Microstructure And Metallurgical Quality Impact On The Fatigue Properties Of The Microalloying Non-quenched And Tempered Steel

microalloyed medium carbon steels is an efficient energy-saving steel which developed with a shortage of international energy. There're many advantages such as reducing the heat treatment cost of parts, simplifing or omitting process, saving energy and so on. Now they have been paid attention to by more and more countries. They're widely used in parts and structural parts such as automotive connecting rods, crankshafts and steering knuckle axis. So it is very important to enhance the fatigue performance of microalloyed medium carbon steels.In this thesis, effect of microstructure and oxygen contene in the fatigue properties of microalloyed medium-carbon steel were studied by rotating-bending fatigue tes,scanning electron microscopy (SEM). Using kind of microalloyed medium-carbon steel,45MnV, 38MnS,38MnVS and C 70S6.The high-cycle fatigue properties of two heats microalloyed medium-carbon steels F45MnV with different oxygen content at various conditions (as-normalized, quenched and tempered (QT) and forged) were studied by using rotating bar bending fatigue test machine. The results show that for the QT specimens with uniform tempered martensite microstructure, the fatigue property of the low oxygen content steel 148 is higher than that of the high oxygen content steel 149, whereas for the normalized specimens with ferrite plus pearlite microstructure, there are no significant difference of both the fatigue property and fatigue crack growth rate da/dN for the two heats steels used. The low oxygen steel in the as-forged condition 148-F has much higher fatigue property than that in the as-normalized condition 148-N for the uniform and finer ferrite and pearlite microstructure of the former, which is close to that of the as-QT condition 148-QT. It is suggested that the different effects of non-metallic inclusion on the fatigue fracture behaviors of the tested steels with different microstructures are the main reason for the above results. Moreover, the da/dN of ferrite plus pearlite microstructure is a little lower than that of tempered martensite of both steels used.Studied the high cycle fatigue properties of different non-tempered steels with several carbon content which used in connecting rod. The traditional medium carbon steel contain vanadium 38MnVS has the best fatigue properties, and the non-tempered steel 38MnS without micro-alloying element which is close to the tempered steel takes second place, but the fatigue properties of high carbon steel C70S6 used for cracking connecting rod is the worst which should be improved. Because the low cost of 38MnS without micro-alloying element at the forged state has no effect of precipitation strengthening and grain refining strengthening, whose fatigue properties is little higher than normalizing state. For the slow cooling annealing, whose structure is close to state of equilibrium with the lowest strength and fatigue properties. And the key is how to improve the low cost non-tempered steels'fatigue properties for much more application in industrial area.