Effect of Cu Addition on Surface Morphology of Bake Hardenable Galvannealed Steel Sheets

Abstract

The effect of chemical composition on the surface characteristics of the hot-dip galvannealed (GA) steel sheets with a carbon content as low as 15 ppm strengthened by Cu and/or Mn sulfides instead of Ti and/or Nb carbides has been studied. The most important goal for developing the new type of steel sheets is focused on the achievement of the extreme surface quality to fulfill the high aesthetical requirements for defect-free automotive outer panels. The shape and distribution of the surface enrichments of Cu, Mn, Al and O on the GA steel sheets were characterized by OM, SEM and TEM. Especially, formation mechanism of the narrow streaky mark with the width of 50–100 µ m occasionally observed on the surface of GA steel sheets will be presented.