Analysis of the influence of electromagnetic stirring of the copper mould tube on the quality of continuous casting slabs

Abstract: Continuous casting billet is a product obtained by casting molten steel in a steel-making furnace through a continuous casting machine. Its application fields are very wide, and continuous casting billet parts are used in mechanical engineering equipment at home and abroad. Among them, some continuous casting billets for steel can be directly rolled into pipes, plates, sections, etc. After the continuous casting billet is electromagnetic stirred in the copper mould tube, some defects can be effectively improved. Based on this, this article briefly analyzes the electromagnetic stirring of the copper mould tube, the concept of continuous casting billet, and related experiments.

Keywords: copper mould tube; electromagnetic stirring; continuous casting billet

The most critical problem in continuous casting slabs is that central segregation, inclusions and central shrinkage holes seriously affect the internal quality of the slab. Electromagnetic stirring is the most commonly used continuous casting production technology. It uses electromagnetic force to optimize the elimination of superheat of molten steel in the mold. The equiaxed crystal ratio of the cast slab will be significantly improved after electromagnetic stirring, thereby obtaining a cast slab with good solidification structure and improving the performance of the finished product. It can effectively solve problems such as shrinkage holes and purity in the center of continuous casting billets.

Overview of copper mould tube electromagnetic stirring and continuous casting billet

Continuous casting billet is a steel billet cast by molten steel through a continuous steel casting machine. Continuous steel casting technology can simplify the entire process from producing molten steel to steel billets, without the need for a rough rolling process. Therefore, continuous casting billet has a series of advantages such as low production cost, high metal yield and good working conditions. At present, continuous casting billet has become an important raw material for steel rolling production. However, continuous casting billet also has certain defects. For example, general porosity, central porosity, ingot-type segregation, general point-like segregation, edge segregation, subcutaneous bubbles, internal bubbles, shrinkage cavity residues, skinning, white spots, axial crystal cracks, non-metallic inclusions and core cracks, etc. . During low-magnification inspection, defects such as center porosity, shrinkage holes, center segregation, surface corner cracks, and surface edge cracks will appear.

Electromagnetic stirring uses the electromagnetic force induced in the liquid phase cavities of the cast slab to enhance the movement of molten steel in the liquid phase cavities. This strengthens the convection, heat transfer and mass transfer processes of molten steel, thereby controlling the solidification process of the slab, which has a positive effect on improving the quality of the slab. Among them, copper mould tube electromagnetic stirring is currently the most common device suitable for various types of continuous casting machines. It has a significant effect on improving the surface quality of the slab, refining the grains, and reducing internal inclusions and center looseness of the slab. Under normal circumstances, in order to avoid affecting the use of the liquid level automatic control device, it is usually installed at the lower part of the copper mould tube. The functions of electromagnetic stirring in the copper mould tube are as follows: First, it improves the surface quality of the slab. The surface of the slab is in a solidified state under the copper mould tube. At this time, the stirrer can be placed on the meniscus of the copper mould tube to “clean” the surface of the slab before solidification begins. Second, improve the internal quality of the cast slab. It can improve the equiaxed crystal area, refine the grain size, reduce center porosity, reduce center segregation, and even eliminate the white bright band phenomenon on the sample. The following points should be noted when using electromagnetic stirring in the mold: First, the mold powder may be involved, resulting in more inclusions in the slab. The electromagnetic stirring of the copper mould tube can effectively improve the quality of the cast slab. The effect will be more significant when the intensity of stirring increases. However, the strength must also be controlled within a fixed range. If the strength is too high, mold powder will be involved in the billet, which will increase inclusions and have a counterproductive effect. At the same time, electromagnetic stirring is also a direct factor affecting the detection effect of the copper mould tube liquid level. Currently, it is generally adopted to lower the installation position of the stirring coil to solve this problem, but it also limits the metallurgical effect of electromagnetic stirring of the copper mould tube. Second, electromagnetic stirring of the copper mould tube is difficult to improve the central carbon segregation of high carbon steel. Although electromagnetic stirring of the copper mould tube can greatly reduce the deviation value and peak value of central segregation, the average value of central segregation does not change significantly. Therefore, it is difficult to fundamentally improve the central segregation of carbon using only electromagnetic stirring of the copper mould tube.

Electromagnetic stirring experiment of continuous casting machine

Baosteel round billet continuous casting machine was selected for the experiment. The relevant parameters of the selected continuous casting machine are:. The arc-shaped continuous casting machine has a bending radius of ten meters, casting sections of 53 mm, 78 mm, and 60 mm respectively, a metallurgical length of 17.35 meters, and a mold height of 780 mm. For the experiment on the impact of central shrinkage cavities, St45 continuous-cast round billet was selected. The central shrinkage cavities were classified into grades. No shrinkage cavities were set as level 0, and they were arranged in this order. The central segregation effect experiment selected 82B high carbon steel continuous casting billet. The head billet, middle billet and tail billet were selected as samples from the experimental billets, and their chemical compositions were analyzed. Since steel is an alloy (mainly iron and carbon), its melting temperature is a range, which consists of the starting melting temperature and the melting end temperature. Among them, the temperature at which melting begins is the solid phase temperature, and steel is solid below this temperature. The temperature at which melting ends is the liquidus temperature, above which the steel is in a liquid state. In the continuous casting process, molten steel superheat control can also be added. First, minimize the temperature drop in the steel-making process; second, stabilize the steelmaking operation; and finally, strengthen production scheduling. On this basis, it is combined with electromagnetic stirring. The working principle of electromagnetic stirring is: First, according to the law of electromagnetic induction, when the magnetic flux in the closed loop changes, the closed loop will generate an induced electromotive force. The alternating electromagnetic field (B) generated by the electromagnetic stirrer changes around the conductive metal melt, causing relative motion between the magnetic field and the molten metal. The magnetic flux in the conductive loop changes because the magnetic field cuts the metal melt in the alternating magnetic field at a certain speed (V), causing an induced current inside it. The current interacts with the magnetic field to produce electromagnetic force, which acts on the metal melt to drive the movement of the metal melt. Mechanical effects of electromagnetic stirring: First, it can promote the formation and dissociation of crystals on the wall and increase the number of crystal nuclei. Second, when the stirring intensity is small (laminar flow), the dendrites will tilt toward the direction of flow. Third, under the action of a magnetic field, the erosion speed of molten steel increases, the solidification front is not smooth, the forced convection flow becomes turbulent, and dendrites are greatly suppressed. A part of it can not only cut off and erode the crystal arms of columnar crystals, but also form a large number of dendrite fragments that serve as the nuclei of equiaxed crystals, causing the grains to grow exponentially, which is beneficial to the refinement of grains in the solidification structure. Thermal effect of electromagnetic stirring:. First, the forced flow caused by electromagnetic stirring accelerates the heat transfer of the molten steel, causing the superheat in the molten steel to disappear quickly. The crystal nuclei formed in the initial stage can be preserved; together with the crystal nuclei formed by electromagnetic stirring, they are broken, eroded and multiplied. Second, when the overheat is released and the temperature of the molten steel drops to between the liquidus temperature and the solidus temperature, some small crystal nuclei will appear and remain in the liquid. It grows with further cooling and finally solidifies in the form of volume crystallization, forming a solidified structure with a fine equiaxed crystal structure.

Analysis of experimental results

(1) Effect of electromagnetic stirring of the copper mould tube on shrinkage cavities of continuous casting billet

The influence of the electromagnetic stirring intensity of the copper mould tube on the shrinkage cavities of the continuous casting billet becomes larger with the increase of the electromagnetic stirring intensity at the same frequency. If the occurrence frequency of level 0 and level 1 shrinkage cavities increases, it means that the improvement of the central shrinkage cavity is good. When it reaches a certain value, the effect of improving shrinkage cavities is not obvious as the strength increases, and it is even detrimental to improving shrinkage cavities. Increasing the stirring force will cause the temperature of the molten steel in the copper mould tube to dissipate faster, making the temperature in the copper mould tube more evenly distributed. Therefore, it is beneficial to improve the central shrinkage cavity of the continuous casting billet. The fluctuating meniscus will make the temperature in the copper mould tube uneven, which is not conducive to improving shrinkage cavities and may even promote the formation of shrinkage cavities.

(2) Effect of electromagnetic stirring in the copper mould tube on center segregation of continuous casting slabs

The central segregation problem of continuous cast high carbon steel billets is a fatal defect that has a serious impact on the performance of wire rods. Its occurrence will cause the wire rods to break during drawing and stranding. Taking 82B high carbon steel as the target steel type, the effect of electromagnetic stirring in the mold on the center segregation of elements was studied. The continuous casting billet with a cross-sectional size of 160mmx160mm was sampled, and the cuttings were drilled with an electric drill and analyzed for their composition. Carbon is analyzed using infrared and other elements are analyzed using spectroscopy. The test results show that the carbon in the continuous casting billet without electromagnetic stirring has serious central segregation. After electromagnetic stirring in the copper mould tube, there is an obvious improvement, and the stronger the intensity, the better the effect.

(3) Effect of electromagnetic stirring of the copper mould tube on the purity of continuous casting billet

First of all, the impact of electromagnetic stirring in the copper mould tube on the total oxygen content of the continuous casting billet is very significant, mainly due to the gradual accumulation of oxides during the electromagnetic stirring process to form inclusions and float up. The electromagnetic stirring of the copper mould tube has no obvious effect on the total nitrogen content of the continuous casting billet, mainly because the content of nitride elements in the molten steel is low; secondly, the electromagnetic stirring of the copper mould tube has a certain impact on the inclusions in the continuous casting billet. In the experiment, the 82B continuous casting billet was processed to standard specifications, and the inclusions were separated through a series of treatments such as oil quenching and electrolytic extraction, and their contents were analyzed. It can be judged that the electromagnetic stirring of the copper mould tube has an obvious effect on the oxides in the continuous casting billet, but has no obvious effect on the sulfide.

Generally speaking, if the superheat of molten steel is too small, it will be easily contaminated by inclusions, while if the superheat is too high, it will cause serious central segregation. Therefore, during construction, emphasis should be placed on controlling the optimal value of superheat. This ensures that the quality of the cast slab is effectively improved.

Conclusion

In summary, electromagnetic stirring of the copper mould tube is a technology that can effectively solve problems such as shrinkage cavities in the center of continuous casting billets and center segregation of high carbon steel. The quality of the cast slabs obtained by using the improved electromagnetic stirring technology of the copper mould tube will also be greatly improved. Moreover, the copper mould tube electromagnetic stirring technology can remove oxide impurities present in molten steel. The higher the stirring intensity, the better the removal effect. In short, the electromagnetic stirring of the mold has more positive effects on the continuous casting billet and can be put into use more effectively.

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