Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
TILTE OF THE INVENTION:
COLD-ROLLED ALUMINUM ALLOY SHEET FOR FORMING BOTTLES
EXCELLENT IN FORMABILITY IN FORMING NECK
AND METHOD OF MANUFACTURING THE SAME
TECHNICAL FIELD
[0001]
The present invention relates to a cold-rolled aluminum
alloy sheet (bottle-forming sheet), for forming a body of a
bottle (beverage can), excellent in formability in forming a
neck of a bottle. A cold-rolled aluminum alloy sheet mentioned
herein is a rolled sheet (cold-rolled sheet) manufactured by
hot rolling and subsequent cold rolling. Hereinafter, an
aluminum alloy will be referred to as an Al alloy.
BACKGROUND ART
[0002]
Most aluminum beverage cans are two-piece aluminum cans
each formed by fastening a lid (end wall) to a can body by a
seaming process. Most two-piece aluminum cans are made by
forming a can body by processing a prescribed aluminum sheet
by a cupping process and a DI process (drawing-with-ironing
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process) , and subjecting the can body to a neck forming process
to form an end part of a diameter smaller than that of the can
body. Hereinafter, such two-piece cans will be referred to as
two-piece aluminum cans.
[0003]
A neck can be comparatively easily formed in such a
two-piece aluminum can because the neck can be formed by drawing
the can body at a comparatively low drawing ratio, namely, the
ratio of the diameter of an end part to that of the body.
[0004]
Hard Al-Mg-Mn alloy sheets, such as 3004 and 3104 series,
are widely used as cold-rolled sheets for forming can bodies.
The 3004 and 3104 alloys are excellent in ironing formability
and can exhibit comparatively satisfactory formability even
when those materials are cold rolled at a high draft to enhance
the strength, and hence those alloys are suitable for forming
DI can bodies.
[0005]
Demands for bottle-shaped aluminum cans (hereinafter,
referred to as "metal bottles") having a body, an opening part,
and a screw cap has increased in recent years. A drawing ratio
at which the body is drawn to form the opening part, namely,
the ratio of the diameter of the opening part to that of the
body, to form the metal bottle is high as compared with the
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drawing ratio used for forming the neck of the two-piece
aluminum can. Therefore, creases and cracks are liable to form
in the neck by a neck forming process.
[0006]
Such metal bottles include three-piece metal bottles
having, as principal parts, a body, a bottom, namely, a member
separate from the body, and a screw cap, and two-piece metal
bottles having a bottomed body, and a screw cap.
[0007]
Generally, some steps of a conventional two-piece metal
bottle manufacturing method are used for manufacturing the
three-piece metal bottle. As mentioned in Patent documents 1
and 2, a prescribed aluminum sheet is processed by a cupping
and DI processes, a baking process, a trimming process, a
printing process, a baking process, a necking process (top
forming process: neck forming process) in that order. The
necking process forms a neck in a bottomed end part of the body,
and then the end wall of the neck is opened to form a mouth.
A screw thread for a screw cap is formed in the neck by a threading
process. A flange is formed in an open end part opposite the
mouth, and a bottom member is fastened to the flange. by a seaming
process to form a bottom.
[0008]
The neck of the three-piece metal bottle is formed in the
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bottomed end part of the bottomed body formed by DI process.
Therefore, neck can be comparatively easily formed even if the
ratio of the diameter of the neck to that of the body is high.
[0009]
Demand for two-piece metal bottles has been progressively
increased instead of demand for three-piece metal bottles from
the viewpoint of cost reduction and recycling facility.
Generally, most two-piece metal bottles are manufactured by a
conventional two-piece aluminum can manufacturing method. A
conventional die neck forming process and a conventional spin
neck forming process are employed just as they are.
[0010]
As disclosed in Patent documents 1 and 2, the two-piece
metal bottle manufacturing method processes a prescribed
aluminum sheet A by cupping and DI processes to form a body and
a bottom. Then an open end part of the body is processed by
a die neck-forming process or a spin neck-forming process to
form a neck. The open end of the neck is a mouth. A thread
with which a cap is engaged is formed in a part of the neck around
the mouth to complete a two-piece metal bottle.
[0011]
Since the neck of the two-piece metal bottle is formed
by processing the open end part of the body by the die
neck-forming process or the spin neck-forming process, it is
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difficult to reduce the diameter of the opening part at a high
drawing ratio.
[0012]
When the neck of a two-piece metal bottle of a hard sheet
of the 3000 series aluminum alloy mentioned above is formed by
drawing at a high drawing ratio, creases and cracks are liable
to form in the neck. Therefore, it has been difficult to apply
a drawing ratio used for forming the three-piece metal bottles
to forming the conventional two-piece metal bottles.
[0013]
To solve problems in the two-piece metal bottle, Patent
document 1 proposes a technique that specifies proper ranges
respectively for the Fe, the Si, the Mn and the Mg content and
the offset yield strength (0.2% offset yield strength) after
baking of the aluminum alloy sheet of a 3000 series aluminum
alloy so that formability, such as DI formability or neck
formability, of the aluminum sheet may be improved and the neck
can be formed by drawing the body at a high drawing ratio.
[0014]
Similarly, Patent document 2 proposes a technique that
specifies proper ranges respectively for the Fe, the Si, the
Mn, the Mg and the Cu content and the offset yield strength (0. 2 0
offset yield strength) after baking of the aluminum alloy sheet
of a 3000 series aluminum alloy so that formability, such as
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DI formability or neck formability, of the aluminum sheet may
be improved and the neck can be formed by drawing the body at
a high drawing ratio, namely, a ratio of the diameter of the
mouth to that of the body.
[0015]
Various techniques that control grain structure have been
proposed to improve formability. Patent document 3 controls the
content of Mn in solid solution and grain size in the hot-rolled
sheet so as to be in predetermined ranges, respectively, stably
maintains earing ratio in the range of 3 to 6%, and subject the
hot-rolled sheet directly to a cold-rolling process without
processing the same by an annealing process to keep the earing
ratio of cold-rolled sheet formed by the cold-rolling process
stably in the range of 0 to 2%.
Patent document 1: JP 2002-256366 A
Patent document 2: JP 2004-250790 A
Patent document 3: JP 2003-342657 A
DISCLOSURE OF THE INVENTION
PROBLEM TO BE SOLVED BY THE INVENTION
[0016]
The size and diameter of two-piece metal bottles trend
tofdecrease with the recent growing demand for small metal
bottles. Requirement for improvement of the sealing per-
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formance of a cap screwed onto a threaded part of a small
two-piece metal bottle, for retort pouch, has been increased.
To meet such a requirement, screwing force for screwing the cap
on the threaded part is increased and hence the body of the
two-piece bottle trends to have strength enough to withstand
the increased screwing force.
[0017]
However, if the strength of the material of the bottle
is increased by controlling the compositions of the aluminum
alloy sheet of the 3000 series aluminum alloy and the yield
strength after baking, there is a tendency that creases and
cracks are liable to form in a neck formed by a neck forming
process or a spin neck forming process, and in a threaded part
for a screw cap formed in a part of a neck around a mouth.
[0018]
Cost reduction through the reduction of the amount of
metal needed to form a can and the weight of a can required of
all types of cans is required also of small two-piece metal
bottles. To meet such a requirement, improvement of the
strength of a material of cans is inevitable.
[0019]
The present invention has been made in view of such
problems and it is therefore an object of the present invention
to provide a cold-rolled aluminum alloy sheet, for forming a
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small two-piece metal bottle, excellent in formability when
shaped by a neck forming process and a threaded part forming
process, and a method of manufacturing the cold-rolled aluminum
alloy sheet.
MEANS FOR SOLVING THE PROBLEM
[0020]
To achieve the object, the present invention provides a
cold-rolled aluminum alloy sheet, for forming a metal bottle,
excellent in neck formability and having a composition
containing 0.7 to 1. 5% by mass (hereinafter, referred to as "%")
Mn, 0.8 to 1.7% Mg, 0.1 to 0.7% Fe, 0.05 to 0.5% Si, 0.1 to 0.60
Cu, and Al and inevitable impurities as other elements, wherein
50 to 400 particles of sizes of 0.05 to 1}lm are dispersed in
an area of 300 um2 when observed under a TEM at a magnification
in the range of 5, 000x to 15, 000x magnification, and the ratio
of the number of the dispersed particles of sizes of 0.3 pm or
above to the number of all the dispersed particles is in the
range of 15 to 70%.
[0021]
A method of manufacturing a cold-rolled aluminum alloy
sheet, for forming a metal bottle, excellent in neck formability
or a preferable cold-rolled aluminum alloy sheet includes the
steps of: annealing a ingot at 550 C or above; slowly cooling
the ingot at a cooling rate of 25 C/hr or below to a temperature
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in the range of 450 C to 550 C; and hot rolling and cold rolling
the ingot such that 50 to 400 particles of particle sizes in
the range of 0.05 to 1 pm are dispersed in an area of 300 um2
when observed under a TEM at a magnification in the range of
5,000 to 15,000 magnification, and the ratio of the number of
the dispersed particles of sizes of 0. 3 pm or above to the number
of all the dispersed particles is in the range of 15 to 70%.
EFFECT OF THE INVENTION
[0022]
As mentioned above, it is required to reduce the thickness
of the DI body of a metal bottle mainly with a view to reduce
the manufacturing cost and the weight of the metal bottle. The
strength of a cold-rolled aluminum alloy sheet, namely, the
material of the metal bottle, needs to be increased so that the
buckling strength of the DI body may not be reduced. To reduce
the thickness of the DI body it is strongly required that a DI
process can be achieved at a low earing ratio. When the earing
ratio is low in the DI process, the yield of the DI process can
be increased, and the break of the body due to edge crack can
be prevented.
[0023]
As mentioned above, it is well known conventionally to
control the microstructure of the cold-rolled aluminum alloy
sheet for aluminum DI can bodies for bottles with lower
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anisotropy (earing) There are typical methods of controlling
the grain size, the number density and the size of intermetallic
compounds such as Mg2Si, micro segregation, the content of alloy
elements such as Mn and crystal orientation such as Cube
orientation.
[0024]
Control of the number and size of particles dispersed in
a cold-rolled aluminum alloy sheet, such as MgZSi, and
precipitates is the same as a conventional metallurgical
control of structure.
[0025]
Whereas the conventional idea reduces sizes of dispersed
particles to the least possible extent, the present invention
allows dispersed particles to grow to some extent in uniform
size so that a fixed amount (fixed number) of such particles
may be formed.
[0026]
The present inventors found that coarse dispersed
particles reduce the effect on the pinning force of fine
particles which makes recrystallization retard, thereby, the
hot-rolled sheet has the isotropic grain microstructure (lower
anisotropy), that is, lower "earing".
[0027]
When sizes of dispersed particles are reduced to disperse
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fine particles, the dispersed particles have strong pinning
effect, original soft PFZ recrystallizes easily during hot
rolling, and precipitation bands are liable to recrystallize
to form large crystal grains. Therefore, when fine particles
are dispersed according to the conventional idea, some large
crystal grains formed by recrystallization are contained in the
dispersed fine particles. Consequently, the structure in-
cludes large and small crystal grains, and the uniformity and
isotropy of crystal grains are liable to be lost.
[0028]
Consequently, earing ratio is reduced, and creases and
cracks are liable to form in a neck formed in an open end part
of the body of a small two- [piece metal bottle by a die neck
forming process or a spin neck forming process, and in a part
near the mouth when a screw thread onto which a screw cap is
screwed is formed therein.
The present invention grows a certain amount of dispersed
particles to some extent in uniform particle size.and grows
isotropic crystal grains not having anisotropy in a hot-rolled
sheet.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029]
Fig. 1 is a photograph of an aluminum alloy in a first
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embodiment according to the present invention containing
distributed particles;
Fig. 2 is a photograph of an aluminum alloy in a second
embodiment according to the present invention containing
distributed particles; and
Fig. 3 is a development of a cup formed by processing a
sheet by a DI process.
BEST MODE FOR CARRYING OUT THE INVENTION
[0030]
Composition of Cold-rolled Aluminum Alloy Sheet
Description will be made of a preferable chemical
composition (unit: percent by mass) of a cold-rolled aluminum
alloy sheet of the present invention meeting properties
including strength and formability required of a material for
forming two-piece metal bottles, and restrictive reasons for
elements.
[0031]
A cold-rolled aluminum alloy sheet excellent in
high-temperature characteristics for forming metal bottles has
a composition containing 0.7 to 1.50 (percent by mass) Mn, 0.8
to 1.7% Mg, 0.1 to 0.7% Fe, 0.05 to 0.5% Si, 0.1 to 0.6% Cu,
and Al and inevitable impurities as other elements.
[0033]
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Mn Content: 0.7 to 1.5%
Mn is effective in improving strength and formability.
A material (cold-rolled sheet) according to the present
invention for forming two-piece metal bottles is subjected to
an ironing process in a DI process, a neck forming process, and
a screw thread forming process. Therefore, Mn is a very
important element.
[0033]
Manganese (Mn) is used for making various Mn-containing
intermetallic compounds including an Al-Fe-Mn-Si intermet-
allic compound ((x phase) Formability and workability can be
improved by properly distributing the a phase. Usually, an
emulsion-type lubricant is used for ironing an aluminum sheet.
The lubricating effect of the emulsion-type lubricant is
insufficient if the amount of the a phase is small, and hence
it is possible that defects that spoil appearance, such as
galling including local welding and scratches, are formed.
ThusMn is indispensable to avoid forming surface defects during
an ironing process by producing the a phase.
[0034]
If the Mn content is excessively low, Mn cannot exercise
an effect of improving formability and workability. Therefore,
the Mn content be 0.7% or above, preferably, 0.8% or above,
desirably, 0.85% or above, more desirably, 0.9% or above.
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[0035]
If the Mn content is excessively high, gigantic Mn-Al
primary crystal grains crystallize to deteriorate formability.
Therefore, an upper limit Mn content is 1. 5 0, preferably, 1. 3%,
desirably, 1.1%, more desirably, 1.0%.
[0036]
Mg Content: 0.8 to 1.7%
Magnesium (Mg) is effective in improving strength. When
Mg is contained in the cold-rolled sheet in combination with
Cu, softening of the cold-rolled sheet can be suppressed when
the cold-rolled sheet is subjected to a final annealing process
(finish annealing process) that heats the cold-rolled sheets
.at temperatures in the range of about 100 C to about 150 C for
a time in the range of about 1 to about 2 hr, and a can made
of the cold-rolled sheet is subjected to a print-baking process.
When the cold-rolled sheet contains both Mg and Cu, Al-Cu-Mg
grains precipitate to suppress softening when the can is
subjected to a print-baking process.
[0037]
If the Mg content is excessively low, the foregoing effect
cannot be exhibited. Therefore, the Mg content be 0. 8 0 or above,
preferably, 0.9% or above, desirably, 1.0% or above.
[0038] If the Mg content is excessively high, work
hardening is liable to occur and hence formability deteriorates.
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Therefore, an upper limit Mg content is 1.7%, preferably, 1.6%,
desirably, 1.35%.
[0039]
Mg affects the amount of precipitated Mn and the Mn content
in solid solution. The larger amount of Mg suppress the
precipitation of Al-Fe-Mn-Si (a phase) more effectively, and
hence the Mn content in solid solution tends to increase. Thus
it is preferable to determine the Mg content in connection with
the amount of Mn in the solid solution.
[0040]
Fe Content: 0.1 to 0.7%
Iron (Fe) has an effect of decreasing the grain size and
improves formability by forming the Al-Fe-Mn-Si intermetallic
compound ((x phase). Iron (Fe) is useful for promoting the
crystallization and precipitation of Mn and in controlling the
Mn content of the aluminum and the dispersion of a Mn in-
termetallic compound in the aluminum base. If the aluminum alloy
containing Mn has an excessively high Fe content, gigantic
intermetallic compound primary crystal grains are liable to
form, which is possible to deteriorate formability.
[0041]
Therefore, the Fe content can be determined according to
the Mn content. A preferable ratio in mass of Fe to Mn (Fe/Mn
ratio) is, for example, in the range of 0.1 to 0.7, desirably,
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0.2 to 0.6, more desirably, 0.3 to 0.5.
[0042]
When the Mn content is in the foregoing Mn content range,
a lower limit Fe content is 0. 1 0 or above, preferably, 0. 2 0 or
above, desirably, 0.3% or above. Preferably, an upper limit
Fe content is 0.7% or below, desirably, 0.6% or below, more
desirably, 0.5% or below.
[0043]
Si Content: 0.05 to 0.5%
Silicon (Si) is useful for producing dispersed particles
of Mg2Si intermetallic compound and the Al-Fe-Mn-Si inter-
metallic compound (a phase) . Formability improves when those
dispersed particles are distributed in an appropriate dis-
tribution specified by the present invention.
[0044]
The Si content is 0. 05 0 or above, preferably, 0. 1 0 or above,
desirably, 0.2% or above. A excessively high Si content
obstructs recrystallization during finish hot rolling, in-
creases 45 ears, and deteriorates formability. Therefore, an
upper limit Si content is 0. 5 0, preferably, 0.45%, desirably,
0.4%.
[0045]
Cu Content: 0.1 to 0.6%
Copper (Cu) produces an Al-Cu-Mg intermetallic compound
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when a can formed by processing a cold-rolled sheet is subj ected
to a baked-finishing process. When Cu contained in combination
with Mg in the aluminum alloy sheet suppresses softening. A
lower limit Cu content is 0.1% or above, preferably, 0.15% or
above, desirably, 0.2% or above. Whereas age hardening can be
readily achieved, the aluminum alloy sheet becomes excessively
hard, formability deteriorates and corrosion resistance
deteriorates if the Cu content is excessively high. An upper
limit Cu content is 0. 6 0, preferably, 0. 5 0, desirably, 0. 35 0.
[0046]
Elements having the same strength improving effect as Cu
are Cr and Zn. The aluminum alloy sheet may contain, in addition
to Cu, Cr and Zn or may selectively contain Cr or Zn.
[0047]
Cr Content: 0.001 to 0.3%
To improve strength, the Cr content is 0.001% or above,
preferably, 0. 002 0 or above. If the Cr content is excessively
high, gigantic crystal grains forms and formability dete-
riorates. An upper limit Cr content is 0.3%, preferably, 0.25%.
[0048]
Zn Content: 0.05 to 1.0%
Precipitation of Al-Mg-Zn particles occurs and strength
improves when the aluminum alloy sheet contains Zn. To make
Zn exhibit this effect, the Zn content is 0.05% or above,
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preferably, 0.06% or above. An excessively high Zn content
deteriorates corrosion resistance. Therefore, an upper limit
Zn content is 0.5%, preferably, 0.45%.
[0049]
Ti Content: 0.005 to 0.2%
Titanium (Ti) has a crystal grain micronizing effect.
The aluminum alloy sheet contains Ti selectively when such an
effect is necessary. To make Ti exhibit such an effect, the
Ti content is 0.005% or above, preferably, 0.01% or above,
desirably, 0.015% or above. If the Ti content is excessively
high, gigantic Al-Ti intermetallic compound grains crystallize
and deteriorate formability. An upper limit Ti content is 0.2%,
preferably, 0.1%, desirably, 0.05%.
[0050]
The aluminum alloy sheet may contain Ti singly or in
combination with a small amount of B. When the aluminum alloy
sheet contains Ti and B in combination, the crystal grain
micronizing effect of Ti improves still further. When B is used,
the B content is 0.0001% or above, preferably, 0.0005% or above,
desirably, 0.0008% or above. If the B content is excessively
high, large Ti-B particles are produced to deteriorate
formability. An upper limit B content is 0.05%, preferably,
0.01%, desirably, 0.005%.
[0051]
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The aluminum alloy sheet contains inevitable impurities
in additions to the foregoing elements. Basically, lower
impurity contents are desirable to avoid deteriorating the
properties of the aluminum alloy sheet. However, the aluminum
alloy sheet may contain impurities in impurity contents that
will not deteriorate the desired properties of the aluminum
alloy sheet and not exceeding upper limit element contents
specified for 3000 series aluminum alloys in JIS.
[0052]
Dispersed Particles
The structure of the cold-rolled aluminum alloy sheet of
the present invention will be described.
As mentioned above, the present invention allows
dispersed particles of intermetallic compounds, such as Mg2Si
and Al-Fe-Mn-Si intermetallic compound (a phase), and pre-
cipitates, to grow to some extent in uniform size so that a fixed
amount (fixed number) of such particles may be formed. Thus
the pinning effect of the dispersed particles is moderated to
form uniform, isotropic crystal grains not having direc-
tionality and anisotropy in the hot-rolled sheet and to improve
earing ratio.
[0053]
More concretely, dispersed particles of sizes (bary-
centric diameters) in the range of 0.05 to 1 um are contained
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in the structure of a cold-rolled aluminum alloy sheet as
observed under a TEM at a magnification in the range of, 5000x
to 15, 000x in a density in the range of 50 to 400 particles per
3000 um2. The ratio of the number of the dispersed particles
of sizes not smaller than 0.3 pm to the number of all the
dispersed particles is in the range of 15 to 70%. Preferably,
a lower limit to the ratio of the number of the dispersed
particles of sizes not smaller than 0.3 pm to the number of all
the dispersed particles is 20% or above, desirably, 25% or above.
Preferably, the ratio is in the range of 20 to 70%, desirably,
25 to 70%.
[0054]
Figs. 1 and 2 are TEM photographs of the structure of
cold-rolled aluminum alloy sheets of the present invention
taken at 10, 000x magnification. In Figs. 1 and 2, white parts
are those of the matrix, and black parts are dispersed particles
of compounds, such as Mg2Si, and precipitates. Figs. 1 and 2
show the structure of a cold-rolled aluminum alloy sheets in
Examples 1 and 2 shown in Table 3, respectively.
[0055]
It is known from the comparative observation of Figs. 1
and 2 that the structure shown in Figs. 1 and 2 contains dispersed
particles of sizes in the range of 0.05 to 1 pm in a density
in the range of 50 to 400 particles per 300 Pm2 The dispersed
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particles in Fig. 1 are comparatively large and are uniformly
distributed as compared with those shown in Fig. 2.
[0056]
In the structure according to the present invention shown
in Fig. 1, the ratio of the number of the comparatively large
dispersed particles of sizes in the range of 0.3 to 1 pm to the
number of all the dispersed particles is high; that is, the ratio
of the number of the comparatively large dispersed particles
to the number of all the dispersed particles is 48%. The
comparatively large dispersed particles uniform in size are
uniformly distributed.
[0057]
In the structure in Example 2 shown in Fig. 2, the ratio
of the number of the comparatively large dispersed particles
of sizes in the range of 0.3 to 1 pm to the number of all the
dispersed particles is 20%; that is, the ratio of the number
of the comparatively small dispersed particles to the number
of all the dispersed particles is high and dispersed particles
in a wide range of size are distributed.
[0058]
If the ratio of the number of comparatively small
dispersed particles to the number of all the dispersed particles
is higher than that in the structure according to the present
invention shown in Fig. 2 or particles of different sizes are
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dispersed and the ratio of the number of the dispersed particles
of sizes not smaller than 0.3 pm to the number of all the
dispersed particles is below 15%, the mode of dispersion of the
dispersed particles is the same as that in the structure of the
conventional sheet. In such a case, original soft PEZ re-
crystallizes easily during hot rolling. Consequently,
precipitation bands are liable to recrystallize to form large
crystal grains, and cube orientation is liable to develop.
Therefore, although the mean grain size of the crystal grains
is small similarly to that of crystal grains in the structure
of the conventional sheet, some large crystal grains formed by
recrystallization are contained in the dispersed fine particles.
Consequently, the structure includes large and small crystal
grains, and the uniformity and isotropy of crystal grains are
liable to be lost.
[0059]
Thus the earing ratio decreases and there is a tendency
that creases and cracks are liable to form in a neck formed by
a neck forming process or a spin neck forming process, and in
a threaded part for a screw cap formed in a part of a neck around
a mouth in forming a small two-piece bottle.
[0060]
As mentioned above, the present invention allows
dispersed particles to grow to some extent in uniform size so
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that a fixed amount (fixed number) of such particles may be
formed, and grows uniform, isotropic crystal grains not having
directionality and anisotropy in the hot-rolled sheet, and
improves the earing ratio of the cold-rolled sheet.
[0061]
Dispersed particles to be analyzed and measured are those
that can be observed under a TEM at 5,000x to 15,000x mag-
nification and have sizes (barycentric diameters) of 0.05 pm
or above. The dispersed particles having sizes of 0.05 }im or
above have significant influence on formability and dispersed
particles having sizes below 0.05 }im have insignificant
influence on formability. It is difficult to observe and
measure small dispersed particles having sizes below 0.05 pm
under a TEM and measured sizes are distributed in a wide range.
Therefore, the present invention does not deal with those small
particles and omits the same from subjects of measurement.
[0062]
Measurement of Particle Size and Number of Particles
Particle sizes of the dispersed particles are measured
through the observation of the structure of the sheet under a
TEM (transmission electron microscope) . Specimens are sampled
from a middle part, with respect to thickness of the sheet, and
a rolled upper surface of the sheet. The specimens are mirror
finished by polishing. The structure of ten fields of about
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um x 15 pm in the polished surface of each specimen was
observed under a TEM, such as field-emission transmission
electron microscope (HF-2000, Hitachi) at 5,000x to 15,000x
magnification.
[0063]
Reflected electron images are observed for the clear
observation of the dispersed particle phase (intermetallic
compound phase ). Parts of Al are represented by white images.
The image of the dispersed particle phase is clearly contrasted
with the white image. Outlines of the images of the dispersed
particles are traced. Image-ProPlus (MEDIACYBERNETICS),
namely, image analyzing software, was used to determine the mean
barycentric diameter through image analysis.
[0064]
The number of the dispersed particles of sizes in the range
of 0.05 to 1 pm was counted, and the number of the dispersed
particles of sizes in the range of 0.05 to 1pn in 300 um2 was
calculated. The mean of the numbers of the dispersed particles
in the ten fields was calculated.
[0065]
The number of the dispersed particles of sizes of 0.3 pm
or above among those of sizes in the range of 0.05 to 1 pm was
counted. The number of the disperse particles of sizes in the
range of 0.05 to 1 um was counted. The ratio (%) of the number
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of the dispersed particles of sizes of 0.3 pm or above to the
number of the dispersed particles of sizes in the range of 0.05
to 1 pm was calculated.
[0066]
Mean Aspect Ratio of Crystal Grains
Preferably, a mean aspect ratio of grains in the
cold-rolled sheet is 2 or above, which means the grains
stretched in the rolling direction, not equiaxial grains.
This brings an effect on the maintenance of the material
strength after heat treatment at higher temperature for a
shorter time, which enables to speed up the heat treatment,
because of being suppressed the thermal deformation of the
cold-rolled aluminum sheet during a print-baking process. In
other words, the stretched grains in the rolling direction in
the cold-rolled aluminum alloy sheet contributes to maintain
the high formability during DI processing, and the appropriate
composition, the distribution of precipitation and solute
content, as mentioned later, contributes to maintain the
strength of a bottle can after the heat treatment. In addition,
these factors suppressed the thermal deformation during the
baking processing.
[0067]
If the mean aspect ratio of the crystal grains is below
2, the crystal grains do not differ a great deal from equiaxial
CA 02625098 2008-04-09
crystal grains and are deficient in the foregoing effects. A
cold-rolled aluminum alloy sheet having such crystal grains
cannot suppress thermal deformation during the baking process
and cannot ensure the strength of cans after the baking process.
Thus greater stretching of the crystal grains greatly in the
rolling direction is desirable. Preferably, the crystal
grains are stretched so that the mean aspect ratio is 2.1 or
above.
[0068]
The aspect ratios of the crystal grains are dependent on
the crystalline structure of the hot-rolled sheet and the
rolling reduction in the cold rolling process. An upper limit
mean aspect ratio is determined on the basis of the limit of
the stretching ability of the manufacturing process, such as
the hot-rolling process or the cold-rolling process. The upper
limit mean aspect ratio is on the order of 4.
[0069]
Method of Measuring Mean aspect Ratio
The mean aspect ratio of crystal grains is determined
through the observation (observation under polarized light) of
the upper surface of a middle part of the sheet with respect
to the thickness. The rolled surface of a middle part, with
respect to the thickness, of the sheet processed by a tempering
process and not yet processed by a bottle forming process is
26
CA 02625098 2008-04-09
observed under polarized light after finishing the surface by
mechanical polishing or electrolytic polishing, and anodizing
using a Barker solution.
[0070]
When the crystalline structure of the upper surface of
the middle part is observed under polarized light, crystals
respectively having different crystal orientations appear in
black and white. The maximum length in the rolling direction
and the maximum length in the direction of the width of the sheet
of each of crystal grains having a clearly recognizable outline
in the observation field are measured. Then, the aspect ratio
of each crystal grain is calculated by using: (Aspect ratio)
= (Maximum length in the rolling direction)/ (Maximum length
in the direction of width) . Suppose that 100 crystal grains
are observed under an optical microscope at 100x magnification.
Then, the mean aspect ratio of the respective aspect ratios of
the 100 crystal grains is calculated. The mean crystal grain
size may be the mean of the maximum lengths in the rolling
direction of the 100 crystal grains.
[0071]
Manufacturing Method
The cold-rolled aluminum alloy sheet of the present
invention can be m'anufactured by the conventional manuf acturing
method including soaking, hot-rolling, and cold-rolling
27
CA 02625098 2008-04-09
processes without introducing many changes into the con-
ventional manufacturing method, except that the ingot needs to
be heated at a temperature not lower than 550 C by a soaking
process, and then cooled slowly at a cooling rate of 25 C/hr
or below to a temperature in the range of 450 C to 550 C to form
the dispersed particle structure specified by the present
invention by hot-rolling and cold-rolling the ingot , and to
provide a cold-rolled aluminum alloy sheet with basic requisite
properties, such as earing ratio, strength, formability and
ironing formability necessary for forming metal bottles.
[0072]
Conditions for Homogenizing Heat Treatment
A homogenizing temperature for the homogenizing heat
treatment (soaking treatment) is in the range of 550 C to 650 C.
The homogenizing heat treatment takes long time and reduces
productivity if the homogenizing temperature is excessively low.
Bulges form in the surface of the ingot if the homogenizing
temperature is excessively high. A homogenizing temperature
in the foregoing temperature range is used. Preferably, the
homogenizing temperature is in the range of 580 C to 615 C, more
desirably, in the range of 590 C to 610 C.
[0073]
A shorter soaking time (homogenizing time) is desirable,
provided that the ingot can be homogenized in the soaking time.
28
CA 02625098 2008-04-09
For example, it is desirable that the soaking time is 6 hr or
below. According to the present invention, the ingot processed
by the soaking process needs to be slowly cooled, which takes
a long cooling time. Therefore, the shortest possible soaking
time is preferable from the view point of productivity and the
efficiency of the soaking process.
[0074]
Conditions for Cooling after Soaking Process
As mentioned above, the ingot needs to be cooled slowly
at a cooling rate of 25 C/hr or below to a temperature in the
range of 450 C to 550 C after being processed by the soaking
process under the foregoing conditions to form the dispersed
particle structure specified by the present invention in the
cold-rolled aluminum alloy sheet processed by hot-rolling and
cold-rolling the sheet, and to provide the cold-rolled aluminum
alloy sheet with basic properties necessary for forming a metal
bottle. Preferably, the ingot processed by the soaking process
is cooled by furnace cooling for such slow cooling.
[0075]
Cooling rate necessarily exceeds the upper limit cooling
rate of 25 C/hr if the ingot processed by the soaking process
is cooled by natural cooling outside the soaking furnace or by
forced cooling by using a fan. Then, the dispersed particle
structure specified by the present invention cannot form, the
29
CA 02625098 2008-04-09
ratio of the number of comparatively small dispersed particles
to the number of all the dispersed particles increases beyond
that of the number of the small dispersed particles in the
structure shown in Fig. 2 or particles of different sizes are
dispersed, and the ratio of the number of the dispersed
particles of sizes not smaller than 0.3 pm to_that of all the
dispersed particles decreases below 15%. Consequently, the
mode of dispersion of the dispersed particles becomes the same
as the conventional one.
[0076]
The soaking process may be divided into a plurality of
soaking stages. At least a cooling process subsequent to the
final soaking stage is a slow cooling process that cools the
ingot at the abovementioned cooling rate.
[0077]
Hot Rolling Starting Conditions
The ingot processed by the soaking process may be cooled,
and subjected to rough hot rolling after reheating or may be
subjected to rough hot rolling without being excessively cooled.
The ingot is cooled by slow cooling at the abovementioned
cooling rate to a rough hot rolling starting temperature after
the soaking process in such a case as well.
[0078]
Conditions for Rough Hot Rolling
CA 02625098 2008-04-09
When hot rolling includes rough hot rolling and finish
hot rolling continuous with rough hot rolling, cracks are liable
to form in the edges of a sheet during finish hot rolling due
to low rolling temperature if the temperature of the sheet after
the rough hot rolling state is excessively low. If the
temperature of the sheet at the end of the rough hot rolling
is excessively low, an unrecrystallized phase remains in the
sheet due to insufficient heat in the sheet or the quality of
the surface of the sheet is deteriorated by increased rolling
force. Therefore, it is preferable that the temperature of the
sheet at the end of rough hot rolling is 420 C or above, desirably,
430 C or above, more desirably 440 C or above, and 470 C or below,
desirably, 460 C or below.
[0079]
Preferably, the temperature of the sheet at the start of
rough hot rolling is, for example, in the range of about 490
to about 550 C, desirably, in the range of about 495 C to about
540 C, more desirably, in the range of about 500 C to about 530 C
to obtain the sheet of a temperature in the range of about 420 C
to 480 C at the end of rough hot rolling. Oxidation of the
surface of the hot-rolled sheet can be prevented, and formation
of large recrystallized grains can be prevented to improve the
formability still further when the temperature of the sheet at
the start of rough hot rolling is 550 C or below.
31
CA 02625098 2008-04-09
[0080]
It is desirable to process an aluminum alloy sheet
processed by rough hot rolling immediately by finish hot rolling.
Recovery of a strain caused in the aluminum alloy sheet during
rough hot rolling can be prevented and the strength of the
aluminum alloy sheet formed by cold rolling subsequent to the
hot rolling can be enhanced when the aluminum alloy sheet
processed by rough hot rolling is processed immediately to
finish hot rolling after rough hot rolling. Preferably, the
aluminum alloy sheet rolled by rough hot rolling is subjected
to finish hot rolling in, for example 5 min, desirably, in 3
min.
[0081]
Conditions for Finish Hot Rolling
Preferably, the temperature of the sheet at the end of
finish hot rolling is in the range of 310 C to 350 C. Finish
hot rolling forms a sheet in a predetermined size. Heat
generated in the finish hot rolled sheet forms recrystallized
structure. Therefore, the heat of the sheet at the end of finish
hot rolling affects the recrystallized structure. When the
temperature of the sheet at the end of the finish hot rolling
is 310 C or above, the final structure having crystal grains
stretched in the rolling direction in an aspect ratio of 3 or
above can be easily formed by cold rolling subsequent to the
32
CA 02625098 2008-04-09
final hot rolling. If the temperature of the sheet at the end
of the finish hot rolling is below 310 C, it is difficult to
form crystal grains having high mean aspect ratio even if the
sheet is processed by cold rolling at a high rolling reduction.
[0082]
If the temperature of the sheet at the end of the finish
hot rolling is above 350 C, final structure includes crystal
grains stretched in the rolling direction in a mean aspect ratio
of 3 or above, large Mg25i grains precipitate and it is difficult
to form distributed particle structure specified by the present
invention. The temperature of the sheet at the end of finish
hot rolling is in the range of 310 C to 350 C, preferably, in
the range of 320 C to 340 C.
[0083]
Type of Finish Hot-rolling Mills
A tandem hot-rolling mill having three or more roll stands
is used for finish hot rolling. In the tandem hot-rolling mill
having three or more roll stands, the rolling reduction in each
of the roll stands may be low, the surface quality of the
hot-rolled sheet can be maintained, and strain can be ac-
cumulated. Consequently, the respective strengths of a
cold-rolled sheet and an object formed by subjecting the
cold-rolled sheet to a DI process can be enhanced still further.
(0084]
33
CA 02625098 2008-04-09
Total Rolling Reduction in Finish Hot Rolling
Desirably, the total rolling reduction of the finish hot
rolling is 80% or above. When a sheet hot-rolled at a total
reduction of 80% or above, structure including crystal grains
stretched in the rolling direction in a mean aspect ratio of
3 or above can be formed and a sheet of dispersed particle
structure specified by the present invention can be easily
produced by cold rolling, and the respective strengths of the
cold-rolled sheet and an object formed by subjecting the
cold-rolled sheet to a DI process can be enhanced.
[0085]
Thickness of Hot-rolled Sheet
Desirably, the thickness of an alloy sheet produced by
finish hot rolling is in the range of about 1.8 to about 3 mm.
When a sheet is hot-rolled in a thickness of 1.8 mm or above,
the deterioration of surface quality thereof, such as formation
of galling and surface roughening, and thickness profile can
be prevented. When a sheet is hot-rolled in a thickness of 3
mm or below, it is possible to avoid cold-rolling the sheet at
an excessively high rolling reduction in manufacturing a
cold-rolled sheet having a thickness in the range of about 0.28
to 0. 35 mm, and earing ratio after a DI process can be suppressed.
[0086]
Cold Rolling -
34
CA 02625098 2008-04-09
It is desirable that a cold-rolling process rolls a
hot-rolled sheet at a total rolling reduction in the range of
77% to 90% by the so-called direct rolling process using a
plurality of passes and omitting intermediate annealing. When
a sheet is cold-rolled at a total rolling reduction of 77% or
above without using intermediate annealing, structure in-
cluding crystal grains stretched in the rolling direction in
a mean aspect ratio of 3 or above can be formed, dispersed
particle structure specified by the present invention can be
formed, and cans having an increased compressive strength can
be formed. If the sheet is subjected to intermediate annealing
during the cold-rolling process or when the total rolling
reduction is low, equiaxial crystal grains are liable to
crystallize and it is difficult for stretched crystal grains
to crystallize.
[0087]
Although the mean aspect ratio of crystal grains is high
when the rolling reduction is above 90%, 45 ears grow ex-
cessively during a DI process, the strength is excessively high
and, consequently, it is highly likely that cupping cracks and
bottom cracks form during a DI process.
[0088]
The thickness of the cold-rolled sheet for forming metal
bottles is in the range of about 0.28 to about 0.35 mm.
CA 02625098 2008-04-09
[0089]
Desirably, the cold-rolling process uses a tandem rolling
mill formed by arranging two or more rolling stands in a line.
The tandem rolling mill, as compared with a single rolling mill
including a single rolling stand and reducing the thickness of
a sheet to a predetermined thickness by passing the sheet
repeatedly through the single rolling stand, can roll a sheet
at the same total rolling reduction as the single rolling mill
by a number of passes smaller than that of the single rolling
mill. The tandem rolling mill can roll a sheet at a high rolling
reduction by a single pass.
[0090]
A sheet can be easily formed in final structure containing
crystal grains stretched in the rolling direction in a mean
aspect ratio of 3 or above.
[0091]
The cold-rolling process, as compared with the con-
ventional cold-rolling process that uses a single rolling mill
and processes a cold-rolled sheet by finish annealing, can cause
continuous recovery at low temperatures and can produce
subgrains. A rolling mill other than the tandem rolling mill
may be used, provided that the rolling mill can cause recovery
by cold rolling and can produce sufficient subgrains.
[0092]
36
CA 02625098 2008-04-09
When a sheet is cold-rolled by the tandem rolling mill,
the amount of heat generated by one pass is large because the
rolling reduction of each pass is high. If an excessively large
amount of heat is generated, it is possible that the grain sizes
of dispersed particles increase.
[0093]
Preferably, an aluminum sheet processed by a cold-rolling
process using the tandem rolling mill is cooled forcibly upon
the rise of the temperature of the aluminum sheet to a maximum
temperature so that the temperature of the aluminum sheet at
the end of cold rolling may not rise to a temperature above 200 C.
[0094]
To cool the aluminum sheet forcibly during the
cold-rolling process, an emulsion of a water-soluble oil or a
water-soluble lubricant may be used instead of a rolling oil
not containing water. It is preferable to cool the aluminum
sheet efficiently without reducing lubricating performance by
using the emulsion.
[0095]
When necessary, the cold-rolled sheet may be processed
by a finish annealing process (final annealing process) at a
temperature below a recrystallization temperature to improve
DI formability and bottom formability by recovering a de-
formation texture. The temperature of finish annealing is in
37
CA 02625098 2008-04-09
the range of, for example, about 100 C to about 150 C, desirably,
in the range of about 115 C to about 150 C. Annealing at 100 C
or above can satisfactorily recover the deformation texture.
Annealing at 150 C or below can prevent the excessive pre-
cipitation of the elements of the solid solution and can improve
DI formability and flange formability still further.
[0096]
Desirably, the duration of finish annealing is 4 hr or
below (particularly, in the range of 1 to 3 hr) . The excessive
precipitation of the elements of the solid solution can be
prevented and DI formability can be improved still more by
avoiding excessively long annealing.
[0097]
Since cold rolling using the tandem rolling mill can
produce subgrains by causing continuous recovery at lower
temperatures, basically, a sheet produced by the tandem rolling
mill does not need to be subjected to a finish annealing process.
[0098]
The present invention will be more concretely described
in terms of examples thereof. It goes without saying that the
following examples are not restrictive and changes and
variations may be made therein in light of the forgoing and the
following teachings without departing from the technical cope
of the present invention.
38
CA 02625098 2008-04-09
Examples
[0099]
Aluminum bullion and scrap cans were used as source
materials. The source materials were melted to obtain molten
aluminum alloys A to N respectively having compositions shown
in Table 1. The molten aluminum alloys were cast by a DC casting
method (Direct chill cast) to make ingots of 600 mm in thickness
and 2100 mm in width. Aluminum alloys A to D are examples of
the present invention, and aluminum alloys E to N are com-
parative examples. In Table1,"-"indicatesan element content
below a detection limit.
[0100]
As shown in Table 1, each of those ingots of aluminum
alloys A to N contains Zr, Bi, Sn, Ga, V, Co, Ni, Ca, Mo, Be,
Pb and W as inevitable impurities in a total impurity content
of 0.03% or above.
[0101]
The ingots respectively having those compositions were
subjected to a soaking process under conditions shown in Tables
2 and 4. The soaking process heated the ingots from 300 C to
soaking temperatures at heating rates shown in Tables 2 and 4.
The ingots processed by the soaking process were cooled from
the soaking temperatures to temperatures to hot-rolling
starting temperatures above 450 C in the range of 450 C to 550 C
39
CA 02625098 2008-04-09
at cooling rates shown in Tables 2 and 4, respectively.
[0102]
The ingots cooled respectively at cooling rates of
25 C/hr or below among those in examples of the present
invention and the comparative examples were furnace cooled in
a soaking furnace. The ingots in comparative examples cooled
respectively at cooling rates above of 25 C/hr were cooled by
natural cooling outside the soaking furnace.
[0103]
Each of the ingots processed by the soaking process was
hot-rolled by using a single-stand reversing rolling mill for
rough hot-rolling, and by using a tandem hot rolling mill having
four roll stands for finishot-rolling. The ingot was subjected
to finish hot rolling within 3 min after the ingot had been
processed by rough hot rolling. All the ingots were processed
by finish hot rolling to produce hot-rolled aluminum alloy
sheets having a thickness of 2.5 mm.
[0104]
The hot-rolled sheets were cold-rolled by a tandem or
single rolling mill to produce cold-rolled sheets having a
thickness of 0.3 mm intended for forming metal bottles. The
cold-rolled sheets were not processed by finish annealing
(final annealing).
[0105]
= CA 02625098 2008-04-09
All the sheets shown in Table 2 were not processed by
intermediate annealing and were produced by single-pass cold
rolling by a tandem rolling mill having two roll stands. All
the sheets shown in Table 4 were produced by four-pass cold
rolling by a single rolling mill having a single roll stand
and were processed by final annealing at 150 C for 1 hr.
[0106]
Cold rolling by the tandem rolling mill for producing the
sheets shown in Table 2 cooled the aluminum sheets by forced
cooling using an aqueous emulsion so that the temperature of
the aluminum sheets might not rise beyond 250 C.
[0107]
Specimens were sampled from rolls of the cold-rolled
sheets for forming metal bottles. The specimens were examined
by the abovementioned measuring method under a TEM at 10, 000x
magnification. The number of dispersed particles of particle
sizes in the range of 0.05 to 1 pm in an area of 300 um2 of each
specimen was counted. The ratio (%) of the number of dispersed
particles having particle sizes not smaller than 0.3 pm among
those dispersed particles to the number of those dispersed
particles was calculated, and the metal aspect ratio of the
disperse particles was calculated. The measurements and those
calculated values are shown in Table 3 continued from Table 2,
and Table 5 continued from Table 4.
41
CA 02625098 2008-04-09
[0108]
Mechanical Properties
The respective 0.2% offset yield strengths of the
specimens were measured by a tensile test method specified in
Z 2201, JIS. Test specimens were formed in the shape of a test
specimen No. 5 specified in JIS such at the length thereof
extends in the rolling direction. The cross head was moved at
a fixed speed of 5 mm/min until the test specimen broke.
[0109]
The respective 0.2% offset yield strengths of specimens
were measured after heating the specimens at 200 C for 20 min
by a heating process simulating a baking process for baking
printed cans. The 0.2% offset yield strengths of the specimens
kept at a room temperature and those of the specimens processed
by the heating process were compared to determine 0.2% offset
yield strength reductions. Those data are shown in Table 3
continued from Table 2, and Table 5 continued from Table 4.
[0110]
Earing ratio indicating formability to be basically
satisfied by a sheet for forming metal bottles, and formability
required by each of forming processes for forming two-piece
metal bottles were measured and evaluated. Measured results
are showri in Table 3.
[0111]
42
CA 02625098 2008-04-09
Earing ratio
A blank cut out from the sheet for forming metal bottles
was coated with a lubricant (Naruko 6461, D. A. Stuart) and was
drawn in a cup by an Erichsen tester for a 40% deep-drawing test.
These conditions were the diameter of the blank: 66.7 mm, the
diameter of the punch: 40 mm, the radius R of the rounded side
shoulder of the die: 2.0 mm, the radius R of the rounded edge
of the punch: 3.0 mm, and blank holder pressure: 400 kgf.
[0112]
Shapes of ridges and valleys in the edge of the open end
of the cup at eight angular positions with respect to the rolling
direction, namely, directions at 0 , 45 , 90 , 135 , 180 , 225 ,
270 and 315 to the rolling direction, were measured, and
earing ratio was calculated.
[0113]
A method of calculating earing ratio will be explained
with reference to Fig. 3. Fig. 3 is a development of a cup
obtained by processing the sheet for forming a metal bottle by
a DI process. The respective heights Ti, T2, T3 and T4 from
the bottom of the cup of negative ears extending in directions
at 0 , 90 , 180 and 270 to the rolling direction, and the
respective heights Yl, Y2, Y3 and Y4 from the bottom of the cup
of positive ears extending in directions at 45 , 135 , 225 and
315 to the rolling direction were measured. An earing ratio
43
CA 02625098 2008-04-09
was calculated by using the measured values and the following
expression.
Earing ratio = [{(Yl + Y2 + Y3 + Y4) - (T1 + T2 + T3 +
T4) }/{1/2 x(Y1 + Y2 + Y3 + Y4 + Tl + T2 + T3 + T4) }] x 100 (o)
[0114]
When the earing ratio is nearly equal to 0, the cold-rolled
sheet of the present invention suppresses the growth of the four
positive ears Y1 to Y4 and the two negative ears T2 and T4 in
the directions at 90 and 270 , but has difficulty in sup-
pressing the growth of the two negative ears T1 and T3 in the
directions at 0 and 180 shown in Fig. 3. If the absolute value
of the earing ratio is simply reduced, for example, if the earing
ratio is between -2 and 2%; that is, if the absolute value of
the earing ratio is 2% or below, the suppression of the negative
ears T1 and T3 shown in Fig. 3 is insufficient even if the earing
ratio is between -2 and 0%. Consequently, the blank holder
pressure is concentrated on the two negative ears Ti and T3 shown
in Fig. 3 causing edge rise and edge crack to cause problems
in production. If the earing ratio is between 0 and 2%, the
two negative ears T1 and T3 shown in Fig. 3 can be satisfactorily
suppressed and hence the fracture of the body due to edge crack
can be prevented. According to the present invention, an
allowable earind ratio is in the range of 0 to +3.5%.
[0115]
44
CA 02625098 2008-04-09
Ironing Formability
The ironing formability of the sheet for forming metal
bottles was evaluated. Blanks of 160 mm in diameter were
punched our from the sheet for forming metal bottles, and the
blanks were formed in cups of 92 mm in diameter. DI bottle
bodies for metal bottles were manufactured at a manufacturing
rate of 300 bottle bodies/min by processing the cups by a
redrawing process, an ironing process and a trimming process.
The DI bottle bodies were 66 mm in inside diameter, 170 mm in
height, 115 }.zm in side wall thickness, and 190 mm in
end-of-side-wall thickness. The ironing ratio of the final
ironing process, namely, a third ironing process, was 4 0%. The
number of broken cans (broken bottle bodies) among 50, 000 cans
was counted to evaluate the formability.
[0116]
The formability of the sheet was rated excellent and was
marked with a double circle when none of the cans formed by
processing the sheet was broken, the formability of the sheet
was rated good and was marked with a circle when four or less
cans among the cans formed by processing the sheet were broken,
and the formability of the sheet was rated unacceptable and was
marked with a cross when five or more cans among the cans formed
by processing the sheet were broken.
[0117]
CA 02625098 2008-04-09
Neck Formability
Neck formability of the sheet for forming metal bottles
was evaluated. The nondefective ones, namely, not broken ones,
of the DI can bodies formed for the formability evaluation were
used. A neck was formed by processing a part of each of the
DI can bodies near the open end by a die neck forming process
to shape the open end in a mouth. The outside diameter of the
can body was 66.2 mm. The neck was formed in four steps. The
outside diameter of the end of the neck was 60.3 mm. The necks
of 10,000 cans were inspected for wrinkles to evaluate neck
formability.
[0118]
The necks of 100 cans were inspected for wrinkles to
evaluate the neck formability of the sheet. The sheet was rated
acceptable and marked with a circle when the number of cans
having a wrinkled neck among the 100 cans was one or zero, and
the sheet was rated unacceptable and marked with a cross when
the number of cans having a wrinkled neck among the 100 cans
was two or greater.
[0119]
Threaded Neck Formability
Threaded neck formability of the sheet for forming metal
bottles was evaluated. 'A screw thread, with which a screw cap
is to be engaged, was formed in a part near the mouth of the
46
CA 02625098 2008-04-09
unwrinkled neck of a two-piece metal bottle to evaluate the
threaded neck formability of the sheet.
[0120]
The threaded necks of 9, 000 metal bottles were examined.
The sheet was rated excellent and marked with a double circle
when all the 9,000 threaded necks were satisfactory in di-
mensional accuracy and did not have any partial deformation at
all. The sheet was rated good and marked with a circle when
one threaded neck among the 9, 000 threaded necks was defective
in shape. The sheet was rated unacceptable and marked with a
cross when three or more threaded necks among the 9, 000 threaded
necks were defective in shape.
[0121]
Buckling Strength of Threaded Neck
Axial compressive load was placed on each of ten two-piece
metal bottles each having the threaded neck. The mean buckling
load of ten measured buckling loads at which the threaded necks
buckled was calculated. Threaded necks having a buckling
strength not lower than 1,500 N are practically acceptable.
[0122]
As obvious from Tables 3 and 5, cold-rolled aluminum alloy
sheets in Examples 1 to 5 shown in Table 3 and those in Examples
20 to 24 shown in Table 5 have compositions specified by the
present invention, and the ratio of the number of the dispersed
47
CA 02625098 2008-04-09
particles of sizes of 0.3 pm or above to the number of all the
dispersed particles of sizes in the range of 0.05 to 1}am in
those cold-rolled aluminum alloy sheets are within the range
specified by the present invention.
[0123]
The cold-rolled aluminum alloy sheets in Examples 1 to
and 20 to 24 are excellent in earing ratio, neck formability
and threaded neck formability required by the two-piece metal
bottle forming processes, and excellent in the buckling
strength of the threaded neck.
[0124]
The cold-rolled aluminum alloy sheets in Examples 1 to
5 shown in Table 3 have crystal grains having aspect ratios of
3 or above, have the offset yield strength that is reduced only
a little by baking and have excellent high-temperature
characteristics.
[0125]
Although the cold-rolled aluminum alloy sheets in
Comparative examples 6 to 9 shown in Table 3 and in Comparative
examples 25 to 28 shown in Table 5 have compositions cor-
responding to those specified by the present invention, the
cooling rate after the soaking process is higher than 25 C/hr.
Therefore, those cold-rolled aluminum alloy sheets do not have
the dispersed particle structure specified by the present
48
CA 02625098 2008-04-09
invention, the ratio of the number of small dispersed particles
to the number of all the dispersed particles is high, dispersed
particles having different particle sizes are dispersed, and
the ratio of the number of the dispersed particles of sizes of
0.3 um or above to the number of all the dispersed particles
in those cold-rolled aluminum alloy sheets is below 15%.
[0126]
The cold-rolled aluminum alloy sheet in Comparative
example 10 shown in Table 3 has an excessively high Mn content,
contains gigantic crystal grains and does not have a dispersed
particle composition required by the present invention.
Therefore many cracks formed when the cold-rolled aluminum
alloy sheet was processed to form metal bottles. The
cold-rolled aluminum alloy sheet in Comparative example 11 has
an excessively low Mn content. Therefore the buckling strength
of the neck of the bottle formed by processing the same is
insufficient.
The cold-rolled aluminum alloy sheet in Comparative
example 12 has an excessively high Mg content. Therefore the
formability, particularly, ironing formability, is dete-
riorated by high work hardening.
The cold-rolled aluminum alloy sheet in Comparative
example 13 has an excessively low Mg content. Therefore, the
buckling strength is insufficient.
49
CA 02625098 2008-04-09
The cold-rolled aluminum alloy sheet in Comparative
example 14 has an excessively high Cu content and hence
unsatisfactory in formability.
The cold-rolled aluminum alloy sheet in Comparative
example 15 has an excessively low Cu content and hence the
buckling strength is insufficient.
The cold-rolled aluminum alloy sheet in Comparative
example 16 has an excessively low Si content and hence large
positive ears form. Since the oc phase is insufficient, the
ironing formability is inferior.
The cold-rolled aluminum alloy sheet in Comparative
example 17 has an excessively high Si content and large positive
ears forms owing to unrecrystallized grains remaining therein.
The cold-rolled aluminum alloy sheet in Comparative
example 18 has an excessively low Fe content, contains un-
recrystallized grains, does not have many crystallized grains,
and has inferior ironing formability.
The cold-rolled aluminum alloy sheet in Comparative
example 19 has an excessively high Fe content, forms large
positive ears, has excessive crystallized grains, and
propagation of cracks therein is promoted to deteriorate
ironing formability.
[0127]
Those cold-rolled aluminum alloy sheets in comparative
CA 02625098 2008-04-09
examples are unsatisfactory in earing ratio, ironing form-
ability, neck formability and threaded neck formability, which
are essential properties needed to form two-piece metal bottles,
and form threaded necks having insufficient buckling strength.
[0128]
The cold-rolled aluminum alloy sheets in Comparative
examples 10 to 19 shown in Table 3 are cooled after the soaking
process at a desirable cooling rate. However, the respective
alloy compositions of those cold-rolled aluminum alloy sheet
do not correspond to the dispersed particle structure specified
by the present invention and, even if those cold-rolled aluminum
alloy sheets have the disperse particle structure specified by
the present invention, the ironing formability, neck form-
ability and threaded neck formability necessary for forming
two-piece metal bottles thereof are inferior.
[0129]
The critical significance of the present invention can
be known from the abovementioned results.
51
CA 02625098 2008-04-09
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CA 02625098 2008-04-09
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CA 02625098 2008-04-09
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CA 02625098 2008-04-09
[0135]
As apparent from the foregoing description, the present
invention provides a cold-rolled aluminum alloy sheet excellent
in neck formability and threaded neck formability for forming
metal bottles. Thus the cold-rolled aluminum alloy sheet is
suitable for uses that requires severe requisite properties,
such as excellent formability necessary for forming small
two-piece metal bottles having a small wall thickness and
capable of maintaining strength when processed by a heat
treatment.
57
