Study On The Strengthening Of U75V Heavy Rail Steel Based On The Smelting Composition Control And Supersonic Air-cooling Heat Transfer Enhancement

As the main heavy rail steel in China's railway,the continuous development of high-speed and heavy-duty transportation has put forward higher requirements on microstructure and property of U75V heavy rail steel.At present,the composition range of U75V heavy rail steel specified in the standard is relatively wide.It is found that narrowing the composition range can further reduce the critical cooling rate,improve the hardenability of steel and the stability of undercooled austenite.Meanwhile,the air cooling after rolling is a widely used heat treatment method for U75V heavy rail steel,but its cooling rate is relatively low(only2?3?/s).Strengthening heat transfer during air cooling can further refine the pearlite lamellar spacing.Therefore,in this paper,based on the actual strengthening demand of U75V heavy rail steel,the strengthening scheme of U75V heavy rail steel based on the smelting composition control and supersonic air-cooling heat transfer enhancement is preliminarily explored by the combination of theoretical analysis,numerical simulation and experimental test,in order to further improve the steel performance.The main achievements are as follows:By studying the formation and evolution of typical inclusions in U75V heavy rail steel and the process conditions of RH vacuum dehydrogenation and denitrification,the accurate control model of two typical inclusions modification of MNS and Al₂O₃,and the prediction formula of degassing rate and argon flowrate are established.Based on the actual smelting process and composition requirements of U75V heavy rail steel and the study of thermodynamic law of each equilibrium precipitate phase in the steel,it is concluded that the change of smelting composition content of C,Si,Mn and V would have an important influence on the strengthening of U75V heavy rail steel.The influence of the changes in the content of each smelting composition on the critical cooling rate of U75V heavy rail steel is analyzed theoretically.On this basis,the range of optimal narrow composition control of U75V heavy rail steel is preliminarily determined as C0.77?0.79wt%,Si0.50?0.53wt%,Mn1.00?1.05wt%,and V0.04?0.06wt%to achieve the minimum critical cooling rate.Meanwhile,a dynamic prediction model for the critical cooling rate of U75V heavy rail steel based on the content of smelting components is established to maximize the supersonic air-cooling strengthening effect of the steel.In order to better solve the problem of supersonic air-cooling heat transfer enhancement,the governing equations of two-dimensional axisymmetric fluid flow and heat transfer in general curvilinear coordinate system are derived.Based on the steady-state incompressible pressure-based SIMPLEM algorithm,the AUSM+flux interpolation scheme in the density-based algorithm is introduced into the traditional Rhie-Chow momentum interpolation scheme,while a smooth Mach-weighted function is constructed to achieve the organic integration of the two interpolation schemes.Then the Mach-weighted SIMPLEM algorithm is established,which can effectively solve the complicated full-speed flow problem with subsonic dominance in supersonic air-cooling heat transfer enhancement.To tackle the overall solution problem of fluid and solid domains in supersonic air-cooling heat transfer enhancement,the near-wall turbulent flow is equivalently treated as laminar flow by introducing the effects of turbulence on fluid flow and heat transfer into the wall-adjacent equivalent generalized diffusion coefficients,while a unified solution method for fluid-solid conjugate heat transfer is constructed according to the principle of equal heat flux.On this basis,the numerical analysis model of heat transfer enhancement of the supersonic air cooling is established.Then the distribution characteristics of flow field,the instantaneous characteristics of temperature field and convective heat transfer coefficient and the influence of jet parameters including the inlet pressure,temperature and turbulence intensity on the characteristics of heat transfer enhancement are systematically studied.For analyzing the effect of high-temperature oxidation behavior on heat transfer enhancement of supersonic air cooling,the formulas of ion diffusion and film growth of multi-layer high-temperature oxidation is derived based on the Wagner's theory of high-temperature oxidation of metals.Then,under consideration of the heterogeneity of interfacial temperature and oxygen partial pressure,the multi-layer non-isothermal high-temperature oxidation kinetics model is established by applying the isothermal division method.To effectively solve the problem of the whole-field solution difficulty caused by the introduced high-temperature oxidation and surface radiation,a unified interface treatment for complex transient conjugate heat transfer including the high-temperature oxidation,surface radiation and turbulent flow is purposed,and a heat transfer enhancement analysis model of supersonic air cooling with the non-isothermal high-temperature oxidation and surface radiation is established.Then the non-isothermal high-temperature oxidation characteristics and its influence on the characteristics of heat transfer enhancement during the supersonic air cooling are analyzed.The correlation formula for the heat transfer enhancement characteristics of supersonic air-cooling with high-temperature oxidation correction coefficient is constructed by using the binary high-order polynomial.And the heat transfer characteristics at different Prandtl numbers are expounded in detail based on the correlation.Simultaneously,the influence of cooling rate on the microstructure and property of U75V heavy rail steel is analyzed by the thermo-mechanical test and theoretical calculation.The change law of the microstructure and lamellar spacing of U75V heavy rail steel is obtained under different cooling rates.Then,the mathematical model between the pearlite lamellar spacing and phase transformation undercooling of U75V heavy rail steel is established.Compared with the traditional air-cooling process,the heat treatment of U75V heavy rail steel with supersonic air cooling can increase the phase transformation undercooling by about 1.25 times,and refine the pearlite lamellar spacing by about 1.71 times,thereby effectively realizing the strengthening of U75V heavy rail steel with better technical advantages.The obtained research results can provide certain theoretical support for the further strengthening of U75V heavy rail steel.