Transformation behaviour in thermomechanically processed C–Mn–Si TRIP steels with and without Nb

Two 0.2wt.% C–1.55wt.% Mn–1.5wt.% Si steels with and without the addition of 0.039 wt.% Nb were studied by laboratory simulations of controlled thermomechanical processing in a quench deformation dilatometer. The microstructures were characterised using optical metallography, image analysis and scanning electron microscopy techniques. The effects of recrystallised and non-recrystallised austenite and discontinuous paths on the final microstructure were studied. The results have shown that the highest volume fraction of retained austenite is associated with a 400°C isothermal bainite transformation temperature and the presence of approximately 50% polygonal ferrite and acicular ferrite as a dominant second phase.