Computational Materials Design of Medium Mn Steels

Abstract: Medium Mn steels (~ 3–10 mass % Mn), a new category of advanced high strength steels, attracted worldwide research interests recent years due to their excellent mechanical properties and low cost. These steels have fine microstructures and contain large fraction of metastable retained austenite (~ 30 volume %), therefore exhibit excellent strength and elongation. The fine microstructure is mainly introduced by an intercritical annealing process.To accelerate the design of such steels, materials design is applied. The materials design concept is a systematic method. Contrary to conventional methods largely based on trial and error, it is based on the classical processing–structure–properties relationships and a quantitative knowledge of each relation represented by a mathematical model, so-called linkage model. Such models are thus an essential part in materials design.The present thesis aims to develop a framework used for materials design of medium Mn steel. The development of models which serve as linkage tools is thus the focus. Tensile properties, i.e. strength and elongation, are set as the design objectives driven by the industrial application.The major part is concentrated on the linkage tools of processing–structure, i.e. models and simulations to predict the microstructure evolution associated with processing. These linkage tools are based on thermodynamic calculations and kinetic simulations using the commercially available Thermo-Calc and DICTRA software. To be specific, the processing involves austenitization and quenching as well as intercritical annealing and quenching; while the associated structure involves transformation of austenite to martensite and reversion of martensite to austenite. Therefore the following aspects have been studied:martensite fraction with undercooling;austenite reversion during intercritical annealing;influence of austenite grain size on martensite start temperature;mechanical stability of retained austenite.Besides these, prediction of tensile properties is studied in the last part, which serves as an example of a linkage tool of structure–properties.Via integrating these models, to achieve certain tensile properties, the required microstructure and the associated processing can be traced back.