Effect of Al-Ti complex deoxidation mode on steel cleanliness during RH refining of IF steels

Abstract

The deoxidation in Ti containing Al killed IF steels is a vital parameter that directly influences the steel cleanliness. In this study, the evolution of steel compositions and inclusion characteristics during RH refining of IF steels were investigated by industrial trials involving three Al-Ti deoxidation modes, which were adding Ti-Fe 2 min after Al deoxidation in Mode I, adding Al and Ti-Fe simultaneously in Mode II, and adding Ti alloys with two equal batches at 3.2 min and 5 min after Al deoxidation in Mode III. The results showed that nitrogen pickup of several ppm was observed with Ti-Fe alloy addition. Therefore, nitrogen content in alloys should be strictly controlled. The main inclusions were single alumina particles, alumina clusters, alumina cluster with Al-Ti oxides, and Al-Ti complex oxides. Mode III had less amount of Al-Ti oxides compared with Mode I, and Mode II had a much higher proportion of Ti bearing inclusions than Mode I and Mode III at RH end. For Mode I & Mode III, the inclusion removal rate was 0.33–0.35 min−1 before adding Ti, and slowed down to 0.14 min−1 for Mode I and 0.22 min−1 for Mode III after Ti-Fe addition. While for Mode II, the rate was only 0.18 min−1. Mode III was the recommended mode for its excellent steel cleanliness and high Ti yield with slight elongation of RH treating time.