On the process, structure, property relationship for the optimization of strength and bendability of martensitic stainless steels

Abstract

Progresses in the structural engineering of materials for automotive is essential to ensure safe and sustainable means of transportation. The development cycle of new materials can be shortened through the application of design tools that enable rapid identification of pathways for materials optimization. This thesis investigates the property - structure - processing relationship of a novel Niobium-added martensitic stainless steel, with a special focus on the enhancement of crash performance. The first part of the thesis unravels the relationship between properties and structures of the alloy. A multi-scale approach combining mechanical characterizations at the macroscale with finite element simulations at the microscale allows identifying critical microstructural features that control strength, ductility and bendability. Above all, the presence of undissolved Cr-rich carbides and residual islands of soft ferrite, is found detrimental for the bendability of the steel. On the other hand, the presence of thin films of retained austenite have a beneficial effect on the overall mechanical properties. In the second part of this work, CALPHAD-based methods are used to understand the thermodynamics and kinetics of the physical processes leading to the formation of those critical microstructural features. On the one hand, the presence of clusters of Cr-rich carbides contributes to the stabilization of ferrite even after prolonged heat treatments at high temperatures. On the other hand, the quenching rate significantly influences the presence of austenite in the room temperature microstructures. Finally, different original modelling tools are proposed, which can serve for the design of novel heat treatments and alloy compositions, contributing to the development of high strength stainless steel grades with improved mechanical properties.(FSA - Sciences de l'ingénieur) -- UCL, 201