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Effect of boron on microstructure and mechanical properties of low carbon microalloyed steels

Low carbon bainitic steels microalloyed with Nb, Ti and V are widely used for the pipeline, construction and automobile industries because of their excellent combination of strength, toughness and weldability. Boron as another major alloying element has been also frequently used in this type of steels since the 1970s. The purpose of adding boron is to improve the hardenability of the steel by promoting bainite formation. / It has been realized that Boron can only be effective as a strengthening element when it is prevented from forming BN and/or Fe23(C, B) 6 precipitates. Therefore, Boron is always added together with other alloying elements which are stronger Nitride or Carbide formers, such as Ti and Nb. However, the formation of complex bainitic structures and the interaction with precipitates at industrial coiling temperature are not adequately understood. / In this study, the effect of boron on the microstructure and mechanical properties of a low carbon Nb-B steel was studied by a hot compression test (50% reduction at 850°C) followed by quenching samples into a salt bath. The microstructures of the tested samples were examined through optical microscopy and SEM; and the mechanical properties of these samples were investigated by micro-hardness and shear punch tests. / The results indicate that during thermo-mechanical controlled rolling (TCR), the final properties of the products not only depend on the applied deformation but also depend on the coiling temperature where phase transformation takes place. According to the investigation, two strengthening mechanisms are responsible for the strength of the steel at the coiling temperature: phase transformation and precipitation. Under optical microscopy, the microstructures of all specimens appear to be bainite in a temperature range from 350°C to 600°C without distinct differences. However, the SEM micrographs revealed that the microstructures at 550°C are very different from the microstructures transformed at the other holding temperatures. / Two strength peaks were observed at 350°C and 550°C in the temperature range studied. It is believed that the NbC precipitates are the main contributor to the peak strength observed at 550°C because the kinetics of NbC is quite rapid at this temperature. The strength peak at 350°C is mainly due to the harder bainitic phase, which formed at relatively lower temperature.

Identiferoai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.112575
Date January 2007
CreatorsLu, Yu, 1977-
PublisherMcGill University
Source SetsLibrary and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada
LanguageEnglish
Detected LanguageEnglish
TypeElectronic Thesis or Dissertation
Formatapplication/pdf
CoverageMaster of Engineering (Department of Mining and Materials Engineering.)
RightsAll items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated.
Relationalephsysno: 002712534, proquestno: AAIMR51466, Theses scanned by UMI/ProQuest.

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