Hot Deformation Behaviors, Microstructures And Properties Of Microalloyed Forging Steel

The advantages of the microalloyed forging steels lie in the facts that they can save the energy, reduce the production cost, shorten the production cycle and increase the labor productivity and the quality of the products. So they have found wide applications in the automobile and mechanical manufacturing industries. In recent years, researchers have paid much attention to the development and application of microalloyed forging steel in China. But, the application of domestic high strength microalloyed forging steel is still limited and it is necessary to do more research works. In this dissertation, the chemical composition of microalloyed forging steel for heavy truck front axle use was firstly designed and the experimental microalloyed forging steel was then smelted and forged. The microstructure, conventional mechanical properties and fatigue properties of microalloyed forging steels were investigated in detail. In order to study the effect of microalloying elements on the microalloyed forging steels and provide theoretical basis for formulating hot working technology, the continuous cooling transformation behaviors and hot deformation behaviors of Nb+V and V microalloyed forging steels were also investigated. The dynamic recrystallization behaviors, static recrystallization behaviors and precipitation behaviors were investigated. At last, in order to provide evaluation on industrial application, the heavy truck front axle manufactured by microalloyed forging steel was finally investigated by bench test. The main research results are as follows.(1) Based on C-Mn-Si-Cr series steel, the chemical composition of microalloyed forging steel for heavy truck front axle was designed by adding suitable content of Nb/V. The experimental results show that the mechanical properties of newly developed Nb+V and V microalloyed forging steels can meet the requirement of heavy truck front axle.(2) The fatigue behaviors of Nb+V and V microalloyed forging steels for heavy truck front axle were investigated by three-point-bending fatigue test, and the effects of specimen size and Nb-V microalloying on three-point-bending fatigue behaviors were also investigated. The results show that the three-point-bending fatigue limit of small specimen is higher than that of large specimen. The reason for this phenomenon is that there exist stress gradient in specimen for three-point-bending fatigue test. Compared to V-microalloyed forging steel, forging steel microalloyed with Nb+V has higher three-point-bending fatigue limit, and this is due to the finer microstructure by adding Nb in Nb+V microalloyed forging steel.(3) The continuous cooling transformation behaviors of Nb+V and V microalloyed forging steels were investigated, and the effects of deformation temperature and cooling rate on continuous cooling transformation behaviors were investigated. With increasing the cooling rate, the bainitic transformation temperature decreases. And proeutectoid ferrite, granular bainite, lath bainite and martensite transformation take place in turn. Through lowering deformation temperature, the transformation temperature of proeutectoid ferrite and bainite increases, and the cooling rate range corresponds to proeutectoid ferrite and bainite transformation broadens.(4) The dynamic recrystallization (DRX) and dynamic precipitation behaviors of Nb+V and V microalloyed forging steels were investigated by using single pass compression test, metallographic examination and TEM analysis. The critical strain for DRX was accurately determined based on the principles of irreversible thermodynamics and high order polynomial fitting for flow stress curves, and the model of critical strain for DRX was established. The interaction between DRX and dynamic precipitation was also investigated. The results show that the dynamic precipitation can only delay the onset of the DRX. And once the DRX take place, the dynamic precipitation could not prevent DRX from occurring.(5) By using double-hit compression test, the static recrystallization behaviors of Nb+V and V microalloyed forging steel were investigated. The static recrystallization critical temperature (SRCT) of experimental steel was determined and the activation energy for static recrystallization was calculated when the deformation temperature was higher than SRCT. The results show that the static recrystallization is inhibited by strain-induced precipitates when the deformation temperature is lower than SRCT and the activation energy for static recrystallization is increased rapidly with decreasing deformation temperature. At last, the interaction between static recrystallization and strain-induced precipitation was further discussed.(6) The V-microalloyed forging steel was forged into heavy truck front axle under industrial conditions and the fatigue behaviors of the heavy truck front axle were investigated by bench test. The results show that the mechanical properties of heavy truck front axle manufactured by V-microalloyed forging steel can meet the standard, and the fatigue life of heavy truck front axle is more than15.4×104cycle, which can meet the requirement of bench test. The fracture of heavy truck front axle was caused by surface defects during machining, so these kinds of defects should be avoided in industrial production.