Hot and Cold Rolling. Thick cast metal slabs usually are hot-rolled to a minimum practical limit and then cold-rolled further to reduce thickness.
Both hot and cold rolling are accomplished by reducing the thickness between pairs of large-diameter rolls or between smaller-diameter rolls supported against the deflection by larger rolls. Regardless of their diameter, the rolls will be deflected by the large forces required to reduce material thickness. The forces can be in the millions of pounds.
If straight rolls deflect, then the metal in the longitudinal center of the coil will be thicker than at the edges. The thinner the material gets during rolling, the longer it will be. The thinner, longer edges of the coil cannot "run away" from the center, so they will take a wavy shape.
To counteract the deflection, the center of the rolls are ground to larger diameters, creating a crown on the barrel-shaped rolls. If the increase in diameter is twice the anticipated deflection, then the working side of the roll will become straight under the load. Coil rolled under this condition will have the same thickness at any point.
The actual deflection, however, can be controlled by adjusting the gap between the rolls. Rolling temperature, uniformity of the starting thickness, and distribution of alloying elements will change the force required for rolling and, hence, the deflection.
As forces change, deflection fluctuates. If the force is higher than the crown is prepared for, the center of the coil will be thicker. If the actual force is less than the anticipated one, the center of the sheet will be thinner and the edges will be thicker. If the edges are thin, a visible edge wave results; if the center is thin, visible center wave (oil canning) can develop.
If an oil-canning sheet is slit in half longitudinally, the long center part will create an outward camber in the sheet. If the edges are wavy, then the camber will be in the opposite direction. Slitting will not take out the camber from these coils.
