Study On Phase Transformation And Microstructure Of Bainitic Rail Steel

Due to the increasing demand for railway transportation,the requirements for load capacity are getting higher.The service conditions of the rails continue to deteriorate.The damage of the rails in service is getting more and more serious.The research of rails with better performance is the focus of the current problems.The microstructure plays a decisive role in the performance of steels,which is determined by both the composition and the process.It is of great significance for the study of phase transformation of steels.In this paper,we focused on the Mn-Si-Cr bainite steel.The microstructure of the controlled cooling rail was characterized in detail.Based on this research,an optimization method for cryogenic treatment and microalloying was proposed.The phase transformation process and the strengthening mechanism was studied in depth.The main contents were as follows:The microstructure of the bainitic rail steel at different tempering temperatures was observed.The tempering temperatures of the rail were determined to be 260 ?,300 ?,320 ?,and 360 ?.The effects of different tempering temperatures on the microstructure of rail steel were studied.The results showed that at 300 ? tempering temperature the microstructure was finer and more uniform,and the average width of the bainitic laths was smaller.The cryogenic treatment process of steel was studied and the transformation of blocky retained austenite was analyzed in detail.The results showed that the blocky retained austenite in the steel decomposed,and the performance of the steel was improved after cryogenic treatment.Both K-S and N-W oriented relationships between ferrite and austenite were observed in the microstructure formed from blocky retained austenite after cryogenic treatment.N-W orientation relationship was the main orientation relationship.Microstructure that deviated from the K-S and the N-W orientation relationships was observed in the ACT(A: Austempering,C: Cryogenic treatment,T: Tempering)samples.Nb microalloying was studied to improve the performance of steel.The results showed that the addition of trace Nb element could effectively refine the microstructure and improve the strength-toughness performance of the present steel.The composition of the precipitate contains Nb,Ti,and C.They distributed in the body-centred-cubic matrix and could improve the performance through grain refinement and dispersion strengthening.Nb microalloyed steel showed fine filmy retained austenite,which was beneficial to the toughness of the steel.