The latest publications regarding the development of technology to control inclusion compositions focusing on MgO · Al 2 O 3 spinel inclusions were summarized in this review article. The problems caused by spinel inclusions, which affect practice as well as products were shown. The formation mechanism of MgO · Al 2 O 3 spinel inclusions is secondly explained thermodynamically from the view points of chemistries of molten steels and slag compositions. Furthermore, crystallization behaviour of spinel was introduced. Countermeasures conducted in practices and laboratories were shown along with some problems still left that should be solved in the future.

Experiments were carried out to determine the effect of silica in the slag of CaO-SiO 2 -Al 2 O 3 -MgO-F system on the formation of MgO · Al 2 O 3 spinel inclusion in 304 stainless steel deoxidized with Al. Immediately after the addition of Al into the molten steel, alumina clusters formed. Simultaneously, reduction of MgO in the slag occurred to raise Mg content in the steel. This resulted in the change in inclusion composition to MgO · Al 2 O 3 spinel which did not further change. In the previous experiments with CaO-Al 2 O 3 -MgO-F slag, however, spinel inclusions changed to MgO or liquid CaO-Al 2 O 3 -MgO system. The difference in behavior was caused by the existence of silica in the slag. Silica in the slag was considered to prevent the extensive reduction of MgO or CaO in the slag by Al to supply soluble Mg or Ca into the molten steel. A stability diagram of inclusions corresponding to Mg and Al contents in the steel was calculated employing available thermodynamic data. The inclusion compositions experimentally obtained well agreed with the diagram. This implies that spinel inclusions were the most stable in the molten 304 stainless steel deoxidized with Al under the presence of 10 mass% silica in the slag. As a result, it was concluded that silica in the slag enhanced the formation of spinel inclusions.

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Solubility of nitrogen in liquid Fe-Cr-Ni-Mo stainless steel was measured by sampling method in the temperature range from 1 723 K to 1 923 K under a N2 gas of 1 atm. As a result, the solubility increased with increasing Cr, Mo and Mn content, whilst it decreased with increasing Ni content. In addition, the solubility increased with decreasing temperature. Thermodynamic analysis was carried out taking Fe-20 mass% Cr alloy as a solvent referring to the manner proposed by Anson. The first attempt showed that the calculated solubility, applying the reported first-order interaction coefficients of N against Cr, Ni, Mo and Mn, did not agree with the measured values. This inconsistency was found to be attributed to the interaction coefficient of N against Ni ( ). Therefore, reassessment to derive available up to 30 mass% Ni has been made with the data obtained by us and Anson. Thereby the corresponding coefficient was derived as 0.0063 with which the solubility could be well predicted.

The maximum size of single inclusion particles and clusters in an Fe-10 mass% Ni alloy deoxidized with Al or Ti/Al were examined using extreme value analysis. The results obtained from conventional twodimensional observations of inclusions on a polished cross section of metal sample (the CS-method) were compared to those from three-dimensional investigations of inclusions on a film filter after electrolytic extraction (the EE-method). It was found that the EE-method can successfully be used as a reference method for estimation of the probable maximum size of single inclusion particles and clusters by using an extreme value distribution (EVD). The EVD results for single inclusion particles obtained from the EEmethod agreed satisfactorily well with those from a conventional CS-method. However, this required identification as well as neglect of pores on an investigated cross section of a metal sample. The predicted maximum size of single inclusion particles in a 1 mm 3 volume was confirmed by results from the EEmethod.