Steel MAking: Lecture Vacuum Degassing, VOD, Ladle-De-S, ESR

MY: 301 Steel Making Processes Lecture No: 20 21, 14 Feb 2011

Vacuum treatment:

Vacuum treatment of molten steel decreases the partial pressure of CO, which results in shifting equilibrium of the reaction of carbon oxidation.

Bubbles of carbon monoxide form in the liquid steel float up and then they are removed by the vacuum system.

In addition to deoxidation vacuum treatment helps to remove Hydrogen dissolved in liquid steel. Hydrogen diffuses into the CO bubbles and the gas is then evacuated by the vacuum pump.

Movement of the molten steel caused by CO bubbles also results in refining the steel from non-metallic inclusions, which agglomerate, float up and are absorbed by the slag.

CO bubbles also favor the process of floating and removal of nitride inclusions and gaseous Nitrogen.

Steels refined in vacuum are characterized by homogeneous structure, low content of non-metallic inclusions and low gas porosity.

Vacuum degassing methods are used for manufacturing large steel ingots, rails, ball bearings and other high quality steels.

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http://www.substech.com/dokuwiki/doku.php?id=hydrogen&DokuWiki=c6f2be733c623189b295a7609ef29b2c

http://www.substech.com/dokuwiki/doku.php?id=nitrogen&DokuWiki=c6f2be733c623189b295a7609ef29b2c


Vacuum Oxygen Decarburization (VOD):

VOD system is consisting of…

i. Vacuum tank ( with Oxygen lancing Facility)

ii. Ladle furnace (with Stirring facility)

Process flow = EAF VOD IC or CC

The ladle has a free board of about 1 to 1.5 meter to contain violent (vigorous) agitation of the bath during lancing.

The charge ingredients are similar to the AOD process. The charge is melted in EAF and transferred to the VOD system.

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Oxygen blowing from lid of the vacuum tank and argon bubbling from the ladle bottom are started when required vacuum is established.

Argon stirring is essential otherwise decarburization is delayed due to lack of mass transport of carbon from bottom portion to the surface where carbon oxygen reaction is greatly achievable.

The carbon can be lowered to around 0.02% at around 16-18%Cr. At the end of the refining the vacuum is broken and the heat/bath is

deoxidized with Al/Fe-Si – or Fe-Si/Cr-Si(for chromium recovery) Then de-sulphurisation is carried out by putting synthetic slag

(Cao,SiO2,Al2O3 or CaF2 = e.g. 40: 40: 20) to the molten steel surface of about 2-3% by weight of the molten steel and argon Stirring of the melt through the porous bottom plug results in deep desulfurization of the steel.

The total VOD cycle is around 2-2.5Hours.Benefits of Vacuum Oxygen Decarburization (VOD):

-Deep carbon removal (ultra low carbon steel can produced -Low losses of chromium in treatment of stainless steels; -Hydrogen removal (degassing); -Sulfur removal (desulfurization); -Precise alloying; -Reheating; -Non-metallic inclusions (oxides and nitrides) removal; -Temperature and chemical homogenizing.

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Since many steels are required to be vacuum treated to decrease the gas content, the vacuum system can be easily done without extra additional investment.

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Ladle desulfurization by injection of active agents

Injection of desulfurizing agents (Ca, Mg, CaSi, CaC2, CaF2+CaO) to molten steel is the most effective method of sulfur removal.

Injection methods usually combine supply of a disperse desulfurizing agent (powder) with stirring by argon blowing.

A ladle with deoxidized molten steel is transported to the injection stand where it is placed under a cover, through which the injection lance may lower and immerse into the melt.

Steel treated in the stand is covered by a layer of desulfurizing slag having high solubility of sulfur and capable to absorb sulfides formed as a result of active agents injection.

Desulfurization agents are injected in argon stream. Argon bubbles produce stirring of the molten steel and the slag promoting desulfurization. Stirring also provides thermal and chemical homogenization of the melt.

When the desulfurizing agents are injected into molten steel in form of a cored wire containing powder of desulfurizing agent stirring by argon bubbling from the porous plug mounted in the ladle bottom is used.

Fumes formed during the operation are extracted through the cover. Injection of desulfurizing agents allows achieving ultra-low

concentrations of sulfur in steel (0.0002%).

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http://www.substech.com/dokuwiki/doku.php?id=desulfurization_of_steel&DokuWiki=c6f2be733c623189b295a7609ef29b2c#desulfurization_of_steel_by_slags

http://www.substech.com/dokuwiki/doku.php?id=deoxidation_of_steel&DokuWiki=c6f2be733c623189b295a7609ef29b2c

http://www.substech.com/dokuwiki/doku.php?id=desulfurization_of_steel&DokuWiki=c6f2be733c623189b295a7609ef29b2c#desulfurization_of_steel_by_injection_of_active_agents


Benefits of Ladle desulfurization by injection of active agents: Deep sulfur removal (desulfurization); Temperature and chemical homogenizing; Non-metallic inclusions removal.

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Electroslag Remelting Electroslag Remelting (ESR) is a process of remelting a consumable

electrode utilizing the heat generated by an electric current passing through a molten slag between the electrode and the solidifying ingot.

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The consumable electrods are as cast or forged cylindrical parts made of an alloy to be remelt.

An electroslag remelting process (ESR) starts when the lower tip of a consumable electrode is immersed into a pool of molten slag.

The premelted slag possessing electrical conductivity is located on the water-cooled mold base connected to a power supply.

The electric current (commonly AC) passing through the the slag keeps it at high temperature, which is about 360ºF (200ºC) higher than the melting point of the remelted metal.

The electrode tip is heated by the hot slag and starts to melt forming droplets of liquid metal, which disconnect from the electrode and sink through the slag layer.

The slag composition is based on calcium fluoride (CaF2), lime (CaO) and alumina (Al2O3).

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Steel MAking: Lecture Vacuum Degassing, VOD, Ladle-De-S, ESR

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