Effect of Nitriding Temperature on Nitride Precipitation Behavior of High Induction Oriented Silicon Steel with Low-temperature Reheating

Scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and selected area electron diffraction (SEAD) were used to characterize and analyze the precipitated phases of high induction oriented silicon steel with low-temperature reheating under nitriding process, and to explore the mechanism of nitride precipitation and transformation during the nitriding process. The results indicate that before nitriding treatment, the inherent nitrides in the steel are mainly AlN, AlN+MnS, and AlN+Cu_xS, with a size distribution of 40–150 nm. The distribution of precipitates in the entire matrix is relatively dispersed, mainly within the grains, and the precipitation amount on boundaries is relatively small. After nitriding treatment at 750 ℃ and 900 ℃, a large number of newly precipitated nitrides appear on the surface of the nitriding strip, and the precipitated phases are distributed at the grain boundaries and their vicinity, as well as inside the grains. With the increase of nitriding temperature, the type and morphology of precipitates become more complex and diverse. The main range of phase size distribution on surface chromatography decreases from 50–150 nm to 20–100 nm, and the distribution density of precipitates below 120 nm increases from 9.449×10⁸ particles/cm² to 1.649×10⁹ particles/cm². The average size decreases from 90 nm to 55 nm, and the volume fraction decreases from 5.55% to 3.63%. Although the precipitation distribution of the obtained nitrides is uneven along the thickness direction of the nitrided strip, the distribution density and content of the precipitated phases in the central layer of the nitrided strip at 900 ℃ are significantly higher, and the average size is significantly smaller, indicating that increasing the nitriding temperature can significantly improve the uniformity of nitrogen content and nitride distribution in the thickness direction of the nitrided strip. The nitriding temperature increases from 750 ℃ to 900 ℃, and the type of nitride precipitation changes from amorphous structure rich in Mn Si₃N₄ to orthogonal crystal structure MnSiN₂ or (Si, Mn)N→(Si, Mn, Al)N or (Si, Al, Mn)N → (Al, Si, Mn)N or (Al, Si)N. Therefore, the influence of nitriding temperature on the thermal stability of nitrides, the diffusion path and diffusion coefficient of nitrogen atoms is the main reason for the changes in the type and distribution of nitrides.