By analyzing the existing production conditions and iron consumption control level of a certain converter smelting, we explore control measures to promote converter smelting to reduce iron consumption indicators. The implementation effect of the control measures taken was verified through production practice. By collecting test data and comparing the iron consumption indicators before and after, effective control measures were solidified to reduce the iron consumption indicators and improve production capacity.
Keywords: converter production; smelting; low iron consumption
Converter smelting molten iron consumption (hereinafter referred to as iron consumption) is an important process indicator. The level of iron consumption not only determines the converter workshop’s ability to consume scrap steel, but also reflects the heat utilization efficiency of the converter smelting process. Especially for steel joint enterprises with limited hot metal production capacity, promoting the reduction of iron consumption can effectively increase the company’s production capacity.
A converter workshop is equipped with two converters with a nominal capacity of 120 tons. The charging span is provided by two 30+30t cranes and two 180t cranes to provide scrap steel and molten iron. The upper process is provided by two 1,250 cubic blast furnaces with molten iron. Behind the furnaces are two online CAS argon blowing mixing stations and one offline LF electrode ladle furnace. The continuous casting includes three 5-machine and 5-strand billet continuous casters. With the successful improvement of the billet section of the continuous caster in the steelmaking plant from 150mm to 155mm, and the internal optimization of the connection time of each process in the steelmaking plant, the consumption capacity of molten iron in the steelmaking plant has reached more than 9,000 tons per day. The daily molten iron output of the iron smelting plant is maintained between 8,000 and 8,200 tons, in order to ensure that the converter has sufficient operation rate and complete the company’s annual output target. Based on converter smelting heat balance and optimized production organization, steelmaking plants have actively carried out various forms of exploration and practice to reduce iron consumption.
Current status of molten iron consumption control
In 2019, the company purchased high-quality scrap steel from external sources, and the structure of loading raw materials in the converter workshop underwent major changes. In the second half of 2019, the steelmaking plant optimized the connection time by sorting out the production organization. The temperature reduction work of the smelting end-point tapping temperature system was promoted, which lowered the end-point tapping temperature and increased the proportion of scrap steel entering the furnace, effectively reducing iron consumption. The iron consumption level and process conditions in the second half of 2019 are shown in Table 1:Although the iron consumption index improved significantly in the second half of 2019, the iron consumption control was reduced to 850kg/t at the beginning of 2020. However, compared with the same industry, iron consumption control still has a large room for reduction, so further improvements in smelting control are needed to ensure that iron consumption indicators are further reduced.
Table 1 Control average levels in the second half of 2019
|Mouth||Molten iron entering the furnace (t)||Scrap steel entering the furnace (t)||Iron consumption (kg/t)||Tapping temperature (℃)|
Exploration and practice of reducing iron consumption
Improve the method of adding scrap steel
Increasing the heat of molten iron entering the furnace and increasing scrap steel consumption are the most direct ways to reduce iron consumption. Starting from 2019, the conditions for using molten iron in a steel plant have been basically stable at: the furnace entry temperature is 1280°C~1320°C, and the silicon content of the molten iron entering the furnace is at the level of 0.45%~0.65%. The above conditions for the molten iron are based on the consumption of 20~26t of scrap steel. There is still a surplus of heat, but due to the limited size of the scrap tank and the limited width of the feeding span, the amount of scrap steel added at a time is very limited. Without adding pig iron and heavy scrap as counterweight, the weight of scrap steel added at a time is about 15 tons. In this regard, the converter workshop actively carried out methods to optimize the addition of scrap steel. The main effects include the following two points.
Adjust the production organization, stagger the production rhythm, and add double-channel scrap steel to single-furnace smelting
The production rhythm of a single converter smelting furnace is about 30 minutes. Under the premise of staggering the production rhythm, the feeding method of adding one pour of molten iron and two troughs of scrap steel can be used to increase the proportion of scrap steel entering the furnace and reduce smelting iron consumption. Through on-site tracking, on the premise that the starting blowing time of the two converters is staggered by no less than 10 minutes, the production organization model of adding two slots of scrap steel to both converters can be realized. Due to the addition of one more slot of scrap steel, the production cycle of a single furnace is 40s~60s longer. Add double trough scrap steel into the molten iron scrap steel structure table 2 of the furnace.
Table 2 Comparison of the amount of molten iron scrap entering the furnace from single and double troughs
|Molten iron (t)||Scrap steel (t)||Feeding time (s)|
Add scrap steel to the molten iron tank and bake it before tapping
Due to the limited scrap steel added in front of the furnace, when the hot metal heat conditions are good, even if 30t of scrap steel/furnace steel is consumed in front of the furnace, there is still a surplus of heat in the converter. Therefore, it is considered to advance the scrap steel adding operation to the time when the hot metal tank is returned to the tank and waiting for the iron to be tapped. Use a steel grab machine to add scrap steel into the hot metal tank, and then use gas to bake and heat the working layer of the hot metal tank and the scrap steel in the tank. This method can effectively make up for the limited amount of scrap steel added before the furnace. The scrap steel added to the hot metal tank is mainly pig iron and steel bar briquettes. In order to ensure that after the scrap is added, the molten iron has enough temperature to be transferred to the converter in front of the furnace, the company has implemented a series of optimizations for the operation of the molten iron tank. This mainly includes investing in hot metal tank baking equipment, reducing the number of online hot metal tank dumps, and shortening the hot metal tank dump cycle. Since the production organization of back-transporting molten iron has been optimized, the amount of scrap steel added before tapping has gradually increased, and 10 to 13 tons of molten iron can be stably added to a single tank of molten iron. That is, the total scrap steel consumed in one furnace of converter smelting can be greater than 39 tons, and the scrap steel ratio is 27 %~30%. Comparison of the amount of molten iron scrap entering the furnace before and after adjustment is shown in Table 3.
Table 3 Comparison of adding scrap steel before tapping or not
|Molten iron (t)||Scrap steel (t)||Amount of flushing (t)||Temperature of molten iron (%)|
Operation with less slag volume improves heat utilization efficiency
The amount of slag in the converter has a very important impact on temperature control. On the premise of meeting the requirements of P removal and furnace protection, operation with a small amount of slag can significantly improve the heat utilization efficiency of the converter, improve the converter’s ability to consume scrap steel, and reduce iron consumption. purpose. The following two methods are mainly used to operate with a small amount of slag:
Control the final slag amount and improve converter heat utilization efficiency
A large amount of final slag can improve the slag splashing effect to a certain extent, but when the amount of final slag is too large, it will easily lead to a long slag splashing time and is not conducive to the heat utilization of the subsequent smelting furnace. After the steel is discharged, 1/3~2/3 of the slag is poured out depending on the amount of slag in the furnace. This not only ensures rapid slag removal in the early stage of smelting in the next furnace, but also controls the overall slag amount and makes full use of the heat in the furnace to increase the temperature of the molten steel. Through experimental tracking, if 1/2 of the slag is poured into each furnace, the molten iron consumption can be reduced by 7kg/t under the premise of stable tapping temperature.
Control the amount of auxiliary materials (lime dolomite) added to reduce heat waste
Changes in converter production conditions create conditions for reducing the amount of auxiliary materials added to the converter. The main changes are as follows: ① The amount of final slag is reduced, process slag overflow is alleviated, the utilization rate of lime and dolomite added to the furnace is increased, and the efficiency of P removal in smelting is improved. ; ② The silicon content of the molten iron entering the furnace is stable, and the high-silicon molten iron is reduced. When the amount of lime added is appropriately reduced, the alkalinity of the final slag can be maintained within the range of 2.8~3.0. The changes in the addition of auxiliary materials before and after adjustment are shown in Table 4: the production of auxiliary materials into the furnace is reduced by 2~2.5t/furnace. Under the premise of stable tapping temperature, molten iron consumption can be reduced by 19kg/t.
Table 4 Changes in the amount of excipients added
|Lime (t)||Dolomite (t)||Raw white (t)||Limestone (t)|
Improve processes and equipment to reduce process temperature
Optimizing on-site process equipment conditions can create production conditions that increase scrap consumption. At the beginning of 2019, the converter tapping time was controlled between 4 and 4 and a half minutes, and the tapping temperature of ordinary carbon steel dropped between 65°C and 70°C. By optimizing the tapping operation, the temperature loss during the tapping process was reduced. On the premise of ensuring that the casting temperature is met, the end tapping temperature is reduced to promote the reduction of iron consumption. The main optimization measures include:
Alloy baking furnace heating alloy
An alloy baking furnace is added after the furnace. Before converter blowing, the alloy is added to the baking furnace and baked to ensure that the alloy temperature is around 300°C when tapping and adding the alloy to the ladle, thereby reducing the temperature drop of the tapping.
Improve the tapping port and shorten the tapping time
By optimizing the tapping bricks and modifying the inner diameter of the tapping port, the inner diameter of the tapping port is expanded by 25 mm, shortening the tapping time and reducing the temperature drop during the tapping process. Table 5: Comparison of tapping time before and after tapping operation optimization and tapping temperature drop before and after. On the premise of sufficient superheat in subsequent processes, the tapping temperature drop can be reduced by 10°C, that is, the tapping temperature can be reduced by 10°C, and the iron consumption can be reduced by about 11kg/t.
Table 5 Comparison before and after optimization of tapping operation
|Alloy adding temperature (℃)||Tapping time (s)||Tapping temperature drop (℃)|
Optimize operation and reduce iron consumption
Double slag operation to reduce slag overflow in the process
Operation with a small amount of slag can reduce slag overflow in the process. However, when the silicon content is greater than 0.55%, the carbon-oxygen reaction is violent in the middle stage of blowing, which can easily lead to splashing and slag spillage, which directly results in reduced steel tapping, high steel material consumption, and indirectly affects the molten iron consumption index. In order to control splashing and slag overflow, a double slag operation method is adopted. Pour the preliminary slag once every 4 minutes during blowing. On the one hand, the slag amount is controlled to ensure the end-point tapping temperature. On the other hand, it reduces the process splashing and overflowing slag, reduces metal consumption, and indirectly improves the iron consumption index.
Improve the end-point carbon pulling level and improve thermal efficiency
The control of the end point C content can intuitively reflect the heat utilization efficiency. On the one hand, high end point C content is not conducive to the full utilization of heat in the furnace. On the other hand, high C content increases the risk of P high steel, and the increase in slag viscosity is not conducive to slag splashing and furnace protection. By increasing the furnace length to check the carbon level, ensure that the end-point C content is stable between 0.05% and 0.07%. For every 0.01% reduction in end-point C content, iron consumption can be reduced by about 9kg/t.
(1) By optimizing the production organization and smelting operations, the iron consumption index can be effectively reduced without increasing the temperature of the molten iron entering the furnace and the silicon content.
(2) As the production cycle and the connection time of each production link are shortened, the production efficiency is improved, the process temperature loss is reduced, and the iron consumption index can be effectively reduced.
(3) Equipment improvement can effectively reduce temperature loss during the production process and improve the ability to consume scrap steel. Subsequent equipment upgrades can be carried out, such as: Equipment modifications such as hot metal tank capping, ladle capping, and auxiliary material (lime dolomite) preheating can further reduce iron consumption.
(4) When the heat is sufficient, the method of adding cold materials can be diversified. Methods such as adding small pieces of soybean steel with high iron content into the furnace at a high position and adding steel bars with the required size during the tapping process can fully utilize the heat of the converter while increasing the output of a single furnace and reducing iron consumption.