Friction Forces between Mould and Strand Shell during Billet Casting

During continuous casting processes lubrication between mould and strand shell is very important for reaching and maintaining a good surface quality of both the as‐cast strand and the finished product. The lubrication is influenced by the properties of the mould powder but also by the friction forces caused by the periodic movements of mould and the descending strand shell. In the present research measurements were carried out to investigate the friction forces during 150 mm sq. billet casting of different steel grades using mould powder or granules and submerged entry nozzles. The friction forces reached values up to 6 kN. Since friction forces are depending on upward and downward movements within the mould‐strand‐system, the evaluation of a rheogram shows the typical change of pressure and tension. Maximum friction forces arise during the positive strip time of the rising mould, introducing tensions and cracks in the as‐cast strand. On the other hand lowest frictions forces are generated by the descending mould within the negative strip time leading to the healing of faults in the as‐cast strand. These industrial experiments indicate a correlation between friction forces and surface quality of as‐cast strands.     

基于液压振动的结晶器非正弦振动

 基于宝钢板坯连铸的实测数据,对液压振动装置下结晶器非正弦振动模型相关特性进行了测试和分析。探讨了不同偏斜角度下非正弦振动波形的特点及振动装置驱动力的变化趋势,同时对拉速-振频反向控制模型下,正弦与非正弦振动方式的负滑脱等参数计算结果进行了分析,并对结晶器摩擦力的实测结果进行了讨论。结果表明,结晶器非正弦振动进一步优化了工艺参数,比传统的正弦振动更适合高速板坯连铸生产。     

Optimization of Oscillation Model for Slab Continuous Casting Mould Based on Mould Friction Measurements in Plant Trial

Lubrication and friction between the mould and strand are strongly influenced by mould oscillation, and play an important role in slab quality and operating safety during continuous casting processes. Investigation of mould oscillation is therefore essential for getting a better online control of the mould processes. A feasible approach for the development and optimization of mould oscillation was put forward, which combined online measurement of mould friction, design of negative oscillating parameters and evaluation for powder consumption. Three different control models including sinusoidal and non-sinusoidal oscillation for mould oscillations were developed to investigate and evaluate the effects of oscillation on mould friction and powder lubrication. For the purpose of investigating mould friction between mould and strand, online measurement was carried out on a slab continuous caster equipped with a hydraulic oscillator. Also the comparison of the mould friction in sinusoidal and non-sinusoidal mould oscillation was made for subsequent analysis. The industrial experiment result shows that the combination of inverse control model and non-sinusoidal oscillation mode will contribute to the proper powder consumption, leading to a suitable effect of friction force on strand surface, especially for high speed continuous casting. The proposed method provides reliable basis for guiding and optimizing mould oscillation among control models, sinusoidal oscillation and non-sinusoidal oscillation.     

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。     

Mould lubrication and friction behaviour with hydraulic oscillators in slab continuous casting

Abstract

It is very important to obtain reliable lubrication from casting powder both at the meniscus and in the gap between strand and mould as it affects slab surface quality and caster productivity. With knowledge of mould friction, a quantitative insight into the behaviour of powder during caster operation is possible. In the present research, the friction was studied based on a slab continuous caster equipped with hydraulic oscillators. The effects of mould oscillation and the abrupt change of casting speed on mould friction force were evaluated, and the characteristics of lubrication behaviour in a casting sequence were investigated. In particular, a comparison between the mould friction force between sinusoidal oscillation mode and non-sinusoidal oscillation mode was made. Finally, the characteristics of friction before a breakout are discussed. The experimental and analytical results may contribute to the development of mould friction online measurement and more clearly learn the lubrication behaviour in different conditions.     

液压振动下板坯连铸结晶器摩擦力检测实验研究

结晶器和铸坯之间的润滑与摩擦行为是影响连铸坯质量及生产效率的重要因素.基于宝钢板坯连铸试验平台,开展了液压振动下的结晶器摩擦力检测实验研究.通过检测不同振动参数下,冷态和拉坯时振动系统输出力的变化情况,计算出结晶器与铸坯间摩擦阻力.分析了摩擦力的周期变化行为,讨论了拉坯过程中相关参数的变化特征.为探究结晶器与铸坯间摩擦行为、优化与开发连铸工艺等提供实验基础与技术支持.     

Mechanism of explaining liquid friction and flux consumption during non‐sinusoidal oscillation in slab continuous casting mould

Abstract

The mechanism of explaining liquid friction and flux consumption in a non‐sinusoidal oscillation mould is expounded by proposing a series of indices for estimating interface phenomena between mould and metal in meniscus and calculating the effect of non‐sinusoidal oscillation parameters on these indices. The results show that the liquid flux is periodically infiltrated into the gap between oscillating mould and withdrawing slab by the negative pressure caused by widening flux channel from the last stage of positive strip time to the last stage of negative strip time. The surface cracks on the slab, even breakout, are formed during the positive strip time and existing surface cracks can be healed within the cumulative negative strip time. The casting performances were improved by decreasing oscillation frequency and increasing amplitude and non‐sinusoidal oscillation factor.

On explique la friction liquide et la consommation de flux dans une coquille à oscillation non‐sinusoïdale en proposant une série d’indices pour estimer les phénomènes d’interface entre la coquille et le métal dans le ménisque et pour calculer l’effet des paramètres d’oscillation non‐sinusoïdale sur ces indices. Les résultats montrent que le flux liquide s’infiltre périodiquement dans l’espace entre la coquille oscillante et la brame en retrait par la pression négative causée par l’élargissement du canal de flux à partir de la dernière étape de temps positif de démoulage jusqu’à la dernière étape de temps négatif de démoulage. Les fissures de surface de la brame, et même la percée, sont formées lors du temps positif de démoulage et les fissures de surface existantes peuvent être réparées lors du temps négatif cumulatif de démoulage. Le rendement du moulage s’améliorait en diminuant la fréquence d’oscillation et en augmentant l’amplitude et le facteur d’oscillation non‐sinusoïdale.     

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。     

Solidification and Particle Entrapment during Continuous Casting of Steel

Avoiding particle entrapment into the solidifying shell of a steel continuous caster is important to improve the quality of the continuous cast product. Therefore, the fluid flow dynamics in the steel melt and mushy zone, heat transfer and solidification of the steel shell, as well as the motion and entrapment of inclusion particles during the casting process were investigated using computational models. Solidification of the strand shell is modelled with an enthalpy‐formulation by assuming a columnar morphology in the mushy zone. The motion of particles is tracked with a Lagrangian approach. When the particles reach the solidification front, they can be entrapped/engulfed into the solid shell or pushed away from the solidification front, depending on the mushy zone morphology and the forces acting on them. The current paper focuses on the mould region at a steel continuous caster, including the submerged entry nozzle (SEN) and 1.2 m length of the strand. The results are validated with plant measurements and demonstrate the potential of the model to predict fluid flow, shell growth and the positions and the amount of entrapped/engulfed particles in the solidifying strand.     

伺服电机驱动的结晶器振动集散控制系统

 伺服电机驱动的连铸结晶器非正弦振动控制相比于目前现场采用的电液伺服驱动控制具有较多优点，但两种驱动方式的控制系统通常采用以高性能控制器作为控制核心的集中控制方式，如果控制器某一部件出现故障，将影响所有连铸流的正常工作，存在一定的安全风险。针对此问题，提出了一种伺服电机驱动的连铸结晶器非正弦振动集散控制系统设计方案，针对多流连铸生产过程进行集散控制系统总体设计及上下位两级计算机控制系统的设计,每一连铸流由一台西门子高性能运动控制器SIMOTION D425单独控制，各流相互独立、互不影响，从而可避免集中控制器某部件故障而影响所有连铸流生产的情况，提高了系统的安全性、可靠性。最后通过试验研究验证了所设计的集散控制系统的可行性。     

Online Measurement for Transient Mold Friction Based on the Hydraulic Oscillators of Continuous-Casting Mold

The interaction of the strand shell surface and mold copper plates has significant effects on the slab surface quality and casting productivity. This article focuses on developing a reliable approach to measure the transient friction force between the slab and the mold for the purpose of the investigation of lubrication and friction behavior inside a mold. This method is presented to monitor transient mold frictions for the slab continuous caster equipped with hydraulic oscillators. A mathematical model is also developed to calculate the empty working force of the no casting state, and a new algorithm, based on the particle swarm optimization, is proposed to predict the dynamic characteristic parameters of mold oscillation. The results have shown that the method has a sufficient sensitivity to variation, especially to the periodical variation of the mold friction, and it has been identified that the transient mold friction can be used as an effective index with regard to detecting mold oscillation and optimizing the casting parameters for process control. It may lay the practical foundation for the online detection of powder lubrication and the visualization of the continuous-casting mold process.     

Experimental research on effect of control model on lubrication for slab continuous casting mould

Abstract

In casting the control model relation between casting speed and frequency plays an important role in the optimisation of process parameters and the improvement of lubrication condition. The mould friction between mould and strand can directly describe the powder lubrication situation in a mould. In the present research, based on the slab continuous caster equipped with hydraulic oscillators, the transient friction forces were obtained by the differences between the monitored driving forces of casting and the calculated driving forces of no casting at same oscillation parameters. According to the trends of friction force with different control models, the characteristics and disadvantages of the traditional positive control model, and the improved positive control model and the inverse control model are discussed. This experimental study contributes to studying the frictional behaviour in mould under different control models, as well as selecting reasonable control model for high speed continuous casing.     

板坯结晶器钢液流动传热和摩擦行为的数值模拟

结晶器摩擦力对连铸顺行非常重要,而钢液流动行为会影响铸坯温度场和保护渣分布,可能对铸坯摩擦力产生一定的影响.利用Fluent软件建立结晶器内钢液流动、传热三维数学模型,并将温度场数据导入Ansys计算铸坯应变,然后根据铸坯-保护渣-结晶器摩擦行为数学模型讨论了渣膜润滑情况,对比了不同水口底部形状下结晶器内液态、固态和总摩擦力.结果表明:不考虑水口射流时结晶器总摩擦力比考虑水口时增大约29.4％;浸入式水口底部形状分别为凸底、平底和凹底3种情况下铸坯窄面中心温度依次降低,凝固壳厚度依次增大,固态摩擦力依次增大,总摩擦力依次增大,液态摩擦力相差较小.     

板坯连铸结晶器摩擦力周期行为的研究

 结晶器摩擦力是反映连铸生产状况的重要参数,研究结晶器摩擦力的周期行为特点对于理解铸坯与结晶器间的相互作用,评价保护渣的润滑特点有着重要的意义。基于液压振动装置的板坯连铸机,对结晶器摩擦力进行了瞬态检测,分析了结晶器摩擦力的周期行为特点。研究结果表明：在稳定拉坯阶段,结晶器摩擦力在两种振动模式下具有相似的周期变化过程;周期内摩擦力最值的出现位置呈现出一定的规律性和不确定性;开浇阶段摩擦力会在长时间内维持正值且波动剧烈。     

Simulation of Nonmetallic Inclusions in a Continuous Casting Strand

The liquid steel flow and the particle transport of non‐metallic inclusions in the liquid domain inside the strand of a continuous steel casting system are numerically investigated. The whole liquid domain inside the solidified shell is simulated, and the loss of liquid steel due to solidification is considered. Assuming a constant drift velocity, a kind of Euler‐Euler multiphase model is implemented to calculate the inclusion behaviour inside the liquid steel, respectively the inclusion removal at the boundaries. The turbulent flow of the liquid steel and the inclusion behaviour are calculated. The resulting distribution of the inclusions in the slab is compared with measurements. The influence of inclusion size (0 to 1 mm) and casting speed on the inclusion distribution is considered.     

非正弦振动在板坯连铸机上的应用

分析了结晶器非正弦振动的振动特性,介绍了安钢引进的VAI板坯连铸机液压振动的振动特点,讨论了非正弦反向振动的各项振动参数,降低结晶器摩擦阻力、提高保护渣耗量,而且可以在低拉速下保证较低的振痕深度,又能在高拉速下保证操作的安全性。并结合生产实践,指出该振动形式对防止粘结漏钢和改善铸坯表面质量有明显作用。     

板坯连铸结晶器/铸坯瞬态摩擦行为的实验研究

合理控制结晶器与铸坯间的润滑和摩擦状况是实现高效连铸技术的关键环节之一,摩擦力作为表征润滑效果的重要参数已受到广泛关注。以板坯连铸机为研究对象,对液压振动条件下结晶器和铸坯间的瞬态摩擦力进行测试和分析。探讨了瞬态摩擦力及相关参数的变化特点,并对正弦、非正弦振动方式下的摩擦力检测结果进行了对比分析,同时讨论了正常工况与漏钢发生前摩擦力的变化特点。为结晶器/铸坯摩擦行为的深入研究和优化连铸工艺等提供了实验基础和技术支持。     

连铸结晶器非正弦振动参数及同步控制模型的研究

给出了一种用于连铸生产的非正弦振动波形,并对其波形参数和振动工艺参数进行了讨论。结果表明：这种非正弦振动波形具有较好的拉坯速度适应范围,能够满足连铸生产的要求。另外,还对其波形参数的选择方法进行了探讨,导出了该非正弦振动的同步控制模型。     

结晶器非正弦振动波形及参数研究

提出了一种既具有非正弦振动的全部特征，又具有正弦振动优点的非正弦振动变形；给出了确定振动参数的方法。     

Some new aspects on inclusion separation in tundishes

The removal of non‐metallic inclusions due to buoyancy forces in tundishes in continuous casting systems is considered. The maximal theoretical removal rate is determined by the flow rate through the tundish, the magnitude of the tundish fluid surface area and the particle terminal rising velocity, depending on the particle size. Two reasons, why the particle separation is worse than the maximal possible one are an unsuitable fluid flow pattern and the turbulent particle diffusion. An analytical discussion of simple flow patterns (parallel flow with different velocity profiles, dead regions, swirling flows) and diffusion shows how they influence the particle removal. Using these simple considerations, it is demonstrated that the often used RTD (residence time distribution)‐curves are inappropriate to estimate the particle separation in tundishes; only the direct measurement and CFD‐simulation of particle removal should be used. The common CFD‐methods are affected with numerical errors such as numerical diffusion for particle concentration simulations with finite volume methods and interpolation errors for particle trajectory calculations. These errors influence significantly the calculated particle removal curves; they are non‐systematic and difficult to quantify.