Effects of prestrain and grain boundary segregation of impurity atoms on bake hardening behaviors of Ti+V-bearing ultra-low carbon bake hardening steel

The distribution pattern of solute atoms, interaction between C atoms and dislocations, and bake hardening behaviors of the P-added and P-free ultra-low carbon bake hardening (ULC-BH) steels were investigated, which were annealed at 810 °C for 2 min and subsequently treated with prestrains and baking. The three-dimensional atomic probe (3DAP) and internal friction results revealed that C atoms were homogeneously distributed in the matrix of the P-added ULC-BH steel, but they were strongly segregated in the P-free ULC-BH steel after annealing at 810 °C for 2 min. The solute C concentration of the P-added ULC-BH steel was higher than that of the P-free ULC-BH steel. The addition of P greatly strengthened the BH behavior of the ULC-BH steel. The variation in the BH behavior with the prestrain was apparently different between the P-added and P-free ULC-BH steels. The BH value of the P-added ULC-BH steel gradually increased with an increase in the prestrain ranging from 1% to 10%. However, the value of the P-free ULC-BH steel increased by increasing the prestrain up to 8% and decreased subsequently. The BH behavior showed higher sensitivity to the solute C concentration for the ULC-BH steel than for the low carbon (LC) steel. The BH value either increased or decreased with an increase in the prestrain, which mainly depended on the solute C concentration, i.e., whether the solute C concentration was sufficient or not to saturate the dislocation generated by the prestrain during the baking process.