Abstract: As the world’s main method for producing high-quality steel, converter steelmaking has played a significant role in promoting energy conservation, emission reduction, cost reduction, and efficiency improvement in the steel industry through technological advancements and development. This paper introduces the research and progress of several key technologies in converter steelmaking in my country. The melting behavior of scrap steel was studied through both theoretical analysis and practical exploration, revealing the melting process. The effects of injection methods and bottom blowing processes on the flow performance of the converter molten pool were analyzed and summarized. The current status and future prospects of converter terminal control technology were introduced from both metallurgical and data modeling perspectives. Finally, from the perspective of overall optimization of the metallurgical process, the progress and problems of integrated steelmaking-refining-continuous casting technology were introduced, from mathematical modeling to platform simulation. The standardization of converter steelmaking requires more support in terms of equipment, methods, and technologies. Standardized converter steelmaking is a comprehensive practical application case involving multiple disciplines such as metallurgy, automation, and computer science.
Green development is one of the world’s most important development trends. Under the “dual carbon” (carbon dioxide, carbon emissions, and carbon sequestration) background, the steel industry, with its high energy consumption and high pollution emissions, faces significant energy conservation challenges. China’s steel industry can achieve green manufacturing by developing green technologies and improving product quality. Converter steelmaking+ is our steelmaking method and a key achievement in long steel production processes; its greening and horizontal transformation has played a crucial role in optimizing and upgrading efficient steel structures. This article systematically reviews and summarizes the achievements and progress of several key technologies, including converter utilization of scrap steel, optimization of molten pool flow characteristics and steelmaking processes, and refining and chemical refining, and also provides an outlook on future steel development trends.
High-efficiency converter technology for scrap steel
To achieve the “dual carbon” targets in the steel industry, overall control of crude steel production and adjustment of process structure are important approaches, while breakthroughs in key common technologies are crucial support. The continuous growth of scrap steel resources in recent years has provided the possibility of adjusting the raw material structure of steel production and realizing steel recycling. Statistics show that replacing 1 ton of molten iron with 1 ton of scrap steel in converter smelting can save 500 kg of standard coal; for every 1% increase in the scrap steel ratio in converter smelting, the loss of non-ferrous elements such as manganese and chromium will decrease by about 0.04%; and a 10% increase in the scrap steel ratio in converter steelmaking can reduce emissions by 6% from the increased emission targets. Therefore, utilizing scrap steel in converters has become one of the important development directions for optimizing converter steelmaking processes.
In industrial practice, steel companies improve the scrap combustion ratio in converters. On the one hand, this increases the heat income of converter smelting, mainly through measures such as increasing the temperature of molten iron, pre-processing scrap, retaining converter slag, secondary technologies, fuel addition technologies, and converter bottom powder injection technologies. On the other hand, it reduces the heat expenditure of converter steelmaking by developing technologies such as ladle covering and molten iron ladle covering, as well as improving the level of production organization and smelting operations, thereby reducing heat loss during transportation.
In theoretical research, scholars believe that the melting rate of scrap steel is a key factor in increasing the scrap steel ratio in converters. Some studies have pointed out that the melting of scrap steel in the iron-carbon molten pool is a melt, and by carburizing the surface of the scrap steel to lower the melting point of the steel, the carburized layer of the scrap steel is induced to melt, thereby creating a new surface and a continuous phase transformation process, as shown in Figure 1;
The melting of scrap steel occurs due to the generation of a temperature and concentration boundary layer on the surface of molten iron and scrap steel, which is affected by the heat and mass transfer between molten iron and scrap steel, as shown in Figure 2.
To study the melting behavior of scrap steel in an iron-carbon molten pool, researchers conducted laboratory thermal simulation experiments, water model experiments, numerical simulations, and industrial experiments. To further elucidate the scrap steel melting behavior, researchers analyzed the microstructure changes and carbon infiltration melting phenomena during the scrap steel melting process under different operating conditions. Figure 3 shows the scrap steel temperature distribution obtained through numerical simulation, the scrap steel melting process in water simulation, the scrap steel melting model obtained through thermal simulation, and the carbon melting phenomenon during the scrap steel melting process, respectively.
Literature analysis shows that, considering the concentration of scrap steel, many believe that the shape, size, and carbon content of the scrap steel affect its melting time in the converter: increasing the carbon content and ratio of scrap steel can promote melting; some scholars have pointed out that the carbon content of scrap steel also affects its melting state. When the centerness of the scrap steel is 0.82, the melting time of multi-grade scrap steel and single scrap steel is the same. Furthermore, some scholars have indicated that the silicon content in the scrap steel is inversely proportional to the melting time in the converter.
From the perspective of the converter molten pool, the composition, temperature, and kinetic conditions of the converter molten pool affect the heat and mass transfer between scrap steel and the molten pool. Some scholars believe that the carbon content, temperature, and stirring intensity of the molten pool can accelerate the heat and mass transfer between scrap steel and the molten pool, thereby reducing the melting time of scrap steel: an increase of 50℃ in the molten pool temperature or a 0.6% increase in the carbon fraction of the molten pool increases the heat and mass transfer time of scrap steel by 1 mm. In addition, the scrap steel distribution method and the gas supply flow rate have a direct impact on the mixing time and flow properties of the molten pool. The mixing time of uniform and tilted steel distribution is about 20%~40% longer than that of concentrated distribution. When the bottom blowing flow rate increases from 15L/min to 50L/min, the volume ratio of the low-speed zone decreases by 89.46%. Figures 5b and 5c show the scrap steel distribution methods in numerical simulation and physical simulation, respectively.
Technical Research on the Flow Characteristics of Converter Molten Pool
Thorough stirring in a converter provides favorable melt kinetics, thereby increasing the speed and area of gas stirring in the molten steel and reducing the mixing time of the converter pool per week, which is often weaker. The converter’s blowing method and bottom blowing process also affect the gas flow, blowing area, and mixing time.
Study on the effect of injection method on the flow characteristics of molten pool
The blowing methods for converters include top blowing, bottom blowing, top and bottom combined blowing, and top and bottom combined blowing plus side blowing. Different blowing methods result in different molten pool mixing effects.
In the study of a 300t top-and-bottom combined blowing converter, mathematical calculations showed that optimizing only the top-blown oxygen lance structure reduced the molten pool mixing time to a maximum of 6 seconds, compared to 108-126 seconds for a pure top-blown converter. However, by increasing the bottom-blown flow rate from 0.04 m³/(t·min) to 0.2 m³/(t·min) on top-blown basis, the molten pool mixing time approached 50 seconds, and the mixing time after top-and-bottom combined blowing was 37-88 seconds. Research on an 80t top-and-bottom combined blowing converter showed that the molten pool mixing time in the top-and-bottom combined blowing converter was 25 seconds less than that in a pure top-blown converter. Physical simulations indicated that when a 200t converter only uses pure bottom blowing, optimizing the bottom blowing process resulted in a minimum mixing time of approximately 27 seconds, while with top-and-bottom combined blowing, the mixing time could be maintained up to approximately 22 seconds. It is evident that bottom blowing has a more significant effect on the mixing time of the local molten pool. However, due to the influence of high temperature and high impact force on the bottom blowing nozzle, improving the sniping effect of the bottom blowing nozzle is one of the urgent problems to be solved in the converter.
After adding side-blowing lances to the simulated 30t and 100t top-and-bottom combined blowing converters, the mixing time of the molten pool was reduced by an average of 60% compared to the original top-and-bottom combined blowing converters. Through water simulation experiments, the mixing time of the molten pool was reduced by 15-18 seconds after adding side-blowing lances to the top-and-bottom combined blowing converters.
Study on the effect of oxygen lance structure and oxygen supply system on molten pool flow characteristics
Oxygen blown from the top of the converter impacts the surface of the molten pool after passing through the oxygen lance, causing molten movement and forming molten pool pits of varying depths and diameters. Due to the advantages of Laval nozzles—high jet velocity, stable jet, and strong capacity—the molten pool is well agitated, and generally, converter oxygen lances use Laval nozzles. Their structure and oxygen supply mechanism have a significant impact on the direct effect on the molten pool, the impact depth, and the impact diameter. Figure 6 shows the structures of three typical oxygen lance nozzles.
Studies show that the mixing time is directly proportional to the top-blown flow rate and inversely proportional to the oxygen lance position, but both have critical values. A water model study of a 210t combined-blown converter indicates that the top-blown gas flow rate has the greatest impact on the mixing time. Studies of an 80t combined-blown converter show that the top-blown flow rate has the greatest impact on the mixing time. The critical value for the mixing time is 78.94 m²/h; after this maximum critical value, the decrease in the mixing time of the molten pool becomes slower with increasing top-blown flow rate. A water simulation experiment of a 100t converter shows that when the lance position is 1.7m, the minimum mixing time of the molten pool is approximately 23 s.
Furthermore, increasing the number of oxygen lance injection holes from 4 to 5 increased the mixing time of the molten pool by approximately 30 seconds. Studies on a 300t combined blowing converter show that when the lance’s Mach number is 2.10, the oxygen jet’s impact on the molten pool is greatest, both in terms of the oxygen nozzle and the impact area.
Recently, scholars have studied dual-structure oxygen lances, arranging them in staggered patterns with different shapes and designing varying inclination angles, throat diameters, and outlet diameters for the nozzles. Research on the dual-structure oxygen lance structure in a 260t combined blowing converter shows that, with the same nozzle flow rate, the impact depth of the dual-structure oxygen lance is increased by up to 17.5% compared to the traditional oxygen lance, and the width of the impact pit is reduced by at least 1.6%. Literature indicates that the mixing time can be increased by up to 18% per week, and the mixing time can be increased by up to 17% per full week.
Research on the effect of bottom blowing process on the flow properties of the molten pool
In order to optimize the kinetic conditions of the converter molten pool, many scholars have studied the influence of the converter bottom blowing process on the converter flow characteristics.
- Bottom blowing flow rate. The bottom blowing rate is inversely proportional to the uniformity time of the molten pool. However, if the bottom blowing flow rate is too high, the energy supplied by the bottom blowing gas will cause the molten pool to shake and generate liquid splash. Therefore, reasonable energy output is an important condition for achieving stable converter smelting. The range of bottom blowing flow rate studied is generally 0.02~0.35 m²/(t.min).
- Bottom-blown gas supply methods. There are two types of bottom-blown gas supply methods: uniform and non-uniform, as shown in Figure 7. Studies have shown that adopting a non-uniform gas supply mode can effectively avoid the deterioration of converter kinetics caused by blockage of some bottom-blowing elements. Research results show that the coordination time of non-uniform gas supply can be reduced by up to 51.5%.
3) Mixing time of bottom-blowing elements. Some researchers believe that a higher number of elements can shorten the mixing time of the molten pool. For example, in a study of a 200t combined blowing converter, 8-12 bottom-blowing elements resulted in a mixing time 6-11 seconds shorter than 4 elements, while the mixing time of a purely bottom-blowing element in physical simulations ranged from 14 to 49 seconds. Some scholars, through water simulation experiments, have shown that when using 8 bottom-blowing elements in a 300t converter, the shortest mixing time of the molten pool is approximately 23 seconds. Other scholars believe that a lower number of elements can increase the flow rate of a single bottom-blowing element, thus promoting faster mixing of the converter molten pool.
4) Construction method of bottom-blowing components. The construction method includes the construction radius, angle, and arrangement, as shown in Figure 8. Literature indicates that the construction methods for bottom-blowing components in converters of different tonnages are: radius within 0.30D~0.55D, angle between 25° and 60°, and arrangement methods including non-preset and pre-set arrangements, as shown in Figure 9. Studies have shown that non-preset construction results in poor mixing.
The above analysis shows that bottom blowing in converters has a better stirring effect. Top blowing oxygen, in addition to creating a significant horizontal flow with bottom blowing to optimize the molten pool flow field, also provides better slag formation. Therefore, the combined top and bottom blowing capability of converters makes their advantage in smelting high-quality steel even more pronounced. However, the nozzle life of bottom blowing elements in converters is short, necessitating the development of high-temperature resistant, corrosion-resistant, and impact-resistant bottom blowing materials to provide technical guidance for further improving the efficiency of bottom blowing in converters.
Converter Endpoint Control Technology
The level of converter endpoint control technology is crucial for achieving high-efficiency and stable production in converter steelmaking, and is also a key factor in realizing the commercialization of converter steelmaking. Currently, more research focuses on the accurate prediction of converter endpoints, the judgment of actual situations after prediction, and further precise control requires more equipment, methods, and technical support.
The accuracy of converter endpoint prediction models is a crucial indicator of the effectiveness of instantaneous converter endpoint prediction. In recent years, many scholars have conducted extensive research to improve converter prediction accuracy, yielding preliminary results and progress: The first is a static model based on metallurgy, but due to its numerous assumptions and consideration of only the initial and final states, its accuracy in predicting carbon temperature is very low, limiting it to theoretical reference only. The second is a data model established based on data processing and algorithm optimization. Its main shortcomings include:
1) The accuracy is affected by the high dependence on the quantity and quality of data. For example, the sub-lance model cannot collect process data for converters below 150t due to the limitation of furnace opening size; while the data in the furnace gas analysis model is lagging and the scrap steel composition is unknown, affecting the accuracy of the model. In addition, the data information collected by the furnace opening template analysis model is limited to some furnace opening information, which is one-sided.
2) The algorithm relies excessively on data and lacks process flow analysis, resulting in low accuracy in field applications. To address these issues, some scholars have improved the prediction accuracy of converter endpoints through convergent models and data models. By numerically simulating some process parameters of the top steelmaking process and using them as input to the data model, the data dimensionality of the model is increased, thus improving the prediction accuracy of the converter endpoint. Additionally, some scholars have established multi-task prediction models by analyzing the correlation between temperature and carbon content through metallurgical analysis, compensating for the severe data dependence and providing methods and ideas for improving the prediction accuracy of converter endpoints. However, to improve the control level and coloration degree of converter steelmaking, data modeling analysis and decision-making should be based on metallurgical ignition. Only in this way can converter steelmaking truly achieve self-sensing, self-decision-making, self-learning, intelligence, and self-execution.
Integrated steelmaking-refining-continuous casting technology
Therefore, the degree of integration of steelmaking, refining and continuous casting has an important impact on improving the production rhythm of steel plants, ensuring the continuous operation of material flow, efficient production cycle, and thus reducing production costs. This is of fundamental significance for the green and low-carbon development of steel plants.
By using mathematical models, the layout scheduling of steel refining and continuous casting is transformed into a mathematical problem. Theoretical research was conducted on the optimization scheduling scheme for the case of single product and fixed production process, and the average waiting time, production cycle and other indicators were optimized.
A dynamic scheduling strategy for experts was developed by combining expert experience rules with heuristic experience knowledge. This approach is universal and has greater guiding significance for on-site operations, providing a reference for on-site production command, safety early warning, and labor reduction. A 3D simulation platform was used to construct an integrated optimized scheduling system for steelmaking, refining, and continuous casting. This method allows for a more intuitive understanding and analysis of the interactions between various relevant variables in the steel manufacturing process, simulating different production strategies and plans, evaluating their impact on production efficiency and product quality, and thus dynamically adjusting production planning and decisions to improve the overall efficiency of steel manufacturing. While the mathematical model employs extensive imagination and provides theoretical guidance for integrated scheduling of steelmaking, refining, and continuous casting in steel plants, the rule extraction efficiency of expert system-based methods is low and cannot meet the real-time requirements of on-site operations. Simulation optimization methods tend to solve production scheduling problems in specific steel plants in a more general way, lacking universality. To improve the integration level of steelmaking, refining, and continuous casting in steel plants and increase its applicability, a laminar flow model can be considered first to simplify complex processes and reduce path continuity. Furthermore, when processing time changes occur during operations, overhead cranes and other transportation tools can be fully utilized for buffering.
Conclusions and Outlook
Against the backdrop of green and low-carbon development, my country’s steel industry faces the challenge of transformation and upgrading. This article analyzes, discusses, and summarizes several key aspects of converter steelmaking technology during this important development period, and proposes future prospects.
1) Efficient utilization of converter scrap steel requires attention to the classification and processing of scrap steel, and it is necessary to measure and judge basic parameters such as the recovery rate of various types of scrap steel in the market. Accurately establishing the link between various types of scrap steel and the molten pool composition, temperature and smelting time in the converter steelmaking process, and carrying out relevant technical explorations can provide practical reference for the rational utilization of scrap steel in converters.
2) The bottom blowing stirring efficiency of the converter determines the mixing of the molten pool and the metallurgical effect. We should actively develop refractory materials with high temperature resistance, impact resistance and chemical resistance to increase the life of bottom blowing elements, and carry out relevant basic technology research and engineering practice exploration.
3) The research and development of key technology units such as the accurate acquisition and transmission of converter steelmaking data, the analytical coupling of metallurgical process reports and big data simulation models, and the production scheduling level of the steelmaking continuous casting prediction process have played a crucial role in further promoting the progress of converter steelmaking coloring.
