An advanced model of lubricated cold rolling with its comprehensive pilot mill validation

Abstract

The increasing demand for thinner and harder steel strips requires the introduction of flexible lubrication systems that continuously adapt the lubrication conditions in existing rolling mills to not saturate the stand capacity by excessive friction. To design these systems, this article introduces one of the most advanced models of lubricated cold rolling since it combines the following features in a single model: elasto-thermo-viscoplasticity of the strip, mixed lubrication by a thermo-piezoviscous lubricant, full-flooded lubrication or starvation, and a complete formulation of non-circular roll flattening. This model is then validated by a new semi-industrial data set, which is one of the most comprehensive ones since it includes: roughness measurements of the rolls and the strips, hardening laws of the strips by plane-strain compression tests, thermo-piezoviscous material laws of the lubricants and a large design space to isolate the influence of individual operating parameters. The results indicate that the changes of the rolling force and the forward slip with the rolling speed and the reduction can be quantitatively predicted except for the decreasing forward slip with the reduction. These predictions require to calibrate the coefficients of boundary friction, thermoplasticity and viscoplasticity for each rolled product as well as the inlet film thickness at each rolling speed, if starvation occurs. Once calibrated, the model therefore allows to predict the influences of various operating conditions, e.g. by how much the rolling force can be reduced if more lubricant becomes available at the entry of the roll bite.