U.S. patent application number 14/650712 was filed with the patent office on 2016-01-14 for hot press molding and manufacturing method therefor.
This patent application is currently assigned to KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.). The applicant listed for this patent is KABUSHIKI KAISHA KOBE SEIKO SHO(KOBE STEEL, LTD.). Invention is credited to Shushi IKEDA, Toshio MURAKAMI, Junya NAITOU, Keisuke OKITA.
Application Number | 20160010171 14/650712 |
Document ID | / |
Family ID | 51391123 |
Filed Date | 2016-01-14 |
United States Patent
Application |
20160010171 |
Kind Code |
A1 |
NAITOU; Junya ; et
al. |
January 14, 2016 |
HOT PRESS MOLDING AND MANUFACTURING METHOD THEREFOR
Abstract
A hot press molding including: a first forming region exhibiting
a metal structure, which contains 80-97 area % of martensite and
3-20 area % of retained austenite, respectively, and in which the
residual structure is 5 area % or less; and a second forming region
exhibiting a metal structure, which contains 70-97 area % of
bainitic ferrite, 27 area % or less of martensite, and 3-20 area %
of retained austenite, respectively, and in which the residual
structure is 5 area % or less. As a result, hot press moldings,
which have at least a region corresponding to a shock-resistant
area and a region corresponding to an energy-absorbing area in a
single molding and in which a high level of balancing of high
strength with elongation according to the respective region can be
achieved, are provided without using a welding method.
Inventors: |
NAITOU; Junya; (Kobe-shi,
JP) ; MURAKAMI; Toshio; (Kobe-shi, JP) ;
IKEDA; Shushi; (Nagoya-shi, JP) ; OKITA; Keisuke;
(Kobe-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA KOBE SEIKO SHO(KOBE STEEL, LTD.) |
Kobe-shi, Hyogo |
|
JP |
|
|
Assignee: |
KABUSHIKI KAISHA KOBE SEIKO SHO
(KOBE STEEL, LTD.)
Kobe-shi, Hyogo
JP
|
Family ID: |
51391123 |
Appl. No.: |
14/650712 |
Filed: |
February 7, 2014 |
PCT Filed: |
February 7, 2014 |
PCT NO: |
PCT/JP2014/052948 |
371 Date: |
June 9, 2015 |
Current U.S.
Class: |
148/643 ;
148/333 |
Current CPC
Class: |
C21D 6/008 20130101;
C22C 38/002 20130101; C21D 8/0205 20130101; C21D 6/005 20130101;
C22C 38/58 20130101; C21D 6/002 20130101; C21D 2211/008 20130101;
C21D 8/0294 20130101; C22C 38/38 20130101; C22C 38/00 20130101;
C22C 38/28 20130101; C22C 38/32 20130101; C21D 2211/002 20130101;
C22C 38/06 20130101; B21D 22/022 20130101; B21D 22/208 20130101;
C22C 38/02 20130101; C21D 2221/00 20130101; C22C 38/001 20130101;
C21D 2211/001 20130101; C21D 1/673 20130101; C22C 38/04
20130101 |
International
Class: |
C21D 8/02 20060101
C21D008/02; C22C 38/28 20060101 C22C038/28; C22C 38/06 20060101
C22C038/06; B21D 22/02 20060101 B21D022/02; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C21D 6/00 20060101
C21D006/00; C22C 38/32 20060101 C22C038/32; C22C 38/04 20060101
C22C038/04 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 21, 2013 |
JP |
2013-032615 |
Claims
1. A hot press molded article formed by hot press molding of a thin
steel plate, comprising: a first molding region exhibiting a metal
structure which contains 80-97 area % of martensite and 3-20 area %
of retained austenite and which has a residual structure at 5 area
% or less; and a second molding region exhibiting a metal structure
which contains 70-97 area % of bainitic ferrite, 27 area % or less
of martensite, and 3-20 area % of retained austenite and which has
a residual structure at 5 area % or less.
2. The hot press molded article according to claim 1, wherein the
first and second molding regions have an identical chemical
component composition, and steel of each component region contains,
in units of mass %, 0.15-0.3% of C, 0.5-3% of Si, 0.5-2% of Mn,
0.05% or less of P, 0.05% or less of S, 0.01-0.1% of Al, 0.01-1% of
Cr, 0.0002-0.01% of B, [N].times.4-0.1% of Ti, and 0.001-0.01% of
N, where 0% is not inclusive for the P and the S, and [N] denotes
an N content in units of %, and the steel of each component region
has a residual consisting of iron and an inevitable impurity.
3. The hot press molded article according to claim 2, wherein the
steel further contains, as other element, one or more selected from
a group consisting of Cu, Ni, and Mo in a total amount of 1% or
less, where 0% is not inclusive.
4. The hot press molded article according to claim 2, wherein the
steel further contains, as other element, at least one of V or Nb
in a total amount of 0.1% or less, where 0% is not inclusive.
5. A method for producing the hot press molded article according to
claim 1 by forming a thin steel plate so as to divide the thin
steel plate into a plurality of regions including at least first
and second molding regions, the method comprising: after the thin
steel plate is heated to a temperature of an Ac3 transformation
point or higher and 1000.degree. C. or lower, starting cooling at
an average cooling rate of 20.degree. C./sec or higher and molding
by pressing at least the first and second molding regions together
with a die; and terminating, in the first molding region, the
molding at equal to or lower than a temperature lower than a
martensite transformation start temperature by 50.degree. C., and
performing, in the second molding region, the cooling to a
temperature range of equal to or lower than a temperature lower
than a bainite transformation start temperature by 100.degree. C.
and equal to or higher than the martensite transformation start
temperature and terminating the molding after a lapse of a stay
time of 10 seconds or longer within the temperature range.
Description
TECHNICAL FIELD
[0001] The present invention relates to a hot press molded article
which is used for a structural member of an automobile component
and whose strength and ductility can be adjusted according to
different regions in the molded article and to the method for
producing such a hot press molded article. More specifically, the
present invention relates to a hot press molded article exhibiting
strength and ductility according to different regions in such a
manner that when a pre-heated steel plate (blank) is molded into a
predetermined shape, thermal treatment is applied at the same time
as shaping, and also relates to the useful method for producing
such a hot press molded article.
BACKGROUND ART
[0002] Development has been made to reduce the weight of vehicle
bodies as one of the measures for improvement of the fuel
efficiency of automobiles, such a measure stemming from global
environmental issues. A highest possible strength has been required
for steel plates used for the automobiles. However, with an
increase in the strength of the steel plates for the purpose of
reducing the weight of the automobiles, an elongation EL
(elongation) and an r value (Lankford value) decrease, resulting in
a lower press formability and a lower shape fixability.
[0003] In order to solve the above-described problem, the following
hot press molding method has been employed for component
production. After a steel plate is heated to a predetermined
temperature (e.g., a temperature at which an austenite phase is
exhibited) to lower strength (i.e., to facilitate molding), molding
is, for shaping, performed using a die having a lower temperature
(e.g., a room temperature) than that of the thin steel plate. At
the same time, rapid-cooling thermal treatment (quenching) is
performed using a temperature difference between the die and the
steel plate, thereby ensuring strength after molding.
[0004] According to the above-described hot press molding method,
since molding is performed in a low-strength state, springback is
small (favorable shape fixability is obtained). In addition, since
a material containing, e.g., alloy elements of Mn and B and
exhibiting favorable hardenability is used, a 1500 MPa class
strength in terms of tensile strength can be obtained by rapid
cooling. Note that the above-described hot press molding method is,
in addition to hot pressing, called as various names such as hot
forming, hot stamping, a hot stamp method, and die quenching.
[0005] FIG. 1 is a schematic view illustrating a die structure for
performing hot press molding (hereinafter sometimes represented by
"hot stamping") as described above. In FIG. 1, a reference numeral
"1" denotes a punch, a reference numeral "2" denotes a die, a
reference numeral "3" denotes a blank holder, a reference numeral
"4" denotes a steel plate (blank), reference characters "BHF"
denote wrinkle pressing force, reference characters "rp" denote a
punch shoulder radius, reference characters "rd" denote a die
shoulder radius, and reference characters "CL" denote a clearance
between the punch and the die. Of these components, the punch 1 and
the die 2 are formed respectively therein passages 1a, 2a through
which a corresponding one of cooling media 5a, 6a (e.g., water) is
able to pass. Configuration is made such that the cooling media 5a,
6a pass through these passages to cool the punch 1 and the die
2.
[0006] In hot stamping (e.g., hot deep drawing) using the
above-described die, molding begins in such a state that the steel
plate (blank) 4 is heated to a single-phase temperature range of an
Acs transformation point or higher and then, is softened. That is,
in the state in which the steel plate 4 in a high-temperature state
is interposed between each pair of the die 2 and the blank holder
3, the punch 1 pushes the steel plate 4 (in the direction indicated
by an arrow A) into a hole of the dies 2 (the space between the
dies 2, 2 in FIG. 1). While the outer diameter of the steel plate 4
is being reduced, the steel plate 4 is molded into a shape
corresponding to the outer shape of the punch 1. The punch 1 and
the dies 2 are cooled in parallel with molding to draw heat from
the steel plate 4 to the die (the punch 1 and the dies 2), and are
further held and cooled at a lower dead point in molding (the point
at which a punch tip end is positioned innermost: the state
illustrated in FIG. 1) to quench the raw material (the steel plate
4). Such a molding method can be performed to obtain a 1500 MPa
class molded article with favorable dimension accuracy. Moreover,
since a forming load can be reduced as compared to the case of
molding a component of the same strength class as the
above-described molded article in a cold state, the capacity of a
pressing machine can be smaller.
[0007] Steel plates containing 22MnB5 steel as a raw material have
been known as widely-used current steel plates for hot stamping.
These steel plates have a tensile strength of 1500 MPa and an
elongation of about 6-8%, and have been applied to shock-resistant
members (members least deforming and rupturing in collision).
Moreover, further development has been made to increase a C content
and increase strength (1500 MPa or higher, a 1800 MPa class) with
22MnB5 steel being used as a base material.
[0008] However, in current situation, steel grades other than
22MnB5 steel have been little applied, and little study has been
made on steel grades and methods in order to control the strength
and elongation of a component (e.g., strength reduction: a 980 MPa
class, elongation enhancement: 20%) to expand the range of
application beyond application to the shock-resistant members.
[0009] In passenger cars of a medium size or larger, components
such as B pillars (center pillars), rear side members, and front
side members may sometimes have both functions of a shock-resistant
area and an energy-absorbing area, considering compatibility (the
function of also protecting a collision partner in collision with a
small-sized vehicle) in lateral collision or rear collision. In
order to produce these members, the following method has been
mainly employed: super-high tensile steel having a high strength of
a 980 MPa class and high tensile steel having an elongation of a
440 MPa class are, for example, laser-welded (form a tailor welded
blank (TWB)) to press-mold the TWB in a cold state. However, in
recent years, development has been made on the technique of
forming, by hot stamping, a component with portions different from
each other in strength.
[0010] For example, Non-Patent Document 1 proposes the method for
laser-welding (forming a tailor welded blank (TWB)) 22MnB5 steel
for hot stamping and a material whose strength is not increased by
quenching in a die and then, performing hot stamping. Different
portions are formed in a component such that a tensile strength is
1500 MPa (an elongation of 6-8%) on a high-strength side (a
shock-resistant area side) and that the tensile strength is 440 MPa
(an elongation of 12% or higher) on a low-strength side (an
energy-absorbing area side). From a similar point of view, the
technique as described in Non-Patent Document 2 has been also
proposed.
[0011] In the techniques of Non-Patent Documents 1 and 2, on the
energy-absorbing area side, the tensile strength is 600 MPa or
lower, and the elongation is about 12-18%. It is required to
perform laser-welding (form the tailor welded blank (TWB)) in
advance, leading to an increase in the number of steps and an
increase in a cost. Moreover, an energy-absorbing area which
intrinsically does not need to be quenched is heated, and
therefore, these techniques are not preferable considering heat
consumption.
[0012] In addition, e.g., the techniques as described in Non-Patent
Documents 3 and 4 have been also proposed as the technique of
forming portions different from each other in strength in a
component. In the technique described in Non-Patent Document 3, the
portions different from each other in strength are formed in such a
manner that a temperature difference (distribution) is given to a
blank in a heating furnace. Since 22MnB5 steel is used as a base
material, poor robustness of the post-quenching strength against
heating in a two-phase temperature range is exhibited due to
addition of boron. Moreover, it is difficult to control the
strength on the energy-absorbing area side, and only an elongation
of about 15% can be obtained.
[0013] On the other hand, in the technique described in Non-Patent
Document 4, the portions different from each other in strength are
formed in such a manner that a cooling rate is changed in a die (by
heating part of the die with a heater or using materials different
from each other in coefficient of thermal conductivity). Since
22MnB5 steel is used as a base material, it is not rational in
terms of controlling (die cooling control) such that 22MnB5 steel
intrinsically having favorable hardenability is not quenched.
CITATION LIST
Non-Patent Document
[0014] Non-Patent Document 1: Klaus Lamprecht, Gunter Deinzer,
Anton Stich, Jurgen Lechler, Thomas Stohr, Marion Merklein,
"Thermo-Mechanical Properties of Tailor Welded Blanks in Hot Sheet
Metal Forming Processes," Proc. IDDRG2010, 2010. [0015] Non-Patent
Document 2: Usibor1500P(22MnB5)/1500
MPa8%-Ductibor500/550.about.700 MPa17% [searched on Apr. 27, 2011],
Internet
<http://www.arcelomittal.com/tailoredblanks/pre/seifware.pl>
[0016] Non-Patent Document 3: 22MnB5/above AC3/1500 MPa8%-below
AC3/Hv190Ferrite/Cementite Rudiger Erhardt and Johannes Boke,
"Industrial application of hot forming process simulation," Proc,
of 1st Int. Conf. on Hot Sheet Metal Forming of High-Performance
steel, ed. By Steinhoff, K., Oldenburg, M, Steinhoff, and Prakash,
B., pp 83-88, 2008. [0017] Non-Patent Document 4: Begona Casas,
David Latre, Noemi Rodriguez, and Isaac Valls, "Tailor made tool
materials for the present and upcoming tooling solutions in hot
sheet metal forming," Proc, of 1st Int. Conf. on Hot Sheet Metal
Forming of High-Performance steel, ed. By Steinhoff, K., Oldenburg,
M, Steinhoff, and Prakash, B., pp 23-35, 2008.
SUMMARY OF THE INVENTION
Technical Problems
[0018] The present invention has been made in view of the foregoing
situation, and is intended to provide hot press molded articles
having, without application of welding, at least regions
corresponding respectively to a shock-resistant area and an
energy-absorbing area in a single molded article and exhibiting a
high-level balance between high strength and elongation according
to each region and to provide the useful method for producing such
a hot press molded article.
Solution to Problems
[0019] The hot press molded article of the present invention
capable of accomplishing the above-described objective is a hot
press molded article formed by hot press molding of a thin steel
plate, which includes a first molding region exhibiting a metal
structure which contains 80-97 area % of martensite and 3-20 area %
of retained austenite and which has a residual structure at 5 area
% or less; and a second molding region exhibiting a metal structure
which contains 70-97 area % of bainitic ferrite, 27 area % or less
of martensite, and 3-20 area % of retained austenite and which has
a residual structure at 5 area % or less.
[0020] In the hot press molded article of the present invention,
the chemical component composition thereof is not limited. However,
examples of the chemical component composition include a chemical
component composition in which the first and second molding regions
have an identical chemical component composition, and steel of each
component region contains, in units of mass %, 0.15-0.3% of C,
0.5-3% of Si, 0.5-2% of Mn, 0.05% or less of P, 0.05% or less of S,
0.01-0.1% of Al, 0.01-1% of Cr, 0.0002-0.01% of B, [N].times.4-0.1%
of Ti, and 0.001-0.01% of N, where 0% is not inclusive for the P
and the S and [N] denotes an N content in units of %, and the steel
of each component region has a residual consisting of iron and an
inevitable impurity.
[0021] In the hot press molded article of the present invention, it
is useful that as necessary, the steel further contains, as other
element, (a) one or more selected from a group consisting of Cu,
Ni, and Mo in a total amount of 1% or less, where 0% is not
inclusive, and that the steel further contains, as other element,
(b) at least one of V or Nb in a total amount of 0.1% or less,
where 0% is not inclusive. Depending on the types of elements to be
contained, the characteristics of the hot press molded article are
further improved.
[0022] The method of the present invention is the method for
producing the above-described hot press molded article by forming a
thin steel plate so as to divide the thin steel plate into a
plurality of regions including at least first and second molding
regions, which includes after the thin steel plate is heated to a
temperature of an Acs transformation point or higher and
1000.degree. C. or lower, starting cooling at an average cooling
rate of 20.degree. C./sec or higher and molding by pressing at
least the first and second molding regions together with a die; and
terminating, in the first molding region, the molding at equal to
or lower than a temperature lower than a martensite transformation
start temperature by 50.degree. C., and performing, in the second
molding region, the cooling to a temperature range of equal to or
lower than a temperature lower than a bainite transformation start
temperature by 100.degree. C. and equal to or higher than the
martensite transformation start temperature and terminating the
molding after a lapse of a stay time of 10 seconds or longer within
the temperature range.
Effects of the Invention
[0023] According to the present invention, in the hot press molding
method, the conditions therefor are properly controlled according
to each region of the molded article. This can adjust the metal
structure of each region while a proper amount of retained
austenite is present. Moreover, the hot press molded article can be
achieved, which has a higher intrinsic ductility (residual
ductility) as compared to a conventional case of using 22MnB5
steel. Further, combination of thermal treatment conditions and a
pre-molded steel plate can properly control the strength and the
elongation according to each region.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a schematic view illustrating a die configuration
for performing hot press molding.
[0025] FIG. 2 is a schematic view illustrating a die used in an
example.
[0026] FIGS. 3(a) and 3(b) are schematic views illustrating the
shape of a press molded article formed in the example.
DESCRIPTION OF EMBODIMENTS
[0027] The inventor(s) of the present invention has conducted study
from various angles in order to realize a hot press molded article
which when a thin steel plate is heated to a predetermined
temperature to produce a molded article by hot press molding,
exhibits strength corresponding to the demand characteristics of
each of different regions and exhibits favorable ductility
(elongation) after molding.
[0028] As a result, the present invention has been accomplished
based on the following findings. When a thin steel plate is
press-molded using a die for press molding to produce a hot press
molded article, a heating temperature and the conditions for each
molding region in molding are properly controlled, and the
structure of each molding region is adjusted so as to contain 3-20
area % of retained austenite. This realizes a hot press molded
article exhibiting a strength-elongation balance according to each
molding region.
[0029] The reasons for setting the range of each structure (a basic
structure) in each molding region of the hot press molded article
of the present invention are as follows.
[0030] (Structures of First Molding Region)
[0031] High-strength martensite is used for a main structure of a
first molding region, thereby ensuring a high strength in a
particular region of the hot press molded article. From this point
of view, it is necessary that the area fraction of the martensite
is 80 area % or greater. However, when such a fraction exceeds 97
area %, the area fraction of retained austenite (a retained
austenite fraction) is insufficient, and ductility (residual
ductility) is lowered. The lower limit of the martensite fraction
is preferably 83 area % or greater (more preferably 85 area % or
greater), and the upper limit of the martensite fraction is
preferably 95 area % or less (more preferably 93 area % or
less).
[0032] The retained austenite has the effect of transforming into
the martensite during plastic deformation to increase a work
hardening rate (transformation-induced plasticity) and improving
the ductility of the molded article. In order to produce such an
effect, it is necessary that the retained austenite fraction is 3
area % or greater. A greater retained austenite fraction results in
better ductility. However, in a composition used for steel plates
for automobiles, the retained austenite which can be ensured is
limited, and the upper limit thereof is about 20 area %. The lower
limit of the retained austenite fraction is preferably 5 area % or
greater (more preferably 7 area % or greater).
[0033] In addition to the above-described structures, ferrite,
pearlite, bainite, etc. may be contained as a residual structure.
These structures are softer than the martensite, and less
contribute to the strength as compared to other structures. For
this reason, these structures are preferably contained in the
minimum possible amount. Note that these structures can be
contained up to 5 area %. The residual structure is more preferably
3 area % or less, and much more preferably 0 area %.
[0034] Since the structures of the first molding region are formed
as described above, a portion (e.g., a shock-resistant area of an
automobile component) where the strength (a tensile strength TS) is
1500 MPa or greater and the elongation (a total elongation EL) is
10% or greater can be formed.
[0035] (Structures of Second Molding Region)
[0036] Since high-strength bainitic ferrite having sufficient
ductility is used for a main structure of a second molding region,
both of a high strength and a high ductility of the hot press
molded article can be realized. From this point of view, it is
necessary that the area fraction of the bainitic ferrite (a
bainitic ferrite fraction) is 70 area % or greater. However, when
such a fraction exceeds 97 area %, the retained austenite fraction
is insufficient, and the ductility (the residual ductility) is
lowered. The lower limit of the bainitic ferrite fraction is
preferably 75 area % or greater (more preferably 80 area % or
greater), and the upper limit of the bainitic ferrite fraction is
preferably 95 area % or less (more preferably 90 area % or
less).
[0037] The high-strength martensite can be partially contained to
increase the strength of the hot press molded article. However, a
greater amount of martensite results in a lower ductility (a lower
residual ductility). From this point of view, it is necessary that
the area fraction of the martensite (the martensite fraction) is 27
area % or less. The lower limit of the martensite fraction is
preferably 5 area % or greater (more preferably 10 area % or
greater), and the upper limit of the martensite fraction is
preferably 20 area % or less (more preferably 15 area % or
less).
[0038] Because of the reasons similar to those of the first molding
region, the retained austenite fraction is 3 area % or greater and
20 area % or less. The preferable lower limit of the retained
austenite fraction is similar to that of the first molding
region.
[0039] In addition to the above-described structures, ferrite,
pearlite, bainite, etc. may be contained as a residual structure.
These structures are softer than the martensite, and less
contribute to the strength as compared to other structures. For
this reason, these structures are preferably contained in the
minimum possible amount. Note that these structures can be
contained up to 5 area %. The residual structure is more preferably
3 area % or less, and much more preferably 0 area %.
[0040] Since the structures of the second molding region are formed
as described above, a portion (e.g., an energy-absorbing area of
the automobile component) where the strength (the tensile strength
TS) is 1100 MPa or greater and the elongation (the total elongation
EL) is 15% or greater can be formed.
[0041] The molded article of the present invention includes at
least the first and second molding regions, but does not
necessarily include only two molding regions. The molded article of
the present invention may further include a third or fourth molding
region. These molding regions can be formed according to a
production method described later.
[0042] In production of the hot press molded article of the present
invention, a thin steel plate (having the same chemical component
composition as that of the molded article) may be formed so as to
be divided into a plurality of regions including at least the first
and second molding regions. Specifically, the above-described thin
steel plate may be heated to a temperature of an Acs transformation
point or higher and 1000.degree. C. or lower. Then, for at least
the first and second molding regions, cooling at an average cooling
rate of 20.degree. C./sec or higher and molding may begin by
pressing of the first and second molding regions together with the
die. In the first molding region, molding may be terminated at
equal to or lower than a temperature (hereinafter sometimes
referred to as "Ms point--50.degree. C.") lower than a martensite
transformation start temperature by 50.degree. C. In the second
molding region, cooling may be performed to a temperature range of
equal to or lower than a temperature (hereinafter sometimes
referred to as "Bs point--100.degree. C.") lower than a bainite
transformation start temperature by 100.degree. C. and equal to or
higher than the martensite transformation start temperature (the Ms
point), and molding may be terminated after a lapse of a stay time
of 10 seconds or longer within the above-described temperature
range. The reasons for setting each requirement in this method are
as follows. Note that the phrase of "molding is terminated"
basically means the state at a lower dead point in molding (the
point at which a punch tip end is positioned innermost: the state
illustrated in FIG. 1). However, when the die needs to be cooled to
a predetermined temperature in such a state, the phrase of "molding
is terminated" also means the state until the die is detached after
the die is maintained cooled.
[0043] According to the above-described method, the steel plate is
divided into at least two molding regions (e.g., a high-strength
region and a low-strength region), and the production conditions
are controlled according to each region. Thus, the molded article
exhibiting the strength-ductility balance according to each region
can be obtained. The production conditions for forming each region
will be described.
[0044] (Production Conditions for First Molding Region
(High-Strength Region))
[0045] In order to properly adjust the structures of the hot press
molded article, it is necessary to control the heating temperature
within a predetermined range. By proper control of the heating
temperature, a predetermined amount of retained austenite can be
ensured at a subsequent cooling step. Meanwhile, the first molding
region can transform into the structure mainly containing the
martensite. As a result, the final hot press molded article can be
formed with desired structures. If the heating temperature of the
thin steel plate is lower than the Ac.sub.3 transformation point, a
sufficient amount of austenite cannot be obtained in heating, and a
predetermined amount of retained austenite cannot be ensured at the
final structure (the structures of the molded article). On the
other hand, if the heating temperature of the thin steel plate
exceeds 1000.degree. C., the particle size of the austenite
increases in heating, and the martensite transformation start
temperature (the Ms point) and a martensite transformation end
temperature (an Mf point) increase. Thus, the retained austenite
cannot be ensured in quenching, and as a result, favorable
moldability cannot be achieved. The heating temperature is
preferably (Acs transformation point+50.degree. C.) or higher and
950.degree. C. or lower.
[0046] The cooling conditions in molding and a molding end
temperature need to be properly controlled according to each
region. First, in a steel plate region (hereinafter sometimes
referred to as a "first steel plate region") corresponding to the
first molding region of the molded article, it is necessary that an
average cooling rate of 20.degree. C./sec or higher is ensured at
the die in molding and that molding is terminated at a temperature
of (Ms point--50.degree. C.) or lower.
[0047] In order that the austenite formed at the above-described
heating step has a desired structure (a structure mainly containing
the martensite) with generation of the structures such as ferrite,
pearlite, and bainite being blocked, the average cooling rate in
molding and the molding end temperature need to be properly
controlled. From this point of view, the average cooling rate in
molding is 20.degree. C./sec or higher, and the molding end
temperature is (Ms point--50.degree. C.) or lower. In particular,
in the case where a steel plate with a high Si content is targeted,
the mixed structure of the martensite and the retained austenite
can be formed by cooling under the above-described conditions. The
average cooling rate in molding is preferably 30.degree. C./sec or
higher (more preferably 40.degree. C./sec or higher).
[0048] While the molding end temperature in the first steel plate
region is cooled to a room temperature at the above-described
average cooling rate, molding may be terminated. However, after the
molding end temperature is cooled to (Ms point--50.degree. C.) or
lower (preferably to a temperature of Ms point--50.degree. C.),
cooling (two-step cooling) may be performed to 200.degree. C. or
lower at an average cooling rate of 20.degree. C./sec or lower.
Such addition of the cooling step allows thickening of carbon of
the martensite in untransformed austenite, and therefore, the
amount of retained austenite can be increased. In such two-step
cooling, the average cooling rate in a second cooling step is
preferably 10.degree. C./sec or lower (more preferably 5.degree.
C./sec or lower).
[0049] (Production Conditions for Second Molding Region
(Low-Strength Region))
[0050] On the other hand, in order to properly adjust the
structures of the second molding region of the hot press molded
article, it is necessary to control the heating temperature of a
steel plate region (hereinafter sometimes referred to as a "second
steel plate region") corresponding to the second molding region
within a predetermined range. By proper control of the heating
temperature, a predetermined amount of retained austenite can be
ensured at a subsequent cooling step. Meanwhile, the second molding
region can transform into the structure mainly containing the
bainitic ferrite. As a result, the final hot press molded article
can be formed with desired structures. If the heating temperature
of the thin steel plate is lower than the Acs transformation point,
a sufficient amount of austenite cannot be obtained in heating, and
a predetermined amount of retained austenite cannot be ensured at
the final structure (the structures of the molded article). On the
other hand, if the heating temperature of the thin steel plate
exceeds 1000.degree. C., the state similar to that of the first
steel plate region is brought about (the preferable temperature
range is also similar to that of the first steel plate region).
[0051] In order that the austenite formed at the above-described
heating step has a desired structure (a structure mainly containing
the bainitic ferrite) with generation of the structures such as
ferrite and pearlite being blocked, the average cooling rate in
molding and a cooling end temperature need to be properly
controlled. From this point of view, the average cooling rate in
molding needs to be 20.degree. C./sec or higher, and the cooling
end temperature needs to be (Bs point--100.degree. C.) or lower and
the martensite transformation start temperature (the Ms point) or
higher (such a temperature range is hereinafter sometimes referred
to as a "cooling rate change temperature"). The average cooling
rate is preferably 30.degree. C./sec or higher (more preferably
40.degree. C./sec or higher).
[0052] Cooling is temporarily stopped within the above-described
temperature range (the cooling rate change temperature), and such a
state stays for 10 seconds or longer within the above-described
temperature range (i.e., a temperature range of (Bs
point--100.degree. C.) or lower and the martensite transformation
start temperature Ms point or higher). In this manner, bainite
transformation proceeds in supercooled austenite so that the
structure mainly containing the bainitic ferrite can be formed. A
stay time in this state is preferably 50 seconds or longer (more
preferably 100 seconds or longer). If the stay time is too long,
the austenite begins decomposing, and the retained austenite
fraction cannot be ensured. For this reason, the stay time is
preferably 1000 seconds or shorter (more preferably 800 seconds or
shorter).
[0053] As long as the stay step as described above is performed
within the above-described temperature range, the stay step may be
any of an isothermal holding step, a monotonic cooling step, or a
re-heating step. Moreover, regarding the relationship between the
above-described stay step and molding, the stay step as described
above may be added after molding is terminated, or the holding step
may be added within the above-described temperature range in the
middle of termination of molding. After molding is terminated as
described above, cooling may be performed to the room temperature
by cold heat radiation at a proper cooling rate.
[0054] Control of the average cooling rate in molding can be
achieved by a unit such as (a) a unit for controlling the
temperature of the die (the cooling media illustrated in FIG. 1)
and (b) a unit for controlling the coefficient of thermal
conductivity of the die (the same applies to cooling in the
later-described method). In the method of the present invention,
the cooling conditions in molding vary according to each steel
plate region. The control units such as the units (a) and (b) may
be provided separately in a single die, and cooling control may be
performed corresponding to each steel plate region in the single
die.
[0055] The method for producing the hot press molded article
according to the present invention is applicable not only to the
case (the direct method) of producing a hot press molded article in
a simple shape as illustrated in FIG. 1, but also to the case of
producing a molded article in a relatively-complicated shape. Note
that in the case of the complicated component shape, it might be
difficult to form the final shape of the product by a single
process of press molding. In this case, the method (called an
"indirect method") for performing cold press molding as a preceding
process of hot press molding can be employed. In this method, a
portion which is difficult to be molded is pre-molded into an
approximate shape by cold working, and the other portion is
hot-press-molded. According to such a method, e.g., when a
recessed-raised portion (ridge portion) is formed at three sections
of a molded article, the first and second recessed-raised portions
are formed by cold press molding, and then, the third
recessed-raised portion is formed by hot press molding.
[0056] The present invention is intended for a hot press molded
article made of a high-strength steel plate. Although the steel
grade of such a steel plate may include steel grades of
high-strength steel plates with typical chemical component
compositions, C, Si, Mn, P, S, Al, Cr, B, Ti, and N are preferably
adjusted to suitable ranged. From this point of view, the
preferable ranges of these chemical components and the reasons for
limiting these ranges are as follows.
[0057] (C: 0.15-0.3%)
[0058] C is an essential element (the low-strength region) in
strength improvement made by micronizing the bainitic ferrite
generated at the cooling process and increasing a dislocation
density in the bainitic ferrite. Moreover, C is also an essential
element (the high-strength region) in control of the strength of
the martensite structure. A less C content results in insufficient
strength even in full martensite. C is an element heavily involved
with hardenability. An increase in the C content produces the
effect of reducing formation of other soft structures such as
ferrite during cooling after heating. Further, C is also an element
necessary for ensuring the retained austenite. If the C content is
less than 0.15%, the bainite transformation start temperature Bs
increases, and a high strength of the hot press molded article
cannot be ensured. On the other hand, if the C content becomes
excess and exceeds 0.3%, the strength becomes too high, and
favorable ductility cannot be obtained. The lower limit of the C
content is more preferably 0.18% or greater (much more preferably
0.20% or greater), and the upper limit of the C content is more
preferably 0.27% or less (much more preferably 0.25% or less).
[0059] (Si: 0.5-3%)
[0060] Si produces the effect of forming the retained austenite in
quenching. Moreover, Si also produces the effect of increasing, by
solid solution strengthening, the strength without lowering the
ductility much. If a Si content is less than 0.5%, a predetermined
amount of retained autstenite cannot be ensured, and favorable
ductility cannot be obtained. On the other hand, if the Si content
becomes excess and exceeds 3%, the degree of solid solution
strengthening becomes too high, and the ductility is significantly
lowered. The lower limit of the Si content is more preferably 1.15%
or greater (much more preferably 1.20% or greater), and the upper
limit of the Si content is more preferably 2.7% or less (much more
preferably 2.5% or less).
[0061] (Mn: 0.5-2%)
[0062] Mn is a useful element in reduction of formation of ferrite
and pearlite during primary cooling. Moreover, Mn is also a useful
element in micronizing the structure unit of the bainitic ferrite
by lowering (Bs point--100.degree. C.), or in enhancement of the
strength of the bainitic ferrite by increasing a dislocation
density in the bainitic ferrite. Further, Mn is also a useful
element in stabilizing the austenite to increase the amount of
retained austenite. In order to produce these effects, Mn is
preferably contained at 0.5% or greater. Only considering
characteristics, a great Mn content is preferable. However, since
the cost for alloy addition increases, the Mn content is preferably
2% or less. Moreover, with significant improvement of the strength
of the austenite, a hot rolling load increases, and it is difficult
to produce a steel plate. For this reason, it is not preferable
that the Mn content exceeds 2%, considering productivity. The lower
limit of the Mn content is more preferably 0.7% or greater (much
more preferably 0.9% or greater), and the upper limit of the Mn
content is more preferably 1.8% or less (much more preferably 1.6%
or less).
[0063] (P: 0.05% or Less (0% is not Inclusive))
[0064] Although P is an element inevitably contained in steel, P
lowers the ductility. For this reason, P is preferably reduced as
much as possible. However, significant reduction results in an
increase in a steel production cost, and in production, it is
difficult to make a P content 0%. Thus, the P content is preferably
0.05% or less (0% is not inclusive). The upper limit of the P
content is more preferably 0.045% or less (much more preferably
0.040% or less).
[0065] (S: 0.05% or Less (0% is not Inclusive))
[0066] As in P, S is also an element inevitably contained in steel,
and lowers the ductility. For this reason, S is preferably reduced
as much as possible. However, significant reduction results in an
increase in the steel production cost, and in production, it is
difficult to make an S content 0%. Thus, the S content is
preferably 0.05% or less (0% is not inclusive). The upper limit of
the S content is more preferably 0.045% or less (much more
preferably 0.040% or less).
[0067] (Al: 0.01-0.1%)
[0068] Al is useful as a deoxidizing element, and is also useful in
ductility improvement because Al fixes, as AlN, solid liquid N
present in steel. In order to effectively produce these effects, an
Al content is preferably 0.01% or greater. However, if the Al
content becomes excess and exceeds 0.1%, Al.sub.2O.sub.3 is
excessively generated, and the ductility is lowered. Note that the
lower limit of the Al content is more preferably 0.013% or greater
(much more preferably 0.015% or greater), and the upper limit of
the Al content is more preferably 0.08% or less (much more
preferably 0.06% or less).
[0069] (Cr: 0.01-1%)
[0070] Cr has the effect of reducing ferrite transformation and
pearlite transformation. Thus, Cr is an element preventing
formation of ferrite and pearlite during cooling and contributing
to ensuring the retained austenite. In order to produce these
effects, Cr is preferably contained at 0.01% or greater. Even if Cr
is excessively contained at greater than 1%, a cost increases.
Moreover, since Cr significantly increases the strength of the
austenite, a hot rolling load increases, and it is difficult to
produce a steel plate. For this reason, it is not preferable that a
Cr content exceeds 1%, considering the productivity. The lower
limit of the Cr content is more preferably 0.02% or greater (much
more preferably 0.05% or greater), and the upper limit of the Cr
content is more preferably 0.8% or less (much more preferably 0.5%
or less).
[0071] (B: 0.0002-0.01%)
[0072] B has the effect of increasing the hardenability and
reducing ferrite transformation and pearlite transformation. Thus,
B is an element preventing formation of ferrite and pearlite during
primary cooling after heating and contributing to ensuring the
bainitic ferrite and the retained austenite. In order to produce
these effects, B is preferably contained at 0.0002% or greater. If
B is excessively contained, at greater than 0.01%, the effect
thereof is saturated. The lower limit of a B content is more
preferably 0.0003% or greater (much more preferably 0.0005% or
greater), and the upper limit of the B content is more preferably
0.008% or less (much more preferably 0.005% or less).
[0073] (Ti: [N].times.4-0.1%)
[0074] Ti produces the effect of improving the hardenability by
fixing N and maintaining B at a solid solution state. In order to
produce such an effect, Ti is preferably contained at at least
equal to or greater than four times as great as an N content [N].
However, if a Ti content becomes excessive and exceeds 0.1%, a
great amount of TiC is formed. In addition, the strength increases
due to precipitation strengthening, but the ductility is lowered.
The lower limit of the Ti content is more preferably 0.05% or
greater (much more preferably 0.06% or greater), and the upper
limit of the Ti content is more preferably 0.09% or less (much more
preferably 0.08% or less).
[0075] (N: 0.001-0.01%)
[0076] Since N is an element capable of reducing a hardenability
improvement effect by fixing B as BN, N is preferably reduced as
much as possible. However, reduction of N is limited in an actual
process, and for this reason, the lower limit of the N content is
preferably 0.001%. If the N content becomes excessive, coarse TiN
particles are formed, and such TiN functions as a starting point
for destruction to lower the ductility. For this reason, the upper
limit of the N content is preferably 0.01%. The upper limit of the
N content is more preferably 0.008% or less (much more preferably
0.006% or less).
[0077] The basic chemical components in the press molded article of
the present invention are as described above. The residual
substantially consists of iron. Note that the phrase of
"substantially consists of iron" can means, in addition to iron,
not only a slight amount of components (e.g., Mg, Ca, Sr, Ba, REM
such as La, and carbide-forming elements such as Zr, Hf, Ta, W, and
Mo) not inhibiting the characteristics of the steel material of the
present invention, but also inevitable impurities (e.g., O and H)
other than P and S.
[0078] It is useful for the press molded article of the present
invention to further contain, as necessary, (a) one or more
selected from the group consisting of Cu, Ni, and Mo in the total
amount of 1% or less (0% is not inclusive) and (b) at least one of
V or Nb in the total amount of 0.1% or less (0% is not inclusive),
for example. Depending on the types of the elements to be
contained, the characteristics of the hot press molded article are
further improved. The preferable ranges of these contained elements
and the reasons for limiting these ranges are as follows.
[0079] (One or More Selected from the Group Consisting of Cu, Ni,
and Mo in the Total Amount of 1% or Less (0% is not Inclusive))
[0080] Since Cu, Ni, and Mo reduce ferrite transformation and
pearlite transformation, Cu, Ni, and Mo effectively function to
prevent formation of ferrite and pearlite during primary cooling
and to ensure the retained austenite. In order to produce these
effects, Cu, Ni, and Mo are preferably contained at the total
amount of 0.01% or greater. Only considering characteristics, a
great content is preferable. However, since the cost for alloy
addition increases, the total amount is preferably 1% or less.
Moreover, since Cu, Ni, and Mo have the effect of significantly
increasing the strength of the austenite, a hot rolling load
increases, and it is difficult to produce a steel plate. For this
reason, it is preferable that the total amount is 1% or less,
considering the productivity. The lower limit of the total content
of these elements is more preferably 0.05% or greater (much more
preferably 0.06% or greater), and the upper limit of the total
content of these elements is more preferably 0.9% or less (much
more preferably 0.8% or less).
[0081] (At Least One of V or Nb in the Total Amount of 0.1% or Less
(0% is not Inclusive))
[0082] V and Nb have the effect of forming fine carbide particles
and micronizing a structure by a pinning effect. In order to
produce these effects, at least one of V or Nb is preferably
contained at the total amount of 0.001% or greater. However, if the
content of these elements becomes excess, coarse carbide particles
are formed, this serves as a starting point for destruction to
lower the ductility. For this reason, the total content is
preferably 0.1% or less. The lower limit of the total content of
these elements is more preferably 0.005% or greater (much more
preferably 0.008% or greater), and the upper limit of the total
content of these elements is more preferably 0.08% or less (much
more preferably 0.06% or less).
[0083] According to the present invention, the press molding
conditions (the heating temperature and the cooling rate according
to each steel plate region) can be properly adjusted to control the
characteristics, such as the strength and the elongation, of the
molded article according to each molding region. In addition, the
hot press molded article can be obtained with a high ductility (a
high residual ductility). Thus, the present invention is also
applicable to a portion (e.g., a member requiring both of shock
resistance and energy absorption reduction) that has been difficult
to apply in a conventional hot press molded article, and is
significantly useful in enlargement of the scope of application of
the hot press molded article. Further, the molded article of the
present invention has a higher residual ductility as compared to
that of a molded article whose structure is adjusted by typical
annealing performed after cold press molding.
[0084] Advantages of the present invention will be more
specifically described below with reference to an example, but the
later-described example does not limit the scope of the present
invention. In light of the description made above and later, any
design changes may be made within the technical scope of the
present invention.
[0085] This application claims the benefit of and priority to
Japanese Patent Application No. 2013-032615 filed on Feb. 21, 2013,
the disclosure of which is hereby incorporated by reference in its
entirety in this application.
Example
[0086] A steel material having a chemical component composition
shown in Table 1 below was vacuum-fused, thereby forming an
experimental slab. Then, after hot rolling was performed, the slab
was cooled and rolled up. Further, cold rolling was performed,
thereby forming a thin steel plate. Note that an AC3 transformation
point, an Ms point, and (Bs point--100.degree. C.) as shown in
Table 1 were obtained using Expressions (1)-(3) described below
(e.g., see "The Physical Metallurgy of Steels," Maruzen Co., Ltd.,
1985).
AC.sub.3 Transformation Point (.degree.
C.)=910-203.times.[C].sup.1/2+44.7.times.[Si]-30.times.[Mn]+700.times.[P]-
+400.times.[Al]+400.times.[Ti]+104.times.[V]-11.times.[Cr]+31.5.times.[Mo]-
-20.times.[Cu]-15.2.times.[Ni] (1)
Ms point (.degree.
C.)=550-361.times.[C]-39.times.[Mn]-10.times.[Cu]-17.times.[Ni]-20.times.-
[Cr]-5.times.[Mo]+30.times.[Al] (2)
Bs point (.degree.
C.)=830-270.times.[C]-90.times.[Mn]-37.times.[Ni]-70.times.[Cr]-83.times.-
[Mo] (3)
[0087] Note that [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. If an element(s) shown as
each term in Expressions (1)-(3) is not contained, calculation is
made without taking such an element(s) into consideration.
TABLE-US-00001 TABLE 1 Ac.sub.3 Bs Point Steel Chemical Component
Composition* (mass %) Transformation Ms Point -100.degree. C. Grade
C Si Mn P S Cr Al Ti B N Point (.degree. C.) (.degree. C.)
(.degree. C.) A 0.232 1.19 1.41 0.014 0.0021 0.21 0.053 0.027
0.0033 0.0047 863 409 526 B 0.232 0.18 1.41 0.014 0.0021 0.21 0.053
0.027 0.0033 0.0047 817 409 526 *Residual: iron and inevitable
impurities other than P and S
[0088] While the heating temperature was changed in each steel
plate region of the obtained steel plate, molding and cooling
treatment were performed. Specifically, press molding was performed
using a bending die having a hat channel shape (a HAT shape)
illustrated in FIG. 2. Note that in FIG. 2, a reference numeral
"10" denotes an upper die (equivalent to the punch 1 illustrated in
FIG. 1), and a reference numeral "11" denotes a lower die
(equivalent to the die 2 illustrated in FIG. 1). Moreover, in this
die, a pad 12 is provided, and is configured such that press
molding is performed with a steel plate 4 being interposed between
the pad 12 and the upper 11 while pressure (pad pressure) is being
applied (at a pad pressure of 9800 N).
[0089] The heating temperature and the average cooling rate in each
steel plate region are shown in Table 2 below (the molding end
temperature (a die-detaching temperature) was 200.degree. C. in any
of the regions). The steel plate size in molding and cooling was
220 mm.times.500 mm (a plate thickness of 1.4 mm) (the area ratio
between the first steel plate region and the second steel plate
region was 1:1). The shape of a molded press molded article is
illustrated in FIGS. 3(a) and 3(b) (FIG. 3(a) is a perspective
view, and FIG. 3(b) is a view schematically illustrating the cross
section). In FIG. 3(a), a reference numeral "15" denotes the first
steel plate region (corresponding to the first molding region of
the molded article), and a reference numeral "16" denotes the
second steel plate region (corresponding to the second molding
region of the molded article). Note that "Average Cooling Rate 1"
of the first steel plate region as shown in Table 2 is an average
cooling rate from the heating temperature to (Ms point--50.degree.
C.) or lower (the molding end temperature), and "Average Cooling
Rate 2" of the first steel plate region is an average cooling rate
from the molding end temperature to 200.degree. C. or lower.
TABLE-US-00002 TABLE 2 Production Conditions Second Steel Plate
Region Average First Steel Plate Region Cooling Average Steel Plate
Average Average Rate Cooling Rate Cooling Rate Stay Time at Heating
Cooling Molding End Cooling (.degree. C./sec) Change (.degree.
C./sec) [Bs -100.degree. C. Test Steel Temperature Rate 1
Temperature Rate 2 in Primary Temperature Retention in Secondary to
Ms Point] No. Grade (.degree. C.) (.degree. C./sec) (.degree. C.)
(.degree. C./sec) Cooling (.degree. C.) Time (sec) Cooling (sec) 1
A 930 40 200 15 40 N/A N/A N/A 3 2 A 930 40 200 15 40 480 0 5 15 3
A 930 40 200 15 40 420 0 20 5 4 A 930 40 200 15 40 420 10 50 15 5 A
930 40 200 15 40 600 10 5 23.4 6 B 930 40 200 15 40 480 N/A 5
15
[0090] For each steel plate subjected to the foregoing processes
(heating, molding, and cooling), a tensile strength (TS) and an
elongation (a total elongation EL) were measured in the following
manner, and metal structures (the fraction of each structure) were
observed in the following manner.
[0091] (Tensile Strength (TS) and Elongation (Total Elongation EL))
A tension test was conducted using a JIS 5 test piece, thereby
measuring the tensile strength (TS) and the elongation (the total
elongation EL). At this point, a strain rate in the tension test
was 10 mm/sec. In the present invention, evaluation was made as
"successful" when (a) in the first region, the tensile strength
(TS) satisfies 1500 MPa or greater and the elongation (the total
elongation EL) satisfies 10% or greater and (b) in the second
region, the tensile strength (TS) satisfies 1100 MPa or greater and
the elongation (the total elongation EL) satisfies 15% or
greater.
[0092] (Observation of Metal Structures (Fraction of Each
Structure))
[0093] (1) For the structures of martensite, ferrite, and bainitic
ferrite in the steel plate, the steel plate was corroded with
nital, and then, the fraction (the area ratio) of each structure
was obtained by scanning electron microscope (SEM) observation (a
magnification of 1000-power or 2000-power) with the ferrite and the
bainitic ferrite being discriminated from each other.
[0094] (2) A retained austenite fraction (an area ratio) in the
steel plate was measured in such a manner that after the steel
plate was ground to the quarter of the thickness of the steel
plate, chemical polishing was performed, and then, X-ray
diffractometry was performed (e.g., ISJJ Int., Vol. 33, 1933, No.
7, P. 776).
[0095] (3) The area ratio of the martensite (as-quenched
martensite) was measured as follows. The steel plate was subjected
to Repera corrosion. Then, the area ratio of a white contrast as
the mixed structure of the as-quenched martensite and the retained
austenite was measured by SEM observation. The retained austenite
fraction obtained by X-ray diffractometry was subtracted from the
area ratio of the white contrast, thereby measuring the as-quenched
martensite fraction.
[0096] The measurement results of the metal structures in each
region of the molded article are shown in Table 3 below, and the
mechanical characteristics in each region of the molded article are
shown in Table 4 below.
TABLE-US-00003 TABLE 3 Structures of Molded Article (area %) First
Molding Region Second Molding Region Test Steel Retained Other
Bainitic As-Quenched Retained Other No. Grade Martensite Austenite
Structure Ferrite Martensite Austenite Structure 1 A 95 5 0 0 95 5
0 2 A 95 5 0 85 7 8 0 3 A 95 5 0 90 8 2 0 4 A 95 5 0 87 4 9 0 5 A
95 5 0 40 30 10 20 (Ferrite) 6 B 100 0 0 95 5 0 0
TABLE-US-00004 TABLE 4 Mechanical Characteristics First Molding
Region Second Molding Region Tensile Tensile Test Steel Strength
Elongation Strength Elongation No. Grade TS (MPa) EL (%) TS (MPa)
EL (%) 1 A 1550 10 1515 10 2 A 1550 10 1203 17 3 A 1550 10 1220 13
4 A 1550 10 1198 18 5 A 1550 10 980 10 6 B 1545 7 1098 13
[0097] The following consideration was made based on these results.
Test Nos. 2 and 4 were examples satisfying the requirements defined
in the present invention, and the results show that in each of Test
Nos. 2 and 4, the molded article exhibiting high performance, i.e.,
a high strength-ductility balance, in each region was obtained.
[0098] On the other hand, Test Nos. 1, 3, 5, and 6 were comparative
examples not satisfying any of the requirements defined in the
present invention, and any of the characteristics was lowered. That
is, in Test No. 1, the stay time at (Bs--100.degree. C.) to the Ms
point was short in the second steel plate region, the fraction of
the bainitic ferrite in the structure of the second region of the
molded article was low, the fraction of the martensite in the
structure of the second region of the molded article was high, and
only a low elongation (a low total elongation EL) was obtained in
the second region.
[0099] In Test No. 3, the cooling rate change temperature was
proper in the second steel plate region, but the stay time at
(Bs--100.degree. C.) to the Ms point was short. Although a proper
fraction of the bainitic ferrite in the structure of the second
region of the molded article was ensured, the amount of retained
austenite was small. Thus, only a low elongation (a low total
elongation EL) was obtained in the second region.
[0100] In Test No. 5, the cooling rate change temperature was high
in the second steel plate region. The ferrite was formed, and the
amount of bainitic ferrite was not ensured. Thus, only a low
strength and a low elongation (a low total elongation EL) were
obtained in the second region. In Test No. 6, a Si content was
small in a steel component. Thus, even if the cooling conditions
were proper, the amount of retained austenite was not formed in any
of the regions of the molded article, and only a low elongation (a
low total elongation EL) was obtained (the strength in the second
region was also low).
INDUSTRIAL APPLICABILITY
[0101] The press molded article of the present invention includes
the first molding region exhibiting the metal structure which
contains 80-97 area % of the martensite and 3-20 area % of the
retained austenite and which has the residual structure at 5 area %
or less; and the second molding region exhibiting the metal
structure which contains 70-97 area % of the bainitic ferrite, 27
area % or less of the martensite, and 3-20 area % of the retained
austenite and which has the residual structure at 5 area % or less.
As a result, at least regions corresponding respectively to a
shock-resistant area and an energy-absorbing area can be, without
application of welding, formed in a single molded article, and a
high-level balance between high strength and elongation can be
achieved according to each region.
EXPLANATION OF REFERENCE NUMERALS
[0102] 1 Punch [0103] 2 Die [0104] 3 Blank Holder [0105] 4 Steel
Plate (Blank)
* * * * *
References