Development of Secondary Refining Process. and Its Application to Production of Clean Steel

Recently the needs of ultra low [S] and ultra low [O] steel for many kinds of products increase. Secondary refining process to achieve less than 5 ppm sulfur and less than 10 ppm oxygen in molten steel was investigated. In order to produce ultra low sulfur steel the optimum slag composition was determined as 60%CaO-32%Al2O3-8%SiO2 and the deviation of final slag composition could be decreased by the method of 2 times flux addition. In the case of producing ultra low oxygen steel, it was important for increasing deoxidation rate that the total content of FeO+MnO in slag before RH process was less than 1 wt% and then deoxidizing products were removed through RH process. On the basis of these results, the refining process, BOF-VSC-NKAP-GI-RH process, was established. By this process it became possible to achieve less than 5 ppm sulfur and less than 10 ppm oxygen in steel.