U.S. patent application number 14/131824 was filed with the patent office on 2014-05-22 for high-strength steel sheet for warm forming and process for producing same.
This patent application is currently assigned to JFE Steel Corporation. The applicant listed for this patent is Yoshimasa Funakawa, Noriaki Kosaka, Hidekazu Okubo, Masato Shigemi. Invention is credited to Yoshimasa Funakawa, Noriaki Kosaka, Hidekazu Okubo, Masato Shigemi.
Application Number | 20140141280 14/131824 |
Document ID | / |
Family ID | 47557861 |
Filed Date | 2014-05-22 |
United States Patent
Application |
20140141280 |
Kind Code |
A1 |
Kosaka; Noriaki ; et
al. |
May 22, 2014 |
HIGH-STRENGTH STEEL SHEET FOR WARM FORMING AND PROCESS FOR
PRODUCING SAME
Abstract
Provided is a high-strength steel sheet having good warm press
formability and excellent strength and ductility after warm press
forming, and a method for manufacturing such. The high-strength
steel sheet has a tensile strength at room temperature not less
than 780 MPa, a yield stress at a heating temperature range of
400.degree. C. to 700.degree. C. not more than 80% of the yield
stress at room temperature, total elongation at the heating
temperature range not less than 1.1 times the total elongation at
room temperature, yield stress and total elongation after the steel
sheet is heated to the heating temperature range, subjected to a
strain of not more than 20%, and cooled from the heating
temperature to room temperature, not less than 70% of the yield
stress and total elongation, respectively, at room temperature
before the heating.
Inventors: |
Kosaka; Noriaki;
(Fukuyama-shi, JP) ; Funakawa; Yoshimasa;
(Chiba-shi, JP) ; Shigemi; Masato; (Chiba-shi,
JP) ; Okubo; Hidekazu; (Chiba-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kosaka; Noriaki
Funakawa; Yoshimasa
Shigemi; Masato
Okubo; Hidekazu |
Fukuyama-shi
Chiba-shi
Chiba-shi
Chiba-shi |
|
JP
JP
JP
JP |
|
|
Assignee: |
JFE Steel Corporation
Tokyo
JP
|
Family ID: |
47557861 |
Appl. No.: |
14/131824 |
Filed: |
July 11, 2012 |
PCT Filed: |
July 11, 2012 |
PCT NO: |
PCT/JP2012/004462 |
371 Date: |
January 9, 2014 |
Current U.S.
Class: |
428/659 ;
148/507; 420/120; 420/122; 420/124; 420/126; 420/8; 72/364 |
Current CPC
Class: |
C22C 38/12 20130101;
C21D 8/0263 20130101; C22C 38/04 20130101; B21B 1/026 20130101;
Y10T 428/12799 20150115; C22C 38/001 20130101; C21D 6/005 20130101;
C21D 9/46 20130101; C22C 38/06 20130101; C22C 38/14 20130101; C22C
38/002 20130101; C22C 38/02 20130101 |
Class at
Publication: |
428/659 ; 72/364;
420/8; 420/120; 420/126; 420/122; 420/124; 148/507 |
International
Class: |
C22C 38/14 20060101
C22C038/14; C21D 8/02 20060101 C21D008/02; C22C 38/00 20060101
C22C038/00; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; B21B 1/02 20060101
B21B001/02; C22C 38/12 20060101 C22C038/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 20, 2011 |
JP |
2011-158508 |
Claims
1. A high-strength steel sheet for warm press forming, wherein: the
tensile strength at room temperature is not less than 780 MPa, the
yield stress at a heating temperature range of 400.degree. C. to
700.degree. C. is not more than 80% of the yield stress at room
temperature, the total elongation at the heating temperature range
is not less than 1.1 times the total elongation at room
temperature, the yield stress of the steel sheet after the steel
sheet is heated to the heating temperature range, subjected to a
strain of not more than 20% and cooled from the heating temperature
to room temperature is not less than 70% of the yield stress at
room temperature before the heating, and the total elongation of
the steel sheet after the steel sheet is heated to the heating
temperature range, subjected to a strain of not more than 20% and
cooled from the heating temperature to room temperature is not less
than 70% of the total elongation at room temperature before the
heating.
2. The high-strength steel sheet for warm press forming according
to claim 1, wherein the steel sheet has a chemical composition
containing, in mass %: C: not less than 0.03% and not more than
0.14%, Si: not more than 0.3%, Mn: above 0.60% and not more than
1.8%, P: not more than 0.03%, S: not more than 0.005%, Al: not more
than 0.1%, N: not more than 0.005%, and Ti: not more than 0.25%,
the balance comprising Fe and inevitable impurities, and satisfying
Expressions (1) and (2) below, and wherein the steel sheet includes
a microstructure which has a matrix having a ferrite grain diameter
of not less than 1 .mu.m and a ferrite phase area fraction of not
less than 95% and in which a carbide having an average particle
diameter of not more than 10 nm is precipitated in the matrix:
([Ti]/48+[V]/51+[Mo]/96+[W]/184)>0.0031 (1)
0.8.ltoreq.([C]/12)/([Ti]/48+[V]/51+[Mo]/96+[W]/184).gtoreq.1.20
(2) wherein [C], [Ti], [V], [Mo] and [W] are contents (mass %) of
respective elements.
3. The high-strength steel sheet for warm press forming according
to claim 2, wherein the chemical composition further contains, in
mass %, one, or two or more of V: not more than 0.5%, Mo: not more
than 0.5% and W: not more than 1.0%.
4. The high-strength steel sheet for warm press forming according
to claim 1, wherein the steel sheet has a coating layer on the
surface.
5. The high-strength steel sheet for warm press forming according
to claim 4, wherein the coating layer is a hot-dip galvanized layer
or a galvannealed layer.
6. A method of working high-strength steel sheets for warm press
forming, comprising heating the high-strength steel sheet for warm
press forming described in claim 1 to a heating temperature range
of 400.degree. C. to 700.degree. C. and subjecting the steel sheet
to a strain of not more than 20%.
7. A method for manufacturing high-strength steel sheets for warm
press forming, comprising: heating a steel slab to a temperature of
not less than 1100.degree. C. and not more than 1350.degree. C.,
hot rolling the steel slab to a steel sheet at a finishing
temperature of not less than 820.degree. C., starting cooling
within 2 seconds after the hot rolling, cooling the steel sheet at
an average cooling rate of not less than 30.degree. C./s in the
temperature range from a temperature of not less than 820.degree.
C. to a coiling temperature, and coiling the steel sheet into a
coil at a coiling temperature of not less than 550.degree. C. and
not more than 680.degree. C., the steel slab having a chemical
composition containing, in mass %: C: not less than 0.03% and not
more than 0.14%, Si: not more than 0.3%, Mn: above 0.60% and not
more than 1.8%, P: not more than 0.03%, S: not more than 0.005%,
Al: not more than 0.1%, N: not more than 0.005%, and Ti: not more
than 0.25%, the balance comprising Fe and inevitable impurities,
the chemical composition satisfying Expressions (1) and (2) below:
([Ti]/48+[V]/51+[Mo]/96+[W]/184)>0.0031 (1)
0.8.ltoreq.([C]/12)/([Ti]/48+[V]/51+[Mo]/96+[W]/184).gtoreq.1.20
(2) wherein [C], [Ti], [V], [Mo] and [W] are contents (mass %) of
respective elements.
8. The method for manufacturing high-strength steel sheets for warm
press forming according to claim 7, wherein the chemical
composition further contains, in mass %, one, or two or more of V:
not more than 0.5%, Mo: not more than 0.5% and W: not more than
1.0%.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is the U.S. National Phase application of
PCT/JP2012/004462, filed Jul. 11, 2012, and claims priority to
Japanese Patent Application No. 2011-158508, filed Jul. 20, 2011,
the disclosures of each of these applications being incorporated
herein by reference in their entireties for all purposes.
FIELD OF THE INVENTION
[0002] The present invention concerns with steel sheets useful for
warm press forming at a forming temperature range of 400.degree. C.
to 700.degree. C. The invention relates to a high-strength steel
sheet for warm press forming which has a tensile strength (TS) at
room temperature of not less than 780 MPa, which exhibits such a
good ductility that the steel sheet can be worked even under severe
forming conditions at the above forming temperature range, and
which shows small changes in mechanical characteristics between
before and after warm press forming, and to a method for
manufacturing such steel sheets.
BACKGROUND OF THE INVENTION
[0003] From the viewpoint of global environmental conservation, the
automobile industry as a whole recently aims at improving the fuel
efficiency of automobiles in order to reduce CO.sub.2 emissions.
Improvements in fuel efficiency can be attained most effectively by
making automobiles lighter through reducing the thickness of parts
to be used. However, the thinning of parts lowers the
crashworthiness of automobiles and thus results in a decrease in
safety. Accordingly, the weight reduction of automobile bodies
entails that parts are reduced in thickness and are increased in
strength. Because a lot of automobile parts are manufactured by
forming steel sheets into desired shapes, however, higher strength
of steel sheets being formed increases the probability of the
occurrence of problems such as deterioration in shape fixability,
overloads to molds, and the occurrence of cracks, necking and
wrinkles.
[0004] As an approach to solving the above problems, Patent
Literature 1 proposes a technique in which a steel sheet is heated
to an austenitic range, starts to be formed with a mold at a
temperature of not less than the Ac.sub.3 transformation point, and
is quenched simultaneously with the forming by removing heat
through the mold and is hardened by martensite transformation. This
technique thus provides steel sheets exhibiting hardenability after
hot press forming and excellent impact characteristics. Further,
Patent Literature 2 proposes a steel sheet for warm press forming
which has a microstructure containing not less than 10% by volume
of a bainite phase with a high solute carbon content and a high
dislocation density, not more than 10% by volume of a total of a
pearlite phase and a martensite phase, and the balance being a
ferrite phase. It is described that when a steel sheet having this
microstructure is subjected to warm press forming at temperatures
of not less than 250.degree. C., a large amount of strain aging
hardening can be obtained during the forming as well as the
subsequent cooling with the result that the warm press formed steel
sheet exhibits markedly improved strength.
PATENT DOCUMENTS
[0005] PTL 1: Japanese Unexamined Patent Application Publication
No. 2004-211197 [0006] PTL 2: Japanese Unexamined Patent
Application Publication No. 2002-256388
SUMMARY OF THE INVENTION
[0007] Steel sheets having a tensile strength at room temperature
of not less than 780 MPa are very difficult to form into a desired
shape by cold press forming because the steel sheets being formed
still have high strength and low shape fixability to cause the
occurrence of spring back. Further, such forming of steel sheets
keeping high strength incurs a heavy load to the mold and shortens
the life of the mold.
[0008] According to the hot press forming technique proposed in
Patent Literature 1, the formed steel sheets exhibit poor ductility
because the martensite phase which is hard and poor in ductility is
utilized. Thus, forming of such steel sheets into a desired shape
cannot produce automobile parts having high strength and excellent
ductility. Since automobile parts are required to exhibit desired
impact absorption performance in case of crash, automobile parts
with insufficient ductility are problematic in that the impact
absorption performance during crash is low. In addition, because
the technique proposed in Patent Literature 1 entails heating of
steel sheets to an austenitic range during forming, mass production
of automobile parts utilizing the technique has a concern that high
energy costs are incurred in the forming step.
[0009] On the other hand, in warm press forming, a steel sheet as a
workpiece is heated before forming to lower the strength of the
steel sheet and to increase the ductility so that the steel sheet
is formed while deformation resistance is lowered and shape
fixability is improved. Thus, warm press forming can suppress the
occurrence of spring back and reduces the gall of the mold.
Further, the enhancement in ductility by heating allows steel
sheets to be formed into complicated shapes. If tensile strength
and ductility are not decreased after warm press forming, the
impact absorption performance of formed parts is not deteriorated.
In addition, warm press forming is advantageous also in terms of
energy costs because the above effects are obtained by heating at a
lower temperature than in the technique of Patent Literature 1.
[0010] In the technique related to warm press forming proposed in
Patent Literature 2, however, the microstructure of the steel sheet
includes a bainite phase which is hard and poor in ductility. In
addition, the strength of the steel sheet is increased by strain
aging, and this further reduces the ductility and causes the
problematic occurrence of cracks or mold damages during warm press
forming.
[0011] Further, because automobile parts and the like are used in a
severely corrosive environment, coating treatments such as hot-dip
galvanization and galvannealing are frequently carried out in the
production of those parts from steel sheets in order to achieve
corrosion resistance. It is therefore necessary that steel sheets
to be used for such parts as automobile parts do not suffer
significant deteriorations in characteristics after coating
treatments. However, the techniques proposed in Patent Literatures
1 and 2 involve steel sheets including a martensite or bainite
phase which is largely deteriorated in quality by heat. That is,
when these steel sheets are subjected to coating treatments with
heating such as hot-dip galvanization and galvannealing, the heat
history due to such coating treatments causes a change in
characteristics, for example, a decrease in the strength of the
steel sheets.
[0012] The present invention advantageously solves the above
problems encountered in the art. It provides a high-strength steel
sheet suited for warm press forming which is excellent in
workability (formability) during warm press forming and is
applicable to warm press forming even under severe conditions and
which has a small change in quality by heat and thus ensures minor
deteriorations in strength and ductility after warm press forming,
as well as to provide a method for manufacturing such high-strength
steel sheets and a method of use of such high-strength steel
sheets.
[0013] In order to solve the aforementioned problems, the present
inventors carried out extensive studies on various factors that
would affect the warm press formability (such as ductility and
strength before, during and after heating) of high-strength steel
sheets. As a result, the present inventors have found that as long
as the yield stress at a prescribed heating temperature range (warm
press forming temperature range) is not more than 80% of the yield
stress at room temperature and the total elongation at the heating
temperature range is not less than 1.1 times the total elongation
at room temperature, even a high-strength steel sheet having a
tensile strength at room temperature of not less than 780 MPa shows
excellent warm press formability by exhibiting a lowered
deformation resistance as well as an increased ductility at the
warm press forming temperature range and can be formed into a
complicated shape. Further, the inventors have found that such
steel sheets also exhibit excellent shape fixability. Furthermore,
the inventors have found that strength and ductility required for
automobile parts can be ensured even after warm press forming as
long as steel sheets are such that the yield stress and the total
elongation after the steel sheets are heated to the heating
temperature range, subjected to a strain of not more than 20% and
cooled to room temperature are respectively not less than 70% of
the yield stress and the total elongation at room temperature
before the heating.
[0014] The present inventors then studied microstructures and
chemical compositions that would allow steel sheets to exhibit the
above characteristics.
[0015] First, the present inventors focused on a ferrite phase
having excellent ductility and a small change in quality by heat,
and came up with a configuration in which the microstructure of a
steel sheet is controlled to be substantially a ferrite single
phase before, during and after warm press forming. Further, the
present inventors have found that a steel sheet substantially
composed of a ferrite single phase in which a dislocation movement
in the ferrite phase is easily activated by heating achieves
improvements in warm press formability and in shape fixability
because such a steel sheet exhibits a lowered deformation
resistance as well as an enhanced ductility when heated to a warm
press forming temperature of not less than 400.degree. C., and have
further found that such a steel sheet exhibits excellent ductility
even after warm press forming.
[0016] In view of the fact that sufficient strength of steel sheets
cannot be obtained with a ferrite single phase, the present
inventors studied approaches to increasing the strength of steel
sheets substantially composed of a ferrite single phase. Although
strain aging hardening due to solute carbon and nitrogen generated
during warm press forming can increase the strength of steel sheets
after warm press forming, the ductility of steel sheets exhibited
during and after warm press forming is insufficient. Further, an
approach to increasing strength by grain refining strengthening is
not suited for materials to be subjected to warm press forming
because grains are grown during heating.
[0017] The present inventors then arrived at the use of
precipitation strengthening by the dispersion of fine carbides.
Further, the present inventors have found that in order to improve
warm press formability as well as strength and ductility after warm
press forming, it is appropriate to increase the strength of steel
sheets by precipitating fine titanium carbide or further vanadium
carbide, molybdenum carbide and tungsten carbide in a matrix
substantially composed of a ferrite single phase. According to the
studies carried out by the present inventors, these carbides do not
become coarse at a warm press forming temperature range (a heating
temperature range) of not more than 700.degree. C. and remain
finely precipitated even after warm press forming. That is, the
present inventors have found that steel sheets exhibiting excellent
strength even after warm press forming can be obtained by
precipitating these carbides in a matrix substantially composed of
a ferrite single phase.
[0018] Furthermore, the present inventors have found that in order
to obtain the above desired microstructure of steel sheets, it is
important to control the contents of the elements forming the
carbides, namely, the content of titanium or the contents of
titanium, vanadium, molybdenum and tungsten in appropriate ranges
as well as to control the content of titanium or the contents of
titanium, vanadium, molybdenum and tungsten relative to the content
of carbon in an appropriate range. Furthermore, the present
inventors have found that controlling the conditions in cooling and
coiling after hot rolling in appropriate ranges is important in the
production of steel sheets having the above desired microstructure,
in particular, in order to suppress the coarsening of the
carbides.
[0019] The present invention has been completed based on the above
findings. A summary of embodiments of the invention is as
follows.
[0020] [1] A high-strength steel sheet for warm press forming,
characterized in that the tensile strength at room temperature is
not less than 780 MPa, the yield stress at a heating temperature
range of 400.degree. C. to 700.degree. C. is not more than 80% of
the yield stress at room temperature, the total elongation at the
heating temperature range is not less than 1.1 times the total
elongation at room temperature, the yield stress of the steel sheet
after the steel sheet is heated to the heating temperature range,
subjected to a strain of not more than 20% and cooled from the
heating temperature to room temperature is not less than 70% of the
yield stress at room temperature before the heating, and the total
elongation of the steel sheet after the steel sheet is heated to
the heating temperature range, subjected to a strain of not more
than 20% and cooled from the heating temperature to room
temperature is not less than 70% of the total elongation at room
temperature before the heating.
[0021] [2] The high-strength steel sheet for warm press forming
described in [1], wherein the steel sheet has a chemical
composition containing, in mass %:
[0022] C: not less than 0.03% and not more than 0.14%, Si: not more
than 0.3%,
[0023] Mn: above 0.60% and not more than 1.8%, P: not more than
0.03%,
[0024] S: not more than 0.005%, Al: not more than 0.1%,
[0025] N: not more than 0.005%, and Ti: not more than 0.25%,
[0026] the balance comprising Fe and inevitable impurities, and
satisfying Expressions (1) and (2) below, and wherein the steel
sheet includes a microstructure which has a matrix having a ferrite
grain diameter of not less than 1 .mu.m and a ferrite phase area
fraction of not less than 95% and in which a carbide having an
average particle diameter of not more than 10 nm is precipitated in
the matrix:
Expressions
[0027] ([Ti]/48+[V]/51+[Mo]/96+[W]/184)>0.0031 (1)
0.8.ltoreq.([C]/12)/([Ti]/48+[V]/51+[Mo]/96+[W]/184).gtoreq.1.20
(2) [0028] ([C], [Ti], [V], [Mo] and [W]: contents (mass %) of
respective elements).
[0029] [3] The high-strength steel sheet for warm press forming
described in [2], wherein the chemical composition further
contains, in mass %, one, or two or more of V: not more than 0.5%,
Mo: not more than 0.5% and W: not more than 1.0%.
[0030] [4] The high-strength steel sheet for warm press forming
described in any of [1] to [3], wherein the steel sheet has a
coating layer on the surface.
[0031] [5] The high-strength steel sheet for warm press forming
described in [4], wherein the coating layer is a hot-dip galvanized
layer or a galvannealed layer.
[0032] [6] A method of working high-strength steel sheets for warm
press forming, including heating the high-strength steel sheet for
warm press forming described in any of [1] to [5] to a heating
temperature range of 400.degree. C. to 700.degree. C. and
subjecting the steel sheet to a strain of not more than 20%.
[0033] [7] A method for manufacturing high-strength steel sheets
for warm press forming, including heating a steel slab to a
temperature of not less than 1100.degree. C. and not more than
1350.degree. C., hot rolling the steel slab to a steel sheet at a
finishing temperature of not less than 820.degree. C., starting
cooling within 2 seconds after the hot rolling, cooling the steel
sheet at an average cooling rate of not less than 30.degree. C./s
in the temperature range from a temperature of not less than
820.degree. C. to a coiling temperature, and coiling the steel
sheet into a coil at a coiling temperature of not less than
550.degree. C. and not more than 680.degree. C., the steel slab
having a chemical composition containing, in mass %:
[0034] C: not less than 0.03% and not more than 0.14%, Si: not more
than 0.3%,
[0035] Mn: above 0.60% and not more than 1.8%, P: not more than
0.03%,
[0036] S: not more than 0.005%, Al: not more than 0.1%,
[0037] N: not more than 0.005%, and Ti: not more than 0.25%,
[0038] the balance comprising Fe and inevitable impurities, the
chemical composition satisfying Expressions (1) and (2) below:
Expressions
[0039] ([Ti]/48+[V]/51+[Mo]/96+[W]/184)>0.0031 (1)
0.8.ltoreq.([C]/12)/([Ti]/48+[V]/51+[Mo]/96+[W]/184).gtoreq.1.20
(2) [0040] ([C], [Ti], [V], [Mo] and [W]: contents (mass %) of
respective elements).
[0041] [8] The method for manufacturing high-strength steel sheets
for warm press forming described in [7], wherein the chemical
composition further contains, in mass %, one, or two or more of V:
not more than 0.5%, Mo: not more than 0.5% and W: not more than
1.0%.
[0042] According to the present invention, high-strength steel
sheets having excellent warm press formability can be obtained
which have a tensile strength of not less than 780 MPa and can be
warm press formed with a low press load into parts with complicated
shapes. In addition to excellent warm press formability, the
high-strength steel sheets of the invention have minor decreases in
strength and ductility after warm press forming, and are therefore
suitable for applications such as automobile parts requiring impact
absorption performance in case of crash. Further, the high-strength
steel sheets of the invention include a microstructure having a
relatively small change in quality by heat, and consequently the
characteristics of the steel sheets are not substantially altered
even when the steel sheets have a heat history due to treatments
such as coating treatments. Accordingly, the inventive steel sheets
may be also applicable to the manufacturing of parts required
coating treatment from the viewpoint of corrosion resistance. Thus,
the invention achieves marked industrial effects.
DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0043] Hereinbelow, the present invention will be described in
detail with reference to exemplary embodiments.
[0044] High-strength steel sheets for warm press forming according
to the invention are steel sheets preferably having a tensile
strength at room temperature of not less than 780 MPa. In the
invention, the term "room temperature" indicates 22.+-.5.degree.
C.
[0045] A high-strength steel sheet for warm press forming according
to an embodiment of the invention is characterized in that the
tensile strength at room temperature is not less than 780 MPa, the
yield stress at a heating temperature range of 400.degree. C. to
700.degree. C. is not more than 80% of the yield stress at room
temperature, the total elongation at the heating temperature range
is not less than 1.1 times the total elongation at room
temperature, the yield stress of the steel sheet after the steel
sheet is heated to the heating temperature range, subjected to a
strain of not more than 20% and then cooled from the heating
temperature to room temperature is not less than 70% of the yield
stress at room temperature before the heating, and the total
elongation of the steel sheet after the steel sheet is heated to
the heating temperature range, subjected to a strain of not more
than 20% and then cooled from the heating temperature to room
temperature is not less than 70% of the total elongation at room
temperature before the heating.
[0046] In the invention, warm press forming at temperatures of
400.degree. C. to 700.degree. C. is assumed. Thus, the invention
specifies characteristics of steel sheets at a heating temperature
range of 400.degree. C. to 700.degree. C.
[0047] In the case of a steel sheet having a tensile strength at
room temperature of not less than 780 MPa, the deformation
resistance of the steel sheet exhibited during warm press forming
cannot be reduced sufficiently if the yield stress at the heating
temperature range of 400.degree. C. to 700.degree. C. exceeds 80%
of the yield stress at room temperature. Consequently, the press
load during warm press forming has to be increased to cause a
problematic decrease in mold life. The application of a high press
load naturally involves a large press machine. However, a large
press machine makes it difficult to perform warm press forming at a
desired temperature because the temperature of a steel sheet heated
to a warm press forming temperature is decreased during the travel
to the press machine. Further, such a steel sheet is not
sufficiently improved in terms of shape fixability and fails to
achieve the aforementioned merits of warm press forming.
[0048] In the case of a steel sheet having a tensile strength at
room temperature of not less than 780 MPa, the formability of the
steel sheet exhibited during warm press forming is not sufficiently
improved if the total elongation at the heating temperature range
of 400.degree. C. to 700.degree. C. is less than 1.1 times the
total elongation at room temperature. As a result, problematic
defects such as cracks occur during forming.
[0049] Warm press forming of a steel sheet often results in a
decrease in the strength of the warm press formed steel sheet
primarily due to heating of the steel sheet. Further, when a steel
sheet is subjected to warm press forming, the ductility of the
steel sheet after the warm press forming is sometimes lowered
problematically due to the strain aging or work hardening.
[0050] In the warm press forming of a steel sheet into a
(automobile) part, the steel sheet is usually strained about 1 to
10% in terms of equivalent plastic strain. Thus, the present
invention assumes warm press forming at the temperature range of
400.degree. C. to 700.degree. C. with a strain of 20% at a maximum.
That is, the present invention specifies the yield stress and the
total elongation of a steel sheet after the steel sheet is heated
to the heating temperature range of 400.degree. C. to 700.degree.
C., subjected to a strain of not more than 20% and then cooled from
the heating temperature to room temperature. From the view point of
maintaining ductility between before and after warm press forming,
the strain applied is desirably not more than 15%.
[0051] In the invention, the "strain" applied to a steel sheet
heated to the heating temperature range of 400.degree. C. to
700.degree. C. indicates an equivalent plastic strain (.epsilon.)
and is usually represented by the following equation as described
in, for example, Non Patent Literature 1.
= 2 3 { ( xx p ) 2 + ( yy p ) 2 + ( zz p ) 2 } + 1 3 { ( .gamma. xy
p ) 2 + ( .gamma. yz p ) 2 + ( .gamma. zx p ) 2 } [ Math . 1 ]
##EQU00001##
[0052] NPL 1: Husahito YOSHIDA, "Dansosei Rikigaku no Kiso (Basics
of elastic plastic dynamics)", first edition, third printing,
published by KYORITSU SHUPPAN CO., LTD., Oct. 5, 1999, p. 155.
[0053] In the case of a steel sheet having a tensile strength at
room temperature of not less than 780 MPa, the strength and the
total elongation of the steel sheet after warm press forming are
insufficient if the yield stress and the total elongation after the
warm press forming are each less than 70% of the yield stress and
the total elongation at room temperature before heating (before the
warm press forming). If such a steel sheet is warm press formed
into an automobile part with a desired shape, the impact absorption
performance during crash is insufficient and the reliability as an
automobile part is deteriorated.
[0054] Thus, the present invention provides that the yield stress
and the total elongation of a steel sheet after the steel sheet is
heated to the heating temperature range of 400.degree. C. to
700.degree. C., subjected to a strain of not more than 20% and then
cooled from the heating temperature to room temperature are
preferably not less than 70% of the yield stress and the total
elongation at room temperature before the thermal forming.
[0055] In order for a steel sheet to exhibit the above
characteristics, it is preferable that the steel sheet have a
chemical composition containing, in mass %, C: not less than 0.03%
and not more than 0.14%, Si: not more than 0.3%, Mn: above 0.60%
and not more than 1.8%, P: not more than 0.03%, S: not more than
0.005%, Al: not more than 0.1%, N: not more than 0.005%, and Ti:
not more than 0.25%, the balance being Fe and inevitable
impurities, and satisfying Expressions (1) and (2) below, as well
as that the steel sheet include a microstructure which has a matrix
having a ferrite grain diameter of not less than 1 .mu.m and a
ferrite phase area fraction of not less than 95% and in which a
carbide having an average particle diameter of not more than 10 nm
is precipitated in the matrix:
Expressions
[0056] ([Ti]/48+[V]/51+[Mo]/96+[W]/184)>0.0031 (1)
0.8.ltoreq.([C]/12)/([Ti]/48+[V]/51+[Mo]/96+[W]/184).gtoreq.1.20
(2) [0057] ([C], [Ti], [V], [Mo] and [W]: contents (mass %) of
respective elements).
[0058] First, there will be described the reasons why the
microstructure and the carbides are limited.
[0059] If a steel sheet includes hard phases such as martensite
phase and bainite phase during and after warm press forming, it
becomes difficult to obtain desired ductility (total elongation).
Thus, it is preferable in the invention that the matrix of a steel
sheet be substantially a ferrite single phase. When a steel sheet
has the above chemical composition and when the matrix of the steel
sheet before the steel sheet is heated to a warm press forming
temperature is substantially a ferrite single phase, the matrix of
the steel sheet substantially remains a ferrite single phase even
when the steel sheet is heated to the heating temperature range
(warm press forming temperature range) of 400.degree. C. to
700.degree. C. The ductility is increased as the steel sheet is
heated so that the total elongation at the heating temperature
range of 400.degree. C. to 700.degree. C. can be brought to not
less than 1.1 times the total elongation at room temperature.
[0060] Further, when a steel sheet having the above chemical
composition is warm press formed at the temperature range of
400.degree. C. to 700.degree. C., there is substantially no
decrease in ductility during the warm press forming because the
recovery of dislocation takes place during the forming of the steel
sheet. Further, because the microstructure is not changed by the
cooling of the warm press formed steel sheet to room temperature,
the matrix of the steel sheet substantially remains a ferrite
single phase and the steel sheet exhibits excellent ductility.
Accordingly, configuring the matrix of a steel sheet (before warm
press forming) to be substantially a ferrite single phase ensures
that the total elongation of the steel sheet after the steel sheet
is heated to the heating temperature range of 400.degree. C. to
700.degree. C., subjected to a strain of not more than 20% and then
cooled from the heating temperature to room temperature is not less
than 70% of the total elongation at room temperature before the
thermal forming (before the warm press forming).
[0061] Heating the ferrite phase to not less than 400.degree. C.
lowers the deformation resistance because a dislocation movement is
activated with an increase in temperature, resulting in a decrease
in the yield stress of the steel sheet. Thus, the yield stress of
the steel sheet at the heating temperature range of 400.degree. C.
to 700.degree. C. becomes not more than 80% of the yield stress of
the steel sheet at room temperature.
[0062] The ferrite grain diameter is preferably not less than 1
.mu.m. If the ferrite grain diameter is less than 1 .mu.m, grain
growth easily occurs during warm press forming and the stability of
the quality of the warm press formed steel sheet is deteriorated.
If the ferrite grain diameter is excessively large, however, it may
be sometimes difficult to obtain a desired strength of the steel
sheet because the amount of grain refining strengthening is small.
Thus, it is preferable that the ferrite grain diameter be not more
than 15 .mu.m, and more preferably not less than 1 .mu.m and not
more than 12 .mu.m.
[0063] In order to achieve excellent ductility or to suppress a
change in quality by heat, it is preferable that the matrix of a
steel sheet be a ferrite single phase. If hard phases such as
bainite phase and martensite phase are mixed in the ferrite phase,
warm press formability may be lowered because these hard phases and
ferrite phase have a large difference in hardness. Even if the
matrix is not a perfect ferrite single phase, however, the steel
sheet can exhibit sufficient ductility during and after warm press
forming and can be kept from a change in quality by heat as long as
the matrix is substantially a ferrite single phase, that is, the
area fraction of the ferrite phase is not less than 95% relative to
the area of the entirety of the matrix.
[0064] In the steel sheet of the invention, exemplary metallic
microstructures other than the ferrite phase include cementite,
pearlite, bainite phase, martensite phase and retained austenite
phase. The presence of these phases is acceptable as long as the
total area fraction thereof is not more than 5% relative to the
entire microstructure.
[0065] As discussed above, sufficient ductility (total elongation)
of a steel sheet during and after warm press forming can be ensured
by configuring the matrix of the steel sheet before the warm press
forming to be substantially a ferrite single phase. However, it is
difficult to obtain the desired strength of the steel sheet
(tensile strength: not less than 780 MPa) with the ferrite single
phase.
[0066] Thus, the present invention aims at increasing the strength
of the steel sheet by precipitating fine carbides, namely, titanium
carbide or further vanadium carbide, molybdenum carbide and
tungsten carbide in the matrix substantially composed of a ferrite
single phase. Here, the desired strength of the steel sheet
(tensile strength: not less than 780 MPa) cannot be obtained if the
average particle diameter of the carbides exceeds 10 nm. Thus, the
average particle diameter of the carbides is specified to be not
more than 10 nm, and preferably not more than 7 nm.
[0067] Carbides present in a steel sheet are usually coarsened
during heating and lower their precipitation strengthening
performance. However, the above carbides (titanium carbide or
further vanadium carbide, molybdenum carbide and tungsten carbide)
having an average particle diameter of not more than 10 nm are not
coarsened and maintain an average particle diameter of not more
than 10 nm as long as the heating temperature is not more than
700.degree. C. That is, the steel sheet having a matrix which is
substantially a ferrite single phase and which includes the
carbides (titanium carbide or further vanadium carbide, molybdenum
carbide and tungsten carbide) with an average particle diameter of
not more than 10 nm is heated to the heating temperature range of
400.degree. C. to 700.degree. C. and warm press formed while a
decrease in the strength of the steel sheet after the warm press
forming is significantly suppressed because the coarsening of the
carbides is suppressed. Accordingly, the configuration in which the
steel sheet has a microstructure having a matrix which is
substantially a ferrite single phase and which includes the
carbides with an average particle diameter of not more than 10 nm
ensures that the yield stress of the steel sheet after the steel
sheet is heated to the heating temperature range of 400.degree. C.
to 700.degree. C., subjected to a strain of up to 20% and then
cooled from the heating temperature to room temperature is not less
than 70% of the yield stress at room temperature before the thermal
forming (before the warm press forming).
[0068] Next, there will be described the reasons why the chemical
composition is limited. The term "%" in the following chemical
composition of components indicates mass % unless otherwise
mentioned.
[0069] C: not less than 0.03% and not more than 0.14%
[0070] Carbon forms carbides with titanium or further vanadium,
molybdenum and tungsten, and is finely dispersed in steel. Thus,
this element is essential in order to increase the strength of
steel sheets. In order to obtain a steel sheet having a tensile
strength of not less than 780 MPa, the steel preferably contains
carbon in at least 0.03% or more. On the other hand, if the C
content exceeds 0.14%, toughness is markedly deteriorated and the
steel sheet fails to exhibit good impact absorption performance
(represented by, for example, TS.times.El wherein TS: tensile
strength and El: total elongation). Thus, the C content is
preferably not less than 0.03% and not more than 0.14%, and more
preferably not less than 0.04% and not more than 0.13%.
[0071] Si: not more than 0.3%
[0072] Silicon is a solid solution strengthening element and lowers
warm press formability by inhibiting the decrease in strength at
the heating temperature range. It is therefore preferable that
silicon be reduced as much as possible. However, a Si content of up
to 0.3% is acceptable. Thus, the Si content is preferably not more
than 0.3%, and more preferably not more than 0.1%.
[0073] Mn: above 0.60% and not more than 1.8%
[0074] Manganese is an element which contributes to strengthening
by lowering the transformation point of steel and facilitating the
occurrence of fine precipitates. Thus, it is preferable that the Mn
content be in excess of 0.60%, and more preferably not less than
0.8%. If the Mn content exceeds 1.8%, however, the workability of
steel sheets is markedly deteriorated. Thus, the Mn content is
preferably not more than 1.8%, and more preferably not more than
1.5%.
[0075] P: not more than 0.030%
[0076] Phosphorus is an element which has very high solid solution
strengthening performance and inhibits the decrease in the strength
of steel sheets during warm press forming. Further, phosphorus is
an element which segregates at grain boundaries to lower ductility
during and after warm press forming. Thus, phosphorus is preferably
reduced as much as possible, and the P content is preferably not
more than 0.030%.
[0077] S: not more than 0.005%
[0078] Sulfur is a harmful element which is present as an inclusion
in steel. In particular, this element bonds to manganese to form a
sulfide and lowers ductility at warm temperatures. Thus, sulfur is
preferably reduced as much as possible, and the S content is
preferably not more than 0.005%.
[0079] Al: not more than 0.1%
[0080] Aluminum is an element which acts as a deoxidizer. In order
to obtain this effect, the Al content is preferably not less than
0.02%. At the same time, however, aluminum lowers ductility by
forming oxides. If the Al content exceeds 0.1%, the inclusions come
to exert considerable adverse effects on ductility at warm
temperatures. Thus, the Al content is preferably not more than
0.1%, and more preferably not more than 0.07%.
[0081] N: not more than 0.005%
[0082] Nitrogen bonds to titanium and vanadium in the steel making
process to form coarse nitrides, thereby significantly lowering the
strength of steel sheets. Thus, nitrogen is preferably reduced as
much as possible, and the N content is preferably not more than
0.005%.
[0083] Ti: not more than 0.25%
[0084] Titanium is an element which contributes to strengthening of
steel sheets by forming a carbide with carbon. Titanium is an
element which contributes to strengthening of steel sheets by
forming a carbide with carbon. In order to obtain this effect, the
Ti content is preferably not less than 0.01%. In the case where
vanadium, molybdenum and tungsten described later are not added,
the Ti content is preferably not less than 0.13%, and more
preferably not less than 0.15% in order to obtain a steel sheet
strength of not less than 780 MPa. If the Ti content exceeds 0.25%,
however, coarse TiC remains during the heating of a slab before hot
rolling to cause the formation of microvoids. Thus, the Ti content
is preferably not more than 0.25%, and more preferably not more
than 0.20%.
[0085] While a preferred basic chemical composition in the
invention is described above, the steel may further contain one, or
two or more of V: not more than 0.5%, Mo: not more than 0.5% and W:
not more than 1.0% in addition to the basic chemical
composition.
[0086] V: not more than 0.5%, Mo: not more than 0.5% and W: not
more than 1.0%
[0087] Similarly to titanium, vanadium, molybdenum and tungsten are
elements which contribute to strengthening of steel sheets by
forming carbides. Thus, these elements may be optionally added in
the case where a further increase in the strength of steel sheets
is required. In order to obtain this effect, it is preferable that
the V content be not less than 0.01%, the Mo content 0.01%, and the
W content not less than 0.01%.
[0088] However, any V content exceeding 0.5% causes the facilitated
coarsening of the carbide. Thus, the carbide is coarsened at the
heating temperature range of 400.degree. C. to 700.degree. C. and
will hardly have an average particle diameter of not more than 10
nm after cooled to room temperature.
[0089] Thus, the V content is preferably not more than 0.5%, and
more preferably not more than 0.35%.
[0090] If the Mo content and the W content exceed 0.5% and 1.0%,
respectively, ferrite transformation is extremely delayed. As a
result, a bainite phase and a martensite phase come to be mixed in
the microstructure of the steel sheet and make it difficult for the
microstructure to be substantially a ferrite single phase. Thus,
the Mo content and the W content are preferably not more than 0.5%
and not more than 1.0%, respectively, and more preferably not more
than 0.4% and not more than 0.9%, respectively.
[0091] In order for a steel sheet with the above chemical
composition to have a tensile strength at room temperature of not
less than 780 MPa, exhibit excellent ductility during warm press
forming and achieve excellent strength and ductility after warm
press forming, Expressions (1) and (2) described below need to be
satisfied.
([Ti]/48+[V]/51+[Mo]/96+[W]/184)>0.0031 (1)
0.8.ltoreq.([C]/12)/([Ti]/48+[V]/51+[Mo]/96+[W]/184).gtoreq.1.20
(2)
[0092] In Expressions (1) and (2), [C], [Ti], [V], [Mo] and [W] are
the contents (mass %) of the respective elements. In the case where
[V], [Mo] and [W] are each less than 0.01% or the elements are
absent, these contents are regarded as zero in the calculation
using the above Expressions.
([Ti]/48+[V]/51+[Mo]/96+[W]/184)>0.0031 (1)
[0093] In an embodiment of the invention in which the steel sheet
has a matrix that is substantially a ferrite single phase, as
already described above, the strength of the steel sheet is
increased by precipitation strengthening in which carbides,
specifically, titanium carbide or further vanadium carbide,
molybdenum carbide and tungsten carbide, having an average particle
diameter of not more than 10 nm are finely dispersed in the matrix.
Thus, it is necessary that the steel contain titanium or further
vanadium, molybdenum and tungsten as carbide-forming elements in
required amounts in order to increase the tensile strength of the
steel sheet. Regarding the contents of titanium or further
vanadium, molybdenum and tungsten as the carbide-forming elements,
the amounts of carbides precipitated in the matrix become
insufficient and it is difficult for the steel sheet to have a
tensile strength of not less than 780 MPa if
([Ti]/48+[V]/51+[Mo]/96+[W]/184) is 0.0031 or less. Thus, when the
aforementioned chemical composition of steel is adopted,
([Ti]/48+[V]/51+[Mo]/96+[W]/184) is specified to be more than
0.0031, and preferably more than 0.0033.
0.8.ltoreq.([C]/12)/([Ti]/48+[V]/51+[Mo]/96+[W]/184).gtoreq.1.20
(2)
[0094] If the steel sheet contains a large amount of solute carbon,
strain aging occurs during warm press forming and the ductility of
the steel sheet during and after the warm press forming is
deteriorated. Further, the presence of hard and micrometer-order
cementite in the steel sheet causes a decrease in the ductility of
the steel sheet during and after warm press forming because
microvoids are formed at the interface between the ferrite phase
and the cementite during the warm press forming.
[0095] That is, in order for a steel sheet with the above chemical
composition to have a tensile strength at room temperature of not
less than 780 MPa, exhibit excellent ductility during warm press
forming and achieve excellent strength and ductility after warm
press forming, it is preferable that the fine carbides be actively
precipitated in the steel sheet as well as that the amount of
carbon which is not involved in the formation of carbides be
controlled so as to reduce the amounts of solute carbon and
cementite in the steel sheet to a minimum.
[0096] Thus, in the case where the aforementioned chemical
composition of steel is adopted, the content of titanium or further
the contents of vanadium, molybdenum and tungsten relative to the
content of carbon are controlled.
[0097] If ([C]/12)/([Ti]/48+[V]/51+[Mo]/96+[W]/184) becomes less
than 0.8, the carbide-forming elements are not sufficiently
precipitated as carbides and the steel sheet fails to achieve a
tensile strength at room temperature of not less than 780 MPa.
[0098] On the other hand, if
([C]/12)/([Ti]/48+[V]/51+[Mo]/96+[W]/184) exceeds 1.2, excess
carbon will be present as solute carbon or cementite without
forming bonds with carbides with the result that good ductility
cannot be obtained during heating at the heating temperature range
of 400.degree. C. to 700.degree. C. (during warm press forming) or
after the warm press forming.
[0099] Thus, in the case where the aforementioned chemical
composition of steel is adopted,
([C]/12)/([Ti]/48+[V]/51+[Mo]/96+[W]/184) is controlled to satisfy
Expression (2), namely, to be not less than 0.8 and not more than
1.20.
[0100] In the invention, the balance after the deduction of the
aforementioned elements is typically iron and inevitable
impurities. Examples of the inevitable impurities include elements
which are not specified in the present invention such as O
(oxygen), Cu, Cr, Ni and Co. The presence of such elements is
acceptable as long as the total content thereof is not more than
0.5%.
[0101] As mentioned above, the steel sheet having a matrix which is
substantially a ferrite single phase and in which fine carbides are
precipitated can be heat treated without suffering adverse effects
on its quality by the heat treatment as long as the heating
temperature is up to 700.degree. C. Thus, the steel sheet can be
subjected to a coating treatment to form, on its surface, a coating
layer such as an electroplating layer, an electroless plating layer
or a hot-dip plating layer. The alloy components forming the
coating layers are not particularly limited, and zinc coatings and
zinc alloy coatings may be used.
[0102] As mentioned above, the steel sheet of the invention can
exhibit excellent warm press formability and can also exhibit
excellent strength and ductility after the warm press forming when
the steel sheet has been subjected to an equivalent tensile strain
of not more than 20% at the heating temperature range of
400.degree. C. to 700.degree. C. Thus, the high-strength steel
sheet for warm press forming according to the invention is
preferably made into a part such as an automobile part by being
heated to the heating temperature range of 400.degree. C. to
700.degree. C. and being warm press formed through working which
applies a strain of not more than 20%.
[0103] Next, a method for manufacturing the high-strength steel
sheets for warm press forming according to an embodiment of the
invention will be described.
[0104] For example, the inventive high-strength steel sheet for
warm press forming may be obtained by producing a molten steel
having the aforementioned composition to made into a steel slab,
heating the steel slab to a temperature of not less than
1100.degree. C. and not more than 1350.degree. C., then hot rolling
the steel slab to a steel sheet at a finishing temperature (the
temperature of the steel sheet at the completion of the hot
rolling) of not less than 820.degree. C., starting cooling within 2
seconds after the hot rolling, cooling the steel sheet at an
average cooling rate of not less than 30.degree. C./s in the
temperature range from a temperature of not less than 820.degree.
C. to a coiling temperature, and coiling the steel sheet into a
coil at a coiling temperature of not less than 550.degree. C. and
not more than 680.degree. C.
[0105] In the invention, the steel may be produced by melting by
any method without limitation. For example, a steel having the
desired chemical composition may be produced by melting in a
furnace such as a converter or an electric furnace, and by
subsequent secondary refining in a vacuum degassing furnace. The
molten steel is made into a steel slab by a known casting method,
and preferably by a continuous casting method in view of
productivity and quality. After being cast, the steel slab is
heated and hot rolled in accordance with the inventive method.
[0106] Temperature for heating steel slab: not less than
1100.degree. C. and not more than 1350.degree. C.
[0107] In the heating before hot rolling, it is necessary that a
substantially homogeneous austenite phase is formed in the steel
slab and coarse carbides in the steel slab be dissolved. Heating
the steel slab at a temperature of less than 1100.degree. C. cannot
dissolve coarse carbides, and consequently the amount of carbides
finely dispersed in the final steel sheet obtained is reduced,
resulting in a marked decrease in the strength of the steel sheet.
On the other hand, heating at a temperature exceeding 1350.degree.
C. results in the occurrence of scale inclusion, and consequently
surface quality is deteriorated. Thus, the temperature for heating
the steel slab is specified to be not less than 1100.degree. C. and
not more than 1350.degree. C., and preferably not less than
1150.degree. C. and not more than 1300.degree. C.
[0108] When the steel slab, that is after casting, has the above
heating temperature (not less than 1100.degree. C. and not more
than 1350.degree. C.), the steel slab may be directly rolled
without being heated. In the practice of hot rolling of the steel
slab by rough rolling and finish rolling, the rough rolling may be
performed under any conditions without limitation.
[0109] Finishing temperature: not less than 820.degree. C.
[0110] If the finishing temperature is less than 820.degree. C.,
elongation of ferrite grains occurs in the microstructure and
further a mixed grain microstructure having ferrite grain diameters
significantly different each other is generated, causing a marked
decrease in the strength of steel sheets. In order to obtain a
microstructure having a ferrite grain diameter of not less than 1
.mu.m, it is necessary that the number of nucleation sites during
ferrite transformation be not excessively large. The number of
nucleation sites is closely related to the strain energy
accumulated in the steel sheet during rolling. If the finishing
temperature is less than 820.degree. C., excessive accumulation of
strain energy cannot be prevented and it becomes difficult to
obtain a microstructure having a ferrite grain diameter of not less
than 1 .mu.m. Thus, the finishing temperature is specified to be
not less than 820.degree. C., and preferably not less than
860.degree. C.
[0111] Time from completion of hot rolling to initiation of
cooling: not more than 2 seconds
[0112] Immediately after finish rolling, a large amount of strain
energy is accumulated in the austenite phase in the steel. As a
result, strain-induced precipitation occurs in the steel
immediately after finish rolling. The carbides resulting from this
strain-induced precipitation tend to become coarse because the
precipitation occurs at a high temperature. Thus, the generation of
large amounts of carbides by the strain-induced precipitation makes
it difficult to realize fine precipitation of carbides in the final
steel sheet obtained. In the present invention, therefore, it is
preferred that cooling be initiated as quickly as possible after
the completion of hot rolling so as to suppress the occurrence of
strain-induced precipitation. Thus, the present invention specifies
that cooling is initiated preferably within 2 seconds after the hot
rolling.
[0113] Average cooling rate in temperature range from temperature
of not less than 820.degree. C. to coiling temperature: not less
than 30.degree. C./s
[0114] Similarly as described above, the coarsening of carbides
generated by strain-induced precipitation proceeds easily as the
steel is held at a high temperature for a longer time. It is
therefore necessary that the steel be quenched after the finish
rolling. In order to suppress the coarsening of carbides, the steel
sheet needs to be cooled at an average cooling rate of not less
than 30.degree. C./s, and desirably not less than 50.degree. C./s
in the temperature range from a temperature of not less than
820.degree. C. to a coiling temperature.
[0115] Coiling temperature: not less than 550.degree. C. and not
more than 680.degree. C.
[0116] If the coiling temperature is less than 550.degree. C., the
amount of carbides precipitated in the steel sheet becomes
insufficient to cause a decrease in the strength of the steel
sheet. On the other hand, coiling at a temperature of above
680.degree. C. causes the precipitated carbides to become coarse,
resulting in a decrease in the strength of the steel sheet. Thus,
the coiling temperature is specified to be not less than
550.degree. C. and not more than 680.degree. C., and preferably not
less than 575.degree. C. and not more than 660.degree. C.
[0117] After the hot rolling, the characteristics of the steel
sheet are not changed irrespective of whether the steel sheet has
scales attached on its surface or the steel sheet has been descaled
by pickling.
[0118] The steel sheet obtained above may be subjected to a coating
treatment to form, on the surface of the steel sheet, a coating
layer such as a hot-dip galvanized layer or a galvannealed layer.
The coating layer may be formed by a known coating method, for
example, by dipping the steel sheet into a plating bath. The
coating amount (the thickness of the coating layer) is variable
depending on the temperature of the plating bath and the duration
of soaking in the bath as well as the speed of lifting from the
bath. It is preferable that the thickness of the coating layer be
not less than 4 .mu.m, and more preferably not less than 6 .mu.m.
An alloying treatment for forming a galvannealed layer may be
carried out in a furnace capable of heating the surface of the
steel sheet, such as a gas furnace, after the coating
treatment.
EXAMPLES
[0119] Steels Nos. A to L which had chemical compositions described
in Table 1 were produced in a converter and then cast into steel
slabs. The steel slabs were heated and soaked at temperatures set
out in Table 2, and were hot rolled under conditions described in
Table 2 to produce coils of hot-rolled steel sheets (sheet
thickness 1.6 mm) Nos. 1 to 18. Of the steel sheets (the hot-rolled
steel sheets) described in Table 2, the steel sheets Nos. 9, 11 and
13 (test pieces Nos. o, q and s set out in Table 3 described later)
were passed through a continuous hot-dip galvanization line in
which they were heated to 700.degree. C., soaked in a hot-dip
galvanization bath at 460.degree. C. and subjected to an alloying
treatment at 500.degree. C., thereby forming a galvannealed layer
with a thickness of 7 .mu.m on the surface of each of the steel
sheets. Some of the steel sheet No. 2 was treated in the same
manner as above to form a galvannealed layer (test pieces Nos. b to
e set out in Table 3 described later), and the other was not passed
through the continuous hot-dip galvanization lines, namely, any
coating layer was not formed (test pieces Nos. f to h set out in
Table 3 described later).
TABLE-US-00001 TABLE 1 Chemical composition (mass %) Expression
Expression Steel No. C Si Mn P S Al N Ti Mo V W (1) *1 (2) *2 A
0.042 0.252 1.75 0.012 0.0025 0.045 0.0048 0.152 -- -- -- 0.0032
1.11 B 0.041 0.010 1.24 0.015 0.0023 0.041 0.0032 0.114 -- 0.05 --
0.0034 1.02 C 0.084 0.025 1.08 0.012 0.0024 0.042 0.0035 0.145 0.20
0.15 -- 0.0080 0.87 D 0.071 0.011 0.86 0.013 0.0022 0.041 0.0033
0.092 0.28 0.11 -- 0.0070 0.85 E 0.124 0.022 1.35 0.012 0.0023
0.052 0.0041 0.151 0.08 0.30 -- 0.0099 1.05 F 0.096 0.015 1.52
0.015 0.0021 0.045 0.0038 0.141 0.05 -- 0.80 0.00781 1.02 G 0.086
0.012 1.25 0.015 0.0026 0.038 0.0029 0.028 0.12 0.32 -- 0.0081 0.88
H 0.032 0.010 1.18 0.016 0.0022 0.039 0.0033 0.012 0.11 0.13 --
0.0039 0.68 I 0.043 0.029 0.57 0.016 0.0021 0.043 0.0041 0.153 --
-- -- 0.0032 1.12 J 0.041 0.016 1.35 0.017 0.0018 0.041 0.0027
0.080 0.05 0.04 -- 0.0030 1.15 K 0.065 0.016 1.35 0.017 0.0018
0.041 0.0027 0.145 0.05 0.03 -- 0.0041 1.31 L 0.091 0.023 1.55
0.018 0.0025 0.029 0.0028 0.012 0.12 0.02 1.17 0.0083 0.92 *1:
Value of ([Ti]/48 + [V]/51 + [Mo]/96 + [W]/184) *2: Value of
([C]/12)/([Ti]/484 + [V]/51 + [Mo]/96 + [W]/184)
TABLE-US-00002 TABLE 2 Steel Steel Slab heating Finishing Time from
completion of finish Average cooling Coiling sheet No. No. temp.
(.degree. C.) temp. (.degree. C.) rolling to initiation of cooling
(s) rate (.degree. C./s) temp. (.degree. C.) Remarks 1 A 1250 890
1.1 74 590 Inv. Ex. 2 B 1250 890 0.9 85 610 Inv. Ex. 3 1070 860 1.2
81 600 Comp. Ex. 4 1240 810 1.0 79 610 Comp. Ex. 5 1250 860 2.6 94
570 Comp. Ex. 6 1260 900 1.3 27 600 Comp. Ex. 7 1250 910 0.9 84 710
Comp. Ex. 8 1250 910 0.9 83 540 Comp. Ex. 9 C 1250 890 0.8 85 610
Inv. Ex. 10 D 1250 870 0.7 81 580 Inv. Ex. 11 E 1260 880 0.9 86 600
Inv. Ex. 12 F 1250 900 1.1 82 610 Inv. Ex. 13 G 1250 900 0.9 85 600
Inv. Ex. 14 H 1250 890 1.0 86 630 Comp. Ex. 15 I 1250 900 1.2 84
580 Comp. Ex. 16 J 1250 900 0.8 85 580 Comp. Ex. 17 K 1260 900 1.3
75 590 Comp. Ex. 18 L 1250 900 1.1 76 600 Comp. Ex.
[0120] Test pieces were sampled from the obtained hot-rolled steel
sheets and were subjected to a tensile test, microstructure
observation, precipitate observation, and an enlarge test at a warm
press forming temperature range to determine the tensile strength
at room temperature, the yield stress and the total elongation at
the warm press forming temperature range, and the yield stress and
the total elongation after the test pieces had been subjected to a
strain (up to 15% strain) described in Table 3 at the warm press
forming temperature range and cooled to room temperature. Further,
test pieces were sampled from the obtained hot-rolled steel sheets
and were analyzed to determine the ferrite grain diameter, the
ferrite phase area fraction and the average particle diameter of
carbides before the steels were heated to the warm press forming
temperature range, as well as to determine the hole expanding ratio
at the warm press forming temperature range. Testing methods were
as described below.
(i) Tensile Test
[0121] 13-B tensile test pieces specified in JIS Z 2201 (1998) were
sampled from the obtained hot-rolled steel sheets in a direction
perpendicular to the rolling direction, and a tensile test was
performed in accordance with JIS G 0567 (1998) to determine the
average yield stress (YS-1), tensile strength (TS-1) and total
elongation (El-1) at room temperature (22.+-.5.degree. C.) as well
as to determine the average yield stress (YS-2), tensile strength
(TS-2) and total elongation (El-2) at temperatures in the
temperature range of 400 to 800.degree. C. Further, test pieces
were sampled in the same manner as above and were subjected to a
tensile test under the same conditions as those in the above
elevated temperature tensile test to introduce a strain described
in Table 3 at each of the temperatures; thereafter, the test pieces
were cooled to room temperature (22.+-.5.degree. C.) at a cooling
rate described in Table 3. The resultant test pieces were tensile
tested at room temperature to determine the average yield stress
(YS-3), tensile strength (TS-3) and total elongation (El-3).
[0122] All the above tensile tests were performed at a cross head
speed of 10 ram/min. In the elevated temperature tensile test in
the heating temperature range, the test pieces were heated in an
electric furnace to a temperature set out in Table 3 and were held
for 15 minutes after the temperature of the test pieces became
stable in the testing temperature .+-.3.degree. C.
(ii) Microstructure Observation
[0123] Test pieces were sampled from the hot-rolled steel sheets. A
central portion along the sheet thickness in a cross section
(L-cross section) parallel to the rolling direction was etched with
5% Nital and the exposed microstructure was observed with a
scanning electron microscope (SEM) at .times.400 magnification. Ten
fields of view were photographed.
[0124] To determine the ferrite phase fraction (area fraction), the
(SEM) images of the microstructure obtained above were analyzed to
separate the ferrite phase from other phases, and the area fraction
of the ferrite phase relative to the observed fields of view was
obtained. While the ferrite phase is characteristic in that
corrosion marks are not observed in the grains and the grain
boundaries are seen as smooth curves, grain boundaries observed as
linear shape were counted as part of the ferrite phase.
[0125] The ferrite grain diameter was measured by a linear
intercept method in accordance with ASTM E112-10 with respect to
the images of the microstructure obtained above.
[0126] To determine the average particle diameter of carbides, a
sample was prepared by a thin-film method from a central portion
along the sheet thickness of the hot-rolled steel sheet, and was
observed with a transmission electron microscope (magnification:
.times.120000), and the diameters of at least 100 particles (100 to
300 particles) of carbides were measured, the results being
averaged. In the calculation of the particle diameters of carbides,
particles larger than the micrometer order, namely, coarse
cementite larger than 1 .mu.m and nitrides were excluded.
(iii) Enlarge Test at Warm Press Forming Temperature Range (Warm
Press Formability)
[0127] Testing temperature: An enlarge test was performed at
550.degree. C., and warm press formability was evaluated based on
the obtained hole expanding ratio.
[0128] The enlarge test was carried out in accordance with
standards by The Japan Iron and Steel Federation (T1001-1996). In
detail, a 100 W.times.100 L mm test piece was sampled from the
hot-rolled steel sheet, and a 10 mm diameter hole was formed by
punching in the center of the test piece with a clearance of 12%.
Next, the test piece was heated and soaked at 600.degree. C. in a
heating furnace, and a cylindrical base as a punch was inserted
into the hole of the test piece at 550.+-.25.degree. C. The hole in
the test piece was enlarged until the hole expanding ratio
calculated by Expression (3) below became 80%.
(Hole expanding ratio)=(diameter of hole after test-diameter of
hole before test(=10 mm))/(diameter of hole before test).times.100
(3)
[0129] After the enlarge test, each test piece was inspected for
the presence or absence of a crack running through the edge face of
the hole. Further, part of the test piece was cut after the test,
and a central portion along the sheet thickness of the exposed
cross section was subjected to a Vickers test. The testing load in
the Vickers test was 1 kgf, and the hardness was measured with
respect to 5 points.
[0130] Warm press formability was evaluated to be good
(.largecircle.) when there was no crack running through the edge
face of the hole and the Vickers hardness of the test piece was not
less than 260 HV. Warm press formability was evaluated to be poor
(x) when there was a crack running through the edge face of the
hole or when the Vickers hardness of the test piece was less than
260 HV.
[0131] The obtained results are set out in Tables 3 and 4.
TABLE-US-00003 TABLE 3 Microstructure of steel sheet Mechanical
Tensile conditions Ferrite Ferrite Precipitate characteristics at
elevated temp. Test Steel grain phase area particle at room temp.
Heating piece sheet diameter fraction diameter YS-1 TS-1 El-1 temp.
Strain No. No. (.mu.m) (%) (nm) (MPa) (MPa) (%) (.degree. C.) (%) a
1 3.5 99 3 708 795 18.2 500 10 b 2 3.6 100 4 757 836 18.0 400 5 c
500 8 d 600 10 e 700 10 f 800 10 g 500 8 h 500 26 i 3 6.9 94 3 604
746 20.6 500 10 j 4 5.8 98 8 653 768 18.4 500 12 k 5 4.3 100 12 642
774 19.4 500 15 l 6 8.6 100 11 623 742 20.1 500 15 m 7 5.9 100 14
603 726 21.1 500 10 n 8 3.1 94 3 623 769 19.7 500 5 o 9 3.8 100 4
987 1085 14.8 570 2 p 10 3.6 100 4 804 887 16.7 690 15 q 11 3.5 100
3 1104 1187 12.6 540 10 r 12 3.2 100 5 996 1071 14.7 500 5 s 13 3.6
100 4 1006 1093 14.3 550 10 t 14 3.8 100 5 713 767 19.5 500 15 u 15
3.2 99 13 639 743 21.3 500 12 v 16 3.3 98 4 702 771 19.4 500 15 w
17 3.5 92 3 707 803 17.3 400 10 x 18 3.6 74 6 885 1092 9.8 500 10
Tensile conditions Mechanical Mechanical at elevated temp.
characteristics characteristics Test Rate of during heating after
heating piece cooling after YS-2 TS-2 El-2 YS-3 TS-3 El-3 No.
tension (.degree. C./s) (MPa) (MPa) (%) (MPa) (MPa) (%) Remarks a
12 502 518 38.2 711 782 20.3 Inv. Ex. b 35 552 571 25.2 768 839
19.8 Inv. Ex. c 95 522 538 39.6 748 822 20.3 Inv. Ex. d 82 462 475
50.4 745 828 20.7 Inv. Ex. e 45 409 415 63.0 567 630 21.3 Inv. Ex.
f 22 351 368 79.2 507 603 10.5 Comp. Ex. g 150 523 534 39.7 742 825
20.4 Inv. Ex. h 20 524 541 39.1 763 838 10.1 Comp. Ex. i 15 508 526
30.9 578 598 16.7 Comp. Ex. j 13 457 471 42.3 540 626 18.3 Comp.
Ex. k 14 469 484 40.7 628 748 19.6 Comp. Ex. l 14 449 462 44.2 602
743 19.7 Comp. Ex. m 13 440 457 44.3 574 699 20.4 Comp. Ex. n 15
529 538 27.6 678 779 13.1 Comp. Ex. o 15 622 638 42.9 938 1020 18.4
Inv. Ex. p 14 571 585 61.8 611 670 17.5 Inv. Ex. q 15 740 763 30.2
938 1031 13.4 Inv. Ex. r 15 727 741 30.9 1006 1105 14.6 Inv. Ex. s
16 654 672 36.6 945 1039 15.8 Inv. Ex. t 14 521 530 41.0 606 666
20.7 Comp. Ex. u 10 454 573 46.9 703 764 23.0 Comp. Ex. v 15 498
517 44.6 631 686 20.3 Comp. Ex. w 11 643 681 17.5 777 845 18.1
Comp. Ex. x 25 797 948 8.5 593 689 10.2 Comp. Ex.
TABLE-US-00004 TABLE 4 Test Steel piece sheet TS-1 Changes in
quality relative to quality at room temp. Warm press formability
No. No. (MPa) (YS-2)/(YS-1) (El-2)/(El-1) (YS-3)/(YS-1)
(El-3)/(El-1) Cracks Hardness (HV) Evaluation Remarks a 1 795 0.71
2.1 1.00 1.12 .largecircle. .largecircle. .largecircle. Inv. Ex. b
2 836 0.73 1.4 1.01 1.10 .largecircle. .largecircle. .largecircle.
Inv. Ex. c 0.69 2.2 0.99 1.13 .largecircle. .largecircle.
.largecircle. Inv. Ex. d 0.61 2.8 0.98 1.15 .largecircle.
.largecircle. .largecircle. Inv. Ex. e 0.54 3.5 0.75 1.18
.largecircle. .largecircle. .largecircle. Inv. Ex. f 0.46 4.4 0.67
0.68 .largecircle. X X Comp. Ex. g 0.69 2.2 0.98 1.13 .largecircle.
.largecircle. .largecircle. Inv. Ex. h 0.69 2.2 1.01 0.56 X
.largecircle. X Comp. Ex. i 3 746 0.84 1.7 0.76 0.81 X X X Comp.
Ex. j 4 768 0.70 2.4 0.71 0.99 .largecircle. X X Comp. Ex. k 5 774
0.73 2.3 0.83 1.01 .largecircle. X X Comp. Ex. l 6 742 0.72 2.5
0.80 0.98 .largecircle. X X Comp. Ex. m 7 726 0.73 2.5 0.76 0.97
.largecircle. X X Comp. Ex. n 8 769 0.85 1.5 0.90 0.66 X X X Comp.
Ex. o 9 1085 0.63 2.9 0.95 1.24 .largecircle. .largecircle.
.largecircle. Inv. Ex. p 10 887 0.71 3.7 0.76 1.05 .largecircle.
.largecircle. .largecircle. Inv. Ex. q 11 1187 0.67 2.4 0.85 1.06
.largecircle. .largecircle. .largecircle. Inv. Ex. r 12 1071 0.73
2.1 1.01 0.99 .largecircle. .largecircle. .largecircle. Inv. Ex. s
13 1093 0.65 2.6 0.94 1.10 .largecircle. .largecircle.
.largecircle. Inv. Ex. t 14 767 0.73 2.1 0.85 1.06 .largecircle. X
X Comp. Ex. u 15 743 0.71 2.2 1.10 1.08 .largecircle. X X Comp. Ex.
v 16 771 0.71 2.3 0.90 1.05 .largecircle. X X Comp. Ex. w 17 803
0.91 1.0 1.10 1.05 X X X Comp. Ex. x 18 1092 0.90 0.9 0.67 1.04 X X
X Comp. Ex.
[0132] For all the steel sheets in Inventive Examples (the test
pieces Nos. a, b, c, d, e, g, o, p, q, r and s), the tensile
strength at room temperature (TS-1) was not less than 780 MPa, the
yield stress of the steel sheet heated to the temperature range of
400.degree. C. to 700.degree. C. (YS-2) was not more than 80% of
the yield stress at room temperature (YS-1), and the total
elongation of the steel sheet heated to the temperature range of
400.degree. C. to 700.degree. C. (El-2) was not less than 1.1 times
the total elongation at room temperature (El-1). Further, for all
the steel sheets in Inventive Examples, the yield stress (YS-3) and
the total elongation (El-3) after the steel sheet was subjected to
a strain of not more than 20% at the above heating temperature
range and cooled to room temperature were each not less than 70% of
the yield stress (YS-1) and the total elongation (El-1) at room
temperature (before the introduction of the strain). Furthermore,
all the steel sheets in Inventive Examples exhibited good warm
press formability.
[0133] On the other hand, the steel sheets in Comparative Examples
(the test pieces Nos. f, h, i, j, k, l, m, n, t, u, v, w and x),
that is, the steel sheets which fail to satisfy the inventive range
in terms of any of the tensile strength at room temperature (TS-1),
the yield stress (YS-2) or the total elongation (El-2) of the steel
sheet heated to the temperature range of 400.degree. C. to
700.degree. C., and the yield stress (YS-3) or the total elongation
(El-3) after the steel sheet was subjected to a strain of not more
than 20% at the above heating temperature range and cooled to room
temperature, exhibited poor warm press formability.
[0134] When the steel sheets were worked under conditions outside
the warm press forming conditions according to the invention (the
test pieces Nos. f and h), the yield stress after the steel sheet
was cooled to room temperature (YS-3) failed to be not less than
70% of the yield stress at room temperature before heating (YS-1),
or the total elongation after the steel sheet was cooled to room
temperature (El-3) failed to be not less than 70% of the total
elongation at room temperature before heating (El-1) as a
result.
[0135] Because the testing temperature (the heating temperature) in
the elevated temperature tensile test for the test piece No. f in
Comparative Example had exceeded 700.degree. C., an austenite phase
was formed and carbides became coarse during heating, resulting in
a marked deterioration in mechanical characteristics after
heating.
[0136] Because an excessively large strain was applied to the test
piece No. h in Comparative Example, the dislocation was not fully
recovered during heating and the steel sheet cooled to room
temperature after heating exhibited poor ductility.
[0137] For the test pieces Nos. i and j in Comparative Examples,
the tensile strength at room temperature (TS-1) did not reach 780
MPa because of the low temperature for heating the slab and because
of the low finishing temperature, respectively.
[0138] In the test pieces Nos. k, l and m in Comparative Examples,
the average particle diameter of carbides was above 10 nm because
of the excessively long exposure to a high temperature after finish
rolling or because the average cooling rate or the coiling
temperature had been outside the inventive range. Consequently, the
tensile strength at room temperature (TS-1) did not reach 780
MPa.
[0139] For the test piece No. n in Comparative Example, a
sufficient amount of carbides was not obtained because of the low
coiling temperature. Consequently, the tensile strength at room
temperature (TS-1) did not reach 780 MPa. Further, because much
carbon was present in the form of solute carbon instead of being
precipitated as carbides, the strain aging precipitation of solute
carbon occurred during heating with the results that the decrease
in stress and the increase in ductility at the time of heating were
suppressed as well as that the steel sheet cooled to room
temperature after heating exhibited poor ductility.
[0140] For the test piece No. t in Comparative Example, the tensile
strength at room temperature (TS-1) did not reach 780 MPa because
Expression (2) failed to be satisfied and the balance among the
contents of carbide-forming elements, namely, carbon, titanium,
vanadium, tungsten and molybdenum, was not appropriate.
[0141] In the test piece No. u in Comparative Example, the tensile
strength at room temperature (TS-1) did not reach 780 MPa because
the Mn content was so low that carbides were precipitated at a high
temperature and became coarse.
[0142] For the test piece No. v in Comparative Example, the tensile
strength at room temperature (TS-1) did not reach 780 MPa because
Expression (1) was not satisfied and the amount of precipitated
carbides was insufficient.
[0143] The test piece No. w in Comparative Example failed to
satisfy Expression (2) and contained a large amount of carbon which
was not involved in the formation of carbides. As a result, strain
aging occurred during heating for warm press forming, the yield
stress at the heating temperature range (the warm press forming
temperature range) (YS-2) was high, and the total elongation at the
heating temperature range (the warm press forming temperature
range) (El-2) was insufficient. Thus, the steel sheet was shown to
be unsuited for warm press forming.
[0144] In the test piece No. x in Comparative Example, ferrite
transformation was delayed and the ferrite phase area fraction was
small because of the high W content. Consequently, deteriorations
were observed in mechanical characteristics at room temperature
after heating.
[0145] Next, among the steel sheets described in Table 2, the steel
sheets corresponding to Inventive Examples (Nos. 1, 2, 9, 10, 11,
12 and 13) were tensile tested in the same manner as described
above (the elevated temperature tensile test and the tensile test
after cooling to room temperature) to determine relations between
mechanical characteristics (yield stress and total elongation) at
the heating temperature range of 400 to 700.degree. C. as well as
the mechanical characteristics after the steel sheets were
subjected to a strain of not more than 20% at the heating
temperature range and cooled to room temperature, and the
mechanical characteristics at room temperature before heating.
[0146] In detail, a tensile test was carried out at a testing
temperature of 400.degree. C. or 650.degree. C. to determine the
average yield stress (Y2-2) and total elongation (El-2);
separately, test pieces were subjected to a tensile test at
400.degree. C. or 650.degree. C. in which a strain of not more than
20% described in Table 5 was applied to the test piece, and were
thereafter cooled to room temperature at a cooling rate described
in Table 5, and the resultant test pieces were tensile tested at
room temperature to determine the average yield stress (YS-3) and
total elongation (El-3). The results are described in Table 5.
TABLE-US-00005 TABLE 5 Microstructure of Mechanical Tensile
conditions steel sheet characteristics at elevated temp. Ferrite
grain Ferrite phase Precipitate at room temp. Rate of cooling Steel
diameter area fraction particle diameter YS-1 TS-1 El-1 Heating
temp. Strain after tension sheet No. (.mu.m) (%) (nm) (MPa) (MPa)
(%) (.degree. C.) (%) (.degree. C./s) 1 3.5 99 3 708 795 18.2 400 1
12 10 15 650 1 16 10 20 2 3.6 100 4 757 836 18.0 400 1 50 10 60 650
1 60 15 60 9 3.8 100 4 987 1085 14.8 400 1 15 10 16 650 1 14 18 15
10 3.6 100 4 804 887 16.7 400 1 52 10 55 650 1 59 18 58 11 3.5 100
3 1104 1187 12.6 400 1 16 10 15 650 1 16 18 15 12 3.2 100 5 996
1071 14.7 400 1 65 10 68 650 1 65 18 66 13 3.6 100 4 1006 1093 14.3
400 1 16 10 16 650 1 15 18 16 Mechanical Mechanical characteristics
characteristics Changes in quality relative during heating after
heating to quality at room temp. Steel YS-2 El-2 YS-3 El-3 (YS-2)/
(El-2)/ (YS-3)/ (El-3)/ sheet No. (MPa) (%) (MPa) (%) (YS-1) (El-1)
(YS-1) (El-1) Remarks 1 562 24.8 721 18.5 0.8 1.4 1.0 1.0 Inv. Ex.
711 16.5 1.0 0.9 415 54.6 715 19.1 0.6 3.0 1.0 1.0 705 20.3 1.0 1.1
2 552 25.2 762 18.6 0.7 1.4 1.0 1.0 Inv. Ex. 748 16.8 1.0 0.9 433
55.8 761 18.8 0.6 3.1 1.0 1.0 703 26.7 0.9 1.5 9 658 20.8 997 15.3
0.7 1.4 1.0 1.0 Inv. Ex. 964 12.6 1.0 0.9 521 49.9 991 15.7 0.5 3.4
1.0 1.1 829 22.5 0.8 1.5 10 586 22.1 807 16.4 0.7 1.3 1.0 1.0 Inv.
Ex. 784 14.9 1.0 0.9 502 50.2 806 17.2 0.6 3.0 1.0 1.0 706 24.4 0.9
1.5 11 763 16.7 1115 12.3 0.7 1.3 1.0 1.0 Inv. Ex. 1085 11.8 1.0
0.9 541 39.1 1098 13.4 0.5 3.1 1.0 1.1 982 20.2 0.9 1.6 12 624 20.5
1002 14.5 0.6 1.4 1.0 1.0 Inv. Ex. 976 12.8 1.0 0.9 535 49.0 994
14.5 0.5 3.3 1.0 1.0 847 22.6 0.9 1.5 13 654 18.4 1015 14.1 0.7 1.3
1.0 1.0 Inv. Ex. 990 12.9 1.0 0.9 539 47.2 994 15.0 0.5 3.3 1.0 1.0
845 22.0 0.8 1.5
[0147] In all the steel sheets according to the present invention,
as shown in Table 5, the tensile strength at room temperature
(TS-1) was not less than 780 MPa, the yield stress of the steel
sheet heated to the heating temperature range of 400.degree. C. to
700.degree. C. (YS-2) was not more than 80% of the yield stress at
room temperature (YS-1), the total elongation of the steel sheet
heated to the heating temperature range of 400.degree. C. to
700.degree. C. (El-2) was not less than 1.1 times the total
elongation at room temperature (El-1), the yield stress (YS-3) and
the total elongation (El-3) after the steel sheet was subjected to
a strain of not more than 20% at the heating temperature range
stated above and cooled to room temperature were each not less than
70% of the yield stress (YS-1) and the total elongation (El-1) at
room temperature (before the introduction of the strain).
[0148] In Inventive Examples where the microstructures and the
chemical compositions of the steel sheets were controlled to be the
preferred microstructures and chemical compositions, the
microstructures remain substantially a ferrite single phase at the
heating temperature range of 400.degree. C. to 700.degree. C., and
the state of carbides in the steel sheets does not change at the
heating temperature range to such an extent that the quality of the
steel sheets is adversely affected. Thus, the steel sheets which
have been heated to the heating temperature range (warm press
forming temperature range) and subjected to warm press forming may
be cooled to room temperature at any cooling rate without suffering
any adverse effects on the quality of the steel sheets after warm
press forming. Accordingly, the inventive high-strength steel
sheets for warm press forming can be applied to warm press forming
in a facility fitted with a quenching apparatus which rapidly cools
the steel sheets after warm press forming. It is needless to
mention that the inventive high-strength steel sheets for warm
press forming can also be applied to warm press forming in a
facility which is not fitted with such a quenching apparatus.
* * * * *