Simulation of Fluid Flow and Oscillation of the Argon Oxygen Decarburization (AOD) Process
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IN 2008, around 26 million tons of stainless steel were produced around the world. Stainless steel contains a minimum of 10 pct chromium and various amounts of Ni, Mo, Mn, and other elements to arrive at the desired material properties. The corrosion resistance is caused by a thin layer of passive, stable chromic oxide, which reacts slowly. The layer adheres extremely well and, when combined with oxygen from the air, is self-healing. To produce stainless steel, a two-stage process is employed. Scrap and alloying elements, sometimes together with hot metal, are molten down in the electric arc furnace. Then the molten metal is refined in the argon oxygen decarburization (AOD) converter. In the AOD process, a highly chromium-alloyed melt is decarburized by injecting and blowing oxygen and inert gases (N2, Ar) through submerged side-wall nozzles and a top lance. Depending on the mix of charge materials, HANS-JUERGEN ODENTHAL, Deputy General Manager, R&D Division—Fundamentals and Models Melting/SAF, UWE THIEDEMANN, Deputy General Manager, Steelmaking/Continuous Casting Technology Division—Product Development Steelmaking, UDO FALKENRECK, General Manager, R&D Division, and JOCHEN SCHLUETER, Vice President Special Technologies, are with the SMS Siemag AG, Eduard-Schloemann-Straße 4, 40237 Duesseldorf, Germany. Contact e-mail: hans-juergen.odenthal@ sms-siemag.com Manuscript submitted May 5, 2009. Article published online January 26, 2010. 396—VOLUME 41B, APRIL 2010
the beginning carbon contents are between 1.5 pct and 4.5 pct, and the chromium contents are between 10 pct and 25 pct, depending on the steel grade to be produced. A specific feature in the decarburization of highchromium melts is that chromium itself has a high affinity to oxygen, which requires special process-related measures to limit its conversion into the slag. The competitive situation of carbon and chromium in contact with oxygen is demonstrated by the following reaction equations: 2½C þ fO2 g ! 2fCOg
½1
4½Cr þ 3fO2 g ! 2ðCr2 O3 Þ
½2
where [ ] means it is dissolved in the melt, ( ) means it is included in the slag, and { } means it is gaseous. At ambient pressure, for every temperature, an equilibrium exists between the carbon and chromium content of the melt and the oxygen supplied (Richardson– Ellingham diagram). As the temperature increases, the free reaction enthalpy DG0 for Eq. [1] decreases, although it increases for the slagging reaction in Eq. [2]. Equations [1] and [2] can be combined as follows: 2½Cr þ 3fCOg $ 3½C þ ðCr2 O3 Þ
½3
Equation [3] makes it clear that the equilibrium between carbon and chromium in the melt depends on the CO partial pressure in the gas bubbles. To supply as much oxygen as possible to the carbon while at the METALLURGICAL AND MATERIALS TRANSACTIONS B
same time minimizing the oxidation of the chromium, the reaction must proceed toward the left. This sequence takes place preferably at high-carbon and low-chromium contents, at high temperatures, and above all, at a low-CO partial pressure pCO. Increasing the
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