Aiming at the defects in the design of the sliding gate mechanism of the 130t ladle and the quality issues of the slide plate. According to the process and technical conditions of the production site, the surface pressure load mode, sliding stroke, and sliding plate installation method of the sliding nozzle mechanism are designed and modified, the performance of the sliding nozzle refractory material is improved, and the refining process is optimized. Comprehensive technology is adopted to realize the continuous use of 4 furnaces of the slide plate.
Keywords: ladle; sliding nozzle; slide plate; joint technology
With the development of fast-paced and continuous production of steelmaking and the need to continuously reduce costs, ladle turnover must be accelerated. In order to achieve rapid turnover of ladles, it is necessary to realize the continuous use of multiple furnaces of the slide plate. Laigang Yinshan Section Steelmaking Plant was completed and put into operation in July 2004. The current main process equipment is: 3 120t top and bottom double-blown converters, 3 120tLF refining furnaces, 2 130tRH refining furnaces, 3 slab continuous casters and One special-shaped billet continuous casting machine has formed a 120t top-bottom double-blown converter-LF(RH) refining-continuous casting production process. With the acceleration of the pace of steelmaking and the development of steel types such as X70 and J55, increasingly stringent requirements have been placed on the sliding nozzle of the ladle. Especially with the application of molten steel calcium treatment technology, the original sliding nozzle mechanism and refractory materials used in ladles can no longer meet the needs of production. To this end, learn from domestic and foreign advanced technologies to carry out technical transformation, realize the multi-furnace use of slide gate plate, and at the same time, meet the performance requirements of slide gate plate for the production of low-carbon and low-silicon steel.
Analysis of the main factors affecting the joint use of slide gate plate
Main factors affecting the joint use of slide gate plate
The original design of the 130t ladle was the YNHK-4D ladle sliding nozzle mechanism produced by a certain company. The aluminum carbon slide was fired. The main problems were as follows, which affected the joint use of the slide.
1)The sliding gate mechanism has poor rigidity and is easily deformed in high temperature environments, which reduces the safety and reliability of the mechanism and is prone to steel penetration accidents. 2) In a high temperature environment, the pressure on the sliding surface provided by the spring is unstable. The YNHK-4D ladle sliding nozzle mechanism provides a pressure point on the slide plate surface that continuously changes as the nozzle opens and closes, reflecting that the stress around the nozzle hole also changes constantly, making it easy to cause accidents such as steel penetration between the slide plates and loss of flow control. 3) Sliding nozzle bricks have poor resistance to the erosion of high-temperature molten steel, and the diameter of the casting hole expands quickly. The sliding working surface is eroded by molten steel, which can easily lead to out-of-control flow control or steel penetration on the sliding surface. 4) The slide gate plate has low high-temperature strength and poor thermal shock resistance. After one use, cracks exceeding 0.5mm will appear, and cracks can easily penetrate the steel. 5) The anti-oxidation performance of the working surface of the slide gate plate is poor. After one use, the sliding surface will appear “raw”, which will intensify when the molten steel penetrates and slides again, which will increase the frictional resistance between the slide gate plate and easily lead to loss of control of the steel penetration between the slide gate plate and the nozzle. 6) When producing low-carbon and low-silicon steel, the slide plate has obvious “horseshoe-shaped” dissolution loss and cannot achieve effective flow control.
Analysis of slide gate plate damage mechanism
Although slide gate plate have different corrosion methods and degrees due to different casting steel types and casting methods, the main damage forms are basically similar. The erosion damage of slide gate plate mainly includes thermomechanical erosion and thermochemical erosion.
The first thing that occurs when a slide gate plate is used is thermomechanical corrosion. The temperature of the sliding plate was very low before work (300-400℃). When pouring steel, it suddenly came into contact with high-temperature molten steel (1540-1620℃), resulting in a temperature difference of 1200-1300℃ on the working surface. Therefore, tensile stress exceeding the strength of the slide plate is generated along the outside of the casting hole, forming radial microcracks centered on the casting hole. The emergence of cracks leads to the diffusion and penetration of foreign impurities, which accelerates chemical erosion. In turn, the chemical reaction promotes the formation and expansion of cracks. This cycle causes the cast hole of the skateboard to expand and be damaged.
Thermochemical corrosion is another main cause of slide gate plate damage. Al₂O₃-C slide gate plate are exposed to high-temperature molten steel and steel slag during use, and a series of chemical reactions occur, causing chemical corrosion.
1) Oxidation of carbon and graphite:
2)Decomposition of mullite:
3) SiO₂ reacts with FeO and MnO in steel and slag to form low melting point 2FeO·SiO₂ (1205℃) and MnO·SiO₂ (1291℃).
4) Al₂O₃ and SiO₂ react with CaO in steel and slag to form low melting point 2CaO·Al₂O₃·SiO₂ (1327 ℃) and 12CaO·Al₂O₃.
Main technical measures
Improvement in design of sliding nozzle mechanism
According to the on-site process technology conditions, hydraulic cylinder and connection port parameters, compressed air source conditions, ladle size, ladle long nozzle, nozzle seat brick matching size, etc. After repeated research and demonstration, the outer dimensions of the sliding nozzle mechanism and supporting refractory materials were designed.
1) The surface pressure loading method of the mechanism is improved. The air trigger compresses the springs on both sides of the mechanism through four surface pressure bolts cooled by compressed air, causing the mechanism to generate surface pressure and stabilize it at about 8t, ensuring the safety of continuous use in high temperature environments.
2) The installation method of the upper and lower slide plates has been improved. The slide gate plate is fixed in the mold frame through four corners of brick-pressed iron blocks with screws, which can inhibit the cracks of the slide gate plate along the direction of the slide and extend the service life of the bricks.
3) Increase the sliding stroke of the slide gate plate. The sliding stroke of the slide gate plate is increased from 165mm to 200mm, which improves the safety factor and the number of times of effective flow control of the sliding nozzle. According to the average digestion value of the effective stroke of multi-continuous sliding, it is generally 15~22mm/time, and when the remaining stroke of the skateboard is 1.5 times the average digestion value, the slide gate plate can be used once more. That is to say, the stroke of the slide gate plate is increased by 30~35mm, and the slide gate plate can be used for an extended period of time. To this end, it is determined that the sliding stroke of the slide gate plate is increased from 165mm to 200mm.
Performance improvements of sliding nozzle refractory materials
1) The slide gate plate adopts new anti-oxidation technology. Add a new type of antioxidant to the ingredients, and apply a layer of about 0.3mm new anti-oxidation paint on the working surface of the slide gate plate to improve the high-temperature oxidation resistance of the slide gate plate.
2) The slide gate plate, upper nozzle, and lower nozzle adopt low-silicon, low-carbon ingredients, and high-pressure molding processes to increase the compressive strength and improve corrosion resistance and thermal shock stability. The main physical and chemical indicators of the slide plate and the nozzle are shown in Table 1 (the sintering strength of the nozzle fire clay at 1400 ℃ is ≥ 4.5 MPa).
Table 1 Main physical and chemical indicators of slide gate plate and nozzles
|slide gate plate
|nozzle fire clay
3) Improved slide gate plate structure. The back of the slide gate plate is pasted with an asbestos board and an iron plate, and is fastened with iron rings around it. The asbestos board has a buffering and heat-insulating effect between the slide gate plate and the mechanism; the thin iron plate can prevent sintering with the mechanism mold frame, and the iron ring prevents cracks in the slide gate plate from expanding.
4) Improvement of the nozzle structure. An iron ring is tightened around the female port of the water supply port to prevent cracks in the water supply port from expanding.
Operation and maintenance technology for skateboard joint use
1) When blowing oxygen to clean the nozzle hole, keep the slide gate plate in a fully open state, shorten the oxygen burning time as much as possible, and excessive oxygen blowing is strictly prohibited.
2) Every time you install a slide gate plate, apply high-temperature lubricating oil to the surface pressure bolts and sliding rails to extend their service life.
3) Strictly implement the technical standards for determining the usage limit of sliding nozzle refractory materials and the daily maintenance and periodic performance testing system of the organization to prevent the occurrence of steel leakage accidents.
4) The pouring flow control operation should minimize the number of pulls and strokes of the slide plate, and strictly prevent the occurrence of slag from the large package when the steel is poured.
Optimization and improvement of LF refined calcium treatment process
Aluminum deoxidized steel is easy to form a large amount of Al₂O₃ in the steel. Al₂O₃ is difficult to remove from the steel. During continuous casting, it is easy to adhere to the nozzle wall and cause the nozzle to be clogged. Ca treatment is usually carried out at the end of refining, and Ca alloys, such as Ca-Si wire and Ca-Fe wire, are added. It reacts with the Al₂O₃ contained in the steel to form low melting point compounds, thereby changing the form of the aluminum oxide inclusions, and the molten steel is discharged as the bottom-blown argon bubbles rise. However, when the Ca alloy is added in excess, that is, the amount added exceeds the amount required to react with Al₂O₃ in molten steel, the excess [Ca] will accelerate the erosion of the slide plate. The erosion process is as follows: Al₂O₃ in the slide plate is first reduced to CaO and Al by [Ca] in the molten steel, and then the generated CaO reacts with Al₂O₃ in the slide plate to form Al₂O₃-CaO series low melting point compounds which are washed away by the molten steel.
Research and practice have proven that when the [Ca] content (mass fraction, the same below) in molten steel is <30×10-⁶. It mainly produces CaO·3Al₂O₃ (melting point 1850℃) and CaO·2Al₂O₃ (melting point 1750℃) with high melting point, which have weak erosion effect on the slide plate. When the [Ca] content in molten steel is (30~50)×10*. Some high melting point CaO·3Al₂O₃, CaO·2Al₂O₃ and some low melting point CaO·Al₂O₃ (melting point 1600℃) and 12CaO·7Al₂O₃ (melting point 1415℃) are produced, which intensifies the erosion of the slide plate. When the [Ca] content in molten steel is >50×10-6, a large amount of low-melting-point 12CaO·7Al₂O₃ and part of CaO·Al₂O₃ are produced, causing “horseshoe-shaped” dissolution loss of the slide plate.
Main optimization and improvement measures: 1) Optimize and improve the LF refining process, control the [Ca] content in molten steel within 30×10-6, and the maximum does not exceed 35×10-6. 2) It is strictly forbidden to stir the ladle with large amounts of argon during the wire feeding process. 3) Strengthen information transmission and adjust the wire feeding amount in a timely manner when the steel tapping amount fluctuates.
After design, testing, improvement and adaptive operation skills training for nozzle operators, the promotion and application of slide gate plate multi-slide technology began in August 2006. After more than 3 years of practice, the use results are good.
1) The improved sliding gate mechanism is easy to operate, and the “surface pressure” control device is reliable, ensuring stable surface pressure during the continuous use of the slide plate, and achieving leak-free casting. Since the test began in May 2006, there has been no steel penetration accident at the sliding nozzle. At the same time, the service life of the mechanism body has been increased from 500 heats to more than 2,000 heats, and the life of the mechanism spring has been increased from 400 heats to more than 1,500 heats, which reduces the maintenance cost of machine parts.
2) The improved slide plate has strong corrosion resistance, erosion resistance and oxidation resistance, allowing the slide plate to be used in four furnaces. The average diameter expansion of the casting hole is 0.8~0.9 mm/time, and the slide plate has good resistance to thermal mechanical erosion and thermochemical erosion. The erosion conditions of the slide plate and drain outlet when four furnaces are used together are shown in Table 2.
3) The combined use of the slide gate plate speeds up the online ladle turnover, shortens the ladle packaging time from 35 minutes to 15 minutes, reduces the number of online ladle turnover by 15%, and realizes the red envelope release of steel. The combined use of sliding plates not only improves the steelmaking process conditions, greatly reduces the labor intensity of nozzle operators, but also lays the foundation for the development and production of various types of steel.
Table 2 Diameter expansion conditions of four furnaces when the slide plate and drain outlet are used together mm
|Upper slide gate plate
|Lower slide gate plate
|Slide gate plate residual travel