Case study 3: Manufacture of can body stock — 2. Rolling

Abstract

The manufacture of can body stock includes both hot and cold rolling. Hot rolling usually utilizes an integrated hot line. The breakdown mill is usually a four-high mill capable of reversing direction. The ingot is passed back and forth through the mill about 15 times, causing a reduction in thickness from about 60 cm to about 4 cm. The breakdown mill requires d.c. motors of several thousand kilowatts, capable of applying the large reductions necessary, and quickly reversing direction. As during breakdown, rolling the ingot (now being rolled into plate) thickness is relatively large, inhomogeneous deformation occurs through the plate thickness. Consequently, unless the thickness of the ingot/plate during each pass through the breakdown mill is controlled appropriately, the end of the ingot may alligator, potentially preventing the ingot/plate from entering the mill for successive passes. To ensure the ends of the plate remain square and true, they may be cropped during breakdown rolling in a plate shear. During hot rolling, a grain structure with a preferred orientation develops. To control the grain orientation or texture, the coils of hot rolled sheet are annealed to a fully recrystallized state prior to cold rolling. Finally, the annealed sheet is cold rolled to a final gauge of about 0.3 mm. This reduction may be achieved by multiple passes through a single-stand cold mill or tandem cold mills.