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 ωCaO/ωAl2O3(C/A)=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-Tix 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.