Study Of Air-Cooling Slack-Quenching Process For 100m U75V Rail Based On Fluctuation Of Alloy Composition Content

As one of the main consumable parts of the railway,the rail bears all the pressure of the train.In addition,the high-speed and heavy-load trend and the high-density transportation organization mode of railway put forward higher requirements for the rail performance.U75V rail with 100-meter length is the main rail of high-speed and heavy-load railway in China.Air slack-quenching is the main strengthening and toughening method of U75V rail,which is a heat treatment process that sprays compressed air onto the rail surface to accelerate its cooling,and finally obtains lamellar pearlite structure(martensite or bainite structure is not allowed).The problems of the air slack-quenching of the 100-meter U75V rail are:1)The influence of the alloy composition content and the cooling procedure of the rail on its solid phase transformation characteristics is not clear;2)The air jet cooling technology has the shortcomings of weak heat transfer capacity and poor heat transfer uniformity.In this thesis,orthogonal optimization design,thermal simulation experiment,numerical simulation combined with experimental verification have been conducted to study the influence of alloy composition content on the solid phase transformation characteristics of U75V rail steel,the influence of cooling regulations on the structure and properties of U75V rail,the transient heat transfer characteristics of supersonic air jet impingement,and the design of rail supersonic profiling nozzle.The purpose is to provide basis for the optimization design of alloy composition and slack-quenching process parameters of U75V rail,and further improve the rail performance.The main conclusions are as follows:(1)The critical cooling rate of pearlite transformation(Vc)of U75V rail steel fluctuates in the range of 6～9℃/s with the change of alloy composition content.The undercooling of pearlite transformation of U75V steel is particularly significant affected by the content of Mn and Si,while the effect of C content is significant.The undercooling increases obviously with the increase of Mn content,decreases with the increase of Si content,and increases with the increase of C content.The Vc prediction model based on alloy composition content is:(2)The cooling rate in the phase transformation stage(Vs)and the initiaS quenching temperature(Ti)have a significant effect on pearlite lamellar spacing(S)of U75V steel.On the premise of not exceeding Vc,taking higher Vs and Ti are conducive to refining S and improving rail performance.The final cooling temperature of phase transformation(Tf)and the cooling rate after phase transformation(Va)have no significant effect on S.The two-stage cooling procedure of "slow-fast ",that is,taking the cooling rate close to but not exceeding Vc before the temperature reaches Tf(about 500℃),and then taking the cooling rate as fast as possible(but not exceeding 10℃/s),is beneficial to shorten the residence time of the rail in the heat treatment unit without reducing the performance of the rail.On the one hand,it can improve production efficiency.In addition,it can reduce the Ti difference between the front end and rear end of the rail,which is conducive to improving the uniformity of the longitudinal performance of the 100-meter rail.(3)The discretization-method is designed to coping with the problem of the inverse heat transfer method that requires installing multiple thermocouples inside the solid material.By separating the solid domain into independent test cylinders,the transient heat transfer characteristics of jet impingement can be studied without installing thermocouples into the solid domain,and the conjugate effect of transverse heat conduction inside the solid on the heat transfer characteristics can also be reduced.(4)Supersonic flow can significantly improve the heat transfer capacity of the air jet.The heat transfer capacity of the jet center is the strongest,and decreasing from the center to the outside.The heat transfer capacity of the jet center is most affected by the jet distance.The average heat transfer capacity is the strongest when H/D=4,and the heat transfer uniformity is the best when H/D=5.H/D=4 is the most suitable distance for the rail air-slack quenching process.(5)In order to improve the heat transfer capacity and uniformity of air jet impingement,a three-chamber supersonic profiling nozzle(OSP nozzle)was designed.Compared with the existing circular constant velocity nozzle,circular supersonic nozzle and constant velocity profiling nozzle,the heat transfer capacity is increased by 54.22%,13.88%and 47.01%respectively,and the heat transfer uniformity is improved by 31.11%,31.11%and 6.06%respectively.The time required to cooling the rail head surface to below 500℃ can be reduced to 43 s by using the OSP nozzle.Compared with the constant velocity profiling nozzle,the average hardness of the hardened layer of the experimental rail can be increased by 3.6%.(6)During production,adjust the inlet pressure of the OSP nozzle(between 0.2～0.4 MPa)to cool the rail surface at a colling rate close to but not greater than Vc until the temperature is lower than 500℃;Afterwards,increase the inlet pressure as much as possible,which can improve the rail movement speed without increasing the length of the heat treatment unit,thus reducing the Ti difference at different positions of the rail,ultimately improving the uniformity of the longitudinal performance of the rail,and improving the production efficiency.