Evolution of Inclusions and Their Statistics in Low-Carbon Aluminum-Killed (LCAK) Steel Treated with Mischmetal

The influence of mischmetal, an alloy of two major rare-earth (RE) elements Ce and La, on alumina inclusions’ modification in low-carbon aluminum-killed (LCAK) steel was investigated in a series of laboratory experiments. Four different samples were prepared containing 0, 160, 260, and 441 ppm mischmetal. Inclusion analysis was carried out using automated FESEM–EDS feature analysis and inclusion classifier software. In untreated sample, clustered alumina inclusions were observed, while inclusions were de-clustered and homogeneously distributed in mischmetal-treated samples. The final transformed inclusion was found to be a composite inclusion with alumina at the core and RE-rich sulfide at the periphery as sulfur ring, which corroborates with thermodynamic analysis by FactSage software. However, low addition (160 ppm) of RE resulted in incomplete alumina modification (incomplete coverage of alumina by Sulfur ring), while high addition (410 ppm) resulted in the formation of bigger composite inclusions. On statistical analysis, the mean aspect ratio, mean length, and mean ECD (equivalent circular diameter) values were reduced significantly in RE-treated sample. 260 ppm RE content in LCAK steel was found to be optimum for evolution of spherical inclusion with a Sulfur ring. A shrinking core model was developed based on the FactSage-predicted inclusion evolution with temperatures to understand the mechanism of inclusion modification. The kinetic result also indicated a critical radius beyond which inclusion modification was not possible during steel-processing period.