U.S. patent application number 11/663581 was filed with the patent office on 2008-01-03 for high strength thin-gauge steel sheet excellent in elongation and hole expandability and method of production of same.
Invention is credited to Koichi Goto, Toshiki Nonaka, Hirokazu Taniguchi.
Application Number | 20080000555 11/663581 |
Document ID | / |
Family ID | 36142775 |
Filed Date | 2008-01-03 |
United States Patent
Application |
20080000555 |
Kind Code |
A1 |
Nonaka; Toshiki ; et
al. |
January 3, 2008 |
High Strength Thin-Gauge Steel Sheet Excellent in Elongation and
Hole Expandability and Method of Production of Same
Abstract
The present invention provides high strength thin-gauge steel
sheet with excellent elongation and hole expandability having a
tensile strength of 500 MPa or more and a method of production of
high strength thin-gauge steel sheet with excellent elongation and
hole expandability enabling production of this on an industrial
scale, that is, high strength thin-gauge steel sheet comprised of,
by mass %, C: 0.03 to 0.25%, Si: 0.4 to 2.0%, Mn: 0.8 to 3.1%,
P.ltoreq.0.02%, S.ltoreq.0.02%, Al.ltoreq.2.0%, N.ltoreq.0.01%, and
a balance of Fe and unavoidable impurities and having a
microstructure comprised of ferrite with an area fraction of 10 to
85% and residual austenite with a volume fraction of 1 to 10%, an
area fraction of 10% to 60% of tempered martensite, and a balance
of bainite.
Inventors: |
Nonaka; Toshiki; (Aichi,
JP) ; Taniguchi; Hirokazu; (Chiba, JP) ; Goto;
Koichi; (Aichi, JP) |
Correspondence
Address: |
KENYON & KENYON LLP
ONE BROADWAY
NEW YORK
NY
10004
US
|
Family ID: |
36142775 |
Appl. No.: |
11/663581 |
Filed: |
October 5, 2005 |
PCT Filed: |
October 5, 2005 |
PCT NO: |
PCT/JP05/18724 |
371 Date: |
March 30, 2007 |
Current U.S.
Class: |
148/328 ;
148/621 |
Current CPC
Class: |
C21D 8/0426 20130101;
C22C 38/18 20130101; C22C 38/002 20130101; C21D 8/0436 20130101;
C22C 38/12 20130101; C21D 2211/002 20130101; C22C 38/04 20130101;
C22C 38/32 20130101; C21D 8/041 20130101; C22C 38/28 20130101; C22C
38/26 20130101; C21D 2211/001 20130101; C22C 38/001 20130101; C22C
38/005 20130101; C21D 2211/005 20130101; C21D 1/25 20130101; C22C
38/22 20130101; C22C 38/02 20130101; C22C 38/38 20130101; B21B 3/02
20130101; C22C 38/14 20130101; C21D 9/46 20130101; C21D 8/0473
20130101; C21D 2211/008 20130101; C22C 38/06 20130101 |
Class at
Publication: |
148/328 ;
148/621 |
International
Class: |
C22C 38/06 20060101
C22C038/06; C21D 9/46 20060101 C21D009/46 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 6, 2004 |
JP |
2004-293990 |
Claims
1. High strength thin-gauge steel sheet with excellent elongation
and hole expandability characterized by being comprised of, by mass
%, C: 0.03 to 0.25%, Si: 0.4 to 2.0%, Mn: 0.8 to 3.1%,
P.ltoreq.0.02%, S.ltoreq.0.02%, Al.ltoreq.2.0%, N.ltoreq.0.01%, and
a balance of Fe and unavoidable impurities and having a
microstructure comprised of ferrite with an area fraction of 10 to
85% and residual austenite with a volume fraction of 1 to 10%, an
area fraction of 10% to 60% of tempered martensite, and a balance
of bainite.
2. High strength thin-gauge steel sheet with excellent elongation
and hole expandability according to claim 1 characterized by
further including as chemical ingredients one or more of V: 0.005
to 1%, Ti: 0.002 to 1%, Nb: 0.002 to 1%, Cr: 0.005 to 2%, Mo: 0.005
to 1%, B: 0.0002 to 0.1%, Mg: 0.0005 to 0.01%, REM: 0.0005 to
0.01%, and Ca: 0.0005 to 0.01%.
3. High strength thin-gauge steel sheet with excellent elongation
and hole expandability according to claim 1 characterized by
further satisfying the following formula (A): (0.0012.times.[TS
target value]-0.29)/3<[Al]+0.7[Si]<1.0 (A) TS target value is
design value of strength of steel sheet in units of MPa, [Al] is
mass % of Al, and [Si] is mass % of Si,
4. A method of production of high strength thin-gauge steel sheet
with excellent elongation and hole expandability characterized by
producing a slab comprised of, by mass %, C: 0.03 to 0.25%, Si: 0.4
to 2.0%, Mn: 0.8 to 3.1%, P.ltoreq.0.02%, S.ltoreq.0.02%,
Al.ltoreq.2.0%, and N.ltoreq.0.01% and, further, when necessary,
one or more types of V: 0.005 to 1%, Ti: 0.002 to 1%, Nb: 0.002 to
1%, Cr: 0.005 to 2%, Mo: 0.005 to 1%, B: 0.0002 to 0.1%, Mg: 0.0005
to 0.01%, REM: 0.0005 to 0.01%, and Ca: 0.0005 to 0.01%, and a
balance of Fe and unavoidable impurities, heating it in a range of
1150 to 1250.degree. C., then hot rolling it in a temperature range
of 800 to 950.degree. C., coiling it at 700.degree. C. or less,
then pickling it as normal, then cold rolling by a reduction rate
of 30 to 80%, then, in a continuous annealing process, soaking it
at 600.degree. C. to the Ac.sub.3 point+50.degree. C. for
recrystallization annealing, cooling to 600.degree. C. to the Ar3
point by an average cooling rate of 30.degree. C./s or less, then
cooling to 400.degree. C. or less by an average cooling rate of 10
to 150.degree. C./s, then holding at higher than a cooling end
temperature of said cooling and 150 to 400.degree. C. for 1 to 20
minutes, then cooling to thereby obtain a metal structure having a
microstructure comprised of ferrite with an area fraction of 10 to
85% and residual austenite with a volume fraction of 1 to 10%, an
area fraction of 10% to 60% of tempered martensite, and a balance
of bainite.
5. A method of production of high strength thin-gauge steel sheet
with excellent elongation and hole expandability according to claim
4 characterized by, in the continuous annealing process, soaking at
600.degree. C. to the Ac.sub.3 point+50.degree. C. for
recrystallization annealing, cooling by an average cooling rate of
10 to 150.degree. C./s to 400.degree. C. or less, then heating and
holding a first time at 150 to 400.degree. C. for 1 to 20 minutes,
then heating and holding a second time at a temperature 30 to
300.degree. C. higher than the first heating and holding
temperature to 500.degree. C. for 1 to 100 seconds, then
cooling.
6. A method of production of high strength thin-gauge steel sheet
with excellent elongation and hole expandability according to claim
4 characterized by, in the continuous annealing process, soaking at
600.degree. C. to the Ac.sub.3 point+50.degree. C. for
recrystallization annealing, cooling by an average cooling rate of
10 to 1 50.degree. C./s to 400.degree. C. or less, then heating and
holding a first time at 150 to 400.degree. C. for 1 to 20 minutes,
cooling to the martensitic transformation point or less, heating
and holding a second time at the cooling end temperature to
500.degree. C. for 1 to 100 seconds, then cooling.
Description
TECHNICAL FIELD
[0001] The present invention relates to high strength thin-gauge
steel sheet excellent in elongation and hole expandability and a
method of production thereof.
BACKGROUND ART
[0002] Recently, due to the need for reducing the weight of
automobiles and improving collision safety, high strength steel
sheet excellent in formability into chassis frame members and
reinforcement members, seat frame parts, and the like are being
strongly demanded. From the aesthetic design and chassis design
requirements, complicated shapes are sometimes demanded. High
strength steel sheet having superior working performance is
therefore necessary.
[0003] On the other hand, due to the increasingly higher strength
of steel sheet, the working method is frequently shifting from the
conventional drawing using wrinkle elimination to simple stamping
and bending. Especially, when the bending ridge is an arc or other
curve, stretch flanging where the end face of the steel sheet is
elongated is sometimes used. Further, there are also quite a few
parts which are worked by burring to expand a worked hole
(preparatory hole) to form a flange. The amount of the expansion in
the large case is up to 1.6 times the diameter of the preparatory
hole.
[0004] On the other hand, the phenomenon of springback or other
elastic recovery after working a part occurs more readily the
higher the strength of the steel sheet and obstructs securing the
precision of the part.
[0005] In this way, these working methods require stretch
flangeability, hole expandability, bendability, and other local
formability of the steel sheet, but conventional high strength
steel sheet do not have sufficient performance, cracks and other
defects occur, and stable working of the products is not
possible.
[0006] Therefore, up to now, high strength steel sheet improved in
stretch flangeability has been proposed in Japanese Patent
Publication (A) No. 9-67645, but there has been a remarkable
increase in the need for improvement in workability, in particular
hole expandability and therefore further improvement enabling
simultaneous improvement in elongation as well.
DISCLOSURE OF INVENTION
[0007] The present invention has as its object to solve the
problems of the prior art as explained above and realize high
strength thin-gauge steel sheet with excellent elongation and hole
expandability and a method of production for the same on an
industrial scale. Specifically, it has as its object to realize
high strength thin-gauge steel sheet exhibiting the above
performance by a tensile strength of 500 MPa or more and a method
of production of the same on an industrial scale.
[0008] The inventors studied the methods of production of high
strength thin-gauge steel sheet with excellent elongation and hole
expandability and as a result discovered that to further improve
the ductility and hole expandability of steel sheet, in the case of
high strength cold rolled steel sheet with a tensile strength of
steel sheet of 500 MPa or more, the form and balance of the metal
structure of the steel sheet and the use of tempered martensite are
important. Furthermore, they discovered steel sheet establishing a
specific relationship between the tensile strength and Si and Al so
as to secure a suitable ferrite area fraction and avoid
deterioration of the chemical conversion ability and plating
adhesion and controlling precipitates and other inclusions
contained inside by the addition of Mg, REM, and Ca so as to
improve the local formability and thereby improve the press
formability to an unparalleled level and a method of production of
the same.
[0009] (1) High strength thin-gauge steel sheet with excellent
elongation and hole expandability characterized by being comprised
of, by mass %, C: 0.03 to 0.25%, Si: 0.4 to 2.0%, Mn: 0.8 to 3.1%,
P.ltoreq.0.02%, S.ltoreq.0.02%, Al.ltoreq.2.0%, N.ltoreq.0.01%, and
a balance of Fe and unavoidable impurities and having a
microstructure comprised of ferrite with an area fraction of 10 to
85% and residual austenite with a volume fraction of 1 to 10%, an
area fraction of 10% to 60% of tempered martensite, and a balance
of bainite.
[0010] (2) High strength thin-gauge steel sheet with excellent
elongation and hole expandability according to (1) characterized by
further including as chemical ingredients one or more of V: 0.005
to 1%, Ti: 0.002 to 1%, Nb: 0.002 to 1%, Cr: 0.005 to 2%, Mo: 0.005
to 1%, B: 0.0002 to 0.1%, Mg: 0.0005 to 0.01%, REM: 0.0005 to
0.01%, and Ca: 0.0005 to 0.01%.
[0011] (3) High strength thin-gauge steel sheet with excellent
elongation and hole expandability according to (1) or (2)
characterized by further satisfying the following formula (A):
(0.0012.times.[TS target value]-0.29)/3<[Al]+0.7[Si]<1.0
(A)
[0012] TS target value is design value of strength of steel sheet
in units of MPa, [Al] is mass % of Al, and [Si] is mass % of
Si,
[0013] (4) A method of production of high strength thin-gauge steel
sheet with excellent elongation and hole expandability
characterized by producing a slab comprised of, by mass %, C: 0.03
to 0.25%, Si: 0.4 to 2.0%, Mn: 0.8 to 3.1%, P.ltoreq.0.02%,
S.ltoreq.0.02%, Al.ltoreq.2.0%, and N.ltoreq.0.01% and, further,
when necessary, one or more types of V: 0.005 to 1%, Ti: 0.002 to
1%, Nb: 0.002 to 1%, Cr: 0.005 to 2%, Mo: 0.005 to 1%, B: 0.0002 to
0.1%, Mg: 0.0005 to 0.01%, REM: 0.0005 to 0.01%, and Ca: 0.0005 to
0.01%, and a balance of Fe and unavoidable impurities, heating it
in a range of 1150 to 1250.degree. C., then hot rolling it in a
temperature range of 800 to 950.degree. C., coiling it at
700.degree. C. or less, then pickling it as normal, then cold
rolling by a reduction rate of 30 to 80%, then, in a continuous
annealing process, soaking it at 600.degree. C. to the Ac.sub.3
point+50.degree. C. for recrystallization annealing, cooling to
600.degree. C. to the Ar.sub.3 point by an average cooling rate of
30.degree. C./s or less, then cooling to 400.degree. C. or less by
an average cooling rate of 10 to 150.degree. C./s, then holding at
150 to 400.degree. C. for 1 to 20 minutes, then cooling to thereby
obtain a metal structure having a microstructure comprised of
ferrite with an area fraction of 10 to 85% and residual austenite
with a volume fraction of 1 to 10%, an area fraction of 10% to 60%
of tempered martensite, and a balance of bainite.
[0014] (5) A method of production of high strength thin-gauge steel
sheet with excellent elongation and hole expandability according to
(4) characterized by, in the continuous annealing process, soaking
at 600.degree. C. to the Ac.sub.3 point+50.degree. C. for
recrystallization annealing, cooling by an average cooling rate of
10 to 150.degree. C./s to 400.degree. C. or less, then heating and
holding a first time at 150 to 400.degree. C. for 1 to 20 minutes,
then heating and holding a second time at a temperature 30 to
300.degree. C. higher than the first heating and holding
temperature to 500.degree. C. for 1 to 100 seconds, then
cooling.
[0015] (6) A method of production of high strength thin-gauge steel
sheet with excellent elongation and hole expandability according to
(4) characterized by, in the continuous annealing process, soaking
at 600.degree. C. to the Ac.sub.3 point+50.degree. C. for
recrystallization annealing, cooling by an average cooling rate of
10 to 150.degree. C./s to 400.degree. C. or less, then heating and
holding a first time at 150 to 400.degree. C. for 1 to 20 minutes,
cooling to the martensitic transformation point or less, heating
and holding a second time at the cooling end temperature to
500.degree. C. for 1 to 100 seconds, then cooling.
BEST MODE FOR CARRYING OUT THE INVENTION
[0016] The biggest characteristic of the structure of a high
strength thin-gauge steel sheet according to the present invention
is that by performing the necessary heat treatment after an
annealing and quenching process, a metal structure containing
ferrite, residual austenite, tempered martensite, and bainite in a
good balance can be obtained and a material having extremely stable
ductility and hole expandability can be obtained.
[0017] Next, the limitations of the chemical ingredients of the
present invention will be explained.
[0018] C is an important element for improving the strengthening
and hardenability of the steel and is essential for obtaining a
composite structure comprised of ferrite, martensite, bainite, etc.
To obtain the bainite or tempered martensite advantageous for
obtaining TS.gtoreq.500 MPa and local formability, 0.03% or more is
necessary. On the other hand, if the content becomes greater, the
cementite or other iron-based carbides easily become coarser, the
local formability deteriorates, and the hardness after welding
remarkably rises, so 0.25% was made the upper limit.
[0019] Si is an element preferable for raising the strength without
lowering the workability of the steel. However, if less than 0.4%,
a pearlite structure harmful to the hole expandability is easily
formed and, due to the drop in the solution strengthening of the
ferrite, the hardness difference between the formed structures
becomes greater and deterioration of the hole expandability is
invited, so 0.4% was made the lower limit. If over 2.0%, due to the
rise in the solution strengthening of ferrite, the cold rollability
drops and the Si oxides formed at the steel sheet surface cause a
drop in the chemical conversion ability. Further, the plating
adhesion and weldability also drop, so 2.0% was made the upper
limit.
[0020] Mn is an element which has to be added from the viewpoint of
securing the strength and, further, delaying the formation of
carbides and is an element effective for formation of ferrite. If
less than 0.8%, the strength is not satisfactory. Further,
formation of ferrite becomes insufficient and the ductility
deteriorates. If over 3.1%, the martensite becomes excessive, a
rise in strength is invited, and the workability deteriorates, so
3.1% was made the upper limit.
[0021] P, if over 0.02%, results in remarkable solidification
segregation of the time of casting, invites internal cracking and
deterioration of the hole expandability, and causes embrittlement
of the weld zone, so 0.02% was made the upper limit.
[0022] S is a harmful element since it remains as MnS and other
sulfide-based inclusions. In particular, the higher the matrix
strength, the more remarkable the effect. If the tensile strength
is 500 Mpa or more, it should be suppressed to 0.02% or less.
However, if Ti is added, precipitation as a Ti-based sulfide
occurs, so this restriction is eased somewhat.
[0023] Al is an element required for deoxidization of steel, but if
over 2.0% increases the alumina and other inclusions and impairs
the workability, so 2.0% was made the upper limit. To improve the
ductility, addition of 0.2% or more is preferable.
[0024] N, if over 0.01%, degrades the aging behavior and
workability of the matrix, so 0.01% was made the upper limit.
[0025] To obtain high strength steel sheet, generally large amounts
of additive elements are necessary and formation of ferrite is
restrained. For this reason, the ferrite fraction of the structure
is reduced and the fraction of the second phase increases, so
especially at 500 MPa or more, the elongation falls. For
improvement of this, normally addition of Si and reduction of Mn
are frequently used, but the former degrades the chemical
conversion ability and plating adhesion, while the latter makes
securing the strength difficult, so these cannot be utilized in the
steel sheet intended by the present invention. Therefore, the
inventors engaged in in-depth studies and as a result discovered
the effects of Al and Si. They discovered that when there is a
balance of Al, Si, and TS satisfying the relationship of formula
(A), a sufficient ferrite fraction can be secured and excellent
elongation can be secured. (0.0012.times.[TS target
value]-0.29)/3<[Al]+0.7[Si]<1.0 (A)
[0026] where the TS target value is the design value of the
strength of the steel sheet in units of MPa, [Al] is the mass % of
Al, and [Si] is the mass % of Si
[0027] If the amounts of Al and Si added are (0.0012.times.[TS
target value]-0.29)/3 or less, they are insufficient for improving
the ductility, while if 1.0 or more, the chemical conversion
ability and plating adhesion deteriorate.
[0028] Next, the optional elements of the present invention will be
explained.
[0029] V, for improving the strength, can be added in the range of
0.005 to 1%.
[0030] Ti is an element effective for the purpose of improving the
strength and for forming Ti-based sulfides with relatively little
effect on the local formability and reducing the harmful MnS.
Further, it has the effect of suppressing coarsening of the welded
metal structure and making embrittlement difficult. To exhibit
these effects, less than 0.002% is insufficient, so 0.002% is made
the lower limit. However, if excessively added, the coarse and
angular TiN increases and reduces the local formability. Further,
stable carbides are formed, the concentration of C in the austenite
falls at the time of production of the matrix, the desired hardened
structure cannot be obtained, and the tensile strength also can no
longer be secured, so 1.0% was made the upper limit.
[0031] Nb is an element effective for the purpose of improving the
strength and forming fine carbides suppressing softening of the
weld heat affected zone. If less than 0.002%, the effect of
suppressing softening of the weld heat affected zone cannot be
sufficiently obtained, so 0.002% was made the lower limit. On the
other hand, if excessively added, the increase in the carbides
causes the workability of the matrix to decline, so 1.0% was made
the upper limit.
[0032] Cr can be added as a strengthening element, but if less than
0.005, has no effect, while if over 2%, degrades the ductility and
chemical conversion ability, so 0.005% to 2% was made the
range.
[0033] Mo is an element which has an effect on securing the
strength and on the hardenability and further makes a bainite
structure easier to obtain. Further, it also has the effect of
suppressing the softening of the weld heat affected zone.
Copresence together with Nb etc. is believed to increase this
effect. If less than 0.005%, this effect is insufficient, so 0.005%
is made the lower limit. However, even if excessively added, the
effect becomes saturated and becomes economically disadvantageous,
so 1% was made the upper limit.
[0034] B is an element having the effect of improving the
hardenability of the steel and interacting with C to suppress
diffusion of C at the weld heat affected zone and thereby suppress
softening. To exhibit this effect, addition of 0.0002% or more is
necessary. On the other hand, if excessively added, the workability
of the matrix drops and embrittlement of the steel or a drop in the
hot workability is caused, so 0.1% was made the upper limit.
[0035] Mg bonds with oxygen to form oxides upon addition, but the
MgO and the complex compounds of Al.sub.2O.sub.3, SiO.sub.2, MnO,
Ti.sub.2O.sub.3, etc. including MgO are believed to precipitate
extremely finely. These oxides finely and uniformly dispersed in
the steel, while not certain, are believed to have the effect of
forming fine voids at the time of stamping or shearing at the
stamped or sheared cross-section forming starting points of cracks
and suppressing stress concentration at the time of later burring
or stretch flanging so as to prevent growth of the cracks to large
cracks. Due to this, it becomes possible to improve the hole
expandability and stretch flangeability, but if less than 0.0005%,
this effect is insufficient, so 0.0005% was made the lower limit.
On the other hand, addition over 0.01% not only results in
saturation of the amount of improvement with respect to the amount
of addition, but also conversely degrades the cleanliness factor of
the steel and degrades the hole expandability and stretch
flangeability, so 0.01% was made the upper limit.
[0036] REM are believed to be elements with a similar effect as Mg.
While not sufficiently confirmed, they are believed to be elements
promising an improvement in the hole expandability and stretch
flangeability due to the effect of suppression of cracks by the
formation of fine oxides, but if less than 0.0005%, this effect is
insufficient, so 0.0005% was made the lower limit. On the other
hand, with addition over 0.01%, not only does the amount of
improvement with respect to the added amount become saturated, but
also this conversely degrades the cleanliness factor of the steel
and degrades the hole expandability and stretch flangeability, so
0.01% was made the upper limit.
[0037] Ca has the effect of improving the local formability of the
matrix by control of the form of the sulfide-based inclusions
(spheroidization), but if less than 0.0005%, the effect is
insufficient, so 0.0005% was made the lower limit. Further, if
excessively added, not only is the effect saturated, but also the
reverse effect due to the increase in inclusions (deterioration of
local formability) occurs, so the upper limit was made 0.01%.
[0038] In the present invention, the reason for making the
structure of the steel sheet a composite structure of ferrite,
residual austenite, tempered martensite, and bainite is to obtain
steel shape excellent in strength and also elongation and hole
expandability. The "ferrite" indicates polygonal ferrite and
bainitic ferrite.
[0039] Furthermore, in the present invention, the biggest feature
in the metal structure of the high strength thin-gauge steel sheet
is that the steel contains tempered marensite in an area fraction
of 10 to 60%. This tempered martensite is tempered and becomes a
tempered martensite structure by heat treatment comprising cooling
the martensite formed in the cooling process of the annealing to
the martensitic transformation point or less, then holding at 150
to 400.degree. C. for 1 to 20 minutes or by holding at a
temperature 50 to 300.degree. C. higher than the holding
temperature to 500.degree. C. for 1 to 100 seconds. Here, if the
area fraction of the tempered martensite is less than 10%, the
hardness difference between the structures will become too large
and no improvement in the hole expansion rate will be seen, while
if over 60%, the strength of the steel sheet will drop too much.
Further, it may be considered that by making the ferrite an area
fraction of 10 to 85% and the residual austenite an area fraction
of 1 to 10% for a good balance in the steel sheet, the elongation
and hole expansion rate would be remarkably improved. If the
ferrite area fraction is less than 10%, the elongation cannot be
sufficient secured, while if the ferrite area fraction is over 85%,
the strength becomes insufficient, so this is not preferable.
Moreover, in the process of the present invention, 1% or more
residual austenite remains. With over a 10% residual austenite
volume fraction, the residual austenite will transform to
martensite transformation by working. At that time, voids or a
large number of dislocations will occur at the interface of the
martensite phase and the surrounding phases. Hydrogen will
accumulate at such locations resulting in inferior delayed fracture
characteristics, so this is not desirable.
[0040] Note that the bainite of the remaining structure can include
untempered martensite in an area fraction of 10% or less with
respect to the entire structure without any major effect on the
quality.
[0041] Next, the method of production will be explained.
[0042] First, a slab comprised of the above composition of
ingredients is produced. The slab is inserted into a heating
furnace while at a high temperature or after cooling down to room
temperature, heated at a temperature range of 1150 to 1250.degree.
C., then hot finished rolled a temperature range of 800 to
950.degree. C. and coiled at 700.degree. C. or less to obtain a hot
rolled steel sheet. If the hot rolled final temperature is less
than 800.degree. C., the crystal grains become mixed grains and the
workability of the matrix is lowered. If over 950.degree. C., the
austenite grains become coarse and the desired microstructure
cannot be obtained. A lower coiling temperature enables the
formation of a pearlite structure to be suppressed, but if
considering the cooling load as well, the temperature is preferably
made a range of 400 to 600.degree. C.
[0043] Next, the sheet is pickled, then cold rolled and annealed to
obtain a thin-gauge steel sheet. The cold rolling rate is
preferably a range of 30 to 80% in terms of rolling load and
material quality.
[0044] The annealing temperature is important in securing a
predetermined strength and workability of high strength steel sheet
and is preferably 600.degree. C. to Ac.sub.3+50.degree. C. If less
than 600.degree. C., sufficient recrystallization does not occur
and the workability of the matrix itself is hard to stably obtain.
Further, if over Ac.sub.3+50.degree. C., the austenite grains
coarsen, formation of ferrite is suppressed, and the desired
microstructure becomes hard to obtain. Further, to obtain the
microstructure prescribed by the present invention, the method of
continuous annealing is preferable.
[0045] Next, the sheet is cooled to 600.degree. C. to Ar.sub.3 at
an average cooling rate of 30.degree. C./s or less to form ferrite.
If less than 600.degree. C., pearlite precipitates and the quality
degrades, so this is not preferred. If over Ar.sub.3, the
predetermined ferrite area fraction cannot be obtained. Further,
even if the average cooling rate is over 30.degree. C./s, the
predetermined ferrite area fraction cannot be obtained, so the
average cooling rate was made 30.degree. C./s or less, more
preferably 10.degree. C./s or less.
[0046] Next, securing tempered martensite with an area fraction of
10% to 60% effective for improving the hole expandability and
stretch flangeability more will be explained.
[0047] After the above annealing and subsequent cooling, the sheet
is cooled by an average cooling rate of 10 to 150.degree. C./s to
400.degree. C. or less. If less than 10.degree. C./s, the majority
of the untransformed austenite is transformed to bainite, so the
subsequent formation of martensite is not sufficient and the
strength becomes inadequate. If over 150.degree. C./s, the shape of
the steel sheet is remarkably degraded, so this is not desirable.
Further, if over 400.degree. C., the amount of martensite cannot be
sufficiently secured and the strength becomes inadequate. To enable
efficient production by a production line working the present
invention connected to a continuous annealing line, 100 to
400.degree. C. or the martensitic transformation point temperature
to 400.degree. C. is preferable. Note that the martensitic
transformation point Ms is found by Ms(.degree.
C.)=561-471.times.C(%)-33Mn(%)-17.times.Ni(%)-17.times.Cr(%)-21.times.Mo(-
%).
[0048] Next, the sheet is treated by a heating and holding process
in which it is held at a temperature range of 150 to 400.degree. C.
for 1 to 20 minutes. If less than 150.degree. C., the martensite
will not be tempered and the hardness difference between the
structures will become large. Further, the bainite transformation
will also be insufficient and the predetermined ductility and hole
expandability will not be obtained. If over 400.degree., the sheet
will be overly tempered and the strength will falls, so this is not
desirable.
[0049] Further, to secure tempered martensite in the heating and
holding process, the upper limit is preferably made the martensitic
transformation point or less.
[0050] Further, to secure the bainite in the heating and holding
process, the lower limit is preferably over the martensitic
transformation point.
[0051] If the holding time is less than 1 minute, the tempering and
transformation do not progress much at all or remain incomplete,
and the ductility and hole expansion rate are not improved. If over
20 minutes, the tempering and transformation substantially end, so
there is no effect even with extending the time.
[0052] Note that the heating and holding process may be one
connected to the continuous annealing line or may be a separate
line, but one connected to the continuous annealing facility or one
performed in an averaging oven of the continuous annealing line is
preferable in terms of productivity.
[0053] Further, to reliably secure bainite, then secure tempered
martensite, it is preferable to make the above heating and holding
process a first heating and holding process of heating and holding
at 150 to 400.degree. C. and holding for 1 to 20 minutes, then a
second heating and holding process of heating to a temperature 30
to 300.degree. C. higher than the holding temperature of the first
heating and holding process to 500.degree. C. for 1 to 100 seconds,
then cooling.
[0054] If the temperature of the second heating and holding process
is less than the holding temperature of the first heating and
holding process +30.degree. C., the martensite is not tempered, the
hardness difference between the structures becomes large, and the
predetermined ductility and hole expandability cannot be obtained.
If the temperature of the second heating and holding process is
over the holding temperature of the first heating and holding
process +300.degree. C., the sheet will be overly tempered and the
strength will fall, so this is not preferable.
[0055] If the holding time is less than 1 second, the tempering
will not proceed much at all or will remain incomplete and the
ductility and hole expansion rate will not be improved. If over 100
seconds, the tempering substantially ends, so there is no effect
even with extending the time.
[0056] Further, to reliably secure bainite, then convert the
untransformed austenite to martensite and secure tempered
martensite, it is preferable to make the heating and holding
process a first heating and holding process of heating and holding
at 150 to 400.degree. C. and holding for 1 to 20 minutes, then
cooling to the martensitic transformation point or less, holding at
the cooling end temperature to 500.degree. C. for 1 to 100 seconds
for second heating and holding, then cooling. If the temperature of
the second heating and holding process is made the cooling end
temperature when cooling to the martensitic transformation point or
less +50 to 300.degree. C. to 500.degree. C. or less, tempered
martensite can be reliably secured, so this is preferable.
[0057] If the temperature of the second heating and holding process
is less than the cooling end temperature, the martensite will not
be tempered, the hardness difference between the structures will
become large, and the predetermined ductility and hole
expandability cannot be obtained. The lower limit of the
temperature of the second heating and holding process is more
preferably the cooling end temperature +50.degree. C. and the
martensitic transformation point or more. If the cooling end
temperature +300.degree. C., it is more preferable. If the
temperature of the second heating and holding process is over
500.degree. C., the sheet is overly tempered and the strength
drops, so this is not preferable.
[0058] When the holding time is less than 1 second, the tempering
does not progress much at all or remains incomplete and the
ductility and hole expanding rate are not improved. If over 100
seconds, the tempering substantially ends, so there is no effect
even with extending the time.
[0059] Further, the steel sheet may also be cold rolled steel sheet
or plated steel sheet. Further, the plating may be ordinary
galvanization, aluminum plating, etc. The plating may be either hot
dipping or electroplating. Further, the steel sheet may be plated,
then alloyed. It may also be plated by multiple layers. Further,
even steel sheet comprised of non-plated steel sheet or plated
steel sheet on which a film is laminated is not outside the present
invention.
EXAMPLES
[0060] Steel of each of the composition of ingredients shown in
Table 1 was produced in a vacuum melting furnace, cooled to
solidify, then reheated to 1200 to 1240.degree. C., final rolled at
880 to 920.degree. C. (to sheet thickness of 2.3 mm), cooled, then
held at 600.degree. C. for 1 hour so as to reproduce the coiling
heat treatment of the hot rolling. The obtained hot rolled sheet
was descaled by grinding, cold rolled (to 1.2 mm), then annealed at
750 to 880.degree. C..times.75 seconds using a continuous annealing
simulator.
[0061] After this, the sheet was cooling, heated, and held under
the conditions of [8] (comparative example) and [2] and [6]
(invention examples) of Table 2.
[0062] Furthermore, the steel type G described in Table 1 was used
for comparison while changing the heating and holding conditions of
the annealing by the conditions of [1] and [5] (invention examples)
and [3], [4], and [7] (comparative examples) of Table 2.
TABLE-US-00001 TABLE 1 TS Steel target type value C Si Mn P S Al N
Mg Ti Nb V Cr Mo B A 590 0.058 0.171 2.06 0.016 0.007 0.970 0.003
0.010 0.015 0.020 0.110 B 590 0.058 0.160 1.10 0.019 0.002 0.896
0.008 C 600 0.071 0.196 1.42 0.017 0.003 0.547 0.005 0.0011 0.029
0.160 0.0008 D 650 0.082 0.089 1.15 0.016 0.004 1.139 0.005 0.026
0.089 E 690 0.082 0.081 2.63 0.019 0.001 1.049 0.003 F 710 0.093
0.055 1.84 0.007 0.006 0.500 0.007 0.010 0.280 G 780 0.100 0.013
1.10 0.002 0.008 0.815 0.004 0.022 0.017 0.210 0.0009 H 800 0.110
0.122 2.64 0.018 0.009 0.731 0.020 I 820 0.120 0.084 1.17 0.010
0.010 0.866 0.004 0.0022 0.130 J 860 0.120 0.148 1.19 0.016 0.008
1.000 0.006 0.045 0.033 0.210 K 920 0.134 0.047 1.19 0.016 0.010
1.114 0.007 0.0033 0.032 0.052 L 940 0.140 0.042 1.71 0.014 0.006
0.780 0.005 0.020 M 970 0.142 0.116 1.27 0.018 0.007 0.850 0.006
0.015 0.021 0.0008 N 1000 0.150 0.107 1.76 0.019 0.006 0.880 0.009
O 1210 0.150 0.107 2.65 0.059 0.006 0.880 0.009 0.012 0.150 P 1230
0.195 0.299 1.20 0.019 0.005 0.600 0.002 0.0041 0.0010 Q 1500 0.196
0.187 1.95 0.018 0.004 0.019 0.009 0.150 a 450 0.025 0.177 1.11
0.016 0.009 0.953 0.005 0.021 0.020 0.0007 b 1350 0.255 0.176 2.73
0.018 0.008 0.850 0.004 0.0020 0.019 0.150 c 970 0.090 2.050 2.70
0.015 0.003 0.040 0.004 0.001 0.0008 d 1500 0.193 0.220 2.53 0.005
0.003 2.030 0.002 0.031 0.023 Formula (A) Sheet Steel Formula (A)
Left side right Right side thickness type REM Ca [Al] + 0.7[Si]
left side judgement side judgement (mm) Class A 1.0897 0.1393 Good
1.0 Poor 1.2 Inv. range B 1.0080 0.1393 Good 1.0 Poor 1.4 Inv.
range C 0.0010 0.6842 0.1433 Good 1.0 Good 1.2 Inv. range D 0.0007
0.0008 1.2013 0.1633 Good 1.0 Poor 1.0 Inv. range E 1.1057 0.1793
Good 1.0 Poor 0.8 Inv. range F 0.5385 0.1873 Good 1.0 Good 1.6 Inv.
range G 0.8241 0.2153 Good 1.0 Good 1.4 Inv. range H 0.8164 0.2233
Good 1.0 Good 3.4 Inv. range I 0.9248 0.2313 Good 1.0 Good 1.4 Inv.
range J 1.1036 0.2473 Good 1.0 Poor Inv. range K 1.1469 0.2713 Good
1.0 Poor Inv. range L 0.0012 0.8094 0.2793 Good 1.0 Good Inv. range
M 0.9312 0.2913 Good 1.0 Good Inv. range N 0.9549 0.3033 Good 1.0
Good Inv. range O 0.0012 0.0013 0.9549 0.3873 Good 1.0 Good Inv.
range P 0.8093 0.3953 Good 1.0 Good Inv. range Q 0.1499 0.5033 Poor
1.0 Good Inv. range a 0.0008 1.0769 0.0833 Good 1.0 Poor 1.2 Comp.
Ex. b 0.9732 0.4433 Good 1.0 Good Comp. Ex. c 0.0010 1.4750 0.2913
Good 1.0 Poor Comp. Ex. d 2.1840 0.5033 Good 1.0 Poor Comp. Ex.
[0063] TABLE-US-00002 TABLE 2 Average First heating and holding
Second heating and holding cooling Cooling Holding Cooling Cooling
Temper Experiment rate end temp. Temp. time temp. Temp. Holding
temp. rolling no. (.degree. C./s) (.degree. C.) (.degree. C.) (min)
(.degree. C.) Cooling (.degree. C.) time (s) (.degree. C.) rate (%)
[1] 150 300 330 3 Room -- -- -- -- 1 Inv. Ex. temp. [2] 120 330 3
Room -- -- -- -- Inv. Ex. temp. [3] 120 120 3 Room -- -- -- --
Comp. Ex. temp. [4] 120 620 3 Room -- -- -- -- Comp. Ex. temp. [5]
300 300 3 Room Ms point or 380 30 Room temp. Inv. Ex. temp. less
[6] 120 300 3 Room Ms point or 380 30 Room temp. Inv. Ex. temp.
less [7] 300 300 3 Room Ms point or 620 30 Room temp. Comp. Ex.
temp. less [8] 80 -- -- -- -- -- -- -- Comp. Ex. [9] 300 300 3 Room
-- 380 30 Room temp. Inv. Ex. temp.
[0064] Note that the various test methods used in the present
invention are as shown below.
[0065] Tensile characteristics: Evaluated by running tensile test
in direction perpendicular to rolling direction of JIS No. 5
tensile test piece
[0066] Hole expansion rate: Hole expansion test method of Japan
Iron and Steel Federation standard JFST1001-1996 employed.
[0067] A conical punch with a 60.degree. apex angle was forced
through a .phi.10 mm punched hole (die inside diameter of 10.3 mm,
clearance 12.5%) to form a burr of the hole in the outside
direction by a speed of 20 mm/min:
[0068] Hole expansion rate .lamda. (%)={(D-Do)/Do}.times.100
[0069] D: Hole diameter when crack penetrates sheet thickness
[0070] Do: Initial hole diameter (10 mm)
[0071] Metal structure:
[0072] Ferrite area fraction: Ferrite observed by Nital
etching.
[0073] The ferrite area fraction is quantified by polishing a
sample by Nital etching (alumina finish), dipping it in corrosive
solution (mixture of pure water, sodium pyrosulfite, ethyl alcohol,
and picric acid) for 10 seconds, then polishing again, rinsing,
then drying the sample by cooling air. After drying, a 100
.mu.m.times.100 .mu.m area of the structure of the sample is
measured for area by a Luzex system at a power of 1000 to determine
the area% of the ferrite. In each table, this ferrite area fraction
is shown as the ferrite area %.
[0074] Tempered martensite
[0075] Area rate: Observation by optical microscope and observation
of martensite by LePera etching.
[0076] The tempered martensite area fraction is quantified by
polishing a sample by LePera etching (alumina finish), dipping it
in corrosive solution (mixture of pure water, sodium pyrosulfite,
ethyl alcohol, and picric acid) for 10 seconds, then polishing
again, rinsing, then drying the sample by cooling air. After
drying, a 100 .mu.m.times.100 .mu.m area of the structure of the
sample is measured for area by a Luzex system at a power of 1000 to
determine the area % of the tempered martensite. In each table,
this tempered martensite area fraction is shown as the tempered
martensite area %.
[0077] Residual austenite volume fraction: The residual austenite
is quantized by MoKa beams from the (200), (210) area strength of
the ferrite and the (200), (220), and (311) area strength of the
austenite at the surface of the supplied sheet chemically polished
to 1/4 the thickness from the surface and used as the residual
austenite volume fraction. A residual austenite volume fraction of
1 to 10% or more is deemed good.
[0078] In each table, the residual austenite volume fraction is
expressed as the residual y-volume % and rate.
[0079] The test results of comparative examples of Experiment No.
[8] shown in Table 2 of Example 1 are shown in Table 3. Further,
the test results of Experiment No. [2] of the present invention are
shown in Table 4, those of Experiment No. [6] are shown in Table 5,
and those of Experiment No. [9] are shown in Table 6. Further, the
test results of Example 2 are shown in Table 7.
Example 1
[0080] Comparing Experiment No. [8] with the same operating
conditions as the past as a comparative example and Experiment Nos.
[2], [6], and [9] of invention examples, it is learned that the
invention examples exhibit better values of the hole expansion rate
and elongation.
[0081] Further, as a comparison of sheets with the same level of
tensile strength and generally the same ingredients, but satisfying
formula (A) and not satisfying it, among the steel types B and C, E
and F, and K and L, the C, F, and L satisfying formula (A)
exhibited larger ferrite area fractions and better elongation.
Example 2
[0082] Further changing and comparing the tempering conditions, the
drop in strength was large and the elongation also conversely
dropped. The drop in elongation is believed due to the formation of
pearlite. Experiment Nos. [1], [2], [5], [6], and [9] of the
invention examples all exhibited good results. TABLE-US-00003 TABLE
3 (Example 1) Experiment No. [8] (Comparative Examples) Underlined,
bold-face, italics indicate rejection Hole Tempered Steel TS
expansion Ferrite Residual .gamma. martensite Other type (MPa) EL
(%) TS .times. EL rate area (%) vol. (%) area (%) composition Class
A 598 30.9 18478 41 81.8 3.6 .ltoreq.0.1 Mainly Comp. Ex. B 602
30.2 18180 40 84.1 2.9 .ltoreq.0.1 martensite Comp. Ex. C 613 32.3
19800 40 84.3 3.6 .ltoreq.0.1 Comp. Ex. D 665 29.2 19418 38 73.0
2.7 .ltoreq.0.1 Comp. Ex. E 703 27.1 19051 38 62.1 3.7 .ltoreq.0.1
Comp. Ex. F 722 28.6 20649 38 66.8 2.7 .ltoreq.0.1 Comp. Ex. G 799
24.7 19735 38 59.3 3.3 .ltoreq.0.1 Comp. Ex. H 811 23.6 19140 37
58.6 2.9 .ltoreq.0.1 Comp. Ex. I 836 21.8 18225 34 57.1 3.1
.ltoreq.0.1 Comp. Ex. J 875 20.7 18113 33 52.3 2.7 .ltoreq.0.1
Comp. Ex. K 931 19.6 18248 33 37.7 4 .ltoreq.0.1 Comp. Ex. L 956
20.5 19598 32 44.3 3.4 .ltoreq.0.1 Comp. Ex. M 984 18.8 18499 30
35.5 3.6 .ltoreq.0.1 Comp. Ex. N 1021 18.3 18684 27 32.5 2.9
.ltoreq.0.1 Comp. Ex. O 1223 14.6 17856 24 28.3 2.8 .ltoreq.0.1
Comp. Ex. P 1243 14.4 17899 22 29.4 3.4 .ltoreq.0.1 Comp. Ex. Q
1521 14.2 21598 20 21.5 3.1 .ltoreq.0.1 Comp. Ex. a 453 31.2 14134
62 87.1 1.8 .ltoreq.0.1 Mainly Comp. Ex. b 1367 11.6 15857 19 26.4
2.4 .ltoreq.0.1 martensite Comp. Ex. c 985 16.0 15760 27 30.2 2.3
.ltoreq.0.1 Comp. Ex. d 1523 9.7 14773 18 19.8 3.1 .ltoreq.0.1
Comp. Ex.
[0083] TABLE-US-00004 TABLE 4 Experiment No. [2] (Invention)
Underlined, bold- face, italics indicate rejection Hole Tempered
Steel TS expansion Ferrite Residual .gamma. martensite Other type
(MPa) EL (%) TS .times. EL rate area (%) vol. (%) area (%)
composition Class A 568 33.1 18783 74 80.2 4.1 12.3 Mainly Inv. B
572 32.0 18308 72 80.7 3.2 13.6 bainite Inv. C 582 35.2 20503 72
82.6 4.4 15.4 Inv. D 632 31.2 19738 69 70.1 3.1 19.8 Inv. E 668
28.7 19185 68 60.9 4.1 20.4 Inv. F 686 31.2 21382 68 64.1 3.3 22.1
Inv. G 759 26.4 20061 68 58.1 3.8 25.8 Inv. H 770 25.0 19274 66
56.3 3.2 29.4 Inv. I 794 23.8 18872 61 55.9 3.8 30.9 Inv. J 831
22.1 18411 56 50.2 3.1 34.1 Inv. K 884 20.8 18375 56 36.9 4.4 37
Inv. L 908 22.3 20294 55 42.6 4.1 38.6 Inv. M 935 20.1 18804 51
34.8 4.1 42.7 Inv. N 990 19.4 19211 46 31.2 3.2 45.9 Inv. O 1162
15.9 18490 40 21.8 3.4 47.7 Inv. P 1181 15.4 18195 38 28.8 3.9 49.3
Inv. Q 1445 15.1 21749 34 20.6 3.4 52.9 Inv. a 430 34.0 14635 86
85.4 2.2 8.9 Mainly Comp. Ex. b 1299 12.4 16119 28 25.3 2.8 55.7
bainite Comp. Ex. c 936 17.0 15870 41 29.6 2.6 49.6 Comp. Ex. d
1447 10.6 15298 26 19.0 3.8 62.3 Comp. Ex.
[0084] TABLE-US-00005 TABLE 5 Experiment No. [6] (Invention)
Underlined, bold- face, italics indicate rejection Hole Ferrite
Tempered Steel TS EL expansion area Residual .gamma. martensite
Other type (MPa) (%) TS .times. EL rate (%) vol. (%) area (%)
composition Class A 540 35.4 19093 85 77.7 4.7 13.8 Mainly Inv. B
549 33.9 18630 84 76.7 3.7 15.5 bainite Inv. C 542 38.4 20784 84
79.3 5.4 17.4 Inv. D 600 33.4 20064 79 68.0 3.5 22.2 Inv. E 641
30.4 19522 79 57.8 4.8 23.3 Inv. F 638 34.0 21675 79 61.6 4.0 25.0
Inv. G 721 28.3 20392 78 56.4 4.3 28.9 Inv. H 740 26.5 19613 77
53.5 3.7 33.5 Inv. I 739 25.9 19130 71 53.7 4.6 34.9 Inv. J 790
23.7 18715 65 48.7 3.5 38.2 Inv. K 849 22.0 18699 66 35.1 5.2 42.2
Inv. L 845 24.4 20572 64 40.9 5.1 43.6 Inv. M 888 21.5 19115 59
33.7 4.7 47.8 Inv. N 951 20.6 19549 54 29.6 3.7 52.3 Inv. O 1081
17.3 18743 47 20.9 4.2 53.9 Inv. P 1122 16.5 18495 44 27.9 4.4 55.2
Inv. Q 1387 16.0 22132 40 19.6 4.0 60.0 Inv. a 400 37.1 14836 89
82.0 2.7 9.8 Mainly Comp. Ex. b 1234 13.3 16385 30 24.6 3.1 60.7
bainite Comp. Ex. c 898 18.0 16150 42 28.2 3.0 55.6 Comp. Ex. d
1346 11.5 15507 29 18.3 4.6 67.9 Comp. Ex.
[0085] TABLE-US-00006 TABLE 6 Experiment No. [9] (Invention)
Underlined, bold- face, italics indicate rejection Hole Tempered
Steel TS EL expansion Ferrite Residual .gamma. martensite Other
type (MPa) (%) TS .times. EL rate area (%) vol. (%) area (%)
composition Class A 528 35.7 18866 77 76.9 4.6 12.9 Mainly Inv. B
543 34.3 18610 75 75.9 3.7 14.1 bainite Inv. C 536 38.7 20749 74
78.5 5.3 15.9 Inv. D 588 33.7 19825 72 67.3 3.4 20.8 Inv. E 634
30.7 19501 70 57.2 4.7 21.2 Inv. F 631 34.3 21639 70 60.9 4.0 22.8
Inv. G 706 28.5 20149 71 55.8 4.2 27.1 Inv. H 732 26.8 19592 69
52.9 3.7 30.6 Inv. I 731 26.1 19098 63 53.1 4.5 31.8 Inv. J 773
23.9 18492 59 48.2 3.4 35.8 Inv. K 840 22.2 18679 58 34.7 5.1 38.5
Inv. L 836 24.6 20537 57 40.4 5.0 39.8 Inv. M 869 21.7 18887 54
33.4 4.6 44.8 Inv. N 941 20.8 19528 48 29.3 3.7 47.7 Inv. O 1069
17.5 18712 42 20.7 4.1 49.1 Inv. P 1098 16.6 18275 40 27.6 4.3 51.8
Inv. Q 1373 16.1 22108 35 19.4 3.9 55.0 Inv. a 396 37.4 14811 87
81.1 2.6 9.2 Mainly Comp. Ex. b 1208 14.0 16939 29 24.3 3.0 56.8
bainite Comp. Ex. c 889 19.0 16886 41 27.9 2.9 51.6 Comp. Ex. d
1331 12.3 16326 27 18.1 4.5 64.2 Comp. Ex.
[0086] TABLE-US-00007 TABLE 7 (Example 2) The effects of the
operational conditions will be seen by the Steel Type G. Hole
Residual Tempered Exp. TS expansion Ferrite .gamma. vol. martensite
Other no. (MPa) EL (%) TS .times. EL rate area (%) (%) area (%)
composition Class [1] 791 24.8 19617 52 45.0 4.0 21.3 Mainly Inv.
Ex. [2] 759 26.4 20061 68 58.1 3.8 25.8 bainite Inv. Ex. [3] 806
23.9 19263 45 45.5 3.0 3.2 Comp. Ex. [4] 697 19.9 13870 49 40.8 3.8
28.3 Comp. Ex. [5] 766 27.4 20988 56 44.2 3.6 25.3 Inv. Ex. [6] 721
28.3 20392 78 56.4 4.3 28.9 Inv. Ex. [7] 691 19.7 13613 48 41.2 5.1
27.9 Comp. Ex. [8] 799 22.8 18217 45 46.7 3.3 .ltoreq.0.1 Comp. Ex.
[9] 706 28.5 20149 71 55.8 4.2 27.1 Inv. Ex.
INDUSTRIAL APPLICABILITY
[0087] According to the present invention, it is possible to
provide high strength thin-gauge steel sheet with excellent
elongation and hole expandability used for auto parts etc. and a
method of production of the same and has extremely great industrial
value.
* * * * *