Retransformation of dynamically induced ferrite during physical simulation of Steckel mill hot rolling

It has been well established that austenite can partially transform into ferrite during thermomechanical processing. The laboratory-scale strip and plate rolling simulations in the literature suggest that both forward and backward dynamic transformation (DT) can occur above the equilibrium Ae3 temperature. In this work, a five-pass Steckel mill rolling schedule (isothermal deformation) of an X70 high Nb steel was simulated using a torsion testing machine. Four different roughing schedules with a strain of 0.4 per pass were employed at 1100 °C before the application of finishing passes. A total of five finishing passes were simulated at 900 °C with pass strains of 0.3, strain rate of 1.0 s−1, and interpass time of 10 s, followed by isothermal holding varying from 1 to 270 s. The level of the flow curves depicted activation of softening phenomena, which was shown to be the formation of DT ferrite by means of optical and scanning electron microscopy images. The volume fraction of DT ferrite increases with increasing the number of roughing passes due to retained work hardening. Moreover, the DT ferrite retransforms back into austenite during the 10 s pass interval and holding after last strains, as shown by phase fraction measurement before and after each pass. Thus, the interpass time and holding intervals after the simulations play a significant role in reducing the amount of ferrite after the process. It seems that the presence of Nb in the alloy affects both the forward and backward transformation. The present investigation can be used to estimate the amount of formed and retransformed ferrite during Steckel mill operations in order to have a better metallurgical understanding of the process. This may result in better dimensional and quality control during the manufacturing of X70 pipeline steels.