Dynamic Transformation of an X70 Steel under Plate Rolling Conditions

Abstract

Torsion simulations of plate rolling were carried out on an X70 steel at 900°C; pass strains of 0.4 and a strain rate of 1 s⁻¹ were employed together with interpass times of 1.0 s, 10.0 s, 20.0 s and 30.0 s. The mean flow stresses (MFS`s) applicable to each pass were calculated by integration of the individual flow curves. These decreased with pass number, indicating that both dynamic transformation and dynamic recrystallization were taking place during straining. The critical strains for the initiation of dynamic transformation were determined by double differentiation and fell in the range 4–5% from the second to the last pass. The volume fraction of dynamically formed ferrite retained after a simulated rolling pass decreased with interpass time. This was due to the increased amount of ferrite retransformed into austenite during the longer interpass times. The forward transformation is considered to occur displacively while the retransformation into austenite during holding takes place by a diffusional mechanism.