The continuous cooling transformation (CCT) curves of round casting bloom of pipeline steel L360NB (/%: 0. 14C

0. 32Si

0. 14Mn

0. 020Ti

0. 010V

0. 038Nb

0. 040Al) has been measured by a Gleeble-1500 thermal simulation machine

and the structure of steel with cooling rate 0. 1 〜50 °C/s is observed by optical microscope. Results show that with cooling rate 0.1 °C the main structure of steel is lath bainite + quasi-polygonal ferrite with minor acicular ferrite

with cooling rate ≥l.0°C/s the quasi-polygonal ferrite decreases and the lath bainite and acicular ferrite increase

as cooling rate ≥10°C /s the main structure in specimen is lath bainite + acicular ferrite with minor quasi-polygonal ferrite

as 50 °C/s the block quasi-polygonal ferrite is not observed in specimen

it is indicated the Nb-V-Ti compound alloying promotes the formation of bainite.