Application-oriented development of high-strength, high-Mn steels and their utilization

Wesselmecking, Sebastian; Bleck, Wolfgang (Thesis advisor); Krupp, Ulrich (Thesis advisor); De Moor, Emmanuel (Thesis advisor)

Aachen : RWTH Aachen University (2022, 2023)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022, Kumulative Dissertation

Abstract

Specialization of engineering sciences accelerates, by ever-shorter development cycles, resulting in growing complexity in research. Consequently, highly specialized experts work either in fundamental research or in applied engineering. To counteract this division of expertise, fundamental scientists and engineers have started to cooperate in the collaborative research center "Steel ab initio". The aim was to develop new and easy to-apply methodologies to characterize and improve the properties of steels. These methods were then applied to the new class of High Manganese Steels (HMnS), which is the focus of this dissertation. The work can be subdivided into three thematic areas: the characterization and exploitation of the Short-Range Ordering (SRO), the adaptation of the mechanical properties by exploiting the specific deformation mechanisms and the transfer of the specific properties of HMnS into the application. To exploit SRO, it first had to be characterized in detail. Thus, SRO formation was analyzed during strain aging at temperatures between room temperature and 200 °C. The kinetics of SRO formation were successfully described by small angle neutron scattering. The formation of SRO was shown to be a result of aging after pre-deformation and was accompanied by increasing yield strength, decreasing deformability and inhomogeneous deformation. The mechanical properties of HMnS are further highly affected by the specific deformation mechanisms. The activation of the TWIP effect increased the twin density and led to a material with very high strength, comparable to martensitic steels. The approach was developed on the basis of the temperature dependence of the Stacking Fault Energy (SFE) and the specific characteristics of the TWIP and the TRIP effect. TWIP and TRIP were activated in a sequential manner. The phase transformations, were tracked by means of Synchrotron XRD. Here, the deformation induced evolution of the martensite in the strongly twinned microstructure was monitored. The resulting material showed a high yield strength and high deformability. Since utilization of HMnS not only involves material properties but also the component-design, a new two-stage crash box was developed. An example of a comprehensive design-process was shown. A TWIP steel was adapted to an automotive crash box by adjusted annealing strategies. Conversely, the crash box was modified to the characteristics of the recrystallized TWIP steel. The study included the simulation, production and testing of the two-stage crash box. The simulations showed that the specific energy absorption of TWIP steels, exceeds the energy absorption of those currently in use.The work extends the understanding of the specific properties (SRO, SFE, TWIP and TRIP) of HMnS and their characterization. Scientific as well as engineering approaches were successfully combined, enabling an application-oriented development of high-strength HMnS.

Institutions

• Division of Materials Science and Engineering [520000]
• Chair of Materials Engineering of Metals and Department of Ferrous Metallurgy [522110]