| The removal of inclusions by bubble is one of the effective ways to improve the removal of inclusions in molten steel. Generating a large number of dispersive and tiny gas bubbles in molten steel is the key to remove the inclusions. In this paper, the inclusion removal by fine bubbles is a new technique for micro non-metallic inclusion removal in molten steel. Its foundation principle is that pretreating liquid steel in an environment of high nitrogen or hydrogen partial pressure, making nitrogen or hydrogen dissolving in liquid steel, and then treating the steel in vacuum environment, letting supersaturated gas precipitating and forming a large amount of dispersive and fine gas bubbles on the surface of inclusions, and finally, the bubbles floating up with the inclusions and capturing more inclusions in the flotation process. In this way, the removal of micro inclusions in molten steel can be significantly promoted. However, the theoretical basis of this technique is scarce. In view of the situation, this article mainly studies five aspects:?1? Thermodynamics of bubble nucleation in liquid steel; ?2? The evolution mechanism of bubble in molten steel; ?3? Bubble growth dynamics in molten steel; ?4? Distribution of bubbles in liquid steel during the process of nitrogen evolution; ?5? Effect of different technological factors on micro inclusions in molten steel during the process of nitrogen adding and releasing. Through the above research, the following conclusions are obtained.?1? Based on the theory of classical solidification nucleation, the nucleation mechanism of bubbles in nitrogen or hydrogen supersaturated molten steel was studied. The calculation expressions of critical radii for homogeneous and heterogeneous nucleation were derived respectively, and the effects of the melt depth, pre nitrogen or hydrogen treatment pressure, and post vacuum treatment pressure on critical nucleation radius were investigated. The results show that the critical radii of homogeneous and heterogeneous nucleation are identical, but the bubble volume of heterogeneous nucleation is only a part of the spherical bubble volume of homogeneous nucleation, and therefore, heterogeneous bubble nucleation on the surface of inclusions with poor wettability with molten steel is more easily to take place. The greater melt depth, the greater critical nucleation radius, and the influence of melt depth on critical nucleation radius are gradually strengthened with increasing melt depth. With higher pre nitrogen or hydrogen treatment pressure, the critical melt depth for spontaneous nucleation and formation of bubble nuclei in different sizes are higher. However, the effect of post vacuum treatment pressure on the critical melt depth for spontaneous nucleation and formation of bubble nuclei in different sizes are not notable.?2? A water model experiment was used to study the evolution process of the supersaturated gas in a molten steel/?N2, H2? supersaturation system. It is found that in the late vaccum process, the inclusion of single or aggregates can act as gas bubble nucleation sites. Bubbles grow rapidly on the surface of inclusions, and always keep the ball in the process of growing up.?3? A water/carbon dioxide system was used to study the bubble growth dynamics in a molten steel/?N2, H2? supersaturation system. It is found that when using the technology of inclusion removal by fine bubbles, the increase of pre nitrogen or hydrogen treatment pressure has a significant effect on bubble growth. The increase of post vacuum treatment pressure has a block effect on bubble growth, and with the increase of post vacuum treatment pressure, the influence is gradually strengthened. The increase of melt depth has a block effect on bubble growth, and with the increase of the melt depth, the influence is gradually weakened. Compared with nitrogen bubbles, hydrogen bubbles in molten steel grow faster.?4? Distribution of gas bubbles in liquid steel during the process of nitrogen evolution was studied. The experimental results show that after nitrogen releasing and quenching treatment of liquid steel containing nitrogen, bubbles appear on the cross section of ingot at different distances from the top surface of the ingot, and the inclusion adhering to bubble is mainly a single angular pure alumina, with size of 10-20 micron. The metal density near the top surface of the ingot is the minimum, and on the other location the difference between the values of metal density is small. With the increase of the distance from the top surface of ingot, the average size of bubble decreases and the number of bubble increases.?5? The results of micro inclusions removal in liquid steel during the process of nitrogen adding and releasing show that the removal efficiency of total oxygen and micro non-metallic inclusions in steel is remarkable after liquid steel treated by the nitrogen absorbing and releasing. The longer vacuum processing time is, the content of total oxygen and amount of micro non-metallic inclusions in steel will be lower, and the removal rate of total oxygen reach to 81.6% after the steel is treated by nitrogen of 0.5×105 Pa and vacuum for 30 min, with the total oxygen content from 38×10-6 to 7×10-6 in steel. |
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