Research Organization And Fatigue Fracture Behavior Of Tempered Steel Carbon BAINITIC

Compared with the conventional quenched and tempered (QT) low alloy steel, the non-quenched and tempered steel, as one of the high-technology steels with high technology content, strong driving force and high influence power, exhibit plenty of advantages, including saving energy, improving produce efficiency, rate, good properties of production, short production cycle, low cost and reducing environmental pollution, which could be used to replace considerable amount of quenched and tempered (QT) low alloy steel. The conventional non-quenched and tempered steel with the disadvantage of low strength and toughness, is hard to meet the requirements of strength and toughness for the security components of automobile. Microalloyed bainitic steel for hot forging have attracted lots of attentions from the researchers at home and abroad, due to the good strength and toughness. The fatigue failure is more harmful for the security components of automobile. Consequently, in this work, the influence of microalloying on the microstructure and mechanical properties of microalloyed bainitic steel was investigated and the fatigue failure behaviour of microalloyed bainitic steel was discussed theoretically. Results as follows:The microstructure and mechanical properties of different vanadium microalloyed bainitic steels for hot forging were studied and results showed that the vanadium had important effect on the microstructure transition of bainitic steel. Bainite could be obtained in a wider cooling-rate range by the experimental steel with vanadium. However, the delay effect was not obvious for the phase transition temperature of bainite. Dissolved vanadium element presented ferrite nuclear in the boundaries and promoted the bainitic transition. The microstructure of the vanadium microalloyed bainitic steel was finer than that of experimental steel without vanadium. With the increase of V content, the structure refinement became more obvious. The microstructure of Ti-V microalloy steel was the finest, and microstructure of Nb-V microalloy steel was relatively large. The strength and yield strength ratio was enhanced with increasing V content, and the impact toughness was improved slightly. The improvement of mechanical properties of the vanadium microalloyed bainitic steel was mainly attributed to the microstructure refinement and precipitation strengthening by vanadium microalloying. The mechanical properties of the two composite microalloying bainitic steels for hot forging were were improved. In the state of forging, the mechanical properties of Ti-V microalloy steel were greatly enhanced. According to the calculation and analysis of precipitated phases for the experimental steel containing vanadium, the driving force of vanadium in austenite was controlled by nitrogen and vanadium content, and the content of C had little effect on the beginning precipitation temperature of V in austenite. The increase of N and V content both could increase the precipitation content of V in the experimental materials.High-cycle fatigue fracture behavior of three microalloyed bainitic steels with different microstructures and ferrite-pearlite type microalloyed steel F38MnVS and quenched and tempered (QT) low alloy steel42CrMo for comparisons were studied using rotating-bending fatigue test. The results of indicated that microalloyed bainitic steels had higher fatigue strength than ferrite-pearlite type microalloyed steel and the fatigue strength of two experimental steels with higher strength and quenched and tempered steel42CrMo was comparable. The high cycle fatigue properties of microalloyed bainitic steel with lower strength was almost the same as that of QT low alloy steel, more lower than that of the ferrite-pearlite type microalloyed steel. The fatigue strength of two experimental steels with higher strength was as high as the QT steel42CrMo, but the fatigue properties was lower than that of QT steel with the same strength. The fatigue cracks of microalloyed bainitic steels initiated along the bainitic ferrite laths in the specimen surface and were easy to initiate in the boundaries of M-A island and bainitic ferrite, preferred to propagate along the bainitic ferrite laths at a certain force direction.Notched fatigue fracture behavior of three microalloyed bainitic steels with different microstructures and ferrite-pearlite type microalloyed steel F38MnVS for comparisons were studied using rotating-bending fatigue test. The results of indicated that the fatigue strength of notched specimen was much lower than that of smooth sample and the decrease extent of fatigue strength was slower with the increase of stress concentration factor. At the same strength level, the fatigue strength of microalloyed bainitic steel(both notched and smooth specimens) was higher than that of ferrite-pearlite type microalloyed steel. The fatigue limit ratio σ-1/Rm of smooth sample was lower than that of ferrite-pearlite type microalloyed steel and the fatigue limit ratio of notched sample for the microalloyed bainitic steel was nearly the same as the ferrite-pearlite type microalloyed steel. As a result, the microalloyed bainitic steel possessed smaller fatigue notch sensitivity. The notched fatigue strength of two microalloyed bainitic steels with higher strength was comparable to that of the microalloyed bainitic steel with lower strength. Thus, with the enhanced of strength, the fatigue notch sensitivity of experimental steels was increased. Further SEM examination of fatigue specimen fracture surfaces revealed that the notched fatigue cracks in the experimental steels initiated in the notch root matrix and obvious plastic deformation was observed in the notch root. The fatigue cracks in the propagation region of fatigue fracture surface propagated in forms of quasi-cleavage. The results of fatigue crack propagation rate indicated that the fatigue crack propagation rate of microalloyed bainitic steel da/dN was obviously slower than that of ferrite-pearlite type microalloyed steel. M-A island and residual austenite blocked the fatigue crack propagation. The rate of fatigue crack propagation for three microalloyed bainitic steels was increased with the increase of the grain size.