Note: Descriptions are shown in the official language in which they were submitted.
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Method and installation for producing aluminum can sheet
TECHNICAL FIELD AND PRIOR ART
The present invention relates to a method for producing aluminum can sheet and
to an installation
configured to perform the method.
When aluminum can sheet is formed into cup-shaped articles, a phenomenon known
as "earing"
usually occurs to some extent. Earing can be observed as a wave-shaped
appearance around
the top edge of the formed cup. The wave-like protruding portions, also known
as "ears", are
formed during the deep drawing step in the fabrication of the cup and
represent an undesirable
feature of the article. In aluminum can body stock (CBS), the cup is
subsequently ironed in
multiple rings which can accentuate the wavy ears. High earing can create
transport problems
with the cup as well as insufficient trim after ironing, clipped ears, and
trimmer jams. These
artefacts are not desirable in aluminum can manufacturing. Thus, it is desired
to minimize earing
in order to avoid these problems and to increase the quality of the cup.
It is known that can body stock material such as AA3004, AA3104 or other
aluminum alloy is
basically suitable for making aluminum can sheet with low earing
characteristics provided that a
suitable manufacturing process can be established.
There is a well-known process established in the aluminum industry for
producing aluminum strip
suitable for can body stock. This process includes hot rolling of an aluminum
ingot through a
rougher mill and then through a multi stand hot rolling mill, usually exiting
at a high temperature
to ensure fully recrystallized material obtained through a self-anneal
process. This well-known
method produces a final product with low earing and desirable mechanical
characteristics.
However, the installation and operation of such a hot continuous mill requires
a major capital
expenditure.
Modifications have already been proposed in the past in order to produce can
sheet with
commercially acceptable earing characteristics from a single stand reversing
mill (see e.g. US
5,362,340 and US 5,362,341). According to the method of US 5,362,340 an
aluminum alloy ingot
is provided and is heated to a temperature between about 527 C to 571 C. After
this, the ingot is
hot rolled in a single-stand reversible hot mill to produce an intermediate
gauge sheet. The
intermediate gauge sheet, which is self-annealed or batch annealed, is then
cold rolled to produce
a final gauge aluminum can sheet having low earing characteristics. The
relative low temperature
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homogenization (527 C to 571 C) is applied in order to avoid uncontrolled
recrystallization during
hot rolling in the single stand reversing mill.
Patent application US 2002/0062889 Al discloses a process and a plant for
producing hot-rolled
aluminum strip for can making. The plant includes a reversing roughing stage
for the feed
material, which is used hot, and immediately thereafter finishing rolling of
the strip, which is
followed by heat treatment of the strip coiled up into coils. During the last
finishing rolling passes,
recrystallization in the rolled material is suppressed by means of controlled
temperature
management of the hot strip. In the embodiment temperature is maintained in
the noncritical
temperature range from 260' C. to 280' C to avoid recrystallization. The
recrystallization is brought
about only outside the rolling train. For this purpose, the hot rolled
material is transferred to a
continuous furnace directly following the finishing rolling. The direct
transfer brings about the
advantage that a furnace used for recrystallization only has to apply a
relatively small temperature
difference (e.g. about 40 C ¨ 60 C) between the rolling temperature and the
recrystallization
temperature, and thus achieves a favorable energy balance.
International patent application published as WO 2015/140833 Al discloses
aluminum alloy
sheets having a low earing rate suitable for making aluminum can bodies. The
alloys mentioned
for this purpose include type A3004 and A3104 alloys. A preferred process
includes the process
steps of casting the ingot, homogenizing the ingot, hot rolling, primary cold
rolling, intermediate
annealing, and secondary cold rolling. The hot rolling step is divided into
two separate steps,
namely "hot rough rolling step" and "hot finish rolling step". In the hot
finish rolling stage, the end
temperature is preferably between 330 C and 380 C. It is observed that the
driving force of
recrystallization is insufficient if the end temperature is less than 330 C.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method and an installation for
producing aluminum can
sheet suitable for making aluminum cans, wherein the aluminum sheet exhibits
favorable earing
characteristics after a deep drawing step and further allows producing stable
cans with thin wall
thickness.
This object is solved by a method comprising the features of claim 1 and an
installation comprising
the features of claim 9. Preferred embodiments are defined in the dependent
claims.
According to the method for producing aluminum can sheet, a body (also denoted
as ingot) made
of an aluminum alloy is provided. The aluminum alloy is selected so that it is
suitable for making
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aluminum can sheet. Specifically, the aluminum alloy is of type AA3004, AA3104
or other
aluminum alloy suitable for making aluminum can sheet, such as AA3204 alloy.
Typical requirements for aluminum alloys suitable for making aluminum can
sheet are described,
for example, in the article "AlMn1Mg1 for Beverage Cans" by J. Hirsch in:
"Virtual Fabrication of
Aluminium Products" Wiley-VCH 2006 (ISBN: 3-527-31363-X), chapter 1-4. In
general, the
material must provide an optimum combination of strength and sufficient
forming properties. For
aluminium (aluminum) strength is achieved by the combination of appropriate
alloy addition for
best solid solution hardening (e.g. by Mg and Mn) and pre-deformation (i.e.
highly rolled sheet).
Furthermore, strength must remain sufficiently high also after the subsequent
paint baking cycles.
Good formability is achieved by an optimum combination of alloy additions for
good work
hardening (Mg) with some particle strengthening effects (Mn). The latter also
maintains
homogeneous deformation and even provides a cleaning effect of the dies,
preventing harmful
oxide build up and galling. As a consequence, the common aluminium alloys used
for the
production of can bodies are AlMg1Mn1 = EN-AW 3004 and AlMg1Mn1(Cu) = EN-AW
3104,
which meet best the requirements for can strength and formability.
In preferred embodiments aluminum alloys comprising the following chemical
compositions are
used (all numbers in wt%): about 0.05 - 0.60 wt% Si (Silicon), preferably 0.15
¨ 0.5 wt% Si; about
0.10 - 0.80 wt% Fe (Iron), preferably 0.25 - 0.70 wt% Fe; about 0.70¨ 1.50 wt%
Mn (Manganese),
preferably 0.80 ¨ 1.40 wt% Mn; about 0.80 ¨ 1.50 wt% Mg (Magnesium),
preferably 0.90 ¨ 1.30
wt% Mg; about 0.05 ¨ 0.25 wt% Cu (Copper), preferably 0.10 ¨ 0.25 wt% Cu; up
to 0.10 wt% Ti
(Titanium); up to 0.25 wt% Zn (Zinc); and up to 0.15 wt% impurities,
preferably each of the
impurities with less than 0.05 wt%; with the remainder as Al (Aluminum).
On the other hand, many aluminum alloys optimized for other purposes are not
considered
suitable for making aluminum can sheet in the context of this application.
Those include, for
example 1XXX series alloys (essentially pure aluminium with a minimum 99%
aluminium content
by weight), 2XXX series alloys alloyed with copper as a basic alloying element
and capable of
being precipitation hardened to strengths comparable to steel, 4XXX series
alloys alloyed with
silicon as a basic alloying element, 5XXX series alloys alloyed with magnesium
as a basic alloying
element to offer superb corrosion resistance, 6XXX series alloys alloyed with
magnesium and
silicon as basic alloying elements, 7XXX series alloys alloyed with zinc as a
basic alloying element
and capable of precipitation hardening, or 8XXX series are alloyed with other
elements which are
not covered by other series, such as Aluminium-lithium alloys.
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In general, chemical compositions of AA3004, AA3104, AA3204 or other aluminum
alloy suitable
for making aluminum can sheet as well as other aluminum alloys are known to a
person skilled in
the art and are available e.g. in the Teal sheets of the Aluminum Association.
The body can be made of cast aluminum, which has subsequently been scalped to
obtain a body
suitable for further processing. The body is heated to a homogenization
temperature. The main
purpose of this heating step is to homogenize the material. Homogenization
temperatures may
be in the range from about 500 C to about 600 C, for example depending on the
desired
temperature for the next process step. The body may be cooled down to
temperatures suitable
for hot rolling.
In a next step, the body is hot rolled in a hot rolling mill to produce a hot
rolled sheet. The hot
rolled sheet exiting the hot rolling mill exits the hot rolling mill at a hot
rolling exit temperature. The
hot rolling step produces a hot rolled sheet having a hot mill exit gauge,
which is the thickness of
the rolled aluminum sheet after hot rolling. In the hot rolling step,
temperature control is made
such that the hot rolling exit temperature is selected so as to substantially
avoid recrystallization
of the hot rolled sheet. In the context of this application, the term
"recrystallization" refers to a
process by which deformed grains in a metallic body are replaced by a new set
of grains that are
essentially free of defects and nucleate and grow until the original grains
have been entirely
consumed. Recrystallization reduces the strength and hardness of the material
while at the same
time the ductility is increased. In the present process, the hot rolling exit
temperature is selected
such that the sheet exiting the hot rolling mill exhibits a high density of
defects, such as
dislocations, etc. and relative high strength and hardness, while at the same
time ductility may be
relatively low.
As a guideline, the substantially un-recrystallized sheet after hot rolling
may exhibit a tensile
strength in the range from 190 MPa to 240 MPa, for example, while the same
material would
exhibit significantly lower tensile strength values in a recrystallized state,
for example down to
about 150 MPa for the fully recrystallized material. Hardness values may be
determined by the
Vickers hardness test and may then be expressed as the Vickers Pyramid Number
(HV) given in
MPa (or N/mm2). Hardness can also be approximated from ultimate tensile
strength (UTS) values
by the well-known relation for aluminum alloys UTS 3*HV.
In a next step, the hot rolled sheet is cold rolled in a cold rolling mill.
The purpose of this process
step is to achieve a cold reduction, meaning that the gauge (or thickness) of
the sheet is further
reduced. The cold reduction is performed to produce a cold rolled sheet having
a cold mill exit
gauge which is smaller than the hot mill exit gauge. Cold rolling follows the
hot rolling step, after
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the sheet has cooled down to temperatures of approximately 100 C or lower,
e.g. as low as 50 c
to 60 C.
The cold rolled sheet (having the cold mill exit gauge) is then transferred to
a furnace to anneal
the cold rolled sheet in an intermediate temperature range with temperatures
selected to allow
recrystallization of the cold rolled sheet. The annealing step results in a
recrystallized sheet having
the cold mill exit gauge. The microstructure of the recrystallized sheet
typically exhibits a new set
of relative defect-free grains replacing the defective microstructure obtained
by cold rolling. In
embodiments, tensile strength values may be in the range from 150 M Pa to
about 200 M Pa, for
example.
In a subsequent step, the recrystallized sheet is cold rolled to apply a cold
reduction to produce
a cold rolled sheet with a final gauge, the final gauge being smaller than the
cold mill exit gauge.
When developing a new process, the inventors have identified certain
shortcomings of
conventional methods and now propose a new way of producing aluminum can sheet
in an
economic way avoiding shortcomings of the prior art. For example, studying the
process disclosed
/5 in US 5,362,340, it has been found that the relative low temperature
homogenization treatment,
in combination with the chemical composition of the aluminum alloy, could
produce strong cube
texture upon annealing (either self-annealing or batch-annealing at hot mill
exit gauge) which in
some cases the cold rolling process that follow the annealing cannot balance.
This may result in
aluminum can sheets with 0 / 90 earing or very low 45 earing. This earing
characteristic may
produce, during subsequent drawing and ironing processes, cans with pinched
ears at 0 / 180
with respect to the rolling direction as well as increased tear-off cans and
low performance at the
can-makers.
Additionally, some limitations of single stand reversing mills may cause
problems in conventional
processes. The hot rolling exit gauge from a single stand reversing mill may
typically range down
to values about 2.0 mm. Producing lower exit gauge from a single stand
reversing mill is generally
difficult and may not be feasible due to difficulties in controlling crown,
wedge and flatness of the
sheet. On the other hand, the tendency of the can-makers is to reduce the
thickness of the can
sheet, this tendency also known as "down-gauging". If it is desired to produce
a lower thickness
final product with similar earing and strength properties when compared to
nowadays usual
thicknesses it is required to keep the same total cold reduction applied to
the material after
intermediate annealing at hot gauge thickness (either self-annealing or batch
annealing).
Achieving this goal would require lowering the hot mill exit gauge to values
significantly below 2
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mm. The new process is capable of substantially avoiding these problems
identified in
conventional processes.
The process according to the above formulation of the invention introduces a
cold rolling step
inserted between the preceding hot rolling step and the subsequent
intermediate annealing step.
The new sequence of steps has at least two significant effects. A first effect
may be understood
considering the final product, the other effect may be understood when
considering the thermo-
mechanical process itself.
It has been found that the final product generally exhibits relatively low
earing values. The
resulting ears are more pronounced at about 45 (relative to the rolling
direction). This earing
orientation is usually preferable from the final customer's point of view,
i.e. from the point of view
of the can maker. The new method generally avoids or reduces high ears at 0 /
90 which are
not desirable from the can maker's point of view and which are very likely
obtained with the
process described in the prior art, such as US 5,362,340. From a metallurgical
point of view it is
believed that the cold reduction introduced after hot rolling and performed on
a material which is
essentially un-recrystallized can enhance the particle stimulated nucleation
(PSN) mechanism
which lowers the cube texture density that the material will have after the
intermediate annealing.
The lower cube texture after annealing will result in an earing tending
towards 45 instead of 0 /
90 to the final product.
Regarding the second effect (on the capability of the thermal-mechanical
process) it is observed
that the final strength of the material and the earing is highly dependent
from the amount of cold
work after intermediate annealing at hot gauge. For example, if, in a present
conventional
process, a material with final gauge 0.26 mm is produced, the intermediate
annealing may be
performed at about 2 mm gauge. Therefore, the total cold reduction is about
87%. Consider now
a case where the final customer asks for 0.24 mm final gauge. In order to
produce the same
earing and properties it would be necessary to make the intermediate annealing
at about 1.85
mm. This relatively small thickness often cannot be achieved satisfactorily in
a single stand
reversing mill due to flatness and thickness range limitations. These
limitations do not exist in the
new method. Applying the new method enables a producer to produce thicker
material from the
hot mill (for example about 2.5 mm), make a light cold reduction to the
required intermediate
annealing gauge (1.85 mm in this hypothetical example), and anneal the sheet
at intermediate
annealing at this gauge to make the material fully soft before it is cold
rolled to the final gauge. In
other words: Some limitations of using a single stand reversing mill as a hot
rolling mill do no
longer limit the capabilities of the overall process. If a single stand
reversing mill is used as a hot
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rolling mill, the method can also increase a lot the output of the single
stand hot mill, since it is
producing thicker gauge.
From another point of view, advantages of the new process result at least
partly from the fact that
cold rolling is performed in two separate steps, wherein the first cold
rolling step is performed after
hot rolling and before intermediate annealing (on the un-recrystallized
material) and the second
cold rolling step is performed after the recrystallization annealing (at
intermediate temperature)
on a material which is recrystallized. As a result, preferable earing
characteristics and strength as
well as small final gauges can be obtained even when hot rolling is performed
with a single stand
reversing mill.
Considering the advantages of the process described above, a single stand
reversing mill is used
as a hot rolling mill in a preferred embodiment of the process and
installation. While a tandem mill
can be used instead of a single stand reversing mill for performing the hot
rolling step, use of a
single stand reversing mill is typically much less expensive so that the final
product can be made
in an economical fashion.
In preferred embodiments the single stand reversing mill is utilized in two
different operation
modes, wherein a first operation mode includes one or more flat passes and a
second operation
mode, utilized after the first operation mode, includes one or more coiling
passes producing coiled
sheet having the hot mill exit gauge.
The hot rolling step shall be performed such that recrystallization of the hot
rolled sheet is
substantially avoided. In preferred processes, the hot rolling exit
temperature is in a range from
about 200 C to about 320 C, with preferred hot rolling exit temperatures being
lower than 290 C.
For aluminum alloys of type AA3004, AA3104 or other aluminum alloys suitable
for making
aluminum can sheet these temperatures are usually suitable to avoid
recrystallization completely,
which enhances the advantages of the overall process. The correct temperatures
to avoid
recrystallization completely may be selected depending on the alloy type and
may differ from alloy
to alloy.
When designing the cold rolling step it has been found that a cold reduction
between 5% and 70%
is preferably applied in the cold rolling mill rolling the hot rolled sheet.
Cold reductions in this range
are particularly capable of enhancing the particle stimulated nucleation (PSN)
which is believed
to lower the cube texture density in the annealed material.
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The cold rolling step can be performed at least in the last rolling passes so
that coils of cold rolled
sheet are obtained in the single stand reversing mill. In this case, it may be
preferable that
annealing the cold rolled sheet is performed in a batch furnace. As an
alternative, a continuous
furnace may be used for the annealing step in the intermediate temperature
range to obtain the
recrystallized sheet.
As the overall process allows high degrees of total reduction, a total
reduction of more than 70%
is applied to the aluminum sheet between the hot mill exit gauge and the final
gauge. The total
reduction may be 80% or more or even 85% or more. This is partly due to the
fact that cold rolling
to reduce the gauge is performed in two steps instead of one single step.
The invention also relates to an installation for producing aluminum can
sheet, the installation
being configured to perform the method according to the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
In the following, an embodiment of the invention will be described in detail
with reference to the
drawings.
/5 Fig. 1 shows a schematic drawing of a portion of an installation
configured to manufacture
aluminum can sheet suitable for making cup-shaped articles;
Fig. 2 shows a diagram illustrating the relation between the degree of
recrystallization of the
sheet material after the initial hot rolling step and the amount and type of
earring after
applying cold reduction to the final gauge; and
Fig. 3 shows a diagram illustrating the influence of cold reduction prior to
the intermediate
annealing and the effect on the type and degree of earing after cold reduction
to the final
gauge.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
Sufficiently high strength and formability (incl. limited earing) are amongst
the major requirements
for aluminum can body sheet. High strength is needed to achieve sufficient
structural stability and
to avoid buckling of the can base (dome reversal) under high internal
pressure. High strength is
also needed to obtain stable cans with very thin can wall after ironing. Good
Formability is required
as the material undergoes heavy forming operations. Anisotropic material flow
due to the texture
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of the sheet - controlled by balancing the hot strip cube and cold rolling
texture - always forms an
uneven rim of the can during the deep drawing and ironing operations. This
unevenness is also
known as "earing". Highly uneven cup rims are detrimental for transport of the
can bodies or affect
the whole process when ears are stretched and clipped off during ironing,
leading to machine
down time, reducing efficiency.
Embodiments of the invention are capable of addressing both requirements in a
satisfactory way
using an economically feasible production process.
Figure 1 shows a schematic drawing of a portion of an installation 100
configured to manufacture
aluminum can sheet suitable for making cup-shaped articles. The schematic
figure shows only
some of the devices utilized in the production route.
The production installation typically includes casting devices to produce
large cast ingots from
aluminum alloy melt. The cast ingots typically consist of coarse grains with
dendrite structure and
random texture. Precipitates comprising aluminum and other constituents, such
as Fe, Mn, and
Si are typically distributed inhomogeneously in the cast ingot.
/5 In a next step, the cast ingots are homogenized in a homogenization
furnace (also denoted as
preheating furnace, not shown in Fig. 1). The homogenization treatment is
typically accompanied
by characteristic changes of the solute content and the precipitation
microstructure later affecting
recrystallization, grain size and texture during the sheet production.
The homogenized ingots are then transferred to the hot rolling stage. A single
stand reversing mill
120 is used for hot rolling in the preferred installation. The single stand
reversing mill 120 is
capable of being operated in two different operation modes drawn separately in
schematic Fig. 1.
In a first operation mode HR-FP (shown on the left hand side of single stand
reversing mill 120),
the incoming ingots are reduced in thickness using several flat passes where
the material is rolled
back and forth without being coiled on either side of the rolls. In a second
operation mode HR-
CP, shown on the right-hand side of the drawing representing the single stand
reversing mill 120,
coiling reels CR on either side of the mill stand MS are used to coil the
sheet SH between coiling
passes performed in mutually opposite rolling directions. In either coiling
pass, one of the reels is
operating as pay-off reel providing an incoming strip to the rolling gap
formed in the mill stand.
The other reel is used as a tension reel coiling the outgoing strip after the
rolling path. Since
single-stand reversing mills are generally known in the art, a detailed
description is considered as
not necessary in this application.
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The hot rolled material is then ¨ after cooling down - transferred as a coil
to a cold rolling stage
130 arranged downstream of the hot rolling stage in the material flow
direction. The cold rolling
mill could be a single stand (as shown) or a multiple stands cold mill.
A batch furnace 140 is arranged downstream of the cold rolling stage 130. The
batch furnace is
configured to receive multiple coils CL after cold rolling and to perform
intermediate annealing of
the cold material to achieve full recrystallization of the sheet material.
A further cold rolling stage 150 is arranged downstream of the intermediate
annealing batch
furnace 140 to apply cold rolling to the recrystallized material to obtain
cold rolled material at the
final gauge desired for further processing steps, e.g. as a H1X material or,
more specifically, as
a H19 material. The cold rolling mill 150 comprises a single stand in the
embodiment of Fig. 1.
An exemplary process for producing aluminum can sheet on the installation 100
was performed
as follows.
In a preparatory step, an aluminum alloy was cast to form a casting and
subsequently scalped to
obtain a body of cast and scalped aluminum alloy suitable for further
processing. This body is
also denoted as ingot in the following. The aluminum alloy can be a can body
stock material such
as AA3004, AA3104 or other aluminum alloy basically suitable for making
aluminum can sheet.
The aluminum alloy used in exemplary processes comprised about 0.30 wt% Si,
about 0.50 wt%
Fe, about 0.95 wt% Mn, about 1.10 wt% Mg, about 0.20 wt% Cu, less than 0.05
wt% Ti, less than
0.10 wt% Zn; and up to 0.15 wt% impurities, preferably each of the impurities
with less than 0.05
wt%, with the remainder as Al.
After casting and scalping, the ingot was homogenized at about 500 - 595 C
with soaking time
e.g. from 5 to 20 hours, followed by ingot cooling down to about 490 - 530 C.
The homogenized ingot (aluminum body) was then transferred to the hot rolling
mill without
significant intermediate cooling so that hot rolling of the ingot started at
about this temperature,
i.e. at about 490 - 530 C. A single stand-reversing mill 120 was utilized as
hot rolling mill in this
installation setup.
Several flat passes were carried out, down to about 25 to 45mm gauge. The
ingot temperature
after the last flat pass was between about 290 and 350 C. The number of flat
passes may range,
for example, from 15 to 50.
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After the flat passes, the thickness of the material was further reduced with
hot rolling on the same
single stand-reversing mill 120, with the difference that the material was
coiled after each pass
(coiling passes). The number of coiling passes was from 2 to 8.
The thickness of the material after the last coiling pass was from about 1.7mm
to about 5mm. In
the experiments reported here, the exit temperature of the material after hot
rolling, i.e. the hot
rolling exit temperature THREx, was low enough to ensure the absence of
recrystallization.
Typically, the hot rolling exit temperature was in a range from about 200 C to
about 340 C and
preferably between about 220 C and about 280 C. The reduction of each coiling
pass was
between 20 and 70%.
/0 The hot rolled material was cooled down and then transferred to a cold
rolling mill.
A cold reduction from 5% to 70% was applied to the material in the cold
rolling mill directly at the
hot band not recrystallized material.
The cold rolled sheet was then transferred in coiled form to a batch furnace
140 for intermediate
annealing. An intermediate annealing step was then applied to the cold rolled
sheet. Annealing
temperatures and annealing times were selected so that the annealed material
was allowed to
become fully recrystallized and to develop a strong cube texture. A typical
range of annealing
temperature is from 280 C to 450 C with 1 to 12 hours holding time.
The recrystallized annealed sheet was then subject to cold rolling to apply a
cold reduction
suitable to produce a cold rolled sheet with a final gauge. Preferably, cold
rolling from 70% to 95%
reduction was applied to the recrystallizes sheet, giving the material the
required strength and
balancing the cube texture with rolling texture. In case of recrystallization
(partial or full) at the
thickness of the hot band (either self-annealing or after batch anneal), the
cube texture developed
after annealing was weak and the final product had high 45 earing.
With the method described above, the un-recrystallized hot band undergoes a
relative low cold
reduction and then an intermediate annealing is applied to the material to
become fully soft. With
this method, there is an intermediate annealing thickness reduction with cold
rolling without
deterioration of the strong cube texture after annealing.
The combination of the low cold reduction to the un-recrystallized structure
directly after hot rolling
and batch annealing to produce fully recrystallized material could be applied
also to the
conventional method of producing can body stock through a tandem hot rolling
mill. In other
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words, in an alternative embodiment a tandem hot rolling mill may be used
instead of a single
stand reversing mill to perform the hot rolling step preceding the cold
rolling step.
In the following, some characteristic aspects of the new, beneficial process
are explained in
connection with the schematic diagrams of Figs. 2 and 3. Fig. 2 schematically
illustrates the
technical connection between the degree of recrystallization of the sheet
material after the initial
hot rolling step and the amount and type of earring after applying cold
reduction to the final gauge.
Fig. 3 illustrates the importance of the step of cold reduction prior to the
intermediate annealing
and the effect on the type and degree of earring after cold reduction to the
final gauge.
In each diagram of Figs. 2 and 3, the x-axis represents the degree of cold
reduction (in percent)
applied after the intermediate annealing. In other words, the x-axis
represents the amount of cold
reduction achieved in the cold rolling mill 150 situated downstream of the
intermediate annealing
furnace 140. The y-axis represents the type and amount of earring (in
percent). The area above
the baseline BL corresponds to 0 ¨ 900 earring, whereas the area below the
baseline BL
represents 450 earring. The absolute distance of a data point from the
baseline in the y-direction
of the diagram represents the amount or strength of the respective earring,
meaning that a point
on the baseline BL corresponds to a sheet showing no earring at all. The
curves of the diagram
represent general trends established in a high number of experiments. The
schematic box plots
BP in Fig. 3 indicate that the trends represented by the lines are considered
to be significant.
Fig. 2 basically illustrates the importance of the requirement that the hot
rolling exit temperature
should be selected such that any recrystallization of the hot rolled sheet
should be avoided as
much as possible.
The solid line represents a case where the rolled sheet is substantially un-
recrystallized after
finishing the hot rolling operation. This is an embodiment of the claimed
invention. For
comparison, the lower curve (dashed line) represents reference cases where the
sheets were
partially recrystallized after finishing the hot rolling step which, in other
words, means that the
recrystallization was not sufficiently avoided in the presented reference
processes. The solid line
shows that there is a high degree of 0 ¨ 90 earring at the fully
recrystallized material after
intermediate annealing and before the cold reduction starts (at value of cold
reduction = 0%). As
cold reduction is increased, the degree of 0 ¨ 90 earring is continuously
decreased so that shortly
before obtaining the final gauge (at the highest point of cold reduction)
there is no discernible
earring (solid curve crosses the baseline). In the final product after the
full cold reduction is applied
to the sheet, a certain amount of 45 earring is discernible, but the degree
of earring is low in
absolute terms.
CA 03172760 2022-08-23
WO 2021/175761
PCT/EP2021/054999
- 13 -
In contrast, where the material shows a significant amount of
recrystallization after finishing the
hot rolling step (dashed line), the degree of 0 ¨ 900 earring is lower than in
cases according to
embodiments of the invention. As cold reduction is increased, the degree of 0
¨ 90 earring
decreases and would vanish completely at a cold reduction which is not
sufficient to obtain the
thinner final gauge. As the amount of cold reduction is increased to obtain
the thinner final gauge
the character of the earring changes from 0 - 90 earing to predominantly 450
earring and the
amount of 450 earring increases to a level much higher in absolute terms than
in the material
according to the claimed process (solid line). This shows that the degree of
recrystallization after
the hot rolling step has a significant influence on the amount and character
of earring in the final
product.
The diagram in Fig. 3 can be read in a similar way. The diagram illustrates
the importance of the
step of cold reduction applied prior to the immediate annealing. In the
diagram, the upper curve
(dashed line) corresponds to a case where no cold reduction was applied prior
to annealing. This
could be a process similar to the processes described in the prior art
mentioned in the beginning
of this application. It is seen that a high degree of 0 - 90 earring is
present immediately after the
intermediate annealing. When the material is finally cold rolled to the final
gauge (maximum
amount of cold reduction) there is almost no or very little earring in the
final product. If a certain
amount of 45 earring is present, the absolute amount is small.
In contrast to that, the dotted line below the dashed line represents
processes according to
embodiments of the invention where a cold reduction is applied prior to the
intermediate annealing
in a cold mill rolling the (essentially un-recrystallized) material exiting
the hot rolling state before
the material is transferred to the intermediate annealing. In the beginning,
before cold reduction
is applied, the amount of 0 ¨ 90 earring is less than in the case of no cold
reduction prior to
annealing. Once the sheet is reduced in thickness to the final gauge (at
maximum cold reduction),
there is a significant amount of 45 earring, which is a property desired by
many can makers
working with a very thin aluminum sheet.
The disclosure of this patent application also relates to a method for making
an aluminum can
which comprises the method steps of the method for producing aluminum can
sheet, wherin the
cold rolled sheet with the final gauge is formed into a cup-shaped article
suitable for making an
aluminum can.
