Abstract: Nb+V and V microalloyed forging steels are usually applied to structural parts and security parts of automobiles, and fatigue fracture is their main failure mode, so it is necessary to investigate in depth their fatigue fracture behaviors. By using LEICA Q550IW optical microscope, AG-25TA electronic tensile testing machine and DYNATUP 9250HV floor-type impact testing machine, the microstructures and mechanical properties of a Nb-V microalloyed forging steel and a V microalloyed forging steel are studied, the three-point-bending fatigue behaviors of them are compared and studied by using FTM-100 hydraulic servo fatigue testing machine, and S-N curves are obtained. The fracture characteristics of the fatigue specimen are observed by using FEI QUANTA600 scanning electron microscope. The results show that when the mass fractions of microalloying elements are basically the same, Nb-V microalloyed forging steel has superior strength and toughness, and its three-point-bending fatigue limit is far higher than that of V microalloyed forging steel because Nb-V microalloyed forging steel has finer grain structures.

Key words: Fatigue limit, Mechanical properties, Microalloyed forging steel, Microstructures, Nb-V microalloying