Precipitation behavior of Nb in hot-rolled Nb-microalloyed TRIP steel during high-temperature deformation

Hot-rolled TRIP steel with 0.038% Nb was thermomechanically processed on a Gleeble simulation test machine. The microstructural evolution and the precipitation behavior of Nb were analyzed by optical microscopy, scanning electron microscopy, and transmission electron microscopy. The amount of Nb in solution/precipitation was quantitatively measured by inductively coupled plasma optical emission spectrometry (ICP-OES). After austenization at 1250℃ for 5 rain, about 70% of added Nb is dissolved into austenite. During hot deformation in the austenitic recrystallized region above 1000℃, only 3% of dissolved Nb precipitates, which is too little to retard static recrystallization of austenite, and then the austenite grain size is markedly refined. Before hot deformation in the austenitie non-recrystallized region at 900℃, the fraction of precipitated Nb is up to 9%, which retards static recrystallization of austenite after hot deformation and results in pancaked austenite grains. Hot deformation in the austenitic non-recrystallized region accelerates the γ→α transformation and refines the ferrite grains. After controlled-cooling to 650℃, there are still 48% and 40% of added Nb in solution after thermomechnical processing for reerystallized austenite and deformed austenite, respectively.