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 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 a flux, oxides solely constituted of CaO or oxides conatining 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 um to 1000 um; and carrying out the desulfurization treatment by controllably maintaining, provided that the 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.