EFFECT OF SILICON CONCENTRATION ON THE CAST MACROSTRUCTURE OF Fe-Si STEELS FOR ELECTRIC APPLICATION

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1 EFFECT OF SILICON CONCENTRATION ON THE CAST MACROSTRUCTURE OF Fe-Si STEELS FOR ELECTRIC APPLICATION J. Alejandro GARCIA H., E. Candido ATLATENCO T., Hector CRUZ M., A. Abril BALANDRA A., Gabriela GONZÁLEZ F., a Yvan HOUBAERT b a Departamento de Ingenieria Metalurgica, Facultad de Quimica, Universidad Nacional Autonoma de Mexico, jagarcia58@yahoo.com.mx b Departamento de Ciencia e Ingenieria de Materiales, Universidad de Gante Belgica, Abstract In this paper was studied the effect of silicon content from 0.5 to 3 wt% Si on the macrostructure of casting ingots. Fe-Si alloys with low contents of impurities were produced in electric induction furnace under inert atmosphere conditions. Castings of 12.5 cm thick, 25 cm long and 30 cm high were obtained of each alloy poured in metallic mould. The ingots obtained were sectioned into slices of 12 cm wide, 25 cm high and 2 cm thick, the central slice of each ingot was prepared metallographically to reveal the macrostructure of the six cast alloys. The results indicate that alloys with low silicon levels (0.5 and 1.0 wt% Si) and with small solidification intervals have relatively fine equiaxed grain, while alloys with higher silicon content and a higher solidification intervals show predominantly grains. These macrostructures are not the usually structures linked to short and long freezing range. Another important result is the absence of dendritic structure usually present in cast alloys. Keywords: Fe-Si steel, macrostructure, electrical steel, solidification range 1. INTRODUCTION Iron-silicon steels with a high Si content are an important material to develop superior magnetic properties and to be applied in magnetic devices due to its high permeability, high electrical resistivity and near zero magnetostriction, which low core loss and low device noise [1, 2]. However the workability of these materials is hard during the rolling processes because of its embrittlement at room temperature linked to the formation of intermettallic phases like B2 and DO 3 [3]. Several methods have been developed in order to avoid this problem such as: rapid quenching [4], hot dipping diffusion [5]. However the most practical is the casting process followed by hot or cold rolling. Directional solidification of grain improve the ductility of this material because large amount of low energy grain boundaries with strong resistance to intergranular fracture are formed. Therefore grain with high grain ratio long/width or L/W is preferred to increase the amount of the low energy grain boundaries [6, 7]. In this paper Fe-(0.5.3) wt% Si were produced in metallic moulds in order to determinate the relationship of the formed macrostructure with the silicon amount, also its grain ratio (L/W) and the hardness of the alloy were evaluated, because these characteristics are important to improve the workability of Fe-Si steels. 2. EXPERIMENTAL Iron ingots were melted in an electric induction furnace under inert atmosphere of argon gas, the final balance of the chemical composition was made with ferrosilicon (FeSi75). Cast ingots in cast nodular iron metallic molds were obtained with a total weight of 100 Kg including the feeding system, useful casting weight just was 65 kg. Ingot dimensions were 250X125X300 mm, Figure 1. All alloys were solidified and cooled under similar conditions. The chemical compositions of the experimental alloys were from 0.5 to 3 wt% Si, in order to determine the effect of silicon on the macrostructure of these steels. Chemical composition of the ingots was analyzed by Atomic Emission Spectrometry it is shown in table 1.Table 1. Chemical composition of the experimental alloys (wt %) Alloy Chemical composition*, wt %

2 (%Si) C Si Mn P S Al N Cr < < , < < < *Fe Balance The ingots were cut into slices of 12.5 cm width, 2.5 cm thickness and 30 cm high. The central slice was prepared by conventional metallographic techniques and etched with 1:3 fresh solution of HNO 3 :H 2 O to reveal the macrostructure of each experimental alloy. The liquidus and solidus temperatures were calculated by ThermoCalc software for each alloy. The solidification range or solidification interval (SR) was calculated by the difference between the liquidus and solidus temperatures. The experimental solidification pattern was determined based on the type of macrostructure, the presence of grains indicate a short solidification pattern. Macrostructures with equiaxed and grain the solidification follow an intermediate solidification pattern, finally the presence of just equiaxed grains the alloy follows a long solidification pattern. For the evaluation of the long/wide grain ratio (L/W), in the equiaxed structures the diameter and the wide of 15 grains were measured, five grains located in the low zone of the casting slide, five in the half zone and five in the high zone. Table 2 show the average value of the three evaluated zones. For the case of the alloys with structure, the following procedure was applied: the grain long and the average wide grain (two positions of the grain) were measured, this was applied in 15 grains of each alloy, five grains located in the low zone, five in the intermediate zone and five in the high zone. A similar procedure for each one of the zones of the sample with 1.5% was applied. Hardness Brinell Number(HBN) was determined applying 3000 Kg of load with10 mm diameter hard ball during 15 seconds. Five values of hardness through all the slide casting were determined, in order to having the hardness average of the material including the grain borders. Finally the Hardness Rockwell B (HRB) was determined individually for 10 equiaxed grains in the case of the alloys with 0.5 and 1.0 wt%si. For the alloy with 1.5% was applied the same procedure in the zone equiaxial. In the case of the alloys with 2, 2.5 and 3.0% with structure, lineal measurements of hardness along the grains in five positions of each grain were carried out. These profiles of hardness were obtained for seven grains of each one of these alloys. Fig 1. Casting dimensions and experimental casting 3. RESULTS AND DISCUSSION

3 Table 2 summarizes the results obtained of the liquidus and solidus temperatures, the solidification range (SR), the experimental solidification pattern, grain ratio (L/W), the average hardness Brinell Number (HBN) and hardness Rockwell B (HRB) of each alloy. Table 2. Solidification parameters (liquidus and solidus temperatures), solidification range (SR), grain ratio (L/W), HBN and HRB. (1) Alloy Tl ( C) Ts ( C ) SR Fe-0.5 %Si Fe-1 %Si Fe-1.5 % Si Fe-2 % Si Experimental pattern (grain) Long equiaxial Ratio L/W (mm/mm) 0.95±0.02 Hardness (HBN) 95.26±0.03 Hardness (HRB) 48.4± Long equiaxial 0.93± ± ± Fe-2.5 %Si Fe-3 %Si Mixed 0.94 (equiaxed ±0.08 equiaxial and zone) 7.16±0.05 ( zone ) Short: 8.26± ±0.07 Short 10.15± ±0.25 Short 10.83± ±0.38 Macrostructures of each alloy can be seen in figures 2a to 2f (a) (b) (c) 66.6± ± ± ±0.04

4 (d) (e) (f) Fig 2. Macrostructures of Fe-Si castings (a) 0.5 wt%si, (b) 1.0 wt%si, (c) 1.5 wt% Si, (d) 2 wt%si, (e) 2.5 wt% Si and (f) 3.0 wt% Si. The macrostructures obtained under the experimental condition in this work do not match with the patterns defined for the conventional mechanisms of macrostructure formation phenomenon (8, 9), which defines the macrostructure as a function of chemical composition related to solidification range (difference between liquidus and solidus temperatures), short solidification range the alloys must have grains, related to the small zone of liquid-solid coexistence in front of the solidification interface, while for longer intervals the macrostructure must be predominantly equiaxed grain. This essential dependence of the ingot macrostructure has been updated by several studies that determine the -equiaxed transition (10,11), in which the product of the gradient temperature (G) by the growth interface rate (V) called GV or cooling rate define the magnitude of each of these zones in casting solidified by several manufacturing processes. These parameters however are affected by different variables associated with the manufacturing conditions such as: pouring temperature, alloy composition, the presence of impurities and/or nucleating agents, mold thickness and geometry, mold material, etc., as well as the particular characteristics of the alloy as are the liquidus and solidus temperatures, undercooling degree, etc. In the case of electrical steels the grain ratio (L/W) in solidification conditions is important because ideal crystallographic texture in plane perpendicular to the solidification direction can be obtained. Langraf (7) reported that relationships L/W from 7.3 to 10 for steels with 3.5 wt% Si obtained by directional solidification have better magnetic properties when it is compared with commercial steels with 2.5% Si and 3.5% Si steels obtained by strip casting processes. In another study done by Liang (6) concludes that there is a close relationship between the directional solidification conditions controlled by the parameter GV with the parameter L/W. For GV values between 5.6 and 5.88 (K/s ) were obtained under a L/W ratio between 18 and 15, which are associated with values of 360 to 375 Hardness Vickers (HV), under these conditions the material reach the best rolling workability. In this work for Fe-Si alloys with 2.0, 2.5 and 3.0 wt%si were obtained L/W ratio from 8,26 to 10.83, these values are close to the results reported by Liang [6], considering that the ingots under this study were manufactured by conventional casting process instead of the directional solidification process. This L/W ratio would indicate that the alloys with more than 2 wt% Si will be easier to rolling due to the directions of growth associates to the crystals during the solidification stage. While the alloys with equiaxial and equiaxial- macrostructures would present smaller workability and magnetic properties.

5 Brinell Hardness Number is a global hardness, because the mark covers medium zones and boundaries grain, indicating a high homogeneity of these materials, the BHN is increased when the silicon content is increased, this is associated with the amount of silicon into α phase at room temperature, this is agree with dates reported by Shin [4]. The hardness profiles obtained by Hardness Rockwell B on the coarse grains, the small variation of the hardness grain profile, as well as the variation in hardness between grain and grain indicates the presence of a high homogeneity of the silicon as main alloying element, however the hardness HRB decreased with increasing amount of silicon. 4. CONCLUSIONS The macrostructure of the alloys show an anomalous behavior when only is considered the chemical composition and the solidification range (solidification pattern). This anomaly could be explained based on the parameters GV, which was not evaluated in this study. Alloys with greater contents than 2 wt% Si grains are presented throughout the casting, while more dilute alloys with silicon equal or less than 1 wt% showed equiaxed fine grain structures. The alloy with 1.5% Si shows a mixed macrostructure associate to intermediate solidification pattern. The high grain ratio L/W for the alloys with 2 or more wt% Si, suggested to be suitable materials for reach high magnetic properties, associated with the shape and growth direction of the grain. The hardness-gv parameter suggested by some authors to relate to the rolling workability, according the result workability could be better for the alloys with 2 or more wt% Si. ACKNOWLEDGEMENT This work was supported by ACERLECT FONCICYT-CONACYT No in collaboration with GU Ghent University, UANL Universidad Autonoma de Nuevo Leon REFERENCES [1.] Beclekey P., Electrical Steels for Rotating Machines, IEE Power and Energy Series 37, UK [2.] AK Steel Selection of Electrical Steels for Magnetic Cores, Armco Inc [3.] Ruiz D., Ros Y.T., Cuello G.B., Houbaert Y., Physica 2006 B [4.] Shin J.K., Lee Z.H., Lee T.D., Lavernia E.J. The effect of Casting Method and Heat Treating Condition on Cold Workability of High-Si Electrical Steel, Scripta Materialia 45 (2001), page 725. [5.] Ros Y.T., Houbaert Y., Rodriguez V.G., 2002 J. Applied Phys [6.] Liang Y.F., Zheng Z.L., Lin J.P., Ye F., Chen G.L., The Effect of the Directionally Solidified Microstructures on Ductility of Fe-6.5 wt% Si Alloy, J. of Physics: Conference Series 144 (2009) IOP Pu. [7.] Landgraf J.P.V., Yonamine T., Taanohashi R., Silva F.Q, Journal of Magnetism and Magnetic Materials (2003), page 364. [8.] Davies G.J., Solidification and Casting, Applied Sc. Pu LTD London [9.] Chalmers B., Principles of Solidification, John Wiley and Sons, [10.] Rappaz M., Gandin Ch. A., Desbilles J.L., Thevoz Ph., Prediction of Grain Structure in Various Solidification Processes, Metallurgical and Materials Transaction A, Vol. 27A, March 1996, page 695. [11.] Shibata H., Itoyama S., Khisimoto Y., Takeuchi S., Sekiguchi H., Prediction of the Equiaxed Crystal Ratio in Continuously Cast Steel Slab by Simplified Columnar-to-Equiexed Transition Model, ISIJ International, Vol. 46 (2006), No. 6 page 921.