Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.

Steel making


Published on

The class of NITRKL steel tech 2012-2014 batch.

Published in: Education

Steel making

  1. 1. Smarajit SarkarDepartment of Metallurgical and Materials Engineering NIT Rourkela
  2. 2.  Ahindra Ghosh and Amit Chatterjee: Ironmaking and Steelmaking Theory and Practice, Prentice- Hall of India Private Limited, 2008 Anil K. Biswas: Principles of Blast Furnace Ironmaking, SBA Publication,1999 R.H.Tupkary and V.R.Tupkary: An Introduction to Modern Iron Making, Khanna Publishers. R.H.Tupkary and V.R.Tupkary: An Introduction to Modern Steel Making, Khanna Publishers. David H. Wakelin (ed.): The Making, Shaping and Treating of Steel (Ironmaking Volume), The AISE Steel Foundation, 2004. Richard J.Fruehan (ed.): The Making, Shaping and Treating of Steel (Steeelmaking Volume), The AISE Steel Foundation, 2004. A.Ghosh, Secondary Steel Making – Principle & Applications, CRC Press – 2001. R.G.Ward: Physical Chemistry of iron & steel making, ELBS and Edward Arnold, 1962. F.P.Edneral: Electrometallurgy of Steel and Ferro-Alloys, Vol.1 Mir Publishers,1979 B. Ozturk and R. J. Fruehan,: "Kinetics of the Reaction of SiO(g) with Carbon Saturated Iron": Metall. Trans. B, Vol. 16B, 1985, p. 121. B. Ozturk and R. J. Fruehan: "The Reaction of SiO(g) with Liquid Slags,” Metall. Trans.B, Volume 17B, 1986, p. 397. B. Ozturk and R. J. Fruehan:”.Transfer of Silicon in Blast Furnace": , Proceedings of the fifth International Iron and Steel Congress, Washington D.C., 1986, p. 959. P. F. Nogueira and R. J. Fruehan:” Blast Furnace Softening and Melting Phenomena - Melting Onset in Acid and Basic Pellets", , ISS-AIME lronmaking Conference, 2002, pp. 585.
  3. 3.  There are as many as two thousand odd varieties of steels in use. These specifically differ in their chemical composition. However, a couple of hundred varieties are predominantly in use. The chemical composition of steels broadly divide them into two major groups, viz. (i) plain carbon steels and (ii) alloy steels.
  4. 4.  The plain carbon steels are essentially alloys of iron and carbon only whereas, if one or more of elements other than carbon are added to steel in significant amounts to ensure specific better properties such as better mechanical strength, ductility, electrical and magnetic properties, corrosion resistance and so on it is known as an alloy steel. These specifically added elements are known as alloying additions in steels.
  5. 5.  Steels may contain many other elements such as AI, Si, Mn, S, P, etc. which are not added specifically for any specific purpose but are inevitably present because of their association in the process of iron and steelmaking and can not be totally eliminated during the known process of iron and steelmaking. These are known as impurities in steel. Every attempt is made to minimise them during the process of steelmaking but such efforts are costly and special tech-niques are required for decreasing their contents below a certain level in the case of each element.
  6. 6.  For cheaper variety of steels therefore their contents at high levels are tolerated. These high. levels are however such that the properties of steels are not signifi-cantly adversely affected. These tolerable limits of impurities are considered as safe limits and the impurity levels are maintained below these safe limits. For example, for ordinary steels sulphur contents up to 0.05% are tolerable ,whereas for several special steels the limit goes on decreasing to as low as 0.005% or even lower. For most high quality steels now the total impurity level acceptable is below 100 ppm and the aim is 45 ppm.
  7. 7. Plain carbon steels are broadly sub-divided into fourmajor types based on their carbon contents. These arenot strict divisions based on carbon contents but aregenerally broad divisions as a basis of classification.This division is definitely useful. These are:(i) Soft or low carbon steels up to 0·15% C(ii) Mild steels in the range 0·15-0·35% C(iii) Medium carbon steels in the range 0·35-0·65% C(iv) High carbon steels in the range 0·65-1·75% C
  8. 8. The alloy steels are broadly sub-divided into three groupson the basis of the total alloying elements present. Thisdivision is also only a broad division and not a rigid one.This is :(i) Low alloy steels up to 5% total alloying contents(ii) Medium alloy steels 5-10% total alloying(iii) High alloy steels above 10% total alloying
  9. 9. The products in the above reactions are only those which arestable at steelmaking temperatures. The oxides which are notthermodynamically stable at steelmaking temperatures need notbe considered here. Except the sulphur reaction all the rest are oxidation processesand are favoured under the oxidizing condition of steelmaking .In the case of oxidation of carbon the product, being a gas,passes off into the atmosphere but the rest of the oxide productsshall remain in contact with the iron melt in the form of a slagphase.
  10. 10. I n steelmaking the reactions should move to the right inpreference to the oxidation of iron and that the danger ofreversion of an impurity to the metal phase is as remote aspossible. From the point of view of law of mass action the requiredconditions can be achieved by increasing the activities of thereactants and decreasing those of the products.For a given composition of iron melt the activity of the impurityis fixed and hence can not be increased.The oxidising potential of an oxidising agent can be increased.
  11. 11. The oxidising potential of an oxidising agent can be increased by usingatmospheric air (ao = 0·21) in place of iron oxide in slag phase and pureoxygen (ao = 1) in place of air. But once the nature of the oxidizing agent ischosen it cannot be increased.The activity of the product can however be decreased by combining it withoxide of opposite chemical character, i.e. an acid oxide product is mixedwith basic oxide and vice-versa.As far as the physical requirement of the oxide product is concerned itshould be readily separable from the iron melt.This is achieved by keeping the slag and the metal both as thin liquids sothat the metal being heavier settles down and the slag floats on top in theform of two immiscible liquids which can be separated readily.
  12. 12.  If the oxide products of iron refining reactions are examined, silicon and phosphorus form acid oxides and hence a basic flux is needed to form a suitable slag for their effective removal. The higher the proportion of base available the lesser will be the danger of back­ward reaction. For manganese elimination, since manganese oxide is basic, an acid flux will be required. The nature of the process itself has made the task little simpler. During refining, being the largest bulk, iron itself gets oxidised to some extent as (FeO) which is basic in nature. It is possible to adjust the contents of silicon and manga­nese in pig iron such that the amounts of (FeO + MnO) formed during refining would be able to form a slag essentially of the type FeO­MnO­Si02 and fix up silica in it.
  13. 13.  In such a slag P20S is not stable because (FeO + MnO) together are not strong enough bases to fix it up in slag. In order to oxidise phosphorus in preference to iron, a strong external base like CaO and/or MgO is needed in sufficient proportion to form a basic slag to hold P20S without any danger of its reversion. Phosphorus is best eliminated by a slag of the type CaO­FeO­ P205 It is quite interesting to note that such a slag is also capable·of removing sulphur from iron melt to a certain extent.
  14. 14. The steelmaking processes can now be divided into two broad categories :(i) when silicon is the chief impurity to be eliminated from iron and that phosphorus and sulphur need not be eliminated and,(ii) when phosphorus and, to some extent, ,sulphur are the chief impurities to be eliminated along with even silicon. The elimination of manganese will take place under both the categories. In the finished steel, except a few exceptions, phosphorus and sulphur each must be below 0·05%. If phosphorus is above this limit, steel becomes cold­short and if sulphur is more it becomes hot­short. Higher sulphur contents are recommended for free­cutting variety of steels and a slightly high phosphorus level is desirable for efficient pack rolling of steel sheets.
  15. 15.  If the pig iron composition is such that phosphorus and sulphur both are below 0·05% and, therefore, need not be eliminated it is possible to remove silicon along with manganese in such a way that slag of the type MnO­FeO­Si02 is formed without the necessity of addition of an external flux. Such a process of steelmaking is called acid steelmaking process which is carried out in an acid brick lined furnace. On the other hand, to eliminate phosphorus and sulphur, the reverse reaction rate can only be suppressed if the slag contains a good amount of stronger base than as is internally available in the form of FeO and MnO. External CaO (and also MgO) is used as a flux and slag of the type CaO­FeO­P205 is made. Such a process is called basic steelmaking process. The furnace lining in this case has to be basic in nature.
  16. 16.  In brief the composition of pig iron is the only factor that determines the acid or the basic character of the process to be adopted for steelmaking. In an acid process slag is acidic and the furnace lining has to be acidic to withstand the slag. Similarly in a basic process the slag contains excess basic oxide and the furnace lining should be basic in nature. If the lining is of opposite chemical charac­ter slag will readily react with the lining and cause damage to the furnace. Besides the acid or the basic nature, the slag needs to possess many other physical and chemical properties to carry out refining efficiently.
  17. 17. By Dr. Smarajit Sarkar Associate ProfessorDept. of Metallurgical and Materials Engg. National institute of Technology, Rourkela
  18. 18.  Introduction to metallurgical slag Structure of pure oxide ◦ Role of ionic radii ◦ Metal – oxygen bond Structure of slag Properties of slag ◦ Basicity ◦ Oxidising power ◦ Sulphide capacity ◦ Electrical and thermal conductivity ◦ Viscosity ◦ Surface tension Constitution of slag
  19. 19.  The slag comprising of simple and/or complex compounds consists of solutions of oxides from gangue minerals, sulphides from the charge or fuel and in some cases halides added as flux. Slag cover protects the metal and from oxidation and prevents heat losses due to its poor thermal conductivity. It protects the melt from contamination from the furnace atmosphere and from the combustion products of the fuel In primary extraction, slags accept gangue and unreduced oxides, whereas in refining they act as reservoir of chemical reactant(s) and absorber of extracted impurities.
  20. 20.  In order to achieve these objectives, slag must possess certain optimum level of physical properties: ◦ Low melting point, ◦ Low viscosity, ◦ Low surface tension, ◦ High diffusivity and chemical Properties: ◦ Basicity, ◦ Oxidation potential and ◦ Thermodynamic properties The required properties of slags are controlled by the composition and structure.
  21. 21.  There are two principal types of bonds found in crystals: electrovalent and covalent. Electrovalent bond strength is lower than the covalent bond. High temperature is required to destroy the covalent bond. However, oxides exhibit varying proportion of both ionic and covalent bonding in slag. Ionic bond fraction indicates the tendency to dissociate in liquid state.
  22. 22. Relative dimensions of cations and anions and type of bondsbetween them are important factors in controlling the structure ofpure oxides
  23. 23.  TiO2, SiO2 and P2O5, bonding is mainly covalent and the electrovalent proportion is strong due to small cations carrying higher charge with a coordination number of 4. These simple ions combine to form complex anions such as SiO4-4 and PO3-4 leading to the formation of stable hexagonal network in slag systems. Hence they are classified as ‘network formers’ or “acidic oxides”. For example SiO2 + 2O2- = SiO4-4 P2O5 + 3O2- = 2(PO3-4)
  24. 24.  The oxides with high ionic fraction form simple ions on heating beyond the melting point or when incorporated into a liquid silicate slag. For example : CaO→Ca2+ + O2- Na2O → 2Na+ + O2- As they destroy the hexagonal network of silica by breaking the bond they are called ‘network breakers’or‘basic oxides.
  25. 25. Oxide z/(Rc+Ra) Ionic Coordination Nature of the fraction number Oxide of bond Solid- -LiquidNa2O 0.18 0.65 6 6 to 8BaO 0.27 0.65 8 8 to 12SrO 0.32 0.61 8 Network breakersCaO 0.35 0.61 6 orMnO 0.42 0.47 6 6 to 8 Basic oxidesFeO 0.44 0.38 6 6 Oxides like Fe2O3, Cr2O3 andZnO 0.44 0.44 6 Al2O3 are known to beMgo 0.48 0.54 6 amphoteric due to their dualBeO 0.69 0.44 4 characteristics because they…………. ……………... ……………... …… ……... …………………... behave like acids in basic slagCr2O3 0.72 0.41 4 and as bases in acidic slag.Fe2O3 0.75 0.36 4 Amphoteric oxidesAl2O3 0.83 0.44 6 4 to 6…………. ……………... …………….. …….. ………. …………………...TiO2 0.93 0.41 4 Network formersSiO2 1.22 0.36 4 4 orP2O5 1.66 0.28 4 4 Acidic oxides
  26. 26.  It is well known that most of the slags are silicates. When a basic oxide is incorporated in to the hexagonal network of silica it forms two simple ions. The fraction of basic oxide, expressed as O/Si ratio plays an important role in destroying the number of Si-O joints. O/Si Formula Structure 2/1 Si O2 Silica tetrahedra form a perfect three dimensional hexagonal network 5/2 MO.2 One vertex joint in each tetrahedron SiO2 breaks to produce two-dimensional lamellar structure. 3/1 MO. Si O2 Two vertex joints in each tetrahedron break to produce a fibrous structure 7/2 3MO. Three vertex joints in each tetrahedron 2SiO2 break 4/1 2MO.SiO2 All the four joints break
  27. 27. A knowledge of various chemical and physical properties of slag is essential in order to adjust them according to the need of extraction and refining processes. 1. Basicity of Slags In slag systems, a basic oxide generates O2- anion while an acidic oxide forms a complex by accepting one or more O2 anions: Base ↔ acid + O2-
  28. 28.  For example, SiO2, P2O5, CO2, SO3 etc are acidic oxides because they accept O2- anions as per the reaction: (SiO2) + 2 (O2-) = SiO44- On the other hand basic oxides like CaO, Na2O, MnO etc. generate O2- anions: (CaO) ↔ Ca2+ +O2- The amphoteric oxides like Al2O3, Cr2O3 Fe2O3 behave as bases in the presence of acid (s) or as acids in presence of base (s): (Al2O3) + (O2-) = 2 (Al O2-) or (Al2 O4 2- ) (Al2O3) = 2(Al3+) + 3(O2-)
  29. 29.  In a binary slag viz. CaO-SiO2 the basicity index (I) is given as: I = wt % CaO / wt % SiO2 For example a complex slag consisting of CaO, MgO, SiO2 and P2O5 employed in dephosphorisation of steel, basicity index 2 is estimated as %CaO + 2 3 wt%MgO wt follows: I= wt%SiO 2 + wt%P2 O 5
  30. 30.  Oxidizing power means the ability of the slag to take part in smooth transfer of oxygen from and to the metallic bath. The oxidizing power of the slag depends on the activity of the iron oxide present in the slag. The equilibrium between iron oxide in slag and oxygen dissolved in metal is represented as: (FeO) = [ Fe ] + [ O ] K= [ a ][ a ] Fe O Thus [ a O ] ∝ ( a FeO ) (a ) FeO
  31. 31.  Since slags are employed to remove sulphur from metal, chemistry of sulphur in silicate slags becomes interesting. Sulphide is soluble in silicate melts but elemental sulphur does not dissolve to any appreciable extent. 1 1 S 2 ( g ) + (O 2 − ) = O2 ( g ) + ( S 2 − ) (18) 2 2 (a )  p S 2−   1 2 x S 2− .γ S 2−  p O2    1 2 (a )  p O2 K= = (19)  x  pS  O 2− H2  O 2−  2 
  32. 32.  The sulphur affinity of a slag, presented as molar sulphide capacity is defined by the equation: 1  pO  2  x 2−  ′ = x 2−  2 CS  = K O  (20) S  pS   γ 2−   2   S  or a more useful term wt % sulphide capacity5 for technologist is defined as 1  p O2  2 C S = (wt% S)   (21)  pS   2  Thus under similar conditions a slag with a high Cs will definitely hold sulphur more strongly than the other with a low Cs and hence will prove to be a better desulphuriser in a metallurgical process.
  33. 33.  Molten silica is a poor electrical conductor3. However its conductivity increases to a great extent by addition of basic oxides e.g. CaO, FeO or MnO as flux. This increase is due to the formation of ions. The conductivity values serve as a measure of degree of ionization of the slag. The electrical conductivity of slags depends on the number of ions present and the viscosity of liquid slag in which they are present. Thus conductivity will be greater in liquid state and further increases with the temperature. In general thermal conductivity of slag is very low but heat losses are much higher due to convection.
  34. 34.  Viscosity of slags are controlled by composition and temperature. The viscosity , of a slag of a given composition decreases exponentially with increase of temperature according to the Arrhenius equation: η = A exp (E η/ RT) Basic oxides or halides with large ionic bond fraction are more effective in reducing viscosity than those with smaller bond fraction by breaking bonds between the silica tetrahedra.
  35. 35. Effect of addition of flux on activationenergy
  36. 36.  Viscosity decreases rapidly with temperature for both basic as well as acid slags. But basic slags with higher melting points are more sensitive to temperature. This indicates that activation energy for viscous flow of basic slags is much lower than for acid slags.
  37. 37.  Use of CaF2 as flux is more effective in reducing viscosity of basic slags than that of acidic slags. This may be due to ability of F- ions to break the hexagonal network of silica and the low melting point of undissociated CaF 2.
  38. 38.  Figure shows that addition of Al2O3 to a basic slag increases viscosity by acting as network former. Addition of Al2O3 to an acidic slag reduces viscosity because it now acts as network breaker.
  39. 39.  The high rates of reaction in basic oxygen converters is due to the physical conditions of the metal, slag and gaseous phases in the converter. The theories regarding rapid reaction rates rely heavily on the formation of slag – metal emulsion and slag foams leading to creation of the large required reaction surface. The most important feature of emulsion and foam is the considerable increase of the interfacial area between the two phases leading to the high rate of reaction.
  40. 40.  As surface tension is the work required to create unit area of the new surface, the necessary energy for emulsifying a liquid or a gas in another liquid increases with increasing surface tension value. In a similar manner energy is liberated when interfacial area decreases. Hence a low interfacial tension favors both formation and retention of emulsion.
  41. 41.  On this basis slag / metal and slag /gas systems are not suitable for emulsification because of the high equilibrium slag/metal interfacial tension. However the slag/metal interfacial tension is considerably lowered to 1/100 of the equilibrium value due to mass transfer. Addition of SiO2 or P2O5 to a basic oxide lowers3 the surface tension due to the absorption of a thin layer of anions, viz. SiO44- , PO43- on the surface. It has been reported that lowering of surface tension of FeO by excess oxygen.
  42. 42.  The major constituents of iron blast furnace slags can be represented by a ternary system: SiO2 – CaO – Al2O3. On the other hand all the steelmaking and many nonferrous slags are represented by the ternary system: SiO2- CaO – FeO.
  43. 43. 1.Basic open hearth steel furnace2.Acid open hearth steel furnace3.Basic oxygen converter4.Copper reverberatory5.Copper oxide blast furnace6.Lead blast furnace7.Tin smelting
  44. 44.  As surface tension is the work required to create unit area of the new surface, the necessary energy for emulsifying a liquid or a gas in another liquid increases with increasing surface tension value. In a similar manner energy is liberated when interfacial area decreases. Hence a low interfacial tension favors both formation and retention of emulsion.
  45. 45.  On this basis slag / metal and slag /gas systems are not suitable for emulsification because of the high equilibrium slag/metal interfacial tension. However the slag/metal interfacial tension is considerably lowered to 1/100 of the equilibrium value due to mass transfer. Addition of SiO2 or P2O5 to a basic oxide lowers3 the surface tension due to the absorption of a thin layer of anions, viz. SiO44- , PO43- on the surface. It has been reported that lowering of surface tension of FeO by excess oxygen.
  46. 46.  The major constituents of iron blast furnace slags can be represented by a ternary system: SiO2 – CaO – Al2O3. On the other hand all the steelmaking and many nonferrous slags are represented by the ternary system: SiO2- CaO – FeO.
  47. 47. 1.Basic open hearth steel furnace2.Acid open hearth steel furnace3.Basic oxygen converter4.Copper reverberatory5.Copper oxide blast furnace6.Lead blast furnace7.Tin smelting
  48. 48.  Introduction Changing Pattern of Steel Making Modern steel making – BOF / LD steel making Silicon Reaction Manganese reaction Phosphorous Reaction Carbon Reaction Vacuum Degassing
  49. 49.  Steelmaking is conversion of pig iron containing about 10 wt weight of carbon , silicon, manganese, phosphorus, sulphur etc to steel with a controlled amount of impurities to the extent of about 1 weight percent. With the exception of sulphur removal of all other impurities is favored under oxidizing conditions. In the case of oxidation of carbon the product, being a gas, passes off into the atmosphere but rest of the oxide products shall remain in contact with the iron melt in the form of a slag phase.
  50. 50.  SiO2, MnO and P2O5 formed by oxidation of Si, Mn and P, respectively will join the slag phase. The formation of these oxides can be facilitated by decreasing their activities which is possible by providing oxides of opposite chemical character serving as flux. As SiO2 and P2O5 are acid oxides a basic flux is required for formation and easy removal of the slag. A strong basic slag is formed by addition of CaO and / or MgO to absorb P2O5 and SiO2. The removal of carbon will take place in the form of gaseous products (CO).
  51. 51.  During refining, controlled oxidation of the impurities in hot metal (with the exception of sulphur) takes place once oxygen is blown at supersonic speeds onto the liquid bath. The interaction of the oxygen jet(s) with the bath produces crater(s) on the surface, from the outer lip(s) of which, a large number of tiny metal droplets get splashed. These droplets reside for a short time in the slag above the bath. Therefore, the existence of a metal-slag-gas emulsion within the vessel, virtually during the entire blowing/refining period is an integral part of BOF steelmaking.
  52. 52.  This is the reason why the slag-metal reactions like dephosphorisation and gas--metal reactions like decarburisation proceed so rapidly in the BOF process The droplets ultimately return to the metal bath. The extent of emulsification varies at different stages of the blowing period, as depicted schematically . A minimum amount of slag, with the desired characteristics, is necessary for ensuring that the emulsion is stable, i.e. the slag should not be too viscous, or too watery. Only in this way can the kinetics of the removal of the impurities be enhanced.
  53. 53.  For encouraging quick formation of the appropriate type of slag, lime/dolomite/other fluxing agents with adequate reactivity are added right from the beginning of the blow. The reactivity of the fluxing agents, primarily lime (consumption 60-100 kg/tls), determines how quickly slag is formed (typically within 4-5 minutes after the commencement of the blow). The rate at which oxygen is blown through the lance, the number of openings (holes) on the lance tip, the distance between the lance tip and the bath surface (lance height), the characteristics of the oxygen jets as they impinge on the bath surface, the volume, basicity and fluidity of the slag, the temperature conditions in the bath and many other operational variables influence the refining.
  54. 54. There are two distinct zones of refining in a LD vessel viz. thereactions in the emulsion and in the bulk phase. Thecontribution of bulk refining, i.e. refining in impact zone and atthe bulk slag-metal interface, is dominant in the beginningsince emulsion is yet to form properly. It has also beenbelieved that substantial decarburisation of droplets can occurbecause of its free exposure to an oxidising gas, particularlyin the beginning. As the emulsion builds up the emulsionrefining attains a dominant role. The bulk phase refiningdominates again towards the end when the emulsioncollapses.
  55. 55.  Conditions for dephosphorisation are that the slag should be basic, thin and oxidising and, that the temperature should be low. Dephosphorisation, therefore, does not take place efficiently until such a slag is formed. Such a slag is formed in LD process only after the initial 4-6 minutes of blowing. The rate of dephosphorisation picks up concurrently with the rate of decarburisation. For efficient decarburisation as well as dephosphorisation the slag should, therefore, form as early as possible in the process. If a preformed slag is present as in a double slag practice wherein the second, slag is retained in the vessel in part or full, the decarburisation rate curve rises more steeply in the beginning
  56. 56. Dephosphorisation is very rapid in the emulsion because of the increased interfacial area and efficient mass transport. Phosphorus should, therefore, be fully eliminated before the emulsion collapses. If this is not achieved the heat will have to be kept waiting for dephosphorisation to take place and, in the bulk phase, it is extremely slow as compared to that in the emulsion. In general dephosphorisation should be over by the time carbon is down to 0·7-1·0%, i.e. well ahead of the collapse of emulsion which begins at around 0·3%C.
  57. 57.  The relative rates of dephosphorisation and decarburisation can be controlled by adjusting the lance height or by adjusting the flow rate of oxygen. Raising the height of the lance or decreasing the oxygen pressure decreases the gas-metal reactions in the emulsion (i.e. decarburisation) and vice versa. The dephosphorisation reaction is thus relatively increased by the above change and vice versa. Towards the end when temperature is high the danger of phosphorus reversion does exist but it can be prevented by maintaining a high basicity of the slag.
  58. 58. The process of decarburization includes at least three stages: supply of reagents - carbon and oxygen - to the reaction site; the reaction [c] + [0] proper; and evolution of reaction products - CO bubbles into the gaseous phase .. The apparent activation energy of the reaction [C] + [0] = CO is relatively small (according to various researchers, E = 80000- 120000 J/mol), which suggests that the reaction occurs practically instantaneously. The solubility of CO in molten steel is also negligible. Accordingly, the process can be limited by either the first or the third stage.
  59. 59.  The nature of kinetic curves of carbon burning-off at its various concentrations is different: on attaining a certain critical’ level of concentration of carbon (0.15-0.35%), the rate of carbon oxidation is observed to drop noticeably. It has also been established in experiments that the critical carbon concentration is determined by the intensity of supply of oxidant to the bath (it increases with increasing intensity of oxygen supply and decreases during bath boil or metal stirring).
  60. 60.  Thus, at carbon concentrations above the ‘critical value’, the intensity of decarburisation reaction is determined by the supply of the oxidant and at those below the critical value, by carbon diffusion to the reaction place . This means practically that, if the carbon content of the metal is sufficiently high, the rate of carbon oxidation will be higher at a higher intensity of oxygen supply. At low concentrations of carbon, however, a higher level of intensity of oxygen supply will not produce the desired effect and the bath should be agitated forcedly (in order to intensify the supply of carbon to the reaction place) so as to increase the rate of carbon oxidation.
  61. 61.  The rate of decarburization can also be limited by the third stage, the evolution of CO. For a bubble of CO to form in metal, It must overcome the pressure of the column of metal (pm), slag (psl), and of the atmosphere (pat) above the bubble and also the forces of the cohesion with the liquid, 2σ/r i.e. pCOev ≥ pm + psl + pat + 2σ/r The value of 2σ/r becomes practically sensible at low values of bubble radius: at r > I mm it can be neglected. Formation of bubbles in the bulk of liquid metal is practically impossible.. They can only form on interfaces between. phases, such as slag - metal, non-metallic inclusion - metal, gas bubble - metal or lining - metal. The most favorable conditions for the nucleation of CO bubbles exist on boundaries between the metal and refractory lining which has a rough surface and is poorly wettable by the metal
  62. 62. Slag evolutionDuring Blow
  63. 63.  High silicon pig iron is required in the acid steelmaking processes to make relatively acid slag to ensure longer life of the refractory lining. Oxidation of silicon also generates sufficient heat required in case of the Bessemer process. However basic steelmaking processes need low silicon iron because the entire amount of acid silica due to the oxidation of silicon has to be neutralized by lime to produce slag with basicity (CaO / SiO2 ratio) between 2 and 4 needed for effective desulphurisation and dephosphorisation.
  64. 64.  Due to the strong attraction between iron and silicon, the Fe-Si system exhibits large negative deviation from the Raoults low. The activity coefficient of silicon in iron in presence of other elements is given by : log fSi = 0.18×%C + 0.11×% Si + 0.058×% Al -0.058 × %S + 0.025 × % V + 0.014 × % Cu + 0.005 × % Ni + 0.002 × % Mn – 0.0023 × % Co – 0. 23 × %O Oxidation of silicon is an exothermic reaction and provides some of the heat necessary for rise of temperature of the bath during blowing.
  65. 65.  Si –O reaction is governed by ∆G0 vs T equation: [Si ]+ 2 [O] = (SiO2 ), Go = -14200 + 55.0 T cals. The activity coefficient of oxygen decreases and that of silicon increases with increasing silicon content in iron. Silica is a very stable oxide, hence once silicon is oxidised to SiO2 the danger of its reversion does not arise.
  66. 66. a SiO 2 a SiO 2K= = ( 20 ) [ a Si ][ aO ] 2 [ f Si .% Si ][ f O .% O ] 2 a SiO 2  a SiO 2 ∴ [ % Si ][ % O ] = 2 = 2.8 × 10 −5   ( 21) f Si f O2 . K  f Si f O2 
  67. 67.  The extremely low activity of silica in basic steelmaking slag poses no danger of preferential reduction of silica like that of phosphorus removal. In basic steelmaking process the silicon content of pig iron should be kept as low as possible to decrease the lime consumption with the prime objective of controlling the required basicity for phosphorus removal at a minimum slag volume. In case of high silicon entering the basic steelmaking furnace double slag practice has to be adopted. Alternatively, external desiliconisation of the hot metal has to be done outside the blast furnace before charging it in a basic steelmaking furnace.
  68. 68.  About 50 to 75% of the manganese in the burden gets reduced along with the pig iron resulting its manganese content between 0.5 to 2.5%. During steelmaking major amount of manganese is lost into the slag and very little is utilized to meet the specifications. Some manganese is required to control the deleterious effects of sulphur and oxygen and also for improvement of mechanical properties of the steel.
  69. 69.  Hence conditions for maximum recovery of manganese can be derived by considering the equilibria: (Fe2+) + [Mn] = (Mn2+) +[Fe] (FeO) +[ Mn] = (MnO) + [Fe] ( a Mn 2 + ) [ a Fe ] ( χ Mn 2 + ) f Fe [ % Fe ] K= = ( 23) ( a Fe 2 + ) [ a Mn ] ( χ Fe 2 + ) f Mn [ % Mn ] ( χ Mn 2 + ) [ % Fe ] or K ′ = ( 24 ) ( χ Fe 2 + ) [ % Mn ] At equilibrium the Mn slag-metal distribution relation is( χ given by( χ Fe 2 + ) ) Mn 2 + = K′ ( 25)[ % Mn ] [ % Fe ]
  70. 70.  From the equation it is apparent that the conditions for the highest possible recovery of Mn i.e. minimum slag- metal distribution ratio are  i) min (χFe2+), requiring a low FeO content in the slag.  ii) min K’ requires a low SiO2 content and a high temperature as evident from the relation showing effect various anions in the slag.log K ′ = 3.1 χ SiO 4 − + 2.5 χ PO 3 − + 2.4 χ O 2 − + 1.5 χ F − ( 26 ) 4 4
  71. 71. From the figure it is evidentthat for slags containingabout 20% MnO, amaximum of 0.1% Mn isfound in metal. The slag containing 50%SiO 2 (the rest being FeOand MnO), with increasingMn content of the metal the(MnO) content of the slagincreases whereas theoxygen content of the metaldecreases and siliconcontent increases.
  72. 72.  Despite its very low boiling point significant amount of P gets dissolved in pig iron due to strong attraction for iron. Making use of the interaction coefficients for the effect of various elements on the activity coefficient of phosphorus in iron, the activity of P can be estimated by the expression: logfP = 0.13×%C + 0.13×%O + 0.12×%Si + 0.062× %P + 0.024×%Cu + 0.028×%S + 0.006×%Mn – 0.0002×%Ni – 0.03×%Cr
  73. 73.  A very close stability of FeO, Fe2O3 and P2O5 is evident from the iron and phosphorus lines in the Ellingham diagram. Hence practically all the phosphorus present in the ore gets reduced along with iron in the blast furnace and joins pig iron. During steelmaking the activity of P2O5 in the slag of basicity 2.4 is reduced drastically to 10-15-10-20. Activity of P2O5 in steelmakig slag is very low even if it contains 25% P2O5.
  74. 74. •i.e. 2[P] + 5[O] + 3(O2-) = 2 (PO43-) (12) Thus for effective removal of phosphorus basic steelmaking processes have to employ slags of high basicity. The distribution of phosphorus between slag and metal can be dessribed as 2[P] + 5(FeO) + 3 (CaO) = (3 CaO.P2O5) + 5[Fe] i.e. 2[P] + 5[O] + 3(O2-) = 2 (PO43-) 2 a PO 3−K= 4 Applying Temkin rule : (13) a[2P ] .a[5O ] .a (3O 2 − ) χ PO 3− 2 = 4 (14 ) [ f P % P] [ fO % O] 5 χ O 2 3 2−The dephosphorising index, D P which is the ratio of phosphorus in slag to that in metal, is given as ( χ PO 3− ) [ % O]5/ 2 ( χ O 1/ 2∴ DP = 4 = K′ )3 / 2 (15) [ % P] 2−
  75. 75. From the figure it is clear that D Pincreases with increase in the (FeO)content upto 15% due to the highoxidizing power.Beyond this D P decreases due todecrease in the lime proportion.Dephosphorisation is more effectiveat lower temperature because D Pincreases with decrease oftemperature.
  76. 76.  The soda ash is 100 times more effective compared to lime on molar basis but it is avoided in practice due to its severe corrosive action on furnace lining. The magnesia content of a basic steelmaking slag reaches equilibrium with the lining hence not under control and MnO depends on charge and hence not much adjustable. The steel maker has the option of controlling lime, silica and FeO. For charges containing high % P more than one slags are made to dephosphorise metal bath to the desired level. In brief ,high basicity, low temperature, and FeO content around 15% favour dephosphorisation of metal by basic slags.
  77. 77. The optimum conditions for dephosphorisation can bederived from the equation defining the index: ( χ PO 3− ) [ %O]5/ 2 (χO 1/ 2 DP = 4 = K′ )3 / 2 [ % P] 2−1. Basic slag gives a high value of χO2-2. High lime content – lime is the divalent oxide making the largest contribution to K’ (log K = 21NCa++ + 18 NMg++ + 13NMn++ + 12 NFe++3. Ferrous oxide close to 15% .4. Low temperature gives a high value of K‘.
  78. 78.  In refining of steel oxidation of Si, Mn and P takes place at the slag-metal interface. The oxidation of carbon practically does not take place at the slag-metal interface because of the difficulty of nucleation of CO bubbles there. C-O reaction takes place at the gas –metal interface since it eliminates the necessity of nucleating gas bubbles. During refining of steel oxygen has to dissolve first in the bath before it reacts with the dissolved impurities. In the absence of other slag forming constituents at 1600oC liquid iron can dissolve oxygen up to at 0.23 wt. %
  79. 79.  In steel making the reaction between carbon and dissolved oxygen is of utmost importance. Generally pig iron contains about 4 wt% carbon. The solubility of carbon in steel is effected by the presence of impurities and alloying elements. Presence of Nb, V, Cr, Mn and W increase solubility of carbon in iron where as presence of Co, Ni, Sn and Cu decrease it.
  80. 80. •Thus solubility of carbon in steel can be calculated by combining the binary data from the following equation:
  81. 81.  Oxidation of Carbon can be discussed according to the reaction: C + O = CO, ΔG0= -5350 – 9.48T cals. pCO pCO K= = a c aO [ fc%C ][ fo%O ] pCO pCO ∴[%C ][%O ] = = K fc fo K At any chosen pressure of CO, % C vs % O indicates inverse hyperbolic relationship
  82. 82. During oxidation period oxygen is continuously transferred fromthe slag to the bath, where it continuously reacts with carbon togive CO.The main resistance to the oxygen flow is the slag–metal and themetal–gas interfaces, whereas inside the steel bath the transferof dissolved oxygen is very fast.
  83. 83.  The activity coefficient of carbon in iron increases with increasing carbon content and that of oxygen decreases with increasing carbon content. The net result is that the product [% C] [% O] for a given pCO decreases slightly with increasing carbon content as shown in Figure
  84. 84.  Since steel making is a dynamic process, the concentration of carbon and oxygen in the bulk metal phase is not in equilibrium with the prevailing CO- pressure in the bubbles. At the gas bubble–metal interface the reaction is close to equilibrium. The experimentally observed excess oxygen and carbon in the bulk metal phase is thus helpful in transfer of the reactants by diffusion to the gas-metal interface in the violently stirred metal bath.
  85. 85.  As [% O] increases with (aFeO) in slag and [% O] decreases with [% C] in the bath. it follows that the iron oxide contents of the slag increases with decreasing carbon in steel during refining. Hence there is a general trend in the variation of slag composition with the carbon content of the metal. For a given total iron oxide in a slag, a lower carbon in the steel corresponds to a higher sum of (% SiO2 + % P2O5) in the slag.
  86. 86.  Within the range of basic slags, for a given sum of % CaO + % MgO + % MnO the carbon content of steel does not vary much with the FeO content of the slag.
  87. 87.  During steelmaking i.e. refining of pig iron where impurities like carbon, silicon, manganese and phosphorus are eliminated to the required level oxygen, nitrogen and hydrogen get dissolved as harmful impurities. As solubility decreases with decrease of temperature excess gases dissolved in steel are liberated during solidification. The evolution of the gas gives rise to the formation of skin or pin holes, blow holes, pipes etc. The unsoundness caused by these cavities affect the mechanical properties of steel
  88. 88.  Nitrogen pick up during steel making: ◦ open atmosphere ◦ raw material charged ◦ during melting and/or refining Effect of nitrogen in steel: ◦ yield-point phenomena ◦ AlN causes intergranular fracture ◦ nitrogen stabilizes the austenitic structure Factors affecting the nitrogen solubility in steel. ◦ partial pressure of nitrogen in the blast ◦ time of contact ◦ length of after blow and ◦ the bath temperature
  89. 89. [wt.%H] = Since nitrogen dissolves atomically in liquid iron and steel in very small proportion its solubility can be discussed in terms of Sievert’s and Henry’s laws There is slow rise in solubility in solid state with increasing temperature but at the melting point it increases very rapidly. It also rises in liquid steel but at a slow rate. Presence of vanadium, niobium, tantalum, chromium, manganese, molybdenum, and tungsten increases nitrogen solubility in iron whereas it decreases in presence of nickel, cobalt, silicon and carbon
  90. 90.  Hydrogen pick up steel making: ◦ wet solid and rusty charge materials ◦ atmospheric humidity ◦ wet refractory channels, runners and containers Effect of hydrogen in steel ◦ Decreases ductility ◦ Appearance of hairline cracks seriously affect the mechanical properties ◦ Formation of blow holes and pin holes.
  91. 91.  Water vapour coming in contact with steel or slag leads to the formation of hydrogen which gets dissolved in steel melt as per reaction: H2O (g) = 2[H]+ [0] At the melting point of iron solubility in delta iron is approximately 10 mL/ 100g. Beyond this hydrogen will be rejected during solidification to produce unsound porous ingots due to gas evolution.
  92. 92.  Thus partial pressure of hydrogen, and composition of steel and its temperature control the hydrogen content of steel. According to Sievert’s law solubility of hydrogen in pure iron is expressed as: Presence of niobium, tantalum, titanium and nickel increases the solubility of hydrogen in iron whereas it decreases in pressure of carbon, silicon, chromium and cobalt.
  93. 93.  The objectives of vacuum degassing include removal of hydrogen from steel to avoid long annealing treatment, removal of oxygen as carbon monoxide and production of steels with very low carbon content (< 0.03%). The principle is based on the usefulness of the Sievert’s law relationship. The equation demonstrates that subjecting the molten steel to vacuum will decrease the hydrogen, nitrogen as well as the oxygen content of the steel according to the following reasons:
  94. 94. 2[H] = H2 (g) 2[N] = N2 (g) [C] + [O] = CO (g) The effectiveness of vacuum treatment increases with increase in the surface area of liquid steel exposed to vacuum. For this purpose metal is allowed to flow in the form of thin stream or even fall as droplets to accelerate the degassing process.
  95. 95. A number of methods available on commercial scale forvacuum treatment of steel may categorized into three groups :1. Ladle Degassing The teeming ladle filled with steel to one fourth of its height is placed inside a vacuum chamber. the melt is stirred either by bubbling argon or by electromagnetic induction Introduction of gas for stirring provides interface which facilitates degassing. In general pumping is carried out to attain the ultimate vacuum of 1-10 mm Hg. which is supposed to be adequate for degassing.
  96. 96. 2. Stream Degassing In this case molten steel is allowed to flow down under vacuum as a stream from the furnace to ladle to another ladle or a mould. A very high rate of degassing is achieved due to large increase in surface area of molten steel in the form of falling droplets. Thus choice of proper vacuum pump and vacuum chamber is important to achieve the adequate level of degassing.
  97. 97. 3. Circuilation DegassingR-H degassing processThe average rate of circulation is12 tons/min.Twenty minutes are required totreat 100 tons of steel to bringdown 90% reduction ofhydrogen content.
  98. 98. D-H Vessel.The chamber is moved through50-60 cm with a cycle time of20 sec. 10-15% steel isexposed at a time.7-10 cycles are required toexpose the entire steel once.Adequate degassing isobtainedin 20-30 cycles in 15-20minutes.
  99. 99.  High carbon steels like rail steels (0.65%-0.74% C, 0.6%-1.0% Mn, 0.27-0.30% Si), ball-bearing steel (1.0% C, 1.2% Cr), etc. are also manufactured in the LD converter by the catch carbon technique. In this technique, dephosphorization is accelerated and completed before decarburization. Extra lime and fluorspar are charged and the lance is raised to a higher position for maintaining a soft blow condition till phosphorus removal is completed. Thereafter, decarburization is continued by a harder blow till the bath carbon content drops to the desired level.
  100. 100.  Alternatively, blowing may be continued to complete both dephosphorization and decarburization. Required amount of carburizer is then added to the low carbon steel bath to raise the carbon content to the desired level. However, this method involves a risk of increasing the inclusion and nitrogen contents in the steel. These are picked up from the carburizer (e.g., petroleum coke or graphite). For production of low alloy steel, the alloying elements are usually added in the ladle during tapping the steel.
  101. 101.  As will be evident from the discussion [Mn] from the bath is lost in the slag. (MnO) thus formed quickly combines with (SiO 2 to form (2MnO· Si02). Thus, there is a reduction in the Mn content in the bath in the initial period of the blow. As the slag basicity increases due to lime dissolution, (MnO) is gradually released and is reduced by carbon during intensive carbon oxidation according to the following reactions: (MnO) +[C] → [Mn]+{CO} [Mn] content in the bath increases again. As the intensity of the carbon-oxygen reaction decreases towards the end of the blow,. manganese is reoxidized from the bath. As a result, the bath manganese content drops again. This accounts for the characteristic manganese hump in the LD converter reaction diagram.
  102. 102.  A basic and highly reactive slag is necessary to attain desulphurization and dephosphorization in LD steel making at the turndown stage. Hence the physical and chemical characteristics of the lime used are of utmost importance. Some common quality criteria for steel making lime are listed below: Chemical composition Size distribution Reactivity Loss on ignition Moisture content Si02 in the lime reduces the CaO activity due to the formation of larger amount of slag by fixing up about two times its mass of CaO. This is detrimental both from "yield" and "cost" points of view.
  103. 103.  The sulphur content in lime should be as low as possible. An MgO content of approximately 3.5% in lime is thought to be beneficial because an MgO content of around 5% in the slag has been found to hinder the formation of dicalcium silicate, thereby ensuring a faster lime dissolution in the slag. However, lowering of melting point and the viscosity of slag due to increased proportion of MgO can result in early slopping. An adequate level of MgO in slag also ensures less corrosion of the vessel refractories because of its known properties of neutralizing the FeO level of the bath.  
  104. 104.  Formation of slag as early as possible during the blow requires a uniform and rapid dissolution of lime. A size range of +8 to -40 mm, minimum proportion of fines in the lime charge and soft burnt lime promote early slag formation. A soft burnt lime is highly porous, having a large specific area. This results in its favorable reactivity. Thermal dissociation reaction of unreacted CaC03 is endothermic. It adversely affects the heat balance of the converter and leads to operating problems. Similarly, a moisture content in lime directly affects the heat balance of the vessel because of temperature losses during its disintegration. It also acts as a potent source of hydrogen in steel. Hence both loss on ignition (LOI) and moisture content of lime should be low.
  105. 105. The lining of oxygen converters is usually made up of three layers of bricks. First an inner layer of magnesite or burnt dolomite brick is made. Gaps between the brick and the shell are filled with tardolomite ramming mass. The same ramming composition is used for making up the second intermediate layer. The upper working layer is made of magnesia carbon brick. The performance of refractories is generally evaluated by the life of the lining or by the consumption of refractories per ton of steel produced. However, this is greatly influenced by the severity of service conditions that prevail during operation. In brief, these are: Furnace atmosphere, Composition of slag, Mechanical stresses ,Thermal shock, Effect of high temperature, Geometry of the vessels, Operational procedure or the blowing technique Quality of hot metal, Quality of refractories.
  106. 106.  A rapid sequence of blows, without pause, increases the lining life. A high silicon hot metal produces a silica rich slag which increases the wear of basic lining. At high temperature, the corrosive attack of the slag is enhanced. Combustion of the CO generated inside the vessel also raises the temperature in the upper zone of the furnace. This enhances lining wear in the region. The distance of the oxygen lance from the bath has a considerable effect on the refractory wear. Usually, a high position of the lance leads to a reduced wear of the furnace bottom, but it increases wear at the top and upper part of converter. However, with the introduction of the multi-hole lance nozzles, the oxygen is evenly distributed on the bath surface. This has solved the problem of preferential bottom or top lining wear.
  107. 107. The early refractory lining for LD vessel was based generallyon doloma, magnesia or magnesia-chrome of the same qualityas used in the earlier steel making processes, e.g., Bessermer,open hearth, etc. However, the high basicity of the LDconverter slag and the high temperature of the bath promotedrapid wear of the refractories. Modern LD converters are,therefore, lined with magnesia-carbon refractories. The totalFe20s, Si02 and Al20s content in the magnesia refractory shouldbe low-definitely less than 4.0%-to improve its resistance toslag attack. Sea water magnesia is usually added along withnatural magnesia to enrich the MgO content in the brick
  108. 108. Care would be taken to lower the B20s content in seawater magnesia to a level at which it does not affectthe high temperature properties. The presence ofsubmicroscopic carbon particles in magnesiacarbon refractories inhibits penetration of slag intothe refractory.
  109. 109. The capacity of graphite to reduce wear is based upon itslarge wetting angle for oxide melts. The melt canpenetrate the bricks only when the graphite is burntaway. near the hot face owing to diffusion of oxygen inbetween blows during a campaign. Thus, the infiltrationzone progressively advances, resulting in a continuouswear of the lining. The slag resistance of magnesiaparticles is improved by its high bulk density, lowimpurity content and large crystal size of MgO particles.
  110. 110. LD refractory lining life has been greatly enhanced in recentyears by adopting the slag splashing technology. In thistechnology, a portion of the slag is retained in the vessel aftertapping. A low FeO and a high MgO slag is desirable for slagsplashing. Such improvement in slag condition is achievedthrough addition of dolomite lime after tapping. Slag splashingis accomplished by injecting nitrogen into a conditioned slag ata given flow-rate and lance height. The existing oxygen-lancingequipment is used.
  111. 111.  By varying lance height and nitrogen flow-rates, slag can be selectively targeted and blown into particular areas of the furnace. This is schematically illustrated in Figure given below. The process time for slag splashing is between 1 and 4 minutes. A well-designed nitrogen slag splashing programme can extend furnace lining life to 8,000 heats. Once slag splashing is started, it would be done on a regular basis. Slag splashing presents some operating challenges like lance shell
  112. 112. DEOXIDATION METHODS AND PRACTICES By Dr. S.Sarkar Associate Professor Dept. of Metallurgical and Materials Engg. National institute of Technology, Rourkela
  113. 113. PLAN OF PRESENTATION Introduction Deoxidation methods Choice of deoxidisers Removal of deoxidation product Deoxidation equilibria Silicon – manganese deoxidation Complex deoxidisers Deoxidation practices
  114. 114. INTRODUCTION Contrary to iron making steelmaking is practiced in oxidizing conditions. In all the steelmaking processes either air or oxygen is blown or surplus air/oxygen is provided to facilitate quick oxidation of impurities. Under these conditions oxygen easily gets dissolved in the steel melt. During solidification of steel castings excess oxygen is evolved because of very low solid solubility and is one of causes of defective casting. This excess oxygen has to be eliminated for production of sound casting. The process of removal of residual oxygen of the refined steel called deoxidation
  115. 115. CONT…
  116. 116. DEOXIDATION METHODS1. Diffusion deoxidation When dissolved oxygen is lowered down by diffusion of oxygen from the steel melt to the slag in the steelmaking furnace, the method is called Diffusion deoxidation. This can also be done outside the furnace under vacuum according to the reaction: 2[O] → O2 (g) But the method can be used effectively to a limited extent.
  117. 117. CONT…
  118. 118. DEOXIDATION METHODS2. Precipitation deoxidation The residual oxygen is allowed to react with elements having higher affinity for oxygen (compared to what iron has for oxygen) to form oxide products. The product being lighter than steel rises to the top surface and can be easily removed. Precipitation deoxidation is practiced extensively because it is very effective in decreasing oxygen content of steel.
  119. 119. PRECIPITAION DEOXIDATION -CHOICES OF DEOXIDISER Thermodynamically best deoxidinsing element (deoxidiser) should have the least amount of dissolved oxygen [O] left in equilibrium with its own lowest concentration in the steel melt. Al and Si are very effective in deoxidation of steel and hence they are used extensively. Al, Si and Mn are reasonably cheap and hence used as common deoxidizers.
  120. 120. CHOICE OF DEOXIDISER Some times Zr, Ti, V, Nb etc. are used in deoxidation of steel but they are costlier than common deoxidisers. The residual content of the deoxidiser in steel after deoxodation should not adversely affect the mechanical properties of steel. The rate of deoxidation i.e. formation of oxide products must be fast. Since kinetic data on deoxidation are very limited thermodynamic consideration play major role in selection of deoxidisers and estimation of residual content of the deoxidisers in steel at the end of deoxidation.
  121. 121. REMOVAL OF DEOXIDATIONPRODUCTS The mechanically entrapped oxide products in steel are called nonmetallic inclusions which deteriorate the mechanical properties. Size, shape, distribution and chemical composition of inclusions make effective contribution in controlling the properties of steel. This makes it essential to remove the deoxidation products from the steel melt to get clean steel. Thus from cleanliness point of view a gaseous product of deoxidation would be most appropriate.
  122. 122. REMOVAL OF DEOXIDATIONPRODUCTS Only carbon produces gaseous deoxidation product under reduced pressure according to the reaction: [ C ] + [ O ] = CO ( g ) Though the reaction is favoured under reduced pressure but economics do not permit for vacuum treatment. Hence carbon cannot be used as a deoxidiser for production of clean steel. Deoxidisers other than carbon form liquid or solid products.
  123. 123. REMOVAL OF DEOXIDATIONPRODUCTS Formation of a solid deoxidation product will give rise to a new phase which will grow during the course of deoxidation and has to rise to surface of the melt for elimination. Otherwise it will disperse in the melt and on solification may be entrapped in steel as nonmetallic inclusions. For nucleation and growth of deoxidation products required interface may be provided by inhomogenities, for example formation of Al2O3/steel interface while deoxidising steel with aluminium at the beginning.
  124. 124. REMOVAL OF DEOXIDATIONPRODUCTS of the decoxidation product (v) in a quiet The rate of rise bath may be estimated from Stoke’s law: Where g, r, ρliq, ρdp and η stand respectively for acceleration due to gravity, radius of the deoxidation product, densities of the liquid metal and the deoxidation product and the viscosity of the liquid metal. that r2 factor plays an important role in controlling the time required for the particles to rise to the surface of the metallic bath.
  125. 125. REMOVAL OF DEOXIDATIONPRODUCTS On the basis of Stoke’s law it can be demonstrated that particles of deoxidation product less than 0.001cm radius will not move to the surface of the metallic bath in a usual ladle within the normal holding time of 20 minutes, whereas larger particles ( radius greater than 0.01cm) should be completely eliminated. These figures emphasise the significance of coalescence of deoxidation products in formation of particles of larger radius to facilitate rapid rise to the surface of the steel melt
  126. 126. REMOVAL OF DEOXIDATIONPRODUCTS Since coalescence of the deoxidation product is more likely in liquid state, deoxidation is often carried out to obtain liquid products. The rate of removal is also affected by the interfacial energy between the liquid metal and the deoxidation product. High interfacial energy will enhance the rate of removal of the product by lowering the dragging affect.
  127. 127. REMOVAL OF DEOXIDATIONPRODUCTS The rate of rise of the decoxidation product (v) in a quiet bath may be estimated from Stoke’s law: Where g, r, ρliq, ρdp and η stand respectively for acceleration due to gravity, radius of the deoxidation product, densities of the liquid metal and the deoxidation product and the viscosity of the liquid metal. that r2 factor plays an important role in controlling the time required for the particles to rise to the surface of the metallic bath.
  128. 128. for which the equilibrium constant is given as :DEOXIDATION EQUILIBRIA A generalised form of chemical equilibrium dealing with the deoxidation product in contact with steel melt may be represented as: x [M] + y [O] = MxOy (s, l ) By and large all the solid deoxidation products except Fe(Mn)O have stoichiometric compositions. Since we are dealing with infinitely dilute solutions of deoxidisers in the melt according to Henry’s law we can write
  129. 129. DEOXIDATION EQUILIBRIA The activity coefficient of oxygen decreases and that of alloying element increase, with increases in concentration of the alloying element. However the minimum oxygen content decreases with the increasing stability of the deoxidation product.
  130. 130. SILICON MANGANESEDEOXIDATION widely carried out by common Deoxidation is most deoxidisers like silicon and manganese. The deoxidation with manganese giving rise to the formation of liquid or solid solution of FeO and MnO may be represented as: [Mn] + (FeO) ( s, l ) = [Fe] ( l ) + (MnO) ( s, l ) Deoxidation by silica is given by [Si] + 2 [O] = (SiO2) Deoxidation with silicon is much more effective as compared to manganese but simultaneous deoxidation by both the elements leaves much lower residual oxygen in the melt due to reduced activity of SiO2 in FeO – MnO – SiO2 slag.
  131. 131. SILICON MANGANESEDEOXIDATION Assuming that the deoxidation product is pure manganese silicate and the sum of the deoxidation reactions by silicon and manganese are represented as: [Si] + 2 (MnO) = 2 [Mn] + ( SiO2 ) The figure highlights the role of manganese in boosting5 the deoxidising power of silicon with increasing silicon content. For example at 0.05% Si in solution, the residual oxygen is lowered from 0.023% to 0.016% when the manganese content is increased from zero to 0.8% ; while at 0.2% Si, a similar increase in manganese lowers the residual oxygen from 0.0104% to 0.0094%”6.
  132. 132. SILICON MANGANESEDEOXIDATION Simultanious deoxidation by silicon and manganese at 1600oC.
  133. 133. SILICON MANGANESEDEOXIDATION Residual oxygen and silicon contents of iron after deoxidation of 0.10 % oxygen steel at 1650oC at various residual manganese contents from 0.2 to 0.6 % Mn.
  134. 134. SILICON MANGANESEDEOXIDATION From the figure it is evident that at all temperatures for the metal compositions lying above the curve, manganese does not take part in deoxidation reaction and solid silica is formed. On the other hand metal composition lying below the curve the deoxidation product is liquid manganese silicate whose composition is controlled by the ratio [% Si]/[% Mn]2 in the metal. From the above discussion it is clear that silicon alone is a very effective deoxidiser but it produces solid product which poses problems in separation from the steel melt.
  135. 135. SILICON MANGANESEDEOXIDATION Though manganese is not effective it produces liquid deoxidation product. Both silicon and manganese used together give better result.
  136. 136. SILICON MANGANESEDEOXIDATION Deoxidation first carried out by addition of ferromanganese in steel melt produces FeO –MnO liquid slag which dissolves SiO2 when ferrosilicon deoxidises the melt in second step. In the resulting slag FeO – MnO – SiO2 the activities of SiO2 and MnO are much lower than when Fe–Mn and Fe–Si are used separately for deoxidation. Lowering of activity improves their effectiveness in reducing the residual oxygen in steel when Mn and Si are added in correct proportion.
  137. 137. SILICON MANGANESEDEOXIDATION (Mn/Si) is normally maintained In practice the ratio between 7 and 4 to obtain a thin liquid slag as the deoxidation product. At 16000C the equilibrium oxygen level is approximately 0.1% with 0.5% Mn but addition of 0.1% Si reduces residual oxygen to 0.015%.
  138. 138. OTHER DEOXIDISERS deoxidiser as it has more Aluminum is even more effective affinity for oxygen compared to silicon and manganese. But it cannot be used alone to deoxidise steel completely because the deoxidation product, Al2O3 is solid at the steelmaking temperature. While using along with manganese and silicon alumina will dissolve in the liquid slag product of deoxidation. Boron, titanium and zirconium are also very effective deoxidisers. The extent of deoxidation achieved by 8% Si can be easily obtained by 0.7% B, or 0.1% Ti or 0.002% Al or 0.00003% Zr.
  139. 139. COMPLEX DEOXIDISERS The rare earth elements or alloys based on them are employed in conjunction with common deoxidisers for bringing down sulphur and oxygen to a low desired level. A commercial rare earth mixture, known as “REM” containing 48-50% Ce, 32-34% La, 13-14% Nd, 4-5% Ps, and 0.6-1.6% higher lanthanides has been reported. For achieving low residual oxygen in steel the complex deoxidisers must exhibit  low vapour pressure  Liquid deoxidation products
  140. 140. COMPLEXin steel calcium silicide reacts with oxygen Dissolution DEOXIDISERS to form molten calcium silicate slag which can flux alumina inclusion. Possessing similar characteristics an alloy of Ca, Si, Al and Ba is a good deoxidiser to produce clean steel. Occasionally the deoxidation products are beneficial if they remain entrapped in a very finely dispersed form. For example, very fine dispersion of Al2O3 particles without coagulation provides the possible nucleation sites during solidification of steel resulting in a very fine grain structure of steel.
  141. 141. DEOXIDATION PRACTICE On industrial scale there are three methods of deoxidation. After refining, molten steel can be deoxidized either inside the furnace, called furnace deoxidation or during tapping in a ladle, called ladle deoxidation. For production of fine grained steel or in case of inadequate deoxidation a small portron of total deoxidation may be done in the ingot moulds.
  142. 142. DEOXIDATION PRACTICE As deoxidation lowers the oxidizing potential of the bath there is a fair chance of reversion of the refining reactions if oxidised refining slag is present in contact with the metal. Stable oxides like SiO and MnO are not prone to 2 reversion in acid steelmaking processes. However P O in basic steelmaking is very easily 2 5 reduced from the slag to the metal phase on drop of oxygen potential.
  143. 143. DEOXIDATION PRACTICE In general the refining slag is flushed off in basic process and deoxidation may be carried out partly in the furnace and major part in the ladle. As products of deoxidation in a furnace get more time to reach the surface of the bath furnace deoxidation is useful in production of clean steel.
  144. 144. CONTROL OF INGOT STRUCTURE The final structure of an ingot is entirely determined by the degree of deoxidation carried out prior to solidification of steel in a mould. The residual oxygen in the steel at the end of refining is determined by the steel making practice and the type of steel produced. For a given type of steel the steel making and deoxidation practices have to properly adjusted to finally obtain the desired ingot structure.
  145. 145. RIMMING STEEL Rimming steel require a lot of gas evolution during solidification. The steel, therefore, must contain enough dissolved oxygen and which is possible only in low carbon steel (<0.15%). The heat must be finished in the furnace in such a way that the bath contains desired level of oxygen having carbon level < 0.15%. In general, no deoxidation is carried out inside the furnace. Only a small amount of deoxidation is carried out in the ladle using Fe-Mn and Al.
  146. 146. RIMMING STEEL The zone between the primary and secondary blow holes is called rim which is characteristics of rimming ingots. Rimming ingot is relatively cleaned due to less inclusions and brisk evolution of gas in the beginning of solidification.
  147. 147. SEMI-KILLED STEEL These are partially deoxidised steel such that only small amount of gas is evolved during solidification. The carbon content has been in the range of 0.15- 0.30%. Partial deoxidation is carried out in the furnace itself using Fe-Mn and Al. The gas is evolved towards the end of the solidification. The blow holes are therefore, present in the middle and top of the ingot.
  148. 148. SEMI-KILLED STEEL Aluminium is put into the ladle toward the end of pouring to completely deoxidises the top of the ingot to compensate the pipe formation.
  149. 149. KILLED STEEL No gas evolution take place in killed steel during solidification. All steels containing 0.3% C are killed. The heat is worked in such a way that by the time carbon level drops close to specification level the refining should be over. In general the heat is then blocked by adding Fe- Si, Fe-Mn and high silicon pig iron. Blocking stops the carbon oxygen reaction by lowering oxygen content of the bath
  150. 150. KILLED STEEL Deoxidation product should be given sufficient time to rise to the surface otherwise it will form nonmetallic inclusions in steel. Solidification of Killed steel is accompanied by V or A type seggregation
  151. 151. ADVANCES IN STEELMAKINGAND SECONDARYSTEELMAKING Smarajit Sarkar Department of Metallurgical and Materials Engineering NIT Rourkela
  152. 152. As a standard guide the temperature rise attainable byoxidation of 0·01 % of each of the element dissolved inliquid iron at 1400°C by oxygen at 25°C is calculatedassuming that no heat is lost to the surroundings andsuch data are shown below .
  154. 154. BOTTOM BLOWING VS TOP BLOWING Oxidation of carbon : Bottom blowing increases sharply the intensity of bath stirring and increases the area of gas-metal boundaries (10-20 times the values typical of top blowing) . Since the hydrocarbons supplied into the bath together with oxygen dissociate into H2, H2O and CO2 gas bubbles in the bath have a lower partial pressure of carbon monoxide (Pco ) All these factors facilitate substantially the formation and evolution of carbon monoxide, which leads to a higher rate of decarburization in bottom blowing
  155. 155. CONT.. The degree of oxidation of metal and slag Removal of phosphorous: Since the slag of the bottom- blown converter process have a low degree of oxidation almost during the whole operation, the conditions existing during these periods are unfavorable for phosphorus removal
  157. 157. SLOPPING Problems arise when the layer of foaming slag created on the surface of the molten metal exceeds the height of the vessel and overflows, causing metal loss, process disruption and environmental pollution. This phenomenon is commonly referred to as slopping.
  158. 158. METALLURGICAL FEATURES OF BATHAGITATED PROCESS:  Better mixing and homogeneity in the bath offer the following advantages: Less slopping, since non-homogeneity causes formation of regions with high supersaturation and consequent violent reactions and ejections. Better mixing and mass transfer in the metal bath with closer approach to equilibrium for [C]-[O]-CO reaction, and consequently, lower bath oxygen content at the same carbon content
  159. 159.  Better slag-metal mixing and mass transfer and consequently, closer approach to slag-- metal equilibrium, leading to: o lower FeO in slag and hence higher Fe yield o transfer of more phosphorus from the metal to the slag (i.e. better bath dephosphorisation) o transfer of more Mn from the slag to the metal, and thus better Mn recovery o lower nitrogen and hydrogen contents of the bath. More reliable temperature measurement and sampling of metal and slag, and thus better process control Faster dissolution of the scrap added into the metal bath
  160. 160. HYBRID BLOWING•A small amount of inert gas, about 3% of the volume of oxygenblown from top, introduced from bottom, agitates the bath soeffectively that slopping is almost eliminated.•However for obtaining near equilibrium state of the systeminside the vessel a substantial amount of gas has to beintroduced from the bottom.•If 20-30% of the total oxygen, if blown from bottom, can causeadequate stirring for the system to achieve near equilibriumconditions. The increase beyond 30% therefore contributesnegligible addition of benefits.
  161. 161. CONT..• The more the oxygen fraction blown from bottom the less is the post combustion of CO gas and consequently less is the scrap consumption in the charge under identical conditions of processing.• Blowing of inert gas from bottom has a chilling effect on bath and hence should be minimum. On the contrary the more is the gas blown the more is the stirring effect and resultant better metallurgical results. A optimum choice therefore has to be made judiciously.
  162. 162. CONT.. As compared to top blowing, the hybrid blowing eliminates the temperature and concentration gradients and effects improved blowing control, less slopping and higher blowing rates. It also reduces over oxidation and improves the yield. It leads the process to near equilibrium with resultant effective dephosphorisation and desulphurisation and ability to make very low carbon steels.
  163. 163.  What is blown from the bottom, inert gas or oxygen? How much inert gas is blown from the bottom? At what stage of the blow the inert gas is blown, although the blow, at the end of the blow, after the blow ends and so on? What inert gas is blown, argon, nitrogen or their combination? How the inert gas is blown, permeable plug, tuyere, etc.? What oxidising media is blown from bottom, oxygen or air? If oxygen is blown from bottom as well then how much of the total oxygen is blown from bottom ?
  165. 165. HYBRID BLOWING The processes have been developed to obtain the combined ad-vantages of both LD and OBM to the extent possible. Therefore the metallurgical performance of a hybrid process has to be evaluated in relation to these two extremes, namely the LD and the OBM. The parameters on which this can be done are : Iron content of the slag as a function of carbon content of bath Oxidation levels in slag and metal Manganese content of the bath at the turndown Desulphurisation efficiency in terms of partition coefficient Dephosphorisation efficiency in terms of partition coefficient Hydrogen and nitrogen contents of the bath at turndown Yield of liquid steel
  166. 166. DEOXIDATION OF STEELThe oxidizing conditions of a heat in a steelmaking plant, thepresence of oxidizing slag, and the interaction of the metal with thesurrounding atmosphere at tapping and teeming - all these factorsare responsible for the fact that the dissolved oxygen in steel has adefinite, often elevated, activity at the moment of steel tapping. Theprocedure by which the activity of oxygen can be lowered to therequired limit is called deoxidation. Steel subjected to deoxidation istermed deoxidized. If deoxidized steel is quiet during solidificationin moulds, with almost no gases evolving from it, it is called killedsteel.
  167. 167.  If the metal is tapped and teemed without being deoxidized, the reaction [O] + [C] = COg will take place between the dissolved oxygen and carbon as the metal is cooled slowly in the mould. Bubbles of carbon monoxide evolve from the solidifying metal, agitate the metal in the mould vigorously, and the metal surface is seen to boil. Such steel is called wild; when solidified, it will be termed rimming steel . In some cases, only partial deoxidation is carried out, i.e. oxygen is only partially removed from the metal. The remaining dissolved oxygen causes the metal to boil for a short time. This type of steel is termed semi-killed.
  168. 168.  Thus, practically all steels are deoxidized to some or other extent so as to lower the activity of dissolved oxygen to the specified limit. The activity of oxygen in the metal can be lowered by two methods: (I) by lowering the oxygen concentration, or (2) by combining oxygen into stable compounds. There are the following main practical methods for deoxidation of steel: (a) precipitation deoxidation, or deoxidation in the bulk; (b) diffusion deoxidation; (c) treatment with synthetic slags; and (d) vacuum treatment.
  169. 169.  The upper part containing the exposed pipe in killed steels has to be rejected and this decreases the yield to about 80 %. The yield from a rimmed ingot is higher. Only a killed steel can be continuously cast. In contrast to ingot steel, the yield in continuous casting is more than 90 %. A rimmed steel cannot be continuously cast, as the rimming action can puncture holes through the thin solidified layer of the cast slab and the liquid steel may pour out uncontrollably. The turbulence during gas evolution in a rimmed ingot physically transports the metal to different parts, causing macrosegregation to a greater extent. 
  170. 170. CONTINUOUS CASTING The advantages of continuous casting (over ingot casting) are: It is directly possible to cast blooms, slabs and billets, thus eliminating blooming, slabbing mills completely, and billet mills to a large extent. Better quality of the cast product. Higher crude-to-finished steel yield (about 10 to 20% more than ingot casting). Higher extent of automation and process control.
  172. 172. THE MAJOR REQUIREMENTS OF CONTINUOUSCASTING Solidification must be completed before the withdrawal rolls. The liquid core should be bowl-shaped as shown in the Figure and not pointed at the bottom (as indicated by the dotted lines), since the latter increases the tendency for undesirable centerline (i.e. axial) macro-segregation and porosity The solidified shell of metal should be strong enough at the exit region of the mould so that it does not crack or breakout under pressure of the liquid.
  173. 173. METALLURGICAL COMPARISON OFCONTINUOUS CASTING WITH INGOTCASTING The surface area-to-volume ratio per unit length of continuously cast ingot is larger than that for ingot casting. As a consequence, the linear rate of solidification (dx/dt) is an order of magnitude higher than that in ingot casting. The dendrite arm spacing in continuously cast products is smaller compared with that in ingot casting.
  174. 174. CONT… Macro-segregation is less, and is restricted to the centreline zone only. Endogenous inclusions are smaller in size, since they get less time to grow. For the same reason, the blow holes are, on an average, smaller in size. Inclusions get less time to float-up. Therefore, any non- metallic particle coming into the melt at the later stages tends to remain entrapped in the cast product.
  175. 175.    In addition to more rapid freezing, continuous casting differs from ingot casting in several ways. These are noted below. Mathematically speaking, continuously cast ingot is infinitely long. Hence, the heat flow is essentially in the transverse direction, and there is no end-effect as is the case in ingot casting (e.g. bottom cone of negative segregation, pipe at the top, etc.). The depth of the liquid metal pool is several metres long. Hence, the ferrostatic pressure of the liquid is high during the latter stages of solidification, resulting in significant difficulties of blow-hole formation.
  176. 176.  Since the ingot is withdrawn continuously from the mould, the frozen layer of steel is subjected to stresses. This is aggravated by the stresses arising out of thermal expansion/ contraction and phase transformations. Such stresses are the highest at the surface. Moreover, when the ingot comes out of the mould, the thickness of the frozen steel shell is not very appreciable. Furthermore, it is at around 1100-1200°C, and is therefore, weak. All these factors tend to cause cracks at the surface of the ingot leading to rejections. Use of a tundish between the ladle and the mould results in extra temperature loss. Therefore, better refractory lining in the ladles, tundish, etc. are required in order to minimise corrosion and erosion by molten metal.
  177. 177. SECONDARY STEELMAKING Smarajit Sarkar Department of Metallurgical and Materials Engineering NIT Rourkela
  178. 178. SECONDARY STEELMAKINGPrimary steelmaking is aimed at fast meltingand rapid refining. It is capable of refining ata macro level to arrive at broad steelspecifications, but is not designed to meetthe stringent demands on steel quality, andconsistency of composition and temperaturethat is required for very sophisticated gradesof steel. In order to achieve suchrequirements, liquid steel from primarysteelmaking units has to be further refined inthe ladle after tapping. This is known asSecondary Steelmaking.
  179. 179. RESORTED TO ACHIEVE ONE OR MOREOF THE FOLLOWING REQUIREMENTS : improvement in quality improvement in production rate decrease in energy consumption use of relatively cheaper grade or alternative raw materials use of alternate sources of energy higher recovery of alloying elements.
  180. 180. QUALITY OF STEEL Lower impurity contents . Better cleanliness. (i.e. lower inclusion contents) Stringent quality control. (i.e. less variation from heat-to-heat) Microalloying to impart superior properties. Better surface quality and homogeneity in the cast product.
  181. 181. CLEAN STEEL The term clean steel should mean a steel free of inclusions. However, no steel can be free from all inclusions. Macro-inclusions are the primary harmful ones. Hence, a clean steel means a cleaner steel, i.e., one containing a much lower level of harmful macro-inclusions.)
  182. 182. INCLUSIONS In practice, it is customary to divide inclusions by size into macro inclusions and micro inclusions. Macro inclusions ought to be eliminated because of their harmful effects. However, the presence of micro inclusions can be tolerated, since they do not necessarily have a harmful effect on the properties of steel and can even be beneficial. They can, for example, restrict grain growth, increase yield strength and hardness, and act as nuclei for the precipitation of carbides, nitrides, etc.
  183. 183. MACRO AND MICROINCLUSIONS The critical inclusion size is not fixed but depends on many factors, including service requirements. Broadly speaking, it is in the range of 5 to 500 µm (5 X 10-3 to 0.5 mm). It decreases with an increase in yield stress. In high- strength steels, its size will be very small. Scientists advocated the use of fracture mechanics concepts for theoretical estimation of the critical size for a specific situation.
  184. 184. SOURCES OF INCLUSIONS Precipitation due to reaction from molten steel or during freezing because of reaction between dissolved oxygen and the deoxidisers, with consequent formation of oxides (also reaction with dissolved sulphur as well). These are known as endogenous inclusions. Mechanical and chemical erosion of the refractory lining Entrapment of slag particles in steel Oxygen pick up from the atmosphere, especially during teeming, and consequent oxide formation. Inclusions originating from contact with external sources as listed in items 2 to 4 above, are called exogenous inclusions.
  185. 185. REMOVAL OFINCLUSIONSWith a lower wettability (higher value of σMe –inc ), an inclusion can be retained in contactwith the metal by lower forces, and therefore,can break off more easily and float up in themetal. On the contrary, inclusion which arewetted readily by the metal, cannot break offfrom it as easily.
  186. 186. CLEANLINESS CONTROL DURINGDEOXIDATION  Carryover slag from the furnace into the ladle should be minimised, since it contains high percentage of FeO + MnO and makes efficient deoxidation fairly difficult.  Deoxidation products should be chemically stable. Otherwise, they would tend to decompose and transfer oxygen back into liquid steel. Si02 and Al203 are preferred to MnO. Moreover the products should preferably be liquid for faster growth by agglomeration and hence faster removal by floatation. Complex deoxidation gives this advantage. 
  187. 187. CONT…  Stirring of the melt in the ladle by argon flowing through bottom tuyeres is a must for mixing and homogenisation, faster growth, and floatation of the deoxidation products. However, very high gas flow rates are not desirable from the cleanliness point of view, since it has the following adverse effects: o Too vigorous stirring of the metal can cause disintegration of earlier formed inclusion conglomerates. o Re-entrainment of slag particles into molten steel. o Increased erosion of refractories and consequent generation of exogenous inclusions. o More ejection of metal droplets into the atmosphere with consequent oxide formation.