Study On TRIP Effect Of Super Bainite

The 60Mn2SiCr steel which is previously austenitized at 900℃ for 30 minutes and then isothermal treated at 260℃ for 12 hours in a salt bath furnace is developed for the research of the phase transformation of retained austenite induced by stress in super bainite microstructure and its effect on the mechanical property of the steel. The microstructure of the experimental steel after isothermal transformation is the typical super bainite microstructure (BF+AR). Bainite ferrite (BF) is lath or fine needle structure and retained austenite (AR) is thin film structure. The phase composition, microstructure morphology and mechanical property of the sample are analyzed by the Optical microscope(OM), Scanning electron microscope(SEM), Transmission electron microscope(TEM), X-ray diffracmeter(XRD) and tensile testing machine.The percentage composition of retained austenite in super bainite has been calculated on the basis of theoretical analysis on morphology characteristic of super bainite. Ultra-fine substructure unit existes in bainite ferrite, and retained austenite in thin film distributes in bainite ferrite which is lath and substructure unit. The volume fraction of retained austenite is 9.4% and the average carbon content is determined as 1.296% after above isothermal transformation. Accordingly, the rockwell hardness is 50 HRC, and the tensile strength is 1816 MPa with the elongation of 7.4%. Impact toughness 22.82 J/cm2 were reached.The microstructure morphology, phase composition and mechanical property of the loaded sample on the fatigue testing machine are analyzed. After the loading test, results of XRD and TEM show that the volume fraction of retained austenite drops to 4.6%. Meanwhile, part of the retained austenite in bainite ferrite lathes transformed into twin martensite. The tensile strength is 2000 MPa with the elongation of 8.86%. This illustrates that the mechanical property of the steel is improved obviously by the loading stress owning to the transformation induced plasticity (TRIP) effect of retrained austenite in super-bainite microstructure. The lack of chemical driving force of the transformation of retained austenite is made up by mechanical force, so retrained austenite transformed at room temperature. TRIP effect is influenced by the volume fraction and stability of retained austenite at different isothermal temperature and time. Simultaneously, the loading force, the loading time and the loading way of sample also have certain influence to TRIP effect.