1、Steelmaking,Part II:EAF processContentsIntroductionConfiguration of the EAFTypical EAF steelmaking processRaw materialsDescription of the EAF steelmaking processNew technologiesIntroductionOver the past 20 years the use of the electric arc furnace(EAF)for the production of steel has grown considerab
2、ly.Many reasons(product cost and technology development):-The capital cost per ton of annual installed capacity generally runs in the range of$140200/ton for an EAF based operation.For a similar blast furnace BOF based operation the cost is approximately$1000 per annual ton of installed capacity.-Re
3、sidual elements levels in the steel and dissolved gases in the steel(nitrogen,hydrogen,oxygen).Crude steel production by process for the World from 1970 to 1996.Source:International Iron and Steel Institute.Configuration of the EAFEAF is composed of several components(as shown in the Figure):-Shell
4、-Refractories -Electrode -Taphole -Roof etc.Schematic of a typical AC electric arc furnaceTypical EAF steelmaking processA typical 60 minute tap-to-tap cycle is:(1)first charge 3 minutes (2)first meltdown 20 minutes (3)second charge 3 minutes (4)second meltdown 14 minutes (5)refining 10 minutes (6)t
5、apping 3 minutes (7)turnaround 7 minutes Total 60 minutesThe electric arc furnace operates as a batch process.Each batch of steel that is produced is known as a heat.The electric arc furnace operating cycle is known as the tap-to-tap cycle.The tap-to-tap cycle is made up of the following operations:
6、furnace charging,melting,refining,de-slagging,tapping and furnace turnaround.Modern operations aim for a tap-to-tap cycle of less than 60 minutes.With the advance of EAF steelmaking into the flat products arena,tap-to-tap times of 3540 minutes are now being sought with twin shell furnace operations.
7、Raw MaterialsThe main raw material for EAF steelmaking is steel scrap.Primarily from three main sources:-Reclaimed scrap(also known as obsolete scrap)which is obtained from old cars,demolished buildings,discarded machinery and domestic objects;-Industrial scrap(also known as prompt scrap)which is ge
8、nerated by industries using steel within their manufacturing processes;and -Revert scrap(also known as home scrap)which is generated within the steelmaking and forming processes(e.g.crop ends from rolling operations,metallic losses in slag etc.).The latter two forms of scrap tend to be clean,i.e.the
9、y are close in chemical composition to the desired molten steel composition and thus are ideal for recycle.Reclaimed/obsolete scrap frequently has a quite variable composition and quite often contains contaminants that are undesirable for steelmaking.Levels of residual elements such as Cu,Sn,Ni,Cr,a
10、nd Mo are high in obsolete scrap and can affect casting operations and product quality if they are not diluted.The method is to use a combination of the contaminated obsolete scrap along with what are generally referred to as clean iron units or virgin iron units.These are materials which contain li
11、ttle or no residual elements.Clean iron units are typically in the form of direct reduced iron(DRI),hot briquetted iron(HBI),iron carbide,pig iron,and molten pig iron(hot metal).Other undesirable components including,oil,grease,paint coatings,zinc coatings,water,oxidized material and dirt.These unde
12、sirable components may result in higher energy requirements and environmental problems.Thus the decision for scrap mix to be used within a particular operation will frequently depend on several factors including availability,scrap cost,melting cost,yield,and the effect on operations(based on scrap d
13、ensity,oil and grease content,etc.).In practice,most operations buy several different types of scrap and blend them to yield the most desirable effects for EAF operations.In addition,scrap is also classified based on its physical size,its source and the way in which it is prepared.Fluxes and Additiv
14、esCarbonLimeCarbonCarbon is one of the key elements which give various steel grades their properties.Carbon is also important in steelmaking refining operations and can contribute a sizable quantity of the energy required in steelmaking operations.In electric furnace steelmaking,some carbon will be
15、contained in the scrap feed,in DRI,HBI or other alternative iron furnace feeds.The amount of carbon contained in these EAF feeds will generally be considerably lower than that contained in hot metal and typically,some additional carbon is charged to the EAF.In the past carbon was charged to the furn
16、ace to ensure that the melt-in carbon level was above that desired in the final product.As higher oxygen utilization has developed as standard EAF practice,more carbon is required in EAF operations.The reaction of carbon with oxygen within the bath to produce carbon monoxide results in a significant
17、 energy input to the process and has lead to substantial reductions in electrical power consumption in EAF operations.The generation of CO within the bath is also key to achieving low concentrations of dissolved gases(nitrogen and hydrogen)in the steel as these are flushed out with the carbon monoxi
18、de.In addition,oxide inclusions are flushed from the steel into the slag.In oxygen injection operations,some iron is oxidized and reports to the slag.Oxy-fuel burner operations will also result in some scrap oxidation and this too will report to the slag once the scrap melts in.Dissolved carbon in t
19、he steel will react with FeO at the slag/bath interface to produce CO and recover iron units to the bath.Typical charge carbon rates for medium carbon steel production lie in the range of 212 kg per ton of liquid steel.Generally,the three types of carbonaceous material used as charge carbon in EAF o
20、perations are -anthracite coal,-metallurgical coke,and -green petroleum coke.Increased coal input to the EAF will require additional lime addition in order to maintain the desired basicity in the slag.The best grades of anthracite coal have fixed carbon contents of 8789%.Low grade anthracite coals m
21、ay have fixed carbon levels as low as 50%.Metallurgical coke is produced primarily in integrated steel operations and is used in the blast furnace(some coke is also used as EAF charge carbon).Generally,it has a composition of 12%moisture content,13.5%volatile material,8688%fixed carbon,912%ash conte
22、nt and 0.881.2%sulfur.Foamy slag practicesAt the start of meltdown the radiation from the arc to the sidewalls is negligible while the electrodes are surrounded by the scrap.As melting proceeds the efficiency of heat transfer to the scrap and bath drops off and more heat is radiated from the arc to
23、the sidewalls.By covering the arc in a layer of slag,the arc is shielded and the energy is transferred to the bath.Oxygen is injected with coal to foam the slag by producing CO gas in the slag.In some cases only carbon is injected and the carbon reacts with FeO in the slag to produce CO gas.When foa
24、med,the slag cover increases from 10 to 30 cm thick.In some cases the slag is foamed to such an extent that it comes out of the electrode ports.Claims for the increase in energy transfer efficiency range from an efficiency of 6090%with slag foaming compared to 40%without.It has been reported that at
25、 least 0.3%carbon should be removed from the bath using oxygen in order to achieve a good foamy slag practice.LimeLime is the most common flux used in modern EAF operations.Most operations now use basic refractories and as a result,the steelmaker must maintain a basic slag in the furnace in order to
26、 minimize refractory consumption.Slag basicity has also been shown to have a major effect on slag foaming capabilities.Thus lime tends to be added both in the charge and also via injection directly into the furnace.Lime addition practices can vary greatly due to variances in scrap composition.As ele
27、ments in the bath are oxidized(e.g.P,Al,Si,Mn)they contribute acidic components to the slag.Thus basic slag components must be added to offset these acidic contributions.If silica levels in the slag are allowed to get too high,significant refractory erosion will result.In addition,FeO levels in the
28、slag will increase because FeO has greater solubility in higher silica slags-lead to lower yield.Slagging offThe slag will be removed from the steel by pouring slag out of the furnace through the slag door which is located at the back of the EAF.This is known as slagging off.If the slag is not remov
29、ed but is instead allowed to carry over to the ladle it is possible for slag reversion to take place.When steel is tapped it is frequently killed by adding either silicon of aluminum during tapping.The purpose of these additions is to lower the oxygen content in the steel.If however P2O5 is carried
30、over into the ladle,it is possible that it will react with the alloy additions producing silica or alumina and phosphorus which will go back into solution in the steel.Sometimes magnesium lime is added to the furnace either purely as MgO or as a mixture of MgO and CaO.Basic refractories are predomin
31、antly MgO,thus by adding a small amount of MgO to the furnace,the slag can quickly become saturated with MgO and thus less refractory erosion is likely to take place.Furnace ChargingThe first step in any tap-to-tap cycle is charging of the scrap.The scrap yard operators will prepare the batch bucket
32、s of scrap to ensure proper melt-in chemistry and to ensure good melting conditions.The scrap must be layered in the bucket according to size and density to ensure rapid formation of a liquid pool in the hearth while also providing protection of the sidewalls and roof from arc radiation.Other consid
33、erations include minimization of scrap cave-ins which can break electrodes and ensuring that large heavy pieces of scrap do not lie directly in front of burner ports which would result in blow-back of the flame onto the water-cooled panels(add some lime and carbon in the scrap bucket).The roof and e
34、lectrodes are raised and are swung out to the side of the furnace to allow the scrap charging crane to move a full bucket of scrap into place over the furnace.The roof and electrodes swing back into place over the furnace.The commence of the melting portion of the cycle(The roof is lowered and the e
35、lectrodes are lowered to strike an arc on the scrap).The number of charge buckets of scrap required to produce a heat of steel is dependent primarily on the volume of the furnace and the scrap density.Most modern furnaces are designed to operate with a minimum of backcharges in order to reduce the d
36、ead time and increase the productivity of the furnace as well as minimisation of the energy loss.Continuous charging operations such as Consteel eliminate the charging cycle.MeltingThe melting period is the heart of EAF operations.Melting is accomplished by supplying electrical or chemical energy to
37、 the furnace interior.Electrical energy is supplied via the graphite electrodes and is usually the largest contributor in melting operations.Usually,light scrap is placed on top of the charge to accelerate bore-in.After a few minutes,the electrodes will have penetrated the scrap sufficiently that a
38、long arc(high voltage)tap can be used without fear of radiation damage to the roof.The long arc maximizes the transfer of power to the scrap and a liquid pool of metal will form in the furnace hearth.Approximately 15%of the scrap is melted during the initial bore-in period.At the start of melting th
39、e arc is erratic and unstable.As the furnace atmosphere heats up the arcing tends to stabilize and once the molten pool is formed,the arc becomes quite stable and the average power input increases.Chemical energy can be supplied via several sources such as oxy-fuel burners and oxygen lancing.Heat is
40、 transferred to the scrap by radiation and convection,and by conduction within the scrap.In some operations,oxygen is used to cut scrap.Once a molten pool of steel is generated in the furnace,oxygen can be lanced directly into the bath.This oxygen will react with several components in the bath inclu
41、ding,aluminum,silicon,manganese,phosphorus,carbon and iron.All of these reactions are exothermic and will supply energy to aid in the melting of the scrap.Once enough scrap has been melted to accommodate the second charge,the charging process is repeated.After the final scrap charge is melted,the fu
42、rnace sidewalls can be exposed to high radiation from the arc.As a result,the voltage must be reduced.Alternatively,creation of a foamy slag will allow the arc to be buried and will protect the furnace shell.A bath temperature and a chemical analysis sample will be taken.The melter can also start to
43、 arrange for the bulk tap alloy additions to be made.RefiningRefining operations in the electric arc furnace have traditionally involved the removal of phosphorus,sulfur,aluminum,silicon,manganese and carbon,and recently the removal of dissolved gases(nitrogen and hydrogen)in the bath.Most impuritie
44、s such as phosphorous,sulfur,silicon,aluminum and chromium are partially removed by transfer to the slag.The equilibrium partition ratios of these elements between metal and slag can be calculated as functions of slag chemistry and temperature.The slag in an EAF operation will have a lower basicity
45、than that for oxygen steelmaking.Typical Slag ConstituentsP removalPhosphorus retention in the slag is a function of the bath temperature,the slag basicity and FeO levels in the slag.At higher temperature or low FeO levels,P will revert from the slag back into the bath.High slag basicity(i.e.high li
46、me content)is also beneficial for phosphorus removal but care must be taken not to saturate the slag with lime.This will lead to an increase in slag viscosity which will make the slag less effective for phosphorus removal.Fluorspar is added to help fluidize the slag.Gas stirring is also beneficial b
47、ecause it will renew the slag/metal interface which will improve the reaction kinetics.The partition ratio of phosphorus in the slag to phosphorus in the bath ranges from 5.015.0.Usually the phosphorus is reduced by 2050%in the EAF.The phosphorous in the scrap is low compared to hot metal and theref
48、ore this level of removal is acceptable.For oxygen steelmaking higher slag basicity and FeO levels give a partition ratio of 100 and with greater slag weight up to 90%of the phosphorous is removed.Sulfur removalSulfur is removed mainly as a sulfide dissolved in the slag.The sulfur partition between
49、the slag and metal(typically 3.05.0 in EAF practice)is dependent on the chemical analysis and temperature of the slag(high basicity is better,low FeO content is better),slag fluidity(high fluidity is better),the oxidation level of the steel(which should be as low as possible),and the bath compositio
50、n.Metallic componentsControl of the metallic constituents in the bath is important as it determines the properties of the final product.Oxygen reacts with aluminum,silicon and manganese to form metallic oxides which are slag components.These metallics tend to react before the carbon in the bath begi
