Abstract
In the process of making structural parts, spheroidization treatment of the rolled bars is applied to obtain good cold forgeability. In this study, the effect of using controlled rolling and cooling process on the subsequent spheroidization behavior of medium carbon steel was investigated using a hot deformation simulater and transmission electron microscope.
The following results were obtained.
(1) By controlling the cooling rate to 10°C/s immediately after controlled rolling, the post dynamically recrystallized fine austenite grains did not grow and transformed to a fine and globular ferrite-pearlite microstructure. This microstructure revealed an extremely high volume fraction of ferrite compared with a microstructure transformed from coarse austenite grains.
(2) By using transmission electron microscope, fine ferrite-pearlite microstructure transformed from fine dynamic-recrystallized austenite grain was observed. It was found that the extrinsic grain boundary dislocations were inherited from residual strain due to rapid cooling during the post dynamic recrystallization state. Furthermore, cementite films, which suggest an enhanced diffusion of carbon along the nonequilibrium boundary structure, were formed along each phase boundary.
The discussion is also made about the enhanced spheroidization during intercritical annealing on the basis of the behavior of grain boundary dislocations during reheating.
