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Measures to Improve the Qualification Rate of Molten Steel Temperature in Tundish

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In view of the large temperature fluctuations of the molten steel tundish in the steelmaking plant and the low qualification rate of the tundish temperature, various control links that affect the temperature such as ladle refining, continuous casting, and production organization have been optimized. The pass rate of temperature control in the molten steel tundish increased from about 80% to more than 95%.

Keywords: tundish; water temperature; superheat; process control

Foreword

The appropriate superheat of the molten steel in the continuous casting tundish is an important factor in ensuring a stable steelmaking-continuous casting production process and stable slab quality. When the superheat of the molten steel in the tundish is properly controlled, the equiaxed grain area of the cast slab will grow and the structural structure of the cast slab will become denser. This will help reduce segregation and porosity in the center of the cast slab, improve the quality of the cast slab and increase the output of the casting machine. When the temperature of the molten steel in the tundish is too high, it will intensify the development of columnar crystals in the continuous casting billet, leading to serious central segregation problems. It will also thin the billet shell and increase the risk of leakage. On the contrary, if the temperature of the molten steel in the tundish is too low and the fluidity of the molten steel is poor, it will cause the risk of interrupting the continuous casting process. Therefore, it is of great significance to stably control the superheat of molten steel in the tundish within an appropriate range.

Equipment conditions and requirements for superheat of molten steel in the tundish

A steelmaking plant currently has three 120t top and bottom combined blowing converters, three sets of online argon blowing wire feeding devices, and two 120tLF refining furnaces. 1 RH vacuum furnace, equipped with 3 slab continuous casters and 1 five-machine and five-stream billet continuous caster.

The smelting varieties mainly include high-strength ship plates, pressure vessel steels, low-alloy high-strength structural steels, pipeline steels, PC steel rods, etc. The superheat of molten steel has a great impact on the quality of the cast slab, and the superheat of molten steel in the continuous casting tundish is required. : Slab regulations: 10℃ ≤ continuous pouring furnaces ≤ 30℃; five machines and five streams 170 mm × 170mm. Billet regulations: 15℃ ≤ continuous pouring furnaces ≤ 35℃. The superheat of the first furnace of the tundish is increased by 5℃.

Analysis of factors affecting the temperature drop of molten steel and existing problems

Current status of medium package temperature hit rate and statistics on reasons for missing hits

Affected by various reasons, the temperature drop in each link of the plant fluctuates greatly, and the temperature of the tundish fluctuates greatly. The average temperature difference of the tundish during the pouring process reaches 20°C. In 2008, the temperature hit rate of the tundish in the steelmaking plant was only about 80%. The statistics of the reasons affecting the failure of the tundish temperature in December 2008 are shown in Table 1. Whether the tundish temperature is successful or not is mainly affected by the temperature drop control fluctuation of the molten steel process.

Table 1 Statistics of reasons affecting medium package temperature misses

Causes of impactNumber of furnaces/seatProportion/%
The argon station handles molten steel and the tap temperature of the converter is low5514.47
Class C and D ladles have low baking temperature and large temperature drop10928.68
Converter tapping temperature is low and refining heating time is long5514.47
Poor bottom blowing effect of ladle82.11
Operation error and high control307.89
The tapping temperature is high and the argon station has no time to cool down.4110.79
Unreasonable production organization and short refining time328.42
The refining bottom blow is not controlled properly and the temperature measurement is not representative.205.26
Continuous casting downshift drawn steel or billet four-flow drawn steel307.89
total380100

The influence of ladle baking, thermal insulation performance and use on the temperature drop of molten steel

One of the main factors affecting the temperature drop of the molten steel process is the thermal insulation performance of the ladle, which depends on the ladle capacity, ladle lining material, number of uses, intermittent time and baking temperature, etc. Good thermal insulation performance of the ladle can reduce and stabilize the temperature drop of the molten steel from tapping to pouring, maintain and stabilize the appropriate pouring temperature of the molten steel, and improve the temperature hit rate of the ladle. A large amount of heat is lost from filling the molten steel into the ladle to the casting process, and the heat storage loss of the ladle lining is the main one. The baking conditions and thermal insulation performance of the ladle lining have a great influence on the temperature drop, and play a key role in the temperature hit of the middle ladle. . Due to the shortage of molten iron in the steelmaking plant, the production rhythm fluctuates greatly, which often results in a large number of ladles being baked without a roaster. When the production rhythm changes from slow to fast, a large number of ladles with baking temperatures that do not meet the standard are put on line, causing a large temperature drop in the process and the temperature of the middle ladle does not hit the target.

The permanent layer of the ladle was originally “lightweight fiberboard + semi-lightweight castable”. The surface temperature of the ladle shell was as high as 300°C, and the ladle’s thermal insulation performance was poor. Due to the influence of various factors in the production process, some ladles take too long to turn around and the ladle lining turns black. The original traditional roaster has poor baking effect and cannot quickly bake and raise the temperature of the ladles. Due to the low baking temperature of Class C and D ladles, the proportion of middle ladle temperature misses is the highest. The influence of the ladle lining baking conditions and thermal insulation performance on the tapping temperature drop of the plain carbon series steel converter is shown in Table 2. The use of ladles is unreasonable. The proportion of Class A and B ladles in the total number of used ladles is too low (see Table 3). The temperature drop of Class C and D ladles during tapping is large, and the temperature drop fluctuations are also large. Therefore, the poor baking effect of the ladle lining, poor thermal insulation performance and unreasonable use of the ladle are important reasons for the low temperature hit rate of the ladle.

Analysis of the influence of converter tapping temperature and tapping temperature drop

Large fluctuations in the molten iron temperature, molten iron composition, and scrap steel composition cause large fluctuations in the tapping temperature. The qualification rate of the converter tapping temperature is only about 60%, resulting in a low qualification rate of the temperature of the molten steel arriving at the argon station, which greatly affects the temperature hit of the tundish.

Table 2 Ladle category and converter tapping temperature drop ℃

Ladle categoryABCD
Average tapping temperature drop before improvement55606575
Fluctuation of tapping temperature drop before improvement45~6050~6555~7560~90
After improvement, the tapping temperature dropped evenly50535863
Fluctuation of tapping temperature drop after improvement45~5545~6050~6550~70

Table 3 Ladle categories and their proportion %

Ladle categoryABCD
Proportion before improvement3330289
Improved proportion55.535.4.84.2

The argon station handles the molten steel converter tap temperature at 1650-1690°C, which is appropriately increased by 10-15°C for the first furnace, new ladle, intermediate repair ladle, etc. The tap temperature drop of the converter generally fluctuates greatly between 45-90°C. After the molten steel is refined in the LF furnace, the converter tapping temperature is 1620-1690°C. Add 400 kg of ladle lime and 200 kg of refining slag to the converter tapping slag, and the converter tapping temperature drops by 60-100°C. The large fluctuations in converter tapping temperature drop are mainly affected by the tapping temperature, the number of tapping ports, the shape of the tapping ports, the baking conditions of the ladle, and the amount of alloy and slag added. The large fluctuations in converter tapping temperature and tapping temperature drop have a great impact on the temperature of the molten steel in the tundish when it passes through the argon station. The argon station has no heating facilities. When the temperature of the molten steel reaches the argon station is low, it will inevitably cause the tundish temperature to be too low. When the temperature of the molten steel reaches the argon station is too high, the argon station can cool down appropriately, but the processing time of the argon station is short. When the temperature is too high, the argon station does not have enough time to cool down, resulting in high tundish temperature.

Analysis of the influence of temperature drop on argon blowing wire feeding station

When processing molten steel in an argon station, argon is generally blown at the argon station for 8 to 16 minutes. The time is short, the temperature of the ladle lining is different, and the fluctuation of heat storage in the ladle wall is the main reason for the fluctuation of the temperature drop of the molten steel in the argon station. During the argon blowing and stirring process, the heat absorption of argon gas has little impact on the temperature drop of the molten steel. The main reason why the temperature of the molten steel drops due to argon blowing is that the argon blowing and stirring eliminate the temperature stratification of the molten steel in the ladle and increase the heat conduction ability of the molten steel to the ladle wall. Second, when argon is blown and stirred, the molten steel surface is exposed and heat dissipation increases. When the temperature of the molten steel at the station is too high, pure light scrap steel can be added into the ladle while stirring with argon blowing to lower the temperature of the molten steel. The temperature drop of the argon station treatment is mainly affected by the ladle baking effect, ladle insulation performance, tapping slag amount and argon blowing time.

The quality of the taphole casing fluctuates, resulting in poor slag-retaining effect at the tap and affecting the temperature drop of the molten steel. The thickness of the slag layer has a greater impact on the heat dissipation on the slag surface. The more slag is added, the thicker the slag layer is, and the less heat is lost from the surface of the molten steel. The less slag is added, the thinner the slag layer is, the greater the surface heat dissipation is.

 Analysis of the impact of temperature drop from tapping to refining

When molten steel is processed in the argon station, since it only takes 1 minute after tapping from the converter to the argon station, this part of the temperature drop is generally not considered. After tapping from the converter, the average temperature of the molten steel drops by about 2℃/min before being processed in the LF furnace. The temperature of the molten steel when it reaches the LF furnace is generally 1540~1570℃. The temperature drop from the completion of tapping to before refining is mainly affected by the ladle baking effect, the thermal insulation performance of the ladle and the time from the completion of tapping to the lifting of the molten steel to the LF furnace. The temperature drop from the completion of tapping to before refining generally fluctuates greatly from 10 to 50°C.

Effect and analysis of temperature drop during molten steel refining process

Judging from production practice, the refining heating temperature rise rate is related to ladle wall heat storage, foaming slag, alloy addition, bottom blowing argon size, slag fluidity, etc. These factors cause the heating rate to fluctuate, and the heating results are quite different from the expected target, thus affecting whether the temperature of the middle package is hit or not.

When processing molten steel in LF, the refining cycle is generally 37 minutes. Due to the heat absorption of slag melting and the heat storage of the ladle wall, the temperature drop of the molten steel in the early stage of refining is large. The temperature after 6 minutes of heating at the 6th gear in the early stage of the LF furnace is equivalent to the temperature before starting treatment. After 35 minutes of normal ladle refining and slagging, the temperature drop decreases and stabilizes, and the average temperature of the molten steel drops by about 0.5°C/min. However, the temperature drop of the molten steel in the new package and the repair package is large during the refining process, and the temperature drop amplitude fluctuates greatly. The refining temperature increases by about 10°C when leaving the station. After refining, the temperature of the molten steel fluctuates greatly, often causing the temperature of the middle package to miss.

The temperature drop for calcium treatment and soft blowing is generally 10-12°C. This temperature drop is related to the fluidity of the slag and the baking conditions of the ladle.

Effect of argon blowing at the bottom of ladle on temperature drop

Since the single breathable brick blows argon, the area around the nozzle on the other side of the ladle becomes a dead zone. The stirring ability of the molten steel is weak, and the uniformity of composition and temperature is poor, resulting in poor temperature measurement representativeness and easily leading to the failure of the middle ladle temperature.

Effect of temperature drop of molten steel from LF to RH and temperature drop during RH process

The temperature drop of molten steel from LF to RH is generally 10 to 15°C. The main influencing factors are the transportation time of molten steel and the condition of the ladle.

The temperature drop during the RH process is mainly affected by the temperature of the tank. After the tank has not been used for a long time, the temperature drop will be large for the first two times before reuse. Generally, the RH temperature will be about 20°C higher than normal.

The temperature drop for calcium treatment and soft blowing is generally about 10℃.

Effect and analysis of temperature drop of continuous casting steel water

The temperature drop of continuous casting molten steel from the ladle to the tundish is mainly affected by the condition of the ladle, the condition of the tundish baking ladle, the continuous casting speed, the ladle capping and the insulation performance of the tundish. The poor insulation effect of the middle ladle and the poor baking effect of the middle ladle result in a large temperature drop of 25 to 50°C from the slab ladle to the middle ladle and a large fluctuation, which is not conducive to the prediction and control of the temperature of the molten steel.

Optimization measures to improve tundish temperature hit rate

Ladle baking, heat preservation performance and usage management optimization measures

In view of the poor thermal insulation performance of the ladle, measures to improve the permanent layer of the ladle are taken. The permanent layer of the steel ladle was changed from lightweight fiberboard + semi-lightweight castable to high-strength and high-insulation mullite castable to form the construction. The thickness of the knotted ladle bottom is 170mm and the cladding wall is 100mm. The physical and chemical indicators of high-strength and high-insulation mullite casting materials are shown in Table 4. The temperature of the surface of the ladle shell is measured with an infrared thermometer to reflect the thermal insulation effect of the ladle. The selected temperature is the reaching temperature of the shell surface of the ladle when returning to the hot repair construction site after continuous casting and steel pouring. For each heat, three temperature measurement points of the ladle slag line, ladle wall and ladle bottom are selected to measure the surface of the ladle shell. See the table for specific comparison data5.

Table 4 Physical and chemical indicators of high-strength and high-insulation mullite casting materials

Thermal conductivity W/m.k (350℃)w(Al₂O₃)/%Thermal shock stability (air cooling)Linear change rate/% Volume density (g·cm-3)Normal temperature compressive strength/MPa
≤0.5≥60≥18-0.4-0.1≤1.6≥18

Table 5 Comparison of steel ladles before and after using high-strength and high-insulation mullite castables and lightweight fiberboards for the permanent layer of the ladle

LadleLightweight fiberboard+Semi-lightweight castableHigh strength and high thermal insulationMullite castable
Temperature measurement partSlag line bag bodybag bottomSlag line bag bodybag bottom
Cladding temperature/℃294288285227246226

The temperature drop of pure argon blowing is calculated at the argon blowing wire feeding station. Table 6 shows the temperature drop data of pure argon blowing of molten steel per minute.

Table 6 Temperature drop data of pure argon blowing of molten steel per minute ℃/min

lightweight fiberboardSteel typemullite castable
Overall average temperature drop2.512.78
HRB335 temperature drop2.692.77
Q195LB temperature drop2.192.65
Q235B temperature drop2.612.81

When returning to the hot repair construction site, the surface of the ladle shell using high-strength and high-insulation mullite castables is about 65°C lower than that using lightweight fiberboards. The temperature drop of pure argon blowing of molten steel in the test ladle argon blowing wire feeding station per minute is lower than that of the non-test ladle to varying degrees. Therefore, a permanent layer of high-strength and high-insulation mullite castable is used to reduce heat dissipation from the ladle wall and greatly improve the thermal insulation performance of the ladle.

In view of the poor ladle baking effect, the factory changed all ladle roasters to regenerative ladle roasters. The regenerative ladle roaster adopts regenerative combustion technology, which improves the thermal energy utilization rate of the fuel, increases the average flame temperature in the ladle, speeds up the baking speed, and ensures the baking effect of the ladle lining. Generally speaking, within 10 minutes of online ladle baking, a ladle with an initial temperature of 800°C can be heated to about 1000°C. The offline package will raise the cold package to about 1000°C within 30 minutes.

Classification of ladles and optimization measures for use and turnover. Ladles are classified according to the length of emptying time and baking conditions: A, B, C, and D 4 categories. Category A: The time from pouring the steel to tapping is less than or equal to 60 minutes, and is baked online. Category B: 60 to 90 minutes from pouring to tapping, and online baking. Category C: 90 minutes from pouring to tapping, and heating for 2 to 8 hours. Category D: Ladles for major, medium and minor repairs, heated for more than 8 hours. Through production organization, we can speed up the turnover of ladles, reduce the number of turnover ladles, and increase the utilization rate of ladles A and B.

Ladle double ventilation modification

In order to better remove inclusions, uniform temperature and composition in the steel, the factory transformed the ladle’s single breathable bricks into double breathable bricks. The double ventilation transformation of the ladle makes the temperature uniformity of the steel sample and temperature more representative, and the refining composition and temperature hit rate are improved. Table 7 is the temperature drop data of pure argon blowing for 80-furnace carbon steel. It can be seen from Table 4 that after using double breathable brick ladle, the temperature drop is basically the same as that of single breathable brick, and has little effect on the temperature drop.

Table 7 Comparison of steel water temperature drop between single breathable brick argon blowing and double breathable brick argon blowing

Argon blowing processArgon blowing time/min Temperature drop rate/℃.min
Double breathable brick argon blowing15.82.67
Single breathable brick argon blowing15.32.60

Optimization measures for converter and argon blowing wire feeding station

The converter adjusts operations in a timely manner based on the temperature of the molten iron, the composition of the molten iron, and the composition of the scrap steel. Increase the reward and assessment of the converter end-point hit rate indicator, increase the pass rate of the converter tapping temperature by about 10%, and increase the pass rate of the temperature of the molten steel to the argon station, which is conducive to temperature control. .

Improve the quality of the taphole casing and repair the taphole in time according to the condition of the taphole to ensure the roundness of the steel flow during the tapping process and reduce the heat dissipation of the steel flow. The shape of the tap opening ensures that the hit rate of converter tapping slag is improved, and the impact of slag on the temperature drop of the molten steel in the argon station is ensured, making the temperature drop of the argon station more stable and conducive to temperature control.

When processing molten steel in an argon station, since the fluctuation of heat storage in the ladle wall is the main reason for the fluctuation of the temperature drop of the molten steel in the argon station, the standard should be modified to appropriately extend the argon blowing time to allow the heat storage in the ladle wall to absorb as much heat as possible, which will help stabilize the temperature drop of the ladle in the later period. Due to the poor ability of the argon station to actively control temperature, avoid using Class C and D ladles with large process temperature drop fluctuations in the argon station to process molten steel. Try to use Class A and B ladles in the argon station because the temperature drop is stable and conducive to temperature control.

Make full use of networks at all levels to optimize production organization, ensure stable argon station processing time, and facilitate argon station temperature control.

LF furnace optimization measures

Maintaining a sufficient amount of slag during refining is firstly beneficial to the heat preservation of molten steel. Studies have shown that as the thickness of the slag layer increases, the radiation heat dissipation effect on the slag surface decreases; secondly, it is beneficial to ensure the submerged arc effect.

The double ventilation modification of the ladle and the optimization of the bottom blowing argon system make the temperature uniform and the temperature measurement representative.

It is necessary to wash all the slag from the converter tapping of molten steel for refining, which is beneficial to heat preservation from the completion of tapping the converter to before refining, and is beneficial to quickly create foamed slag submerged arc in the refining.

Optimizing the refining slag system is conducive to foaming during the refining process and ensures the submerged arc effect. Producing foamed slag in the LF furnace is conducive to improving heating efficiency and stabilizing the heating temperature rise rate. When the foamed slag is submerged well in the middle and late stages of refining, the 4-speed temperature rise is 5°C/min, which is beneficial to the end temperature of the LF furnace reaching the expected target or close to the expected target. In view of the fact that the temperature drop of molten steel during the refining process when using new ladles and intermediate repair ladles is larger than that of normal ladles, attention should be paid to treating the temperature drop differently during the refining process, and the outgoing temperature should be increased appropriately.

Make full use of networks at all levels to optimize production organization and ensure that the refining time is greater than 35 minutes, which is conducive to ensuring the quality of refined molten steel. When using new ladles or mid-repair ladles, the refining time should be appropriately extended to facilitate heat storage in the ladle wall to fully stabilize the temperature drop. It is beneficial to stabilize the time control from the end of refining to the start of continuous casting.

Calcium treatment plus soft blowing temperature drop is mainly to control the fluidity of the slag and pay attention to the baking condition of the ladle. The temperature drop can be better predicted.

RH optimization measures

In response to the large temperature drop after reuse for the first time, measures were taken: Strengthen the management of trough baking to ensure the trough baking temperature. If the tank is empty for more than 5 hours, wash the tank first to allow the tank to fully absorb heat and reduce the temperature drop during normal processing. When the RH temperature is too high, add cooling material to reduce the temperature. The final temperature of the molten steel that has been passed through the LF furnace and then RH is 5°C lower than that of the molten steel that is only passed through the LF furnace.

Continuous casting optimization measures

Poor baking of the tundish will cause the temperature of the molten steel in the tundish to drop greatly and quickly. Optimize the tundish baking system to ensure the tundish baking effect. Strengthen the insulation work of the molten steel surface in the middle package. Do a good job in insulating the middle bag, and discard the bag cover in time when it is deformed. Use asbestos cloth to pad it before placing the bag cover to ensure that there is no gap between the bag cover and the middle bag. The permanent layer of the tundish uses high-strength and high-insulation mullite castable to reduce heat dissipation from the tundish wall, which can effectively reduce the temperature drop of molten steel in the tundish.

Continuous casting implements typical casting speed control, and stabilizing the pouring time is conducive to stabilizing the prediction and control of the temperature drop from the large ladle to the middle ladle and the temperature of the molten steel.

Temperature system optimization

After the implementation of the above optimization measures, the temperature drop and fluctuation range of each link of the molten steel process became smaller. The optimization of the temperature system has a very good guiding significance for on-site temperature control and improves the level of process temperature control.

The effect after optimization

The baking temperature of ladles has been greatly increased, and the lining temperature of Class C and D ladles reaches 1000°C before being placed in the ladle. Through online baking, the temperature of the ladle lining reaches more than 1100°C before tapping. The tapping temperature of Class C and D ladle converters drops by more than 10°C. The temperature drop of the molten steel when it is left standing after refining is about 20%.

The utilization rate of Class A and B ladles has increased from 63% to 91%. The molten steel ladles processed by the argon station are basically Class A and B ladles. Class C and D ladles only account for 1.1% of the total number of ladles, which greatly reduces the impact of the large temperature drop of C and D ladles on the temperature hit rate of the middle ladle.

After taking optimization measures, the temperature drop from the slab ladle to the middle ladle is reduced to 15-35°C, with small fluctuations. The temperature drop in other links is also reduced and relatively stable, which is conducive to the prediction and control of the molten steel temperature. The fluctuation range of the temperature of the molten steel in the tundish is reduced, the temperature difference in the tundish is reduced to about 10°C during the slab pouring process, and the pass rate of the temperature control of the molten steel in the tundish is increased from about 80% to more than 95%. The optimization effect is obvious.

Conclusion

1) The double ventilation modification of the ladle is beneficial to the uniform temperature of the molten steel and the control of the process temperature.

2) The permanent layer of the ladle and tundish is made of high-strength and high-insulation mullite castable, which can improve the thermal insulation performance of the ladle and tundish and effectively reduce the temperature drop of the molten steel.

3) Improve ladle baking and strengthen production organization to increase the utilization rate of Class A and B ladles, which is beneficial to process temperature control.

4) Make full use of networks at all levels to optimize production organization and ensure that the argon station processing and refining time is conducive to stable temperature drop.

5) Optimize process temperature control from the above aspects, and the pass rate of molten steel tundish temperature control increases from about 80% to more than 95%.

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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