【作者】 于湛；

【导师】 任忠鸣；

【作者基本信息】 上海大学 ， 钢铁冶金， 2010， 博士

【摘要】 结晶器是连铸机的心脏,结晶器内的冶金过程是改善连铸坯质量的关键环节。大量的研究表明：铸坯的质量与结晶器内钢液的流动密切相关。随着铸坯拉速的提高,水口出流的流量和流速增加,结晶器内的钢液流动加速,流动的稳定性变差；表面流速过大容易发生卷渣,引起液位波动加剧,使铸坯产生表面缺陷；钢液流速过大对初生坯壳的冲刷增强,坯壳厚度均匀性降低、表面振痕增加；钢液的冲击深度同时增加、夹杂物和气泡难以上浮,影响铸坯的内部质量。因此,研究和控制结晶器内钢液的流动具有重要的意义。为了解决高速连铸中出现的这些问题,电磁技术在连铸工艺中得到发展,例如电磁制动和电磁搅拌技术。本文对电磁场控制板坯、方坯和双辊薄带连铸结晶器内流动进行研究,并结合现场生产中出现的质量问题,分析了结晶器内流动对铸坯质量的影响。并根据三种不同连铸结晶器内钢液的流动特点,分别施加不同的电磁场,以改善结晶器内的钢液流动。研究的主要内容包括：在高速板坯连铸方面：采用水银为介质,结合目前钢铁企业普遍使用的全幅一段和FC-Mold两种电磁制动技术,模拟结晶器内钢液的流动,使用超声波多普勒测速仪测量结晶器内液体的流速和液面的波动,分析磁场对流体流动的影响,研究磁感应强度和磁铁的位置对结晶器内金属液流动的影响。结果表明：施加磁场抑制了浸入式水口射流的运动,改变了结晶器内动能和湍流度的分布,结晶器内金属液流动的稳定性增强。全幅一段电磁制动的研究发现：当Bmax大于0.29T时,液面波动减弱,液面水平流动稳定,改善弯月面处的流动条件；金属液向下的冲击强度减弱,冲击深度减小；对结晶器壁的冲刷减弱。模型中Bmax=0.29T对应原型Bp=0.16T,所以在实际生产中,磁感应强度大于0.16T时,更有利于优化和控制结晶器内的流动,提高铸坯质量。磁铁靠近浸入式水口,有利于改善上部环流区的流动、液面的波动和流动的稳定性；磁铁下移远离浸入式水口有利于减小流体的冲击强度和冲击深度,因此,实际生产中则应考虑连铸工艺、设备等因素确定磁场的位置。与全幅一段电磁制动相比,FC-Mold更能有效地改善上部环流区的流动,减小液面的波动和液面处水平流速,提高液面的流动的稳定性；但对向下运动流股的冲击强度和冲击深度控制方面,全幅一段电磁制动的效果更好。实验结果表明,磁场不仅具有制动钢液流的作用,还具有改变流动方向,分配钢液流量的作用。因此,通过优化磁场可实现结晶器内钢液流场的优化。在小方坯连铸方面：针对小方坯连铸的直通水口,开发了电磁旋流水口技术,并通过在浸入式水口内安装旋流转子模拟旋转电磁搅拌,运用水力学模拟结合数学模拟的方法,进行连铸结晶器内流动的研究。结果表明：旋流式水口有利于改善结晶器内的流动,加强了结晶器上部液体的流动,增大液流向弯月面区域的回流,使液面形成水平环流,进而提高液面的活跃度,并增强钢液表面熔渣效果；减弱了下部流动,有效降低冲击深度,使结晶器内横截面上的流动变得均匀；在旋流水口条件下,结晶器上部湍流度增大,下部湍流度减小,这样的流动有利于改善结晶器上部温度的分布,促进结晶器下部夹杂物和气泡的上浮；转子位置靠近水口出口,出流的冲击深度降低,液面流动增强,因此,理论上电磁搅拌器的位置越靠近水口出口其效果越好,而在实际生产中则应考虑连铸工艺、装备和操作性等因素确定搅拌器的位置。低熔点合金的试验表明：旋转磁场能够使浸入式水口的金属液产生旋转流动。对转子叶片角度的研究表明：当叶片的角度减小到某一值时,再减小其角度,水口出口的切向速度不再增大,旋转流动对浸入式水口内壁的冲刷增强；对无量纲旋度的研究表明：旋度大于0.5时,增大旋度,对冲击深度影响不大。在电磁搅拌条件下,电磁搅拌的强度决定水口出口的切向速度和旋度,可见,电磁搅拌的强度存在一个最佳值,因此,在实际生产中应该考虑拉速等连铸操作因素确定电磁搅拌的强度。在双辊薄带连铸方面：采用静磁场耦合直流电对双辊薄带浸入式水口(布流器)内金属液的流动进行控制,利用水银模拟钢液,分别研究磁铁位置和电流强度对熔池内弯月面处流动以及自由液面波动的影响。结果表明：水口电磁制动技术有利于改善熔池内液面的流动,减小自由液面的波动,从而改善初始凝固的条件,有利于提高铸坯的质量,同时也为其他技术的应用提供良好的工作条件；磁体安放在距水口出口较近的位置有利于减小自由液面的波动；采用直流电耦合静磁场较只使用静磁场更有助于改善熔池内的流动,减小液面附近的流动和自由液面的波动。更多还原

【Abstract】 The mold is the key part of the continuous caster, in which the metallurgical processes are very critical to improvement of slab quality. A lot of researches showed that most of the defects affecting steel quality in the process were closely associated with fluid flow in the mold. With increase of casting speed, both of the flow rate and flux of the flow discharged in the submerged entry nozzle (SEN) increased and the flow of liquid steel was accelerated in the mold, Moreover, the flow stability of liquid steel was deteriorated. Excessive surface velocity could entrain mold flux and cause surface level variations and fluctuations that produced surface defects. Excessive velocity of liquid steel could strengthen scouring action to the initial solidification shell, degraded the uniformity of the shell, increased the depth of the mark and deep penetration of the jet entering the mold and hindered the inclusions and bubbles float upward that produce the inter defects. Accordingly it is extremely important to study and control the flow in the mold. To solve these problems, electromagnetic techniques have been developed and applied to the continuous casting process, such as electromagnetic brake and electromagnetic stirring. In the paper, the flows in mold of the continuous casting slab, billet and two- roll strip with electromagnetic fields have been studied, and the effects of the flows on the quality of steel have been analyzed combined with the questions in the industry. The different electromagnetic fields have been applied to improving the flows in three kinds of molds according to their flow characteristics. The main studies are given as follows.For the high slab continuous casting, mercury was used to simulate the flow liquid steel with electromagnetic brake ruler (EMBR) and Flow Control Mold (FC-Mold), which are widely used in the industry at present. The flow in the mold and fluctuation of meniscus were measured by the DOP2000 velocimeter. The effects of magnetic fields on the flow in the mold have been analyzed. In addition, the effects of magnetic flux density and location of the magnets on the flow in the mold have been studied as well. The results showed that the flow discharged from the SEN was suppressed, both of the distribution of the kinetic energy and the turbulence intensity were changed, and the flow stability of liquid metal in the mold was enhanced. In electromagnetic brake ruler when Bmax was more than 0.29T, the surface level fluctuations were weakened, the flow at the meniscus became stable and the flow at the meniscus were improved, and at the same time the impact action of liquid metal was weakened, and the penetration depth was reduced. The Bp=0.16T in the prototype is corresponding to Bmax=0.29T in the model. So in the industry when the Bmax is more than 0.16T, it is beneficial to optimize and control the flow in the mold and improve the quality of steel. It was beneficial to improve the flow in the upper eddy, fluctuation of meniscus and stability of the flow when the magnet was located near the SEN. However, it was good to reduce the impact action and penetration depth when the magnet was far from the SEN. Hence improvement of the flow relies on many factors, e.g., continuous casting process and equipments in the industry. Compared with EMBR, it was more effective to use FC-Mold for improving the flow in the upper eddy, fluctuation of meniscus and stability of the flow. Nevertheless, it was more effective to reducing the impact action and penetration depth in the lower mold using EMBR. The experiment results showed that a magnetic field could not only damp flow, but also change the flow direction and distribute the flux of liquid steel. Thus, the flow in the mold could be improved by optimizing the distribution of magnetic field.For billet continuous casting, the electromagnetic swirling nozzle has been exploited on the base of straight nozzle. The physical and numerical modeling has been carried out to study the flow patterns in continuous casting billet mold using swirling flow nozzle, and the effect of electromagnetic stirring in SEN was simulated by setting a swirl blade in SEN. The results indicated that the swirling flow nozzle technique is conducive to improve the flow field in the mold, enhance the flow in the upper mold, and increase the velocity on the free surface. The activity of liquid level and the effectiveness of molten slag increased. The flow was weakened in the lower mold and the penetration depth was effectively reduced. The flow in the horizontal section became uniform. The turbulent intensity in upper mold increased and reversely the turbulent intensity in the lower mold was reduced, so it was beneficial to improve the temperature distribution in the upper mold and promote the float of inclusions and bubbles in the lower mold. When the blade was near the outlet of nozzle, the penetration depth was reduced and the flow on the free surface was enhanced. Thus, the effect is more obvious when the electromagnetic stirrer is more closely near the outlet of the nozzle theoretically, but its location should be determined according to the continuous casting process, equipments and operation parameters in the industry. The experiment results of low melting point alloys showed that the swirling flow in the SEN could be generated by the electromagnetic stirrer. Investigation of the angle of the blade showed that the tangential velocity did not increase any more when the angle of the blade was less than a certain value; the scour action to the inner wall of the nozzle was enhanced by the swirling flow. The dimensionless swirling number showed that the effect of swirling number on penetration depth was not obvious by increasing swirling number when swirling number was more than 0.5. In the case of electromagnetic stirring, the intensity of electromagnetic stirring determined the tangential velocity and swirling number at the outlet of the nozzle, and there was the optimization intensity of electromagnetic stirring. Consequently, the intensity of electromagnetic stirring should be determined according to operation parameters in continuous casting, e.g., casting speed, in the industry.For two roll strip casting, the flow in twin roll pool was controlled by imposing static magnetic fields coupled with direct current in the nozzle. The effects of location of magnets and intensity of electric currents on surface fluctuation and flow behaviors near the meniscus have been investigated using mercury, respectively. The experimental results showed that it was beneficial for improvement of the flow at the meniscus and the free surface fluctuation to apply the nozzle electromagnetic brake technology, which would be good for improvement of the conditions of initial solidification and the quality of steel. Additionally, it could provide good conditions for other technologies. When the magnet was placed nearer the outlet of the nozzle, it is beneficial to reduce the free surface fluctuation. The magnetic field coupled with direct current is more effective for suppression of the flow near the meniscus and the free surface fluctuation in twin-roll strip casting caster compared with only imposing a magnetic field.更多还原