Compositions Design, Microstructures And Properties Of Weathering Bridge Steel

With the rapid development of bridge construction in the coastal region in domestic, it is obvious to increase the demand for bridge steel. Meanwhile there is higher requirement for comprehensive performance of the bridge steel, which means that the bridge steel has high strength, high toughness, favorable weldability and corrosion resistance. Since the marine atmosphere contains abundant chloride ions which can easily permeate the rust layer, the conventional weathering steel is difficult to form stable and compact rust layer under marine atmosphere, which means that the weathering steel does not effectively play its corrosion resistance. So it is necessary to study the corrosion behavior of bridge steel under marine atmosphere and to develop weathering bridge steel which can be safely in service under marine atmosphere for a long time. In this dissertation, the chemical compositions of weathering bridge steels were firstly designed and the experimental steels were then smelted and rolled. Then the microstructures, mechanical properties and corrosion resistance of the experimental steels were studied to determine the compositions of the steels with favorable comprehensive performance. On the basis above, the environmental suitability of weathering bridge steels was studied to analyze the control link of their atmospheric corrosion process. In order to improve the performance of the steels, hot deformation and continuous cooling transformation behaviors of a typical weathering bridge steel were also investigated to provide theoretical basis for formulating hot working technology. The main research contents and results obtained in this paper are as follows:(1) Based on the existing standards, performance requirement, cost factor and production condition, the chemical compositions of weathering bridge steels were designed and the experimental steels were then smelted.(2) The effect of cooling rate and alloy elements on continuous cooling transformation was investigated to provide theoretical basis for reasonable cooling schedule after the experimental steels were rolled. The experimental results show that the phase transformation is from proeutectoid ferrite and pearlite to bainite and the cooling rate range corresponding to proeutectoid ferrite decreases with increasing the cooling rate, which indicates that the increase of cooling rate suppresses phase transformation in diffusion mechanism. Mn content significantly affects microstructure organization at the cooling rate of 0.5-1?/s. When the cooling rate is from 2?/s to 10?/s, Cr, Mo and Mn elements effectively inhibit the generation of ferrite. However, Cr, Mo, Ni and Mn elements have little effect on microstructure types of the steels at the cooling rate of 20?/s.(3) In order to determine the reasonable composition of the steel, the effect of C, Mn, Cr, Ni and Mo elements on microstructure and mechanical properties of the bridge steels and mechanism of corrosion resistance at different stage were systemastically investigated. The experimental results show that the content of Ni and Mn in rust layers is smaller than that in steel matrix. When the content of Ni in rust layers exceeds 1.82%, the rust layer formed on the steel starts to possess evident cation-selectivity. With increasing Mn content, compactness of the rust layer decreases. The steels with the addition of 0.43% Cr or 0.15% Mo form more compact rust layer. With increasing Ni content, strength of the steel also increases. Adding 0.43% Cr to steel suppresses ferrite generation, and improves tensile strength of the steel, but reduces ductility and toughness. As Mn content increases from 0.75% to 1.46%, the microstructures are from proeutectoid ferrite and pearlite to sole bainite, strength of the steel increases, but ductility and toughness decrease.(4) The effect of rust layer compositions and structure on corrosion behavior of weathering and carbon steels was studied to reveal the corrosion mechanism of weathering steel in different environments. The results are as follows:? The compositions of rust layers are all Fe3O4, ?-FeOOH,?-FeOOH, ?-FeOOH and Amorphous in the environment containing different concentration of chloride ions. With the increase of chloride ion concentration in the experimental environment, the content of Fe3O4, ?-FeOOH and ?-FeOOH in rust layers increases, but Amorphous content decreases, the rust layer growth is promoted to an internal development, the corrosion rate increases. In addition, the protective rust layers are both formed on carbon and weathering steels in the environment containing low concentration of chloride ions. However, the protective rust layer is difficult to form on carbon steel in the environment containing high concentration of chloride ions. In the environments containing different concentration of chloride ions, the corrosion rate of the weathering steel is significantly lower than that of the carbon steel.? Ni, Cu, Cr and Mo elements in the weathering steel suppress generation of Fe3O4, ?-FeOOH and ?-FeOOH and crystallization of the rust layer in the environment containing chloride ions, and hinder the rust layer growth to an internal development and promote the formation of protective rust layers. ?-FeOOH is distributed in the outer rust layer, but ?-FeOOH is located in the inner rust layer, the distribution of corrosion products in the rust layers is influenced by the growth pattern of rust layers.? The compositions of rust layers are Fe3O4, ?-FeOOH, ?-FeOOH and ?-FeOOH in the simulated industrial environment. The distribution of ?-FeOOH and ?-FeOOH is uniform, but ?-FeOOH is distributed in the outer rust layer. There are many differences in the morphologies of inner and outer rust layers, and the morphology of outer rust layer is significantly affected by the external environment, but the structure of the inner rust layer is influenced by both the chemical compositions of steel and the external environment.(5) The thermal deformation behavior of austenite in a typical weathering bridge steel was investigated by hot compression test. The relationships between critical strain, critical stress and Z parameter of the bridge weathering steel were established, and Avrami equation was used to quantitatively analyze factors for the recrystallization kinetics. The results show that both the deformation temperature and strain rate influence DRX rate of the weathering bridge steel, where the strain rate increases an order of magnitude, there is around 0.85 orders of magnitude increment in the rate of DRX. Z parameter has much effect on recrystallized austenite microstructure, and the relationship between steady recrystallized grain size and Z parameter is as follow: dDRX=146487.3·Z-0.207(6) By the thermal simulation experiment, the influence of deformation parameters on ferrite and bainite transformations was investigated. The results show that the starting transformation temperature of austenite to ferrite and bainite increases with strain value changing from 0 to 0.4, the cooling rate range corresponding to proeutectoid ferrite transformation broadens, and the ferrite fraction increases, but the ferrite grain size decreases. As the deformation temperature decreases from 850? to 800?, the transformation temperature of proeutectoid ferrite increases, but that of bainite decreases, and the cooling rate range corresponding to proeutectoid ferrite transformation broadens, the ferrite fraction increases, but the ferrite grain size decreases.