【作者】 张小伟；

【导师】 邓康；

【作者基本信息】 上海大学 ， 钢铁冶金， 2011， 博士

【摘要】 滑板在水口中以其准确控制钢水的浇注流量,易于实现自动控制,且操作简便、安全等优点,被板坯连铸机广泛使用。然而引入滑板控流系统后,由于滑板对钢液的抑制、挤压作用,使水口内的钢水出现挤压又突然扩展的现象,进而先后产生压缩、突扩、分离等流态,使浸入式水口内部、水口出口射流,以及结晶器内部的钢液流动变得十分复杂,在结晶器中形成非对称和非稳定流态,直接影响钢水中夹杂物、气泡等的去除效率和效果,同时对铸坯初凝壳的形成和均衡生长产生重要影响(如无规律或规律不明显的表面裂纹及局部漏钢等)。本文围绕滑动水口的浇注工艺,研究了不同湍流模型对水口内部和水口出口射流的适应性,以及不同连铸工艺条件下,水口出流和结晶器内钢水流动的流态、流速分布,及其物理特征;讨论了不同湍流模型对滑板控流的水口和结晶器内部流动行为和状态的预测能力,展示了标准k-ε模型和雷诺应力模型(RSM)的基本假定及特点,以及在计算连铸浇注和结晶器内钢水运动方面的适应性和准确度等问题;同时,采用水力学物理模拟和超声波多普勒测速等方法定量地研究了水口滑板在不同开度和/或水口部分堵塞的条件下,各工艺(如拉速)和结构参数(如滑板下方浇管长度)等对水口射流流态(如旋转或平直出流)、结晶器内非稳定/非对称流场、液面涡流及其形成规律的影响,其评价指标包括流速分布、液面水平流动、液流对结晶器窄面的冲击和冲刷强度与位置等。在此基础上,总结各种工艺和结构参数对板坯结晶器内非稳定/非对称流场的作用规律,为优化操作工艺、改善铸坯质量提供理论指导或参考。文中首先讨论了标准k-ε模型和雷诺应力模型(RSM)在模拟滑板沿垂直于结晶器宽面开启条件下,水口内部和水口出口射流特征的适应性,进而研究水口入口速度、滑板开度和水口浇管长度等因素对水口出口射流特征的影响规律。结果表明:与标准k-ε模型相比,RSM能够准确地预测水口出口射流的旋转特征,并与物理模拟实验中超声波流速测量的结果相一致,其计算准确度优于前者,具有较强的适应性;水口入口速度对水口出口的射流特征不会产生大的影响;在滑板开度小于35%时,滑板下方浇管内先后出现二次流和分离流,浇管较长(600 mm)时,水口底部出现自滑板堵塞侧经由水口底部向滑板开启侧流动的旋转射流,该射流的旋转方向与水口浇管长度有关,当浇管长度减小到400 mm时,射流的旋转方向与前者相比将发生逆转;在滑板开度为35~40%之间时,在浇管中的液流在流体惯性力、湍流脉动性和流线扩张效应之间出现动态平衡点,届时水口出口射流表现为对称/平滑出流;以后,随滑板开度增大,射流的旋转方向也随之改变,即滑板开度为40~65%的区间范围内,射流由滑板开启侧经由水口底部向滑板堵塞侧旋转,且该旋转方向不再受浇管长度影响;当滑板开度在65~70%之间时,水口内的液流面临第二个对称/平滑出流的动态平衡点,此后,随着浇管长度减小,水口内滑板所引起的扩张流指向滑板堵塞侧内壁方向的速度分量的作用逐渐增强,在浇管长度较小的情况下,将出现滑板开度小于35%时同向的旋转射流;而当水口滑板开度超过90%后,水口出口射流的旋转现象基本消失,液流表现为准对称/平滑出流状态。其次,文中讨论了湍流模型在不同滑板开启条件下对结晶器内部钢水流动的模拟和预测能力,并采用水力学物理模拟实验和超声波多普勒流速测量的方法研究多种操作工艺参数对结晶器内非对称流等的影响规律。这里,通过调节水口滑板的开度或部分堵塞水口出口来形成结晶器内液流的非对称流态,并根据物理模拟实验结果分析和总结滑板开度与水口堵塞度等对结晶器内非对称流场的影响规律。结果表明:(1)在滑板沿垂直于结晶器宽面开启条件下,水口的旋转出流使结晶器两边宽壁面附近及两侧的流动出现不对称,对此,RSM三维计算能够相对准确地反映结晶器上部的流态、流速分布和流动特征,但对结晶器下部的计算结果与物理模拟实验实测的结果仍有差距;在该种滑板开启方式下,水口出口一侧堵塞会对结晶器内流场的对称性产生严重影响。(2)在滑板沿平行于结晶器宽面开启条件下,其不全开启使水口左右两侧出流不平衡,呈现出两侧射流的冲击深度和射流角不对称,导致结晶器内流场呈现左右不平衡状态;对此,标准k-ε模型和RSM均能够计算和预测出结晶器内钢水的基本流动特征,而RSM的结果与物理模拟实验实测的结果更为接近,显示出其在模拟该类问题时的优势。(3)水口的浸入深度和结晶器宽度对结晶器内流场非对称的影响不大;增加拉速,可使水口两侧出流的冲击点位置上移,结晶器内流场的不对称度将减弱;另外,增加水口滑板下方的浇管长度,有利于消除滑板不全开启时水口出流旋转等非对称现象。最后,本文采用物理模拟实验和流速超声波实测研究板坯结晶器内出现液面涡流的规律和分布情况,以及连铸操作工艺对其的影响。在板坯连铸结晶器中,若液面流不均衡,水口两侧的液面来流(上回流)会在水口一侧某点相遇并发生剪切,形成旋转的流态,进而引发周围液流呈圆周运动,并在离心力作用下形成螺旋形水涡(又称液面涡流),该涡流是铸坯液面卷渣、吸气、二次氧化的主要原因。本文进行了不同工艺参数组合条件下的物理模拟实验,考察和分析工艺参数对涡流形成的影响。结果表明:形成液面涡流的原因,一是表面流的湍动性造成的水口两侧表面流动量不对称;二是操作工艺导致的水口两侧表面流不对称。在结晶器液面,涡流出现形式分为水口一侧单涡流、水口一侧双涡流和水口附近区域对角双涡流三类,均与水口周围的液流流态有关。涡流“强度”(文中用涡流直径和长度表征),以及涡流出现的频度均随表面液流的动量增大而增加,而连铸中增大水口浸入深度或增加水口出流的射流角度均有利于减小液面涡流出现的频度和强度。更多还原

【Abstract】 Slide gate control system, with the advantages of accurate control precision of molten steel flow rate, automatic control being easily realized, simple operation and safety, consequently, it is widely employed in continuous casting of steel. However, due to the suppression and extrusion effect of slide gate to the molten steel, the sudden expansion phenomenon appears after extrusion by slide gate, and some flow patterns are emerged, such as contraction flow, sudden expansion flow, separated flow, and so on. Thus, the flow in the SEN, the jet characteristic at nozzle port and the flow in the mold become more complex. The asymmetric and unsteady flow state in the mold directly affects the removal efficiency of inclusions and gas bubbles, and it has a great influence to the formation and uniform growth of initial solidification shell (rule-less surface crack or breakout). Based on pouring technique of slide gate, the adaptability of turbulence models for the fluid flow in the SEN and nozzle port has been discussed; the flow state, velocity distribution and physical characteristic of jet exiting from nozzle port under different process parameters have been studied. And then, the prediction ability of turbulence models for the flow behavior and flow states in the SEN and mold have been discussed, and the basic hypothesis and specialty of standard k-εmodel and RSM are displayed, together with the adaptability and accuracy of these two turbulence models for the simulation of fluid flow in the continuous casting system. Meanwhile, by means of physical simulation of water model and velocity measurement using Ultrasonic Doppler Velocimeter (UDV), the effect of process (casting speed, etc.) and structural (SEN length under slide gate and so on) parameters on the jet flow status (swirl flow or smooth flow), unsteady/asymmetric flow in the mold, surface vortex and its formation rules are studied quantitatively when slide gate is not fully opened and/or nozzle port is clogged, where the evaluation indicators include velocity distribution in the mold, surface flow velocity, and the intensity and position of impingement and scour of jet to the mold narrow wall. On the basis of above research results, the influence rules of different process and structural parameters on the unsteady/asymmetric flow in the mold are summarized, and some theoretical guidance and references are provided to optimize process and improve slab quality.In the first part of the paper, under the condition of slide gate moving perpendicular to the mold wide wall, the adaptability of turbulence models for the fluid flow in the SEN and nozzle port has been discussed. In addition, the influence rules of inlet velocity at SEN, opening ratio of slide gate and SEN length on the swirl characteristic of nozzle jet have been studied. The study indicates that, compared to standard k-εmodel, RSM can predict the swirl characteristic of jet at nozzle port accurately, and the simulated result agrees to the UDV measurement, so that the calculation accuracy of RSM is superior to the former, and it has strong adaptability; and it is found that, the velocity at nozzle inlet cannot make effect on the jet swirl characteristic at nozzle port. When the opening ratio of slide gate is smaller than 35 pct, a secondary flow and a separated flow caused by slide gate appear under the slide gate, in the circumstances of a longer SEN (600 mm), a swirl flow appears at nozzle port, with the swirl direction of that from the clogging side via nozzle bottom to the opening side of the slide gate, and the swirl direction is affected by the SEN length, when the length of SEN is decreased to 400 mm, the swirl direction is changed to the opposite mode; when the opening ratio of slide gate is in the range of 35 pct to 40 pct, a dynamic equilibrium point of inertia force of molten steel, fluctuation of turbulence and expansion effect of streamline is emerged, the jet presents a symmetry/smooth outflow state; and then, with the opening ratio of slide gate increasing, the jet swirl direction is changed to the opposite swirl direction, as is described when the opening ratio of slide gate is in the range of 40 pct to 65 pct, the swirl direction of jet is from the clogging side via nozzle bottom to the opening side of the slide gate, and it is not affected by the SEN length; when the opening ratio of slide gate is in the range of 65 pct to 70 pct, the second dynamic equilibrium point is emerged, and then, with the SEN length decreasing, the influence of velocity component towards to the SEN wall of the slide gate clogging side is gradually strengthened, in the condition of shorter SEN, the jet swirl direction will change to the pattern as described when the opening ratio of slide gate is smaller than 35 pct; when the opening ratio of slide gate is larger than 90 pct, the swirl phenomenon almost disappears, and the jet presents to a quasi-symmetry/smooth outflow stare.In the second part of the paper, the prediction ability of turbulence models for the flow of molten steel in the mold has been discussed under different slide gate opening mode, and the physical simulation and UDV measurement are conducted to study the effect of process and operation parameters on unsteady flow of molten steel in the mold when slide gate is not full opened or outlet is clogged, where the asymmetry fluid flow in mold is obtained by SEN slide gate and clogging of outlet, and the influence rules of opening ratio of slide gate and nozzle port clogging on the asymmetric flow are analyzed and summarized based on above experimental results. (1) Under the condition of slide gate moving perpendicular to the mold wide wall, the swirl flow at nozzle port will cause asymmetric velocity distributions near the mold wide wall, and the RSM can predict the flow state in the upper part of mold, but the simulation result in the lower part of mold disagrees to the measurement result; and under the same opening pattern of slide gate, the clogging at one nozzle port has serious effect on the bias flow in the mold. (2) In the circumstances of slide gate moving parallel to the mold wide wall, the partial opening slide gate bring to unbalanced flow rate of molten steel at two nozzle ports, presenting impact depth and jet angle of two streams asymmetry, which will cause non-homogeneous fluid flow in mold; for the flow status in the mold, both standard k-εmodel and the RSM can predict the basic flow characteristic in the mold, but the result simulated by RSM is close to the UDV result, it is shown that RSM has an advantage to simulate such kind flow. (3) The SEN immersion depth and slab width has little influence on the asymmetric flow in the mold; with the increasing of casting speed, the impingement points of two jet shift up and the asymmetry of fluid flow become weakened. If the SEN length is long enough, the asymmetry outflow of two nozzle port almost disappears. In addition, increasing the SEN length under slide gate, it is useful to eliminate the asymmetric phenomenon at nozzle port when the slide gate is partial opened.In the last part of the paper, physical experiment and UDV measurement have been used to study the rules and distributions of surface vortex, and the influences of operating process on the vortex. In slab continuous casting mold, if the surface streams, coming from two upper recirculation zones, are imbalance, they encounter and shear at a point on one side of the SEN, and cause a swirl flow at free surface. It may bring to circular motion of around fluid and on the effect of descent force a spiral eddies of water called vortex is forming, and it is believed to have significant contribution to mold powder entrapment, air entrapment and re-oxidation of molten steel. Various experiments with different parameters have been done in lab to analyze the effect of these parameters on vortex. The study indicates that the cause of vortex lies in two aspects, one is that the turbulence of surface flow gives rise to asymmetry of flow momentum near the SEN, and the other is operating parameters cause asymmetry of the surface flow besides the SEN. There are three types of vortex which appears on surface, a single vortex on one side of SEN, two-vortex on one side of SEN, and two-vortex in diagonal position of SEN, but they are all related to the surface flow near the SEN. The vortex strength (vortex tube diameter and working depth) and frequency increase as the raise of surface flow kinetic energy. The increase of SEN immerging depth and slab width will decrease the strength and frequency of vortex.更多还原