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Temperature drop factors and control measures in each process of converter steelmaking

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Analyze the temperature drop of molten steel during the converter steelmaking process and its influencing factors: tapping temperature, ladle operating time, alloy, auxiliary material addition amount and argon blowing and feeding line operation are the main factors affecting the temperature drop of the molten steel process. Through the study of the temperature drop of molten steel, we have mastered the temperature change of the whole process of molten steel, and by studying the factors that affect the temperature drop of molten steel, we have proposed measures to reduce the temperature drop of molten steel, such as lowering the tapping temperature and strengthening ladle management. The logistics time of the whole steelmaking process is shortened, the temperature drop of the molten steel process is reduced, and the pass rate of the tundish temperature is greatly improved. After the pass rate of the tundish temperature is increased, the pouring becomes more stable, and the quality pass rate of the slab is also significantly improved.

Keywords: control measures; molten steel temperature; temperature control

In modern continuous casting production, stable molten steel temperature conditions are the guarantee of stable continuous casting production. Appropriate molten steel superheat is an important condition for obtaining high-quality cast slabs, and effective control of molten steel temperature throughout the entire process is the key to ensuring a smooth and orderly production rhythm. Improving the operating level of molten steel temperature control in the entire process from converter tapping to continuous casting rotary table, and preparing the molten steel temperature conditions before continuous casting are important aspects that reflect the above technical ideas. By understanding the temperature control function of each process step and the thermal status of the ladle and tundish turnover process, we can formulate a reasonable target temperature of molten steel in the ladle and tundish, and control it within a narrow range, thereby reducing the tapping temperature and realizing steelmaking. →Secondary refining→Optimization of molten steel temperature during continuous casting.

1 Analysis of temperature control of molten steel process

1.1 Temperature control situation

A factory uses a process flow (200 t converter): converter → small platform → LF refining → continuous casting, producing a total of 525 furnaces of Q235B steel. The temperature control of the molten steel throughout the process is shown in Table 1.

Table 1 Temperature control conditions of the entire molten steel process /℃

projectEntering the furnaceTapping steelArgon preAfter argonLF pit stopLF outboundmid-package
average value1 2941 6801 6271 6121599   1 6011 547
minimum value1 2001 6511 5801 5731556   1 5751 516
maximum value1 4361 7101 6681 6501624   1 6461 603

Figure 1 Tapping temperature distribution

Judging from the tapping temperature distribution in Figure 1, the proportion of heats with relatively high tapping temperatures (≥1 690°C) has reached 22.5%. Excessively high tapping temperature causes serious erosion of converter refractory materials and increases production costs.

1.2 Temperature drop in each process

1.2.1 Temperature drop during tapping process

The tapping process is a process in which the temperature of the molten steel decreases. The temperature drop during the tapping process is affected by the tapping time, alloy and slag materials added during tapping. The temperature drop during the tapping process of the same steel type is mainly affected by the tapping time. The temperature drop of Q235B during the tapping process is shown in Table 2.

Table 2 Temperature drop during tapping process

projectTemperature drop during tapping process/℃ Tapping time/min Tapping temperature drop rate/(℃·min-¹)
average value52.36.48.2
minimum value427.25.8
maximum value719097.2

As can be seen from Table 2, the average temperature drop during the tapping process of Q235B steel is 52.3°C, the average tapping time is 6.4 min, and the average temperature drop rate during the tapping process is about 8°C/min. When the life of the tapping port is basically the same, the temperature drop distribution during the tapping process is shown in Figure 2.

Figure 2 Temperature drop distribution during tapping process

It can be seen from Figure 2 that when the tapping time changes little, the tapping temperature drop fluctuates greatly. Among them, the proportions of temperature drops less than 40°C and greater than 60°C reached 23.3% and 35.1% respectively. It can be seen that the condition of the ladle is the main reason for the large fluctuations in the tapping temperature drop.

1.2.2 Temperature drop during small platform processing

The temperature drop during the small platform argon blowing treatment process is shown in Table 3. The small platform treatment process is a process in which the temperature of the molten steel decreases.

Table 3 Temperature drop during small platform processing

projectTreatment temperature drop/℃Treatment time/minTemperature drop rate during treatment/(℃ ·min- 1)
average value15.45.53.0
minimum value3.02.10.6
maximum value459.98.3

As can be seen from Table 3, the average processing time of the small platform is 5.5 minutes, and the average process temperature drop is 15.4°C. When the processing time is equivalent, the temperature drop distribution of the small platform is shown in Figure 3.

It can be seen from Figure 3 that when the treatment time is equivalent, if the argon blowing intensity is too large, the temperature drop will be greater than 20°C, and the proportion is as high as 23.8%; if the argon blowing intensity is small, the temperature drop will be less than 10°C, and the proportion will be 22. 1% ,

Figure 3 Temperature drop distribution during tapping process

It can be seen that the argon blowing intensity is too high during the argon blowing treatment on the small platform, causing the molten steel to violently churn and cause the temperature drop to be too large. Therefore, the argon blowing operation on the small platform must be standardized to reduce the temperature drop of molten steel.

1.2.3 Temperature drop from small platform to LF

The temperature changes during the standing process of molten steel from the small platform to the LF station are shown in Table 4.

Table 4. Temperature drop of standing molten steel at small platform-LF station

projectTemperature drop during tapping process/℃ Tapping time/min Tapping temperature drop rate/(℃·min-¹)
average value17.113.51.2
minimum value77.20.8
maximum value30202.5

As can be seen from Table 4, although the molten steel is not processed during the refining process from the small platform to the LF, the process temperature drop is still large due to the heat storage and radiation heat dissipation of the ladle. The average refining time from the small platform to the LF is 13.5 minutes. The average process temperature drop is 17.1℃, and the temperature drop rate is 1.2℃/min.

1.2.4 Temperature drop from LF to CC

The temperature drop of molten steel from the end of LF refining to the start of molten steel pouring is shown in Table 5.

Table 5 LF outlet-temperature drop of steel water at the beginning of continuous casting

projectTemperature drop during tapping process/℃ Tapping time/min Tapping temperature drop rate/(℃·min-¹)
average value52.2183.1
minimum value2111.51.4
maximum value8429.77.3

As can be seen from Table 5, the process takes an average of 18 minutes, the average temperature drop of the molten steel reaches 52.2°C, the temperature drop rate is 3.1°C/min, and the temperature loss is serious. The cooling distribution is shown in Figure 4.

Figure 4 Temperature drop distribution from the end of LF refining to the middle packaging process

As can be seen from Figure 4, the proportion of temperature drops greater than 60°C from the LF refining exit to the opening of the intermediate package reaches 25%, which shows that the temperature drop fluctuates greatly.

In the normal continuous production state, the middle and full ladles are cast, the production cycle and continuous casting rhythm are relatively stable, and the temperature drop is also relatively stable. Therefore, the argon blowing of molten steel, the temperature drop of the molten steel from the large ladle to the tundish ladle and the condition of the ladle are the main reasons for the large temperature drop. The process temperature drop can be reduced by adjusting the argon blowing flow rate, selecting the covering material of the tundish ladle and maintaining the ladle. .

1.3 Problems in molten steel temperature control

Through the investigation and analysis of the current situation of molten steel temperature in the whole steelmaking process, it can be seen that the main problems in molten steel temperature control are as follows: (1) The tapping temperature of the converter is too high; (2) The argon blowing of molten steel and the wire feeding operation are not standardized; (3) Ladle conditions such as ladle baking and turnover time vary greatly; (4) A complete molten steel temperature control system has not been established.

Therefore, in order to establish a stable and reasonable temperature control system, it is necessary to conduct research from the above aspects and explore methods for stable temperature control of the molten steel process.

2 Analysis of factors affecting the process temperature of molten steel

2.1 Effect of tapping temperature on temperature drop during tapping process

The temperature drop of Q235B steel in different tapping temperature ranges is shown in Table 6.

  Table 6 Temperature drop in different tapping temperature ranges

Tapping temperature/℃Temperature drop during tapping process/℃
AverageMinimum Maximum
≥1680572085
<1680522080

It can be seen from Table 6 that the temperature drop during the tapping process of heats with a tapping temperature higher than 1680°C is 5°C higher than that of heats with a tapping temperature lower than 1680°C. Therefore, appropriately lowering the tapping temperature in converter smelting is not only beneficial to the quality of molten steel, but also beneficial to reducing the temperature drop during the tapping process.

2.2 Effect of ladle operating time on temperature drop of molten steel process

The temperature drop of the molten steel at different operating times from the Q235B small steel platform to the intermediate ladle is shown in Table 7.

Table 7 Temperature drop in different ladle operating time intervals

Running time/minProcess temperature drop/℃
AverageMinimum Maximum
9~15492574
15~25523578
>25584486

It can be seen from Table 7 that the temperature drops of molten steel are 49°C, 52°C and 58°C respectively in the three time intervals of 9min~15min, 15min~25min and more than 25min. It is further verified that as the ladle operating time increases, the temperature drop of the molten steel process increases. Therefore, the production logistics organization should be strengthened to reduce the running time of the ladle, thereby reducing the temperature drop of the molten steel process.

2.3 Effect of alloy elements on molten steel temperature

The effects of alloying elements on the temperature of molten steel during alloying are shown in Table 8.

It can be seen from Table 8 that not all alloying elements will cool down the molten steel after being added. The addition of aluminum in the aluminum material used for deoxidation will heat up the molten steel.

Table 8 Effect of alloy elements on temperature

Alloying elementsSiMnAlC rMo
Temperature drop/°C0.01310.00360.0022-0.04280.00440.0012

Note: “-” means to heat up the molten steel after adding

2.4 The influence of argon blowing and wire feeding operations on the temperature drop of molten steel

In order to examine the temperature drop of molten steel under different argon blowing intensities, three different argon blowing methods were carried out under the same treatment time: no argon blowing, small flow argon blowing (the molten steel surface moved slightly), and large flow argon blowing (the molten steel turned over greatly). Experiments were conducted on the changes in the temperature drop rate of molten steel, and the results are shown in Figure 5.

Figure 5 Temperature drop rate of molten steel under different argon blowing methods

It can be seen from Figure 5 that the average temperature drop rate of molten steel is 1.1°C/min without argon blowing, the average temperature drop rate of molten steel is 2.4°C/min when argon is blown at a small flow rate, and the average temperature drop rate is as high as 6.2°C/min when argon is blown at a large flow rate. min. The temperature drop of large-flow argon blowing is too large, and the exposure of the molten steel surface to the air can easily cause secondary oxidation of the molten steel. Moreover, there are reports in the literature that excessive argon blowing flow is not conducive to the floating of small inclusions in the molten steel. Therefore, the intensity of argon blowing should be controlled during argon blowing refining, so that the molten steel surface moves slightly and the molten steel is not exposed. The temperature drop of molten steel at different wire feeding speeds during calcium treatment is shown in Figure 6.

Figure 6 Temperature drop of molten steel at different wire feeding speeds during calcium treatment

It can be seen from Figure 6 that under the same bottom blowing air supply intensity, as the wire feeding rate increases, the temperature drop of the molten steel process increases. When the wire feeding rate is greater than 4m/s, the temperature drop of the molten steel becomes larger. Therefore, controlling the bottom blowing air supply intensity and controlling the wire feeding speed to no more than 4 m/s during the wire feeding process will help reduce the temperature drop during the wire feeding process, and at the same time help prevent secondary oxidation of the molten steel.

3. Technical measures and effects of temperature control in molten steel process

3.1 Reduce the tapping temperature and reduce the tapping temperature drop

In order to reduce the temperature drop during the tapping process and improve the accuracy of converter end-point temperature control, the heat balance of the converter smelting process was calculated, and the scrap adding mode under different furnace entry conditions was established. It effectively improves the accuracy of converter end temperature control and reduces the tapping temperature and temperature drop during the tapping process.

3.2 Strengthen ladle management and reduce process temperature drop

Studies have shown that the heat dissipation ratio of each surface of the ladle is about 70% to 80% of the heat dissipation on the upper surface of the steel slag, about 20% of the heat dissipation of the ladle wall, and < 5% of the heat dissipation of the bottom of the ladle. Moreover, the heat energy loss of ladles is relatively large during transportation. To this end, measures have been formulated to enhance ladle turnover and reduce process temperature drop:

(1) Strengthen ladle baking and reduce the temperature drop of molten steel in the process;

(2) Red envelope tapping reduces the temperature of the tapping process;

(3) Strengthen ladle turnover, shorten operating time, and reduce the temperature drop of the molten steel process;

(4) Ladle classification management and formulation of special ladle temperature control requirements.

3.3 Effect of molten steel temperature control

Through the implementation of various measures, the average tapping temperature of the converter has been significantly reduced to <1 660°C, and the number of heats with relatively high tapping temperatures (≥1 690°C) has been reduced to <7.5%.

4 Conclusion

(1) Research shows that tapping temperature, ladle operating time, alloy, auxiliary material addition, and argon blowing and feeding line operations are the main factors that affect the temperature drop of the molten steel process.

(2) By controlling the tapping temperature, strengthening ladle management and other measures to reduce the temperature drop in the molten steel process, the proportion of ladle turnover time less than 60 minutes increased from 44% to 46%.

(3) By establishing a full-process temperature control system for different steel types, the operating time is shortened and the implementation of measures to reduce the process temperature drop is implemented. The cost of converter refractory materials has been significantly reduced, the pass rate of the tundish temperature has been greatly improved, and the pass rate of the cast slab has been greatly improved.

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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