U.S. patent application number 11/885804 was filed with the patent office on 2008-07-10 for galvannealed steel sheet and method for producing the same.
Invention is credited to Yutaka Awajiya, Takayuki Futatsuka, Hiroshi Matsuda, Yasunobu Nagataki, Tatsuya Nakagaito.
Application Number | 20080163961 11/885804 |
Document ID | / |
Family ID | 37053523 |
Filed Date | 2008-07-10 |
United States Patent
Application |
20080163961 |
Kind Code |
A1 |
Nakagaito; Tatsuya ; et
al. |
July 10, 2008 |
Galvannealed Steel Sheet and Method for Producing the Same
Abstract
A galvannealed steel sheet contains, by % by mass, about 0.05 to
about 0.25% of C, about 2.0% or less of Si, about 1 to about 3% of
Mn, about 0.1% or less of P, about 0.01% or less of S, about 0.3 to
about 2% of Al, less than about 0.005% of N, about 1% or less of
Cr, about 1% or less of V, about 1% or less of Mo, less than about
0.005% of Ti, and less than about 0.005% of Nb, and satisfies the
relations, Si +Al >0.6% and Cr +V +Mo =0.1 to 2%, the balance
being Fe and inevitable impurities.
Inventors: |
Nakagaito; Tatsuya;
(Hiroshima, JP) ; Futatsuka; Takayuki; (Hiroshima,
JP) ; Matsuda; Hiroshi; (Chiba, JP) ; Awajiya;
Yutaka; (Hiroshima, JP) ; Nagataki; Yasunobu;
(Chiba, JP) |
Correspondence
Address: |
IP GROUP OF DLA PIPER US LLP
ONE LIBERTY PLACE, 1650 MARKET ST, SUITE 4900
PHILADELPHIA
PA
19103
US
|
Family ID: |
37053523 |
Appl. No.: |
11/885804 |
Filed: |
March 31, 2006 |
PCT Filed: |
March 31, 2006 |
PCT NO: |
PCT/JP2006/307406 |
371 Date: |
September 6, 2007 |
Current U.S.
Class: |
148/537 ;
148/330; 148/331; 148/334 |
Current CPC
Class: |
C21D 9/46 20130101; C22C
38/22 20130101; C22C 38/38 20130101; C23C 2/02 20130101; C22C 38/02
20130101; C23C 2/28 20130101; C22C 38/002 20130101; C22C 38/24
20130101; C22C 38/06 20130101; B32B 15/013 20130101; C22C 38/04
20130101 |
Class at
Publication: |
148/537 ;
148/334; 148/330; 148/331 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C21D 1/26 20060101 C21D001/26; C22C 38/22 20060101
C22C038/22 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 2005 |
JP |
2005-103832 |
Mar 3, 2006 |
JP |
2006-058458 |
Claims
1. A galvannealed steel sheet including, by % by mass, about 0.05
to about 0.25% of C, about 2.0% or less of Si, about 1 to about 3%
of Mn, about 0.1% or less of P, about 0.01% or less of S, about 0.3
to about 2% of Al, less than about 0.005% of N, about 1% or less of
Cr, about 1% or less of V, about 1% or less of Mo, less than about
0.005% of Ti, and less than about 0.005% of Nb, and satisfying the
relations, Si+Al.gtoreq.0.6% and Cr+V+Mo=0.1 to 2%, the balance
being Fe and inevitable impurities.
2. A galvannealed steel sheet including, by % by mass, about 0.05
to about 0.25% of C, about 2.0% or less of Si, about 1 to about 3%
of Mn, about 0.1% or less of P, about 0.01% or less of S, about 0.3
to about 2% of Al, less than about 0.005% of N, about 1% or less of
Cr, about 1% or less of V, about 1% or less of Mo, less than about
0.005% of Ti, and less than about 0.005% of Nb, and satisfying the
relations, Si+Al.gtoreq.0.6%, N.ltoreq.0.007%-(0.003.times.Al)%,
and Cr+V+Mo=0.1 to 2%, the balance being Fe and inevitable
impurities.
3. The galvannealed steel sheet according to claim 1, further
comprising, by % by mass, at least one of about 0.005% or less of B
and about 1% or less of Ni.
4. The galvannealed steel sheet according to claim 2, further
comprising, by % by mass, at least one of about 0.005% or less of B
and about 1% or less of Ni.
5. The galvannealed steel sheet according to claim 1, further
comprising, by % by mass, at least one of Ca and REM in a total of
about 0.01% or less.
6. The galvannealed steel sheet according to claim 2, further
comprising, by % by mass, at least one of Ca and REM in a total of
about 0.01% or less.
7. The galvannealed steel sheet according to claim 3, further
comprising, by % by mass, at least one of Ca and REM in a total of
about 0.01% or less.
8. The galvannealed steel sheet according to claim 4, further
comprising, by % by mass, at least one of Ca and REM in a total of
about 0.01% or less.
9. The galvannealed steel sheet according to claim 1, including a
residual austenite phase at a volume ratio of about 3 to about
20%.
10. A method for producing a galvannealed steel sheet, comprising:
annealing, in a temperature range of about 730.degree. C. to about
900.degree. C., a cold-rolled steel sheet containing, by % by mass,
about 0.05 to about 0.25% of C, about 2% or less of Si, about 1 to
about 3% of Mn, about 0.1% or less of P, about 0.01% or less of S,
about 0.3 to about 2% of Al, less than about 0.005% of N, about 1%
or less of Cr, about 1% or less of V, about 1% or less of Mo, less
than about 0.005% of Ti, and less than about 0.005% of Nb, and
satisfying the relations, Si+Al.gtoreq.0.6% and Cr+V+Mo=0.1 to 2%,
the balance being Fe and inevitable impurities; cooling the
annealed cold-rolled steel sheet at a cooling rate of about 3 to
about 100.degree. C./second; retaining the cooled cold-rolled steel
sheet in a temperature range of about 350.degree. C. to about
600.degree. C. for about 30 to about 250 seconds; hot-dip
galvanizing the cold-rolled steel sheet after the retention; and
alloying the hot-dip galvanized cold-rolled steel sheet at a
temperature of about 470.degree. C. to about 600.degree. C.
11. A method for producing a galvannealed steel sheet, comprising:
annealing, in a temperature range of about 730.degree. C. to about
900.degree. C., a cold-rolled steel sheet containing, by % by mass,
about 0.05 to about 0.25% of C, about 2% or less of Si, about 1 to
about 3% of Mn, about 0.1% or less of P, about 0.01% or less of S,
about 0.3 to about 2% of Al, less than about 0.005% of N, about 1%
or less of Cr, about 1% or less of V, about 1% or less of Mo, less
than about 0.005% of Ti, and less than about 0.005% of Nb, and
satisfying the relations, Si+Al.gtoreq.0.6%,
N.ltoreq.0.007%-(0.003.times.Al)%, and Cr+V+Mo=0.1 to 2%, the
balance being Fe and inevitable impurities; cooling the annealed
cold-rolled steel sheet at a cooling rate of about 3 to about
100.degree. C./second; retaining the cooled cold-rolled steel sheet
in a temperature range of about 350.degree. C. to about 600.degree.
C. for about 30 to about 250 seconds; hot-dip galvanizing the
cold-rolled steel sheet after the retention; and alloying the
hot-dip galvanized cold-rolled steel sheet at a temperature of
about 470.degree. C. to about 600.degree. C.
12. The method according to claim 10, wherein the cold-rolled steel
sheet further contains, by % by mass, at least one of about 0.005%
or less of B and about 1% or less of Ni.
13. The method according to claim 11, wherein the cold-rolled steel
sheet further contains, by % by mass, at least one of about 0.005%
or less of B and about 1% or less of Ni.
14. The method according to claim 10, wherein the cold-rolled steel
sheet further contains, by % by mass, at least one of Ca and REM in
a total of about 0.01% or less.
15. The method according to claim 11, wherein the cold-rolled steel
sheet further contains, by % by mass, at least one of Ca and REM in
a total of about 0.01% or less.
16. The method according to claim 12, wherein the cold-rolled steel
sheet further contains, by % by mass, at least one of Ca and REM in
a total of about 0.01% or less.
17. The method according to claim 13, wherein the cold-rolled steel
sheet further contains, by % by mass, at least one of Ca and REM in
a total of about 0.01% or less.
18. The method according to claim 10, wherein the galvannealed
steel sheet contains a residual austenite phase at a volume ratio
of about 3 to about 20%.
Description
RELATED APPLICATION
[0001] This is a .sctn.371 of International Application No.
PCT/JP2006/307406, with an international filing date of Mar. 31,
2006 (WO 2006/104282 A1published Oct. 5, 2006), which is based on
Japanese Patent Application Nos. 2005-103832, filed Mar. 31, 2005,
and 2006-058458, filed Mar. 3, 2006.
TECHNICAL FIELD
[0002] This disclosure relates to a low-yield-ratio, high-strength
galvannealed steel sheet used in application as an automobile steel
sheet and a method for producing the same.
BACKGROUND
[0003] In recent years, from the viewpoint of conservation of the
global environment, improvement in mileage of automobiles has
become an important problem. Therefore, attempts have been made
actively to increase the strength of car body materials to thin the
materials and decrease the weights of car bodies. However,
increases in strength of steel sheets decrease ductility, i.e.,
decrease workability, and thus there have been desired materials
having both high strength and high workability.
[0004] For such a requirement, there have been developed various
composite structure steels such as ferrite-martensite dual phase
steel (Dual-Phase steel) and TRIP steel (transformation induced
plasticity steel) using transformation-induced plasticity of
residual austenite.
[0005] In some cases, the surfaces of these steel sheets are
galvanized for improving rust proofness in practical use. As such
galvanized steel sheets, galvannealed steel sheets subjected to
heat treatment for diffusing Fe of the steel sheets into plating
layers after hot-dip galvanization are widely used from the
viewpoint of securing press property, spot weldability, coating
adhesion. With respect to the galvannealed steel sheets, various
proposals have been made.
[0006] For example, in Japanese Unexamined Patent Application
Publication No. 11-279691, there has been proposed a galvannealed
steel sheet with excellent workability in which residual y is
secured by adding a large amount of Si to the steel sheet, thereby
achieving high ductility. However, Si decreases the plating
properties, and thus a complicated process of Ni pre-plating,
applying a special chemical, reducing an oxide layer on the surface
of a steel sheet, and/or appropriately controlling the thickness of
an oxide layer is required for adhesion of a zinc plating to such
high-Si steel.
[0007] In Japanese Unexamined Patent Application Publication No.
2002-030403, there has been proposed a galvannealed steel sheet
with excellent ductility in which instead of Si, Al with a small
adverse effect on the plating properties is added to the steel
sheet, thereby improving wettability and anti-powdering property.
In actual press forming, improvement in ductility as well as
improvement in shape fixability has become a large problem.
[0008] When the strength of a steel sheet is increased, yield
strength is also increased, and the amount of spring back in press
forming is increased to decrease the shape fixability. Such a
decrease in shape fixability can be improved by decreasing yield
ratio. In Japanese Unexamined Patent Application Publication No.
2002-317249, a low-yield-ratio cold-rolled steel sheet has been
proposed. However, when this steel sheet is applied to an alloyed
hot-dip steel sheet, it is difficult to achieve a low yield ratio
because of the zinc bath temperature of as high as over 450.degree.
C. and the need for alloying at over 500.degree. C.
[0009] Further, in Japanese Unexamined Patent Application
Publication No. 2004-115843, there has been proposed a hot-dip
galvanized steel sheet in which the amounts of Si, Al, and Mn are
balanced, and the steel sheet is maintained at a low temperature
for a short time after annealing to form a martensite phase
containing a large amount of C, thereby achieving a low yield
ratio. However, the proposed technique relates to DP steel which
cannot utilize improvement in ductility (TRIP effect) due to
strain-induced transformation of residual austenite. Therefore, the
steel sheet cannot be recognized as having sufficient
ductility.
SUMMARY
[0010] We provide a high-strength galvannealed steel sheet capable
of achieving good alloying hot-dip galvanization properties without
the need of a complicated process and achieving excellent ductility
and a low yield ratio after alloying hot-dip galvanization, and a
method of producing the steel sheet. As sometimes used hereinafter,
"high strength" means a TS of 340 MPa or more.
[0011] We found that the yield ratio of the galvannealed steel
sheet can be significantly decreased by adding Cr, V, and Mo in
combination with Al, thereby achieving a yield ratio of about 55%
or less. In addition, when the amounts of C, Si, Mn, and Al are
appropriately controlled, the amount of residual austenite can be
increased without decreasing the alloying hot-dip galvanization
properties, achieving excellent ductility.
[0012] Although the reason why the yield ratio can be decreased by
adding Cr, V, and Mo in combination with Al is not entirely
understood, a conceivable reason is as follows: Al discharges C
dissolved in ferrite into a second phase and effectively functions
to purify ferrite, thereby decreasing the yield ratio. On the other
hand, the addition of Cr, V, and Mo permits residual austenite to
be formed by austempering at a high temperature with a short time.
Therefore, the formed residual austenite has a small amount of
dissolved C and is transformed to martensite with small strain,
forming a strain field around it and decreasing yield stress. It is
thought that the yield stress is more effectively decreased due to
the formation of a strain field around ferrite which is purified by
adding Al to decrease the amount of dissolved C.
[0013] Thus, we provide the following items (1) to (18): [0014] (1)
A galvannealed steel sheet including, by % by mass, about 0.05 to
about 0.25% of C, about 2.0% or less of Si, about 1 to about 3% of
Mn, about 0.1% or less of P, about 0.01% or less of S, about 0.3 to
about 2% of Al, less than about 0.005% of N, about 1% or less of
Cr, about 1% or less of V, about 1% or less of Mo, less than about
0.005% of Ti, and less than about 0.005% of Nb, and satisfying the
relations, Si +Al.gtoreq.0.6% and Cr+V+Mo=0.1 to 2%, the balance
being Fe and inevitable impurities. [0015] (2) A galvannealed steel
sheet containing, by % by mass, about 0.05 to about 0.25% of C,
about 2.0% or less of Si, about 1 to about 3% of Mn, about 0.1% or
less of P, about 0.01% or less of S, about 0.3 to about 2% of Al,
less than about 0.005% of N, about 1% or less of Cr, about 1% or
less of V, about 1% or less of Mo, less than about 0.005% of Ti,
and less than about 0.005% of Nb, and satisfying the relations,
Si+Al.gtoreq.0.6%, N.ltoreq.0.007%-(0.003.times.Al)%, and
Cr+V+Mo=0.1 to 2%, the balance being Fe and inevitable impurities.
[0016] (3) The galvannealed steel sheet described in (1), further
containing, by % by mass, at least one of about 0.005% or less of B
and about 1% or less of Ni. [0017] (4) The galvannealed steel sheet
described in (2), further containing, by % by mass, at least one of
about 0.005% or less of B and about 1% or less of Ni. [0018] (5)
The galvannealed steel sheet described in (1), further containing,
by % by mass, at least one of Ca and REM in a total of about 0.01%
or less. [0019] (6) The galvannealed steel sheet described in (2),
further containing, by % by mass, at least one of Ca and REM in a
total of about 0.01% or less. [0020] (7) The galvannealed steel
sheet described in (3), further containing, by % by mass, at least
one of Ca and REM in a total of about 0.01% or less. [0021] (8) The
galvannealed steel sheet described in (4), further containing, by %
by mass, at least one of Ca and REM in a total of about 0.01% or
less. [0022] (9) The galvannealed steel sheet described in any one
of (1) to (8), having a metal structure containing a residual
austenite phase at a volume ratio of about 3 to about 20%. [0023]
(10) A method for producing a galvannealed steel sheet, comprising
the steps of: [0024] annealing, in the temperature range of about
730.degree. C. to about 900.degree. C., a cold-rolled steel sheet
containing, by % by mass, about 0.05 to about 0.25% of C, about 2%
or less of Si, about 1 to about 3% of Mn, about 0.1% or less of P,
about 0.01% or less of S, about 0.3 to about 2% of Al, less than
about 0.005% of N, about 1% or less of Cr, about 1% or less of V,
about 1% or less of Mo, less than about 0.005% of Ti, and less than
about 0.005% of Nb, and satisfying the relations, Si+Al.gtoreq.0.6%
and Cr+V+Mo=0.1 to 2%, the balance being Fe and inevitable
impurities; [0025] cooling the annealed cold-rolled steel sheet at
a cooling rate of about 3 to about 100.degree. C./second; [0026]
retaining the cooled cold-rolled steel sheet in the temperature
range of about 350.degree. C. to about 600.degree. C. for about 30
to about 250 seconds; [0027] hot-dip galvanizing the cold-rolled
steel sheet after the retention; and [0028] alloying the hot-dip
galvanized cold-rolled steel sheet at a temperature of about
470.degree. C. to about 600.degree. C. [0029] (11) A method for
producing a galvannealed steel sheet, comprising the steps of:
[0030] annealing, in the temperature range of about 730.degree. C.
to about 900.degree. C., a cold-rolled steel sheet containing, by %
by mass, about 0.05 to about 0.25% of C, about 2% or less of Si,
about 1 to about 3% of Mn, about 0.1% or less of P, about 0.01% or
less of S, about 0.3 to about 2% of Al, less than about 0.005% of
N, about 1% or less of Cr, about 1% or less of V, about 1% or less
of Mo, less than about 0.005% of Ti, and less than about 0.005% of
Nb, and satisfying the relations, Si+Al.gtoreq.0.6%,
N.ltoreq.0.007%-(0.003.times.Al)%, and Cr+V+Mo=0.1 to 2%, the
balance including Fe and inevitable impurities; [0031] cooling the
annealed cold-rolled steel sheet at a cooling rate of about 3 to
about 100.degree. C./second; [0032] retaining the cooled
cold-rolled steel sheet in the temperature range of about
350.degree. C. to about 600.degree. C. for about 30 to about 250
seconds; [0033] hot-dip galvanizing the cold-rolled steel sheet
after the retention; and [0034] alloying the hot-dip galvanized
cold-rolled steel sheet at a temperature of about 470.degree. C. to
about 600.degree. C. [0035] (12) The method for producing the
galvannealed steel sheet described in (10), wherein the cold-rolled
steel sheet further contains, by % by mass, at least one of about
0.005% or less of B and about 1% or less of Ni. [0036] (13) The
method for producing the galvannealed steel sheet described in
(11), wherein the cold-rolled steel sheet further contains, by % by
mass, at least one of about 0.005% or less of B and about 1% or
less of Ni. [0037] (14) The method for producing the galvannealed
steel sheet described in (10), wherein the cold-rolled steel sheet
further contains, by % by mass, at least one of Ca and REM in a
total of about 0.01% or less. [0038] (15) The method for producing
the galvannealed steel sheet described in (11), wherein the
cold-rolled steel sheet further contains, by % by mass, at least
one of Ca and REM in a total of about 0.01% or less. [0039] (16)
The method for producing the galvannealed steel sheet described in
(12), wherein the cold-rolled steel sheet further contains, by % by
mass, at least one of Ca and REM in a total of about 0.01% or less.
[0040] (17) The method for producing the galvannealed steel sheet
described in (13), wherein the cold-rolled steel sheet further
contains, by % by mass, at least one of Ca and REM in a total of
about 0.01% or less. [0041] (18) The method for producing the
galvannealed steel sheet described in any one of (10) to (17),
wherein the galvannealed steel sheet contains a residual austenite
phase at a volume ratio of about 3 to about 20%.
[0042] We can obtain sufficient alloying hot-dip galvanization
properties without passing through a complicated process, and
excellent ductility and a low yield ratio of about 55% or less can
be achieved after alloying hot-dip galvanization.
DETAILED DESCRIPTION
[0043] First, the reasons for specifying the composition of the
galvannealed steel sheet will be described. Hereinafter, "%"
represents "% by mass." C: about 0.05 to about 0.25%
[0044] C is an element for stabilizing austenite and a necessary
element for securing residual austenite. When the C amount is less
than about 0.05%, it is difficult to simultaneously secure the
strength of the steel sheet and the amount of residual austenite to
achieve high ductility. On the other hand, when the C amount
exceeds about 0.25%, a welded portion and a heat-affected portion
are significantly hardened, thereby impairing weldability.
Therefore, the C amount is in the range of about 0.05 to about
0.25%.
Si: about 2.0% or less
[0045] Si is an element effective in strengthening steel. Si is
also a ferrite forming element which promotes the concentration of
C in austenite and suppresses the formation of a carbide and thus
has the function of promoting the formation of residual austenite.
The Si amount is preferably about 0.01% or more. However, when the
Si amount exceeds about 2.0%, plating properties are degraded.
Therefore, the Si amount is about 2.0% or less and preferably about
0.5% or less.
Mn: about 1 to about 3%
[0046] Mn is an element effective in strengthening steel. Mn is
also an element for stabilizing austenite and an element necessary
for increasing residual austenite. However, when the Mn amount is
less than about 1%, these effects cannot be easily obtained. On the
other hand, when the Mn amount exceeds about 3%, a second phase
fraction is excessively increased, and the amount of solid-solution
strengthening is increased, thereby significantly increasing
strength and decreasing ductility. Therefore, the Mn amount is in
the range of about 1 to about 3%.
P: about 0.1% or less
[0047] P is an element effective in strengthening steel. However,
when the P amount exceeds about 0.1%, embrittlement is caused by
grain boundary segregation to impair impact properties. Therefore,
the P amount is about 0.1% or less.
S: about 0.01% or less
[0048] S forms an inclusion such as MnS and causes deterioration in
impact resistance and cracking along a metal flow of a welded
portion. Therefore, the S amount is preferably as small as
possible. However, from the viewpoint of production cost, the S
amount is about 0.01% or less.
Al: about 0.3 to about 2%
[0049] Al effectively functions to purify ferrite and decrease the
yield ratio of steel. However, when the Al amount is less than
about 0.3%, the effect is insufficient. On the other hand, when the
Al amount exceeds about 2%, the amount of the inclusion in a steel
sheet is increased to degrade ductility. Therefore, the Al amount
is in the range of about 0.3% to about 2%.
Si+Al.gtoreq.0.6%
[0050] Like Si, Al is a ferrite forming element which promotes the
concentration of C in austenite and suppresses the formation of a
carbide and thus has the function of promoting the formation of
residual austenite. When the total of Al and Si is less than about
0.6%, the effect is insufficient, and sufficient ductility cannot
be obtained. Therefore, the total of Si+Al is about 0.6% or more
and preferably about 3% or less.
N: less than about 0.005%
[0051] N is an inevitable impurity and forms a nitride. When the N
amount is about 0.005% or more, ductility at high and low
temperatures is decreased by the formation of a nitride. Therefore,
the N amount is less than about 0.005%.
N.ltoreq.0.007%-(0.003.times.Al)%
[0052] When the amount of AlN precipitate is increased with an
increase in the N amount, cracking in a slab easily occurs in
continuous casting. When it is necessary to avoid such cracking in
a slab in continuous casting, in order to avoid this, the N amount
is less than about 0.005%, and the relational expression,
N.ltoreq.0.007%-(0.003.times.Al)%, is satisfied.
Cr, V, Mo: each about 1% or less Cr+V+Mo: about 0.1 to about 2%
[0053] Cr, V, and Mo are elements effective in decreasing the yield
ratio of steel. The effect becomes significant when these elements
are added in combination with Al. Even when each of these elements
is added in an amount of over 1%, the effect is saturated. In
addition, the effect is insufficient when the total of Cr, V, and
Mo is less than about 0.1%. Conversely, when the total exceeds
about 2%, strength may be excessively increased to decrease
ductility and degrade the plating properties. Therefore, the amount
of each of Cr, V, and Mo is about 1% or less, and the total is
about 0.1 to about 2% and preferably about 0.15 to about 1.3%.
Ti, Nb: each less than about 0.005%
[0054] Ti and Nb precipitate as carbonitrides to strengthen steel.
However, such precipitation strengthening increases yield stress
and is thus disadvantageous for decreasing the yield ratio. When
the amount of each of the elements added is about 0.005% or more,
the yield stress is increased. Therefore, the amount of each of Ti
and Nb is less than about 0.005%.
B: about 0.005% or less
[0055] B is effective in strengthening steel and can thus be added
according to demand. When the B amount exceeds about 0.005%,
strength is excessively increased to decrease workability.
Therefore, when B is added, the amount is about 0.005% or less.
Ni: about 1% or less
[0056] Ni is an austenite-stabilizing element which causes
austenite to remain and is effective in increasing strength, and
thus can be added according to demand. However, even when the
amount of Ni exceeds about 1%, the effect is saturated, and
conversely the cost is increased. Therefore, when Ni is added, the
amount is about 1% or less.
Ca and REM: at Least One in Total of about 0.01% or less
[0057] Ca and REM have the function to control the form of a
sulfide inclusion and thus have the effect of improving elongation
and flange properties of a steel sheet, and thus can be added
according to demand. When the total of these elements exceeds about
0.01%, the effect is saturated. Therefore, when Ca and REM are
added, the total of at least one of the elements is about 0.01% or
less.
[0058] Besides the above-described elements and Fe in the balance,
various impurities in the production process and trace amounts of
essential elements added in the production process are inevitably
mixed. However, these inevitable impurities are permissible because
they have no particular influence on the advantage of our steel
sheets.
[0059] Next, the metal structure of the steel sheet will be
described.
Residual Austenite Phase: Volume Ratio of about 3 to about 20%
[0060] A residual austenite phase is essential for effectively
utilizing strain-induced transformation and obtaining high
ductility. Therefore, it is very important to control the volume
ratio of the residual austenite. From the viewpoint of securing
high ductility, the ratio of the residual austenite phase is
preferably at least about 3% or more. On the other hand, when the
ratio of the residual austenite phase exceeds about 20%, a large
amount of martensite is formed after molding to increase
brittleness. Since it may be necessary to suppress brittleness in a
permissible range, therefore, the ratio of the residual austenite
phase is preferably about 20% or less. The metal structure of the
steel sheet includes a ferrite main phase and a second phase
including a residual austenite phase. However, the volume ratio of
the ferrite phase is preferably about 40 to about 90% from the
viewpoint of securing high ductility. Examples of a metal structure
other than the residual austenite phase in the second phase include
a bainite phase, a martensite phase and a pearlite phase. The total
volume ratio of these phases is preferably about 7 to about
50%.
[0061] Next, the conditions for producing the galvannealed steel
sheet will be described.
[0062] Steel having the above-described composition is melted and
continuously cast to form a cast slab, and then the slab is
hot-rolled and cold-rolled. However, the conditions for these
processes are not particularly limited. Then, in a continuous
hot-dip plating line, the steel sheet is annealed in a temperature
range of about 730.degree. C. to about 900.degree. C., cooled at
about 3 to about 100.degree. C./s, retained in a temperature range
of about 350.degree. C. to about 600.degree. C. for about 30 to
about 250 seconds, hot-dip galvanized, and then alloyed at about
470.degree. C. to about 600.degree. C. Annealing temperature: about
730 to about 900.degree. C.
[0063] Annealing is performed in an austenite single-phase zone or
a two-phase zone including an austenite phase and a ferrite phase.
When the annealing temperature is lower than about 730.degree. C.,
in some cases, a carbide is not sufficiently dissolved in the steel
sheet, or recrystallization of ferrite is not completed, thereby
failing to obtain intended properties. On the other hand, when the
annealing temperature exceeds about 900.degree. C., austenite
grains are significantly grown, and the number of ferrite
nucleation sites formed from the second phase by subsequent cooling
may be decreased. Therefore, the annealing temperature is about
730.degree. C. to about 900.degree. C.
Cooling Rate: about 3 to about 100.degree. C./s
[0064] When the cooling rate is less than about 3.degree. C./s, a
large amount of pearlite precipitates, the amount of C dissolved in
untransformed austenite is significantly decreased, and thus the
intended structure cannot be obtained. When the cooling rate
exceeds about 100.degree. C./s, growth of ferrite is suppressed to
significantly decrease the volume ratio of ferrite, and thus
sufficient ductility cannot be secured. Therefore, the cooling rate
is preferably about 3 to about 100.degree. C./s.
Retention Temperature: about 350.degree. C. to about 600.degree.
C.
[0065] When the retention temperature exceeds about 600.degree. C.,
a carbide precipitates from untransformed austenite. When the
retention temperature is lower than about 350.degree. C., a
car-bide precipitates in bainitic ferrite due to lower bainite
transformation, thereby failing to sufficiently obtain stable
residual austenite. Therefore, the retention temperature is about
350.degree. C. to about 600.degree. C. In order to stably produce
residual austenite, the retention temperature is preferably about
500.degree. C. or less.
Retention Time: about 30 to about 250 seconds
[0066] The retention time pays a very important role for
controlling residual austenite. Namely, when the retention time is
less than about 30 seconds, stabilization of untransformed
austenite does not proceed, and thus the amount of residual
austenite cannot be secured, thereby failing to obtain desired
properties. On the other hand, when the retention time exceeds
about 250 seconds, an austenite phase containing a small amount of
dissolved C cannot be obtained, and it becomes difficult to
transform to a martensite phase with a small amount of strain and
achieve low yield stress by a strain field formed around the
martensite phase. Therefore, the retention time is about 30 to
about 250 seconds. From the viewpoint of stabilization of
untransformed austenite, the retention time preferably exceeds
about 60 seconds and more preferably exceeds about 90 seconds. In
order to decrease yield stress, the retention time is preferably
about 200 seconds or less.
Alloying Temperature: about 470.degree. C. to about 600.degree.
C.
[0067] The alloying temperature after the retention and hot-dip
galvanization must be higher than the plating bath temperature, and
the lower limit is about 470.degree. C. When the alloying
temperature exceeds about 600.degree. C., like in the case where
the retention temperature exceeds about 600.degree. C., a carbide
precipitates from untransformed austenite, and thus stable residual
austenite cannot be obtained. Therefore, the alloying temperature
is about 470.degree. C. to about 600.degree. C.
[0068] In the production conditions, the specified annealing
temperature, retention temperature, and alloying temperature need
not be constant as long as they are in the above respective ranges.
In addition, the cooling rate may be changed during cooling as long
as it is in the above range. Further, the plating conditions may be
in a usual operation range, i.e., METSUKE may be about 20 to about
70 g/m.sup.2, and the amount of Fe in a plating layer may be about
6 to about 15%.
EXAMPLE
[0069] An example of our steel sheets will be described.
[0070] Steel having each of the compositions shown in Table 1 was
molten by a converter and continuously cast to form a cast slab.
The occurrence of cracking in the slab is shown in Table 1. The
occurrence of cracking was determined by visual observation as well
as color check after the slab was cooled to room temperature.
[0071] The resulting slab was heated to 1250.degree. C. and then
hot-rolled at a finish rolling temperature of 900.degree. C. to
prepare a hot-rolled steel sheet having a thickness of 3.0 mm.
After hot-rolling, the hot-rolled steel sheet was pickled and
further cold-rolled to prepare a cold-rolled steel sheet having a
thickness of 1.2 mm. Then, in a continuous hot-dip galvanization
line, each cold-rolled steel sheet was heat-treated under the
conditions shown in Table 2, plated at 50/50 g/m.sup.2, and then
alloyed so that the Fe amount in the plating layer was 9%.
[0072] Further, each of the resulting steel sheets was
temper-rolled by 0.5% to examine mechanical properties. As the
mechanical properties, yield stress YS, tensile strength TS, and
elongation EL were measured using a JIS No. 5 tensile specimen
obtained from each steel sheet in a direction perpendicular to the
rolling direction. A tensile test was conducted at a strain rate of
6.7.times.10.sup.-3 s.sup.-1. The measured values, yield ratios YR,
and values of TS.times.EL are also shown in Table 2.
[0073] Table 2 indicates that steel sheet Nos. 1, 2, 5 to 8, 11 to
16, 18, 21, 22, 24, and 28 satisfy our composition and production
conditions and have yield ratios as low as about 55% or less and
satisfactory values of tensile strength TS and elongation EL. On
the other hand, comparative steel sheet Nos. 3, 4, 9, 10, 17, 19,
20, 23, 25 to 27, and 29 to 38 not satisfying our composition and
production conditions are out of the preferred range of at least
one of yield ratio YR, tensile strength TS, elongation EL, and
balance therebetween. Table 1 indicates that among our steel
sheets, steel sheet Nos. A to L satisfying
N.ltoreq.0.007%-(0.003.times.Al)% caused no cracking in the
slabs.
* * * * *