METHOD FOR MELTING EXTREMELY LOW NITROGEN STEEL

PURPOSE: To reduce nitrogen concn. in molten steel and to stably melt an extremely low nitrogen steel by blowing a specific atmospheric pressure gas to a refractory in a vacuum vessel of an RH degassing apparatus and preventing the invasion of outer gas in a treating system. CONSTITUTION: A ladle 4 is carried just below the RH degassing apparatus 10 and the immersion tubes 24, 25 in a lower vessel 21 are dipped into non- deoxidizing molten steel 5 in the ladle 4. An upper vessel 11 having alloy charging holes 13 and a gas exhaust passage 14 and the lower vessel 21 are connected with joints 12 and the inert gas is blown through a gas blowing pipe 28 while evacuating this degassing apparatus 10 to circulate the molten steel 5. At this time, the inert gas pressurized at >=1atm. is blown from two nozzles 27 arranged at the suitable position of the lower vessel 21 so as to substantially uniformize to the lining refractory 18 of the lower vessel 21. By this method, the invasion of the outer air into the treating system is prevented, and the extremely low nitrogen steel having < about 25ppm nitrogen content is stably obtd.