Theoretical And Experimental Research On The Rail Universal Rolling

Since the first high-speed railway had been accomplished in Japan in 1964, the high-speed railway becomes the most important vehicle all of the world rapidly. For satisfying the requirement of expanding population and the rapidly economic development, the development of world's railway presents the tendency of high-speed and heavy-load. At present, the process of building the high-speed railway has been accelerated by our country. However the speed of train had been increased many times based on the previous running speed, its absolute velocity has not reached the level of developed countries yet. The reason is that the size precision, mechanical performance, strength of rail is the key factor to restrict the running speed of train.The hot rolling process of rail does not only influence the surface quality, size precision and straightness of the rail, but also influence the residual stress and internal defect greatly. Moreover, the technological parameter, such as the rolling temperature, rolling velocity and cooling temperature, will influence the microstructure and grain size of rail directly. Therefore, the mechanical property of rail will be influenced by the hot rolling process of rail. For obtaining the high-precision and high-performance rail, it is not good enough to depend on the numerical simulation, empirical data and experimental research except for applying refining method in the process of steelmaking and developing new rail steel. So, the deformation mechanism of rail rolling needs to be analyzed deeply and a correct theoretical model for rail rolling can be built. Based on this theoretical model, if the rolling experiments and FEM numerical simulation is considered and integrated into that, a rational and optimized rolling procedure can be determined, and the rolling process and cooling process can be controlled accurately. Otherwise, with the development and maturity of universal continuous rolling technology and automatic control technology, it is urgent to build an accurate theoretical model for rail rolling.A complete theoretical model for rail universal rolling should consists of the law of metal flow, resistance of deformation, temperature variation, forward-slip, force-energy parameter.The deformation mechanism of rail universal rolling is very complex. During the rail universal rolling, the variation of depress ratio between the web, head and base of rail influences the metal flow greatly. So, the cooperation parameter, considered the effect of variation of depress ratio, is lead in the calculation of spread of the base and head of rail, and the spread formula of .the base and head of rail is obtained. Based on the spread formula, the mean elongation coefficient of the web, head and base of rail is derived according to the incompressibility condition of metal. Moreover, the law of metal flow about the different parts of rail is analyzed by qualitative method.The model of resistance of deformation about plain carbon rail steel and plain alloyed rail steel can be obtained by the experimental formula from Guan kezhi, and the new alloyed rail steel and rail-earth rail steel can be obtained by the updated Misaka formula derived by Scililiano and Jonas.For building the theoretical model more easily, the cross-section of rail is simplified firstly, and the vertical roll with box pass for rolling the head of rail is simplified as an equivalent flat roll. The simplified three-dimensional geometrical model of rail cross-section is determined, and the possible velocity field satisfying the kinematic principle is obtained.For building the model of temperature variation during the rail universal rolling, the energy conservation principle,heat equilibrium equation and the basic law of heat transfer is applied in the paper,and the mean temperature of the web, head and base of rail is set as the subject investigated. The temperature drop for the convection, radiation, descaling, contact and the temperature rise for the plastic deformation and friction between the workpiece and the roll are considered, and the model of temperature variation is obtained.For building the forward-slip model, the neural angle formula about the deformation zone of the web, head and base of rail is derived, and the neutral line formula about the contact zone on the flank of the horizontal roll is also derived according to the law of metal flow per second conservation. Based on the method of force equilibrium, the forward-slip model is built.When building the model of force-energy parameter, the upper-bound principle about rigid plastic material is applied in this paper. Based on the possible velocity field satisfying the kinematic principle, the approximate solution of rolling force and rolling torque is obtained.During simulating the process of rail universal rolling by Rigid-Plastic FEM software DEFORM-3D V5.0, the influence of different specification and material of rail is considered, and the hot rolling process of 60kg/m rare-earth rail BNbRe, 18kg/m plain carbon rail Q235 is simulated individualy.For verifying the theoretical model and implementing the comparative research with simulation results, the hot rolling experiments of 18kg/m plain carbon rail Q235 is accomplished on the three-stand universal rolling mills in the Rolling Lab of Yanshan University. In the experimental process, the effect of the rolling temperature, rolling velocity and depress ratio is considered and some different rolling procedures are adopt. During the process of experiments, the experimental data about rolling force , rolling torque and the temperature on the surface of worpiece are collected, andFor analyzing the microstructure and the metallic phase of rolled workpiece, the bottom of rolled workpiece is quenched. The test samples are gained on the different place of the web, the head, and the base of rail, and the corresponding cross-section of the test sample is polished and corroded to get the microstructure and grain size. Based on the results of metallographical experiments, the effect of the different quenching temperature and the different rolling procedure on the microstructure of rolled workpiece is analyzed.The research results of this paper is significant for producing the rail by the universal rolling mills. The theoretical model of rail universal rolling can provide a important theoretical support for setting a rational rolling procedure, and this model make it easier to program and realize the automatic control and the control of rail continuous rolling. Moreover, it can provide a significant reference to control and improve the quality of rail. By the metallographical analysis, a rational cooling technology can be obtained, and it is useful to improve the structure property and mechanical property.