The production practice of continuous casting has proved that low molten steel superheat can refine the grains of continuous casting slab and reduce center segregation; constant temperature and low superheat casting can reduce and maintain the heat flux density, thereby preventing breakout and corner cracks of the slab; only available The system of tundish molten steel heating measures can ensure low overheating: constant temperature continuous casting can effectively increase the number of continuous casting and reduce the advantages of continuous casting accidents, so it is highly valued by steel workers.
For continuous casting without tundish molten steel heating equipment, in order to ensure the number of continuous casting and reduce continuous casting accidents, the high temperature route is generally taken, so the temperature of molten steel has a particularly high range.
From the late 1980s to the late 1990s, about 40 steel plants at home and abroad installed tundish plasma heating systems. But because of expensive equipment, low heating efficiency of the system, and cost of use, it was abandoned by manufacturers.
System comprehensive upgrade research and development test
Taking full account of the shortcomings and shortcomings of the use of water-cooled metal guns in the tundish plasma heating system in the 1980s and 1990s, Beijing Aobang New Materials Co., Ltd., in conjunction with Beijing University of Science and Technology, redesigned the tundish molten steel plasma heating system:
The water-cooled metal gun is replaced by a graphite electrode; argon gas is drilled in the middle of the graphite electrode, which can stabilize the arc, extend the life of the graphite electrode and protect the molten steel in contact with the arc; the electrode forms an electrical circuit, eliminating The disadvantage of the bottom electrode of the masonry on the tundish.
The original system was redesigned and reformed, and the laboratory test was carefully arranged. After obtaining good test results, a set of 1250KW industrial-grade multi-electrode plasma heating system was manufactured. The system uses 3 hollow graphite electrodes and is not packaged. Masonry bottom electrode. In 2016, the system was installed on the 40-ton tundish of the No. 4 continuous caster of Wuhu Company of the New Cast Pipe Group. From 2016 to December 2017, the system has been running continuously in the plant, and it is easy to operate. It has repeatedly recovered low-temperature molten steel on site and avoided cast-breaking in continuous casting. It is welcomed by steel mills.
At the end of 2018, a new 2500KW tundish molten steel plasma heating system with a second improvement was installed on the two-strand slab continuous caster of Baowu Group Meishan Iron and Steel Company. As of July 2020, it has been successfully operated for 18 months. Practice has proved that the system is reliable, high heating efficiency (heating rate 0.7-2°C/min), low operating cost, simple operation, and multi-point heating can ensure the uniform temperature of the molten steel in the tundish; the temperature of the molten steel in the tundish is reduced by 10 -20°C, the comprehensive calculation cost is reduced by 20 yuan/ton steel. For billet casters or manufacturers whose management needs to be optimized, lower the tapping temperature(Temperature of molten steel in tundish) 10-50°C, the range of cost reduction will be greater.
Description of the new multi-electrode tundish molten steel plasma heating system
The new multi-electrode tundish molten steel plasma heating system is composed of AC transformer, short network, DC thyristor power supply system, PLC control system, 3 sets of multi-dimensional electrode blessing robot system, reactor and controller, graphite electrode, etc. Calculate the height of the molten steel in the tundish by using the arc length and voltage value, and connect to the existing continuous temperature measurement system of the molten steel in the tundish, which can realize the one-key start heating and automatic stop heating operation.
Since multiple graphite electrodes form a self-contained electrical circuit, there is no need to build bulky bottom electrode devices in the tundish lining. After equipped with a new type of multi-electrode tundish molten steel plasma heating system, there is no need to make any modification to the tundish structure.
1. The general layout of the new tundish plasma molten steel heating system is shown in Figure
2. Tundish heating operation
The multi-graphite electrode tundish heating system is easy to operate, and there is no water leakage or misoperation to insert molten steel explosion accident. Using argon as the working gas can stabilize the arc and obtain a longer arc.
According to different factors such as temperature, steel grade, and drawing speed, PLC can automatically change the heating power. The ultimate goal is to balance the casting temperature and improve the quality of the billet. The PLC control idea is to keep the arc voltage always stable, which requires constant change of the electrode position.
The factors that affect the arc voltage are the change of the atmosphere in the tundish and the height of the molten steel level in the gas composition in the tundish. At this time, the PLC will automatically adjust the electrode position to compensate for arc voltage fluctuations. However, practice shows that the change value of the electrode position with the atmosphere is very small compared with the absolute value of the arc length. The electrode can accurately and automatically track the change of the steel liquid level.
3. Consumption of graphite electrodes
When the tundish is heated, the graphite electrode consumption is 0.04 yuan/t steel. Production practice shows that the consumption of graphite electrodes mainly comes from side oxidation and head erosion, and the side is dominant.
4.Changes in the carbon, oxygen and nitrogen content of molten steel
The molten steel heated by plasma was sampled and analyzed in the tundish. On average, after using tundish plasma heating, the carbon content of molten steel only increases by 5-10ppm, with an average of 8ppm. For the carbon content of 0.03% of ultra-low carbon steel, an increase of 0.0008% of carbon is not a major problem.
The nitrogen content in the cast slab varies between 1-3 ppm before and after heating, which is within the error range. Therefore, plasma heating basically has no effect on the nitrogen content in the cast slab.
After heating, the oxygen content of the molten steel in the tundish shows a downward trend, and the range of decline is between 1-16 ppm. The DC power supply and plasma heating control system designed by the company can ensure that the graphite electrode does not need to be immersed in the molten steel to start the arc smoothly. This technology is also an important condition for the molten steel to not increase carbon significantly. The working state of the plasma arc during the heating process is shown in Figure .
5.Operating cost of plasma heating with multiple graphite electrodes
The use of multi-graphite electrode plasma heating system can greatly reduce the superheat of the molten steel in the tundish. According to this system-a reduction of 10°C as an example, only calculating the temperature reduction item can achieve a cost reduction of 11.5 yuan/t; if calculated by a reduction of 20°C, a cost reduction of 23 yuan/t can be achieved. 1 Note that the graphite electrode heating system has low operating cost, easy operation, basically no vulnerable and consumable parts, and few equipment units, so the failure rate is extremely low, and the equipment utilization rate is high. It can be seen from the table that there are many ink electrodes and other plasma heating The operating cost of the system mainly comes from electricity consumption, which is 1.63 yuan/t; it is far lower than the cost reduction caused by reducing the superheat of the molten steel in the tundish.
6. Use effect
The use of a steel plant in Baowu Group shows that with a new multi-electrode plasma heating system, the temperature of the molten steel in the tundish can be reduced by up to 20°C, and the heating rate is 0.7-2°C/min.
In addition, plasma heating plays an important role in reducing accidents. In a converter maintenance section in October 2019, a major accident in which the temperature of 5 ladle molten steel was too low and usually must be returned to the furnace to increase temperature was successfully saved.
Plasma heating equipment is especially effective in eliminating large molten steel temperature fluctuations between different furnaces and different casting moments in the same furnace; it plays a very reliable role in achieving the target temperature hit rate of the tundish molten steel;
The temperature of molten steel can always be controlled within + 5°C of the target value. Figure 7 shows the change of the temperature of the molten steel in the tundish with and without plasma heating.
Comparison of new multi-electrode plasma heating and induction heating
In the past 5 years, some steel mills have installed induction heating equipment for continuous casting tundishes. Production practice shows that the system is Compared with plasma heating, there are the following disadvantages:
1） The new type of plasma heating does not require modification of the tundish multi-electrode self-contained electrical circuit; while the induction heating equipment is reducing
Inter-packet capacity 10-20%;
2） Induction heating requires special refractory materials immersed in molten steel for a long time, which are generally imported and have a low life span, resulting in high operating costs.
3） Induction heating requires a bulky electric circuit plugged into the tundish body; if an accident occurs, the tundish car will be fast
Remove, easy to damage and cause loss.
4）Plasma heating takes up some space above the tundish, but does not affect operation.
5） Induction heating requires continuous heating, and the operating cost is high (compared to Meishan Steel’s parameters, 10 yuan/t more than plasma heating). Plasma heating only heats 5-10 minutes per ladle of molten steel; the operating cost is 1.63 yuan/t.
6） The new type of plasma heating uses argon as the working gas, and the process does not increase carbon and ammonia (only carbon increase 8PPM), which can reduce the oxygen content of molten steel.
1） Successfully realized that the bottomless multi-graphite electrode plasma heating system was put into production; using graphite electrode heating, the carbon increase of molten steel is between 5-10ppm, not more than 10ppm, which has no effect on the production of all steel grades of Meishan Iron and Steel; before and after heating of molten steel The oxygen content can be reduced; the nitrogen content can be increased by 1-3PPM.
2） The operating cost of the plasma heating system is 1.63 yuan/t steel; it mainly comes from the consumption of electric energy; the consumption of graphite electrodes
The consumption is only 0.04 yuan/t.
3） The system is easy to operate; it realizes-key heating; the equipment failure rate is low.
4） It can reduce the superheat of molten steel in the tundish by about 10-50C
5） During a continuous casting cycle, the temperature fluctuation of molten steel in the tundish can be controlled within +5°C of the target value.
6） The system is easy to operate without requiring dedicated operators.
7） Plasma heating can be started in the ladle-pouring, which can effectively avoid the problem of ladle replacement or the steep drop in the temperature of the molten steel in the first tundish of continuous casting, and can minimize the superheat of molten steel, that is, reduce the tapping temperature;
8） One-time equipment investment can be recovered within half a year.
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