Effect of Nb microalloying on the microstructure and mechanical properties of high-Al cold-rolled TRIP steel

The effects of 0.025% Nb microalloying on the microstructure and mechanical properties of high-Al cold-rolled TRIP steel were investigated by means of two-stage salt bath heat treatment,optical microscopy(OM),scanning electron microscopy(SEM),X-ray diffraction(XRD) and tensile test.The results indicate that the addition of Nb refines the microstructure after continuous annealing and increases the volume fraction and carbon content of retained austenite.The Nb-microalloyed high-Al TRIP steel treated at 370℃ or 400℃ has very excellent mechanical properties,the tensile strength is higher than 650 MPa and the total elongation is above 35%.The best isothermal bainitic transformation process of the Nb-alloyed TRIP steel is from 370℃ to 400℃,which is wider than that of the TRIP steel without Nb,at about 400℃.The consistency of the process is enhanced.