STEELMAKING AND CASTING Renaissance of the VHD (VAD) plant Following re-evaluation of the VHD (VAD) vacuum heating degassing (vacuum arc degassing) process, INTECO has made a number of design modifications to improve its operation and effectiveness, making it a competitor with secondary steelmaking plants with separate ladle furnace and vacuum degassing units. Authors: Johannes Obitz, Dieter Brück and Roland Krist Inteco The original VAD development work at A Finkl & Sons, USA, started in 1964. Following continuous trials, numerous patents were issued from 1970 onwards. At that time the process met all the demands for high productivity and steel quality thanks to the versatility of the process. The majority of VAD plants were installed between 1970 and 1990 in combination with a basic oxygen converter or an electric arc furnace. Today, however, there are only approximately eight VHD plants still in operation. The main reason for low interest in the recent past can be found in the improvement of the conventional ladle furnace design and operation. INTECO special melting technologies, Austria, has been evaluating VAD technology in recent years and believes that improvements it has made in design and operation will revive interest in this process for secondary steelmaking. One of the eight plants is an INTECO-designed plant in China commissioned in early 2011. GENERAL PROCESS DESCRIPTION The VHD process offers the possibility of applying a variable sequence of metallurgical processes to liquid steel in a ladle before casting, the choice of which is determined by the metallurgical and operational requirements. Completely closed treatment stations in conjunction with steam ejector vacuum pumps create an environmentally favourable facility where heating, degassing, slag treatment and alloy adjustment take place without interruption under vacuum conditions. The melt (15-200t) is tapped into a standard ladle with a freeboard of approximately 500-800mm which is then lowered into the vacuum vessel by a shop crane. The vacuum cover, supported by three hydraulic cylinders contained in a lifting frame, is equipped with three adjustable electrodes in the cover centre and is then placed on the vessel flange (see Figure 1). The vacuum inside the vessel is produced by means of a multi-stage steam ejector vacuum pump. During the treatment, inert gas is introduced through the ladle bottom to provide circulation of the steel. Having reached the vacuum required, the electrodes are adjusted to the r Fig 1 Cross-section through unit showing main components desired position above the molten steel and the electrode circuit is closed. The graphite electrodes are installed inside a vacuumtight telescopic tube system. It is usually not necessary to adjust the distance between the electrodes and the surface of the melt because an electric arc is longer under vacuum. As a result, the electrodes are kept in good condition since they do not come in contact with droplets of molten steel or slag, and are not oxidised by atmospheric oxygen. As a minimum the electric arc compensates the melt from heat losses, but additional heating is also possible so that the process can be continued as long as necessary for operational or metallurgical reasons. Above the electric arc there is a heat shield in the vacuum vessel to prevent excessive heating of the vessel cover. The energy applied to the surface of the melt pushes the slag aside so that the gas can readily escape from the melt. The treatment is a 79
Module Fixed installed vacuum vessel Vessel cover transfer and lifting system Vessel transfer system Vessel cover lifting system Hydraulic system Electrode lifting system by plunger cylinder Electrode lifting system by hydraulic cylinder Electrode lifting system by winch system Electrode guiding by guided plunger cylinder Electrode guiding by guide column High current tubes on supporting arms Copper plated support arms Electrode telescopic system Combined Cu-graphite electrode system Temperature and sampling system Alloying system Vacuum system, mechanical vacuum pumps Vacuum system, steam ejector vacuum pump Electric and instrumentation Level 2 automation system Utilities supply and discharge Steel structure and buildings Design Under floor or on floor level Travelling or swivelling Under floor or on floor level Hydraulic or mechanical Combined or separate systems Stroke approx. 1,600mm Stroke approx. 1,600mm Stroke approx. 2,500mm Guiding frame with rollers Guiding column and carriage with rollers 1 to 4 tubes per phase Under examination Inner/outer telescopic tube with sealing Sealing on Cu-tube Under atmospheric condition Under vacuum or atmospheric condition No steam required Steam and condenser cooling water used r Table 1 Module design options continued until the melt corresponds to the metallurgical requirements and the desired gas content is reached. ADVANTAGES The most important advantages of an arc heating system are as follows: ` Lower tapping temperatures and consequently, a longer life of refractory lining both in the electric arc furnace and the ladle ` Larger quantities of alloying agents can be added ` Shorter primary furnace time, hence higher production of the plant and lower cost Additional benefits of treatment under vacuum include: ` Treatment can be continued as long as necessary to achieve lowest possible gas values ` The possibility of longer treatment cycles under vacuum enables steelmakers to leave the ladle in the heating station as long as necessary, for instance, in the event of delays at the casting facilities STATE OF DEVELOPMENT IN DESIGN AND ENGINEERING The design of secondary metallurgical facilities is governed by a number of factors. Those of greatest importance are: Steelworks layout The facility has to be integrated into the layout of the existing steelworks. Ladle size, column spacing, and height of overhead cranes are all conditions which influence the layout of secondary steel making equipment. Material flow The facility has to be integrated into the existing material flow logistics with the operating cycle times of upstream and downstream units playing a major role in the design of the facility components. Availability of equipment The demand for high availability is one of the essential design factors to be considered by the plant builder. The extent to which the mechanical and electrical equipment will be available for operation is largely predictable if the facility is appropriately engineered with special know-how, and provided that the user has sufficient experience and skill in operation of similar equipment. The modular concept has proved to be useful in the selection of suitable equipment for each application. Each module comprises a number of facility components which in their interaction can be regarded as an independent functional group and part of the facility as a whole. The main modules of VHD units which a standard turnkey order may comprise are shown in Figure 1. Engineering such units therefore consists of appropriately selecting and assembling the modules best suitable for the specific requirements (see Table 1). 80
STEELMAKING AND CASTING r Fig 2 Vessel cover transfer and lifting system Design and main sub-assemblies r Fig 3 Hydraulic lifting system r Fig 4 Telescopic tube system a Vessel transfer system A vessel transfer system is employed where the treatment station is outside the range of the shop crane approach or in the case of a combination of VHD with VD/VOD units. The shop crane places each ladle on the ladle supports inside the vacuum vessel. The car then travels to a position under the vessel cover and the cover is lowered onto the vessel flange which is equipped with a vacuum seal. The design of the vessel transfer car is adapted to the ladle size. Vessel cover transfer and lifting system A vessel cover transfer and lifting system (see Figure 2) is employed where it is possible to use a stationary vacuum vessel. After the shop crane has placed the ladle into position, the cover together with the electrode heating system, is moved above the vacuum vessel by means of a traversing mechanism and is lowered onto the vessel flange by use of a lifting and lowering mechanism. Vessel cover traversing is generally by use of electro-mechanical drives, but hydraulic drives are also used. Vessel cover lifting and lowering system The lifting systems for the VHD cover in the past were mainly actuated either directly or indirectly by oil hydraulics. Most of the systems had three cylinders, and synchronisation was made by stroke measurement and proportional valves. Some plant users, however, regarded oil hydraulic systems as a potential safety hazard, therefore designers had to find safer technical solutions. Two electro-mechanical concepts are: ` The counterweight system Lifting and lowering is by a gear motor as well as a sprocket and chain arrangement which act on a hollow shaft positioned transversely to the cover car runway. A sprocket mounted on each end of the hollow shaft supports 81
r Fig 5 High current bus-bars and supporting arms r Fig 6 Contact heads a chain which is connected to a counterweight at one end and a chain at the other end. Each of the cross beams on which the vessel cover brackets rest is suspended by the hoist for lifting and lowering. The use of counterweights significantly reduces the electrical power required for lifting and lowering because it is not the cover weight but only the friction losses and the inertia of the masses that have to be considered in the power requirement calculation. The counterweight represents a relative simple technical solution which has proved its value especially for smaller cover loads and lower strokes. ` Spindle system The spindle system is a successful concept for greater cover loads and higher strokes. A central drive acting through cardan shafts and angular gears actuates the spindle, which produces the lifting and lowering motions. The spindles are suitable for the highest loads. They feature additional mechanical safeguards. Synchronisation of the system is mechanical and no space has to be reserved for a hydraulic room. Electrode lifting system A common oil hydraulic system is employed for non-simultaneous cover lifting and electrode positioning. The plunger cylinder operates with a pressure up to 160 bar. The plunger cylinder with the guiding frame moves the supporting arm directly together with the telescopic tube. The stroke of this system is limited to approximately 1,600mm (see Figure 3). As a technical alternative, a normal standard hydraulic cylinder can be used for electrode positioning. In this case the hydraulic cylinder moves a carrier guided by a column which is mounted on the vessel cover. The electrode stroke can be increased to approximately 2,500mm. The stainless steel ground plate for the support of the telescopic tubes has been modified for a reliable cooling water circulation and the sealing system for telescopic tubes has been modified for easier maintenance. An alternative system is an electro-mechanical winch in which the same guiding method is used for a standard hydraulic cylinder. The output signals from the electrode controller are supplied to a variable speed drive which operates a winch and therefore affects lifting or lowering of the electrodes. Direction of movement and speed are determined by the intensity of the magnetic field generated in the variable drive coupling. The winch system in addition permits various arrangements to suit the space situation of the melt shop. For instance, it is possible to install the winch above the vessel cover or on the shop floor. In this case no space is needed for a hydraulic room. Telescopic tube system The telescopic tube system is a proven electrode sealing system for many VHD facilities with transformer capacities up to 24MVA and an electrode diameter up to 450mm. In the latest design (see Figure 4) the external tube is fixed while the internal tube is mounted underneath the electrode clamping device. This construction is suitable for any of the electrode lifting systems described above. To provide high availability and low maintenance time the telescopic sealing device is equipped with an automatic lubrication system. For dust protection the surface of the inner tube is protected by textile compensators. The compensator and lubrication 82
STEELMAKING AND CASTING system has been improved and additional attention has been devoted to the design for dust protection. High current bus-bars and supporting arms The contact jaw is connected to the high current tubes arranged on the supporting arm (see Figure 5). The size and number of high current tubes are in relation to the transformer size and secondary current. INTECO is currently investigating the possibility of using copper-plated arms as used on modern electric arc furnaces. This offers the benefits of low maintenance cost while improving electrical efficiency. Contact heads The copper contact head (see Figure 6) consists of a double-walled, water-cooled housing connected to the lifting arm. An adaption socket, prepared for bolting to the corresponding electrode arm on site, is positioned on the connection side of the contact head. The contact clamp consists of forged copper with drilled cooling channels to achieve a good heat transfer. The cooling water supply to the contact clamp is assured by a flange connection. This guarantees a good current transfer from the contact clamp into the electrode. The electrode Please visit us at METEC on 28.06-02.07.2011 clamping device is also integrated in the clamping head and is thereby protected. The necessary high clamping pressure is achieved by a disc spring package. The release of the clamping system is done by means of pneumatic cylinders. The electrode holder is electrically insulated against the arm body and electrode. CONCLUSIONS A significant number of design changes have improved the operating performance of the VHD and put it in renewed competition as a useful and modern alternative to a combined ladle furnace and vacuum degassing facility. A new INTECO-designed plant has recently been commissioned in China and metallurgical results will be published in due course. MS Johannes Obitz, Dieter Brück and Roland Kristl are with Inteco, Bruck an der Mur, Austria. Contact: Johannes.Obitz@inteco.at