结晶器渣圈对渣道压力、振痕及渣耗影响的分析

对比分析了三种类型结晶器渣圈,阐明了渣圈对连铸坯生产过程的影响。结果表明,当渣圈存在时,渣道动态压力变化幅度显著增大,最大正压由1.373 k Pa提高到21 k Pa,压力增大导致振痕产生,渣圈越厚,振痕越深;同时,渣圈会影响保护渣的消耗量,无渣圈时,最大渣耗量为0.009 7 kg/(m·s),渣圈存在时,最大渣耗量降至0.007 kg/(m·s),较厚的渣圈会使渣道宽度变窄,在振动负滑脱中期降低保护渣的消耗量。     

高频磁场下连铸保护渣润滑行为数学模型

摘要：为了研究高频磁场下连铸保护渣在结晶器内的润滑状况，建立了高频磁场下连铸保护渣润滑行为数学模型，并应用该数学模型研究了初始凝固时磁场作用下渣道宽度、弯月面高度、渣道动压、渣耗、摩擦力等因素对保护渣润滑行为的影响。结果发现，磁场的作用拓宽了保护渣渣道宽度，增大了弯月面高度，使保护渣渣道入口及出口宽度增加，使初始凝固点下移，改善了传热条件有利于铸坯表面质量提升；磁场的作用减小了因结晶器振动而产生的正压和负压，并且正、负压都是随着磁场强度的增大而减小，但磁场强度存在一个最佳值；磁场的作用增大了渣耗量，改善了铸坯与结晶器之间的润滑；铸坯与结晶器之间摩擦力随着磁场强度的增大而减小，当磁场强度为40mT时，总摩擦力减小趋于平缓，因此磁场强度为40mT左右时对减小摩擦力的作用效果较好。     

连铸漏钢原因分析

通过对某厂连铸漏钢时结晶器内残留坯壳的剖析和工艺分析,查明漏钢的原因是结晶器内卷渣造成局部坯壳过薄,该处坯壳在拉出结晶器后被撕裂所致.采用预熔性保护渣并改善工艺操作后避免了漏钢事故的发生.     

保护渣对430不锈钢铸坯边部凹陷缺陷的影响

针对太钢430不锈钢铸坯边部凹陷严重的问题,采用Gleeble 3800、高温原位分析仪、黏度分析仪等技术手段,系统研究430铸坯边部凹陷缺陷的产生机理和保护渣性能对边部凹陷的影响规律。研究结果表明,430铸坯边部凹陷缺陷的主要原因与保护渣的控制传热有关。保护渣碱度过小,结晶能力弱,坯壳在结晶器内冷却强度大,凝固收缩带来较大的角部扭动力而产生边部凹陷,此时铸坯边部凹陷主要发生在结晶器内;保护渣碱度过大,结晶能力强,铸坯冷却强度不够,出结晶器的坯壳厚度薄,在钢水静压力的作用下铸坯宽度产生延展效应,导致后续产生较大的凝固收缩而形成边部凹陷,此时铸坯边部凹陷主要发生在二冷阶段。保护渣碱度控制为1.00,保护渣的结晶能力适宜,既避免了结晶器内强冷带来的铸坯凹陷,又保证了出结晶器坯壳足够的厚度和强度,最终使铸坯边部凹陷深度由1.26 mm降低至0.30 mm,显著改变了铸坯表面质量。     

结晶器/铸坯气隙内渣膜润滑行为的数学模拟

建立了连铸保护渣润滑行为的数学模型,对不同连铸工艺参数对结晶器内铸坯摩擦力的影响进行了预测和讨论.结果表明,对于一给定的保护渣,存在一个最佳拉坯速度,低于或高于这一速度都会使摩擦力增大;增大结晶器振动的振幅、频率和结晶器倒锥度,铸坯所受到的摩擦力增大.     

连铸坯表面夹渣的原因分析及解决措施

表面夹渣是连铸坯常见的表面缺陷,直接影响成品的表面质量.通过分析连铸坯表面夹渣问题产生的原因,制订实施了保持结晶器内钢水的稳定性、选择合适的保护渣及结晶器的振幅、振频等措施,使连铸坯表面夹渣的问题得到了有效的控制.     

含TiO2无氟保护渣的传热研究及生产实践

在对结晶器无氟保护渣熔粘特性研究的基础上,研究了结晶器无氟保护渣渣膜的传热.即在保护渣熔粘特性的基础上利用添加TiO2来降低和控制铸坯及结晶器之间的热传递.结果表明:采用碱度在0.95～1.05范围内、TiO2含量为2.5%的结晶器无氟保护渣,不仅可保证连铸生产所要求的正常渣耗和结晶器进出水温差,而且铸坯表面质量良好.     

中厚板表面夹渣缺陷分析与研究

通过对夹渣缺陷处夹杂物的扫描电镜分析,铸坯皮下夹渣的产生原因是结晶器流场不合理,保护渣随结晶器内钢液流动卷入铸坯所致。通过改进浸入式水口形状、尺寸,优化浸入式水口插入深度,优化结晶器保护渣理化指标等工艺措施,可以有效减少连铸坯夹渣类缺陷。     

结晶器与铸坯间气隙内液态保护渣流动行为的数学解析

应用粘性流体动力学原理及有限差分析方法，对结晶器与铸坯间气隙内的液态保护渣流动行为进行了数值计算，分析了结晶器振动参数对液渣流动行为的影响。结果表明，液渣在一个振动周期内流入，流出状态和流动速度与结晶器振动参数有关，低振幅，低频率使液渣平均流动速度提高，为满足渣耗量，高速连铸工艺应采用低振幅，低频率的正弦波或非正弦波振动形式。     

连续铸钢结晶器与保护渣渣膜间的界面热阻

结晶器弯月面附近凝固坯壳与结晶器间的传递在连铸产品表面裂纹的形成中起着重要作用,保护渣膜与铜结晶器壁间的界面热阻对热传递影响很大,因此,建立一个由坯壳、保护渣膜和铜壁组成的模型模拟连铸中结晶器弯月面附近的热传递。通过测量模拟器铜模内的梯度确定热阻,以前的研究认为,界面热阻与渣膜厚度无关,而经研究发现,界面热阻随潭膜厚度增加而增加,界面热阻点热传递总热阻的５０％,对于相同潭膜厚度的保护渣,当浇注结晶     

连铸结晶器瞬态摩擦阻力的实验研究

应用连铸结晶器凝固结构模拟装置,研究了结晶器瞬态摩擦阻力随结晶器振的变化规律。结晶显示：在结晶器上部,连铸坯与结晶器之间以粘性摩擦为主,摩擦阻力的主烨规律与结晶器振动的速度波形一致。     

Transient Thermo-fluid Model of Meniscus Behavior and Slag Consumption in Steel Continuous Casting

The behavior of the slag layer between the oscillating mold wall, the slag rim, the slag/liquid steel interface, and the solidifying steel shell, is of immense importance for the surface quality of continuous-cast steel. A computational model of the meniscus region has been developed, that includes transient heat transfer, multi-phase fluid flow, solidification of the slag, and movement of the mold during an oscillation cycle. First, the model is applied to a lab experiment done with a “mold simulator” to verify the transient temperature-field predictions. Next, the model is verified by matching with available literature and plant measurements of slag consumption. A reasonable agreement has been observed for both temperature and flow-field. The predictions show that transient temperature behavior depends on the location of the thermocouple during the oscillation relative to the meniscus. During an oscillation cycle, heat transfer variations in a laboratory frame of reference are more severe than experienced by the moving mold thermocouples, and the local heat transfer rate is increased greatly when steel overflows the meniscus. Finally, the model is applied to conduct a parametric study on the effect of casting speed, stroke, frequency, and modification ratio on slag consumption. Slag consumption per unit area increases with increase of stroke and modification ratio, and decreases with increase of casting speed while the relation with frequency is not straightforward. The match between model predictions and literature trends suggests that this methodology can be used for further investigations.     

高拉速连铸结晶器振动参数对板坯表面裂纹形成的影响

 结晶器振动导致初凝坯壳受力和变形是产生铸坯表面裂纹的主要原因。通过计算2.0 m/min拉速时弯月面区最大液体摩擦力和最大渣道动态压力,分析了高拉速下结晶器振动参数变化对板坯表面纵裂纹和横裂纹形成的影响,并结合振动参数对结晶器润滑和振动状态的影响,阐明减少表面裂纹的振动参数控制措施。研究结果表明：提高振频和振幅均增大铸坯表面裂纹形成的可能,振频影响强于振幅;增大非正弦振动因子降低了坯壳撕裂可能性,且对润滑有利,但使振痕加深,振动冲击加剧;适当降低振频,增大振幅和非正弦振动因子可抑制表面裂纹形成。     

Optimization of mold oscillation for high speed casting—new criteria for mold oscillation

Consistent and uniform lubrication in the mold during casting is an issue especially with high speed casting. However, criteria of mold oscillation related to lubrication, has not been clarified for critical conditions such as thin slab casting. This paper discusses the mechanism of powder penetration into the meniscus, in terms of negative strip area ratio and positive strip time, including the case of non-sinusoidal oscillation which is effective for better mold lubrication. In this context, a lubrication index is proposed as a new criteria which can be used to estimate mold powder consumption rate.     

Modeling transient slag-layer phenomena in the shell/mold gap in continuous casting of steel

Mold-slag friction and fracture may cause heat-transfer variations in continuous casting, which leads to steel shell temperature and stress variations, resulting in surface cracks. Analytical transient models of liquid slag flow and solid slag stress have been coupled with a finite-difference model of heat transfer in the mold, gap, and steel shell to predict transient shear stress, friction, slip, and fracture of the slag layers. The models are validated by comparing with numerical models and plant measurements of mold friction. Using reported slag-fracture strength and time-temperature-transformation (TTT) diagrams, the models are applied to study the effect of casting speed and mold-powder viscosity properties on slag-layer behavior between the oscillating mold wall and the solidifying steel shell. The study finds that liquid-slag lubrication would produce negligible stresses. A lower mold-slag consumption rate leads to high solid friction and results in solid-slag-layer fracture and movement below a critical value. Crystalline slag tends to fracture near the meniscus and glassy slag tends to fracture near the mold exit. A medium casting speed may be the safest to avoid slag fracture, due to its having the lowest critical lubrication consumption rate. The high measured friction force in operating casters could be due to three sources: an intermittent moving solid slag layer, excessive mold taper, or mold misalignment. Other symbols are defined in Table I     

A Study for Initial Solidification of Sn-Pb Alloy During Continuous Casting: Part II. Effects of Casting Parameters on Initial Solidification and Shell Surface

The initial shell solidification of liquid steel in the mold has significant influence on both surface and internal quality of the final slab, and it is mainly determined by the high transient high temperature thermodynamics occurring in the mold. This study investigated the effects of casting parameters like casting temperature, mold oscillation frequency, and stroke on the initial solidification of a Sn-Pb alloy through the use of a mold simulator to allow the clear understanding of the inter-relationship between irregular shell solidification, heat transfer, negative strip time (NST), and casting conditions. Results suggested that the shell surface oscillation marks (OMs) are strongly depending upon the fluctuations of meniscus responding temperatures and heat flux. An abrupt sudden fluctuation of high frequency temperature and heat flux at the meniscus during the NST would deteriorate the shell surface and leads to deep OMs. The fluctuations of responding temperature and heat flux are determined by the NST, meniscus solidification, and oil infiltration, which in turn are influenced by casting conditions, like casting temperature, oscillation frequency, stroke, etc.     

结晶器非正弦振动理论的研究

建立了非正弦振动波形的计算方法,首次提出了一个基于电液伺服振动系统的振幅和振动频率可调的分级匹配控制模型,使正常拉坯时,振动频率为１００～１２０ｍｉｎ＾－１负滑动率为－１０％～－２０％,负滑动时间稳定在０．０６ｓ左右,开发了电液伺服结晶器振动系统,热试结果表明,应用非正弦振动可使初始凝固坯壳在正滑动其期间所受的摩擦阻力降低４０％,皱痕深度大为减轻,铸坯表面质量明显提高。     

A new mechanism of hook formation during continuous casting of ultra-low-carbon steel slabs

The initial stages of solidification near the meniscus during continuous casting of steel slabs involve many complex inter-related transient phenomena, which cause periodic oscillation marks (OMs), subsurface hooks, and related surface defects. This article presents a detailed mechanism for the formation of curved hooks and their associated OMs, based on a careful analysis of numerous specially etched samples from ultra-low-carbon steel slabs combined with previous measurements, observations, and theoretical modeling results. It is demonstrated that hooks form by solidification and dendritic growth at the liquid meniscus during the negative strip time. Oscillation marks form when molten steel overflows over the curved hook and solidifies by nucleation of undercooled liquid. The mechanism has been justified by its explanation of several plant observations, including the variability of hook and OM characteristics under different casting conditions, and the relationships with mold powder consumption and negative/positive strip times.