DEGASSING PROCESSES OF STEEL | Cleanliness Steel Refining

Degassing Processes

Degassing consist of exposing the molten metal stream to vacuum and removal of dissolved gases in it. When metal steam sudden exposes to vacuum it explodes in to fine droplets and surface area increase. Thus increase in surface area, increases the surface relation process and removal of gases to vacuum.

There are three type of degassing I.e
1. Ladle degassing
2. Stream degassing
3. Circulation degassing

Ladle Degassing :

Ladle containing molten steel is placed in a chamber which is then taken out, after refined. After that it teemed for casting.

● Ladle is provided with a porous plug at its bottom to purge argon gas. In a vacuum chamber the ladle is placed. The vacuum chamber is equipped with a hopper so as to make addition of element as and when it needed. For effective, fully killed steel is necessary to purge argon through the bottom of ladle. Stirring the bath enhances rate of gas removal, vigorous removal of gases causes metal splashing too. So that ladle is not filled completely and nearly 20 to 15% of its height is kept as free board for splashed metal droplets.(required pressure -- 1mmHg to 10mmHg).

● During degassing addition are made for deoxidation and alloying purpose. But in certain cases ladle is heated to compensate for the loss of heat in the time of degassing. Stirring gas is introduce either from top through the roof by a submerged refractory Lance or through the porous plug from bottom.

● Electro magnetic stirring is employed for degassing. For this purpose ladle has to be made of non magnetic austenitic stainless steel or stainless windows. For certain grades of alloy steels both induction stirring and arc heating are employed for degassing.

● The final content of gas in steel depends on degree of vacuum and time of treatment. Hydrogen and nitrogen in steel also reduced, the pick up N2 from atmospheric air may occur during open pouring. Where ladle lining with high alumina bricks at upper slag zone and rest are magcarbon with fireclay bricks.

Stream Degassing :

In this degassing technology molten steel is teemed in to another vessel, which is under vacuum. Sudden exposure of molten stream in vacuum leads to very rapid degassing due to increase surface area create by break up of stream in to droplets. The major amount of degassing occurs during the fall of molten stream. Height of the pouring is an important design. It is two types
1. Ladle to mould
2. Ladle to ladle

Ladle to Mould :
Slab caster - ladle to mould
Ladle to mould

Preheat mould with hot top is placed in vacuum chamber. Above the chamber a tundish is placed. Steel tapped in the ladle at superheat equivalent to 30℃ to 40℃ is placed above the tundish. Steel is bottom poured in the tundish, which is goes to the mould through the path of sen. The sen is made up of refractory lined pipe, which avoid the atmospheric air reaction to the liquid steel. The surface of the mould is covered with mould powder to avoid the heat loss and reaction from atmosphere.

Ladle to Ladle :

● In ladle to ladle degassing, a ladle with the stopper rod is placed in a vacuum chamber. Ladle containing molten steel from BOF or EAF is placed on top of the vacuum chamber and the gap is vacuum sealed. Alloy addition are made under vacuum. Stream is allowed to fall in the ladle where molten steel is degassed and alloy addition also done.

● In some cases degassing is done during tapping. In this arrangement molten steel from EAF and BOF is tapped in to tundish or pony ladle. From the pony ladle molten stream is allowed to fall in to a ladle which is evacuated. Ladle is closed from top with a special cover which contain exhaust opening. Steel with 30℃ to 40℃ superheat is tapped in to ladle.

● In some case degassing is done after taping in ladle from BOF and EAF in to another ladle. It is done because of chemical imbalance in ladle or some problems in ladle like ladle puncher, purging failure etc. Basically in steel industry used circulation degassing processes for better inclusion and gas free steel. Among of them RH degassing is popular in which very ultra low carbon up to 0.005% steel can be produces.

References :
1. Modern Steel Making :  Dr R.H. Tupkary and V.R. Tupkary.
2. Ironmaking and Steelmaking Theory an d Practice : A. Ghosh and A. Chatterjee
3. Steel Making : A.K.Chakravorty

4. Sr. Professors from IIT's & BPUT. (INDIA)

Author :
Subir Kumar Sahu

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