U.S. patent application number 12/668057 was filed with the patent office on 2010-12-30 for method for producing low yield strength cold rolled steel sheet excellent in uniformity.
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 | 20100326572 12/668057 |
Document ID | / |
Family ID | 40228711 |
Filed Date | 2010-12-30 |
![](/patent/app/20100326572/US20100326572A1-20101230-D00001.png)
United States Patent
Application |
20100326572 |
Kind Code |
A1 |
Ono; Yoshihiko ; et
al. |
December 30, 2010 |
METHOD FOR PRODUCING LOW YIELD STRENGTH COLD ROLLED STEEL SHEET
EXCELLENT IN UNIFORMITY
Abstract
A method produces a high-strength cold-rolled steel sheet
includes hot-rolling and cold-rolling steel having a composition
which contains, by % by mass, over 0.01% to less than 0.08% of C,
0.2% or less of Si, 0.8% to less than 1.7% of Mn, 0.03% 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 0.4% to 2% of Cr, and which satisfies 1.9<[Mneq]<3
and 0.34.ltoreq.[% Cr]/[% Mn], the balance being composed of iron
and inevitable impurities; 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.; annealing at an annealing temperature of over
740.degree. C. to less than 820.degree. C.; cooling at an average
cooling rate of 2 to 30.degree. C./sec in a temperature range of
the annealing temperature to 650.degree. C.; cooling at an average
cooling rate of 10.degree. C./sec or more in the temperature range
of 650.degree. C. to Tc.degree. C.
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: |
40228711 |
Appl. No.: |
12/668057 |
Filed: |
July 10, 2008 |
PCT Filed: |
July 10, 2008 |
PCT NO: |
PCT/JP2008/062873 |
371 Date: |
January 28, 2010 |
Current U.S.
Class: |
148/645 |
Current CPC
Class: |
C21D 1/26 20130101; C21D
1/185 20130101; C21D 2211/004 20130101; C21D 8/0247 20130101; C21D
9/46 20130101; C22C 38/04 20130101; C22C 38/18 20130101; C21D 9/48
20130101; C21D 8/0447 20130101 |
Class at
Publication: |
148/645 |
International
Class: |
C21D 8/02 20060101
C21D008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 11, 2007 |
JP |
2007-181947 |
Claims
1. A method for producing a high-strength cold-rolled steel sheet
comprising: hot-rolling and cold-rolling steel having a composition
which contains, by % by mass, over 0.01% to less than 0.08% of C,
0.2% or less of Si, 0.8% to less than 1.7% of Mn, 0.03% 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 0.4% to 2% of Cr, and which satisfies 1.9<[Mneq]<3
and 0.34.ltoreq.[% Cr]/[% Mn], the balance being composed of iron
and inevitable impurities; 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.; annealing at an annealing temperature of over
740.degree. C. to less than 820.degree. C.; cooling at an average
cooling rate of 2 to 30.degree. C./sec in a temperature range of
the annealing temperature to 650.degree. C.; cooling at an average
cooling rate of 10.degree. C./sec or more in a temperature range of
650.degree. C. to Tc.degree. C. represented by equation (1) below;
and cooling at an average cooling rate of 0.2 to 10.degree. C./sec
in a temperature range of Tc.degree. C. to 200.degree. C.,
Tc=410-40.times.[% Mn]-30.times.[% Cr] (1) wherein [Mneq]
represents the Mn equivalent shown by [Mneq]=[% Mn]+1.3.times.[%
Cr] and [% Mn] and [% Cr] represent the contents of Mn and Cr,
respectively.
2. The method according to claim 1, wherein during annealing,
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.
3. The method according to claim 1, wherein the steel satisfies
0.55.ltoreq.[% Cr]/[% Mn].
4. The method according to claim 1, wherein the steel further
comprises, by % by mass, 0.005% or less of B.
5. The method according to claim 1, wherein the steel further
comprises, by % by mass, at least one of 0.1% or less of Mo and
0.2% or less of V.
6. The method according to claim 1, wherein the steel further
comprises, 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. The method according to claim 2, wherein the steel satisfies
0.55.ltoreq.[% Cr]/[% Mn].
8. The method according to claim 2, wherein the steel further
comprises, by % by mass, 0.005% or less of B.
9. The method according to claim 3, wherein the steel further
comprises, by % by mass, 0.005% or less of B.
10. The method according to claim 7, wherein the steel further
comprises, by % by mass, 0.005% or less of B.
11. The method according to claim 2, wherein the steel further
comprises, by % by mass, at least one of 0.1% or less of Mo and
0.2% or less of V.
12. The method according to claim 3, wherein the steel further
comprises, by % by mass, at least one of 0.1% or less of Mo and
0.2% or less of V.
13. The method according to claim 4, wherein the steel further
comprises, by % by mass, at least one of 0.1% or less of Mo and
0.2% or less of V.
14. The method according to claim 2, wherein the steel further
comprises, 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.
15. The method according to claim 3, wherein the steel further
comprises, 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.
16. The method according to claim 4, wherein the steel further
comprises, 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.
17. The method according to claim 5, wherein the steel further
comprises, 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.
Description
RELATED APPLICATIONS
[0001] This is a .sctn.371 of International Application No.
PCT/JP2008/062873, with an international filing date of Jul. 10,
2008 (WO 2009/008548 A1, published Jan. 15, 2009), which is based
on Japanese Patent Application Nos. 2007-181947, filed Jul. 11,
2007, and 2008-177468, filed Jul. 8, 2008, the subject matter of
which is incorporated by reference.
TECHNICAL FIELD
[0002] This disclosure relates to a method for producing a
high-strength cold-rolled steel sheet for press forming which is
used for automobiles, home electric appliances, and the like
through a press forming process.
BACKGROUND
[0003] BFI 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 are required sufficient dent resistance.
[0004] In recent years, regarding the increasing requirement of
further weight reduction of car bodies, new attempt of applying
steel sheet with higher strength and superior dent resistance has
been carried out to reduce thickness of the steel sheet for outer
panel. 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.
[0005] 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 that
application is desired that YP after press forming and baking
finish treatment is increased more than YP of a conventional steel
sheet while maintaining extremely low YP before press forming.
[0006] 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 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. 2006-233294 discloses a method
for producing a high-strength cold-rolled steel sheet having high
BH by annealing steel which contains 0.01% to 0.04% of C, 0.3 to
1.6% of Mn, 0.5% or less of Cr, and 0.5% or less of Mo and which
satisfies 1.3.ltoreq.Mn+1.29Cr+3.29Mo.ltoreq.2.1% and cooling at a
cooling rate of 100.degree. C./sec or more in the temperature range
of at least 550.degree. C. or lower to increase the amount of
dissolved C in the steel. Japanese Unexamined Patent Application
Publication No. 2006-52465 discloses a method for producing a
high-strength cold-rolled steel sheet including ferrite and a
low-temperature transformed phase and having high BH and excellent
surface appearance quality after press forming, the method
including annealing steel which contains 0.0025% to less than 0.04%
of C, 0.5 to 2.5% of Mn, and 0.05% to 2.0% of Cr, cooling at a
cooling rate of 15 to 200.degree. C./sec in the temperature range
of 650.degree. C. to 450.degree. C., and further cooling at a
cooling rate of less than 10.degree. C./sec in the temperature
range of 200.degree. C. to near 300.degree. C.
[0007] However, the high-strength cold-rolled steel sheets produced
by the methods described in Japanese Examined Patent Application
Publication No. 62-40405 and Japanese Unexamined. Patent
Application Publication Nos. 2006-233294 and 2006-52465 have the
following problems: [0008] i) YP is not sufficiently decreased, and
thus press-forming into a door panel or the like produces a large
amount of surface distortion as compared with 340BH. [0009] ii) In
such dual phase high-strength cold-rolled steel sheets, hard
martensite is dispersed as a second phase for strengthening, and
thus fluctuations in mechanical properties easily occur. For
example, the volume fraction of a second phase significantly
influenced by changes in the C content of several tens ppm in steel
and the annealing temperature of 20 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.
[0010] It could therefore be helpful to provide a method for
producing a high-strength cold-rolled steel sheet with low YP and
excellent uniformity.
SUMMARY
[0011] We conducted investigations on methods for further
decreasing YP while maintaining high BH equivalent to or higher
than a general value and decreasing variation in mechanical
properties with respect to a dual phase high-strength cold-rolled
steel sheet. As a result, the following findings were obtained:
[0012] (I) By appropriately controlling the composition ranges of
Mn and Cr and performing slow heating in a predetermined
temperature range during annealing, an attempt can be made to
coarsely and uniformly disperse a second phase, thereby decreasing
YP and suppressing YP variation with annealing temperature. [0013]
(II) By appropriately controlling the composition ranges of Mn and
Cr, excessive decrease in the amount of dissolved C can be
suppressed, thereby achieving high BH.
[0014] We thus provide a method for producing a high-strength
cold-rolled steel sheet, the method including hot-rolling and
cold-rolling steel having a composition which contains, by % by
mass, over 0.01% to less than 0.08% of C, 0.2% or less of Si, 0.8%
to less than 1.7% of Mn, 0.03% 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 0.4% to 2% of
Cr, and which satisfies 1.9<[Mneq]<3 and 0.34.ltoreq.[%
Cr]/[% Mn], the balance being composed of iron and inevitable
impurities; 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.; annealing at an annealing temperature of over
740.degree. C. to less than 820.degree. C.; cooling at an average
cooling rate of 2 to 30.degree. C./sec in the temperature range of
the annealing temperature to 650.degree. C.; cooling at an average
cooling rate of 10.degree. C./sec or more in the temperature range
of 650.degree. C. to Tc.degree. C. represented by the equation (1)
below; and cooling at an average cooling rate of 0.2 to 10.degree.
C./sec in the temperature range of Tc.degree. C. to 200.degree.
C.
Tc=410-40.times.[% Mn]-30.times.[% Cr] (1)
[0015] Here, [Mneq] represents the Mn equivalent shown by [Mneq]=[%
Mn]+1.3.times.[% Cr] and [% Mn] and [% Cr] represent the contents
of Mn and Cr, respectively.
[0016] In the method for producing the high-strength cold-rolled
steel sheet, heating is preferably performed at an average heating
rate of less than 2.degree. C./sec in the temperature range of
680.degree. C. to 740.degree. C. during annealing.
[0017] Further, preferably, steel satisfying 0.55.ltoreq.[% Cr]/[%
Mn] is used, and 0.005% by mass or less of B is 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.
[0018] A high-strength cold-rolled steel sheet with low YP and
excellent uniformity can be produced. The high-strength cold-rolled
steel sheet produced by the method has excellent resistance to
surface distortion and excellent dent resistance and is thus
suitable for strengthening and thinning automotive parts.
BRIEF DESCRIPTION OF DRAWINGS
[0019] FIG. 1 is a graph showing a relationship between YP and the
average heating rate in annealing.
DETAILED DESCRIPTION
[0020] 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: Over 0.01% to Less Than 0.08%
[0021] C is an element necessary for securing a predetermined
amount of a second phase. When the C content is excessively low,
the second phase cannot be secured in a sufficient amount, and low
YP cannot be achieved. Further, sufficiently high BH cannot be
secured, and the anti-aging property is degraded. The C content is
required to exceed 0.01% to secure a sufficient amount of the
second phase. On the other hand, when the C content is 0.08% or
more, the ratio of the second phase is excessively increased with a
result that YP increases. Therefore, the upper limit of the C
content is less than 0.08%. The C content is preferably less than
0.06% for achieving lower YP and more preferably less than 0.04%
for achieving further lower YP.
Si: 0.2% or Less
[0022] 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 further homogenizing and coarsening the
microstructure of a steel sheet, and the effect of improving
seizing to a mold (mold galling) in press forming. Therefore, Si
can be added from this viewpoint. However, Si has a large
solution-hardening ability and thus has the large effect of
increasing YP. Therefore, the Si content is in the range of 0.2% or
less which causes little influence on an increase in YP. The C
content is preferably 0.1% or less.
Mn: 0.8% to Less than 1.7%
[0023] Mn can enhance hardenability and decrease the amount of
dissolved C in a predetermined range by appropriately controlling
the Mn content to decrease YP and increase BH. When the Mn content
is 0.8% or less, the amount of dissolved C is excessively increased
in a cooling step of annealing, and a large amount of dissolved C
precipitates in strain around martensite during overaging treatment
in the temperature range of less than 400.degree. C., causing
difficulty in sufficiently decreasing YP. In addition, when the
amount of dissolved C is excessively increased, anti-aging property
is degraded. On the other hand, when the Mn content is 1.7% or
more, the amount of dissolved C is excessively decreased, thereby
decreasing BH. Further, solid-solution hardening of Mn is
increased, and a second phase is made fine increasing YP and cause
variation of YP with annealing temperature. Therefore, the Mn
content is 0.8% to less than 1.7%.
P: 0.03% or Less
[0024] P has a large solution hardening ability and is preferably
added in as small an amount as possible from the viewpoint of
decreasing YP. However, P has the effect of further coarsening the
microstructure of a steel sheet and the effect of improving seizing
to a mold (mold galling) in press forming. Therefore, the P content
is 0.03% or less which has a small adverse effect on an increase in
YP.
S: 0.02% or Less
[0025] 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
[0026] 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 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%, castability 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
[0027] 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 0.4% to 2%
[0028] Cr is the most important element. Since Cr has a small
amount of solid-solution hardening and the effect of making fine
martensite as the second phase and enhancing hardenability, Cr is
an element effective in decreasing YP and decreasing variation in
material quality. It is necessary to control the Mn equivalent and
the composition ratio to Mn to exhibit these effects, which will be
described below, and the Cr content is necessary to exceed 0.4%. 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.
1.9<[Mneq]<3
[0029] When the Mn equivalent, i.e., the [Mneq], is controlled to
exceed 1.9 by controlling the cooling rate in annealing, the amount
of dissolved C is decreased to a proper range, and the formation of
pearlite and bainite is suppressed to decrease YP and increase BH.
Further, from the viewpoint of decreasing YP, [Mneq] preferably
exceeds 2.1 and more preferably exceeds 2.2. On the other hand,
when [Mneq] is excessively increased, BH is decreased, and the cost
is increased. Therefore, [Mneq] is less than 3.
0.34.ltoreq.[% Cr]/[% Mn]
[0030] When the ratio of the Cr content to the Mn content, i.e., [%
Cr]/[% Mn], is controlled to 0.34 or more while [Mneq] is constant,
the second phase can be coarsened and solid-solution hardening of
Mn can be decreased, thereby decreasing YP and material quality
variation. To further decrease YP and material quality variation,
0.55.ltoreq.[% Cr]/[% Mn] is preferred.
[0031] The balance includes iron and inevitable impurities, but the
elements below may be contained at predetermined contents.
B: 0.005% or Less
[0032] Similarly, B is an element for enhancing hardenability and
has the function to fix N as BN to improve the grain growth
property. However, when B is excessively added, the second phase is
inversely made fine by the influence of residual dissolved B.
Therefore, the B content is preferably 0.005% or less. 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, the B content preferably exceeds 0.001%.
Mo: 0.1% or Less
[0033] Like Mn and Cr, Mo is an element for enhancing hardenability
and can be added for the purpose of improving hardenability.
However, when Mo is excessively added, like Mn, the second phase is
made fine and hard, increasing YP. Therefore, Mo is preferably
added in the range of 0.1% or less which has the small influence on
an increase in YP. 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
[0034] Similarly, V is an element for enhancing hardenability.
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%
[0035] 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%
[0036] Nb has the effect of delaying recrystallization in hot
rolling controlling 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 and increase YP. Therefore, Nb is preferably added in the
range of less than 0.01% which has a small influence on an increase
in YP.
Cu: 0.3% or Less
[0037] Cu is an element mixed when scraps 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
[0038] 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 defects due to heterogeneity of scales are produced.
Therefore, the Ni content is preferably 0.3% or less.
2) Production Condition
[0039] As described above, the production method includes
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 the temperature range of 680.degree.
C. to 740.degree. C., annealing at an annealing temperature of over
740.degree. C. to less than 820.degree. C., cooling at an average
cooling rate of 2 to 30.degree. C./sec in the temperature range of
the annealing temperature to 650.degree. C., cooling at an average
cooling rate of 10.degree. C./sec or more in the temperature range
of 650.degree. C. to Tc.degree. C. represented by the equation (1)
described above, and cooling at an average cooling rate of 0.2 to
10.degree. C./sec in the temperature range of Tc.degree. C. to
200.degree. C.
Hot Rolling
[0040] 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. Preferably, the slab heating
temperature is 1200.degree. C. or less, and the finish rolling
temperature is 840.degree. C. or less to obtain beautiful plating
appearance quality for an outer panel. 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. In particular, 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 .gamma. in the subsequent
annealing step is improved, contributing to a decrease in YP. To
decrease in-plane anisotropy of r value of the steel sheet and
suppress YP in a direction at 45.degree. with the rolling
direction, the cooling rate after finish rolling is preferably as
large as 40.degree. C./sec or more.
Cold Rolling
[0041] The rolling rate of cold rolling may be 50% to 85%.
Annealing
Average Heating Rate in Annealing: Less Than 3.degree. C./sec
[0042] It is effective to control the heating rate in the
temperature region of 680.degree. C. to 740.degree. C. to uniformly
disperse the coarse second phase after annealing and decrease YP
and variation in material quality. This is because in a component
system with [Mneq] of over 1.9, the second phase after annealing is
easily made fine. It is considered that, when the Mn content is
high, the Ac.sub.1 transformation temperature is excessively
decreased, and y grains are formed in unrecrystallized ferrite
grain boundaries before the completion of recrystallization. Even
when recrystallization is completed, .gamma. grains are produced in
fine ferrite grain boundaries immediately after recrystallization.
Therefore, YP of a steel sheet is easily increased.
[0043] Steel containing 0.028% of C, 0.01% of Si, 1.6% of Mn, 0.01%
of P, 0.01% of S, 0.04% of sol. Al, 0.8% 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 reduction of 67%. The
resultant cold-rolled sheet was annealed at 780.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 at
an average cooling rate of 7.degree. C./sec in the temperate range
from the annealing temperature to 650.degree. C., cooled at
25.degree. C./sec in the temperature range from 650.degree. C. to
300.degree. C., cooled at 0.5.degree. C./sec in the temperature
range from 300.degree. C. to 200.degree. C., and then air-cooled to
room temperature. A JIS No. 5 tensile test piece was collected from
the resultant steel sheet and subjected to a tensile test
(according to JISZ2241, the tensile direction perpendicular to the
rolling direction) and SEM observation of the structure.
[0044] FIG. 1 shows a relation between YP and the average heating
rate in the temperature region of 680.degree. C. to 740.degree. C.
during annealing. At the heating, rate of less than 3.degree.
C./sec, YP of 200 MPa or less can be obtained, while at the heating
rate of less than 2.degree. C./sec, YP of 195 MPa or less can be
obtained. In this case, it was confirmed by SEM that the second
phase is more coarsely and uniformly dispersed. Further, the
influence on variation in material quality was examined for steel
sheets annealed at various heating rates. Namely, the annealing
temperature of each steel sheet was changed from 760.degree. C. to
810.degree. C. to examine a variation .DELTA.YP of YP with a change
of 50.degree. C. in the annealing temperature. As a result, it was
found that in a sample subjected to annealing at a heating rate of
20.degree. C./sec in the range of 680.degree. C. to 740.degree. C.,
.DELTA.YP is 20 MPa, while in a steel sheet subjected to annealing
at a heating rate of less than 3.degree. C./sec, .DELTA.YP is
decreased to less than 15 MPa. Therefore, a steel sheet having low
YP and low .DELTA.YP with annealing temperature can be obtained by
controlling the heating rate in a predetermined range.
Annealing Temperature: Over 740.degree. C. to Less Than 820.degree.
C.
[0045] At the annealing temperature of 740.degree. C. or less, the
second phase cannot be stably secured because of the 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 after
annealing. 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.
Average Cooling Rate (Primary Cooling Rate) in Temperature Range of
Annealing Temperature to 650.degree. C.: 2 to 30.degree. C./sec
[0046] To concentrate Mn and Cr in .gamma. grains during cooling to
enhance hardenability of the .gamma. grains and decrease YP, the
average cooling rate in the temperature range of the annealing
temperature to 650.degree. C. is necessary to be 2 to 30.degree.
C./sec.
Average Cooling Rate (Secondary Cooling Rate) in Temperature Range
of 650.degree. C. to Tc.degree. C. Represented By Equation (1)
Described Above: 10.degree. C./sec or More
[0047] When cooling is performed at an average cooling rate of
10.degree. C./sec or more in the temperature range from 650.degree.
C. to Tc.degree. C. near the Ms point in which pearlite and bainite
are easily produced, the formation of pearlite and bainite is
suppressed, thereby achieving sufficiently low YP.
Average Cooling Rate (Tertiary Cooling Rate) in Temperature Range
of Tc.degree. C. to 200.degree. C.: 0.2 to 10.degree. C./sec
[0048] When cooling is performed at an average cooling rate of 0.2
to 10.degree. C./sec in the temperature range from Tc.degree. C. to
200.degree. C., dissolved C which excessively remains in ferrite is
precipitated to decrease YP and increase ductility.
[0049] The high-strength cold-rolled steel sheet produced by the
method has yield point elongation (YPEI) of less than 0.5% and
sufficiently decreased YP in an annealed state and thus can be used
directly as a steel sheet for press forming. However, from the
viewpoint of controlling surface roughness and stabilizing press
formability by flattening a shape of steel sheet, skin pass rolling
may be performed. In this case, from the viewpoint of decreasing YP
and increasing El and WH, the elongation is preferably 0.3% to
0.5%.
Example
[0050] Steel of each of Steel Nos. A to BB 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, I, R to V, and X to BB) or 880.degree. C. (Steel Nos.
E to H, J to Q, and W). Then, the steel sheet was cooled at an
average cooling rate of 20.degree. C./sec and coiled at a coiling
temperature of 540.degree. C. to 640.degree. C. The resultant
hot-rolled sheet was cold-rolled with a rolling reduction of 67% to
78% after pickling to form a cold-rolled sheet of 0.75 mm in
thickness. The resultant cold-rolled sheet was annealed at the
average heating rate in the temperature range of 680.degree. C. to
740.degree. C., the annealing temperature, the primary average
cooling rate in the temperature range of the annealing temperature
to 650.degree. C., the secondary average cooling rate in the
temperature range of 650.degree. C. to Tc.degree. C., and the
tertiary average cooling rate in the temperature range of
Tc.degree. C. to 200.degree. C., which are shown in Tables 2 and 3.
A JIS No. 5 test piece was collected from the resultant annealed
steel sheet, i.e., the steel sheet not having undergone skin pass
rolling, in each of 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. Further,
prestrain of 2% was applied to the same test piece to determine an
increase in YP after heat treatment at 170.degree. C. for 20
minutes, i.e., BH.
[0051] The results are shown in Tables 2 and 3.
[0052] Our steel sheets exhibit low YP, i.e., low YR, as compared
to a material in the same TS level. Our steel sheets also have
.DELTA.YP and are thus excellent in YP stability. In particular, in
the steel sheet in which [Mneq] and [% Cr]/[% Mn] are appropriately
controlled to over 2.1 and 0.55 or more, respectively, and the
heating rate of annealing is controlled to less than 3.degree.
C./sec, solution hardening by Mn and dissolved C is decreased, and
the second phase is uniformly coarsened, thereby decreasing YP and
.DELTA.YP. For example, in the steel of Steel Nos. B, C, and D,
[Mneq] is increased as compared with the steel of Steel No. A, but
[% Cr]/[% Mn] is in the range of 0.34 to 0.41. Therefore, the
amounts of pearlite and bainite produced are decreased with
increase in [Mneq], and the amount of dissolved C is decreased.
However, the second phase is made fine to exhibit YP in the range
of 191 to 197 MPa and .DELTA.YP in the range of 7 to 9 MPa with
annealing temperature under the conditions with a heating rate of
1.5.degree. C./sec and an annealing temperature of 780.degree.
C.
[0053] On the other hand, in the steel of Steel Nos. E, F, G, and H
in each of which [Mneq] is increased to over 2.1 and [% Cr]/[% Mn]
is controlled to 0.55 or more, YP and .DELTA.YP with annealing
temperature are in the range of 172 to 198 MPa and the range of 4
to 6 MPa, respectively, and very low under the same conditions as
Steel Nos. A, B, C, and D. In addition, an increase in YP due to an
increase in C is extremely small, and Steel No. K in which the C
content is increased to 0.058% has TS of 490 MPa and YP of as low
as 208 MPa. Further, Steel No. L in which the C content is
increased to 0.072% has TS of 541 MPa and YP of as low as 230 MPa.
Namely, even when the C content is changed, a steel sheet with
small .DELTA.YP and low YR can be stably obtained. Further, since
the composition ranges of Mn and Cr are appropriately controlled,
high BH is achieved in spite of low YP.
[0054] However, a steel sheet in which [Mneq] and the heating rate
and cooling rate in annealing arc not appropriately controlled has
high YR as compared to our steel sheets in the same strength level.
For example, Steel Nos. S and V in which [% Cr]/[% Mn] is not
appropriately controlled have the fine second phase and the large
amount of solution hardening and thus have high .DELTA.YP and YP
and low BH. Steel No. T containing Mo has the tendency to form a
fine microstructure, increasing YP and .DELTA.YP. With Steel No. U
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 second phase is
coarsened, but low YP cannot be achieved because the amount of
solid-solution hardening is excessively increased. Therefore, with
conventional steel, a steel sheet satisfying low YP, small
.DELTA.YP, and high BH cannot be obtained.
TABLE-US-00001 TABLE 1 Steel [% Cr]/ Tc No. C Si Mn P S sol. Al N
Cr others [Mneq] [% Mn] (.degree. C.) Remarks A 0.032 0.01 1.35
0.008 0.002 0.02 0.0018 0.46 -- 1.95 0.34 342 Invention steel B
0.031 0.01 1.39 0.008 0.002 0.02 0.0018 0.55 -- 2.11 0.40 338
Invention steel C 0.029 0.01 1.48 0.004 0.002 0.02 0.0018 0.61 --
2.27 0.41 333 Invention steel D 0.025 0.01 1.60 0.007 0.003 0.03
0.0020 0.65 -- 2.45 0.41 327 Invention steel E 0.034 0.02 1.28
0.006 0.001 0.04 0.0018 0.71 -- 2.20 0.55 338 Invention steel F
0.037 0.01 1.08 0.008 0.003 0.04 0.0018 0.96 -- 2.33 0.89 338
Invention steel G 0.031 0.01 0.96 0.006 0.003 0.04 0.0016 1.20 --
2.52 1.25 336 Invention steel H 0.030 0.01 0.80 0.008 0.003 0.04
0.0014 1.40 -- 2.62 1.75 336 Invention steel I 0.014 0.02 1.58
0.008 0.001 0.02 0.0018 0.90 -- 2.75 0.57 320 Invention steel J
0.048 0.01 1.20 0.009 0.014 0.05 0.0012 1.02 -- 2.53 0.85 331
Invention steel K 0.058 0.01 1.04 0.009 0.012 0.04 0.0010 1.14 --
2.52 1.10 334 Invention steel L 0.072 0.01 1.20 0.009 0.006 0.03
0.0019 1.20 -- 2.76 1.00 326 Comparative steel M 0.038 0.08 1.08
0.008 0.015 0.07 0.0022 1.02 -- 2.41 0.94 336 Invention steel N
0.039 0.01 1.00 0.008 0.003 0.04 0.0018 0.96 B: 0.0030 2.25 0.96
341 Invention steel O 0.037 0.01 1.01 0.009 0.005 0.03 0.0025 0.95
Mo: 0.07, V: 0.1 2.25 0.94 341 Invention steel P 0.038 0.02 1.01
0.007 0.004 0.07 0.0025 0.98 Ti: 0.01, B: 0.001 2.28 0.97 340
Invention steel Q 0.036 0.01 1.02 0.006 0.004 0.04 0.0026 0.98 Cu:
0.1, Ni: 0.1, Nb: 0.003 2.29 0.96 340 Invention steel R 0.029 0.01
1.60 0.008 0.004 0.04 0.0018 0.18 -- 1.83 0.11 341 Comparative
steel S 0.019 0.01 1.88 0.009 0.005 0.02 0.0018 0.40 -- 2.40 0.21
323 Comparative steel T 0.025 0.01 1.60 0.009 0.004 0.04 0.0018
0.55 Mo: 0.28 2.32 0.34 330 Comparative steel U 0.006 0.01 1.30
0.010 0.004 0.04 0.0020 0.82 -- 2.37 0.63 333 Comparative steel V
0.038 0.01 2.15 0.010 0.006 0.03 0.0027 0.30 -- 2.54 0.14 315
Comparative steel W 0.045 0.01 0.60 0.010 0.008 0.02 0.0028 1.00 --
1.90 1.67 356 Comparative steel X 0.033 0.01 1.52 0.035 0.004 0.04
0.0022 0.80 -- 2.56 0.53 325 Comparative steel Y 0.035 0.25 1.52
0.006 0.004 0.04 0.0033 0.78 -- 2.53 0.51 326 Comparative steel Z
0.031 0.01 1.32 0.022 0.008 0.07 0.0018 0.46 B: 0.0026 1.92 0.35
343 Invention steel AA 0.030 0.01 1.31 0.015 0.002 0.06 0.0025 0.47
B: 0.0015, Ti: 0.005 1.92 0.36 344 Invention steel BB 0.033 0.01
1.24 0.008 0.002 0.10 0.0018 0.69 B: 0.0019 2.14 0.56 340 Invention
steel
TABLE-US-00002 TABLE 2 Annealing condition Primary Secondary
Tertiary Average average average average Steel heating Annealing
cooling cooling cooling Mechanical properties sheet Steel rate
temperature rate rate rate YP TS YR .DELTA.YP BH No. No. (.degree.
C./sec) (.degree. C.) (.degree. C./sec) (.degree. C./sec) (.degree.
C./sec) (MPa) (MPa) (%) (MPa) (MPa) Remarks 1 A 1.5 740 7 30 0.6
212 438 48 -- 47 Comparative example 2 1.5 780 7 30 0.6 197 450 44
7 62 Invention example 3 1.5 800 7 30 0.6 199 452 44 -- 63
Invention example 4 1.5 830 7 30 0.6 214 448 48 -- 59 Comparative
example 5 2.3 780 7 30 0.6 199 452 44 10 63 Invention example 6 5.0
780 7 30 0.6 206 454 45 15 64 Comparative example 7 B 1.5 780 7 30
0.6 192 450 43 7 60 Invention example 8 C 1.5 775 7 30 0.6 191 450
42 7 56 Invention example 9 5.0 775 7 30 0.6 202 454 44 16 58
Comparative example 10 D 1.5 770 7 30 0.6 193 450 43 9 51 Invention
example 11 5.0 770 7 30 0.6 205 455 45 17 52 Comparative example 12
E 1.5 780 7 30 0.6 188 450 42 6 67 Invention example 13 2.7 780 7
30 0.6 195 453 43 9 67 Invention example 14 2.7 810 7 30 0.6 198
455 44 -- 68 Invention example 15 5.0 780 7 30 0.6 203 456 45 15 68
Comparative example 16 2.7 830 7 30 0.6 205 450 46 -- 62
Comparative example 17 2.7 780 40 40 0.6 205 456 45 -- 70
Comparative example 18 2.7 780 7 5 0.6 228 436 52 -- 58 Comparative
example 19 2.7 780 7 30 50 205 458 45 -- 70 Comparative example 20
F 0.8 780 7 30 0.6 183 451 41 5 70 Invention example 21 1.5 780 7
30 0.6 184 452 41 5 70 Invention example 22 2.7 780 7 30 0.6 188
454 41 8 71 Invention example 23 15 780 7 30 0.6 204 458 45 15 72
Comparative example 24 G 1.5 785 7 30 0.6 178 428 42 4 68 Invention
example 25 H 1.5 790 7 30 0.6 172 408 42 4 75 Invention example
TABLE-US-00003 TABLE 3 Annealing condition Primary Secondary
Tertiary Average average average average Steel heating Annealing
cooling cooling cooling Mechanical properties sheet Steel rate
temperature rate rate rate YP TS YR .DELTA.YP BH No. No. (.degree.
C/sec) (.degree. C.) (.degree. C./sec) (.degree. C./sec) (.degree.
C./sec) (MPa) (MPa) (%) (MPa) (MPa) Remarks 26 I 1.5 770 15 40 1.5
184 430 43 7 60 Invention example 27 J 1.5 780 7 30 0.6 198 482 41
12 62 Invention example 28 K 1.5 780 7 30 0.3 208 490 42 16 60
Invention example 29 L 1.5 780 10 40 0.6 230 541 43 24 50 Invention
example 30 M 1.5 780 7 30 0.6 193 452 43 5 74 Invention example 31
N 1.5 780 5 20 0.4 182 444 41 5 75 Invention example 32 O 1.5 780 7
30 0.6 196 450 44 9 76 Invention example 33 P 1.5 780 7 30 0.6 192
442 43 8 74 Invention example 34 Q 1.5 780 7 30 0.6 198 450 44 10
76 Invention example 35 R 1.5 775 7 30 0.6 214 448 48 14 49
Comparative example 36 5.0 775 7 30 0.6 223 451 49 18 51
Comparative example 37 S 1.5 775 7 30 0.6 205 454 45 15 47
Comparative example 38 5.0 775 7 30 0.6 219 460 48 20 48
Comparative example 39 T 2.0 775 7 30 0.6 204 458 45 15 52
Comparative example 40 U 1.5 780 7 30 0.6 240 410 59 25 48
Comparative example 41 V 1.5 775 7 30 0.6 234 494 47 36 39
Comparative example 42 V 5.0 780 7 30 0.6 250 502 50 45 41
Comparative example 43 W 1.5 790 7 30 0.6 205 440 47 14 70
Comparative example 44 W 5.0 780 7 30 0.6 212 444 48 18 70
Comparative example 45 X 1.5 780 7 30 0.6 212 460 46 5 55
Comparative example 46 Y 1.5 780 7 30 0.6 210 452 46 5 55
Comparative example 47 Z 1.5 780 7 30 0.6 189 452 42 6 66 Invention
example 48 2.8 780 7 30 0.6 196 455 43 9 65 Invention example 49 10
780 7 30 0.6 207 463 45 15 66 Comparative example 50 AA 1.5 780 7
30 0.6 191 454 42 6 67 Invention example 51 BB 1.5 780 7 30 0.6 182
449 41 5 69 Invention example 52 2.8 780 7 30 0.6 190 452 42 5 69
Invention example 53 10 780 7 30 0.6 203 454 45 15 69 Comparative
example
* * * * *