In-situ observation of abnormal grain growth in a low-alloyed carbon steel using SEM-EBSD

Because abnormal grain growth (AGG) degrades mechanical properties of industrial polycrystalline materials such as steel, understanding and controlling AGG are important. In this study, AGG of Al-Nb-microalloyed lowcarbon steel was investigated at 1100 degrees C using in-situ scanning electron microscopy-electron backscatter diffraction (SEM-EBSD). Owing to the pinning particles of AlN and Nb(C,N), fine austenite grains formed initially, and AGG appeared owing to the dissolution of the pinning particles at high temperatures. Relatively large grains invaded the surrounding smaller grains with a size ratio of approximately 0.3, resulting in AGG. We developed the in-situ observation method to investigate the AGG of the carburization process using a diffusion couple of high- and lowcarbon steels. The carbon diffusion into the low-carbon steel from the high-carbon steel enhanced the grain growth of the low-carbon steel. Although the detailed mechanism is still unclear, we clearly showed that carburization can reduce the pinning force. When focusing on the misorientation of the grain boundary, grain boundaries with misorientation angles of 50-59 degrees remained during AGG, which is explained by their lower mobility owing to their lower grain boundary energy. These results suggest that the difference in grain-boundary mobility can induce duplex grain growth with a bimodal distribution, resulting in AGG.

Automated summary

This study investigated the abnormal grain growth (AGG) process in Al-Nb-microalloyed low carbon steel at high temperatures, revealing that the dissolution of pinning particles, enhanced grain growth due to carburization, and lower mobility of grain boundaries with specific misorientation angles are key factors influencing AGG. The findings suggest that differences in grain boundary mobility can induce duplex grain growth, resulting in AGG.