The structure and mechanical properties of 15 mm plate of bridge steel Q690q (/% : 0. 05C

0. 30Si

1. 40Mn

1. lOCu

0. 50Cr

0. 80Ni

0. 07V

0. 55Mo

weld cold-cracking sensitivity index P

cm

≤0. 267) processed by both technology i. e. the thermal-mechanical control rolling process (TMCP) - finishing rolling at 920 ℃

cooling at finishing temperature 830 T with 12℃ /s to 590 Y

air cooling

and the process of TMCP + quenching at 940 ℃

tempering at 630 ℃. have been studied. Results show that the main structure of bridge steel Q690q produced by TMCP process consists of granular bainite and minor ferrite

and the structure of steel Q690q produced by TMCP + quenching-tempering treatment is polygonal ferrite and minor cement

with yield strength R

P0. 2

- 845 〜870 MPa

tensile strength R

m

-895 ~900 MPa

impact energy at -20 ℃ 153 〜186 J

at -40℃ 141 〜155 J. Due to quenching and tempering treatment the M/A island size and the dislocation density in structure of steel products decrease thereby to get higher strength and better impact toughness of steel Q690q.