| Electromagnetic soft-contact continuous casting technology is a novel efficient continuous casting technology. It can improve the surface quality of the continuous casting billet dramatically which can make it possible that the continuous casting billet can be directly rolled without surface conditioning. When the technology is applied, the process can be shortened which is good for saving energy and improving the metallic yield. In this thesis, some key problems such as the distribution of magnetic field in the mold, the meniscus behavior as well as the effect of electromagnetic field on the liquid steel and the mechanism thereof were investigated. These investigations gave a firm protection to a successful implement of the plant trail. Moreover, some results of the plant trial were presented in the thesis.Firstly, theoretical deduction was used to investigate the distribution of magnetic field in the induction coil without load and the propagation characteristic of the temporarily harmonious electromagnetic wave in the liquid steel. The electromagnetic body force and electromagnetic pressure subjected to the liquid steel in the mold were deduced based on three hypotheses. The results of theoretical analysis show that the magnetic field in a space point is not only related to the coil current and the position of the space point, but also related to the configuration parameters of the induction coil, i.e., radius, pitch and number of the coil. The temporarily harmonious electromagnetic wave in the liquid steel attenuates exponentially so do the electromagnetic body force and electromagnetic pressure from the mold wall to the center. And they attenuate more rapidly as the frequency of the magnetic field increases. The analysis on the meniscus equation shows that the meniscus height depends on the electromagnetic force in the triple point, interfacial tension and the density difference between liquid steel and mold flux. When the term of electromagnetic force is eliminated, the meniscus equation comes to be the Bikerman’s equation. The equation of Joule heat generated in the mold within the influence range of the electromagnetic field was deduced by changing the integration limit of the original equation referred by the textbook. From the modified Joule heat equation, the result that the Joule heat increases as the increase of the frequency of magnetic field. For this reason, the frequency of magnetic field can’t be too high in practical use. Otherwise, too much Joule heat will hinder the growth of the equiaxed crystal.Secondly, distribution of magnetic field in the mold was measured using electromagnetic induction method. The results show that magnetic field in the mold concentrates on the common area of the induction coil and the load along the casting direction, and the peak value locates10-15mm below the coil center, while the magnetic induction density becomes more and more weak as the distance of the tested point from the coil center increases. Along the circumferential direction, the magnetic induction density measured in the mold slit is bigger than it measured in the adjacent segment center. When the power increases, the magnetic induction density in the mold increases, however, the homogeneity along the circumferential direction becomes worse. The magnetic induction density in the mold decreases as the increase of frequency of the magnetic filed, but when it exceeds40kHz, the frequency has minor influence on the peck value. The distribution of magnetic filed moves with synchronization as the installation site of the induction coil changes. When the coil center and load level locate in the center of the mold slit, the peak value becomes to be the maximum. So, when the induction coil is set at this optimal position, favourable metallurgical effect can be obtained.Thirdly, dip process was used to measure the meniscus shape under the high frequency electromagnetic field using low melting alloy Sn-Pb-Bi as the experimental material. The result show that the radius curvature and height of the meniscus increase when the power increase, and the triple point and the contact angle between the meniscus and the mold wall decrease. The meniscus deforms dramatically when the induction coil is set at the optimal position recommended by the investigation of the distribution of magnetic field in the mold. In this case, the mold flux channel between the mold wall and the initial solidified shell is widened, which is good for lubrication. The meniscus rises remarkably at the frequency range of20-30kHz. For this reason, we recommend20-30kHz as the optimal frequency range to be used in the plant trial. Check experiment was carried out using liquid steel, as a result, similar phenomenon mentioned above was observed. So Sn-Pb-Bi which is more safe and controllable can be used to simulate the meniscus behavior of liquid steel, and certainly the findings obtained from this method can be used to explore the electromagnetic soft-contact technology. In addition, laser displacement sensor was used to measure the surface fluctuation of Sn-Pb-Bi under high frequency electromagnetic filed. The result shows that the surface fluctuation is related to the coil current and the frequency of magnetic field, i.e. the fluctuation extent increases as the increase of coil current, but decreases as the increase of frequency of magnetic field.Laboratory sale electromagnetic soft-contact continuous casting experiments were carried out at a multi-functional continuous caster with carbomsteel Q235B and pearlitic heat resistant steel15CrMo, according to the research results mentioned above. The results show that defects such as oscillation marks, depressions and cracks are produced on the billet under the conventional continuous casting condition. But when high frequency electromagnetic field is applied on the initial solidifying area, the oscillation marks is decreased, and when the power comes to an optimal value, the oscillation marks is totally eliminated. However, when the power exceeds the optimal value, oscillation marks appears again in a wavy pattern. The theoretical analysis shows that there are mainly two reasons why the surface quality of the continuous casting billet is improved, including the Lorenz force effect and Joule heat effect. When the power is excessive, the homogeneity of magnetic field along the circumferential direction of the mold becomes worse. Therefore the starting point during solidification presents a wavy pattern along the circumferential direction of the mold. Additionally, the surface fluctuation of liquid steel becomes more acutely when the power is excessive. Thus, wavy oscillation marks are produced due the two reasons.In March,2008, commercial sale electromagnetic soft-contact continuous casting experiments were successfully carried out at a billet continuous caster of Baosteel in China. Low alloy steel with0.18wt%C was employed for the casting material and16.6kHz was chosen as the frequency of magnetic field. The depth of the oscillation marks was detected with a high-precision laser displacement sensor, and the macrostructure was tested with cold acid etching method. The results show that the mold flux consumption is increased by11.9%in the electromagnetic soft-contact continuous casting, compared with conventional continuous casting. The depth of oscillation mark on the billet casted in the conventional way is0.65mm on average, while in the electromagnetic soft-contact continuous casting the oscillation mark is totally eliminated when the power is129kW. At last, we also find that high frequency electromagnetic field has minor influence on the inner quality of billet. |
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