Cold-strip reduction mills
A tandem cold rolling mill usually consists of three to five stands of rollers, each of which reduces the strip thickness by about 20 to 40% so that the total reduction in one pass may be by a factor ten.
It has long been recognized that some, if not most, of the variations in thickness of strip produced by a cold-reduction mill have their origin in the hot rolling process. For example, it has been shown that in many coils of metal produced by a cold mill several heavy areas can be identified which result from water-cooled skids in the slab-heating furnace preceding the hot-strip mill
The first step in the control of a tandem mill is to eliminate variations in thickness in the incoming strip. If the strip entering the second stand of the mill has a uniform thickness, then the second and all succeeding stands require little adjustment. The best way to achieve the required adjustment is to apply automatic screw control to the first stand.At this point the steel is still relatively soft and moves slowly so that the mill screws have time to act.
Many tandem mills have contactor-operated, fixed-speed, screw-down motors, but on the most modern plant the screw motors are actuated by variable-speed systems incorporating rotary or magnetic amplifiers. Such plants lend themselves to fast, accurate control with gauges.
Several variables are, in principle, available to the control engineer for applying corrective action; for instance the screw setting on each stand and the inter-stand tension. In practice all but one or two of these are preset and thickness gauges are included only on the closed loop to control the remaining one or two variables. In the U.S.A. a method has been developed which uses a radioisotope instrument after the first stand to control the screws of this stand and a second gauge after the last stand to control the inter-stand tension. The first successful installation using automatic control of the first stand was made in 1953 and dual control of first and last stands was accomplished in 1955.
