Effects of Heat-treatment Processes on Microstructures and Properties of a 1 000 MPa Grade Vanadium-alloyed High Strength Steel

The 0.24%C-1.5%Si-2.0%Mn-0.16%V cold-rolled steel was heat treated at 780 ℃ intercritical annealing for 180 s,then quenching into 350～380 ℃ for 360 s,or quenching into 380 ℃ for holding 60～1 200 s.The microstructures and properties after heat treatments were tested by tension test,OM,SEM and XRD analysis.The results show that the good comprehensive property with tensile strength of 1 029 MPa,product of strength and elongation of 20.1 GPa·%,work hardening index of 0.22,is obtained by quenching into 380 ℃ for 360 s.The higher or lower quenching temperature,the higher tensile strength and the lower elongation are.As the holding time prolongs to 1 200 s,the strength and elongation are not varying much,but the yield platform appears.The volume fraction of retained austenite（RA） can be greatly affected by equal quenching temperature and time.The volume fraction of RA increases from 2.58% to 3.86% as isothermal the temperature increasing from 350 ℃ to 410 ℃,and then rapidly decreases in higher temperatures since martensite transformation is hindered and bainite transformation appears.And the RA maintains about 3.5% as the carbon has been enriched in austenite under isothermal quenching for 180 s.