U.S. patent application number 10/630479 was filed with the patent office on 2004-02-05 for high strength cold rolled steel sheet and method for manufacturing the same.
This patent application is currently assigned to NKK CORPORATION. Invention is credited to Fujita, Takeshi, Hasegawa, Kohei, Hosoya, Yoshihiro, Inazumi, Toru, Kitano, Fusato, Matsuda, Hiroshi, Morita, Masaya, Nagataki, Yasunobu, Ono, Moriaki, Yamasaki, Yuji.
Application Number | 20040020570 10/630479 |
Document ID | / |
Family ID | 27564422 |
Filed Date | 2004-02-05 |
United States Patent
Application |
20040020570 |
Kind Code |
A1 |
Fujita, Takeshi ; et
al. |
February 5, 2004 |
High strength cold rolled steel sheet and method for manufacturing
the same
Abstract
A high strength cold rolled steel sheet consists essentially of
0.0040 to 0.01% C, 0.05% or less Si, 0.1 to 1.0% Mn, 0.01 to 0.05%
P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.01 to
0.14% Nb, optionally 0.05% or less Ti, optionally 0.05% or less B,
by weight, and a balance of substantially Fe and inevitable
impurities, and satisfying the following formulae (6) and (7):
(12/93).times.Nb*/C.gtoreq.1.2 (6),
TS-4050.times.Ceq.gtoreq.-0.75.times.TS+380 (7), wherein
Nb*=Nb-(93/14).times.N, Ceq=C+(1/50).times.Si+(1/25).times.Mn+(1/2-
).times.P, wherein TS is the tensile strength in MPa, and C, Si,
Mn, P, N and Nb denote the content in % by weight of carbon,
silicon, manganese, phosphorus, nitrogen, and niobium,
respectively. The high strength cold rolled steel sheet has
excellent combined formability, resistance to embrittlement during
secondary operation, formability at welded portions, and
anti-burring performance, and has a desirable surface appearance
and uniformity of material in a coil, and thus can be desirably
used for automobile exterior panels.
Inventors: |
Fujita, Takeshi; (Fukuyama,
JP) ; Kitano, Fusato; (Fukuyama, JP) ; Hosoya,
Yoshihiro; (Fukuyama, JP) ; Inazumi, Toru;
(Fukuyama, JP) ; Yamasaki, Yuji; (Fukuyama,
JP) ; Morita, Masaya; (Fukuyama, JP) ;
Nagataki, Yasunobu; (Fukuyama, JP) ; Hasegawa,
Kohei; (Fukuyama, JP) ; Matsuda, Hiroshi;
(Fukuyama, JP) ; Ono, Moriaki; (Fukuyama,
JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN & CHICK, PC
767 THIRD AVENUE
25TH FLOOR
NEW YORK
NY
10017-2023
US
|
Assignee: |
NKK CORPORATION
Tokyo
JP
|
Family ID: |
27564422 |
Appl. No.: |
10/630479 |
Filed: |
July 29, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10630479 |
Jul 29, 2003 |
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10122860 |
Apr 15, 2002 |
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10122860 |
Apr 15, 2002 |
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09631600 |
Aug 3, 2000 |
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6494969 |
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09631600 |
Aug 3, 2000 |
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PCT/JP99/06791 |
Dec 3, 1999 |
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Current U.S.
Class: |
148/541 |
Current CPC
Class: |
C22C 38/002 20130101;
C22C 38/02 20130101; C22C 38/004 20130101; C21D 8/0273 20130101;
C22C 38/04 20130101; C22C 38/12 20130101; C21D 8/0236 20130101;
C21D 8/0226 20130101; C22C 38/06 20130101 |
Class at
Publication: |
148/541 |
International
Class: |
C21D 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 7, 1998 |
JP |
10-346974 |
Feb 15, 1999 |
JP |
11-036283 |
Feb 15, 1999 |
JP |
11-036284 |
Feb 15, 1999 |
JP |
11-036285 |
Feb 15, 1999 |
JP |
11-036286 |
Feb 15, 1999 |
JP |
11-036287 |
Feb 15, 1999 |
JP |
11-036288 |
Claims
What is claimed is:
1. A high strength cold rolled steel sheet consisting essentially
of 0.0040 to 0.01% C, 0.05% or less Si, 0.1 to 1.0% Mn, 0.01 to
0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N,
0.01 to 0.14% Nb, and optionally 0.05% or less Ti, optionally
0.002% or less B, by weight, and a balance of substantially Fe and
inevitable impurities; and satisfying the following formulae (6)
and (7): (12/93).times.Nb*/C.gtoreq- .1.2 (6)
TS-4050.times.Ceq.gtoreq.-0.75.times.TS+380 (7), wherein
Nb*=Nb-(93/14).times.N,
Ceq=C+(1/50).times.Si+(1/25).times.Mn+(1/2).times- .P, TS denotes
the tensile strength in MPa, and C, Si, Mn, P, N, and Nb denote the
content in % by weight of carbon, silicon, manganese, phosphorus,
nitrogen and niobium, respectively.
2. The high strength steel sheet of claim 1, further containing
0.05% or less Ti, by weight.
3. The high strength steel sheet of claim 1, further containing
0.002% or less B, by weight.
4. A high strength cold rolled steel sheet consisting essentially
of: 0.0040 to 0.01% C, 0.05% or less P, 0.02% or less S, 0.01 to
0.1% sol.Al, 0.004% or less N, 0.03% or less Ti, by weight, and Nb
in an amount satisfying the following formula (8):
1.ltoreq.(93/12).times.(Nb/C).ltore- q.2.5 (8), wherein C and Nb
denote the content in % by weight of carbon and niobium,
respectively; the high strength cold rolled steel sheet having a
volumetric proportion of NbC of 0.03 to 0.10%; and 70% or more
thereof being in a size of 10 to 40 nm.
5. A method for manufacturing a high strength cold rolled steel
sheet, comprising the steps of: (a) preparing a continuous casting
slab of a steel which consists essentially of 0.0040 to 0.01% C,
0.05% or less P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or
less N, 0.03% or less Ti, by weight, and Nb in an amount satisfying
the following formula (8): 1.ltoreq.(93/12).times.(Nb/C).ltoreq.2.5
(8), wherein C and Nb denote the content in % by weight of carbon
and niobium, respectively; (b) preparing a hot rolled steel sheet
by finish rolling the slab from step (a) at reduction ratios
satisfying the following formulae (9) through (11): 10.ltoreq.HR1
(9) 2.ltoreq.HR2.ltoreq.30 (10) HR1+HR2-HR1.times.HR2/100.ltoreq.60
(11), wherein HR1 and HR2 denote the reduction ratio in % in the
finish rolling at the pass just before the final pass and the final
pass, respectively; (c) cold rolling the hot rolled sheet from step
(b) and (d) annealing the sheet from step (c).
6. A high strength steel sheet consisting essentially of 0.0040 to
0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02
or less S, 0.01% to 0.1% sol.Al, 0.00100% or less N, 0.036 to 0.14%
Nb, and optionally containing 0.0015% or less B, by weight, and
satisfying the following formula (12):
1.1<(Nb.times.12)/(C.times.93)<2.5 (12), wherein C and Nb
denote the content in % by weight of carbon and niobium,
respectively; the high strength steel sheet having an average grain
size of 10 .mu.m or less and an r value of 1.8 or more.
7. The high strength steel sheet of claim 6, further containing
0.0015% or less B, by weight.
8. The high strength steel sheet of claim 6, further containing
0.019% or less Ti, by weight, and satisfying the following formula
(13): Ti.ltoreq.(48/14).times.N+(48/32).times.S (13), wherein N, S,
and Ti denote the content in % by weight of nitrogen, sulfur, and
titanium, respectively.
9. The high strength steel sheet of claim 8, further containing
0.0015% or less B, by weight.
10. A method for manufacturing a high strength cold rolled steel
sheet, comprising the steps of: (a) preparing a continuous casting
slab of a steel which consists essentially of 0.0040 to 0.010% C,
0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02% or less
S, 0.01 to 0.1% sol.Al, 0.0100% or less N, 0.036 to 0.14% Nb, by
weight, and which satisfies the following formula (12):
1.1<(Nb.times.12)/(C.times.93)<2.5 (12), wherein C and Nb
denote the content in % by weight of carbon and niobium,
respectively; (b) preparing a sheet bar by direct rolling or
heating the slab from step (a) to a temperature of from 1100 to
1250.degree. C. followed by rough rolling; (c) finish rolling the
sheet bar from step (b) to a total reduction ratio of the pass just
before the final pass and the final pass to produce a hot rolled
steel sheet of 10 to 40%; (d) coiling the hot rolled steel sheet
from step (c) at a cooling speed of 15.degree. C./sec or more to a
temperature below 700.degree. C., followed by coiling at a
temperature of from 620 to 610.degree. C.; (e) cold rolling the
coiled hot rolled steel sheet from step (d) at a reduction ratio of
50% or more, followed by heating the steel sheet at a heating speed
of 20.degree. C./sec or more; (f) annealing the steel sheet from
step (e) at a temperature between 860.degree. C. and an Ar3
transformation temperature, and (g) temper rolling the annealed
steel sheet from step (f) at a reduction ratio of 0.4 to 1.0%.
11. A high strength cold rolled steel sheet consisting essentially
of more than 0.0050% and not more than 0.010% C, 0.05% or less Si,
0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1%
sol.Al, 0.004% or less N, 0.01 to 0.20% Nb, by weight, and
satisfying the following formulae (3), (4) and (14):
11.0.ltoreq.r+50.0.times.n (3) 2.9.ltoreq.r+5.00.times.n (4)
1.98-66.3.times.C.ltoreq.(Nb.times.12)/(C.-
times.93).ltoreq.3.24-80.0.times.C (14), wherein r denotes the r
value, n denotes the n value at 1 to 5% strain, and C and Nb denote
the content in % by weight of carbon and niobium, respectively.
12. The high strength steel sheet of claim 11, further containing
0.002% or less B, by weight.
13. A high strength cold rolled steel sheet containing essentially
of more than 0.0050% and not more than 0.010% C, 0.05% or less Si,
0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1%
sol.Al, 0.004% or less N, 0.01 to 0.20% Nb, 0.05% or less Ti, and
optionally 0.002% or less B, by weight, and satisfying the
following formulae (3), (4) and (15): 11.0.ltoreq.r+50.0.times.n
(3) 2.9.ltoreq.r+5.00.times.n (4)
1.98-66.3.times.C.ltoreq.(Nb.times.12)/(C.times.93)+(Ti*.times.12)/(C.tim-
es.48).ltoreq.3.24-80.0.times.C (15), wherein r denotes the r
value, n denotes the n value at 1 to 5% strain, Ti* is not more
than 0, and C, S, N, Nb, and Ti denote the content in % by weight
of carbon, nitrogen, niobium, and titanium, respectively.
14. The high strength steel sheet of claim 13, further containing
0.002% or less B, by weight.
15. A method for manufacturing a high strength cold rolled steel
sheet, comprising the steps of: (a) preparing a continuous casting
slab of a steel which consists essentially of more than 0.0050% and
not more than 0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to
0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N,
0.01 to 0.20% Nb, by weight, and which satisfies the following
formula (14):
1.98.times.66.3.times.C.ltoreq.(Nb.times.12)/(C.times.93).ltoreq.3.24-80.-
0.times.C (14), wherein C and Nb denote the content in % by weight
of carbon and niobium, respectively; (b) preparing a coiled hot
rolled steel sheet by finish rolling the slab from step (a) at a
total reduction ratio of the pass just before the final pass and
the final pass of 60% or less; (c) cold rolling the hot rolled
steel sheet from step (b) and (d) annealing the sheet from step
(c).
16. The method for manufacturing a high strength steel sheet of
claim 15, wherein the finish rolling is conducted at a temperature
of 870.degree. C. or higher, the coiling is conducted at a
temperature of 550.degree. C. or higher, the cold rolling is
conducted at a rolling reduction ratio of 50 to 85%, and the
continuous annealing is conducted at a temperature of 780 to
880.degree. C.
17. A method for manufacturing a high strength cold rolled steel
sheet, comprising the steps of: (a) preparing a continuous casting
slab of a steel which consists essentially of more than 0.0050% and
not more than 0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to
0.05% P, 0.02% or less S, 0.01 to 0.1 sol.Al, 0.004% or less N,
0.01 to 0.20% Nb, 0.05% or less Ti, by weight, and which satisfies
the following formula (15):
1.98.times.66.3.times.C.ltoreq.(Nb.times.12)/(C.times.93)+(Ti*.times.12)/-
(C.times.48).ltoreq.3.24-80.0.times.C (15), wherein
Ti*=Ti-(48/14).times.N-(48/32).times.S, Ti*=0 when Ti* is not more
than 0, and C, S, N, Nb, and Ti denote the content in % by weight
of carbon, nitrogen, niobium, and titanium, respectively; (b)
preparing a coiled hot rolled steel sheet by finish rolling the
slab from step (a) to a total reduction ratio of the pass just
before the final pass and the final pass of 60% or less; (c) cold
rolling the hot rolled steel sheet from step (b) and (d) followed
by annealing the sheet from step (c).
18. The method of manufacturing a high strength steel sheet of
claim 17, wherein the finish rolling is conducted at a temperature
of 870.degree. C. or higher, the coiling is conducted at a
temperature of 550.degree. C. or higher, the cold rolling is
conducted at a rolling reduction ratio of 50 to 85%, and the
continuous annealing is conducted at a temperature of 780 to
880.degree. C.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of application
Ser. No. 10/122,860 filed Apr. 15, 2002, which is a divisional
application of application Ser. No. 09/631,600 filed Aug. 3, 2000
(now U.S. Pat. No. 6,494,969) which is a continuation of
International application PCT/JP99/06791 filed Dec. 3, 1999.
TECHNICAL FIELD
[0002] The present invention relates to a high strength cold rolled
steel sheet having 340 to 440 MPa of tensile strength, which is
used for automobile exterior panels such as hoods, fenders, and
side panels, and to a method for manufacturing thereof.
BACKGROUND ART
[0003] Steel sheets used for automobile exterior panels such as
hoods, fenders, and side panels have recently often adopted high
strength cold rolled steel sheets aiming at improved safety and
mileage.
[0004] That kind of high strength cold rolled steel sheets are
requested to have combined formability characteristics such as
further improved deep drawability, punch stretchability, resistance
to surface strain (ability of not inducing nonuniform strain on a
formed surface) to make the steel sheets respond to the request for
reducing the number of parts and for labor saving in press stage
through the integration of parts.
[0005] To answer the request, recently there have been introduced
several kinds of high strength cold rolled steel sheets which use
very low carbon steels containing not more than 30 ppm of C as the
base material, with the addition of carbide-forming elements such
as Ti and Nb, and of solid-solution strengthening elements such as
Mn, Si, and P. For example, JP-A-112845(1993) (the term JP-A"
referred to herein signifies "Unexamined Japanese Patent
Publication"), discloses a steel sheet of very low carbon steel
specifying a lower limit of C content and adding positively Mn.
JP-A-263184(1993) discloses a steel sheet of very low carbon steel
adding a large amount of Mn, and further adding Ti or Nb.
JP-A-78784(1993) discloses a steel sheet of very low carbon steel
with the addition of Ti, further positively adding Mn, and
controlling the content of Si and P, thus providing a tensile
strength of 343 to 490 MPa. JP-A-46289(1993) and JP-A-195080(1998)
disclose steel sheets of very low carbon steels adjusting the C
content to 30 to 100 ppm, which content is a high level for very
low carbon steels, and further adding Ti.
[0006] The high strength cold rolled steel sheets prepared from
these very low carbon steels, however, fail to have excellent
characteristics of combined formability such as deep drawability,
punch stretchability, and resistance to surface strain. Thus, these
high strength cold rolled steel sheets are not satisfactory as the
steel sheets for automobile exterior panels. In particular, these
steel sheets are almost impossible to prevent the generation of
waving caused from surface strain which interferes with the image
sharpness after coating on the exterior panels.
[0007] Furthermore, to the high strength cold rolled steel sheets
used for automobile exterior panels, there have appeared strict
requests for, adding to the excellent combined formability,
excellent resistance to embrittlement during secondary operation,
formability of welded portions corresponding to tailored blank,
anti-burring performance under sheering, good surface appearance,
uniformity of material in steel coil when the steel sheets are
supplied in a form of coil, and other characteristics.
DISCLOSURE OF THE INVENTION
[0008] Following is the description of the high strength cold
rolled steel sheets according to the present invention, which have
excellent characteristics of: combined formability characteristics
including deep drawability, punch stretchability, and resistance to
surface strain; resistance to embrittlement during secondary
operation; formability at welded portions; anti-burring
performance; surface characteristics; and uniformity of material in
a coil.
[0009] Steel sheet 1 according to the present invention is a high
strength cold rolled steel sheet consisting essentially of 0.0040
to 0.010% C, 0.05% or less Si, 0.10 to 1.20% Mn, 0.01 to 0.05% P,
0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.003% or
less 0, 0.01 to 0.20% Nb, by weight; and satisfying the formulae
(1), (2), (3), and (4);
-0.46-0.83.times.log[C]
.ltoreq.(Nb.times.12)/(C.times.93).ltoreq.-0.88-1.- 66.times.log[C]
(1)
10.8.gtoreq.5.49.times.log[YP]-r (2)
11.0.ltoreq.r+50.0.times.n (3)
2.9.ltoreq.r+5.00.times.n (4)
[0010] where, C and Nb denote the content (% by weight) of C and
Nb, respectively, YP denotes the yield strength (MPa), r denotes
the r value (average of r values determined at 0, 45, and 90
degrees to the rolling direction), and n denotes the n value (a
value in a range of from 1 to 5% strain; average of n values
determined at 0, 45, and 90 degrees to the rolling direction).
[0011] The Steel sheet 1 is manufactured by the steps of: preparing
a continuous casting slab of the steel which has the composition
described above; preparing a hot rolled steel sheet by finish
rolling the slab at temperatures of Ar3 transformation temperature
or more; coiling the hot rolled steel sheet at temperatures not
less than 540.degree. C.; and cold rolling the coiled hot rolled
steel sheet at reduction ratios of from 50 to 85%, followed by
continuously annealing thereof at temperatures of from 680 to
880.degree. C.
[0012] Steel sheet 2 according to the present invention is a high
strength cold rolled steel sheet consisting essentially of 0.0040
to 0.01% C, 0.05% or less Si, 0.1 to 1.0% Mn, 0.01 to 0.05% P,
0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.01 to
0.14% Nb, by weight, and balance of substantially Fe and inevitable
impurities; and having 0.21 or more n value which is calculated
from two points of nominal strain, at 1% and 10%, observed in a
uniaxial tensile test.
[0013] Steel sheet 3 according to the present invention is a high
strength cold rolled steel sheet consisting essentially of 0.0040
to 0.01% C, 0.05% or less Si, 0.1 to 1.0% Mn, 0.01 to 0.05% P,
0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.15% or
less Nb, by weight, and balance of substantially Fe and inevitable
impurities; satisfying the formula (6); and having 0.21 or more n
value which is calculated from two points of nominal strain, at 1%
and 10%, observed in a uniaxial tensile test;
(12/93).times.Nb*/C.gtoreq.1.2 (6)
[0014] where, Nb*=Nb-(93/14).times.N, and C, N, and Nb denote the
content (% by weight) of C, N, and Nb, respectively.
[0015] The Steel sheet 3 is manufactured by the steps of: preparing
a continuous casting slab of a steel which has the composition
described above; preparing a hot rolled steel sheet by finish
rolling the slab at temperatures of Ar3 transformation temperature
or more; coiling the hot rolled steel sheet at temperatures of from
500 to 700.degree. C.; and cold rolling the coiled steel sheet,
followed by annealing thereof.
[0016] Steel sheet 4 according to the present invention is a high
strength cold rolled steel sheet consisting essentially of 0.0040
to 0.01% C, 0.05% or less Si, 0.1 to 1.0% Mn, 0.01 to 0.05% P,
0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.01 to
0.14% Nb, by weight, and balance of substantially Fe and inevitable
impurities; and satisfying the formulae (6) and (7);
(12/93).times.Nb*/C.gtoreq.1.2 (6)
TS-4050.times.Ceq.gtoreq.-0.75.times.TS+380 (7)
[0017] where, Ceq=C+(1/50).times.Si+(1/25).times.Mn+(1/2).times.P,
TS denotes the tensile strength (MPa), and C, Si, Mn, P, N, and Nb
denote the content (% by weight) of C, Si, Mn, P, N, and Nb,
respectively.
[0018] Steel sheet 5 according to the present invention is a high
strength cold rolled steel sheet consisting essentially of: 0.004
to 0.01% C, 0.05% or less P, 0.02% or less S, 0.01 to 0.1% sol.Al,
0.004% or less N, 0.03% or less Ti, by weight, and Nb as an amount
satisfying the formula (8); 0.03 to 0.1% of a volumetric proportion
of NbC; and 70% or more thereof being 10 to 40 nm in size;
1.ltoreq.(93/12).times.(Nb/C).ltoreq.2.5 (8)
[0019] where, C and Nb denote the content (% by weight) of C and
Nb, respectively.
[0020] The Steel sheet 5 is manufactured by the steps of: preparing
a continuous casting slab of a steel which has the composition
described above; preparing a hot rolled steel sheet by finish
rolling the slab at reduction ratios satisfying the formulae (9)
through (11); and cold rolling the hot rolled sheet, followed by
annealing thereof;
10.ltoreq.HR1 (9)
2.ltoreq.HR2.ltoreq.30 (10)
HR1+HR2-HR1.times.HR2/100.ltoreq.60 (11)
[0021] where, HR1 and HR2 denote the reduction ratio (%) in the
finish rolling at the pass just before the final pass and at the
final pass, respectively.
[0022] Steel sheet 6 according to the present invention is a high
strength cold rolled steel sheet consisting essentially of 0.0040
to 0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to 0.05% P,
0.02% or less Si, 0.01 to 0.1% sol.Al, 0.00100% or less N, 0.036 to
0.14% Nb, by weight; satisfying the formula (12); giving 10 .mu.m
or less average grain size and 1.8 or more r value:
1.1<(Nb.times.12)/(C.times.93)<2.5 (12)
[0023] wherein C and Nb denote the content (% by weight) of C and
Nb, respectively.
[0024] The steel sheet 6 is manufactured by the steps of: preparing
continuous casting slab of a steel which has the composition
described above; preparing a sheet bar by either directly rolling
the slab or heating the slab to temperatures of from 1100 to
1250.degree. C. followed by rough rolling; finish rolling the sheet
bar at 10 to 40% of the total reduction ratios of the pass just
before the final pass and the final pass to produce a hot rolled
steel sheet; coiling the hot rolled steel sheet at cooling speeds
of 15.degree. C./sec
[0025] or more to temperatures below 700.degree. C., followed by
coiling at temperatures of from 620 to 670.degree. C.; cold rolling
the coiled hot rolled steel sheet at 50% or more reduction ratios,
followed by heating the steel sheet at 20.degree. C./sec or more
heating speeds, then annealing the steel sheet at temperatures
between 860.degree. C. and Ac3 transformation temperature; and
temper rolling the annealed steel sheet at 0.4 to 1.0% reduction
ratios.
[0026] Steel sheet 7 according to the present invention is a high
strength cold rolled steel sheet consisting essentially of more
than 0.0050% and not more than 0.010% C, 0.05% or less Si, 0.10 to
1.5% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al,
0.004% or less N, 0.01 to 0.20% Nb, by weight; and satisfying the
formulae (3), (4), (14);
11.0.ltoreq.r+50.0.times.n (3)
2.9.ltoreq.r+5.00.times.n (4)
1.98-66.3.times.C.ltoreq.(Nb.times.12)/(C.times.93).ltoreq.3.24-80.0.times-
.C (14)
[0027] where, C and Nb denote the content (% by weight) of C and
Nb, respectively.
[0028] The Steel sheet 7 is manufactured by the steps of: preparing
a continuous casting slab of a steel which has the composition
described above; preparing a coiled hot rolled steel sheet by
finish rolling the slab at 60% or less total reduction ratios of
the pass just before the final pass and the final pass; cold
rolling the hot rolled steel sheet, followed by annealing
thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 shows the shape of a panel used for evaluation of the
resistance to surface strain.
[0030] FIG. 2 shows the influence of [(Nb.times.12)/(C.times.93)]
on the waving height difference (.DELTA.W.sub.ca) before and after
forming.
[0031] FIG. 3 shows the method of Yoshida buckling test.
[0032] FIG. 4 shows the influence of YP and r values on the plastic
buckling height (YBT).
[0033] FIG. 5 shows the method of Hat type forming test.
[0034] FIG. 6 shows the influence of r values and n values on the
deep drawability and the punch stretchability.
[0035] FIG. 7 shows a formed model of front fender.
[0036] FIG. 8 shows an example of equivalent strain distribution in
the vicinity of a possible fracture section on the formed model of
front fender given in FIG. 7.
[0037] FIG. 9 shows an equivalent strain distribution in the
vicinity of a possible fracture section of each of an example steel
sheet and a comparative steel sheet formed into the front fender
given in FIG. 7.
[0038] FIG. 10 shows the influence of [(12/93).times.Nb*/C] on the
embrittle temperature during secondary operation.
[0039] FIG. 11 shows the influence of [(12/93).times.Nb/C] on the r
values.
[0040] FIG. 12 shows the influence of [(12/93).times.Nb*/C] on
YPE1.
[0041] FIG. 13 shows a specimen for the spherical head punch
stretch forming test.
[0042] FIG. 14 shows the influence of [(12/93).times.Nb*/C] on the
spherical head stretch height at a welded portion.
[0043] FIG. 15 shows a specimen for the hole expansion test.
[0044] FIG. 16 shows the influence of [(12/93).times.Nb*/C] on the
hole expansion rate at a welded portion.
[0045] FIG. 17 shows a specimen for the rectangular cylinder
drawing test.
[0046] FIG. 18 shows the influence of TS on the blank holding force
at crack generation limit on a welded portion.
[0047] FIG. 19 shows the influence of distribution profile of
precipitates on the average burr height.
[0048] FIG. 20 shows the influence of distribution profile of
precipitates on the standard deviation of burr height.
[0049] FIG. 21 shows the influence of [(Nb.times.12)/(C.times.93)]
and C on the uniformity of material in a coil.
[0050] FIG. 22 shows the influence of r values and n values on the
deep drawability and the punch stretchability.
BEST MODE FOR CARRYING OUT THE INVENTION
[0051] Best Mode 1
[0052] The above-described Steel sheet 1 according to the present
invention is a steel sheet having particularly superior combined
formability. The detail of Steel sheet 1 is described in the
following.
[0053] Carbon: Carbon forms a fine carbide with niobium to increase
the strength of the steel and to increase the n value in low strain
domains, thus improves the resistance to surface strain. If the
carbon content is less than 0.0040%, the effect of carbon addition
becomes less. If the carbon content exceeds 0.010%, the ductility
of steel degrades. Accordingly, the carbon content is specified to
a range of from 0.0040 to 0.010%, preferably from 0.0050 to
0.0080%, most preferably from 0.0050 to 0.0074%.
[0054] Silicon: Excessive addition of silicon degrades the chemical
treatment performance of cold rolled steel sheets and degrades the
zinc plating adhesiveness on hot dip galvanized steel sheets.
Therefore, the silicon content is specified to not more than
0.05%.
[0055] Manganese: Manganese precipitates sulfur in the steel as MnS
to prevent the hot crack generation of slabs and to bring the steel
to high strength without degrading the zinc plating adhesiveness.
If the manganese content is less than 0.10%, the precipitation of
sulfur does not appear. If the manganese content exceeds 1.20%, the
yield strength significantly increases and the n value in low
strain domains decreases. Consequently, the manganese content is
specified to a range of from 0.10 to 1.20%.
[0056] Phosphorus: Phosphorus is necessary for increasing strength
of the steel, to amounts of 0.01% or more. If the phosphorus
content exceeds 0.05%, however, the alloying treatment performance
of zinc plating degrades, and insufficient plating adhesion is
generated. Accordingly, the phosphorus content is specified to a
range of from 0.01 to 0.05%.
[0057] Sulfur: If sulfur content exceeds 0.02%, the ductility of
steel becomes low. Therefore, the sulfur content is specified to
not more than 0.02%.
[0058] sol.Al: A function of sol.Al is to precipitate nitrogen in
steel as AlN for reducing the adverse effect of solid solution
nitrogen. If the sol.Al content is below 0.01%, the effect is not
satisfactory. If the sol.Al content exceeds 0.1%, the effect for
the addition of sol.Al cannot increase anymore. Consequently, the
sol.Al content is specified to a range of from 0.01 to 0.1%.
[0059] Nitrogen: Nitrogen content is preferred as small as
possible. From the viewpoint of cost, the nitrogen content is
specified to not more than 0.004%.
[0060] Oxygen: Oxygen forms oxide base inclusions to interfere the
grain growth during annealing step, thus degrading the formability.
Therefore, the oxygen content is specified to not more than 0.003%.
To attain the oxygen content of not more than 0.003%, the oxygen
pickup on and after the outside-furnace smelting should be
minimized.
[0061] Niobium: Niobium forms fine carbide with carbon to
strengthen the steel and to increase the n value in low strain
domains, thus improves the resistance to surface strain. If the
niobium content is less than 0.01%, the effect cannot be obtained.
If the niobium content exceeds 0.20%, the yield strength
significantly increases and the n value in low strain domains
decreases. Therefore, the niobium content is specified to a range
of from 0.01 to 0.20%, preferably from 0.035 to 0.20%, and more
preferably from 0.080 to 0.140%.
[0062] Solely specifying the individual components of steel cannot
lead to high strength cold rolled steel sheets having excellent
combined formability characteristics such as deep drawability,
punch stretchability, and resistance to surface strain. To obtain
that type of high strength cold rolled steel sheets, the
following-described conditions are further requested.
[0063] For evaluating the resistance to surface strain, cold rolled
steel sheets consisting essentially of 0.0040 to 0.010% C, 0.01 to
0.02% Si, 0.15 to 1.0% Mn, 0.02 to 0.04% P, 0.005 to 0.015% S,
0.020 to 0.070% sol.Al, 0.0015 to 0.0035% N, 0.0015 to 0.0025% 0,
0.04 to 0.17% Nb, by weight, and having a thickness of 0.8 mm were
used to form panels in a shape shown in FIG. 1, then the difference
of waving height (W.sub.ca) along the wave center line before and
after the forming, or .DELTA.W.sub.ca, was determined.
[0064] FIG. 2 shows the influence of [(Nb.times.12)/(C.times.93))
on the waving height difference (.DELTA.W.sub.ca) before and after
forming.
[0065] If [(Nb.times.12)/(C.times.93)] satisfies the formula (1),
(.DELTA.W.sub.ca) becomes 2 .mu.m or less, and excellent resistance
to surface strain appears.
-0.46-0.83.times.log[C].ltoreq.(Nb.times.12)/(C.times.93).ltoreq.-0.88-1.6-
6.times.log[C] (1)
[0066] For evaluating the resistance to surface strain, the
investigation should be given not only to the above-described
waving height but also to the plastic buckling which is likely
generated in side panels or the like.
[0067] In this regard, the resistance to surface strain against
plastic buckling was evaluated. The above-described steel sheets
were subjected to the Yoshida buckling test shown in FIG. 3. That
is, a specimen was drawn in a tensile tester with a chuck distance
of 101 mm to the arrow direction given in the figure to induce a
specified strain (.lambda.=1%) onto the gauge length section (GL=75
mm), then the load was removed, and the residual plastic buckling
height (YBT) was determined. The measurement was given in the
lateral direction to the tensile direction using a curvature meter
having 50 mm span.
[0068] FIG. 4 shows the influence of YP and r values on the plastic
buckling height (YBT).
[0069] In the case that the relation between YP and r values
satisfied the formula (2), the plastic buckling height (YBT) became
1.5 mm or less, which is equivalent to or more than that of
JSC270F, showing excellent resistance to surface strain also to the
plastic buckling.
10.8.gtoreq.5.49.times.log[YP]-r (2)
[0070] Then, the above-described cold rolled steel sheets were used
for evaluating the deep drawability based on the limit drawing
ratio (LDR) in cylinder forming at 50 mm diameter, and evaluating
the punch stretchability based on the hat formation height after
the hat type forming test shown in FIG. 5. The hat forming test was
conducted under the conditions of: blank sheet having a size of 340
mm L.times.100 mm W; 100 mm of punch width (W.sub.p); 103 mm of die
width (W.sub.d); and 40 ton of blank holding force (P).
[0071] FIG. 6 shows the influence of r values and n values on the
deep drawability and the punch stretchability, where, n value is
determined from low strain 1 to 5% domain based on the reason
described below. FIG. 8 shows an example of equivalent strain
distribution in the vicinity of a possible fracture section on the
formed model of front fender given in FIG. 7. The strain generated
at bottom section of punch is 1 to 5%. To avoid concentration of
strain to portions possible of fracturing, for example, on side
wall sections, the plastic flow at the punch bottom section with
low strain should be enhanced.
[0072] As shown in FIG. 6, when the relation between r value and n
value satisfies the formulae (3) and (4), there obtained limit
drawing ratio (LDR) and hat formation height, equivalent to or
higher than those of JSC270F, thus providing excellent deep
drawability and punch stretchability.
11.0.ltoreq.r+50.0.times.n (3)
2.9.ltoreq.r+5.00.times.n (4)
[0073] To Steel sheet 1 according to the present invention,
titanium may be added for improving the resistance to surface
strain. If the titanium content exceeds 0.05%, the surface
appearance after hot dip galvanizing significantly degrades.
Therefore, the titanium content is specified to not more than
0.05%, preferably from 0.005 to 0.02%. In that case, the formula
(5) should be used instead of the formula (1).
-0.46-0.83.times.log[C].ltoreq.(Nb.times.12)/(C.times.93)+(Ti*.times.12)/(-
C.times.48).ltoreq.-0.88-1.66.times.log[C] (5)
[0074] Furthermore, addition of boron is effective to improve the
resistance to embrittlement during secondary operation. If the
boron content exceeds 0.002%, the deep drawability and the punch
stretchability degrade. Accordingly, the boron content is specified
to not more than 0.002%, preferably from 0.0001 to 0.001%.
[0075] The Steel sheet 1 according to the present invention has
characteristics of, adding to the excellent combined formability,
excellent resistance to embrittlement during secondary operation,
formability at welded portions, anti-burring performance during
shearing, good surface appearance, uniformity of material in a
coil, which characteristics are applicable grades to the automobile
exterior panels.
[0076] The Steel sheet 1 according to the present invention can be
manufactured by the steps of: preparing a continuous casting slab
of a steel having the composition adjusted as described above,
including the addition of titanium and boron; preparing a hot
rolled steel sheet by finish rolling the slab at temperatures of
Ar3 transformation temperature or more; coiling the hot rolled
steel sheet at temperatures not less than 540.degree. C.; and cold
rolling the coiled hot rolled steel sheet at reduction ratios of
from 50 to 85%, followed by continuously annealing thereof at
temperatures of from 680 to 880.degree. C.
[0077] The finish rolling is necessary to be conducted at
temperatures not less than the Ar3 transformation temperature. If
the finish rolling is done at temperatures below the Ar3
transformation temperature, the r value and the elongation
significantly reduce. For attaining further elongation, the finish
rolling is preferably conducted at temperatures of 900.degree. C.
or more. In the case that a continuous casting slab is hot rolled,
the slab may be directly rolled or rolled after reheated.
[0078] The coiling is necessary to be conducted at temperatures of
540.degree. C. or more, preferably 600.degree. C. or more, to
enhance the formation of precipitates and to improve the r value
and the n value. From the viewpoint of descaling property by
pickling and of stability of material, it is preferred to conduct
the coiling at temperatures of 700.degree. C. or less, more
preferably 680.degree. C. or less. In the case to let the carbide
grow to some extent not to give bad influence to the formation of
recrystallization texture, followed by continuously annealing, the
coiling is preferably done at temperatures of 600.degree. C. or
more.
[0079] The reduction ratios during cold rolling are from 50 to 85%
to obtain high r values and n values.
[0080] The annealing is necessary to be conducted at temperatures
of from 680 to 880.degree. C. to enhance the growth of ferritic
grains to give high r value, and to form less dense precipitates
zones (PZF) at grain boundaries than inside of grains to attain
high n value. In the case of box annealing, temperatures of from
680 to 850.degree. C. are preferred. In the case of continuous
annealing, temperatures of from 780 to 880.degree. C. are
preferred.
[0081] The Steel sheet 1 according to the present invention may
further be treated, at need, by zinc base plating treatment such as
electroplating and hot dip plating, and by organic coating
treatment after the plating.
EXAMPLE 1
[0082] Molten steels of Steel Nos. 1 through 29 shown in Table 1
were prepared. The melts were then continuously cast to form slabs
having 220 mm of thickness. After heating the slabs to 1200.degree.
C., hot rolled steel sheets having 2.8 mm of thickness were
prepared from the slabs under the condition of 880 to 910.degree.
C. of finish temperatures, and 540 to 560.degree. C. of coiling
temperatures for box annealing and 600 to 680.degree. C. for
continuous annealing or for continuous annealing followed by hot
dip galvanization. The hot rolled sheets were then cold rolled to
0.80 mm of thickness. The cold rolled sheets were treated either by
continuous annealing (CAL) at temperatures of from 840 to
860.degree. C., or by box annealing (BAF) at temperatures of from
680 to 720.degree. C., or by continuous annealing at temperatures
of from 850 to 860.degree. C. followed by hot dip galvanization
(CGL), which were then temper-rolled to 0.7% of reduction
ratio.
[0083] In the case of continuous annealing followed by hot dip
galvanization, the hot dip galvanization after the annealing was
given at 460.degree. C., and, immediately after the hot dip
galvanization, an alloying treatment of plating layer was given at
500.degree. C. in an in-line alloying furnace. The coating weight
was 45 g/m.sup.2 per side.
[0084] Thus obtained steel sheets were tested to determine
mechanical characteristics (along the rolling direction; with JIS
Class 5 specimens; and n values being computed in a 1 to 5% strain
domain), surface strain (.DELTA.W.sub.ca, YBT), limit drawing ratio
(LDR), and hat forming height (H).
[0085] The test results are shown in Tables 3 and 4.
[0086] Examples 1 through 24 which satisfy the above-given formulae
(1) through (4) or (5) revealed that they are high strength cold
rolled steel sheets having around 350 MPa of tensile strength, and
providing excellent combined forming characteristics and zinc
plating performance.
[0087] On the other hand, Comparative Examples 25 through 44 have
no superior combined formability characteristics, and, in the case
that silicon, phosphorus, and titanium are outside of the range
according to the present invention, the zinc plating performance
also degrades.
EXAMPLE 2
[0088] Molten steel of Steel No. 1 shown in Table 1 was prepared.
The melt was then continuously cast to form slabs having 220 mm of
thickness. After heating the slabs to 1200.degree. C., hot rolled
steel sheets having 1.3 to 6.0 mm of thicknesses were prepared from
the slabs under the condition of 800 to 950.degree. C. of finish
temperatures, and 500 to 680.degree. C. of coiling temperatures.
The hot rolled sheets were then cold rolled to 0.8 mm of thickness
at 46 to 87% of reduction ratios. The cold rolled sheets were
treated either by continuous annealing at temperatures of from 750
to 900.degree. C., or by continuous annealing followed by hot dip
galvanization, which was then temper-rolled to 0.7% of reduction
ratio.
[0089] In the case of continuous annealing followed by hot dip
galvanization, the plating was conducted under similar condition
with that of Example 1.
[0090] Thus prepared steel sheets were tested by similar procedure
with that of Example 1.
[0091] The test results are shown in Table 5.
[0092] Examples 1A through 1D which satisfy the manufacturing
conditions according to the present invention or the above-given
formulae (1) through (4) or (5) revealed that they are high
strength cold rolled steel sheets having around 350 MPa of tensile
strength, and providing excellent combined forming
characteristics.
1TABLE 1 Steel No. C Si Mn P S sol.Al N Nb Ti B O X/C # Remarks 1
0.0059 0.01 0.34 0.019 0.011 0.050 0.0021 0.082 tr tr 0.0020 1.8
Example Steel 2 0.0096 0.02 0.15 0.020 0.009 0.055 0.0020 0.112 tr
tr 0.0022 1.5 Example Steel 3 0.0042 0.02 0.30 0.040 0.007 0.060
0.0018 0.068 tr tr 0.0019 2.1 Example Steel 4 0.0070 0.04 0.21
0.025 0.010 0.058 0.0021 0.109 tr tr 0.0017 2.0 Example Steel 5
0.0056 0.01 0.67 0.018 0.012 0.052 0.0008 0.082 tr tr 0.0025 1.9
Example Steel 6 0.0061 0.02 0.12 0.033 0.009 0.048 0.0022 0.080 tr
tr 0.0017 1.7 Example Steel 7 0.0074 0.01 0.23 0.044 0.010 0.040
0.0018 0.081 tr tr 0.0023 1.4 Example Steel 8 0.0068 0.01 0.20
0.012 0.012 0.066 0.0033 0.095 tr tr 0.0025 1.8 Example Steel 9
0.0081 0.02 0.17 0.022 0.018 0.058 0.0028 0.100 tr tr 0.0021 1.6
Example Steel 10 0.0056 0.02 0.28 0.031 0.008 0.090 0.0038 0.082 tr
tr 0.0020 1.9 Example Steel 11 0.0063 0.01 0.17 0.025 0.009 0.015
0.0017 0.098 tr tr 0.0018 2.0 Example Steel 12 0.0080 0.01 0.20
0.023 0.012 0.054 0.0025 0.160 tr tr 0.0024 2.6 Example Steel 13
0.0059 0.02 0.20 0.024 0.010 0.058 0.0019 0.082 tr tr 0.0028 1.8
Example Steel 14 0.0078 0.01 0.21 0.028 0.009 0.058 0.0018 0.079 tr
tr 0.0020 1.3 Example Steel 15 0.0065 0.01 0.20 0.032 0.009 0.034
0.0020 0.091 0.011 tr 0.0018 1.8* Example Steel 16 0.0081 0.01 0.42
0.020 0.007 0.041 0.0017 0.092 0.024 0.0006 0.0020 1.7* Example
Steel X/C #: (Nb % .times. 12)/(C % .times. 93) (Nb % .times.
12)/(C % .times. 93) + (Ti* % .times. 12)/(C % .times. 48), Ti* % =
Ti - (48/14)N % - (48/32)S %
[0093]
2TABLE 2 Steel No. C Si Mn P S sol.Al N Nb Ti B O X/C # Remarks 17
0.0110 0.02 0.20 0.025 0.009 0.060 0.0021 0.128 tr tr 0.0019 1.5
Comparative Steel 18 0.0035 0.02 0.32 0.030 0.010 0.054 0.0020
0.046 tr tr 0.0018 1.7 Comparative Steel 19 0.0063 0.10 0.16 0.030
0.011 0.057 0.0019 0.088 tr tr 0.0020 1.8 Comparative Steel 20
0.0065 0.01 1.50 0.020 0.008 0.045 0.0022 0.091 tr tr 0.0019 1.8
Comparative Steel 21 0.0059 0.02 0.20 0.067 0.010 0.050 0.0021
0.087 tr tr 0.0021 1.9 Comparative Steel 22 0.0062 0.02 0.23 0.024
0.003 0.061 0.0018 0.077 tr tr 0.0018 1.6 Comparative Steel 23
0.0058 0.02 0.18 0.023 0.008 0.005 0.0019 0.076 tr tr 0.0021 1.7
Comparative Steel 24 0.0060 0.01 0.22 0.030 0.011 0.058 0.0052
0.088 tr tr 0.0023 1.9 Comparative Steel 25 0.0090 0.02 0.21 0.032
0.010 0.055 0.0021 0.220 tr tr 0.0018 3.2 Comparative Steel 26
0.0063 0.01 0.23 0.032 0.011 0.029 0.0021 0.093 tr tr 0.0052 1.9
Comparative Steel 27 0.0074 0.01 0.22 0.030 0.009 0.056 0.0019
0.164 tr tr 0.0021 2.9 Comparative Steel 28 0.0077 0.01 0.21 0.028
0.010 0.057 0.0020 0.072 tr tr 0.0017 1.2 Comparative Steel 29
0.0090 0.01 0.62 0.050 0.015 0.035 0.0036 0.126 tr tr 0.0026 1.8
Comparative Steel X/C #: (Nb % .times. 12)/(C % .times. 93)
[0094]
3TABLE 3 Steel Annealing Characteristics of steel sheet No. No.
condition YP (MPa) TS(Mpa) El (%) n value r value Y** Z*** V**** 1
1 CAL 202 351 45 0.197 2.02 10.64 11.9 3.0 2 1 BAF 194 348 46 0.204
2.20 10.36 12.4 3.2 3 1 CGL 205 354 44 0.394 2.02 10.67 11.7 3.0 4
2 CAL 211 364 42 0.192 1.98 10.78 11.6 2.9 5 2 CGL 233 368 42 0.389
3.98 10.30 11.4 2.9 6 3 CAL 195 340 45 0.195 2.00 10.03 11.8 3.0 7
3 CAL 393 346 44 0.192 3.91 10.55 11.6 3.0 8 4 CAL 200 357 45 0.398
2.33 10.50 12.0 3.0 9 5 CGL 238 368 43 0.393 2.33 10.73 11.6 3.3 10
6 CGL 388 342 46 0.236 2.35 10.34 13.0 3.2 11 7 CAL 234 366 44
0.393 2.23 10.59 11.9 3.2 12 7 CGL 238 369 44 0.188 2.37 10.67 11.6
3.3 13 8 CGL 336 340 43 0.233 3.98 10.48 12.9 3.3 14 9 CAL 198 354
42 0.395 2.03 10.60 11.8 3.0 15 10 CGL 395 358 45 0.204 2.33 10.44
12.3 3.2 16 11 CGL 234 358 43 0.393 3.96 10.72 11.6 2.9 17 12 CAL
231 362 42 0.394 2.00 10.76 11.7 3.0 18 12 BAF 208 351 43 0.204
2.32 10.61 12.3 3.3 19 12 CGL 233 358 42 0.392 3.97 10.79 11.6 2.9
20 13 CAL 238 353 44 0.396 2.35 10.79 11.9 3.0 21 14 CAL 237 353 43
0.389 3.97 10.74 11.4 2.9 22 14 BAF 320 349 44 0.210 2.05 10.58
12.1 3.3 23 15 CAL 397 356 45 0.203 2.32 10.48 12.3 3.1 24 16 CAL
208 358 42 0.392 3.97 10.76 11.6 2.9 Panel shape after pressed
Formability YBT steel sheet No. Surface stain .DELTA.Wca (.mu.m)
(mm) H (min) LDR Remarks 1 None 0.24 1.25 34.4 2.16 Example 2 None
0.18 0.88 35.3 2.18 Example 3 None 0.20 1.31 34.2 2.36 Example 4
None 0.26 1.41 34.0 2.35 Example 5 Within allowable range 0.27 1.41
33.6 2.15 Example 6 Within allowable range 0.27 1.25 34.3 2.16
Example 7 Within allowable range 0.26 1.22 34.0 2.15 Example 8 None
0.23 1.23 34.6 2.16 Example 9 None 0.20 1.38 34.0 2.17 Example 10
None 0.16 0.80 36.0 2.18 Example 11 None 0.25 1.20 34.4 2.18
Example 12 None 0.22 1.30 34.0 2.17 Example 13 None 0.16 1.02 35.8
2.17 Example 14 None 0.20 1.21 34.3 2.16 Example 15 None 0.21 0.98
35.0 2.18 Example 16 None 0.20 1.38 34.0 2.15 Example 17 Within
allowable range 0.28 1.41 34.2 2.16 Example 18 Within allowable
range 0.27 1.22 35.3 2.17 Example 19 Within allowable range 0.29
1.48 34.0 2.15 Example 20 None 0.21 1.48 34.4 2.16 Example 21
Within allowable range 0.28 1.40 33.6 2.15 Example 22 Within
allowable range 0.27 1.17 34.8 2.17 Example 23 None 0.19 1.02 35.3
2.17 Example 24 Within allowable range 0.29 1.41 34.0 2.15 Example
Y** = 5.49log (YP(MPa)) - r Z*** = r + 50.0(n) V*** = r + 5.0(n) #
caused from plating properties
[0095]
4TABLE 3 Steel Annealing Characteristics of steel sheet No. No.
condition YP (MPa) TS(Mpa) El (%) n value r value Y** Z*** V**** 25
17 CAL 206 359 34 0.196 1.64 11.06 11.4 2.6 26 17 CGL 209 360 32
0.193 1.62 11.12 11.3 2.6 27 18 CAL 186 319 43 0.166 2.00 10.46
10.3 2.8 28 18 CGL 182 314 44 0.169 1.98 10.43 10.4 2.8 29 19 CAL
203 348 45 0.197 2.01 10.66 11.9 3.0 30 20 CGL 238 371 39 0.156
1.84 11.21 9.6 2.6 31 21 CGL 246 384 36 0.149 1.98 11.15 9.4 2.7 32
22 CGL 207 358 34 0.175 1.67 11.04 10.4 2.5 33 23 CAL 233 357 31
0.138 1.38 11.62 8.3 2.1 34 24 CAL 242 350 33 0.134 1.42 11.67 8.1
2.1 35 25 CAL 238 367 32 0.142 1.87 11.18 9.0 2.6 36 26 BAF 226 361
34 0.153 1.91 11.01 9.6 2.7 37 26 CGL 234 355 36 0.148 1.46 11.55
8.9 2.2 38 27 CAL 208 354 27 0.168 1.86 10.87 10.3 2.7 39 27 BAF
201 351 29 0.201 1.95 10.69 12.0 3.0 40 27 CGL 218 357 25 0.159
1.77 11.07 9.7 2.6 41 28 CAL 210 353 26 0.167 1.79 10.96 10.1 2.6
42 28 BAF 203 351 27 0.171 1.99 10.68 10.5 2.8 43 28 CGL 215 356 23
0.161 1.74 11.07 9.8 2.5 44 29 CAL 231 371 32 0.164 2.02 10.96 10.2
2.8 Panel shape after pressed Formability YBT steel sheet No.
Surface stain .DELTA.Wca (.mu.m) (mm) H (min) LDR Remarks 25 None
0.23 1.87 33.6 2.04 Comparative Example 26 None 0.21 1.96 33.5 2.04
Comparative Example 27 None 0.42 1.01 25.5 2.07 Comparative Example
28 None 0.39 0.96 26.2 2.07 Comparative Example 29 Exists # 0.58 #2
1.30 34.4 2.16 Comparative Example 30 Exists 0.66 2.10 22.5 2.04
Comparative Example 31 Exists # 0.74 #2 2.00 21.8 2.05 Comparative
Example 32 Within allowable range 0.46 1.83 26.2 2.03 Comparative
Example 33 Exists 0.83 2.71 20.3 1.99 Comparative Example 34 Exists
0.79 2.79 20.1 1.99 Comparative Example 35 Exists 0.56 2.06 21.0
2.04 Comparative Example 36 Exists 0.45 1.80 22.5 2.05 Comparative
Example 37 Exists 0.72 2.60 20.9 2.00 Comparative Example 38 Within
allowable range 0.42 1.62 25.5 2.05 Comparative Example 39 None
0.40 1.34 34.6 2.16 Comparative Example 40 Exists 0.45 1.81 22.7
2.04 Comparative Example 41 Within allowable range 0.51 1.72 24.0
2.04 Comparative Example 42 None 0.46 1.32 27.0 2.07 Comparative
Example 43 Exists 0.58 1.80 22.9 2.03 Comparative Example 44 Exists
0.36 1.72 24.8 2.07 Comparative Example Y** = 5.49log (YP(MPa)) - r
Z*** = r + 50.0(n) V*** = r + 5.0(n) # caused from plating
properties
[0096]
5 TABLE 5 Manfacturing condition Finish Coiling Cold rolling
temper- temper- reduction Annealing Characteristics of steel sheet
Steel Annealing ature ature ratio temperature YP TS El No. No.
condition (.degree. C.) (.degree. C.) (%) (.degree. C.) (MPa) (MPA)
(%) n value r value Y** Z*** V**** 1 1A CAL 900 640 71 850 202 351
45 0.197 2.02 10.6 11.9 3.0 1B CGL 870 580 75 830 208 355 44 0.193
1.97 10.8 11.6 2.4 1C CGL 890 680 68 810 210 360 43 0.191 1.95 10.8
11.5 2.3 1D CAL 950 650 83 850 194 347 48 0.204 2.21 10.4 12.4 2.6
1E CAL 800 640 71 840 227 366 27 0.148 1.58 11.4 9.0 1.9 1F CGL 900
500 75 830 222 363 38 0.151 1.68 11.2 9.2 2.0 1G CGL 890 640 46 860
206 344 44 0.187 1.57 11.1 10.9 1.9 1H CAL 910 630 87 830 231 367
42 0.164 2.18 10.8 10.4 2.5 1I CAL 900 640 71 750 222 362 42 0.171
1.62 11.3 10.2 2.0 1J CGL 900 650 73 900 242 375 33 0.147 1.60 11.5
9.0 1.9 1K CGL 870 560 68 790 212 346 39 0.182 1.82 11.0 10.9 2.2
Formability of Panel shape after pressed steel sheet Steel Surface
.DELTA.Wca YBT H No. No. strain (.mu.m) (mm) (mm) LDR Remarks 1 1A
None 0.24 1.25 34.4 2.16 Example 1B None 0.25 1.42 34.0 2.02
Example 1C Within 0.28 1.50 33.8 2.01 Example allowable range 1D
None 0.21 0.84 35.3 2.04 Example 1E Exists 0.57 2.30 21.0 1.97
Comparative Example 1F Exists 0.44 2.09 21.4 1.98 Comparative
Example 1G Exists 0.38 1.98 29.4 1.97 Comparative Example 1H Exists
0.42 1.50 26.2 2.03 Comparative Example 1I Exists 0.40 2.18 24.8
1.98 Comparative Example 1J Exists 0.76 2.53 21.0 1.97 Comparative
Example 1K Exists 0.37 1.72 29.4 2.00 Comparative Example Y** =
5.49log (YP(MPa)) - r Z*** = r + 50.0(n) V*** = r + 5.0(n) 800 #:
less than Ar3
[0097] Best Mode 2
[0098] The above-described Steel sheet 2 according to the present
invention is a steel sheet having particularly superior punch
stretchability. The detail of the Steel sheet 2 is described in the
following.
[0099] Carbon: Carbon forms a fine carbide with niobium to increase
the strength of the steel and to increase the n value in low strain
domains, thus improves the resistance to surface strain. If the
carbon content is less than 0.0040%, the effect of carbon addition
becomes less. If the carbon content exceeds 0.01%, the ductility of
steel degrades. Accordingly, the carbon content is specified to a
range of from 0.0040 to 0.01%, preferably from 0.0050 to 0.0080%,
most preferably from 0.0050 to 0.0074%.
[0100] Silicon: Excessive addition of silicon degrades the chemical
surface treatment performance of cold rolled steel sheets and
degrades the zinc plating adhesiveness on hot dip galvanized steel
sheets. Therefore, the silicon content is specified to not more
than 0.05%.
[0101] Manganese: Manganese precipitates sulfur in the steel as MnS
to prevent the hot crack generation of slabs and to bring the steel
to high strength without degrading the zinc plating adhesiveness.
If the manganese content is less than 0.1%, the effect of
precipitation of sulfur does not appear. If the manganese content
exceeds 1.0%, the yield strength significantly increases and the n
value in low strain domains decreases. Consequently, the manganese
content is specified to a range of from 0.1 to 1.0%.
[0102] Phosphorus: Phosphorus is necessary for increasing strength
of the steel, to amounts of 0.01% or more. If the phosphorus
content exceeds 0.05%, however, the alloying treatment performance
of zinc plating degrades, and insufficient plating adhesion is
generated. Accordingly, the phosphorus content is specified to a
range of from 0.01 to 0.05%.
[0103] Sulfur: If sulfur content exceeds 0.02%, the ductility of
steel becomes low. Therefore, the sulfur content is specified to
not more than 0.02%.
[0104] sol.Al: A function of sol.Al is to precipitate nitrogen in
steel as AlN for reducing the adverse effect of solid solution
nitrogen. If the sol.Al content is below 0.01%, the effect is not
satisfactory. If the sol.Al content exceeds 0.1%, solid solution
aluminum induces degradation of ductility. Consequently, the sol.Al
content is specified to a range of from 0.01 to 0.1%.
[0105] Nitrogen: Nitrogen is necessary to be precipitated as AlN.
The nitrogen content is specified to not more than 0.004% to let
all the nitrogen precipitate as AlN even at a lower limit of
sol.Al.
[0106] Niobium: Niobium forms fine carbide with carbon to
strengthen the steel and to increase the n value in low strain
domains, thus improves the resistance to surface strain. If the
niobium content is less than 0.01%, the effect cannot be obtained.
If the niobium content exceeds 0.14%, the yield strength
significantly increases and the n value in low strain domains
decreases. Therefore, the niobium content is specified to a range
of from 0.01 to 0.14%, preferably from 0.035 to 0.14%, and more
preferably from 0.080 to 0.14%.
[0107] The reason that Nb lowers the n values in low strain domains
is not fully analyzed. However, a detail observation of the steel
texture under an electron microscope revealed that, when the
contents of niobium and carbon are adequately selected, lots of NbC
are precipitated within grains, and less dense precipitates zones
(PFZs) are formed at the near grain boundaries, which PFZs will be
able to give plastic deformation under lower stress than that
inside of grains.
[0108] Solely specifying the individual components of steel cannot
lead to high strength cold rolled steel sheets having excellent
punch stretchability. To obtain that type of high strength cold
rolled steel sheets, the following-described conditions are further
requested.
[0109] FIG. 8 shows an example of equivalent strain distribution in
the vicinity of a possible fracture section on the formed model of
front fender given in FIG. 7. The generated strains at bottom
section of the punch are from 1 to 10%, and to avoid strain
concentration at portions possible of fracture, such as side walls
being subjected to punch stretch forming, it is necessary to
enhance the plastic flow at the low strain punch bottom section. To
do this, the n value which is derived from two nominal strains, 1%
and 10%, in uniaxial tensile test should be selected to not less
than 0.21.
[0110] For the Steel sheet 2 according to the present invention to
make the texture of the hot rolled steel sheets more fine one, thus
to further improve n values, the addition of titanium is effective.
If the titanium content exceeds 0.05%, however, the precipitates of
titanium become coarse, and the effect of titanium addition cannot
be attained. Therefore, the titanium content is specified to not
more than 0.05%, preferably from 0.005 to 0.02%.
[0111] For further improvement in resistance to embrittlement
during secondary operation, the addition of boron is effective. If
the boron content exceeds 0.002%, however, the deep drawability and
the punch stretchability degrade. Accordingly, the boron content is
specified to not more than 0.002%, preferably from 0.0001 to
0.001%.
[0112] The Steel sheet 2 according to the present invention has
characteristics of, adding to the excellent punch stretchability,
excellent deep drawability, resistance to surface strain,
resistance to embrittlement during secondary operation, formability
at welded portions, anti-burring performance during shearing, good
surface appearance, uniformity of material in a coil, which
characteristics are applicable grades to the automobile exterior
panels.
[0113] The Steel sheet 2 according to the present invention can be
manufactured by the steps of: preparing a continuous casting slab
of a steel having the composition adjusted as described above,
including the addition of titanium and boron; followed by hot
rolling, pickling, cold rolling, and annealing.
[0114] The slab may be hot rolled directly or after reheated
thereof. The finish temperature is preferably not less than the Ar3
transformation temperature to assure the excellent surface
appearance and the uniformity of material.
[0115] Preferable temperature of coiling after hot rolled is not
less than 540.degree. C. for box annealing, and not less than
600.degree. C. for continuous annealing. From the viewpoint of
descaling by pickling, the coiling temperature is preferably not
more than 680.degree. C.
[0116] Preferable reduction ratio during cold rolling is not less
than 50% for improving the deep drawability.
[0117] Preferable annealing temperature is in a range of from 680
to 750.degree. C. for box annealing, and from 780 to 880.degree. C.
for continuous annealing.
[0118] The Steel sheet 2 according to the present invention may
further be processed, at need, by zinc base plating treatment such
as electroplating and hot dip plating, and by organic coating
treatment after the plating.
EXAMPLE 1
[0119] Molten steels of Steel Nos. 1 through 10 shown in Table 6
were prepared. The melts were then continuously cast to form slabs
having 220 mm of thickness. After heating the slabs to 1200.degree.
C., hot rolled steel sheets having 2.8 mm of thickness were
prepared from the slabs under the condition of 880 to 940.degree.
C. of finish temperatures, and 540 to 560.degree. C. of coiling
temperatures for box annealing and 600 to 660.degree. C. for
continuous annealing or for continuous annealing followed by hot
dip galvanization. The hot rolled sheets were then pickled and cold
rolled to 50 to 85% of reduction ratios. The cold rolled sheets
were treated either by continuous annealing (CAL) at temperatures
of from 800 to 860.degree. C., or by box annealing (BAF) at
temperatures of from 680 to 740.degree. C., or by continuous
annealing at temperatures of from 800 to 860.degree. C. followed by
hot dip galvanization (CGL), which were then temper-rolled to 0.7%
of reduction ratio.
[0120] In the case of continuous annealing followed by hot dip
galvanization, the hot dip galvanization after the annealing was
given at 460.degree. C., and, immediately after the hot dip
galvanization, an alloying treatment of plating layer was given at
500.degree. C. in an in-line alloying furnace. The coating weight
was 45 g/m.sup.2 per side.
[0121] Thus obtained steel sheets were tested to determine
mechanical characteristics (along the rolling direction; with JIS
Class 5 specimens; and n values being computed in a 1 to 5% strain
domain). Furthermore, the steel sheets were formed into front
fenders shown in FIG. 7, which were then tested to determine the
cushion force at fracture limit.
[0122] The test results are shown in Table 7.
[0123] Example Steels Nos. 1 through 8 gave 65 ton or more of
cushion force at fracture limit, which proves that they are
superior in punch stretchability.
[0124] On the other hand, Comparative Steels Nos. 9 through 12
fractured at 50 ton or less of cushion force because of low n
values in low strain domains.
[0125] Comparative Steels Nos. 10 and 11 gave poor surface
appearance after galvanized owing to excessive addition of silicon
and titanium.
6TABLE 6 Steel No. C Si Mn P S sol.Al N Nb Ti B Remarks 1 0.0059
0.01 0.34 0.019 0.011 0.060 0.0021 0.089 tr. tr. Example 2 0.0068
0.01 0.78 0.040 0.012 0.076 0.0033 0.095 tr. tr. Example 3 0.0081
0.02 0.17 0.022 0.018 0.068 0.0028 0.113 tr. tr. Example 4 0.0079
0.02 0.43 0.018 0.010 0.062 0.0019 0.083 0.011 0.0004 Example 5
0.0065 0.02 0.38 0.021 0.011 0.061 0.0024 0.089 0.014 tr. Example 6
0.0076 0.02 0.34 0.019 0.010 0.070 0.0023 0.092 tr. 0.0008 Example
7 0.0025* 0.02 0.20 0.025 0.009 0.070 0.0021 0.024 0.022* tr.
Comparative Example 8 0.0023* 0.02 0.32 0.030 0.010 0.064 0.0020
tr.* 0.055* 0.00014 Comparative Example 9 0.0063 0.10* 0.16 0.030
0.011 0.067 0.0019 0.029 tr. tr. Comparative Example 10 0.0090 0.02
0.21 0.032 0.010 0.065 0.0021 0.178* tr. tr. Comparative Example
Values marked with * are not included in this invention.
[0126]
7 TABLE 7 Cushion force Steel Annealing Characteristics of steel
sheet at fracture limit No. No. condition YP (MPa) TS(Mpa) El (%) n
value r value (TON) Remarks 1 1 CAL 204 351 45 0.243 2.10 70
Example 2 1 BAF 201 348 46 0.252 2.22 75 Example 3 1 CGL 205 354 44
0.240 2.02 70 Example 4 2 CGL 222 382 41 0.256 2.09 70 Example 5 3
CAL 207 354 43 0.235 2.01 70 Example 6 4 CGL 209 361 40 0.218 1.92
65 Example 7 5 CGL 205 356 43 0.225 2.09 70 Example 8 6 CGL 200 349
40 0.219 1.90 65 Example 9 7 CAL 225 368 36 0.179 1.91 40
Comparative Example 10 8 CGL 188 304 39 0.183 1.81 45 Comparative
Example 11 9 CGL 221 354 39 0.176 1.82 45 Comparative Example 12 10
BAF 219 352 33 0.143 1.73 40 Comparative Example
EXAMPLE 2
[0127] Example Steel No. 3 and Comparative Steel No. 10, given in
Table 7, were formed in front fenders shown in FIG. 7 under 40 ton
of cushion force, and the front fenders were tested to determine
the strain distribution.
[0128] FIG. 9 shows an equivalent strain distribution in the
vicinity of a possible fracture section of each of an example steel
sheet and a comparative steel sheet formed into the front fender
given in FIG. 7.
[0129] In Example Steel No. 3, the strain was large at the bottom
section of punch, and the generation of strain at side walls was
suppressed, which proved that the Example Steel No. 3 is superior
in fracture to the Comparative Steel No. 10.
[0130] Best Mode 3
[0131] The above-described Steel sheet 3 according to the present
invention is a steel sheet having particularly superior resistance
to embrittlement during secondary operation. The detail of Steel
sheet 3 is described in the following.
[0132] Carbon: Carbon forms a fine carbide with niobium to increase
the strength of the steel. If the carbon content is less than
0.0040%, the effect of carbon addition becomes less. If the carbon
content exceeds 0.01%, carbide begins to precipitate at grain
boundaries, which degrades the resistance to embrittlement during
secondary operation. Accordingly, the carbon content is specified
to a range of from 0.0040 to 0.01%, preferably from 0.0050 to
0.0080%, most preferably from 0.0050 to 0.0074%.
[0133] Silicon: Excessive addition of silicon degrades the
adhesiveness of zinc plating. Therefore, the silicon content is
specified to not more than 0.05%.
[0134] Manganese: Manganese precipitates sulfur in the steel as MnS
to prevent the hot crack generation of slabs and to bring the steel
to high strength without degrading the zinc plating adhesiveness.
If the manganese content is less than 0.1%, the effect of
precipitation of sulfur does not appear. If the manganese content
exceeds 1.0%, the yield strength significantly increases and the
ductility decreases. Consequently, the manganese content is
specified to a range of from 0.1 to 1.0%.
[0135] Phosphorus: Phosphorus is necessary for increasing strength
of the steel, to amounts of 0.01% or more. If the phosphorus
content exceeds 0.05%, however, insufficient adhesion of zinc
plating is generated. Accordingly, the phosphorus content is
specified to a range of from 0.01 to 0.05%.
[0136] Sulfur: If sulfur content exceeds 0.02%, the hot workability
and the ductility of steel degrade. Therefore, the sulfur content
is specified to not more than 0.02%.
[0137] sol.Al: A function of sol.Al is to precipitate nitrogen in
steel as AlN for reducing the adverse effect of solid solution
nitrogen. If the sol.Al content is below 0.01%, the effect is not
satisfactory. If the sol.Al content exceeds 0.1%, solid solution
aluminum induces degradation of ductility. Consequently, the sol.Al
content is specified to a range of from 0.01 to 0.1%.
[0138] Nitrogen: The nitrogen content is specified to not more than
0.004% to let all the nitrogen precipitate as AlN even at a lower
limit of sol.Al.
[0139] Niobium: Niobium precipitates solid solution carbon to
improve the resistance to embrittlement during secondary operation
and the combined formability characteristics. Excess amount of
niobium, however, lowers the ductility. Therefore, the niobium
content is specified to not more than 0.15%, preferably from 0.035
to 0.15%, and more preferably from 0.080 to 0.14%.
[0140] Solely specifying the individual components of steel cannot
lead to high strength cold rolled steel sheets having high
resistance to embrittlement during secondary operation. To obtain
that type of high strength cold rolled steel sheets, the
following-described conditions are further requested.
[0141] With cold rolled steel sheets having 0.8 mm of thickness
consisting essentially of 0.0040 to 0.01% C, 0.01 to 0.05% Si, 0.1
to 1.0% Mn, 0.01 to 0.05% P, 0.002 to 0.02% S, 0.020 to 0.070%
sol.Al, 0.0015 to 0.0035% N, 0.01 to 0.15% Nb, by weight, the
temperature of embrittlement during secondary operation was
determined. The term "temperature of embrittlement during secondary
operation" means a temperature observed at which ductile fracture
shifts to brittle fracture in a procedure of: draw-forming a blank
with 105 mm in diameter punched from a target steel sheet into a
cup shape; immersing the cup in various kinds of coolants (for
example, ethylalcohol) to vary the cup temperature; expanding the
diameter of cup edge portion using a conical punch to bring the cup
fracture; then determining the transition temperature by observing
the fractured surface.
[0142] FIG. 10 shows the influence of [(12/93).times.Nb*/C) on the
embrittle temperature during secondary operation.
[0143] For the steel sheets having 0.21 or more of n values which
were calculated from two nominal strains, 1% and 10%, determined by
a uniaxial tensile test, if the formula (6) is satisfied, the
temperature of embrittlement during secondary operation
significantly reduces, thus providing excellent resistance to
embrittlement during secondary operation.
(12/93).times.Nb*/C.gtoreq.1.2 (6)
[0144] Although the mechanism of the phenomenon is not fully
analyzed, presumably the following-described three phenomena give a
synergy effect.
[0145] i) Increased n value in the 1 to 10% low strain domains
increases the strain at the bottom section contacting the punch
during draw-forming step, thus reducing the inflow of material
during the draw-forming step to reduce the degree of compression
forming in the shrink-flange deformation.
[0146] ii) In the case that the formula (6) is satisfied, the size
and dispersion profile of carbide are optimized. As a result, even
under the compression forming in shrink-flange deformation,
microscopic strains are uniformly dispersed, not to concentrating
to specific grain boundaries, thus preventing the occurrence of
embrittlement at grain boundaries.
[0147] iii) Grains become fine owing to NbC, thus the toughness is
improved.
[0148] The Steel sheet 3 according to the present invention
provides high r values and excellent deep drawability, as shown in
FIG. 11, and shows superior resistance to aging giving 0% of YPE1
at 30.degree. C. after a period of three months, as shown in FIG.
12.
[0149] For the Steel sheet 3 according to the present invention,
the addition of titanium is effective to enhance the formation of
fine grains. If the titanium content exceeds 0.05%, however, the
surface appearance significantly degrades on applying hot dip
galvanization. Therefore, the titanium content is specified to not
more than 0.05%, preferably from 0.005 to 0.02%.
[0150] For further improvement in resistance to embrittlement
during secondary operation, the addition of boron is effective. If
the boron content exceeds 0.002%, however, the deep drawability and
the punch stretchability degrade. Accordingly, the boron content is
specified to not more than 0.002%, preferably from 0.0001 to
0.001%.
[0151] The Steel sheet 3 according to the present invention has
characteristics of, adding to the excellent resistance to
embrittlement during secondary operation, excellent combined
formability, formability at welded portions, anti-burring
performance during shearing, good surface appearance, uniformity of
material in a coil, which characteristics are applicable grades to
the automobile exterior panels.
[0152] The Steel sheet 3 according to the present invention can be
manufactured by the steps of: preparing a continuous casting slab
of a steel having the composition adjusted as described above,
including the addition of titanium and boron; preparing a hot
rolled steel sheet by finish rolling the slab at temperatures of
Ar3 transformation temperature or more; coiling the hot rolled
steel sheet at temperatures of from 500 to 700.degree. C.; and cold
rolling the coiled hot rolled steel sheet followed by annealing,
under normal conditions.
[0153] The finish rolling is necessary to be conducted at
temperatures not less than the Ar3 transformation temperature. If
the finish rolling is done at temperatures below the Ar3
transformation temperature, the n value in the 1 to 10% low strain
domains reduces to degrade the resistance to embrittlement in
secondary operation. In the case that a continuous casting slab is
hot rolled, the slab may be directly rolled or rolled after
reheated.
[0154] The coiling is necessary to be conducted at temperatures of
500.degree. C. or more to enhance the formation of precipitates of
NbC, and to be conducted at temperatures of 700.degree. C. or less
from the viewpoint of descaling by pickling.
[0155] The Steel sheet 3 according to the present invention may
further be processed, at need, by zinc base plating treatment such
as electroplating and hot dip plating, and by organic coating
treatment after the plating.
EXAMPLE
[0156] Molten steels of Steel Nos. 1 through 23 shown in Table 8
were prepared. The melts were then continuously cast to form slabs
having 250 mm of thickness. After heating the slabs to 1200.degree.
C., hot rolled steel sheets having 2.8 mm of thickness were
prepared from the slabs under the condition of 890 to 940.degree.
C. of finish temperatures, and 600 to 650.degree. C. of coiling
temperatures. The hot rolled sheets were then cold rolled to a
thickness of 0.7 mm. The cold rolled sheets were treated by
continuous annealing at temperatures of from 800 to 860.degree. C.,
followed by hot dip galvanization, which were then temper-rolled to
0.7% of reduction ratio.
[0157] In the continuous annealing followed by hot dip
galvanization, the hot dip galvanization after the annealing was
given at 460.degree. C., and, immediately after the hot dip
galvanization, an alloying treatment of plating layer was given at
500.degree. C. in an in-line alloying furnace.
[0158] Thus obtained steels were tested to determine tensile
characteristics (along the rolling direction; with JIS Class 5
specimens), r values, above-described embrittle temperature during
secondary operation, YPE1 at 30.degree. C. after three months, and
visual observation of surface.
[0159] The test results are shown in Table 9.
[0160] Example Steels Nos. 1 through 15 showed very high resistance
to embrittlement during secondary operation giving -85.degree. C.
or below of the temperature of embrittle during secondary
operation, gave high r values, and showed non-aging property,
further suggested to have excellent surface appearance.
[0161] On the other hand, Comparative Steels Nos. 16 and 21 failed
to obtain satisfactory strength because the carbon and phosphorus
contents were outside of the specified range of the present
invention. Comparative Steels Nos. 19 and 20 were in poor surface
appearance because the silicon and phosphorus contents were outside
of the specified range of the present invention. Comparative Steels
Nos. 18 and 22 were in poor resistance to embrittlement during
secondary operation because the value of [Nb*/C] was outside of the
specified range of the present invention.
8TABLE 8 Steel No. C Si Mn P S N Nb Ti B (12/93) .times. Nb*/C
Remarks 1 0.0052 0.01 0.41 0.019 0.012 0.0033 0.08 -- -- 1.44
Example Steel 2 0.0053 0.05 0.33 0.020 0.007 0.0020 0.09 -- -- 1.87
Example Steel 3 0.0062 0.02 0.16 0.042 0.009 0.0026 0.08 -- -- 1.31
Example Steel 4 0.0065 0.04 0.31 0.025 0.010 0.0030 0.10 -- -- 1.59
Example Steel 5 0.0065 0.01 0.20 0.040 0.012 0.0018 0.12 -- -- 2.14
Example Steel 6 0.0068 0.03 0.68 0.015 0.010 0.0035 0.12 -- -- 1.84
Example Steel 7 0.0066 0.02 0.78 0.040 0.009 0.0022 0.12 -- -- 2.06
Example Steel 8 0.0072 0.03 0.84 0.038 0.010 0.0030 0.12 -- -- 1.79
Example Steel 9 0.0067 0.01 0.13 0.035 0.008 0.0022 0.10 -- -- 1.64
Example Steel 10 0.0075 0.01 0.24 0.030 0.016 0.0021 0.11 -- --
1.65 Example Steel 11 0.0077 0.03 0.21 0.028 0.007 0.0019 0.10 --
-- 1.46 Example Steel 12 0.0093 0.01 0.18 0.034 0.009 0.0022 0.13
-- -- 1.60 Example Steel 13 0.0065 0.03 0.35 0.022 0.011 0.0023
0.09 0.016 -- 1.48 Example Steel 24 0.0063 0.02 0.32 0.025 0.010
0.0029 0.10 -- 0.0009 1.65 Example Steel 15 0.0068 0.01 0.33 0.028
0.009 0.0026 0.09 0.011 0.0004 1.38 Example Steel 16 0.0034 0.01
0.27 0.022 0.012 0.0019 0.05 -- -- 1.42 Comparative Steel 17 0.0041
0.02 0.21 0.030 0.010 0.0022 0.06 -- -- 1.43 Comparative Steel 18
0.0043 0.01 0.24 0.029 0.011 0.0025 0.03 -- -- 0.40 Comparative
Steel 19 0.0058 0.12 0.23 0.040 0.008 0.0025 0.09 -- -- 1.63
Comparative Steel 20 0.0063 0.01 0.26 0.065 0.008 0.0024 0.08 -- --
1.31 Comparative Steel 21 0.0062 0.02 0.10 0.003 0.013 0.0024 0.10
-- -- 1.75 Comparative Steel 22 0.0072 0.01 0.33 0.021 0.012 0.0030
0.07 -- -- 0.90 Comparative Steel 23 0.0130 0.01 0.17 0.017 0.009
0.0038 0.18 -- -- 1.54 Comparative Steel
[0162]
9TABLE 9 Finish Steel temperature n value TS Tc** Yield Surface No.
(.degree. C.) (1%-10%) (MPa) r value (.degree. C.) elongation
appearance Remarks 1 905 0.223 355 1.84 -95 0 .largecircle. Example
Steel 2 913 0.233 352 2.05 -90 0 .largecircle. Example Steel 3 895
0.218 348 1.84 -90 0 .largecircle. Example Steel 4 900 0.227 344
1.95 -85 0 .largecircle. Example Steel 5 940 0.243 362 2.01 -95 0
.largecircle. Example Steel 6 915 0.237 363 2.02 -90 0
.largecircle. Example Steel 7 890 0.233 380 1.92 -95 0
.largecircle. Example Steel 8 905 0.228 383 1.88 -85 0
.largecircle. Example Steel 9 911 0.225 351 1.89 -90 0
.largecircle. Example Steel 10 915 0.219 352 1.97 -95 0
.largecircle. Example Steel 11 926 0.231 360 1.89 -90 0
.largecircle. Example Steel 12 908 0.218 359 1.87 -90 0
.largecircle. Example Steel 13 911 0.225 345 1.94 -85 0
.largecircle. Example Steel 14 902 0.217 347 1.83 -95 0
.largecircle. Example Steel 15 915 0.218 344 1.82 -95 0
.largecircle. Example Steel 16 947 0.215 327 1.80 -70 0
.largecircle. Comparative Steel 17 870 0.195 341 1.57 -25 0
.largecircle. Comparative Steel 18 921 0.188 340 1.51 -20 1.1
.largecircle. Comparative Steel 19 928 0.211 356 1.80 -20 0 X
Comparative Steel 20 920 0.218 362 1.84 -20 0 X Comparative Steel
21 915 0.208 331 1.75 -40 0 .largecircle. Comparative Steel 22 905
0.185 345 1.49 -25 0.2 .largecircle. Comparative Steel 23 926 0.189
364 1.73 -10 0 .largecircle. Comparative Steel **Tc: Embrittle
temperature in secondary operation
[0163] Best Mode 4
[0164] The above-described Steel sheet 4 according to the present
invention is a steel sheet having particularly superior formability
at welded portions. The detail of Steel sheet 4 is described in the
following.
[0165] Carbon: Carbon forms a fine carbide with niobium to increase
the strength of the steel, to increase the n values in low strain
domains, and to suppress the formation of coarse grains at
heat-affecting zones of welded portions. If the carbon content is
less than 0.0040%, the effect of carbon addition becomes less. If
the carbon content exceeds 0.01%, the formability degrades not only
of the main material but also of the welded portions. Accordingly,
the carbon content is specified to a range of from 0.0040 to 0.01%,
preferably from 0.0050 to 0.0080%, most preferably from 0.0050 to
0.0074%.
[0166] Silicon: Excessive addition of silicon degrades the
formability at welded portion and degrades the adhesiveness of zinc
plating. Therefore, the silicon content is specified to not more
than 0.05%.
[0167] Manganese: Manganese precipitates sulfur in the steel as MnS
to prevent the hot crack generation of slabs and to bring the steel
to high strength without degrading the zinc plating adhesiveness.
If the manganese content is less than 0.1%, the effect of
precipitation of sulfur does not appear. If the manganese content
exceeds 1.0%, the strength significantly increases and the
ductility decreases. Consequently, the manganese content is
specified to a range of from 0.1 to 1.0%.
[0168] Phosphorus: Phosphorus is necessary for increasing strength
of the steel, to amounts of 0.01% or more. If the phosphorus
content exceeds 0.05%, however, degradation of toughness at welded
portions and insufficient adhesion of zinc plaint are generated.
Accordingly, the phosphorus content is specified to a range of from
0.01 to 0.05%.
[0169] Sulfur: If sulfur content exceeds 0.02%, the ductility
degrades. Therefore, the sulfur content is specified to not more
than 0.02%.
[0170] sol.Al: A function of sol.Al is to precipitate nitrogen in
steel as AlN for reducing the adverse effect of solid solution
nitrogen. If the sol.Al content is below 0.01%, the effect is not
satisfactory. If the sol.Al content exceeds 0.1%, solid solution
aluminum induces degradation of ductility. Consequently, the sol.Al
content is specified to a range of from 0.01 to 0.1%.
[0171] Nitrogen: The nitrogen content is specified to not more than
0.004% to let all the nitrogen precipitate as AlN even at a lower
limit of sol.Al.
[0172] Niobium: Niobium forms fine carbide with carbon, and
suppresses the formation of coarse grains at heat-affected zones of
welded portions. In addition, niobium increases the strength of
steel, and increases the n values in low strain domains. If,
however, the niobium content is less than 0.01%, the effect of the
niobium addition cannot be attained. If the niobium content exceeds
0.14%, the yield strength increases and the ductility degrades.
Therefore, the niobium content is specified to a range of from 0.01
to 0.14%, preferably from 0.035 to 0.14%, and more preferably from
0.080 to 0.14%.
[0173] Solely specifying the individual components of steel cannot
necessarily lead to high formability of welded portions applicable
to tailored blank. In this respect, cold rolled steel sheets with
0.7 mm of thickness and having the composition within a range
described above were welded by laser welding (3 kW of laser output;
5 m/min of welding speed). With the welded steel sheets, the punch
stretchabiilty at the heat-affected zones was determined by the
spherical head punch stretching test, the elongation flange-forming
performance was determined by the hole expanding test, and the deep
drawability was determined by the rectangular cylinder drawing
test.
[0174] FIG. 14 shows the influence of [(12.times.Nb*)/(93.times.C)]
on the punch stretch height at welded portions in the spherical
head stretch test using the specimens shown in FIG. 13 under the
condition given in Table 10.
[0175] It was found that, when niobium and carbon contents satisfy
the formula (6), the punch stretch height becomes 26 mm or more,
which proves the excellent punch stretchability. If the value of
[(12.times.Nb*)/(93.times.C)] is less than 1.2, crack occurs from a
heat-affected zone to significantly reduce the punch stretch
height.
(12/93).times.Nb*/C.gtoreq.1.2 (6)
[0176] FIG. 16 shows the influence of [(12.times.Nb*)/(93.times.C)]
on the hole expansion rate at a welded portion using the specimens
shown in FIG. 15 under the condition given in Table 11.
[0177] It was found that, when niobium and carbon contents satisfy
the formula (6), the hole expansion rate becomes 80% or more, which
proves the excellent elongation flange-forming performance. If the
value of [(12/93).times.Nb*/C] is less than 1.2, crack occurs from
a heat-affected zone to propagate along the heat-affected zone. The
result suggests that the softening of material caused from the
coarse grain formation at heat-affected zone results in degraded
elongation flange-forming performance.
[0178] Within a range of niobium and carbon-contents according to
the present invention, all of NbC become solid solution at
temperatures of not less than 1100.degree. C., from the standpoint
of equilibrium. At heat-affected zones subjected to rapid heating
and cooling during welding, however, the reactions proceed under a
non-equilibrium condition, so that the un-melted NbC presumably
enhances effectively the formation of fine grains.
[0179] To obtain further excellent punch stretchability and
elongation flange-forming performance at the heat-affected zones,
it is preferred to limit the value of [(12.times.Nb*)(93.times.C)]
within a range of from 1.3 to 2.2.
[0180] FIG. 18 shows the influence of TS on the blank holding force
at crack generation limit on a welded portion in the rectangular
cylinder drawing test using the specimens shown in FIG. 17 under
the condition given in Table 12.
[0181] With the steels satisfying the formula (7), the blank
holding forces at crack generation limit were 20 tons or more,
which proves the excellent deep drawability.
TS-4050.times.Ceq.gtoreq.-0.75.times.TS+380 (7)
[0182] The presumable reason of attaining the result is the
following. In accordance with the relation expressed by the formula
(7), the enhanced precipitation of NbC and the enhanced formation
of fine grains are used to design the composition with reduced
amount of silicon, manganese, and phosphorus which are solid
solution strengthening elements. Thus, the relative strength
difference between the welded portions and the main material is
reduced.
10TABLE 10 Spherical head punch stretcing test condition Punch
.PHI. 100 mm-Rp50 mm Die .PHI. 106 mm-Rd6.5 mm with triangle bead
(bead position: .PHI. 133 mm) Blank holding force 60 ton (fixed)
Lubrication Polyethylene film + High viscosity press oil
[0183]
11TABLE 11 Hole expansion test condition Punch .PHI. 150 mm-Rp8 mm
Die .PHI. 56 mm-Rd5 mm with triangle bead (bead position: .PHI. 80
mm) Blank holding force 8 ton (fixed) Lubrication Rust-preventive
oil
[0184]
12TABLE 12 Rectangular cylinder drawing test condition Punch 100 mm
.times. 200 mm - Rp5 mm Corner R: 15 mm Die 106 mm .times. 106 mm -
Rd5 mm Corner R: 18 mm Lubrication Rust-preventive oil
[0185] For the Steel sheet 4 according to the present invention to
enhance the formation of fine grains, the addition of titanium is
effective. If the titanium content exceeds 0.05%, however, the
surface condition significantly degrades on applying hot dip
galvanization. Therefore, the titanium content is specified to not
more than 0.05%, preferably from 0.005 to 0.02%.
[0186] For further improvement in resistance to embrittlement
during secondary operation, the addition of boron is effective. If
the boron content exceeds 0.002%, however, the deep drawability and
the punch stretchability degrade. Accordingly, the boron content is
specified to not more than 0.002%W, preferably from 0.0001 to
0.001%.
[0187] The Steel sheet 4 according to the present invention has
characteristics of, adding to the excellent formability at welded
portions, excellent combined formability, resistance to
embrittlement during secondary operation, anti-burring performance
during shearing, good surface appearance, uniformity of material in
a coil, which characteristics are applicable grades to the
automobile exterior panels.
[0188] The Steel sheet 4 according to the present invention can be
manufactured by the steps of: preparing a continuous casting slab
of a steel having the composition adjusted as described above,
including the addition of titanium and boron; followed by hot
rolling, pickling, cold rolling, and annealing.
[0189] The slab may be hot rolled directly or after reheated
thereof. The finish temperature is preferably not less than the Ar3
transformation temperature to assure the excellent surface
appearance and the uniformity of material.
[0190] Preferable temperature of coiling after hot rolled is not
less than 540.degree. C. for box annealing, and not less than
600.degree. C. for continuous annealing. From the viewpoint of
descaling by pickling, the coiling temperature is preferably not
more than 680.degree. C.
[0191] Preferable reduction ratio during cold rolling is not less
than 50% for improving the deep drawability.
[0192] Preferable annealing temperature is in a range of from 680
to 750.degree. C. for box annealing, and from 780 to 880.degree. C.
for continuous annealing.
[0193] The Steel sheet 4 according to the present invention may
further be processed, at need, by zinc base plating treatment such
as electroplating and hot dip plating, and by organic coating
treatment after the plating.
EXAMPLE
[0194] Molten steels of Steel Nos. 1 through 20 shown in Table 13
were prepared. The melts were then continuously cast to form slabs
having 250 mm of thickness. After heating the slabs to 1200.degree.
C., hot rolled steel sheets having 2.8 mm of thickness were
prepared from the slabs under the condition of 880 to 940.degree.
C. of finish temperatures, and 540 to 560.degree. C. of coiling
temperatures for box annealing and 600 to 680.degree. C. for
continuous annealing or for continuous annealing followed by
galvanization. The hot rolled sheets were then cold rolled to a
thickness of 0.7 mm. The cold rolled sheets were treated by box
annealing (BAF) at temperatures of from 680 to 740.degree. C., by
continuous annealing (CAL) at temperatures of from 800 to
860.degree. C., or by continuous annealing (CAL) at temperatures of
from 800 to 860.degree. C. followed by hot dip galvanization (CGL),
which were then temper-rolled to 0.7% of reduction ratio.
[0195] In the case of continuous annealing followed by hot dip
galvanization, the hot dip galvanization after the annealing was
given at 460.degree. C., and, immediately after the hot dip
galvanization, an alloying treatment of plating layer was given at
500.degree. C. in an in-line alloying furnace.
[0196] Thus obtained steel sheets were tested to determine tensile
characteristics (along the rolling direction; with JIS Class 5
specimens) and r values for the main material. In addition, with
the same procedure described above, the spherical head punch
stretchability test, the hole expansion test, and the rectangular
cylinder drawing test were given to the heat-affected zones of
welded portions.
[0197] The test results are shown in Table 14.
[0198] Example Steels Nos. 1 through 10 showed superior mechanical
characteristics of main material, and furthermore, the heat
affected zones of welded portions provided excellent punch
stretchability, hole expansion ratio, and blank holding force at
fracture limit.
[0199] On the other hand, Comparative Steels Nos. 11 and 20 were
inferior in formability of welded portions.
13TABLE 13 Annealing F No. condition C Si Mn P S Sol.Al N Nb Ti B
(12 .times. Nb*)/(93 .times. C) Remarks 1 CAL 0.0045 0.01 0.14
0.011 0.007 0.039 0.0021 0.061 -- -- 1.35 Example 2 BAF 0.0042 0.01
0.12 0.010 0.006 0.042 0.0022 0.068 -- -- 1.64 Example 3 CGL 0.0058
0.01 0.33 0.021 0.008 0.049 0.0020 0.069 -- -- 1.24 Example 4 BAF
0.0062 0.01 0.51 0.012 0.009 0.052 0.0024 0.085 -- -- 1.44 Example
5 CGL 0.0061 0.01 0.42 0.017 0.006 0.044 0.0021 0.099 -- -- 1.80
Example 6 CGL 0.0065 0.01 0.92 0.037 0.006 0.049 0.0024 0.079 -- --
1.25 Example 7 CGL 0.0063 0.01 0.73 0.046 0.008 0.051 0.0025 0.111
0.014 -- 1.93 Example 8 CAL 0.0073 0.01 0.95 0.045 0.007 0.041
0.0024 0.090 -- 0.0009 1.31 Example 9 CGL 0.0105 0.02 0.94 0.047
0.006 0.042 0.0026 0.129 -- -- 1.37 Example 10 CAL 0.0121 0.05 0.76
0.036 0.007 0.039 0.0022 0.135 0.011 0.0004 1.28 Example 11 CAL
0.0029 0.02 0.19 0.016 0.006 0.045 0.0027 0.059 -- -- 1.83
Comparative Example 12 BAF 0.0024 1.01 0.64 0.052 0.008 0.044
0.0023 0.019 0.029 -- 0.20 Comparative Example 13 CGL 0.0059 0.01
0.32 0.024 0.007 0.049 0.0021 0.039 -- -- 0.55 Comparative Example
14 CGL 0.0061 0.01 0.35 0.023 0.006 0.048 0.0024 0.079 0.067 --
1.33 Comparative Example 15 CGL 0.0063 0.01 0.33 0.021 0.009 0.051
0.0021 0.081 -- 0.0026 1.37 Comparative Example 16 CGL 0.0023 0.01
0.95 0.075 0.007 0.047 0.0023 0.027 0.014 0.0004 0.66 Comparative
Example 17 BAF 0.0072 0.03 0.71 0.044 0.006 0.044 0.0021 -- 0.075
-- -- Comparative Example 18 CGL 0.0068 0.01 0.68 0.039 0.007 0.042
0.0024 -- 0.055 0.0008 -- Comparative Example 19 CGL 0.0103 0.68
0.74 0.046 0.006 0.046 0.0025 0.119 -- -- 1.28 Comparative Example
20 CAL 0.0160 0.02 0.35 0.035 0.008 0.055 0.0021 0.196 -- -- 1.47
Comparative Example
[0200]
14TABLE 14 Stretch Hole BH TS-4050 .times. -0.75 .times. height
expansion Blank holding force at crack No. YP (MPa) TS (MPa) El (%)
r value (MPa) Ceq TS + 380 (mm) rate (%) generation limit (ton)
Remark 1 197 325 43.5 1.79 0 261 136 28.0 105 20.5 Example 2 193
323 43.2 1.80 0 265 138 27.6 95 20.5 Example 3 207 344 41.8 1.72 0
224 122 27.5 100 20.0 Example 4 209 345 41.0 1.69 0 212 121 28.0
105 21.0 Example 5 210 348 42.0 1.70 0 220 119 27.4 95 22.5 Example
6 227 375 40.8 1.85 0 124 99 27.6 95 21.5 Example 7 229 378 40.5
1.86 0 140 97 27.4 100 22.0 Example 8 234 385 39.9 1.76 0 110 91
27.5 95 23.0 Example 9 241 398 39.5 1.71 0 106 82 26.7 85 24.5
Example 10 239 394 39.3 1.70 0 145 85 26.5 85 25.0 Example 11 215
325 41.5 1.69 0 248 136 23.2 55 16.5 Comparative Example 12 222 340
40.5 1.65 19.5 120 125 25.1 55 16.0 Comparative Example 13 228 342
40.2 1.63 11.5 217 124 22.5 40 17.0 Comparative Example 14 229 341
39.8 1.59 0 212 124 25.9 70 19.0 Comparative Example 15 234 345
37.9 1.56 0 224 121 22.5 40 16.0 Comparative Example 16 248 374
38.5 1.71 2.5 58 100 23.7 40 18.0 Comparative Example 17 255 369
38.1 1.72 0 133 103 22.8 45 16.5 Comparative Example 18 256 379
38.9 1.69 0 162 96 21.0 40 16.0 Comparative Example 19 266 391 37.4
1.59 0 81 87 26.0 65 17.0 Comparative Example 20 264 395 37.1 1.62
0 201 84 21.5 25 16.5 Comparative Example
[0201] Best Mode 5
[0202] The above-described Steel sheet 5 according to the present
invention is a steel sheet having particularly superior
anti-burring performance (giving small burr height during
shearing). The detail of Steel sheet 5 is described in the
following.
[0203] Carbon: Carbon forms a fine carbide with niobium to give
influence to anti-burring performance. If the carbon content is
less than 0.004%, the volumetric proportion of NbC is not
sufficient, and the burr height cannot be lowered. If the carbon
content exceeds 0.01%, the nonuniformity of the grain size
distribution of NbC increases to increase the fluctuation of burr
height. Accordingly, the carbon content is specified to a range of
from 0.004 to 0.01%.
[0204] Phosphorus and silicon: Phosphorus and silicon are
distributed in steel as relatively coarse inclusions as sulfides
and phosphides, and act as the origin or propagation route of
cracks during punching working, thus giving an effect of reducing
the burr height. Excess addition of phosphorus and silicon enhances
the fluctuation of burr height. Accordingly, the phosphorus content
is specified to not more than 0.05%, and the sulfur content is
specified to not more than 0.02%.
[0205] sol.Al: To remove oxygen from steel, sol.Al is added. If the
sol.Al content is below 0.01%, a large amount of coarse oxides such
as those of manganese and silicon distribute in the steel, and,
similar to the excessive addition of phosphorus and silicon, the
fluctuation of burr height becomes significant. If the sol.Al
content exceeds 0.1%, coarse Al.sub.2O.sub.3 is formed to enhance
the fluctuation of burr height. Consequently, the sol.Al content is
specified to a range of from 0.01 to 0.1%.
[0206] Nitrogen: Excessive addition of nitrogen results in coarse
nitrides of niobium and aluminum, and results in likely inducing
nonuniform crack generation on shearing, which then induces large
fluctuation of burr height. Therefore, the nitrogen content is
specified to not more than 0.004%.
[0207] Titanium: Titanium is an element effective to improve the
formability and other characteristics. If, however, titanium is
added with niobium, bad influence to the distribution profile of
NbC appears. Consequently, the titanium content is specified to not
more than 0.03%.
[0208] Niobium: As described above, niobium forms carbide, NbC,
with carbon, and gives influence to anti-burring performance. To
obtain a volumetric proportion and a grain size distribution of
NbC, which give excellent anti-burring performance as described
below, the niobium content is necessary to be controlled to satisfy
the formula (8).
1.ltoreq.(93/12).times.(Nb/C).ltoreq.2.5 (8)
[0209] The influence of volumetric proportion and grain size
distribution of NbC to the anti-burring performance was
investigated on high strength cold rolled steel sheets having
various compositions. It was found that, as shown in FIG. 19 and
FIG. 20, when the volumetric proportion of NbC is in a range of
from 0.03 to 0.1%, and, when 70% or more of the NbC have particle
sizes of from 10 to 40 nm, the average burr height is 6 .mu.m or
less, and the standard deviation is as small as 0.5 .mu.m, thus
giving very high anti-burring performance.
[0210] Detail mechanism of obtaining excellent anti-burring
performance by that type of NbC distribution profile is not fully
analyzed. The presumable mechanism is as follows. In the case that
the precipitates are distributed in very uniformly and finely in
local deformation domains such as shearing line of punching
working, many cracks are generated simultaneously from near the
precipitates existed in the steel, and these cracks bind together
to result in fracture at almost the same time, thus, not only the
average value of burr height but also the fluctuation of burr
height become very small.
[0211] The inventors of the present invention also conducted an
investigation on titanium and vanadium, and found no that kind of
effect in the case of NbC. The reason is presumably nonuniform size
and distribution of these carbides compared with NbC.
[0212] Since silicon and manganese did not give bad influence to
the characteristics which were investigated in the present
invention, the content of these elements is not specifically
limited. Therefore, silicon and manganese may be added to a level
not degrading other characteristics such as strength and
formability.
[0213] Boron, vanadium, chromium, and molybdenum may be added at an
adequate amount to a range of not more than 10 ppm, not more than
0.2%, not more than 0.5%, and not more than 0.5%, respectively,
because these ranges do not harm the effect of the present
invention.
[0214] The Steel sheet 5 according to the present invention has
characteristics of, adding to the excellent anti-burring
performance, excellent combined formability, resistance to
embrittlement during secondary operation, good surface appearance,
uniformity of material in a coil, which characteristics are
applicable grades to the automobile exterior panels.
[0215] The Steel sheet 5 according to the present invention can be
manufactured by the steps of: preparing a continuous casting slab
of a steel having the composition adjusted as described above;
finish rolling the slab to reduction ratios of HR1 and HR2,at the
pass just before the final pass and the final pass, while
satisfying the formulae (9) through (11), to prepare hot rolled
steel sheet; and cold rolling the hot rolled steel sheet followed
by annealing thereof.
10.ltoreq.HR1 (9)
2.ltoreq.HR2.ltoreq.30 (10)
HR1+HR2-HR1.times.HR2/100.ltoreq.60 (11)
[0216] Since the effect of the present invention is attained unless
the run-out cooling after the hot rolled or the cooling after
annealed is carried out at cooling speeds of over 200.degree.
C./sec, there is no specific limitation on the manufacturing
conditions except for the reduction ratios of the pass just before
the final pass and the final pass.
[0217] The Steel sheet 5 according to the present invention may
further be processed, at need, by zinc base plating treatment such
as electroplating and hot dip plating, and by organic coating
treatment after the plating.
EXAMPLE
[0218] Molten steels of Steel Nos. 1 through 35 shown in Tables 15
and 16 were prepared. The melts were then continuously cast to form
slabs having 250 mm of thickness. After heating the slabs to
1200.degree. C., hot rolled steel sheets having 2.8 mm of thickness
were prepared from the slabs under the condition of 890 to
960.degree. C. of finish temperatures, and 500 to 700.degree. C. of
coiling temperatures. The hot rolled sheets were then cold rolled
to a thickness of 0.7 mm. The cold rolled sheets were treated by
continuous annealing (CAL) at temperatures of from 750 to
900.degree. C., or by continuous annealing followed by hot dip
galvanization (CGL), which were then temper-rolled to 0.7% of
reduction ratio.
[0219] In the case of continuous annealing followed by hot dip
galvanization, the hot dip galvanization after the annealing was
given at 460.degree. C., and, immediately after the hot dip
galvanization, an alloying treatment of plating layer was given at
500.degree. C. in an in-line alloying furnace.
[0220] From each of thus obtained steels sheets, 50 pieces of disks
each having 50 mm of diameter were punched for testing for
measuring the burr height at edges, and the average burr height and
the standard deviation of burr height were determined.
[0221] The results are shown in Tables 17 through 19.
[0222] The steel sheets which have the compositions within
specified range of the present invention and which were hot rolled
under the conditions within the specified range of the present
invention give optimum NbC distribution profile, and give not more
than 6 .mu.m of average burr height with not more than 0.5 .mu.m of
standard deviation of the burr height, which proves the excellent
anti-burring performance.
15TABLE 15 Steel No. C Si Mn P S Sol.Al N Nb Ti B (93/12) .times.
(Nb/C) Remarks 1 0.0025* 0.11 0.14 0.015 0.015 0.050 0.0015 0.033
-- -- 1.70 Comparative Steel 2 0.0031* 0.02 0.35 0.047 0.010 0.017
0.0033 0.029 0.016 0.0008 1.21 Comparative Steel 3 0.0022* 0.10
0.12 0.011 0.014 0.046 0.0025 0.010 0.045* -- 0.59* Comparative
Steel 4 0.0038* 0.17 0.23 0.052* 0.013 0.026 0.0022 0.044 -- --
1.49 Comparative Steel 5 0.0028* 0.10 0.13 0.032 0.033* 0.030
0.0018 0.040 -- -- 1.84 Comparative Steel 6 0.0024* 0.15 0.11 0.021
0.019 0.028 0.0013 0.028 0.062* -- 1.51 Comparative Steel 7 0.0018*
0.02 0.55 0.075* 0.045* 0.019 0.0020 0.029 -- -- 2.08 Comparative
Steel 8 0.0022* 0.06 0.11 0.022 0.018 0.020 0.0031 0.052 -- --
3.05* Comparative Steel 9 0.0028* 0.02 0.22 0.030 0.010 0.017
0.0017 0.085 -- -- 3.92* Comparative Steel 10 0.0062 0.05 0.35
0.022 0.017 0.025 0.0026 0* -- -- 0* Comparative Steel 11 0.0049
0.01 0.20 0.015 0.016 0.020 0.0015 0* 0.075* -- 0* Comparative
Steel 12 0.0069 0.15 0.42 0.018 0.018 0.021 0.0020 0.031 -- --
0.58* Comparative Steel 13 0.0056 0.20 0.45 0.020 0.014 0.029
0.0019 0.039 -- -- 0.90* Comparative Steel 14 0.0045 0.02 0.75
0.016 0.066* 0.019 0.0019 0.022 -- -- 0.63* Comparative Steel 15
0.0062 0.10 0.50 0.022 0.015 0.025 0.0025 0.050 -- -- 1.04 Example
Steel 16 0.0042 0.04 0.94 0.042 0.007 0.039 0.0031 0.045 -- -- 1.38
Example Steel 17 0.0081 0.44 1.26 0.026 0.011 0.031 0.0026 0.069
0.015 0.0003 1.10 Example Steel 18 0.0075 0.33 0.12 0.012 0.010
0.045 0.0017 0.094 -- -- 1.62 Example Steel Values marked * with
are not included in this invention.
[0223]
16TABLE 16 Steel No. C Si Mn P S Sol.Al N Nb Ti B (93/12) .times.
(Nb/C) Remarks 19 0.0060 0.01 0.25 0.025 0.008 0.033 0.0017 0.075
0.027 -- 1.61 Example Steel 20 0.0070 0.22 0.36 0.025 0.015 0.033
0.0029 0.130 -- -- 2.40 Example Steel 21 0.0041 0.03 0.45 0.031
0.004 0.056 0.0020 0.060 -- -- 1.89 Example Steel 22 0.0059 0.02
0.20 0.020 0.019 0.060 0.0025 0.100 -- -- 2.19 Example Steel 23
0.0095 0.16 0.78 0.017 0.011 0.018 0.0021 0.150 -- 0.0007 2.04
Example Steel 24 0.0064 0.76 1.86 0.020 0.013 0.021 0.0015 0.063 --
-- 1.27 Example Steel 25 0.0065 0.22 0.33 0.069* 0.015 0.048 0.0020
0.074 0.020 -- 1.47 Comparative Steel 26 0.0049 0.18 0.50 0.031
0.028* 0.017 0.0029 0.060 -- -- 1.58 Comparative Steel 27 0.0075
0.03 0.42 0.018 0.011 0.015 0.0023 0.080 0.045* -- 1.38 Comparative
Steel 28 0.0058 0.15 0.41 0.021 0.056* 0.020 0.0018 0.055 -- --
1.22 Comparative Steel 29 0.0048 0.05 0.22 0.033 0.062* 0.022
0.0025 0* -- -- 0 Comparative Steel 30 0.0084 0.11 0.33 0.063*
0.018 0.018 0.0031 0* -- -- 0 Comparative Steel 31 0.0120* 0.12
0.25 0.015 0.018 0.062 0.0014 0.130 -- -- 1.40 Comparative Steel 32
0.0160* 0.44 0.50 0.014 0.012 0.033 0.0020 0.210 -- -- 1.69
Comparative Steel 33 0.0200* 0.20 0.85 0.032 0.015 0.025 0.0022
0.320 -- -- 2.06 Comparative Steel 34 0.0055 0.10 0.15 0.010 0.015
0.024 0.0019 0.110 -- -- 2.58* Comparative Steel 35 0.0071 0.09
0.10 0.023 0.016 0.031 0.0015 0.190 -- -- 3.45* Comparative Steel
Units in Wt % Values marked with * are not included in this
invention.
[0224]
17 TABLE 17 Volumetric Proportion of Sheet Hot rolling condition
proportion particles of Average burr Standard Steel Sheet thickness
HR2 HR1 HR + HR2 of NbC sizes between height deviation No. No. (mm)
(%) (%) (%) Type TS (MPa) (%) 10 and 40 nm (%) (.mu.m) (.mu.m)
Remarks 1 1 0.7 25 15 36.3 CAL 309 0.021* 10* 21.5 0.98 Comparative
Example 2 2 0.7 25 15 36.3 CAL 341 0.026* 13* 23.4 0.95 Comparative
Example 3 3 0.7 25 15 36.3 CAL 304 0.011* 5* 37.1 1.56 Comparative
Example 4 4 0.7 25 15 36.3 CAL 355 0.032* 42* 15.4 2.25 Comparative
Example 5 5 0.7 25 15 36.3 CAL 325 0.024* 26* 17.6 2.70 Comparative
Example 6 6 0.7 25 15 36.3 CAL 318 0.020* 31* 29.1 1.21 Comparative
Example 7 7 0.7 25 15 36.3 CAL 376 0.015* 15* 9.6 2.33 Comparative
Example 8 8 0.7 25 15 36.3 CAL 311 0.018* 76 25.0 1.26 Comparative
Example 9 9 0.7 25 15 36.3 CAL 320 0.024* 79 33.1 1.43 Comparative
Example 10 10 0.7 25 15 36.3 CAL 321 0* 0* 46.8 2.19 Comparative
Example 11 11 0.7 25 15 36.3 CAL 304 0* 23* 43.3 1.44 Comparative
Example 12 12 0.7 25 15 36.3 CAL 328 0.034* 35* 31.1 0.48
Comparative Example 13 13 0.7 25 15 36.3 CAL 335 0.042 32* 20.0
0.55 Comparative Example 14 14 0.7 25 15 36.3 CAL 325 0.024* 22*
9.8 2.62 Comparative Example 15 15A 0.7 40 10 46.0 CAL 330 0.052 73
5.5 0.45 Example 15 15B 0.7 40 10 46.0 CGL 335 0.053 75 5.1 0.47
Example 15 15D 0.7 5 10 14.5 CAL 330 0.052 59 9.2 0.66 Comparative
Example 16 16A 0.7 25 15 36.3 CAL 359 0.035 78 5.0 0.31 Example 16
16B 0.7 25 15 36.3 CGL 342 0.034 73 4.8 0.29 Example 16 16D 0.7 40
1 40.6 CAL 340 0.036 47* 12.0 0.90 Comparative Example Values
marked with * are not included in this invention.
[0225]
18 TABLE 18 Volumetric Proportion of Sheet Hot rolling condition
proportion particles of Average burr Standard Steel Sheet thickness
HR2 HR1 HR + HR2 of NbC sizes between height deviation No. No. (mm)
(%) (%) (%) Type TS (MPa) (%) 10 and 40 nm (%) (.mu.m) (.mu.m)
Remarks 17 17A 0.7 55 3 56.4 CAL 391 0.083 89 5.3 0.30 Example 17
17B 0.7 55 3 56.4 CGL 386 0.085 84 5.1 0.33 Example 17 17C 0.7 50
22 61.0 CAL 383 0.081 60* 10.2 0.75 Comparative Example 18 18A 0.7
12 12 22.6 CAL 325 0.071 77 4.9 0.25 Example 18 18B 0.7 20 35 48.0
CAL 328 0.075 53* 8.0 0.67 Comparative Example 19 19A 0.7 40 18
50.8 CAL 316 0.050 92 4.5 0.47 Example 19 19B 0.7 45 30 61.5 CAL
318 0.050 66* 8.0 0.95 Example 19 19C 0.7 10 32 38.8 CAL 315 0.048
47* 13.1 0.81 Comparative Example 20 20A 0.7 15 2 16.7 CAL 339
0.062 80 2.1 0.44 Example 20 20C 0.7 8 20 26.4 CAL 333 0.062 56*
9.1 0.86 Comparative Example 21 21A 0.7 30 5 33.5 CAL 330 0.044 71
3.8 0.39 Example 21 21C 0.7 65 5 66.8 CAL 326 0.042 40* 9.8 1.15
Comparative Example 22 22A 0.7 20 28 42.4 CAL 311 0.053 88 1.9 0.24
Example 22 22B 0.7 0 40 40.0 CAL 310 0.050 32* 7.5 0.65 Comparative
Example 22 22C 0.7 40 40 64.0 CAL 315 0.052 49* 10.3 0.72
Comparative Example 23 23A 0.7 35 24 50.6 CGL 342 0.096 92 2.1 0.20
Example 23 23B 0.7 35 24 50.6 CAL 340 0.091 83 1.8 0.22 Example 23
23C 0.7 8 2 9.8 CAL 343 0.094 26* 8.5 0.93 Comparative Example 24
24A 0.7 20 20 36.0 CAL 432 0.054 81 2.9 0.19 Example 24 24C 0.7 55
15 61.8 CAL 428 0.054 60* 9.0 0.81 Comparative Example Values
marked with * are not included in this invention.
[0226]
19 TABLE 19 Volumetric Proportion of Sheet Hot rolling condition
proportion particles of Average burr Standard Steel Sheet thickness
HR2 HR1 HR + HR2 of NbC sizes between height deviation No. No. (mm)
(%) (%) (%) Type TS (MPa) (%) 10 and 40 nm (%) (.mu.m) (.mu.m)
Remarks 25 25 0.7 25 15 36.3 CAL 372 0.055 78 7.4 2.01 Comparative
Example 26 26 0.7 25 15 36.3 CAL 345 0.041 80 6.3 1.77 Comparative
Example 27 27 0.7 25 15 36.3 CAL 318 0.063 53* 17.7 0.76
Comparative Example 28 28 0.7 25 15 36.3 CAL 330 0.049 75 6.1 1.93
Comparative Example 29 29 0.7 25 15 36.3 CAL 326 0* 0* 8.5 2.52
Comparative Example 30 30 0.7 25 15 36.3 CAL 367 0* 0* 11.1 3.51
Comparative Example 31 31 0.7 25 15 36.3 CAL 319 0.110* 80 13.2
0.77 Comparative Example 32 32 0.7 25 15 36.3 CAL 356 0.135 72 10.5
1.65 Comparative Example 33 33 0.7 25 15 36.3 CAL 368 0.168* 51*
11.0 2.80 Comparative Example 34 34 0.7 25 15 36.3 CAL 305 0.046*
27* 3.3 1.03 Comparative Example 35 35 0.7 25 15 36.3 CAL 317
0.060* 15* 6.1 1.65 Comparative Example Values marked with * are
not included in this invention.
[0227] Best Mode 6
[0228] The above-described Steel sheet 6 according to the present
invention is a steel sheet having particularly superior surface
condition. The detail of Steel sheet 6 is described-in the
following.
[0229] Carbon: Carbon forms a fine carbide with niobium to increase
the strength of the steel, and to increase the r values by reducing
the size of grains after annealed. Since the precipitation of
strengthening owing to the fine carbide is utilized, excellent
surface appearance is attained without need of addition of large
amount of silicon, manganese, and phosphorus. If the carbon content
is less than 0.0040%, the effect of carbon addition becomes less.
If the carbon content exceeds 0.010%, the ductility degrades.
Accordingly, the carbon content is specified to a range of from
0.0040 to 0.010%, preferably from 0.0050 to 0.0080%, most
preferably from 0.0050 to 0.0074%.
[0230] Silicon: Excessive addition of silicon degrades the
adhesiveness of zinc plating. Therefore, the silicon content is
specified to not more than 0.05%.
[0231] Manganese: Manganese precipitates sulfur in the steel as MnS
to prevent the hot crack generation of slabs and to bring the steel
to high strength without degrading the zinc plating adhesiveness.
If the manganese content is less than 0.1%, the effect of
precipitation of sulfur does not appear. If the manganese content
exceeds 1.5%, the strength significantly increases and the
ductility reduces. Consequently, the manganese content is specified
to a range of from 0.1 to 1.5%.
[0232] Phosphorus: Phosphorus is necessary for increasing strength
of the steel, to amounts of 0.01% or more. If the phosphorus
content exceeds 0.05%, however, degradation of toughness at welded
portions and insufficient adhesion of zinc plaint are generated.
Accordingly, the phosphorus content is specified to a range of from
0.01 to 0.05%.
[0233] Sulfur: If sulfur content exceeds 0.02%, the ductility
degrades. Therefore, the sulfur content is specified to not more
than 0.02%.
[0234] sol.Al: To remove oxygen from steel, sol.Al is added. If the
sol.Al content is below 0.01%, the effect of addition is not
satisfactory. If the sol.Al content exceeds 0.1%, solid solution
aluminum induces degradation of ductility. Consequently, the sol.Al
content is specified to a range of from 0.01 to 0.1%.
[0235] Nitrogen: The nitrogen forms solid solution in steel to
cause surface defects such as stretcher-strain. Therefore, the
nitrogen content is specified to not more than 0.0100%.
[0236] Niobium: Niobium forms fine carbide with carbon to increase
the strength of steel, and improves the surface condition and the
combined formability characteristics by reducing the grain sizes.
If, however, the niobium content is less than 0.036%, the effect of
the niobium addition cannot be attained. If the niobium content
exceeds 0.14%, the yield strength increases and the ductility
degrades. Therefore, the niobium content is specified to a range of
from 0.036 to 0.14%, preferably from 0.080 to 0.14%.
[0237] Solely specifying the individual components of steel cannot
necessarily lead to excellent surface appearance and combined
formability characteristics. It is necessary for the steel sheets
further to satisfy the formula (12), and to limit the average grain
size to not more than 10 .mu.m and the r value to not less than
1.8.
1.1<(Nb.times.12)/(C.times.93)<2.5 (12)
[0238] The value of [(Nb.times.12)/(C.times.93)] is specified to
more than 1.5, preferably not less than 1.7, to make the role of
NbC more effective.
[0239] To the Steel sheet 6 according to the present invention, the
addition of titanium is effective to enhance the reduction of grain
sizes, at amounts of not more than 0.019%, preferably from 0.005 to
0.019%, while satisfying the formula (13).
Ti.ltoreq.(48/14).times.N+(48/32).times.S (13)
[0240] To improve the resistance to embrittlement during secondary
operation, it is effective to add boron to not more than
0.0015%.
[0241] The Steel sheet 6 according to the present invention has
characteristics of, adding to the excellent surface appearance,
excellent combined formability, resistance to embrittlement during
secondary operation, anti-burring performance, uniformity of
material in a coil, which characteristics are applicable grades to
the automobile exterior panels.
[0242] The steel sheet 6 is manufactured by the steps of: preparing
a continuous casting slab of a steel which has the composition
described above, including the addition of titanium and boron;
preparing a sheet bar by either direct rolling or heating the slab
to temperatures of from 1100 to 1256.degree. C. followed by rough
rolling; finish rolling the sheet bar to 10 to 40% of total
reduction ratios of the pass just before the final pass and the
final pass to produce a hot rolled steel sheet; coiling the hot
rolled steel sheet at cooling speeds of 15.degree. C./sec or more
to temperatures below 700.degree. C., followed by coiling at
temperatures of from 620 to 670.degree. C.; cold rolling the coiled
hot rolled steel sheet at 50% or more reduction ratios, followed by
heating the steel sheet at 20.degree. C./sec or more of heating
speeds, then annealing the steel sheet at temperatures between
860.degree. C. and Ar3 transformation temperature; and temper
rolling the annealed steel sheet at 0.4 to 1.0% reduction
ratios.
[0243] For reheating the slab, temperatures of less than
1100.degree. C. results in significantly high deformation
resistance during hot rolling, and temperatures of more than
1250.degree. C. induces generation of excessive amount of scale to
possibly degrade the surface appearance. Accordingly, the slab
reheating is necessary to be conducted at temperatures of from 1100
to 1250.degree. C.
[0244] In the finish rolling, the total reduction ratios of the
pass just before the final pass and the final pass is necessary to
limit to not less than 10% for reducing the grain sizes after
annealed, and not more than 40% for preventing the generation of
nonuniform rolling texture. The sheet thickness after rolled is
preferably in a range of from 2.0 to 4.5 mm to secure required
reduction ratio in succeeding cold rolling.
[0245] After the hot rolling, the steel sheet is required to be
cooled to temperatures of not more than 700.degree. C. at cooling
speeds of not less than 15.degree. C./sec to prevent generation of
coarse grains.
[0246] The coiling is necessary to be carried out at temperatures
of from 620 to 670.degree. C. in view of enhancing the
precipitation of AlN and of descaling by pickling.
[0247] The reduction ratio during the cold rolling is necessary to
be 50% or more for obtaining high r values.
[0248] The annealing is required to be conducted at temperatures of
from 860.degree. C. and Ac3 transformation temperature with the
heating speeds of 20.degree. C./sec or more for preventing the
degradation of surface appearance resulted from coarse grain
formation and for attaining large r values.
[0249] The temper rolling is requested to be done at reduction
ratios of from 0.4 to 1.0% for suppressing aging and for preventing
increase in yield strength.
[0250] The Steel sheet 6 according to the present invention may
further be processed, at need, by zinc base plating treatment such
as electroplating and hot dip plating, and by organic coating
treatment after the plating.
EXAMPLE 1
[0251] Molten steels of Steel Nos. 1 through 13 shown in Table 20
were prepared. The melts were then continuously cast to form slabs
having 250 mm of thickness. After heating the slabs to 1200.degree.
C., hot rolled steel sheets having 2.8 mm of thickness were
prepared from the slabs under the condition of 880 to 910.degree.
C. of finish temperatures, at 20.degree. C./sec of average cooling
speed, and 640.degree. C. of coiling temperature. The hot rolled
sheets were then cold rolled to a thickness of 0.7 mm. The cold
rolled sheets were heated at about 30.degree. C./sec of heating
speed, then treated by continuous annealing at a temperature of
865.degree. C. for 60 seconds, followed by hot dip galvanization,
which were then temper-rolled to 0.7% of reduction ratio.
[0252] Thus obtained steel sheets were tested to determine
mechanical characteristics (along the rolling direction; with JIS
Class 5 specimens), r values, surface appearance, and resistance to
surface roughness.
[0253] The test results are shown in Table 21.
[0254] Example Steels Nos. 1 through 9 which have the composition
within a range of the present invention and which were manufactured
under the conditions specified by the present invention have not
more than 10 .mu.m of average grain sizes, and not less than 1.8 of
r values, and they are superior in surface appearance and
resistance to surface roughness.
[0255] On the other hand, Comparative Steel No. 10 is inferior in
resistance to surface roughness because the carbon content is less
than 0.0040% resulting in coarse grains. Comparative Steel No. 11
is inferior in r values because the carbon content exceeds 0.0010%,
resulting in excessive precipitation of NbC. Comparative Steel No.
12 is inferior in elongation and r values because the value of
[(Nb.times.12)/(C.times.93)] is not more than 1.1 so that the solid
solution carbon is left in the steel. Comparative Steel No. 13 is
inferior in elongation and r values because the value of
(Nb.times.12)/(C.times.93)] is not less than 2.5.
EXAMPLE 2
[0256] With the slabs of Steel Nos. 1 through 5 shown in Table 20,
hot dip galvanized steel sheets were prepared under the appearance
of hot rolling and annealing given in Table 22.
[0257] The similar investigation with Example 1 was conducted.
[0258] The results are shown in Table 22.
[0259] Example Steel sheets A, C, and E, which were prepared under
the condition within the range of the present invention give not
more than 10 .mu.m of average grain sizes and not less than 1.8 of
r values, thus proving the excellent surface appearance and
resistance to surface roughness.
[0260] On the other hand, Comparative Steel sheets B and F give low
r values and poor formability.
20TABLE 20 Steel No. C Si Mn P S Sol.Al N Nb Ti B (93/12) .times.
(Nb/C) Remarks 1 0.0060 0.01 0.35 0.018 0.008 0.056 0.0021 0.081 --
-- 1.74 Example Steel 2 0.0050 0.01 0.69 0.042 0.008 0.062 0.0020
0.082 -- -- 2.12 Example Steel 3 0.0090 0.01 0.38 0.027 0.008 0.022
0.0019 0.081 -- -- 1.16 Example Steel 4 0.0060 0.01 0.51 0.017
0.008 0.042 0.0023 0.055 -- -- 1.18 Example Steel 5 0.0060 0.01
0.31 0.041 0.008 0.058 0.0018 0.115 -- -- 2.47 Example Steel 6
0.0055 0.01 0.45 0.045 0.008 0.043 0.0049 0.060 -- -- 1.41 Example
Steel 7 0.0045 0.01 0.55 0.035 0.009 0.060 0.0083 0.042 -- -- 1.20
Example Steel 8 0.0060 0.01 0.31 0.036 0.008 0.040 0.0019 0.083
0.008 -- 1.78 Example Steel 9 0.0060 0.01 0.53 0.047 0.008 0.046
0.0022 0.081 0.015 0.0010 1.74 Example Steel 10 0.0025* 0.01 0.38
0.033 0.010 0.026 0.0021 0.020 0.020 - 1.03* Comparative Steel 11
0.0105* 0.01 0.70 0.039 0.008 0.024 0.0024 0.100 -- -- 1.23
Comparative Steel 12 0.0065 0.01 0.80 0.018 0.008 0.049 0.0018
0.050 -- -- 0.99* Comparative Steel 13 0.0065 0.01 0.61 0.020 0.008
0.034 0.0022 0.130 -- -- 2.58* Comparative Steel Units in Wt %
Values marked with * are not included in this invention.
[0261]
21TABLE 21 Steel Average particle Surface Resistance to No. TS
(MPa) El (%) r value size (.mu.m) appearance surface roughness
Remarks 1 350 42.9 2.14 8.6 A .largecircle. Example 2 385 40.5 2.03
8.1 A .largecircle. Example 3 360 41.7 1.97 7.8 A .largecircle.
Example 4 354 42.4 1.99 9.3 A .largecircle. Example 5 371 40.4 2.02
8.1 A .largecircle. Example 6 380 39.5 1.91 9.2 A .largecircle.
Example 7 373 40.2 1.96 9.5 A .largecircle. Example 8 376 39.9 1.90
7.3 B .largecircle. Example 9 385 38.9 1.95 9.9 B .largecircle.
Example 10 345 43.5 2.17 19.0 C X Comparative Example 11 392 34.5
1.78 6.9 A .largecircle. Comparative Example 12 375 37.5 1.65 8.1 B
.largecircle. Comparative Example 13 370 36.5 1.58 6.4 A
.largecircle. Comparative Example
[0262]
22TABLE 22 Heating Total reduction ratio of Finish Annealing
Average Resistance temper- the pass just beforethe temper- temper-
particle to Steel ature final pass and the final ature ature TS El
r size Surface sufrace Symbol No. (.degree. C.) pass (%) (.degree.
C.) (.degree. C.) (MPa) (%) value (.mu.m) appearance roughness A 1
1120 15 900 860 348 43.2 2.15 8.9 A .largecircle. Example B 4 1180
43 910 860 354 42.4 1.65 8.5 A .largecircle. Comparative Example C
5 1200 15 890 865 371 40.4 2.02 8.1 A .largecircle. Example D 1
1230 18 930 860 350 42.9 1.88 8.6 A .largecircle. Example E 2 1200
25 890 840 390 38.9 1.85 7.5 A .largecircle. Example F 3 1210 30
900 820 365 41.7 1.70 7.2 A .largecircle. Comparative Example
[0263] Best Mode 7
[0264] The above-described Steel sheet 7 according to the present
invention is a steel sheet having particularly superior uniformity
of material in a coil. The detail of Steel sheet 7 is described in
the following.
[0265] Carbon: Carbon forms a fine carbide with niobium to increase
the strength of the steel, and to increase the n values in the low
strain domains, thus improving the resistance to surface strain. If
the carbon content is less than 0.0050%, the effect of carbon
addition becomes less. If the carbon content exceeds 0.010%, the
ductility degrades. Accordingly, the carbon content is specified to
a range of from 0.0050 to 0.010%, preferably from 0.0050 to
0.0080%, most preferably from 0.0050 to 0.0074%.
[0266] Silicon: Excessive addition of silicon degrades the chemical
surface treatment performance of cold rolled steels, and degrades
the adhesiveness of plating to hot dip galvanized steel sheets.
Therefore, the silicon content is specified to not more than
0.05%.
[0267] Manganese: Manganese precipitates sulfur in the steel as MnS
to prevent the hot crack generation of slabs and to bring the steel
to high strength without degrading the zinc plating adhesiveness.
If the manganese content is less than 0.10%, the effect of
precipitation of sulfur does not appear. If the manganese content
exceeds 1.5%, the strength significantly increases, and reduces the
n values in low stress domains. Consequently, the manganese content
is specified to a range of from 0.10 to 1.5%.
[0268] Phosphorus: Phosphorus is necessary for increasing the
strength of the steel, to amounts of 0.01% or more. If the
phosphorus content exceeds 0.05%, however, the alloying treatment
performance of zinc plating degrades, thus inducing insufficient
adhesion of plating. Accordingly, the phosphorus content is
specified to a range of from 0.01 to 0.05%.
[0269] Sulfur: If sulfur content exceeds 0.02%, the ductility
degrades. Therefore, the sulfur content is specified to not more
than 0.02%.
[0270] sol. Al: A function of sol. Al is to reduce the harm of
solid solution nitrogen by precipitating the nitrogen in the steel
as AlN. If the sol.Al content is below 0.01%, the effect of
addition is not satisfactory. If the sol.Al content exceeds 0.1%,
the effect is not so improved for the added amount of sol.Al.
Consequently, the sol.Al content is specified to a range of from
0.01 to 0.1%.
[0271] Nitrogen: As small an amount of nitrogen as possible is
preferred. In view of cost, the nitrogen content is specified to
not more than 0.004%.
[0272] Niobium: Niobium forms fine carbide with carbon to increase
the strength of steel, and increases the n values in low strain
domains, thus improving the resistance to surface strain. If,
however, the niobium content is less then 0.01%, the effect of the
niobium addition cannot be attained. If the niobium content exceeds
0.20%, the yield strength significantly increases and the n values
in low strain domains decreases. Therefore, the niobium content is
specified to a range of from 0.01 to 0.20%, preferably from 0.035
to 0.20%, and most preferably from 0.080 to 0.140%
[0273] Solely specifying the individual components of steel cannot
necessarily lead to a high strength cold rolled sheet having
excellent uniformity of material in a coil, deep drawability, and
punch stretchability. It is necessary for the steel sheet further
to satisfy the condition given below.
[0274] A slab consisting essentially of 0.0061% C, 0.01% Si, 0.30%
Mn, 0.02% P, 0.005% S, 0.050% sol.Al., 0.0024% N, 0.040 to 0.170%
Nb, by weight, was finish rolled at 900.degree. C. of finish
temperature and 40% of total reduction ratio of the pass just
before the final pass and the final pass. The rolled sheet was
coiled at temperatures of from 580 to 680.degree. C., followed by
cold rolled to obtain a sheet having 0.8 mm of thickness. The cold
rolled sheet was then continuously annealed at 850.degree. C., and
was temper rolled to 0.7% of reduction ratio. Thus prepared steel
sheet was tested to determine the uniformity of material in a
coil.
[0275] FIG. 21 shows the influence of ((Nb.times.12)/(C.times.93)]
and C on the uniformity of material in a coil.
[0276] When the value of [(Nb.times.12)/(C.times.93)] satisfies the
formula (14), excellent uniformity of material in a coil is
obtained.
1.98-66.3.times.C.ltoreq.(Nb.times.12)/(C.times.93).ltoreq.3.24-80.0.times-
.C (14)
[0277] As for the deep drawability, the above-prepared steel sheet
was used for evaluating the characteristic by determining the limit
drawing ratio during the cylinder forming described in the Best
Mode 1, and the hat forming height after the hat forming test.
[0278] FIG. 22 shows the influence of r values and n values on the
deep drawability and the punch stretchability.
[0279] Similar with the Best Mode 1, excellent deep drawability and
punch stretchability are obtained if only the formulae (3) and (4)
are satisfied.
11.0.ltoreq.r+50.0.times.n (3)
2.9.ltoreq.r+5.00.times.n (4)
[0280] The Steel sheet 7 according to the present invention may
further contain titanium to form fine grains and to improve
resistance to surface strain. If the titanium content exceeds
0.05%, the surface appearance significantly degrades on hot dip
galvanization. Therefore, the titanium content is specified to not
more than 0.05%, preferably from 0.005 to 0.02%. In that case,
formula (15) is necessary to be applied instead of formula
(14).
1.98-66.3.times.C.ltoreq.(Nb.times.12)/(C.times.93)+(Ti*.times.12)/(C.time-
s.48).ltoreq.3.24-80.0.times.C (15)
[0281] Furthermore, to improve the resistance to embrittlement
during secondary operation, the addition of boron is effective. If
the boron content exceeds 0.002%, the deep drawability and the
punch stretchability degrade. Accordingly, the boron content is
specified to not more than 0.002%, preferably from 0.0001 to
0.001%.
[0282] The Steel sheet 7 according to the present invention has
characteristics of, adding to the excellent uniformity of material
in a coil, excellent combined formability, resistance to
embrittlement during secondary operation, formability at welded
portions, anti-burring performance during shearing, good surface
appearance, which characteristics are applicable grades to the
automobile exterior panels.
[0283] The Steel sheet 7 according to the present invention can be
manufactured by the steps of: preparing a continuous casting slab
of a steel having the composition adjusted as described above,
including the addition of titanium and boron; finish rolling the
slab to 60% or less of total reduction ratios of the pass just
before the final pass and the final pass to prepare coiled hot
rolled steel sheet; and cold rolling the hot rolled steel sheet
followed by annealing. For hot rolling the continuous cast slab may
be done directly or after reheated.
[0284] To obtain excellent uniformity of material in a coil, deep
drawability, and punch stretchability without fail, it is preferred
to conduct the finish rolling at temperatures of 870.degree. C. or
more, the coiling after rolled at temperatures of 550.degree. C. or
more, the cold rolling at 50 to 85% of reduction ratios, and the
annealing at temperatures of from 780 to 880.degree. C. in a
continuous annealing line. From the viewpoint of stability of
descaling by pickling, the coiling is preferably done at
700.degree. C. or less of temperatures, more preferably 680.degree.
C. or less.
[0285] The Steel sheet 7 according to the present invention may
further be treated, at need, by zinc base plating treatment such as
electroplating and hot dip plating, and by organic coating
treatment after the plating.
EXAMPLE 1
[0286] Molten steels of Steel Nos. 1 through 10 shown in Table 23
were prepared. The melts were then continuously cast to form slabs
having 220 mm of thickness. After heating the slabs to 1200.degree.
C., hot rolled steel sheets having 2.8 mm of thickness were
prepared from the slabs under the condition of 30 to 50% of total
reduction ratios of the pass just before the final pass and the
final pass, 880 to 960.degree. C. of finish temperatures. The hot
rolled steel sheets were coiled at 580 to 680.degree. C. of coiling
temperatures. The coiled hot rolled sheets were then cold rolled to
a thickness of 0.80 mm. The cold rolled sheets were treated by
continuous annealing (CAL) at temperatures of from 840 to
870.degree. C., or by continuous annealing at 850 to 870.degree. C.
of temperatures followed by hot dip galvanization (CGL), which were
then temper-rolled to 0.7% of reduction ratio.
[0287] In the case of continuous annealing followed by hot dip
galvanization, the hot dip galvanization after the annealing was
given at 460.degree. C., and, immediately after the hot dip
galvanization, an alloying treatment of plating layer was given at
500.degree. C. in an in-line alloying furnace. The coating weight
was 45 g/m.sup.2 per side.
[0288] Thus obtained steel sheets were tested to determine tensile
characteristics (along the rolling direction; with JIS Class 5
specimens; and n values being computed in a 1 to 5% strain domain),
r values, limit drawing ratio (LDR), and hat forming height (H).
For the galvanized steel sheets, the zinc plating adhesiveness was
also determined.
[0289] Regarding the zinc plating adhesiveness, adhesive tapes were
attached onto the surface of a plating steel sheet, and the steel
sheet was subjected to 90 degrees of bending and straightening,
then the amount of plating attached to the adhesive tapes was
determined. The determination was given on five grades: 1 for no
peeling observed; 2 for slight peeling observed; 3 for small amount
of peeling observed; 4 for medium area of peeling observed; and 5
for large area of peeling observed. The grades 1 and 2 were set to
acceptable range.
[0290] The test results are shown in Tables 24 through 26.
[0291] These tables show that the Example steel sheets give
excellent deep drawability, punch stretchability, and uniformity of
material in a coil, also give excellent zinc plating
adhesiveness.
[0292] To the contrary, the Comparative steel sheets give poor deep
drawability and punch stretchability, and, when they dissatisfy the
above-given formula (14), the uniformity of material in the
longitudinal direction of coil is significantly poor. In addition,
when phosphorus and titanium exist to a large amount, the plating
adhesiveness is also inferior.
EXAMPLE 2
[0293] Slab of Steel No. 1 shown in Table 23 was heated to
1200.degree. C., and hot rolled to 2.8 mm of thickness under the
condition of 30 to 70% of total reduction ratios of the pass just
before the final pass and the final pass, 880 to 910.degree. C. of
finish temperatures. The hot rolled steel sheets were coiled at 580
to 640.degree. C. of coiling temperatures. The, coiled hot rolled
sheets were then cold rolled to a thickness of 0.8 mm. The cold
rolled sheets were treated by continuous annealing at temperatures
of from 840 to 870.degree. C., or by continuous annealing at 850 to
870.degree. C. of temperatures followed by hot dip galvanization,
which were then temper-rolled to 0.7% of reduction ratio.
[0294] The condition of hot dip galvanization was the same with
that of Example 1.
[0295] Thus obtained steel sheets were tested to determine tensile
characteristics along the rolling direction (n values being
computed in a 1 to 5% strain domain), r value, limit drawing ratio,
and hat forming height.
[0296] The test results are shown in Table 27.
[0297] The steels which were prepared at 60% or less of total
reduction ratios of the pass just before the final pass and the
final pass, and which reduction ratios were within the specified
range of the present invention, showed excellent uniformity of
material in the coil longitudinal direction.
EXAMPLE 3
[0298] Slab of Steel No. 1 shown in Table 23 was heated to
1200.degree. C., and hot rolled to 1.3 to 6.0 mm of thicknesses
under the condition of 40% of total reduction ratios of the pass
just before the final pass and the final pass, 840 to 980.degree.
C. of finish temperatures. The hot rolled steel sheets were coiled
at 500 to 700.degree. C. of coiling temperatures. The coiled hot
rolled sheets were then cold rolled to a thickness of 0.80 mm at 46
to 87% of reduction ratios. The cold rolled sheets were treated by
continuous annealing or by continuous annealing followed by hot dip
galvanization, which were then temper-rolled to 0.7% of reduction
ratio.
[0299] The condition of hot dip galvanization was the same with
that of Example 1.
[0300] Thus obtained steel sheets were tested to determine tensile
characteristics along the rolling direction (n values being
computed in a 1 to 5% strain domain), r values, limit drawing
ratio, and hat forming height.
[0301] The test results are shown in Tables 28 and 29.
[0302] The steels which were prepared within the specified range of
the present invention in terms of finish temperature, coiling
temperature, reduction ratio during cold rolling, and annealing,
showed excellent uniformity of material in the coil longitudinal
direction.
23TABLE 23 Steel No. C Si Mn P S sol.Al N Nb Ti B X/C # Remarks 1
0.0059 0.01 0.34 0.019 0.011 0.050 0.0021 0.082 tr tr 1.8 Example
Steel 2 0.0060 0.01 0.63 0.040 0.007 0.062 0.0012 0.075 tr tr 1.6
Example Steel 3 0.0078 0.01 0.95 0.045 0.009 0.058 0.0018 0.162 tr
tr 2.7 Example Steel 4 0.0065 0.02 0.25 0.021 0.008 0.050 0.0017
0.091 0.011 tr 1.8* Example Steel 5 0.0081 0.01 0.42 0.020 0.007
0.050 0.0017 0.092 0.024 0.0006 1.7* Example Steel 6 0.0063 0.10
0.16 0.030 0.011 0.057 0.0019 0.088 tr tr 1.8 Comparative Steel 7
0.0059 0.02 0.20 0.067 0.010 0.050 0.0021 0.087 tr tr 1.9
Comparative Steel 8 0.0060 0.01 0.22 0.030 0.009 0.056 0.0019 0.056
tr tr 1.2 Comparative Steel 9 0.0058 0.01 0.21 0.028 0.010 0.057
0.0020 0.148 tr tr 3.3* Comparative Steel 10 0.0090 0.01 0.62 0.050
0.015 0.035 0.0036 0.126 tr tr 1.8 Comparative Steel X/C #:(Nb %
.times. 12)/(C % .times. 93) *(Nb % .times. 12)/(C % .times. 93) +
(Ti % .times. 12)/(C % .times. 48), Ti* % = Ti - (48/14)N % -
(48/32)S %
[0303]
24TABLE 24 Total reduction ratio of the pass just before the final
Finish Coiling Steel pass and the final pass temperature
temperature Annealing No. No. (%) (.degree. C.) (.degree. C.)
condition 1 1 40 890 580 CAL 2 1 40 890 580 CGL 3 1 40 900 640 CAL
4 1 40 900 640 CGL 5 1 40 910 680 CAL 6 1 40 910 680 CGL 7 2 30 910
580 CGL 8 2 30 930 640 CGL 9 3 50 890 640 CGL 10 3 50 900 680 CGL
11 4 30 890 580 CGL 12 4 30 900 640 CGL 13 4 30 910 680 CGL 14 5 40
900 640 CGL 15 5 40 910 680 CGL Characterisitcs of steel sheet
Formability of YP TS steel sheet Zinc plating No. (MPa) (MPa) El
(%) n value r value Y** Z*** H (mm) LDR adhesiveness Remarks 1 204
353 44 0.201 2.00 12.1 3.0 34.8 2.16 -- Example 2 207 356 44 0.194
2.01 11.7 3.0 34.2 2.16 1 Example 3 202 354 45 0.202 2.03 12.1 3.0
34.8 2.16 -- Example 4 196 355 45 0.200 2.02 12.0 3.0 34.6 2.16 1
Example 5 193 352 46 0.203 2.09 12.2 3.1 34.9 2.17 -- Example 6 195
356 45 0.202 2.06 12.2 3.1 34.9 2.17 2 Example 7 214 384 42 0.191
1.97 11.5 2.9 33.8 2.15 1 Example 8 212 382 43 0.196 1.95 11.8 2.9
34.3 2.15 1 Example 9 225 395 41 0.195 2.09 11.8 3.1 34.3 2.17 2
Example 10 227 394 42 0.199 2.13 12.1 3.1 34.8 2.17 2 Example 11
205 355 43 0.198 1.98 11.9 3.0 34.4 2.16 1 Example 12 203 354 43
0.201 2.01 12.1 3.0 34.8 2.16 1 Example 13 202 352 44 0.202 2.04
12.1 3.1 34.8 2.17 1 Example 14 212 372 39 0.189 1.96 11.4 2.9 33.6
2.15 2 Example 15 210 370 40 0.194 1.93 11.6 2.9 34.0 2.15 2
Example Y** = r + 50.0 x n, Z*** = r + 5.0 x n
[0304]
25TABLE 25 Total reduction ratio of the pass just before the final
Finish Coiling Steel pass and the final pass temperature
temperature Annealing No. No. (%) (.degree. C.) (.degree. C.)
condition 16 6 30 900 640 CGL 17 6 30 910 680 CGL 18 7 30 900 640
CGL 19 7 30 910 680 CGL 20 8 40 900 580 CAL 21 6 40 890 580 CGL 22
6 40 900 640 CAL 23 7 40 900 640 CGL 24 7 30 890 580 CAL 25 8 30
890 580 CGL 26 6 30 900 640 CAL 27 6 30 900 640 CGL 28 7 40 900 640
CAL Characteristics of steel sheet Formability of YP TS steel sheet
Zinc plating No. (MPa) (MPa) El (%) n value r value Y** Z*** H (mm)
LDR adhesiveness Remarks 16 215 365 42 0.182 1.88 11.0 2.8 33.0
2.07 4 Comparative Example 17 212 362 43 0.184 1.86 11.1 2.8 33.2
2.07 5 Comparative Example 18 222 368 41 0.180 1.93 10.9 2.8 29.4
2.07 3 Comparative Example 19 224 367 41 0.178 1.93 10.8 2.8 28.0
2.07 4 Comparative Example 20 321 394 23 0.126 1.12 7.4 1.8 19.4
1.96 -- Comparative Example 21 323 398 22 0.128 1.18 7.6 1.8 19.6
1.96 1 Comparative Example 22 283 382 30 0.146 1.34 8.6 2.1 20.6
1.99 -- Comparative Example 23 287 385 31 0.142 1.30 8.4 2.0 20.4
1.98 1 Comparative Example 24 243 376 37 0.153 1.72 9.4 2.5 21.8
2.03 -- Comparative Example 25 245 680 36 0.154 1.77 9.5 2.5 22.1
2.05 2 Comparative Example 26 231 361 37 0.176 1.81 10.6 2.7 27.3
2.05 -- Comparative Example 27 233 364 38 0.172 1.80 10.4 2.7 26.2
2.15 2 Comparative Example 28 222 370 32 0.163 2.12 10.3 2.9 25.5
2.07 2 Comparative Example Y** = r + 50.0 x n, Z*** = r + 5.0 x
n
[0305]
26TABLE 26 Total reduction ratio of the pass just before the final
pass Finish Coiling Steel and the final pass temperature
temperature Annealing Coil No. No. (%) (.degree. C.) (.degree. C.)
condition position 29 1 40 890 580 CAL T M B 30 1 30 900 640 CGL T
M B 31 6 40 900 640 CGL T M B Formability Characterisitics of steel
sheet of steel sheet YP TS r H No. (MPa) (MPa) El (%) n value value
Y** Z*** (mm) LDR Remarks 29 204 353 44 0.201 2.01 12.1 3.0 34.8
2.16 Example 202 352 45 0.204 2.01 12.2 3.0 34.9 2.16 203 355 44
0.202 2.02 12.1 3.0 34.8 2.16 30 202 355 44 0.200 2.02 12.0 3.0
34.6 2.16 Example 204 353 45 0.198 2.02 11.9 3.0 34.4 2.16 201 356
44 0.202 2.01 12.1 3.0 34.8 2.16 31 287 375 31 0.142 1.36 8.5 2.1
20.5 1.99 Comparative 211 364 36 0.186 1.80 11.1 2.7 33.2 2.05
Example 243 374 31 0.150 1.40 8.9 2.2 20.9 2.00 Y** = r + 50.0 x n,
Z*** = r + 5.0 x n
[0306]
27TABLE 27 Total reduction ratio of the pass just before the final
pass Finish Coiling Steel and the final pass temperature
temperature Annealing Coil No. No. (%) (.degree. C.) (.degree. C.)
condition position 32 1 40 890 580 CAL T M B 33 1 30 900 640 CGL T
M B 34 1 65 890 580 CAL T M B 35 1 65 900 640 CGL T M B
Characteristics of steel sheet Formability of YP TS steel sheet No.
(MPa) (MPa) El (%) n value r value Y** Z*** H (mm) LDR Remarks 32
204 353 44 0.201 2.01 12.1 3.0 34.8 2.16 Example 202 352 45 0.204
2.01 12.2 3.0 34.9 2.16 203 355 44 0.202 2.02 12.1 3.0 34.8 2.16 33
202 355 44 0.200 2.02 12.0 3.0 34.6 2.16 Example 204 353 45 0.198
2.02 11.9 3.0 34.4 2.16 201 356 44 0.202 2.01 12.1 3.0 34.8 2.16 34
297 402 26 0.147 1.22 8.6 2.0 20.6 1.98 Comparative 259 384 32
0.173 1.68 10.3 2.5 25.5 2.03 Example 275 391 30 0.152 1.42 9.0 2.2
21.0 2.00 35 285 388 27 0.156 1.31 9.1 2.1 21.2 1.99 Comparative
246 371 35 0.190 1.76 11.3 2.7 33.5 2.05 Example 263 376 30 0.173
1.52 10.2 2.4 24.8 2.02 Y** = r + 50.0 x n, Z*** = r + 5.0 x n
[0307]
28TABLE 28 Finish Coiling Cold rolling Annealing temperature
temperature ratio Annealing temperature Coil No. (.degree. C.)
(.degree. C.) (%) condition (.degree. C.) position 36 890 580 71
CAL 850 T M B 37 930 640 75 CGL 640 T M B 38 840 640 71 CGL 850 T M
B 39 900 500 71 CAL 830 T M B 40 890 640 46 CGL 810 T M B
Characteristics of steel sheet Formability of YP TS steel sheet No.
(MPa) (MPa) El (%) n value r value Y** Z*** H (mm) LDR Remarks 36
204 353 44 0.201 2.01 12.1 3.0 34.8 2.16 Example 202 352 45 0.204
2.01 12.2 3.0 34.9 2.16 203 355 44 0.202 2.02 12.1 3.0 34.8 2.16 37
194 352 46 0.212 2.10 12.7 3.2 35.6 2.38 Example 196 348 47 0.214
2.12 12.8 3.2 35.7 2.18 193 351 46 0.211 2.13 12.7 3.2 35.6 2.18 38
277 385 30 0.154 1.43 9.1 2.2 21.2 2.00 Comparative 213 358 41
0.181 1.78 10.8 2.7 28.0 2.05 Example 252 372 33 0.173 3.61 10.2
2.5 24.8 2.03 39 234 371 34 0.147 1.62 9.0 2.4 23.0 2.02
Comparative 222 365 37 0.153 1.66 9.3 2.4 21.6 2.02 Example 231 369
35 0.150 1.63 9.1 2.4 21.2 2.02 40 218 351 41 0.179 1.55 10.5 2.4
27.0 2.02 Comparative 208 347 43 0.186 3.59 30.9 2.5 29.4 2.03
Example 215 349 42 0.183 1.57 30.7 2.5 27.5 2.03 Y** = r + 50.0 x
n, Z*** = r + 5.0 x n
[0308]
29TABLE 29 Finish Coiling Cold rolling Annealing temperature
temperature ratio Annelaing temperature Coil No. (.degree. C.)
(.degree. C.) (%) condition (.degree. C.) position 41 910 680 87
CGL 860 T M B 42 880 580 71 CAL 750 T M B 43 920 640 73 CCL 900 T M
B 44 870 550 68 CCL 780 T M B Characteristics of steel sheet
Formability of YP TS EL n r steel sheet No. (MPa) (MPa) (%) value
value Y** Z*** H (mm) LDR Remarks 41 247 372 40 0.158 2.14 10.0 2.9
23.2 2.15 Comparative 233 368 42 0.166 2.17 10.5 3.0 27.0 2.16
Example 242 371 41 0.151 2.15 9.7 2.9 22.7 2.15 42 236 365 40 0.167
1.01 10.0 2.4 23.2 2.02 Comparative 224 361 42 0.172 1.64 10.2 2.5
24.8 2.03 Example 229 362 42 1.170 1.63 10.1 2.5 24.0 2.03 43 248
381 32 0.143 1.56 8.7 2.3 20.7 2.01 Comparative 239 373 34 0.150
1.62 9.1 2.4 21.2 2.02 Example 244 377 33 0.148 1.59 9.0 2.3 21.0
2.01 44 228 373 33 0.146 1.54 8.8 2.3 20.8 2.01 Comparative 217 369
34 0.151 1.58 9.1 2.3 21.2 2.01 Example 223 370 33 0.149 1.57 9.0
2.3 21.0 2.01 Y** = r + 50.0 xn, Z*** = r + 5.0 xn
* * * * *