Interplay between precipitate evolution and abnormal grain growth in electrical steel during high temperature annealing

To correlate the abnormal grain growth with the precipitate evolution, a nitrided electrical steel was investigated at different annealing temperatures up to 1250 °C. Scanning/Transmission electron microscopy (S/TEM) results show that three types of precipitates are present in the steel: type I - large irregular core-shell (Mn,Cu)S-(Al,Si)N precipitates; type II - medium-size six-sided prismatic AlN-based precipitates ((Al,Si)N-(Mn,Si)N or (Al,Si)N–CuS); type III - small spherical Cu_2-xS (0<x < 0.6) precipitates. A novel method (30 kV high angle annular dark field (HAADF) - SEM image stitching in transmission mode) is employed to quantitatively obtain accurate size distribution of type II precipitates between 10 and 100 nm over large areas, and distinct areas with high and low number densities of type II precipitates are observed. Type III precipitates first dissolve between 830 °C and 900 °C and then precipitate out again as Cu_2-xS precipitates during (slow) cooling from annealing above 900 °C. The results demonstrate that the dissolution of type II precipitates is responsible for activating the abnormal grain growth process, whereas type III precipitates only indirectly affect this process by acting as nuclei on which type II precipitates can grow. This work provides an in-depth understanding of precipitate (composition and size) evolution during high temperature annealing and illuminates the role precipitates with different compositions and sizes play in the abnormal grain growth process.