Post-calcium Treatment Evolution of Alumina (Al2O3) Inclusions Morphology

Abstract The post-calcium morphological evolution of Al 2 O 3 inclusions formed under high-oxygen supersaturation (~1000 ppm) was investigated through time-resolved sampling and quenching of aluminum-killed, calcium-treated steel. SEM-EDS analysis revealed the formation of angular two-phase CaO·Al 2 O 3 –CaS inclusions within 1 minute, gradually transforming into spherical liquid 12CaO·7Al 2 O 3 and CaO·Al 2 O 3 with a thin CaS layer around it. The 3D morphology of the CaS phase of the complex CaO·Al 2 O 3 –CaS inclusions observed at 1 minute appeared as a cone-cap attached to the partially modified CaO·Al 2 O 3 . This cone-cap CaS phase then transformed into a spherical CaS phase around fully modified CaO·Al 2 O 3 inclusions 2 minutes after calcium addition. At 2 minutes, increased CaO content due to reoxidation and facilitated the conversion of CaO·Al 2 O 3 –CaS inclusions into liquid calcium aluminates, driven by rapid calcium diffusion through the partially modified calcium aluminate layer. Thermodynamic calculations agreed with the compositional evolution of the inclusions observed from the experiment. Additionally, the evolution of the morphologies of the Al 2 O 3 and calcium aluminates inclusions were classified and studied. The formation and disappearance rate of spherical Al 2 O 3 and calcium aluminates inclusions post-calcium treatment were steady and lower than that for angular inclusions.