Causes and preventive measures for bonded steel breakouts in slab continuous casting machines

This paper analyzed the caster breakouts produced by the No. 9 slab continuous caster of Kunming Iron and Steel Plant in the production of Q345 low alloy steel, and found the specific reasons for the bonded steel breakouts. By formulating and implementing corresponding process and equipment improvement measures, The frequency of bonding breakouts of low-alloy steel is reduced, ensuring the smooth flow of continuous casting production.


1 Introduction

The No. 9 slab continuous caster in the second continuous casting operation area of Kunming Iron and Steel Co., Ltd. was completed and put into operation at the end of June 2002. Since it was put into operation, a total of 66 steel breakouts have occurred. Among them, there were 40 bonded steel breakouts, accounting for 60.61% of the total steel breakouts. From January to July 2007, there were 12 steel breakouts in the slab, and the low alloy steel Q345, which accounted for 4.35% of the slab output, had 5 bonded steel breakouts. Accounting for 41.67% of the total number of steel breaks from January to July 2007. It can be seen that the bonded steel breakout of low alloy steel Q345 has become a restrictive link in slab production, especially the production of Q345 low alloy steel, which has had a great impact on normal production. It has had a greater impact on the further improvement of steelmaking plant production capacity and the optimization of economic and technical indicators such as steel material consumption. To this end, a research team was established to reduce bonding breakouts of Q345 low-alloy steel, and achieved good results by adopting a series of improvement measures.


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2 Kunming Iron & Steel slab continuous caster equipment and main technical parameters

Table 1 Main process parameters of slab continuous casting machine

serial number project Technical Parameters
1 model Straight mold arc slab continuous casting machine
2 Number of continuous casting machines × number of streams 1×1
3 The basic radius of continuous casting machine/m 8
4 Metallurgical length of continuous casting machine/m 29.4
5 Crystallizer length/mm 900
6 Straight line length/mm 200
7 Casting machine speed range/(m·min~¹) 0.2~2.5
8 Section specifications 200,230)×(900~1600)
9 Secondary cooling system Air-water atomization cooling system
10 Vibration device Four-eccentric high-frequency small-amplitude vibration system

3 Analysis of the causes of bonding steel leakage

A bonded breakout is a breakout caused by the bonding between the solidified billet shell and the inner wall of the crystallizer, which increases the drawing resistance and causes the bonding point to be pulled apart. Except for Q345, the molten steel of low alloy steel undergoes δ→γ phase transformation during the solidification process. In addition to the strong volume shrinkage, high crack sensitivity, and easy surface cracks, the influencing factors of bonded steel leakage are comprehensive, including the performance of the mold slag and the influence of process and operating factors.

3.1 Mold powder performance

The type of mould flux used in the casting of Q345 steel in the steelmaking plant is LB-22. The mold powder used during the two periods when the low alloy steel bonding breakouts occurred more intensively was sampled and submitted for on-site inspection. Comparing the test results of physical and chemical indicators and moisture (Table 2 and Table 3), it can be seen that the physical and chemical indicators meet the technical requirements, while the moisture content is higher than the required 0.5%. Through investigation, it was found that the main reasons for the high moisture content are: ① Due to the small output of Q345 (monthly output is 1000~3000 tons, the dosage of protective slag per ton of steel is 0.5~0.7kg/t), and each purchase cannot be less than 10 tons. Unused protective slag can only be stored in the main warehouse, resulting in increased moisture due to long storage time. ② Due to the gap at the edge of the slab mold cover, water vapor in the secondary cooling chamber overflows during the pouring process, causing the mold powder being used to become damp.

Due to the high moisture content of the mold slag, the slag agglomerates and the liquid slag layer becomes sticky. A large amount of cold steel is formed, which affects the flow of the liquid slag. The lubrication effect is poor and bonded steel leakage occurs.

Table 2 Chemical composition of LB-22 mold powder

  SiO₂ CaO Al₂O₃ MgO Na₂O R Csolid
Require 30±5 30±5 ≤8 ≤4.5 3~10 0.8~1.4 3~8
Used in June 2007 31.2 30.5 3.1 4.0 6.7 0.98 6.0
Used in July 2007 32.8 32.3 4.7 3.2 5.8 0.98 5.7

Table 3 Physical indicators of LB-22 mold powder

project name Indicator Used in June 2007  Used in July 2007
Melting point/℃ 1100~1150 1112 1142
Melting speed s/1350℃ 40+10 / /
Viscosity poise/1300℃ 2.2~3.2 2.7 2.6
Moisture/% ≤0.5 1.05 0.9
Particle size (0.15~1mm) ≥85% / /
Bulk weight/(g/cm³) 0.5+0.9 / /

3.2 Crystallizer liquid level fluctuation

During the production process, it was found that due to fluctuations in argon gas pressure and flow, abnormalities in the tension and leveling machine, abnormal automatic liquid level control and manual pouring, etc., the operator did not properly control the liquid level, causing large fluctuations in the liquid level. Causing bonded steel leakage. In addition, frequent adjustment of the pulling speed during the steel pouring operation causes the liquid level of the mold to fluctuate violently, and the slag condition easily deteriorates, causing the slag film between the mold wall and the billet shell to break and cause bonding leakage.

3.3 Molten steel temperature

Since the liquidus temperature of Q345 low alloy steel is low, and the amount of ferromanganese alloy added during the smelting process is large, the tapping temperature drops greatly, so Q345 low alloy steel is prone to low molten steel temperature. The casting temperature of molten steel is too low, causing the temperature of the molten steel at the meniscus of the mold to be too low, forming cold steel on the liquid surface of the mold. The protective slag liquid slag layer is affected by the low temperature, resulting in poor slag formation, increased viscosity, and slag bonding, resulting in Bonding of broken steel.

3.4 Insertion depth of immersed nozzle

If the insertion depth of the immersed nozzle is too shallow, the molten steel flowing out of the diverter hole will divert upward too much, which will cause slag entrainment caused by excessive surface fluctuations. The insertion depth of the immersed nozzle is too deep, the upward diversion of the molten steel flowing out of the diverter hole is too small, and the surface heating is insufficient, which will cause poor melting of the mold slag, uneven inflow of the mold slag, and prone to bonding and steel leakage.

3.5 Slag fishing operation

Improper slag fishing operation. Liquid slag easily forms slag rings (slag strips) of different sizes around the crystallizer wall. If the slag strips are removed when the temperature in the mold is low, and the melting effect of the mold slag is poor, it will be difficult for the liquid slag to flow continuously and stably between the mold and the shell, and it will not be able to provide lubrication. The shell and the crystal wall are bonded together. If the slag strips are removed when the molten steel level rises, the slag strips will be disconnected from the solid slag film, which will cause the molten steel to directly contact and bond with the crystallizer wall. In addition, due to the operator’s perspective, the outer arc slag strips are often easily discovered and removed in time, while the inner arc slag strips are not easily discovered, and there are more slag strips forming. This is also the main reason for the large proportion of internal arc bonded steel breakouts.

3.6 Slag changing operation

After pouring for a period of time, the mold slag in the mold absorbed a large number of inclusions, and the operator failed to replace the slag in time. Some of these inclusions have a higher melting point and cannot be melted in time, and the liquid slag formed flows into the space between the shell and the crystallizer wall. After slowly enriching, it not only hinders the flow of liquid slag, but also deteriorates the performance of the mold slag, leading to the eventual formation of bonding.

4 improvement measures

4.1 Strengthen the use and management of mold powder

There are strict packaging requirements for protective slag. The pouches are double-packed, with the inner bag packed in a fully sealed plastic bag and the outer bag packed in a woven bag. The molding powder that is not used temporarily or remains after pouring shall be stored in the baking room until it is used in the next production. The molding powder that is not in use shall not be placed on the pouring platform for more than 16 hours. , the display time should not exceed 4 hours during the rainy season (June to August). The moisture content of the mold powder should be measured on-site at least once a week and used in batches. If the batch is changed, the moisture content of the mold powder should be measured promptly. Cover the crystallizer cover with asbestos cloth to reduce the risk of water vapor rising during pouring and making the protective slag damp.

4.2 Reduce crystallizer liquid level fluctuations

Improve the control level of the main pourer on liquid level fluctuations. When manual pouring must be used, ensure that the liquid level fluctuations are as small as possible, no more than 10mm. It is not allowed to adjust the pulling speed in a wide range to stabilize the liquid level. When adjusting the liquid level, the pulling speed fluctuation should be ≤0.1m/min. Standardize the operation to avoid frequent changes in the pulling speed. The lifting and lowering speed range is preferably 0.05~0.10m/min to reduce the fluctuation of the crystallizer liquid level.

4.3 Provide qualified molten steel temperature

Implementing the continuous casting policy of constant temperature and constant drawing speed, all Q345 steel passes through the refining furnace. While ensuring the internal quality of the molten steel, it is more important to stabilize the temperature of the molten steel. Ensure that the temperature of the middle package is 1540~1550℃ and the pulling speed is constant between 1.1~1.2m/min.

4.4 Select the appropriate insertion depth of the immersed nozzle

A reasonable nozzle insertion depth is an important condition for giving full play to the role of protective slag. After production practice and water model testing, it is proposed that under the existing cross-section and nozzle size, the appropriate immersion depth is 90~110mm.

4.5 Strengthen the online management of crystallizers

Strictly inspect the mold before pouring steel to ensure that scratches on the surface of the mold copper plate above 300mm are ≤0.3mm, and scratches on the surface of the copper plate below 300mm are ≤0.7mm. The gap between the wide and narrow surfaces of the copper plate is ≤0.3mm; the copper plate of the crystallizer must be replaced when it expires. If the copper plate is found to be discolored, it must also be replaced.

4.6 Correct use of mold powder

4.6.1 Correct handling of mold slag rings

If the exposed height of the slag ring is only 25mm, use a rod to break it into pieces. If it is >25mm, you can use a rod to bend it down, and then cover it with newly added protective slag. Do not lift the slag ring upward to prevent damage to the slag film between the crystallizer and the shell, causing adhesion.

4.6.2 Slag fishing operation

When the liquid level is low, fish out the slag; on the movable side, fish out from the center to both sides, and on the fixed side, fish out from both sides to the center; the action should be gentle, and the slag should be fished out cleanly every time to prevent the slag ring from being broken and unable to be fished out. It is strictly prohibited to pick the slag ring when adjusting width, adjusting taper or changing pulling speed. When the performance of the mold slag is stable and the liquid level is stable, there is usually no need to perform frequent slag fishing operations. Instead, new slag needs to be continuously replenished to ensure that enough liquid slag continues to flow stably between the mold and the shell.

4.6.3 Replace slag

When the number of continuous pouring furnaces increases, the viscosity of the mold slag increases and the fluidity deteriorates, the slag should be replaced in time. The slag replacement operation is performed every 4 furnaces to prevent bonding and steel leakage.

By taking the above improvement measures and after more than one year of production practice, the leakage accident of low alloy steel bonding has been effectively suppressed, and the breakage rate has been reduced from 1.5% from January to July 2007 to 0.15% in 2008.

5 Conclusion

1) Analysis of the formation mechanism of bonded steel breakouts based on the characteristics of low alloy steel. It can be confirmed that during the pouring process of slab low-alloy steel, ensuring that the meniscus of the mold is continuously lubricated by sufficient liquid slag with good fluidity is the key problem in solving the bonding leakage of slab low-alloy steel.

2) The physical and chemical properties of the mold powder, especially the moisture content, are directly related to whether the mold powder can form a good three-layer structure in the crystallizer. The moisture content of the mold powder should be strictly controlled at ≤0.5%. Ensure that the mold slag has a liquid slag layer thickness of 10-12mm during the pouring process, and can continuously and stably flow into the space between the shell and the crystallizer wall.

3) When the performance of the mold powder is stable and the process and equipment conditions remain unchanged, operation is the biggest influencing factor in the occurrence of bonded steel breakouts. The correct operation method of adding and removing slag, timely understanding of the slag condition in the crystallizer, timely replacement of slag, and stable control of the liquid level in the crystallizer are all crucial.

4) When pouring Q345 with a large cross-section of 200mm×1520mm, under the existing nozzle size, the appropriate nozzle insertion depth is 90~110mm, and the bonding probability increases when it reaches 120mm or more.

5) The precision of the mold and the accuracy and stability of the mold vibration, as well as the appropriate molten steel temperature, stable pulling speed and smooth liquid level, are not only the prerequisite for avoiding steel breakouts, but also the guarantee for the stable and smooth continuous casting production. Steel accidents have been effectively suppressed, with the steel breakout rate reduced from 1.5% in January to July 2007 to 0.15% in 2008.

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