The continuous cooling transformation (CCT) curves of S690 steel tube (/% : 0.15C

0.25Si

1.32Mn

0.012P

0. 005S

0.20Cr

0.04Al

0.08V) with cooling rate 0. 01 ~30℃/s are measured and the effect of quenching at 890 〜970 ℃ and tempering at 600 〜690℃ on structure and mechanical properties of the steel has been studied. Results show that the phase transformation points of steel S690 seamless tube are Ac

3

828 °C

Ac

1

753 ℃

Ms 395 ℃

and the critical cooling rate of steel is 13 ℃/s

and there are ferrite

pearlite

bainite and martensite four transformation zone； with selecting quenching at 910 ~930 ℃

the structure of steel tube is uniform and fine with average grain size 13.9μm

and the steel has higher hardness with HRC value 42.5 ；with tempering at 615 - 675 ℃，the steel tube has uniform

tempered sorbite structure and better comprehensive mechanical properties.