| Heavy-haul railway is one of the important directions of railway development in China,and it is also the most effective way to improve freight capacity.At present,the rail in service can not fully meet the requirements of ductility,wear resistance and fatigue resistance.Bainitic rail,known as "rail of the 21st Century",after reasonable strengthening and toughening methods,its strength and toughness matching,wear resistance were better than traditional rails and meet service requirements.In this paper,the Mn bainitic rail was taken as the research object.The relationship between process parameters and microstructure and performance was studied by optimizing the heat treatment process.Then,the optimal strength-toughness matching of rail was achieved,and the prototype bainitic rail for heavy-haul railway was trial-produced in the production site of enterprise with reasonable process.There is segregation phenomenon of Mn element in the experimental rail.By measuring the proportion of the segregation bands,the average band width and the quantitative analysis of the element content by electron probe,it was determined that Mn was the main element that caused the segregation bands.Different degrees of segregation had different effects on the phase transition,in order to study the effect of component segregation on phase transition,after continuous cooling transformation and isothermal transformation experiment,it was found that the incubation period of isothermal transformation was the longest at 15mm distance from tread and the hardness of the position was the largest in both the isothermal transformation process and the continuous cooling process.In the isothermal transformation process,the bainitic transformation at any position from the tread decreased with the increase of isothermal temperature,and the bainitic transformation at 15mm was the least.According to Mn element segregation effect on rail phase transition point,three groups of different heat treatment process were designed:BQPT1(quenching-partitioning),BQPT2 and BQ&DP(quenching-dynamic partitioning).By analyzing the corresponding microstructure and testing mechanical properties of the three groups of processes,it was found that the hardness and impact toughness of the rail treated by BQPT1 process decrease with the increase of the partitioning temperature,which was not conducive to the improvement of the comprehensive strength and toughness of the bainitic rails;with the decrease of quenching temperature,the strength and impact toughness of the rail treated by BQPT2 process increase,the retained austenite content increased first and then decreased,when the quenching temperature was 200 ?,the retained austenite content was the highest;the strength of rail treated by BQ&DP process increase with the decrease of quenching and dynamic partitioning temperature,but the impact toughness tend to decrease;and the results showed that the best matching process of strength and toughness was BQ&DP.In order to investigate the mechanism of the strength and toughness of rail treated by BQ&DP process,EBSD(Electron Backscattered Diffraction)technologywas used to characterize multi-scale microstructure and the relationship between strength and toughness and microstructure distribution,morphology and size was studied.The results showed that the yield strength at different quenching temperatures of the BQ&DP process includes the strength of retained austenite and bainite,martensite,among which the latter contributes the most to the yield strength.The size of lath in bainite and martensite determines the the yield strength,and the yield strength decreases with the increases of lath size.Comparing the actual test yield strength with the estimated yield strength,it was found that the variation of the two strength was consistent,both of which decrease with the increase of quenching temperature and the error was less than 5%.The BQ&DP process was used to trial-produce the rail in the industrial site,and the sample of the actual bainitic rail was taken,and the microstructure and mechanical properties were compared with those of the air-cooling rail.The microstructure distribution,strength,toughness and hardness of BQ&DP process were better than those of traditional air cooling +tempering process. |
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