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8/28/03

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Page 483

Marangoni Flow at the Gas/Melt Interface of Steel HONGBIN YIN and TOSHIHIKO EMI Direct observation with a scanning laser microscope was made to determine the direction and velocity of surface flow of steel melt in the vicinity of the solid/melt (S/M) interface. During solidification, a fast solutal Marangoni flow moving away from the S/M interface was confirmed to exist on steel melts containing oxygen and sulfur of 10 to 105 ppm. Even in such a low range of oxygen and sulfur content, the solutal Marangoni flow can be very fast, carrying inclusion particles up to the free surface along the S/M interface. During heating and holding, however, a thermal Marangoni flow combined with convective flow generated a reverse flow directed toward the S/M interface. These features have important relevance to inclusion entrainment and solute segregation during the solidification of steel.

I.

INTRODUCTION

A gradient of surface tension, s, on the steel melt surface causes a Marangoni flow pointed from low s to high s area. The s value of the melt is sensitive to temperature and surface-active solute concentration. Accordingly, the existence of the thermal and/or solutal gradient on the melt surface is the major factor that creates the Marangoni effect. The impact of the Marangoni flow on the bulk flow of a melt is normally weak compared with the bulk convection at the same thermal or solutal gradient. Under special circumstances, where the bulk natural convection is eliminated, e.g., in a microgravity space environment, and where the thermal/solutal gradient is extremely high, e.g., in the welding and electron-beam melting of metals, the Marangoni effect may greatly influence the bulk flow. Effort was put forth to study the Marangoni effect and its impact on materials properties at high- and lowgravity environments.[1–10] However, investigation techniques were limited, and direct observation of the surface flow of metal melt at elevated temperatures was hardly achieved. Information provided by indirect methods was also limited. Recently, we developed a high-temperature confocal scanning laser microscope combined with an infrared image furnace (CSLM-IIF) for direct observation of the solid and liquid metal surfaces.[11–15] This new facility enabled us to clearly distinguish fine nonmetallic inclusion particles from steel melt owing to the great difference in laser reflection. Taking the fine particles as markers, weak surface flow (mm/s scale) has been visualized and studied at high magnification (up to 2300 times). Surface flows of steel melt containing sulfur and oxygen were observed with the CSLM-IIF at temperatures up to 1827 K in a HONGBIN YIN, Staff Engineer, is with Research and Development, ISPAT Inland Inc., East Chicago, IN 46312. Contact e-mail: hongbin. [email protected] TOSHIHIKO EMI, Visiting Professor, formerly with the School of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea, is with the Graduate School of Iron and Steel Technology (GSIST), Pohang Unive