Thermodynamic Modelling on Nanoscale Growth of Magnesia Inclusion in Fe-O-Mg Melt

Nano-magnesia is the intermediate product during the growth of magnesia inclusion in Mg-deoxidized steel. Understanding the thermodynamics on nano-magnesia is important to explore the relationship between magnesia product size and deoxidation reaction in molten steel. In this work, a thermodynamic modeling is developed to study the Mg-deoxidation reaction between nano-magnesia inclusions and liquid iron. The thermodynamic results based on the first principle method show that the Gibbs free energy change for the forming magnesia product decrease gradually with the increasing nano-magnesia size in liquid iron. The published experimental data about Mg-deoxidation equilibria in liquid iron are scattered across the region between the thermodynamic curves of 2 nm magnesia and bulk-magnesia. It is suggested that these scattered experimental data of Mg-deoxidized liquid iron are in different thermodynamic states. Some of these experiments are in equilibrium with bulk-magnesia, while most of these experiments do not reach the equilibrium state between bulk magnesia and liquid iron, but in quasi-equilibria between nano-magnesia and liquid iron. This is the reason that different researchers gave different equilibrium constants. Furthermore, the behavior of the metastable magnesia is one of the most important reasons for the supersaturation ratio or the excess oxygen for MgO formation in liquid iron.