Experimental Study on the Mechanism of Pinching in Cold-Rolling Processes

Herein, an experimental investigation into the occurrence of "pinching" defects during cold-rolling of thin metal sheets is presented. Pinched strips are typically characterized by repetitive ripples and local ruptures, which can cause strip breaks. Even though pinches are a widely experienced phenomenon in both hot- and cold-rolling of steel strips, no previous studies are known that have investigated the underlying mechanism for pinching during continuous rolling processes. Therefore, a set of rolling experiments is performed in a single-stand pilot mill to create pinches by applying sudden perturbations in the process conditions. For the rolling settings chosen in the performed experiments, it is found that disruptions in the lubrication state are a powerful approach to induce shape defects, which develop as pinches. Herein, a first extensive description of the pinching mechanism is provided, as observed during the trials, by monitoring the strip's behavior and analyzing the damaged sheets. It is shown that ripples in these pinched strips are the result of folds, which form in the roll bite. The folds originate from the waviness of the strip upstream of the bite, being created by nonuniform conditions over the width of the strip.