U.S. patent application number 14/433869 was filed with the patent office on 2015-09-10 for cold-rolled steel sheet with excellent shape fixability and method of manufacturing the same.
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is JFE STEEL CORPORATION. Invention is credited to Koichiro Fujita, Taro Kizu, Hideharu Koga, Masahide Morikawa, Kenji Tahara.
Application Number | 20150252456 14/433869 |
Document ID | / |
Family ID | 50477012 |
Filed Date | 2015-09-10 |
United States Patent
Application |
20150252456 |
Kind Code |
A1 |
Kizu; Taro ; et al. |
September 10, 2015 |
COLD-ROLLED STEEL SHEET WITH EXCELLENT SHAPE FIXABILITY AND METHOD
OF MANUFACTURING THE SAME
Abstract
A steel material having a chemical composition contains 0.0010%
to 0.0030% C, 0.05% or less Si, 0.1% to 0.5% Mn, 0.021% to 0.060%
Ti, and 0.0005% to 0.0050% B on a mass basis such that B/C
satisfies 0.5 or more, whereby a resulting cold-rolled steel sheet
has a microstructure dominated by ferrite with an average grain
size of 10 .mu.m to 30 .mu.m, a proportional limit of 100 MPa or
less, and excellent shape fixability.
Inventors: |
Kizu; Taro; (Tokyo, JP)
; Fujita; Koichiro; (Tokyo, JP) ; Koga;
Hideharu; (Tokyo, JP) ; Morikawa; Masahide;
(Tokyo, JP) ; Tahara; Kenji; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Chiyoda-ku, Tokyo |
|
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Chiyoda-ku, Tokyo
JP
|
Family ID: |
50477012 |
Appl. No.: |
14/433869 |
Filed: |
October 11, 2012 |
PCT Filed: |
October 11, 2012 |
PCT NO: |
PCT/JP2012/006532 |
371 Date: |
April 7, 2015 |
Current U.S.
Class: |
148/603 ;
148/330 |
Current CPC
Class: |
C21D 8/0236 20130101;
C22C 38/32 20130101; C21D 8/0226 20130101; C22C 38/001 20130101;
C22C 38/04 20130101; C21D 1/26 20130101; C21D 9/46 20130101; C22C
38/002 20130101; C21D 8/0273 20130101; C22C 38/004 20130101; C22C
38/02 20130101; C21D 2211/005 20130101; C22C 38/26 20130101; C21D
8/0263 20130101; C22C 38/06 20130101; C22C 38/14 20130101; C22C
38/18 20130101; C22C 38/28 20130101; C21D 6/002 20130101; C21D
6/008 20130101; C22C 38/12 20130101; C21D 6/005 20130101 |
International
Class: |
C22C 38/32 20060101
C22C038/32; C21D 9/46 20060101 C21D009/46; C21D 1/26 20060101
C21D001/26; C21D 6/00 20060101 C21D006/00; C22C 38/00 20060101
C22C038/00; C22C 38/26 20060101 C22C038/26; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C21D 8/02 20060101 C21D008/02; C22C 38/28 20060101
C22C038/28 |
Claims
1-20. (canceled)
21. A cold-rolled steel sheet with excellent shape fixability,
having a chemical composition containing 0.0010% to 0.0030% C,
0.05% or less Si, 0.1% to 0.5% Mn, 0.05% or less P, 0.02% or less
S, 0.10% or less Al, 0.0050% or less N, 0.021% to 0.060% Ti, and
0.0005% to 0.0050% B on a mass basis such that B/C satisfies 0.5 or
more, the remainder being Fe and incidental impurities; a
microstructure dominated by ferrite with an average grain size of
10 .mu.m to 30 .mu.m and a proportional limit of 100 MPa or
less.
22. The cold-rolled steel sheet according to claim 21, further
containing 0.009% or less Nb on a mass basis in addition to the
chemical composition.
23. The cold-rolled steel sheet according to claim 21, further
containing 0.06% or less Cr on a mass basis in addition to the
chemical composition.
24. The cold-rolled steel sheet according to claim 21, further
containing 0.009% or less Nb and 0.06% or less Cr on a mass basis
in addition to the chemical composition.
25. The cold-rolled steel sheet according to claim 22, wherein the
content of Nb is 0.001% to 0.009% on a mass basis.
26. The cold-rolled steel sheet according to claim 23, wherein the
content of Cr is 0.001% to 0.06% on a mass basis.
27. The cold-rolled steel sheet according to claim 21, wherein the
B/C is greater than or equal to 0.5 and less than or equal to
5.
28. The cold-rolled steel sheet according to claim 27, wherein the
B/C is greater than or equal to 1.0 and less than or equal to
3.3.
29. The cold-rolled steel sheet according to claim 28, wherein the
B/C is greater than or equal to 1.5 and less than or equal to
3.3.
30. The cold-rolled steel sheet according to claim 21, wherein the
proportional limit is greater than or equal to 40 MPa and less than
or equal to 100 MPa.
31. The cold-rolled steel sheet according to claim 21, wherein the
microstructure dominated by ferrite contains 95% or more ferrite in
terms of area fraction.
32. A method of manufacturing a cold-rolled steel sheet with
excellent shape fixability, comprising subjecting a steel material
to a hot-rolling step, a pickling step, a cold-rolling step, and an
annealing step in that order, wherein the steel material has a
composition containing 0.0010% to 0.0030% C, 0.05% or less Si, 0.1%
to 0.5% Mn, 0.05% or less P, 0.02% or less S, 0.10% or less Al,
0.0050% or less N, 0.021% to 0.060% Ti, and 0.0005% to 0.0050% B on
a mass basis such that B/C satisfies 0.5 or more, the remainder
being Fe and incidental impurities; the hot rolling step is a step
in which the steel material is heated, roughly rolled,
finish-rolled at a finishing delivery temperature of 870.degree. C.
to 950.degree. C., and coiled at a coiling temperature of
450.degree. C. to 630.degree. C.; the cold-rolling step is a step
in which cold rolling is performed at a rolling reduction of 90% or
less; and the annealing step is a step in which heating is
performed up to a holding temperature in the range of 700.degree.
C. to 850.degree. C. at an average heating rate of 1.degree. C./s
to 30.degree. C./s in a temperature region not lower than
600.degree. C., retention is performed at the holding temperature
for 30 s to 200 s, and cooling is performed at a cooling rate of
3.degree. C./s or more in a temperature region down to 600.degree.
C.
33. The method according to claim 32, wherein the chemical
composition further contains 0.009% or less Nb on a mass basis.
34. The method according to claim 32, wherein the chemical
composition further contains 0.06% or less Cr on a mass basis.
35. The method according to claim 32, wherein the chemical
composition further contains 0.009% or less Nb and 0.06% or less Cr
on a mass basis.
36. The method according to claim 33, wherein the content of Nb is
0.001% to 0.009% on a mass basis.
37. The method according to claim 34, wherein the content of Cr is
0.001% to 0.06% on a mass basis.
38. The method according to claim 32, wherein the B/C is greater
than or equal to 0.5 and less than or equal to 5.
39. The method according to claim 38, wherein the B/C is greater
than or equal to 1.0 and less than or equal to 3.3.
40. The method according to claim 39, wherein the B/C is greater
than or equal to 1.5 and less than or equal to 3.3.
Description
TECHNICAL FIELD
[0001] This disclosure relates to a cold-rolled steel sheet
suitable for members of parts requiring strict dimensional accuracy
in the electrical, automotive, building material, and other fields
and which has excellent shape fixability and also relates to a
method of manufacturing the same. The disclosure particularly
relates to the enhancement of shape fixability.
BACKGROUND
[0002] In recent years, to protect the global environment,
reduction of automotive fuel consumption has been required from the
viewpoint of reducing CO.sub.2 emissions. For such a request to
reduce fuel consumption, reduction in weight of automotive bodies
has been attempted. Furthermore, demands to reduce the gauge of
steel and the amount of steel have been growing in association with
a requirement for cost reduction. However, reduction in gauge of
steel materials (steel sheets) reduces the rigidity of parts to
cause problems such as deflections, dents and warpage of the parts.
Furthermore, in the field of consumer electrical appliances such as
AV devices and OA machines, requirements for dimensional accuracy
of parts have become strict and therefore demands for steel sheets
with excellent shape fixability have been increasingly growing.
[0003] For such requirements, for example, WO 00/06791 discloses a
ferritic steel sheet with excellent shape fixability. In a
technique described in WO '791, steel having a composition
containing 0.0001% to 0.05% C, 0.01% to 1.0% Si, 0.01% to 2.0% Mn,
0.15% or less P, 0.03% or less S, 0.01% or less Al, 0.01% or less
N, and 0.007% or less O on a mass basis is hot-rolled such that the
sum of rolling reductions at a temperature of not lower than the
Ar.sub.3 transformation temperature to 950.degree. C. is 25% or
more and the coefficient of friction during hot rolling at
950.degree. C. or lower is 0.2 or less, hot rolling is completed at
a temperature not lower than the Ar.sub.3 transformation
temperature, and coiling is performed at a temperature not higher
than a predetermined critical temperature after cooling, whereby a
steel sheet in which the ratio of the {100} plane to {111} plane
parallel to a sheet surface is 1.0 or more is obtained. In the
steel sheet, a slip system can be controlled during bending and
springback can be suppressed during bending-dominated forming.
[0004] Japanese Unexamined Patent Application Publication No.
2002-66637 discloses a method of press-forming a formed product
with excellent dimensional accuracy. In a technique described in JP
'637, forming is performed using a steel sheet in which the ratio
of the {100} plane to {111} plane parallel to a sheet surface is
1.0 or more such that a tensile stress equal to 40% to 100% of the
tensile strength of material is applied to a vertical wall portion
of a hat-shaped member. According to that technique, a member
having significantly increased hat bendability, small springback,
and excellent shape fixability can be provided.
[0005] However, the technique described in WO '791 has problems
such as: the degree of improvement in shape fixability is small in
performing press forming other than bending, and springback may be
large due to the influence of grain boundary sliding or the like
even when performing bending. Furthermore, the technique described
in JP '637 has a problem that the effect of improving the
dimensional accuracy of a formed product is not obtained in
performing press forming other than hat forming and a problem that
the blank holding pressure needs to be large to apply stress to a
vertical wall in performing hat forming and therefore the power of
a press needs to be increased, leading to an increase in cost.
[0006] It could therefore be helpful to provide a cold-rolled steel
sheet having excellent shape fixability and causing no significant
strain in a flat portion of a formed member and a method of
manufacturing the same.
SUMMARY
[0007] We discovered that the strain of a flat portion of a formed
member is significantly affected by the proportional limit of a
steel sheet used. We also found that the strain of a flat portion
of a formed member is significantly increased particularly when the
proportional limit is more than 100 MPa. We further found that an
ultra-low carbon based chemical composition essentially containing
Ti and B needs to be adjusted such that the ratio, B/C, of the
content of B to the content of C satisfies 0.5 or more such that
the proportional limit is 100 MPa or less.
[0008] We thus provide: [0009] (1) A cold-rolled steel sheet with
excellent shape fixability, has a chemical composition containing
0.0010% to 0.0030% C, 0.05% or less Si, 0.1% to 0.5% Mn, 0.05% or
less P, 0.02% or less S, 0.10% or less Al, 0.0050% or less N,
0.021% to 0.060% Ti, and 0.0005% to 0.0050% B on a mass basis such
that B/C satisfies 0.5 or more, the remainder being Fe and
incidental impurities; a microstructure dominated by ferrite with
an average grain size of 10 .mu.m to 30 .mu.m; and a proportional
limit of 100 MPa or less. [0010] (2) The cold-rolled steel sheet
specified in (1) further contains 0.009% or less Nb on a mass basis
in addition to the chemical composition. [0011] (3) The cold-rolled
steel sheet specified in (1) further contains 0.06% or less Cr on a
mass basis in addition to the chemical composition. [0012] (4) The
cold-rolled steel sheet specified in (1) further contains 0.009% or
less Nb and 0.06% or less Cr on a mass basis in addition to the
chemical composition. [0013] (5) In the cold-rolled steel sheet
specified in (2), the content of Nb is 0.001% to 0.009% on a mass
basis. [0014] (6) In the cold-rolled steel sheet specified in (3),
the content of Cr is 0.001% to 0.06% on a mass basis. [0015] (7) In
the cold-rolled steel sheet specified in (1), the B/C is greater
than or equal to 0.5 and less than or equal to 5. [0016] (8) In the
cold-rolled steel sheet specified in (7), the B/C is greater than
or equal to 1.0 and less than or equal to 3.3. [0017] (9) In the
cold-rolled steel sheet specified in (8), the B/C is greater than
or equal to 1.5 and less than or equal to 3.3. [0018] (10) In the
cold-rolled steel sheet specified in (1), the proportional limit is
greater than or equal to 40 MPa and less than or equal to 100 MPa.
[0019] (11) In the cold-rolled steel sheet specified in (1), the
microstructure dominated by ferrite contains 95% or more ferrite in
terms of area fraction. [0020] (12) A method of manufacturing a
cold-rolled steel sheet with excellent shape fixability includes
subjecting a steel material to a hot-rolling step, a pickling step,
a cold-rolling step, and an annealing step in that order. The steel
material has a chemical composition containing 0.0010% to 0.0030%
C, 0.05% or less Si, 0.1% to 0.5% Mn, 0.05% or less P, 0.02% or
less S, 0.10% or less Al, 0.0050% or less N, 0.021% to 0.060% Ti,
and 0.0005% to 0.0050% B on a mass basis such that B/C satisfies
0.5 or more, the remainder being Fe and incidental impurities. The
hot rolling step is a step in which the steel material is heated,
is roughly rolled, is finish-rolled at a finishing delivery
temperature of 870.degree. C. to 950.degree. C., and is coiled at a
coiling temperature of 450.degree. C. to 630.degree. C. The
cold-rolling step is a step in which cold rolling is performed at a
rolling reduction of 90% or less. The annealing step is a step in
which heating is performed up to a holding temperature in the range
of 700.degree. C. to 850.degree. C. at an average heating rate of
1.degree. C./s to 30.degree. C./s in a temperature region not lower
than 600.degree. C., retention is performed at the holding
temperature for 30 s to 200 s, and cooling is then performed at a
cooling rate of 3.degree. C./s or more in a temperature region down
to 600.degree. C. [0021] (13) In the method of manufacturing the
cold-rolled steel sheet specified in (12), it further contains
0.009% or less Nb on a mass basis in addition to the chemical
composition. [0022] (14) In the method of manufacturing the
cold-rolled steel sheet specified in (12), it further contains
0.06% or less Cr on a mass basis in addition to the chemical
composition. [0023] (15) In the method of manufacturing the
cold-rolled steel sheet specified in (12), it further contains
0.009% or less Nb and 0.06% or less Cr on a mass basis in addition
to the chemical composition. [0024] (16) In the method of
manufacturing the cold-rolled steel sheet specified in (13), the
content of Nb is 0.001% to 0.009% on a mass basis. [0025] (17) In
the method of manufacturing the cold-rolled steel sheet specified
in (14), the content of Cr is 0.001% to 0.06% on a mass basis.
[0026] (18) In the method of manufacturing the cold-rolled steel
sheet specified in (12), the B/C is greater than or equal to 0.5
and less than or equal to 5. [0027] (19) In the method of
manufacturing the cold-rolled steel sheet specified in (18), the
B/C is greater than or equal to 1.0 and less than or equal to 3.3.
[0028] (20) In the method of manufacturing the cold-rolled steel
sheet specified in (19), the B/C is greater than or equal to 1.5
and less than or equal to 3.3.
[0029] A cold-rolled steel sheet having a significantly reduced
proportional limit and excellent shape fixability after forming can
be readily manufactured at low cost. This is industrially
particularly advantageous. Furthermore, there is an effect that the
reduction in gauge of a member can be accelerated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a schematic view showing a test specimen for punch
stretch forming and a flange-suppressing region (hatched portion)
during a forming test.
[0031] FIG. 2 is a schematic view showing a method of measuring the
maximum strain height after a punch stretch forming test.
[0032] FIG. 3 is a graph showing the relationship between the
proportional limit and the maximum strain height.
[0033] FIG. 4 is a graph showing the relationship between B/C and
the proportional limit.
DETAILED DESCRIPTION
[0034] First, reasons for limiting the composition (chemical
composition) of a cold-rolled steel sheet are described.
Incidentally, mass percent is hereinafter simply represented by %
unless otherwise specified.
C: 0.0010% to 0.0030%
[0035] C is an element which forms a solid solution to promote
formation of coarse B precipitates and which contributes to a
reduction in proportional limit. Such an effect is remarkable when
the content thereof is 0.0010% or more. However, when the content
thereof is high, more than 0.0030%, the reduction of ductility is
caused because the amount of solute C and/or carbides is large and
the strength is excessively high. Therefore, C is limited to
0.0010% to 0.0030%. It is preferably 0.0020% or less.
Si: 0.05% or Less
[0036] When a large amount of Si is contained, workability is
deteriorated by hardening, and Si oxides are produced during
annealing and thereby wettability is impaired. Furthermore, since
high Si content increases the austenite (.gamma.)-to-ferrite
(.alpha.) transformation temperature, it is difficult to complete
rolling in a .gamma.-region during hot rolling. Therefore, Si is
0.05% or less.
Mn: 0.1% to 0.5%
[0037] Mn combines with S, where S significantly reduces hot
ductility and is harmful, in steel to form MnS, contributes to
rendering S harmless, and has the effect of hardening steel. The
content thereof needs to be 0.1% or more to achieve such effects.
However, when the content thereof is high, more than 0.5%,
ductility is reduced by hardening and recrystallization of ferrite
is suppressed during annealing. Therefore, Mn is 0.1% to 0.5%. It
is preferably 0.3% or less and more preferably 0.2% or less.
P: 0.05% or Less
[0038] P segregates at grain boundaries and has the function of
reducing ductility. Therefore, P is preferably minimized and up to
0.05% is acceptable. Hence, P is 0.05% or less. It is preferably
0.03% or less and more preferably 0.02% or less.
S: 0.02% or Less
[0039] S is an impurity element and is preferably minimized. S
significantly reduces hot ductility, causes hot cracking,
significantly deteriorates surface properties, and has adverse
influences. Furthermore, S hardly contributes to strength and forms
coarse MnS to reduce ductility. This becomes significant when S is
more than 0.02%. Therefore, S is 0.02% or less. It is preferably
0.01% or less.
Al: 0.10% or Less
[0040] Al is an element acting as a deoxidizer. 0.02% or more is
preferably contained to achieve such an effect. On the other hand,
Al has the function of increasing the .gamma.-to-.alpha.
transformation temperature of steel. Therefore, when the content is
high, more than 0.10%, it is difficult to complete rolling in a
.gamma.-region during hot rolling. Therefore, Al is 0.10% or
less.
N: 0.0050% or Less
[0041] N is an element which combines with a nitride-forming
element to form a nitride and has the function of hardening steel
by precipitation hardening. When the content is high, more than
0.0050%, not only a reduction in ductility but also slab cracking
during hot rolling are caused and many surface flaws may possibly
be caused. Therefore, N is 0.0050% or less. It is preferably
0.0030% or less and more preferably 0.0020% or less.
Ti: 0.021% to 0.060%
[0042] Ti is an element which fixes N in the form of a nitride and
has the function of suppressing hardening and aging deterioration
due to solute N. 0.021% or more needs to be contained to achieve
such effects. However, when the content is high, more than 0.060%,
the precipitation of carbides is promoted and the amount of solute
C is reduced. Hence, production of coarse B precipitates containing
C and Fe is suppressed. Therefore, a desired reduction in
proportional limit cannot be achieved. Thus, Ti is 0.021% to
0.060%. It is preferably 0.050% or less.
B: 0.0005% to 0.0050%
[0043] B is an important element and forms coarse B precipitates to
contribute to a reduction in proportional limit. 0.0005% or more
needs to be contained to achieve such an effect. However, when the
content is high, more than 0.0050%, slab cracking is caused.
Therefore, B is 0.0005% to 0.0050%. It is preferably 0.0010% or
more, more preferably 0.0020% or more, and further more preferably
0.0030% or more.
B/C: 0.5 or More
[0044] C and B are contained in the above ranges and the contents
of C and B are adjusted such that the ratio, B/C, of the content of
B to the content of C satisfies 0.5 or more. When B/C is less than
0.5, it is difficult to form coarse B precipitates. Therefore, B/C
is limited to 0.5 or more. Incidentally, it is preferably 1.0 or
more, more preferably 1.5 or more, and further more preferably 2.0
or more.
[0045] The above components are fundamental components. 0.009% or
less Nb and/or 0.06% or less Cr may be contained as a selective
element in addition to the fundamental components as required.
Nb: 0.009% or Less
[0046] Nb, as well as Ti, is an element which combines with N to
form a nitride, which fixes N, which suppresses hardening and aging
deterioration due to solute N, and contributes to enhancement of
shape fixability and may be contained as required. 0.001% or more
is preferably contained to achieve such effects. However, the
content is high, more than 0.009%, grains become fine. Therefore,
when Nb is contained, Nb is preferably 0.009% or less.
Cr: 0.06% or Less
[0047] Cr is an element which destabilizes C in a solid solution to
promote production of coarse B precipitates containing C and may be
contained as required. 0.001% or more is preferably contained to
achieve such an effect. However, when the content of Cr is high,
more than 0.06%, the production of the coarse B precipitates
containing C is inhibited instead. Therefore, when Cr is contained,
Cr is preferably 0.06% or less. The remainder other than the above
components are Fe and incidental impurities.
[0048] Next, reasons for limiting the microstructure of the
cold-rolled steel sheet are described.
[0049] The cold-rolled steel sheet has a microstructure dominated
by ferrite with an average grain size of 10 .mu.m to 30 .mu.m. The
microstructure dominated by ferrite allows the steel sheet to be
soft and therefore allows workability thereof to be enhanced. The
term "microstructure dominated by ferrite" as used herein refers to
a microstructure in which ferrite (polygonal ferrite) accounts for
95% or more, and more preferably 100%, in terms of area fraction. A
secondary phase other than ferrite is preferably cementite or
bainite. If the average grain size of ferrite is 10 .mu.m or more,
the concentration of strain at grain boundaries can be suppressed,
strain can be concentrated around precipitates, and the
proportional limit can be reduced. However, when the average grain
size of ferrite is large, more than 30 .mu.m, surface markings such
as orange peeling become obvious during press working. Therefore,
the average grain size of ferrite is 10 .mu.m to 30 .mu.m. It is
preferably 15 .mu.m to 25 .mu.m.
[0050] Next, a preferred method of manufacturing the cold-rolled
steel sheet is described.
[0051] A steel material (slab) with the above composition is used
as a starting material.
[0052] A method of manufacturing the steel material is not
particularly limited. Molten steel with the above composition is
preferably produced in a regular converter, an electric furnace, or
the like and is then solidified into a slab (steel material) by a
continuous casting process or an ingot casting-blooming process. If
the slab is manufactured by continuous casting, the slab is
preferably directly hot-rolled without cooling the slab to room
temperature when having heat sufficient for hot rolling.
Alternatively, the slab is preferably hot-rolled after the slab is
temporally charged into a furnace and is heat-retained or the slab
is cooled to room temperature and is then reheated to a temperature
of 1,100.degree. C. to 1,250.degree. C. by charging the slab into a
furnace.
[0053] The heated steel material is subjected to a hot rolling
step.
[0054] In the hot rolling step, hot rolling including rough rolling
and finish rolling is performed and coiling is then performed.
[0055] In rough rolling, conditions are not particularly limited as
far as a sheet bar having a desired size and shape is obtained.
Next, the sheet bar is finish-rolled, whereby a hot-rolled sheet is
obtained.
[0056] Finish rolling is performed at a finishing delivery
temperature of 870.degree. C. to 950.degree. C.
[0057] When the finishing delivery temperature is low, lower than
870.degree. C., the microstructure is transformed from austenite
into ferrite in the course of rolling and therefore it is difficult
to control the load of a rolling machine. Hence, the risk of
causing fracture or the like during processing increases.
Incidentally, if rolling is performed from the finishing entry side
in a ferrite region, the fracture or the like during processing can
be avoided. However, there is a problem in that the microstructure
of the hot-rolled sheet is transformed into unrecrystallized
ferrite because of the decrease of the rolling temperature and
therefore the load for cold rolling is increased. On the other
hand, when the finishing delivery temperature is high, higher than
950.degree. C., the hot-rolled sheet has a large ferrite grain
size. Therefore, a cold-rolled annealed sheet has an excessively
large ferrite grain size. Thus, the finishing delivery temperature
is 870.degree. C. to 950.degree. C. After finish rolling is
completed, the hot-rolled sheet is coiled. Cooling until coiling
after finish rolling is not particularly limited and it is
sufficient that the rate of cooling is higher than that of air
cooling. There is no particular problem even if quenching is
performed at 100.degree. C./s or more as required.
[0058] The coiling temperature after the completion of finish
rolling is 450.degree. C. to 630.degree. C.
[0059] When the coiling temperature is lower than 450.degree. C.,
acicular ferrite is produced and a steel sheet is hardened. Hence,
the load for subsequent cold rolling is increased and, also, leads
to the difficulty in operating hot rolling. However, when the
coiling temperature is high, higher than 630.degree. C., the
precipitation of carbides is promoted, the amount of solute C is
reduced and, therefore, a desired amount of solute C cannot be
ensured during hot rolling process. Thus, the coiling temperature
is 450.degree. C. to 630.degree. C.
[0060] The coiled hot-rolled sheet is subjected to an ordinary
pickling step and then subjected to a cold-rolling step, whereby a
cold-rolled sheet is obtained.
[0061] In the cold-rolling step, the cold-rolled sheet is obtained
by performing cold rolling at a cold-rolling reduction of 90% or
less.
[0062] When the cold-rolling reduction is large, more than 90%,
recrystallized ferrite grains after annealing become fine. At the
same time, the load for cold rolling is increased, leading to
difficulty in operating cold rolling. Thus, the cold-rolling
reduction is limited to 90% or less. It is preferably 80% or less.
The lower limit of the cold-rolling reduction is not particularly
limited. However, when the cold rolling reduction is low, the
thickness of the hot-rolled sheet needs to be reduced with respect
to the predetermined thickness of products and, therefore,
productivity of hot rolling and pickling is reduced. Hence, the
cold-rolling reduction is preferably 50% or more.
[0063] The cold-rolled sheet is subjected to an annealing step,
whereby a cold-rolled annealed sheet is obtained.
[0064] The annealing step is a step in which heating is performed
up to a holding temperature of 700.degree. C. to 850.degree. C. at
an average heating rate of 1.degree. C./s to 30.degree. C./s in a
temperature region not lower than 600.degree. C., retention is
performed at the holding temperature for 30 s to 200 s, and cooling
is then performed at a cooling rate of 3.degree. C./s or more down
to 600.degree. C. or lower. In the annealing step, cold-rolled
worked ferrite is recrystallized to have a desired average grain
size and coarse B precipitates containing C and Fe are distributed
at grain boundaries and in grains. Heating rate: 1.degree. C./s to
30.degree. C./s
[0065] When the average heating rate in a temperature region
ranging from 600.degree. C. to the holding temperature is less than
1.degree. C./s, ferrite grains grow significantly and therefore
ferrite with a desired average grain size cannot be obtained.
However, when the heating rate is high, more than 30.degree. C./s,
TiC is precipitated during heating instead of the production of B
precipitates and therefore it is difficult to form desired coarse B
precipitates. Thus, the average heating rate in a temperature
region not lower than 600.degree. C. is limited to 1.degree. C./s
to 30.degree. C./s. It is preferably 5.degree. C./s or more and
more preferably 10.degree. C./s or more.
Holding Temperature: 700.degree. C. to 850.degree. C.
[0066] In the annealing step, the holding temperature is
700.degree. C. or higher because the recrystallization of
cold-worked ferrite needs to be completed. However, when the
holding temperature is high, higher than 850.degree. C., ferrite
grains become coarse and therefore ferrite with a desired average
grain size cannot be obtained. Thus, the holding temperature is
700.degree. C. to 850.degree. C.
Holding Time: 30 s to 200 s
[0067] The holding time is 30 s or more to complete the
recrystallization of cold-worked ferrite. When the holding time is
short, the recrystallization thereof is not completed or ferrite
grains remain fine. However, when the holding time is long, more
than 200 s, ferrite grains grow excessively. Thus, the holding time
is of 30 s to 200 s.
Cooling Rate: 3.degree. C./s or More
[0068] Growth of ferrite grains is promoted when the cooling rate
after holding is low. Thus, the average cooling rate in a
temperature region ranging from the holding temperature to
600.degree. C. is 3.degree. C./s or more. The upper limit of the
cooling rate need not be particularly limited and is determined
depending on the capacity of a cooling facility. In ordinary
cooling facilities, the upper limit of the cooling rate is about
30.degree. C./s.
[0069] Coarsening of the microstructure due to growth of ferrite
grains can be suppressed by cooling to 600.degree. C., whereby a
microstructure dominated by ferrite with a desired average grain
size can be obtained. Conditions for cooling to 600.degree. C. or
less need not be particularly limited and arbitrary cooling is not
particularly problematic.
[0070] After cooling is stopped, galvanizing may be performed at
about 480.degree. C. as required. After galvanizing, galvannealing
may be performed by reheating to 500.degree. C. or higher. Thermal
history including retention during cooling may be performed.
Furthermore, temper rolling may be performed at about 0.5% to 2% as
required. When not performing plating, electrogalvanizing may be
performed for the purpose of enhancing corrosion resistance.
Furthermore, a coating may be provided on the cold-rolled steel
sheet or a plated steel sheet using chemical conversion or the
like.
[0071] Our steel sheets and methods are further described below in
detail on the basis of examples.
Examples
[0072] First, experiment results underlying our steel sheets and
methods are described.
[0073] Steel materials (slabs) having a composition containing
0.0010% to 0.035% C, 0.01% to 0.03% Si, 0.10% to 0.45% Mn, 0.03% to
0.08% Al, 0.022% to 0.060% Ti, 0.0003% to 0.0048% B, and 0.0015% to
0.0040% N on a mass basis were subjected to hot rolling and cold
rolling and further subjected to annealing under various heating,
holding, and cooling conditions, whereby cold-rolled annealed
sheets were obtained.
[0074] A JIS #5 test specimen was taken from each obtained
cold-rolled annealed sheet such that a tensile direction coincided
with a rolling direction, followed by determining the proportional
limit thereof. A 5 mm strain gauge was attached to a parallel
portion of the tensile test specimen and tensile testing was
performed at a cross head speed of 1 mm/min. The stress at which
the slope of the stress-strain curve thereof began to decrease was
defined as the proportional limit thereof.
[0075] A test specimen (a size of 120 mm.times.120 mm) was taken
from each obtained cold-rolled annealed sheet and then punch
stretch formed. Punch stretch forming was performed by press
forming such that a central portion of the test specimen was
stretched by 8 mm using a spherical punch with a diameter of 20 mm.
In punch stretch forming, a region (hatched portion) with a
diameter of 28 mm to 54 mm was pressed with a load of 100 kN and
formed as shown in FIG. 1. Next, as shown in FIG. 2, the formed
test specimen was placed on a platen and a flange portion thereof
was measured for maximum strain height. Observation of the obtained
cold-rolled annealed sheets showed that all the cold-rolled
annealed sheets had a microstructure dominated by ferrite.
[0076] The obtained results are shown in FIGS. 3 and 4. FIG. 3
shows the relationship between the proportional limit and maximum
strain height of each flange portion. FIG. 4 shows the relationship
between B/C and the proportional limit.
[0077] As is clear from FIG. 3, as the proportional limit exceeds
100 MPa, the maximum strain height of the flange portion increases
sharply. As is clear from FIG. 4, to adjust the proportional limit
to 100 MPa or less, B/C needs to be 0.5 or more.
[0078] From this, we found that the shape fixability of a pressed
part is increased and particularly the strain of a flat portion of
a formed member is significantly reduced by using a steel sheet
having a composition which essentially contains Ti and B and in
which B/C is 0.5 or more, a microstructure dominated by ferrite,
and a proportional limit of 100 MPa or less as material. We also
found that it is effective in enhancing shape fixability that hot
rolling conditions are controlled such that C forms a solid
solution, cold rolling is performed, and coarse B precipitates
containing C and Fe are formed at grain boundaries and also in
grains during annealing. We further found that, in such a
microstructure, distributed coarse B precipitates adequately anchor
dislocations during press forming to concentrate strain around the
precipitates and suppress intertwining of dislocations by
preventing the dislocations from gathering at grain boundaries,
whereby springback is significantly reduced, the proportional limit
is reduced, and shape fixability is remarkably enhanced.
[0079] Then, in particular Examples, steel materials (slabs) having
a chemical composition shown in Table 1 were used as starting
materials. After the slabs were heated to 1,200.degree. C., the
slabs were subjected to a hot-rolling step, a pickling step, a
cold-rolling step, and an annealing step in that order, whereby
cold-rolled annealed sheets were obtained. In the hot-rolling step,
each steel material was roughly rolled into a sheet bar and the
sheet bar was finish-rolled at a finishing delivery temperature
equal to a temperature (FT) shown in Table 2 and was then coiled at
a coiling temperature (CT) shown in Table 2, whereby a hot-rolled
sheet with a thickness shown in Table 2. Next, after the hot-rolled
sheet was subjected to the pickling step, the hot-rolled sheet was
subjected to cold rolling at a cold-rolling reduction shown in
Table 2, whereby a cold-rolled sheet with a thickness shown in
Table 2 was obtained.
[0080] Next, the cold-rolled sheet was subjected to the annealing
step, whereby a cold-rolled annealed sheet was obtained. In the
annealing step, annealing was performed at a heating rate, a
holding temperature, a holding time, and a cooling rate as shown in
Table 2. Cooling from 600.degree. C. or lower to room temperature
was performed at a similar cooling rate. After the annealing step
was performed, temper rolling was performed at a rolling reduction
of 1.0%.
[0081] The obtained cold-rolled annealed sheets (cold-rolled steel
sheets) were subjected to microstructure observation, a tensile
test, and a punch stretch forming test. Testing methods were as
described below.
(1) Microstructure Observation
[0082] A test specimen for microstructure observation was taken
from each obtained cold-rolled annealed sheet; a cross section
(L-cross section) in a rolling direction was polished and etched;
the microstructure thereof was observed and photographed using an
optical microscope (a magnification of 100 times) and a scanning
electron microscope (a magnification of 1,000 times); and the
average grain size of ferrite, the fraction of ferrite, and the
type and fraction of a secondary phase were determined by image
analysis. For ferrite, the average intercept length of ferrite
grains in a 300 .mu.m.times.300 .mu.m region was determined in the
rolling and thickness directions and the value of 2/(1/A+1/B) was
defined as the average grain size, where A is the average intercept
length of the ferrite grains in the rolling direction and B is the
average intercept length of the ferrite grains in the thickness
direction. The fraction of ferrite was measured in a 300
.mu.m.times.300 .mu.m region.
(2) Tensile Test
[0083] A JIS #5 test specimen was taken from each obtained
cold-rolled annealed sheet such that a tensile direction coincided
with the rolling direction, followed by determining the
proportional limit thereof. A strain gauge was attached to a
parallel portion of the tensile test specimen and tensile testing
was performed at a cross head speed of 1 mm/min, whereby tensile
properties (proportional limit, tensile strength, and elongation)
were determined. Incidentally, the proportional limit was defined
as the stress at which the slope of the stress-strain curve thereof
began to decrease.
(3) Punch Stretch Forming Test
[0084] A test specimen (a size of 120 mm.times.120 mm) was taken
from each obtained cold-rolled annealed sheet and was then punch
stretch formed. Punch stretch forming was performed by press
forming such that a central portion of the test specimen was
stretched by 8 mm using a spherical punch with a diameter of 20 mm.
In punch stretch forming, a region (hatched portion) with a
diameter of 28 mm to 54 mm was depressed with a load of 100 kN and
formed as shown in FIG. 1. After forming, as shown in FIG. 2, the
test specimen was placed on a platen and a flange portion thereof
was measured for maximum strain height. Obtained results are shown
in Table 3.
TABLE-US-00001 TABLE 1 Chemical components (weight percent) Steel
Material ID C Si Mn P S Al N Ti B Nb Cr B/C Remarks A 0.0015 0.01
0.15 0.01 0.01 0.03 0.0020 0.040 0.0029 -- -- 1.9 Adequate Example
B 0.0013 0.03 0.35 0.04 0.01 0.05 0.0040 0.022 0.0018 0.005 1.4
Adequate Example C 0.0016 0.02 0.45 0.02 0.02 0.08 0.0030 0.058
0.0009 0.008 0.01 0.6 Adequate Example D 0.0028 0.05 0.25 0.01 0.01
0.04 0.0020 0.035 0.0048 -- 0.05 1.7 Adequate Example E 0.0012 0.01
0.15 0.01 0.01 0.05 0.0015 0.031 0.0025 -- -- 2.1 Adequate Example
F 0.0013 0.01 0.15 0.01 0.01 0.04 0.0025 0.055 0.0035 -- 0.01 2.7
Adequate Example G 0.0012 0.01 0.10 0.01 0.01 0.05 0.0015 0.060
0.0040 -- -- 3.3 Adequate Example H 0.0025 0.01 0.10 0.01 0.01 0.04
0.0020 0.035 0.0023 -- -- 0.9 Adequate Eample I 0.0015 0.01 0.15
0.01 0.01 0.05 0.0020 0.045 0.0008 -- -- 0.5 Adequate Example J
0.0035 0.02 0.25 0.02 0.01 0.05 0.0025 0.032 0.0015 -- -- 0.4
Comparative Example K 0.0010 0.01 0.20 0.02 0.01 0.06 0.0021 0.025
0.0003 -- -- 0.3 Comparative Example L 0.0020 0.01 0.18 0.01 0.02
0.05 0.0023 0.035 0.0010 0.003 -- 0.5 Adequate Example M 0.0011
0.02 0.15 0.02 0.01 0.04 0.0030 0.030 0.0020 -- -- 1.8 Adequate
Example N 0.0025 0.02 0.20 0.01 0.01 0.04 0.0030 0.040 0.0020 -- --
0.8 Adequate Example O 0.0015 0.01 0.15 0.01 0.01 0.04 0.0030 0.005
0.0030 -- -- 2.0 Comparative Example
TABLE-US-00002 TABLE 2 Hot-rolling step Cold-rolling step Annealing
step Finishing Cold- Heating Steel Steel Heating delivery Coiling
Thick- rolling Thick- rate Holding Holding Cooling sheet Material
temperature temperature temperature ness reduction ness (.degree.
C./ temperature time rate ID No. (.degree. C.) (.degree. C.)
(.degree. C.) (mm) (%) (mm) s)* (.degree. C.) (s) (.degree. C./s)**
Remarks 1 A 1200 890 560 2.5 76 0.6 11 770 130 20 Example 2 B 1200
920 620 2.7 78 0.6 6 720 40 5 Example 3 C 1200 940 460 1.5 60 0.6 3
840 180 12 Example 4 D 1200 900 500 1.3 55 0.6 20 780 80 25 Example
5 E 1200 890 600 2.0 70 0.6 28 800 100 15 Example 6 F 1200 930 580
2.4 75 0.6 15 830 150 10 Example 7 G 1200 920 570 2.9 79 0.6 12 850
180 8 Example 8 H 1200 910 580 2.4 75 0.6 10 800 150 10 Example 9 I
1200 890 560 2.7 78 0.6 10 800 130 18 Example 10 J 1200 890 600 2.4
75 0.6 12 830 130 10 Comparative Example 11 K 1200 880 590 2.5 76
0.6 10 820 120 11 Comparative Example 12 L 1200 910 650 2.7 78 0.6
15 800 140 15 Comparative Example 13 M 1200 890 590 2.4 75 0.6 0.4
860 150 10 Comparative Example 14 N 1200 880 560 2.2 73 0.6 12 750
20 15 Comparative Example 15 O 1200 890 560 2.4 75 0.6 10 750 100
15 Comparative Example *Average in a temperature region not lower
than 600.degree. C. **Average from a holding temperature to
600.degree. C.
TABLE-US-00003 TABLE 3 Steel Microstructure Tensile properties
Shape fixability sheet Ferrite Proportional limit Tensile strength
Elongation Maximum strain No. Type* Average grain size (.mu.m)
Fraction (area percent) (MPa) TS (MPa) El (%) height (mm) Remarks 1
F 16 100 80 330 50 0.4 Example 2 F 12 100 85 340 49 0.6 Example 3 F
+ C 11 98 100 350 48 0.7 Example 4 F 13 100 80 355 47 0.5 Example 5
F 16 100 70 320 51 0.3 Example 6 F 23 100 50 310 51 0.2 Example 7 F
28 100 40 300 52 0.2 Example 8 F 12 100 95 330 50 0.7 Example 9 F
13 100 100 320 51 0.8 Example 10 F 10 100 125 360 46 2.0
Comparative Example 11 F 12 100 130 320 51 2.2 Comparative Example
12 F 11 100 120 340 49 1.9 Comparative Example 13 F 35 100 100 290
53 0.8 Comparative Example 14 F + C 8 97 130 330 50 2.3 Comparative
Example 15 F 15 100 140 340 48 2.4 Comparative Example *F
represents ferrite, C represents cementite, and B represents
bainite.
[0085] In all our Examples, cold-rolled steel sheets have excellent
shape fixability with a low proportional limit of 100 MPa or less
and flat portions of punch stretch formed members having a maximum
strain height of 0.8 mm or less. However, in Comparative Examples
which are outside our scope, the proportional limit is more than
100 MPa or the maximum strain height is large, more than 0.8 mm,
and shape fixability is low.
* * * * *