This paper briefly introduces the control technology of the end-point hit rate of converter steelmaking, analyzes the influencing factors of the end-pointhit rate of converter steelmaking, and proposes strategies to improve the end-point hit rate of converter steelmaking and improve the production quality and efficiency of converter steelmaking.
Keywords: converter steelmaking; endpoint hit rate; control technology; control measures
Converter steelmaking is currently the steelmaking process with the widest range of applications and the highest application frequency. The endpoint hit rate is one of the important indicators to measure the quality and economic benefits of converter steelmaking. The control effect of the endpoint hit rate directly determines the cost of converter steelmaking, Production efficiency and molten steel quality. In order to ensure the technical level and economic benefits of converter steelmaking, it is necessary for steelmaking plants and technicians to analyze the influencing factors of end-point hit rate control from the perspectives of equipment, raw materials, processes, etc., and on this basis, optimize the original production process. Looking at the current status of the end-point hit rate control of converter steelmaking, it can be found that some steelmaking plants are still unable to fully improve the end-point hit rate. For this reason, the control measures to improve the end-point hit rate of converter steelmaking are analyzed.
Control technology of converter steelmaking end point hit rate
Manual control technology
Carbon pulling and supplementary blowing is a common artificial experience converter steelmaking end point hit rate control technology. It requires technicians to have certain professional abilities and experience. They can judge whether oxygen blowing is required based on parameters such as the carbon content requirements of converter steelmaking. If carbon steel or high carbon steel is produced, technicians also need to add target carbon content and oxidation speed to the above judgment indicators. The operation process of carbon pulling and supplementary blowing technology is relatively simple, it will not cause excessive loss of converter steelmaking raw materials, and can achieve accurate control of carbonization rate and oxygen consumption, but the requirements for technical personnel’s capabilities are too high.
Static control technology
The application of static control technology in the end-point hit rate control of converter steelmaking needs to consider the current status of end-point static control, raw material status and steelmaking types at the same time, so as to clarify the application amount of raw materials such as scrap iron, scrap steel, molten iron and alloys, and calculate the raw material requirements amount of oxygen. In the static control process, technicians need to master the upper and lower limits of the manual control static point, so as to control the end point hit rate through non-modification of the converter steelmaking process. Generally, the end point hit rate can be increased to about 80%. The main Internet models of static control technology are empirical models and mechanism models. Mechanism models are more frequently used in converter steelmaking end-point hit rate control, but the operation process is complex and easily affected by external factors. Technicians need to pay special attention to the accuracy of material balance and heat balance calculation results.
Dynamic control technology
The secondary gun dynamic key control technology requires technicians to grasp the converter steelmaking operation data in real time based on the molten pool temperature and other secondary gun detection data, and make effective corrections, which is conducive to improving the stability of the converter steelmaking, thereby accurately controlling the end point hit rate.
Furnace gas analysis dynamic end-point control technology is frequently used in end-point hit rate control of converter steelmaking. Technicians need to judge whether the decarburization speed of the molten pool and the composition of molten steel have reached the end-point temperature requirements based on the furnace gas composition on the furnace mouth table. Compared with manual control, dynamic control is more accurate and can optimize the technical parameters of converter steelmaking. This requires technicians to have high control capabilities.
Automatic control technology
Automatic control is mainly oriented to the carbon content and temperature control of converter steelmaking. Technicians need to control the endpoint hit rate through computers and related software, and at the same time realize real-time monitoring of the endpoint temperature. The essence of automatic control technology is online control technology. Technicians can use automatic control technology to view the converter steelmaking process in real time, compare carbon content and temperature data in real time, and achieve effective quality score management. With the support of automatic control technology, the traditional manual scheduled furnace blowing, furnace sweeping, and furnace pouring operations can be transformed into automated operations, and the converter steelmaking operation efficiency and end-point hit rate control rate can be increased to about 85%.
Factors influencing the end-point hit rate of converter steelmaking
Carbon and oxygen content
During the converter steelmaking operation, C and O in the molten pool will produce strong chemical reactions, and the reaction equation is shown in Formula 1.
[C]+[O]=COΔGθ=-22 364-39.63,
Among them: ΔGθ is the standard Gibbs free energy; T is the temperature of the molten steel; PCO is the partial pressure of CO; aC is the activity of carbon in the molten steel, mol/L; aO is the activity of oxygen in the molten steel, mol/L.
It can be seen from Equation 1: According to thermodynamic theory, temperature and PCO are the main factors affecting the carbon oxygen product. At a certain temperature and PCO, the carbon oxygen product is a constant. There is an inverse correlation between the end point w (O) and w (C) of converter steelmaking, that is, when the end point w (C) is the same, the end point w (O) will decrease as the carbon oxygen product decreases. Converter steelmaking operations generally use standard atmospheric pressure as PCO. It can be seen that the final factor affecting the carbon and oxygen content is temperature.
According to the kinetic theory, proper stirring operation is conducive to accelerating the carbon-oxygen reaction in the molten pool, thereby reducing carbon-oxygen accumulation. Specific stirring operations include bottom-blowing gas stirring, top-blowing oxygen gun air flow impact, and carbon-oxygen reaction gas stirring.
Tapping temperature
The heat in the converter steelmaking operation is mainly the chemical heat generated by the oxidation reaction of the chemical elements of the molten iron and the physical heat of the molten iron itself. The composition and temperature changes of the conventional molten iron are shown in Table 1. During the continuous operation of converter steelmaking, there is a certain surplus of chemical heat and physical heat. Therefore, technicians need to add a certain amount of coolant during the blowing operation to effectively control the end point temperature, thereby improving the end point hit rate.
| Table 1 Statistics of molten iron composition and temperature in steelmaking plants | |||||
| w(Si)/% | w(Mn)/% | w(P)/% | w(S)/% | w(C)/% | Temperature/°C |
| 0.35～0.80 | 0.10～0.30 | 0.100～0.140 | 0.010～0.045 | 4.0～4.8 | 1300～1450 |
It should be noted that if the converter is shut down for a long time, the heat generated by the blowing operation may not be enough to reach the tapping temperature requirements. At this time, technicians need to add heat to increase the temperature to ensure the end temperature. However, increasing the temperature of supplementary heat may cause new problems such as increased furnace lining erosion, increased iron loss, reduced purity of molten steel, and reduced alloy recovery rate. In addition, splashing caused by the converter blowing operation will cause a large amount of heat loss and will also adversely affect the end temperature.
Dephosphorization conditions
According to the oxygen potential diagram, the oxidation law is selected. When the temperature in the early stage of converter blowing is low and the slagging FeO content is high, the dephosphorization reaction efficiency is higher than the carbon-oxygen reaction. As the molten pool temperature continues to rise, lime is added to the converter steelmaking operation, the melting efficiency begins to increase, and the alkalinity of the slag formation also increases.
2P+5FeO+4CaO=4CaO·P2O5+5Fe. (2)
It can be seen from Equation 2 that conditions that can push the dephosphorization reaction equation to the right can enhance the dephosphorization reaction effect. According to the kinetic theory, the slagging maintains sufficient fluidity. Therefore, the dephosphorization reaction conditions can be summarized as higher slagging FeO content, higher slag basicity, lower molten pool temperature and good steel-slag reaction interface.
Control measures to improve the end-point hit rate of converter steelmaking
Improve the slag retention operation
In order to meet the dephosphorization reaction conditions, technicians can carry out scientific and reasonable residue retention operations. Specifically, the desulfurization and dephosphorization effects obtained by leaving slag in the converter are relatively ideal, and it has many application advantages such as less slag pouring time, higher early slag formation efficiency and less lime consumption. However, too much slag will cause the problem of low-temperature slag splashing. If the splashed slag cannot be dried in time, it will lead to abnormal ignition of the converter blowing. Therefore, technicians need to effectively control the amount of residue left.
Different molten iron components correspond to different ideal slag amounts. Technicians often need to determine the slag amount based on the number of slag-forming cycles and the silicon content of the molten iron. If the w (Si) in the molten iron is low, the amount of raw materials such as white raw material, cold slag, and lime added will be reduced, and technicians need to appropriately increase the amount of slag left. If w (Si) in the molten iron is high, the amount of the above raw materials added will increase, and technicians need to appropriately reduce the amount of slag left to ensure a stable level of the amount of slag formed in the converter. If w (Si) in the molten iron exceeds 1.0%, slag retention does not need to be performed.
Reduce tapping temperature
The oxygen content of molten steel will increase as the end temperature of the converter increases, and the amount of deoxidizer applied will also increase. In order to meet the dephosphorization reaction conditions, the tapping temperature can be reduced. When the converter heat is insufficient, a lower tapping temperature can improve the end-point hit rate. Common measures to reduce tapping temperature include:
1) Shorten the converter steelmaking cycle and improve converter steelmaking efficiency.
2) Reduce the number of ladle turnover, improve ladle turnover efficiency, and ensure that the ladle lining temperature can reach 1000°C.
3) Add alloys during converter steelmaking operations.
4) Insulate the outbound ladles.
5) Develop plans for each converter heat.
The above tapping temperature reduction measures can reduce the end-point temperature of converter steelmaking by about 20 to 30°C, increase the end-point hit rate by about 12% to 13%, and at the same time, reduce the consumption of converter steelmaking.
Control blowing intensity
The blowing intensity control methods for converter steelmaking mainly include variable gun constant pressure, variable gun variable pressure, and constant gun variable pressure. The gun position changes mainly include early gun position changes, mid-term gun position changes, and late gun position changes. The ideal blowing intensity corresponding to different molten iron compositions and temperatures is different. If w (Si) in the molten iron is high, technicians can lower the gun position in the early stage, increase the gun position in the middle stage, and keep the gun position moderate in the later stage. If w (Si) in the molten iron is low, technicians can increase the early gun position, keep the mid-term gun position moderate, and lower the late gun position. If the temperature of the molten iron is low, technicians can increase the early gun position, keep the mid-term gun position moderate, and keep the late gun position moderate. If the molten iron temperature is high, technicians can lower the gun position in the early stage, increase the gun position in the middle stage, and keep the gun position moderate in the later stage.
In addition, the oxygen lance nozzle structure can be improved and the number of nozzles can be increased to change the impact flow field of the oxygen lance on the molten pool, reduce the probability of blowing splashing problems, shorten the converter steelmaking and blowing cycle, and ultimately improve the end point hit rate.
Supplementary heat and temperature rise
Temperature adjustment is the basic way to control the carbon and oxygen content at the end point, so supplementary heat can be used to increase the temperature. Specifically, iron oxide is the main element to achieve heating of the molten pool, but the application of iron oxide increases the cost of converter steelmaking. In order to further improve the production efficiency of the steelmaking plant, technicians can choose to add coke or ferrosilicon in the early stage of blowing, which can not only achieve supplementary heat and temperature rise, but also control the cost of converter steelmaking.
Of course, the addition amount of coke or ferrosilicon needs to be calculated, and the sub-gun model can be applied specifically. Technicians need to clarify the conditions for adding coke or ferrosilicon. For example, if w (Si) in the molten iron exceeds 0.35%, adding 2,000 kg of coke still cannot achieve effective supplementary heat and temperature rise. Technicians can only add ferrosilicon to continue supplementary heat and temperature rise. , the amount of ferrosilicon added cannot exceed 0.60%. If w (Si) in the molten iron does not exceed 0.35%, the addition of ferrosilicon with a mass fraction of 0.60% still cannot achieve effective supplementary heat and temperature rise. Technicians can only add coke to continue supplementary heat and temperature rise. The amount of coke added cannot exceed 2,500 kg. . Technicians need to know the time to add coke or ferrosilicon, which is usually added after the scrap addition is completed, after the slag splashing is completed or after the blowing is completed. If the blowing oxygen consumption exceeds 2 500 m3, it is prohibited to continue adding ferrosilicon.
Conclusion
By analyzing the converter steelmaking end-point hit rate control technology, a technical foundation has been laid for improving the converter steelmaking end-point hit rate.
1) Converter steelmaking end-point hit rate control technology mainly includes manual control technology, static control technology, dynamic control technology and automatic control technology.
2) Factors affecting the end-point hit rate of converter steelmaking mainly include carbon and oxygen content, tapping temperature and dephosphorization conditions. The final factor that affects carbon and oxygen content is temperature. Effectively controlling the endpoint temperature can improve the endpoint hit rate. The dephosphorization reaction conditions are higher slag FeO content, higher slag alkalinity, lower molten pool temperature and good steel-slag reaction interface.
3) The control measures to improve the end-point hit rate of converter steelmaking mainly include improving the slag retention operation, lowering the tapping temperature, controlling the blowing intensity and carrying out supplementary heating to increase the temperature, and ultimately improve the end-point hit rate.