Application and development of induction melting technology in cast iron production

Smelting technology is one of the key links in the production of cast iron, and its equipment generally uses more cupola. In the past one period, induction melting furnaces have developed rapidly, and in the case of sufficient power, they have gradually replaced the cupola. At present, the capacity of coreless power frequency induction furnace has reached 110 t, and the capacity of core power frequency induction furnace is as high as 270 t. In recent years, the development of the medium frequency induction furnace and its superiority have injected new vitality into the production of cast iron.

1 2 induction melting furnaces

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

The power frequency induction furnace can melt and heat the metal, and the heating is evenly burned less, which is convenient for adjusting the composition and pollution of the molten iron. However, the power frequency induction furnace melts the cold material slowly, is not conducive to slag formation, requires the start block to start the furnace, and the production is not flexible enough, so it is commonly used for remelting and heating of metals and alloys. In addition, the power frequency induction furnace has a low power factor and requires a large number of compensation capacitors, which also increases the floor space and equipment investment.

The medium frequency induction furnace has high electrical efficiency and thermal efficiency, short melting time, power saving, less floor space, low investment, easy process automation and production flexibility.

The medium frequency induction furnace is suitable for melting cast iron, and is especially suitable for melting alloy cast iron, ductile iron and compacted graphite cast iron. It is also more adaptable to the charge, and the variety and block size of the charge can be varied within 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 iron liquid storage, heat preservation, adjustment of alloy composition and overheating, especially as a blast furnace. Double furnaces for smelting furnaces such as cupolas are still widely used. The world's most famous centrifugal ductile iron pipe manufacturer, France's Musongqiao company uses the power frequency induction furnace and blast furnace double process, while Japan's Kubota company uses the power frequency induction furnace and cupola double process. As early as the early 1980s, Shougang Foundry first adopted the power frequency induction furnace and the blast furnace or cupola double process in the country, and succeeded in the continuous casting of as-cast ductile iron pipe, and provided high-temperature high-quality molten iron in the future production. It has been used until now and the effect is good.

2 Development of medium frequency induction furnace

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

2.1 Static frequency converter

The development of the medium frequency induction furnace benefits from the use of static frequency converters. Compared with magnetic frequency converters, the efficiency of these inverters is as high as 95% to 98%. The rated power of the inverter used as the induction furnace is constantly increasing. Recently, the 9 000 kW inverter has been put into production, and it is connected to a furnace with a capacity of 12 t. The productivity of molten iron can reach 18 t/h; the medium frequency induction furnace The power density per ton of melting capacity is increased to 1 000 kW, which shortens the melting period to 35 min. The melting rate of the induction furnace varies with the capacity of the furnace. Generally, the melting rate of the cast iron in the medium frequency induction furnace is 0.4 to 35 t/h. For example, using a 2 t capacity furnace, a melting rate of 2 to 2.38 t/h can be obtained, and a 12 t furnace can achieve a melting rate of 18 to 21 t/h; while a power frequency induction furnace is used to melt the melting material. The rate is 1.55 t/h for a 1.5 t furnace, 1.5 t/h for a 3 t furnace, 2.5 t/h for a 5 t furnace, and 4 t/h for a 10 t furnace. It can be seen that the melting rate of the medium frequency induction furnace far exceeds the power frequency induction furnace, which can be used to replace the larger capacity power frequency induction furnace with a smaller capacity medium frequency induction furnace when selecting the cast iron production melting equipment. Condition, the medium frequency furnace replaces the power frequency furnace, which not only reduces the land use, but also reduces the investment, and also ensures the continuous supply of molten iron. It is very beneficial for the continuous operation and the production of cast iron with large production capacity. The medium frequency induction furnace is used for the smelting of molten iron produced by continuous casting or centrifugal casting of ductile iron pipe, and it can be used to replace the cupola or double with the blast furnace and the cupola, and its production capacity can be fully utilized. For example, there is a manufacturer of centrifugal ductile iron pipes in China, which uses a 10 t medium frequency induction furnace and a blast furnace double process to raise the temperature and adjust the composition of the molten iron, and heat the stored blast furnace molten iron from 1300 °C to 1520 °C. It takes about 27 minutes. The furnace has a frequency of 100 to 200 Hz and a power of 2,500 kW.

The medium frequency induction furnace has high electrical efficiency and thermal efficiency, which not only increases the melting rate, shortens the melting time, but also reduces the unit power consumption. Compared with the power frequency induction furnace, the power consumption can be reduced from 700 kW.h/t to 515-580 kW.h/t. The relevant data show that the unit power consumption is about 582 kW.t/h when the medium frequency induction furnace is cold-started, and the unit power consumption is 505-545 kW when the medium-frequency induction furnace is cold-started, considering the energy loss required for the melting and superheating of the slag. .h/t, if continuous feeding operation, the unit power consumption is only 494 kW.h/t.

2.2 Production flexibility

The medium frequency induction furnace has greater flexibility in production scheduling, and the melting operation has greater flexibility. For power frequency induction furnaces, continuous operation is required continuously, because intermittent cooling increases cold start, cold start not only increases melting time and energy consumption, but also requires a starting block every time. When the medium frequency induction furnace is used for a short time, the cold start can be performed without using the starter block. It is very convenient to replace the charge, and the iron liquid can be completely emptied, which can realize material replacement in a short time, which is beneficial to the production organization. This brings favorable conditions for single-piece small batch production. For example, the industrial machine repair factory in the iron and steel metallurgical enterprise mainly undertakes the production of spare parts, spare parts and bulk consumables required by the iron and steel enterprises, and undertakes the manufacture of some non-standard equipments, and most of them are single-piece small-scale production, with more varieties. , material requirements often change. In this respect, the production flexibility of the medium frequency induction furnace makes up for the shortage of the power frequency induction furnace and other melting furnaces.

2.3 Furnace structure

With the continuous improvement of the power density of the medium frequency induction furnace, the requirements for the safe operation of the furnace, the improvement of the life of the lining and the reduction of noise are becoming higher and higher, and the rationality of the structure of the furnace is becoming more and more important. Steel shell furnaces have many advantages such as high durability, high efficiency, high productivity, low noise, and easy maintenance.

Unlike the frame furnace, the heavy-duty steel shell furnace is a high-strength ring-shaped steel shell with a plurality of large inspection ports. When the furnace is running, the inspection port is closed, and each inspection port can be opened during maintenance. The internal structure of the heavy-duty steel shell furnace is strong, which can avoid the deformation caused by the inclined iron casting, prolong the life of the lining, and the working noise is greatly reduced due to the closed strong furnace shell and the interior thereof, which can increase the sound absorbing insulation material. The sturdy steel shell also effectively protects the induction coil from the risk of splashing metal, giving the furnace maximum safety during operation. In order to effectively insulate and improve the life of the lining, the heavy-duty steel shell furnace is also provided with a cooling ring at the top and the bottom respectively, which serves to uniform the lining temperature and reduce the thermal expansion. Low energy, high strength stainless steel cooling ring greatly improves furnace efficiency.

The heavy-duty steel shell furnace not only has a strong steel shell, but also designs a thick-walled tube structure coil specially designed for induction melting, and the coil has the highest conversion efficiency and the smallest resistance by correctly selecting the inter-turn distance of the induction coil. On the one hand, the thick-walled induction coil has a large current-carrying cross section, uniform wall thickness, and is less prone to damage caused by arcing and expansion. On the other hand, the coil adopts a special support system, and each one is firmly locked and has good rigidity. The vibration caused by the electromagnetic force is reduced, and the service life of the lining is greatly extended. In addition, the induction coil is covered with a 50% yoke, which effectively changes the distribution of the magnetic field. The isolation of the cushion also reduces noise and vibration, increasing the efficiency of the yoke. The heavy-duty steel shell furnace also adopts an elongated coil design in which the induction coil extends and exceeds the furnace substrate portion, so that the coupling between the charge and the magnetic field from the top to the bottom of the furnace body is very uniform, the energy conversion efficiency is improved, and the stirring due to the magnetic field is also reduced. The effect of force on the bottom lining.

A reasonable furnace structure is very important, and a heavy steel shell furnace is only one of them. Many manufacturers are working hard to improve the structure of the furnace to adapt to the application and development of medium frequency induction furnaces.

2.4 Deslagging and repairing furnace

The medium frequency induction furnace has high productivity and the melting period can be shortened to about 35 minutes. In order to maximize the power of the furnace, it is necessary to remove the slag as quickly as possible. The use of skimmers or manual slag removal has a poor effect and a long time, and the working conditions are poor. In order to solve this very time-consuming and cumbersome work, there is a method of pouring the slag after the furnace, that is, the slag is poured into the transport vehicle through the slot at the back of the top of the furnace body, which is quick and convenient. The angle of the furnace back tilt is generally 20 ° ~ 25 °.

After the medium frequency induction furnace works for one furnace, it must be dismantled and repaired. In order to shorten the shutdown time of replacing the refractory lining, mechanization must be considered, and the vibration building furnace and the lining extruder are the main accessories of the large induction furnace. The lining pusher pushes the refractory lining out before it is completely cooled, further reducing the time required to repair the furnace and improving the working environment.

2.5 dual power supply and control system

A unique development of the IF induction furnace is to supply power to two furnaces with one power supply system, thus realizing a working system with no production interval. Practice has proved that the total effective power of an induction furnace is usually not fully utilized during the entire melting period. In the determination of molten iron temperature, sampling, slag removal, tapping, especially in the case of pouring, power is reduced or the power is cut off. . If the casting cycle is long, the utilization rate is only about 50%, which indicates that to achieve the required productivity, the rated power of the power supply equipment should be 1.8 times the utilization rate of 90%. In order to solve this problem, a dual power system was successfully developed. The system uses two identical inverters and capacitor banks, one for each furnace, but all powered by a common rectifier and transformer, each of which can be individually controlled to provide the total effective power in any ratio Assigned to 2 furnaces. In addition to providing sufficient power for the insulation of one furnace, the system can be used for the second furnace for molten iron. Once the smelting furnace is ready for pouring, the power of the converter is immediately reduced to the holding level; at this time, the other furnace is emptied, and then the cold material is refilled, the power of the converter is increased, and new smelting is started. If necessary, the total effective power can be connected to one furnace and the other furnace can be deactivated; or the total effective power can be evenly distributed to two furnaces for simultaneous melting. For example, the United States should reach the company's DUAL-TRAK dual power supply and control system, which can supply power to two furnaces at the same time, and the power delivered to the two furnaces is steplessly distributed. With this power supply, the melting power can be delivered to one furnace at the same time, and the insulation power can be transmitted to the other furnace, so that the power utilization rate reaches 100%. This kind of power supply simultaneously delivers power to two furnaces, completely avoiding the switch or adding another set of power supply. It is not necessary to switch the power supply to the holding furnace during smelting to maintain the necessary pouring temperature, so that the intermediate frequency furnace realizes melting and heat preservation. 2 functions. When one furnace is in maintenance, the power supply can be isolated from the furnace, and only one furnace can be powered, which also increases safety.

It should be mentioned that the medium frequency induction furnace equipped with dual power supply and control system can guarantee the normal production of the other furnace even in the case of one furnace, thus realizing the timely and continuous supply of molten iron. The frequency induction furnace is unmatched. The practice of 1 change 2 furnace of Shougang Foundry proved that two sets of 10 t power frequency induction furnaces use one set of power supply. When one furnace is used, the other one cannot be produced. If the first oven takes about 3 days, it will not be produced in about 1 month in one year, which has a great impact. In order to achieve the purpose of continuous production, 1 is to increase the number of power frequency induction furnaces, and 2 is to increase 1 set of oven power supply equipment. When constructing the centrifugal ductile iron pipe workshop, Shougang Foundry added one power frequency induction furnace and corresponding power supply equipment on the basis of the original 1 change 2 furnace; the other domestic centrifugal ductile iron pipe manufacturer in the original 1 change 2 On the basis of the furnace, it is proposed to add 1 set of oven power supply equipment. Regardless of the measures mentioned above, it is necessary to increase the corresponding equipment and facilities, which will increase the land occupation and investment. The application of the dual-supply power supply and control system of the medium frequency induction furnace not only ensures the smelting and heat preservation, but also realizes that the oven does not stop production.

2.6 degree of automation

The medium frequency induction furnace adopts a computer control system with a high degree of automation. For high-powered medium-frequency induction furnaces, the melting period is very short and the superheat rate is very high. Only the use of computer control systems can realize the potential of the equipment to provide the most effective protection for the factory and operators. The furnace that overheats the furnace charge at a rate of 25 to 30 ° C/min is unattended for a few minutes at full power and is likely to cause a catastrophic accident. At present, the large-scale medium frequency induction furnace has been equipped with a relatively complete computer-aided monitoring and control system. The main contents include:

(1) When the molten iron in the furnace reaches the pre-programmed temperature, it is guaranteed to automatically reduce the furnace power;

(2) The furnace is fixed on the weight measuring instrument, and the weight measuring instrument provides the computer with the quality data of the metal charging material in the furnace;

(3) monitoring, calculating and controlling the temperature of the molten iron;

(4) Providing an automatic sintering procedure for the new furnace lining;

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

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

In addition, the installation of automated mechanical charging systems is also indispensable for the full potential of high-power IF induction furnaces. A vibrating feed system that is generally mounted on the track for lateral and longitudinal movement or pivoting is most suitable.

The melting process of the high-efficiency intermediate frequency induction furnace is the function and operating procedure required for the entire melting process to be monitored by the melt processor. The processing system consists of a microprocessor with a color monitor. In operation, under the joint action of the furnace charging system and the weighing system, the power required to melt to the pouring temperature is automatically calculated, and the absorption of energy required for the melting process of the furnace during the melting period is controlled. Under the action of the bridge, the automatic charging of the melting process of the furnace is realized; after the melting, the molten iron reaches the pouring temperature, the processing system can automatically convert the heat preservation process, and the expected superheat can be achieved with the minimum energy; In the case of protecting the lining, the cold start of the furnace and the sintering process of the furnace lining refractory are automatically controlled.

The melt processing system provides full self-diagnosis of the power supply and the furnace. If a system alarm is activated in an abnormal state, the processing system can provide the problem and store the fault information or connect to the printer for print reservation.

3 Conclusion

The medium frequency induction furnace has more advantages than the power frequency induction furnace. It is suitable for melting various cast irons, and is particularly suitable for melting alloy cast iron and ductile iron. In recent years, it has been widely used in cast iron production. The furnace has high productivity, energy saving and large production flexibility. It is very advantageous for industrial machine repair factories such as iron and steel metallurgy enterprises and other single-piece small batch production; it is highly automated and equipped with dual power supply and control system. It has the functions of melting and heat preservation, and is also suitable for continuous operation such as cast iron pipe production, and its application prospect is inevitably optimistic.

The new development of medium frequency induction furnace should attract the attention and attention of China's induction furnace design and manufacturer, and strive to increase the intensity of self-development, appropriately supplemented by introduction and digestion, improve innovation, and improve China's induction furnace melting technology and equipment as soon as possible. A new level.