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UBC Theses and Dissertations

Laser ultrasonics measurement of ferrite formation during stepped cooling Hayashi, Nobumasa


Dual phase steels consist of soft ferrite and hard martensite phases and have been the most used advanced high strength steels in automotive design. The control of the phase transformations during processing is essential to achieve their optimal mechanical properties. Thus, sensor technology which can in-situ monitor phase transformation is of great interest. Recently, laser ultrasonics (LUS) has been gaining attention as an in-situ monitoring technique for the microstructure evolution. In this work, phase transformation kinetics of two low carbon steels with potential dual phase chemistries has been investigated using LUS for thermal treatments conditions relevant for run-out table cooling in hot strip mills. Comparing the ultrasound velocity changes during continuous cooling with conventional dilatometry, it was confirmed that LUS can successfully monitor the phase transformation in the present steels. For the industrially relevant stepped cooling transformation tests, the fraction transformed concluded from ultrasound velocity agrees well with the ferrite phase fraction in the final microstructure as obtained from ex-situ metallography. The hardness has a linear relationship with the fraction transformed, which is consistent with literature data for dual phase steels. The evolution of the normalized velocity change for a given isothermal holding temperature can be described by the JMAK approach with the exponent and the rate parameters being consistent with the literature data for the austenite-to-ferrite transformation. Depending on the heat treatment conditions, banded or non-banded microstructures were observed. FEM analysis confirmed that the geometric configuration of phases has a negligible effect on the LUS phase transformation measurement, indicating the robustness of the LUS method.

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