Vigorously promote no thermal barrier, high thermal conductivity cast steel cooling stave

Vigorously promote no thermal barrier, high thermal conductivity cast steel cooling stave

Reduce the cost of blast furnace construction and greatly extend the life of the blast furnace

 

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

 

 

　　Abstract: the iron cooling stave life are hard to meet the requirements of Generation ofblast furnace life-span, invention of copper cooling stave made of blast furnace life be 15 years,but popularization is difficult with high price. The steel cooling stave with no clearance, highthermal conductivity be invented， which made it become reality that cooling stave with lowinvestment and high life.

　　Keywords: thermal barrier metallurgical combination

 

 

Efficient and long-life blast furnace is the common goal pursued by ironmaking enterprises. The lifespan of blast furnaces in foreign developed countries has generally reached about 15 years, and the lifespan of most domestic blast furnaces is less than 10 years. After adopting advanced furnace bottom and hearth structures, as well as high thermal conductivity and high-quality refractory materials, in terms of furnace bottom and hearth life, my country's blast furnaces have basically solved the problem of more than 15 years of life. The short service life of the blast furnace stave has become a realistic key factor restricting the longevity of blast furnaces in my country.

The successful application of copper stave as a high-efficiency cooling equipment has made it possible for China's blast furnace life to catch up with developed countries, but the high investment in copper stave restricts its popularization and application (especially in small and medium blast furnaces); The commonly used ferritic ductile iron cooling stave generally has a service life that is difficult to meet the requirements of about 15 years; as a thermal shock resistance, long service life, relatively low investment, no thermal barrier, high thermal conductivity cast steel cooling stave, Its excellent cost performance will inevitably become an excellent cooling equipment for blast furnaces at home and abroad.

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

　　No thermal barrier, high thermal conductivity cast steel cooling stave is a cast steel cooling stave that truly realizes the metallurgical combination of cooling water pipes and cast steel matrix. Its high thermal conductivity is achieved by the real metallurgical combination of cooling water pipe and cast steel matrix.

1. Renewal of the longevity concept of cooling stave

　　 The life of the furnace belly, the furnace waist, and the cooling stave under the furnace body is the key to determining the life of the blast furnace body. The lower part of the blast furnace is located in the high temperature and slag iron melting zone. The cooling stave of these parts not only has to withstand the erosion and erosion of high-speed gas flow and liquid slag iron, but also the abrasion of high temperature coke. The early solution to the problem is to extend the life of refractory materials to delay the damage time of the cooling stave. However, the facts show that whether it is silicon carbide bricks or corundum bricks and silicon carbide combined with silicon nitride bricks, the brick lining life is very short. , The protection effect of the cooling stave is not ideal. As a result, people’s technical ideas gradually

Gradually transformed into the establishment of a non-overheating cooling system in the lower part of the furnace body. That is to improve the cooling capacity of the cooling stave, so that it has good slagging ability, and use the slag skin to protect the cooling stave. Under normal working conditions, the actual working temperature of the cooling stave does not exceed the temperature allowed by its material; When it falls off, slag can be quickly formed in a short time. The practice in recent years has made everyone fully aware that the slag skin is the best protective material for the cooling stave. The ability to build such a cooling stave without overheating system is an effective way to pursue the longevity and high efficiency of the blast furnace.

Second, the characteristics of different materials of cooling stave

1. Pure copper cooling stave

　　 is sufficient to achieve the functional requirements for establishing a cooling system without overheating. The advantage is that it has excellent cooling capacity and thermal shock resistance, long service life, and good long-term benefits; but on the other hand, it has low strength and easy deformation. It has been in service for 4 years and severe bending deformation can reach 50mm/m. There are more serious distortions, and the one-time investment is high. This is a difficult choice for investors who wish to reduce investment, quickly output, recover their investment as soon as possible, and generate benefits.

2. Ferrite-based ductile iron cooling stave

Compared with gray cast iron cooling stave (HT) and heat-resistant cast iron cooling stave (RTCr), 　　 ductile iron cooling stave is widely used in ironmaking blast furnaces because of its high strength, high toughness and excellent comprehensive performance. However, ductile iron cooling stave also has obvious defects, which determines that it is difficult to establish a non-overheating cooling system in the furnace body, furnace waist, and lower part of the furnace body:

First, the thermal conductivity of ferritic ductile iron is low, which is more obvious at lower ambient temperatures. For example, 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%] λ=50.5W/m·K at 100°C, λ=42.7W/m·K at 400°C, while TU 2 rolled copper plate corresponds to the normal temperature The thermal conductivity is as high as 370 W/m·K, and λ=340W/m·K at 200°C. The above data reflects the thermal conductivity of ferritic ductile iron. At lower ambient operating temperatures, there is still a certain gap compared with low-carbon cast steel, and a farther difference compared with rolled oxygen-free copper plates.

Second, the thermal barrier between the cooling water pipe and the cast iron matrix. For the cast iron stave, when there is no scale in the pipe, the thermal resistance of the air gap in the cast iron stave accounts for about 86% of the total thermal resistance. This thermal barrier has a great influence on the cast iron stave, which mainly comes from two Aspects: First, in order to prevent the high temperature liquid pig iron from carburizing the surface of the steel pipe during the casting process, a 0.075-0.15mm thick anti-carburizing paint is applied to the outer surface of the steel pipe. Most of the physical material components of this coating are thermal insulation materials with a thermal conductivity of about 1 to 2 W/m·K, such as zircon powder (ZrO 2 ), quartz powder (SiO 2 ), chromium oxide, silicon carbide, etc. The insulation material will remain on the outer surface of the water pipe after the cooling stave is cast, forming a thermal barrier coating for the water pipe. The second is that there is an air gap of 0.1-0.3mm between the cooling water pipe and the cast iron stave body. This air gap layer is unavoidable in the traditional casting process. It is not only because of the different expansion coefficients of low carbon steel and cast iron, but also Moreover, when the steel pipe encounters liquid metal, it will produce linear expansion due to rapid heating. After cooling to room temperature, it will produce the same amount of linear shrinkage. The gap between the steel pipe and the cast iron matrix caused by the expansion and contraction of the steel pipe itself, It is twice the expansion of the steel pipe itself; the cast iron matrix will produce two shrinkages from liquid to cooling to normal temperature solid state, one is liquid shrinkage, the liquid shrinkage rate of ductile iron under restraint is 0.8%, and the other is from liquid to normal temperature solid The resulting solid state shrinkage. Above, because of the objective expansion and contraction of the steel pipe and the cast iron matrix, there is an air gap of 0.1-0.3mm or more between the cooling water pipe and the cast iron stave body. This air gap cannot be eliminated without special process measures. . Because of the existence of the coating thermal barrier and the air gap thermal barrier, the thermal conductivity of the cast iron cooling stave is greatly reduced, and it will also cause local overheating of the cooling stave. The local overheating causes the cooling stave to generate huge thermal stress, which is frequently affected by the thermal alternating stress. Cracks are formed on the surface of the cooling stave and the cracks continue to extend, which will eventually cause obvious longitudinal cracks in the cooling stave parallel to the water pipe. The expansion of the cracks will cause the stave wall to break and fall off, expose the cooling water pipe, or pull the steel pipe to cause water leakage. , Eventually leading to premature failure of the cooling stave.

Third, the safe working temperature of ferritic ductile iron is 450℃. Below 450℃, the pearlite in the ductile cast iron is stable. When the temperature exceeds this temperature, the pearlite will be granulated; if the temperature continues to increase, the volume expansion will be caused by graphitization. This is also the ductile iron cooling stave will grow up in the later period of service Phenomenon, an important cause of stress and cracks due to graphitization expansion and graphite oxidation. Because of the second reason mentioned above, the cooling capacity of ductile iron stave is poor, so that the stave will withstand high temperature for a long time after the slag skin falls off and before the slag skin is rebuilt. In some cases, this process may take up to four or five hours (while the copper stave only takes 15 minutes). During the period of loss of slag protection, the temperature of the hot surface of the cooling stave is usually above 400℃, and the temperature can even reach above 1100℃ in a short time. For some cooling staves, even when the brick lining is completely etched, the surface temperature of the cooling stave is generally 600-700℃ during normal operation (that is, most of the time), and the temperature at the same temperature measurement point in a short period of time will be Up to 1200℃, as shown in the table below: The measured value of the thermocouple temperature of the 7th layer (measurement points 1-6), ℃

Even number Normal temperature average value Short-term temperature value

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; there is no data for thermocouples 1-4.

 

　　 The peak temperature of the above table can be explained by the loss of the brick lining at this part, and the formed slag skin has fallen off. The period of this situation varies in length, and can occur 1-2 times a day in a short period of time.

 

Under the above working conditions, the working temperature of the cooling stave often exceeds (sometimes more than most of the time) the stable working temperature of nodular cast iron of 450℃. Therefore, the cooling stave made of nodular cast iron is selected for this part, and its service life is difficult to satisfy Required by the first generation of furnace service.

Fourth, nodular cast iron has poor thermal shock resistance. When the ferritic ductile iron is heated and cooled repeatedly between 650℃ and 20℃, the number of thermal fatigue cracks between the two holes of the flat sample is 200-500 times, which is located in the furnace belly, furnace waist, and lower part of the furnace body. The cooling stave is always in the cycle of slag skin formation-shedding-regeneration-shedding again. The material of the cooling stave must have good thermal shock resistance. Obviously, the cooling stave made of ductile iron is selected for this part. Life is difficult to meet the requirements.

Fifth, the ductile iron cooling stave core structure has low elongation. The elongation of the surface layer and core of ductile iron cooling stave is very different. This is caused by the spheroidization and recession of thick ductile iron parts during the casting process. So far, there has been no successful solution for thick ductile iron parts. Reports of spheroidization decline. When designing ductile iron stave, the elongation index of single cast test block is ≥18%, and the attached test block is ≥12%, while the elongation rate of ductile iron stave core sampling is only required to be ≥6%. As a manufacturer In other words, it is not easy for the elongation of the core structure to reach 6% or more stably.

　　 In addition, the cast iron cooling stave, which is often in contact with the high-temperature slag iron, will cause serious carburization, sulphurization and phosphorus on the surface of the cooling stave, which will increase the brittleness under the erosion of the high carbon, sulfur and phosphorus slag iron. The metallographic inspection and chemical analysis of the broken ductile iron staves of No. 4 blast furnace 7, 8, and 9 of Wuhan Iron and Steel Co., Ltd. showed that the changes in the metallographic structure and the increase in brittleness caused by such carburization and oxidation are serious and inevitable. Yes, this is the source of cracks in the cooling stave. The carburizing depth of the surface layer of the hot surface of the cooling stave is not very large. The deepest one that has been studied is about 15mm. As long as the internal material of the casting body is excellent, the crack will not expand rapidly, and it will not be important to the service life of the entire cooling stave. influences. However, due to the inevitable spheroidization and recession of ductile iron stave, the elongation rate of the inner layer of the stave is mostly 4%-8%. According to the internal test, the elongation rate of the core structure is only 2%-3%, and a large amount of pearlite and ledeburite structure in the metallographic structure causes the toughness of the cooling stave to decrease. When the wall temperature of the cooling stave exceeds 450°C, under the multiple actions of graphitization and expansion, precipitation and oxidation of graphite balls, alternating thermal stress and stress wedge, the crack will extend and expand, and finally penetrate the stave, cooling The wall was divided into many pieces and fell off, leaving the cooling wall water pipes exposed, worn through and scrapped.

1.3 Cast steel cooling stave

Using low-carbon cast steel or low-alloy cast steel as the material, the production of blast furnace stave is an innovation in process manufacturing technology. To summarize the failure modes of common cast iron cooling staves, they are generally caused by cooling stave cracks, exposed water pipes caused by the falling of the matrix, and water leakage in the cooling water pipes. The reasons are as described above, mainly due to the low working temperature of ductile iron, poor thermal shock resistance, and core Caused by the poor toughness of the organization. But low-carbon cast steel is different. Take 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. The first is its high working temperature. Under the cooling and reinforcement of the cast steel body, the high temperature creep strength of the stave body is not a major problem. Even if the working temperature is higher than the phase change working temperature of 736℃, theoretically, the working temperature of the hot surface should be lower than The melting point temperature of cementite of 1227°C is safe and reliable. For cast iron cooling staves, this temperature is the peak temperature that may be encountered on the surface of the hot surface in the case of poor heat conduction and slag peeling; the second is its strong thermal shock resistance, good impact toughness, and chemical composition Evenly. For low carbon steel, its physical performance index does not have the concept of the number of thermal shocks under unstressed conditions. Under the use environment of cooling stave, there will be no cracking of the cast steel body, and there will be no surface structure and The problem of the difference in core structure and performance; the third is the size stability, because its carbon content is very low, there is no problem of graphite precipitation and growth. In view of the above-mentioned advantages of low-carbon cast steel, during its use, there will be no cooling stave cracks, cooling stave matrix structure falling off in blocks, and water pipes being exposed.

The blast furnace cooling stave made of low carbon cast steel or low alloy cast steel has its unique advantages:

Compared with pure copper cooling stave:

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 as large as that of copper stave, and its resistance to deformation is unmatched by copper stave;

3) Higher cost performance, because the price of cast steel stave is cheap, the unit price is about one-sixth of copper stave, one-time investment is small;

4) It is safer to use. The melting point of steel is 400°C higher than that of copper. Practice has proved that when the cooling water system stops water for a certain period of time due to a fault, the copper stave may not be able to withstand the high temperature and will be scrapped, while the steel stave will not have problems under the same circumstances;

5) Although the thermal conductivity is not as good as the copper stave, the cooling capacity of the cast steel stave can fully meet the requirements of the blast furnace to establish a non-overheating system under the condition of excellent production. Some people in the industry believe that the copper stave has a problem of excessive cooling capacity due to its good thermal conductivity, but the cast steel stave does not exist;

6) Because the hot surface of the cooling stave is exposed to slag iron with high carbon content for a long time, or is in a reducing atmosphere with high carbon potential and high concentration C for a long time, the surface carburization is definitely inevitable. This carburizing effect will generate a large amount of hard and wear-resistant Fe 3 C cementite in the superficial layer (depth <15mm) of the hot surface of the stave, which will increase the hardness of the surface of the stave, which effectively resists the cast steel stave. High speed dusty gas flow and high

The erosion and abrasion of warm coke is very advantageous.

It is compared with ductile iron cooling stave:

1) Although the thermal conductivity of cast steel and ductile iron is similar, the cooling capacity of cast steel stave is good. After special measures are taken for the cast steel stave, the metallurgical combination of the water pipe and the stave body can be realized, while it is much more difficult for the ductile iron stave to achieve this combination. In other words, the traditional ductile iron stave has a large air gap thermal resistance, and the cast steel stave can eliminate this thermal resistance

2) Cast steel cooling stave has good toughness and strong thermal shock resistance, and will not crack and fail due to alternating thermal stress during use;

3) The use temperature is high. The safe use temperature of ductile iron is 450°C, while the cast steel can withstand higher temperatures. For example, 880°C-910°C is only equivalent to the annealing temperature of cast steel. This temperature not only affects the cooling stave body. In addition, it can eliminate the coarse Widmanite structure produced by casting, refine the crystal grain of the cast steel body, and improve the mechanical performance. Therefore, even if there is an air gap thermal resistance between the cast steel stave water pipe and the body, the casting

In the case of poor cooling of the steel stave, it can also withstand the impact of the peak temperature after the slag skin falls off, and will not cause too much damage to the cast steel stave; actual use has proved that there is no water leakage in the cast steel stave water pipe In the case of, no matter the wall temperature measured by the thermocouple is higher or lower than the cast iron stave, its service life is longer than the ductile iron stave;

4) The size is stable, because the carbon content of the cast steel body is very low, there will be no graphite precipitation and expansion, and no cracks due to graphitization expansion;

5) High strength. Under the same internal (thermal) stress, the cast steel stave has a longer service life than the ductile iron stave;

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

7) The surface layer of the hot surface of the cooling wall will increase the hardness and wear resistance due to carburization during use, which is very beneficial to improve the wear resistance of the corners of the cooling wall;

8) Because of the good cooling capacity, the slag skin regeneration capacity is strong, which will form a more effective protection for the cooling stave.

3. The status quo and existing technical problems of cast steel cooling stave

1) Current status of cast steel cooling stave

Because of the excellent comprehensive performance and high cost performance of cast steel staves, countries all over the world are committed to the research of steel staves. Since the mid-1990s in my country, in the Ninth Five-Year Plan’s key research projects, the country has regarded the research and manufacture of cast steel cooling staves as a sub-topic of blast furnace longevity research. Many domestic universities, research institutions, and enterprises have done a lot of work. Some achievements have been made, but in the manufacturing process of cast steel cooling staves, the core difficulty is still difficult to break through: it is to realize the metallurgical combination of water pipes and the parent body without melting the cooling water pipe cast into the steel parent body. Only to achieve the metallurgical combination of water pipe and parent body and eliminate the thermal resistance of the air gap of the cooling stave is the only way to greatly improve the cooling capacity of the cast steel cooling stave and give full play to the excellent comprehensive performance of the cast steel cooling stave. The technological bottleneck that is difficult to break through in the process. Because of this difficult technical problem, the cast steel cooling staves on the market now generally have the problems of poor cooling capacity, easy cracking of the cooling water pipe, and short service life. The specific reasons are analyzed as follows:

1. The fundamental reason for the poor cooling capacity of the existing cast steel stave: it is because there is a large air gap thermal resistance between the outer surface of the steel pipe cast into the stave and the stave matrix. Compared with the cast iron stave, the air gap thermal resistance of the cast steel stave is much larger than the latter, that is, the gap between the steel pipe and the cast steel matrix is ​​much larger than that of the ductile iron stave. This 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 molten iron. The solidus temperature of commonly used cast steel is about 1450℃, and the solidus temperature of pig iron is 1153℃. The difference between the two is 297℃. Low carbon cast steel [ω(C)=0.25%] The linear shrinkage coefficient at 20℃—600℃ is 14.41×10-6 K -1, the linear shrinkage coefficient of ferritic ductile iron at 20℃—600℃ is 13.5×10 -6 K -1, under such a large temperature difference, the gap caused by only the shrinkage of the solid line is about 121% higher than that of ductile iron; it can be seen that the gap formed by the liquid shrinkage of the steel and the cumulative shrinkage of the solid line is 2.71 times that of the sphere. If the gap between the ductile iron stave water pipe and the wall is 0.3mm, then the gap between the cast steel stave water pipe and the wall body is at least 0.81mm, which is the thermal resistance of this air gap, which is at least 2.71 times that of the ductile iron stave. Tests show that an air gap of 2mm will cause the temperature difference between the mother body and the water pipe to reach 240℃, causing a heat flow of 29W/㎡

2. The cracking and water leakage of the cooling water pipe is the main reason for the failure of the current cast steel cooling stave. After anatomical examination, there is a 0.5-1mm gap between the cast steel cooling stave water pipe and the parent body, and some are even larger. According to the existing cast steel stave, the hot surface peak temperature of the cast steel matrix is ​​740℃, and the normal working temperature is 200℃, the length of the cast steel stave is 1800mm, and the expansion of the hot surface of the cast steel matrix is ​​calculated when the cast steel matrix changes in this temperature range. Up to 21.09mm; and the water pipes cast into the cooling stave always have cooling water flowing. When the hot surface of the cooling stave undergoes a large temperature change, under the heat insulation effect of the air gap thermal barrier and the cooling action of the cooling water, the cooling The temperature of the water pipe will not change too much, that is, the linear size of the cooling water pipe in the cooling wall body will not change much. When the wall of the cooling stave produces about 21mm linear contraction or linear expansion, the cast steel matrix will inevitably "hold" the cooling water pipe by this strong internal stress to expand or contract by 19-20mm. This amount of expansion or contraction is not a big problem in the length direction of a steel pipe with good toughness without joints, but because there is a gap between the steel pipe and the stave matrix, the focal point of the deformation stress of the stave matrix must be at U The two R parts of the U-shaped water pipe and the riser of the U-shaped pipe make the two R parts of the U-shaped pipe continuously bear the repeated action of alternating stress, which will inevitably lead to the water pipe cracking and leaking at the two elbows. 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 stave after two years of use; what’s more, the cast steel stave was forced to be scrapped and dismantled due to water leakage after several months of use.

This is why the cast steel cooling stave has not been widely promoted at present.

It is important and the only way to avoid water leakage of the cooling stave water pipe, improve the thermal conductivity of the cooling stave, and form a reliable metallurgical bond between the water pipe and the cast steel matrix.

2) Technical problems in the manufacture of cast steel cooling stave

In the casting process of the cast steel cooling stave, it is a technical problem that has not been broken through before at home and abroad to ensure that the steel pipe is not melted through, but also to realize the metallurgical combination of the steel pipe and the cast steel matrix.

The solidus of [ω(C)=0.15-0.30%] of general cast steel is 1450℃ and the liquidus temperature is 1525℃. In order to prevent ladle and formation of cold barriers during the casting process, the casting temperature of molten steel is generally 1530 ℃ above. If the steel pipe is not properly protected, the huge system heat of molten steel can easily melt the steel pipe through. In order to protect the pipe from being melted through, two methods are generally used: cold iron welding on the steel pipe and forced cooling of the steel pipe. It is expected that the steel pipe can be metallurgically combined with the parent body while ensuring the integrity of the steel pipe. Years of production practice has proved that this kind of good idea is impossible to achieve. The reasons for the failure of the two solutions to achieve metallurgical integration are analyzed as follows

1. The method of welding inner cold iron on steel pipe

Using the principle of fusion internal cooling iron heat balance, according to the formula G cold = fv 0 ρ cold (M 0-M r)/ M 0, the weight of the internal cooling iron is calculated, and the internal cooling iron or spiral internal cooling iron is welded on the outer wall of the steel pipe. Yes

1) The internal cold iron absorbs the heat of the molten steel system and melts the cold iron to protect the water pipe;

2) When the inner cooling iron absorbs enough heat and is melted, the remaining heat of the molten steel is not too much or less, which just melts the outer wall of the cooling wall water pipe to realize the metallurgical combination of 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°C and the outer surface temperature of the steel pipe is above 1485°C. At this time, the steel pipe can also be regarded as an inner cooling iron. Experiments have shown that only when the temperature of the inner cooling iron rises above 1485°C, can it fuse with the casting. Obviously this goal cannot be achieved: First, in the direction of the 12mm wall thickness of the steel pipe (if the cooling water pipe is selected as φ64×12mm), no matter what cooling measures are used for the steel pipe, the temperature gradient of the inner and outer surface of the steel pipe cannot exceed 35℃. , Can only be much smaller than this temperature difference. This is determined by the properties of carbon steel as a good conductor of heat. Second, it is difficult to accurately control the temperature of the molten steel system, that is, it is difficult for the steel pipe to be accurately heated by the molten steel to above 1485℃ and not much. Steel pipes below 1485°C will not fuse with the castings, but slightly higher than 1525°C may melt all the steel pipes. Obviously, it is very important to accurately calculate the heat balance during the pouring process so that the steel pipe can be accurately heated by molten steel to a temperature above 1485°C and not too much. However, the entire pouring system is extremely complex, with too many variable and uncontrollable factors. From the above formula, we can see that the calculation of heat balance only considers the heat injected into the molten metal during the casting process, as well as the heat absorbed by the cold iron and the heat absorbed by the molding sand. no consideration. However, the amount of heat absorbed by the molding sand is often the key to the melting and non-melting of the fusible cold iron. It is impossible to calculate how much heat the molding sand absorbs. The molding sand box cannot be the same, and the type and particle size of the molding sand are different. This determines that the temperature field of the cavity system has a wide range of changes. This temperature field can be accurately calculated and controlled. Is unlikely. If it is a large-volume product, with a fixed shape and size, using a fixed sandbox, molding sand, etc., it can be calculated through theoretical calculations, and after many practical corrections, the best casting process parameters can be found and accurately controlled. But the stave is not a mass product. The stave of a blast furnace using the same drawing is usually dozens of pieces. Regardless of the cost or time, it is impossible to be like a mass product. The best pouring process is to be explored. After the parameters, start the production of cooling stave. As analyzed above, even if the internal cooling iron is calculated accurately, the 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. In view of the good thermal conductivity of carbon steel, the steel pipe It is difficult to maintain a temperature difference of tens of degrees between the inner and outer surfaces. Theory and practice have proved that it is impossible to realize a large-area metallurgical combination between the water pipe and the cooling stave body by using the cast steel stave which is poured by the method of welding cold iron. In actual production, in order to ensure that the steel pipe is not melted through and to ensure the yield of cast steel staves, generally stave manufacturers have to choose more cold iron to ensure the integrity of the water pipes. However, a large amount of unmelted chilled iron exists between the base of the cast steel stave and the water pipe, which splits the body of the stave and produces a large amount of Widmanstatten structure, leaving hidden dangers for cracks in the stave in the future. The cooling stave manufactured by this process has been leaking after half a year of use.

2. The method of passing a large amount of strong cooling medium into the steel pipe

For the stave manufactured by this process, the strength of the stave matrix is ​​better, but because of the super cooling effect of the cooling medium, the steel pipe cannot be heated by molten steel to above 1485°C. Not only can the steel pipe and the casting be fused, but also A larger gap will be formed between the cooling stave body and the steel pipe, and a larger air gap thermal barrier will be formed. Viewed from the cut end of the cooling stave, the water pipe can be easily taken out of the cooling stave by hand. This is because the temperature of the water pipe is lower under the forced cooling of the strong cooling medium during the pouring process, and the temperature of the molten steel is very high. The molten steel in contact with the low-temperature steel pipe solidifies and shells instantaneously. In the constrained state during solidification, it will shrink at a rate of 1.2%, plus the solid-state shrinkage of the cooling stave from high temperature to normal temperature, so a gap of not less than 0.5mm or even greater is produced between the steel pipe and the cast steel matrix. No metallurgical bond. This air gap layer becomes a much larger thermal resistance layer than the cast iron stave, which greatly reduces the thermal conductivity of the cast steel stave, and becomes a major obstacle to the difficulty of large-scale promotion of the cast steel stave. The measured temperature of Jinan Steel No. 1 Ironmaking 5#350m³ blast furnace in February 2001 showed that the wall temperature of the cast steel stave was much higher than that of the ductile iron stave at the same position, indicating that the cast steel stave produced by this process method The thermal resistance of the air gap formed is much larger than that of ductile iron cooling stave. This makes people deeply realize that whether a metallurgical bond can be formed between the water pipe and the cooling stave is a key factor affecting the service life of the cast steel cooling stave. At present, countries all over the world are committed to solving the problem of metallurgical bonding between the cast steel stave body and the steel pipe.

Fourth, the characteristics of high thermal conductivity cast steel cooling wall

The successful development of high thermal conductivity, no thermal barrier cast steel cooling stave has achieved a leap in the quality of cast steel cooling stave. It does not add any spiral cold iron around the water pipe, and on the premise of ensuring the integrity of the cooling water pipe, a reliable metallurgical bond is formed between the water pipe and the cast steel matrix. Because of the metallurgical combination, the air gap between the water pipe and the parent body is eliminated, the thermal conductivity of the cast steel stave is greatly improved, and the service life of the cast steel stave is greatly extended.

1. Calculation of hot surface temperature of high thermal conductivity cast steel stave

1.1 At the peak heat flux density, the hot surface temperature of the cast steel stave

1.1.1 When the thickness of the hot surface of the cooling stave is 200mm

Take the peak heat flux density of the hot surface of the stave: q=58.2 KW.m-2;

Cooling medium temperature: t j= 25℃;

Inner diameter of cooling stave water pipe: d= 40mm;

Tile thickness: b=200mm;

Half of the thickness of the cooling stave: a =100mm;

Centerline spacing of water pipes: L=200mm;

Heat transfer coefficient of water: α=2.326 W.(㎡. ℃) -1

Thermal conductivity of cast steel (at 200℃): λ=48.6 W.(m. ℃) -1;

Thermal conductivity of refractory bricks: λ s =15 W.(m. ℃) -1

Based on the above data, it can be calculated that the average temperature of the heating surface of the cooling stave is 583 ℃ when the thickness of the tiles on the hot surface of the stave reaches 200mm and the heat flux reaches the peak.

1.1.2 After the tile on the hot surface of the cooling stave is worn to 0mm (the refractory brick lining disappears)

In the harsh environment inside the furnace, the refractory embedded in the cooling stave is difficult to last. When the covered refractory bricks are worn out to expose the dovetail of the cooling stave (usually only 6 months), take the thickness of the inlaying brick b=0, other parameters are the same as 1.1.1, and calculate when the brick lining embedded on the hot surface of the stave disappears Later, when the heat flux reaches its peak, the average temperature of the heated surface of the stave is 215°C.

1.2 Under normal working conditions, the hot surface temperature of the stave

When the brick lining inlaid on the hot surface of the stave disappears, the temperature of the hot surface of the cast steel stave is calculated to be 163℃ when the stave is in a normal working state with a heat flux density of 34.9 KW.m-2. When the ductile iron stave is selected, after the brick lining of the hot surface of the stave disappears (excluding the refractory bricks in the dovetail trough), when the heat flux reaches the peak, the average temperature of the heating surface of the ductile iron stave is calculated to be 265℃. It is only 50°C higher than the cast steel stave temperature under the same conditions. From the calculation data, whether the cooling stave material is cast steel or cast iron, the average temperature of the hot surface is not much different. But in fact, the cast iron stave temperature calculated by the above formula is very different from the actual situation. The main reason is that the calculation formula does not consider the air gap between the water pipe and the wall and the thermal resistance of the coating layer. The hot surface temperature of the cast steel stave calculated by the above formula is only suitable for the metallurgical combination of the water pipe and the casting body. The successful development of high thermal conductivity, no thermal barrier cast steel cooling stave, because it has broken through the worldwide problem of metallurgical bonding between water pipes and castings, and realized the integration of the pipe wall, so it can completely solve the poor cooling of cast steel cooling stave , And the major problem of water pipes being prone to cracking and water leakage under the action of alternating stress, the cast steel stave has a qualitative leap, and the service life of the cast steel stave can be greatly extended.

 

Picture of cast steel cooling stave of Shandong Tianming Metallurgical Equipment Co., Ltd. after dissection

It can be seen from the picture: the water pipe and the cooling stave body have realized a complete metallurgical combination

 

2. Life expectancy of high thermal conductivity cast steel stave

If the copper stave achieves a service life of 15 years, because the high thermal conductivity cast steel stave has better strength, better resistance to deformation and wear resistance, it also has good thermal conductivity and slag regeneration ability, and at the same time With good thermal shock resistance, there is reason to believe that the service life of high thermal conductivity cast steel cooling stave should also be more than 15 years. No thermal barrier, high thermal conductivity cast steel cooling stave, with its good thermal conductivity, good physical and mechanical properties, low price, high cost performance, it will surely be available in large, medium and small blast furnaces at home and abroad. More and more applications.

references

1. Liu Qi: Use copper stave to extend the service life of the blast furnace. Collection of technical seminars on copper stave. 2003.5

2. Jingpeng She and Keshi She: Ways to ensure the quality and reduce cost of copper stave. Collected articles on technical seminars on copper stave. 2003.

3. Casting Handbook, Cast Iron Section. Casting Branch of Chinese Mechanical Engineering Society

4. Casting Handbook, Cast Steel Section. China Mechanical Engineering Society Casting Branch

5. Liu Quanwu. Thermal state test of blast furnace cooling stave: [Master's thesis]. Beijing: University of Science and Technology Beijing. 1997

6. Casting Manual Cast Iron. Chapter 5 Ductile Cast Iron. Foundry Branch of Chinese Mechanical Engineering Society

7. "Iron Making" Issue 3, 2004, Analysis of the Erosion Status of the No. 5 Blast Furnace Body of Jinan Iron and Steel Author: Jinan Iron and Steel Li Dawei 8. Casting Handbook, Cast Iron Section. Chapter 5, Ductile Iron. P335. Foundry Branch of China Mechanical Engineering Society

9. "Ironmaking" No. 5, 2008. Causes of damage to the cooling stave of WISCO's No. 4 blast furnace. Wuhan Iron and Steel Group. Chen Lingkun and Song Musen

10. "Ironmaking" 2009 No. 3. Investigation on the damage of the No. 6 blast furnace of Jiuquan Steel. Jiuquan Iron and Steel Group Co., Ltd. Ma Xinlin

11. L. Bout, H. Delenghe, M. Depamelare, et al. Installing Cupper Staves and Operational Practice at

Sidmar. Iron and Steel Engineer, 1999

12. Casting Handbook, Cast Steel Section P430. Casting Branch of Chinese Mechanical Engineering Society

13. Blast Furnace Ironmaking Production Technology Manual Zhou Chuandian Chief Editor Metallurgical Industry Press 2008