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Development of bimodal grain structures and their effect on toughness in HSLA steel

Chakrabarti, Debalay (2007)
Ph.D. thesis, University of Birmingham.

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To understand the formation of bimodal ferrite grain structures (i.e. mixed coarse + fine grain sizes) in TMCR HSLA steel plates, as-continuously cast HSLA steel slabs with varying microalloying additions (Ti, Nb and V) were reheated to temperatures in the range 1000-1300 °C and deformed at 1110 °C and 980 °C temperatures in Gleeble 3500. The interdendritic segregation during continuous casting and the consequent inhomogeneous microalloying precipitate distributions (with interdendritic regions being the precipitate-rich regions) has resulted in severely bimodal austenite grain structures under certain reheat conditions (due to the significantly higher pinning force in solute-rich regions compared to solute-poor regions). The segregation of microalloying elements can also promote bimodality during deformation by affecting the local recrystallisation kinetics. Notch-bend fracture tests were performed at –160 °C to investigate the effect of a bimodal grain size distribution on fracture toughness by comparing local fracture stress values for uniformly fine, uniformly coarse and bimodal ferrite grain structures. Analysis of local fracture stress values suggests that bimodality can raise the scatter in the fracture test results and therefore, it is undesirable. Current methods of measuring bimodality are not useful at consistently quantifying small differences in bimodality between microstructures of steel, and hence, two easy-to-measure parameters (peak height ratio, PHR and peak grain size range, PGSR) have been suggested in this study to quantify bimodality in HSLA steels.

Type of Work:Ph.D. thesis.
Supervisor(s):Davies, Claire
School/Faculty:Colleges (2008 onwards) > College of Engineering & Physical Sciences
Department:Department of Metallurgy and Materials
Subjects:TN Mining engineering. Metallurgy
Institution:University of Birmingham
ID Code:989
This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder.
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