High-temperature deformation behavior of high-Nb microalloyed steel during plate rolling simulation

When austenite is deformed above the Ae₃ temperature, partial amount of it transforms dynamically into ferrite by a displacive mechanism. After unloading, it retransforms back into the stable austenite by a diffusional process. This phenomenon influences the rolling load, which either decreases under isothermal rolling condition or does not progressively increase with decreasing temperature during cooling. In this work, plate rolling simulations employing isothermal and continuous cooling conditions were carried out on a 0.09 wt.% Nb X-70 steel. Pass strains in the range of 0.2-04, interpass times between 10- 30 s and strain rate of 1s^-1 were employed. The results showed that the critical strains for the initiation of dynamic transformation fell around 0.06 while the critical strain for dynamic recrystallization was 0.12. Under cooling conditions, the ferrite formation is favored as the temperature approaches the Ae₃ line. Such ferrite forms when the driving force for dynamic transformation is higher than the total free energy barrier. Increasing the holding time after rolling increases the amount of austenite available for microstructure control on subsequent stages. The dynamic transformation (DT) mechanism can be used to design improved rolling schedules. (author)