【作者】 董安平；

【导师】 孙宝德；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2009， 博士

【摘要】 热镀锌钢板大量应用于汽车内外板,高表面质量是其必需具备的基本条件。镀锌液中的锌渣是影响镀锌板表面质量的最重要因素,由锌渣引起的各种缺陷造成了巨大的经济损失。目前防止锌渣缺陷产生的措施主要是通过镀锌工艺控制和设备改造,抑制锌渣的形成。随着材料电磁加工技术的发展,电磁净化已经成为具有应用前景的新型金属熔体净化工艺。尝试将电磁净化技术应用于热镀锌液中锌渣的去除,揭示外加电磁场作用下锌渣相的电磁分离机理和分布变化,不仅具有重要的理论价值,而且对于高质量热镀锌产品的生产也具有显著的应用价值。针对热镀锌液中锌渣去除的研究现状,本文通过电磁分离热镀锌液中锌渣这一新工艺开展了如下研究:(1)揭示了交变磁场作用下锌渣的电磁分离机理;(2)通过数值分析研究了锌渣去除效率的影响因素;(3)建立了热镀锌液电磁分离的实验装置,并进行了热镀锌液静态分离实验和连续分离实验;(4)设计了大流量的热镀锌液电磁净化中试试验装备,并开展了热镀纯锌(GI)和合金化(GA)钢板镀锌液体系的中试试验研究;(5)通过数值模拟研究了中试锌锅及工业锌锅中的流场、温度场和锌渣浓度场在锌液体外循环净化下的变化规律。通过上述研究提出了热镀锌液电磁净化的技术原型,为热镀锌液电磁净化技术的工业应用奠定了实验和理论基础。通过建立锌渣电磁分离的计算模型,研究了锌渣电磁分离过程中的运动规律、锌渣分布变化以及去除效率与主要工艺参量的变化关系。由于锌渣相的导电率与锌液存在数量级的差异,锌渣颗粒的受力为一个与锌液所受电磁力方向相反的挤压力作用,其大小在数值上等于等体积锌液所受电磁力的0.75倍。因此,通过在锌液熔体内施加定向电磁力作用即可实现热镀锌液中锌渣的定向分离。锌渣的去除效率与电磁力施加时间、磁感应强度、磁场频率和分离器尺寸有关。锌渣的去除效率随平均停留时间和磁感应强度的增大而增大;当磁场频率一定时,分离器尺寸与集肤深度的比值a/δ在2附近时锌渣去除效率最佳,随着分离器尺寸的进一步增大锌渣去除效率将减小;在分离器尺寸一定时,锌渣去除效率随磁场频率的增大而增大。根据分离器单个通道的尺寸,合适的磁场频率应使得集肤深度δ=2.5 mm,因此,实际磁场频率f可选择为15 kHz-20 kHz。在分离器尺寸、磁场频率和锌液在电磁感应线圈中平均停留时间一定的条件下,磁感应强度值不小于0.06T,净化电源的额定功率为50kW。以电磁参数计算结果为理论依据建立了热镀锌液静态电磁净化装置并开展了相关实验研究。实验结果表明:在磁场频率为17.5kHz、磁感应强度均方根值为0.05T时,电磁作用15 s可有效去除不同铝含量的热镀锌液中粒度大于10μm的锌渣。设计了热镀锌液连续净化实验装置,并进行了锌液连续流动条件下的电磁净化实验,对锌渣去除效率随工艺参数的变化规律进行分析,实验结果表明:当磁场频率为17.5kHz,有效磁感应强度为0.05-0.1T,电磁力施加时间为0.6-2.5s,陶瓷管横截面的锌液通道尺寸为10×10mm,10μm粒度的锌渣分离效率变化范围从43.76%到98.63%,实验结果与数值计算结果吻合较好。为了提高分离效率,充分利用方形孔陶瓷管内锌液的二次扰动,研制了多级分离器,利用数值模拟对锌渣去除效率进行了计算,由于第一级分离器每个通道的中心位置现在被壁面占据,而原本处于分离“弱区”的锌渣在第二级分离器内恰恰处在靠近壁面的“强区”,锌渣的电磁分离效率可提高10%左右,并通过实验加以验证。在现有电磁净化实验装置的基础上,设计了一套具有自主知识产权的热镀锌液电磁净化中试装备。本装置在国内首次使用了热镀锌液体外循环净化技术,成功实现了锌液体外连续电磁净化,成为该技术工业应用的装备雏形。在中试平台上进行的GA镀锌液优化试验结果表明,通过延长电磁净化时间、优化工艺参数和改变陶瓷管分离器结构可明显改善净化效果,锌锅内所有取样点的平均锌渣去除效率可达89.52%,电磁净化流槽中陶瓷分离器前后的锌渣去除效率约为75%左右,粒度大于20μm的锌渣经过电磁净化可完全去除;中试平台上进行的GI镀锌液电磁净化试验表明,锌渣去除效率可达70%,电磁净化流槽中粒度小于15μm的锌渣去除效率仅为42.1%,而大于15μm的锌渣去除效率为91.67%,直径大于20μm的锌渣可全部去除。中试锌锅的数值模拟结果表明,在热镀锌液体外循环电磁净化的前提下,综合考虑实验平台内锌液的流动和除渣率两方面因素,当回锌管液流速度较低时,宜取较短的回锌管长度;而当回锌管液流速度较高时,宜取较长的回锌管长度。回锌管液流速度或净化效率较高时,实验平台内锌渣浓度总体上均处于较低水平,除渣效果较好。锌渣粒径对锌渣浓度场和除渣率影响不大。工业在役锌锅的数值模拟结果表明,回锌管或抽锌管的放置与否主要影响锌锅沉没辊上方的V形区及其附近区域,而对锌锅内锌液的总体流动影响甚小。循环净化条件下锌锅中锌渣的浓度随处理时间的增加而逐渐降低。相比于普通锌锅,循环净化方式的引入有利于热镀锌板质量的提高。更多还原

【Abstract】 The hot-dip galvanizing steel sheet has been widely used in automotive exposed and unexposed body panel application, which requires premium surface quality. The most important factor that impairs the surface quality of the hot dip galvanizing steel sheet is the dross defect, causing a dramatic economical loss. So far, the method to eliminate the dross defect is to restrict the dross formation by processing control and equipment upgrade. Recently, with the rapid progress of electromagnetic processing of materials (EPM), electromagnetic separation technology has been developed and become a novel melt purification method with much prospect in application. It is of a great value to make electromagnetic separation technology applied in removing zinc dross from hot dip galvanizing zinc melts, clarifying the separation mechanism and distribution evolution of dross phases under electromagnetic field.Based on the state of the art of zinc dross removing from zinc melts, the following research works have been carried out on the new technology of removing zinc dross by using electromagnetic separation: (1) The zinc dross separation mechanism has been investigated by using alternating magnetic field; (2) Study on influence factors of zinc dross removal efficiency through numerical analysis; (3) The laboratory-scale experimental apparatus of electromagnetic separation have been set up, and experiments to static and continuous separation deleterious zinc dross from zinc bath were conducted on a laboratory-scale apparatus; (4) The establishment of a series of pilot-plant-scale equipment, GA and GI zinc melts purification were carried out on this equipment; (5) By numerical simulation of the pilot-plant-scale and industrial zinc pot, the change laws of velocity field, temperature field and concentration field of zinc dross were studied when the zinc melts pumped out the zinc pot. All of these investigations provided experimental and theoretical basis for hot-dip galvanizing electromagnetic purification technology applied in industry.Through the establishment of the electromagnetic separation calculation model, their kinematic behaviors of zinc dross in the electromagnetic separation process, zinc dross distribution and the relation between the removal efficiency and the main operation parameters have been investigated. The conductivity of zinc dross is far less than the zinc melts, so the zinc dross experienced the repulsive force opposite to the electromagnetic force on the melt under electromagnetic field, and the repulsive force is 3/4 times that of the electromagnetic force for same volume of zinc melts. Accordingly, those particles can be successful separated from the metal melt under electromagnetic force. Removal efficiency of zinc dross is related to the electromagnetic force and the imposing time, magnetic flux density, magnetic field frequency and size of the separator. The zinc dross removal efficiency increased with an average residence time in the ceramic pipe and magnetic flux intensity increases when square separator used for zinc melts purification. It can achieve the highest removal efficiency when the frequency of magnetic field fixed and the ratio of the separator size and skin depth is about 2 times, the removal efficiency of zinc dross will be decreased with the separator size further increase; The removal efficiency increases with the frequency of magnetic filed increased when the separator size is fixed. The electromagnetic parameters and operation parameters of the electromagnetic separation apparatus are calculated, the skin depthδshould be 2.5mm when we selected suitable magnetic frequency in accordance with the size of single-channel separator, therefore, the actual frequency of the magnetic field is set to 15-20 kHz. When the separator size, frequency of the magnetic field, and the average residue time inside the separator are set as fixed value, the magnetic flux density is no less than 0.06T, the calculated rated power of the electromagnetic apparatus is 50kW. According to all above calculated results, we established electromagnetic purification equipment. Experiments to static separate deleterious zinc dross causing surface defects of galvanizing steel sheets from zinc bath were conducted on this apparatus by using alternating magnetic field. Experimental results show that for the zinc dross particles with diameter larger than 10μm can be removed when the magnetic frequency is 17.5 kHz, the effective magnetic flux density is 0.05-0.1 T, imposed time is 15 s, and the cross section of the ceramic square pipe is 5×5 mm.Experiments to continuously separate Fe-Al-Zn dross phase from hot dip galvanizing zinc melt were conducted on a laboratory-scale apparatus by using high-frequency alternating magnetic field too. When the magnetic frequency is 17.5 kHz, effective magnetic flux intensity is 0.1T, the cross section of the ceramic square pipe is 10×10 mm, and the processing time is 0.6-2.5 s, the separation efficiency of zinc dross varies from 43.76% to 98.63%, and the experimental results are almost in good agreement with theoretical results. In order to improve the removal efficiency and make full use of the secondary disturbance inside the ceramic pipe, a new multi-stage separator is developed and the removal efficiency of multi-stage separator is calculated. As the center of each channel of the first stage separator is now occupied by the wall, and zinc dross was originally located in a "weak areas" of the first stage separator successfully changed to the "strong areas" when these dross entered the second stage separator, the zinc dross electromagnetic separation efficiency can be increased by 10% or so, and these numerical calculation results has been verified through experiments.A series of pilot-plant-scale equipment which owns independent intellectual property rights for zinc melt electromagnetic purification was developed. A new cardiopulmonary bypass zinc melts purification technology was first introduced in this patent equipment, the successful implementation of a continuous electromagnetic purification outside the zinc pot, this equipment can be seen as the prototype of the equipment for industrial applications. The GA zinc bath purification experiment results show that through extending the process time, optimize the process parameters and change the structure of ceramic separator can significantly improve the purification effect, the average zinc dross removal efficiency is up to 89.52% for all samples, the removal efficiency after separation in the launder was about 75%, zinc dross can be removed completely which particle size larger than 20μm; GI zinc bath purification experiment results show that zinc dross removal efficiency up to 70%, the removal efficiency of zinc dross less than 15μm was only 42.1% in the launder, and the removal efficiency of dross particle size larger than 15μm was 91.67%, but zinc dross can be removed completely which particle size larger than 20μm as same as in GA purification.Numerical simulation results of pilot-plant-scale zinc pot show that the return pipe should be shorted when the return flow rate of zinc is low, and vice versa. The removal efficiency increases with the increased of the flow rate of zinc bath. The particle size and concentration fields of zinc dross have little effect on the separation results.Numerical simulation results of zinc pot applied in industry show that the location of return pipe and pump pipe has effect on the“V”shaped area, but has little effect on the bulk flow. The dross concentration in the bath decreases with the processing time in the case of circulating purification. Compared to zinc pot in general, the introduction of circulating purification can significantly improve the surface quality of galvanizing steel sheet.更多还原