Effect of competing hardening and softening mechanisms on the flow stress curve modeling of ultra-low carbon steel at high temperatures

An ultra-low carbon steel was deformed in torsion over a temperature range of 650–1100 °C and diagrams of the work-hardening rate vs. applied stress were drawn up for the hardening region of the flow stress curves. The work-hardening rate decreased continuously as the applied stress increased in the austenite domain, displaying an inflexion point near the peak stress while, in the ferrite region, where the work-hardening rate decreased linearly as the applied stress increased, no inflexion point was observed. These data were compared with one-parameter models, in which the total dislocation density was selected as the determining microstructural characteristic. The difference in the expected microstructural evolution is discussed and equations are given for the flow stress curve modeling for both phases.