Mِssbauer study of the retained austenitic phase in multiphase steels

Samples of steels with composition 0.30%C-1.5%Mn-1.5%Si-0.5%Al-0.5%Mo (wt.%) were subjected to different thermomechanical treatments to produce ferrite/pearlite/bainite (FPB), spheroidized (ESF) and martensite (MAR) microstructures. Subsequently they underwent a two stage annealing to obtain a final structure comprising of ferrite, bainite, martensite and austenite. The samples were studied by means of Mِssbauer spectroscopy (transmission and conversion electron Mِssbauer spectrocopy (CEMS)), X-ray diffraction (XRD), and metallographic analysis. Austenite contents were found to be the same for all samples except for the spheroidized sample annealed at 750°C that showed an increase of the austenite with increasing temperature of the treatment. Mِssbauer spectroscopy and quantitative XRD analysis exhibited significant discrepancies ascribed to texture effects. It is shown that the thermal treatment was successful in retaining significant quantities of the austenite phase for steels of this composition.