Behavior of alumina-magnesia complex inclusions and magnesia inclusions on the surface of molten low-carbon steels
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INTRODUCTION
AMONG various types of nonmetallic inclusions, oxide and sulfide inclusions have been thought harmful for common steels. However, there are some positive roles for these inclusions. Oxide inclusions act as the trapping sites of hydrogen atoms in enameled steel and prevent the coating film from stripping off. Sulfide inclusions also improve the machinability of free-cutting steels. Furthermore, it has been well known in welding that the tiny oxide inclusions act as the nucleation sites for very fine acicular ferrite crystals in the bond metal, giving a very good ductility.[1–5] In the rolled steel products too, many fine intragranular ferrite crystals nucleate at some oxide inclusions inside of austenite grains. This technique is based on the concept of oxides metallurgy,[6] in which an important role is given to oxide inclusions as inoculants for the heterogeneous nucleation of the phase transformation and the precipitation. It has certainly been known in principle that the heterogeneous nucleation gives rise to the grain refinement, and that the precipitation often occurs at the nucleation sites of inclusions. However, in the concept of oxide metallurgy, the the oxide particles are intentionally controlled, from the beginning of the steelmaking process, for the subsequent nucleation. This concept is new, particularly from the viewpoint of the new overall processing of the heterogeneous nucleation, combining the steelmaking process with the following heat-treatment and rolling processes. In order to make oxide inclusions useful for this purpose, the composition, size, and distribution must be controlled properly. For example, alumina inclusions appearing in the normal aluminum-killed steel have a strong attractive force SEI KIMURA, Research Fellow dispatched from Kakogawa Works, is with Kobe Steel Co. Ltd., Kakogawa 675-0023, Japan. K. NAKAJIMA, Associate Professor, and S. MIZOGUCHI, Professsor, are with the Institute for Advanced Materials Processing,Tohoku University, Sendai 980-8577, Japan Manuscript submitted April 3, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
between them and show a remarkable tendency to coagulate and to form clusters.[7] Alumina inclusions also show a weak capability for MnS to precipitate on alumina inclusions.[8] In view of such behavior, alumina inclusions seem not to be interesting. On the other hand, alumina-magnesia complex inclusions have a much weaker tendency to coagulate and to form clusters than alumina inclusions.[9] In this study, the attractive force between a pair of inclusions such as aluminamagnesia complex inclusions and magnesia inclusions is measured by using a confocal scanning laser microscope. The reason why the behavior of these inclusions is entirely different from that of alumina inclusions will be explained further, in detail, through morphological analysis and theoretical calculations. II. EXPERIMENTS A. Specimen Two samples were used. Sample A is a Mg-added Alkilled steel with a high oxygen content. A 100 g portion of a low-carbon aluminun-kil
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