In order to investigate the source of oxide inclusions in steel by mineralo-chemical method, mineral composition and microscopic structure of ingot scums and oxide inclusions, appeared in some bottom-and top-teeming ingots produced in our plant, were investigated.
The results were summarized as follows.
1. In bottom-teeming ingot scums, many kinds of constituent mineral, mainly composed of FeO-MnO-Al2O3-SiO2 system with a small quantity of slag mineral, were found. It is characteristic that a large quantity of α-quartz as silica mineral were observed.
2. Oxide inclusions in the bottom-teeming ingot were rich in Al2O3 compared with ingot scums, and consist of aluminate minerals (CaO·6Al2O3, MgO·Al2O3) with a small quantity of silicate matrix.
3. In top-teeming ingot scums, many kinds of constituent mineral, originated from slag, refractories, deoxidation products etc. were found. It is characteristic that a large quantity of α-cristobalite were observed instead of α-quartz as silica mineral.
4. Mineral composition and microscopic structure of oxide inclusions in the top-teeming ingot were similar to that in the bottom-teeming ingot. However, in macroscopic inclusions, three typical forms, -(1) multi-component silicate (spinel, corundum and α-cristobalite with a fine crystal matrix mainly composed of CaO-Al2O3-SiO2 system), (2) silicious silicate, (3) special type (chrome spinel) were observed.
5. Spinel (MgO·Al2O3), a main constituent mineral of oxide inclusions, was observed regardless of Aldeoxidation in the ladle. Moreover, in the top-teeming ingot, crystal growth of it was observed comparatively remarkable, coagulating with matrix minerals mainly composed of CaO-Al2O3-SiO2 system.
6. On the basis of these investigation, the source of oxide inclusions was discussed.