Effect of Mold Design on Microstructure and Macrosegregation in a 5.5-Ton Steel Ingot Using a Three-Phase Mixed Columnar-Equiaxed Model

Macrosegregation, as a typical internal metallurgical defect of steel ingots, determines the mechanical properties and service life of the finished steel. Currently, the mold design is the fundamental approach to effectively reduce macrosegregation during the solidification process of steel ingot. To analyze the effect of the mold design on segregation, a three-phase mixed columnar-equiaxed model that couples melt flow, heat transfer, microstructure evolution, and solute transport was established in this work based on the volume-averaged Eulerian-Eulerian approach. After actually dissecting a 5.5-ton steel ingot, the calculated microstructure distribution and macrosegregation could be well verified by the metallographic observation at low-magnification. For the structure design of the 5.5-ton square ingot, increasing the ingot height-diameter ratio is beneficial to reducing the negative segregation at the ingot bottom. The smaller the height-diameter ratio of the steel ingot is, the more severe the positive segregation will be, and the more conducive it is to the nucleation and growth of equiaxed grains. As the height-diameter ratio of the steel ingot increases, the A-type segregation index decreases gradually, the space occupied by A-type segregation becomes smaller. With the increase in taper, the larger the volume fraction of the equiaxed grains, and the more severe the negative segregation in the corresponding area. The greater the taper, the more the solidification rate in the upper middle part of the steel ingot slows down, which would also lead to a wider CET zone.