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The microstructure of steels treated by Q&P(quenching and partitioning) process was characterized,a method of controlling retained austenite fraction based on inhomogeneous martensitic transformation was proposed,and the mechanical properties of steels treated by Q&P process were measured.The results show that the microstructure of the studied steels is mainly composed of initial martensite,fresh martensite and retained austenite.The initial marteniste formed at the first quenching step is easily etched;the fresh martensite formed at the final quenching step looks like ’blocky’ type phase,and the retained austenite is mainly located on the packet boundary and initial austenite grain boundary.The inhomogeneous microstructure causes the experimental optimum quenching temperature corresponding to maximum retained austenite fraction to be higher than the calculation based on CPE(constrained paraequilibrium) model.The product of tensile strength and total tensile elongation is 47.5 GPa%,and tensile strength of 1760 MPa was obtained for the steel with carbon content of 0.51 wt%.The TRIP(transformation induced plasticity) effects of the large fractioned metastable austenite make a main contribution to the high ductility improvement,and the martensitic matrix provides high strength.
The microstructure of steels treated by Q & P (quenching and partitioning) process was characterized, a method of controlled austenite fraction based on inhomogeneous martensitic transformation was proposed, and the mechanical properties of steels treated by Q & P process were measured. Results show that the microstructure of the studied steels is mainly composed of initial martensite, fresh martensite and retained austenite.The initial marteniste formed at the first quenching step is easy etched; the fresh martensite formed at the final quenching step looks like ’blocky’ type phase, and the retained austenite is mainly located on the packet boundary and initial austenite grain boundary. the inhomogeneous microstructure causes the experimental optimum quenching temperature corresponding to currently retained austenite fraction to be higher than the calculation based on CPE (constrained paraequilibrium) model. The product of tensile strength and total tensile elongation is 47.5 GPa%, and The tensile strength of 1760 MPa was obtained for the steel with carbon content of 0.51 wt%. The TRIP (transformation induced plasticity) effects of the large fractioned metastable austenite make a main contribution to the high ductility improvement, and the martensitic matrix provides high strength.