Macro–micro coupled solute transport and microsegregation in continuously cast slabs under dynamic cooling conditions

Non-uniform heat extraction and mushy-zone solute transport strongly affect microsegregation during slab continuous casting. Although detailed macro–micro coupling approaches have been developed, reduced-order dendrite-scale models often use constant or averaged cooling rates and treat the dendritic unit as a closed domain, limiting their application under spatially and temporally varying casting conditions. In this study, a one-way macro–micro coupled solute-transport framework was developed to predict microsegregation in continuously cast slabs. The macro-scale model was first used to calculate the local cooling-rate history and the relationship between cell-averaged carbon concentration and solid fraction. During the micro-scale calculation, the cooling rate was updated according to the macro-scale time history to determine the temperature evolution and effective partition coefficient. Meanwhile, the macro-scale carbon concentration was mapped to the dendrite-scale model through solid-fraction matching, and an equal-volume liquid-exchange term was introduced to describe solute transfer between the bulk and the interdendritic liquid while the mushy-zone liquid channels remained connected. Carbon was selected as the representative solute because the macro-scale solute-transport calculation was primarily established for carbon and because carbon strongly affects steel solidification and the δ/γ transformation. The model was evaluated by comparison with EPMA-derived carbon segregation indices. For the investigated Ti-bearing microalloyed steel, the predicted carbon segregation ratios at the quarter-thickness position and slab center showed relative deviations of 7.88% and 8.64%, respectively, lower than those obtained using the classical Ueshima model and the model considering dynamic cooling alone. The results indicate that the dynamic cooling history mainly modifies the local partitioning and redistribution path, whereas macro–micro liquid exchange suppresses excessive terminal solute accumulation caused by the closed-cell assumption. The proposed framework provides a computationally efficient route for introducing macro-scale thermal and solute-transport histories into dendrite-scale microsegregation calculations under practical continuous-casting conditions.