Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
Title of Invention: HIGH-STRENGTH STEEL SHEET FOR WARM PRESS
FORMING AND METHOD FOR MANUFACTURING THEREOF
Technical Field
[0001]
The present invention concerns with steel sheets useful
for warm press forming at a forming temperature range of
400 C to 700 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 Art
[0002]
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 CO2 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
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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.
[0003]
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 Ac3
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
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when a steel sheet having this microstructure is subjected
to warm press forming at temperatures of not less than 250 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.
Citation List
Patent Literature
[0004]
PTL 1: Japanese Unexamined Patent Application
Publication No. 2004-211197
PTL 2: Japanese Unexamined Patent Application
Publication No. 2002-256388
Summary of Invention
Technical Problem
[0005]
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.
[0006]
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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.
[0007]
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
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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.
[0008]
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.
[0009]
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
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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.
[0010]
The present invention advantageously solves the above
problems encountered in the art. It is an object of the
invention to provide 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.
Solution to Problem
[0011]
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
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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.
[0012]
The present inventors then studied microstructures and
chemical compositions that would allow steel sheets to
exhibit the above characteristics.
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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 C, and have further found that such a steel sheet
exhibits excellent ductility even after warm press forming.
[0013]
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
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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.
[0014]
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 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.
[0015]
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
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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.
[0016]
The present invention has been completed based on the
above findings. A summary of the invention is as follows.
[1] A steel sheet for warm press forming the steel
sheet 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% and not
less than 0.02%,
N: not more than 0.005%, and Ti: not more than 0.25%,
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%,
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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 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
([Ti]/48 + [V]/51 + [Mo]/96 + [W]/184) > 0.0031 === (1)
0.8 ([C]/12)/([Ti]/48 + [V]/51 + [Mo]/96 + [W]/184) 1.20
=== (2)
([C], [Ti], [V], [Mo] and [W]: contents (mass%) of
respective elements).
[0017]
[2] The steel sheet for warm press forming according to
[1], wherein the tensile strength of the steel sheet at room
temperature of not less than 780 MPa, the yield stress of
said steel sheet at a heating temperature range of 400 C to
700 C is not more than 80% of the yield stress at room
temperature, the total elongation of said steel sheet 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
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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.
[0018]
[3] The steel sheet for warm press forming according to
any one of [1] or [2], wherein the steel sheet has a coating
layer on the surface.
[0019]
[4] The steel sheet for warm press forming according to
[3], wherein the coating layer is a hot-dip galvanized layer
or a galvannealed layer.
[0020]
[5] A method of working steel sheets for warm press
forming, comprising heating the high-strength steel sheet
for warm press forming described in any one of [1] to [4] to
a heating temperature range of 400 C to 700 C and subjecting
the steel sheet to a strain of not more than 20%.
[0021]
[6] A method for manufacturing steel sheets for warm
press forming, comprising heating a steel slab to a
temperature of not less than 1100 C and not more than
1350 C, hot rolling the steel slab to a steel sheet at a
finishing temperature of not less than 820 C, starting
cooling within 2 seconds after the hot rolling, cooling the
steel sheet at an average cooling rate of not less than
30 C/s in the temperature range from a temperature of not
less than 820 C to a coiling temperature, and coiling the
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steel sheet into a coil at a coiling temperature of not less
than 550 C and not more than 680 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% and not
less than 0.02%,
N: not more than 0.005%, and Ti: not more than 0.25%,
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%
the balance comprising Fe and inevitable impurities,
the chemical composition satisfying Expressions (1) and (2)
below:
Expressions
([Ti]/48 + [V]/51 + [Mo]/96 + [W]/184) > 0.0031 === (1)
0.8 ([C]/12)/([Ti]/48 + [V]/51 + [Mo]/96 + [W]/184) 5_ 1.20
=== (2)
([C], [Ti], [V], [Mo] and [W]: contents (mass%) of
respective elements).
Advantageous Effects of Invention
[0024]
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
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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 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.
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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
[0025]
Hereinbelow, the present invention will be described in
detail.
High-strength steel sheets for warm press forming
according to the invention are steel sheets having a tensile
strength at room temperature of not less than 780 MPa. In
the invention, the term "room temperature" indicates 22
C.
A high-strength steel sheet for warm press forming
according to 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 C to 700 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
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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.
[0026]
In the invention, warm press forming at temperatures of
400 C to 700 C is assumed. Thus, the invention specifies
characteristics of steel sheets at a heating temperature
range of 400 C to 700 C.
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 C to
700 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
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not sufficiently improved in terms of shape fixability and
fails to achieve the aforementioned merits of warm press
forming.
[0027]
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 C to 700 C is less
than 1.1 times the total elongation at room temperature. As
a result, problematic defects such as cracks occur during
forming.
[0028]
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.
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 C to 700 C with a strain of 20% at
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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 C to 700 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%.
[0029]
In the invention, the "strain" applied to a steel sheet
heated to the heating temperature range of 400 C to 700 C
indicates an equivalent plastic strain (c) and is usually
represented by the following equation as described in, for
example, Non Patent Literature 1.
[0030]
[Math. 1]
E {(E P )2 +(EP Y + (EP. )2 } -"OP +(y.,PzY + )2
3
3 '3'
[0031]
NPL 1: Husahito YOSHIDA, "Dansosei Rikigaku no Kiso
(Basics of elastic plastic dynamics)", first edition, third
printing, published by KYORITSU SHUPPAN CO., LTD., October 5,
1999, p. 155.
[0032]
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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.
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 C to 700 C, subjected to a strain of not more than 20%
and then cooled from the heating temperature to room
temperature are not less than 70% of the yield stress and
the total elongation at room temperature before the thermal
forming.
[0033]
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
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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 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
nm is precipitated in the matrix:
Expressions
([Ti]/48 + [V]/51 + [Mo]/96 + [W]/184) > 0.0031 === (1)
0.8 ([C]/12)/([Ti]/48 + [V]/51 + [Mo]/96 + [W]/184) __ 1.20
=== (2)
([C], [Ti], [V], [Mo] and [W]: contents (mass%) of
respective elements).
[0034]
First, there will be described the reasons why the
microstructure and the carbides are limited.
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
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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 C to 700 C. The ductility is increased as the
steel sheet is heated so that the total elongation at the
heating temperature range of 400 C to 700 C can be brought
to not less than 1.1 times the total elongation at room
temperature.
[0035]
Further, when a steel sheet having the above chemical
composition is warm press formed at the temperature range of
400 C to 700 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 C to 700 C,
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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).
[0036]
Heating the ferrite phase to not less than 400 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 C to 700 C becomes not more than 80% of the
yield stress of the steel sheet at room temperature.
[0037]
The ferrite grain diameter is preferably not less than
1 m. If the ferrite grain diameter is less than 1 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 m, and more preferably not less than 1 m and not
more than 12 m.
[0038]
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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.
[0039]
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.
[0040]
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
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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.
[0041]
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.
[0042]
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 C. That is, the
steel sheet having a matrix which is substantially a ferrite
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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 C to 700 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 C to 700 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).
[0043]
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.
C: not less than 0.03% and not more than 0.14%
Carbon forms carbides with titanium or further vanadium,
CA 02840724 2013-12-30
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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 x 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%.
[0044]
Si: not more than 0.3%
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%.
[0045]
Mn: above 0.60% and not more than 1.8%
Manganese is an element which contributes to
strengthening by lowering the transformation point of steel
CA 02840724 2013-12-30
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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%.
[0046]
P: not more than 0.030%
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%.
[0047]
S: not more than 0.005%
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%.
[0048]
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Al: not more than 0.1%
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%.
[0049]
N: not more than 0.005%
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%.
[0050]
Ti: not more than 0.25%
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
CA 02840724 2013-12-30
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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%.
[0051]
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.
V: not more than 0.5%, Mo: not more than 0.5% and W:
not more than 1.0%
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%.
[0052]
However, any V content exceeding 0.5% causes the
CA 02840724 2013-12-30
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facilitated coarsening of the carbide. Thus, the carbide is
coarsened at the heating temperature range of 400 C to 700 C
and will hardly have an average particle diameter of not
more than 10 nm after cooled to room temperature.
Thus, the V content is preferably not more than 0.5%,
and more preferably not more than 0.35%.
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.
[0053]
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 ([C]/12)/([Ti]/48 + [V]/51 + [Mo]/96 + [W]/184)
1.20 === (2)
CA 02840724 2013-12-30
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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.
[0054]
([Ti]/48 + [V]/51 + [Mo]/96 + [W]/184) > 0.0031 =-- (1)
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,
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([Ti]/48 + [V]/51 + [Mo]/96 + [W]/184) is specified to be
more than 0.0031, and preferably more than 0.0033.
[0055]
0.8 __. ([C]/12)/([Ti]/48 + [V]/51 + [Mo]/96 + [W]/184)
1.20 === (2)
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.
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.
[0056]
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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.
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.
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 C to 700 C (during warm press forming) or after the warm
press forming.
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.
[0057]
In the invention, the balance after the deduction of
the aforementioned elements is iron and inevitable
impurities. Examples of the inevitable impurities include
elements which are not specified in the present invention
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such as 0 (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%.
[0058]
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 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.
[0059]
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 C to 700 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
CA 02840724 2013-12-30
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range of 400 C to 700 C and being warm press formed through
working which applies a strain of not more than 20%.
[0060]
Next, a method for manufacturing the high-strength
steel sheets for warm press forming according to the
invention will be described.
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 C and not more than 1350 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 C, starting cooling within
2 seconds after the hot rolling, cooling the steel sheet at
an average cooling rate of not less than 30 C/s in the
temperature range from a temperature of not less than 820 C
to a coiling temperature, and coiling the steel sheet into a
coil at a coiling temperature of not less than 550 C and not
more than 680 C.
[0061]
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
CA 02840724 2013-12-30
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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.
[0062]
Temperature for heating steel slab: not less than
1100 C and not more than 1350 C
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 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 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 C and
not more than 1350 C, and preferably not less than 1150 C
and not more than 1300 C.
When the steel slab, that is after casting, has the
above heating temperature (not less than 1100 C and not more
CA 02840724 2013-12-30
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than 1350 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.
[0063]
Finishing temperature: not less than 820 C
If the finishing temperature is less than 820 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 I tun, 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 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 Rm. Thus, the
finishing temperature is specified to be not less than 820 C,
and preferably not less than 860 C.
[0064]
Time from completion of hot rolling to initiation of
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cooling: not more than 2 seconds
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 necessary 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 within 2 seconds after the hot rolling.
[0065]
Average cooling rate in temperature range from
temperature of not less than 820 C to coiling temperature:
not less than 30 C/s
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
CA 02840724 2013-12-30
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coarsening of carbides, the steel sheet needs to be cooled
at an average cooling rate of not less than 30 C/s, and
desirably not less than 50 C/s in the temperature range from
a temperature of not less than 820 C to a coiling
temperature.
[0066]
Coiling temperature: not less than 550 C and not more
than 680 C
If the coiling temperature is less than 550 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 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 C and not more than 680 C, and preferably not
less than 575 C and not more than 660 C.
[0067]
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.
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
CA 02840724 2013-12-30
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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 m, and more preferably not less than 6 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
[0068]
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 C,
CA 02840724 2013-12-30
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soaked in a hot-dip galvanization bath at 460 C and
subjected to an alloying treatment at 500 C, thereby forming
a galvannealed layer with a thickness of 7 f.tm 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).
- 42 -
[0069]
[Table 1]
Chemical composition (mass%)
Steel No.
Expression Expression
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
n
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
0
I.)
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 co
0
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
IV
FP
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 I.)
0
H
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 UJ
I
H
I 0.043 0.029 0.57 0.016 0.0021 0.043 0.0041 0.153 -
- - 0.0032 1.12 I.)
,
UJ
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 0
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)/([T0/48 + [V]/51 + [Mo]/96 + [W]/184)
- 43 -
[0070]
[Table 2]
Steel Steel Slab heating temp. Finishing
Time from completion of finish Average cooling rate Coiling
R
sheet No. No. CC) temp. ( C)
rolling to initiation of cooling (s) CC/s) temp. CC) emark s
1 A 1250 890 1.1
74 590 Inv. Ex.
2 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.
B 1250 860 2.6 94
570 Comp. Ex. n
6 1260 900 1.3
27 600 Comp. Ex. 0
I.)
0
7 1250 910 0.9
84 710 Comp. Ex. '
0
-,1
8 1250 910 0.9
83 540 Comp. Ex. I.)
a,
I.)
9 C 1250 890 0.8
85 610 Inv. Ex. 0
H
CA
D 1250 870 , 0.7 81
580 Inv. Ex. HI
I \ )
I
11 E 1260 880 0.9
86 600 Inv. Ex. u.)
0
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.
I 1250 900 1.2 84
580 Comp. Ex.
16 2 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.
CA 02840724 2013-12-30
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[0071]
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.
[0072]
(i) Tensile test
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
CA 02840724 2013-12-30
- 45 -
(1998) to determine the average yield stress (YS-1), tensile
strength (TS-1) and total elongation (E1-1) at room
temperature (22 5 C) as well as to determine the average
yield stress (YS-2), tensile strength (TS-2) and total
elongation (E1-2) at temperatures in the temperature range
of 400 to 800 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 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 (E1-3).
[0073]
All the above tensile tests were performed at a cross
head speed of 10 mm/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 C.
[0074]
(ii) Microstructure observation
CA 02840724 2013-12-30
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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 x400 magnification. Ten fields of view
were photographed.
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.
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.
[0075]
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: x120000), and the diameters of at
CA 02840724 2013-12-30
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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
m and nitrides were excluded.
[0076]
(iii) Enlarge test at warm press forming temperature range
(warm press formability)
Testing temperature: An enlarge test was performed at
550 C, and warm press formability was evaluated based on the
obtained hole expanding ratio.
The enlarge test was carried out in accordance with
standards by The Japan Iron and Steel Federation (T1001-
1996). In detail, a 100 W x 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 C in a heating furnace, and a cylindrical base
as a punch was inserted into the hole of the test piece at
550 25 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) x 100=== (3)
CA 02840724 2013-12-30
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[0077]
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.
Warm press formability was evaluated to be good (D)
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.
The obtained results are set out in Tables 3 and 4.
- 49 -
[0078]
[Table 3]
MechanicalMechanical
Mechanical
Tensile conditions at elevated
Microstructure of steel sheet characteristics at room
characteristics during characteristics after
Test Steel temp. temp.
heating heating
piece sheet FerritePrecipitate
Remarks
No. No. Ferrite Heating Rate of
grain particle YS-1 TS-1 El-1 Strain YS-
2 TS-2 El-2 YS-3 TS-3 El-3
phase area temp.
cooling after
diameter diameter (MPa) (MPa) (%) (%)
(MPa) (MPa) (%) (MPa) (MPa) (%)
fraction (%) ( C)
tension (t/s)
(,um) (nm)
.
a 1 3.5 99 3 708 795 18.2 500 10
12 502 518 38.2 711 782 20.3 Inv. Ex.
b 400 5 35
552 571 25.2 768 839 , 19.8 Inv. Ex.
c 500 8 95
522 538 39.6 748 822 20.3 Inv. Ex.
d 600 10 82
462 475 50.4 745 828 20.7 Inv. Ex. 0
I.)
e 2 3.6 100 4 757 836 18.0 700
, 10 45 409 415 63.0 567 630 21.3 Inv. Ex.
co
a,.
f 800 10 22
351 368 79.2 507 603 10.5 Comp. Ex. 2
& 500 8 150
523 534 39.7 742 825 20.4 Inv. Ex.
h 500 26 20
524 541 39.1 763 838 10.1 Comp. Ex. 1,;)
_
-
_
i 3 6.9 94 3 604 746 20.6 500 , 10
15 508 526 30.9 578 598 16.7 Comp. Ex. ro
1
j 4 5.8 98 8 653 768 , 18.4 , 500
12 13 = 457 471 42.3 540 626 18,3 Comp. Ex. r)
k 5 4.3 100 12 642 774 19.4 , 500 15 ,
14 469 484 , 40.7 628 748 19.6 Comp. Ex.
I 6 8.6 100 11 623 742 20.1 , 500 ,
15 14 449 462 44.2 602 . 743 19.7 Comp. Ex.
m 7 5.9 100 , 14 603 726 21.1 500 10 1
13 440 457 44.3 574 699 20.4 Comp. Ex.
n , 8 3.1 94 3 _ 623 769 19.7 500 5
15 529 538 27.6 678 779 13.1 Comp. Ex.
o 9 3.8 100 4 987 1085 14.8 570 2
15 622 638 42.9 938 1020 18.4 Inv. Ex.
. .
p 10 3.6 100 4 804 887 16.7 690 15 14
571 , 585 , 61.8 611 670 17.5 Inv. Ex.
q 11 3.5 100 3 1104 1187 12.6 540 10 15
740 763 30.2 938 1031 13.4 Inv. Ex.
r 12 3.2 100 . 5 996 1071 , 14.7 500 5 15
727 741 30.9 1006 1105 14.6 Inv. Ex.
$ 13 3.6 100 4 1006 1093 14.3 550
10 16 654 672 =36.6 945 1039 15.8 Inv. Ex.
_ .
_
t 14 3.8 100 5 713 767 19.5 500 15 14
521 530 41.0 606 666 20.7 Comp. Ex.
_ .
u 15 3.2 99 , 13 639 743 21.3 500 12
10 454 573 46.9 703 764 23.0 Comp. Ex.
/ 16 . 3.3 98 4 702 771 19.4 500 15
15 498 517 44.6 631 686 = 20.3 Comp. Ex.
õ _ _
w 17 3.5 92 _ 3 707 803 17.3 400 10
11 643 681 17.5 777 845 18.1 Comp. Ex.
_
x 18 3.6 74 6 _ 885 1092 9.8 500 10 25
797 948 8.5 593 689 10.2 Comp. Ex.
- 50 -
[0079]
[Table 4]
Test Steel Changes in quality relative to quality at room temp.
Warm press formability
TS-1
piece sheet Remarks
(MPa) (YS-2)/(YS-1) (EI-2)/(E1-1) (YS-3)/(YS-1) (El-3)/(EI-1) Cracks Hardness
(HV) Evaluation
No. No.
a 1 795 0.71 2.1 1.00 1.12 0
0 0 Inv. Ex.
b 0.73 1.4 1.01 1.10 0
0 0 Inv. Ex.
c 0.69 2.2 0.99 1.13 0
0 0 Inv. Ex.
d 0.61 2.8 0.98 1.15 0
0 0 Inv. Ex.
e 2 836 0.54 3.5 0.75 1.18
0 0 0 Inv. Ex. r)
f 0.46 4.4 0.67 0.68 0
x x Comp. Ex.
g 0.69 2.2 0.98 1.13 0
0 0 Inv. Ex. 0
"
0
h 0.69 2.2 1.01 0.56 x
0 x Comp. Ex. a,
0
i 3 746 0.84 1.7 0.76 0.81 x
x x Comp. Ex.
"
FP
J 4 768 0.70 2.4 0.71 0.99 0
x x Comp. Ex. I.)
k 5 774 0.73 2.3 0.83 1.01 0
x x Comp. Ex. 0
H
CA
I 6 742 0.72 2.5 0.80 0.98 0
x x Comp. Ex. HI
m 7 726 0.73 2.5 0.76 0.97 0
x x Comp. Ex. "
1
u.)
n 8 769 0.85 1.5 0.90 0.66
x x x Comp. Ex. 0
o 9 1085 0.63 2.9 , 0.95 1.24
0 0 0 Inv. Ex.
P 10 887 0.71 3.7 0.76 1.05 0 _
0 0 Inv. Ex.
a 11 1187 0.67 2.4 , 0.85 1.06 0
, 0 0 Inv. Ex.
/ 12 1071 0.73 2.1 1.01 0.99
0 0 0 Inv. Ex.
s 13 1093 0.65 2.6 0.94 1.10 0
0 0 Inv. Ex.
t 14 767 0.73 2.1 0.85 1.06 0
x x Comp. Ex.
u 15 743 0.71 2.2 1.10 , 1.08
0 x x Comp. Ex.
/ 16 771 0.71 2.3 0.90 1.05
0 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.
CA 02840724 2013-12-30
- 51 -
[0080]
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 C to 700 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 C to 700 C (E1-2) was not less than
1.1 times the total elongation at room temperature (E1-1).
Further, for all the steel sheets in Inventive Examples, the
yield stress (YS-3) and the total elongation (E1-3) 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 (E1-1) at room
temperature (before the introduction of the strain).
Furthermore, all the steel sheets in Inventive Examples
exhibited good warm press formability.
[0081]
On the other hand, the steel sheets in Comparative
Examples (the test pieces Nos. f, h, i, j, k, 1, 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
CA 02840724 2013-12-30
- 52 -
total elongation (E1-2) of the steel sheet heated to the
temperature range of 400 C to 700 C, and the yield stress
(YS-3) or the total elongation (E1-3) 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.
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 (E1-3)
failed to be not less than 70% of the total elongation at
room temperature before heating (E1-1) as a result.
[0082]
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 C, an austenite phase was formed and carbides became
coarse during heating, resulting in a marked deterioration
in mechanical characteristics after heating.
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
CA 02840724 2013-12-30
- 53 -
cooled to room temperature after heating exhibited poor
ductility.
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.
[0083]
In the test pieces Nos. k, 1 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.
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
CA 02840724 2013-12-30
- 54 -
after heating exhibited poor ductility.
[0084]
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.
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.
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.
[0085]
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) (E1-2) was
CA 02840724 2013-12-30
- 55 -
insufficient. Thus, the steel sheet was shown to be
unsuited for warm press forming.
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.
[0086]
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 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.
In detail, a tensile test was carried out at a testing
temperature of 400 C or 650 C to determine the average yield
stress (Y2-2) and total elongation (E1-2); separately, test
pieces were subjected to a tensile test at 400 C or 650 C in
which a strain of not more than 20% described in Table 5 was
CA 02840724 2013-12-30
- 56 -
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 (E1-3). The results are described in Table
5.
- 57 -
[0087]
[Table 5]
Mechanical
Mechanical Mechanical
Tensile conditions at elevated Changes in quality relative to quality at
Microstructure of steel sheet characteristics at room
characteristics characteristics
temp.
room temp.
temp.
during heating after heating
Steel
Remarks
sheet No. Ferrite Precipitate
Ferrite Heating Rate of cooling
grain particle YS-1 TS-1 El-1 Strain YS-2
YS-3 (YS-2)/ (El-2)/ (YS-3)/ (El-3)/
phase area temp. I%) after
tension (MPa) El-2 (%) (MPa) El-3 (%)
diameter diameter (MPa) (MPa) (%)
(YS-1) (El-1) (YS-1) (El-1)
( # m) fraction (%) (nm) CC) ' (
C/s)
1 12
721 18.5 1.0 1.0
400 562 24.8 0.8 1.4
10 15 711 16.5 1.0 0.9
1 3.5 99 3 708 795 18.2
Inv. Ex
1 16
715 191 1.0 1.0 = =
650 415 54.6 0.6 3Ø 0
10 20 705 20.3 1.0 1.1 I.)
m
1 50
762 186 1.0 1.0
400 552 25.2 0.7 1.4. 0
10 60 748 16.8 1.0 0.9
2 3.6 100 4 757 836 18.0
Inv. Ex. "
1 60 761 188 1.0 1.0 .1,
650 433 55.8 0..6 3.1
I.)
15 60 703 26.7 0.9 1.5 0
H
1 15 997 15.3 1.0 1.0 u.)
400 658 20.8 0.7 1.4 1
10 16 964 12.6 1.0 0.9 H
9 3.8 100 4 987 1085 14.8
Inv. Ex. T
1 14 991 15.7 1.0 1.1 u.)
650 521 49.9 0.5 3.4 0
18 15 829 22.5 0.8 1.5
1 52 807 16.4 1.0 1.0
400 586 22.1 0.7 1.3
10 55 784 14.9 1.0 0.9
3.6 100 4 804 887 16.7
= Inv. Ex.
1 59 806 17.2 1.0 1.0
650 502 50.2 0.6 3.0
18 58 706 24.4 0.9 1.5
1 16 1115 12.3 1.0 , 1.0
400 763 16.7 0.7 1.3
10 15 1085 11.8 1.0 0.9
11 3.5 100 3 1104 1187 12.6
Inv, Ex,
1 16 1098 13.4 1.0 1.1
650 541 39.1 0.5 3.1
18 15 982 20.2 0.9 1.6
,
.
1 65 1002 14.5 1.0 1.0
400 624 20.5 0.6 1.4
10 68 976 12.8 1.0 0.9
12 3.2 100 5 996 1071 14.7
Inv. Ex.
1 65 994 14.5 1.0 1.0
650 535 49.0 0.5 3.3
18 66 847 22.6 0.9 1.5
_
.
1 16 1015 14.1 1.0 1.0
400 654 18.4 0.7 1.3
10 16 990 12.9 1.0 0.9
13 3.6 100 4 1006 1093 14.3
Inv. Ex.
1 15 994 15.0 1.0 1.0
650 539 47.2 0.5 3.3
18 16 845 22.0 0.8 1.5
CA 02840724 2013-12-30
- 58 -
[0088]
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 C to 700 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 C to 700 C (E1-2) was not less than
. 1.1 times the total elongation at room temperature (E1-1),
the yield stress (YS-3) and the total elongation (E1-3)
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 (E1-1) at
room temperature (before the introduction of the strain).
[0089]
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 C to 700 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.
CA 02840724 2013-12-30
- 59 -
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.
