Vacuum Degassing (VD) treatment is used to reduce the dissolved gas content (e.g. hydrogen, oxygen, nitrogen) of the steel to enhance the steels capabilities. The liquid steel arrives from the primary metallurgy production steps in ladles of typically 20 t to 200 t size. The steel is covered with a slag material to protect it from the atmopshere. Some slags are also "active" which means they provide a certain chemical reaction with the steel.
Inside the tank degasser the ladles are exposed to a vacuum of typically < 1 mbar . When the steel surface gets in contact with the vacuum, the volatile, solved gases are removed. To refresh the melt surface, the liquid bath is stirred by Argon which is bubbling upwards from a "porous plug" at the bottom of the ladle. The uprising Argon bubbles get saturated with the dissolved gases which supports the degassing clearly.
As the VD system does not include any heating device, the melt cools down continuously. A short and uninterupted treatment is therefore necessary.
The vacuum system must handle a high gas-flow plus significant amounts of dust, created by condensed outgassing of the melt.
• Typical pumping speed demand at the degassing pressure (~0.67 mbar) is between 50,000 m³/h to 250,000 m³/h
• Required evacuation time: typically 4-6 min.
• Treatment time under vacuum: ~10-20 min.
Fast evacuation is important, as the melt is continuously cooling down during the treatment. But, too fast evacuation might lead to slagfoaming, heavy gas outbursts and splashing, this is why a controlled pumpdown process is often necessary.
Dust filtration is required to trap the partly huge amounts of dust created during the process. As the dust particles are often pyrophoric, the filtration system must be designed accordingly. Successfully used filter designs therefore typically use "inert-gas blow-back cleaning" steps to clean the filter surface.