Internal Friction Study On Effect Of Annealing Process And Pre-deformation On The Bake-hardening Properties In Low Carbon Steel

High strength steel plates are being applied more extensively in automobile industry under higher demands of weight reduction, safety, and environmental protection. As one of the high strength steel plates for automobile body panel, bake-hardening steel plate is featured by low yield strength and good formability during stamping, and is particularly featured by improved yield strength through following paint baking process, exhibiting higher strength and anti-dent ability in service. Bake hardening is closely related to interactions between interstitial atoms and crystal defects during baking process. In this study, bake-hardening mechanisms in low carbon steel are studied under different annealing temperatures, different cooling methods and different deformations by measuring and analyzing stress-strain curves and bake-hardening values(BH value) in baked conditions, and internal friction curves in both deformed and baked conditions. The results show:(1) When the annealing temperature increases gradually from 750 to 880 ℃, the stress strain curve exhibit discontinuous yielding behavior with stronger serration of yield platform and continuous elongation of yield point. With the increase of the annealing temperature from 750 to 780 ℃, BH value decreases; difference between Snoek peak values in deformed and baked conditions increases; SKK peak value decreases; and Kê peak value does not change significantly, which indicates that solid solution strengthening dominates the bake hardening. When the annealing temperature increases from 780 to 880 ℃, BH value continuously increases; difference between Snoek peak values in deformed and baked conditions gradually decreases; relaxation strength of SKK peak gradually increases; and Kê peak is stable, which indicates that Cottrell atmosphere strengthening is playing an increasing role in the bake hardening. After baking, the amount of C atoms segregation at the grain boundary increases in low carbon steel by different annealing temperature treatment.(2) The stress-strain curves in 3 cooling methods after baking show discontinuous yielding phenomenon with stronger serration of yield platform and continuous elongation of yield point. Serrated yield platform and yield point elongation of air cooling sample is the most obvious. Plasticity of air cooling sample is the best and plasticity of water-cooling sample is the worst, plasticity of furnace cooling sample is between water and air.BH value of furnace cooling sample is positive. The mechanism combined contributed by precipitation strengthening and solid solution strengthening, precipitation strengthening is the leading role; BH value of air cooling and water cooling sample are negative. Comparison between air cooling and water cooling, the difference of Snoek peak height between deformation state and baking state is smaller and the relaxation strength of SKK peak is weaker. The Cottrell atmosphere strengthening has a weak effect on bake hardening.(3) Coarse grain bake hardening properties of low carbon steel(BH) is negative. Deformation in 2%~5%, BH value increases with the increasing of deformation, deformation in 5%~10%, the value of BH decreases with increasing the deformation, when the pre deformation is 5% when the BH value reached the maximum value.(4) In the experiment of baking time, with the increase of baking time, BH values decrease gradually.