Tools and techniques for use in development of mould powders

Abstract

The tools and techniques used within Corus UK to improve mould powder performance and product quality are reviewed. Mould thermal monitoring, which allows real time assessment of heat transfer, is a critical tool in the development of mould powders. Thermal analysis, viscosity measurements, flux–mould plate interactions, steel velocity profiles in the mould and slag film properties also play a part in continuing studies. Case studies of improvements in process performance achieved using these methods are presented.     

Water modelling of level fluctuation in thin slab continuous casting mould

Abstract

A water modelling experiment was conducted to study the meniscus instability in a continuous thin slab casting mould using particle image visualisation. The results show that the level fluctuation, circulation centre position and jet impinging depth are unsteady and periodic with a similar period. The probability distributions of the fluctuating meniscus and wave height have been obtained with the highest frequency near the average position. The flow pattern and meniscus profile may be momentarily asymmetrical, and the phase difference of level fluctuation in the two sides of mould centreline is a half period. The average meniscus profile, the highest and lowest meniscus positions are generally symmetrical about the mould centreline. The wave height mainly depends on the jet impinging depth and circulation centre position. The wave height increases as the jet impinging position rises and the circulation centre approaches to the submerged entry nozzle.     

Study on cascaded whole-leaf spring oscillation mechanism for mould in continuous casting

Abstract

A design method of a cascaded whole-leaf spring mechanism is proposed, which is a new oscillation guidance device for the mould in continuous casting. Then its prototype designed in this paper is produced in the lab, of which kinematics and dynamics simulations are carried out based on the rigid–flexible coupling virtual model. Simulation curves of the displacement and velocity of the mould are almost consistent with the ideal ones, which verifies the model built in this paper is rational. Furthermore, natural frequencies and mode shapes of the mechanism are calculated by dynamics simulation, and forces applied on leaf springs and revolute joints are analysed and effects of the basic parameters on these forces are also studied, which establish the basis for further studies and next application of this mechanism.     

Combination of inverse problem and neural network for thermal behaviour calculation of mould process based on temperature measurements in plant trial

Abstract

Heat transfer between mould and strand has a critical influence on billet quality, caster productivity and operating safety. It is very important to obtain the correct distributions of temperature and heat flux, and many studies are made on the calculation methods of heat transfer between strand and mould, aiming to reduce the computation time and improve the calculation accuracy. In the present paper, based on measured data of temperature and heat flux during round billet continuous casting, the calculation method which combines the online measurement data and numerical simulation was investigated. Through identifying the local thermal resistance and its distribution between the mould and the strand by an inverse heat transfer model, the heat flux and shell thickness profiles were calculated. To avoid the iterative solution by inverse model, a faster alternative model using an artificial neural network was developed to predict the thermal resistance from the measured temperature. After training, there is an exact correspondence between the observed temperature values and the thermal resistance. The calculation results obtained by the combination of neural network and numerical simulation can correctly reflect the characteristics of non-uniform heat transfer around the mould circumference, which provides a worthwhile and applicable method for online calculation and visual technology of heat transfer and solidification in continuous casting mould.     

Manufacture of centrifugal cast high speed steel rolls for wire rod mills

Abstract

By means of a centrifugal casting method, high speed steel (HSS) rolls with excellent wear resistance have been manufactured. The hardness of the HSS rolls was 65–67 HRC, the range of hardness variation was smaller than 2 HRC and the impact toughness exceeded 15 J. The wear rate of the rolls was 2.5 × 10^?4 mm t^?1 of steel passing through the prefinishing stands of the high speed hot wire rod rolling mill. Casting cracking of the HSS rolls was definitely decreased and the rejection rate owing to the presence of cracks was less than 3%, as a result of the use of a two layer paint and varying the rotation rate. In addition, the segregation of alloying elements could be decreased by adding a suitable amount of niobium (0.6–2.0%).     

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.     

Viscosity effect of titanium pickup by mould fluxes for stainless steel

Abstract

Mould flux samples from an operating continuous caster showed a steady state increase in titanium oxide content of 3–4% for a titanium stabilised ferritic stainless steel and about 6% for a titanium stabilised austenitic stainless steel. Owing to the reduction of silica the flux basicity increased from 0.8 to 0.9, and from 0.95 to 1.2, for the ferritic and austenitic steels, respectively. Similar changes in composition were found for mould shims, which had been recovered from the mould after casting the ferritic steel. The increased titanium oxide content caused a decrease in the flux viscosity, for the original basicity. However, when the flux basicity was increased from 0.95 to 1.2, the apparent viscosity of the mould flux of the austenitic steel increased markedly, for 10% TiO₂ and temperatures of 1250°C and below. Precipitation of perovskite was found to be responsible for the higher apparent viscosity.     

Effect of exciting current of high frequency electromagnetic field on initial solidification of steel during electromagnetic continuous casting

Abstract

A three-dimensional finite element model of the electromagnetic field and temperature field of electromagnetic continuous casting (EMCC) process was developed. The aim was to investigate the effects of induction heat of high frequency electromagnetic field on the early solidification process of molten steel in mould under various conditions of exciting current parameters. The results show that the induction heat has significant effects on the early solidification process, which appear as increasing the billet surface temperature, thinning the initial solidified shell and lowering the starting point of the initial solidification. The increases in exciting current frequency and density make the effects of induction heat on solidification process increase remarkably. Especially, with the exciting current frequency increase, the early solidification process and shell growth become non-uniform in billet circumferential direction. Furthermore, if the exciting current density exceeds a certain value, there occurs a high temperature region in the top of molten steel column, and then the initial solidification rate is greatly decreased. As a conclusion, the effects of induction heat on initial solidification process must be considered when the exciting current frequency and density are adjusted during the electromagnetic continuous casting process.     

Thermal behaviour of moulds with different water channels and their influence on quality in continuous casting of beam blanks

Abstract

Two-dimensional finite element heat transfer models have been developed to predict temperature distribution in beam blank moulds with large and small hole water cooling channels. The effects of water channel design and grinding thickness on transverse temperature profile in meniscus region were analysed in detail. The effects of both moulds during plant trials are also compared. The results show that the peak temperature is found in the fillet area of the large hole mould and is 20°C higher than that of the small hole mould. With increasing grinding thickness, peak temperature in both moulds decreases linearly, and when the grinding thickness reaches 9 mm, the peak temperature of the small hole mould exceeds that of the large hole mould. The transverse temperature uniformity of the hole mould is superior to that of the large hole mould. It is also found that longer mould life, better strand surface quality and more homogeneous surface microstructure are obtained when using the small hole mould.     

High speed continuous casting of steel billets: Part 2: Mould heat transfer and mould design

Abstract

A high speed instrumented mould trial was conducted under industrial conditions to study the heat transfer at the midfaces and corners of the mould and to clarify mould taper requirements in high speed continuous casting. The predicted peak heat transfer in this high speed system was found to be up to 2·5 times that reported for conventional speeds, and up to 1·5 times that for other reported high speed systems. The average heat transfer was found to be up to 45% greater than in conventional systems, and comparable with other high speed systems. The effect of casting speed was analysed in detail and was found to be dependent on carbon content. Increased casting speed was also found to increase the metal level standard deviation but to have less of an impact on the heat transfer than similar changes at conventional speeds. A mathematical billet thermal and solidification model was applied to these heat transfer results to determine the response of the current mould to high speed casting conditions. Using this assessment of the mould distortion and billet dimensions, new mould tapers were designed on the basis of minimising any mould-strand interaction and/or binding. New mould taper designs for high and low carbon grades were recommended for casting speeds of 3·0, 3·5, 4·0, and 4·5 m min ^-1. The design sensitivity to changes in casting speed is discussed.     

高效连铸抛物线锥度结晶器的研究与设计

应用凝固理论对高效连铸条件下结晶器内初生坯壳生长规律、铸坯的凝固收缩特性进行了系统研究。利用结晶器锥度设计的最小气隙原则,建立结晶器设计计算模型,开发出高效连铸抛物线锥度结晶器设计系统。该系统可依据工艺条件设计出满足不同钢种和拉速条件的结晶器锥度、内腔曲线方程、上下口以及任意间位置的内腔尺寸,为结晶器的生产提供操作数据和理论依据。最后分别讨论了钢种、结晶器长度、拽坯速度、铸坯断面尺寸与结晶器锥度、凝固收缩系数的关系。     

Model based optimisation of continuous annealing operation for bundle of packed rods

Abstract

A typical industrial thermal processing operation has multifold complexity, with varying charge dimensions, multiple grades and inconsistent loading patterns as well as the absence of in situ sensors. These operational characteristics and restrictions invariably lead to empirical design for the temperature time cycles, which often results in suboptimal operation in terms of higher energy consumption, inconsistent quality and lower productivity. In the present work, a process model is proposed for designing the heating cycles for bundles of packed rods with different rod diameters, bundle diameters and packing fractions in a continuous annealing furnace. The process model has the capability of predicting spatial and temporal evolution of temperature and hardness in the bundle as it traverses through the furnace. Interestingly, the model based process cycles are found to be counterintuitive as compared with the empirically designed cycles normally employed in the plant. It is shown that instead of designing the process cycles on the basis of rod diameters, which is the general practice in the plant, it should be based on bundle characteristics, such as bundle diameter and packing fraction. These concepts have been implemented in an industrial operation resulting in around 20% energy reduction and 15% productivity enhancement.     

Influence of casting parameters on void volume and centre segregation in continuously cast 100Cr6 blooms

Abstract

Continuously cast ball and roller bearing steel 100Cr6 exhibits comparatively strong segregation which poses a quality handicap. In the present work which was carried out in the time between 1992 and 1996 the void formation which is related to the centre segregation and the centre segregation were measured on 2 m long segments of 240 × 200 mm² blooms cast at the steel plant of Georgsmarienhütte GmbH. It was found that the segregation of carbon expressed with the ratio of C _max/C _o increases with casting speed and superheat. Many experiments were carried out to elucidate the effect of electromagnetic stirring along the lower part of the metallurgical length (F-EMS). But, unfortunately, a decrease in segregation could not be achieved by F-EMS. The only method to decrease the centre segregation in continuously cast blooms of 100Cr6 seems to be casting at low speeds with low superheat.     

高拉速下凝固进程的试验测量与模型预测

 根据京唐高拉速试验特点,结合板坯连铸机设备和洁净钢连铸工艺特点和要求,建立连铸板坯凝固传热模型并结合射钉法测量综合预测了1.9～2.4 m/min高拉速条件下铸坯的凝固坯壳厚度和凝固终点位置。综合研究表明,结合射钉试验和数值模拟能更精确跟踪铸坯的凝固进程,为高拉速试验提供准确的凝固信息,并能为评价连铸机综合冷却能力、优化二冷制度和轻压下工艺提供合理的参考信息。     

Melting speed of mould powders: determination and application in casting practice

Abstract

Controlled melting of mould powder on top of the meniscus is essential for efficient mould flux performance. This paper presents two complementary methods to determine the melting speed of a mould powder. One method measures the displacement of a prepressed cylinder of mould powder at a fixed temperature. This method yields qualitative, but reproducible, results which can be related to flux composition. In the second method, a sample of mould powder is melted on top of a steel bath. The data are interpreted with the help of an improved theoretical melting model, resulting in quantitative values of melting speed and thickness of the molten flux layer. The results of both methods agree well with measured plant data.     

连续铸钢结晶器钢水液位自动控制及其实践

介绍连续铸钢结晶器钢水液位自动控制技术的国内外状况，包括液位检测技术、控制算法和执行器等在实际生产过程中的应用情况及存在的问题。针对上述状况进行了评估并提出解决问题的建议。     

结晶器角部锥度对方坯温度及应力场的影响

通过调整气隙厚度模拟结晶器角部锥度变化,使用有限单元法计算出不同锥度下方坯坯壳内的温度及应力场,明确了结晶器角部锥度对方坯温度及应力分布的影响.     

低碳钢薄板坯高速连铸保护渣研究与优化

 针对低碳钢薄板坯高速连铸过程中保护渣液渣层过薄、黏结报警频发、铸坯表面纵裂纹过多等问题,在充分考虑高拉速下低碳钢凝固收缩特性的基础上,确定了保护渣润滑与传热性能的优化方向并开展了工业试验。将保护渣碱度从1.10提高到1.30,Li₂O质量分数从0.57%提高到1.06%,Na₂O质量分数从5.48%提高到8.16%,碳质量分数由7.71%降低到6.72%。对2种保护渣的流变性能和渣膜3层结构进行了深入研究,发现优化后保护渣渣膜中的液渣层比例增加,渣膜润滑系数α增大;同时,渣膜中的结晶层比例也有一定程度的提高,渣膜热阻系数β增大,从而使保护渣的润滑性能和控制传热能力均得到改善。从矿相分析结果看出,保护渣碱度的提高在一定程度上会促进硅灰石的析出,导致渣膜结晶率提高、热阻增大,进而起到控制传热的目的。生产实践表明,在拉速提高后,使用新型保护渣基本避免了黏结和裂纹的产生,生产效率和铸坯质量均得到显著提高。     

Coordinating optimisation of F-EMS and soft reduction during bloom continuous casting process for special steel

To control the halfway cracks and shrinkage cavities during bloom continuous casting (CC) process, final electromagnetic stirring (F-EMS) and soft reduction techniques have been coordinately optimised. The halfway crack and shrinkage cavity can be successfully controlled by adopting the CC technique parameters described as follows: (1) casting speed is 0.62 m min-1, (2) secondary cooling water ratio is 0.2?L?kg^?1, (3) reduction amount is 18?mm; (4) reduction region ranges from 19.9?m (location of M3 roller) to 25.4?m (location of M9 roller) distance from meniscus; and (5) corresponding distributions of reduction amount for each roller are 1, 1, 2, 3, 4, 4, and 3?mm along the casting direction. As compared to origin scheme for bearing steel, the difference between the maximum and minimum segregation degrees at the strand centre can be reduced from 0.35 to 0.09 under the optimal case.     

高碳高铬冷作模具钢连铸应用开发

高碳高铬冷作模具钢由于其高度的裂纹敏感性,基本都采用模铸生产。宝钢特钢金属板带产线,利用立式连铸机成功地实现了连铸+热轧生产冷作模具钢。立式连铸机具有对称凝固好、组织均匀、夹杂物容易上浮等特点,由于连铸机没有弯曲和矫直的变形应力,特别适合生产裂纹敏感性钢种。以钢种Cr12MoV为例,通过对材料特性研究、成分优化设计以及冷却速度对凝固组织的影响等基础研究,结合连铸过程生产实践,介绍了宝钢特钢冷作模具钢连铸应用技术的开发过程。