Effect Of Trace Rare Earth Elements On Microstructure And Properties Of High Strength Weathering Bridge Steel

In recent years,the promotion of steel for long-span bridge structures in China has been commonly used in highway transportation in coastal areas and the construction of cross-sea bridges.The requirements for mechanical properties and corrosion resistance of steel for bridges are more stringent.At present,the traditional weathering bridge steel is mainly added a large number of valuable alloying elements such as Cr,Cu,Ni and other elements in carbon steel to improve the corrosion resistance,which greatly increases the production cost of steel.A large number of studies have confirmed that the addition of rare earth elements in steel can improve the comprehensive performance of steel.However,most scholars generally add a large amount of rare earth.Although it can improve the comprehensive performance of steel,there are large particles of rare earth inclusions in the process of continuous casting process to block the casting nozzle and residual in the steel to deteriorate the impact performance,which is not suitable for industrial mass production of continuous casting.Therefore,the addition of trace rare earth elements（less than 20 ppm）in steel to improve the comprehensive performance has practical application value,and its mechanism is not yet perfect.The high strength weathering bridge steel Q370q ENH with different trace rare earth content（less than 20 ppm）produced by a steel plant is used as the prototype steel.The mechanism and law of rare earth on the test steel are compared and analyzed,which has theoretical guiding significance for promoting industrial application.The phase transformation points of three groups of test steels were tested by automatic phase transformation instrument.Combined with the original austenite grain size of the test steel,the effect of trace rare earth elements on the microstructure was studied.The experimental results show that compared with the blank experimental steel,the Ac₁ of the experimental steel containing 6ppm RE and 10ppm RE is reduced by 11°C and 14°C respectively,and the Ac₃ is increased by 19°C and 14°C respectively,that is,the austenite zone is expanded,the original austenite grains are refined,the large-size grains are reduced,and the grains are finer and more uniform.In addition,the morphology of inclusions was observed by scanning electron microscopy.The results showed that the addition of trace rare earth elements in steel modified the long strip Mn S and irregular polygonal Al₂O₃ inclusions into elliptical rare earth composite inclusions,which significantly reduced the size of inclusions（<3μm）,reduced the corrosion source in steel,reduced the occurrence of pitting corrosion,and inhibited the corrosion rate of steel.At the same time,the stress concentration in the steel is reduced.The low temperature impact results of the samples at-40°C,-60°C and-80°C show that the fracture dimples of the three groups of experimental steel gradually become shallow with the decrease of temperature,and the impact absorption energy of the samples without rare earth is the lowest at each temperature.At-80°C,the fracture morphology has many uneven cleavage surfaces and tear dimples.The fracture of the experimental steel with rare earth is mainly centered on rare earth inclusions to form large dimples,which hinders crack propagation and improves low temperature toughness.Among them,the low temperature impact performance of the experimental steel with 10 ppm RE is the best,and the impact absorption energy is increased by 18%,30%and 40%respectively.Through the salt spray experiment to simulate the marine atmospheric environment corrosion experiment and combined with the outdoor atmospheric exposure corrosion experiment,the corrosion rate,the macro morphology and micro morphology of the rust layer of the three groups of samples with different corrosion cycles were compared.EPMA cross-section element analysis and XRD phase composition test,and electrochemical corrosion research on rusted samples.The results show that the addition of trace rare earth elements reduces the corrosion rate of steel and improves the protectiveα-Fe OOH conversion of rust layer.At the same time,it promotes the enrichment of alloying elements in the inner rust layer and hinders the invasion of corrosive ions.In addition,the higher the rare earth content,the more positive the electrochemical corrosion potential,the larger the impedance radius,and the lower the corrosion current density,indicating that the addition of trace rare earth elements can improve the corrosion resistance of high-strength weathering bridge steel.