| With Chinese sixth large-scale railway speed increase,a large number of trains with a speed of 200 km or above have been put into use.The high speed of the train not only provides convenience for the traveling of people,but also puts forward higher requirements for the safety of the train operation,axle as the key to ensure the safety of train operation components,its strength and toughness is particularly important in the ensure the safety of the train.At present,EA4T steel is the main material of axle for high-power locomotives,high-speed rail trains and subway trains in China.In this paper,the chemical composition,microstructure and inclusion of EA4T steel for imported axle were systematically analyzed by means of direct reading spectrometer,metallographic microscope,scanning electron microscope and energy spectrometer,and the banding microstructure defect in the steel was studied.The results showed that the chemical composition and mechanical properties of EA4T steel were in accordance with the European standard EN13261,and the microstructure was mainly tempered martensite+bainite.The number and size of the non-metallic inclusions along the surface to the core show an increasing trend,and the main categories are D type inclusions,B type inclusions and DS type inclusions.The inclusions are mainly calcium sulfide,aluminum oxide and a small amount of silicate.In addition,there is strip structure caused by alloying element segregation in the longitudinal direction of axle,which can be completely eliminated by isothermal annealing process.Based on the analysis of EA4T steel used for imported axle,the QPT process was studied.In this paper,the selection of isothermal quenching temperature,the selection of partition temperature,the one-step QPT process and the two-step QPT process are designed,and the influence of the process parameters on the microstructure and properties of QPT process in each stage is explored and analyzed,and the microstructure transformation process and fracture mechanism of QPT process are discussed.Results show that the distribution of temperature is 400?,as the isothermal quenching temperature,the steel strong plasticity can change present two stages,namely between280?~300?,the tensile strength decreases at the same time elongation is obviously reduced;Between 320?~360?,the tensile strength increased,but elongation decreases gradually,this is because with the isothermal quenching temperature increase of martensite content caused by the lower content of austenite increase.When best isothermal quenching temperature 260?,with the increase of temperature distribution,tensile strength and elongation are showed a trend of reducing the rise,then it mainly by distribution in the process of carbide nucleation to gather in the process of growing up.At 400?for partitioning,the carbon atom diffusion faster and can get a suitable amount of residual austenite stability.When isothermal quenching temperature is 260?and the partition temperature is 400?,with the extension of the partition time,tensile strength increased and elongation decreased gradually.This is because the carbides precipitated from the steel grow larger with the lengthening of the partition time,and martensite with higher carbon content is formed in the final water quenching process.Compared with traditional QT process,one-step QPT process has higher strength and lower plasticity,but the final strong plastic product is larger,indicating that one-step QPT process can obtain better comprehensive performance.In addition,compared with one-step QPT process,two-step QPT process can significantly improve the strength and toughness of EA4T steel.Through the two-step QPT optimum process:the austenitizing temperature is 900?+260?isothermal quenching+the partition temperature is 400?+high temperature tempering at 600?,which EA4T steel tensile strength increased to 1640.75MPa,the elongation increased to 17.2%,product of strength and ductility is 28220.9MPa·%,strong close to low carbon steel's plastic limit 3×10~4 MPa·%. |
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