Note: Descriptions are shown in the official language in which they were submitted.
CA 02923582 2016-03-07
DESCRIPTION
TITLE OF THE INVENTION:
METHOD FOR MANUFACTURING PRESS-MOLDED ARTICLE, AND PRESS-
MOLDED ARTICLE
TECHNICAL FIELD
[0001]
The present invention relates to a press-formed article to be used when
manufacturing an automotive structural component, and a method for
manufacturing
such a press-formed article. More specifically, the present invention relates
to a
press-formed article manufactured by applying, when forming a previously
heated
steel sheet (blank) into a predetermined shape, a press forming method of
imparting a
shape together with applying a heat treatment to obtain a predetermined
strength, and a
method useful for the manufacture of such a press-formed article.
BACKGROUND ART
[0002]
As one of the measures for automotive fuel economy improvement triggered
by global environmental problems, weight saving of a vehicle body is
proceeding, and
in turn, the strength of a steel sheet used for automobiles must be increased
as much as
possible. On the other hand, when the strength of a steel sheet is increased,
the shape
accuracy during press forming decreases.
[0003]
For this reason, a component is manufactured by employing a hot-press
forming method where a steel sheet is heated to a given temperature (e.g., a
temperature for forming an austenite phase) to lower the strength and then
formed with
a mold at a temperature (e.g., room temperature) lower than that of the steel
sheet to
impart a shape and, perform rapid-cooling heat treatment (quenching) by making
use
of a temperature difference therebetween so as to ensure the strength after
forming.
Such a hot-press forming method is referred to by various names such as hot
forming
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method, hot stamping method, hot stamp method and die quenching method, in
addition to hot-pressing method.
[0004]
FIG. 1 is a schematic explanatory view showing the mold configuration for
carrying out the above-described hot-press forming. In FIG. 1, 1 is a punch, 2
is a
die, 3 is a blank holder, 4 is a steel sheet (blank), BHF is a blank holding
force, rp is a
punch shoulder radius, rd is a die shoulder radius, and CL is a punch-to-die
clearance.
Of these parts, the punch 1 and the die 2 are configured such that passages la
and 2a
allowing for passing of a cooling medium (e.g., water) are formed in
respective insides
and the members are cooled by passing a cooling medium through the passage.
[0005]
When hot-press forming (for example, hot deep drawing) is performed using
such a mold, the forming is started in a state where the steel sheet (blank) 4
is softened
by heating at a two-phase zone temperature of (Aci transformation point to Ac3
transformation point) or a single-phase zone temperature equal to or more than
Ac3
transformation point. More specifically, in the state of the steel sheet 4 at
a high
temperature being sandwiched between the die 2 and the blank holder 3, the
steel sheet
4 is pushed into a hole of the die 2 (between 2 and 2 in FIG. 1) by the punch
1 and
formed into a shape corresponding to the outer shape of the punch 1 while
reducing the
outer diameter of the steel sheet 4. In addition, heat is removed from the
steel sheet 4
to the mold (the punch and the die) by cooling the punch and the die in
parallel with
forming, and quenching of the material is carried out by further holding and
cooling
the steel sheet at the forming bottom dead center (the point when the punch
head is
positioned at the deepest part: the state shown in FIG. 1). By carrying out
such a
forming method, a formed article of 1500 MPa class can be obtained with high
dimensional accuracy and moreover, the forming load can be reduced as compared
with a case of forming a component of the same strength class by cold working,
so that
the volume required of the pressing machine can be small.
[0006]
As the steel sheet for hot-pressing which is widely used at present, a steel
sheet using 22MnB5 steel as the material is known. This steel sheet has a
tensile
strength of 1,500 MPa and an elongation of approximately from 6 to 8% and is
applied
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to an impact-resistant member (a member that undergoes as little a deformation
as
possible at the time of collision and is not fractured). However, its
application to a
component requiring a deformation, such as energy-absorbing member, is
difficult
because of low elongation (ductility).
[0007]
As the steel sheet for hot-pressing which exerts good elongation, the
techniques of, for example, Patent Documents 1 to 4 have also been proposed.
In
these techniques, the carbon content in the steel sheet is set in various
ranges to adjust
the fundamental strength class of respective steel sheets, and the elongation
is
enhanced by introducing a ferrite having high deformability and reducing the
average
particle diameters of ferrite and martensite. The techniques above are
effective in
enhancing the elongation but in view of elongation enhancement according to
the
strength of the steel sheet, it is still insufficient. For example, the
elongation EL of a
steel sheet having a tensile strength TS of 1,270 MPa or more is about 12.7%
at the
maximum, and further improvement is demanded.
[0008]
On the other hand, an automotive component needs to be joined mainly by
spot welding, but in a hot-stamped formed article having a microstructure
mainly
including martensite, it is known that strength in the weld heat affected zone
(HAZ) is
reduced significantly and the welded joint is subject to a strength reduction
(softening)
(for example, Non-Patent Document 1).
RELATED ART
PATENT DOCUMENT
[0009]
Patent Document 1: JP-A-2010-65292
Patent Document 2: JP-A-2010-65293
Patent Document 3: JP-A-2010-65294
Patent Document 4: JP-A-2010-65295
NON-PATENT DOCUMENT
[0010]
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Non-Patent Document 1: Hirosue et al. "Nippon Steel Technical Report", No.
378, pp. 15-20 (2003)
SUMMARY OF THE INVENTION
PROBLEMS THAT THE INVENTION IS TO SOLVE
[0011]
The present invention has been made under these circumstances, and an object
thereof is to provide: a method useful for manufacturing a press-formed
article which
is capable of achieving a high-level balance between high strength and
elongation and
has good anti-softening property in HAZ; and a press-formed article which
exerts the
above properties.
MEANS FOR SOLVING THE PROBLEMS
[0012]
In the method for manufacturing a press-formed article in the present
invention, which can attain the object above, a steel sheet for hot-pressing
is heated at
900 C or more and 1,100 C or less, the steel sheet for hot-pressing including:
C: from 0.15 to 0.5% (mass%; hereinafter, the same applies to the chemical
component composition),
Si: from 0.2 to 3%,
Mn: from 0.5 to 3%,
P: 0.05% or less (exclusive of 0%),
S: 0.05% or less (exclusive of 0%),
Al: from 0.01 to 1%,
B: from 0.0002 to 0.01%,
Ti: equal to or more than 3.4[N]+0.01% and equal to or less than 3.4[N]+0.1%
[wherein [N] indicates a content (mass%) of N], and
N: from 0.001 to 0.01%, with the remainder being iron and unavoidable
impurities, in which an average equivalent-circle diameter of a Ti-containing
precipitate having an equivalent-circle diameter of 30 urn or less among Ti-
containing
precipitates contained in the steel sheet is 6 nm or less, and a precipitated
Ti amount
and a total Ti amount in a steel satisfy the following formula (1),
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and thereafter press forming is started and the steel sheet is cooled to a
temperature equal to or less than a temperature 100 C below a bainite
transformation
starting temperature Bs and equal to or more than a martensite transformation
starting
temperature Ms, while ensuring an average cooling rate of 20 C/sec or more in
a mold
during forming and after the completion of forming, and thereafter the steel
sheet is
cooled to 200 C or less at an average cooling rate of less than 20 C/sec.
Here, the
"equivalent-circle diameter" is the diameter of a circle having the same area
as the size
(area) of a Ti-containing precipitate (e.g., TiC) when the precipitate is
converted to a
circle ("the average equivalent-circle diameter" is the average value
thereof).
Precipitated Ti amount (mass%)-3.4[N] < 0.5x[total Ti amount (mass%)-
3.4[N]] (1)
(in the formula (1), [N] indicates the content (mass%) of N in the steel).
[0013]
In the steel sheet for hot-pressing to be used in the manufacturing method in
the present invention, it is also useful to contain, as the other element(s),
at least one of
the following (a) to (c), if desired. The properties of the press-formed
article are
further improved according to the kind of the element that is contained
according to
need.
(a) One or more kinds selected from the group consisting of V, Nb and Zr, in
an amount of 0.1% or less (exclusive of 0%) in total
(b) One or more kinds selected from the group consisting of Cu, Ni, Cr and
Mo, in an amount of 1% or less (exclusive of 0%) in total
(c) One or more kinds selected from the group consisting of Mg, Ca and
REM, in an amount of 0.01% or less (exclusive of 0%) in total
[0014]
In the press-formed article obtained by this manufacturing method, the metal
microstructure of the press-formed article includes bainitic ferrite: from 60
to 97
area%, martensite: 37 area% or less, retained austenite: from 3 to 20 area%,
and
remainder microstructure: 5 area% or less, the average equivalent-circle
diameter of
Ti-containing precipitate having an equivalent-circle diameter of 30 nm or
less among
Ti-containing precipitates contained in the press-formed article is 10 nm or
less, and a
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relationship of the formula (1) is satisfied, and thus, a high-level balance
between high
strength and elongation can be achieved as uniform properties in the formed
article.
ADVANTAGE OF THE INVENTION
[0015]
According to the present invention, a steel sheet where the chemical
component composition is strictly specified, the size of the Ti-containing
precipitate is
controlled and the precipitation rate of Ti not forming TiN is controlled is
used, so that
by hot-pressing the steel sheet under predetermined conditions, the strength-
elongation
balance of the formed article can be made to be a high-level balance and the
anti-
softening property in HAZ is improved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016]
[FIG. 1] A schematic explanatory view showing the mold configuration for
carrying out hot-press forming.
MODE FOR CARRYING OUT THE INVENTION
[0017]
The present inventors have made studies from various aspects to realize a
press-formed article which ensures that, in the manufacture of a press-formed
article
by heating a steel sheet at a predetermined temperature and then hot-press
forming the
steel sheet, a press-formed article exhibiting good ductility (elongation) is
obtained
while assuring high strength after press forming.
[0018]
As a result, it has been found that when the chemical component composition
of the steel sheet for hot-pressing is strictly specified and the size of the
Ti-containing
precipitate as well as the precipitated Ti amount are controlled and when the
steel sheet
is hot-press formed under predetermined conditions, a predetermined amount of
retained austenite is ensured after press forming and a press-formed article
having
increased intrinsic ductility (residual ductility) and good anti-softening
property in
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HAX is obtained. The present invention has been accomplished based on this
finding.
[0019]
In the steel sheet for hot-pressing to be used in the present invention, the
chemical component composition needs to be strictly specified, and the reason
for
limiting the range of each chemical component is as follows.
[0020]
(C: from 0.15 to 0.5%)
C is an important element in lowering the bainite transformation starting
temperature Bs to refine bainitic ferrite produced in the cooling process, and
increasing
the dislocation density in bainitic ferrite to enhance the strength. In
addition, the
amount of fine retained austenite formed between bainitic ferrite laths is
increased, and
a high-level balance between high strength and elongation can be ensured. If
the C
content is less than 0.15%, the bainite transformation starting temperature Bs
elevates
to bring about coarsening of bainitic ferrite and reduction in the dislocation
density,
and the strength of a hot press-formed article cannot be ensured. If the C
content is
too large and exceeds 0.5%, the strength is excessively high, and good
ductility is not
obtained. The lower limit of the C content is preferably 0.18% or more (more
preferably 0.20% or more), and the upper limit is preferably 0.45% or less
(more
preferably 0.40% or less).
[0021]
(Si: from 0.2 to 3%)
Si exerts an effect of suppressing cementite formation due to decomposition
of retained austenite formed between bainitic ferrite laths during cooling of
mold
quenching, and forming retained austenite thereby. In order to exert such an
effect,
the Si content must be 0.2% or more. If the Si content is too large and
exceeds 3%,
ferrite is readily formed, making it difficult to produce a single phase of
austenite
during heating, and the fraction of a microstructure other than bainitic
ferrite and
retained austenite in the steel sheet for hot-pressing exceeds 5 area%. The
lower limit
of the Si content is preferably 0.5% or more (more preferably 1.0% or more),
and the
upper limit is preferably 2.5% or less (more preferably 2.0% or less).
[0022]
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(Mn: from 0.5 to 3%)
Mn is an element effective in enhancing the quenchability and suppressing the
formation of a soft microstructure such as ferrite and pearlite during cooling
of mold
quenching. In addition, this is an important element in lowering the bainite
transformation starting temperature Bs to refine bainitic ferrite produced in
the cooling
process and increasing the dislocation density in bainitic ferrite to enhance
the
strength. Furthermore, this is an element capable of stabilizing austenite and
is an
element contributing to an increase in the retained austenite amount. In order
to exert
such effects, Mn must be contained in an amount of 0.5% or more. In the case
of
considering only the properties, the Mn content is preferably larger, but
since the cost
of alloying addition rises, the content is set to 3% or less. The lower limit
of the Mn
content is preferably 0.7% or more (more preferably 1.0% or more), and the
upper
limit is preferably 2.5% or less (more preferably 2.0% or less).
[0023]
(P: 0.05% or less (exclusive of 0%))
P is an element unavoidably contained in the steel but deteriorates the
ductility and therefore, the P content is preferably reduced as much as
possible.
However, an extreme reduction causes an increase in the steelmaking cost, and
it is
difficult in terms of manufacture to reduce the content to 0%. For this
reason, the
content thereof is set to 0.05% or less (exclusive of 0%). The upper limit of
the P
content is preferably 0.045% or less (more preferably 0.040% or less).
[0024]
(S: 0.05% or less (exclusive of 0%))
S is an element unavoidably contained in the steel, as with P, and
deteriorates
the ductility and therefore, the S content is preferably reduced as much as
possible.
However, an extreme reduction causes an increase in the steelmaking cost, and
it is
difficult in terms of manufacture to reduce the content to 0%. For this
reason, the
content thereof is set to 0.05% or less (exclusive of 0%). The upper limit of
the S
content is preferably 0.045% or less (more preferably 0.040% or less).
[0025]
(Al: from 0.01 to 1%)
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Al is useful as a deoxidizing element and allows the solute N present in the
steel to be fixed as AIN, which is useful in enhancing the ductility. In order
to
effectively exert such an effect, the Al content must be 0.01% or more.
However, if
the Al content is too large and exceeds 1%, A1203 is excessively produced to
deteriorate the ductility. The lower limit of the Al content is preferably
0.02% or
more (more preferably 0.03% or more), and the upper limit is preferably 0.8%
or less
(more preferably 0.6% or less).
[0026]
(B: from 0.0002 to 0.01%)
B is an element having an action of suppressing ferrite transformation and
pearlite transformation, and therefore, contributes to preventing the
formation of
ferrite, pearlite and bainite during cooling after heating at a two-phase zone
temperature of (Aci transformation point to Ac3 transformation point), and
ensuring
retained austenite. In order to exert such effects, B must be contained in an
amount of
0.0002% or more, but even when this element is contained excessively over
0.01%, the
effects are saturated. The lower limit of the B content is preferably 0.0003%
or more
(more preferably 0.0005% or more), and the upper limit is preferably 0.008% or
less
(more preferably 0.005% or less).
[0027]
(Ti: equal to or more than 3.4[N]+0.01% and equal to or less than 3.4[N]+0.1%:
[N] is
the content (mass%) of N)
Ti exerts an effect of improving the quenchability by fixing N and
maintaining B in a solid solution state. In order to exert such an effect, it
is important
to contain this element in an amount larger than the stoichiometric ratio of
Ti and N
(3.4 times the N content) by 0.01% or more. In addition, when Ti added
excessively
relative to N is caused to be present in a solid solution state in a hot-stamp
formed
article and the precipitated compound is finely dispersed, the strength
reduction in
HAZ can be suppressed by virtue of precipitation strengthening due to
formation, as
TiC, of Ti dissolved in solid during welding of the hot-stamp formed article
or by
virtue of an effect such as delaying increase of the dislocation density due
to the
dislocation movement-preventing effect of TiC. However, if the Ti content is
too
large and exceeds 3.4[N]+0.1%, the Ti-containing precipitate (e.g., TiN)
formed is
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coarsened to deteriorate the ductility of the steel sheet. The lower limit of
the Ti
content is more preferably 3.4[N]+0.02% or more (further preferably
3.4[N]+0.05% or
more), and the upper limit is more preferably 3.4[N]+0.09% or less (further
preferably
3.4[N]+0.08% or less).
[0028]
(N: from 0.001 to 0.01%)
N decrease the improvement effect of the hardenability during quenching by
fixing B as BN, and thus, the content thereof is preferably reduced as much as
possible,
but the reduction in an actual process is limited and therefore, the lower
limit is set to
0.001%. If the N content is too large, the Ti-containing precipitate (e.g.,
TiN) formed
is coarsened, and this precipitate works as a fracture origin to deteriorate
the ductility
of the steel sheet. For this reason, the upper limit is set to 0.01%. The
upper limit of
the N content is preferably 0.008% or less (more preferably 0.006% or less).
[0029]
The basic chemical components in the steel sheet for hot-pressing to be used
in the present invention are as described above, and the remainder is iron and
unavoidable impurities (e.g., 0, H) other than P, S and N. In the steel sheet
for hot-
pressing to be used in the present invention, it is also useful to further
contain, as the
other element(s), at least one of the following (a) to (c), if desired. The
properties of
press-formed article are further improved according to the kind of the element
that is
contained according to need. In the case of containing such an element, the
preferable range and the reason for limitation on the range are as follows.
(a) One or more kinds selected from the group consisting of V, Nb and Zr, in
an amount of 0.1% or less (exclusive of 0%) in total
(b) One or more kinds selected from the group consisting of Cu, Ni, Cr and
Mo, in an amount of 1% or less (exclusive of 0%) in total
(c) One or more kinds selected from the group consisting of Mg, Ca and
REM, in an amount of 0.01% or less (exclusive of 0%) in total
[0030]
(One or more kinds selected from the group consisting of V, Nb and Zr, in an
amount
of 0.1% or less (exclusive of 0%) in total)
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V, Nb and Zr have an effect of forming fine carbide and refining the
microstructure by a pinning effect. In order to exert such an effect, these
elements are
preferably contained in an amount of 0.001% or more in total. However, if the
content of these elements is too large, coarse carbide is formed and works out
to a
fracture origin to conversely deteriorate the ductility. For this reason, the
content of
these elements is preferably 0.1% or less in total. The lower limit of the
content of
these elements is more preferably 0.005% or more (still more preferably 0.008%
or
more) in total, and the upper limit is more preferably 0.08% or less (still
more
preferably 0.06% or less) in total.
[0031]
(One or more kinds selected from the group consisting of Cu, Ni, Cr and Mo: 1%
or
less (exclusive of 0%) in total)
Cu, Ni, Cr and Mo suppress ferrite transformation and pearlite transformation,
and therefore, effectively act to prevent the formation of ferrite and perlite
during
cooling after heating and ensure retained austenite. In order to exert such an
effect,
these are preferably contained in an amount of 0.01% or more in total. In the
case of
considering only the properties, the content is preferably larger, but since
the cost of
alloying addition rises, the content is preferably 1% or less in total. In
addition, these
elements have an action of greatly increasing the strength of austenite and
put a large
load on hot rolling, making it difficult to manufacture a steel sheet.
Therefore, also
from the standpoint of manufacturability, the content is preferably 1% or
less. The
lower limit of the content of these elements is more preferably 0.05% or more
(still
more preferably 0.06% or more) in total, and the upper limit is more
preferably 0.5%
or less (still more preferably 0.3% or less) in total.
[0032]
(One or more kinds selected from the group consisting of Mg, Ca and REM, in an
amount of 0.01% or less (exclusive of 0%) in total)
These elements refine the inclusion and therefore, effectively act to enhance
the ductility. In order to exert such an effect, these elements are preferably
contained
in an amount of 0.0001% or more in total. In the case of considering only the
properties, the content is preferably larger, but since the effect is
saturated, the content
is preferably 0.01% or less in total. The lower limit of the content of these
elements
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is more preferably 0.0002% or more (still more preferably 0.0005% or more) in
total,
and the upper limit is more preferably 0.005% or less (still more preferably
0.003% or
less) in total.
[0033]
In the steel sheet for hot-pressing to be used in the present invention, (A)
the
average equivalent-circle diameter of Ti-containing precipitates having an
equivalent-
circle diameter of 30 nm or less among Ti-containing precipitates contained in
the steel
sheet is 6 nm or less, and (B) the relationship of "precipitated Ti amount
(mass%)-
3.4[N] < 0.5x[total Ti amount (mass%)-3.4[N]]" (the relationship of the
formula (1)) is
satisfied, are also important requirements.
[0034]
The Ti-containing precipitate and formula (1) is controlled for preventing
softening of HAZ and such a control is originally a control required of a
formed article,
but these values are little changed between before and after hot-press
forming.
Therefore, the control needs to be already done at the stage before forming
(the steel
sheet for hot-pressing). When excessive Ti relative to N in the steel sheet
before
forming is cause to be present in a solid solution state or refined state, the
Ti-
containing precipitate can be maintained in a solid solution state or refined
state during
heating of hot pressing. As a result, the amount of Ti precipitated in the
press-formed
article can be controlled to not more than a predetermined amount, and
softening in
HAZ can be prevented, whereby the joint properties can be improved.
[0035]
From such a standpoint, Ti-containing precipitates needs to be finely
dispersed and to this end, the average equivalent-circle diameter of Ti-
containing
precipitates having an equivalent-circle diameter of 30 nm or less among Ti-
containing
precipitates contained in the steel sheet must be 6 nm or less (requirement of
(A)
above). The size (average equivalent-circle diameter) of the Ti-containing
precipitate
is preferably 5 nm or less, more preferably 3 nm or less. Examples of the Ti-
containing precipitate targeted in the present invention include TiC, TiN and
other Ti-
containing precipitates such as TiVC, TiNbC, TiVCN and TiNbCN.
[0036]
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As described later, the average equivalent-circle diameter of Ti-containing
precipitates in the press-formed article is specified to be 10 nm or less,
whereas that
before forming (steel sheet for hot-pressing) is specified to be 6 nm or less.
The
reason why the size of the precipitate is specified to be larger in the formed
article than
in the steel sheet is that Ti is present as a fine precipitate or in a solid
solution state in
the steel sheet and when heated at near 800 C for 15 minutes or more, the Ti-
containing precipitate is slightly coarsened. In order to ensure the
properties as a
formed article, the average equivalent-circle diameter of Ti-containing
precipitates
must be 10 nm or less, and for realizing this precipitation state in a hot-
stamp formed
article, it is necessary that in the state of the steel sheet for hot-
stamping, the average
equivalent-circle diameter of fine precipitates of 30 nm or less is adjusted
to 6 nm or
Less and many of Ti is caused to be present in a solid solution state.
[0037]
In addition, in the steel sheet for hot-pressing, the majority of Ti except
for Ti
to be used for precipitating and fixing N must be caused to be present in a
solid
solution state or refined state. To this end, the amount of Ti present as a
precipitate
other than TiN (i.e., precipitated Ti amount-3 .4[N]) needs to be an amount
smaller than
0.5 times the remainder after deduction of Ti that forms TiN from total Ti
(i.e.,
0.5x[(total Ti amount)-3.4[N]]) (requirement of (B) above). The "precipitated
Ti
amount-3.4[M" is preferably 0.4x[(total Ti amount)-3.4[N]] or less, more
preferably
0.3x[(total Ti amount)-3.4[N]] or less.
[0038]
For manufacturing the above steel sheet (steel sheet for hot-pressing), a slab
prepared by melting a steel material having the above-described chemical
component
composition may be hot-rolled at a heating temperature: 1,100 C or more
(preferably
1,150 C or more) and 1,300 C or less (preferably 1,250 C or less) and a finish
rolling
temperature of 850 C or more (preferably 900 C or more) and 1,000 C or less
(preferably 950 C or less), and immediately after that, it may be cooled
(rapid cooling)
at an average cooling rate of 20 C/sec or more (preferably 30 C/sec or more)
until
500 C or less (preferably 450 C or less) and after that, it may be wound at a
temperature of 350 C or more (preferably 380 C or more) and 450 C or less
(preferably 430 C or less).
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[0039]
In the method above, (1) rolling is terminated in a temperature region where a
dislocation introduced into austenite by hot rolling remains, (2) rapid
cooling is
performed immediately thereafter to allow a Ti-containing precipitate such as
TIC to be
finely formed on the dislocation, and (3) rapid cooling is further performed,
followed
by winding, whereby bainite transformation or martensite transformation is
controlled
to occur.
[0040]
The steel sheet for hot-pressing which has the above-described chemical
component composition and Ti-precipitation state may be directly used for the
manufacture by hot pressing or may be subjected to cold rolling at a rolling
reduction
of 10 to 80% (preferably from 20 to 70%) after pickling and then used for the
manufacture by hot pressing. The steel sheet for hot-pressing or a cold rolled
material thereof may be subjected to a heat treatment including heating at 830
C or
more (preferably 850 C or more and 900 C or less), then rapid cooling at a
cooling
rate of 20 C/sec or more (preferably 30 C/sec or more) until 500 C or less
(preferably
450 C or less), and then holding at 500 C or less for 10 seconds or more and
1,000
seconds or less, or tempering at a temperature of 500 C or less. In addition,
the
surface of the steel sheet for hot-pressing (the surface of the base steel
sheet) in the
present invention may be subjected to plating containing one or more kinds of
Al, Zn,
Mg and Si.
[0041]
Using the above-described steel sheet for hot-pressing, the steel sheet is
heated at a temperature of 900 C or more and 1,100 C or less, and after press
forming
is started, the steel sheet is cooled to a temperature equal to or less than a
temperature
100 C below the bainite transformation starting temperature Bs (Bs-100 C) and
equal
to or more than the martensite transformation starting temperature Ms, while
ensuring
an average cooling rate of 20 C/sec or more in a mold during forming as well
as after
the completion of forming, and then cooled to 200 C or less at an average
cooling rate
of less than 20 C/sec, whereby an optimal microstructure as a formed article
with
predetermined strength and high ductility (microstructure mainly including
bainitic
14
CA 02923582 2016-03-07
ferrite) can be produced in a press-formed article having a single property.
The
reason for specifying each requirement in this forming method is as follows.
[0042]
If the heating temperature of the steel sheet is less than 900 C, a sufficient
amount of austenite cannot be obtained during heating, and the martensite
fraction is
too large in the final microstructure (microstructure of a formed article). If
the
heating temperature of the steel sheet exceeds 1,100 C, the austenite grain
size grows
during heating, the martensite transformation starting temperature Ms and
martensite
transformation finishing temperature Mf are elevated, retained austenite
cannot be
ensured during quenching, and good formability is not achieved. The heating
temperature is preferably 950 or more and 1,050 C or less. At this time, if
the
heating time is too long, the Ti-containing precipitate in the steel sheet can
be hardly
refined, and a Ti-containing precipitate even in a small amount is formed
during
heating and coarsened to reduce the effect of improving weldability. For this
reason,
the heating time is preferably shorter. The heating time is preferably 3,600
seconds or
less, and more preferably 20 seconds or less.
[0043]
For allowing austenite formed in the heating step above to be a desired
microstructure (microstructure mainly including bainitic ferrite) while
impeding
production of a microstructure such as ferrite or pearlite, the average
cooling rate
during forming as well as after forming and the cooling finishing temperature
must be
appropriately controlled. From such a standpoint, it is necessary that the
average
cooling rate during forming is 20 C/sec or more and the cooling finishing
temperature
is equal to or less than a temperature 100 C below the bainite transformation
starting
temperature Bs and equal to or more than martensite transformation starting
temperature Ms. The average cooling rate during forming is preferably 30 C/sec
or
more (more preferably 40 C/sec or more). When the cooling finishing
temperature is
equal to or less than a temperature 100 C below the bainite transformation
starting
temperature Bs, austenite present during heating is transformed to bainite
while
impeding production of a microstructure such as ferrite or pearlite, whereby
fine
austenite is caused to remain between bainitic ferrite laths and a
predetermined amount
of retained austenite is assured while ensuring the amount of bainitic
ferrite.
CA 02923582 2016-03-07
[0044]
If the cooling finishing temperature exceeds the temperature 100 C below the
bainite transformation starting temperature Bs or the average cooling rate is
less than
20 C/sec, a microstructure such as ferrite and pearlite is formed, and a
predetermined
amount of retained austenite cannot be ensured, resulting in deterioration of
the
elongation (ductility) in a formed article. When the cooling is performed to a
temperature less than the martensite transformation starting temperature Ms,
the
production amount of martensite is increased and the elongation (ductility) of
the
formed article is deteriorated.
[0045]
After reaching a temperature equal to or less than a temperature 100 C below
the bainite transformation starting temperature Bs and equal to or more than
the
martensite transformation starting temperature Ms, rapid cooling is stopped,
and
cooling to 200 C or less is performed at an average cooling rate of less than
20 C/sec.
By adding such a cooling step, bainitic ferrite transformation is promoted. If
the
average cooling rate here is 20 C/sec or more, martensite is formed and
although the
strength may be increased, good elongation is not obtained. The average
cooling rate
is preferably 15 C/sec or less, more preferably 10 C/sec or less. The reason
why the
steel sheet is cooled to 200 C or less in this cooling is that the amount of
retained
austenite remaining at room temperature is increased by distributing carbon
from
bainitic ferrite to untransformed austenite.
[0046]
After performing the above-described two-stage cooling, fundamentally, the
average cooling rate need not be controlled, but the steel sheet may be cooled
to room
temperature at an average cooling rate of, for example, from 1 C/sec or more
and
100 C/sec or less. The control of the average cooling rate during press
forming as
well as after the completion of forming can be achieved by a technique of, for
example, (a) controlling the temperature of the forming mold (the cooling
medium
shown in FIG. 1), or (b) controlling the thermal conductivity of the mold.
[0047]
In the press-formed article obtained by this manufacturing method, the metal
microstructure includes bainitic ferrite: from 60 to 97 area%, martensite: 37
area% or
16
CA 02923582 2016-03-07
less, retained austenite: from 3 to 20 area%, and remainder microstructure: 5
area% or
less, and the amount of carbon in the retained austenite is 0.50% or more, so
that a
high-level balance between high strength and elongation can be achieved as a
uniform
property in a formed article. The reason for setting the range of each
requirement (the
amount of carbon in basic microstructure and retained austenite) in this hot
press-
formed article is as follows.
[0048]
When the main microstructure of a press-formed article is high-strength
bainitic ferrite rich in ductility, both of high strength and high ductility
of a press-
formed article can be satisfied. From such a standpoint, the area fraction of
bainitic
ferrite must be 60 area% or more. However, if this fraction exceeds 97 area%,
the
retained austenite fraction is insufficient, and the ductility (residual
ductility) is
reduced. The lower limit of the bainitic ferrite fraction is preferably 65
area% or
more (more preferably 70 area% or more), and the upper limit is preferably 95
area%
or less (more preferably 90 area% or less).
[0049]
The strength of a hot press-formed article can be increased by partially
incorporating high-strength martensite, but if the amount thereof is large,
the ductility
(residual ductility) is reduced. From such a standpoint, the area fraction of
martensite
must be 37 area% or less. The lower limit of the martensite fraction is
preferably 5
area% or more (more preferably 10 area% or more), and the upper limit is
preferably
30 area% or less (more preferably 25 area% or less).
[0050]
Retained austenite has an effect of increasing the work hardening ratio
(transformation induced plasticity) and enhancing the ductility of the press-
formed
article by undergoing transformation to martensite during plastic deformation.
In
order to exert such an effect, the retained austenite fraction must be 3 area%
or more.
The ductility is more improved as the retained austenite fraction is higher.
In the
composition to be used for an automotive steel sheet, the assurable retained
austenite is
limited, and the upper limit is about 20 area%. The lower limit of the
retained
austenite is preferably 5 area% or more (more preferably 7 area% or more).
[0051]
17
CA 02923582 2016-03-07
As for the microstructure other than those described above, ferrite, pearlite,
and the like may be contained as a remainder microstructure, but such a
microstructure
is inferior to other microstructures in terms of contribution to strength or
contribution
to ductility, and it is fundamentally preferable not to contain such a
microstructure
(may be even 0 area%). However, an area fraction up to 5 area% is acceptable.
The
area fraction of the remainder microstructure is preferably 4 area% or less,
more
preferably 3 area% or less.
[0052]
In the press-formed article above, the average equivalent-circle diameter of
Ti-containing precipitates having an equivalent-circle diameter of 30 nm or
less among
Ti-containing precipitates contained in the press-formed article (i.e., in the
steel sheet
constituting the press-formed article) is 10 nm or less. When this requirement
is
satisfied, a press-formed article capable of achieving a high-level balance
between high
strength and elongation can be obtained. The average equivalent-circle
diameter of
the Ti-containing precipitate is preferably 8 nm or less, more preferably 6 nm
or less.
[0053]
In addition, in the press-formed article, the amount of Ti present as a
precipitate other than TiN (i.e., precipitated Ti amount-3.4[N]) is smaller
than 0.5
times the remainder Ti after deduction of Ti that forms TiN from total Ti
(i.e., smaller
than 0.5x[total Ti amount (%)-3.4[N]]). When this requirement is satisfied, Ti
dissolved in solid during welding is finely precipitated in HAZ or the
existing fine Ti-
containing precipitate suppresses recovery, etc. of the dislocation, and as a
result,
softening in HAZ is prevented, and the weldability is improved. The
"precipitated Ti
amount-3.4[N]" is preferably 0.4x[total Ti amount)-3.4[N]] or less, more
preferably
0.3x[total Ti amount)-3.4[N]] or less.
[0054]
According to the method in the present invention, the properties such as
strength and elongation of a formed article can be controlled by appropriately
adjusting
the press-forming conditions (heating temperature and cooling rate) and
moreover, a
press-formed article having high ductility (residual ductility) is obtained,
making its
application possible to a site (e.g., energy absorption member) to which the
18
CA 02923582 2016-03-07
conventional hot press-formed article can be hardly applied. This is very
useful in
expanding the application range of a hot press-formed article.
[0055]
The effects in the present invention are described more specifically below by
referring to Examples, but the present invention is not limited to the
following
Examples, and all design changes made in light of the gist described above or
later are
included in the technical range in the present invention.
EXAMPLES
[0056]
Steel materials (Steel Nos. 1 to 31) having the chemical component
composition shown in Table 1 below were melted in vacuum to make an
experimental
slab, then hot-rolled to prepare a steel sheet, followed by cooling and
subjecting to a
treatment simulating the winding (sheet thickness: 3.0 mm). As to the method
for
treatment simulating the winding, the sample was cooled to a winding
temperature, and
put in a furnace heated at the winding temperature, followed by holding for 30
minutes
and then cooling in the furnace. The manufacturing conditions of the steel
sheets are
shown in Table 2 below. Here, in Table 1, Ac3 transformation point, Ms point,
and
Bs point were determined using the following formulae (2) to (4) (see, for
example,
The Physical Metallurgy of Steels, Leslie, Maruzen, (1985)). In addition,
treatments
(1) and (2) shown in Remarks of Table 2 mean that each treatment (rolling,
cooling
and alloying) described below was performed.
[0057]
Ac3 transformation point ( C) = 910-203x[C]I/2+44.7x[Si]-
30x [Mn]+700x [P]+400 x[A1]+400x[Ti]+104x [V]-11 x [Cr]+31.5x [Mo]-20x [Cul-
15.2x[Ni] (2)
Ms point ( C) = 550-361x[C]-39x[Mn]-10x[Cu]-17x[Ni]-20x[Cr]-
5x[Mo]+30x[Al] (3)
Bs point ( C) = 830-270x[C]-90x[Mn]-37x[Ni]-70x[Cr]-83x[Mo] (4)
wherein [C], [Si], [Mn], [P], [Al], [Ti], [V], [Cr], [Mo], [Cu] and [Ni]
represent the
contents (mass%) of C, Si, Mn, P, Al, Ti, V, Cr, Mo, Cu and Ni, respectively.
In the
19
CA 02923582 2016-03-07
case where the element shown in each term of formulae (2) to (4) is not
contained, the
calculation is done assuming that the term is not present.
[0058]
Treatment (1): The hot-rolled steel sheet was cold-rolled (sheet thickness:
1.6
mm), then heated at 800 C for simulating continuous annealing in a heat
treatment
simulator, held for 90 seconds, cooled to 500 C at an average cooling rate of
20 C/sec,
and held for 300 seconds.
Treatment (2): The hot-rolled steel sheet was cold-rolled (sheet thickness:
1.6
mm), then heated at 860 C for simulating a continuous hot-dip galvanizing line
in a
heat treatment simulator, cooled to 400 C at an average cooling rate of 30
C/sec, held,
further held under the conditions of 500 Cx10 seconds for simulating immersion
in a
plating bath and alloying treatment, and thereafter cooled to room temperature
at an
average cooling rate of 20 C/sec.
_
[0059]
[Table 1]
Steel Chemical Component Composition* (mass%)
No. C Si Mn P S Al B Ti N
V Nb Cu
, 1
0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 - - -
2 0.150 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 - - -
3 0.220 0.05 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 - - -
4 0.220 0.25 1.20 0.0050 0.0020 0.030 0.0020 ,
0.044 0.0040 - - -
5 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 - - -
6 0.220 , 1.20 1.20 0.0050 0.0020 0.030 0.0020
0.044 0.0040 - - -
7 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 - - - P
8 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 - - -
9 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 - - -
r.9
10 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 - - -
,
11 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 - - - .
,
w
12 0.220
1.20 , 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 - - -
13 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 - - -
14 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 - - -
15 0.220 2.00 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 - - -
16 0.350 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 - - -
*Remainder: Iron and unavoidable impurities except for P, S and N.
21
LTable 1] (continued)
Chemical Component Composition* (mass%) Ac3
Steel .
Bs-100 C Ms Point
transformation
No. Ni Zr Ca Mg REM Cr Mo
( C) ( C)
point ( C) ,
1 - - - . - . - 845
563 425
. .
2 - - - - - 0.20 - 880
568 446
3 - - - - - 0.20 - 812
549 421
. .
4 - . - - - 0.20 . 821
549 421
- - - - - 0.20
- 853 549 , 421
6 - - - - - 0.20 - 863
549 421
_
7 - - - - - 0.20 - 863
549 421
- .
8 - - - - - 0.20 - 863
549 421 . 0
9 - - - -- 0.20 - 863
549 421 2
- -
2
- - - - - 0.20 - 863 549
421
.
co
11 - -- - - 0.20 - 863
549 421
_ - _
_
12 - - - 0.20 - 863
549 421 13
.
,,
13 - - - - - 0.20 - 863
549 421
1:'
_
14 - - - - - 0.20 - 863
549 421
_
_ .
- - . - - 0.20 - 899 549
421 .
16 - - - - - 0.20 - 838
514 374 _
*Remainder: Iron and unavoidable impurities except for P, S and N.
22
'Table 1] (continued)
Steel Chemical Component Composition* (mass%)
No. C Si Mn P S Al B Ti N V Nb Cu
17 0.720 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 -
- -
18 0.220 1.20 0.80 0.0050 0.0020 0.030 0.0020 0.044 0.0040 -
- -
19 0.220 1.20 2.40 0.0050 0.0020 0.030 0.0020 0.044 0.0040 -
- -
20 0.220 1.20 1.20 _ 0.0050 0.0020 0.030 0.0020
0.100 0.0040 - - -
21 0.220 1.20 1.20 , 0.0050 0.0020 0.030 0.0020
0.200 0.0040 - - -
22 0.220 0.50 1.20 0.0050 0.0020 0.40 0.0020 0.044 0.0040 -
- -
23 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 0.030 -
-
24 0.220 1.20 1.20 _ 0.0050 0.0020 0.030 0.0020
0.044 0.0040 - 0.020 - P
25 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 -
- 0.20 ,9
26 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 -
- -
2
27 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 - , -
-
,
28 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 -
- -
,
29 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 -
- - ow'
30 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 -
- -
31 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.040
_
*Remainder: Iron and unavoidable impurities except for P, S and N.
23
,
[Table 1] (continued)
Chemical Component Composition* (mass%) Ac3
_
Steel
Bs-100 C Ms Point
transformation
No. Ni Zr Ca Mg REM Cr Mo
( C) ( C)
point (C)
. ,
.
17- - - - - 0.20 - 786
414 240
_
_
_
18 - - - - - 0.20 - 875
585 436
19 - - - - - 0.20 - 827
441 374
20 - - - - - 0.20 - 886
549 421
21 - - - - - 0.20 - 926
549 421
22 - - - - - 0.20 - 980
549 432
23 - - - - - 0.20 - 866
549 421
P
24 - - - - - 0.20 -
863 549 421
,
r.,0
25 - - - - - 0.20 - 859
549 419
_
26 0.20 - - - - 0.20 860
541 417 r.,0
N)
27 - - - - -
0.20 0.20 , 869 532 420
,
28 - 0.015 - - 0.20 - 863
549 421 s'
,
29 - - 0.002 - - 0.20 - 863
549 421 2
30 - - - 0.002 - 0.20 - 863
549 421
31 - - - - 0.002 0.20 - 863
549 421
*Remainder: Iron and unavoidable impurities except for P, S and N.
24
[0060]
-ITable 2]
Steel Sheet Manufacturing Conditions
Finish Average Cooling Rate from
Steel Heating
Rolling Finish Rolling Temperature to Winding
No. Temperature
Remarks
( C) Temperature Winding Temperature Temperature ( C)
( C) ( C/sec) .
1 1200 950 20 500
-
2 1200 950 20 500
-
3 1200 950 20 500
-
4 1200 950 20 500
- P
.
.
1200 950 20 500 -
."
"
6 1200 950 20 500
- w
2
_
7 1200 800 20 500
- 0"
.
,
8 1200 950 20 500
treatment (1) Z
,õ
,
9 1200 950 20 500
treatment (2) -
,
1200 950 20 500 -
11 1200 950 20 500
-
_
12 1200 950 20 500
-
_
13 1200 950 20 500
-
14 1200 950 20 500
-
_
1200 950 20 500 -
16 1200 950 20 500
-
[Table 2] (continued)
Steel Sheet Manufacturing Conditions
_
Finish Average Cooling Rate from
Steel Heating
Rolling Finish Rolling Temperature to Winding
No. Temperature
Remarks
( C) Temperature Winding Temperature Temperature ( C)
( C) ( C/sec) -
. 17 1200 950 20 500 -
18 1200 950 20 500 -
19 1200 950 20 500 -
20 1200 950 20 500 -
21 1200 950 20 500 -
22 1200 980 20 500 -
P
2
23 1200 950 20 500 -
24 1200 950 20 500 -
2
N)
25 1200 950 20 500 -
0
,
,
26 1200 950 20 500 -
0
,õ
,
0
27 1200 950 20 500 -
,
28 1200 950 20 500 -
29 1200 950 20 500 -
30 1200 950 20 500 -
31 1200 950 20 500
26
CA 02923582 2016-03-07
[0061]
With respect to the steel sheets (steel sheets for press-forming) obtained,
analysis of the Ti precipitation state ("precipitated Ti amount-3.4[N]" and
average
equivalent-circle diameter of Ti-containing precipitates) was performed in the
following manner. The results obtained are shown in Table 3 below together
with the
calculated value of 0.5x[total Ti amount-3.4[N]].
[0062]
(Analysis of Ti Precipitation State of Steel Sheet)
An extraction replica sample was prepared, and a transmission electron
microscope image (magnifications: 100,000 times) of Ti-containing precipitates
was
photographed using a transmission electron microscope (TEM). At this time, the
Ti-
containing precipitate (those having an equivalent-circle diameter of 30 nm or
less)
was identified by the composition analysis of precipitates by means of an
energy
dispersive X-ray spectrometer (EDX). At least 100 pieces of Ti-containing
precipitates were measured for the area by image analysis, the equivalent-
circle
diameter was determined therefrom, and the average value thereof was defined
as the
precipitate size (average equivalent-circle diameter of Ti-containing
precipitates). As
for the "precipitated Ti amount-3.4[N]" (the amount of Ti present as a
precipitate),
extraction residue analysis was performed using a mesh having a mesh size of
0.1 gm
(during extraction treatment, a fine precipitate resulting from aggregation of
precipitates could also be measured), and the "precipitated Ti amount-3.4[N]"
was
determined. In the case where the Ti-containing precipitate partially
contained V or
Nb, the contents of these precipitates were also measured.
[0063]
27
, CA 02923582 2016-03-07
,
[Table 3]
Steel Sheet for Press-Forming
Steel Precipitated TiAverage Equivalent-Circle
0.5x[Total Ti Amount-
No. Amount-3.4[N] Diameter
of Ti-Containing
3.4[N] (mass %)
(mass %) Precipitates (nm)
1 0.009 0.015 4.0
2 0.008 0.015 2.5 .
3 0.008 0.015 2.3
4 0.006 0.015 3.3
0.001 0.003 3.0
6 0.008 0.015 3.2
_ 7 0.018 0.015 9.2
8 0.008 0.015 3.4
9 0.010 0.015 3.2
_ 10 0.008 0.015 2.8
11 0.008 0.015 2.8
12 0.008 0.015 2.8
_ 13 0.008 0.015 2.8
_ 14 0.008 0.015 2.8
0.009 0.015 3.9
_ 16 0.009 0.015 3.7
_ 17 0.008 0.015 3.2
18 0.008 0.015 2.7
19 0.006 0.015 2.1
, _
0.025 0.043 3.1
_ 21 0.128 0.093 10.8
22 0.008 0.015 2.7
23 0.009 0.015 3.1
24 0.006 0.015 3.6 _
0.007 0.015 3.6 _
26 0.008 0.015 3.6 _
27 0.008 0.015 2.7 _
_ 28 0.006 0.015 3.0 _
29 0.007 0.015 3.3 _
0.006 0.015 3.2 .
31 0.006 0.015 3.0 _
[0064]
Each of the steel sheets above (1.6 mmtx150 mmx200 mm) (the thickness t of
those other than the treatment (1) and (2) was adjusted to 1.6 mm by hot
rolling) was
28
CA 02923582 2016-03-07
heated at a predetermined temperature in a heating furnace, followed by
subjecting to
press forming and cooling treatment using a hat-shaped mold (FIG. 1) to obtain
a
formed article. The press forming conditions (heating temperature, heating
time
average cooling rate, and rapid cooling finishing temperature during press
forming) are
shown in Table 4 below.
[0065]
29
, CA 02923582 2016-03-07
,
[Table 4]
Press-Forming Conditions
Cooling Rate
Steel Heating Heating Average Rapid Cooling
After Finishing
No. Temperature time Cooling Rate Finishing
Rapid Cooling
( C) (sec) ( C/sec) Temperature ( C) ( C/sec)
1 900 600 40 450 3
2 900 15 40 450 3
3 900 15 40 450 3
4 900 15 40 450 3
900 15 40 _ 450 3
6 900 15 40 450 3
7 900 15 40 450 3
8 900 15 40 450 3
9 900 15 40 450 3
900 15 40 450 3
11 900 15 40 450 25
12 800 15 40 300 3
13 900 15 5 450 3
14 900 15 40 600 3
900 15 40 450 3
16 900 15 40 450 3
17 900 15 40 400 3
18 900 15 40 450 3
19 900 15 40 400 3
-
900 15 , 40 450 3
21 900 15 40 450 3
22 900 15 40 450 3
23 900 15 40 450 3
_
24 900 15 40 450 3
_
900 15 40 450 3
26 _ 900 15 40 450 3
27 900 15 40 450 3
28 900 15 40 450 3
29 900 15 40 450 3
_
900 15 40 450 3
_
31 900 15 40 450 3
[0066]
=
CA 02923582 2016-03-07
With respect to the press-formed articles obtained, the tensile strength (TS),
elongation (total elongation EL), observation of metal microstructure
(fraction of each
microstructure), and hardness reduction amount after heat treatment were
measured by
the following methods, and the Ti precipitation state was analyzed by the
method
described above.
[0067]
(Measurement of Tensile Strength (TS) and Elongation (Total Elongation EL))
A tensile test was performed using a JIS No. 5 test piece, and the tensile
strength (TS) and elongation (EL) were measured. At this time, the strain rate
in the
tensile test was set to 10 mm/sec. In the present invention, the test piece
was rated
"passed" when a tensile strength (TS) of 1,180 MPa or more and an elongation
(EL) of
12.0% or more were satisfied and the strength-elongation balance (TSxEL) was
16,000
(MPa.%) or more.
[0068]
(Observation of Metal Microstructure (Fraction of Each Microstructure))
(1) With respect to the microstructures of bainitic ferrite, martensite and
ferrite in the formed article, the steel sheet was corroded with nital and
after
distinguishing bainitic ferrite, martensite and ferrite from each other by SEM
observation (magnifications: 1,000 times or 2,000 times), the fraction (area
ratio) of
each microstructure was determined.
(2) The retained austenite fraction in the formed article was measured by X-
ray diffraction method after the steel sheet was ground to 1/4 thickness and
then
subjected to chemical polishing (for example, ISJJ Int. Vol. 33. (1933), No.
7, P. 776).
[0069]
(Hardness Reduction Amount After Heat Treatment)
As the heat history based on spot welding, the hardness reduction amount
(AHv) relative to the original hardness (Vickers hardness) was measured after
heating
to 700 C at an average heating rate of 50 C/sec in a heat treatment simulator
and then
cooling at an average cooling rate of 50 C/sec. The anti-softening property in
HAZ
was judged as good when the hardness reduction amount (AHv) was 50 Hv or less.
[0070]
31
CA 02923582 2016-03-07
The observation results (fraction of each microstructure, Ti precipitation
state,
and precipitated Ti amount-3.4[N]) of the metal microstructure are shown in
Table 5
below. In addition, the mechanical properties (tensile strength TS, elongation
EL,
TSxEL, and hardness reduction amount AHv) of the press-formed article are
shown in
Table 6 below. Here, the value of "precipitated Ti amount-3.4[M" in the press-
formed article is slightly different from the value of "precipitated Ti amount-
3.4[M" in
the steel sheet for press-forming, but this is a measurement error.
32
[0071]
[Table 5]
Metal Microstructure of Press-Formed Article
Steel Bainitic Ferrite Martensite
Average Equivalent-Circle Precipitated Ti .
Ferrite Fraction Retained Austenite
No. Fraction Fraction Diameter of
Ti-Containing Amount-3.4[N]
(area%) Fraction (area %)
(area %) (area%)
Precipitates (nm) (mass%)
1 87 8 0 5 2.7
0.010
2 81 8 5 6 2.5
0.010
3 94 6 0 0 3.0
0.010
4 89 7 0 4 2.5
0.009
86 6 0 8 3.5
0.000 P
6 86 7 0 7 3.3
0.011 2
N)
7 85 8 0 7 8.0
0.023 w
8 86 7 0 7 2.9
0.010 rõ
0
9 87 6 0 7 2.1
0.011 ,
,
0
88 5 0 7 3.0
0.011 ,õ
I
c,
,
11 6 88 0 6 3.4
0.011
12 0 92 0 8 3.9
0.011
13 26 5 48 6 3.2
0.011
14 55 6 33 6 3.4
0.012
91 0 0 9 2.4
0.013
,
16 86 7 0 7 2.3
0.012
33
ITable 5] (continued)
,
Metal Microstructure of Press-Formed Article
.
Steel Bainitic Ferrite Martensite
Average Equivalent-Circle Precipitated Ti
Ferrite Fraction Retained Austenite
No. Fraction Fraction Diameter of Ti-
Containing Amount-3.4[N]
(area%) Fraction (area %)
'
(area %) (area%)
Precipitates (nm) (mass%)
17 72 8 8 12 2.8
0.013
, 18 88 5 0 7 2.4
0.008
19 87 6 0 7 3.0
0.012
20 88 6 0 6 3.8
0.021
_
21 87 6 0 7 13.8
0.180
_ 22 85 7 0 8 2.1
0.012
23 , 88 6 0 6 3.1
0.011 p
24 86 7 0 7 3.6
0.009 ,9
_
r: ,
25 85 7 0 8 3.0
0.012 ,õ
2
_ 26 89 5 0 6 2.0
0.013 r.,
.
27 87 7 0 6 2.0
0.011
_
I
,õ0
28 85 8 0 7 3.6
0.008 ,,,I
_,
-
39 84 9 0 7 3.0
0.010
30 86 7 0 7 3.3
0.009
31 85 7 0 8 3.0
0.011
34
0 = CA 02923582 2016-03-07
[0072]
[Table 6]
Mechanical Properties of Press-Formed Article
Steel
Tensile Strength TS Elongation EL TSxEL Hardness
Reduction
No.
(MPa) (%) (MPa.%)
Amount AHv (Hv)
1 1217 13.3 16151 42
2 1189 15.5 16188 38
3 1239 10.1 12551 38
4 1210 13.3 16115 35
1254 13.0 16296 35
6 1263 12.8 16220 39
7 1241 11.9 14786 68
8 1213 13.6 16488 44
9 1218 13.4 16263 36
1206 13.3 16057 42
11 1495 10.6 15847 44
12 1532 10.3 15735 40
13 890 18.1 16096 40
14 955 15.2 14516 43
1260 13.0 16329 42
16 1320 12.3 16187 38
17 1992 3.5 6972 40
18 1217 13.4 16311 41
19 1245 13.2 16481 38
, 1215 13.3 16160 44
21 1253 12.9 16174 98
22 1266 12.9 16275 37
23 1221 13.3 16256 40
24 1220 13.2 16063 40
1224 13.2 16136 42
26 1244 13.1 16271 41
27 1232 13.2 16236 40
28 1233 13.5 16646 39
29 1250 13.2 16479 42
1244 13.1 16271 40
31 1229 13.2 16196 41
[0073]
These results allow for the following consideration. It is found that in the
case of Steel Nos. 1, 2, 4 to 6, 8 to 10, 15, 16, 18 to 20, and 22 to 31,
which are
= CA 02923582 2016-03-07
Examples satisfying the requirements specified in the present invention, a
formed
article having a good strength-ductility balance and a good anti-softening
property is
obtained.
[0074]
On the other hand, in the case of Steel Nos. 3, 7, 11 to 14, 17 and 21, which
are Comparative Examples failing in satisfying any of the requirements
specified in the
present invention, any of the properties is deteriorated. More specifically,
in the case
of Steel No. 3 where a steel sheet having a small Si content is used, the
retained
austenite fraction is not ensured in the press-formed article and since only
low
elongation EL is obtained, the strength-elongation balance (TSxEL) is
deteriorated.
In the case of Steel No. 7 where the finish rolling temperature in the
manufacture of a
steel sheet is low, the relationship of the formula (1) is not satisfied, and
not only the
Ti-containing precipitate is coarsened to reduce the strength-elongation
balance
(TSxEL) but also the anti-softening property is deteriorated.
[0075]
In the case of Steel No. 11 where the cooling rate after rapid cooling during
press forming is high, martensite is excessively produced and not only the
strength is
too high, resulting in obtaining only low EL, but also the strength-elongation
balance
(TSxEL) is deteriorated. In the case of Steel No. 12 where the rapid cooling
finishing
temperature during press forming is low, martensite is excessively produced
and not
only the strength is too high, resulting in obtaining only low EL, but also
the strength-
elongation balance (TS xEL) is deteriorated.
[0076]
In the case of Steel No. 13 where the average cooling rate during press
forming is low, the area ratio of bainitic ferrite cannot be ensured and not
only the
strength is too low but also the strength-elongation balance (TSxEL) is
deteriorated.
In the case of Steel No. 14 where the rapid cooling finishing temperature
during press
forming is high, the area ratio of bainitic ferrite cannot be ensured due to
production of
ferrite and not only the strength is too low but also the strength-elongation
balance
(TSxEL) is deteriorated.
[0077]
36
= CA 02923582 2016-03-07
In the case of Steel No. 17 where a steel sheet having an excessive C content
is used, the strength of a formed article is high, but only low elongation EL
is obtained.
In the case of Steel No. 21 where a steel sheet having an excessive Ti content
is used, a
press-formed article does not satisfy the relationship of the formula (1), and
not only
the Ti-containing precipitate in the press-formed article is coarsened but
also the anti-
softening property is deteriorated.
INDUSTRIAL APPLICABILITY
[0078]
In the present invention, a steel sheet for hot-pressing which has a
predetermined chemical component composition, where the equivalent-circle
diameter
of Ti-containing precipitates having an equivalent-circle diameter of 30 nm or
less
among Ti-containing precipitates contained in the steel sheet is 6 nm or less
and the
precipitated Ti amount and the total Ti amount in the steel satisfy a
predetermined
relationship, is heated at a temperature of 900 C or more and 1,100 C or less,
and after
press forming is started, the steel sheet is cooled to a temperature equal to
or less than a
temperature 100 C below the bainite transformation starting temperature Bs and
equal
to or more than the martensite transformation starting temperature Ms, while
ensuring
an average cooling rate of 20 C/sec or more in a mold during forming as well
as after
the completion of forming, and then cooled to 200 C or less at an average
cooling rate
of less than 20 C/sec, whereby a press-formed article capable achieving a high-
level
balance between high strength and elongation can be obtained and moreover, a
press-
formed article having good anti-softening property in HAZ can be realized.
DESCRIPTION OF REFERENCE NUMERALS AND SIGNS
[0079]
1: Punch
2: Die
3: Blank holder
4: Steel sheet (blank)
37
