Analysis and Significance of Non-metallic Inclusions in Non Grain-oriented Electrical Steel

Abstract

NMI in electrical steels are known to have a very complex chemical makeup and history of formation, and exert a large influence on the handling of this steel in secondary metallurgy and casting. We have developed routines to analyse the non-metallic inclusion contents of non grain-oriented electrical steels by automated FEG-SEM analysis. We automatically analyze NMI of sizes down to 0.08 μm2 in a steel that has significant amount of alloyed Si (2.3 wt%) by employing a matrix spectrum subtraction routine that leaves only the signal of the NMI itself to be quantified. We describe data reduction procedures for the NMI populations that consist out of duplexing oxides, sulfides, and nitrides. The chemical complexity can be represented and understood in terms of using a multicomponent projection technique, and based on the analysis of the particles it is possible to calculate quantitatively the amount of elements contained in the NMI assemblage of a steel sample. We find that this mass balance gives results in good to excellent agreement with bulk steel analyses for elements that are dominantly inclusion bound, such as Ca, if the complete inclusion size range above 0.08 μm2 is taken into account. Based on these data analysis methods, we compare the NMI development in two heats through the secondary metallurgy process. Although the same steel composition was alloyed, the NMI developed vastly different depending on the details of treatment on the ladle furnace.