U.S. patent application number 14/113761 was filed with the patent office on 2014-02-27 for hot press-formed product, process for producing same, and thin steel sheet for hot press forming.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). The applicant listed for this patent is Shushi Ikeda, Toshio Murakami, Junya Naitou, Keisuke Okita. Invention is credited to Shushi Ikeda, Toshio Murakami, Junya Naitou, Keisuke Okita.
Application Number | 20140056753 14/113761 |
Document ID | / |
Family ID | 47296191 |
Filed Date | 2014-02-27 |
United States Patent
Application |
20140056753 |
Kind Code |
A1 |
Naitou; Junya ; et
al. |
February 27, 2014 |
HOT PRESS-FORMED PRODUCT, PROCESS FOR PRODUCING SAME, AND THIN
STEEL SHEET FOR HOT PRESS FORMING
Abstract
There is provided a hot press-formed product, including a steel
sheet formed by a hot press-forming method, and having a metallic
structure that contains ferrite at 30% to 80% by area, bainitic
ferrite at lower than 30% by area (not including 0% by area),
martensite at 30% by area or lower (not including 0% by area), and
retained austenite at 3% to 20% by area, whereby balance between
strength and elongation can be controlled in a proper range and
high ductility can be achieved.
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 |
Naitou; Junya
Murakami; Toshio
Ikeda; Shushi
Okita; Keisuke |
Kobe-shi
Kobe-shi
Nagoya-shi
Kobe-shi |
|
JP
JP
JP
JP |
|
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi, Hyogo
JP
|
Family ID: |
47296191 |
Appl. No.: |
14/113761 |
Filed: |
June 8, 2012 |
PCT Filed: |
June 8, 2012 |
PCT NO: |
PCT/JP2012/064850 |
371 Date: |
October 24, 2013 |
Current U.S.
Class: |
420/90 ; 420/104;
420/106; 420/119; 420/124; 420/8; 72/342.5 |
Current CPC
Class: |
B21D 22/022 20130101;
C22C 38/06 20130101; B21D 37/16 20130101; C21D 1/673 20130101; B21D
22/22 20130101; C22C 38/32 20130101; C22C 38/08 20130101; C22C
38/04 20130101; C22C 38/18 20130101; C21D 2211/002 20130101; C21D
2211/005 20130101; C22C 38/12 20130101; C22C 38/02 20130101; C22C
38/28 20130101; C21D 2211/008 20130101; C22C 38/14 20130101; C22C
38/20 20130101; C22C 38/22 20130101; C22C 38/001 20130101; C22C
38/16 20130101; C21D 9/46 20130101; C22C 38/002 20130101 |
Class at
Publication: |
420/90 ;
72/342.5; 420/106; 420/119; 420/8; 420/104; 420/124 |
International
Class: |
B21D 22/02 20060101
B21D022/02; C22C 38/02 20060101 C22C038/02; C22C 38/04 20060101
C22C038/04; C22C 38/06 20060101 C22C038/06; C22C 38/08 20060101
C22C038/08; C22C 38/32 20060101 C22C038/32; C22C 38/14 20060101
C22C038/14; C22C 38/16 20060101 C22C038/16; C22C 38/20 20060101
C22C038/20; C22C 38/22 20060101 C22C038/22; C22C 38/28 20060101
C22C038/28; C22C 38/00 20060101 C22C038/00; C22C 38/12 20060101
C22C038/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 10, 2011 |
JP |
2011-130636 |
Claims
1. A hot press-formed product, comprising a steel sheet formed by a
hot press-forming method, and having a metallic structure that
contains ferrite at 30% to 80% by area, bainitic ferrite at lower
than 30% by area (not including 0% by area), martensite at 30% by
area or lower (not including 0% by area), and retained austenite at
3% to 20% by area.
2. The hot press-formed product according to claim 1, having the
following chemical element composition: C at 0.1% to 0.3% (where
"%" means "% by mass", and the same applies to the below with
respect to the chemical element composition); Si at 0.5% to 3%; Mn
at 0.5% to 2%; P at 0.05% or lower (not including 0%); S at 0.05%
or lower (not including 0%); Al at. 0.01% to 0.1%; and N at 0.001%
to 0.01%, and the remainder consisting of iron and unavoidable
impurities.
3. The hot press-formed product according to claim 2, further
comprising, as additional elements, B at 0.01% or lower (not
including 0%) and Ti at 0.1% or lower (not including 0%).
4. The hot press-formed product according to claim 2, further
comprising, as additional elements, one or more selected from the
group consisting of Cu, Ni, Cr, and Mo at 1% or lower (not
including 0%) in total.
5. The hot press-formed product according to claim 2, further
comprising, as additional elements, V and/or Nb at 0.1% or lower
(not including 0%) in total.
6. A process for producing a hot press-formed product as set forth
in claim 1, comprising: heating a hot-rolled steel sheet having a
metallic structure that contains ferrite at 50% by area or higher,
or heating a cold-rolled steel sheet at a reduction of 30% or
higher, to a temperature not lower than Ac.sub.1 transformation
point and not higher than (Ac.sub.1 transformation
point.times.0.3+Ac.sub.3 transformation point.times.0.7); and then
starting the forming of the hot-rolled steel sheet or the
cold-rolled steel sheet with a press tool to produce the hot
press-formed product, during which forming an average cooling rate
of 20.degree. C./sec or higher is kept in the press tool, and which
forming is finished at a temperature not higher than (bainite
transformation starting temperature Bs-100.degree. C.).
7. A process for producing a hot press-formed product as set forth
in claim 1, comprising: heating a thin steel sheet to a temperature
not lower than Ac.sub.3 transformation point and not higher than
1000.degree. C.; cooling the thin steel sheet to a temperature not
higher than 700.degree. C. and not lower than 500.degree. C. at an
average cooling rate of 10.degree. C./sec or lower; and then
starting the forming of the thin steel sheet with a press tool to
produce the hot press-formed product, during which forming an
average cooling rate of 20.degree. C./sec or higher is kept in the
press tool, and which forming is finished at a temperature not
higher than (bainite transformation starting temperature
Bs-100.degree. C.).
8. The process according to claim 6, wherein the forming finishing
temperature is controlled in a temperature range of not higher than
(bainite transformation starting temperature Bs-100.degree. C.) and
not lower than martensite transformation starting temperature Ms,
in which temperature range the steel sheet is retained for 10
seconds or longer, followed by the forming.
9. A thin steel sheet for hot press forming, which is intended for
use in producing a hot press-formed product as set forth in claim
1, and which is a hot-rolled steel sheet having a metallic
structure that contains ferrite at 50% by area or higher, or a
cold-rolled steel sheet at a reduction of 30% or higher.
Description
TECHNICAL FIELD
[0001] The present invention relates to a hot press-formed product
required to have high strength, such as used for structural members
of automobile parts, a process for producing the same, and a thin
steel sheet for hot press forming. In particular, the present
invention relates to a hot press-formed product that can be
provided with a prescribed shape and at the same time heat treated
to have prescribed strength when a preheated steel sheet (blank) is
formed into the prescribed shape, a process for producing such a
hot press-formed product, and a thin steel sheet for hot press
forming.
BACKGROUND ART
[0002] As one of the measures for fuel economy improvement of
automobiles beginning from global environmental problems,
automobile body lightening has proceeded, and steel sheets to be
used for automobiles need to be strengthened as highly as possible.
However, highly strengthening of steel sheets for automobile
lightening lowers elongation EL or r value (Lankford value),
resulting in the deterioration of press formability or shape
fixability.
[0003] To solve such a problem, a hot press-forming method has been
adopted for production of parts, in which method a steel sheet is
heated to a prescribed temperature (e.g., a temperature for change
in austenite phase) to lower its strength (i.e., make it easily
formable) and then formed with a press tool at a temperature (e.g.,
room temperature) lower than that of the thin steel sheet, whereby
the steel sheet is provided with a shape and at the same time heat
treated by rapid cooling (quenching), which makes use of a
temperature difference between both, to secure its strength after
forming.
[0004] According to such a hot pressing method, a steel sheet is
formed in a state of low strength, and therefore, the steel sheet
has decreased springback (favorable shape fixability). In addition,
the use of a material having excellent hardenability, to which
alloy elements such as Mn and B have been added, thereby obtaining
a strength of 1500 MPa class in terms of tensile strength by rapid
cooling. Such a hot press-forming method has been called with
various names, in addition to a hot press method, such as a hot
forming method, a hot stamping method, a hot stamp method, and a
die quench method.
[0005] FIG. 1 is a schematic explanatory view showing the structure
of a press tool for carrying out hot press forming as described
above (hereinafter represented sometimes by "hot stamp"). In this
figure, reference numerals 1, 2, 3, and 4 represent a punch, a die,
a blank holder, and a steel sheet (blank), respectively, and
abbreviations BHF, rp, rd, and CL represent a blank holding force,
a punch shoulder radius, a die shoulder radius, and a clearance
between the punch and the die, respectively. In these parts, punch
1 and die 2 have passage 1a and passage 2a, respectively, formed in
the inside thereof, through which passages a cooling medium (e.g.,
water) can be allowed to pass, and the press tool is made to have a
structure so that these members can be cooled by allowing the
cooling medium to pass through these passages.
[0006] When a steel sheet is subjected to hot stamp (e.g., hot deep
drawing) with such a press tool, the forming is started in a state
where steel sheet (blank) 4 is softened by heating to a temperature
within two-phase region, which is from Ac.sub.1 transformation
point to Ac.sub.3 transformation point, or a temperature within
single-phase region, which is not lower than Ac.sub.3
transformation point. More specifically, steel sheet 4 is pushed
into a cavity of die 2 (between the parts indicated by reference
numerals 2 and 2 in FIG. 1) by punch 1 with steel sheet 4 in
high-temperature state being sandwiched between die 2 and blank
holder 3, thereby forming steel sheet 4 into a shape corresponding
to the outer shape of punch 1 while reducing the outer diameter of
steel sheet 4. In addition, heat is removed from steel sheet 4 to
the press tool (punch 1 and die 2) by cooling punch 1 and die 2 in
parallel with the forming, and the hardening of the material is
carried out by further retaining and cooling steel sheet 4 at the
lower dead point in the forming (the point of time when the punch
head is positioned at the deepest level: the state shown in FIG.
1). Formed products with high dimension accuracy and strength of
1500 MPa class can be obtained by carrying out such a forming
method. Furthermore, such a forming method results in that the
volume of a pressing machine can be made smaller because a forming
load can be reduced as compared with the case where parts of the
same strength class are formed by cold pressing.
[0007] As steel sheets for hot stamp, which have widely been used
at present, there are known steel sheets based on 22MnB5 steel.
These steel sheets have tensile strengths of 1500 MPa and
elongations of about 6% to 8%, and have been applied to
impact-resistant members (members neither deformed nor fractured as
much as possible at the time of impact). In addition, some
developments have also proceeded for C content increase and further
highly strengthening (in 1500 to 1800 MPa class) based on 22MnB5
steel.
[0008] However, there is almost no application of steel grades
other than 22MnB5 steel. One can find a present situation where
little consideration is made on steel grades or methods for
controlling the strength and elongation of parts (e.g., strength
lowering to 980 MPa class and elongation enhancement to 20%) to
extend their application range to other than impact-resistant
members.
[0009] In middle or higher class automobiles, taking into
consideration compatibility (function of, when a small class
automobile comes to collide, making safe of the other side) at the
time of side or back impact, both functions as an impact-resistant
portion and an energy-absorbing portion may sometimes be provided
in parts such as B pillars or rear side members. To produce such
members, there has mainly been used so far, for example, a method
in which ultra-high tensile strength steel sheets having high
strength of 980 MPa class and high tensile strength steel sheets
having elongation of 440 MPa class are laser welded (to prepare a
tailor welded blank, abbreviated as TWB) and then cold press
formed. However, in recent years, the development of a technique
has proceeded, in which parts are each provided with different
strengths by hot stamp.
[0010] For example, Non-patent Document 1 has proposed a method of
laser welding 22MnB5 steel for hot stamp and a material that does
not have high strength even if quenched with a press tool (to
prepare a tailor welded blank, abbreviated as TWB), followed by hot
stamp, in which method different strengths are provided so that
tensile strength at a high strength side (i.e., impact-resistant
portion side) becomes 1500 MPa (and elongation becomes 6% to 8%)
and tensile strength at a low strength side (i.e., energy-absorbing
portion side) becomes 440 MPa (and elongation becomes 12%). In
addition, as the technique of providing parts each with different
strengths, some techniques have also been proposed, such as
disclosed in Non-patent Documents 2 to 4.
[0011] The techniques disclosed in Non-patent Documents 1 and 2
provide a tensile strength of not higher than 600 MPa and an
elongation of about 12% to 18% at an energy-absorbing portion side,
in which techniques, however, laser welding (to prepare a tailor
welded blank, abbreviated as TWB) is needed previously, thereby
increasing the number of steps and resulting in high cost. In
addition, it results in the heating of energy-absorbing portions,
which need not to be hardened originally. Therefore, these
techniques are not preferred from the viewpoint of energy
consumption.
[0012] The technique disclosed in Non-patent Document 3 is based on
22MnB5 steel, in which boron addition; however, adversely affects
the robustness of strength after quenching against heating to a
temperature within two-phase region, making difficult the control
of strength at an energy-absorbing portion side, and further making
it possible to obtain only an elongation as low as 15%.
[0013] The technique disclosed in Non-patent Document 4 is based on
22MnB5 steel, and therefore, this technique is not economic in that
control is made in such a manner that 22MnB5, which originally has
excellent hardenability, is not hardened (control of press tool
cooling).
PRIOR ART DOCUMENTS
Non-Patent Documents
[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)/1500MPa-8%-Ductibor500/550-700 MPa-17%
[searched on Apr. 27, 2013] Internet
<http://www.arcelormittal.com/tailoredblanks/pre/seifware.pl>
[0016] Non-patent Document 3: 22MnB5/above AC3/1500MPa-8%-below
AC3/Hv190-Ferrite/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., pp83-88, 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., pp23-35, 2008.
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0018] The present invention has been made in view of the
above-described circumstances, and its object is to provide a hot
press-formed product in which balance between strength and
elongation can be controlled in a proper range and high ductility
can be achieved, a process useful for producing such a hot
press-formed product, and a thin steel sheet for hot press
forming.
Means for Solving the Problems
[0019] The hot press-formed product of the present invention, which
can achieve the above object, is a hot press-formed product,
characterized by comprising a thin steel sheet formed by a hot
press-forming method, and having a metallic structure that contains
ferrite at 30% to 80% by area, bainitic ferrite at lower than 30%
by area (not including 0% by area), martensite at 30% by area or
lower (not including 0% by area), and retained austenite at 3% to
20% by area.
[0020] In the hot press-formed product of the present invention,
the chemical element composition thereof is not particularly
limited, typical examples of which may include the following
chemical element composition: C at 0.1% to 0.3% (where "%" means "%
by mass", and the same applies to the below with respect to the
chemical element composition); Si at 0.5% to 3%; Mn at 0.5% to 2%;
P at 0.05% or lower (not including 0%); S at 0.05% or lower (not
including 0%); Al at 0.01% to 0.1%; and N at 0.001% to 0.01%, and
the remainder consisting of iron and unavoidable impurities.
[0021] In the hot press-formed product of the present invention, it
is also useful to allow additional elements to be contained, when
needed; for example, (a) B at 0.01% or lower (not including 0%) and
Ti at 0.1% or lower (not including 0%); (b) one or more selected
from the group consisting of Cu, Ni, Cr, and Mo at 1% or lower (not
including 0%) in total; and (c) V and/or Nb at 0.1% or lower (not
including 0%) in total. Depending on the kind of element to be
contained, the hot press-formed product may have further improved
characteristics.
[0022] When the hot press-formed product of the present invention
is produced, the following steps may be used, i.e., heating a
hot-rolled steel sheet having a metallic structure that contains
ferrite at 50% by area or higher, or a cold-rolled steel sheet at a
reduction of 30% or higher, to a temperature not lower than
Ac.sub.1 transformation point and not higher than (Ac.sub.1
transformation point.times.0.3+Ac3 transformation point.times.0.7);
and then starting the forming of the hot-rolled steel sheet or the
cold-rolled steel sheet with a press tool to produce the hot
press-formed product, during which forming an average cooling rate
of 20.degree. C./sec or higher is kept in the press tool, and which
forming is finished at a temperature not higher than (bainite
transformation starting temperature Bs-100.degree. C.). The forming
finishing temperature may preferably be controlled in a temperature
range of not higher than (bainite transformation starting
temperature Bs-100.degree. C.) and not lower than martensite
transformation starting temperature Ms point, in which temperature
range the steel sheet may preferably be retained for 10 seconds or
longer, followed by the forming
[0023] Alternatively, the following method may be adopted as the
other method. When a thin steel sheet is press formed with a press
tool, the thin steel sheet may be heated to a temperature not lower
than Ac.sub.3 transformation point and not higher than 1000.degree.
C., and then cooled to a temperature not higher than 700.degree. C.
and not lower than 500.degree. C. at an average cooling rate of
10.degree. C./sec or lower, and then the forming of the thin steel
sheet may be started, during which forming an average cooling rate
of 20.degree. C./sec or higher may be kept in the press tool, and
which forming may be finished at a temperature not higher than
(bainite transformation starting temperature Bs-100.degree. C.).
Also in this method, the forming finishing temperature may
preferably be controlled in a temperature range of not higher than
(bainite transformation starting temperature Bs-100.degree. C.) and
not lower than martensite transformation starting temperature Ms
point, in which temperature range the steel sheet may preferably be
retained for 10 seconds or longer, followed by the forming.
[0024] The present invention further includes a thin steel sheet
for hot press forming, which is intended for producing a hot
press-formed product as described above, and this thin steel sheet
is characterized by being a hot-rolled steel sheet having a
metallic structure that contains ferrite at 50% by area or higher,
or a cold-rolled steel sheet at reduction of 30% or higher.
Effects of the Invention
[0025] The present invention makes it possible that: retained
austenite can be allowed to exist at a proper fraction to adjust
the metallic structure of a hot press-formed product by properly
controlling the conditions of a hot press-forming method; a hot
press-formed product having more enhanced ductility (retained
ductility) inherent to the formed product as compared with the case
where conventional 22MnB5 steel is used; and strength and
elongation can be controlled by a combination of heat treatment
conditions and pre-forming steel sheet structure (initial
structure). In addition, the control of heating temperature within
two-phase region makes it possible to provide different strengths
and elongations freely.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a schematic explanatory view showing the structure
of a press tool for carrying out hot press forming.
MODE FOR CARRYING OUT THE INVENTION
[0027] The present inventors have studied from various angles to
realize a hot press-formed product having prescribed strength and
further exhibiting excellent ductility (elongation) after forming
when a steel sheet is heated to a prescribed temperature and then
hot press formed to produce the formed product.
[0028] As a result, the present inventors have found that a hot
press-formed product having excellent balance between strength and
ductility can be achieved when the type of a steel sheet, heating
temperature, and forming conditions are properly controlled so that
its structure is controlled to contain retained austenite at 3% to
20% by area in the hot press forming of a steel sheet with a press
tool, thereby completing the present invention.
[0029] The reasons for setting the ranges of the respective
structures (basic structures) in the hot press-formed product of
the present invention are as follows:
[0030] [Ferrite at 30% to 80% by Area]
[0031] High ductility of a hot press-formed product can be achieved
by making its structure composed mainly of fine and high-ductility
ferrite. From this viewpoint, the area fraction of ferrite should
be controlled to 30% by area or higher. However, when this area
fraction is higher than 80% by area, prescribed strength becomes
not secured. The fraction of ferrite may preferably be not lower
than 40% by area as the preferred lower limit (more preferably not
lower than 45% by area) and not higher than 70% by area as the
preferred upper limit (more preferably not higher than 65% by
area).
[0032] [Bainitic Ferrite at Lower than 30% by Area (Not Including
0%)]
[0033] Bainitic ferrite is effective for strength improvement, but
it causes a slight lowering of ductility. Therefore, the fraction
of bainitic ferrite should be controlled to lower than 30% by area
as the upper limit The fraction of bainitic ferrite may preferably
be not lower than 5% by area as the preferred lower limit (more
preferably not lower than 10% by area) and not higher than 25% by
area as the preferred upper limit (more preferably not higher than
20% by area).
[0034] [Martensite at 30% by Area or Lower (Not Including 0%)]
[0035] Martensite is effective for strength improvement, but it
causes a considerable lowering of ductility. Therefore, the
fraction of martensite should be controlled to not higher than 30%
by area as the upper limit. The fraction of martensite may
preferably be not lower than 5% by area as the preferred lower
limit (more preferably not lower than 10% by area) and not higher
than 25% by area as the preferred upper limit (more preferably not
higher than 20% by area).
[0036] [Retained Austenite at 3% to 20% by Area]
[0037] Retained austenite is transformed into martensite during
plastic deformation, thereby having the effect of increasing work
hardening rate (transformation-inducing plasticity) to improve the
ductility of a formed product. To make such an effect exhibited,
the fraction of retained austenite should be controlled to 3% by
area or higher. When the fraction of retained austenite is higher,
ductility becomes more excellent. In a composition to be used for
automobile steel sheets, retained austenite that can be secured is
limited, of which upper limit becomes about 20% by area. The
fraction of retained austenite may preferably be not lower than 5%
by area as the preferred lower limit (more preferably not lower
than 7% by area) and not higher than 15% by area as the preferred
upper limit (more preferably not higher than 10% by area).
[0038] When the hot press-formed product of the present invention
is produced, a hot-rolled steel sheet having a metallic structure
that contains ferrite at 50% by area or higher, or a cold-rolled
steel sheet at a reduction of 30% or higher, may be used, and when
the hot-rolled steel sheet or the cold-rolled steel sheet is press
formed with a pres tool, the hot-rolled steel sheet or the
cold-rolled steel sheet may be heated to a temperature not lower
than Ac.sub.1 transformation point and not higher than (Ac.sub.1
transformation point.times.0.3+Ac.sub.3 transformation
point.times.0.7), and then the forming of the hot-rolled steel
sheet or the cold-rolled steel sheet may be started, during which
forming an average cooling rate of 20.degree. C./sec or higher may
be kept in the press tool, and which forming may be finished at a
temperature not higher than (bainite transformation starting
temperature Bs-100.degree. C.). The reasons for defining the
respective requirements in this process are as follows:
[0039] [Using a Hot-Rolled Steel Sheet Having a Metallic Structure
that contains Ferrite at 50% by Area or Higher, or a Cold-Rolled
Steel Sheet at a Reduction of 30% or Higher]
[0040] To obtain ferrite structure, which has high contributions to
ductility, during heating to a temperature within two-phase region,
the type of a steel sheet (steel sheet for forming) should properly
be selected. When a hot-rolled steel sheet is used as the steel
sheet for forming, it is important to achieve that the fraction of
ferrite is high and ferrite is retained during heating to a
temperature within two-phase region. From this viewpoint, the
hot-rolled steel sheet to be used may preferably have a metallic
structure that contains ferrite at 50% by area or higher. The
fraction of ferrite may preferably be not lower than 60% by area as
the preferred lower limit (more preferably not lower than 70% by
area). When the fraction of ferrite in the hot-rolled steel sheet
becomes too high, the fraction of ferrite in the formed product
becomes too high. Therefore, the fraction of ferrite in the
hot-rolled steel sheet may preferably be not higher than 95% by
area, more preferably not higher than 90% by area.
[0041] On the other hand, a cold-rolled steel sheet is used, it
becomes an important requirement that recrystallization occurs
during heating to form dislocation-free ferrite, and therefore,
rolling (cold rolling) should be carried out at a prescribed
reduction or higher so that recrystallization occurs. In the case
of a cold-rolled steel sheet, it may have any structure. From this
viewpoint, when a cold-rolled steel sheet is used, it is preferable
to use a cold-rolled steel sheet at a reduction of 30% or higher.
The reduction may preferably be 40% or higher, more preferably 50%
or higher. The "reduction" as used herein is a value determined by
formula (1) below.
Reduction (%)=[(thickness of steel sheet before cold
rolling-thickness of steel sheet after cold rolling)/thickness of
steel sheet before cold rolling].times.100 (1)
[0042] [Heating a Steel Sheet to a Temperature Not Lower than
Ac.sub.1 Transformation Point and Not Higher than (Ac.sub.1
Transformation Point.times.0.3+Ac.sub.3 Transformation
Point.times.0.7), and then Starting the Forming]
[0043] To cause the partial transformation, while retaining, of
ferrite, which is contained in the steel sheet, into austenite, the
heating temperature should be controlled in a prescribed range. The
proper control of the heating temperature makes it possible to
cause transformation into retained austenite or martensite in the
subsequent cooling step to provide the final hot press-formed
product with a desired structure. When the heating temperature of
the steel sheet is lower than Ac.sub.1 transformation point, a
sufficient fraction of austenite cannot be obtained during heating,
and therefore, a prescribed fraction of retained austenite cannot
be secured in the final structure (the structure of a formed
product). When the heating temperature of the thin steel sheet is
higher than (Ac.sub.1 transformation point.times.0.3+Ac.sub.3
transformation point.times.0.7), the fraction of transformed
austenite is increased too highly during heating, and therefore, a
prescribed fraction of ferrite cannot be secured in the final
structure (the structure of a formed product).
[0044] [During Forming, an Average Cooling Rate of 20.degree.
C./sec or Higher is Kept in the Press Tool, and the Forming is
Finished at a Temperature Not Lower than (Bainite Transformation
Starting Temperature Bs-100.degree. C.)]
[0045] To change the austenite, which was formed in the above
heating step, into a prescribed fraction of retained austenite,
while preventing the formation of cementite, the average cooling
rate during forming and the forming finishing temperature should
properly be controlled. From this viewpoint, the average cooling
rate during forming should be controlled to 20.degree. C./sec or
higher, and the forming finishing temperature should be controlled
to a temperature not higher than (bainite transformation starting
temperature Bs point-100.degree. C., sometimes abbreviated as
"Bs-100.degree. C"). The average cooling rate during forming may
preferably be 30.degree. C./sec or higher (more preferably
40.degree. C./sec or higher). With respect to the forming finishing
temperature, the forming may be finished, while cooling to room
temperature at an average cooling temperature as described above.
Alternatively, the cooling is stopped after the cooling to a
temperature not higher than Bs-100.degree. C., and then the forming
may be finished. The control of the average cooling rate during
forming can be achieved by a means of, for example, (a) controlling
the temperature of a press tool (using a cooling medium shown in
FIG. 1 above) or (b) controlling the thermal conductivity of a
press tool (the same applies to the cooling in the method described
below).
[0046] As another method for producing the press-formed product of
the present invention, when a steel sheet is press formed with a
press tool, the thin steel sheet may be heated to a temperature not
lower than Ac.sub.3 transformation point and not higher than
1000.degree. C., and then the thin steel sheet is cooled to a
temperature not higher than 700.degree. C. and not lower than
500.degree. C. at an average cooling temperature of 10.degree.
C./sec or lower, and then the forming of the thin steel sheet may
be started, during which forming an average cooling rate of
20.degree. C./sec or higher may be kept in the press tool, and
which forming may be finished at a temperature not higher than
(bainite transformation starting temperature Bs-100.degree. C.).
The reasons for defining the respective requirements in this
process are as follows (the same as described above applies to the
cooling finishing temperature):
[0047] [Heating a Thin Steel Sheet to a Temperature Not Lower than
Ac.sub.3 Transformation Point and Not Higher than 1000.degree.
C.]
[0048] To properly adjust the structure of a hot press-formed
product, the heating temperature should be controlled in a
prescribed range. The proper control of the heating temperature
makes it possible to cause transformation into a structure composed
mainly of ferrite while securing a prescribed fraction of retained
austenite in the subsequent cooling step to provide the final hot
press-formed product with a desired structure. When the heating
temperature of the thin steel sheet is lower than Ac.sub.3
transformation point, a sufficient fraction of austenite cannot be
obtained during heating, and therefore, a prescribed fraction of
retained austenite cannot be secured in the final structure (the
structure of a formed product). When the heating temperature of the
thin steel sheet is higher than 1000.degree. C., the grain size of
austenite becomes increased during heating, and therefore, ferrite
cannot be formed in the subsequent cooling.
[0049] [Cooling to a Temperature Not Higher than 700.degree. C. and
Not Lower than 500.degree. C. at an Average Cooling Rate of
10.degree. C./sec or Lower, and then Starting the Forming]
[0050] This cooling step is an important step for forming ferrite
during cooling. When the average cooling rate in this cooling step
becomes higher than 10.degree. C./sec, a prescribed fraction of
ferrite cannot be secured. The average cooling rate may preferably
be 7.degree. C./sec or lower, more preferably 5.degree. C./sec or
lower. The cooling stopping temperature in this cooling step (this
temperature may sometimes be referred to as the "cooling rate
changing temperature") should be controlled to not higher than
700.degree. C. and not lower than 500.degree. C. When the cooling
stopping temperature becomes higher than 700.degree. C., a
sufficient fraction of ferrite cannot be secured. When the cooling
stopping temperature becomes lower than 500.degree. C., the
fraction of ferrite becomes too high, and therefore, prescribed
strength cannot be secured. The cooling stopping temperature may
preferably be not higher than 680.degree. C. as the preferred upper
limit (more preferably not higher than 660.degree. C.) and not
lower than 520.degree. C. as the preferred lower limit (more
preferably not lower than 550.degree. C.).
[0051] In any of these methods, the forming finishing temperature
should be controlled to not higher than (Bs-100.degree. C.), but
may preferably be controlled in a temperature range of not lower
than martensite transformation starting temperature Ms (a
temperature in this range may sometimes be referred to as the
"cooling temperature changing temperature), in which temperature
range retention may preferably be carried out for 10 seconds or
longer. The bainite transformation can proceed from super-cooled
austenite to form a structure composed mainly of ferrite by
retention in the above temperature range for 10 seconds or longer.
The retention time may preferably be 50 seconds or longer (more
preferably 100 seconds or longer). When the retention time becomes
too long, austenite starts to decompose, so that the fraction of
retained austenite cannot become secured. Therefore, the retention
time may preferably be 1000 seconds or shorter (more preferably 800
seconds or shorter).
[0052] Retention as described above may be any of isothermal
retention, monotonic cooling, and re-heating step, so long as it is
in the above temperature range. With regard to a relationship
between such retention and forming, retention as described above
may be added at the stage when forming is finished. Alternatively,
a retention step may be added within the above temperature range
during the finish of forming. After forming is finished in such a
manner, the steel sheet may be left as it is for cooling or cooled
at a proper cooling rate to room temperature (25.degree. C.).
[0053] The process for producing the hot press-formed product of
the present invention can be applied, not only to the case where a
hot press-formed product having a simple shape as shown in FIG. 1
above is produced (i.e., direct method), but also to the case where
a formed product having a relatively complicated shape is produced,
even if any of the methods described above is adopted. However, in
the case of a complicated product shape, it may be difficult to
provide a product with the final shape by a single press forming
step. In such a case, there can be used a method of cold press
forming in a step prior to hot press forming (this method has been
referred to as "indirect method"). This method includes previously
forming a difficult-to-form portion into an approximate shape by
cold processing and then hot press forming the other portions. When
such a method is used to produce, for example, a formed product
having three projections (profile peaks) by forming, two
projections are formed by cold press forming and the third
projection is then formed by hot press forming.
[0054] The present invention is intended for a hot press-formed
product made of a high-strength steel sheet, the steel grade of
which is acceptable, if it has an ordinary chemical element
composition as a high-strength steel sheet, in which, however, C,
Si, Mn, P, S, Al, and N contents may preferably be controlled in
their respective proper ranges. From this viewpoint, the preferred
ranges of these chemical elements and the grounds for limiting
their ranges are as follows:
[0055] [C at 0.1% to 0.3%]
[0056] C is an important element for securing retained austenite.
The concentration of austenite during heating at a temperature
within two-phase region or at a temperature within single-phase
region, which is not lower than Ac.sub.3 transformation point,
allows the formation of retained austenite after quenching. It
further contributes to an increase of martensite fraction. When C
content is lower than 0.1%, a prescribed fraction of retained
austenite cannot be secured, making it impossible to obtain
excellent ductility. When C content becomes higher than 0.3%, it
results in that strength becomes too high. C content may more
preferably be not lower than 0.15% as the more preferred lower
limit (still more preferably not lower than 0.20%) and not higher
than 0.27% as the more preferred upper limit (still more preferably
not higher than 0.25%).
[0057] [Si at 0.5% to 3%]
[0058] Si suppresses austenite after heating at a temperature
within two-phase region or at a temperature within single-phase
region, which is not lower than Ac.sub.3 transformation point, from
being formed into cementite, and exhibits the action of increasing
the fraction of retained austenite. It further exhibits the action
of enhancing strength by solid solution enhancement without
deteriorating ductility too much. When Si content is lower than
0.5%, retained austenite cannot be secured at a prescribed
fraction, making it impossible to obtain excellent ductility. When
Si content becomes higher than 3%, the degree of solid solution
enhancement becomes too high, resulting in the drastic
deterioration of ductility. Si content may more preferably be not
lower than 1.15% as the more preferred lower limit (still more
preferably not lower than 1.20%) and not higher than 2.7% as the
more preferred upper limit (still more preferably not higher than
2.5%).
[0059] [Mn at 0.5% to 2%]
[0060] Mn is an element to stabilize austenite, and it contributes
to an increase of retained austenite. To make such an effect
exhibited, Mn may preferably be contained at 0.5% or higher.
However, when Mn content becomes excessive, the formation of
ferrite is prevented, thereby making it impossible to secure a
prescribed fraction of ferrite, and therefore, Mn content may
preferably be 2% or lower. In addition, a considerable improvement
of austenite strength increases a hot rolling load, thereby making
it difficult to produce steel sheets, and therefore, even from the
viewpoint of productivity, it is not preferable that Mn is
contained at higher than 2%. Mn content may more preferably be not
lower than 0.7% as the more preferred lower limit (still more
preferably not lower than 0.9%) and not higher than 1.8% as the
more preferred higher limit (still more preferably not higher than
1.6%).
[0061] [P at 0.05% or Lower (Not Including 0%)].
[0062] P is an element unavoidably contained in steel and
deteriorates ductility. Therefore, P content may preferably be
reduced as low as possible. However, extreme reduction causes an
increase of steel production cost, and reduction to 0% is difficult
in the actual production. Therefore, P content may more preferably
be controlled to 0.05% or lower (not including 0%). P content may
more preferably be not higher than 0.045% as the more preferred
upper limit (still more preferably not higher than 0.040%).
[0063] [S at 0.05% or Lower (Not Including 0%)]
[0064] S is also an element unavoidably contained in steel and
deteriorates ductility, similarly to P. Therefore, S content may
preferably be reduced as low as possible. However, extreme
reduction causes an increase of steel production cost, and
reduction to 0% is difficult in the actual production. Therefore, S
content may preferably be controlled to 0.05% or lower (not
including 0%). S content may more preferably be not higher than
0.045% as the more preferred upper limit (still more preferably not
higher than 0.040%).
[0065] [Al at 0.01% to 0.1%]
[0066] Al is useful as a deoxidizing element and further useful for
fixation of dissolved N in steel as AlN to improve ductility. To
make such an effect effectively exhibited, Al content may
preferably be controlled to 0.01% or higher. However, when Al
content becomes higher than 0.1%, it results in the excessive
formation of Al.sub.2O.sub.3 to deteriorate ductility. Al content
may more preferably be not lower than 0.013% as the more preferred
lower limit (still more preferably not lower than 0.015%) and not
higher than 0.08% as the more preferred upper limit (still more
preferably not higher than 0.06%).
[0067] [N at 0.001% to 0.01%]
[0068] N is an element unavoidably incorporated in steel, and a
reduction of N content may be preferred, which has, however, a
limitation in actual process. Therefore, the lower limit of N
content was set to 0.001%. When N content becomes excessive,
ductility is deteriorated by strain aging, or the addition of B
causes deposition of N as BN, thereby lowering the effect of
improving hardenability by solid solution of B. Therefore, the
upper limit of N content was set to 0.01%. N content may more
preferably be not higher than 0.008% as the more preferred upper
limit (still more preferably not higher than 0.006%).
[0069] The basic chemical components in the press-formed product of
the present invention are as described above, and the remainder
consists essentially of iron. The wording "consists essentially of
iron" means that the press-formed product of the present invention
can contain, in addition to iron, minor components (e.g., besides
Mg, Ca, Sr, and Ba, REM such as La, and carbide-forming elements
such as Zr, Hf, Ta, W, and Mo) in such a level that these minor
components do not inhibit the characteristics of the steel sheet of
the present invention, and can further contain unavoidable
impurities (e.g., O, H) other than P, S, and N.
[0070] It is also useful to allow the press-formed product of the
present invention to contain additional elements, when needed; for
example, (a) B at 0.01% or lower (not including 0%) and Ti at 0.1%
or lower (not including 0%); (b) one or more selected from the
group consisting of Cu, Ni, Cr, and Mo at 1% or lower (not
including 0%) in total; and (c) V and/or Nb at 0.1% or lower (not
including 0%) in total. The press-formed product may have further
improved characteristics depending on the kinds of elements
contained. When these elements are contained, their preferred
ranges and grounds for limitation of their ranges are as
follows:
[0071] [B at 0.01% or Lower (Not Including 0%) and Ti at 0.1% or
Lower (Not Including 0%)]
[0072] B is an element to prevent the formation of cementite during
cooling after heating, thereby contributing to the securement of
retained austenite. To make such an effect exhibited, B may
preferably be contained at 0.0001% or higher, but even if B is
contained beyond 0.01%, the effect is saturated. B content may more
preferably be not lower than 0.0002% as the more preferred lower
limit (still more preferably not lower than 0.0005%) and not higher
than 0.008% as the more preferred upper limit (still more
preferably not higher than 0.005%).
[0073] On the other hand, Ti fixes N and maintains B in solid
solution state, thereby exhibiting the effect of improving
hardenability. To make such an effect exhibited, Ti may preferably
be contained at-least 4 times higher than N content. However, when
Ti content becomes excessive beyond 0.1%, it results in excessive
formation of TiC, thereby causing an increase of strength by
precipitation enhancement but a deterioration of ductility. Ti
content may more preferably be not lower than 0.05% as the more
preferred lower limit (still more preferably not lower than 0.06%)
and not higher than 0.09% as the more preferred higher limit (still
more preferably not higher than 0.08%).
[0074] [One or More Selected from the Group Consisting of Cu, Ni,
Cr, and Mo at 1% or Lower (Not Including 0%) in Total]
[0075] Cu, Ni, Cr, and Mo prevent the formation of cementite during
cooling after heating, and effectively act the securement of
retained austenite. To make such an effect exhibited, these
elements may preferably be contained at 0.01% or higher in total.
Taking only characteristics into consideration, their content may
be preferable when it is higher, but may preferably be controlled
to 1% or lower in total because of a cost increase by alloy element
addition. In addition, these elements have the action of
considerably enhancing the strength of austenite, thereby
increasing a hot rolling load so that the production of steel
sheets becomes difficult. Therefore, even from the viewpoint of
productivity, their content may preferably be controlled to 1% or
lower. These elements' content may more preferably be not lower
than 0.05% as the more preferred lower limit (still more preferably
not lower than 0.06%) in total and not higher than 0.9% as the more
preferred upper limit (still more preferably not higher than 0.8%)
in total.
[0076] [V and/or Nb at 0.1% or Lower (Not Including 0%) in
Total]
[0077] V and Nb have the effect of forming fine carbide and make
structure fine by pinning effect. To make such an effect exhibited,
these elements may preferably be contained at 0.001% or higher in
total. However, when these elements' content becomes excessive, it
results in the formation of coarse carbide, which becomes the
origin of fracture, thereby deteriorating ductility in contrast.
Therefore, these elements' content may preferably be controlled to
0.1% or lower in total. These elements' content may more preferably
be not lower than 0.005% as the more preferred lower limit (still
more preferably not lower than 0.008%) in total and not higher than
0.08% as the more preferred upper limit (still more preferably not
higher than 0.06%) in total.
[0078] The thin steel sheet for hot press forming of the present
invention may be either a non-plated steel sheet or a plated steel
sheet. When it is a plated steel sheet, the type of plating may be
either ordinary galvanization or aluminium coating. The method of
plating may be either hot-dip plating or electroplating. After the
plating, alloying heat treatment may be carried out, or additional
plating may be carried out as multilayer plating.
[0079] According to the present invention, the characteristics of
formed products, such as strength and elongation, can be controlled
by properly adjusting press forming conditions (heating temperature
and cooling rate), and in addition, hot press-formed products
having high ductility (retained ductility) can be obtained, so that
they can be applied even to parts (e.g., energy-absorbing members),
to which conventional hot press-formed products have hardly been
applied; therefore, the present invention is extremely useful for
extending the application range of hot press-formed products. The
formed products, which can be obtained in the present invention,
have further enhanced residual ductility as compared with formed
products, of which structure was adjusted by ordinary annealing
after cold press forming.
[0080] The following will describe the advantageous effects of the
present invention more specifically by way of Examples, but the
present invention is not limited to the Examples described below.
The present invention can be put into practice after appropriate
modifications or variations within a range capable of meeting the
gist described above and below, all of which are included in the
technical scope of the present invention.
[0081] The present application claims the benefit of priority based
on Japanese Patent Application No. 2011-130636 filed on Jun. 10,
2011. The entire contents of the specification of Japanese Patent
Application No. 2011-130636 filed on Jun. 10, 2011 are hereby
incorporated by reference into the present application.
EXAMPLES
[0082] Steel materials having respective chemical element
compositions shown in Table 1 below were formed into slabs for
experimental use by a vacuum fusion method, after which the slabs
were hot rolled, followed by cooling, and then wound. These rolled
sheets were further cold rolled into thin steel sheets, followed by
quench treatment so that they had respectively prescribed initial
structures. In Table 1, Ac.sub.1 transformation point, Ac.sub.3
transformation point, Ms point, and (Bs-100.degree. C.) were
determined respectively on the basis of formulas (2) to (5)
described below (see, e.g., the Japanese translation of "The
Physical Metallurgy of Steels" originally written by William C.
Leslie, published by Maruzen, 1985). Table 1 further shows the
calculated values of (Ac.sub.1 transformation
point.times.0.3+Ac.sub.3 transformation point.times.0.7) (these
calculated values may hereinafter be referred to as "A
values").
Ac.sub.1 transformation point (.degree.
C.)=723+29.1.times.[Si]-10.7.times.[Mn]+16.9.times.[Cr]-16.9.times.[Ni]
(2)
Ac.sub.3 transformation point (.degree. C.)
=910-203.times.[C].sup.1/2+44.7.times.[Si]-30.times.[Mn]+700.times.[P]+40-
0.times.[Al]+400.times.[Ti]+104.times.[V]-11.times.[Cr]+31.5.times.[Mo]-20-
.times.[Cu]-15.2.times.[Ni] (3)
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] (4)
Bs point (.degree.
C.)=830-270.times.[C]-90.times.[Mn]-37.times.[Ni]-70.times.[Cr]-83.times.-
[Mo] (5)
where [C], [Si], [Mn], [P], [Al], [Ti], [V], [Cr], [Mo], [Cu], and
[Ni] indicate C, Si, Mn, P, Al, Ti, V, Cr, Mo; Cu, and Ni contents
(% by mass), respectively. When some element indicated in a certain
term of formulas (2) to (5) above is not contained, calculation is
carried out under the assumption that the term does not exist in
the formula.
TABLE-US-00001 TABLE 1 Chemical element composition* Steel (% by
mass) grade C Si Mn P S Cu Ni Cr Mo V Nb Ti A 0.232 1.19 1.41 0.014
0.0021 B 0.231 1.21 1.39 0.014 0.0021 0.21 C 0.222 1.20 1.29 0.014
0.0021 0.21 0.027 D 0.225 1.31 1.33 0.014 0.0021 0.15 0.027 E 0.234
1.10 1.52 0.014 0.0021 0.22 0.027 F 0.229 1.04 1.41 0.014 0.0021
0.07 0.18 0.027 G 0.219 1.20 1.14 0.014 0.0021 0.15 0.03 0.027 H
0.225 1.23 1.26 0.014 0.0021 0.17 0.027 I 0.217 1.41 1.44 0.014
0.0021 0.20 0.03 0.027 J 0.230 0.89 1.37 0.014 0.0021 0.19 0.03
0.027 K 0.047 1.22 1.25 0.014 0.0021 0.19 0.027 L 0.230 0.01 1.22
0.014 0.0021 0.19 0.027 M 0.311 1.20 1.29 0.014 0.0021 0.21 0.027 N
0.232 0.18 1.41 0.014 0.0021 0.21 0.027 Chemical element Ac.sub.1
Ac.sub.3 composition* transformation transformation Ms Bs- A Steel
(% by mass) point point point 100.degree. C. value grade B Al N
(.degree. C.) (.degree. C.) (.degree. C.) (.degree. C.) (.degree.
C.) A 0.053 0.047 743 854 413 540 821 B 0.053 0.047 747 854 410 528
822 C 0.0033 0.053 0.047 748 869 417 539 832 D 0.0033 0.053 0.047
747 871 417 550 834 E 0.0033 0.053 0.047 735 854 404 522 818 F
0.0033 0.053 0.047 741 856 410 529 821 G 0.0033 0.053 0.047 748 876
425 555 837 H 0.0033 0.053 0.047 745 878 420 542 838 I 0.0033 0.053
0.047 752 878 413 528 840 J 0.0033 0.053 0.047 737 851 411 531 817
K 0.0033 0.053 0.047 748 923 482 592 870 L 0.0033 0.053 0.047 713
816 417 545 785 M 0.0033 0.053 0.047 748 851 385 515 820 N 0.0033
0.053 0.047 717 817 409 526 787 *The remainder consists of iron and
unavoidable impurities other than P, S, and N.
[0083] The steel sheets thus obtained were heated under the
respective conditions shown in Table 2 below, and then subjected to
forming and cooling treatment using a high speed heat treatment
testing system for steel sheets (CAS series, available from
ULVAC-RIKO, Inc.), which can control an average cooling rate. The
steel sheets to be subjected to cooling treatment had a size of 190
mm.times.70 mm (and a sheet thickness of 1.4 mm). Test Nos. 1 to
14, 17 to 19, and 21 to 25 were the cases where hot-rolled steel
sheets were used as steel sheets for forming. Test Nos. 15, 16, and
20 were the cases where cold-rolled steel sheets were used steel
sheets for forming. The term "cooling 1" shown in Table 2 indicates
cooling from a heating temperature to a temperature of 700.degree.
C. to 500.degree. C. The term "cooling 2" shown in Table 2
indicates cooling from then to a temperature range of
[(Bs-100.degree. C.) to Ms point] (In Test Nos. 19 to 23, forming
was started at this stage). When needed, the steel sheet was
subjected to hot-dip galvanization to obtain a hot-dip galvanized
steel sheet (Test No. 25).
[0084] For the respective steel sheets after the above treatments
(heating, forming, and cooling), measurement of tensile strength
(TS) and elongation (total elongation EL), and observation of
metallic structure (fraction of each structure), were carried out
by the methods described below.
[0085] [Tensile Strength (TS) and Elongation (Total Elongation
EL)]
[0086] JIS No. 5 specimens were used for tensile tests to measure
tensile strength (TS) and elongation (EL). At that time, strain
rate in the tensile tests was set to 10 mm/sec. In the present
invention, the specimens were evaluated as "passing" when
fulfilling any of the conditions that: (a) tensile strength (TS) is
from 780 to 979 MPa and elongation (EL) is 25% or higher; and (b)
tensile strength (TS) is from 980 to 1179 MPa and elongation (EL)
is 15% or higher.
[0087] [Observation of Metallic Structure (Fraction of Each
Structure)]
[0088] (1) For ferrite and bainitic ferrite structures in the steel
sheets, the steel sheets were each subjected to nital etching, and
then observed by SEM (with a magnification of 1000.times. or
2000.times.), in which ferrite and bainitic ferrite were
distinguished to determine their respective fractions (area
fractions).
[0089] (2) For the fraction (area fraction) of retained austenite
in the steel sheets, the steel sheets were each measured by an
X-ray diffraction method, after grinding to one-quarter thicknesses
of the steel sheets and subsequent chemical polishing (see, e.g.,
ISJJ Int. Vol. 33 (1933), No. 7, p. 776).
[0090] (3) For the area fraction of martensite (as-quenched
martensite), the steel sheets were each subjected to repera
etching, and assuming white contrast as a mixed structure of
martensite (as-quenched martensite) and retained austenite by SEM
observation, the area fraction of the mixed structure was measured.
The fraction of as-quenched martensite was calculated by
subtracting the fraction of retained austenite, which had been
determined by an X-ray diffraction method, from the area fraction
of the mixed structure.
[0091] These results are shown in Table 3 below, together with the
types of pre-forming steel sheets (fraction of ferrite, and
reduction of cold-rolled steel sheet).
TABLE-US-00002 TABLE 2 Production conditions Cooling rate Retention
time Steel sheet for forming Average changing temperature Average
at Forming Fraction of Heating cooling rate from cooling 1 cooling
rate [Bs - 100.degree. C. finishing Test Steel ferrite Reduction
temperature in cooling 1 to cooling 2 in cooling 2 to Ms point]
temperature Plated or No. grade (% by area) (%) (.degree. C.)
(.degree. C./sec) (.degree. C.) (.degree. C./sec) (sec) (.degree.
C.) non-plated 1 A 60 -- 800 40 -- -- 3.2 300 Non-plated 2 B 60 --
800 40 -- -- 3.0 300 Non-plated 3 C 60 -- 800 40 -- -- 3.1 300
Non-plated 4 D 60 -- 800 40 -- -- 3.3 300 Non-plated 5 E 60 -- 800
40 -- -- 2.9 300 Non-plated 6 F 60 -- 800 40 -- -- 3.0 300
Non-plated 7 G 60 -- 800 40 -- -- 3.3 300 Non-plated 8 H 60 -- 800
40 -- -- 3.0 300 Non-plated 9 I 60 -- 800 40 -- -- 2.9 300
Non-plated 10 J 60 -- 800 40 -- -- 3.0 300 Non-plated 11 K 60 --
800 40 -- -- 2.7 300 Non-plated 12 L 60 -- 800 40 -- -- 3.2 300
Non-plated 13 M 50 -- 800 40 -- -- 3.3 300 Non-plated 14 N 60 --
800 40 -- -- 2.9 300 Non-plated 15 C 30 50 800 40 -- -- 3.1 300
Non-plated 16 C 30 20 800 40 -- -- 3.1 300 Non-plated 17 C 60 --
720 40 -- -- 3.1 300 Non-plated 18 C 60 -- 900 40 -- -- 3.1 300
Non-plated 19 C 60 -- 900 5 600 40 3.1 300 Non-plated 20 C 30 20
900 5 600 40 3.1 300 Non-plated 21 C 60 -- 800 40 450 3 13.3 300
Non-plated 22 C 60 -- 900 15 600 40 3.1 300 Non-plated 23 C 60 --
900 5 450 40 3.1 300 Non-plated 24 C 60 -- 800 40 -- -- 3.1 600
Non-plated 25 C 60 -- 800 40 -- -- 3.1 300 Plated
TABLE-US-00003 TABLE 3 Tensile Structure of formed product strength
Elongation Test (% by area) TS EL No. Steel grade Ferrite Bainitic
ferrite Martensite Retained austenite Others* (MPa) (%) 1 A 41 25
26 8 994 17 2 B 43 29 22 6 1020 16 3 C 48 23 21 8 994 17 4 D 49 23
21 7 1002 17 5 E 49 24 20 7 1023 17 6 F 48 24 21 7 1031 17 7 G 44
25 23 8 1028 17 8 H 46 25 22 7 1011 17 9 I 45 24 23 8 1019 17 10 J
48 24 22 6 1018 17 11 K 49 26 24 1 1022 12 12 L 51 25 22 2 1302 14
13 M 36 28 28 8 1095 16 14 N 45 27 28 0 989 13 15 C 48 24 20 8 1023
17 16 C 25 45 25 5 1082 13 17 C 81 -- 15 -- .theta.: 4 745 14 18 C
-- -- 95 5 1523 10 19 C 65 8 20 7 984 17 20 C 62 9 22 7 999 17 21 C
43 26 22 9 1032 18 22 C 12 61 20 7 1233 12 23 C 83 17 -- -- 921 14
24 C 56 20 -- -- P: 24 893 14 25 C 47 23 23 8 994 17 *.theta. and P
indicate cementite and pearlite, respectively.
[0092] From these results, discussions can be made as follows: Test
Nos. 1 to 10, 13, 15, 19 to 21, and 25 are Examples fulfilling the
requirements defined in the present invention, thereby indicating
that parts having satisfactory balance between strength and
ductility were obtained.
[0093] In contrast, Test Nos. 11 to 12, 14, 16 to 18, and 22 to 24
are Comparative Examples not fulfilling any of the requirements
defined in the present invention, thereby deteriorating any of the
characteristics. More specifically, Test No. 11 was the case where
steel having insufficient C content (steel grade K shown in Table
1) was used, so that retained austenite was not secured, thereby
obtaining only low elongation (EL). Test No. 12 was the case where
steel having insufficient Si content (steel grade L shown in Table
1), so that retained austenite was not secured, thereby obtaining
only low elongation (EL).
[0094] Test No. 14 was intended for conventional 2MnB5 equivalent
steel (steel grade N shown in Table 1), so that retained austenite
was not secured, thereby obtaining only low elongation (EL),
although high strength was obtained. Test No. 16 was the case where
cold-rolled steel sheet having low reduction was used, so that the
formed product had a structure containing ferrite at 25% by area,
thereby lowering elongation (EL). Test No. 17 was the case where
the heating temperature was lower than Ac.sub.1 transformation
point, so that the formed product had a structure containing
ferrite at 81% by area (the remainder was martensite and cementite)
and retained austenite was not secured, thereby lowering elongation
(EL) and tensile strength. Test No. 18 was the case where the
heating temperature was higher than A value, so that ferrite and
bainitic ferrite were not secured by excessive formation of
martensite, thereby lowering elongation (EL).
[0095] Test No. 22 was the case where the average cooling rate in
cooling 1 was high, so that ferrite was not secured by the
formation of bainitic ferrite, thereby lowering elongation (EL).
Test No. 23 was the case where the average cooling rate in cooling
1 was low and the cooling rate changing temperature was low, so
that the formed product had a structure containing ferrite at 83%
by area (the remainder was bainitic ferrite) and retained austenite
was not secured, thereby lowering elongation (EL). Test No. 24 was
the case where the forming finishing temperature was high, so that
pearlite was formed in the structure of the formed product and
retained austenite was not secured, thereby lowing elongation
(EL).
INDUSTRIAL APPLICABILITY
[0096] The present invention makes it possible to provide a hot
press-formed product, including a steel sheet formed by a hot
press-forming method, and having a metallic structure that contains
ferrite at 30% to 80% by area, bainitic ferrite at lower than 30%
by area (not including 0%), martensite at 30% by area or lower (not
including 0%), and retained austenite at 3% to 20% by area, whereby
balance between strength and elongation can be controlled in a
proper range and high ductility can be achieved.
DESCRIPTION OF REFERENCE NUMERALS
[0097] 1 Punch [0098] 2 Die [0099] 3 Blank holder [0100] 4 Steel
sheet (Blank)
* * * * *
References