Strengthening of steel Austenite and new multiphase microstructures

Plastic deformation mechanisms are a direct function of the crystalline structure of the metals under consideration. Steels, depending on their chemical composition and thermomechanical treatments, present themselves at room temperature as two allotropic varieties, ferrite with a face-centered cubic structure and austenite with a face-centered cubic structure. After describing the role of microstructure in the hardening of ferritic steel grades [M 4 341] , this article looks at the deformation of austenitic grades, of which stainless steels are emblematic. The excellent combination of high mechanical strength and high elongation at break has led to a marked revival of interest in chromium-free austenitic grades. The nature and content of additive elements control the austenite's mechanical behavior and stability. In addition to dislocation sliding, plastic deformation of these steels at room temperature is accompanied by maclage and induced martensitic transformation, depending on the austenite's (meta)stability.

Austenite stability depends on its chemical composition. By adjusting its chemical composition, austenite can be partially transformed into martensite during plastic deformation, leading to a two-phase microstructure. It is also possible to directly obtain multiphase grades composed of two or more of the steels' constituents. The behavior of these multiphase grades then depends on the nature and volume fraction of each phase, as well as their possible evolution during deformation. These new microstructures open the way to a very wide variety of mechanical behaviors, and above all help to limit the drop in ductility as maximum mechanical strength increases.

This article presents new avenues for the development of ultra-high-strength steels, and gives some examples of new grades. It concludes with elements for modeling the behavior of multiphase steels, highlighting the relationships between microstructures and mechanical properties.