Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~5~32~L
. ~
Aluminum alloy can stock and process of manufacture
This invention relates to a process for making
aluminum alloy can stock, viz., aluminum alloy sheet for
forming one-piece drawn and ironed can bodies, and to the
product of such process.
Present-day metal cans as used for beverages such
as soft drinks, beer and the like are commonly constituted
of a seamless one-piece body (which includes the bottom end
and cylindrical side wall of the can) and a top end bearing
a ring or other opening device. The body is produced from
a blank of cold-rolled aluminum alloy sheet (having a gauge,
for example, of about 0.014 inch) by a now-conventional
forming technique known as drawing and ironing, which involves
drawing the blank into a cup and then passing it through a
succession of dies to achieve the desired elongated
cylindrical body configuration, with a side wall of reduced
thickness relative to the bottom end. The top end is
separately produced from another sheet aluminum aIloy blank,
by different but also conventional forming opexations, and
is secured around its circumference to the top edge of the
side wall of the body to provide a complete can.
The severity of the forming procedure employed
in producing a drawn-and-ironed can body as described above,
and in particular the reduction in thickness of the can side
wall (which must nevertheless be able to withstand the
internal and external forces exerted on it in use), as well
~3S8~
...--
as the fact that the formed can is usually lacquered in an
operation necessitating a strength-reducing exposure to heat,
require a special combination of strength, formability, and
tool wear properties in the alloy sheet from which the can
body is made. Significant among these properties are
ultimate tensile strength, yield strength, elongation, and
earing. Attainment of the requisite combination of
properties is dependent on alloy composition and on the
processing conditions used to produce the sheet.
Heretofore, a conventional sheet for can body
blanks has been constituted of the alloy having the
Aluminum Association (AA) designation 3004, and has been
produced from conventionally direct-chill-cast ingot up to
24 inches thick by scalping and homogenizing the ingot, and
successively hot rolling and cold rolling to the desired
final gauge; often an anneal treatment is used between the
hot and cold rolling operations, with the annealing gauge
so selected that the amount of cold reduction to final gauge
; after annealing is about 85%, thereby to provide can body
blanks in H19 (extra hard) temper. Copending Canadian
Patent ~pplication Serial No 390,957, filed November 26, 1981,
by Paul W. Jeffrey (applicant herein) and John C. Blade
for Aluminum Alloy Can Stock and assigned to the same
assignee as the present application, describes can body
stock comprising aluminum alloy sheet at an intermediate
temper and directly formable by drawing and ironing into a
one-piece can body, containing 0.45 - 0.3~ Mn and 1.5 -
2.2% Mg, with the following properties: ultimate tensile
strength, at least about 38 thousand pounds/in.2 (k.p.s.i.);
yield strength, at least about 35 k.p.s.i.; elongation, at
least about 1%; earing, not more than about 4%. It will be
understood that all composition percentages above and else-
where herein are expressed as percentages by weight.
It would be desirable to utilize, e.y. in the
manufacture of can body stock, so-called continuous strip
easting techniques in place of eonventional direet-chill
easting of relatively thick ingots. Continuous strip
casting is performed by supplying molten metal to a cavity
defined between chilled, moving casting surfaces such as
S8%~
" ~
substantially parallel, extended plan~r runs of a pair of
chilled endless metal belts, thereby to produce a thin
(typically less than one inch thick) continuous cast strip.
Belt-casting apparatus for such casting of strip is
described, for example, in U.S. patents Nos. 4,061,177 and
4,061,178. Advantages of continuous strip casting (as
compared with direct chill casting of thick ingots) for
production of sheet aluminum alloy products include enhanced
efficiency and economy, especially in that the -thinness of
the as-cast strip significantly lessens the extent to which
the cast body must be reduced by rolling to a desired sheet
gauge. Heretofore, however, it has not be feasible to
produce sheet for one-piece can bodies from belt-cast strip
because AA 3004 alloy rolled from such strip to provide sheet
of can body stock gauge at Hl~ ~emper does not possess
satisfactory properties for commercial drawing and ironing
into one-piece can bodies, owing to differences in work-
hardening rate, earing, and required annealing temperature
between strip-cast and direct chill-cast AA 3004 products.
U.S. patents No. 4,235,646 and No. 4,238,248
describe procedures for producing can body stock of various
aluminum alloys from strip continuously cast in a casting
machine, preferably of the type having a plurality of
continuously moving chilling blocks arranged in two sets
rotating in opposite senses to form a casting cavity to
which the aluminum alloy is supplied for solidification in
contact with the blocks. In these procedures, the cast
strip is subjected to a holding period at elevated temperature
before hot rolling. The patents further describe the cast
3a strip as having a cell size or dendritic arm spacing
preferably of about 5 - 15 microns in the region of the strip
surface and preferably of about 50 - 80 microns in the
center of the strip thickness. After the holding period,
the strip is initially reduced by hot rolling under conditions
su~h that the temperature of the strip at the end of the hot
rolling step is at least 280C, and is then further reduced
to can stock gauge by cold rolling.
~9S82~
Summary oE the Invention
The present invention is directed to improvements
in a process for making can stock comprisiny cold-rolled
sheet of an aluminum alloy consisting essenkially of 0.45 -
0.8% Mn, 1.1 - 2.2% Mg, 0.3 - 1.2~ Fe, 0.1 - 0.50% Si, up
to 0.05% Ti, up to 0.15~ each of Cu and Cr, other elements
up to 0.05% each and up to 0.1% total, balance Al, the
combined content of Mn -t Mg bein~ more than 1.9% and the
combined content of Fe ~ Mn ~ Mg being at least about 2.5%,
such process including the steps of continuously strip-
casting an ingot of the allo~ and rolling the ingot to
produce the sheet. In particular, the invention contemplates
improvements in this process which comprise performing the
step of casting the ingot by continuously supplying the alloy
in molten state to a casting space defined between facing
extended planar s~lrfaces of a pair of chilled, thermally
conductive endless belts continuously moving so as to
advance the supplied allo~ through the casting space as a
solidifying strip ingot in extended contact with the belt
surfaces while maintaining a heat flux of at least about
40 cal./cm2/sec. through the belts, thereby to achieve
controlled, rapid solidification, for producing an ingot
which becomes fully solidified while in contact with the belt
surfaces within the casting space and which as cast has a
cell size of about 10 - 15 microns at its surfaces and of
about 23 - 30 microns at the center of its thickness.
~ he rolling step includes at least a final cold-
rolling operation for producing cold-rolled aluminum alloy
sheet directly formable, by drawing and ironing, into a one-
piece can body. As used herein~ the term "directly formable"means sheet characterized by a gauge and properties such that
it can be cut into blanks and drawn and ironed without any
further reduction or thermal treatment. Further in
accordance with the invention, the conditions of the afore-
mentioned casting step are such as to provide, in the finalcold-rolled sheet, a constituent particle size o~ not more
than about 2 microns at its surfaces and not more than about
4 microns at the center of its thickness.
~ ~9~8~
Preferably, in the castiny step, the heat flux
through the belts is between about ~0 and ~bout 90 cal./cm. /
sec. It is especially preferred to perform the casting
step in a twin-belt casting machine of the type shown and
described in the aEorementioned U.S. patents No. 4,061,177
and No. ~,06],178, wherein the casting space is defined
between runs of the bel-ts each having a surface facing away
from the casting space, and wherein the castiny step
comprises chilling the belts by direct impingement of coolant
on the last-mentioned surfaces of the belt runs.
In a further aspect, the invention additionally
embraces can stock produced by the foregoing process.
The production of sheet in accordance with the
invention provides can stock that is fully satisfactory for
use in making drawn and ironed can bodies, and realizes the
benefits of continuous strip casting, indeed with special
advantages. In particular, the defined casting step
affords features of microstructure including a difference
between center and surface cell size of beneficially reduced
magnitude in the as-cast strip, and an advantageously smaller
constituent size in the center of the final cold-rolled sheet~
i.e. as compared to the microstructure attained with
previously known techniques utilizing strip casting in the
manufactur~ of can body stock. These features of micro-
structure are desirable from the standpoint of product
properties, and, very importantly, they enable can body
stock to be produced from continuously cast strip without
the necessity of providing special temperature conditions
after the casting and/or hot-rolling steps.
Further features and advantages of the invention
will be apparent from the detailed description hereinbelow
set forth, together with the accompanying drawing.
~rief Description of the Drawing
The single figure is a simplified side elevational
view of casting apparatus suitable for use in the practice
of the process of the present invention.
S~32~
Detailed Description
Referring to the drawing, there is shown a twin-
belt casting machine 10 o~ the type described in the afore-
mentioned U.S. patents No. 4,061,177 and No. 4,0~1,178 for
casting a more or less wide continuous strip of an aluminum
alloy. This machine includes a pair of resiliently flexible,
heat-conducting endless belts 20 and 21, e.g. metal bel-ts,
arranged to be continuously drawn (in ro-tational senses
opposite to each other) through a region in which they have
runs substantially parallel to each other, with some degree
of convergence. The runs of the two belts in the last-
mentioned region have facing, extended planar surfaces which
cooperatively define a casting space 22. Molten metal is
continuously supplied into this casting space while the
last-mentioned runs of the belts are chilled at their
reverse faces, i.e. the surfaces facing away from the casting
space, by direct impingement of coolant liquid on the latter
belt surfaces.
In the illustrated apparatus, the path of the
metal being cast is substantially horizontal with a small
degree of downward slope from entrance to exit of the casting
space. Thus the upper and lower endless belts 20 and 21 are
arranged so that their facing runs are substantially parallel
to each other through the region where they define the
casting space 22 from its entrance 24 to its exit 26, the
belts being guided through looped return paths between the
localities 26 and 24. Suitable means (including a driving
pulley 28 for the upper belt and a similar driving pulley,
not shown, for the lower belt) are provided for continuously
advancing both belts. The path of metal through the casting
apparatus is indicated by arrows 40. The belts themselves
are constructed in appropriate manner for casting apparatus
of this type, being advantageously o~ metal, e.g. suitably
flexible but stiffly resilient steel of appropriately high
strength and of such nature that it can be sufficiently
tensioned without inelastic yield. The apparatus as shown
includes fluid cylinder means for positionally adjusting
~l~S82~:'
the shafts ~8 and 50 of the driving pulleys, such means being
indicated at 58.
Molten metal is supplied to the casting space 22
by a suitable launder or trough (not shown) which is
5 disposed at the entrance end 24 of the casting space. As is
usual in belt-casting machines, the apparatus is provided
with edge dams (not shown), e.g. of conventional character,
at each side so as -to complete the enclosure of the cas-ting
space 22 at its edges. Suitable means are provided for
cooling and supporting the belts 20 and 21 along the length
of the casting space 22, such means being represented
schematically at 80 and including nozzles or the like (not
shown) for directing coolant water over the surfaces of the
belts facing away from the casting space, all as fully
described in the aEorementioned U.S. patents No. 4,061,177
and No. 4,061,178. It will be understood that in the
operation of the appara-tus, molten metal supplied to the
aforementioned inlet launder feeds against the belts 20 and
21 converging in their curved paths to the casting space
entrance 24; the metal enters that space as a substantially
parallel-faced liquid body, and in its advance through the
casting space 22 to the exit end 26 (such advance being
effeeted by the eontinuous motion of the belts), the metal
being cast becomes progressively solidified from its upper
and lower faces inward, heat from the metal belng trans-
ferred through the belts and removed therefrom by the coolant
supplied by means &0; throughout the extended length of the
casting zone, the metal being cast is in contact with the
surfaces of the belts and becomes fully solidified before
reaching the exit end of the casting space~ emerging from the
exit end as a continuous, solid, cast strip.
In currently preferred embodiments of the present
invention, the casting step is performed in a casting
machine of the above-described type, having the further
features set forth in more detail in U.S. patents No. 4,061,177
and No. 4,061,173, such apparatus being found to be
especially effective in achieving satisfactory performance
of the special casting step of this process.
~9S8Z~
. ~
Within the bro~d limits of composition hereinabove
set forth, preferred alloys ~or the practice of the
invention include those described in the aforementioned
copending Canadian patent application Serial No. 390,957,
and, in particular, an alloy composition consisting
essentially of 0.5 - 0.8% Mn, 1.5 - 2.2% Mg., 0.1 - 0.50% Si,
0.3 - 1.0% Fe, up to 0.15% Cu, 0.015 - 0.025~ Ti, o-ther
elements less than 0.05% each, balance Al, with a combined
content of Mn and Mg of not less than about 2.2%. A
presently especially preferred composition consists
essentially of the following:
Range or Maximum (%~ Nominal (%)
Mn 0.65 - 0~75 0.70
Mg 1.70 - 1.90 1.80
Si 0.12 - 0.18 0.15
Fe 0.45 - 0.60 0.50
Cu 0.06 - 0.10 0.08
Ti 0.015 - 0.025 0.020
; other elements
(total) 0.10
Al balance
Further in accordance with presently preferred
practice, to produce can body stock with the process of
the invention, an alloy having a composition as just
described is prepared, and suitably degassed and filtered
to ensure a high quality of metal being supplied to the
; casting machine. This alloy is continuously cast into
strip having a thickness of 1/2 inch in a twin-belt casting
machine of the above-described type using steel casting
belts e.g. 0.040 inch thick, so arranged that their surfaces
defining the casting space converge 0.010 inch over a
casting space length of 40 inches. The belts have surfaces
shot-blasted to a roughness of 210 microinches RMS and
subsequently brushed with a sillcon-carbide-loaded brush
for 14 - 20 belt revolutions, i.e. prior to casting. To
these belts there is applied a parting layer comprising
polybutenes with 25% lecithin and sufficient freon to make
~5l3~
the parting layer composition satisfactori~y sprayable. The
amount of parting layer used is on the order of 0.1 to
0.2 mg/cm2 of belt area as a precoat with uniEorm recoating
provided by continuous spraying onto the belts during
casting; the amount of respray is only a fraction of the
precoat, and is adjusted -to maintain a minimum heat flux
through the belts of 40 - 60 cal./cm.2/sec. Preferably, the
heat flux is maintained at a value of 70 - 80 cal./crn. /sec.
To avoid belt distortion, the heat flux is kept below an upper
limit of 85 - 90 cal./cm.2/sec. During casting, the belts
are brushed continuously with ro-tating brushes to maintain
uniformity of oil distribution over the belt surface.
The casting speed, for a 1/2-inch-thick ~ngot, is
preferably in a range of 20 - 30 feet per minu-te and is
adjusted as indicated by ingot surface appearance and to
achieve a desired ingot average exit temperature from the
casting machine. An exit temperature of 450 - 480C is
preferred.
The as-cast ingot is fed directly from the casting
machine into a hot rolling mill at an ingoing temperature
of between 3~0 and 450C; it is typically subjected to a
total hot reduction of about 72 to about 82%, leaving the
hot mill at an exit temperature of about 150 - 200C, and
is then coiled.
Thereafter, the hot-rolled coil (herein termed
"reroll") is cold rolled to a final can body stock gauge,
e.g. a final gauge of 0.013 - 0.015 inch, with an anneal
performed at a gauge such that the amount of cold reduction
after annealing (i.e. to reduce the coil from the annealing
gauge to the final can body stock gauge)is between 40 and 65go
using a batch anneal or 30 - 65g6 using a flash anneal,
thereby to provide can body stock at an intermediate temper.
In a typical e~ample of half-inch cast strip hot-rolled to
a gauge of 0.090 inch, the reroll is reduced from the latter
gauge to 0.040 inch in an initial cold-rolling operation,
then batch-annealed for two hours at 400 - 420C, and then
further cold rolled to a final gauge of 0.015 inch.
-- 10 --
The can body stock thus prod~ced can be cut into
suitable blanks and forrned directly, by drawing and ironing,
into one-piece can bodies. Properties of the can body stock
i.e. in final cold-rolled gauge, include an ultimate tensile
strength of at l~ast about 38 k.p.s.i. (but no-t more than
about 45 k.p.s.i.), yield stxength of at least about 35
k.p.s.i. (but not more than about ~4 k.p.s.i.), at least
about 1% elongation, and not more than about ~% earing.
Referriny further to the above-described strip-
casting step of the present process, a relatively highheat transfer rate for the duration of the solidification
is achieved in the specified casting machine by maintenance
of a high heat transfer rate to the coolant water through
the use of thin steel belts and high water velocities against
the reverse surfaces of the belts, together with the
convergence of the casting surfaces of the belts which
assures good contact of the belts with the solidifying strip
ingot throughout the solidification interval. This desired
high heat transfer rate is controlled at the belt/ingot
interfaces by the use of a liquid oil formulation and a
randomly rough controlled texture of the belt surfaces.
Uniformity of oil (parting layer) over the belt
surfaces is extremely important to satisfactory performance
of the casting step. Spray guns, reciprocating at
controlled speeds and synchronized with the belt motion, are
currently preferred for application of the parting layer to
provide the requisite macro-uniformity of the parting layer,
while the aforementioned rotating brushes mainkain micro-
uniformity of parting layer distribution. Thereby, there is
achieved a desired consistency of metallurgical ingot quality
during continuous strip casting. Deviations of heat fluxes
from area to area can lead to some variations of ingot
structure and surface blemishes as the slower solidifying
areas exhibit coarser cell size, grain size, and porosity;
the porosity results from feed metal being drawn from these
areas as a result of shrinkage contraction in higher heat
transfer areas, and must be avoided. The described
controlled but high heat transfer rates, and controlled belt
s~
proximity and flatness relatlve to the solidifying strip
ingot surface, minimiz~ the coarsening tendencies of ingot
structure from ingo-t surface -to ingot center.
This minimization of coarsening tendencies is
evident from a comparison of measured dendritic cell size
(or dendrite arm spacing) in -the as~cast strip ingot of the
present process, with known or reported values for
conventional, 20-inch-thick direct-chill-cast ingot and
strip ingot produced on a block casting machine:
Average Dendrite Arm Spacing tmicrons)
Ingot Surface Ingot Center
Conventional D.C. Ingot 30 70 or more
Present Ingot 10 - 15 23 - 30
Block-Caster Ingot 5 - 15 50 - 80
(preferred) (preferred)
and, further, f.rom measurements of the variation of dendritic
cell size through the thickness of a l/2-inch-thick as-cast
strip ingot of the above-described preferred alloy, cast in
accordance with the casting step of the present process, viz:
20Distance from Ingot Average Dendritic
Top Surface Cell Size (microns)
(% of thickness)
0~5 11
13.0 19
2535.0 17
~9.0 24
The fine cell size of the alpha aluminum phase, attained in
the practice of the present invention, r~sults in a fine
distribution of soluble and insoluble eutectic phases, which
in turn provides advantageously fine constituent particle
sizes in the rolled sheet products, as compared with sheet
products obtained from conventional direct-chill-cast ingot:
Size of I.argest Constituent
Particles (microns)
35 Sheet Produced From: Sheet Surface Sheet Center
Conventional D.C. Ingot 10 25 - 30
Present Ingot 2 3 - 4
~S8~L
,..~
- 12 -
The very fine constituent size achieved with the present
process affords desi.rably improved formability and
mechanical properties.
By way of further illustration of the invention,
reference may be made to the following specific example:
wo alloys were prepared respectively having the
following percentaye contents of alloying elements (balance
essentially aluminum):
Alloy I Alloy II
Mn 1.20 0.66
Mg 0.99 1.60
Si 0.17 0.13
Fe 0.53 0.51
Cu 0.07 0.09
Ti 0.010 0.012
Alloy I was an AA 3004-type alloy, and alloy II had a
composition in accordance with the present invention.
Each alloy was continuously cast as 1/2-inch-thick
strip on a belt caster of the type referred to above, and
rolled to can body stock gauge. One coil of each alloy was
homogenized for 8 hours at 575C (at 0.090 inch gauge for
alloy I and at 0.060 inch gauge for alloy II) while another
coil of each alloy was simply annealed for 2 hours at 470C
(alloy I) or 440C (Alloy II).
Pertinent treatments and properties of the coils
of can body stock gauge sheet thus produced are as follows:
Longitudinal Tensile Properties
Ult.
Final Tensile Yield Buckle
Heat Cold Strength Strength Elongation 45 Earing Pressure**
Alloy Treatment* Work (%) (k.p.s.i.) (k.p.s.i.) (%) (%) (p.s.i.)
I A 63 41.0 39.4 2.3 3.5 92
H 83 42.7 41.9 1.8 3.7 96
II A 50 39.5 36.1 4.0 1.5 92
H 75 41.3 39.9 2.8 3.9 94
'~1
*A - annealed ~ -
H - homogenized
** - adjusted for gauge
~S~
-
- 14 -
About 60 one-piece can bodies were formed, by
drawing and ironing, from each coil, wlth no scoring
problems. The coil of alloy II with 50% reduction a~ter
annealing, demonstrated preferred properties, although its
yield strength was below that typically shown by conventional
can stock materials, the ~uckle pressure satisfactorily
exceeded the minimum standard of 90 p.s.i. generally
required by can manu~acturers.
The remaining three coils exhibited unduly high
earing in the drawing-and-ironing operation, as would be
expected from the earing levels recorded above.
The batch annealing temperature of 470C
required by Alloy I led to una~ceptably high levels of
oxidation and staining and the problem cannot be avoided by
flash-annealing at economically acceptable rates.
Although the annealing temperature of 440C applied
to Alloy II leads ~o barely acceptable levels of oxidation
and staining, it has been found possible to lower the
annealing temperature for Alloy II to 410-420C, at which
the staining and oxidation is greatly reduced without
adverse effects on the earing characteristics. Large scale
trials have been carried out successfully on sheet of a
composition similar to Alloy II (but having a Mg content of
1.8~) and annealed at 410 - 420C.
It is to be understood that the invention is not
limited to the features and em~odiments hereinabove specif-
ically set forth, but may be carried out in other ways with-
out departure from its spirit.
