Low temperature microstructure and mechanical properties of a low carbon alloy steel by quenching-partitioning-tempering process

Fe-0.25C-1.5Mn-1.2Si-1.5Ni-0.05Nb(mass fraction,%) steel subjected to a novel heat treatment process designated as quenching-partitioning-tempering(Q-P-T) process exhibits high strength(over 1250 MPa) and substantial ductility(elongation over 17%),for which the microstructure consists of lath martensite,dispersive distributed carbides and thin film-like retained austenite.The mechanical properties of this Q-P-T steel at-85-25 ℃ were investigated by tensile tests,and the microstructure of the samples before and after tensile tests at 25 ℃ and-85 ℃ were characterized by transmission electron microscopy(TEM),respectively.The results show that Q-P-T steel possesses excellent mechanical properties at-70 ℃ to 25 ℃,while the elongation decreases obviously when the temperature drops below-70 ℃.The microstructure characterization indicates that retained austenite,as the soft phase in Q-P-T steels,presents good stability at low temperatures before plastic deformation.However,it will transform into martensite during plastic deformation,showing the transformation induced plasticity(TRIP) effect,which has been considered as the crucial role on the excellent combination of strength and ductility.