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Three dimensional atom probe(3DAP)and transmission electron microscopy(TEM)were applied to investigate the microstructure evolution of a Nb-Mo-V micro-alloyed steel tempered at 650℃ for 10,30,120,240,600 and 1800min after solution treatment at 1200℃ for 30min.The results indicated that,with the tempering time prolonged,the microstructure evolved from dislocated lath-like martensite to polygonal ferrite.Meanwhile,fine carbides were found heterogeneously distributed in ferrite matrix and interface of the original martensite.The carbides detected in the samples tempered for 10,120 and 240 min,were in the stage of nucleation,growth and coarsening,respectively,with atomic composition of Fe57.19 Mo3.12 Nb2.41 V21.50 C13.77,Fe66.82 Mo3.88 Nb2.77 V11.43 C10.71,Fe51.61 Mo1.01 Nb1.70 V16.77 C18.35 and average radius of(0.67±0.29),(1.11±0.40),(1.85±1.36)nm.The highest number density of the carbides was obtained in the sample tempered for 120 min,contributing to the strongest secondary harden effect.