The mechanical properties and microstructure of 1,000 MPa grade Cu-Ni TRIP steel at different bainitic isothermal temperatures

For developing TRIP steel with good coordination on strength and plasticity, a TRIP steels containing copper and nickel at different bainite isothermal temperatures (350-470°C) were investigated to explore the best process. The microstructure of the tested steel was investigated by X-ray diffraction, transmission electron microscope, scanning electron microscope, and electron backscatter diffraction, as well as its mechanical properties were tested and analysed by uniaxial tensile test. It can conclude that when holding bainitic isothermal temperature at 410°C, the steel obtained the excellent mechanical properties, e.g., the elongation is 16.88%, the strength of extension is 1,088 MPa and the strength and ductility product is 18,365 MPa%. The crystallite dimension and carbon level of retained austenite play an important role in its stability. Moreover, in terms of morphology, the film-like austenite makes an important impact in the gentle transition during deformation and the stability of retained austenite is affected by its film thickness.