Effect of micro-defects on the surface brightness of cold-rolled stainless-steel strip

Abstract

Cold-rolled stainless-steel strip can be produced efficiently by high-speed rolling in a cluster-type rolling mill with small-diameter work rolls, or in a tandem cold-rolling mill with large-diameter work rolls. The surface brightness of the strip is deteriorated by high-speed rolling, and the surface brightness of a strip rolled by large-diameter work rolls becomes worse than that of a strip rolled by small-diameter work rolls. In this study, the effect of micro-defects on surface brightness is examined with laboratory-scale and actual rolling mills, and the mechanisms for the occurrence of micro-pits in cold rolling are discussed. The surface brightness was affected strongly by micro-defects on the surface, the brightness being improved for decreasing surface-area ratio of the micro-defects. Micro-defects can be reduced to four types: micro-pits originating from the surface roughness of the mother sheet; oil-pits formed during cold rolling; grooves formed by inter-granular corrosion during pickling; and scratches due to the surface roughness of the rolls. The micro-pits remaining on the surface of cold-rolled sheet were affected by the diameter of the work rolls, the surface roughness of the mother sheet, and the rolling reduction. The effects of rolling speed and of the viscosity of the rolling oil were minor under the experimental conditions investigated.