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Production of Steel in Induction Furnace


Production of Steel in Induction Furnace

Medium frequency coreless induction furnace is generally used for the production of steel in the steel melting shops of low capacity. The induction furnace is equipped with a converter for producing the necessary medium frequency from the 50 Hz frequency of the power supply. For this, a direct voltage is produced in a rectifier, and is fed to the inverter via a smoothing choke, and a medium frequency voltage is produced in the inverter with the aid of compensating capacitors and the inductivity of the furnace coil. The regulation of the converter is carried out by the built-in control electronics. The control of the furnace is carried out using the devices in the operating cabinet and if necessary with the aid of a processor.

A transformer is used for the energy supply. The furnace transformer is connected to the power supply network. The transformer converts the supply voltage to the voltage required for the operation of the furnace which is generally 770 V for medium frequency induction furnace. The transformer is usually equipped with the built-in monitoring devices such as thermometers, oil filling level monitoring, Buchholz relays and air de-humidifiers.

The smelting is carried out in the refractory crucible made normally with either acidic (silica based) or neutral (alumina based) monolithic refractories. The crucible is heated by an induction furnace coil surrounding the crucible.

Making of a heat in an induction furnace consists of certain cyclic activities. These activities are known as ‘heat cycle’ or ‘production cycle’. A heat cycle has two components namely (i) melt cycle, and (ii) non-production cycle. The melt cycle is the period when maximum power is continuously applied to the furnace and the charge is added. The non-production cycle is when no or reduced power is being applied, such as when the initial charge is being added, when slag is being removed, when a temperature dip or analysis sample is being taken, waiting for an analysis result, and tapping of the furnace empty etc. The furnace utilization is the melt cycle divided by the heat cycle expressed as a percentage. If the melt cycle is of 80 minutes and the non-production cycle is of 40 minutes, then the heat cycle is 120 minutes. The 80 minute of melt cycle divided by the 120 minute of the heat cycle times 100 gives a utilization of 66.67 %. If in induction furnace, it is a process which requires 10 tons of liquid steel to be tapped per heat and the heat cycle is such that it can only achieve 66.67 % utilization, then it is necessary to have power supply capable of melting 15 tons per heat.

The induction furnace for melting sponge iron is required to have a large ratio of cross sectional area to volume so that the heat transfer is high and to keep the slag hot and fluid.

The induction furnace uses the transformer principle of induction, i.e. when an electrical conductor is placed in a fluctuating magnetic field then a voltage is induced in the conductor. In crucible furnaces, this voltage causes strong eddy currents, which due to the resistance of the material, cause it to be heated and ultimately to melt. The water is used for the cooling of the coil. The cooling water lines are monitored with regard to volume and temperature.

During the production of steel, substantial quantity of electrical energy is needed. Besides the theoretical energy required for producing steel, energy is also required for compensating the losses which are taking place while producing steel. The energy losses increase the specific energy consumption and decrease the furnace efficiency. The losses which take place during the production of steel are (i) thermal losses, (ii) furnace coil losses, (iii) capacitor bank losses, (iv) convertor losses, and (v) losses on main side transformer.  Thermal losses are the main losses and contribute maximum towards loss of energy. The major thermal losses in induction furnace (Fig 1) are (i) radiation loss from the furnace top, (ii) conduction losses from the refractory lining, (iii) heat losses in the cooling water of the coil, (iv) heat carried by the removed slag, and (v) heat carried by the gases being emitted from the furnace top. Further, during the making of a heat, the furnace is constantly losing heat both to the cooling water and by radiation from the shell and the exposed metal surface at the top. Electrical energy is required to be spent to substitute these heat losses. Hence longer is the heat time the greater is the energy consumption and lower is the furnace inefficiency.

Fig 1 Major thermal losses in induction furnace

The factors which affects the consumption of electrical energy in the furnace (Fig 2) include (i) dirt going in the furnace with scrap, (ii) rusty charge material, (iii) low bulk density of the scrap, (iv) recarburizing of steel when the steel is almost ready, (v) not using full power for melting, (vi) excessive formation of slag, (vii) excessive generation of fumes and emissions, (viii) excessive losses of metal due to spillage and splashing, (viii) time of making a heat since longer production cycle means higher thermal losses due to radiation and conduction, and (ix) holding of the completed heat in the furnace.

Fig 2 Factors affecting consumption of energy in induction furnace

Metal losses for metallic charge materials depend upon the physical size of the component and their quality, but are normally less than 5 %, with a fair proportion of this loss being due to spillage and splash during the de-slagging and pouring operations. The one factor which has the maximum effect on the energy consumption is the level of the furnace utilization. Higher is the utilization means energy efficient production cycle.                        

Charge materials

Charge materials used for making a heat are important for controlling the quality of steel being made in the induction furnace. The materials ensure that the liquid steel made has the aimed mechanical properties and chemical composition after its casting and is free from defects. Besides the quality of steel, the charge materials also affect (i) volume of slag produced, (ii) life of refractory lining, and (iii) safety of both the plant and the working personnel. Further, the charge materials along with the charging practice have a considerable influence on the specific consumption of electrical energy and the furnace productivity.

In induction furnace, the main charge materials are metallics consisting of scrap and sponge iron. Both the steel scrap and iron scrap is used. Iron scrap brings carbon to the furnace bath. Pig iron is also sometimes used in some furnaces for the purpose of introducing carbon to the bath. The ratio of these materials used for producing a heat depends on their relative availability at the economic cost at the plant location. In case of induction furnaces using high sponge iron to scrap ratio, a carburizer (e.g. anthracite coal or petroleum coke) is also added for controlling carbon content of the bath. Metallics are charged in the furnace either mechanically or manually.

The control of the melting operation in the furnace and the chemistry of the liquid steel are dependent on the degree at which the mix of metallics can be optimized. The qualities of metallics are required to be known for proper charge mix for efficient operation of the furnace.  

For improving the quality of produced steel, input scrap quality is required to be controlled. The important parameters needed to be controlled in scrap charge are (i) size, (ii) bulk density, (iii) chemical composition, (iv) cleanliness of the scrap materials meaning that they are to be free of contamination such as rust, scale, sand, dirt, oils/grease, and (v) non-metallic coatings such as zinc, tin, and chromium etc.

The most troublesome residual elements (such as copper, cobalt, tin, arsenic, antimony, nickel, and molybdenum etc.) from scrap are ultimately concentrated in steel. Their presence in steel induces undesirable resistance to deformation, hot shortness, and mechanical defects.

If the scrap sections are long and extend out of the top of the furnace, these, though ultimately melt but take time, and hence influence the furnace utilization. The size of the scrap is important to ensure the charge does not bridge. On an average, each piece is not to have a dimension greater than 33 % of the furnace diameter and no dimension is to exceed 50 % of the furnace diameter. The feed rate of the system is to be able to deliver the full charge into the furnace within 65 % to 70 % of the actual melt cycle.

The initial materials are required to be charged in the furnace as quickly as possible and of sufficient density to allow maximum power. For optimum performance, the density of the charge materials is needed to be high and is not to be less than 1.3 tons per cubic metre. The quantity of initial furnace charge materials is to constitute a substantial percentage of the rated capacity of the furnace.

During the melting of steel scrap, most of the scrap is suspended with air inside the furnace. As the induction field raises the temperature of the scrap, it now must go all the way to the melting point of steel, because there is no carbon present to lower the melting point. Hence, this requires more energy and time for the initial melting. In addition, once the steel reaches a temperature of around 700 deg C the increase in oxidation becomes dramatic and during the heat up from 700 deg C to around 1540 deg C, the surface of the steel scrap continues to oxidize at a higher and higher rate. Once molten, the droplets of steel continue to oxidize as they fall down the charge until they reach the bottom of the furnace and join the molten bath with hopefully higher carbon. The carbon in the bath stops the oxidation of the iron. The thinnest steel scrap can go from room temperature to glowing cherry red colour within just one or two minutes increasing the oxidation. The oxides of iron increases the amount of slag formed. All of this oxidation produces a highly reactive FeO slag.

Cleanliness of the scrap is very important since dirty or contaminated scrap tends to deposit a slag layer on the furnace refractory. This occurs at, or just below, the liquid level in the crucible and restricts the quantity of power which is drawn by the furnace. The effective reduction in the internal diameter of the furnace can also be there which makes the charging more difficult and protracted. This again affects the energy efficiency of the furnace. Further, rusty scrap takes more time to melt. It also contains less metal per charging. Dirty metallics charge results into higher volume of slag which means higher specific consumption of power. For every 1 % slag formed at 1500 deg C energy loss is 10 kWh per ton.

The sponge iron charge in the furnace is characterized by (i) high porosity, (ii) low density, (iii) low thermal conductivity, (iv) high specific surface area, (v) high oxygen content, and (vi) intermediate carbon content. Sponge iron has uniform chemical and physical characteristics. It has low percentage of tramp metallic elements (around 0.02 %) and low sulphur content but usually is associated with high phosphorus content. Sponge iron with higher carbon content is preferred since it reduces the requirement of the carburizer in the furnace.

The melting process of sponge iron is considerably influenced by the physical, chemical, and thermal characteristics of the sponge iron. Some of these characteristics are shape, size, density, chemical analysis, and degree of metallization. Other parameters such as the method of charging, the type of furnace, the temperature of the bath, the chemical composition of liquid metal in the furnace, and the flow of fluid inside the furnace and around the particles are also of appreciable importance.

The gangue content and unreduced iron oxide content of the sponge iron is needed to be as low as possible. Low iron oxide content is important for safety reasons as well as for energy consumption reasons. If a large quantity of unreduced iron oxide is introduced into a high carbon bath at high temperature, there is a vigorous carbon boil which can be extremely dangerous.

Advantages of using sponge iron in induction furnace are (i) no additional desulphurization is needed and at the same time the low sulphur content in the steel can be achieved, (ii) final product contains low amount of residual metals like chromium, copper, molybdenum, tin etc., (iii) charging time decreases which also reduces the overall heat loss, and (iv) improves the product quality consistency.

The role of carburizer during steelmaking in the induction furnace is to remove oxygen from the sponge iron which is present in the form of FeO and to provide carbon pick up in the liquid steel to the desired level. Anthracite coal and petroleum coke are the two popular carburizers being used during steelmaking in the induction furnace. Recovery of carbon depends on the size and quality of the carburizer, method of addition, and time of addition. It can be expected to be within a range of 85 % to 95 %. Higher ash content in the carburizer lowers the carbon being added to the bath, while increasing the generation of slag. Carbon input in the bath through pig iron or cast iron scrap is more desirable in order to have better recovery of carbon. Use of very fine particle size of the carburizer is to be avoided because of excessive loss. Other carburizers which can be used are metallurgical coke, iron carbide and metallurgical silicon carbide (63 % silicon and 31 % carbon). Silicon carbide is normally charged with scrap and has the advantages of (i) faster absorption, (ii) acts also as a de-oxidizer, and (iii) improves the lining life.

Besides metallics and carburizer, deoxidizers are used for making of steel in an induction furnace. Deoxidizers are ferro alloys (silico-manganese, ferro-manganese, and ferro-silicon), and aluminum. The yield of the ferro alloys depends on their specification (size, grading, and composition).

An accurate calculation of the charge-mix based on material analyses is necessary. Also, a precise weight determination and metering of the charge materials and additives (carburizer and deoxidizers) are basic prerequisites for minimizing melting times and power needs besides ensuring proper composition of the liquid steel. The use of clean and dry charge materials is necessary for better result.

Regardless of whether the furnace is to be charged manually or mechanically, the charge materials are required to be weighed and the materials are to fit into the furnace. A crane scale can be used to weigh the charge material.

Charging and melting operation

Medium frequency coreless Induction furnaces for making steel are operated without a sump (heel). The material is charged into the empty furnace up to the upper edge of the furnace coil.

Immediately after the tapping of the previous heat, the condition of the lining material need to be inspected and then the scrap charging is to start. With the start of scrap charging, the heat cycle starts. The quality of charge materials, sequence of their charging has substantial influence on the heat cycle.

As soon as the first lot of scrap is charged in the furnace, power is switched on and current starts flowing at a high rate and a comparatively low voltage through the induction coils of the furnace, producing an induced magnetic field inside the central space of the coils where the crucible is located. The induced magnetic fluxes are thus generated through-out the available charge in the crucible. As the magnetic fluxes generate through the scrap and complete the circuit, they generate and induce eddy current in the scrap. This induced eddy current, as it flows through the highly resistive bath of scrap, generates tremendous heat and melting starts. It is thus apparent that the melting rate depends primarily on two things namely (i) the density of magnetic fluxes, and (ii) compactness of the charge. The denser is the charge and occupying more space in the furnace, it reduces the melting time and hence the energy consumption.

The heating of the scrap starts as soon as sufficient charge material is in the furnace to enable power to be applied. The goal is to get the energy into the charge as quickly and efficiently as possible. A power supply able to deliver maximum power throughout the heat cycle, always achieves the best melt rate. As the charge goes through the melting process, the voltage applied to the coil is allowed to increase. This increase gives two advantages namely (i) it ensures maximum kilowatts are continuously applied to the coil, and (ii) a high coil voltage means that the voltage induced into the charge is higher and hence the contact heating in the charge is more efficient. Typically, this results in a 10 % improvement in the melting rate as compared to a power supply where the power draw drops as the charge passes through the melting process.

In a medium frequency furnace, the heat is developed mainly in the outer rim of the metal in the charge but is carried quickly to the centre by conduction. Soon a pool of liquid metal is formed in the bottom causing the charge to sink. The melting material settles together, and the furnace can be recharged with more material. In the medium frequency furnaces, the material is not charged into the liquid bath, but onto the still solid material.

At this point, additional charging is to be done gradually. The eddy current, which is generated in the charge, has other uses. It imparts a molten effect on the liquid steel, which is thereby stirred and mixed and heated more homogeneously. This stirring effect is inversely proportional to the frequency of the furnace. The melting continues till around one half of furnace volume is filled with the liquid steel. At this point a sample is taken for the analysis and the furnace is deslagged in a slag pot by tilting. Slags generally developed in medium frequency coreless furnaces are not fluid and is quite heavy and sticky and often dry and in the form of a dross. During the removal of the slag, the power is to be off to ensure all the slag floats to the surface and can be removed. The longer the power is off the greater is the effect on the overall furnace utilization.

Based on the analysis results, the requirement of further charge of scrap, sponge iron and carburizer is determined and the charging is continued. In case the bath develops a convex surface, then the power input is decreased temporarily to flatten the convexity and to reduce the circulation rate.

Sponge iron can be added directly into the liquid metal when the stirring action accelerates the transfer of heat to it and promotes the melting. Care is required to be taken to have enough molten pool before adding sponge iron.

When sponge iron is charged in the furnace, continuous removal of slag is required for smoothening of the melting operation. This is because slag gets solidifies on top of the liquid bath and hinders further melting of the sponge iron. The continuous removal of slag is carried out by scooping the slag out of the furnace. Removal of the slag is generally facilitated by the use of de-slagging spoons fitted with long steel bars. These spoons are specially made for the purpose. The slag removal with spoon is possible since the slag is thick at this stage and its viscosity is high. The manual slag removal is a hard and unpleasant job.

Manual removal of slag can be enhanced by using a slag coagulant. The slag coagulant exfoliates to tie the slag pieces together so they can be lifted off. If slag coagulants are used to aid the removal of the slag, their use is to be strictly controlled to prevent chemical attack on the furnace lining material.

Slag volumes can be reduced by selecting clean and proper charge materials and with sponge iron having higher percentage of total iron. The enhancing of the melting rate also reduces the slag formation.

In case of higher amount of sponge iron in the charge, there is a need for carbon (anthracite coal or petroleum coke) addition to the bath for the removal of oxygen. The oxygen present in the sponge iron is in the form of FeO, which reacts vigorously with carbon in the liquid bath and improves heat transfer, slag-metal contact and homogeneity of the bath.

Irrespective of charging mode, sponge iron is always charged after initial formation of molten pool (i.e. hot heel) by melting of steel scrap. Melting of sponge iron is greatly influenced by factors like carbon content of the liquid bath and degree of metallization of sponge iron. Carbon content of the liquid bath reacts with unreduced iron oxide content of the sponge iron giving evolution of CO and CO2 gases from liquid bath i.e. carbon boil takes place, which results into subsequent removal of hydrogen and nitrogen gases, ultimately producing clean steel. Carbon boil occurs at slag metal interface by the reaction 3 FeO + 2C = 3 Fe + CO + CO2. 

Carbon content in the liquid bath is to be kept at a proper level in order to maintain appropriate carbon boil during the melting period. The amount of carbon required (C, in kg) to reduce the FeO content of the sponge iron is given by the equation C = 1.67 [100 – % M–{(% Slag /100) x % Fe}]. Here, M is degree of metallization and Fe is amount of iron in the slag.

Making the heat ready, tapping, and emptying of the furnace

When the liquid filling level reaches around the upper edge of the coil, i.e. heat is about to be completed, bath analysis sample and bath temperature is taken with the help of dip probes. For this activity power is kept under hold. Immediately after the temperature dip and analysis sample are taken, holding power is restored to the furnace. For the sake of accuracy and speed, spectrographic analysis is usually done.

Based on analysis results trimming additions are carried out in the baths for adjustment of bath analysis. The trimming addition material is melted, and the bath temperature is brought up to a temperature of 80 deg C to 100 deg C below the tapping temperature. The carburizer used for trimming needs to be small-grained to increase its surface area as this ensures that it goes into solution quickly.

When the tapping ladle is ready, the furnace is skimmed and brought up to the tapping temperature. In the case of medium frequency furnaces, 2 to 5 minutes are needed for this activity. The tapping temperature is to be decided taking into account, the chilling effect of the ferro-alloy addition. Before tapping a small amount of ferro-alloys are charged in the furnace so as to avoid any boiling action during tapping.

In the teeming ladle, the required amount of ferro-alloys and carburizer (if required) is put in the ladle bottom and the metal is tapped. During tapping, the faster the furnace is emptied the better it is. The time taken for emptying of the furnace affects the furnace utilization.

Precautions required and safety issues

The smelting process is always associated with dangers due to molten material which cannot always be accurately estimated in advance. It is often said that known dangers are no dangers, or at least dangers which can be anticipated and counteracted. The important safety related issues during steelmaking in induction furnaces are due to the ejection of molten metal in the form of splashes, small and large drops, heat radiation from the melting bath and water vapour explosions. These occurrences are explained here.

Metal splashes with a relatively low volume of melt are created when very small metal parts come into contact with the melting bath and are ejected from the melt. If these parts are also wet or damp, this leads to the ejection of small and large drops. The operator on the operating floor is exposed to a great deal of heat. If the operator is not using proper protective equipment (PPE), this can lead to burns on the skin and damage to the eyes.

Water vapour explosions always occur when liquids get under the surface of the bath. In extreme cases, 1 cc (cubic centimetre) of water penetrating deep below the surface can expand in a moment to 1,600 times its original volume. Water can get into the melting bath during the melting process from the materials charged or by damp or wet tools.

When operating the induction furnace, it can happen that the ramming mix has suffered damage, and the melt has been moved forward up to the coil. If this condition leads to a blockage of the windings and the release of water, water can also penetrate under the melt, resulting in a sudden upward ejection of the melt. This can cause powerful water vapour explosion causing the melt thrown out onto the furnace platform.

The important precautions required and the safety issues are described below.

  • Neatness and tidiness of the workplace which means that the furnace platform is to be tidy at all times, with the necessary tools ready to hand in their proper places. Any other materials or objects lying around are required to be removed without delay.
  • Adequate lighting at the workplace ensures that irregularities or problems on the furnace platform can be recognized and rectified in time.
  • Damage to equipment, operating switches, electrical and hydraulic lines are to be noted in the log book and reported to maintenance so that the repairs can be carried out. Indicating lights are safety devices, and need to be tested in planned intervals.
  • The condition of the crucible is required to be inspected visually after every emptying or every tapping. Possible cracks in the crucible wall are indicated by dark traces, which can then be inspected more closely.
  • The materials to be charged are to be inspected when being prepared. Pipes, tubes or hollow components are to be sorted out by hand, and checked to ensure that they do not hold any water since it can lead to water vapour explosions.
  • Visitors or personnel from other areas are to be made aware of the dangers and they are to be told to remain at a safe distance.
  • The minimum PPEs required by the personnel at the furnace operating floor are safety helmet, safety shoes, long trousers, cotton clothes, and protective goggles with side protection.
  • The emergency outlet channel must be kept dry and clean at all times.
  • The furnace body is to be inspected once every week, and cleaned every month of dust, small particles of scrap and other impurities.
  • Any oil which has leaked out is to be picked up and the spot is covered with sand. The leak is to be located and repaired.
  • Two emergency escape routes are to be available from the furnace platform in the event of accidents. These routes are to be kept clear at all times, and are not to be blocked even for short periods.
  • When working with metal tools in the melting bath, and with the furnace switched on, the tools are to be earthed, or the operator is to at least wear dry leather gloves. Such work is only to be carried out with the furnace switched off. The tools are to be warmed up over the bath before immersion, in order to remove any damp or humidity.
  • The formation of bridges is to be avoided in order to prevent the unforeseen breakthrough of molten material to the outside. If a bridge has formed, the furnace is to be switched off and tilted, so that contact with the melt can be made using a thin handspike. In some cases, the bridge can be melted with the furnace at low power and in the tilted position and the furnace then recharged with more material through this opening in the basic position, and then fully melted.
  • In the event of a power failure when the furnace contains a full melt, and it is not known how long it will take to correct the problem, the further procedure must be established. There are two options – either to allow the melt to solidify, or to empty the crucible.
  • The electrical insulation of the live components against earth is measured with the aid of an earthing relay. If the melt at earth potential approaches the coil, the resistance is going to fall, and the system is to be switched off.
  • If work is to be carried out with the furnace in the tilted position, the furnace is required to be secured against tipping. The furnace is also to be secured when pushing out the crucible.

The condition of the crucible is needed to be inspected visually, and the remaining wall thickness determined with the aid of measuring devices. An assessment of the average remaining wall thickness can be made from the frequency display.

 

 

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