Abstract:It is increasingly demanded to improve the fatigue properties of medium-carbon microalloyed forging steels which are increasingly used to replace conventional quenched and tempered (QT) medium-carbon steels for the manufacture of fatigue design components. For ferrite-pearlite steel, owing to the existence of soft phase of ferrite, it is revealed that fatigue crack preferentially initiated and propagated along the ferrite/pearlite boundary in the early stage of fatigue crack propagation. Therefore, the fatigue property of ferrite-pearlite type microalloyed forging steel could be improved by increasing ferrite strength through precipitation strengthening and controlling the distribution of ferrite and refining microstructure. Further investigation shows that ferrite-pearlite steel with film-like ferrite along coarse prior austenite grain boundary and pearlite microstructure or coarse bainite microstructure has lower fatigue property. However, ferrite-pearlite steel with much fine and uniform ferrite and pearlite microstructure has higher fatigue property. Microalloying element V can significantly improve the fatigue properties of ferrite-pearlite steel mainly through precipitation strengthening and grain refining mechanism in as-forged condition. Unlike that of tempered martensite structure, there is little effect of oxygen content on the fatigue properties of ferrite-pearlite steel. In conclusion, the fatigue properties of microstructure carefully controlled ferrite-pearlite steel could be as high as that of QT steel.