The characteristics of inclusions in as-cast FeCrAl-(La) alloys were investigated by using scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and confocal laser scanning microscopy (CLSM). The types, morphologies and behaviour of inclusions were examined, and further discussion focused on the analysis of thermodynamic calculations. The results showed that the characteristics of Al 2 O 3 and AlN in FeCrAl alloys were different from those in regular Al-killed steels. The Al-O equilibrium diagram is utilized to understand Al 2 O 3 formation in high-Al steels. The formation of AlN in the liquid phase and at the solidifying front of FeCrAl alloys was investigated based on the theoretical calculations and the in-situ observations at high temperatures. La addition affected the formation of AlN and also generated La-containing inclusions as well. Based on the inclusion observations and the thermodynamic calculations, the predominant formations and possible precipitation path of La-S-O precipitates was theoretically predicted.

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The effect of cooling rate on the evolution of AlN inclusions precipitated during solidification in FeCrAl stainless steel was investigated using an experimental study and thermodynamic and kinetic calculations. The number and size of AlN inclusions precipitated under different cooling rates were examined with the feature function of the field-emission scanning electron microscope. A model combining micro-segregation and the diffusion-controlled growth model was set up to determine the mechanism of AlN particle growth. The results showed that AlN precipitates in the mushy zone. The size of AlN particles decreases and the number of AlN particles increases with increasing cooling rate, whereas the volume fraction is essentially unchanged. The AlN particles grow during solidification after the content of solutes in molten steel has exceeded the concentration in equilibrium with AlN. The nitrogen content varies significantly with the cooling rate during solidification. Increasing the cooling rate and reducing the nitrogen content in the molten steel can reduce the AlN particle size in FeCrAl alloys as the growth time decreases.