<p>Since the railway axles generally need to bear heavy dynamic loads of impaction, bending, torsion, and vibration under high speed and heavy transporting, it is of great significance to control the microstructure of the railway axle steel. In this study, the material behaviors including hot deformation and microstructure evolution of LZ50 steel are tested. According to the stress–strain and grain size of basic experiments, a mechanism-based constitutive model considering internal state variables of dislocation density, recrystallization fraction, and average grain size (AGS) is established to theoretically describe the macroscopical deformation and microstructure evolution of LZ50 steel. Subsequently, a finite element (FE) model of flexible skew rolling (FSR) bar is further developed via compiling the mechanism-based constitutive model into FE software by user-defined subroutine and its reliability has been verified by comparing the size of geometry and grain of experimental and FE results. The FE results show that the microstructure evolution mechanism of FSR rolling LZ50 steel is that hot plastic deformation leads to dislocation multiplication resulting in dynamic recrystallization and ultimately grain refinement. As the bar is formed by continuous FSR rolling, its microstructure is relatively uniform with the grain size of the outer layer slightly smaller than that of the inner layer because of the more severe deformation of the outer metal. The influence of FSR parameters on grain size reveals that the microstructure of the rolled bar is generally uniform in various situations and the parameter conditions of larger area reduction, longer sizing length, larger forming angle, and smaller skewing angle can improve the microstructure and properties of the FSR rolled bar by grain refinement.</p>