Dissolved Oxygen in Steel

Precipitation of Oxides / Inclusions

Formation of Pores / blow holes

HindersDe- Sulpherisation

Effects Nitrogen Removal

The main sources of Oxygen in steel are as follows:

1. Oxygen blowing (example: Basic Oxygen Furnace);

2. Oxidizing slags used in steel making processes (example: Electric Arc Furnaces);

3. Atmospheric oxygen dissolving in liquid steel during pouring operation;

4. Oxidizing refractories (lining of furnaces and ladles);

5. Rusted and wet scrap.

Diatomic gases like O2, N2, H2, dissolve in liquid and solid metals in the atomic form

½X2 (g) = [X]

For which the isothermal equilibrium constant is

K = [%X] / (pX2)1/2

For ideal solutions, the concentration of X is directly proportional to the square root of

the equilibrium gas partial pressure; this is known as Sievert’s law.

For calculating the solute content (in ppm) of H2, N2, and O2 by mass at a given

temperature (oK) and gas pressure (in atm.), refer the following equations: -

Log [ppm H]

= -1900

+ 2.423

(pH2)1/2 T

Log [ppm N]

= -188

+ 2.760

(pN2)1/2 T

Log [ppm O]

= -6046

+ 4.242

(pO2)1/2 T

By above formula, the solubility of oxygen at various temperatures in liquid steel was calculated.

S. No. Temperature (oC) O2 Content (ppm)

1 600 0.046 2 800 1.32 3 1000 10 4 1200 222 5 1400 700 6 1550 1125 7 1700 2000

Solubility of oxygen in molten steel is around 0.20% at 1700oC. It decreases during cooling downIt drops sharply during solidification reaching 0.003% in solid steel.

Oxygen liberated from the solid solution leads to: -

• Oxidation of the steel components (C, Fe, alloying elements)

• Formation gas pores (blowholes)

• Entrapment of non-metallic inclusions within the cast structure

Both blowholes and inclusions adversely affect the steel quality.

There are three principal deoxidation methods:

1. Deoxidation by metallic deoxidizers

2. Deoxidation by vacuum

3. Diffusion deoxidation.

The most popular deoxidation method. It uses elements forming strong and stable oxides. Manganese (Mn), silicone (Si), aluminum (Al), 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 aO

k) 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 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

Deoxidizer Reaction A BEquilibrium constant at 1873

°K

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

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

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 does not cause formation of carbon monoxide (CO).Ingots and castings of killed steel have homogeneous structureNo gas porosity (blowholes).

Semi-killed steels

Incompletely deoxidized steels Contains some amount of excess oxygenCarbon monoxide liberated during last stages of solidification.

Rimmed steels

Partially deoxidized or non-deoxidized low carbon steels Evolve sufficient amount of carbon monoxide during solidification.Ingots of rimmed steels are characterized by good surface quality Considerable quantity of blowholes.

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 results in: -

Decreases the partial pressure of CO.

Shifts equilibrium of the reaction of carbon oxidation.

Formation of bubbles of carbon monoxide in the liquid steel.

Floatation and removal of bubbles by the vacuum system.

Removal of Hydrogen dissolved in liquid steel.

Hydrogen diffuses into the CO bubbles and 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

• Low gas porosity.

Vacuum treatment is used for manufacturing large steel ingots, rails, ball bearings

and other high quality steels.

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)

ora[O] = KFeO*a(O)

Deoxidation of slag results in deoxidation

of the steel.

That’s why there is the quote “A good slag maker is a good steel maker”

Reduced oxygen activity in slag

Oxygen ions diffusion from steel to Slag

Attaining equilibrium conditions

Addition of deoxidizers

Diffusion deoxidation

Elements used for slag deoxidation: -

1. Carbon (coke)

2. Silicone

3. Aluminum and others.

Diffusion deoxidation allows to produce steel less contaminated by non-metallic

inclusions as in the diffusion method deoxidizers are not introduced directly into the

steel melt, oxide non-metallic inclusions do not form