Guidelines for long ultrafine grained steel production and application to the automotive sector (LUNA) : final report

The main objectives of this project were to supply the EU Steelmaking and Automotive Industry with guidelines for the production using existing productive plants and best utilisation of ultrafine grained carbon steel long products for mechanical applications. A ferrite grain size in the range of 1÷4 μm and a steel microstructure characterised by a mixture of ferrite-pearlite and/or martensite, bainite, microstructure, can give a very good combination of mechanical (strength, ductility, toughness, fatigue) and technological properties (machinability, cold/ warm metal forming, etc.) for final application to automotive components. In particular the project objectives were: - Definition of the most suitable thermomechanical process for long ultrafine grained (UFG) steel production; - Identification of merits, in terms of microstructural and mechanical properties of UFG long steels in as-rolled conditions and after cold/warm forming; - Production of components or component-like dummies in UFG long products and evaluation of their mechanical performances. To give guidelines for automotive UFG long products applications. On the basis of the results of the project DIFT and Heavy γ deformation mechanisms were identified as suitable to be exploited to industrially produce UFG long product steels and the main parameters that control the formation of ultrafine ferrite were assessed. The use of UFG steel showed to allow the improvement of cold forming for fastener production, while no significant advantage was found for powertrain applications since, after their fabrication, they require a further heat treatment that modifies the previous UFG microstructure.