Effect of niobium, molybdenum and boron on the mechanical properties and microstructures of direct quenched ultra-high-strength steels.

Thesis event information

Date and time of the thesis defence

Place of the thesis defence

Linnanmaa, auditorium IT116

Topic of the dissertation

Effect of niobium, molybdenum and boron on the mechanical properties and microstructures of direct quenched ultra-high-strength steels.

Doctoral candidate

Master of Science Jaakko Hannula

Faculty and unit

University of Oulu Graduate School, Faculty of Technology, Materials and Mechanical Engineering Research Group

Subject of study

Materials Engineering

Opponent

Professor Hardy Mohrbacher, KU Leuven

Second opponent

Professor Pasi Peura, Tampere University

Custos

Professor Jukka Kömi, University of Oulu

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Improving the properties of ultra-high-strength steels by microalloying.

During the last ten years, the demand for ultra-high-strength steels has grown significantly. More strict environmental criteria and more demanding applications have accelerated the development of ultra-high-strength steels, and also improved the properties of these steels. Especially ultra-high-strength steels produced with direct quenching technology have become an important topic in the steel research.

Common applications for ultra-high-strength steels are crane booms, transportation equipment, heavy machinery and forest machines. In many of these applications, by replacing the traditional low-strength steel with ultra-high-strength steel, a considerable weight saving can be achieved, which leads to for example lower fuel consumption and higher load capacity.

The chemical composition of direct-quenched steels, together with the processing parameters, significantly affect the properties of the steel. The use of various microalloying elements has increased, especially in steels designed for demanding applications. These microalloying elements can clearly improve the mechanical properties of steel. Therefore, the aim of this work is to increase the understanding of microalloying in ultra-high-strength steels, especially produced by direct quenching technology. This thesis focuses on the use of niobium, molybdenum and boron in ultra-high-strength steels.

The results of the dissertation showed that phase transformations, microstructures and mechanical properties of investigated steels can be influenced by microalloying. The proper microalloying combined with the proper process parameters can improve the mechanical properties of steel, however without a well-planned manufacturing process, the benefits of microalloying can be negligible. Especially in the tempering treatment, microalloying plays a significant role in achieving the sufficient strength level. The obtained results of the dissertation can help to optimize the chemical compositions of ultra-high-strength steels and to develop new steel grades.
Last updated: 15.8.2022