Dynamic recrystallisation in steels

The stress-strain behaviour observed during the hot deformation of steels can be subdivided into three successive domains. Monotonic strain hardening which occurs at low deformations is replaced at large deformations by a steady state regime, the two being separated by a transition zone in which the stress-strain relation can take different forms. Microstructural observations reveal the permanent competition between the deformation mechanisms (dislocation mouvement and muliplication) and the mechanisms of minimising the stored energy (recovery and recrystallization). The disappearance of strain hardening corresponds to the onset of dynamical recrystallization. Depending on the crystal structure, the composition of the steel and the hot deformation conditions, dynamical recrystallization can be qualified as either discontinuous (microstructurally analogous to discontinuous recristallization of pre-strained material after the applied stress is removed) or continuous, the latter corresponding to a prolongation of the process of dynamic recovery and polygonisation simultaneously at all points in the metal. The present paper presents a brief description of the elementary micromecanismes involved in the structural evolution of different types of steel (austenitic and ferritic stainless steels, plain carbon austenite and ferritic silicon steels) during dynamical recrystallization and an outline of the mechanical modelling of the stress-strain behaviour. The consequences for the industrial processing of steel in relation to the optimisation of properties are also invoked. (orig.)