Optimum variant for closing the orifice in a steel-pouring nozzle

Abstract

When the stopper of a steel-pouring ladle equipped with a slide gate is replaced, it is difficult to ensure that the molten steel will pour freely through the orifice in the gate when it is opened.The problem stems from the fact that molten steel being tapped from the fumace comes into contact with steel that is already in the orifice from the previous heat. Since these two materials differ appreciably in density, some of the residual steel is washed out of the orifice. The newly free volume of the orifice is filled with the steel from the current heat, which in turn forms a "plug" inside the orifice when it solidifies. A slag skin is also formed at the site where the two materials come into contact, the thickness of this skin being determined by the density and refractoriness of the lining formed by the particles of the material from the previous heat.To break up the metal-slag plug, it is necessary to "wash" the orifice with oxygen. This operation is performed with an L-shaped steel pipe, the short end of which has a length of 0.6-0.8 m. Such washing destroys the cylindrical shape of the orifice, which leads to an increase in the number of ingots that are cast with an unorganized stream of metal (fan-shaped, slanting, etc.) and with the orifice not completely closed as casting proceeds from one ingot mold (fountain) to the next. This leads to "seepage."Based on the above, the main requirement that must be met by the material used to close the orifice in a steel-pouring nozzle is that it form a slag skin which is easily broken up either by the pressure of the molten steel after the slide gate has been opened or at least by a single "washing" with oxygen.One method that has been proposed to prevent material from being washed out of the nozzle orifice involves completely filling the orifice with a refractory. The material of the upper layer expands when heated, leading to the formation of a slag skin [1]. The problem with this method is that it is necessary to heat the ladle after the nozzle orifice has been closed. We tried a simpler method of closing the orifice -placing a metal plate on top of the orifice after it has been filled with the refractory. Table 1 presents data on the results of tests performed with different materials to close the orifice in the nozzle of a 250-ton steel-pouring ladle.An analysis shows that the worst results in terms of burning through the hole, determined by the number of bendings of the oxygen pipe, were obtained by using coke fines, a molding-sand mixture, and natural sand. The poor results in this case are attributable to the fact that coke fines are easily washed from the orifice, resulting in the formation of a relatively thick metallic plug; the natural sand and the molding-sand mixture, characterized by a lower refractoriness compared to the other materials, form a thick slag skin. Better results were obtained with the use of coke fines, mill scale, and powdered fireclay, which is due to the fact that these materials "pack" the orifice more tightly and prevent its penetratio...