[Designation of Document] SPECIFICATION
[Title of the Invention] HOT STAMPED ARTICLE, METHOD OF PRODUCING
HOT STAMPED ARTICLE, ENERGY ABSORBING MEMBER, AND METHOD OF
PRODUCING ENERGY ABSORBING MEMBER
[Technical Field]
[0001]
The present invention relates to a hot stamped article excellent in local
deformability, a method of producing the hot stamped article, an energy absorbing
member having a difference in tensile strength by 200 MPa or more in a member, and a
method of producing the energy absorbing member.
[0002]
Priority is claimed on Japanese Patent Application No. 2011-108397, filed on
May 13, 2011, Japanese Patent Application No. 2011-108564, filed on May 13, 2011,
Japanese Patent Application No. 2011-198160, filed on September 12, 2011, and
Japanese Patent Application No. 2011-198261, filed on September 12, 2011, the
contents of which are incorporated herein by reference.
[Background Art]
[0003]
In recent years, an examination for applying a high-strength steel sheet to the
vehicle body has been actively made to reduce the weight of a vehicle body fi-om the
viewpoint of global environment protection, and thus strength demanded for a steel
material has been increasing. However, workability of a steel sheet deteriorates as the
strength of the steel sheet increases, and thus the shape-freezing properties need to be
considered.
[0004]
- 1 -
On the other hand, in commonly used press working, a forming load gradually
increases, and thus there is a significant problem with improvement in pressing
capability in terms of being put into practical use.
[0005]
In a hot stamping technology, press forming is carried out after heating a steel
sheet to a high temperature of an austenite range. Accordingly, the forming load is
greatly reduced compared to common press working that is carried out at room
temperature.
[0006]
In addition, in the hot stamping technology, a hardening treatment is carried
out concurrently with the press working by cooling the steel sheet in a die, and thus
strength corresponding to the content of C in steel may be obtained. Accordingly, the
hot stamping technology has attracted attention as a technology of making the shape
freezing properties and the strength compatible with each other.
[0007]
Patent Document 1 discloses a method of obtaining a hot stamped article
having tensile strength of 980 MPa or more as a hot stamping technology. However,
in this method, it is difficult to obtain a hot stamped article having tensile strength
lower than 980 MPa.
[0008]
Patent Document 2 and Patent Document 3 disclose a technology related to a
member using a hot stamping material with low tensile strength, and a production
method thereof, and a technology related to a member by a tailored blank to which the
technology is applied. However, in these technologies, consideration is not made for
delayed fracture characteristics and toughness, and thus it is difficult to say that
performance as a member is sufficient.
[Prior Art Document]
[Patent Document]
[0009]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. 2005-097725
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2005-248320
[Patent Document 3] Japanese Unexamined Patent Application, First
Publication No. 2006-200020
[Disclosure of the Invention]
[Problem that the Invention is to solve]
[0010]
Vehicle parts, particularly, parts such as a frame, a member, and reinforcement
are classified into (1) parts that efficiently absorb energy during collision, and (2) parts
that have a sufficient proof stress and transmit energy without deformation during
collision according to functions.
[0011]
Particularly, demanded strength for the frame and member gradually increases,
and a member having both characteristics of axial compression deformation and
bending deformation is demanded. As a method of realizing this, utilization of hot
stamping is considered.
[0012]
That is, it is necessary to construct a portion with low strength in a member by
adjusting a component composition in order for a difference in strength to occur after
hardening with hot stamping by utilizing a tailored blank material.
[0013]
A problem to be solved by the present invention is to realize the abovedescribed
configuration, particularly, when considering the axial compression
deformation, and an object of the present invention is to provide a hot stamped article
that has tensile strength less than 980 MPa and is excellent in local deformability, a
method of producing the hot stamped article, an energy absorbing member having a
difference in strength in a member, and a method of producing the energy absorbing
member.
[Means for Solving the Problems]
[0014]
The present inventors have extensively studied to accomplish the abovedescribed
object. As a result, the present inventors have found that when a
component composition of steel and a condition of hot stamping are optimized, the
above-described object may be accomplished due to synergism of these.
[0015]
The present invention has been made on the basis of the above-described
finding, and the gist thereof is as follows.
[0016]
(1) According to a first aspect of the present invention, there is provided a hot
stamped article that is obtained by hot stamping a steel sheet for hot stamping. The
hot stamped article has a component composition containing, in terms of %> by mass,
0.002% to 0.1% of C, 0.01% to 0.5% of Si, 0.5% to 2.5% of Mn+Cr, 0.1% or less of P,
0.01% or less of S, 0.05% or less of t-Al, 0.005% or less of N, and 0.0005% to 0.004%
of B which is optionally contained in a case where the Mn+Cr is 1.0%) or more, the
- 4 -
remainder being Fe and unavoidable impurities. The hot stamped article has a
microstructure composed of, in terms of an area ratio, 0% or more and less than 90%
of martensite, 10%) to 100%) of bainite, and less than 0.5%) of imavoidable inclusion
structures, or a microstructure composed of, in terms of an area ratio, 99.5%> to 100%)
of bainitic ferrite, and less than 0.5%) of unavoidable inclusion structures.
(2) In the hot stamped article according to (1), a plated layer may be provided
on a surface of the hot stamped article.
(3) In the hot stamped article according to (1) or (2), the component
composition may further contain one or more kinds selected from, in terms of %> by
mass, 0.001% to 0.1% of Ti, 0.001% to 0.05% of Nb, 0.005% to 0.1% of V, and 0.02%
to 0.5% of Mo.
(4) In the hot stamped article according to any one of (1) to (3), in a case
where the Mn+Cr is less than 1.0%), the component composition may further contain,
in terms of % by mass, 0.0005% to 0.004% of B.
(5) According to a second aspect of the present invention, there is provided an
energy absorbing member including the hot stamped article according to any one of (1)
to (4), and a joint member which is joined to the hot stamped article and has tensile
strength of 1180 MPa or more. A difference in tensile strength between the hot
stamped article and the joint member is 200 MPa or more.
(6) According to a third aspect of the present invention, there is provided a
method of producing a hot stamped article. The method includes: a heating process
of heating a slab in order for a surface temperature to be in a temperature range of Ar3
point to 1400°C, the slab having a component composition containing, in terms of %>
by mass, 0.002% to 0.1% of C, 0.01% to 0.5% of Si, 0.5% to 2.5% of Mn+Cr, 0.1% or
less of P, 0.01% or less of S, 0.05% or less of t-Al, 0.005% or less of N, and 0.0005%
to 0.004% of B which is optionally contained in a case where the Mn+Cr is 1.0% or
more, the remainder being Fe and unavoidable impurities; a hot rolling process of
subjecting the heated slab to finish rolling in which a total rolling reduction at a final
stand and an immediately previous stand of the final stand is set to 40% or more in a
temperature range state in which the surface temperature is Ar3 point to 1400°C, and
initiating cooling within one second after the finish rolling to produce a hot-rolled steel
sheet; a coiling process of coiling the hot-rolled steel sheet in a temperature range of
650°C or lower; and a hot stamping process of using the hot-rolled steel sheet as a steel
sheet for hot stamping, forming the steel sheet for hot stamping using a die in a state in
which the steel sheet is heated to a temperature of Ac3 point or higher, cooling the steel
sheet for hot stamping in the die at a cooling rate exceeding 100 °C/second in a case
where the Mn+Cr is less than 1.0%, or cooling the steel sheet for hot stamping in the
die at a cooling rate of 10 °C/second to 100 °C/second in a case where the Mn+Cr is
1.0% or more to produce the hot stamped article having a microstructure composed of,
in terms of an area ratio, 0% or more and less than 90% of martensite, 10% to 100% of
bainite, and less than 0.5% of unavoidable inclusion structures, or a microstructure
composed of, in terms of an area ratio, 99.5% to 100% of bainitic ferrite, and less than
0.5% of unavoidable inclusion structures.
(7) The method of producing a hot stamped article according to (6) may
fiirther include a plating process of carrying out a plating treatment with respect to the
hot-rolled steel sheet before the hot stamping process. In the hot stamping process,
the hot-rolled steel sheet to which the plating treatment is carried out may be used as
the steel sheet for hot stamping.
(8) The method of producing a hot stamped article according to (6) may
further include a cold rolling process of producing a cold-rolled steel sheet by carrying
out cold rolling with respect to the hot-rolled steel sheet before the hot stamping
process. In the hot stamping process, the cold-rolled steel sheet may be used as the
steel sheet for hot stamping.
(9) The method of producing a hot stamped article according to (6) may
further include a cold rolling process of producing a cold-rolled steel sheet by carrying
out cold rolling with respect to the hot-rolled steel sheet before the hot stamping
process, and a plating treatment process of carrying out a plating treatment with respect
to the cold-rolled steel sheet. In the hot stamping process, the cold-rolled steel sheet
to which the plating treatment is carried out may be used as the steel sheet for hot
stamping.
(10) The method of producing a hot stamped article according to (6) may
further include a cold rolling process of producing a cold-rolled steel sheet by carrying
out cold rolling with respect to the hot-rolled steel sheet before the hot stamping
process, and a continuous annealing process of carrying out continuous annealing with
respect to the cold-rolled steel sheet. In the hot stamping process, the cold-rolled
steel sheet to which the continuous annealing is carried out may be used as the steel
sheet for hot stamping.
(11) The method of producing a hot stamped article according to (6) may
further include a cold rolling process of producing a cold-rolled steel sheet by carrying
out cold rolling with respect to the hot-rolled steel sheet before the hot stamping
process, a continuous annealing process of carrying out continuous annealing with
respect to the cold-rolled steel sheet, and a plating treatment process of carrying out a
plating treatment with respect to the cold-rolled steel sheet to which the continuous
annealing is carried out. In the hot stamping process, the cold-rolled steel sheet to
which the continuous annealing and the plating treatment are carried out may be used
- 7 -
as the steel sheet for hot stamping.
(12) In the method of producing a hot stamped article according to any one of
(6) to (11), the slab may further contain one or more kinds selected from, in terms of %
by mass, 0.001% to 0.1% of Ti, 0.001% to 0.05% of Nb, 0.005% to 0.1% of V, and
0.02% to 0.5% of Mo.
(13) In the method of producing a hot stamped article according to any one of
(6) to (12), in a case where the Mn+Cr is less than 1.0%, the slab may contain, in terms
of % by mass, 0.0005% to 0.004% of B.
(14) According to a fourth aspect of the present invention, there is provided a
method of producing an energy absorbing member. The method includes: a joining
process of joining the steel sheet for hot stamping according to any one of (6) to (13) to
a steel sheet for joint to produce a joined steel sheet; and a hot stamping process of
forming the joined steel sheet using a die in a state in which the joined steel sheet is
heated to a temperature of Ac3 point or higher, and cooling the joined steel sheet in the
die at a cooling rate exceeding 100 °C/second in a case where the Mn+Cr is less than
1.0%, or cooling the joined steel sheet in the die at a cooling rate of 10 °C/second to
100 °C/second in a case where the Mn+Cr is 1.0% or more so as to set a difference in
tensile strength between a portion corresponding to the steel sheet for hot stamping and
a portion corresponding to the steel sheet for joint in the joined steel sheet to 200 MPa
or more.
[Advantage of the Invention]
[0017]
According to the present invention, in a case of producing parts utilizing a
tailored blank, strength after hot stamping may be suppressed to be low with respect to
an axially compression-deformed portion, and thus local deformability may be applied
- 8 -
to the parts. As a result, a member, which is excellent in energy absorbing
characteristics during axial compression deformation and bending deformation, may be
produced.
[Brief Description of the Drawing]
[0018]
FIG. 1 is a diagram illustrating a relationship between the content of C and
tensile strength of a hot stamped article.
FIG. 2 is a diagram illustrating a relationship between a cooling rate during
hot stamping and tensile strength of the hot stamped article.
FIG. 3 is a diagram illustrating a shape of a test specimen for delayed fracture
evaluation.
FIG. 4 is a diagram illustrating a member in which a backboard is attached to
a hat type joint member obtained by hot stamping a joined steel sheet (tailored blank
material), a weld line position in the joined steel sheet, and a load direction during
axial compression deformation.
[Description of Embodiment]
[0019]
First, experiments carried out to complete the present invention will be
described.
[0020]
The present inventors have focused on the content of Mn+Cr which has a
great effect on hardenability, and have carried out the following experiments with
respect to each of a component composition in which the content of Mn+Cr is less
(less than 1.0% by mass), and a component composition in which the content of
Mn+Cr is much (1.0% by mass or more).
9 -
[0021]
The present inventors have investigated a relationship between the content of
C and tensile strength (TS) of steel during a heat treatment under conditions of
reproducing thermal history in hot stamping, that is, conditions of heating to 900°C
and then cooling to room temperature at 200 °C/second by using cold-rolled annealed
sheets shown in Table 1, which have component compositions in which the content of
Mn+Cr is less than 1.0% and boron is not contained, and which have a sheet thickness
of 1.6 mm.
In addition, the present inventors have investigated a relationship between the
content of C and tensile strength (TS) during a heat treatment under conditions of
reproducing thermal history in hot stamping, that is, conditions of heating to 900°C
and then cooling to room temperature at 50 °C/second by using cold-rolled annealed
sheets shown in Table 2, which have component compositions in which the content of
Mn+Cr is 1.0% or more and boron is contained, and which have a sheet thickness of
1.6 mm. In addition, in the component compositions shown in Table 2, an
appropriate amount of boron is added to obtain a sufficient hardening effect even at a
cooling rate (50 °C/second) that is set to be slower compared to the cooling rate of 200
°C/second.
- 10
MIWBfap-'*"'"—»«
[0022]
No.
1
2
3
4
5
6
7
8
9
10
11
[Table 1]
C 1 Si
0.017
0.023
0.035
0.049
0.057
0.079
0.097
0.115
0.151
0.188
0.212
0.22
0.14
0.15
0.13
0.14
0.15
0.15
0.13
0.16
0.15
0.14
Mn
0.75
0.98
0.81
0.58
0.81
0.75
0.68
0.74
0.88
0.88
0.48
Cr
0.22
0.01
0.18
0.25
0.18
0.19
0.22
0.22
0.05
0.11
0.49
P 1 S
mass%
0.008
0.012
0.014
0.015
0.015
0.012
0.016
0.014
0.015
0.015
0.014
0.0042
0.0018
0.0025
0.0027
0.0021
0.0018
0.0019
0.0023
0.0015
0.0018
0.0021
t-Al 1 B 1 N
0.028
0.035
0.027
0.034
0.032
0.026
0.033
0.028
0.025
0.031
0.033
-
-
---------
0.0025
0.0028
0.0034
0.0029
0.0032
0.0022
0.0033
0.0029
0.0033
0.0029
0.0031
Mn+Cr
0.97
0.99
0.99
0.83
0.99
0.94
0.90
0.96
0.93
0.99
0.97
Ac3
°C
888
878
875
878
867
856
857
846
832
825
831
M
0
0
70
60
60
80
85
100
100
100
100
Microstructure (area ratio)
B
100
100
30
40
40
20
15
0
0
0
0
Others
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
M:martensite, B: bainite. Others: unavoidable inclusion structures
[0023]
No.
1'
2'
3'
4'
5'
6'
7'
8'
9'
10'
11'
[Table 2]
C Si
0.017
0.023
0.035
0.049
0.057
0.079
0.097
0.115
0.151
0.188
0.212
0.22
0.14
0.15
0.13
0.14
0.15
0.15
0.13
0.16
0.15
0.14
Mn
0.75
1.24
1.35
1.27
1.08
1.35
1.23
1.32
1.05
1.22
1.19
Cr
0.37
0.22
0.18
0.25
0.37
0.19
0.22
0.31
0.72
0.24
0.36
P
mass
0.008
0.012
0.014
0.015
0.015
0.012
0.016
0.014
0.015
0.015
0.014
S
%
0.0042
0.0018
0.0025
0.0027
0.0021
0.0018
0.0019
0.0023
0.0015
0.0018
0.0021
t-Al
0.028
0.035
0.027
0.034
0.032
0.026
0.033
0.028
0.025
0.031
0.033
B
0.0009
0.0015
0.0008
0.0012
0.0007
0.0017
0.0010
0.0011
0.0008
0.0017
0.0012
N
0.0025
0.0028
0.0034
0.0029
0.0032
0.0022
0.0033
0.0029
0.0033
0.0029
0.0031
Mn+Cr
1.12
1.46
1.53
1.52
1.45
1.54
1.45
1.63
1.77
1.46
1.55
Ac3
"C
888
871
859
857
859
838
841
828
827
815
810
M
0
0
70
60
60
80
85
100
100
100
100
Microstructure (area ratio)
B
100
100
30
40
40
20
15
0
0
0
0
Others
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
M: martensite, B: bainite, Others: unavoidable inclusion structures
11 -
[0024]
No. 5 test specimens were prepared from a steel sheet after being subjected to
a heat treatment on the basis of JIS Z 2241 (2011), and a tensile test was carried out.
Results that were obtained are shown in FIG. 1. In FIG. 1, "o" represents a resuh of
steel corresponding to Table 1, and " • " represents a result of steel corresponding to
Table 2.
[0025]
From Table 1, Table 2, and FIG. 1, it was found that it is necessary to set the
content of C in steel to 0.1% by mass or less so as to make tensile strength after hot
stamping less than 980 MPa. When confirming a microstructure of a test specimen in
which tensile strength after hot stamping was less than 980 MPa, it was found that the
microstructure was composed of less than 90% of martensite, 10% or more of bainite,
and less than 0.5% of unavoidable inclusion structures.
[0026]
Furthermore, a steel sheet of No. 5 in Table 1 and a steel sheet of No. 5' in
Table 2 were used. These steel sheets were heated to 900°C at a heating rate of 10
°C/second and were heat-retained for 20 seconds, and then were immediately cooled to
room temperature at various cooling rates. Then, a tensile test was carried out by the
same method as the above-described tensile test, and hole expansibility that exhibited a
good correlation with local deformability was examined.
[0027]
The examination of the hole expansibility was carried out by a method
described in JIS Z 2256 (2010). That is, a hole with a diameter 10 mm (do) was
punched in each of the steel sheets, and the hole was expanded by using a conical
punch of 60° in such a manner that a burr was formed at an outer side. Then, a hole
- 12 -
diameter (d) at the point of time at which cracking penetrates through a sheet thickness
was measured, and evaluation was carried out by A, (= ((d-do)/do)xlOO).
[0028]
A relationship between the cooling rate and the tensile strength after the hot
stamping is shown in FIG. 2. In FIG. 2, steel sheets, which are evaluated as X>50%,
are plotted with rectangles (a case in which Mn+Cr is less than 1.0%: n, and a case in
which Mn+Cr is 1.0% or more: • ) , steel sheets, which are evaluated as A,<50%, are
plotted with triangles (a case in which Mn+Cr is less than 1.0%: A, and a case in which
Mn+Cr is 1.0% or more: A).
[0029]
As can be from FIG. 2, in a component composition in which Mn+Cr is less
than 1.0% (plotted with n and A), in a case where the cooling rate is 100 °C/second or
less, a structure becomes "ferrite + pearlite" or "ferrite + bainite", and the hole
expansibility deteriorates due to a difference in hardness in the structure, and thus the
local deformability is not sufficient. As a result, particularly, stable deformation
behavior may not be obtained during axial compression deformation.
[0030]
In addition, in a component composition in which Mn+Cr is less than 1.0%
(plotted with n and A), when a steel sheet is cooled at a cooling rate exceeding 100
°C/second, a structure including "bainite", "martensite", or "bainite + martensite" may
be obtained, and thus tensile strength exceeding 450 MPa may be obtained, and X is
50% or more. Accordingly, particularly, a stable deformation behavior may be
obtained during axial compression deformation.
[0031]
Furthermore, as can be seen from FIG. 2, in a component composition in
- 13 -
which Mn+Cr is 1.0% or more (plotted with • and A), in a case where the cooling rate
is less than 10 °C/second, a structure becomes "ferrite + pearlite" or "ferrite + bainite",
and the hole expansibility deteriorates due to a difference in hardness in the structure,
and thus the local deformability is not sufficient. As a result, particularly, a stable
deformation behavior may not be obtained during axial compression deformation.
Therefore, it can be understood that it is necessary to set the lower limit of the cooling
rate to 10 °C/second, and preferably 30 °C/second. On the other hand, when the steel
sheet is cooled at a cooling rate exceeding 100 °C/second, tensile strength exceeding
980 MPa is obtained, and thus particularly, stable deformation behavior may not be
obtained during axial compression deformation. Accordingly, it can be understood
that it is necessary to set the upper limit of the cooling rate to 100 °C/second, and
preferably 70 °C/second.
[0032]
On the basis of the experimental results, the present inventors have found that
when the component composition of the hot stamped article is controlled to obtain a
microstructure composed of, in terms of an area ratio, 0% or more and less than 90%
of martensite, 10% to 100% of bainite, and less than 0.5%) of unavoidable inclusion
structures, or a microstructure composed of, in terms of an area ratio, 99.5%) to 100%
of bainitic ferrite, and less than 0.5% of unavoidable inclusion structures, excellent
local deformability may be applied to the hot stamped article. Hereinafter, the present
invention accomplished on the basis of the above-described finding will be described
in detail with reference to embodiments.
[0033]
(First Embodiment)
The first embodiment of the present invention relates to a hot stamped article
- 14 -
that may be obtained by hot-stamping a steel sheet for hot stamping.
[0034]
First, a microstructure of the hot stamped article according to this embodiment
will be described. % related to the microstructure represents an area ratio. In
addition, with regard to each structure, the area ratio is calculated by carrying out
image analysis with respect to a scanning electron microscope (SEM) photograph.
[0035]
(Martensite: 0% or more and less than 90%)
The microstructure of the hot stamped article according to this embodiment
contains less than 90% of martensite. When martensite is set to 90% or more, the
tensile strength of the hot stamped article may not be suppressed to 980 MPa or less.
On the other hand, an area ratio of martensite may be 0%. It is preferable that the
area ratio of martensite be 85% or less, and more preferably 80% or less.
[0036]
(Bainite: 10% to 100%)
The microstructure of the hot stamped article according to this embodiment
contains 10% to 100% of bainite in addition to 0% or more and less than 90% of
martensite. Since a difference in hardness between martensite and bainite is small,
even when both of these are mixed in, there is no great effect on the hole expansibility.
That is, satisfactory local deformability may be obtained. In a case where bainite is
less than 10%, since martensite as the remainder increases, it is difficult to suppress the
tensile strength of the hot stamped article to 980 MPa or less. Therefore, it is
preferable that the lower limit of the area ratio of bainite be 15%, and more preferably
20%. On the other hand, it is preferable that the upper limit of the area ratio of
bainite be 100%. However, the upper limit may be 99.5% when considering
- 15 -
unavoidable inclusion structures to be described later.
[0037]
(Bainitic Ferrite: 99.5% to 100%)
In addition, in a case of using steel having a component composition in which
the content of C is 0.01% or less, an amount of cementite that precipitates by hot
stamping is not sufficient, and thus it is difficult to obtain a bainitic structure.
Therefore, the microstructure of the hot stamped article according to this embodiment
may be a microstructure that is substantially composed of bainitic ferrite, that is, a
microstructure including 99.5% or more of bainitic ferrite. In a case where the area
ratio of the bainitic ferrite is less than 99.5%, there is a concern that the hole
expansibility may decrease due to a difference in hardness with other structures, and
thus the lower limit is set to 99.5%.
[0038]
(Unavoidable Inclusion Structures: less than 0.5%)
The microstructure of the hot stamped article according to this embodiment
may contain structures such as ferrite (ferrite other than bainitic ferrite) and pearlite as
long as the structures are contained in a ratio of 0.5% or less. However, these
structures have a large difference in hardness with martensite, and apply a difference in
hardness to the inside of the hot stamped article. Therefore, the hole expansibility
deteriorates, thereby leading to a deterioration in the local deformability. Therefore,
it is preferable to reduce the structures as much as possible.
[0039]
As described above, the hot stamped article according to this embodiment has
a microstructure composed of, in terms of an area ratio, 0% or more and less than 90%
of martensite, 10% to 100% of bainite, and less than 0.5% of unavoidable inclusion
- 16 -
structures, or a microstructure composed of, in terms of an area ratio, 99.5% to 100%
of bainitic ferrite, and less than 0.5% of unavoidable inclusion structures.
[0040]
Next, a component composition of the hot stamped article (and a slab that is a
raw material thereof) according to this embodiment will be described. In addition, %
related to the component composition represents % by mass.
[0041]
(C: 0.002% to 0.1%)
C is an element that determines strength, and is an element that has a great
effect on strength, particularly, after hardening. In the present invention, the tensile
strength of the hot stamped article is set to be less than 980 MPa, and thus the upper
limit of the content of C is set to 0.1%, preferably 0.06%, and more preferably 0.05%.
On the other hand, when decarburization is carried out to a low carbon range, the
decarburization cost increases, and it is difficult to obtain necessary strength within a
range less than 980 MPa. Therefore, the lower limit of the content of C is set to
0.002%, preferably 0.005%, and more preferably 0.01%.
[0042]
(Si: 0.01% to 0.5%)
Si is a solid-solution strengthening element, and thus Si is added in a ratio of
0.01% or more. However, when Si is added in a ratio of more than 0.5%, plating
properties deteriorate, and thus the upper limit thereof is set to 0.5%. It is preferable
that the lower limit of the content of Si be 0.05%, and more preferably 0.1%. In
addition, it is preferable that the upper limit of the content of Si be 0.4%, and more
preferably 0.3%.
[0043]
- 17 -
(Mn+Cr: 0.5% to 2.5%)
Mn and Cr are elements that are added to secure hardenability. When the
content of Mn+Cr is less than 0.5%, sufficient hardenability may not be secured.
Therefore, the lower limit of the content of Mn+Cr is set to 0.5%, preferably 0.6%, and
more preferably 0.7%). On the other hand, when the content of Mn+Cr exceeds 2.5%),
hardenability increases, and thus it is difficult to suppress tensile strength to be low.
Therefore, the upper limit of Mn+Cr is set to 2.5%, preferably 2.3%), and more
preferably 2.0%).
[0044]
As described later, when the content of Mn+Cr is less than 1.0%), a
microstructure composed of, in terms of an area ratio, 0% or more and less than 90%
of martensite, 10%) to 100% of bainite, and less than 0.5%) of unavoidable inclusion
structures, or a microstructure composed of, in terms of an area ratio, 99.5%) to 100%
of bainitic ferrite, and less than 0.5% of unavoidable inclusion structures is made by
performing cooling at a cooling rate exceeding 100 °C/second during hot stamping.
When using this cooling condition, it is preferable that the content of Mn+Cr be 0.9%o
or less, and more preferably 0.5%) or less so as to suppress formation of ferrite to the
utmost.
On the other hand, when the content of Mn+Cr is 1.0% or more, the
microstructure composed of, in terms of an area ratio, 0% or more and less than 90%)
of martensite, 10%) to 100% of bainite, and less than 0.5%) of unavoidable inclusion
structures, or a microstructure composed of, in terms of an area ratio, 99.5% to 100%o
of bainitic ferrite, and less than 0.5%) of unavoidable inclusion structures is made by
performing cooling at a cooling rate of 10 °C/second to 100 °C/second during hot
stamping. When using this cooling condition, it is preferable that the content of
- 18 -
Mn+Cr be 1.4% or more, and more preferably 1.5% or more.
[0045]
The lower limit of the content of Mn may be set to 0.1%, and preferably 0.5%,
and the upper limit may be set to 1.5%.
The lower limit of the content of Cr may be set to 0.01%, and preferably 0.2%,
and the upper limit may be set to 1.5%.
[0046]
(P: 0.1% or less)
P is a solid-solution strengthening element, and may increase strength of a
steel sheet at relatively low cost. However, P is an element that has a tendency to
precipitate at a grain boundary, and causes low-temperature embrittlement in a case
where strength is high. Therefore, the content of P is limited to 0.1 %> or less. It is
preferable that the content of P be limited to 0.020% or less, and more preferably
0.015% or less. It is preferable that the content of P be as small as possible, but
reduction of P to less than 0.001% may cause an increase in the dephosphorization cost,
and thus the content of P may be set to 0.001% or more.
[0047]
(S: 0.01% or less)
S is an element that deteriorates hot workability, and deteriorates workability
of a steel sheet. Therefore, the content of S is limited to 0.01 %> or less. The content
of S is preferably limited to 0.005%) or less. It is preferable that the content of S be as
small as possible, but reduction of S to less than 0.001%) may cause an increase in the
desulfiirization cost, and thus the content of S may be set to 0.001% or more.
[0048]
(t-Al: 0.05% or less)
- 19
Al is an element that is commonly added for deoxidation. When the content
of t-Al is less than 0.005%), deoxidation is not sufficient, and a large amount of oxides
remain in steel, thereby causing deterioration of local deformability. Therefore, the
content of Al is preferably 0.005%) or more. On the other hand, when the content of
Al exceeds 0.05%), a large amount of oxides mainly composed of alumina remain in
steel, thereby causing deterioration of local deformability. Therefore, it is preferable
that the content of Al be 0.05%) or less, and more preferably 0.04%) or less. In
addition, t-Al represents total aluminum.
[0049]
(N: 0.005% or less)
N is an element which is preferable as less as possible, and N is limited to
0.005% or less. Reduction of the content of N to less than 0.001%) may cause an
increase in the refining cost, and thus the content of N may be set to 0.001%) or more.
On the other hand, when the content of N exceeds 0.003%, precipitates are generated,
and toughness after hardening deteriorates, and thus the content of N is preferably
0.003% or less.
[0050]
(In a case where Mn+Cr is 1.0%) or more, B: 0.0005%o to 0.004%))
In a case where the content of Mn+Cr is 1.0%) or more, B is added in a range
of 0.0005%) to 0.004%. When B is added, even when cooling is carried out at a
cooling rate of 100°C/second or less during hot stamping, hardenability may be
secured.
The lower limit of the content of B may be set to 0.0008%), and preferably
0.0010%) so as to obtain the addition effect of B. However, when the content of B
exceeds 0.004%, the addition effect is saturated, and thus the upper limit of the content
- 20 -
of B is 0.004%, and preferably 0.002%.
In addition, as described later, even in a case in which the content of Mn+Cr is
less than 1.0%, B may be added.
[0051]
The component composition of the hot stamped article according to this
embodiment may contain at least one kind selected from a group consisting of B, Ti,
Nb, V, and Mo as a selective element. That is, the present invention includes a case in
which these elements are 0%.
[0052]
(In a case where Mn+Cr is less than 1.0%, B: 0% to 0.004%)
B is an element that improves hardenability, and thus even in steel in which
the content of C is small, B is added to allow the structure of steel to be composed of
bainite or martensite so as to secure necessary strength.
Accordingly, even in a case where Mn+Cr is less than 1.0%, the lower limit of
the content of B may be set to 0.0005%) to obtain the addition effect of B, and
preferably 0.0008% or 0.0010%. However, when the content of B exceeds 0.004%,
the addition effect is saturated, and thus the upper limit of the content of B is 0.004%,
and preferably 0.002%.
[0053]
(Ti:0%to0.1%)
(Nb: 0% to 0.05%)
Ti and Nb are elements that form fine carbides, and make the grain size of
prior-austenite after hot stamping fine. To obtain an addition effect, the lower limit of
each of Ti and Nb may be set to 0.001%, and preferably 0.01%). On the other hand,
when these elements are excessively added, the addition effect is saturated, and the
- 21 -
production cost increases. Therefore, with regard to the content of Ti, the upper limit
thereof is set to 0.1%, and preferably 0.08%, and with regard to the content of Nb, the
upper limit thereof is set to 0.05%, and preferably 0.03%.
[0054]
(V:0%to0.1%)
V is an element that forms carbides and makes a structure fine. When a steel
sheet is heated to an Ac3 point or higher, fine V carbides suppress recrystallization and
grain growth, thereby making austenite grains fine and improving toughness. When
the content of V is less than 0.005%, the addition effect may not be obtained, and thus
the lower limit of V is set to 0.005%), and preferably 0.01%. On the other hand, when
the content of V exceeds 0.1%, the addition effect is saturated, and the production cost
increases. Therefore, the upper limit of the content of V is set to 0.1%, and preferably
0.07%.
[0055]
(Mo: 0% to 0.5%)
Similar to Ti, Nb, and V, Mo is an element which also forms fine carbides
when a steel sheet is heated to the Ac3 point or higher, suppresses recrystallization and
grain growth, makes austenite grains fine, and improves toughness. When the content
of Mo is less than 0.02%), the addition effect may not be obtained, and thus the lower
limit of the content of Mo may be set to 0.02%», and preferably 0.08%. On the other
hand, when the content of Mo exceeds 0.5%), the addition effect is saturated, and the
production cost increases. Therefore, the upper limit of the content of Mo is set to
0.5%, and preferably 0.3%.
[0056]
In addition, the hot stamped article of the present invention may contain Cu,
- 22 -
Sn, Ni, and the like, which are mixed-in from scrap or the like during a steel-making
stage, in a range not deteriorating the effect of the present invention. In addition, the
hot stamped article may contain Ca that is used as a deoxidizing element, and a REM
including Ce and the like within a range not deteriorating the effect of the invention.
Specifically, the hot stamped article may contain 0.1% or less of Cu, 0.02% or less of
Sn, 0.1% or less of Ni, 0.01% or less of Ca, and 0.01% of REM as unavoidable
impurities.
[0057]
Hereinafter, a method of producing the hot stamped article according to this
embodiment will be described in detail.
[0058]
The method of producing the hot stamped article according to this
embodiment includes at least a heating process, a hot rolling process, and a hot
stamping process. That is, a microstructure composed of, in terms of an area ratio,
0% or more and less than 90% of martensite, 10% to 100% of bainite, and less than
0.5% of unavoidable inclusion structures, or a microstructure composed of, in terms of
an area ratio, 99.5% to 100% of bainitic ferrite, and less than 0.5% of unavoidable
inclusion structures is made by appropriately controlling heating conditions, hot rolling
conditions, and hot stamping conditions.
[0059]
(Heating Process)
In the heating process, a slab having the above-described component
composition is heated in order for a surface temperature to be in a temperature range of
Ar3 point to 1400°C. This is because it is necessary to make a grain size of prioraustenite,
which is obtained after hot stamping, as small as possible from the viewpoint
- 23 -
#
of securing necessary delayed fracture characteristics and toughness. That is, to make
a structure of a hot-rolled sheet stage fine, the heating temperature is set to 1400°C or
lower, and preferably 1250°C or lower. On the other hand, in a case where the
surface temperature exceeds 1400°C, rolling properties deteriorate, and thus the upper
limit of the heating temperature is set to 1400°C.
[0060]
In addition, a method of producing a steel slab that is provided to hot rolling is
not limited to a continuous casting method. A common continuous casting method,
or a method of casting a thin slab having a thickness of 100 mm or less may be
employed.
[0061]
(Hot Rolling Process)
In the hot rolling process, the heated slab is subjected to finish rolling in
which a total rolling reduction at a final stand and an immediately previous stand of the
final stand is set to 40% or more in a temperature range state in which the surface
temperature is Ar3 point to 1400°C, and cooling is initiated within one second after the
finish rolling. According to this, a hot-rolled steel sheet which is used as a steel sheet
for hot stamping is produced.
[0062]
(Coiling Process)
In the coiling process, the hot-rolled steel sheet is coiled in a temperature
range of 650°C or less. In a case of coiling the hot-rolled steel sheet in a temperature
range exceeding 650°C, coil deformation (coil buckling) has a tendency to occur after
coiling, and 650°C is set as the upper limit.
In addition, when the hot-rolled steel sheet is coiled at a temperature lower
- 24 -
J^'
than 400°C, the strength of the hot-rolled steel sheet increases too much, and thus the
coiling temperature is preferably 400°C or higher. However, after being coiled at a
temperature lower than 400°C, the hot-rolled steel sheet may be reheated for the
purpose of softening.
[0063]
(Hot Stamping Process)
In the hot stamping process, the above-described hot-rolled steel sheet is used
a steel sheet for hot stamping, and the steel sheet for hot stamping is formed using a die
in a state in which the steel sheet is heated to a temperature of Ac3 point or higher. In
addition, the steel sheet for hot stamping is cooled in the die at a cooling rate
exceeding 100 °C/second in a case where the Mn+Cr is less than 1.0%, or the steel
sheet for hot stamping is cooled in the die at a cooling rate of 10 °C/second to 100
°C/second in a case where the Mn+Cr is 1.0% or more. When the hot stamping is
carried out under these temperature conditions, a hot stamped article having a
microstructure composed of, in terms of an area ratio, 0% or more and less than 90%
of martensite, 10% to 100% of bainite, and less than 0.5% of unavoidable inclusion
structures, or a microstructure composed of, in terms of an area ratio, 99.5% to 100%
of bainitic ferrite, and less than 0.5% of unavoidable inclusion structures is produced.
[0064]
In addition to using the hot-rolled steel sheet as a steel sheet for hot stamping,
various kinds of steel sheets, which may be obtained by appropriately carrying out cold
rolling, annealing, a plating treatment, and the like with respect to a hot-rolled steel
sheet, may be used as the steel sheet for hot stamping. Each condition of the cold
rolling, annealing, and plating is not particularly defined, and may be a common
condition. The cold rolling may be carried out within a range of a common cold-
- 25 -
#
rolling reduction ratio, for example, 40% to 80%. The plating is carried out after hot
rolling, cold rolling, or recrystallization annealing, but heating conditions or cooling
conditions are not particularly defined. As the plating, Zn plating or Al plating is
mainly preferable. With regard to the Zn plating, an alloying treatment may be
carried out or may not be carried out. With regard to the Al plating, even when Si is
contained in plating, this does not have an effect on the present invention. Rough
rolling of a hot-rolled steel sheet, a cold-rolled steel sheet, an annealed steel sheet, and
a plated steel sheet may be appropriately carried out to appropriately adjust a shape.
[0065]
In the hot stamping process, the steel sheet for hot stamping is heated to an
Ac3 point or higher. When the heating temperature is lower than the Ac3 point, a
region which is not austenized partially occurs. In this region, bainite or martensite is
not generated, and thus sufficient strength across the entirety of a steel sheet may not
be obtained.
[0066]
However, the heating temperature has a great effect on the grain size of prioraustenite,
and when the heating temperature exceeds 950°C, the grain size of the prioraustenite
is enlarged, and thus the heating temperature is preferably 950°C or lower.
[0067]
In addition, the heating time is preferably 5 seconds to 600 seconds. When
the heating time is shorter than 5 seconds, remelting of carbides is not sufficient, and it
is difficult to secure solid-solution C in an amount sufficient for securing strength.
On the other hand, when the heating time exceeds 600 seconds, the grain size of prioraustenite
is enlarged, and thus the local deformability has a tendency to decrease.
[0068]
- 26 -
w
In a case where the content of Mn+Cr is less than 1.0%, the cooling during
hot stamping is carried out at a cooling rate exceeding 100 °C/second. This is
because when the cooling rate is 100 °C/second or less, ferrite or pearlite is generated,
a uniform structure is not obtained, 50% or more of A, is not obtained, and local
deformability deteriorates.
On the other hand, in a case where the content of Mn+Cr is 1.0% or more, the
cooling during hot stamping is carried out at a cooling rate of 10 °C/second to 100
°C/second. This is because when the cooling rate is less than 10 °C/second, ferrite or
pearlite is generated, a uniform structure is not obtained, 50% or more of A, is not
obtained, and local deformability deteriorates. The cooling rate is preferably 25
°C/second or more. When the cooling rate exceeds 100 °C/second, tensile strength
may exceed 980 MPa in some cases, and thus the upper limit of the cooling rate is set
to 100 °C/second. The upper limit is preferably 85 °C/second or less.
[0069]
In addition, it is necessary to carry out the cooling after the heating from a
temperature exceeding the Ar3 point. When the cooling is initiated from a
temperature of Ar3 point or lower, ferrite is generated, a uniform structure is not
obtained. A, becomes low, and local deformability deteriorates.
[0070]
(Second Embodiment)
The second embodiment of the present invention relates to an energy
absorbing member including a buckling deformation portion having tensile strength of
less than 980 MPa, which corresponds to the hot stamped article described in the first
embodiment, and a deformation suppressing portion having tensile strength of 1180
MPa or more. That is, in the energy absorbing member, a difference in tensile
- 27 -
w
strength between the buckling deformation portion and the deformation suppressing
portion is designed to be 200 MPa or more.
For example, the energy absorbing member is applied to a member such as a
front frame which is accompanied with particularly, axial compression deformation,
and a member such as a lower portion of a center pillar which is a bending deformation
portion but requires flat deformation to the some degree, among vehicle parts. The
member accompanied with the axial compression deformation includes an energy
absorbing portion (portion corresponding to the steel sheet for hot stamping) by
buckling deformation, and a portion (portion corresponding to steel sheet for joint)
such as a kick-up portion which suppresses deformation to the utmost.
[0071]
The tensile strength of the buckling deformation portion (portion
corresponding to the steel sheet for hot stamping) is lower than that of the deformation
suppressing portion (portion corresponding to the steel sheet for joint) by 200 MPa or
more so as to allow the deformation to progress in a compact mode. Even in a
member in which flat deformation is necessary, tensile strength of less than 980 MPa is
preferable so as to allow flat deformation to progress in the bending deformation
portion.
[0072]
The energy absorbing member according to this embodiment may be obtained
by carrying out a hot stamping treatment by using a joined steel sheet, which is
obtained by joining a steel sheet for joint to the steel sheet for hot stamping such as the
hot-rolled steel sheet, the cold-rolled steel sheet, the annealed steel sheet, and the
plated steel sheet which are described in the first embodiment, as a steel sheet for hot
pressing.
28 -
[0073]
That is, the energy absorbing member according to this embodiment is
produced as follows.
(1) A slab having a component composition described in the first embodiment
is heated in order for a surface temperature to be in a temperature range of Ar3 point to
1400°C,
(2) The heated slab is subjected to finish rolling in which a total rolling
reduction at a final stand and an immediately previous stand of the final stand is set to
40% or more in a temperature range state in which the surface temperature is Ar3 point
to 1400°C, and cooling is initiated within one second after the finish rolling to produce
a hot-rolled steel sheet,
(3) The hot-rolled steel sheet is coiled in a temperature range of 650°C or
lower,
(4) The hot-rolled steel sheet is joined to a steel sheet for joint to produce a
joined steel sheet,
(5) The joined steel sheet is formed by a die in a state in which the joined steel
sheet is heated to a temperature of Ac3 point or higher,
(6) The joined steel sheet is cooled in the die at a cooling rate exceeding 100
°C/second in a case where the Mn+Cr is less than 1.0%, or the joined steel sheet is
cooled in the die at a cooling rate of 10 °C/second to 100 °C/second in a case where
the Mn+Cr is 1.0% or more to form a microstructure composed of, in terms of an area
ratio, 0% or more and less than 90% of martensite, 10% to 100% of bainite, and less
than 0.5% of unavoidable inclusion structures, or a microstructure composed of, in
terms of an area ratio, 99.5% to 100% of bainitic ferrite, and less than 0.5% of
unavoidable inclusion structures. In addition, an object, which is obtained by joining
- 29 -
a steel sheet obtained by subjecting the hot-rolled steel sheet to any one kind or more
of a cold rolling process, a continuous annealing treatment, and a plating treatment
with respect to a steel sheet for joint, may be used as the joined steel sheet.
[Examples]
[0074]
Next, examples of the present invention will be described, but a condition in
the examples is only a conditional example employed to confirm reproducibility and an
effect of the present invention, and the present invention is not limited to the
conditional example. The present invention may employ various conditions as long
as the object of the present invention may be accomplished without departing from the
gist of the present invention.
[0075]
(Example a l)
Molten steel having a component composition shown in Table 3 was taken
from a converter to form a slab, and the slab was subjected to hot rolling under hot
rolling conditions (a heating temperature: 1220°C, a finish temperature: 870°C, a total
rolling reduction at a final stand and an immediately previous stand of the final stand:
65%, a time taken from finish rolling termination to cooling initiation: 1 second, and a
coiling temperature: 630°C) of the present invention, thereby obtaining a hot-rolled
steel sheet having a sheet thickness of 3 mm.
30
[0076]
[Table 3]
Steel
A-1
B-1
C-1
D-1
E-1
F-1
G-1
H-1
I-l
J-1
K-1
L-1
M-1
N-1
0-1
C Si Mn Cr P S t-Al Ti Nb V Mo B N Others Mn+Cr
mass%
0.0025
0.018
0.021
0.028
0.038
0.048
0.052
0.062
0.077
0.082
0.097
0.0015
0.109
0.048
0.076
0.02
0.14
0.28
0.12
0.34
0.18
0.15
0.12
0.46
0.21
0.23
0.15
0.23
0.72
0.21
0.92
0.87
0.82
0.77
0.66
0.85
0.45
0.86
0.45
0.65
0.56
0.98
0.62
0.69
0.35
0.05
0.12
0.05
0.21
0.33
0.12
0.45
0.12
0.52
0.33
0.32
0.01
0.17
0.28
0.08
0.082
0.006
0.008
0.008
0.005
0.007
0.011
0.013
0.011
0.009
0.014
0.007
0.011
0.009
0.005
0.0021
0.0028
0.0034
0.0051
0.0032
0.0027
0.0037
0.0033
0.0071
0.0037
0.0024
0.0093
0.0035
0.0021
0.0077
0.037
0.029
0.038
0.034
0.028
0.031
0.041
0.028
0.038
0.041
0.022
0.028
0.038
0.047
0.039
0.021
-
0.048
-
0.014
0.072
0.015
0.037
-
0.067
0.045
0.015
0.024
-
0.027
0.022
-
0.034
0.042
0.071
0.054
0.085
0.052
-
-
0.076
0.015
-
-
0.009
-
-
-
-
-
-
0.07
-
-
0.08
-
0.05
-
-
-
-
-
-
0.08
-
0.22
-
0.47
-
0.38
-
-
0.24
-
-
0.0007
0.0008
-
-
-
0.0006
-
-
0.0015
-
-
0.0017
-
-
0.0018
0.0015
0.0021
0.0022
0.0015
0.0029
0.0018
0.0023
0.0018
0.0021
0.0023
0.0022
0.0024
0.0018
0.0018
0.0015
-
-
Cu:0.11Ni:0.04Sn:0.013
-
-
Cu:0.09Ni:0.05Sn:0.013
Cu:0.08Ni:0.04Sn:0.012
-
-
-
-
-
Cu:0.10Ni:0.05Sn:0.010
-
Cu:0.10Ni:0.07Sn:0.013
0.97
0.99
0.87
0.98
0.99
0.97
0.90
0.98
0.97
0.98
0.88
0.99
0.79
0.97
0.43
Ac3
°C
945
879
908
877
886
894
894
874
884
891
877
907
867
902
883
Ar3
°C
751
765
830
802
787
697
774
752
756
797
770
758
816
852
866
31
mmmmimmimm
^
[0077]
The hot-rolled steel sheet was subjected to cold rolling to obtain a cold-rolled
steel sheet of 1.4 mm, and then continuous annealing, or annealing and a plating
treatment after the annealing were carried out under conditions shown in Table 4. The
plating treatment was set to hot-dip zinc plating (GI (without an alloying treatment)/GA
(with an alloying treatment)), or hot-dip aluminizing (Al) containing 10% of Si. In
addition, after the annealing or the plating treatment, skin pass rolling was carried out
with a rolling reduction shown in Table 4.
- 32
iWiiMMM
"tp'
[0078]
[Table 4]
steel
A-1
B-l
C-1
D-1
E-l
F-1
G-1
H-1
I-l
J-1
K-1
L-1
M-1
N-1
0-1
Annealing
temperature
"C
800
750
770
780
750
750
780
780
770
750
800
780
790
780
750
Plating
Al
Not
perfor
med
Al
Al
Not
perfor
med
Not
perfor
med
Al
Not
perfor
med
Zn
(GA)
Zn (01)
Not
perfor
med
Al
Zn
(GA)
Zn
(GA)
Zn (GI)
Microstructure (area ratio)
M
0
0
0
0
0
70
0
75
80
85
88
0
100
0
0
B
0
100
100
100
100
30
100
25
20
15
12
0
0
70
55
BF
100
0
0
0
0
0
0
0
0
0
0
100
0
0
0
F
0
0
0
0
0
0
0
0
0
0
0
0
0
30
45
Others
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0,5
<0.5
<0.5
<0.5
<0.5
Skin
pass
%
0.5
0.5
0.5
1.0
1.0
1.0
1.0
1.0
0.5
1.2
1.0
0.7
1.2
1.5
0.8
TS before heat
treatment
MPa
441
374
388
367
367
385
379
388
394
411
386
338
421
384
395
TS after
cooling
MPa
601
511
524
571
632
711
768
831
891
931
975
421
1205
697
542
X
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
NO
NG
Plating
properties
OK
-
OK
OK
-
-
OK
-
OK
OK
-
OK
OK
NG
OK
Delayed
fracture
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
Toughn
ess
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
Remark
Steel of present
invention
Steel of present
invention
Steel of present
invention
Steel of present
invention
Steel of present
invention
Steel of present
invention
Steel of present
invention
Steel of present
invention
Steel of present
invention
Steel of present
invention
Steel of present
invention
Comparative Steel
Comparative Steel
Comparative Steel
Comparative Steel
M: martensite, B: bainite, BF:bainitic ferrite, F: ferrite. Others: unavoidable inclusion structures
- 33 -
Ir
[0079]
Each of the cold-rolled and annealed steel sheet, and the aluminized steel sheet
were heated to 900 °C in a heating furnace, and were interposed in a die provided with a
water supply inlet through which water is ejected from the surface, and a water drain
outlet which sucks in the water. Then, the steel sheet was cooled to room temperature
at a cooling rate of 200 °C/second, thereby simulating thermal history during hot
stamping.
[0080]
Each of the GI steel sheet and the GA steel sheet was heated to 870°C by
electrical heating at a heating rate of 100 °C/second, was heat-retained for
approximately five seconds, and then was cooled with air to Ar3 point + 10°C.
Similarly, each of the GI steel sheet and the GA steel sheet was interposed in a die
provided with a water supply inlet through which water is ejected from the surface, and
a water drain outlet which sucks in the water. Then, the steel sheet was cooled to room
temperature at a cooling rate of 200 °C/second, thereby simulating thermal history
during hot stamping.
[0081]
The tensile strength after the heat treatment was evaluated by preparing No. 5
test specimen and by performing a tensile test on the basis of JIS Z 2241 (2011). The
local deformability was evaluated as k by examining the hole expansibility by a method
described in JIS Z 2256 (2010) as described above. A case in which A, was 50% or
more was regarded as "pass (OK)". In addition, the delayed fracture characteristics
and low-temperature toughness were also evaluated.
[0082]
With regard to the delayed fracture characteristics, a V-notched test specimen
- 34 -
shown in FIG. 3 was used, the test specimen was immersed in an aqueous solution,
which was obtained by dissolving 3g/l of ammonium thiocyanate in 3% salt solution, at
room temperature for 100 hours, and evaluation was carried out by presence or absence
of rupture in a state in which a load of 0.7 TS (after a heat treatment) was applied
(without rupture: OK, with rupture: NG).
With regard to low-temperature brittleness, a Charpy test was carried out at -
40°C, and a case in which percent ductile fracture of 50% or more was obtained was
regarded as "pass (OK)", and a case in which the percent ductile fi-acture was less than
50% was regarded as "failure (NG)".
[0083]
Results that were obtained are collectively shown in Table 4. In steel (A-1
steel to K-1 steel) according to the present invention, excellent local deformability in
which TS was 490 MPa to 980 MPa was obtained, and there was no problem in the
delayed fracture characteristics or the low-temperature toughness.
[0084]
In L-1 steel in which the content of C was low, and deviated from the range of
the present invention, the tensile strength after a heat treatment corresponding to the hot
stamping was low. In M-1 steel in which the content of C was high, and deviated fi'om
the range of the present invention, the tensile strength exceeded 1180 MPa, and
buckling deformation was unstable during axial compression deformation, and thus
there was a concern about a decrease in energy absorbing characteristics.
[0085]
In N-1 steel in which the content of Si exceeded the range of the present
invention, and in 0-1 steel in which the content of Mn+Cr deviated from the range of
the present invention toward a lower side, ferrite was generated, and a structure became
- 35 -
4
ununiform, and thus X was lower than 50%. Therefore, there was a concern about a
decrease in energy absorbing characteristics due to a decrease in the local deformability.
In addition, in the N-1 steel, the content of Si deviated from the range of the present
invention toward a higher side, and thus plating properties were poor.
[0086]
(Example a2)
With regard to K-1 steel shown in Table 3, a hot-rolled steel sheet having a
sheet thickness of 2 mm was obtained under hot rolling conditions within a range of the
present invention (a heating temperature: 1250°C, a finish temperature: 880°C, a total
rolling reduction at a final stand and an immediately previous stand of the final stand:
60%, a time taken from finish rolling termination to cooling initiation: 0.8 seconds, and
a coiling temperature: 550°C), and then the hot-rolled steel sheet was subjected to
pickling.
[0087]
The steel sheet after the pickling was heated to 880°C in a heating furnace, and
then was interposed in a die provided with a water supply inlet through which water is
ejected from the surface, and a water drain outlet which sucks in the water. The steel
sheet was cooled to room temperature at various cooling rates, thereby simulating the
thermal history during hot stamping. Furthermore, the steel sheets after the pickling
were subjected to zinc plating (GI, GA), or hot-dip aluminizing containing 10% of Si,
and then were subjected to the same heating and cooling treatments.
[0088]
With regard to the K-1 steel shown in Table 3, a hot-rolled steel sheet having a
sheet thickness of 3.2 mm was obtained under hot rolling conditions within a range of
the present invention (a heating temperature: 1250°C, a finish temperature: 890°C, a
- 36 -
#
total rolling reduction at a final stand and an immediately previous stand of the final
stand: 45%, a time taken fi-om finish rolling termination to cooling initiation: 0.5
seconds, and a coiling temperature: 500°C), the hot-rolled steel sheet was subjected to
pickling, and a cold-rolled steel sheet of 1.6 mm was obtained at a cold rolling reduction
of 50%.
[0089]
The cold-rolled steel sheet was heated to 900°C in a heating fiamace, and then
was interposed in a die provided with a water supply inlet through which water is
ejected from the surface, and a water drain outlet which sucks in the water. The coldrolled
steel sheet was cooled to room temperature at various cooling rates, thereby
simulating the thermal history during hot stamping.
[0090]
Steel sheet, which was obtained by subjecting the cold-rolled steel sheet to zinc
plating (GI, GA), was heated to 870°C by electrical heating for five seconds, and was
heat-retained for approximately five seconds, and then was cooled with air to 650°C.
Then, the steel sheet was interposed in a die provided with a water supply inlet through
which water is ejected from the surface, and a water drain outlet which sucks in the
water. Then, the steel sheet was cooled to room temperature at various cooling rates,
thereby simulating thermal history during hot stamping.
[0091]
The same heating and cooling treatments were also carried out with respect to
the steel sheet subjected to the hot-dip aluminizing containing 10% of Si. In addition,
after the hot rolling, the annealing, or the plating treatment, skin pass was carried out
with a rolling reduction shown in Table 4. Material characteristics of the steel sheets
that were obtained were evaluated in the same manner as Example a l . Results are
- 37 -
#
shown in Table 5.
38 -
[0092]
[Table 5]
Meth
od
a
b
c
d
e
f
g
h
i
;
Kinds of
steel
K-1
K-1
K-1
K-1
K-1
K-1
K-1
K-1
K-1
K-1
Cold
rolling
Not
performed
Not
performed
Not
performed
Not
performed
Not
performed
Performed
Performed
Performed
Performed
Performed
Plating
Not
performed
GI
Al
Not
performed
GA
Not
performed
GI
Al
Not
performed
GA
Skin
pass
%
1.0
1.2
1.5
2.0
0.8
1.0
1.2
1.5
2.0
0.8
TS before
heat
treatment
MPa
378
367
369
372
372
381
365
372
380
381
Cooling
temperature
"C/sec
300
200
150
110
50
300
200
150
110
50
TS after
cooling
MPa
938
926
915
922
425
952
941
933
931
410
Microstructure
(*)
M
85
80
75
85
0
88
85
80
85
0
B
15
20
25
15
0
12
15
20
15
0
F
0
0
0
0
30
0
0
0
0
35
P
0
0
0
0
70
0
0
0
0
65
Othe
rs
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
X
OK
OK
OK
OK
NG
OK
OK
OK
OK
NG
Delayed
fracture
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
Toughn
ess
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
Remark
Method of present
invention
Method of present
invention
Method of present
invention
Method of present
invention
Comparative Method
Method of present
invention
Method of present
invention
Method of present
invention
Method of present
invention
Comparative Method
^
M: martensite, B: bainite, F: ferrite, P: pearlite. Others: unavoidable inclusion structures
39
#
[0093]
In examples of a method a, a method b, a method c, a method d, a method f, a
method g, a method h, and a method i according to methods of the invention, excellent
local deformability may be obtained, and there is no problem in the delayed fracture
characteristics or the low-temperature toughness.
[0094]
On the other hand, in examples of a method e and a method j in which the
cooling rate deviates from the range of the present invention toward a lower side, ferrite
and pearlite were generated in a structure after the heat treatment, and thus strength after
hot stamping was low, and X was lower than 50%. Therefore, there was a concern
about a decrease in energy absorbing characteristics due to a decrease in the local
deformability.
[0095]
(Example a3)
To prepare a member having a shape shown in FIG. 4 by hot stamping, the I-l
steel that is steel of the invention in Example a l or 0-1 steel of comparative steel was
disposed at an axial compression deformation portion 1, a cold-rolled sheet of, in terms
of % by mass, 0.21% C-0.2% Si-1.4% Mn-0.0025% B, which had a sheet thickness of
1.4 mm, was disposed at a portion 2 in which tensile strength after hot stamping was
1180 MPa or more, and both steel sheets were laser-welded at a location of a laser
welding portion 3.
[0096]
The welded member was heated to 900°C by an electric furnace, was heatretained
for 60 seconds, and was interposed in a die provided with a water supply inlet
through which water is ejected from the surface, and a water drain outlet which sucks in
40 -
the water. The laser welded member was simultaneously subjected to press forming
and cooling to prepare a member having a shape shown in FIG. 4. Then, a backboard 4
having tensile strength of 590 MPa was disposed and was joined to the member by spot
welding.
[0097]
Small-sized tensile test specimens were prepared from the members 1 and 2,
and tensile strength was measured by a tensile test. As a result, in a case of using the
I-l steel at the portion corresponding to the member 1, the tensile strength was 880 MPa,
and in a case of using the 0-1 steel, the tensile strength was 520 MPa. On the other
hand, the tensile strength of the portion corresponding to the member 2 was 1510 MPa.
[0098]
A drop weight test was carried out with respect to the member shown in FIG. 4.
Deformation was applied to the member shown in FIG. 4 from a direction of a load
direction 5 during axial compression deformation, which is shown in FIG. 4, with a load
of 150 kg at a speed of 15 m/second. In the member using the I-l steel that is steel of
the invention, buckling deformation occurred without occurrence of cracking, but in the
member using the 0-1 steel of comparative steel, cracking occurred at a buckling
deformation portion, and thus an amount of energy absorption decreased.
[0099]
(Example a4)
When preparing a member having the shape shown in FIG. 4 by hot stamping,
the A-1 steel and H-1 steel that are steels of the invention in Example a l were used.
Each of the members was heated to 950°C, and was heat-retained for 60 seconds.
Then, similar to Example a3, the member was interposed in a die provided with a water
supply inlet through which water is ejected from the surface, and a water drain outlet
- 41 -
which sucks in the water. The member was simultaneously subjected to press forming
and cooling.
[0100]
A drop weight test was carried out to evaluate a deformation behavior of the
member. With regard to axial compression deformation, a load of 150 kg was applied
from a direction of the load direction 5 during axial compression deformation which is
shown in FIG. 4 at a speed of 15 m/second. With regard to bending deformation,
deformation was applied to the member from a load direction 6 during bending
deformation at a speed of 5 m/second. It was confirmed that each of the members was
deformed without rupture in any deformation mode, and had sufficient energy absorbing
performance.
[0101]
(Example pi)
Molten steel having a component composition shown in Table 6 was emitted
from a converter to form a slab, and the slab was subjected to hot rolling under hot
rolling conditions (a heating temperature: 1220°C, a finish temperature: 870°C, a total
rolling reduction at a final stand and an immediately previous stand of the final stand:
65%, a time taken from finish rolling termination to cooling initiation: 1 second, and a
coiling temperature: 630°C) of the present invention, thereby obtaining a hot-rolled
steel sheet having a sheet thickness of 3 mm.
42 -
[0102]
[Table 6]
Steel
A-2
B-2
C-2
D-2
E-2
F-2
G-2
H-2
1-2
J-2
K-2
L-2
M-2
N-2
0-2
P-2
C Si Mn Cr P S t-Al Ti Nb V Mo B N Others Mn+Cr
mass%
0.0025
0.018
0.021
0.028
0.038
0.048
0.052
0.062
0.077
0.082
0.097
0.0015
0.109
0.048
0.039
0.038
0.02
0.14
0.28
0.12
0.34
0.18
0.15
0.12
0.46
0.21
0.23
0.15
0.23
0.72
0.21
0.22
1.52
1.12
1.08
1.75
1.32
1.11
1.12
1.25
0.51
0.87
1.18
1.25
1.21
1.32
0.72
1.25
0.05
0.25
0.52
0.02
0.33
0.85
0.55
0.04
1.35
0.78
0.32
0.25
0.33
0.24
0.15
0.26
0.082
0.006
0.008
0.008
0.005
0.007
0.011
0.013
0.011
0.009
0.014
0.007
0.011
0.009
0.005
0.004
0.0021
0.0028
0.0034
0.0051
0.0032
0.0027
0.0037
0.0033
0.0071
0.0037
0.0024
0.0093
0.0035
0.0021
0.0077
0.0029
0.037
0.029
0.038
0.034
0.028
0.031
0.041
0.028
0.038
0.041
0.022
0.028
0.038
0.047
0.039
0.031
0.021
-
0.048
-
0.014
0.072
0.002
0.037
-
0.067
0.045
0.015
0.024
-
0.027
0.024
0.022
-
0.002
0.042
0.071
0.054
0.085
0.052
-
-
0.076
0.015
-
-
0.009
-
-
-
-
-
-
-
0.07
-
-
0.08
-
0.05
-
-
-
-
-
-
-
0.03
-
0.22
-
0.47
-
0.38
-
-
0.24
-
-
0.38
0.0007
0.0008
0.0011
0.0015
0.0008
0.0006
0.0014
0.0008
0.0010
0.0008
0.0007
0.0015
0.0008
0.0011
0.0018
;
0.0015
0.0021
0.0022
0.0015
0.0029
0.0018
0.0023
0.0018
0.0021
0.0023
0.0022
0.0024
0.0018
0.0018
0.0015
0.0023
-
-
Cu:0.09Ni:0.04Sn:0.013
-
-
Cu:0.11 Ni:0.05 Sn:0.013
Cu:0.08Ni:0.05Sn:0.011
-
-
-
-
-
Cu:0.10Ni:0.04Sn:0.012
-
Cu:0.12Ni:0.07Sn:0.015
-
1.57
1.37
1.60
1.77
1.65
1.96
1.67
1.29
1.86
1.65
1.50
1.50
1.54
1.56
0.87
1.51
Ac3
"C
927
871
901
848
866
886
869
862
882
885
858
833
849
883
888
868
Ar3
°C
703
734
717
663
654
618
632
647
685
663
641
717
676
724
762
769
- 43 -
[0103]
The hot-rolled steel sheet was subjected to cold rolling to obtain a cold-rolled
steel sheet of 1.4 mm, and then continuous annealing, or annealing and a plating
treatment after the annealing were carried out under conditions shown in Table 7. The
plating treatment was set to hot-dip zinc plating (GI (without an alloying treatment)/GA
(with an alloying treatment)), or hot-dip aluminizing (Al) containing 10% of Si. In
addition, after the annealing or the plating treatment, skin pass rolling was carried out
with a rolling reduction shown in Table 7.
44
[0104]
Ste
el
A-2
B-2
C-2
D-2
E-2
F-2
G-2
H-2
1-2
J-2
K-2
L-2
MI
2
N-2
0-2
M
[Table 7]
Annealing
temperature
"C
800
750
770
780
750
750
780
780
770
750
800
780
790
780
750
790
Plating
Al
Not
performed
Al
Al
Not
performed
Not
performed
Al
Not
performed
Zn (GA)
Zn (GI)
Not
performed
Al
Zn (GA)
Zn (GA)
Zn (GI)
Not
performed
Microstructure (area ratio)
M
0
0
0
0
0
50
0
75
80
80
85
0
100
0
0
0
B
0
100
100
100
100
50
100
25
20
20
15
0
2
85
70
50
BF
100
0
0
0
0
0
0
0
0
0
0
100
0
0
0
0
F
0
0
0
0
0
0
0
0
0
0
0
0
0
15
30
50
Others
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5'
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
Skin
pass
%
0.5
0.5
0.5
1.0
1.0
1.0
1.0
1.0
0.5
1.2
1.0
0.7
1.2
1.5
0.8
1.0
TS before heat
treatment
MPa
457
374
388
367
367
385
379
388
394
411
386
338
421
384
395
368
TS after
cooling
MPa
594
498
516
556
612
694
752
814
865
910
964
408
1192
688
522
791
X
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
NG
NG
NG
Plating
properties
OK
-
OK
OK
-
-
OK
-
OK
OK
-
OK
OK
NG
OK
-
Delayed
fracture
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
Toughn
ess
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
Remark
Steel of present
invention
Steel of present
invention
Steel of present
invention
Steel of present
invention
Steel of present
invention
Steel of present
invention
Steel of present
invention
Steel of present
invention
Steel of present
invention
Steel of present
invention
Steel of present
invention
Comparative Steel
Comparative Steel
Comparative Steel
Comparative Steel
Comparative Steel
M: martensite, B: bainite, BF: bainitic ferrite, F: ferrite, Others: unavoidable inclusion structures
45
[0105]
Each of the cold-rolled and annealed steel sheet, and the alvraiinized steel sheet
was heated to 900°C in a heating furnace, and was interposed in a die. Then, the steel
sheet was cooled to room temperature at a cooling rate of 50 °C/second, thereby
simulating thermal history during hot stamping.
[0106]
Each of the GI steel sheet and the GA steel sheet was heated to 870°C by
electrical heating at a heating rate of 100 °C/second, was heat-retained for
approximately five seconds, and then was cooled with air to Ar3 point + 10°C.
Similarly, each of the GI steel sheet and the GA steel sheet was interposed in a die.
Then, the steel sheet was cooled to room temperature at a cooling rate of 50 °C/second,
thereby simulating thermal history during hot stamping.
[0107]
The tensile strength after the heat treatment was evaluated by preparing No. 5
test specimen and by performing a tensile test on the basis of JIS Z 2241 (2011). The
local deformability was evaluated as X by examining the hole expansibility by a method
described in JIS Z 2256 (2010) as described above. A case in which 'k was 50% or
more was regarded as "pass (OK)". In addition, the delayed fracture characteristics
and low-temperature toughness were also evaluated,
[0108]
With regard to the delayed fracture characteristics, a V-notched test specimen
shown in FIG. 3 was used, the test specimen was immersed in an aqueous solution,
which was obtained by dissolving 3g/l of ammonium thiocyanate in 3% salt solution, at
room temperature for 100 hours, and determination was carried out by presence or
absence of rupture in a state in which a load of 0.7 TS (after a heat treatment) was
- 46 -
^
applied (without rupture: OK, with rupture: NG).
[0109]
With regard to low-temperature brittleness, a Charpy test was carried out at -
40°C, and a case in which percent ductile fracture of 50% or more was obtained was
regarded as "pass (OK)", and a case in which the percent ductile fi'acture was less than
50% was regarded as "failure (NG)".
[0110]
Results that were obtained are collectively shown in Table 7. In steels (A-2
steel to K-2 steel) according to the present invention, excellent local deformability in
which TS was 490 MPa to 980 MPa was obtained, and there was no problem in the
delayed fracture characteristics or the low-temperature toughness.
[0111]
In L-2 steel in which the content of C was low, and deviated from the range of
the present invention, the tensile strength after a heat treatment corresponding to the hot
stamping was low. In M-2 steel in which the content of C was high, and deviated from
the range of the present invention, the tensile strength exceeded 1180 MPa, and
buckling deformation was unstable during axial compression deformation, and thus
there was a concern about a decrease in energy absorbing characteristics.
[0112]
In N-2 steel in which the content of Si exceeded the range of the present
invention, in 0-2 steel in which the content of Mn+Cr was low due to a cooling rate of
50 °C/second, and in P-2 steel in which the content of Mn+Cr was 1.0%) or more, and B
was not added, ferrite was generated, and a structure became nonuniform, and thus X
was lower than 50%. Therefore, there was a concern about a decrease in energy
absorbing characteristics due to a decrease in the local deformability. In addition, in
- 47 -
the M-2 steel, the content of Si deviated from the range of the present invention toward
a higher side, and thus plating properties were poor.
[0113]
(Example P2)
With regard to K-2 steel shown in Table 6, a hot-rolled steel sheet having a
sheet thickness of 2 mm was obtained under hot rolling conditions within a range of the
present invention (a heating temperature: 1250°C, a finish temperature: 880°C, a total
rolling reduction at a final stand and an immediately previous stand of the final stand:
60%, a time taken from finish rolling termination to cooling initiation: 0.8 seconds, and
a coiling temperature: 550°C), and then the hot-rolled steel sheet was subjected to
pickling.
[0114]
The steel sheet after the pickling was heated to 880°C in a heating fiimace, and
then was interposed in a die. The steel was cooled to room temperature at various
cooling rates, thereby simulating the thermal history during hot stamping.
Furthermore, the steel sheets after the pickling were subjected to zinc plating (GI, GA),
or hot-dip aluminizing containing 10% of Si, and then were subjected to the same
heating and cooling treatments.
[0115]
With regard to the K-2 steel shown in Table 7, a hot-rolled steel sheet having a
sheet thickness of 3.2 mm was obtained under hot rolling conditions within a range of
the present invention (a heating temperature: 1250°C, a finish temperature: 890°C, a
total rolling reduction at a final stand and an immediately previous stand of the final
stand: 45%), a time taken from finish rolling termination to cooling initiation: 0.5
seconds, and a coiling temperature: 500°C), the hot-rolled steel sheet was subjected to
- 48 -
pickling, and a cold-rolled steel sheet of 1.6 mm was obtained at a cold rolling reduction
of 50%.
[0116]
The cold-rolled steel sheet was heated to 900°C in a heating ftimace, and then
was interposed in a die. The cold-rolled steel sheet was cooled to room temperature
at various cooling rates, thereby simulating the thermal history during hot stamping.
Furthermore, steel, which was obtained by subjecting the cold-rolled steel sheet to zinc
plating (GI, GA), was heated to 870°C by electrical heating for five seconds, and was
heat-retained for approximately five seconds, and then was cooled with air to 650°C.
Then, the steel was interposed in a die. Then, the steel was cooled to room
temperature at various cooling rates, thereby simulating thermal history during hot
stamping.
[0117]
The steel, which was subjected to the hot-dip aluminizing containing 10% of Si,
was heated to 880°C in a heating ftamace, and was interposed in a die, and was cooled
to room temperature at various cooling rates, thereby simulating thermal history during
hot stamping. In addition, after the hot rolling, the annealing, or the plating treatment,
skin pass was carried out with a rolling reduction shown in Table 8.
[0118]
Material characteristics of the steel sheets that were obtained were evaluated in
the same maimer as Example P1. Results that were obtained are shown in Table 8.
49 -
[0119]
[Table 8]
Meth
od
a'
b'
c'
d'
s!
f
g'
h'
i'
l
Kinds of
steel
K-2
K-2
K-2
K-2
K-2
K-2
K-2
K-2
K-2
K-2
Cold
rolling
Not
performed
Not
performed
Not
performed
Not
performed
Not
performed
Performed
Performed
Performed
Performed
Performed
Plating
Not
performed
GI
Al
Not
performed
GA
Not
performed
GI
Al
Not
performed
GA
Skin
pass
%
1.0
1.2
1.5
2.0
0.8
1.0
1.2
1.5
2.0
0.8
TS before heat
treatment
MPa
378
367
369
372
372
381
365
372
380
381
Cooling
temperature
"C/sec
100
50
25
10
5
100
50
25
10
5
TS after
cooling
MPa
958
924
931
927
457
955
941
936
911
451
Microstructure
(*)
M
85
80
75
70
0
88
85
80
70
0
B
15
20
25
30
0
12
15
20
30
0
F
0
0
0
0
50
0
0
0
0
45
P
0
0
0
0
50
0
0
0
0
55
Others
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
X
OK
OK
OK
OK
NG
OK
OK
OK
OK
NG
Delayed
fracture
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
Tough
ness
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
Remark
Method of present
invention
Method of present
invention
Method of present
invention
Method of present
invention
Comparative
Method
Method of present
invention
Method of present
invention
Method of present
invention
Method of present
invention
Comparative
Method
M: martensite, B: bainite, F: ferrite, P: pearlite. Others: unavoidable inclusion structures
50
#
[0120]
In examples of a method a', a method b', a method c', a method d', a method f,
a method g', a method h', and a method i' according to methods of the invention,
excellent local deformability may be obtained, and there is no problem in the delayed
fracture characteristics or the low-temperature toughness.
[0121]
On the other hand, in examples of a method e' and a method]' in which the
cooling rate deviates from the range of the present invention, ferrite and pearlite were
generated in a structure after the heat treatment, and thus strength after hot stamping
was low, and A, was lower than 50%. Therefore, there was a concern about a decrease
in energy absorbing characteristics due to a decrease in the local deformability.
[0122]
(Example P3)
To prepare a member having a shape shown in FIG. 4 by hot stamping, a steel
sheet of the 1-2 steel that is steel of the invention in Example pi or 0-2 steel of
comparative steel was disposed at the axial compression deformation portion 1, a coldrolled
steel sheet of, in terms of % by mass, 0.21% C-0.2% Si-2.4% Mn-0.0025% B,
which had a sheet thickness of 1.4 mm, was disposed at the portion 2 in which tensile
strength after hot stamping was 1180 MPa or more, and both steel sheets were laserwelded
at a location of the laser welding portion 3.
[0123]
The welded member was heated to 900°C by an electric furnace, was heatretained
for 60 seconds, and was interposed in a die. The welded member was
simultaneously subjected to press forming and cooling to prepare a member having a
shape shown in FIG. 4. Then, a backboard 4 having tensile strength of 590 MPa was
- 51 -
m^
disposed and was joined to the member by spot welding.
[0124]
Small-sized tensile test specimens were prepared from the members 1 and 2,
and tensile strength was measured by a tensile test. As a result, in a case of using the
1-2 steel at the portion corresponding to the member 1, the tensile strength was 880 MPa,
and in a case of using the 0-2 steel, the tensile strength was 520 MPa. On the other
hand, the tensile strength of the portion 2 corresponding to the member 2 was 1510 MPa.
Accordingly, a difference (ATS) in tensile strength after hot stamping was 200 MPa or
more.
[0125]
A drop weight test was carried out with respect to the member shown in FIG. 4.
Deformation was applied to the member shown in FIG. 4 from a direction of the load
direction 5 during axial compression deformation, which is shown in FIG. 4, with a load
of 150 kg at a speed of 15 m/second. In the member using the 1-2 steel that is steel of
the invention, buckling deformation occurred without occurrence of cracking.
However, in the member using the 0-2 steel of comparative steel, ferrite and bainite
were generated, and a microstructure became ununiform. According to this, cracking
occurred at the buckling deformation portion, and an amount of energy absorption
decreased.
[0126]
(Example p4)
When preparing a member having the shape shown in FIG. 4 by hot stamping,
the A-2 steel and H-2 steel that are steel of the invention in Example pi were used.
Each steel sheet of the members was heated to 950°C, and was heat-retained for 60
seconds. Then, similar to Example P3, the steel sheet was interposed in a die. The
52 -
^
steel sheet was simultaneously subjected to press forming and cooling.
[0127]
A drop weight test was carried out to evaluate a deformation behavior of the
member. With regard to axial compression deformation, a load of 150 kg was applied
from a direction of the load direction 5 during axial compression deformation which is
shown in FIG. 4 at a speed of 15 m/second. With regard to bending deformation,
deformation was applied to the member from a load direction 6 during bending
deformation at a speed of 5 m/second. It was confirmed that each of the members was
deformed without rupture in any deformation mode, and had suflFicient energy absorbing
performance.
[Industrial Applicability]
[0128]
As described above, according to the present invention, in a case of producing
parts utilizing a tailored blank material, with respect to an axial compression
deformation portion, tensile strength after hot stamping may be suppressed to be low,
and thus local deformability may be applied to the parts. As a result, a member which
is excellent in energy absorbing characteristics during axial compression deformation
and bending deformation may be produced. Accordingly, the present invention has
high applicability in mechanical part production industry.
[Description of Reference Numerals and Signs]
[0129]
1: Axial compression deformation portion
2: Portion in which tensile strength after hot stamping > 1180 Mpa
3: Laser welded portion
4: Backboard
53 -
5: Load direction during axial compression deformation
6: Load direction during bending deformation
- 54 -

[Designation of Document] CLAIMS
[Claim 1]
A hot stamped article that is obtained by hot stamping a steel sheet for hot
stamping, the hot stamped article having a component composition containing, in terms
of % by mass:
0.002% to 0.1% of C;
0.01% to 0.5% of Si;
0.5%to2.5%ofMn+Cr;
0.1% or less of P;
0.01% or less of S;
0.05% or less of t-Al;
0.005% or less of N; and
0.0005% to 0.004% of B which is optionally contained in a case where the
Mn+Cr is 1.0% or more, remainder being Fe and unavoidable impurities,
wherein the hot stamped article has a microstructure composed of, in terms of
an area ratio, 0% or more and less than 90% of martensite, 10% to 100% of bainite, and
less than 0.5% of unavoidable inclusion structures, or a microstructure composed of, in
terms of an area ratio, 99.5% to 100% of bainitic ferrite, and less than 0.5% of
unavoidable inclusion structures.
[Claim 2]
The hot stamped article according to Claim 1,
wherein a plated layer is provided on a surface of the hot stamped article.
[Claim 3]
The hot stamped article according to Claim 1,
wherein the component composition further contains one or more kinds
- 55 -
selected from, in terms of % by mass,
0.001% to 0.1% of Ti,
0.001% to 0.05% of Nb,
0.005% to 0.1% of V, and
0.02% to 0.5% of Mo.
[Claim 4]
The hot stamped article according to Claim 1,
wherein in a case where the Mn+Cr is less than 1.0%, the component
composition further contains, in terms of %> by mass, 0.0005% to 0.004%) of B.
[Claim 5]
An energy absorbing member, comprising:
the hot stamped article according to any one of Claims 1 to 4; and
a joint member which is joined to the hot stamped article and has tensile
strength of 1180 MPa or more,
wherein a difference in tensile strength between the hot stamped article and the
joint member is 200 MPa or more.
[Claim 6]
A method of producing a hot stamped article, the method comprising:
a heating process of heating a slab in order for a surface temperature to be in a
temperature range of Ar3 point to 1400°C, the slab having a component composition
containing, in terms of % by mass, 0.002% to 0.1% of C, 0.01% to 0.5% of Si, 0.5% to
2.5% of Mn+Cr, 0.1% or less of P, 0.01% or less of S, 0.05% or less of t-Al, 0.005% or
less of N, and 0.0005% to 0.004%) of B which is optionally contained in a case where
the Mn+Cr is 1.0% or more, remainder being Fe and unavoidable impurities;
a hot rolling process of subjecting the heated slab to finish rolling in which a
- 56 -
total rolling reduction at a final stand and an immediately previous stand of the final
stand is set to 40%) or more in a temperature range state in which the surface
temperature is Ar3 point to 1400°C, and initiating cooling within one second after the
finish rolling to produce a hot-rolled steel sheet;
a coiling process of coiling the hot-rolled steel sheet in a temperature range of
650°C or lower; and
a hot stamping process of using the hot-rolled steel sheet as a steel sheet for hot
stamping, forming the steel sheet for hot stamping using a die in a state in which the
steel sheet is heated to a temperature of Ac3 point or higher, cooling the steel sheet for
hot stamping in the die at a cooling rate exceeding 100 °C/second in a case where the
Mn+Cr is less than 1.0%, or cooling the steel sheet for hot stamping in the die at a
cooling rate of 10 °C/second to 100 °C/second in a case where the Mn+Cr is 1.0% or
more to produce a hot stamped article having a microstructure composed of, in terms of
an area ratio, 0% or more and less than 90% of martensite, 10% to 100%) of bainite, and
less than 0.5%) of unavoidable inclusion structures, or a microstructure composed of, in
terms of an area ratio, 99.5%) to 100% of bainitic ferrite, and less than 0.5%) of
unavoidable inclusion structures.
[Claim 7]
The method of producing a hot stamped article according to Claim 6, the
method fiirther comprising:
a plating process of carrying out a plating treatment with respect to the hotrolled
steel sheet before the hot stamping process,
wherein in the hot stamping process, the hot-rolled steel sheet to which the
plating treatment is carried out is used as the steel sheet for hot stamping.
[Claim 8]
- 57 -
The method of producing a hot stamped article according to Claim 6, the
method further comprising:
a cold rolling process of producing a cold-rolled steel sheet by carrying out
cold rolling with respect to the hot-rolled steel sheet before the hot stamping process,
wherein in the hot stamping process, the cold-rolled steel sheet is used as the
steel sheet for hot stamping.
[Claim 9]
The method of producing a hot stamped article according to Claim 6, the
method further comprising:
a cold rolling process of producing a cold-rolled steel sheet by carrying out
cold rolling with respect to the hot-rolled steel sheet before the hot stamping process;
and
a plating treatment process of carrying out a plating treatment with respect to
the cold-rolled steel sheet,
wherein in the hot stamping process, the cold-rolled steel sheet to which the
plating treatment is carried out is used as the steel sheet for hot stamping.
[Claim 10]
The method of producing a hot stamped article according to Claim 6, the
method further comprising:
a cold rolling process of producing a cold-rolled steel sheet by carrying out
cold rolling with respect to the hot-rolled steel sheet before the hot stamping process;
and
a continuous annealing process of carrying out continuous annealing with
respect to the cold-rolled steel sheet,
wherein in the hot stamping process, the cold-rolled steel sheet to which the
- 58 -
^B
continuous annealing is carried out is used as the steel sheet for hot stamping.
[Claim 11]
The method of producing a hot stamped article according to Claim 6, the
method further comprising:
a cold rolling process of producing a cold-rolled steel sheet by carrying out
cold rolling with respect to the hot-rolled steel sheet before the hot stamping process;
a continuous armealing process of carrying out continuous annealing with
respect to the cold-rolled steel sheet; and
a plating treatment process of carrying out a plating treatment with respect to
the cold-rolled steel sheet to which the continuous annealing is carried out,
wherein in the hot stamping process, the cold-rolled steel sheet to which the
continuous annealing and the plating treatment are carried out is used as the steel sheet
for hot stamping.
[Claim 12]
The method of producing a hot stamped article according to Claim 6,
wherein the slab further contains one or more kinds selected from, in terms
of % by mass, 0.001% to 0.1% of Ti, 0.001% to 0.05% of Nb, 0.005% to 0.1% of V, and
0.02% to 0.5% of Mo.
[Claim 13]
The method of producing a hot stamped article according to Claim 6,
wherein in a case where the Mn+Cr is less than 1.0%, the slab further contains,
in terms of % by mass, 0.0005% to 0.004% of B.
[Claim 14]
A method of producing an energy absorbing member, the method comprising:
a joining process of joining the steel sheet for hot stamping according to any
one of Claims 6 to 13 to a steel sheet for joint to produce a joined steel sheet; and
a hot stamping process of forming the joined steel sheet using a die in a state in
which the joined steel sheet is heated to a temperature of Ac3 point or higher, and
cooling the joined steel sheet in the die at a cooling rate exceeding 100 °C/second in a
case where the Mn+Cr is less than 1.0%, or cooling the joined steel sheet in the die at a
cooling rate of 10 °C/second to 100 °C/second in a case where the Mn+Cr is 1.0% or
more so as to set a difference in tensile strength between a portion corresponding to the
steel sheet for hot stamping and a portion corresponding to the steel sheet for joint in the
joined steel sheet to 200 MPa or more.