Steel Making: Lecture deoxidation

Steel Making: Lecture deoxidation

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  MY: 301 SteelMaking Processes Dr.R.H Tupkary, An Introduction to Modern Steel Making (Chapter No 04 page nos: 58-64) Deoxidation of Steel: (Why Deoxidized) • Steel making is carried out under oxidizing condition, since oxygen is bound to dissolve in it. • The solubility of Oxygen in liquid steel is 0.23% at 16000 C and rise 0.48% at 18000 C, but in solid it is only 0.003% OR • The solubility of Oxygen in liquid steel is 0.16%, but in solid it is only 0.003% www.steeluniversity.org (Referance) • Therefore, steps have to be taken to reduce the excess oxygen content (deoxidized) of the steel before it solidifies (casting) in order to prevent blowholes formation during casting or large quantities of FeO being precipitated. • The removal of residual oxygen content of refined steel is known as deoxidation or killing of steel. it can be done as precipitation deoxidation: Page 1 of 8
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  MY: 301 SteelMaking Processes Precipitation Deoxidation: • The principle behind precipitation deoxidation is to use elements (called Deoxidizers) having higher affinity for oxygen than iron are added to the melt. • It leads to the formation of oxide Product as [X] + [O] (XO) any phase • 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. 1. Those elements are preferred which are more stable and have highest possible deoxidizing ability. 2. Those elements to be preferred that form low melting oxides (passing rapidly into the slag) 3. Those elements to be used that form oxides that have lowest possible densities (come out from melt and join slag phase) 4. Insoluble in melt (steel) and do not revert (go) back to melt. Page 2 of 8
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  MY: 301 SteelMaking Processes Thermodynamics of Deoxidation: • Thermodynamically the best deoxidizers should have the minimum % [O] in equilibrium with its own minimum contact in steel. • Use of Ellingham Diagram to help you decide which is suitable and cheapest. • It indicates that Al, SI, Mn and C are commonly used deoxidizers for being reasonably cheap. • It may be noted that Mn, Si and C are elements oxidized earlier as impurities from iron and the same element are used later to remove excess oxygen from the refined steel. • AL, Si and Mn are the most deoxidizers used in steel making. the chemical reactions associated are. 2[Al] +3[O]  (Al2O3) [Si] + 2[O]  (SiO2) [Mn] + [O]  (MnO) • Elements like Zr,Ti,B,V etc. may be used but these are costlier than common deoxidezers. Page 3 of 8
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  MY: 301 SteelMaking Processes • These element must be in the range of chemical specifications at the end of deoxidation. Effectiveness of Deoxidizers • Equilibrium concentration of oxygen under different concentration of Mn. Si and Al. Target dissolved oxygen/ppm Equilibrium Concentration/% Temperature/0 C Mn Si Al 1000 0.5 0.003 4.5 x 10-6 1560100 5 0.3 0.00014 10 - - 0.0045 • Al is more powerful deoxidizers than Si and Mn. Al will also reduce Si and Mn oxidize if these are present in the refractories. Therefore for maximum and predictable deoxidation by Al, fireclay refractories (which are Si rich) should not be used. Page 4 of 8
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  MY: 301 SteelMaking Processes CALCULATING AL ADDITIONS • The weight percentage of aluminum required for deoxidation is therefore: • When calculating the total aluminum addition required, this value must be added to the aim (or residual) Al composition of steel • Page 5 of 8
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  MY: 301 SteelMaking Processes Example A 250 tonne of steel having an oxygen content of 450 ppm is to be Al-deoxidized at tap. Assuming an Al recovery rate of 60 % and an aim Al composition of 0.04 %, calculate the amount of 98 % Al alloy addition that is required. Page 6 of 8
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  MY: 301 SteelMaking Processes • Kinetics of deoxidation: • Kinetically the deoxidizers should be quick in action to obtain its high percentage utilization for deoxidation reaction. • Such data however largely unknown and thermodynamically rather than kinetics consideration dictate the choice of a deoxidizers. • It is important that the oxide product (particles) of deoxidation reaction should not remain in steel. • These particles are known as non-metallic inclusions which impair the mechanical properties also this can cause nozzle blockage during casting. • The mechanical properties vary with number, size, shape, distribution and composition of these inclusions. • The kinetics of deoxidation reaction is not so much important as is the kinetics of elimination of the product of deoxidation produce relatively cleaner steel. Page 7 of 8
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  MY: 301 SteelMaking Processes Alumina Modification /Ca Treatment • The removal of oxides from liquid to the slag is most effective if the oxide products are liquid. • This is because liquid particle are less dense and can combine to form large particles, which will float to the surface. • Since the melting point of alumina is 20380 C any particle will be solid at typical casting temperature. This can cause nozzle blockage during casting with a consequential reduction in casting speed and possibly the formation of large aggregated alumina clusters in the solidified product. • Calcium treatment is carried out to produce liquid calcium aluminates, which reduce nozzle blockage. Page 8 of 8