Researches On Microstructures And Performances Of Carbide Free Bainitic Steel Containing Al Used For Rail

Bainitic steels have been widely used in various industrial applications due to their excellent properties of high strength and compatible toughness.Compared with the lower bainitic steels,carbide-free bainitic steel exhibits a better matching of strength and toughness,because it does not involve the precipitation of the hard and brittle carbides.Thus,carbide-free bainitic steel holds more application prospects.As the main alloy element,Si element can inhibit the carbide precipitation in the carbide-free bainitic steel.A large number of studies show that Al element not only has the effect on inhibiting the carbide precipitation,but also can reduce the sensitivity of hydrogen embrittlement and accelerate the transformation of bainite.Therefore,Al element has been widely used in the carbide-free bainitic steel in recent years.In this dissertation,a series of carbide-free bainitic steels alloyed with Al were designed,for the use in railway rails.The microstructures,mechanical properties,hydrogen embrittlement characteristics of steel and the mechanical stability of the retained austenite were studied.Meanwhile,the hot deformation behavior,fatigue,wear and other properties of carbide-free bainitic steels alloyed with Al were also studied.Results showed that the carbide-free bainitic steel,30Mn2CrNiMoAlSi,could obtain a better comprehensive performances of strength and toughness undergoing the annealing process of tempering at the relative lower temperature?320??after an ordinary normalizing treatment,or directly undergoing the isothermal quenching process at 320?.The retained austenite exhibited a relatively high mechanical stability after undergoing annealing process of tempering or isothermal quenching at higher temperature,which resulted from the higher diffusion rate of C atom into the retained austenite at higher temperature.Before the normalizing treatment,the bainitic steel underwent a deformation process with a degree of 20%at 930?.This process could accelerate the phase transformation of bainite and refine the microstructure,therefore leading to a higher toughness.The retained austenite after this treatment also possessed higher mechanical stability.Retained austenite in the steel alloyed with Al exhibited higher mechanical stability compared with that in the steel alloyed with Si.The hydrogen embrittlement behaviour of 30Mn2CrNiMoAlSi carbide-free bainitic steel treated with different cold rolling degrees was studied.After deformation,the retained austenite was transformed into martensite which had a high hydrogen embrittlement sensitivity.Meanwhile,the deformation also increased the internal stress and the defect density.These factors intensified the enrichment of H atoms,leading to the dramatical increase of the hydrogen embrittlement sensitivity of bainitic steel.Theoretical analysis showed that,when Al element was added into the steel instead of Si element,it could not only partially suppress the reduction of the unit cell binding energy that induced by the addition of hydrogen,but also further improve the diffusion barrier of hydrogen in the unit cell,finally reducing the hydrogen embrittlement sensitivity of the carbide-free bainitic steel.Based on the high temperature flow curves of 34Mn2CrNiMoAlSi carbide-free bainitic steel,the hot deformation constitutive equation with an accuracy of 98.4%was built,as well as the hot working diagram during high temperature deformation with different deformation degrees.The optimal hot working parameters to obtain fine uniform austenitic microstructures were inferred:989-1012?,0.01-0.02 s^-1 and1034-1066?,0.07-0.22 s^-1.The results provide a theoretical support for the hot work forming of bainitic steel crossings.A carbide-free bainitic rail steel,24CrMnSiAlMoNi,was prepared.The wear resistance of this steel with bainite microstructure obtained during air cooling was found to be equal to that of the traditional pearlite steel of U75V.Under the same strain amplitude,samples of the carbide-free bainite exhibited longer fatigue life than that of the pearlite steel.24CrMnSiAlMoNi rail steel possessed a relatively reasonable combination of wear and fatigue properties compared to U75V rail steel.