Estudio y modelización de la recristalización estática de aceros ferríticos laminados en frío

Abstract

New generation ferritic steels for the automobile industry are designed to obtain, and even improve, the toughness and strength properties of common steel sheets, but with lower thickness. Therefore, the steels are subjected to a large and complex processing route, which includes hot-rolling in the austenitic and/or ferritic range, cold-rolling to obtain the required thickness of the sheets, annealing at temperatures below Ac1, temper rolling to grant the material its _nal mechanical properties and, eventually, a galvanisation process to increase the corrosion resistance. One of the most interesting investigation topics in these materials consists in substituting the annealing and subsequent temper-rolling process by a controlled annealing stage. The monitoring of the volume fraction of recrystallised ferrite with regard to the annealing temperature and time allows to obtain an optimum balance of toughness and strength without applying a temper-rolling. Due to the high importance of microstructural characterisation of these steels during controlled annealing, in this work an optimisation has been performed of the metallographic techniques that are used to reveal and describe accurately the evolution of the microstructure during this annealing stage. The most important contribution of this work constitutes the design of a recrystallisation model that includes, for the _rst time, the role of the presence and morphology of cementite particles. This is based on the observation that carbide precipitates accelerate the recrystallisation process, due to the increase of the driving force for this transformation. Moreover, it has been observed that this process is increasingly relevant at higher particles sizes. The global recrystallisation model presented in this work correctly predicts the evolution of the microstructure during annealing.