Abstract

The influence of MgO and sulphur contents on the shape and state of oxide and oxide/sulphide inclusions was studied by analysing the aspect ratios and shape factors of inclusions in the steel samples taken from the mould in continuous casting. Aspect ratio (AR) refers to the ratio between the longest and the shortest length of each inclusion (in the direction of polished surface in the samples), whereas the shape factor (SF) describes the roundness of each inclusion. Low values (i.e. values close to 1) of AR and SF indicate shape of at least partially molten inclusions.

Based on the values of AR and SF the inclusions were categorized in four different categories:

- AR < 1.2 and SF < 1.2

- AR < 1.2 and SF > 1.2

- AR > 1.2 and SF < 1.2

- AR > 1.2 and SF > 1.2

Furthermore, inclusions were divided into large (length > 5 μm) and small (length < 5 μm) inclusions, which resulted in eight different categories in total. Inclusions smaller than 3 μm in diameter were excluded from this study in order to avoid the weakened clarity of results due to excessive amount of inclusions. Similar results (but with less clarity) were obtained while taking also smaller inclusions into account. Compositions of inclusions belonging to different categories were compared by plotting the inclusion compositions in a (pseudo)ternary diagram describing the CaO-Al₂O₃-CaS(-MgO) system.

According to the results, most of the inclusions that could be considered as molten (i.e. inclusions with low values of both AR and SF) were located in a composition range between binary CaO-Al₂O₃ liquid window and CaS. On the other hand, most inclusions outside this area were not molten. The amount of MgO in inclusions did not seem to have an influence on the shape and state of inclusions, although it should be noted that the variation of MgO-content in the inclusions studied in this research was not very large.

Finally, the influence of the boundary values given to AR and SF on the results was estimated by changing the above mentioned limits and comparing the results obtained by using different boundaries as starting points. Values from 1.1 to 1.3 were tested for both AR and SF.