【作者】 王朝阳；

【导师】 赫冀成； 王恩刚；

【作者基本信息】 东北大学 ， 热能工程， 2009， 博士

【摘要】 钢的电磁软接触连铸技术是通过在结晶器外施加交变电磁场来控制钢液的初始凝固过程来提高铸坯的表面质量,近年来该技术得到了广泛的研究,研究的焦点主要集中在高频电磁场下的软接触连铸技术。但是,由于高频电磁软接触电源和结晶器的制造难度大、造价高,高频交变磁场的致穿透性差,能量损耗高,对铸机电子设备的干扰大,工业推广困难等问题,提出了以中频电磁场代替高频磁场的电磁软接触连铸技术。研究和探讨电参数、结晶器结构参数和工艺参数条件下中频电磁软接触连铸结晶器内的磁场、流场、传热和凝固和弯月面行为变化规律,为后续工业用中频电磁软接触连铸技术的应用提供理论和技术支持。本文在理论分析的基础上建立了中频电磁软接触连铸结晶器内的三维电磁场有限元模型,揭示了不同参数下结晶器内磁感应强度、电磁体积力、感生电流和焦耳热的三维分布规律。结果表明：在中频电磁场作用下,结晶器内沿拉坯方向上的磁场分量远大于铸坯径向和周向上的分量,铸坯中沿径向上电磁压力的分量要远大于其它两个方向上的分量,表明施加中频磁场具有与高频磁场相同的以电磁压力为主的磁场特性；沿拉坯方向上的磁场作用范围主要集中在线圈高度范围内,在线圈中心高度附近出现磁感应强度峰值,向结晶器两端迅速减小；当频率为2500Hz,电流强度为800A时,结晶器内最大磁感应强度达0.07261T,满足软接触技术的要求；结晶器切缝中心与分瓣体中心两处的磁感应强度分布趋势相同,分瓣体中心处的磁感应强度峰值相对较小；结晶器内磁感应强度随线圈电流强度的增加线性增加；随电源频率的提高,最大磁感应强度增加,增幅却在减小,结晶器内周向上磁感应强度的均匀性会逐渐变差；切缝宽度对结晶器内拉坯方向上的磁感应强度的分布影响不大,建议切缝宽度选择0.5～0.8mm；随着切缝数的增加,结晶器对中频磁场的屏蔽减小,钢液外表面的磁感应强度增大,但增幅逐渐减小；当分瓣数为8时,中频(2500Hz)电磁软接触连铸结晶器内周向上磁场的均匀度与高频磁场(20.5kHz)下分瓣数为12的结晶器内周向上的磁场均匀度相当；金属液面高度自线圈顶端向下移动时,钢液表面磁感应强度先增大后减小,在自由液面位于感应线圈中心高度位置时获得的磁感应强度最大,随着自由液面高度的下降沿拉坯方向上磁场分布不均匀性加大；随着线圈相对位置的下降,结晶器内沿拉坯方向上的磁场分布规律不变,但磁场作用的有效区域整体随之下降,最大磁感应强度也随之有所减小。在连续介质模型基础上,建立了中频电磁软接触连铸结晶器内钢液流动、传热和凝固的耦合计算模型,揭示了不同参数下结晶器内钢液的流动和温度分布规律,结果表明：中频电磁场主要对弯月面区域的钢液有明显力的作用,使弯月面区域的流速加快,温度升高,初始凝固点位置略微下降,初始凝固壳厚度变薄,结晶器下端几乎不受电磁场的影响；自由表面上钢液流动速度按切缝、分瓣体中心反向对称分布,使自由表面产生凸凹变形；钢液纵截面内流动呈现双回流,较常规连铸结晶器在弯月面区域多产生一个由电磁力作用而产生的回流区；随电流强度的增大,结晶器内上部熔池的流体速度和温度增大,电源频率改变会影响钢液上部涡流区域速度的大小和方向,弯月面趋于平稳,自由表面三相点附近钢液的温度略有升高,提高电源频率有抑制自由表面波动的作用。在数值计算的基础上,对φ100mm圆坯中频电磁软接触连铸结晶器内的电磁场分布、弯月面行为进行了实验研究,实验结果表明：圆坯中频电磁软接触结晶器内沿周向上的弯月面变形较均匀；在结晶器的分瓣体中心处和切缝处弯月面高度相当,分瓣体中心处的弯月面高度较切缝处稍低；随着电源功率增大,弯月面高度增大,但是弯月面波动逐渐加剧,初始凝固三相点的稳定性变差,不利于获得表面质量良好的连铸坯；在保证适宜弯月面高度的情况下,在2500Hz、30kW条件下,只需高频电磁软接触近一半的功率就能获得与之相当的弯月面高度,大幅节约能源；当初始液面在线圈中心位置时,弯月面的高度最大；当自由液位平行感应线圈顶端和底端时,弯月面的波动都相对较弱,液位越靠近感应线圈中心高度时波动越剧烈,偏离线圈中心高度后波动都会有所减弱；在拉坯过程中,采用合适功率时应将自由液面控制在线圈中心高度偏下附近,以在保障软接触效果的前提下达到节能的效果；随着频率的增加,弯月面的高度几乎成线性关系减小,但弯月面的波动减小,趋于稳定；且随着频率的增加,波动方差的衰减幅度减小。综合考虑结晶器内磁场强度的分布情况、弯月面高度和弯月面波动等情况,中频电磁软接触连铸适宜的频率为2500Hz。更多还原

【Abstract】 The technology of soft-contact electromagnetic continuous casting of steel is used to improve the surface quality of billets by imposed an alternating electromagnetic field to control the initial solidification of liquid steel. In recent years, the technology research has been focus on the soft contact electromagnetic continuous casting under high-frequency electromagnetic field. However, the manufacture of high-frequency power and mold are very difficult and high cost, high-frequency alternating magnetic field has a strong skin effect which causes poor penetrating and huge energy losing, disturb the normal working of the electronic devices of the caster, the industrial promotion of soft-contact electromagnetic continuous casting very difficult, and so inadequate. In this paper, the technology of using intermediate frequency electromagnetic fields in place of high-frequency electromagnetic field has been put forward and carry out the investigation of the electromagnetic field, flow field, heat transfer, solidification, meniscus behavior and the influence of different electrical parameters, structure parameters of mold and process parameters on them in order to provide theoretical and technical fundamention for industrial applications.Based on the theoretical analysis, the three-dimensional electromagnetic field finite element model of intermediate frequency electromagnetic soft-contact continuous casting mold is developed and the three-dimensional distribution of magnetic induction intensity, electromagnetic volume force, induced current and Joule heat in mold are revealed. The results show that:under the intermediate frequency magnetic field, the magnetic induction intensity component along the casting direction in the mold is much larger than ones in the direction of radial component and circumferential component, so that the magnetic pressure component along the radial is much larger than that in the other two directions, which shows that the characteristic of magnetic filed in the mold with imposed intermediate frequency magnetic fields has the same characteristics of electromagnetic pressure as high-frequency magnetic field. The magnetic field mainly concentrated in the high range of the coil along the casting direction, the position of maximum magnetic induction intensity is near the center height of the coil, and the magnetic induction intensity rapid decrease along the both ends of mold. With the frequency of 2500Hz and current intensity of 800A, the maximum magnetic induction intensity is 0.07261T, which meets the requirements of soft-contact technology. The distributions of magnetic induction intensity are same in slit and segment center, but the maximum magnetic induction intensity in segment center is slightly less than that in slit. The magnetic induction intensity linear increases with the increasing of current intensity in coils, it also increases with the increasing of frequency, but the increasing amplitude decrease. When the frequency is above 2500Hz, the uniformity of magnetic induction intensity along the circumferential direction will gradually deteriorate. The slits width has little effect on the distribution of magnetic induction intensity along the casting direction in mold, so the slits width of 0.5mm-0.8mm is recommended. With the increasing of slits number, the shielding of mod to intermediate frequency magnetic field is reduced, the magnetic induction intensity in outer surface of molten steel increases, but the growth rate gradually decrease. When the slits number is 8, the uniformity of magnetic fields along the circumferential direction of mold under intermediate frequency (2500Hz) is as the same as that in mould with slits number of 12 under high-frequency (20.5 kHz). With the height of free surface declining from the top of coils, the magnetic induction intensity in outer surface of molten steel firstly increases and then decreases. The maximum magnetic induction intensity can be gotten when the position of free surface is at the center height of coils. With the height of free surface declining, the non-uniformity of magnetic field distribution increases. With the relative position of the coils declining, the distribution of magnetic field along the casting direction doesn’t change, but the effective region of magnetic field declines, the maximum magnetic induction intensity decreases slightly.From the Bennon’s continuum model, a three dimensional coupled mathematical model of flow field, heat transfer and solidification in mold under intermediate frequency electromagnetic soft-contact continuous casting is developed. The fluid flow and temperature distribution of molten steel are revealed under different parameters. The results show that: intermediate frequency electromagnetic field mainly has effects on the meniscus zone of billet, which induce the incensement of fluid velocity and temperature of molten steel, the position declining of initial solidification point and thin shell thickness of initial solidification, and it cannot act on molten steel in the lower part of mold. The flow velocities are periodical antisymmetric distribution at the center of slit and segment which results in the concavo-convex deformation at free surface. There are two vortexes forming in longitudinal section compared to the one vortex in conventional continuous casting. The flow velocity and temperature increase with the current intensity increasing in the upper part of mold. When frequency increases, the magnitude and direction of fluid velocity vector also altered, meniscus gradually stabilizes, the temperature of molten steel in three-phase-point near the free surface increases slightly, the fluctuation of free surface of liquid steel could be retrained under the higher frequency electromagnetic field.Based on the numerical simulation, compared the calculation results, experimental study of electromagnetic field distribution and meniscus behavior was carried out forφ100mm round billet under intermediate frequency soft contact electromagnetic continuous casting, revealed the rule of electromagnetic field distribution, meniscus deformation and meniscus fluctuations under different parameters. The experimental results show that:the meniscus height was very closed along the circumferential direction, meniscus height in segment slightly lower than that in the slit, the meniscus height increased with the power increasing and the fluctuations of meniscus become more intense, the initial solidification environmental of three-phase point is degradation, all of these are not conducive to obtain good surface quality billet. With the conditions of appropriate meniscus height, frequency of 2500Hz, power of 30kW, the meniscus height is almost as same as that under high-frequency soft contact electromagnetic continuous casting with frequency of 20.5 kHz and power of 67.2kW, a significant energy is saved. When the position of free surface is in the center height of the coil, the meniscus height is maximum. When the free surface paralleled the top and bottom of induction coil, meniscus fluctuations is relatively stable. When the position of free surface was more closed to the induction coil centers, the meniscus fluctuations become more intense. In the casting process, under the appropriate power, the free surface should be controlled near the center under partial of coils in order to ensure the effectiveness of soft contact and energy-saving. With the frequency increasing, the meniscus height decreases almost linear relationship, the meniscus fluctuation reduce, the attenuation range of fluctuation variance reduce. Considered the distribution of magnetic induction intensity, meniscus height and fluctuations, the frequency of 2500Hz is suggested to the appropriate parameter of intermediate frequency soft contact electromagnetic continuous casting更多还原