Abstract: The new technologies of efficient continuous casting mainly include: hydraulic non-sinusoidal vibration, online width adjustment of crystallizer, electromagnetic braking of crystallizer, dynamic soft pressure, and dynamic secondary cooling control. The development status of efficient continuous casting technology is analyzed, and various new continuous casting technologies and their applications in production are introduced. The development and wide application of new continuous casting technologies have promoted the process of efficient continuous casting and promoted the rapid development of the steel industry.
Keywords: efficient continuous casting; casting speed; billet quality; copper mould tube
In recent years, with the rapid growth of the economy and the acceleration of industrial and infrastructure construction, China’s steel industry has developed rapidly. Continuous steel casting technology has played a very important role in the transformation of the production process of the steel industry, the improvement of product quality and the optimization of structure. The technical level of continuous casting has become an important indicator of the degree of modernization of a country’s steel industry.
For continuous casting production, on the premise of meeting the product variety and quality, improving the casting speed and operation rate of the continuous casting machine means increasing production and increasing benefits. High-efficiency continuous casting technology is a continuous casting system technology that takes high casting speed as the core and high-quality, defect-free billet production as the basis to achieve high continuous casting rate and high operating rate. Therefore, the development of high-efficiency continuous casting technology has become the main direction for exerting the potential of continuous casting technology. It has been highly valued by major steel companies, engineering companies, and equipment manufacturers in the world, and has a very broad development prospect.
1 Development status of high-efficiency continuous casting
With the rapid development and increasing maturity of continuous casting technology, the stability of production operations has been greatly improved, but there is still a lot of room for the promotion of high-efficiency continuous casting technology. Since the efficiency of continuous casting must comprehensively consider the type of steel cast and its thermal sensitivity, thermal conductivity, center organization quality and other factors, and at the same time, it is necessary to create suitable conditions in the process. The proportion of high-efficiency continuous casting in continuous casting production is still relatively low. At present, the development of high-efficiency continuous casting technology mainly focuses on improving the productivity of continuous casting machines and improving the quality of continuous casting billets.
1.1 Improving the productivity of continuous casting machines
The improvement of the productivity of continuous casting machines is actually the comprehensive improvement of casting speed, casting machine operating rate, and number of continuous casting furnaces under the premise of ensuring the quality of billets. China’s steel production structure is more long profiles, and the plate is relatively small, about 40%. For small square billet continuous casting machines, the core of improving productivity is to increase the drawing speed. For slab continuous casting machines, the core of improving productivity is to increase the continuous casting machine operation rate. This is because the drawing speed of the slab continuous casting machine is limited by the furnace machine matching conditions and the metallurgical length of the casting machine itself. The damage of steel leakage caused by excessive drawing speed has a much greater impact on the slab continuous casting machine than on the small square billet continuous casting machine.
Table 1 Comparison of technical indicators after the continuous casting machine adopts high-efficiency transformation
| Billet type
| Before renovation | After renovation | Internationally advanced | |||
| Single-stream output/(10,000 t·a-¹) | Pulling speed/(m·mn-¹) | Single-stream output/(10,000 t·a-¹) | Pulling speed/(m·mn-¹) | Single-stream output/(10,000 t·a-¹) | Pulling speed/(m·mn-¹) | |
| Small square billet | 5～8 | 1.5～2.0 | 12～15 | 2.5～3.5 | – | 3.0～3.5 |
| Slab | 50 | 1.2 | 100 | 1.8 | 150～200 | 2.0～2.5 |
Table 1 is a comparison of technical indicators such as the casting speed and output of the casting machine after the high-efficiency transformation. It can be seen that the domestic small square billet high-efficiency continuous casting technology has basically approached the international advanced level, but the slab high-efficiency continuous casting technology still has a certain gap with the international advanced level. In the future, in-depth research work will be carried out in terms of improving the slab casting speed and casting machine output.
1.2 Improving the quality of continuous casting billets
The quality of the billet determines the quality of the final product. Improving the quality of the billet is one of the goals that steel companies strive to achieve. Improving the quality of continuous casting billets includes: improving the cleanliness of the billet to control the number, type, size, distribution, and shape of non-metallic inclusions in steel; improving the surface quality of the billet (reducing longitudinal cracks, transverse cracks, star-shaped cracks, and slag inclusions) and improving the internal quality of the billet (reducing middle cracks, corner cracks, centerline cracks, looseness, shrinkage, and segregation).
2 New technologies for efficient continuous casting
2.1 Hydraulic non-sinusoidal vibration
The purpose of mold vibration is to prevent the billet from sticking to the inner wall of the copper tube of the crystallizer during solidification, resulting in hanging, cracking or leakage accidents, thereby ensuring the surface quality of the billet and reducing the depth of vibration marks.
Since its invention, the hydraulic vibration of the crystallizer has been rapidly promoted because of its ability to adjust the amplitude, frequency and negative slip parameters online to keep the negative slip rate and negative slip time within the optimal range. In the high-speed, high-quality and efficient continuous casting production, in order to obtain a good crystallizer vibration process effect, it is hoped that the crystallizer vibration mode is short down vibration time, fast speed, long up vibration time, slow speed. The non-sinusoidal vibration mode has the characteristics of a longer positive sliding time and a smaller difference between the crystallizer vibration speed and the billet drawing speed.
The crystallizer adopts hydraulic non-sinusoidal vibration, which can not only adjust the amplitude and positive and negative slip ratio online, but also facilitate the uniform penetration of protective slag into the gap between the crystallizer and the solidified billet shell, improve the lubrication effect of the inner wall of the crystallizer, reduce the depth of vibration marks on the surface of the billet, reduce cracking and leakage, and improve the surface quality of the billet. The hydraulic non-sinusoidal vibration of the crystallizer provides a reliable guarantee for continuous casting production. It is one of the key technologies for efficient continuous casting.
Meigang uses the crystallizer hydraulic vibration technology on the No. 2 efficient continuous casting machine. The billet section is 1320mm×230mm), and the maximum working speed of low-carbon aluminum-killed steel reaches 2.4m/nin; the maximum working speed of medium-carbon steel reaches 2.2m/min, which ensures the stable and smooth operation of the efficient continuous casting machine. At the same time, the depth of vibration marks of the billet is significantly shallower.
After the hydraulic non-sinusoidal vibration of the crystallizer was adopted in the third steelmaking plant of Wuhan Iron and Steel, the depth of vibration marks was significantly reduced, the surface quality of the billet was improved, and the risk of leakage was reduced. The reduction of vibration mark depth has greatly reduced the transverse cracks of the ingot, and the percentage of peritectic alloy steel has been reduced from 35.64% to 17.96%, and the percentage of medium carbon steel has been reduced from 24.37% to 4.68%.
2.2 Online width adjustment of crystallizer
In order to meet the needs of slabs of different widths, the width of the crystallizer must be adjusted with the width of the slab. Online width adjustment of the crystallizer is to change the width of the ingot without stopping the billet drawing.
The traditional method of adjusting the width of the crystallizer must pull out the tail billet of the first two steel grades and then re-adjust the width of the crystallizer, reopen the sliding gate of the middle package, and each change in the cross-section of the slab requires stopping production for more than 2 hours. The use of online width adjustment technology can continuously cast ingots of different widths without stopping the machine. Online width adjustment saves manpower and material resources, and also greatly improves the production rhythm and efficiency. Especially when realizing the production needs of hot delivery and continuous casting and rolling, online width adjustment technology is particularly important.
As continuous casting production increasingly emphasizes the maximization of production capacity, the width adjustment speed and casting speed have become serious limiting links. For this reason, the newly developed S-shaped mode of hot width adjustment of the crystallizer presents an S shape when adjusting the width. The corresponding parameters when casting carbon steel on a traditional slab continuous casting machine are: maximum pulling speed 2m/min, maximum width adjustment speed of each side 50 mm/nin. The high-speed online width adjustment technology developed by Nippon Steel in Japan has a single-side moving speed of 100mm/min when adjusting the width. When the width is adjusted from 630mm to 1340mm, it can be achieved on a billet length of 7.6m. When the width is reduced, the bulging shape and internal quality of the billet are good.
2.3 Electromagnetic braking of the crystallizer
The kinetic energy is also increasing, and the impact on the narrow surface of the crystallizer is intensified, increasing the risk of leakage. The bubbles and non-metallic inclusions entrained in the molten steel are also difficult to float and remove due to the increase in the immersion depth. At the same time, the upward reflux of the molten steel also causes the fluctuation of the meniscus of the molten steel in the crystallizer to intensify, and the surface defects of the billet caused by slag rolls are greatly increased.
Electromagnetic braking uses the upward electromagnetic force to stop the molten steel flowing out of the submerged nozzle and change its direction, thereby reducing the penetration depth of the molten steel, causing the inclusions to float up and separate, and at the same time, suppressing the fluctuation of the meniscus and preventing slag rolling.
In the 1980s, the slab crystallizer electromagnetic braking technology jointly developed by Swedish ASEA and Japanese Kawasaki was tested at Kawasaki and achieved good results. In the 1990s, two-stage electromagnetic braking technology was successfully developed, with the upper stage used to suppress the fluctuation of the meniscus and the lower stage used to brake high-speed streams. In recent years, a full-width three-stage electromagnetic braking technology has been developed, which uses the lower magnetic field for secondary braking. After adopting electromagnetic braking, Kawasaki Steel of Japan has found that even during high-speed continuous casting at more than 2.5m/min, the protective slag in the crystallizer will not be rolled into the molten steel. After using electromagnetic braking, Berkedey Company in the United States reduced the impact depth of molten steel from 15mm to 5mm when the pulling speed was 5rn/min, reducing the possibility of mold slag and ingot cracks, reducing the defects caused by the slag entanglement by 90%, and reducing the longitudinal crack index by 80%.
In China, electromagnetic braking technology has also been widely used. The full-width two-stage electromagnetic braking technology is used in the No. 2 continuous casting machine of Meishan Iron and Steel. The use effect shows that after using electromagnetic braking, the fluctuation amplitude of the crystallizer liquid level is significantly reduced, the number of non-metallic inclusions in the steel is small and the size is small, which has achieved good results in increasing the pulling speed and changing the surface quality of the ingot.
2.4 Dynamic Soft Reduction
Dynamic soft reduction is a new continuous casting technology that is being vigorously developed internationally. It tracks the thermal state of the ingot online and applies appropriate pressure reduction to the ingot according to the actual solidification end position to hinder the directional flow of enriched segregation elements, reduce or eliminate center segregation, and offset the volume shrinkage at the end of solidification of the ingot, avoiding the formation of center shrinkage and center porosity.
Dynamic soft reduction technology was developed in the mid-to-late 1990s. In 1997, VAI took the lead in using dynamic soft reduction technology for the transformation of slab casting machines, and achieved the effect of correcting the deformation of the slab during the bending and straightening process, eliminating indentations, and improving the surface quality of the slab19. After Posco, South Korea, adopted the soft reduction technology, the center segregation of the ingot was reduced from 1.6 to 1.1, the phosphorus segregation was reduced from 3.7 to 1.8, the manganese segregation was reduced from 1.5 to 1.25, and the break rate of P70 wire was reduced from 10% to 4.3%.
The comparison of the ingots produced by the No. 3 wide and thick plate casting machine using the dynamic light reduction process at Wuhan Iron and Steel No. 3 Steelmaking Plant with the ingots produced by the No. 1 and No. 2 conventional slab continuous casting machines shows that the ingots produced by the No. 3 continuous casting machine eliminate the A-level center segregation, and the probability of C-level 1.0 and below 1.0 is increased by 9.6% and 6.1% respectively compared with the No. 1 and No. 2 casting machines.
For the practical application of dynamic light reduction, it is necessary to rely on the dynamic secondary cooling control model to calculate and track the end position of solidification in real time, and determine the reasonable reduction position and reduction amount. Therefore, further improving the calculation accuracy and speed of the dynamic secondary cooling control model will continue to be a hot spot and difficulty in current research.
2.5 Dynamic secondary cooling control
The high efficiency of continuous casting promotes the structural optimization of the steel industry, and the high operating rate of the casting machine and the high quality of the ingot are closely related to the solidification process of the molten steel. The secondary cooling of continuous casting is to continue to strengthen the cooling of the ingot out of the crystallizer. By improving the secondary cooling system and optimizing the secondary cooling water distribution, the cooling of the ingot can be uniform. At the same time, whether the secondary cooling system is reasonable or not is crucial to the implementation of the continuous casting dynamic light reduction process and the final quality of the billet.
The secondary cooling water distribution of continuous casting has gone through the static water distribution method of artificial water distribution, water meter proportional control, and drawing speed parameter control method to the dynamic control process of measured surface temperature feedback control based on heat transfer mathematical model, billet age control method and target surface temperature control method. The dynamic secondary cooling control method can overcome the shortcomings of traditional water volume adjustment based on drawing speed, especially the large fluctuation of billet surface temperature.
At present, the static secondary cooling water distribution model is very mature and widely used in domestic indium plants. The control model based on billet heat transfer calculation is the hot spot of current dynamic secondary cooling research. VAI (Vaisteel)’s DYNACS dynamic secondary cooling water model simulates and calculates the casting strand temperature field, and integrates multiple factors such as tundish molten steel temperature, drawing speed, secondary cooling water flow rate, billet surface temperature, steel grade characteristics (such as chemical composition, solid-liquid phase temperature, heat capacity), etc., to perform secondary cooling dynamic control, and has achieved good practical application results.
At present, although most dynamic secondary cooling studies have optimized the control parameters, it is difficult to ensure that the control system always maintains good performance and stability in the actual production process with many steel types, frequent changes, and complex process factors. Therefore, the development of advanced dynamic secondary cooling control models is still a hot topic and trend in current research.
3 The impact of new technologies on efficient continuous casting
Table 2 The impact of new technologies on efficient continuous casting
| Technical means
| Main effects
| Efficiency Impact | |
| Productivity | Billet quality | ||
| Hydraulic non-sinusoidal vibration | Prevent the ingot from cracking or leaking, and reduce the depth of vibration marks. | √ | √ |
| Online width adjustment of crystallizer | Save manpower and material resources, improve production rhythm and efficiency, | √ | |
| Electromagnetic brake of crystallizer | Reduce the penetration depth of molten steel, promote the floating and separation of inclusions, suppress the fluctuation of the meniscus, and prevent slag rolling. | √ | √ |
| Dynamic soft pressing | Reduce or eliminate center segregation, avoid the formation of center shrinkage and center looseness, | √ | |
| Dynamic control of secondary cooling | Make the cooling of the ingot uniform. | √ | √ |
The impact of new technologies on efficient continuous casting is shown in Table 2. In general, the development of efficient continuous casting technology is based on ensuring the quality of continuous casting billets and improving the productivity of continuous casting machines. With the development and widespread application of new continuous casting technologies, it not only promotes the process of efficient continuous casting, but also plays an indispensable supporting role in the development and stability of continuous casting varieties and the improvement of quality.
4 Conclusion
Efficient continuous casting not only improves the productivity and output of continuous casting machines, but also further improves the quality of billets, which plays a very important role in the development of the steel industry. The development and application of new continuous casting technologies such as hydraulic non-sinusoidal vibration, online width adjustment of crystallizer, electromagnetic braking of crystallizer, dynamic light pressure, and dynamic secondary cooling control have achieved remarkable results and promoted the continuous promotion of efficient continuous casting. However, efficient continuous casting is a systematic project. The comprehensive application of various continuous casting technologies and continuous development and innovation can promote the further development of efficient continuous casting.
