After the mold is installed, in order to check the sealing condition of the mold during the assembly process, it is necessary to conduct a water flow test to see whether the connection and coordination between the various parts are normal, and whether the seal is leaking. I often observe the mold installation on site, talk with workers, understand the situation of spare parts on site, watch the fouling of copper pipes, and ask the workers who install the mold on site to suppress the water intake after the mold is installed, and how much pressure to maintain the bar In some steel mills, after the mold is installed, the water is tested, the pressure of the inlet water is higher, and some press the test water pressure to 10bar, 16bar or even higher pressure levels, and worry that water leakage cannot be detected, so the pressure must be maintained For a long time, Figure 1 shows that the assembled crystallizer has been placed on the vibration table. (Picture 1, Picture 2)
Although the design of the mold is different, the heat transferred from the casting slab during the first cooling process can only be the cooling water in the narrow water gap. Under static conditions, the pressure of the water is increased to more than 12 bar. Imagine 12 bar of water. When the pressure enters the mold, it is equivalent to creating a strong hydraulic pressure in the mold. Although it seems that the pressure per unit area is not very large, the huge cavity causes a lot of destructive force. Fragile copper tubes are often damaged to a certain extent at this moment. Figure 2 shows the photos of the on-site pressing equipment and the use of a taper to test the deformation of the copper pipe during the pressing. For larger specifications of copper pipe testing, the copper pipe center deformation reaches 0.5mm when the test pressing pressure is 10-12 bar. The suppression pressure continues to rise, and this value will continue to rise. The copper tube undergoing such a large deformation cannot but cause the concern of continuous casting workers.
Pressure on the copper mould tube
Take the installation of a 180x220mm crystallizer on site in a steel plant as an example. The size of the inner and outer arcs of the lower outlet of the copper pipe is 150mm, the inner cavity distance between the two side arcs is 226mm, and the thickness of the copper pipe is assumed to be 18mm, so the outer surface area of the copper pipe is : F=((18+1.8)+(22.6+1.8))*2*83=7337cm2
Note that the unit used here is square centimeters, and the length of the copper pipe is 850mm, but the lower port is side seal and the upper port is top seal. The actual compression length is 830mm.
I asked the on-site staff and found that the pressure after the installation of the mold is 12 bar, which is 12kgf/cm2, and the hydraulic pressure of the water on the outer surface of the copper pipe is: Q=7337*12=88044kgf=88 tons
Such a fragile copper pipe needs to accept such a large forging pressure, which is equivalent to an average of nearly 22 tons of pressure on each side, and the wide-face size is subject to greater pressure. I don’t believe that copper pipes can withstand this pressure. Getting hurt.
If it is a 150mm billet copper tube, when it is pressed to 12 bar in a static state, the copper tube is subjected to 70 tons of hydraulic pressure. Because the billet copper tube has a smaller and thinner wall, it is more susceptible to damage under higher pressure. The inside of the copper pipe is a cavity. When the outside is subjected to such a large pressure, the copper pipe will inevitably be deformed in the direction of the inner cavity, restrained at both ends of the upper wire, and forged by hydraulic pressure along the entire length, causing two problems:
(1) Change of taper: Because it is a rectangular billet, the squeeze on the wide side is more serious. The static pressure can only be resisted by the copper tube wall on the narrow side of the copper tube, causing slight pressure on the narrow side copper tube Unstable bending, the wide surface of the copper tube is recessed inward, and the irregularity of the copper tube taper is increased.
(2) Spatial deformation of copper pipes: The production of standard copper pipes should require a space curve according to the radius of R10 or R8 meters. Because the copper pipe is subjected to huge water pressure during the suppression, the copper pipe is deformed, forming a curve that is difficult to describe with a model , Which caused unpredictable extra resistance to the slab descending during a cooling process.
(3) Coating loss: The plasticity of the copper tube is much higher than that of the coating. The extra lateral pressure generated by the copper tube causes plastic deformation of the copper tube. The coating with poor plasticity is first torn, causing the coating to fall off easily. In particular, the plating layer on the lower part of the copper pipe is most likely to fall off. The deformation of the copper pipe formed by excessive pressure is one of the reasons why the plating layer is easy to fall off.
From the above three points, the copper tube is deformed by the huge water pressure, the taper changes and the spatial curve change, which causes the casting billet to be subjected to additional resistance downwards. The casting billet cannot be drawn down according to the designer’s requirements. Has a certain impact on the cooling process. Although the adaptability of the slab is also very strong and the production can be maintained, it is necessary to overcome all unfavorable factors in the production of high-quality steel so that the slab can be used in the continuous casting according to the wishes of the designer and the continuous casting operator. The radius of the machine is continuously drawn down to maintain the normal and stable solid billet thickness, so as to leave the mold with a safe thickness of the cold billet. At the same time, the quality of the primary billet produced is intact. As a continuous casting engineer, it must be Keep the continuous casting machine mold at the designed level and precision, so as to create favorable conditions for the production of defect-free billets.
Why use such a high water pressure test? There is a misunderstanding in this. It is believed that the water inlet pressure during the first cooling process of the mold is about 8 bar, so the pressure of the water pressure in the narrow water gap on the copper pipe is the data of 12 bar. In order to prevent the mold seal from leaking, Therefore, after the installation of the mold is completed, the corresponding 12 bar water pressure is used for the pressure holding test. If there is no leakage, the installation is in good condition.
The water pressure of this water pressure sample is not high. I have seen it in a famous large state-owned enterprise that the upper flange bolts often cracked during the pressure test. They thought that the mold design was unreasonable. I described their situation. Very surprised, I immediately asked them what the water pressure of the pressure test was after the mold installation was completed. They answered that it was above 16 bar and the highest reached 26 bar. What do I say is the basis for selecting this pressure value? According to the requirements of the pressure vessel, the pressure of the pressure test is 1.6 times. The pressure they used on site has reached more than 20 bar. The drawings they brought to me showed me that the mold copper pipe cloth drawing requires a pressure of 12 bar. Tell them that the fragile copper pipe is not a seamless steel pipe or a pressure vessel. What can not stand the toss is that the copper pipe is not a circular structure. The copper pipes with square and rectangular cavity structures cannot be equal to the strength of the steel pipe material. Than. In the hydrostatic pressure holding test, there is a hydraulic cylinder inside the mold barrel, and the upper flange of the mold bears huge pressure, so when the water pressure continues to increase, the bolt will break when the strength limit is reached.
I installed a crystallizer with the workers on site. The workers were surprised to see that I was skilled in the installation. Could it be possible that engineers can install the crystallizer like us? I said I can design the crystallizer. Why can’t we install it? It is a laborer. It is a basic skill for engineers to install this equipment. The workers said that our steel company has never had engineers and leaders to install and disassemble molds with us. I said that your level is high, and the leaders are assured of your work, so you don’t need it.
I talked about this principle with the employees at the scene, pointed out the evil results of this pressure-holding method, and hoped that they would not use this wrong concept. (image 3)
For copper pipes with relatively large width and thickness, such as small slab products above 160*380mm, the wide surface size is large. During the pressure test of the mold, the huge pressure often causes the wide surface to deform seriously and the center of the wide surface The sealing rings are all away from the outer wall of the copper pipe, which can not play a good sealing role, causing a water leakage accident. Modern technology continues to develop, and even rectangular billets such as 165×590 use copper tube structures, that is, small slab copper tube molds, as shown in Figure 4. More attention should be paid to the pressure test of such small slab copper tubes. Excessive water pressure cannot be used for the test. If the water inlet and outlet valves installed on the mold are not opened properly, the wide-face center deformation and water leakage will occur immediately. The author and the engineers and technicians of Dashan Mould Factory have met and dealt with such things, so it has a deeper impact, and the engineers and technicians of Dashan are level.
The pressure of the copper pipe during the first cooling process
How much pressure does the mold copper tube in normal production receive? People often think that it is the mold inlet pressure. For example, the mold inlet pressure of some steel plants is 10 bar, so the outer wall of the copper pipe must withstand this pressure. In order to achieve the conditions suitable for production, the mold test pressure is increased To 12~16 bar, or even higher.
In fact, it is not. According to the waterway layout of the general steel plant, the water pressure entering the lower mouth of the mold is 10 bar, and the flow rate of a cold water in the pipeline is 3~4m/s, but it enters the water jacket and copper in the mold. After the narrow water gap formed by the outer wall of the pipe, the flow rate of the cold water is greatly increased, generally reaching 10m/s. The increase in the flow rate of water actually converts the static pressure of a cold water into dynamic pressure, and the energy is conserved. , The pressure of cold water in the narrow water gap at high flow rate is relatively small, so after this static pressure is converted into dynamic pressure, the pressure of cold water is greatly reduced.
The pressure acting on the copper pipe is basically the back pressure, that is, the return water pressure. The general return pressure is about 1 bar. This value is very small. According to this calculation, the total pressure on the copper pipe is only 7333kg. If it is this Value, don’t worry about it, after all, it’s an order of magnitude! In this case, the working external conditions of the copper tube are much better, at least there will be no major deformation.
There is a small trend now. The pressure of the first cooling water of the crystallizer continues to increase, reaching a pressure of 1 MPa. There is still an inlet pressure of 12 bar in the field observation. In fact, this is not necessary. I think that 0.8 MPa is enough, and it is completely satisfactory. The flow rate required on site meets the high flow rate requirements in the narrow water gap, so that the motor power of the pump room can be reduced, and it is unnecessary to output such high water pressure. The energy saving effect of the pump is obvious. This is to start from the design. . The current design department is blindly conservative, preferring high rather than low, so that insurance, and no responsibility, in fact, it must be carefully designed, fully considering various factors, to save money for the enterprise and the boss, and to save energy for environmental protection. Start with drops. Of course, the relationship between the flow rate of cold water in the narrow water gap and the heat taken away must consider the level of cooling water treatment. Many steel plants are difficult to meet the water treatment requirements. The meniscus of the copper pipe shows a black heart shape, indicating this A cold water did not take away the heat transferred from the copper pipe in time, and formed heat to accumulate on the surface, and the temperature was high for a long time to form cuprous oxide. In some steel plants, even if the external slotting of the copper pipe is used to increase the cooling area, if the water treatment is not in place, scaling and burning will still occur like the outer wall of the original flat copper pipe, as shown in Figures 2~3. I think that it is not necessary to groove the copper tube of the mold which is not particularly high-speed, and it is not necessary to produce high-quality special steel at a lower speed, otherwise the processing cost of the copper tube is too high. (Figure 4)
A cold water pump design principle
One cold water requires a large flow rate, such as a seven-machine seven-strand continuous casting machine, each flow requires a cold flow rate of 150m3/h for normal production, a total flow rate of 1050m3/h, and a pressure of 0.8Mpa to a cold valve station. The pumps should be arranged according to the dual-use and one-backup plan. For example, the flow of a single pump is 525m3/h, and the required flow and pressure are formed when two pumps work at the same time. Even if one pump fails, the other pump can still work , Forming a water volume of 525m3/h. Although it cannot meet the needs of normal production, it can still maintain a cold and normal heat transfer in a short time. At this time, immediately turn on the standby pump to form the flow and pressure required for normal production. The design is more reasonable. Please also note that when a pump is working, the water pressure formed must be greater than the water pressure formed by the accident water tower, so as to keep the one-way valve from being opened and the accident water not to leak. Generally speaking, the height of the accident water tower is 30 meters, and the use point of the crystallizer is 10 meters. This forms a pressure of 2 bar. Although the flow of a single pump is greatly reduced, the pressure is at least 4 bar, which can fully meet the design requirements. In the water supply design of the water pump, the one-use one-backup solution should be avoided. Once the water pump fails, it is too late to turn on the standby water pump, and the crystallizer cooling water will dry up and vaporize, forming a large amount of steam explosion and causing an accident. In production practice, even if it is a dual-use and one-backup pump arrangement, it is necessary to ensure that the standby pump can be turned on in time, or turned on in turn, and checked regularly. There must be safeguards in the system.
Crystallizer water valve operation
After the continuous casting maintenance and replacement work is completed, before informing the pump room to send water, the mold return valve should be opened, or the mold a cold water inlet and outlet valve can be opened at the same time, and then adjust the amount of cold water required in production. Remember not to close the return valve, open the inlet valve, and then gradually open the return valve. This operation will cause a high pressure of cold water to be held in the narrow water gap. At this time, the pressure of the narrow water gap is a cold inlet. The water pressure, to prevent the damage caused by this high pressure to the copper pipe, is the same as the excessively high pressure holding test after the mold is installed.
When replacing a certain stream or several streams of crystallizer online, the pump room did not turn off the pump at this time. Before lifting the crystallizer that needs to be replaced, first close a cold water inlet valve of the flow, and then close the return valve, so that no pressure will be generated. After replacing the crystallizer, first open the return valve of the flow, and then open the inlet valve. In the same way, no pressure will be generated. Pay attention to this sequence process in operation. Do not make mistakes. It seems that the relationship does not seem big. However, it has adversely affected the copper tube of the mold. (Figure 5)
I started with the Wuxi Xuefeng Great Round Continuous Casting Machine in 2005. The continuous casting machine I design and manufacture basically uses a pneumatic film valve to adjust a cold water volume. This improves the precision of the mold cooling water adjustment and is convenient for employees to operate. If it is a seven-strand continuous casting machine that closes all the water inlet and outlet valves once and then opens them again, a considerable amount of labor is required, and sometimes the strenuous operation of the gate valve is even more troublesome.
How much water pressure will be suppressed after the mold is installed?
I think that only 3 bar of water pressure is enough. It is enough to observe that there is no water leakage at the upper and lower flange seals of the mold. In fact, it is the leakage of water on the copper pipe side of the lower flange, which has no effect on the use of the mold. Remember that copper pipes are not steel pipes, let alone pressure vessels.
I saw the mold installer in the steel plant assembling the mold at the site, communicated with them, told them the truth, corrected the incorrect mold pressure holding test, in fact, our employees are very good, as long as they say something Reason, they can accept it. The distance between the cavities of the copper tube was measured with a vernier caliper after different water test pressures on site. It was found that as the water pressure increased, the distance changed. The greater the pressure, the more serious the central depression of the copper tube cavity. The test water pressure is gradually reduced, and the deformation of the copper pipe is gradually restored. After the pressure is relieved, the copper pipe is measured to return to the original cavity size, but the deformation of the copper pipe in the space is unknown.
I happened to see the production of 150mm billet with high back pressure in foreign countries, and I knew that this was intentional at the site, saying that it was to resist the static pressure of molten steel and deform the copper tube so that the solid billet shell can be close to the copper tube. Inner wall. I think this approach has more harm than good. The air gap caused by the shrinkage during the cold process is eliminated by drawing and regularity. If the copper pipe is required to deform to compensate for the air gap, it can only be at the center, and the corners of the copper pipe deform inward. Almost none, I do not agree with this approach. According to the production at that time, no abnormality was found, but it was always felt that this pressure separated the copper pipe from the inner water jacket and destroyed the conditions of the narrow water gap design.
The author has observed many scenes, and always mentioned that the coating on the lower part of the mold copper tube is severely peeled off, and there is a phenomenon of copper tube loss, as shown in Figure 6. I have seen a lot of large areas of the lower part of the coating falling off, but also accompanied by this phenomenon of copper tube meat loss. Excessive pressure on the copper tube causes the deformation of the copper tube body, while the brittle coating material has poor plasticity. In the case of copper tube compression and deformation The bottom is severely deformed, and the coating is easy to fall off during the production process. The coating near the lower mouth of the copper tube cavity is easy to fall off. In addition to the huge friction generated by the starter, there is no restricted lower mouth.
Copper pipes are easy to deform, causing damage to the combination with the coating material, or the coating itself is affected by the stress of the deformation of the copper pipe and brittle cracks. Therefore, during the on-site mold installation and pressure test, care must be taken not to increase the water pressure at will, otherwise the coating will fall off. We have to go to the manufacturer again. We should first look for the shortcomings of our own use, and then find the faults of others. The lower mouth part of the copper pipe is unconstrained, and it is easy to deform compared to the upper middle part. From the results of the pressure measurement of the copper pipe cavity on site, it is also the serious deformation of the lower mouth of the copper pipe. Figure 6 reflects the meat loss from the lower part of the round copper tube. (Image 6)
Water jacket in crystallizer
The water jacket in the crystallizer and the copper tube together form a narrow water gap. The design of this component is also very important. Because the water flows in the narrow water gap at a high speed, the water pressure has little effect on the cavity pressure of the inner water jacket. In the water inlet cavity of the device, the outer wall of the lower part of the water jacket is affected by the water inlet pressure, which also exerts a huge compression effect on the inner water jacket. Therefore, the stainless steel water jacket needs to have a certain rigidity to resist deformation. Its design and manufacture The wall thickness should not be too thin, otherwise it will cause deformation of the water jacket and destroy the uniformity of the water gap. At present, the processing method of this water jacket is gradually transitioning from drawing forming to finishing water jacket. This processing method determines that the water jacket has a considerable thickness and greatly improves the rigidity. It is a water jacket that works under higher pressure. The parts are not deformed under operating conditions. Figure 7 shows the stainless steel finishing inner water jacket designed and manufactured by Dalian Dashan 15 years ago. It uses thick-walled seamless steel pipes and the inner cavity is formed by CNC machining. This is a high-level design and manufacturing, and it has been used for many years to maintain a very high precision.