The influence of carbon content on the mechanical properties of high yield strength bridge steel has been in- vestigated. The results show that the excellent mechanical properties and corrosion resistance are obtained for the steel with carbon content of 0.03% -0. 05% （mass percent）. According to the results, a new weathering bridge steel plate with carbon content of 0. 045% （mass percent） has been developed. The appropriate controlled cooling process should be taken due to the results of CCT （continuous cooling transformation） and TTT （time-temperature-transformation） to ensure both microstructure and mechanical properties. CCT curve of the newly developed steel shows that when accelerated cooling speed is higher than 5 ℃/s, the intermediate transformation products can be formed. The TTT curve displays that the intermediate transformation temperature ranges from 600 to 530 ℃. Yield strength of the newly developed steels reaches 500 MPa, and their elongation and toughness are excellent.
