Research On High-strength And Resistance Corrsion Rail

For a long time, the steel corrosion studies have been mainly focused on low carbon andultra-low carbon steel which have approximate single phase. As one kind of rails that can formmany micro-cells, few studies have been carried out on corrosion resistant of high carbonpearlitic rail. According to the requirements of harsh environment such as tunnels and marineclimate to steel rail, it has important practical significance to develop the high strength andcorrosion resistant rail from the view of steel material.In this paper, the C-Si-Mn alloy system is basis, the experimental steels (Cu-Cr, Cu-Nb,Cr-Cu-Nb) with different C content are smelted in ZGJ0.05-100-2.5A vacuum inductionsmelting furnace, and then rolling on the Φ800mm two roller reversing hot rolling mill, in orderto simulate industrial cooling process, steel plates with rolling size of16×200×L mmeventually are stacked and slow cooled, the cooling rate is controlled below0.8℃/s. Thecomposition range of high strength and corrosion resistant rail has been obtained on the basis oflaboratory researches, thus3furnace experimental steels are produced following the industrialprocesses.Austenitic continuous cooling transformation and true stress-strain curve of experimental steelare tested by Gleeble1500thermal simulation machine, the SEM, TEM, OM and mechanicalproperties testing equipments are used to conduct the research about influence of Cr, Nb onstrength and toughness of high carbon pearlitic steel. The results show that the increase of Crcontent can reduce the lamellar separation of pearlite and impede the hardness, when Cr contentis over0.46%, the influence of Cr on austenitic continuous cooling transformation reduces; Nbhas the same effect as Cr, but overmuch Nb will promote the generation of proeutectoid ferrite,which can reduce hardness of steel; the test of austenitic continuous cooling transformationshows that the phase structure of experimental steel with low cooling rate is sorbite, and therewill appear bainite transformation when cooling rate is over0.73℃/s, thus the cooling rate ofindustrial processes is controlled below0.73℃/s.According to the high quality requirements of high speed rail, the desulfurization and noaluminum deoxidation processes, form controlling process of oxide inclusions and large squarebland continuous-casting process are comprehensively studied. The results indicate that thequality of high strength and corrosion resistant rail can meet the requirements of high speed rail,the oxygen content is10ppm, aluminum content is30ppm. The quantity and pattern of inclusionsin steel are controlled at high level, the pass rate of inclusions is over99%, the center rarefactionof casting blank≤1.0level, center segregation≤1.0level, center shrinkage void≤1.0level,center crack≤0.5level, corner crack≤0.5level, equiaxed grain rate reaches50%to70%, thepass rate of continuous casting blank is over99.9%.The change process of MnS inclusions in9#heavy rail casting slab was in-situ observed byconfocal laser scanning microscope. The result shows the MnS has shape evolution owing to theinfluence of the diffusion and dissolution with the increase of the temperature. The small-sizedMnS inclusion with spindle and nearly spherical shapes in continuous slabs are obtained withproper heating temperature between700℃and900℃or1100℃and1200℃, and the excessive heating temperature can cause size increase of MnS inclusion.The cyclic slaking corrosion test is carried out to test the corrosion resistance of experimentalsteel in3%NaCl solution, the corrosion rates of normal rail (0#) and corrosion resistant rail (7#,8#and9#) are studied. Results indicate that the corrosion rates of normal rail and corrosionresistant rail have similar change tendency, the relative corrosion rate decreases with the prolongof time. The highest relative corrosion rate of corrosion resistant rail can reach58%, whichimproved by40%than normal rail.The AFM is used to obtain the3D pattern diagrams and surface roughness curves of0#and9#steels under different slaking time, the results show that when slaking time is10min, the surfaceof0#steel is rough,9#steel only bleak than initial; when slaking time is30min, many corrosionpits appear on the surface of0#steel, and the peak value of surface roughness curve reaches110nm, but9#steel just continue to dark and no obvious corrosion pits appear, the surfaceroughness curve of9#steel has no obvious change; when slaking time is60min, there areobvious corrosion pits both on the surface of0#and9#steels, and corrosion pits in0#steel arebigger and deeper than that in9#steel.The microstructures, phases and composition of rusty scale are analyzed with the help of SEM,XRD and electron probe, results show that the pattern of rusty scale surface is group ball, theinitial rusty scale grain of9#steel is finer than0#steel, at the fifth period, the rusty scale grain of9#steel is also fine, but he rusty scale grain of0#steel is bulky with the appearance of crack; Cr,Cu, Nb content of9#steel continuously increase from the outside rusty scale to inside rusty scale,it can find obvious Cr beneficiation around the cracks of inside rusty scale, and Cr shows thesurface distribution, Cu shows the point distribution in inside rusty scale, Nb in inside rusty scaleshows the chain-like distribution, however, Cu, Nb and Cr can not be found in0#steel; theoutside rusty scale of0#and9#steels include both γ-FeOOH, Fe3O4, α-Fe2O3and a small amountof α-FeOOH, in addition, the inside rusty scale contains β-FeOOH, the diffraction peak intensityof9#steel is higher than0#steel with the prolong of time, the smaller half width and the moresharp diffraction peak indicate that generated phases are stability.The corrosion potential, polarization curve, electrochemical resistance curve and noise ofexperimental steel are studied by electrochemical method, results indicate that the corrosionpotential of9#steel is slightly above0#steel at the initial stage of corrosion, potential of0#and9#steels is maintained at about-0.66V, with the prolong of corrosion time, the corrosionpotential of0#steel reduces sharply, meanwhile the corrosion potential and current density of9#steel decrease slowly, and the current density of0#steel increases sharply; There is a greatdifference between surface polarization resistance of0#(1049·cm~2) and9#steels (3121·cm~2),and9#steel shows a more stable resistance peak (1000Ω) than0#steel, compared to0#steel, it ismore easy for9#steel to form stable rusty scale, and9#steel has a better corrosion-resistanceproperty than0#steel.