Carbide-free bainitic steels have an extensive application in the railway and automotive industries because of excellent mechanical properties, such as strength, ductility, and toughness. [1][2][3][4][5][6][7] Based on the outstanding properties of this class of steel, some traditional dual-phase (DP) and conventional transformationinduced plasticity (TRIP) steels are expected to be replaced. [7][8][9] The typical microstructure of these steels is mainly composed of fine bainitic ferrite containing high density of dislocations, metastable retained austenite, and/or hard martensite. [1][2][3][4][5][6][7] The high strength is attributed to the fine scale of bainitic ferrite, high density of dislocations, and/or hard martensite; and the TRIP effect caused by the transformation of austenite to martensite induced by monotonic tensile strain provides the possibility for simultaneously strengthening and toughening of carbidefree bainitic steels. [10][11][12] So far, many investigations focus on the effect of the content and stability of retained austenite on mechanical properties of carbide-free bainitic steels. However, the tensile properties and corresponding monotonic deformation behaviors closely associated with the microstructural evolution of carbidefree bainitic steels are rarely reported.In practical applications, most of structural components in railway and automotive fields, such as rails/crossings, chassis, axles and suspension arms, and so on, are usually damaged or fatigue-failed owing to tension-compression cyclic deformation with large strains. [13][14][15][16][17][18] Therefore, understanding the low-cycle fatigue (LCF) properties and their influencing factors of structural components are very important for investigating the safe service condition. According to previous publications, austenite-to-martensite transformation can occur at accumulated plastic strain during cyclic loading, thereby influencing the cyclic deformation behavior and fatigue life. [15][16][17][18] As reported, the martensitic transformation from austenite induced by strain helps to promote cyclic hardening due to the TRIP effect, but shorten the fatigue life because of the existence of freshly brittle martensite, especially for austenitic stainless steels with larger amounts of retained austenite; [19][20][21] in the literature, however, it has been reported that the martensitic transformation from austenite in austenitic stainless steels with more retained austenite can not only promote cyclic hardening, but also contribute to increasing the fatigue life due to the TRIP effect. [22][23][24] Differently, several authors suggested that the transformation of austenite to martensite is not the main influencing factor of cyclic hardening for traditional DP and conventional TRIP steels with less amount of retained austenite, while it is beneficial to increase the fatigue life of these steels due to the TRIP effect. [25][26][27] In addition, these above reports also suggested that cyclic softening resulted from dislocation rearrangement a...