The continuous cooling transformation (CCT) curves of 18 mm plate of X80 pipeline steel (% : 0. 05C

0. 17Si

1.78Mn

0. 40Mo

0.08Nb

0.03V

0. 03Ti) with 1 s

-1

and 40% deforming at 1 100°C

then 5 T/s cooling to 850 °C

1 s

-1

and 40% deforming and 1 ~40 °C/s cooling to ambient temperature were determined by Gleeble-1500 ther- mo-simulition machine and the effect of cooling rate on structure and HV hardness of steel was studied. Results show that with increasing cooling rate

the grains fine

the proportion of acicular ferrite in structure increases and the HV hardness of structure also increases. In order to get better structure

the cooling rate of hot-deformed steel should be ≥15 °C/s.