Development of high strength hot rolled strip steel products with bainitic microstructures

Du, Jinlong (2017). Development of high strength hot rolled strip steel products with bainitic microstructures. University of Birmingham. Ph.D.

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High strength, low alloy, hot rolled strip steels with yield stresses in the range 700 to 1300MPa are required for the Lifting and Excavating product sector. Improved combination of strength and toughness in these high strength steels is desired, requiring a detailed understanding of the relationship between microstructure and mechanical properties.
In this work 12mm thick 700MPa yield stress strip steels with fully bainitic microstructures, with different compositions and/or processing conditions, have been studied. The microstructures of the steels were investigated with both optical microscopy and SEM. Micro-hardness and Charpy impact tests (at different temperatures) were carried out to investigate the mechanical properties, followed up with fracture surface analysis and unit crack path (UCP) analysis.
Three types of bainitic microstructures were identified and quantified, including upper bainite, lower bainite and granular bainite. The fracture surface and UCP analysis indicating that granular bainite is detrimental to toughness, but cannot necessarily be avoided in the steels investigated; while the presence of a small amount of lower bainite (above 5%) was found to improve the impact toughness significantly.
Continuous cooling transformation (CCT) diagrams for steels with different compositions were produced, which allowed investigation of the phase transformation behaviour and selection of optimum chemical composition/coiling temperature for improved properties. The effects of alloying elements (B, Mo and V) on the transformation behaviour have been confirmed through the experiments. An optimised combination of alloying and coiling temperature has been proposed and validated via investigating plant trial products with similar alloying and processing parameters.

Type of Work: Thesis (Doctorates > Ph.D.)
Award Type: Doctorates > Ph.D.
College/Faculty: Colleges (2008 onwards) > College of Engineering & Physical Sciences
School or Department: School of Metallurgy and Materials
Funders: None/not applicable
Subjects: Q Science > QD Chemistry
T Technology > TN Mining engineering. Metallurgy


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