Tests on corundum ? Graphite immersible nozzles in an MNLZ

Abstract

At the Nizhnetagil ~ Metallurgical Combine (NTMC), in pouring 160-ton melts of killed steel St3sp in a curvilinear MNLZ, 2-3 chamotte-graphite deep-bottomed nozzles, 360 mm high, are consumed.With the aim of assessing the feasibility of replacing the chamotte-graphite nozzles with more stable ones, at the NTMC we have tested deep-bottomed corundum-graphite nozzles made at the Novomoskovsk Refractories Works.The corundum-graphite nozzles, 600 mm high with a wall thickness of 40 mm and an outlet channel hole diameter of 60 or 75 mm, were made by hydrostatic pressing in a machine built at the All-Union ScientificResearch and Design Institute of Metallurgical Machine Construction (VNIImetmash). The graphite-containing aluminum-silicate mass included 55% of white synthetic corundum {with 96-98% A1203) , 20% ZT graphite, 70/0 silicon carbide, 15% of Druzhkovka clay DN-1, and 370 of crystalline silicon [1, 2], The open porosity was 15-18%, and the apparent density was 2.60-2.63 g/cm 3, while the compressive strength was 28-32 MPa and the tensile strength was 3.0-3~ MPa.In contrast with the chamotte-graphite nozzles 360 mm high, the corundum-graphite ones can pour steel in a totally enclosed stream, reducing oxidation of the steel on the way from the intermediate ladle to the crystallizer. The corundum-graphite nozzles were tested on a one-armed curvilinear MNLZ for pouring steels St3sp, M16s, and low-alloy steel 14G2 {Table 1).In all we tested 30 nozzles. The results of tests on six nozzles are listed in Table 2. One melt was also poured through each of the remaining 24 nozzles. The nozzles were warmed up by a gas flame to about 1000~ for 2-4 h. Nozzle No. 6 was fitted when insufficiently warmed up. The temperature of the steel in the intermediate ladle was 1540-1560~ the rate of pouring was 0.5-0.6 m/rain, the duration of pouring was 100-110 min, the length of the cast slab was 55-58 m, and the cross section of the slab was 1500 • 250 ram. The steel was deoxidized with aluminum. The composition of the fluoride mix added to the crystallizer was as follows: 46% portland cement; 24% nepheline; 67o graphite; 670 silicate clods; 18~0 fluorspar. The chemical composition of the fluoride mixture was as follows: 34-3~0 CaO; 26-31% SIO2; 7-1270 A1203; 6-8% CaF2; 5.9-7.3% Na20 + K20.The changes in the holes in the outlet channels of the nozzle were characterized by the relative overgrowth or erosion K on a side per meter of cast slab per unit time: