RATIONAL SELECTION OF THE REFRACTORIES FOR SLIDE GATE PLATES

During teeming in operation of the gate the temperature field changes, but for a long time the temperature gradient changes but exceeds the limiting value. The greatest variations in temperature occur in the supporting portion in relation to the metal of the lower movable plate, which is responsible for reliable shutting off of the stream. The more shut-offs during teeming, the more intense the thermal action of the molten metal on the contact layer of the refractory. Corundum refractories are 1.5-2 times more heat resistant than periclase. Therefore, for gates for teeming on a continuous billet casting machine it is desirable to use periclase refractories, and in top teeming into molds corundum refractories. The essence of the corrosive action of molten steel on the gate plate material consists of spontaneous liberation on the contact surface of endogenic oxides of the primary oxide phase of the molten metal and subsequent development of the chemisorption process. The composition of the primary oxide phase is determined by the type of steel melted and the method of deoxidation of it. Depending upon the composition of the primary oxide phase the oxides precipitated may be in the liquid or solid state [2]. Steels containing flowable components of the primary oxide phase of the FeO-MnO-Si02 system, which are characteristic of rimmed and semikilled steels, are the most aggressive toward the refractory. Corundum refractories possess a lower resistance toward the corrosive action of such steels than periclase. Therefore, for teeming of them it is desirable to use periclase refractories. Since with an increase in temperature and contact time the corrosive action increases, for teeming on a continuous billet casting machine and teeming of large heats it is also desirable to use periclase refractories. Killed carbon steels are deoxidized with a somewhat larger quantity of silicon and sometimes aluminum is used. The oxide components of such molten steels are most frequently solid phases. In these cases there is not capillary penetration causing significant development of the depth of the reaction layer, and corrosion of the refractory is practically absent. In teeming of such steels the selection of the refractory is not signficant. Steels with an increased manganese content are distinguished by a significant quantity of its oxide in the primary oxide phase. In contrast to a periclase refractory, at the teeming temperatures a corundum refractory forms with this oxide a broad area of l~-melting compositions of secondary phases. Therefore in teeming of such steels it is desirable to use periclase refractory. An effective measure preventing penetration of the components of the primary oxide phase into the refractory is the introduction of carbon into the pores such as by impregnation by a coking hydrocarbon with subsequent heat treatment. It is desirable to use refractories with such a treatment for teeming of steels with fluid components of the primary oxide phase. Since corundum refractories are corroded more by these steels, treatment of them is more effective.