| Lots of mechanical component accidents frequently happened because of fatigue crack, so it is important and necessary to study the fatigue strength of materials for improving the safety and fatigue-life of mechanical components. In iron & steel materials, especially in high strength structural steels, the ductility always decreases when the strength is improved. TRIP (transformation-induced plasticity) steel, as a new kind of steel, has the excellent properties of high strength and high ductility. However, the conventional TRIP steels are almost medium or low carbon Si-Mn alloy steels, and they can not be used for high strength or ultra high strength mechanical components due to their low strength.Therefore, a new type of steel with an optimized matching of tensile strength, ductility and toughness was developed by designing chemical composition and improving heat treatment technique. It is the first time to apply the TRIP effect to high strength (≥1000 MPa) mechanical structural steels in this paper, so that both tensile strength and fatigue life would be improved.The effects of austempering (AT) treatment and quenching-tempering (QT) treatment with two tensile strength levels (1100 MPa and 1300 MPa) on the fatigue behavior of a 0.4C-1.5Si-1.5Mn type TRIP steel was investigated by using rotating-bending fatigue test and fatigue crack growth (FCG) test. The fatigue fracture mechanism was discussed by SEM observation of the fatigue crack. Moreover, the fatigue fracture behaviors were compared with those of medium-carbon high strength steel (42CrMo).The general mechanical properties of QT specimen show that the changes of tempering treatment are fundamentally same with those of conventional medium and high-carbon steels. And the effects of austenitized temperature, isothermal temperature and isothermal time on mechanical properties of AT specimen are remarkable. The tensile strength and toughness decrease and the ductility increases with the austenitized temperature increasing. As the isothermal temperature and time increase, the tensile strength decreases while ductility and toughness increase. In addition, the multiphase microstructures with a balanced matching of tensile strength, ductility and toughness was obtained by austenitizing at 900℃and austempering at 400℃for 600s in followed, and the content of retained austenite is 19%, which possesses an obvious TRIP effect. Furthermore, the yield strength of AT specimen is lower than that of QT specimen, while ductility is obviously higher than that of QT specimen at the same tensile strength level.The results of rotating-bending fatigue test (10'cyc) show that fatigue strength and fatigue life of AT specimens are better than those of QT specimens and 42CrMo at the strength levels of 1100 MPa and 1300 MPa, that is, the AT specimens with multiphase microstructures have outstanding fatigue properties with high cycle regime. And the difference in fatigue strength between AT and QT condition decreased with the increase of tensile strength from 1100 MPa to 1300 MPa. The ratios of fatigue strength (σ-1) to tensile strength (Rm) of AT specimens can reach 0.56 at these two strength levels, which are higher than those of QT specimens (0.51～0.52) and 42CrMo (0.50). Moreover, all of fatigue cracks of AT and QT specimens initiated from surface matrix, while some of fatigue cracks of 42CrMo initiated from surface inclusion. That, the fatigue cracks initiate from microstructure or inclusion depends on a competition mechanism of them.The TEM observations of AT specimens show that various forms of martensite exist in fatigue plasticity area after rotating-bending fatigue test, while the retained austenite is hardly to be observed in those areas. After certain number of cycles, about 4% of retained austenite transformed to martensite in AT-1100 specimens, while about 2% of retained austenite transformed to martensite in AT-1300 specimens. This suggests that TRIP effect could be induced in AT specimens after certain number of stress cycles, and it is more obvious in low strength level (1100 MPa)The FCG results show that the fatigue crack growth rates (da/dN) of AT specimens are lower than those of QT specimens in two strength levels of 1100 MPa and 1300 MPa. It also means that the fatigue properties of AT specimens are better than those of QT specimens.However, the difference in da/dN value between AT and QT specimens is smaller in1300 MPa, i.e., the rate is reduced with the increase of tensile strength.The SEM observations of the initiation and propagation of AT specimens with different tensile strength show that the fatigue cracks mechanism of high strength TRIP steel is influenced by the multiphase microstructures, especially by the location, size, and initial volume fraction of the soft-phase microstructure (retained austenite and lower bainite) and the transformation rate of retained austenite. Fatigue cracks of AT specimens mainly initiated from the concentration of plasticity strain, and enlarged along with the interface among upper bainitie and soft-phase, and terminated in the retained austenite which existed at the tip of cracks. Furthermore, the selectivity of crack propagation direction on soft-phase, the resistance of crystal boundaries of multiphase structures to crack propagation, and the initiation and propagation of quadratic crack consumed lots of expansion energy and relaxed strain concentration, which reduced the crack growth rates.
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