Electric Arc and Ladle Furnaces

Electric Arc and Ladle Furnaces

Are trademarks of the Techint Group

As part of the Techint Group that operates worldwide through more than 100 companies, Techint Technologies provides a wide range of products and services for the steel and related industry. Exchanging of know-how and expertise is a common practice within the Techint Group:Tagliaferri and Lectromelt, well known trademarks in the electric furnace activities, work in close association with engineering, construction and steel-making experts of the Techint companies. Our main offices are located in Italy and USA.

Tradition, expertise and steelmaking background

Tagliaferri and Lectromelt have designed and manufactured electric arc furnaces since the beginning of electrometallurgy. Process and product know-how have increased continuously, leading to the top technologies and equipment for successful steelmaking. As a steelmaker,Techint holds in-house know-how of steel making and is able to fulfil the customer’s most demanding expectations.Techint Technologies is the ideal partner for steelmaking equipment and technologies.

Clear design philosophy

The design concept is intrinsically simple, for reliable, very sturdy and long-life furnaces.Actually, they must withstand extreme stresses as well as high thermal loads in order to work properly in a very severe environment. Design simplicity also means customer’s satisfaction in terms of easy daily handling of the furnace. Operators can readily understand the furnace working mode and the related circuits.When maintenance is required, it can be performed simply and fast. All modern technologies for steelmaking are implemented in Tagliaferri-Lectromelt furnaces.This enables efficient process and a large charge-mix range. In other words, low cost production and high quality products.

Always innovative furnace design: some highlights

Lectromelt pioneered the first top charge furnace. Lectromelt developed the first power clamp. Lectromelt supplied the largest electric arc furnace ever built with a 38 ft (11,6 m) shell diameter. Tagliaferri was the first to introduce the hydraulic electrode regulation system for fast accurate and reliable electrode positioning. Tagliaferri’s Bottom Tapping Mechanism (TBT) is of original design, providing tangential flap rotation for extremely safe and reliable tapping. The furnace basic concept is unique, being characterized by a roof lifting and swinging system, which is completely independent from the shell supporting platform.

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Controlled environmental impact

Whether it is dust, fume, gaseous emissions, noise or electric network disturbances,Tagliaferri and Lectromelt provide all of the most advanced technologies in the field of environmental impact control.

Engineering services

Techint Technologies supplies engineering of tailormade solutions, including retrofits, upgradings, revampings or re-utilisation of existing equipment for optimized-cost solutions. Techint Technologies engineers insertion, modification and/or replacement of single parts, for example the electrode arms, the electrode regulation system or the TBT system.

Training programs, technical assistance and after sales service

Training programs are directed to personnel in charge of the operation and maintenance of the supplied equipment.They include practical and theoretical sessions to give the trainees knowledge of technologies, equipment functions, maintenance and operational procedures. Fast commissioning and quick learning curve can be guaranteed by Techint Technologies’ expertise and assistance. Assistance continues after sales through a comprehensive spare parts service.A dedicated internal office is in charge of the satisfaction of the clients’ requests.

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COMPLETE FURNACE PACKAGES

Techint Technologies supplies complete furnace packages consisting of technology and the equipment for all of the charging mix alternatives: = 100% scrap, whether batch charged or continuously Consteel® charged = up to 100% DRI or HBI continuously charged = up to 50% hot metal, fed by special devices A dedicated thermo-chemical package for each installation, including: = high thermal yield post combustion systems = side-wall, roof or door jet-burners = water-cooled and consumable lances for oxygen and carbon injection = bottom injection and melt stirring systems All the auxiliary plants in the meltshop area: = tailor-made slag formers and ferroalloys addition systems. = automatic manipulators for sampling and temperature measurement. = systems for uninterrupted foamy slag monitoring and slag composition adjustment. = specially suited electrode nippling and handling devices. = refractory gunning and fettling equipment.

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AC and DC Electric Arc Furnaces

Tagliaferri-Lectromelt can provide both AC and DC electric arc furnaces. The selection between the two different solutions depends mainly on the investment and operation costs. Moreover, when a weak electric power supply grid is available, the choice can be highly influenced by other elements such as the need of an additional Static Var Compensator (SVC). DC furnace shows proven advantages in terms of low graphite electrode consumption and reduced network disturbances (flicker level). The last generation of AC furnaces has also significantly reduced their flicker disturbances through the introduction of the high impedance technology. Additionally the flicker level is dramatically reduced when the electric arc furnace,AC or DC, is operating with a continuous scrap feeding system as the Consteel® provided by Techint Technologies.

High impedance AC furnaces

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High impedance technology improves arc stability, allows stable long arc operation, reduces electrode consumption and limits short circuit currents intensities. Series reactors on the primary side of the furnace transformer increase the circuit inductance.

Compact and rigid furnace design concept

The success of Tagliaferri-Lectromelt electric arc furnaces is built on its sturdy mechanical structure. In fact, it is well known that a rigid system is the prerequisite for a successful performance in a harsh environment.As vibrations and flexions are minimised, the electrode regulation is fast, of quick response and accurate. Moreover, electrodes are not subjected to the bending stress of oscillating holding arms. The shell supporting platform has a very reduced extension.The shell rests on it centered by pivots but without attachment.This allows for free and independent expansion and contraction of the platform and of the shell.

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To open the furnace, the superstructure is lifted up from the shell by an independent swivelling system. As the rotation axis of this assembly is far from the furnace shell, the roof rotation angle is small, ranging from 55 up to 60°. Thus, roof opening is fast.Thanks to the small angle, the electric cable twisting is avoided, too. Cable life is greatly extended and

Charging

ING.

LEONE

TAGLIAFERRI S.p.A

MILANO

Melting and superheating

After charging has been done and melting starts only the furnace superstructure is hooked directly to the shell platform, the roof swing and lift system remains disconnected so that furnace rotation during tilting can take place without interlock to it. Electrode arms are short and their elastic inflexion negligible, thus allowing an accurate electrode regulation. They are built from thick bimetallic sheets. Steel (inside) is in charge of the mechanical resistance while copper (outside) is the current conductor.Their

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no articulation in the cables is needed. The furnace is designed to always return by gravity to the deslagging position, thus it can be kept in level position by two simple and heavy rest bumpers.The rest stops, anchored to the concrete foundations, act directly as rockers’ stops and prevent the tilting cylinders from shocks or impact loads during scrap bucket charging. The roof lifting and swinging structure do not experience shocks during fall of scrap as it is disconnected from the shell carrying platform during charging.

closed section is completely water-cooled and houses the electrode clamping system, the pipe for spray water electrode cooling and the air blowing system for clamp cleaning. The central arm body design provides triangular arrangement in order to minimise the phase reactance unbalance and achieve well distributed power input of the electric arcs. The electrode pitch diameter is extremely small.Therefore, the arc is far away from the shell walls so that they are less affected by radiation.

Tapping

There is no need to lock the roof lifting and swinging structure during tapping or deslagging as it does not participate to platform tilting. The Tagliaferri Bottom Tapping system (TBT) is sturdy, simple and accurate. A swinging flap, pneumatically driven through a cooled shaft, allows tight closure, easy opening and reliable operation. The flap consists of just a stiff steel plate rotating tangentially to the furnace bottom surface so that it always follows its thermal expansion.The driving system

can be easily disconnected from the plug when removing the shell so that no second driver is needed for the spare shell. Residuals around the tap hole are smoothly cut by the tangential flap motion at taphole closing. Multiple mechanical interlocks prevent from accidental taphole opening. Moreover, there is a dedicated air tank, which acts as a buffer for emergency situations. Backtilting of the furnace is simply accomplished by gravity. Fast return makes sure slag free tapping is achieved.

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Maintenance

The upper shell and furnace roof are built with tubular water-cooled panels.Tubes are generally made of steel, except for the panels just above the slag line, which are made of copper to enhance heat transfer to the cooling water in the extremely heat exposed areas.The panels are arranged in a cage, which simultaneously is the manifold of the cooling water circuit. The water flow and outlet temperature of each panel can be constantly monitored. The water-cooled upper shell can be easily split from the lower shell containing the refractory lining.The lower shell simply rests on the rocker platform, centered by means of a couple of pivots. Therefore, shell removal and replacement, if intended or

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needed, can be easily accomplished in just a couple of hours. The roof is supported by water-cooled horizontal arms, mounted on the furnace superstructure. It is connected to the arms by means of four adjustable tierods and extractable pins. If maintenance operations must be performed on the roof, pins are extracted and the superstructure is lifted up. The fume elbow lifted by the ascending superstructure.The refractory delta replacement can be easily accomplished. Roof lifting cylinders are of original design and are characterised by ease of maintenance because the V-seal can be replaced without disassembly of the cylinder.

Electrode supporting and bearing system

Tagliaferri and Lectromelt have coupled water incompressibility in the hydraulic system with the right mechanics of the electrode bearing system to have a package of high rigidity and high resonance frequency. Vertical electrode shift is obtained by a single-acting hydraulic cylinder, incorporated in the electrode mast. Electrode masts slide vertically, accurately guided by two series of rollers. Moreover, a number of eccentrics allow column alignment by easy and accurate setting. Electrode masts are provided with holes and relevant pins for their mechanical locking during furnace shutdown. One centralized greasing system is provided for the column guiding rollers. Just a small number of other lubricated points requires manual greasing. Electrode clamps are made of forged copper to allow high clamping forces for high pressure contact and, therefore, low electric contact resistance.Watercooling channels are drilled inside the clamp body for efficient heat dissipation.The electrodes clamping force is achieved by a series of spring washers. It can be hydraulically released for electrode slipping or electrode column change.

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Furnace hydraulics

Tagliaferri pioneered the hydraulic electrode regulation of the furnace.A single circuit takes care of the electrode regulation as well as of all the other hydraulically driven furnace movements.The working fluid is simple: = 95% water + 5% emulsifying oil for low pressure operation (typically 64 bar) = water glycol for pressure operation higher than 120 bar All working surfaces of the hydraulic components are hard-chromed in order to achieve long life. The simplicity and reliability of the circuit is evident by looking at its scheme.There is just one level of pressure to manage, either low or high. Accumulators are included in the circuit to guarantee the maximum operational regularity and to assure remaining operability in case of power failure.Thus, either the electrodes and the roof can still be raised or the furnace still be tilted. As water compressibility is lower than oil, the stiffness of the hydraulic circuit is greater. Consequently, accuracy of electrode control is outstanding and all hydraulically driven movements are of extremely quick response. There are other relevant advantages related to the use of water in the meltshop area as it is environment friendly, cheap and not flammable.

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Electrode regulation system

Successful electrode regulation enables efficient and smooth energy input. In fact, efficient power transfer through the arc is realized provided the arc is stable. Arc stability depends on a great extent on a successful electrode regulation. The core of the electrode regulation system is Tagliaferri’s specially designed hydraulic distributor, controlled by a servovalve, which receives the signals from a microcomputer. Because the upward and downward electrode speed is proportional to the fluid flow in the cylinder located in the electrode column, fluid flow is of fundamental importance.The distributor makes sure that, being its displacement proportional to the error signal, the area available for the flow changes in accordance to the needs. Thus, in an error signal current/speed pattern the speed curve is flat at small deviations from the set point but steep in case the correcting signals are high.This allows a very sensitive electrode regulation.

EAF ONE LINE DIAGRAM PROTECTION AND MEASUREMENT

DIGITAL ELECTRODE REGULATOR CONTROL FUNCTION CONSTANT CURRENT 13

Top level digital electronic regulation

Tagliaferri’s-Lectromelt’s digital regulator makes sure that immediately after the feedbacks of electrode voltage and current signals are analogically collected and sent to a fast sampling input card, all calculations between feedback and output signals are of digital type. Adoption of digital components in the regulation loop provides several advantages. Setting of all set points and coefficients of the P.I.D. regulation loops is accomplished through a MMI (Man Machine Interface) video terminal. Additionally, auto-diagnosis and alarm signalling in case of fault, whether in hardware or in software components are of high quality. Finally, the system flexibility is greatly enhanced as future new functions can be implemented by simply adding new programme lines. The furnace can be operated whether with constant impedance, constant active

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power input or constant power factor. All the components of the regulation system are assembled in a water and dust proofed control panel and supplied with its own air conditioning system. The regulator includes several auxiliary functions: = Full downward electrode speed until approaching the arc zone. = Electrode fast rise system at short circuit situations. = Electrode rise in case of electrode-touch on nonconductive material. = Automatic current reduction at transformer tap changer movement. = Self-adaption of the operating electrical set points to the primary voltage fluctuations. = Autocheck of the zero position of the hydraulic regulator and consequent correction through an offset signal.

Automation and process control

The architecture of the system is hierarchical. All levels are interconnected: =Level 0: Field equipment =Level 1: Basic Automation (Level 1/A) and Process Supervision (Level 1/B) =Level 2: Process control =Level 3: Plant Management and Production Planning Communication among levels is accomplished by means of serial lines or high-speed data links (e.g. Local Area Network). Level 1/A interacts directly with the process, either autonomously or according to the directions from Level 1/B or Level 2. It includes PLCs, dedicated microprocessors and smart instrumentation.The basic automation carries out functions of primary process data acquisition (status, measurements, etc.), sequential logic, alarm processing and handling, regulation and direct actuation towards the process. At Level 1/B, the automation implements the operator interface (MMI), i.e. a set of functions which enable the operators in the control room to handle and control the Level 1/A devices in charge of each plant.Workstations include printers, colour video terminals or TFT technology, as well as control desks and panels. Level 2 coordinates and/or controls the Level 1 equipment to which it is connected. The Process Control system enables the operator to control in a centralized way the functions performed by the lower-level subsystems.The level includes PCs, mini computers and the related peripherals. Process Control supports technological and production data acquisition/elaboration, pre-set and set of machines/plant, working parameters prediction, operator guidance, numerical models and interchange of information with an eventual higher level system or external system.

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Level 1 PLCs operate and interlock the EAF

PLCs activate logic sequences for the operations of various parts of the EAF. These operations are interlocked to ensure safety and correct sequencing. In addition, PLCs carry out several automation functions: = collect signals from the field = control signal transfer to the field, typically to

= =

=

= =

=

MMIs guide the operator during the heat

MMI functions are grouped in the following classes: =

=

=

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supervision of equipment status through various production phases operator guidance by producing pages of summarized data or operating practice pages for each steel grade to be manufactured data base storage, e.g. steel

electrovalves and motors communicate with MMI receive the liquid steel temperature measurement results collect cooling water temperatures and flows data collect EAF operating electrical parameters collect signals related to the energies (electrical and thermal) entering the EAF collect alarm signals

grades data base, power profiles and setting parameters. In particular, steel grades tables contain the main parameters of the steel to be manufactured like range for chemical components, range of tapping temperature, basic power profile, basic additions and tables of operating practices.

The melting and refining process is continuously followed and controlled

The Process Control monitors the different phases of the melting process, from charge preparation to tapping. Main functions of the controller are: =

= =

Heat and production reports are generated

automatic meltdown control, in terms of generation of a suitable melting power profile. Power profile is divided in a number of melting steps, with associated transformer tap changer position, electrode current set points and stability factor of the regulation loop. charge composition monitoring calculation of electric and thermal energy consumptions, including gas and oxygen as well as slag formers

Data stored during melt down and refining operations are summarized at the end of the heat in a general report. The report includes, for example, basic heat information, codes, summary of the charges, energy consumption records for each phase, consumption

= =

= =

= = = =

calculation of stirring gas consumption liquid steel temperature measurement and calculation.The calculation is made taking into account the electric energy input and the losses due to fumes, water cooling and heat transfer through the walls. analysis collection and recording voltage and current set points to electrode regulator power on/power off times recording calculation of furnace electrical parameters signalling of particular events to the operator feedback signals from the EAF, for example bath temperature, foamy slag, etc.

indexes, chemical analysis and steel temperature, productivity indexes and interruption/faults records. Each report is transmitted to a central computer for production accounting over a time period.

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The DC bottom electrode system

A design of a bottom electrode has been patented to ensure that high electric performances are in optimized interaction with the refractory material. The consumable part of each of the bottom electrodes consists of a steel tube with a refractory core.The steel pipe melts but the refractory core avoids quick melting and lowering of the molten front; a steady state is reached.The steel tube is welded to a water cooled copper head, which carries the current to the furnace bottom. Electrode consumption is monitored by thermocouples on the shell bottom plates around the electrodes.Also, the cooling water temperature is monitored. If molten lead penetrates between the bottom electrode and the refractory material, it is drained away from the bottom electrode insulation in order to avoid electrical contact between the electrode and the furnace shell. Basically, flexible cables and current bus tubes are arranged underneath the shell bottom in a symmetric pattern, namely four perpendicular ways. Therefore electromagnetic forces are self-balanced and the electric arc is vertical. No correcting coils are required underneath the shell. The four electrodes have independent current regulation.This is accomplished by four thyristor rectifier banks and four current regulation loops. Residual arc deflection can be corrected by unbalancing the current intensities. Furnace refractory and bottom electrode maintenance is very simple.The shell is interchangeable and can be removed without unscrewing the electric connection under the bottom.This is possible as electric contact between bottom busbars and cable heads is assured simply by shell resting on the tilting platform.The time required for a shell replacement is the same as the one for an AC furnace. Refractory materials employed for the furnace bottom and walls are similar to those of AC furnaces. The electrode refractory rings dismounting and installation practice is similar to the procedure of exchange of an eccentric bottom taphole refractory set.

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Ladle Furnaces Know-how for ladle metallurgy

Ladle metallurgy stations fulfil several functions after tapping in order to prepare the melt to be processed in the Continuous Casting Machine: =

= =

Temperature and composition homogenization Carburization Desulphurization

=

Alloys addition Adjustment of the chemical composition = Setting the content and shape of inclusions = Accurate setting of the casting temperature Simultaneously, ladle furnaces act as a buffer between EAF and CCM, thus smoothing and optimizing the process flow. =

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Ladle furnace installations

Tagliaferri and Lectromelt offer complete ladle furnace packages including electric power supply, ladles, ladle transfer cars, additives charging and injection systems, dedusting and all the auxiliary equipment for the related area, e.g. the electrode make-up stations.

Facilities for flexible ladle metallurgy

Each heating station is equipped with the units for an enhanced ladle metallurgy: =gas

stirring system injection machines =temperature, carbon and oxygen measurement lances Layouts are studied to allow installation of additional components, typically a Vacuum Degassing (VD) unit. =wire

Ladle furnace design concept Completely water-cooled furnace hood

During the heating phase, the ladle is covered with a water cooled roof of tubular type. Water temperature of each section is monitored by temperature sensors on the outlet circuits. The roof is movable in the vertical direction, in order to allow proper positioning of the ladle under the furnace. Roof rise is carried out by a single hydraulic cylinder through chains and pulleys. By this way, the hood is perfectly horizontal when approaching the ladle. Due to the principle of indirect fume suction, a gap is intentionally provided between the roof and ladle,

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so that correct positioning is also accomplished in case the ladle borders are not totally clean. A pressure sensor in the working cylinder makes sure the hood is placed as close as possible to the ladle border. Several openings are provided in the ceiling of the roof in order to allocate the various accessories of the ladle furnace.These accessories include: chute for alloys addition, lances for sampling and temperature measurement, inspection door, emergency top stirring lance and wire injection system. Openings are provided with remote control closing flap to prevent air intake or fume escape.

Electrode positioning

Extremely small electrode pitch diameter

Ladle furnace hydraulics

Electrode vertical movement is provided by a single-acting cylinder incorporated in each electrode mast. Masts are guided by adjustable rollers at the top and bottom of each column. The guide rollers are

lubricated by a centralized system and shielded from grit and dust.

The electrodes are arranged on a small diameter circle, thanks to the special arrangement of electrode holding arms.The arms are completely water-cooled and their triangulation leads to a well balanced power input.

The electrode clamping mechanism is housed in the arm body.The electrodes are clamped by springs and hydraulically released, for safety in case of power failure.

Electrodes are independently powered by the Tagliaferri hydraulic regulator, which is electronically piloted.

An integrated compressed air blowing system provides clamp cleaning before clamping the electrodes.

Furnace movements and electrode regulation are both carried out by means of hydraulic cylinders.The working fluid is water-based, synthetic and nonflammable.The hydraulic components’ working surfaces are thickly chromed. In case of electric power failure, the electrodes are automatically raised to the top position. 21

Emission controls

Indirect fume evacuation protects from steel bath oxidation

The fume suction system has been specially designed to strictly avoid the circulation of air inside the ladle furnace. In principle, fumes are not sucked directly from the furnace. Conversely, evacuation is operated on the surplus fumes leaving the furnace, either from the roof gap or from the electrode holes. Successful fume evacuation is achieved through a careful

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balance of the depressions in the following areas: = =

circular duct around the ladle roof cylindrical hoods around the electrodes

The cylindrical electrode hood can be either watercooled or provided with an internal refractory lining. Refractory rings are arranged around the electrodes to maximize hood sealing.

LF Electrode regulation system

Successful electrode regulation enables efficient and smooth energy input. In fact, efficient power transfer through the arc is realized provided the arc is stable. Arc stability depends on a great extent on a successful electrode regulation. The core of the electrode regulation system is Tagliaferri’s specially designed hydraulic distributor, controlled by a servovalve, which receives the signals from a microcomputer. Because the upward and downward electrode speed is proportional to the fluid flow in the cylinder located in the electrode column, fluid flow is of fundamental importance.The distributor makes sure that, being its displacement proportional to the error signal, the area available for the flow changes in accordance to the needs. Thus, in an error signal current/speed pattern the speed curve is flat at small deviations from the set point but steep in case the correcting signals are high.This allows a very sensitive electrode regulation.

Top level digital electronic regulation

Tagliaferri’s-Lectromelt’s digital regulator makes sure that immediately after the feedbacks of electrode voltage and current signals are analogically collected and sent to a fast sampling input card, all calculations between feedback and output signals are of digital type. Adoption of digital components in the regulation loop provides several advantages. Setting of all set points and coefficients of the P.I.D. regulation loops is accomplished through a MMI (Man Machine Interface) video terminal. Additionally, auto-diagnosis and alarm signalling in case of fault,

whether in hardware or in software components are of high quality. Finally, the system flexibility is greatly enhanced as future new functions can be implemented by simply adding new programme lines. The furnace can be operated whether with constant impedance, constant active power input or constant power factor. All the components of the regulation system are assembled in a metallic board, water and dust proofed and supplied with its own air conditioning system.

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Techint Technologies Complete Systems for the Metal Industry Electric Arc Furnaces are the core of a mini mill supplied by Techint Technologies. Additionally,Techint Technologies is the combined strength of leading trademarks in the field of equipment for the total package of steelmaking, casting, rolling, reheating and product processing.

Techint Group through the DST companies (Dalmine, Siderca and Tamsa) is the world leader in seamless tubes’ manufacturing. Moreover, Siderar and Sidor are major producers of flat steel products in Latin America.Worldwide the Group produces over 8 million tons of steel per year.

Techint companies also provide a full range of services, including feasibility studies, project financing, project management, basic and detail engineering, procurement, construction, start-up, and technical assistance in operation and maintenance.

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For further information please contact

Steel Plant Division Via Leonardo da Vinci, 20 21053 Castellanza (VA) / Italy phone: +39 0331 444 557 fax: +39 0331 444 547 E-mail: [email protected] Techint Technologies Inc. Cherrington Corporate Center 100 Corporate Center Drive Coraopolis, PA 15108-3185 / USA phone : +1 412 262 2240 fax: +1 412 262 6090 E-mail: [email protected] www.techint-tech.com

Are trademarks of the Techint Group Techint Compagnia Tecnica Internazionale S.p.A. Headquarters:Via Monte Rosa, 93 - 20149 Milano, Italy phone: +39 02 4384.1- fax: +39 02 4693026 - E-mail: [email protected]