The continuous cooling transformation ( CCT) curve

microstructure and mechanical properties of Q500qE low yield ratio high strength steel 60 mm thickness plate have been studied by JMatpro software

scanning electron microscope and mechanical property tests. The results show that by control rolling and control cooling process: the final rolling temperature 800-840 °C

the into water temperature 660-680 X. and the final cooling temperature 400-450 °C

the microstnicture of steel is composed of ferrite

bainite and MA( martenite/austenite) island

and many large angle grain boundaries exist in junction of two phases and in bainite. The properties of the steel at 1/4 and 1/2 thickness are yield ratio 0.78-0.79

yield strength ≥500 MPa

tensile strength ≥640 MPa

charpy impact energy of 1/2 thickness at -40 °C ≥ 200 J

for HAZ charpy impact test at -40 °C

the charpy impact energy of HAZ ≥100 J.