Mechanical properties and microstructure of continuous annealing TRIP steel without Si

A kind of TRIP steel without Si was heat-treated on a Gleeble3500 thermal simulation testing machine. Its mechanical properties,microstructure,and phase transformation were studied by tensile machine,scanning electron microscope(SEM),transmission electron microscope(TEM),X-ray diffraction(XRD),and thermal dilatometer.The effects of bainite transformation temperature and time on the mechanical properties and retained austenite in the Si-free TRIP steel were analyzed.It is found that the Si-free TRIP steel has excellent overall mechanical properties with the tensile strength of 740 to 810 MPa and the elongation above 25%,which can reach up to 32%.The best comprehensive mechanical properties can be achieved when the isothermal bainite transformation temperature is 420℃.At this temperature,the tensile strength decreases and the elongation increases with the increase of bainite transformation time, but the yield strength has no significant change.The size of ferrite grains in the Si-free TRIP steel is 3 to 4μm,smaller than that in the Si-containing TRIP steel.The volume fraction of retained austenite is 8%to 10%,about 3%lower than that in the Si-containing TRIP steel.Bainite transitions in the Si-free TRIP steel almost complete at 420℃for 300 s, but the diffusion of C still continues.Compared to the Si-containing TRIP steel,bainite transitions in the Si-free TRIP steel are much faster and the bainite transformation amount is bigger.