What is Argon oxygen decarburization (AOD) in Secondary Steel Making?

 

Argon Oxygen Decarburization (AOD)

Argon oxygen decarburization offers improved metal cleanliness, which is measured by low unwanted residual element contents and gas contents; this ensures superior mechanical properties. The AOD process is duplexed, with molten metal supplied from a separate melting source to the AOD refining unit (vessel). The source of the molten metal is usually an electric arc furnace or a coreless induction furnace. Foundries and integrated steel mills utilize vessels ranging in nominal capacity from 1 to 160 Mg (1 to 175 tons).

Although the process was initially targeted for stainless steel production, argon oxygen decarburization is used in refining a wide range of alloys, including:

v  Stainless steels

v  Tool steels

v  Silicon (electrical) steels

v  Carbon steels, low-alloy steels, and high-strength low-alloy steels

v  High-temperature alloys and superalloys

Fundamentals

In the AOD process, oxygen, argon, and nitrogen are injected into a molten metal bath through submerged, side-mounted tuyeres. The primary aspect of the AOD process is the shift in the decarburization thermodynamics that is afforded by blowing with mixtures of oxygen and inert gas as opposed to pure oxygen.

To understand the AOD process, it is necessary to examine the thermodynamics governing the reactions that occur in the refining of stainless steel, that is, the relationship among carbon, chromium, chromium oxide (Cr3O4), and carbon monoxide (CO). The overall reaction in the decarburization of chromium-containing steel can be written as:

The equilibrium constant, K, is given by:

Where a and P represent the activity and partial pressure, respectively.

At a given temperature, there is a fixed, limited amount of chromium that can exist in the molten bath that is in equilibrium with carbon. By examining Eq 2, one can see that by reducing the partial pressure of CO, the quantity of chromium that can exist in the molten bath in equilibrium with carbon increases. The partial pressure of CO can be reduced by injecting mixtures of oxygen and inert gas during the decarburization of stainless steel. Figure 1 illustrates the relationship among carbon, chromium, and temperature for a partial pressure of CO equal to 1 and 0.10 atm (1000 and 100 mbar, or 760 and 76 torr). The data shows that diluting the partial pressure of CO allows lower carbon levels to be obtained at higher chromium contents with lower temperatures.

Fig. 1 Carbon-chromium equilibrium curves.

In refining stainless steel, it is generally necessary to decarburize the molten bath to less than 0.05% C. Chromium is quite susceptible to oxidation; therefore, prior to the introduction of the AOD process, decarburization was accomplished by withholding most of the chromium until the bath had been decarburized by oxygen lancing. After the bath was fully decarburized, low-carbon ferrochromium and other low-carbon ferroalloys were added to the melt to meet chemical specifications.

Dilution of the partial pressure of CO allows the removal of carbon to low levels without excessive chromium oxidation. This practice enables the use of high-carbon ferroalloys in the charge mix, avoiding the substantially more expensive low carbon ferroalloys.

Fig. 2 Composition changes in refining type 304-L stainless steel using electric arc furnace practice and argon oxygen decarburization

Equipment

The processing vessel consists of a refractory-lined steel shell mounted on a tiltable trunnion ring (Fig. 2). With a removable, conical cover in place, the vessel outline is sometimes described as pear shaped. Several basic refractory types and various quality levels of the refractories have gained widespread acceptance (Ref 4, 5). Dolomite refractories are used in most AOD installations; magnesite chromium refractories are predominant in small (<9 Mg, or 10 ton) installations.

Fig. 2 Schematic of argon oxygen decarburization vessel.

Processing

1.      Stainless Steels

Charge materials (scrap and ferroalloys) are melted in the melting furnace. The charge is usually melted with the chromium, nickel, and manganese concentrations at midrange specifications. The carbon content at meltdown can vary from 0.50 to 3.0%, depending on the scrap content of the charge. Once the charge is melted down, the heat is tapped, and the slag is removed and weighed prior to charging the AOD vessel.

In the refining of stainless steel grades, oxygen and inert gas are injected into the bath in a stepwise manner. The ratio of oxygen to inert gas injected decreases (3:1, 1:1, 1:3) as the carbon level decreases. Once the aim carbon level is obtained, a reduction mix (silicon, aluminum, and lime) is added. If extra-low sulfur levels are desired, a second desulfurization can be added. Both of these steps are followed by an argon stir. After reduction, a complete chemistry sample is usually taken and trim additions made following analysis.

Carbon and Low-Alloy Steels

The refining of carbon and low-alloy steels involves a two-step practice: a carbon removal step, followed by a reduction/heating step. The lower alloy content of these steels eliminates the need for injecting less than a 3:1 ratio of oxygen to inert gas. Once the aim carbon level is obtained, carbon steels are processed similarly to stainless steels. Figure 5 illustrates carbon content and temperature relationship for the AOD refining of carbon and low-alloy steels. Because the alloy content of these grades of steel is substantially lower than that of stainless steel and because the final carbon levels are generally higher, there is no thermodynamic or practical reason for using an oxygen, inert-gas ratio of less than 3:1.

Oxidation measurements indicate that all of the oxygen reacts with the bath and that none leaves the vessel unreacted. By monitoring and recording the oxygen consumption during refining, very close control of end point carbon is achieved. Because the oxygen and inert gases are introduced below the bath and at sonic velocities, there is excellent bath mixing and intimate slag/metal contact. As a result, the reaction kinetics of all chemical processes that take place within the vessel are greatly improved.

Decarburization

In both stainless and low-alloy steels, the dilution of oxygen with inert gas results in increased carbon removal efficiencies without excessive metallic oxidation. In stainless grades, carbon levels of 0.01% are readily obtained.

Chemistry Control

The excellent compositional control of AOD-refined steel is indicated in Table 2 for a ten-heat series of high-strength low-alloy steel. The injection of a known quantity of oxygen with a predetermined bath weight enables the steelmaker to obtain very tight chemical specifications.

Nitrogen Control

    Degassing in argon oxygen decarburization is achieved by inert gas sparging. Each argon and CO bubble leaving the bath removes a small amount of dissolved nitrogen and hydrogen. Final nitrogen content can be accurately controlled by substituting nitrogen for argon during refining. Nitrogen levels as low as 25 to 30 ppm can be obtained in carbon and low-alloy steels, and 100 to 150 ppm N can be obtained in stainless steels. The ability to obtain aim nitrogen levels substantially reduces the need to use nitrided ferroalloys for alloy specification, and this also minimizes the use of argon. Hydrogen levels as low as 1.5 ppm can be obtained.