Development of Hot Metal Dephosphorization with Lime Powder Top Blowing: Part 2. High Blowing Rate

Phosphorus in steel is detrimental element for mechanical properties and dephosphorization treatment of hot iron is necessary to produce high-grade steel products. However, it is difficult to perform dephosphorization treatment to attain lower phosphorus content using CaO-SiO2-FeO flux without fluorite of which use is limited owing to stringent environmental regulations. It is because increasing slag basicity is difficult without slagging promotion agent. In the present work, addition of ladle slag and lime powder top blowing method were applied in 2 t test converter to obtain the higher slag basicity. Obtained results are as follows: (1) phosphorus content of hot iron decreased to 0.001 mass% under the condition of (CaO)/(SiO2)=3. (2) Rate constant, K, of dephosphorization reaction below [P] = 0.02 mass% increased with increasing bottom gas flow rate, but K decreased under the condition of too much bottom gas flow rate. (3) Phosphorus distribution in the final stage of blowing was much higher than predicted values on a basis of thermodynamic relation between top slag and hot iron. (4) It is suggested that the FeO-CaO melt formed at hot spot is capable of decreasing the phosphorus content of hot iron to the ultralow range even at high temperature at hot spot.