rate up to the heating temperature Tl
- 37 -
may be 1 to 30 °C/s.
The Ac3 point can be obtained from Expression (2).
[0091]
Ac3 point (°C) = 912- 230.5 x C + 31.6 x Si- 20.4 x Mn- 14.8 x Cr + 16.8 x
Mo ... Expression (2)
Each element symbol in Expression (2) indicates the amount (mass%) of the
corresponding element. In a case where the corresponding elements are not contained,
0 is substituted.
[0092]
The steel sheet for hot stamping heated to the heating temperature T 1 is held
at the heating temperature Tl. The retention time is not limited, but is preferably set
to 60 seconds to 20 minutes. In a case where the retention time is shorter than 60
seconds, the re-dissolving of carbides does not proceed, coarse carbides remain
undissolved, and the number density of the carbides becomes too high, so that there are
cases where a desired microstructure cannot be obtained. In a case where the
retention time is longer than 20 minutes, the prior austenite grains may be excessively
coarsened, the proportion of high angle grain boundaries may be reduced, so that there
are cases where a desired microstructure cannot be obtained.
[0093]
(First Heat Treatment) Average Cooling Rate to Cooling Stop Temperature:
10 °C/s to 500 °C/s
Cooling is performed so that the average cooling rate from the heating
temperature Tl to a cooling stop temperature, which will be described later, is 10 oc/s
to 500 °C/s. By this cooling, the microstructure has martensite as the primary phase,
so that a large amount of high angle grain boundaries are introduced into the prior
- 38 -
austenite grains. Fine austenite is present at a block interface, which is the high angle
grain boundary, and this has a strong effect on the refinement of austenite during the
second heat treatment and a reduction in the Mn concentration at the prior austenite
grain boundaries. That is, since this high angle grain boundary serves as a diffusion
path for Mn of the prior austenite grain boundaries in the second heat treatment, the
high angle grain boundary plays an important role in reducing the Mn concentration at
the prior austenite grain boundaries.
[0094]
In a case where the average cooling rate from the heating temperature T1 to
the cooling stop temperature described later is slower than 10 ac/s, a soft phase such as
ferrite may be formed, and the introduction of high angle grain boundaries becomes
insufficient. As a result, the reduction in the Mn concentration at the prior austenite
grain boundaries in the second heat treatment becomes insufficient, and there are cases
where the average Mn concentration at the prior austenite grain boundaries cannot be
reduced to 1.0 mass% or less. Therefore, the average cooling rate is set to 10 °C/s or
faster. The average cooling rate is preferably 20 ac/s or faster. On the other hand,
in a case where the cooling rate exceeds 500 °C/s, an internal stress associated with
martensitic transformation increases, and there are cases where a crack occurs in a
cooling process to room temperature. Therefore, the average cooling rate is set to
500 ac/s or slower. The average cooling rate is preferably 300 ac/s or slower.
[0095]
(First Heat Treatment) Cooling Stop Temperature: 250°C to 400°C
In the cooling of the first heat treatment, it is necessary not only to simply
form martensite but also to allow austenite to remain at the block interface of
martensite. This is because, as described above, this remaining austenite serves as a
- 39 -
diffusion path for Mn in the second heat treatment. In order to achieve stabilization
of austenite, it is necessary to promote the diffusion of C from martensite into
untransformed austenite. Therefore, cooling is stopped in a temperature range of
250°C to 400°C. In a case where the cooling stop temperature is lower than 250°C,
the diffusion of C from martensite into untransformed austenite does not proceed.
Therefore, the cooling stop temperature is set to 25ooc or higher. The cooling stop
temperature is preferably 260°C or higher. In a case where the cooling stop
temperature exceeds 400°C, carbides are generated and the stabilization of residual
austenite between blocks does not proceed. Therefore, the cooling stop temperature
is set to 400°C or lower.
[0096]
(First Heat Treatment) Average Cooling Rate at Cooling Stop Temperature or
Lower: Slower Than 10 °C/s
In order to allow austenite which serves as a diffusion path for Mn in the
second heat treatment to remain, it is necessary to control the cooling rate to the
cooling stop temperature or lower to promote the diffusion of carbon from martensite
into untransformed austenite so that austenite is stabilized. In order to exhibit this
action, the average cooling rate to the cooling stop temperature or lower is controlled
to slower than 10 °C/s. The average cooling rate is preferably 8 °C/s or slower. In a
case where the cooling rate to the cooling stop temperature or lower is 10 oc/s or faster,
the diffusion of carbon from martensite into untransformed austenite does not proceed,
the stability of austenite decreases, so that residual austenite cannot remain.
Therefore, there are cases where austenite grains become coarse in the heating process
during the second heat treatment and the Mn concentration at the prior austenite grain
boundaries cannot be reduced.
- 40 -
[0097]
(Second Heat Treatment) Average Heating Rate: 10 °C/s to 1,000 °C/s
For the steel sheet for hot stamping subjected to the first heat treatment, in
order to refine the prior austenite grains and reduce the Mn concentration at the prior
austenite grain boundaries, the average heating rate of the heating (second heat
treatment) during the hot stamping is controlled. By setting the average heating rate
of the second heat treatment to 10 °C/s or faster, the grain growth of the prior austenite
grains can be suppressed. In addition, the diffusion of Mn from the prior austenite
grain boundaries to the high angle grain boundaries with the high angle grain
boundaries introduced in the first heat treatment as the diffusion path can proceed. As
a result, the prior austenite grains can be refined and the Mn concentration at the prior
austenite grain boundaries can be reduced. Accordingly, the toughness of the hotstamping
formed body can be improved. Therefore, the average heating rate is set to
10 oC/s or faster. The average heating rate is preferably 30 oc /s or faster. On the
other hand, when the average heating rate exceeds 1,000 °C/s, it becomes difficult to
control the heating temperature of the hot-stamping formed body, and there are cases
where the average grain size of the prior austenite grains cannot be 5.0 11m or less
depending on the portion. As a result, there are cases where the toughness of the hotstamping
formed body deteriorates. Therefore, the average heating rate is set to
1,000 oC/s or slower. The average heating rate is preferably 700 oc /s or slower.
[0098]
(Second Heat Treatment) Heating Temperature T2: Ac3' Point to Ac3' Point +
Mn is concentrated in residual austenite formed by the first heat treatment.
Since Mn is an austenite stabilizing element, the Ac3 point is lower than that of the first
- 41 -
heat treatment. This lowered Ac3 point is referred to as an "Ac3' point", and a heating
temperature during the second heat treatment is referred to as T2.
[0099]
By setting the heating temperature T2 during the second heat treatment to the
Ac3' point to the Ac3' point+ 100°C, Mn concentrated in the prior austenite grain
boundaries in the first heat treatment with the high angle grain boundaries in the prior
austenite grains as the diffusion path is diffused. Accordingly, the Mn concentration
at the prior austenite grain boundaries is reduced. In a case where the heating
temperature T2 is lower than the Ac3' point, Mn is not sufficiently diffused from the
prior austenite grain boundaries, and there are cases where the Mn concentration at the
prior austenite grain boundaries exceeds 1.0 mass%. As a r esult, there are cases
where the toughness of the hot-stamping formed body deteriorates. Therefore, the
heating temperature T2 is set to Ac3' point or higher. The heating temperature T2 is
preferably Ac3' + 20oc or higher. On the other hand, in a case where the heating
temperature T2 exceeds the Ac3' point + 100°C, the grain growth of the prior austenite
grains proceeds, and there are cases where the average grain size of the prior austenite
grains exceeds 5.0 11m. As a result, there are cases where the toughness of the hotstamping
formed body deteriorates. Therefore, the heating temperature T2 is set to
the Ac3' point+ 100°C or lower. The heating temperature T2 is preferably the Ac3'
point + soac or lower.
[0100]
Regarding the Ac3' point, the steel sheet for hot stamping after the first heat
treatment is subjected to a thermal expansion measurement, a temperature at which the
microstructure is completely austenitized is obtained from a change in the amount of
thermal expansion during heating, and this temperature is determined as the AC3' point.
- 42 -
An apparatus used for the thermal expansion measurement may be any apparatus that
can continuously measure the amount of thermal expansion during heating, and for
example, a thin sheet Formaster tester manufactured by Fuji Electronic Industrial Co.,
Ltd. may be used.
[0101]
The retention time at the heating temperature T2 is set to longer than 10
seconds and 60 seconds or shorter. When the retention time is 10 seconds or shorter,
the diffusion of Mn from the prior austenite grain boundaries into the high angle grain
boundaries does not proceed sufficiently, so that there are cases where the amount of
Mn of the prior austenite grain boundaries cannot be reduced. When the retention
time exceeds 60 seconds, the growth of the prior austenite grains proceeds, and there
are cases where the toughness deteriorates. A preferable retention time considering
the balance between the refinement of the prior austenite grains and the diffusion of
Mn from the austenite grain boundaries into the high angle grain boundaries is 20
seconds or longer and 30 seconds or shorter.
[0102]
Furthermore, the difference (T2 - cooling stop temperature) between the
cooling stop temperature during the first heat treatment and the heating temperature T2
during the second heat treatment is set to lower than 600°C. When the T2 - cooling
stop temperature is 6oooc or higher, the grain growth of austenite proceeds in the
heating stage during the second heat treatment, and there are cases where the average
grain size of the prior austenite grains exceeds 5.0 11m and/or the average Mn
concentration at the prior austenite grain boundaries increases. More preferably, the
difference (T2 - cooling stop temperature) between the cooling stop temperature during
the first heat treatment and the heating temperature T2 during the second heat
- 43 -
treatment is 57ooc or lower.
[0103]
FIG. 2 is a diagram showing the relationship between T2- cooling stop
temperature and the average Mn concentration at the grain boundaries of the prior
austenite grains in examples. FIG. 3 is a diagram showing the relationship between
T2 - cooling stop temperature and the average grain size of the prior austenite grains in
the examples.
As shown in FIG. 2, it can be seen that by setting T2 - cooling stop
temperature to lower than 600°C, the average Mn concentration at the grain boundaries
of the prior austenite grains becomes 1.0 mass% or less. In addition, as shown in FIG.
3, it can be seen that by setting T2- cooling stop temperature to lower than 600°C, the
average grain size of the prior austenite grains becomes 5.0 Jlm or less.
Invention examples and comparative examples of FIGS. 2 and 3 are an
extraction of some of all the invention examples and all the comparative examples in
the examples.
[0104]
FIG. 4 is a diagram showing the relationship between the retention time at the
heating temperature T2 and the average Mn concentration at the grain boundaries of
the prior austenite grains in the examples. FIG. 5 is a diagram showing the
relationship between the retention time at the heating temperature T2 and the average
grain size of the prior austenite grains in the examples.
As shown in FIG. 4, it can be seen that by setting the retention time at the
heating temperature T2 to longer than 10 seconds and 60 seconds or shorter, the
average Mn concentration at the grain boundaries of the prior austenite grains becomes
1.0 mass% or less. In addition, as shown in FIG. 5, it can be seen that by setting the
- 44 -
retention time at the heating temperature T2 to longer than 10 seconds and 60 seconds
or shorter, the average grain size of the prior austenite grains becomes 5.0 11m or less.
Invention examples and comparative examples of FIGS. 4 and 5 are an
extraction of some of all the invention examples and all the comparative examples in
the examples.
[0105]
The steel sheet for hot stamping heated to and held at the heating temperature
T2 is formed into a hot-stamping formed body by hot stamping, and is cooled at the
following cooling rate.
[0106]
(Second Heat Treatment) Average Cooling Rate in Temperature Range to
200°C after Hot-Stamping Forming: 10 oc/s to 500 oc/s
By controlling the average cooling rate in a temperature range to 200°C after
hot-stamping forming to 10 °C/s to 500 °C/s, the microstructure of the hot-stamping
formed body contains martensite (including fresh martensite and tempered martensite)
as the primary phase. In a case where the average cooling rate is slower than 10 oc/ s,
hardening is not sufficiently achieved, a soft phase such as ferrite is formed in the
microstructure, and the toughness of the hot-stamping formed body deteriorates.
Therefore, the average cooling rate is set to 10 °C/s or faster. The average cooling
rate is preferably 30 oc/s or faster. On the other hand, in a case where the average
cooling rate exceeds 500 °C/s, the self-tempering of martensite does not proceed
sufficiently, the internal stress in the microstructure increases, and there are cases
where the toughness of the hot-stamping formed body deteriorates. Therefore, the
average cooling rate is set to 500 °C/s or slower. The average cooling rate is
preferably 300 °C/s or slower.
- 45 -
[0107]
After the hot-stamping forming, for the purpose of adjusting the strength,
tempering may be performed by heating to a temperature range of 1 oooc to 6oooc and
holding in the temperature range. In addition, for the purpose of improving the
deformability of the hot-stamping formed body, a softened region may be formed in a
portion of the hot-stamping formed body after hot stamping and cooling. The
softened region mentioned here means a region formed by irradiating only a portion
(for example, a flange portion) of the hot-stamping formed body with a laser and
tempering the portion.
[Examples]
[0108]
Next, the examples of the present invention will be described. However, the
conditions in the examples are one example of conditions adopted to confirm the
feasibility and effects of the present invention, and the present invention is not limited
to this one example of conditions. The present invention can adopt various conditions
as long as the object of the present invention is achieved without departing from the
gist of the present invention.
[0109]
Steels having the chemical compositions shown in Tables 1 to 3 were melted
and continuously cast to obtain steel pieces. The steel piece was heated to 1, 150°C,
held in the temperature range for 30 minutes, and then hot-rolled so that the fini sh
rolling temperature was 940°C, thereby obtaining a hot-rolled steel strip. The
obtained hot-rolled steel strip was coiled into a coil shape at 580°C. The hot-rolled
steel strip was cold-rolled under the condition that the cumulative rolling reduction was
50%, thereby obtaining a steel sheet for hot stamping (cold-rolled steel sheet) having a
- 46 -
thickness of 1.4 mm.
[0110]
Some of the steel sheets for hot stamping were hot-dip galvanized to obtain
plated steel sheets for hot stamping. The amount of plating adhered was set to 10 to
200 g/m2 per surface. For the steel sheets for hot stamping that had been hot-dip
galvanized, "Present" is described in the "Plating" column in Tables 4 to 8.
[0111]
Each of the steel sheets for hot stamping and the plated steel sheets for hot
stamping (hereinafter collectively referred to as "steel sheets for hot stamping") were
subjected to the first heat treatment (pre-heat treatment) and the second heat treatment
shown in Tables 4 to 8 and subjected to hot stamping to obtain hot-stamping formed
bodies. In Tables 4 to 8, "Cooling 1" indicates cooling from the heating temperature
T1 to the "cooling stop temperature of 250°C to 400°C", "Cooling 2" indicates cooling
in a temperature range to the cooling stop temperature or lower, and "Cooling 3"
indicates the average cooling rate in a temperature range to 200°C after hot-stamping
forming.
[0112]
In addition, some of the hot-stamping formed bodies were tempered by
heating to a temperature range of 100°C to 600°C and holding for the purpose of
adjusting the strength. For the hot-stamping formed bodies that had been tempered,
"Present" is described in the "Annealing" column in Tables 4 to 8.
Furthermore, for some of the hot-stamping formed bodies, a portion of the
hot- stamping formed body was irradiated with a laser to be heated to 200°C, thereby
forming a partially softened region. Regarding the hot-stamping formed bodies in
which the partially softened region was formed, "Present" is described in the "Partially
- 47 -
softened region" column in Tables 9 to 13.
[0113]
The microstructure of the steel sheets for hot stamping and the hot -stamping
formed bodies was measured by the above-mentioned measurement methods. In
addition, the mechanical properties of the hot-stamping formed body were measured.
The results are shown in Tables 9 to 13. The mechanical properties of the hotstamping
formed body were measured and evaluated by the following methods.
In Test No. 66 in Tables 6 and 11, the cooling rate during the first heat
treatment was too fast and a crack had occurred, so that the microstructure and the like
of the hot-stamping formed body were not observed.
[0114]
"Tensile Strength"
The tensile strength of the hot-stamping formed body was obtained in
accordance with the test method described in JIS Z 2241:2011 by producing a No.5
test piece described in JIS Z 2241:2011 from a position as flat as possible in the hotstamping
formed body. In a case where the tensile strength was 2,000 MPa or more,
having excellent strength and being acceptable was determined. On the other hand, in
a case where the tensile strength was less than 2, 000 MPa, not having excellent
strength and being unacceptable was determined.
[0115]
"Hardness"
A test piece was cut out from any position (a position avoiding the end
portion) of the hot-stamping formed body so that a cross section (sheet thickness cross
section) perpendicular to the surface could be observed. The length of the test piece
was set to about 10 mm. The sheet thickness cross section of the test piece was
- 48 -
polished using #600 to #1500 silicon carbide paper and thereafter mirror-finished using
a liquid obtained by dispersing a diamond powder having a particle size of 1 to 6 J.lm in
a diluted solution such as alcohol or pure water. This sheet thickness cross section
was used as a measurement surface. Using a Micro Vickers hardness tester, Vickers
hardnesses were measured at intervals of three or more times an indentation under a
load of 1 kgf at a t/4 thickness position (a region from a t/8 thickness depth from the
surface to a 3t/8 thickness depth from the surface) of the measurement surface. By
measuring 20 points in total and calculating the average value thereof, the average
value (average hardness) of the Vickers hardnesses was obtained. The average
hardness obtained by this method was used for toughness evaluation described below.
In a case where the average hardness is 650 Hv or more, having sufficient
hardness can be determined.
[0116]
"Toughness"
The toughness of the hot-stamping formed body was evaluated by early
fracture properties and hardness variation (~Hv). A value obtained by dividing the
tensile strength (unit: MPa) of the hot-stamping formed body by a value obtained by
multiplying an average hardness (unit: Hv) by 3.3 was determined as a value which is
an index of the early fracture properties. The tensile strength and the average
hardness are values obtained by the above methods.
The value obtained by multiplying the average hardness by 3.3 is a tensile
strength which is estimated from the hardness. When an actual measurement value of
the tensile strength is 0.60 MPa/Hv or more times the estimated tensile strength,
excellent early fracture properties can be determined.
[0117]
- 49 -
"Hardness Variation (~Hv)"
In a hot-stamping formed body having a tensile strength of 2,000 MPa or
more, in a case where deformation (stress) occurs from the outside, a stress
concentration occurs when the hardness variation (~Hv) is large in the hot-stamping
formed body, and there are cases where the toughness deteriorates. The toughness
deteriorates in a case where the hardness variation (~Hv) exceeds 50 Hv.
The hardness variation (~Hv) was defined as the difference between the
maximum value and the minimum value of the Vickers hardnesses at the 20 points,
which were obtained when the average hardness was obtained by the above method.
[0118]
In a case where the value as an index of the early fracture properties was 0.60
MPa/Hv or more and the hardness variation (~Hv) was 50 Hv or less, being excellent
in toughness and being acceptable was determined. In a case where either one was
not satisfied, being inferior in toughness and being unacceptable was determined.
- 50 -
[0119]
[Table 1]
Steel Chemical composition (mass%) of steel sheet for hot stamping, remainder consistin of Fe and impurities Ac,
Note
No. c Si Mn Sol.Al Ti Cr B p s N Nb Mo v Ni REM Mg Ca Co ("C)
1 0.62 0.08 1.36 0.054 0.060 0.15 0.0031 0.013 0.0030 0.0030 742 Invention Steel
2 0.40 0.50 1.80 0.033 0.010 0.01 0.0010 0.010 0.0016 0.0032 0.020 799 Invention Steel
3 0.59 0.22 2.30 0.061 0.040 0.01 0.0021 0.006 0.0016 0.0016 0.055 0.38 742 Invention Steel
4 0.37 0.20 0.40 0.030 0.020 0.20 0.0015 0.010 0.0008 0.0030 822 Comparative Steel
5 0.40 0.14 1.30 0.040 0.020 0.20 0.0019 0.016 0.0007 0.0021 795 Invention Steel
6 0.55 0.22 0.40 0.035 0.020 0.21 0.0020 0.008 0.0030 0.0020 781 Invention Steel
7 0.70 0.50 0.40 0.350 0.020 0.30 0.0030 0.010 0.0003 0.0024 754 Invention Steel
8 0.72 0.26 0.42 0.100 0.024 0.05 0.0014 0.018 0.0010 0.0030 745 Comparative Steel
9 0.45 0.005 0.50 0.030 0.021 0.25 0.0015 0.011 0.0008 0.0020 795 Comparative Steel
10 0.47 0.010 0.60 0.100 0.022 0.36 0.0013 0.016 0.0010 0. 0018 786 Invention Steel
11 0.44 0.70 0.50 0.040 0.030 0.30 0.0018 0.015 0.0020 0.0022 818 Invention Steel
12 0.45 1.21 0.45 0.031 0.021 0.10 0.0020 0.010 0.0007 0.0030 836 Invention Steel
13 0.50 1.40 1.20 0.100 0.024 0.20 0.0012 0.010 0.0009 0.0015 814 Comparative Steel
14 0.40 0.20 0.30 0.040 0.025 0.30 0.0016 0.010 0.0008 0.0020 816 Comparative Steel
15 0.44 0.22 0.40 0.073 0.024 0.40 0.0014 0.020 0.0006 0. 0018 803 Invention Steel
16 0.47 0.24 1.70 0.035 0.025 0.35 0.0024 0.010 0.0008 0. 0019 771 Invention Steel
17 0.45 0.30 3.00 0.100 0.028 0.05 0.0011 0.020 0.0006 0. 0015 757 Invention Steel
18 0.45 0.26 3.20 0.044 0.022 0.20 0.0011 0.018 0.0010 0.0021 749 Comparative Steel
19 0.45 0.10 0.45 0.045 0.020 0.45 0.0010 0.001 0.0006 0.0018 795 Invention Steel
20 0.42 0.03 0.73 0.020 0.027 0.16 0.0017 0.010 0.0016 0.0024 799 Invention Steel
21 0.46 0.18 0.40 0.030 0.020 0.02 0.0020 0.100 0.0005 0.0018 804 Invention Steel
22 0.44 0.30 0.45 0.100 0.022 0.20 0.0012 0.110 0.0009 0.0015 808 Comparative Steel
23 0.44 0.30 0.50 0.045 0.024 0.24 0.0011 0.020 0.0001 0.0015 806 Invention Steel
24 0.44 0.25 0.51 0.044 0.023 0.20 0.0015 0.010 0.0050 0.0020 805 Invention Steel
25 0.44 0.10 0.46 0.100 0.020 0.50 0.0010 0.018 0.0100 0.0018 797 Invention Steel
26 0.44 0.30 0.45 0.080 0.024 0.21 0.0012 0.010 0.0110 0.0015 808 Comparative Steel
27 0.45 0.30 0.50 0.100 0.005 0.40 0.0010 0.012 0.0010 0.0018 801 Comp arative Steel
28 0.46 0.22 0.80 0.073 0.010 0.24 0.0012 0.010 0 .0003 0.0024 792 Invention Steel
29 0.47 0.30 0.45 0.047 0.050 0.20 0.0019 0.010 0 .0005 0.0020 801 Invention Steel
30 0.47 0.26 0.46 0.100 0.100 0.24 0.0015 0.012 0.0005 0.0021 799 Invention Steel
Underline means outside the range specified in the present invention.
- 51 -
[0120]
[Table 2]
Steel Chemical composition mass%) of steel sheet for hot stamping, remainder consisting of Fe and impurities Ac,
Note
No. c Si Mn Sol.Al Ti Cr B p s N Nb Mo v Ni REM Mg Ca Co (OC)
31 0.46 0.31 0.60 0.040 0.130 0.10 0.0021 O.Dll 0.0008 0.0040 802 Comparative Steel
32 0.47 0.25 0.45 0.030 0.030 0.005 0.0017 0.010 0.0007 0.0025 802 Comparative Steel
33 0.47 0.30 0.45 0.031 0.021 0.010 0.0021 0.010 0.0005 0.0032 804 Invention Steel
34 0.46 0.33 0.45 0.030 0.022 0.42 0.0020 0.011 0.0008 0.0026 801 Invention Steel
35 0.47 0.30 0.45 0.030 0.020 0.79 0.0020 0.011 0.0003 0.0037 792 Invention Steel
36 0.45 0.30 1.30 0.100 0.020 0.85 0.0015 0.012 0.0006 0.0030 778 Comparative Steel
37 0.40 0.10 0.48 0.058 0.028 0.36 0.0001 0.018 0.0006 0.0015 808 Comparative Steel
38 0.47 0.18 0.46 0.030 0.020 0.36 0.0005 0.020 0.0006 0.0015 795 Invention Steel
39 0.45 0.20 0.40 0.080 0.040 0.50 0.0053 0.010 0.0009 0.0011 799 Invention Steel
40 0.46 0.80 1.40 0.059 0.028 0.20 0.0100 0.020 0.0005 0.0009 800 Invention Steel
41 0.45 0.22 1.20 0.060 0.030 0.30 0.0150 0.010 0.0020 0.0025 786 Comparative Steel
42 0.45 0.31 0.80 0.0005 0.023 0.20 0.0020 0.014 0.0009 0.0030 799 Comparative Steel
43 0.46 0.30 0.60 0.002 0.024 0.28 0.0013 0.010 0.0009 0.0015 798 Invention Steel
44 0.45 0.20 1.10 0.250 0.025 0.20 0.0020 0.015 0.0010 0.0027 789 Invention Steel
45 0.45 0.55 2.00 0.500 0.024 0.32 0.0014 0.020 0.0006 0.0024 780 Invention Steel
46 0.46 0.18 0.46 0.550 0.030 0.24 0.0011 0.018 0.0007 0.0021 798 Comparative Steel
47 0.46 0.20 0.48 0.030 0.020 0.20 0.0014 0.012 0.0005 0.0002 800 Invention Steel
48 0.47 0.30 1.00 0.040 0.030 0.30 0.0024 0.012 0.0007 0.0050 788 Invention Steel
49 0.46 0.26 0.42 0.100 0.080 0.28 0.0011 0.020 0.0003 0 .0100 800 Invention Steel
50 0.44 0.30 0.44 0.030 0.028 0.32 0.0012 0.014 0.0006 0.0150 806 Comparative Steel
51 0.46 0.24 0.41 0.031 0.030 0.29 0.0025 0.014 0.0007 0.0023 0.01 801 Invention Steel
52 0.47 0.20 0.43 0.037 0.022 0.30 0.0022 0.015 0.0008 0.0026 0.48 797 Invention Steel
53 0.46 0.19 0.44 0.046 0.020 0.22 0.0021 0.008 0.0006 0 .0027 0.45 807 Invention Steel
54 0.45 0.17 0.48 0.031 0.027 0.25 0.0017 0.011 0.0011 0 .0023 0.90 815 Invention Steel
55 0.45 0.19 0.41 0.046 0.026 0.27 0.0023 0.007 0.0014 0.0016 0.050 802 Invention Steel
56 0.46 0.17 0.42 0.040 0.023 0.27 0.0018 0.009 0.0011 0.0024 0.090 799 Invention Steel
57 0.46 0.21 0.41 0.045 0.026 0.22 0.0023 0.011 0.0009 0 .0029 0.050 801 Invention Steel
58 0.47 0.23 0.45 0.045 0.021 0.25 0.0022 0.013 0.0007 0 .0023 0.100 798 Invention Steel
59 0.46 0.19 0.47 0.045 0.025 0.26 0.0022 0.010 0.0011 0 .0026 0.0050 799 Invention Steel
60 0.46 0.21 0.47 0.049 0.023 0.29 0.0024 0.014 0.0014 0 .0021 0.0040 799 Invention Steel
Underline means outside the range specified in the present invention.
- 52 -
[0121]
[Table 3]
Steel Chemical com osition (mass%) of steel sheet for hot stamping, remainder consisting of Fe and impurities AI;;,
Note
No. c Si Mn Sol.Al Ti Cr B p s N Nb Mo v Ni REM Mg Ca Co ("C)
61 0.46 0.21 0.48 0.038 0.023 0.29 0.0022 0.010 0.0012 0.0026 0.0080 799 Invention Steel
62 0.45 0.15 0.49 0.045 0.030 0.26 0.0030 0.005 0.0014 0.0022 4.00 799 Invention Steel
63 0.70 1.00 0.45 0.100 0.021 0.35 0.0015 0.010 0.0002 0.0015 768 Invention Steel
64 0.31 0.20 1.30 0.030 0.030 0.20 0.0020 0.011 0.0007 0.0026 817 Comparative Steel
65 0.46 0.43 0.41 0.030 0.028 0.27 0.0021 0.007 0.0003 0.0030 0 .020 0.19 810 Invention Steel
66 0.46 0.21 0.42 0.043 0.025 0.21 0.0020 0.010 0.0007 0.0027 0.049 0.20 804 Invention Steel
67 0.47 0.35 0.60 0.020 0.024 0.31 0.0020 0.010 0.0007 0.0020 0.020 0.25 802 Invention Steel
68 0.46 0.36 0.80 0.035 0.019 0.24 0.0015 0.010 0.0004 0.0025 0.020 0.20 800 Invention Steel
69 0.46 0.36 1.00 0.030 0.028 0.30 0.0021 0.011 0.0006 0.0043 0.021 0.30 798 Invention Steel
70 0.46 0.40 1.40 0.030 0.021 0.20 0.0015 0.018 0.0003 0.0013 0.020 0.10 789 Invention Steel
71 0.46 1.25 0.69 0.016 0.010 0.42 0.0006 0.016 0.0330 0.0024 0.055 0.49 833 Comparative Steel
Underline means outside the range specified in the present invention.
- 53 -
[0122]
[Table 4]
First h eat treatment Second heat treatment
Hot
stamping
Cooling 1
Cooling
Cooling 3
Test Steel
Plating Average Heating Retention 2 Average Heating Retention T2 - cooling Annealing Note
No. No. h eating Ac3 temp erature time Average Cooling Average heating Ac3 temperature time stop
Average
rate T1 cooling stop cooling r ate T2 temperature
(°C/s) (OC) (OC) (s) rate temperature rate CC/s) (OC) (°C) (s) CC)
cooling rate
COC/s) (OC) (oC/s) (°C/s)
1 1 Absent 5 742 730 120 15 250 5 50 715 850 30 600 45 Absent Comparative Example
2 2 Absent 4 799 970 300 30 180 7 50 795 840 35 660 50 Absent Comparative Example
3 3 Absent 5 742 900 240 40 450 5 20 740 820 30 370 40 Absent Comparative Example
4 4 Absent 4 820 950 118 78 290 5 43 784 800 30 510 60 Absent Comparative Example
5 5 Absent 12 795 940 137 30 270 6 41 770 790 30 520 50 Absent Invention Example
6 6 Absent 6 781 940 169 38 260 7 49 763 800 30 540 50 Absent Invention Example
7 7 Absent 4 754 940 300 32 250 6 51 745 820 30 570 50 Absent Invention Example
8 8 Absent 9 745 900 139 60 250 5 100 738 800 40 550 50 Absent Comparative Example
9 .2 Absent 6 795 900 223 74 300 6 48 789 850 30 550 60 Absent Comparative Example
10 10 Present 4 786 950 149 30 280 7 59 777 820 30 540 50 Absent Invention Example
11 11 Absent 5 818 950 151 39 280 7 31 800 830 30 550 60 Absent Invention Example
12 12 Absent 8 836 950 102 40 290 8 47 820 850 20 560 60 Absent Invention Example
13 13 Absent 15 814 950 155 34 250 8 500 815 840 30 590 50 Absent Comparative Example
14 14 Absent 7 816 900 198 36 280 5 45 805 830 40 550 100 Absent Comparative Example
15 15 Absent 4 803 950 126 67 360 6 41 785 810 30 450 450 Absent Invention Example
16 16 Absent 8 771 950 300 60 250 4 100 760 790 30 540 50 Present Invention Example
17 17 Absent 10 757 950 600 15 250 3 58 750 810 30 560 50 Absent Invention Example
18 18 Absent 10 749 940 250 42 250 5 42 740 800 30 550 50 Present Comparative Example
19 19 Absent 5 795 950 197 50 270 5 46 779 810 30 540 60 Absent Invention Example
20 20 Present 4 799 950 241 43 280 6 57 782 820 30 540 60 Absent Invention Example
21 21 Absent 14 804 950 150 70 280 6 52 786 820 30 540 60 Absent Invention Example
22 22 Absent 7 808 950 212 34 280 6 31 790 820 30 540 60 Absent Comparative Example
23 23 Absent 14 806 960 160 70 290 5 53 782 820 30 530 60 Absent Invention Example
24 24 Absent 4 805 950 160 70 290 6 48 785 820 30 530 60 Absent Invention Example
25 25 Absent 14 797 950 192 50 310 6 58 779 820 30 510 60 Absent Invention Example
Underline means outside the range specified in the present invention or outside the manufacturing c onditions recommended in the present invention.
- 54 -
[0123]
[Table 5]
First heat treatment Second heat treatment Hot stamping
Average Heating Retention
Cooling 1 Cooling 2
Average Heating Retention
T2 - Cooling 3
Test Steel Plating cooling heating Acl temperature time Average Cooling Average heating Ac3' temperature time Average Annealing Note
No. No. cooling stop cooling stop
rate T1 rate T2 cooling rate
(°C/s) ec> (OC) (s) rate temperature rate (°C/s) (OC) (OC) (s) temperature
(°C/s)
("Cis) ("C) ("Cis) ("C)
26 26 Absent 12 808 950 169 75 290 6 70 786 820 30 530 60 Absent Comparative Example
27 27 Absent 5 801 950 135 80 290 5 50 794 870 30 580 60 Absent Comparative Example
28 28 Absent 11 792 950 279 50 290 7 41 775 810 30 520 50 Absent Invention Example
29 29 Absent 6 801 950 231 53 280 6 31 790 820 30 540 60 Absent Invention Example
30 30 Absent 4 799 950 295 50 300 5 60 782 820 30 520 60 Absent Invention Example
31 31 Absent 7 802 960 164 65 290 6 49 785 820 30 530 80 Absent Comparative Example
32 32 Absent 11 802 950 283 50 280 6 32 788 820 30 540 80 Absent Comparative Example
33 33 Absent 8 804 950 250 50 290 5 43 780 820 30 530 80 Absent Invention Example
34 34 Absent 10 801 950 177 80 290 5 60 774 820 30 530 80 Absent Invention Example
35 35 Absent 12 792 950 297 72 280 7 56 783 820 30 540 60 Absent Invention Example
36 36 Absent 8 778 950 200 34 290 8 36 770 820 30 530 50 Absent Comparative Example
37 37 Absent 12 808 950 205 76 290 5 200 803 840 30 550 20 Absent Comparative Example
38 38 Absent 8 795 950 165 70 290 8 60 786 820 30 530 60 Absent Invention Example
39 39 Absent 8 799 950 240 60 290 6 51 780 820 30 530 80 Absent Invention Example
40 40 Absent 4 800 950 235 44 270 8 100 773 810 30 540 50 Absent Invention Example
41 41 Absent 13 786 950 232 50 280 8 50 772 820 30 540 50 Absent Comparative Example
42 42 Absent 9 799 950 241 29 300 7 45 787 810 30 510 60 Absent Comparative Example
43 43 Absent 11 798 940 185 26 290 6 42 790 820 30 530 60 Present Invention Example
44 44 Absent 5 789 950 157 64 300 7 43 771 820 30 520 60 Absent Invention Example
45 45 Absent 13 780 970 400 50 270 5 300 765 800 30 530 15 Absent Invention Example
46 46 Absent 11 798 950 180 50 290 7 65 780 810 30 520 70 Absent Comparative Example
47 47 Absent 10 800 950 205 60 290 7 60 788 820 30 530 70 Absent Invention Example
48 48 Absent 5 788 960 250 65 300 6 50 770 810 30 510 70 Absent Invention Example
49 49 Absent 10 800 950 270 76 300 4 70 781 810 30 510 70 Absent Invention Example
50 50 Absent 13 806 950 200 60 280 5 50 793 820 30 540 70 Absent Comparative Example
Underline means outside the range specified in the present invention or outside the manufacturing c onditions recommended in the present invention.
- 55 -
[0124]
[Table 6]
First heat treatment Second heat t reatment Hot stamging
Average Heating Retention
Cooling 1 Cooling 2
Average Heating Retention T2 ~ c ooling
Cooling 3
Test Steel Average Cooling Average
Plating heating Ac3 temperature time heating Ac,' temperature time stop Average Annealing Note
No. No. rate T1 cooling stop cooling rate T2 temperature cooling rate
(°C/s) ("C) (OC) (s) rate temperature r ate (°C/s) (OC) (OC) (s) (oC) CC/s)
CCISJ (oCJ COCisJ
51 51 Absent 4 801 950 250 70 290 6 32 788 820 30 530 60 Absent Invention Example
52 52 Absent 6 797 950 150 30 300 7 50 780 820 30 520 70 Absent Invention Example
53 53 Absent 8 807 960 182 70 260 7 50 800 830 30 570 70 Absent Invention Example
54 54 Absent 4 815 950 250 60 320 5 70 797 820 30 500 60 Absent Invention Example
55 55 Present 12 802 960 243 60 300 5 56 785 820 30 520 60 Present Invention Example
56 56 Absent 8 799 950 200 70 290 6 42 787 820 30 530 60 Absent Invention Example
57 57 Absent 14 801 930 191 70 300 5 100 780 810 30 510 70 Absent Invention Example
58 58 Absent 5 798 940 240 50 290 6 12 783 820 30 530 70 Absent Invention Example
59 59 Absent 6 799 970 200 50 300 4 975 778 810 20 510 100 Absent Invention Example
60 60 Absent 5 799 990 185 72 300 7 200 788 840 30 540 60 Absent Invention Example
61 61 Absent 14 799 950 200 480 290 8 20 790 820 30 530 60 Absent Invention Example
62 62 Absent 10 799 805 300 250 370 4 54 774 830 30 460 480 Absent Invention Example
63 16 Absent 5 771 760 240 70 260 5 100 766 820 40 560 50 Absent Comparative Example
64 16 Absent 5 771 1000 240 70 260 5 20 764 840 30 580 50 Absent Comparative Example
65 16 Absent 5 771 900 120 5 260 5 100 765 830 30 570 50 Absent Comparative Example
66 16 Absent 5 771 900 120 1000 250 5 ~ Absent Comparative Example
67 16 Absent 5 771 900 120 70 200 5 100 769 810 30 610 50 Absent Comparative Example
68 16 Absent 5 771 900 140 60 500 5 100 770 820 30 320 60 Absent Comparative Example
69 16 Absent 5 771 920 200 60 260 15 20 769 830 30 570 60 Absent Comparative Example
70 16 Absent 5 771 950 200 60 270 4 5 765 820 30 550 60 Absent Comparative Examp le
71 16 Absent 5 771 920 240 60 270 5 1100 767 830 30 560 60 Absent Comparative Example
72 16 Absent 5 771 920 240 60 280 4 50 763 740 20 460 60 Absent Comparative Example
73 16 Absent 5 771 920 240 60 260 5 50 765 950 40 690 60 Absent Comparative Example
74 63 Absent 5 768 930 290 55 250 3 50 760 930 30 680 60 Absent Comparative Example
75 64 Absent 5 817 930 240 80 300 5 45 780 850 30 550 60 Absent Comparative Example
Underline means outside the range specified in the present invention or outside the manufacturing c onditions recommen ded in the present invention .
- 56 -
[0125]
[Table 7]
First heat treatment Second heat treatment Hot
stamping
Test Steel Average Heating Retention
Cooling 1 Cooling 2
Average Heating Retention T2- cooling
Cooling 3
No. No. Plating Average Cooling Average Annealing Note heating Ac3 temperatme time heating Ac3 temperature time stop Average
rate T1 cooling stop cooling rate T2 temperatme cooling r ate
CC/s) (OC) CC) (s) (orCat/es ) tem(poeCra)t me (°rCat/es ) (°C/s) (oC) CC) (s) CC) CC/s)
76 65 Absent 10 811 930 210 50 310 5 75 782 820 30 510 60 Absent Invention Example
77 66 Absent 15 805 930 200 50 300 5 80 784 820 30 520 70 Absent Invention Example
78 67 Absent 10 802 940 200 40 310 5 50 7 93 820 30 510 60 Absent Invention Example
79 68 Absent 11 800 950 280 60 300 6 40 773 820 30 520 60 Absent Invention Example
80 69 Absent 10 798 950 250 40 305 5 50 779 820 30 515 50 Absent Invention Examp le
81 70 Absent 10 789 950 240 40 280 6 90 769 810 30 530 50 Absent Invention Example
82 70 Absent 10 789 950 240 50 230 6 80 775 820 30 590 60 Absent Comparative Example
83 45 Absent 15 780 970 400 50 236 5 300 771 810 30 574 15 Absent Comparative Example
84 40 Absent 5 800 950 235 45 210 8 100 786 830 30 620 60 Absent Comparative Example
85 70 Absent 10 789 950 240 50 407 5 80 770 820 40 413 50 Absent Comparative Example
86 45 Absent 15 780 950 400 50 410 5 50 769 820 40 410 15 Absent Comparative Example
87 65 Absent 10 811 930 210 50 326 5 75 779 820 30 494 60 Absent Invention Example
88 65 Absent 10 811 930 210 50 345 5 75 776 820 30 475 60 Absent Invention Example
89 21 Absent 15 804 950 150 70 350 6 50 783 820 30 470 60 Absent Invention Example
90 5 Absent 12 795 940 140 30 265 6 45 772 871 30 606 50 Absent Comparative Example
91 16 Absent 8 771 950 300 60 250 5 100 7 59 855 30 605 50 Absent Comparative Example
92 44 Absent 5 789 950 160 60 265 7 45 776 875 30 610 60 Absent Comparative Example
93 44 Absent 5 789 950 160 60 260 6 50 777 875 30 615 60 Absent Comparative Example
94 44 Absent 5 789 950 160 60 270 7 45 771 830 8 560 60 Absent Comparative Example
95 40 Absent 4 800 950 240 45 270 8 100 775 810 5 540 50 Absent Comparative Example
96 2 Absent 4 799 970 300 40 290 5 45 786 820 1 530 50 Absent Comparative Example
97 5 Absent 12 795 940 140 40 270 7 45 770 790 9 520 50 Absent Comparative Example
98 7 Absent 4 754 940 300 35 270 6 30 745 840 65 570 50 Absent Comparative Example
99 24 Absent 4 805 950 160 70 290 7 48 7 87 840 70 550 60 Absent Comparative Example
100 47 Absent 10 800 950 205 60 280 6 60 789 830 100 550 50 Absent Comparative Example
Underline means outside the range specified in the present invention or outside the manufacturing conditions recommended in the present invention.
- 57 -
[0126]
[Table 8]
First heat treatment Second heat treatment Hot stamping
Average Heating Retention
Cooling 1 Cooling 2
Average Heating Retention T2 - cooling
Cooling 3
Test Steel
Plating heating Ac, temperature time
Average
Cooling stop Average heating Ac3 temperature time stop Average Annealing Note
No. No. cooling
rate T1 temperature cooling r ate rate T2 temperature cooling rate
(•Cis) (OC) (DC) (s)
rate (•Cis) (DC) (OC) (s) (OC)
(•Cis)
(OC) (°C/s) (°C/s)
101 5 Absent 12 795 940 140 40 270 6 45 770 860 300 590 50 Absent Comparative Example
102 66 Absent 15 805 930 200 50 300 5 80 753 820 20 520 70 Absent Invention Example
103 44 Absent 5 789 950 160 65 300 6 45 771 820 25 520 60 Absent Invention Example
104 16 Absent 8 771 950 300 60 270 4 105 760 790 20 520 50 Present Invention Example
105 53 Absent 8 807 960 190 70 300 7 50 800 830 20 530 70 Absent Invention Example
106 10 Present 4 786 950 150 30 280 7 59 778 820 45 540 50 Absent Invention Example
107 21 Absent 15 804 950 150 70 280 6 52 786 820 55 540 60 Absent Invention Example
108 68 Absent 11 800 950 280 60 300 6 40 772 820 50 520 60 Absent Invention Example
109 44 Absent 5 789 950 160 65 300 7 43 771 820 12 520 60 Absent Invention Example
110 69 Absent 10 798 950 250 40 305 5 50 779 820 15 515 50 Absent Invention Example
111 44 Absent 5 789 950 160 64 275 7 43 771 860 30 585 60 Absent Invention Example
112 11 Absent 10 833 950 120 40 300 7 50 805 850 30 550 60 Absent Comparative Example
113 71 Absent 30 833 900 10 50 250 9 1000 810 850 0 .1 600 100 Absent Comparative Example
114 11 Absent 30 833 900 lQ 50 250 8 1000 810 870 30 22Q 60 Absent Comparative Example
115 71 Absent 30 833 900 !Q 50 260 8 1000 809 850 !Q 590 60 Absent Comparative Example
116 71 Absent 30 833 900 1Q 50 260 8 1000 809 850 ~ 590 60 Absent Comparative Example
Underline means outside the range specified in the present invention or outside the manufacturing conditions recommended in the present invention.
- 58 -
[0127]
[Table 9]
Steel sheet for hot stamping Hot -stamping formed body Mechanical properties
Microstructure Density of Microstructure
Proportion of high carbides having Average AverageMn
Density of carbides
Test Steel having circle Partially Tensile Average Hardness
angle grain Residual circle equivalent grain size concentration of prior Early fracture Note
No. No. Plating Others Martensite Others Total equivalent diameter softened strength hardness va.riation Miv
boundaries austenite diameter of 0.20 ofprioq 1 grain boundaries evaluation
(%) (area%)
(area%) (area%) (area%) (area%)
()llll) (mass%)
of 0. 20 )llll or more region (MPa) (Hv) (Hv)
JlillOr rmre
(/)llli)
(/)llll2)
1 1 Absent 30 5 6 0.2 98 2 100 3.3 1.3 0.2 Absent 2ill 880 0.31 60 Comparative Example
2 2 Absent so 0 1 0.4 100 0 100 5.0 1.5 0 .3 Absent 1,304 670 0.59 51 Comparative Example
3 3 Absent 47 0 1 1.0 100 0 100 8.9 2.0 0 .3 Absent 677 855 0.24 63 Comparative Example
4 4 Absent 33 3 5 0.3 100 0 100 4.1 0.5 0.1 Absent 1 888 596 0.96 15 Comparative Example
5 5 Absent 40 4 0 0.4 100 0 100 3.5 0.9 0.2 Absent 2,008 676 0.90 23 Invention Example
6 6 Absent 53 3 0 0.2 100 0 100 3.6 0.3 0.1 Absent 2,219 810 0.83 47 Invention Example
7 7 Absent 60 4 1 0.3 100 0 100 4.5 0.2 0 Absent 2,371 971 0.74 48 Invention Example
8 8 Absent 59 2 1 1.2 100 0 100 3.0 0.3 1.0 Absent 1,637 992 O.SO 49 Comparative Example
9 9 Absent 42 1 0 0.9 100 0 100 4.8 0.5 0.8 Absent 1396 717 0.59 25 Comparative Example
10 10 Present 46 3 1 0.4 100 0 100 4.2 0.6 0.2 Absent 2,335 737 0.96 26 Invention Example
11 11 Absent 44 5 0 0.2 100 0 100 4.5 0.4 0 Absent 2,321 725 0.97 22 Invention Example
12 12 Absent 45 8 0 0.1 100 0 100 4.7 0.4 0 Absent 2,337 730 0.97 21 Invention Example
13 13 Absent 49 0 2 0.2 100 0 100 3.5 1.2 0 .1 Absent 1236 780 0.48 57 Comparative Example
14 14 Absent 35 1 4 0.4 98 2 100 4.9 0.3 0.2 Absent 1 158 605 0.58 13 Comparative Example
15 15 Absent 45 3 1 0.2 100 0 100 3.7 0.4 0.1 Absent 2,237 706 0.96 19 Invention Example
16 16 Absent so 4 0 0.2 100 0 100 3.2 0.8 0.1 Present 2,286 745 0.93 30 Invention Example
17 17 Absent 47 4 0 0.1 100 0 100 3.4 0.9 0 .1 Absent 2,162 720 0.91 23 Invention Example
18 18 Absent 47 3 1 0.2 100 0 100 2.9 1.0 0 .1 Present 860 724 0.36 48 Comparative Example
19 19 Absent 43 2 2 0.2 100 0 100 3.8 0.3 0 .1 Absent 2,323 711 0.99 20 Invention Example
20 20 Present 41 1 1 0.2 100 0 100 4.5 0.6 0.1 Absent 2,128 686 0.94 17 Invention Example
21 21 Absent 44 3 0 0.3 100 0 100 4.2 0.4 0.2 Absent 2,287 722 0.96 22 Invention Example
22 22 Absent 42 2 1 0.3 100 0 100 4.6 0.4 0.2 Absent 1332 708 0.57 27 Comparative Example
23 23 Absent 43 3 0 0.2 100 0 100 4.1 0.4 0 .1 Absent 2,257 705 0.97 21 Invention Example
24 24 Absent 43 2 2 0.3 100 0 100 4.3 0.5 0.2 Absent 2,154 702 0.93 23 Invention Example
25 25 Absent 42 3 0 0.2 100 0 100 4.0 0.4 0 .2 Absent 2,207 704 0.95 25 Invention Example
Underline means outside the range specified in the present invention or that the target performance is not satisfied.
- 59 -
[0128]
[Table 10]
Steel sheet for bot stamping Hot-stamping formed body Mechanical properties
Microstructure Density of carbides Microstructure Average Mn Density of carbides
Proportion of high Hardness
Test Steel having circle Average grain concentration of having circle Partially Tensile Average E arly
angle grain Residual variation Note
No. No. Plating Others equivalent diameter Martensite Others Total size of prior r prior r grain equivalent diameter of softened strength hardness fr acture
boundaries austenite I'.Hv
(are a%) of0.20 )llllor more (area%) (area%) (area%) ()llll) boundaries 0 .20 )lill or more region (MPa) (Hv) evaluation
(%) (area%)
(/)llll2) (mass%) (/)llll2)
(Hv)
26 26 Absent 41 2 1 0.3 100 0 100 4.5 0.4 0.2 Absent 1,178 700 0.51 18 Comparative Example
27 27 Absent 43 3 0 0.3 100 0 100 12.3 0 .5 0.1 Absent 1275 690 0.56 20 Comparative Example
28 28 Absent 45 4 0 0.2 100 0 100 3.9 0 .6 0.1 Absent 2,292 7 31 0 .95 27 Invention Example
29 29 Absent 44 3 1 0.2 100 0 100 4.3 0.4 0.1 Absent 2,362 7 38 0 .97 22 Invention Example
30 30 Absent 45 4 0 0.1 100 0 100 3.8 0 .3 0.1 Absent 2,403 743 0 .98 21 Invention Example
21 31 Absent 46 3 1 0.2 100 0 100 4 .1 0 .5 0.2 Absent 1 328 745 0.54 23 Comparative Example
32 32 Absent 44 2 0 1.0 100 0 100 4.4 0.4 0.6 Absent 1,430 747 0 .58 24 Comparative Example
33 33 Absent 44 3 0 0.4 100 0 100 3.8 0.3 0.2 Absent 2,372 741 0 .97 20 Invention Example
34 34 Absent 43 3 0 0.2 100 0 100 3.5 0.3 0.1 Absent 2,419 748 0 .98 16 Invention Example
35 35 Absent 46 5 0 0.1 100 0 100 3.7 0.4 0 Absent 2,213 745 0 .90 19 Invention Example
2§. 36 Absent 44 6 0 0.1 100 0 100 4.0 1.2 0 Absent 1121 7 55 0.45 53 Comparative Example
37 37 Absent 37 2 8 0.5 84 16 100 4 .8 0 .5 0.4 Absent 1 104 587 0.57 18 Comparative Example
38 38 Absent 47 4 0 0.3 100 0 100 3.7 0.4 0.2 Absent 2,271 740 0 .93 22 Invention Example
39 39 Absent 42 3 0 0.2 100 0 100 3.5 0 .3 0.1 Absent 2,289 715 0 .97 17 Invention Example
40 40 Absent 46 5 0 0.1 100 0 100 2.6 0 .5 0 Absent 2,053 7 32 0 .85 29 Invention Example
41 41 Absent 42 4 1 0.2 100 0 100 3.7 1.1 0.1 Absent 1AQ2 7 36 0 .58 55 Comparative Example
42 12 Absent 44 3 0 0.3 100 0 100 4 .2 0 .6 0.2 Absent ~ 723 fU2 30 Comparative Ex ample
43 43 Absent 43 2 1 0.2 100 0 100 4 .6 0 .5 0.1 Present 2,268 7 31 0 .94 23 Invention Example
44 44 Absent 42 3 0 0.2 100 0 100 3.5 0 .8 0.2 Absent 2,1 61 744 0 .88 36 Invention Example
45 45 Absent 46 6 0 0.1 100 0 100 2.6 1.0 0 Absent 2,028 723 0 .85 34 Invention Example
46 46 Absent 44 4 1 0.2 100 0 100 4.4 0.4 0.2 Absent 1 205 745 Qd2 20 Comparative Ex ample
47 47 Absent 42 2 0 0.3 100 0 100 4 .6 0.4 0.2 Absent 2,344 740 0 .96 18 Invention Example
48 48 Absent 43 2 0 0.3 100 0 100 3.3 0 .7 0.2 Absent 2,002 749 0 .81 32 Invention Example
49 49 Absent 42 3 0 0.2 100 0 100 4 .1 0 .3 0.1 Absent 2,388 746 0 .97 19 Invention Example
50 50 Absent 41 2 1 0.2 100 0 100 4.9 0.4 0.2 Absent 1.161 718 0.49 21 Comparative Ex ample
Underline means outside the range specified in the present invention or that the target performance is not satisfied.
- 60 -
[0129]
[Table 11]
Steel sheet for hot stamping Hot-stamping formed body Mechanical properties
Microstructure Density of carbides Microstructure Average Mn Density of carbides
Proportion of Hardness
Test Steel having circle equivalent Average grain concentration of having circle Partially Tensile Average Early
high angle g~:ain Residual variation Note
No. No. Plating Others diameter of 0. 20 IJlll or Martensite Others Tot al size of prior r prior r g~:ain equivalent diameter of softened strength hardness fracture
boundaries austenite Mlv
( area%) more (area%) (area%) (area%) (IJlll) boundaries 0.20 IJlllor more region (MPa) (Hv) evaluation
(%) (area%)
(/fliiJ') (mass%) (/1Jlll2)
(Hv)
51 51 Absent 41 3 0 0.1 100 0 100 4.5 0.3 0 .1 Absent 2,359 737 0.97 20 Invention Example
52 52 Absent 43 5 0 0.1 100 0 100 4.4 0.3 0.1 Absent 2,320 740 0.95 24 Invention Example
53 53 Absent 44 4 0 0.1 100 0 100 4.7 0.4 0.1 Absent 2,332 736 0.96 27 Invention Example
54 54 Absent 45 5 0 0.2 100 0 100 3.9 0.3 0 .1 Absent 2,372 741 0.97 22 Invention Example
55 55 Present 44 4 0 0.1 100 0 100 4.1 0.3 0.1 Present 2,326 742 0.95 16 Invention Example
56 56 Absent 46 4 0 0 100 0 100 3.8 0.3 0 Absent 2,332 744 0.95 15 Invention Example
57 57 Absent 48 4 0 0.1 100 0 100 2.5 0.3 0 Absent 2,323 749 0.94 18 Invention Example
58 58 Absent 49 5 0 0.2 100 0 100 3.4 0.3 0.1 Absent 2,421 741 0.99 15 Invention Example
59 59 Absent 41 3 1 0.2 100 0 100 2.5 0.4 0.2 Absent 2,314 738 0.95 24 Invention Example
60 60 Absent 42 3 1 0.2 100 0 100 3.3 0.3 0 .1 Absent 2,388 746 0.97 26 Invention Example
61 61 Absent 44 3 0 0 100 0 100 4.6 0.4 0 Absent 2,383 737 0.98 25 Invention Example
62 62 Absent 39 2 4 0.1 100 0 100 3.5 0.4 0.1 Absent 2,292 739 0.94 19 Invention Ex~p~e
63 16 Absent 25 1 10 0.5 89 11 100 4.3 1.6 0.4 Absent 1.290 674 0.58 51 Comparative Example
64 16 Absent 40 2 0 0.1 100 0 100 9 .0 1.2 0.1 Absent 618 720 0.26 52 Comparative Example
65 16 Absent 23 1 12 0.4 100 0 100 7.3 1.5 0.3 Absent 700 731 Q.22. 53 Comparative Example
66 16 Absent 57 3 0 0 - - - - Absent - - - Comparative Example
QL 16 Absent 53 0 0 0.1 100 0 100 5.0 1.6 0.1 Absent 1 244 711 .Q.2J. ll Comparative Example
68 16 Absent 46 0 2 1.0 95 5 100 10.4 1.7 0.8 Absent 582 705 0.25 52 Comparative Example
69 16 Absent 52 0 0 0.1 100 0 100 9 .5 1.6 0.1 Abse nt 639 717 0.27 54 Comparative Example
70 16 Absent 49 5 0 0.2 100 0 100 8.7 1.0 0.1 Absent 689 720 0.29 48 Comparative Example
71 16 Absent 49 4 0 0.2 100 0 100 6.1 1.0 0.2 Absent 1 322 732 0.55 46 Comparative Example
Il 16 Absent 49 5 0 0.1 100 0 100 3.1 Ll. 0.2 Abse nt 1..d22 750 ~ ~ Comparative Example
73 16 Absent so 4 0 0.1 100 0 100 12.7 1.0 0.1 Absent 714 698 0.31 42 Comparative Example
74 63 Absent 65 6 1 0.4 100 0 100 13.0 0.3 0 Absent 2,044 1050 Q.22. so Comparative Example
75 64 Absent 58 5 1 0.3 100 0 100 4.7 0.8 0.1 Absent 1,863 576 0.98 36 Comparative Example
Underline means outside the range specified in the present invention or that the target performance is not satisfied.
- 61 -
[0130]
[Table 12]
Steel sheet for hot stamping Hot-stamping formed body Mechanical properties
Proportion Microstructure Density of carbides Microstructure AverageMn
Density of carbides having circle Hardness
Test Steel of high angle having circle Average grain size concentration of Partially Tensile Average Early
Residual equivalent diameter of 0 .20 ~Jill variation Note
No. No. Plating grain Others equivalent diameter Martensite Others Total of prior r prior r grain softened strength hardness fracture
austenite or IIDre L\.Hv
boundaries (area%) of 0.20 )1IIl or more (area%) (area%) (area%) (~Jill) boundaries
(/)llll2)
region (MPa) (Hv) evaluation
(%)
(area%)
(/)llll2) (mass%)
(Hv)
76 65 Absent 49 4 0 0.1 100 0 100 2.3 0.3 0.1 Absent 2,426 750 0.98 19 Invention Example
77 66 Absent 50 4 0 0.1 100 0 100 2.1 0.3 0.1 Absent 2,422 749 0.98 20 Invention Example
78 67 Absent 46 5 0 0.1 100 0 100 3.0 0.4 0.1 Absent 2,347 741 0.96 24 Invention Example
79 68 Absent 48 6 0 0.2 100 0 100 3.0 0.5 0.1 Absent 2,353 735 0.97 25 Invention Example
80 69 Absent 49 4 0 0 .2 100 0 100 2.9 0.6 0.1 Absent 2,386 753 0.96 27 Invention Example
81 70 Absent 47 6 0 0.2 100 0 100 2.2 0.4 0.1 Absent 2,367 755 0.95 23 Invention Example
82 70 Absent 50 0 0 0.1 100 0 100 3.1 1.1 0.1 Absent 1 482 761 0.59 54 Comparative Example
83 45 Absent 52 0 0 0.1 100 0 100 3.3 1.3 0 Absent 1,186 719 0.50 52 Comparative Example
84 40 Absent 53 0 0 0.1 100 0 100 3.5 1.2 0 Absent 1220 7 11 0.52 51 Comparative Example
85 70 Absent 41 5 1 0.3 100 0 100 3.4 1.1 0.1 Absent 1428 746 0.58 53 Comparative Example
86 45 Absent 42 3 2 0.1 100 0 100 3.6 1.4 0 Absent 1065 717 0.45 51 Comparative Example
87 65 Absent 47 6 0 0.1 100 0 100 2.2 0.3 0 Absent 2,457 752 0.99 17 Invention Example
88 65 Absent 45 7 0 0.1 100 0 100 2.1 0.2 0.1 Absent 2,485 753 1.00 14 Invention Example
89 21 Absent 42 4 0 0.1 100 0 100 3.9 0.1 0.2 Absent 2,375 727 0.99 12 Invention Example
2Q 5 Absent 41 3 0 0.4 100 0 100 4.5 1.2 0.2 Absent .1...112. 674 0.53 51 Comparative Example
21 16 Absent 48 4 0 0.2 100 0 100 4.9 1.3 0.1 Present .!..122 739 Qd2. 53 Comparative Example
.2£ 44 Absent 46 1 0 0.1 100 0 100 22 1.1 0.2 Absent .l.ID 740 ~ ~ Comparative Example
.22 44 Absent 45 1 0 0 .1 100 0 100 5.7 1.1 0.2 Absent 1 364 738 0.56 52 Comparative Example
94 44 Absent 46 1 0 0 .1 100 0 100 3.2 1.1 0.1 Absent 1430 747 0.58 51 Comparative Example
95 40 Absent 46 3 0 0.1 100 0 100 2.5 1.2 0.3 Absent 1,234 733 0.51 52 Comparative Example
.22 2 Absent 48 5 0 0 100 0 100 4.6 1.5 0.4 Absent 1 004 676 0.45 54 Comparative Example
'il 5 Absent 41 3 0 0 .3 100 0 100 3.4 1.1 0.1 Absent 1 277 679 ~ ~ Comparative Example
~ 7 Absent 55 5 1 0.4 100 0 100 5.5 0.3 0.2 Absent 1552 960 Qd2. 48 Comparative Example
22 24 Absent 44 1 1 0 .2 100 0 100 5.6 0.5 0.2 Absent 1 315 699 0.57 21 Comparative Example
100 47 Absent 44 1 0 0.2 100 0 100 5.3 0.4 0.1 Absent 1,437 738 0.59 24 Comparative Example
Underline means outside the range specified in the present invention or that the target performance is not satisfied.
- 62 -
[0131]
[Table 13]
Steel sheet for hot stamping Hot-stamping forrred body Mechanical properties
Microstructure Density of Micro structure Density of
Proportion of
carbides having Average
Average Mn
carbides having Hardness
Test Steel high angle concentration of Partially Tensile Average Early
Residual c ircle equivalent grain size circle equivalent variation Note
No. No. Plating grain Others Martensite Others Total prior 1 grain softened strength hardness fracture
austenite diameter of 0.20 of prior r diarreter of 0 .20 M!v
boundaries (area%) (area%) (area%) (area%) boundaries region (MPa) (Hv) evaluation
(%)
(area%) J.illl or more (fllll)
(mass%)
J.UUDr ODre (Hv )
(/~) (/fllll2)
101 5 Absent 40 3 0 0.2 100 0 100 7.8 0.9 0 .1 Absent 1 308 672 Q.2l. 40
Comparative
Example
102 66 Absent 50 4 0 0.1 100 0 100 1.9 0.2 0 .1 Absent 2,478 751 1.00 15 Invention Example
103 44 Absent 41 4 0 0.1 100 0 100 3.1 0.7 0 .2 Absent 2,363 746 0.96 30 Invention Example
104 16 Absent 48 5 0 0.2 100 0 100 3.0 0.6 0 .1 Prese nt 2,391 747 0.97 28 Invention Example
105 53 Absent 42 6 0 0.1 100 0 100 4.0 0.3 0 .1 Absent 2,439 739 1.00 18 Invention Example
106 10 Present 46 3 1 0.3 100 0 100 4.5 0.5 0 .2 Absent 2,377 735 0.98 23 Invention Example
107 21 Absent 44 3 0 0.2 100 0 100 4.7 0.3 0 .2 Absent 2,328 720 0.98 20 Invention Example
108 68 Absent 48 5 0 0.1 100 0 100 3.9 0.4 0 .1 Absent 2,395 733 0.99 24 Invention Example
109 44 Absent 42 1 0 0.2 100 0 100 3.1 0.9 0 .2 Absent 2,142 746 0.87 41 Invention Example
110 69 Absent 49 4 0 0 .2 100 0 100 2.6 0.7 0 .1 Absent 2,358 752 0.95 36 Invention Example
111 44 Absent 42 1 0 0.2 100 0 100 4.9 0.9 0 .1 Absent 2,122 739 0.87 39 Invention Example
Absent Comparative
ill 11 Absent 49 9 0 0.1 100 0 100 2.2 0.2 0 .1 2,475 750 1.00 13
Example
113 11 Absent 60 0 1 0 .6 100 0 100 4.5 11 0.3
Absent
2,591 785 1.00 QQ
Comparative
Example
114 71 Absent 59 0 1 0 .1 100 0 100 5.3 1.2 0.4
Absent
2,496 764 0.99 57
Comparative
- - - - - Example
-115 -7 1 Absent 58 1 1 0.1 100 0 100 5.0 0.4
Absent
771 62
Comparative
-1.1 2,493 0.98 - Example
Absent Comparative
-116 -7 1 Absent 58 1 1 0.1 100 0 100 -5.6 -1.2 0 .4 2,433 760 0.97 -55 Example
Underline means outside the range specified in the present invention or that the target performance is not satisfied.
- 63 -
[0132]
As shown in Tables 1 to 13, the invention examples satisfying the chemical
composition and microstructure specified in the present invention were excellent in
mechanical properties. The comparative examples that did not satisfy the chemical
composition and microstructure specified in the present invention were inferior in
mechanical properties.
[Industrial Applicability]
[0133]
According to the above aspect according to the present invention, it is
possible to provide a hot-stamping formed body having excellent strength and
toughness.

WE CLAIMS

1. A hot-stamping formed body comprising, as a chemical composition, by
C: 0.40% to 0.70%;
Si: 0.010% to 1.30%;
Mn: 0.40% to 3.00%;
sol. Al: 0.0010% to 0.500%;
Ti: 0.010% to 0.100%;
Cr: 0.010% to 0.80%;
B: 0.0005% to 0.0100%;
P: 0.100% or less;
S: 0.0100% or less;
N: 0.0100% or less;
Nb: 0% to 0.100%;
Mo: 0% to 1.00%;
V: 0% to 0.100%;
Ni: 0% to 0.50%;
REM: 0% to 0.0100%;
Mg: 0% to 0.0100%;
Ca: 0% to 0.0100%;
Co: 0% to 4.00%; and
a remainder consisting of Fe and impurities,
wherein an average grain size of prior austenite grains in a microstructure is
5.0 11m or less, and
an average Mn concentration at grain boundaries of the prior austenite grains
- 65 -
is 1.0 mass% or less.
2. The hot-stamping formed body according to claim 1 comprising, as the
chemical composition, by mass%, one or two or more elements selected from:
Nb: 0.010% to 0.100%;
Mo: 0.01% to 1.00%;
V: 0.001% to 0.100%;
Ni: 0.001% to 0.50%;
REM: 0.0010% to 0.0100%;
Mg: 0.0010% to 0.0100%;
Ca: 0.0010% to 0.0100%; and
Co: 0.10% to 4.00%.
3. The hot-stamping formed body according to claim 1 or 2, further
compnsmg:
a plating layer on a surface of the hot-stamping formed body.
4. The hot-stamping formed body according to any one of claims 1 to 3,
wherein a portion of the hot-stamping formed body has a softened region.