EP0851943B1

EP0851943B1 - A method for making beverage can sheet

Info

Publication number: EP0851943B1
Application number: EP96935838A
Authority: EP
Inventors: Edwin J. Westerman; Gavin F. Wyatt-Mair
Original assignee: Alcoa Inc
Current assignee: Howmet Aerospace Inc
Priority date: 1995-09-18
Filing date: 1996-09-17
Publication date: 2003-05-21
Anticipated expiration: 2016-09-17
Also published as: DE69628312D1; ES2196183T3; BR9611416A; AU7362596A; CA2232436A1; CN1200771A; CN1085743C; MX9802071A; JP3878214B2; CA2232436C; EP0851943A1; AU722391B2; JPH11511389A; WO1997011205A1; DE69628312T2

Description

The present invention relates to a process for making aluminum alloy beverage containers, and more particularly, to a process for making such can bodies, ends and tabs for such containers allowing them to be produced more economically and efficiently.

It is now conventional to manufacture beverage containers from aluminum alloys. An aluminum alloy sheet stock is first blanked into a circular configuration and then cupped. The side wall are ironed by passing the cup through a series of dies having diminishing bores. The dies thus produce an ironing effect which lengthens the sidewall to produce a can body thinner in dimension than its bottom.

Thus, formability is a key characteristic of aluminum alloy to be used in manufacturing cans. Such cans are most frequently produced from aluminum alloys of the 3000 series. Such aluminum alloys contain alloy elements of both magnesium and manganese. In general, the amount of manganese and magnesium used in can body stock is generally present at levels less than about 1% by weight.

In the manufacture of such beverage containers, it has been the practice in the industry to separately form both a top lid of such cans and tabs for easy opening of such lids separately and using different alloys. Such lids and tabs are then shipped to the filler of the beverage can and applied once the containers has been filled by a filler. The requirements for can ends and tabs are generally quite different than those for can bodies. In general, greater strength is required for can ends and tabs, and that requirement for greater strength has dictated that such can ends and tabs be fabricated from an aluminum alloy. One such alloy commonly used is alloy AA5182, an aluminum alloy containing relatively high amounts of magnesium to provide the added strength necessary for can ends and tabs. AA5182 typically contains magnesium in an amount ranging from 4.4% by weight, thus adding to the cost of the alloy for can ends and tabs.

It has been proposed to employ, as the aluminum alloy used in the fabrication of can ends and tabs, alloy from the 3000 series, such as AA3104. Because such alloys generally have diminished strength as compared to AA5187, it has been necessary to employ can ends fabricated from AA3104 which have a greater thickness and thus are more expensive.

It is accordingly an object of the present invention to provide can end and tab stocks and can ends and tabs made therefrom which overcome the foregoing disadvantages.

It is more specifically an object of the present invention to provide can ends and tabs and a method for fabricating same in which use is made of aluminum alloys containing less alloying elements without sacrificing strength.

It is a more specific object of the present invention to provide can ends and tabs therefor and a method for fabricating them which can be employed with aluminum alloys containing less than 2% magnesium without sacrificing the necessary strength of the can ends and tabs.

These and other objects and advantages of the invention appear more fully hereinafter from a detailed description of the invention.

The present invention provides the method of independent claim 1. The dependent claims specify preferred but optional steps.

The concepts of the present invention reside in the discovery that aluminum alloys containing lesser amounts of alloying elements can, nonetheless, be used in fabricating can ends and tabs without sacrificing strength by utilizing a fabrication process in which the aluminum alloy, preferably containing less than 2% by weight of magnesium as an alloying element, is formed into sheet stock for making can bodies, ends and tabs. In accordance with one embodiment of the invention, the aluminum alloy is strip cast between a pair of continuous moving metal belts to form a hot strip cast feedstock, and then the feedstock is rapidly cooled to prevent substantial precipitation of aluminum alloying elements as intermetallic compounds.

It has been unexpectedly found that such a fabrication process provides an aluminum alloy feedstock having equal or better metallurgical characteristics as compared to aluminum alloys conventionally used in forming can ends and tabs.

It has been found in accordance with the preferred embodiment of the present invention that the fabrication process can be applied to alloys of the 3000 series such as AA3104 without the need to increase the thickness of the can ends and tabs to achieve comparable strips. Without limiting the present invention as to theory, it is believed that the techniques of strip casting followed by rapid cooling provide an alloy sheet stock having improved strength by reason of its eutectic constituents which provide increased strengths. In addition, it is believed, once again, without limiting the present invention as to theory, that formability of the sheet stock of this invention used in forming can ends and tabs is improved over aluminum alloys containing greater qualities of alloying elements because it is unnecessary, in the practice of the invention, to use an annealing step typically used by the prior art. Thus, the present invention allows can ends and tabs to be produced from less expensive aluminum alloys without sacrificing the metallurgical properties of those more expensive alloys.

In one preferred embodiment of the invention, the sequence of steps of strip casting, cooling and rolling is preferably greater within a continuous, in-line sequence. That has a further advantage of eliminating process and material handling steps typically employed in the prior art. The strip casting can be used to produce a cast strip having a thickness less than 25.4 mm (1.0 inches), and preferably within the range of 0.254 mm (0.01) to 5.08 mm (0.2 inches). In addition, in accordance with the most preferred embodiment of the invention, the width of the strip is narrow contrary to conventional wisdom. That facilitates ease of in-line threading and processing and allows production lines for the manufacture of can ends and tabs to be physically located with or as part of a can making facility. A filler location that has the further advantage of eliminating additional handling and shipping costs, thus promoting the overall economics of a can making operation.

In co-pending Application Serial No. 07/902,936, filed June 23, 1992, there is disclosed a new concept in the processing of aluminum alloys in the manufacture of aluminum can stock. It has been discovered that it is possible to combine casting, hot rolling, annealing, solution heat treating, quenching and cold rolling into one continuous, in-line operation in the production of aluminum alloy can body stock. One of the advantages afforded by the process of the foregoing application is that it is possible to operate the continuous, in-line sequence of steps at very high speeds, of the order of several hundred feet per minute where 1 foot is 0.3048 m. One of the disadvantages that has been discovered in connection with the process of the foregoing application is that the intermediate annealing step, which provides re-solution of soluble elements and earing control through recrystallization of the sheet, may be a limiting factor on the speed at which the process can be operated. Thus, as production speed increases, the continuous annealing furnace preferably used in the practice of the process disclosed in the foregoing application must be made longer and be run at higher energy levels, representing an increase in the cost of capital equipment and the cost in operating the process. It would, therefore, be desirable that the continuous annealing step be avoided.

It is possible to produce aluminum alloy sheet stock, and preferably aluminum alloy can body stock having desirable metallurgical properties by using, in one continuous sequence of steps, the steps of providing a hot aluminum alloy feedstock which is subjected to a series of rolling steps to rapidly and continuously cool the feedstock to the thickness and metallurgical properties without the need to employ an annealing step conventionally used in the prior art. In similar prior art processes, such as that described in U.S. Patent No. 4,282,044, it has been suggested that aluminum alloy can body stock can be produced by strip casting, followed by rolling and coiling whereby the rolled feedstock in the form of coils is allowed to slowly cool. Thereafter, the coil is later annealed to improve the metallurgical properties of the sheet stock.

It has been found, in accordance with the present invention, that when the feedstock is rapidly cooled following casting, it is unnecessary to employ annealing steps to attain the desired metallurgical properties resulting from solution of soluble elements. Without limiting the present invention as to theory, it is believed that the rapid cooling effected by the continuous, in-line rolling operations is carried out in a sufficiently short period of time to prevent precipitation of alloying elements contained in the aluminum feedstock as intermetallic compounds. That precipitation reaction is a diffusion-controlled reaction, requiring the passage of time. Where the feedstock is rapidly cooled during rolling, there is insufficient time to permit the diffusion-controlled precipitation from occurring. That, in turn, not only facilitates in-line processing of the aluminum alloy to minimize the number of materials handling steps, so too does the rapid cooling prevent substantial precipitation of alloying elements, making it unnecessary to utilize a high temperature annealing step to attain the desired strength in the final can product.

The feedstock produced by the method of the present invention is characterized as being produced in a highly economical fashion without the need to employ a costly annealing step. As will be understood by those skilled in the art, annealing has been used in the prior art to minimize earing. It has been found, in accordance with the practice of this invention, that, the conditions (time and temperature) of hot rolling, the thickness of the alloy as strip cast and the speed at which it is cast can be used to control earing. For example, casting the aluminum alloy at reduced thickness is believed to reduce earing; similarly, casting at higher speeds can likewise reduce earing. Nonetheless, where use is made of processing conditions which tend to yield an aluminum alloy strip having a tendency toward higher earing, that phenomenon can be controlled by means of an alternative embodiment.

In accordance with that alternative embodiment of the invention, the high earing that can occur on the feedstock can be compensated for by cutting the processed feedstock into non-circular blanks prior to cupping, using what has become known in the art as convoluted die. The use of a convoluted die compensates for any earing tendencies of the sheet stock, by removing metal from those peripheral portions of the blank which would be converted to ears on cup-drawing. Thus, the convoluted die offsets any earing that would otherwise be caused by the omission of high temperature annealing.

In accordance with a preferred embodiment of the invention, the strip is fabricated by strip casting to produce a cast thickness less than 25.4 mm (1.0 inches), and preferably within the range of 0.254 to 5.08 mm (0.01 to 0.2 inches).

In another preferred embodiment, the width of the strip, slab or plate is narrow, contrary to conventional wisdom; this facilitates ease of in-line threading and processing, minimizes investment in equipment and minimizes cost in the conversion of molten metal to can body stock.

Fig. 1 is a schematic illustration showing a process utilizing a continuous in-line sequence of steps for producing aluminum alloy sheet stock.

Fig. 2 is a schematic illustration of preferred strip casting apparatus used in the practice of the invention.

Fig. 3 is a generalized time-temperature transformation diagram for aluminum alloys illustrating how rapid heating and quenching serves to eliminate or at least substantially minimize precipitation of alloying elements in the form of intermetallic compounds.

Fig. 4 is a drawing of a blank produced with a convoluted die to control earing in accordance with the invention.

The preferred process of the present invention involves a new method for the manufacture of aluminum alloy cups and can bodies utilizing the following process steps in one, continuous in-line sequence:

(a) In the first step, a hot aluminum feedstock is provided, preferably by strip casting; and

(b) The feedstock is, in the preferred embodiment, subjected to rolling to rapidly and continuously cool the sheet stock to the desired thickness and attain the desired strength properties.

The cooled feedstock can then be either formed into a coil for later use or can be further processed to form non-circular blanks by means of a convoluted die to effect earing control, in accordance with conventional procedures.

It is an important concept of this embodiment of the invention that the rolling of the freshly cast strip be effected rapidly, before there is sufficient time for the diffusion-controlled reaction by which alloying elements are precipitated from solid solution as intermetallic compounds. In that way, the process of the present invention makes it possible to omit high temperature annealing as is required in the prior art to effect solution of soluble alloying elements. In general, the cast feedstock must be cooled to cold rolling temperatures in less than 30 second, and preferably in less than 10 seconds.

In a preferred embodiment, the overall process of the present invention embodies characteristics which differ from the prior art processes:

(a) The width of the can body stock product is narrow;

(b) The can body stock is produced by utilizing small, in-line, simple machinery;

(c) The tendency of the non-annealed aluminum alloy to exhibit high earing is offset through the use of a convoluted die while achieving desirable strength properties; and

(d) The said small can stock plants are located in or adjacent to the can making plants, and therefore packaging and shipping operations are eliminated.

It has become the practice in the aluminum industry to employ wider cast strip or slab for reasons of economy. In the preferred embodiment of this invention, it has been found that, in contrast to this conventional approach, the economics are best served when the width of the cast feedstock 4 is maintained as a narrow strip to facilitate ease of processing and enable use of small decentralized strip rolling plants. Good results have been obtained where the cast feedstock is less than 610 mm (24 inches) wide, and preferably is within the range of 50.8 to 508 mm (2 to 20 inches) wide. By employing such narrow cast strip, the investment can be greatly reduced through the use of small, in-line equipment. Such small and economic in-line equipment of the present invention can be located near the points of need, as, for example, can-making facilities. That in turn has the further advantage of minimizing costs associated with packaging, shipping of products and customer scrap.

Thus, the in-line arrangement of the processing steps in a narrow width (for example, 305 mm (12 inches)) makes it possible for the process to be conveniently and economically located in or adjacent to can production facilities. In that way, the process of the invention can be operated in accordance with the particular technical and throughput needs for can stock of can making facilities.

In the preferred embodiment of the invention as illustrated in Fig. 1, the sequence of steps employed in the practice of the present invention is illustrated. One of the advances of the present invention is that the processing steps for producing can body sheet can be arranged in one continuous line whereby the various process steps are carried out in sequence. Thus, numerous handling operations are entirely eliminated.

In the preferred embodiment, molten metal is delivered from a furnace 1 to a metal degassing and filtering device 2 to reduce dissolved gases and particulate matter from the molten metal, as shown in Fig. 1 The molten metal is immediately converted to a cast feedstock 4 in casting apparatus 3. As used herein, the term "feedstock" refers to any of a variety of aluminum alloys in the form of ingots, plates, slabs and strips delivered to the hot rolling step at the required temperatures. Herein, an aluminum "ingot" typically has a thickness ranging from about 152 mm (6 inches) to about 762 mm (30 inches), and is usually produced by direct chill casting or electromagnetic casting. An aluminum "plate", on the other hand, herein refers to an aluminum alloy having a thickness from about 12.7 mm (0.5 inches) to about 152 mm (6 inches), and is typically produced by direct chill casting or electromagnetic casting alone or in combination with hot rolling of an aluminum alloy. The term "slab" is used herein to refer to an aluminum alloy having a thickness ranging from 9.53 mm (0.375 inches) to about 76.2 mm (3 inches), and thus overlaps with an aluminum plate. The term "strip" is herein used to refer to an aluminum alloy, typically having a thickness less than 9.53 mm (0.375 inches). In the usual case, both slabs and strips are produced by continuous casting techniques well known to those skilled in the art.

The feedstock employed in the practice of this embodiment of the present invention can be prepared by any of a number of casting techniques well known to those skilled in the art, including twin belt casters like those described in U.S. Patent No. 3,937,270 and the patents referred to therein. In some applications, it is preferable to employ as the technique for casting the aluminum strip the method and apparatus described in co-pending Application Serial Nos. 184,581, filed June 21, 1994, 173,663, filed December 23, 1993 and 173,369, filed December 23, 1990 . The strip casting technique described in the foregoing co-pending applications which can advantageously be employed in the practice of this invention is illustrated in Fig. 2 of the drawing.

As there shown, the apparatus includes a pair of

endless belts

10 and 12 carried by a pair of

upper pulleys

14 and 16 and a pair of corresponding

lower pulleys

18 and 20. Each pulley is mounted for rotation, and is a suitable heat resistant pulley. Either or both of the

upper pulleys

14 and 16 are driven by suitable motor means or like driving means not illustrated in the drawing for purposes of simplicity. The same is true for the

lower pulleys

18 and 20. Each of the

belts

10 and 12 is an endless belt and is preferably formed of a metal which has low reactivity with the aluminum being cast. Low-carbon steel or copper are frequently preferred materials for use in the endless belts.

The pulleys are positioned, as illustrated in Fig. 2, one above the other with a molding gap therebetween corresponding to the desired thickness of the aluminum strip being cast.

Molten metal to be cast is supplied to the molding gap through suitable metal supply means such as a tundish 28. The inside of the tundish 28 corresponds substantially in width to the width of the

belts

10 and 12 and includes a metal supply delivery casting nozzle 30 to deliver molten metal to the molding gap between the

belts

10 and 12.

The casting apparatus also includes a pair of cooling means 32 and 34 positioned opposite that position of the endless belt in contact with the metal being cast in the molding gap between the belts. The cooling means 32 and 34 thus serve to cool

belts

10 and 12, respectively, before they come into contact with the molten metal. In the preferred embodiment illustrated in Fig. 2,

coolers

32 and 34 are positioned as shown on the return run of

belts

10 and 12, respectively. In that embodiment, the cooling means 32 and 34 can be conventional cooling devices such as fluid nozzles positioned to spray a cooling fluid directly on the inside and/or outside of

belts

10 and 12 to cool the belts through their thicknesses Further details respecting the strip casting apparatus may be found in the cited co-pending applications.

The feedstock 4 is moved through optional pinch rolls 5 into one or more hot rolling stands 6 where its thickness is decreased. In addition, the rolling stands serve to rapidly cool the feedstock to prevent or inhibit precipitation of the strengthening alloying components such as manganese, copper, magnesium and silicon present in the aluminum alloy.

It will be appreciated by those skilled in the art that there can be expected some insignificant precipitation of intermetallic compounds that do not affect the final properties. Such minor precipitation has no affect on those final properties by reason of the fact that the volume and type of the intermetallic compounds have a negligible effect on the final properties. As used herein, the term "substantial" refers to precipitation which affects the final sheet properties.

As will be appreciated by those skilled in the art, use can be made of one or more rolling steps which serve to reduce thickness of the strip 4 while simultaneously rapidly cooling the strip to avoid precipitation of alloying elements. The exit temperature from the strip caster 3 varies within the range of about 371°C (700°F) to the solidus temperature of the alloy. The rolling operations rapidly cool the temperature of the cast strip 4 to temperatures suitable for cold rolling, below 177°C (350°F) in less than 30 seconds, and preferably in less than 10 seconds, to ensure that the cooling is effected sufficiently rapidly to avoid or substantially minimize precipitation of alloying elements from solid solution. The effect of the rapidly cooling may be illustrated by reference to Fig. 3 of the drawing, showing the formation of intermetallic precipitates in aluminum as a function of temperature and time. Such curves, which are generally knwon in the art as time-temperature transformation or "C" curves, show the formation of coarse and fine particles formed by the precipitation of alloying elements as intermetallic compounds as an aluminum alloy is heated or cooled. It is important in the practice of the present invention to rapidly cool the feedstock during the rolling operations so that the strip 4 is cooled along a temperature time line that does not intersect the curves shown on Fig. 3 of the drawing. The prior art practice of allowing a slow cool of, for example, a coil, results in a temperature time line which intersects those curves, maintaining that the slow cooling causes precipitation of alloying elements as intermetallic compounds.

In a preferred embodiment of the invention, the feedstock is passed from where it is cooled to one or more cold rolling stands in which the feedstock is worked to harden the alloy and reduce its thickness to finish gauge. In the preferred practice of the invention, it is sometimes desirable, after cold rolling to age the cold roll strip at an elevated temperature, preferably at temperatures within the range of 149-191°C (300-375°F) for about 1 to about 10 hours. Because the strip has been cooled so as to substantially minimize precipitation of alloying elements as intermetallic compounds, the cast strip has an unusually high level of solute supersaturation. Thus, the aging step causes the ultimate tensile strength and yield strength to increase along with formability.

Thereafter, the cast strip which has been aged can either be coiled until needed or it can be immediately formed into can ends and/or tabs using conventional techniques.

As will be appreciated by those skilled in the art, it is possible to realize the benefits of the present invention without carrying out the cold rolling step in the cold mill as part of the in-line process. Thus, the use of the cold rolling step is an optional process step of the present invention, and can be omitted entirely or it can be carried out in an off-line fashion, depending on the end use of the alloy being processed.

The effect of the reductions in thickness likewise effected by the rolling operations are subject to wide variation, depending upon the types of feedstock employed, their chemistry and the manner in which they are produced. For that reason, the percent reduction in thickness of the rolling operations is not critical to the practice of the invention. In general, good results are obtained when the rolling operation effects a reduction in thickness within the range of 40 to 99 percent of the original thickness of the cast strip.

Alternatively, it is preferred to immediately cut blanks using a convoluted die and produce cups for the manufacture of cans instead of coiling the strip or slab 4. Convoluted dies useful in the practice of the present invention are known to the art, and are described in U.S. Patent Nos. 4,711,611 and 5,095,733. Such dies are now conventional and well known to those skilled in the art. The convoluted dies used in the practice of this invention may be used to form a non-circular blank having the configuration shown in Fig. 4 which in turn can be used to form a cup having the configuration shown in the same Figure. Thus, the convoluted die can be used, where necessary, to minimize earing tendencies of the sheet stock.

As will be appreciated by those skilled in the art, it is also possible, before treating the sheet stock with a convoluted die, to coil the sheet stock.

The concepts of this embodiment of the present invention are applicable to a wide range of aluminum alloys for use as can body stock. In general, alloys suitable for use in the practice of the present invention are those aluminum alloys containing from about 0 to about 0.6% by weight silicon, from 0 to about 0.8% by weight iron, from about 0 to about 0.6% by weight copper, from about 0.2 to about 1.5% by weight manganese, from about 0.2 to about 4% by weight magnesium, from about 0 to about 0.25% by weight zinc, with the balance being aluminum with its usual impurities. Representative of suitable alloys include aluminum alloys from the 3000 and 5000 series, such as AA 3004, AA 3104 and AA 5017.

Having described the basic concepts of this embodiment of the invention, reference is now made to the following examples which are provided by way of illustration of the practice of the invention.

Example 1

A sheet of finish gauge can stock which was not annealed was formed into a cup using a conventional round die. The earing was measured as 6.6%.

An adjacent sheet from the same processing (still without an anneal) was formed into a cup with a convolute cut edge on the blanking die. The earing was measured as 3.1%.

Example 2

A thin strip of metal 2.286 mm (0.09 inch) thick was cast at 91.44 m (300 feet) per minute and immediately rolled in three passes at high speed from 2.286 mm (0.090 inch) thick to 0.2896 mm (0.0114 inch) thick while decreasing in temperature during rolling from 482°C (900°F) to 149°C (300°F). The earing of the sheet so produced was 3.8%. The ultimate tensile strength of the sheet was 299,234 kN.m^-2 (43,400 psi) and the elongation 4.4%.

Claims

A method of making aluminum alloy beverage containers by manufacturing an aluminum alloy sheet stock in a continuous, in-line sequence comprising the following steps:

(a) providing hot aluminum alloy feedstock in the form of ingots, plates, slabs and strips in a casting apparatus, wherein the exit temperature of the feedstock from the casting apparatus is from 371°C (700°F) to the solidus temperature of the alloy; and

(b) hot rolling the feedstock to reduce its thickness and to rapidly cool the feedstock sufficiently to substantially avoid substantial precipitation of alloying elements in solid solution; wherein said hot rolling cools the temperature of the cast feedstock to below 177°C (350°F) in less than 30 seconds.
A method according to claim 1, wherein the cast feedstock is a strip having a thickness before hot rolling of less than 1.0 inch (25 mm).
A method according to claim 2, wherein the strip cast feedstock has a thickness before hot rolling of within the range of 0.01 to 0.2 inches (0.254 to 5.08mm).
A method according to any preceding claim, wherein the rolling reduces the thickness of the feedstock by 40 to 99%.
A method according to claim 1, wherein the feedstock is rolled to about 0.0114 inch (2.90 mm).
A method according to any preceding claim, wherein the rolling to cool the feedstock is carried out in less than 10 seconds.
A method according to any preceding claim, wherein the aluminum alloy is a can body stock alloy.
A method according to any preceding claim, wherein the feedstock is provided by strip casting which is carried out using a continuous belt caster, the moving belts of which are cooled before contacting the molten aluminum.
A method according to any preceding claim, wherein the feedstock is an aluminum alloy containing from 0 to 0.6% by weight silicon, from 0 to 0.8% by weight iron, from 0 to 0.6% by weight copper, from 0.2 to 1.5% by weight manganese, from 0.2 to 4% by weight magnesium, from 0 to 0.25% by weight zinc, from 0 to 0.1 % by weight chromium with the balance being aluminum and its usual impurities.
A method according to claim 9, wherein the aluminum alloy feedstock is selected from AA 3004, AA 3104 and AA 5017.
A method according to any preceding claim which includes the further step of forming cups from the cooled rolled feedstock by use of a convoluted blanking die.
A method according to any preceding claim, wherein the rolled feedstock is coiled after rolling.