12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Effects of Si content on the selective oxidation and reactive wetting of two grades of Fe-0.1C-2Mn-xSi (wt.%) (x=1.3 and 1.7 wt.%) advanced high strength steel arising from continuous galvanizing heat treatments were investigated in this research. Experimental steels were intercritically annealed for 120 s in a N2-5 vol.% H2 gas atmosphere at dew points of –50 °C, –30 C and +5 C and continuously galvanized in a 0.2 wt.% dissolved Al bath. A well-developed  Fe2Al5ZnX interfacial layer, representative of improved reactive wetting, was observed on both grades after being annealed under the highest pO2 process atmosphere of +5 °C dew point. This was due to the transition from external to internal oxidation resulting in morphological alterations of the surface oxides from thick film-like MnO, MnSiO3, Mn2SiO4 and SiO2 oxides at –50 °C and –30 C dew points to MnSiO3, Mn2SiO4 and MnO particles and thin films at +5 °C dew point. Although increasing the alloy Si content to 1.7 wt.% deteriorated the reactive wetting during continuous galvanizing due to the formation of thicker SiO2 and MnSiO3, Mn2SiO4 surface oxides, the addition of 0.05 wt.% Sn to this steel effectively changed the morphology of the surface oxides to widely distributed particles and thinner inter-particle films via reducing the kinetics of the external oxidation and altering the surface interfacial tensions. Reaping the benefits of increasing the annealing atmosphere pO2 and Sn addition, considerable improvements to coating quality were achieved on the Sn-added-1.7 wt.% Si steel annealed under the +5 °C dew point.