Abstract: Microstructure transformation of a low yield ratio 890 MPa grade high strength steel during intercritical QT (quenching and tempering) treatment and its effect on mechanical properties were investigated. The heat-treated microstructure was observed by scanning electron microscope, and the effect of tempering temperature on microstructure and properties was analyzed via mechanical properties and XRD dislocation density calculation. The effect of quenching temperature on microstructure and properties was further investigated by Gleeble thermal simulation experiment. The results show that the intercritical QT treated microstructure is ferrite and tempered lath structure. With the increase of tempering temperature, the lath structure gradually decomposes, and the dislocation strengthening increment decreases, which lead to the decrease of tensile strength. The yield strength increases firstly under the coordination deformation effect of soft and hard phases, and then decreases with the decrease of overall dislocation density. The yield strength changes a little leading to the increase of yield ratio. With the increase of quenching temperature, the dislocation strengthening increment increases and the tensile strength increases, the dilation change during phase transition increases, the proportion of soft phase ferrite decreases, which leads to the increase of yield strength and yield ratio. The increasing proportion of hard phase tempered lath structure makes the low-temperature toughness decrease as the quenching temperature rises.

Key words: low alloy high strength steel, low yield ratio, intercritical QT treatment, dislocation density, thermal simulation experiment