Method of desulfurizing steel

#This application claims priority to U.S. patent application Ser. No. 13/007,163 filed Jan. 14, 2011, now abandoned the disclosure of which is incorporated herein by reference.

#Certain commercial steel compositions require relatively low amounts of carbon (less than 0.05%), nitrogen (less than 50 ppm), and sulfur (less than 30 ppm). In the past, methods of producing these low carbon and low sulfur steels used a combination of processes in a steelmaking furnace and a degasser. The prior method involved reducing the carbon levels in the steel composition in the steelmaking furnace, such as an electric arc furnace (EAF), making alloy additions during the tapping process to desulfurize and alloy the steel, and then shipping the steel to the degasser such as a vacuum tank degasser (VTD). This processing route was simple and quite straightforward.

#To achieve the steel composition requirements of such commercial grades in the past, steel with very low carbon levels, such as less than 0.025%, was tapped at the steelmaking furnace. The dissolved oxygen levels associated with these low carbon amounts were in the order of 1200 ppm to 1400 ppm in the furnace before tapping. Where the degasser was a distance from the furnace, the steel was tapped at approximately 1700° C. to compensate for temperature losses during transportation to the degasser. During the tapping process, the steel was deoxidized with aluminum and ferrosilicon (FeSi). Lime and aluminum dross were also added to create a fluid, deoxidized, desulfurizing slag. By these additions, the desulfurizing reaction was started in the ladle during shipping to the degasser. At the degasser further additions of aluminum, lime, calcium aluminate and dolomitic lime were made to ensure desired sulfur removal during the degassing cycle.

#FIG. 1 is a diagrammatical flowchart of processes used in the present method of desulfurizing a silicon killed steel,

#FIG. 2 is a chart showing a distribution of tap temperatures for a plurality of heats,

#FIG. 3 is a chart showing a distribution of tap oxygen content for a plurality of heats,

#In a process for preparing steel for casting, as shown diagrammatically in FIG. 1, the steel typically progresses from an electric arc furnace (EAF) to a vacuum tank degasser (VTD) to a ladle metallurgical furnace (LMF) before casting. The steel grades produced by the presently disclosed method typically have carbon less than about 0.05% by weight. The steel grade also typically has low nitrogen less than about 0.005% by weight and low sulfur less than about 0.003% by weight, or less than about 0.0015% by weight.

#A steel melt shop typically has one or more electric arc furnaces, such as a 120 ton (metric ton) EAF equipped with 110 megavolt-ampere (MVA) transformers. Such electric arc furnaces may be between about 30 ton and 400 ton capacity, but are generally for continuous casting between 60 and 120 tons capacity. Each furnace may contain gas injection lances, such as a More brand gas injector system, through which a combination of oxygen and natural gas can be blown into the furnace, as well as a carbon injection pipe. These may be used to inject carbon and oxygen at different ratios throughout the heating of the steel to create a foamy EAF slag. Preparation of a foamy slag has been described in Pretorius, E. B. and R. C. Carlisle, Foamy Slag Fundamentals and Their Practical Application to Electric Furnace Steelmaking, ISS-AIME, EF Conference Proceedings, 1998, pp. 275-291. The EAF may also be equipped with a top feed flux system through which lime, dolomite and carbon can be fed to control the slag basicity and viscosity for better slag foaming while melting. The present process may be used with other steelmaking furnaces, such as a basic oxygen furnace.

#The molten steel in the heat of an EAF is tapped open when the oxygen content and temperature of the steel composition are within desired parameter ranges. As the EAF may be removed from the VTD, the tapping temperature of the steel composition at the EAF is selected to allow for a temperature decrease during transit from the EAF to the VTD to provide a desired temperature at the VTD for decarburization and nitrogen and sulfur reduction. The steel composition may be tapped open from the furnace into a ladle through an eccentric bottom tap hole (EBT) system, which is typically designed to minimize carry-over slag from the EAF into the ladle during tapping. For example, in a 120 ton capacity EAF, about 95 ton of steel is tapped from the furnace into the ladle.