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Deoxidation of steel

Dr. Dmitri Kopeliovich

The main sources of Oxygen in steel are as follows:

Solubility of oxygen in molten steel is 0.23% at 3090°F (1700°C). However it decreases during cooling down and then drops sharply in Solidification reaching 0.003% in solid steel.

Oxygen liberated from the solid solution oxidizes the steel components (C, Fe, alloying elements) forming gas pores (blowholes) and non-metallic inclusions entrapped within the ingot structure. Both blowholes and inclusions adversely affect the steel quality.

In order to prevent oxidizing of steel components during solidification the oxygen content should be reduced.

Deoxidation of steel is a steel making technological operation, in which concentration (activity) of oxygen dissolved in molten steel is reduced to a required level.

There are three principal deoxidation methods:

Deoxidation by metallic deoxidizers

This is the most popular deoxidation method. It uses elements forming strong and stable oxides. Manganese (Mn), silicone (Si), aluminum (Al), cerium (Ce), calcium (Ca) are commonly used as deoxidizers.
Deoxidation by an element (D) may be presented by the reaction:

n[D] + k[O] = (DnOk)

The equilibrium constant KD-O of the reaction is:

KD-O = aox/(aDn x aOk)
or
log KD-O = log aox - n*log aD - k*log aO

where:
aox - activity of the oxide (DnOk) in the resulted non-metallic inclusion;
aD - activity of the deoxidizer in liquid steel;
aO - activity of oxygen in liquid steel.

Thermodynamic activity of a solute in a solution is a parameter related to the solute concentration. Activity substitutes concentration in thermodynamic equations describing chemical reactions in non-ideal solutions (activities of solutes in a diluted solution are close to their concentrations).

The equilibrium constant of a deoxidation reaction is determined by the steel temperature:

log KD-O = AD/T - BD

where:
AD, BD - characteristic parameters determined for the particular deoxidizer D;
T - steel temperature, °K

The table presents parameters of the deoxidation reactions for some metallic oxidizers:

Deoxidizer Reaction A B Equilibrium constant at 1873 °K (2912°F, 1600°C)
Manganese [Mn] + [O] = (MnO) 12440 5.33 1.318
Silicone [Si] + 2[O] = (SiO2) 30000 11.5 4.518
Aluminum 2[Al] + 3[O] = (Al2O3) 62780 20.5 13.018


Values of the equilibrium constant parameters are used for calculation of equilibrium concentrations of oxygen and the deoxidizer by the equation:

AD/T - BD = log aox - n*log aD - k*log aO

In the simplest case aox=1, aD=[D], aO=[O], therefore:

AD/T - BD = n*log [D] - k*log [O]

According to the degree of deoxidation Carbon steels may be subdivided into three groups:

  • Killed steels - completely deoxidized steels, solidification of which does not cause formation of carbon monoxide (CO). Ingots and castings of killed steel have homogeneous structure and no gas porosity (blowholes).
  • Semi-killed steels - incompletely deoxidized steels containing some amount of excess oxygen, which forms carbon monoxide during last stages of solidification.
  • Rimmed steels - partially deoxidized or non-deoxidized low carbon steels evolving sufficient amount of carbon monoxide during solidification. Ingots of rimmed steels are characterized by good surface quality and considerable quantity of blowholes.

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Deoxidation in vacuum

Method of deoxidation in vacuum utilizes carbon dissolved in steel as the deoxidizer according to the equation:

[C] + [O] = {CO}

where:
[C] and [O] - carbon and oxygen dissolved in liquid steel; {CO} - gaseous carbon monoxide.

The equilibrium constant of this chemical reaction is expressed as follows:

KCO = pCO/(aC x aO)

where:
pCO - partial pressure of carbon monoxide in the atmosphere; aC and aO - activities of carbon and oxygen in liquid steel.

Temperature dependence of KCO is insufficient.
For approximate calculations the following equation may be used:

[C]*[O] = 0.0025*pCO at 2948°F (1620°C)
According to the above expressions the oxygen activity (concentration) is proportional to the partial pressure of carbon monoxide therefore decrease of the latter will cause reduction of the oxygen activity.
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.
Vacuum deoxidation is used mainly in Ladle refining.

Steels deoxidized in vacuum are characterized by homogeneous structure, low content of non-metallic inclusions and low gas porosity.
Vacuum treatment is used for manufacturing large steel ingots, rails, ball bearings and other high quality steels.

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Diffusion deoxidation

Oxygen dissolves in both steel and slag. Equilibrium between the two systems may be presented by the equation:

[O] = (O)

The equilibrium constant of the reaction:

KFeO = a[O]/a(O)
or
a[O] = KFeO*a(O)

Thus reduction of the oxygen activity (concentration) in steel may be achieved by decreasing the oxygen activity in the slag.
When the oxygen activity in the slag is reduced oxygen ions dissolved in steel begin to diffuse from the steel into the slag, and the equilibrium conditions are restored. In other words, deoxidation of slag results in deoxidation of the steel.
Carbon (coke), silicone, aluminum and other elements are used for slag deoxidation.
Since deoxidizers in the difusion method are not introduced directly into the steel melt, oxide non-metallic inclusions do not form.
Diffusion deoxidation allows to produce steel less contaminated by non-metallic inclusions.

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deoxidation_of_steel.txt · Last modified: 2012/05/31 by dmitri_kopeliovich
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