Abstract: Many experiments and industrial practices show that it is hard to achieve the thermodynamic equilibrium during metal deoxidization reaction to form solid inclusions in liquid steel. The deoxidizing products cannot transform completely into the most stable structure of crystal or solid inclusions, possibly to form the metastable phase whose stability is lower than that of solid crystal. The metastable phase may be mesoscale phase, the size of which is between atoms and macroscopic particles, while the structure is in the evolution state from liquid (including amorphous state) to stable solid (including crystalline state). The computation by first principles shows that the mesoscale metastable phase in deoxidation system includes oxide clusters that formed with deoxidizer atoms and oxygen and aggregates before nucleation, critical nucleus and nanosized oxide inclusions after nucleation. The computation of equilibrium thermodynamics shows that the metastable phase is in equilibrium with deoxidizer and oxygen in the deoxidation system. The nucleation of inclusions during deoxidation is done in two steps. The first step is the reaction between deoxidizer atoms and oxygen to form clusters, which determines the dissolved oxygen content. In this step, the thermodynamic tendency is big and the reaction rate is high. The second step is the aggregation of clusters into nucleus, which is a limit step to determine the nucleation rate of inclusions that includes the diffusion of clusters and the transformation behavior of liquidlike into solid or crystal. Therefore, it is important to study the related factors affecting the second step and the control methods to control the size distribution of inclusions.

Key words: deoxidation, inclusion, mesoscale, metastable phase, twostep nucleation