Correlations of microstructural and mechanical anisotropy of ultra-high strength steels

Raphael Esterl

    Research output: ThesisDoctoral Thesis

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    Abstract

    For the construction of mobile crane arms, ultra-high strength steels (UHSS) come to application to fulfil the requirements of an increased payload to weight ratio. Steels produced via thermomechanical processing and direct quenching (TM + DQ) provide several advantages over conventional quenching and tempering (Q+T) steels through the systematic combination of micro-alloying elements (MAE), process parameters and the subsequent quenching from the rolling heat. For a given carbon content, clear strength benefits can be reached by the advanced TM process route. Materials properties may be adjusted by an additional tempering heat treatment, which is accompanied by an asset in ductility. Nonetheless, the combination of MAE and low finish rolling temperatures (FRT) promotes certain properties that adversely affect the isotropy of the material. The microstructural anisotropy by the highly deformed austenite grain yields in an orientation dependence of the mechanical properties, which is expressed by differences between the longitudinal and transversal bendability and impact toughness. In order to establish a correlation between the microstructure and the mechanical properties of UHS steels, the effect of the processing route and MAE on the condition of the prior austenite grain was studied. By this means, the goal of the present work was to characterize the underlying microstructural processes and reveal the reasons for the mechanical anisotropy. A metallographic technique was developed to visualize the prior austenite grains and characterize their size and elongation. Double-hit deformations on a dilatometer were applied to study the influence of the MAE on the microstructural processes, which were visualized with this established metallographic method. Based on these observations, test alloys were developed to analyze the effect of MAE and process parameters on the mechanical properties. The investigations showed, that a decrease in the FRT lead to an augmented orientation dependence of the mechanical properties. However, a correlation between the pancaking and the mechanical anisotropy was not found, as a globular γ grain through re-austenitization exhibits similar anisotropy. Micro-alloying with Nb and V, both contribute to an increase of temper resistance, yet, the strengthening through V is accompanied by an embrittlement and significant decrease of the impact toughness. It was found further, that intensive annealing in the austenite region optimizes isotropy, however, certain microstructural features such as texture components are inherited so that a complete extinction of the mechanical anisotropy cannot be achieved. Overall, knowledge on the correlations of process route, micro-alloying elements and microstructure on the resulting mechanical properties of UHS steels could be acquired.
    Translated title of the contributionZusammenhänge zwischen der mikrostrukturellen und mechanischen Anisotropie von ultrahochfesten Stählen
    Original languageEnglish
    QualificationDr.mont.
    Awarding Institution
    • Montanuniversität
    Supervisors/Advisors
    • Sonnleitner, Markus, Co-Supervisor (external), External person
    • Buchmayr, Bruno, Assessor B (internal)
    • Schnitzer, Ronald, Assessor A (internal)
    Publication statusPublished - 2019

    Bibliographical note

    embargoed until null

    Keywords

    • Thermomechanical Processing
    • Direct Quenching
    • Micro-alloying
    • recrystallization
    • Ultra-high strength steels
    • mechanical anisotropy

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