Microstructural changes in equal channel angular pressed low carbon steel by static annealing

The thermal stability of ultrafine grained low carbon steel manufactured by the equal channel angular pressing technique was examined by imposing the static annealing treatment for 1h at various temperatures of 693–873K. The initial grain size of the ferrite phase of ∼30μm was refined to ∼0.2μm by applying four passes of the equal channel angular pressing in which a single passage yielded an effective strain of ∼1. At annealing temperatures of 693–783K, ultrafine ferrite grains were relatively stable with little grain growth but microstructural examination revealed that a recovery process was active, showing the decrement of dislocation density, the restoration of well-defined grain boundary and the existence of a dislocation cell. Above 783K, the ferrite phase consisted of coarse recrystallized grains which were observable with optical microscopy and ultrafine unrecrystallized grains. The microstructural change of ultrafine ferrite with annealing was examined by estimating the activation energy for grain growth and by comparing it with the activation energies for processes associated with ferrite grain growth. Under the present annealing conditions, microstructure of pearlite phase was characterized by the enhanced spheroidization of cementite compared with the unpressed sample. The enhanced spheroidization behavior of the present pearlite phase was discussed in terms of easy carbon dissolution from cementite into pearlitic ferrite due to intensive plastic deformation of cementite.