Year

2013

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials and Mechatronic Engineering

Abstract

To evaluate the role of niobium (Nb) segregation and dynamic recrystallisation in the development of non-uniform ferrite microstructures in microalloyed steels, two Nb microalloyed steels with Mn contents of 1.2 and 0.5wt %, and Non-Nb steel were selected. Different processing parameters, such as the austenitising/deformation temperature, inter-pass time, pass strain, and strain sequence, were investigated to study the behaviour of Nb and its effect on austenite recrystallisation and the ferrite microstructure homogeneity. High and Low Mn steels were austenitised at two temperatures (below and above the grain coarsening temperatures and deformed at three temperatures (below, near, and above the non-recrystallisation temperature (Tnr). The influence of the Mn content and inter-pass time upon deformation at high temperature has also been studied.

The addition of 0.06 wt% of Nb increased the Tnr by 75ºC compared to the Non-Nb steel. Conversely, an increase in Mn led to a lower by 25°C Tnr and by 50°C austenite to ferrite transformation temperatures. Increase in the austenitising temperature resulted in a stronger grain boundary pinning effect from Nb solute atoms and the precipitates, but this pinning effect became weaker when the content of Mn was reduced. The presence of co-clusters of Nb-C solute atoms and fine precipitates in sizes < 8 nm suppressed austenite recrystallisation either fully or partially, near and below Tnr. Above Tnr, an increase in the inter-pass time resulted in austenite grain growth due to ineffective pinning from Nb solute atoms in the absence of fine precipitates.

The ferrite inhomogeneity level was decreased by lowering the austenitising temperature for High and Low Mn steel when they were also deformed at a high temperature, i.e. above Tnr. Also, more Nb precipitation occurred in ferrite when deformation occurred at high temperature, where there was either no or limited strain induced precipitation, than when the deformation was applied below Tnr. An increase in the inter-pass time had a minor effect on the inhomogeneity of the ferrite grain structure for High and Low Mn steel when deformed near or below Tnr.

There was a remarkable improvement in the ferrite microstructure homogeneity when the fine and uniform austenite grain size was achieved after the deformations above Tnr, which was followed by the deformation in non-recrystallisation region. This was a result of the thermo-mechanical processing schedule, which involved lower austenitising temperature and increased cooling rate to prevent grain growth between the roughing and finishing stages.

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