Abstract: Nozzle injection modes, together with electromagnetic stirring, play a key role on the as-cast macrostructure and the distribution of macro-segregation in bloom continuous castings, and then reach to the soundness and compositional homogeneity of the final as-rolled products. Based on the production process of a middle carbon steel bloom casting and its heavy section bars, various flow control modes have been adopted in casting production to study their effect on the semi-product solidification structure, carbon distribution across the bloom section and the following rolled bars. By the conventional nozzle with a single straight port, it shows that the equiaxed crystal ratio in the casting can only increase from 6.06% to 11.71% while M-EMS intensity changes from 0A to 800A, in which there still exists the obvious shrinkage cavity and macro-segregation along the centerline. While the novel five ports nozzle has been adopted, the equiaxed crystal ratio can reach to 23.1% even with M-EMS power off，and the center cavity index drop down to grade 1.0 or below, which meet the requirement of micro-void flaw detection for the bar products. Additionally, it is observed that the carbon segregation across the bloom cross section looks like a M-shaped curve with maximum carbon segregation index in the CET zone instead of the popular center region. For the heavy section bars rolled from the bloom casting, similar carbon distribution pattern as cast is observed after the hot working but with even higher centerline segregation index. Accordingly, it is pointed that, for the consideration of any special following hot working quality requirement, the macrostructure and both the pattern and intensity of macro-segregation should be regulated from very beginning casting with a reasonable flow control mode as mentioned in the study.