Improvement of the wettability of SiMn IF-HSS by liquid zinc by controlling the dew point of the annealing gas atmosphere

The wettability of low-carbon, 0.3 wt%Si-0.4 wt%Mn interstitial-free steel by liquid zinc at 450 °C was investigated using the dispensed sessile drop method. Before the wetting tests, the steel samples were annealed in a 15%H ₂-Ar gas atmosphere at three different dew points, namely -60, -40, and 0 °C. It was found that as the dew point was increased from -60 to -40 °C, the wettability became poorer. However, as the dew point was increased further to 0 °C, the wettability was dramatically improved and was better than that of -60 °C. In order to understand the dramatic change in wettability, the surfaces of the steel samples after annealing were analyzed with SEM and TEM. It was found that the surface oxide changed from randomly distributed hemisphere particles of 20-30-nm high on a very thin oxide film to a film-like layer ∼15-nm thick as the dew point was increased from -60 to -40 °C, and at the dew point of 0 °C, internal oxidation was so pronounced that a very thin surface oxide layer 1-2-nm thick was formed. It was believed that the improvement of the wettability at the dew point of 0 °C was caused by the short diffusion distance in the surface oxide layer.