Microstructure evolution of V-containing low carbon ultra-fine bainitic steel during medium-temperature tempering and strengthening-toughening mechanism

In the present study, the microstructure evolution and the corresponding mechanical properties of V-containing low carbon ultra-fine bainitic steel during tempering were studied. The results show that during the initial stages of tempering, part of retained austenite in V-containing ultra-fine bainite steel transforms into bainite. The stability of the retained austenite improves when the tempering period is increased to 5 hours. Simultaneously, nano-sized VC precipitates are precipitated in the microstructure. On the one hand, these phases can give full play to the transformation-induced plasticity (TRIP) effect by modulating the carbon content of the retained austenite. On the other hand, they can also reduce the lattice distortion in the matrix and combine it with the lower dislocation density, thereby improving the plasticity of the specimen. At the same time, the fine precipitated phase can also effectively improve the toughness of the cleavage crack region in the transition zone, preventing the micro-cracks from generating at the interface of the precipitated phase and then extending to the adjacent matrix to cause fracture, so that the uniform elongation, total elongation, impact toughness at room temperature and impact toughness at low temperature of the 5 h specimen are as high as 8.8±0.2%、16.2±0.7%、113±4 J/cm2 and 76±2 J/cm2, respectively. Furthermore, the primary contribution of the strengthening mechanism in the microstructure has shifted from fine grain strengthening and dislocation strengthening to precipitation strengthening, and part of the retained austenite undergoes a martensitic transformation during cooling, resulting in the highest yield strength of 1086MPa for the 5 h specimen. The tensile strength is effectively maintained at 1402 MPa by increasing the tempering period, which has good tempering resistance, to achieve the comprehensive performance optimization of bainitic steel.