The resistance of magnesian spinellide refractories to molten slag attack is studied and effects associated with the material composition and the microstructure of test specimens are discussed. The mechanisms of corrosion wear and chemical reactions involved are considered in detail. Slags with a basicity of 1.9 -2.1 react with periclase to yield spinel, calcium orthosilicate, and monticellite. To prevent the chemical corrosion of periclase, sintered periclase or alumina as additions to the slag are recommended for use.
In efforts to reduce metal costs, it is important to extend the lining life in metallurgical systems [1]. In particular, the linings of vacuum units account for a considerable portion of total refractory costs.Three groups of factors affect the lining life in cir culatory vacuum units [2]:(1) structural factors (the shape and size of the metal framework, the lining design, the format of the refractory pieces, and so on);(2) the physicochemical properties of the refracto ries, which depend on the quality of the raw materials, the manufacturing technology, and so on; (3) technological factors (the operating conditions of the metallurgical system, the slag composition and properties).Specialists at OAO EVRAZ NTMK have been studying these factors, in order to improve the refrac tory components and optimize their operating condi tions. The results assist in the development and intro duction of new lining configurations with improved performance and longer life.On that basis, measures have been taken to improve the operation of the vacuum units at OAO EVRAZ NTMK. STRUCTURAL FACTORSIn the converter shop at OAO EVRAZ NTMK, the lining configuration illustrated in Fig. 1 is employed in the vacuum unit. A deficiency of this configuration is that the junction of the refractory ring and the refrac tory concrete at the end is rapidly worn, as is the region where the transporting gas is supplied (Fig. 2). During operation, the liquid steel penetrates into the damaged surface and erodes the metallic plate that supports the refractory rings at the end of the tube. That leads to settling of the lower ring of the tube and the formation of a gap between the rows of components. In subse quent operation, metal melt leaks into the gaps, pro ducing local erosion around the circumference (Fig. 3). The vacuum chamber requires premature repair [2,3]. This problem at the junction of the refractory concrete and the refractory rings may be eliminated by adopting the design in Fig. 4. In that case, there is no horizontal junction between the concrete and the refractory ring, and the lower ring is rigidly fixed. In the course of operation, the modified structure does not permit per ceptible settling of the refractory rings. The mean life in experimental vacuum chambers was 106.0 melts (as against 102.0 melts for mass produced components). PROPERTIES OF THE REFRACTORIESSpecialists at OAO EVRAZ NTMK pay great attention to the refractory components obtained from suppliers and, in particular, their susceptibility to wear [4]. Research shows that the unit wear of refractories in the discharge tube does not depend on the overall properties but is largely determined by the characteris tics of the intake refractories. The operating condi tions are most intense at the intakes: additional cool ing of the refractory surface in the period between melts by neutral gas and considerable turbulence of the metal, which tends to erode the brick.It is evident from the table that the components dif fer considerably in properties and working life. For more detaile...
The results of an integrated study of external factors on wear resistance of periclase-chromite refractories in vacuum chamber lining are discussed. It is established that the mineral composition and structure of magnesian-spinellide refractories are the least liable to transformations.The existing technology of melting transport metal at the Nizhnii Tagil Metallurgical Works includes treatment of steel in a circulation vacuum chamber intended for degassing and refining steel. Reliable performance of the vacuum chamber, which depends on the state of its lining, is essential to implement a steady degassing procedure. The lining of a vacuum chamber consists of three layers: the heat-insulating (calcium silicate plates), reinforcing (lightweight, high-grade, fireclay brick), and working layer (periclase-chromite brick based on a melted or sintered material). The most expensive is the working lining, especially in the lower part of the vacuum chamber and in the branch pipes. Therefore, the development of high-quality resistant refractories for steel degassing is an important and promising line of research.To determine the most significant factors affecting the wearing rate of refractories, we used the service parameters of forty complete campaigns of vacuum chambers lined with refractories produced by the same company. In processing these service parameters, we analyzed the dependence of the specific wear of refractories on 11 factors and then selected the most significant ones using the pairwise correlation method: average degassing duration; residual pressure in the chamber during metal degassing; the number of gun-concreting operations; the average consumption of gunite per operation. After data processing, the latter factor was excluded from analysis as being non-significant. Then we calculated a multiple regression with correlation coefficient 0.67 characterizing the degrees of the mutual impact of the considered parameters. The regression equation has the following form:where X 1 is the average degassing duration, X 2 is the residual pressure (<300 Pa) in the chamber during steel degassing; X 3 is the number of gun-concreting operations.The equation reflects the influence of the service conditions on specific wear and does not include an estimate of the product quality. Therefore, we compared the resistance and specific wear of three types of experimental refractory samples (Nos. 1, 2, and 3) that differed in their qualitative parameters. The results are shown in Tables 1 and 2. The data in Tables 1 and 2 show that the specific wear of refractories in the discharge pipe depends not so much on the qualitative parameters but to a great extent on the quality of products in the inlet branch pipe. It is here that refractories experience the most stringent service conditions: additional cooling of the product surface by a neutral gas between the heats and perceptible turbulence of metal, which causes additional erosion of the products. These reasons are largely responsible for the increased wear of refractories in th...
Literature data on measurements of slag temperature in EAFs and ladle-furnaces are examined together with the author's own such measurements. Known recommendations on improving the foamability of the slag are also discussed. Results are presented from studies of technological features of "good" and "poor" slag-foaming regimes in a high-power electric-arc furnace.There have been reports that the temperature of the slag cover in high-power electric-arc furnaces used to make steel sometimes exceeds the temperature of the metal itself [1][2][3]. Values of slag temperature measured at different points in the cover at the end of the melting period and values of the temperature of the molten metal measured throughout the heat are within the range 1650-1800°C [1,3]. It has also been reported that the formation of a slag cover whose thickness is greater than the length of the electric arc creates a closed circuit inside the slag, this circuit by-passing both the metal and the arc. Thus, the temperature of the slag can rise to 1800°C, regardless of the metal's temperature [2]. The process that unfolds in the cover therefore becomes similar to the electroslag process, complete with high temperatures (1700-1800°C) in the slag bath.In addition, the source of the high temperatures may not be related to the arcs. A "hot spot" -a reaction zone where the metal bath undergoes intensive oxidation and which releases heat that may not be fully transferred to the volume of the metal -is created in the region where the oxygen jet comes into contact with the bath [4,5]. The surface of the melt inside the reaction zone is heated to the boiling point of iron, since excess heat that may not be transferred deep into the bath is expended on the vaporization of iron and oxides [4,5]. The temperature of the surfaces of the hot spots can be estimated based on the evaporation point of the components of the slag and metal (2500-3500°C) [6][7][8]. Since the hot spots are in a layer of foaming slag or in the immediate vicinity of the layer, this may not have any effect on its temperature. A temperature that is determined by an automated probe and is taken as the average temperature of the semifi nished product probably cannot truly be considered the average temperature of the slag cover or even an effective value of that temperature.It is obviously incorrect to analyze slag-foaming processes within a temperature range that is in fact not characteristic of foaming slags.We compared experimental results from measurements of the temperatures of the metal and slag during a heat against literature data [1, 3] on the temperatures of metal and foamed slag in a high-power electric-arc furnace (Fig. 1). Our measurements confi rmed the known fact that the slag is heated to temperatures higher than the metal and they indicated that the temperature of the slag in the region above the location where the temperature of the semifi nished product is traditionally measured is roughly 70° higher than the temperature of the semifi nished product and ranges from 1650 ...
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