Processing and Properties of Medium-Mn TRIP Steel to Obtain a Two-Stage TRIP Behavior

Two medium-Mn steels of nominal composition 0.15C-2.0Si-10.5Mn-(0.75, 1.5) Al-0.04Nb-balFe (wt pct) were processed to produce sub-micron grain sizes of 0.65 and 0.80 mu m. Mechanical testing was performed in three successive conditions: hot rolled, intercritically annealed, and cold rolled with subsequent intercritical annealing. Intercritical annealing was performed at 923 K for 20 hours. Electron-backscattered diffraction and X-ray diffraction were utilized to characterize the microstructure, consisting of alpha-ferrite,alpha-martensite,epsilon-martensite, and gamma-austenite. Microstructural constituents were tracked during tensile deformation and it was found that both steels exhibited two-stage TRIP with gamma-austenite martensitically transforming first to epsilon-martensite and as strain increased epsilon-martensite transformed to alpha-martensite.