PROCESS OF FABRICATION OF ALUMINUM SHEET

Abstract: A process for the production of strip stock from an alloy is provided. The strip stock produced from the alloy is suitable for the fabrication of both container ends and container bodies in thinner gauges than are typically employed, has low earing characteristics and may be derived from recycled aluminum scrap. The alloy preferably has a magnesium concentration of from about 1.9 to about 2.8 weight percent and a manganese concentration of from about 0.9 to about 1.6 weight percent. The process preferably includes continuous chill block casting the alloy melt into a strip, hot rolling the strip to a first thickness, annealing the hot rolled strip and then cold rolling the annealed strip to a final thickness. Cold rolling preferably includes two stages, with an intermediate anneal step between the two stages. The process increases tensile and yield strength while decreasing earing texture, even in very thin gauges, such as 0.010 inches.

Description: PROCESS OF FABRICATION OF ALUMINUM SHEET Technical Field of the Invention This invention relates to a process for production of aluminum sheet from an aluminum alloy composition, the sheet having reduced earing and improved strength and being is suitable for conversion into useful products, such as container ends and container bodies. Background of the Invention In recent years, substantial effort has been made to produce an aluminum alloy sheet which is suitable without modification for the manufacture of both container bodies and container ends. Aluminum beverage containers are generally made in two pieces, one piece forming the container sidewalls and bottom (collectively referred to herein as "container body") and a second piece forming the container top. Using methods well known in the art, a container body is formed by cupping a circular blank of aluminum sheet and then drawing and ironing the cupped sheet by subsequently extending and thinning the sidewalls by passing the cup through a series of dies with diminishing bores. The result is an integral body with sidewalls thinner than the bottom. A common alloy used to produce container bodies is AA 3004 (an alloy registered with the Aluminum Association) whose characteristics are appropriate for the drawing and ironing process due primarily to low magnesium (Mg) and manganese (Mn) concentrations. However, alloys such as AA 3O04 having low magnesium content usually possess insufficient strength to be used for the fabrication of container ends with easy open "ring pulls" or the like. Therefore, alloys with a higher magnesium concentration, such as AA 5082 or AA 5182 alloys, are used for container ends. Table 1 provides a comparison of the major components of alloys AA 3004, 5082 and 5182, as well as other alloys discussed herein. TABLE 1 (WEIGHT %) * * The remainder being aluminum. A completed container (a body together with an end) must be able to withstand an internal pressure of at least about 60 pounds if it is to contain unpasteurized beer and at least about 90 pounds if it is to contain pasteurized beer, soda pop, or any beverage having similarly high carbonation levels. Currently, containers fabricated from AA 3004 body alloy and AA 5082 end stock are able to withstand 90 pounds of internal pressure if fabricated from aluminum sheet having a gauge of about 0.0116 inches. Containers made from thinner gauges employ less sheet material than those made from thicker gauges and are therefore less expensive to produce. However, containers made from thinner gauge stock, such as 0.0110 inches, have not been sufficiently strong to withstand 90 pounds of internal pressure or have not been sufficiently strong to survive the rigors encountered during long distance transportation. Another desirable characteristic of an aluminum alloy sheet which is to be drawn and ironed is that the sheet have a low earing percentage. As used herein, the term "earing percentage" (also referred to herein as "earing") refers to the 45° earing or 45° rolling texture. This value is determined by measuring the height of ears which stick up in a drawn cup minus the height of valleys between the ears. This difference is divided by the height of the valleys times 100 to convert to a percentage. The 45° earing is measured at 45° to the longitudinal axis of the strip. Due to this earing, the rim of the shell often becomes deformed and takes on a scalloped appearance. Because this earing must be removed before the container body is completed, waste occurs. Furthermore, excessive earing, greater than about 2 percent as measured by the Olsen cup test, may also interfere with the drawing apparatus. Minimizing earing helps to minimize waste and simplifies the production process. One step that has been used to reduce earing is to reduce the cold work percentage (or the percent thickness reduction during the step of cold rolling an alloy sheet) . As illustrated in Figure 1, when AA 5017 alloy is employed, earing decreases as the cold work percentage decreases. However, as further illustrated in Figure 1, the yield strength also decreases as the cold work percentage decreases. Therefore, increasing the cold work to form stock with thinner gauges or greater strength produces unacceptably high earing. Conversely, reducing the earing by reducing the cold work results in thicker stock with relatively low strength. Aluminum alloys may be produced by direct chill casting of molten alloy into ingots which are then rolled into strips or may be produced by a continuous strip casting process. Apparatus for continuous strip block casting are described in U.S. Patent Nos. 3,709,281, 3,744,545, 3,747,666, 3,759,313 and 3,774,670. Although there exist numerous variations of the continuous block casting process, all of the processes generally include the steps described hereinbelow. Molten aluminum alloy is injected through a nozzle or distributor tip into a cavity formed between two sets of oppositely rotating chilled blocks. While in the cavity, the alloy cools and solidifies to form an aluminum sheet. The aluminum sheet then passes between rollers to further reduce the thickness of the strip. This is typically referred to as hot rolling. As the continuous strip comes out of the hot rolling step, it is coiled and allowed to cool. The cooled coil is then cold rolled to reduce its thickness still further. Often, the strip will be cold rolled in several passes with an intermediate annealing step between each cold rolling pass. When the alloy strip has been reduced to its final thickness, it can be cut into appropriate shapes for the production of useful products, such as container bodies or container ends. Typically, at various stages of the process, scrap is produced (plant scrap) . Several patents pertain to low earing aluminum alloys or processes for their production. For example, U.S. Patent No. 4,238,248 by Gyongyos et al., issued on December 9, 1980, discloses a process for producing a low earing aluminum alloy. A melt of 3004 alloy, or an alloy in which the combined concentration of manganese and magnesium is between 2 percent and 3.3 percent (unless otherwise indicated, all percentages will be weight percent) and in which the ratio of magnesium:manganese is between 1.4:1 and 4.4:1, is cast and then held for 2 to 15 minutes between 400°C and the alloy's liguidus temperature (the temperature at which the alloy's phase changes between a liquid and a solid/liquid composition, in this case, approximately 600°C) . It is then hot-rolled at a temperature between 300°C and the non-equilibrium solidus temperature (the temperature at which the alloy's phase changes between the solid/liquid composition and a completely solid composition) , coiled and cooled to room temperature. A first cold rolling stage reduces the thickness by at least 50 percent and is followed by a flash annealing stage at 350°C to 500°C for less than 90 seconds. A second cold rolling stage results in further reduction of up to 75 percent. U.S. Patent No. 3,560,269 by Anderson et al., issued on February 2, 1971, discloses an aluminum alloy, the composition of which is set forth in Table 1. An ingot is cast by direct chill casting, heated to 800°F, and held at that temperature for 24 hours. The ingot is hot-rolled and the resulting strip is annealed at 700°F. A first cold rolling stage reduces the thickness by at least 85 percent and is followed by annealing at 600°F. An optional second cold rolling stage provides further reduction of at least 30 percent to a final thickness. The resulting sheet is described as having earing of not more than 3 percent, an amount which, according to the inventors, is acceptable. As noted above, the required characteristics of alloy for container ends differ from those of container bodies; melting recycled aluminum containers (a combination of ends and bodies) produces a melt which may be unsatisfactory for the production of either container bodies or container ends. The weight percents of the components of a typical melt of recycled aluminum comprising approximately 25 percent container ends and 75 percent container bodies are shown in Table 1. Efforts have been made to produce an alloy from recycled aluminum containers which is suitable for both container bodies and container ends. U.S. Patent Nos. 4,411,707 by Brennecke et al., issued on October 25, 1983; 4,282,044 by Robertson et al., issued on August 4, 1981; 4,269,632 by Robertson et al. issued on May 26, 1981; 4,260,419 by Robertson et al. issued on April 7, 1981; and 4,235,646 by Neufeld et al. issued on November 25, 1980 disclose related methods for processing recycled aluminum containers. All begin with an initial melt of approximately 25 weight percent container ends and approximately 75 weight percent container bodies, as shown in Table 1. The initial melt is then adjusted, generally by the addition of pure aluminum, to form an alloy whose composition is also shown in Table 1. The combined concentration of manganese and magnesium is within the range of 2.0 to 3.3 percent and the ratio magnesium: anganese is within the range of 1.4:1 to 4.4:1. The differences among the foregoing patents occur in the way the alloy is cast and processed after being adjusted to the desired composition. U.S. Patent Nos. 4,235,646, 4,260,419 and 4,282,044 each disclose a continuous strip casting process in which the alloy strip (having the composition previously described) is held at a temperature between 400°C and 600°C for 2 to 15 minutes after it has been cast. It is then hot-rolled for a thickness reduction of at least 70 percent, coiled and allowed to cool to room temperature. The strip is then uncoiled and cold rolled to a final thickness in either one or two steps. If cold rolling occurs in two steps, the first results in a reduction of at least 50 percent and is followed by a flash anneal in which the alloy is heated to between 350°C and 500°C and then cooled down to room temperature, all within a period not exceeding 90 seconds. The alloy is cold rolled a second time producing additional reduction of 75 percent or less. U.S. Patent Nos. 4,269,632 and 4,260,419 disclose direct chill casting methods of the melt described above in which the resulting cast ingot is held at a temperature between 550°C and 600 β C for 4 to 6 hours and then allowed to cool. It is hot-rolled when its temperature is between 450°C and 510°C producing a thickness reduction of between 40 percent and 96 percent. The resulting strip is hot- rolled a second time for an additional reduction of between 70 percent and 96 percent. The strip is coiled and then annealed in one of two ways. It may be flash annealed for 30 to 90 seconds between 350°C and 500°C or, it may be annealed for 2 to 4 hours between 315°C and 400°C. After annealing, the strip is allowed to cool and is then cold rolled in one or more stages to produce a total reduction of approximately 89 percent in thickness. After each cold rolling stage, the alloy is annealed using either a flash or conventional method. U.S. Patent No. 4,411,707 discloses a process for producing container ends from the previously described scrap melt using a variation of the continuous chill roll casting method. The molten alloy, between 682°C and 710°C, is cast to a thickness between 0.23 and 0.28 inches and then rolled to reduce the thickness to approximately 25 percent. The strip is coiled and allowed to cool to room temperature after which it is cold rolled in at least two stages. In the first, a reduction of at least 60 percent in thickness occurs and in the second, a reduction of at least 85 percent occurs. The alloy is annealed for approximately 2 hours at 440 β C to 483°C between the two cold rolling stages. Additional cold rolling/annealing stages can be used if desired. U.S. Patent No. 3,787,248 by Setzer et al., issued on January 22, 1974, also discloses a process for producing an alloy from a melt of recycled aluminum containers which is suitable for both container ends and container bodies. The composition of the alloy is set forth in Table 1. Any conventional casting method may be used (although a preference is stated for direct chill casting) after which -li¬ the alloy is homogenized for 2 to 24 hours between 850°F and 1150°F. The metal is then hot-rolled at least twice, the first time achieving at least a 20 percent reduction in thickness at a temperature between 650 β F and 950°F and the second, also achieving at least a 20 percent reduction, between 400 β F and 800°F. A third rolling operation (comparable to cold rolling) , at a temperature less than 400°F, achieves at least a 20% reduction to the final thickness. The alloy is then annealed between 200°F and 450°F for a period greater than 5 seconds (preferably between 30 minutes and 8 hours) . Instead of a single cold rolling step, the aluminum strip may be cold rolled and annealed two or three times to obtain the final thickness. U.S. Patent No. 4,318,755 by Jeffrey et al., issued on March 9, 1982 discloses an aluminum alloy, the composition of which is set forth in Table 1, suitable for container bodies made from recycled containers using continuous strip casting methods. The strip exits the caster at 380°C to 450°C and is hot-rolled to reduce the thickness between 72 percent and 82 percent; the strip exits the hot-roller between 150°C and 200°C and is coiled. The strip is then cold rolled to its final thickness and is either annealed for 2 hours between 400°C and 420°C or flash annealed. It would be useful to provide a process for the fabrication of an aluminum alloy sheet which has a low earing percentage, which possesses good strength characteristics in thinner gauges than are presently employed and which is suitable for use in the production of both container bodies and container ends. It would also be useful to fabricate such a sheet from an alloy which can be produced substantially from recycled aluminum containers. Summary of the Invention In accordance with the present invention, a process is provided for the production of aluminum sheet from an aluminum alloy. The aluminum sheet (also known as strip stock) is suitable for the fabrication of both container ends and container bodies in gauges thinner than typically currently employed, has low earing properties and can be formed at least in part from recycled aluminum scrap. An initial alloy melt may be formed from aluminum scrap, including plant, container and consumer scrap, which is then adjusted to form the alloy composition of the present invention. This composition preferably comprises: about 1.9 percent to about 2.8 percent magnesium; about 0.9 percent to about 1.6 percent manganese; the balance being essentially aluminum. The adjusted melt is preferably cast into strips and is hot rolled to a first thickness. The hot rolled strip is annealed and then cold rolled in at least one pass to a final gauge. In one embodiment, annealing the hot rolled strip comprises coiling the strip and heating it for about three hours or less at a temperature in the range from about 775°F to about 830°F and then cooling the annealed strip to about 500°F. In another embodiment, annealing the hot rolled strip comprises self annealing wherein the strip is tightly coiled and allowed to cool from its hot rolled exit temperature to ambient temperature over an extended period of time. Cold rolling the annealed strip comprises the steps of reducing the thickness of the strip by at least 35 percent in a first stage and then subjecting the strip to intermediate annealing. After the intermediate annealing step, the strip is cold rolled in a second stage to a final gauge with a reduction of about 45 percent to about 65 percent. The final gauge can be very thin, for example 0.010 inches, while maintaining the strength and low earing properties of the strip. In one embodiment of the present invention, cold rolling also includes the step of using a water-based rolling emulsion which allows the cold rolled strip to attain a higher temperature than would be possible if an oil-based rolling emulsion was used. The process of the present invention has the technical advantage of providing low earing aluminum sheet which is suitable for fabrication of both container ends and container bodies in thinner gauges than are possible using prior known alloys and processes. The process of the present invention has the further technical advantage of permitting the aluminum alloy sheet to be derived from aluminum scrap. Brief Description of the Drawings Figure 1 is a graph illustrating relationships between yield strength and cold work, and earing and cold work; Figures 2 and 2a are a flowchart of embodiments of the process of the present invention; and Figure 3 is a chart illustrating the effect of altering the manganese and magnesium concentrations on strength and earing characteristics of alloy sheets fabricated in accordance with the present invention. Figure 4 is a chart illustrating the effect of cold work on the earing percentage of a direct cast prior art alloy and a block cast alloy of the present invention. Detailed Description of the Invention In accordance with the present invention, a process for producing aluminum strip or sheet stock from an alloy is provided. As used herein, the term "sheet" refers to a cast piece of aluminum having a length and width substantially longer than the thickness, and includes coils, strips, flat stock, can blanks, and lid stock. The sheet stock has a reduced earing percentage and improved strength in thinner gauges than aluminum sheet that is presently fabricated. The alloy comprises a composition which can be derived, at least in part, from recycled aluminum scrap. The process can include the steps of casting, hot-rolling, annealing and cold rolling. The resulting aluminum sheet is especially suitable for use in the fabrication of deep drawn and ironed articles, such as beverage container bodies, as well as beverage container ends. According to the present invention, it is preferable to utilize a block casting technique to process an aluminum alloy composition. A block casting technique is shown graphically in the flowchart of Figures 2 and 2a. The block caster is preferably of the type disclosed in U.S. Patent Nos. 3,709,281, 3,744,545, 3,747,666, 3,759,313 and 3,774,670. A melt of the proper alloy composition is formed and is pre arably cast through a nozzle with, for example, a 16 millimeter tip. The melt is cast in a casting cavity formed by opposite pairs of rotating blocks, preferably to a thickness of less than about 0.8 inches (20 mm) , and more preferably from about 0.6 to about 0.8 inches (15.2 mm to 20 mm) . The strip of metal travels as it cools and solidifies along with the chilling blocks until the strip exits the casting cavity where the chilling blocks separate from the cast strip and travel to a cooler where the chilling blocks are cooled. The rate of cooling as the cast strip passes through the casting cavity of the chill block casting machine is controlled by various process and product parameters. These parameters include the composition of the material being cast, the strip gauge, the chill block material, the length of the casting cavity, the casting speed and the efficiency of the chill block cooling system. It is preferred that the cast strip be as thin as possible. This minimizes subsequent working of the strip. Normally, a limiting factor in obtaining minimum strip thickness is the size of the distributor tip of the caster. In the preferred embodiment of the present invention, the strip is cast at a thickness of from about 0.6 to about 0.8 inches (about 15.2 mm to 20 mm) . However, thinner strip can be cast. The cast strip normally exits the block caster in the temperature range from about 850°F to about 1100°F (about 450° C to 595° C) . Upon exiting the caster, the cast strip is then subjected to a hot rolling operation in a hot mill. The cast strip preferably enters the first hot rollers at a temperature in the range from about 880°F to about 1000°F (about 470° C to 525°C) , and more preferably in the range from about 900°F to about 975°F (about 480° C to 525° C) . The hot rollers preferably reduce the thickness of the strip by at least about 70 percent and more preferably by at least about 80 percent. It is preferred to maximize the percentage reduction in the hot mill. It has been unexpectedly found that strip product having improved properties can be obtained if, in addition to the other process steps indicated herein, the temperature of the strip exiting the hot mill is minimized. To obtain the desired product properties, the exit temperature from the hot mill should be no more than about 660°F (345°C) , and is preferably from about 630°F to about 660°F (about 330°C to 345°C) . While this temperature is preferably minimized, an increased percentage of hot mill reduction will lead to higher exit temperatures. The strip is preferably held at the hot mill exit temperature for a period of time, coiled and then annealed (also known as heat treatment) . It is believed that this annealing step is critical to reducing the earing in the final strip stock. Preferably, the coiled strip is annealed for at least about 3 hours, preferably at a temperature from about 820°F to about 830°F. In one embodiment, the coiled strip is annealed for less than about 3 hours at a temperature from about 775°F to about 830°F (410°C to 445 β C) . The temperature of the coil upon exiting the annealing step is preferably about 500°F (260°C) , and it is allowed to cool to ambient temperature. In an alternative embodiment, if the strip has sufficient mass, such as greater than about 13,000 pounds, it may be self-annealed by coiling the strip very tightly and allowing it to cool slowly to ambient temperature. This process may take as long as two days or more, but is advantageous since no additional heat is necessary to anneal the strip and thus energy costs are reduced. After the annealed coil has cooled to ambient temperature, it is cold rolled to a final gauge in at least one stage of cold roll passes, and preferably in two stages. In the first cold rolling stage, the thickness is preferably reduced by about 40 percent to about 80 percent. In one embodiment, the first cold rolling stage includes a single cold roll pass. In a more preferred embodiment, at least two cold roll passes are employed, the first pass causing a thickness reduction of up to about 40 percent and the second cold roll pass causing an additional reduction of about 35 percent to about 70 percent. It has been found that cold rolling using at least two cold roll passes in the first cold rolling stage produces a cast strip having better uniformity. The temperature of the strip upon its exit from each cold rolling pass is approximately 150°F to 200°F (65°C to 95°C) due to the friction of the rollers on the alloy strip. Following the first cold rolling stage, the strip is preferably annealed for about 3 hours at from about 650°F to about 700°F (about 340°C to 375°C) , more preferably at from about 670°F to about 680°F (about 355°C to 360°C) . This intermediate anneal improves the formability and earing characteristics of the final strip. After the cold rolled and annealed strip has cooled to ambient temperature, it goes through a second cold rolling stage in which the thickness is further reduced. The final cold rolling stage is a significant factor in controlling the earing of the product. The amount of reduction in thickness needed in the final cold roll stage, i.e., the final cold work percentage, determines the amount of reduction in thickness required in the first cold rolling stage. The preferred final cold work percentage is that point at which the optimum balance between the yield strength and earing are obtained. This point can be readily determined for a particular alloy composition by plotting each of the yield strength and earing values against the cold work percentage. Once this preferred cold work percentage is determined for the final cold rolling stage, the gauge of the strip during the intermediate annealing stage and, consequently, the cold working percentage for the initial cold roll stage, can be determined. The final cold work percentage required to minimize earing is dependent upon the composition of the particular alloy. It is expected that aluminum alloys with higher magnesium content have higher cold work percentages. According to the present invention, the thickness is reduced in the second cold rolling stage by about 35 percent to about 70 percent, preferably by about 45 percent to about 65 percent, and more preferably by about 50 percent to about 60 percent, to a final gauge of, for example, less than about 0.0116 inches (about 0.29 mm). The second stage can include a single cold rolling pass or can include two or more passes, and the final gauge can be, for exε le, 0.010 inches (0.254 mm). The second cold rolling stage can include stabil-'ing the cold rolled strip by employing a water-based rolxing emulsion during the cold rolling process. The amount of reduction which is possible during cold rolling utilizing an oil-based emulsion is limited by the flash point of the emulsion. Greater reduction creates greater friction which increases the exit temperature of the strip. If the temperature rises above the flash point of the emulsion, a fire can occur. Consequently, the reduction must be limited such that the heat generated remains below the flash point of the oil-based emulsion. By contrast, stabilizing during cold rolling by utilizing a water-based rolling emulsion reduces the chance of a fire. Therefore, greater thickness reductions may occur in each pass with temperatures as high as 300°F to 350°F (145°C to 180°C), temperatures which are much greater than would be safely possible with an oil-based emulsion. By stabilizing, the mechanical properties will be reduced during cold rolling so that the aluminum sheet will not experience any substantial decrease in strength during subsequent processing. After the final cold rolling pass, the strip can be subjected to a tension leveling step to achieve a more uniform flatness. This is accomplished by pulling or stretching the strip between rollers. The aluminum alloy sheet produced according to the process of the present invention is useful for a number of applications. These applications include, but are not limited to, cable sheathing, Venetian blind stock, and other building products. The alloy sheet produced according to the present invention is particularly useful for drawn and ironed container bodies and for container tops. When the aluminum alloy sheet is to be fabricated into container tops, the intermediate anneal step is preferably not performed. Before baking or stabilization, the alloy sheet has a yield strength greater than about 38 ksi (262 MPa) , preferably greater than about 42 ksi (290 MPa) and more preferably greater than about 44 ksi (304 MPa) . The as- rolled alloy sheet has a tensile strength preferably greater than about 46 ksi (318 MPa) and more preferably greater than about 48 ksi (332 MPa) . To produce drawn and ironed container bodies, the aluminum alloy sheet is cut into substantially circular blanks. The blanks are then shaped with a die to form a cup. The cup is drawn and ironed into a container body by forcing the cup through a series of dies having progressively smaller diameters. Typically, after the container has been drawn and ironed, it is washed to remove any impurities. After washing, the container body is typically placed in a drying oven to remove moisture. The drying oven will typically be at a temperature of approximately 400 β F (204°C) and the container will typically stay within the oven for about 3.5 minutes. Following the drying step, the container can be internally coated and painted on the exterior. After coating and painting, the container is again subjected to baking for about 3.5 minutes at about 400°F (204°C) to cure the paint and the coating. According to the present invention, an aluminum alloy composition especially suitable for the manufacture of aluminum sheet of the present invention preferably includes at least about 0.9 weight percent manganese, more preferably from about 1.1 weight percent to about 1.6 weight percent manganese, and most preferably from about 1.1 weight percent to about 1.2 weight percent manganese. The alloy composition further includes from about 1.9 weight percent to 2.8 weight percent magnesium, preferably from about 2.0 weight percent to about 2.6 weight percent magnesium. Certain narrower embodiments wherein these ranges are preferred, as will be discussed hereinbelow. In addition to the manganese and magnesium, the aluminum alloy preferably includes: from about 0.13 weight percent to about 0.25 weight percent silicon, more preferably from about 0.15 weight percent to about 0.20 weight percent silicon; from about 0.15 weight percent to about 0.28 weight percent copper, more preferably from about 0.20 weight percent to about 0.25 weight percent copper; and from about 0.25 weight percent to about 0.40 weight percent iron, more preferably from about 0.30 weight percent to about 0.35 weight percent iron; the balance being essentially aluminum. The foregoing constitute the primary alloying elements of the aluminum alloy. In addition to these primary aluminum alloying agents, traces of other elements, such as titanium, chromium and zinc, may be present in the composition. It is preferred that the level of zinc not exceed about 0.12 weight percent, that the level of titanium not exceed about 0.03 weight percent and that the total chromium not exceed about 0.05 weight percent. It is preferable that any additional impurities do not exceed a total of about 0.2 weight percent, and that none of the impurity elements comprise more than about 0.05 weight percent individually. According to the present invention, the amounts of magnesium and manganese can vary within the above-described ranges, and an alloy suitable for the manufacture of drawn and ironed container bodies will still result. However, there exists certain preferred ranges for the magnesium and manganese alloying elements ^ within these broad ranges. These preferred alloys are listed in Table 2. Table 2 Mg (Wt. %) Mn (Wt. %) Alloy A 2.6-2.8 0.9-1.0 Alloy B 2.6-2.8 1.3-1.5 Alloy C 2.0-2.1 1.5-1.6 Alloy D 1.9-2.3 0.95-1.2 Alloy E 2.3-2.6 0.95-1.2 More preferably. Alloy D includes from about 2.0 to about 2.1 weight percent magnesium. Alloy E includes from about 2.45 weight percent to about 2.55 weight percent magnesium, and both alloys preferably include from about 1.1 weight percent to about 1.2 weight percent manganese. While all of the alloys in Table 2 are useful for manufacturing drawn and ironed container bodies. Alloys D and E are most preferred. This is because too high a strength can lead to "wrinkling" of the can container body during the drawing and ironing steps of the manufacturing process, which can lead to an increased amount of scrap. It has been found advantageous to minimize the ratio of magnesium to manganese within these ranges. Accordingly the ratio of magnesium to manganese is preferably less than about 3.2:1, more preferably less than about 2.2:1. It has been found that decreasing the ratio of magnesium to manganese (that is, increasing the amount of manganese relative to the magnesium, or decreasing the amount of magnesium relative to the manganese) permits a hot rolled strip of the present alloy to tolerate greater cold work, thus increasing the strength and reducing the thickness, without increasing the earing. While not wishing to be bound by theory, it is believed that each 0.1 weight percent increase in the concentration of manganese increases the yield strength of an aluminum sheet formed from the alloy by approximately 660 psi (4.5 MPa). Increasing the cold work percentage during processing may also increase the yield strength; however, cold working also tends to increase the earing percentage when an alloy blank is drawn and ironed into a beverage container. Figure 1 graphically illustrates these relationships for an AA 5017 alloy. The strip stock produced from the alloy and process of the present invention can advantageously provide increased yield strength by increasing the amount of manganese in the alloy, but maintains a low earing percentage. The alloy used to fabricate aluminum sheet of the present invention may be obtained by melting the primary constituents together or may be obtained by adjusting the composition of a melt of scrap aluminum. As used herein, the term scrap aluminum refers to aluminum that may comprise plant, container and consumer scrap in which container body alloy, eg. AA 3004, and container end alloy, eg. AA 5082 and AA 5182, are present in a weight ratio of approximately 3:1. As previously noted, such a scrap melt will typically have a manganese content of approximately 0.8 weight percent and a magnesium content of approximately 1.5 weight percent. Adjustment to provide the composition of the present invention can involve the addition of unalloyed aluminum, manganese, magnesium or combinations of the three. A technique useful for measuring the strength of a container body is to measure the dome strength of the container. The dome strength is the internal pressure that a container can withstand before the dome at the bottom of the container yields, or deforms. Containers formed from a sheet of the alloy according to the present invention having a thickness from about 0.011 inches (0.28 mm) to about 0.0123 inches (0.31 mm), have a minimum dome strength of at least about 90 psi (0.62 MPa), more preferably at least about 96 psi (0.66 MPa) and most preferably at least 100 psi (0.69 MPa) . To produce a 90 pound container, suitable for soda and other highly carbonated beverages, it is preferable that the container maintain a yield strength of between about 38 ksi (262 MPa) and about 42 ksi (290 MPa) after the final baking process described above. The aluminum alloy sheets produced according to the process of the present invention preferably have a yield strength greater than about 38 ksi (262 MPa) after baking, and more preferably between about 42 ksi (290 MPa) and about 46 ksi (318 MPa) after baking. Additionally, the alloy sheet produced according to the process of the present invention preferably has a 45° earing percentage of less than about 2 percent, more preferably less than about 1.8 percent, and most preferably less than about 1.7 percent. This low earing characteristic facilitates the manufacture of drawn and ironed container bodies, reduces the labor required during the drawing and ironing, and minimizes plant scrap. EXAMPLES Example 1: As an example of the application of the process of the present invention, a melt derived from scrap aluminum was adjusted to have a manganese concentration of 1.0 weight percent and a magnesium concentration of 2.8 weight percent. The resulting alloy composition was cast as a strip in a continuous chill block caster through a 16 mm distributor tip. Hot rolling reduced the cast strip to a gauge of 0.085 inches (2.16 mm) with an exit temperature of between 620° F and 640° F (325 β C to 340°C) . The hot rolled strip was subsequently annealed (heat treated) for about three hours at 825° F (440°C) . Following the annealing were two cold rolling stages. The first stage included two cold roll passes, the first pass reducing the strip to a gauge of 0.055 inches (1.40 mm) and the second reducing the strip to a gauge of 0.017 inches (0.43 mm) . The cold rolled strip was then intermediate annealed at 650° F to 700° F (340°C to 375°C) and cold rolled in a second stage, comprising a single pass, to a final gauge of 0.0110 inches (0.28 mm). Testing of the resulting strip stock demonstrated a tensile strength of 46.5 to 51.3 ksi (320 MPa to 355 MPa), a yield strength of 43.6 to 46.8 ksi (300 MPa to 323 MPa) and a percent elongation of 2 to 4 percent. The 45° earing texture was 2.2 percent and the dome strength was 97 psi. Example 2: Table 3 illustrates the results of tests showing the effect of increasing the final cold work percentage on ultimate tensile strength (UTS) , yield tensile strength (YTS) and 45° earing texture of a sheet fabricated from Alloy A in accordance with the process of the present invention: TABLE 3 Cold UTS YTS Earing Work (ksi) (ksi) (%) 45% 46.5 44.4 1.8 55% 49.5 45.9 2.4 Increasing the cold work increases the strength but also increases the earing. By comparison, a sheet fabricated from Alloy C in accordance with the process of the present invention with cold work of 55 percent has a tensile strength of about 48.7 'ksi (336 MPa), a yield strength of about 46.1 ksi (318 MPa) and a 45° earing texture of about 1.7 percent. Example 3: Figure 3 graphically illustrates the effect of changes in the amounts of manganese and magnesium on ultimate tensile strength (UTS) , yield strength and earing percentage in aluminum alloy sheets fabricated in accordance with the process of the present invention. The alloys identified as R-16, R-22 and U-03 are AA 5017 alloys and the alloy identified as C-10 is Alloy A of the present invention (from Table 2 above) . The concentrations of manganese and magnesium in each of the alloys is set forth in Table 4: c It can be seen that increasing the manganese and magnesium concentrations from the amounts in the AA 5017 alloys to the amounts in the C-10 alloy causes an increase in both tensile strength and yield strength. It also causes some increase in earing, although the earing percentage does not exceed the desirable 2 percent limit. Example 4: The following example illustrates the high strength of containers fabricated in accordance with the process of the present invention. Aluminum alloy sheets were produced using Alloy A, having 1.0 weight percent manganese and 2.8 weight percent magnesium, in accordance with the process of the present invention. During the process, some of the sheets were stabilized during cold rolling, while the others were not. The sheets were cold rolled to three gauges and fabricated into two-piece aluminum beverage containers which were then subjected to dome strength testing to measure the maximum internal pressure which a sealed container can withstand. The results are shown in Table 5: TABLE 5 The term "3 sigma low" in Table 4 refers to three standard deviations and indicates the lowest dome strength statistically predictable. As indicated in Table 5, containers fabricated with the process of the present invention have sufficient strength to withstand the internal pressures generated by pasteurized beer and other highly carbonated beverages even in thin gauges. Example 5: As an example of the production of aluminum sheet of the present invention, a melt derived from scrap aluminum is adjusted to have a manganese concentration of 1.0 weight percent and a magnesium concentration of 2.5 weight percent. The resulting alloy composition is cast as a strip in a continuous chill block caster. Hot rolling reduces the cast strip to a gauge of about 0.085 inches (2.16 mm) with an exit temperature of from 620°F to about 640°F (325°C to 340°C) . The hot rolled strip is subsequently annealed (heat treated) for about three hours at 825°F (440°C) . The percentage of cold work on the hot rolled strip is varied and the results are plotted in Figure 4. The results are plotted against a direct cast alloy having a composition comprising 1.1 percent magnesium and 0.8 percent manganese. As can be seen in Figure 4, a cold work percentage of less than about 62 percent on the alloy of the present invention results in an earing of less than about 1.8 percent. To achieve a 45° earing of less than about 1.8 percent on the direct cast alloy requires a cold work percentage of greater than 80 percent. Exa ple 6 : Table 6 lists the manganese and magnesium concentrations for two examples of prior art alloys, and three examples of alloys according to the present invention. Example A is an AA 5017 alloy. Example B is an AA 3104 alloy, and Examples C, D & E are according to the present invention. Table 7 shows the results of block casting these compositions. As can be seen from Table 7, the block caster produced sheets of 5017 and 3104 alloys that have an after bake yield strength below the 38 ksi minimum required to meet the minimum average 90 psi buckle strength for beverage cans. However, Examples C, D and E according to the present invention have the desired mechanical property limits after stabilization and baking. TS = Tensile Strength (ksi) YS = Yield Strength (ksi) While various embodiments of the present invention have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. For example, the process of the present invention can be used to produce sheets from alloys other than the alloys disclosed. It is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present invention.

Claims: What Is Claimed Is: 1. A process for the production of aluminum alloy sheet, comprising the steps of: a) forming an aluminum alloy melt composition comprising from about 0.9 to about 1.6 weight percent manganese and from about 1.9 to about 2.8 weight percent magnesium; b) casting said aluminum alloy to form a cast strip; c) hot rolling said cast strip to reduce the thickness of said cast strip by at least about 70 percent and form a hot rolled strip; d) annealing said hot rolled strip to form an annealed strip; and e) cold rolling said annealed strip to form an aluminum alloy sheet. 2. A process as recited in Claim 1, wherein said cast strip has a thickness from about 0.60 inches to about 0.80 inches. 3. A process as recited in Claim 1, wherein said casting step comprises casting said aluminum alloy composition in a block caster. 4. A process as recited in Claim 1, wherein said hot rolling reduces the thickness of said cast strip by at least about 75 percent. 5. A process as recited in Claim 1, wherein said hot rolled strip ha? a thickness of less than about 0.080 inches. 6. A process as recited in Claim 1, wherein said hot rolled strip has a thickness of less than about 0.070 inches. 7. A process as recited in Claim 6, wherein the hot roll exit temperature of said hot rolled strip is less than about 600°F. 8. A process as recited in Claim 1, wherein the hot roll exit temperature of said hot rolled strip is from about 630°F to about 660°F. ___ 9. A process as recited in Claim 1, wherein said annealing step comprises maintaining said hot rolled strip at a temperature from about 775°F to about 830°F. 10. A process as recited in Claim 1, wherein said annealing step comprises coiling said hot rolled strip into a coil and allowing said coil to cool to about room temperature. 11. A process as recited in Claim 1, wherein said annealing step comprises annealing said hot rolled strip for less than about 3 hours. 12. A process as recited in Claim 1, wherein said cold rolling step includes a first stage of cold rolling comprising the steps of: a) cold rolling said annealed strip to form a cold rolled strip having a first cold roll gauge; and b) cold rolling said cold rolled strip having a first cold roll gauge to form a cold rolled strip having a second cold roll gauge. 13. A process as recited in Claim 12, wherein step a) reduces the thickness of the strip by less than about 40 percent and step b) further reduces the thickness of the strip from about 35 percent to about 70 percent. 14. A process as recited in Claim 1, further comprising the steps of: a) annealing the cold rolled strip; and b) further cold rolling said annealed strip to a final gauge. 15. A process as recited in Claim 14, wherein said annealing step comprises heating said cold rolled strip to a temperature from about 650 β F to about 700°F. 16. A process recited in Claim 1, wherein said cold rolling step comprises cold rolling said annealed strip to reduce the thickness of the annealed strip by at least about 35 percent. 17. A process as recited in Claim 1, wherein said cold rolling step comprises the steps of: a) cold rolling said annealed strip to a strip having a first cold roll gauge; b) annealing said cold rolled strip having a first cold roll gauge; and c) further cold rolling said strip to an aluminum alloy sheet of final gauge; wherein the final gauge of the aluminum alloy sheet is from about 35 percent to about 70 percent of the first cold roll gauge. 18. A process as recited in Claim 17, wherein said final gauge is less than about 0.0118 inches. 19. A process as recited in Claim 1, wherein said aluminum alloy melt composition further comprises: a) from about 0.13 to about 0.25 weight percent silicon; b) from about 0.25 to about 0.40 weight percent iron; and c) from about 0.15 to about 0.28 weight percent copper. 20. A process as recited in Claim 19, wherein said aluminum alloy melt composition comprises: a) from about 2.6 to about 2.8 weight percent magnesium; and b) from about 1.3 to about 1.5 weight percent manganese. 21. A process as recited in Claim 19, wherein said aluminum alloy melt composition comprises: a) from about 2 to about 2.1 weight percent magnesium; and b) from about 1.5 to about 1.6 weight percent manganese. 22. A process as recited in Claim 19, wherein said aluminum a3.1oy melt composition comprises: a) from about 2.6 to about 2.8 weight percent magnesium; and b) from about 0.9 to about 1.0 weight percent manganese. 23. A process as recited in Claim 19, wherein said aluminum alloy melt composition comprises: a) from about 1.9 to about 2.3 weight percent magnesium; and b) from about 0.95 to about 1.2 weight percent manganese. 24. A process as recited in Claim 23, wherein said aluminum alloy melt composition comprises: a) from about 2.0 to about 2.1 percent magnesium; and b) from about 1.1 weight percent to about 1.2 weight percent manganese. 25. A process as recited in Claim 19, wherein said aluminum alloy melt composition comprises: a) from about 2.3 to about 2.6 weight percent magnesium; and b) from about 0.95 to about 1.2 weight percent manganese. 26. A process as recited in Claim 25, wherein said aluminum alloy melt composition comprises: a) from about 2.45 to about 2.55 weight percent magnesium; and b) from about 1.1 to about 1.2 weight percent manganese. 27. A process as recited in Claim 1, further comprising the step of tension leveling said cold rolled strip. 28. A process as recited in Claim 1, wherein said cold rolling step comprises cold rolling with water cooled rollers to stabilize said cold rolled strip. 29. A process as recited in Claim 1, wherein said aluminum alloy sheet has a yield strength of at least about 38 ksi. 30. A process as recited in Claim 1 wherein said aluminum alloy has an ultimate tensile strength of at least about 46 ksi. 31. A process for the production of aluminum alloy sheet having a yield strength of at least about 40 ksi and a 45° earing percentage of less than about 2 percent, comprising the steps of: a) casting an aluminum alloy melt composition in a continuous caster to form a cast strip, said aluminum alloy melt composition comprising; i) from about 2.0 to about 2.8 weight percent magnesium, ii) from about 0.9 to about 1.6 weight percent manganese, iii) from about 0.13 to about 0.20 weight percent silicon, iv) from about 0.25 to about 0.35 weight percent iron, and v) from about 0.20 to about 0.25 weight percent copper; b) hot rolling said cast strip to reduce the thickness of said cast strip by at least about 70 percent and form a hot rolled strip, wherein the cast strip exits said hot rolling step at a temperature less than about 650°F c) annealing said hot rolled strip at a temperature of from about 820°F to about 830°F to form an annealed strip; d) cold rolling said annealed strip to a first cold rolled gauge; e) cold rolling said cold'rolled strip of a first cold rolled gauge to a second cold rolled gauge; f) annealing said cold rolled strip of a second cold rolled gauge at a temperature from about 650°F to about 700°F to form an annealed cold rolled strip; and g) cold rolling said annealed cold rolled strip to reduce the thickness of the annealed cold rolled strip by at least about 35 percent. 32. A process as recited in Claim 31, wherein said casting step comprises continuously casting said aluminum alloy in a block caster. 33. A process for the production of an aluminum alloy container body, comprising the steps of: a) forming an aluminum alloy melt composition comprising from about 0.9 to about 1.6 percent manganese and from about 2.0 to about 2.8 percent magnesium; b) casting said aluminum alloy to form a cast strip; c) hot rolling said cast strip to reduce the thickness of said cast strip by at least about 70 percent and form a hot rolled strip wherein the exit temperature of the hot rolled strip is less than about 650°F; d) annealing said hot rolled strip to form an annealed strip; e) cold rolling said strip to reduce the thickness of said annealed strip and form an aluminum alloy sheet having a thickness of less than about 0.018 inches; f) removing a portion of said aluminum alloy sheet to form a blank; g) cupping said blank on a cupping mold; and h) drawing and ironing said blank to form an aluminum alloy container body. 34. A process as recited in Claim 33, wherein said container body has a dome strength of at least about 90 psi.

Inventors: MARSH IVAN M, MCAULIFF DONALD C, MARSH, IVAN, M., MCAULIFF, DONALD, C.
Applicants: GOLDEN ALUMINUM CO, GOLDEN ALUMINUM COMPANY
Application year: 1991
Main classification: C22F1
IPC codes: C22C21, C22F1