[Technical Field of the Invention]
[0001]
The present invention relates to a hot-stamping formed body.
Priority is claimed on Japanese Patent Application No. 2020-084591, filed
May 13, 2020, 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 the
vehicle body of a vehicle in terms of environmental protection and resource saving,
and a high strength steel sheet has been applied to vehicle members. Vehicle
members are manufactured by press forming, but not only a forming load is increased
but also the formability deteriorates as the strength of a steel sheet is increased. For
this reason, the formability of the high strength steel sheet into a member having a
complicated shape becomes an issue.
[0003]
In order to solve this issue, the application of hot stamping technique in which
press forming is performed after a steel sheet is heated up to a high temperature of an
austenite range where the steel sheet softens is in progress. Hot stamping is attracting
attention as technique that achieves both the formability of a steel sheet into a vehicle
member and the strength of the vehicle member by performing the hardening of the
steel sheet in a die at the same time as press working.
[0004]
In order to obtain a higher effect of reducing the weight of a vehicle body
- 1 -
from a vehicle member into which a steel sheet is formed by hot stamping, it is
necessary to obtain a member that has high strength and is also excellent in collision
characteristics. As a technique for improving the collision characteristics of a vehicle
member, particularly, a technique for improving the bendability of the vehicle member
is being studied.
[0005]
Patent Document 1 discloses a high strength pressed component having
excellent impact absorption characteristics, in which the hardness of the pressed
component in the sheet thickness center is Hv400 or more, a soft layer having a
hardness of Hv300 or less is provided in a surface layer of the pressed component, and
the thickness of the soft layer is 20 to 200 J.lm.
[0006]
Patent Document 2 discloses a high strength cold-rolled steel sheet having
excellent uniform elongation and hole expansibility, in which the texture in the center
portion of the steel sheet is controlled.
[0007]
At the time of bending distortion, distortion starts from the surface of a
vehicle member, and the distortion gradually progresses toward the inside of the
vehicle member. Therefore, in order to further improve the bendability of the vehicle
member, it is effective to enhance the bending distortion capability of the surface layer
of the vehicle member and then enhance the bending distortion capability of the inside
of the vehicle member. In Patent Documents 1 and 2, improvement in the bending
distortion capabilities of both the surface layer area and the inside of the vehicle
member are not taken into account.
[0008]
- 2 -
In addition, when the surface layer of a vehicle member is softened in order to
improve the bendability of the vehicle member, there is a problem of the deterioration
of the ductility.
[Prior Art Document]
[Patent Document]
[0009]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. 2015-30890
[Patent Document 2] PCT International Publication No. W020121144567
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0010]
The present invention has been made in view of the above-mentioned problem.
An object of the present invention is to provide a hot-stamping formed body having
excellent strength, bendability, and ductility.
[Means for Solving the Problem]
[0011]
The gist of the present invention is as follows.
(1) A hot-stamping formed body according to an aspect of the present
invention contains, as a chemical composition, by mass%,
C: 0.15 to 0.50%,
Si: 0.0010% to 3.000%,
Mn: 0.30% to 3.00%,
Al: 0.0002% to 2.000%,
P: 0.100% or less,
- 3 -
S: 0.1000% or less,
N: 0.0100% or less,
Nb: 0% to 0.15%,
Ti: 0% to 0.15%,
V: 0% to 0.15%,
Mo: 0% to 1.0%,
Cr: 0% to 1.0%,
Cu: 0% to 1.0%,
Ni: 0% to 1.0%,
B: 0% to 0.0100%,
Ca: 0% to 0.010%,
REM: 0% to 0.30%, and
a remainder consisting of Fe and an impurity,
in which the hot-stamping formed body has a metallographic structure
consisting of, by area ratio, a total of 10% to 30% of ferrite and granular bainite and a
remainder in microstructure consisting of one or more of martensite, bainite, and
tempered martensite,
in a texture between a surface and a sheet thickness 1/4 position from the
surface, a ratio between a pole density of an orientation group consisting of { 001 } < 1-
1 0> to { 001 } < -1-1 0> and a pole density of an orientation group consisting of { 111 }
<1-10> to { 111} <-1-12> is less than 1.8, and
in a texture between the sheet thickness 114 position from the surface and a
sheet thickness 1/2 position from the surface, a ratio between a pole density of an
orientation group consisting of {001} <1-10> to {001} <-1-10> and a pole density of
an orientation group consisting of { 111 } < 1-10> to { 111 } < -1-12> is less than 2.3.
- 4 -
(2) The hot-stamping formed body according to (1) may further contain, as the
chemical composition, by mass%, one or more of the group consisting of
Nb: 0.05% to 0.15%,
Ti: 0.05% to 0.15%,
V: 0.05% to 0.15%,
Mo: 0.05% to 1.0%,
Cr: 0.05% to 1.0%,
Cu: 0.05% to 1.0%,
Ni: 0.05% to 1.0%,
B: 0.0001% to 0.0100%,
Ca: 0.001% to 0.010%, and
REM: 0.001% to 0.30%.
(3) The hot-stamping formed body according to (1) or (2), in which a
decarburization index may be 0.085 or more.
[Effects ofthe Invention]
[0012]
According to the above-mentioned aspect of the present invention, it is
possible to provide a hot-stamping formed body having excellent strength, bendability,
and ductility.
[Embodiments of the Invention]
[0013]
The present inventors studied a method enabling not only for a tensile
(maximum) strength of 1.5 to 2.5 GPa and excellent bendability to be obtained but also
for the deterioration of ductility to be suppressed after hot stamping. As a result, the
present inventors found that, in a hot-stamping formed body, when the surface layer of
- 5 -
the steel sheet is softened, and furthermore, the texture at a predetermined position in
the sheet thickness direction is controlled, it is possible to obtain a high strength and
superior bendability than ever and to suppress the deterioration of ductility.
[0014]
The texture is affected by the texture and the carbon concentration of the
metallographic structure before hot stamping. Therefore, the present inventors found
that, in order to obtain a desired texture in the hot-stamping formed body, it is effective
to control the texture in the steel sheet after hot rolling and, furthermore, to reduce the
amount of carbon in the surface layer of the steel sheet during the subsequent
annealing.
[0015]
Hereinafter, a steel sheet for hot stamping for manufacturing a hot-stamping
formed body according to the present embodiment by hot stamping will be described
in detail. First, the reasons for limiting the chemical composition of the steel sheet
for hot stamping will be described.
[0016]
Numerical limiting ranges expressed below using "to" include the lower limit
and the upper limit in the ranges. Numerical values expressed with 'more than' and
'less than' are not included in numerical ranges. Regarding the chemical composition,
"%" indicates "mass%" in all cases.
[0017]
The steel sheet for hot stamping for manufacturing the hot-stamping formed
body according to the present embodiment by hot stamping contains, as a chemical
composition, mass%, C: 0.15% to 0.50%, Si: 0.0010% to 3.000%, Mn: 0.30% to
3.00%, Al: 0.0002% to 2.000%, P: 0.100% or less, S: 0.1000% or less, N: 0.0100% or
- 6 -
less, Nb: 0% to 0.15%, Ti: 0% to 0.15%, V: 0% to 0.15%, Mo: 0% to 1.0%, Cr: 0% to
1.0%, Cu: 0% to 1.0%, Ni: 0% to 1.0%, B: 0% to 0.0100%, Ca: 0% to 0.010%, REM:
0% to 0.30%, and a remainder consisting of Fe and an impurity.
Hereinafter, each element will be described.
[0018]
C: 0.15% to 0.50%
Cis an element that improves the strength of the hot-stamping formed body.
In a case where the C content is less than 0.15%, the desired strength of the hotstamping
formed body cannot be obtained. For this reason, the C content is set to
0.15% or more. The C content is preferably 0.17% or more, 0.20% or more, or
0.23% or more. On the other hand, when the C content is more than 0.50%, it is not
possible to obtain excellent bendability. For this reason, the C content is set to 0.50%
or less. The C content is preferably 0.46% or less or 0.43% or less.
[0019]
Si: 0.0010% to 3.000%
Si is an element that improves the strength of the hot-stamping formed body
by solid solution strengthening. When the Si content is less than 0.0010%, it is not
possible to obtain a desired strength. For this reason, the Si content is set to 0.0010%
or more. The Si content is preferably 0.050% or more, 0.100% or more, 0.300% or
more, or 0.500% or more. On the other hand, when the Si content is more than
3.000%, the amount of ferrite increases, and it is not possible to obtain a desired
metallographic structure. For this reason, the Si content is set to 3.000% or less.
The Si content is preferably 2.700% or less or 2.500% or less.
[0020]
Mn: 0.30% to 3.00%
- 7 -
Mn is an element that improves the hardenability of steel. In order to
improve the hardenability and thereby obtain a desired amount of martensite after hot
stamping, the Mn content is set to 0.30% or more. The Mn content is preferably
0.50% or more, 0. 70% or more, or 1.00% or more. On the other hand, when the Mn
content is more than 3.00%, cracking attributed to Mn segregation is likely to occur,
and it is not possible to obtain excellent bendability. For this reason, the Mn content
is set to 3.00% or less. The Mn content is preferably 2.70% or less, 2.50% or less, or
2.30% or less.
[0021]
Al: 0.0002% to 2.000%
Al is an element that improves the distortion capability by deoxidizing molten
steel to suppress the formation of oxide serving as the origin of fracture and improves
the bendability of the hot -stamping formed body. When the Al content is less than
0.0002%, deoxidation is not sufficiently performed, and a coarse oxide is formed,
which makes it impossible to obtain the above-mentioned effect. For this reason, the
Al content is set to 0.0002% or more. The Al content is preferably 0.001% or more.
On the other hand, when the Al content exceeds 2.000%, a coarse oxide is formed in
steel, and the bendability of the hot-stamping formed body deteriorates. For this
reason, the Al content is set to 2.000% or less. The Al content is preferably 1.700%
or less or 1.500% or less.
[0022]
P: 0.100% or less
Pis an impurity element and serves as the origin of fracture by being
segregated at a grain boundary. For this reason, the P content is limited to 0.100% or
less. The P content is preferably 0.050% or less. The lower limit of the P content is
- 8 -
not particularly limited, but reduction of the P content to less than 0.0001%
significantly increases the dephosphorization cost, which is not preferable
economically. For this reason, the P content may be set to 0.0001% or more.
[0023]
S: 0.1000% or less
S is an impurity element and forms an inclusion in steel. Since this inclusion
serves as the origin of fracture, the S content is limited to 0.1000% or less. The S
content is preferably 0.0500% or less or 0.0300% or less. The lower limit of the S
content is not particularly limited, but reduction of the S content to less than 0.0001%
significantly increases the desulfurization cost, which is not preferable economically.
For this reason, the S content may be set to 0.0001% or more.
[0024]
N: 0.0100% or less
N is an impurity element and forms nitride in steel. Since this nitride serves
as the origin of fracture, theN content is limited to 0.0100% or less. TheN content is
preferably 0.0050% or less. The lower limit of theN content is not particularly
limited, but reduction of the N content to less than 0.0001% significantly increases the
denitrification cost, which is not preferable economically. For this reason, theN
content may be set to 0.0001% or more.
[0025]
The remainder of the chemical composition of the steel sheet for hot stamping
may be Fe and impurities. Elements, which are unavoidably mixed from a steel raw
material or scrap and/or during the manufacture of steel and are allowed in a range
where the characteristics of the hot-stamping formed body according to this
embodiment do not deteriorate, are exemplary examples of the impurities.
- 9 -
[0026]
The steel sheet for hot stamping may contain the following elements as
arbitrary elements instead of a part of Fe. The contents of the following arbitrary
elements, which are obtained in a case where the following arbitrary elements are not
contained, are 0%.
[0027]
Nb: 0% to 0.15%
Ti: 0% to 0.15%
V: 0% to 0.15%
Nb and Ti have an effect on improvement in the strength of the hot-stamping
formed body by precipitation hardening by forming a carbonitride in steel. In order
to reliably exhibit this effect, the content of even one ofNb, Ti, and Vis preferably set
to 0.05% or more. On the other hand, in a case where the content of even one of Nb,
Ti, and Vis set to more than 0.15%, a large amount of a carbonitride is formed in steel,
and the ductility of the hot-stamping formed body deteriorates. Therefore, the Nb
content, Ti content, and V content are each set to 0.15% or less.
[0028]
Mo: 0% to 1.0%
Cr: 0% to 1.0%
Cu: 0% to 1.0%
Ni: 0% to 1.0%
Mo and Cr have an action of increasing the strength of the hot-stamping
formed body by forming a solid solution in prior austenite grains during heating before
hot stamping. In order to reliably obtain this effect, the content of even one ofMo, Cr,
Cu, and Ni is preferably set to 0.05% or more. On the other hand, since the effect is
- 10 -
saturated even when a large amount of Mo, Cr, Cu, and Ni are contained, the Mo
content, the Cr content, the Cu content, and the Ni content are each preferably set to
1.0% or less.
[0029]
B: 0% to 0.0100%
B is an element that improves the hardenability of steel. In order to reliably
obtain this effect, the B content is preferably set to 0.0001% or more. On the other
hand, even when the B content is set to more than 0.0100%, the effect on improvement
in the hardenability is saturated. For this reason, the B content is set to 0.0100% or
less.
[0030]
Ca: 0% to 0.010%
REM: 0% to 0.30%
Ca and REM are elements that improve the distortion capability by
suppressing the formation of an oxide serving as the origin of fracture and improve the
bendability of the hot-stamping formed body. In order to reliably obtain this effect,
the content of even one of Ca and REM is preferably set to 0.001% or more. On the
other hand, since the effect is saturated even when a large amount of Ca and REM are
contained, theCa content is set to 0.010% or less, and the REM content is set to 0.30%
or less.
[0031]
In this embodiment, REM refers to a total of 17 elements that are composed
of Sc, Y, and lanthanoid and the REM content refers to the total content of these
elements.
[0032]
- 11 -
The above-mentioned chemical composition of the steel sheet for hot
stamping may be measured by an ordinary analysis method. For example, the
chemical composition of the above-mentioned hot-stamping formed body may be
measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES).
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. In a case where
a plating layer is provided on the surface of the steel sheet for hot stamping, the
chemical composition may be analyzed after the plating layer is removed by
mechanical grinding.
[0033]
Next, the metallographic structure of the steel sheet for hot stamping for
manufacturing the hot-stamping formed body according to the present embodiment by
hot stamping will be described.
The steel sheet for hot stamping has a metallographic structure consisting of,
by area ratio, a total of 20% to 80% of ferrite, granular bainite, bainite, and martensite
and the remainder in microstructure consisting of pearlite and a carbide. Regarding
the metallographic structure to be described below, "%" indicates "area%" in all cases.
[0034]
Ferrite, granular bainite, bainite, and martensite: 20% to 80%
Ferrite, granular bainite, bainite, and martensite are necessary structures to
obtain a desired texture in a hot-stamping formed body. When the total area ratio of
these structures is less than 20%, it is not possible to obtain a desired texture in the hotstamping
formed body. For this reason, the area ratio of the ferrite is set to 20% or
more. The area ratio of the ferrite is preferably 30% or more or 40% or more. On
the other hand, when the area ratio of these structures is more than 80%, carbon is
- 12 -
concentrated in pearlite, which is the remainder, it becomes difficult for a carbide to
dissolve during hot stamp heating, and the carbide serves as the origin of cracking
during distortion. Therefore, the area ratio is set to 80% or less. The area ratio is
preferably 70% or less or 60% or less.
[0035]
Remainder in microstructure: Pearlite and carbide
The remainder in microstructure of the metallographic structure of the steel
sheet for hot stamping consists of pearlite and a carbide. In the metallographic
structure of the steel sheet for hot stamping, structures other than the above-mentioned
structure, pearlite, and the carbide are not contained, the area ratio of the remainder in
microstructure may be set to 20% to 80%.
[0036]
Measurement method of metallographic structure of steel sheet for hot
stamping
A sample is cut out from an arbitrary position away from an end surface of the
steel sheet for hot stamping by a distance of 50 mm or more (a position that avoids an
end portion in a case where the sample cannot be collected at this position) so that a
sheet thickness-cross section parallel to a rolling direction can be observed. The size
of the sample also depends on a measurement device, but is set to a size that can be
observed by about 10 mm in the rolling direction.
[0037]
After being polished using silicon carbide paper having a grit of #600 to
#1500, the cross section of the sample is finished as a mirror surface using liquid in
which diamond powder having a grain size in the range of 1 J.lm to 6 J.lm is dispersed in
diluted solution of alcohol or the like or pure water and finish-polished using a
- 13 -
colloidal silica solution. Next, analysis is performed in a region that has a length of
50 11m and is present between a depth corresponding to 118 of the sheet thickness from
the surface and a depth corresponding to 3/8 of the sheet thickness from the surface at
an arbitrary position on the cross section of the sample in a longitudinal direction at an
analysis rate of 200 to 300 points/second using an EBSD analyzer including a schottky
emission scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and
an EBSD detector (DVC 5-type detector manufactured by TSL Solutions). The area
ratio of a region where the crystal structure is bee is calculated using a "Phase Map"
function installed in software "OIM Analysis (registered trademark)" included in an
EBSD analyzer, whereby the total area ratio of the ferrite, the granular bainite, the
bainite, and the martensite can be obtained.
[0038]
The pearlite and the carbide can be identified by the following method.
After being polished using silicon carbide paper having a grit of #600 to #1500, the
cross section of the sample is finished as a mirror surface using liquid in which
diamond powder having a grain size in the range of 1 11m to 6 11m is dispersed in
diluted solution of alcohol or the like or pure water and Nital etching is performed.
Then, photographs having a plurality of visual fields are taken using a schottky
emission scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) in a
region that has a length of 50 11m and is present between a depth corresponding to 1/8
of the sheet thickness from the surface and a depth corresponding to 3/8 of the sheet
thickness from the surface at an arbitrary position on the cross section of the sample in
a longitudinal direction. Evenly spaced grids are drawn in the taken photographs, and
structures at grid points are identified. The number of grid points corresponding to
each structure is obtained and is divided by the total number of grid points, so that the
- 14 -
area ratio of each structure is obtained. The area ratio can be more accurately
obtained as the total number of grid points is larger. In this embodiment, grid
spacings are set to 2 Jlm x 2 Jlm and the total number of grid points is set to 1500.
Particles with bright brightness are regarded as the carbide, and a region where regions
with bright brightness are disposed in a granular or sheet shape and in a lamellar shape
is regarded as the pearlite.

CLAIMS
mass%:
1. A hot-stamping formed body comprising, as a chemical composition, by
C: 0.15 to 0.50%;
Si: 0.0010% to 3.000%;
Mn: 0.30% to 3.00%;
Al: 0.0002% to 2.000%;
P: 0.100% or less;
S: 0.1000% or less;
N: 0.0100% or less;
Nb: 0% to 0.15%;
Ti: 0% to 0.15%;
V: 0% to 0.15%;
Mo: 0% to 1.0%;
Cr: 0% to 1.0%;
Cu: 0% to 1.0%;
Ni: 0% to 1.0%;
B: 0% to 0.0100%;
Ca: 0% to 0.010%;
REM: 0% to 0.30%; and
a remainder consisting of Fe and an impurity,
wherein the hot-stamping formed body has a metallographic structure
consisting of, by area ratio, a total of 10% to 30% of ferrite and granular bainite and a
remainder in microstructure consisting of one or more of martensite, bainite, and
tempered martensite,
- 57 -
in a texture between a surface and a sheet thickness 1/4 position from the
surface, a ratio between a pole density of an orientation group consisting of { 001 } < 1-
1 0> to { 001 } < -1-1 0> and a pole density of an orientation group consisting of { 111 }
<1-10> to { 111} <-1-12> is less than 1.8, and
in a texture between the sheet thickness 1/4 position from the surface and a
sheet thickness 1/2 position from the surface, a ratio between a pole density of an
orientation group consisting of {001} <1-10> to {001} <-1-10> and a pole density of
an orientation group consisting of { 111 } < 1-10> to { 111 } < -1-12> is less than 2.3.
2. The hot-stamping formed body according to claim 1, further comprising,
as the chemical composition, by mass%, one or more of the group consisting of:
Nb: 0.05% to 0.15%,
Ti: 0.05% to 0.15%,
V: 0.05% to 0.15%,
Mo: 0.05% to 1.0%,
Cr: 0.05% to 1.0%,
Cu: 0.05% to 1.0%,
Ni: 0.05% to 1.0%,
B: 0.0001% to 0.0100%,
Ca: 0.001% to 0.010%, and
REM: 0.001% to 0.30%.
3. The hot-stamping formed body according to claim
wherein a decarburization index is 0.085 or more.