Slag Capability Optimizing Technology for Preventing Slag Foaming Phenomenon during VTD (Vacuum Tank Degasser) Operation

摘要

Tank type vacuum degasser which is used all around the world, including POSCO's VTD (Vacuum Tank Degasser), has critical problems during operation, so called slag foaming phenomenon. Slag foaming refers to considerable decrease in the original removal ability of [S], [N], [H] and steelmaking inclusions from molten steel that everybody would expect when they adopt this powerful secondary refining equipment at the first stage. We have witnessed even bigger problems arising, especially when we produce the line pipe steels and higher-grade of structural plate steels. Unfortunately, we could not improve our productivity either because VTD has too much net treatment time due to slag foaming. Therefore, POSCO formed a task force team of steelmaking engineers and researchers. First of all, we studied variety of possible factors that could cause slag foaming during the VTD operation. Based on this, we found out slag foaming control technologies not only by controlling the surface tension, viscosity and basicity of ladle slag, but also by optimizing the point of time that we make artificial slag during the secondary refining process. As a result, we could decrease the HIC (Hydrogen Induced Crack) defect ratio of API steels for sour service by improving steel cleanliness with minimizing [S], [N], [H] and decreasing total numbers, sizes of steelmaking inclusions. Moreover, we also could increase steelmaking productivity by shortening VTD net treatment time by more than 15 minutes.