U.S. patent application number 12/667707 was filed with the patent office on 2010-12-30 for galvanized steel sheet excellent in uniformity and method for producing the same.
This patent application is currently assigned to JFE STEEL CORPORATION. Invention is credited to Takeshi Fujita, Hideyuki Kimura, Kaneharu Okuda, Yoshihiko Ono, Michitaka Sakurai.
Application Number | 20100330392 12/667707 |
Document ID | / |
Family ID | 40228714 |
Filed Date | 2010-12-30 |
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United States Patent
Application |
20100330392 |
Kind Code |
A1 |
Ono; Yoshihiko ; et
al. |
December 30, 2010 |
GALVANIZED STEEL SHEET EXCELLENT IN UNIFORMITY AND METHOD FOR
PRODUCING THE SAME
Abstract
A high-strength galvanized steel sheet has a steel composition
which contains, by % by mass, 0.01 to 0.12% of C, 0.2% or less of
Si, less than 2% of Mn, 0.04% or less of P, 0.02% or less of S,
0.3% or less of sol. Al, 0.01% or less of N, and over 0.3% to 2% of
Cr, and which satisfies 2.1.ltoreq.[Mneq].ltoreq.3 and
0.24.ltoreq.[% Cr]/[% Mn], the balance being composed of iron and
inevitable impurities, and has a steel microstructures containing
ferrite and a second phase.
Inventors: |
Ono; Yoshihiko; (Tokyo,
JP) ; Kimura; Hideyuki; (Tokyo, JP) ; Okuda;
Kaneharu; (Tokyo, JP) ; Fujita; Takeshi;
(Tokyo, JP) ; Sakurai; Michitaka; (Tokyo,
JP) |
Correspondence
Address: |
IP GROUP OF DLA PIPER LLP (US)
ONE LIBERTY PLACE, 1650 MARKET ST, SUITE 4900
PHILADELPHIA
PA
19103
US
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
40228714 |
Appl. No.: |
12/667707 |
Filed: |
July 10, 2008 |
PCT Filed: |
July 10, 2008 |
PCT NO: |
PCT/JP2008/062876 |
371 Date: |
January 26, 2010 |
Current U.S.
Class: |
428/659 ;
148/533 |
Current CPC
Class: |
C22C 38/001 20130101;
C22C 38/48 20130101; C23C 2/02 20130101; C23C 2/28 20130101; C22C
38/22 20130101; C22C 38/04 20130101; C21D 8/0247 20130101; C22C
38/18 20130101; C22C 38/02 20130101; C22C 38/28 20130101; C22C
38/42 20130101; Y10T 428/12799 20150115; C22C 38/24 20130101; C21D
9/46 20130101; C23C 2/06 20130101; C22C 38/32 20130101; B32B 15/013
20130101; C22C 38/06 20130101; C22C 38/38 20130101; C22C 38/002
20130101; C21D 2211/005 20130101 |
Class at
Publication: |
428/659 ;
148/533 |
International
Class: |
C22C 38/38 20060101
C22C038/38; C22C 38/58 20060101 C22C038/58; C23C 2/02 20060101
C23C002/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 11, 2007 |
JP |
2007-181945 |
Claims
1. A high-strength galvanized steel sheet comprising steel having a
composition which contains, by % by mass, 0.01 to 0.12% of C, 0.2%
or less of Si, less than 2% of Mn, 0.04% or less of P, 0.02% or
less of S, 0.3% or less of sol. Al, 0.01% or less of N, and over
0.3% to 2% of Cr, and which satisfies 2.1.ltoreq.[Mneq].ltoreq.3
and 0.24.ltoreq.[% Cr]/[% Mn], the balance being composed of iron
and inevitable impurities, the steel having a microstructure
containing ferrite and a second phase, wherein the area ratio of
the second phase is 2 to 25%, the area ratio of pearlite or bainite
in the second phase is 0% to less than 20%, the average grain
diameter of the second phase is 0.9 to 7 .mu.m, and the area ratio
of grains with a grain diameter of less than 0.8 .mu.m in the
second phase is less than 15%, wherein [Mneq] represents the Mn
equivalent shown by [Mneq]=[% Mn]+1.3[% Cr] and [% Mn] and [% Cr]
represent the contents of Mn and Cr, respectively.
2. The high-strength galvanized steel sheet according to claim 1,
wherein 2.2<[Mneq]<2.9 is satisfied.
3. The high-strength galvanized steel sheet according to claim 1,
wherein 0.34.ltoreq.[% Cr]/[% Mn] is satisfied.
4. The high-strength galvanized steel sheet according to claim 1,
further comprising, by % by mass, 0.005% or less of B.
5. The high-strength galvanized steel sheet according to claim 1,
further comprising, by % by mass, at least one of 0.15% or less of
Mo and 0.2% or less of V.
6. The high-strength galvanized steel sheet according to claim 1,
further comprising, by % by mass, at least one of less than 0.014%
of Ti, less than 0.01% of Nb, 0.3% or less of Ni, and 0.3% or less
of Cu.
7. A method for producing a high-strength galvanized steel sheet
comprising: hot-rolling and cold-rolling a steel slab having the
composition according to claim 1; heating at an average heating
rate of less than 3.degree. C./sec in a temperature range of
680.degree. C. to 740.degree. C. in a continuous galvanizing line
(CGL); annealing at an annealing temperature of over 740.degree. C.
to less than 820.degree. C.; cooling from the annealing temperature
at an average cooling rate of 3 to 20.degree. C./sec; dipping in a
galvanization bath or dipping in the galvanization bath and further
alloying the coating; and cooling at an average cooling rate of 7
to 100.degree. C./sec.
8. The method according to claim 7, wherein heating is performed at
an average heating rate of less than 2.degree. C./sec in a
temperature range of 680.degree. C. to 740.degree. C. in the
CGL.
9. The method according to claim 7, wherein hot rolling is
performed by starting cooling within 3 seconds after hot rolling,
cooling to 600.degree. C. or less at an average cooling rate of
40.degree. C./sec or more, and coiling at a coiling temperature of
400.degree. C. to 600.degree. C., and then cold rolled with a
rolling reduction of 70 to 85%
10. The high-strength galvanized steel sheet according to claim 2,
wherein 0.34.ltoreq.[% Cr]/[% Mn] is satisfied.
11. The high-strength galvanized steel sheet according to claim 2,
further comprising, by % by mass, 0.005% or less of B.
12. The high-strength galvanized steel sheet according to claim 3,
further comprising, by % by mass, 0.005% or less of B.
13. The high-strength galvanized steel sheet according to claim 10,
further comprising, by % by mass, 0.005% or less of B.
14. The high-strength galvanized steel sheet according to claim 2,
further comprising, by % by mass, at least one of 0.15% or less of
Mo and 0.2% or less of V.
15. The high-strength galvanized steel sheet according to claim 3,
further comprising, by % by mass, at least one of 0.15% or less of
Mo and 0.2% or less of V.
16. The high-strength galvanized steel sheet according to claim 4,
further comprising, by % by mass, at least one of 0.15% or less of
Mo and 0.2% or less of V.
17. The high-strength galvanized steel sheet according to claim 2,
further comprising, by % by mass, at least one of less than 0.014%
of Ti, less than 0.01% of Nb, 0.3% or less of Ni, and 0.3% or less
of Cu.
18. The high-strength galvanized steel sheet according to claim 3,
further comprising, by % by mass, at least one of less than 0.014%
of Ti, less than 0.01% of Nb, 0.3% or less of Ni, and 0.3% or less
of Cu.
19. The high-strength galvanized steel sheet according to claim 4,
further comprising, by % by mass, at least one of less than 0.014%
of Ti, less than 0.01% of Nb, 0.3% or less of Ni, and 0.3% or less
of Cu.
20. The method according to claim 8, wherein hot rolling is
performed by starting cooling within 3 seconds after hot rolling,
cooling to 600.degree. C. or less at an average cooling rate of
40.degree. C./sec or more, and coiling at a coiling temperature of
400.degree. C. to 600.degree. C., and then cold rolled with a
rolling reduction of 70 to 85%
Description
RELATED APPLICATIONS
[0001] This is a .sctn.371 of International Application No.
PCT/JP2008/062876, with an international filing date of Jul. 10,
2008 (WO 2009/008551 A1, published Jan. 15, 2009), which is based
on Japanese Patent Application Nos. 2007-181945, filed Jul. 11,
2007, and 2008-177467, filed Jul. 8, 2008, the subject matter of
which is incorporated by reference.
TECHNICAL FIELD
[0002] This disclosure relates to a high-strength galvanized steel
sheet for press forming which is used for automobiles, home
electric appliances, and the like through a press forming process,
and to a method for manufacturing the steel sheet.
BACKGROUND
[0003] BH steel sheets with 340 MPa grade in tensile strength
(bake-hardenable steel sheets, simply referred to as "340BH"
hereinafter) and IF steel sheets with 270 MPa grade in tensile
strength (Interstitial Free steel sheets, simply referred to
as"270IF" hereinafter), which is ultra-low-carbon steel containing
carbide/nitride-forming elements such as Nb and Ti to control the
amount of dissolved C, have been applied to automotive outer
panels, such as hoods, doors, trunk lids, back doors, and fenders,
which require sufficient dent resistance. In recent years,
regarding the increasing requirement of further weight reduction of
car bodies, new attempts of applying steel sheets with higher
strength and superior dent resistance have been carried out to
reduce thickness of the steel sheet for outer panels. Also
investigations to improve dent resistance and to decrease the
temperature and time of a baking finish process while maintaining
the current thickness have progressed in view of applying higher
strength steel sheet.
[0004] However, when a solution-hardening element such as Mn, P, or
the like is further added to 340BH with a yield strength (YP) of
230 MPa or 270IF with a YP of 180 MPa to strengthen and thin a
steel sheet, surface distortion occurs. The term "surface
distortion" represents micro wrinkles or wavy patterns produced in
a press-formed surface due to an increase in YP. The occurrence of
surface distortion impairs the design or design property of a door,
a trunk lid, or the like. Therefore, the steel sheet for such
application is desired wherein YP after press forming and baking
finish treatment is increased over the YP of conventional steel
sheets while maintaining extremely low YP before press forming.
[0005] In such a background, for example, Japanese Examined Patent
Application Publication No. 62-40405 discloses a method for
producing a steel sheet having low YP, high work-hardenability WH,
and high BH by appropriately controlling the cooling rate after
annealing of steel to form a dual phase mainly composed of ferrite
and martensite, the steel containing 0.005 to 0.15% of C, 0.3 to
2.0% of Mn, and 0.023 to 0.8% of Cr. In addition, Japanese
Unexamined Patent Application Publication No. 2004-307992 discloses
a method for satisfying both surface distortion resistance and
anti-cracking property of steel containing 0.005 to 0.05% of C and
3% or less of Mn by adjusting the average grain diameter of
martensite to 1.5 .mu.m or less, the ratio of martensite in a
second phase to 60% or more, and the ratio of the number of
martensite grains to 0.7 to 2.4 relative to the number of ferrite
grains. Japanese Unexamined Patent Application Publication No.
2001-207237 discloses that a steel sheet with high ductility and
low yield ratio YR is produced by appropriately controlling a
cooling rate after annealing of steel containing 0.010 to 0.06% of
C, 0.5 to 2.0% of Mn, and 1% or less of Cr and increasing the ratio
of martensite in a second phase to 80% or more. Further, Japanese
Unexamined Patent Application Publication No. 2001-303184 discloses
that a low-YP dual phase steel sheet composed of ferrite and
martensite is produced by decreasing the C content to 0.02 to
0.033% in steel containing 1.5 to 2.5% of Mn and 0.03 to 0.5% of
Cr.
[0006] However, in the dual phase steel sheets described in
Japanese Examined Patent Application Publication No. 62-40405 and
Japanese Unexamined Patent Application Publication Nos.
2004-307992, 2001-207237 and 2001-303184, hard martensite is
dispersed as a strengthening phase, and thus fluctuations of
mechanical properties easily occur. For example, the volume
fraction of a hard second phase significantly varies with several
tens ppm of C content in steel and the transition of annealing
temperature of 30 to 50.degree. C., and thus mechanical properties
significantly vary as compared with conventional 340BH and 270IF
which are solid-solution-hardened with Mn and P.
[0007] A steel sheet of 490-590 MPa in TS grade is required to
decrease temperature and time of the baking finish process while
reducing the thickness of the sheet. But, such a high-strength
steel sheet has the problem of extremely large variation in
mechanical properties.
[0008] The steel sheets described in Japanese Examined Patent
Application Publication No. 62-40405 and Japanese Unexamined Patent
Application Publication Nos. 2004-307992, 2001-207237 and
2001-303184 show 440 MPa in TS and 210 to 260 MPa in YP, and thus
YP is suppressed to a low level as compared with the conventional
YP level of 320 MPa in solid-solution hardening 440 MPa grade IF
steel. The surface distortion in these steel sheets is improved
compared with conventional 440 MPa grade IF steel with YP of 320
MPa. However, in actual press-forming into a door or the like
produces, a larger surface distortion still arises as compared to
340BH. Therefore, it is also desired to decrease the absolute value
of YP.
[0009] It could therefore be helpful to provide a high-strength
galvanized steel sheet with low YP and excellent uniformity and a
method for producing the same.
SUMMARY
[0010] We conducted investigations on a method for decreasing YP to
a general value or lower and decreasing variation .DELTA.YP in YP
with changes in a production factor with respect to a conventional
high-strength galvanized steel sheet having a dual phase. As a
result, we found: [0011] (i) By appropriately controlling the Mn
equivalent and the composition ranges of Mn and Cr and further
appropriately controlling the heating rate and cooling rate of
annealing in a continuous galvanizing line (CGL), an attempt can be
made to coarsen a second phase and homogenize a dispersion form
even in a thermal history of CGL in which slow cooling is performed
after annealing, thereby decreasing both YP and variation of
mechanical properties. [0012] (ii) By applying rapid cooling after
hot rolling and appropriately controlling a cold rolling reduction,
YP in a direction at 45.degree. with the rolling direction can be
decreased to a level equivalent to YP in the rolling direction and
in a direction perpendicular to the rolling direction, thereby
effectively decreasing surface distortion in the surrounding of a
car door knob where surface distortion easily occurs.
[0013] We thus provide a high-strength galvanized steel sheet
including steel having a composition which contains, by % by mass,
0.01 to 0.12% of C, 0.2% or less of Si, less than 2% of Mn, 0.04%
or less of P, 0.02% or less of S, 0.3% or less of sol. Al, 0.01% or
less of N, and over 0.3% to 2% of Cr, and which satisfies
2.1.ltoreq.[Mneq].ltoreq.3 and 0.24.ltoreq.[% Cr]/[% Mn], the
balance being composed of iron and inevitable impurities, and
having a microstructure containing ferrite and a second phase,
wherein the area ratio of the second phase is 2 to 25%, the area
ratio of pearlite or bainite in the second phase is 0% to less than
20%, the average grain diameter of the second phase is 0.9 to 7
.mu.m, and the area ratio of grains with a grain diameter of less
than 0.8 .mu.m in the second phase is less than 15%. Herein, [Mneq]
represents the Mn equivalent shown by [Mneq]=[% Mn]+1.3 [% Cr] and
[% Mn] and [% Cr] represent the contents of Mn and Cr,
respectively.
[0014] The high-strength galvanized steel sheet preferably
satisfies 2.2<[Mneq]<2.9 and 0.34.ltoreq.[% Cr]/[% Mn].
[0015] Further, 0.005% by mass or less of B is preferably
contained. In addition, at least one of 0.15% by mass or less of Mo
and 0.2% by mass or less of V is preferably contained. Further, at
least one of less than 0.014% by mass of Ti, less than 0.01% by
mass of Nb, 0.3% by mass or less of Ni, and 0.3% by mass or less of
Cu is preferably contained.
[0016] The high-strength galvanized steel sheet can be produced by
a method for producing a high-strength galvanized steel sheet, the
method including hot-rolling and cold-rolling a steel slab having
the above-described composition, heating at an average heating rate
of less than 3.degree. C./sec in a temperature range of 680.degree.
C. to 740.degree. C. in CGL, annealing at an annealing temperature
of over 740.degree. C. to less than 820.degree. C., cooling from
the annealing temperature at an average cooling rate of 3 to
20.degree. C./sec, dipping in a galvanization bath or dipping in
the galvanization bath and further alloying the coating, and then
cooling at an average cooling rate of 7 to 100.degree. C./sec.
[0017] In the method for producing the high-strength galvanized
steel sheet, heating is preferably performed at an average heating
rate of less than 2.degree. C./sec in a temperature range of
680.degree. C. to 740.degree. C. in CGL. Further, hot rolling is
preferably performed by starting cooling within 3 seconds after hot
rolling, cooling to 600.degree. C. or less at an average cooling
rate of 40.degree. C./sec or more, and coiling at a coiling
temperature of 400.degree. C. to 600.degree. C., and then cold
rolling is preferably performed with a rolling reduction of 70 to
85%.
[0018] A high-strength galvanized steel sheet with low YP excellent
in uniformity can be produced. The high-strength galvanized steel
sheet is excellent in resistance to surface distortion and is thus
suitable for strengthening and thinning automotive parts.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a graph showing a relation between YP and the area
ratio of grains with a grain diameter of less than 0.8.mu.m in a
second phase.
[0020] FIG. 2 is a graph showing a relation between YP and the
heating rate in annealing.
DETAILED DESCRIPTION
[0021] Our steels and methods will be described in detail below.
"%" indicating the content of a component represents "% by mass"
unless otherwise specified.
1) Composition
C: 0.01 to 0.12%
[0022] C is an element necessary for securing a predetermined area
ratio of a second phase. When the C content is excessively low, the
second phase cannot be secured at a sufficient area ratio, and low
YP cannot be achieved. Further, sufficient BH cannot be secured,
and the anti-aging property is degraded. The C content is required
to be 0.01% or more to secure a sufficient area ratio of the second
phase. On the other hand, when the C content exceeds 0.12%, the
area ratio of the second phase is excessively increased to increase
YP, and .DELTA.YP with the annealing temperature is also increased.
In addition, weldability is also degraded. Therefore, the C content
is 0.12% or less. The C content is preferably less than 0.08% for
achieving lower YP and more preferably less than 0.06% for
achieving further lower YP.
Si: 0.2% or Less
[0023] Si has the effect of delaying scale formation in hot rolling
and improves surface appearance quality when added in a small
amount, the effect of appropriately delaying an alloying reaction
between ferrite and zinc in a galvanization bath or galvannealing
treatment, and the effect of further homogenizing and coarsening
the microstructure of a steel sheet. Therefore, Si can be added
from this viewpoint. However, when Si is added in an amount
exceeding 0.2%, the surface appearance quality is impaired and
cause difficulty in application to outer panels, and YP is
increased. Therefore, the Si content is 0.2% or less.
Mn: Less Than 2%
[0024] Mn enhances hardenability, suppresses the formation of
pearlite and bainite in cooling and in allying treatment after
annealing, and decreases the amount of dissolved C in ferrite.
Therefore, from the viewpoint of decreasing YP, Mn is added.
However, when the Mn content is excessively high, the second phase
is made fine and heterogeneous, and .DELTA.YP with respect to the
annealing temperature is increased. That is, when the Mn content is
excessively increased, the recrystallization temperature is
decreased, and .gamma. grains are finely and nonuniformly produced
in fine ferrite grain boundaries immediately after
recrystallization or boundaries of recovered grains during
recrystallization, thereby increasing the area ratio of the second
phase grains with a grain diameter of less than 0.8 .mu.m, which
will be described below, in the structure after annealing. As a
result, reduction in YP and .DELTA.YP is inhibited. The Mn content
is necessary to be less than 2% to decrease YP and .DELTA.YP with
respect to the annealing temperature. From the viewpoint of more
decreasing .DELTA.YP and YP, the Mn content is preferably less than
1.8%. From the viewpoint of further decreasing .DELTA.YP and YP,
the Mn content is preferably less than 1.6%. Although the lower
limit of the Mn content is not particularly determined, the Mn
content preferably exceeds 0.1% because the Mn content of 0.1% or
less causes red shortness due to MnS precipitation and easily
causes surface defects.
P: 0.04% or Less
[0025] P has the effect of appropriately delaying an alloying
reaction between ferrite and zinc in a galvanization bath or
galvannealing treatment and the effect of further coarsening the
microstructure of a steel sheet. From this viewpoint, P can be
added. However, P has a large solution hardening ability and thus
significantly increases YP when excessively added. Therefore, the P
content is 0.04% or less which has a small adverse effect on an
increase in YP.
S: 0.02% or Less
[0026] S precipitates as MnS in steel but decreases the ductility
of a steel sheet and decreases press formability when added in a
large amount. In addition, hot ductility is decreased in hot
rolling of a slab, and thus surface defects easily occur.
Therefore, the S content is 0.02% or less but is preferably as low
as possible.
sol. Al: 0.3% or Less
[0027] Al is used as a deoxidizing element or an element for
improving the anti-aging property by fixing N as AlN. However, Al
forms fine AlN during coiling or annealing after hot rolling to
suppress the growth of ferrite grains and slightly inhibit
reduction in YP. From the viewpoint of decreasing oxides in steel
or improving anti-aging property, Al is preferably added in an
amount of 0.02% or more. On the other hand, from the viewpoint of
improving the grain growth property, the ferrite grain growth
property is improved by increasing the coiling temperature to
620.degree. C. or more, but the amount of fine AlN is preferably as
small as possible. Therefore, preferably, the sol. Al content is
0.15% or more, and AlN is coarsely precipitated during coiling.
However, since the cost is increased when the sol. Al content
exceeds 0.3%, the sol. Al content is 0.3% or less. In addition,
when the sol. Al content exceeds 0.1%, castabiliy is impaired to
cause deterioration of the surface appearance quality. Therefore,
the sol. Al content is preferably 0.1% or less for application to
exterior panels which are required to be strictly controlled in
surface appearance quality.
N: 0.01% or Less
[0028] N precipitates during coiling or annealing after hot rolling
to form fine AlN and inhibit the grain growth property. Therefore,
the N content is 0.01% or less but is preferably as low as
possible. In addition, an increase in the N content causes
deterioration of the anti-aging property. From the viewpoint of
improving the grain growth and anti-aging property, the N content
is preferably less than 0.008% and more preferably less than
0.005%.
Cr: Over 0.3% to 2%
[0029] Cr is the most important element in our steels. Since Cr has
a small amount of solution hardening and the small effect of making
fine martensite and can impart high hardenability, Cr is an element
effective in decreasing the absolute value of YP and decreasing
.DELTA.YP with respect to the annealing temperature. As described
below, the Cr content is required to be controlled according to the
Mn content so that [Mneq] and [% Cr]/[% Mn] are in the
above-described respective ranges, but the Cr content is necessary
to exceed at least 0.3%. In addition, when [Mneq] and [% Cr]/[% Mn]
are controlled to [Mneq]>2.2 and [% Cr]/[% Mn].gtoreq.0.34,
respectively, which are particularly preferred for decreasing YP
and quality variation .DELTA.YP described below, the Cr content is
required to exceed at least 0.5%. On the other hand, when the Cr
content exceeds 2%, the cost is increased, and the surface
appearance quality of a galvanized steel sheet is degraded.
Therefore, the Cr content is 2% or less.
[Mneq]: 2.1 to 3
[0030] It is necessary to control [Mneq] to 2.1 or more to suppress
the formation of pearlite and bainite in cooling and alloying
treatment after annealing to decrease YP and .DELTA.YP with respect
to the annealing temperature. Further, from the viewpoint of
decreasing the pearlite formation to decrease YP, [Mneq] preferably
exceeds 2.2. To substantially disappear pearlite and bainite in the
thermal history of CGL to decrease YP, [Mneq] more preferably
exceeds 2.3. On the other hand, when [Mneq] is excessively
increased, the surface appearance quality of the coating is
degraded, and the cost is increased by adding large amounts of
alloy elements. Therefore, [Mneq] is 3 or less and preferably less
than 2.9.
[% Cr]/[% Mn]: 2.4 or More
[0031] From the viewpoint of decreasing YP by coarsening the second
phase and homogenizing the dispersion form thereof, it is necessary
to control [% Cr]/[% Mn] to 0.24 or more. Further, .DELTA.YP with
the annealing temperature is decreased by controlling [% Cr]/[% Mn]
to this range. Namely, the formation of pearlite and bainite is
suppressed by controlling [Mneq], but when the Mn content is
increased even with constant [Mneq], the structure is made fine,
failing to decrease variation of mechanical properties. On the
other hand, the microstructure can be homogenized and coarsened by
controlling [% Cr]/[% Mn] to the predetermined range, and thus a
strength change with a change in area ratio of the second phase due
to a change in the annealing temperature can be suppressed to a low
level. From the viewpoint of decreasing YP and further decreasing
.DELTA.YP with the annealing temperature, [% Cr]/[% Mn].gtoreq.0.34
is preferred, and a more preferred range is [% Cr]/[%
Mn].gtoreq.0.44.
[0032] The balance includes iron and inevitable impurities, but the
elements below may be contained at predetermined contents.
B: 0.005% or Less
[0033] Similarly, B can be utilized as an element for enhancing
hardenability. Also B has the function to fix N as BN to improve
the grain growth property. In particular, the effect of improving
the ferrite grain growth property can be sufficiently exhibited by
adding over 0.001% of B, thereby achieving extremely low YP.
Therefore, B is preferably added in an amount of over 0.001%.
However, when B is excessively added, the microstructure is
inversely made fine by the influence of residual dissolved B.
Therefore, the B content is preferably 0.005% or less.
Mo: 0.15% or Less
[0034] Like Mn and Cr, Mo is an element for enhancing hardenability
and can be added for the purpose of improving hardenability or the
purpose of improving the surface appearance quality of a galvanized
steel sheet. However, when Mo is excessively added, like Mn, the
structure is made fine and hard, increasing .DELTA.YP with the
annealing temperature. Therefore, Mo is preferably added in the
range of 0.15% or less which has the small influence on an increase
in YP and .DELTA.YP with the annealing temperature. From the
viewpoint of further decreasing YP and .DELTA.YP, the Mo content is
preferably less than 0.02% (not added).
V: 0.2% or Less
[0035] Similarly, V is an element for enhancing hardenability and
can be added for the purpose of improving the surface appearance
quality of a galvanized steel sheet. However, when V is added in an
amount exceeding 0.2%, the cost is significantly increased.
Therefore, V is preferably added in the range of 0.2% or less.
Ti: Less Than 0.014%
[0036] Ti has the effect of improving the anti-aging property by
fixing N and the effect of improving castability. However, Ti forms
fine precipitates of TiN, TiC, Ti(C, N), and the like in steel to
inhibit the grain growth property. Therefore, from the viewpoint of
decreasing YP, the Ti content is preferably less than 0.014%.
Nb: Less Than 0.01%
[0037] Nb has the effect of delaying recrystallization in hot
rolling to control the texture and decrease YP in a direction at 45
degrees with the rolling direction. However, Nb forms fine NbC and
Nb(C, N) in steel to significantly degrade the grain growth
property. Therefore, Nb is preferably added in the range of less
than 0.01% which has the small influence on an increase in YP.
Cu: 0.3% or Less
[0038] Cu is an element mixed when craps or the like are positively
utilized and a recycled material can be used as a raw material when
Cu is allowed to be mixed, thereby decreasing the production cost.
Cu has a small influence on the material quality, but mixing of
excessive Cu causes surface flaws. Therefore, the Cu content is
preferably 0.3% or less.
Ni: 0.3% or Less
[0039] Ni also has a small influence on the material quality of a
steel sheet, but Ni can be added from the viewpoint of decreasing
surface flaws when Cu is added. However, when Ni is excessively
added, surface defecting due to heterogeneity of scales is
promoted. Therefore, the Ni content is preferably 0.3% or less.
2) Microstructure
[0040] The steel sheet is mainly composed of ferrite, martensite,
pearlite, and bainite and contains small amounts of retained
.gamma. and carbides. The method of measuring the morphology of the
microstructures is first described.
[0041] The area ratio of the second phase was determined by
observing a L section (vertical section parallel to the rolling
direction) of the steel sheet, which was prepared by polishing and
etching with natal, in 12 fields of view with SEM with a
magnification of 4000 times power and then image processing of
structure photographs. In a structure photograph, a region with a
light black contrast was regarded as ferrite, regions including
lamellar or dot-sequential carbides formed therein were regarded as
pearlite and bainite, and grains with a white contrast were
regarded as martensite or retained .gamma.. In addition, dot-like
fine grains with a diameter of 0.4 .mu.m or less which were
observed in a SEM photograph were mainly composed of carbides
according to TEM observation. These fine grains were considered to
little influence the material quality because of the very low area
ratio, and grains with a diameter of 0.4 .mu.m or less were
excluded from evaluation of the area ratio and average grain
diameter. Thus, the area ratio and average grain diameter were
determined for a structure containing white-contrast grains mainly
composed of martensite and lamellar or dot sequential carbides
composed of pearlite and bainite. The area ratio of the second
phase shows a total area ratio of these microstructures. For
spherical grains, the diameters thereof were used for the average
grain diameter, while for elliptical grains on a SEM screen, long
axis a and single axis b perpendicular to the long axis were
measured to determine (a.times.b).sup.0.5 as an equivalent grain
diameter. Grains having a slightly rectangular shape were handled
in the same manner as elliptical grains, and the long axis and the
single axis were measured to determine a grain diameter according
to the above equation. When second phases were adjacent to each
other, the phases in contact with the same width as a grain
boundary were separately counted, while the phases in contact with
a larger width than a grain boundary, i.e., in contact with a
certain width, were counted as one grain.
Ferrite and Second Phase
[0042] The steel sheet has a structure mainly composed of ferrite
and containing as the second phase martensite, pearlite, bainite, a
small amount of retained .gamma., and carbides. The area ratio of
the carbides is as small as less than 1%. When ferrite grains are
excessively coarsened, surface roughness occurs in press forming.
Therefore, the ferrite grain diameter is preferably 4 to 15
.mu.m.
Area Ratio of Second Phase: 2 to 25%
[0043] It is necessary for the area ratio of the second phase to be
2% or more to decrease YPEl of the steel sheet to sufficiently
decrease YP. This can impart functions required for exterior
panels, such as high WH, high BH, and excellent anti-aging
property. However, when the area ratio of the second phase exceeds
25%, sufficiently low YP cannot be achieved, and .DELTA.YP with the
annealing temperature is increased. Therefore, the area ratio of
the second phase is in the range of 2 to 25%.
Average Grain Diameter of Second Phase: 0.9 to 7 .mu.m
[0044] As described above, the steel sheet has a structure composed
of ferrite, martensite, pearlite, bainite, and retained .gamma.,
but mostly composed of ferrite and martensite. When martensite is
finely and nonuniformly dispersed, YP is increased. TEM observation
indicated that many dislocations imparted by quenching are
introduced in the periphery of martensite, and when martensite is
finely and nonuniformly dispersed, regions around martensite where
dislocations are introduced overlap each other. Such dislocations
around martensite have already been entangled, and it is thus
considered that the dislocations hardly contribute to initial
deformation at low stress.
[0045] To decrease YP, preferably, the grains in the second phase
are as large as possible in diameter and dispersed as uniformly as
possible. It is necessary that the average grain diameter of the
second phase is at least 0.9 .mu.m or more to sufficiently decrease
YP and .DELTA.YP with the annealing temperature of the high-[Mneq]
steel sheet. On the other hand, when the grain diameter of the
second phase exceeds 7 .mu.m, it is necessary to significantly
coarsen ferrite grains, and surface roughness may occur in press
forming. Therefore, the grain diameter of the second phase is 7
.mu.m or less.
[0046] In CAL capable of rapid cooling to less than 400.degree. C.
after annealing, the structure can be frozen by rapid cooling from
near 700.degree. C., and the second phase can be relatively
coarsely dispersed. However, in the thermal history of CGL in which
galvanizing is performed, slow cooling at an appropriate cooling
rate is required after annealing, and thus .gamma..fwdarw..alpha.
transformation proceeds during slow cooling in the temperature
range of 700.degree. C. to 500.degree. C. with the result of a fine
second phase. The second phase can be coarsely dispersed even in
the thermal history of CGL by appropriately controlling the Mn
equivalent, the Cr and Mn composition ranges, and the heating rate
in annealing.
Area Ratio of Pearlite or Bainite in Second Phase: 0% to Less Than
20%
[0047] When slow cooling is performed after annealing, particularly
when alloying treatment is also performed, fine pearlite or bainite
is formed mainly adjacent to martensite unless [Mneq] is
appropriately controlled, thereby causing variation in .DELTA.YP
with the annealing temperature. When the area ratio of pearlite or
bainite in the second phase is 0% to less than 20%, sufficiently
low YP can be achieved with the result that decreasing .DELTA.YP.
Further, the area ratio is preferably 0 to 10%. The area ratio of
pearlite or bainite in the second phase represents the area ratio
of pearlite or bainite relative to the area ratio of 100 of the
second phase.
Area Ratio of Grains with Diameter of Less Than 0.8 .mu.m in Second
Phase: Less Than 15%
[0048] As a result of detailed examination on a structure factor
for decreasing YP and .DELTA.YP, it was found to be necessary for
YP and .DELTA.YP to control the average grain diameter of the
second phase and control the presence frequency of the fine second
phase. Namely, correlation is observed between YP and .DELTA.YP and
the average grain diameter of the second phase. However, for steel
sheets having substantially the same second phase grain diameter, a
steel sheet having large variation of mechanical properties and a
steel sheet having small variation of mechanical properties may be
produced. The detail examination of a microstructure of such steel
sheets indicates that a steel sheet having a cluster distribution
of fine second phase grains with a diameter of less than 0.8 .mu.m
exhibits large YP and .DELTA.YP. It was also found that a
microstructure in which cluster-like second phases are finely
dispersed can be decreased by controlling the heating rate of
annealing. This is because when .alpha..fwdarw..gamma.
transformation proceeds before the complete completion of
recrystallization, the second phase is produced preferentially
adjacent to fine recovered grains and fine ferrite grain boundaries
immediately after recrystallization. However, when the heating rate
is decreased, the second phase is produced after recrystallization
is sufficiently completed, and thus the second phase is mainly
produced at a triple point of ferrite grain boundaries and
uniformly and coarsely dispersed.
[0049] Steel containing 0.024% of C, 0.01% of Si, 1.8% of Mn, 0.01%
of P, 0.01% of S, 0.04% of sol. Al, 0.55% of Cr, and 0.003% of N
was molten in a laboratory to produce a slab of 27 mm in thickness.
The slab was heated to 1250.degree. C., hot-rolled to 2.3 mm at a
finish rolling temperature of 830.degree. C., and then coiled for 1
hour at 620.degree. C. The resultant hot-rolled sheet was
cold-rolled to 0.75 mm with a rolling rate of 67%. The resultant
cold-rolled sheet was annealed at 770.degree. C. for 40 seconds at
an average heating rate changed from 0.3 to 20.degree. C./sec in
the range of 680.degree. C. to 740.degree. C., cooled from the
annealing temperature to 470.degree. C. (galvanization bath
temperature) at an average cooling rate of 6.degree. C./sec,
galvanized, galvaneealed of 530.degree. C..times.20 sec by heating
from 470.degree. C. to 530.degree. C. at 15.degree. C./sec, and
then cooled to a temperature region of 100.degree. C. or lower at
an average cooling rage of 30.degree. C./sec. A JIS No. 5 tensile
test piece was obtained from the resultant steel sheet and
subjected to a tensile test (according to JISZ2241, the tensile
direction perpendicular to the rolling direction). In addition, the
microstructure was observed with SEM to determine the area ratio of
the second phase.
[0050] FIG. 1 shows a relation between YP and the area ratio of
grains with a diameter of less than 0.8 .mu.m in the second phase.
When the area ratio of grains with a diameter of less than 0.8
.mu.m in the second phase is less than 15%, YP is decreased to 210
MPa or less, while when the area ratio is less than 12%, YP is
decreased to 205 MPa or less. Further, as a result of measurement
of .DELTA.YP by changing the annealing temperature from 760.degree.
C. to 810.degree. C., it was found that in a sample (heating rate:
20.degree. C./sec) containing of grains with a diameter of less
than 0.8 .mu.m at an area ratio of 26% in the second phase,
.DELTA.YP is 24 MPa, while.in a sample (heating rate: less than
3.degree. C./sec) containing of grains with a diameter of less than
0.8 .mu.m at an area ratio of less than 15% in the second phase,
.DELTA.YP is decreased to 15 MPa.
[0051] Therefore, a steel sheet with low YP and low .DELTA.YP with
the annealing temperature can be produced by decreasing the amount
of grains of less than 0.8 .mu.m produced. Therefore, the area
ratio of grains with a diameter of less than 0.8 .mu.m in the
second phase is less than 15%. From the viewpoint of further
decreasing YP and .DELTA.YP with the annealing temperature, the
area ratio is preferably less than 12%. Like in the above-mentioned
measuring method, the area ratio of grains with a diameter of less
than 0.8 .mu.m in the second phase represents the area ratio of
grains with a diameter of less than 0.8 .mu.m relative to the area
ratio of 100 of the second phase.
3) Production Condition
[0052] As described above, the steel sheet can be produced by the
method including hot-rolling and cold-rolling a steel slab having
the above-described composition, heating at an average heating rate
of less than 3.degree. C./sec in a temperature range of 680.degree.
C. to 740.degree. C. in CGL, annealing at an annealing temperature
of over 740.degree. C. to less than 820.degree. C., cooling from
the annealing temperature at an average cooling rate of 3 to
20.degree. C./sec, dipping in a galvanization bath or dipping in
the galvanization bath and further alloying the coating, and then
cooling at an average cooling rate of 7 to 100.degree. C./sec.
Hot Rolling
[0053] The slab can be hot-rolled by a method of rolling the slab
after heating, a method of directly rolling the slab without
heating after continuous casting, or a method of rolling the slab
by heating for a short time after continuous casting. The hot
rolling may be performed according to a general method, for
example, at a slab heating temperature of 1100.degree. C. to
1300.degree. C., a finish rolling temperature of Ar.sub.3
transformation point or more, an average cooling rate after finish
rolling of 10 to 200.degree. C./sec, and a coiling temperature of
400.degree. C. to 720.degree. C. To obtain excellent zinc coating
appearance quality for an automotive outer panel, preferably, the
slab heating temperature is 1200.degree. C. or less, and the finish
rolling temperature is 840.degree. C. or less. In addition,
descaling is preferably sufficiently performed for removing primary
and secondary scales formed on the surface of the steel sheet. From
the viewpoint of decreasing YP, the coiling temperature is
preferably as high as possible and 640.degree. C. or more. When the
coiling temperature is 680.degree. C. or more, Mn and Cr can be
sufficiently concentrated in the second phase in the state of the
hot-rolled sheet, and stability of y in the subsequent annealing
step is improved, contributing to a decrease in YP.
[0054] On the other hand, when the steel sheet is applied to outer
panels, such as a door panel with a door knob, having a shape in
which material inflow and material shrinkage in a direction at
45.degree. greatly influence the surface distortion of an embossing
periphery, it is considered effective for decreasing the surface
distortion to suppress YP in the direction at 45.degree.. In this
application, preferably, cooling in hot rolling is started within 3
seconds after finish rolling and performed to 600.degree. C. or
less at an average cooling rate of 40.degree. C./sec or more,
followed by coiling at a coiling temperature of 400.degree. C. to
600.degree. C. Under these hot rolling conditions, a fine
low-temperature transformed phase mainly composed of bainite can be
produced at an area ratio of 30% or more, and the development of a
texture in which YP in the direction at 45.degree. is relatively
suppressed can be promoted. When a dual phase steel sheet composed
of C, Mn, and Cr is produced according to a general method, YP
(YP.sub.D) in the direction at 45.degree. generally tends to be 5
to 15 MPa higher than YP (YP.sub.L) in the rolling direction and YP
(YP.sub.C) in a direction perpendicular to the rolling direction.
However, the above-described hot-rolling conditions can suppress to
the range of -10.ltoreq.YP.sub.D-YP.sub.C.ltoreq.5 MPa.
Cold Rolling
[0055] The rolling reduction of cold rolling may be 50% to 85%.
YP.sub.C is decreased by decreasing the rolling rate to 50% to 65%.
However, YP in the direction at 45.degree. is relatively increased
by decreasing the rolling rate, thereby increasing anisotropy.
Therefore, for a steel sheet for application such as a door knob,
the rolling rate is preferably 70% to 85%.
CGL
[0056] The steel sheet after cold rolling is annealed and
galvanized in CGL. To uniformly disperse the coarse second phase
after annealing and decrease .DELTA.YP with the annealing
temperature, it is effective to control the heating rate in the
temperature region of 680.degree. C. to 740.degree. C. during
annealing.
[0057] FIG. 2 shows a relation between YP and the heating rate in
the temperature region of 680.degree. C. to 740.degree. C. during
annealing. The results of FIG. 2 were obtained by arranging data of
an experiment conducted for leading to the results shown in FIG. 1.
FIG. 2 indicates that at the heating rate of less than 3.degree.
C./sec, YP of 210 MPa or less can be obtained, while at the heating
rate of less than 2.degree. C./sec, YP of 205 MPa or less can be
obtained. When the heating rate is less than 3.degree. C./sec, the
formation of .gamma. grains in ferrite grain boundaries which
remain unrecrystallized can be suppressed, and the formation of the
fine second phase can be suppressed, thereby decreasing YP. When
the heating rate is less than 2.degree. C./sec, nucleation of
.gamma. from unrecrystallized ferrite can be suppressed, and
recrystallized ferrite grains can be sufficiently grown. Therefore,
the structure is further homogenized and coarsened, thereby further
decreasing YP and .DELTA.YP.
[0058] The annealing temperature is over 740.degree. C. to less
than 820.degree. C. At the annealing temperature of 740.degree. C.
or lower, the area ratio of the second phase cannot be secured
because of insufficient solid solution of carbides. At the
annealing temperature of 820.degree. C. or more, the .gamma. ratio
is excessively increased in annealing, and elements such as Mn, C,
and the like are not sufficiently concentrated in .gamma. grains,
thereby failing to achieve sufficiently low YP. This is possibly
because when elements are not sufficiently concentrated in .gamma.
grains, strain is not sufficiently applied to the periphery of
martensite, and pearlite and bainite transformation easily occurs
in the cooling step. The holding time during annealing is
preferably 20 seconds or more in the temperature range of over
740.degree. C. which corresponds to usual continuous annealing, and
is more preferably 40 seconds or more. After soaking, cooling is
performed at an average cooling rate of 3 to 20.degree. C./sec from
the annealing temperature to the temperature of the galvanization
bath generally kept at 450.degree. C. to 500.degree. C. When the
cooling rate is lower 3.degree. C./sec, large amounts of pearlite
and bainite are formed in the second phase because of the passage
through the pearlite nose in the temperature region of 550.degree.
C. to 650.degree. C., thereby failing to achieve sufficiently low
YR On the other hand, when the cooling rate is larger than
20.degree. C./sec, elements such as Mn, Cr, C, and the like are not
sufficiently concentrated in .gamma. grains during
.gamma..fwdarw..alpha. transformation in the temperature region of
the annealing temperature to 650.degree. C., and pearlite is easily
produced in alloying treatment. In addition, decrease of the
dissolved C in ferrite resulted from .gamma..fwdarw..alpha.
transformation and carbide precipitation in the temperature region
of 480.degree. C. to 550.degree. C. cannot be sufficiently
promoted, and YP cannot be sufficiently decreased.
[0059] Then, the steel sheet is dipped in the galvanization bath,
and if required, alloying treatment can be also performed by
keeping the sheet in the temperature region of 500.degree. C. to
650.degree. C. for 30 seconds or less. With a conventional steel
sheet in which [Mneq] is not appropriately controlled, the
mechanical properties are significantly degraded by the
galvannealing treatment. However, with our steel sheet, YP is
slightly increased, and good mechanical properties can be obtained.
After dipping in the galvanization bath or galvannealing treatment,
cooling is performed at an average cooling rate of 7 to 100.degree.
C./sec. At the cooling rate of lower than 7.degree. C./sec,
pearlite is produced near 550.degree. C., and bainite is produced
in the temperature region of 400.degree. C. to 450.degree. C.,
increasing YP. On the other hand, at the cooling rate of higher
than 100.degree. C./sec, self-tempering of martensite which takes
place in continuous cooling is insufficient, and thus martensite is
excessively hardened, thereby increasing YP and decreasing
ductility.
[0060] The resultant galvanized steel sheet has YPEl of less than
0.5% and sufficiently decreased YP in a galvanized state and thus
can be used directly as a steel sheet for press forming as long as
the area ratio of the second phase, the average grain diameter of
the second phase, the area ratio of grains with a grain diameter of
less than 0.8 .mu.m in the second phase, and the area ratio of
pearlite and bainite are controlled. However, as described above,
from the viewpoint of controlling surface roughness and stabilizing
press formability by flattening a shape of steel sheet, skin-pass
rolling is generally performed. In this case, from the viewpoint of
decreasing YP and increasing El and WH, the elongation is
preferably 0.3% to 0.5%.
Examples
Example 1
[0061] Steel of each of Steel Nos. A to CC shown in Table 1 was
molten and continuously cast into a slab of 230 mm in thickness.
The slab was reheated to 1180.degree. C. to 1250.degree. C. and
hot-rolled at a finish rolling temperature of 830.degree. C. (Steel
Nos. A to D, G to U, and X to CC), 870.degree. C. (Steel Nos. E and
V), or 900.degree. C. (Steel Nos. F and W). Then, the hot rolled
band was cooled at an average cooling rate of 20.degree. C./sec and
coiled at a coiling temperature of 640.degree. C. The resultant
hot-rolled sheet was cold-rolled with a rolling reduction of 67% to
form a cold-rolled sheet of 0.75 mm in thickness. The resultant
cold-rolled sheet was annealed in CGL at the average heating rate
in the temperature range of 680.degree. C. to 740.degree. C., the
annealing temperature AT, and the cooling rate shown in Tables 2
and 3 and galvanized in a cooling step. Cooling from the annealing
temperature AT to the galvanization bath temperature of 460.degree.
C. is primary cooling, and cooling from the galvanization bath
temperature or the alloying temperature when alloying was performed
is second cooling. Tables 2 and 3 show the average cooling rate of
each of the primary cooling and second cooling. Alloying treatment
was performed by heating to 510.degree. C. to 530.degree. C. at an
average heating rate of 15.degree. C./sec after dipping in the
galvanization bath and maintaining for 10 to 25 seconds so that the
Fe content in the coating was in the range of 9 to 12%. Both
surfaces were coated with a coating weight of 45 g/m.sup.2 per one
side. The resultant galvanized steel sheet was sampled in an
untempered state (without skin pass rolling).
[0062] The obtained sample was examined with respect to the area
ratio of the second phase, the average grain diameter of the second
phase, the area ratio of pearlite or bainite in the second phase,
and the area ratio of grains with a grain diameter of less than 0.8
.mu.m in the second phase. Further, a JIS No. 5 test piece was
collected in the rolling direction and the perpendicular direction
and subjected to a tensile test (according to JISZ2241) to evaluate
YP and TS. In addition, the annealing temperature for the steel
sheet with each of the compositions was changed in the range of
760.degree. C. to 810.degree. C. to measure the maximum and minimum
of YP and determine variation .DELTA.YP of YP.
[0063] The results are shown in Tables 2 and 3.
[0064] Our steel sheets exhibit small .DELTA.YP as compared with a
material in the same TS level. Our steel sheets also have YP which
is equivalent to or lower than YP of conventional steel, i.e., low
YR. For example, in the 440-MPa TS class (440 to less than 490)
steel sheet, .DELTA.YP is suppressed to 15 MPa or less, and YP is
also as low as 206 MPa. In the 490-MPa TS class (440 to less than
540) steel sheet, .DELTA.YP is suppressed to 20 MPa, while in the
590-MPa TS class steel sheet, .DELTA.YP is suppressed to 32 MPa. In
particular, in the steel sheet in which [Mneq] and [% Cr]/[% Mn]
are appropriately controlled to over 2.2 and 0.34 or more,
respectively, the ratio of fine grains with a grain diameter of
less than 0.08 .mu.m in the second phase is decreased, the
formation of pearlite and bainite is suppressed, and solution
hardening by Mn and dissolved C is decreased, thereby decreasing YP
and .DELTA.YR For example, in the steel of Steel No. B, [Mneq] is
increased as compared with the steel of Steel No. A, but [% Cr]/[%
Mn] is in the range of 0.27 to 0.33. Therefore, although the
amounts of pearlite and bainite produced are decreased with
increase in [Mneq], the microstructure is made fine and YP is in
the range of 202 to 203 MPa and .DELTA.YP in the range of 11 to 15
MPa under the conditions with a heating rate of 1.5.degree. C./sec
and an annealing temperature of 775.degree. C. On the other hand,
in the steel of Steel Nos. C, E, F, Z, and AA in each of which
[Mneq] is increased to over 2.2 and [% Cr]/[% Mn] is controlled to
0.34 or more, YP and .DELTA.YP are in the range of 182 to 198 MPa
and the range of 5 to 9 MPa, respectively, and very low under the
conditions including a heating rate of 1.5.degree. C./sec, an
annealing temperature of 775.degree. C. to 805.degree. C., and a
primary cooling rate of 4 to 5.degree. C./sec. Further, when [Mneq]
is constant, .DELTA.YP decreases as [% Cr]/[% Mn] increases.
[0065] In such steel, a change in YP due to the alloying treatment
is significantly suppressed. For example, in the steel sheet
prepared from Steel No. C at a heating rate of 1.5.degree. C./sec
and an annealing temperature of 775.degree. C., a change in YP due
to alloying treatment is as low as 2 MPa, and an increase in YP by
alloying treatment is suppressed. Namely, our steel sheets exhibit
good mechanical properties even after galvannealing treatment and
are suitable for applications in which alloying treatment is
performed. In addition, an increase in YP due to an increase in C
is extremely small, and in Steel Nos. H and I, YP is suppressed to
219 MPa or less even when C is increased to 0.051%. Further, in
590-MPa TS class Steel No. J containing 0.108% of C, YP is
suppressed to 262 MPa, and a steel sheet with low YR can be stably
obtained. Further, in Steel Nos. F, BB, and CC containing over
0.001% of B, ferrite grains and the second phase are coarsened, and
YP (or YR) and .DELTA.YP are suppressed to low levels. For example,
in a comparison between AA and BB, [% Cr]/[% Mn] is substantially
the same, but YP and .DELTA.YP of BB containing B are lower than AA
in spite of low [Mneq].
[0066] However, a steel sheet in which the heating rate and cooling
rate in annealing are not appropriately controlled to produce a
large amount of fine grains with a diameter of less than 0.8 .mu.m
in the second phase and a steel sheet in which pearlite and bainite
are produced in large amounts have large .DELTA.YP and large YP
absolute value as compared with our steel sheets in the same
strength level. For example, in Steel Nos. P and W having low
[Mneq], large amounts of pearlite and bainite are produced, and YP
and .DELTA.YP are large as compared with the steel sheet of an
example of the present invention in the same strength level.
Further, in Steel Nos. Q, T, and U in which [% Cr]/[% Mn] is not
appropriately controlled while [Mneq] is in the predetermined
range, martensite is fine and the amount of solution hardening is
large, thereby increasing YP and .DELTA.YP. Steel No. R containing
Mo has the tendency to form a fine microstructure, increasing
.DELTA.YP. With Steel No. S in which the C content is out of the
predetermined range, and consequently the area ratio of the second
phase is out of the predetermined range, low YR cannot be achieved.
In Steel Nos. X and Y containing large amounts of P and Si, the
microstructure is coarsened, but the absolute value of YP is
increased because the amount of solid-solution hardening is
excessively increased.
TABLE-US-00001 TABLE 1 (% by mass) Steel No. C Si Mn P S sol. Al N
Cr others [Mneq] [% Cr]\[% Mn] Remarks A 0.024 0.01 1.53 0.006
0.002 0.02 0.0020 0.51 -- 2.19 0.33 Invention steel B 0.020 0.01
1.80 0.007 0.003 0.03 0.0021 0.49 -- 2.44 0.27 Invention steel C
0.029 0.02 1.48 0.008 0.001 0.04 0.0022 0.74 -- 2.44 0.50 Invention
steel D 0.029 0.01 1.46 0.004 0.010 0.03 0.0022 1.08 -- 2.86 0.74
Invention steel E 0.030 0.01 1.02 0.009 0.003 0.04 0.0020 1.20 --
2.58 1.18 Invention steel F 0.042 0.01 0.65 0.010 0.005 0.03 0.0025
1.42 B: 0.0015 2.50 2.18 Invention steel G 0.013 0.02 1.69 0.008
0.012 0.08 0.0026 0.66 -- 2.55 0.39 Invention steel H 0.017 0.02
1.52 0.008 0.001 0.02 0.0015 0.81 -- 2.57 0.53 Invention steel I
0.051 0.01 1.48 0.009 0.014 0.05 0.0012 0.82 -- 2.55 0.55 Invention
steel J 0.108 0.10 1.42 0.011 0.012 0.04 0.0009 1.08 B: 0.0010 2.82
0.76 Invention steel K 0.029 0.08 1.45 0.011 0.015 0.07 0.0015 0.82
-- 2.52 0.57 Invention steel L 0.031 0.01 1.22 0.011 0.003 0.04
0.0015 0.88 B: 0.0028 2.36 0.72 Invention steel M 0.029 0.01 1.40
0.009 0.005 0.03 0.0018 0.80 Mo: 0.07, V: 0.1 2.44 0.57 Invention
steel N 0.030 0.02 1.35 0.008 0.004 0.07 0.0016 0.85 Ti: 0.01, B:
0.0009 2.46 0.63 Invention steel O 0.030 0.01 1.40 0.007 0.004 0.04
0.0018 0.80 Cu: 0.1, Ni: 0.1, Nb: 0.003 2.44 0.57 Invention steel P
0.029 0.01 1.52 0.012 0.004 0.04 0.0035 0.30 -- 1.91 0.20
Comparative steel Q 0.019 0.01 2.05 0.010 0.005 0.02 0.0025 0.40 --
2.57 0.20 Comparative steel R 0.023 0.01 1.62 0.011 0.004 0.04
0.0025 0.55 Mo: 0.28 2.34 0.34 Comparative steel S 0.008 0.01 1.42
0.012 0.004 0.04 0.0020 0.78 -- 2.43 0.55 Comparative steel T 0.039
0.01 2.08 0.013 0.006 0.03 0.0029 0.18 -- 2.31 0.09 Comparative
steel U 0.085 0.01 2.08 0.013 0.006 0.03 0.0029 0.18 -- 2.31 0.09
Comparative steel V 0.037 0.01 1.00 0.010 0.008 0.02 0.0018 0.84 --
2.09 0.84 Comparative steel W 0.045 0.01 0.20 0.006 0.008 0.04
0.0024 1.35 -- 1.96 6.75 Comparative steel X 0.029 0.01 1.50 0.045
0.005 0.04 0.0022 0.80 -- 2.54 0.53 Comparative steel Y 0.035 0.25
1.52 0.006 0.004 0.04 0.0035 0.78 -- 2.53 0.51 Comparative steel Z
0.024 0.01 1.51 0.008 0.004 0.04 0.0024 0.57 -- 2.25 0.38 Invention
steel AA 0.023 0.01 1.61 0.008 0.004 0.04 0.0024 0.63 -- 2.43 0.39
Invention steel BB 0.030 0.01 1.40 0.022 0.004 0.04 0.0024 0.55 B:
0.0028 2.12 0.39 Invention steel CC 0.031 0.01 1.40 0.010 0.004
0.04 0.0024 0.57 B: 0.0018, Ti: 0.003 2.14 0.41 Invention steel
TABLE-US-00002 TABLE 2 Microstructure Annealing condition Average
grain Area ratio of Steel Heating Primary Secondary Presence of
Area ratio of diameter of pearlite and/or Sheet Steel rate AT
cooling rate cooling rate alloying second phase second phase
bainite in second No. No. (.degree. C./sec) (.degree. C.) (.degree.
C./sec) (.degree. C./sec) treatment (%) (.mu.m) phase (%) 1 A 1.5
740 5 30 Yes 1 1.1 15 2 1.5 775 5 30 Yes 3 1.3 15 3 1.5 800 5 30
Yes 4 1.3 18 4 1.5 850 5 30 Yes 4 1.4 25 5 1.5 775 5 30 No 3 1.3 15
6 5.0 775 5 30 Yes 3 1.2 15 7 5.0 800 2 30 No 3 1.2 12 8 B 1.5 775
4 30 Yes 5 1.0 4 9 5.0 775 4 30 Yes 5 0.9 4 10 C 1.5 740 4 30 Yes 1
1 3 11 0.8 776 4 30 Yes 5 1.3 3 12 1.5 775 4 30 Yes 5 1.2 3 13 2.8
775 4 30 Yes 5 1.2 3 14 15 775 4 30 Yes 5 1.2 3 15 1.5 800 4 30 Yes
6 1.4 6 16 1.5 775 2 30 Yes 4 0.8 22 17 1.5 12 30 Yes 6 1.6 5 18
1.5 45 30 Yes 7 2.0 20 19 1.5 4 3 Yes 4 1.1 25 20 1.5 4 30 No 5 1.3
0 21 D 1 775 4 30 Yes 5 1.3 0 22 15 775 4 30 Yes 5 1.1 0 23 E 0.8
790 5 30 Yes 4 2.4 2 24 1.5 790 5 30 Yes 4 2.3 3 25 2.5 790 5 30
Yes 4 2.1 3 Microstructure Area ratio of Ferrite Steel second phase
grain grain Mechanical properties Sheet of less than 0.8 .mu.m
diameter YP TS YR .DELTA.YP No. in diameter (%) (.mu.m) (MPa) (MPa)
(%) (MPa) Remarks 1 10 9 225 428 53 -- Comparative example 2 10 9
202 440 46 11 Invention example 3 9 10 206 442 47 -- Invention
example 4 8 11 230 432 53 -- Comparative example 5 10 9 196 440 45
8 Invention example 6 15 9 214 444 48 18 Comparative example 7 17 9
208 435 48 18 Comparative example 8 14 8 203 442 46 15 Invention
example 9 18 8 212 449 47 19 Comparative example 10 10 8 213 435 49
-- Comparative example 11 8 10 194 451 43 6 Invention example 12 9
9 195 451 43 8 Invention example 13 13 8 203 453 45 10 Invention
example 14 23 8 219 460 48 20 Comparative example 15 9 9 197 451 44
-- Invention example 16 14 9 217 443 49 20 Comparative example 17 9
9 203 457 44 10 Invention example 18 7 9 226 470 48 24 Comparative
example 19 15 9 223 438 51 20 Comparative example 20 9 9 193 452 43
6 Invention example 21 8 10 188 447 42 7 Invention example 22 20 9
212 455 47 17 Comparative example 23 6 12 182 422 43 5 Invention
example 24 8 12 184 424 43 5 Invention example 25 11 11 186 426 44
8 Invention example
TABLE-US-00003 TABLE 3 Microstructure Annealing condition Average
grain Area ratio of Steel Heating Primary Secondary Presence of
Area ratio of diameter of pearlite and/or Sheet Steel rate AT
cooling rate cooling rate alloying second phase second phase
bainite in second No. No. (.degree. C./sec) (.degree. C.) (.degree.
C./sec) (.degree. C./sec) treatment (%) (.mu.m) phase (%) 26 F 1.5
805 5 30 Yes 6 5.1 2 27 G 1.5 775 6 30 Yes 2 1.1 2 28 H 1.5 775 4
30 Yes 3 1.3 3 29 I 1.5 775 4 30 Yes 11 1.3 2 30 J 1.5 775 7 30 Yes
19 1.3 3 31 K 1.5 775 4 30 Yes 5 1.3 2 32 L 1.5 770 4 30 Yes 4 1.5
2 33 M 1.5 775 4 30 Yes 5 1.1 2 34 N 1.5 775 4 30 Yes 5 1.2 2 35 O
1.5 775 4 30 Yes 5 1.0 2 36 P 1.5 775 4 30 Yes 5 1.2 35 37 5.0 775
4 30 Yes 5 1.2 35 38 5.0 775 4 30 No 5 1.2 20 39 Q 1.5 770 4 20 Yes
4 0.8 6 40 5.0 770 4 20 Yes 4 0.8 5 41 5.0 770 4 20 No 4 0.8 5 42 R
2.0 775 4 20 Yes 4 0.8 5 43 S 1.5 775 4 20 Yes 1 1.1 0 44 T 1.5 775
4 20 Yes 10 1.0 10 45 T 5.0 775 4 20 Yes 10 0.9 10 46 U 1.5 775 7
20 Yes 18 1.0 10 47 V 1.5 770 4 20 Yes 4 2.2 21 48 V 5.0 770 4 20
Yes 4 1.9 21 49 W 1.5 800 6 30 Yes 4 4.5 25 50 X 1.5 775 4 20 Yes 4
1.7 6 51 Y 1.5 775 4 20 Yes 4 1.7 4 52 Z 1.5 775 4 20 Yes 4 1.3 13
53 AA 1.5 775 4 20 Yes 4 1.2 2 54 BB 1.5 775 4 20 Yes 4 1.5 1 55 CC
1.5 775 4 20 Yes 4 1.5 0 Microstructure Area ratio of Ferrite Steel
second phase grain grain Mechanical properties Sheet of less than
0.8 .mu.m diameter YP TS YR .DELTA.YP No. in diameter (%) (.mu.m)
(MPa) (MPa) (%) (MPa) Remarks 26 3 13 182 386 47 5 Invention
example 27 12 8 193 430 45 10 Invention example 28 10 10 186 431 43
8 Invention example 29 9 9 219 495 44 20 Invention example 30 8 8
262 594 44 32 Invention example 31 9 9 194 457 42 8 Invention
example 32 8 10 184 436 42 6 Invention example 33 12 9 202 459 44
10 Invention example 34 10 10 196 455 43 9 Invention example 35 18
8 204 459 44 12 Invention example 36 10 8 230 440 52 20 Comparative
example 37 15 8 240 446 54 25 Comparative example 38 15 8 230 446
52 20 Comparative example 39 18 9 218 453 48 20 Comparative example
40 25 8 230 465 49 30 Comparative example 41 25 8 228 467 49 24
Comparative example 42 16 8 217 455 48 18 Comparative example 43 10
10 265 418 63 24 Comparative example 44 18 8 238 492 48 40
Comparative example 45 22 8 250 505 50 54 Comparative example 46 16
8 290 603 48 48 Comparative example 47 8 13 215 416 52 20
Comparative example 48 8 13 224 422 53 26 Comparative example 49 6
14 200 370 54 18 Comparative example 50 10 9 228 458 50 6
Comparative example 51 10 9 216 450 48 6 Comparative example 52 10
9 198 445 44 9 Invention example 53 11 9 194 446 43 9 Invention
example 54 8 11 188 445 42 7 Invention example 55 8 11 190 448 42 7
Invention example
Example 2
[0067] The slab having the composition of Steel No. C shown in
Table 1 was heated to 1200.degree. C., hot-rolled at a finish
rolling temperature of 830.degree. C., maintained for various times
shown in Table 4 to control the cooling start time, cooled to 600
.degree. C. at various cooling rates shown in Table 4, and coiled
at the coiling temperature CT shown in Table 4. Each of the
resultant hot-rolled band was cold-rolled with a rolling rate of
77% and, in CGL, heated at a heating rate of 1.5.degree. C./sec,
annealed at 775.degree. C., cooled at an average primary cooling
rate of 4.degree. C./sec, galvanized, alloyed at 520.degree. C. for
20 seconds, and then cooled at an average secondary cooling rate of
30.degree. C./sec. JIS No. 5 tensile test pieces were collected
from the result steel sheet in the rolling direction, the
perpendicular direction (C direction), and the direction at
45.degree. with the rolling direction and subject to a tensile
test.
[0068] The results are shown in Table 4.
[0069] When rapid cooling is started at the cooling rate of
40.degree. C./sec or more within 3 seconds after finish rolling, YP
in the direction at 45.degree. with the rolling direction is
decreased. Our steel sheets which have extremely low YP in the
direction at 45.degree. with the rolling direction and which are
obtained by controlling the hot rolling conditions can possibly
effectively decrease surface distortion in the surrounding area of
a door knob.
TABLE-US-00004 TABLE 4 Steel Hot rolling condition Mechanical
properties sheet Steel Cooling start Cooling rate CT YPn YPc
YPn-YPc No. No. time (sec) (.degree. C./sec) (.degree. C.) (MPa)
(MPa) (MPa) Remarks 56 C 2.2 20 530 195 188 7 Invention example 57
2.2 40 530 192 190 2 Invention example 58 2.2 70 500 191 196 -5
Invention example 59 2.2 150 490 189 196 -7 Invention example 60 5
40 540 193 187 6 Invention example
* * * * *