Retained austenite (RA) transformation and its role in the mechanical properties of three low-carbon micro-alloyed quenching and partitioning (Q&P) steels was investigated utilizing in situ tensile tests and electron microscopy. Meanwhile, RA’s strain-induced martensite transformation (SIMT) was analyzed and discussed in terms of the strengthening mechanism. The results show that the ductility of the Q&P steels relies on the size and morphology of RA. In addition, both affect RA’s mechanical or thermostability. Dislocation density and carbon trapping should be considered in estimating the yield strength in the two-step Q&P process. V and Nb-Ti elements promote the formation of blocky RA. Ti accelerates the formation of film-like RA. For experimental Q&P steels with different processes and compositions, the true stress always keeps a linear relationship with the amount of transformed martensite, i.e., 30.38~46.37 MPa per vol. 1% transformed martensite, during the in situ tensile deformation.

Three C-Si-Mn Q&P steels with different V addition after one-step and two-step quenching and partitioning (Q&P) processes were investigated by means of optical microstructure observation, X-ray diffraction (XRD) measurement, transmission electron microscopy (TEM) characterization and particle size distribution (PSD) analysis. The effect of V addition on strength and ductility of the steels was elucidated by comparative analysis on the microstructure and mechanical properties as functions of partitioning time and temperature. For one-step Q&P treatment, the mechanical properties were mainly controlled by the tempering behavior of martensite during partitioning. V addition was helpful to mitigate the deterioration of mechanical properties by precipitation strengthening and grain refinement strengthening. For two-step Q&P treatment, the satisfying plasticity was attributed to the transformation-induced plasticity (TRIP) effect of retained austenite maintaining the high work hardening rate at high strain regime. The higher volume fraction of retained austenite with high stability resulted from the refined microstructure and the promoted carbon partitioning for the steel with 0.16 wt% V addition. However, the carbon consumption due to the formation of VC carbides led to the strength reduction of tempered martensite.

Two experimental steels with chemical composition of 0.24%C-1.5%Si-1.90%Mn (0V steel) and 0.24%C-1.52%Si-1.83%Mn-0.16%V (0.16V steel), were annealed at intercritical temperature of 780℃ for 180s, followed by austempering at temperatures from 350℃ to 480℃ for 360s or austempering at 380℃ for 60s to 1200s. After the heat treatments, their microstructures and mechanical properties were investigated. The results show that the microstructures are mainly composed of ferrite, bainite or blocky martensite, and M/A islands. The difference between the 0V steel and 0.16V steel at the same austempering process is that the M/A islands are refined and the bainite seems to be retarded by vanadium alloying. The tensile strength of 0.16V steel is about 100MPa higher than that of 0V steel austempering at 380℃ for its much more blocky martensitevolume fraction and VC precipitation, and its yield ratio can be adjusted by austempering for different times, as maintained the tensile strength above 1000MPa and the total elongation about 19%. The better work hardening behavior with n-value of 0.24 in the 0.16V steel was observed under austempering at 380℃, which is contributed to VC precipitation and retained austenite stabilities.