Surface Defect Generation and Recovery in Cold Rolling of Stainless Steel Strips

Previously, researchers investigated the mechanism of surface defect evolution in rolling. It was highlighted how the lubricant plays an essential role for the final strip surface quality. In some cases the lubricant can be entrapped in pits or in other defects where hydrostatic pressure tends to prevent its elimination; however, when some favorable conditions are satisfied, the lubricant can be drawn out by hydrodynamic actions and defects can be recovered. This mechanism has been described as microplastohydrodynamic lubrication (MPHL) and recent studies report a suitable parameter (the ratio of the oil drawn out from the pit to the initial pit volume) as MPHL characterization coefficient. The present paper deals with the recovery of isolated surface defects in the Sendzimir rolling process of AISI 304 stainless steel; the analyses have been conducted on two rolling conditions, which although quite similar, regularly showed opposite capability of defect recovery, moreover, with a trend that is in contrast with the predictions made by standard MPHL. Two effects, which are usually ignored in literature modeling, have been considered in this work: The former is the back-tension, which has relevant outcome on the contact pressure and the latter is the position of the neutral point, which cannot be assumed to lie at the end of the roll bite. The analytical treatment was supported by FEM simulations, which permitted to put realistic data into the MPHL equations, thus, to explain the experimental behavior. The analysis was then validated with two further rolling schedules that seem to confirm the proposed approach.