Microstructure characterization of streaky defects on hot dip galvannealed IF steel

Streaky mark defects on hot dip galvannealed Ti-IF sheets were analyzed using SEM, EBSD, FIB, and TEM. Results revealed that bright streak had slightly larger skin-passed area in correspondence to more uniform GA coating. After the removal of GA coating, more coarse grains on the substrate surface, primarily elongated grains along the rolling direction with the normal direction//<100>, were found in the bright streak area. These coarse grains accelerate the Fe-Zn diffusion reaction during galvannealing and result in a flatter coating surface and an even coating thickness. The source of these coarse grains at the uppermost surface could be traced back to the hot rolling.