Microstructure and mechanical properties of dual phase strip steel in the overaging process of continuous annealing

Microstructure and properties of DP590 steel in overaging process of continuous annealing were investigated in the laboratory. Effects of the overaging temperature (OT) in conventional process (Process A) and reheated overaging temperature (ROT) of the proposed process (Process B) on microstructure and mechanical properties were analyzed. The results showed that, in process A, with the increase of the OT from 280 °C to 400 °C, the amount of granular retained austenite increases, the tensile strength decreases, the yield strength and elongation both increase. In process B, with the increase of the ROT from 310 °C to 400 °C, the tensile strength decreases, whereas the yield strength changes little. Compared with the conventional process, the tensile strength of the experimental steel increases obviously and the ratio of yield-to-tensile strength decreases in process B. The variation of instantaneous work hardening index (n⁎) with tensile strain can be divided into three stages. The increase in the OT or ROT leads to the continuous increase of n⁎ in the third stage.