Microstructure And Mechanical Properties Optimization Of Bainitic Rail Steels

The "composition-process-microstructure-properties" systematic research has been worked on a 20Mn2SiCrMo bainitic rail steel.Based on static/dynamic CCT and TTT curves,the effect of chemical composition and key processing parameters on microstructure and properties was studied by combination of simulation and experiments.For the isothermal platform(about 15 min duration)during industrial cooling process of bainitic rail,the influence of platform temperature was studied.Furthermore,retained austenite(RA)and mechanical property changes in tempering course and deep cryogenic treatment(DCT)were analyzed.On the basis of above results,the wear resistance and rolling contact fatigue performance of optimized bainitic rail steel were compared with that of pearlitic rail steel.The main results were listed as following:1.As an outstanding hardening element,1.9wt.%content of Mn in employed bainitic rail chemical composition system could avoid proeutectoid ferrite formation during air-cooling.Simulation results agreed well with experiments and so,a systematic approach of "Calculation-Experiment-Analysis-Simulation-Feedback" was obtained.2.An interesting phenomenon that there existed an isothermal platform for about 15 minutes during bainitic rails' air-cooling after rolling process was observed.The platform temperature had an important effect on the rail properties.With the isothermal temperature rising,granular bainite and M/A constituents size increased gradually.Furthermore,micro structure refinement and performance improvement could be obtained by controlling isothermal temperature.The tensile strength,elongation and impact energy(20?)was 1391 MPa,15%and 142 J respectively when the isothermal temperature was 325?.The impact energy here was 46%higher than that at 350?.Therefore,we can predict transformation behavior,to control micro structure and properties of as-treated bainitic rails by further research on the relationship between phase transformation and isothermal parameters(temperature,time duration etc).3.The RA in bainite/martensite multiphase microstructure during tempering was a critical factor affecting steel properties.As tempering temperature increased,the decomposition degree of martensite/austenite(M/A)constituents was aggravated and RA volume percentage decreased.Stable RA is beneficial to properties of bainitic rail steel and RA stability could be improved by tempering treatment.The toughness of 280?-tempered sample was about 90%and 10%higher than that at as-rolled state and tempered at 250??respectively.4.Deep cryogenic treatment was a potential process to optimize the properties of bainitic rail steels.The treatment made RA more stable,and higher strength,elongation as well as hardness could be obtained for as-treated bainitic rail steel.5.The relative wear resistance of employed rail steels was closely related to the applied load in sliding wear and impact wear.Wear resistance of bainitic rail steel was superior than that of pearlitic under high experimental load.However,the pearlitic showed better when the load was small.Compared with bainite/martensite multiphase microstructure,pearlitic microstructure tended to form hardened layer easily in rolling contact fatigue experiments.The interface between the brittle layer and matrix may be the potential sites for crack initiation and propagation.6.Failure analysis showed that filmy M/A constituents were helpful to hinder the propagation of micro-cracks in bainitic rail.