Study on the growth and morphology evolution of titanium oxide clusters in molten iron with molecular dynamics simulation

Formation of nano-scale titanium oxides is a desirable result in the deoxidation process of steelmaking. However, the nucleation of nano-scale titanium oxide inclusions remains unknown up to now because of the difficulty in observing and detecting inclusions in steel melt. In this work, we studied the formation and evolution of titanium oxygen clusters in molten iron by molecular dynamics (MD) simulation using empirical atomic interaction potentials. The structures of small titanium oxygen clusters in iron are reasonable compared to the first-principles simulation results. The growth process of small clusters into larger clusters was simulated and it is found the clusters grow through the collision mechanism, with the intermediate products exhibiting chain structures. The iron environment was found to play an important role in the structural form of the titanium oxygen clusters. This study is useful to provide the details of formation and the growth mechanism of titanium oxygen clusters and to provide a valuable picture for the nucleation mechanism of titanium oxide in molten steel.

Structural evolution of calcia during calcium deoxidation in Fe-O-Ca melt

The crystallization process of CaO in iron melt begins with nucleation, which determines the structure and size of the CaO inclusions; thus, it is important to investigate the mechanism of inclusion modification by calcium treatment. In this study, a two-step nucleation method was used to investigate the behavior during the early stages of CaO inclusion crystallization. The first principles method was applied to calculate the structures and properties of CaO crystals and CaO clusters. Then, the nucleation mechanism of CaO in the Fe-O-Ca melt has been discussed. The numerical results show that CaO clusters with cubic structures and appropriate variations are the lowest energy structures and are more stable than other isomers. The stability of the cubic (CaO)_n clusters increases with the increase in size and gradually approaches that of the CaO crystal. CaO clusters can form spontaneously in the Fe-O-Ca melt, while the transformation reaction of the CaO clusters in the Fe-O-Ca melt deeply depends on the supersaturation ratio of [Ca] and [O]. CaO clusters may remain as suspended CaO inclusions in the iron melt for a long time, and these suspensions of CaO clusters are the source of excess oxygen in the iron melt.

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.

Nucleation and growth for magnesia inclusion in Fe-O-Mg melt

The crystallization process of magnesia in iron melt begins with nucleation, which determines the structure and size of magnesia inclusions. Thus, it is necessary to have a deep insight into the crystallization of magnesia by two-step nucleation mechanisms. In this work, the two-step nucleation method was used to investigate the behavior during the early stages of magnesia inclusions crystallization. A first principles method was applied to calculate the thermodynamic properties of magnesia crystal from various cluster structures for the formation of magnesia inclusions. Based on the numerical results, the nucleation mechanism of magnesia in liquid iron has been discussed. The magnesia clusters appear as the structural units for Mg-deoxidation reaction in the liquid iron, and the residual magnesia clusters are the reason for the supersaturation ratio or the excess oxygen for MgO formation in the liquid iron. Based on the comparison between Mg-deoxidation equilibrium experiments and numerical results, the previous experiments may be in a different thermodynamic state. The equilibrium reaction product should be not only magnesia clusters but also bulk-magnesia in those equilibrium experiments.

The crystallization process of magnesia involves two steps.