The effects of overageing （OA） and vanadium addition on microstructure and properties of cold-rolled lowcarbon Si-Mn dual phase （DP） steel sheets are studied. The results show that the microstructure and mechanical properties of DP steels are greatly affected by OA temperature. When OA is done at low temperatures （473 -- 573 K）, the elongation rate increases sharply while both the yield strength and tensile strength decrease significantly, When OA is done at high temperatures （〉573 K）, the elongation does not further increase, but the yield strength increases and tensile strength decreases, which deteriorates the mechanical properties of the steel. The results also show that both the strength and the tempering stability can be significantly improved by vanadium microalloying. Finally, the optimum OA temperature for the traditional steel appears to be around 523 K, while it is around 573 K for the V-containing steel, resulting in the best overall mechanical properties.
Tensile tests of TWIP steels of two compositions are performed in the strain rate range of 10^-5-10^3 s^-1. Results indicate that steel 1^# does not exhibit TWIP effect but deformation-induced martensitic transformation appears only. There exists TWIP effect in steel 3^# . Tensile properties at room temperature are sensitive to strain rate in the studied strain rate ranges. Analysis on the relationship between strain-hardening exponent and strain rates shows that strain induced martensitic transformation and formation of twins during deformation have significant in- fluence on their strain-hardening behavior.
XIONG Rong-gang FU Ren-yu SU Yu LI Qian WEI Xi-cheng LI Lin
