2019 Volume 105 Issue 7 Pages 676-682
In the process of hot-dip galvanizing of steel, alloying elements such as Si and Mn are easily oxidized by H2O in the annealing atmosphere, causing coating defects. Because this selective oxidation depends on the annealing heat pattern, i.e., the soaking temperature and time, basic research on the kinetics of selective oxidation is important for clarifying the phenomenon of selective oxidation. In this study, the effects of the annealing temperature and dew point on the kinetics and compounds of Mn external oxidation were investigated experimentally, and the Mn external oxidation rate was estimated based on a diffusion equation and thermodynamic equilibrium, considering the diffusion coefficient and the activity coefficient of Mn in steel. The amount of Mn oxide increased in proportion to the square root of the soaking time. This result suggests that Mn oxidation is a diffusion limited process. The Mn oxidation rate increased with increasing temperature and reached a peak value, and at higher temperatures, the Mn oxidation rate became dramatically slower. The peak value also depended on the dew point. To clarify the reason for this slowdown of Mn oxidation, the Mn oxidation rate was estimated. Considering the activity coefficient and the diffusion coefficient of Mn in steel, the calculated Mn oxidation rate was consistent with the measured value. It is thought that the Mn oxidation rate slows at high temperature because the gradient of the Mn concentration around the steel surface becomes small at high temperatures near the equilibrium temperature of Mn/MnO.