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

Abstract

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 (Ds). 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 .

Interaction between recrystallization and strain-induced precipitation in a high Nb- and N-bearing austenitic stainless steel: Influence of the interpass time

“…As it is well known, one of the approaches for obtaining fine grains is through fine precipitation occurring during the recrystallization process [12][13][14]. This will hinder the mobility of boundaries recently formed by dynamic recrystallization and even during time intervals between deformation passes, where static and metadynamic recrystallization can take place.…”

Theoretical and Experimental Nucleation and Growth of Precipitates in a Medium Carbon–Vanadium Steel

“…The nucleation time calculated at any temperature means the time necessary for the formation of a number N of nuclei, of size R 0 , whose growth continues when the precipitate radius reaches a size of 1.05R 0 . The experimental nucleation time refers to the time necessary for the pinning forces to exceed the driving forces for recrystallization, temporarily inhibiting the progress of recrystallization [34]. On the other hand, the carbon extraction replica technique does not easily detect precipitates with sizes of less than 1 nm.…”

Thermomechanical Processing of Structural Steels with Dilute Niobium Additions

“…The number densities of the precipitates were also calculated (Table 5 -5). Then the volume fraction of the NbC precipitates were determined using the following equation proposed by Ashby and Ebeling [202], which has been adopted in a number of studies [35,37,93,139,203].…”