Thermomechanical processing during warm deformation in a medium C microalloyed steel for developing spheroidised and ultrafine grained microstructures

The microstructure development of 55VNb1 microalloyed steel after warm deformation via multi-pass biaxial compression tests was studied, and the effect of thermomechanical conditions on spheroidisation of cementite lamellae and ferrite recrystallisation for a range of deformation temperatures (600-700 °C), cooling/soaking time (water quenching, air cooling, 10 and 30 min of soaking time) and interpass time (0-10 s) was analysed. During deformation, the spheroidisation of pearlite is dynamically accelerated mainly by boundary splitting mechanism together with the rapid dissolution of cementite, while ferrite softening is attributed to dynamic recovery and continuous dynamic recrystallisation. The strong microstructural evolution during cooling/soaking time indicates that deformation energy accumulated is sufficient to activate metallurgical phenomena in both phases also statically. Static spheroidisation is a diffusive process, with rate controlled by the diffusion of vacancies, as suggested by the estimated activation energy. Ferrite refinement is the result of the evolution of continuous recrystallisation and pinning effect exerted by fine, globulised and homogeneously dispersed cementite particles. Increasing temperature causes accelerated kinetics in metallurgical phenomena; therefore, cooling/soaking time becomes key parameters to achieve ultrafine grained and spheroidised microstructures. Interpass time favours spheroidisation and promotes continuous recrystallisation; however, it must be carefully controlled to find a balance between recrystallisation and Ostwald ripening to optimise microstructural development.