Effect of hot compression and warm deformation on the microstructure of medium carbon steel

Multi-pass hot compression tests were conducted on medium carbon steel using a Gleeble-3500 thermal simulation testing machine at a strain rate of 20 s^-1, aiming to simulate its warm rolling process in the warm deformation region of 650-730 ℃. Scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) were employed to systematically investigate the effects of different deformation temperatures, compression amounts, and warm rolling temperatures on the microstructural evolution of ferrite and cementite in the tested steel. The results show that a decrease in warm deformation temperature is conducive to the equiaxed distribution of ferrite grains, while the cementite morphology tends to be spherical and granular. At a warm deformation temperature of 650 ℃, with the increase in compression amount, the spheroidization degree of cementite intensifies, and it aggregates and distributes at the ferrite grain boundaries. Furthermore, as the warm rolling temperature decreases, the cementite morphology transforms from an alternating lamellar and chain-like distribution to a chain-like and discontinuous distribution. Meanwhile, this leads to the average grain size of ferrite reaching the ultrafine-grained ferrite level (0.45 μm), which is mainly related to the high-density dislocations introduced by the decrease in warm rolling temperature.