It is known that standard alloys, such as AA5052, AA5754, or AA5182, which have alloy proportions as specified above, have a tendency to form stretcher strains, particularly flow lines, during deep drawing.
Such stretcher strains are highly undesirable for high demands of the surface of exterior car body parts, as they are still visible after painting.
Moreover, various approaches are known from prior art, which result in the reduction or the complete avoidance of the undesired flow lines after deforming and deep drawing, respectively. These include in particular the addition of Zn and/or Cu, the omission of the intermediate soft annealing, and/or the final soft annealing in the continuous furnace. The setting of the grain size by means of the addition of Zn and/or Cu leads to the increased risk of creating a so-called orange skin during the deforming and the deep drawing, respectively. If the intermediate soft annealing is omitted, increased demands are created of the cold roll process or of the preliminary warm roll process, since the reductions per pass are limited with cold rolling. Finally, the use of a continuous furnace involves high initial purchase costs.
Furthermore, for avoiding flow lines during the deforming or deep drawing of semi-finished products, a method for the production of semi-finished products is known from U.S. Pat. No. 4,151,013, in which an ingot made of aluminum alloys is cold rolled into a semi-finished product directly after the hot rolling, or after an intermediate soft annealing at a reduction of thickness of at least 40%, mostly 60%-80%, the semi-finished product is then subjected to a final soft annealing in a continuous furnace, and is finally stretch-formed by 0.25% to 1%. It has been shown, however, that semifinished products produced by means of the known method do not safely avoid flow lines, for example during subsequent deep drawing.