Recycling Steels by Selective Precipitation Treatment Modelling – RESSET

At first sight there is limited room for steel recycling improvement as the recycling rate is already quite high (90%). However, this rate does not account for downcycling aspects. Indeed, recycling a material does not mean that its application is as prestigious as originally. New alloys are thus processed to fulfill the need for structural and functional materials, generating a large part of the CO2 emission over the world. The fact that materials are not simply reused is partly related to sorting issues. The industrial technology lacks maturity to completely differentiate the huge diversity of grades ending up to scrap dealers. Consequently, steel scrap becomes a mix of grades that does not have the composition the research and material development have focused on so far. In turn, the scrap is either mixed with primary chemical elements from ores to return to a well-known composition, or transformed into material parts for which the specifications are less strict. From a sustainability perspective, it is needed to take care of this scrap.
The RESSET project aims at avoiding steel scrap downcycling by exploring new thermomechanical routes leading to improved mechanical properties in presence of contaminants, like Cu, and using materials design concepts. The RESSET framework specifically intends for precipitation sequences able to strengthen the material. To fulfill this objective, modelling tools describing phase transformation kinetics depending on processing conditions and for the constrained scrap composition will be developed. Thermodynamics and kinetics-based derivations will provide criteria for precipitation initiation, equations for precipitate growth and coarsening, and will be coupled with other phase transformations of interest in steels, namely the martensitic transformation or the austenite reversion. To make the framework fully generic, governing equations will be coupled with thermodynamic and kinetic databases. The best processing conditions leading to selective precipitation of contaminants and material strengthening will be found with an optimization algorithm. Experimentally obtained microstructure of a model alloy subjected to a thermomechanical treatment and which composition will be fixed by a typical scrap composition will be investigated to get insights into the microstructure evolution during the processing to help the fundamental model development. These microstructures will finally be compared to the modelling tool predictions in order to validate it and to use it to help steel recycling by providing new solutions.