1995 Volume 81 Issue 1 Pages 70-75
The effect of Al content in the Zn bath on the unevenness of the coated surface of galvannealed steel sheet was investigated by observing the growth of the Zn-Fe alloy during galvannealing process. In the case of 0.13 mass% Al, Zn-Fe alloy initiated homogeneously on the substrate surface grows uniformly during galvannealing and this results in an even coated surface. In contrast, with a high Al content (≥0.17 mass%), Zn-Fe alloy initiates preferentially at the substrate grain boundary at an early stage of galvannealing and this brings about the formation of an outburst structure. As galvannealing progresses, this structure predominantly grows outwards by withdrawing the surrounding molten zinc. The coated surface, thereby, turns out to be uneven. An increase in the unevenness of coated surface increases the coefficient of friction and the exfoliation resistance deteriorates when the sheet undergoes the sliding action with ironing.
