It is well known that alumina inclusions on the surface of molten Al-killed steel quickly attract each other to form clusters. On the other hand, alumina-magnesia complex inclusions on the surface of molten low-carbon steel with a high oxygen content have a much weaker tendency to form clusters. In the present work, the reason for the different behaviors of the two types of inclusions was analyzed in detail. A confocal scanning laser microscope was used to carry out the experiment of in-situ observation of the two types of inclusion on the molten pool. The first type of inclusion was 93 mass pct alumina-7 mass pct magnesian, obtained in a Mg-added Al-killed steel. The second type of inclusion was nearly pure magnesia, obtained in a Mg-killed steel. The attractive force between a pair of inclusions, for both cases, was found to be approximately 1O Ϫ17 to 10 Ϫ16 N and one-tenth of that between a pair of alumina inclusions. The various effects of contact angle, surface tension, and oxygen content of the steel melt on the attractive force are discussed in detail from the viewpoint of the capillary force.

The present study is concerned with the interaction phenomena of nonmetallic inclusions in front of a moving solid-liquid interface. The in situ observation was done in a high-temperature experiment by using a laser microscope. Alumina inclusions in an aluminum-killed steel with low oxygen content exhibited the well-known clustering behavior. The velocity of the advancing interface first increased while approaching the particle, but became stagnant during engulfment and increased again after that. Alumina-magnesia complex inclusions in a magnesium-added steel with high oxygen content were very finely dispersed in the molten pool. These inclusions escaped from the advancing interface during solidification, but gathered again at the retreating interface during remelting. The tiny inclusions were thought to behave just as tracer particles of a local flow. The velocity of particles was measured on a video image, and the significant acceleration or deceleration was found near the interface. It was concluded that the flow was induced by the Marangoni effect due to the local difference in temperature and oxygen content in front of the interface, particularly in the case of a higher oxygen content. However, the flow was weak in the case of a low oxygen content.

The in-situ observation of MnS precipitation was made on cooling using a confocal laser microscope. It was found that two types of precipitates appeared at the different temperatures. The first type precipitated at the nucleation sites of MgO or MnO-TiOx inclusions in ␥ -Fe in the temperature range between 1500 and 1200 K. The shape was triangular or rodlike, and the size was between 1 and 5 m. The second type of MnS drastically precipitated in ␣ -Fe at 1100 K just after the Ar 3 transformation. The shape was triangular or polygonal, and the size was between 0.1 and 0.5 m, much smaller than that of the first type. No significant precipitation was observed in the intermediate temperature range between 1200 and 1100 K. The reasons why MnS precipitation took place twice at the different temperature ranges and why no precipitation occurred between the two temperature ranges were discussed from the thermodynamic and kinetic viewpoints. The diffusion coefficients of Mn in ␥ -and ␣ -Fe and the supercooling are the key factors which explain the reason.

No abstract

An efficient continuous desiliconization process equipped with a mechanical stirrer in a hot metal runner was newly developed. The facility was installed during the revamping of No.3 blast furnace at Kobe Works, and the commercial operation started up successfully in January 2008. Before the installation of the commercial facility, the reaction behaviour was investigated under various experimental conditions for the application of a mechanical stirring method to continuous desiliconization treatment in the hot metal runner. Hot metal experiments at laboratory scale showed that the stirring intensity was an important factor for the process performance, and the mechanical stirring method was available for the improvement of reaction efficiency. As a result of plant tests, it was confirmed that a higher oxygen efficiency of desiliconization was achieved by the combination of runner arrangement and mechanical stirrer compared with the conventional injection of the desiliconizing agent. According to the reaction analysis of continuous desiliconization in the hot metal runner using the semi‐batch reaction model, it was estimated that the average slag‐metal residence time in the reaction region is improved due to an increased entrainment of foamed slag into the stirred metal bath in the mechanical stirring method, and therefore, it leads to a high desiliconization efficiency. Based on the experimental results, the equipment specifications and the runner design for this process were determined.