Description：Aiming at the common problem of copper tube blackening in the mold of continuous casting machine, the influencing factors of copper tube blackening caused by the cooling water system of the mold were analyzed, and a solution to the problem was put forward in a targeted manner. The service life of the copper pipe is improved, and the quality of the billet product is also improved.
Keyword: Billet continuous casting; Mold copper tube; Blackened copper tube
Foreword
The high-temperature molten steel of about 1500 ℃ produced by steelmaking and smelting needs to be rapidly cooled by the continuous casting machine to form a billet. The mold is a key component in the continuous casting equipment and is called the heart of the continuous casting machine. The key equipment of the billet is the copper tube of the mold of the continuous casting machine. The heat transfer effect of the copper tube directly affects the quality of the billet product. Affected by various factors such as copper pipes, it is easy to appear blackening and scaling on the surface. Blackening and scaling of copper tubes will inevitably affect the heat transfer effect and affect the quality of cast billet products. By analyzing and eliminating the reasons for the blackening and scaling of the copper tubes of the crystallizer, the causes of the blackening and scaling of the copper tubes of the crystallizer are analyzed and found in practice, and measures are taken to control and slow down the blackening and scaling of the copper tube surface, so as to Meet billet cooling requirements.
Brief Introduction of Mold Copper Tube Cooling Water System
Huaigang Company has a total of 6 billet continuous casting machines. The billet section is mainly 150×150 and 200×200. The molds of the 6 continuous casting machines are cooled by open circulation of demineralized water. The water process is basically the same, that is, the pressure of qualified cooling water can reach 1.2~1.3 MPa through the water pump, and the water flow rate is different according to the flow number of the casting machine. After that, it is sent to the continuous casting machine distributor and distributed to the crystallizer copper tubes of each flow. After the cooling water passes through the copper tubes for heat exchange, the high-temperature return water enters the cooling tower to cool down with the residual pressure, and then returns to the pool. There is an automatic dosing system and a water quality testing and sampling device near the pool. The pool is equipped with a lift pump for closed-circuit cleaning and filtration, and the treated water passes through the pump again for cooling and recycling of the crystallizer. Among them, the circulating water volume of the 1# billet casting machine system is 1300m3/h, the retained water volume is about 800 m3, the water gap width of the mold copper pipe is 4 mm, the copper pipe outlet water flow rate is 127~135 m3/h, and the cooling water flow rate is 11.6~12.3 m/s , the copper pipe cooling water inlet temperature is less than 34 ℃, the water outlet temperature is 43 ℃, the number of heterotrophic bacteria in the system circulating water is controlled at less than 5×104/mL, and the indicators are strictly controlled within the standard range during the operation of the water system. Table 1 is Operational control index of copper pipe water system of 6 sets of continuous casting machines.
Table 1 Operation indicators of water quality data in soft water system
| system name | pH | Turbidity/(mg/L) | Hardness/(mg/L) | Total alkali(mg/L) | Cl-/(mg/l) | Total iron/(mg/L) | Conduc ivity/(uS/cm) | oil content/(mg/L) |
| Softened water requirement index | 7~9 | ≤10 | <20 | <300 | <150 | <1.0 | <1000 | 0 |
| Supplemental Water Requirement Index | 7~9 | ≤10 | <3 | <200 | <150 | <1.0 | <300 | 0 |
| Actual index of softened water | 8.61 | 5.37 | 6.5 | 216 | 92 | 0.65 | 853 | 0 |
Current status of mold copper tube use
Under the normal use of the copper pipes of the continuous casting machine of Huaigang Company, the technological requirement is to control the amount of passing steel between 5000 and 6000 t. The actual passing amount of copper pipes can meet the technical requirements of the process. The surface of more than 80% is smooth and clean, and the effect is good. Occasionally, blackening and scaling of copper tubes may occur. When observing the blackening and scaling of off-line copper tubes, it is found that the blackening and scaling of copper tubes and the blackened area are not the same. Not exactly the same, sometimes the same casting machine only has blackening of a single copper tube in a single flow, and it is basically regular, there is a circle of blackening on the lower half of the copper tube upwards; sometimes the same casting machine has multiple flows The copper tubes turn black one after another, and in severe cases, the whole body even turns black; sometimes only the local high-temperature area of the inverted triangle turns black. For details, see three representative pictures of blackened copper tubes in Figure 1.
Cause analysis and investigation of the blackening of the copper tube of the crystallizer
Check the operating parameters of the water system
Analyze and check the blackened and scaled copper tubes, and check the operating parameters of the cooling water system that cause the blackened and scaled copper tubes in turn; check whether the pressure and flow of the cooling water of the copper tubes meet the process requirements; check whether the water temperature is lower than 34℃, Whether the water from the accident water tower has entered the cooling water system; at the same time, check the change of the crystallizer cooling water flow rate. The investigation results show that the operating parameters of the water system are basically normal. Sometimes the copper pipe cooling water system has accidents and water enters the system with impurities; the flow rate of the copper pipe cooling water is 11.2 m/s, which does not reach the recommended flow rate (12m/s). In the above two abnormal situations, the accidental water entering the system will cause blackening and scaling of the copper pipe due to the large amount of impurities; as for the problem of small flow rate, although the large flow rate is conducive to the formation of turbulent flow, reducing the thickness of the laminar flow zone and accelerating heat exchange, Avoiding the overheating of the copper tube surface has a great impact on the effective control of the blackening of the copper tube surface. However, the cooling water flow rate of the continuous casting machine of Huaigang Company has been in this state, and the copper tube film boiling has not occurred, resulting in local overheating of the copper sleeve, and even copper However, the intermittent blackening of copper pipes in this investigation may be one of the reasons for the blackening and scaling of copper pipes due to the problem of cooling water flow rate.
Cooling water index
Check the operation process according to the requirements of crystallizer cooling water quality indicators. The pH, calcium ions, chloride ions, conductivity, and total alkali in the water are all in a qualified state. The turbidity and iron content are sometimes at the upper limit of index control, and the phosphorus content in water sometimes fluctuates. Oil was detected in the copper pipe blackening water system. Sampling and inspection of the water system that caused serious blackening of copper pipes revealed that the number of heterotrophic bacteria in the water was 4.96×104/mL, which was within the upper limit although it was not beyond the control range. Based on the above investigation results, it can be known that phosphorus, heterotrophic bacteria, oil, iron ions, etc. may be the cause of blackening and scaling of copper tubes.
Phosphorus
The cooling water system of the crystallizer adopts the treatment plan of phosphate chemicals. Due to fluctuations in the amount of chemicals added, the total phosphorus in the system water also fluctuates. Phosphorus is prone to generate calcium phosphate scale in high temperature parts of the system. Nutrients can promote the rapid reproduction of heterotrophic bacteria, which may cause blackening and scaling of copper pipes.
Heterotrophic bacteria
Heterotrophic bacteria in the cooling water can easily lead to the formation of a mucus layer on the surface of the copper tube of the crystallizer to cover the equipment and reduce the heat transfer efficiency. , reduce the pH and cause corrosion at the same time. At the same time, heterotrophic bacteria can easily convert organic matter in the cooling water system into acid, further reducing the pH value to cause corrosion. After the carbon steel in the system is corroded, more iron ions are produced in the water, and these iron ions combine After the ferric oxide passes through the surface of the copper tube, it is easy to form deposits and adhere to the surface of the copper tube to produce black scale. The scale is first likely to be produced at the highest temperature part of the inverted triangle of the copper tube. In June, when analyzing the cause of the blackening of copper tubes in Figure 1(b), the content of heterotrophic bacteria in the cooling water system of the crystallizer of three casting machines was selectively analyzed, and it was found that 1# casting machine had blackened copper tubes (Figure 2) The number of heterotrophic bacteria is 49 600/mL, and the 3# casting machine and the electric furnace No. mL, the heterotrophic bacteria content varied greatly. Judging from the analysis data.It is concluded that the main reason for the blackening of copper pipes in Figure 1(b) is caused by heterotrophic bacteria, and then the treatment plan for killing heterotrophic bacteria in water is adopted, and the side flow is used to filter the slime of heterotrophic bacteria to reduce turbidity And other measures, after the heterotrophic bacteria are killed, the blackening of the copper tube is eliminated. Figures 2 to 4 are the pictures of analyzing the number of heterotrophic bacteria at that time.
Oil control situation
The cooling water of the copper tube of the crystallizer has no oil under normal circumstances. When the reducer of the circulating water cooling tower fails, the lubricating oil leaks and enters the cooling water. After a small amount of oil enters the system water, it is quickly emulsified. After passing through the surface of the copper tube of the crystallizer, the oil is carbonized on the surface of the high-temperature copper tube, forming black scale and adhering to the surface of the copper tube. According to the difference of the oil content in the water, the amount and position of the attachment are also different. The black scale first appears on the copper tube. The triangular high temperature part is generated. In November, casting machine #1 reappeared blackened copper pipes similar to those in Figure 1(b). At that time, a rapid investigation of the number of heterotrophic bacteria was carried out, and it was found that the number of heterotrophic bacteria was almost zero, and the above-mentioned problem of heterotrophic bacteria did not occur. The water samples were analyzed for oil, and the cooling water of the crystallizer of three casting machines was selected for the detection of oil content, see Table 2. It was found that the 1# casting machine oil that caused the blackening of the copper tube had a content of 0.195 mg/L, and the 2# and 3# casting machine oil contents were all zero. The investigation found that the 1# casting machine circulating cooling tower reducer oil leaked, and then immediately took measures to remove the oil in the water , and soon the blackening of the copper tubes off the assembly line was eliminated.
Table 2 Detection of oil content in crystallizer cooling water
| sample date | Sample serial number | Test result/(mg/L) | ||
| total oil | Petro | animal and vegetable oils | ||
| 2020.11.27 | 1# machine soft water | 0.195 | 0.02 | 0.175 |
| 2020.11.27 | 2# machine soft water | NO | NO | NO |
| 2020.11.27 | 3# machine soft water | NO | NO | NO |
Turbidity and suspended solids
Most of the water deposition comes from suspended matter. Although turbidity and suspended matter are two different concepts, they are still related. When the turbidity in circulating water is high, the suspended matter will also increase. Although the turbidity of the water system does not exceed the standard during the operation, when the accident water enters the cooling water system, the content of suspended solids will increase, and the suspended solids will exceed the standard, which means that the content of slime, biological fouling, and algae in the water will change. Of course, the suspended matter will also contribute some minerals (CaCO3). The suspended matter will bond and accumulate on the surface of the copper tube passing through the crystallizer. After sintering the high-temperature copper tube, it will form black scale and adhere to the surface of the copper tube. After long-term observation, it is found that such suspended matter The effect is similar to that of Figure 1. Copper hairblack phenomenon.
Check the operation of crystallizer supporting process equipment
The blackening of the copper tube of the crystallizer is not only related to the quality of the cooling water, but also related to the material of the copper tube, the water gap, the flow rate of the cooling water and other factors. Regardless of the open system or the closed system, in the range of 0-80 °C, the corrosion rate of carbon steel increases with the increase of temperature, which is mainly because, with the increase of temperature, the dissolution of dissolved oxygen to the surface of carbon steel The rate of diffusion increases, thereby accelerating the depolarization of oxygen. The cooling effect of the copper tube is also affected by the convective heat transfer coefficient between the copper tube and the cooling water interface, and the convective coefficient is related to the flow rate of the cooling water, the width of the water gap and other factors; min billet continuous casting machine, the total length of the copper tube is only 1.0 m, the cooling time of the molten steel in the copper tube of the mold is less than 40 s, and the mold needs to take away more than 20% of the molten steel in this short period of time The heat must be required to have a sufficiently large flow rate of cooling water. After analyzing and comparing the cooling water flow rate of the three casting machines, it is found that under the same water quality, the electric furnace continuous casting machine with the highest cooling water flow rate among the six casting machines has a cooling water flow rate of 13 m/s. The copper tube is always significantly better than the other casting machines; it is obvious that the cooling water flow rate is small, which easily leads to insufficient cooling strength of the copper tube, and affects the gradual oxidation of copper in high temperature and corrosive water environments, forming black cuprous oxide, oxidized The formation of substances further promotes the decline of thermal conductivity, causing the temperature to rise, forming a vicious circle.
After the crystallizer is maintained, water pressure testing is required. The process requires that the remaining water in the water gap of the copper pipe of the crystallizer needs to be blown dry if the mold is repaired. If the residual water in the water gap of the copper pipe is not cleared, the crystallizer should be stored for more than one week Or longer, stagnant water in the water joints is likely to produce heterotrophic bacteria, rust and other attachments on the surface of copper pipes. Once such copper pipes are used online, they are prone to regular blackening and scaling similar to Figure 1.
Scale on copper pipe surface
In order to verify the cause of the blackened and scaled copper tubes, the surface attachments of the blackened and scaled copper tubes were analyzed by scanning electron microscopy. The test results are shown in Table 3. It can be seen from the analysis results of the copper tube surface attachment in Table 3 that the main component of the copper tube surface attachment is 75% of iron oxide, and the content of iron in the crystallizer surface deposit is very high, indicating that the system pipeline is heavily corroded; Burnt slime-like organic matter, although the weight percentage of organic matter is only 13%, but due to the small proportion of organic matter itself, the volume ratio of organic matter with a content of 13% in the copper tube surface attachment is already very high, which should be caused by copper. Another important factor for the blackening of copper tubes is that the number of heterotrophic bacteria in the laboratory exceeds the standard. The combination of the two can be further verified. Heterotrophic bacteria are also a major factor in the blackening of copper tubes. After the slime adheres to the copper tube, the cooling effect of the copper tube is not good, and the copper on the surface of the copper tube is oxidized and corroded to form black copper oxide deposited on the surface of the crystallizer.
Table 3 Weight percentage of copper pipe attachments
| serial number | name | wt% weight percentage |
| 1 | Fe2O3 | 75% |
| 2 | SiO2 | 4% |
| 3 | CaO | 2% |
| 4 | CuO | 2% |
| 5 | Al2O3 | 1% |
| 6 | Na2O | 1% |
| 7 | SO3 | 1% |
| 8 | CO2 | 1% |
| 9 | Burning loss at 925°C | 13% |
Improvement measures
In order to keep the copper tube of the continuous casting mold with good thermal conductivity and stabilize the quality of the cast slab, it is necessary to reduce or eliminate the formation of scale on the surface of the copper tube as much as possible. The cause of the formation of scale substances has been preliminarily identified. In practice, it is necessary to take targeted solutions to the causes of blackening and scaling of copper pipes.
First of all, ensure that the water pump, cooling tower, bypass filter, dosing device and other equipment meet the process requirements for operation, and the outlet flow and pressure of the water pump are greater than the process requirements, so as to ensure the flow rate of copper pipe cooling water; regularly check the cooling tower cooling fan, On the one hand, it prevents oil leakage, on the other hand, it ensures the cooling effect, so that the water temperature is always lower than 34°C. At the same time, it regularly maintains the cooling tower to prevent the growth of heterotrophic bacteria around; Heterotrophic growth.
Secondly, to control the growth of heterotrophic bacteria, on the one hand, strengthen the frequency of laboratory inspection of heterotrophic bacteria, and immediately kill heterotrophic bacteria when they appear; on the other hand, control the source of heterotrophic bacteria from the source of water supply.
Strengthen water quality management and control water quality indicators within the range of process requirements. In addition, gradually optimize the quality of chemicals and gradually use phosphorus-free chemicals; pay close attention to changes in total iron content in water to prevent oily substances from entering and polluting cooling water.
Conclusion
After taking targeted solutions to the causes of blackening and scaling of copper tubes, the copper tubes of continuous casting crystallizers were blackened and scaled.The problem has been effectively controlled, and there is no obvious blackening or scaling on the surface of the off-line copper pipe.
