Entwicklung hochfester Quenching & Partitioning Stähle

Translated title of the contribution: Development of high-strength Quenching & Partitioning steels

Christoph Kickinger

    Research output: ThesisDoctoral Thesis


    The rising demands on safety and weight reduction of modern automobiles are the main driving forces for the development of new Advanced High Strength Steels (AHSS) in the automotive industry. One possibility to achieve these demands is the use of Quenching & Partitioning (Q&P) -steels. Q&P-steels consist of a martensitic-austenitic microstructure and therefore exhibit a very high strength and a good combination of local and global formability due to the Transformation Induced Plasticity (TRIP) -effect. For this purpose, the austenite must have a balanced stability against stress and strain induced transformation. To enable the stabilization of the austenite by the partitioning of carbon, Si has to be alloyed. However, Si reduces the weldability of galvanized steels by promoting Liquid Metal Embrittlement (LME). Furthermore, Si plays a role in the build up of oxides on the furnace rolls during the heat treatment of the steel. Roll oxidation leads to increased maintenance costs of the annealing lines and deteriorates the surface quality of the steel strips. Due to the high alloying contents of Q&P-steels, a reduction in strength combined with a simultaneous increase in ductility is only possible by introducing additional phases into the microstructure. For this purpose, the addition of ferrite has already been proposed in literature. The aim of this thesis is the investigation of the influence of alloying elements on microstructure and mechanical properties of Q&P-steels with reduced sensitivity to LME and surface oxide formation. For this purpose, 18 different alloys with varying amounts of Si, Al, Mn, C and B were produced and heat-treated on a simulator. The phase transformation and grain growth behavior was investigated by dilatometry and laser confocal microscopy. The mechanical properties were analyzed by tensile and hardness tests, and the microstructures by light optical and electron microscopy. Another aim of the work is the examination of partially ferritic Q&P-steels. For this purpose, industrially produced material was heat treated on a simulator to analyze the influence of the initial microstructure and the ferrite morphology on the mechanical properties of the steels, as well as to determine the producibilty on existing industrial annealing lines. A replacement of Si by Al leads to lower strength and global formability at the same time. This follows from an increase in grain size and an overstabilization of the retained austenite (¿R), which leads to a reduced TRIP-effect. Only at very low Si-contents, Al increases the ¿R-amount and thus enhances the global formability. Both C and Mn lead to higher strength by solid solution strengthening and by the delaying of the ¿-¿-transformation, as well as to better global formability by increasing the amount of ¿R with sufficient stability. B only has a marginal effect on the mechanical properties of Q&P-steels. It delays the bainite formation during the partitioning-step and hence, results in higher ¿R-amounts and a better ductility. The insert of ferrite into the microstructure is possible by intercritical annealing as well as slow cooling after complete austenitization. Both strategies lead to a reduction in strength and a considerable enhancement in global ductility. However, partial austenitization is to prefer because it results in a finer microstructure, which yields a better combination of strength and ductility. An additional annealing step can also extend the spectrum of properties to higher ductility at lower strength. In contrast, a variation of the warm rolled microstructure only has little impact on the properties of Q&P-steels.
    Translated title of the contributionDevelopment of high-strength Quenching & Partitioning steels
    Original languageGerman
    Awarding Institution
    • Montanuniversität
    • Schnitzer, Ronald, Supervisor (internal)
    • Sommitsch, Christof, Assessor B (external)
    • Clemens, Helmut, Co-Supervisor (internal)
    • Suppan, Clemens, Co-Supervisor (external), External person
    • Pippan, Reinhard, Assessor A (internal)
    Publication statusPublished - 2022

    Bibliographical note

    embargoed until 01-12-2027


    • Quenching and Partitioning (Q&P)
    • Advanced high strength steel (AHSS)
    • influence of alloying elements
    • microstructure
    • mechanical properties
    • partially ferritic
    • morphology
    • Q&Q&P
    • influence of starting microstructure

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