Effect of vanadium microalloying on phase transformation and strengthening mechanism of 1000 MPa low carbon bainitic steel

To develop an ultrahigh-strength hot-rolled strip, the yield strength of low-carbon bainitic steel was increased from 804 to 1074 MPa via V microalloying. The effects of V microalloying on the microstructure and mechanical properties of the alloys are discussed in detail. The microstructures of the test steel containing 0.2 wt % V element is finer, and the Degenerate Upper Lath Bainite (DUB) is replaced by Lower Lath Bainite (LLB). This was clearly due to the addition of V, which promoted the formation of LLB. In this study, the effect of V dissolved in the matrix on the phase transformation kinetics was investigated. This is because it is difficult for V to precipitate during quenching and non-isothermal partitioning (QNP) processes, and it mainly exists in the matrix in the form of a solid solution. The main effect of V is to reduce the activation energy Q* of bainite by 1.76 kJ/mol, increase the nucleation rate of lath bainite, and further refine the bainitic lath. However, a high dislocation density exists in the bainite laths, and the comprehensive strengthening effect of lath strengthening and dislocation strengthening can explain the change in the mechanical properties of the 0.2V steel.