[0001] The present invention is directed to a process for making aluminium alloy strip stock
and in particular to a process for preparing continuous strip cast aluminium alloy
suitable for use in the manufacture of deep drawn and wall-ironed articles such as
cans and the like.
[0002] In recent years, aluminium alloys such as the Aluminium Association Specification
3004 have been successfully fabricated into two piece beverage cans by deep drawing
and ironing. The expanding use of two piece aluminium cans has created a need for
aluminium alloy sheet for forming the can body that not only possesses the required
combination of formability and strength properties but is also economical to manufacture.
[0003] Typically the aluminium alloy sheet useful in the production of deep drawn and ironed
beverage cans is cast by direct chill casting an ingot having a thickness of 50.8-63.5
cm (20-25 inches). The ingot is homogenized at 510-607°C (950-1125°F) for 4-24 hours
and then subjected to hot rolling wherein the ingot is passed through a series of
breakdown rolls maintained at a temperature of 204-482°C (400-900°F) to reduce the
ingot in thickness to a reroll gauge of about 0.033 cm (0.0130 inch).
[0004] Thereafter, the reroll stock is subjected to an annealing step wherein the stock
is heated at 316-482°C (600-900°F) for 0.5―3 hours to effect recrystallization of
the metal structure. The annealed reroll stock is subjected to a final work hardening
step wherein the reroll stock is cold rolled (room temperature rolling) to a final
gauge of about 0.033 cm (0.013 inch) or about 90% of its original thickness to produce
the substantially full hard (H19) temper required for two-piece can body stock.
[0005] In spite of the successful use in can-making of direct chill ingot cast aluminium
alloy, economic and energy considerations would favour the manufacture of the aluminium
sheet by continuous strip casting. In this process the molten aluminium is cast and
solidified into a thin web of 2.54 cm (one inch) or less in thickness so that subsequent
rolling is reduced to a minimum and the costly step of hot rolling is eliminated.
[0006] In the manufacture of continuous strip cast aluminium alloy for can manufacture,
the thin, e.g. 0.51-2.54 cm (0.2-1.0 inch) solidified cast web is typically reduced
in thickness to a gauge of about 0.033 cm (0.130 inch) by cold rolling with an intermediate
recrystallization anneal at 316―482°C (600-900°F). Thereafter, as in the manufacture
of direct chill ingot cast stock, the thinned, annealed stock is subjected to a final
work hardening step by cold rolling to a final gauge of about 0.033 cm (0.013 inch)
to produce the H19 temper required for can body manufacture.
[0007] Although the continuous strip cast aluminium alloy is advantageously utilized for
many fabricated products, such stock has not been used extensively for the manufacture
of drawn and wall-ironed can bodies.
[0008] In the production of two-piece drawn and wall-ironed beverage cans, circular discs
or blanks are cut or punched from the cold worked (H19) sheet for deep drawing into
the desired shape. Deep drawing is a process for forming sheet metal between punch
and die to produce a cup or shell-like part. When a deep drawn shell with a heavy
bottom and thin sidewalls is desired, wall-ironing is used in conjunction with deep
drawing. The blank is first drawn to approximately the final diameter cup. The sidewalls
are then reduced in thickness in one or more ironing operations.
[0009] Because of the nature of the working stresses incurred during wall-ironing of the
deep drawn shell, when continuous strip cast aluminium alloy such as 3004 is subjected
to wall-ironing, scoring may occur on the die surface; alternately, deep grooves may
appear on the finished can which is referred to in the art as "galling". Galling adversely
affects the acceptability of the can product and the effectiveness of the can manufacturing
process. Galling is not normally observed during wall-ironing aluminium sheets of
the same alloy composition produced from direct chill ingot casting.
[0010] In spite of the economic advantage of the strip casting process, due to the drawback
of not being gall-free when subjected to severe mechanical operations such as wall-ironing
operations in two-piece aluminium can making, the utility and applicability of continuous
strip cast aluminium alloy for can making has been extremely limited.
[0011] It is known to fabricate aluminium alloy strip stock by continuously casting an aluminium
alloy in strip form; homogenizing the alloy strip; and then cold rolling the homogenized
strip, as defined in the pre-characterizing portion of Claim 1.
[0012] The art has addressed the problem of providing continuous strip cast aluminium alloys
which have the capability to be gall-free when subjected to the severe mechanical
working conditions of can making. For example, U.S. 4,111,721 discloses a process
for imparting an anti-galling character to a continuous strip cast aluminium alloy
wherein the aluminium strip is heat treated at a temperature of at least 482°C (900°F)
and advantageously at about 621°C (1150°F) for a period of time between 16 to 24 hours
prior to its final cold reduction pass.
[0013] The art prior to U.S. 4,111,721, namely U.S. 3,930,895 disclosed that in the process
of making continuous strip cast aluminium alloy suitable for can making, the cast
strip, before cold rolling, is homogenized at a temperature of 510 to 566°C (950 to
1050°F) for 8 to 16 hours.
[0014] Although the art reported that gall-free continuous strip cast aluminium alloy had
been produced, the strip has remained substantially unacceptable for can making stock
because of the problem of "earing" which manifests itself as a scalloped appearance
around the top edge of the cup during the deep drawing cup formation step of the drawn
and wall-iron processing of the aluminium sheet.
[0015] The scallops, or ears, represent an almost universally undesirable feature of the
cup as the ears must be removed in order to present a smooth or flat upper lip on
the cup. This of course necessitates cup trimming prior or subsequent to wall-ironing
with an attendant increase in production costs and material waste.
[0016] The level of earing in a drawn cup is determined by the following equation:
where he is the distance between the bottom of the cup and the peak of the ear and
ht is the distance between the bottom of the cup and the valley of the ear.
[0017] To be acceptable for can making, the aluminium alloy sheet when processed into a
cup must exhibit a low level of earing. The level of earing experienced with commercially
available continuously cast strip of 3004 aluminium alloy is generally in the range
of 5% or more. The present invention aims to provide lower levels of earing than the
range of 5% or more. Advantageously, for optimum commercial use the level of earing
should be reduced to no more than about 3.5% and preferably less than about 3% earing.
[0018] It is evident, therefore, that the reduction of the degree of earing during deep
drawing of continuous cast aluminium strip to a level of about 3.5% or less represents
a major contribution to the art of manufacture of continuous cast aluminium strip
can stock.
[0019] Another problem encountered with continuous strip cast aluminium alloy 3004 is that
the alloy sheet when fabricated into a two piece drawn and wall-ironed can exhibits
a marginal level of buckle strength, that is, the ability of the can to withstand
high internal pressure without bottom inversion.
[0020] Buckle strength is determined by applying pressure within a drawn and wall-ironed
can and then gradually increasing the pressure until the bottom end of the can deforms
and bulges out, i.e., it buckles. The pressure at which the bottom buckles is then
designated as the buckle strength. To be acceptable as can body stock a can formed
from the alloy sheet must exhibit a buckle strength of at least 620
X10
3 N/m
2 (90 pounds per square inch (psi)), and preferably between 655x10
3 and 689x10
3 N/m
2 (95 and 100 psi). Cans drawn and wall ironed from a hard temper sheet of the continuous
strip cast aluminium alloy 3004 homogenized at 566―593°C(1050―1100°F) to eliminate
galling exhibit a buckle strength of about 586x 1 03 N/m
2 (85 psi).
[0021] The present invention relates to a process for the preparation of non-galling, low
earing can stock from continuously cast aluminium strip suitable for deep drawing
and wall-ironing into hollow articles.
[0022] The present invention provides a process for fabricating aluminium alloy strip stock
suitable for the manufacture of drawn and wall-ironed articles, the process comprising
the steps of: continuously casting an aluminium alloy in strip form; homogenizing
the alloy strip; and then cold rolling the homogenized strip; characterised in that:
the continuous cast strip has a thickness of up to 2.54 cm (one inch); said homogenizing
step is carried out at a temperature of 510 to 621°C (950 to 1150°F) for up to 50
hours; said cold rolling step is carried out to yield at least a 25% reduction in
thickness; and thereafter heating the cold rolled strip to a recovery temperature
of from 177 to 288°C (350 to 550°F); cold rolling the strip to a second reduction
in thickness of at least 10%; heating the cold rolled strip to a recrystallization
temperature of from 316 to 428°C (600 to 900°F); and then cold rolling the recrystallized
strip to a final gauge having a total reduction in thickness of at least about 50%.
[0023] In accordance with the present invention, the molten aluminium material is cast by
continuous strip casting into a web generally of 2.54 cm (one inch) or less in thickness.
The strip material is heated to a temperature of from 510 to 621°C (950 to 1150°F)
for a time sufficient to homogenize the alloy. The homogenized strip material is cold
rolled to effect a first reduction in sheet thickness of at least 25%. The cold rolled
sheet is heated to a recovery temperature of from 177°C (350°F) to 288°C (550°F),
and subjected to a second cold rolling to effect a reduction in thickness of at least
10%. The cold rolled sheet product is heated to effect recrystallization of the grain
structure and then subjected to effect a final reduction in thickness which is at
least about 50% and is preferably at least 75% of the original thickness of the sheet
to impart an H19 temper to the sheet.
[0024] To effect the most advantageous reduction in earing, the sheet is subjected to a
second recovery heating of from 177°C (350°F) to 288°C (550°F) intermediate between
the second cold reduction and the recrystallization heating step.
[0025] As will hereinafter be illustrated, it has been determined that in the fabrication
of strip cast aluminium sheet suitable for the production of drawn and wall-ironed
beverage containers, control of the homogenization step within the parameters set
forth above will render the sheet resistant to galling when subjected to drawing and
ironing operations. Control of the cold roll and recovery heating parameters set forth
above prior to the recrystallization heating step, will result in the fabrication
of an aluminium sheet exhibiting low earing properties as well as non-galling characteristics.
[0026] Generally in affecting homogenization to prepare an aluminium alloy sheet product
in accordance with the present invention, the continuous cast web is heated at 510
to 621°C (950 to 1150°F) and preferably 537 to 593°C (1000 to 1100°F) for a period
of time up to about 50 hours and preferably 10 to 25 hours. Advantageously, the homogenization
treatment is conducted at a temperature of about 593°C (1100°F) for at least about
10 hours. It is recognized that several hours are required to heat the metal to reach
the temperature at which homogenization is effected.
[0027] In the event that the cast aluminium web is subjected to homogenization temperatures
while in coil form, it has been determined that the coil be heated in a slow, pre-programmed
manner for time periods ranging from 2 to 10 hours at increasing temperatures to avoid
incipient melting of the alloy which will otherwise cause the coil layers to fuse
and weld together and render the coiled product unsuitable for subsequent use. A programmed
heating sequence which has been found advantageous for the homogenization of the continuous
cast aluminium coil is as follows: Temperature of the web is raised from ambient 24°C
(75°F) to 537°C (1000°F) over a 5 hour period.
[0028] Temperature of the web is raised from 537 to 566°C (1000 to 1050°F) over a 3 hour
period.
[0029] Temperature of the web is raised from 566 to 593°C (1050 to 1100°F) over a 5 hour
period.
[0030] Web is homogenized at 593±5.5°C (1100±10°F) for 20 hours.
[0031] The homogenization step of the process of the present invention imparts a very critical
change in the microstructure of the alloy primarily in the size, shape and distribution
of the intermetallic particles present in the alloy matrix. It has been determined
that the change in intermetallic particle disposition is dependent upon the temperature
as well as the time of the homogenization treatment and that the degree of galling
is inversely dependent upon the intermetallic particle size.
[0032] Examination of photomicrographs of 3004 aluminium alloy subjected to the homogenization
sequence of the present invention indicates that the secondary constituents in the
aluminium alloy, e.g. (MnFeSi) Al, are caused to agglomerate whereby they change their
shape substantially and increase in size. The net effect of this is the development
of intermetallic particles approaching a globular shape having a particle size of
1 to 3 microns. These relatively large, globular shaped particles are believed to
act as galling-resistant bearings for the strip cast stock during the severe mechanical
working encountered in the wall-ironing operations of two piece can manufacture.
[0033] For example, continuous cast 3004 aluminium alloy strip cold rolled and size-reduced
to 0.034 cm (0.0135 inch) gauge to H-19 temper by conventional practice typically
has an intermetallic particle size in the order of 0.3-0.7 microns. As already indicated,
this strip when subjected to ironing operations encounters severe galling. However,
if the aluminium web is subjected to the homogenization step, as previously described,
prior to cold rolling, the inter-metallic particle size increases with increasing
homogenization temperature which results in a proportionate decrease in galling when
the homogenized strip is subjected to wall-ironing conditions.
[0034] The relationship between homogenization temperature, intermetallic particle size
and galling is summarized in the Table below:
[0035] Although the aluminium web when homogenized at 510-621°C (950-1150°F) will encounter
no galling during wall-ironing a cup formed from the web, it will after being subjected
to drawing operations, exhibit unacceptably high earing.
[0036] By following the cold roll/recovery-recrystallization heating sequence described
herein there is attained a reduction in earing to levels required for commercial acceptance
of the drawn and wall-ironed container.
[0037] Thus, after the aluminium alloy stock has been produced by continuous strip casting
and homogenized in accordance with the parameters disclosed above, the cooled web
which has a thickness of up to one inch and typically 0.64 to 1.27 cm (0.25 to 0.50
inch) in thickness is subjected to a first cold rolling step to effect a total gauge
reduction in excess of about 25% and preferably 50 to 75%. Thereafter, the cold rolled
sheet is heated to a recovery temperature level.
[0038] The term "recovery temperature" as it is used in the art means the temperature at
which the rolled metal is heated whereby it is softened without forming a new grain
structure. For aluminium alloys of the 3004 type the recovery temperature is in the
range of 177 to 288°C (350 to 550°F). In accordance with the present invention, the
cold rolled strip is heated to a recovery temperature of from 177 to 288°C (350 to
550°F). Preferably, the recovery temperature to which the cold rolled web is heated
after the first cold roll reduction is 204 to 246°C (400 to 475°F) for 2 to 6 hours,
and more preferably from 218 to 246°C (425 to 475°F) for 2 to 4 hours.
[0039] After being heated at the recovery temperature the heated web is cooled to ambient
temperature and subjected to a second cold rolling step to effect a total reduction
in thickness of the web of at least 10% and preferably from 10 to 25%.
[0040] As will hereinafter be illustrated, heating the web to a recovery temperature intermediate
between the two cold rolling steps is critical to imparting a low earing characteristic
to the aluminium sheet.
[0041] After the second cold roll step, the temperature of the cold rolled web is raised
to the "recrystallization temperature" level.
[0042] The term "recrystallization temperature", as it is used in the art, means the temperature
at which the rolled metal web softens simultaneously with the formation of a completely
new grain structure. In the case of 3004 alloy, the grain structure changes from a
substantially elongated structure to an equiaxed structure when the alloy is heated
at the recrystallization temperature.
[0043] In the practice of the present invention, the recrystallization temperature is in
the range of 316 to 482°C (600 to 900°F), the heating being effected for 1 to 4 hours
and preferably at a temperature from 371 to 427°C (700 to 800°F) for 2 to 3 hours.
[0044] After heating at the recrystallization temperature for the prescribed time period,
the recrystallized web is cooled to ambient temperature and then cold rolled, e.g.,
to at least about 50% and preferably 60 to 90%, to the final gauge dictated by can
performance requirements, e.g., 0.030 to 0.037 cm (0.012 to 0.0145 inch) and H19 temper.
[0045] To achieve an optimum reduction in earing the aluminium web is heated a second time
to a recovery temperature, the second recovery heating occurring between the second
cold rolling step and the recrystallization heating step. The second recovery heating
is effected at a temperature from 232 to 288°C (450 to 550°F) for 0.5 to 3 hours and
preferably from 246 to 274°C (475 to 525°F) for 0.75 to 1.25 hours.
[0046] In effecting the second recovery heating, the web may be cooled to room temperature
between the second recovery heating step and the recrystallization step. Preferably
the recrystallization heating is carried out without prior cooling to room temperature
by direct heating from the second recovery temperature to the recrystallization temperature.
[0047] It has been further determined that to achieve a consistency in earing reduction
results it is advantageous that, after the homogenization step of the process of the
present invention the web is cooled in a controlled stepped manner, i.e., at a cooling
rate of no more than 42°C/hr (75°F/hr). A preferred sequence of cooling is summarized
as follows:
[0048] An aluminium alloy preferred in the practice of the present invention is a 3004 aluminium
alloy having incorporated therein 0.1-0.4% by weight chromium. Sheet formed from the
chromium modified alloy 3004 when fabricated into a two piece drawn and wall-ironed
can exhibits an improved level of buckle strength, that is, the ability of the can
to withstand high internal pressure without bottom inversion.
[0049] The chromium modified aluminium alloy 3004 preferred in the practice of the present
invention has the following range of constituents expressed in percent by weight:
0.5 to 1.5% magnesium, 0.5 to 1.5% manganese, 0.1 to 1.0% iron, 0.1 to 0.5% silicon,
0.0 to 0.25% zinc, 0.0 to 0.25% copper, 0.1 to 0.4% chromium, the balance being aluminium
and incidental elements and impurities.
[0050] For sheet formed from the chromium modified alloy 3004 to perform as desired, it
is essential that it be in the state resulting from a cold roll reduction of at least
50% of the material in the recrystallized state. The sheet in this state can exhibit
tensile yield strengths in the range of 275.6x 10
6 to 310.1 × 10° N/m
2 (40,000 to 45,000 psi) and total elongation, measured in 5.1 cm (2 inches) gauge
length samples, of 1.5% or more. A tensile yield strength of 275.6x 10
6 to 310.1 x 1 0
6 N/m
2 (40,000 to 45,000 psi) in the sheet material has been found, when such sheet is drawn
and wall ironed into a two piece beverage container, to correlate with a can buckle
strength of at least 675x10
3 N/
M2 (98 psi).
[0051] The improved properties imparted to alloy 3004, and particularly the high tensile
yield strengths, by the incorporation therein of 0.1 to 0.4% by weight chromium is
totally unexpected when viewed against the teachings of the prior art.
[0052] Thus, U.S. 4,111,721 teaches that additaments to alloy 3004 such as chromium should
be limited to trace amounts in the order of several hundred thousands of a weight
percent or less as such additaments tend to have profound effects on the intermetallic
particle sizes in the alloy. U.S. 3,834,900 teaches that the presence of chromium
in the strip cast aluminium alloy should be minimized, i.e., limited to a concentration
of less than 0.001% by weight, to avoid casting defects.
[0053] The processing limitations of the present invention, and preferably the compositions
of the preferred embodiments, should be closely followed in order to achieve the required
high tensile yield strength properties which characterise the sheet prepared from
continuous strip cast modified alloy of the present invention. Improved results may
be obtained by the preferred process of the present invention when the chromium concentration
in the Aluminium 3004 alloy is from 0.1 to 0.4 wt%. For example, if the maximum chromium
concentration levels are exceeded, problems such as fracturing during can forming
may result. If chromium levels of less than about 0.1 % by weight are incorporated
in the alloy, the tensile yield strength of sheet fabricated from the continuous strip
case alloy can fall below the minimum requirements for beverage can performance.
[0054] In converting the chromium modified alloy composition of the present invention into
sheet material by strip casting, the aluminium and alloying elements are charged into
a melting furnace from which a stream of alloy is fed to a conventional strip caster
which solidifies a web of 2.54 cm (an inch) or less in thickness preferably 0.64 to
1.27 cm (0.25 to 0.50 inch) in thickness. The strip cast web is fabricated into sheet
having non-galling, low earing and high strength characteristics by employing the
homogenization and cold roll/anneal process conditions of the process of the present
invention.
[0055] A more thorough understanding of the present invention may be attained by reference
to the following specific examples of the practice of the invention.
Example I
[0056] A series of strip-cast aluminium alloys having varying alloy constituents including
those within the Aluminium Association Specification 3004 aluminium alloy range were
evaluated for use in the fabrication of drawn and wall-ironed can bodies. The composition
of the alloys is summarized in Table I below:
[0057] 30.5 cm (one foot) wide by 91.4 cm (three feet) long sections of the cast aluminium
strip having a thickness of 1.22 cm (0.48 inch) were placed in a furnace in a nitrogen
atmosphere, brought up rapidly to the desired temperature, and held for 10 to 40 hours
at homogenization temperatures varying from 590°C to 610°C (1094 to 1130°F). Thereafter,
the strips were removed from the furnace and cooled to ambient temperature by blowing
cold compressed air on the strips. The homogenization conditions used in the series
of runs are summarized in Table II as follows:
[0058] The cooled strips were rolled in successive passes using a commercial rolling mill
until the strip was reduced to varying degrees of thickness ranging from 66 to 75%
(0.41 to 0.30 cm) (0.160 to 0.120 inch).
[0059] The reduced thickness strips were subjdcted to a first recovery temperature wherein
the strips were placed in a furnace previously heated to 232°C (450°F) and held for
3 hours after which time the strips were removed from the furnace and allowed to cool
to room temperature.
[0060] After being subjected to the first cold roll/recovery temperature treatment, the
strips were subjected to a second cold roll reduction by being passed successively
through a pair of reduction rolls until the strip was reduced 10-25% in thickness
to 0.30 cm (0.120 inch).
[0061] After the second cold roll reduction the strips were subjected to a second recovery
heating at 260°C (500°F) for one hour and then annealed at a recrystallization temperature
of 427°C (800°F) for 2 hours.
[0062] The cold roll/recovery-recrystallization (anneal) conditions to which the series
of strips were subjected are summarized in Table III below.
[0063] For purposes of contrast, the cold roll/anneal conditions of Example I were repeated
with the exception that no recovery temperature heating was effected between the cold
roll reduction step and the recrystallization step. These contrasting conditions are
summarized in Table III below designated by the symbols "C
1" and "C
2".
[0064] The recrystallized strips were cooled to ambient temperature and then work hardened
by passing the strips successively in a commercial rolling mill until the strip was
reduced about 88% in thickness (H19 temper) to 0.0340 to 0.0376 cm (0.0134 to 0.0148
inch).
[0065] The H19 temper strips were examined under a scanning electron microscope in the back
scattering mode and found to have an intermetallic particle size in the 1 to 3 microns
range indicating that no galling would occur when the strips were subjected to the
wall-ironing conditions of can making.
[0066] To determine the extent of earing which would occur when the strips were subjected
to the drawing operations of can making, circular blanks 5.59 cm (2.20 inch) diameter
were cut from the H19 hardened strips and deep drawn into shallow cups of 3.35 cm
(1.32 inch) diameter with a resultant 39% reduction in diameter. The tooling used
for deep drawing 0.0343 cm (0.0135 inch) sheet was designed to yield about a 3.5%
positive clearance (0.00127 cm) (0.0005 inch) between the walls of the punch and die.
A die clearance of 5% or less and a reduction in diameter of 39% is typically required
in this standard test for canstock earing which simulates the drawing step of the
can making process. Cupping speed and blank clamping pressure were adjusted for each
test to obtain a fracture and wrinkle-free cup.
[0067] The results of the earing tests using strips of the alloy compositions of Table I
which had been subjected to the homogenization and cold roll/anneal conditions disclosed
in Tables II and III are summarized in Tables IV and V below. Each earing test result
represents an average of three tests.
[0068] The results of earing tests on aluminium strips subjected to comparative cold roll/anneal
cycles C, and C
2 are summarized in Table VI below.
[0069] By reference to the earing data summarized in Tables IV and V, and comparing such
data to the comparative earing data in Table VI, it is readily apparent that aluminium
strip treated in accordance with cold roll/anneal cycles 1 and 2 produce lower earing
when compared to comparative cold roll/anneal cycles C
1 and C
2. The data indicates that cold roll/anneal cycles 1 and 2 which involve one or more
recovery heating steps prior to recrystallization heating can be more effective in
reducing earing than anneal cycles C
1 and C
2 in which there are one or more recrystallization heating steps but no recovery heating
step. Cold roll/anneal cycle 1 produces superior earing results when compared to cold
roll/anneal cycle 2; cycle 1 having a lower second rolling reduction (10%) than cycle
2 (25%), indicating that a low (10%) second rolling reduction is desirable in reducing
earing. However, the use of a second recovery treatment in which the strip is heated
to a second recovery temperature after the second cold roll and prior to heating the
strip to the recrystallization temperature is not essential to the present invention
but is merely preferred in order to give reduced earing.
Example II
[0070] The procedure of Example I was repeated with the exception that there was simulated
the heating and cooling conditions that would be expected to occur in a commercially
produced 1.02x 104 to 1.53x 10
4 kg (10-15 ton) coil of continuous strip cast aluminium alloy 3004 of about 1.27cm
(0.50 inch) thickness which had been subjected to the heating sequence described above.
[0071] The programmed heating and cooling sequences outlined herein were used to achieve
strip homogenization in these coil simulation tests. The time and temperature used
in the heating and cooling sequences are summarized in Table VII below:
[0072] The strips homogenized in accordance with Table VII were then cooled in accordance
with the following schedule:
[0073] At 191°C (375°F) the furnace was shut off and the strips allowed to cool to room
temperature.
[0074] The cooled strips were then cold rolled/annealed in the manner of Example I using
the cold roll/anneal conditions summaried in Table VIII below.
[0075] For purposes of contrast, the cold roll/anneal conditions of Example II were repeated
with the exception that no recovery temperature heating was effected between the cold
roll reduction step and the recrystallization step. This contrasting condition is
summarized in Table VIII below designated by the symbol C
3.
[0076] The heating and cooling conditions that would be expected to occur in processing
a commercial coil were used in each recovery and recrystallization step. These conditions
are summarized in Table IX below:
[0077] The cooled recrystallized strips of Table IX were work hardened to H19 temper and
reduced in thickness to 0.0340 to 0.0376 cm (0.0134 to 0.0148 inch).
[0078] The H19 temper strips were examined under a scanning electron microscope in the back
scattering mode and found to have an intermetallic particle size in the 1 to 3 microns
range, indicating that no galling would occur when the strips were subjected to the
wall-ironing conditions of can making.
[0079] The results of earing tests using strips of the alloy compositions of Table I which
had been subjected to the homogenization and cold roll/anneal conditions as disclosed
in Tables VIII and IX are summarised in Tables X-XIII below. Each earing test result
represents an average of 3 tests.
[0081] By reference to the data summarized in Tables X-XIII and comparing such data to that
in Table XIV, it is readily apparent that the largest reduction in earing occurs when
cold roll/anneal cycle 5, which employs two recovery heatings prior to recrystallization
is used.
[0082] Cold roll/anneal cycle 6 which is identical to cycle 5, except that a second cold
roll reduction of 25% is used instead of 10%, produces a reduction in earing, but
the reduction achieved is less than that achieved using cycle 5, indicating that a
second cold roll reduction of 10% is more advantageous in effecting a reduction in
earing.
[0083] Cold roil/anneal cycle 7 which utilizes a single recovery heating/single recrystallization
heating sequence does not achieve the earing reduction level of cycle 5 but does produce
a superior reduction in earing when compared to the single recrystallization heating
of cold roll/anneal cycle C
3.
[0084] The double recovery heating/recrystallization heating of cycle 8 produces a reduction
in earing when compared to control cycle C
3, but does not provide an advantage over cycle 5 which utilizes only one recrystallization
heating.
Example III
[0085] A strip-cast aluminium alloy having the alloy composition of the present invention
designated by the symbol "I" was prepared as well as alloy compositions having varying
alloy constituents within the 3004 specification range designated by the symbol "A".
These alloys were then evaluated for use in the fabrication of drawn and wall-ironed
can bodies. The composition of the alloys is summarized in Table XV below:
[0086] 30.5 cm (one foot) wide by 91.4 cm (three feet) long sections of the cast aluminium
strip having a thickness of 1.22 cm (0.48 inch) were placed in a furnace in a nitrogen
atmosphere and heated for 10 to 40 hours at homogenization temperatures varying from
590°C to 600°C (1094°F to 1112°F). The heating and cooling conditions that would be
expected to occur in a commercially produced 1.02x 10
4 to 1.5x10
4 kg (10-15 ton) coil of a strip of continuous cast aluminium alloy of about 1.27 cm
(0.50 inch) thickness when subjected to the programmed heating and cooling sequences
preferred for homogenization and outlined in the Preferred Embodiments of this application
were simulated to achieve strip homogenization. The time and temperature used in the
heat and cooling sequences are summarized in Table XVI below:
[0087] The strips homogenized in accordance with the conditions in Table XVI were then cooled
in accordance with the following schedule:
[0088] At 191°C (375°F) the furnace was shut off and the strips allowed to cool to room
temperature.
[0089] The cooled strips were rolled in successive passes using a commercial rolling mill
until the strip was reduced to varying degrees of thickness ranging from 66 to 75%
(0.040 cm to 0.030 cm) (0.160 to 0.120 inch).
[0090] In a first series of cold roll/recovery-recrystallization heatings the reduced (66-72%)
thickness strips were subjected to a first recovery temperature wherein the strips
were heated in a furnace to 232°C (450°F) and held for 3 hours. After being subjected
to the first cold roll/recovery temperature treatment, the strips were then subjected
to a second cold roll reduction by being passed successively through a pair of reduction
rolls until the strip was reduced 10-25% in thickness to 0.030 cm (0.120 inch).
[0091] After the second cold roll reduction the strips were subjected to a second recovery
heating at 260°C (500°?) for one hour and then heated to recrystallization temperature
of 427°C (800°F) for 2 hours.
[0092] The first series of cold roll recovery-recrystallization heatings was varied whereby
in a first variation the second cold reduction was eliminated and recrystallization
carried out immediately after the first recovery heating. In a second variation, the
recovery heating was eliminated and recrystallization was carried out immediately
after the cold reduction.
[0093] The cold roll/anneal conditions to which the series of strips were subjected are
summarized in Table XVII below.
[0094] The heating and cooling conditions that would be expected to occur in processing
a commercial coil were used in each recovery and recrystallization step. These conditions
are summarized in Table XVIII below.
[0095] The recrystallized strips were cooled to room temperature and then were hardened
by passing the strips successively in a commercial rolling mill until the strip was
reduced about 88% in thickness (H19 temper) to 0.0338 to 0.0376 cm (0.0133 to 0.0148
inch).
[0096] The H19 tempered strips were examined under a scanning electron microscope in the
back scattering mode and found to have an intermetallic particle size in the 1 to
3 microns range, indicating that no galling would occur when the strips were subjected
to the wall-ironing conditions of can making.
[0097] To determine the level of earing that would occur when the strips were subjected
to the drawing operations of can making, circular blanks 5.59 cm (2.20 inch) diameter
were cut from the H19 hardened strips and deep drawn into shallow cups of 3.35 cm
(1.32 inch) diameter with a resultant 39% reduction in diameter. The tooling used
for deep drawing was designed to yield about a 3.5% positive clearance (0.00127 cm)
(0.005 inch) between the walls of the punch and die. A die clearance of 5% or less
and reduction in diameter of 39% is typically required in this standard test for earing
which simulates the drawing step of the can making process. Cupping speed and blank
clamping pressure were adjusted for each test to obtain a fracture and wrinkle-free
cup.
[0098] The results of the earing tests using strips of the alloy compositions of the Table
XV which had been subjected to the homogenization and cold roll/anneal conditions
disclosed in Tables XVI and XVII are summarised in Tables XIX-XXI below. Each earing
test result represents an average of three tests.
[0099] The mechanical properties of the H19 hardened strips in tension, i.e. yield strength,
ultimate strength and tensile total elongation were determined in accordance with
the ASTM Test Procedure Number E-8 using 5.1 cm (2 inches) gauge length test specimens.
Each mechanical test result represents an average of six tests, three measured in
the direction longitudinal and three in transverse to the rolling direction. The results
of these tests are also recorded in Tables XIX-XXI below.
[0100] It had been previously determined that the buckle strength of cans formed from continuous
strip cast aluminium alloy 3004 correlates closely with the tensile yield strength
of the H19 temper sheet. The correlation between buckle strength and tensile yield
strength is summarized in Table XXII below.
[0101] The tensile ultimate strength, along with the tensile total elongation, is a measure
of sheet formability. To be suitable for can body manufacture, the sheet must have
a tensile ultimate strength of at least 289x10
6 N/m
2 (42,000 psi).
[0103] By reference to Table XIX it is immediately apparent that the incorporation of 0.11%
by weight chromium in aluminium alloy 3004 improves the tensile yield strength and
thereby the corresponding buckle strength without any deleterious effect on the can
formability of sheet formed from the alloy. Thus the tensile yield strength of Alloy
I generally exceeds 275.6x10
6 N/m
2 (40,000 psi) reflecting a buckle strength in excess of 675.2x10
3 N/m
2 (98 psi). Similarly, the total elongation of Alloy I is in excess of the minimum
requirement of 1.5%.
[0104] By comparing the data recorded in Tables XX and XXI with that of Table XIX it is
immediately apparent that conventional 3004 alloy, such as alloys A, and A
2, when processed in accordance with the same conditions of Alloy I have buckle strength
substantially lower than that of Alloy I.
Example IV
[0105] A second series of strip cast aluminium alloys were evaluated for use in the fabrication
of drawn and wall ironed can bodies. The composition of the alloys is summarized in
Table XXIII below:
[0106] Copper was incorporated in the alloys to simulate aluminium can scrap which has been
found to contain 0.1 to 0.2 percent by weight copper.
[0107] The aluminium alloys were continuously cast, using a Hunter type twin roll caster
into sheet 0.66 cm (0.26 inches) thick which were wound into 2270 kg (5000 pound)
coils. The coils were allowed to reach room temperature over a 48 hour period. The
cooled coils were then placed in a furnace and homogenized in a nitrogen atmosphere.
The coil was brought up to 580± 3.9°C (1076°F±7°F) over a 12 hour period and held
at that temperature for 16 hours. Thereafter, the coils were allowed to cool in the
furnace to 93°C (200°F) over a 32 hour period. The cooled coils were removed from
the furnace and further allowed to cool to room temperature over the next 48 hours.
[0108] The room temperature cooled coils were subjected to a first cold roll/recovery temperature
treatment wherein the cooled coils were rolled in successive passes using commercial
rolling equipment until each of the coils was reduced to varying degrees of thickness
varying from 83 to 85% (0.132 to 0.15 cm) (0.052 to 0.059 inches).
[0109] The reduced thickness coils were subjected to a first recovery temperature wherein
the coils were placed in a furnace and heated to 232±1.7°C (450°F±3°F) over a 4 hour
period and held at this temperature for 4 hours whereupon the coils were allowed to
cool in the furnace to 149°C (300°F) over a period of nine hours. The coils were removed
from the furnace and allowed to cool to room temperature over the next 48 hours.
[0110] After being subjected to the first cold roll/recovery temperature treatment, the
coils were subjected to a second cold roll reduction by being passed successively
through a pair of reduction rolls until each of the coils was reduced 25% in thickness
0.099 to 0.112 cm (0.039 to 0.044 inches).
[0111] After the second cold roll reduction, the coils were placed back in the furnace and
subjected to a second recovery heating by raising the temperature of the furnace to
260°C (500°F) over a 3.5 hour period, and holding at that temperature for 1.5 hours.
The coils were annealed at a recrystallization temperature by raising the temperature
of the furnace to 427°C (800°F) over a 6 hour period and held at this temperature
for 3 hours. The coils were allowed to cool in the furnace to 149°C (300°F) over a
14 hour period and then removed from the furnace and allowed to cool to room temperature
over the next 48 hours.
[0112] The recrystallized coils were then work hardened by passing the coils successively
in a commercial rolling mill until the coil was reduced 65 to 67% in thickness to
0.0343 cm (0.0135 inches).
[0113] The work hardened coils were then fabricated into two-piece aluminium beverage cans
on a commercial drawn and wall ironing manufacturing line, about 5000 cans being fabricated
from each coil. No galling was encountered. Earing ranged from 2.0 to 2.6%.
[0114] The cans were also evaluated for buckle strength, i.e., ability of the can to withstand
high internal pressure without buckling.
[0115] Buckle strength is determined by applying pressure within a drawn and wall-ironed
can and then gradually increasing the pressure until the bottom end of the can deforms
and bulges out, i.e. it buckles. The pressure at which the bottom buckles is then
designated as the buckle strength. To be acceptable as can body stock, a can formed
from the alloy sheet must exhibit a buckle strength of at least 620.1 x 1 03 N/m
2 (90 pounds per square inch (psi)).
[0116] The average buckle strength for cans fabricated from alloys A, B and C in the above
manner are recorded in the Table XXIV below:
1. A process for fabricating aluminium alloy strip stock suitable for the manufacture
of drawn and wall-ironed articles, the process comprising the steps of: continuously
casting an aluminium alloy in strip form; homogenizing the alloy strip; and then cold
rolling the homogenized strip; characterized in that: the continuous cast strip has
a thickness of up to 2.54 cm (one inch); said homogenizing step is carried out at
a temperature of 510 to 621°C (950 to 1150°F) for up to 50 hours; said cold rolling
step is carried out to yield at least a 25% reduction in thickness; and thereafter
heating the cold rolled strip to a recovery temperature of from 177 to 288°C (350
to 550°F); cold rolling the strip to a second reduction in thickness of at least 10%;
heating the cold rolled strip to a recrystallization temperature of from 316 to 482°C
(600 to 900°F); and then cold rolling the recrystallized strip to a final gauge having
a total reduction in thickness of at least about 50%.
2. A process according to claim 1, characterized in that the continuous cast aluminium
strip has a thickness of from 0.635 to 1.27 cm (0.25 to 0.50 inch).
3. A process according to claim 1, characterized in that the strip is homogenized
at a temperature from 537 to 593°C (1000 to 1100°F) for from 10 to 25 hours.
4. A process according to claim 1, characterized in that the first cold roll reduction
effects a reduction in thickness of from 50 to 75%.
5. A process according to claim 1, characterized in that the strip is heated at a
recovery temperature from 204 to 246°C (400 to 475°F) for from 2 to 6 hours.
6. A process according to claim 1, characterized in that the strip is heated at a
recovery temperature of from 218 to 246°C (425 to 475°F) for from 2 to 4 hours.
7. A process according to claim 1, characterized in that the cold rolled strip is
heated to a recrystallization temperature of from 371 to 454°C (700 to 850°F) for
from 2 to 3 hours.
8. A process according to claim 1, characterized in that the second cold roll reduction
effects a reduction in thickness of from 10 to 25%.
9. A process according to claim 1, characterized in that the strip is heated to a
second recovery temperature after the second cold roll and prior to heating the strip
to the recrystallization temperature, the second recovery temperature being in the
range of from 232 to 288°C (450 to 550°F), the heating being effected for from 0.5
to 3 hours.
10. A process according to claim 1, characterized in that the recrystallized strip
is cold rolled to a final gauge having a total reduction in thickness of from 85 to
90%.
11. A process according to claim 1, characterized in that the aluminium alloy is Aluminium
Association Specification 3004 aluminium alloy.
12. A process according to claim 1, characterized in that the aluminium alloy includes
from 0.5 to 1.5% by weight magnesium, from 0.5 to 1.5% by weight manganese, from 0.1
to 1.0% by weight iron, from 0.1 to 0.5% by weight silicon, from 0.0 to 0.25% by weight
zinc, from 0.0 to 0.25% by weight copper and from 0.10 to 0.4% by weight chromium.
13. A process according to claim 1, characterized in that the cold rolled strip is
heated prior to recrystallization to a recovery temperature of from 177 to 288°C (350
to 550°F) for at least 2 hours and then cold rolled to a second reduction in thickness
of at least 10%.
14. A process according to claim 1, characterized in that the first cold roll reduction
effects a reduction in thickness of from 50 to 85%.
15. A process according to claim 1, characterized in that the second cold roll reduction
effects a reduction in thickness of from 10 to 50%.
16. A process according to claim 1, characterized in that the recrystallized strip
is cold rolled to a final gauge having a total reduction in thickness of from 50 to
90%.
17. A process according to claim 12, characterized in that the chromium incorporated
in the alloy 3004 is in the range of from 0.11 to from 0.25% by weight.
1. Verfahren zur Herstellung von Streifenmaterial aus Aluminiumlegierung, das sich
für die Fertigung von Gegenständen im Abstrecktiefziehverfahren eignet, wobei das
Verfahren folgende Schritte umfaßt: Stranggießen einer Aluminiumlegierung in Streifenform;
Diffusionsglühen des Legierungsstreifens; und anschließend Kaltwalzen des diffusionsgeglühten
Streifens; dadurch gekennzeichnet, daß der stranggegossene Streifen eine Dicke von
bis zu 2,54 cm (1 Zoll) aufweist; daß der Schritt des Diffusionsglühens bei einer
Temperatur von 510-621°C (950-1150°F) über einen Zeitraum bis zu 50 Stunden durchgeführt
wird; daß der Schritt des Kaltwalzens derart ausgeführt wird, daß sich eine Reduzierung
der Dicke um mindstens 25% ergibt; und daß daraufhin der kaltgewalzte Streifen auf
eine Erholungsglühtemperatur von 177-288°C (350-550°F) erhitzt wird; daß der Streifen
bis zu einer zweiten Reduzierung der Dicke um mindestens 10% kaltgewalzt wird, daß
der kaltgewalzte Streifen auf eine Rekristallisationstemperatur von 316-482°C (600-900°F)
erhitzt wird; und daß sodann der rekristallisierte Streifen auf ein Endmaß mit einer
Gesamtreduzierung der Dicke von mindestens ca. 50% kaltgewalzt wird.
2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß der stranggegossene Aluminiumstreifen
eine Dicke von 0,635-1,27 cm (0,25-0,50 Zoll) aufweist.
3. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß der Streifen bei einer Temperatur
von 537-593°C (1000-1100°F) zwischen 10 und 25 Stunden diffusionsgeglüht wird.
4. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß der erste Kaltwalzvorgang
eine Reduzierung der Dicke von 50-75% bewirkt.
5. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß der Streifen auf eine Erholungsglühtemperatur
von 204-246°C (400-475°F) zwischen 2 und 6 Stunden erhitzt wird.
6. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß der Streifen auf eine Erholungsglühtemperatur
von 218-246°C (425―475°F) zwischen 2 und 4 Stunden erhitzt wird.
7. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß der kaltgewalzte Streifen
auf eine Rekristallisationstemperatur von 371-454°C (700-8500F) zwischen 2 und 3 Stunden erhitzt wird.
8. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß der zweite Kaltwalzvorgang
eine Reduzierung der Dicke von 10-25% bewirkt.
9. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß der Streifen auf eine zweite
Erholungsglühtemperatur nach dem zweiten Kaltwalzvorgang und vor dem Erhitzen des
Streifens auf die Rekristallisationstemperatur erhitzt wird, wobei die zweite Erholungsglühtemperatur
im Bereich zwischen 232 und 288°C (450 und 550°F) liegt und die Erhitzung zwischen
0,5 und 3 Stunden lang durchgeführt wird.
10. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß der rekristallisierte Streifen
auf ein Endmaß mit einer Gesamtreduzierung der Dicke von 85-90% kaltgewalzt wird.
11. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß die Aluminiumlegierung
eine Legierung gemäß der Aluminium-Association-Norm 3004 ist.
12. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß die Aluminiumlegierung
im Gewichtsanteil von 0,5 bis 1,5 Magnesium, von 0,5 bis 1,5% Mangan, von 0,1 bis
1,0% Eisen, von 0,1 bis 0,5% Silizium, von 0,0 bis 0,25% Zink, von 0,0 bis 0,25% Kupfer
und von 0,10 bis 0,4% Chrom enthält.
13. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß der kaltgewalzte Streifen
vor dem Rekristallisationsvorgang auf eine Erholungsglühtemperatur von 177 bis 288°C
(350 bis 550°F) über einen Zeitraum von mindestens 2 Stunden erhitzt und sodann bis
zu einer zweiten Reduzierung der Dicke von mindestens 10% kaltgewalzt wird.
14. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß der erste Kaltwalzvorgang
eine Reduzierung der Dicke von 50 bis 85% bewirkt.
15. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß der zweite Kaltwalzvorgang
eine Reduzierung der Dicke von 10 bis 50% bewirkt.
16. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß der rekristallisierte Streifen
auf ein Endmaß mit einer Gesamtreduzierung der Dicke von 50 bis 90% kaltgewalzt wird.
17. Verfahren nach Anspruch 12, dadurch gekennzeichnet, daß das in die Legierung 3004
eingebrachte Chrom einen Gewichtsanteil von 0,11 bis 0,25% cinnimmt.
1. Procédé pour la fabrication d'une bande en un alliage d'aluminium, convenant à
la fabrication d'articles étirés et à parois étirées, procédé comprenant les étapes
consistant: à couler en continu un alliage d'aluminium sous forme de bande; à homogénéiser
la bande d'alliage; puis à laminer à froid la bande homogénéisée; caractérisé en ce
que la bande ayant subi la coulée continue a une épaisseur jusqu'à 2,54 cm (un pouce);
que ladite étape d'homogénéisation est mise en oeuvre à une température de 510 à 621°C
(950 à 1150°F) pendant jusqu'à 50 heures; que ladite étape de laminage à froid est
mise en oeuvre pour donner une réduction d'au moins 25% de l'épaisseur; les opérations
ultérieures consistant à chauffer la bande laminée à froid jusqu'à une température
de restauration de 177 à 288°C (350 à 550°F), à laminer la bande à froid jusqu'à une
deuxième réduction d'épaisseur d'au moins 10%; à chauffer la bande laminée à froid
à une température de recristallisation de 316 à 482°C (600 à 900°F), puis à laminer
à froid la bande recristallisée jusqu'à une jauge finale correspondant à une réduction
totale d'épaisseur d'au moins environ 50%.
2. Procédé selon la revendication 1, caractérisé en ce que la bande d'aluminium ayant
subi une coulée continue a une épaisseur de 0,635 à 1,27 cm (0,25 à 0,50 pouce).
3. Procédé selon la revendication 1, caractérisé en ce que la bande est homogénéisée
à une température de 537 à 593°C (1000 à 1100°F) pendant 10 à 25 heures.
4. Procédé selon la revendication 1, caractérisé en ce que la première réduction par
laminage à froid réalise une réduction d'épaisseur de 50 à 75%.
5. Procédé selon la revendication 1, caractérisé en ce que le bande est chauffée à
une température de restauration de 204 à 246°C (400 à 475°F) pendant 2 à 6 heures.
6. Procédé selon la revendication 1, caractérisé en ce que la bande est chauffée à
une température de restauration de 218 à-246°C (425 à 475°F) pendant 2 à 4 heures.
7. Procédé selon la revendication 1, caractérisé en ce que la bande laminée à froid
est chauffée à une température de recristallisation de 371 à 454°C (700 à 850°F) pendant
2 à 3 heures.
8. Procédé selon la revendication 1, caractérisé en ce que la deuxième réduction par
laminage à froid réalise une réduction d'épaisseur de 10 à 25%.
9. Procédé selon la revendication 1, caractérisé en ce que la bande est chauffée à
une deuxième température de restauration après le deuxième laminage à froid et avant
chauffage de la bande à la température de recristallisation, la deuxième température
de restauration étant dans l'intervalle de 232 à 288°C (450 à 550°F), le chauffage
étant mis en oeuvre pendant 0,5 à 3 heures.
10. Procédé selon la revendication 1, caractérisé en ce que la bande recristallisée
est laminée à froid jusqu'à une jauge finale correspondant à une réduction totale
d'épaisseur de 85 à 90%.
11. Procédé selon la revendication 1, caractérisé en ce que l'alliage d'aluminium
est l'alliage d'aluminium 3004 selon l'Aluminium Association Specification.
12. Procédé selon la revendication 1, caractérisé en ce que l'alliage d'aluminium
comprend de 0,5 à 1,5% en poids de magnésium, de 0,5 à 1,5% en poids de manganèse,
de 0,1 à 1,0% en poids de fer, de 0,1 à 0,5% en poids de silicium, de 0,0 à 0,25%
en poids de zinc, de 0,0 à 0,25% en poids de cuivre et de 0,10 à 0,4% en poids de
chrome.
13. Procédé selon la revendication 1, caractérisé en ce que la bande laminée à froid
est chauffée avant recristallisation à une température de restauration de 177 à 288°C
(350 à 550°F) pendant au moins 2 heures, puis laminée à froid jusqu'à une deuxième
réduction d'épaisseur d'au moins 10%.
14. Procédé selon la revendication 1, caractérisé en ce que la première réduction
par laminage à froid réalise une réduction d'épaisseur de 50 à 85%.
15. Procédé selon la revendication 1, caractérisé en ce que la deuxième réduction
par laminage à froid réalise une réduction d'épaisseur de 10 à 50%.
16. Procédé selon la revendication 1, caractérisé en ce que la bande recristallisée
est laminée à froid jusqu'à une jauge finale correspondant à une réduction totale
d'épaisseur de 50 à 90%.
17. Procédé selon la revendication 12, caractérisé en ce que le chrome incorporé dans
l'alliage 3004 y est contenu en une quantité de 0,11 à 0,25% en poids.