The carefully controlled forging of as-cast 30Cr2Ni4MoV steel was investigated as a mean to refine its coarse grain structure and crush the dendrites that cause segregation. This found that a fine-grained (∼50 um) structure could be achieved by deformation at 1000°C as a result of DRX. Grain boundary segregation of Mo caused a significant precipitation of MoC that could effectively pinned grain boundaries and suppressed the grain growth. In contrast, forging at temperatures above 1000°C led to a significant increase in grain size. Furthermore, the grain boundary segregation of sulfur resulted in the formation of low melting point sulfides near the grain boundaries that provided additional initiation points for intergranular cracking, thereby reducing the mechanical properties of the alloy. The results of this study therefore provide a basis for optimizing the forging process so as to improve the microstructure of heavy forgings, and sheds new light on the mechanism of grain refinement and crack initiation for 30Cr2Ni4MoV steel.