Previous part reading：Effects of high scrap steel ratio on Steel making process steps
Control measures for high scrap ratio scrap steel entering the furnace
According to the production practice of 120t top and bottom combined blown converter in Yulin Steel, in order to maximize the scrap ratio, process adjustments and improvements are mainly made from the following aspects.
(1) Optimization of scrap material type. In order to ensure the loading of scrap steel in a single bucket and maximize the scrap ratio, it is crucial to increase the purchase ratio of heavy scrap steel or reduce the size of scrap steel to increase the pile density. The purchase size of the purchased scrap steel used in the 120t converter is recommended to be controlled within 1000mm×600mm, and the pile density reaches more than 1.15t/m3, which can meet the requirements of single bucket loading of scrap steel.
(2) Adding scrap steel to the blast furnace molten iron tank. The thermal balance of the converter depends on the difference in heat income and heat expenditure of the charge. Making full use of the physical heat and chemical heat of the molten iron to optimize the structure of the metal material entering the furnace is an important way to improve the scrap ratio. Yulin Steel adopts a molten iron one-pot smelting process. The turnover rate of the molten iron pot is about 2h/time. The blast furnace tapping temperature is 1510℃～1540℃, and the converter molten iron temperature is 1350℃～1450℃. The molten iron has excess heat. Before the molten iron pot is assigned to the blast furnace, 5t～6t of scrap steel is added to the molten iron pot. The heat and impact force during tapping are used to melt part of the scrap steel in the molten iron pot and directly convert it into liquid iron, thereby reducing the converter molten iron temperature, making the converter smelting process optimal, effectively improving the scrap steel ratio, further reducing iron consumption and increasing steelmaking capacity.
(3) Adding scrap steel to the ladle after the furnace. Purchase some steel bar cut ends or use pure scrap steel produced by rolling mills. The length is controlled within 10cm. After this part of scrap steel is baked in the post-furnace baking furnace, it is added to the ladle along with the steel flow during the converter tapping process. The amount added per furnace is controlled at about 1t, and the temperature loss of molten steel is about 10℃, which can maximize the scrap steel ratio.
Control measures for steel material consumption indicators under high scrap steel ratio mode
The scrap steel recovery rate is lower than the molten iron recovery rate, and the high scrap steel ratio is bound to affect the increase of steel material consumption indicators. Under the high scrap steel ratio mode, the converter end point FeO can be reduced to less than 12% through measures such as less slag and slag retention in the converter and the end point gun pressure time of not less than 90s. At the same time, the appropriate amount of auxiliary iron-containing materials such as dust removal ash balls and sludge balls can not only help the process slag and prevent the loss of return drying and splashing, but also recover a certain proportion of metal to reduce the steel material consumption index, so that the steel material consumption index is controlled at a reasonable level.
Control measures for high scrap ratio smelting process
(1) Optimize the converter slag making process. According to the high scrap ratio smelting practice of Yulin Steel’s 120t top and bottom combined blown converter, the converter process temperature of the high scrap ratio smelting mode is not surplus, and the smelting process adopts a slag-retaining process with less slag. The auxiliary materials are only active lime and magnesium balls for slag making. The consumption of magnesium balls is controlled at 6kg/t to 8kg/t. After each furnace of steel smelting is completed and slag splashing is completed, 25% to 100% of the slag is retained in the furnace according to the final slag conditions for the next furnace of steel smelting. This can not only alleviate the problem of low molten pool temperature and difficulty in slag making in the early stage of converter smelting, but also make full use of calcium oxide in the slag of the slag-retaining furnace to reduce the consumption of active lime. The high scrap ratio smelting mode is characterized by low temperature and difficulty in slag making in the early stage. The use of iron slag making path is conducive to scrap steel melting and rapid slag making. In the actual smelting process, slag-forming agents need to be added in batches. The first batch is mainly magnesium materials, and the addition time is delayed by 1min to 2min, which is conducive to the melting of scrap steel and the rapid heating in the early stage. In the early stage, the MgO content is increased to protect the splashing slag layer, and the FeO content is increased to promote slag. In the future, slag materials are added in batches according to the slag-forming situation, and the CaO content in the slag is gradually increased.
(2) Optimization of the first batch of auxiliary materials and oxygen supply system. The amount of scrap steel entering the converter increases. In order to ensure the temperature balance and good slag-forming during the smelting process, the first batch of auxiliary materials and oxygen supply intensity are adjusted as follows.
Control of the first batch of auxiliary materials. While ensuring that the total amount remains unchanged, add them in batches. It is strictly forbidden to add the first batch of auxiliary materials to the furnace at one time; the subsequent auxiliary materials are added according to the principle of adding small amounts in multiple batches, and the amount of each batch is controlled at 500kg to 800kg.
Gun position control. During the ignition of the lower gun, the gun position height is controlled according to 800mm of the measured liquid level; after the fire is ignited and the first auxiliary material is added, the oxygen gun position is controlled according to 100mm of the measured liquid level; during the smelting period from the start of the carbon-oxygen reaction to carbon pulling, the oxygen gun position is controlled according to 200mm to 300mm of the measured liquid level; during the final carbon pulling, the oxygen gun position is controlled according to the measured gun position.
In terms of oxygen flow control. Combined with the characteristics of dry dust removal, the oxygen flow during ignition is adjusted to 18000Nm3/h to 22000Nm3/h to prevent explosion during ignition. The oxygen flow during ignition is adjusted to 30000Nm3/h. Increasing the oxygen flow increases the impact depth and strengthens the stirring, which is conducive to rapid heating in the early stage to ensure the first batch of auxiliary materials are thoroughly melted. After the carbon-oxygen reaction begins, the oxygen flow rate is adjusted to 28000Nm3/h ~ 28500Nm3/h, reducing the oxygen flow rate, slowing down the carbon-oxygen reaction rate, making the molten pool evenly heated, ensuring the FeO content in the slag, and alleviating the dry-out phenomenon. After 12 minutes of smelting oxygen supply time, the oxygen flow rate is adjusted to 30000Nm3/h to strengthen the stirring of the molten pool, uniform composition and temperature, stabilize the flame, facilitate carbon pulling, reduce the FeO content in the slag, and improve the metal recovery rate.
(3) After the scrap ratio is increased, in order to improve the hit rate of the converter terminal carbon temperature, the first step is to reduce the tapping temperature. Through measures such as expanding the tapping port, covering the ladle, baking the alloy scrap steel after the furnace, and nano-insulation of the ladle, the tapping temperature can be reduced by 15°C. The second is to establish a smelting model under different scrap steel ratio modes. According to the temperature of the molten iron entering the furnace, the timing of adding auxiliary materials and the position of the oxygen lance blowing gun are reasonably selected to ensure the slag-removing effect of the smelting process and the control accuracy of the slag system components, which is more conducive to the control of the converter smelting process.
Control measures for converter lining with high scrap ratio
According to the practice of high scrap ratio smelting in the 120t top and bottom combined blowing converter of Yulin Steel, under the high scrap ratio mode, firstly, the operation of leaving slag and less slag is adopted, and the slag blocks are melted in the early stage of the slag blowing. After the magnesium material is added in the early stage, the slag is quickly melted. The MgO and CaO content of the slag is high, which inhibits the erosion of the furnace lining in the early stage; secondly, magnesium balls with high MgO content are used to replace light-burned dolomite, reducing the total slag amount of the converter by about 8kg/t, reducing the temperature loss caused by the large amount of auxiliary materials added during the converter smelting process, and reducing the converter tapping temperature by about 15℃ through measures such as expanding the diameter of the steel outlet, covering the ladle, and preheating the alloy and scrap steel baking furnace after the furnace, reducing the thermal erosion of the furnace lining; thirdly, pig iron blocks, rolled steel cutting heads and tails, and steel bar cutting heads are used to make large-scale slag filling in front of the converter, 3 to 4 times a day for a single furnace, reducing the impact of the converter adding scrap steel process on the furnace lining; fourthly, the low basicity and low MgO furnace protection process is studied, and the auxiliary material structure is continuously optimized. The basicity of the converter final slag is controlled at 2.3 times~ 2.5 times, the final slag MgO is controlled at 5% ~ 7%, and the slag FeO is controlled not more than 12% on the basis of ensuring the stability of the converter liquid level. The furnace operation is stable, and the converter life can reach more than 20,000 furnaces under the converter direct-to-continuous casting machine organization mode.
Quality Control Measures for Molten Steel with High Scrap Ratio
Strictly controlling the quality of scrap steel is one of the key measures in the high scrap steel ratio smelting process. The chemical composition of scrap steel is very important for the control of smelting reactions. In the smelting process of high scrap steel ratio, adding appropriate amounts of protective agents and alloying elements is very important for controlling smelting reactions and improving product quality. Scrap steel may contain some oxides, impurities and other harmful components, which can have an adverse effect on the quality of steel during the smelting process. In order to reduce the impact of these substances on the smelting process, operators need to add appropriate protective agents such as silicon, aluminum, calcium, etc. These protective agents can react with harmful components such as oxides to form stable compounds or inclusions, thereby reducing their impact on steel.
In addition, according to the requirements of the product and the required composition and properties of the steel, some alloying elements may need to be added. These alloying elements can alloy with other elements in the steel during the smelting process to adjust the composition and properties of the steel. For example, adding manganese can improve the strength and toughness of steel, adding chromium can increase corrosion resistance, and adding nickel can improve plasticity and low temperature resistance, etc. By adding appropriate amounts of protective agents and alloying elements, the smelting reaction can be effectively controlled and the quality of the product can be improved. This requires operators to make reasonable selections and controls based on the specific scrap steel composition and process requirements. At the same time, close furnace condition monitoring and real-time adjustment are also key to ensuring the stability of the smelting process and consistency of product quality.
Through chemical analysis methods, such as spectral analysis or chemical analysis, the main element content of scrap steel, including iron, carbon, silicon, manganese, etc., can be determined. These data can be used to adjust the steelmaking process to ensure appropriate smelting reactions. There may be impurities in scrap steel that are not conducive to smelting and product quality. For example, excessive sulfur content may lead to the formation of sulfides during smelting and affect the quality of steel.
Therefore, impurity detection, such as sulfur content, oxide content, and non-metallic inclusions, is required. The particle size distribution of scrap steel has an impact on the melting and reaction rate during steelmaking. Methods such as screening or laser particle size analysis can be used to test the particle size of scrap steel to ensure that it is within a reasonable range to facilitate uniform melting and smelting reactions in the furnace.
In addition to testing the quality of scrap steel, appropriate screening measures should be taken, such as selecting a reliable scrap steel supplier and ensuring that its quality control system is perfect to obtain high-quality scrap steel raw materials. Scrap steel is classified according to its type and characteristics in order to better control its use in the smelting process. Different types of scrap steel may have different effects on the smelting process, so they need to be classified and processed according to their needs. By strictly controlling the quality of scrap steel and taking necessary testing and screening measures, it can be ensured that the scrap steel meets the requirements and minimizes its impact on the steelmaking process. This will help maintain the stability of the smelting process and the consistency of product quality.
Results achieved
Through the above process optimization and improvement, the scrap ratio of the 120t top and bottom blown converter of Yulin Steel reached more than 26% without adding coke powder, ferrosilicon and other exothermic agents and without preheating scrap steel. The converter smelting indicators were effectively improved. After process optimization, the scrap steel consumption increased by 55kg/t, and the indicators such as the converter terminal carbon temperature hit rate, terminal phosphorus content, final slag FeO, and abnormal composition furnaces were significantly improved. The terminal resulfurization and nitrogen increase were effectively controlled. In the converter direct continuous casting machine mode, the converter furnace condition was stable, and the converter furnace life exceeded 20,000 furnaces.
Conclusion
Through summarizing and analyzing the problems of low iron consumption and high scrap ratio in Yulin Steel’s steelmaking process, control measures were formulated to optimize the converter steelmaking process. The specific conclusions are as follows.
(1) In order to solve the problem of difficulty in adding scrap steel to the converter under the high scrap ratio mode, the first is to optimize the scrap steel material size, the size of the purchased scrap steel shall not exceed 1000mm×600mm, and the bulk density shall not be less than 1.15t/m3; the second is to add a part of small and medium-sized scrap steel into the molten iron tank before the molten iron is tapped, and the blast furnace tapped iron flow is used to stir and melt, so as to control the impact on the temperature of the molten iron entering the furnace. According to the steelmaking practice of Yulin Steel, the temperature of the molten iron before adding 6t of scrap steel to a single tank before entering the furnace is reduced by 20℃～30℃. Subsequently, the temperature loss in the process can be reduced by covering the molten iron tank or preheating the scrap steel in the molten iron tank, and the scrap ratio can be further improved; the third is to add 1t of baked steel bar cut heads during the steel tapping process after the furnace, and the temperature loss is about 10℃. It can be selectively added according to the final temperature of the converter, which fundamentally solves the problem of loading in the converter under the condition of high scrap ratio.
(2) Under the high scrap ratio mode, the converter adopts the slag-retaining slag mode, which is conducive to the operation of the converter process and the improvement of indicators. According to the end point control situation, after the slag splashing is completed, 25% to 100% of the slag is left in the furnace for use in the next furnace. On the one hand, it can promote the rapid slagging in the early stage of converter smelting. The CaO and MgO content of the early slag is high, which not only inhibits the corrosion of the early acidic slag on the furnace condition, but also has a significant dephosphorization effect, and the FeO content of the final slag is reduced by about 2%; on the other hand, through the operation of less slag and slag retention, the consumption of active lime and magnesium balls is minimized to the greatest extent, the lime consumption is reduced by about 5kg/t, and the magnesium ball consumption is reduced by about 3kg/t. Under the premise of ensuring the stability of the smelting process, the cost of converter smelting auxiliary materials is effectively reduced, and the economy is stronger.
(3) In the high scrap steel ratio smelting mode, the converter only uses active lime and magnesium balls to make slag, the final slag basicity is controlled at 2.3 times to 2.5 times, and the final slag MgO is controlled at 5% to 7%. The liquid level of the 120t top and bottom combined blowing converter of Yulin Steel is controlled to be no less than 1.8m, the converter furnace condition is stable, and the furnace life can exceed 20,000 furnaces.
