Reconstructing the vacuum-treatment system in the oxygen-converter shop at OAO MMK

Abstract

139 In the second half of 1996, a combined system for vacuum treatment of steel went into operation in the oxygen-converter shop at OAO MMK. This system was designed by the All-Russian Scientific-Research Institute of Metallurgical Machinery, with the participation of Magnitogorsk State Scientific-Research Institute of Metallurgical-Plant Design, and manufactured at the Azovmash plant. The liquid metal is vacuum-treated in a 380-t steel-casting ladle, by a portion-by-portion method (DH process) or by a circulatory method (RH process), depending on the type of vacuum-chamber floor [1]. Up to 2000, portion-by-portion vacuum treatment predominated. Only 163 melts underwent circulatory treatment, on account of the poor durability of the Russian refractory lining of the suction pipe and the vacuum-chamber floor. The working life of the refractory floor is no more than 58 melts for circulatory treatment and 131 melts for portion-by-portion treatment. The mean working life of Russian refractory floors is 83 melts for portional treatment and 40 melts for circulatory treatment. In portional vacuum treatment, each portion in the vacuum chamber is 8‐21 t (13.4 t on average, which corresponds to 3.7% of the melt mass). Because the metal portion is small, the number of vacuum-decarburization cycles must be increased, with corresponding increase in the duration of vacuum treatment; this impairs the durability of the refractory cladding on the replaceable vacuum-chamber floor. The number of vacuum-treatment cycles is 45‐130 (76 on average). The treatment time in this method is 22.3 min, on average, and the carbon content in the metal obtained is 0.023%, on average. In circulatory vacuum treatment, the treatment time is less: 17.5 min, on average. Decarburization is improved in circulatory treatment. The carbon content in the metal obtained is 0.013% on average; this increases the yield of low-carbon metal in extrusion groups OSV and VOSV by 8.5% [2]. The widespread use of circulatory vacuum treatment was impeded by the lack of high-quality refractories at MMK. Therefore, at the end of 1999, imported Chinese refractories were purchased. This increased the working life of the floor to 135 melts, and the results more closely matched those of portional vacuum treatment. Table 1 compares the basic characteristics of circulatory and portional treatment. It is evident that, in circulatory treatment, the assimilation of aluminum on introduction in the vacuum chamber is better than for portional treatment (the aluminum consumption is reduced by ~0.25 kg/t); the vacuum-treatment time is less by 6 min on average; and the carbon content in the metal obtained is less by 0.01%, on average. The least carbon content in the metal obtained by circulatory vacuum treatment is 0.003%, as against 0.008% in portional treatment. The prevalence of different carbon contents in the metal obtained is shown in the figure. The results obtained indicate that circulatory vacuum treatment is better in the production of superlowcarbon steel and should always be used for this purpose. With the introduction of IF steel and the need to ensure that its carbon content is ≤ 0.004%, the vacuumtreatment system had to be reconstructed.