Abstract: The causes of inadequate resistance to hydrogen-induced cracking (HIC) in X65MS pipeline steel were systematically investigated. By correlating the findings with the distribution characteristics of carbonitrides in the continuous casting slab, it clearly revealed the evolution process and underlying mechanism of "casting segregation → formation of precipitates and detrimental microstructures → initiation of hydrogen-induced cracks". The experimental results demonstrated that the enrichment of solute elements such as Nb, Ti, C, and N in the centerline segregation zone of the slab provided the driving force for the precipitation and growth of coarse carbonitrides. These coarse carbonitrides were inherited into the steel plates, acting as irreversible hydrogen traps that promoted HIC nucleation and propagated along the banded hard/brittle microstructure induced by casting segregation. Thermodynamic calculations indicated that the onset precipitation temperature of TiN was approximately 1479.0 ℃, close to the solidus temperature, suggesting its initial precipitation in the liquid or interdendritic regions during the final stage of solidification. In contrast, the onset precipitation temperature of NbC was about 1134 ℃, indicating its propensity to precipitate in the slab core where the defect density was high. By precisely adjusting the contents of key elements such as Ti, N, and Nb, the precipitation temperature and amount of carbonitrides could be actively controlled. To improve the HIC resistance of X65MS pipeline steel, the following integrated measures are recommended. During continuous casting, adopt low casting speed and low superheat combined with enhanced soft reduction to alleviate centerline segregation; during rolling, control appropriate reheating temperature and deformation in the non-recrystallization region, and optimize cooling procedures to refine the microstructure and suppress harmful precipitates. Meanwhile, nitrogen content should be strictly controlled, and Ti/Nb contents may be appropriately reduced. Through coordinated regulation across the entire process route, the HIC resistance of the steel is expected to be improved, providing a valuable reference for industrial production.

Key words: X65MS steel; TiN; NbC; segregation; thermodynamic