Smarajit SarkarDepartment of Metallurgical and Materials Engineering NIT Rourkela
The OBM vessel is essentially a Bessemer-like converter fitted with a special bottom . The tuyeres are inserted from the bottom in such a way that the oxygen would be surrounded by a protective hydrocarbon gas like propane. On entry propane cracks down in an endothermic reaction and takes up some of the heat-gene-rated by the entry of oxygen. The relative feed rates of these two fluids are adjusted to obtain optimum temperatures at the tuyere tip and thereby ensure its reasonable life as well as speed of refining. The deposition of carbon, which is a product of cracking, also helps to protect the bottom from heat generated due to the refining reactions at the tips of tuyeres.
Inorder to promote turbulence in the bath and thereby ensure good slag-metal contact, the tuyeres are arranged only on half the converter bottom. Experience dictated that provision of a few bigger tuyeres is better than large number of fine tuyeres. Maintenance problems are minimised without loosing in terms of metallurgical requirements of turbulence. By this arrangement, it is ensured that the direction of metal circulation is upwards in the tuyere half of the vessel, and downwards in the other half. This arrangement is also helpful in minimising the damage to tuyeres while charging scrap, since it can now be charged on that part where there are no tuyeres.
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
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
Almost 98% oxygen being reacted with metal in OBM and hence that much scrap rate is lower in the OBM. If scrap is cheaper the top blowing can offer some cost advantage in this respect. The iron losses in top blown are nearly 5% more than those in OBM. Very low carbon steers are achievable in top blowing only at the expense of extra iron loss in slag. But this is readily achievable in OBM.
This also, leads to situation wherein higher carbon levels can be obtained by catch carbon techniques easily in LD than in OBM, at low P contents. The stirring intensity, which is estimated to be nearly ten times more in OBM than in LD gives better partition of phosphorus and sulphur, higher manganese and lower oxygen at turndown result-ing in better ferroalloy recovery.
Since the slag of the bottom-blown converter process have a lowdegree of oxidation almost during the whole operation, theconditions existing during these periods are unfavorable forphosphorus removal. Only at the end of blowing, when the bath islow in carbon, the oxidation degree of the slag increasessharply, thus favouring dephosphorization. At thatmoment, phosphorus passes intensively to slag. When using lumpylime in the charge, it is difficult to make medium or high carbonsteels with a low content of phosphorus. The metal must be blownto a low carbon content, so as to form an oxidizing slag at the endof heat, and then carburized in the ladle.
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.
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
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
•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.
• 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.
As compared to top blowing, the hybrid blowingeliminates the temperature and concentrationgradients and effects improved blowing control, lessslopping and higher blowing rates. It also reduces overoxidation and improves the yield. It leads the processto near equilibrium with resultant effectivedephosphorisation and desulphurisation and ability tomake very low carbon steels.
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 ?
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
The 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.
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.
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.
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.
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.
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.
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.
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.
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.
Smarajit SarkarDepartment of Metallurgical and Materials Engineering NIT Rourkela
Primary 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.
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.
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.
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.)
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.
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.
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.
With a lower wettability (higher value of σMe – inc), an inclusion can be retained in contact with themetal by lower forces, and therefore, can breakoff more easily and float up in the metal. On thecontrary, inclusion which are wetted readily by themetal, cannot break off from it as easily.
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.
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.
The varieties of secondary steelmaking processes that have proved to be of commercial value can broadly be categorised as under: Stirring treatments Synthetic slag refining with stirring Vacuum treatments Decarburisation techniques Injection metallurgy Plunging techniques Post-solidification treatments.
It is a simple ladle like furnace provided with bottom plug for argon purging and lid with electrodes to become an arc furnace for heating the bath. Another lid may be provided to connect it to vacuum line, if required. Chutes are provided for additions and an opening even for injection. In short it is capable of carrying out stirring, vacuum treatment, synthetic slag refining, plunging, injection etc. all in one unit without restraint of temperature loss, since it is capable of being heated independently.
Every ladle furnace need not be equipped withall these arrangements. As per the requirementsof refining the ladle furnace may be providedwith the necessary facilities. For example if gascontent is no consideration, vacuum attachmentmay be eliminated. The principal component ofthe facilities are shown in next slideschematically.
The ASEA-SKF furnace is a special variety of LF furnace only. The SKF furnace is essentially a teeming ladle for which additional fittings are provided. The metal in the ladle is stirred by an electromagnetic stirrer provided from outside. The ladle shell is made of austenitic stainless steel for this reason. Two ladle covers are employed. One of these fits tightly on to the ladle forming a vacuum seal, and is connected to a steam ejector unit for evacuation of the ladle chamber. For vacuum decarburisation oxygen lance is introduced through a vacuum sealed port located in the cover.
When the decarburisation and vacuum degassing is over the first cover is replaced by the second cover which contains three electrodes. Final alloying and temperature adjustments are then made. Steel can also be desulphurised by preparing a reducing basic slag under the electrode cover. The process is schematically shown in next slide. The nearly re-fined steel in only one of the primary steelmaking processes can be treated in this furnace by carrying out the following operations :
Tapping primary furnace into the SKF ladle directly . Controlled stirring during the entire secondary processing Vacuum treatment including minor decarburisation Extensive decarburisation for stainless steelmaking. Deoxidation. Desulphurisation and deslagging. Alloying to desired extent. Temperature adjustment. Teeming from the same SKF ladle.
Quality improvement of steel can also be brought about after steel is refined and cast into ingots from the primary refining furnace, by remelting and casting once again. Typical examples of this type is zone refining which is adopted to produce purer metals. The other two techniques that have been developed are meant for the production of, not pure metals, but alloy steels of better cleanliness and low sulphur contents. The vacuum arc remelting, VAR(750kWh/ton) for short and the electro slag refining, ESR (900-1300kWh/ton) for short, are commercially used for further refining of steels after these are cast into ingots.
In both of these processes the steel ingot produced by the primary refining forms the electrode to be drip-melted into a water cooled copper mould. In VAR melting is carried out under vacuum and in ESR it is in open atmosphere. In VAR arc is struck between the electrode and the mould and it generates the heat required for melting the electrode. In ESR a slag layer is used to act as a resistor between the electrode and the mould and which is responsible for melting the electrode. The slag also acts as a refining agent.
In both of these processes the electrode melts progressively and is resolidified on the mould, nearly unidirectionally. Because of the high temperature, small pool of molten metal and almost unidirectional solidification, both of these processes can produce sound ingots of high density. The composition of the product is nearly the same as that of the original material but with improved cleanliness, decreased segregation and with practically no cavities. The ingot size ranges from about 200 to 1500mm on industrial level
The product of both of these processes is exceptionally suited for the production of forgings of high alloy steels. But because of high cost of such a process, applications are limited to specialty products like turbo rotor shafts and so on. In VAR the hydrogen and oxygen contents are very low but in ESR they are like ordinary steels. In ESR the choice of the slag composition is fairly critical since it has to act as a resistor as well as a refin-ing agent. These are essentially oxy-fluoride type reducing slag like CaO-CaF2·
The ESR however has some advantages over VAR and these are given below: Multiple electrode can be melted into a single electrode. Spacing between the mould wall and the electrode is not critical. Surface quality is superior requiring little or no conditioning. Steel can be desulphurised to as low as 0·002% sulphur. Round, square, hollow and rectangular shapes of ingots can be produced. Ingots of much larger weight can be produced.
Molten steel is contained in the ladle. The two legs of the vacuum chamber (known as Snorkels) are immersed into the melt. Argon is injected into the up leg. Rising and expanding argon bubbles provide pumping action and lift the liquid into the vacuum chamber, where it disintegrates into fine droplets, gets degassed and comes down through the down leg snorkel, causing melt circulation. The entire vacuum chamber is refractory lined. There is provision for argon injection from the bottom, heating, alloy additions, sampling and sighting as well as video display of the interior of the vacuum chamber.
Why RH-OB Process?To meet increasing demand for cold-rolled steel sheets with improvedmechanical properties, and to cope with the change from batch-type tocontinuous annealing, the production of ULC steel (C < 20 ppm) isincreasing. A major problem in the conventional RH process is that the timerequired to achieve such low carbon is so long that carbon content atBOF tapping should be lowered. However, this is accompanied byexcessive oxidation of molten steel and loss of iron oxide in the slag. It adversely affects surface the quality of sheet as well.
Hence, decarburization in RH degasser is to be speeded up. This is achieved by some oxygen blowing (OB) during degassing. The RH-OB process, which uses an oxygen blowing facility during degassing, was originally developed for decarburization of stainless steel by Nippon Steel Corp., Japan, in 1972. Subsequently, it was employed for the manufacture of ULC steels. The present thrust is to decrease carbon content from something like 300 ppm to 10 or 20 ppm within 10 min. Cont…
Ferrochrome, which contains about 55 to 70% chromium is the principal source of Chromium. This ferroalloy can be classified into various grades, based primarily on their carbon :ontent, such as: Low carbon ferrochrome (about 0.1 % C). Intermediate carbon ferrochrome (about 2% C). High carbon ferrochrome (around 7% C). Amongst these grades, the high carbon variety has the drawback that though it is the least expensive, it raises the carbon content of the melt. This is undesirable, since all SS grades demand carbon contents less than 0.03%. Chromium forms stable oxides. Hence, the removal of carbon from the bath by oxidation to CO is associated with the problem of simultaneous oxidation of chromium in molten steel.
The higher the temperature, the greater is the tendency for preferential oxidation of carbon rather than chromium. From this point of view, higher bath temperatures are desirable; however, too high a temperature in the bath gives rise to other process problems. The dilution of oxygen with argon lowers the partial pressure of CO, which helps in preferential removal of CO without oxidising bath chromium. Attempts were made to use this in the EAF, but the efforts did not succeed. Hence, as is the case with the production of plain carbon steels, the EAF is now basically a melting unit for stainless steel production as well. Decarburisation is carried out partially in the EAF, and the rest of the carbon is removed in a separate refining vessel. In this context, the development of the AOD process was a major breakthrough in stainless steelmaking.
AOD is the acronym for Argon-Oxygen Decarburisation. The process was patented by the Industrial Gases Division of the Union Carbide Corporation In an AOD converter, argon is used to dilute the other gaseous species (02, CO, etc.). Hence, in some literature, it is designated as Dilution Refining Process. After AOD, some other dilution refining processes have been developed. Lowering of the partial pressures, such as the partial pressure of carbon monoxide, is achieved either by argon or by employing vacuum
The combination of EAF and AOD is sufficient for producing ordinary grades of stainless steels and this combination is referred to as a Duplex Process. Subsequent minor refining, temperature and composition adjustments, if required, can be undertaken in a ladle furnace. Triplex refining, where electric arc furnace melting and converter refining are followed by refining in a vacuum system, is often desirable when the final product requires very low carbon and nitrogen levels. About 65-70% of the worlds total production of stainless steel is in the austenitic variety, made by the duplex EAF-AOD route. If the use of AOD converters even in the triplex route is included, the share of AOD in world production would become as high as 75-80%.
Conventional AOD, no top blowing is involved. Only a mixture of argon and oxygen is blown through the immersed side tuyeres. However, the present AOD converters are mostly fitted with concurrent facilities for top blowing of either only oxygen, or oxygen plus inert gas mixtures using a supersonic lance as in BOF steelmaking.
Initially, when the carbon content of the melt is high, blowing through the top lance is predominant though the gas mixture introduced through the side tuyeres also contains a high percentage of oxygen. However, as decarburisation proceeds, oxygen blowing from the top is reduced in stages and argon blowing increased. As stated earlier, some stainless steel grades contain nitrogen as a part of the specifications, in which case, nitrogen is employed in place of argon in the final stages.
This process produces molten iron in a two-step reduction melting operation. One reactor is melter-gasifier and the other is pre- reducer. In the pre-reducer, iron oxide is reduced in counter-flow principle. The hot sponge is discharged by screw conveyors into the melting reactor. Coal is introduced in the melting-gassifying zone along with oxygen gas at the rate of 500-600 Nm3/thm. The flow velocity is chosen such that temperature in the range of 1500-1800 C is main-tained. The reducing gas containing nearly 85% CO is hot dedusted and cooled to 800-900 C before leading it into the pre- reducer
In the FINEX Process fine ore is preheated and reduced to DRI in a train of four or three stage fluidized bed reactors. The fine DRI is compacted and then charged in the form of Hot Compacted Iron (HCI) into the melter gasifier. So, before charging to the melter- gasifier unit of the FINEX unit, this material is compacted in a hot briquetting press to give hot compacted iron (HCI) since the melter- gasifier can not use fine material (to ensure permeability in the bed). Non-coking coal is briquetted and is fed to the melter gasifier where it is gasified with oxygen
As a standard guide the temperature riseattainable by oxidation of 0·01 % of each of theelement dissolved in liquid iron at 1400°C byoxygen at 25°C is calculated assuming that noheat is lost to the surroundings and such data areshown below .
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Smarajit SarkarDepartment of Metallurgical and Materials Engineering NIT Rourkela