MICROSTRUCTURAL EVOLUTION DURING THERMOMECHANICAL PROCESSING OF A TI-NB INTERSTITIAL-FREE STEEL JUST BELOW THE AR-3 TEMPERATURE

Laboratory thermomechanical processing (TMP) experiments have been car ried out to study the austenite transformation characteristics, precip itation behavior, and recrystallization of deformed ferrite for an int erstitial-free (IF) steel in the temperature range just below Ar-3. Fo r cooling rates in the range 0.1 degrees Cs-1 to 130 degrees Cs-1, aus tenite transforms to either polygonal ferrite (PF) or massive ferrite (MF). The transformation temperatures vary systematically with cooling rate and austenite condition. There is indirect evidence that the tra nsformation rates for both PF and MF are decreased by the presence of substitutional solute atoms and precipitate particles. When unstable a ustenite is deformed at 850 degrees C, it transforms to an extremely f ine strain-induced MF. Under conditions of high supersaturation of Ti, Nb, and S, (Ti,Nb)(x)S-y precipitates form at 850 degrees C as coprec ipitates on pre-existing (Ti,Nb)N particles and as discrete precipitat es within PF grains. Pre-existing intragranular (Ti,Nb)(x)S-y precipit ates retard recrystallization and grain coarsening of PF deformed at 8 50 degrees C and result in a stable? recovered subgrain structure. The results are relevant to the design of TMP schedules for warm rolling of IF steels.