Application of electromagnetic stirring technology in continuous casting machine and its impact on slab quality

The continuous casting electromagnetic stirring device can effectively improve the internal structure of the cast slab, improve the surface quality, and reduce center segregation and center porosity. It basically eliminates central shrinkage holes and cracks and greatly increases the equiaxed grain rate. It is a necessary equipment for the production of high carbon steel, so it is widely used in various billet continuous casting machines. Electromagnetic stirring enables contactless energy conversion. That is, the electromagnetic energy can be converted into the kinetic energy and partial thermal energy of the molten steel without touching the molten steel, and the direction of the electromagnetic flow and the stirring direction of the molten steel can be artificially adjusted. Thereby producing slabs that meet the needs of different steel types, which plays an important role in improving slab quality. In view of this, this article analyzes and discusses the application of electromagnetic stirring technology in continuous casting machines and its impact on the quality of cast slabs.

Keywords: electromagnetic stirring technology; continuous casting machine; secondary cooling water distribution; slab quality

Principles and classification of electromagnetic stirring technology

The electromagnetic stirrer works on the same principle as a three-phase asynchronous motor. The three-phase power supply provides electrical support and forms a rotating magnetic field in the magnetic poles. By stirring the molten steel in the device, the magnetic field will generate an electromagnetic torque that acts on the molten steel and rotates around the axis of the injection section. The direction of the electromagnetic force is determined by the changing direction of the magnetic poles of the magnetic field. The direction of the electromagnetic force can be changed by exchanging the boundaries of any two-phase power supply. The direction of the electromagnetic stirring can be flexibly adjusted based on the requirements of the stirring process. By controlling the convection, heat transfer and mass transfer processes of molten steel, the superheat of molten steel is promoted to be uniform, dendrites are broken, bubbles and impurities in the molten steel are promoted to float, and the formation of equiaxed crystals is intensified. In this way, the problems of central porosity, shrinkage cavities and central segregation can be improved, and the internal quality and surface quality of the slab can be effectively improved. As far as the types of electromagnetic stirrers are concerned, they are divided into three types according to different installation locations: ① Secondary cooling zone electromagnetic stirrers are installed in the secondary cooling section of the continuous casting machine and have foot-roller stirrers. ② The electromagnetic stirrer of the crystallizer is installed at the position of the crystallizer of the continuous casting machine. The stirrer that spans the foot roller and the crystallizer also belongs to this category. ③ The electromagnetic stirrer at the solidification end is installed near the solidification end area of the continuous casting machine. ④Electromagnetic stirrer for heating the tundish. This type of electromagnetic stirrer is used in the continuous casting machine to keep the temperature of the molten steel always within the range of 10~25℃ of the liquid phase temperature of the tundish. In a wide range of applications, both investment and cost are far less than plasma heating methods, and the secondary metallurgical effect is considerable.

The influence of electromagnetic stirring process on continuous casting process

With the application of electromagnetic stirring device, the cast slab can obtain an equiaxed crystal band with a wider center, which has a significant effect on improving problems such as center segregation and center porosity. The fundamental reason is that due to the application of the electromagnetic stirrer, the movement speed of the molten steel is accelerated, the dendrites at the solidification front are broken, and the bottom of the liquid phase cavity can be made smooth and wide, inhibiting the growth of columnar crystals, avoiding the formation of solidification bridges, and improving the center of the cast slab. The effect is remarkable in terms of loosening problems. In addition, the structural forming of small steel ingots can be improved and the problem of central segregation of the slab can be avoided.

Improve the problem of uneven shell thickness of continuous casting billets

Combined with the characteristics of the continuous casting process, the solidification of the continuous casting billet is a complex process. It undergoes heat transfer and mass transfer processes. The molten steel only passes through the crystallizer about 1m in 20 to 80 seconds, but more than 28% of the heat passes through the crystallizer. Outgoing, this process is essentially a heat dissipation process. Affected by factors such as water flow distribution in the cooling system, mold structure, and protective slag, the heat transfer effect of the mold does not meet the expected requirements. The cooling intensity of the molten steel is also different, and the problem of uneven thickness of the slab shell is inevitable. Non-contact stirring of molten steel helps to further improve heat transfer efficiency. The electromagnetic stirrer is installed outside the crystallizer. Through the combined action of the magnetic field and the induced current, the molten steel in the crystallizer is rotated, and the heat is quickly transferred to the outside of the crystallizer, making the cooling intensity of the molten steel more uniform and improving the uneven thickness of the continuous casting shell. , improve the quality of cast slabs. Compared with traditional technology or cast slabs that do not use electromagnetic stirring, the equiaxed grain rate of the electromagnetic stirring process is increased by more than 10% to 20%. It has a significant effect in refining the grains. The cross-section of the cast slab is The grains are uniform and small. During the growth of columnar crystals, electromagnetic stirring is used for the liquid phase. The growth direction of the columnar crystals will change and grow obliquely in the upstream direction.

Improve the problem of difficulty in floating inclusions in continuous casting slabs

During continuous casting production, since the solidification of molten steel in the crystallizer is a rapid process, the molten steel will continue to solidify as the temperature gradually decreases. Before solidification, the gas is saturated, and the gas precipitates and adheres to impurities, causing bubbles. The generation of bubbles is mostly caused by poor deoxidation. If the bubbles are not treated in time during rolling, they will cause surface defects of the cast slab, and these harmful defects will further expand. At the same time, these bubbles are easily captured by dendrites, which is a main reason for the formation of inclusions. Even in the ordinary continuous casting process, some inclusions float up with the movement of molten steel and are absorbed by the mold slag, but most of the inclusions cannot float to the surface. Relying on electromagnetic stirring technology to promote the improvement and innovation of the continuous casting process, non-contact electromagnetic force is used to promote the rotational movement of molten steel, speed up the up and down flow of molten steel, and promote inclusions to float under the action of electromagnetic force to facilitate the full absorption of slag, effectively Improve the problem of subcutaneous air bubbles.

Center segregation problem of continuous casting billet

The problem of segregation in the center of the continuous casting billet occurs because during the continuous casting production process, high-intensity cooling is selected to speed up the solidification speed of the billet and advance the solidification interface in parallel to promote the single-sided heat dissipation of the billet. The temperature gradient in the cross section of the continuous casting slab is relatively high. After gradual cooling, the temperature gradient at the front edge of the solidification interface becomes higher and higher, which promotes the formation of columnar crystals. At the end of solidification, the enriched elements that have not yet solidified still rotate and flow in the molten steel, causing central segregation problems. To address such problems, electromagnetic stirring technology is used to generate powerful electromagnetic stirring force, speed up the rotation speed of molten steel, and increase the heat exchange rate of molten steel, which facilitates more uniform cooling of molten steel, reduces the temperature gradient of the slab section, and effectively improves the quality of central segregation. question. The electromagnetic stirring process has a greater impact on composition homogenization, which is specifically reflected in the correlation between the cooling speed of the molten steel and the flow direction. Based on the cast slab processed by the electromagnetic stirring process, the composition homogenization effect is more obvious. At the same time, due to the refined grains of the cast slab, the tensile strength of the cast slab is greatly improved. If the strength is controlled within a specific range, the surface of the cast slab will be smoother, which will reduce the stress concentration and improve the plasticity of the cast slab.

Application of electromagnetic stirring technology in continuous casting machines

Single mixing process

Whether the mixing method selected on the continuous casting machine is reasonable is directly related to the quality of the slab. It is common to choose a single mixing process, which does not have strict requirements on segregation. It can improve the internal quality and surface quality of the slab and meet the needs of protected casting and open casting. The surface and subcutaneous area of the cast slab are formed in the electromagnetic stirrer installed in the crystallizer. Under the action of electromagnetic force, the molten steel is caused to rotate, scouring the inner surface of the solidified shell, and gas is precipitated, which can improve the surface temperature of the cast slab. Effectively grasp the size and density of the shell to improve problems such as skinning, slag inclusions and air bubbles. Choosing the electromagnetic stirring process in the secondary cooling section can refine the equiaxed crystal area of the grains. The width needs to be adjusted according to the width of the liquid core at the installation position of the stirrer. This kind of mixer can provide spiral stirring motion. The spiral stirring method has outstanding advantages and can meet the needs of billet operations, while the traveling wave stirring method is suitable for application in slabs and blooms. In addition, there is a new propeller-type mixing process that can achieve the superposition of the above two mixing processes. However, the investment cost is high and the energy consumption is high, so it is difficult to promote and apply it in practice. For the application of electromagnetic stirring technology in the secondary cooling section, the selected modes vary depending on the application conditions.

From a structural perspective, it includes box-type electromagnetic stirrers, roller-type electromagnetic stirrers and plug-in electromagnetic stirrers. ①Box-type electromagnetic stirrer. In practical applications, the magnetic support roller on the inner arc side of the continuous casting machine is replaced with a non-magnetic support roller, and a traveling wave magnetic field electromagnetic stirrer is equipped to form a magnetic field and promote the movement of molten steel. Due to the large air gap of the device, the energy consumption increases, coupled with the size and weight, the equipment installation and later inspection and maintenance are difficult, and need to be fully considered in the design. ②Roller electromagnetic stirrer. One or two support rollers are removed from both sides of the sector-shaped section of the slab, and electromagnetic stirring rollers with the same outer diameter are installed on the sector-shaped section for support and stirring. Due to the characteristics of this device, the electromagnetic stirring roller coil is installed inside the roller sleeve, and the iron core is relatively close to the surface of the cast slab, so the operating power of the device is small. Choosing an electromagnetic stirring roller continuous casting machine will cost less to modify and does not require too much modification. It is relatively easy to change the installation position of the agitator. On non-driven rollers with the same diameter and different diameters, the best mixing position must be selected based on comprehensive considerations. This method is the best choice for the secondary cooling section mixing process. ③Plug-in electromagnetic stirrer. Remove a pair of support rollers on both sides of the slab, choose two pairs of non-magnetic small rollers to support the slab, and install plug-in slab electromagnetic stirrers between the small rollers on both sides. Compared with the other two methods, this method has a greater power of the electromagnetic stirrer, and the iron core is in close contact with the surface of the slab, so the stirring effect is considerable. However, the equipment is difficult to install and maintain. Once it fails, it may cause production stagnation, waste resources, and increase costs.

Combined mixing process

If high alloy steel and high carbon steel are cast, there may be problems with high overheating and rapid casting, or if there are special requirements for the continuous casting process, it is difficult to ensure the quality of the slab with just a single stirring process. Based on this, you can choose to use two mixers with a certain distance apart. The combined stirring process can improve the problem of alloy elements gathering in the axial direction, but it is difficult to improve the segregation problem of steel types with high carbon content. The S2+FEMS combined stirring process can be implemented. With the help of the stirrer installed at the end position, the grains in the billet are moved, the metal elements are mixed and stirred, and the solidification medium forms a good organizational structure, and the uniformity meets the requirements. For some steel types that are more difficult to cast, such as ball bearing steel, which may be affected by objective factors and cause special metallurgical problems, three mixer combination processes are implemented to fully integrate the advantages of different processes. It helps to improve the internal quality of the slab to the greatest extent and needs to be selected flexibly according to the actual situation.

Conclusion

In summary, the metallurgical effect of the application of electromagnetic stirring technology in slab continuous casting is obvious. With the intensification of competition in the domestic and foreign steel industry, the production of high-quality high-end products is bound to be a trend. Electromagnetic continuous casting technology is to install appropriate types of electromagnetic stirring devices in different parts of the continuous casting machine. During the continuous casting process, the electromagnetic force generated by the electromagnetic stirring is used to stir the molten metal in the cast slab to resist the gravity effect. This form of electromagnetic stirring is efficient and non-contact. During the solidification process of the metal melt, it can effectively achieve uniform flow, heat and mass transfer within the metal melt, thereby obtaining an ideal temperature field and flow field. It can effectively reduce solidification defects such as uneven distribution of alloy elements in the melt, refine the grain size of the slab, improve the quality of the continuous casting slab, and then produce slabs with excellent properties.

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