The effect of surface movements on nitrogen mass transfer in liquid iron
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INTRODUCTION
THE demand for high-quality steels, such as interstitialfree steels, places stringent requirements on the amounts of dissolved carbon and nitrogen in steel. These impurities are traditionally removed via gas-metal reactions. Thus, numerous refining processes which have focused on the free surface have been developed. These include the injection of oxygen for decarburization,m the injection of inert gases for good mixing and surface renewal,C2] vacuum degassing operations for reducing the partial pressure of CO,t3] the introduction of new fluxes for decarburization,t4~ and the application of electromagnetic forces to enhance stirring and surface renewal.t 1] Despite these latest refining technologies, relatively lengthy treatment time is still necessary to attain the required carbon and nitrogen levels. This results in low productivity and high production costs. The basic theme in all of the previous refining technologies is the enhancement of the overall rate of gas-metal reactions. In the case of the decarburization process, the rate of this reaction is believed to be controlled by mass transfer of carbon in the diffusion layer of the molten metal phase. [5,6,71For nitrogen removal, the rate-determining step N. HIRASHIMA, Assistant Manager, is with the Steelmaking Division, Yawata Works, Tobata-ku, Kitakyushu 804, Japan. R.T.C. CHOO, formerly Research Associate, Department of Metallurgy and Materials Science, University of Toronto, is with Concurrent Technologies Corp., Johnstown, PA 15904. J.M. TOGURI, 1NCO/NSERC Professor, is with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON Canada M5S 1A4. K. MUKAI, Professor, is with the Department of Materials Engineering, Kyushu Institute of Technology, Tobato, Kitakyushu 804, Japan. Manuscript submitted May 5, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS B
appears to be the combined chemical reaction of nitrogen desorption at the free surface and the mass transfer of nitrogen in the concentration boundary layer of molten iron.tS] Thus, methodologies that can enhance mass-transfer-controlled processes should be developed as they possess the advantage of simultaneously tackling both nitrogen and carbon removal. One way to accelerate the diffusion of the impurity across the concentration botmdary layer at the free surface is by utilizing the principle of Marangoni convection. Richardson[9] and Mukai Et~ have pointed out that Marangoni convection, induced by either a surface or an interfacial tension gradient, may promote mass transfer across the surface/interface and thereby enhances the rate of reactions. Marangoni convection is especially important since it is localized at the free surface or interface. Such localized processing methodology can be seen to be cost effective in addressing gas-metal reactions which are interface controlled. Surface or interfacial tension gradients can be produced by temperature, concentration, or electric potential gradients. In this article, we shall focus on solutal-capillary flow; more
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