With the implementation of the steel production restriction policy, the shortage of molten iron has become more serious, and the amount of scrap steel has increased year by year. At the same time, with the large-scale consumption of high-grade iron ore resources and high-quality coking coal resources at home and abroad and the increasing pressure of energy conservation and emission reduction, the production cost of blast furnace molten iron has gradually increased.
Therefore, how to maximize the steel production without increasing the iron production has become the primary consideration of the steelmaking system. Considering the factors such as converter loading, smelting heat balance, converter lining maintenance, product quality and comprehensive cost, a high scrap steel ratio smelting plan is planned to reduce production costs and increase economic benefits.
Effect of high scrap ratio on converter smelting process
Effect of high scrap ratio on converter charging
High scrap ratio means the maximum consumption of scrap steel. The scrap steel trough is basically used for adding scrap steel to the converter. First, different types of scrap steel are loaded into the scrap steel trough by matching, and the scrap steel trough is hoisted by the charging cross crane to add the scrap steel into the converter. With the continuous increase of scrap steel ratio, the scrap steel trough of some enterprises cannot meet the requirements of single bucket loading. The scrap steel trough is expanded, heightened and lengthened. Although the amount of scrap steel added has increased, the total tonnage of scrap steel + trough exceeds the design tonnage of the charging cross crane, which does not meet the regulatory requirements. Considering the high investment cost of modifying the crane, the crane has not been modified, which limits the improvement space of the scrap steel ratio;
Another part of the enterprises choose to add scrap steel with double buckets. Although this mode solves the problem of crane overload, it increases the converter smelting cycle and seriously affects the production efficiency of the converter. Therefore, in order to fundamentally solve the problem of scrap steel entering the furnace under the high scrap steel mode, other ways of adding scrap steel should be considered in addition to directly adding scrap steel to the converter.
The impact of high scrap ratio on steel material consumption indicators
The scrap steel ratio is a key factor affecting the steel material consumption index. Through the high scrap steel ratio smelting practice of Yulin Iron and Steel’s 120t top and bottom double-blown converter, the recovery rates of different types of scrap steel are different. According to the proportion of scrap steel purchased by Yulin Iron and Steel, the scrap steel recovery rate is calculated to be about 87.27%. According to the molten iron composition and the total slag amount of the converter, the molten iron recovery rate is calculated to be about 91.85%. Because the molten iron recovery rate is greater than that of scrap steel, the steel material consumption index increases by about 0.0458kg/t for every 1kg/t increase in scrap steel consumption.
Effect of high scrap ratio on converter smelting process
（1）The melting of scrap steel in the converter pool needs to go through three processes: surface molten iron condensation, condensation layer melting, and scrap steel carburization melting. According to the smelting practice of Yulin Steel’s 120t top and bottom double-blowing converter, under the high scrap steel ratio mode, the scrap steel entering the converter is not less than 32t. In the early stage of blowing, small and medium-sized scrap steel and plate-shaped scrap steel with large surface area melt quickly, and the converter pool temperature drops significantly in the early stage of blowing. The dust removal system of Yulin Steel’s steelmaking converter is dry dust removal. In order to control the explosion during opening and unloading, low oxygen pressure ignition is adopted for dry dust removal. The phenomenon that the converter lower gun fails to ignite within 50s of oxygen supply is quite frequent.
（2）When molten iron is added to the converter, some scrap steel is melted. In the early stage of blowing, the temperature of the molten pool is low, the heat and mass transfer of the molten pool is slow, and the temperature of the molten pool rises slowly. In addition, other scrap steel melts quickly, which slows down the heating rate of the converter. It is difficult to slag after the first batch of auxiliary materials are added to the converter, and the dephosphorization efficiency of the converter decreases in the early stage of blowing.
A steel plant uses a one-can system to load molten iron into steel. Affected by the turnover rate of the molten iron tank, the temperature of the molten iron in a single tank varies greatly. In the early stage of smelting, it is necessary to flexibly adjust the amount and timing of auxiliary materials according to the process temperature. The temperature control during the smelting process is difficult. Low-temperature slag overflow and unstable dephosphorization rate are prone to occur in the early stage of smelting. Poor slag formation in the middle stage is prone to dryness, resulting in a lower end carbon temperature hit rate and an increase in the number of overoxidation furnaces.
Effect of high scrap ratio on converter lining
According to the practice of 120t top and bottom blown converter smelting in steel mills, in order to increase the scrap steel ratio, the scrap steel material type is mainly heavy scrap steel above 6mm. During the process of scrap steel entering the furnace, the impact on the large lining of the front of the converter is large, causing certain damage to the large lining of the front of the converter.
During the smelting process, the oxides and non-metallic impurities in the scrap steel have a certain erosion effect on the refractory materials of the converter. At the same time, the molten pool temperature is low in the early stage of smelting, slag is difficult, the slag basicity is low, and the lining is seriously eroded.
Due to the low calorific value of scrap steel, high scrap steel ratio loading cannot provide enough heat, the converter process temperature is not surplus, the terminal temperature of some furnaces is insufficient, and the converter tapping temperature standard is not met. Spot blowing is required for tapping. The carbon content at the end of the spot blowing furnace is low, the molten steel is highly oxidizable, the FeO content in the slag is high, the converter splashing slag protection effect is poor, and the converter furnace condition maintenance is more difficult.
Effect of high scrap ratio on molten steel quality
The scrap steel entering the converter is mainly purchased scrap steel. The types of purchased scrap steel are diverse and difficult to distinguish. The trace elements such as sulfur, chromium and copper in the scrap steel are unstable. High scrap steel ratio smelting increases the amount of sulfur returned in the converter. At the same time, the nitrogen content of the molten steel at the end tends to increase. High sulfur and chromium content at the converter end often occur, affecting the quality of the steel billet and the rolling performance of the steel.
Analysis on the control measures of the influence of high scrap steel on steelmaking process
Control measures for high scrap steel ratio scrap steel entering the furnace
According to the production practice of 120t top and bottom blown converter in steel mill, in order to maximize the scrap ratio, process adjustments and improvements are mainly made in the following aspects.
（1）Optimize scrap steel material type. To ensure the loading of scrap steel in a single bucket and maximize the scrap steel 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 loading scrap steel in a single bucket.
（2）Add scrap steel to the blast furnace hot metal tank. The thermal balance of the converter depends on the difference between the heat income and heat expenditure of the charge. Making full use of the physical heat and chemical heat of the hot metal to optimize the structure of the metal material entering the furnace is an important way to increase the scrap steel ratio.
The steel mill adopts a hot metal tank smelting process. The turnover rate of the hot metal tank is about 2h/time. The blast furnace tapping temperature is 1510℃～1540℃, and the converter hot metal temperature is 1350℃～1450℃. The hot metal has excess heat. Before the hot metal tank is assigned to the blast furnace, 5t～6t of scrap steel is added to the hot metal tank. The heat and impact force during tapping are used to melt part of the scrap steel in the hot metal tank and directly convert it into liquid iron, reducing the hot metal temperature entering the converter, making the converter smelting process optimal, effectively improving the scrap steel ratio, and further reducing iron consumption and increasing steelmaking output.
（3）Add scrap steel to the ladle after the furnace. Purchase some steel bar cut ends or use pure scrap steel produced by rolling mill, the length of which is controlled within 10cm. This part of scrap steel is baked in the baking furnace after the furnace, and added to the ladle along with the steel flow during the steel tapping process of the converter. The amount added in a single 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 steel ratio smelting practice of Yulin Steel’s 120t top and bottom combined blowing converter, the converter process temperature of the high scrap steel ratio smelting mode is not surplus, and the smelting process adopts the less slag slag retention process. 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, according to the final slag conditions, 25% to 100% of the slag is retained in the furnace for the next furnace of steel smelting. It can not only alleviate the problem of low slag slag in the early stage of converter smelting, but also make full use of calcium oxide in the slag of the slag furnace to reduce the consumption of active lime.
The high scrap steel ratio smelting mode is characterized by low temperature and difficulty in slag slag in the early stage. The use of iron slag forming path is conducive to scrap steel melting and rapid slag forming. 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 rapid heating in the early stage. In the early stage, the MgO content is increased to protect the slag splashing layer, and the FeO content is increased to promote slagging. Subsequently, according to the slagging situation, slag materials are added in batches to gradually increase the CaO content in the slag.
（2）Optimization of the first batch of auxiliary materials and oxygen supply system. The amount of scrap steel entering the converter increased. In order to ensure the temperature balance and good slag formation during the smelting process, the first batch of auxiliary materials and oxygen supply intensity were 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 in accordance with the principle of adding multiple batches in small quantities, and the amount of each batch is controlled at 500kg to 800kg.
Control of gun position. During the ignition of the lower gun, the gun position height is controlled at 800mm of the measured liquid level; after the fire is ignited and the first auxiliary material is added, the oxygen gun position is controlled at 100mm of the measured liquid level; during the smelting period from the start of the carbon-oxygen reaction to the carbon pulling, the oxygen gun position is controlled at 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.
Control of oxygen flow. Combined with the characteristics of dry dust removal, the oxygen flow rate during ignition is adjusted to 18000Nm3/h ~ 22000Nm3/h to prevent explosion during ignition. The oxygen flow rate of ignition is adjusted to 30000Nm3/h. Increasing the oxygen flow rate 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 fully 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 carbon temperature at the converter end, the first step is to reduce the tapping temperature. The tapping temperature is reduced by 15°C 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 second step is to establish a smelting model under different scrap 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 smelting high scrap ratio in 120t top and bottom double-blown converter in steelmaking, under the high scrap ratio mode,
First, adopt the operation of leaving slag and less slag, leave slag furnace to blow the early slag block, add magnesium material in the early stage to quickly slag, the slag has high MgO and CaO content, and inhibits the erosion of the furnace lining in the early stage;
Second, use magnesium balls with high MgO content to replace light-burned dolomite, reduce the total slag of the converter by about 8kg/t, reduce the temperature loss caused by the large amount of auxiliary materials added during the converter smelting process, and reduce the converter tapping temperature by about 15℃ through the measures of expanding the steel outlet, covering the ladle, and preheating the alloy and scrap steel baking furnace after the furnace, and reduce the thermal erosion of the furnace lining;
Third, use pig iron blocks, rolled steel cutting heads and tails, and steel bar cutting heads to make large-scale slag filling in front of the converter, 3 to 4 times a day for a single furnace, to reduce the impact of the converter adding scrap steel process on the furnace lining; Fourth, study the low basicity and low MgO The furnace protection process continuously optimizes the auxiliary material structure, controls the basicity of the final slag of the converter at 2.3 times to 2.5 times, and the MgO of the final slag at 5% to 7%. On the basis of ensuring the stability of the liquid level of the converter, the slag FeO is controlled not to be greater than 12%, and the furnace operation is stable. Under the organizational mode of the converter directly connected to the continuous casting machine, the converter life can reach more than 20,000 furnaces.
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 reaction. In the smelting process of high scrap steel ratio, adding appropriate amount of protective agent and alloying elements is very important for controlling smelting reaction and improving product quality.
Scrap steel may contain some oxides, impurities and other harmful components, which will 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, adding nickel can improve plasticity and low temperature resistance, etc. By adding the right amount 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 adjustments are also the key to ensuring the stability of the smelting process and the 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 the appropriate smelting reaction. Scrap steel may contain impurities that are not conducive to smelting and product quality. For example, too high a sulfur content may lead to the formation of sulfides during the smelting process and affect the quality of the steel.
Therefore, it is necessary to detect impurities, such as sulfur content, oxide content and non-metallic inclusions. The particle size distribution of scrap steel affects the melting and reaction rate during the steelmaking process. The particle size of scrap steel can be tested by screening or laser particle size analysis 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 reliable scrap steel suppliers and ensuring that their quality control systems are sound, in order 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 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
Through the above process optimization and improvement, a steel plant’s 120t top and bottom combined blown converter has achieved a scrap ratio of more than 26% without adding coke powder, ferrosilicon and other exothermic agents and without preheating scrap steel, and the converter smelting indicators have been effectively improved. After the process optimization, the scrap steel consumption increased by 55kg/t, the converter end carbon temperature hit rate, end phosphorus content, end slag FeO, abnormal composition heats and other indicators were significantly improved, and the end sulfur regeneration and nitrogen increase were effectively controlled. In the direct-on continuous casting machine mode, the converter operates stably and the converter life exceeds 20,000 furnaces.
in conclusion
By summarizing and analyzing the problems of low iron consumption and high scrap steel ratio in steelmaking, control measures were formulated and the converter steelmaking process was optimized. The specific conclusions are as follows.
(1) To address the difficulty in adding scrap steel to the converter under the high scrap ratio mode,
First, optimize the size of scrap steel. The size of purchased scrap steel should not exceed 1000mm×600mm, and the bulk density should not be less than 1.15t/m3;
Second, add a portion of small and medium-sized scrap steel into the molten iron tank before tapping, and use the blast furnace tapping iron flow to stir and melt, so as to control the impact on the temperature of the molten iron entering the furnace. According to the steelmaking production practice of Yulin Steel, the temperature of the molten iron before adding 6t of scrap steel to a single tank is 20℃～30℃ lower than that before entering the furnace. 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, so as to further improve the scrap ratio;
Third, add 1t of baked steel bar cut head during the 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) The converter adopts the low slag retention mode under the high scrap ratio mode, which is conducive to the converter process operation and index improvement. According to the end point control, 25% to 100% of the slag is left in the furnace for use in the next furnace after the slag splashing is completed.
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 low slag retention operation, 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 ratio smelting mode, the converter only uses active lime and magnesium balls to make slag, the final slag basicity is controlled at 2.3 to 2.5 times, the final slag MgO is controlled at 5% to 7%, and the liquid level of the 120t top and bottom blown converter of Yulin Steel is controlled to be no less than 1.8m. The converter operation is stable and the service life can exceed 20,000 furnaces.
