中间包上水口环形吹氩下气泡大小和迁移行为研究

摘要：通过水模型实验研究了上水口环形吹氩工艺下中间包和结晶器内气泡形貌,并结合数值模拟分析了透气砖位置、拉坯速度和吹氩量对中间包和结晶器内气泡尺寸、气泡迁移和中间包近液面钢液流动的影响。结果表明：上水口环形吹氩形成以塞棒为中心的圆台状气泡羽流,气泡浓度沿径向向外逐渐减少;附壁效应使得气泡羽流偏向塞棒壁面流动,增大气泡的碰撞聚并概率和近塞棒壁面的羽流上升速度,对中间包液面产生较大冲击作用;同时,部分细小气泡会随钢液进入水口及结晶器内部;增大吹氩量,中间包内环形气泡羽流中气泡数目明显增多,中间包近液面钢液上升速度增大;增大拉坯速度,环形气泡羽流的宽度和气泡数量逐渐减小,近液面速度减小;增大透气环距水口中心距离,中间包内气泡弥散度增大,环形气泡羽流宽度也随之增大,气泡羽流对中间包液面冲击作用减弱;增大吹氩量和拉坯速度、减小透气环距水口中心距离,进入结晶器的气量和气泡尺寸逐渐增大。实验条件下,透气环内外径为110mm/140mm、拉坯速度为1.2m/min时,吹氩量为4L/min较为合适。     

Study on dimension and migration behavior of bubble under annular argon blowing at tundish upper nozzle

The water model experiments were carried out to study the bubble morphology in the tundish and mold with the process of annular argon blowing at tundish upper nozzle. The effects of the position of gas permeable brick, the casting speed and the argon flow rate on the bubble size distribution, the bubble migration behavior and the flow behavior of liquid steel near the liquid level in tundish were further investigated, coupled with the numerical simulation. The results show that with the process of annular argon blowing at tundish upper nozzle, a frustum cone shaped bubble plume can be formed around the stopper rod. The concentration of argon bubbles gradually decreases outward along the radial direction of the stopper rod. Owing to the wall attached effect, the bubble plumes float upward along the stopper rod, which can increase the collision probability between bubbles and the velocity of bubble plumes, causing a larger impact strength on the liquid level in tundish. In addition, a part of small bubbles are wrapped into the nozzle and the mold due to the drag force of liquid steel. With increasing argon flow rate, the number of bubbles in annular bubble plumes and the vertical velocity of liquid steel near the liquid level in tundish increase significantly. With increasing casting speed, the width and the bubble number of annular bubble plumes gradually decrease, leading to a decrease of the vertical velocity of liquid steel near the liquid level in tundish. Increasing the distance between the annular gas permeable brick and the center of tundish upper nozzle, the dispersion of bubbles and the width of bubble plumes increase, and the impact strength of bubbles acting on the liquid level in tundish becomes weaker. As the argon flow rate and the casting speed increase, and the distance between the gas permeable brick and the center of tundish upper nozzle decreases, the gas volume and bubble size in the mold increase. Under the experimental conditions, when the inner and outer diameters of the annular gas permeable brick are 110mm and 140mm, respectively, and the casting speed is 1.2m/min, the appropriate argon flow rate is 4L/min.     

上水口环形吹氩对中间包内渣眼形成的影响

 中间包上水口环形吹氩可以在塞棒周围形成清洗钢液的环形气幕,同时部分氩气泡随钢液进入上水口内,可以减少非金属夹杂物在水口内壁的黏附,起到防止水口堵塞的作用。然而,不合理的吹氩量会导致中间包内液面渣层受过强的气液羽流冲击而形成渣眼,使得钢液裸露并发生二次氧化,严重影响铸坯质量。采用标准 k ε 湍流模型研究中间包内流体流动,采用DPM模型和VOF模型耦合方法,研究上水口环形吹氩条件下渣眼的形成及演化规律。结果表明,上水口环形吹氩在塞棒周围形成较强的上升流,塞棒上部邻近区域存在多个涡流区;在钢液涡流的影响下,中间包液渣下层远离塞棒区域,上层向塞棒区域迁移;随着吹氩量的增大,平均湍动能增大,塞棒附近钢液速度逐渐增大,钢渣界面钢液速度先增大后减小,渣眼边缘钢液速度先增大后减小然后再增大,速度与垂直方向夹角逐渐减小;增大吹氩量,中间包熔池液面形成以塞棒为中心的渣眼,渣眼面积逐渐增大。试验条件下不产生渣眼的临界吹氩量为4.2 L/min,对应的钢渣界面最大速度为0.247 m/s,与垂直方向夹角为70°。     

Modelling performance of four‐strand, 12 t,delta shaped continuous casting tundish fitted with different flow modifying arrangements for better steel quality

Abstract

The performance of a 12?t, delta shaped, four‐strand, billet casting tundish was investigated using a full scale (1∶1) isothermal model using water as the simulating fluid for steel. Three different modelling experiments were carried out. Quantifiable parameters like ‘number of slag beads’ in transient physical modelling of slag entrainment, or ‘residual ratio of inclusions’ (RRI %), in steady state mathematical modelling of inclusion removal and ‘mean residence time’, ‘dead volume fraction’, etc., in steady state mathematical computation of residence time distributions, were used as the performance indicators. Results for three different flow modifying dam arrangements were considered and compared with those of a bare tundish. Computational fluid dynamic analysis showed that different flow modifying dam arrangements significantly alters the flow pattern within the tundish. On the basis of these performance indicators, the best arrangement was identified. The assumption in this paper that similar conclusions can be drawn from experiments carried out either in transient or in steady state conditions was verified. It was shown that both transient physical modelling experiments and the steady state mathematical predictions, point to the same conclusion.     

几种中间包控流装置的作用及比较

 中间包内设置控流装置对钢水流动行为和夹杂物去除影响显著。采用物理模拟的方法,系统研究和比较了常规双挡墙、湍流控制器、导流挡板和透气砖吹氩对中间包流体流动特征的影响。结果表明,湍流控制器可起到减小钢水喷溅、防止二次氧化和改善流体运动路线的作用,并减少短路流的发生。导流挡板与双挡墙比较,最短停留时间变化不大,但死区体积更小。与双挡墙相比,透气砖吹氩的最短停留时间提高不多,但峰值时间增加明显,死区体积大大降低。     

Liquid steel flow in continuous casting machine: modelling and measurement

Abstract

Single phase (liquid steel) and two-phase (liquid steel and argon bubbles) three-dimensional computational fluid dynamic and heat transfer models were developed for the continuous casting machines of ArcelorMittal. The computational domains include tundishes, slide gates, submerged entry nozzles and moulds. The effects of buoyancy, tundish design, tundish practices, nozzle design and caster practices on flow structure were investigated. Mathematical modelling is discussed in detail. In addition, submeniscus velocity measurements in the slab caster mould are performed with the method of torque measurement. A consumable probe is inserted into the liquid steel meniscus from the top of the mould through mould powder and slag layer. The liquid steel flow applies a drag force to the probe, which then generates a torque. This torque value is measured and then converted back to velocity. The concept and challenges of the technique are discussed, and the effects of casting parameters on mould flow structure are investigated. Product quality in relation to real time meniscus velocity measurements is also discussed.     

长水口吹氩生成微小气泡工业实验研究

在连铸生产中采用大流量长水口吹氩,并采用“冷钢片沾钢法”沾取中间包钢液试样,成功沾取了中间包钢液中微小氩气泡。冷钢片沾样表面气泡为中间包上部钢/渣界面和炉渣中氩气泡,尺寸主要位于1.0~3.0 mm,但该尺寸不能反映中间包钢液内部长水口吹氩生成气泡,冷钢片沾样内部气泡为钢液内部长水口吹氩生成的气泡。结合扫描电镜和共聚焦显微镜对沾取试样内部气泡形貌、尺寸和数量进行了分析,结果表明大部分气泡为独立圆形气泡,偶见少量粘连和聚合气泡;钢液内部氩气泡尺寸主要位于100~1000 μm,平均尺寸为500 μm左右;气泡在长水口出口及其下方较为弥散,气泡数量可达15.2 cm?2。采用扫描电镜结合能谱分析,发现部分气泡内粘附有夹杂物,有些气泡粘附多个夹杂物;气泡粘附Al₂O₃夹杂物的几率高于粘附CaO(?MgO)?Al₂O₃?SiO₂复合夹杂物的几率。      

Study of fluid flow in tundishes due to different types of inlet streams

Flow pattern and residence time distribution induced by different methods of pouring of molten steel into a tundish are studied by means of a water model with and without flow modifiers (FM). It was found that the submerged stream without FMs produces short circuiting and argon shrouded stream leads to excessive surface directed flow of liquid in a tundish. Open stream pouring generates surface turbulence and does not produce short circuiting. The effect of FM is found to be different for different methods of pouring, e.g. dam eliminates short circuiting and produces surface directed flow for submerged stream, whereas weir limits the surface turbulence within the inlet region for open stream pouring. Dam is found to reduce the dead volume for argon shrouded pouring.     

Particle Distribution and Separation in Continuous Casting Tundish

The tundish as a part of a continuous casting machine combines the discontinuous ladle metallurgy with the continuous solidification of slabs in the mould. The tundish plays a major role in the challenging task of “clean steel” production. That means the smallest number of inclusions and high cleanliness in all steel grades after changing the conditions at the inlet of the tundish. Inclusions hinder the metal forming process and lead often to fatigue. The cleanliness of steels is important to fulfil the customer's requirements. In the present study inclusion removal was simulated in a 1:3 scaled water model of a single‐strand tundish for the production of stainless steels with a particle counter. The particle counter is capable of counting a large number of particles with a wide range of diameters. The separation rate for particle diameters from d_P = 1 ‐ 250 μm was determined with a counter for the water model tundish. With similarity conditions for the particles this deposition rate can be transformed to the melt flow in a steel tundish. The separation rate was measured for different flow rates in the water model tundish. A larger flow rate decreased the separation rate. Additionally, the separation rate for the tundish fitted with an impact pad was measured and showed a significant increase of separation for particles with a smaller diameter. Furthermore, the particle distribution in the tundish for different size groups of particles was investigated with and without an impact pad. Numerical simulations were carried out with the finite‐volume commercial code FLUENT using the realizable k‐ε turbulence model. A special boundary condition for the separation of particles at the surface was implemented.     

通道式感应加热五流中间包流场的水力学模拟

 通道式感应加热能有效补偿连铸过程中间包钢水的温降,且具加热效率高、设备简单等优点,是近年来得到快速推广应用的中间包新技术。配置感应加热器后的中间包结构与常规中间包差异较大,其加热通道的设计对中间包流场具有明显影响。针对实际应用的某5流直通道感应加热中间包边部流和中间流温差大、钢水浇铸过热度偏高的问题,通过水模拟试验对该中间包流场进行优化,提出了一种新型的分口通道方案。该方案可将中间包的死区比例由原型的29.50%降低到20.33%,钢水平均停留时间较原型延长了40 s。工业试验表明,分口式新型通道设计中间包各流浇铸温度一致性得到改善,其中边部流和中间流的平均温差较原型降低了3.6 ℃,实现了感应加热中间包应用效果的进一步提升。     

板坯连铸中间包气幕挡墙夹杂物去除的研究

分析了中间包不加气幕挡墙和加气幕挡墙吹氩时夹杂物上浮速度与气泡直径、夹杂物直径和密度的关系。气泡尺寸对夹杂物的上浮速度影响比较明显,夹杂物的密度对上浮速度的影响不大。进行了中间包气幕挡墙的工业试验,试验表明:中间包底吹气对钢液中小夹杂物的去除作用不明显,而对大型的夹杂物去除效果显著。     

低碳铝镇静钢表面缺陷控制

介绍了低碳铝镇静钢轧后容易出现的几种主要钢质缺陷,并对缺陷的产生原因进行了初步分析,通过改进钢水的化学成份,降低钢水氧化性,适当延长钢水炉外处理时间和降低浇铸时中间包水口吹氩量,可有效减少低碳铝镇静钢的轧后缺陷率.     

210 t BOF-RH-CC炼钢过程无取向硅钢XG800WR洁净度的试验研究

两炉次无取向硅钢XG800WR(/%:0.003～0.004C、0.71～0.75Si、0.32～0.33Mn、0.004～0.007S、0.016P)的炼钢流程为铁水预处理(KR)-210 t顶底复吹转炉-钢包吹氩-RH脱碳精炼-230 mm×1220 mm板坯连铸。53 t中间包钢水过热度为25～30℃,钢包到中间包采用长水口全程吹氩保护浇铸,中间包至结晶器采用浸入式水口浇铸。结果表明,在RH、中间包、结晶器过程中钢中总氧以及夹杂物数量和尺寸均明显降低;但在钢包到中间包过程T[O]、[N]和钢中夹杂物数量增加,说明长水口浇铸过程存在二次氧化。连铸坯中T[O]、[N]平均他分别为11×10^-6和30×10^-6,显微夹杂物数量平均为4个/mm~2。铸坯中的显微夹杂物主要为3～5 μm的AIN,同时存在少量的MnS、Al₂O₃·AIN和Al₂O₃·MgO·MnS。     

Effect of asymmetric casting of conticaster on liquid steel in tundish

The flow characteristic and temperature characteristic of steel liquid in tundish at asymmetric casting were studied using SolidWorks, ANSYS, etc.It has an important impact on flow field in tundish for asymmetry casting of conticaster, which can lead to the change of tundish temperature distribution. When a strand of conticaster on one side is closed, the flow rate of steel liquid can be reduced and more backflow regions can be formed in this side of tundish. Simultaneously, the region of lower temperature steel liquid in middle part of tundish moves to the other side obviously, and the phenomenon is more noteworthy when an edge strand of conticaster is closed.     

Effects of clogging, argon injection, and continuous casting conditions on flow and air aspiration in submerged entry nozzles

The inter-related effects of nozzle clogging, argon injection, tundish bath depth, slide-gate opening position, and nozzle-bore diameter on the steel flow rate and pressure in continuous-casting slide-gate nozzles are quantified using computational models of three-dimensional (3-D) multiphase turbulent flow. The results are validated with measurements on operating steel continuous slab-casting machines and are presented for practical conditions with the aid of an inverse model. Predictions show that initial clogging may enhance the steel flow rate due to a potential streamlining effect before it becomes great enough to restrict the flow channel. The clogging condition can be detected by comparing the measured steel flow rate to the expected flow rate for those conditions, based on the predictions of the inverse model presented here. Increasing argon injection may help to reduce air aspiration by increasing the minimum pressure, which is found just below the slide gate. More argon is needed to avoid a partial-vacuum effect at intermediate casting speeds and in deeper tundishes. Argon flow should be reduced during shallow tundish and low casting speed conditions (such as those encountered during a ladle transition) in order to avoid detrimental effects on flow pattern. Argon should also be reduced at high casting speed, when the slide gate is open wider and the potential for air aspiration is less. The optimal argon flow rate depends on the casting speed, tundish level, and nozzle-bore diameter and is quantified in this work for a typical nozzle and range of bore diameters and operating conditions.     

高压气瓶钢34Mn2V 200 mm×200 mm轧坯角部拉裂缺陷的控制工艺

通过工艺对比分析了34Mn2V高压气瓶用钢中间包浇铸温度、结晶器液面波动、[Als]、中问包炉次和280 mm×380 mm连铸坯拉速波动对轧坯角部拉裂的影响。提出减小RH加Al量,按[Als]0.01%控制;RH后喂Ca-Si线;适当提高连铸钢液温度,控制中间包钢水温度1 520～1 530℃;控制浇铸过程塞棒吹氩量≤10 L/min;结晶器液位波动±3 mm等工艺措施。应用结果表明,轧坯角部拉裂缺陷率由原来的23.57%降至1.21%。     

汽车钢DC03夹杂物产生原因分析及改进措施

通过采用扫描电镜、金相分析等手段,结合现场实际生产过程探讨了邯宝炼钢厂生产汽车钢DC03产生夹杂的原因分析,并针对其原因提出了改进措施。研究结果表明：DC03钢板产生的夹杂有两类：一类是钙铝酸盐夹杂,主要原因是来源于炼钢过程中大颗粒耐火材料脱落物或中间包卷渣;另一类是伴有钠、氟元素氧化铝夹杂,主要原因是保护渣卷入。为此,采取了优化保护渣成分、增加浇次开浇中间包质量、改进中间包包盖的吹氩管路、优化中间包覆盖剂成分和中间包吹氩量等措施,取得了良好的效果,低碳钢平均夹杂率由改进前的0.47%减少至0.32%。     

浸入式水口吸入空气机理及吹氩量控制模型

 连铸过程浸入式水口本身及其与上水口连接处容易吸入空气,导致钢水二次氧化,随之可能造成水口结瘤及断浇,导致严重的连铸事故。通过向水口内部吹入氩气可以防止空气吸入,吹氩量控制不当容易引起铸坯缺陷。基于伯努利原理和质量守恒定律建立了从中间包到结晶器的速度-质量模型,探讨了以上部件吸入空气的机理。首先研究了理想与考虑压损2种情况下水口直径、水口浸入深度、中间包液位、铸坯宽度和拉坯速度对水口入口的钢液横截面积与出口的钢液横截面积之比(A_A/A_P)的影响,然后对水口结构进行了优化,并建立了吹氩量控制模型。结果表明,为防止水口吸入空气,应尽量减小水口直径、降低中间包液位和水口浸入深度的高度差、增大拉坯速度和铸坯断面宽度。其中水口直径提高10%,A_A/A_P从2.15增大至2.62;铸坯宽度和拉坯速度对A_A/A_P影响略低于水口直径,同样提高10%,A_A/A_P均从2.15降低至1.96;中间包液位和浸入深度对A_A/A_P影响最小。基于此研究结果,水口结构优化为符合钢液流束的圆台形,并结合水口内真空区体积确定了吹氩量控制模型,使得水口内始终保持微正压。本研究结果为减小甚至消除水口的空气吸入、控制氩气吹入量提供了理论基础,对高品质钢的生产及节能降耗具有重要意义。     

Fluid Dynamics of Vortex Formation in Tundish Operations: Physical Modelling

The present study focuses on the fluid dynamics of vortex formation in tundish operations using a physical water analogue model and Particle Image Velocimetry (PIV) measurements. The aim is to get fundamental knowledge of the nature of vortex formation that clearly represents this flow phenomenon. The results indicate that vortex formation is directly related to the highly turbulent flows present in the tundish. Great changes of velocity fields during short times and large distances between the upper edge of the dam and the free surface of the bath enhance the formation of vortex flows. Therefore, increasing tundish capacity does not guarantee steel cleanliness through long residence times if the dam height is not adjusted to the new level of liquid in the tundish.