The evolution of surface morphologies and microstructures during cold rolling after hydrogen reduction

Abstract

The removal of oxide scale on a cold-rolled strip surface can completely eliminate environmental pollution by hydrogen reduction descaling instead of the traditional pickling descaling. However, the cold-rolled strip surface morphologies after hydrogen reduction are different from those that result from pickling. To investigate the effects of cold rolling on the surface morphologies, microstructures and properties of cold-rolled strip steel, a cold rolling experiment was performed using a 4-high mill after hydrogen reduction and after pickling. The surface morphologies and cross-section microstructures of the samples were observed by SEM, and the surface roughnesses were measured by a TR200 profilometer before and after cold rolling deformation. The evolution of the surface morphologies and cross-section microstructures of the samples after cold rolling were analyzed. The results show that the oxide scale formed on the surface turns into a metallic iron layer, and a large number of micro cracks, pores and bubbles appear on the surface of the sample after hydrogen reduction. The pores, bubbles and some smaller cracks were roll-flattened, and the larger cracks were turned into micro-traces remaining on the sample surface after five passes of cold rolling. Compared with the rolled sample after pickling, more micro-traces remained on the surface of the rolled sample after hydrogen reduction. The surface roughnesses of the reduced samples are slightly greater than that of the pickled samples. A metallic iron layer and a decarburization layer appear on the sample surface after hydrogen reduction. After the fifth pass, both layers became thinner, and the metallic iron layer is characterized by non-uniform thickness and discontinuity. The work hardening degree and mechanical properties of the reduced sample are similar to that of the pickled sample after cold rolling.