Deoxidation

Deoxidation

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  Deoxidation Madhava . J, R161529, B.TechIInd year, Metallurgical and Materials Engineering
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  Introduction to deoxidation Atfirst, refining of hot metal to steel is done under oxidizing atmosphere. During refining oxygen dissolves in steel and solubility of oxygen in steel in negligibly small. At the end of refining, a considerable amount of oxygen (0.05-0.10%) is left in liquid steel. If a steel with 0.05% oxygen is cast, the residual oxygen is evolved in the form of gases will leads to blow holes and non metallic inclusion. The removal of residual oxygen content of refined steel is known as deoxidation or killing of steel. This defects of considerable effect on mechanical properties of steel.
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  Sources of oxygenin steel ❖ Rust on steel, it is one of the oxide form of iron and forms when oxygen reacts with iron or steel. ❖ Oxygen blowing, it is to remove the unwanted elements like Si, Mn, C, P which appears in various states of manufacturing process. ❖ Steel making slag, such that this unwanted elements in the form oxides in the slag may result in excess oxygen. ❖ Atmospheric oxygen dissolved in during teeming. ❖ Oxidizing refractories
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  Kinetics of deoxidation Kineticsof deoxidation before elimination : ➔ It consists of dissolution of deoxidisers into molten steel, chemical reaction between dissolved oxygen and the deoxidizing element, nucleation and intial growth of the deoxidation product. In detail : ● Mechanism of dissolution depends on melting point. In this case the aluminium is expecte dmelt faster than iron or steel ● Nucleation of solid becomes easier if interface is present. Here the deoxidation of Al produces solid Al2O3 and this interface is useful nucleation
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  ● Deoxidation withsingle elements like Al, Si etc., produce solid deoxidation product at the steelmaking temperature. ● As we know that a liquid product can easily coalesce when compared to the solid product ● Deoxidation with ferro silicon + ferro manganese produces liquid deoxidation product. ● Boron, titanium, zirconium are also quite effective deoxidizers but expensive. Kinetics of elimination of deoxidising products : ❏ It consists of further growth of deoxidation products by agglomeration and their elimination from liquid steel. ❏ This is a slow process and it is important to removal deoxidation product it is achieved by floatation and absorption into a slag.
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  ❏ The followingsteps are important for removal of deoxidation products from steel. 1. Growth of deoxidation product. 2. Movement through molten steel to surface. 3. Absorption of inclusion by a suitable designed synthetic slag ❏ For the removal, flotation of deoxidation product depends on physical properties of steel(laminar flow) it can be described at steady state by stoke’s law Vt = terminal velocity(m/s) of the inclusion ᑭᑭ = difference in the density of steel and deoxidation product and ŋ is the viscocity of steel (kg./m.s) ❏ Degree of stirring in the melt is important, stirring may help floating of deoxidation product.
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  Deoxidisers Aluminum : ➔ Itis effective deoxidiser which is used in most steel making operations. ➔ It is used in the form of rods, pellets, powders etc., ➔ Usually the aluminium deoxidation is carried out in the ladle. ➔ It is an alloying addition in the heat resistant steels.
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  Silicon : ➢ Itis used as primary deoxidising agent in a furnace where a reducing slag is to be made. ➢ It is used in the form of ferro-silicon. ➢ Silicon is used as alloying element for better strength, hardenability and electrical properties. Manganese : ● It is a weaker deoxidiser than silicon. ● As an alloying element it gives strength and toughness. ● It is used in the form ferro-manganese of various grades
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  ❖ Aluminum isvery effective since Al2O3 is far more stable oxide than SiO2, MnO, etc. ❖ But Al2O3 is solid even at steel making temperatures and hence cannot be used alone to deoxdise steel completely. ❖ It is generally used along Mn and Si when aluminium has a chance to join the liquid slag product of deoxidation. ❖ B, Ti, Zr are also effective deoxidisers. They aren’t used primarily.
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  Inclusion control ➔ Wecan’t avoid the inclusion formation in steels. The only thing is to modify or minimise them such that they won’t act as stress raisers during rolling. Calcium injection : ❏ Much cleaner steels can get from the Ca injection. ❏ It is either pure Ca metal ar as Ca-Fe-Al alloy with 32% Ca and 40% Al in powder form. ❏ Ca must injected deep so it can improve it’s efficiency. ❏ After that it reacts with sulphides and silicate inclusions to form complex sulphide-oxide. ❏ The inclusions are modified step by step and by assimilation of CaO in Al2O3 particles.
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  ❏ It convertssome of the solid alumina into liquid alumina particles(liquid phase inclusions) and others will be semi-solid state and other oxides also. ❏ This inclusions surround solid alumina to form inclusions with solid core and liquid periphery. ❏ The size of alumina inclusion increases slightly when it gets enveloped by the liquid phase. ❏ Thus it changes the morphology of sulphide and alumina inclusions and rises them to join the slag at the surface for the cleaner steels.
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  Thank you