Computational Fluid Dynamics Modeling of Macrosegregation and Shrinkage in Large-Diameter Steel Roll Castings
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IN this study, a framework for studying macrosegregation in ingot processes was developed using a computational fluid dynamics (CFD) code by solving for the temperature, flow, and solute balance in multicomponent alloy systems. The relevant literature work on the micro- and macrosegregation was reviewed in detail for this work. A good reference book that describes most of the key aspects of modeling of macromass transport in the context of casting solidification was written by Stefanescu.[1] Several numerical macrosegregation models have been proposed in the literature.[2–7] These models were validated only for binary alloys and they do not take into account all of the transport phenomena at both micro- and macrolevel that can take place during ingot casting. The macrosegregation models developed by Chang and Stefanescu[6] can directly be reduced to the local solute redistribution equation when the assumptions made by Flemings and Nereo[8] are used. In addition, under the assumption of equal solid and liquid LAURENTIU NASTAC, Professor, is with the Department of Metallurgical and Materials Engineering, University of Alabama, Tuscaloosa, AL 35487-0202. Contact e-mail: [email protected] Manuscript submitted March 24, 2011. Article published online June 8, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS B
density, the model reduces to the Scheil or Gulliver– Scheil equation.[9,10] The ingot segregation model was customized for roll ingots. The multiphase modeling development effort includes up-hill teeming of the base alloy, submerged entry nozzle (SEN) top pouring of the dilute alloy, species transfer during filling, and multicomponent segregation during solidification. Customization also includes the development and implementation of boundary conditions, mesh, thermophysical, and solidification/segregation properties of the base and diluted steel alloys. A main objective of the study is to optimize SEN solidification dynamics to avoid the formation of remelting problems in the fusion zone between the base and the dilute alloy materials, as illustrated in Figure 1.
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MODEL DESCRIPTION
A comprehensive analysis tool that can model macrosegregation during ingot casting and solidification of multicomponent alloys was developed and implemented into a CFD code. The viscous standard k-epsilon with standard wall functions, species transport, and modified Scheil-based solidification models were used. The Scheil microsegregation equation was corrected to account for diffusion in the solid phase (e.g., back-diffusion) in a VOLUME 42B, DECEMBER 2011—1231
Fig. 1—Cross section of production roll ingot illustrating fusion zone and solidification shrinkage flaws including transverse macrosection of the fusion zone.
multicomponent system based on the microstructure characteristics of the alloy under consideration and the cooling conditions.[11–14] Thus, a framework for studying macrosegregation in various casting processes was developed by solving in a fully coupled mode for the temperature, flow, and solute balance in the system. Specia
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