Modified constitutive approach to flow stress, activation energy evolution, and processing map in a low-carbon bainitic steel

In this study, the hot deformation behavior of a low-carbon bainitic steel is systematically examined within a temperature range of 1073–1373 K (800–1100 °C) and for strain rates from 0.01 to 10 s − 1 using isothermal uniaxial compression testing. Commonly-used constitutive models typically assume constant material parameters during deformation; however, material constants can vary during deformation. To address this, a modified hyperbolic sine constitutive model is implemented to more accurately predict flow stress behavior under varying thermo-mechanical conditions. The modified model exhibited strong predictive capability with experimental data, achieving a low average absolute relative error of approximately 5% and a correlation coefficient of 0.98. Activation energy analysis revealed a clear temperature dependence, with activation energy decreasing at higher temperatures due to enhanced dislocation mobility. The activation energy was also shown to progressively decrease with increasing strain, due to dynamic recrystallization mechanisms. Processing maps were developed for different strain levels based on the dynamic materials model, revealing the effect of strain on instability zones. The optimal hot deformation conditions were identified to be in the range of 1250–1350 K (977–1077 °C) at 0.02 to 0.12 s − 1 , to facilitate dynamic recrystallization, leading to formation of a uniformly refined prior austenite grain microstructure.