The present invention relates to a steel sheet for hot stamping, a method for
manufacturing the same, a hot stamped component, and a method for manufacturing the
same.
The present application claims priority based on Japanese Patent Application
No. 2020-174457 filed on October 16, 2020, the content of which is incorporated herein
by reference.
[Related Art]
[0002]
In recent years, there has been a demand for further improvement in fuel
efficiency of vehicles from the viewpoint of regulations on greenhouse gas emissions
associated with global warming. Most vehicle structures are made of steel, especially,
steel sheets. Therefore, the steel sheet is thinned to reduced weight, thereby reducing
the weight of a vehicle body. As a result, fuel efficiency is improved. However,
when the thickness of the steel sheet is simply reduced to reduce the weight of the steel
sheet, there is concern that the strength of the structure will be reduced and collision
safety will be reduced. Therefore, in order to reduce the thickness of the steel sheet, it
is necessary to increase the mechanical strength of the steel sheet used such that the
strength of the structure is not reduced. Therefore, in recent years, the application of
high strength steel sheets to components for a vehicle has been expanding more and
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more in order to reduce the weight of the vehicle body and to ensure collision safety.
[0003]
Similarly, vehicles are being converted into electric vehicles (EVs) from the
viewpoint of global warming countermeasures. The EVs are generally much heavier
than gasoline vehicles. Therefore, there is a high demand for application of high
strength steel sheets. In addition, in recent years, colli sion regulations have been
strengthened. Therefore, for EV s, the application of ultra-high tensile strength steel
(steel material having a tensile strength of 980 MPa or more) to collision deformation
components has been examined.
[0004]
For example, Patent Document 1 discloses a method for manufacturing a high
strength steel sheet that has a good strength-ductility balance, a good strength-hole
expansibility balance, and very high stretch flangeability. Patent Document 1 also
discloses that tensile strength is equal to or greater than 980 MPa.
[0005]
However, in the case of the EVs, some vehicle types are heavy. Therefore,
even when ultra-high tensile strength steel is used, a sheet thickness of 3 mm or more
may be required from the viewpoint of rigidity and the like. It is impossible to form
the ultra-high tensile strength steel with this thickness using cold press forming.
[0006]
Therefore, as a method for reducing load during press forming, hot stamping
which performs hot forming and die quenching is attracting attention. In the hot
stamping, a material to be formed is heated to a high temperature. Then, the material
softened by heating is pressed and formed or is cooled at the same time as the
formation. That is, the material is heated to a high temperature to be softened. Then,
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the material is pressed in a softened state. Therefore, it is possible to easily press the
material.
However, in Patent Document 1, it is not considered that hot stamping is
performed on the cold-rolled steel sheet in the subsequent process.
[0007]
For example, Patent Document 2 discloses that hot stamping is performed on a
steel sheet having a tensile strength of 500 to 600 MPa and a thickness of 1.0 to 1.8 mm
to obtain a component having a tensile strength of 1400 MPa or more.
[0008]
However, in a case in which a steel sheet having a large sheet thickness as
described above is formed by hot stamping, in the hot stamping according to the related
art as in Patent Document 2, the steel sheet is not sufficiently quenched, and sufficient
strength is not obtained in the steel sheet after hot stamping (hot stamped component).
In addition, even in the case of a steel sheet having a small sheet thickness, when the
thin steel sheet is joined to steel sheets with different sheet thicknesses by tailor welded
blank (TWB), it is difficult to manage the clearance of a die and punch in a portion in
which the sheet thickness changes, and sufficient quenching may not be performed.
For this reason, the steel sheet to be subjected to hot stamping is required to
have sufficient strength after hot stamping even in a case in which a cooling rate is slow
during hot stamping.
However, this study has not been conducted in the related art including Patent
Document 2.
[0009]
In addition, in the related art, the hot stamping is mainly applied to a nondeformable
portion of a hot stamped component (a component whose deformation is
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suppressed to secure the safety of passengers), and the application of the hot stamping
to a deformable portion is not considered. That is, as described above, in a case in
which the application of the hot stamping to a collision deformation component is
considered, characteristics that can suppress cracks at the time of large deformation are
required. However, in the hot stamped component according to the related art, the
securing of these characteristics is hardly considered.
[Prior Art Document]
[Patent Document]
[0010]
[Patent Document 1] Japanese Patent No. 5407168
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2002-102980
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0011]
The invention has been made in view of the above problems. An object of the
invention is to provide a steel sheet for hot stamping that has high hardenability during
hot stamping (obtains high strength after hot stamping even in a case in which a cooling
rate during hot stamping is relatively slow) and good collision characteristics after hot
stamping (obtains characteristics that can suppress cracks during large deformation) and
a method for manufacturing the same. In addition, an object of the invention is to
provide a hot stamped component obtained by using the steel sheet for hot stamping as a
material and a method for manufacturing a hot stamped component using the steel sheet
for hot stamping.
[Means for Solving the Problem]
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[0012]
The inventors conducted thorough studies in order to obtain a steel sheet (steel
sheet for hot stamping) having high strength and good collision characteristics after hot
stamping. As a result, it was found that, when a microstructure of a steel sheet was
mainly composed of upper bainite and carbides with a predetermined size were present,
the hardenability of the steel sheet was improved, and hot stamping was performed on
the steel sheet to obtain a hot stamped component having high strength and good
collision characteristics (had high deformability and was hard to fracture at the time of
collision).
[0013]
The invention has been made based on the above findings and has the
following gist.
[1] According to one aspect of the invention, there is provided a steel sheet for
hot stamping including, as a chemical composition, by mass%: C: 0.060% to 0.120%;
Si: 0% to 0.70%; Mn: 1.60% to 3.00%; P: 0.100% or less; S: 0.0100% or less; N:
0.0100% or less; Al: 0.001% to 0.100%; Ti: 0.005% to 0.050%; B: 0.0005% to
0.0100%; Nb: 0% to 0.100%; V: 0% to 0.100%; W: 0% to 0.100%; Ni: 0% to 2.00%;
Cu: 0% to 2.00%; Cr: 0% to 2.00%; Mo: 0% to 2.00%; Sn: 0% to 0.200%; Ca: 0% to
0.0500%; Mg: 0% to 0.0500%; REM: 0% to 0.0500%; and a remainder of Fe and
impurities. A microstructure includes 70% or more of upper bainite by area ratio, and
a number density of iron carbides having a major axis of0.11lm or more included in the
upper bainite is equal to or greater than 4/llm2.
[2] In the steel sheet for hot stamping according to [1], the chemical
composition may contain, by mass%, one or more selected from the group consisting of:
Nb: 0.005% to 0.100%; V: 0.005% to 0.100%; W: 0.005% to 0.100%; Ni: 0.01% to
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2.00%; Cu: 0.01% to 2.00%; Cr: 0.01% to 2.00%; Mo: 0.01% to 2.00%; Sn: 0.005% to
0.200%; Ca: 0.0003% to 0.0500%; Mg: 0.0003% to 0.0500%; and REM: 0.0003% to
0.0500%.
[3] In the steel sheet for hot stamping according to [1] or [2], a tensile strength
may be less than 980 MPa.
[4] In the steel sheet for hot stamping according to any one of [1] to [3], the
steel sheet for hot stamping may have a plating layer on a surface thereof.
[5] In the steel sheet for hot stamping according to [4], the plating layer may be
a hot-dip galvanized layer, a hot-dip galvannealed layer, an electrogalvanized layer, or
anAl plating layer.
[6] According to another aspect of the invention, there is provided a method for
manufacturing a steel sheet for hot stamping. The method includes: a heating process
of heating a steel ingot or a slab having the chemical composition according to [1] to
1,15ooc to 1,300°C directly or after cooling the steel ingot or the slab; a hot rolling
process of performing hot rolling on the steel ingot or the slab after the heating process
such that a finishing temperature is equal to or higher than 85ooc to obtain a hot-rolled
steel sheet; a coiling process of coiling the hot-rolled steel sheet after the hot rolling
process at 640oc to 450°C; a holding process of holding the hot-rolled steel sheet after
the coiling process in a temperature range of 500°C to 450°C for 1.0 hour or longer; and
a cooling process of cooling the hot-rolled steel sheet after the holding process to room
temperature.
[7] The method for manufacturing a steel sheet for hot stamping according to
[6] may further include a cold rolling process of performing cold rolling on the hotrolled
steel sheet after the holding process at a cumulative rolling reduction of 30% to
70% to obtain a cold-rolled steel sheet.
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[8] According to still another aspect of the invention, there is provided a
method for manufacturing a steel sheet for hot stamping. The method includes: a
heating process of heating a steel ingot or a slab having the chemical composition
according to [1] to 1,150°C to 1,300°C directly or after cooling the steel ingot or the
slab; a hot rolling process of performing hot rolling on the steel ingot or the slab after
the heating process such that a finishing temperature is equal to or higher than 85ooc to
obtain a hot-rolled steel sheet; a coiling process of coiling the hot-rolled steel sheet after
the hot rolling process at 700°C to 500°C; a cooling process of cooling the hot-rolled
steel sheet after the coiling process to room temperature; a pickling process of pickling
the hot-rolled steel sheet after the cooling process; a cold rolling process of performing
cold rolling on the hot-rolled steel sheet after the pickling process at a cumulative
rolling reduction of 30% to 70% to obtain a cold-rolled steel sheet; an annealing process
of heating the cold-rolled steel sheet to an annealing temperature range of 840°C to
900°C and holding the cold-rolled steel sheet in the annealing temperature range for 10
to 2,000 seconds; and a heat treatment process of cooling the cold-rolled steel sheet
after the annealing process to a temperature range of 400°C to 600°C, holding the coldrolled
steel sheet in the temperature range for 100 to 1,000 seconds, and cooling the
cold-rolled steel sheet to room temperature.
[9] The method for manufacturing a steel sheet for hot stamping according to
[8] may further include a plating process of immersing the cold-rolled steel sheet after
the heat treatment process in a plating bath to form a plating layer on a surface of the
cold-rolled steel sheet.
[ 1 0] The method for manufacturing a steel sheet for hot stamping according to
[9] may further include an alloying process of holding the cold-rolled steel sheet after
the plating process in an alloying temperature range of 450°C to 6oooc to alloy the
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plating layer.
[11] The method for manufacturing a steel sheet for hot stamping according to
[8] may further include a plating process of immersing the cold-rolled steel sheet after
the annealing process and before the heat treatment process in a plating bath to form a
plating layer on a surface of the cold-rolled steel sheet.
[ 12] The method for manufacturing a steel sheet for hot stamping according to
[ 11] may further include an alloying process of holding the cold-rolled steel sheet after
the plating process and before the heat treatment process in an alloying temperature
range of 450°C to 600°C to alloy the plating layer.
[13] According to yet another aspect of the invention, there is provided a hot
stamped component including, as a chemical composition, by mass%: C: 0.060% to
0.120%; Si: 0% to 0.70%; Mn: 1.60% to 3.00%; P: 0.100% or less; S: 0.0100% or less;
N: 0.0100% or less; Al: 0.001% to 0.100%; Ti: 0.005% to 0.050%; B: 0.0005% to
0.0100%; Nb: 0% to 0.100%; V: 0% to 0.100%; W: 0% to 0.100%; Ni: 0% to 2.00%;
Cu: 0% to 2.00%; Cr: 0% to 2.00%; Mo: 0% to 2.00%; Sn: 0% to 0.200%; Ca: 0% to
0.0500%; Mg: 0% to 0.0500%; REM: 0% to 0.0500%; and a remainder of Fe and
impurities. A microstructure includes 90% or more of tempered martensite by area
ratio.
[14] In the hot stamped component according to [13], the chemical
composition may contain, by mass%, one or two or more selected from the group
consisting of: Nb: 0.005% to 0.100%; V: 0.005% to 0.100%; W: 0.005% to 0.100%; Ni:
0.01% to 2.00%; Cu: 0.01% to 2.00%; Cr: 0.01% to 2.00%; Mo: 0.01% to 2.00%; Sn:
0.005% to 0.200%; Ca: 0.0003% to 0.0500%; Mg: 0.0003% to 0.0500%; and REM:
0.0003% to 0.0500%.
[15] According to still yet another aspect of the invention, there is provided a
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method for manufacturing a hot stamped component. The method includes a hot
stamping process of heating the steel sheet for hot stamping according to any one of [1]
to [5] using a heating furnace at an ambient temperature of 85ooc to 950°C for 3
minutes or longer and cooling the steel sheet for hot stamping to a martensitic
transformation start temperature or lower at a cooling rate of 10 °C/sec or faster.
[ 16] In the method for manufacturing a hot stamped component according to
[ 15], the cooling rate in the hot stamping process may be 10 to 20 °C/sec.
[Effects of the Invention]
[0014]
According to the present invention, it is possible to provide a steel sheet for hot
stamping which has high hardenability during hot stamping and good collision
characteristics after hot stamping, a method for manufacturing the same, a hot stamped
component using the steel sheet for hot stamping, and a method for manufacturing the
same.
[Embodiments ofthe Invention]
[0015]
A steel sheet for hot stamping according to an embodiment of the invention
(hereinafter, referred to as a steel sheet according to this embodiment), a method for
manufacturing the same, a hot stamped component obtained by performing hot
stamping on the steel sheet according to this embodiment (a hot stamped component
according to this embodiment), and a method for manufacturing a hot stamped
component using the steel sheet according to this embodiment will be described.
[0016]
First, the steel sheet according to this embodiment will be described. The
steel sheet according to this embodiment has a predetermined chemical composition.
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A microstructure includes 70% or more of upper bainite by area ratio, and the number
density of iron carbides having a major axis of 0.1 J.lm or more included in the upper
bainite is equal to or greater than 4/J.lm2.
[0017]

[Microstructure Includes 70% or More of Upper Bainite by Area Ratio]
[Number Density of Iron carbides Having Major Axis of 0.1 J.lm or More
Included in Upper Bainite Is Equal to or Greater Than 4/J.lm2
]
In general, steel sheets for hot stamping are designed to be easily processed by,
for example, cutting, and to have high strength after hot stamping. Therefore, in many
cases, the microstructure of the steel sheet for hot stamping (before hot stamping) is a
structure including a large amount of soft ferrite (for example, a ferrite-pearlite
structure). On the other hand, in a case in which a large amount of alloying element is
added to increase hardenability during hot stamping heat treatment, the strength of the
steel sheet for hot stamping also increases, causing problems such as poor cuttability
and difficulty in correction with a leveler. Further, the addition of a large amount of
alloying element enhances the hardenability of the steel sheet and lowers a Ms point.
Therefore, the microstructure of a hot-stamping formed body is mainly composed of
fresh martensite (martensite without including carbide). In this case, deformability at
the time of collision is degraded.
In contrast, the inventors found that a configuration in which the area ratio of
upper bainite in a microstructure of a steel sheet for hot stamping was equal to or greater
than 70% and the number density of iron carbides having a major axis of 0.1 J.lm or
more included in the upper bainite was equal to or greater than 4/).lm2 made it possible
to achieve both hardenability for obtaining sufficient strength after hot stamping and
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collision characteristics after hot stamping while suppressing the hardness of the steel
sheet for hot stamping in order to secure workability including cutting even in a case in
which a large amount of alloying element was included.