U.S. patent application number 14/195598 was filed with the patent office on 2014-06-26 for method for manufacturing a high-strength steel sheet for a can.
This patent application is currently assigned to JFE Steel Corporation. The applicant listed for this patent is JFE Steel Corporation. Invention is credited to Makoto ARATANI, Toshikatsu KATO, Katsuhito KAWAMURA, Masaki KOIZUMI, Katsumi KOJIMA, Kaku SATO, Shigeko SUJITA, Takumi TANAKA.
Application Number | 20140174609 14/195598 |
Document ID | / |
Family ID | 41135701 |
Filed Date | 2014-06-26 |
United States Patent
Application |
20140174609 |
Kind Code |
A1 |
ARATANI; Makoto ; et
al. |
June 26, 2014 |
METHOD FOR MANUFACTURING A HIGH-STRENGTH STEEL SHEET FOR A CAN
Abstract
A method for manufacturing a high-strength steel sheet for a
can, including (a) hot rolling a steel slab at a slab extraction
temperature of 1200.degree. C. or more and a finish rolling
temperature of (Ar3 transformation temperature-30).degree. C. or
more, the steel slab containing, on the basis of mass percent, more
than 0.02%, but 0.10% or less of C, 0.10% or less of Si, 1.5% or
less of Mn, 0.20% or less of P, 0.20% or less of S, 0.10% or less
of Al, 0.0120% to 0.0250% of N, and the balance being Fe and
incidental impurities; (b) coiling at a temperature of 650.degree.
C. or less; (c) pickling; (d) carrying out a first cold rolling;
(e) continuously annealing; and (f) carrying out a second cold
rolling at a reduction ratio of 10% or more and less than 20%.
Inventors: |
ARATANI; Makoto; (Chiba,
JP) ; KATO; Toshikatsu; (Chiba, JP) ;
KAWAMURA; Katsuhito; (Chiba, JP) ; TANAKA;
Takumi; (Fukuyama, JP) ; KOJIMA; Katsumi;
(Fukuyama, JP) ; SATO; Kaku; (Chiba, JP) ;
SUJITA; Shigeko; (Chiba, JP) ; KOIZUMI; Masaki;
(Chiba, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE Steel Corporation |
Tokyo |
|
JP |
|
|
Assignee: |
JFE Steel Corporation
Tokyo
JP
|
Family ID: |
41135701 |
Appl. No.: |
14/195598 |
Filed: |
March 3, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12935564 |
Dec 2, 2010 |
|
|
|
PCT/JP2009/057153 |
Apr 1, 2009 |
|
|
|
14195598 |
|
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|
Current U.S.
Class: |
148/537 ;
148/603; 72/66 |
Current CPC
Class: |
C21D 8/0263 20130101;
C22C 38/04 20130101; C21D 8/0426 20130101; C22C 38/02 20130101;
C21D 8/0473 20130101; C22C 38/06 20130101; C21D 2211/004 20130101;
C21D 8/0442 20130101; C22C 38/001 20130101; C21D 8/0436 20130101;
B21B 1/26 20130101 |
Class at
Publication: |
148/537 ;
148/603; 72/66 |
International
Class: |
C21D 8/02 20060101
C21D008/02; B21B 1/26 20060101 B21B001/26; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 3, 2008 |
JP |
2008-097011 |
Claims
1. A method for manufacturing a high-strength steel sheet for a
can, comprising: (a) hot rolling a steel slab at a slab extraction
temperature of 1200.degree. C. or more and a finish rolling
temperature of (Ar3 transformation temperature-30).degree. C. or
more, the steel slab containing, on the basis of mass percent, more
than 0.02%, but 0.10% or less of C, 0.10% or less of Si, 1.5% or
less of Mn, 0.20% or less of P, 0.20% or less of S, 0.10% or less
of Al, 0.0120% to 0.0250% of N, and the balance being Fe and
incidental impurities; (b) coiling at a temperature of 650.degree.
C. or less; (c) pickling; (d) carrying out a first cold rolling;
(e) continuously annealing; and (f) carrying out a second cold
rolling at a reduction ratio of 10% or more and less than 20%.
2. The method for manufacturing a high-strength steel sheet for a
can according to claim 1, wherein a soaking temperature of the
continuous annealing is equal to or higher than the Ar1
transformation temperature.
3. The method for manufacturing a high-strength steel sheet for a
can according to claim 1, further comprising plating after carrying
out the second cold rolling.
4. The method for manufacturing a high-strength steel sheet for
cans according to claim 2, further comprising plating after
carrying out the second cold rolling.
5. The method for manufacturing a high-strength steel sheet for
cans according to claim 1, wherein the steel sheet has a thickness
of 0.3 mm or less.
6. The method for manufacturing a high-strength steel sheet for
cans according to claim 3, wherein the steel sheet has a thickness
of 0.3 mm or less.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of Ser. No.
12/935,564 filed Dec. 2, 2010, which is the United States national
phase application of International, application PCT/JP2009/057153
filed Apr. 1, 2009. The entire contents of each of application Ser.
No. 12/935,564 and International application PCT/JP2009/057153 are
incorporated by reference herein.
TECHNICAL FIELD
[0002] The present invention relates to a high-strength steel sheet
for cans, which is suitable as a material for cans in which the
diameter shape is reduced or increased after three-piece
processing, such as welding, or two-piece processing, such as
drawing and ironing, and to a method for manufacturing the
high-strength steel sheet for cans.
BACKGROUND ART
[0003] In recent years, for the purpose of reducing costs and for
the purpose of reducing materials to be used and environmental
load, product development has been performed to decrease the
thickness of a product formed of a steel material (steel
sheet).
[0004] Since a reduction in the thickness of a product sheet
results in lower rigidity, the strength of steel must be increased
to compensate for the reduction in rigidity. However, since
higher-strength steel is hard, cracking may occur during a flanging
or necking process. To address this problem, various manufacturing
methods have been proposed.
[0005] For example, Patent Document 1 proposes a method of
performing hot rolling at a temperature of (Ar3 transformation
point-30.degree. C.) or more, performing cold rolling, and then
performing continuous annealing, while controlling the steel
components within a certain range.
[0006] However, in the method according to Patent Document 1, in
which P is 0.02% by weight or less to prevent deterioration in
flanging and necking formability and corrosion resistance, and the
draft in second cold rolling ranges from 15% to 30%, it is
difficult to efficiently treat and manufacture thin products, and
defective appearance tends to occur. Furthermore, stable
manufacturing is difficult to achieve, and therefore some
improvements are needed.
[0007] Patent Document 2 proposes a method for manufacturing a
steel sheet for cans that has a yield stress of at least 550 MPa
after coating and baking, the method including hot rolling at a
temperature of (Ar3 transformation point-30.degree. C.) or more,
predetermined cooling, coiling, water cooling, cold rolling, and
subsequent continuous annealing in a predetermined heating pattern,
while the steel components and dissolved N are controlled within a
certain range.
[0008] However, in the method according to Patent Document 2,
semi-very low carbon material is used, the continuous annealing
temperature is increased to secure predetermined dissolved N, and
the heating pattern is difficult to control precisely. These make
the manufacture difficult. Furthermore, only by securing dissolved
N in an amount of at least 80% of N in steel, it is difficult to
stably manufacture a steel sheet having a specified strength,
because the N content in steel varies. Thus, some improvements are
needed. Furthermore, the method according to Patent Document 2
provides small total elongation, resulting in poor
processability.
[0009] The following methods are proposed as representative methods
for manufacturing a high-strength steel sheet for cans, and have
been appropriately used depending on the type of annealing (for
example, Non-patent Document 1).
[0010] Hot rolling.fwdarw.pickling.fwdarw.cold rolling.fwdarw.box
annealing (BAF).fwdarw.second cold rolling (draft: 20% to 50%)
[0011] Hot rolling.fwdarw.pickling.fwdarw.cold
rolling.fwdarw.continuous annealing (CAL).fwdarw.second cold
rolling (draft: 20% to 50%)
[0012] However, in these methods, since the draft in the second
cold rolling is as high as 20% to 50%, the high rolling load
results in low operational efficiency. Furthermore, since various
viscous rolling oils are used to improve lubricity in rolling,
inconsistencies in concentration of the rolling oil and partial oil
deposition cause defective appearance after rolling. A high draft
results in small total elongation and low processability.
Elongation of a steel sheet in rolling increases a difference
between the proof stress in the width direction and the proof
stress in the longitudinal direction in accordance with the machine
direction and processing direction of a material.
[0013] To address this problem, a method in which the draft in the
second cold rolling is reduced is conceivable. However, at a low
draft, it is difficult to achieve a desired proof stress. [0014]
Patent Document 1: Japanese Patent No. 3108615 [0015] Patent
Document 2: Japanese Unexamined Patent Application Publication No.
2001-107187 [0016] Non-patent Document 1: "Wagakuniniokeru
kanyohyomensyorikohan no gijutushi (Technography of surface-treated
steel sheets for cans in Japan)", The Iron and Steel Institute of
Japan, Oct. 30, 1998, p. 188
[0017] Thus, there is presently no method for manufacturing a thin
steel sheet for cans that satisfies both strength and productivity,
and such a method is desired.
[0018] In view of the situations described above, it is an object
of the present invention to provide a steel sheet for cans that has
a yield stress YP of at least 500 MPa after coating and baking and
a method for manufacturing the steel sheet for cans.
DISCLOSURE OF INVENTION
[0019] The present invention is as follows:
[0020] [1] A high-strength steel sheet for cans, containing, on the
basis of mass percent, C: more than 0.02% and 0.10% or less, Si:
0.10% or less, Mn: 1.5% or less, P: 0.20% or less, S: 0.20% or
less, Al: 0.10% or less, N: 0.0120% to 0.0250%, dissolved N being
0.0100% or more, and the balance being Fe and incidental
impurities.
[0021] [2] The high-strength steel sheet for cans according to [1],
wherein the high-strength steel sheet has a plated layer on the
surface thereof.
[0022] [3] A method for manufacturing a high-strength steel sheet
for cans, including: hot rolling a steel slab at a slab extraction
temperature of 1200.degree. C. or more and a finish rolling
temperature of (Ar3 transformation temperature-30).degree. C. or
more, the steel slab containing, on the basis of mass percent, C:
more than 0.02% but 0.10% or less, Si: 0.10% or less, Mn: 1.5% or
less, P: 0.20% or less, S: 0.20% or less, Al: 0.10% or less, N:
0.0120% to 0.0250%, and the balance being Fe and incidental
impurities; coiling at a temperature of 650.degree. C. or less;
pickling; cold rolling; and then continuously annealing.
[0023] [4] The method for manufacturing a high-strength steel sheet
for cans according to [3], further including second cold rolling at
a reduction ratio of 10% or more and less than 20% after the
continuous annealing.
[0024] [5] The method for manufacturing a high-strength steel sheet
for cans according to [3] or [4], wherein the soaking temperature
of the continuous annealing is equal to or higher than the Ar1
transformation temperature.
[0025] [6] The method for manufacturing a high-strength steel sheet
for cans according to any one of [3] to [5], further including
plating after the continuous annealing or the second cold
rolling.
[0026] In the present specification, all the percentages of
components of steel are based on mass percent. The term
"high-strength steel sheet for cans", as used herein, refers to a
steel sheet for cans that has a yield stress YP of at least 500 MPa
after coating and baking.
[0027] A high-strength steel sheet for cans according to the
present invention is intended for a material for cans. Whether the
presence or absence of surface treatment, the high-strength steel
sheet for cans may be subjected to tin plating, nickel-tin plating,
chromium plating (so-called tin-free plating), or organic coating,
and can be used in a wide variety of applications.
[0028] The thickness of the steel sheet is not limited to a
particular value, but is preferably 0.3 mm or less and more
preferably 0.2 mm or less to get the most out of the present
invention. 0.170 mm or less is particularly preferred.
BEST MODES FOR CARRYING OUT THE INVENTION
[0029] As a result of diligent research to solve the problems
described above, the present inventors have obtained the following
findings.
[0030] The present inventors have found that a high-strength
material can be obtained by using a low carbon material as the
composition, maintaining the absolute quantity of dissolved N at a
certain value or more, and performing hardening by quench aging and
strain aging, for example, in a printing process, a film lamination
process, or a drying and baking process before can
manufacturing.
[0031] Thus, in the present invention, a high-strength steel sheet
for cans has been completed by controlling the components on the
basis of these findings.
[0032] The present invention will be described in detail below.
[0033] A high-strength steel sheet for cans according to the
present invention is a steel sheet for cans that has a yield stress
YP of at least 500 MPa after coating and baking.
[0034] In the present invention, by using a low carbon material,
maintaining the absolute quantity of dissolved N at a certain value
or more, and performing age hardening after coating and baking, a
high-strength steel sheet for cans can be efficiently produced
without second cold rolling or by second cold rolling at a low
draft. A steel sheet for cans manufactured without second cold
rolling, that is, manufactured by temper rolling of about 1% after
continuous annealing has a total elongation El of at least 20%
after coating and baking. A steel sheet for cans subjected to
second cold rolling at a draft of 10% or more but less than 15% has
a total elongation El above 10% after coating and baking.
[0035] The composition of a steel sheet for containers according to
the present invention will be described below.
C: More than 0.02% but 0.10% or Less
[0036] C is effective in increasing the strength of steel by solid
solution strengthening, but forms carbide, thus reducing the
ductility and therefore the processability of a steel sheet. A
higher amount of C component results in a hard steel sheet after
second cold rolling, thus reducing can productivity and necking
formability. C greatly increases the hardness of a weld, thus
causing HAZ cracking in flanging. Because more than 0.10% C
significantly exhibits these effects, C is 0.10% or less. On the
other hand, at an excessively low C content, the strength is
secured only at a draft in second cold rolling as high as 20% or
more. Thus, C is more than 0.02%. Preferably, C ranges from 0.03%
to 0.05%.
Si: 0.10% or Less
[0037] Si increases the strength of steel by solid solution
strengthening. However, the addition of a large amount of Si causes
problems, such as deterioration in surface treatment and corrosion
resistance. Thus, Si is limited to 0.10% or less. When particularly
high corrosion resistance is required, Si is preferably 0.02% or
less.
Mn: 1.5% or Less
[0038] Mn is effective in preventing hot tearing caused by S. Mn is
appropriately added depending on the S content to effectively
prevent cracking. To produce these effects, at least 0.20% Mn is
preferably added. Mn also has an effect of reducing the size of
crystal grains. However, the addition of a large amount of Mn tends
to cause a deterioration in corrosion resistance, provides a steel
sheet that is harder than required, and causes a deterioration in
flanging and necking formability. Thus, the upper limit is 1.5%.
Preferably, Mn ranges from 0.20% to 0.30%.
P: 0.20% or Less
[0039] P provides very hard steel, causes a deterioration in
flanging and necking formability, and causes a considerable
deterioration in corrosion resistance. Thus, in the present
invention, P is limited to 0.20% or less. Preferably, P ranges from
0.001% to 0.018%.
S: 0.20% or Less
[0040] S exists as an inclusion in steel, reduces the ductility of
a steel sheet, and causes a deterioration in corrosion resistance.
Thus, S is 0.20% or less. Preferably, S ranges from 0.001% to
0.018%.
Al: 0.10% or Less
[0041] Al combines with dissolved N to form AlN and is effective in
reducing the amount of dissolved N. An increase in Al content
results in an increase in recrystallization temperature. Thus, the
annealing temperature must be increased. In high-temperature
annealing, the formation of AlN reduces the amount of dissolved N,
reduces age hardening, and therefore reduces the strength of a
steel sheet. In a low carbon material, such a phenomenon is
noticeable at an Al content above 0.10%. Thus, Al is limited to
0.10% or less. Al is desirably 0.020% or more in view of stable
operation of a steel melting process. Preferably, Al ranges from
0.020% to 0.060%.
N: 0.0120% to 0.0250%
[0042] N promotes age hardening. Thus, in the present invention, N
is positively included. In a low carbon material, age hardening is
highly promoted at a N content of 0.0120% or more. However, at a N
content above 0.0250%, the risk of cracking in rolled material
(slab) increases considerably. Thus, N ranges from 0.0120% to
0.0250%.
Dissolved N in Steel Sheet for Cans (Cold-Rolled Steel Sheet):
0.0100% or More
[0043] To achieve substantial age hardening, which is
characteristic of the present invention, the amount of dissolved N
in a steel sheet for cans (cold-rolled steel sheet) must be 0.0100%
or more. This is the most important requirement in the present
invention.
[0044] A cold-rolled steel sheet according to the present invention
is preferably manufactured by pickling a hot-rolled sheet,
performing cold rolling, performing continuous annealing, and, if
necessary, performing second cold rolling. In this continuous
annealing process, because AlN tends to be precipitated out, it is
important to control a process in which the amount of dissolved N
in a steel sheet for cans (cold-rolled steel sheet) is not less
than 0.0100%. In the present invention, the amount of N in the form
of AlN (hereinafter referred to as "N as AlN") is determined by an
extraction analysis after dissolution in bromine ester as usually
performed, and the N as AlN is subtracted from the total N content
to determine the amount of dissolved N.
[0045] Preferably, the total of the amount of dissolved N and the
amount of dissolved C is 0.0150% or more. The amount of dissolved C
may be determined by internal friction measurement or by
subtracting the C content in a precipitate extracted from a steel
sheet from the total C content.
[0046] The remainder are Fe and incidental impurities.
[0047] The remainder other than the components described above are
Fe and incidental impurities. For example, incidental impurities
may be 0.01% or less Sn.
[0048] A method for manufacturing a high-strength steel sheet for
cans according to the present invention will be described
below.
[0049] A high-strength steel sheet for cans according to the
present invention is manufactured by the following method. First, a
molten steel having the composition described above is melted by a
generally known melting method using a converter or the like and is
formed into a rolled material (slab) by a generally known casting
method, such as a continuous casting method. The rolled material is
then hot-rolled into a hot-rolled sheet. At that time, the slab
extraction temperature is 1200.degree. C. or more, and the finish
rolling temperature is (Ar3 transformation temperature-30).degree.
C. or more (suitably, at least Ar3 transformation temperature). The
hot-rolled sheet is then coiled at 650.degree. C. or less, is
pickled, is cold-rolled, and is continuously annealed. If
necessary, second cold rolling is further performed at a draft of
10% or more but less than 20% (suitably, 10% or more but less than
15%). Plating may also be performed.
[0050] The following is a detailed description.
Slab Extraction Temperature: 1200.degree. C. Or More
[0051] To achieve 0.0100% or more dissolved N in a steel sheet for
cans, a slab is heated in a furnace and is extracted at a
temperature of 1200.degree. C. or more. This aims at promoting the
decomposition of AlN to achieve a predetermined amount of dissolved
N. Preferably, a slab is introduced into and heated in a furnace
maintained at this temperature.
Finish Rolling Temperature: (Ar.sub.3 Transformation
Point-30.degree. C.) or More
[0052] In the present invention, the finish rolling temperature in
hot rolling is (Ar.sub.3 transformation point-30.degree. C.) or
more to effectively reduce the precipitation of AlN and to prevent
a deterioration in anisotropy and processability. At a finish
rolling temperature below (Ar.sub.3 transformation point-30.degree.
C.), AlN is precipitated out considerably, dissolved N decreases,
and anisotropy and processability deteriorates. Preferably, the
finish rolling temperature is at least the Ar.sub.3 transformation
point.
[0053] After finish rolling, preferably, forced cooling is
performed by water cooling. This can reduce the precipitation of
AlN.
Coiling Temperature: 650.degree. C. Or Less
[0054] The coiling temperature is 650.degree. C. or less to reduce
nitrogen fixation by Al. At a coiling temperature above 650.degree.
C., the precipitation of AlN increases considerably, and dissolved
N decreases. Thus, the intended age hardening cannot be achieved.
More preferably, the coiling temperature is 600.degree. C. or less
to consistently achieve substantial age hardening.
[0055] In the present invention, after coiling, air cooling or
water cooling is preferably performed in a coiled state. While
water cooling can increase productivity, air cooling is preferred
in terms of uniform quality of a steel sheet in the sheet width
direction and the longitudinal direction.
Pickling and Cold Rolling
[0056] A hot-rolled sheet thus manufactured is pickled and
cold-rolled into a cold-rolled sheet. Pickling follows a routine
procedure, and surface scale is removed with an acid, such as
hydrochloric acid or sulfuric acid. The cold-rolling draft also
follows a routine procedure and increases with decreasing sheet
thickness.
Soaking Temperature in Continuous Annealing: 600.degree. C. Or More
(Suitable Conditions)
[0057] In a continuous annealing process, soaking is preferably
performed at a temperature of 600.degree. C. or more. At a soaking
temperature of 600.degree. C. or more, recrystallization proceeds
fast, strain caused by cold rolling does not remain, and a sheet
has high ductility and is suitable for press working. Soaking at
the Ar1 transformation point or higher can further improve the
strength and is therefore preferred. It is surmised that soaking at
the Ar1 transformation point or higher partly forms a pearlite
structure, which contributes to the improved strength.
[0058] As long as being within this temperature range, there is no
need to maintain a constant temperature. In view of operational
stability, a substantial soaking time of at least 10 s is
sufficient.
[0059] After continuous annealing, temper rolling of about 1% is
preferably performed to control the surface roughness and the
hardness.
[0060] A cold-rolled steel sheet manufactured through these
processes has a total elongation El of at least 20% after coating
and baking and is a steel sheet for cans that has excellent
processability.
[0061] After continuous annealing, second cold rolling at a draft
of 10% or more but less than 20% may be performed. The second cold
rolling mainly aims at further increasing the strength. At a draft
of 10% or more, the strength can be further increased. At a draft
below 20%, the strengthening effect can be achieved while the
elongation (the total elongation El in the range of 8% to 15% after
coating and baking) and the processability are maintained. In
particular, at a draft of 10% or more but less than 15% in the
second cold rolling, the total elongation El above 10% after
coating and baking can be achieved.
[0062] At a draft below 20%, the strengthening effect can be
achieved while the elongation and the processability are
maintained. A cold-rolled steel sheet subjected to the second cold
rolling at a draft of 10% or more but less than 20% has a total
elongation El in the range of 8% to 15% after coating and baking
and is a very high strength steel sheet for cans that has excellent
processability. The draft is preferably 10% or more but less than
15%, and a cold-rolled steel sheet having a total elongation El of
10% or more after coating and baking can be manufactured.
[0063] A cold-rolled steel sheet is manufactured through these
processes. The cold-rolled steel sheet is a hard material due to
coating and baking before can manufacturing (before press working),
and its superiority is more effectively demonstrated when the
cold-rolled steel sheet is applied to an ultrathin steel sheet
having a thickness of 0.3 mm or less. The cold-rolled steel sheet
manufactured through the processes described above is a
high-strength steel sheet for cans that contains 0.0100% or more
dissolved N and that has a yield stress YP of at least 500 MPa
after coating and baking. Furthermore, a steel sheet for cans
according to the present invention can exhibit large elongation and
therefore has excellent processability.
[0064] A steel sheet for cans according to the present invention
has undergone substantial age hardening because of dissolved N.
Thus, a steel sheet for cans according to the present invention has
a yield stress YP of at least 500 MPa after coating and baking, and
the reduction in the thickness of the steel sheet can be achieved
advantageously. A cold-rolled steel sheet according to the present
invention, effectively utilizing the effects of dissolved N, has an
increased strength after reflowing after plating, and a substantial
age hardening phenomenon may occur during a coating and baking
process after press forming, thus resulting in a tremendous
increase in the strength of cans.
[0065] In the present invention, a plated layer may be formed on a
surface (at least one side) of a cold-rolled steel sheet thus
manufactured to form a plated steel sheet. As the plated layer
formed on the surface, any plated layer to be applied to a steel
sheet for cans is applicable. Examples of the plated layer include
tin plating, chromium plating, nickel plating, and nickel chromium
plating. After the plating, coating or an organic resin film may be
applied without any problem.
Example 1
[0066] A steel composed of the components shown in Table 1 was
melted in a converter and was formed into a slab by a continuous
casting method. The slab was then hot-rolled under the conditions
shown in Table 2 to form a hot-rolled sheet having a thickness of
2.0 mm. The hot-rolled sheet thus formed was then descaled by
pickling, was cold-rolled, and was continuously annealed under the
conditions shown in Table 2. Part of the sheets were subjected to
second rolling. Thus, a cold-rolled steel sheet having a final
thickness of 0.17 mm was manufactured.
TABLE-US-00001 TABLE 1 Chemical components (% by mass) Steel No. C
Si Mn P S Al N A 0.044 0.01 0.25 0.017 0.010 0.035 0.0148 B 0.015
0.02 0.30 0.012 0.017 0.009 0.0160
TABLE-US-00002 TABLE 2 Hot rolling Second Ar3 Slab Finish Cold Ar1
Continuous annealing cold transformation extraction rolling Coiling
rolling transformation Annealing Soaking rolling Steel point
temperature temperature temperature Draft point temperature time
Draft sheet No. Steel No. .degree. C. .degree. C. .degree. C.
.degree. C. % .degree. C. .degree. C. s % 1* A 860 1210 890 590
91.4 723 680 14 -- 2* A 860 1150 890 590 91.4 723 680 14 -- 3* B
870 1210 890 590 91.4 723 760 14 -- 4 A 860 1200 900 600 90.0 723
690 15 15 5* A 860 1230 890 590 91.4 723 740 14 -- 6 A 860 1250 900
590 90.5 723 690 15 10 *Temper rolling at a draft of 1.1% was
performed after continuous annealing.
[0067] The cold-rolled steel sheet thus manufactured was subjected
to the measurement of the amount of dissolved N and a tensile test
before and after a bake hardenability test.
(i) Analysis of the Amount of Dissolved N
[0068] The N content in a cold-rolled steel sheet was analyzed by
chemical analysis, and the amount of N in the form of AlN was
determined by an extraction analysis after dissolution in bromine
ester. The amount of dissolved N in the cold-rolled steel sheet was
calculated by ((the N content in the cold-rolled steel sheet)-(the
amount of N in the form of AlN)).
(ii) Tensile Test
[0069] A JIS 13-B test piece for tensile test was sampled in the
rolling direction from the central part of a cold-rolled steel
sheet in the width direction and was subjected to a tensile test at
a strain rate crosshead speed of 10 m/s to determine the yield
stress YP and the total elongation El. The tensile test was
performed within one day after manufacture. The JIS 13-B test piece
for tensile test was selected to minimize the breakage outside the
gage marks.
(iii) Bake Hardenability Test
[0070] A JIS 13-B test piece for tensile test was sampled in the
rolling direction from the central part of a cold-rolled steel
sheet in the width direction, was subjected to 2% tensile
prestraining, was then temporarily unloaded, and was subjected to
heat treatment at 210.degree. C. for 20 min, which corresponded to
coating and baking. Before and after this test, the tensile test
described in (ii) was performed.
[0071] Table 3 shows the results.
TABLE-US-00003 TABLE 3 After coating Cold-rolled steel sheet and
baking Total Total Steel Dis- Strength elongation Strength
elongation sheet solved N YP El YP El No. % MPa % MPa % Note 1
0.0106 490 23 510 23 Example 2 0.0055 435 25 450 23 Compar- ative
example 3 0.0104 470 22 490 21 Compar- ative example 4 0.0102 530
12 540 10 Example 5 0.0103 510 24 530 22 Example 6 0.0112 520 13
530 12 Example
[0072] Table 3 shows that Nos. 1, 4, 5, and 6 according t working
examples had a sufficient yield stress YP and total elongation El
after coating and baking and had a sufficient strength and
processability, for example, for three-piece processing.
[0073] No. 6 according to a working example, which was subjected to
second cold rolling at a draft of 10%, had a total elongation El
above 10%, that is, 12% after coating and baking, in spite of the
second cold rolling.
[0074] By contrast, Nos. 2 and 3 according to comparative examples
had an insufficient yield stress YP, did not have the strength and
processability required for three-piece processing, and therefore
cannot be subjected to specified processing.
[0075] Thus, the present invention can provide a high-strength
steel sheet for cans that has a yield stress YP of at least 500 MPa
after coating and baking.
[0076] Furthermore, in the present invention, by using a low carbon
material, maintaining the absolute quantity of dissolved N in a
cold-rolled steel sheet at a certain value or more by (1)
containing a predetermined amount of N as a component, (2) setting
the slab extraction temperature at 1200.degree. C. or more to
decompose AlN produced in slab casting, and (3) coiling a
hot-rolled coil at 650.degree. C. or less to reduce the
precipitation of AlN, and performing age hardening after coating
and baking, a high-strength steel sheet for cans can be efficiently
produced without second cold rolling or by second cold rolling at a
low draft.
[0077] A high-strength steel sheet for cans according to the
present invention manufactured without second cold rolling, that
is, manufactured by temper rolling of about 1% after continuous
annealing has a total elongation El of at least 20% after coating
and baking. In a high-strength steel sheet for cans subjected to
second cold rolling, the draft in the second cold rolling can be
set at a suitable range of 10% or more but less than 15% to achieve
the total elongation El above 10% after coating and baking.
INDUSTRIAL APPLICABILITY
[0078] In a steel sheet for cans according to the present
invention, coating and baking after forming greatly increases the
yield stress and accordingly the strength of cans, thus
contributing to the reduction in the thickness of the steel
sheet.
* * * * *