HEAT-TRANSFER CONDITIONS IN ROLL GAP IN HOT STRIP ROLLING

An extensive amount of literature exists concerning the thermal modell ing of flat rolled steel sheet for both the hot and cold processes. Th is reflects the importance attached to being able to predict the tempe rature profile down the mill from the furnace or continuous easter to final coiling. From a metallurgical point of view, the whole process i s thermally activated, requiring control within definite temperature b ands and cooling rates in order to hit target properties. Of equal imp ortance is roll thermal camber growth, which requires on line predicti on and dynamic control to achieve strip shape and profile within toler ance. These aspects are still actively researched, but what have recei ved considerably less attention are the physics of the small scale hea t transfer mechanisms and the links with traction in the roll gap. A c onsideration is important since both thermal contact and friction are bounding conditions to the process and together determine the strip fl ow behaviour as it passes through the roll gap; the manner in which th e strip material is worked directly determines product quality. These issues are complicated by the presence of scale and entrapped fluid. F or exam pie, the conductivity for the scale is considerably lower than for the base material and scale breakup, because of surface expansion , adds a random input to the conditions. Researchers have found from l ow speed laboratory trials that the heat transfer coefficient can rapi dly change along the arc of contact. It is sensitive to pressure, lubr ication conditions, temperature, and process parameters such as strip velocity and percentage reduction. For the present discussions, curren t research on these focused aspects are reviewed, Finite element model ling is also described which has been used to examine the roll gap con ditions allowing for temperature dependent plasticity and material pro perties, frictional heating, and volumetric fluxes arising from plasti c work. This approach has offered a window on the process that is prac tically extremely difficult to examine, particularly for present indus trial production rates which are continuing to increase. The small sca le numerical analysis of the surface interactions has allowed further insight into the contact mechanisms of the process. (C) 1996 The Insti tute of Materials.