The continuous cooling transformation (CCT) curves of Φ406.4 mm x 16.5 mm X70 seamless steel tube for deep-sea pipe-line (/% : 0. 07C

0. 70Si

0. 80Mn

0.008P

0. 002S

0. 25Ni

0. 25Cr

0. 20Mo

0. 10Cu

0. 06V

0. 03Nb

0. 04Alt

0.006 ON) have been measured by using Gleeble-3500 thermo-mechanical simulator

and the structure feature of the steel tube with cooling rate 0. 1 ~ 100 ℃/s is analyzed by optical microscope

scanning electron microscope and HV hardness meter. Results show that with cooling rate 0.15℃ /s the main structure in steel is ferrite + pearlite

as cooling rate more than 5 ℃/s the structure in steel is bainite + ferrite ； with increasing cooling rate the structure of steel is fined and the hardness of steel increases ； with cooling rate 30 〜50 °C /s the suitable amount of lath bainite structure is obtained therefore to insure getting excellent mechanical properties of pipe-line steel.