technology for electroslag remelting with rolation of the consunable electrode

Metallurgist,Vol. 45,Nos. 3–4,2001TECHNOLOGY FOR ELECTROSLAG REMELTINGWITH ROTATION OF THE CONSUMABLEELECTRODEI. V. Chumanov and V. I. Chumanov UDC 669.56.001It is known that metal which is produced by the method of electroslag remelting (ESR) has service properties supe-rior to those of metal made by open refining and casting. This can be attributed to the better conditions which exist for refin-ing in the specially chosen slag and for the crystallization of the metal in the case of ESR. However,the need for a two-stage metallurgical conversion (essentially requiring repeat refining) makes ESR metal more expensive than metal made by elec-tric-arc refining. By resorting to out-of-furnace treatments,the content of harmful impurities in the latter product can be reduced to nearly the same level as in ESR metal. In addition,the consumption of electric power in the production of ESR ingots is more than three times greater than the level characteristic of steelmaking in electric furnaces. It is this circumstance that is presently impeding the wider use of ESR [1]. The costs of producing ESR ingots could be reduced significantly by using an electroslag remelting technology that involves rotation of the consumable electrode around its axis.The General Metallurgy Department of South-Ural State University has already had many years of experience in developing and studying an ESR process that takes place in a centrifugal force field [2–8]. Such a technology has several important advantages. Most significantly,it makes it possible to appreciably increase the productivity of the process (more than 25% in some cases) without increasing the amount of power that must be supplied to the slag bath. This is accomplished through a change in the hydrodynamic situation in the bath,a reduction in the height of the bath,and the forced removal of liquid metal from the electrode.The hydrodynamic pattern which exists during the melting of a rotating electrode is different from the pattern char-acteristic of an electrode that moves only vertically. An ascending slag flow is created in the zone near the consumable elec-trode as it rotates. While rising,the slag is heated and reaches its highest temperature in the immediate vicinity of the elec-trode end that is being melted (Fig. 1b). In the standard technology,slag washes over the melting conical portion of the elec-trode after it gives up heat to the water-cooled wall of the mold. Moving through the near-electrode region,the slag reaches its highest temperature at the boundary with the liquid metal bath (Fig. 1a).The removal of liquid metal from the electrode as the latter rotates occurs mainly under the influence of centrifugal forces rather than gravitational forces,which is typical of the given (standard) remelting technology. When the electrode is melted in a centrifugal force field,the liquid phase (drops) are more shielded from the melting surface than when the elec-trode is not rotated.The melting end of the consumable electrode is flat when it is rotated. The slag level in the mold is thus lower than in the standard technology. In the latter,the slag bath is higher because slag is displaced by the conical end immersed in it. The decrease in the area of contact between the slag and the water-cooled wall of the mold also reduces heat losses by 9–13%, the exact reduction depending on the parameters of the remelting process. The presence of a flat end makes it possible to ensure that the required distance is maintained between the electrode and the liquid bath with a 10–15% decrease in the amount of flux which is used.In addition,rotation of the consumable electrode significantly improves the refining ability of the remelting process. This is due to the fact that the thickness of the layer of liquid metal (film) on the end of the electrode is uniform and mini-South Ural State University (Zlatoust Campus). Translated from Metallurg,No. 3,pp. 40–41,March,2001.0026-0894/01/0304-0125$25.00 ©2001 Plenum Publishing Corporation125mal (commensurate with the size of the nonmetallic inclusions),i.e.,this boundary is the limiting factor in the refining oper-ation. The drops of metal leaving the end of the electrode under the influence of centrifugal forces are very small,which increases the likelihood of nonmetallic inclusions being located at the phase boundary and subsequently being assimilated by the flux [8].An important aspect of the removal of nonmetallic inclusions from the metal during ESR is the length of time that drops are in (or pass through) the refining flux. The centrifugal forces created by the rotation of the electrode ensure radial flow of the liquid metal on the melting electrode surface. When the drops of metal separate from the electrode,they move along a trajectory that depends on the rotational velocity of the electrode. The path followed by the drops in this case is an average of twice as long as in the case of remelting without rotation. The drops determine the geometry of the metal bath as they fall into it. Because the rotation of the electrode disperses the drops and causes them to strike the bath at different loca-tions,we achieve a more planar form for the bottom of the liquid bath (and,thus,for the crystallization front) during the entire remelting process. This in turn improves the quality of the crystalline structure of the ingot.The above advantages caused us to reexamine the issue of controlling the remelting rate by changing the speed of rotation of the consumable electrode. The relations obtained to describe the effect of the rotational velocity of the electrode on the rate of its remelting for different process parameters were studied experimentally and were determined to be of an extreme nature (Fig. 2). The extremum corresponds to the maximum productivity of the furnace and the best refining con-ditions for the given process parameters. Additive expressions that were obtained previously [4,5] make it possible to reli-ably determine the optimum rotational velocity of the consumable electrode.The quality of the metal in ESR refining and the uniformity of its properties depend largely on the constancy of the temperature regime of the slag bath. However,the length of the consumable electrode decreases during the remelting oper-ation. This decreases the resistivity of the electrode and the height of the slag bath,since some of the slag goes into the crust that is formed and since the electrode is overheated to some extent. These changes lead to an increase in the power supplied 126Fig. 1. Hydrodynamics of the slag bath in the existing ESR technology (a ) and theESR technology with a rotating consumable electrode (b ):ω) speed of rotation ofthe electrode; T max ) maximum temperature of the slag bath; H 1,h 1) height of theslag and metal baths,respectively,in the existing ESR technology; H 2,h 2) heightof the slag and metal baths,respectively,in the ESR technology with a rotatingelectrode; d 1,d 2) thickness of the slag crust in the existing ESR technology andthe technology with the rotating electrode,respectively.127Fig. 2. Dependence of the rate of melting of the consumable electrode on its speed ofrotation in the electroslag remelting of steel 12Kh13 (electrode diameter 80 mm,molddiameter 150 mm):1) under flux ANF-6,current 1.8 kA,voltage 32 V; 2)under fluxANF-6,current 2.3 kA,voltage 32 V; 3) under flux AN-291,current 1.4 kA,voltage 41 V .Fig. 3. Change in electrical and process characteristics during electroslag remelting:P ) power delivered to the slag bath; t ) heat time; ω) speed of rotation of consumableelectrode; R el1,R sl ,R in ) resistance of the electrode,slag bath,and ingot,respectively;I ) furnace current; U ) working voltage of the furnace transformer.to the slag bath,so that remelting rate gradually increases. Thus,there is a change in the crystallization rate and the condi-tions under which the physico-chemical processes take place.An analysis of the changes in the electrical characteristics during the refining operation (Fig. 3) and experimental data on the effect of the rotational velocity of the consumable electrode on the rate of its melting made it possible to devel-op a technology in which the electrode initially rotates at a linear velocity that maximizes the productivity of the process for the given electrical and slag regimes. During the remelting operation,the speed of rotation of the electrode is uniformly decreased in proportion to the decrease in resistance on the slag bath and the electrode.We used the proposed technology to make a series of heats of steel 12Kh2N4A on electroslag unit A-550. The diam-eter of the consumable electrode was 80 mm,the diameter of the mold was 150 mm,the flux used as ANF-6,and the elec-trical parameters were as follows:I= 2.3 kA,U= 32 V. Using additive expressions [4,5],for specific experimental condi-tions we determined the dependence of remelting rate on the speed of rotation of the electrode:the speed ensuring the max-imum productivity was determined to be 80 rpm. The existence of this relation allowed us organize the melting operation in such a way that the drop in resistance would be offset by a decrease in the speed of the consumable electrode from 80 to 60 rpm for the duration of the heat. We constantly monitored the linear remelting rate,and when it increased we decreased elec-trode speed so as to return to the originally specified productivity. We thus succeeded in maintaining a constant thermal regime in the slag bath without changing the electrical parameters of the heat.Study of the quality of the metal that was produced showed that use of the new technology ensures the production of ingots with a defect-free macrostructure and a high degree of chemical homogeneity along and across the ingot. One char-acteristic feature of the macrostructure of the experimental ingots is the more pronounced orientation of the principal axes of the dendrites parallel to the axis of the ingot.REFERENCES1. A. E. V olkov,A. V. Laktionov,and A. G. Shalimov,“Prospects of electroslag technologies,”Elektrometallurgiya,No.1,35–37 (1998).2.V. I. 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