Effect of Molten Steel Composition on Inclusion Modification by Calcium Treatment in Al-Killed Tinplate Steel

Abstract

The effects of oxygen, magnesium, and sulfur content in Al-killed tinplate steel on the “liquid zone” of inclusion modification by calcium treatment were clarified through industrial experiments and thermodynamic calculation, and the characteristics of inclusions modification were studied. The results show that the inclusions in molten steel before calcium treatment are mainly Al₂O₃ inclusions. The inclusions in molten steel are CaO·Al₂O₃ after calcium treatment with a holding time of 10 min, while the inclusions are mainly 3CaO·Al₂O₃ with a holding time of 30 min. And 12CaO·7Al₂O₃ inclusions are observed in molten steel when T.O content increases to 40 ppm after calcium treatment with a holding time of 30 min. As the increase of T.O content from 10 ppm to 40 ppm, the difference between the upper and lower limits of the critical calcium content corresponding to the “liquid zone” increases from 5 ppm to 17 ppm. The increase of T.O content in molten steel will enlarge the “liquid zone” range of inclusion modification by calcium treatment, and increase the critical calcium content. With the increase of magnesium content in molten steel, the liquid phase ratio of inclusions modification by calcium treatment decreases. To obtain the liquid phase ratio of inclusions at least 50% in molten steel, not only the calcium content in steel should be strictly controlled, but also the magnesium content in steel should not be larger than 15 ppm. With the increase of sulfur content in molten steel, the “liquid zone” range of inclusion modification by calcium treatment becomes narrow.