Effect of bainitic transformation temperature on the microstructures and mechanical properties of the hot rolled trip steel containing Ti and Mo and its precipitation characteristics

With the increasing consciousness for reducing fuel consumption and improving automobiles safety, the automotive industry is urgent to develop a new-type of steel with high strength and excellent formability. Among many high strength steels, the transformation induced plasticity (TRIP) steel may be a good candidate for automotive applications, as it drastically improves the balance between strength and ductility compared to precipitation hardened and solution hardened steels. While the tensile strength of conventional hot rolled TRIP steels are usually between 500 and 600 MPa, the TRIP steel with higher tensile strength, especially in excess of 750 MPa, is becoming increasingly important for the automotive industry. Thus, many strengthening mechanisms, such as precipitation strengthening, solution strengthening, refinement strengthening and dislocation strengthening, have been employed to improve the strength of the TRIP steel. Among them, microalloying with Nb, V and Ti, may provide effective means for further strengthening via grain refinement and precipitation strengthening. So far, many researches about the Ti-microalloyed high strength low alloy (HSLA) steel have been reported. However, the influences of alloying elements Ti and Mo on the hot rolled TRIP steel, especially the precipitation characteristics and their effects on mechanical properties, were rarely reported. Therefore, in this work the microstructure, retained austenite contents, mechanical properties and precipitation characteristics of the hot rolled TRIP steel containing Ti and Mo after bainitic transformation at different temperatures, were studied by using SEM, XRD and HRTEM. The results show that the bainitic transformation temperature has a significant effect on organizational morphology, retained austenite contents and mechanical properties of the TRIP steel. The optimal bainitic transformation temperature is 400°C, in which the retained austenite content and the balance of strength and ductility are 17.13% and 23.87 GPa·%, respectively. In addition, through HRTEM analysis, it was observed that the larger (Ti, Mo)C carbides over 20 nm in size exhibited the relationship ((100)(Ti, Mo)C//(110)α-Fe, [011](Ti,Mo)C//[1̄11]α-Fe) with ferrite matrix, and the smaller (Ti, Mo)C carbides less than 5 nm in size satisfied the Baker-Nutting orientation relationship: (100)(Ti,Mo)C//(100)α-Fe, [011](Ti,Mo)C//[001]α-Fe.