Development of an integrated method for fully characterizing multiphase inclusions and its application to calcium-treated steels

An integrated technique for the full characterization of multiphase inclusions has been developed. It is based on the positioning of the beam of a scanning electron microscope along the skeleton of each phase present in the inclusions. It requires the integration of the signals generated by the scanning electron microscope, the X-ray spectrometer, and the image analyzer. This technique was used to characterize inclusions found in commercial heats of steel treated with calcium. An optimum CaS ratio of 0.70 allows modification of the mean shape factor of MnS inclusions, which then changes from 0.25 to 0.73. The optimum calcium concentration in the sulfide phase is ≈15wt.%. An increase of this value beyond 15wt.% does not translate into any change in the shape factor of the inclusions. Finally, an empirical model for the transformation of inclusions, as a result of the calcium treatment, is proposed. It specifies that Ca-aluminates act as nucleation sites for the solidification of (Mn,Ca)S inclusions.