Retransformation Behavior of Dynamically Transformed Ferrite during the Simulated Plate Rolling of a Low C and an X70 Nb Steel

Abstract

Plate rolling simulations were carried out on an X70 Nb and a low C steel by means of torsion testing. A seven-pass rolling schedule was employed where the last pass was always applied above the respective Ae₃ temperature of the steel. Interpass intervals of 10 and 30 s were employed, which corresponded to cooling rates of 1.5 and 0.5 C/s. The mean flow stresses (MFS`s) applicable to each schedule increased less rapidly than expected from the decreases in temperature due to the dynamic transformation (DT) that took place during straining. The amounts of ferrite that retransformed into austenite during holding were determined by optical metallography. These increased with length of the interpass intervals and were reduced in the X70 steel due to the presence of Nb. The holding times after rolling required to increase the amount of austenite available for microstructure control on subsequent cooling were also determined for the two steels.