METHOD FOR REFINING MOLTEN STEEL TO PRODUCE ULTRA LOW SULFUR STEEL BY USING RECIRCULATION VACUUM-DEGASSING SYSTEM

摘要

An object of the present invention is to efficiently refine molten steel by using a recirculation vacuum-degassing system to produce ultra low sulfur steel having sulfur content £ 10 ppm, prevent temperature of the molten steel from dropping too much, and suppress attachment of skull onto inner walls of a vacuum chamber and damage by erosion of refractory materials as best as possible. Specifically, the present invention provides a method for desulfurizing molten steel by using a recirculation vacuum-degassing system, comprising the steps of: employing, as the flux, oxides solely constituted of CaO or oxides containing CaO as primary component, said oxides containing no fluorine-based component and the average particle diameter of said oxides being in the range of 75 mm to 1000 mm; and carrying out the desulfurization treatment by controllably maintaining, provided that amount of molten steel in the ladle is W (t) and circulation rate of molten steel is Q (t/min), ratio W/Q to be in the range of 1.0 min. £ W/Q £ 2.0 min.