Improving the Mechanical Properties of Prestrained CP800 Steel by Annealing-Induced Carbides

Different degrees of strain occur during the manufacturing of CP800 torsion beams, leading to a decrease in mechanical properties and the formation of residual tensile stress in local regions. Thus, the performance and fatigue resistance of torsion beams are negatively affected. Prestrained CP800 steel samples with tensile strains of 0 or 7% are annealed from 400 to 700 °C in the present study. The mechanical properties of the above samples are measured by tensile testing, and the microstructure is characterized by scanning electron microscopy, transmission electron microscopy, x-ray stress meter and atom probe tomography. The results show that after proper annealing at 575-650 °C, the product of the strength and elongation of the CP800 samples increases to 14.2-16.1 GPa%, which is higher than that of the original hot-rolled sheet (13.3 GPa%). This improvement is attributed to the change in strengthening mechanisms. During the proper annealing process, a large number of dispersive carbides emerge in the matrix of the prestrained samples. The precipitation strengthening which has less damage to ductility has been introduced into the matrix, while the solution strengthening is weakened.