Splitting of needle-like precipitates in grain-oriented silicon steel manufactured by the acquired inhibitor method

Highlights: • A sawtooth splitting of a needle-like precipitate in HGO silicon steel was observed for the first time. • The composition, structure and formation of the needle-like precipitate have been analyzed. • Based on the analysis of the evolution process, the splitting mechanism of the needle-like precipitate was clarified. • The inhibitor observed after nitriding cannot represent the actual state during high-temperature annealing due to splitting. • The evolution of the inhibitor formed by nitriding is more complex than that of inherent inhibitor of HGO silicon steel. Needle-like precipitates are formed in high-permeability grain-oriented (HGO) silicon steel manufactured by the acquired inhibitor method after nitriding. A sawtooth splitting of the needle-like precipitate was observed in high-temperature annealing. To understand this, the needle-like precipitates at different temperatures have been analyzed using scanning electron microscope (SEM) and transmission electron microscope (TEM). The needle-like precipitate is defined as (Al,Si,Mn)N having a hexagonal crystal structure similar to AlN, and its longer direction correspond to the [0001] (c-axis) direction. Its needle shape is closely related to the growth conditions including high supersaturation of nitrogen and low nitriding temperature. With the increase of temperature, the needle-like (Al,Si,Mn)N gradually transforms to AlN, and it splits along the gaps at c-axis edges into 6–8 hexagonal regular particles finally. In this process, the crystal structure and orientation stay the same. The evolution of morphology shows the sawtooth unique characteristic because the dissolution of unstable (Al,Si,Mn)N precipitate occurs simultaneously with the formation of regular AlN precipitates under the near-equilibrium condition. The splitting results in the dispersing and increasing of precipitates in steel, which is significant for studying secondary recrystallization