Dynamic Transformation during the Simulation of Plate Rolling in an X70 Steel

Abstract

A five-pass rolling schedule is simulated by means of torsion testing. The first and last pass temperatures are 920 and 860 °C with 15° of cooling between passes. All of the rolling is carried out above the Ae3 temperature of 845 °C that applies to this steel. Interpass times of 10, 20, and 30 s are employed, which correspond to cooling rates of 1.5, 0.75, and 0.5 °C s−1, 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 data also allow for estimation of the amounts of ferrite that retransform to austenite during the time between passes. The volume fractions of ferrite and martensite (prior austenite) were determined by optical metallography. The amount of dynamic ferrite formed and retained increased with pass number. The volume fraction of ferrite that retransformed increased with pass number, averaging about 4, 6, and 9% for the 10, 20, and 30 s interpass times, respectively. The simulations indicate that ferrite is unavoidably produced during plate rolling and that the microstructure present at the initiation of accelerated cooling does not consist solely of austenite.