Evolution of Surface Pits on Stainless Steel Strip in Cold Rolling and Strip Drawing

Theoretical models are presented for describing the evolution of pits in the inlet and work zone during cold rolling and strip drawing of shot-blast stainless steel under ‘mixed’ lubrication. Results shows that the rough shot-blast surface is flattened rapidly in a short inlet zone, thereby entrapping the lubricant in surface pits. The subsequent evolution of these surface pits in the work zone can be explained by micro-plasto-hydrodynamic-lubrication (MPHL) models described previously. A development of these models is presented which takes into account the effects of the oil film entrained in the inlet, an oil film penetrating from adjacent pits and the finite depth of the pits. The role of an inlet oil film and penetrating MPHL oil film is to limit the potential reduction of pit size. Lubrication regime maps are constructed which describe the evolution of the surface pits for a range of pit geometries. Results explain the experimental observation that some pits survive even after a multi-pass schedule. Predictions of the pit area show good agreement with measurements on samples obtained in strip drawing or rolled under industrial conditions.