Effect of Heating Rate during Continuous Annealing on Microstructure and Mechanical Properties of High-Strength Dual-Phase Steel

Abstract

The study concerns the influence of heating rate during continuous annealing on microstructure and mechanical properties of high-strength ferrite–martensite dual-phase (DP) steel. The study suggests that the superheat (ΔT), carbon diffusion and distribution were strongly influenced by increasing the heating rate, leading to different microstructure and mechanical properties of DP steel. The morphology of martensite was observed to shift from network along ferrite grain boundaries to banded structure to uniformly dispersed, and the average ferrite grain size was refined from 5.3 to 2.0 µm. When the heating rate was 300 °C/s, the steel exhibited excellent mechanical properties with tensile strength (UTS) of 982 MPa, total elongation (TEL) of 17.5%, strength–elongation balance (UTS × TEL) of 17.2 GPa%, because of finer microstructure, uniform martensite distribution and low carbon content of martensite. Moreover, the precipitation strengthening effect of NbC is an important reason for the high-strength DP steels. In addition, the variation in strain-hardening behavior and fracture mechanism of steel subjected to different heating rates is discussed in relation to the microstructure.