Steel Making Fundamentals and Applications

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~A r c e l o r M i t t a l

STE LMAKINGFUNDAMENTALS AND APLICATION

ArcelorMittal Point LisasLong Carbon Steel

.ArcelorMittal Training Advantage Programme

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SUMMARY

ELETRIC ARC FURNACE1. STEELMAKING MAIN ROUTES ........................................................................................ .............................................2. INTRODUCTIONTO THE PROCESS 73. MAINEQUIPMENT ......................................................................................................... ................................. 94. ELECTRICAL CIRCUITOFAN ELECTRICARC FURNACE......................................................................................85 INPUTSAND RAW MATERIALS ..................................... .............................................................................. 496. STEEL MANUFACTURINGSTEPS ..........................................................................................................................57. ELECTRICARC FURNACETHERMAL BALANCE 728. REFININGMECHANISMS REACTIONSINTERFACES) 759. REFININGREACTIONS ................................................................................................. ..............................................5

SECONDARY REFINING

1. INTRODUCTION 912. CHEMICAL HEATINGPROCESS .................................................................................................................. ...............63. LADLE FURNACE .......................................... ............................................................................. ............. 1 44. VACUUM PROCESS ..................................................................................... ........................................................ 85 REMELTINGAND PROGRESSIVESOLIDIFICATIONPROCESSES ........................................................... 1276. ANALYSIS OF TECHNIQUES USED FOR SECONDARY REFINING 12961INTRODUCTION ................. .................................... .............................................................. 12962 OXIDATION LEVEL CONTROL ..................... .............................................................. 136 3 CHEMICAL COMPOSITION ADJUSTMENT ........ ..................................................... 1316 4METALLIC BATH HEATING ...................................................................................... 1377. STEEL CLEANLINESS CONTROL.......................................................................................... .................... 148. CHARACHTERISTICSOF THESLAG USED INSECONDARYREFINING 1469. METALIC BATH AGITATION 1491 DESULPHURIZATION AND DEPHOSPHORIZATION ..................................................... ...... 15411VACUUMMETALLURGY ............................................................................. .............................................................55

THE CONTINUOUS CASTING

3. THE CONTINUOUSCASTING 1653.1. INTRODUCTION ..................................................................................................................... 1673.2. MCONTINUOUSCASTINGMACHINES ....................................................................................................... 16933 THE TUNDISH .......................................................... ..................................................... ..........................................753.4. THEMOULD ........................................................................................................................................... 1893.5.THE ROLLS ...................................................................... ........................................................................................964. ECASTINGMOULD LUBRICANTS ........................................................................................................ 2034 1FUNCTIONS .............................................. .............................................................................................................03


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1.STEELMAKING MAIN ROUTESFigure 2.3 presents a flowchart of the steel refining staeps to illustrate the positioning of theprimary refining processes in the steel mills.

Th e r m o el ed r icOKYUenConverter or furnaces Foundry

Torpedo car wtth hotmetal

C o n v e n ti o na lC ast ing

steel Rolling or ForgingScrap

Flux(/'J-~ t.Jr .A,,~~~.

Sponge iron Electrical ArcFurnaee(EAF) Block s B il lets Shapes

C on t i n u o u s Ca s t in gSlag (eo.product) Seco n da r y R ef in in g P r oc ess

Figure 2.3 - Schematic flowchart of the steel refining step in steelmills.

2. INTRODUCTION TO THE PROCESSThe forms of converting electric energy into heat which are of interest to the processes ofmaking steel are:- through heat transfer radiation from an electric arc generated by the flow of current through anionized gas; , '

- through the resistance to the flow of electrical current in a solid conductor;- electromagnetic induction.The first two methods promote surface heating, i.e., from inside out instead of internal heating,from outside in, provoked by the last method. The kind of furnace described next is the directelectric arc and uses the resistance to electrical current of a solid conductor as heating source.Electric arc, also named voltaic arc is generated by the flow of electric current through ionizedair (plasma) which separates the two points between which the arc is formed. In this case, one

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of the points is the electrode and the second point is the material being heated, which should ~necessarily be an electrical conductor. Electric arc can jump between these two spots. Thus, the,~heat transfer is done directly to the load. Electric current also flows through the metallic load, 'generating additional heating due to its inherent resistance during its flow through the conductor.This heating is not as great as the one generated by the voltaic arc. The arc temperature ~~corresponds to the boiling point of the material that makes the electrode. In the case of carbon ~electrodes this temperature is 4,19JOC (one can estimate that temperature inside the arc is::between 10,000 to 18,000C). These furnaces are subdivided in:Alternating current: electrical current passes from an electrode, through the electric arc, stops Cthe charge and flows through the arc to the other electrode. Polarity of these two hot spotsinverts at each current semi-cycle of the network frequency. (60 cycles/second in the case of2BRAZIL). Electrodes are installed through the furnace roof. In the case of steel fabrication it is-=:;usual the use of a three electrodes design (three-phase alternating current direct-arc- AC). :::;

\Direct current: the electric current flows from an electrode to the load and then to the other '-'\electrode which is installed in the furnace bottom. In steelmaking it is usual to have one1electrode in the roof and the other in the bottom (direct current direct-arc - DC).Electric arc is characterized by presenting a great concentration of power in a small volume.2-The average temperature in the furnace is lower, making a great temperature gradient between~the arc and the load to be heated. The dimensions of the electric arc depend on the electricalr>parameters and the mechanism that regulates the distance of the electrodes from the charge.=-The multiplication of the voltage in the arc by the power supplied by each arc constitutes a-:;measurement of the irradiated heat. In electric arc furnaces using alternating current, three:::;electrodes introduced from the furnace roof, are used. Conventional AC electric arc furnaces')~.operate with specific power, that is, power by ton of steel produced in the range of 300 to 750kVAlt. Newer furnaces can operate with spec if ic nominal power in the range of 800 to 1,000kVAlt. Energy consumption is being reduced from 400 to 500 kVAlt (1990) to around 350 to 400,

,-,kVAlt of steel produced (1999) due to the use of burners and the injection of O2. One of the '. ~.factors influencing the power consumption is the composition of the charge, because the use of 'sponge iron tends to increase consumption due to higher slag formation. '

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electrodes

Slag Pot

Figure 2.1 - Schematic representation of several kinds of electric arc furnaces: indirect or free arc .

3- MAIN EQUIPMENTThe body of electrical arc furnaces is built with steel and is generally circular or slightly ellipticalin shape. The body may be lined with bricks and REFRACTORY MASS (Figure 3.1) or to beequipped with cooling systems through jackets with piping for water circulation (cooling panels)that may also have a refractory lining in its lower part (Figure 3.2) which is replaced over timeby adhered slag. Although these panels decrease the efficiency of heat transfer to the charge,its use increases the lifetime of the refractory lining, which can reach over 10,000 heats. Thepresence of slag/refractory lining reduces the thermal loss.The panels may be built with steel or copper piping. It is recommended to use copper coolingpanels in the regions nearer to the hot metal or electric arc and steel panels in the furnaceupper regions. Copper, thanks to its high thermal conductivity, supplies a more efficient coolingfor the panel, allowing its use very near to the hot metal level (around 250 mm). Thermocouplesmay be used for monitoring the cooling panels' performance. In older electric arc furnaces,internal cooling panels may be used over the steel body, substituting the walls of refractorybricks. In newer models, the panels may replace part of the metallic body. The use of panelsgives an increase of the furnace available volume when compared with the traditional lining inthe side and roof walls.

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The reduction of downtime for replacement of old refractory lining also contributes to increase ~the availability of the furnace to production, also contributing for higher energy efficiency. As ..Jalready mentioned the cooling panels may be covered with the slag of the process which '-solidifies and adheres to them. This lining reduces the radiation loss to the walls by around ..J70%. The body base named hearth presents a flattened curve to avoid that the hot metal-:-presents an exaggerated depth, allowing the acceleration of charge heating by heat radiation . . . . . . ,from the arc and from the refractory walls of the furnace upper region. The hearth has one ormore layers of refractory lining. The furnace has two side openings, one for the removal of--slag, inspection of the furnace's internal parts, addition of materials and oxygen injection by __-lance, if necessary. In older furnaces (Figure 3.1) the other opening is the heat tapping hole todrain the liquid metal. In some instances, the opening corresponding to the tapping hole may betilted upwards to reduce the height of the work floor in relation to the furnace platform or mayhave its slope reduced in a way that it becomes submerged (submerged tapping hole) avoiding '


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electrodes

R ef r ac t o r y li n in gwfth bricks

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nc e for oxygenInj ec ti o n

Slag Pot

Figure 3.1 - Schematics of a traditional furnace with tapping chute and submerged tapping hole.

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Addition ofalloys or fluxingagents

Collectionoffumes

Gasses'o analyser

Burner - - - - -- - - - -

,J ,-.r-,,-.

Operating the oxygen ~or sampling lance~'

Slag Pot ~

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Figure 3.2 - Schematics of an EST AC current furnace.

Figures 3.3 to 3.5 show images of bodies, roofs, and cooling panels of eletric arcfurnaces.

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Figure 3.3 - Images of electric arc furnaces bodies highlighting the background image of a body withinjection of gasses through the bottom.

Figure 3.4 - Images with examples of the distribution of water cooled copper and steel panels, with carbonand oxygen injection nozzles and panels with slag adhered through use. (Images supplied by Voest-AlpineIndustria Uda.).

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Figure 3.5 - Examples of water cooled panel's roofs.

The use of EBT system has the advantage of requiring just little tilting (around 10 to 15) to ~completely tap the hot metal, smaller heat loss during tapping due to the more compact ,......stream and shorter tapping times. The use of this tapping hole also reduces the wear of 'the refractory in the slag line region, besides making possible to cover a bigger area of the 'furnace body with cooling panels. The reduction of the length of electrical conductors, due ro-to the reduced tilting angle, lessening energy losses along these conductors is an ,-...additional advantage. ~The EBT system tapp ing hole may be replaced from time to time (150 heats is a typical

datum). Considering that the maintenance of the hole may be needed at every heat byfilling it with sand, the EBT furnace may present the inconvenience of higher downtime .....-

between heats, but this depends upon the operational practice of each mill and the use of ::::'a system to access the hole through an opening and a balcony for the positioning of the :'operator (Figure 3.2). It may also be necessary to remove all slag before starting the ,,--tapping or to retain some hot metal in the furnace. To avoid cold air from entering the ~furnace during the oxygen injection through lances in the side openings, air nozzles can be r=;installed vertically to the side openings to make an air curtain in this region. This curtain ~also avoids the gasses generated in the furnace from escaping, directing them to thecollection system installed in the furnace roof, mainly when a bigger volume of gas is 'generated by the use of a larger quantity of sponge iron as raw material. The oxidation of ,.....-,the electrodes' surface is also reduced. 'The tilting of the furnace is done through electrical or hydraulic systems, which allow a45 tilt for the tapping of steel in furnaces with a tapping chute and up to 20 backwards totap the slag. In EBT furnaces the maximum tilting angle is 15. The roof is moved in arotary movement to allow charging, normally done through the furnace upper part, or for

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maintenance or inspection operations. For such, the roof is normally supported by metallicbeams. The roof diameter should be enough for it to rest on the metallic body and not onthe refractory lining, lessening the mechanical strain on it and thermal losses. Since thiscomponent wears relatively quickly, it might be interesting to have a spare roof to shortenfurnace downtime.To increase protection of the roof region, the furnace is designed to have a large distancefrom the roof to the hot metal and thanks to this the ratio between useful volume andactual volume of the furnace is relatively small, around 20%. Depending on the furnacecharge, this ratio can change, increasing in the case of scrap metal due to its higherapparent density. This low use rate occurred because, in the past the furnaces weredesigned with relatively big diameters, in the case of walls made only by refractory, so therefractory wear was reduced. Nowadays, furnaces tend to be higher to avoid thermallosses (smaller roof area) and to have more space to increase the use of gasses in post-combustion. A higher body allows charging 70% of the charge at once and the remainderis charged continuously, without opening the furnace thus reducing the tap-to-tap time andenergy consumption.The roof has holes to allow the passage of electrodes, addition of materials, gasses outletducts, oxygen injection lance or lime fines, depending on the design of the furnace. Theroof may be built with refractory material externally cladded with steel sheeting or to havea body-like structure and cooling panels with piping for cooling water circulation andstuffed with refractory material. There are also roof cooling systems based on the use of aseries of sprays for water aspersion over the metallic sheeting. Figure 3.7 shows typical ACEAF installations. Figure 3.8 shows an example of an electric arc furnace inside a metallicstructure (dog house) aiming to reduce sound and environment pollution and to makeoperation safer.

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It is extremely important that the supporting system regulates itself in order to reduce thepossibility of breakage, even in the case of contact with a conductive or non-conductivematerial. The regulat ing system should be adequate for its complete integration in theautomation process environment. The integrated calculus program initially determines theworking points based on the desired current flow and the length of the arc, what can alsobe done through the electrical diagram. The process monitoring and control system shouldguarantee that the working points are maintained.Traditionally, the electrodes' support ing arms are made of steel to supply the mechanicalresistance needed to support the electrodes. Each arm supports one electrode or phase,in a way that they may be moved independently to make the necessary adjustments to thearc. On these arms, the tubular copper electrical conductors are assembled (Figure 3.9).The arms should be electrically insulated from these conductors. Recent developmentsaim at substituting the steel structure by solid aluminium arms to transfer the electricalcurrent and to support the electrodes. Another option is the use of a steel structure linedwith copper plates, easing the connections with the electrodes' claws. In the arms' endsthere are claws with forged or cast copper shoes, internally cooled by water andelectrically insulated from the electrodes by a layer of mica (Figure 3.9). The electrodesare vertically placed inside the claws where they are clamped by a spring system hydraulicdriven. To allow adjustment of the vertical position of consumable electrodes, the contactsystem with conductors is not rigid and may be disconnected by remotely operatedhydraulic cylinders.

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Figure 3.9 - Examples of arms and support claws of internally water cooled electrodes for tri-phase furnaces. r-,(Images supplied by Voest-Alpine Industria Ltda;Concast AG).

r>Electrodes are normally made of graphite or amorphous carbon. These materials are carefully chosen according to the characteristics required to make an electrode:

do not melt, to resist oxidation and chemical attack, to have high resistance to rthermal shocks; to have high thermal condutibility and low electrical resistance toavoid its heating and to reduce voltage drop to a minimum, to be easily machinableto allow the making of proper joints preserving the best electrical qualities.

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Amorphous carbon electrodes are most used in submerged arc furnaces, used in the ,.-,manufacture of ferroailloys or calcium carbide. Raw material may be the coal named r-anthracite, coke and tar based binding agents. They are extruded followed by curing in a rr .heating furnace with temperatures in the range of 1,300C during several days. After a slow cooling they can be machined to make joining threads. 'The graphite electrode has a higher resistance to compression and higher electricalconductivity than the amorphous carbon ones. The chemical composition of the electrodecorrespond to more than 99% of carbon and the remainder is constituted by Fe, Si, Ca, Sand P. Actual density is in the range of 2.22 NO 2.25 tlm3. Although found in Nature,natural graphite, due to the quantity of impurities is not adequate to the direct making ofelectrodes. The manufacturing process, in general lines, follows the route described below(Figure 3.11) considering the use of raw materials which are sources of amorphouscarbon.

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The raw materials, oil coke (previously calcinated) or the coke based on mineral coalpitch and a binding agent as pitch (obtained from the distillation of tar) are mixed with acontrolled granulometry.The mix is extruded to promote the orientation of coke grains in a parallel direction to theelectrical current direction.A first heating for curing is done at temperature ranges of 850 -1,250C to eliminatevolatile materials and to cokefy the pitch to promote coke particles agglomeration.In the case of electrodes that will be used for the manufacture of nipples or connectors, anadditional impregnation with pitch promotes the increase of some electrical andmechanical properties .Graphitization (transformation of amorphous carbon in a crystalline carbon) is done in afurnace with electrical resistances at temperatures of 2,400-2,500C.After a slow cooling the electrodes may be machined to ensure that the desired finaldiameter is achieved and to make the joining threads.

One can notice that a great amount of electrical energy is used for the manufacture ofelectrodes.

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coke

pitch

..,~ grinding

mixing~ . ,. . . . . , . , . -

Impregnation withpitch

extrusion

Cure heating. . ,.

Machininq

Figure 3.11 - Electrodes manufacturing process (images courtesy of SGL Carbon).

The electrode diameter is defined as a function of the intensity and density of the currentflowing through them. This diameter can vary from 75 to 750 mm in lengths up to 2.8 m

~ ~ ~~ ~?~d_w_e- ig1-h_t_u...p_t_o_2_to_n_s_.lectrodes may be coated with a layer made with aluminium and-refractory materials, deposited by melting, which has high electrical conductivity in relation

to graphite, as a way to reduce carbon oxidation and to reduce the current intensity Ithrough the inner parts of the electrode. '\Electrodes Consumption

The consumption of electrodes is one of the operational indicators which is highlyinfluenced by the operation electrical parameters. Till the 80's specific consumption of

melting furnaces were from 4 to 6 kg of graphite per ton of steel produced. At the end of ~90's, consumption was around 1.1 to 1.4 kg/t of steel produced. One of the factors r>contributing to this reduction in electrodes consumption was the increase of the ratios r>voltage/current. The furnaces had their transformers modified to operate with higher '\

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voltages (up to a little above 1,000 V). One should guarantee in this case, a more efficientinsulation between the electrodes arm and the mast.

During the manufacture of steels, electrodes are consumed. This consumption can bedivided in two main categories:- normal consumption;- sudden consumption.Normal consumption encompasses two types: side consumption and linear or verticalconsumption. Side consumption is responsible for making the electrode funnel-shaped,while linear consumption shortens the electrode. It occurs during all the time that theelectrode remains hot and the main reason for that is due to oxidation through thereaction:

The oxidation rate of the graphite electrode depends, to a certain extent, on the quality ofthe graphite and the temperature in the surface of the electrode, velocity and turbulence ofthe gasses flowing from the furnace, as well as the oxygen contents of these gasses. Toreduce the side face wear by oxidation, one can use devices for cooling its surface, thusdecreasing the speed of the reaction of carbon with oxygen inside or outside the furnace.The increase in the steel manufacturing time and the intensity of the electrical current alsocause an increase in the electrode wear by oxidation. Air intake in the furnace, incorrectpositioning of the oxygen lance and a more intense flow of gasses moving to the exhaustsystem passing near the electrode, also increases the oxidation on its surface.The consumption of the electrode extremity or linear consumption occurs mainly when thefurnace is operating. Its main causes are the action of the voltaic arc and the erosion bymetal and slag. The wear of the electrode tip occurs by sublimation (passage from thesolid state directly to the gaseous state) of carbon in the electric arc, erosion of graphiteparticles (caused by the projection of metal in the electrode), graphite chemical at tack(absorption) by slag and steel. The agitation of the hot metal and slag provokes anincrease in the interaction electrode tip/hot metal and slag, increasing the wear bychemical attack.Contrasting with normal consumption, which is essentially a surface phenomenon, the

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sudden consumption could end in a sudden variation in the length of the electrode, mainly ,-...by cracks and breakage. The causes for this type of consumption may be found in the (-furnace operational area and normally involve poor preparation of the scrap and thermal r>shock in the electrode. The improper preparation of the scrap may cause its movement r--inside the furnace with consequent impacts in the electrode and its possible rupture as


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why the use of a cooling system to avoid premature wear acquires more importance toprevent premature wear.

Table 3.1 shows an example of the most important properties of graphite electrodes. Alldata presented in this table were measured at room temperature. Flexibility increaseswith temperature. Resistivity decreases with temperature increase, but starts to rise againat higher temperatures, being 600C the temperature above which this inversion occurs.The higher the resistivity, the higher the resistance to thermal shocks and to electrodeoxidation. Thermal conductivity decreases with temperature increase.As electrodes wear, new electrodes may be joined to the ones in operation throughappropriate connectors (niples). The assembling can be made by removing and placingthe set in the melting shop floor, or in the upper part of the column, without removing theset from the furnace. Electrodes can also be joined by male-female joints without the useof connectors. In all cases, joining is done through threads, using pitch to cement the jointto avoid the slackening of the joint by the vibrations in the electric arc furnace.

Table 3.1 Graphite electrodes most important propertiesVariable Unit ValueDiameter mm >600Apparent density q/crn 1.66-1.73Porosity 0 / 0 17-20Specific Electric Resistance QI- lm 4.6-5.2Flexibility MPa 9-12Thermal Conductivity W/(K.m) 200-250Thermal Expansion ,Llm/(K.m) 0.3 - 0.8Coefficient

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Refractory LiningRefractory lining has had an extraordinary evolution over steelmaking history. By beingan important variable for the operational cost of a steel mill it is fundamental that it doesnot suffer premature wear problems or breakages that may hinder the availability of thefurnace for production. The temperature for making steel, which has been increasing dueto ladle metallurgy and the introduction of continuous casting, is an item of concernbecause it increases the wear of the refractory lining. Another important item is the marketdemand for lower phosphorus contents.Refractory lining may be acid or basic.

'Acid lining is only used in the re-melting of steel in foundries. In this case, steel was ~\already produced in a previous step and the main objective is the use of molten steel for - r - ,filling the die of a part, with minimum alteration in metallurgical properties which might end '\in the formation of a slag that would be very aggressive to the acid lining. '-

Basic or alkaline lining is used in furnaces used in the primary refining of steel. In this ,case, one can choose to lay a lining of acid brick (silicon-aluminium) over the metallic ~sheeting and covering it with the basic lining (bricks and mortar lining made of MgO - r>:magnesia or dolomite - MgOCaO - with or without the addition of carbon). In designing '\the refractory lining of an electric arc furnace, first there is the division of the lining in rr>;,different zones according to the wear profile present in the inspections, measurements or repairs carried out during the furnace campaign and after its end. The intensity and kind of ,wear supply data that makes possible to determine the type of request present in each r>.

-region and the wear speed, allowing to establish, a priori a wear standard and the r>corresponding selection of adequate materials to stand such wear, thus avoiding frequent -:downtime for repairs or even prem~ture stops in the furnace campaign, or worse, a more ~. 0serious situation, accidents caused by cracks in the metallic body.

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Lining WearThe wear of the lining starts when the furnace starts to be used. This wear is of thermal,chemical, and mechanical strain nature and is inter-related what makes the accurateanalysis of the phenomenon responsible for the wear, more difficult.

The main causes for refractory lining wear are:

- mechanical impact by loaded scrap;- attack by the slag formed in the process;- energy irradiated by the voltaic arc.Mechanical impacts caused by scrap are controlled through an adequate preparation ofthis material, followed by appropriate charging sequence. Slag attack depends on itschemical composition. The addition of fluxing agents can adjust this composition andminimize the refractory attack.Refractory wear caused by energy irradiated by the voltaic arc depends upon theoperational parameters of the furnace, specially voltage and current, which affect thelength of the arc. A shorter arc makes the energy received by refractory smaller. One wayto lessen this kind of wear is to work with the electrode tip inside the solid charge and aftermelting, inside the slag. It is important to note that the internal diameter of the furnaceincreases with the refractory wear, without the consequent increase of the furnacecapacity. In some cases, it may be encountered capacity increase of up to 10% from startto end of the campaign.There are however, other secondary causes described below:Considering the thermal aspects, one aspect that exerts strong influence in the refractorylining campaign in an EAF is the obedienced to the limits established as maximumtemperature when selecting the refractory. If, during operation one avoids to cause suddenvariations of temperature, this attitude will contribute to reduce the wear of the lining.Among the chemical phenomena, oxidation, formation of compounds with low melting

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points and corrosion may be mentioned. Mechanical strains which cause refractory wear ''are related mainly with erosion, abrasion, mechanical removal, impacts, and tensions due / \to materials' dilation. Sudden temperature variations provoke heterogeneous dimensional - r - ,variations in the refractory bricks and mortar, which due to having its displacement ,-.restricted by neighboring bricks, produce local tension which may exceed the rupture limit,causing the formation and later propagation of cracks in the refractory structure, causing the loss of fragments in the refractory surface. This wearing mechanism is known as / \thermoclass.

r>.Corrosion comprises several wear mechanisms of MgO-C refractory, starting with the 'oxidation of carbon by the oxygen which is present in the furnace atmosphere or in slag's 'omponents, followed by the infiltration of slag in the brick lattice, and the formation of ~compounds with low melting points in the magnesia matrix resulting in its melting and the-removal of magnesia grains by lack of retention. Every time a layer thickness is lost the

process is started over. The slag line is more subject to the wear by corrosion mainly due to ~the carbon's higher tendency to oxidation due to the oxygen contained in the FeO and MnO


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The knowledge of these wear mechanisms is of essential importance to the selection of themost adequate refractory materials for each region, justifying the diversity of regionspresented in Table 3.1. To ensure a balanced wear, combinations of bricks, mortar andcooling panels are used, in conformity with the zone. of higher or lower wear. In zones withhigh strain special bricks of several kinds may be used with higher cost bricks in criticalspots, to optimize the cost of refractory by ton of steel produced.Besides the division of the electrical furnace in several regions, refractory lining can befurther divided in permanent l ining or safety lining, meaning a lining which is onlyremoved after the closing of the campaign and the work lining, that is the lining whichwears out and that may be partially repaired throughout the campaign, and that iscompletely changed after the closing of the campaign.The separation of refractory in EAF as permanent and work lining is necessary to allow theselection of the most adequate material for the walls, aiming to simultaneously comply withsafety and performance requirements. For example, the two layers corresponding topermanent and wear lining comprise two different lines of bricks, mortar, or blocks, thusreducing the possibility of coincident joints, increasing the tightness of the lining. The heattransfer from the interior of the furnace to the body made in steel sheeting should be theminimum possible, demanding a smaller thermal conductivity for the bricks in the safetylining. This conductivity should be higher in the wear lining to avoid cracks in therefractory due to highly heterogeneous deformations which might occur caused by non-uniform heating along the wall. The wear lining comes into contact with the hot metal, withslag, with the gasses and particulates and should have adequate characteristics to resistto chemical attacks and strains of abrasion, erosion and mechanical shocks which are notdemanded in the safety lining.Another improvement introduced in the design of refractory projects of EAF is the cuttingand pre-assembly of certain regions that require more frequent repairs, such as tappingholes and the tuyeres region, which by this improvement may be quickly changed duringdowntime of the equipment for maintenance or repairs. A tapping hole is normally changedafter being used in a range of 150 to 250 heats.The regions of furnaces corresponding to the slagline, hot metal line, hot spots andtapping holes (in EAF) normally use MgO-C bricks, pitch bonded or resin bonded or

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bricks with addition of electro fused grains. Dolomite-magnesia mortar can be used in the - r - ,building of hearths and in hearth and ramps hot repairs. The sub-regions of the side wall rr-.called hot spots correspond to the regions in front of the electrodes, whose surface is r\directly hit by the electric arc flames and the impact of solid metal, electrodes and slag ''particles, moving at high speed, causing wear by erosion.The roof lining may be basic, of the magnesium-chromium type. It can also be of thebasic aluminous type. In this situation one should use a material with chromite orchromium-magnesia base in the last regions of the furnace's side walls, which avoid the ~excessive wear due to acid-base reaction between the roof and side walls refractory. The

rr-.remainder of the roof is normally lined with a refractory material alumina-based (90 to\95%). Some companies use a refractory lining in front of the copper panels near the slag

line (bottom part of the panels) to reduce the loss and temperature in this region.The refractory lining of the roof is highly demanded due to the high temperature in this region -r-,and thermal shocks arising from side dislocation during the furnace loading. In the r>.assembly of the roof acid refractory lining, silica bricks are normally used. Due to the fact .r-..,that these bricks present a high thermal dilation they are assembled with spacers of .r-..,cardboard or wood which by burning during heating allow the expansion of the brick ~without introducing thermal tensions. A typical value for the lifetime of the roof lining is (,around 100 heats with consumption from 20 to 40 tons per ton of steel produced. In some

~\

instances, the roof might be constituted by water cooled panels, with a refractory lining or - - -- - - ..non-magnetic water cooled metallic ring in the holes for the passage of electrodes, to

r\avoid the voltaic arc between the electrode and the roof. In this case, a lifetime over 1,000 .r>heats may be achieved.

Refractory material used in EAF may be supplied as shaped material or as non-shaped ---,/material. As an example of shaped parts it can be mentioned: bricks, tuyeres, refractory r>concrete side wall panels, tapping holes and tapping chutes, sealing rings for electrode's ' - A .openings in the roof and the inner parts of the roof (Figure 3.17). As examples of non- - ~I.shaped materials mortars and plastic refractory may be mentioned. Non-shaped or (non- U t ..molded) plastic mixture refractory present a high MgO content (91 to 96%)\and are used ),

-- -- c fin the assembly of new linings (correction of warps in the body, junctions of the work ,..-,linings) as well as in maintenance through hot repairs. Refractory mortars have a lower KMgO content (around 80%) and are used in the assembly of the safety lining joints. ~

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Molded bricks in EAF may be manufactured from different refractory materials,depending upon cost and its availability at the place where the steel mill is located.Dolomite, fused magnesia, high purity magnesia sinter and other magnesia materials arerefractory's main raw materials. All are basic and contain additives to increase itsresistance to wear. MgO - Cr203 may be used in EAF used in the production of stainlesssteels.When the refractory bricks used in the safety lining are of MgO base they may be pre-burnt in a way that all chemical bonds are well developed and stable regarding volumevariation, thus reducing thermo-mechanical tensions over the adjacent layers of the worklining and steel body lessening the possibility of cracks in the equipment. These bricksnormally have inferior mechanical properties when compared with the wear lining, due tothe absence of additives so as not to increase the undesirable effect of thermalconductivity increase. In relation to the refractory bricks used in the wear l in ing layers,one can consider that, if the installed capacity is higher than projected production; theproduction losses with downtime needed to re-apply a new lining or to make repairs do nothave an impact on the desired production, one can thus choose the dolomite (MgO > 30%and CaO > 55%) due to its lower cost. In other cases MgO lining combined or not withother additives is used. Among main additives we can mention carbon, bonding agentsand anti-oxidation additives. Magnesia can be electro fused or sintered.In the case of sintered bricks, magnesium oxide or magnesia (MgO) is sintered attemperatures from 1,700 to 2,1 OOC, with steps of 5 to 12 hours, achieving the followingproperties: rock-like or dense ceramic material aspect, high mechanical resistance, highmelting point, high apparent specific gravity, low apparent porosity and compatibility withFeO. However, this magnesia sinter shows deficiencies as high thermal expansion, highspecific heat and a relatively high wetability by metal and/or slag.The combination of MgO with carbon is adequate to increase resistance to corrosion andinfiltration by slag, resulting from the lesser wetability of carbon by the oxides and by theformation of a magnesium oxide layer in the hot face of the brick. Generally, it may beconsidered that MgO refractory results from a certain chemical incompatibility betweenmagnesium oxide and carbon, which can be evidenced by the non-existent formation of aliquid phase (eutectics) and by both constituents not being thermo-chemically stable athigh temperatures. This favors the formation of magnesium oxide in dense layers in the

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~ArcelorMittal Point Lisas Limited, - . . . .

refractory structure itself (hot face) originating from the reduction reactions. Antioxidantadditives such as aluminium, magnesium, silicon, and boron carbide are used to avoid ,,-....the carbon oxidation in the brick and to reduce porosity, helping to avoid the penetration of .-...,slag. They also promote an increase (flexural strength) in the mechanical properties at '-'high temperatures. However, they increase cost, the rigidity of bricks and an undesirablechange in the chemical composition of the slag.From the refractory's consumption standpoint, a good quality slag is the one having thefollowing characteristics: low total iron contents (below 20%), high alkalinity (in the 3.0 to -5.5 range) and high MgO contents (above 6.0% in some companies). This is necessary to ~

- - - - .increase the slag melting point to above 1,700oe, preventing the slag film adhered to the 'refractory wall from melting during steelmaking. The wetability, measured by its fluidity

related to slag temperature, total Fe contents, alkalinity and MgO contents is also an :'important variable, because it determines the capacity of the slag to infiltrate in the micro r - -pores of the refractory bricks. The higher the FeO and MnO contents in the slag the higher ,the tendency to form a fluid slag increasing the wear of the refractory due to higher r :prenetration. This is due to the fact that these substances reduce the slag melting point. - r - ,The wear of the refractory lining varies along the various regions of the furnace, beinghigher in the side walls in contact with the slag and lower in the hearth. Typical values r>demanding the change of the work lining are around 150 heats for the slag line, chutes ,-and tapping holes, 80 heats for the roof and 5,000 heats for the hearth (without the use of r=:oxygen lance). A research published by IISI (International Iron and Steel Institute) in 2000, -r - -shows that the average consumption of refractory fell from 6.9 kg/t (1990) to 3.1 kg/t '~(1999) in 35 furnaces researched in various countries. However, these numbers may bealtered by a series of factors, for instance:- the kind of steel being made (tapping temperature, alloy elements added, etc.);- heat time (affected by the use or not of secondary refining, burners, oxygen injection, pre-heating of the metallic charge,etc);- chemical composition of the metallic charge used, highlighting phosphorus contents;-the company policy in relation to the preservation of the lining or steel production and '.

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- the materials (chemical composition, manufacturing method and microstructure) used inthe ligning;

- the use or not of a practice with foaming slag, as well as the duration of it;- kind of furnace (AC or DC) and the adjustment of operational electric parameters;- electrode cooling method;For a better understanding of the influence of refractory performance in the furnace,refractory consumption can be sub-divided in consumption of bricks for the walls andhearth, bricks for the roof, and ~atching mix .llie. In general, around ~ofrefractory consumption corresponds to patching mixture,.20 to 30% of bricks for walls andhearth and 5 to 10% of bricks to roofs. We should stress that the cost per kg of these~materials and the cost of assembly varies a lot and normally the lowest cost is for thepatching mixture. Thus, one should not be fooled by only analyzing the final refractoryconsumption per ton of steel produced. Refractory consumption for storage and transportladles for liquid steel and liquid pig iron ladles and refractory materials employed in theconventional or continuous casting area should also be considered in the analysis ofrefractory consumption per ton of steel cast.In repairing refractory lining hot and cold patching techniques can be used. One hot repairtechnique is called gunning. In this technique the material is injected in the furnace throughpneumatic transport, allowing its use in a hot furnace, aiming to the formation of thin layersthat are sintered during application. Such procedure represents an increase in productivityand higher energy savings. However, this technique has a disadvantage of low densityand heterogeneity caused by segregation during the gunning. Furthermore, thegranulometry and porosity control of the lining to allow water steam escape and thermalaccommodation is more difficult. The binding agents used in the patching mixture can besilicates, phosphates or chromium based.In terms of the refractory ..lining c am pa ig . f 1 .. 1 1ean ing the number of heats in the electricfurnace that would deteriorate the lining in such a way that would require the total stop ofthe equipment for the complete replacement of both work and safety refractory linings,5,000 to 7,000 heats may be achieved or even more, if the maintenance of the lining takesprecedence over other parameters related to production.

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The monitoring of the refractory lining wear in an electric furnace may be done through theuse of two distinct and complementary methodologies: visual inspection and laser ,-..,inspection. The obtained data are used to estimate the lifetime of the refractory lining and r\countermeasures to correct high wear in certain regions. ,-'\The visual inspection can be done after each heat. The furnace interior inspection is '-,normally done through the work door, inside the furnace next to the tapping hole and -- - . . .complemented by the operator positioned in a platform from where he can see the wholefurnace interior. The tapping chutes and work door normally receive refractory

~maintenance with higher frequency (every 2 or 4 heats). Visual inspection may becomplemented with the monitoring of the furnace body temperature through optical C'yrometers, thermographs, or thermocouples installed at specific points. This monitoringallows the identification of high temperature spots in the metallic sheeting which indicate a : = 'severe wear in the refractory lining or faults in the cooling panels. .--,Laser inspection is done using equipment that allows the scanning of the refractory \surface through a laser beam, allowing the measurement of the remaining refractory lining ~by comparing the current measurement with the value obtained at the start of the -campaign. Data acquired are processed in a computer allowing a more accurate mappingof the refractory wear. Laser measurement may be done weekly or at higher frequency r--according to operational needs.Gunning systems were developed to make repairs in electric arc furnaces linings ''considering the mapping acquired by laser data. The gunning equipment may be remote ;decreasing the time needed for the repair and increasing its accuracy. ,h dJ~..,.,...:;: '~ ~~,. escort

. . . .. ,'

',-. .,-'\

'

, . .. .

(b) , - - .- - -


electrode ~ Eletrodo ~electric arc

hot metal Carga mstahca ou ago Hquido

/-,Figure 4.2 - Schematics of the alteration in the electric arcs profiles due to interaction between relectric/magnetic fields in a thriphase AC furnace and the consequent alteration of the wear profile: (a)only one electrode in a DC furnace and (b) deformation of the arc in an AC electric furnace. \------:/t:-----jFeSi Fe AI 0

captation

s

Figure 3.1. Schematic representation of a ladle furnace instalation.

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~ArcelorMittal Point Lisas LimitedThe term ladle furnace may not be the most appropriate, because, actually, thesecondary refining installation consists basically of a roof equipped with electrodes(usually, three electrodes are used .in AC triphase furnaces), the alloy addition andtemperature and chemical composition sampling systems, the bath agitation deviceswith inert gas injection or the electromagnetic equipment, the fumes capitation systemand auxiliary devices for moving the laddie car (Figure 3. 1). Figures 3.2 and 3.3 showimages of a ladle furnace station.

The ladle furnace is a device whose characteristics make it possible to carry out aseries of operations such as:

Temperature control Adjustment of chemical composition Deoxidation Desulphurization Chemical composition and temperature homogenization Control of morphology and removal of non-metallic inclusions.

The sequence of operations normally used is shown below:- the ladle containing the liquid steel from the primary refining furnace is transported tothe ladle furnace.- the addition of ferroalloys or pure metals for chemical composition correction arecarried out based on the sample taken after the tapping.- the deoxidation by silicon and/or aluminum can be done.- the addition of lime or synthetic slag to correct the slag volume in the laddie preventingthe exposure of the electric arc during the heating .- the preparation of steel with higher demands for quality requires that the slag of the

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~ArcelorMittol Point Lisas Limitedtreatment is conducive to the capture of inclusions and/or steel desulphurization,making it necessary to remove the remaining slag (through scraping) from the primaryfurnace tapping, replacing it with a more appropriate slag.- the furnace is started for the first heating and the tendency of sharp temperature fallcaused by the completion of the additions reverts; however, the ladle has not yet hadthe suitable thermal soaking time and the rate of the liquid steel global warming variesfrom 1.5 to 2.5C/minute (although in the last few minutes rates of 3 to 4C/minute canbe achieved). The turned time of the furnace can vary from 10 to 15 minutes andusually does not exceed this limit. This is due to the fact that the slag temperature iscontinually increased, and could harm the slag line edge and the furnace roof edge.Observe that in the case of slag removal, there is a greater fall in temperature and it isnecessary to heat the steel for a longer period.- after these initial treatments the steel is sampled (temperature and chemicalcomposition) and the deoxidation is carried out in case it has not been done yet, and thealloys corrective additions.- then the second heating is started, and its time is calculated based on the targetedfinal temperature. In this phase, the increase of temperature is constant and dependingon the power employed it may reach from 3.5 to 6C/minute.- in case a small adjustment of the chemical composition is necessary, the heating canbe interrupted or the additions can be made with the furnace turned on, depending onthe chemical element to be corrected.

~r=~~r..,

- - - - -r..,

.r-...

~

- to complete the process a final sample is collected and the steel is released for casting nor for treatment complementation in other secondary refining equipment (vacuum 'furnaces), if new corrections are not needed. 'For the preparation of steels with more stringent quality requirements a calcium alloyinjection treatment can be carried out before the release of the heat for casting tocontrol the inclusions morphology and the agitation to increase steel cleanliness byspeeding up the flotation of the inclusions still present. In companies that carry out theproduction of steel with the addition of alloy elements, such as lead and bismuth, toincrease steel machinability these elements can be added at the secondary refining

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~ArcelorMittal Point Lisas Limitedstation or in another station equipped with a fumes capture system, because the vaporsof these materials are harmful to health. Ladle furnaces treatment total time varies from30 to 70 minutes depending on the operations required.For example, the use of devices that minimise the passage of the primary furnace slagto the ladle prevents the skimming of the remaining slag and temperature loss isreduced. In normal quality steels thre is no agitation to promote flotation of inclusions,saving energy and time.To minimize the risks of overflowing and overheating the roof, it is usually operated witha free edge range of 600 to 1400 mm. Ladle furnaces are normally operated withshorter electric arcs than electric furnaces. This procedure provides a reduction in thewear of the ladle slag line and furnace roof refractory linings. The ladle furnaces thatwork with electromagnetic agitator require that the ladle shells are changed in theregion near the agitator. In this region it is necessary to use a non-magnetic steel-typein order to avoid induction heating of the shell and reduce the losses in the magneticfield. When using the agitation through the injection of inert gases it is important toelectrically insulate the porous brick with a metal liner to prevent current leakage, whichcould cause damage to the brick region and led to the accidental leakage of liquid steel.The electrodes used in the ladle furnace are similar to those used in the arc electricfurnace for primary refining. They are made of graphite and when worn can be restoredthrough the use of niples. They must present an ability to conduct higher current toprevent the formation of a long electric arc. This is one of the reasons justifying itssmaller diameter. They are subject to lower mechanical work so as not to melt thescrap. Moreover, the environment in the furnace is reductant, thereby reducing the wearby oxidation of the electrode surface. The basic principle of the furnaces laddies powersupply can be described as an electr ical c ircu it formed at one end by a source(distribution system, circuit breaker, transformer, etc.) linked to one or more conductorsat the other end, causing short-circuits which generate and concentrated enoughelectricity to heat the liquid metal charge. One of the main features of the electric energyneeded to cause these short-circuits is the combination of low voltage and high current(Figure 3.4).

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Arm Support orcontact clawsbus bar

t

Generaltransformer(substation)

Figure 3.4 - Schematic representation of the main components of the power supplycircuit of a AC type furnace ladle.

4 - VACUUM PROCESSESThe possibility of applying pressure below atmospheric pressure to treat the liquid steelin the secondary metallurgy stage was proposed initially by the pioneer of the steelproduction by pneumatic processes, Henry Bessemer in 1865. Bessemer suggested adevice to make the steel casting in a mould in vacuum. However, at that time thetechnical conditions were not available, particularly for the generation of vacuum,because, the vacuum pumps had not been invented yet. However, after thedevelopment of vacuum pumps, steam blowers and water sealed pumps in the 50's, themetallurgical processes using vacuum degassing techniques became possible.Approximately 100 years after Henry Bessemer's suggestion, the first processes inindustrial scale for completion of the vacuum degassing of the steels were presented.The first practical tests on the liquid steel vacuum treatment were conducted in steel

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~ArcelorMittal Point Lisas Limitedworks in the river Ruhr region in Bochum, Dortmund and Hattingen (Germany).The development of the first industrial scale procedures in vacuum treatment occurredin the 50's. The first vaccum secondary refining plants were designed only fordegassing. In 1955 came the process of degassing in the liquid steel jet during thetapping a process known as SD (Stream Degassing). In 1956 the DH (Dortmund-HerderHuettenunion) process was presented followed in 1957 by the RH (Ruhrstahl andHeraeus) process and in 1960 by VD (Vacuum Degassing) processes. The VOD(Vacuum Oxygen Descarburization) and VAD (Vacuum Arc Degassing) processes werepresented in 1965. Currently, the main techniques used to perform degassingprocesses in the presence of vacuum can be divided into two basic categories:- with liquid steel circulation through a container outside the ladle (HR and DHprocesses and their variants RH-KTB, RH-TOP, RD-KTB and RH-OB);- without liquid steel recirculation out of the ladle, making the entirely process in thesteel ladle, called by some authors as the degassing tank (ASEA-SKF, VD, VAD, VOD,AOD, ASM).Thus, for didactic purposes and without strict classification, a summary of some of thedegassing processes available in the market is presented in Table 4.1.The most recently developed vacuum degassing processes were the DH and RHprocesses. These methods were created aimed at reducing the levels of hydrogen,oxygen, nitrogen and carbon in high volumes in only one stage of treatment, allowing ahigh annual production in the secondary refining stations. These methods allow for thetreatment of high weight heats; the upper limit is around 400 t. and for this reason theyare applied mainly in integrated steel plants. Because it is possible to make a moreefficient decarburation through the use of vacuum associated with the liquid steelrecirculation RH and DH processes allow the production of steel with low levels ofcarbon, through the removal of this element as CO gas. The RH process developmentstarted in the headquarters of the Ruhrstahl HenrichshOtte company in Hattingen, whichnow belong to the Thyssen group. The need to increase the size of the ingots used asraw materials for forging the axes of the transmission lines of hydroelectric plants led toan increase in the cost of annealing treatment. To eliminate this treatment, A. Lorenz a

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~ArcelorMittal Point Lisas Limitedresearcher from Heraeus had the idea of degassing the liquid metal, a method that waspatented in 1957. This process was later called RH process, initials of Ruhrstahl andHeraeus.Table 4.1.Summary of some of the degassing processes available in the market

VAD DH (RUI~~tahf (~l~~~~~f VDvaccum (Dor1nullnJ-


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~ArcelorMittal Point Lisas Limitedbased electrical resistance. After heating, the blowtorch is removed from the vessel.Currently, the MgO or MgO-CrO based refractory materials are used to line the RHladle. Depending on the RH vessel location, the refractory material may differ regardingraw material composition, grain structure and agglomeration agents. One of the maingoals of the oxygen injection through lances placed at the top of the RH treatmentvessel is the supply of oxygen in the cases when the amount of this element dissolvedin the liquid steel from the primary refining furnace, do not favour decarburationthermodynamics and kinetics. The oxygen injection can also be necessary to allow thesteel heating through the use of exothermal reactions with AI and Si mainly. Theselances are similar to the oxygen injection ones. used in LD converters, with theexception of having a single hole. Where it is not necessary to use the lance, it remainsstationary at the top of the vessel with a minimum flow of inert gas to prevent theobstruction of the gas output hole due to liquid steel projection.The placement of a lance for oxygen blowing at the top of the RH process vessel wasimplemented by the Thyssen RH company in Germany in the late 60's RH-TOPprocess). In the 80's Kawasaki Steel Corporation developed the RH-KTB processwhich combines the oxygen blowing RH-TOP system with the introduction of heatingusing the of CO post-combustion effect released by the liquid steel converting it intoCO2, or through the addition of AI to the metallic bath. The RH process can be used toperform the following types of operations:- reduction of Hand N content in the steel;- reduction of carbon content in the steel;- control of the liquid steel temperature;- adjustment of chemical composition;- deoxidation of liquid steel;- desulphurization of liquid steel;- morphology control and removal of non-metallic inclusions.

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Addition ofterre-alloys -

Nozzle forinjection ofAlrlN,

(a)

Oxygen lanceBurner- (KTB lance)

Gas suction(hot off take)Oxygenr - - temperature\ measurementand sampling

Vase

Rising snorkel(injection ofAir/N,)

Busbar -te)

Figure 4.1 - (a) Schematic representation of the main equipment of an vacuumdegassing type secondary refining unit (b) image of a whole treatment vessel and snorkelsuspended to make its exchange, (c) image of a new snorkel and (d) snorkel removedfrom the ladle after treatment is completed.

However, it is considered that the primary purpose of degassing is producing liquid steelwith a minimum of dissolved gases in the bath as possible. The process allows that,the application of vacuum decreases gases content to values below 2 ppm for hydrogen

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~ArcelorMittal Point Lisas Limitedand below 20 ppm for nitrogen. These are indicative values, eventually reaching levelseven lower with a stricter control of the process.Because the equipment requires a higher investment than secondary refining stations ofthe chemical heating type or ladle furnace, this equipment shall be used for thepreparation of steels that can not be processed in those processes. Therefore, priorityshoul be given to the preparation of steels that require the first two before mentionedoperations, the reduction of Hand N content in the steel and the reduction of carboncontent in the steel. The other operations are necessary to compensate the temperatureloss or chemical composition changes, inherent to the RH process or required by thesteel final chemical specification.The process of vacuum degassing in the systems that carry out the recycling of steel ina vessel above the liquid steel ladle, usually follows the steps presented below:- The liquid steel ladle is placed in the RH station;- The snorkels are introduced in the metallic steel bath;- The pressure in the higher vessel is reduced, so the liquid steel bath is forced to rise,reaching a height around 1.40 m above the original surface level;- The inert gas (usually argon) is injected at the bottom of the first tube, near the steelsurface in the ladle (Figure 4.1a), the injection of inert gas can be started a little earlierto prevent the obstruction of the injection nozzles; this tube is called upward snorkel,and consequently the other is called downward snorkel.- The injected gas rises and, thus causes the acceleration of the liquid steel in theupward snorkel, in addition it causes a reduction in the liquid steel density, which helpsits upward movement;- Forced by temperature increase, pressure decrease and steel gases release at the topof the tube, the molten metal disintegrates into small droplets inside the vessel, return tothe surface and goes down to the other leg of the snorkel, promoting the liquid metalcirculatory movement.It can be considered that a regular RH type facility requires an investment 3 times

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~ArcelorMittal Point Lisas Limitedbigger than a chemical heating station.As the RH process treatment is carried out in a separate vessel the steel ladle needs alower free edge, which can be regarded as an advantage in relation to other chemicalheating processes or the ladle furnace, for example. The introduction of pipes or legs(snorkels) in the liquid steel is carried out through ladle elevation (usually in steelworkswho work alternating two RH vessels) or, more commonly, by lowering the vessel. Insome older facilities a single vacuum system with a dual vessel system moving on railsis employed to enable maintenance in the refractories of the vessel legs. However,more recent facilities tend to have a single vessel system associated with devices thatallow fast movement (few minutes); usually the complete unit is replaced by another(standby) at regular intervals and the repairs are carried out in less than one hour. Thepipes, which a few years ago were composed of various sections, are currently made ofa single piece that is welded to the vessel. The developments obtained in refractoryperformance enabled this breakthrough regarding the lifespan of the pipes (over 600heats).The phenomenon corresponding to the increase in the liquid steel bath level due to thegeneration of vacuum in the above the snorkels is called barometric over height.According to results of mathematical and physical models, it was determined that amixture containing a part of steel and ten parts of gas (volume basis), allows the steellevel to reach 1m above the barometric height and may even occur steel projections atthe top of the vacuum vessel, if this proportion is exceeded. The circulation rate, i.e.,the amount of steel that goes through the vacuum chamber per unit of time, isdetermined by the capacity of lifting liquid steel in the upward snorkel and the metallicbath level in the degassing vessel, being one of the main operating parameters of theRH process. This movement rate can not be directly measured. However,measurements were made using radioactive tracers in experimental conditions or coldmodels, allowing to infer that the rates vary between 10 and 85 tlmin depending on theparameters of each equipment or operational procedures adopted. Normally, thegreater the liquid steel movement rate, the most efficient the treatment process will beand the shorter the time required to achieve the desired results.The amount of steel going through the vacuum vessel can not be directly measured;

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~ArcelorMittal Point Lisas Limitedhowever, measurements made using radioactive gold showed that for a 20-cm entrypipe diameter the flow rates obtained were between 10 and 20tlmin, according to thequantity of gas introduced. It is possible to complete the steel refining in the RH withoutbeing necessary to send it to another secondary refining station, avoiding compromisingthe steelworks synchronism. Examples of activities complementary to the process ofdegassing are the calcium basis composite injection or steel deoxidation. To illustratethis point, it can be mentioned that if a ladle containing IF1 (interstitial free) steelprepared at a RH secondary refining and released to the continuous casting machinehas to be heated due to a long waiting time as a result of a stop in the machine shouldpreferably be directed to the chemical or electrical (ladle furnace) stations. So the RHstation is released for the preparation of other heats with the above mentionedcharacteristics.The steel quantity through the vacuum vessel can not be directly measured. However,measurements made using radioactive gold showed that, for a diameter of 20 cm of theentry tube, flow rates between 10 and 20 tlmin were obtained, according to the quantityof introduced gas. It is possible to complete the steel refining in the RH without beingnecessary to send it to another secondary refining station, avoiding compromise thesteelworks synchronism. it has to experience a warming due to a long waiting time,according to the machine stop, should preferably be directed to the chemical orelectrical (ladle furnace) stations. So the RH station is released for the preparation ofother heats that display the characteristics listed above.Steel cleanliness, on micro and macroscopic scales, can be improved by the RHprocess treatment by two methods:

Removal of oxygen via carbon through the gas reaction that forms COpreventing the formation of oxides inclusion.

Flotation of solid or liquid inclusions as a result of the agitation of the liquid steelin the vessel and in the ladle; the inclusions agglomerate and move upwards to the slagof the ladle, where they are fixate.The addition of alloy elements in the RH vessel during treatment, both for largeadditions or for small chemical composition adjustments has some advantages over the

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~ArcelorMittal Point Lisas Limitedaddition of alloys during casting or during ladle bubbling:- Better yield, especially in the case of oxidizable elements.- It is possible to achieve high alloy addition rates.- It is possible to obtain narrow bands of alloys.- It is faster to homogenize chemical composition and temperature.- There is no obstruction by the slag layer.An addition system of an RH station includes alloy silos that feed a weighing device,quantifying the additions to be made. A system of conveyor belts transports the materialto vacuum silos and through the use of vibration feeders, the alloys are added to theliquid steel bath circulating through the vacuum vessel.The ideal point of impact for the added alloys in the vessel is located in the upward flowarea of the snorkel because of the violent movement of the steel in this area. But if thereis an exchange of upward and downward steel flow in the snorkels, the impact pointshould be positioned at the center of the vessel.During the degassing, the flow of liquid steel through the vessel (movement rate) isconstant. The alloys portions are added at a feeding rate also constant and proportionalto the metal flow rate. However, some adjustments can be made depending on thedensity of the alloys added, the treatment stage or the level of dissolution andabsorption by the heat, and the granulometry of the alloys. Several tests were madewith model and industrial stations to analyze the time of homogenization and mixing ofalloys during treatment in the RH process. It was observed that mixture times fordifferent combinations are almost the same, depending mainly, on the circulation speedcaused by the different injection rates of the dragging gas. Due to non oxidation by theatmosphere and reduced amount of slag in the RH vessel, the conditions to achieve ahigh yield by adding the alloys during RH process treatment are highly favourable.However, it must be considered that possible sources of oxygen in the steel, in the ladleslag or in the ladle refractory, as well as the oxidized skull in the RH process vessel mayreduce alloy yield. Moreover, alloy elements fines may be suctioned by the hot off-take

116

r

'

- --r-,

r--., ,-. .,

~- '

,. . - -~r


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~ArcelorMittal Point Lisas Limitedpiping reducing alloys yield. To prevent suction from the hot off take (device for suckingprocess generated gas), it is necessary to use a granulometry of at least 3mmespecially for lightweight materials such as carbon and aluminum. Another factor thatmay cause a decrease in alloys yield is the high steam pressure from some elementssuch as manganese, calcium and nitrogen.If medium and high carbon content steels are produced in the RH process it may benecessary to add large quantities of carbon suppliers (coke or graphite) because aftertapping in the primary refining furnaces the steel carbon content in the ladle is usuallylow. In RH processes the yield of carbon addition is around 95%, which does notpresent problems to the inclusion of this element in steel composition. The combineduse of vacuum and oxygen blow enables the production of ultra-low carbon steel 20ppm or 0.0020% in carbon weight). See below a summarized description of the carbonremoval treatment procedures in the RH. The steel must be prepared in primaryrefining furnaces with a proper combination of temperature and composition to facilitatesubsequent decarburation in the RH station. The manganese can be added in thetapping jet through the use of the Fe-Mn ferroalloy with high carbon content, because itis cheaper than the Fe-Mn ferroalloy with low carbon content. If the carbon content inthe tapping is too low to optimize the decarburation, high-carbon manganese mayslightly increase the level of carbon in the steel. Ideal values of original carbon contentto optimize the RH process decarburation are between 0.04 to 0.06% in weight (400 to600 ppm) or lower ranges by other authors (350 to 400 ppm). This range depends onthe availability of lances for oxygen injection in the RH process to make thedescarburation stage. It was observed that the higher the carbon content, the higher thetreatment and agitation time in the vessel.

The amount of slag from the primary refining furnace must be reduced to preventphosphorus from reversing to the metallic bath and to reduce the problems inherent tothe recirculation of slag inside the RH station vessel. The injection of inert gases in theladle (argon) still in the primary refining furnace is usually necessary to achieve oftemperature and chemical composition homogenization. A sample is usually taken atthis stage and sent to the laboratory for the determination of the steel chemicalcomposition. In the case of basic lining ladles the steel surface can be covered withlime. During the transfer of the heat to the RH station, the sample is analyzed in the

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~ArcelorMittal Point Lisas Limitedchemical testing laboratory. When the ladle arrives at the treatment position in the RH,the vacuum pumps are turned on in sequence to pre-empty the pump system volume to100-200 mbar. Before the treatment is started, it is recommended that the slag and thefree edge thickness are measured to define the depth of the legs immersion preventingthat an incomplete immersion results in air suction, dragging steel and slag to thecooling gas system. The temperature is also measured and a new sample of the liquidsteel bath can be taken for chemical analysis in the laboratory. Besides that, themeasurement of oxygen content is taken through the use of appropriate systems thatcombine samplers with process computers calibrated for this purpose. After these initialsteps, the ladle is raised while the dragging gas is changed from nitrogen to argon. Thesnorkels are submerged in the steel to a desired level. The vacuum valve is thenopened. After approximately one minute, steel rises to the vessel and starts to circulatebetween the ladle and the vessel. If necessary, manganese should be added at thebeginning of this stage in order to perform the decarburation of the carbon normallycontained in the Fe-Mn ferroalloy. The steel decarburation process begins due to thedecrease in pressure inside the RH vessel.A careful procedure to increase inert gas flow and to reduce the pressure must be doneto avoid projections related to the decarburation reaction. The process can bemonitored by a video camera installed inside the treatment vessel. To achieve carboncontent below 20 ppm, it is necessary to reduce the pressure to 2 mbar or less.Decarburation discontinuation is done through the addition of aluminum which willpreferably react with oxygen. Some RH stations have installed a computerized controlof pressure reduction through the recirculation of output gas to the condenser. To makethe treatment of various types of steel, various pressure reduction curves can beapplied.Decarburation speed is basically dependent on steel circulation rate in the vessel,directly affected by argon injection rate through the upward snorkel, because itincreases the argon/liquid steel interface participation in the CO bubbles nucleation.After a treatment period of approximately 15 to 18 minutes, the desired carbon contentis usually achieved. A measurement can be performed aiming to determine the oxygencontent, which is usually between 250 and 450 ppm, depending of course, on the finalcarbon content achieved. Based on the measured value for the oxygen content, a

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~ArcelorMittal Point Lisas Limitedquantity of aluminum necessary for deoxidation and reaching the right composition iscalculated and subsequently added to the vessel. The aluminum oxidation is a highlyexothermal reaction, so that the resulting increase in temperature should be consideredat the beginning of the treatment. If the steel is overheated, it may need a largerquantity of strip scrap during the first few minutes of treatment.To promote the flotation of inclusions, it is advisable to maintain a high flow of argon.The pressure can be higher than the pressure applied during the decarburation stage toreduce the loss of manganese by evaporation. If necessary, titanium, niobium or boronis added to the vessel as ferroalloys. After the final addition, a homogenization timeshould be considered. After this period the temperature can be measured and treatmentis ended by closing the vacuum valve to remove the vacuum in the RH vessel byflooding it with nitrogen when atmospheric pressure is reached in the RH vessel and thesnorkels can be removed from the steel. At this moment the dragging gas is againchanged to nitrogen and the injection rate is reduced to 800 Nl/min. At the end of thetreatment the temperature is measured and a sample of the steel is tanken. The ladlecan be closed before its transfer to the casting area.Immediately after the ladle leaves the RH station, the snorkels must be internally andexternally inspected. Cracks and erosion points should be repaired by the projectionwith special refractory material. The dragging gas injection nozzles should also beinspected.When the ladle arrives at the RH station the procedures are similar to those employedfor liquid steel decarburation. After the snorkels are submersed in the steel, the mainvacuum valve is opened. Due to the pressure balance between the pre-vacuum of thevacuum pump and RH vessel, a pressure of approximately 500 mbar is immediatelyobtained in the vessel. Due to the fact that steel is deoxidized by aluminum, only a smallamount of CO gas is formed, and the steam ejectors can be turned on without delay.After one or two minutes the steel circulation begins. After three minutes of treatmentthe pressure in the vessel is reduced to less than 4 mbar, which means that the time foreffective treatment for reducing hydrogen began. At the same time, the dragging gasinjection rate is increased, creating conditions for the addition of alloy elements.Comparatively, we need a lower pressure to reduce hydrogen content than for

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~ArcelorMittal Point Lisas Limiteddecarburation. To compensate the fact that in some cases the pressure is notsufficiently reduced, the treatment time and the dragging gas injection rate can beincreased. For the example mentioned above, a certain amount of carbon andmanganese were added. After 5 minutes of homogenization, the treatment is finished byclosing the main vacuum valve. The total treatment time is around 25 minutes for ladlesof approximately 315 tonnes of liquid steel. The vessel is flooded with nitrogen and steelreturns to the ladle when atmospheric pressure is reached in the vacuum chamber. Thetotal drop in temperature during RH process treatment is approximately 40C, in theexample mentioned above.The reduction in the hydrogen content is favoured by the presence of dissolved oxygenin the metallic bath. This is due to hydrogen being a reducing gas and also to CObubbles formation which facilitate the formation of H2 bubbles. However, H must beremoved when the steel is already deoxidized in the primary refining unit, becausesteels with low hydrogen requirements, are steels that should contain high content ofalloy elements (structural steels).If chemical composition correction to incorporate the necessary chemical elements weredone in the secondary refining stage, it would take a long time due to the large volumeof additions and also to make the corresponding samplings/analysis/corrections, as wellas it would cause excessive temperature loss, requiring a high steel tappingtemperature in the primary refining furnaces. Typical values of hydrogen in liquid steelare from 4 to 6 ppm before treatment and 1.5 to 2.5 ppm after hydrogen contentreduction in the RH process.The reduction in nitrogen content is done by following procedures similar to those inthe reduction of hydrogen content. However, the final nitrogen content reached ishigher. These differences will be presented later.The VD process (Vacuum Degassing), also called Tank Degassing basically consistsof placing a steel ladle in a container that is then hermetically sealed, initiating thedegassing process by starting the vacuum system. Figure 4.5 shows schematically, thetypical arrangement of VD process equipment.The tank is a container lined with refractory material normally of the silicous aluminous

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~ArcelorMittal Point Lisas Limitedtype, being dimensioned to receive and support the steel ladle during the treatment(Figure 4.5). It must have safety mechanisms against leaks or operational accidents,such as low melting point plugs.

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Figure 4.5 - Schematic representation of a VD (Vacuum Degassing) type secondaryrefining station and images of industrial facilities (image (b) Danieli do Brasil Uda. andimage (c) SMS DEMAG Uda.).

Constructively, vessels or tanks may be installed in transfer cars, which make possibleto position and remove the ladle, or the vessels can be fixed with a movable cap to sealthe tank. The operation to position and remove the ladle inside the tank is performedusing an overhead crane. The tank is closed at the top with a cap, which can be linedwith refractories or water cooled panels. To ensure the sealing of the tank water and/orrubber rings are used to ensure tightness. Ducts are installed on the cap or on the sideof the tank to connect with the vacuum generation system, in addition to the auxiliaryequipment. The cap movement is usually performed vertically through chains andhorizontally through sliding mechanisms, both driven by hydraulic systems. Some plantsmay use shields for protection under the cap, usually composed of water cooled panels,in order to minimize heat radiation from the liquid steel bath surface and slag to vesselwalls and from them to the external surfaces of the ladle. The vacuum system consistsof a series of pumps, usually water ring and/or ejectors which operate in sequence. The

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~Arce/orMitta/ Point Lisas Limiteddimension of the draining system with the definition of suction capacity (kg/h) as well asspecific consumption is done considering the internal volume of the vessel, the valuestargeted for deep vacuum (usually less than 1 mbar), the volume of inert gas bubledduring treatment and the loss of charge due to the construction limitations. Usually, thewater ring pumps are turned on during the initial stage of the process and are designedto reduce internal pressure to close to 100 rnbar. They have the advantage of beingmore economical than the ejectors, especially due to their lower energy cost, as it doesnot consume steam. Examples of typical VD auxiliary equipment can be:- Observation windows on the cover, usually two; one of them holding a video cameraan essential dvice for process control.- Lance for temperature measurement and for environment pressure and/or undervacuum sampling.- Silos, storage stalls and scales for alloy dosage, which may have manual or fullyautomated operation and be remotely monitored.- Device for alloy addition under vacuum.- Machines for alloy injection in wire shape.- Agitation and regulation control systems by inert gas injection or electromagneticfields.A point worth mentioning is the fact that, due to the intense agitation of the bath theinterface ladle refractory/slag/steel is highly significant. As this contact is not limited tothe slag line, the ladle lining has to be strengthened. Together with the requirement interms of greater free edge, the installation of VD process stations requires changes inthe steel ladles as an increase in its height/diameter ratio. Given that the largest cost inthe installation of a VD process is the vacuum system, some plants have two tanks for asingle vacuum system. The works in this case, is done in parallel, meaning that as aladle is being positioning or as another preparation stage is being carried out, thevacuum system is us

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Steel Making Fundamentals and Applications