For its application development, the medium manganese lightweight steel with 3 wt.% and 10 wt.% Mn contents was galvanized in continuous hot dip galvanizing (HDG) simulator and the process parameters on the production line were adopted. Combined with the experimental analysis and thermodynamic calculation, the effect of dew point and alloy composition on the reactive wetting of the steel was investigated. It was shown that MnO existed as a stable oxide for the medium Mn steel with 5 wt.% Al as long as Mn content exceeded 5.1 wt.%. The galvanizability of the steel with 10 wt.% Mn was deteriorated resulting from the formation of a thick and continuous external MnO layer, which had adverse effects on the wettability. MnO particles in the form of unstable phase can be found at the surface of 3Mn steel galvanized at dew point +10 °C. It distributed sparsely and the reactive wettability can be obtained by “bridging connection”, which mitigated the damage of external oxidation. Moreover, the lower dew point, the less tendency to form external oxide. Although the decrease of dew point to −30 °C had a certain benefit for coating quality, the galvanizing quality of 10Mn steel could not be improved due to the formation of a thick MnO layer. Therefore, the Mn content played a stronger role than dew point on the reactive wetting of hot dip galvanized medium manganese lightweight steel.
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