Evolution of non-metallic inclusions in an Al-killed, Ca-treated, low-S steel during industrial secondary refining

Abstract The evolution of non-metallic inclusions during secondary refining of an Al-killed, Ca-treated, low-S steel was characterized at eight stages from deoxidation to the tundish. Automated SEM–EDS analysis was applied down to an equivalent circle diameter of 0.5 μm. The modification path proceeded from alumina via spinel to Ca-aluminates. Ca-treatment affected the pre-existing oxides differently: within approximately 85 s of CaSi wire injection, the spinel inclusions were consumed, whereas a small alumina population persisted. A predominantly submicroscopic, transient CaO–CaS population formed and converted to Ca-aluminates within minutes, with the CaS component consumed preferentially. During subsequent ladle holding, reoxidation generated new Ca-poor spinel and alumina alongside the modified inclusions, as the Ca fade progressed from 32 to 17 ppm. Between the final ladle sample and the tundish, the mean number density decreased from 291.4 to 96.4 mm - ², the size distributions shifted toward larger diameters in every heat, and the total O content decreased from 29 to 13 ppm, indicating agglomeration-assisted removal. Despite differences during treatment, all four heats converged at the tundish, centered on the low-melting 12CaO·7Al 2 O 3 composition. For the investigated steel and process route, tundish samples therefore provide a reliable basis for cleanness assessment, whereas single ladle samples may not be representative.