Converter Smelting Technology in the Iron and Steel Industry: The Evolution from Traditional to Intelligent
As the "backbone" of the industrial system, the iteration of iron and steel production technology is directly related to the improvement of a country's industrial strength. Among the numerous iron and steel smelting processes, converter smelting technology has become the core link in the modern steelmaking process due to its advantages of high efficiency and low cost. From the early air side-blown converters to today's intelligent oxygen top-bottom combined blown converters, this technology has undergone a century of evolution. It not only supports the continuous growth of global iron and steel output but also makes continuous breakthroughs in fields such as green low-carbon development and high-quality steel grade research and development, becoming a key driving force for the transformation and upgrading of the iron and steel industry.
I. Development History of Converter Smelting Technology: The Leap from "Primitive" to "Precise"
The origin of converter smelting technology can be traced back to the mid-19th century. Its development history is essentially a history of technological exploration focused on "how to more efficiently use oxygen to decarbonize molten iron," which can be roughly divided into three key stages:
(1) Traditional Side-Blown Converters: The Starting Point of Industrial Scale Production
In 1856, British engineer Henry Bessemer invented the Bessemer converter, which for the first time realized the industrial production of removing carbon from molten iron by blowing air into it. Early converters adopted the side-blown method, in which air was forced into the converter through tuyeres on the side of the furnace body. The heat released by the reaction between oxygen in the air and elements such as carbon, silicon, and manganese in the molten iron maintained the smelting process (no additional heating was required). The smelting cycle was only 15-20 minutes, which was nearly 10 times more efficient than the open-hearth steelmaking method at that time.
However, the technology at this stage had obvious defects: the nitrogen brought by air would lead to excessively high nitrogen content in the molten steel, affecting the toughness of the steel; at the same time, the iron oxide generated by the reaction between air and molten iron would cause iron loss (oxidation and loss of iron elements), resulting in low resource utilization. Therefore, side-blown converters were mainly used to produce low-quality carbon steel and could hardly meet the high-quality requirements of fields such as machinery manufacturing and construction.
(2) Oxygen Top-Blown Converters: The "Founder" of Modern Steelmaking
In the 1950s, the Austrian company Linz-Donawitz (LD) developed the oxygen top-blown converter, which completely changed the technical pattern of converter smelting. This technology replaced air with high-purity oxygen (purity > 99.5%) and injected oxygen into the molten iron at high speed through an oxygen lance at the top of the furnace. This not only greatly reduced the pollution of nitrogen to the molten steel but also significantly improved the efficiency of decarburization and dephosphorization.
The birth of the oxygen top-blown converter marked the maturity of modern converter smelting technology, and its advantages are mainly reflected in three aspects: first, the smelting cycle is short (20-30 minutes per furnace), and the output per furnace can reach the 100-ton level, meeting the needs of large-scale production; second, the quality of molten steel is stable, and it can produce various steel grades such as carbon steel and low-alloy steel; third, the energy consumption is low, and the smelting temperature can be maintained by using the heat of chemical reactions without a large amount of external heat supply. By the end of the 20th century, the oxygen top-blown converter had become the mainstream steelmaking process in the world, accounting for more than 80% of the global converter steel output.
(3) Top-Bottom Combined Blown Converters: The "Upgraded Version" for Precise Control
In the 1980s, to further improve the quality of molten steel and the flexibility of smelting, the top-bottom combined blown converter technology emerged. On the basis of oxygen top-blowing, this technology added a bottom gas supply system — inert gas (such as argon, nitrogen) or weakly oxidizing gas (such as carbon dioxide) is blown into the bottom of the molten iron through porous plugs or nozzles at the bottom of the furnace, forming a composite stirring mode of "top-blown oxygen + bottom-blown gas."
The core advantage of top-bottom combined blowing lies in "precisely regulating the stirring intensity of the molten pool": the stirring effect of the bottom gas can break the oxide slag layer on the surface of the molten iron, enabling full contact between oxygen and molten iron, reducing local hot spots, and lowering the oxygen content and the number of inclusions (such as oxides and sulfides) in the molten steel; at the same time, by adjusting the type and flow rate of the bottom-blown gas, segmented control of the dephosphorization, desulfurization, and decarburization processes can be realized, meeting the smelting requirements of high-quality steel (such as automotive steel and pipeline steel). At present, almost all newly built converters in the world adopt the top-bottom combined blowing technology, and some traditional top-blown converters have also been transformed and upgraded to the combined blowing process, becoming the core equipment for the production of high-end steel.
II. Core Processes of Converter Smelting: The "Transformation" from Molten Iron to Molten Steel
The essence of converter smelting is a physical and chemical process that "uses oxygen to remove impurities (carbon, silicon, manganese, phosphorus, sulfur, etc.) from molten iron, adjusts the composition and temperature of the molten steel, and finally obtains qualified molten steel." Its core processes can be divided into three stages: charging, blowing, and tapping. Each stage requires strict control of parameters to ensure the quality of molten steel.
(1) Charging Stage: "Precise Proportioning" is the Foundation
Charging is the preparation link of converter smelting, and its core is to load molten iron, scrap steel, and slag-forming agents into the converter in a specific proportion to lay the foundation for subsequent blowing. The specific process includes:
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Molten Iron Charging: Molten iron comes from the blast furnace, with a temperature of about 1250-1350°C and a carbon content of 3.5%-4.5% (it is the main raw material for steelmaking). The charging amount accounts for 70%-80% of the total raw materials. Before charging, the molten iron needs to be pretreated (such as desulfurization and desiliconization) to reduce the impurity content — for example, by adding lime or magnesium-based desulfurizers to the molten iron, the sulfur content can be reduced from 0.05% to less than 0.01%, meeting the requirements of low-sulfur steel grades.
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Scrap Steel Charging: Scrap steel mainly comes from iron and steel waste (such as waste steel billets and waste steel materials), and the charging amount accounts for 20%-30% of the total raw materials. Its functions are: first, to adjust the furnace temperature (the melting of scrap steel absorbs heat to avoid excessive temperature during the blowing process); second, to reduce the consumption of molten iron and save resources. The quality of scrap steel must be strictly controlled to avoid mixing with non-ferrous metals (such as copper and zinc) or harmful elements (such as lead and arsenic), otherwise, the composition of the molten steel will exceed the standard.
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Slag-Forming Agent Charging: Slag-forming agents mainly include lime (CaO), dolomite (CaCO₃·MgCO₃), fluorite (CaF₂), etc. The charging amount is determined according to the composition of the molten iron. Their core function is to react with impurities such as phosphorus and sulfur in the molten iron to form slag (such as calcium phosphate and calcium sulfide) and separate the slag from the molten steel — for example, calcium phosphate generated by the reaction between lime and phosphorus has a low melting point and low density, which will float on the surface of the molten steel and be finally discharged with the slag to achieve the purpose of dephosphorization.
(2) Blowing Stage: "Oxygen and Temperature Control" is the Key
Blowing is the core link of converter smelting, with a duration of about 15-25 minutes. It is divided into three stages: early, middle, and late. Each stage has different reaction focuses and control objectives:
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Early Stage (Desiliconization and Dephosphorization Stage): After the start of blowing, the oxygen lance is lowered to a position 1.5-2.0 meters away from the surface of the molten iron. High-speed oxygen first reacts with silicon (Si) and manganese (Mn) in the molten iron (Si + O₂ = SiO₂, Mn + O₂ = MnO), releasing a large amount of heat and rapidly increasing the furnace temperature to above 1400°C. At the same time, the lime in the slag-forming agent begins to melt, forming alkaline slag, which reacts with phosphorus (P) in the molten iron to generate calcium phosphate (2P + 5FeO + 3CaO = 3CaO·P₂O₅ + 5Fe), realizing dephosphorization. In this stage, the oxygen flow rate should not be too high to avoid "slag drying" (the slag temperature is too low and the fluidity is poor, making it impossible to fully react with impurities).
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Middle Stage (Decarburization Stage): As the furnace temperature rises, the reaction between oxygen and carbon (C) in the molten iron becomes the main reaction (C + O₂ = CO, C + FeO = CO + Fe). A large amount of carbon monoxide gas escapes from the molten steel, forming a violent "boiling" phenomenon (molten pool stirring). This stage is the key period for decarburization. The carbon content decreases from 3%-4% to less than 0.5%, and the furnace temperature rises to 1550-1600°C. It is necessary to adjust the height of the oxygen lance (lowering the oxygen lance can enhance the oxygen penetration and accelerate decarburization) and the flow rate of the bottom-blown gas to ensure uniform stirring of the molten pool and avoid "overoxidation" caused by excessively low local carbon content (excessively high oxygen content in the molten steel affects the toughness of the steel).
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Late Stage (Composition Fine-Tuning Stage): When the carbon content drops to the target range (such as 0.1%-0.2%), the late stage of blowing begins. At this time, the oxygen flow rate needs to be reduced to slow down the decarburization speed. At the same time, samples are taken from the furnace mouth to analyze the composition of the molten steel (such as carbon, manganese, and phosphorus content) and temperature. According to the analysis results, alloys (such as ferromanganese and ferrosilicon to adjust the composition of the molten steel) or coolants (such as scrap steel and iron ore to control the furnace temperature) are added. The core goal of this stage is to accurately control the composition and temperature of the molten steel within the requirements for tapping. For example, cold-rolled steel for automobiles requires the carbon content to be controlled below 0.05% and the temperature to be controlled at 1600-1650°C.
(3) Tapping Stage: "Slag Blocking and Purity Preservation" is the Core
When the blowing reaches the target, the converter is tilted, and the molten steel is poured from the tapping hole into the ladle. At the same time, a "slag blocking" operation (such as using a slag ball or a slag cone) is carried out to prevent the slag from entering the ladle — the slag contains a large number of oxides and harmful elements. If it is mixed into the molten steel, it will increase the number of inclusions and reduce the quality of the steel.
During the tapping process, "molten steel refining" is also required: deoxidizers (such as aluminum and calcium-silicon alloys) are added to the ladle to remove excess oxygen in the molten steel (2Al + 3FeO = Al₂O₃ + 3Fe), and desulfurizers (such as calcium carbide and magnesium powder) are added to further reduce the sulfur content. In addition, some high-end steel grades also need to transfer the ladle to an LF furnace (ladle furnace) or an RH furnace (vacuum circulation degassing device) for secondary refining to achieve deeper degassing and inclusion removal, and finally obtain high-quality molten steel.
III. Key Technological Breakthroughs in Converter Smelting: Greenization and Intelligence in Parallel
With the advancement of global "dual carbon" (carbon peaking and carbon neutrality) goals and the growth of the iron and steel industry's demand for high-quality steel, converter smelting technology is making breakthroughs in two major directions: "green low-carbon" and "intelligent precision," and a number of representative key technologies have emerged.
(1) Green Low-Carbon Technologies: Reducing Energy Consumption and Emissions
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Converter Gas Recovery and Utilization Technology: In the past, the carbon monoxide (CO) gas generated during the blowing process (i.e., converter gas) was mostly directly burned and discharged, which not only wasted energy but also produced carbon dioxide. Today, through a "dry dust removal + gas recovery" system, the CO concentration in converter gas can be purified to 60%-80%. After recovery, it is used as fuel for heating furnaces and boilers, or as a chemical raw material for methanol production. At present, the converter gas recovery rate of advanced domestic iron and steel enterprises has reached 100-120 m³ per ton of steel, which can meet 20%-30% of the enterprise's fuel demand and reduce carbon dioxide emissions by hundreds of thousands of tons annually.
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Slag Resource Utilization Technology: In the past, converter slag (100-150 kg generated per ton of steel) was mostly stored as waste, occupying land and polluting the environment. Now, through "water quenching treatment + magnetic separation," iron metal can be recovered from the slag (recovery rate > 80%), and the remaining slag can be processed into construction aggregates (such as concrete admixtures), road base materials, or cement raw materials. For example, Baowu Group uses converter slag in the concrete of high-speed rail track bases, which not only meets the strength standards but also realizes "turning waste into treasure," reducing solid waste emissions by more than 10 million tons annually.
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Low-Carbon Smelting Processes (e.g., Exploration of "Zero-Carbon" Blowing in Converters): Some enterprises are testing the use of clean energy such as hydrogen and natural gas to replace part of the oxygen, reducing carbon dioxide generated by carbon oxidation reactions. For example, the "HYBRIT" project jointly developed by Sweden's SSAB, LKAB, and Vattenfall blows hydrogen into the converter to convert carbon in the molten iron into methane (instead of carbon dioxide), and then recovers hydrogen through methane cracking for recycling. At present, small-scale low-carbon steel production has been realized, and commercial application is planned for 2030.
(2) Intelligent Control Technologies: Improving Precision and Efficiency
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Dynamic Steelmaking Model: Based on big data and artificial intelligence technology, a dynamic correlation model of "molten iron composition - oxygen blowing amount - furnace temperature - molten steel composition" is established. During the blowing process, real-time data on furnace temperature and exhaust gas composition (such as CO and CO₂ concentrations) are collected through a furnace mouth infrared thermometer and an exhaust gas analyzer. The model automatically calculates the required oxygen lance height, bottom-blown gas flow rate, and alloy addition amount to achieve "real-time regulation and precise targeting." For example, the intelligent converter control system of Ansteel Group has increased the hit rate of molten steel composition from 85% to 98%, shortened the smelting cycle by 2-3 minutes per furnace, and reduced alloy consumption by more than 1,000 tons annually.
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Machine Vision and Robot Application: Through a furnace mouth camera and machine vision algorithms, the boiling state of the molten pool and the slag thickness in the furnace are monitored in real time, replacing manual visual observation and reducing human misjudgment; at the same time, robots are used to complete dangerous operations such as furnace mouth sampling, temperature measurement, and slag addition, improving production safety. For example, the converter workshop of Baosteel Co., Ltd. has realized "unmanned sampling" — the robot inserts a sampling lance into the furnace through a mechanical arm, automatically completes sampling, and sends the sample to the laboratory for analysis. The entire process requires no manual intervention, the sampling efficiency is increased by 50%, and the harm of high temperature and dust to operators is avoided.
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Digital Twin Technology: A digital twin model of the converter is built to map the operating parameters of physical equipment (such as furnace temperature, oxygen lance life, and bottom-blown porous plug status) to the virtual model in real time. Equipment failures and smelting effects are predicted through simulation. For example, the converter digital twin system of HBIS Group can predict the blockage risk of bottom-blown porous plugs 72 hours in advance with an accuracy rate of 92%, avoiding furnace shutdown accidents caused by porous plug failures and increasing equipment utilization by more than 15%.
IV. Challenges and Future Outlook: The "Second Half" of Converter Smelting Technology
Although converter smelting technology has made significant progress, it still faces challenges in green low-carbon development, high-end steel production, and equipment life: first, the carbon dioxide emission intensity is still relatively high (about 1.8-2.0 tons of carbon dioxide per ton of converter steel), and there is still a large gap from the "carbon neutrality" goal; second, for ultra-pure steel (such as bearing steel and nuclear power steel), the precision of inclusion control in converter smelting needs to be further improved; third, the service life of key components such as the converter furnace body, oxygen lance, and bottom-blown porous plug is short (for example, the furnace lining life is about 1,500-3,000 furnaces), and frequent replacement increases production costs.
Looking to the future, converter smelting technology will make breakthroughs in the following directions:
Deeper Low-Carbonization: On the one hand, promote the large-scale application of clean energy such as hydrogen and natural gas in converters, and explore the hybrid smelting process of "converter + electrolytic reduction" to reduce carbon emissions from the source; on the other hand, strengthen the research and development of carbon capture, utilization, and storage (
