DEOXIDATION OF STEEL

Deoxidation


●  Steel making is carried out under oxidizing condition where oxygen is bound to dissolved in iron melt. The solubility if oxygen in pure iron at 1600℃ is 0.23% and rised to 0.48% at 1800℃.

●  The oxygen content in iron varies inversely with the impurities content in melt and carbon content in particular. As refining progress the oxygen content in the melt increase and at the end of refining a considerable amount of oxygen up to 0.05 to 0.10% is left in liquid steel.

●  Thus the removal of residual oxygen content of refinded steel is known as deoxidation or killing of steel. It can be two types:

Diffusion deoxidizer :  The dissolved oxygen content is lower by including it to diffuse in to slag inside steel making furnace itself or under vacuum separately outside. There is a limit to which this can be made use of effectively.

Precipitation deoxidation :  Elements having higher affinity for oxygen are added to the melt. A reaction of the this leads to the formation of oxide product. This method is generally adopted as it is very effective in decreasing the oxygen content of steel. The choice of a deoxidizer depends on a number of factors.

THERMODYNAMICS OF DEOXIDATION :

●  The equilibrium state for the common deoxidation reaction pertaining to steel making conditions. The equilibrium oxygen content for various deoxidation reaction given pure oxide product like Zr, Al, Ti, B, Si, C, V, Cr and Mn etc.

●  It indicates that aluminium is a very effective deoxidizer and quite extensively used in practice. Silicon is also reasonable effective, Al, Si, Mn, C are commonly used as deoxidizer for being reasonable cheap. The elements are oxidised earlier as impurities form the iron and the same elements are used later to remove excess oxygen from the refined steel.

●  Occasionally Zr, Ti, B, V, Nb etc may be used as deoxidizer but these are little bit costlier than other. The residual deoxidizer content in steel should not lead to any adverse effects on the properties of steel.



KINETICS OF DEOXIDATION :

●  The deoxidizer should be quick in action to obtain it's high percentage utilisation for deoxidation reaction. Oxide product of the deoxidation reaction should not remain as mechanically entrapped practice in steel which is known as non-metallic inclusion, impair the mechanical properties of steel.

●  The mechanical properties varies with the number, size, shape, distribution and composition of these inclusion. Every attempt is therefore made to eliminate the product of deoxidation from the melt and thereby obtain clean steel.

●  A gaseous product of deoxidation is carbon alone, which produce gas in deoxidation reaction at atmospheric pressure. The physics of carbon monoxide bubble formation may be referred to again to understand deoxidation of steel by carbon.

●  The kinetics of the deoxidation reaction required the understanding of the mechanism of precipitation deoxidation, which referred to only as deoxidation. Since almost the entire deoxidation is invariably achieved through precipitation deoxidation.


MECHANISM OF DEOXIDATION :

●  Dissolution and homogenization of the deoxidizer in the steel melt so as to make the deoxidation reaction move in the direction of oxide formation.

●  Formation of critical nuclei of the deoxidation products in a homogeneous medium since it involves formation of a new phase.

●  Progress of deoxidation resulting in a growth of the reaction products.

●  Separation of the product of the deoxidation by the way their flotation from the steel melt to improve clean lines.


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