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FORMATION of porosity (referred to as bubbles here on out) is related to gas content of the system (composition), system’s capacity to hold gas elements in solution (solubility), and volume change (shrinkage) in the system during phase change. In steels, bubbles can be divided into two categories based on their origin. One is the bubble that enters steel in gas phase and the other is one that precipitates from liquid. Precipitation is said to occur when the content of the dissolved gas in the phase, solid or liquid, is larger than solubility of the phase and is also large enough to overcome the barriers for formation, e.g., surface tension and ambient pressure. The solubility is dependent on temperature and composition. For steels, the solubility gradually decreases with temperature in liquid and solid phases. There is a great difference between the solubility values in different phases. Solubility in liquid steel is larger than in solid steel. Coupling the difference in solubility values with shrinkage and segregation, steels are most susceptible to bubble formation during solidification.[21,31,33] Bubbles move in the liquid but are stationary in the solid state. The motion of a bubble in liquid is governed by gravity (buoyancy force) and surface tension, among others. The motion of a bubble due to gravity is caused by the difference between the density of gas and liquid. Bubbles move due to gravity in gravity vector direction; however, the motion of the bubble due to surface tension does not have a predefined direction. In steels, surface tension is dependent on temperature and

ARASH SAFAVI NICK, PhD Candidate, MICHAEL VYNNYCKY, Senior Researcher, and HASSE FREDRIKSSON, Professor Emeritus, are with the Department of Materials Science and Engineering, KTH Royal Institute of Technology, Brinellva¨gen 23, 100 44 Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted September 25, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A

concentration of surface-active elements. Hence, a gradient in temperature or composition creates a gradient in surface tension. The gradient of surface tension acts as the force for the movement of the bubble. The direction of the motion would be along the direction of the gradient. The dominant surface-active element that is present in most steels is sulfur. Surface tension and surface tension gradients are functions of sulfur concentration and temperature in steels.[32] The temperature derivative of surface tension changes sign, from negative to positive, by increasing sulfur content above around 30 ppm. The change in the direction of surface tension gradient with temperature is speculated to cause a change in the direction of bubble motion.[18] In this study, it is shown that the change in the direction of temperature derivative of surface tension does not cause a change in the direction of a bubble moving through the mush area. Liquid motion could also have an effect on the motion of bubbles. Another aspect of bubble motion in molten steel is the interaction with the solid