Study On Heat Treatment Process And Optimization Of 20Mn2 Axle Steel

In this paper, the heat treatment processes and microstructure of 20Mn2 axle steel of a steel company. were analyzed by production analysis and process experiment. The phase transition characteristics and continuous cooling transformation of 20Mn2 steel were studied. Using numerical simulation method, the cooling condition at normalizing process of 20Mn2 axle steel were simulated and analyzed, and the relevant conditions replacing tempering process with normalizing process were obtained to provide the theory basis of process improvement on axle heat treatment production.Using Gleeble3500 thermal simulation testing machine, the heat treatment phase transition temperatures of 20Mn2 axle steel were measured by analysis expansion curve and microstructure. The results show that Ac1, Ac3, Ar3 and Ar1 are respectively715℃, 849℃, 722℃ and 550℃, and the coarsening temperature of austenite are between 980℃~1070℃, therefore the quenching temperature of 20Mn2 axle steel should be 880℃~900℃.By means of optical metallographic microscope and scanning electron microscopy, combined with laboratory experiments, the microstructure,performance and existing problems of the axle are analyzed in the actual heat treatment production. The results show that the main microstructure of the axle are tempered troostite and tempered sorbite, existing reticular ferrite and feathery bainite microstructure defects precipitating along the original austenite grain boundary. By adding the intermediate link of releasing high temperature steam in quenching, the microstructure and mechanical properties of axle after quenched and tempered can be improved.The thermal simulation experiment results show that the microstructure transformation rules of the axle steel during continuous cooling are as follows: the microstructure are mainly constituted of block or multilateral ferrite, quasimultilateral ferrite and pearlite at 0.5~2℃/s cooling rate, and a small amount of granular bainite appearing when more than 1℃/s. The granular bainite content increases obviously and the microstructure gradually are acicular ferrite with ferrite size decreasing when cooling speed increasing to 2~5℃/s. At 10~20℃/s, the matrix are mainly constituted of acicular ferrites refining and crossing perspective each other,with the pearlitic disappearing, multilateral ferrite amount greatly reducing, and asmall amount of martensite appearing. At 30℃/s, the granular bainite ferrite laths significantly elongate with orientation enhancing, the islands get from granular to strip, and the number of lath martensite obviously increase. When controlling continuous cooling rate in 10℃~30℃/s, the mixed microstructure are mainly constituted of granular bainite, a small amount of quasi-multilateral ferrite and a small amount of lath martensite, with the hardness values are 250-290 HV, which meet both the factory and the national standard requirements.Normalizing cooling process was analyzed in numerical simulation with finite element analysis MARC software. The results showed the lower limit and upper limit of water spray on normalizing are respectively 20.3L/(m2?s) and 63.5L/(m2?s) for controlling cooling rate of the axle pipe section at 10 to 30℃/s. the cooling speeds in the tube outer wall, the center and inner wall were respectively 13.8, 10.7, 10.1℃/s and 10.1, 16.1,14.7℃/s, when cooling with lower limit and upper limit spraying water.Increasing the water spray can improve the steel pipe cooling capacity, but increase the cooling rate difference between the districts of axle pipe. Changing of normalizing temperature little effect the lower or upper spray water limit and the cooling velocity distribution, while the pipe thickness effects relatively obviously.