Microstructure,mechanical properties and strengthening mechanism of Nb-bearing TRIP steel after continuous annealing process

The continuous annealing processing of transformation-induced plasticity (TRIP) steel was simulated in lab.The microstructure and mechanical properties were analyzed by optical microscopy (OM),scanning electron microscopy (SEM),transmission electron microscopy (TEM),electron-backscatter diffraction (EBSD),X-ray diffraction (XRD) techniques and tensile test.The factors influencing the stabilization of retained austenite and the strengthening mechanism of TRIP steel were analyzed.The experimental results showed that fine grains in the microstructure of Nb-bearing TRIP steel formed during continuous annealing processing because of the existence of Nb.The Nb-bearing TRIP steel has more retained austenite and more carbon in retained austenite than the TRIP steel without niobium.It is found that retained austenite in the Nb-bearing TRIP steel is bulky or thin filmy at ferrite and bainite grain boundaries and few fine globular ones exist in the ferrite matrix.Fe₃C and (Nb,Ti)(C,N) are the main precipitates in the Nb-bearing TRIP steel after hot rolling.After continuous annealing,(Nb,Ti)(C,N) is the only main precipitates.Obviously the yield strength and tensile strength increase with increasing annealing temperature,which results from the precipitation strengthening of fine Nb-bearing precipitates.