Prediction of Macrosegregation in Steel Ingots: Influence of the Motion and the Morphology of Equiaxed Grains
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ONE of the major goals of the steel industry is the manufacture of products with a minimum number of defects; an important parameter of this is chemical homogeneity. Despite good control of the steel grade and chemical homogeneity of liquid steel, chemical heterogeneities develop during the solidification stage. They can be classified into three types, depending on their scale: (1) at the dendrite scale: microsegregation, due to the difference in the solubility of chemical species in the solid and liquid phases; (2) at the scale of the product: macrosegregation, due to the relative motion of the solid and liquid phases; and (3) at an intermediate scale: mesosegregations (e.g., A segregates, freckles), due to localized flow phenomena.
HERVE´ COMBEAU, Professor, and MIHA ZALOZˇNIK, Postdoctoral Fellow, are with the Laboratoire de Science et Ge´nie des Mate´riaux et de Me´tallurgie (LSG2M), Ecole des Mines de Nancy, Nancy-Universite´, Parc de Saurupt, CS 14234, F-54042 Nancy Cedex, France. Contact e-mail: [email protected] STE´PHANE HANS, Melting and Casting Process Development Engineer, and PIERRE EMMANUEL RICHY, Research and Development Engineer, are with Aubert & Duval, BP 1, F-63770 Les Ancizes, France. This article is based on a presentation given at the International Symposium on Liquid Metal Processing and Casting (LMPC 2007), which occurred in September 2007 in Nancy, France. Article published online October 21, 2008. METALLURGICAL AND MATERIALS TRANSACTIONS B
The typical segregation pattern of a steel ingot[1,2] generally consists of a negative segregation in the bottom part of the ingot and a positive in the top part. Moreover, depending on the steel grade and the size and shape of the ingot, A segregates can be observed; these correspond to highly segregated channels a few millimeters in diameter. In the center of the product, V segregates, another type of mesosegregate, can be encountered. The main phenomena responsible for these macrosegregations and mesosegregations were identified a long time ago (for example, in Reference 1): the shrinkage, the thermal and solutal natural convection of the liquid, and the motion of the equiaxed grains. The importance of each of these phenomena was not known, however, and a first generation of models appeared with the aim of estimating the effect of one or two of these phenomena.[2–5] Mehrabian and co-workers[3] studied the combined effect of the shrinkage and the natural convection of the liquid phase, for a case in which the gravity is perpendicular to the direction of solidification. In this case, they showed that the macrosegregation pattern strongly depends on the sense of variation in the density of the interdendritic liquid vs the solid fraction. They also proposed the hypothesis that A segregates develop as the result of a flow instability that occurs at a critical flow condition. Chuang and Schwerdtfeger[4] developed a two-dimensional model that considered both the vertical motion of the solid and liquid phases in the central region of the ingot an
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