Refining of intermediate transformation microstructure by relaxation processing

Abstract

The influence of RFC (relaxation-precipitation-controlling phase transformation) processing parameters on the microstructure was studied by thermo-simulation for a low carbon Nb and Ti containing microalloyed steel. The semi-quantitative statistic work of the packet size was carried out by the aid of optical microscope. TEM, EBSD and SEM were applied to investigate the microstructure refinement, precipitation, the evolution of crystal defects configuration and their interaction during the relaxation. The results demonstrate that the steel is composed of ultra-fine bainite/martensite composite microstructure, and the microstructure could be refined markedly by RFC processing. The best thermo-simulation process for refinement in this experiment is deformation by 30% at 850°C, and then relaxing at this temperature for 60 to 200s. Increasing the reduction ratio from 30 to 60 % or decreasing the deformation temperature to 800°C would cause the optimized relaxation time to become shorter, and increasing the deformation temperature to 900°C would cause the refinement effect to be weaker. It is also indicated that the nucleation, growing and coarsening of precipitates, and the dislocations polygonizing occur during the relaxation. Both two processes could be helpful to refine the intermediate transformation microstructure, and when these two processes corporate and promote each other, the optimized processing is obtained.