Influence Of Bake Hardened Ability For Low-Carbon Steels Under Annealing And Baking States

This paper focus on Bake Hardened ability for low carbon steels deeply, explore the formation of Cottrell atmosphere at initial aging stage, precipitation of carbide after baking a long time and Bake Hardened ability under annealing state. BH value, stress-strain curves and internal friction spectrums have been measured to reveal bake hardened mechanism from atomic level. The main results are as follows:(1) Under 2% prestrain, samples bake at 100 ℃, 170 ℃, 200 ℃ and 250 for ℃0.5 min, 1 min, 2 min, 5 min, 8 min, 10 min, 15 min, 20 min, 50 min, 100 min and 200 min respectively, found that BH value increase gradually with the increase of baking time at 100 ℃, 170 ℃ and 200 ℃. On the contrary, BH value increase at first and then reduce at 250 ℃. Discontinuous yielding phenomenon emerges earlier with the increase of baking temperature and serrated yielding appear. Bake Hardening mechanism at 100 ℃, 170 and 200 is ℃ ℃ Cottrell atmosphere strengthening, but at 250 is ℃ Cottrell atmosphere strengthening and precipitation strengthening together.(2) Under 2%, 4% and 6% prestrain respectively, samples based on baking rate of 3 ℃/min bake from 15 ℃ to 400 ℃ and then hold for 10 min in internal friction furnace. Repeat above operation for 5 times, found that with the increase of baking times, BH value decrease at the same prestrain. It is illustrated that Bake Hardening phenomenon appears. BH value decrease with the increase of prestrain at the same baking times. Serrated yielding phenomenon becomes unobvious with the increase of baking times at the same prestrain and length of YPE become short. Yielding platform become higher and shorter with the increase of prestrain at the same baking times. Bake Hardened mechanism is Cottrell atmosphere strengthening and precipitation strengthening together with the increase of baking times at the same prestrain.(3) The sample annealed at 750 ℃ and 780 ℃ and then holding for 2 min, 5 min and 20 min and cooling in the water, the air and the furnace respectively, found that BH value are negative under different annealing process, it is said that Baking Softening phenomenon appear after prestrain and baking. BH value decrease with the increase of annealing time at the same annealing temperature and method. In addition, with the increase of cooling speed, BH value decrease rapidly and then increase slightly. Under different annealing process, discontinuous yielding phenomenon occur, YPE become short with the increase of annealing time at the same annealing temperature and cooling speed. Uniform elongation rate decrease with the increase of cooling speed at the same annealing temperature and time. Bake hardened mechanism under different annealing process is solid solution strengthening.