[0001]The present invention relates to a hot-stamped product, a steel sheet for hot
stamping, and manufacturing methods thereof.
Priority is claimed on Japanese Patent Application No. 2019-070211, filed
April 1, 2019, the content of which is incorporated herein by reference.
[Background Art]
[0002]
Today the industrial technology field is highly divided, and materials used in
each technical field are required to have special and high performance. For example,
steel sheets for a vehicle are required to have high strength in order to improve fuel
efficiency by reducing the weight of the vehicle body in consideration of the global
environment. In a case where a high strength steel sheet is applied to the vehicle
body of a vehicle, a desired strength can be imparted to the vehicle body while
reducing the sheet thickness of the steel sheet and reducing the weight of the vehicle
body.
[0003]
However, in press forming, which is a process for forming a vehicle body
member of a vehicle, cracks and wrinkles are more likely to occur as the thickness of
the steel sheet used decreases. Therefore, the steel sheet for a vehicle also requires
excellent press formability.
[0004]
- 1 -
Securing the press formability and high-strengthening of the steel sheet are
contradictory elements, and it is difficult to satisfy these properties simultaneously.
In addition, when a high strength steel sheet is press-formed, the shape of the member
is greatly changed by springback that occurs when the member is taken out from the
die, so that it is difficult to secure the dimensional accuracy of the member. As
described above, it is not easy to manufacture a high strength vehicle body member by
press forming.
[0005]
Hitherto, as a method of manufacturing an ultrahigh-strength vehicle body
member, for example, as disclosed in Patent Document 1, a technique for pressforming
a heated steel sheet using a low-temperature press die has been proposed.
This technique is called hot stamping or hot pressing, and in this technique, a steel
sheet which is heated to a high temperature and is thus in a soft state is press-formed,
so that a member having a complex shape can be manufactured with high dimensional
accuracy. In addition, since the steel sheet is rapidly cooled by contact with the die, it
is possible to significantly increase the strength by hardening at the same time as press
forming. For example, Patent Document 1 describes that a member having a tensile
strength of 1400 MPa or more is obtained by performing hot stamping on a steel sheet
having a tensile strength of 500 to 600 MPa.
[0006]
Among vehicle body members, in frame structure components such as a
center pillar and a side member, a hard portion and a soft portion are provided in the
member in order to control the deformation state of the member at the time of a
collision of a vehicle.
[0007]
- 2 -
As a method of manufacturing a member having a soft portion by hot
stamping, Patent Document 2 discloses a method in which a heating temperature of a
steel sheet is partially changed by induction heating or infrared heating in order to
soften a portion heated to a low temperature.
Patent Document 3 discloses a method in which a heat insulating material is
attached to a portion of a steel sheet when the steel sheet is subjected to furnace
heating to partially reduce the heating temperature and soften the steel sheet.
[0008]
Patent Documents 4 and 5 disclose a method in which the cooling rate of a
steel sheet is partially changed by changing the contact area between the steel sheet
and a die at the time of forming in order to soften a portion having a low cooling rate.
Patent Document 6 discloses a technique of performing hot stamping using a
so-called tailored blank material in which two element sheets are connected to each
other by welding.
[0009]
In hot stamping, a steel sheet is usually heated to an austenite region and then
cooled at a cooling rate equal to or higher than the critical cooling rate to form a single
martensite structure for high-strengthening. On the other hand, in the methods
described in Patent Documents 2 to 5, as described above, the heating temperature or
cooling rate of the steel sheet is partially reduced to partially generate structures other
than martensite, thereby softening the steel sheet. However, since the fractions of the
structures other than martensite change sensitively in response to the heating
temperature and the cooling rate, the methods of Patent Documents 2 to 5 have a
problem that the strength of the soft portion is not stable.
[0010]
- 3 -
In the technique described in Patent Document 6, a soft portion can be formed
under predetermined heating and cooling conditions by using a steel sheet having low
hardenability as one element sheet. However, although the microstructure and
strength properties of the soft portion greatly depend on the composition of the steel
sheet, Patent Document 6 does not provide any consideration for the composition of
the steel sheet having low hardenability.
[0011]
Regarding such problems, Patent Documents 7 and 8 disclose a method of
stabilizing the strength of a soft portion in a hot stamping member consisting of a hard
portion and a soft portion or a hot stamping member that is soft as a whole.
[0012]
Specifically, Patent Document 7 discloses a high strength member for a 600 to
1200 MPa class vehicle and a manufacturing method thereof, in which the C content is
limited to a low level and hardening elements are contained in a predetermined amount
or more to suppress the formation of ferrite, pearlite, and martensite during cooling.
In addition, Patent Document 8 discloses a hot stamping member having a
tensile strength of 500 MPa or more and a manufacturing method thereof, in which the
C content is limited to a low level and Ti is contained to control the amount of
martensite generated.
[0013]
According to the techniques described in Patent Documents 7 and 8, it is
possible to increase the strength and the uniformity of elongation in the member.
However, according to the examination by the present inventors, it was found that in
the techniques described in Patent Documents 7 and 8, since the microstructure
contains hard structures such as bainite and martensite, the thermal stability is low, and
- 4 -
there are cases where the strength decreases when the member is subjected to a paint
baking treatment. Since vehicle members are often subjected to the paint baking
treatment, there remains room for improvement in the techniques described in Patent
Documents 7 and 8.
[Prior Art Document]
[Patent Documents]
[0014]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. 2002-102980
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2005-193287
[Patent Document 3] Japanese Unexamined Patent Application, First
Publication No. 2009-61473
[Patent Document 4] Japanese Unexamined Patent Application, First
Publication No. 2003-328031
[Patent Document 5] PCT International Publication No. W02006/38868
[Patent Document 6] Japanese Unexamined Patent Application, First
Publication No. 2004-58082
[Patent Document 7] Japanese Unexamined Patent Application, First
Publication No. 2005-248320
[Patent Document 8] PCT International Publication No. W02008/132303
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0015]
As described above, it is not easy to manufacture a soft member or a member
- 5 -
including a soft portion by hot stamping. In particular, it has been difficult in the
related art to manufacture a low strength hot stamping member (formed article) which
includes a soft portion partially or entirely and has excellent thermal stability by hot
stamping.
[0016]
An object of the present invention is to solve the above-described problems
and provide a hot-stamped product which is excellent in thermal stability, and more
specifically, has a portion with small fluctuation in strength (tensile strength) before
and after a paint baking treatment caused by the paint baking treatment, and a tensile
strength of less than 700 MPa, a steel sheet for hot stamping suitable as a base thereof,
and manufacturing methods thereof.
[Means for Solving the Problem]
[0017]
The present invention has been made in order to solve the above-described
problems, and the gist thereof is a hot-stamped product, a steel sheet for hot stamping,
and manufacturing methods thereof described below.
[0018]
(1) A hot-stamped product, an entirety or a part of the hot-stamped product
including, as a chemical composition, by mass%: C: 0.001% or more and less than
0.090%; Si: 2.50% or less; Mn: 0.01% or more and less than 0.50%; P: 0.200% or less;
S: 0.0200% or less; sol. Al: 0.001% to 2.500%; N: 0.0200% or less; Cr: 0.01% or more
and less than 2.00%; Ti: 0% to 0.300%; Nb: 0% to 0.300%; V: 0% to 0.300%; Zr: 0%
to 0.300%; Mo: 0% to 2.00%; Cu: 0% to 2.00%; Ni: 0% to 2.00%; B: 0% to 0.0200%;
Ca: 0% to 0.0100%; Mg: 0% to 0.0100%; REM: 0% to 0.1000%; Bi: 0% to 0.0500%;
and a remainder: Fe and impurities, in which a microstructure of the hot-stamped
- 6 -
product contains, by area%, ferrite: more than 60.0%, martensite: 0% or more and less
than 20.0%, and bainite: 0% or more and less than 20.0%, a tensile strength of the hotstamped
product is less than 700 MPa, and ~TS, which is a decrease in the tensile
strength when a heat treatment at 170aC for 20 minutes is performed, is 100 MPa or
less.
(2) The hot-stamped product according to (1), including, as the chemical
composition, by mass%, one or two or more selected from the group consisting of: Ti:
0.001% to 0.300%; Nb: 0.001% to 0.300%; V: 0.001% to 0.300%; Zr: 0.001% to
0.300%; Mo: 0.001% to 2.00%; Cu: 0.001% to 2.00%; Ni: 0.001% to 2.00%; B:
0.0001% to 0.0200%; Ca: 0.0001% to 0.0100%; Mg: 0.0001% to 0.0100%; REM:
0.0001% to 0.1000%; and Bi: 0.0001% to 0.0500%.
(3) The hot-stamped product according to (1) or (2), including, as the
chemical composition, by mass%: Mn: 0.01% or more and less than 0.30%.
(4) The hot-stamped product according to any one of (1) to (3), including: a
plating layer on a surface of the hot-stamped product.
(5) A steel sheet for hot stamping including, as a chemical composition, by
mass%: C: 0.001% or more and less than 0.090%; Si: 2.50% or less; Mn: 0.01% or
more and less than 0.50%; P: 0.200% or less; S: 0.0200% or less; sol. Al: 0.001% to
2.500%; N: 0.0200% or less; Cr: 0.01% or more and less than 2.00%; Ti: 0% to
0.300%; Nb: 0% to 0.300%; V: 0% to 0.300%; Zr: 0% to 0.300%; Mo: 0% to 2.00%;
Cu: 0% to 2.00%; Ni: 0% to 2.00%; B: 0% to 0.0200%; Ca: 0% to 0.0100%; Mg: 0%
to 0.0100%; REM: 0% to 0.1000%; Bi: 0% to 0.0500%; and a remainder: Fe and
impurities, in which a microstructure of the steel sheet for hot stamping contains iron
carbides, and a Mn content and a Cr content in the iron carbides satisfy Expression (i)
[Mn]e + [Cr]e > 0.8 ... (i)
- 7 -
where meaning of each symbol in the expression is as follows:
[Mn]e: the Mn content by at% in the iron carbides when a total amount of Fe,
Mn, and Cr contained in the iron carbides is 100 at%; and
[Cr]e: the Cr content by at% in the iron carbides when the total amount of Fe,
Mn, and Cr contained in the iron carbides is 100 at%.
(6) The steel sheet for hot stamping according to (5), including, as the
chemical composition, by mass%, one or two or more selected from the group
consisting of: Ti: 0.001% to 0.300%; Nb: 0.001% to 0.300%; V: 0.001% to 0.300%;
Zr: 0.001% to 0.300%; Mo: 0.001% to 2.00%; Cu: 0.001% to 2.00%; Ni: 0.001% to
2.00%; B: 0.0001% to 0.0200%; Ca: 0.0001% to 0.0100%; Mg: 0.0001% to 0.0100%;
REM: 0.0001% to 0.1000%; and Bi: 0.0001% to 0.0500%.
(7) The steel sheet for hot stamping according to (5) or (6), including, as the
chemical composition, by mass%: Mn: 0.01% or more and less than 0.30%.
(8) The steel sheet for hot stamping according to any one of (5) to (7),
including: a plating layer on a surface of the steel sheet for hot stamping.
(9) A manufacturing method of the hot-stamped product according to any one
of (1) to (3), including: a heating step of heating the steel sheet for hot stamping
according to any one of (5) to (7) to a heating temperature T°C, which is higher than an
Ac3 point; and a hot stamping step of starting hot stamping on the steel sheet for hot
stamping after the heating step at a temperature of (T - 80tC or higher.
(10) A manufacturing method of the hot-stamped product according to any
one of (1) to (3), including: a joining step of joining the steel sheet for hot stamping
according to any one of (5) to (7) to a steel sheet for joining to obtain a joined steel
sheet; a heating step of heating the joined steel sheet after the joining step to a heating
temperature rc, which is higher than an Ac3 point of the steel sheet for hot stamping;
- 8 -
and a hot stamping step of starting hot stamping on the joined steel sheet after the
heating step at a temperature of (T- 80)°C or higher.
(11) A manufacturing method of the hot-stamped product according to (4),
including: a heating step of heating the steel sheet for hot stamping according to (8) to
a heating temperature T°C, which is higher than an Ac3 point; and a hot stamping step
of starting hot stamping on the steel sheet for hot stamping after the heating step at a
temperature of (T - 80)°C or higher.
(12) A manufacturing method of the hot-stamped product according to (4),
including: a joining step of joining the steel sheet for hot stamping according to (8) to a
steel sheet for joining to obtain a joined steel sheet; a heating step of heating the joined
steel sheet after the joining step to a heating temperature rc, which is higher than an
Ac3 point of the steel sheet for hot stamping; and a hot stamping step of starting hot
stamping on the joined steel sheet after the heating step at a temperature of (T - 80)°C
or higher.
( 13) A manufacturing method of the steel sheet for hot stamping according to
any one of (5) to (8), including: a hot rolling step of performing hot rolling on a slab
containing, as a chemical composition, by mass%, C: 0.001% or more and less than
0.090%, Si: 2.50% or less, Mn: 0.01% or more and less than 0.50%, P: 0.200% or less,
S: 0.0200% or less, soL Al: 0.001% to 2.500%, N: 0.0200% or less, Cr: 0.01% or more
and less than 2.00%, Ti: 0% to 0.300%, Nb: 0% to 0.300%, V: 0% to 0.300%, Zr: 0%
to 0.300%, Mo: 0% to 2.00%, Cu: 0% to 2.00%, Ni: 0% to 2.00%, B: 0% to 0.0200%,
Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, REM: 0% to 0.1000%, Bi: 0% to 0.0500%,
and a remainder: Fe and impurities, and performing coiling in a temperature range of
800°C or lower to obtain a hot-rolled steel sheet; a hot-rolled sheet annealing step of
performing hot-rolled sheet annealing of heating the hot-rolled steel sheet to a
- 9 -
temperature range of higher than 65ooc to obtain a hot -rolled and annealed steel sheet;
and a cold rolling step of performing cold rolling on the hot-rolled and annealed steel
sheet to obtain a cold-rolled steel sheet.
(14) The manufacturing method of the steel sheet for hot stamping according
to (13), including: a plating step of performing plating on the cold-rolled steel sheet
after the cold rolling step after optionally performing continuous annealing.
[Effects ofthe Invention]
[0019]
According to the present invention, it is possible to obtain a hot- stamped
product which has a portion with small strength fluctuation caused by a paint baking
treatment (excellent thermal stability), and a tensile strength ofless than 700 MPa.
[Brief Description of the Drawings]
[0020]
FIG. 1 is a schematic view illustrating the shape of a hot-stamped product
manufactured in Example 1.
FIG. 2 is a schematic view illustrating the shape of a hot-stamped product
manufactured in Example 2.
[Embodiments of the Invention]
[0021]
The present inventors intensively studied a method for suppressing a decrease
in strength at the time of coating baking for a hot-stamped product having a tensile
strength of less than 700 MPa. As a result, the following findings were obtained.
[0022]
(A) When the microstructure of a hot- stamped product contains a large
amount of hard structures such as martensite or bainite, the tensile strength of the
- 10 -
formed article is greatly reduced by a paint baking treatment. It is considered that this
is because the hard structures are tempered and softened.
[0023]
(B) On the other hand, even with a hot-stamped product having a
microstructure which has a low fraction of hard structures and primarily includes a soft
structure containing ferrite, there are cases where the tensile strength is greatly reduced
by a paint baking treatment depending on the chemical composition.
[0024]
(C) In a hot-stamped product having a microstructure primarily including a
soft structure containing ferrite, a reduction in the tensile strength due to a paint baking
treatment is suppressed by limiting the Mn content to a low level, including a desired
amount of Cr, and controlling the Mn content and Cr content in iron carbides to desired
amounts in a steel sheet before hot stamping.
[0025]
The reason for this is not clear, but the present inventors presume that it is due
to the following reason. (a) When the Mn content is excessive, the transformation
temperature from austenite to ferrite decreases, fine iron carbides or fine iron-carbon
clusters are generated in ferrite during a cooling process after hot stamping, and the
ferrite is hardened. (b) By controlling the Mn content and the Cr content in iron
carbides to desired amounts, the iron carbides are stabilized, and the generation of fine
iron carbides or fine iron-carbon clusters in ferrite is suppressed. (c) Fine iron
carbides or fine iron-carbon clusters present in ferrite are changed to coarse iron
carbides by a heat treatment during coating baking and thus the strength of ferrite is
reduced.
[0026]
- 11 -
(D) In a step of performing hot stamping, a decrease in tensile strength due to
a paint baking treatment is suppressed by raising the temperature at which hot
stamping is started.
The reason for this is not clear, but the present inventors presume that it is due
to the following reason. (a) When the start temperature of hot stamping is high, the
amount of carbon in a solid solution state contained in ferrite in a hot-stamped product
decreases. (b) Solid solution carbon in ferrite precipitates as coarse iron carbides by a
heat treatment at the time of coating baking, resulting in a decrease in the strength of
ferrite.
[0027]
From the findings of (A) to (D) described above, the present inventors found
that by starting hot stamping at a desired temperature in a hot stamping step using a
steel sheet for hot stamping in which the Mn content and Cr content in iron carbides
are controlled to desired amounts by limiting the Mn content to a low level and
including a desired amount of Cr, it is possible to manufacture a hot-stamped product
which has a microstructure primarily containing ferrite, excellent thermal stability, and
a small reduction in strength by a paint baking treatment.
[0028]
Hereinafter, a hot-stamped product according to an embodiment of the present
invention (a hot-stamped product according to the present embodiment), a steel sheet
for hot stamping suitable as a base thereof (a steel sheet for hot stamping according to
the present embodiment), and manufacturing methods thereof will be described in
detail. However, the present invention is not limited to the configuration disclosed in
the present embodiment, and various modifications can be made without departing
from the gist of the present invention.
- 12 -
[0029]

The entirety or a part of the hot-stamped product according to the present
embodiment has a chemical composition described below. The reasons for limiting
each element are as follows. In the following description, all "%" regarding the
contents of the chemical composition mean "mass%". In a case where the hotstamped
product includes a portion having a tensile strength of less than 700 MPa and
a portion having a tensile strength of 700 MPa or more, at least the portion having a
tensile strength of less than 700 MPa may have the following chemical composition.
[0030]
C: 0.001% or More and Less Than 0.090%
C is an element having an effect of increasing the tensile strength of a steel
sheet (a steel sheet provided in the hot-stamped product) after hot stamping. When
the C content is less than 0.001%, an increase in the tensile strength due to hot
stamping cannot be expected. Therefore, the C content is set to 0.001% or more. A
preferable C content is 0.010% or more, 0.020% or more, or 0.030% or more.
On the other hand, when the C content is 0.090% or more, the area ratio of
martensite and/or bainite in the microstructure after hot stamping increases, and the
tensile strength of the hot-stamped product becomes 700 MPa or more. In this case,
even if the Mn content and the Cr content are adjusted as will be described later, the
thermal stability of the hot-stamped product cannot be secured. Therefore, the C
content is set to less than 0.090%. A preferable C content is less than 0.085%, less
than 0.080%, less than 0.070%, or less than 0.060%.
[0031]
Si: 2.50% or Less
- 13 -
Si is an element contained as an impurity in steel. When the Si content
exceeds 2.50%, the weldability deteriorates, the transformation point becomes too high,
and it becomes difficult to heat the steel sheet to a temperature equal to or higher than
the transformation point during a heating process of the hot stamping. Therefore, the
Si content is set to 2.50% or less. A preferable Si content is 2.00% or less, 1.50% or
less, 1.00% or less, or 0.50% or less. In a case where a plated steel sheet is used as
the steel sheet for hot stamping, the Si content is set to preferably less than 0.50%,
more preferably less than 0.40%, and even more preferably less than 0.30% in order to
secure platability.
The lower limit of the Si content is not particularly limited, but an excessive
decrease in the Si content causes an increase in steelmaking cost. Therefore, the Si
content is preferably set to 0.001%. In addition, Si has an action of increasing the
tensile strength of the steel sheet after hot stamping and thus may be contained
positively. From the viewpoint of high-strengthening, a preferable Si content is
0.10% or more, 0.20% or more, or 0.30% or more.
[0032]
Mn: 0.01% or More and Less Than 0.50%
Mn is an element that deteriorates the thermal stability of the hot-stamped
product. In particular, when the Mn content is 0.50% or more, the thermal stability of
the formed article after hot stamping significantly deteriorates. Therefore, the Mn
content is set to less than 0.50%. The Mn content is preferably less than 0.40%, less
than 0.35%, less than 0.30%, less than 0.25%, or less than 0.20%.
On the other hand, Mn is an element that has an action of suppressing the
embrittlement of steel caused by the inclusion of S by being bonded to S as an impurity
and forming MnS. In order to obtain this effect, the Mn content is set to 0.01% or
- 14 -
more. The Mn content is preferably 0.05% or more, 0.10% or more, or 0.15% or
more.
[0033]
P: 0.200% or Less
P is an element contained in steel as an impurity. When the P content
exceeds 0.200%, the weldability and toughness after hot stamping significantly
deteriorate, so that the P content is set to 0.200% or less. A preferable P content is
0.100% or less, 0.050% or less, or 0.020% or less.
The lower limit of the P content is not particularly limited, but an excessive
decrease in the P content causes an increase in steelmaking cost. Therefore, the P
content is preferably set to 0.001% or more. In addition, P has an action of increasing
the tensile strength of the formed article after hot stamping and thus may be contained
positively. From the viewpoint of high-strengthening, a preferable P content is
0.010% or more, 0.020% or more, or 0.030% or more. In a case where a plated steel
sheet is used as the steel sheet for hot stamping, the P content is set to preferably
0.050% or less, and more preferably 0.040% or less in order to secure platability.
[0034]
S: 0.0200% or Less
S is an element contained in steel as an impurity and embrittles the steel.
Therefore, the smaller the S content is, the more preferable it is. However, when the
S content exceeds 0.0200%, the embrittlement of the steel becomes significant.
Therefore, the S content is set to 0.0200% or less. A preferable S content is 0.0100%
or less, 0.0050% or less, or 0.0030% or less.
The lower limit of the S content is not particularly limited, but an excessive
decrease in the S content causes an increase in steelmaking cost. Therefore, the S
- 15 -
content is preferably set to 0.0001% or more.
[0035]
sol. Al: 0.001% to 2.500%
Al is an element having an action of deoxidizing molten steel. When the sol.
Al content (acid-soluble Al content) is less than 0.001%, deoxidation is insufficient.
Therefore, the sol. Al content is set to 0.001% or more. The sol. Al content is
preferably 0.010% or more, 0.020% or more, or 0.040% or more.
On the other hand, in a case where the sol. Al content is too large, the
transformation point rises, and it becomes difficult to heat the steel sheet to a
temperature equal to or higher than the transformation point during the heating process
of the hot stamping. Therefore, the sol. Al content is set to 2.500% or less. The sol.
Al content is preferably 1.000% or less, 0.500% or less, 0.100% or less, or 0.060% or
less.
[0036]
N: 0.0200% or Less
N is an element which is contained in steel as an impurity and forms nitrides
during continuous casting of the steel. Since these nitrides deteriorate the toughness
after hot stamping, theN content is preferably low. When theN content exceeds
0.0200%, the deterioration of the toughness becomes significant. Therefore, theN
content is set to 0.0200% or less. TheN content is preferably less than 0.0100%, less
than 0.0080%, or less than 0.0050%.
The lower limit of theN content is not particularly limited. However, since
an excessive decrease in the N content causes an increase in steelmaking cost, it is
preferable that theN content is set to 0.0010% or more.
[0037]
- 16 -
Cr: 0.01% or More and Less Than 2.00%
Cr is an element having an action of improving the thermal stability of the
hot-stamped product (the steel sheet after hot stamping) having a microstructure
primarily containing ferrite. In a case where the Cr content is less than 0.01 %, the
above effect cannot be sufficiently obtained even if the hot stamping start temperature
in the hot stamping step is adjusted as described later. Therefore, the Cr content is set
to 0.01% or more. The Cr content is preferably 0.05% or more, 0.10% or more,
0.15% or more, or 0.20% or more.
On the other hand, when the Cr content is 2.00% or more, the area ratio of
martensite and/or bainite contained in the microstructure of the hot-stamped product
becomes excessive, and the thermal stability of the hot -stamped product deteriorates.
Therefore, the Cr content is set to less than 2.00%. In order to increase the yield ratio
of the hot-stamped product and improve the impact absorbability, the Cr content is set
to preferably less than 0.30%, and more preferably less than 0.25%.
[0038]
Ti: 0% to 0.300%
Nb: 0% to 0.300%
V: 0% to 0.300%
Zr: 0% to 0.300%
Ti, Nb, V, and Zr are elements that have an action of increasing the tensile
strength of the hot-stamped product by refining the microstructure. In order to obtain
this effect, one or more selected from Ti, Nb, V, and Zr may be contained as necessary.
Since these elements do not have to be contained, the lower limit of the amounts of
these elements is 0%.
[0039]
- 17 -
In a case where it is desired to obtain the above effect, it is preferable that one
or more selected from Ti, Nb, V, and Zr are each contained in 0.001% or more.
Moreover, it is more preferable to include any one or more of 0.005% or more of Ti,
0.005% or more ofNb, 0.010% or more of V, and 0.005% or more of Zr.
[0040]
In a case where Ti is contained, the Ti content is set to more preferably
0.010% or more, and particularly preferably 0.020% or more.
In a case where Nb is contained, the Nb content is set to more preferably
0.020% or more, and particularly preferably 0.030% or more.
In a case where Vis contained, the V content is more preferably set to 0.020%
or more.
In a case where Zr is contained, the Zr content is more preferably set to
0.010% or more.
[0041]
On the other hand, in a case where the amounts of Ti, Nb, V, and Zr each
exceed 0.300%, the effect is saturated and the manufacturing costs of the steel sheet
mcrease. Therefore, even in a case where the above elements are contained, the
amounts of Ti, Nb, V, and Zr are each set to 0.300% or less.
[0042]
Moreover, in a case where the amounts of Ti, Nb, V, and Zr are large, there
are cases where carbides of these elements precipitate in a large amount and the
toughness after hot stamping are impaired.
Therefore, the Ti content is preferably less than 0.060%, and more preferably
less than 0.040%.
The Nb content is preferably less than 0.060%, and more preferably less than
- 18 -
0.040%.
0.100%.
0.100%.
The V content is preferably less than 0.200%, and more preferably less than
The Zr content is preferably less than 0.200%, and more preferably less than
[0043]
Mo: 0% to 2.00%
Cu: 0% to 2.00%
Ni: 0% to 2.00%
Mo, Cu, and Ni have an action of increasing the tensile strength of the hotstamped
product (the steel sheet after hot stamping). Therefore, one or more selected
from Mo, Cu, and Ni may be contained as necessary. Since these elements do not
have to be contained, the lower limit of the amounts of these elements is 0%.
[0044]
In a case where it is desired to obtain the above effect, it is preferable that one
or more selected from Mo, Cu, and Ni are each contained in 0.001% or more. A
more preferable Mo content is 0.05% or more, a more preferable Cu content is 0.10%
or more, and a more preferable Ni content is 0.10% or more.
[0045]
On the other hand, when the amounts of Mo, Cu, and Ni each exceed 2.00%,
the area ratio of martensite and/or bainite contained in the microstructure of the hotstamped
product becomes excessive, and the thermal stability of the hot-stamped
product deteriorates.
Therefore, even in a case where the above elements are contained, the
amounts of Mo, Cu, and Ni are each set to 2.00% or less. A preferable Mo content is
- 19 -
0.50% or less, a preferable Cu content is 1.00% or less, and a preferable Ni content is
1.00% or less.
[0046]
B: 0% to 0.0200%
B is an element having an action of segregating to grain boundary and
improving the toughness of the steel sheet after hot stamping. In order to obtain this
effect, B may be contained as necessary. Since B does not have to be contained, the
lower limit of the B content is 0%.
[0047]
In a case where it is desired to obtain the above effect, the B content is
preferably 0.0001% or more. The B content is more preferably 0.0006% or more, and
even more preferably 0.0010% or more.
[0048]
On the other hand, in a case where the B content exceeds 0.0200%, the area
ratio of martensite and/or bainite contained in the microstructure of the hot-stamped
product becomes excessive, and the thermal stability of the hot-stamped product
deteriorates. Therefore, even in a case where B is contained, the B content is set to
0.0200% or less. The B content is preferably 0.0050% or less, and more preferably
0.0030% or less.
[0049]
Ca: 0% to 0.0100%
Mg: 0% to 0.0100%
REM: 0% to 0.1000%
Ca, Mg, and REM are elements having an action of improving the toughness
of the steel sheet after hot stamping (hot-stamped product) by adjusting the shape of
- 20 -
inclusions. Therefore, one or more selected from Ca, Mg, and REM may be
contained as necessary. Since these elements do not have to be contained, the lower
limit of the amounts of these elements is 0%.
[0050]
In a case where it is desired to obtain the above effect, it is preferable that one
or more selected from Ca, Mg, and REM are each contained in 0.0001% or more.
On the other hand, in a case where theCa or Mg content exceeds 0.0100%, or
in a case where the REM content exceeds 0.1000%, the above effect is saturated and
the manufacturing cost of the steel sheet increases. Therefore, even in a case where
the above elements are contained, theCa and Mg contents are each set to 0.0100% or
less, and the REM content is set to 0.1000% or less.
[0051]
In the present embodiment, REM refers to a total of 17 elements of Sc, Y, and
lanthanoids, and the REM content means the total amount of these elements.
Lanthanoids are industrially added in the form of mischmetal.
[0052]
Bi: 0% to 0.0500%
Bi is an element having an action of improving the toughness of the steel
sheet after hot stamping (hot-stamped product) by refining a solidification structure.
Therefore, Bi may be contained as necessary. Since Bi does not have to be contained,
the lower limit of the Bi content is 0%.
[0053]
In a case where it is desired to obtain the above effect, the Bi content is
preferably 0.0001% or more. The Bi content is more preferably 0.0003% or more,
and even more preferably 0.0005% or more.
- 21 -
On the other hand, in a case where the Bi content exceeds 0.0500%, the above
effects are saturated and the manufacturing cost of the steel sheet increases.
Therefore, even in a case where Bi is contained, the Bi content is set to 0.0500% or
less. The Bi content is preferably 0.0100% or less, and more preferably 0.0050% or
less.
[0054]
In the above chemical composition, the remainder consists of Fe and
impurities. Here, the "impurities" mean elements that are incorporated due to various
factors including raw materials such as ore and scrap and the manufacturing process
when the steel sheet is industrially manufactured, and are acceptable in a range without
adversely affecting the hot-stamped product according to the present embodiment.
[0055]
The chemical composition of the hot-stamped product described above may
be measured by a general analytical method. For example, the chemical composition
may be measured using inductively coupled plasma-atomic emission spectrometry
(ICP-AES). In addition, sol. Al may be measured by ICP-AES u sing a filtrate
obtained by heating and decomposing a sample with an acid. C and S may be
measured using a combustion-infrared absorption method, and N may be measured
using an inert gas fusion-thermal conductivity method.
[0056]

The microstructure (microstructure) of the hot-stamped product according to
the present embodiment will be described. The entirety or a part of the hot-stamped
product according to the present embodiment has a microstructure containing ferrite,
martensite, and bainite in amounts described below. In the following description
- 22 -
regarding the microstructure, "%"means "area%".
[0057]
Ferrite: More Than 60.0%
When the area ratio of ferrite is 60.0% or less, the tensile strength of the
formed article after hot stamping (hot-stamped product) becomes 700 MPa or more,
and thermal stability cannot be secured. Therefore, the area ratio of ferrite is set to
more than 60.0%. The area ratio of ferrite is preferably more than 70.0%, and more
preferably more than 80.0%.
The upper limit of the area ratio of ferrite does not need to be particularly
determined, but is set to preferably less than 98.0%, more preferably less than 96.0%,
and even more preferably less than 94.0% in order to increase the strength of the hotstamped
product.
[0058]
In the present embodiment, the ferrite includes, in addition to polygonal
ferrite, pseudo-polygonal ferrite and granular bainitic ferrite having a higher
dislocation density than polygonal ferrite, and acicular ferrite having serrated grain
boundaries.
From the viewpoint of thermal stability, the ratio of polygonal ferrite to the
entire ferrite is preferably 5.0% or more by area ratio.
[0059]
Martensite: 0% or More and Less Than 20.0%
Bainite: 0% or More and Less Than 20.0%
When the microstructure contains a large amount of martensite and bainite,
the thermal stability of the hot- stamped product deteriorates. Therefore, the area
ratios of both of martensite and bainite are set to less than 20.0%. The area ratios of
- 23 -
both of martensite and bainite are set to preferably less than 10.0%, more preferably
less than 5.0%, and even more preferably less than 2.0%.
[0060]
Since martensite and bainite do not have to be necessarily contained, the
lower limits of the area ratios of martensite and bainite are both 0%.
However, martensite and bainite have an action of increasing the strength of
the hot-stamped product and thus may be contained in the microstructure within the
above ranges. When the area ratios of both of martensite and bainite are less than
0.1 %, the effect by the above action cannot be sufficiently obtained. Therefore, in a
case of increasing the strength, the lower limits of the area ratios of martensite and
bainite are both set to preferably 0.1% or more, and more preferably 0.5% or more.
[0061]
In the present embodiment, martensite includes fresh martensite and tempered
martensite. Fresh martensite is martensite that has not been tempered, and tempered
martensite is martensite that has been self-tempered and/or tempered.
[0062]
The remainder of the microstructure may contain pearlite or residual austenite,
and may further contain precipitates such as cementite. Since it is not necessary to
positively contain pearlite, residual austenite, and precipitates, the lower limits of the
area ratios of pearlite, residual austenite, and precipitates are all 0%.
[0063]
Since pearlite has an action of increasing the strength of the hot-stamped
product, in a case of increasing the strength, the area ratio of pearlite is set to
preferably 1.0% or more, more preferably 2.0% or more, and even more preferably
5.0% or more.
- 24 -
On the other hand, in a case where pearlite is excessively contained, the
toughness after hot stamping deteriorates. Therefore, the area ratio of pearlite is set to
preferably 20.0% or less, and more preferably 10.0% or less.
[0064]
Residual austenite has an action of improving the impact absorbability of the
hot-stamped product. Therefore, in a case of obtaining this effect, the area ratio of
residual austenite is set to preferably 0.5% or more, and more preferably 1.0% or more.
On the other hand, when residual austenite is excessively contained, the
toughness after hot stamping decreases. Therefore, the area ratio of residual austenite
is set to preferably 5.0% or less, and more preferably 3.0% or less.
[0065]
In the present embodiment, the area ratio of each microstructure is obtained as
follows.
First, a test piece is collected from the hot-stamped product, and a sheet
thickness cross section (longitudinal section of the steel sheet) is polished. Thereafter,
in a case of a non-plated steel sheet, at a 1/4 depth position of the sheet thickness of the
steel sheet from the surface of the steel sheet (a region between a 1/8 depth of the sheet
thickness from the surface of the steel sheet and a 3/8 depth of the sheet thickness from
the surface of the steel sheet), and in a case of a plated steel sheet, at a 1/4 depth
position of the sheet thickness of the steel sheet as a substrate, from the boundary
between the steel sheet as the substrate and a plating layer (a region between a 1/8
depth of the sheet thickness of the steel sheet as the substrate from the boundary and a
3/8 depth of the sheet thickness of the steel sheet as the substrate from the boundary),
structure observation is performed. In a case where the hot-stamped product has a
portion having a tensile strength of less than 700 MPa and a portion having a tensile
- 25 -
strength of 700 MPa or more, the test piece is collected from the portion having a
tensile strength of less than 700 MPa and observed.
[0066]
Specifically, the polished sheet thickness cross section is subjected to nital
etching or electrolytic polishing, structure observation is then performed using an
optical microscope and a scanning electron microscope (SEM), and image analysis is
performed on the obtained structure photograph, whereby the area ratio of each of
ferrite, pearlite, bainite, and tempered martensite is obtained. Thereafter, LePera
corrosion is applied to the same observation position, structure observation is then
performed using the optical microscope and the scanning electron microscope (SEM),
and image analysis is performed on the obtained structure photograph, whereby the
total area ratio of residual austenite and fresh martensite is calculated.
[0067]
In addition, regarding the same observation position, the sheet thickness cross
section is subjected to electrolytic polishing, and then the area ratio of residual
austenite is measured using SEM provided with an electron beam backscattering
pattern analyzer (EBSP).
[0068]
Based on these results, the area ratio of each of ferrite, pearlite, bainite,
martensite, and residual austenite is obtained.
Tempered martensite can be distinguished from fresh martensite by the
presence of iron carbides inside, and can be distinguished from bainite by the fact that
iron carbides present inside are stretched in a plurality of directions. In the present
embodiment, the area ratio of martensite is obtained by calculating the total area ratio
of fresh martensite and tempered martensite.
- 26 -
[0069]

The entirety or a part of the hot-stamped product according to the present
embodiment has a tensile strength of less than 700 MPa. That is, the tensile strength
of the entirety or a part of the base steel sheet of the hot -stamped product according to
the present embodiment is less than 700 MPa. This is because when the tensile
strength of the entirety of the hot-stamped product is 700 MPa or more, the thermal
stability of the hot-stamped product cannot be secured. Preferably, the tensile
strength is less than 600 MPa or less than 560 MPa in the entirety or a part of the hotstamped
product. On the other hand, in order to improve the impact absorbability of
the hot-stamped product, the tensile strength of the hot-stamped product is preferably
set to 300 MPa or more, 340 MPa or more, 390 MPa or more, 440 MPa or more, 460
MPa or more, or 490 MPa or more.
[0070]
In order to further improve the impact absorbability of the hot-stamped
product, in a portion of the hot -stamped product where the tensile strength is less than
700 MPa, the yield ratio (yield ratio of the steel sheet after hot stamping provided in
the hot-stamped product) is set to preferably 0.65 or more, and more preferably 0.67 or
more. The yield ratio is obtained by dividing the yield stress by the tensile strength
(yield stress I tensile strength). The yield stress is set to an upper yield point in a case
where the steel sheet after hot stamping yields discontinuously, and to 0.2% proof
stress in a case where the steel sheet yields continuously.
[0071]
In the hot-stamped product according to the present embodiment, a soft
portion having a tensile strength of less than 700 MPa and a hard portion having a
- 27 -
tensile strength of 700 MPa or more may be mixed in the hot-stamped product. By
providing the portions having different strengths, it is possible to control the
deformation state of the hot-stamped product at the time of a collision, and the impact
absorbability of the hot-stamped product can be improved. As will be described later,
the hot-stamped product having the portions with different strengths can be
manufactured by joining two or more kinds of steel sheets having different chemical
compositions and performing hot stamping thereon.
[0072]

In the hot-stamped product according to the present embodiment, a decrease
in tensile strength (~TS) when a heat treatment is performed at 170°C for 20 minutes
from the tensile strength before the heat treatment is 100 MPa or less. ~ TS is
preferably 60 MPa or less, and more preferably 30 MPa or less. The lower limit of
~TS is not particularly limited. However, in order to significantly reduce ~TS, it is
necessary to excessively increase the Mn content and the Cr content in iron carbides in
a steel sheet for hot stamping described later, which impairs the manufacturability of
the steel sheet manufacturability. Therefore, ~ TS is preferably 1 MPa or more, 5
MPa or more, or 10 MPa or more.
[0073]
It is considered that the reason why the strength during coating baking
decreases in the hot-stamped product having a structure primarily containing ferrite
(more than 60.0% by area ratio) is that due to the paint baking treatment, fine iron
carbides or fine iron-carbon clusters present in the ferrite are changed to coarse iron
carbides, and solid solution carbon in the ferrite precipitates as coarse iron carbides.
Although it is not easy to directly and quantitatively evaluate the presence state of the
- 28 -
fine iron carbides, fine iron-carbon clusters, and solid solution carbon, the evaluation
can be indirectly achieved by a decrease in tensile strength (fl.TS) when the heat
treatment is performed at 170ac for 20 minutes. When fl. TS is 100 MPa or less, it
can be determined that the generation of fine iron carbides or fine iron-carbon clusters
in the ferrite and an increase in the amount of solid solution carbon are suppressed, so
that excellent thermal stability is achieved.
[0074]
The tensile strength is obtained by collecting a JIS No. 13B tensile test piece
and conducting a tensile test at a tensile speed of 10 mm/min.
[0075]

The hot-stamped product according to the present embodiment may have a
plating layer on the surface. By providing the plating layer on the surface, it is
possible to prevent the generation of scale during hot stamping and to further improve
the corrosion resistance of the hot-stamped product. The kind of plating is not
particularly limited as long as the above purpose is satisfied. As will be described
later, the hot-stamped product having the plating layer can be formed by hot stamping
using a plated steel sheet. Examples of the hot-stamped product having a plating
layer include hot-stamped products having a zinc-based plating layer or an aluminiumbased
plating layer obtained by hot stamping using a zinc-plated steel sheet or an
aluminium-plated steel sheet, specifically, a hot-dip galvanized steel sheet, a
galvannealed steel sheet, a hot-dip aluminium-plated steel sheet, a hot-dip Zn-Al alloy
plated steel sheet, a hot-dip Zn-Al-Mg alloy plated steel sheet, a hot-dip Zn-Al-Mg-Si
alloy plated steel sheet, an electrolytic zinc-plated steel sheet, an electrolytic Ni-Zn
alloy plated steel sheet, or the like. The plating layer may be formed on one side or
- 29 -
both sides.
[0076]
Next, a steel sheet for hot stamping suitable for manufacturing the above hotstamped
product will be described.
[0077]

Since the chemical composition is not substantially changed by hot stamping,
the steel sheet for hot stamping has the same chemical composition as that of the
above-described hot-stamped product.
[0078]

The microstructure of the steel sheet for hot stamping according to the present
embodiment contains iron carbides, and the chemical composition of the iron carbides
(the Mn content and the Cr content in the iron carbides) satisfies Expression (i).
[0079]
[Mn]e + [Cr]e > 0.8 ... (i)
Here, the meaning of each symbol in the above expression is as follows.
[Mn]e: Mn content (at%) in the iron carbides when the total amount of Fe, Mn,
and Cr contained in the iron carbides is 100 at%
[Cr]e: Cr content (at%) in the iron carbides when the total amount of Fe, Mn,
and Cr contained in the iron carbides is 100 at%
[0080]
As the chemical composition of the iron carbides contained in the
microstructure of the steel sheet for hot stamping satisfies Expression (i), the thermal
stability of the steel sheet after hot stamping can be improved. When the value on the
- 30 -
left side of Expression (i) is 0.8 or less, the thermal stability of the hot-stamped product
cannot be secured even if the manufacturing conditions of the hot-stamped product are
adjusted as described later. The value on the left side of Expression (i) is preferably
more than 1.0, more preferably more than 1.2, and even more preferably more than 1.4.
[0081]
On the other hand, in order to increase the Mn content and the Cr content in
the iron carbides, it is necessary to anneal a hot-rolled steel sheet at a high temperature
in a hot-rolled sheet annealing step, which will be described later, which impairs the
manufacturability of the steel sheet. Therefore, the value on the left side of
Expression (i) is preferably less than 5.0, and more preferably less than 4.0.
[0082]
In the present embodiment, the chemical composition of the iron carbides is
measured by the following procedure.
First, a test piece is collected from any position of the steel sheet, and a sheet
thickness cross section (longitudinal section) parallel to a rolling direction of the steel
sheet is polished. Thereafter, precipitates are extracted at a 1/4 depth position of the
sheet thickness from the surface of the steel sheet (a region between a 1/8 depth of the
sheet thickness from the surface of the steel sheet and a 3/8 depth of the sheet thickness
from the surface of the steel sheet) by a replica method. These precipitates are
observed using a transmission electron microscope (TEM), and identification of the
precipitates and composition analysis are performed by electron beam diffraction and
energy dispersive X-ray spectroscopy (EDS).
[0083]
Quantitative analysis of the iron carbides by the EDS is performed on the
three elements Fe, Mn, and Cr, and the Mn content (at%) and the Cr content (at%)
- 31 -
when the total amount thereof is 100 at% are respectively obtained as [Mn]e and [Cr] e.
This quantitative analysis is performed on a plurality of iron carbides, and the average
value thereof is taken as the Mn content and the Cr content in the iron carbides in the
steel sheet. The number of iron carbides to be measured is set to 10 or more, and the
larger the number of iron carbides measured, the more preferable. The iron carbides
include cementite that is present in isolation in the microstructure in addition to
cementite contained in pearlite.
[0084]
In the present embodiment, in a case of a cold-rolled steel sheet, or an
annealed steel sheet, at a 1/4 depth position of the sheet thickness from the surface of
the steel sheet (a region between a 1/8 depth of the sheet thickness from the surface of
the steel sheet and a 3/8 depth of the sheet thickness from the surface of the steel sheet),
and in a case of a plated steel sheet, at a 114 depth position of the sheet thickness of the
steel sheet as a substrate from the boundary between the steel sheet as the substrate and
the plating layer (a region at a 118 depth of the sheet thickness of the steel sheet as the
substrate from the boundary and a 3/8 depth of the sheet thickness of the steel sheet as
the substrate from the boundary), the above-described microstructure is specified.
[0085]
The area ratio of the iron carbides does not need to be particularly determined.
However, in order to increase the tensile strength by refining the microstructure after
hot stamping, the area ratio of the iron carbides is set to preferably 1% or more, and
more preferably 3% or more.
On the other hand, when the area ratio of the iron carbides is excessive, the
tensile strength of the steel sheet after hot stamping becomes too high, and the thermal
stability is impaired. Therefore, the area ratio of the iron carbides is set to preferably
- 32 -
20% or less, and more preferably 15% or less.
[0086]
The remainder of the microstructure of the steel sheet for hot stamping
according to the present embodiment primarily contains ferrite, but may contain
martensite (including fresh martensite and tempered martensite), bainite, and residual
austenite as the remainder, and may further contain precipitates other than the iron
carbides. However, since martensite, bainite, and residual austenite deteriorate the
toughness after hot stamping, the area ratios of these structures are preferably small.
The area ratios of martensite (including fresh martensite and tempered martensite),
bainite, and residual austenite are all preferably less than 1.0%, and more preferably
less than 0.5%.
[0087]
The area ratio in the microstructure of the steel sheet for hot stamping can be
obtained by the same method as in the case of the hot-stamped product.
The tensile strength of the steel sheet for hot stamping is not particularly
limited, but is preferably 300 MPa or more or 340 MPa or more from the viewpoint of
manufacturability of the steel sheet, and preferably 650 MPa or less or less than 590
MPa from the viewpoint of the steel sheet.
[0088]

Preferable manufacturing methods of the hot-stamped product according to
the present embodiment and the steel sheet for hot stamping according to the present
embodiment will be described.
[0089]
[Manufacturing Method of Hot-Stamped Product]
- 33 -
A manufacturing method of the hot-stamped product according to the present
embodiment includes a heating step of heating a steel sheet for hot stamping having the
above-described chemical composition and the microstructure, and a hot stamping step
of performing hot stamping on the heated steel sheet for hot stamping. In the hot
stamping step, cooling and forming are performed by a die, thereby obtaining a hotstamped
product.
[0090]
In the heating step of heating the steel sheet for hot stamping, a heating
temperature T (°C) is preferably set to higher than an Ac3 point. The Ac3 point is the
temperature at which ferrite disappears in the microstructure when a base steel sheet is
heated, and can be obtained from a change in thermal expansion of the steel sheet in
the heating step. When the heating temperature T (°C) is the Ac3 point or lower, the
generation of martensite or bainite is promoted in the microstructure of the hotstamped
product, and the amount of solid solution carbon in ferrite increases, so that
the thermal stability of the hot-stamped product deteriorates. The heating temperature
T CC) is preferably (Ac3 point+ 50tC or higher, and more preferably (Ac3 point+
1 00)°C or higher.
In addition, the steel sheet for hot stamping subjected to the heating preferably
has the above-mentioned structure.
[0091]
The upper limit of the heating temperature T (°C) is not particularly limited.
However, when the heating temperature T (°C) is too high, austenite becomes coarse
and the strength of the hot-stamped product decreases. Therefore, the heating
temperature T (°C) is preferably 11 00°C or lower, more preferably 1 000°C or lower,
and even more preferably 95oac or lower.
- 34 -
In addition, a preferable retention time of the hot stamping at a heating
temperature T (°C) is one to five minutes.
[0092]
In the hot stamping step of performing hot stamping on the heated steel sheet
for hot stamping, the start temperature of the hot stamping is set to (T - 80)°C or higher.
T is the heating temperature T CC). When the start temperature of the hot stamping
is lower than (T- 80)°C, the amount of solid solution carbon in ferrite in the
microstructure of the hot-stamped product increases, and the thermal stability of the
formed article deteriorates. The start temperature of the hot stamping is preferably (T
- 50tC or higher.
[0093]
In order to suppress the generation of martensite or bainite in the
microstructure of the hot-stamped product and improve the thermal stability of the hotstamped
product, the start temperature of the hot stamping is preferably set to (T-
80) oc or higher and higher than the Ar3 point.
The Ar3 point is the temperature at which ferrite starts to be generated in the
microstructure when the base steel sheet is cooled, and is obtained from a change in
thermal expansion when the steel sheet is cooled after the heating step.
[0094]
Another manufacturing method of the hot-stamped product according to the
present embodiment includes a joining step of joining a steel sheet (steel sheet for hot
stamping) having the above-described chemical composition and the microstructure to
a steel sheet for joining to obtain a joined steel sheet, a heating step of heating the
joined steel sheet, and thereafter a hot stamping step of performing hot stamping on the
heated joined steel sheet. As the joining method, for example, a method of butting
- 35 -
and overlapping the steel sheet for hot stamping and the steel sheet for joining and
joining the resultant by welding can be adopted.
[0095]
In the hot stamping step, the heating temperature T CC) of the joined steel
sheet is set to higher than the Ac3 point of the steel sheet for hot stamping, and the start
temperature of the hot stamping is set to (T - 80tC or higher. In this case, a
preferable heating temperature T (°C) is (Ac3 point + 50)°C or higher of the steel sheet
for hot stamping, and a more preferable heating temperature T CC) is (Ac3 point
+ 1 00)°C or higher of the steel sheet for hot stamping. In addition, the heating
temperature T (°C) is preferably 11 ooac or lower, more preferably 1 oooac or lower,
and even more preferably 950°C or lower.
A preferable start temperature of the hot stamping is (T - 50tC or higher, and
a more preferable start temperature is (T - 80)°C or higher and higher than the Ar3
point.
In addition, a preferable retention time of the hot stamping at a heating
temperature T (°C) is one to five minutes.
The reasons for the above are the same as the above-described manufacturing
method of the hot-stamped product that does not include the j oining step.
[0096]
The chemical composition and mechanical properties of the steel sheet for
joining are not particularly limited. However, in order to increase the impact
absorbed energy of the hot-stamped product, the steel sheet for joining preferably has a
tensile strength of 700 MPa or more after hot stamping. A more preferable tensile
strength of the steel sheet for joining after hot stamping is more than 1000 MPa, more
than 1200 MPa, or more than 1500 MPa.
- 36 -
[0097]
In order to secure the tensile strength of the steel sheet for joining after hot
stamping, the C content of the steel sheet for joining is preferably 0.090% or more.
The C content is more preferably 0.100% or more, 0.120% or more, or 0.200% or more.
For the same reason, the Mn content of the steel sheet for joining is preferably 0.50%
or more. The Mn content is more preferably 0.80% or more, 1.00% or more, or
1.20% or more.
[0098]
The steel sheet used as the base (steel sheet for hot stamping) is subjected to
hot-rolled sheet annealing as described later, and is further subjected to cold rolling
after the hot-rolled sheet annealing. After cold rolling, optionally continuous
annealing may be further performed. On the other hand, as the steel sheet for joining,
any of a hot-rolled steel sheet, a cold-rolled steel sheet obtained by cold-rolling a hotrolled
steel sheet, a hot-rolled and annealed steel sheet obtained by annealing a hotrolled
steel sheet, and a cold-rolled annealed steel sheet obtained by annealing a coldrolled
steel sheet rna y be used.
[0099]
In order to improve the corrosion resistance of the hot-stamped product, a
plated steel sheet of which the surface is plated may be used as the steel sheet for hot
stamping and the steel sheet for joining. The kind of plated steel sheet is not
particularly limited, and examples thereof include a hot -dip galvanized steel sheet, a
galvannealed steel sheet, a hot-dip aluminium-plated steel sheet, a hot-dip Zn-Al alloy
plated steel sheet, a hot-dip Zn-Al-Mg alloy plated steel sheet, a hot-dip Zn-Al-Mg-Si
alloy plated steel sheet, an electrolytic zinc-plated steel sheet, and an electrolytic Ni-Zn
alloy plated steel sheet.
- 37 -
[0100]
[Manufacturing Method of Steel Sheet for Hot Stamping]
The manufacturing method of the steel sheet for hot stamping according to the
present embodiment includes a hot rolling step of hot-rolling a slab having the abovedescribed
chemical composition and thereafter performing coiling in a temperature
range of 800°C or lower to obtain a hot-rolled steel sheet, a hot-rolled sheet annealing
step of performing hot-rolled sheet annealing of heating the hot-rolled steel sheet to a
temperature range of higher than 65oac to obtain a hot -rolled and annealed steel sheet,
and a cold rolling step of cold-rolling the hot-rolled and annealed steel sheet.
[0101]
In the hot rolling step, the coiling temperature after the hot rolling is set to
800°C or lower. When the coiling temperature is higher than 800°C, the
microstructure of the hot-rolled steel sheet becomes excessively coarse, and the tensile
strength of the steel sheet after hot stamping decreases. The coiling temperature is
preferably lower than 650°C, more preferably lower than 600°C, and even more
preferably lower than 55oac.
When the coiling temperature is too low, the hot-rolled steel sheet becomes
full hard and it becomes difficult to perform cold rolling. Therefore, the coiling
temperature is preferably 400°C or higher.
[0102]
A manufacturing method of the slab provided for the manufacturing method
of the steel sheet for hot stamping according to the present embodiment is not
particularly limited. In a preferable manufacturing method of the slab exemplified, a
steel having the above-described chemical composition (composition) is melted by a
known method, thereafter made into a steel ingot by a continuous casting method, or
- 38 -
made into a steel ingot by any casting method, and then made into a steel piece by a
blooming method or the like. In the continuous casting step, in order to suppress the
occurrence of surface defects due to inclusions, it is preferable to cause an external
additional flow such as electromagnetic stirring to occur in molten steel in a mold.
The steel ingot or steel piece may be reheated after being cooled once and subjected to
hot rolling, or the steel ingot in a high temperature state after the continuous casting or
the steel piece in a high temperature state after the blooming may be subjected to hot
rolling as it is, after being kept hot, or after being subjected to auxiliary heating. In
the present embodiment, the steel ingot and the steel piece are collectively referred to
as a "slab" as the material of hot rolling.
[0103]
The temperature of the slab to be subjected to hot rolling is set to preferably
lower than 1250°C, and more preferably lower than 1200°C in order to prevent
coarsening of austenite. Hot rolling is preferably completed in a temperature range of
the Ar3 point or higher in order to refine the microstructure of the hot-rolled steel sheet
by transforming austenite after completion of rolling.
[0104]
In a case where the hot rolling includes rough rolling and finish rolling, the
rough-rolled material may be heated between the rough rolling and the finish rolling in
order to complete the finish rolling at the above temperature. At this time, it is
desirable to suppress temperature fluctuation over the entire length of the rough-rolled
material at the start of the finish rolling to 140°C or lower by performing heating such
that the rear end of the rough-rolled material has a higher temperature than the front
end. This improves the uniformity of product characteristics in the coil after the
coiling step.
- 39 -
[0105]
A heating method of the rough-rolled material may be performed using a
known method. For example, a solenoid induction heating device may be provided
between a roughing mill and a finishing mill, and an increase in the heating
temperature may be controlled based on the temperature distribution and the like in the
longitudinal direction of the rough-rolled material on the upstream side of the
induction heating device.
[0106]
The hot-rolled and coiled steel sheet is annealed after being subjected to a
treatment such as degreasing according to a known method, as necessary. Annealing
performed on a hot-rolled steel sheet is called hot-rolled sheet annealing, and a steel
sheet after being subjected to the hot-rolled sheet annealing is called a hot-rolled and
annealed steel sheet. Before the hot-rolled sheet annealing, descaling by pickling or
the like may be performed.
[0107]
The heating temperature in the hot-rolled sheet annealing step is set to higher
than 650°C. This is to increase the Mn content and the Cr content in the iron carbides
in the microstructure of the hot-rolled and annealed steel sheet. When the heating
temperature is 650°C or lower, the Mn content and the Cr content in the iron carbides
do not satisfy Expression (i), and the thermal stability of the hot-stamped product is not
secured. The heating temperature in the hot-rolled sheet annealing step is preferably
higher than 680°C, and more preferably higher than 700°C.
On the other hand, when the heating temperature in the hot-rolled sheet
annealing step becomes too high, the microstructure of the hot-rolled and annealed
steel sheet becomes coarse, and the tensile strength after hot stamping decreases.
- 40 -
Therefore, the heating temperature in the hot-rolled sheet annealing step is preferably
lower than 750°C, and more preferably lower than 720°C.
[0108]
In order to sufficiently obtain the effect of hot-rolled sheet annealing, it is
preferable to perform holding at the heating temperature for 30 minutes or longer. On
the other hand, when the retention time is too long, the microstructure of the hot-rolled
and annealed steel sheet becomes coarse, and the tensile strength after hot stamping
decreases. Therefore, the retention time at the heating temperature in the hot-rolled
sheet annealing step is preferably shorter than 10 hours, more preferably shorter than
five hours, and even more preferably shorter than two hours.
[0109]
After the hot-rolled sheet annealing step, the hot-rolled and annealed steel
sheet may be cold-rolled into a cold-rolled steel sheet. This is to refine the
microstructure after hot stamping and increase the tensile strength. In order to reduce
the weight of the hot-stamped product, the sheet thickness of the cold-rolled steel sheet
is preferably 2.8 mm or less, more preferably 2.0 mm or less, even more preferably 1.8
mm or les s, and particularly preferably 1.6 mm or less. From the viewpoint of the
manufacturability of the steel sheet, the sheet thickness of the cold-rolled steel sheet is
preferably 0.6 mm or more.
[0110]
Cold rolling may be performed according to a typical method, and descaling
by pickling or the like may be performed before the cold rolling. In order to
sufficiently obtain the effect of cold rolling, the cold rolling reduction (cumulative
rolling reduction in cold rolling) is set to preferably 30% or more, and more preferably
40% or more. When the cold rolling reduction is too high, the toughness after hot
- 41 -
stamping deteriorates, so that the cold rolling reduction is set to preferably 65% or less,
and more preferably 60% or less. As will be described later, in a case where
continuous annealing is performed after the cold rolling, in order to refine the
microstructure of the annealed steel sheet, the cold rolling reduction is set to preferably
60% or more, and more preferably 70% or more.
[0111]
After the cold rolling step described above, continuous annealing may be
performed on the cold-rolled steel sheet to obtain an annealed steel sheet. The
continuous annealing may be performed according to a typical method, and a treatment
such as degreasing may be performed by a known method before performing the
continuous annealing. In order to refine the microstructure of the annealed steel sheet
by recrystallization, a soaking temperature during the continuous annealing is
preferably set to 600°C or higher, 650°C or higher, or 700°C or higher.
[0112]
On the other hand, during the continuous annealing, when the heating rate is
too slow, the soaking temperature is too high, or the soaking time is too long, the
microstructure of the annealed steel sheet becomes coarse due to grain growth, and
there are cases where the tensile strength of the steel sheet after hot stamping decreases
and the impact absorbability decreases. Therefore, the average heating rate up to the
soaking temperature in the continuous annealing is preferably set to 1 ac/sec or faster,
the soaking temperature is preferably set to 800°C or lower or 760°C or lower, and the
soaking time is preferably set to shorter than 300 seconds or shorter than 120 seconds.
[0113]
The cold-rolled steel sheet, and the annealed steel sheet obtained as described
above may be subjected to temper rolling according to a typical method.
- 42 -
[0114]
The steel sheet for hot stamping according to the present embodiment may be
provided with a plating layer on the surface layer for the purpose of preventing the
generation of scale during hot stamping and improving the corrosion resistance of the
steel sheet after the hot stamping. The kind of plating is not particularly limited as
long as the above-mentioned purpose is satisfied, and examples thereof include a hotdip
galvanized steel sheet, a galvannealed steel sheet, a hot-dip aluminium-plated steel
sheet, a hot-dip Zn-Al alloy plated steel sheet, a hot-dip Zn-Al-Mg alloy plated steel
sheet, a hot-dip Zn-Al-Mg-Si alloy plated steel sheet, an electrolytic zinc-plated steel
sheet, and an electrolytic Ni-Zn alloy plated steel sheet.
[0115]
In a case of manufacturing a hot-dip plated steel sheet, the cold-rolled steel
sheet, or the annealed steel sheet manufactured by the method described above as a
base steel sheet may be plated according to a typical method. In a case of using the
cold-rolled steel sheet as the base steel sheet, the soaking temperature in the annealing
process of continuous hot-dip plating is preferably set to 6ooac or higher, 65oac or
higher, or 700°C or higher in order to refine the microstructure of the plated steel sheet
by recrystallization.
[0116]
On the other hand, when the soaking temperature is too high, the
microstructure of the annealed steel sheet becomes coarse due to grain growth.
Therefore, regardless of the kind of the base steel sheet, the soaking temperature in the
annealing process of continuous hot-dip plating is preferably set to 800°C or lower or
760°C or lower. An alloying treatment may be performed by reheating the steel sheet
after the hot-dip plating.
- 43 -
[0117]
In a case of manufacturing an electro plated steel sheet, the cold-rolled steel
sheet, or the annealed steel sheet manufactured by the above-described method as a
base steel sheet may be subjected to electro plating according to a typical method after
being subjected to a known pretreatment for cleaning and adjusting the surface as
necessary. The plated steel sheet obtained as described above may be subjected to
temper rolling according to a typical method.
[0118]
Hereinafter, the present invention will be described more specifically with
reference to examples, but the present invention is not limited the examples.
[Examples]
[0119]
(Example 1)
Molten steel was cast using a vacuum melting furnace to manufacture Steels
A to 0 having the chemical composition shown in Table 1. Ac1 point and Ac3 point
in Table 1 were obtained from changes in thermal expansion when the cold-rolled steel
sheets of Steels A to 0 were heated at 2 °C/sec. In addition, Ar3 point in Table 1 was
obtained from a change in thermal expansion when the cold-rolled steel sheets of
Steels A to 0 were heated to 920°C and then cooled at 10 °C/sec.
- 44 -
Steel
A
B
c
D
E
F
G
H
I
J
K
L
M
N
0
[0120]
[Table 1]
c Si
0.049 0.08
0.056 0.10
0.058 0.11
0.053 0.04
0.055 0.04
0.103 0.46
0.052 0.22
0.053 0.10
0.055 0.11
0.057 0.09
0.051 0.21
0.053 0.08
0.050 0.23
0.069 0.05
0.057 0.45
Mn
0.12
0.19
0.11
0.38
0.59
0.45
0.12
0.12
0.16
0.15
0.11
0.14
0.15
0.46
0.28
Chemical composition (mass%, remainder: Fe and impurities)
p s sol.Al N Cr
0.011 0.0018 0.043 0.0024 0.08
0.010 0.0020 0.044 0.0023 0.11
0.010 0.0018 0.045 0.0023 0.27
0.012 0.0019 0.042 0.0028 0.39
0.011 0.0020 0.044 0.0023 0.11
0.010 0.0019 0.045 0.0027 0.21
0.010 0.0020 0.045 0.0024 0.08
0.009 0.0021 0.046 0.0024 0.10
0.008 0.0021 0.043 0.0023 0.11
0.011 0.0019 0.042 0.0025 0.09
0.012 0.0020 0.045 0.0023 0.08
0.010 0.0021 0.044 0.0025 0.11
0.011 0.0022 0.046 0.0048 2 .04
0.010 0.0018 0.046 0.0025 0.12
0.010 0.0019 0.042 0.0024 0.11
Underline indicates outside the range of the present invention.
- 45 -
Transformation point ec)
Note
Others Ac 1 Ac1 Arl
732 885 844 Present fuvention Steel
Ti:0 .023 739 889 850 Present fuvention Steel
Ti:0.029 Nb: 0.032 752 896 858 Present fuvention Steel
Ti:0.028 Nb: 0.033 749 888 854 Present fuvention Steel
Ti:0.028 Nb: 0.033 742 889 853 Comparative Steel
Ti:0 .060 B:0.0006 743 898 846 Comparative Steel
Mo:0.09 733 892 846 Present fuvention Steel
Cu:0.20 Ni:0.11 725 876 838 Present fuvention Steel
Ca:0.0004 Mg :0.0005 729 879 842 Present fuvention Steel
Bi:0.0020 REM:0.0005 724 882 843 Present fuvention Steel
B:0.0004 736 897 845 Present fuvention Steel
V:0.010 Zr:0.011 726 882 848 Present fuvention Steel
Ti:0 .060 769 892 831 Comparative Steel
Ti:0 .061 B:0.0009 739 895 849 Present fuvention Steel
Nb:0.032 744 893 856 Present fuvention Steel
[0121]
[Table 2]
Hot rolling Hot-rolled sheet Cold rolling Annealing
conditions annealing conditions conditions conditions
Test
Steel Coiling Heating Retention Sheet thickness Soaking Note
No.
temperature temperature time after cold r olling temperature
(OC) (OC) (hr) (mm) (OC)
1 A 540 710 1 1.4 - Present Invention Example
2 A 540 710 1 1.4 - Present Invention Example
3 A 540 710 1 1.4 - Present Invention Example
4 A 540 710 1 1.4 730 Present Invention Example
5 A 540 710 1 1.4 - Present Invention Example
6 A 540 710 12 1.4 730 Present Invention Example
7 A 540 620 1 1.4 - Comparative Example
8 A 580 - - 1.4 - Comparative Example
9 B 540 710 1 1.4 730 Present Invention Example
10 B 540 710 1 1.4 730 Present Invention Example
11 B 540 710 1 1.4 - Present Invention Example
12 B 540 710 1 1.4 - Present Invention Example
13 B 580 - - 1.4 - Comparative Example
14 c 540 710 1 1.4 750 Present Invention Example
15 c 540 710 1 1.4 750 Present Invention Example
16 c 540 710 1 1.4 - Present Invention Example
17 c 540 710 1 1.4 - Present Invention Example
18 c 580 - - 1.4 - Comparative Example
19 D 540 710 1 1.4 740 Present Invention Example
20 E 540 710 1 1.4 - Comparative Example
21 F 540 710 1 1.4 - Comparative Example
22 G 540 710 1 1.4 730 Present Invention Example
23 H 540 710 1 1.4 720 Present Invention Example
24 I 540 710 1 1.4 720 Present Invention Example
25 J 540 710 1 1.4 720 Present Invention Example
26 K 540 710 1 1.4 - Present Invention Example
27 L 540 710 1 1.4 - Present Invention Example
28 M 540 710 1 1.4 - Comparative Example
29 A 580 - - 1.4 730 Comparative Example
30 B 580 - - 1.4 730 Comparative Example
31 N 540 660 1 1.4 730 Present Invention Example
32 N 540 660 1 1.4 730 Present Invention Example
33 0 540 710 1 1.4 740 Present Invention Example
Underline indicates outside the range of the present invention.
[0122]
Steels A to 0 were heated to 1200oc and held for 60 minutes, and then
subjected to hot rolling under the hot rolling conditions shown in Table 2.
Specifically, Steels A to 0 were rolled in 10 passes in a temperature range of the An
- 46 -
point or higher into hot-rolled steel sheets having a thickness of 3.6 mm. After the
hot rolling, the hot-rolled steel sheet was cooled to 540°C to 580°C with water spray,
the cooling finishing temperature was set to a coiling temperature, the hot-rolled steel
sheet was loaded into an electric heating furnace held at the coiling temperature and
held for 60 minutes, the hot-rolled steel sheet was then subjected to furnace cooling to
room temperature at an average cooling rate of 20 oC/hr, and slow cooling after coiling
was simulated.
[0123]
After slow cooling, some of the hot-rolled steel sheets were subjected to hotrolled
sheet annealing. Specifically, the hot-rolled steel sheet was heated to 620oc to
71 ooc at an average heating rate of 50 °C/hr using the electric heating furnace, held for
1 to 12 hours, and subsequently cooled at an average cooling rate of 20 ac/hr, whereby
a hot-rolled and annealed steel sheet was obtained.
[0124]
The hot-rolled steel sheet and the hot-rolled and annealed steel sheet were
pickled to obtain base metal for cold rolling, and cold rolling was performed thereon at
a rolling reduction of 61%, whereby cold-rolled steel sheets having a thickness of 1.4
mm were obtained. In addition, some of the cold-rolled steel sheets were heated to a
soaking temperature for annealing shown in Table 2 at an average heating rate of
10 ac/sec using a continuous annealing simulator and soaked for 60 seconds.
Subsequently, the resultant was cooled to 400°C, held for 180 seconds, and cooled to
room temperature, whereby an annealed steel sheet was obtained. The obtained
annealed steel sheet was described as "ACR" in the "Kind of steel" column and"-" in
the "Kind of plating" column in Table 3. The cold-rolled steel sheet was described as
"CR" in the "Kind of steel" column and"-" in the "Kind of plating" column in Table 3.
- 47 -
[0125]
In addition, some of the cold-rolled steel sheets were heated to a soaking
temperature for annealing shown in Table 2 at an average heating rate of 10 ac/sec
using a hot-dip plating simulator and soaked for 60 seconds. Subsequently, the
resultant was cooled, immersed in a hot-dip galvanizing bath or a hot-dip aluminium
plating bath, and subjected to hot-dip galvanizing or hot-dip aluminium plating,
whereby a hot-dip galvanized steel sheet or a hot-dip aluminium-plated steel sheet was
obtained. After the hot-dip galvanizing, some of the steel sheets were heated to
520°C and subjected to an alloying treatment to obtain a galvannealed steel sheet.
The obtained plated steel sheet was described as "ACR" in the "Kind of steel" column
and "GI", "GA", or "AL" in the "Kind of plating" column in Table 3.
[0126]
From the cold-rolled steel sheets, annealed steel sheets, hot-dip galvanized
steel sheets, galvannealed steel sheets, and hot-dip aluminium-plated steel sheets (these
steel sheets are collectively referred to as steel sheets for hot stamping) obtained as
described above, test pieces for structure observation were collected, and structure
observation was performed.
[0127]
Specifically, after polishing the sheet thickness cross section parallel to the
rolling direction, in a case of a non-plated steel sheet (the cold-rolled steel sheet and
the annealed steel sheet), from a 1/4 depth position of the sheet thickness of the steel
sheet from the surface of the steel sheet, and in a case of a plated steel sheet (a region
between a 1/8 depth of the sheet thickness from the surface of the steel sheet and a 3/8
depth of the sheet thickness from the surface of the steel sheet), from a 1/4 depth
position of the sheet thickness of the steel sheet as the substrate from a boundary
- 48 -
between the steel sheet as the substrate and the plating layer (a region between a 1/8
depth of the sheet thickness of the steel sheet as the substrate from the boundary and a
3/8 depth of the sheet thickness of the steel sheet as the substrate from the boundary),
precipitates were extracted by a replica method, and iron carbides were identified using
TEM. Ten iron carbides were quantitatively analyzed for the three elements Fe, Mn,
and Cr using EDS. When the total amount of Fe, Mn, and Cr was 100 at%, the Mn
content (at%) and the Cr content (at%) in the iron carbides were respectively indicated
as [Mn]e and [Cr]e, and the average value of the sum of [Mn]e and [Cr]e was obtained.
[0128]
Furthermore, a JIS No. 13B tensile test piece was collected from the steel
sheet for hot stamping along a direction perpendicular to the rolling direction, and a
tensile test was conducted at a tensile speed of 10 mrnlmin to obtain a tensile strength.
Table 3 shows the observation results of the microstructure of the steel sheet for hot
stamping and the examination results of the mechanical properties of the steel sheet for
hot stamping.
- 49 -
[0129]
[Table 3]
Microstructure of
Mechanical
steel sheet for hot
properties of steel
Test
Steel
Kind of Kind of
stamping
sheet for hot
No. steel# I plating#2 stamping
[Mn]a + [Cr]a (at%)
Tensile strength
(MPa)
1 A CR - 0.9 522
2 A CR - 0.9 522
3 A CR - 0.9 522
4 A ACR GI 0.9 341
5 A CR - 0.9 522
6 A ACR GI 1.0 317
7 A CR - 0.5 531
8 A CR - 0.3 536
9 B ACR GA 1.1 357
10 B ACR GA 1.1 357
11 B CR - 1.1 538
12 B CR - 1.1 538
13 B CR - 0.4 550
14 c ACR AL 1.7 373
15 c ACR AL 1.7 373
16 c CR - 1.7 552
17 c CR - 1.7 552
18 c CR - 0.4 567
19 D ACR AL 3.2 382
20 E CR - 2.3 568
21 F CR - 2.4 652
22 G ACR GA 0.9 369
23 H ACR AL 0.9 354
24 I ACR - 1.0 346
25 J ACR - 0.9 343
26 K CR - 0.9 538
27 L CR - 1.0 531
28 M CR - 9.1 669
29 A ACR GI 0.3 349
30 B ACR GA 0.4 366
31 N ACR GA 0.9 405
32 N ACR GA 0.9 405
33 0 ACR AL 1.4 360
Underline indicates outside the range of the present invention.
#1 CR: Cold-rolled steel sheet, ACR: Annealed steel sheet
#2 GI: Hot-dip galvanized steel sheet, GA: Galvannealed steel sheet,
AL: Hot-dip aluminum-plated steel sheet,- Non-plated steel sheet
- 50 -
Note
Present Invention Example
Present Invention Example
Present Invention Example
Present Invention Example
Present Invention Example
Present Invention Example
Comparative Example
Comparative Example
Present Invention Example
Present Invention Example
Present Invention Example
Present Invention Example
Comparative Example
Present Invention Example
Present Invention Example
Present Invention Example
Present Invention Example
Comparative Example
Present Invention Example
Comparative Example
Comparative Example
Present Invention Example
Present Invention Example
Present Invention Example
Present Invention Example
Present Invention Example
Present Invention Example
Comparative Example
Comparative Example
Comparative Example
Present Invention Example
Present Invention Example
Present Invention Example
[0130]
An element sheet for hot stamping having a width of 240 mm and a length of
170 mm was collected from the steel sheet for hot stamping, and a hat member having
the shape shown in FIG. 1 was manufactured by hot stamping. In the hot stamping
step, the element sheet was heated at the heating temperature shown in Table 4 for four
minutes using a gas heating furnace, thereafter taken out of the heating furnace and
subjected to air cooling, and sandwiched between dies provided with a cooling
apparatus to be subjected to hat forming at the start temperature shown in Table 4.
[0131]
Some of the obtained hat members (hot-stamped products) were subjected to a
heat treatment corresponding to a paint baking treatment at 170°C for 20 minutes using
an electric heating furnace.
[0132]
A test piece for SEM observation was collected from a punch bottom portion
of the hat member before the heat treatment, a sheet thickness cross section of the test
piece parallel to the rolling direction of the steel sheet was polished, and thereafter the
sheet thickness cross section was subjected to nital etching and LePera corrosion. In
a case of a non-plated steel sheet, at a 1/4 depth position of the sheet thickness of the
steel sheet from the surface of the steel sheet (a region between a 118 depth of the sheet
thickness from the surface of the steel sheet and a 3/8 depth of the sheet thickness from
the surface of the steel sheet), and in a case of a plated steel sheet, at a 1/4 depth
position of the sheet thickness of the steel sheet as the substrate from the boundary
between the steel sheet as the substrate and the plating layer (a region between a 118
depth of the sheet thickness of the steel sheet as the substrate from the boundary and a
3/8 depth of the sheet thickness of the steel sheet as the substrate from the boundary),
- 51 -
the microstructure was observed. Using the above-described method, the area ratios
of ferrite, martensite, and bainite were measured by image processing. Table 4 shows
the results. In Table 4, in test numbers satisfying the regulations of the present
invention, the proportion of polygonal ferrite in ferrite in the microstructure of the hotstamped
product was 5.0% or more.
[0133]
Furthermore, a JIS No. 13B tensile test piece was collected from a punch
bottom portion of the hat member before the heat treatment along the longitudinal
direction of the member, and a tensile test was conducted at a tensile speed of 10
mrn!min to obtain a tensile strength, a yield stress, and a yield ratio. The yield stress
was set to an upper yield point in a case of discontinuous yield, and to 0.2% proof
stress in a case of continuous yield.
[0134]
A JIS No. 13B tensile test piece was similarly collected from the punch
bottom portion of the hat member after the heat treatment, and a tensile test was
conducted in the same manner to obtain a tensile strength. The difference (fl TS)
between the tensile strength of the hat member not subjected to the heat treatment and
the tensile strength of the hat member subjected to the heat treatment was obtained,
and when fl TS was 100 MPa or less, the thermal stability of the hat member was
determined to be good.
[0135]
A case where the tensile strength before the heat treatment was less than 700
MPa and fl TS was 100 MPa or less was determined to be acceptable as satisfying the
regulations of the present invention. A case where the yield ratio before the heat
treatment was 0.65 or more was determined to be superior in impact absorbability.
- 52 -
[0136]
Table 4 shows the observation results of the microstructure of the hat member
and the evaluation results of the mechanical properties of the hat member. In Table 4,
underlined numerical values mean outside the range of the present invention.
- 53 -
[0137]
[Table 4]
Hot stamping conditions Microstructure of hot-stamped product Mechanical properties of hot -stamped product
Test
Steel Heating Start Other Tensile strength Yield stress before Yield ratio Tensile strength Ferrite Martensite Bainite t..TS Note No. temperature T temperature tructures b efore heat treatment heat treatment before heat after heat treatment
. (oC) (oc) (area%) (area%) (area%)
(area%) (MPa) (MPa) treatment (MPa)
(MPa)
1 A 920 840 90.4 0.3 2.0 7.3 466 317 0.68 431 35 Present Invent ion Example
2 A 920 750 92.0 0.0 0.0 8.0 429 297 0.69 311 118 Comparative Example
3 A 920 870 85.3 0.6 4.4 9.7 492 330 0.67 463 29 Present Invention Example
4 A 920 840 89.4 0.3 2.6 7.7 454 305 0.67 421 33 Present Invent ion Example
5 A 850 790 90.9 0.7 2.3 6.1 477 315 0.66 363 114 Comparative Example
6 A 920 840 89.6 0.2 2.5 7.7 437 293 0.67 407 30 Present Invent ion Example
7 A 920 840 90.3 0.2 1.9 7.6 483 329 0.68 378 105 Comparative Example
8 A 920 840 90.6 0.2 2.2 7.0 491 339 0.69 359 132 Comparative Example
9 B 920 840 88.7 0.6 2.8 7.9 459 308 0.67 433 26 Present Invention Example
10 B 920 750 91.6 0.0 0.0 8.4 427 304 0.71 320 107 Comparative Example
11 B 920 840 89.2 0.6 2.4 7.8 469 315 0.67 447 22 Present Invent ion Example
12 B 850 790 90.0 0.9 2.0 7.1 488 318 0.65 378 110 Comparative Example
13 B 920 840 89.8 0.4 2.8 7.0 494 346 0.70 365 129 Comparative Example
14 c 920 850 89.8 0.7 2.3 7.2 466 313 0.67 447 19 Present Invention Example
15 c 920 750 92.2 0.0 0.0 7.8 435 305 0.70 333 102 Comparative Example
16 c 920 850 90.3 0.6 2.0 7.1 474 318 0.67 454 20 Present Invent ion Example
17 c 850 790 91.1 1.0 1.6 6.3 485 321 0.66 381 104 Comparative Example
18 c 920 850 90.3 0.7 1.7 7.3 498 334 0.67 385 113 Comparative Example
19 D 920 840 87.7 2.4 3.7 6.2 481 298 0.62 468 13 Present Invention Example
20 E 920 840 87.9 2.3 3.8 6.0 499 315 0.63 394 105 Comparative Example
21 F 920 850 48.3 21.5 24.8 5.4 875 542 0.62 709 166 Comparative Example
22 G 920 840 86.8 1.6 4.6 7.0 508 331 0.65 470 38 Present Invention Example
23 H 920 840 85.0 0.7 4.9 9.4 497 329 0.66 471 26 Present Invention Example
24 I 920 840 88.7 0.4 2.9 8.0 453 304 0.67 424 29 Present Invention Example
25 J 920 840 87.4 0.3 3.4 8.9 460 309 0.67 425 35 Present Invention Example
26 K 920 840 85.3 1.8 5.3 7.6 519 338 0.65 479 40 Present Invent ion Example
27 L 920 840 88.0 0.4 3.8 7.8 463 311 0.67 439 24 Present Invent ion Example
28 M 920 790 56.1 27.7 12.5 3.7 722 433 0.60 556 166 Comparative Example
29 A 920 840 89.6 0.3 2.5 7.6 482 327 0.68 380 129 Comparative Example
30 B 920 840 89.1 0.4 3.0 7.5 485 334 0.69 361 124 Comparative Example
31 N 920 840 80.1 2.6 11.7 5.6 623 406 0.65 535 88 Present Invention Example
32 N 920 820 86.3 4.1 4.6 5.0 611 378 0.62 477 134 Comparative Example
33 0 920 850 89.4 0.6 2.9 7.1 464 307 0.66 439 25 Present Invention Example
Underline indicates outside the range of the present inventwn.
- 54 -
[0138]
In all of Test Nos. 1, 3, 4, 6, 9, 11, 14, 16, 19, 22 to 27, 31, and 33 satisfying
the regulations of the present invention, the tensile strength of the hot-stamped product
was less than 700 MPa, foJ. TS was 100 MPa or less, and good thermal stability was
exhibited.
[0139]
In all of Test Nos. 1, 3, 4 , 6, 9, 11, 14, 16, 22 to 27, 31, and 33 in which the Cr
content in the chemical composition of the hot-stamped product was less than 0.30%,
the yield ratio of the hot-stamped product was 0.65 or more, and the strength properties
were particularly good.
In all of Test Nos. 1, 3, 4 , 9, 11, 14, 16, 22 t o 27, 31, and 33 in which the
retention time at the heating temperature in the hot-rolled sheet annealing step is
shorter than 10 hours, the tensile strength of the hot-stamped product was 440 MPa or
more, and the strength properties were particularly good.
[0140]
Contrary to this, in Test Nos. 20, 21 and 28 of comparative examples in which
the chemical composition was outside the range of the present invention, the tensile
strength of the hot-stamped product was 700 MPa or more and !oJ.TS was 100 MPa or
more, or foJ. TS was 100 MPa or more, so that the thermal stability was inferior.
[014 1]
Specifically, in Test No. 20 using Steel E, the Mn content of the steel was too
high, so that foJ. TS was large.
[0142]
In Test No. 21 using Steel F, the C content of the steel was too high, so that
the area ratio of ferrite was insufficient in the microstructure of the hot -stamped
- 55 -
product, the area ratio of martensite and the area ratio of bainite became excessive, the
tensile strength of the hot-stamped product (before the heat treatment) was 700 MPa or
more, and ~TS was large.
[0143]
In Test No. 28 using Steel M, the Cr content of the steel was too high, so that
the area ratio of ferrite was insufficient in the microstructure of the hot -stamped
product, the area ratio of martensite was excessive, the tensile strength of the hotstamped
product (before the heat treatment) was 700 MPa or more, and ~ TS was large.
[0144]
In Test Nos. 2, 5, 7, 8, 10, 12, 13, 15, 17, 18, 29, 30, and 32 of comparative
examples in which the chemical composition was within the range of the present
invention but the microstructure of the steel sheet for hot stamping or the
manufacturing method of the hot-stamped product deviated from the range of the
present invention, ~TS of the hot-stamped product was 100 MPa or more, and the
thermal stability was inferior.
[0145]
Specifically, in Test No. 8 using Steel A, Test No. 13 using Steel B, Test No.
18 using Steel C, Test No. 29 using Steel A, and Test No. 30 using Steel B, the hotrolled
sheet annealing was not performed, so that the sum of the Mn content and the Cr
content in iron carbides in the microstructure of the steel sheet for hot stamping was
low, and ~TS in the hot-stamped product was large.
[0146]
In Test No. 7 using Steel A, the heating temperature in the hot-rolled sheet
annealing step was too low, so that the sum of the Mn content and the Cr content in
iron carbides in the microstructure of the steel sheet for hot stamping was low, and
- 56 -
~ TS in the hot-stamped product was large.
[0147]
In Test No.2 using Steel A, Test No. 10 using Steel B, Test No. 15 using
Steel C, and Test No. 32 using Steel N, the hot stamping start temperature in the hot
stamping step was too low, so that ~TS in the hot-stamped product was large.
[0148]
In Test No.5 using Steel A, Test No. 12 using Steel B, and Test No. 17 u sing
Steel C, the heating temperature in the heating step was too low, so that ~ TS in the
hot-stamped product was large.
[0149]
(Example 2)
Molten steel was cast using a vacuum melting furnace to manufacture Steels
A to C having the chemical composition shown in Table 1 in Example 1. Using
Steels A to C, in the same manner as in Example 1, under the conditions shown in
Table 5, hot rolling, hot-rolled sheet annealing, cold rolling, and annealing were
performed, and a plating treatment was thereafter performed, whereby a hot-dip
galvanized steel sheet, a galvannealed steel sheet, and a hot-dip aluminium-plated steel
sheet (steel sheets for hot stamping) were manufactured.
- 57 -
[0150]
[Table 5]
Hot rolling conditions
Hot-rolled sheet Cold rolling Annealing
annealing conditions conditions conditions
Test Sheet
Sheet
No.
Steel
thickness
Coiling Heating Retention thickness Soaking Note
after rolling
temperature temperature time after cold temperature
(mm)
(OC) (OC) (hr) rolling (OC)
(mm)
Present
34 A 3.6 540 710 1 1.4 730 Invention
Example
Present
35 B 3.6 540 710 1 1.4 730 Invention
Example
Present
36 c 3.6 540 710 1 1.4 730 Invention
Example
[0151]
The microstructure and mechanical properties of these steel sheets for hot
stamping were examined in the same manner as in Example 1. Table 6 shows the
observation results of the microstructure of the steel sheet for hot stamping and the
examination results of the mechanical properties of the steel sheet for hot stamping.
[0152]
[Table 6]
Microstructure of
Mechanical
steel sheet for hot
properties of
Test No. Steel
Kind of Kind of
stamping
steel sheet for
steel113 plating114 hot stamping
[Mn]e + [Cr]s Tensile strength
(at%) (MPa)
34 A ACR GA 0.9 343
35 B ACR AL 1.1 355
36 c ACR GI 1.7 379
#3 ACR: Annealed steel sheet
#4 GI: Hot-dip galvanized steel sheet, GA: Galvannealed steel sheet,
AL: Hot-dip aluminum-plated steel sheet
[0153]
Note
Present Invention Example
Present Invention Example
Present Invention Example
From these steel sheets for hot stamping, an element sheet for hot stamping
- 58 -
having a thickness of 1.4 mm, a width of 240 mm, and a length of 170 mm was
collected. This element sheet was joined to a steel sheet for joining having the same
dimensions by laser welding to manufacture a joined steel sheet having a thickness of
1.4 mm, a width of 240 mm, and a length of 340 mm. As the steel sheet for joining, a
cold-rolled steel sheet containing, as a chemical composition, by mass%, 0.21% of C,
0.13% ofSi, 1.31% ofMn, 0.012% ofP, 0.0018% ofS, 0.043% of soL Al, 0.0030% of
N, 0.21% ofCr, and 0.0018% ofB was used.
[0154]
The joined steel sheet was hot-stamped in the same manner as in Example 1
under the conditions shown in Table 7, whereby a hat member having the shape shown
in FIG. 2 was manufactured. Thereafter, some of the hat members were subjected to
a heat treatment corresponding to a paint baking treatment at 170ac for 20 minutes
using an electric heating furnace.
[0155]
Then, in the hat member before and after the heat treatment, the
microstructure and mechanical properties of portions made of Steels A to C were
examined in the same manner as in Example 1. Table 7 shows the observation results
of the microstructure of the hat member (hot-stamped product) and the evaluation
results of the mechanical properties of the hat member.
- 59 -
[0156]
[Table 7]
Hot stamping conditions Microstructure of hot-stamped product Mechanical properties of hot-stamped product
Tensile Yield stress
Tensile
Test
Steel
Heating Start
Ferrite Martensite Bainite
Other
strength before before heat
Yield ratio strength
~TS Note
No. temperature temperature structures before heat after heat
(OC) (OC) (area%) (area%) (area%) heat treatment treatment (MPa) (area%)
(MPa) (MPa)
treatment treatment
(MPa)
Present
34 A 920 840 88.1 0.4 3.6 7.9 462 307 0.66 434 28 Invention
Example
Present
35 B 920 840 87.3 0.8 4.3 7.6 473 322 0.68 444 29 Invention
Example
Present
36 c 920 850 88.2 1.0 3.7 7.1 479 327 0.68 459 20 Invention
Example
- 60 -
[0157]
In any test results of Test Nos. 34 to 36, TS of the hot-stamped product
(before the heat treatment) was less than 700 MPa, f1. TS was 100 MPa or less, and
good thermal stability was exhibited. The microstructure of a portion of the steel
sheet for joining of the hat member was a single structure of martensite, and the tensile
strengths of Test Nos. 34 to 36 were 1580 MPa, 1583 MPa, and 1575 MPa,
respectively.
[Industrial Applicability]
[0158]
According to the present invention, it is possible to obtain a hot-stamped
product which has a portion with small strength fluctuation caused by a paint baking
treatment and a tensile strength of less than 700 MPa and is thus excellent in thermal
stability, a steel sheet for hot stamping suitable as a base thereof, and manufacturing
methods thereof.

WE CLAIMS

1. A hot-stamped product, an entirety or a part of the hot-stamped product
comprising, as a chemical composition, by mass%:
C: 0.001% or more and less than 0.090%;
Si: 2.50% or less;
Mn: 0.01% or more and less than 0.50%;
P: 0.200% or less;
S: 0.0200% or less;
sol. Al: 0.001% to 2.500%;
N: 0.0200% or less;
Cr: 0.01% or more and less than 2.00%;
Ti: 0% to 0.300%;
Nb: 0% to 0.300%;
V: 0% to 0.300%;
Zr: 0% to 0.300%;
Mo: 0% to 2.00%;
Cu: 0% to 2.00%;
Ni: 0% to 2.00%;
B: 0% to 0.0200%;
Ca: 0% to 0.0100%;
Mg: 0% to 0.0100%;
REM: 0% to 0.1000%;
Bi: 0% to 0.0500%; and
a remainder: Fe and impurities,
- 62 -
wherein a microstructure of the hot-stamped product contains, by area%,
ferrite: more than 60.0%,
martensite: 0% or more and less than 20.0%, and
bainite: 0% or more and less than 20.0%,
a tensile strength of the hot -stamped product is less than 700 MPa, and
~TS, which is a decrease in the tensile strength when a heat treatment at
170°C for 20 minutes is performed, is 100 MPa or less.
2. The hot-stamped product as claimed in claim 1, comprising, as the
chemical composition, by mass%, one or two or more selected from the group
consisting of:
Ti: 0.001% to 0.300%;
Nb: 0.001% to 0.300%;
V: 0.001% to 0.300%;
Zr: 0.001% to 0.300%;
Mo: 0.001% to 2.00%;
Cu: 0.001% to 2.00%;
Ni: 0.001% to 2.00%;
B: 0.0001% to 0.0200%;
Ca: 0.0001% to 0.0100%;
Mg: 0.0001% to 0.0100%;
REM: 0.0001% to 0.1000%; and
Bi: 0.0001% to 0.0500%.
3. The hot-stamped product as claimed in claim 1 or 2, comprising, as the
- 63 -
chemical composition, by mass%:
Mn: 0.01% or more and less than 0.30%.
4. The hot-stamped product as claimed in any one of claims 1 to 3,
compnsmg:
mass%:
a plating layer on a surface of the hot-stamped product.
5. A steel sheet for hot stamping comprising, as a chemical composition, by
C: 0.001% or more and less than 0.090%;
Si: 2.50% or less;
Mn: 0.01% or more and less than 0.50%;
P: 0.200% or less;
S: 0.0200% or less;
soL Al: 0.001% to 2.500%;
N: 0.0200% or less;
Cr: 0.01% or more and less than 2.00%;
Ti: 0% to 0.300%;
Nb: 0% to 0.300%;
V: 0% to 0.300%;
Zr: 0% to 0.300%;
Mo: 0% to 2.00%;
Cu: 0% to 2.00%;
Ni: 0% to 2.00%;
B: 0% to 0.0200%;
- 64 -
Ca: 0% to 0.0100%;
Mg: 0% to 0.0100%;
REM: 0% to 0.1000%;
Bi: 0% to 0.0500%; and
a remainder: Fe and impurities,
wherein a microstructure of the steel sheet for hot stamping contains iron
carbides, and a Mn content and a Cr content in the iron carbides satisfy Expression (i)
[Mn]e + [Cr]e > 0.8 ... (i)
where meaning of each symbol in the expression is as follows:
[Mn]e: the Mn content by at% in the iron carbides when a total amount of Fe,
Mn, and Cr contained in the iron carbides is 100 at%; and
[Cr]e: the Cr content by at% in the iron carbides when the total amount of Fe,
Mn, and Cr contained in the iron carbides is 100 at%.
6. The steel sheet for hot stamping as claimed in claim 5, comprising, as the
chemical composition, by mass%, one or two or more selected from the group
consisting of:
Ti: 0.001% to 0.300%;
Nb: 0.001% to 0.300%;
V: 0.001% to 0.300%;
Zr: 0.001% to 0.300%;
Mo: 0.001% to 2.00%;
Cu: 0.001% to 2.00%;
Ni: 0.001% to 2.00%;
B: 0.0001% to 0.0200%;
- 65 -
Ca: 0.0001% to 0.0100%;
Mg: 0.0001% to 0.0100%;
REM: 0.0001% to 0.1000%; and
Bi: 0.0001% to 0.0500%.
7. The steel sheet for hot stamping as claimed in claim 5 or 6, comprising,
as the chemical composition, by mass%:
Mn: 0.01% or more and less than 0.30%.
8. The steel sheet for hot stamping as claimed in any one of claims 5 to 7,
comprising:
a plating layer on a surface of the steel sheet for hot stamping.
9. A manufacturing method of the hot-stamped product as claimed in any
one of claims 1 to 3, comprising:
a heating step of heating the steel sheet for hot stamping as claimed in any one
of claims 5 to 7 to a heating temperature rc, which is higher than an Ac3 point; and
a hot stamping step of starting hot stamping on the steel sheet for hot
stamping after the heating step at a temperature of (T - 80)°C or higher.
10. A manufacturing method of the hot-stamped product as claimed in any
one of claims 1 to 3, comprising:
a joining step of joining the steel sheet for hot stamping as claimed in any one
of claims 5 to 7 to a steel sheet for joining to obtain a joined steel sheet;
a heating step of heating the joined steel sheet after the joining step to a
- 66 -
heating temperature rc, which is higher than an Ac3 point of the steel sheet for hot
stamping; and
a hot stamping step of starting hot stamping on the joined steel sheet after the
heating step at a temperature of (T- 80tC or higher.
11. A manufacturing method of the hot-stamped product as claimed in claim
4, comprising:
a heating step of heating the steel sheet for hot stamping as claimed in claim 8
to a heating temperature rc, which is higher than an Ac3 point; and
a hot stamping step of starting hot stamping on the steel sheet for hot
stamping after the heating step at a temperature of (T - 80)°C or higher.
12. A manufacturing method of the hot-stamped product as claimed in claim
4, comprising:
a joining step of joining the steel sheet for hot stamping as claimed in claim 8
to a steel sheet for joining to obtain a joined steel sheet;
a heating step of heating the joined steel sheet after the joining step to a
heating temperature rc, which is higher than an Ac3 point of the steel sheet for hot
stamping; and
a hot stamping step of starting hot stamping on the joined steel sheet after the
heating step at a temperature of (T - 80)°C or higher.
13. A manufacturing method of the steel sheet for hot stamping as claimed
in any one of claims 5 to 8, comprising:
a hot rolling step of performing hot rolling on a slab containing, as a chemical
- 67 -
composition, by mass%, C: 0.001% or more and less than 0.090%, Si: 2.50% or less,
Mn: 0.01% or more and less than 0.50%, P: 0.200% or less, S: 0.0200% or less, sol.
Al: 0.001% to 2.500%, N: 0.0200% or less, Cr: 0.01% or more and less than 2.00%,
Ti: 0% to 0.300%, Nb: 0% to 0.300%, V: 0% to 0.300%, Zr: 0% to 0.300%, Mo: 0% to
2.00%, Cu: 0% to 2.00%, Ni: 0% to 2.00%, B: 0% to 0.0200%, Ca: 0% to 0.0100%,
Mg: 0% to 0.0100%, REM: 0% to 0.1000%, Bi: 0% to 0.0500%, and a remainder: Fe
and impurities, and performing coiling in a temperature range of 800°C or lower to
obtain a hot-rolled steel sheet;
a hot-rolled sheet annealing step of performing hot-rolled sheet annealing of
heating the hot-rolled steel sheet to a temperature range of higher than 65oac to obtain
a hot-rolled and annealed steel sheet; and
a cold rolling step of performing cold rolling on the hot-rolled and annealed
steel sheet to obtain a cold-rolled steel sheet.
14. The manufacturing method of the steel sheet for hot stamping as claimed
in claim 13, comprising:
a plating step of performing plating on the cold-rolled steel sheet after the
cold rolling step after optionally performing continuous annealing.