Instantaneous transition of composition and morphology of inclusions with an initial Al2O3 composition in the molten steel during calcium treatment

• In-situ observation reveals that Al2O3 inclusions undergo instantaneous morphological transition to spherical calcium aluminates during calcium treatment, with overall regularity increasing over time. • An empirical formula linking inclusion composition (CaO/Al2O3 ratio) to overall regularity enables real-time prediction of inclusion modification state. • A CaO/Al2O3 mass ratio of 0.451 corresponds to ideal spherical calcium aluminate inclusions with overall regularity of 1, providing a quantitative target for process control. • Smaller Al2O3 particle sizes accelerate the transformation to spherical calcium aluminates, highlighting the importance of inclusion size control in steel refining.