Reasons Affecting Mold Life and Improving Measures

Through the investigation and analysis of the problem of low service life of the crystallizer of the 3# continuous casting machine in the steelmaking plant, measures such as reducing the wear of the chrome plating layer of the copper tube of the mold, improving the quality of the first cold water, and reforming the fixing method of the copper tube of the mold were put forward. The service life of the crystallizer is significantly improved.

Key words: copper mould tube; wear; service life; transformation

The mold is the most important process equipment in continuous casting production. The molten steel flows from the ladle through the tundish and then flows into the mold. After being cooled and solidified by the mold, it enters the secondary cooling zone to continue cooling. The main heat of the molten steel is transferred to the copper tube of the mold through the close contact area with the copper tube of the mold, the solidified billet shell and the air gap area, and then the copper tube of the mold transfers the heat to the cooling water. The performance and service life of the crystallizer directly affect the operation rate, slab quality, and yield of the continuous casting machine, which in turn affects the production cost of the continuous casting process. The pouring section of the 3# continuous casting machine in the steelmaking plant is 150mmX150mm, mainly producing low-alloy steel and ordinary carbon steel. Recently, the off-line rate of the crystallizer has been increasing, and its service life has been significantly reduced. For this reason, investigations and research have been carried out, and effective measures to improve the service life of the crystallizer have been taken.

Reason Analysis

 Mold copper tube wear

After on-site investigation, there are three main reasons for the wear of the copper tube of the crystallizer: first, the inaccurate alignment of the foot roll of the mold causes the wear of the copper tube of the mold; The friction with the copper tube of the crystallizer leads to serious wear at the lower part of the copper tube of the mold; the third is that the back taper of the mold is too large, and the purpose of using the back taper is to reduce the thickness of the air gap and improve the cooling effect, but the back taper is too large, The drawing resistance is large, and it is easy to cause the lower part of the copper tube to wear.

Effect of crystallizer cooling water on crystallizer life

When the molten steel enters the crystallizer, its temperature and the wall temperature of the copper tube form a temperature difference of more than 1300°C. The driving force of the lateral heat transfer is relatively large. The heat of the molten steel temperature drop is taken away by the cooling water of the copper tube to maintain the thickness of the copper tube. The temperature difference is stable at a certain level, thus forming a stable cold heat transfer process. In the actual production process, some scale is often produced on the copper tube wall of the crystallizer. Due to the uneven distribution of heat flow on the wall of the crystallizer, the thickness of the scale is also different everywhere, resulting in local overheating of the copper tube, and in severe cases, it protrudes inward and deforms. The formation of scale reduces the heat transfer capacity of the crystallizer, thereby greatly reducing its service life. Different casting speeds have different requirements for crystallizer cooling water. In the actual production process, due to some reasons, the casting speed will fluctuate to a certain extent, but the strength of crystallizer cooling water will not change automatically with the fluctuation of casting speed. Yes, this change will also affect the service life of the crystallizer.

Too much stress at the meniscus of the mold copper tube

During continuous casting production, when the molten steel enters the mold, the temperature of the hot surface of the copper tube of the mold rises sharply, resulting in thermal expansion; while the temperature of the cold surface of the mold is much lower than the temperature of the hot surface, which is connected in the mold by adjustable bolts On the water jacket, the expansion is hindered; the stress between the hot and cold surfaces increases, thereby reducing the service life of the crystallizer.

Poor sealing

After on-site investigation, some crystallizers were forced to go off-line due to poor sealing and water leakage from the flange. According to the analysis, the main reasons for the poor sealing and water leakage of the flange are as follows: the sealing ring was scratched and twisted during the assembly process; the sealing ring was aging and was not replaced in time; The flange sealing ring is burned out, etc.

Improvement measures

Reduce the wear of mold copper tube

Formulate maintenance standards, improve maintenance quality, ensure that mold foot roller arc alignment accuracy meets requirements, and at the same time ensure that crystallizer back taper is within the allowable range (from theoretical analysis, crystallizer back taper should change with the change of casting speed, but in practice It is impossible in production, so it should be kept within an allowed range). In addition, build a maintenance ledger and strengthen the inspection and acceptance of crystallizers, so that every repaired crystallizer can meet the maintenance standards. At the same time, in order to avoid the deviation of the dummy spindle, the maintenance of the dummy rod centering device is increased.

Reduce fouling on crystallizer wall

After analysis, it was found that the cause of scaling was mainly due to the high hardness of circulating water. The crystallizer is a heat exchanger with high heat load. In order to prevent the heat transfer of the crystallizer from being affected by the precipitation of carbonate precipitation due to high water temperature, the water needs to be softened. Control the hardness of water within 20mg/L, and control the turbidity of water quality within 50mg/L. The water quality is greatly improved, and the scale phenomenon on the outer wall of the copper pipe and the water jacket is reduced. Strictly operate in accordance with standardized operations to reduce fluctuations in pulling speed.

Modification of mold copper tube fixing method

The mold copper pipe and inner water jacket of the 3# continuous casting machine are fixed by the water seam adjustment bolts on the inner water jacket of the mold. Before the transformation, the fixing positions of the inner and outer arc adjustment bolts at the upper opening of the copper pipe were all at the meniscus of the copper pipe (Fig. 1).

Figure 1 Adjustable bolt fixing form before modification

The copper tube here has a high heat flux density, concentrated heat, and the largest thermal expansion deformation, which is likely to cause stress concentration, increase the drawing resistance, and aggravate the wear of the chrome plating layer of the copper tube. In order to avoid stress concentration here, it is necessary to stagger the positions of the inner and outer arc water seam adjustment bolts, and move the inner arc surface adjustment bolts downward by 150mm (Figure 2).

Figure 2 Adjustable bolt fixing form after modification

Strengthen supervision and management

Formulate the mold maintenance and management system, implement every part of the mold maintenance to everyone, so that everyone is responsible, so as to improve the quality of mold maintenance and prevent unqualified molds from going online; the aging sealing ring must be replaced in time, Avoid water leakage from the crystallizer; standardize the process operation, requiring operators to operate in strict accordance with the process regulations, and control the height of the liquid steel level in the crystallizer.

Conclusion

After the implementation of the above measures, the amount of steel passing through the copper tube of the 3# continuous casting machine mold has increased from 8000t to 12000t, and the online use time has been greatly improved, which has greatly improved the service life of the mold; at the same time, it has saved equipment maintenance costs and costs, and reduced The steel breakout accident caused by the crystallizer was eliminated.

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