Study on quasi-in-situ tensile deformation behavior in medium-carbon carbide-free bainitic steel

摘要

Carbide-free bainitic steel is one of the most widely manufactured and applied materials. In this paper, its behavior in uniaxial tensile deformation is studied in quasi-in-situ conditions. The microstructure evolution of bainitic ferrite and retained austenite during tensile deformation was investigated by scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and X-ray diffraction. Results indicate that the sequence of the multiscale retained austenite transformed to martensite from easy to difficulty is blocky (micron level), flake (sub-nano level) and film-like (nano-size). The blocky retained austenite undergoes a martensitic transformation at the elastic stage, where a "bainitic ferrite-soft austenite-hard martensite" structure is formed. With increasing strain, the flake retained austenite between the bainitic ferrite plates plays a positive role, which is accompanied by continuous strain-induced martensitic transformation. Finally, a refined microstructure consisting of "bainitic ferrite-soft austenite-hard martensite-soft austenite" is formed. In the final non-uniform strain fracture process, the nano film-like retained austenite finally begins to transform to martensite owing to its extreme mechanical stability. In addition, the film-like retained austenite itself becomes part of the combination structure of "bainitic ferrite and austenite".