Investigation On The Application Basis For Developing Bainitic Rail For Usage Of High-speed And Over Loading Railroad

By various test means such as smelting and rolling in laboratory, and combined with industrial production, investigations on the optimization of the chemical composition, the production processes and the their adaptability, the bainitic microsrructure and the performances as well as the relationship between them for the carbide-free bainitic rail with1200MPa strength level were carried out. Further more, the researches on the weldability of bainitic rail including contact resistance welding, gas-pressure welding, heat treatment after welding were also carried out. The welded bainitic rails developed in the present study were laid in existing railway for trial, and were used in frog wing and track switch split. Based on the above researches, the investigation on further optimization of strength and toughness of bainitic rail was carried out. The above research shows that:Because of adopting the design on the composition and the microstructure of carbide-free bainite, the excellent strength and toughness matching of bainitic rail was realized. On the basis of C, Si, Mn, Mo composition series, addition of0.5%Ni can improve the stability of the residual austenite in the steel to a certain extent, but is not enough to cancel the tempering treatment of the rail. On the designed level of C, Si, Mn conttent, the Mo content more than0.30%is need. Lowering the finishing temperature and increasing the cooling rate after rolled can make the carbide-free bainite microstructure become lathy and fine obviously, and better strength and toughness matching can be obtained.The microstructure on the full section of different specifications of the bainitic steel rails-60kg/m,75kg/m and60AT are carbide-free bainite, and the tensile properties, hardness and impact toughness on whole section are uniform. The fracture toughness of the steel is far higher than that of quenched pearlitic rails, and the rate in fatigue crack propagation reached the level of high speed rail.The research also shows that:the plasticity of bainitic rail steel at high temperature meets the requirement of continuous casting process. The difference between the microstructures on the full sections as well as the performance uniformity of steel rails produced through the process conditions of hot rolling-tempering-straightening and those through the process conditions of hot rolling-straightening-tempering is not big, but the strength of the steel rail produced through the hot rolling-straightening-tempering was increased and the toughness of the steel rail was lowered to some extent. The tempering at the temperature range of300℃-500℃has no significant effect on the strength of bainitic rail. As the tempering temperature is higher than400℃, the toughness of the steel rail was lowered to some extent accompaning with the precipitation of carbide. The stability of residual austenite in the carbide-free bainitic rail produced by the process of hot-rolling and air cooling was obviously improved throughout low temperature tempering. Under the same plastic deformation conditions, the stability even at low temperature of-20℃of residual austenite in the bainitic rail tempered at low temperature,is obviously higher than that at room temperature of the bainitic rail without tempering. The performance of rail after multiple tempering or tempering for prolonged time is stable. Although the online flat and vertical complex straightening on hot-rolled-air-cooled bainitic rail makes it generate about350MPa residual tensile stress at the center of rail bottom, the microstructure of residual austenite in the steel after fatigue test which simulated rail operations at low temperature (-40℃) environment is basically stable yet. The researches on contact welding, gas-pressure welding and other welding processes show that the welding performance of bainitic rail is good. Welded bainitic rails were successfully trial-laid, their service life is prolpnged more than1times than that of the queched pearlitic rails, and the service life of forge and welding frog using the bainitic wing rail is prolonged about ten times than that of frog wing manufactured by using manganese steel; Bainitic AT split rails were successfully trial-laid on Datong-Qinhuangdao railway line, their average service life could reach4times of that of split rails manufactured by using U75V material.It also shows thatthe strength and toughness of bainitic rail could be further improved and optimized through the microalloying or quenching treatment.