Development Of High Strength Heavy Plate For Bridge Construction

The rapid development of traffic and transportation industries leads to the increase of bridge construction, and increases the requirement of bridge steel. However, the traditional bridge steel has lower strength, weldability and weather resistant properties and can not well satisfy the requirement of bridge construction. Aiming at the subject, State Key Laboratory of Rolling and Automation of Northeastern University and Capital Steel Co. developed the high strength bridge steel together. The goal was to turn out the plate with yield strength≥460MPa and thickness≤60mm by composition designing combined with controlled rolling and controlled cooling process and to satisfy the requirement of bridge steel of the nation. In this paper, based on the yield strength, tensile strength, toughness, weldability and weather resistant property, the chemical composition was designed and the rolling schedules were determined. In the meantime, the industry trial-production of Q460q steel was performed in 4300mm heavy plate mill. The results indicate that all indexes achieve the new national standard for bridge steel. The chief work of this paper is as follows:(1) The chemical composition of 460MPa grade bridge steel was designed by the concept of low carbon and microalloying. Nb, V, Ti were added in Q460q steel development and Cu, Cr, Ni were added in Q460qNH steel to increase the weather resistant property.(2) With Gleeble 2000, the dynamic recovery and dynamic recrystallization during deformation were investigated through single pass compression experiments under different deformation parameters. The results indicate that the austenite recrystallization is mainly affected by deformation temperature and strain rate, and it is difficult for recrystallization to take place when the deformation temperature is lower or the strain rate is higher, especially when the strain rate is more than 5s-1. The static recovery and static recrystallization after deformation were investigated through double-hit compression experiments under different deformation parameters. Temperature affects the static softening process greatly. The higher the temperature is, the higher the softening rate of austenite is. On the basis of experiments, the mathematical models for static recrystallization of test steels were built.(3) According to hot dilatometric value and micrographs observation, the deformation conditions in austenite region were investigated. The results show that:a) From the CCT curves, the ferrite region is enlarged after deformed in austenite region. So fine ferrite can be obtained, and fine grain strengthening and bainitic transformation can be used together in the production of bridge steel.b) The deformation in austenite non-recrystallization region can lower the starting temperature of bainitic transformation by destroying the grain boundary and tangling dislocation which stifle nucleation and growing of bainite. And the prior deformation affects the structure of granular bainite greatly. The granular bainite grain is refined. The distribution of M/A constituent is random and the chord of M/A constituent is shortened.(4) Taking Q460q steel as research object, the trial-production was done in 4300mm heavy plate mill. The results indicate that yield strength is 490-540MPa, tensile strength is 580-645MPa, elongation is 22.0-29.0%, impact energy at -40℃is not less than 115J, which arrive the preplanned research target.(5) The fatigue S-N curves of Q460q and Q460qNH steel were determined with sine wave (R>0). The results indicate that the fatigue values are 492MPa and 470MPa respectively. And Q460q steel was welded using submerged-arc welding. The results indicate that Q460q steel has better service weldability. The strength of the weld joint is not lower than the base metal, and the welding seam and heat affected zone have good toughness at -40℃-20℃.(6) The results of wet/dry cyclic corrosion test and electrochemical impedance spectroscopy indicate that the corrosion rates of SPA-H steel and Q460qNH steel are lower than that of Q460q steel. And the difference is more obvious at the later stage of corrosion. The corrosion rate of SPA-H steel is lower than that of Q460qNH steel. By observing the sectional configuration of corrosion rust, it can be known that the rust layer of Q460qNH steel and SPA-H steel consist of outer layer and inner layer. The inner layer is dense, but the outer one is loose and multihole. However, those of Q460q steel are similar that both the outer layer and inner layer are loose and multihole. The inner rust layer in weathering steel can protect matrix of steel from continuous corrosion.