Numerical Simulation And Experimental Research On The Controlled Cooling Process Of Bainitic Rail

With the development of railway transportation in the direction of high speed and heavy load,the stress of the rails increases greatly and the running condition deteriorates continuously.Due to abrasion and fatigue,the damage of the pearlite rails,such as spalling,hidden damage and even fractures,have significantly increased.Therefore,it is urgent to improve the mechanical properties of rail steels to meet the requirements of railway development.In this paper,Mn-Si-Cr bainitic rail steel was taken as the research object.Through the numerical simulation and the experimental research,the effects of finish cooling temperature on the temperature field,microstructure evolution and mechanical properties of rails were investigated.The crystallographic characterization of the rail steels was also performed.Besides,the strengthening and toughening mechanisms of the controlled cooling rails were analyzed and a prediction model of the yield strength was established.The main contents are as follows:According to the production process,the numerical simulation schemes of the quenching final cooling temperature were determined to be 230℃,250℃and 280℃.The finite element simulation was used to explore the temperature change of the 60 kg/m rail during the controlled cooling process.There were two types of cooling for rail head when the fast quenching was stopped.One type was return temperature-slow cooling and the other type was direct slow cooling.The phenomenon of return temperature appeared on the surface of the rail head and the return temperature of the tread was about 50℃.And the relationships among temperature,microstructure and hardness were quantitatively analyzed.Based on the simulated temperature curve of the rail tread,two types dynamic-partitioning processes were designed to investigate the influence of different finish cooling temperatures on the microstructure evolution and mechanical properties.Compared with the sample of the return temperature-slow cooling process,the sample of the direct slow cooling process had higher content of retained austenite,higher tensile strength and lower toughness and the martensite laths were clear and fine.The crystallographic features of the parallel nano-sized retained austenite inside the martensite lath,perpendicular to the lath boundary,were quantitatively analyzed.The long axis direction is[-3-5 1]_γ.The stacking faults in the nano-sized retained austenite were connected with the dislocation outcrops on the interfaces at both ends,consistent with the situation when a perfect dislocation separated into two partial dislocations with a layer of stacking fault interposed between the dislocations.The near-coincidence-site model was used to accurately characterize and analyze the crystal plane index of the nano-sized retained austenite with flat facets.Because the interface covering many near-coincidence-site clusters had the smallest interface energy and low mismatch,the preferred facets tended to choose the plane containing high-density near-coincidence-site clusters.Based on the numerical simulation and heat treatment experiments,the trial production of controlled cooling rails with finish quenching temperatures of 230℃,250℃and 280℃was carried out.The microstructure of the rail-head surface was mainly martensite,with a small amount of lath bainite.The cooling speed of the rail tread was the fastest,but tensile strength and hardness of the rail tread were lower than that of the center.The phenomenon of return temperature resulted in the tempering and softening effect of martensite,reducing internal stress and rearranged the dislocations,which eventually led to the reduction of the tensile strength and the hardness.From the tread to the center,the yield strength continued to decline,and the impact toughness decreased first and then tended to be stable.The strengthening and toughening mechanisms of the controlled cooling rail was quantitatively studied,and a prediction model of the yield strength was established.The microstructure evolution and the strengthening and toughening mechanisms of the Mn-Si-Cr bainitic rail material were systematically studied.It has certain reference value for the research and the performance optimization of high strength and toughness rails.