Argon oxygen decarburization converter

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Argon oxygen decarburization (AOD) is a process primarily used in stainless steel making (secondary steelmaking) and other high grade alloys with oxidizable elements such as chromium and aluminum. After initial melting of the metal in an electric arc furnace or an induction furnace, it is then transferred to an AOD vessel where it will be subjected to three steps of refining; decarburization, reduction, and desulfurization. (which became known as Praxair in 1992). AOD is widely used for the production of stainless steels and specialty alloys such as silicon steels, tool steels, nickel-base alloys and cobalt-base alloys. The process helps to utilize lower cost charge materials, improve mechanical properties of the metal, recover nearly 100% of metallic charge elements, and remove dissolved gases, as well as most of the carbon and sulfur.

The Process

An AOD vessel is a brick lined steel shell with submerged tuyers that introduce oxygen and inert gases (argon or nitrogen) below the surface of the molten metal.

Argon Oxygen Decarburizer (Source: IspatGuru)

Decarburization stage involves reduction of carbon level to specification level, control of bath temperature, and additions to adjust heat weight, bath and slag composition, and control temperature. It is a common step for carbon reduction in the steelmaking practice and involves oxygen injection to reduce the carbon content in the steel melt. Once the liquid steel containing iron, chromium, carbon, and nickel is transferred from an electric arc furnace or induction furnace to the AOD converter, high carbon ferro-chrome is added and blowing of inert gas (argon, nitrogen) and oxygen mixture is started. In the initial stage, oxygen to argon ratio is maintained between 5:1 to 3:1. The ratio is lowered as decarburization progresses. Argon makes it possible to carry out the decarburization at a lower temperature. When carbon reduces to 30 % of the original value, the ratio of oxygen to argon is changed to 2:1. As the dilution process continues and when the oxygen to inert gas ratio reaches 1:1, oxidation of carbon continues, but oxidation of chromium is limited. This is because of the very low oxygen potential of the gas mixture, which minimizes chromium oxidation. The blow is continued to achieve 0.09 to 0.012 % Carbon.

Reduction stage involves recovering virtually all oxidized metallics. In this step, the reduction additions comprising of silicon alloys, such as ferro-silicon or chromium-silicon, and / or aluminum along with fluxing agents such as lime, dolomitic lime, and fluorspar are charged into the converter. The reduction mix helps reduction of metallic oxides from the slag. The bath is then stirred with inert gas, typically for around five to eight minutes. Here the chromium oxides get reduced. Any chromium oxide not ends up in the slag, which can form a complex spinel. The effectiveness of the reduction step is dependent on several factors including slag basicity and composition, temperature, mixing conditions in the converter and solid addition dissolution kinetics.

Desulfurization involves sulphur control to any level from 0.001 % to 0.02 % with a single or double slag practice. The desulfurization step is achieved by introducing lime in the metal bath. The introduced lime dilutes and reduces the sulfur content in the metal bath. Aluminum or silicon may also be added to remove oxygen, and additional alloys are added at the end to fine-tune the final steel chemistry.

External links

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