Development of Hot Metal Dephosphorization with Lime Powder Top Blowing: Part 1. Low Blowing Rate

Abstract

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. 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) By simultaneously adopting lime powder top blowing and ladle slag addition, phosphorus content of hot iron decreased to 0.01 mass% under the condition of (CaO)/(SiO₂)=2.

(2) By comparison of the results of the heats with and without ladle slag, the ratio of dephosphorization at hot spot by lime top blowing was estimated to be 35%.

(3) (FeO) content of CaO–FeO melt at hot spot was estimated 37% on a basis of oxygen flow rate except to be consumed for decarburization and lime feeding rate. CaO–FeO phase diagram shows that the melt is liquid at hot spot but solid in hot iron. It is suggested that low ratio of dephosphorization at hot spot is attributed to this result.