Tempering Behavior of Ductile 1700 MPa Mn-Si-Cr-C Steel Treated by Quenching and Partitioning Process Incorporating Bainite Formation

We obtained a good combination of strength and ductility in a 0.4C-2.0Mn-1.7Si-0.4Cr(wt%) steel,namely,~1.7 GPa of ultimate tensile strength and ~26% of elongation,by conducting a Q-P-T(quenching-partitioning- tempering) process incorporating the formation of carbide-free bainite. The tempering behavior of this steel was discussed by using experimental finding(scanning electron microscopy,X-ray diffraction(XRD),transmission electron microscopy and dilatometer) and CCE(constrained carbon equilibrium) modeling. The XRD results combined with CCE calculation prove that carbon partitioning from martensite to austenite occurs during tempering. Consequently,the thermodynamic stability of retained austenite is enhanced. This idea can be utilized to design novel Q-P-T processes in future.