Constitutive Modeling for Flow Behavior of Medium-Carbon Bainitic Steel and Its Processing Maps

The hot deformation behavior of a medium-carbon bainitic steel was studied in a temperature range of 900–1100 °C and a strain rate range of 0.01–10 s⁻¹. With increasing strain, the flow stress displays three tendencies: a continuous increase under most conditions and a peak stress with and without a steady-state region. Accurate constitutive modeling was proposed and exhibits a correlation coefficient of 0.984 and an average absolute relative error of 0.063 between the experimental and predicted stress values. The activation energy of the steel increased from 393 to 447 kJ/mol, when the strain increased from 0.1 to 0.4, followed by a slight fluctuation at higher strain. Finally, processing maps under different strains were constructed and exhibit a varied instability region with increasing strain. Microstructural observations show that a mischcrystal structure formed in the specimens that worked on the instability regions, which resulted from the occurrence of flow localization. Some deformation twins were also observed in certain specimens and were responsible for negative m-values. The optimum hot working processing parameters for the studied steel were 989–1012 °C, 0.01–0.02 s⁻¹ and 1034–1066 °C, 0.07–0.22 s⁻¹, and a full dynamic recrystallization structure with fine homogeneous grains could be obtained.