Steelmaking in Electric Arc Furnace
Steelmaking in Electric Arc Furnace
Though De Laval had patented an electric furnace for the melting and the refining iron in 1892 and Heroult had demonstrated electric arc melting of ferro-alloys between 1888 and 1892, the first industrial electric arc furnace (EAF) steelmaking process only came into operation in 1900. Development was rapid and there was a ten-fold increase in production from 1910 to 1920, with over 500,000 tons being produced in 1920, though this still only represented a very small proportion of the global steel production at the time.
Initially, EAF steelmaking was developed for producing special grades of steels using solid forms of feed such as scrap and ferro-alloys. Solid material were firstly melted through direct arc melting, refined through the addition of the appropriate fluxes and tapped for further processing. Till the late twentieth century, tap to tap times over three hours were common and power usage was frequently well over 600 kilowatt hour per ton (kWh/t), nearly twice the thermodynamic requirement. For much of the twentieth century EAF steelmaking was viewed as an expensive and slow process’ only suitable for high value steels.
EAFs are sometimes classified based on the power supplied per ton of furnace capacity. For several EAFs, the design allows for at least 500 kVA per ton of capacity. There are EAFs which allow 900 kVA to 1000 kVA per ton of furnace capacity. Majority of EAFs operate at a maximum power factor of around 0.85. Hence, the transformer ratings correspond to a maximum power input of around 0.75 to 0.85 MW per ton of furnace capacity.
The emerging new technology of EAF started in the beginning of the twentieth century when wide-ranging generation of relatively cheap electric energy started at that time. First-generation EAFs had a capacity in between 1 ton (t) and 15 t. The EAF had Bessemer / Thomas converters and Siemens-Martin furnaces as strong competitors, initially. But its niche was the production of special steels needing high temperature, ferroalloy melting, and long refining times. EAF based operations have gradually moved into production of steels which were traditionally made through the integrated route. The first of these areas was long products (reinforcement bar, merchant bar, and wire rod). In the 1960s, with the advent of billet casting, the EAF occupied a new niche and became the melting unit of choice for the so called mini mills, feeding billet casting machines for the production of long products. This was followed by advances into heavy structural and plate products and by 1985, a new niche for electric steelmaking began to be taken to the flat products area with the advancement of thin slab casting and direct rolling process.
In the following two decades, to better support the short tap-to-tap time needed by billet casting machines, the EAF reinvented itself as a melting-only unit. Steel refining was left for the recently introduced ladle furnace. Large transformers were introduced; ultrahigh-power (UHP) furnaces were developed, which were made possible by adopting foaming slag practice. This way, tap-to-tap time became close to casting time.
Since the 1960s, the technology has undergone rapid development moving from a ‘boutique’ technology to the second largest steelmaking technology behind basic oxygen steelmaking (BOS) technology. Since the late twentieth century, the production of steel by the EAF has grown considerably. There have been several reasons for this but primarily they all relate back to product cost and advances in technology. Fig 1 shows evolution of EAF technology during the period 1965 to 2010.
Fig 1 Evolution of EAF technology during the period 1965 to 2010
The traditional EAF steelmaking process with all or high ratio of scraps as metallic raw materials can result in a higher content of harmful heavy metal elements to deteriorate aimed steel specifications or grades. Hence, the normal flowchart and organization of EAF steelmaking can be normally disturbed by charging excessive scraps with complex chemical composition. It has been widely proposed that one of the effective measures to avoid and solve this problem is to increase hot metal (HM) charge ratio. However, excessive charging HM can lead to prolonging the tap-to-tap time of EAF steelmaking process, since large carbon (C) load from the excessively charged HM can overburden the decarburization operation during the EAF steelmaking process
As the EAF steelmaking attempt to further displace steelmaking through the integrated route, several issues come into play such as residual levels in the steel (essentially elements contained in the steel which are not removed during melting or refining) and dissolved gases namely nitrogen (N2), hydrogen (H2), and oxygen (O2) in the steel. Both of these have a large effect on the quality of the steel and are to be controlled carefully, if EAF steelmaking is to compete for the production of higher quality steels.
Advances in EAF steelmaking
There have been several advances in EAF technology which have allowed the EAF to compete successfully with the integrated production route. Majority of these have dealt with increases in productivity leading to lower cost of steel production. Present day EAFs can produce any type of steel grades which include the general-purpose construction steel grade, the TiNb (titanium-niobium) ultra-low C steel for automotive body parts having a C specification of less than 0.0035 % in order to optimize formability, the line-pipe steel for gas distribution which is a very demanding grade as the combination of high strength and high fracture toughness demands extremely low levels of impurities (phosphorus, sulphur, H2, O2, and N2), and the engineering steel which is a heat-treatable low alloy steel grade. It contains considerable chromium (Cr) and molybdenum (Mo) additions.
The developments in the EAF technologies since 1965, have promoted lower electric energy consumption, shorter tap-to-tap time, and less electrode consumption. EAF size has enlarged up to 350 t maximum, which together with the shortening of tap-to-tap time, made possible to have more than 1 million tons per annum (Mtpa) capacity with just one furnace. Electric energy consumption with 100 % scrap operations has decreased to 350 kWh/t. Chemical energy increased at levels not far from those of BOS process. Refractory consumption has fallen down because of the replacement by cooled roof and panels, foaming slag, and refractory quality improvement. Power-off time is now less than 10 minutes (min) for the best operated furnaces. For a large number of EAFs, average electrode consumption is around 1.43 kilograms per ton (kg/t).
The present day UHP EAF has several sub-processes which have contributed to its remarkable productivity and efficiency. The phenomena which occur in the furnace include (i) heating of the scrap by the oxy-fuel burners, (ii) melting of the scrap by the electric arcs, (iii) liquid slag formation, (iv) O2 and C injection for foaming of the slag, (v) movement of the scrap pieces during melting, (vi) O2 lancing for scrap cutting and decarburization, (viii) chemical reaction between slag and metal, and (ix) post-combustion of the CO (carbon mono-oxide) evolved from the reactions. These phenomena are inter-related, so that changes in operational practices probably change more than one of these phenomena. This makes the prospect for process control very challenging.
The shift towards EAF steelmaking has been accompanied by technical advances which have allowed large decreases in power requirements and increased productivity. Some of the major technical advances are (i) foamy slag practice in which a foaming slag is used to ‘bury’ the arc and reduce refractory damage and heat loss from the arc region, (ii) hot heel operation in which liquid steel is left in the bottom of the furnace to assist in the melting of fresh solid feed entering the furnace, (iii) post combustion, in which CO generated during decarburization is burnt through O2 injection from lances inside the furnace and the energy liberated is used either heating the bath directly or preheating the incoming feed material, (iv) oxy-fuel burners in which auxiliary burners are used to improve melting rates and to provide more even heat distribution throughout the furnace, (v) preheating of scrap in which a number scrap preheating systems utilizing the heat associated with the exhaust gases have been developed, some based on batch bucket systems and others on continuous shaft systems, and (vi) furnace electrics where large improvements in control ·and energy efficiency have been accompanied the development of power supplies with higher operating voltages.
The increase in furnace electric power has been the key factor in the development of EAF technology. As in the 1960s, a common EAF power was 250 kVA/t of liquid steel (kVA/tLS) to 300 kVA/tLS, while today standard UHP EAFs have 900-1000 kVA/tLS available in the transformers. These furnaces are equipped with water-cooled panels and EBT tapping. EBT stands for eccentric bottom tapping, a system of tapping which yields a uniform steel jet falling into the ladle, with slag carry over controlled to a certain extent.
The present day EAF includes three water-cooled parts namely (i) roof, (ii) panels, and (iii) exhaust gas duct. Although some heat is lost due to the heat extraction by the cooling water, this design makes possible less refractory consumption (since they replace refractory linings) and the use of high power. At the time the panels were first introduced, some fears arose on safety risks, but after realizing the cost advantage, almost all EAFs adopted them. The panels can be made of steel or copper (much longer life) and with different designs (conventional, flip and turn etc.). Recently, more attention has been paid to safety with water cooling. Firstly, to detect, limit, and avoid the possibility of water leakage and secondly, to cut the need of repairing work in the hot furnace. Exhaust gas analysis, when H2 is included, is a useful tool to detect leakage. To limit leakage and maintenance work, solid cast or machined water panes have been introduced. Split shell, with spray-cooled upper shell means less risk as non-pressurized water tends to penetrate less in case of leakage.
Today, EAF process is an essential part of steelmaking. Melting is the main task of EAF process, and it is accomplished by energy supply which includes both electric power and chemical energy. The electric power is supplied by the graphite electrodes which is the main energy supplier. Chemical energy is supplied through several sources which mainly include oxy-fuel burners and O2 injection.
At first glance, the basic processes of the EAF appear quite straight forward consisting of simply providing enough electrical energy to heat and melt the steel scrap. However, the whole process takes place under conditions of extreme temperature, which complicate maintenance of the furnace, and the correction of any problems. For example, to preserve the refractory lining in the furnace, water cooling panels are built-in in the furnace walls. Without careful control, these can overheat and the process temperature has to be adjusted accordingly. During the process overall, the electrical input has to be balanced to make the best use of the power supply, which is controlled by the operator.
The electrical power is distributed among the three electrodes, which melt the scrap by creating an arc between themselves and the scrap. The electrodes are consumed during the process with a progressive wear by individual rate during power on. Some adjustments of the electrode positions need to be done to make sure that all three electrodes are in contact with the material so that energy is efficiently transferred.
The electrodes hold brittle properties (limited mechanical toughness) and are consumables with a high cost. Hence, the electrodes need to be handled carefully. If the furnace is filled with an excessive amount of coarse scrap, the electrodes need to be lowered very carefully in order to avoid expensive breakages. The electrodes progressively wear during heating and melting and measures have to be taken to avoid ‘short electrodes’. These are examples of some of the several events which can arise during the EAF process. Fig 2 shows schematics of EAF.
Fig 2 Schematics of EAF
Conventional EAFs today have 30 % to 50 % of their energy requirements met by the chemical energy, and it appears that the scrap melting is rather to be flexible in respect to from where the energy comes from. This is a distinct advantage for the EAF steelmaking for several reasons. This has led the equipment manufacturers to develop a number of new furnaces which have improved energy efficiency and / or flexibility.
The main raw material for EAF steelmaking is steel scrap. Today, the EAF is the most common way to recycle steel from scrap. There is a broad variety of steel scraps, both in terms of composition (from plain C steel through to highly alloyed tool steel) and geometry (from finely shredded sheet through to large beams). By melting the scrap in the EAF with the help of electrodes and an electrical current, new, functional steel can be produced from old products. Instead of deploying ore based raw material resources, basic steel elements and valuable alloy steels can be reused, which is beneficial from both an economic and environmental point of view.
Scrap is energy intensive and valuable commodity and comes primarily from three main sources namely (i) reclaimed scrap (also known as obsolete scrap) which is obtained from old cars, demolished buildings, discarded machinery and domestic objects, (ii) industrial scrap (also known as prompt scrap), and (iii) revert scrap (also known as internal or home scrap) which is generated within the steelmaking and forming processes (e.g. crop ends from rolling operations, and metallic losses in slag etc.). The latter two forms of scrap tend to be clean, i.e. they are close in chemical composition to the desired liquid steel composition and hence are ideal for recycle.
Reclaimed / obsolete scrap frequently has a quite variable composition and quite frequently contains contaminants which are undesirable for steelmaking. Levels of residual elements such as Cr, Mo, Cu (copper), Sn (tin), and Ni (nickel) are high in obsolete scrap and can affect casting operations and product quality if they are not diluted. Hence, a production shop which has a need for very low residual levels in the steel, is forced to use higher quality prompt scrap but at a much higher cost. The alternative is to use a combination of the contaminated obsolete scrap along with what are normally referred to as clean iron (Fe) units or virgin Fe units. These are materials which contain little or no residual elements. Clean Fe units are typically in the form of direct reduced iron (DRI), hot briquetted iron (HBI), iron carbide, liquid iron (hot metal), and pig iron. It is possible to use lower grade scrap which contains residual elements, if this scrap is blended with clean Fe units so that the resulting residual levels in the steel following melting meet the requirements for flat rolled products. Obsolete scrap is much more readily available than prompt scrap and hence the demand of clean Fe units is always there.
There are EAFs in which 100 % steel scrap is the most common charge. The situation is not the same in all the EAFs in which alternative materials like DRI, HBI, HM and pig iron take part in the charge. World-wide, scrap covers around 75 % of the metallics for EAFs, while DRI / HBI covers around 15 % and the balance is covered by HM and pig iron. It is well known that DRI melting consumes more electric energy than scrap. This has to do with the presence of acid gangue in the iron ore, which follows to DRI and is then to be neutralized by lime addition and increased amount of slag compared with conventional melting of scrap. A high percentage of metallization helps in keeping energy consumption under control.
Inherent to the coal-based direct reduction process is the low C content in DRI compared with gas-based DRI (0.1 % coal-based against around 2 % for gas-based DRI). C in the DRI is important since some of this C means an external energy source which combined with the O2 injection in the EAF promotes a natural slag foaming by the CO produced by the reaction of unreduced iron oxide and C in the DRI, which helps to decrease the electric energy consumption. In the EAFs where the charge consists of 100 % DRI / HBI, the charging is done through the fifth hole. Charging of high proportion of HM or pig iron in the EAF is practiced in some of the EAFs.
In addition to classification of scrap into the above three groups, scrap is also classified based on its physical size, its source, and the way in which it is prepared. For example the categories normally used are No. 1 bundles, No. 1 factory bundles, No. 1 shredded, No. 1 heavy melt, No. 2 heavy melt, No. 2 bundles, No. 2 shredded, busheling, turnings, shredded auto, structural / plate 1 meter (m), structural / plate 1.5 m., rail crops, and rail wheels.
In addition to the residual elements contained in the scrap, there are also several other undesirable components including, oil, grease, paint coatings, zinc coatings, water, oxidized material, and dirt. The lower the grade of scrap, the more likely it is to contain higher quantities of these materials. As a result this scrap can be cheaper but the yield of liquid steel can be considerably lower than that achieved when using a higher grade scrap. In addition, these undesirable components can result in higher energy requirements and environmental problems. Hence the decision for scrap mix to be used within a particular production shop frequently depend on several factors including availability, scrap cost, melting cost, yield, and the effect on operations (based on scrap density, oil and grease content, etc.). In practice, majority of the production shops buy several different types of scrap and blend them to get the most desirable effects for EAF operations.
C is necessary for the production of steel. C is one of the key elements which give different steel grades their properties. C is also important in steelmaking refining operations and can contribute a sizable quantity of the energy required during the steelmaking process. In BOS process, C is present in the HM which is produced in the blast furnace (BF). In the EAF steelmaking, some C is available in the scrap feed, in DRI, HBI or other alternative iron furnace feeds. The quantity of C contained in these EAF feeds is normally considerably lower than that contained in the HM and hence, some additional C is needed to be charged to the EAF.
In the past, C was charged to the EAF to ensure that the melt-in C level was above the level desired in the final product. As higher O2 utilization has developed as standard EAF practice, more C is needed in EAF steelmaking process. The reaction of C with O2 within the bath to produce CO results in a considerable energy input to the process and has led to substantial reductions in electrical power consumption. The generation of CO within the bath is also a key to achieving low concentrations of dissolved gases (N2 and H2) in the steel as these are flushed out with the CO. In addition, oxide inclusions are flushed from the steel into the slag.
In O2 injection operations, some Fe is oxidized to FeO (ferrous oxide) and goes to the slag. Oxy-fuel burner operations also result in some scrap oxidation and this also goes to the slag once the scrap melts in. Dissolved C in the steel reacts with FeO at the slag / bath interface to produce CO and recover Fe units to the bath.
The quantity of charge C needed is dependent on several factors which include (i) C content of scrap feed, (ii) projected O2 consumption, (iii) desired C at the tapping, and (iv) the economics of Fe yield compared with C cost. In general, the quantity needed corresponds to a C / O2 balance as the furnace operator try to maximize the Fe yield. Typical charge C rates for medium C steel production lie in the range of 2 kilograms per ton of liquid steel (kg/tLS) to 12 kg/tLS.
The three types of carbonaceous materials which are normally used as charge C are (i) anthracite coal, (ii) metallurgical coke, and (iii) green petroleum coke. Anthracite coal typically has moisture content of 3 % to 8 %, ash content of 11 % to 18%, and sulphur (S) content of 0.4 % to 0.7 %. The high variation in ash content translates into wide variations in fixed C content and hence, for use in EAF the ash content is to be kept to a minimum. The ash consists primarily of silica (SiO2). Hence increased ash input to the EAF needs additional lime (CaO) addition in order to maintain the desired basicity ratio in the slag. The best grades of anthracite coal have fixed C contents of 87 % to 89 %. Low grade anthracite coals can have fixed C levels as low as 50 %. Anthracite coal is available in a wide variety of sizes ranging from 100 mm – 200 mm down to 1.2 mm – 100 mesh. The most popular sizes for use as charge C are of nut size (20 mm to 40 mm), pea size (15 mm to 20 mm), and buckwheat size (8 mm to 15 mm).
Metallurgical coke is produced primarily in integrated steel plants and is used in the BF. However, some coke is used as charge C in EAF. Normally, metallurgical coke has a composition consisting of moisture content 1 % to 2 %, volatile matter 1 % to 2 %, ash content 9 % to 12 %, fixed C 86 % to 88 %, and S content 0.88 % to 1.2 %. Normally coke breeze with a size of minus 10 mm is used as charge C. Coarser material can be used but has higher cost.
Green petroleum coke is a byproduct of crude oil processing. Its properties and composition vary considerably and are dependent on the crude oil feedstock from which it is derived. Several coking processes are used in commercial operation and these produce considerably different types of petroleum coke.
Sponge coke results from delayed coking operations and is porous in nature. It can be used as a fuel or can be processed into electrodes or anodes depending on the S content and impurity levels. This material is sometimes available as charge C.
Needle coke is produced using a special application of the delayed coking process. It is made from high grade feedstocks and is the prime ingredient for the production of C and graphite electrodes. This material is normally very expensive to be used as charge C. Shot coke is a hard, pebble-like material resulting from delayed coking operation under conditions which minimize coke byproduct generation. It is normally used as a fuel and is cost competitive as charge C. Fluid coke is produced in a fluid coking process by spraying the residue onto hot coke particles. It is normally high in S content and is used in anode baking furnaces. It can also be used as a recarburizer if it is calcined.
Operation of EAF
The EAF is a batch melting process which produces batches of liquid steel (LS) known ‘heats. The tap-to-tap cycle of the EAF is made up of several steps consisting of (i) furnace charging, (ii) melting, (iii) refining, (iv) de-slagging, (v) tapping, and (vi) furnace turn-around. The present day EAFs aim for a tap-to-tap time of less than 60 minutes (min). Some EAFs with twin shells are achieving tap-to-tap times of 35 min to 40 min.
Today, a large number of EAFs operate with the condition called ‘hot heel’, meaning that almost 15 % to 20 % of metal from the previous heat has remained at the bottom of furnace after tapping. Hot heel increases productivity by allowing use of high electric power since it decreases the possibility of damaging refractory by powerful arcs, and it also affects the melting rate of the scrap.
Furnace charging – The first step in the production of any heat is to select the grade of steel to be made. Normally a schedule is developed prior to each production shift. Hence, the operators (normally called melters) know in advance the schedule for their shift. The scrap yard operator prepares buckets of scrap according to the needs of the EAF operator. Preparation of the charge bucket is an important operation, not only to ensure proper melt-in chemistry but also to ensure good melting conditions. The scrap is to be layered in the bucket according to size and density to promote the rapid formation of a liquid pool of steel in the hearth while providing protection for the sidewalls and roof from electric arc radiation.
Other considerations 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 results in the blow-back of the flame onto the water cooled panels. The charge can include lime and C or these can be injected into the furnace during the heat. Many operators add some lime and C in the scrap bucket and supplement this with injection.
The first step in any tap-to-tap cycle is ‘charging’ of the scrap in the EAF. The roof and electrodes are raised and are swung 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 bucket bottom is normally of a clam shell design, i.e. the bucket opens up by retracting two segments on the bottom of the bucket. The scrap falls into the furnace and the scrap crane removes the scrap bucket. The roof and electrodes swing back into place over the furnace. The roof is lowered and then the electrodes are lowered to strike an arc on the scrap. This commences the melting portion of the cycle. The number of charge buckets of scrap needed to produce a heat of steel is dependent primarily on the volume of the furnace and the scrap density.
Present day EAFs are designed to operate with a minimum of back-charges. This is advantageous since charging is a dead-time where the EAF does not have power on and hence is not melting. Minimizing of these dead-times helps to maximize the productivity of the EAF. In addition, energy is lost every time the furnace roof is opened. For each occurrence, the energy loss is between 10 kWh/t and 20 kWh/ton. Majority of the EAF operators aim for 2 buckets to 3 buckets of scrap per heat and they attempt to blend the scrap for meeting this requirement. Some EAF operations achieve a single bucket charge. Continuous charging operations such as ‘CONSTEEL’ and the ‘Fuchs shaft furnace’ eliminate the charging cycle.
Melting – The melting period is the heart of EAF operations. The EAF has evolved into highly efficient melting equipment and the present day designs are focused on maximizing the melting capacity of the EAF. The heat needed to melt steel scrap is provided by electric arcs, created between the electrodes and scrap in the furnace. The electrical power of normal EAFs lies in the range of 50 MW to 120 MW, depending on the size of the furnace. Melting of the scrap occurs at a temperature range of 1,500 deg C to 1,550 deg C, depending upon the composition of the steel scrap.
Melting is accomplished by supplying energy to the furnace interior. This energy can be electrical or chemical. Electrical energy is supplied through the graphite electrodes and is normally the largest contributor in melting operations. Initially, an intermediate voltage tap is selected until the electrodes bore into the scrap. For accelerating the bore-in, normally light scrap is placed on top of the charge. Around 15 % of the scrap charged is melted during the initial bore-in period.
After a few minutes, the electrodes have penetrated the scrap sufficiently so that a 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 is formed in the furnace hearth. At the start of melting the arc is erratic and unstable. Wide swings in current are observed accompanied by rapid movement of the electrodes. As the furnace atmosphere heats up, the arc stabilizes and once the liquid pool is formed, the arc becomes quite stable and the average power input increases.
Although UHP furnaces are used, fast melting by using only electric power is difficult and not the most economic practice either. Importing extra energy and assisting melting technique can greatly accelerate scrap melting and bring economic benefits. Accordingly, the present state of the art in EAF steelmaking is to use as much as possible chemical energy, besides electric energy, to accommodate tap-to-tap times to the pace of the downstream continuous casting process.
Chemical energy is to be supplied through several sources including oxy-fuel burners and O2 lances. In the oxy-fuel burners fuel gas is burned using O2 or a blend of O2 and air. Heat is transferred to the scrap by flame radiation and convection by the hot products of combustion. Heat is transferred within the scrap by conduction. Large pieces of scrap take longer to melt into the bath than smaller pieces. In some of the EAFs, O2 is injected through a consumable pipe lance to ‘cut’ the scrap. The O2 reacts with the hot scrap and burns Fe to produce intense heat for cutting the scrap. Once a liquid pool of steel is generated in the furnace, O2 can be lanced directly into the bath. This O2 reacts with several components in the bath which include C, Fe, Al (aluminum), Si (silicon), Mn (manganese), and P (phosphorus). All of these reactions are exothermic (i.e. they generate heat) and supply additional energy to aid in the melting of the scrap. The metallic oxides which are formed end up in the slag. The reaction of O2 with C in the bath produces CO, which either burns in the furnace if there is sufficient O2, and / or is exhausted through the direct evacuation system where it is burned and conveyed to the pollution control system.
Once enough scrap has been melted to accommodate the second charge, the charging process is repeated. Once the final scrap charge is melted, the furnace sidewalls are exposed to intense radiation from the arc. As a result, the voltage is to be reduced. Alternatively, creation of a foamy slag allows the arc to be buried and protect the furnace shell. In addition, a higher quantity of energy is to be retained in the slag and is transferred to the bath resulting in higher energy efficiency.
One of the most perplexing aspects about scrap is the way it moves during the melting process, and this causes operational problems, such as ‘cave-ins’, in which the scrap suddenly collapses onto the electrode. These events disrupt the electrical operation, necessitating the raising of the electrodes, or in extreme cases, precipitating breakage of electrodes. Further, the solid scrap is introduced into a liquid steel pool when the charge is dropped into the liquid heel, and also when scrap starts to ’cave in’ to the pool which is formed by the arcs during the melting process. EAF operators frequently talk about icebergs or ’steelbergs’ that form by the bridging of several pieces of scrap, which can take longer to melt than the individual pieces. However, it is difficult to understand the contributing factors to this problem.
Once the final scrap charge is fully melted, flat bath conditions are reached. At this point, a bath temperature and sample is taken. The analysis of the bath chemistry allows the operator to determine the quantity of O2 to be blown during the refining. At this point, the operator can also start to arrange for the tap bulk ferro-alloy additions to be made. These quantities are finalized after the refining period.
Refining – After the scrap has been melted, the temperature is normally increased so that refining reactions can be carried out. O2 and C can be injected into the steel phase and slag phase respectively. However, the reactions can also create products which are detrimental to the steel quality and which therefore need to be handled carefully. To do that, a slag is formed with the help of slag forming agents, such as lime, calcined dolomite and fluorspar. Slag, having a lower density than steel, normally floats on the steel surface. In addition to absorbing impurities from the steel, the slag also protects the steel from the atmosphere. In addition, it protects the furnace walls from the arcs, thereby increasing the electrical efficiency. It is hence of great importance to maintain a high slag quality and provide it with foaming properties.
Slag properties like viscosity, S capacity, and P capacity etc. vary with composition and temperature. One of the main tasks during the refining operation is to maintain adequate slag properties by adding slag forming agents such as lime, calcined dolomite and / or fluorspar. Some of the metallic oxides which end up in the slag are acidic. Hence adding basic slag forming agents helps to keep the basicity of the slag at an appropriate level. High slag basicity (i.e. high CaO / SiO2 ratio) is also beneficial for the removal of P but care is needed not to saturate the slag with CaO as this leads to an increase in slag viscosity, which makes the slag less effective.
Refining operations in the EAF have traditionally involved the removal of P, S, Al, Si, Mn, and C from the steel. In recent times, dissolved gases, especially H2 and N2, have been recognized as a concern. Traditionally, refining operations are carried out following meltdown i.e. once a flat bath is achieved. These refining reactions are all dependent on the availability of O2. O2 is lanced at the end of meltdown to lower the bath C content to the desired level for tapping. Majority of the compounds which are to be removed during refining have a higher affinity for O2 than that the C. Hence the O2 preferentially reacts with these elements to form oxides which float out of the steel and into the slag.
Decarburization is carried out as a refining task. If the charge is 100 % scrap, C in the metallic charge is relatively low. This is not the case when charging a high percentage of DRI (if produced in a gas-based unit), HBI, HM, or pig iron, with C contents varying between 1.6 % and 4.5 %.Normally, due to the productivity reasons, EAF operators prefer to achieve a C content of around 0.05 % before tapping. Lower contents can imply a too high oxidation level. An alternative strategy is ‘catch-carbon’ practice in which C content is targeted at somewhat under the final value, to be reached with ferro-alloys addition. This practice can bring material savings in alloying and deoxidation, but needs strict individual process control for each heat. For this reason, it is not so good for productivity. Further, if final C is maintained high, dephosphorization can be problematic.
In the present day EAFs, especially those operating with a ‘hot heel’ of liquid steel and slag retained from the previous heat, O2 can be blown into the bath throughout most of the heat. As a result, some of the melting and the refining operations occur simultaneously. P and S normally occur in the EAF charge in higher concentrations than are normally permitted in steel and are to be removed. Unfortunately the conditions favourable for removing P are the opposite of those promoting the removal of S. Hence, once these materials are pushed into the slag phase, they can revert back into the steel.
P 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, the P reverts from the slag back into the bath. P removal is normally carried out as early as possible in the heat. Hot heel practice is very beneficial for the removal of P since O2 can be lanced into the bath while its temperature is quite low. Early in the heat, the slag contains high FeO levels carried over from the previous heat hence aiding in the removal of P. High slag basicity (i.e. high CaO content) is also beneficial for the P removal but care is to be taken not to saturate the slag with CaO which leads to an increase in slag viscosity, and hence makes the slag less effective.
Sometimes fluorspar is added to help fluidize the slag. Stirring the bath with inert gas is also beneficial since it renews the slag / metal interface hence improving the reaction kinetics. In general, if low P levels are a requirement for a particular steel grade, the scrap is selected to give a low level at melt-in. The partition of P in the slag to P in the bath ranges from 5 to 15. Normally P is reduced by 20 % to 50 % in the EAF.
S is removed mainly as a sulphide dissolved in the slag. The S partition between the slag and metal is dependent on slag chemistry and is favoured at low steel oxidation levels. Removal of S in the EAF is difficult especially in the present day practices where the oxidation level of the bath is quite high. Normally the partition ratio is between 3 and 5 for EAF operations. Majority of EAF operators find it more effective to carry out desulphurization during the reducing stage of the steelmaking. This means that desulphurization is performed during tapping (where a calcium aluminate slag is built) and during ladle furnace operations. For reducing conditions where the bath has a much lower O2 activity, distribution ratios for S between 20 and 100 can be achieved.
Control of the metallic constituents in the bath is important since it determines the properties of the final product. Normally, the operator aims at lower levels in the bath than are specified for the final product. O2 reacts with Al, Si, and Mn to form metallic oxides, which are slag components. These metallics tend to react with O2 before the C. They also react with FeO resulting in a recovery of Fe units to the bath. For example Mn + FeO = MnO + Fe. Mn is typically lowered to around 0.06 % in the bath.
The reaction of C with O2 in the bath to produce CO is important since it supplies a less expensive form of energy to the bath, and performs several important refining reactions. In the present day EAF operations, the combination of O2 with C can supply between 30 % and 40 % of the net heat input to the furnace. Evolution of CO is very important for slag foaming. Coupled with a basic slag, CO bubbles are tapped in the slag causing it to ‘foam’ and helping to bury the arc. This gives greatly improved thermal efficiency and allows the furnace to operate at high arc voltages even after a flat bath has been achieved. Burying the arc also helps to prevent N2 from being exposed to the arc where it can dissociate and enter into the steel.
If the CO is evolved within the steel bath, it helps to strip N2 and H2 from the steel. N2 levels in steel as low as 50 ppm (parts per million) can be achieved in the furnace prior to tapping. Bottom tapping is beneficial for maintaining low N2 levels since tapping is fast and a tight tap stream is maintained. A high O2 potential in the steel is beneficial for low N2 levels and the heat is to be tapped open as opposed to blocking the heat.
At 1,600 C, the maximum solubility of N2 in pure Fe is 450 ppm. Typically, the N2 levels in the steel following tapping are ranging from 80 ppm to 100 ppm. Decarburization is also beneficial for the removal of H2. It has been demonstrated that decarburizing at a rate of 1 % per hour can lower H2 levels in the steel from 8 ppm down to 2 ppm in 10 minutes. At the end of refining, a bath temperature measurement and a bath sample are taken. If the temperature is too low, power can be applied to the bath. This is not a big concern in the present day steel melting shops where temperature adjustment is carried out in the ladle furnace.
For most steelmaking operations, refining in the EAF is limited to dephosphorization, decarburization, and temperature adjustment. P in the charge can be at higher than usual levels for DRI (direct reduced iron), HBI (hot briquetted iron), pig iron, and HM, depending on the iron ore source. Some steels require particularly low P levels to avoid too high ductile / brittle transition temperature or tempering brittleness. From the equilibrium point of view, lower temperatures, high slag basicity, and high oxidation of the bath favour dephosphorization. Slag / metal interaction is important from a kinetic point of view but is not always attainable in the EAF.
Reversion of P from slag to steel can take place when heating to the aimed temperature, close to the end of the process. Then, if there is some slag carry over to the ladle, and steel and slag have a low oxidation level, P reversion is again possible. Hence, in such P critical cases slag carry over to the ladle is to be carefully prevented. This can be done by slag detection-slag stopping system or by EBT (eccentric bottom tapping), which is stopped before the end of the steel. In such a ‘hot heel’ practice, a considerable fraction of liquid steel is left in the furnace for the next heat. Hot heel practice not only prevents steel from rephosphorization, but also speeds up the melting process. However, it has a smaller charge weight in the ladle as a drawback.
The conditions favourable for removing P are the opposite of those promoting the removal of S. Hence, even after these elements have been transferred into the slag phase, they can revert back into the steel. Retention of P in the slag depends on the temperature and O2 activity of the liquid steel and on the basicity and FeO content of the slag. At higher temperatures or lower FeO levels, the P reverts from the slag back into the liquid steel. Hence, P removal is normally carried out as early as possible in the heat when the temperature is low. In order to remove S from the liquid steel, it is necessary to use a sulphide-forming agent such as a Ca (calcium) compound. The sulphide-forming reactions are promoted in a reducing atmosphere, at a low O2 level, high slag mass and high temperature. All of these are normally achieved later in the heat.
The refining step normally does not need full power, which with already flat bath can be dangerous for the lining. At that time, the foaming of the slag is a must. For the slag to foam, the production of CO gas is necessary, by means of the injection of C and O2 through lances or burners. For foaming purposes, several carbonaceous materials are useful, depending on local cost and availability these are anthracite, petroleum coke, and coke breeze.
In the EAFs, the melt can flow due to mechanisms taking place during the melting process, but thermal stratification does not disappear, so it takes normally a long time for large pieces of scrap to be melted. Concentration gradient in the bath has also been reported which causes the reaction rates in the bath to decrease and also the reactions do not take place uniformly in all parts of the bath. Thermal stratification is a more common problem in UHP furnaces where there is a high superheat. Hence, it is important that the melt dripping down from the top is convected to reduce stratification.
The idea of stirring or forced convection in the bath has been introduced some years ago to improve the melt flow and eventually decrease the energy consumption. The first electromagnetic coils were used in a laboratory scale in 1933, and the studies showed that the flow velocities for stainless steels are lower than the C steels when electromagnetic stirring was used.
It has been shown that stirring is more important in production of low C steels since when the content of C goes down to below 0.2 %, decarburization due to O2 blowing is not that effective. Hence, a powerful driving force is needed to transfer C to the reaction zone, where O2 is blowing. In forced convection; an external driving force is used to make the fluid flow, and the magnitude of velocity and rate of heat transfer increase in comparison to the natural convection.
CO bubbles, formed during formation of the foaming slag, contribute to bath circulation, but an additional amount of gas can be injected into the bath to increase the intensity of bath stirring. Inert gas, N2, Ar (argon), or mixture of both can be injected through tuyeres or porous plugs submerged within the bottom refractory of the furnace. The number of tuyeres or plugs used is dependent on their size as well as the size of the furnace. The blowing is normally done with the low intensity of 0.1 cum/ton.min.
De-slagging – De-slagging operations are carried out to remove impurities from the furnace. During the melting stage and refining operation, some of the undesirable materials within the bath are oxidized and enter the slag phase. It is advantageous to remove as much P into the slag as early in the heat as possible (i.e. while the bath temperature is still low). The furnace is tilted backwards and slag is poured out of the furnace through the slag door. Removal of the slag eliminates the possibility of P reversion.
During slag foaming operations, C can be injected into the slag where it reduces FeO to metallic Fe and in the process produce CO which helps to foam the slag. If the high P slag has not been removed prior to this operation, P reversion occurs. During slag foaming, slag can overflow the sill level in the EAF and flow out of the slag door. Tab 1 shows the typical constituents of the EAF slag.
|Tab 1 Typical constituents of EAF slag|
|Component||Source||Composition range, %|
|CaO||Charged lime||40- 60|
|CaF2||Charged fluorspar – slag fluidizer|
|MnO||Oxidation product||2 -5|
|S||Absorbed from steel|
Tapping – Once the desired steel composition and temperature are achieved in the furnace, the tap-hole is opened for tapping the heat. Tapping can either be through a spout, or through a tap hole positioned at the bottom of the furnace. In case of spout tapping, the furnace is tilted, and the steel pours into a ladle for transfer to the next batch operation (normally a ladle furnace). During the tapping process, bulk alloy additions are made based on the bath analysis and the desired steel grade. De-oxidizers can be added to the steel to lower the O2 content prior to further processing. This is normally referred to as ‘blocking the heat’ or ‘killing the steel’. Common de-oxidizers are Si in the form of ferrosilicon or silico-manganese and Al. Majority of the C steel operations aim for minimal slag carry-over. A new slag cover is ‘built’ during tapping. For ladle furnace operations, a calcium aluminate slag is a good choice for the S control. Slag forming compounds are added in the ladle at tapping so that a slag cover is formed prior to transfer to the ladle furnace. Additional slag materials can be added at the ladle furnace if the slag cover is insufficient.
Furnace turn-around – Furnace turn-around is the period following completion of tapping until the furnace is recharged for the next heat. During this period, the electrodes and roof are raised and the furnace lining is inspected for refractory damage. If necessary, repairs are made to the hearth, slag-line, tap-hole, and spout. In the case of a bottom-tapping furnace, the tap hole is filled with sand.
Repairs to the EAF are made using guniting refractories or mud slingers. In the majority of the present day EAFs, the increased use of the water-cooled panels has reduced the quantity of patching or ‘fettling’ needed between heats. Several operators now switch out the furnace bottom on a regular basis (2 weeks to 6 weeks) and perform the hearth maintenance off-line. This reduces the power-off time for the EAF and maximizes furnace productivity. Furnace turn-around time is normally the largest dead time (i.e. power off) period in the tap-to-tap cycle. With advances in furnace practices this has been reduced from 20 minutes to less than 5 minutes in some newer installations.
Chemical reactions in EAF
During and after the melt down of the materials, several compounds and elements begin to react with each other. In order to reach the required quantities of the respective elements in the steel, the reaction processes are to be facilitated by injection of C and O2 through the lance. By injecting C into the steel phase and O2 into the slag phase CO bubbles are formed and a foaming slag is created.
One of the oxide reactions forms CO gas, which is especially important for the formation of a foaming slag. C injection into the slag phase through the lance creates CO bubbles through a reaction with O2. These bubbles then help to cause the slag to ‘foam’. A foaming slag protects the liquid steel from reacting with the atmosphere and also increases the electrical efficiency by burying the arc from the electrodes. This provides thermal efficiency and allows the furnace to operate at higher voltages without damaging the furnace walls and roof. Burying the arc also helps to prevent N2 from being exposed to the arc where it can dissociate and enter into the steel.
Due to the high affinity of O2 to the elements such as Al, Si, Cr, C, P, and Fe, metallic oxides are easily formed and because of their relatively low density these float up to the slag phase. All of these reactions are exothermic, i.e. they provide heat to the system and hence supply additional energy to melting and heating the scrap. The thermodynamic and kinetic of the process assumes that the reaction which provides the lowest dissolved O2 content controls the dissolved O2 activity in the steel. The reactions are highly exothermic are (i) 2 Al + 3O = Al2O3, (ii) C + O = CO (g), (iii) 2 Cr + 3O = Cr2O3, (iv) Fe + O = FeO, and (v) Si + 2O = SiO2.
There are several chemical reactions which are taking place during the steelmaking in the EAF. These reactions are between charge materials and elements and substances added to a melt in order to prepare steel of a required composition in the EAF. These chemical reactions generate a lot of chemical energy which can also improve the efficiency and reduce the time of EAF process. In EAFs, chemical reactions can affect the temperature of steel which is one of the most important factors for the steelmaking. The chemical reactions taking place in the EAF can be grouped into four categories namely (i) chemical reaction of scrap components, (ii) chemical reactions proceeding on adding fluxes, (iii) chemical reactions during deoxidation, and (iv) chemical reactions with alloying additions.
Chemical reaction of scrap components – The chemical reaction of scrap components are (i) 2[Fe] + 3[O] = (Fe2O3), (ii) [Fe] + [O] = (FeO), (iii) [C] + [O] = CO, (iv) [Si] + 2[O] = (SiO2), (v) [Mn] + [O] = (MnO), (vi) 2[P] + 5[O] = (P2O5), and (vii) 2[Cr] + 3[O] = (Cr2O3).
Chemical reactions proceeding on adding fluxes – The chemical reactions proceeding on adding fluxes are (i) (CaO) + [S] = (CaS) + [O], (ii) (CaCO3) = (CaO) + CO2, (iii) (MgCO3) = (MgO) + CO2, and (iv) 2[Al] + 3[O] = (Al2O3).
Chemical reactions during deoxidation – The chemical reactions during deoxidation are (i) [Mn] + [O] = (MnO), (ii) 2[Al] + 3[O] = (Al2O3), (iii) [Si] + 2[O] = (SiO2), (iv) 2[Fe] + 3[O] = (Fe2O3), (v) [Fe] + [O] = (FeO), and (vi) [C] + [O] = CO.
Chemical reactions with alloying additions – The chemical reactions with alloying additions are (i) [Mn] + [O] = (MnO), (ii) 2[Fe] + 3[O] = (Fe2O3), (iii) [Fe] + [O] = (FeO), (iv) [Si] + 2[O] = (SiO2), (v) [C] + [O] = CO, (vi) 2[P] + 5[O] = (P2O5), (vii) (CaO) + [S] = (CaS) + [O], (viii) 2[Al] + 3[O] = (Al2O3), (ix) 2[Cr] + 3[O] = (Cr2O3), (x) [Zn] + [O] = [ZnO], and (xi) 2[B] + 3[O] = (B2O3).
Chemical energy is very important for melting steel in an EAF. Energy of exothermic reactions is a secondary energy source in an EAF. However, these forms of energy are basic in an EAF. Electrical energy comprises of the order of around 60 % of the overall expenditure and chemical energy of the order of around 30 % of energy required in an EAF. The balance energy comes from the input through oxy-fuel burners.
Chemical energy is introduced by O2, carbonaceous materials, and fuel gas, more and more through injectors rather than lances. The energy generating reactions are (i) C+ 1/2O2 = CO (2.55 kWh/kg C), (ii) C + O2 = CO2 (9.1 kWh/kg C), (iii) CO + 1/2O2 = CO2 (2.81 kWh/kg C), and (iv) Fe + 1/2O2 = FeO (1.32 kWh/kg Fe).
Certain grades of steel, such as those used for gas and oil pipelines need very low levels of S to provide better welding and forming properties. Desulphurization is driven by an exchange of S between the liquid steel and the slag. The reactions which take place are governed by the dissolved content of Al and S in the steel and the content of CaO, Al2O3, and calcium sulphide (CaS) in the slag. Normally this is described by the reaction 3(CaO) + 2[Al] + 3[S] = 3(CaS) + (Al2O3). In practice, desulphurization in the EAF is achieved by (i) adding a synthetic CaO based desulphurizing slag at furnace tapping, and (ii) Al deoxidizing the steel to very low O2 activity (otherwise the Al reacts preferentially with O2).
Role of slag
An important part of steelmaking is the formation of slag, which floats on the surface of the liquid steel. Slag normally consists of metal oxides, and acts as a destination for oxidized impurities. Protection of cooling panels and refractories, arc stability, dephosphorization, thermal insulation, and heat transfer are some of the expected tasks for the slag. The main slag formers in the EAF are lime, calcined dolomitic, oxides in scrap, gangue in DRI, oxidation products from metallic charge, ash of carbonaceous additions, MgO picked-up from refractories as well as the slag remaining from the previous heat. Some plants recycle scale or other wastes, hence adding to the slag. The main EAF slag components are CaO, SiO2, FeO, and MgO and other components are Al2O3, MnO, and P2O5.
Majority of EAFs use basic refractories and as a result, it is necessary to maintain a basic slag in the furnace in order to minimize the refractory consumption. Slag basicity has also been shown to have a major effect on slag foaming capabilities. Hence, lime is to be added both in the charge and also through injection directly into the furnace. Lime addition practices can vary greatly due to the variances in scrap composition. As elements in the bath are oxidized (e.g. P, Al, Si, Mn) they contribute acidic components to the slag. Hence basic slag components are to be added to offset these acidic contributions. If silica levels in the slag are allowed to get too high, considerable refractory erosion results. In addition, FeO levels in the slag increase since FeO has greater solubility in higher SiO2 slags. This can lead to higher yield losses in the EAF.
The generation of slag also allows these materials which have been stripped from the bath to 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 deslagging or slagging off. If the slag is not removed but is instead allowed to carry over to the ladle it is possible for slag reversion to take place. This occurs when metallic oxides are reduced out of the slag by a more reactive metallic present in the steel. When steel is tapped, it is frequently killed by adding either Si or Al during tapping. The purpose of these additions is to lower the O2 content in the steel. If however P2O5 is carried over into the ladle, it is possible that it reacts with the alloy additions producing SiO2, or Al2O3 and P which goes 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 predominantly MgO, hence by adding a small quantity of MgO to the furnace, the slag can quickly become saturated with MgO and hence less refractory erosion is likely to take place.
In the ternary diagram of Fig 3, including MgO saturation lines, it is possible to evaluate MgO needed for saturation of a slag of a given basicity and FeO content. Control of foaming slag remains a matter of continuous development. Majority of EAFs incorporate tools to evaluate slag foaming, based on noise, vibration, and harmonics. In majority of cases, this is used as an off-line help for the furnace operator. But some EAFs have recently introduced online control of the C and O2 injection for slag forming, based on the mentioned measurements.
Fig 3 Isothermal cross section of CaO-SiO2-FetO diagram
A mass balance, based on the chemistry and weight of those slag formers, is helpful for the comparison with measured chemistry, for a better understanding of attack on refractory linings, lime fines lost to the exhaust gas, incidence of changes in weight, chemistry of additions, etc. For foaming purposes it is important to maintain slag basicity (% CaO / % SiO2) between 1.2 and 2.5. To decrease the need of MgO for slag saturation, slag basicity is to be at least 2.
At present, The EAF electric arc furnace has become a very complex process, with oxy-fuel burners, C and O2 lances, post-combustion injectors, foamy slag practices and long arc operation. These developments have reduced the cost of production and increased productivity. Because of the complexity, it is difficult for the operators to know how these various process features interact with each other and to know how to optimize them.