Influence of Different Additions on the Behavior of Mold Fluxes

Transcription

1 Influence of Different Additions on the Behavior of Mold Fluxes Elena Brandaleze 1, Alejandro Martin 1, Jorge Madias 1, Edgardo Benavidez 2 and Carlos González Oliver 3 (1) Instituto Argentino de Siderurgia Av. Central y 19 Oeste - Barrio Somisa, 2900 San Nicolás, Pcia. Buenos Aires, Argentina Phone Fax: brandaleze@siderurgia.org.ar (2) Universidad Tecnológica Nacional - FRSN Colón 332, 2900 San Nicolás, Pcia. Buenos Aires, Argentina Phone Fax: ebenavidez@frsn.utn.edu.ar (3) Centro Atómico Bariloche (CNEA) Av. Bustillo Km. 9,5 (8400) Bariloche, Argentina Fax: gon@cab.cnea.gov.ar Key Words: mold flux, crystallization, fluidity, heat transfer, surface tension, tracers. INTRODUCTION BaO and CeO 2 have been added to ladle slag and/or tundish slag to trace the origin of macroinclusions [1]. They may also be added to casting powders for the same purpose. ZrO 2 has been used as a tracer to mold fluxes to study phenomena occurring during casting [2]. ZrO 2 may also be added to mold fluxes in order to minimize SEN slag line attack [3] or to bring about more uniform heat transfer [4]. It is known that mold powder infiltrated between the mold and the solidifying strand is liquid with or without crystals, steel and the mold. Viscosity and crystallization of this layer play an important role in the lubrication and heat extraction during casting. The influence of the ZrO 2, BaO and CeO 2 on the physical and chemical properties at operating temperatures constitutes an interesting point of study. The crystallization tendency is relevant, because it is the key to mold heat transfer. Previous papers by the same authors detailed results of physical properties and crystallization tendency of mold powders used for different steel grades [3, 5]. In this paper, information on powders with addition of ZrO 2, CeO 2 and Ba carbonate is presented. The interest was to evaluate the influence of these oxides on fluidity and heat transfer. Inclined plane tests were carried out to obtain results on fluidity evolution. Samples with different quantity and compounds were evaluated. The influence on viscosity was calculated using the Riboud model. Wettability represents other important factor in inclusions entrainment. In this paper partial results of contact angle measurements on mold fluxes with ZrO 2 at different temperatures are presented. Finally, the crystallization tendency was determined through observation with light microscope and SEM. EXPERIMENTAL In order to analyze the influence of ZrO 2, CeO 2 and Ba carbonate addition effects on mold flux behavior, samples with different oxides and carbonate addition to the same type of mold powder were prepared. The detail of these samples is presented in table I.

2 Table I. Samples prepared with different oxide/carbonate additions. Addition (%) BaCO 3 CeO 2 ZrO 2 0 B0 C0 Z0 3 B1 C1 Z1 8 B2 C2 Z2 11 B3 C3 Z3 18 B4 C4 Z4 Fluidity Inclined plane tests were carried out on samples with different percentage of Ba carbonate (B0 B4), CeO 2 (C0 C4) and ZrO 2 (Z0 Z4) to determine the fluidity evolution at 1300 ºC. During the test, 20 g of each sample with and without addition were melted in a platinum crucible and then poured on the plane, inclined 10. The length of the layer generated was measured by a digital caliber. Each value of layer length was determined by the average of ten tests. The results obtained for all the cases are presented in figure 1. Layer length (mm) LB LC LZ Percentage (%) Viscosity Figure 1. Layer length in inclined plane tests, for a mold powder with additions of BaCO3, CeO 2 y ZrO 2. Viscosity values were estimated by the Riboud model, considering the flux chemical composition with and without the different percentage of addition in one case of BaO and another case of ZrO 2. The Ba addition was considered as BaO because of at 1200 ºC the carbonate is not stable. The behavior of BaO is considered similar as Na 2 O that cause the silicate chain break. In figure 2 is possible to compare the viscosity evolution with different contents of the mentioned oxides.

3 2,5 2,0 η (dpas) 1,5 1,0 0,5 Ba Zr 0, Adition (%) Figure 2. Flux viscosity behavior with different percentage of additions. BaO and ZrO 2 have the opposite effect on the viscosity. With the higher BaO percentage, the lower viscosity is obtained. ZrO 2 causes an important increase in viscosity; the viscosity obtained with the higher addition makes the powder nonusable. Wettability In order to determine the effect of ZrO 2 addition on wettability, measurements of contact angle at temperatures between 1000 and 1080 ºC were carried out by hot stage microscopy (HSM) on samples of the flux without Zr0 2 and with 3 % of ZrO 2 (Z0 and Z1). In figure 3, is possible to compare the contact angle differences between both samples at the same three temperatures. (a) ZrO 2 = 0%, θ = 33 at 1030 ºC (b) ZrO 2 = 0%, θ = 36 at 1040 ºC (c) ZrO 2 = 0%, θ = 37 at 1045 ºC (d) ZrO 2 = 3%, θ = 17 at 1030 ºC (e) ZrO 2 = 3%, θ = 20 at 1040 ºC (f) ZrO 2 = 3%, θ = 25 at 1045 ºC Figure 3. Contact angle comparison between Z0 and Z1 samples at three different temperatures. The results presented in figure 4 indicates that this oxide cause a low contact angle increase at temperatures higher than 1050 ºC with respect to the flux without this oxide addition. When θ = 180º, the liquid is wholly non-wetting with respect

4 to the particular solid and for systems in which θ = 0º the solid is completely wetted. At the conditions analyzed, there is a decrease in wettability. θ (Degree) With ZrO 2 Without ZrO 2 Temperature (ºC) Figure 4. Evolution of contact angle with temperature. Crystallization tendency The crystal phases evolution were established by light and electronic microscopy on samples melted at 1300 C and quenched in air in all the cases simulating cooling during casting operation in the mold. At first, a description of the crystallization tendency in the flux without additions is presented for a comparison with the results of the same flux with the different compounds and quantity of addition. The characteristic of this mold powder is to develop low crystallization. In figure 5, it is possible to observe the structural characteristics of quenched sample. A thin crystal layer of 30 µm is generated in the surface; the rest of the sample is glassy. Figure 5. Flux layer structural aspect without additions. a) Barium carbonate additions. The quenched samples of the flux with 3% - 18 % of barium carbonate were observed by light microscopy. It was possible to evaluate that crystallization tendency became lower. The higher percentage of

5 barium carbonate is the thinner crystalline proportion is. In the sample with 11 % of Ba carbonate addition, the crystal layer was very thin (see figure 6) and in the case of 18 % of Ba carbonate the samples was totally glassy. Figure 6. Flux layer structural aspect with 11 % of barium carbonate addition. b) CeO 2 oxide additions. The observation of the samples with 3 %, 8 %, 11 % of CeO 2 oxide permits to determine an increase of the crystallization with the oxide content (figure 7a, 7b, 7c and 7d). The principal crystal phase present was cuspidine and constitute crystal chains in the surface and in the inner of the sample. The flux with 18 % of CeO 2 became totally crystallized and white particles were observed between the cuspidine crystals. (a) Mold flux with 3 % CeO 2 (b) Mold flux with 8 % CeO 2 (c) Mold flux with 11 % CeO 2 (d) Mold flux with 18 % CeO 2 Figure 7. Crystallization evolution with different contents of CeO 2.

6 By electronic microscopy the sample was analyzed and it was possible to identify that the white particles composition is CeO 2. Figure 8 CeO 2 Figure 8. White particles identified in the cuspidine crystal phases with 18 % CeO 2 addition. b) ZrO 2 oxide additions. The addition of ZrO 2 was carried out in the same percentage (3%, 8%, 11% and 18 %) as the other compounds used in this study. In all the samples, the crystal structure generated was evaluated as in the previous cases by light and electronic microscopy. In the figure 9, cuspidine crystal chains were developed. Figure 9. Crystalline structure developed in the flux with 3 % of ZrO 2 addition. The samples were analyzed by EDS to determine the dendrite and matrix chemical composition. Elements distribution on the sample was identified and an important increase of cuspidine crystals was observed. The crystallization tendency developed with the ZrO 2 addition on the flux, increase with higher percentage of the oxide. RESULTS AND DISCUSSION In the casting of calcium-treated aluminum-illes steel, ladle, tundish and mold slags, after reaction with the dissolved aluminum in the steel, yield very similar types of oxide inclusions containing calcium, magnesium, aluminum, sulfur and occasionally sodium. Tracers are considered an important tool to differentiate them.

7 Cramb report results of the use of CeO 2 oxide as tracer in the tundish and found that long open pours at any tundish level gave significant slag entrainment. In the study also includes the BaO addition in the slag laddle [1]. The tracer study had indicated the source of slag entrainment. CeO 2 oxide is found in the mold slag only after the open pouring period of the ladle change. This indicated that open poring into tundish was a major source of entrainment slag inclusions. Also was observed that long open pours at any tundish level gave significant tundish slag entrainment. The paper inform that if the tracers elements are present in the mold slag, they are also present in the product [1]. Hooli comments about tests results using BaO with other oxides as tracer during the final 15 minutes of casting and not change in mold lubrication was detected [2]. Lubrication and heat transfer in the mold are determined by chemical composition and structural evolution during casting operation [3]. To prevent strand breakouts or surface defects while using tracers, it is important to increase the knowledge on the incidence of the addition on the flux behavior at operating temperatures. Analyzing the obtained results of the different compounds added on the flux samples it is possible to describe an opposite influence on physical properties (fluidity and viscosity) at operation temperature range, between the addition of Ba carbonate and both oxides (ZrO 2 and CeO 2 ). Although barium carbonate causes a decrease in viscosity and as a consequence an increase in fluidity, these variations seem not to be a drastic change on the mold operation conditions. But in the case of ZrO 2 and CeO 2 the results indicate a relevant effect on both physical properties. Mills determine the same effect of the BaO for different additive content on the viscosity and fluidity at high temperatures [6]. The effects are justified because BaO oxide decrease the degree of polymerization of the melt as Na 2 O, CaO or LiO 2 [7, 8] but the other oxides (ZrO 2 and CeO 2 ) promotes the silicates polymerization. Fluidity degree is determined by structural, chemical, particle size aspects and thermal history. On the base of literature considerations, ZrO 2 and CeO 2 have similar effects on the viscosity and surface tension. Both oxides also could have the similar effect on the physical properties mentioned, as Al 2 O 3 [9]. The contact angle evolution, determined by hot stage microscopy, indicate a slight wettability decrease with 3 % of ZrO 2 addition, the change in the behavior was detected at higher temperatures than 1050 ºC, but the change of this property could be considered negligible. The similar behavior of CeO 2 and ZrO 2 on viscosity and fluidity suggests a similar effect of CeO 2 on wettability. Measurements of contact angle evolution with this oxide addition will be carried out in the near future. With regards to crystallization tendency, BaO turned the flux structure slightly glassy during cooling, decreasing crystal nucleation, as it was visualized by light microscopy. Otherwise, ZrO 2 and CeO 2 promote the cuspidine crystallization and crystal chains are generated in the surface and into the layer. For example, with 18 % of CeO 2 the flux became totally crystalline. Both tendencies are consistent with the physical properties evaluated. The results obtained reveal that CeO 2 and ZrO 2 oxides promote crystal nucleation during cooling. The crystallization occurred on ZrO 2 particles by homogeneous nucleation and diffusion-controlled growth. The oxide particle size and the cooling rate establish the proportion of the crystal phase generated on the sample at test conditions. In casting conditions, a high crystalline proportion developed in the flux layer during casting, decreases heat extraction rate because of the increment caused on heat conduction component respect to the radiation one. If ZrO 2 or CeO 2 content is increased in the melt flux composition is important to take in consideration the risk of change in the heat transfer. For the results obtained in the condition analyzed, Ba carbonate produces no relevant changes on flux behavior.

8 CONCLUSIONS The obtained results are summarized as follows: 1) Barium carbonate addition causes slight variations on the viscosity and fluidity of the melt flux and also produces a lower decrease of the crystallization tendency. Nevertheless, it is possible to use this compound as a tracer in the mold because not considerable effects on lubrication and heat transfer are expected. 2) Small content variation of ZrO 2 and CeO 2 on the melt flux should be controlled if these oxides are present in the flux chemistry as a consequence of tracers practice in the continuous casting or entry from other sources. Both oxides drastically increase the viscosity and decrease fluidity promoting the risk of sticking problems and also an important change of heat extraction is expected because the crystallization tendency is considerable increased. 3) The comparison of the contact angle evolution of the melt flux with and without ZrO 2 addition, permits to think that the wettability behaviour of the melt flux should not present great changes. REFERENCES [1] Cramb A. W., Byrne M., Tundish Slag Entrainment at Bethlehem s Burns Harbor (Indiana) Slab Caster, Iron & Steelmaker, Vol. 15, 1988, Issue 12, pp [2] Hooli P.O., Mould Flux Film between Mould and Steel Shell, Ironmaking and Ssteelmaking, Vol. 29, 2002, Issue 4, pp [3] Brandaleze E., Di Gresia G., Benavidez E. and González Oliver C., Evaluation of mechanical and chemical properties of mold fluxes and zircon addition, 15 th Steelmaking Conference, 2005, Argentina, pp [4] Shibata H., Emi T., Waseda Y., Kondo K., Ohta H., Nakajima K. Thermal diffusivities of continuous casting mold fluxes for steel in the glassy and crystalline states, Tetsu-to-Hagane, Vol. 82, No. 6, pp , June [5] Brandaleze E., Benavidez E., Castellá L. and Madías J., Behaviour of continuous casting fluxes during heating and cooling, VII International Conference on Molten Slags, Fluxes and Salts, Cape Town, South Africa, 2004, pp [6] Mills K., Curso sobre polvos coladores y colada continua, Instituto Argentino de Siderurgia, San Nicolás, Argentina, [7] Orling O., Sridhar S., Cramb A.W. In situ observation of the role of alumina particles on crystallization behavior of slags, ISIJ International, Vol 40, 2000, Issue 9, pp [8] R. Hevia, N. Centritto, P.A. Novaes Oliveira, A. M. Bernardin y A. Duran, Introducción a los esmaltes cerâmicos, Edit.: Faenza Editrice Ibérica S. Ll. (2001). [9] Riboud P.V. Continuous casting slags. Seminaire: formation de la première peau en coulèe continue, Institut de Recherches de la Sidérurgie Francaise IRSID, (1985). [10] Soares R.W., Araújo Fonseca M.V., Colli L., Souza J.L., Estudo da influéncia dos teores de alumina e zircônia na alteração da T c de fluxantes utilizados em lingotamento contínuo, 51º Congresso Anual da ABM, Porto Alegre,1996, pp