01 Background and problems of the project
The main indicators of high-efficiency billet continuous casting are embodied in high-speed casting, high-quality stability, and high casting machine operating rate. Among them, high-speed casting is the core of high-efficiency continuous casting. In the 1990s, my country used the high-efficiency billet continuous casting technology as a national key research project during the "Ninth Five-Year Plan" period. Taking the 150×150mm cross-section as an example, the goal is: the drawing speed is greater than 3.0m/min, and the casting machine The operating rate is greater than 80-85%, and the defect-free rate of slabs is greater than 90%. This project has enabled my country's continuous casting technology to make great progress, and this goal has basically been achieved by the beginning of this century.
In the past ten years, my country's high-efficiency billet continuous casting technology has progressed slowly. At present, the continuous casting speed of ordinary construction steel in my country is basically between 2.8-3.5m/min, which is far behind the advanced level of foreign countries. During the period, the European Sandalian Casting Engineering Co., Ltd. successively introduced high-speed billet casting machine technology, which can increase the billet continuous casting speed to 5.0-6.0m/min and above, and move towards realizing the billet endless continuous casting. The rolling process is advancing.
High-speed continuous casting can not only increase the output of single-strand continuous casting, reduce the number of casting machines, but also increase the surface temperature of the red billet after cut-to-length cutting, and increase the utilization of the latent heat of the red billet. The reduction of the casting machine flow not only saves one investment, but also reduces the consumption of manpower, materials, energy and waste; the increase of the surface temperature of the continuous casting red billet is for the subsequent processes of hot delivery and hot charging, direct delivery, and direct rolling. The realization of head rolling provides a strong guarantee. In addition, in my country, converter steelmaking production still dominates. Under the development trend of high-efficiency converter steelmaking rhythm and large-scale converter, the number of matching single billet continuous casters cannot be increased indefinitely. Stream output is the only path, and the current level of billet continuous casting is increasingly difficult to adapt to these changes in the smelting system. Therefore, the development of high-speed continuous casting technology and the improvement of single-strand continuous casting production are also urgent needs of the market. The development and application of high-speed and high-efficiency billet continuous casting technology meets the industrial development direction and requirements of my country's energy-saving emission reduction and high-quality development, and has good economic and social benefits.
The main difficulties of high-speed and high-efficiency billet continuous casting are: ① With the increase of the drawing speed, the probability of production accidents such as steel breakout, squaring and bulging is greatly increased; ② Slag inclusions and cracks on the surface of the slab are prone to occur. The quality deteriorates rapidly and so on. Therefore, the realization of high drawing speed is a comprehensive technological improvement that integrates process technology, equipment technology and production operation technology.
02 Technical ideas to solve the problem
1. General technical ideas:
Billet continuous casting is a process of continuously casting liquid molten steel into a square or rectangular solid cast slab. Any factors that affect the initial formation of the molten steel shell, the smooth release of the shell, the uniform growth of the shell, and the complete solidification of the cast slab It may cause quality problems or failures in each link in this process. As the pulling speed increases significantly, the heat exchange load increases, the transmission rate increases, and the sensitivity of various influencing factors increases. Therefore, the overall direction of this research lies in the relevant equipment performance, cooling form and rate, heat transfer and demoulding scheme, stress on the blank shell, solidification characteristics of the blank shell, and production operation from the pouring of the tundish to the cut-to-length cutting of the billet. Research on stability and other aspects.
2. Technical solutions:
Develop the following core technologies from the mold to the tension leveler to solve the key issues affecting high-speed continuous casting.
(1) Development of high-efficiency cooling crystallizer. The primary cooling technology that meets high heat exchange capacity and more uniform cooling, while ensuring the stability of the mold structure performance under high heat flux, such as service life, sealing performance, etc.
(2) Development of high-speed mold vibration. To meet the demoulding requirements of high-temperature thin-shell casting slabs, a mold vibration device with smaller vibration deflection, no high-frequency resonance phenomenon, resistance to high-frequency shock loads, and accurate high-frequency vibration parameter follow-up is required.
(3) Development of high-efficiency secondary cooling technology. Provides a secondary cooling technology with greater cooling strength, higher heat transfer efficiency, more uniform surface water distribution, and more reasonable cooling gradient, which reduces the solidification thermal stress, promotes the rapid and uniform growth of the shell, and obtains a more uniform solidification structure.
(4) Development of high-speed secondary cooling guide system. Prevent the billet with liquid core from bulging, deviation, etc., and at the same time minimize the mechanical clamping stress of the billet shell.
(5) Development of non-steady state precision control technology for secondary cooling. It can solve the problem of changing the production conditions such as high-speed continuous casting's pulling speed fluctuations, molten steel superheat changes and other production conditions, and accurately adjust the water volume in each cooling zone in time to avoid excessive cooling and reduce surface temperature return.
(6) Development of high-speed mold flux lubrication technology. Develop high-speed mold fluxes with different melting speeds and lubricating properties to meet the requirements of different steel grades and different drawing speed levels, with good heat transfer and lubrication effects between the billet shell and the copper tube, and avoid adhesion.
03 Main innovative results
1. Efficient cooling crystallizer
The traditional billet continuous casting mold adopts water gap cooling. This cooling mode has defects such as difficulty in ensuring the uniformity of water gaps, and two-dimensional heat transfer at the corners leading to overcooling of the corners of the billet shell, which limits the further improvement of the drawing speed. It is easy to cause surface cracks, detachment, and even leakage accidents.
A large number of numerical simulations and actual production tests are carried out through the cooling and heat transfer of the billet crystallizer, such as the installation of thermocouples on the copper tube to obtain actual production heat transfer data to verify the calculation model and guide the process and equipment design. On this basis, develop a high-efficiency cooling crystallizer, which has the following characteristics:
(1) The high-efficiency cooling mold copper tube with higher heat exchange efficiency and better copper tube-blank bonding characteristics-"Plum-shaped copper tube", which has a special three-dimensional cavity and taper.
(2) Forced uniform cooling of the crystallizer, canceled the water gap cooling mode, increased the cooling area of the copper tube, and enhanced the cooling uniformity of the corners and the face.
(3) The new copper tube base material and coating technology, while ensuring the heat conduction performance of the copper tube, enhance the strength of the copper tube and the wear resistance of the coating.
(4) More suitable copper tube length design.
(5) The maximum temperature of the hot surface of the mold copper tube has been reduced by nearly 100°C, and the temperature difference between the corner temperature and the middle part has been reduced from about 100°C to about 5℃.
(6) The average heat flux density of the hot surface of the mold has increased by nearly 10%.
2. High-speed vibration device
The mold vibrating device moves up and down to make the mold copper tube and the blank shell form a relative movement, avoiding the phenomenon of bonding between the primary blank shell at the meniscus and the inner wall of the copper tube, causing the blank shell to be torn when the blank is drawn. And a steel breakout accident occurred.
As the pulling speed increases, the amplitude and frequency of the mold vibration need to be increased, which leads to an increase in the speed and acceleration of the vibration, which greatly increases the shock load of the vibration. Conventional vibration has problems such as poor structural rigidity, weak impact resistance, insufficient stability, large deflection, low arc simulation accuracy, insufficient vibration waveform and parameter accuracy, etc., and when the vibration frequency reaches more than 180 times per minute, it is different. The vibration frequency points will produce resonance, the higher the vibration frequency, the denser the natural resonance points. Therefore, the traditional vibrating structure is difficult to meet the requirements of large amplitude and ultra-high frequency vibration required for high-speed continuous casting.
Through the modal analysis and strength calculation of the high-frequency vibration device, the structural optimization research on the load center of the device, the guide link support, etc., makes the load center closer to the support guide center, and designs and analyzes the driving form and driving ability , Developed a new type of high-speed mold vibration device, which has the following characteristics:
(1) The overall box structure, the load center of gravity is located at the center of the arc-like guide support, the mechanism has good rigidity, high vibration stability, high accuracy of arc-like, and the natural resonance point is greatly improved.
(2) The whole leaf spring is connected and guided, there is no running deviation caused by joint wear, the amount of deflection is small, and the impact load is strong.
(3) Hydraulic cylinder drive can provide greater driving force, adapt to high impact load and accident load conditions, and also has the advantages of dynamic fitting of vibration curves, high amplitude accuracy, and long service life.
(4) Vibration deflection is less than 0.10mm.
The designed maximum amplitude is 10mm, and the natural resonance frequency exceeds 350 times/minute, which is outside the frequency of 200-300 times/minute required for high-speed vibration.
3. Efficient secondary cooling system
With the increase of the continuous casting speed, the heat transferred from the slab per unit time increases, and the steam film on the surface of the slab becomes thicker. The secondary cooling requires greater cooling strength, greater impact force and longer cooling length. However, the conventional secondary cooling water nozzle has low working pressure and small particles, and the striking force is not enough to eliminate the barrier of the steam film to the heat exchange, which limits the heat transfer effect of the secondary cooling. The core of the high-efficiency secondary cooling system is high-pressure and strong cooling, which can meet the requirements of the billet continuous casting 6.5m/min drawing speed condition. The main innovations of the system include:
(1) The working pressure of the nozzle reaches above 1.0MPa, and the pressure of the cooling water system reaches 2.5MPa;
(2) The wide-angle rectangular nozzle is adopted, the nozzle usage is small, the flow rate of a single nozzle is large, the coverage area is wide, and the striking force is strong;
(3) The secondary cooling length and control circuit partition suitable for high drawing speed, fully embody the surface temperature gradient control law, and control the surface temperature between secondary cooling zones and after entering air cooling to be less than 50℃;
(4) The unique spray header design can achieve the effects of small cooling water pressure loss, high centering accuracy, anti-deformation, and easy installation and unloading.
4. High-speed secondary cold support guiding system
This system divides the casting billet clamping, supporting and guiding equipment behind the mold into several sections to facilitate the realization of different functions required in different areas. The main function of the upper area near the mold is to prevent the billet from bulging, and the liquid-core cast billet needs to be clamped, such as the foot roll area, the sector 1 section and/or the sector 2 section; the main function of the lower area near the tension leveler is To prevent the cast slab from deviating, it is necessary to guide the two sides and inner arc of the cast slab, such as fan-shaped 2 sections and/or fan-shaped 3 sections.
As high-speed continuous casting is difficult to prevent steel breakout accidents, the development and design of the guiding system must also have the characteristics of online rapid replacement, small equipment maintenance, and easy accident handling.
5. Other related technologies
In order to realize the high casting speed of the casting machine, related supporting technologies are also essential, such as the secondary cooling dynamic control system, the automatic control technology of the mold liquid level detection, the high drawing speed mold powder and its automatic adding technology, and the high drawing speed electromagnetic stirring Technology, slab quality tracking and determination system, slab surface temperature full-process control system, etc.
In short, in order to achieve stable production at high casting speeds of the caster, we must work hard around every link in the process of molten steel pouring, slab solidification and stretching, instead of relying on one of the technologies to achieve this goal in isolation.
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