Study of the Macrosegregation of Silicon in Steels for Electrical Applications
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Study of the Macrosegregation of Silicon in Steels for Electrical Applications V. A. Aranda Villada1, J. A. García Hinojosa1, H. Cruz Mejía1, A. A. Balandra Aranzueta1., M. G. González F1., Y. Houbaert2 1
Departamento de Ingeniería Metalúrgica, Facultad de Química, Edif. D, UNAM, Ciudad Universitaria, Coyoacán, México D. F. CP 04510; 2 Department of Metallurgy and Materials Science, Ghent University, B-9052ԜZwijnaarde, Belgium ABSTRACT Macrosegregation is harmful to the material properties (strength, corrosion resistance, etc.) and it must be avoided or eliminated from castings. In this work, macrosegregation behavior is evaluated in terms of silicon concentration in cast ingots of steels with 0.5, 1.5 and 3 wt% Si. The dimensions of the castings are 25x30x12.5 cm. The Si segregation profiles are evaluated in three zones: upper, middle and bottom of each ingot. The characterization is carried out by energy dispersive spectrometry in at least 10 positions of each zone of the ingot. Silicon analysis is evaluated from the surface to the centre of each ingot. The experimental results are compared with predictions using an Experimental Segregation Index (ESI) and the Scheil Segregation Index (SSI). The experimental results show a strong deviation from the profiles predicted by the Scheil model. The evaluation of the degree of segregation by "ESI" indicates little variation of silicon from the surface to the center of the ingot showing a uniform distribution across the ingot analyzed. INTRODUCTION Macrosegregation is a phenomenon characterized by variations of concentration in different zones of ingots and castings, particularly from their surface to the center. Macrosegregation patterns are affected by conditions prior to solidification, where gravity macrosegregation [1] and the Ludwig-Soret [2] effect are important, and during solidification. In this case, macrosegregation patterns are classified as normal [3], inverse [4,5], freckles [6], banded [7] and ingot macrosegregation [8]. In general, segregation occurs as a consequence of periodic solute enrichment or depletion, caused by the advance of the solidification front under non-stationary state conditions. These events define the magnitude of solute segregation reached at the end of solidification. In addition, fluid flow in the liquid phase and the value of the equilibrium distribution coefficient Ko of the alloy have a significant role in the evolution of macrosegregation. The chemical composition of a specific region in the ingot can be determined qualitatively or semi quantitatively by Energy Dispersive Spectroscopy (EDS) [9]. Anisotropic properties can be observed after hot rolling as result of macrosegregation in Fe-Si steels. In order to reduce or eliminate these problems, an annealing treatment is suggested before the hot rolling process. In this paper, the composition profiles of three different steels Fe-Si (Fe-0.5%wt, Fe-1.5%wt and Fe-3%wt Si) are evaluated experimentally. The experimental results are compared with predictions performed using the Scheil–
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