Mitigation of uncommon peel-off defects in galvanized steel sheets

Abstract

Galvanizing defects occurring in hot-and cold-rolling procedures were studied by scanning electron microscopy and focused ion beam, and analyzed via the JMP software. The results show that, among the 200 bright spots dispersed along the hot dipping steel plate, the defect may be misidentified as zinc slag by the representation of coating surface, but there exists a tiny peel off on the matrix without oxide particles under each spot. Upon partial removal of zinc coating, a tiny iron film outcropped from the coating was revealed. As some of iron films are not adhered to the matrix, such outcrops are not detected. The cross-sectional morphology shows the formation of inhibition layer, with no oxide between the peel off and matrix. The relative cold and hot rolling plates are also analyzed to trace the source, that the defect is inherited from hot rolling process according to the results. By using JMP to calculate the relationship of hot rolling parameters with the defect, slab heating temperature is concluded as a strong correlation condition. The high slab heating temperature ensures smooth rolling procedure, but the intergranular binding force is too low, which enhances the coating surface separation from the matrix. After reducing the slab heating temperature from 1350 to 1300°C or lower, and optimization of descale and pickling parameters, the occurrence ratio of bright spot defects caused by peel-off was significantly reduced.