The Effect of Stress on Ferrite Transformation During the Continuous Annealing of DP780 High-Strength Steel

Continuous cooling transformation curves for DP780 high-strength steel were drawn by combining measured thermal expansion curves, microstructures and hardness values. Annealing experiments for DP780 highstrength steel under three different stresses of 0 MPa, 10 MPa and 20 MPa were carried out using Gleeble 3500, and a transformation kinetics model of austenite to ferrite was established by considering the effect of stress at low cooling rates. Then, based on the finite element platform and secondary development, the continuous annealing process of DP780 high-strength steel under various stress states was simulated. The results show that the applied stress has significant effects on the ferrite transformation of DP780 high-strength steel. With increasing tensile stress, the transformation plasticity of ferrite becomes more obvious. The simulation results coincide with the measurement. It is thus proven that the numerical model can correctly reflect the deformation and transformation laws during the heat-treatment process and lay a foundation for further numerical simulations of the heat-treatment process of high-strength steel.