In the rolling process the generation of heat through friction and deformation in the roll bite, presents the principle challenge in achieving optimum rolling performance.
Because of the ever changing demands on rolling mills to produce much lighter exit gauges from increasingly thicker hot strip input, much greater reductions are required on individual mill stands.
Such higher reductions at a nominal width result in a larger area of contact between the work rolls and the strip with correspondingly higher rolling forces, friction and heat generation and heat transfer into the rolls.
These high reduction schedules combined with the requirement to produce a widening range of material cross-sections with a more diverse range of softer and harder materials, also result in increasingly greater challenges in the control of roll temperature and the effective transfer / extraction of heat in the roll stack.
The frictional / deformation heat is partially absorbed by the work rolls in varying degrees based on the reduction / drafting pattern which causes localised, irregular and variable thermal expansion of the work rolls.
The changing profile of the work rolls is transferred to the strip and if not selectively controlled (cooled) results in out of shape, non-flat sheet being produced with wavy edges and buckles ("bad shape").
Establishing uniform homogenous cooling across the roll with a uniform and acceptable thermal distribution transversely across the work roll, without uncontrolled thermal profiles or asymmetrical gradients, is the ultimate goal in terms of cooling, to assure:
The most important considerations in production are: