LABORATORY VERIFICATION OF RESISTANCE OF REFRACTORY MATERIALS FOR CERAMIC FILTERS

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1 LABORATORY VERIFICATION OF RESISTANCE OF REFRACTORY MATERIALS FOR CERAMIC FILTERS Ladislav SOCHA a, Jiří BAŽAN a, Ludvík MARTÍNEK b, Pavel FILA b, Martin BALCAR b, Přemysl LEV c a VSB - Technical University of Ostrava, Faculty of Metallurgy and Materials Engineering, Department of Metallurgy, 17. listopadu 15/2172, Ostrava - Poruba, Czech Republic, ladislav.socha@vsb.cz, jiri.bazan@vsb.cz b ZDAS, a.s., Strojirenska 6, Zdar nad Sazavou, Czech Republic, ludvik.martinek@zdas.cz, pavel.fila@zdas.cz, martin.balcar@zdas.cz c KERAMTECH, s.r.o., Horska 139, Zacler, Czech Republic, lev@keramtech.cz Abstract The paper is devoted to laboratory verification of resistance of developed refractory materials designed for manufacture of ceramic filters used for filtration of steels. Today, ceramic filters are used for improvement of steel purity, for example at ingots bottom casting, but also in tundish at continuous steel casting. It means that refractory materials must withstand very demanding conditions. Exacting requirements comprise e.g. ability to remove impurities, resistance to sudden temperature changes, resistance to corrosion and erosion by metal. Use of ceramic filters should not lead to decrease of temperatures of cast steel due to cooling effect of refractory material, since it could lead to its solidification and filters clogging. That s why development focused on reduction of thermal capacity of refractory material was realised by the company KERAMTECH, s.r.o. It was reduced by increase of porosity of refractory material by starch addition. At development several materials were proposed, in which porosity was purposefully reduced by starch addition into the mixture for filters production in ratios 3; 5; 7.5 and 10 wt.%. In this manner, mass and cooling effect of filters refractory material on liquid steel was reduced. Experimental resistance verification of developed materials with increased porosity to corrosion and erosion was realised in the laboratory of the Department of Metallurgy at the Technical University of Mining and Metallurgy in Ostrava (VSB-TU Ostrava). These experiments were aimed at simulation of industrial operational conditions and determination of probable behaviour of developed refractory materials. Keywords: steel, filtration, refractory material, resistance, corrosion, erosion 1. INTRODUCTION Today, ceramic filters are used for improvement of steel purity in steelworks. Use of ceramic filters in gating system is one of possibilities for increase of purity and quality at ingots casting. Application of this technology enables increase of the cast steel purity, reduction of non-metallic inclusions occurrence, costs reduction for repairs of defects, etc. However, the general rule should be that filtration of liquid metal should not be a substitution for an inappropriate production technology of liquid metals, but it should be rather an addition to a technology, and it is important to create optimal conditions. Nevertheless, technology of steel filtration is complicated by more exacting technical requirements at preparation of filtration system, necessity of control and regulation of casting temperature and by other factors. Method of molten steel filtration represents a perspective technology, it can be used to increase the macro-purity and the micro-purity of final product [1].

2 2. USE OF CERAMIC FILTERS AT CASTING OF STEEL INGOTS Technology of steel filtration at ingots bottom casting via gating system in order to eliminate occurrence of inclusions and to ensure high purity of steel was tested in industrial conditions of the steelwork in ZDAS, a.s. The gating system for ingots casting consisted of gate stick, stool and ingot-mould with sink head. Application of filtration system for ingots casting consisted in use of the series of filters embedded in ceramic cartridge behind another. The cartridge is placed in the gating system in the enhanced channel of the stool. Steelwork at ZDAS, a.s. participated in project and realisation of technical solution, including manufacture of cartridges and filters. Foam ceramic filters mm based on ZrO 2, SiO 2 +SiC and carbon, Al 2 O 3 and SiO 2 based materials were tested there. Filtration cartridges were due to the problems at casting (mechanical damage) modified in collaboration with the company KERAMTECH, s.r.o. during the next phase and mullite (RK-5) based ceramic strainer filters with dimensions mm and mm were used. Filtration cartridge is made of fireclay material with share of Al 2 O 3 > 61 wt.% [2]. It was found during industrial applications that this arrangement is satisfactory. However, the molten steel flow got cooled during its passage through the filter channel and thus caused the problems with freezing of steel in this case too. In order to minimise this phenomenon, the company KERAMTECH, s.r.o. developed and then tested a refractory material, the porosity of which was purposefully increased (up to 10 wt.% of material) and its mass and cooling effect of the material were reduced. 3. DEVELOPMENT AND TESTING OF REFRACTORY MATERIALS FOR NEW CERAMIC FILTERS Development of refractory materials was realised by the company KERAMTECH, s.r.o. and refractory material representing mullite-corundum mass based on Al 2 O 3 (75 wt.%) SiO 2 (21 wt.%) with an increased refractoriness named RK-5 was chosen for the above mentioned modification. In order to decrease the thermal capacity of this material, its porosity was increased by starch addition in ratios of 3; 5; 7.5 and 10 wt.%. Specific thermal capacity of material RK-5 is 1800 kj K -1 kg -1. Starch addition of 1 wt.% to the basic mass decreases the mass of the final product by 2 % due to the increased porosity and decreases therefore also its thermal capacity. The modified refractory materials with increased porosity were tested by experimental heats at the laboratory of VSB-TU Ostrava in order to verify the erosion and corrosion effects and to determine resistance and service life of the new ceramic filters. Two types of steel were used for all experimental heats ordinary carbon steel CSN (T L = 1495 C) and high-manganese steel CSN (C = 1.3 wt.%, Mn = 13 wt.%, T L = 1375 C). Samples with dimensions mm were prepared from the modified refractory material supplied by KERAMTECH, s.r.o. Experimental heats were performed in induction furnace connected to the highfrequency generator GV 22. Subsequently, two series of experiments for evaluation of corrosion and erosion effects with use of carbon and manganese steel at temperatures of 1560 C, 1600 C and 1680 C for 20 minutes were realised. Porosity of tested refractory materials was increased by various starch additions [3].

3 4. EVALUATION OF REFRACTORY MATERIALS Evaluation of stressed samples was made in several parts. First, the visual evaluation of the whole stressed samples after laboratory experiments was realised. Afterwards, the evaluation of cross-sections with focus on structure and surfaces (edges) of samples was done. 4.1 Visual evaluation of erosion and corrosion Visual evaluation of stressed samples was made with use of photographs of the whole samples taken after the experiment. In the first series of experiments, refractory materials were tested from the viewpoint of carbon and manganese steel influence at extreme temperature of 1680 C for 20 minutes with starch content 3; 5; 7.5 and 10 wt.%. The results of the first series of experiments are given in Fig. 1. Starch Addition 3 wt.% 5 wt.% 7,5 wt.% 10 wt.% 3 wt.% 5 wt.% 7,5 wt.% 10 wt.% Carbon Steel Manganese Steel Fig. 1. Photos of refractory material after exposition in carbon and manganese steel at temperature 1680 C for a period of 20 min It is evident from this figure that addition of starch in the quantity of up to approx. 3 wt.% did not have any significant influence on wear of refractory materials. However, further additions of starch up to 10 wt.% manifested themselves both in carbon and manganese steel by distinct wear of refractory material and even by its deformation. It is also obvious that its corrosion effect on tested refractory material was substantially higher in case of manganese steel use than in case of use of carbon steel heats. In the second series of experiments, modified tests were made on the basis of previous results, again with starch content 3; 5; 7.5 and 10 wt.% in refractory materials with use of carbon and manganese steel for 20 minutes of interaction, but at temperatures of 1560 C and 1600 C. Purpose of these tests was not only to simulate temperatures used in practical conditions of bottom steel casting into the ingot-moulds, but also to test influence of various starch additions on wear at these reduced temperatures. The results of these experiments are given in Fig. 2. and Fig. 3.

4 Starch Addition 3 wt.% 5 wt.% 7,5 wt.% 10 wt.% 3 wt.% 5 wt.% 7,5 wt.% 10 wt.% Temperature t = 1560 C Temperature t = 1600 C Fig. 2. Photos of refractory material after exposition in carbon steel at temperatures 1560 C and 1600 C for a period of 20 min Starch Addition 3 wt.% 5 wt.% 7,5 wt.% 10 wt.% 3 wt.% 5 wt.% 7,5 wt.% 10 wt.% Temperature t = 1560 C Temperature t = 1600 C Fig. 3. Photos of refractory material after exposition in manganese steel at temperatures 1560 C and 1600 C for a period of 20 min It was determined by an analysis of these figures that temperature of experiments 1560 C seems to be too low for carbon steel. The calculated temperature of liquidus according to the chemical composition of steel is 1495 C. In this case, steel froze on the walls of ceramic samples during tests. This meant in case of industrial application that it was necessary to increase the temperature of casting from usual 1560 C (temperature of casting at the ZDAS, a.s.) to approx C (i.e. by approx C). However, the refractory materials showed minimum wear up to the content of 5 wt.% of starch, and slightly higher wear at the content of 7.5 wt.% of starch in case of the same steel and temperature of experiments 1600 C. Nevertheless, higher content of starch (10 wt.%) had already negative impacts at this temperature. When manganese steel was used at the temperature of 1560 C and calculated temperature of liquidus was 1375 C, the degree of wear was higher when the content of starch was more than 7.5 wt.%. For this reason, the experiments at the temperature of 1600 C did not use the sample with 10 wt.% of starch. At the temperature of 1600 C a minimum loss was found at 3 wt.% of starch in the same (manganese) steel. 4.2 Evaluation of cross-sections The cross-sections apart from visual evaluation of the samples after experiments were also evaluated. The evaluation was made on the basis of comparison of photo-documentation, which consisted of the photos

5 taken by stereo-microscope Olympus and scanning microscope Tescan Vega working in the mode fish eye. The pictures taken by the stereo-microscope make it possible to determine the depth of penetration, structure of material (cracks, fissures, structural failures) and also losses of material. For this paper only the results for carbon and manganese steel for 20 minutes at temperature of 1600 C with starch content of 3; 5; 7.5 and 10 wt.% were chosen [4]. Fig. 4. shows the photos of samples subjected to the effect of carbon steel and temperature 1600 C. It is evident from photo-documentation that character and morphology of refractory materials' surfaces are similar. Refractory materials up to the starch content of 5 wt.% showed a minimum wear, and slightly increased wear at the starch content from 7.5 wt.% up. Fig. 5. shows the photos of samples subjected to the effect of manganese steel and temperature 1600 C. It is evident from photo-documentation that an addition of starch > 5 wt.% at the temperature of 1600 C had negative effect in case of this steel. This does not concern only the surface layers of refractory material, but even its central parts. Higher additions had very negative influence on material structure. Starch Addition 3 wt.% 5 wt.% 7,5 wt.% 10 wt.% Photos of Stereomicroscope Photos of Scanning Microscope Fig. 4. Comparison of cross-sections' photos stressed at temperature 1600 C in carbon steel made by the help of stereo-microscope and scanning microscope Starch Addition 3 wt.% 5 wt.% 7,5 wt.% 10 wt.% Photos of Stereomicroscope Photos of Scanning Microscope Fig. 5. Comparison of cross-sections' photos stressed at temperature 1600 C in manganese steel made by the help of stereo-microscope and scanning microscope

6 5. CONCLUSIONS It is possible to formulate the following findings on the basis of realised laboratory experiments: in case of manganese steel use, the corrosion effect on tested refractory materials was much higher than in case of carbon steel heats, addition of starch up to approx. 3 wt.% in tested samples of the first series had no distinct influence on wear of refractory materials at their contact with carbon and manganese steel at the temperature of 1680 C for 20 minutes. However, further starch additions up to 10 wt.% resulted in case of both steels in significant wear of refractory material and also in deformation, in case of samples of the second series, the refractory materials were in contact with carbon and manganese steel for 20 minutes, but at lower temperatures of 1560 C and 1600 C. Temperature of experiments 1560 C seemed to be too low for carbon steel, since during the tests the steel froze on the walls of the samples. In case of the same steel and temperature of 1600 C, the samples showed a minimum wear up to the starch content up to 5 wt.%. Nevertheless, higher starch content of 10 wt.% had at the temperature of 1600 C already negative effect, in case of manganese steel use and temperature of 1560 C, the starch content higher than 7.5 wt.% was manifested by an increased degree of wear. For this reason, the experiments at the temperature of 1600 C were made without the sample containing 10 wt.% of starch. The samples subjected to influence of the same steel showed at the temperature of 1600 C and starch content of 3 wt.% a minimum loss of refractory material, it was determined from the photos of cross-sections at use of carbon steel and temperature of 1600 C, that the samples of refractory material showed a minimum wear up to the 5 wt.% of starch, and wear slightly increased at the starch content above 7.5 wt.%. However, at use of manganese steel even the addition of starch > 5 wt.% had negative impact. This phenomenon has occurred both in surface layers and also in central area of the refractory material, on the basis of the obtained results the company KERAMTECH, s.r.o. has already started production of ceramic filters made of material RK-5 with starch addition of 5 wt.% under the name RK-5/5. ACKNOWLEDGEMENTS The work was prepared within the framework of the project EUREKA E!4092 MICROST and projects MPO No. FI-IM3/034 and FR-TI1/222. LITERATURE REFERENCES [1] STRÁNSKÝ, K., BAŽAN, J., HORÁKOVÁ, D. Filtrace taveni železa v průmyslové praxi. vyd. Ostrava: VŠB-TU Ostrava, s. ISBN [2] FILA, P., BALCAR, M., MARTÍNEK, L., LEV, P. Nové zkušenosti s využitím keramických filtrů při odlévání ocelových ingotů. In Sborník z konference Hutní keramika. Ostrava: TANGER, 2009, s ISBN [3] BAŽAN, J., SOCHA, L., LEV, P. Vliv pórovitosti žáromateriálů na jejich korozi a erozi ocelí. In Sborník z konference Hutní keramika. Ostrava: TANGER, 2009, s ISBN [4] Protokol č. LP/09 E006/124. Hutní a chemické laboratoře. ArcelorMittal, a.s., 2009, 19 s.