Abstract: Improving the hardenability of steels for ultra-heavy forgings is crucial for achieving uniform strength and toughness across the entire cross-section. Based on a simulation of the thermal history at the core of a 370 mm thick cylindrical shell forging, the effects of carbon and nickel enrichment on the microstructure and mechanical properties of a 20MnMoNb steel were systematically compared. The results show that although both C and Ni enhance hardenability, their underlying mechanisms of microstructural control are fundamentally different. After heat treatment, the C-enriched steel developed extensive regions characterized by low precipitate density and an absence of sub-grain boundaries. Its Charpy impact energy at 0 ℃ dropped sharply from 128.7 J after tempering at 640 ℃ to 62.3 J upon increasing the tempering temperature to 650 ℃, while its room-temperature tensile strength underwent accelerated softening in the higher tempering range. Analysis reveals that the morphological difference of martensite-austenite (M-A) constituents in the as-quenched microstructure causes temper carbides to precipitate preferentially within lath-like bainite regions. The resulting chemical potential gradient drives the directional migration of carbon from granular bainite regions toward lath-like bainite regions, which is the key mechanism responsible for the formation of the low-precipitate, sub-grain-free zones. Carbon enrichment intensifies this effect, leading to a more heterogeneous spatial distribution of carbides and a heightened sensitivity of strength and toughness to tempering temperature. In the Ni-enriched steel, a lower bainite transformation temperature promotes a more uniform as-quenched microstructure, narrowing the temporal disparity in M-A decomposition during tempering and thereby suppressing the carbon migration effect. Combined with the solid-solution toughening conferred by Ni, this results in stable strength-toughness synergy across the entire tempering temperature range, providing an effective alloy design strategy for ultra-heavy wall forgings.