Patent ID: 8523977

Claim:
A method of desulfurizing a silicon killed steel comprising the steps of: forming a slag over a molten metal, drawing a vacuum to less than 5 torr over the combination of slag and molten metal, stirring the molten metal and slag, deoxidizing and desulfurizing the molten metal and slag to degas the steel reducing at least sulfur, nitrogen, oxygen, and hydrogen contents, and reducing activity of oxygen in the molten metal to less than 30 ppm, and forming a slag composition after degassing the steel comprising: CaO between about 50 and 70% by weight, SiO 2 between about 20 and 28% by weight, CaF 2 between about 5 and 15% by weight, MgO not more than 8% by weight, Al 2 O 3 not more than 1% by weight, and a combination of FeO+MnO not more than 2% by weight, where the sum of CaO+CaF 2 +SiO 2 +MgO is at least 85% by weight.