Lining of tap holes of large-tonnage electric steel melting furnaces

Abstract

The increasing requirements for electric melted steel quality and improvements in methods of ladle treatment have made it necessary to search for methods of protection of the molten metal from oxidation and contamination by nonmetallic inclusions. This in turn is responsible for attempting to more completely separate the molten steel from the furnace slag in tapping of it into the steel teeming ladle. The design of the tap hole has a significant influence on solution of this problem.Most common is tapping of steel through a spout (Fig. I). The design is material consuming, does not eliminate entry of furnace slag into the ladle, and is characterized by a long distance from the point of exit of the steel from the furnace to the ladle with cooling of it and oxidation of the surface of the stream of metal.Siphon tapping of steel is assumed to be more progressive. The design of the siphon tap hole (Fig. 2) includes preparation in the furnace wall of a tap hole of refractory rings with the start of it below the level of the pool of the bath. Such a design of tap hole and spout reduces the distance from the furnace to the pool of metal in the ladle only a little but provides interception of the slag from the steel being tapped in operation of the furnace with a residue of molten metal.Recently bottom tapping of steel has become more common. Initially the tap hole was located on the axis of the furnace (Fig. 3) [i]. The design significantly reduced the time for tapping of steel, decreased the distance from the tap hole to the level of metal in the ladle, and made it possible to significantly increase the area of water-cooled panels in the wall lining. However, central bottom tapping did not provide interception of the slag and permitted a portion of the molten metal to remain in the furnace. In addition servicing of the tap hole was difficult and repair of it required additional clumsy equipment. Therefore subsequently the bottom tap hole was moved to the edge of the bath in the projection (Fig. 4). While possessing the advantages of center bottom tapping, this design makes it possible to eliminate its disadvantages with respect to interception of the slag and eases servicing of the hole.These three types of tap holes differ both in the conditions of flow of the molten metal and in the design of the lining, the method of preparation of it, the consumption of refractories, the labor requirement for servicing, and the effectiveness of use.For example, while the time for tapping steel through a siphon or spout is 8-10 min, through the bottom hole it is not more than 2-3 min. In addition the length of the interrepair operating time of the spout is 30-40 heats, of the siphon assembly 40-60, and of the bottom projection 80-100 heats.Service of the spout lining requires repair of its operating surface each heat with the use of various compounds for this. The furnace lining in the region of the joint of the spout with the bath does not require special repair each heat other than more careful patching during the interheat period...