The microstructure and properties of high performance steels with low yield-to-tensile ratio

► We have produced a 600MPa grade steel with yield ratio lower than 0.8. ► We have found the controlling method for remained austenite by high temperature tempering. ► The amount of remained austenite is the key effect to influence the yield ratio of steels. The microstructure and properties of 600-800MPa grade steels produced by TMCP and relaxation–precipitation-controlled phase transformations (RPC) has been studied. The influence of tempering process on the yield-to-tensile ratio of steels has been investigated by means of optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The results show that after the RPC processing the microstructures of steels are mainly composed of bainite. The bainite has been refined markedly after the relaxation processing, therefore the properties of steels has been improved evidently. In order to decrease the yield-to-tensile ratio the steels underwent high temperature tempering. It has been found that increasing tempering temperature the yield-to-tensile ratio of steels decreased. The XRD and TEM results show that the amount of the retained austenite increases when the tempering temperature is higher than 680°C. The more the amount of the retained austenite is and the less the yield-to-tensile ratio of steels is.