Solidification Microstructure and Segregation in the Medium-carbon Steel Cast with a Laboratory-scale Local-chilled Mold

In the industrial steel manufacturing process, such as continuous casting and ingot casting, macrosegregation occurs due to the effect of bridging and contraction flow during the middle and end periods of solidification. Since the macrosegregation results in a nonuniform structure and eventually cracks, there has been a demand for technological development that does not cause macrosegregation in the casting process. In this study, model experiments using the medium-carbon steel cast with a laboratory-scale local-chilled mold at different superheating have been carried out to investigate the relationship between solidified structure and macrosegregation occurred by local bridging during casting. The morphology of shrinkage porosities and dendrite structures was observed. The concentrations of the alloying elements were analyzed for macrosegregation by an electron probe micro-analyzer. Chill plates successfully formed the columnar dendrite bridging area and the columnar dendrite shell in the sample with high superheating. During solidification, the negative pressure of the region below the bridging increased, and the concentrated contraction flow flowed into the bottom of the large shrinkage porosity. Finally, V segregation was formed in the bridging area, large shrinkage porosities remained below the bridging area, and point-like or band-like positive macrosegregation occurred in the interdendritic region between columnar dendrites and equiaxed dendrites below the bridging. In comparison, a lower casting temperature increased the grain density and formed shrinkage porosities that were smaller in size but larger in number and more dispersed.