A study on quantitative evaluation of soft reduction amount for CC bloom by thermo-mechanical FEM model

摘要

Soft reduction has been proved to be the most promising way to minimize center segregation together with the strand center porosity elimination. The negative pressure driven residual melt flow could be balanced by a reasonable reduction amount to compensate the volume shrinkage in the crater mushy zone. The optimum soft reduction amount to bloom casting for given heavy railway steel has been studied in the paper from the point of view of the volume shrinkage compensation. A solidification analysis model was developed to determine the reduction amount based on the local shrinkage in the final solidification stage. The volume shrinkage in the center inter-dendritic region has been quantitatively calculated through the evaluation to the changing melt density. Another thermo-mechanical coupled FEM model was developed to investigate the bloom deformation behavior upon soft reduction operation. The squeezing effect of the soft reduction on the center mushy zone at various solid fractions was revealed quantitatively as well. The results shown that the theoretical soft reduction amount should decrease from 12.2 mm to 4.0 mm corresponding to the solid fraction from 0.3 to 0.9. A new definition for the calculation of the internal soft reduction efficiency was proposed and compared with the conventional one. The nomogram for the determination of optimum soft reduction has been presented for the bloom casting at various center solid fraction. Accordingly, an optimized soft reduction practice was determined for quality high speed railway steel production with clearly improved strand soundness.