Macrostructure and macrosegregation behavior of bloom products under various flow control modes of the casting process

Owing to the large cross-section and wide solidification-temperature zone, bloom castings of medium- and high-carbon steels are prone to severe central shrinkage and macrosegregation defects. Flow control technologies such as nozzle injection mode and electromagnetic stirring, together with the casting speed, play a key role in the as-cast macrostructure and macrosegregation distribution in bloom castings achieving soundness and compositional homogeneity of the final as-rolled products. Based on the production process of a medium-carbon-steel bloom casting and its heavy section bars, various flow control modes have been adopted in the casting production to study their effects on the semiproduct solidification structure and the carbon distribution across the bloom section and the following rolled bars. The conventional nozzle with a single straight port shows that the equiaxed crystal ratio in the casting process can only increase from 6.06% to 11.71% with the M-EMS intensity changes from 0 to 800 A, in which a shrinkage cavity and macrosegregation exist along the centerline on the strand casting. While the novel five-port nozzle has been adopted, the equiaxed crystal ratio can reach 23.1% even with the M-EMS power off, and the center cavity index drops down to grade 1.0 or below, meeting the requirement of microvoid flaw detection for the bar products. Additionally, the carbon segregation across the bloom cross-section is observed to resemble an M-shaped curve with a maximum carbon segregation index in the columnar to equiaxed transition zone instead of the popular center region. For the heavy section bars rolled from bloom casting, a similar carbon distribution pattern as the cast is observed after hot working but with an even higher centerline segregation index. Therefore, considering the special quality requirements for the subsequent hot processing, the macrostructure and pattern and intensity of macro-segregation should be regulated from the beginning of casting with a reasonable flow control mode as mentioned in the study.