PROCESS AND APPARATUS FOR VACUUM DEGASSING MOLTEN STEEL

摘要

Molten steel is efficiently vacuum treated in a vacuum treatment vessel 9 provided with a top blow lance 1 capable of injecting an oxygen gas 6 and a fuel gas 8 at desired flow rates, respectively, onto molten steel on the top of the vacuum treatment vessel 9 in a freely vertically movable manner, by conducting an appropriate combination of a step of setting the lower end of the top blow lance 1 to a level of not more than 2 m from the surface of molten steel bath and injecting only an oxygen gas onto the molten steel and a step of setting the lower end of the top blow lance to a level of 1.0 m or more from the surface of molten steel bath and injecting both of oxygen gas and a fuel gas from the top blow lance onto the molten steel, thereby preventing a decrease in the temperature of molten steel during the vacuum treatment and also preventing deposition of molten steel on the inside wall of the vacuum treatment vessel without using a large scale heater of electrical resistance type. The top blow lance 1 comprises an oxygen injection region comprising a throat part 2 and a tapered region 3 connected to the lower end of the throat part 2, provided along the axial center line of the lance, and a plurality of fuel gas supply ports 4 provided in the tapered region 3.