Study On V-microalloyed Medium-carbon Steel And Tailored Forgings

There has been an increasing demand of tailored steels and forging components due to increased automobile performance.In this paper,medium-carbon forging steels with different V additions(0-0.45%)were used to study the microstructure and properties oriented controlling technology for the purpose of manufacturing tailored forgings.Firstly,hot deformation and phase transformation behaviors of V-microalloyed forging steels were analyzed by using thermal simulator.The influence of austenitizing temperature on grain growth,and the effect of forging parameters especially post-forging cooling rate on microstructure and hardness were systematically investigated.Then the precipitation hardening behavior of V(C,N)precipitates was analyzed for the purpose of controlling the fatigue properties of medium-carbon forging steels.Finally,pilot manufacturing of tailored forgings of connecting rod and long-arm steering knuckle were successfully conducted.The following conclusions could be obtained:The hot deformation and dynamic recrystallization behavior of the tested medium-carbon forging steels were studied by using thermal simulator,and constitutive equations and deformation activation energy were obtained to express their hot forging characteristics.The mechanism of V on dynamic recrystallization kinetics was verified to be mainly through solute-drag effect.The dependence of dynamic austenite grain size on Z parameter was established.The dynamic CCT curves were established for the tested forging steels,and it was found that the fields of transformation products especially bainite were shifted to the right side of the diagram with increasing of V content,and this effect is more noticeable with increasing V content.The effects of different forging and cooling parameters on the microstructural characteristics and mechanical properties of the tested steels were investigated systematically,The results show that the austenite grain grows up exponentially with the increase of reheating temperature,while austenite grains grow up in a parabolic way as the soaking time is prolonged,and a mathematical model which could predict the austenite grain growth behavior was established.Besides influencing austenite grain size,the reheating temperature also notably influence the dissolution of V(C,N)particles in austenite during austenitizing and thus the precipitation hardening effect as well as the volume fraction of pearlite,which all remarkably effect the final strength level of forgings.It is revealed that post-forging cooling strategy have significant influences,that is to say,by controlling microstructure especially the precipitation of fine V(C,N)particles through adjusting post-forging cooling,the strengthening and gradient function in one hot-forging part could be obtained.A direct cooling strategy after finish forging is proposed,which includes accelerated cooling with forced air in ferrite field,following by short time isothermal holding or very slow cooling and then air cooling.The results of physical-chemical phase analysis show that the amount of V(C,N)precipitates increases with increasing V content,and about 34-57%of the precipitates are less than 10 nm and that only 48-64%of total V is in the V(C,N)precipitates.It is thus concluded that through appropriate controlling of reheating temperature,post-forging cooling rate to control microstructure especially the precipitation of fine V(C,N)particles,tailored properties in one forgings could be obtained.Efforts were also conducted to controlling the high-cycle fatigue properties of the tested steels.It was found that both fatigue strength and fatigue strength ratio increase with increasing V content mainly through precipitation strengthening,whereas this improving tendency tends to slow down when V content is too high.SEM analysis of fracture surfaces reveals that fatigue crack initiation is not controlled by inclusion like that for the medium-carbon forging steels while microstructure plays a controlling role.The influence of V on fatigue crack growth rate depends on the applied stress intensity factor range.It is concluded that the fatigue properties of microalloyed forging steel equal or evern higher than that of quenched and tempered medium carbon steels through microstructure controlling.The loading behaviors of a kind of new long-arm steering knuckle and connecting rod were determined using CAE analysis and it was found that maximum stresses are located at the arm root and middle part,respectively.Therefore,these parts were enhanced strengthened(an increase of at least 10%)through application of tailored cooling strategy,and comprehensive analyses and bench scale tests of these forgings confirmed the effectiveness of tailored forging.The new tailored connecting rod was lighted about 19.6%as well as a 10%increase of fatigue safety factor,and the tailored steering knuckle possesses enhanced hardness at its arm root due to its comparatively large size.