Evolução estrutural no processamento termomecânico para reversão de um aço inoxidável austenítico ASTM 201LN com efeito TRIP

Abstract

Austenitic stainless steels (ASS’s) are known to have properties such as good mechanical strength, ductility and corrosion resistance. Among the most used ASS’s is the 300 series. However, the steels of this series have a higher cost due to the high Ni content it has. Therefore, the ASS’s of the 200 series gain prominence, since this has a lower Ni content and a lower cost. Another advantage of these materials is that they are able to exhibit the TRIP effect ( textit Transformation Induced Plasticity), in which metastable austenite is transformed into martensite during the cold deformation process. This phenomenon contributes to the increase in hardening capacity due to the increase in mechanical strength with a reduction in the material’s ductility. Another way to promote improvements in the mechanical properties of these steels is through grain refinement, but with maintenance of ductility, which is through a thermomechanical processing, which consists of subjecting the material to cold deformation and, subsequently, to an annealing heat treatment for martensitic reversion. Therefore, this doctoral thesis aimed to carry out a study of the structural evolution of ASTM 201LN steel throughout the thermomechanical processing consisting of cold rolling, in which a thickness reduction greater than 60% was obtained, followed by a heat treatment annealing at 900 °C for between 15 and 180 seconds. The results obtained induced that cold rolling was promising in promoting the TRIP effect in this material, in which a percentage around 40% was added for a thickness reduction of 66%. It was also verified that the conditions adopted in the annealing were sufficient to promote an almost complete reversion of the martensite induced in the cold rolling into austenite. In the sample with a thickness reduction of 66% annealed at 180 seconds, a percentage of residual martensite below 2% was observed. The results obtained in the microstructural characterization defined that the more nucleation sites for reversed austenite, which provided the final annealing treatment, with the martensite reversal process together with the recrystallization of the hardened austenite, obtaining a more refined austenitic microstructure that contributed to a significant increase in mechanical strength, compared to as-received condition.