Patent Publication Number: US-6668610-B2

Title: Method for continuous tension leveling of aluminum strip

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for continuous tension leveling of thin aluminum strip, particularly 6000 series aluminum alloy strip. 
     2. Description of the Prior Art 
     For the leveling and flattening of metal strip, various straightening processes are used, e.g. stretch and tension straightening, tension leveling, tension-bend leveling, etc. 
     Generally these straightening or leveling processes are carried out continuously by means of tension stretching arrangements using a set of pull rollers and a set of brake rollers and between these sets of rollers stretch-forming forces are applied. During the stretching procedure, the strip is subjected to plastic elongation resulting in a reduction in the strip width and thickness. 
     It is also known to apply bending forces to the strip under tension, e.g. by bending and reverse bending over a series of small rollers positioned between the pull rollers and the brake rollers. Some prior studies have found that these multiple bendings result in an accumulation of plastic strains causing substantial work hardening. This typically leads to an increase in yield strength and a reduction in formability. 
     One device for stretching a continuously moving thin metal strip is described in Noé, U.S. Pat. No. 3,867,826. A further system for continuous tension stretching of thin metal strips is described in Noé et al. U.S. Pat. No. 5,182,931. A general review of the leveling of aluminum strip can be found in A. L. Noé, “New Developments in the Leveling of Aluminum Strip”, Second International Aluminum Conference and Trade Exposition, Atlanta, Ga., 503-529 (1997). 
     These prior studies have generally found that softer aluminum alloy strips such as those formed of series AA 1000 and AA 3000 alloys can be run in a pure stretch mode, although harder AA 5000 series alloys are more successfully run in the tension leveling mode with flex rolls. 
     Yet another study on the action of roller levelers may be found in N. H. Polakowski, “Restoration of Ductility of Cold-Worked Aluminum, Copper and Low-Carbon Steel by Mechanical Treatment”, Proceedings, Am. Soc. Testing Mats., pp 1086-1097 (1952). Here tests were conducted on the bending of aluminum rods. This researcher found that cold-worked rod material made from the softer AA 1000 and AA 3000 series aluminum alloys tended to soften when subjected to reverse bending cycles. 
     Thus, while there appears to be inconsistencies among the studies, in the tension leveling of thin aluminum alloy strips, the prior art has generally concluded that the softer 1000 and 3000 alloys can be run in pure stretch mode while the harder 5000 series alloys require the tension leveling mode. 
     SUMMARY OF THE INVENTION 
     According to the present invention, it has surprisingly been discovered that thin aluminum alloy strip or sheet of the AA 6000 series does not behave in the expected manner. The 6000 series aluminum alloy contains both magnesium and silicon as main alloying elements and is generally considered to be harder than the 5000 series alloy which contains only magnesium as the main alloying element. It was therefore expected that 6000 series aluminum alloy sheet would require more reverse bending action during the leveling process than the 5000 series sheet. However, it has been found that the 6000 series aluminum ally sheet or strip achieves the best combination of flatness and formability when the pure stretch mode is used or tension leveling with a low degree of reverse bending. 
     Thus, the present invention in its broadest aspect relates to a method for continuous tension leveling of AA 6000 series aluminum alloy sheet in which the sheet is passed through a set of pull rollers and a set of brake rollers to subject the sheet to longitudinal tension therebetween. A series of small rollers may also be located between the pull rollers and the brake rollers for subjecting the sheet under longitudinal tension to bending and reverse bending over the rollers. Best results are obtained with the AA 6000 series alloy when the system is operated in a pure stretch mode (0° wrap angle) or at a wrap angle about the small rollers of no more than about 12°. 
     For tension leveling, the alloy strip typically has a thickness in the range of about 0.1 to 4.0 mm, preferably about 0.8 to 2.1 mm. The strip is subjected to a longitudinal stretch of about 0.4 to 1.0%, preferably 0.8 to 1.0%. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings which illustrate this invention: 
     FIG. 1 is an elevation view of a tension leveling device; 
     FIG. 2 is an elevation view of the bending rollers at a wrap angle of 11.5°; 
     FIG. 3 is an elevation view of the bending rollers at a wrap angle of 23.5°; 
     FIG. 4 is a schematic view of the bending rollers; 
     FIGS. 5A and 5B are plots of bend vs stretch for a first quench process; 
     FIGS. 6A and 6B are plots of bend vs stretch for a second quench process; and 
     FIGS. 7A and 7B are plots of bend vs stretch for a third quench process. 
    
    
     As seen in FIG. 1, the device for carrying out the invention comprises three main components, namely a drag bridle  10 , a tension leveler  11  and a pull bridle  12 . The drag bridle  10  comprises essentially brake rollers and the incoming aluminum alloy sheet  13  passes around rollers  14 ,  15 ,  16  and  17  from which it enters into the tension leveler  11 . 
     The tension leveler  11  includes a pair of upper small roller sets  18   a  and a pair of lower small roller sets  18   b  and the degree of wrap of the sheet  13  about the tension leveler rollers is determined by the location of the upper roller sets  18   a . It will be seen that these upper roller sets  18   a  are mounted on a pivotal support frame  19  which can be pivoted upwardly into position shown at  19   a.    
     The pull bridle  12  comprises a series of pull rollers  20 ,  21 ,  22  and  23  and the sheet  13  emerges from the tension leveler over guide roll  25  and travels around these pull rolls in sequence. 
     FIG. 2 shows the pivotal frame  19  and the upper tension leveler where rollers  18   a  are positioned for a roll wrap angle of about 11.5°. 
     FIG. 3 shows the frame  19  and upper rolls  18   a  moved further in a downward direction relative to the lower rolls  18   b  thereby increasing the wrap angle. In FIG. 3 the wrap angle is approximately 23.5°. 
     The different wrap angles can also be expressed in terms of “mesh” and this is shown schematically in FIG.  4 . Thus, the mesh is the actual vertical distance of adjustment of the upper rolls  18   a  with one location being adjusted downwardly by a distance of 47 mm from the passline and the second location being a downward adjustment by a distance of 95 mm from the passline. The 47 mm mesh is approximately equivalent to a wrap angle of about 11.5° and the 95 mm mesh is approximately equivalent to a wrap angle of about 23.5°. The wrap angle is defined as the sum of the two angles made by the sheet as it is deflected away from the passline on both sides of a roll. The relationship between the angle of the sheet and the mesh is shown in Table 1 below: 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Angle (°)* 
                 Mesh (mm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 5.75 
                 47.00 
               
               
                   
                 8.76 
                 70.58 
               
               
                   
                 9.68 
                 78.13 
               
               
                   
                 11.75 
                 95.00 
               
               
                   
                   
               
               
                   
                 *The wrap angle is double the sheet angle.  
               
            
           
         
       
     
     FIG. 4 schematic also shows an arrangement with upper rollers  18   a  out of contact with sheet  13 , this representing a pure stretch mode or 0° wrap angle. 
     EXAMPLE 1 
     A series of tests were conducted on AA 6111 aluminum alloy sheet. The alloy has the typical composition: 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
                   
               
               
                   
                 Cu 
                 Fe 
                 Mg 
                 Mn 
                 Si 
                 Ti 
                 Zn 
                 Cr 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 min 
                 0.5 
                   
                 0.50 
                 0.15 
                 0.70 
                   
                   
                   
               
               
                 max 
                 0.9 
                 0.40 
                 1.00 
                 0.45 
                 1.1 
                 0.1 
                 0.15 
                 0.1 
               
               
                   
               
            
           
         
       
     
     and the balance aluminum and incidental impurities. The alloy was cast and rolled to a sheet thickness of about 0.93 mm. The sheet was solution heat treated at about 550° C. and thereafter cooled under three different cooling procedures, designated as quench “A”, quench “B” and quench “C”. 
     The tension leveler used for the tests contained bending rolls having diameters of 1.75 inches with a horizontal distance center to center spacing of the roll groups of 18 inches. Tensions were applied to the aluminum alloy sheet to provide high and low stretch values of 1.0% and 0.4% and the bending rolls were positioned to provide wrap angles of about 11.5° and 23.5°. 
     The results obtained are shown in FIGS. 5A,  5 B,  6 A,  6 B,  7 A and  7 B. A quite similar pattern was observed with three different methods of quench for the aluminum alloy sheet. The general pattern observed is that with the higher degree of elongation of 1%, there was either no change in yield strength or a slight decrease in yield strength for bending roll wrap angles of up to about 12°. At wrap angles beyond about 12° there was a significant increase in yield strength. 
     At a lower degree of elongation of 0.4%, there was a consistent rise in yield strength with increasing degrees of wrap angle on the bending rolls. 
     This shows that for the AA 6000 series aluminum alloy formability responds to the degree of stretch rather than the bending action and that the best results were obtained when the bending action is at a wrap angle of no more than about 12°.