An Experimental Investigation of Surface Pit Evolution During Cold-Rolling or Drawing of Stainless Steel Strip

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Abstract
This paper presents an experimental investigation of the mechanisms of pit elimination in strip drawing and rolling of stainless steel strips. Strip drawing tests with artificial indents confirm the role of micro-plasto-hydrodynamic lubrication (MPHL) in allowing pits to be reduced in size and depth. The similarity of results for two oils, which differ in viscosity by a factor of 10, is attributed to the fact that oil is drawn out of the pits rather easily, so that the behavior tends to the unlubricated case. Similar behavior is observed for strip drawing of shot blast white hot band. For much smoother bright anneal strip, it is suggested that the presence of an oil film in the unpitted region prevents generation of pressure differences between the pits and the unpitted regions. A comparison of strip-drawn and cold-rolled stainless steel samples show that the change in pit area and Rq roughness varies with overall reduction in a remarkably similar way. The reason for such similar behavior is attributed to the absence of hydrodynamic action in preventing pit elimination, albeit for opposite reasons. The similar rate of pit evolution in both cases confirms the usefulness of the strip drawing rig as a tool to model the change of surface topography during rolling, as long as care is taken in matching the regimes of lubrication.

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