Effect of strain and morphology of the bainitic microstructure on the retained austenite stability and mechanical properties of thermomechanically processed C-Mn-Si(-Nb) TRIP steels

摘要

The relationships between the morphology and distribution of the bainitic structure and the stability of retained austenite have been studied at different amounts of strain. Two 0.2%C-1.55%Mn-1.5%Si steels (% referring to mass content) with, and without the addition of 0.039%Nb have been examined after simulated thermomechanical processing (IMP). A laboratory simulation of discontinuous cooling during IMP was performed using a hot rolling mill. An isothermal bainite transformation at 450℃ was used to produce a carbide-free bainitic microstructure. All samples after thermomechanical processing were characterised using optical microscopy. The volume fraction of retained austenite was measured using X-ray diffraction. Room temperature mechanical properties have been determined by tensile test. Specimens were sectioned parallel to the deformation direction to examine the dynamic of retained austenite-martensite transformation during straining. An original heat tinting method has been utilised to reveal the microstructural constituents such as martensite, retained austenite and martensite/retained austenite constituent. Image analysis software was then used to measure the size and volume fraction of retained austenite and martensite according to its location within microstructure. After that, the two-dimensional observation of multilevel micro-structural evolution was performed by progressive removal of the microstructural layers. The results have shown that the final micro-structures of the thermomechanically processed TRIP steels consisted of ～50% of polygonal ferrite, 12-14% of retained austenite, non-carbide bainite and martensite. The microstructure-property investigations revealed the relationship between mechanical properties of TRIP steels and chemical and mechanical stability of retained austenite. The results have demonstrated that the best combination of strength (～1100MPa) and elongation (～40%) was found in the steel without Nb and was due to the formation of refined acicular ferrite and granular bainite microstructure with ～14% of stable retained austenite. Furthermore, it has been found that the optimum combination of morphology and volume fraction of bainitic ferrite as well as its behaviour during straining plays an important role in the stabilisation of retained austenite. The importance of the spatial location on the stability of retained austenite has been revealed.