Dynamic Transformation and Retransformation During the Simulated Plate Rolling of an X70 Pipeline Steel

The controlled rolling of pipeline steels involves pancaking the austenite and then subjecting it to accelerated cooling. However, the formation of ferrite during rolling decreases the amount of austenite available for microstructure control. Here the formation of ferrite during rolling is simulated using a five-pass rolling schedule applied by means of torsion testing. The first and last pass temperatures were 920 and 860 °C with 15° of cooling between passes. All of the rolling was carried out above the Ae₃ temperature of 845 °C that applies to this steel. Interpass times of 10 and 30 s were employed, which corresponded to cooling rates of 1.5 and 0.5 °C/s, respectively. Samples were quenched before and after the first, third, and fifth passes in order to determine the amount of dynamic ferrite produced in a given pass. The amounts of dynamic ferrite formed and retained increased with pass number. The amounts of ferrite that retransformed increased with pass number. The simulations indicate that ferrite is unavoidably produced during plate rolling and that the microstructures present at the initiation of accelerated cooling do not consist solely of austenite.