There are two types of electric furnaces: direct current (DC) and alternating current (AC). The DC electric furnace uses one electrode and the AC electric furnace uses three electrodes. Therefore, the unit consumption of DC electric furnace electrodes is lower and the dissolution heat efficiency is better. , low noise.
Croman revives the power of DC electric furnaces
The reconstruction of Croman electric furnaces has attracted special attention in the industry. This is because this project confirms the strong vitality of the conductive refractory furnace bottom of Concast (formerly ABB) DC electric furnace. Thanks to the new equipment structure and the modern ConsoTech injection device, Croman DC electric furnaces are completely reborn. The rebuilt electric furnace can produce 34 furnaces of steel per day, making it one of the best DC electric furnaces in the world. Concast's new technology brings the efficiency of Croman DC electric furnaces to new heights, while also significantly reducing the furnace's power consumption.
Croman is one of the important steel mills in Türkiye. The plant has an 80-ton ABB electric furnace and a ladle furnace, and mainly produces medium and low carbon steel, medium and low alloy steel and spring steel. A 6-strand billet continuous casting machine is used to cast billets ranging from 100×100 to 180×180mm.
In order to meet users' demand for products, improve product quality and expand varieties, Croman's decision-makers decided to upgrade its electric furnaces. The identified goals are:
1. Improve the furnace Output;
2. Reduce various consumption;
3. Improve production operations;
4. Enhance equipment reliability;
5. Strengthen process control.
The essence of the above goal is to significantly reduce production costs by improving furnace operation and expanding its flexibility.
In order to achieve the above goals, the following measures must be taken:
1. Improve the furnace production capacity;
2. Reduce the number of steel scrap charging tanks per furnace from 3 cans reduced to 2 cans;
3. Increase electric power input;
4. Reasonably input chemical energy;
5. Improve furnace utilization and strengthen Its reliability;
6. Minimize furnace downtime.
Implementing the above measures means rebuilding a new electric furnace, with only the original anode and cathode parts available for use.
New electric stove. The newly installed Concast electric furnace adopts a platform structure and a bridge-type electrode lifting device. This structure is compact and sturdy, and is most suitable for small sites and narrow circuit routing space constraints.
In order to save infrastructure and installation costs and avoid long-term production shutdown caused by changing the furnace foundation, the copper conductive parts at the bottom of the original conductive furnace are retained.
The new furnace shell is the starting point for this upgrade. On the one hand, the new furnace shell needs to ensure the capacity required for increased production and a two-can furnace for scrap steel. On the other hand, it needs to support the input of more electrical and chemical energy and minimize the wear of the furnace body and lining.
In this way, the furnace shell diameter will be expanded from the original 5900mm to 6300mm. At the same time, the new furnace shell is equipped with an EBT tapping system, which can tap steel without slag. The furnace shell is equipped with steel/copper water-cooled furnace plates, and each plate has independent water supply and temperature control.
In order to speed up various operations, new high-pressure hydraulic devices were installed and the original hydraulic pipelines were modified.
Adhere to DC operation. In the early 1990s, the development momentum of DC operation of electric furnaces declined. Despite this, Croman decided to stick to the DC solution for this electric furnace reconstruction because DC operation of the electric furnace has obvious advantages over AC in terms of flexibility, economy, and maintenance.
Croman’s management and employees have a deep understanding of the above views. With several years of experience, the operation of DC electric furnaces is no longer a mystery to them.
Conductive furnace bottom. The furnace bottom that can carry electricity is the core of DC electric furnace steelmaking. Concast DC electric furnace is different from other electric furnaces in that it uses a patented conductive furnace bottom. The bottom of this spherical conductive furnace is made of steel, and the inside is covered with a number of annular copper plates. There are four through holes through which the terminals pass and extend out of the furnace bottom.
The terminal post is connected to the anode busbar under the furnace through a flexible connector.
Refractory furnace lining. The surface of the furnace bottom copper plate is paved with conductive refractory bricks, and the brick surface is smoothed with graphite.
The bottom of the furnace uses forced ventilation to dissipate heat.
Under normal circumstances, the furnace bottom requires neither maintenance nor replacement. If necessary, only the top refractory brick layer can be repaired. It is repaired with a magnesium mixture filled with carbon to ensure the required conductivity.
The new furnace requires only minor repairs. This is an important advantage of Concast DC electric stove. After minor repairs, the stove can be turned on immediately without preheating. After about 5,000 furnaces of smelting, the top floor bricks may be worn beyond repair. At this time, the entire furnace bottom (copper plates and steel plates) connected together by flanges can be removed and replaced with a new integral furnace bottom.
Furnace bottom insulation. The bottom of the DC electric furnace is connected to the anode circuit of the silicon controlled rectifier and is the electrical operating part of the furnace, so it must be insulated from the furnace shell and grounded. Insulation method: The bottom of the Concast DC electric furnace is equipped with an annular flange, and the flange is supported on a non-conductive ceramic block in an annular channel steel welded to the furnace shell. The voids in the channel steel are filled with insulating material.
Quick power control. Concast DC electric furnaces are characterized by extremely fast power control. During the upgrade, Croman installed a new transformer with higher power (2×45MVA) and a 10-position voltage conversion switch. The thyristor adopts 12-pulse wiring method.
The control response of the DC electric furnace power control system is rapid and accurate. At the same time, this control is extremely flexible, and the optimal parameters can be adjusted at any time according to the needs of the operation.
The transformer can use taps to increase the voltage or current to ensure the output of the rated power. Full power operation with various arc resistors. A high arc resistance turns into a long arc with high voltage and low current; a low arc resistance turns into a short arc with low voltage and high current. Determine the most appropriate arc based on operational needs. The newly designed Concast DC power system has sufficient flexibility to ensure the optimal arc resistance required for specific operations.
Use a silicon controlled rectifier device to control the operating current. Use the selector switch on the control panel to set 10 levels of current from full current to zero current.
The electrode hydraulic control system is used to control the firing angle signal of the silicon controlled rectifier to adjust the voltage. In addition to logic functions, it is also equipped with a synchronous voltage monitor and current limiter to prevent system overload.
Zero arc bias is extremely important to ensure symmetrical melting and avoid overheating of the slag line. The Concast DC furnace makes the arc almost completely vertical.
The main advantages of electric furnaces using DC operation are:
1. Low electrode consumption;
2. Low power consumption;
3. Reasonable heat distribution;
4. Light refractory abrasion;
5. Real-time smelting control can be implemented using electric current;
6. Less maintenance;
7. With less flicker, the system can smoothly connect to the local power grid.
Concast injection system
Another key link in the Croman electric furnace upgrade is the complete update of the furnace's chemical energy supply system to meet the following main requirements:
1. Improve the chemical energy input effect;
2. Ensure the formation and control of foam slag;
3. Integrated control of chemical energy and electrical energy input;
4. Flexibly input electric energy to make it suitable for different furnace raw materials;
5. Ensure the repeatability of the smelting process.
To achieve the above purpose, the original external spray gun was replaced with a ConsoTech wall-mounted burner and spray device. The advantages of wall-mounted injectors are shown in Table 1.
Table 1 Features and advantages of injectors installed on the furnace wall
Features and advantages
Mounted on the furnace wall requires little maintenance and low operating costs
The spray is distributed at several points of the molten steel to decarburize quickly, the oxygen utilization rate is high, and the molten steel is easy to uniform
Fully automatic operation can continuously control the foamed slag, and the process is highly repeatable
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Injection with the slag door closed reduces the intrusion of air and absorbs less nitrogen
The utilization rate of oxygen injection kinetic energy and high oxygen is significantly improved
Development of ConsoTech multi-functional injection The purpose of the device is to integrate various spraying operations into one and use a single device to perform spraying. Therefore, the ConsoTech injector has multiple functions. It can be used as a burner and secondary combustion oxygen lance, as well as a high-kinetic oxygen injector and carbon supply burner.
The layout of Croman’s new injection system is: 3 ConsoTech injectors are installed on the furnace wall and 1 ConsoTech injector is installed on the EBT wall plate. In addition, four slag carbon injectors are used to create foamed slag and carburize the molten pool.
The shape of the injector flame can be easily changed to match parameters such as the type of scrap used in the melting stage and the remaining life of the water-cooled surface.
The ConsoTech oxygen ejector is based on the working principle of De Laval ultrasonic burner. It is effective in accelerating melting, decarburization, stirring the molten pool more evenly and making the foamed slag more ideal. It is suitable for every type of equipment. needs, such as adjusting flow rate, compensating pipeline pressure drop and ensuring TOP gas transmission pressure, etc.
The above layout can ensure precise carbon injection, maintain ideal metallurgical balance and effectively generate foamy slag.
Ensure repeatability
While the electric furnace is being fully updated, Croman also adopts a new complete set of Level I automation system. This new system guarantees absolute repeatability and optimal castability from furnace to furnace, allowing for fully automatic control of various furnace parameters.
The Croman Level I control system has programmable chip control functions and can provide real-time images of the operating status of the furnace. Advanced mathematical screening procedures compensate for variability in input data so that the furnace always meets metallurgical targets.
Production results
The renovation project was completed in just 28 days. The furnace was put into operation quickly and reached production smoothly in less than a month. The overall efficiency of the new furnace has been significantly improved, and various consumptions have been reduced.