Mathematical model for nitrogen control in oxygen steelmaking
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I. INTRODUCTION
CONTROL of the nitrogen content of steels during steelmaking has become more important because of the diversity in nitrogen requirements for different steel grades. For example, low carbon sheet steels, intended for special applications such as automobile body panels, require very low nitrogen contents (20 to 50 ppm),[1] whereas tin plate requires high nitrogen levels (60 to 120 ppm).[2] In order to meet these strict specifications, special attention is given to identifying the sources of nitrogen in the steelmaking process and to evaluating possible methods for controlling the nitrogen content of steel. Several investigators have identified the operational factors that affect the nitrogen content of steel produced by electric arc furnace (EAF) and oxygen steelmaking (OSM) processes. These factors are summarized as follows.[3,4,5] (1) Carbon oxidation: Decarburization is one of the most effective ways of reducing the nitrogen content of steel during steelmaking. The CO gas produced during carbon oxidation flushes the nitrogen out of the metal bath and also creates a protective atmosphere over the melt that reduces nitrogen pickup from air. (2) Nitrogen content in hot metal: Hot metal charged into the BOF generally contains 70 to 80 ppm of nitrogen. During decarburization, the nitrogen from the hot metal is removed by the CO gas. Consequently, the final nitrogen content of steel is not strongly affected by the nitrogen present in the hot metal.[3] (3) Nitrogen content in scrap: Scrap is an important source of nitrogen during steelmaking, and its nitrogen content varies from 40 to 220 ppm.[6] Heavy pieces of scrap are difficult to melt and remain unmelted until the end of the blow. At this time, little CO is produced and the nitrogen from scrap remains in the liquid steel produced.
D.A. GOLDSTEIN, Research Engineer, is with the Homer Research Laboratory, Bethlehem Steel Corporation, Bethlehem, PA 18016. R.J. FRUEHAN, Professor, is with the Materials Science and Engineering Department Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted August 20, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
(4) Direct reduced iron: Direct reduced iron (DRI) and hot briquetted iron (HBI) generally have lower nitrogen levels than scrap and also melt faster than scrap. Therefore, when DRI products are used as coolants instead of scrap, steels with lower nitrogen contents are produced.[7] (5) Oxygen purity: Nitrogen present as an impurity in the oxygen blown for decarburization can increase the final nitrogen content in the steel produced.[3] (6) Combined gas blowing: The gases used for bottom stirring to improve slag-metal mixing and scrap melting include N2, Ar, CO2, O2, and CO. When nitrogen is used as a stirring gas, it increases the nitrogen content of the steel produced. Therefore, if low nitrogen steels are to be produced, it is necessary to switch from nitrogen to argon in order to avoid excessive nitrogen pickup into the metal.[3] Other factors that increase the nitrogen content of steel
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