Influence of Top Slag Chemistry on Micro Inclusion Characteristics and Sulfur Refining for LCAK Steel

摘要

The influence of top slag chemistry has been studied on a low-carbon aluminium killed (LCAK) steel grade has been studied at the SIDERAR steel plant. Samples of slag and steel were collected and the temperature was measured for five heats at four stages during ladle furnace treatment. Cleanness was determined using optical microscopy for number and size distribution and Scanning Electron Microscopy (SEM) for the composition. The results from the evaluation show that the best removal of inclusions is obtained during the sulfur refining period. This is probably due to strong stirring by gas that promotes the growth of inclusions by turbulent collisions and separation of inclusions due to Stoke's flotation as well as bubble flotation. It was also shown that the number of inclusions increased during the final stirring period due to a late addition of aluminum and a too short stirring period. The alumina inclusions were found to contain more CaO and MgO for those heats where the sulfur refining was good. This indicates that the good mixing between slag and steel that is necessary for an effective sulfur refining also affects the inclusion compositions. A theoretical parameter study focusing on the optimum thermodynamic conditions for sulfur refining based on results from plant trials was also carried out. The sulfur distribution was determined by calculating the sulfide capacity using a model developed at KTH and by using the well established theories for sulfur refining. It was found that the equilibrium sulfur distribution is increased when i) the (%FeO)/(%Al_2O_3) ratio increase ii) the aluminum content in the steel increase, and iii) the temperature decrease.