Effect Of The Heat Treatment Process In The Bainitic Region On The Microstructure And Mechanical Properties Of Low Carbon Si-Mn TRIP Steel

TRIP(Transformation Induced Plasticity)steel is the most potential automobile steel at present. Because of Transformation Induced Plasticity Effect, TRIP steel has the high strength and high ductility at the same time. The low carbon Si-Mn TRIP steel which is studied in the paper, its tensile strength 500MPa-700MPa, elongation rate at around 30%, can be used for automobile, petroleum exploration fields.The low carbon Si-Mn TRIP steel which is usually a multi-phase steel, made up of ferrite(50%～60%), bainite(25%～40%), retained austenite(5%～15%) and a small amount of martensite. The effect of Transformation Induced Plasticity is come from the transformation of retained austenite to martensite, aimed at strengthening and toughening the steel.In the paper, different microstructures are obtained by using the different heat treatments, and the microstructures are examined and analyzed by the optical microscope (OM), scanning electron microscope (SEM) and also the transmission electron microscope (TEM). The experimental TRIP steels after the heat treatments are tested by tensile test. The mainly factor which influence the mechanical properties of low carbon Si-Mn TRIP steel is mechanical stability of the retained austenite,which is affected by the carbon content of retained austenite. In this paper, the volume fraction and carbon content of retained austenite is investigated by means of X-ray diffraction (XRD). And, using high frequency resistance welding (ERW), the TRIP steel is welded into the expansion tube. This paper focuses on the microstructure of the welding zone. The welding zone tempered at different temperatures was observed and the hardness was tested. The effect of tempering on the structure and the mechanical properties of the welding zones are also analyzed in the end.