Effect of Coating Structure on Surface Color of Galvanized Hot Forming Steel

The galvanized 22MnB5 steel plate was heat-treated by different processes. The color change of the coating on the surface of the steel plate was analyzed. The coating structure and migration of elements in the coating were studied by means of scanning electron microscope (SEM), electron probe microanalyzer (EPMA) and glow discharge spectrometer (GDOES). The results show that with the increase of heat-treatment holding time, the color difference increased, but the brightness decreased for the coating surface. The color difference at the heat-treatment temperature of 890 ℃ was larger than that at 880 ℃, when the holding time was the same. The coating thickness was 15-35 μm, the cracks in the coating did not penetrate into the substrate, and the structure was composed of α-Fe(Zn) solid solution and Γ phase. During heat treatment, Zn and Fe diffused, the surface oxide layer thickened. There was no Fe-Zn intermetallic compounds observed in the coating. The change of color difference and brightness of the coating surface were the joint result of increase of coating thickness and surface roughness and the accumulation of surface oxides.