Investigation of the combined effect of friction and interstand tension on the operative conditions of a two-stands reversing cold mill

Abstract

Cold rolling is one of the fundamental processes for the manufacturing of steel plates and sheets intended for several applications. The quality of the rolled strips, among other factors, is strictly related to the friction conditions within the roll bite, initially unknown. The present paper proposes an accurate approach to inversely calibrate the coefficient of friction (CoF) at the roll/strip interface in a two-stands reversing cold mill (RCM) by combining numerical simulation, training of accurate regression model and a final optimization step managed by a multi-objective genetic algorithm. The process was reproduced in the virtual environment of the commercial code Abaqus by modelling both the blank and the rolls as deformable bodies. A set of simulations, arranged according to a full factorial plan, were run by changing the CoF of the first (µ1) and second stand (µ2). The discrepancy between numerical results (rolling forces and interstand tension) and the correspondent data acquired from the in-situ measurements allowed to train accurate regression models, subsequently used in an optimization managed by the MOGA-II genetic algorithm. The methodology was applied to evaluate the tribological performance of two lubricants, a natural based (GR) and a semi-synthetic (SS) one. The numerical simulations of the process highlighted a cross interaction between the rolling conditions of the two stands: in fact, the increase of the CoF in the first stand affects the interstand tension that, in turns, influences the distribution of the contact pressure in the roll bite of the second stand. Therefore, a precise calibration of the friction coefficients was possible only by looking simultaneously at the rolling forces and at the interstand tension. It was eventually demonstrated that GR lubricant showed comparable tribological performance to the SS one but with a non-negligible reduction on the rolling forces and a sensibly lower impact from the environmental perspective.