BAKE HARDENING OF A LOW CARBON STEEL FOR AUTOMOTIVE APPLICATIONS

Bake hardening is an advanced processing technique to produce low carbon steels, used for car bodies, with high strength. An optimized batch annealing treatment is necessary in order to have enough carbon in solution required for bake hardening. This makes automotive bodies and panels strengthened after paint baking treatment. This effect is due to pinning of dislocations by solute carbon atoms which refers to cottrell's atmosphere. In the present work, the influence of batch annealing parameters (annealing temperature and time and cooling rate) and the amount of prestrain as well as baking temperature on bake hardenability of St14 steel was investigated. Various annealing treatments were performed at temperatures of 680, 730 and 780 degrees C with the soaking times of 10, 20, and 30 minutes. Employed cooling media were furnace, still air and water. Aging index tests, 8 percent tensile prestrain and one hour baking at 100 degrees C, were carried out for assessing the bake hardenability of annealed specimens. After employing the optimum annealing cycle (780 degrees C for 30 minutes), the samples were subjected to 4, 6 and 8 percent prestrain followed by baking at temperatures of 150, 180 and 210 degrees C for 20 minutes. The results show that the highest hardenability will be obtained for the samples annealed at the highest time, temperature and cooling rates. Besides, the higher the prestrain and baking temperature, the higher the increment of strength due to bake hardening.