X-ray observation of phosphorus vaporization from steelmaking slag and suppression method of phosphorus reversion in liq
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I. INTRODUCTION
PHOSPHORUS adversely affects the mechanical properties of steels due to segregation at grain boundaries during solidification and heat treatment. Therefore, high-quality steel products require extremely low phosphorus contents. Also, the decrement of the phosphorus content in slag is very important for the recycling of basic oxygen-furnace (BOF) slag. There is about 400 kg of slag generated per ton of steel product in integrated plants. It is composed of 300 kg of blastfurnace slag and 100 kg of BOF slag. Almost all of the blast-furnace slag can be utilized as raw material for cement, road construction materials, fertilizer, and other uses. However, less than 50 pct of BOF slag is utilized, and the remainder is dumped. Its recycling is limited to the separation of iron from slag as scrap, as a raw material for the sintering plant, and for cement production. The other possible method to recycle BOF slag is by using it as an iron source in the blast furnace or as a refining flux. The BOF slag has high phosphorus content of 2 to 3 wt pct P2O5. When BOF slag is used in the blast furnace, all the phosphorus in the slag is reduced and reverts back to molten iron. For this reason, the amount that can be recycled to BOF slag is limited. In order to increase the recycling of BOF slag, phosphorus should MIN-OH SUK, Graduate Student, SUNG-KOO JO, Research Associate, and SEON-HYO KIM, Professor, are with the Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, 790-784, Korea. Contact e-mail: [email protected] KAE-YOUNG LEE, Senior Research Engineer, and JONG-MIN PARK, Chief Research Engineer, are with the Technical Research Laboratories, POSCO, Pohang, 790-785, Korea. Manuscript submitted January 20, 2005 METALLURGICAL AND MATERIALS TRANSACTIONS B
be eliminated from the slag. The methods of reducing phosphorus which have been studied include (1) the slow cooling method, which separates the phosphorus-rich slag,[1] (2) reducing the BOF slag by using solid carbon,[2] and (3) increasing the activity coefficient of phosphorus to prevent the reduced phosphorus from being absorbed into molten metal.[3,4] Moreover, the phosphorus content in slag causes negative affects in ultralow-carbon steelmaking processes. The FetO content in slag should be lowered to less than 3 wt pct for effective desulfurization and the suppression of steel reoxidation. Deoxidation of slag by a CaCO3 Al mixture is a common method for decreasing the FetO content in slag. However, P2O5 in slag is inevitably reduced during the treatment process, causing the increment of phosphorus content in the hot metal. Therefore, it is necessary to find a method to remove P2O5 in slag without causing other harmful effects such as an increase of phosphorus in the metal, in order to activate BOF slag recycling and achieve steel cleanliness. It was reported that P2O5 in slag is removed as gas into the air during the reduction process from the mass-balance results between slag and metal.[5] The mass
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