DESCRIPTION
TITLE OF INVENTION: HOT-PRESSED STEEL SHEET MEMBER,
METHOD OF MANUFACTURING THE SAME, AND
STEEL SHEET FOR HOT PRESSING
TECHNICAL FIELD
[0001] The present invention relates to a hotpressed
steel sheet member used for a mechanical
structural component and the like, a method of
manufacturing the same, and a steel sheet for hot
pressing.
BACKGROUND ART
[0002] For reduction in weight of an automobile,
efforts are advanced to increase the strength of a
steel material'bsed for an automobile body and to
reduce the weight of steel material used. In a thin
steel sheet widely used for the automobile, press
formability thereof generally decreases with an
increase in strength, thus making it difficult to
manufacture a component having a complicated shape.
For example, a highly processed portion fractures
with a decrease in ductility, and springback becomes
prominent to deteriorate dimensional accuracy.
Accordingly, it is difficult to manufacture
components by performing press-forming on a highstrength
steel sheet, in particular, a steel sheet
having a tensile strength of 980 MPa or more. It is
easy to process the high-strength steel sheet not by
press-forming but by roll-forming, but its
application target is limited to a component having a
uniform cross section in a longitudinal direction.
[0003] Methods called hot pressing intended to
obtain high formability in the high-strength steel
sheet are described in Patent Literatures 1 to 4. By
the hot pressing, it is possible to form the highstrength
steel sheet with high accuracy to obtain a
high-strength hot-pressed steel sheet member.
[0004] On the other hand, the hot-pressed steel
sheet member is required to be improved also in
ductility. However, steel microstructure of the
steel sheet obtained by the methods described in
Patent Literatures 1 to 4 is substantially a
martensite single phase, and thus it is difficult for
the methods to improve in ductility.
[0005] Hot-pressed steel sheet members intended to
improve in ductility are described in Patent
Literatures 5 to 7, but it is also difficult for
these conventional hot-pressed steel sheet members to
balance strength and ductility.
[0006] A hot-pressed steel sheet member intended to
improve in ductility is described also in Patent
Literature 8. However, manufacture of the hotpressed
steel sheet member requires complicated
control and thus has other problems such as decrease
in productivity and increase in manufacturing cost.
CITATION LIST
PATENT LITERATURE
[0007] Patent Literature 1: U.K. Patent No. 1490535
Patent Literature 2: Japanese Laid-open Patent
Publication No. 10-96031
- 2 -
Patent Literature 3: Japanese Laid-open Patent
Publication No. 2009-197253
Patent Literature 4: Japanese Laid-open Patent
Publication No. 2009-35793
Patent Literature 5: Japanese Laid-open Patent
Publication No. 2010-65292
Patent Literature 6: Japanese Laid-open Patent
Publication No. 2010-65293
Patent Literature 7: Japanese Translation of PCT
International Application Publication No. 2010-521584
Patent Literature 8: Japanese Laid-open Patent
Publication No. 2010-131672
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0008] An object of the present invention is to
provide a hot-pressed steel sheet member capable of
obtaining excellent strength and ductility without
performing complicated control, a method of
manufacturing the same, and a steel sheet for hot
pressing.
SOLUTION TO PROBLEM
[0009] As a result of earnest studies to solve the
above problems, the inventors of the present
application have found that a hot-pressed steel sheet
member having a steel microstructure being a multiphase
microstructure containing ferrite and
martensite can be obtained without performing
complicated control as described in Patent Literature
8, by treating a steel sheet for hot pressing having
a chemical composition containing specific amounts of
C and Mn and relatively large amount of Ti, and
having a specific steel microstructure including hot
pressing under specific conditions. The inventors of
the present application have also found that the hotpressed
steel sheet member has a high tensile
strength of 980 MPa or more and excellent ductility.
The inventors of the present application have reached
various aspects of the invention described below.
[00101 (1)
A hot-pressed steel sheet member, including:
a chemical composition represented by, in mass%:
C: 0.10% to 0.24%;
Si: 0.001% to 2.0%;
Mn: 1.2% to 2.3%;
sol. Al: 0.001% to 1.0%;
Ti: 0.060% to 0.20%;
P: 0.05% or less;
S: 0.01% or less;
N: 0.01% or less;
Nb: 0% to 0.20%;
V: 0% to 0.20%;
Cr: 0% to 1.0-%;
Mo: 0% to 0.15%;
Cu: 0% to 1.0%;
Ni: 0% to 1.0%;
Ca: 0% to 0.01%;
Mg: 0% to 0.01%;
REM: 0% to 0.01%;
Zr: 0% to 0.01%;
B: 0% to 0.005%;
Bi: 0% to 0.01%; and
balance: Fe and impurities; and
a steel microstructure represented by, in area%:
ferrite: 10% to 70%;
martensite: 30% to 90%; and
a total area ratio of ferrite and
martensite: 90% to loo%,
wherein 90% or more of all Ti in steel
precipitates, and
wherein a tensile strength of the hot-pressed
steel sheet member is 980 MPa or more.
[00111 ( 2 )
The hot-pressed steel sheet member according to
(I), wherein the chemical composition contains one or
more selected from the group consisting of, in mass%:
Nb: 0.003% to 0.20%;
V: 0.003% to 0.20%;
Cr: 0.005% to 1.0%;
M0: 0.005% to 0.15%;
Cu: 0.005% to 1.0%; and
Ni: 0.005% to 1.0%.
[00121 (3)
The hot-pressed steel sheet member according to
(1) or ( 2 ) , wherein the chemical composition contains
one or more selected from the group consisting of, in
mass% :
Ca: 0.0003% to 0.01%;
Mg: 0.0003% to 0.01%;
REM: 0.0003% to 0.01%; and
Zr: 0.0003% to 0.01%.
[00131 (4)
The hot-pressed steel sheet member according to
any one of (1) to (3), wherein the chemical
composition contains, in mass%, B: 0.0003% to 0.005%.
[00141 (5)
The hot-pressed steel sheet member according to
any one of (1) to ( 4 ) , wherein the chemical
composition contains, in mass%, Bi: 0.0003% to 0.01%.
[00151 ( 6 )
A steel sheet for hot pressing, including:
a chemical composition represented by, in mass%:
c: 0. 10% to 0.24%;
Si: 0.001% to 2.0%;
Mn: 1.2% to 2.3%;
sol. Al: '0.001% to 1.0%;
Ti: 0.060% to 0.20%;
P: 0.05% or less;
S: 0.01% or less;
N: 0.01% or less;
Nb: 0% to 0.20%;
V: 0% to 0.20%;
Cr: 0% to 1.0%;
M0: 0% to 0.15%;
Cu: 0% to 1.0%;
Ni: 0% to 1.0%;
Ca: 0% to 0.01%;
Mg: 0% to 0.01%;
REM: 0% to 0.01%;
Zr: 0% to 0.01%;
B: 0% to 0.005%;
Bi: 0% to 0.01%; and
balance: Fe and impurities,
wherein 70% or more of all Ti in steel
precipitates.
to0161 (7)
The steel sheet for hot pressing according to
(6), wherein the chemical composition contains one or
more selected from the group consisting of, in mass%:
Nb: 0.003% to 0.20%;
V: 0.003% to 0.20%;
Cr: 0.005% to 1.0%;
Mo: 0.005% to 0.15%;
Cu: 0.005% to 1.0%; and
Ni: 0.005% to 1.0%.
[00171 (8)
The steel sheet for hot pressing according to (6)
or (7), wherein the chemical composition contains one
or more selected from the group consisting of, in
mass% :
Ca: 0.0003% to 0.01%;
Mg: 0.0003% to 0.01%;
REM: 0.0003% to 0.01%; and
Zr: 0.0003% to 0.01%.
100181 (9)
The steel sheet for hot pressing according to any
one of (6) to (8), wherein the chemical composition
contains, in mass%, B: 0.0003% to 0.005%.
[0019] (10)
The steel sheet for hot pressing according to any
one of (6) to ( 9 ) , wherein the chemical composition
contains, in mass%, Bi: 0.0003% to 0.01%.
[0020] (11)
A method of manufacturing a hot-pressed steel
sheet member, including:
heating the steel sheet for hot pressing
according to any one of (6) to (10) in a temperature
zone of an Ac3 temperature to the A c ~te mperature +
100°C for 1 minute to 10 minutes; and
hot pressing after the heating,
wherein the hot pressing includes:
first cooling in a temperature zone of 60OoC
to 750°C; and
second cooling in a temperature zone of 150°C
to 60OoC,
wherein an average cooling rate is 3 "Clsecond to
200 "Clsecond so as to cause ferrite to start to
precipitate in the temperature zone of 600°C to 750°C
in the first cooling, and
wherein the average cooling rate is 10 "Clsecond
to 500 "Clsecond in the second cooling.
ADVANTAGEOUS EFFECTS OF INVENTION
[0021] According to the present invention, it is
possible to obtain excellent ductility while
obtaining high tensile strength without performing
complicated control.
BRIEF DESCRIPTION OF DRAWINGS
[0022] [Fig. 1 1 Fig. 1 is a view illustrating a
metal microstructure photograph of a hot-pressed
steel sheet member according to an embodiment.
DESCRIPTION OF EMBODIMENTS
[0023] Hereinafter, embodiments of the present
invention will be described. The embodiments of the
present invention relate to a hot-pressed steel sheet
member having a tensile strength of 980 MPa or more.
[0024] First, chemical compositions of the hotpressed
steel sheet member (hereinafter, sometimes
referred to as a "steel sheet member") according to
the embodiment of the present invention and a steel
sheet for hot pressing used for manufacturing the
same will be described. In the following
description, " % " being a unit of content of each
element contained in the steel sheet member or the
steel sheet for hot pressing means "mass%" unless
otherwise specified.
[0025] The chemical compositions of the steel sheet
member according to the embodiment and the steel
sheet for hot pressing used for manufacturing the
same are represented by, in mass%: C: 0.10% to 0.24%;
Si: 0.001% to 2.0%; Mn: 1.2% to 2.3%; sol. Al: 0.001%
to 1.0%; Ti: 0.060% to 0.20%; P: 0.05% or less; S:
0.01% or less; N: 0.01% or less; Nb: 0% to 0.20%; V:
0% to 0.20%; Cr: 0% to 1.0%; Mo: 0% to 0.15%; Cu: 0%
to 1.0%; Ni: 0% to 1.0%; Ca: 0% to 0.01%; Mg: 0% to
0.01%; REM: 0% to 0.01%; Zr: 0% to 0.01%; B: 0% to
0.005%; Bi: 0% to 0.01%; and balance: Fe and
impurities. Examples of the impurities include ones
contained in raw materials such as ore and scrap, and
ones mixed in during a manufacturing process.
[0026] (C: 0.10% to 0.24%)
C is a very important element which increases
hardenability of the steel sheet for hot pressing and
mainly determines the strength of the steel sheet
member. When the C content of the steel sheet member
is less than 0.10%, it may be difficult to secure the
tensile strength of 980 MPa or more. Accordingly,
the C content is 0.10% or more. When the C content
of the steel sheet for hot pressing is more than
0.24%, a steel microstructure of the steel sheet
member may become a martensitic single phase, and
there is remarkable deterioration in ductility.
Accordingly, the C content is 0.24% or less. The C
content of the steel sheet member is preferably 0.21%
or less, and more preferably 0.18% or less from the
viewpoint of weldability.
[0027] (Si: 0.001% to 2.0%)
Si is an element effective in improving the
strength and ductility of the steel sheet member.
When the Si content is less than 0.001%, it may be
difficult to obtain the above-described effects.
Accordingly, the Si content is 0.001% or more. When
the Si content is more than 2.0%, the above-described
effects may be saturated to result in economical
disadvantage, and plating wettability significantly
decreases to frequently cause unplating.
Accordingly, the Si content is 2.0% or less. From
the viewpoint of further improving the ductility, the
Si content is preferably 0.05% or more. From the
viewpoint of improving the weldability, the Si
content is preferably 0.2% or more. From the
viewpoint of relatively lowering a temperature at
- lo -
which the steel microstructure becomes an austenite
single phase during hot pressing, the Si content is
preferably 0.6% or less. When the temperature is the
relatively low temperature, effects such as reduction
in heating time, improvement in productivity,
decrease in manufacturing cost, and suppression of
damage to a heating furnace can be obtained.
[0028] (Mn: 1.2% to 2.3%)
Mn is an element very effective in improving the
hardenability of the steel sheet for hot pressing and
in securing the strength of the steel sheet member.
When the Mn content is less than 1.2%, it may be
difficult to obtain the above-described effects.
Accordingly, the Mn content is 1.2% or more. When
the Mn content is more than 2.3%, the steel
microstructure of the steel sheet member may become a
martensitic single phase, and there is remarkable
deterioration in ductility. Accordingly, the Mn
content is 2.3% or less. From the viewpoint of
relatively lowering a temperature (for example, 860°C
or lower) at which the steel microstructure becomes
an austenite single phase during hot pressing, the Mn
content is preferably 1.4% or more. From the
viewpoint of preventing the steel microstructure of
the steel sheet member from becoming a conspicuous
banded microstructure to thereby obtain excellent
bendability, the Mn content is preferably 2.2% or
less, and more preferably 2.1% or less.
[0029] (Sol. A1 (acid-soluble Al) : 0.001% to 1.0%)
A1 is an element having an effect of deoxidizing
steel to make steel material better. A1 also has an
effect of improving the yield of a carbonitride
forming element such as Ti or the like. When the
sol. A1 content is less than 0.001%, it may be
difficult to obtain the above-described effects.
Accordingly, the sol. Al content is 0.001% or more.
In order to more surely obtain the above-described
effects, the sol. Al content is preferably 0.015% or
more. When the sol. Al content is more than 1.0%,
the weldability significantly may decrease, oxidebased
inclusions may increase, and the surface
property may significantly deteriorate. Accordingly,
the sol. A1 content is 1.0% or less. In order to
obtain better surface property, the sol. A1 content
is preferably 0.080% or less.
[0030] (Ti: 0.060% to 0.20%)
Ti is an element accelerating ferrite
transformation during hot pressing. The acceleration
of the ferrite transformation significantly improves
the ductility of the steel sheet member. Further, Ti
finely precipitates as a carbide, a nitride or a
carbonitride to make the steel microstructure of the
steel sheet member finer. When the Ti content is
less than 0.060%, the ferrite transformation is not
sufficiently accelerated, and the steel
microstructure of the steel sheet member is likely to
become a martensitic single phase, failing to obtain
sufficient ductility. Accordingly, the Ti content is
0.060% or more. From the viewpoint of further
improving the ductility, the Ti content is preferably
- 12 -
0.075% or more. When the Ti content is more than
0.20%, a coarse carbonitride may be formed during
casting and during hot-rolling for obtaining the
steel sheet for hot pressing, and there is remarkable
deterioration in toughness. Accordingly, the Ti
content is 0.20% or less. From the viewpoint of
securing excellent toughness, the Ti content is
preferably 0.18% or less, and more preferably 0.15%
or less.
[00311 (P: 0.05% or less)
P is not an essential element and is contained,
for example, as an impurity in steel. From the
viewpoint of weldability, a lower P content is
better. In particular, when the P content is more
than 0.05%, the weldability may significantly
decrease. Accordingly, the P content is 0.05% or
less. In order to secure better weldability, the P
content is preferably 0.018% or less. On the other
hand, P has an effect of enhancing the strength of
the steel by solid solution strengthening. To obtain
the effect, 0.003% or more of P may be contained.
[0032] (S: 0.01% or less)
S is not an essential element and is contained,
for example, as an impurity in steel. From the
viewpoint of the weldability, a lower S content is
better. In particular, when the S content is more
than 0.01%, the weldability may significantly
decrease. Accordingly, the S content is 0.01% or
less. In order to secure better weldability, the S a
content is preferably 0.003% or less, and more
preferably 0.0015% or less.
[0033] (N: 0.01% or less)
N is not an essential element and is contained,
for example, as an impurity in steel. From the
viewpoint of the weldability, a lower N content is
better. In particular, when the N content is more
than 0.01%, the weldability may significantly
decrease. Accordingly, the N content is 0.01% or
less. In order to secure better weldability, the N
content is preferably 0.006% or less.
[0034] Nb, V, Cr, Mo, Cu, Ni, Ca, Mg, REM, Zr, Band
Bi are not essential elements, and are arbitrary
elements which may be appropriately contained, up to
a specific amount as a limit, in the steel sheet
member and the steel sheet for hot pressing.
[0035] (Nb: 0% to 0.20%, V: 0% to 0.20%, Cr: 0% to
1.0%, Mo: 0% to 0.15%, Cu: 0% to 1.0%, Ni: 0% to
1.0%)
Each of Nb, V, Cr, Mo, Cu, and Ni is an element
which increases hardenability of the steel sheet for
hot pressing and has an effect in stably securing the
strength of the steel sheet member. Accordingly, one
or more selected from the group consisting of these
elements may be contained. However, regarding Nb and
V, when any of their contents is more than 0.20%, not
only hot-rolling and cold-rolling for obtaining the
steel sheet for hot pressing may become difficult,
but also the steel microstructure of the steel sheet
member may become a martensitic single phase, and
there is remarkable deterioration in ductility.
Accordingly, each of the Nb content and the V content
is 0.20% or less. Regarding Cr, when its coptent is
more than 1.0%, it may become difficult to stably
secure strength. Accordingly, the Cr content is 1.0%
or less. Regarding Mo, when its content is more than
0.15%, the steel microstructure of the steel sheet
member may become a martensitic single phase, and
there is remarkable deterioration in ductility.
Accordingly, the Mo content is 0.15% or less.
Regarding Cu and Ni, any of their contents is 1.0%,
the above-described effects may be saturated to
result in economical disadvantage, and hot-rolling
and cold-rolling for obtaining the steel sheet for
hot pressing become difficult. Accordingly, each of
the Cu content and the Ni content is 1.0% or less.
I n order to stably secure the strength of the steel
sheet member, each of the Nb content and the V
content is preferably 0.003% or more, and each of the
Cr content, the Mo content, the Cu content, and the
Ni content is preferably 0.005% or more. More
specifically, it is preferable to satisfy at least
one of "Nb: 0.003% to 0-.20%", "V: 0.003% to 0.20%",
"Cr: 0.005% to 1.0%", "Mo: 0.005% to 0.15%", "Cu:
0.005% to 1.0%", and "Ni: 0.005% to 1.0%".
[0036] (Ca: 0% to 0.01%, Mg: 0% to 0.01%, REM: 0% to
0.01%, Zr: 0% to 0.01%)
Each of Ca, Mg, REM, and Zr is an element which
has an effect of contributing to control of
inclusions, in particular, fine dispersion of
inclusions to enhance the toughness. Accordingly,
one or more selected from the group consisting of
these elements may be contained. However, when the
content of any one of them is more than 0.01%, the
deterioration in surface property may become obvious.
Accordingly, each of the Ca content, the Mg content,
the REM content, and the Zr content is 0.01% or less.
In order to improve the toughness, each of the Ca
content, the Mg content, the REM content, and the Zr
content is preferably 0.0003% or more. More
specifically, it is preferable to satisfy at least
one of "Ca: 0.0003% to 0.01%", "Mg: 0.0003% to
0.01%", "REM: 0.0003% to 0.01%", and "Zr: 0.0003% to
0.01%".
[0037] REM (rare-earth metal) indicates 17 kinds of
elements in total of Sc, Y, and lanthanoid, and the
"REM content" means a total content of these 17 kinds
of elements. Lanthanoid is industrially added as a
form of, for example, misch metal.
[0038] (B: 0% to 0.005%)
B is an element which has an effect of enhancing
the toughness of the steel sheet. Accordingly, B may
be contained. However, when the B content is more
than 0.005%, the steel microstructure of the steel
sheet member may become a martensitic single phase,
and there is remarkable deterioration in ductility.
Further, hot workability deteriorates, and hotrolling
for obtaining the steel sheet for hot
pressing may become difficult. Accordingly, the B
content is 0.005% or less. In order to enhance the
toughness, the B content is preferably 0.0003% or
more. More specifically, the B content is preferably
0.0003% to 0.005%.
[0039] (Bi: 0% to 0.01%)
Bi is an element which has an effect of
uniforming the steel microstructure to enhance the
ductility. Accordingly, Bi may be contained.
However, when the Bi content is more than 0.01%, the
hot workability deteriorates, and hot-rolling for
obtaining the steel sheet for hot pressing may become
difficult. Accordingly, the Bi content is 0.01% or
less. In order to enhance the ductility, the Bi
content is preferably 0.0003% or more. More
specifically, the Bi content is preferably 0.0003% to
0.01%.
[0040] Next, the steel microstructure of the steel
sheet member according to the embodiment and
precipitates in the steel sheet member will be
described. The steel sheet member includes a steel
microstructure represented by, in area%: ferrite: 10%
to 70%; martensite: 30% to 90%; and a total area
ratio of ferrite and martensite: 90% to 100%.
Further, 90% or more of all Ti in steel precipitates.
Each of numerical values relating to the steel
microstructure is, for example, an average value of
the whole of the steel sheet member in a thickness
direction, but the average value may be represented
by a numerical value relating to the steel
microstructure at a point where the depth from a
surface of the steel sheet member is 1/4 of the
thickness of the steel sheet member (hereinafter,
this point is sometimes referred to as a "1/4 depth
position"). For example, when the thickness of the
steel sheet member is 2.0 mm, the average value may
be represented by a numerical value at a point where
the depth from the surface is 0.50 mm. This is
because the steel microstructure at the 1/4 depth
position indicates an average steel microstructure in
the thickness direction of the steel sheet member.
[0041] (Area ratio of ferrite: 10% to 70%)
The ferrite precipitated in a network form
contributes to improvement in ductility of the steel
sheet member. When the area ratio of ferrite is less
than lo%, the ferrite is less likely to constitute
the network, and sufficient ductility may not be
obtained. Accordingly, the area ratio of ferrite is
10% or more. When the area ratio of ferrite is more
than 70%, the area ratio of martensite necessarily
becomes less than 30%, and it may be difficult to
secure the tensile strength of 980 MPa or more in the
steel sheet member. Accordingly, the area ratio of
ferrite is 70% or less.
[0042] (Area ratio of martensite: 30% to 90%)
The martensite is important in increasing the
strength of the steel sheet member. When the area
ratio of martensite is less than 30%, it may be
difficult to secure the tensile strength of 980 MPa
or more in the steel sheet member. Accordingly, the
area ratio of martensite is 30% or more. When the
area ratio of martensite is more than 90%, the area
ratio of ferrite necessarily becomes less than lo%,
and sufficient ductility may not be obtained.
Accordingly, the area ratio of martensite is 90% or
less.
LO0431 (Total area ratio of ferrite and martensite:
90% to 100%)
The steel microstructure of the hot-pressed steel
sheet member according to the embodiment is
preferably composed of ferrite and martensite,
namely, the total area ratio of ferrite and
martensite is preferably 100%. However, depending on
the manufacturing conditions, one or more selected
from the group consisting of bainite, retained
austenite, cementite, and pearlite may be contained
as a phase or microstructure other than ferrite and
martensite. In this case, when the area ratio of the
phase or microstructure other than ferrite and
martensite is more than lo%, target properties may
not be obtained in some cases due to the influence of
the phase or microstructure. Accordingly, the area
ratio of the phase or microstructure other than
ferrite and martensite is 10% or less. That is, the
total area ratio of ferrite and martensite is 90% or
more.
[0044] As a method of measuring the area ratio of
each phase in the above steel microstructure, a
method well-known to the skilled person in the art
may be employed. Each of the area ratios is
obtained, for example, as an average value of a value
measured in a cross section perpendicular to a
rolling direction and a value measured in a cross
section perpendicular to a sheet width direction (a
direction perpendicular to the rolling direction).
In other words, the area ratio is obtained, for
example, as an average value of area ratios measured
in two cross sections.
[0045] (Percentage of precipitated Ti: 90% or more)
The precipitate of Ti contributes to stable
securement of the tensile strength of the steel sheet
member. As described above, the steel sheet member
contains 0.060% to 0.20% of Ti, and when the
percentage of precipitated Ti is less than 90%, it
may be difficult to obtain the above-described
effects. Accordingly, the percentage of the
precipitated Ti of all Ti in steel is 90% or more in
the steel sheet member. The precipitate of Ti is
contained, for example, as a carbide, a nitride or a
carbonitride, in the steel sheet member. The amount
of Ti precipitated in the steel sheet member can be
specified by inductively coupled plasma (ICP)
analysis of residue obtained by electroextraction of
the steel sheet member.
[0046] The steel sheet member can be manufactured by
treating a specific steel sheet for hot pressing
under specific conditions.
[0047] Here, the steel sheet for hot pressing used
for manufacturing the steel sheet member according to
the embodiment will be described. In the steel sheet
for hot pressing, 70% or more of all Ti in steel
precipitates.
[0048] The steel microstructure of the steel sheet
- 20 -
for hot pressing is not particularly limited. This
is because the steel sheet for hot pressing is heated
up to a temperature of an Ac3 temperature or higher
during hot pressing as will be described later.
[DO491 (Percentage of precipitated Ti: 70% or more)
When the percentage of precipitated Ti of all Ti
contained in the steel sheet for hot pressing is less
than 70%, the ferrite transformation is less likely
to occur during hot pressing, and it may be difficult
to obtain the steel sheet member having a desired
steel microstructure. Accordingly, in the steel
sheet for hot pressing, the percentage of
precipitated Ti of all Ti in steel is 70% or more.
[0050] Next, a method of manufacturing the steel
sheet member according to the embodiment, namely, a
method of treating the steel sheet for hot pressing
will be described. In the treatment of the steel
sheet for hot pressing, the steel sheet for hot
pressing is heated in a temperature zone of the Ac3
temperature to the Ac3 temperature + 100°C for 1
minute to 10 minutes, and is subjected to hot
pressing after the heating. In the hot pressing,
first cooling is performed in a temperature zone of
600°C to 750°C, and second cooling is performed in a
temperature zone of 150°C to 600°C. In the first
cooling, an average cooling rate is 3 OC/second to 200
OC/second to cause ferrite to start to precipitate in
the temperature zone of 600°C to 750°C. In the second
cooling, the average cooling rate is 10 "C/second to
500 "C/second.
[0051] (Heating temperature of the steel sheet for
hot pressing: a temperature zone of Ac3 temperature to
Ac3 temperature + 100°C)
The steel sheet to be supplied to hot pressing,
namely, the steel sheet for hot pressing is heated in
a temperature zone of the Ac3 temperature to the Ac3
temperature + 100°C. The Ac3 temperature is a
temperature (unit: OC) at which the steel
microstructure becomes an austenite single phase,
which is calculated by the following empirical
formula (i) .
[0052] Ac3 = 910 - 203 x (cO.') - 15.2 x Ni + 44.7
x Si + 104 x V + 31.5 x Mo - 30
x Mn - 11 x Cr - 20 x Cu + 700
x P + 400 x A1 + 50 x Ti . . . (i)
Here, the element symbol in the above formula
indicates the content (unit: mass%) of each element
in a chemical composition of the steel sheet.
[0053] When the heating temperature is lower than
the Ac3 temperature, the steel microstructure of the
steel sheet member is likely to become non-uniform,
and the steel sheet member is not stable in tensile
strength and may deteriorate in ductility.
Accordingly, the heating temperature is the Ac3
temperature or higher. When the heating temperature
is higher than the Ac3 temperature + 100°C, the
stability of an austenite grain boundary excessively
increases and the ferrite trans'formation becomes less
likely to be accelerated. As a result, the steel
microstructure of the steel sheet member becomes a
martensitic single phase, and the ductility
significantly deteriorates. Further, when the Ti
content is less than 0.08%, the precipitate of Ti
becomes likely to dissolve. Accordingly, the heating
temperature is the Ac3 temperature t 100°C or lower.
From the viewpoint of suppressing damage to a heating
furnace and improving the productivity, the heating
temperature is preferably 86OoC or lower.
Appropriately controlling the composition of the
steel sheet for hot pressing makes it possible to
make the steel microstructure into an austenite
single phase at a temperature of 860°C or lower.
[0054] (Heating time of the steel sheet for hot
pressing: 1 minute to 10 minutes)
When the heating time is less than 1 minute, the
single phase microstructure of austenite is likely to
be non-uniform, and it may be difficult to stably
secure strength. Accordingly, the heating time is 1
minute or more. When the heating time is more than
10 minutes, the ferrite transformation is less likely
to occur during cooling thereafter, and the steel
microstructure of the steel sheet member may become a
martensitic single phase and significantly
deteriorate in ductility. Further, the decrease in
productivity may become remarkable. Accordingly, the
heating time is 10 minutes or less.
[0055] The heating time is a time period from the
time at which the temperature of the steel sheet
reaches the Ac3 temperature to a heating end time.
The heating end time, specifically, is the time at
- 23 -
which the steel sheet is taken out of the heating
furnace in the case of furnace heating, and is the
time at which induction or the like is turned off in
the case of electric resistance heating or induction
heating .
[0056] An average heating rate in the heating up to
the temperature zone of the Ac3 temperature to the Ac,
temperature + 100°C is preferably 0.2 OC/second to 100
OC/second. Setting the average heating rate to 0.2
OC/second or more makes it possible to secure higher
productivity. Further, setting the average heating
rate to 100 OC/second or less makes it easy to control
the heating temperature when it is heated by using an
ordinary furnace. In the case of performing highfrequency
heating or electric resistance heating,
even when the average heating rate is more than 100
OC/second, the control of the heating temperature is
easy, so that the average heating rate may be more
than 100 OC/second. The average heating rate in a
temperature zone of 700°C to the Ac3 temperature is
preferably 1 OC/second to 10 OC/second. When the
average heating rate in this temperature zone is
within this range, the steel microstructure of the
steel sheet member can be made further uniform and
further improved in ductility.
[0057] (Ferrite precipitation start temperature:
600°C to 750°C)
The precipitation start temperature of ferrite in
hot pressing affects the quality of ferrite. When
ferrite starts to precipitate over 750°C, the ferrite
may become coarse and the toughness may be
deteriorated. When ferrite starts to precipitate
below 600°C, the dislocation density in ferrite may
increase and the ductility may be deteriorated.
Accordingly, in the first cooling, ferrite is caused
to start to precipitate in a temperature zone of 600°C
to 750°C.
[0058] (Average cooling rate in the first cooling: 3
OC/second to 200 OC/second)
A temperature at which ferrite is caused to start
to precipitate, namely, a precipitation start
temperature of ferrite can be controlled by adjusting
the average cooling rate in hot pressing. For
example, the first cooling is preferably performed
under the conditions obtained by analysis of a
thermal expansion curve. However, when the average
cooling rate in the first cooling is less than 3
OC/second even when the precipitation start
temperature of ferrite is in the range of 600°C to
75OoC, the ferrite transformation excessively
progresses, so that it is difficult to make the area
ratio. of martensite in the steel sheet member to 30%
or more and a tensile strength of 980 MPa or more may
not be obtained. It may be difficult to control the
average cooling rate to less than 3 "C/second only by
air cooling or by forced air cooling. Accordingly,
the average cooling rate in the first cooling is 3
"C/second or more. This average cooling rate is
preferably G°C/second or more. Further, when the
average cooling rate in the first cooling is more
than 200 OC/second even when the precipitation start
temperature of ferrite is in the range of 600°C to
750°C, it may be difficult to make the area ratio of
ferrite in the steel sheet member to 10% or more and
excellent ductility may not be obtained.
Accordingly, the average cooling rate in the first
cooling is 200 "C/second or less. This average
cooling rate is preferably 60 OC/second or less.
[00591 In the case of using the steel sheet for hot
pressing having the above-described chemical
composition and 70% or more of the precipitated Ti of
all Ti in steel, ferrite starts to precipitate in the
temperature zone of 600°C to 750°C when the average
cooling rate in the temperature zone of 600°C to 750°C
is 3 OC/second to 200 "C/second.
[0060] (Average cooling rate in the second cooling:
10 'C/second to 500 OC/second)
It is important to make diffusional
transformation unlikely to occur in the cooling in a
temperature zone of 150°C to 60OoC. When the average
cooling rate in this temperature zone is less than 10
OC/second, bainite t.ransformation being the
diffusional transformation is likely to occur, so
that it may be difficult to make the area ratio of
martensite in the steel sheet member to 30% or more
and it may be difficult to secure the tensile
strength of 980 MPa or more. Accordingly, the
average cooling rate in the second cooling is 10
OC/second or more. From the viewpoint of more surely
securing a higher area ratio of martensite, the
average cooling rate is preferably 15 'C/second or
more. It may be difficult to make the average
cooling rate in the second cooling to more than 500
OC/second in an ordinary facility. Accordingly, the
average cooling rate in the temperature zone is 500
OC/second or less. From the viewpoint of more stable
cooling, the average cooling rate is preferably 200
'C/second or less.
[0061] Between the first cooling and the second
cooling, a steel microstructure in which fine ferrite
is distributed in a network form as illustrated in
Fig. 1 is obtained. Such a steel microstructure is
effective in improving the ductility.
[0062] In the second cooling, heat generation by
phase transformation is likely to extremely increase
after the temperature reaches 600°C. Therefore, when
the cooling in the temperature zone of lower than
600°C is performed by the same method as the cooling
in the temperature zone of 600°C or higher, it may be
difficult to secure a sufficient average cooling rate
in some cases. It is preferable to perform the
second cooling from 600°C to 150°C more forcibly than
the first cooling to 600°C. For example, it is
preferable to employ the following method.
[0063] Generally, the cooling in the hot pressing is
performed by setting a die made of steel used for
forming a heated steel sheet to normal temperature or
a temperature of about several tens of degrees
centigrade in advance and bringing the steel sheet
into contact with the die. Accordingly, the average
cooling rate can be controlled, for example, by
change in heat capacity with the change in size of
the die. The average cooling rate can be also
controlled by changing the material of the die to a
different metal (for example, Cu or the like). The
average cooling rate can be also controlled by using
a water-cooling die and changing the amount of
cooling water flowing through the die. The average
cooling rate can be also controlled by forming a
plurality of grooves in the die in advance and
passing water through the grooves during hot
pressing. The average cooling rate can be also
controlled by raising a hot pressing machine in the
middle of the hot pressing and passing water through
its space. The average cooling rate can be also
controlled by adjusting a die clearance and changing
a contact area of the die with the steel sheet.
[0064] Examples of the method of increasing the
cooling rate in the temperature zone of 600°C or lower
include the following three kinds.
(a) Immediately after reaching 60OoC, the steel
sheet is moved to a die different in heat capacity or
a die at room temperature.
(b) A water-cooling die is used and the water
flow rate through the die is increased immediately
after reaching 600°C.
(c) Immediately after reaching 600°C, water is
passed between the die and the steel sheet. In this
method, the cooling rate may be further increased by
increasing the quantity of water according to
- 28 -
temperature.
[0065] The mode of the forming in the hot pressing
in the embodiment is not particularly limited.
Examples of the mode of the forming include bending,
drawing, bulging, hole expansion, and flanging. The
mode of the forming may be appropriately selected
depending on the kind of a target steel sheet member.
Representative examples of the steel sheet member
include a door guard bar, a bumper reinforcement and
the like which are automobile reinforcing components.
The hot forming is not limited to the hot pressing as
long as the steel sheet can be cooled simultaneously
with forming or immediately after forming. For
example, roll forming may be performed as the hot
forming .
[0066] Such a series of treatments are performed on
the above-described steel sheet for hot pressing,
namely, a steel sheet for hot pressing having
specific contents of C, Mn and Ti, whereby the steel
sheet member according to the embodiment can be
manufactured. In other words, it is possible to
obtain a hot-pressed steel sheet member having a
desired steel microstructure, a tensile strength of
980 MPa, and excellent strength and ductility,
without performing complicated control.
[0067] For example, the ductility can be evaluated
by a total elongation (EL) in a tensile test, and the
total elongation in the tensile test is preferably
10% or more in the embodiment. The total elongation
is more preferably 14% or more.
- 29 -
[ 0 0 6 8 ] After the hot pressing and cooling, shot
blasting may be performed. By the shot blasting,
scale can be removed. The shot blasting also has an
effect of introducing a compressive stress into the
surface of the steel sheet member, and therefore
effects of suppressing delayed fracture and improving
fatigue strength can also be obtained.
[0069] In the above-described method of
manufacturing the steel sheet member, the steel sheet
for hot pressing is heated in the temperature zone of
the Ac3 temperature to the Ac3 temperature + 100°C to
cause austenite transformation, and then is formed.
Accordingly, the mechanical properties of the steel
sheet for hot pressing at room temperature before
heating are not,important. Therefore, as the steel
sheet for hot pressing, for example, a hot-rolled
steel sheet, a cold-rolled steel sheet, a plated
steel sheet and the like may be used. Examples of
the cold-rolled steel sheet include a full hard
material and an annealed material. Examples of the
plated steel sheet include an aluminum plated steel
sheet and a zinc plated steel sheet. Their
manufacturing methods are not particularly limited.
[ 0 0 7 0 ] The steel sheet member according to the
embodiment can also be manufactured through hot
pressing accompanied by preforming. For example, in
a range where the above-described conditions of the
heating and the cooling are satisfied, the hotpressed
steel sheet member may be manufactured by
preforming by press working of the steel sheet for
- 30 -
hot pressing using a die in a specific shape, putting
it into the same type of die, applying a pressing
force thereto, and rapidly cooling it. Also in this
case, the kind of the steel sheet for hot pressing
and its steel microstructure are not limited, but it
is preferable to use a steel sheet that is soft and
.has ductility as much as possible in order to
facilitate the preforming. For example, the tensile
strength is preferably 700 MPa or less. A coiling
temperature after the hot-rolling of the hot-rolled
steel sheet is preferably 450°C or higher in order to
obtain a soft steel sheet, and is preferably 700°C or
lower in order to reduce scale loss. In the coldrolled
steel sheet, annealing is preferable to obtain
a soft steel sheet, and the annealing temperature is
preferably the Acl temperature to 90OoC. The average
cooling rate down to room temperature after annealing
is preferably an upper critical cooling rate or
lower.
[0071] It should be noted that the above embodiments
merely illustrate concrete examples of implementing
the present invention, and the technical scope of the
present invention is not to be construed in a
restrictive manner by these embodiments. That is,
the present invention may be implemented in various
forms without departing from the technical spirit or
main features thereof.
EXAMPLE
[0072] Next, the experiment performed by the
inventors of the present application will be
- 31 -
described. In this experiment, first, 23 kinds of
steel materials having chemical compositions listed
in Table 1 were used to fabricate 30 kinds of sample
materials each having a thickness of 1.2 mm listed in
Table 2. The balance of each steel material was Fe
and impurities.
[0073] In fabrication of each of the sample
materials, a slab prepared in a laboratory was hotrolled
and cold-rolled. In fabrication of Sample
Material No.1, a cold-rolled steel sheet obtained by
cold-rolling was subjected to A1 plating of a coating
weight per side of 120 g/m2. In fabrication of Sample
Material No.2, a cold-rolled steel sheet obtained by
cold-rolling was subjected to hot-dip galvanizing of
a coating rsieight per side of 60 g/m2, and then
subjected to alloying treatment. An Fe content in a
hot-dip galvanized film became 15 mass% by the
alloying treatment. The A1 plating and the hot-dip
galvanizing were performed by using a plating
simulator, and an annealing temperature in the
plating simulator rias 820°C, and the average cooling
rate from 820°C to 500°C was 5 "C/second.
[0074] After the fabrication of each sample
material, a steel piece having a thickness of 1.2 mm,
a width of 100 mm, and a length of 200 mm was cut out
of each sample material, and heat-treated (heating
and cooling) under the conditions listed in Table 2.
In the thermal treatment, while a thermocouple was
attached to the steel piece, the average cooling rate
in the first cooling and the average cooling rate in
the second cooling were measured. Further, the
precipitation start temperature of ferrite was
obtained from the'analysis result of the dilatometry
curve.
[0075] [Table 11
100761 [Table 21
[ 0 0 7 7 ] After the thermal treatment, the tensile test
and microstructural observation of the specimen were
performed on each of the steel pieces. In the
tensile test, the tensile strength ( T S ) and the total
elongation (EL) were measured. In the measurement of
the tensile strength and the total elongation, a JIS
No. 5 tensile test piece obtained from each steel
piece was used. In the microstructural observation
of the specimen, the area ratio of ferrite and the
area ratio of martensite were found. These area
ratios are each an average value calculated by
performing image analysis of electron micrograph
observation images in two cross sections, that is, a
cross section perpendicular to a rolling direction
and a cross section perpendicular to a sheet width
direction (a direction perpendicular to the rolling
direction). The area of a field of view of the
electron micrograph observation was 8 mm2. These
results are listed in Table 3. Hot pressing was not
performed on the steel piece being the target of the
tensile test and the microstructural observation of
the specimen, but the mechanical properties of the
steel piece reflect the mechanical properties of the
hot-pressed steel sheet member fabricated receiving,
during forming, the same thermal history as that of
the thermal treatment of this experiment. In other
words, as long as-the thermal history is
substantially the same, the mechanical properties
thereafter become substantially the same regardless
of presence or absence of hot pressing accompanied by
forming .
[ 0 0 7 8 ] [Table 31
TABLE 3
I I STEEL MICROSTRUCTURE ] I I I 1
NOTE I
UNOERUNE INDICATES THAT VALUE IS OUTSIDE THE RNiGE OF THE PfESEt~INVENnON
[0079] As listed in Table 3, Sample Materials No. 1,
No. 4, No. 6, No. 8, No. 11, No. 15, No. 16, No. 18,
No. 20. No. 22, No. 24, No. 26, No. 27, and No. 29
were invention examples each of which exhibited
excellent tensile strength and ductility.
[00801 On the other hand, Sample Materials No. 2,
No. 3, and No. 30 each failed to obtain sufficient
tensile strength because the manufacturing conditions
were outside the range of the present invention and
the steel microstructure after the thermal treatment
was also outside the range of the present invention.
Sample Materials No. 5, No. 14, No. 17, No. 19, No.
21, No. 23, and No. 28 each failed to obtain
sufficient ductility because the chemical composition
of the steel material was outside the range of the
present invention and the steel microstructure after
the thermal treatment was also outside the range of
the present invention. Sample Material No. 7 failed
to obtain sufficient ductility because the chemical
composition of the steel material was outside the
range of the present invention. Sample Materials No.
9, No. 10, and No. 12 each failed to obtain
sufficient ductility because the manufacturing
conditions were outside the range of the present
invention and the steel microstructure after the
thermal treatment was also outside the range of the
present invention. Sample Material No. 25 failed to
obtain sufficient tensile strength because the
chemical composition of the steel material was
outside the range of the present invention and the
steel microstructure after the thermal treatment was
also outside the range of the present invention.
INDUSTRIAL APPLICABILITY
[ 0 0 8 1 ] The present invention may be used for, for
example, industries of manufacturing and using
automobile body structural components and so on in
which importance is placed on excellent tensile
strength and ductility. The present invention may be
used also for industries of manufacturing and using
other machine structural components, and so on.
CLAIMS
[Claim 11 A hot-pressed steel sheet member,
comprising:
a chemical composition represented by, in mass%:
C: 0.10% to 0.24%;
Si: 0.001% to 2.0%;
Mn: 1.2% to 2.3%;
sol. Al: 0.001% to 1.0%;
Ti: 0.060% to 0.20%;
P: 0.05% or less;
S: 0.01% or less;
N: 0.01% or less;
Nb: 0% to 0.20%;
V: 0% to 0.20%;
Cr: 0% to 1.0%;
M0: 0% to 0.15%;
Cu: 0% to 1.0%;
Ni: 0% to 1.0%;
Ca: 0% to 0.01%;
Mg: 0% to 0.01%;
REM: 0% to 0.01%;
Zr: 0% to 0.01%;
B: 0% to 0.005%;
Bi: 0% to 0.01%; and
balance: Fe and impurities; and
a steel microstructure represented by, in area%:
ferrite: 10% to 70%;
martensite: 30% to 90%; and
a total area ratio of ferrite and
martensite: 90% to loo%,
wherein 90% or more of all Ti in steel
precipitates, and
wherein a tensile strength of the hot-pressed
steel sheet member is 980 MPa or more.
[Claim 21 The hot-pressed steel sheet member
according to claim 1, wherein the chemical
composition comprises one or more selected from the
group consisting of, in mass%:
Nb: 0.003% to 0.20%;
V: 0.003% to 0.20%;
Cr: 0.005% to 1.0%;
Mo: 0.005% to 0.15%;
Cu: 0.005% to 1.0%; and
Ni: 0.005% to 1.0%.
[Claim 31 The hot-pressed steel sheet member
according to claim 1 or 2, wherein the chemical
composition comprises one or more selected from the
group consisting of, in mass%:
Ca: 0.0003% to 0.01%;
Mg: 0.0003% to 0.01%;
REM: 0.0003% to 0.01%; and
Zr: 0.0003% to 0.01%.
[Claim 41 The hot-pressed steel sheet member
according to any one of claims 1 to 3, wherein the
chemical composition comprises, in mass%, B: 0.0003%
to 0.005%.
[Claim 51 The hot-pressed steel sheet member
according to any one of claims 1 to 4, wherein the
chemical composition comprises, in mass%, Bi: 0.0003%
to 0.01%.
[Claim 61 A steel sheet for hot pressing,
comprising:
a chemical composition represented by, in mass%:
C: 0. 10% to 0.24%;
Si: 0.001% to 2.0%;
Mn: 1.2% to 2.3%;
sol. Al: 0.001% to 1.0%;
Ti: 0.060% to 0.20%;
P: 0.05% or less;
S: 0.01% or less;
N: 0.01% or less;
Nb: 0% to 0.20%;
v: 0% to 0.20%;
Cr: 0% to 1.0%;
Mo: 0% to 0.15%;
Cu: 0% to 1.0%;
Ni: 0% to 1.0%;
Ca: 0% to 0.01%;
Mg: 0% to 0.01%;
REM: 0% to 0.01%;
Zr: 0% to 0.01%;
B: 0% to 0.005%;
Bi: 0% to 0.01%; and
balance: Fe and impurities,
wherein 70% or more of all Ti in steel
precipitates
[Claim 73 The steel sheet for hot pressing according
to claim 6, wherein the chemical composition
comprises one or more selected from the group
consisting of, in mass%:
Nb: 0.003% to 0.20%;
V: 0.003% to 0.20%;
Cr: 0.005% to 1.0%;
Mo: 0.005% to 0.15%;
Cu: 0.005% to 1.0.%~; and
Ni: 0.005% to 1.0%.
[Claim 81 The steel sheet for hot pressing according
to claim 6 or 7, wherein the chemical composition
comprises one or more selected from the group
consisting of, in mass%:
Ca: 0.0003% to 0.01%;
Mg: 0.0003% to 0.01%;
REM: 0.0003% to 0.01%; and
Zr: 0.0003% to 0.01%.
[Claim 91 The steel sheet for hot pressing according
to any one of claims 6 to 8, wherein the chemical
composition comprises, in mass%, B: 0.0003% to
0.005%.
[Claim 101 The steel sheet for hot pressing
according to any one of claims 6 to 9, wherein the
chemical composition comprises, in mass%, Bi: 0.0003%
to 0.01%.
[Claim 111 A method of manufacturing a hot-pressed
steel sheet member, comprising:
heating the steel sheet for hot pressing
according to any one of claims 6 to 10 in a
temperature zone of an Ac3 temperature to the Ac3
temperature + 100°C for 1 minute to 10 minutes; and
hot pressing after the heating,
wherein the hot pressing comprises:
first cooling in a temperature zone of 600°C
to 750°C; and
second cooling in a temperature zone of 150°C
to 600°C,
wherein an average cooling rate is 3 OC/second to
200 OC/second so as to cause ferrite to start to '1
precipitate in the temperature zone of 600°C to 750°C
in the first cooling, and
wherein the average cooling rate is 10 "C/second
to 500 'C/second in the second cooling.