INTERNAL FRICTION STUDY OF MECHANISM OF BAKE-HARDENING ON 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(BH) is closely related to interactions between interstitial atoms and crystal defects during baking process. In this work, BH mechanisms in low carbon steel are studied under different annealing temperatures by measuring and analyzing stress- strain curves and BH values in baked conditions, and internal friction curves in both deformed and baked conditions. The results show that when the annealing temperature increases gradually from 750 to 880 ℃, the stress-strain curves 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 Ke peak value does not change significantly, which indicates that solid solution strengthening dominates the BH. 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 Ke peak is stable, which indicates that Cottrell atmosphere strengthening is playing an increasing role in the BH. The BH is due to a combined mechanism contributed by solid solution strengthening Cottrell atmosphere strengthening and precipitation strengthening.