Steelmaking in Induction Furnace

Steelmaking in Induction Furnace

Coreless induction furnaces have been used in the ferrous industry for over 50 years and are now one of the most popular means of melting and holding ferrous materials. Induction melting had dramatic growth during the 1960s based on line frequency technology, and later with the large-scale introduction of medium frequency power supply during the 1980s. Making of mild steel in the induction furnace was first experimented during early 1980s and it gained popularity when the production of sponge iron utilizing coal based process of rotary kilns became popular.

Induction furnace is a type of electric melting furnace which uses electric current to melt metal. The principle of induction melting is that a high voltage electrical source from a primary coil induces a low voltage, high current in the metal (secondary coil). Induction heating is simply a method of transfer of the heat energy. Two laws which govern induction heating are (i) electromagnetic induction, and (ii) the joule effect.

Coreless induction furnace comprises a relatively thin refractory crucible encircled by a water cooled copper coil excited from a single AC supply. When the coil is energized, the fluctuating axial magnetic field causes a current to flow in electrically conducting pieces of charge material within the crucible. The power induced in the charge depends on the physical properties of the material, the flux linking it and its geometric shape. Dependent on the resistivity of the material being melted, the coreless induction furnace converts electrical energy to heat the charge at an efficiency of between 50 % and 85 %, although furnace efficiency is further reduced by thermal losses from radiation from the melt surface and conduction through the furnace lining.

Medium frequency induction furnaces which are commonly used for steelmaking use the heat produced by eddy currents generated by a high frequency alternating field. The inductor is usually made of copper in order to limit the electric losses. The inductor is water cooled. The furnace consists of a crucible made of a suitable refractory material surrounded by a water cooled copper coil. In this furnace type, the charge is melted by heat generated from an electric arc. The coil carries the high frequency current. The alternating magnetic field produced by the high frequency current induces powerful eddy currents in the charge resulting in very fast heating.

Electrical energy needed for heating one ton of iron to 1500 deg C is 396 kWh. In furnace several losses takes place which increases the specific energy consumption. The losses consists of (i) thermal losses, (ii) furnace coil losses, (iii) capacitor bank losses, (iv) convertor losses, and (v) losses on main side transformer. The higher the losses lower is the furnace efficiency. Thermal losses contributes maximum towards loss of energy. The major thermal losses are (i) radiation loss from furnace top, (ii) conduction losses from refractory lining, (iii) heat losses in cooling water of the coil, and (iv) heat carried by the removed slag.

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. Electrical energy is required to be spent to substitute this heat loss. Hence longer the heat time the greater is the furnace inefficiency.

Raw materials

Coreless induction furnace is usually regarded as ‘dead melting’ unit, where effectively only minimal changes occur during the process. Hence the raw materials play an important role during steelmaking.

Raw materials for making a heat in the furnace is to be selected and controlled to ensure that the liquid steel made has the aimed mechanical properties and chemical composition after its casting in the continuous casting machine and is free from defects. Besides the quality of steel produced raw materials also affect (i) volume of slag produced, (ii) refractory lining life, and (iii) safety of both the plant and the working personnel. Further, raw materials along with their charging practice have a considerable influence on the specific consumption of electric energy and the furnace productivity.

The important parameters to be controlled in raw materials are (i) size, (ii) bulk density, (iii) chemical composition, (iv) cleanliness, amount of contamination, and freedom from rust, scale, sand, dirt, oils/grease, and (v) non-metallic coatings. Raw material charges with bulk density greater than 1 ton/cum give lower energy consumption than the charge materials with lower bulk density of around 0.5 ton/cum.

Amongst the various raw materials used for making a heat, metallics take the lion’s share both in terms of technology and economics. The main raw materials for steelmaking in induction furnace are (i) steel scrap, (ii) iron scrap or/and pig iron, (iii) sponge iron, (iv) carburizer, and (v) additives. Out of these the first three are metallics.

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 make the charging more difficult and protracted. This again affects the energy efficiency of the furnace.

Rusty scrap not only takes more time to melt but also contains less metal per charging. Scrap is to be checked to ensure that pre-coated steels such as tinned plate and zinc coated are not included, since these materials produce excessive amounts of metallurgical fume and slag. For every 1 % slag formed at 1500 deg C energy loss is 10 kWh per ton.

Unlike steel scrap, iron scrap, and pig iron, the sponge iron 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.

Additives used for making of steel in an induction furnace are normally ferro alloys. Ferro alloys are to be checked that they comply with the specification for size, grading and composition. In particular, they are to be checked for fines as this is a source of high losses and hence variation in the expected chemistry of steel made in the heat.

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. Petroleum coke and anthracite coal are two popular carburizers being used during steelmaking in the induction furnace. However 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 as an de-oxidizer, and (iii) improves lining life.

An accurate calculation of the necessary charge -mix based on material analyses, and a precise weight determination and metering of charge materials and additives (carburizer and additives) 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.

Efficient operation of the induction furnace depends primarily on implementation of the operating practices. The steps involved in operation of induction furnace are shown in Fig 1.

Fig 1 Stages of operation during steelmaking in an induction furnace

Charge preparation and charging

Energy consumption is significantly increased by incorrect charging practices. The worst practice is to charge a small amount and wait for melting to occur before adding further material. The best practice is to add charge to the level of the top of the power coil and to top up as the charge sink downs.

The raw materials are required to be weighed and arranged on the operating floor near the furnace before starting a heat. The raw materials to be charged are stored in suitable containers and are to be ready for charging by the chosen method.  The carburizer and additives are to be weighed accurately and handled properly to avoid wastages during handling.

The maximum size of single piece of metal/scrap is to be less than 0.4 times the diameter of the furnace crucible. It avoids problem of bridging. Further each charge of metal/scrap is to be around 10 % of the volume of the furnace crucible. Also, it is to be ensured that there is practically no sharp edges since this can damage the refractory.

Medium frequency coreless furnaces are operated without a sump (heel). Charging methods for these furnaces depend upon several factors which include (i) furnace size, (ii) furnace throughput, and (iii) charge materials used.

Both the mechanical or manual methods can be used but the three factors given above normally decide the method. For magnetic materials such as steel scrap, cast iron, pig iron and mill returns, overhead crane fitted with electromagnet is used for direct charging of the furnace. Sponge iron can be charged both by overhead cranes fitted with electromagnets or/and by manual methods.

Manual charging methods are only really suited to smaller furnaces. Where throughputs are high or the operating conditions are difficult, charge materials are added to the furnace by drop bottom buckets or vibratory chargers, which often incorporate weighing devices to ensure correct charge make-up.

Furnace is never to be charged beyond the coil level, i.e. charging the furnace to its capacity. Further it is to be understood that as the furnace lining wears out, the quantity of charge materials is to increase accordingly. Proper sequence of charging is to be followed. Charging of the light scrap at the bottom followed by heavier scrap at top protects the bottom lining from damage during charging. Charging of wet or damp material in the melt can cause explosion and is to be avoided.

Melting and slag removal

The material is charged into the empty furnace up to the upper edge of the furnace coil. When the electrical power supply is switched on, a voltage is induced in the charged material, which causes strong eddy currents. Due to the high electric current and the resistance of the material, the material is heated up to the point of melting.

The melting material settles together, and the furnace can be recharged with more material. In medium frequency furnaces, the material is not charged into the liquid bath, but onto the still solid material.

In case of sponge iron in the charge, 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.

For smoothening of the melting operation, periodical removal of slag is required as it gets solidifies on top of the liquid bath and hinders further melting of the sponge iron. 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 needed to ensure that there is enough liquid pool before adding sponge iron.

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.

Carbon in the form of anthracite or petroleum coke is normally added throughout the metallic charging period to improve mixing and reduce the amount of trim additions to be made to the fully molten bath. Medium frequency furnaces show less vigorous stirring action as the operating frequency increases. This in turn makes the addition of carbon to the fully molten bath more difficult.

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. Removal of the slag therefore is generally facilitated by the use of de-slagging spoons fitted with long steel bars. These spoons are specially made for the purpose.

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.

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. 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 %.

Making the heat ready, tapping and emptying the furnace

When the liquid filling level has reached around the upper edge of the coil, the sample is taken and the material for the final analysis is added to the furnace. This material is now melted, and the melt brought up to a temperature of 80 deg C to 100 deg C below the tapping temperature.

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 liquid temperature is measured with a dip thermocouple. 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.

Process control and automation

The modern concept of steelmaking shops involves control of all of the functions taking place so that a detailed knowledge of quality of liquid steel and costs can be collected. Earlier, only the larger steelmaking shops had a form of furnace control. However, the low cost of computers and programmable logic control (PLC) devices now allows control systems of varying degrees of complexity to be economically installed in smaller installations. These systems perform several functions which can be classified under the headings of (i) process automation, (ii) process monitoring, (iii) information display and recording, and (iv) interfacing with other furnaces and control systems.

Process automation – The most advanced automation systems can control the steelmaking cycle from the selection of charge materials to the tapping of the liquid steel and also interface with other management systems. Simpler systems only control the steelmaking operation. For functioning, these systems need information on charge weight, time and power input. The charge weight is obtained from load cells or input from the operator while time is known from the internal clock of the device which is reset at the start of the each heat. Power is derived from the voltage and current measurements for the furnace coil. The energy input is then calculated and compared with a set value which is determined from the experience of the manufacturer with similar furnaces and can be altered by the operator to suit the individual case. When the set value is reached the furnace is automatically switched off and the charge is molten at around the target temperature. Measurement of these parameters is reasonably accurate, however variation in the charge and how it lies in the furnace results in varying induced energy so that the temperature obtained varies between the heats. The next stage is to superheat the metal to the set tapping temperature which can only be achieved if the starting temperature is known. This is provided by ensuring that an accurate dip measurement of the liquid metal is taken, with the result either being directly fed to the control system or entered by the operator.

Between melting and superheating, the metal is normally deslagged, sampled and composition altered to meet specification. If required, the control system holds the temperature at any set value and calculates the optimum power level to do it. In this way an accurate control is kept on the energy supplied, avoiding high energy cost and excessive temperature.

The control systems can be used for other automatic operations such as (i) cold starting furnace, and (ii) sintering of a new lining. In these cases temperature data is provided by thermocouples and the system controls the temperature by varying the power input.

Process monitoring – While controlling the steelmaking operation, the system can also monitor the auxiliaries such as water, hydraulics, power supply and fume extraction system. When a problem occurs, an alarm display alerts the operator. A long term record can be kept of the coil current and its trends at a particular voltage as any increase can indicate lining wear. Hence, the system can provide the operator an indication when the refractory needs replacing.

Information display and recording – The control system provides the information and the more complex systems do it at all levels from operator to management. A visual display unit (VDU) gives information on energy consumption, power, temperature and metal weight in the furnace during melting, holding and superheating. The data is frequently shown in a graphical form to assist in reading the information. There are different menu screens for different functions such as (i) to indicate alarms, (ii) to fit the lining, or (iii) to tap. A slave monitor can duplicate the display away from the furnace platform. The simpler systems can have a liquid crystal display (LCD) which can provide the same general data but not graphically.

The operator can communicate with the system to input information or alter the settings, with keypads, light pens or touchscreens depending upon the manufacturer’s preference and the sophistication of the system. Records can also be provided for a heat, a shift or a month.

Interfacing with other furnaces and control systems – Systems are also designed to control the steelmaking operations with more than one furnace. The system then controls all the furnaces and optimizes the melting to give the required metal output from the entire steelmaking installation. It takes account of the power limitations imposed at some periods of the day by the contract with the electricity suppliers and optimize the steelmaking to give the best melting rate.

Systems have also been developed to report to and receive information from a control network operating at a higher level. It can act on information from the network such as a change to metal pouring temperature, change in alloy composition, raw material availability and charge weight and operate the furnaces accordingly.

The control systems described above are a method of providing automatic control of melting, holding and tapping. They can start the furnace from cold, control the lining sintering cycle and make a continuous diagnostic check on the power supply, the furnace components, the auxiliary systems and the furnace lining. Any fault or failure can be identified and drawn to the attention of the operator. Comprehensive records can be kept of all the data monitored for management control. The systems can control more than one furnace and be part of a larger network controlling all the functions of the melting shop. The most sophisticated systems are not cost effective for the small steelmaking shops. Some systems can be retro-fitted to existing equipment.