CHARACTERISATION OF A COLD ROLLED 2101 LEAN DUPLEX STAINLESS STEEL

摘要

Duplex stainless steels (DSS) may be defined as a category of steels having a two phase ferritic-austenitic microstructure, which allow combining good mechanical and corrosion properties. However, due to the large amount of alloying element content, to the instability of the ferritic matrix at the high temperatures and to the presence of the metastable austenitic phase, these steels can undergo significant microstructural modification, with formation of new phases, as a consequence of either thermo-mechanical treatments or during plastic deformation. Type and number of these new phases are strictly correlated to chemical composition, heat treatment temperature range and strain associated to deformation process. In relation to the different conditions at which the steel is subjected, ferrite decomposition (which leads to σ- and χ-phase formation), carbides precipitation and martensite formation from austenite can occur. The latter takes place after plastic deformation at room temperature and is referred to the so-called strain-induced martensite formation. Identification of the new phases is not always easy: especially for non-diffusive transformation, where no chemical composition differences are expected, and for low new phase content, making difficult the use of massive analyses such as X-ray diffraction (XRD) techniques. On the contrary, electron backscattered diffraction (EBSD), being a local technique, could help in the identification of microstructural features observed after thermo-mechanical processing. In the present work, a 2101 lean duplex stainless steel was subjected to cold rolling at various degree of deformation (from 10 % to 85 % thickness reduction). The steel was analysed by means of a scanning electron microscope combined with EBSD in order to investigate the microstructural changes after the plastic deformation. XRD measurements, hardness tests and magnetic investigations were also performed to better characterize and understand the martensitic transformation. Cold rolling increases the crystals disorder and thus hinders the EBSD investigations: in fact, samples more intensively rolled exhibited low pattern quality. Moreover, deformation is not homogeneous: the austenite phase seems to undergo a more intense shear, acting as a soft phase inside a harder ferrite matrix. Consequently, austenite decomposition is not easily observable. Furthermore, a direct relationship between microstructure and magnetic properties was revealed. In particular the reciprocal of relative magnetic permeability and the coercivity increased with martensite content and the amount of cold deformation.