Study on reducing coke ratio of blast furnace

　　Abstract: the ladle, an important tool for storing, transporting, refining and casting molten steel in the steel plant, is introduced in detail, and the key points and difficulties in the use, turnover and operation of the ladle are objectively pointed out. The main economic and technical indicators of the ladle, such as ladle age, ladle natural pouring rate, ladle bottom blowing unblocked rate, are systematically analyzed, and the methods and ideas for improving ladle use efficiency and operation quality in daily practice are shared.

　　Key words: ladle; function; practice
 

　　1. Ladle overview

　　As an important tool for steel-making enterprises to transport molten steel, ladle not only affects the safety production and balance and stability of steel-making plants, but also affects the cleanliness of molten steel and product quality. Therefore, improving the service life of ladle and ensuring its related economic and technical indicators is also an important task for steel mills.
 

　　2. Ladle age

　　Ladle is responsible for receiving, storing and transporting molten steel, refining molten steel and casting molten steel. With the continuous increase of steel output, the accelerating pace of steelmaking and the continuous development of refining technology outside the furnace, the demand for ladle life is further increased, which requires efforts on ladle masonry and service conditions.

　　2.1 ladle masonry

　　2.1.1 permanent layer pouring

　　First, a layer of fiber heat insulation board shall be pasted on the inner wall of the ladle shell, and the mold shall be installed after the mold is evenly coated with a layer of grease and covered with plastic film. The mold shall be placed stably and firmly, and then the permanent layer of tamping material shall be evenly added from all around, and the vibrator shall be used for full mixing until it is completely bleeding. The permanent layer requires one-time construction and natural curing for a certain period of time to make the water volatilize evenly. The permanent layer forms a skeleton and is baked with low fire after demoulding. When the permanent layer reaches a certain strength, it is baked with high fire to the best strength, creating good conditions for the masonry of the working layer.

　　2.1.2 masonry of working layer

　　During the masonry of the working layer, the four sides of the base brick shall be placed in parallel, the width of the surrounding tamping joint shall be 80-120 mm, the bottom covering brick shall be parallel and vertical to the base brick, and the brick joint shall be less than 1 mm. After the bottom covering masonry is completed, the joints around the base brick and the bottom covering brick shall be filled with materials, and the joints of each base brick shall be completed at one time; Before laying the first layer of ladle molten pool, lay a layer of dry materials for leveling, and then start the laying from the ladle trunnion. During the laying, use a level ruler and other tools to ensure the flatness. After each 3~4 bricks are laid, use a rubber hammer to knock tightly to reduce the brick joints; The vertical brick joints above the second layer shall be staggered, the surface of the brick shall be cleaned, and the dislocation of the upper and lower layers shall not be greater than 10 mm; The position of the joint bricks is located at both sides of the ladle trunnion that deviates 45 °. Before the joint of a ring of bricks is not made, it is allowed to make another ring at most; After each two layers of bricks are laid, the position of joint bricks shall be adjusted to the other side, and the adjacent two layers of joint bricks shall be staggered above two bricks; The size of the big end and small end of the joint brick must be greater than 1/2 of the corresponding size of the whole brick; After jointing, use special tools to pry and tighten, and fill the back seam with mud; When pouring the base brick and the surrounding of the bottom with self flowing material, it is strictly prohibited to dump the material directly on the bottom brick; When lifting ladle bricks, mud, masonry platform and other items, it is strictly prohibited to bump against the completed working layer.

　　2.2 influence of material and process conditions on ladle life

　　2.2.1 material

　　Permanent layer castable material, ladle brick material and auxiliary material material material are also important factors affecting ladle age.

　　For example, the aluminum magnesium carbon steel ladle bricks jointly developed by luonaiyuan and Baosteel have been used in Baosteel 300 T continuous casting ladle, and the ladle age has increased from more than 20 times of using first-class high aluminum bricks to more than 80 times, up to 126 times; The 200 t ladle of Angang No. 3 steelmaking plant uses alumina magnesia carbon bricks, with an average life of 64 times and a maximum life of 73 times. According to the actual situation of the enterprise, many steel-making plants throughout the country have successively improved the castable materials of permanent layer, ladle brick materials and auxiliary materials, so that the service life of ladle can be significantly improved.

　　2.2.2 process conditions

　　The change of process conditions has a great influence on the ladle life. The factors such as steel grade, molten steel temperature and refining time have an influence on the ladle life Especially in RH, v0d, LF and other ladle refining processes, it will be greatly tested, mainly in the following aspects: melting loss of ladle lining materials caused by strong arc radiation; Slag intrusion will deteriorate the refractory surface, reduce the fire resistance, and cause melting loss and spalling; The temperature changes sharply, and the online soaking time of ladle is too long.

　　Therefore, the refractory materials must have the following characteristics: high temperature resistance, can withstand the long-term action of high-temperature molten steel without melting and softening; Heat resistant impact, can repeatedly bear the loading and unloading of molten steel without cracking and peeling; The material is resistant to the corrosion of molten slag and can withstand the corrosion of molten slag on the ladle lining; The safety factor is high, and it is required that the ladle shall not be red and steel leakage during use to ensure safe operation; The ladle lining has certain expansibility. Under the action of high temperature molten steel, the lining is closely integrated into a whole; It has sufficient high-temperature mechanical strength and can withstand the agitation and scouring of molten steel. When the ladle is in use, the temperature of the steel shell shall not be too high to prevent the deformation of the steel shell and ensure the safety of people and equipment working around the ladle. It is generally required that the temperature shall not be greater than 300 ℃. The temperature of the steel clad shall be less than the creep temperature of the clad material, generally less than 300-350 ℃ (the creep temperature of ordinary carbon steel is 300-350 ℃, and the creep temperature of alloy steel is 350~400 ℃). At present, due to the needs of smelting low carbon steel and ultra-low carbon steel and improving the service life of ladle, the working layer of ladle for out of furnace refining is mainly composed of magnesia carbon brick, which greatly improves the service life of ladle [1].
 

　　3. Ladle self opening rate

　　The ladle self opening rate is also an important quality index of the steelmaking plant. If the molten steel cannot be automatically opened, the drainage will affect the quality of molten steel, and the continuous production will be affected if the molten steel cannot be automatically opened. The material, adding method and using condition of drainage sand are the key factors affecting the self opening rate of ladle.

　　Natural ladle pouring refers to the automatic outflow of molten steel from the ladle after the ladle sliding plate is opened. The basic principle is that after the molten steel contacts with the drainage sand, the high-temperature molten steel will make the upper drainage sand form a very thin sintering layer. When the sliding plate is opened, the drainage sand under the sintering layer flows out along the water outlet under the action of gravity, making the sintering layer an empty shell. Then, under the static pressure of molten steel, the molten steel will automatically flow out of the shell.

　　3.1 selection of drainage sand material

　　At present, the drainage sand commonly used in steel mills is of high quality, and its automatic pouring rate is high. The main components are Luo ore sand, quartz sand and a certain amount of additives. After the drainage sand contacts with molten steel, it generally forms a three-layer structure of melting layer, sintering layer and loose layer. The molten layer is produced because the drainage sand is melted by the high-temperature molten steel after contacting the molten steel, which is not easy to be too much. Therefore, the sawn drainage sand is required to have relatively high-temperature resistance. The sintering layer is under the melting layer, which is also formed due to high-temperature sintering, which can prevent the continuous downward penetration of molten steel. The process of natural pouring is the process of crushing the sintering layer by the pressure generated by the self weight of molten steel. It is required that the sintering layer is not easy to be too thick and has a high sintering temperature. Loose layer requires proper particle ratio and good fluidity. It is best that the drainage sand is approximately round, with good fluidity, low thermal conductivity, small expansion value and not easy to crack. In general, when selecting the drainage sand, it is required that the drainage sand should have good dryness, fluidity, strong chemical stability, excellent heat insulation performance, moderate sintering and high fire resistance.

　　3.2 adding method of drainage sand

　　The drainage sand is added through the conduit with lifting device, which not only ensures the sand adding amount, but also ensures the positioning sand adding. There is little chance to bring the surrounding waste slag into the nozzle, which is easy to accumulate and form a cone, and the natural pouring rate is high. However, in practice, in order to reduce the amount of labor and add sand quickly, the sand filling personnel often use the throwing method, which can not be piled into a cone, and the positioning and quantification are difficult to ensure. They also often bring waste slag into the nozzle, which is an important reason for the low natural pouring rate, which can not be ignored.

　　3.3 impact of operation process

　　3.3.1 cleaning of ladle upper nozzle

　　If the cold steel inside the upper nozzle is not cleaned up, the diameter of the upper nozzle will be reduced, and the addition and falling speed of the drainage sand will be affected; The waste steel around the nozzle block is not cleaned up. When the ladle is baked online, the waste steel will adhere to the drainage sand as the ladle temperature increases, preventing the drainage sand from flowing out; After the hot repair of the ladle, the waste slag from other parts will fall into the water inlet and mix with the drainage sand to form a solidification block with high strength and block the water inlet; After the sliding plate is installed, the water inlet will be blocked if the excess mud is not cleaned up; When installing a new upper nozzle, sand is added directly without baking. The upper nozzle is not completely dry, and the combination of water and drainage sand leads to the thickening of the sintering layer. Based on the above reasons, the following points should be paid attention to in the process of ladle hot repair: the scrap around the nozzle and seat brick should be cleaned; After the installation of the sliding plate, the excess mud shall be cleaned and thoroughly purged with air duct; During sand filling, carefully check whether there is any foreign matter at the nozzle. If there is any foreign matter or cannot be confirmed, the package must be turned over. Sand filling can be carried out after the nozzle is confirmed again.

　　3.3.2 impact of production conditions

　　The ladle on-line temperature, molten steel tapping temperature and the position of the steel flow direction relative to the ladle nozzle during tapping are all factors affecting the ladle self opening rate.

　　By reforming the ladle roaster and optimizing the baking mode, the ladle on-line temperature can be guaranteed, which is beneficial to improve the self pouring rate. Due to the limitations of the properties and materials of the drainage sand, under the condition that the quality of the drainage sand cannot be improved and the thickness of the sintering layer cannot be reduced, the high-temperature steel and low-temperature steel will not be produced. The steel production process should not directly face the nozzle and the brick area, so as to reduce the scouring of the drainage sand during the steel production process. In addition, the production rhythm, the residence time of molten steel in the ladle, and the sedation time from the end of refining to the start of continuous casting will also affect the self opening rate of the ladle to varying degrees. For example, the stainless steel production line in the north area of No. 2 Steelmaking Plant of TISCO currently has a self opening rate of about 98%. However, when the molten steel is online for more than 4h30min, the drainage rate increases by 20% and the self opening rate decreases to about 78%; When the sedation time was controlled at about 30 minutes, the self opening rate was more than 98%. When the sedation time was more than 40 minutes, the drainage rate increased by 30% and the self opening rate decreased to about 75%.
 

　　4. Ladle bottom blowing unblocked rate

　　Poor bottom blowing of the ladle will prolong the treatment time of the ladle in the refining process, and also lead to secondary temperature rise or ladle switching, affecting the normal use of the ladle and product quality. Ensuring the smooth bottom blowing rate of the ladle is also an important quality index of the ladle.

　　4.1 bottom blowing leakage

　　Before the ladle is put on the line, it is necessary to carefully check whether the bottom blowing pipeline has cracks and trachoma, and whether the quick connector of the bottom blowing pipe is firm. Whether the hose between the bottom blowing pipeline and the bottom blowing core is damaged, and whether the screw thread of the hose and the bottom blowing core tailpipe is tightened, etc. regular patrol inspection and spot inspection shall also be carried out during the on-line use.

　　4.2 insufficient bottom blowing cleaning

　　When cleaning the bottom blowing operation, the bottom blowing operator shall clean up the steel slag covered on the surface of the bottom blowing core and the surrounding area, otherwise the steel slag is not cleaned up, which will block the slit channel of the bottom blowing core and affect the smooth bottom blowing of the ladle.

　　4.3 penetration of molten steel in bottom blowing slit

　　As the bottom blowing gas source is cut off, the pipeline pressure is suddenly released, and the huge pressure in the breathable brick air chamber is instantly zero, forming a negative pressure, so that the molten steel is sucked into the slit of the bottom blowing brick. When it reaches a certain depth, the molten steel forms a steel sheet, blocking the slit channel, resulting in a small bottom blowing core ventilation flow. At present, measures such as bottom blowing delay closing device and installing check valve in ladle tail pipe are mainly adopted to prevent bottom blowing slit from penetrating molten steel.

　　4.4 low ladle temperature

　　When the ladle temperature is low, especially when the ladle bottom temperature is low, when the molten steel enters the ladle, in the area where the bottom blowing core is lower than the surrounding block, the molten steel will penetrate into the bottom blowing slit and form a thick steel shell on the surface of the bottom blowing core, preventing the bottom blowing gas from flowing out and affecting the bottom blowing effect. Therefore, the ladle temperature shall not be lower than 850 ℃ when the ladle is online.

　　4.5 production rhythm

　　When the molten steel stays online or the refining process takes a long time, it will also affect the unblocked rate of ladle bottom blowing. On the contrary, if the ladle bottom blowing is not smooth and serious, the production will be interrupted, the normal production order will be disrupted, and even the safety production will be affected.
 

　　5. Conclusion

　　Increasing the ladle life can reduce the frequency of ladle up and down, reduce the labor intensity of cold repair operators, and have a strong practical significance for effectively reducing the steel-making cost and promoting the stable and smooth production. According to the current actual situation, the main work directions are as follows: first, select high-quality strategic suppliers to provide high-quality ladle bricks, auxiliary materials and accessories; Second, select a well managed and professional masonry personnel team; Third, do a good job in the baking management before the ladle is put on line, and the ladle on line temperature must meet the standard; Fourth, in the process of use, the ladle condition, ladle bottom, molten pool, slag line and ladle mouth shall be checked one by one; Fifth, pay close attention to molten steel temperature, refining treatment time and residence time of molten steel in ladle; Sixthly, the abnormal state of the ladle shall be repaired off the production line in time, and the use with injury or risk shall be resolutely prohibited. The ladle users shall overcome the fluke mentality and prevent the occurrence of ladle accidents from the source.