Study On The Technology Of Inclusion Removal In High Quality Special Steel With Microbubble Filtration

Non-metallic inclusions affect the quality of special steel, such as fatigue life of bearing steel, and the numbers, sizes and distributions of non-metallic inclusions are important quality indexes of bearing steel. In the conventional production process of high quality special steel, non-metallic inclusions are mostly controlled by the LF, RH or VD, but they take long time to remove non-metallic inclusions, and their production efficiency is very low. Therefore, as a sub-topics of the national science and technology support program of "twelfth five-years", how to improve the efficiency of removal inclusion in refining process of high quality special steel is the key and difficulty of the research. The research will provide laboratory datum and theoretical basis for the inclusion removal technologies and equipments of high quality special steel.The mechanisms and characteristics of inclusion removal by single bubble floating through bubble adhesion and wake flow capture were analyzed and studied, and the processes, laws and factors of removing inclusions through bubble wake flow were more comprehensively studied. At the same time, the laws, factors and mechanisms of inclusion removal through gas-liquid two-phase flow of big bubbles and diffuse small bubbles in conventional bottom blowing were deeply analyzed. On the basis, the effects of inclusion removal by larger zone with diffuse small bubbles were researched, its mechanisms were also analyzed, and finally the technology of rapid and efficient inclusion removal with large zone of diffuse small bubbles in ladle bath was designed. The conclusions are as follows:(1) For a single bubble, the small bubble is beneficial to the adhesion of inclusions, and the big bubble is easily to capture inclusions through wake flow. Bubble and inclusion sizes, particle concentration greatly impact on the adhesion rates and capture rates of inclusions.(2) It is shows that under the same flow rate, the number of diffuse small bubbles in gas-liquid two-phase flow is larger, the dispersity and specific surface area are also greater, so the removal efficiency with diffuse small bubbles is significantly higher than that with big bubbles in gas-liquid two-phase flow. When the flow rates is more than a certain critical value, diffuse small bubbles generared by porous brick begin to collide and aggregate into big bubbles, which makes the removal effect get bad with diffuse small bubbles. There is an inadequacy of removing inclusions in conventional process that is the zones of diffuse small bubbles are limited.(3) It was found that increasing the zone proportion of diffuse small bubbles in gas-liquid two-phase flow of the bath can greatly improve the efficiency of inclusion removal. Under the same flow rate, the inclusion removal rate of the mode of scattering full bath with diffuse small bubbles is 1.7 times more than that of the common single blowing with slit element. The inclusion removal rate can reach 97% in the process at 4 min with the mode.(4) Inclusions are mainly removed by slag after the inclusions are brought into the interface between molten steel and slag through bubble adhesion and liquid flow in the process of bottom blowing argon. Under the same flow rate, the diameter of slagging surface with diffuse small bubbles is smaller, and the degree of agitation to the interface of molten steel and slag is also lighter, so the efficiency of inclusion removal with diffuse small bubbles is higher. The region that inclusions contact slag layer with the highest frequency is at the mouth of the steel and slag interface. With increasing the number of porous bricks, the influence region of bubbles increase and the turbulence intensity on the interface of liquid steel and slag decreases, these are beneficial to remove inclusions.(5) Combining the ladle structural and operational characteristics in the actual production, the programs of rapid and efficient inclusion removal with large zone of diffuse small bubbles were put forward. Under the same flow rates, the inclusion removal rate of the optimizing mode is 1.6 times more than that of slit element in the common single bottom blowing. The inclusion removal rate can reach 95.2% at 9 min in the process of the mode 4.(6) The laws of fast and efficient removal inclusions were studied with dimensional analysis. The empirical relationships between the efficiencies of inclusion removal with diffuse small bubbles by large zones and the impact factors were established through a large number of statistics. These provide a theoretical basis for the further study and engineering design.