A modelled constitutive equation derived from hot working tests to predict hot rolling mill loads is proposed and validated against industrial hot rolling data for AISI 321 stainless steel. Good correlation is found between the predicted mean flow stress, the Zener–Hollomon Z parameter and actual industrial mill load values from mill logs if allowances are made for differences in von Mises plane strain conversion, friction and front or back end tension. The multipass hot working behaviour of this steel was simulated through Gleeble thermomechanical compression testing with the deformation temperature varying between 1200°C down to 800°C and the strain rate between 0.001 s −1 and 5 s −1 . At strain rates greater than 0.05 s −1 , dynamic recovery as a softening mechanism was dominant, increasing the dynamic recrystallisation to dynamic recovery transition temperature to higher temperatures. This implies that through extrapolation to typical industrial strain rates of about 60 s −1 , most likely no dynamic recrystallisation in plant hot rolling occurs in this steel but only dynamic recovery. Grain refinement by dynamic recrystallisation is, therefore, unlikely in this steel under plant hot rolling conditions. Finally, mill load modelling using the hot working constitutive constants of the near-equivalent AISI 304 instead of those specifically determined for AISI 321, introduces measurable differences in the predicted mill loads. The use of alloy-specific hot working constants even for near-equivalent steels is, therefore, recommended.