EFFECT OF AL CONTENT IN BATH ON COATING S TRUCTURE AND EXFOLIATING RESISTANCE OF GALVANNEALED STEEL SHEET

The effect of Al content in the Zn bath on the unevenness of the coate d surface of galvannealed steel sheet was investigated by observing th e 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 e ven coated surface. In contrast, with a high Al content (greater than or equal to 0.17 mass%), Zn-Fe alloy initiates preferentially at the s ubstrate grain boundary at an early stage of galvannealing and this br ings about the formation of an outburst structure. As galvannealing pr ogresses, this structure predominantly grows outwards by withdrawing t he surrounding molten zinc. The coated surface, thereby, turns out to be uneven. An increase in the unevenness of coated surface increases t he coefficient of friction and the exfoliation resistance deteriorates when the sheet undergoes the sliding action with Ironing.