Microstructural Evolution during Q&P Processing and Effects on the Mechanical Properties

Research output: ThesisDoctoral Thesis

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Abstract

Advanced high strength steels (AHSS) are highly sophisticated materials that have attracted growing interest for automotive applications due to their lightweight potential and improved safety performance. Especially the current development of the third generation of AHSS entails a further improvement in terms of a well-balanced strength-ductility combination. Among them, the quenching and partitioning (Q&P) approach, which combines a strong martensitic matrix with an increased fraction of metastable retained austenite to ensure high formability, is one of the most promising concepts. The present thesis aims to reveal the relationship between process parameters, microstructure and mechanical properties of Q&P steels. Since conventional characterization methods are mostly limited to the final microstructure, in-situ high-energy X-ray diffraction was employed to directly access the transformation kinetics during the heat treatments. In combination with additional high-resolution methods, it was possible to establish a better understanding of the interactions between carbon partitioning, carbide precipitation and austenite decomposition occurring during the Q&P process. For example, it could be shown that carbon rapidly enriches in the austenite at the very beginning of partitioning at elevated temperatures. Furthermore, austenite decomposition due to bainite formation could not be suppressed irrespective of the partitioning parameters, while the precipitation of carbides occurred only in certain cases. The mechanical properties were determined by tensile testing and the obtained results showed a promising combination of strength and ductility. Moreover, in-situ tensile tests were performed to study the austenite to martensite transformation, commonly known as transformation-induced plasticity (TRIP) effect, upon external loading. By this means, it was confirmed that metastable austenite with an adequate stability efficiently contributes to an enhanced work hardening behavior via the TRIP effect. Additionally, the Q&P process was compared with the TRIP-aided bainitic ferrite (TBF) process, which represents another concept for the third generation of AHSS, and the major differences were elaborated. Overall, it could be shown that Q&P is a very robust process with little influence of the applied heat treatment parameters, and moreover the fast carbon partitioning suggests the possibility of short process times regarding an industrial implementation.

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Translated title of the contributionMikrostrukturentwicklung während des Q&P Prozesses und Auswirkungen auf die mechanischen Eigenschaften
Original languageEnglish
QualificationDr.mont.
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Publication statusPublished - 2020