Microstructural evolution during the austenite-to-ferrite transformation from deformed austenite |
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Authors: | R Bengochea B López I Gutierrez B López I Gutierrez |
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Affiliation: | (1) the Department of Materials, CEIT, 20009 San Sebastian, Spain;(2) ESII, 20009 San Sebastian, Spain |
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Abstract: | It is well established that the ferrite grain size of low-carbon steel can be refined by hot rolling of the austenite at temperatures
below the nonrecrystallization temperature (T
nr
). The strain retained in the austenite increases the number of ferrite nuclei present in the initial stages of transformation.
In this work, a C-Mn-Nb steel has been heavily deformed by torsion at temperatures below the determined T
nr
for this steel. After deformation, specimens are cooled at a constant cooling rate of 1 °C/s, and interrupted quenching at
different temperatures is used to observe different stages of transformation. The transformation kinetics and the evolution
of the ferrite grain size have been analyzed. It has been shown that the stored energy due to the accumulated deformation
is able to influence the nucleation for low undercoolings by acting on the driving force for transformation; this influence
becomes negligible as the temperature decreases. At the early stages of transformation, it has been observed that the preferential
nucleation sites of ferrite are the austenite grain boundaries. At the later stages, when impingement becomes important, ferrite
coarsening accompanies the transformation and a significant reduction in the number of the ferrite grains per unit volume
is observed. As a result, a wide range of ferrite grain sizes is present in the final microstructure, which can influence
the mechanical properties of the steel. |
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Keywords: | |
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