Determination of driving and pinning forces for static recrystallization during hot rolling of a niobium microalloyed steel

The hot rolling process of a low Nb-microalloyed steel under different interpass time conditions is simulated by means of hot torsion tests. Subsequent graphic representation of the Mean Flow Stress (MFS) versus the inverse of the absolute temperature for each pass allows us to know the critical rolling temperatures (T-nr, A(r3), A(r1)) and to characterize the progressive strengthening of austenite due to incomplete recrystallization between T-nr and A(r3), thanks to the measurement of a magnitude called accumulated stress (Delta sigma). Optical and electron microscopy studies demonstrate that the evolution of the microstructure and the precipitation state-particularly the mean particle size-over the rolling schedule is strongly dependent on the interpass time. A review is made of the expressions that have been proposed to estimate the values of recrystallization driving (F-R) and pinning forces (F-P). Using these expressions and the experimental data from the hot rolling simulations performed, the evolution of F-R and F-p during rolling is studied. A comparative analysis of hypotheses concerning the interaction between precipitates and migrating grain boundaries is achieved and the methods for estimating the volume fraction of precipitates and the dislocation density are assessed. Though the selected criterion significantly influences the values obtained for both forces, it is found that F-P always grows faster than F-R as the rolling temperature drops, which helps to explain the start of inhibition of the static recrystallization of austenite at temperatures below T-nr.