Thermodynamic analysis and strength-toughness research of nanobainite

In view of slow phase transformation rate in high-carbon,silicon-rich and low-alloy bainite steel(nanostructured bainite steel),the effects of alloy elements on the overall kinetics of low temperature bainite phase transformation were analyzed by using the thermodynamic theory of bainite phase transformation,and a composition of 0.83C-2.44Si-0.43Mn-0.73Al(mass fraction,%) was designed to develop a novel nanostructured bainitic steel. The overall phase transformation kinetics of this new designed nanobainitic steel at different temperatures were investigated by dilatometry. The effects of heat treatment parameters on the microstructure and mechanical properties were analyzed by scanning electron microscopy,X-ray diffraction and electron backscatter diffraction. It is found that the nanobainitic steel,which exhibited excellent mechanical properties with an ultimate tensile strength of 1401 MPa,a total elongation rate of 42.21% and a strength and ductility product of 59136 MPa·%,can be obtained by austempering at 350℃. In addition,transformation-induced plasticity(TRIP) effect appears during tensile testing at room temperature. When austempered at 230℃,the nanobainitic steel has a finer microstructure with the thickness of bainite ferrite laths less than 100 nm,and its tensile strength can reach up to 2169 MPa.