AHSS Through Paraequilibrium Carbon Partitioning and Austenite Stabilization

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Funding provided by this grant will be used to develop a new generation of advanced high strength steel (AHSS) exhibiting exceptional mechanical properties achieved by creating microstructures comprised of martensite and austenite. Determining the maximum amount of austenite that can be retained in relatively lean alloys by fully exploiting the phenomenon of stabilization will be a central goal of the work. A double stabilization processing scheme will be employed. Candidate alloys will be fully austenitized after cold rolling and cooled rapidly enough to circumvent the bainite transformation in route to an initial hold temperature. The first stage of stabilization will occur at that point when the steel is briefly held just above its Ms temperature. The steel then will be cooled to its final quench temperature chosen to cause a prescribed volume fraction of the austenite to transform to martensite. Additional stabilization will occur when the steel is aged at a higher temperature to allow partitioning of carbon from the martensite into the austenite under paraequilibrium, carbide-free conditions. This second stabilization will enable the austenite to have the required resistance to transformation to martensite when the steel is finally cooled to room temperature. The broader impact of the project is the development of a new generation of steel for advanced engineering designs. To realize a viable new structural material a high volume fraction of austenite with optimized stability must be obtained within the constraints of a carbon content low enough to not severely compromise weldability, alloy levels that do not greatly increase the cost of the steel and a processing path consistent with current sheet steel production practice. The strength and ductility combinations that will become available are intended for the production of lighter weight components with increased resistance to fracture enabling greater energy savings and margins of safety.