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.

A novel chamfered mold is developed to solve the problem of corner transverse cracking in micro-alloyed steel slabs. The shape of the slab was changed from four corners and sides to eight corners and sides due to the use of a chamfered mold. Based on numerical simulation, the solidification and heat transfer of different steel grades in the mold are studied. The results reveal rapid solidification shrinkage of molten steel in the upper area of the mold and slow solidification shrinkage in the lower area; thus, a double-taper mold is designed according to these results. The first area of the variable taper falls in the range of 250–400 mm from the top of the mold, and the design method of double inclined water channels in the chamfered face is found to be the most helpful for the formation of a uniform initial shell and reducing hotspots of the mold. Actual production results show that the quality of the slab is better when the heat flux of the narrow face is larger than that of the broad face. Corner transverse cracking in micro-alloyed steels is greatly reduced from an incidence of 4.2% to less than 0.3%. In addition, the chamfered mold is applied in IF (interstitial free) steel production, and the edge quality of hot rolled sheets is found to also be dramatically improved.

To investigate the formation of one kind of typical inter-dendritic crack around triple point region in continuous casting(CC) slab during the operation of soft reduction, fully coupled 3D thermo-mechanical finite element models was developed, also plant trials were carried out in a domestic continuous casting machine. Three possible types of soft reduction amount distribution (SRAD) in the soft reduction region were analyzed. The relationship between the typical inter-dendritic cracks and soft reduction conditions is presented and demonstrated in production practice. Considering the critical strain of internal crack formation, a critical tolerance for the soft reduction amount distribution and related casing parameters have been proposed for better contribution of soft reduction to the internal quality of slabs. The typical inter-dendritic crack around the triple point region had been eliminated effectively through the application of proposed suggestions for continuous casting of X70 pipeline steel in industrial practice.