Induction Furnace and Steelmaking
Induction Furnace and Steelmaking
Induction furnace is a type of furnace for steelmaking which uses electrical energy for its operation. Induction furnace (IF) steelmaking is one of the two electrical steelmaking processes. The other process for electrical steelmaking uses an electric arc furnace (EAF). Though IFs are being used since a long time, the production of mild steel by the IF is relatively not very old phenomenon.
The principle of melting in IF is that a high voltage electrical source from a primary coil induces a low voltage, high current in the metal or secondary coil. IF uses the heat produced by the eddy currents generated by a high frequency alternating field. The alternating magnetic field produced by the high frequency current induces powerful eddy currents in the charge resulting in very fast heating.
The development of IF has started with the discovery of the principle of electromagnetic induction by Michael Faraday. However it was not until the late 1870s when De Ferranti, in Europe began experiments on Induction furnaces. The first IF for melting metals was patented by Edward Allen Colby around 1900. The first practical usage of IF was by Kjellin in Gysinnge, Sweden in 1900. The first steel was made in an IF in the United States in 1907 in a Colby furnace near Philadelphia. The first IF for 3-phase application was built in Germany in 1906 by Rochling-Rodenhauser.
Characteristics of Induction furnace
There are mainly two types of IFs. They are (i) channel IF, and (ii) coreless IF.
The channel IF consists of a refractory lined steel shell which contains the molten metal. Attached to the steel shell and connected by a throat is an induction unit which forms the melting component of the furnace. The induction unit consists of an iron core in the form of a ring around which a primary induction coil is wound. This assembly forms a simple transformer in which the molten metal loops comprise the secondary component. The heat generated within the loop causes the metal to circulate into the main well of the furnace. The circulation of the molten metal causes a useful stirring action in the melt. The channel IF is normally used for melting low melting point alloys, or as a holding and superheating unit for higher melting point alloys such as cast iron. The furnace can be used as holder for metal melted off peak in coreless IF thereby reducing total melting costs by avoiding peak demand charges. Channel IF is not generally used for the steelmaking.
The coreless IF has a fairly simple construction. It basically consists of a refractory vessel and the surrounding coil borne by a steel frame. When an alternating current (AC) flows through the coil, it creates an electromagnetic field which in turn induces eddy currents in the charged material. This charge material gets heated up as per Joule’s law and with further heat the charge material melts.
The main component of the coreless IF consists of a crucible, a power supply unit consisting of transformer, inverter and capacitor bank, the charging arrangement, the cooling system for the power supply and furnace coil, process control system, and the fume extraction equipment. The schematic diagram of a coreless IF is shown in Fig 1.
Fig 1 Schematic diagram of a coreless IF
In case of a coreless IF, the heart of the furnace is the coil, which consists of a hollow section of heavy duty, high conductivity copper tubing which is wound in the form of a helical coil. Coil shape is contained within a steel shell and magnetic shielding is used to prevent heating of the supporting shell. To protect it from overheating, the coil is water-cooled, the water being recirculated after cooling in a cooling tower. The shell is supported on trunnions on which the furnace tilts to facilitate pouring. The crucible is formed by ramming a granular refractory between the coil and a hollow internal former which is melted away with the first heat leaving a sintered lining.
The furnace has two separate electrical systems, one for the cooling system, furnace tilting and instrumentation, and second for the induction coil power. The power for the induction coil is fed from a 3-phase, high voltage, high amperage electrical line. The power unit converts the voltage and frequency of main supply, to that required for electrical melting. Frequencies used in induction melting vary from 50 cycles per second (mains frequency) to 10,000 cycles per second (high frequency). The higher the operating frequency, the greater the maximum amount of power which can be applied to the furnace of given capacity and the lower the amount of turbulence induced.
When the charge material is molten, the interaction of the magnetic field and the electrical currents flowing in the induction coil produce a stirring action within the molten metal (Fig 2). This stirring action forces the molten metal to rise upwards in the centre causing the characteristic meniscus on the surface of the metal. The degree of stirring action is influenced by the power and frequency applied as well as the size and shape of the coil and the density and viscosity of the molten metal. The stirring action within the bath is important as it helps with mixing of alloys and melting of turnings as well as homogenizing of temperature throughout the furnace. Excessive stirring can increase gas pick up, lining wear and oxidation of alloys.
Fig 2 Stirring action generated by magnetic field
The coreless IF is normally used to melt all grades of irons and steels as well as many non-ferrous alloys. The furnace is ideal for remelting and alloying because of the high degree of control over temperature and chemistry while the induction current provides good circulation of the melt. In this furnace, since the charge material gets melted on its own by the generated heat, the emissions created by other types of steelmaking furnaces are not found. Typical installation diagram of an induction melting furnace is shown in Fig 3.
Fig 3 Typical installation diagram of an induction melting furnace
Bath agitation mechanism
The eddy currents induced in the furnace charge and the magnetic induction creates electromagnetic forces. These forces basically run in a radial direction to the furnace axis and press the melt inward away from the furnace wall. Gravity works against these forces and hence a dome (meniscus) is formed on the bath surface. Additionally a bath flow is created in the form of two eddy toroids with opposite direction of the turns. This is attributable to the fact that the radial pressure reaches as maximum around halfway up the coil due to the leakage of the field at the coil end. The power distribution and flow pattern is shown in Fig 4.
Fig 4 Magnetic flux generation by the coil current in a medium frequency crucible
The inductive bath agitation firstly leads to a good homogenization of the molten metal with respect to the temperature and chemical composition. It also stirs the charge materials and creates optimum heat transfer conditions for melting of the charge materials.
Power is supplied to the induction coil through a transformer, a frequency inverter and a capacitor bank. The capacitor bank is to compensate for the reactive power. Further since the induction furnace is switched on via a time ramp, all types of flickers and grid loading through rush currents are avoided. The current fed in by the inverter oscillates with a resonance frequency (within 60 % to 110 % of the nominal frequency) and it helps in constant load regulation in a simple manner.
Electrical energy required for heating one ton of steel to 1500 deg C is around 396 kWh. In the furnace, generally a large number of losses take place which increases the specific energy consumption to above 500 kWh per ton (kWh/t). The losses are mainly the thermal furnace losses, furnace coil losses, capacitor bank losses, convertor losses and losses on main side transformer. Around 20 % to 33 % of the energy losses are absorbed by the cooling water.
In a typical IF, the energy losses in the equipment are between 100 kWh/t to 130 kWh/t. The furnace efficiency is around 62 % to 75 %. With new development in energy efficient coils, new refractory material, reduction of converter and transformer losses, and the state of art furnace equipment, the energy losses are reduced to a level of 60 kWh/t to 90 kWh/t. The new furnaces have efficiency in the range of 81 % to 87 %. The usual energy losses of the coreless IF are shown in the typical Sankey diagram in Fig 5.
Fig 5 Typical Sankey diagram of a coreless induction furnace
Operation of induction furnace
Efficient operation of coreless induction furnace depends primarily on the implementation of good operating practices. The stages of making a heat in the IF are shown in Fig 6.
Fig 6 Stages of making a heat in the IF
Charge preparation and charging – The raw materials are weighed and kept near the furnace on the furnace charging floor before starting the melting. The charge is to be free from all the foreign materials including sand, dirt and oil/grease. Rusty scrap not only takes more time to melt but also contains less metal per charging. For every 1 % slag formed at 1500 deg C, the energy loss is 10 kWh/t. The scrap is to be clean. Exact weight of the ferro-alloys is to be kept ready, since the ferro-alloys are very expensive and their proper handling not only reduces wastage but also reduces the time lost in their addition.
The maximum size of single piece of metal/scrap is not to be more than one-third of the diameter of the furnace crucible. It avoids problem of bridging. Moreover, each charge is to be around 10 % of the crucible volume. Also, there are not to be any sharp edges, particularly in case of heavy and bulky scrap, as this can damage the refractory lining of the furnace. Further, the furnace is not to be charged beyond the coil level, i.e. charging the furnace to its capacity. It is to be understood that as furnace lining wears out, the charging can slightly increase.
Proper charge sequence is to be followed. Bigger size metal is to be charged first followed by charging the smaller size and gaps are to be filled by turnings and boring. The use of baled steel scrap and loose borings (machining chips) is to be controlled. Charge driers and pre-heaters are to be used to remove moisture, pre-heat the charge, and remove any oil or grease. Introduction of wet or damp scrap in the melt is to be avoided as this can cause explosion.
Melting and making ready the heat – It is essential that the furnace is always run with full power. This not only reduces batch duration but also improves energy efficiency. By the use of furnace cover, the radiation heat loss can be reduced substantially. The build-up of slag on furnace walls (Fig 7) is to be avoided. Typical slag build-up takes place near the neck, above coil level where agitation effect is less. Quantity of flux used for slag removal is important. Typically flux consumption is to less than 1 kg per ton of steel. Proper tools are to be used for de-slagging. Tools with flat head are to be used instead of rod or bar for de-slagging. They are more effective and take very less time.
Fig 7 slag build up near furnace crucible neck
Process control through melt processor leads to less interruptions. Typically process control reduces interruptions by 2 minutes to 4 minutes. Spectral testing laboratory is to be located near to steel melting shop to avoid waiting time for the chemical analysis of the heat and slag samples. Un-necessary super-heating of the liquid steel is to be avoided. Superheating by 50 deg C can increase furnace specific energy consumption by 25 kWh/t.
Tapping of the heat – The plant layout plays an important role in determining the distance travelled by the liquid steel in the ladle and the temperature drop. The ladle size is to be optimized to minimize the heat losses and to empty the furnace in the shortest possible time. The melting is needed to be synchronized with the casting of the liquid steel. Liquid steel is not to wait in the furnace. The ladle pre-heater is to be used to avoid drop of the temperature. Use of liquid steel to pre-heat the ladle is quite energy intensive and expensive. The quantity of liquid steel left in the ladle is to be as low as possible. Ladle covering compound is to be used to minimize the temperature drop due to the radiation losses from the ladle top.
Production of mild steel by Induction furnace
A large tonnage of mild steel is made globally through IF route. While producing this steel, the chemistry of end product is controlled. The chemical analysis of all the input materials is to be done to have a decision on the charge mix. After completing 50 % charging of the input materials, a bath sample is analyzed for chemical composition. Based on the chemical analysis of the bath sample at this stage, calculations are made for further additions of the metallics. If the bath sample at this stage shows high percentage of carbon, sulphur and phosphorus, then the sponge iron content of the charge is to be increased. Final bath sample is taken when 80 % melting is completed. Based on the analysis of this sample, there is another adjustment made in the charge. The lower content of carbon in the sample is corrected by increasing the quantity of pig iron/charge iron in the charge. Silicon and manganese in the metal is oxidized by the iron oxide of the sponge iron. Sulphur and phosphorus is also diluted by the sponge iron. Because of the use of sponge iron, the trace elements in the steel made in the IF remains under control.
Monitoring of the parameters and data analysis
Energy monitoring is the first step for achieving energy saving. It is desirable to install separate energy meter for the furnace. The energy consumption is to be monitored on heat to heat basis so that the energy consumption can be analyzed in correlation with the production data to arrive at the specific energy consumption of the furnace on daily basis. Any peak or valley in the data is required to be studied and investigated in conjunction with the tapping temperature and quantity of metal tapped. The water temperature for the coil cooling and panel cooling and the flow rate is required to be monitored. The panel is to be checked on weekly basis and cleaning is to be done on monthly basis. Effective raw material storage is important for optimum performance of the furnace. For example, the scrap, if it is stored on the mud floor, leads to the dust and moisture pick-up.
Lining of induction furnace
Lining is the important part of induction furnace. Furnace performance is directly related to the performance of its lining. Well laid and stabilized lining results in smooth working of furnace, optimum output, and good control of the metallurgical reactions. The lining practice best suited to a particular furnace depends upon the capacity and design of the furnace, operation practice adopted during making of a heat, and furnace output. For successful and consistent performance of the lining, the important aspects are (i) use of proper grade and quality of the lining material, (ii) careful and systematic lining practice, and (iii) consistency in working conditions.
Normally, the selection of refractory for the furnace lining is based on (i) the type and the size of the furnace, (ii) the type of the steel being melted, (ii) temperature of the molten steel, and (iv) the type and the composition of the slag generated during melting. There are three types of ramming masses used for the lining. These are (i) acidic, (ii) basic, and (iii) neutral. If the slag contains high amount of acidic components then a silica (SiO2) lining is used. For slags with a high basicity index, magnesite (MgO) linings are the choice. Neutral refractory has become the new trend for lining in the IFs. The ramming refractory mass used for neutral lining in the IF consists of a mixture of alumina (Al2O3) and sintered MgO blended according to a certain granulometry
For the lining of the IF, the correct lining material is to be selected. The lining thickness at the bottom or the sidewalls is not to be increased since the increase in the lining thickness means reducing capacity of furnace and increasing the power consumption. The furnace is not to be allowed to cool very slowly. Forced air cooling helps in developing cracks of lower depth, this helps in faster cold start cycle. Cold start cycle time is to be ideally not more than 120 % of the normal cycle time. Coil cement is to be smooth, in straight line and having thickness of 3 mm to 5 mm. While performing the furnace lining work, it is to be ensured that each layer is not more than 50 mm. Compaction is better with smaller layer.
Comparison with EAF steelmaking process
Compared with EAF, induction furnaces have the characteristics namely (i) high and relatively narrow melting vessel (large h/d ratio), (ii) low crucible wall thickness, (iii) low slag temperature, and (iv) powerful bath stirring. The comparison of some of the operating parameters of the IF with those of the EAF during the steelmaking process is given in Tab 1.
|Tab 1 Comparison of the operating parameters of IF with EAF|
|10||De-carburizing||Restricted by refractory wear||Possible by O2 blowing and slag reaction|
|13||Electrical supply||Low load||High load|
|Flicker disturbance||No flicker disturbance|
The induction furnace has several technical advantages over the EAF which includes (i) low requirement on the electric grid, (ii) relatively cleaner process and lesser environment related expenditure, (ii) higher yields, (iv) lower consumption of ferro-alloys, (v) no cost on electrodes, (vi) lesser capital expenditure, (vii) lower space requirement, (viii) suitable for charging addition agents any time due to the characteristics of the bath agitation, (ix) has low load and no flicker disturbance, and (x) automated application in a simple way.
The disadvantages of IF over EAF are (i) the requirement of minimal wall thickness of the refractory lining is having risk of crack formation resulting in stoppage of operations, (ii) induction furnaces puts more stringent requirement on the quality of scrap, (iii) de-carburizing, de-sulphurizing and de-phosphorizing is restricted due to refractory wear, (iv) the non metallic component of the charge materials is to be kept under control so that volume of the slag remains under limit and does not have adverse effect on the lining, and (v) compared to EAFs, IFs of very high capacities are not presently available.