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.     

Investigations on the Transient Mould Friction Force in Slab Continuous Casting based on Fast Fourier Transformation

In continuous casting, the friction forces in the mould reflect the lubrication between the mould and slab, moreover, they affect the slab surface quality and caster productivity. In the present research the friction force is calculated by measuring the difference of working forces during HOT and COLD tests, which are carried out on the slab continuous caster equipped with hydraulic oscillators. Through analysing the mould friction using the Fast Fourier Transformation (FFT) method, the effects of mould oscillation mode and casting speed on the mould friction force are evaluated. The characteristics of the friction signal in the frequency domain are also investigated. Especially, the characteristics of friction force fluctuations before breakouts are discussed. The experimental and analytical results may contribute to more clearly understand the transient lubrication behaviour not only in the time domain but also in the frequency domain, which is also useful for the application of FFT in data analysis of mould processes.     

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.     

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。     

正弦振动结晶器摩擦力与负滑脱参数对比分析

结晶器摩擦力是反映结晶器与铸坯间相互作用、评价保护渣润滑性能的重要参数.由于摩擦力的检测与计算比较复杂,一般通过对振动工艺参数的理论分析来估算结晶器与铸坯间的润滑与摩擦.基于液压振动装置的板坯连铸试验平台,对结晶器摩擦力和振动速度进行了检测实验研究.通过对理论与实测计算结果的对比分析,讨论了结晶器摩擦力状态与负滑脱参数的关系及二者随拉速的变化趋势.该研究在制定适宜的振频-拉速匹配关系、定量理解结晶器与铸坯间的瞬态摩擦行为等方面具有一定的指导作用.     

外加液态保护渣连铸生产实践与前景展望

 外加液态保护渣可以提高铸坯质量和可浇性。在某厂板坯连铸机上,分别采用液态保护渣和固态保护渣进行多钢种浇注试验,对比液态保护渣和固态保护渣消耗量、结晶器温度分布、拉坯摩擦力及铸坯质量,并对液态保护渣的应用前景进行展望。结果显示,液态保护渣比固态保护渣消耗量增加60%左右,结晶器温度分布更均匀,连铸拉坯摩擦力降低约15%,铸坯质量有所提高。这说明在连铸过程中,液态保护渣使结晶器和铸坯间传热更均匀;液态保护渣润滑效果更好,可以提高铸坯质量。同时,有望不添加F-、Na+等有害离子,改善环保问题。     

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。     

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.     

Challenge to control mould heat transfer during thin slab casting

Abstract

At the thin slab caster of Tata Steel, IJmuiden, mild cooling mould powders were introduced with the aim to control the mould heat transfer during casting. These mild cooling mould powders are characterised by specific values of basicity, solidification point and chemical composition. Application of these mould powders resulted in a redistribution of mould heat transfer during casting, i.e. a reduced and more stable mould heat transfer in the critical upper part of the mould and an increased mould heat transfer in the lower part of the mould. The average mould heat transfer and hence the shell thickness at mould exit are comparable to the standard powder. The application of mild cooling mould powders also resulted in improved solidification behaviour of the steel shell. A thinner chill zone with smaller thickness variations was observed. Furthermore, it was found that the mould taper required optimisation to match the changes in shrinkage behaviour to ensure uniform solidification. The use of mild cooling powders was observed to give an increase in mould friction. Mould thermal monitoring indicated that the solid slag films fractured (sheeting) in the upper part of the mould. However, no operational problems were reported, which indicate that the first 200 mm under the steel meniscus is essential for initial solidification and for the formation of a homogeneous steel shell. All these findings can be understood by considering the crystallisation properties of the mould slag, which include the cooling rate. Mild cooling has been shown to provide uniform heat transfer and adequate lubrication for high speed thin slab casting.     

Investigation of Friction Force between Mould and Strand Shell under Sinusoidal and Non‐sinusoidal Oscillation in Continuous Slab Casting

Friction force is an important parameter to evaluate powder lubrication and reflect the interaction between mould and strand shell. A non‐sinusoidal oscillation pattern emerges under high speed continuous casting, the characteristic of which is that the time of upward motion is longer than that of the downward motion within one cycle. In the present research experiments were performed on a slab continuous caster with a strand size of 1000 mm ×120 mm. The friction forces of sinusoidal and non‐sinusoidal oscillation under the same parameters were calculated and comparatively analysed. The results show that the friction forces of both oscillation patterns approximately change along with velocity, and present a trend to a trapezoid wave, but there is a phase difference between the friction force and relative velocity. Compared with sinusoidal oscillation, the non‐sinusoidal oscillation pattern exhibits some important advantages as follows; the maximal and average compressive forces and the compressive force work acting on the strand shell are all higher, the maximal tension is lower, and the actual amount of negative strip is higher. These advantages effect better healing of surface cracks and strand demoulds, and reduce the generation and expansion of cracks, consequently improve the surface quality.     

Development of Mould Flux for High Speed Thin Slab Casting

Mould powders impact the stability of the continuous casting process for steel at all casting speeds. The main functions of mould powder are to provide sufficient lubrication and to control the mould heat transfer between the solidifying steel shell and the copper mould. At higher casting speeds associated with thin slab casting, the role of the mould powder is even more important. Actual casting speeds for the thin slab caster at Corus IJmuiden are between 5.4 and 6 m/min; the production level is around 1.3 Mt/year (coils). It has been decided to increase the production of this caster to a level of 1.8 Mt/year (coils). In order to meet this demand, the steel in mould time has to be increased to approximately 85% and the maximum casting speed will be increased to 8 m/min. A collaborative project between Sumitomo Metal Industries (SMI) and Corus IJmuiden was initiated to develop mould powders which facilitate casting speeds up to 8 m/min at the thin slab caster. Main subjects of this project are: mould powder design, characterisation of mould powder and mould slag, trials at the pilot caster of Sumitomo and finally plant trials at the thin slab caster of Corus. A special point of attention is the condition to use mould powder as a granulated material at the thin slab caster. As a consequence, the characterisation work focussed on the choice of raw materials and on the corresponding phase relations at elevated temperatures. Typical of the developed mould powders are so‐called mild cooling properties which will result in a controlled mould heat transfer during casting. In this paper, several aspects of this joint project between Sumitomo and Corus will be described.     

板坯连铸提高拉速工艺研究与实践

对首钢二炼钢板坯铸机提高拉速的工艺进行了研究。为适应1．2～1．3m／min拉速条件下稳定正常的生产,对结晶器冷却水量、浸入式水口参数、二冷比水量、振动参数、保护渣性能指标进行了调整,最高拉速达到了1．4m／min。研究了拉速变化对结晶器热流密度的影响、提高拉速后对结晶器铜板温度的影响、矫直区铸坯表面温度变化、保护渣耗量的变化等。通过采取以上措施,与0．8m／min拉速情况对比,铸坯表面纵裂发生率维持在同一水平,含铌的微合金钢种一检合格率显著上升,铸坯内部质量维持在同一水平。板坯双流铸机改为单流铸机后,由于拉速的提高,板坯月产量达到了9．5万t,达到了双流铸机的产量水平。     

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。     

Mould heat transfer and continuously cast billet quality with mould flux lubrication Part 1 Mould heat transfer

Abstract

With the drive to cast higher quality, many minimills are adopting mould powder as a lubricant for the continous casting of steel billets. Over the past three decades considerable experience has been accumulated on the relationship between mould behaviour and billet quality for oil lubrication, but comparatively few studies have been conducted for mould powder lubrication. This study, conducted at a Canadian minimill, involved instrumenting four faces of a copper mould with thermocouples and monitoring mould temperatures during casting of 208 × 208 mm billets with mould flux lubrication. Billet samples were also taken to coincide with periods of measurements. Mould temperatures were monitored for two different mould powder compositions, for different mould oscillation frequencies, two mould cooling water velocities, and a range of steel compositions. An inverse heat conduction model was developed to calculate mould heat transfer from the measured temperatures. In this paper, which is the first part of a two part series, details of the inverse heat conduction model and mould heat transfer data are presented. The results obtained for mould flux lubrication have been compared with those for mould heat transfer for oil lubrication. For peritectic steels, with carbon content in the range 0·12–0·14%, it was found that lubricant type has little influence on the measured mould heat flux distribution at the centreline of a face. The peak mould heat flux was found to be approximately 2500 kW m^-2 . In contrast, for medium carbon steels, mould heat transfer with mould powder was significantly lower than when oil was employed as a lubricant. For instance, at the meniscus, the peak heat flux with mould powder was approximately 2500 kW m^-2 , which was half that recorded with oil as a lubricant. The influence of oscillation frequency, mould cooling water velocity, and mould powder type on mould heat flux has also been presented.     

宽厚板连铸结晶器摩擦行为在线测试与分析

 结晶器/铸坯摩擦行为是影响并决定铸坯表面质量的重要因素。以宽厚板坯连铸结晶器为对象,在线检测基于液压振动装置的结晶器/铸坯摩擦力,测试和分析浇铸温度、铸坯断面、拉速与结晶器振动方式等主要工艺参数对摩擦力的影响,为考察和调控结晶器/铸坯摩擦行为提供试验基础。相同工艺条件下,摩擦力随浇铸温度的升高而降低,随铸坯断面尺寸的增加而增大。正弦、非正弦振动方式与拉速-振频、振幅振动控制模型的合理匹配,能够显著改善高拉速下的结晶器/铸坯摩擦行为,结晶器振动工艺的开发和优化对于充分发挥液压振动装置设备潜力,稳定高拉速下铸坯表面质量具有积极意义。     

Transient Mould Friction Based on the Wavelet Theory

The mould friction is an important parameter reflecting the initial shell and mould lubrication conditions.The research of mould friction is very important to the optimization and developing of continuous casting.The measured mould friction under hydraulic oscillation mode was researched with wavelet analysis to reveal its time-frequency characteristics.Firstly,the mould friction signals under different production conditions were monitored and the mould friction was calculated.Then,mother wavelet function was selected from three wavelet functions which were chosen preliminary according to the characteristics of mould friction and wavelet theory.Through wavelet transformation,mould friction signal was projected onto the wavelet domain,and the time-frequency characteristics of mould friction under different production conditions were obtained and discussed.Mould friction under different production conditions such as different oscillation mode,casting speed fluctuation,increasing and decreasing stage of casting speed and breakout occurrence was reported in detail in the wavelet time-frequency maps.The characteristics of mould friction were reflected well through wavelet transformation which proved that wavelet analysis had a good feasibility for mould friction study and can further reveal the nature of mould friction.     

Mould Powder Requirements for High‐speed Casting

Mould powders play an important role in the stability of the continuous casting process of steel. The main functions of mould slag (i.e. molten powder) are to provide sufficient lubrication and to control the heat transfer between the developing steel shell and the mould. Sufficient lubrication requires an undisturbed melting of mould powders and uniform infiltration of mould slag. Based on the casting practice in IJmuiden, it is found that these demands become even more important for the applied high casting speeds in thin slab casting at 5 to 6 m/min. At Corus RD&T, mould powders were characterised by X‐ray diffraction and subsequent fully quantitative Rietveld analysis. Additionally, the melting of mould powders has been studied in‐situ using high‐temperature X‐ray diffraction, to gain crucial knowledge about melting relations. Slag rims obtained from the thin slab caster mould were characterised using extended microscopic techniques in order to describe the mechanisms of rim formation and growth. Finally, slag films obtained after casting were characterised. As a result, not only the melting process of mould powder, but also the mechanism of formation and growth of slag rims is much better understood. This knowledge will be applied to define the demands on the composition and properties of mould powder for even higher casting speeds.     

Mould heat transfer and continuously cast billet quality with mould flux lubrication Part 2 Quality issues

Abstract

With many billet producers adopting mould powder lubrication, there is a need to clarify the gains in quality that can be achieved with this practice. Over the past three decades considerable research has been conducted to establish the relationship between mould behaviour and defect formation for billets continuously cast with oil lubrication, but little has been done to compare oil cast billets with powder cast billets. In this study, conducted at a Canadian minimill, four faces of a copper mould were instrumented with thermocouples and mould temperatures and billet quality were monitored with mould powder lubrication during casting of 208 × 208 mm billets. In the first part of this two part series (in Ironmaking & Steelmaking No. 1 2000), the results of the mould heat transfer analysis and the influence of variables were presented, together with a comparison between oil and powder lubrication. In the present paper, Part 2, billet quality is examined in detail. The difference in turbulence at the meniscus between oil and powder lubrication is established, and the need to tune mould level sensors when switching to mould powders is demonstrated. Previous work has shown that mould level fluctuations have a strong influence on defects such as offsquareness and transverse depressions, both of which are markedly reduced when casting with mould powders. The inherent stability of the meniscus is improved when employing mould powder lubrication and a submerged entry nozzle. Furthermore, the significant reduction in mould heat transfer at the meniscus, when mould powders are employed, particularly for medium carbon steels has been shown to correlate well with the observed reduction in offsquareness. The paper also elucidates the reasons for the reduction, and in most cases, elimination of transverse depressions in B–Ti grades when casting with mould powders. The mechanism of longitudinal depression formation and subsurface cracking observed in many of the powder cast, medium carbon billets has also been established.     

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.     

Investigation of mould thermal behaviour by means of mould instrumentation

Abstract

The thermal behaviour of the continuous casting mould has a critical influence on strand surface quality, casting productivity and operating safety. voestalpine Stahl GmbH has long been interested in the field of research into mould behaviour, and began with the investigation of heat flux in 1995, and then of thermal variability in 1999. Since 2000, an online mould thermal monitoring system for heat flux density, thermal variability and friction has been installed. The heat flow density in the mould is determined by measuring the inflow and outflow temperatures and the throughflow volume of the primary cooling water. Temperature as measured by a thermocouple based breakout detection system in the mould copper plate is used to investigate the thermal variability. These online measured values have been employed to examine the influence of casting parameters, steel analysis and casting powder on the heat flux and thermal variability, and the relationship between these variables and cracking. The knowledge gained through these wide ranging plant based investigations has been used as a major tool in the diagnosis of problems (such as breakout and sticking), for optimisation of the process, particularly in the field of casting powders, and for control of slab quality. In particular, this knowledge has been incorporated into online mould thermal monitoring.