Influence of Bridging on Macrosegregation in the Medium-carbon Steel Cast with a Laboratory-scale Middle-chilled Mold

Currently, steel products are manufactured by continuous casting or large-sized ingot casting, and macrosegregation that occurs during the manufacturing process significantly impacts product quality in terms of cracks and deterioration of mechanical properties. To clarify the principle of casting defects, such as shrinkage porosity and macrosegregation due to the formation of bridging of the columnar dendrite of the medium-carbon steel, in this experiment, a laboratory-scale local-chilled mold in which the middle part was forcedly cooled was designed to cause bridging, shrinkage porosities, and macrosegregation. Enough risers were also designed to simulate realistic gravity casting as much as possible. The solidification structure morphology was observed, concentration analysis of alloying elements was performed, and the effect of bridging on macrosegregation was investigated. Solidification proceeded preferentially from the chill plate, and the bridging was formed successfully at a high casting temperature. The high casting temperature condition could cause bridging, but large shrinkage porosities would be formed as well. On the contrary, the lower casting temperature condition could increase the grain density and form shrinkage porosities that are smaller in size but larger in number and more dispersed, compared with the case cast with no chill plate mold. Due to the formation of bridging, macrosegregation was formed, and the difference between positive and negative segregation was increased from the longitudinal center of the sample.