This article analyzes the influence of the flow rate, temperature, flow rate and water quality of cooling water used in continuous casting mold copper tubes on the life of steel pipes. Relevant factors were identified and corresponding countermeasures were proposed to significantly increase the life of the crystallizer copper tube.
Keywords: crystallizer copper tube; lifespan; scale
The crystallizer is a key component in continuous casting equipment and is called the heart of the continuous casting machine. The cooling conditions play a very important role in the service life of the mold copper tube, the production capacity of the continuous casting machine and the quality of the slab. The crystallizer assembly of the billet continuous casting machine is generally composed of seamless arc-shaped copper tubes, molten steel jackets, jackets and foot rollers. Since the No. 1 ROKOP continuous casting machine of the Xinyang Iron and Steel Company’s steelmaking plant was put into operation in 1997, its production capacity and operating level have increased year by year. In 1999, the annual output of single-strand slabs reached 80,000 tons. However, it has also brought about a series of problems, among which the problem of low service life of the crystallizer copper tube is very prominent. For this reason, investigations and studies have been carried out to find technical measures to improve the service life of the crystallizer copper tube.
Basic information introduction
Relevant technical parameters
Table 1 Relevant technical parameters of the old ROKOP continuous casting machine No. 1
|ROKOP two machines and two streams
|Water seam width/mm
|Copper tube length/mm
|Design capacity/10,000 tons/year
|Copper pipe material
|TP2 (phosphorus deoxidized copper)
|Pipe wall thickness/mm
Main problems before tackling the problem
(1) When abnormal steel leakage occurs in the casting machine or continuous cracks occur on the surface of the cast slab, new copper pipes can only be replaced, resulting in a low service life of the copper pipes and an increase in the cost per ton of steel. At the same time, due to frequent replacement, the operating rate of the casting machine is reduced and the labor intensity of the operator is increased. Table 2 shows the consumption and service life of copper pipes from January to April 1999.
Table 2 Copper pipe consumption and average service life records
|Number of copper tubes used
|Average service life t/piece
(2) During the steel pouring process, it is difficult to demould the billet shell in some copper tubes, and in severe cases, the billet shell is broken.
(3) The cold surface of the copper tube is severely scaled, and even after the crystallizer is replaced, the cold surface of the copper tube is completely covered with a layer of scale. The above problems have seriously restricted the smooth flow of continuous casting production and increased the amount of cast blank waste. According to statistics, from January to April 1999, the steel breakout rate was as high as 6.9%, and the cracked waste billet was 248t.
Factors affecting the service life of copper pipes
After analyzing a large number of abnormally replaced copper tubes, in addition to the quality problems of the copper tubes themselves, that is, the phosphorus content in the copper tubes was too low, which failed to significantly increase their recrystallization temperature, and many factors related to the cooling water were also found.
Cooling water quality
Scale has a great impact on the usage and service life of copper pipes. First, it directly changes the working surface temperature of the copper pipe. This article analyzes the components of scale scraped off the cold side of copper pipes and finds that the main components of scale are insoluble salts (CaCO₃, CaSO₄) and oxide precipitates in water. Its thermal conductivity is very poor. For example, the thermal conductivity of CaCO₃ is 4.605W/(M·k), which is much lower than Cu’s thermal conductivity of 1649.6W/(M·k). Due to uneven heat flow distribution on the crystallizer wall, scale everywhere The thickness is also different. The heat flux on the hot surface of the crystallizer is large, located at the meniscus of the molten steel, while the highest temperature on the tube wall is approximately 20mm below the meniscus of the molten steel. The peak temperature of the hot surface of the crystallizer that was just replaced is about 160°C, which is far lower than the softening starting temperature of phosphorus deoxidized copper, the material of the crystallizer, which is 250°C; the peak temperature of the cold surface of the crystallizer is less than 80°C, which will not cause the cooling water to boil, indicating that the crystallizer The heat transfer state is good. The calculation results of the heat conduction of the crystallizer copper tube show that when the scale thickness reaches more than 0.6mm, the temperature of the copper tube working surface has reached 400°C, which is much higher than the TP2 recrystallization temperature of 250°C, resulting in the deterioration of the copper tube. The permanent deformation changes the shape and size of the inner cavity of the copper tube, causing difficulty in demolding the billet and defects in the casting. At the same time, the deformation aggravates the peeling off of Cr plating, and even the copper tube is scrapped. In order to avoid this situation, the cooling water of the crystallizer should be soft water to eliminate scale and maintain good heat transfer status of the crystallizer. Secondly, scale reduces the heat transfer effect of the crystallizer and increases the risk of steel leakage. After analysis, we found out the reasons for scaling, including the following two points: First, the hardness of circulating water is high. Second, the circulating water dosing treatment effect is poor. Crystallizer cooling requires high quality water with a hardness of ≤1.6 mg/L. However, the hardness of new water added in actual production has reached 1.8 mg/L, and the hardness of circulating water is as high as 2.2 to 2.5 mg/L. Moreover, the 508 type scale and corrosion inhibitor added to the water during production is not ideal and cannot inhibit scale.
Cooling water flow
Before 1999, the operation rate of continuous casting machines was low, and the single flow rate was 70~80m³/h, which could basically meet the requirements. However, with the improvement of continuous casting production level and the speed of billet drawing, the cooling water flow rate appears insufficient.
Cooling water flow rate
The influence of different cooling water flow rates on the peak temperature of the hot and cold surfaces of the crystallizer wall was calculated using a mathematical model. The results are listed in Table 3.
It can be seen from the data in Table 3 that when the flow rate is 3m/s, the temperature of the cold surface of the crystallizer wall will exceed 100°C, causing the cooling water to boil, which is detrimental to the continuous casting heat transfer process; when the cooling water flow rate is 6m/s, The heat transfer of the crystallizer is good; when the cooling water flow rate is higher than 9m/s, the heat transfer of the crystallizer is better, which will be suitable for higher pulling speed.
Table 3 Peak temperatures of hot and cold surfaces of the crystallizer wall at different cooling water flow rates
|Cooling water flow rate/m/s
|Peak temperature of cold surface of crystallizer wall/℃
|Peak temperature of hot surface of crystallizer wall/℃
Cooling water temperature entering the crystallizer
An original GBNL₃-250 cooling tower has been in operation for more than two years. It is partially blocked and severely blocked. As a result, part of the hot water flowing out of the crystallizer is forced to flow directly into the drying pool. The cooling effect is poor and the temperature of the water entering the crystallizer is too high. Table 4 shows the circulating water temperature record during a certain pouring time:
Table 4 Circulating water temperature record during a certain pouring
|Water temperature entering the crystallizer
|Water temperature leaving the crystallizer
|Water temperature in the drying pool
The temperature of the circulating water entering the crystallizer is often greater than 45°C, which cannot meet the corresponding technical requirements. According to relevant information 5, when the water temperature exceeds 50°C, the minerals in the water begin to decompose, increasing the possibility of scale formation.
Measures and effects to improve the service life of copper pipes
Use high-quality copper tubes
Actively contacted the manufacturer, notified the copper pipe quality objections, recovered some economic losses, and put forward corresponding improvement suggestions, strengthened the inspection of outsourced copper pipes, and purchased high-quality copper pipes from multiple sources.
Increase cooling water volume
Increase the amount of cooling water per flow of the copper pipe to 100~110m²/h so that the heat on the copper pipe can be taken away in time to avoid an increase in pipe wall temperature caused by heat accumulation.
Improve the crystallizer water jacket
A stainless steel water jacket is used, and its inner corners are changed from right angles to rounded corners, corresponding to the outer rounded corners of the copper tube, which greatly improves the two-dimensional heat transfer effect at the corners of the crystallizer.
Add new cooling water volume
Increase the amount of new water and overflow water added to the continuous casting circulating water to reduce the hardness of the circulating water. Cover and seal the pool to prevent lime dust from falling in the factory area.
Strengthen crystallizer maintenance management
Establish a crystallizer card delivery system to minimize the impact of human factors. In addition, formulating a reasonable temperature system to ensure appropriate superheat of molten steel is also one of the beneficial measures.
Through this stage of efforts, initial results have been achieved. The average service life of copper pipes and the resulting cracked waste products have been greatly reduced. The relevant results are shown in Table 5.
After the first stage of research, the problem of scaling on the copper pipe wall has not been completely solved. After entering 2000, the second phase plan began to be implemented, a new combined 30m²/h soft water treatment device was installed, the addition of chemicals was stopped to treat circulating water, and a closed-circuit circulation of soft water was realized. A new DBNLg-350 cooling tower was installed to ensure that the cooling water inlet temperature of the crystallizer was <40°C. These two measures achieved significant results, and the research achieved the expected goals.
Table 5 Situation of the first and second phases of research
|Average service life U
|Maximum service life/v units
After the completion of the research, nearly 600,000 yuan of benefits were generated that year. If factors such as reducing water consumption, reducing steel breakage rate, and scrap volume are taken into account, the benefits will be even more significant. During the research on how to improve the service life of the crystallizer copper tube, the following conclusions were drawn:
(1) The service life of the crystallizer copper tube is mainly related to factors such as cooling water flow rate, inlet and outlet water temperature, water quality, and copper tube manufacturing quality.
(2) Scale formation on the copper pipe wall is an important reason for copper pipe deformation, difficulty in shelling, and steel leakage. Scale must be avoided. The use of soft water closed-circuit circulation is successful and has good economic and environmental benefits.
(3) Outsourced copper pipes must be strictly controlled and the quality must be up to standard.