Abstract:
Aluminum alloy strips which are 30-150 mμ thick, having a composition (in wt. %): Si&lt;0.4; Fe: 1.5-1.9; Mn: 0.04-0.15; other elements: &lt;0.05 each and 0.15 in total, remainder aluminum. The strips are used, in particular, for the manufacture of trays and dishes for the distribution of foodstuffs and fast food.

Description:
SCOPE OF THE INVENTION  
       [0001]     The invention relates to thin strips, with a typical thickness between 30 and 150 μm, made of an aluminium-iron alloy, suitable for pressing, intended especially for packaging, and more particularly for the preparation of thin food trays and plates for the distribution of foodstuffs and the fast food industry.  
       STATE OF THE ART  
       [0002]     According to the nomenclature of the Aluminum Association, the Al—Fe alloys of the 8000 series are widely used for the production of thin sheets or strips intended for packaging. They may be produced, either in the traditional manner of casting a plate, hot rolling, then cold rolling, with intermediate annealing of the plate one or more times and most often a final annealing, either by means of continuous casting between two rolls, for example, and cold rolling, and the plate may be annealed once or more times.  
         [0003]     The continuous casting of strips makes it possible, for a moderate investment cost, to obtain a fairly wide range of alloys of strips that do not require any further hot rolling. Recently, significant progress has been made by casting machine manufacturers in the reduction of the thickness of the cast strip, which can be reduced in some cases to approximately 1 mm, reducing the amount of cold rolling that has to be done by the same amount.  
         [0004]     The use of continuous casting, to the extent that the conditions of solidification are different from the customary procedure, results in a different microstructure. Thus, U.S. Pat. No. 3,989,548 from Alcan, published in 1976, describes (Example 9) aluminium alloys containing at least one of the elements Fe, Mn, Ni or Si, cast in strips by continuous casting between rolls with a 7-mm thickness. The structure of the cast strip has rods of fragile intermetallic compounds with a diameter between 0.1 and 1.5 μm, which a cold rolling, with at least a 60% reduction, breaks down into fine particles with a diameter of less than 3μ. The resulting strips indicate a good tradeoff between mechanical strength and formability.  
         [0005]     U.S. Pat. No. 5,380,379 from Alcoa Aluminio de Nordeste relates to the production, by means of continuous casting between rolls, of very thin sheets made of alloys containing 1.35 to 1.6% of iron, 0.3 to 0.6% of manganese, 0.1 to 0.4% of copper and less than 0.2% of silicon. The silicon content is limited by the onset of intermetallic phases of the AlFeSi or AlMnSi type, while the presence of copper is required to give the product sufficient mechanical strength.  
         [0006]     Patent application WO 98/52707 by the applicant describes a process for producing aluminium alloy strips containing (by weight) at least one of the elements Fe (between 0.15 to 1.5%) or Mn (between 0.35 to 1.9%) with Fe+Mn&lt;2.5%, and which may contain Si (&lt;0.8%), Mg (&lt;0.2%), Cu (&lt;0.2%), obtained by continuous casting between cooled and reinforced rolls with a thickness between 1 and 5 mm, following by a cold rolling. The strips obtained in this manner have at the same time a yield strength that is higher than that of the strips produced in the conventional manner and good formability.  
         [0007]     The production of thin trays and plates intended for prepared foodstuffs requires strips that have good mechanical strength, good formability, particularly for fairly deep pressed sections, and good isotropy of the mechanical properties, especially for circular products. One alloy frequently used for this application is the alloy 8021B, whose composition is registered with the Aluminum Association as follows (% by weight):  
                                                           Si   Fe   Cu   Mn   Mg   Cr   Zn   Ti                   &lt;0.40   1.1-1.7   &lt;0.05   &lt;0.03   &lt;0.01   &lt;0.03   &lt;0.05   &lt;0.05                  
 
         [0008]     The aim of the invention is to improve the tradeoff between mechanical strength, formability and the isotropy of the mechanical properties in comparison with this reference alloy.  
       PURPOSE OF THE INVENTION  
       [0009]     The subject of the invention is aluminium alloy strips with a thickness between 30 and 150 μm, made of an alloy of composition (% by weight):  
         [0010]     Si: &lt;0.4; Fe: 1.5-1.9; Mn: 0.04-0.15; other elements: &lt;0.05 of each and 0.15 in total, with the remainder consisting of aluminium.  
         [0011]     The subject of the invention is also a process for producing alloy strips with this composition by continuous casting between rolls of a strip with a thickness between 2 and 10 mm, with the possible homogenization of this strip at a temperature setting between 420 and 550° C., cold rolling of this strip until the final thickness is achieved, and possibly an intermediate annealing for 1 to 4 hours at a temperature setting between 300 and 350° C., and a final annealing at a temperature setting between 200 and 430° C. for a period of at least 30 hours.  
         [0012]     Another object of the invention is to use these strips for plates and trays for prepared foodstuffs. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0013]     The alloy used for sheets and strips according to the invention is characterized by an iron content between 1.5 and 1.9%, which is higher than that which is customarily used for the alloy 8021B intended for the production of plates and trays. The advantage of a higher iron content resides in the improvement in mechanical strength; this effect is even more marked when the strips are produced by continuous casting between rolls. The iron content must remain below 1.9% to avoid approaching the eutectic AlFe content, which would cause coarse-grained primary AlFe phases to appear.  
         [0014]     The other characteristic is a manganese content between 0.04 and 0.15%. This addition has a favourable effect on the mechanical strength, while maintaining a high level of elongation, the tradeoff between these properties, which are usually antagonistic, clearly being improved with strips produced by means of continuous casting. If the manganese content is greater than 0.15%, the manganese clearly plays more of an anti-recrystallizing role, which may adversely affect the efficiency of the final annealing required to obtain good isotropy of the mechanical characteristics.  
         [0015]     The production of sheets and strips according to the invention is preferably done by the continuous casting of a strip with a thickness of 2 to 10 mm between two cooled and reinforced rolls (“twin-roll casting”). The cast strip may be homogenized, especially in the case where elongation is favoured over mechanical strength. This homogenization must be carried out at a temperature that is not too high, between 420 and 550° C., in order to prevent too great a degradation of the mechanical strength. If the requirements regarding elongation are less binding, homogenization is not essential.  
         [0016]     The strip is then cold rolled with the necessary number of passes until the final thickness between 30 and 150 μm is reached. The said cold rolling may be done with or without intermediate annealing. If intermediate annealing is necessary, it must be relatively short, of the order of 1 to 4 hours, and carried out at a temperature setting that is not too high, typically between 300 and 350° C., to avoid enlargement of the grain. But it is possible, when very high elongation values are not desired, to avoid both homogenization and intermediate annealing, which makes the production process particularly simple.  
         [0017]     The rolled strip is then annealed at a temperature between 200 and 400° C. for a period of at least 30 hours, in such a manner that a recrystallized structure is obtained. This annealing may be carried out at one or more temperature settings, such as a first setting between 200 and 300° C., and a second setting between 300 and 400° C. The strip may subsequently be cut into sheets if necessary.  
         [0018]     An improvement of the ultimate tensile strength R m  and of the yield strength R 0.2  of the order of 5% compared to the conventional alloy B8021 is thus obtained, with an elongation of the same order and a lower deviation between the Rm and the R 0.2  values measured in the rolling direction (longitudinal direction) and in the perpendicular direction (cross direction). These properties are particularly suited to the production of plates and trays.  
       EXAMPLES  
     Example 1  
       [0019]     On a casting machine 3C® from Pechiney Rhenalu, two strips 7 mm thick were cast in two alloys A (conventional 8021B) and B according to the invention, whose compositions are indicated in Table 1:  
                                                                         TABLE 1                                   Alloy   Fe   Si   Mn   Cu   Ti                                        A   1.25   0.22   0.02   0.005   0.007           B   1.55   0.18   0.085   0.007   0.009                      
 
         [0020]     These strips were cold rolled without intermediate annealing until they reached the final thickness of 58 μm in 9 passes with successive stops at 4.7 mm, 2.7 mm, 1.5 mm, 0.9 mm, 0.6 mm, 0.41 mm, 0.21 mm, 0.12 mm and 0.08 mm. They subsequently underwent an annealing for 20 hours at 260° C., then 65 hours at 340° C.  
         [0021]     Subsequently, the ultimate tensile strength R m  (in MPa) was measured on the two strips, as well as the yield strength R 0.2  (in MPa) and the elongation at rupture A (in %) in the longitudinal direction and in the transverse direction. The results are indicated in Table 2:  
                                                                     TABLE 2                           R m      R 0.2      A   R m      R 0.2      A           L di-   L di-   L    T   T   T       Alloy   rection   rection   direction   direction   direction   direction                                A   138   121   20.5   136   123   21       B   149   130   22.5   145   131   22.5                  
 
         [0022]     It is observed that R m  and R 0.2  are higher for B, that the elongations are also good and that the difference between the results in the L direction and in the T direction is reduced.  
       Example 2  
       [0023]     Two coils made of alloy C of the 8021B type and D according to the invention were cast, whose compositions is indicated in Table 3:  
                                                                         TABLE 3                                   Alloy   Fe   Si   Mn   Cu   Ti                                        C   1.17   0.12   0.005   0.008   0.010           D   1.63   0.04   0.09   0.007   0.006                      
 
         [0024]     The production process is identical to that of Example 1, except that an intermediate annealing for 2 hours at 340° C. was added at the 0.6 mm thickness. The static mechanical characteristics in the L and T directions are indicated in Table 4:  
                                                                     TABLE 4                           R m      R 0.2      A   R m      R 0.2      A           L di-   L di-   L   T   T   T       Alloy   rection   rection   direction   direction   direction   direction                                C   132   119   23   130   120   22       D   143   127   24   146   129   23.5                  
 
         [0025]     The comparison of the results between alloys C and D leads to the same remarks as the preceding example. In addition, it is observed that, although the D alloy contains a slightly higher iron content, the introduction of an intermediate annealing into the process leads, in comparison to alloy B of Example 1, to a slight reduction of R m  and R 0.2  and to a slight increase in the elongation.  
       Example 3  
       [0026]     Two coils were cast in alloys E (8021B) and F (according to the invention), whose compositions are indicated in Table 5:  
                                                                         TABLE 5                                   Alloy   Fe   Si   Mn   Cu   Ti                                        E   1.21   0.08   0.007   0.005   0.007           F   1.72   0.06   0.12   0.009   0.007                      
 
         [0027]     The production process is identical to that of Example 2, with the addition of a homogenization of the cast strip for 10 hours at 520° C. The static mechanical characteristics in the L and T directions are indicated in Table 6:  
                                                                     TABLE 6                           R m      R 0.2      A   R m      R 0.2      A           L di-   L di-   L   T   T   T       Alloy   rection   rection   direction   direction   direction   direction                                E   125   113   24   123   110   23       F   134   121   25   132   114   24.5                  
 
         [0028]     The comparison of the alloys E and F leads to the same remarks as those of the two preceding examples. In addition, the introduction of homogenization for the alloy F, in comparison with the alloy D of Example 2, leads to a slight reduction of R m  and R 0.2 , and a slight improvement in elongation. The introduction of an homogenization and/or an intermediate annealing into the process thus depends upon the desired tradeoff between the mechanical strength and the formability.