Effect of isothermal bainitic processing on microstructures and mechanical properties of novel Mo and Nb microalloyed TRIP steel

Abstract

A transformation induced plasticity (TRIP) assisted steel with a new composition of C–Mn–Si–Al–Cu–Mo–Nb was designed in order to achieve significant improvements in both strength and ductility. In this study, computational thermodynamics calculations and a dilatometric method were used to develop a predictive methodology to determine the processing parameters in order to reach the desired amount of ferrite and retained austenite (RA) fractions during a conventional two-stage heat treatment. The heat treatment at the bainitic temperature affects RA content and carbon concentration in the γ phase. The effect of isothermal temperature on the microstructures and the mechanical properties of the investigated steel were investigated, and the mechanical stability of RA was evaluated using the Ludwigson and Burger relation. Direct experimental evidence by electron probe microanalysis confirms the distribution of alloying elements in different phases, especially C. Most of the C existed in the RA and small M/A islands. Observation by TEM reveals that the multiple additions of Mo and Nb not only refine the final microstructure but also make fully fine and dispersive (Nb,Mo)C carbide precipitate in ferrite grains. The samples annealed at 830°C and isothermally treated at 440°C demonstrated a significant improvement in the ultimate tensile strength (1080 MPa) with good uniform elongation (23%), as compared to conventional TRIP steels.