Estimation of Isothermal Alloying Behavior in Galvannealing Process of Galvanized Sheet Steel

A mathematical technique has been suggested to estimate more adequate isothermal alloying behaviortime and temperature dependence of Fe enrichment-in galvanized coating in galvannealing process. Galvannealing experiments using salt-bath were conducted with mill products of galvanized steel; and Austin-Ricketts' type equations were applied to data-fitting by means of multiple regression analysis so as to linearize the relation of Fe quantity in coating and heating time. The original experimental equations, which involve errors due to unavoidable heating-up periods in heattreatment in salt-bath were adjusted to more adequate ones by applying finite difference approximation and multiple regression analysis repeatedly and alternately. It was proved through correlation between estimated values and observed ones that the adjusted equations are more appropriate than the original ones. Corrected log (Fe quantity in coating) vs. log(t) plots indicated the following: 1) Fe enriching behavior at 450°C is subject to the reverselogarithmic time dependence at the outset of the reaction, and to auto-catalytic time dependence after some latent time. 2)The behavior at 500°C and higher temperatures is subject to the parabolic time dependence.