In the process of hot dip galvanizing high tensile strength sheet steels containing Si and Mn, selective surface oxidation of Si and Mn causes coating defects. One promising method for overcoming this problem is an oxidation-reduction process. When the steel surface is exposed to an oxidizing atmosphere, it will react primarily by forming an Fe oxide, which can be reduced by hydrogen in a reduction process that follows. It has been explained that good wettability can be obtained due to the formation of pure iron. However, the mechanism of suppression of selective surface oxidation has not been clearly understood in detail yet.
In order to reveal this mechanism, the present study focused on both Mn and Fe oxidation behavior during the oxidation-reduction process for a cold-rolled sheet steel containing 0.25mass%Si-1.8mass%Mn. Surface and cross-sectional analyses were performed by using secondary electron microscopy and transmission electron microscopy. Selective surface oxidation behavior was investigated by glow discharge optical emission spectroscopy.
The main results obtained are as follows.
First, selective surface oxidation of Mn was suppressed even if soaking was continued after completion of the reduction of the Fe oxide.
Second, as the reduction process proceeded, Mn was trapped as an internal oxide under the Fe oxide layer. Moreover, depletion of solute Mn was observed in the matrix.
From these results, depletion of solute Mn is supposed to suppress the outer diffusion of Mn during soaking. Therefore, selective surface oxidation of Mn is suppressed even after Fe oxide reduction is completed.