Abstract: The effects of tempering temperatures on microstructure and mechanical properties of the on-line quenched Q690q bridge steel were investigated through optical and scanning electron microscopes and tensile tests. The results show that the microstructure of the on-line quenched steel is primarily the lath bainite. After tempering at 540~650℃， the microstructure is the tempered sorbite. It is also found that with the increase of the tempering temperature， the lath width and the averaged grain size become larger， and the amounts of the carbide precipitations and large-angle grain boundaries increase. Meanwhile， with the tempering temperature increasing， the strength of the tested steel trends to be decreased， in contrast to the plasticity and toughness. At 620℃， the tested steels have the best mechanical properties， in which the yield and tensile strengths are 878 MPa and 992 MPa， the percentage elongation after fracture is 20%， and the impact absorption energy at -40℃ is 143 J.

Key words: Q690q bridge steel; tempering temperature; microstructure; large-angle grain boundary; mechanical properties