Today, there are about 600 converters in the world, and the crude steel output is 450 million tons, accounting for about 60% of the global crude steel output. Since the first converter in the world was put into operation in Aogang, the modern high-efficiency basic oxygen converter is the product of continuous development for more than 50 years, and has made remarkable progress in furnace life, increasing load capacity and reducing maintenance. This kind of equipment is exposed to high temperature environment and subjected to mechanical impact and thermal stress, so its engineering design is very challenging. Suspension system is very important to realize the long life of converter. In order to produce high-quality steel and improve process economy, such as auxiliary grab, bottom mixing device and highly sophisticated and complex automation system have been developed.
Converter design
The technological state of steelmaking process makes it almost impossible to directly observe what is happening in the converter. At present, there is no mathematical model that can completely describe the process of high temperature metallurgy and fluid dynamics. Since its birth, converter steelmaking has been continuously studied and improved, so the understanding of metallurgical reaction is more comprehensive. However, the following two examples clearly show that there is still much research work to be done.
The position of the stirring tuyere at the bottom of the furnace still needs to be optimized. These tuyeres provide better stirring effect for molten steel and reduce carbon content faster, which should shorten the smelting cycle. However, the optimal location and number of tuyeres today is based on experience. In order to have a deeper understanding, some people abroad carried out research work in 2000, and soon found that the description of high-temperature hydrodynamic process is very complicated, which is feasible only by making many assumptions, such as bubbles and their reaction with molten steel can only be approximately described.
The mathematical description of converter swing in blowing process still needs to be elaborated in detail, especially those bottom blowing or side blowing processes, which are very intense. These vibrations are caused by spontaneous processes. The energy introduced during oxygen blowing makes the system swing at a very low Aigen frequency, usually 0.5-2.0Hz. The excavation of mathematical model that can describe this nonlinear chemical/mechanical fluid dynamics system has not been completed yet.
Converter housing
In the mechanical part of the converter, molten steel is contained in a furnace shell lined with refractory materials. These refractories show complex nonlinear thermal viscoelastic shrinkage behavior. Nonlinear contact with steel shell. People know more or less about the behavior of steel shell itself, so it is possible to describe this elastic-plastic material and its creep effect which changes with temperature. But there are many unknown things about the interaction between steel shell and refractory. Converter design is regarded as art rather than science to a greater extent. But the accumulation of experience, the improvement of materials and the application of computer technology are all helpful to better understand and design this mechanism.
There are several criteria for optimizing the design of furnace shell. The most important thing is the internal volume surrounded by refractory materials. In order to have the largest reaction space and realize the best metallurgical process, the volume should be maximized in the available space. The ratio of reaction space to molten steel quality is generally about 1.0m3/t in comparison, but due to the continuous pursuit of improving the productivity of steelmaking equipment with the lowest investment by steel mills, the loading capacity is increased and this ratio is reduced while keeping the original shell unchanged. The result is serious splashing, which often occurs when the furnace volume ratio drops to 0.7-0.8m3/t. Now the shape of converter body, that is, the upper and lower cone angle, the diameter-height ratio and so on. It is determined by the steel plant or the existing equipment, such as the flue gas system, the height of the inclined shaft, the inclined driving device, etc. Therefore, only a few parameters can be changed when designing a new furnace.
The modern converter consists of an upper cone with an iron ring at the end, a barrel-shaped furnace body and a lower cone with a dished bottom. In recent years, the connecting parts between the upper and lower cones and the furnace body, and between the lower cone and the furnace bottom have been removed. Production experience shows that the stress in these areas is not as serious as originally thought, and it can be solved by using high-quality furnace shell materials, so the above practice is feasible.
Design criteria of furnace shell
An important step in the design process is the checking of furnace shell structure, that is, the calculation of stress and deformation, and the comparison with allowable limit values. The design of metallurgical vessels such as converters does not need to meet specific standards. In the evolution of converter design art, the original shell design referred to the design standards of boilers and pressure vessels. The successful production of products designed in this way shows that these standards are also applicable to steelmaking practice. But the converter is not a pressure vessel after all, and its internal pressure comes from the thermal expansion of refractories, not the liquid or gas in the boiler. Moreover, damage such as cracks will not lead to explosion like high-pressure vessels. This is why the design of converter does not completely follow the design standard of pressure vessel.
Furnace shell thickness
The selection of wall thickness of traditional pressure vessels is mainly based on internal pressure. However, on the converter, this pressure cannot be accurately calculated, and the reason is determined by the interaction between refractory and furnace shell and production operation. When determining the thickness of the furnace shell, other loads and factors should also be considered, mainly including: mechanical loads caused by the weight of equipment, refractories and molten steel; Internal pressure generated by the interaction between furnace shell and refractory lining, that is, secondary pressure; Mechanical load caused by external forces such as dynamic mass effect, molten iron mixing, scrap addition and tapping; Temperature and temperature gradient on the furnace shell; The furnace shell is deformed under the action of temperature, which causes mechanical load to the suspension system; Because of the uneven temperature distribution between the furnace shell and the suspension system, the furnace shell produces secondary stress.
AISE Subcommittee 32 tried to give a simple "formula" program for calculating the thickness of furnace shell. However, some studies show that it is impossible to determine a simple procedure or standard when determining the thickness of furnace shell. These standards can be used to determine the furnace shell on the basis of confirmation, however, the introduced force, such as the force from the suspension system, must be calculated in detail by finite element method. The suspension system developed abroad is statically determinate, so all the loads in the system can be calculated accurately. The advantage of this function is that local stress and deformation can be calculated very accurately.
Converter life
World experience shows that the service life of converter is limited due to long-term deformation. When the furnace shell touches the support ring, the converter reaches the end point, which is generally 20 ~ 25a. This deformation is caused by creep. Creep is in high temperature environment (>: 350℃). Creep deformation is related to temperature, stress level and materials used. The only feasible methods to prolong the service life of converter are cooling furnace shell, material selection and production operation.
cooling system
In principle, forced cooling equipment is not absolutely necessary, and natural ventilation cooling is enough. Many practical applications have proved this point. Forced cooling reduces the equipment temperature, which has a positive effect on reducing creep deformation, thus prolonging the life of refractories and ensuring high yield strength at production temperature. Some steel mills have applied cooling systems such as water cooling, forced ventilation and gas-water combined cooling (aerosol cooling) to converter shell. The most effective cooling method is water cooling.
Material selection
Initially, the furnace shell material was mainly high temperature resistant pressure vessel steel. In order to bear many unknown loads and stresses, special emphasis is placed on fine-grained steel. The yield strength of this steel is relatively low, but it has quite high strain hardening ability at the yield point. Its advantage is that when it is overloaded, there will be enough excess strength. Even if cracks appear, brittle crack propagation will not occur, and cracks will either stop developing or grow at a very slow speed. Generally, there are A5 16Cr.60, Aldur4 1, Altherm4l, WStE285, WStE355, P275NH, P355NH, etc. Selected as steel for furnace shell.
This principle is still valid for the new converter, but in recent years 10- 15 years, due to the use of magnesia-carbon brick and slag splashing technology, the life of furnace lining has been prolonged. These changes lead to the increase of furnace shell temperature, promote creep effect and shorten furnace shell life. In order to counteract the creep effect, more anti-creep materials are selected, such as A204Cn60, 16M03, A387Cn 1 1, A387Cr.22, 13CrM044, etc. The disadvantage is that these steels have the same grain size and are difficult to weld.
Suspension system is an important part of converter. The ideal suspension system should not affect the behavior of the furnace shell and does not need maintenance in production. In the past few years, many different converter suspension systems have been developed. At first, the support ring was integrated with the converter, but it soon separated. The principle basis of various suspension systems is different, for example, Japan adopts rigid system, which is contrary to "free converter". Rigid retaining ring inhibits the deformation of furnace shell, but any constraint on thermal expansion will produce high stress and increase the probability of furnace shell cracking.
In order to allow the converter to expand or deform, and the support ring does not produce additional stress, it is necessary to carry out static design of the suspension system. According to this principle, VAI has developed a series of converter suspension systems, such as support system, VAI- Kangpan, VAI- Kanglian, VM-CON Quick and so on. VM-CON Link is a maintenance-free suspension system, and its design has received good application feedback. The typical application is the 160t converter of paulista iron smelting company in Brazil. Its size parameters are: steel water volume 160t, volume 160m3, furnace capacity ratio 1.0m3/t, converter height 8920mm, furnace shell thickness 70mm, bottom cone thickness 55mm, tray bottom thickness 55mm and converter outer diameter 7300mm. The furnace shell is made of Mo alloy steel 16Mo3 (equivalent to GRB). The support ring adopts box-section welding structure, with a gap of 250mm from the furnace shell, so as to be assembled with the cold plate of the furnace body. The upper cone is equipped with a fully verified water cooling system. These two cooling systems mainly extend the life of refractory lining and also cool the furnace shell. The inverter adopts VAI- Kanglian suspension system. For metallurgical reasons, the furnace shell is equipped with six bottom stirring tuyeres.
Converter technology
In addition to converter design, modern advanced converter technology also includes:
* The metallurgical process is improved by bottom stirring with inert gas and less slag operation;
* A great deal of secondary metallurgy has been incorporated into converter technology;
* Computer process automation and related sensor technology improve the quality, production efficiency and production safety, and reduce the production cost;
* Tools and equipment for smooth operation and easy maintenance of equipment, and durable materials with long service life;
:: Systems to improve the environmental compatibility of waste.
The goal of further development of converter technology is to improve process economy, that is, to optimize logistics and equipment operation and process technology. The optimization of process technology is not only limited to target analysis, determination of target temperature and selection of additive materials, but also includes production operations, such as lance position and injection mode of oxygen lance operation, immersion time and depth of sub-lance, addition mode of addition system and stirring mode of furnace bottom stirring system. All these must be standardized before the equipment is put into production and optimized for the steel grades produced during commissioning.
Dynamic process control needs sub-gun system and gas analysis. The auxiliary gun system measures temperature, carbon content and molten pool level, and takes samples during steelmaking. Therefore, measurement can be realized in blowing, and production time will not be lost. The sublance system is fully automatic, and the measuring probe can be replaced within 90 s. In recent years, the development in the field of process automation is to use Dynacon system to realize full dynamic control. Through continuous gas analysis, the system realizes the control of steelmaking process from the starting point to the end point of blowing.
The function of slag stopper is to reduce the slag carrying capacity of steel bucket. Slag blocking operation reduces the consumption of deoxidizing materials, especially in the production of low carbon steel. Another feature is that secondary metallurgy needs desulphurization of ladle slag, and slag retention operation can also reduce the dosage of ladle slag additives. At the same time, the slag removal operation and temperature loss of the steel bucket are also avoided. Ladle slag required for secondary metallurgy is formed during tapping of converter.
According to experience, the slag carrying capacity of tapping without slag stopper is 10- 14 kg/t steel, and it is reduced to 3-5 kg/t steel after slag stopper is used. When used in combination with slag sensor, the bearing capacity of slag can be stably controlled in the range of 2 kg/t or 3 kg/t steel. Another advantage is that the phosphorus content is reduced from about 30ppm to 10ppm. Therefore, the number of heats with unqualified phosphorus content is reduced.
In view of the improvement of metallurgical effects of bottom-blown converters such as OBM/Q-BOP and K-OBM, it is decided to develop inert gas stirring technology for bottom-blown converters. The system should make use of the advantages of bottom blowing to avoid the disadvantages of changing the bottom of the furnace in the middle of furnace service. Taking the No.3 converter plant of Aogang as an example, under the condition of 1650℃ without stirring, the average carbon content at the end of blowing is 0.035% [c] × ao, and it is reduced to 0.0023 when bottom blowing stirring is adopted with the flow rate of 0.08 nm3/min/ ton of steel. If bottom blowing mixing is not used, there will be about 1% iron loss and lime consumption will increase by about 25%. Assume that the slag carrying capacity of ladle is 12 kg. /t steel (without slag), the aluminum consumption per ton of steel will increase by 0.7 kg. In addition, correspondingly, the greater the amount of converter slag, the more refractories can be consumed. It is uneconomical to reach 0.035% at the end of blowing on BOF converter without bottom blowing stirring, and the carbon content is generally limited to 0.045% ~ 0.050%.
Logistics optimization and path algorithm are specially designed for steel plant layout and production equipment to find the best configuration. The user-friendly interface and standardized output make it a very useful tool, which can optimize and simulate the configuration of any steel plant and allow users to test various layout and process options. It enables users to find the best solution in production time management, maintenance, auxiliary equipment capacity and so on.
In order to determine the most economical production mode of different steel grades and use different production equipment, long-term experience accumulation and a large number of calculations are needed to compare various alternative methods. This calculation requires computer-aided tools, such as steelmaking expert system. This tool can be applied to the whole production line.
For reference only, I hope it will help you!