Microstructure and mechanical property of bainitic structure in low carbon steel, 0.13C-1.8Ni-1.6Cr-0.5Mo-0.07V, were studied by optical and electron microscopy and tensile test. Bainitic structure was obtained by isothermal transformation at the temperature of 400℃, 450℃ and 500℃ from austenitizing temperature. Electron microscopic observation revealed that the feature of bainite formation is affected by initial cooling stage, from the austenitizing temperature to the trensformation temperature. In this cooling stage coarse bainitic ferrite lath was formed in the specimens, isothermally transformed at 450℃ and 500℃.
In the specimens transformed at 450℃ and 500℃, mixed structure of bainitic ferrite and retained austenite was found in the initial period of the transformation. As the transformation proceeded, the finer bainitic ferrite lath was formed in the region of retained austenite, and carbide was precipitated in the coarse bainitic ferrite. It was also observed that the subgrain began to be created in this coarse bainitic ferrite due to the dislocation rearrangement. After the longer transformation time the subgrain disappeared and dislocation density was lowered. At the transformation temperature of 400℃, a somewhat different microstructure, so-called granular type bainite, was developed.
Ultimate tensile strength and 0.2% offset yield strength were increased with increasing transformation time but again decreased at the longer time, in the specimens transformed at 450℃ and 500℃. The increase of strength is attributed to the creation of finer bainitic ferrite lath and subgrains and to the precipition of carbides in the coarse ferrite. The decrease of strength at the later stage is related to the recovery phenomena such as dislocation annihilation and elimination of subgrains. In the 400℃ transformed specimens, the strength was decreased with transformation time. This corresponds to the decrease of untransformed area in the granular type bainite.
