The effect of temper rolling (0~3.3%) and natural aging on mechanical properties and bake-hardening performance of 0.7 mm ultra-low carbon bake-hardening steels plate (%:0.0030C

0.008Nb-0.003Ti

0.0030N and 0.0030C

0.012Nb-0.012Ti

0.0042N) annealed at 830 ℃ has been studied. Results show that at beginning with in- creasing temper elongation

due to decrease of Cottrell atmosphere and increase of dislocation density

the yield strength of steels decreases

as the temper elongation of 0.008Nb-0.003Ti steel and 0.012Nb-0.012Ti steel increases respectively up to 0.48% and 0.26%

the yield strength of steels decreases to minimum; then with further increasing temper elongation

due to enhancement of cold work hardening

the yield strength of steel increases. The temper elongation too low and too high lead up to decrease of hake hardening performance. To control temper elongation 0.5%~1.5% is available to get lower yield strength

higher tensile fracture elongation and excellent bake hardening performance of steel. With naturalizing for 3 months

the bake hardening performance and the elongation of steel plate degrade.