Effects of Nb on dynamic recrystallization kinetics and pearlite transformation in high-carbon rail steels

The dynamic recrystallization (DRX) behavior and microstructure evolution of Nb-bearing and Nb-free high-carbon steels were analyzed in the present study through thermal simulation, theoretical calculation, and morphological analysis. The results indicate that 0.027 wt% Nb refined the austenite grains by approximately 57.1%. Based on the calculation of true stress-strain curves at deformation temperatures from 900 to 1150 °C and strain rates of 1 and 0.1/s, Nb addition resulted in the increment of critical strain, peak strain and peak stress of DRX at lower deformation temperatures. It shows that DRX process was hindered at lower deformation temperatures due to Nb addition, but the inhibition effect was progressively weakened with the increase of the deformation temperature. The starting time of DRX was delayed and the activation energy of DRX was increased by 7.2%. In addition, Nb hindered the transformation of the hot-deformed austenite to pearlite due to its solute drag effect. The higher the deformation temperature, the stronger the hindering effect. Moreover, the addition of Nb refined the pearlite nodules and this effect was enhanced by increasing the strain rate at the deformation temperature of 900 °C. It means that the effect of Nb on the pearlite nodules in the high-carbon rail steel was closely related to whether DRX occurred. Meanwhile, Nb addition led to the refinement of pearlite interlamellar spacing at the deformation temperature of 1000 °C, and the refinement effect was almost similar whether DRX occurred or not.