Study of the ferrite grain coarsening behind the transformation front by electron backscattered diffraction techniques

Abstract

The degree of ferrite grain refinement that can be reached in low-carbon microalloyed steels by thermomechanical processing is limited, to a certain extent, by the grain coarsening which can take place behind the transformation front. The coarsening of ferrite grains is the result of two different mechanisms: elimination of ferrite grains produced by normal grain growth after full impingement on the austenite grain boundary plane and/or coalescence between different ferrite grains with close orientation formed from the same crystallographic variant. The lack of experimental data to support either process is due to the experimental difficulties encountered when analyzing the phenomenon. Some transmission electron microscope (TEM) observations reveal that the ferrite grains formed along a prior grain boundary in deformed austenite are separated by a mixture of low and high angle grain boundaries upon impingement. In the present work, the electron backscattered diffraction (EBSD) technique has been applied to investigate the microstructural evolution during transformation, with special emphasis placed on the α-α grain boundary character as a means of investigating the contribution of coalescence/grain growth to coarsening.