Method for making an improved aluminum alloy sheet product

Abstract

An aluminum alloy sheet and a method for producing an aluminum alloy sheet. The aluminum alloy sheet is useful for forming into drawn and ironed container bodies. The sheet preferably has an after-bake yield strength of at least about 37 ksi and an elongation of at least about 2 percent. Preferably, the sheet also has earing of less than about 2 percent.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001] The-present application is a continuation-in-part of U.S. patent application Ser. No. 08 / 401,418 filed Mar. 9, 1995, the disclosure of which is incorporated herein by reference in its entirety.FIELD OF THE INVENTION[0002] The present invention relates generally to aluminum alloy sheet and methods for making aluminum alloy sheet. Specifically, the present invention relates to aluminum alloy sheet and methods for making aluminum alloy sheet wherein the sheet is particularly useful for forming into drawn and ironed container bodies.BACKGROUND OF THE INVENTION[0003] Aluminum beverage containers are generally made in two pieces, one piece forming the container sidewalls and bottom (referred to herein as a "container body") and a second piece forming the container top. Container bodies are formed by methods well known in the art. Generally, the container body is fabricated by forming a cup from a circular blank of aluminum sheet and then exten...

AI Technical Summary

Benefits of technology

[0020] In accordance with the present invention, a continuous process for producing aluminum sheet is provided. In accordance with the process, relatively high reductions in gauge can be achieved in both the hot mill and cold mill. Additionally, due to the fact that greater hot mill and cold mill reductions are possible, the number of hot roll and cold roll passes can be reduced as compared to commercially available continuously cast can body stock. A relatively high proportion of cold work is needed to produce can body stock having acceptable physical properties according to the sheet production process of the present invention, as compared to commercially available continuously cast can body stock. Thus, a reduced amount of work hardening is imparted to the sheet when it is manufactured into items such as drawn and ironed containers, when compared to commercially available continuously cast can body stock.

[0021] In accordance with the present invention, the need for a high temperature soak (i.e., homogenization) can be avoided. When the high temperature homogenization step is performed when the metal is coiled, it can result in pressure welding such that it is impossible to unroll the coil. Also, the need for solution heat treatment after the hot mill (e.g., as disclosed in European Patent Application No. 93304426.5) can be avoided. By avoiding solution heat treatment, the continuous casting process is more economical and results in fewer process control problems.

[0022] In accordance with the present process, high amounts of recycled aluminum can be advantageously employed. For example, 75 percent and preferably up to 95 percent or more of used beverage containers (UBC) can be employed to produce the continuous cast sheet of the present invention. The use of increased amounts of UBC significantly reduces the cost associated with producing the aluminum sheet.

[0023] In accordance with the present invention, a continuous cast alloy is provided which includes relatively high levels of copper (e.g., 0.3 to 0.6 percent). It has surprisingly been found that the copper can be increased to these levels without negatively affecting the earing. If copper is increased in ingot cast processes, the resulting alloy can be too strong for can-making applications. In addition, in accordance with the present invention, relatively low levels of magnesium are used (e.g., 1.0 to 1.5 percent), leading to better can surface finish than commercially available continuously cast can body stock. For example, when drawn and ironed cans manufactured from aluminum sheet according to the present invention are subjected to industrial washing, less surface etching takes place and, therefore, a brighter can results. Also, the relatively low magnesium content decreases the work hardening rate. Also in accordance with the present invention, a relatively high iron content compared to commercially available continuous cast can body stock is employed to increase formability. It is believed that formability is increased because the increased iron changes the microstructure resulting in a finer grain material, when compared to a low iron content continuously cast material. The tolerance of these high iron levels also increases the amount of UBC that can be utilized, since iron is a common contaminant in consumer scrap.

[0034] The elimination of the annealing step directly after the hot rolling step and the performance of two separate annealing steps only after cold rolling steps offer a number of advantages, particularly when the resulting sheet is employed in the fabrication of containers such as cans. The containers produced from the aluminum alloy sheet can have a reduced degree of earing and a reduction in the occurrence of split flanges and sidewalls in containers produced from the sheet. The plug diameter can be within an acceptable tolerance of the specified plug diameter. Containers produced from the sheet can have a significantly reduced incidence of bulging in the container necked / flange sidewalls compared to containers produced from aluminum alloy sheet having different compositions and / or produced by other processes. - It is believed that the alloy sheet of the present invention typically experiences less work hardening during fabrication of containers from the sheet than other continuously cast alloys and comparable to direct chill or ingot cast sheet. For instance, work hardening can occur when cans come off the canmaker and are heated to elevated temperatures to dry the paint on the can. Finally, the annealing of a thinner gauge of sheet (i.e., annealing which is performed only after cold rolling steps) compared to annealing in previous embodiments (i.e., which is performed after casting and before hot rolling and again after cold rolling) increases the amount of reduction which can be satisfactorily achieved with each cold roll pass and thus can eliminate one or more cold rolling passes relative to previous embodiments.

[0039] The aluminum alloy sheet of this embodiment can provide several advantages relative to aluminum alloy sheet having other compositions, especially in the fabrication of containers. The physical properties of the sheet of this embodiment can experience significantly less reduction during fabrication relative to the reduction in physical properties of other alloy sheets during fabrication. In canmaking applications, for example, existing continuously cast alloy sheets can suffer a reduction in physical properties of as much as 4 lbs or more in buckle strength and 20 lbs-or more in column strength, after heating the sheet in deco / IBO ovens.

Problems solved by technology

When the high temperature homogenization step is performed when the metal is coiled, it can result in pressure welding such that it is impossible to unroll the coil.

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