Prediction and control of strip flatness in cold rolling

Abstract

The rolling of thin gauge, hard aluminium alloys to a high standard of flatness on high-speed mills is primarily a problem of thermal-crown control. The temperature distribution in the rolls has been studied experimentally and theoretically and three separate thermal effects are described, each with a different time constant. The short-term transient does not affect the thermal crown directly, while use is made of the medium- and long-term transients to control local and overall shape variations, respectively. Mechanical deflections are then described and it is shown how a 4-high mill can achieve a balance between thermal and mechanical effects for rolling strip of different widths. Strip off-flatness is manifested through a non-uniform tension distribution and this is the basis for flatness measurement by the Alcan-ASEA Automatic Flatness-Control system. The rapid, predictive response of the automatic roll-deflection control system is augmented by the slower response of coolant flow control across the roll face, the former needed during mill acceleration and deceleration, and the latter to take care of longer-term changes in roll shape due to thermal expansion.