CA1201959A - Process for fabricating high strength aluminum sheet - Google Patents

Abstract

PROCESS FOR FABRICATING HIGH STRENGTH ALUMINUM SHEET

ABSTRACT OF THE INVENTION
The yield strength of aluminum sheet is increased by work hardening the aluminum at a temperature in the range of 150°-400°F.

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Description

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PRC)CESS FOR FABRICATING E~IGH STE~ENGTH ALIJMINUM S IEET

:BACKGROUND OF THEINVENTION
Field of Invention This invention relates to a proce.ss for making aluminum sheet of improved properties especially useful as can stock and more particularly to a process for making aluminum sheet of increased strength without sacrifice of ductility properties.
Prlor Art As used herein, the term "aluminum" includes commer-cial grades of the metal itself such as 1100 (Aluminum Associa-tion designation3 as well as aluminum base alloys containing upwards of 90% aluminum by weight. Alloys which are especially responsive to treatment in accordance with the invention hereinafter disclosed include 1100, 3003, 3004 and 3009 and other mangane~e-containing alloys with and without magnesium.
Aluminum beverage cans equipped with easy opening ends have gained wide acceptance by consumers. To conserve our natural resources, efforts are being made on a national and international scale to recycle these cans.
In the manufacture of these cans, different alloys are used for the can body and can end. Thus, alloy 3004 which is used for the can body is not suitable for the manu-facture of the end which reguires high ductility for the forming operations. The aluminum sheet used from which the end is formed must be of high strength so that the end will safely contain the pressurized contents of the cans. Alloy .~, 335~

3004 which has a low magnesium content (1%) does not have the necessary high yield strength in conjunction with high ductility to be useful in the formation of can ends. Alloy 5182 which has a relatively high magnesium content (4-5%) has the requisite ductility and strength to be used as can ends. The typical compositions of these alloys are recorded in the Table below:
Table Metal Con-tent (~O) Alloy Fe Si Cu Mn Mg Cr Zn Ti Al 3004 0.43 0.20 0.15 1.1 1.02 0.03 0.04 0.01 Balance 5182 0.35 0.20 0.15 0.4 4.5 0.10 0.25 0.10 Balance The use, in the manufacture of easy opening cans, of different aluminum alloys in the end and body stock, substantially decreases the desirability of these cans as recyclable scrap, as the remelted product is an alloy of uncertain composition.
It would be highly advantageous if the tensile strength of alloy 3004 could be raised to equal the levels of alloy 5182 so that alloy 3004 could be used both as can body stock as well as end stock. Aluminum sheet for use as can body stock, e.g., alloy 3004 - ~19 temper, is produced from ingot which is subjected to the following mill operations to produce can body stock having a thickness of about 0.013 inches: casting, homogenizing, hot rolling, annealing and cold rolling as, for example, disclosed in U.S. 3,802,931.
In the manufacture o~ aluminum sheet by DC (direct chill) casting, an ingot having a thickness of about 16-24 inches is produced. The inyot is subjectecl to the homogeniz-ing step whereby the ingot is heated at 950~1125F for 4-16 hours. Immediately after the homogenizing step the ingot is subjected to hot rolling wherein the ingot is passed through a series of breakclown rolls maintained at a temperature of 650-950F whereby the ingot is reduced in thickness to a reroll gauge of about 0.130 inch. Thereater the reroll ., stock is subjected to an annealing step wherein the stock is heated at 700-900F for 0.5-4 hours to effect recrystalliza-tion of the metal structure.
The annealed reroll stock is then subjected to a final work hardening step wherein the reroll stock is cold rolled (room temperature rolling) to a final gauge of about 0.013 inch in a 5-stand mill or about 90% of its original thickness to produce substantially full hard (Hl9) temper.
In rolling to Hl9 temper, the yield strength of the work hardened aluminum sheet is increased from about 10 ksi (k=1000 pounds) to about 40-45 ksi, but the percent elongation (a measure of ductility for can making) of the metal decreases from about 25% to ahout 2%.
Vari~us attempts to improve -the yield strength of work hardened aluminum sheet have involved varying the rolling operations but such approaches have accomplished increased yield strength at the expense of reduced tensile elongation. Any reduction in tensile elongation below 1%
renders the aluminum sheet unsuitable for two-piece can forming operations.
It would be highly advantageous to increase the yield strength of work hardened low magnesium content aluminum alloys such as alloy 3004 ~ Hl9 to the level of high magnesium content aluminum alloys such as alloy 5182 without deleterious effect on other metal physical propexties such as elongation.
Increasing the yield strength of alloy 3004-Hl9 to that of alloy 5182 would enable the use o the low magnesium alloy for can end stock with the resultant advantage that the aluminum can ends would have substantially the same composi-tion as the body and be highly desirable as a recyclable canproduct.
The prior art, U.S. Patent 3,787,248, teaches a process for preparing high strength, improved formability aluminum base alloys suitable for use as can end stock having alloy compositions similar to aluminum can body material.
The process involves a heat treatment step after every rolling reduction of 10 to 20% whereby the rolling process is inter~

~0:~359 rupted by a large number of heat treatment steps, a procedure which is difficult to implement in industrial practice.
The present invention replaces the multiple heat treatment steps with a warm work hardening step (rolling at a few hundred degrees above room temperature) and results not only in process simplification but also in greater improvement in strength.
SUMMARY OF THE I~VE~TIO~
In accordance with the present invention, a process is provided of rolling aluminum strip to prepare work hardened sheet for the manufacture of cans, whereby the improvement comprises rolling the strip formed from supersaturated aluminum duri~g work hardening at a temperature of about 150 to about 450F whereby the yield strength of the sheet is substantially increased.
This process is utilized to substantially increase the tensile yield strength of work hardened aluminum alloys, and particularly work hardened low magnesium content aluminum alloys such as alloy 3004, without detriment to other alloy physical properties.
By the practice of the present invention, the yield strength of aluminum alloys conventionally used for the fabrication of can bodies, e.g., alloy 3004, can be raised to a level equaling that of the high strength aluminum alloys, e~g., alloy 5182, the increase in yield strength being accomplished without substantial decrease in % elongation. As will hereinafter be further illus-trated, by work hardening alloy 3004 to Hl9 temper at a temperature between 150-400F, the yield strength of the resultant alloy 3004 - Hl9 sheet is raised from 45 ksi to 65 ksi, a yield strength which is comparable to the 58 ksi yield strength of alloy 5182 in the Hl9 temper.
The improved yield strength imparted to low magnesium content aluminum sheet by the practice of the ,:

~ 4a -present invention enables the sheet to be used for the fabrication of can ends. ~hus, by the practice of the present invention, i-t is possible to manufacture alumi-num beverage cans wherein the end and body portions are composed of the same alloy material, e.g., alloy 3004.
The singular composition of the can components renders the aluminum beverage cans a highly desirable scrap item and thereby promotes the recycling of this packaging product.

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As will hereinafter be also illustrated, the pro-cess of the present invention does not materially affect the ductility of the work hardened alloy, thereby permitting the aluminum alloy to be fabricated into can ends using conven-tional equipment and manufacturing procedures.
PREFERRED EMBOD I MENTS
The aluminum alloys work hardened by the process of the present invention may be cast in any manner. The parti-cular m~thod of casting is not critical and any commercial method may be conveniently employed such as DC casting or continuous strip casting.
Irrespective of the casting method, it has been determined that the effect of the work hardening step of the present invention is enhanced if the alloy is heat treated prior to work hardening to bring any impurities in the alloy in solid solution and retain these impurities in a super-saturated state in the alloy. This may be accomplished by heating the dead soft alloy at a temperature of 800-1100F
and thereafter guenching the heated strip to room tempex~ture by rapid immersion in a suitable fluid, e.g., cold air, water, to retain the dissolved impurities in the supersatur ated state.
If the right t~pe of impurities (Mn, Fe, Ti, Cr, V) and amounts of impurities are present in the alloy and super-saturation has already been developed in the strip as it exits from the hot mill, the heat treatment step to achieve the supersaturated state just described can be avoided. For the purposes of this application, an aluminum alloy is con-sidered to be in the supersakurated state when the amount of Mn in the aluminum alloy is at least 0.4% and the amount of metals selected from the group of Fe, Ti, Cr and V are at least 0.05%.
The work harde~ing step of the process of the pr~sent invention is performed when the aluminum alloy has been previously rolled or cast to a thickness of about 0.1~0.75 inch. In the DC process, the cast aluminum is hot rolled and reduced in thickness from about 16-20 inches to about 0.1 :

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inch before it is worked hardened. In the continuous casting process the strip is cast directly in-to a strip about 0.25-0.75 inch thick which is then homogenized prior to work hardening.
The specific temperature ,at which the aluminum alloy is work hardened and rolled to a thickness of about 0.013 inch, the thickness re~uired for can stock, will vary according to the specific alloy being work hardened. For alloy 3004, optimum results are ohtained when the aluminum strip is work hardened at about 275F. Generally the temper-ature at which work hardening is effected in accordance with the present invention will vary from about 150E to about 400F.
The selection of work hardening temperature will also depend upan the rolling speed at which work hardening is accomplished. Commercial work hardening rolling speeds vary from 1000 to 5,000 feet per minute. In selecting the temper-ature at which work hardening is to be accomplished, th~
temperature selected will vary with the rolling speed, i.e., at the higher levels of the rolling speed range temperatures of about 250 to 400F are used and at lower rolling speeds temperatures in the order of about 200 to 350F are used.
The aluminum alloys which can be work hardened in accordance with the process of the present invention have the following composition ranges:

METAL COMPOSITION RANGE
Mg 0 to 6%
~n 0 to 3%
Cu 0 to 5%
Fe 0 to 1%
Si 0 to 2%
Ti 0 to 1%
Zr 0 to 1%
Cr 0 to 1%
V 0 to 1%
Al Balance ~2~5S~

The following Examples illustrate the practice of the present invention:
EXAMPLE I
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A hot rolled strip (hot bcmd) 0.087 inch thick of alloy 3004 was subjected to a work hardening s-tep in accor-dance with the practice of the present invention.
As received from -the alumi.num mill the s-trip was dead soft., i.e., zero temper, and had a yield strength of 11 ksi and an elongat.ion of 25~. The strip was heated at 875 10F and held for 5 minutes to dissolve impurities (Mn, Fe, Ti, Cr~ in the alloy (Step A). After being heated in this manner, the heated strip was rapidly ~uenched in water (Step B) to trap the impurities in the supersaturated state. The ~uenched strip was heated and held at 275F ~or 5 minutes to bring the strip up to the temperature at which it was to be work hardened (Step C).
To work harden the strip in accordance with the practice of the present invention, the strip at 275F was passed successively at 7 ft./minute through a pair of reduc--tion rolls heated to 275F until the strip was reduced 90% in the thickness (Hl9 temper) to 0.009 inch. To effect this reduction in thickness, five passes khrough the rolls at a speed of 7 feet per minute were re~uired (Step D). After the completion of each pass through the rolls, the strip was heated to 275F and held ~or 30 seconds to reestablish the temperature and to insure that the strip was at 275F at the time it was fed into the .~olls (Step E). The temperature in Steps C, D and E was controlled within ~ 5F.
The yield strength and ~ elongation of the work hardened alloy was determined by performing standard 2 inch gauge length tensile tests on samples of the work hardened aluminum strip. The strength of the alloy as represented by yield strength and the ductility as represented by % elonga-tion are properties of the alloy essential for the manufacture of can ends. The yield strength of the work hardened strip was determined to be 65 ksi and the elongation 2%.

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The same aluminum strip treated in accordance with Steps A and B, when cold-rolled to produce the conven-tional 3004 in Hl9 temper, exhibited a yield strength of 45 ksi and an elongation of 2%. The currently~ used can end material 518~ in H19 temper has a yield strength of 58 ksi and 3%
elongation.
The processing history of the hotband material used in Example I was as follows:
(1) homogenized at 950-1050 F for 1~ hours

(2) air cooled to a rolling temperature of 800-850 F

(3) hot rolled from 231~" to 0.187" gauge.
Microanalysis of the hotband revealed partitioning of iron and manganese between the aluminum matrix and complex intermetallics. The supersaturated aluminum matrix contained 0.58% Mn and 0.09% Fe in solution, the remaining Mn and Fe were present as complex intermetallics (metal compound).
Titanium, chromium and vanadium were not detectable.
For purposes of contrast, an alloy of same composi-tion (3004) was subjected to a different processing procedure as follows:
(1) homogenized at 1070 1120 F for 10 hours ~ 2) furnace cooled at 100 F/hr. to rolling tempera-tures o 800 F
(33 hot rolled from 23~2" to 0.110" gauge.
Microanalysis of the hotband revealed that the aluminum matrix contained 0.33% Mn and undectable Fe (precipi-tated out and was not supersaturated) Ti, Cr and V were also undectable, and the remaining Mn and Fe were present in the complex intermetallics. When subjected to the processing steps of Example I, i.e., Steps, A, B, C, D and E, the alloy exhibited no improvement in yield strength when subjected to work hardening at between 250-300 F at rolling speeds between 7 to 250 ft./minute. Manipulation of Steps A and B also did not effect an improvement in yield strength after work hardening, demonstrating the need for the presence in the alloy of certain supersaturated metal impurities before a work hardening improvement in accordance with the process of the present invention can be achieved.

?19S9 _9_ EXAMPLE II
The procedure of Example I was repeated with the exception that the work hardening (Step D) was performed to Hl~ temper (80% rolling reduction) at varying temperatures.
The results are summarized in the Table below:
TABLE
Work Hardening Yield Strength Temperature ~F) .in H18 Temp~r (ksi) 150 ~7 As is readily e~ident from the Table, the optimum temperature of work hardening is 275F. At lower or higher temperatures, the yield strength after work hardening is lower~
EXAMPLE III
The procedure o~ Example I was repeated with the exception that Steps A, B, C and E were not used in the work hardening procedure, i.e., the strip as received from the mill was work hardened to Hl9 temper by successive passage at 7 ft./minute through the reduction rolls heated to 275F
(Step D). The yield strength of the work hardened strip was determined to be 60.8 ksi and the elongation was 2%.
EXAMPLE IV
The procedure of Example I was repeated with the exception that Steps A and B were not used in the work harden-ing procedure. The yield strength of the work hardened strip was determined to be 58.1 ksi and the elongation was 2%.

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Claims (7)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In the process of rolling aluminum strip to prepare work hardened sheet for the manufacture of cans, the improvement which comprises rolling the strip formed from supersaturated aluminum during work hardening at a temperature of about 150° to about 450°F whereby the yield strength of the sheet is substantially increased.

2. The process of claim 1 wherein the aluminum sheet is fabricated from an aluminum alloy containing at least 0.4% manganese.

3. The process of claim 1 wherein the aluminum sheet is fabricated from an aluminum alloy containing at least 0.05% of a metal selected from the group consisting of iron, titanium, chromium and vanadium.

4. The process of claim 1 wherein the aluminum is alloy 3004.

5. The process of claim 2 wherein the strip is work hardened to H19 temper.

6. The process of claim 3 wherein the strip is heated at 275° ? 5°F.

7. The process of claim 1 wherein the strip is heated at about 800° to 1100°F prior to effecting work harden-ing.