Note: Descriptions are shown in the official language in which they were submitted.
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V
Description
Title of Invention
STEEL PLATE FOR HOT STAMPING
Technical Field
[0001] The present invention relates to a steel plate for hot stamping.
Background Art
[0002] In recent years, there has been a demand for improvement in collision
safety of motor
vehicles, and in association with this, there has been a demand for a further
increase in strength of
steel plates for hot stamping used in parts required to exhibit rigidity of
motor vehicles. However,
when the strength of steel plate is improved, the low temperature toughness
deteriorates and the
balance between strength and toughness is thus lost. In order to cope with
this problem, Non
Patent Literature 1 proposes that the balance between strength and toughness
of a steel plate is
improved by refining the former austenite grains after hot stamping.
[0003] In hot stamping, the cooling velocity inside the steel plate may
decrease by an increase in the
die temperature and the clearance between the die and the steel plate. When
the cooling velocity
of the steel plate is equal to or lower than the critical cooling velocity,
soft phases such as ferrite and
bainite precipitate and the hardness of the steel plate thus decreases. In
particular, as the cooling
velocity at a temperature equal to or lower than the Ms point decreases, auto
tempering is promoted
and this causes a decrease in hardness of the steel plate.
[0004] In Non Patent Literature 2, a change in cooling velocity when changing
the clearance
between the die and the steel plate is examined and it has been indicated that
the cooling velocity
decreases to about 15 C/s when this clearance is 0.4 mm.
[0005] As described in Non Patent Literature 1, there is a method in which the
crystal grains of steel
are refined as a general structure design technology of steel plates for hot
stamping and this method
makes it possible to obtain a steel plate having an excellent balance between
strength and
toughness. As a method for refining the crystal grains, there is a method in
which elements such
as Nb, Ni, and Ti are added, but the economical efficiency of the steel plate
becomes poor in this
case. A steel plate having refined crystal grains exhibits poor hardenability
and thus lacks
hardness stability.
[0006] In order to solve this problem, it is also considered to improve
process problems that cause a
decrease in hardness, such as an increase in die temperature and clearance
between the die and the
steel plate. However, in that case, it is required to repeatedly modify the
die and prepare a special
die, and this requires a great deal of labor and cost. Hence, in the
conventional steel plates for hot
stamping, there is a problem that it is difficult to obtain a member (molded
product) having an
excellent balance between strength and toughness and excellent hardness
stability without
increasing labor and cost.
Citation List
Non Patent Literature
[0007] Non Patent Literature 1: Kazuo Hikida et al., "Development of TS 1800
MPa Grade Hot
1
Stamping Steel Sheet" Materia Vol. 52, No. 2, 2013, pp. 68-70
Non Patent Literature 2: Katsuji Nakashima, "Hardening Technology of Steel by
Die Quenching and Application to Body Parts" CAMP-ISIJ Vol. 17 2004, pp. 980-
983
Summary of Invention
[0008] An object of the present invention is to provide a steel plate for hot
stamping
which can provide a molded product which exhibits excellent hardness stability
in
addition to the balance between strength and toughness while suppressing
increases in
labor and cost in the hot stamping process.
[0009] A steel plate for hot stamping according to an aspect of the present
invention
contains,
in % by mass,
C: 0.25% or more and 0.4% or less,
Si: 1.05% or more and 1.4% or less,
Mn: 0% or more and 1.4% or less,
Cr: 0.6% or more and 3.0% or less,
P: 0% or more and 0.03% or less,
S: 0% or more and 0.02% or less,
Al: 0.01% or more and 1% or less,
N: 0% or more and 0.01% or less,
B: 0.0005% or more and 0.005% or less,
Ti: 0.005% or more and 0.1% or less, and
iron and inevitable impurities as remainder. This steel plate for hot stamping
exhibits excellent hardness stability in addition to a balance between
strength and
toughness as a following relational expression (1) is satisfied, where [C]
denotes a C
content, [Si] denotes a Si content, [Mn] denotes a Mn content, and [Cr]
denotes a Cr
content.
[0010]
[Math. 1]
[C]+-2 [Si]-I- ¨7 ¨8 [Mn]+ [Cr-]- ¨7 >0 = = =(1)
9 9 9 4
[0010a] In yet another aspect, the present invention provides a steel plate
for hot
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stamping comprising:
in % by mass,
C: 0.25% or more and 0.4% or less,
Si: 1.05% or more and 1.4% or less,
Mn: 0% or more and 1.4% or less,
Cr: 0.6% or more and 3.0% or less,
P: 0% or more and 0.03% or less,
S: 0% or more and 0.02% or less,
Al: 0.01% or more and 1% or less,
N: 0% or more and 0.01% or less,
B: 0.0005% or more and 0.005% or less,
Ti: 0.005% or more and 0.1% or less, and
iron and inevitable impurities as remainder,
wherein the steel plate for hot stamping satisfies following relational
expressions
(1) to (4), where [C] denotes a C content, [Si] denotes a Si content, [Mn]
denotes a Mn
content, [Cr] denotes a Cr content, A (J/cm2) denotes absorbed energy in a
Charpy impact
test at ¨40 C when a flat plate is hardened using a die, B(Hv) denotes a
hardness when
the steel plate for hot stamping is heated to an austenite region, then cooled
to room
temperature at a cooling velocity of 10 C/s, and hardened, and C(Hv) denotes a
hardness
when the steel plate for hot stamping is heated to the austenite region, then
cooled to
room temperature at a cooling velocity of 30 C/s, and hardened
[Math. 1]
7
[C]+-2 1 +-8-4 [Si]+[Mn] [Cr]-Ecr1- -- >0 = = =(1)
9 9 9
[Math. 2]
B>-4.0A+627 = = = (2)
[Math. 3]
516 = = = (3)
[Math. 4]
C - B1 35 = = = (4)
[0011] According to the present invention, it is possible to provide a steel
plate for hot
stamping which can provide a molded product which exhibits excellent hardness
stability
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in addition to the balance between strength and toughness while suppressing
increases in
labor and cost in the hot stamping process.
Brief Description of Drawings
[0012] FIG. 1 is a graph illustrating the relation between absorbed energy in
a Charpy
impact test when a flat plate is hardened using a die and hardness when
hardening is
performed at a cooling velocity of 10 C/s.
FIG. 2 is a diagram schematically illustrating a hot stamping process.
FIG. 3 is a schematic diagram illustrating the respective dimensions of a test
piece
used in
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1 '
a Charpy pendulum impact test.
FIG. 4 is a schematic diagram illustrating the respective dimensions of a test
piece used in
a hardness test.
Description of Embodiment
[0013] Hereinafter, a steel plate for hot stamping according to an embodiment
of the present
invention will be described in detail.
[0014] (Steel plate for hot stamping)
The steel plate for hot stamping according to the present embodiment contains,
in % by mass,
C: 0.25% or more and 0.4% or less,
Si: 1.05% or more and 1.4% or less,
Mn: 0% or more and 1.4% or less,
Cr: 0.6% or more and 3.0% or less,
P: 0% or more and 0.03% or less,
S: 0% or more and 0.02% or less,
Al: 0.01% or more and 1% or less,
N: 0% or more and 0.01% or less,
B: 0.0005% or more and 0.005% or less,
Ti: 0.005% or more and 0.1% or less, and
iron and inevitable impurities as remainder.
This steel plate for hot stamping exhibits excellent hardness stability in
addition to a
balance between strength and toughness as a following relational expression
(1) is satisfied, where
[C] denotes a C content, [Si] denotes a Si content, [Mn] denotes a Mn content,
and [Cr] denotes a
Cr content.
[0015]
[Math. 1]
7 7
[C]+-2 [Si1+ ¨9 [MnD- ¨8 [Cr] ¨ ¨4 >0 = = = (1)
9 9
[0016] In order to obtain a steel plate for hot stamping which is excellent in
both the balance
between strength and toughness and the hardness stability, the present
inventors have conducted
extensive studies on the component composition of steel plate. From the
description in Non Patent
Literature 2, it has been expected that the cooling velocity of a normal
member fluctuates in a range
of 30 C/s to 10 C/s in the hot stamping process due to the clearance between
the die and the steel
plate and an increase in the die temperature. For this reason, the present
inventors have focused on
suppression of the variation in hardness even when the cooling velocity
fluctuates in addition to the
balance between strength and toughness, and conducted detailed investigations
on the component
system of steel plate for achieving this. As a result, the present inventors
have newly found out
that the balance between strength and toughness and hardness stability can be
both achieved by
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adjusting the balance among the contents of C, Si, Mn, and Cr so that the
relational expression (1) is
satisfied as well as each component composition in a steel plate satisfies the
above range, and thus
conceived the present invention.
[0017] First, each component composition in the steel plate for hot stamping
according to the
present embodiment will be described in detail.
[0018] [C (carbon): 0.25% by mass or more and 0.4% by mass or less]
The C content determines the strength of the steel plate after die cooling. In
order to
obtain sufficient strength of the steel plate, the C content is 0.25% by mass
or more, preferably
0.255% by mass or more, more preferably 0.260% by mass or more.
[0019] However, when the C content is excessive, the strength of the steel
plate after hot rolling
may increase and this may lead to cracking during cold rolling and
deterioration in weldability.
Hence, the C content is 0.4% by mass or less, preferably 0.38 % by mass or
less, more preferably
0.36 % by mass or less.
[0020] [Si (silicon): 1.05% by mass or more and 1.4% by mass or less]
Si contributes to the hardness stability of the steel plate by increasing the
temper softening
resistance. Si also has an effect of preventing scale peeling off after die
cooling when the surface
of the steel plate is not plated. In order to exert these effects, the Si
content is 1.05% by mass or
more.
[0021] On the other hand, Si facilitates the generation of retained austenite
(y) and promotes a
decrease in yield strength (YS) and segregation of Mn. Hence, the Si content
is 1.4% by mass or
less, preferably 1.35% by mass or less.
[0022] [Mn (manganese): 0% by mass or more and 1.4% by mass or less]
Mn is one of the important elements contained in the steel plate for hot
stamping according
to the present embodiment and contributes to an increase in the strength of
the steel plate after die
cooling by enhancing the hardenability of the steel plate. In order to exert
this effect, the Mn
content is preferably 0.5% by mass or more, more preferably 0.6% by mass or
more, still more
preferably 0.8% by mass or more.
[0023] On the other hand, in the investigations to achieve both the strength
and toughness of the
steel plate after die cooling, it has been confirmed that when Mn is
excessive, coarse carbides
precipitate during die cooling and brittle fracture is caused when shocking
stress is applied to the
steel plate in a low temperature environment. Hence, the Mn content is 1.4% by
mass or less,
preferably 1.35% by mass or less, more preferably 1.30% by mass or less.
[0024] Mn is an element that is inevitably mixed into the steel plate and it
is thus difficult to set the
Mn content to 0% by mass.
[0025] [Cr (chromium): 0.6% by mass or more and 3.0% by mass or less]
Cr is one of the important elements in the steel plate for hot stamping
according to the
present embodiment. In the investigations to achieve both the strength and
toughness of the steel
plate after die cooling, it has been confirmed that Cr contributes to securing
of the hardness at a low
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3
cooling velocity (for example, 10 C/s) as well as suppression of coarse
carbide precipitation during
die cooling and thus suppresses brittle fracture when shocking stress is
applied to the steel plate in a
low temperature environment. In order to exert these effects, the Cr content
is 0.6% by mass or
more, preferably 0.8% by mass or more, more preferably 1.05% by mass or more.
[0026] On the other hand, when Cr is excessively contained in the steel plate,
the strength of the
steel plate after hot rolling increases and this leads to cracking of the
steel plate during cold rolling
and deterioration in pickling property after hot rolling. Hence, the Cr
content is 3.0% by mass or
less, preferably 2.5% by mass or less.
[0027] [P (phosphorus): 0% by mass or more and 0.03% by mass or less]
From the viewpoint of weldability of the member, toughness, and prevention of
surface
flaws, it is required to regulate the upper limit of P content. Hence, the P
content is 0.03% by mass
or less, preferably 0.025% by mass or less, more preferably 0.02% by mass or
less.
[0028] P is an element that is inevitably mixed into the steel plate and it is
thus difficult to set the P
content to 0% by mass.
[0029] [S (sulfur): 0% by mass or more and 0.02% by mass or less]
S forms MnS to decrease the uniformity of the Mn concentration distribution
and also
deteriorate the weldability of the steel plate. Hence, the S content is 0.02%
by mass or less,
preferably 0.018% by mass or less, more preferably 0.015% by mass or less.
[0030] S is an element that is inevitably mixed into the steel plate as P and
it is thus difficult to set
the S content to 0% by mass.
[0031] [Al (aluminum): 0.01% by mass or more and 1% by mass or less]
Al is an element that acts as a deoxidizer. In order to exert this effect, the
Al content is
0.01% by mass or more, preferably 0.015% by mass or more.
[0032] However, when Al is excessively contained in the steel plate, the
hardness after die cooling
decreases and excessive generation of Al2O3 deteriorates the low temperature
toughness. Hence,
the Al content is 1% by mass or less, preferably 0.8% by mass or less, more
preferably 0.1% by
mass or less. The Al content here means the content of Al(sol.A1) in a solid
solution state.
[0033] [N (nitrogen): 0% by mass or more and 0.01% by mass or less]
N is an element that is inevitably mixed into the steel plate. When N is
excessively
contained in the steel plate, the amount of solid solution B in the steel
plate decreases as N forms a
boride and this leads to deterioration in hardenability. Hence, the N content
is 0.01% by mass or
less, preferably 0.008% by mass or less, more preferably 0.005% by mass or
less.
[0034] [B (boron): 0.0005% by mass or more and 0.005% by mass or less]
B is an important element for improving the hardenability of the steel plate.
By adding an
appropriate amount of B to the steel plate, the hardenability is enhanced and
this makes it possible
to stably increase the strength of the steel plate after die cooling. In order
to exert this effect, the B
content is 0.0005% by mass or more, preferably 0.0010% by mass or more, more
preferably
0.0015% by mass or more.
CA 03094926 2020-09-23
[0035] On the other hand, when B is excessively contained in the steel plate,
a coarse iron-boron
compound precipitates and this leads to deterioration in toughness. Hence, the
B content is
0.0050% by mass or less, preferably 0.0045% by mass or less, more preferably
0.0030% by mass or
less.
[0036] [Ti (titanium): 0.005% by mass or more and 0.1% by mass or less]
Ti decreases the amount of BN generated in the steel plate by forming TiN.
This
increases the amount of solid solution B in the steel plate and makes it
possible to enhance the
hardenability improving effect by B. In order to exert this effect, the Ti
content is 0.0050% by
mass or more, preferably 0.010% by mass or more, more preferably 0.015% by
mass or more.
[0037] On the other hand, when Ti is excessively contained in the steel plate,
a carbide precipitates
at the crystal grain boundaries and the hardenability of the steel plate
deteriorates. Hence, the Ti
content is 0.1% by mass or less, preferably 0.08% by mass or less, more
preferably 0.06% by mass
or less.
[0038] The steel plate for hot stamping according to the present embodiment
may further contain
one or more selected from the group consisting of Mo, Nb, and V or one or more
selected from the
group consisting of Cu and Ni in addition to the above component composition.
The ranges of
component compositions of these will be described below. These elements are
not essential
elements in the steel plate for hot stamping of the present invention and may
not be added.
[0039] [Mo (molybdenum): 0% by mass or more and 1.0% by mass or less]
Mo is an element that contributes to the improvement in hardenability of the
steel plate.
In order to exert this effect, the Mo content is preferably 0.01% by mass or
more. However, when
Mo is excessively contained in the steel plate, the strength of the steel
plate before hot molding is
increased. In order to prevent this, the Mo content is preferably 1.0% by mass
or less.
[0040] [Nb (niobium) and V (vanadium): 0% by mass or more and 0.1% by mass or
less]
Nb and V form fine carbides and have the effect of refining the structure of
steel by the
pinning effect. V also has a secondary hardening action by being precipitated
during tempering.
In order to exert these effects, the Nb and V contents are both preferably
0.0008% by mass or more.
[0041] However, when Nb and V are excessively contained in the steel plate,
coarse carbides are
formed and this becomes the starting point of fracture to lead to
deterioration in toughness. Hence,
the Nb and V contents are both preferably 0.1% by mass or less, more
preferably 0.08% by mass or
less, still more preferably 0.07% by mass or less.
[0042] [Cu (copper) and Ni (nickel): 0% by mass or more and 0.5% by mass or
less]
Cu and Ni are preferably added when it is required to improve the delayed
fracture
properties of the member. However, when Cu and Ni are excessively contained in
the steel plate,
flaws may be generated on the surface of the steel plate and finally on the
surface of the member.
Hence, it is preferable that the Cu and Ni contents are each 0.5% by mass or
less and it is more
preferable that the sum of the Cu and Ni contents is 0.5% by mass or less.
[0043] The steel plate for hot stamping according to the present embodiment
exhibits excellent
6
hardness stability in addition to the balance between strength and toughness
as the
following relational expression (1) is satisfied by adjustment of the balance
among the
contents of C, Si, Mn, and Cr. In this relational expression (1), [C] denotes
the C content
(% by mass) in the steel plate for hot stamping. [Si] denotes the Si content
(% by mass)
in the steel plate for hot stamping. [Mn] denotes the Mn content (% by mass)
in the steel
plate for hot stamping. [Cr] denotes the Cr content (% by mass) in the steel
plate for hot
stamping.
[0044]
[Math. 1]
[C]+-2 ¨7 ¨8 [Si]+ [Mn]+ [Cr]¨ ¨7 >0 = = = (1)
9 9 9 4
[0045] As the relational expression (1) is satisfied as well as the respective
component
compositions satisfy the component ranges, the steel plate for hot stamping
according to
the present embodiment exhibits excellent hardness stability as well as is a
steel plate
having an excellent balance between the strength after hardening by die
cooling and the
low temperature toughness. Specifically, the following relational expressions
(2), (3),
and (4) are all satisfied where, A (J/cm2) denotes the absorbed energy in a
Charpy impact
test at ¨40 C when a flat plate is hardened using a die, B(Hv) denotes the
hardness when
the steel plate for hot stamping is heated to the austenite region, then
cooled to room
temperature at a cooling velocity of 10 C/s, and hardened, and C(Hv) denotes
the
hardness when the steel plate for hot stamping is heated to the austenite
range, then
cooled to room temperature at a cooling velocity of 30 C/s, and hardened.
[0046]
[Math. 2]
B>-4.0A+627 = = = (2)
[0047]
[Math. 3]
B 516 = = = (3)
[0048]
[Math. 4]
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IC-BI 35 = = = (4)
[0049] The relational expression (2) is an index of the balance between the
strength and
toughness of the steel plate newly devised by the present inventors and is an
important
concept when considering the balance between the strength and toughness of the
steel
plate for hot stamping. In the course of investigations on the balance between
strength
and toughness, the present inventors have focused on the hardness when the
cooling
velocity is 10 C/s and the toughness after die cooling of a flat plate. In the
die cooling of
a flat plate, ideal cooling conditions in which a
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=
clearance is not generated between the die and the steel plate in the hot
stamping process are taken
into consideration. By using the relational expression (2), it is possible to
more faithfully evaluate
the balance between strength and toughness when the steel plate for hot
stamping is processed into a
member (molded product).
[0050] The graph of FIG. 1 illustrates the relation between the absorbed
energy A (horizontal axis)
in a Charpy impact test at ¨40 C when a flat plate is hardened using a die and
the hardness B
(vertical axis) of the steel plate when being hardened at a cooling velocity
of 10 C/s. The straight
line (1) in this graph corresponds to the relational expression (2). The
straight line (2) in this
graph corresponds to an equation of B = 516.
[0051] The horizontal axis (A) of the graph of FIG. 1 assumes the toughness at
the most brittle
portion of the member after die cooling. In other words, when a flat plate is
subjected to die
cooling, the die and the steel plate are in contact with each other in an
ideal state and the cooling
velocity is thus high. For this reason, the strength after cooling is high
but, on the other hand, the
flat plate is extremely brittle. In other words, this horizontal axis has a
meaning as toughness at
the most brittle portion when the steel plate for hot stamping is molded into
a member (molded
product).
[0052] On the other hand, the vertical axis (B) of the graph of FIG. 1 assumes
the hardness of the
most softened portion of the member after die cooling. As described above, in
the hot stamping
process, a clearance may be generated between the die and the steel plate and
the die temperature
may rise. For this reason, the member after die cooling has a portion that is
cooled at a low
cooling velocity and has a low hardness (strength). From the description in
Non Patent Literature
2, it is assumed that the minimum cooling velocity during die cooling is about
10 C/s. Hence, this
vertical axis has a meaning as hardness (strength) at the most softened
portion of the member
(molded product) after die cooling. Consequently, by using these two axes, the
toughness of the
weakest portion when shocking stress is applied to the member after being
molded and the strength
of the weakest portion when static stress is applied to this member can be
evaluated.
[0053] Usually, in the hardness region in which B is 516 Hv or more, the
strength and toughness of
a steel plate are in a trade-off relation and thus the toughness tends to
deteriorate when the strength
of the steel plate is improved. In other words, it is difficult to improve
both the strength and
toughness of a steel plate and it is normal that the distribution of A and B
exists in the region below
the straight line (1) in the graph of FIG. 1.
[0054] The straight line (2) is one index that indicates the hardness
stability. During continuous
operation of the die in the hot stamping process, the temperature of the die
may rise and a clearance
may be generated between the die and the steel plate. Due to these factors,
the cooling velocity of
the steel plate during hardening decreases and the hardness of the steel plate
after being hardened
decreases as the cooling velocity decreases, Even in the case of a steel plate
in which the balance
between strength and toughness is improved by the refinement of crystal
grains, it is usually
difficult for the hardness when the steel plate is hardened in a low cooling
velocity region (10 C/s)
8
to satisfy a range of 516 Hv or more. Hence, even in the case of a steel plate
in which the
balance between strength and toughness is improved by the refinement of
crystal grains,
it is normal that the distribution of A and B exists in the region below the
straight line (2)
in FIG. 1.
[0055] In contrast, as a result of extensive studies conducted by the present
inventors, it
has been revealed that the distribution of A and B are located in the region
above the
straight lines (1) and (2) in FIG. 1 in the steel plate for hot stamping which
satisfies the
relational expression (1). Hence, the steel plate for hot stamping according
to the present
embodiment exhibits excellent hardness stability in addition to the balance
between
strength and toughness. In other words, this steel plate for hot stamping has
an excellent
balance between strength and toughness that satisfies the relational
expression (2) and
can realize a hardness at a certain degree or more even when being cooled at
the
minimum cooling velocity of 10 C/s.
[0056] The relational expression (4) is another index of the hardness
stability of steel
plate. When the die temperature rises or a clearance is generated between the
die and the
steel plate during hot stamping, the cooling velocity of the steel plate may
decrease and
the hardness of the steel plate after being hardened may become unstable. As
described
above, it is usually difficult to satisfy the relational expression (4) since
the hardness
stability decreases when the crystal grains are refined.
[0057] In contrast, as a result of extensive studies conducted by the present
inventors, it
has been revealed that a hardness exceeding 516 Hv is attained after the steel
plate is
hardened even in a low cooling velocity region of 10 C/s as well as the
difference in
hardness between a case having a cooling velocity of 30 C/s and a case having
a cooling
velocity of 10 C/s is suppressed to 35 Hv or less in the steel plate for hot
stamping in
which the relational expression (1) is satisfied as well as the respective
components
satisfy the component ranges. 30 C/s is an ideal cooling velocity during die
cooling,
which has been confirmed by an experiment and the like while 10 C/s is the
minimum
cooling velocity expected as described above. In other words, the relational
expression
(4) is an index indicating that the difference (variation) in hardness after
hardening is
small between the upper and lower limits of the cooling velocity assumed in
hot
stamping. According to the steel plate for hot stamping of the present
embodiment, it is
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possible to stabilize the hardness of the steel plate after being hardened to
the extent to
which the relational expression (4) is satisfied regardless of the temperature
rise of the die
and the generation of clearance between the die and the steel plate.
[0058] The steel plate for hot stamping of the present invention may be a base
steel plate
having a surface not subjected to a plating treatment or a plated steel plate
having a
surface subjected to a plating treatment.
[0059] (Method for manufacturing steel plate for hot stamping)
Next, a method for manufacturing the steel plate for hot stamping according to
the
present embodiment will be described.
[0060] First, a slab manufacturing process is performed. In this process, a
slab is
obtained by melting steel according to a conventional method, pouring the
molten steel
into a mold, and
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performing continuous casting. In this process, the component composition of
the steel is adjusted
during melting so that the compositions of the respective components contained
in the slab satisfy
the above ranges and the contents of C, Si, Mn, and Cr satisfy the relational
expression (1).
[0061] Next, a hot rolling process is performed. In this process, the slab
obtained in the above
process is first disposed in a heating furnace, heated to a predetermined
temperature (for example,
1200 C), and held at the heating temperature for a predetermined time (for
example, 30 minutes).
[0062] Next, the heated slab is placed upstream of the hot rolling line.
Thereafter, the slab is
rolled into a steel plate having a predetermined thickness by allowing the
slab to sequentially pass
through between the rolls of the rolling stands of the rough rolling mill and
the finishing rolling mill
and allowing the slab to flow downstream. Thereafter, the steel plate after
being hot-rolled is
cooled to a predetermined temperature in a cooling apparatus and then wound by
a coiler.
[0063] Next, a cold rolling process is performed. In this process, the scale
(oxides of iron)
generated on the surface of the steel plate in the hot rolling step is first
washed off with an acid
(pickling) and then the hot-rolled steel plate is further rolled so that the
thickness further decreases.
Specifically, the hot-rolled steel plate after being subjected to pickling is
allowed to pass through
between the rolls of the rolling stands so that the hot-rolled steel plate is
further thinned. The cold-
rolled steel plate obtained by the above processes is the steel plate for hot
stamping according to the
present embodiment.
[0064] (Hot Stamping)
Next, hot stamping performed using the steel plate manufactured by the above
processes
will be described with reference to FIG. 2. First, a steel plate for hot
stamping 1 manufactured by
the above processes is heated in a predetermined heating furnace 2 to a
temperature equal to or
higher than the austenite transformation temperature. Thereafter, the steel
plate for hot stamping 1
after being heated is disposed between dies 3 and 4 and press-molded into a
desired shape by the
dies 3 and 4. At this time, the steel plate for hot stamping 1 is cooled by
coming into contact with
the dies 3 and 4, and hardening is performed at the same time as molding.
Thereafter, the steel
plate after being hardened is taken out from the dies 3 and 4 as a molded
product 5 (molded
member).
[0065] The molded product 5 has the same component composition as that of the
steel plate for hot
stamping 1 according to the present embodiment described above and is one in
which the balance
among the contents of C, Si, Mn, and Cr is adjusted so that the relational
expression (1) is satisfied.
Hence, the molded product 5 exhibits excellent hardness stability in addition
to the balance between
strength and toughness and can be utilized in various applications including
members for motor
vehicles.
[0066] The outline of the above-described embodiment is as follows.
[0067] A steel plate for hot stamping according to the present embodiment
contains,
in % by mass,
C: 0.25% or more and 0.4% or less,
CA 03094926 2020-09-23
Si: 1.05% or more and 1.4% or less,
Mn: 0% or more and 1.4% or less,
Cr: 0.6% or more and 3.0% or less,
P: 0% or more and 0.03% or less,
S: 0% or more and 0.02% or less,
Al: 0.01% or more and 1% or less,
N: 0% or more and 0.01% or less,
B: 0.0005% or more and 0.005% or less,
Ti: 0.005% or more and 0.1% or less, and
iron and inevitable impurities as remainder. This steel plate for hot stamping
exhibits
excellent hardness stability in addition to a balance between strength and
toughness as a following
relational expression (1) is satisfied, where [C] denotes a C content, [Si]
denotes a Si content, [Mn]
denotes a Mn content, and [Cr] denotes a Cr content.
[0068]
[Math. 1]
2
[SO+ ¨7 ¨8 [Mni [Cr]--7 >0 = = = (1)
9 9 4
[0069] The steel plate for hot stamping may contain, in % by mass, one or more
selected from the
group consisting of
Mo: 0% or more and 1.0% or less,
Nb: 0% or more and 0.1% or less, and
V: 0% or more and 0.1% or less.
[0070] The steel plate for hot stamping may contain, in % by mass, one or more
selected from the
group consisting of
Cu: 0% or more and 0.5% or less, and
Ni: 0% or more and 0.5% or less.
Examples
[0071] Hereinafter, the present invention will be described in more detail
based on examples.
However, the present invention is not limited to the following examples, it is
also possible to carry
out the present invention by adding changes within a range that is compatible
with the above-
mentioned and below-mentioned gist, and all of them are included in the
technical scope of the
present invention.
[0072] <Manufacture of steel plate for hot stamping>
First, a slab was manufactured by melting steel (the remainder being iron and
inevitable
impurities) having the component composition shown in Nos. 1 to 17 in the
following Table 1.
This molten slab was heated to 1200 C, held for 30 minutes, and then hot-
rolled. The finishing
temperature was set to 900 20 C, and the finishing plate thickness was set
to 2.8 mm. Thereafter,
the hot-rolled steel plate was cooled to a winding temperature (CT
temperature) at a cooling
11
CA 03094926 2020-09-23
velocity of 20 C/s to 30 C/s, held at 650 C for 30 minutes, and then cooled in
the furnace.
Thereafter, the hot-rolled steel plate was subjected to pickling and cold-
rolled so that the steel plate
had a thickness of 1.4 mm.
[0073] <Charpy impact test>
First, the cold-rolled steel plate fabricated according to the above procedure
was cut and
hardened. Hardening was performed under the following conditions by a die
quench method using
a flat plate simulating a die (testing machine: JIS Charpy impact tester
(300J)).
[0074] [Hardening conditions]
Steel plate dimensions before hardening: 1.4 mm x 70 mm x 150 mm
Steel plate temperature: 900 C
Steel plate temperature holding time after steel plate reaches 900 C: 100
seconds
Cooling time: about 15 seconds
Die quench start temperature: 700 C
Die quench load: 2000 kgf
Bottom dead center holding time: 30 seconds
[0075] Next, a Charpy pendulum impact test was performed using the cold-rolled
steel plate after
being subjected to the hardening. This test was performed in conformity with
JIS 2242 "Charpy
impact test method for metal materials" except the dimensions of the test
piece. The dimensions
of the test piece used in the present test are as follows. The reference
numerals indicating the
respective dimensions correspond to the reference numerals illustrated in FIG.
3.
[0076] [Test piece dimensions]
Test piece height hl: 10 mm 0.05 mm
Test piece length L: 55 mm 0.6 mm
Test piece width b: 1.4 mm 0.05 mm
Notch shape: V notch
Notch angle: 45 2
Notch bottom radius: 0.25 mm 0.025 mm
Height under notch h2: 8 mm 0.05 mm
Angle between test piece longitudinal direction and notch symmetry plane: 90
2
Angle between adjacent surfaces eliminating fracture surface: 90 2
[0077] A test piece having the above dimensions was disposed in liquid
nitrogen adjusted to have a
temperature of ¨40 C 1 C and held for at least 10 minutes. Thereafter, the
test piece was taken
out from the liquid nitrogen and placed on a support, and an impact was made
on the test piece. At
this time, the time until the impact was made after the test piece was placed
on a support was set to
seconds or less.
[0078] A HS Charpy impact tester (300J) was used as a tester, and an impact
blade having a radius
of 2 mm was used. The number of test pieces was two, and the average value of
two measured
values was used for evaluation.
12
CA 03094926 2020-09-23
[0079] <Evaluation on scale adhesive property>
Hardening was performed by the die quench method under the same conditions as
those in
the Charpy impact test described above and then the degree of scale peeling
off on the surface of the
steel plate was visually confirmed to evaluate the adhesive property of scale.
It was evaluated as
"0" when the area ratio of the surface of the steel plate in which scale
peeling off occurred was
14% or less, and it was evaluated as "x" when the area ratio exceeded 14%.
[0080] <Hardness test>
First, the cold-rolled steel plate fabricated according to the above procedure
was processed
into a test piece having a shape illustrated in FIG. 4. In FIG. 4, Li is 10
mm, L2 is 2 mm, L3 is
1.4 mm, L4 is 0.7 mm, L5 is 3 mm, and L6 is 1 mm. Hardening was performed
using this test
piece under the following conditions.
[0081] [Hardening conditions]
Rate of temperature rise when converting to austenite: 10 C/s
High temperature holding: held at 900 C for 100 seconds
Cooling velocity: constant cooling from 900 C to room temperature at 10 C/s or
30 C/s
[0082] A hardness test was performed using the test piece after being
subjected to the hardening in
conformity with the "Vickers hardness test method" prescribed in JIS Z 2244.
In this test, five
points were measured at the positions to be the 1/4 plate thickness from the
surface of the test piece
at a test load of 9.8 N, and evaluation was performed using the average value
of these.
[0083] The following Tables 1 and 2 present each of the component composition
(% by mass),
absorbed energy A (J/cm2) in a Charpy impact test at ¨40 C, Vickers hardness B
(Hv) when the
cooling velocity is 10 C/s, Vickers hardness C (Hv) when the cooling velocity
is 30 C/s, difference
in hardness (Hy) between a case having a cooling velocity of 30 C/s and a case
having a cooling
velocity of 10 C/s, value on the left side of the relational expression (1),
value when the right side
of the relational expression (2) is subtracted from the left side, and
evaluation results on scale
adhesive property for each of Nos. 1 to 17 steel plates.
[0084] In the graph of FIG. 1, the respective data for Nos. 1 to 17 steel
plates are plotted. The data
for Nos. 1 to 9 and 14 to 17 are marked with black circles, and the data for
Nos. 10 to 13 are marked
with white circles.
[0085]
[Table 1]
13
Component composition (% by mass)
Si Mn Cr P S Ai Cu ti Mo Nb Ti N V B 0
No.1 0.345 1.24 1.23 0.82 0.0100 0.0019 0.041 -
- - - 0.022 0.0041 - 0.0021 ,0.0005
, No2 0.351 1.22 1.22 113 0.0090 ,0.0011 0.040
- - - - , 0.022 0.0044 - 0.0020 0.0006
No.3 0.337 1.22 0.81 0.82
0.0070 0.0018 0.041 0.021 0.0038 - 0.0018 0.0006,
NoA 0.336 1.20 0.81 1.21 0.0090 0.0018 0.040 - -
- 0.021 0.0034 - 0.0019 0.0009
No.5 0.342 1.22 0.22 1.21 0.0080 0.0009 0.040 -
- - 0.022 0.0044 - 0.0016 0.0007
No.6 0.326 1.21 0.21 1.52 0.0090 0.0014 0.040 -
- - - 0.022 0.0040 - 0.0020 0.0008
, No.7 , 0.332 1.22 0.21 2.04 , 0.0100 0.0016 0.039 -
- - - 0.022 0.0040 - 0.0020 0.0007_
No.8 0.294 1.19 0.85 0.83 0.0100, 0.0010 0.038 , -
- - , - 0.022 0.0047 0.001 0.0014 -
No.9 0.290 , 1.19 1.23 0.65 0.0100 0.0020 0.038 - -
- , 0.022 0.0039 - 0.0014 -
No.10 0.289 0.01 0.23 , 1.24 0.0040 0.0009 0.040
- - 0.004 0.020 0.0009 0.001 _0.0011 0.0009
No.11 0.315 1.24 1.20 0.01 0.0040 0.0010 0.041 -
- - 0.021 0.0041 - 0.0015 0.0006
No.12 0.326 1.21 1.21 0.01 0.0040 0.0010 0.041 -
- - - , 0.020 0.0045 - 0.0017 0.0009
z: No.13 0.266 1.19 1.21 0.01 0.0040
0.0012 0.041 - - - 0.004 0.020 0.0005 0.001 0.0018
0.0005
No.14 0.271 1.23 1.19 0.60 0.0040 0.0020 0.038 0.10
- - - 0.022 0.0043 - 0.0017 0.0007 ig
No.15 0.294 1.26 1.18 , 0.61 0.0040 0.0010 0.038 -
0.10 - - 0.021 0.0041, - 0.0016 0.0007,
No.16 0.281 1.24 1.18 0.60 0.0040 0.0020 0.038 0.10
0.10 - - 0.021 0.0046 - 0.0018 0.0006
No.17 0.335 1.21 0.82 1.19 0.0040 0.0013 0.036 -
- 0.19 - 0.021 0.0023 - 0.0019_ 0.0007
CA 03094926 2020-09-23
,
. .
[0086]
[Table 2]
V" ________ )
11 1 0 0 0 0 0 0 0 0 0 X 0 0 0 0 0 0 0
J õ
..... ,
2 :g 28 83 3 28 28 I:: 24 EP S; 7 liz g a; g 38
1
. , .
--"P
..L.'"
es
Is'
CO !NI CO === CO Ce) .53- Q3 1'4 r'' in co co In I
tst 71 44 c4 ul
Q oi Q Q 6 cog:pc:61'5 4P? c, czo 4=3 cro
irr's
l'a
..t.
Q .
. õ
4..=
C4 el rs a, .i. C4 CO co 22 w 41) r-
1
".'. "w ". C4 -- ...". C4 ,... = - ea ...... el I- Cft mt. ,...
==== r. ===== ======
1
. 4
1 g 40
1 ig CM ;71 g 2 g 28 114 :i: " :8 ^- (0c" 38 -- 8
E c,, in CO tea tea in to in 74 an .01. in Ira IS rti;
U)ret co
ii" =,
t
"¨___ 4)) CI) 04 00 CO CO C) 00 Oa le) C= 00 CA CO E Z., .104 tki 0,
111 Cb 0) CO rs er in rs ... cv csa C=4 CO r.., .--
co ICI Ile) 10 leb tal lee le) lib let CO =ce V' C1P ir, V,
I
.
.1
. ,
.0 .
,...- .4. 411) sy rs co) era .ler in rs 0 co 4t CO CC! crt '-
yr 0 a 0, u, as ¨ 41, 1`.. CO e4 cr) in .1.- CI) 0.= Csi
CM C67 C., CO hb V) Co, wir r#) co ge Col (0 W) gi" If) Co)
H '
. ,
." ei 0/ .III: tO cO e..: aq at CO ..... CV el 41) It) Ci, .....
1147244 4214 ggggl ggl
=
[0087] <Discussion>
The following can be discussed based on Tables 1 and 2.
[0088] In Nos. 1 to 9 and 14 to 17, the contents of C, Si, Mn, and Cr satisfy
the relational
expression (1) as well as the contents of C, Si, Mn, Cr, P, S, Al, N, B, and
Ti in the steel
plate each satisfy the ranges in the present invention. In this case, the
value of "B + 4A ¨
627" is a positive value, the relational expression (2) is satisfied, and thus
the steel plate
has an excellent balance between strength and toughness. Moreover, in Nos. 1
to 9 and
14 to 17, it is "B 516" and "C ¨ B < 35", the relational expressions (3) and
(4) are also
satisfied, and thus the steel plate also exhibits excellent hardness
stability. This is clear
from the fact that the data (black circles) for Nos. 1 to 9 and 14 to 17 exist
in the regions
above the straight lines (1) and (2) in the graph of FIG. 1. The evaluation
results on the
scale adhesive property are all "0".
[0089] In contrast, in Nos. 10 to 13 that do not satisfy the requirements
regulated in the
present invention, a steel plate excellent in both the balance between
strength and
toughness and the hardness stability is not obtained as to be described below.
As
illustrated in the graph of FIG. 1, the data (white circles) for Nos. 10 to 13
all exist in the
regions below the straight lines (1) and (2).
[0090] In No. 10, the Si content is less than 1.05% by mass and the value of
"[C] +
2/9[Si] + 7/9[Mn] + 8/9[Cr] ¨ 7/4" is a negative value, thus the value of "B +
4A ¨ 627"
is a negative value and the balance between strength and toughness is poor.
The hardness
B when the cooling velocity is 10 C/s is less than 516 Hv, the difference in
hardness
between a case having a cooling velocity of 30 C/s and a case having a cooling
velocity
of 10 C/s also exceeds 35 Hv, and the hardness stability is also poor. The
evaluation
results on the scale adhesive property are also "x".
[0091] In Nos. 11 to 13, the Cr content is less than 0.6% by mass and the
value of ''[C] +
2/9[Si] + 7/9[Mn] + 8/9[Cr] ¨ 7/4" is a negative value, thus the value of "B +
4A ¨ 627"
is a negative value and the balance between strength and toughness is poor.
The hardness
B when the cooling velocity is 10 C/s is less than 516 Hv, the difference in
hardness
between a case having a cooling velocity of 30 C/s and a case having a cooling
velocity
of 10 C/s also exceeds 35 Hy, and the hardness stability is also poor.
16
CA 3094926 2022-01-31
[0092] It should be understood that the embodiments and examples disclosed
herein are
illustrative in all points and not restrictive. The scope of present invention
is shown not
by the above description, and is intended to include meanings equivalent to
all
modifications within the scope.
16a
CA 3094926 2022-01-31
