Vigorously promote non thermal barrier and high thermal conductivity cast steel cooling walls

Vigorously promote non thermal barrier and high thermal conductivity cast steel cooling walls

Reduce the construction cost of blast furnaces and significantly extend their lifespan

Zhou Chuanlu and Jiang Hongjun from Shandong Tianming Metallurgical Equipment Co., Ltd

Abstract: The service life of cast iron cooling walls in the belly, waist, and lower part of the blast furnace cannot meet the requirements of the first generation of blast furnace service. The invention of copper cooling walls has made it possible for the blast furnace to have a service life of more than 15 years, but its high price has become a bottleneck for its promotion and application. The successful development of seamless and high thermal conductivity cast steel cooling walls has made the idea of low investment and high service life of cooling walls impossible to achieve simultaneously become a reality.

Keywords: Thermal barrier metallurgical bonding

Abstract: The iron cooling staff life is hard to meet the requirements of Generation of blast furnace life span, invention of copper cooling staff made of blast furnace life to be 15 years, but popularization is difficult with high price The steel cooling stage with no clarity, high-temperature conductivity is invented, which makes it come true that cooling stage with investment and high life

Keywords: thermal barrier metallic combination

Efficient and long-lasting blast furnace is a common goal pursued by ironmaking enterprises. The lifespan of blast furnaces in developed countries abroad is generally around 15 years, while the lifespan of most blast furnaces in China is below 10 years. After adopting advanced bottom and hearth structures, as well as high thermal conductivity and high-quality refractory materials, China's blast furnaces have basically solved the problem of a lifespan of over 15 years in terms of bottom and hearth lifespan. The short service life of blast furnace cooling walls has become a key factor restricting the longevity of blast furnaces in China.

The successful application of copper cooling walls as efficient cooling equipment has made it possible for China's blast furnace life to surpass that of developed countries. However, the high investment in copper cooling walls has limited its promotion and application, especially in small and medium-sized blast furnaces; The ferrite ductile iron cooling wall commonly used in ironmaking blast furnaces has a service life that is generally difficult to meet the requirement of about 15 years; As a non thermal barrier, high thermal conductivity cast steel cooling wall with strong thermal shock resistance, long service life, and relatively low investment, with its excellent cost-effectiveness, it will inevitably become an excellent cooling equipment chosen by domestic and foreign blast furnaces.

What is a non thermal barrier, high thermal conductivity cast steel cooling wall?

The non thermal barrier and high thermal conductivity cast steel cooling wall is a cast steel cooling wall that truly achieves metallurgical integration between the cooling water pipe and the cast steel matrix. Its high thermal conductivity is achieved through the true metallurgical combination of cooling water pipes and cast steel matrix.

1、 Renewal of the concept of cooling wall longevity

The lifespan of the bosh, waist, and lower cooling wall of the blast furnace is the key to determining the lifespan of the blast furnace body. The lower part of the blast furnace is located in the high-temperature and slag iron melting zone, and the cooling walls in these areas not only have to withstand the erosion and erosion of high-speed gas flow and liquid slag iron, but also the abrasion of high-temperature coke, resulting in extremely harsh working conditions. The initial solution to the problem is to extend the service life of refractory materials to delay the damage time of the cooling wall. However, the fact shows that whether it is silicon carbide bricks, corundum bricks, or silicon carbide bonded silicon nitride bricks, the service life of the brick lining is very short, and the protective effect on the cooling wall is not ideal. So, people's technological concepts gradually evolved

Gradually transitioning to establishing a non overheated cooling system at the bottom of the furnace. That is to improve the cooling capacity of the cooling wall, so that it has good slag hanging ability, and use slag skin to protect the cooling wall. Under normal working conditions, the actual working temperature of the cooling wall does not exceed the temperature allowed by its material; When the slag skin falls off, it can quickly form slag skin in a short period of time. In recent years, practice has made everyone fully realize that slag skin is the best protective material for cooling walls, and establishing a cooling wall without overheating system is an effective way to pursue the longevity and efficiency of blast furnaces.

2、 Characteristics of cooling walls made of different materials

1. Pure copper cooling wall

Sufficient to meet the functional requirements of establishing a non overheated cooling system. The advantages are excellent cooling capacity and thermal shock resistance, long service life, and good long-term benefits; On the other hand, it has low strength and is prone to deformation, with a cumulative service life of 4 years. Severe bending deformation can reach 50mm/m, and some even have more severe twisting deformation, with high one-time investment. This is a difficult decision for investors who hope to invest less, produce quickly, recover investment as soon as possible, and generate benefits.

2. Ferrite based ductile iron cooling wall

Compared with gray cast iron cooling walls (HT) and heat-resistant cast iron cooling walls (RTCr), ductile iron cooling walls are widely used in ironmaking blast furnaces due to their high strength, toughness, and excellent comprehensive performance. However, there are also obvious defects in the ductile iron cooling wall, which makes it difficult to establish a non overheated cooling system in the furnace belly, waist, and lower part of the furnace body:

Firstly, the thermal conductivity of ferritic ductile iron is relatively low, which is more pronounced at lower ambient temperatures. At 100 ℃, ferritic ductile iron λ= 38.69W/m · K at 400 ℃ λ= 38.14W/m · K

Low carbon cast steel [ ω (C) =0.23%] at 100 ℃ λ= 50.5W/m · K at 400 ℃ λ= 42.7W/m · K, while the thermal conductivity of TU 2 rolled copper plate at room temperature is as high as 370 W/m · K, at 200 ℃ λ= 340W/m · K. The above data reflects the thermal conductivity of ferritic ductile iron. At lower ambient working temperatures, there is still a certain gap compared to low-carbon cast steel, and a further difference compared to rolled oxygen free copper plates.

Secondly, there is a thermal barrier issue between the cooling water pipe and the cast iron matrix. For cast iron cooling walls, when there is no scale inside the pipe, the thermal resistance of the air gap inside the cast iron cooling wall accounts for about 86% of the total thermal resistance. This thermal barrier has a significant impact on the cast iron cooling wall, mainly from two aspects: first, to prevent high-temperature liquid pig iron from carburizing the surface of the steel pipe during the casting process, and to apply 0.075-0.15mm thick anti-seepage carbon coating on the outer surface of the steel pipe. The majority of the material components of this coating are insulation materials, with a thermal conductivity of about 1-2 W/m · K, such as zircon powder (ZrO2), quartz powder (SiO2), chromium oxide, silicon carbide, etc. These insulation materials will remain on the outer surface of the water pipe after the cooling wall is cast, forming a thermal barrier coating for the water pipe. Secondly, there is a 0.1-0.3mm air gap between the cooling water pipe and the cast iron cooling wall body, which cannot be avoided by traditional casting processes. This air gap layer is not only due to the different expansion coefficients of low-carbon steel and cast iron materials, but also because the steel pipe will experience linear expansion due to rapid heating when encountering liquid metal. After cooling to room temperature, it will produce the same amount of linear shrinkage. The gap between the steel pipe and the cast iron body caused by the expansion and contraction of the steel pipe itself is twice the expansion amount of the steel pipe itself; The cast iron matrix undergoes two contractions when cooled from liquid to solid at room temperature. The first is liquid shrinkage, with a liquid shrinkage rate of 0.8% for ductile iron in a confined state. The second is solid shrinkage when cooled from liquid to solid at room temperature. Due to the objective expansion and contraction between the steel pipe and the cast iron matrix, there is an air gap of 0.1-0.3mm or even larger between the cooling water pipe and the cast iron cooling wall body. This air gap cannot be eliminated without special process measures. Due to the presence of coating thermal barriers and air gap thermal barriers, the thermal conductivity of cast iron cooling walls is greatly reduced, which can also cause local overheating of the cooling walls. Local overheating causes huge thermal stress on the cooling walls, and under the frequent action of thermal alternating stress, cracks are generated on the surface of the cooling walls. The cracks continue to extend, which will ultimately lead to obvious longitudinal cracks parallel to the water pipes in the cooling walls. The crack propagation causes the wall of the cooling walls to shatter and fall off, exposing the cooling water pipes, or pulling the steel pipes to cause water leakage, ultimately leading to premature failure of the cooling walls.

Thirdly, the safe working temperature of ferritic ductile iron is 450 ℃. At temperatures below 450 ℃, pearlite in ductile iron remains stable, and above this temperature, pearlite becomes granular; If the temperature continues to rise, the volume expansion caused by graphitization is also an important reason for the growth phenomenon of ductile iron cooling walls in the later stage of service, which generates stress and cracks due to graphitization expansion and graphite oxidation. Due to the second reason mentioned above, the cooling capacity of ductile iron cooling walls is poor, causing the cooling walls to withstand high temperatures for a long time after the slag skin falls off and before rebuilding the slag skin. In the case of poor cooling of the cooling walls, this process can take up to four to five hours (while copper cooling walls only take 15 minutes). During the period of losing slag protection, the hot surface temperature of the cooling wall is usually above 400 ℃, and the short-term temperature can even reach over 1100 ℃. Some cooling walls, even when the brick lining is completely corroded, usually have a surface temperature of 600-700 ℃ during normal operation (most of the time). In a short period of time, the temperature at the same temperature measurement point can reach up to 1200 ℃. The following table shows the temperature measurement values of the 7th layer thermocouple (measurement points 1-6), ℃

The average temperature of the even number is normal, and the temperature value is short-term

1 2 3 4 5 6 1 2 3 4 5 6

5 680 740 1120 1198

6 197 182 243 205 247 230 286 205 240 196 247 293

Note: The values in the table are from February 10th to February 15th, 2001; Thermocouples 1-4 have no data.

The peak temperature in the above table can explain the loss of brick lining in this area, and the formed slag has already peeled off. The duration of this situation varies, and it can occur 1-2 times a day in a short period of time.

In the above working conditions, the working temperature of the cooling wall often exceeds (some times exceed) the stable working temperature of ductile iron by 450 ℃. Therefore, the cooling wall made of ductile iron is chosen, and its service life is difficult to meet the requirements of the first generation furnace operation.

Fourthly, ductile iron has poor thermal shock resistance. When ferrite ductile iron is repeatedly heated and cooled between 650 ℃ and 20 ℃, the number of thermal fatigue cracks generated between the two holes of the flat sample is 200-500 times. The cooling walls located in the lower part of the furnace belly, waist, and body are always in the cycle of slag generation, detachment, regeneration, and detachment. It is required that the material of the cooling walls must have good thermal shock resistance. Obviously, it is difficult to meet the requirements for the service life of the cooling walls made of ductile iron in this area.

Fifth, the elongation rate of the core structure of the ductile iron cooling wall is low. The difference in elongation between the surface and core of the ductile iron cooling wall is very large, which is caused by the spheroidization recession of thick and large ductile iron castings during the casting process. So far, there have been no reports of successfully solving the spheroidization recession of thick and large ductile iron castings. When designing ductile iron cooling walls, the elongation index for single cast test blocks is ≥ 18%, and for attached cast test blocks is ≥ 12%. However, the elongation index for sampling the core of ductile iron cooling walls is only required to be ≥ 6%. As a manufacturer, it is not easy for the elongation index of the core structure to stably reach 6% or above.

In addition, cast iron cooling walls that are often in contact with high-temperature slag iron will experience severe carburization, sulfur infiltration, and phosphorus infiltration on the surface of the cooling walls under the erosion of high carbon, sulfur, and phosphorus slag iron, leading to increased brittleness. The metallographic examination and chemical analysis of the damaged ductile iron cooling walls in sections 7, 8, and 9 of the No.4 blast furnace at Wuhan Iron and Steel Group showed that the metallographic changes and increased brittleness caused by carburization and oxidation were severe and inevitable, which was the source of cracks in the cooling walls. The carburization depth of the surface layer on the hot surface of the cooling wall is not very deep, and the deepest layer of about 15mm has been studied. As long as the internal material of the casting is good, this crack will not expand rapidly and will not have a significant impact on the service life of the entire cooling wall. However, due to the inevitable spheroidization and deterioration of the ductile iron cooling wall, the elongation of the inner layer of the cooling wall is mostly between 4% and 8%. In the core testing of the 17th cooling wall in the 4th section of the 6th blast furnace of Jiuquan Iron and Steel, which has been in operation for 3 years and 6 months, the elongation of the core structure is only 2% to 3%. The large amount of pearlite and ledeburite structure in the metallographic structure leads to a decrease in the toughness of the cooling wall. When the temperature of the cooling wall body exceeds 450 ℃, under the multiple effects of graphitization expansion, graphite ball precipitation and oxidation, alternating thermal stress and stress splitting, cracks will extend and expand, ultimately penetrating the cooling wall, dividing it into many fragments and falling off, exposing and grinding through the water pipes of the cooling wall and scrapping them.

1.3 Cast steel cooling wall

The production of blast furnace cooling walls using low-carbon cast steel or low alloy cast steel as materials is an innovation in manufacturing technology. Summarize the common failure forms of cast iron cooling walls, which are generally caused by cracks in the cooling walls, detachment of the substrate, exposure of water pipes, and leakage of cooling water pipes. The reasons are mainly due to the low working temperature, poor thermal shock resistance, and poor toughness of the core structure of ductile iron, as mentioned above. But low-carbon cast steel is different. Taking ZG200-400 as an example, although its thermal conductivity is similar to that of ductile iron, it has many advantages that ductile iron does not have: firstly, its working temperature is high. Under the cooling and strengthening effect of the cooling water pipe on the cast steel body, the high-temperature creep strength of the cooling wall body is not the main problem. Even if the working temperature is higher than the phase change working temperature of 736 ℃, theoretically speaking, as long as the working temperature of the hot surface surface is lower than the melting point temperature of the cementite at 1227 ℃, it is safe and reliable. For cast iron cooling walls, this temperature is the peak temperature that may be encountered on the surface of their hot surface in the event of poor thermal conductivity and slag peeling; The second is his strong thermal shock resistance, good impact toughness, and uniform chemical composition. For low-carbon steel, there is no concept of thermal shock frequency in its physical performance indicators under stress free conditions. In the use environment of cooling walls, there will be no cracking of the cast steel body, nor will there be any differences in surface and core microstructure properties; The third is dimensional stability, as its carbon content is very low and there is no problem of graphite precipitation and growth. Given the above advantages of low-carbon cast steel, it will not experience cracks in the cooling wall, block shedding of the cooling wall matrix structure, or exposed water pipes during use.

Manufacturing blast furnace cooling walls using low-carbon cast steel or low alloy cast steel as materials has its unique advantages:

Compared to pure copper cooling walls:

1) It also has excellent thermal shock resistance, will not crack during use, and the matrix structure will not fall off;

2) Strong resistance to deformation, its strength is much higher than that of pure copper, and its cross-sectional size is generally twice that of copper cooling walls. Its resistance to deformation is incomparable to copper cooling walls;

3) Higher cost-effectiveness, due to the cheap price of cast steel cooling walls, the unit price is about one sixth of that of copper cooling walls, with less one-time investment;

4) It has higher usage safety, and the melting point of steel is 400 ℃ higher than that of copper. Practice has proven that when the cooling water system is shut down for a certain period of time due to a malfunction, the copper cooling wall may not be able to withstand high temperatures and be scrapped, while in the same situation, the steel cooling wall will not have any problems;

5) Although the thermal conductivity is not as good as that of copper cooling walls, the cooling capacity of cast steel cooling walls can fully meet the requirements of establishing a non overheating system in blast furnaces when they are well made. Industry insiders believe that copper cooling walls have excessive cooling capacity due to their excellent thermal conductivity, but cast steel cooling walls do not exist;

6) The surface carburization of the cooling wall hot surface is inevitable due to long-term contact with high carbon content slag iron, or long-term exposure to high carbon potential and high concentration C reduction atmosphere. This carburizing effect will generate a large amount of hard and wear-resistant Fe3C cementite in the shallow surface layer (depth<15mm) of the cooling wall, which increases the hardness of the surface layer structure of the cooling wall. This effectively resists high-speed dusty gas flow and high temperature in the cast steel cooling wall

The erosion and erosion of warm coke are very beneficial.

It has a cooling wall performance ratio to ductile iron:

1) Although the thermal conductivity of cast steel is similar to that of ductile iron, the cooling ability of cast steel cooling walls is good. After special measures are taken for the cast steel cooling wall, it is possible to achieve metallurgical bonding between the water pipe and the cooling wall body, while achieving this bonding in ductile iron cooling walls is much more difficult. That is to say, traditional ductile iron cooling walls have a large air gap thermal resistance, and cast steel cooling walls can eliminate this thermal resistance

2) The cast steel cooling wall has good toughness and strong thermal shock resistance, and will not crack or fail due to the action of alternating thermal stress during use;

3) The safe operating temperature of ductile iron is 450 ℃, while cast steel can withstand higher temperatures. For example, 880 ℃ -910 ℃ is only equivalent to the annealing temperature of cast steel. This temperature is not only harmless to the cooling wall body, but also can eliminate the coarse Weinstein structure produced by casting, refine the grain size of the cast steel body, and improve its mechanical properties. Therefore, even if there is an air gap thermal resistance between the cooling wall water pipe and the body of the cast steel, during casting

In the case of poor cooling of the steel cooling wall, it can also withstand the impact of peak temperature after slag peeling, without causing significant damage to the cast steel cooling wall; It has been proven in practical use that in the case of no water leakage in the cast steel cooling wall water pipe, regardless of whether the wall temperature measured by the thermocouple is high or lower than that of the cast iron cooling wall, its service life is longer than that of the ductile iron cooling wall;

4) Stable in size, due to the low carbon content of the cast steel body, it will not produce graphite precipitation and expansion, nor will it produce cracks due to graphitization expansion;

5) High strength, under the same internal (thermal) stress, cast steel cooling walls have a longer service life than ductile iron cooling walls;

6) Don't worry about the carburization problem on the surface of the steel pipe caused by molten steel during the casting process;

7) The surface of the hot surface of the cooling wall will increase its hardness and wear resistance due to carburization during use, which is beneficial for improving the wear resistance of the edges and corners of the cooling wall;

8) Due to its good cooling ability, the slag skin has a strong regeneration ability, which will provide more effective protection for the cooling wall.

3、 The current situation and existing technical problems of cast steel cooling walls

1） The current situation of cast steel cooling walls

Due to the excellent comprehensive performance and high cost-effectiveness of cast steel cooling walls, countries around the world are committed to the research of steel cooling walls. Since the mid-1990s, China has focused on the research and manufacturing of cast steel cooling walls as a sub project of blast furnace longevity research in the Ninth Five Year Plan. Many universities, research institutions, and enterprises in China have done a lot of work and achieved some achievements. However, in the manufacturing process of cast steel cooling walls, the core difficulty is still difficult to overcome: to achieve the metallurgical combination of water pipes and the mother body while ensuring that the cooling water pipes cast into the steel mother body do not melt through. Only by achieving the metallurgical combination of water pipes and the parent material, and eliminating the thermal resistance of the cooling wall air gap, can the cooling capacity of cast steel cooling walls be significantly improved and the excellent comprehensive performance of cast steel cooling walls be fully utilized. It is also a technical bottleneck that has been difficult to break through in the manufacturing process of cast steel cooling walls for many years. Due to this difficult technical problem, cast steel cooling walls on the market now generally suffer from poor cooling capacity, easy cracking and leakage of cooling water pipes, and short service life. The specific reasons are analyzed as follows:

1. The fundamental reason for the poor cooling ability of the current cast steel cooling wall is that there is a large air gap thermal resistance between the outer surface of the steel pipe cast into the cooling wall and the parent body of the cooling wall. The air gap thermal resistance of cast steel cooling walls is much larger than that of cast iron cooling walls, which means that the gap between the steel pipe and the cast steel matrix is much larger than the gap existing in ductile iron cooling walls. that is because

1) The liquid shrinkage rate of molten steel is 1.2%, while that of ductile iron is 0.8%. The gap caused by liquid shrinkage alone is 50% higher than that of ductile iron;

2) The pouring temperature of molten steel is 200 ℃ higher than that of iron. The solidus temperature of commonly used cast steel is about 1450 ℃, while the solidus temperature of pig iron is 1153 ℃, with a difference of 297 ℃ between the two. Low carbon cast steel [ ω (C) The linear shrinkage coefficient between 20 ℃ and 600 ℃ is 14.41 × 10-6 K-1, while the linear shrinkage coefficient of ferrite ductile iron is 13.5 × 10-6 K-1. Under such a large temperature difference, the gap caused by solid linear shrinkage alone is about 121% higher than that of ductile iron; It can be seen that the gap formed by the accumulation of liquid shrinkage and solid linear shrinkage of steel is 2.71 times that of a sphere. If the gap between the ductile iron cooling wall water pipe and the wall is 0.3mm, then the gap between the cast steel cooling wall water pipe and the wall is at least 0.81mm, which is the air gap thermal resistance, which is at least 2.71 times that of the ductile iron cooling wall. Tests have shown that a 2mm air gap can cause a temperature difference of 240 ℃ between the matrix and the water pipe, resulting in a heat flow of 29W/㎡

2. Cracking and leakage of cooling water pipes are currently the main cause of failure in cast steel cooling walls. After anatomical examination, there is a common gap of 0.5-1mm between the cast steel cooling wall water pipe and the mother body, and some even larger. Considering the peak temperature of the hot surface of the existing cast steel cooling wall, which is 740 ℃ and the normal working temperature is 200 ℃, the length of the cast steel cooling wall is 1800mm. It is calculated that the expansion of the hot surface of the cast steel matrix can reach up to 21.09mm when it changes in this temperature range; The water pipes cast into the cooling wall always have cooling water flowing. When the hot surface of the cooling wall undergoes significant temperature changes, the temperature change of the cooling water pipes will not be too large under the insulation effect of the air gap thermal barrier and the cooling effect of the cooling water. That is to say, the linear size of the cooling water pipes inside the cooling wall will not change significantly. When the cooling wall body experiences a linear contraction or expansion of about 21mm, the cast steel matrix will inevitably use this strong internal stress to "hold" the cooling water pipe for 19-20mm expansion or contraction. This expansion or contraction amount is not a problem when applied in the length direction of seamless and well resilient steel pipes. However, due to the gap between the steel pipe and the cooling wall parent body, the focus of the deformation stress of the cooling wall parent body must be on the two R parts of the U-shaped water pipe and the vertical pipe of the U-shaped pipe, causing the two R parts of the U-shaped pipe to continuously withstand the repeated effects of the alternating stress, ultimately leading to the cracking and leakage of the water pipe at the two elbow parts. If the two elbows of the water pipe cast into the wall are welded, the time for the water pipe to leak will be greatly advanced. In practice, there is a record of water leakage and scrapping of cast steel cooling walls after two years of use; Even worse, the cast steel cooling wall was forced to be scrapped and dismantled due to water leakage after several months of use.

This is why cast steel cooling walls have not been widely promoted at present.

To avoid water leakage in the cooling wall water pipe, improve the thermal conductivity of the cooling wall, and form a reliable metallurgical bond between the water pipe and the cast steel matrix, is an important and only way.

2） Technical difficulties in the manufacturing of cast steel cooling walls

In the casting process of cast steel cooling walls, ensuring that the steel pipe is not melted through and achieving metallurgical bonding between the steel pipe and the cast steel matrix has been a technical challenge that has not been breakthrough both domestically and internationally.

General cast steel [ ω (C) The solidus temperature is 1450 ℃, and the liquidus temperature is 1525 ℃. To prevent hanging and forming cold shuts during the casting process, the pouring temperature of molten steel is generally above 1530 ℃. If the steel pipe is not properly protected, the huge system heat of molten steel can easily melt through the steel pipe. In order to protect the pipe from melting through, two methods are generally adopted: welding cold iron on the steel pipe and forced cooling of the steel pipe. It is expected to achieve metallurgical bonding between the steel pipe and the parent material while ensuring the integrity of the steel pipe. Years of production practice have proven that such a good idea is impossible to achieve. The reasons why neither plan can achieve metallurgical integration are analyzed as follows

1. The method of welding internal cold iron on steel pipes

Using the principle of iron thermal equilibrium in fusion internal cooling, according to the formula G cold=f v 0 ρ   Cold (M 0- M r)/M 0, calculate the weight of the inner chill, weld the inner chill or spiral inner chill on the outer wall of the steel pipe, with the purpose of

1) Absorb the heat from the total system of molten steel through internal cooling iron, and melt the cooling iron to protect the water pipe;

2) When the internal cooling iron absorbs enough heat and melts, the remaining heat of the molten steel is not much, just enough to melt a layer on the outer wall of the cooling wall water pipe, thus achieving metallurgical bonding between the cooling wall and the steel pipe. This scheme is theoretically feasible, but in practice, it must be able to ensure that the inner surface temperature of the steel pipe is below the solidus temperature of 1450 ℃, while also ensuring that the outer layer temperature of the steel pipe is above 1485 ℃. At this point, steel pipes can also be regarded as internal cold iron. Experiments have shown that only when the temperature of the internal cold iron rises to above 1485 ℃, it will fuse with the casting. Obviously, this goal cannot be achieved: firstly, in the direction of the 12mm wall thickness of the steel pipe (assuming a cooling water pipe is selected) φ 64 × 12mm), regardless of the cooling measures used for the steel pipe, the temperature gradient on the inner and outer surfaces of the steel pipe cannot exceed 35 ℃, and can only be much smaller than this temperature difference. This is determined by the property that carbon steel is a good conductor of heat. Secondly, the temperature of the overall molten steel system is difficult to accurately control, which means that the steel pipe is difficult to be accurately heated to above 1485 ℃ by the molten steel and does not exceed much. Steel pipes below 1485 ℃ will not fuse with castings, but slightly above 1525 ℃ may completely melt the steel pipes. Obviously, it is very important to accurately calculate the thermal balance during the pouring process, so that the steel pipe can be accurately heated to above 1485 ℃ by the molten steel and not exceed much. But the entire pouring system is extremely complex, with too many variable and uncontrollable factors. From the above equation, it can be seen that the calculation of thermal balance only considers the heat injected into the molten metal during the casting process and the heat absorbed by the cold iron, without considering the heat absorbed by the molding sand. But the amount of heat absorbed by the molding sand is often the key to the melting and non melting of the fusible cold iron. The amount of heat absorbed by molding sand cannot be calculated, and the sand box used during molding cannot be consistent. The type and particle size of molding sand are also different, which determines that the temperature field of the mold cavity system varies greatly. It is impossible to accurately calculate and control this temperature field. If it is a large batch of products with fixed shapes and sizes, using fixed sandboxes, molding sand, etc., theoretical calculations and multiple practical corrections can be used to find the optimal casting process parameters and accurately control them. But cooling walls are not mass-produced products, and there are usually only a few dozen cooling walls using the same drawing on a blast furnace. Regardless of cost or time considerations, it is impossible to start production of cooling walls like mass-produced products after exploring the optimal pouring process parameters. As analyzed above, even if the calculation of internal cooling iron is very accurate, molten steel can accurately heat the steel pipe to above 1485 ℃ and not much, but the wall thickness of the steel pipe is generally only about 8-12mm. Given the good thermal conductivity of carbon steel, it is difficult to maintain a temperature difference of tens of degrees on the inner and outer surfaces of the steel pipe. Theoretical and practical evidence has shown that it is unlikely to achieve metallurgical bonding between water pipes and the cooling wall body on a large scale using the method of welding cold iron for casting steel cooling walls. In actual production, in order to prevent steel pipes from being melted through and ensure the yield of cast steel cooling walls, most cooling wall manufacturers have to choose to add more cold iron to ensure the integrity of the water pipes. However, a large amount of unmelted cold iron exists between the matrix of the cast steel cooling wall and the water pipe, cutting off the body of the cooling wall and producing a large amount of Weinstein structure, which poses a hidden danger for future cracks in the cooling wall. There have been instances of cooling walls manufactured using this process leaking after six months of use.