Characteristics of the VOD Vacuum Oxygen Decarburisation Process and Overview of Lining Refractory Damage
The VOD (Vacuum Oxygen Decarburisation) furnace, as shown in Figure 1, is a refining apparatus that performs decarburisation by introducing oxygen into molten steel under vacuum or reduced-pressure conditions. It also facilitates desulphurisation, dephosphorisation, degassing, deoxidation, and composition adjustment. The VOD process is primarily employed for refining stainless steel and special steels.
In operation, the molten steel is first placed in a refining ladle (see Figure 1), which is then positioned within a vacuum chamber or serves as the vacuum vessel itself. A porous plug located at the base of the ladle is used to stir the molten steel.
1 – Oxygen; 2 – Argon; 3 – Vent hole; 4 – Metal addition hole
The operational characteristics of the VOD process involve a wide range of slag basicity (from 0.6 to 4, or even higher), high operating temperatures, strong thermal shock resistance, and intense agitation forces. Consequently, the operating conditions for the refractory materials lining VOD ladles are among the most demanding compared with other refining processes.
Furthermore, oxygen-blowing rates are increased to achieve rapid decarburisation, and the injection volume of stirring gases is heightened to improve refining efficiency. As a result, technological developments in the VOD process indicate that it continues to evolve toward increasingly demanding operating conditions.
Figure 2 illustrates the refractory materials used for the VOD ladle lining. The slag line region employs MgO–Cr₂O₃-based refractories or MgO–CaO-based refractories; the side walls typically use MgO–Cr₂O₃-based refractories, MgO–CaO-based refractories, etc.; and the bottom employs MgO–Cr₂O₃-based refractories, MgO–CaO-based refractories, and high-alumina refractories.
VOD Process and Damage to VOD Ladle Linings
The Vacuum Oxygen Decarburisation (VOD) process was developed for stainless steel refining and is particularly suited to producing ultra-low-carbon steels. This is because the molten steel can be agitated by blowing oxygen from the top and argon from the bottom. Consequently, in VOD operations, the ladle serves not only as a vessel for holding and transporting molten steel within the stainless steel production process but also as a key piece of equipment in stainless steel manufacturing (see Figure 1).
The VOD process comprises three stages-oxygen blowing, degassing, and reduction-with refining operations carried out intermittently under high-temperature, gas-stirred conditions. The molten steel remains in the ladle for extended periods, and its lining is subjected to slurry erosion, corrosion, and gas permeation under vacuum conditions. These exceptionally harsh operating conditions expose the VOD ladle lining to severe environments, resulting in rapid deterioration.
Typically, the most severely damaged area of the ladle during vacuum deoxidation is the slag line, indicating that slag erosion is a major factor in this deterioration. As illustrated in Figure 3, the condition of the molten slag changes significantly throughout the vacuum deoxidation refining process.
The refining temperature during vacuum deoxidation operations reaches its peak during decarburisation. However, because the decarburisation slag contains a high proportion of ferro-chromium (25%–35%), it exhibits considerable viscosity. Slag erosion tests confirm that this high-viscosity decarburisation slag causes less erosion to magnesium-chromium bricks compared with reduction slag. Changes in slag composition during the vacuum deoxidation process are illustrated in Figure 4.
Depending on the type of reducing agent, reduction slags comprise two main compositions:
For general stainless steel production using silicon-based reducing agents, the primary constituents of the reduction slag are calcium and silicon dioxide.
For special stainless steel production using aluminium-based reducing agents, refractories are subject to erosion by high-alkalinity slags of the CaO–Al₂O₃ system containing calcium fluoride.
When silicon reduction is employed, the calcium-to-silicon ratio in the final slag varies among manufacturers, ranging from 1.2 to 2.8. Even when the final slag exhibits high basicity, it is inferred that the slag had low basicity during the initial reduction phase, based on the low basicity of the decarburisation slag preceding the reduction period. Slag corrosion test results are shown in Figure 5. Low-alkalinity slag possesses a low melting point and high magnesium solubility. Consequently, the corrosion resistance of ladles to low-alkalinity slag during silicon reduction operations in vacuum deoxidation processes is considered critically important.
Figure 5 presents slag erosion test results for magnesium-chromium bricks using both silicon-reducing and aluminium-reducing slags. Compared to silicon-reducing slag, aluminium-reducing slag exhibited slightly greater erosion depth. Producing special stainless steels requires prolonged smelting, which leads to severe erosion damage at the slag line of ladles during vacuum deoxidation operations involving aluminium reduction.
During erosion tests using aluminium-based reducing slags, a significant observation was made: coarse-grained aggregates retained their original morphology, protruding from the working surface. This was attributed to erosion that had previously destroyed the matrix. Another notable characteristic was the more severe erosion caused by aluminium-based reducing slags compared to silica-based ones. This behavior results from the addition of calcium fluoride, which lowers the slag's melting point and viscosity.
The compositions of these two slags differ significantly. Moreover, as the proportion of special steel melting increases, operating conditions become more demanding. Since CaO–Al₂O₃ system slags exhibit considerably lower viscosity than CaO–SiO₂ system slags, the operating conditions for special steel melting have a significant impact on the durability of refractory materials.
During vacuum deoxidation operations, ladles operate intermittently, making the refractory lining susceptible to spalling and damage. For instance, when comparing usage under identical molten steel conditions, magnesium-chromium bricks in vacuum deoxidation ladles show a higher damage rate than those in continuously operated RH degassing units or AOD ladles. This disparity can be attributed to spalling induced by intermittent operation. Furthermore, increasing the operating rate of ladles during vacuum deoxidation operations has been found to be highly effective in extending refractory service life. The flaking observed in the refractory linings of vacuum deoxidation ladles is more accurately characterized as structural flaking induced by slag penetration, rather than thermal flaking. Consequently, suppressing slag penetration is crucial for enhancing flaking resistance.
Figure 6 illustrates an exemplary ladle design for vacuum deoxidation operations. To facilitate stirring of the molten steel, a vent plug is installed at the ladle base. If the vent plug is positioned near a particular side wall, severe erosion occurs on the refractory lining of that wall. Operational results indicate that the erosion rate of the slag line bricks increases as the distance between the vent plug and the ladle wall decreases.