Effects of Partial Pressure of Water Vapor in Annealing Atmosphere on Wettability of Mn, Si Containing Steel with Molten Zinc

Effects of partial pressure of water vapor (dew point) in annealing atmosphere on wettability of 1.4 mass% Mn and 0.14-0.25 mass% Si containing steel with molten zinc was studied with considering the characteristics of surface oxide layer by cross sectional TEM microscopy.1.4%Mn-0.14%Si exhibited a good wettalbility with molten zinc after annealing in 20%H2-N2 mixed gas at 1133K for 120 s when the dew point of H2-N2gas is 237K. The wettability deteriorated while dew point was lower (209-227K) or higher (248-260K).It is considered that wettability was relevance to the form of oxides on the surface. After annealing in the dew point of 237K while wettability would be good, the oxide particles were dispersed on the surface and metallic surface was partially exposed. The internal oxide layer with small oxide particles (diameter of less than 100nm) was observed. And, Mn depletion was detected in that layer. On the other hand, the continuous oxide layer was observed on the surface having a poor wettability with the molten zinc.Dew point dependency of the forms of oxides was illustrated as a superposition of surface concentration of Mn, Si and total amount of oxides. In the dew point range of this experiment, MnO and MnSiO3 are predicted to generate. At the lower dew point, oxides would exhibit more concentration to the surface because oxidation depth was short. However, total amount of oxide would be increased at the higher dew point. Consequently, the wettability on 1.4%Mn-0.14%Si steel reached the peak at a specific dew point. But 1.4%Mn-0.25%Si steel did not exhibit good wettability in the dew point range of this experiment. This behavior was considered that the volume of MnSiO3 on the surface increased as the Si content.