Abstract:To improve the mechanical properties of HSLA Cu-bearing steel,multi-stage heat treatment (QLT) consisting of quenching,lamellarization and tempering was applied in a low-carbon Ni-Cr-Mo-V-Cu low alloy steel. Superior combination of high strength and low-temperature toughness was achieved and yield strength 895 MPa,tensile strength 950 MPa and low-temperature (-80 ℃) toughness 188 J was obtained. The dual-phase microstructure evolution with inter-critical annealing temperature (within Ac1-Ac3 region) in the tested steel treated by QLT route was invested by methods of SEM,XRD and TEM,and corresponding relationships between microstructure and mechanical properties were also clarified. Yield strength of the QLT specimens shows a quadratic parabolic relationship with the volume percent of the tempered secondary martensite,and the tensile strength exhibits a linear positive relationship with the volume percent of the tempered secondary martensite. Elongation after fracture is positively correlated with the volume percent of inter-critical ferrite. Among all QT and QLT specimens,QL720T one (inter-critical annealing temperature is 720 ℃) shows excellent combination of strength and low-temperature toughness. High strength of QL720T specimen comes from tempered secondary martensite which is strengthened by nanosized MC (M is any combination of Nb,Mo,V and Ti) and Cu particles. Excellent low-temperature toughness of QL720T specimen is caused by following factors, the refining effect of dual-phase microstructure led by the parallel distribution of tempered secondary martensite and inter-critical ferrite; the refined brittle cementite or alloyed cementite caused by lots of heterogeneous-phase interfaces; low strength difference between tempered secondary martensite and inter-critical ferrite.
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