[0001]The present invention relates to a hot-stamped product and a manufacturing
method thereof.
Priority is claimed on Japanese Patent Application No. 2019-070212, 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 440 MPa or more and less than 700 MPa, and a manufacturing method
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, and a manufacturing method
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: less than 0.50%; Mn: 0.50% or more and less than 1.70%; P: 0.200% or
less; S: 0.0200% or less; sol. Al: 0.001% to 2.500%; N: 0.0200% or less; B: 0.0002%
to 0.0200%; Ti: 0% to 0.300%; Nb: 0% to 0.300%; V: 0% to 0.300%; Zr: 0% to
0.300%; Cr: 0% to 2.00%; Mo: 0% to 2.00%; Cu: 0% to 2.00%; Ni: 0% to 2.00%; 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 product
- 6 -
contains, by area%, ferrite: more than 50.0%, tempered martensite: 5.0% or more and
less than 50.0%, martensite: 0% or more and less than 1 0.0%, and bainite: 0% or more
and less than 20.0%, a tensile strength of the hot-stamped product is 440 MPa or more
and 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 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%; Cr: 0.001% to 2.00%; Mo: 0.001% to 2.00%; Cu: 0.001% to 2.00%; Ni:
0.001% to 2.00%; 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: a plating layer
on a surface of the hot-stamped product.
(4) A manufacturing method of the hot-stamped product according to (1) or
(2), including: a heating step of heating a steel sheet for hot stamping having the
chemical composition according to (1) or (2) to a temperature of higher than an Ac3
point; a hot stamping step of starting hot stamping on the steel sheet for hot stamping
after the heating step at a temperature of (An point - 200°C) or higher and lower than
the Ar3 point and subsequently performing cooling to a temperature of lower than
goac; and a reheating step of heating a formed article after the hot stamping step to a
temperature of 1 00°C to 140°C and performing holding at the temperature for 3 to 120
minutes.
(5) A manufacturing method of the hot-stamped product according to (1) or
(2), including: a joining step of joining a steel sheet for hot stamping having the
chemical composition according to (1) or (2) to a steel sheet for joining to obtain a
- 7 -
joined steel sheet; a heating step of heating the joined steel sheet after the joining step
to a temperature of higher than an Ac3 point of the steel sheet for hot stamping; a hot
stamping step of starting hot stamping on the joined steel sheet after the heating step at
a temperature of (An point - 200°C) of the steel sheet for hot stamping or higher and
lower than the An point and subsequently performing cooling to a temperature of
lower than 90°C; and a reheating step of heating a formed article after the hot stamping
step to a temperature of 1 00°C to 140°C and performing holding at the temperature for
3 to 120 minutes.
(6) A manufacturing method of the hot-stamped product according to (3),
including: a heating step of heating a steel sheet for hot stamping having the chemical
composition according to ( 1) or (2) and a plating layer on a surface of the steel sheet
for hot stamping to a temperature of higher than an Ac3 point; a hot stamping step of
starting hot stamping on the steel sheet for hot stamping after the heating step at a
temperature of (An point - 200°C) or higher and lower than the An point and
subsequently performing cooling to a temperature of lower than 90°C; and a reheating
step of heating a formed article after the hot stamping step to a temperature of 1 oooc
to 140°C and performing holding at the temperature for 3 to 120 minutes.
(7) A manufacturing method of the hot-stamped product according to (3),
including: a joining step of joining a steel sheet for hot stamping having the chemical
composition according to ( 1) or (2) and a plating layer on a surface of the steel sheet
for hot stamping, 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 temperature of higher
than an Ac3 point of the steel sheet for hot stamping; a hot stamping step of starting hot
stamping on the joined steel sheet after the heating step at a temperature of (An point -
200°C) of the steel sheet for hot stamping or higher and lower than the An point and
- 8 -
subsequently performing cooling to a temperature of lower than 90°C; and a reheating
step of heating a formed article after the hot stamping step to a temperature of 1 00°C
to 140°C and performing holding at the temperature for 3 to 120 minutes.
[Effects of the 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 of 440 MPa or more and
less 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 440 MPa or more and 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
formed article is greatly reduced by a paint baking treatment. It is considered that this
- 9 -
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 and hot stamping
conditions.
[0024]
(C) A decrease in tensile strength caused by a paint baking treatment is
suppressed by, in a step of performing hot stamping, starting the hot stamping in a
temperature range in which ferrite and austenite coexist, and by reheating the hotstamped
product after the hot stamping step in a predetermined temperature range.
[0025]
The reason for this is not clear, but the present inventors presume that it is due
to the following reason. (a) In a hot-stamped product, carbon in a solid solution state
contained in ferrite precipitates as coarse iron carbides during coating baking, which
causes a decrease in the strength of ferrite. (b) In a hot-stamped product, fine iron
carbides or fine iron-carbon clusters change to coarse iron carbides during coating
baking, which causes a decrease in the strength of ferrite. (c) When hot stamping is
performed in the presence of ferrite, dislocations are introduced into the ferrite in a
hot-stamped product. (d) When a hot-stamped product is reheated, solid solution
carbon in ferrite precipitates on dislocations, the amount of the solid solution carbon
decreases, and fine iron carbides or fine iron-carbon clusters become coarse.
[0026]
(D) When B (boron) is contained in a chemical composition, a decrease in
- 10 -
tensile strength caused by a paint baking treatment is suppressed. The reason for this
is not clear, but it is presumed that when B is contained, the amount of dislocations
introduced into ferrite increases in a hot-stamped product, and the precipitation of
carbides due to reheating is promoted, so that the amount of solid solution carbon
further decreases, and coarsening of iron carbides or iron-carbon clusters is promoted.
[0027]
Based on the above findings (A) to (D), the present inventors found that by
starting hot stamping in a temperature range in which ferrite and austenite coexist
using a steel sheet containing a desired amount of boron (B), and furthermore, by
reheating a hot-stamped product after the hot stamping step, it is possible to
manufacture a hot-stamped product having a microstructure primarily containing
ferrite, having at least a portion with a tensile strength of less than 700 MPa, having
excellent thermal stability, and having a small decrease in strength caused by a paint
baking treatment.
[0028]
Hereinafter, each requirement of a hot-stamped product according to an
embodiment of the present invention (hot-stamped product according to the present
embodiment) and a manufacturing method 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.
[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
- 11 -
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 0.020% or more, 0.030% or more, 0.040% or more, or 0.050%
or more.
On the other hand, when the C content is 0.090% or more, the area ratio of
tempered martensite, 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, so that 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 0.075%, or less than 0.070%.
[0031]
Si: Less Than 0.50%
Si is an element contained as an impurity in steel. When the Si content is
0.50% or more, it becomes difficult to secure the thermal stability of the hot-stamped
product even if the hot-stamped product is reheated as described later. Therefore, the
Si content is set to less than 0.50%. A preferable Si content is less than 0.40%, less
- 12 -
than 0.20%, less than 0.10%, or less than 0.05%. In a case where a plated steel sheet
is used as a steel sheet for hot stamping, the Si content is set to preferably less than
0.40%, and 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% or more. 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, the Si content is
preferably set to 0.10% or more, or 0.20% or more.
[0032]
Mn: 0.50% or More and Less Than 1. 70%
Mn is an element that improves the hardenability of steel, and is contained in
an amount of 0.50% or more in order to obtain a microstructure containing ferrite and
tempered martensite. A preferable Mn content is 0.60% or more, or 0.70% or more.
On the other hand, when the Mn content is 1.70% or more, it becomes
difficult to secure the thermal stability of the hot-stamped product even if the hotstamped
product is reheated as described later. Therefore, the Mn content is set to
less than 1.70%. The Mn content is preferably less than 1.50%, less than 1.20%, less
than 1.00%, or less than 0.80%.
[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.
- 13 -
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
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 0.005% or more, 0.010% or more, or 0.020% or more.
On the other hand, when the sol. Al content is too large, the transformation
- 14 -
point rises, and it becomes difficult to heat the steel sheet to a temperature of higher
than an Ac3 point in a heating step of hot stamping. Therefore, the sol. Al content is
set to 2.500% or less. The sol. Al content is preferably less than 0.500%, less than
0.100%, less than 0.050%, or less than 0.040%.
[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]
B: 0.0002% to 0.0200%
B is an element having an action of improving the thermal stability of a hotstamped
product having a microstructure containing ferrite and tempered martensite.
In a case where the B content is less than 0.0002%, the effect by the above action
cannot be sufficiently obtained. Therefore, the B content is set to 0.0002% or more.
A preferable B content is 0.0006% or more, 0.0010% or more, or 0.0015% or more.
On the other hand, in a case where the B content exceeds 0.0200%, the tensile
strength of the steel sheet after hot stamping becomes too high, and the thermal
stability of the hot-stamped product deteriorates. Therefore, the B content is set to
- 15 -
0.0200% or less. A preferable B content is less than 0.0050%, less than 0.0030%, or
less than 0.0020%.
[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]
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
0.020% or more, and particularly preferably 0.030% 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
- 16 -
0.010% or more.
[0041]
On the other hand, in a case where the amounts ofTi, Nb, V, and Zr each
exceed 0.300%, the effect is saturated and the manufacturing costs of the steel sheet
increase. Therefore, even in a case where these elements are contained, the amounts
ofTi, 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%.
0.040%.
0.100%.
0.100%.
The Nb content is preferably less than 0.060%, and more preferably less than
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]
Cr: 0% to 2.00%
Mo: 0% to 2.00%
Cu: 0% to 2.00%
Ni: 0% to 2.00%
Cr, 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
- 17 -
from Cr, 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 Cr, Mo, Cu, and Ni are each contained in 0.001% or more. A
preferable Cr content is 0.05% or more, a preferable Mo content is 0.05% or more, a
preferable Cu content is 0.10% or more, and a preferable Ni content is 0.10% or more.
[0045]
On the other hand, when the amounts of Cr, Mo, Cu, and Ni each exceed
2.00%, the tensile strength of the steel sheet after hot stamping becomes too high, and
the thermal stability of the hot-stamped product deteriorates.
Therefore, even in a case where the above elements are contained, the
amounts of Cr, Mo, Cu, and Ni are each set to 2.00% or less. A preferable Cr content
is less than 0.50% or less than 0.20%, a preferable Mo content is less than 0.50% or
less than 0.20%, and a preferable Cu content is less than 1.00%, or a preferred Ni
content is less than 1.00%.
[0046]
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
after hot stamping by adjusting the shape of 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%.
- 18 -
[0047]
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.
[0048]
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.
[0049]
Bi: 0% to 0.0500%
Bi is an element having an action of improving the toughness after hot
stamping 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%.
[0050]
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.
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
- 19 -
less. The Bi content is preferably 0.0100% or less, and more preferably 0.0050% or
less.
[0051]
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.
[0052]
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 using 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.
[0053]

The 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, tempered
martensite, martensite, and bainite in amounts described below. In the following
description regarding the microstructure, "%" means "area%".
[0054]
Ferrite: More Than 50.0%
- 20 -
When the area ratio of ferrite is 50.0% or less, the tensile strength of the
formed article after hot stamping becomes 700 MPa or more, and thermal stability
cannot be secured. Therefore, the area ratio of ferrite is set to more than 50.0%.
The area ratio of ferrite is preferably more than 60.0%, and more preferably more than
70.0%, and even 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 95.0%, more preferably less than 90.0%,
and even more preferably less than 85.0% in order to increase the strength of the hotstamped
product.
[0055]
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.
[0056]
Tempered Martensite: 5.0% or More and Less Than 50.0%
Tempered martensite is a structure having an action of increasing the strength
of a hot-stamped product while maintaining the thermal stability of the hot-stamped
product. When the area ratio of tempered martensite is less than 5.0%, the effect of
the above action cannot be sufficiently obtained, and it becomes difficult to secure the
thermal stability of the hot-stamped product and/or the strength of the hot-stamped
product. Therefore, the area ratio of tempered martensite is set to 5.0% or more.
The area ratio of tempered martensite is preferably 8.0% or more, more preferably
10.0% or more, and even more preferably 12.0% or more.
- 21 -
[0057]
On the other hand, when the area ratio of tempered martensite is 50.0% or
more, the tensile strength of the steel sheet after hot stamping becomes too high, and
the thermal stability of the hot-stamped product deteriorates. Therefore, the area r atio
of tempered martensite is set to less than 50.0%. The area ratio of tempered
martensite is preferably less than 40.0%, more preferably less than 30.0%, and even
more preferably less than 20.0%.
[0058]
Martensite: 0% or More and Less Than 10.0%
Bainite: 0% or More and Less Than 20.0%
When the microstructure (microstructure) contains a large amount of
martensite (indicating martensite that has not been tempered, also called fresh
martensite) and bainite, the thermal stability of the hot-stamped product deteriorates.
Therefore, the area ratio of martensite is set to less than 1 0.0%, and the area ratio of
bainite is set to less than 20.0%. The area ratio of martensite is set to preferably less
than 5.0%, more preferably less than 2.0%, and even more preferably less than 1.0%.
The area ratio of bainite is set to preferably less than 10.0%, more preferably less than
5.0%, and even more preferably less than 2.0%.
[0059]
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
- 22 -
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.
[0060]
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%.
[0061]
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.
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.
[0062]
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 less than 3.0%, and more preferably less than 2.0%.
[0063]
In the present embodiment, the area ratio of each microstructure is obtained as
follows.
- 23 -
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 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 a substrate, from the boundary
between the steel sheet as the substrate and a 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),
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
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.
[0064]
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 u sing 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 martensite is calculated.
[0065]
- 24 -
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).
[0066]
Based on these results, the area ratio of each of ferrite, pearlite, bainite,
tempered martensite, martensite, and residual austenite is obtained.
Tempered martensite can be distinguished from 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.
[0067]

The entirety or a part of the hot-stamped product according to the present
embodiment has a tensile strength of 440 MPa or more and less than 700 MPa. For
this purpose, the tensile strength of the entirety or a part of a base steel sheet of the hotstamped
product according to the present embodiment needs to be 440 MPa or more
and less than 700 MPa. This is because when the tensile strength is 700 MPa or more,
the thermal stability of the hot-stamped product cannot be secured. Therefore, the
tensile strength of the entirety or a part of the hot -stamped product is set to less than
700 MPa. Preferably, the tensile strength is less than 650 MPa or less than 600 MPa
in the entirety or a part of the hot-stamped product. On the other hand, in order to
improve the impact absorbability of the hot -stamped product, the tensile strength of the
entirety or a part of the hot-stamped product is set to 440 MPa or more. Preferably,
the tensile strength is 460 MPa or more, 490 MPa or more, or 540 MPa or more in the
entirety or a part of the hot-stamped product.
- 25 -
[0068]
In the hot-stamped product according to the present embodiment, a soft
portion having a tensile strength of 440 MPa or more and less than 700 MPa and a hard
portion having a 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.
[0069]

In the hot-stamped product according to the present embodiment, a decrease
in tensile strength (~TS) when a heat treatment is performed at 170ac 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 les s. The lower limit of
~TS is not particularly limited, but is preferably 1 MPa or more, 5 MPa or more, or 10
MPa or more from the viewpoint of steel sheet manufacturability.
[0070]
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 50.0% by area ratio) is that due to the paint baking treatment, carbon in a
solid solution state present in ferrite precipitates as coarse iron carbides, and fine iron
carbides or fine iron-carbon clusters present in the ferrite are changed to coarse iron
carbides due to a heat treatment during the coating baking. Although it is not easy to
- 26 -
directly and quantitatively evaluate the presence state of the solid solution carbon, fine
iron carbides, and fine iron-carbon clusters, 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 is determined that the amount of
solid solution carbon generated in ferrite and the amount of fine iron carbides or fine
iron-carbon clusters generated are small, and excellent thermal stability is achieved.
[0071]
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.
[0072]

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 r esistance 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
- 27 -
both sides.
[0073]
Next, a steel sheet for hot stamping suitable for manufacturing the above hotstamped
product will be described.
[0074]

Since the chemical composition is not substantially changed by hot stamping,
the chemical composition of the steel sheet for hot stamping has the same chemical
composition as that of the above-described hot-stamped product.
[0075]

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) preferably satisfies Expression
(i).
[0076]
[Mn]e + [Cr]e > 1.7 ... (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%
[0077]
As the chemical composition of the iron carbides contained in the
microstructure of the steel sheet for hot stamping satisfies Expression (i), the thermal
- 28 -
stability of the steel sheet after hot stamping can be further improved. The value on
the left side of Expression (i) is preferably more than 3.0, and more preferably more
than 4.0.
[0078]
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 20.0, and more preferably less than 10.0.
[0079]
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).
[0080]
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%)
when the total amount thereof is 100 at% are respectively obtained as [Mn]e and [Cr]e.
- 29 -
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.
[0081]
In the present embodiment, in a case of a hot-rolled and annealed steel sheet, a
cold-rolled steel sheet, or an annealed steel sheet, at a 114 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 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 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 abovedescribed
microstructure is specified.
[0082]
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 r atio of the iron carbides is set to preferably
- 30 -
20% or less, and more preferably 15% or less.
[0083]
The remainder of the microstructure of the steel sheet for hot stamping
according to the present embodiment preferably primarily contains ferrite, but may
contain martensite, tempered martensite, bainite, and residual austenite as the
remainder, and may further contain precipitates other than the iron carbides.
However, since martensite, tempered 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, tempered martensite, bainite, and
residual austenite are all preferably less than 1.0%, and more preferably less than 0.5%.
[0084]
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.
[0085]

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.
[0086]
[Manufacturing Method of Hot-Stamped Product]
A manufacturing method of the hot-stamped product according to the present
- 31 -
embodiment includes a heating step of heating a steel sheet for hot stamping having the
above-described chemical composition, a hot stamping step of performing hot
stamping on the heated steel sheet for hot stamping and subsequently performing
cooling, and a reheating step of reheating a formed article after the hot stamping step.
In the hot stamping step, cooling and forming are performed by a die, thereby
obtaining a hot-stamped product.
[0087]
In the heating step of heating the steel sheet for hot stamping, a heating
temperature is 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 is the Ac3 point or lower, the dissolution of
carbides during heating becomes insufficient, and the strength of the hot-stamped
product decreases. The heating temperature is preferably (Ac3 point+ 20tC or
higher, and more preferably (Ac3 point+ 40)°C or higher.
In addition, the steel sheet for hot stamping subjected to the heating preferably
has the above-mentioned structure.
[0088]
The upper limit of the heating temperature is not particularly limited.
However, when the heating temperature is too high, austenite becomes coarse and the
strength of the hot-stamped product decreases. Therefore, the heating temperature is
preferably 1000°C or lower, more preferably 950°C or lower, and even more
preferably 900°C or lower.
In addition, a preferable retention time of the hot stamping at a heating
temperature is one to five minutes.
- 32 -
[0089]
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 (An point - 200tC
or higher and lower than the An point. The An point is the temperature at which
ferrite starts to be generated in the microstructure when the base steel sheet is cooled
from a temperature of higher than the AC3 point. The An point is obtained from a
change in thermal expansion when the steel sheet is cooled after the heating step.
When the hot stamping start temperature is the An point or higher, the amount of
dislocations introduced into ferrite is insufficient, and the thermal stability of the hotstamped
product is impaired. When the hot stamping start temperature is lower than
(Ar3 point- 200°C), the area ratio of tempered martensite in the microstructure the hotstamped
product decreases, and the strength of the hot-stamped product becomes
insufficient. A preferable upper limit of the hot stamping start temperature is lower
than (Ar3 point - 20°C), less than (Ar3 points -40°C), or less than (Ar3 points - 60°C).
The preferable lower limit of the hot stamping start temperature is (Ar3 point - 170°C)
or higher, (Ar3 point- 140°C) or higher, or (Ar3 point- ll0°C) or higher.
[0090]
After performing forming by hot stamping, the formed article is cooled to a
temperature of less than 90°C by holding the formed article in the die and/or taking out
the formed article from the die and cooling the formed article by any method. When
the cooling stop temperature is 90°C or higher, the area ratio of tempered martensite
decreases in the microstructure of the hot-stamped product, and the strength of the hotstamped
product becomes insufficient. The cooling stop temperature is preferably
lower than 50°C, and more preferably room temperature. In order to increase
productivity, it is preferable to hold the formed article in the die up to a temperature of
- 33 -
lower than 90°C.
[0091]
In the reheating step of reheating the hot-stamped product, the reheating
temperature is set to 100oc to 140°C, and the retention time at the reheating
temperature is set to 3 to 120 minutes. When the reheating temperature is lower than
1 00°C, fine iron carbides or fine iron-carbon clusters are generated, and the thermal
stability of the hot-stamped product deteriorates. On the other hand, when the
reheating temperature exceeds 140°C, the strength of the hot -stamped product
decreases.
[0092]
When the retention time is shorter than three minutes, a large amount of solid
solution carbon in ferrite remains in the microstructure of the hot- stamped product, and
the thermal stability of the hot-stamped product deteriorates. On the other hand,
when the retention time exceeds 120 minutes, the strength of the hot-stamped product
decreases. The retention time is preferably adjusted according to the reheating
temperature. In a case where the reheating temperature is 1 oooc or higher and lower
than l20°C, the retention time is preferably longer than 60 minutes, longer than 70
minutes, or longer than 80 minutes, and the retention time is preferably shorter than
110 minutes, shorter than 100 minutes, or shorter than 90 minutes. In a case where
the reheating temperature is 120 to 140 minutes, the retention time is preferably longer
than 5 minutes, longer than 7 minutes, or longer than 9 minutes, and the retention time
is preferably shorter than 30 minutes, shorter than 20 minutes, or shorter than 15
minutes. From the viewpoint of productivity, the reheating temperature is preferably
set to l20°C to 140°C.
[0093]
- 34 -
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 to a steel sheet for joining
to obtain a joined steel sheet, a heating step of heating the joined steel sheet, and a step
of thereafter performing hot stamping on the heated joined steel sheet and subsequently
performing cooling, and a step of reheating a hot -stamped product after the hot
stamping step. As the joining method, for example, a method of butting and
overlapping the steel sheet for hot stamping and the steel sheet for joining and joining
the resultant by welding can be adopted.
[0094]
The above-mentioned joined steel sheet is heated to a temperature of higher
than the Ac3 point of the steel sheet for hot stamping, hot stamping is started at a
temperature of (Ar3 point - 200°C) or higher and lower than the Ar3 point of the steel
sheet for hot stamping, and cooling is subsequently performed to a temperature of
lower than 90°C. Thereafter, the hot-stamped product is reheated to a temperature of
100oc to 140°C and held at the temperature for 3 to 120 minutes. A preferable
heating temperature in the step of heating the joined steel sheet, a preferable hot
stamping start temperature and a preferable cooling stop temperature in the step of hotstamping
the joined steel sheet, and a preferable reheating temperature and a preferable
retention time in the step of reheating the hot-stamped product are the same as in the
manufacturing method of the hot-stamped product that does not include the joining
step, and the reasons for these are the same as those in a case of not including the
joined steel sheet.
[0095]
The chemical composition and mechanical properties of the steel sheet for
- 35 -
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 reheating. A more preferable tensile
strength of the steel sheet for joining after reheating is more than 1000 MPa, more than
1200 MPa, or more than 1500 MPa.
[0096]
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.
[0097]
The steel sheet used as the base (steel sheet for hot stamping) is preferably
subjected to hot-rolled sheet annealing as described later. After the hot-rolled sheet
annealing, cold rolling, or cold rolling and 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 hot-rolled steel sheet, a hotrolled
and annealed steel sheet obtained by annealing a hot-rolled steel sheet, and a
cold-rolled annealed steel sheet obtained by annealing a cold-rolled steel sheet may be
used.
[0098]
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
- 36 -
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.
[0099]
[Manufacturing Method of Steel Sheet for Hot Stamping]
It is preferable that the steel sheet for hot stamping according to the present
embodiment is manufactured according to a manufacturing method including a hot
rolling step of hot-rolling a slab having the above-described chemical composition and
thereafter performing coiling in a temperature range of 800°C or lower to obtain a hotrolled
steel sheet, and 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
65ooc to obtain a hot-rolled and annealed steel sheet.
[0100]
In the hot rolling step, the coiling temperature after the hot rolling is
preferably 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. A more preferable
coiling temperature is lower than 650°C, lower than 600°C, or lower than 55ooc.
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.
[0101]
A manufacturing method of the slab provided for the manufacturing method
- 37 -
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 composition (chemical composition) is melted by a
known method, thereafter made into a steel ingot by a continuous casting method, or
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.
[0102]
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.
[0103]
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
- 38 -
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.
[0104]
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.
[0105]
The hot-rolled and coiled steel sheet is preferably 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 hotrolled
and annealed steel sheet. Before the hot-rolled sheet annealing, descaling by
pickling or the like may be performed.
[0106]
The heating temperature in the hot-rolled sheet annealing step is preferably 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. 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
- 39 -
of the hot-rolled and annealed steel sheet becomes coarse, and the tensile strength after
hot stamping decreases. Therefore, the upper limit of the heating temperature in the
hot-rolled sheet annealing step is more preferably lower than 750°C, and even more
preferably lower than 720°C.
[0107]
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.
[0108]
It is preferable that after the hot-rolled sheet annealing step described above,
the hot-rolled and annealed steel sheet is cold-rolled into a cold-rolled steel sheet
having a sheet thickness of 2.8 mm or less. In order to reduce the weight of the hotstamped
product, the sheet thickness of the cold-rolled steel sheet is more preferably
2.3 mm or less, further preferably 2.0 mm or less, particularly more preferably 1.8 mm
or less, and even more 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.
[0109]
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 the cold rolling,
in order to refine the microstructure after hot stamping and increase the tensile strength,
- 40 -
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 stamping deteriorates. Therefore, 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.
[OllO]
The cold-rolled steel sheet may be subjected to continuous annealing 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, 650oc or higher,
or 700°C or higher.
[Oll1]
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, the
tensile strength after hot stamping decreases. Therefore, the average heating rate up
to the soaking temperature in the annealing is preferably set to 1 °C/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.
[0112]
The hot-rolled and annealed steel sheet, the cold-rolled steel sheet, and the
- 41 -
annealed steel sheet obtained as described above may be subjected to temper rolling
according to a typical method.
[0113]
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.
[0114]
In a case of manufacturing a hot-dip plated steel sheet, the hot-rolled and
annealed 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 hotdip
plating is preferably set to 600°C or higher, 650°C or higher, or 700°C or higher in
order to refine the microstructure of the plated steel sheet by recrystallization.
[0115]
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
- 42 -
760ac or lower. An alloying treatment may be performed by reheating the steel sheet
after the hot-dip plating.
[0116]
In a case of manufacturing an electro plated steel sheet, the hot-rolled and
annealed 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.
[0117]
Hereinafter, the present invention will be described more specifically with
reference to examples, but the present invention is not limited the examples.
[Examples]
[0118]
(Example 1)
Molten steel was cast using a vacuum melting furnace to manufacture Steels
A toN 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 toN were heated at 2 ac/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 toN were heated to 920°C and then cooled at 10 °C/sec. Steels A toN were
heated to 1200ac and held for 60 minutes, and then subjected to hot rolling under the
hot rolling conditions shown in Table 2.
- 43 -
Steel
A
B
c
D
E
F
G
H
I
J
K
L
M
N
[0119]
[Table 1]
c Si
0.033 0.02
0.063 0.11
0.062 0.16
0.081 0.22
0.032 0.02
0.098 0.23
0.062 Qj]
0.062 0.14
0.060 0.24
0.061 0.22
0.062 0.21
0.061 0.20
0.067 0.43
0.062 0.21
Mn
0.52
0.79
0.92
1.74
0.43
1.51
0.77
0.81
0.77
0.78
0.77
0.77
0.85
0.76
Chemical composition (mass%, remainder: Fe and impurities)
p s sol.Al N B
0.010 0.0021 0.036 0.0026 0 .0005
0.011 0.0020 0.037 0.0019 0.0012
0.010 0.0018 0.036 0.0018 0.0010
0.012 0.0021 0.036 0.0020 0.0009
0.011 0.0022 0.038 0.0020 0.0004
0.012 0.0020 0.039 0.0019 0.0010
0.010 0.0021 0.037 0.0018 0.0011
0.010 0.0021 0.038 0.0021 0.0001
0.010 0.0019 0.038 0.0020 0 .0010
0.010 0.0018 0.039 0.0018 0.0011
0.011 0.0021 0.039 0.0018 0.0011
0.010 0.0021 0.038 0.0019 0 .0010
0.010 0.0019 0.036 0.0018 0 .0025
0.011 0.0019 0.037 0.0020 0.0010
- 44 -
Transformation p oint (°C)
Note
Others Ac1 Ac3 Ar,
Cr:0.05 734 894 832 Present Invention Steel
Ti:0.031 735 886 820 Present Invention Steel
Ti:0.030 Nb:0.031 744 892 811 Present Invention Steel
Ti:0.061 Nb:0.033 735 878 725 Present Invention Steel
733 898 839 Present Invention Steel
Ti:0.060 725 878 734 Present Invention Steel
744 901 823 Present Invention Steel
Ti:0.031 731 879 836 Present Invention Steel
Cu:0.20 Ni: 0.10 728 877 803 Present Invention Steel
Ca:0.0003 Mg:0.0005 730 880 808 Present Invention Steel
Bi:0.0022 REM:0.0004 731 879 806 Present Invention Steel
V:0.010 Zr:0.011 732 880 810 Present Invention Steel
Ti:0.030 Nb:0.032 750 902 812 Present Invention Steel
734 882 811 Present Invention Steel
[0120]
[Table 2]
Hot rolling Hot-rolled sheet Cold rolling Annealing
conditions annealing conditions conditions conditions
Test
Steel Sheet thickness Soaking Note
No. Coiling temperature Heating temperature
(OC) (OC)
after cold rolling temperature
(mm) ("C)
1 A 580 - 1.4 - Present lnvention Example
2 A 540 710 1.4 - Present lnvention Example
3 A 580 - 1.4 730 Present lnvention Example
4 A 580 - 1.4 730 Present lnvention Example
5 A 580 - 1.4 730 Present lnvention Example
6 A 580 - 1.4 730 Present lnvention Example
7 A 580 - 1.4 730 Present lnvention Example
8 A 580 - 1.4 730 Present lnvention Example
9 B 580 - 1.4 - Present lnvention Example
10 B 540 710 1.4 730 Present lnvention Example
11 B 580 - 1.4 730 Present lnvention Example
12 B 580 - 1.4 730 Present lnvention Example
13 B 580 - 1.4 730 Present lnvention Example
14 B 580 - 1.4 730 Present lnvention Example
15 c 580 - 1.4 - Present lnvention Example
16 c 540 710 1.4 730 Present lnvention Example
17 c 580 - 1.4 730 Present lnvention Example
18 c 580 - 1.4 730 Present lnvention Example
19 c 580 - 1.4 730 Present lnvention Example
20 D 580 - 1.4 - Comparative Example
21 E 580 - 1.4 730 Comparative Example
22 F 580 - 1.4 - Comparative Example
23 G 580 - 1.4 730 Comparative Example
24 H 580 - 1.4 730 Comparative Example
25 I 540 710 1.4 720 Present lnvention Example
26 J 580 - 1.4 - Present lnvention Example
27 K 540 710 1.4 - Present lnvention Example
28 L 580 - 1.4 730 Present lnvention Example
29 A 580 1.4 730 Present lnvention Example
30 A 580 - 1.4 730 Present lnvention Example
31 B 580 - 1.4 730 Present lnvention Example
32 M 580 1.4 730 Present lnvention Example
33 N 580 - 1.4 - Present lnvention Example
Underline indicates outside the range of the present invention.
[0121]
Specifically, Steels A toN were rolled in 10 passes in a temperature range of
the Af3 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
- 45 -
water spray, the cooling finishing temperature was set to a coiling temperature, the hotrolled
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.
[0122]
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 710°C at
an average heating rate of 50 °C/hr using the electric heating furnace, held for one hour,
and subsequently cooled at an average cooling rate of 20 oc/hr, whereby a hot-rolled
and annealed steel sheet was obtained.
[0123]
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 oc/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.
[0124]
In addition, some of the cold-rolled steel sheets were heated to a soaking
- 46 -
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.
[0125]
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.
[0126]
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 114 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 114 depth
position of the sheet thickness of the steel sheet as the substrate from a 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
- 47 -
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.
[0127]
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 mm/min 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.
- 48 -
[0128]
[Table 3]
Microstructure of steel Mechanical properties of
Test Kind of Kind of sheet for hot stamping steel sheet for hot stamping
Steel
No. steel"1 plating"2
[Mn]e + [Cr]e (at%)
1 A CR 0.7
2 A CR 1.8
3 A ACR GI 0.7
4 A ACR GI 0.7
5 A ACR GI 0.7
6 A ACR GI 0.7
7 A ACR GI 0.7
8 A ACR GI 0.7
9 B CR 0.9
10 B ACR AL 2.4
11 B ACR AL 0.9
12 B ACR AL 0.9
13 B ACR AL 0.9
14 B ACR 0.9
15 c CR 1.1
16 c ACR GA 2.8
17 c ACR GA 1.1
18 c ACR GA 1.1
19 c ACR GA 1.1
20 D CR 2.0
21 E ACR GA 0.5
22 F CR 1.7
23 G ACR GI 0.9
24 H ACR GA 0.9
25 I ACR AL 2.3
26 J CR 1.0
27 K CR 2.2
28 L ACR GA 1.0
29 A ACR GI 0.7
30 A ACR GI 0.7
31 B ACR AL 0.9
32 M ACR GA 0.9
33 N CR 1.0
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
- 49 -
Tensile strength
(MPa)
536
528
357
357
357
357
357
357
553
371
386
386
386
388
580
399
408
408
408
642
343
649
405
374
397
548
544
387
357
357
386
417
547
Note
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Comparative Example
Comparative Example
Comparative Example
Comparative Example
Comparative Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
Present fuvention Example
[0129]
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, sandwiched between dies provided with a cooling apparatus to
be subjected to hat forming at the start temperature shown in Table 4, and cooled in the
dies to the cooling stop temperature shown in Table 4. Some of the hat members
were reheated using an electric heating furnace under the conditions shown in Table 4.
RT of Hot stamping conditions in Table 4 indicates room temperature, and "-"
indicates that the reheating step was not performed.
[0130]
Some of the hat members (hot-stamped products) were subjected to a heat
treatment at 170°C for 20 minutes using the electric heating furnace.
[0131]
A test piece for SEM observation was collected from a vertical wall 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 subj ected 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 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 the substrate from the boundary
- 50 -
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),
the microstructure was observed. Using the above-described method, the area ratios
of ferrite, pearlite, residual austenite, tempered martensite, martensite, and bainite were
measured by image processing. The results are shown in Table 4. The remainder of
the structure shown in Table 4 was pearlite, residual austenite, and/or precipitates. In
the table, in test numbers satisfying the regulations of the present invention, the
proportion of polygonal ferrite in ferrite in the microstructure of the hot-stamped
product was 5.0% or more.
[0132]
Furthermore, a JIS No. 13B tensile test piece was collected from a vertical
wall portion of the hat member before and after the heat treatment along the
longitudinal direction of the member, and a tensile test was conducted at a tensile
speed of 10 mm/min 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.
[0133]
A case where the tensile strength before and after the heat treatment was 440
MPa or more and 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. On the other hand,
a case where the tensile strength before the heat treatment was less than 440 MPa or
700 MPa or more and fl. TS was more than 100 MPa was determined to be unacceptable
- 51 -
as not satisfying the regulations of the present invention.
[0134]
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 Tables
1 to 4, underlined numerical values mean outside the range of the present invention, or
outside preferable manufacturing conditions.
- 52 -
[0135]
[Table 4]
Hot stamping conditions Microstructure of hot~ stamped product
Test
No.
Steel Heating Start Cooling stop Reheating Retention Ferrite Tempered
temperature temperature temperature temperature time (area%) martensite
CCC) (OC) (oC)"' (OC) (min) (area%)
1 A 920 800 RT 130 10 85.4 5.6
2 A 920 800 RT 140 10 86.2 5.3
3 A 920 800 RT 140 10 85.7 5.5
4 A 920 790 RT 140 180 88.0 4.7
5 A 920 790 RT 150 120 88.2 4.7
6 A 800 750 RT 140 10 90.1 < 0.1
7 A 920 600 RT 130 10 92.2 < 0.1
8 A 920 760 130 140 10 94.7 2.3
9 B 920 790 RT 140 10 76.4 10.8
10 B 920 790 RT 140 10 76.0 10.6
11 B 920 790 RT 140 10 76.2 10.6
12 B 920 790 RT 110 2 75.8 3.2
13 B 920 790 RT 80 30 76.3 < 0.1
14 B 920 790 RT 110 90 75.9 8.3
15 c 920 780 RT 140 10 75.0 14.5
16 c 920 780 RT 140 10 75.3 14.0
17 c 920 780 RT 140 10 75.1 14.2
18 c 920 830 RT 110 10 50. 3 7 .6
19 c 920 780 RT 74.8 < 0.1
20 D 920 710 RT 140 10 53.2 22.4
21 E 920 800 RT 130 10 87.5 4 .0
22 F 920 720 RT 140 10 47.6 26.2
23 G 920 790 RT 130 10 75.4 3.4
24 H 920 800 RT 110 10 79.5 2.6
25 I 920 780 RT 140 10 79.3 9.0
26 J 920 780 RT 110 90 79.7 6.8
27 K 920 780 RT 130 10 79.4 8.5
28 L 920 780 RT 110 90 80.1 6.6
29 A 920 800 RT 110 120 85.6 5.1
30 A 920 800 80 140 10 85.5 5.2
31 B 920 790 RT 140 6 76.3 10.7
32 M 920 810 RT 140 10 65.6 12.1
33 N 920 780 RT 110 90 79.4 6J!
Underline indicates outside the range of the present invention, or outside preferable manufacturing c onditions.
#1 RT: Room temperature (20 ± 5°C)
- 53 -
Martensite Bainite
(area%) (area%)
< 0.1 4.3
< 0.1 3 .8
< 0.1 4 .0
< 0.1 3.4
< 0.1 3.6
< 0.1 5 .3
< 0.1 3.2
< 0.1 1.9
< 0.1 8.1
< 0.1 8.5
< 0.1 8.3
7.1 8.7
10.4 8.5
2.4 8.8
< 0.1 4 .9
< 0.1 5.2
< 0.1 4 .6
13.6 24.6
14.6 5.0
< 0.1 20.4
< 0.1 4 .5
< 0.1 21.3
9.5 7 .6
4.7 8 .2
< 0.1 6 .5
1.5 6 .7
< 0.1 7 .0
1.6 6 .8
0.4 3.9
< 0.1 4 .0
< 0.1 8.4
< 0.1 17.4
1.7 6 .6
Mechanical properties of hot~stamped
product
Tensile
Tensile strength
strength after h eat L'l.TS
Note
before heat treatment (MPa)
treatment
(MPa) (MPa)
476 432 44 Present Invention Example
448 436 12 Present Invention Example
452 412 40 Present Invention Example
427 400 27 Comparative Example
418 396 22 Comparative Example
398 350 48 Comparative Example
402 360 42 Comparative Example
389 349 40 Comparative Example
612 560 52 Present Invention Example
590 562 28 Present Invention Example
599 552 47 Present Invention Example
639 517 122 Comparative Example
646 501 145 Comparative Example
632 564 68 Present Invention Example
622 567 55 Present Invention Example
603 577 26 Present Invention Example
614 564 50 Present Invention Example
737 583 154 Comparative Example
658 516 142 Comparative Example
715 605 110 Comparative Example
435 387 48 Comparative Example
742 616 126 Comparative Example
628 519 109 Comparative Example
642 538 104 Comparative Example
593 572 21 Present Invention Example
618 573 45 Present Invention Example
607 589 18 Present Invention Example
610 557 53 Present Invention Example
456 410 48 Present Invention Example
445 407 38 Present Invention Example
606 557 49 Present Invention Example
674 591 83 Present Invention Example
615 567 48 Present Invention Example
[0136]
In all of Test Nos. 1 to 3, 9 to 11, 14 to 17, and 25 to 33 satisfying the
regulations of the present invention, the tensile strength of the hot-stamped product
was 440 MPa or more and less than 700 MPa, good strength properties were exhibited,
/1. TS was 100 MPa or less, and good thermal stability was exhibited.
[0137]
In Test Nos. 2, 10, 16, 25, and 27 in which the hot-rolled sheet annealing was
performed in the manufacturing process of the steel sheet for hot stamping, /1. TS of the
hot-stamped product was 30 MPa or less, and the thermal stability was particularly
good.
[0138]
Contrary to this, in Test Nos. 20 to 24 of comparative examples using the steel
sheets in which the chemical composition was outside the range of the present
invention, the tensile strength of the hot-stamped product was less than 440 MPa, the
strength properties were inferior, /1. TS was 100 MPa or more, and the thermal stability
was inferior.
[0139]
Specifically, in Test No. 21 using SteelE, the Mn content of the steel was too
high, so that the area ratio of tempered martensite in the microstructure of the hotstamped
product was insufficient, and the tensile strength of the hot- stamped product
was low.
[0140]
In Test No. 20 using Steel D, the Mn content of the steel was too high, so that
the tensile strength of the hot-stamped product was 700 MPa or more and /I.TS was
large.
- 54 -
[0141]
Specifically, in Test No. 22 using Steel F, the C content of the steel was too
high, so that the area ratio of ferrite in the microstructure of the hot -stamped product
was insufficient, the tensile strength of the hot -stamped product was 700 MPa or more,
and /1;. TS was large.
[0142]
Specifically, in Test No. 23 using Steel G, the Si content of the steel was too
high, so that /1;. TS was large.
[0143]
In Test No. 24 using Steel H, the B content of the steel was too low, so that
/1;. TS was large.
[0144]
In Test Nos. 4 to 8, 12, 13, 18, and 19 of comparative examples in which the
chemical composition was within the range of the present invention but the
manufacturing conditions of the hot-stamped product were outside the range of the
present invention, the tensile strength of the hot-stamped product was less than 440
MPa, the strength properties were inferior or /1;. TS was 100 MPa or more, and the
thermal stability was inferior.
[0145]
Specifically, in Test Nos. 4 and 5 using Steel A, the retention time in the
reheating step was too long or the reheating temperature was too high, so that the
tensile strength of the hot- stamped product was low.
[0146]
In Test No. 6 using Steel A, the heating temperature in the heating step was
too low, so that the tensile strength was low.
- 55 -
In Test Nos. 7 and 8 using Steel A, the forming start temperature in the hot
stamping step was too low or the cooling stop temperature was too high, so that the
tensile strength was low.
[0147]
In Test Nos. 12 and 13 using steel B, the retention time in the reheating step
was too short or the reheating temperature was too low, so that the area ratio of
tempered martensite in the microstructure of the hot-stamped product was insufficient
and /j, TS was large.
[0148]
In Test No. 18 using Steel C, the forming start temperature in the hot
stamping step was too high, so that fj,TS was large. In Test No. 19 using Steel C, the
reheating step was too not performed, so that the area ratio of tempered martensite was
insufficient and /j, TS 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.
- 56 -
[0150]
[Table 5]
Hot rolling Hot-rolled sheet Cold rolling Annealing
conditions annealing conditions conditions conditions
Test
Steel Coiling Sheet thickness Soaking Note
No. Heating temperature
temperature after cold rolling temperature
CC)
(OC)
(mm) CC)
34 A 580 1.4 730
Present Invention
-
Example
35 B 540 710 1.4 730
Present Invention
Example
36 c 580 1.4 730
Present 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 steel
Mechanical properties
Test Kind of Kind of sheet for hot stamping
of steel sheet for hot
No.
Steel steel#3 plating#4 stamping
[Mn]e + [Cr]e Tensile stren gth
(at%) (MPa)
34 A ACR GA 0.7 355
35 B ACR GI 2.5 373
36 c ACR AL 1.1 412
#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
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
- 57 -
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 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.
- 58 -
[0156]
[Table 7]
Hot stamging conditions Microstructme of hot -stamped p oduct Mechanical properties of hot-stamped product
Test
Stee
Heating Start Cooling stop Reheating Retention
Fenite
Tempered
!MartensitE Bainite
Tensile strength before Tensile strength after
6.TS Note
No. temperatme temperatme temperature temperature time martensite heat treatment heat treatment
(oC) (oC) (oC) (oC) (min) area% (area%) (area%) area%) (MPa) (MPa) (MPa
34 A 920 800 RT 140 10 85.5 5.8 < 0.1 4.2 458 420 38 Present :Invention
Example
35 B 920 790 RT 140 10 76.1 10.9 < 0.1 8.0 583 560 23 Present :Invention
Example
36 c 920 780 RT 140 10 74.7 14.5 < 0.1 5.4 619 567 52
Present :Invention
Example
- 59 -
[0157]
In any test results of Test Nos. 34 to 36, the tensile strength of the hotstamped
product was 440 MPa or more and less than 700 MPa, ~ TS was 100 MPa or
less, and good strength properties and thermal stability were exhibited. The tensile
strengths of portions of the steel sheets for joining of the hat members in Test Nos. 34
to 36 were 1545 MPa, 1540 MPa, and 1536 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 440 MPa or more and less than 700 MPa and is thus
excellent in thermal stability.

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: less than 0.50%;
Mn: 0.50% or more and less than 1.70%;
P: 0.200% or less;
S: 0.0200% or less;
sol. Al: 0.001% to 2.500%;
N: 0.0200% or less;
B: 0.0002% to 0.0200%;
Ti: 0% to 0.300%;
Nb: 0% to 0.300%;
V: 0% to 0.300%;
Zr: 0% to 0.300%;
Cr: 0% to 2.00%;
Mo: 0% to 2.00%;
Cu: 0% to 2.00%;
Ni: 0% to 2.00%;
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,
- 61 -
wherein a microstructure of the hot-stamped product contains, by area%,
ferrite: more than 50.0%,
tempered martensite: 5.0% or more and less than 50.0%,
martensite: 0% or more and less than 1 0.0%, and
bainite: 0% or more and less than 20.0%,
a tensile strength of the hot-stamped product is 440 MPa or more and less than
700MPa, 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%;
Cr: 0.001% to 2.00%;
Mo: 0.001% to 2.00%;
Cu: 0.001% to 2.00%;
Ni: 0.001% to 2.00%;
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%.
- 62 -
3. The hot-stamped product as claimed in claim 1 or 2, comprising:
a plating layer on a surface of the hot-stamped product.
4. A manufacturing method of the hot-stamped product as claimed in claim
1 or 2, comprising:
a heating step of heating a steel sheet for hot stamping having the chemical
composition as claimed in claim 1 or 2 to a temperature of higher than an Ac3 point;
a hot stamping step of starting hot stamping on the steel sheet for hot
stamping after the heating step at a temperature of (Af3 point - 200°C) or higher and
lower than the Ar3 point and subsequently performing cooling to a temperature of
lower than gooc; and
a reheating step of heating a formed article after the hot stamping step to a
temperature of 1 oooc to 140°C and performing holding at the temperature for 3 to 120
minutes.
5. A manufacturing method of the hot-stamped product as claimed in claim
1 or 2, comprising:
a joining step of joining a steel sheet for hot stamping having the chemical
composition as claimed in claim 1 or 2 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
temperature of higher than an AC3 point of the steel sheet for hot stamping;
a hot stamping step of starting hot stamping on the joined steel sheet after the
heating step at a temperature of (Ar3 point - 200°C) of the steel sheet for hot stamping
- 63 -
or higher and lower than the Af3 point and subsequently performing cooling to a
temperature of lower than 90°C; and
a reheating step of heating a formed article after the hot stamping step to a
temperature of 1 oooc to 140°C and performing holding at the temperature for 3 to 120
minutes.
6. A manufacturing method of the hot-stamped product as claimed in claim
3, comprising:
a heating step of heating a steel sheet for hot stamping having the chemical
composition as claimed in claim 1 or 2 and a plating layer on a surface of the steel
sheet for hot stamping to a temperature of higher than an Ac3 point;
a hot stamping step of starting hot stamping on the steel sheet for hot
stamping after the heating step at a temperature of (Ar3 point- 200°C) or higher and
lower than the Ar3 point and subsequently performing cooling to a temperature of
lower than 90°C; and
a reheating step of heating a formed article after the hot stamping step to a
temperature of 1 00°C to 140°C and performing holding at the temperature for 3 to 120
minutes.
7. A manufacturing method of the hot-stamped product as claimed in claim
3, comprising:
a joining step of joining a steel sheet for hot stamping having the chemical
composition as claimed in claim 1 or 2 and a plating layer on a surface of the steel
sheet for hot stamping, 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
- 64 -
temperature of higher than an Ac3 point of the steel sheet for hot stamping;
a hot stamping step of starting hot stamping on the joined steel sheet after the
heating step at a temperature of (Ar3 point - 200°C) of the steel sheet for hot stamping
or higher and lower than the Ar3 point and subsequently performing cooling to a
temperature of lower than 90°C; and
a reheating step of heating a formed article after the hot stamping step to a
temperature of 1 00°C to 140°C and performing holding at the temperature for 3 to 120
minutes.