The relationship between microstructures and mechanical properties of vanadium microalloyed cold rolled ultrahigh strength steel treated by austempering

Abstract

Two experimental steels with chemical composition of 0.24%C-1.5%Si-1.90%Mn (0 V steel) and 0.24%C-1.52%Si-1.83%Mn-0.16%V (0.16 V steel), were annealed at intercritical temperature of 780 °C for 180 s, followed by austempering at temperatures from 350 °C to 480 °C for 360 s or austempering at 380 °C for 60 s to 1200 s. After the heat treatments, their microstructures and mechanical properties were investigated. The results show that the microstructures are mainly composed of ferrite, bainite or blocky martensite, and M/A islands. The difference between the 0 V steel and 0.16 V steel at the same austempering process is that the M/A islands are refined and the bainite seems to be retarded by vanadium alloying. The tensile strength of 0.16 V steel is about 100 MPa higher than that of 0 V steel austempering at 380 °C for its much more blocky martensitevolume fraction and VC precipitation, and its yield ratio can be adjusted by austempering for different times, as maintained the tensile strength above 1000 MPa and the total elongation about 19%. The better work hardening behavior with nincr.-value of 0.24in the 0.16 V steel was observed under austempering at 380 °C, which is contributed to VC precipitation and retained austenite stabilities.