Determination of Driving and Pinning Forces for Static Recrystallization during Hot Rolling of a Niobium Microalloyed Steel

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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 (Tnr, Ar3, Ar1) and to characterize the progressive strengthening of austenite due to incomplete recrystallization between Tnr and Ar3, thanks to the measurement of a magnitude called accumulated stress (Δσ). 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 (FR) and pinning forces (FP). Using these expressions and the experimental data from the hot rolling simulations performed, the evolution of FR and FP 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 FP always grows faster than FR as the rolling temperature drops, which helps to explain the start of inhibition of the static recrystallization of austenite at temperatures below Tnr.
社団法人日本鉄鋼協会の論文
2005-11-15

Determination of Driving and Pinning Forces for Static Recrystallization during Hot Rolling of a Niobium Microalloyed Steel

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