Inverse calibration of the friction conditions in cold rolling by means of on-site force monitoring

Lubrication in cold rolling is a key aspect to ensure high quality of the rolled strips and the choice of a suitable lubricant surely has a remarkable influence on the contact conditions at the roll/strip interface. The present work proposes an original methodology based on inverse analysis to evaluate the friction conditions in the cold rolling process. Such a methodology was applied for the investigation of two different lubricants: a commercial semi-synthetic one and a natural-based one (able to reduce the environmental impact). Forces acting on work rolls were measured during the rolling operations in the two-stands reversing cold mill (Marcegaglia plant, Italy) and used as target value for tuning the unknown parameters (coefficients of friction, CoF, and roll’s diameters, D) of the plane-strain analytical model through which each pass was modelled. In particular, an automatic optimization procedure, managed by the NSGA-II algorithm, was used for tuning the parameters CoF and D by iteratively changing their value and minimizing the discrepancy between the analytically calculated and the experimental forces. The optimization results allowed to evaluate the friction coefficients more efficiently than a laboratory test, since being directly calculated using data from the real production. Reliable values of the CoF could be thus determined: the inversely determined coefficients, when implemented in the proposed analytical model, led to a precise estimation of the rolling force (the discrepancy was below 10%). The Pareto designs revealed that the contact conditions at the roll/strip interface could be remarkably improved when adopting the natural-based lubricant (with an average reduction of the friction coefficient of 8%), the contact pressure being considerably reduced. Such a result is coherent with the reduction of rolls’ wear when using the natural based lubricant and it is crucial in limiting the process-related costs.