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A brief discussion on industrial practices and measures to reduce temperature drop in steelmaking systems

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This article focuses on the relevant processes and production optimization practices and measures taken by the Anyuan Steelmaking Plant of Ping’an Steel Company in reducing the temperature drop of the steelmaking system. Match the appropriate furnace type with the charging system, dynamically adjust the heat control means, reduce the tapping temperature and system temperature drop, heat preservation and heating, and optimize the production organization. The temperature drop of the entire process system is controlled within 100°C, and the iron consumption is controlled to within the optimal level of 825kg/t, thereby increasing production and reducing consumption while further reducing cost control.

Keywords: iron loss; heat balance; tapping temperature; production organization; steelmaking cost

The temperature drop of the steelmaking system is an important technical indicator of steelmaking economy and technology. The high-heat system not only has a greater impact on the erosion of the furnace lining and subsequent process casting, but the excessively low-heat system not only has a negative impact on the production organization. Moreover, it will adversely affect the uniform composition and temperature of the argon station blowing gas, resulting in adverse factors for product quality. Therefore, the essence of controlling the temperature drop of the steelmaking system is to reduce the tapping temperature and meet the temperature control requirements of subsequent processes. It is necessary to conduct necessary analysis and research on the temperature drop of the steelmaking system to reduce iron consumption control.

From the above process flow, it can be concluded that the control of the temperature drop of the steelmaking system not only involves the control of the steelmaking material flow, but also is related to the energy flow. That is, the control of the system temperature drop has an optimal functional relationship (1):

In formula (1), f(X1,X2,…) is the material flow equation

Q(Y1,Y2…) is the energy flow equation

In formula (1), f(X1,X2,…) is in the material flow equation. When the input items are subject to certain objective conditions, such as molten iron quality, scrap steel quality, auxiliary material unit consumption, etc., the material flow equation will depend on the energy flow equation. Insufficient heat will affect the iron flow loss and converter end point control level, thereby affecting steel material consumption, alloy consumption level and furnace type fluctuations. Therefore, the principle of energy flow control should be followed in the temperature drop control of the steelmaking system. In the static energy balance equation (equation (2) below), it is directly related to the steel tapping temperature, production rhythm, furnace type control, etc. Therefore, under the condition that there is no scrap heating device, the system temperature drop control is centered on heat control. Lowering the tapping temperature, increasing the converter production rhythm, and stable furnace type control are important aspects of its control.

  Q release = Q physical heat + Q chemical heat = Q absorption + Q dispersion = Q scrap + Q slag + Q molten steel + Q dispersion Formula (2)

1 Focus on heat balance and dynamically adjust the scrap loading system

Ping’an Steel’s Anyuan Steelmaking Plant adopts a one-tank-to-bottom process and adopts a process of adding scrap steel to the molten iron line. The amount of molten iron charged, its composition, and temperature fluctuate greatly, so it is necessary to dynamically adjust the amount of scrap steel charged to meet the heat demand. In actual operation, due to the influence of the unbalanced amount of scrap steel added to the empty tank of the hot metal line and the influence of the filling rate, the temperature of the hot metal entering the furnace fluctuates greatly. The most direct and effective way is to focus on the temperature measurement of molten iron entering the furnace, with a temperature measurement rate of more than 70%, and at the same time, establish a communication channel for molten iron information to ensure that the furnace director and shift leader carry out work around the furnace turnover temperature of 1610-1650°C. According to the molten iron Si , S, as well as the ironmaking tap temperature (temperature measured when entering the furnace), furnace type control on duty and the amount of scrap steel added to the hot metal line to adjust the amount of scrap steel entering the furnace.

In carbon-saturated molten iron, the factors that have the greatest impact on heat are the molten iron Si and the temperature of the molten iron entering the furnace. The static model determined that the theoretical cooling effect of 1 ton of scrap steel is 26°C, the heating effect of 0.1% Si is 16°C, and the molten iron temperature entering the furnace of 10°C affects the end temperature by 10°C. The impact of tons of iron on temperature is calculated based on the iron consumption in the current period corresponding to the unit consumption of scrap steel. It contributes to the appropriate surplus of heat, and takes the method of comparing the upper and lower furnaces as a reference to solve the impact of furnace type and other heat dissipation on the calculated scrap loading amount in the static model.

2 Control of small thermal balance and optimize slagging system

Based on the fact that the thermal balance control in the static model is greatly affected by objective factors, the slagging system was optimized while meeting the requirements of basic slag alkalinity and furnace protection during the operation. The slag alkalinity was ensured to be 2.6~3.0, and the MgO content was 6~8. %. The unit consumption of lime is not allowed to be less than 30kg/t to adjust the heat and ensure the stability of the furnace. When the heat is low, a modifier is used to replace the light-burned dolomite process to reduce the impact of the total input of slag material on the temperature. When the heat is high, iron-containing oxides (stewing pot grade C material) are used to balance the heat, and the ton of steel is controlled not to exceed 3kg/t. Otherwise, adjust the amount of scrap steel loaded into the tank in the maximum heat balance to adjust the heat control to minimize the tapping temperature.

3. Stabilize the furnace type and ensure the rationalization of furnace type parameters.

Stabilizing the furnace type means determining a reasonable furnace volume ratio, reducing slag overflow and splashing, reducing the impact of furnace crisis on production and making full use of heat. Based on the basic charging system of 52±2 tons of molten iron + 13-14 tons of scrap steel, the furnace parameters are set to: 6.3±0.1m, visible stamping position, and furnace bottom gradient of 300mm. Dynamic furnace protection measures are implemented to ensure the alkalinity of the slag system and meet heat control to prevent excessive fluctuations in the furnace shape or loss of control due to low alkalinity. The fine-tuning method mainly focuses on the use of light-burned dolomite and modifiers and the standardization of the slag splashing mode. A two-pronged search can be used to adjust the slag by splashing and slag: 200kg of modifier and 200kg of lightly burned dolomite are not suitable for excessive slag adjustment to avoid poor slag splashing effect caused by clumping of slag and inability to splash slag.

4 Reduce the tapping temperature and minimize iron consumption

4.1 Iron alloy baking use

The baking effect of ferroalloy has a great influence on the tapping temperature drop, and online baking is generally adopted. However, due to space constraints, Anyuan Steelmaking adopts the method of baking off-site and then hauling and adding, and the alloy baking temperature of the original masonry refractory alloy baking furnace is 100~200℃. As a result, the temperature drop of the molten steel caused by the addition of alloy is large. After modification, a more popular baking energy-saving device is adopted. The alloy baking can reach 400°C and above, and a single furnace is used for a single warehouse. Standardize the alloy time node to be 6 to 7 minutes after gun smelting time to promote the effective time of alloy baking, increase the alloy baking temperature, and reduce the impact of alloy on tapping temperature.

4.2 Ladle usage management and baking process

Red envelope tapping is an important measure to reduce the system temperature drop. The on-line temperature of the steel ladle and the normal turnover temperature have always been low: 600~700℃ (measured before tapping). To speed up the ladle turnover, three furnaces and three machines are used to rotate nine ladles. The roaster is fully optimized and innovated. Because the ladle is poured with complete castable material without an insulation layer, the on-line temperature of the ladle is not ideal. Through investigation, a fully automatic capping device of a gear-shaped ladle was adopted to comprehensively improve the thermal insulation performance. After being put into operation, the on-line temperature of the ladle reached more than 1000°C, and the tapping temperature dropped significantly, reaching 10°C.

4.3 Comprehensive improvement of thermal insulation measures

When the molten steel leaves the station, covering agent and carbonized rice husk are added, the large ladle is cast for protection, the liquid level of the middle ladle is thrown into the covering agent or carbonized rice husk, the middle ladle is protected and cast, and the middle ladle is completely sealed. The whole process realizes high-level black surface operation. The temperature drop from the molten steel leaving the station to the middle stage of continuous casting is controlled below 35°C, and the temperature drop throughout the whole process is controlled at around 100°C.

5 Optimize production rhythm and reduce temperature loss

5.1 Improve the rhythm of furnace steel production

5.1.1 Optimize tap hole parameters

The original inner casing of the tapping port of the 60-ton converter is ¢ 140, and the tapping time in the early stage of use is 4′ 30″, which seriously hinders the production rhythm, and the tapping temperature drops greatly. The tapping temperature needs to be increased by 10-20°C. After continuous testing Optimization, on the premise of ensuring the service life, the inner casing of the tapping port is improved. The outer diameter is increased from ¢ 280 to ¢ 300, and the inner diameter is increased from ¢ 140 to ¢ 170. The tapping time in the early stage of use is reduced to within 3′, no need Increase the tapping temperature to meet the large loss of tapping temperature drop.

5.1.2 Optimize converter oxygen supply technology and shorten oxygen supply time

Adopt large throat low-pressure oxygen supply technology to increase the diameter of the oxygen lance nozzle throat, and the oxygen lance throat is gradually optimized from the original 31.5mm to 33mm. The gun body has been optimized from ¢ 194mm to ¢ 219mm, and is equipped with a 34.3mm large throat nozzle. To improve the oxygen supply intensity and shorten the oxygen supply time by more than 30″. 5.1.3 Optimize the furnace mouth diameter and furnace mouth management to reduce the phenomenon of scrap steel stuck in the slot

The size of the furnace mouth tank has been expanded from the original 2.1 meters to 2.3 meters. To reduce the amount of scrap steel loaded under low iron consumption, it will easily cause the scrap steel to block the furnace mouth, which not only affects the production rhythm, but also affects the iron consumption control. Management ensures that the furnace mouth is wide. The furnace mouth excavator and oxygen-enriched furnace mouth technology are used to control the degree of furnace mouth slagging.

5.2 Optimize the continuous casting process, increase the casting speed, implement three furnaces to two machines, shorten the casting cycle and reduce temperature drop losses

The casting cycle of molten steel has a great influence on the temperature. Reducing the casting cycle is conducive to reducing the tapping temperature to the maximum extent. The pulling speed of the three six-stream and six-machine continuous casting machines in Anyuan Steelmaking Plant is generally 2.0~2.2m/min, and the casting cycle is 22~24min, the cycle is longer, the cooling rate is 1.5℃/min, the continuous casting machine speed is optimized to 3~3.5m/min, the casting cycle can be reduced to 12 minutes, the tapping temperature can be reduced by 18℃, and the iron The consumption is about 10kg/t. In actual operation, the pulling speed has been achieved at 2.8~2.9m/min. The cooling mode is adopted to add furnace steel after the furnace or add waste billet to the ladle. After air blowing, the temperature can be reduced to 1545℃. In comparison, In other words, the tapping temperature is reduced by 15°C, laying a solid foundation for reducing iron loss control.

6 Strengthen production organization, control ladle turnover and shorten operating time

(1) Strengthen production coordination management and shorten the molten steel town time. The molten steel town time refers to the time between the time when the molten steel reaches the rotary table and the time when the previous heat of steel is stopped under normal circumstances. Ensure that the sedation time is within 0~6 minutes. If there are other accident-related reasons, only 3 furnaces are allowed to exceed the sedation time.

(2) Strengthen ladle turnover management. Under normal circumstances, three furnaces and three machines are only allowed to use ≤9 ladles for turnover, and two furnaces and two machines are only allowed to use ≤6 ladles for turnover.

7 Implementation results

It can be seen from Table 1 that the tapping temperature gradually decreases, and the average tapping temperature throughout the year is 1640.8°C.

Table 1 Annual tapping temperature results of Anyuan Steelmaking Plant

Pour the temperature16301630







Tapping temperature164616431644164216431637163816391638   1640   1639   1640
before blowing159215901591158815841584158715881591   1591   1589   1588
After blowing157115691570156615641564156615681570   1571   1568   1567
Medium package temperature153515331532153315311529152915281527   1527   1531   1532

8 Conclusion and outlook

Anyuan Steelmaking Plant focuses on heat control, dynamically controls the amount of scrap steel entering the furnace, and establishes a furnace type benchmark mechanism to make the furnace type controllable. We also adopted measures to lower the tapping temperature and reduce system temperature drop losses, improve the insulation and heating effects of subsequent processes, actively carry out production organization optimization, improve the level of production rhythm control, and improve various technical indicators. The key points to reduce the system temperature drop in the next step are: the full use of the ¢219 oxygen lance oxygen supply technology, the full-process capping technology of the ladle, and the high-speed continuous casting machine adaption to the operation of three furnaces and two machines.

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