Abstract: Three-dimensional morphological characterization and automatic statistical quantification of typical inclusions such as large-size Al₂O_3,MnS, conventional oxysulfides and nitrides were conducted in the ultra-low carbon steel, free cutting steel, pipeline steel and spring steel by using electrolytic etching and automated inclusion analysis technique. The in-situ electrolytic etching mechanism and the effects of electrolytic experimental parameters and analysis parameters of scanning electron microscope on the three-dimensional characterization of inclusions were discussed. The results show that different inclusions in the tested steels appear to be protruded on the flat matrix by controlling the electrolytic parameters of the constant potential electrolytic etching of the steel specimens, and then the real three-dimensional morphologies of the inclusions can be observed by the scanning electron microscope, and the inclusions in a certain area can be quantified statistically. When the electrolytic voltage increases or the electrolytic time prolongs properly, the etching of the electrolyte on the matrix increases accordingly, which is beneficial to the more exposure of the inclusions. However, the pearlite, bainite and martensite microstructures of the steel matrix are easy to cause serious interference to the statistical quantitative results of inclusions, therefore some measures such as the filtration of matrix composition or inhibiting the appearance of matrix structure can be taken to prevent its unfavorable effects. Scanning electron microscope parameters such as magnification, image mode, image contrast and inclusion gray threshold also have an important influence on the three-dimensional statistical quantification of inclusions, therefore these parameters should be set appropriately to ensure that inclusions are accurately identified and quantified. The in-situ electrolysis method can quickly obtain the in-situ three-dimensional morphology of non-metallic inclusions in the steels, and realize the automatic statistical quantification of inclusions in the steels with different matrix microstructures. Compared with the two-dimensional analysis, the three-dimensional morphology of inclusions obtained by the in-situ electrolysis method is more complete, and the statistical quantitative data of the inclusions obtained from a certain depth area of the steel specimens are more representative.

Key words: steel, inclusion, three-dimensional morphology, quantitative analysis, electrolytic etching, in-situ characterization