Smelting technology is one of the key links in the production of cast iron, and cupola is widely used. For a period of time, induction melting furnace has been developed rapidly, and cupola is gradually replaced by induction melting furnace with sufficient power. At present, the capacity of coreless induction furnace has reached 110 T, while that of cored induction furnace is as high as 270 t. In recent years, the development of medium frequency induction furnace and its advantages have injected new vitality into the production of cast iron.

1.2 induction melting furnaces

At present, the induction melting furnaces which are most widely used in cast iron production are power frequency induction furnace and medium frequency induction furnace.

The frequency induction furnace can make the metal melt and raise the temperature, and the heating is uniform, the burning loss is small, it is convenient to adjust the composition of molten iron, and the pollution is small. However, the frequency induction furnace is usually used for remelting and heating of metals and alloys because of its slow melting speed, unfavorable slagging, need of starting block and inflexible production. In addition, the power factor of power frequency induction furnace is low, and a large number of compensation capacitors are needed, which also increases the floor area and equipment investment.

The induction furnace has the advantages of high electrical and thermal efficiency, short melting time, low power consumption, less floor space, low investment, easy to realize process automation and production flexibility.

Medium frequency induction furnace is suitable for melting cast iron, especially alloy cast iron, nodular cast iron and vermicular cast iron. It also has strong adaptability to burden, and the variety and lump size of burden can be changed in a wide range. It should be pointed out that although the medium frequency induction furnace has many advantages and develops rapidly, the power frequency induction furnace is still a good equipment in the aspects of liquid iron storage, heat preservation, alloy composition adjustment and overheating, especially the duplex furnace used as blast furnace, cupola and other smelting furnaces. The most famous and largest centrifugal ductile iron pipe manufacturer in the world, France's musongqiao company adopts the dual process of power frequency induction furnace and blast furnace, while Japan's Kubota company adopts the dual process of power frequency induction furnace and cupola. As early as the early 1980s, Shougang foundry first adopted the dual process of power frequency induction furnace and blast furnace or cupola in China, successfully trial produced continuous casting ductile iron pipes, and provided high-temperature and high-quality molten iron in future production, which has been used up to now with good results.

2 development of medium frequency induction furnace

At present, the development of power frequency induction furnace to large capacity has basically stopped, while the development of medium frequency induction furnace is remarkable.

2.1 static frequency converter

The development of medium frequency induction furnace benefits from the use of static frequency converter. Compared with magnetic frequency converter, the efficiency of this frequency converter is as high as 95% ～ 98%. The rated power of frequency converter used as induction furnace has been continuously improved. Recently, the 9 000 kW frequency converter has been put into operation. When it is connected to the furnace with a capacity of 12 T, the productivity of molten iron can reach 18 T / h. If the power density of medium frequency induction furnace is increased to 1 000 kW per ton, the melting period can be shortened to 35 min. The melting rate of induction furnace varies with the capacity of the furnace. Generally, the melting rate of cast iron melted by medium frequency induction furnace is 0.4 ~ 35 t / h. For example, the melting rate of 2-2.38 T / h can be obtained by using 2 T capacity furnace, and 18-21 T / h can be obtained by using 12 T capacity furnace; while the melting rate of 1.5 T furnace is 0.75 T / h, 3 T furnace is 1.5 T / h, 5 t furnace is 2.5 t / h, and 10 t furnace is only 4 T / h. It can be seen that the melting rate of medium frequency induction furnace is much higher than that of power frequency induction furnace, which creates conditions for the selection of melting equipment for cast iron production to replace the large one with small one. Replacing power frequency induction furnace with medium frequency induction furnace not only reduces land use, but also reduces investment, and ensures the continuous supply of molten iron The production of cast iron with large capacity is very favorable. If the medium frequency induction furnace is used for molten iron melting in the production of continuous casting or centrifugal casting ductile iron pipes, its production capacity can be fully developed by replacing cupola or double connecting with blast furnace and cupola. For example, a centrifugal ductile iron pipe manufacturer in China uses a 10 T medium frequency induction furnace and blast furnace dual process to raise the temperature and adjust the composition of the molten iron. It takes about 27 minutes to raise the temperature of the stored molten iron from 1300 ℃ to 1520 ℃. The frequency of the furnace is 100-200 Hz and the power is 2500 kW.

The electric efficiency and thermal efficiency of medium frequency induction furnace are high, which not only improves the melting rate and shortens the melting time, but also reduces the unit power consumption. Compared with the power frequency induction furnace, its power consumption can be reduced from 700 kW. H / T to 515 ~ 580 kW. H / T. Relevant data show that, considering the energy loss required for slag melting and overheating, the unit power consumption of medium frequency induction furnace is about 582 kw.t/h during cold start, 505-545 kw.h/t during hot furnace operation, and only 494 kw.h/t during continuous charging operation.

2.2 production flexibility

The medium frequency induction furnace has great flexibility in production arrangement and melting operation. For the frequency induction furnace, it is required to work continuously, because the interruption will increase the cold start. The cold start not only increases the melting time and energy consumption, but also has to use the starting block every time. When the medium frequency induction furnace is used intermittently for a short time, it can be cold started without using the starting block. It is very convenient to change the burden, and the molten iron can be completely emptied. It can realize the material change in a short time, which is conducive to the production organization and arrangement. This brings favorable conditions for single piece small batch production. For example, the industrial machinery repair plants in iron and steel metallurgical enterprises are mainly responsible for the production of spare parts, spare parts and large consumption parts required by iron and steel enterprises, as well as the manufacturing of some non-standard equipment. They are mostly single piece small batch production, with many varieties, and the material requirements are often changed. In this respect, the production flexibility of medium frequency induction furnace makes up for the shortage of power frequency induction furnace and other smelting furnaces.

2.3 furnace structure

With the continuous improvement of power density of medium frequency induction furnace, the requirements for safe operation of furnace, improvement of lining life and reduction of noise become higher and higher, and the rationality of furnace structure becomes more and more important. Among them, heavy steel shell furnace has many advantages, such as strong durability, high efficiency, high productivity, low noise and easy maintenance.

The heavy-duty steel shell furnace is different from the frame furnace. It is a high-strength annular steel shell with many large inspection ports. When the furnace is running, the inspection ports are closed, and each inspection port can be opened during maintenance. The internal structure of heavy-duty steel shell furnace is solid, which can avoid the deformation caused by inclined cast iron and prolong the service life of furnace lining. Moreover, the sound-absorbing isolation material can be added to the enclosed solid furnace shell and its interior, which greatly reduces the working noise. The strong steel shell can also effectively protect the induction coil from the danger of metal splashing, so that the furnace has the maximum safety in the operation process. In order to effectively insulate and improve the lining life, cooling rings are set at the top and bottom of heavy steel shell furnace, which can achieve uniform lining temperature and reduce thermal expansion. The stainless steel cooling ring with low energy consumption and high strength greatly improves the furnace efficiency.

The heavy-duty steel shell furnace not only has a solid steel shell, but also has a coil with thick wall tube structure specially designed for induction melting. By correctly selecting the turn to turn distance of the induction coil, the coil has the highest conversion efficiency and the lowest resistance. On the one hand, the thick wall induction coil has large current carrying section, uniform wall thickness, and is not easy to be damaged due to arc and expansion. On the other hand, the coil adopts a special support system, with each turn firmly locked and good rigidity, which reduces the vibration caused by electromagnetic force and greatly prolongs the service life of furnace lining. In addition, 50% yoke is covered around the induction coil, which effectively changes the distribution of magnetic field. At the same time, the isolation cushion also reduces the noise and vibration, and increases the efficiency of the yoke. In addition, the induction coil is extended beyond the bottom of the heavy-duty steel shell furnace, so that the coupling between the burden load and the magnetic field from the top to the bottom of the furnace is very uniform, the energy conversion efficiency is improved, and the influence of the stirring force caused by the magnetic field on the bottom lining is reduced.

Reasonable furnace structure is very important, heavy steel shell furnace is only one of them. Many manufacturers are committed to improving the furnace structure to adapt to the application and development of medium frequency induction furnace.

2.4 slag removal and furnace repair

The productivity of medium frequency induction furnace is high, and the melting period can be shortened to about 35 min. in order to maximize the power of the furnace, the slag must be removed as soon as possible. Using skimmer or manual slag removal, the effect is poor, the time is long, and the working conditions are bad. In order to solve this very time-consuming and heavy work, there is a method of slag dumping from the back of the furnace, that is, pouring the slag into the transport vehicle through the slot on the back of the furnace top. This method is fast and convenient. The furnace back tilt angle is generally 20 & deg; ~ 25 & deg;.

The intermediate frequency induction furnace must be dismantled and repaired after one furnace service. In order to shorten the shutdown time of replacing refractory lining, mechanization must be considered. Vibration furnace building and lining pusher have become the main parts of large induction furnace. The lining pusher can push out the refractory lining before it is completely cooled, which further shortens the furnace repair time and improves the working environment.

2.5 dual power supply and control system

A unique development of medium frequency induction furnace is to use a set of power supply system to supply power to two furnaces, realizing the working system of no production interval operation. Practice has proved that the total effective power of an induction furnace can not be fully utilized in the whole melting period, so it is necessary to reduce the power or cut off the power when measuring the molten iron temperature, sampling, slag removal, tapping, especially in the case of pouring. If the pouring cycle is long, the utilization rate is only about 50%, which indicates that the rated power of power supply equipment should be 1.8 times of the utilization rate of 90% in order to achieve the required productivity. In order to solve this problem, a dual power supply system is developed. The system uses two identical converters and capacitor banks, one for each furnace, but they are powered by common rectifiers and transformers. Each converter can be controlled independently, and the total effective power can be distributed to two furnaces in any proportion. This system can provide sufficient power for one furnace, and the rest power can be used for the second furnace for molten iron smelting. Once the smelting furnace is ready for pouring, the power of the converter will be reduced to the insulation level immediately; at this time, the other furnace will be emptied, and then the cold material will be loaded again to increase the power of the converter and start a new smelting. If necessary, the total effective power can be connected to one furnace and the other furnace can be stopped; or the total effective power can be equally distributed to two furnaces to make them melt at the same time. For example, dual-trak dual power supply and control system of American Yingda company can supply power to two furnaces at the same time, and the power transmitted to two furnaces is stepless distribution. By using this power supply, the melting power can be transmitted to one furnace and the heat preservation power can be transmitted to another furnace at the same time, so that the utilization rate of power supply can reach 100%. This kind of power supply supplies power to two furnaces at the same time, which completely avoids switching switch or adding another set of power supply. It is not necessary to switch the power supply to the holding furnace to maintain the necessary pouring temperature during smelting, so that the intermediate frequency furnace can realize two functions of melting and holding. When one furnace is in maintenance state, the power supply can be isolated from the furnace, and only supply power to the other furnace, which also increases the safety.

It should be mentioned that the medium frequency induction furnace equipped with dual power supply and control system can ensure the normal production of the other furnace even when one furnace is drying, so as to realize the timely and continuous supply of molten iron, which is unmatched by the power frequency induction furnace. The practice of No.1 transformer and No.2 furnace in Shougang foundry has proved that two 10 t induction furnaces are powered by one set of power supply. When one furnace is drying, the other furnace can not be produced. If it takes about three days to dry the furnace once, it will not be able to produce for about one month in a year, which has a great impact. In order to achieve the purpose of continuous production, 1 is to increase the number of frequency induction furnace, 2 is to increase a set of oven drying power equipment. When Shougang foundry builds the centrifugal ductile iron pipe workshop, it is to add a set of power frequency induction furnace and corresponding power supply equipment on the basis of the original No.1 transformer and No.2 furnace; another domestic centrifugal ductile iron pipe manufacturer plans to add a set of furnace drying power supply equipment on the basis of the original No.1 transformer and No.2 furnace. No matter which measures mentioned above are adopted, the corresponding equipment and facilities should be increased, which will increase the land occupation and investment. The application of double power supply and control system of medium frequency induction furnace not only ensures the melting and heat preservation, but also realizes the non-stop production of furnace drying.

2.6 degree of automation

The medium frequency induction furnace adopts computer control system with high degree of automation. For the high-power medium frequency induction furnace, the melting period is very short and the overheat rate is very high. Only by using the computer control system can the potential of the equipment be brought into play and the most effective protection be provided for the factory and operators. If the furnace with 25-30 ℃ / min superheating and melting charge is unattended for a few minutes at full power, it may cause catastrophic accident. At present, the large medium frequency induction furnace has been equipped with a relatively complete computer-aided monitoring and control system

(1) When the molten iron in the crucible reaches the pre programmed temperature, the power of the furnace can be reduced automatically;

(2) The furnace is fixed on the weight meter, which provides the quality data of the metal charge in the furnace for the computer;

(3) Monitor, calculate and control the temperature of molten iron;

(4) Provide automatic sintering program for new lining;

(5) Determine the amount of carbon, silicon and other added elements needed to achieve the required composition of molten iron;

(6) Other useful and necessary functions including an automatic cold start device.

In addition, the installation of automatic mechanical charging system is also indispensable for the full potential of high-power medium frequency induction furnace. Generally, the vibration feeding system installed on the track can move horizontally and longitudinally or rotate around the pivot is the most suitable.

The melting processing system of high efficiency medium frequency induction furnace is composed of a microprocessor with a color monitor, which monitors the functions and operating procedures needed in the whole melting process by a melting processor. Under the joint action of furnace charging system and weighing system, the power required for melting to pouring temperature is calculated automatically, and the energy absorption required for melting process is controlled during melting period. Under the action of electric bridge, the automatic charging of furnace melting process can be realized; when the molten iron reaches the pouring temperature after melting, the treatment system can automatically switch the heat preservation process and achieve the expected superheat with minimum energy; through additional program, the cold start of furnace and the sintering process of lining refractory can be automatically controlled under the condition of the most favorable protection of furnace lining.

The melting treatment system can carry out a comprehensive self diagnosis of the power supply and furnace body. If the system alarm is started in an abnormal state, the treatment system can provide the problem, store the fault information, and connect with the printer for printing and reservation.

3 conclusion

Medium frequency induction furnace has more advantages than power frequency induction furnace. It is suitable for melting all kinds of cast iron, especially alloy cast iron and nodular cast iron. In recent years, it has been widely used in cast iron production. The furnace has the advantages of high productivity, energy saving and large production flexibility, which is very beneficial to the machine repair plants of industries such as iron and steel metallurgical enterprises and other small batch production of single piece. It has high degree of automation and is equipped with dual power supply and control system. It also has the functions of melting and heat preservation. It is also suitable for continuous operation such as cast iron pipe production, and its application prospect is bound to be promising.

The new development of medium frequency electric furnace should arouse the attention and concern of the design and manufacture of induction furnace in our country. Efforts should be made to increase the strength of self research and development, supplemented by introduction, digestion and innovation, so as to raise the smelting technology and equipment of induction furnace to a new level as soon as possible.

Yiphee Electric Furnace Co.,Ltd specializes in producing induction furnace.Tel(WhatsApp):+86-13450756789, yiphee@yiphee.cn