Formation of a dark streaky edge defect on galvannealed ultra-high strength steel

Abstract

To analyze the formation mechanism of a dark streaky edge defect on a hot-dip galvannealed (GA) ultra-high strength steel (UHSS) sheet, multiple metallographic characterizations were performed on samples collected from different processes. It was found that this streaky defect was directly caused by differences in the coating morphology. The coating in the edge zone showed a crater morphology, while the coating in the center zone showed a flatter morphology and uniform coating thickness. Corresponding streaky edge defects were found on the annealed steel prior to hot-dipping and on the pickled steel prior to cold rolling. The root cause was finally traced back to the hot-rolled steel after coiling. A formation mechanism of this defect was proposed based on the evolution of surface microstructures in multiple processes. The difference in the amounts of internal oxides formed in the subsurface of different zones of the hot-rolled strip influenced the pickled steel surface, cold-rolled steel surface, annealed steel surface and the galvannealed coating surface.