Electron backscatter diffraction (EBSD) was used to examine the microstructural evolution in a one-step thermomechanically processed 304 austenitic stainless steel specimen during the thermomechanical process of grain boundary engineering. Solution-treated materials were cold-rolled to 3% reduction and subsequently annealed at 1220 K for different annealing times. The EBSD observation of the specimen showed an increase in the frequency of coincident site lattice (CSL) boundaries and a decrease in the percolation probability of random boundaries. Additionally, the specimen exhibited heterogeneous growth of clusters of grains that contained a high frequency of CSL boundaries. These clusters of grains were developed in the entire observed area by strain-induced grain growth according to the results of grain orientation spread analysis. The details of the growth of the clusters and the disconnection of random boundaries were successfully observed in situ using EBSD and a heating stage. The frequency of CSL boundaries increased with the growth of the clusters. Disconnection of random boundaries between the clusters was achieved by the formation of annealing twins through the impingement of the growing clusters during the thermomechanical process. Twin variant selection to introduce CSL boundaries into a random boundary network was observed by the in situ EBSD observation.
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