Note: Descriptions are shown in the official language in which they were submitted.
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PRC)CESS FOR FABRICATING E~IGH STE~ENGTH ALIJMINUM S IEET
:BACKGROUND OF THEINVENTION
Field of Invention
This invention relates to a proce.ss for making
aluminum sheet of improved properties especially useful as
can stock and more particularly to a process for making
aluminum sheet of increased strength without sacrifice of
ductility properties.
Prlor Art
As used herein, the term "aluminum" includes commer-
cial grades of the metal itself such as 1100 (Aluminum Associa-
tion designation3 as well as aluminum base alloys containing
upwards of 90% aluminum by weight. Alloys which are especially
responsive to treatment in accordance with the invention
hereinafter disclosed include 1100, 3003, 3004 and 3009 and
other mangane~e-containing alloys with and without magnesium.
Aluminum beverage cans equipped with easy opening
ends have gained wide acceptance by consumers. To conserve
our natural resources, efforts are being made on a national
and international scale to recycle these cans.
In the manufacture of these cans, different alloys
are used for the can body and can end. Thus, alloy 3004
which is used for the can body is not suitable for the manu-
facture of the end which reguires high ductility for the
forming operations. The aluminum sheet used from which the
end is formed must be of high strength so that the end will
safely contain the pressurized contents of the cans. Alloy
.~,
335~
3004 which has a low magnesium content (1%) does not have the
necessary high yield strength in conjunction with high ductility
to be useful in the formation of can ends. Alloy 5182 which
has a relatively high magnesium content (4-5%) has the requisite
ductility and strength to be used as can ends. The typical
compositions of these alloys are recorded in the Table below:
Table
Metal Con-tent (~O)
Alloy Fe Si Cu Mn Mg Cr Zn Ti Al
3004 0.43 0.20 0.15 1.1 1.02 0.03 0.04 0.01 Balance
5182 0.35 0.20 0.15 0.4 4.5 0.10 0.25 0.10 Balance
The use, in the manufacture of easy opening cans,
of different aluminum alloys in the end and body stock,
substantially decreases the desirability of these cans as
recyclable scrap, as the remelted product is an alloy of
uncertain composition.
It would be highly advantageous if the tensile
strength of alloy 3004 could be raised to equal the levels of
alloy 5182 so that alloy 3004 could be used both as can body
stock as well as end stock. Aluminum sheet for use as can
body stock, e.g., alloy 3004 - ~19 temper, is produced from
ingot which is subjected to the following mill operations to
produce can body stock having a thickness of about 0.013
inches: casting, homogenizing, hot rolling, annealing and
cold rolling as, for example, disclosed in U.S. 3,802,931.
In the manufacture o~ aluminum sheet by DC (direct
chill) casting, an ingot having a thickness of about 16-24
inches is produced. The inyot is subjectecl to the homogeniz-
ing step whereby the ingot is heated at 950~1125F for 4-16
hours. Immediately after the homogenizing step the ingot is
subjected to hot rolling wherein the ingot is passed through
a series of breakclown rolls maintained at a temperature of
650-950F whereby the ingot is reduced in thickness to a
reroll gauge of about 0.130 inch. Thereater the reroll
.,
stock is subjected to an annealing step wherein the stock is
heated at 700-900F for 0.5-4 hours to effect recrystalliza-
tion of the metal structure.
The annealed reroll stock is then subjected to a
final work hardening step wherein the reroll stock is cold
rolled (room temperature rolling) to a final gauge of about
0.013 inch in a 5-stand mill or about 90% of its original
thickness to produce substantially full hard (Hl9) temper.
In rolling to Hl9 temper, the yield strength of the work
hardened aluminum sheet is increased from about 10 ksi (k=1000
pounds) to about 40-45 ksi, but the percent elongation (a
measure of ductility for can making) of the metal decreases
from about 25% to ahout 2%.
Vari~us attempts to improve -the yield strength of
work hardened aluminum sheet have involved varying the
rolling operations but such approaches have accomplished
increased yield strength at the expense of reduced tensile
elongation. Any reduction in tensile elongation below 1%
renders the aluminum sheet unsuitable for two-piece can
forming operations.
It would be highly advantageous to increase the
yield strength of work hardened low magnesium content aluminum
alloys such as alloy 3004 ~ Hl9 to the level of high magnesium
content aluminum alloys such as alloy 5182 without deleterious
effect on other metal physical propexties such as elongation.
Increasing the yield strength of alloy 3004-Hl9 to that of
alloy 5182 would enable the use o the low magnesium alloy
for can end stock with the resultant advantage that the
aluminum can ends would have substantially the same composi-
tion as the body and be highly desirable as a recyclable canproduct.
The prior art, U.S. Patent 3,787,248, teaches a
process for preparing high strength, improved formability
aluminum base alloys suitable for use as can end stock having
alloy compositions similar to aluminum can body material.
The process involves a heat treatment step after every rolling
reduction of 10 to 20% whereby the rolling process is inter~
~0:~359
rupted by a large number of heat treatment steps, a
procedure which is difficult to implement in industrial
practice.
The present invention replaces the multiple
heat treatment steps with a warm work hardening step
(rolling at a few hundred degrees above room temperature)
and results not only in process simplification but also
in greater improvement in strength.
SUMMARY OF THE I~VE~TIO~
In accordance with the present invention, a
process is provided of rolling aluminum strip to prepare
work hardened sheet for the manufacture of cans, whereby
the improvement comprises rolling the strip formed from
supersaturated aluminum duri~g work hardening at a
temperature of about 150 to about 450F whereby the
yield strength of the sheet is substantially increased.
This process is utilized to substantially increase the
tensile yield strength of work hardened aluminum alloys,
and particularly work hardened low magnesium content
aluminum alloys such as alloy 3004, without detriment to
other alloy physical properties.
By the practice of the present invention, the
yield strength of aluminum alloys conventionally used
for the fabrication of can bodies, e.g., alloy 3004, can
be raised to a level equaling that of the high strength
aluminum alloys, e~g., alloy 5182, the increase in yield
strength being accomplished without substantial decrease
in % elongation. As will hereinafter be further illus-
trated, by work hardening alloy 3004 to Hl9 temper at a
temperature between 150-400F, the yield strength of
the resultant alloy 3004 - Hl9 sheet is raised from 45
ksi to 65 ksi, a yield strength which is comparable to
the 58 ksi yield strength of alloy 5182 in the Hl9
temper.
The improved yield strength imparted to low
magnesium content aluminum sheet by the practice of the
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present invention enables the sheet to be used for the
fabrication of can ends. ~hus, by the practice of the
present invention, i-t is possible to manufacture alumi-
num beverage cans wherein the end and body portions are
composed of the same alloy material, e.g., alloy 3004.
The singular composition of the can components renders
the aluminum beverage cans a highly desirable scrap
item and thereby promotes the recycling of this
packaging product.
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As will hereinafter be also illustrated, the pro-
cess of the present invention does not materially affect the
ductility of the work hardened alloy, thereby permitting the
aluminum alloy to be fabricated into can ends using conven-
tional equipment and manufacturing procedures.
PREFERRED EMBOD I MENTS
The aluminum alloys work hardened by the process of
the present invention may be cast in any manner. The parti-
cular m~thod of casting is not critical and any commercial
method may be conveniently employed such as DC casting or
continuous strip casting.
Irrespective of the casting method, it has been
determined that the effect of the work hardening step of the
present invention is enhanced if the alloy is heat treated
prior to work hardening to bring any impurities in the alloy
in solid solution and retain these impurities in a super-
saturated state in the alloy. This may be accomplished by
heating the dead soft alloy at a temperature of 800-1100F
and thereafter guenching the heated strip to room tempex~ture
by rapid immersion in a suitable fluid, e.g., cold air,
water, to retain the dissolved impurities in the supersatur
ated state.
If the right t~pe of impurities (Mn, Fe, Ti, Cr, V)
and amounts of impurities are present in the alloy and super-
saturation has already been developed in the strip as it
exits from the hot mill, the heat treatment step to achieve
the supersaturated state just described can be avoided. For
the purposes of this application, an aluminum alloy is con-
sidered to be in the supersakurated state when the amount of
Mn in the aluminum alloy is at least 0.4% and the amount of
metals selected from the group of Fe, Ti, Cr and V are at
least 0.05%.
The work harde~ing step of the process of the
pr~sent invention is performed when the aluminum alloy has
been previously rolled or cast to a thickness of about 0.1~0.75
inch. In the DC process, the cast aluminum is hot rolled and
reduced in thickness from about 16-20 inches to about 0.1
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inch before it is worked hardened. In the continuous casting
process the strip is cast directly in-to a strip about 0.25-0.75
inch thick which is then homogenized prior to work hardening.
The specific temperature ,at which the aluminum
alloy is work hardened and rolled to a thickness of about
0.013 inch, the thickness re~uired for can stock, will vary
according to the specific alloy being work hardened. For
alloy 3004, optimum results are ohtained when the aluminum
strip is work hardened at about 275F. Generally the temper-
ature at which work hardening is effected in accordance with
the present invention will vary from about 150E to about
400F.
The selection of work hardening temperature will
also depend upan the rolling speed at which work hardening is
accomplished. Commercial work hardening rolling speeds vary
from 1000 to 5,000 feet per minute. In selecting the temper-
ature at which work hardening is to be accomplished, th~
temperature selected will vary with the rolling speed, i.e.,
at the higher levels of the rolling speed range temperatures
of about 250 to 400F are used and at lower rolling speeds
temperatures in the order of about 200 to 350F are used.
The aluminum alloys which can be work hardened in
accordance with the process of the present invention have the
following composition ranges:
METAL COMPOSITION RANGE
Mg 0 to 6%
~n 0 to 3%
Cu 0 to 5%
Fe 0 to 1%
Si 0 to 2%
Ti 0 to 1%
Zr 0 to 1%
Cr 0 to 1%
V 0 to 1%
Al Balance
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The following Examples illustrate the practice of
the present invention:
EXAMPLE I
.
A hot rolled strip (hot bcmd) 0.087 inch thick of
alloy 3004 was subjected to a work hardening s-tep in accor-
dance with the practice of the present invention.
As received from -the alumi.num mill the s-trip was
dead soft., i.e., zero temper, and had a yield strength of 11
ksi and an elongat.ion of 25~. The strip was heated at 875
10F and held for 5 minutes to dissolve impurities (Mn, Fe,
Ti, Cr~ in the alloy (Step A). After being heated in this
manner, the heated strip was rapidly ~uenched in water (Step
B) to trap the impurities in the supersaturated state. The
~uenched strip was heated and held at 275F ~or 5 minutes to
bring the strip up to the temperature at which it was to be
work hardened (Step C).
To work harden the strip in accordance with the
practice of the present invention, the strip at 275F was
passed successively at 7 ft./minute through a pair of reduc--
tion rolls heated to 275F until the strip was reduced 90% in
the thickness (Hl9 temper) to 0.009 inch. To effect this
reduction in thickness, five passes khrough the rolls at a
speed of 7 feet per minute were re~uired (Step D). After the
completion of each pass through the rolls, the strip was
heated to 275F and held ~or 30 seconds to reestablish the
temperature and to insure that the strip was at 275F at the
time it was fed into the .~olls (Step E). The temperature in
Steps C, D and E was controlled within ~ 5F.
The yield strength and ~ elongation of the work
hardened alloy was determined by performing standard 2 inch
gauge length tensile tests on samples of the work hardened
aluminum strip. The strength of the alloy as represented by
yield strength and the ductility as represented by % elonga-
tion are properties of the alloy essential for the manufacture
of can ends. The yield strength of the work hardened strip
was determined to be 65 ksi and the elongation 2%.
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The same aluminum strip treated in accordance with
Steps A and B, when cold-rolled to produce the conven-tional
3004 in Hl9 temper, exhibited a yield strength of 45 ksi and
an elongation of 2%. The currently~ used can end material
518~ in H19 temper has a yield strength of 58 ksi and 3%
elongation.
The processing history of the hotband material used
in Example I was as follows:
(1) homogenized at 950-1050 F for 1~ hours
(2) air cooled to a rolling temperature of 800-850 F
(3) hot rolled from 231~" to 0.187" gauge.
Microanalysis of the hotband revealed partitioning
of iron and manganese between the aluminum matrix and complex
intermetallics. The supersaturated aluminum matrix contained
0.58% Mn and 0.09% Fe in solution, the remaining Mn and Fe
were present as complex intermetallics (metal compound).
Titanium, chromium and vanadium were not detectable.
For purposes of contrast, an alloy of same composi-
tion (3004) was subjected to a different processing procedure
as follows:
(1) homogenized at 1070 1120 F for 10 hours
~ 2) furnace cooled at 100 F/hr. to rolling tempera-
tures o 800 F
(33 hot rolled from 23~2" to 0.110" gauge.
Microanalysis of the hotband revealed that the
aluminum matrix contained 0.33% Mn and undectable Fe (precipi-
tated out and was not supersaturated) Ti, Cr and V were also
undectable, and the remaining Mn and Fe were present in the
complex intermetallics. When subjected to the processing
steps of Example I, i.e., Steps, A, B, C, D and E, the alloy
exhibited no improvement in yield strength when subjected to
work hardening at between 250-300 F at rolling speeds between
7 to 250 ft./minute. Manipulation of Steps A and B also did
not effect an improvement in yield strength after work hardening,
demonstrating the need for the presence in the alloy of
certain supersaturated metal impurities before a work hardening
improvement in accordance with the process of the present
invention can be achieved.
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EXAMPLE II
The procedure of Example I was repeated with the
exception that the work hardening (Step D) was performed to
Hl~ temper (80% rolling reduction) at varying temperatures.
The results are summarized in the Table below:
TABLE
Work Hardening Yield Strength
Temperature ~F) .in H18 Temp~r (ksi)
75O 42
150 ~7
200 50
275 53
350 51
400 47
450 38
500 36
As is readily e~ident from the Table, the optimum
temperature of work hardening is 275F. At lower or higher
temperatures, the yield strength after work hardening is
lower~
EXAMPLE III
The procedure o~ Example I was repeated with the
exception that Steps A, B, C and E were not used in the work
hardening procedure, i.e., the strip as received from the
mill was work hardened to Hl9 temper by successive passage at
7 ft./minute through the reduction rolls heated to 275F
(Step D). The yield strength of the work hardened strip was
determined to be 60.8 ksi and the elongation was 2%.
EXAMPLE IV
The procedure of Example I was repeated with the
exception that Steps A and B were not used in the work harden-
ing procedure. The yield strength of the work hardened strip
was determined to be 58.1 ksi and the elongation was 2%.
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