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Inclusion removal in molten steel by injecting fine gas bubbles in shroud nozzle

Inclusion removal in molten steel by injecting fine gas bubbles in shroud nozzle
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Although many kinds of approaches have been applied to remove inclusions from liquid steel, gas injection is relatively very effective method among those approaches. This method is commonly practiced to secondary refining process because of low cost and simplicity of application. It is important to produce fine gas bubbles less than 5mm in order to increase inclusion removal efficiency, because fine gas bubbles have larger gas/liquid interfacial area and higher attachment probability. Several researches have been conducted by employing water model experiment to investigate inclusion removal using fine gas bubbles, which were created by injecting gas into a shroud nozzle just underneath the slide gate. These works showed that the fine gas bubbles were produced by turbulent flow while the gas was injected into water. The size of bubbles were observed in the range of 1mm to 5mm in diameter and relative removal efficiency is about 60% regardless of gas/water flow rate and slide gate opening rate until 58%. However, in the previous researches, similarity analysis, particle size effect and fine particle less than 50μm were not concerned. According to the similarity analysis in the present study, equivalent particle size in a water model experiment was smaller than inclusion size in molten steel. For instance, equivalent particle size for 50μm inclusion in the molten steel corresponds to 22.5μm particle in the water model study. And, particles larger than 375μm (831μm inclusion) in diameter were floating up almost 100% even without the help by bubble. The average of relative removal efficiency of particle was about 66%. This result agrees with the previous experiment result (60%). Gas flow rate did not affect the removal efficiency if gas/water flow rate ratio was larger than 0.024. Meanwhile, particle size affected the relative removal efficiency. Relative removal efficiency increased as particle size increased. Particle size for average removal efficiency 66% is 116μm (257μm inclusion). This means relative removal efficiency is less than average efficiency if particle size is less than 116μm (257μm inclusion). According to application analysis, the relative removal efficiency of inclusion in molten steel is less than particle in water model of the same size.
용강 내 개재물을 제거하기 위해 가장 널리 사용되는 방법은 용강 내 gas bubble을 투입하여 개재물과 bubble을 attachment시켜 부상시키는 방법이다. 개재물 제거 효율을 극대화하기 위해서는 매우 작은 사이즈의 bubble이 필요하나 일반적으로 제철공정에서 사용되는 방법으로 만들 수 있는 최소 bubble 사이즈는 20~30mm로 높은 제거효율을 기대하기 힘들다. 이에 shroud nozzle slide gate바로 하단부에 강한 난류가 형성되는 곳에 gas를 투입하여 만들어지는 1mm 이하의 매우 작은 bubble로 개재물 제거효율을 극대화시키는 연구가 1997년부터 이루어져 왔다. 본 연구에서는 선행연구 결과를 기초로 추가적으로 Particle의 사이즈에 따라 제거 효율이 어떻게 변하는지 그리고 제철공정에 적용시켰을 때 예상 효과에 대해 알아보았다. 연구 결과, 1. 수 모델에서 bubble size는 약 0.36~0.52mm으로 관찰되어 높은 제거효율을 얻을 수 있는 조건이 된다는 것을 선행적으로 확인하였다. 2. Particle 사이즈가 375μm 이상인 경우에는 거의 100%의 particle이 Ar bubble의 도움 없이 제거되었다. 이 사이즈는 inclusion 375μm 에 해당된다. 3. 평균 상대제거율은 66%로 선행연구 60%와 매우 잘 일치하였다. 4. Gas flow rate 는 선행 연구결과와 같이 상대제거율에 영향을 미치지 않는 것으로 나타났으나, gas/water flow rate ratio가 0.024 이상이 되어야만 평균 상대제거율이 된다는 것을 최초로 알게 되었다. 5. Particle size는 상대제거효율에 영향을 주며 particle 사이즈가 커질수록 상대제거효율은 점점 증가하였다. 평균제거효율에 해당하는 particle 사이즈는 116μm이고 inclusion에 해당하는 사이즈는 257μm 이다. 6. Collection probability 분석결과, 용강에 이 방법을 실제로 적용할 시 수 모델에서 얻은 평균 66%의 제거효율보다는 다소 낮을 것으로 예상되나 정확한 제거효율은 추가 연구가 필요하다고 판단된다.
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