Evolution of Al-Ti Complex Oxide Inclusions in Interstitial-Free Steel Analyzed Using CALPHAD, SEM, EDS, EBSD, and CSLM

The very low C content in interstitial-free (IF) steel is achieved through vacuum degassing and by adding Ti to Al-deoxidized molten steel during the steelmaking process. However, this low C content can lead to reoxidation, which, in turn, generates various oxide inclusions. Despite its significance, a comprehensive understanding of the evolution of these oxide inclusions has been hindered by the scarcity of fundamental information, such as phase equilibria in the relevant system. In this study, the thermodynamics and phase equilibria of the Fe-Al-Ti-O system, along with the evolution of oxide inclusions in the molten steel, were analyzed using various analytical techniques: CALculation of PHAse Diagrams (CALPHAD), scanning electron microscopy, energy dispersive spectroscopy, electron backscatter diffraction, and confocal scanning laser microscopy. High-temperature monitoring of oxide inclusion evolution through in-situ CSLM, SEM, and EBSD revealed that the primary constituents of the major oxide inclusions were alumina (corundum). Notably, $$\text {(Ti, Al, Fe)}_3\text {O}_5$$ , exhibiting a pseudobrookite structure, was identified at higher O potentials, indicative of the reoxidation phenomena in molten steel. To advance the understanding of the stability of oxide inclusions in the molten steel at equilibrium, a self-consistent CALPHAD model was developed. This model provided insights into the stability and evolution of oxide inclusions within this system for the first time. It was demonstrated that the transition from alumina to $$\text {(Ti, Al, Fe)}_3\text {O}_5$$ is influenced not only by the Al and Ti concentrations in the molten steel but also by the O content (or potential) within the system. The presence of $$\text {(Ti, Al, Fe)}_3\text {O}_5$$ as an O carrier in molten steel adversely affects the steel’s cleanliness.