Analysis of the work-hardening behavior of C–Mn steels deformed under hot-working conditions

• A constitutive description of a C–Mn steel under hot-working conditions has been developed.
• Plane strain compression tests were conducted between 850°–1100 °C, at strain rates of 0.3–24 s−1.
• The temperature and strain rate dependence of the flow stress is introduced in two different ways.
• The formulation considers the interaction of dislocations with athermal and thermal barriers.
• The flow stress is computed from the numerical integration of the work-hardening rate equation.

The computation of loads, torques and power consumption of hot rolling processes conducted at industrial scale requires a detailed analysis of the work-hardening rate of the material, as well as a precise description of the changes in flow stress with microstructure and deformation conditions. The present work describes two rational experimental methodologies that can be applied in order to accomplish this analysis, which encompasses the determination of the athermal stress, as well as the temperature and strain rate dependence of both the yield and saturation stresses of the material. The evolution of the flow stress in the course of plastic deformation is determined by means of the numerical integration of the phenomenological exponential-saturation work-hardening law advanced by Sah, J.P., Richardson, G., Sellars, C.M., 1969. Recrystallization during hot deformation of nickel. J. Aust. Inst. Met. 14, 292–297 expressed in differential form. In this way, it is possible to compute the current value of the flow stress in terms of its previous value and to update the changes in deformation temperature and strain rate that occur after each strain interval during the course of plastic deformation, as expected in industrial hot working processes. In the present work, these methodologies are applied to the analysis of the work-hardening transients of a number of stress–strain curves obtained for a C–Mn steel deformed under plane strain compression conditions, in the temperature range of 1123–1373 K, at strain rates in the range of 0.4–24 s−1. The results indicate that both the work-hardening rate and flow stress of the material can be satisfactorily described for most deformation conditions. It is shown that by employing the Sellars–Tegart–Garofalo model and the Zener–Hollomon parameter, in order to account for the temperature and strain rate dependencies of the stress parameters, the accuracy in the description of the experimental flow stress can be improved, but at the expense of an increase in the number of material parameters involved in the analysis. The limitations of employing a single internal state variable for the computation of the work-hardening rate and flow stress have also been discussed.