Effect of Nb microalloying on microstructure evolution and mechanical properties in low carbon medium manganese steel

摘要

The effect of Nb microalloying on microstructure evolution and mechanical properties of 0.02C-7Mn steel was investigated. The results showed that the microstructure was fully quenched martensite after subjected to reheating and quenching (RQ) process. The microstructure consisted of tempered a-martensite, reversed austenite (RA), and athermal ε-martensite after tempered at 620 °C, while the microstructure contained tempered a-martensite, RA, athermal e-martensite, and athermal α-martensite after tempered at 650 °C. The thermal and mechanical stability of both RA and athermal e-martensite decreased after the 0.05% Nb was added to the 0.02C-7Mn steel, and also decreased as the intercritical tempering (IT) temperature increased, which was mainly because the content of solid solution C in RA decreased. The calculated stacking fault energies (SFEs) were all negative, which spontaneously induced athermal martensitic transformation. Multiphase microstructure resulted in the remarkable mechanical property evolution. After tempered at 620 °C, the optical mechanical properties of 7Mn-0.05Nb steel were obtained: yield strength of 720.0 MPa, tensile strength of 819.0 MPa, total elongation of 26.0% and -40 °C impact energy of 266 J, while the yield strength of 675.0 MPa, tensile strength of 776.5 MPa, the total elongation of 29.5% and the -40 ~C impact energy of 274 J appeared in 7Mn-0Nb steel. As the IT temperature increased to 650 °C, the yield strength of both the steels sharply decreased (475.5 MPa for 650 °C-0Nb steel and 460.5 MPa for 650 =C-0.05Nb steel), and the tensile strength of both the steels increased about 56 MPa, while the -40 °C impact energy decreased by 40 and 60 J, respectively, showing that the -40 °C impact energy was not very sensitive to the mechanical stability of RA and athermal e-martensite. The mean size of NbC particles was mainly about 5-7 nm, which significantly improved the precipitation strengthening, but there was just little damage to low-temperature toughness.