Effect of calcium addition timing to liquid steel on inclusion modification of steel heavy plates

Calcium addition to steels normally aims at modifying inclusions to improve castability and cleanliness. This study investigated the inclusion modification efficiency in liquid steel for three conditions varying injection timing: all Ca after RH degasser, all Ca before RH degasser and split addition. Six industrial heats were produced at Usiminas Steelworks, two for each condition. The heats were sampled for automated SEM/EDS inclusion analysis and total oxygen and the results compared to computational thermodynamics simulations. Inclusion modification was most efficient for the split addition condition. This condition was the closest to the calculated castability window, resulting in low inclusion density, a higher percentage of liquid inclusions during casting and lower CaS formation. Furthermore, computational thermodynamic simulations and inclusion analysis presented good agreement. These findings not only enhance the understanding of calcium treatment in steel production but also provide practical insights for optimizing the calcium addition process.