Thermodynamic Analysis of Selective Oxidation Behavior of Si and Mn-added Steel during Recrystallization Annealing

In order to establish the necessary conditions for producing high strength hot-dip Zn galvanized steel sheets for automotive use on a Continuous Galvanizing Line (CGL), a thermodynamic calculation of the selective oxidation behavior of Si, Mn-added high strength steel sheets was introduced, assuming a model in which an equilibrium is reached locally at the outermost surface layer of the steel sheet. The applicability of that model is confirmed by comparison with experimental results. Both the calculated chemical potential diagram for an Fe–Si–Mn–O system and an isothermal pseudo ternary phase diagram for an FeO–SiO2–MnO system, explain the reaction path of the selective oxidation behavior in 1 mass% Si and 0.01–3.01 mass% Mn-added steel. As this simulation model demonstrates good agreement with experimental results, this thermodynamic calculation is extremely appropriate for prediction of the surface oxidation behavior of Si, Mn-added high strength steel sheets. The transition from selective surface oxidation to internal oxidation can be explained by considering the oxygen flux in the oxidation film and the effect of the selective surface oxides on the inward diffusion behavior of oxygen. By thermodynamic calculation of the suppression condition of SiO2 film formation, which deteriorates the molten Zn wettability of the surface of annealed sheets, a process control model for industrially stable production of high strength hot-dip Zn galvanized steel sheets with arbitrary chemical compositions is proposed. Based on this research, a comprehensive surface control technology for high strength steel sheets is established.