Influence of multipass deformation on Ar 3 for niobium bearing steel using torsion simulations

The continuous cooling transformation behaviour of a niobium microalloyed steel is studied by combining multipass deformation and continuous cooling torsion testing and by monitoring the variation of flow stress as afunction of temperature during cooling. Deformation above the non-recrystallisation temperature T_nr slightly increases the Ar₃ temperature and so encourages the austenite to ferrite transformation. Further deformation below T_nr both enhances the Ar₃ temperature and promotes the transformation significantly. Decreasing the interpass time and increasing the pass strain below T_nr increases the Ar₃ temperature. A method is proposed to estimate the stored deformation energy from pass to pass as the deformation proceeds below T_nr based on the course of the mean flow stress curve and on the concept of shear deformation energy. The effect of the process variables, mainly the interpass time below T_nr on the stored deformation energy and consequently on the Ar₃ temperature is discussed.

MST/3198