DESIGN OF TUNDISH FOR VACUUM OVER-PRESSURE INDUCTION MELTING DEVICE

DESIGN OF TUNDISH FOR VACUUM OVER-PRESSURE INDUCTION MELTING DEVICE Zbyněk HUDZIECZEK a), Vladislav KURKA a), Petra BĚDROŇOVÁ a), Jaroslav PINDOR a), Jiří CIENCIALA b) a) MATERIÁLOVÝ A METALURGICKÝ VÝZKUM s.r.o., Pohraniční 693/31, 706 02 Ostrava-Vítkovice, Czech Republic, zbynek.hudzieczek@mmvyzkum.cz, vladislav.kurka@mmvyzkum.cz, petra.bedronova@mmvyzkum.cz, jaroslav.pindor@mmvyzkum.cz b) Vysoká škola podnikání, a.s. Michálkovická 1810/181, 710 00 Ostrava-Slezská Ostrava, Czech Republic, jiri.cienciala@vsp.cz Abstract The presented article deals with the design of a filtering tundish to be used on a Vacuum Pressure Induction Melting furnace (VPIM) and the numerical verification of the character of liquid steel flow inside the tundish. The designed tundish must meet the dimensional limits of placement in VPIM device and must be compatible for use in both vacuum and overpressure. The main benefits of the tundish consist in the separation of refining slag and filtration of liquid steel. In terms of metallurgical aspects, the internal shape of the tundish is designed by MAGMA software. This work is a part of "Regional Materials Science and Technology Centre" project No. CZ.1.05/2.1.00/01.0040, research program No. 6, entitled "Experimental verification of new technological procedures for metallic materials with higher quality parameters." Metallurgical laboratory will strengthen the base of MATERIÁLOVÝ A METALURGICKÝ VÝZKUM s.r.o. with the objective of more sophisticated research and development of metallurgical processes. Keywords: Induction furnace, tundish, steel, ingot casting. INTRODUCTION MATERIÁLOVÝ A METALURGICKÝ VÝZKUM s.r.o. has a laboratory demonstration melting device - Vacuum and Pressure Induction Melting device (hereinafter only VPIM). The restoration is implemented as a part of "Regional Materials Science and Technology Centre" project No.: CZ.1.05/2.1.00/01.0040. VPIM device was included in this project as a part of the built "Laboratory for experimental verification of production technology of new materials (hereinafter the "Metallurgical Laboratory") at MATERIÁLOVÝ A METALURGICKÝ VÝZKUM s.r.o. The modernized VPIM device is also built for the purpose of research and development and physical simulation of technologies of production of molten metals on large metallurgical plants. It is assumed that the equipment will be able to simulate selected metallurgical operations. VPIM device will further allow controlling of purity of molten metal during its casting using the installed ceramic filters. As a part of this work, a design is drawn up of the new tundish for VPIM device. Transport of molten steel from the induction melting device crucible into the sprue holes of the casting set using the designed tundish. Since the sprue pin is placed outside the VPIM outlet axis, development of asymmetrical ladle was decided with particularly the following objectives: transport the steel from the VPIM crucible into the casting kit for bottom casting separate slag from steel

support inclusion flotation and secure contingent filtration of steel by a ceramic filter. 1. DESIGN OF TUNDISH Up to this day, a number of works have been published dealing with the studies of parameters of tundish ladles (particularly concerning tundish ladles for CCM), character of flow in them, removal of nonmetallic inclusions, temperature modes, etc. Compared to the mentioned tundish ladles for CCM, the design has a several different features: the external shape of the tundish ladle must not exceed the internal spatial limits of the VPIM caisson, sufficient area needs to be secured for falling of the metal from the furnace crucible, the outlet from the tundish needs to be spatially fixed to the inlet into the casting kit, the internal shape of the tundish should maximally support removal of nonmetallic inclusions from steel 1.1. Tundish external shape Fig. 1 spatial limits binding for dimensions of the tundish external shape. Red colour marks the caisson shell, induction furnace body and crucible outlet. Green colour marks the possible outline of the considered tundish placed under the crucible outlet. With regard to the internal arrangement of the caisson, the tundish has a trapezoidal shape. The maximum external dimensions of the trapezoidal tundish are 850 mm long, 750 mm wide and 450 mm high. Fig. 1: Illustrations of space possibilities of tundish design

Area 1 15. - 17. 5. 2013, Brno, Czech Republic, EU 1.2. Tundish internal shape In the case of large tundish ladles (particularly CCM tundish ladles), there is a number of ways of directing the flow of liquid steel to support removal of nonmetallic inclusions. The character of flow is affected particularly by the internal shape of the tundish ladle and tundish equipment (dam, weirs, baffle, filters). The summary publication [1] deals with the possibilities and advantages of the placement of the mentioned tundish equipment on the flow character and support to flotation of nonmetallic impurities. Fig. 1 includes even the basic fixed data for creating of the tundish internal shape. The first fixed spot is the impact area of steel flow. The yellow line marks the axis of the furnace outlet, i.e. the assumed direction of flow of steel during tapping. The second fixed point is shown by the blue cross. It is the place where the tundish nozzle is to be placed. From here the metal flow goes to the sprue pin, sprue holes, and finally into the actual mould. Fig. 2 shows the basic internal shape of the tundish consisting of the impact area (called "area 1") which must be large enough to cover the whole area of impact of the casting stream, which changes in time due to the movement of the furnace outlet (when the furnace is tilted) and the intensity of the molten metal flow. The impact area is adjoined by the "channel" (called area 2) which leads the stream of steel towards the nozzle placed above the sprue pin. Moreover, the whole internal area of the tundish is inclined towards the nozzle. Area 2 designed so as to maximally support flotation of inclusions and slag droplets. Area 2 Fig. 2: Specification of tundish zones Fig. 3: Final internal shape of tundish Ceramic strainers and filters help to reduce particularly the number of nonmetallic inclusions and emulsified slag in steel. They are placed mostly in the inlet as close as possible to the casting to eliminate also the inclusions released from the casting kit. The internal equipment of the actual tundish for VPIM device was designed using a dam and offset weirs and subsequently even a filter. It is assumed that the combination of the dam and offset weirs (siphon shaped) will result in separation of furnace slag even in the impact area. Offsetting of weirs should according to assumptions result in directing the flow after the weirs towards the molten metal surface, which should result in separation of coarse inclusions and the fine particles of emulsified slag. The last stage of steel "purification" is implemented by the filter above the nozzle. There is a whole range of types and possibilities when positioning filters and sieves. A sufficient filter area needs to be secured for steel filtration corresponding to the capacity of the VPIM crucible (1750 kg). One of the possibilities is the placement of a vertically positioned plate filter in the channel, or placement of a tubular filter above the nozzle. With the metal level at 20 cm, the plate filter should have the

filtration area of ca 400 cm 2. With the same metal level, a tubular filter with the internal diameter 8 cm has the filtration surface of ca 1000 cm 2. Placement of the filter is also affected by the spatial possibility of filter placement. In the case that the plate filter is selected, the weirs and the dam and also the mentioned ceramic filter would have to be placed in the area between the nozzle and the impact area. For these reasons (lack of space and a greater area of the filter) it was decided that a tubular foam filter would be used. Ceramic filters are used particularly for reducing the number of nonmetallic inclusions and further for steel flow control. In the case that it is possible, it is preferred that the filter be as close as possible to the casting [2]. The authors of the article [3] dealt with the efficiency of removal of nonmetallic inclusions depending on the shape and equipment of the tundish. Large inclusions from 75 to 150 m are removed particularly by flotation. In this way, ca 30 % of large inclusions are removed. On the other hand, small inclusions from 25 to 40 m are removed by sticking to tundish walls. In this way, up to 50 % of small inclusions are removed. The author of publication [4] states that only 10 % of inclusions smaller than 20 µm are removed by flotation. The authors of the article [5] tested ZrO based foam ceramic filters on 718 Nickel molten alloy and FV458 martensitic stainless steel, i.e. metals very similar to the production range of MMV. In the case of the given molten metals, the filter successfully reduced the numbers of particularly oxide inclusions smaller than 10 m. The resistance of filters (and heat resistant materials in general) at temperatures exceeding steel liquidus is a very important parameter. A number of scientific teams deal with the heat-resistance of heat-resistant materials such as [6]. 2. NUMERICAL VERIFY OF FLUID FLOW IN THE TUNDISH Today's research of the character of flow in metallurgical reactors is currently based particularly on process simulation methods. These are especially physical simulations on cold water models (i.e. [7]) and numerical simulations using special software (i.e. [8]). Simulation of real processes allows not only the possibility of understanding the character of flow but also the possibility (based on the obtained results) to adjust the existing equipment as necessary but also to design completely new equipment before it is put into operation. The process of designing a new tundish by means of physical and numerical simulation is described by authors of article [9]. A comprehensive view of tundish design is provided by the summary article [10]. In his article, Mazumdar pays special attention to tundish for continuous casting machines. He assesses the success rate of individual solutions using particularly C and F curves. 2.1. Flow nature in selected parts of the tundish An experiment was conducted within MATERIÁLOVÝ A METALURGICKÝ VÝZKUM s.r.o. consisting in using MAGMA software for numerical simulation of steel flow character and flotation of inclusions and emulsified slag in the tundish designed in chapter 1. Fig. 4 illustrates velocity array in the area 1 complemented with flow vectors in the tundish impact area. Particularly in terms of volume, there are some areas apparent just before the flow impact with lower flow speed volumes. Conversely, at the place of impact there is intensive flow and turbulence activity and after the impact, significant recirculation of molten metal occurs. At the place of metal impact, there is an increased risk of slag emulsification and the following introduction of slag droplets by the metal into the following part of the tundish (area 2).

Fig. 4: Fluid flow in zone 1 Fig. 5 shows molten metal flow from area 1 to area 2 and the subsequent behaviour of molten metal in area 2. Vectors describing the direction of flow behind weirs and dam are directed towards molten metal surface, which confirms the assumption stated in article 1.2 regarding the direction of steel flow after the weirs and dam, when the ascending flow of molten metal supports flotation of tiny particles of emulsified slag and coarse inclusions. Fig. 5 shows slowing of steel flow in the foam filter. Deceleration is represented by flow resistance given by the filter. Fig. 5: Fluid behaviour in zone 2.

2.2. Inclusion and slag particles removal As a part of carried out numerical simulations, behaviour of inclusions and emulsified slag in the designed tundish was examined using MAGMA software. It needs to be highlighted that substantial simplification of the complex process of inclusion removal occurred. Inclusions and emulsified slag were for simplicity's sake defined as tracers. Only size and density properties were assigned to them. Large inclusions were defined as 100 µm. It follows from the simulation results that the greatest risk of introduction of furnace slag into area 2 is at the beginning of casting at the time when the metal level in the tundish is not up to the offset weirs. In the case of large inclusions, there is a possibility that a minimal number of them pass through the weir and the dam. In the case that these large inclusions pass through the weir and dam, the flow of steel directed upwards after the weirs and dam facilitates their flotation. To minimize the intrusion of small inclusions into the casting kit a tubular filter is used as the last element placed above the nozzle to the casting kit. The density of pores of the tubular filter was defined as 10 ppi. Fig. 6: Inclusion behaviour during pouring of steel 3. CONCLUSIONS As a part of this work, a tundish was designed intended for application on the caisson of VPIM device. The tundish was designed with regard to the limited spatial placement in the caisson and its internal shape was designed so as to allow transport of steel into the casting kit but also so as to serve as a refining tundish increasing the purity of the produced metal. The purity of metal will be achieved by the application of weir and dam and also the tubular filter placed above the tundish nozzle. Basic assumptions of steel flow control in the tundish were verified using a numerical simulation method in MAGMA software for the purpose of maximum support to flotation of emulsified slag and nonmetallic inclusions before the actual implementation of tundish production.

ACKNOWLEDGEMENTS Complete evaluation of suitability of used methodology was realised as a part of the project No. CZ.1.05/2.1.00/01.0040 "Regional Materials Science and Technology Centre" within the frame of the operation programme "Research and Development for Innovations" financed by the Structural Funds and from the state budget of the Czech Republic LITERATURE [1] SAHAI, Y. Tundish Technology for Clean Steel Production. 1. vyd. Singapore: World Scientific Publishing Co. Pte. Ltd, 2008. 311 s. ISBN-13 978-981-270-621-8, ISBN-10 981-270-621-6 [2] BAŽAN, J., SOCHA, L., MARTÍNEK, L., et al. Wear of Refractory Materials for Ceramic Filters of Different Porosity in Contact with Hot Metal. MATERIALI IN TEHNOLOGIJE. 2011, vol. 45, no. 6, p. 603-608. ISSN: 1580-2949. [3] SINHA, A., K. et. al. Mathematical Modelling of Inclusion Transport and Removal incontinuous Casting Tundishes. ISIJ International, 1993, No. 5, pp. 556-566, ONLINE ISSN: 1347-5460, PRINT ISSN: 0915-1559 [4] KAUFMANN, B. et. al. Seperation of Nonmetallic Particles in Tundishes. Steel Research, vol. 64, 1993, No. 4, pp. 203-209 [5] BATES, P., KENT, A., P. The Use of Ceramic Foam Filters in the Production of High Integrity Steels and Ni-base Alloys. ISIJ International, 1992, No. 5, pp. 682-684, ONLINE ISSN: 1347-5460, PRINT ISSN: 0915-1559 [6] SOCHA, L., BAŽAN, J., MARTÍNEK, L., et al. Laboratory Verification of Resistance of Refractory Materials for Ceramic Filters. In Metal 2010: 19 th International Metallurgical and Materials Conference: 18.-20. 02. 2010, Hotel Relax, Czech Republic, Ostrava: TANGER Ltd. 2010, p. 90 95. ISBN: 978-80-87294-17-8. [7] GRYC, K., MICHALEK, K., HUDZIECZEK, Z. Physical Modelling of Flow Pattern in 5-Strand Asymmetrical Tundish with Baffles. In Metal 2010: 19 th International Metallurgical and Materials Conference: 18.-20. 02. 2010, Hotel Relax, Czech Republic, Ostrava: TANGER Ltd. 2010, p. 90 95. ISBN: 978-80-87294-17-8. [8] MICHALEK, K., GRYC, K., TKADLEČKOVÁ, M., Model Study of Tundish Steel Intermixing and Operational Verification. ARCHIVES OF METALLURGY AND MATERIALS, Vol. 57, 2012, pp: 291-296 DOI: 10.2478/v10172-012-0025-4 [9] SADUAL, M., R. Recent Trend on Tundish Design. National Conference on Processing and Characterization of Materials (NCPCM-2011), Department of Metallurgical & Materials Engineering, 2-3 December 2011, National Institute of Technology, Rourkela [10] MAZUMDAR, D., GUTHRIE, R., I., L. The Physical and Mathematical Modelling of Continuous Casting Tundish Systems. ISIJ International, 1999, No. 6, pp. 524-547, ISBN-13 978-981-270-621-8, ISBN-10 981-270-621-6