The effect of finishing temperature (800 ~850 °C ) and cooling rate (2 〜20 ℃/s) on structure and mechanical properties of microalloying steel Q55OD plate (/% ： 0. 06C

0. 20Si

I. 60Mn

0. 010P

0. 001S

0. 10Mo

0. 06Nb

0. 01V

0. 02Ti) has been studied by using thermo-mechanical simulator Gleeble-3800. Results show that with decreasing finishing temperature and increasing cooling rate after rolling

the granular bainite in steel gradually decreases

and the lath bainite gradually increases

as a results the yield and tensile strength of steel increasing trend is obvious and the - 20 ℃ toughness is improved but the elongation of steel is decreasing trend ； with finishing temperature 850 °C and cooling rate 15 - 20 ℃/s the steel Q550D has better strength-toughness comprehensive properties

i. e. tensile strength is about 750 MPa

yield strength 650 MPa

elongation 39% and -20 ℃ impact energy 65 J.