Improvement of cleanness and grain size in Ca-treated and Al-deoxidised steels with high S-contents in billet and bloom casting : secondary steelmaking: final report

The present investigation clearly shows that there exists an 'inclusion path' during the ladle treatment largely influencing the formation of liquid Ca aluminates. The inclusion path for the heats with RH treatment markedly differs from the heats without RH treatment. This may be mainly caused by the oxygen level. A very low oxygen level easily leads to the formation of MgO/AI2O3 spinel via the reaction between liquid steel and ladle refractory together with a decrease of the CaO/Al2O3 ratio. Additionally, the spinel is mostly enveloped by CaS. These reactions lead to a poor castability. Therefore, Ca treatment must be corrected taking the above-mentioned phenomena into account. Furthermore, the castability was theoretically analysed with regard to the 'liquid window' and a calculation model has been established to couple with the problem of nozzle clogging. Quality improvement of Al-deoxidised calcium-treated steel grades with high S-contents has been developed through a redesign of the secondary metallurgy and billet continuous casting processes acting on steel final composition and morphology of inclusions. A novel deoxidation technology without using aluminium was investigated to improve castability, grain structure and properties of continuously cast steel. Using the novel deoxidation technology, good castability and grain refinement were reached together with excellent cleanness. The numbers of steel grades being produced via billet casting were significantly enlarged due to the improved knowledge of top slag, Ca treatment, removal of oxygen and castability.