Practice of controlling molten steel temperature in continuous casting tundish

 Based on the actual situation of steelmaking and continuous casting production, the analysis shows that the tapping temperature is the main influencing factor of the molten steel pouring temperature in the continuous casting tundish, but the actual control point lies in the temperature of the molten steel entering the continuous casting table. For high-temperature molten steel, use argon stations or add clean standard pig iron during the tapping process to achieve temperature control. For low-temperature molten steel, measures to increase the drawing speed and shorten the casting cycle can effectively avoid uncastability. When the casting temperature of the molten steel in the tundish is kept within the superheat range, the casting rate reaches more than 90%, which improves the quality of the cast slab and reduces the rolling defective scrap rate to 0.01%.

Keywords: continuous casting tundish; molten steel temperature; casting temperature; control

 Fluctuation rules of molten steel temperature in tundish during casting process

Controlling the molten steel casting temperature in the continuous casting tundish within a suitable superheat range is of great significance to the steelmaking and continuous casting production process and the stability of the slab quality. The control of molten steel temperature in the tundish is affected by various factors, and there is the following relationship:

In actual production, considering the continuous pouring properties of furnace steel, 2X₁×△T; is basically determined, and T out becomes a key factor. Adjustment means and measures must be provided to ensure that the temperature of the molten steel on the upper continuous casting table meets the temperature fluctuations during the entire casting cycle. Within the superheat range, the relationship between the temperature change of the molten steel in the tundish can be simplified as:

Therefore, it is particularly important to control the molten steel temperature in the ladle of the continuous casting station.

The steelmaking plant mainly produces construction steel: HRB400 wire rods and bars, HPB300 wire rods, using a 3×60t converter corresponding to a 3×6R8-170 continuous casting machine. The molten steel sedation time is 1 to 8 minutes; the red envelope tapping time (lining temperature > 850°C), the tapping time is 2 to 4 minutes, and the tapping temperature drops by 45 to 60°C; the argon station blowing time is 6 to 8 minutes, and the blowing temperature drops by 20 to 23 ℃, argon station on the continuous casting table for 1 to 3 minutes; the entire continuous casting process is protected and equipped with a black body cavity continuous temperature measurement device; a T-shaped symmetrical tundish is selected; the casting cycle is 24 to 25 minutes.

The molten steel casting process in the tundish is essentially a dynamic process of balance between heat absorption and heat dissipation. The change in casting temperature must be within a specific superheat range. Through the summary of the continuous temperature measurement curve, the actual change has four basic forms. As shown in Figure 1.

It can be seen from Figure 1 that the range between lines A and B is the normal temperature fluctuation range of casting molten steel. The position of the DE segment above line A is related to the final casting temperature of the molten steel in the front furnace (starting temperature of casting) and the temperature of the molten steel in the large ladle. When the initial casting temperature is too high, or the temperature of the molten steel in the ladle is high, the DE section will move to the left. If it occurs in advance, it will affect the quality of the slab and the casting speed. The lower part of line B is the solid-liquid two-phase area. When the temperature of the molten steel in the ladle is Too low and low casting starting temperature will cause the CF section of line B to shrink to the left, which can easily interrupt continuous casting production and cause sudden shutdown accidents. Therefore, the upper part of line A and the lower part of line B need to be controlled in real time during actual production, and the actual control point is the temperature of the molten steel entering the continuous casting table.

Figure 1 Changes in casting temperature within the superheat range

Measures to control molten steel casting temperature

Add clean standard pig iron

The control of tapping temperature is affected by operating skills, tapping time, ladle conditions and other factors, and the gap is large. The tapping temperature of Anyuan Steelmaking Plant is 1640~1680℃, with an average tapping temperature of 1655℃, with a wide range of fluctuations. Therefore, we choose to selectively add clean standard pig iron in the argon station or tapping process for temperature adjustment. The adjusted temperature standard is related to the temperature of the tundish in the forehearth casting. After a large amount of actual data tracking and comparison, taking HRB400 series steel as an example, the corresponding relationship between the outgoing molten steel temperature of the argon station and the molten steel temperature of the tundish required for the continuous casting temperature of molten steel in the steelmaking plant to fluctuate between lines A and B is shown in Figure 2.

It can be seen from Figure 2 that under the current process requirements, the outgoing molten steel temperature of the argon station is within the standard temperature range of ±10°C, and there will be no breakthrough of line A or penetration of line B.

In actual production, the temperature of the molten steel arriving at the argon station is first measured, and the outgoing station temperature is back-calculated according to the blowing temperature dropped by 20°C. Combined with the temperature of the molten steel in the pouring tundish, if the back-calculated outgoing temperature is higher than the temperature requirement of line A, then add Clean standard pig iron is cooled; or the tapping temperature is obviously high before tapping, and part of the pig iron is delivered during the tapping process. This is not only beneficial to temperature control, but also beneficial to pre-deoxidation of molten steel and increased alloy yield. The amount of pig iron is calculated based on simple heat balance:

Obtained, the specific amount of pig iron is shown in Table 1. After adding pig iron, the temperature error of the outgoing molten steel is ≤5 ℃.

Figure 2 Correspondence between the molten steel temperature in the tundish and the required outbound molten steel temperature at the argon station

Table 1 Effect of adding amount of furnace steel pig iron on temperature

Pig iron addition amount/kg50100150200250300
Cooling value/℃1.534.567.59

Increase the casting speed and shorten the casting cycle

The timing of tapping in the steelmaking system of a steelmaking plant mainly relies on manual judgment, and insufficient tapping temperature is inevitable. According to the ④ line in Figure 1, measures can be taken to shorten the casting cycle. The steelmaking plant has optimized the tundish nozzle. The upper nozzle has a fixed diameter of 18mm. The lower nozzle is configured with different diameters (≤φ18mm). The normal production configuration is to configure the lower nozzle with a fixed diameter of φ16.5mm. When the phenomenon of low-temperature steel occurs, the method of replacing the large-diameter nozzle of the tundish is used to shorten the casting cycle (see Table 2 for specific adjustment relationships). At the same time, the smelting rhythm in front of the furnace is adjusted, and the corresponding relationship between the furnace and the machine is replaced to keep the production rhythm smooth.

Table 2 The corresponding relationship between the adjustment casting cycle of low-temperature steel and the drain outlet

Lower than B line temperature/℃Cycle/minReplace the drain diameter/mm
522Single flow conversion φ17
1018Three-flow exchange for φ17
1516Six-current conversion φ17

Temperature control effect of molten steel casting

Improved the qualification rate of molten steel casting temperature

In actual production, the temperature that exceeds line A and goes below line B in Figure 1 is regarded as an unqualified item, and the casting temperature between line A and line B is regarded as a qualified item. After statistics, the changes in the pass rate of controlled casting temperature and the comparison over the same period are shown in Figure 3.

As can be seen from Figure 3, after taking control measures, the pass rate of molten steel casting temperature in the tundish increased from 75% to 80% in 2016 to 90% to 95% in 2017. The average pass rate of molten steel casting temperature increased by 12.21 %, solving the adverse effects of high and low temperature steel casting on production.

Figure 3 Monthly comparison of tundish temperature qualification rate over the same period

Improved billet quality

The temperature of the molten steel in the tundish is stabilized within the superheat range, which is conducive to improving the internal cracks of the cast slab. However, if the cracks cannot be fused during the rolling process, defects such as peeling and scar spots will occur in the material, which will affect the smooth rolling process. The steelmaking plant tracked the rolling defects and compared the defects before and after stabilizing the molten steel casting temperature in the tundish, as shown in Figure 4.

Figure 4 Comparison of billet scrap rates before and after casting temperature control

Comparing Figure 3 and Figure 4, it is easy to see that when the casting pass rate of the molten steel temperature in the tundish increases, the scrap rate produced during billet rolling is significantly reduced. When the tundish temperature qualification rate is controlled ≥90%, the scrap rate produced by billet rolling is reduced to less than 0.01%. After the steelmaking plant took measures to control the casting temperature of molten steel in the tundish, the quality of the slab was significantly improved.

Conclusion

The steelmaking plant summarized the temperature change data during the casting process of molten steel in the tundish and summarized four main forms. Based on the actual process, based on the existing process flow, by controlling the temperature of the molten steel outgoing from the argon station, the casting rate of the molten steel in the tundish can be kept within the superheat range and the casting rate can reach more than 90%. The stability of the molten steel temperature in the tundish improves the quality of the cast slab, and the rolling defect rejection rate drops to less than 0.01%, meeting the rolling demand.

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