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Discussion on control measures to improve end-point hit rate of converter steelmaking

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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 the most widely used and frequently applied steelmaking process. The endpoint hit rate is a crucial indicator of quality and economic benefits. The control of the endpoint hit rate directly impacts the cost, efficiency, and steel quality.

To maintain technical expertise and economic benefits, steelmaking plants and technicians must analyze factors influencing endpoint hit rate control. They should focus on equipment, raw materials, processes, and optimize the production process accordingly. Despite efforts, some plants still struggle to enhance the endpoint hit rate in converter steelmaking.

Therefore, it is essential to analyze control measures to improve the endpoint hit rate effectively.

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

Technicians applying static control technology in converter steelmaking must consider current end-point control, raw material status, and steelmaking types. They need to determine the amounts of scrap iron, scrap steel, molten iron, and alloys required. They also need to calculate the oxygen needed for these raw materials.

In static control, technicians must know the upper and lower limits of manual control points. This helps control the end-point hit rate without modifying the steelmaking process. Generally, this can increase the hit rate to about 80%.

Static control technology uses empirical and mechanism models. Mechanism models are more common in end-point hit rate control but are complex and influenced by external factors. Technicians must ensure accurate material and heat balance calculations.

Dynamic control technology

Technicians need to monitor real-time operation data in converter steelmaking, including molten pool temperature and secondary gun detection data. They must make timely corrections to enhance stability and accurately control the endpoint 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
0.35~0.800.10~0.300.100~0.1400.010~0.0454.0~4.81300~1450

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. When the w (Si) in the molten iron is low, there is a reduction in the addition of raw materials like white raw material, cold slag, and lime, requiring technicians to adjust the amount of slag left accordingly. Conversely, with high w (Si) in the molten iron, there is an increase in the addition of the mentioned raw materials, and technicians must decrease the amount of slag left to maintain a consistent slag level in the converter. If the w (Si) in the molten iron exceeds 1.0%, slag retention is unnecessary.

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 mentioned tapping temperature reduction methods can lower the end-point temperature of converter steelmaking by approximately 20 to 30°C. This leads to a 12% to 13% increase in the end-point hit rate and reduces converter steelmaking consumption.

Control blowing intensity

Technicians control blowing intensity in converter steelmaking using variable gun constant pressure, variable gun variable pressure, and constant gun variable pressure. Gun position changes can occur early, mid-term, or late.

Ideal blowing intensity varies with molten iron composition and temperature. In cases of high w (Si) in molten iron, technicians adjust the gun position by lowering it early, raising it mid-term, and maintaining it at a moderate level later. Conversely, when w (Si) in molten iron is low, they raise the gun position early, keep it moderate mid-term, and lower it late.

If molten iron temperature is low, technicians raise the early gun position and keep it moderate mid-term and late. If the temperature is high, they lower the early gun position, raise it mid-term, and keep it moderate later.

Furthermore, enhancing the oxygen lance nozzle structure and increasing the number of nozzles can alter the impact flow field on the molten pool. This adjustment reduces the likelihood of blowing splashing issues, shortens the converter steelmaking and blowing cycle, and ultimately enhances the end point hit rate.

Supplementary heat and temperature rise

Adjusting temperature is a fundamental method to regulate carbon and oxygen levels at the endpoint. Supplementary heat can be applied to raise the temperature. Iron oxide is crucial for heating the molten pool, but its use increases converter steelmaking costs. To enhance production efficiency, technicians can opt to introduce coke or ferrosilicon early in the blowing stage. This approach achieves temperature rise, supplementary heat, and cost control.

Technicians must calculate the amount of coke or ferrosilicon added. They can apply the sub-gun model specifically. They need to clarify the conditions for adding these materials. For example, if w (Si) in molten iron exceeds 0.35%, adding 2,000 kg of coke won’t achieve effective heat and temperature rise. Technicians must add ferrosilicon instead. The amount of ferrosilicon added cannot exceed 0.60%.

If w (Si) in molten iron is below 0.35%, adding ferrosilicon at 0.60% mass fraction won’t be effective. Technicians must add coke to achieve the desired heat and temperature rise. The amount of coke added cannot exceed 2,500 kg. Technicians need to know the right time to add these materials. They usually add them after scrap addition, slag splashing, or blowing.

If blowing oxygen consumption exceeds 2,500 m³, adding ferrosilicon is prohibited.

Conclusion

Analyzing converter steelmaking end-point hit rate control technology establishes a technical foundation for enhancement. The technology encompasses manual, static, dynamic, and automatic control methods.

Factors influencing the end-point hit rate include carbon and oxygen levels, tapping temperature, and dephosphorization conditions. Temperature affects carbon and oxygen content, and controlling it can enhance the hit rate. Effective dephosphorization requires specific conditions like higher slag FeO content and good steel-slag reaction interface.

Improving the end-point hit rate involves actions such as enhancing slag retention, reducing tapping temperature, adjusting blowing intensity, and implementing supplementary heating for temperature increase. These measures aim to enhance the end-point hit rate effectively.

LMM YOTAI established in 2007. Our production technology comes from Japanese Yotai. As an experienced and international player in the refractories industry. We have succeeded in expanding both the breadth of its product range and the depth of its services. From raw material selection, refractory portofio & optimization, installation & services & recycle of used refractories on site to further reduce client’s Opex & Capex in refractory consumption per ton steel output, meanwhile improve product quality of client.

Our Product have been supplied to world’s top steel manufacturer Arcelormittal, TATA Steel, EZZ steel etc. We do OEM for Concast and Danieli for a long time

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