Correlation between the Microstructure of Galvannealed Coatings and the Defoliation during Press Forming

Abstract

The defoliation properties of galvannealed coating, so-called powdering and flaking, in a commercial continuous galvanizing line were investigated by scanning electron microscopy, X-ray diffraction, a roughness profiler and deep drawing test. Both aluminum content in molten zinc and galvannealing temperature were found to be critical factors controlling the microstructure and mechanical properties of galvannealed coatings. Below 0.135 wt% Al, the coating surface was composed of a mixture of granular δ_1p and columnar ζ phases, while, above 0.155 wt% Al, the coating surface contained the mixture of granular δ_1p and pan-cake δ_1k phases. The appearance of brittle δ_1k phase approximately 10 μm in size can account for the high amount of defoliation during deep drawing test. The formation of relatively ductile ζ phase and thin Γ phase contributes to improve the press formability. The optimization of the surface microstructure controlling the ratio of granular δ_1p and columnar ζ phases was important to reduce the powdering.