| Effects of both Al concentration in molten Zn and galvannealing temperature on the microstructure and defoliation properties of the galvannealed coating in continuous galvanizing line were investigated by scanning electron microscopy, X-ray diffraction and deep drawing test. The amount of powdering, a measure of defoliation, increased with increasing the Al concentration and was associated with galvannealing temperature. Below 0.135 wt% Al, coating surface consists of the mixture of δ_(1p) and ζ phases. The thickness of Γ phase was measured to be lower than 1 ㎛. It is considered that the formation of relatively ductile ζ phase and thin Γ phase contributes to improve the press formability, i.e. the decrease of the amount of powdering. Above 0.155 wt% Al, coating surface shows the mixture of δ_(1p) and δ_(1k) phases. The appearance of brittle δ_(1k) phase approximately 10 ㎛ in size can account for the high amount of defoliation during deep drawing. The optimum Al concentration in Zn bath of continuous galvanizing line was found to be 0.135 wt% with galvannealing temperature below 520℃. |
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