Study On Microstructure And Properties Of New Ultra-fine Bainite Rail Steel With High Strength And Toughness

The development and application of bainitic steel has a long history,but the application of bainite materials in rail is rarely studied.Since the speed of passenger trains is higher and higher,and the axle load of freight vehicles is increasing,The development of traditional pearlite rail has reached the limit in strength and toughness.Bainite rail have high strength and good toughness,which could meet the development requirements of rail.However,bainite rail is much different from pearlite rail because there are a lot of technical problems in the control of microstructure and properties.Therefore,it has great scientific development and application value to investigate the microstructure and properties of new ultra-fine high strength and toughness bainite rail.The high strength bainite rail with the chemical composition Fe-0.25%C-1.73%Si-1.69%Mn-0.012%P-0.002%S-1.4%Cr-0.49%Mo-0.57%Ni-0.06%was studied.The microstructure mainly contains bainite,and the width of bainite ferrite lath is150?350nm.The tensile strength(Rm)is up to 1510MPa,and the impact energy(KV₂)at room temperature reaches 56J.Compared with pearlite rail(the highest strength level after heat treatment is 1280MPa,and impact energy at room temperature is 8?10J),bainite rail has the higher strength and better toughness.The effects of basic physical parameters,continuous cooling transformation,actual cooling process and tempering process on microstructure and properties of bainite rail were systematically analyzed by ultrasonic method,thermal simulation test and confocal laser scanning microscope test.The microstructure of bainitic steel before and after heat treatment were observed by OM,TEM and EBSD.The following conclusions are obtained:(1)The physical properties of bainite rail were measured,and the results show that at the temperature range of room temperature?800?,the velocity of P-wave and S-wave,elastic modulus,shear modulus,bulk modulus,sonic anisotropy coefficient and Debye temperature of bainite rail were decrease with the increase of temperature,while Poisson's ratio and compression coefficient increase with the increase of temperature.When the tensile temperature increases from 850?to 900?,the tensile strength decreases from100MPa to 75MPa,meaning that the strength decreases by 25%.(2)In the process of continuous cooling transformation of bainitic steel,the microstructure is mainly bainite at the cooling rate of 0.1?/s,whereas at the cooling rate of above 1.0?/s,the microstructure is mainly martensite.Compared thermal simulation tests with confocal laser scanning microscope results,it is found that the temperature difference of the critical temperature is 10?20?.(3)The austenite grain growth of bainite steel in heating process has the migration of austenite grain boundaries,the merging of small grains into large grains,and some divided grains merging to the surrounding grains.The austenite grain size number is 7.5 grade when it was heated to 950?,meaning that it is essential fine grain steel.When the heating temperature increases from 711?to 1150?,the austenite grain size number increases from 8.5 to 6 grade,and some grain size number reached 4 grade because of abnormal growth during holding.In the normal cooling process of rail,the nucleation in the grain with the obvious relief effect should mainly be martensitic transformation,while the transformation of bainitic rail steel is close to diffusion phase transformation.(4)When the tempering temperature is in the range of 300?500?,the microstructure of bainitic steel is not affected by holding time but tempering temperature.With the increase of tempering temperature,the microstructure changes from bainite+martensite to bainite+martensite+pearlite.In addition,with the increase of tempering temperature,the number and total length of grain boundaries and the grains number increase,while the average diameter and the equivalent area of grains decrease,and the grain size increases from 18.7 to 19.2.Moreover,as the tempering temperature increases,the amount of retained austenite decreases.The austenite content decreases sharply from 3%at 300?to0.1%at 500?.At the same tempering temperature,holding time increasing from 30 min to 60 min has no obvious effect on austenite content.(5)During the tempering of bainite steel,with the increase of tempering temperature,the M/A is decomposed and some carbides are observed.The elongation,yield strength,impact energy and tread hardness increase firstly and then decrease with the increase of tempering temperature,whereas the tensile strength decreases initially and then increases.In addition,with the increase of holding time,the yield strength,tensile strength,elongation and impact energy of rail decrease,but the tread hardness has no obvious change.Therefore,connected the microstructure observation and mechanical properties,it is concluded that the best tempering process of bainite rail is 400?holding for 200 min.(6)The theoretical research results are transformed into industrial applications.The production of bainite rail is produced by converter,secondary refining,continuous casting,billet heating,and rolled by universal rolling mill,and then cooled in cooling bed and finally straightened.Hot rolled bainite rail were send to railroad switch mill,and after tempering,they are processed into bainite frogs for use in railway lines.The service life is more than twice that of existing products.