The effect of bainite in producing nano/ultrafine grained steel by the martensite treatment

The martensite treatment including cold rolling and subsequent annealing of microstructures with high percentage of martensite is one of the most effective thermo-mechanical methods for grain refinement in steels. In the present work, the effect of bainite phase on the formation of nano/ultrafine grained structure in a low alloy 0.2% C steel was investigated. The martensitic and bainitic–martensitic microstructures were first produced in the specimens by quenching in water and oil from the austenitizing temperature, respectively. The specimens were subjected to 90% reduction in thickness by cold rolling and subsequent annealing. The formation of the nano/ultrafine grained structure was discussed from the viewpoint of characteristics of the martensite and bainite starting structure. It was found that nano/ultrafine grained structures with similar hardness and mean ferrite grain size can be produced in both specimens by annealing 90% cold rolled specimens at 550 °C for 80 min. However, due to the lower hardness of the bainite phase, cold rolling of the bainitic–martensitic microstructure was much more facile than the martensitic one, resulting in no transverse cracks in the cold rolled specimen.

Research highlights
► Obtaining two specimens with martensitic and bainitic–martensitic microstructures.
► Obtaining the same grain size and hardness in two specimens after martensite treatment.
► The bainitic–martensitic specimen was cold rolled with no cracks.
► The bainitic microstructure had maximum dislocation density in the both specimens.