Renovation and Application of Billet Tubular copper mould tube

Abstract: The article introduces the modification and application of tubular copper mould tube on Baotou Steel’s bloom caster. It improves the surface quality of the billet, eliminates cold scars at the corners, and at the same time meets the requirements for internal quality and external dimensions of the product, improves the operating rate of the casting machine, reduces costs, and achieves good results in production.

Keywords: tubular mold; billet continuous casting machine; surface defects; copper tube deformation

The 280×380 billet is the main specification billet produced by the bloom continuous casting machine. Since it was put into production, the surface quality has been the main issue restricting production. Although surface defects such as scratches and slag pits have been overcome one by one, there has been little progress in eliminating cold scar defects at the corners of billets. Cold scars at the corners are the most serious surface quality defects, and the defect rate remains high, accounting for about 3% of the total number of heavy rail blanks. Only a grinding process can be added to reduce the impact of this defect on the quality of the finished product, which wastes huge manpower and material resources.

The corner seam at the joint of the copper plates of the combined plate mold is a key factor affecting the occurrence of cold scars. Corner seams change in a complex production environment. Often the plate seams are out of tolerance when the steel quantity is not high. At this time, the copper mould tube must be taken offline for repair, which affects the service life of the copper mould tube and increases the cost of spare parts. It also increases the downtime of stopping pouring to check the copper mould tube, which reduces the operation rate of the casting machine. In order to eliminate the occurrence of cold scars, many researches and experiments have been conducted on the original combined plate copper mould tube in terms of equipment and technology over the years, but no satisfactory results have been achieved and cold scars have not been completely eliminated.

With the development of technology and manufacturing, through development and research, China has the ability to manufacture key components of large-section tubular copper mould tubes such as 280×380 parabolic copper tubes and integral water jackets. In this way, the existing combined plate mold can be transformed into a tubular mold, which can fundamentally overcome the cold scar defect of the slab and improve the quality of the slab and the productivity of the casting machine.

Design and modification plan of tubular copper mould tube

The design and transformation requirements meet the existing production process parameters, equipment performance parameters, etc., such as the water speed and flow rate of the water seam, the liquid level detection range and intensity, the central magnetic induction intensity of the electromagnetic stirring, the inertial mass of the vibration, etc. It is guaranteed to be able to produce high-quality, defect-free cast slabs at no less than the existing drawing speed.

Assembly design and modification of 280×380 tubular copper mould tube

Based on the existing 280×380 combined plate copper mould tube, only the embedded tubular copper mould tube assembly structure is redesigned, while the support frame, foot roller assembly, automatic liquid level measurement system, etc. remain unchanged. The tubular copper mould tube assembly must ensure the installation and connection dimensions of each part, meet the requirements for centering and arc adjustment, and install the external electromagnetic stirring coil.

Copper tube design

The inner cavity shape of the modified copper tube is a multi-taper parabolic high-efficiency copper tube.

Since the copper tubes of the tubular mold are disposable, the steel passing capacity must reach more than 15,000 t/tube. After reaching the expected amount of steel, the copper pipe is directly scrapped without repair, so the copper wall thickness of the copper pipe is designed to be thin. Based on the copper plate repair standards for plate copper mould tubes, the minimum thickness from the bottom of the water seam to the hot surface of the copper plate is not less than 20mm, and the wall thickness of the copper tube is determined to be 22mm.

Reasonably control the opening size of the copper pipe and adopt an appropriate shrinkage rate of the cast billet to meet the cold billet size within the range of 280 (-2.0/+1.5) × 380 (-2.0/+1.5).

Water seam design

In order to ensure good heat transfer performance of the copper mould tube, in addition to paying attention to the material, thickness and cooling water quality of the copper wall, the flow rate of the water in the water gap is the most important. Generally, the water velocity is in the range of 6~12m/s. According to tests, increasing water speed can significantly reduce the cold surface temperature of the mold, reduce rhombus deformation and corner cracks of the slab. However, when the water speed exceeds a certain range, the heat flow increases very little as the water speed increases. When the flow speed increases from 6m/s to 12m/s, the total heat transfer coefficient only increases by 3%, but the resistance of the system increases by 4 times. Therefore, there is no need for excessive water speed.

The size of the water gap is based on the principle of ensuring that the cooling water has the required water velocity. The thickness of the copper mould tube water seam is generally: 4mm ~ 6mm. To ensure uniform cooling, the water seams should be symmetrical and even around the perimeter. The stainless steel inner water jacket adopts advanced manufacturing technology to control its deformation, and its cross-sectional error range is within 0.2mm, ensuring the uniformity of the water seam.

The water volume of the wide and narrow surfaces of the plate copper mould tube is: 1250+1650= 2900l/min; the total water gap area is: 5×23×(16×2+11×2)=6210mm²; the water speed is 7.8m/s.

The water gap design of the tubular copper mould tube is: 4±0.5 mm; the total water gap area is: (433+332)×2×4=6120mm²; the water speed is: 7.9m/s. It is consistent with the plate copper mould tube and meets the requirements.

Test and application of tubular copper mould tube

The tubular copper mould tube has passed industrial production tests, and the results show that the service life is greatly improved compared to the plate copper mould tube, with the longest online time reaching 384 hours and the highest steel passing capacity reaching more than 23,400 tons. The application of tubular copper mould tube can significantly improve the surface quality of the cast slab, reduce the surface defect rate, and completely eliminate cold scar defects. The tubular copper mould tube plays a certain role in improving the dimensional qualification rate of the cast slab. Its impact on the internal quality has no obvious change compared with the plate copper mould tube, and it can adapt to the needs of casting machine production. One year after the industrial production test, the tubular copper mould tube was fully put into use.

The only flaw in tubular copper pipes is that they deform after production and the middle of the cross-section of the copper pipe is dented inward. Data statistics show that after the deformation reaches a certain value as the amount of steel increases (usually between 1.5mm and 2.0mm), the deformation is basically stable, and the defect does not affect the outer dimensions and internal quality of the billet. This is a key factor in determining the popularization and use of tubular copper mould tubes.

Figure 1 Deformation diagram of 280×380 copper tube

Figure 2 Relationship between copper pipe deformation and steel amount

Analysis of copper tube deformation

The assembly structures of tubular copper mould tubes and plate copper mould tubes are completely different. The copper plate of the plate copper mould tube is fastened to the outer water jacket with three rows of bolts, and the two adjacent plates are also connected with bolts. The copper plate is thicker than 43mm (including the depth of the water tank) and has high strength. Therefore, the copper plate does not have bending deformation of the copper tube plate surface of the tubular mold. The copper tube of the tubular copper mould tube is positioned and installed in the water jacket by the upper and lower openings. There are no bolts on the pipe wall, so it directly bears water pressure and thermal stress. The copper pipe relies on its own strength to resist deformation. The cross-section is large, and the copper pipe wall thickness is thin (22mm). When the strength is not enough, deformation will occur. The following is a brief analysis through calculation.

The mold copper tube is subjected to complex forces during the production process, including water pressure, thermal stress, casting friction, etc. The environmental conditions are harsh and the temperature field of the copper tube changes greatly. Among them, water pressure and thermal stress are the main factors causing copper pipe deformation. To facilitate analysis, some assumptions are made about the stress on the copper pipe.

Assume that the copper mould tube is a rectangular straight copper tube without taper.

Assume that the hot surfaces in the height direction of the copper plate are all in contact with the molten steel, and the temperature of the hot surfaces is the same everywhere.

Stress and deformation of steel plates under the action of water pressure in water joints

Calculate each side of the copper plate as a rectangular plate in the thin plate. Take the wide surface of the copper tube (inner and outer arc copper walls) as an example. It is simplified as one pair of sides is fixed, the other pair is simply supported, and the continuous load is evenly distributed throughout the plate. As shown in Figure 3.

Figure 3 Schematic diagram of rectangular thin plate

Maximum deformation and maximum stress of a rectangular thin plate (on the center line of width b):

The k and f coefficients are obtained by looking up the value of a/b in the table.

f=0.4968,k=0.0273

In the formula: a——height of steel pipe, a=0.7m

b——Width of steel pipe, b=0.433m

t——Steel pipe wall thickness, t=0.022m

q——water pressure, q=0.8MPa

Calculated: σmax =154Mpa; Wmax=0.77mm

Stress and deformation of copper plate under thermal stress

Under normal water flow conditions, the operating temperature of the inner wall of the copper mould tube is 200°C~300°C, and the temperature of the outer wall does not exceed 100°C. Under special circumstances, the highest temperature can reach 500°C. The maximum hot surface temperature and the maximum cold surface temperature occur near the mold steel liquid level, where periodic thermal fluctuations occur.

Taking the wide copper wall as an example, it is regarded as a statically indeterminate beam (Fig. 4a). Considering that the structure and temperature changes of the beam are both symmetrical to the mid-span section of the beam, it is simplified to a statically indeterminate beam (Fig. 4b). T₁ is the cooling surface, T₂ is the hot surface, and the temperature changes linearly. Bending moment, deflection and stress at the middle position of the copper plate:

Figure 4 Schematic diagram of thermal stress of copper tubes

F1/2-0 means no bending deformation.

σ—M/W

In the formula: α – linear expansion coefficient of copper, α – 17.6×10-6

E—elastic modulus, E=113×109pa

I—moment of inertia, I=ah3/12

a—length of copper plate, a=0.7m

h—copper plate thickness, h=0.022m

T₂, T₁——The temperature of the inner and outer surfaces of the copper plate, take T₂-T₁=100℃ according to the data

l—copper plate width, l=0.433m

W—flexural modulus W=bh²/6

Then: M₂=0.005273×10⁶N·m

σ=93Mpa

Allowable stress and maximum allowable elastic deformation of copper plates

Silver copper Cu-Ag takes the elastic limit op=250Mpa

Maximum allowable elastic deformation:

Summary

From the above analysis, it is known that under the action of water pressure and thermal stress, the stress at the middle position of the wide copper wall is 247Mpa, which basically reaches the elastic limit. Since the stress on the copper pipe is very complicated during the production process, the copper pipe can easily exceed the elastic limit and produce permanent plastic deformation, so internal shrinkage is inevitable. The deformation of the wide copper wall on one side reaches 0.77mm, that is, the copper tube will shrink by 1.54mm in the inner and outer arc direction.

Conclusion

The transformation and application of the tubular copper mould tube on Baotou Steel’s bloom billet continuous casting machine has improved the surface quality of the billet and eliminated the cold scars on the corners. It can also meet the requirements for internal quality and external dimensions of the product and improve the operation rate of the casting machine , reduced costs and achieved good results in production.

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