U.S. patent application number 10/639588 was filed with the patent office on 2004-02-26 for dual phase steel sheet with good bake-hardening properties.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd). Invention is credited to Akamizu, Hiroshi, Ikeda, Shushi, Makii, Koichi.
Application Number | 20040035500 10/639588 |
Document ID | / |
Family ID | 31185190 |
Filed Date | 2004-02-26 |
United States Patent
Application |
20040035500 |
Kind Code |
A1 |
Ikeda, Shushi ; et
al. |
February 26, 2004 |
Dual phase steel sheet with good bake-hardening properties
Abstract
A dual phase steel sheet with good bake-hardening properties is
provided. The steel sheet is characterized in containing (in terms
of percent by mass) C: no less than 0.06% and less than 0.25%;
Si+Al: 0.5 to 3%; Mn: 0.5 to 3%; P: no more than 0.15%; and S: no
more than 0.02%; and also meeting the following condition (in terms
of space factor) that retained austenite is at least 3%, bainite is
at least 30%, and ferrite is no more than 50%, and further
characterized in differing in stress larger than 50 MPa before and
after application of 2% pre-strain and ensuing heat treatment for
paint baking at 170.degree. C. for 20 minutes. The steel sheet has
well-balanced strength and workability, exhibits good
bake-hardening properties at the time of paint baking, and offers
good resistance to natural aging.
Inventors: |
Ikeda, Shushi; (Kobe-shi,
JP) ; Makii, Koichi; (Kobe-shi, JP) ; Akamizu,
Hiroshi; (Kobe-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd)
Kobe-shi
JP
|
Family ID: |
31185190 |
Appl. No.: |
10/639588 |
Filed: |
August 13, 2003 |
Current U.S.
Class: |
148/320 |
Current CPC
Class: |
C22C 38/06 20130101;
C21D 8/02 20130101; C21D 8/0426 20130101; C22C 38/12 20130101; C21D
8/0473 20130101; C21D 2211/001 20130101; C22C 38/002 20130101; C22C
38/02 20130101; C22C 38/001 20130101; C21D 8/0436 20130101; C21D
2211/002 20130101; C21D 2211/005 20130101; C22C 38/04 20130101;
C21D 1/20 20130101 |
Class at
Publication: |
148/320 |
International
Class: |
C22C 038/06 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 20, 2002 |
JP |
2002-239816 |
Claims
What is claimed is:
1. A dual phase steel sheet with good bake-hardening properties
containing (in terms of percent by mass): C: no less than 0.06% and
less than 0.25%; Si+Al: 0.5 to 3%; Mn: 0.5 to 3%; P: no more than
0.15% (excluding 0%); and S: no more than 0.02% (excluding 0%),
wherein said steel sheet comprising (in terms of space factor):
retained austenite: at least 3%; bainite: at least 30%; and
ferrite: no more than 50% (including 0%), and wherein said steel
sheet has difference in stress larger than 50 MPa before and after
ensuing heat treatment for paint baking at 170.degree. C. for 20
minutes, after application of 2% pre-strain.
2. The dual phase steel sheet as defined in claim 1, wherein said
difference in stress is no less than 100 MPa.
3. The dual phase steel sheet as defined in claim 1, wherein the
space factor of bainite is no less than 60%.
4. The dual phase steel sheet as defined in claim 1, further
containing at least one of the following constituents (in terms of
percent by mass): Mo: no less than 0.05% and no more than 1%; Ni:
no less than 0.05% and no more than 0.5%; Cu: no less than 0.05%
and no more than 0.5%; and Cr: no less than 0.05% and no more than
1%.
5. The dual phase steel sheet as defined in claim 1, further
containing at least one of the following constituents (in terms of
percent by mass): Ti: no less than 0.01% and no more than 0.1%; Nb:
no less than 0.01% and no more than 0.1%; and V: no less than 0.01%
and no more than 0.1%.
6. The dual phase steel sheet as defined in claim 1, further
containing (in terms of percent by mass) at least one of Ca: no
less than 3 ppm and no more than 30 ppm and REM: no less than 3 ppm
and no more than 30 ppm.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a dual phase steel sheet
with good bake-hardening properties and, more particularly, to a
dual phase steel sheet having well-balanced strength and forming
properties. This steel sheet has not only good bake-hardening
properties but also good resistance to natural aging. (The term
"bake-hardening properties" implies that the steel sheet improves
in strength upon paint baking. The term "resistance to natural
aging" implies that the steel sheet retains its characteristic
properties (such as forming properties) without deterioration after
aging at room temperature). The dual phase steel sheet according to
the present invention will be widely used in automotive, electric,
and machine industries and other industrial fields. The following
description is mainly concerned with its use in automotive bodies
as a typical example.
[0003] 2. Description of the Related Art
[0004] There has been an increasing demand for steel sheets for
automotive use which are thinner (for improved fuel consumption)
and stronger (for improved collision safety) than before. Such
steel sheets are required to exhibit good forming properties at the
time of forming, such as press working. Unfortunately, improved
strength often has an adverse effect on forming properties. Steel
sheets for automotive use, which undergo complicated forming, are
required to be comparatively soft (for easy forming) at the time of
press working and to become highly strong at the time of heat
treatment to bake the coating thereon which follows press
working.
[0005] The above-mentioned bake-hardening is due to strain aging
that occurs at a high temperature (about 150-200.degree. C.) for
paint baking. Strain aging results from interstitial elements (C
and N) fixing dislocations. Therefore, paint baking offers the
advantage of imparting high strength to the final product.
[0006] Incidentally, strain aging occurs also at normal
temperature, and in this case, dissolved carbon and nitrogen in the
steel migrate to fix dislocations even before paint baking. Any
steel sheet with strain aging at normal temperature is poor in
ductility due to yield elongation, and poor ductility leads to
flaws (such as wrinkles) at the time of press working.
[0007] Consequently, automotive steel sheets are required to
readily undergo strain aging at high temperatures for paint baking,
thereby increasing in strength, and hardly undergo strain aging at
normal temperature. In other words, they are required to be good in
bake-hardening and also in resistance to natural aging.
[0008] Under these circumstances, there have been proposed steel
sheets with improved bake-hardening, such as BH steel of quasi-IF
(Interstitial Free) type. It contains about 30 ppm of dissolved
carbon in the ferrite structure, so that dissolved carbon fix
dislocations, thereby improving the bake-hardening properties. It
is used mainly for the outer panel of automobiles.
[0009] Unfortunately, the BH steel of quasi-IF type mentioned above
has a strength of about 440 MPa at most even after bake-hardening
on account of its low content of dissolved carbon.
[0010] There is a kind of DP steel (Dual Phase Steel) which
contains dislocations introduced into the parent phase ferrite by
martensitic transformation. It has a low value of yield point as
such but has a high value of yield point due to hardening after
paint baking which fixes the above-mentioned dislocations and other
dislocations introduced by working.
[0011] Moreover, there is a kind of so-called TRIP steel which is
designed to improve the bake-hardening properties. TRIP steel is a
steel which contains retained austenite of several to tens of
percent in the metal structure, so that it exhibits high toughness
after plastic forming. For example, Japanese Patent Laid-open No.
11565/2001 discloses a technology for increasing the amount of
bake-hardening. This technology aims at developing a steel sheet
that absorbs a large amount of collision energy to meet both
requirements for safety of passenger cars and weight reduction of
car body.
[0012] Generally, a conceivable mechanism which makes TRIP steel
improve in bake-hardening is the bonding of carbon which originally
exists in the ferrite to dislocations induced by working, as in the
case of above-mentioned dual phase steel. This conception, however,
does not explain why the steel increases in strength by 50 MPa or
more by bake-hardening. Another conceivable mechanism has been
proposed as follows. Retained austenite is transformed into
martensite by plastic forming before bake-hardening. Carbon in the
martensite releases itself at the time of paint baking. This carbon
bonds to the dislocations in ferrite which have been introduced
during working. In this way, hardening takes place.
[0013] Improvement in TRIP steel which inherently has well-balanced
strength and workability has been made to provide a new steel sheet
capable of high bake-hardening at the time of paint baking, as
mentioned above. However, a steel sheet with high bake-hardening
poses problems with increased yield point, decreased elongation,
and aging namely deterioration with time in characteristic
properties. These phenomena could possibly occur as follows. First,
dislocations form from skin pass rolling or martensitic
transformation during production, and then these dislocations catch
carbon which has diffused and migrated from retained austenite
after its decomposition that takes place for one reason or another,
since TRIP steel contains retained austenite with a large amount of
dissolved carbon. As the result, deterioration in characteristic
properties such as increased yield point and decrease of elongation
occurs. Such a steel sheet exhibits good workability immediately
after production but deteriorates with time due to aging when it is
worked by the user. Japanese Patent Laid-open No. 297350/2000
proposes an idea that a steel sheet is improved in bake-hardening
properties and resistance to natural aging when it has the dual
phase structure in which the principal phase is ferrite and the
second phase is at least one of pearlite, bainite, martensite, and
retained austenite, with dissolved nitrogen controlled in amount
and positions where it exists. However, there seems to be room for
further improvement in elongation.
SUMMARY OF THE INVENTION
[0014] The present invention was completed in view of the
foregoing. It is an object of the present invention to provide a
dual phase steel sheet having good bake-hardening properties as
well as good resistance to natural aging.
[0015] The gist of the present invention resides in a dual phase
steel sheet with good bake-hardening properties which is
characterized in containing (in terms of percent by mass):
[0016] C: no less than 0.06% and less than 0.25%,
[0017] Si+Al: 0.5 to 3%,
[0018] Mn: 0.5 to 3%,
[0019] P: no more than 0.15% (excluding 0%), and
[0020] S: no more than 0.02% (excluding 0%), and also meeting the
following condition (in terms of space factor):
[0021] retained austenite: at least 3%,
[0022] bainite: at least 30%, and
[0023] ferrite: no more than 50% (including 0%),
[0024] and further characterized in differing in stress larger than
50 MPa (preferably larger than 100 MPa) before and after ensuing
heat treatment for paint baking at 170.degree. C. for 20 minutes,
after application of 2% pre-strain.
[0025] For better bake-hardening properties, the steel sheet should
preferably have a space factor of bainite more than 60%.
[0026] The preferred embodiments of the present invention include
the following.
[0027] (1) The dual phase steel sheet as defined above which is
characterized in further containing at least one of the following
constituents (in terms of percent by mass):
[0028] Mo: no less than 0.05% and no more than 1%,
[0029] Ni: no less than 0.05% and no more than 0.5%,
[0030] Cu: no less than 0.05% and no more than 0.5%, and
[0031] Cr: no less than 0.05% and no more than 1%.
[0032] (2) The dual phase steel sheet as defined above which is
characterized in further containing at least one of the following
constituents (in terms of percent by mass):
[0033] Ti: no less than 0.01% and no more than 0.1%,
[0034] Nb: no less than 0.01% and no more than 0.1%, and
[0035] V: no less than 0.01% and no more than 0.1%.
[0036] (3) The dual phase steel sheet as defined above which is
characterized in further containing (in terms of percent by
mass):
[0037] Ca: no less than 3 ppm and no less than % and no more than
30 ppm and/or,
[0038] REM: no less than 3 ppm and no more than 30 ppm.
[0039] The present invention mentioned above provides a steel sheet
which has well-balanced strength and workability, exhibits good
bake-hardening properties at the time of paint baking, and offers
good resistance to natural aging, by virtue of its unique structure
in which bainite is the principle constituent and retained
austenite and ferrite are present in a specified amount. This steel
sheet exhibits outstanding workability at the time of forming and
also exhibits high strength after paint baking.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 is an SEM microphotograph showing one example of the
structure of the steel sheet according to the present
invention.
[0041] FIG. 2 is a diagram illustrating the heat treatment carried
out in one Example.
[0042] FIG. 3 is a diagram illustrating the heat treatment carried
out in another Example.
[0043] FIG. 4 is a diagram illustrating the heat treatment carried
out in another Example.
[0044] FIG. 5 is a diagram illustrating the heat treatment carried
out in another Example.
[0045] FIG. 6 is an SEM microphotograph showing the structure of
the steel sheet in experiment No. 3.
[0046] FIG. 7 is an SEM microphotograph showing the structure of
the steel sheet in experiment No. 17.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0047] Under the circumstances mentioned above, the present
inventors carried out extensive studies on the development of a new
dual phase steel sheet with good bake-hardening properties which
keeps good workability without aging and yet increases in strength
upon paint baking.
[0048] The results of the studies revealed that the steel sheet
exhibits better bake-hardening properties than before if it is
composed mainly of bainite and it has a high initial dislocation
density at the time of production. The present invention is based
on a finding that the steel sheet is effectively relieved from age
hardening at normal temperature if it has the structure of
so-called TRIP steel containing retained austenite, with dissolved
carbon bonding to dislocations introduced at the time of
production.
[0049] As mentioned above, the steel sheet of the present invention
has the structure which is composed of at least 3% of retained
austenite, at least 30% of bainite, and no more than 50% (including
0%) of ferrite, in terms of space factor. These space factors were
established for the reasons given below.
[0050] Bainite (at Least 30%)
[0051] The steel sheet of the present invention is characterized
most by being composed mainly of bainite. It differs in structure
from the conventional TRIP steel as follows. Being composed of
ferrite and pearlite as the principal phase, the conventional TRIP
steel has the disadvantage of not keeping sufficient dislocations
at the time of steel sheet production, and the resulting steel
sheet is poor in bake-hardening properties. By contrast, the steel
sheet of the present invention is composed mainly of bainite and it
has a high initial dislocation density. Therefore, it exhibits much
better bake-hardening properties than any other conventional steel
sheets at the time of paint baking, which leads to a greatly
improved strength due to strain aging.
[0052] For the steel sheet to produce such an effect, it should
have the structure in which the amount of bainite is at least 30%,
preferably more than 60%, more preferably more than 70%, and most
preferably more than 80%. Also, for the steel sheet to exhibit good
bake-hardening properties at the time of paint baking and to have
good resistance to natural aging, it should substantially have the
dual-phase structure composed of retained austenite and
bainite.
[0053] Retained Austenite (at Least 3%)
[0054] Retained austenite contributes to improvement in total
elongation. For the steel sheet to produce such an effect, it
should contain retained austenite as much as at least 3%,
preferably more than 5%, more preferably more than 7%, and most
preferably more than 10%, in terms of space factor. The upper limit
should be 30%, preferably 25%; retained austenite with an excess
space factor deteriorates stretch flange formability.
[0055] As mentioned above, the steel sheet of the present invention
keeps as much retained austenite as necessary to hold therein the
dissolved carbon and nitrogen which fix dislocations. In this way
the retained austenite prevents dislocations from being fixed by
dissolved carbon and nitrogen at normal temperature. Therefore, the
steel sheet is relieved from age hardening at normal temperature
even in the case where a large number of dislocations are
introduced at the time of production.
[0056] Retained austenite should preferably contain more than 0.8%
of carbon for better elongation.
[0057] Ferrite (no More Than 50%, Including 0%)
[0058] The point of the present invention lies in the fact that the
steel sheet is composed mainly of bainite so that it has good
bake-hardening properties. The present inventors found that the
object of the present invention is achieved so long as the steel
sheet contains as much bainite and retained austenite as specified
above even though its ferrite content is less than 50%.
[0059] The foregoing is apparent from FIG. 1 which is an SEM
microphotograph (.times.4000) showing the structure of the steel
sheet of the present invention. In this photograph, the black
background represents ferrite and the gray parts represent bainite
or retained austenite. It was found that the steel sheet has good
bake-hardening properties even though its structure is composed
mainly of bainite, with the remainder (45%) being ferrite.
[0060] The steel sheet becomes better in bake-hardening properties
as the amount of ferrite decreases and the amount of bainite
relatively increases. Therefore, the amount of ferrite should be
less than 30%, preferably less than 25%, and more preferably
0%.
[0061] The steel sheet may contain ferrite in an amount more than
10% and less than the upper limit specified above so that it has
good elongation characteristics as well as good workability.
[0062] Others: pearlite and martensite (minimal, including 0% in
terms of space factor)
[0063] The steel sheet of the present invention usually have the
mixed structure mentioned above (which consists of retained
austenite, ferrite, and bainite, or consists of retained austenite
and bainite). However, the mixed structure may additionally contain
pearlite and martensite in an amount not harmful to the desired
characteristic properties. These constituents inevitably enter the
structure in the manufacturing process; their content should
preferably be as little as possible.
[0064] The steel sheet of the present invention is composed of the
basic constituents listed below. The amount of constituents is
expressed in terms of mass %.
[0065] C: no less than 0.06% and less than 0.25%
[0066] Carbon is an element essential for the steel sheet to
exhibit high strength and to contain retained austenite. In other
words, carbon sufficiently existing in the austenite phase permits
the austenite phase to remain as much as desired at normal
temperature. The content of carbon necessary to produce this effect
is no less than 0.06%, preferably no less than 0.10%. However, for
the steel sheet to have good weldability, the content of carbon
should be less than 0.25%, preferably less than 0.20%.
[0067] Si+Al: 0.5-3%
[0068] Silicon and aluminum are elements to prevent retained
austenite from decomposing to give carbides. Silicon plays an
important role in solid solution strengthening. The total amount of
silicon and aluminum necessary for this effect is no less than
0.5%, preferably no less than 0.7%, and more preferably no less
than 1%. However, it should be less than 3%, preferably less than
2.5%, and more preferably less than 2%, because excess silicon and
aluminum more than 3% are wasted and lead to high temperature
brittleness.
[0069] Mn: 0.5-3%
[0070] Manganese stabilizes austenite to give as much retained
austenite as desired. The amount of manganese to produce this
effect is no less than 0.5%, preferably no less than 0.7%, and more
preferably no less than 1%. However, its upper limit should be 3%,
preferably 2.5%, and more preferably 2%, because excess manganese
produces an adverse effect such as ingot cracking.
[0071] P: no more than 0.15% (excluding 0%)
[0072] Phosphorus secures as much retained austenite as desired.
The amount of phosphorus to produce this effect is no less than
0.03%, preferably no less than 0.05%. However, its upper limit is
0.15%, preferably 0.1%, because excess phosphorus adversely affects
secondary workability.
[0073] S: no more than 0.02% (including 0%)
[0074] Sulfur forms sulfide inclusions such as MnS, which bring
about a starting point of cracking, thereby deteriorating
workability. The amount of sulfur should be no more than 0.02%,
preferably no more than 0.015%.
[0075] N: no more than 60 ppm (excluding 0%)
[0076] Excess nitrogen causes a large amount of nitride to
precipitate, thereby deteriorating ductility. Therefore, the amount
of nitrogen should be no more than 60 ppm, preferably no more than
50 ppm, and more preferably no more than 40 ppm. The less the
amount of nitrogen in the steel sheet, the more desirable. However,
the lower limit of the amount of nitrogen is about 10 ppm,
depending on how much of nitrogen the process employed can
reduce.
[0077] The steel sheet of the present invention is made up of the
above-mentioned principal constituents, with the remainder being
substantially iron and inevitable impurities. It may additionally
contain the following components in an amount not harmful to the
effect of the present invention.
[0078] At least any one of:
[0079] Mo: no less than 0.05% and no more than 1%
[0080] Ni: no less than 0.05% and no more than 0.5%
[0081] Cu: no less than 0.05% and no more than 0.5%
[0082] Cr: no less than 0.05% and no more than 1%
[0083] These elements strengthen the steel sheet and stabilize
retained austenite and secure as much retained austenite as
necessary. For these elements to produce their desired effects, it
is recommended that the steel sheet contain each of them in an
amount no less than 0.05%, preferably no less than 0.1%, as
follows.
[0084] Mo: no less than 0.05% (preferably no less than 0.1%); Ni:
no less than 0.05% (preferably no less than 0.1%); Cu: no less than
0.05% (preferably no less than 0.1%); and Cr: no less than 0.05%
(preferably no less than 0.1%).
[0085] Mo and Cr in excess of 1% and Ni and Cu in excess of 0.5%
will be wasted without extra effect. Therefore, their desirable
amounts are as follows.
[0086] Mo: no more than 0.8%; Ni: no more than 0.4%; Cu: no more
than 0.4%; Cr: no more than 0.8%.
[0087] At least any one of:
[0088] Ti: no less than 0.01% and no more than 0.1%
[0089] Nb: no less than 0.01% and no more than 0.1%
[0090] V: no less than 0.01% and no more than 0.1%
[0091] These elements contribute to precipitation strengthening and
fine structure, that is, they make the steel sheet strong. For
these elements to produce their desired effects, it is recommended
that the steel sheet contain each of them in an amount no less than
0.01%, preferably no less than 0.02%, as follows.
[0092] Ti: no less than 0.01% (preferably no less than 0.02%); Nb:
no less than 0.01% (preferably no less than 0.02%); V: no less than
0.01% (preferably no less than 0.02%).
[0093] When used in excess of 0.1%, they will be wasted without
extra effect. Therefore, their desirable amounts are as
follows.
[0094] Ti: no more than 0.08%; Nb: no more than 0.08%; and V: no
more than 0.08%.
[0095] Ca: no less than 3 ppm and no more than 30 ppm, and/or REM:
no less than 3 ppm and no more than 30 ppm
[0096] Ca and REM (rare earth elements) control the form of
sulfides in the steel sheet, thereby improving workability. The
rare earth elements include Sc, Y, and lanthanoid. For these
elements to produce their desired effects, it is recommended that
the steel sheet contain each of them in an amount no less than 3
ppm, preferably no less than 5 ppm. When used in excess of 30 ppm,
they are wasted without extra effect. Therefore, their desired
amount is no more than 25 ppm.
[0097] The steel sheet of the present invention may be produced by
any method without specific restrictions. However, it will have the
structure characteristic of the present invention if hot rolling or
cold rolling is followed by continuous annealing or plating which
is carried out under the following conditions.
[0098] (1) Keep the steel sheet at a temperature higher than
A.sub.3 point for 10-200 seconds.
[0099] (2) Cool the steel sheet to the bainite transformation
temperature (about 500-350.degree. C.) at an average cooling rate
larger than 3.degree. C./s, thereby avoiding pearlite
transformation.
[0100] (3) Keep the steel sheet at said temperature for more than
one second.
[0101] The isothermal treatment at a temperature higher than
A.sub.3 point completely dissolves carbides to form retained
austenite as desired. It also effectively yields bainite with a
high dislocation density in its ensuing cooling step. Heating at
said temperature should last for 10-200 seconds. Excessively brief
heating does not produce the desired effect. Excessively elongated
heating results in coarse crystal grains. An adequate length is
20-150 seconds.
[0102] Subsequently, the steel sheet should be cooled to the
bainite transformation temperature (about 500-350.degree. C.) at an
average cooling rate larger than 3.degree. C./s, preferably larger
than 5.degree. C./s, for avoidance of pearlite transformation.
[0103] The controlled average cooling rate mentioned above helps
introduce a large number of dislocations, thereby imparting the
desired bake-hardening properties (defined by as a difference in
stress larger than 50 MPa before and after ensuing heat treatment
for paint baking at 170.degree. C. for 20 minutes, after
application of 2% pre-strain). Better bake-hardening properties
with a difference in stress larger than 100 MPa may be attained if
cooling is accomplished by using water-cooled rolls, so that the
average cooling rate is greater than 5.degree. C./s. The cooling
rate should be as great as possible to improve the bake-hardening
properties; however, an adequate cooling rate should be established
from the practical point of view.
[0104] The control of the cooling rate specified above should be
maintained until the bainite transformation temperature is reached.
If the control of the cooling at the above specified rate (rapid
cooling) is suspended while the steel sheet is still hotter than
the bainite transformation temperature and is followed by slow
cooling, the resulting steel sheet is poor in bake-hardening
properties due to insufficient dislocations and is also poor in
elongation due to insufficient retained austenite. On the other
hand, if cooling at the above specified rate is continued until a
lower temperature than the bainite transformation temperature, the
resulting steel sheet is liable to age hardening at normal
temperature and is poor in elongation due to insufficient retained
austenite.
[0105] After cooling, the steel sheet should be kept at the
specified temperature for more than one second, so that carbon
efficiently concentrates in retained austenite in a short time,
giving rise to a large amount of stable retained austenite. The
resulting retained austenite greatly contributes to the TRIP
effect. However, an excessively long holding time should be avoided
because the resulting steel sheet is poor in bake-hardening
properties due to recovery, namely decrease of dislocations formed
by cooling.
[0106] To summarize, since the initial dislocations exist in
bainite phase, increase of the ratio of bainite phase itself
provides increase of the initial dislocation density. In addition,
the cooling rate to the bainite transformation temperature (the
higher, the better) and the temperature and time to keep at the
bainite transformation temperature are the factors to effect the
initial dislocation density.
[0107] The above-mentioned heat treatment may be accomplished, for
example, by heating/cooling using a salt bath or CAL simulator, or
by water cooling.
[0108] The cooling to normal temperature after the keeping at the
specified temperature may be accomplished by air cooling or water
cooling without any specific restrictions. Moreover, the steel
sheet may undergo plating or alloying to modify the structure as
desired to such an extent not harmful to the effect of the present
invention.
[0109] The steel sheet of the present invention may be produced by
either of the following steps which include the above-mentioned
steps.
[0110] (a) "Hot rolling step".fwdarw."Continuous annealing step or
plating step"
[0111] (b) "Hot rolling step".fwdarw."Cold rolling
step".fwdarw."Continuou- s annealing step or plating step"
[0112] The hot rolling and cold rolling may be carried out under
ordinary conditions without specific restrictions. However, their
ensuing steps, namely continuous annealing and plating, under
controlled conditions are more influential in formation of the
desired structure in the steel sheet of the present invention.
[0113] To be more specific, the hot rolling step should be
completed at a temperature higher than the A.sub.r3 point. Then the
rolled steel sheet should be cooled at an average cooling rate of
about 30.degree. C./s and finally wound up at about 500-600.degree.
C. In addition, the cold rolling step may be carried out at a draft
of about 30-70%. These conditions are not mandatory, as a matter of
course.
[0114] The invention will be described in more detail with
reference to the following examples, which are not intended to
restrict the scope thereof. The examples may be modified without
altering the scope of the invention.
Examples
[0115] An experimental slab was prepared from a vacuum-melted steel
having the composition shown in Table 1. The slab was made into a
steel sheet, 2.4-3.2 mm thick, by hot rolling under the following
conditions.
[0116] Starting temperature: 1100.degree. C.
[0117] Finishing temperature: 850.degree. C.
[0118] Winding temperature: 600.degree. C.
[0119] After acid pickling, the hot-rolled steel sheet was
cold-rolled (with a draft of 50-75%) for reduction of thickness to
1.0-1.6 mm.
[0120] The cold-rolled steel sheet subsequently underwent heat
treatment as illustrated in FIG. 2 by a continuous annealing line
(CAL). To be more specific, in the Samples Nos. 1 to 14 mentioned
later, the steel sheet was kept at 900.degree. C. for 2 minutes in
a salt bath, quenched in another salt bath at 400.degree. C., kept
at 400.degree. C. for 1 minute in the same salt bath, and finally
air-cooled to room temperature. After cooling, the steel sheet
underwent skin pass rolling, with the reduction of area being
0.5-2%. It was finally wound up.
[0121] The thus obtained steel sheet was examined for structure by
observation under an optical microscope and a scanning electron
microscope (SEM) after Lepera etching. The areal ratio of ferrite
and bainite was obtained from the microphotographs. The space
factor of retained austenite was obtained by X-ray measurement.
[0122] The specimens were further tested for tensile strength (TS),
total elongation (El), bake-hardening properties (BH), and
resistance to natural aging in the following manner.
[0123] In tensile testing, test specimens conforming to JIS No. 5
were used for measurement of tensile strength (TS) and elongation
(El). Bake-hardening properties were determined from
.sigma..sub.2-.sigma..sub.- 1, where .sigma..sub.1 denotes a stress
of a JIS No. 5 specimen under 2% pre-strain, and .sigma..sub.2
denotes a stress of the same specimen measured after load release
and heat treatment at 170.degree. C. for 2 minutes. Resistance to
natural aging was evaluated in the following manner instead of the
ordinary accelerated test (for AI values). Tensile test is
performed on samples of steel sheet immediately after production
and also after aging at room temperature for three months. The
samples are rated as poor in bake-hardening properties in either or
both of the following cases.
[0124] The samples tested after aging are higher than the samples
tested immediately after production in the average value of yield
point (n=2) by more than 30 MPa.
[0125] The samples tested after aging are lower than the samples
tested immediately after production in the average value of
elongation (n=2) by more than 2%.
[0126] The results are shown in Table 2 (in which a x mark
indicates samples with poor bake-hardening properties).
[0127] In this example, continuous annealing was carried out under
the condition different from that shown in FIG. 2. The resulting
steel sheet was evaluated.
[0128] The sample used in this example is a steel sheet, 1.0-1.6 mm
thick, obtained from an experimental slab having the composition
shown in No. 3 of Table 1, by hot rolling and cold rolling under
the same conditions as mentioned above.
[0129] Sample No. 15 underwent heating at about 900.degree. C. for
2 minutes in a salt bath and then water cooling in the continuous
annealing as illustrated in FIG. 3, without keeping at about
400.degree. C. as shown in FIG. 2. Sample No. 16 underwent heating
at about 900.degree. C. for 2 minutes in a salt bath, quenching in
another salt bath at about 400.degree. C., keeping at about
400.degree. C. for 5 minutes, and air cooling to room temperature,
as illustrated in FIG. 4.
[0130] Sample No. 17 underwent heating at about 850.degree. C. for
2 minutes in a salt bath, quenching in another salt bath at about
400.degree. C., keeping at about 400.degree. C. for 1 minute, and
air cooling to room temperature, as illustrated in FIG. 5.
[0131] Sample No. 18 underwent heating at about 900.degree. C. for
2 minutes in a salt bath, cooling to about 400.degree. C. at an
average rate of 5.degree. C./sec, keeping at about 400.degree. C.
for 1 minute, and air cooling to room temperature.
[0132] After air cooling to room temperature, samples Nos. 15 to 17
underwent skin pass rolling, with the reduction of area being
0.5-2%. They were finally wound up.
[0133] The thus obtained samples Nos. 15 to 17 were tested for
tensile strength (TS), total elongation (El), bake-hardening
properties (BH), and resistance to natural aging, in the same way
as for Samples Nos. 1 to 14. The results are shown in Table 2.
1TABLE 1 Steel desig- Chemical Composition (mass %) A.sub.c3
transforma- nation No. C Si Mn P S Al N Others tion point (.degree.
C.) 1 0.033 1.48 1.50 0.03 0.006 0.032 0.0035 -- 894 2 0.096 1.54
1.54 0.03 0.004 0.034 0.0041 -- 870 3 0.157 1.57 1.53 0.02 0.004
0.033 0.0037 -- 854 4 0.204 1.55 1.45 0.04 0.005 0.035 0.0034 --
844 5 0.151 0.48 1.55 0.04 0.005 1.030 0.0042 -- 806 6 0.147 0.30
0.32 0.04 0.004 0.030 0.0029 -- 836 7 0.150 1.46 1.55 0.03 0.005
0.033 0.0036 Mo: 0.2 856 8 0.147 1.52 1.48 0.04 0.005 0.032 0.0035
Ni: 0.2 853 9 0.154 1.44 1.50 0.03 0.006 0.028 0.0037 Cu :0.2 846
10 0.155 1.54 1.52 0.03 0.005 0.033 0.0040 Cr: 0.2 853 11 0.153
1.51 1.55 0.03 0.006 0.032 0.0035 Ti: 0.03 864 12 0.152 1.54 1.52
0.02 0.005 0.033 0.0045 Nb: 0.03 854 13 0.153 1.50 1.54 0.03 0.006
0.033 0.0027 V: 0.03 852 14 0.151 1.53 1.54 0.03 0.004 0.032 0.0039
Ca: 10 ppm 853
[0134]
2 TABLE 2 Structure (areal %) Characteristic Properties Experiment
Steel desig- Retained TS El BH Resistance to No. nation No.
austenite Bainite Ferrite (MPa) (%) (MPa) natural aging 1 1 1 30 70
585 22 58 x 2 2 7 68 25 730 20 88 .smallcircle. 3 3 12 88 0 870 23
105 .smallcircle. 4 4 15 85 0 995 22 133 .smallcircle. 5 5 13 87 0
776 20 102 .smallcircle. 6 6 2 73 25 740 18 68 x 7 7 12 88 0 1030
20 143 .smallcircle. 8 8 12 88 0 983 23 121 .smallcircle. 9 9 13 87
0 885 24 118 .smallcircle. 10 10 13 87 0 910 20 104 .smallcircle.
11 11 13 87 0 921 22 120 .smallcircle. 12 12 12 88 0 933 21 115
.smallcircle. 13 13 13 87 0 915 22 109 .smallcircle. 14 14 14 86 0
864 24 110 .smallcircle. 15 3 1 99 0 1054 6 120 x 16 3 12 88 0 865
24 48 .smallcircle. 17 3 13 27 60 767 26 38 .smallcircle. 18 3 12
43 45 821 24 98 .smallcircle.
[0135] The foregoing results lead to the following conclusion.
Incidentally, No. below denotes experiment No. in Table 2.
[0136] Steel sheets in Nos. 2 to 5, 7 to 14, and 18 exhibit good
characteristic properties because they meet the requirements
specified in the present invention.
[0137] In No. 18, the conditions other than the cooling rate to
400.degree. C. are the same as those of No. 3. The difference in
the cooling rate causes formation of ferrite in the course of
cooling.
[0138] Other samples than mentioned above, which fail to meet any
of the requirements specified in the present invention, have some
flaws as mentioned below.
[0139] No. 1 has insufficient retained austenite but has excess
ferrite on account of low carbon content. Therefore, it is poor in
bake-hardening properties and is liable to strain aging at normal
temperature.
[0140] No. 6 has insufficient retained austenite on account of low
content of (Si+Al) and low content of Mn. Therefore, it is poor in
bake-hardening properties and is liable to strain aging at normal
temperature.
[0141] No. 15 suggests that a prescribed amount of retained
austenite can be secured if the sample is quenched in the
continuous annealing step and then kept at about 400.degree. C. for
a certain period of time.
[0142] No. 16 suggests that keeping the steel sheet at about
400.degree. C. for a long time after quenching from about
900.degree. C. is not desirable for a large number dislocations
necessary for the bake-hardening properties. A probable reason for
this is that dislocations which have resulted from quenching from
about 900.degree. C. recover, resulting in a low dislocation
density, if the steel sheet is kept at about 400.degree. C. for an
excessively long time.
[0143] No. 17 suggests that it is desirable to heat the steel sheet
at a temperature higher than the A.sub.3 point at the beginning of
the continuous annealing process, if the steel sheet is to have a
large number dislocations necessary for the bake-hardening
properties.
[0144] FIG. 6 is an SEM microphotograph (.times.4000) which shows
the structure of No. 3 conforming to the present invention. It is
noted that the sample has the bainite structure. By contrast, FIG.
7 is an SEM microphotograph (.times.4000) which shows the structure
of No. 17 in a comparative example. The black parts represent
ferrite and the gray parts represent retained austenite. It is seen
that ferrite dominates bainite.
* * * * *