In the process of hot dip galvanizing of Si and Mn added high tensile strength sheet steels, the selective surface oxidation of Si and Mn causes coating defects. One of the promising methods overcoming this problem is oxidation-reduction process. The steel surface exposed to an oxidizing atmosphere will react primarily by forming mainly an iron oxide, which can be reduced by hydrogen in the following reduction process. It has been explained that due to the formation of pure iron, good wettability can be obtained. However, the mechanism of the suppression of selective surface oxidation has not been clarified in detail yet.
In order to reveal this mechanism, present study focused on both Mn and Fe oxidation behavior on the way of the oxidation-reduction process of the cold rolled sheet steel contained 0.25mass%Si-1.8mass%Mn. The surface and cross-sectional analysis were performed by using secondary electron microscopy and transmission electron microscopy. The selective surface oxidation behavior was investigated by glow discharge spectroscopy.
The main results obtained are as follows. First, Even if the soaking was continued after Fe oxide's reduction finished, the selective surface oxidation of Mn was suppressed.
Second, Mn was trapped as the internal oxide under Fe oxide layer as reduction was processed. Moreover, the depletion of solute Mn was observed in the matrix.
From these results, the depletion of solute Mn supposes to suppress the outer diffusion of Mn during the soaking. Therefore, even after Fe oxide's reduction completes, the selective surface oxidation of Mn is suppressed.
