Factors affecting the mold life of wide and thick plate continuous casting machines and their solutions

The main factors affecting the mold life of thick slab continuous casting machine were analyzed. By taking effective solutions, the service life of the mold was greatly improved.

Keywords: cooper mould tube; taper; copper plate wear

Preface

The main equipment and technology of the No. 1 continuous caster of the No. 3 Steelmaking Plant of Jinan Iron and Steel Co., Ltd. were all imported from VAI and put into operation in March 2003. The design specifications are: 200/270 mm×(1200~2100)mm. Currently, we mainly produce casting billets with 270 mm×(1600~2100)mm specifications. The continuous casting machine adopts a straight crystallizer and applies advanced technologies such as steel breakage prediction expert system, hydraulic online width adjustment system, and hydraulic vibration.

In 2009, the continuous casting machine had a total of 45 mold replacements, and the average steel flow rate was only 48,300 tons. Among them, only 8 molds were replaced as planned, and the average steel flow rate was 85,000 tons. After statistical analysis, copper plate wear, mold taper deviation, and thermocouple failure are the main factors affecting the life of the mold.

Main factors affecting crystallizer life and improvement measures

Wear of the copper plate of the crystallizer

Cause analysis

During the continuous casting process, due to the movement of the billet and the vibration of the mold, relative friction will occur between the copper plate of the mold, the casting billet and the mold slag. Under the action of friction, the copper plate coating will gradually be consumed until the base material is exposed, causing the copper plate to infiltrate copper into the grain boundaries of the casting billet and cause star cracks. At the same time, the actual inverse taper of the narrow-side copper plate will be reduced, causing a gap between the billet shell and the copper plate, affecting the generation of the billet shell. After the weak shell comes out of the mold, under the action of the static pressure of the molten steel and mechanical stress, it is easy to produce longitudinal cracks at the corners, and in severe cases, it can cause steel leakage accidents.

The main reasons for the wear of the crystallizer copper plate are:

(1) The centering parameters of the wide-surface foot roller of the crystallizer and the copper plate are unreasonably set, and the foot roller does not play a supporting role in the casting slab.

(2) The set value of the reverse taper of the mold is too large, which increases the friction between the billet shell and the copper plate.

(3) The liquid level of the crystallizer is too high during the final pouring and capping, and the capping time is too long, resulting in high hardness of the shell. In addition, a steel pipe is used to stir the liquid level of the crystallizer when capping, which can easily cause the molten steel to overflow the billet shell and enter the gap between the mold and the mold. When the tail billet is pulled out, the mold will be scratched, causing serious wear and tear on the lower part of the mold.

Improvement measures

(1) Optimize the parameters for centering the crystallizer copper plate and the foot roller. When formulating the alignment parameters between the mold copper plate and the foot roller, fully consider the influence of factors such as the bearing clearance of the foot roller, assembly and measurement errors, and the deformation of the foot roller under the action of the static pressure of the molten steel. The foot roller has a good supporting effect on the casting slab, thereby reducing the wear of the casting slab on the copper plate.

(2) Reasonably set the inverse taper of the crystallizer. During the life cycle of the crystallizer, break the convention of a single inverse taper and gradually increase the inverse taper of the crystallizer according to the wear of the copper plate in the early, middle and end stages of the life. In this way, the actual reverse taper of the crystallizer can be guaranteed, and the wear of the copper plate can be slowed down and the service life can be extended.

(3) Optimize the final pouring and capping operation. On the premise of ensuring the safety of the capping, gradually explore the appropriate initial liquid level and capping time. At the same time, the bad operation method of using steel pipes to stir the liquid level is abandoned. After completing the operation of removing slag from the crystallizer, a cooling spring is added to the liquid level of the crystallizer, and then water is pumped for cooling. After a safe billet shell is formed, the tail billet is pulled out.

Changes in crystallizer taper

Cause analysis

It takes a long time to change the package, and the molten steel seeps down causing extrusion.

If the quick-change tundish takes too long, the billet will shrink severely due to long-term cooling in the mold, resulting in a gap of 15 to 30 mm between the billet and the copper plate of the mold. When pouring starts again, the molten steel will flow over the shell into the gap between the slab and the mold, and gradually solidify, thereby increasing the width of the slab. After the tension and leveling is restarted, the force F between the slab and the mold will increase significantly. Under the action of force F, the pressure of the hydraulic oil in the right chamber of the hydraulic cylinder will increase. When the pressure exceeds the set pressure of relief valve 2 by 22 MPa, relief valve 2 will open, and the hydraulic oil in the right chamber of the hydraulic cylinder will overflow back to the tank through relief valve 2. Under the action of the extrusion force F, the piston rod will move with the narrow copper plate of the mold, causing the taper of the mold to deviate. Figure 1 is the schematic diagram of the crystallizer width adjustment hydraulic system.

Crystallizer data distorted during transmission

Since it was put into production, the terminal box and servo valve sealing box of the crystallizer width adjustment system have never been replaced. In the long-term high temperature and humid operating environment, the terminal box and servo valve sealing box have been severely deformed and the seals have aged. Since the working environment of the crystallizer is relatively humid, water vapor will distort the data of the crystallizer width adjustment system during the collection and transmission process. The taper change of the crystallizer during normal steel drawing mostly belongs to this reason.

Figure 1 Principle diagram of the crystallizer width adjustment hydraulic system

The cylinder seal is aging and there is internal leakage

Due to the high temperature and heavy load operating environment of the continuous casting machine, the seals of the hydraulic cylinder are prone to aging and the sealing effect is reduced. During quick change of the tundish or continuous pouring of different steel types, if the force on the narrow surface of the mold increases, internal leakage will occur between the two cavities of the oil cylinder. The piston rod drives the narrow surface of the crystallizer to move, causing the taper to change.

Air entering the hydraulic system

During the preparation and maintenance of the mold hydraulic width adjustment system, improper operation may cause some air to enter the hydraulic pipeline. Because air is highly compressible, once air enters the hydraulic system, the elastic modulus of the hydraulic oil will be greatly reduced. Under the action of high-pressure oil, the air dissolved in the hydraulic oil is sharply compressed, causing the hydraulic cylinder to crawl, causing the inverse taper of the crystallizer to change.

1.2.2 Improvement measures

Optimize package replacement operation and reduce package replacement time

Check the new tundish in advance. When there is 20 t of molten steel remaining in the original tundish, gradually reduce the pulling speed. At the same time, stop the tundish baking, install the immersion nozzle for offline baking, and drive the tundish truck toward the crystallizer to the collision protection limit. When the molten steel in the original tundish drops to the critical liquid level, close the plug rod and blind plate, and drive the tundish truck to the preheating position. At the same time, another tundish truck drives to the pouring position, and then the ladle starts pouring quickly. When the molten steel in the tundish reaches 4 to 5 t, the tundish starts pouring. After the molten steel reaches the normal liquid level, the casting starts. After the optimized operation, the quick-change tundish operation time was reduced from the original 3.5 to 4 minutes to about 2.5 minutes, which greatly reduced the shrinkage of the shell.

Use “V” shaped iron sheet to prevent molten steel from seeping down

In order to prevent molten steel from seeping downward along the gap between the copper plate of the mold and the cast slab during quick change of the tundish, a thin iron sheet with a certain toughness is folded into a “V” shape and plugged into the gap between the copper plate and the cast slab. In this way, when the tundish is quickly changed, the “V” shaped iron sheet will prevent the downward penetration of molten steel, reduce the force between the casting billet and the copper plate of the mold, thereby reducing the chance of taper change.

Improve the sealing effect of each component

Update the transmission line, junction box, and servo valve sealing box of the crystallizer width adjustment system, and adopt new sealing technology to improve the sealing effect. Lay cooling air ducts on the junction box and servo valve sealing box respectively, and let the cooling air flow in when the crystallizer is working. It can not only improve the working environment of the junction box and servo valve sealing box, but also maintain positive pressure in the junction box and servo valve sealing box to prevent water vapor from entering them and minimize the impact of humid environment on data transmission.

Keep hydraulic oil clean

During the use and maintenance of the hydraulic system, prevent debris and air from entering. Set an exhaust point at the top of the crystallizer width-adjusting hydraulic system, and perform venting operations after the crystallizer is maintained. Perform a few more width adjustments before using the crystallizer to further promote the discharge of air in the hydraulic system.

Thermocouple failure

Harm and causes of thermocouple failure

The thermocouple is an important part of the crystallizer expert system. Once it fails, it will lead to the following two situations: First, the faulty thermocouple frequently issues false steel leakage alarms. After the alarm, the tension and straightening system automatically crawls, affecting the normal operation of production; Second, if the bonding point occurs at a faulty thermocouple, the faulty thermocouple cannot accurately send out a steel leakage alarm message, which will cause a steel leakage accident. The causes of thermocouple failure are as follows:

(1) There are quality problems with the thermocouple itself. During use, the performance is unstable or the thermoelectric characteristics are lost, and the temperature curve fluctuates excessively.

(2) The thermocouple assembly and sealing effect is not good, and the measured temperature after water enters has a large deviation from the actual temperature, causing the temperature curve to fluctuate.

(3) During the transmission process of thermocouple signals, data distortion occurs, causing the temperature curve to fluctuate.

Improvement measures

(1) Improve the quality of thermocouples and improve assembly methods. Carefully inspect each thermocouple spare part to strictly control quality and reduce the impact of unstable performance during use. During assembly, each thermocouple is required to be in close contact with the copper plate of the crystallizer, perpendicular to each other, and the fixing bolts must be tightened and not loosen. Use high-quality sealing gaskets and shielding sleeves to prevent water from entering the thermocouple and interference from other signals.

(2) If a single thermocouple is abnormal, the thermocouple can be temporarily shielded to prevent false alarms; by observing the changes in nearby thermocouples, determine whether bonding has occurred. If two thermocouple columns fail at the same time, the crystallizer must be replaced.

Conclusion

After adopting the above measures, the mold life of the continuous casting machine was increased from the original 48,300 tons to 76,000 tons. It is close to the planned life of the crystallizer, which greatly improves the operating rate of the casting machine, reduces accident risks and production costs, and achieves considerable economic benefits.

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