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 1 000 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 1 030 MPa and a total elongation of 14% was obtained after being soaked at 800 qC for 60 s. With the increase of annealing temperature, the yield strength and tensile strength decrease. When the annealing temperature reaches 830℃, the volume fraction of granular non-martensite in the microstructure increases obviously. When the over-aging temperature is lower than 300℃, the yield strength and the tensile strength change slightly. When the over-aging tempe'rature is higher than 300℃, 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℃.