Application of unified constitutive model of 34CrNiMo6 alloy steel and microstructure simulation for flexible skew rolling hollow shafts

Flexible skew rolling (FSR) is a novel rolling process for shaft parts, which is capable of rolling the hollow shafts. The microstructure of shaft parts is very important to its service performance, and the application of finite element (FE) method has been widely used to simulate microstructure evolution. Firstly, hot compression tests were performed to obtain the flow curves and corresponding microstructure states of the steel under the conditions of 950–1100 °C and 0.1–10 s−1. Secondly, the average grain size (AGS) under different conditions was recorded. A unified viscoplastic constitutive model considering recrystallization is established to characterize the high-temperature deformation and microstructure evolution of 34CrNiMo6 alloy steel. A genetic algorithm (GA) is used to solve the material constants of the constitutive model. The error of the predicted flow stress is 3.632%, and the error of the predicted grain size is less than 10%, which indicates that the established constitutive model can precisely predict the flow stress and grain size variation of 34CrNiMo6 alloy steel. The constitutive model was coded and embedded into the FE software Simufact via a user-defined subroutine interface to simulate the grain evolution during a uniaxial hot compression test and microstructure evolution of alloy steel in the FSR process. The FSR trials of hollow shafts were conducted and the microstructure at different positions of rolled-part was observed to validate the correctness of the model. The FE simulation results of grain size are compared with the experimental results, and the results are in good agreement. The internal variable constitutive model established in this paper can predict and reveal the microstructure evolution of FSR hollow shafts very well, and the AGS of shaft parts rolled by FSR are distinctly refined.