Effect of slag composition on steel cleanliness in interstitial-free steel

Ladle slag affects steel cleanliness at the end of the Ruhrstahl-Hereaeus (RH) and holding process. The relationship between composition of ladle slag, total oxygen (TO) and inclusions was investigated using X-ray fluorescence (XRF), infrared absorption, and SEM + EDS methods. The results indicate that TO in steel at the end of RH increases linearly with increasing FeO content in slag. TO is lower when ${\omega }_{\text{CaO}}/{\omega }_{{\text{Al}}_{\text{2}}{\text{O}}_3}\left(\text{C}/\text{A}\right)=1.5-2.0$ than that of C/A = 1.0–1.4 under an approximate content of FeO. During the holding process, irregular Al₂O₃ inclusions are newly generated due to slag reoxidation. Additionally, Al₂O₃-Ti_x O inclusions are newly generated in the steel when the content of FeO is higher. By combining experimental and thermodynamic calculation results, it is determined that the slag has a good melting property within the zone of C/A=1.2–1.8 and adsorption capacity of Al₂O₃ when the content of SiO₂ in slag is controlled at 4%–6%. The increase in the C/A ratio and the decrease of FeO content in slag can slow down the reoxidation rate.