U.S. patent application number 11/909665 was filed with the patent office on 2009-02-26 for aluminum alloy sheet with excellent high-temperature property for bottle can.
This patent application is currently assigned to KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD). Invention is credited to Yasuhiro Aruga, Katsura Kajihara, Kiyohito Tsuruda.
Application Number | 20090053099 11/909665 |
Document ID | / |
Family ID | 37053153 |
Filed Date | 2009-02-26 |
United States Patent
Application |
20090053099 |
Kind Code |
A1 |
Kajihara; Katsura ; et
al. |
February 26, 2009 |
ALUMINUM ALLOY SHEET WITH EXCELLENT HIGH-TEMPERATURE PROPERTY FOR
BOTTLE CAN
Abstract
An aluminum alloy sheet for bottle cans superior in
high-temperature properties and capable of preventing thermal
deformation thereof in coating and heat treatment and securing can
strength after the heat treatment. The aluminum alloy sheet has the
following composition: Mn 0.7-1.5%, Mg 0.8-1.7%, Fe 0.1-0.7%, Si
0.05-0.5%, Cu 0.1-0.6%, with the remainder being Al and inevitable
impurities, and has a crystal structure elongated in a rolling
direction and with an aspect ratio of crystal grains of 3 or more
as determined through an examination from above of a part located
at the center in the through-thickness direction. In the sheet, the
amount of solute Cu is 0.05-0.3%, which means the amount of Cu in a
solution separated from a precipitate exceeding 0.2 m in particle
size by the extracted residue method using hot phenol, and the
amount of solute Mg is 0.75-1.6%, which means the amount of solute
Mg separated from a precipitate exceeding 0.2 m in particle size by
the extracted residue method using hot phenol. The aluminum alloy
sheet can have improved high-temperature properties without
impairing its formability.
Inventors: |
Kajihara; Katsura; (Hyogo,
JP) ; Tsuruda; Kiyohito; (Tochigi, JP) ;
Aruga; Yasuhiro; (Hyogo, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
KABUSHIKI KAISHA KOBE SEIKO SHO
(KOBE STEEL, LTD)
HYOGO
JP
|
Family ID: |
37053153 |
Appl. No.: |
11/909665 |
Filed: |
March 7, 2006 |
PCT Filed: |
March 7, 2006 |
PCT NO: |
PCT/JP2006/304381 |
371 Date: |
September 25, 2007 |
Current U.S.
Class: |
420/535 ;
148/417; 420/534 |
Current CPC
Class: |
C22C 21/00 20130101;
C22C 21/08 20130101 |
Class at
Publication: |
420/535 ;
148/417; 420/534 |
International
Class: |
C22C 21/08 20060101
C22C021/08 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 25, 2005 |
JP |
2005-089369 |
Mar 25, 2005 |
JP |
2005-089370 |
Mar 25, 2005 |
JP |
2005-089371 |
Claims
1. An aluminum alloy sheet for bottle cans superior in
high-temperature properties, the aluminum alloy sheet comprising
the following composition: Mn 0.7-1.5% (mass %, also in the
following), Mg 0.8-1.7%, Fe 0.1-0.7%, Si 0.05-0.5%, Cu 0.1-0.6%,
with the remainder being Al and inevitable impurities, and
comprising a crystal structure elongated in a rolling direction and
with an average aspect ratio of crystal grains of 3 or more as
determined through an examination from above of a part located at
the center in the through-thickness direction, wherein the amount
of solute Cu is 0.05-0.3%, which means the amount of Cu in a
solution separated from a precipitate exceeding 0.2 .mu.m in
particle size by the extracted residue method using hot phenol, and
the amount of solute Mg is 0.75-1.6%, which means the amount of
solute Mg separated from a precipitate exceeding 0.2 .mu.m in
particle size by the extracted residue method using hot phenol.
2. The aluminum alloy sheet for bottle cans superior in
high-temperature properties according to claim 1, wherein the
difference between a maximum value and a minimum value of tensile
strength in 0.degree., 45.degree. and 90.degree. directions
relative to the rolling direction is 25 MPa or less and the
difference between a maximum value and a minimum value of n values
obtained by tensile tests in 0.degree., 45.degree. and 90.degree.
directions relative to the rolling direction is 0.03 or less.
3. The aluminum alloy sheet for bottle cans superior in
high-temperature properties according to claim 1, wherein an
average particle size of 0.5 .mu.m or larger dispersed particles in
the aluminum alloy sheet is 5 .mu.m or smaller, and .DELTA.T
indicative of a solid-liquid coexistence temperature range between
liquid and solid phases of aluminum is 40.degree. C. or less.
4. The aluminum alloy sheet for bottle cans superior in
high-temperature properties according to claim 1, further
comprising 0.001-0.3% of Cr and/or 0.05-1.0% of Zr.
5. The aluminum alloy sheet for bottle cans superior in
high-temperature properties according to claim 1, further
comprising 0.005-0.2% of Ti alone or in combination with
0.0001-0.05% of B.
6. The aluminum alloy sheet for bottle cans superior in
high-temperature properties according to claim 1, wherein the
amount of solute Mn is 0.12-0.38%, which means the amount of solute
M separated from a precipitate exceeding 0.2 .mu.m in particle size
by the extracted residue method using hot phenol.
7. The aluminum alloy sheet for bottle cans superior in
high-temperature properties according to claim 1, wherein when the
aluminum alloy sheet is subjected to a heat treatment at
290.degree. C. for 20 seconds, a change in hardness, .DELTA.Hv, of
the aluminum alloy sheet before and after the heat treatment is 30
Hv or less, and 0.2% proof stress of the aluminum alloy sheet after
the heat treatment is 215 MPa or more.
8. The aluminum alloy sheet for bottle cans superior in
high-temperature properties according to claim 1, which is a cold
rolled aluminum alloy sheet obtained by subjecting a hot rolled
aluminum alloy sheet to cold rolling up to a final sheet without
intermediate annealing.
9. The aluminum alloy sheet for bottle cans superior in
high-temperature properties according to claim 1, wherein when the
aluminum alloy sheet is subjected to a heat treatment at
290.degree. C. for 20 seconds, a change in hardness, .DELTA.Hv, of
the aluminum alloy sheet before and after the heat treatment is 30
Hv or less, and 0.2% proof stress of the aluminum alloy sheet after
the heat treatment is 215 MPa or more.
10. The aluminum alloy sheet for bottle cans superior in
high-temperature properties according to claim 1, which is a cold
rolled aluminum alloy sheet obtained by subjecting a hot rolled
aluminum alloy sheet to cold rolling up to a final sheet without
intermediate annealing.
11. An aluminum alloy sheet for bottle cans superior in
high-temperature properties, the aluminum alloy sheet comprising
the following composition: Mn 0.7-1.5% (mass %, also in the
following), Mg 0.8-1.7%, Fe 0.1-0.7%, Si 0.05-0.5%, Cu 0.1-0.6%,
with the remainder being Al and inevitable impurities, and
comprising a crystal structure elongated in a rolling direction and
with an average aspect ratio of crystal grains of 3 or more as
determined through an examination from above of a part located at
the center in the through-thickness direction, wherein the
difference between a maximum value and a minimum value of tensile
strength in 0.degree., 45.degree. and 90.degree. directions
relative to the rolling direction is 25 MPa or less, and a
difference between a maximum value and a minimum value of n values
obtained by tensile tests in 0.degree., 45.degree. and 90.degree.
directions relative to the rolling direction is 0.03 or less.
12. An aluminum alloy sheet for bottle cans superior in
high-temperature properties, the aluminum alloy sheet comprising
the following composition: Mn 0.7-1.5% (mass %, also in the
following), Mg 0.8-1.7%, Fe 0.1-0.7%, Si 0.05-0.5%, Cu 0.1-0.6%,
with the remainder being Al and inevitable impurities, and
comprising a crystal structure elongated in a rolling direction and
with an aspect ratio of crystal grains of 3 or more as determined
through an examination from above of a part located at the center
in the through-thickness direction, wherein an average particle
size of 0.5 .mu.m or larger dispersed particles in the aluminum
alloy sheet is 5 .mu.m or smaller, and .DELTA.T indicative of a
solid-liquid coexistence temperature range between liquid and solid
phases of aluminum is 40.degree. C. or less.
13. The aluminum alloy sheet for bottle cans superior in
high-temperature properties according to claim 11, further
comprising 0.001-0.3% of Cr and/or 0.05-1.0% of Zn.
14. The aluminum alloy sheet for bottle cans superior in
high-temperature properties according to claim 11, further
comprising 0.005-0.2% of Ti alone or in combination with
0.0001-0.05% of B.
Description
TECHNICAL FIELD
[0001] The present invention relates to an aluminum alloy sheet for
bottle cans (blank for bottle cans) which even when reduced in
thickness to 0.2 mm or less (about 120-130 .mu.m in a
thickness-reduced central portion of a can body) and heat-treated
at a high temperature as a can body material of a bottle can
(beverage can), exhibits little deterioration in strength, can
ensure a high strength and is difficult to be deformed, thus having
high-temperature properties. As the aluminum alloy sheet as
referred to herein, reference will be made below to a rolled sheet
(cold rolled sheet) as an example which has been rolled through hot
rolling and cold rolling. It is applicable widely to aluminum alloy
sheets, including this type of cold rolled sheets. The aluminum
alloy will hereinafter be referred to also as Al alloy.
BACKGROUND ART
[0002] As aluminum beverage cans, 2-piece aluminum cans fabricated
by seaming a can body and a can lid (can end) are popular. The can
body is fabricated by subjecting a cold rolled aluminum sheet to DI
(deep drawing and ironing), followed by trimming into a
predetermined size, subsequent degreasing and washing, further,
painting, printing, baking, and subsequent necking and flanging of
can body edge portions.
[0003] As the cold rolled sheet for can body, hard sheets of, for
example, JIS3004 alloy and 3104 alloy, which are Al--Mg--Mn alloys,
have heretofore been widely used. The JIS3004 alloy and 3104 alloy
are superior in ironing and exhibit a relatively good formability
even when subjected to cold molding at a high draft in order to
enhance the strength and are therefore considered suitable as DI
can body materials.
[0004] On the other hand, in the case of a bottle can, an aluminum
alloy sheet formed on both surfaces thereof with thermoplastic
resin coating layers and with lubricant applied thereto is punched
to obtain a blank, the blank is then subjected to deep drawing into
a cup shape, then this cup-like molding is subjected to again deep
drawing and stretching or ironing (DI working) to form a bottomed
cylindrical can having a body portion of reduced diameter and
thickness. Then, the bottom side of the can is subjected to deep
drawing plural times to form a shoulder portion and an unopened
mouth portion, followed by washing and trimming, subsequent
printing and coating for the can body portion, further, opening the
mounting portion, forming a curled portion and a screw portion
(threading/curling), subsequent neck-in working and flanging, and
sealing a separately-formed can lid by a seamer to afford a bottle
can (see Patent Literature 1).
[0005] Thus, in the case of a 2-piece can, after an aluminum alloy
sheet is subjected to substrate treatment (e.g., chromate
treatment), it is coated with resin (application of resin or film
laminate), then is punched into a circular blank, followed by
forming into a cup, subjected deep drawing and ironing, further,
printing, coating, necking, and trimming.
[0006] In the case of a bottle can with a threaded mouth portion,
an aluminum alloy sheet is subjected to substrate treatment (e.g.,
chromate treatment), followed by resin coating (application of
resin or film laminate), subsequent punching into a circular blank,
forming into a cup, deep drawing and ironing, trimming, printing,
coating, threading/curling, and subsequent neck flanging.
[0007] Just after DI, the bottle can body is usually in a
substantially true circular form in its horizontal section.
However, at the time of printing/coating and heat treatment for
improving the adhesion of laminate film, the can body is heated to
a temperature of 200.degree. C. or higher.
[0008] At this time, the can body itself is in a state in which it
is reduced in thickness from the original 0.3 to 0.4 mm or so as a
cold rolled sheet thickness to 0.2 mm or less. Therefore, when the
can body is heat-treated at such a high temperature as exceeds
200.degree. C., it is released from its work strain and residual
stress induced during DI and softens thermally.
[0009] On this regard, in the case of a material which softens
easily, the degree of softening is marked, with the result that the
strength and hardness of the can are deteriorated markedly, thus
giving rise to the problem that a sufficient can strength can
longer be ensured.
[0010] Besides, since the degree of softening becomes non-uniform
in the circumferential direction of the can, a cross section of the
can body is not a true circle as formed, but is deformed
elliptically, thus giving rise to the problem that the can body
shape becomes non-uniform.
[0011] Recently, due to a demand for reduction of the can weight,
the aluminum can thickness is at the level of 0.2 mm or less and is
becoming more and more small. At the same time, the aforesaid
phenomena caused by thermal softening such as lowering in strength
and hardness of the can body and non-uniforming in shape of the can
body are becoming more and more marked.
[0012] Recently, moreover, from the standpoint of improving the
productivity of cans, the foregoing printing/coating and heat
treatment for improving the adhesion of laminate film are becoming
more and more high in temperature and speed like, for example,
290.degree. C..times.20 seconds. Such a tendency also promote the
lowering in strength and hardness of the can body and
non-uniforming of the shape thereof caused by the aforesaid thermal
softening.
[0013] If the thickness of the can body is increased in an effort
to prevent lowering in strength and deformation of the can body
caused by thermal softening, an increase of the can weight results,
while if the strength of the aluminum material itself is increased
without increasing the sheet thickness, there occurs an
inconvenience such as breakage during the foregoing ironing work.
Thus, with only such selection of can material and method as in the
prior art, it is impossible to cope with the problem in
question.
[0014] Against the non-uniforming in shape of the can body caused
by the foregoing thermal softening there has so far been proposed
an aluminum alloy sheet for DI can capable of preventing thermal
deformation during painting and heat treatment and affording a DI
can high in true circularity (Patent Literature 2). The composition
of such proposed aluminum alloy sheet for DI can is as follows: Mn
0.5-1.3 mass %, Mg 0.5-1.3 mass %, Cu 0.1-0.3 mass %, Fe 0.2-0.6
mass %, Si 0.1-0.5 mass %. When heat treatment is performed at a
baking temperature T(.degree. C.) of 230.degree. to 270.degree. C.
for 20 minutes, it is intended to diminish a change .DELTA.TS in
tensile strength before and after the heat treatment.
[0015] In addition, also as to controlling the structure for
improving the formability into a can, a large number of proposals
have heretofore been made. For example, it has been proposed to
control the amount of solute Mn and crystal grain diameter of a hot
rolled sheet within respective predetermined ranges, thereby making
the earing rate of the hot rolled sheet stably to -3% to 6%, then
subject the sheet to cold rolling without going through
intermediate annealing, thereby making the earing rate of the
resulting cold rolled sheet stably to 0% to 2% (Patent Literature
3).
[0016] Patent Literature 1: Japanese Unexamined Patent Application
Publication No. 2001-162344 (whole sentence)
[0017] Patent Literature 2: Japanese Unexamined Patent Application
Publication No. 2003-277865 (whole sentence)
[0018] Patent Literature 3: Japanese Unexamined Patent Application
Publication No. 2003-342657 (whole sentence)
DISCLOSURE OF THE INVENTION
Problem to be Solved by the Invention
[0019] By only controlling the metallurgical factors on the
structure of aluminum alloy sheets for stabilizing the earing rate
such as controlling the amount of solute Mn and crystal grain
diameter, which has so far been conducted, it is impossible to
prevent thermal deformation during coating and heat treatment.
[0020] Likewise, by only controlling the aluminum alloy
composition, including Mn, Mg, Cu, Fe and Si, a large limit is
encountered in suppressing the lowering in strength and deformation
of the can body caused by the foregoing thermal softening.
[0021] More particularly, the method proposed in Patent Literature
2 may be effective for its prescribed or presumed heat treatment of
230-270.degree. C..times.20 minutes. However, as noted above,
against such a higher temperature and shorter time heat treatment
as 290.degree. C..times.20 seconds, since the heat treatment
temperature is higher and the can body thickness is smaller, it is
impossible to prevent lowering in strength and deformation of the
can body caused by thermal softening.
[0022] The present invention has been accomplished in view of such
problems. On the premise that satisfactory formability in DI, etc.
is to be ensured, it is an object of the present invention to
provide an aluminum alloy sheet for bottle can capable of
preventing thermal deformation during coating and heat treatment,
securing can strength after heat treatment, affording a bottle can
high in true circularity, and being superior in high-temperature
properties.
Means for Solving the Problems
[0023] For achieving the above-mentioned object, in a first aspect
of the present invention there is provided an aluminum alloy sheet
for bottle cans superior in high-temperature properties, the
aluminum alloy sheet comprising the following composition: Mn
0.7-1.5% (mass %, also in the following), Mg 0.8-1.7%, Fe 0.1-0.7%,
Si 0.05-0.5%, Cu 0.1-0.6%, with the remainder being Al and
inevitable impurities, and comprising a crystal structure elongated
in a rolling direction and with an average aspect ratio of crystal
grains of 3 or more as determined through an examination from above
of a part located at the center in the through-thickness direction,
wherein the amount of solute Cu is 0.05-0.3%, which means the
amount of Cu in a solution separated from a precipitate exceeding
0.2 .mu.m in particle size by the extracted residue method using
hot phenol and the amount of solute Mg is 0.75-1.6%, which means
the amount of solute Mg separated from a precipitate exceeding 0.2
.mu.m in particle size by the extracted residue method using hot
phenol.
[0024] As to a DI can body of a bottle can, as noted above, a
further reduction of thickness is desired mainly for the purpose of
reducing the manufacturing cost and weight. For attaining the
thickness reduction it is necessary that the strength of the cold
rolled aluminum alloy sheet as the material be made high so as to
not cause a lowering of buckling strength. For achieving the
thickness reduction it is also strongly desired that the earing
rate in DI be low. If the earing rate in DI is made low, it is
possible to increase the yield in DI and further possible to
prevent the breakage of the can body caused by edge cutting of the
can body.
[0025] Heretofore, as noted above, in order to highly stabilize the
earing rate, there has publicly been known a method wherein
metallurgical factors of the structure of the cold rolled aluminum
alloy sheet as the DI bottle can body material are controlled.
Typical of such controls are crystal grain size microsizing
control, controlling the number and size of a compound such as
Mg.sub.2Si, microscopic segregation suppression for added elements,
controlling the amount of solute alloy elements such as Mn, and
cube orientation control.
[0026] However, a technique for controlling metallurgical factors
of the structure of the rolled aluminum alloy sheet in order to
prevent thermal deformation in coating and heat treatment, which is
to be attained in the present invention, has not substantially been
proposed yet. This is because metallurgical factors of the
structure correlated with thermal deformation in coating and heat
treatment have not been made clear yet. Moreover, by merely
controlling various known metallurgical factors of the structures
for stabilizing the earing rate as referred to above, it is
impossible to prevent thermal deformation in coating and heat
treatment.
[0027] On the other hand, according to the present invention it has
been found out that among various metallurgical factors of the
structure, the form of crystal grain and the amount of solute Cu
and that of solute Mg in the structure are correlated with the can
strength after heat treatment and thermal deformation in coating
and heat treatment.
[0028] Since such metallurgical factors of the structure do not
obstruct the stabilization of the earing rate but rather act to
stabilize the earing rate, it is possible to secure can strength
after heat treatment and suppress thermal deformation in coating
and heat treatment and then secure formability in DI, etc. In other
words, it is possible to obtain an aluminum alloy sheet capable of
securing can strength after heat treatment and suppressing thermal
deformation in coating and heat treatment and then securing
formability in DI, etc.
[0029] By controlling each individual crystal grain of the aluminum
alloy sheet not into an equiaxed grain but into an elongated
structure in the rolling direction with an average aspect ratio of
2 or more, it is possible to suppress thermal deformation in
coating and heat treatment and secure can strength after heat
treatment, thus possible to cope with a high-speed heat treatment
performed at a higher temperature for a shorter time.
[0030] In the present invention, in addition to the above crystal
grain shape control, the amount of solute Cu and that of solute Mg
in the structure are controlled to respective optimum ranges.
[0031] The amount of solute Cu and that of solute Mg exert a great
influence on anti-softening property in high-temperature heat
treatment. Therefore, by securing both such Cu and Mg quantities
present in a solid solution form it is possible to improve the
anti-softening property in high-temperature heat treatment and
suppress elliptic deformation. Besides, the amount of solute Mg
exerts a great influence on the strength property after
high-temperature heat treatment, so by securing the amount of
solute Mg it is possible to also secure strength after
high-temperature heat treatment.
[0032] Controlling the amount of other alloy elements, e.g., Mn,
present in a solid solution form in the prior art described above
makes contribution to improving the formability in DI, etc. such as
lowering the earing rate of the cold rolled sheet. However, in
point of suppressing thermal deformation in coating and heat
treatment and securing can strength after heat treatment, which are
to be attained in the present invention, such solid solution
quantity control for other alloy elements is much less effective
than controlling both Cu and Mg solid solution quantities. Thus,
even if the amount of other alloy elements as Mn present in a solid
solution form are ensured, thermal deformation during coating and
heat treatment is not suppressed and it is impossible to secure can
strength after heat treatment.
[0033] It is the first object of the present invention to suppress
thermal deformation in coating and heat treatment and make it
possible to secure can strength after heat treatment in connection
with a high-speed heat treatment performed at a higher temperature
for a shorter time by controlling each individual crystal grain of
the aluminum alloy sheet not to an equiaxed grain but to an
elongated structure in the rolling direction with an average aspect
ratio of 3 or more. In the present invention, in order to further
ensure this effect, there is made control so as to suppress
anisotropy in the structure. More specifically, among tensile
strengths in 0.degree., 45.degree. and 90.degree. directions
relative to the rolling direction, the difference between the
maximum and the minimum value is 25 MPa or less, and among n values
obtained by tensile tests in 0.degree., 45.degree. and 90.degree.
directions relative to the rolling direction, the difference
between the maximum and the minimum value is 0.3 or less.
[0034] Thus, in a second aspect of the present invention there is
provided an aluminum alloy sheet for bottle cans superior in
high-temperature properties, the aluminum alloy sheet comprising
the following composition: Mn 0.7-1.5% (mass %, also in the
following), Mg 0.8-1.7%, Fe 0.1-0.7%, Si 0.05-0.5%, Cu 0.1-0.6%,
with the remainder being Al and inevitable impurities, and having a
structure elongated in a rolling direction and with an average
aspect ratio of crystal grains of 3 or more as determined through
an examination from above of a part located at the center in the
through-thickness direction, wherein among tensile strengths in
0.degree., 45.degree. and 90.degree. directions relative to the
rolling direction, the difference between maximum and minimum
values is 25 MPa or less, and among n values obtained by tensile
tests in 0.degree., 45.degree. and 90.degree. directions relative
to the rolling direction, the difference between maximum and
minimum values is 0.03 or less.
[0035] According to the second aspect of the present invention, in
a conventional aluminum alloy sheet manufacturing process involving
hot rolling and cold rolling, when a hot rolled sheet is cold
rolled to the final sheet thickness by performing intermediate
annealing (intermediate annealing), the draft in cold rolling
inevitably becomes high, with consequent occurrence of anisotropy
in strength, and there occurs a difference of about 30 MPa or more
among tensile strengths in 0.degree., 45.degree. and 90.degree.
directions relative to the rolling direction. As the anisotropy in
strength becomes higher, the internal stress after cupping and
ironing becomes non-uniform in the circumferential direction, so
that the degree of recovery becomes non-uniform and elliptic
deformation is apt to occur during printing/painting and heat
treatment performed for improving the adhesion of laminate film. It
is for this reason that thermal deformation in coating and heat
treatment cannot be prevented when the conventional hot rolling and
cold rolling are performed.
[0036] Also in the case where a hot rolled sheet is cold rolled
directly to the final sheet thickness without going through
intermediate annealing, the draft in cold rolling inevitably
becomes high and there easily occurs anisotropy in strength.
Consequently, there occurs a difference among tensile strengths in
0.degree., 45.degree. and 90.degree. directions relative to the
rolling direction and hence elliptic deformation is apt to occur.
It is for this reason that thermal deformation in coating and heat
treatment cannot be prevented in the conventional intermediate
annealing-free cold rolling.
[0037] On the other hand, in the present invention, even when not
the conventional cold rolling involving interpass (halfway in cold
rolling) intermediate annealing but cold rolling is performed
directly to the final sheet thickness without intermediate
annealing of a hot rolled sheet, the foregoing thermal deformation
in coating and heat treatment is suppressed and can strength after
heat treatment can be secured.
[0038] Further, in the present invention, as noted above, crystal
grains of the cold rolled aluminum sheet are controlled not to
equiaxed grains but to an elongated structure in the rolling
direction with an average aspect ratio of 3 or more, whereby
thermal deformation in coating and heat treatment is suppressed and
can strength after heat treatment can be secured in connection with
a high-speed heat treatment performed at a higher temperature for a
shorter time. In the present invention, for further ensuring this
effect, dispersed grains in the structure are controlled. More
specifically, an average grain size of dispersed grains is
controlled as fine as 5 .mu.m or less and .DELTA.T, which
represents a solid-liquid coexistence temperature range between
liquid and solid phases of aluminum, is set at 40.degree. C. or
less.
[0039] The larger the solid-liquid coexistence temperature range
.DELTA.T, the larger the solid-liquid coexistence temperature range
in a component system of both dispersed grains of Al (Fe, Mn)-based
intermetallic compounds and the liquid phase of aluminum. That is,
the component system is apt to undergo a change of crystal phase
depending on casting conditions and the form thereof is apt to
scattered, and the structure obtained permits easy formation of
coarse compound grains.
[0040] Conversely, the smaller the solid-liquid coexistence
temperature range .DELTA.T, the smaller the solid-liquid
coexistence temperature range in a component system of both
dispersed particles and the liquid phase of aluminum. It can be
said that in this structure the variations in production of stable
and metastable phases of the intermetallic compound are small and
compound grains are fine particles.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] FIG. 1 is a developed view of a cup formed by Di of a
blank.
[0042] FIG. 2 is a schematic phase diagram showing .DELTA.T defined
in the present invention.
[0043] FIG. 3 is a calculation phase diagram for determining
.DELTA.T by calculation.
BEST MODE FOR CARRYING OUT THE INVENTION
Composition of Cold Rolled Al Alloy Sheet
[0044] First, a preferred chemical components composition (unit:
mass %) of the cold rolled Al alloy sheet according to the present
invention will be described together with the reason for limitation
of each element.
[0045] The composition of the cold rolled aluminum sheet for bottle
can superior in high-temperature properties according to the
present invention is as follows: Mn 0.7-1.5%, Mg 0.8-1.7%, Fe
0.1-0.7%, Si 0.05-0.5%, Cu 0.1-0.6%, with the remainder being Al
and inevitable impurities.
[0046] In the present invention it is preferable to design the
components balance of main constituent elements (Mn, Mg, Fe, Cu,
Si) in such a manner that the required amount of solute Mg and that
of solute Cu can be ensured. By making such a design, not only fine
and stable crystals are formed, but also it is possible to make
control so as to afford a structure of an optimum Cu or Mg solid
solution quantity.
[0047] Mn: 0.7-1.5%
[0048] Mn is an effective element contributing to improvement of
not only strength but also formability. Particularly, since the can
body material (cold rolled sheet) used in the present invention is
subjected to ironing in DI, Mn is an extremely important
element.
[0049] More particularly, Mn affords various Mn-based intermetallic
compounds, e.g., Al--Fe--Mn--Si-based intermetallic compounds
(.alpha.phase). The more appropriately dispersed the .alpha. phase,
the more can be improved the ironing workability. In ironing an
aluminum sheet, there usually is employed an emulsion type
lubricant. But if the amount of the aforesaid .alpha. phase is
small, even if an emulsion type lubricant is used, the lubricity
becomes deficient, with a consequent fear of occurrence of an
appearance defect such as rubbed scratch or seizure called galling.
Thus, Mn is an indispensable element also for formation of .alpha.
phase to prevent a surface defect in ironing.
[0050] If the content of Mn is too low, the above effect will not
be exhibited. Therefore, the content of Mn is 0.7% or more,
preferably 0.8% or more, more preferably 0.85% or more, still more
preferably 0.9% or more.
[0051] On the other hand, if the content of Mn is too high, a giant
metallic compound, MnAl.sub.6, will be formed as a primary crystal,
with consequent deterioration of formability. Therefore, the upper
limit of the Mn content is set at 1.5%, preferably 1.3%, more
preferably 1.1%, still more preferably 1.0%.
Mn Solid Solution Quantity
[0052] As noted above, by combining with cold rolling free of
intermediate annealing, the amount of solute Mn in the cold rolled
aluminum sheet contributes to improving the formability in DI, etc.
such as lowering of the earing ate of the cold rolled sheet.
Therefore, in order to improve the formability in DI, etc., it is
preferable to set the amount of solute Mn at 0.12-0.38%, which
means the amount of Mn (total amount of both dissolved Mn quantity
and the amount of Mn contained in a precipitate of 0.2 .mu.m or
less in particle size) in a solution separated from a precipitate
exceeding 0.2 .mu.m in particle size by the extracted residue
method using hot phenol. If the amount of solute Mn is less than
0.12%, there will be no effect of improving the formability in DI,
etc., while if the amount of solute Mn exceeds 0.38%, the work
hardening in cold rolling will become excessive and rather the
formability in DI, etc. is very likely to be deteriorated.
[0053] Mg: 0.8-1.7%
[0054] Mg is effective in that it can improve its strength by solid
solution hardening. Further, by using it in combination with Cu, as
will be described later, it is possible to prevent softening of the
can body material (cold rolled sheet) of the present invention at
the time of final annealing (also called finish annealing, e.g.,
annealing at a temperature of about 100.degree. to 150.degree. C.
for about 1 to 2 hours) and subsequent forming into a can and
baking. More particularly, in the presence of both Mg and Cu, it is
possible to ensure the required amount of solute Cu at the stage of
fabrication of a hot rolled sheet, and since Al--Cu--Mg is
precipitated at the time of baking, it is possible to suppress
softening during baking.
[0055] If the content of Mg is too low, it is impossible to ensure
the required amount of solute Mg and the effect of improving the
anti-softening property in high-temperature heat treatment is not
exhibited. Therefore, the content of Mg is set at 0.8% or more,
preferably 0.9% or more, more preferably 1.0% or more.
[0056] On the other hand, if the content of Mg is too much, work
hardening is apt to occur and hence the formability is
deteriorated. Therefore, the upper limit of the Mg content is set
at 1.7%, preferably 1.6%, more preferably 1.35%.
[0057] The content of Mg also exerts an influence on the amount of
Mn precipitated and the amount of solute Mn. More particularly, the
higher the Mg content, the more suppressed the amount of
Al--Fe--Mn--Si-based intermetallic compounds (.alpha. phase)
precipitated, so that the amount of solute Mn is apt to become
large. It is preferable to determine the content of Mg in relation
to the amount of solute Mn.
Sum of Mg Solid Solution Quantity and the Amount of Mg Contained in
Fine Precipitate of 0.2 .mu.m or Less
[0058] The sum of the amount of solute Mg and the amount of Mg
contained in a fine precipitate of 0.2 .mu.m or less, like the sum
of the amount of solute Cu and the amount of Cu contained in a fine
precipitate of 0.2 .mu.m or less, exerts a great influence on the
anti-softening property in high-temperature heat treatment.
Heretofore there has been the patent of Patent Literature 3 which
defines both Mn and Cu solid solution quantities for the purpose of
eliminating variations in the earing rate and stabilizing the same
rate. However, in order to suppress elliptic deformation after can
heating in intermediate annealing, the conventional control if
applied alone is insufficient. It is necessary to also control the
state of solid solution and precipitation of Mg. As a result of
detailed investigation about the state of presence of Mg it turned
out that not only Mg was present as a solid solution and a fine
precipitate like that so far mentioned but also it was dissolved in
a coarse Al--Fe--Si-- or Al--Mn--Fe--Si-based precipitate and that
if the amount thereof is large, the amount of dissolved Mg and that
of Mg contained in fine precipitate become smaller and elliptic
deformation is apt to occur. The amount of solute Mg exerts a great
influence also on the strength property after high-temperature heat
treatment. In the present invention, therefore, both Cu and Mg
solid solution quantities required are ensured to not only improve
the anti-softening property in high-temperature heat treatment but
also ensure the required strength after high-temperature heat
treatment.
[0059] Accordingly, in the present invention, the Mg solid solution
quantity is set at 0.75-1.6% as the content of Mg (the total
quantity of both dissolved Mg quantity and the amount of Mg
contained in a precipitate of 0.2 .mu.m in particle size) in a
solution separated from a precipitate exceeding 0.2 .mu.m in
particle size by the extracted residue method using hot phenol.
[0060] Mg present in a coarse precipitate exceeding 0.2 .mu.m in
particle size rather deteriorates the anti-softening property in
high-temperature heat treatment and the strength property after the
same treatment. Thus, ensuring the required amount of solute Mg
also leads to restricting the presence of a coarse precipitate
exceeding 0.2 .mu.m in particle size.
[0061] Not only Mg actually dissolved, but also Mg contained in a
precipitate of 0.2 .mu.m or less in particle size, like dissolved
Mg, also improves the anti-softening property in high-temperature
heat treatment and ensures the required strength after the same
treatment. In the present invention, therefore, the total amount of
both dissolved Mg quantity and the amount of Mg contained in a
precipitate a of 0.2 .mu.m or less in particle size is defined to
be the dissolved Mg quantity. Accordingly, the dissolved Mg
quantity is defined as the amount of Mg contained in a solution
separated from a precipitate exceeding 0.2 .mu.m in particle size
by the extracted residue method using hot phenol.
[0062] If the dissolved Mg quantity is less than 0.75%, the
anti-softening property in high-temperature heat treatment will
become insufficient and deformation of the can will not be
suppressed; besides, the strength after high-temperature heat
treatment will be deteriorated.
[0063] On the other hand, even if the dissolved Mg quantity exceeds
1.6%, the work hardening in cold rolling will become excessive,
rather resulting in deterioration of formability in DI, etc.
[0064] Fe: 0.1-0.7%
[0065] Fe has a function of microsizing crystal grains and produces
the foregoing Al--Fe--Mn--Si-based intermetallic compound
(.alpha.phase), thus contributing to the improvement of
formability. Moreover, Fe is useful also in point of promoting the
crystallization and precipitation of Mn and controlling the amount
of solute Mn in aluminum base and the state of dispersion of
Mn-based intermetallic compounds (e.g., the aforesaid .alpha.
phase). On the other hand, if the content of Fe is too high in the
presence of Mn, a giant intermetallic compound as a primary crystal
is apt to be produced, with consequent fear of formability being
impaired.
[0066] Thus, the content of Fe can be set in accordance with the
content of Mn and the mass ratio of Fe to Mn (Fe/Mn) is, for
example, in the range of 0.1 to 0.7, preferably 0.2 to 0.6, more
preferably 0.3 to 0.5.
[0067] In the case where the content of Mn falls under the above
range, a lower-limit content of Fe is 0.1% or more, preferably 0.2%
or more, more preferably 0.3% or more, while an upper-limit content
of Fe is 0.7% or less, preferably 0.6% or less, more preferably
0.5% or less.
[0068] Si: 0.05-0.5%
[0069] Si produces Al--Fe--Mn--Si-based intermetallic compounds
(.alpha. phase) and controls the state of dispersion Mn-based
intermetallic compounds, thus being a useful element. The more
appropriate the distribution of .alpha. phase, the more can be
improved formability.
[0070] Accordingly, the content of Si is 0.05% or more, preferably
0.1% or more, more preferably 0.2% or more. On the other hand, if
the Si content is too high, the material will become too hard due
to age hardening, resulting in formability being deteriorated.
Therefore, an upper-limit Si content is set at 0.5%, preferably
0.45%, more preferably 0.4%.
[0071] Cu: 0.1-0.6%
[0072] When performing baking in fabrication of the cold rolled
sheet into a can, Al--Cu--Mg precipitates, and when Cu is used in
combination with Mg, softening can be suppressed by the action of
both dissolved Mg and dissolved Cu. Therefore, a lower-limit
content of Cu is set at 0.1% or more, preferably 0.15% or more,
more preferably 0.2% or more. On the other hand, if the Cu content
is too high, hardening proceeds to an excessive degree although age
hardening is attained easily, so that the formability is
deteriorated and so is corrosion resistance. Accordingly, an
upper-limit content of Cu is set at 0.6%, preferably 0.5%, more
preferably 0.35%.
[0073] As other examples of strength improving element exhibiting
the same effect as Cu there are mentioned Cr and Zn. Thus, Cr
and/or Zn may be used selectively in addition to Cu.
Sum of Cu Solid Solution Quantity and the Amount of Cu Contained in
Fine Precipitate of 0.2 .mu.m or Less
[0074] The sum of the amount of solute Cu and the amount of Cu
contained in a fine precipitate of 0.2 .mu.m or less, like the sum
of the amount of solute Mg and the amount of Mg contained in a
precipitate of 0.2 .mu.m or less, exerts a great influence on the
anti-softening property in high-temperature heat treatment.
[0075] In the present invention, the Cu content (total quantity of
both dissolved Cu quantity and the amount of Cu contained in a
precipitate of 0.2 .mu.m or less in particle size) in a solution
separated from a precipitate exceeding 0.2 .mu.m in particle size
by the extracted residue method using hot phenol is set at
0.05-0.3%.
[0076] Cu present in a coarse precipitate exceeding 0.2 .mu.m in
particle size rather deteriorates the anti-softening property in
high-temperature heat treatment and the strength property after the
same treatment. Therefore, ensuring the required dissolved Cu
quantity also leads to restricting the formation of a coarse
precipitate exceeding 0.2 .mu.m in particle size.
[0077] Not only Cu actually dissolved but also Cu contained in a
precipitates of 0.2 .mu.m or less in particle size, like dissolved
Cu, also improves the anti-softening property in high-temperature
heat treatment and ensures the required strength after the same
treatment. In the present invention, therefore, the total amount of
both dissolved Cu quantity and the amount of Cu contained in a
precipitate of 0.2 .mu.m or less in particle size is defined to be
the dissolved Cu quantity. Accordingly, the dissolved Cu quantity
is defined as the amount of Cu contained in a solution separated
from a precipitate exceeding 0.2 .mu.m in particle size by the
extracted residue method using hot phenol.
[0078] If the dissolved Cu quantity is less than 0.05%, the
anti-softening property in high-temperature heat treatment will
become insufficient and deformation of the can will not be
suppressed; besides, the strength after high-temperature heat
treatment will be deteriorated.
[0079] On the other hand, even if the dissolved Cu quantity exceeds
0.3%, the work hardening in cold rolling will become excessive,
rather resulting in deterioration of formability in DI, etc. and
deterioration of corrosion resistance.
[0080] Cr: 0.001-0.3%
[0081] For exhibiting the strength improving effect, the content of
Cr is set at 0.001% or more, preferably 0.002% or more. On the
other hand, if the Cr content is too high, a giant product will be
formed by crystallization, with consequent deterioration of
formability. Therefore, an upper limit of the Cr content is set at
0.3%, preferably 0.25%.
[0082] Zn: 0.05-1.0%
[0083] In the presence of Zn, there occurs age precipitation of
Al--Mg--Zn particles, whereby the strength can be improved. For
exhibition of this effect, the content of Zn is set at 0.05% or
more, preferably 0.06% or more. On the other hand, a too high
content of Zn will result in deterioration of corrosion resistance.
Therefore, an upper limit of the Zn content is set at 0.5%,
preferably 0.45%.
[0084] Ti: 0.005-0.2%
[0085] Ti is a crystal grain microsizing element. When this effect
is to be exhibited, Ti is used selectively. In this case, the
content of Ti is set at 0.005% or more, preferably 0.01% or more,
more preferably 0.015% or more. If the Ti content is too high,
giant Al--Ti-based intermetallic compounds will be formed by
crystallization and impair the formability. Therefore, an upper
limit of the Ti content is set at 0.2%, preferably 0.1%, more
preferably 0.05%.
[0086] Ti may be used alone or in combination with a very small
amount of B. A combined use thereof with B will further improve the
crystal grain microsizing effect. Therefore, the content of B in a
selective use thereof is set at 0.0001% or more, preferably 0.0005%
or more, more preferably 0.0008% or more. On the other hand, a too
high content of B will result in formation of coarse Ti--B
particles and deterioration of formability. An upper limit of the B
content is set at 0.05%, preferably 0.01%, more preferably
0.005%.
[0087] The balance other than the elements described above consists
of inevitable impurities. In order not to impair the sheet
properties described above, it is preferable that the content of
such impurities be as low as possible. But their contents up to
approximately the upper-limit values of elements in 3000 series
aluminum alloys defined in JIS insofar as they do not impair the
above properties.
Structure of Cold Rolled Al Alloy Sheet
[0088] The following description is now provided about the
structure of cold rolled Al alloy sheet of the present
invention.
Average Aspect Ratio of Crystal Grain
[0089] As noted above, by making each individual crystal grain of
the cold rolled aluminum alloy sheet into not the ordinary equiaxed
grain but an elongated crystal grain in the rolling direction with
an average aspect ratio of 3 or more, thermal deformation in
coating and heat treatment is suppressed and the required can
strength after heat treatment can be ensured in connection with
high-speed heat treatment at a higher temperature for a shorter
time.
[0090] That is, by making each individual crystal grain of the cold
rolled aluminum alloy sheet into an elongated grain in the rolling
direction, it is possible to impart ironing processability to the
sheet and ensure the formability in DI, etc. and further possible
to ensure the required can strength after heat treatment under the
above components' composition and the state and texture of solid
solution and precipitation to be described later. As a result,
thermal deformation in coating and heat treatment is also
suppressed.
[0091] If the average aspect ratio of crystal grains is less than
3, there is no great difference from the ordinary equiaxed grains
and the foregoing effect is not attained to a satisfactory extent,
so that it is impossible to suppress thermal deformation in
painting and heat treatment and ensure the required can strength
after heat treatment. In this point, the larger the extension in
the rolling direction of crystal grains, the better. More
preferably, an average aspect ratio of crystal grains is 3.1 or
more.
[0092] Without intermediate annealing, the aspect ratio of each
individual crystal grain depends on the crystal structure of the
hot rolled sheet, as well as the draft in cold rolling and the cold
rolling temperature. In this point, an upper limit of the average
aspect ratio of crystal grains is determined from a capacity limit
of the manufacturing process for making the crystal grains into the
elongated grains, including hot rolling and cold rolling. Its level
is 6 or so.
Method for Measuring Average Aspect Ratio
[0093] An average aspect ratio of crystal grains is measured
through an observation (polarization observation) from above of a
part located at the center in the through-thickness direction. A
rolled upper surface of the central part in the thickness direction
of the sheet after thermal refining (before forming the bottle can)
is examined by polarization observation after mechanical polishing,
electrolytic polishing and anodic oxidation using barker's
liquid.
[0094] When the crystal structure is examined by polarization
observation from the upper surface side of the central part in the
thickness direction of the sheet, there occurs black-white
difference due to the difference in crystal orientation. In this
observation, with respect to crystal grains in a visual field
permitting clear observation of the contour, both maximum length in
the rolling direction and maximum length in the sheet width
direction of each individual crystal grain are measured. In this
case, (maximum length in the rolling direction)/(maximum length in
the sheet width direction) of each individual crystal grain is
calculated as an aspect ratio. Through observation using an optical
microscope of 100 magnifications, the number of crystal grains to
be subjected to measurement is determined to be 100 grins and a
mean value of aspect ratios of those crystal grains is determined
as an average aspect ratio of crystal grains.
Suppressing Anisotropy
[0095] In the present invention, even when the hot rolled sheet is
subjected to cold rolling directly up to the final sheet thickness
without going through intermediate annealing in order to attain the
thus-defined average aspect ratio of crystal grains and improve the
formability of the sheet into a can and the strength thereof, the
foregoing thermal deformation in coating and heat treatment is
suppressed and the required can strength after heat treatment is
ensured.
[0096] To this end, in the present invention, not only the above
crystal grain control is made, but also a control is made so as to
suppress anisotropy in the structure. For this anisotropy control,
two anisotropy indices, which are tensile strength and n value, are
used selectively.
[0097] In connection with the tensile strength as one anisotropy
index, when the tensile strength becomes more and more anisotropic,
as noted above, an internal stress after cupping and ironing
becomes non-uniform in the circumferential direction and the degree
of recovery becomes non-uniform in printing/coating and during heat
treatment performed for improving the adhesion of laminate film,
with the result that elliptic deformation is apt to occur.
[0098] For diminishing the anisotropy of the tensile strength, the
difference between maximum and minimum values among tensile
strengths in 0.degree., 45.degree. and 90.degree. directions
relative to the rolling direction is made as small as possible.
More specifically, this difference is reduced to 25 MPa or less,
preferably 20 MPa or less.
[0099] In addition to the above tensile strength, anisotropy in the
rolling direction of a work hardening index, i.e., n value, is also
important. If the anisotropy of n value is large, even if the
anisotropy of the tensile strength referred to above is small (even
if it is within the prescribed range), an internal stress induced
by cupping and ironing becomes non-uniform in the circumferential
direction, and the degree of recovery becomes non-uniform in
printing/coating and when heat treatment is performed for improving
the adhesion of laminate film, resulting in elliptic deformation
being apt to occur.
[0100] Therefore, in order to diminish the anisotropy of n value,
the difference between maximum and minimum values among n values in
0.degree., 45.degree. and 90.degree. directions relative to the
rolling direction of the aluminum alloy sheet is made as small as
possible. More specifically, this difference is set at 0.03 or
less, preferably 0.028 or less, more preferably 0.025 or less,
still more preferably 0.02 or less, and still more preferably 0.015
or less.
[0101] If the aluminum alloy sheet has one or both of the
anisotropic indices, there will occur the foregoing thermal
deformation in coating and heat treatment even if the foregoing
crystal grain control is made, although the can formability may not
be influenced. More particularly, if the difference between maximum
and minimum values among the foregoing tensile strengths exceeds 25
MPa and/or if the difference between maximum and minimum values
among the foregoing n values exceeds 0.03, there will occur the
foregoing thermal deformation in coating and heat treatment.
Anisotropy Suppressing Method
[0102] Even when the hot rolled sheet is subjected to cold rolling
without intermediate annealing, hot rolling conditions are
controlled in order to satisfy both anisotropic indices. More
specifically, hot finish rolling is performed by a tandem rolling
mill equipped with three to six rolling stands and the coil winding
tension in those final stands is made relatively high to increase
the forward slip of the rolled sheet.
[0103] In this connection, an average coil winding tension in the
final stands is made as high as possible in excess of at least 20
MPa. If an average coil winding tension is 20 MPa or less, crystal
grains such as cube orientation are apt to be formed, and when cold
rolling is performed without intermediate annealing, the sheet
anisotropy becomes conspicuous. An ordinary average coil winding
tension is in the range of 5 to 10 MPa.
[0104] A description will now be given also about controlling
dispersed particles in the cold rolled aluminum alloy structure. In
the present invention, as noted above, in order to ensure the
exhibition of the effect of the foregoing elongated crystal grains,
an average particle size of dispersed particles in this structure
is controlled. More specifically, in the observation of particles
of 0.5 .mu.m or more, an average particle size of dispersed
particles is microsized to 5 .mu.m or less and .DELTA.T which
represents the liquid-solid phase coexistence temperature range of
aluminum is set at 40.degree. C. or lower.
Average Particle Size of Dispersed Particles
[0105] Dispersed particles in the cold rolled aluminum alloy
structure are various intermetallic compound particles, including
the foregoing Al--Fe--Mn--Si-based intermetallic compounds (.alpha.
phase). In this case, the finer the average particle size of the
dispersed particles, the better.
[0106] If the proportion of coarse dispersed particles
(precipitated compounds) exceeding 5 .mu.m in average particle size
increases, they are apt to become the nucleus of recovery and
recrystallization, so that softening at a high temperature becomes
conspicuous and there easily occur elliptic deformation and a
lowering of strength in high-temperature heat treatment. As a
result, the aforesaid effect of the elongated crystal grains is
offset.
[0107] In the present invention, therefore, an average particle
size of dispersed particles in the observation of 0.5 .mu.m or
larger particles is set at 5 .mu.m or less, preferably 4.5 .mu.m or
less.
[0108] It is assumed that the dispersed particles to be analyzed
have a size (barycentric diameter) of 0.5.mu. or more. This is
because the presence of particles of 0.5 .mu.m or larger exerts a
great influence on the anti-softening property as noted above,
while particles smaller than 0.5 .mu.m are little influential.
Moreover, dispersed particles smaller than 0.5 .mu.m are difficult
to be observed and measurement variations in this measurement
become large. Therefore, such small particles are excluded from the
measurement range defined in the present invention.
Measurement of Average Particle Size
[0109] An average particle size of dispersed particles in the
observation of particles of 0.5 .mu.m or more is determined using a
scanning electron microscope (SEM) for the sheet structure. More
specifically, a test piece of an upper rolled surface of a central
sheet portion is mirror-polished and the structure of the polished
surface is observed in ten visual fields each about 200 .mu.m by
about 150 .mu.m in size through an SEM (e.g., Model S4500 FE-SEM:
Field Emission Scanning Electron Microscopy, a product of Hitachi,
Ltd.) having 500 or 1000 magnifications.
[0110] In this case, for observing the dispersed particle phase
(intermetallic compound phase) clearly, the observation is made by
observing a reflected electron image. A black image indicates Al
and different contrasts make the dispersed particle phase clear.
The dispersed particles are traced and an average size (mean value
of barycentric diameters) of the dispersed particles is determined
using Image-ProPlus (a product of MEDIACYBERNETICS Co.) as software
for image analysis. The number of measured dispersed particles is
200 or more as a total in the above ten-visual field structure
observation and calculation was made using a mean value.
Solid-Liquid Coexistence Temperature Range .DELTA.T
[0111] FIG. 2 is a schematic phase diagram of Al--Mg--Mn-based
alloy, showing schematically a relation among a liquidus line of
aluminum, a solidus line of aluminum, and the crystallization
temperature of AlMn-- and Al(Fe, Mn)-based compounds as main
crystals. In FIG. 2, the temperature range (temperature difference)
between the Al liquidus and solidus lines corresponds to the
solid-liquid coexistence temperature range .DELTA.T as referred to
herein.
[0112] There is a tendency that the wider (longer) the .DELTA.T of
a component system, variations in production between stable and
metastable phases of the intermetallic compound become larger,
depending on solidifying and cooling conditions in casting. In this
state of structure, elements other than the constituent elements of
the intermetallic compounds are forcibly dissolved in those
crystals. Therefore, at the stage of the bottle can body, softening
at a high temperature becomes conspicuous and elliptic deformation
and a lowering of strength are apt to occur in high-temperature
heat treatment. Accordingly, as is the case with coarsening in
average particle size of dispersed particles, the effect of the
foregoing elongated crystal grain is offset.
[0113] Moreover, the wider (larger) the .DELTA.T of a component
system, the more easily is formed a coarse compound particle
distribution because intermetallic compounds grow rapidly in liquid
phase. As a result, it becomes impossible to microsize the average
particle size of dispersed particles defined above. Consequently,
as explained above in connection with the dispersed particles,
coarse particles become the nucleus of recovery and
recrystallization and softening of the bottle can body in heat
treatment become more conspicuous, so that elliptic deformation is
apt to occur. Further, the presence itself of a coarse compound is
apt to be a cause of defect of the can surface.
[0114] Strictly speaking, in connection with .DELTA.T, there is a
method for defining the range of crystallization temperature of
Al--Mn-based intermetallic compounds and that of the solid phase
temperature of Al, as noted above. However, in the Al alloy system
according to the present invention, the melting point of the Al
alloy system and the crystallization temperature of Al--Mn-based
compounds exhibit a change of only about 4.degree. to 7.degree. and
thus cannot be an exact index. For this reason, there was used the
range (temperature difference) between the Al liquid phase
temperature and the Al solid phase temperature, which can be an
exact index, with a sufficient difference (margin) found in
.DELTA.T in point of measurement evaluation.
[0115] The narrower (smaller) the .DELTA.T (solid-liquid
coexistence temperature range), the smaller the solid-liquid
coexistence temperature range between dispersed particles and
aluminum liquid phase in the component system concerned, and the
smaller the variations in the production of stable phase and
metastable phase in intermetallic compounds and the finer the
compound particles. Consequently, at the stage of the bottle can
body, the anti-softening property at high temperatures is enhanced
and it is possible to suppress elliptic deformation and a lowering
of strength in high-temperature heat treatment.
[0116] As will be seen in Examples to be described later, as
.DELTA.T becomes larger, the average particle size of dispersed
particles increases as well. Particularly, when the value of
.DELTA.T exceeds 40.degree. C., the average particle size of
dispersed particles shows a coarsening tendency. Thus, the smaller
the value of .DELTA.T in the range of not higher than 40.degree.
C., the better. More preferably, the value of .DELTA.T is
38.degree. C. or less, still more preferably 36.degree. C. or less,
and still more preferably 34.degree. C. or less.
Calculation of Solid-Liquid Coexistence Temperature Range
.DELTA.T
[0117] The value of .DELTA.T is calculated by measuring the melting
point and solid phase temperature of a cold rolled aluminum sheet
(test piece) concerned by differential thermal analysis and thereby
calculating the temperature range .DELTA.T in which the liquid
phase of aluminum is existent. In the range related to the aluminum
alloy system according to the present invention, the melting point
lies in the range of approximately 645.degree. to 660.degree. C.
and a change detected at a temperature of approximately 600.degree.
to 630.degree. C. is the solid phase temperature.
[0118] Tests were conducted using, for example, TG/DTA (TGD7000)
manufactured by ULVAC-RIKO, Inc. and under the following
conditions:
[0119] Heat pattern: RT.about.700.degree. C..about.RT: 10.degree.
C./min
[0120] Atmosphere: Ar (100 ml/min)
[0121] Sample weight: about 500 mg
[0122] Reference: Alumina powder
[0123] Sample container: Alumina (Macro type 8.times.10 mm)
[0124] As a method other than differential thermal analysis,
.DELTA.T may be determined from a calculation phase diagram, but
the method using differential thermal analysis is more accurate.
However, .DELTA.T determined by a thermodynamic equilibrium phase
diagram calculation is useful in case of making alloy design
beforehand so that the value of .DELTA.T becomes 40.degree. C. or
less. FIG. 3 illustrates .DELTA.T in a calculation phase diagram of
C alloy according to an Example of the present invention which is
described in Table 7 to be shown later.
Control of .DELTA.T
[0125] The control of .DELTA.T is basically made by designing the
balance of the main constituent elements (Mn, Mg, Fe, Cu, Si) in
the present invention in such a manner that the solid-liquid
coexistence temperature range .DELTA.T of aluminum becomes
40.degree. C. or less. According to a general trend of the alloy
elements (components), as to Mn and Fe, the value of .DELTA.T
becomes larger as the content thereof increases or decreases from a
median in the defined content range. As to Mg, Cu and Si, the value
of .DELTA.T tends to become larger as their contents increases.
Within the contents' ranges defined in the present invention,
generally, the lower the contents of these alloy elements, the
smaller the value of .DELTA.T.
[0126] However, in order for the cold rolled aluminum alloy sheet
for bottle cans to satisfy the required strength and formability,
it is difficult to simply decrease the respective contents of the
main constituent elements.
[0127] Further, the crystallization temperature of Al (Fe,
Mn)-based intermetallic compound varies depending on multi-element
components, including the foregoing other selective added elements
and impurity elements. Therefore, the value of .DELTA.T also varies
largely depending on such other selective added elements and
impurity elements.
[0128] Particularly, the proportion of can material scraps in
molten material used as can material has been increasing every year
in comparison with base metal and the proportion of inevitable
impurity elements present other than basic component elements is
increasing. Examples of such inevitable impurity elements are Zr,
Bi, Sn, Ga, V, Co, Ni, Ca, Mo, Be, Pb, and W. The total (total
amount) of contents of these elements has heretofore been 0.01% or
less, but with the recent increase of the scrap proportion, it is
now 0.015% or more, or 0.02% or more, as the case may be it is
0.05% or 0.1% or more, as inevitable impurities.
[0129] Consequently, even assuming that the amounts of the main
constituent elements and the amounts of the selective added
elements remains the same, if the total amount of the impurity
elements exceeds 0.01%, the value of .DELTA.T is influenced thereby
and varies greatly. The degree of such influence differs depending
on the kind of alloying elements; besides, in a multi-component
system, the solid-liquid coexistence temperature range varies also
depending on interaction of the constituent components. Therefore,
in the case where the amount of such inevitable impurity elements
becomes large, with a mere control of the content range and the
balance of basic components (e.g., Mg/Mn ratio), it is very
difficult to make a components design for making the solid-liquid
coexistence temperature range into an optimal range, because of
complicatedness of the component system.
[0130] For this reason, when controlling the value of .DELTA.T,
first there is made design of components balance with respect to
the main constituent elements (Mn, Mg, Fe, Cu, Si) and selective
added elements in the present invention and there is made alloy
design so as to satisfy the strength and formability required of
the cold rolled aluminum alloy sheet for bottle cans.
[0131] Then, for calculating the liquid phase temperature and solid
phase temperature, there is made such a thermodynamic equilibrium
phase diagram calculation as shown in FIG. 2 and the alloy design
is modified so that the solid-liquid coexistence temperature range
.DELTA.T of aluminum becomes 40.degree. C. or less. Thereafter, it
is necessary to make a trial manufacture and check beforehand
whether the solid-liquid coexistence temperature range .DELTA.T of
aluminum becomes 40.degree. C. or less under mass-productive
manufacturing conditions to be described later.
Manufacturing Method
[0132] The cold rolled aluminum sheet of the present invention can
be manufactured without greatly changing the conventional soaking,
hot rolling and cold rolling processes. However, in order to attain
the structure defined in the present invention and ensure the basic
material properties (earing rate and strength), as well as
formability and ironing processability, without impairing them, it
is necessary to limit each of the above individual processes to an
optimal condition range and combine those processes.
Soaking Condition
[0133] The soaking temperature is set at 550-650.degree. C. If the
soaking temperature is too low, it will take too much time for
soaking, resulting in productivity being deteriorated, while if the
soaking temperature is too high, there occurs swelling on ingot
surface. Therefore, the soaking temperature is set to a temperature
falling under the above range. A preferred soaking temperature
range is in the range of 580.degree. C. (especially 590.degree. C.)
to 615.degree. C. (especially 610.degree. C.).
[0134] The shorter the soaking time (homogenizing time), the better
insofar as the ingot can be homogenized. For example, the soaking
time is preferably 12 hours or less, more preferably 6 hours or
less. However, if the soaking temperature is set at 550.degree. C.
or higher, the soaking time is required to be 6 hours or more.
Likewise, if the soaking temperature is set at 580.degree. C. or
higher, the soaking time is required to be 5 hours or more, and if
the soaking temperature is set at 590.degree. C. or higher, the
soaking time is required to be 4 hours or more.
[0135] The soaking treatment may be done dividedly in plural
stages. In the soaking treatment, control of the heating speed, of
the soaking temperature (homogenizing temperature) and of the
cooling speed may be done in any stage. Such control may be done in
all the stages, but is preferably done at least in the first
stage.
[0136] In the case where the temperature of the first soaking
treatment is set to a temperature falling under the above range,
the temperatures of the second and subsequent soaking treatments
are in many cases set lower than the temperature of the first
soaking treatment. For example, the temperatures of the second and
subsequent soaking treatments are set lower by about 10.degree. to
100.degree. C., preferably about 50.degree. to 100.degree. C., as
compared with the temperature of the first soaking treatment.
Hot Rolling Start Condition
[0137] After end of the soaking process, the ingot may be once
cooled, then re-heated and thereafter subjected to rough hot
rolling, or without cooling to excess, it may be subjected directly
to rough hot rolling. In case of subjecting the ingot directly to
rough hot rolling without excessive cooling, the sum of the amount
of solute Cu and the amount of Cu contained in a fine precipitate
of 0.2 .mu.m or less can be easily set at a value in the range of
0.05% to 0.3%, which means the amount of Cu in a solution separated
from a precipitate exceeding 0.2 .mu.m in particle size by the
extracted residue method using hot phenol, and the sum of the
amount of solute Mg and the amount of Mg contained in a fine
precipitate of 0.2 .mu.m or less can be easily set at a value in
the range of 0.75% to 1.6%, which means the amount of solute Mg
separated from a precipitate exceeding 0.2 .mu.m in particle size
by the extracted residue method using hot phenol. Moreover, it is
possible to utilize self-heat generation of the ingot after the
soaking process, whereby not only the production time and heat
energy can be saved, but also it is possible to diminish the number
density of alloy elements' precipitates and hence possible to
diminish the earing rate.
[0138] In case of once cooling the ingot and re-heating it, it is
preferable to perform rapid heating at a rate of 30.degree. C./hr
or higher. By this rapid heating it is possible to suppress
dissolving of Mg and Cu to the coarse compounds so far produced or
precipitation at the interface of coarse precipitates, whereby the
sum of the amount of solute Cu and the amount of Cu contained in a
fine precipitate of 0.2 .mu.m or less in particle size can be
easily set at a value in the range of 0.05% to 0.3%, which means
the amount of Cu in a solution separated from a precipitate
exceeding 0.2 .mu.m in particle size by the extracted residue
method using hot phenol, and the sum of the amount of solute Mg and
the amount of Mg contained in a fine precipitate of 0.2 .mu.m or
less in particle size can be easily set at a value in the range of
0.75% to 1.6%, which means the amount of solute Mg separated from a
precipitate exceeding 0.2 .mu.m in particle size by the extracted
residue method using hot phenol. Moreover, it is possible to
prevent an excessive increase in the number density of alloying
elements' precipitates.
Rough Hot Rolling Condition
[0139] When performing hot rolling dividedly into rough rolling and
finish rolling and in a continuous manner, if the end temperature
in rough hot rolling is too low, the rolling temperature will
become low in the next finish hot rolling and an edge crack becomes
easier to occur. Moreover, if the end temperature in rough hot
rolling becomes too low, the self heat required for
recrystallization after finish rolling is apt to become deficient,
so that the crystal grain diameter becomes too small. Therefore, it
is preferable that the end temperature in rough hot rolling be set
at 420.degree. C. or higher, more preferably a temperature in the
range of 430.degree. C. (especially 440.degree. C.) to 470.degree.
C. (especially 460.degree. C.).
[0140] In order to ensure the end temperature in rough hot rolling
of 420.degree. to 480.degree. C. it is preferable that the start
temperature in rough hot rolling be set at, for example, about
490.degree. to 550.degree. C., more preferably about 495.degree. to
540.degree. C., still more preferably about 500.degree. to
530.degree. C. As long as the said start temperature is kept at a
temperature of 550.degree. C. or lower, it is also possible to
prevent surface oxidation of the hot rolled sheet. Further, it is
possible to prevent coarsening of recrystallized grains and hence
possible to further enhance the formability.
[0141] It is preferable that the aluminum alloy sheet having gone
through the rough hot rolling process be subjected to finish hot
rolling rapidly, for example continuously, whereby it is possible
to prevent recovery of the strain accumulated in rough hot rolling
and hence possible to enhance the strength of the cold rolled sheet
to be obtained subsequently. It is preferable that the aluminum
alloy sheet after the end of rough hot rolling be subjected to
finish hot rolling for example within 5 minutes, preferably within
3 minutes.
Finish Hot Rolling Condition
[0142] It is preferable that the end temperature in finish hot
rolling be set at 310.degree. to 350.degree. C. The finish hot
rolling process is a process for finishing the cold rolled alloy
sheet to predetermined dimensions, and since the structure after
the end of rolling becomes a recrystallized structure due to
self-heat generation, the end temperature in finish hot rolling
exerts an influence on the recrystallized structure. By setting the
end temperature in finish hot rolling at 310.degree. C. or higher,
in combination with the cold rolling condition which follows, the
final sheet structure can be made into an elongated structure in
the rolling direction with an aspect ratio of 3 or more and it is
possible to ensure both Cu and Mg solid solution quantities defined
in the present invention. If the end temperature in finish hot
rolling is lower than 310.degree. C., it is difficult to attain the
above structure in the present invention even if the draft in
cooling rolling in the subsequent cold rolling process is set
large.
[0143] On the other hand, if the end temperature in finish hot
rolling exceeds 350.degree. C., it will be impossible to make the
final sheet structure into an elongated structure in the rolling
direction and the desired amount of solute Mg cannot be ensured.
Accordingly, a lower limit of the end temperature in finish hot
rolling is set at 310.degree. C. or higher, preferably 320.degree.
C. or higher, while an upper limit thereof is set at 350.degree. C.
or lower, preferably 340.degree. C. or lower.
Type of Finish Hot Rolling Mill
[0144] As a finish hot rolling mill there is used a tandem hot
rolling mill equipped with three or more stands. By setting the
number of stands at three or more it is possible to diminish the
draft per stand and accumulate strain while ensuring surface
properties of the hot rolled sheet, so that it is possible to
further enhance the strength of the cold rolled sheet and that of
its DI-formed product.
Total Draft in Finish Hot Rolling
[0145] It is preferable that the total draft in finish hot rolling
be set at 80% or more. By setting the total draft at 80% or more,
in combination with cold rolling to be described later, the final
sheet structure can be easily made into an elongated structure in
the rolling direction with an average aspect ratio of 3 or more. It
is also possible to enhance the strength of the cold rolled sheet
and its DI-formed product.
Thickness of Hot Rolled Sheet
[0146] It is preferable that the thickness of the alloy sheet after
hot (finish) rolling be about 1.8 to 3 mm. By setting the sheet
thickness at 1.8 mm or more it is possible to prevent worsening
(e.g., seizure or roughening) of surface properties of the hot
rolled sheet and the sheet thickness profile. On the other hand, by
setting the sheet thickness at 3 mm or less it is possible to
prevent the draft from becoming too high when fabricating the cold
rolled sheet (the sheet thickness is usually about 0.28 to 0.35 mm)
and hence possible to suppress the earing rate after DI.
[0147] In the hot rolled sheet obtained in the above manner, the
amount of solute Cu and that of solute Mg are in a controlled state
to the respective optimum ranges, so that the average earing rate
is controlled to its predetermined range. Therefore, it is possible
to effect cold rolling without intermediate annealing and make the
average earing rate of the cold rolled sheet as small as 0% to
3.5%. Moreover, by combination with cold rolling to be described
later, the final sheet structure can be made into an elongated
structure in the rolling direction with an average aspect ratio of
crystal grains of 3 or more and the sum of the amount of solute Cu
and the amount of Cu contained in a fine precipitate of 0.2 .mu.m
or less in particle size can be easily set at 0.05-0.3%, which
means the amount of Cu in a solution separated from a precipitate
exceeding 0.2 .mu.m in particle size by the extracted residue
method using hot phenol, likewise, the sum of the amount of solute
Mg and the amount of Mg in a fine precipitate of 0.2 .mu.m or less
in particle size can be easily set at 0.75-1.6%, which means the
amount of solute Mg separated from a precipitate exceeding 0.2
.mu.m in particle size by the extracted residue method using hot
phenol.
Cold Rolling
[0148] In the cold rolling process, rolling is performed directly
through plural number of passes without intermediate annealing and
it is preferable that the total draft be 77-90%. By making the
total draft 77% or more without intermediate annealing, the final
sheet structure can be made into an elongated structure in the
rolling direction with an average aspect ratio of crystal grains of
3 or more and the sum of the amount of solute Cu and the amount of
Cu contained in a fine precipitate of 0.2 .mu.m or less in particle
size can be easily set at 0.05-0.3%, which means the amount of Cu
in a solution separated from a precipitate exceeding 0.2 .mu.m in
particle size by the extracted residue method using hot phenol,
likewise, the sum of the amount of solute Mg and the amount of Mg
present in a fine precipitate of 0.2 .mu.m or less can be set at
0.75-1.6%, which means the amount of solute Mg separated from a
precipitate exceeding 0.2 .mu.m in particle size by the extracted
residue method using hot phenol. Besides, it is possible to secure
compressive strength of the can. In the case where intermediate
annealing is performed or where the total draft is low, the crystal
grains are apt to become equiaxed grains and the final sheet
structure is difficult to become an elongated structure in the
rolling direction with an average aspect ratio of crystal grains of
3 or more.
[0149] On the other hand, if the draft exceeds 90%, although the
average aspect ratio of crystal grains can be made large, the plus
earing in DI becomes too large and the strength becomes too high,
so a cupping crack or a can bottom crack is very likely to occur
during DI.
[0150] The sheet thickness after cold rolling is set at about
0.28-0.35 mm in view of formability thereof into a bottle can.
[0151] In the cold rolling process it is preferable to use a tandem
rolling mill with two or more stages of rolling stands arranged in
series. By using such a tandem rolling mill, in comparison with a
single rolling mill having one stage of rolling stand and wherein
cold rolling to a predetermined sheet thickness is performed by
repeated pass (sheet pass), a smaller number of pass suffices even
at the same total draft in cold rolling and it is possible to
increase the draft in one pass.
[0152] Thus, the final sheet structure can be made more easily into
an elongated structure in the rolling direction with an average
aspect ratio of crystal grains of 3 or more.
[0153] Moreover, in comparison with the conventional cold rolling
process using a single rolling mill wherein finish annealing is
performed after the cold rolling, it is possible to let recovery
occur continuously at a lower temperature and produce subgrains.
However, the rolling mill to be used is not limited to the tandem
rolling mill insofar as the rolling mill adopted can cause recovery
in cold rolling and produce subgrains sufficiently.
[0154] In cold rolling using a tandem rolling mill, the amount of
heat generated in one pass is large because the draft in one pass
is high. In the case where the amount of heat generated becomes too
large, the amount of precipitates of Cu and Mg produced, especially
the amount of precipitates at the boundary of coarse precipitates,
increases due to strain introduction and heat generation in cold
rolling. Consequently, there is the possibility that it will be
impossible to ensure the required amount of solute Cu and that of
solute Mg and the required amount of a fine precipitate.
[0155] Accordingly, in cold rolling using a tandem rolling mill,
when the temperature of the aluminum alloy sheet rises to the
greatest extent just after cold rolling in the cold rolling
process, it is preferable to cool the aluminum alloy sheet forcibly
so as to prevent the aluminum alloy sheet temperature after cold
rolling from rising to a level exceeding 200.degree. C.
[0156] As means for cooling the aluminum alloy sheet forcibly in
cold rolling it is preferable to adopt cooling means such that
water-free rolling oil used commonly is changed into an emulsion
type such as water-soluble oil or lubricant and an aqueous solution
of the emulsion is used to strengthen the cooling performance
without deteriorating the lubricating performance.
[0157] After cold rolling, finish annealing (final annealing) may
be done at a temperature lower than the recrystallization
temperature, if necessary. Finish annealing results in recovery of
the structure and improvement of both DI formability and can bottom
formability. It is preferable that the finish annealing temperature
be, for example, about 100-150.degree. C., more preferably
115-150.degree. C. By setting the temperature at 100.degree. C. or
higher, it is possible to effect a satisfactory recovery of the
structure. On the other hand, by setting the temperature at
150.degree. C. or lower, it is possible to prevent excessive
precipitation of dissolved elements and further enhance the DI
formability and flange formability.
[0158] It is preferable that the finish annealing time be 4 hours
or less (especially about 1 to 3 hours). By avoiding too long
annealing it is possible to prevent excessive precipitation of
dissolved elements and further enhance the DI formability.
[0159] However, in cold rolling using the foregoing tandem rolling
mill, finish annealing is basically not required because it is
possible to let recovery occur at a lower temperature and
continuously and product subgrains.
[0160] The present invention will be described below more
concretely by way of Examples, but the present invention is not
limited by the following Examples and changes may be made
appropriately within the scope conforming to the above and
following gists of the present invention, which changes are all
included in the technical scope of the present invention.
EXAMPLE 1
[0161] Using aluminum metal alone as a melting raw material and
using molten metal of Al alloy components in A to N shown in Table
1 below, ingots each 600 mm thick by 2100 mm wide were produced by
the DC casting method. In Table 1, the element content indicated by
"-" represents that it is below the detection limit.
[0162] As shown in Table 1, in both working examples and
comparative examples, the ingots contained inevitable impurity
elements Zr, Bi, Sn, Ga, V, Co, Ni, Ca, Mo, Be, Pb and W in total
contents of 0.01% or less as the total amount of other
elements.
[0163] The ingots were each subjected to a soaking treatment in
accordance with the conditions shown in Table 2. The soaking
treatment was conducted twice. That is, after the first soaking
treatment, each ingot was cooled to room temperature at a cooling
speed shown in Table 2, followed by re-heating to effect the second
soaking treatment. The heating speed as a first soaking condition
indicates a heating speed in the temperature range from 300.degree.
C. to the highest temperature which temperature range substantially
exerts an influence on properties. The cooling speed as a first
soaking condition indicates a cooling speed in the temperature
range from the highest temperature to 300.degree. C. which
temperature range substantially exerts an influence on properties.
After this soaking treatment, rough hot rolling was performed using
a reversing rough hot rolling mill having one stand, followed by
finish hot rolling using a tandem hot rolling mill having four
stands. The finish hot rolling was started within three minutes
after the end of rough hot rolling. In this way there were obtained
hot rolled aluminum alloy sheets 2 to 2.5 mm in thickness as a
common condition after finish hot rolling.
[0164] The hot rolled sheets were then subjected to cold rolling by
only one pass with use of a tandem rolling mill having two stages
of rolling stands without intermediate annealing to afford sheets
(cold rolled sheets) for bottle can body 0.3 mm in final thickness
as a common condition. In the cold rolling by the tandem rolling
mill, the aluminum sheets were cooled forcibly using an aqueous
emulsion so as to prevent the aluminum sheet temperature just after
cold rolling from rising to a temperature exceeding 250.degree. C.
Finish annealing (final annealing) after this cold rolling was not
conducted.
[0165] In only comparative example 10, two passes were performed in
a single rolling mill having one stage of rolling stand and
intermediate annealing of 150.degree. C..times.1 hour was performed
between the first and the second pass for comparison purpose
although the total draft in cold rolling was the same.
[0166] A test piece was sampled from each sheet (coil) for bottle
can body after cold rolling and the structure thereof was checked.
More particularly, an average aspect ratio of crystal grains and
the amount of solute Cu and that of solute Mg were checked by the
methods described above, the results of which are shown in Table
3.
[0167] For determining high temperature properties of the test
piece, hardness and 0.2% proof stress of the test piece surface at
room temperature, as well as hardness and 0.2% proof stress of the
test piece surface in 290.degree. C..times.20 sec. heat treatment
of the test piece, were measured and a change in hardness (hardness
reduction) .DELTA.Hv (Hv) of the test piece before and after the
heat treatment was determined. Further, an elliptic deformation
quantity upon bake-hardening of the can body after forming was
measured. These results are also shown in Table 3.
[0168] (Measurement of 0.2% Proof Stress)
[0169] Tensile test for measuring 0.2% proof stress was conducted
in accordance with JIS Z 2201 and, as the test piece shape, there
was used the shape of JIS No. 5 test piece. The test piece was
obtained in such a manner that its longitudinal direction was
coincident with the rolling direction. Crosshead speed was 5 mm/min
and a constant speed was adopted until breakage of the test
piece.
[0170] (Hardness Measurement)
[0171] Each cold rolled sheet sample was measured for hardness at
four positions under the application of 100 g load by means of a
micro Vickers hardness tester and a mean value of the measured
values was used as the hardness value.
[0172] (Evaluation of Elliptic Deformation)
[0173] For evaluation of elliptic deformation, as will be described
later, a bottle can body obtained by DI of the above sheet for a
bottle can body was washed and then baked under the condition that
the bodily temperature of the can should reach 300.degree. C. in 30
seconds, then the degree of elliptic deformation was checked. For
checking the degree of elliptic deformation, the diameter of the
mouth portion of the bottle can body was checked successively in
the circumferential direction, then an amount obtained by
subtracting the minimum diameter from the maximum diameter was
determined as an elliptic deformation quantity (mm), and evaluation
was made using a mean value from N=10 cans. In the case where the
elliptic deformation quantity is 4 mm or less, it was evaluated
that the elliptic deformability came up to a passing point. If the
elliptic deformation quantity exceeds 4 mm, there will occur
defects such as falling-down and jam during conveyance and necking
as post-steps in the can manufacturing process, thus making
continuous and efficient manufacture of cans difficult.
[0174] Further, as formability which the sheet for bottle can body
should satisfy basically, both earing rate and DI formability (the
number of times of cracking in forming) were measured and
evaluated, the results of which are also shown in Table 3.
[0175] (Earing Rate)
[0176] For checking the earing rate, a blank was sampled from the
sheet for a bottle can body and lubricating oil (Nalco 147, a
product of D.A. Stuart Co.) was applied thereto, then the blank was
subjected to 40% deep drawing test and formed into a cup for
investigation, using an Erichsen tester. The test was performed
under the following conditions: blank dia. 66.7 mm, punch dia. 40
mm, R of the die-side shoulder portion 2.0 mm, punch shoulder R 3.0
mm, blank holder pressure 400 kgf.
[0177] Mountain-valley shapes formed in eight directions
(0.degree., 45.degree., 90.degree., 135.degree., 180.degree.,
225.degree., 270.degree. and 315.degree. directions, assuming that
the rolling direction is 0.degree.) in the peripheral edge portion
of the opening of the cup thus obtained were measured and an
average earing rate was calculated.
[0178] An average earing rate calculating method will now be
described with reference to FIG. 1. FIG. 1 is a developed view of
the cup formed by DI of the sheet for a bottle can body. In this
developed view, earing heights (T1, T2, T3, T4, designated minus
earings) created in 0.degree., 90.degree., 180.degree. and
270.degree. directions, assuming that the rolling direction is
0.degree., are measured and likewise earing heights (Y1, Y2, Y3,
Y4, designated plus earings) created in 45.degree., 135.degree.,
225.degree. and 315.degree. directions are measured. The heights Y1
to Y4 and T1 to T4 are from the bottom of the cup. From the
measured values, an average earing rate is calculated in accordance
with the following equation:
Average Earing
Rate(%)=[{(Y1+Y2+Y3+Y4)-(T1+T2+T3+T4)}/{1/2.times.(Y1+Y2+Y3+Y4+T1+T2+T3+T-
4)}].times.100
[0179] In the cold rolled sheet according to the present invention,
when the average earing rate is set at near 0, the development of
the four plug earings (Y1 to Y4) and the two minus earings (T2 and
T4 in FIG. 1) in 90.degree. and 270.degree. directions are
suppressed, but the development of the two minus earings (T1 and T3
in FIG. 1) in 0.degree. and 180.degree. directions are difficult to
be suppressed. Even if the absolute value of the average earing
rate is merely made small, for example when the average earing rate
is set at -2% to 2% (2% or less in absolute value), the minus
earings (T1 and T3 in FIG. 1) are not suppressed to a satisfactory
extent even if the average earing rate is not less than -2% and
less than 0%, so that the blank holder pressure in deep drawing is
concentrated on the two minus earings (T1 and T3 in FIG. 1),
causing edge rise and edge crack, which are inconvenient to the
manufacture. On the other hand, when the average earing rate is set
at 0-2% (plus side), it is possible to prevent breakage of the can
body caused by edge crack because the remaining two minus earings
(T1 and T3 in FIG. 1) can also be suppressed to a satisfactory
extent. In the present invention, the range of +0% to +3.5% is
defined to be an allowable range.
[0180] (DI Formability)
[0181] A blank 156 mm in diameter was punched from the above sheet
for bottle can body (sheet thickness 0.3 mm). Using the blank, a
cup having a diameter of 92 mm was formed and then subjected to
re-deep drawing, ironing and trimming, whereby DI can bodies for
bottle can (inside diameter 66 mm, height 170 mm, side wall
thickness 103 .mu.m, side wall tip thickness 165 .mu.m, final third
ironing rate 40%) were fabricated at a can manufacturing speed of
300 cans/min. The number of body-cracked cans per 50,000 formed
cans was determined and DI formability was evaluated. The
evaluation was made in accordance with the following criterion.
.circleincircle.: no cracked can (extremely good), .largecircle.:
one cracked can or less (good), .DELTA.: two to four cracked cans
(generally good), X: more than five cracked cans (bad).
[0182] As is apparent from Table 3, working examples 1 to 6 have
compositions according to the present invention, average aspect
ratios of crystal grains therein are 3 or more, Cu solid solution
quantities as determined in the foregoing manner are in the range
of 0.05% to 0.3%, and Mg solid solution quantities are in the range
of 0.75% to 1.6%.
[0183] Thus, in working examples 1 to 6, as is seen from Table 3,
after heat treatment at 290.degree. C. for 20 seconds (after
bake-hardening), the hardness change .DELTA.Hv is 30 Hv or less,
0.2% proof stress is 210 MPa or more, there is little lowering of
hardness and strength, and high-temperature properties are
excellent.
[0184] Working examples 1 to 6 are superior also in earing rate and
DI formability. Thus, it is seen that the improvement of
high-temperature properties in the present invention does not
impair the formability which the sheet for bottle can body should
satisfy basically.
[0185] On the other hand, in comparative examples 7 to 10,
conditions for soaking and hot rolling do not fall under the
foregoing preferred conditions, although the compositions adopted
therein fall under those defined in the present invention, so that
any of the average aspect ratio of crystal grains, the amount of
solute Cu and that of solute Mg deviates from the range defined in
the present invention. As a result, in comparison with the
foregoing working examples, a lowering of hardness and that of
strength are marked and high-temperature properties are
inferior.
[0186] In comparative example 7, the second soaking temperature is
too low, and the end temperature in finish hot rolling is too low.
In comparative example 8, the end temperature in finish hot rolling
is too low. In comparative example 9, the end temperature in finish
hot rolling is too low. In comparative example 10, the rolling mill
used is a single cold rolling mill and intermediate annealing was
performed halfway of cold rolling.
[0187] Comparative examples 11 to 20 follow preferred manufacturing
conditions. However, alloy compositions used therein deviate from
those defined in the present invention. As a result, in comparison
with the foregoing Examples of the present invention, a lowering of
hardness and that of strength are marked and high-temperature
properties are inferior. Formability is also low.
[0188] From the above results, critical meanings of the conditions
defined in the first invention can be understood.
TABLE-US-00001 TABLE 1 Al Alloy Chemical Components (mass %,
provided B is in ppm, balance Al) Total amount of Example Symbol Si
Fe Cu Mn Mg Cr Zn Ti B other elements * Working A 0.26 0.44 0.22
1.04 1.25 -- -- -- -- 0.005 Examples B 0.25 0.44 0.22 1.03 1.22
0.03 0.19 0.03 0 0.005 C 0.24 0.42 0.19 0.91 0.92 0.03 0.2 0.02 10
0.007 D 0.31 0.45 0.10 1.32 1.01 -- -- 0.02 10 0.01 Comparative E
0.35 0.50 0.2 1.6 1.05 -- -- 0.01 40 0.007 Examples F 0.35 0.51 0.2
0.6 1.04 -- -- 0.01 40 0.007 G 0.20 0.31 0.3 1.0 1.8 -- -- 0.03 30
0.005 H 0.20 0.31 0.3 1.0 0.7 -- -- 0.03 30 0.005 I 0.30 0.45 0.7
1.07 0.95 -- -- 0.02 30 0.005 J 0.30 0.46 0.03 1.06 0.93 -- -- 0.02
30 0.005 K 0.03 0.51 0.4 0.96 1.02 -- -- 0.03 30 0.005 L 0.6 0.50
0.4 0.95 1.03 -- -- 0.03 30 0.005 M 0.4 0.05 0.35 1.2 1.25 -- --
0.02 30 0.005 N 0.4 0.80 0.34 1.21 1.24 -- -- 0.02 30 0.005 * Total
amount of other elements (Zr, Bi, Sn, Ga, V, Co, Ni, Ca, Mo, Be,
Pb, and W)
TABLE-US-00002 TABLE 2 1st Soaking 2nd Soaking Rough Hot Rolling
Heating Soaking Holding Cooling Soaking Holding Start End Al Alloy
Speed Temp. Time Speed Temp. Time Temp. Temp. Example Symbol Symbol
(.degree. C./hr) (.degree. C.) (hr) (.degree. C./hr) (.degree. C.)
(hr) (.degree. C.) (.degree. C.) Working 1 A 25 610 6 30 510 7 505
450 Examples 2 A 18 610 6 40 510 15 510 430 3 A 23 610 4 40 510 10
500 425 4 B 21 610 4 45 550 8 550 440 5 B 19 590 6 45 510 9 505 430
6 C 15 610 6 30 550 10 540 465 Comparative 7 A 40 610 10 40 450 6
440 410 Examples 8 C 40 610 10 40 530 4 530 490 9 A 40 610 8 40 500
8 495 380 10 D 40 610 8 40 500 8 490 420 11 E 40 610 8 40 500 8 499
465 12 F 40 610 8 40 500 8 489 447 13 G 40 610 8 40 500 8 478 440
14 H 40 610 8 40 500 8 494 485 15 I 40 610 8 40 500 8 496 469 16 J
40 610 8 40 500 8 472 465 17 K 40 610 8 40 500 8 500 472 18 L 40
610 8 40 500 8 485 445 19 M 40 610 8 40 500 8 494 463 20 N 40 610 8
40 500 8 495 420 Finish Hot Rolling Cold Rolling Total Type Final
Working End Sheet of Sheet Ratio Temp. Thickness Rolling Draft
Thickness Example Symbol (%) (.degree. C.) (mm) Mill (%) (mm)
Working 1 94 339 2.5 Tandem 88 0.3 Examples 2 94 342 2.5 Tandem 88
0.3 3 94 321 2 Tandem 88 0.3 4 94 333 2.5 Tandem 88 0.3 5 94 349
2.5 Tandem 88 0.3 6 94 320 2 Tandem 85 0.3 Comparative 7 94 302 2.5
Tandem 88 0.3 Examples 8 94 360 2.5 Tandem 88 0.3 9 90 291 2.5
Tandem 88 0.3 10 94 332 2 Single 85 0.3 11 94 339 2.5 Tandem 88 0.3
12 94 334 2.5 Tandem 88 0.3 13 94 331 2.5 Tandem 88 0.3 14 94 319
2.5 Tandem 88 0.3 15 94 312 2.5 Tandem 88 0.3 16 94 325 2.5 Tandem
88 0.3 17 94 303 2.5 Tandem 88 0.3 18 94 325 2.5 Tandem 88 0.3 19
94 330 2.5 Tandem 88 0.3 20 94 327 2.5 Tandem 88 0.3
TABLE-US-00003 TABLE 3 Al Alloy Sheet Structure Cu Solid Mg Solid
Mn Solid Al Alloy Sheet High-temp. Properties Average Solution
Solution Solution AS AB Hardness Al Alloy aspect Quantity Quantity
Quantity Hardness Hardness Reduction Example Symbol Symbol ratio
(%) (%) (%) (Hv) (Hv) (.DELTA.Hv) Working 1 A 4.5 0.15 0.15 0.32 99
81 18 Examples 2 A 5.0 0.12 1.0 0.29 103 80 23 3 A 3.5 0.09 1.05
0.36 95 78 17 4 B 5.2 0.10 1.2 0.27 100 85 25 5 B 5.5 0.07 1.0 0.20
99 85 13 6 C 4.5 0.06 0.79 0.15 95 83 12 Comparative 7 A 4.0 0.03
0.7 0.1 105 73 32 Examples 8 C 4.1 0.1 0.95 0.2 103 73 30 9 A 4.3
0.05 0.71 0.21 106 76 30 10 D 2.5 0.08 0.98 0.15 105 71 34 11 E 6.5
0.06 0.78 0.45 121 75 46 12 F 4.8 0.05 0.8 0.09 92 71 21 13 G 5.7
0.15 1.7 0.21 120 76 44 14 H 4.8 0.13 0.6 0.22 95 73 22 15 I 5.0
0.4 0.85 0.28 110 74 36 16 J 4.8 0.01 0.79 0.30 100 71 29 17 K 3.9
0.25 0.88 0.36 95 72 22 18 L 3.7 0.28 0.90 0.33 106 71 35 19 M 3.4
0.23 1.05 0.28 98 72 26 20 N 4.1 0.25 1.08 0.25 105 75 30 Al Alloy
Sheet High-temp. Properties Al Alloy Sheet Formability AS AB 0.2%
Number of times 0.2% 0.2% Proof of cracking Proof Proof Stress
Elliptic Earing occurrence in Stress Stress Reduction Deformation
Rate DI (times/ Formability Example Symbol (MPa) (MPa) (.DELTA.MPa)
(mm) (%) 50,000 cans) Evaluation Working 1 303 225 78 2.0 +3.0 1
.largecircle. Examples 2 310 230 80 1.0 +1.0 0 .circleincircle. 3
302 227 75 1.0 +2.0 0 .circleincircle. 4 302 225 79 1.5 +1.0 0
.circleincircle. 5 296 220 76 1.8 +2.0 0 .circleincircle. 6 291 229
62 2.1 +2.2 1 .largecircle. Comparative 7 316 212 104 5.1 +4.0 8 X
Examples 8 301 225 76 4.8 -1.0 4 .DELTA. 9 311 210 101 4.5 +2.0 4
.DELTA. 10 318 220 98 5.1 +2.0 1 .largecircle. 11 325 224 101 5.1
+4.1 7 X 12 285 202 83 4.5 +0.5 6 X 13 335 230 105 5.0 -1.5 10 X 14
295 210 85 4.8 -0.5 9 X 15 315 220 95 5.5 +3.9 7 X 16 303 211 92
4.2 +2.0 7 X 17 290 203 97 4.5 -1.0 5 X 18 304 222 82 5.0 +2.7 6 X
19 287 201 86 5.1 -0.5 8 X 20 312 223 89 5.0 -3.5 5 X
EXAMPLE 2
[0189] Using aluminum metal alone as a melting raw material and
using molten metal of alloy components in A to N shown in Table 4
below, ingots each 600 mm thick by 2100 mm wide were produced by
the DC casting method. In Table 4, the element content indicated by
"-" represents that it is below the detection limit.
[0190] As shown in Table 4, in both working examples and
comparative example, the ingots contained inevitable impurity
elements Zr, Bi, Sn, Ga, V, Co, Ni, Ca, Mo, Be, Pb and W in total
contents of 0.03% or more as the total amount of other
elements.
[0191] The ingots were each subjected to a soaking treatment in
accordance with the conditions shown in Table 5. The soaking
treatment was conducted twice. That is, after the first soaking
treatment, the ingot was cooled to room temperature at a cooling
speed shown in Table 5, followed by re-heating to effect the second
soaking treatment. The heating speed as a first soaking condition
indicates a heating speed in the temperature range from 300.degree.
C. to the highest temperature which temperature range substantially
exerts an influence on properties. The cooling speed as a first
soaking condition indicates a cooling speed in the temperature
range from the highest temperature to 300.degree. C. which
temperature range substantially exerts an influence on properties.
After this soaking treatment, rough hot rolling was performed using
a reversing rough hot rolling mill having one stand, followed by
finish hot rolling using a tandem hot rolling mill having four
stands. The finish hot rolling was started within three minutes
after the end of rough hot rolling, and for controlling the
foregoing anisotropic index, an average winding tension was
controlled as in Table 5. In this way there were obtained hot
rolled aluminum alloy sheets 2 to 2.5 mm in thickness as a common
condition after finish hot rolling.
[0192] The hot rolled sheets were then subjected to cold rolling by
only one pass with use of a tandem rolling mill having two stages
of rolling stands without intermediate annealing to afford sheets
(cold rolled sheets) for bottle can body 0.3 mm in final thickness
as a common condition. In the cold rolling by the tandem rolling
mill, the temperature of each aluminum sheet just after cold
rolling was controlled to 130-200.degree. C. Finish annealing
(final annealing) after this cold rolling was not conducted.
[0193] In only comparative example 110, two passes were performed
in a single rolling mill having one stage of rolling stand and
intermediate annealing of 150.degree. C..times.1 hour was performed
between the first and the second pass for comparison purpose
although the total draft in cold rolling was the same.
[0194] A test piece was sampled from each sheet (coil) for a bottle
can body after cold rolling and the structure thereof was checked.
More particularly, an average aspect ratio of crystal grains and
tensile properties were checked. These results are shown in Table
6.
[0195] For determining high temperature properties of the test
piece, hardness and 0.2% proof stress of the test piece surface at
room temperature, as well as hardness and 0.2% proof stress of the
test piece surface in 290.degree. C..times.20 sec. heat treatment
of the test piece, were measured and a change in hardness (hardness
reduction) .DELTA.Hv Hv) of the test piece surface before and after
this heat treatment was determined. Tensile test was conducted
under the following conditions and in 0.degree. direction relative
to the rolling direction. Further, an elliptic deformation quantity
upon bake-hardening of the can body after forming was measured.
These results are also shown in Table 6.
[0196] (Anisotropy Measurement by Tensile Test)
[0197] Tensile test of each test piece was conducted in accordance
with JIS Z 2201 and, as the test piece shape, there was used the
shape of JIS No. 5 test piece. Crosshead speed was 5 mm/min and a
constant speed was adopted until breakage of the test piece. In
this case, there were provided test pieces whose longitudinal
directions were 0.degree., 45.degree. and 90.degree. directions
respectively relative to the rolling direction and tensile strength
and n value of each of the test pieces were determined. There were
determined a difference (MPa) between maximum and minimum values of
tensile strength and a difference between maximum and minimum
values of the n values (strain quantities in the range of between
2% and 4%). Mean values in the above directions of both tensile
strength and n values were also determined.
[0198] (Hardness Measurement)
[0199] Each cold rolled sheet sample was measured for hardness at
four positions under the application of 100 g load and a mean value
of the measured values was used as the hardness value.
[0200] (Evaluation of Elliptic Deformation)
[0201] For evaluation of elliptic deformation, as will be described
later, a bottle can body obtained by DI of the above sheet for a
bottle can body was washed and then baked under the condition that
the bodily temperature of the can should reach 300.degree. C. in 30
seconds, then the degree of elliptic deformation was checked. For
checking the degree of elliptic deformation, the diameter of the
mouth portion of the bottle can body was checked successively in
the circumferential direction, then an amount obtained by
subtracting the minimum value from the maximum value was determined
as an elliptic deformation quantity (mm), and evaluation was made
using a mean value from N=10 cans. In the case where the elliptic
deformation quantity is 4 mm or less, it was evaluated that the
elliptic deformability came up to a passing point. If the elliptic
deformation quantity exceeds 4 mm, there will occur defects such as
falling-down and jam during conveyance and necking as post-steps in
the can manufacturing process, thus making continuous and efficient
manufacture of cans difficult.
[0202] Further, as formability which the sheet for bottle can body
should satisfy basically, both earing rate and DI formability (the
number of times of cracking in forming) were measured and
evaluated, the results of which are also shown in Table 6.
[0203] (Earing Rate)
[0204] For checking the earing rate, in the same way as in Example
1, a blank was sampled from the sheet for bottle can body and
lubricating oil (Nalco 147, a product of D.A. Stuart Co.) was
applied thereto, then the blank was subjected to 40% deep drawing
test and formed into a cup for investigation, using an Erichsen
tester. The test was performed under the same conditions as in
Example 1 and an average earing rate was calculated.
[0205] (DI Formability)
[0206] In the same manner as in Example 1 the number of
body-cracked cans per 50,000 formed cans was determined and DI
formability was evaluated. The evaluation was made in accordance
with the following criterion. .circleincircle.: no cracked can
(extremely good), .largecircle.: one cracked can or less (good),
.DELTA.: two to four cracked cans (generally good), X: more than
five cracked cans (bad).
[0207] As is apparent from Table 6, working examples 101 to 106
have compositions according to the present invention, average
aspect ratios of crystal grains therein are 3 ore more, the
difference between maximum and minimum values in tensile strength
in 0.degree., 45.degree. and 90.degree. directions relative to the
rolling direction is 25 MPa or less, and the difference between
maximum and minimum values among n values obtained by tensile tests
in 0.degree., 45.degree. and 90.degree. directions relative to the
rolling direction is 0.03 or less. Thus, the structures obtained in
these working examples are less anisotropic.
[0208] In working examples 101 to 106, as is seen from Table 6,
after heat treatment at 290.degree. C. for 20 seconds (after
bake-hardening), the hardness change .DELTA.HV is 30 Hv or less,
0.2% proof stress is 215 MPa or more, there is little lowering of
hardness and strength, and high-temperature properties are
excellent.
[0209] Working examples 101 to 106 are superior also in earing rate
and DI formability. Thus, it is seen that the improvement of
high-temperature properties in the present invention does not
impair the formability which the sheet for a bottle can body should
satisfy basically.
[0210] From the above results, critical meanings of the conditions
defined in the second invention can be understood.
TABLE-US-00004 TABLE 4 Al Alloy Chemical Components (mass %,
provided B is in ppm, balance Al) Total amount of Example Symbol Si
Fe Cu Mn Mg Cr Zn Ti B other elements * Working A 0.27 0.45 0.23
1.03 1.23 -- -- -- -- 0.03 Examples B 0.27 0.46 0.19 1.02 1.22 0.03
0.19 0.03 0 0.03 C 0.26 0.43 0.21 0.94 0.94 0.03 0.2 0.02 10 0.04 D
0.33 0.44 0.11 1.31 1.00 -- -- 0.02 10 0.05 Comparative E 0.41 0.51
0.41 1.6 1.01 -- -- 0.01 40 0.04 Examples F 0.22 0.45 0.26 0.6 1.01
-- -- 0.01 40 0.04 G 0.13 0.25 0.31 1.01 1.80 -- -- 0.03 30 0.03 H
0.12 0.23 0.33 1.02 0.70 -- -- 0.03 30 0.03 I 0.31 0.47 0.71 1.11
0.96 -- -- 0.02 30 0.03 J 0.14 0.43 0.03 1.05 0.95 -- -- 0.02 30
0.03 K 0.03 0.52 0.42 0.92 1.05 -- -- 0.03 30 0.03 L 0.61 0.42 0.41
0.96 1.04 -- -- 0.03 30 0.03 M 0.34 0.05 0.36 1.21 1.15 -- -- 0.02
30 0.03 N 0.25 0.8 0.33 0.99 1.21 -- -- 0.02 30 0.03 * Total amount
of other elements (Zr, Bi, Sn, Ga, V, Co, Ni, Ca, Mo, Be, Pb, and
W)
TABLE-US-00005 TABLE 5 1st Soaking 2nd Soaking Rough Hot Rolling
Heating Soaking Holding Cooling Soaking Holding Start End Al Alloy
Speed Temp. Time Speed Temp. Time Temp. Temp. Example Symbol Symbol
(.degree. C./hr) (.degree. C.) (hr) (.degree. C./hr) (.degree. C.)
(hr) (.degree. C.) (.degree. C.) Working 101 A 25 610 6 40 510 7
503 460 Examples 102 A 23 610 6 35 510 15 512 440 103 A 25 610 4 45
510 10 500 450 104 B 24 610 6 50 550 4 545 480 105 B 22 590 8 40
510 7 505 466 106 C 16 610 4 35 550 10 543 460 Comparative 107 A 40
610 10 40 510 6 503 435 Examples 108 C 40 610 9 40 530 4 491 430
109 A 40 610 8 40 500 8 491 388 110 D 40 610 8 40 500 8 495 460 111
E 40 610 8 40 500 8 490 470 112 F 40 610 8 40 500 8 490 454 113 G
40 610 8 40 500 8 490 454 114 H 40 610 8 40 500 8 494 488 115 I 40
610 8 40 500 8 495 460 116 J 40 610 8 40 500 8 488 459 117 K 40 610
8 40 500 8 498 462 118 L 40 610 8 40 500 8 482 453 119 M 40 610 8
40 500 8 491 450 120 N 40 610 8 40 500 8 495 420 Finish Hot Rolling
Cold Rolling Total Average Type Sheet Temp. Final Working End
Winding Sheet of just after Sheet Ratio Temp. Tension Thickness
Rolling Draft Cold Rolling Thickness Example Symbol (%) (.degree.
C.) (MPa) (mm) Mil (%) (.degree. C.) (mm) Working 101 94 341 22 2.5
Tandem 88 147 0.3 Examples 102 94 341 25 2.5 Tandem 88 153 0.3 103
94 320 30 2.5 Tandem 88 148 0.3 104 94 332 20 2.5 Tandem 88 145 0.3
105 94 350 25 2.5 Tandem 88 140 0.3 106 94 321 25 2.5 Tandem 88 150
0.3 Comparative 107 94 320 10 2.5 Tandem 88 153 0.3 Examples 108 94
300 20 2.5 Tandem 88 149 0.3 109 90 360 15 2.5 Tandem 88 155 0.3
110 94 333 20 2.5 Tandem 85 80 0.3 111 94 341 20 2.5 Tandem 88 149
0.3 112 94 333 20 2.5 Tandem 88 133 0.3 113 94 335 20 2.5 Tandem 88
149 0.3 114 94 321 20 2.5 Tandem 88 151 0.3 115 94 314 20 2.5
Tandem 88 155 0.3 116 94 324 20 2.5 Tandem 88 141 0.3 117 94 302 20
2.5 Tandem 88 153 0.3 118 94 322 20 2.5 Tandem 88 149 0.3 119 94
328 20 2.5 Tandem 88 153 0.3 120 94 320 20 2.5 Tandem 88 141
0.3
TABLE-US-00006 TABLE 6 Al Alloy Sheet Structure Average Anisotropy
Al Alloy Sheet High-temp. Properties Average Tensile of Tensile
Mean Anisotropy AS AB Hardness Al Alloy aspect Strength Strength n
of n Value Hardness Hardness Reduction Example Symbol Symbol ratio
(MPa) *1 Value *2 (Hv) (Hv) (.DELTA.Hv) Working 101 A 4.7 320 15
0.07 0.01 98 82 16 Examples 102 A 5.1 330 17 0.072 0.015 99 81 18
103 A 3.7 320 20 0.074 0.02 100 79 21 104 B 5.0 326 15 0.076 0.01
99 86 13 105 B 5.6 322 10 0.078 0.004 98 86 12 106 C 4.7 319 11
0.080 0.006 96 85 11 Comparative 107 A 4.1 322 30 0.075 0.035 103
72 31 Examples 108 C 4.3 326 21 0.072 0.032 101 72 29 109 A 4.6 325
35 0.070 0.035 98 70 28 110 D 2.2 328 28 0.078 0.030 104 70 34 111
E 6.3 330 30 0.076 0.030 116 73 43 112 F 4.6 320 27 0.069 0.032 93
70 23 113 G 5.5 340 31 0.072 0.032 110 73 47 114 H 4.5 315 32 0.079
0.033 93 71 22 115 I 5.2 325 32 0.080 0.025 105 72 33 116 J 4.9 322
32 0.070 0.029 99 70 29 117 K 4.0 310 33 0.078 0.024 94 71 23 118 L
3.6 325 31 0.068 0.020 100 70 30 119 M 3.7 311 31 0.071 0.033 94 69
25 120 N 4.0 330 30 0.075 0.035 100 72 28 Al Alloy Sheet High-temp.
Properties Al Alloy Sheet Formability AS AB 0.2% Number of times
0.2% 0.2% Proof of cracking Proof Proof Stress Elliptic Earing
occurrence in Stress Stress Reduction Deformation Rate DI (times/
Formability Example Symbol (MPa) (MPa) (.DELTA.MPa) (mm) (%) 50,000
cans) Evaluation Working 101 294 224 70 1.0 +2.0 0 .circleincircle.
Examples 102 296 228 68 1.1 +1.1 0 .circleincircle. 103 301 225 76
1.2 +2.0 0 .circleincircle. 104 288 224 64 1.8 +1.1 0
.circleincircle. 105 295 223 72 1.4 +2.2 0 .circleincircle. 106 290
226 64 2.0 +2.0 0 .circleincircle. Comparative 107 299 212 77 4.9
+4.1 6 X Examples 108 301 225 76 4.5 -1.0 3 .DELTA. 109 290 210 80
4.0 +3.9 2 .DELTA. 110 308 220 88 5.3 +2.2 1 .largecircle. 111 315
224 91 5.0 +4.2 6 X 112 275 202 73 4.7 +0.6 5 X 113 325 230 95 4.9
-1.7 9 X 114 285 210 75 4.7 -0.6 8 X 115 305 220 85 5.4 +3.8 8 X
116 293 211 82 4.1 +1.8 8 X 117 280 203 87 4.7 -1.1 6 X 118 294 222
72 4.9 +2.8 7 X 119 277 201 76 5.3 -0.6 7 X 120 302 223 79 4.9 +3.7
5 X *1 Anisotropy of Tensile Strength: Difference between maximum
and minimum values of tensile strength in 0.degree., 45.degree. and
90.degree. directions relative to the rolling direction. *2
Anisotropy of n Value: Difference between maximum and minimum
values among n values obtained by tensile tests in 0.degree.,
45.degree. and 90.degree. directions relative to the rolling
direction.
EXAMPLE 3
[0211] Using aluminum metal alone as a melting raw material and
using molten metal of alloy components in A to N shown in Table 7
below, ingots each 600 mm thick by 2100 mm wide were produced by
the DC casting method. In Table 7, the element content indicated by
"-" represents that it is below the detection limit.
[0212] As shown in Table 7, in both working examples and
comparative examples, the ingots contained inevitable impurity
elements Zr, Bi, Sn, Ga, V, Co, Ni, Ca, Mo, Be, Pb and W in total
contents of 0.03% or less as the total amount of other
elements.
[0213] Therefore, first an alloying design satisfying strength and
formability required of the cold rolled aluminum alloy sheet for a
bottle can was made by designing balance of main constituent
elements (Mn, Mg, Fe, Cu, Si) and selective added elements.
Thereafter, a thermodynamic equilibrium phase diagram calculation
in each example was made to effect control of .DELTA.T, thereby
calculating the solid-liquid coexistence temperature range .DELTA.T
of aluminum to make an alloying design or modification. There were
obtained such actual aluminum alloy components' compositions in A
to N as shown in Table 7.
[0214] Ingots of the said compositions were subjected to a soaking
treatment in accordance with the conditions shown in Table 8. The
soaking treatment was conducted twice. More specifically, after the
first soaking treatment, each ingot was cooled to room temperature
at a cooling speed shown in Table 8, followed by re-heating to
effect the second soaking treatment.
[0215] The heating speed as a first soaking condition indicates a
heating speed in the temperature range from 300.degree. C. to the
highest temperature which temperature range substantially exerts an
influence on properties. The cooling speed as a first soaking
condition indicates a cooling speed in the temperature range from
the highest temperature to 300.degree. C. which temperature range
substantially exerts an influence on properties. After this soaking
treatment, rough hot rolling was performed using a reversing rough
hot rolling mill having one stand, followed by finish hot rolling
using a tandem hot rolling mill having four stands. The finish hot
rolling was started within three minutes after the end of rough hot
rolling. In this way there were obtained hot rolled aluminum alloy
sheets 2.5 mm in thickness as a common condition after finish hot
rolling.
[0216] The hot rolled sheets were then subjected to cold rolling by
only one pass with use of a tandem rolling mill having two stages
of rolling stands without intermediate annealing to afford sheets
(cold rolled sheets) for bottle can body 0.3 mm in final thickness
as a common condition. In the cold rolling by the tandem rolling
mill, the aluminum sheets were cooled forcibly using an aqueous
emulsion so as to prevent the aluminum sheet temperature just after
cold rolling from rising to a temperature exceeding 250.degree. C.
Finish annealing (final annealing) after this cold rolling was not
conducted.
[0217] In only comparative example 210, two passes were performed
in a single rolling mill having one stage of rolling stand and
intermediate annealing of 150.degree. C..times.1 hour was performed
between the first and the second pass for comparison purpose
although the total draft in cold rolling was the same.
[0218] A test piece was sampled from each sheet (coil) for a bottle
can body after cold rolling and the structure thereof was checked.
More particularly, an average aspect ratio of crystal grains, an
average size (central part in the through-thickness direction) of
intermetallic compounds of 0.5 .mu.m or more, and the solid-liquid
coexistence temperature range .DELTA.T, were checked by
differential thermal analysis, the results of which are shown in
Table 9.
[0219] For determining high temperature properties of the test
piece, hardness and 0.2% proof stress of the test piece surface at
room temperature, as well as hardness and 0.2% proof stress of the
test piece surface in 290.degree. C..times.20 sec. heat treatment
of the test piece, were measured and a change in hardness (hardness
reduction) .DELTA.Hv (Hv) of the test piece before and after the
heat treatment was determined. Further, an elliptic deformation
quantity upon bake-hardening of the can body after forming was
measured. These results are also shown in Table 9.
[0220] (Measurement of 0.2% Proof Stress)
[0221] Tensile test for measuring 0.2% proof stress was conducted
in the same way as in Example 1 and in accordance with JIS Z 2201
and, as the test piece shape, there was used the shape of JIS No. 5
test piece. The test piece was obtained in such a manner that its
longitudinal direction was coincident with the rolling direction.
Crosshead speed was 5 mm/min and a constant speed was adopted until
breakage of the test piece.
[0222] (Hardness Measurement)
[0223] Each cold rolled sheet sample was measured for hardness at
four positions under the application of 100 g load by means of a
micro Vickers hardness tester and a mean value of the measured
values was used as the hardness value.
[0224] (Evaluation of Elliptic Deformation)
[0225] As will be described later, evaluation of elliptic
deformation was made in the same way as in Example 1.
[0226] Further, as formability which the sheet for bottle can body
should satisfy basically, both earing rate and DI formability (the
number of times of cracking in forming) were measured and
evaluated, the results of which are also shown in Table 9.
[0227] (Earing Rate)
[0228] For checking the earing rate, as in Example 1, a blank was
sampled from the sheet for bottle can body and lubricating oil
(Nalco 147, a product of D.A. Stuart Co.) was applied thereto, then
the blank was subjected to 40% deep drawing test and formed into a
cup for investigation, using an Erichsen tester. The test was
performed under the following conditions: blank dia. 66.7 mm, punch
dia. 40 mm, R of the die-side shoulder portion 2.0 mm, punch
shoulder R 3.0 mm, blank holder pressure 400 kgf.
[0229] Mountain-valley shapes formed in eight directions
(0.degree., 45.degree., 90.degree., 135.degree., 180.degree.,
225.degree., 270.degree. and 315.degree. directions, assuming that
the rolling direction is 0.degree.) in the peripheral edge portion
of the opening of the cup thus obtained were measured and an
average earing rate was calculated.
[0230] How to calculate the earing rate is as described in Example
1.
[0231] (DI Formability)
[0232] In the same way as in Example 1 the number of body-cracked
cans per 50,000 formed cans was determined and DI formability was
evaluated. The evaluation was made in accordance with the following
criterion. .circleincircle.: no cracked can (extremely good),
.largecircle.: one cracked can or less (good), .DELTA.: two to four
cracked cans (generally good), X: more than five cracked cans
(bad).
[0233] As is apparent from Table 9, working examples 201 to 206
have compositions according to the present invention and have
structures such that an average particle size of 0.5 .mu.m or
larger dispersed particles with an aspect ratio of crystal grains
of 3 or more is 5 .mu.m or less, and solid-liquid coexistence
temperature range .DELTA.T of aluminum is 40.degree. or lower.
[0234] Thus, in working examples 201 to 206, as is seen from Table
9, after heat treatment at 290.degree. C. for 20 seconds (after
bake-hardening), the hardness change .DELTA.Hv is 30 Hv or less,
0.2% proof stress is 270 MPa or more, there is little lowering of
hardness and strength, and high-temperature properties are
excellent.
[0235] Working examples 201 to 206 are superior also in earing rate
and DI formability. Thus, it is seen that the improvement of
high-temperature properties in the present invention does not
impair the formability which the sheet for bottle can body should
satisfy basically.
[0236] On the other hand, in comparative examples 207 to 210,
conditions for soaking and hot rolling do not fall under the
foregoing preferred conditions, although the compositions adopted
therein fall under those defined in the present invention, so that
any of the average aspect ratio of crystal grains, the average
particle size of dispersed grains 0.5 .mu.m or more in particle
size, and .DELTA.T, deviates from the range defined in the present
invention. As a result, in comparison with the foregoing working
examples, a lowering of hardness and that of strength are marked
and high-temperature properties are inferior.
[0237] In comparative example 207, the second soaking temperature
is too low and so is the end temperature in finish hot rolling. In
comparative example 208, the end temperature in finish hot rolling
is too low. In comparative example 209, the end temperature in
finish hot rolling is too low. In comparative example 210,
intermediate annealing was performed halfway of cold rolling.
[0238] Comparative examples 211 to 220 follow preferred
manufacturing conditions. However, alloy compositions used therein
deviate from those defined in the present invention. As a result,
any of the average aspect ratio of crystal grains, the average
particle size of 0.5 .mu.m or larger dispersed particles, and
.DELTA.T, deviates from the range defined in the present invention.
Consequently, in comparison with the working examples, a lowering
of hardness and that of strength are marked and high-temperature
properties are inferior. Formability is also low.
[0239] From the above results, critical meanings of the conditions
defined in the present invention can be understood.
TABLE-US-00007 TABLE 7 Al Alloy Chemical Components (mass %,
provided B is in ppm, balance Al) Total amount of Example Symbol Si
Fe Cu Mn Mg Cr Zn Ti B other elements * Working A 0.25 0.44 0.21
1.05 1.2 -- -- -- -- 0.03 Examples B 0.26 0.44 0.21 1.04 1.21 0.03
0.19 0.03 0 0.03 C 0.25 0.42 0.2 0.9 0.9 0.03 0.2 0.02 10 0.04 D
0.3 0.45 0.1 1.3 1 -- -- 0.02 10 0.05 Comparative E 0.4 0.5 0.4 1.6
1 -- -- 0.01 40 0.04 Examples F 0.2 0.4 0.2 0.6 1 -- -- 0.01 40
0.04 G 0.1 0.2 0.3 1 1.8 -- -- 0.03 30 0.03 H 0.1 0.2 0.3 1 0.7 --
-- 0.03 30 0.03 I 0.3 0.4 0.7 1.1 0.9 -- -- 0.02 30 0.03 J 0.12
0.45 0.03 1.05 0.97 -- -- 0.02 30 0.03 K 0.03 0.5 0.4 0.9 1 -- --
0.03 30 0.03 L 0.6 0.4 0.4 0.95 1.05 -- -- 0.03 30 0.03 M 0.3 0.05
0.35 1.2 1.1 -- -- 0.02 30 0.03 N 0.2 0.8 0.33 0.99 1.2 -- -- 0.02
30 0.03 * Total amount of other elements (Zr, Bi, Sn, Ga, V, Co,
Ni, Ca, Mo, Be, Pb, and W)
TABLE-US-00008 TABLE 8 1st Soaking 2nd Soaking Rough Hot Rolling
Heating Soaking Holding Cooling Soaking Holding Start End Al Alloy
Speed Temp. Time Speed Temp. Time Temp. Temp. Example Symbol Symbol
(.degree. C./hr) (.degree. C.) (hr) (.degree. C./hr) (.degree. C.)
(hr) (.degree. C.) (.degree. C.) Working 201 A 21 610 6 40 510 7
505 460 Examples 202 A 20 610 8 35 510 15 510 440 203 A 25 610 4 45
510 10 500 450 204 B 22 610 6 50 510 4 510 470 205 B 20 590 8 40
550 7 550 480 206 C 15 610 6 35 510 10 510 460 Comparative 207 A 40
610 10 40 450 6 450 401 Examples 208 C 40 610 9 40 530 4 490 430
209 A 40 610 8 40 500 8 490 330 210 D 40 610 8 40 500 8 498 450 211
E 40 610 8 40 500 8 500 460 212 F 40 610 8 40 500 8 488 444 213 G
40 610 8 40 500 8 476 443 214 H 40 610 8 40 500 8 497 487 215 I 40
610 8 40 500 8 499 467 216 J 40 610 8 40 500 8 476 460 217 K 40 610
8 40 500 8 501 470 218 L 40 610 8 40 500 8 480 443 219 M 40 610 8
40 500 8 490 460 220 N 40 610 8 40 500 8 495 470 Finish Hot Rolling
Cold Rolling Total Type Final Working End Sheet of Sheet Ratio
Temp. Thickness Rolling Draft Thickness Example Symbol (%)
(.degree. C.) (mm) Mill (%) (mm) Working 201 92 341 2.5 Tandem 88
0.3 Examples 202 92 341 2.5 Tandem 88 0.3 203 92 320 2.5 Tandem 88
0.3 204 92 332 2.5 Tandem 88 0.3 205 92 340 2.5 Tandem 88 0.3 206
92 321 2.5 Tandem 85 0.3 Comparative 207 92 300 2.5 Tandem 88 0.3
Examples 208 92 300 2.5 Tandem 88 0.3 209 88 290 2.5 Tandem 88 0.3
210 92 333 2.5 Tandem 85 0.3 211 92 341 2.5 Tandem 88 0.3 212 92
333 2.5 Tandem 88 0.3 213 92 335 2.5 Tandem 88 0.3 214 92 321 2.5
Tandem 88 0.3 215 92 314 2.5 Tandem 88 0.3 216 92 324 2.5 Tandem 88
0.3 217 92 302 2.5 Tandem 88 0.3 218 92 322 2.5 Tandem 88 0.3 219
92 329 2.5 Tandem 88 0.3 220 92 325 2.5 Tandem 88 0.3
TABLE-US-00009 TABLE 9 Al Alloy Sheet Structure Al Alloy Sheet
High-temp. Properties Average Average Particle AS AB Hardness Al
Alloy aspect Size of Dispersion .DELTA.T Hardness Hardness
Reduction Example Symbol Symbol ratio Particles (.mu.m) (.degree.
C.) (Hv) (Hv) (.DELTA.Hv) Working 201 A 4.1 4.3 36 98 75 23
Examples 202 A 3.9 4.8 36 105 77 28 203 A 3.8 4.3 36 98 80 18 204 B
5.5 3.8 33 103 88 25 205 B 5.4 3.8 33 100 82 28 206 C 4.1 3.6 27 95
80 15 Comparative 207 A 3.9 4.4 36 107 74 33 Examples 208 C 4.2 3.7
27 105 75 30 209 A 4.2 4.0 36 106 75 31 210 D 2.7 4.8 39 105 70 35
211 E 6.3 5.4 43 120 75 45 212 F 4.5 3.7 32 90 70 20 213 G 5.5 5.8
47 125 77 48 214 H 5.0 4.0 33 95 72 23 215 I 4.8 5.5 44 110 75 35
216 J 4.7 3.5 30 100 70 30 217 K 4.0 3.6 31 95 71 24 218 L 3.8 5.9
45 105 70 35 219 M 3.5 4.8 38 95 70 25 220 N 4.0 3.5 30 110 74 31
Al Alloy Sheet High-temp. Properties Al Alloy Sheet Formability AS
AB 0.2% Number of times 0.2% 0.2% Proof of cracking Proof Proof
Stress Elliptic Earing occurrence in Stress Stress Reduction
Deformation Rate DI (times/ Formability Example Symbol (MPa) (MPa)
(.DELTA.MPa) (mm) (%) 50,000 cans) Evaluation Working 201 300 220
80 3.1 +3.2 1 .largecircle. Examples 202 310 225 88 1.9 +1.5 1
.largecircle. 203 300 230 70 2.0 +1.9 0 .circleincircle. 204 300
222 78 2.8 +2 0 .circleincircle. 205 297 220 77 1.5 +3 1
.largecircle. 206 290 230 60 1.1 +1.9 0 .circleincircle.
Comparative 207 315 210 95 4.5 +3.7 7 X Examples 208 300 213 87 5.0
+4.3 3 .DELTA. 209 310 211 89 4.3 +2.5 4 .DELTA. 210 320 210 110
5.0 +1.8 1 .largecircle. 211 325 212 113 6.0 +4 8 X 212 280 200 80
5.0 +1 7 X 213 330 213 117 5.0 -1 9 X 214 290 205 85 5.0 -1 8 X 215
320 210 110 5.3 +3.8 6 X 216 300 210 90 4.5 +2.0 5 X 217 285 200 85
5.2 -1.0 6 X 218 305 212 93 5.2 +2.6 5 X 219 285 200 85 5.2 -0.5 7
X 220 315 215 100 5.0 +3.3 5 X
INDUSTRIAL APPLICABILITY
[0240] According to the present invention, as set forth above, it
is possible to provide a cold rolled aluminum alloy sheet for a
bottle can which, on the premise that formability in DI, etc. is to
be secured, even in a high-speed heat treatment performed at a
higher temperature for a shorter time, can be prevented from being
thermally deformed in coating and heat treatment, can ensure
required can strength after heat treatment, and can afford a bottle
can high in true circularity, and which has excellent
high-temperature properties. Thus, the cold rolled aluminum alloy
sheet of the present invention is suitable for such applications as
require strict properties while retaining formability intact.
* * * * *