Abstract: Effect of V microalloying on microstructure and delayed fracture resistance of the 55SiCr steel was investigated by means of optical microscope (OM), scanning electron microscope (SEM), thermodynamic calculations, physicochemical phase analysis and constant load notch tensile test. The results show that the yield strength and tensile strength of the quenched and tempered steel are increased by 150 MPa and 100 MPa, respectively, due to the addition of V element. At the same time, MC precipitates with a size of 18-36 nm are obtained, which effectively prevents the growth of austenite grains during heating process of quenching, and the prior austenite grain size is reduced from 15.4 μm to 4.7 μm. After adding V element, under the combined action of ultrafine grains and MC-type nano-precipitates as hydrogen traps, better delayed fracture resistance of the 55SiCr steel with high strength can be improved.

Key words: V microalloying, grain refinement, mechanical properties, hydrogen trap, delayed fracture resistance