Dynamic Transformation during the Simulated Plate Rolling of a 0.09% Nb Steel

Plate rolling simulations were carried out on an X70 Nb steel by means of torsion testing. A seven-pass rolling schedule was employed over the finishing temperature range 950-860 degrees C with 15 degrees of cooling between passes. Interpass intervals of both 10 and 30 seconds were employed, which corresponded to cooling rates of 1.5 and 0.5 degrees C/s, respectively. The mean flow stresses (MFS's) applicable to each schedule increased less rapidly than expected from the decreases in temperature. This indicated that both dynamic transformation (DT) and dynamic recrystallization were taking place during straining. The volume fractions of DT ferrite formed were determined by optical metallography and increased with pass number. Shorter interpass times also favored the formation and retention of ferrite as did the approach of the Ae(3). The simulations indicate that ferrite is unavoidably produced during plate rolling and this decreases the amount of austenite available for microstructure control on subsequent cooling.