Microstructure and properties of ultra-high strength cold-rolled dual phase steel

The continuous annealing process of ultra-high strength cold-rolled dual phase steels was simulated using a Gleeble-3500 thermal simulator. The effects of continuous annealing parameters on the microstructure and mechanical properties of the 1000 MPa grade dual phase steel were investigated by optical microscope, scanning electron microscope, transmission electron microscope and tensile tests. It is shown that a dual phase (DP) steel with a tensile strength of 1030 MPa and a total elongation of 14% was obtained after being soaked at 800°C for 60 s. With the increase of annealing temperature, the yield strength and tensile strength decrease. When the annealing temperature reaches 830°C, the volume fraction of granular non-martensite in the microstructure increases obviously. When the over-aging temperature is lower than 300°C, the yield strength and the tensile strength change slightly. When the over-aging temperature is higher than 300°C, the tensile strength decreases strongly, whereas the yield strength reduces first and then increases. There is yield point elongation when the over-aging temperature is 360°C.