The bake hardening values ( BH) of ULC-BH steel 1 mm annealed cold rolled sheet (/%： 0. 002C

0. 008Si

0. 60Mn

0. 043P

0. 009S

0. 031 Al

0.017Ti) with prestraining by 2% ~ 10% and baking al 170 °C for 20 min have been measured

and the effect of Cottrell atmosphere on bake hardening property of ULC-BH sleet is studied by internal friction test getting internal friction spectrum curves at different strain ageing and using X-ray diffraction technique measuring dislocation density. Results show that with increasing prestrain the BH value of ULC-BH steel increases ； and with increasing prestrain the Snoek peak gradually reduces while SKK peak gradually raise； with prestraining by 2%

6% and 10% the dislocation density increases

and that respectively are 3. 9 x 10

10

/cm

2

8. 4 x 10

10

/cm

2

and 6. 8 x 10

11

/cm

2

while corresponding Cottrell atmosphere density gradually decreases.