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.     

F-free mould powders for low carbon steel slab casting – technological parameters and industrial trials

The choice of the mould powder for slab casting is a difficult task because mould powders have many important functions during the continuous casting of steel. CaF₂ is a key ingredient in conventional mould slags since it reduces the viscosity, the liquidus temperature and the break temperature. Fluorine in mould powders is undesirable from the environmental and health points of view due to the following reasons: (i) evolves easily from slags, producing health-injurious gaseous substances, such as hydrofluoric acid; (ii) creates problems for storage and utilisation of solid waste and (iii) causes machinery corrosion. Aim of the present work is to describe the development of a new F-free mould powder for low carbon steel slab casting replacing CaF₂ with B₂O₃. Laboratory tests and industrial trials were performed considering the technological parameters viscosity, break temperature and crystallisation tendency. From laboratory tests it was concluded that important technological parameters are similar when comparing the F-bearing mould powder (reference) and the new F-free B-bearing mould powder for low carbon steel slab casting: viscosity at 1300°C, break temperature, and crystallisation tendency. It was observed during industrial trials that a significant decrease of the submerged entry nozzle erosion was observed. The results were similar when comparing the F-bearing and the F-free performance: slag pool thickness measurements, melting behaviour, Al₂O₃ absorption, mould powder consumption and slabs superficial quality.     

Understanding mould powders for high-speed casting

The development and application of mould powder for high-speed continuous casting of steel is described. For thin slab casting, the main requirements are proper powder melting, undisturbed slag infiltration, adequate strand lubrication and the control of mould heat transfer. For increased casting speeds i.e. up to 8?m/min, slag infiltration and in particular the control of mould heat transfer via crystallisation of the slag film becomes even more important. It was found that a low powder consumption and hence a thin slag film is no restriction for an undisturbed thin slab casting process. Given a stable casting process and machine condition, the mould powder properties are not as critical as widely assumed.     

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.     

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.     

Design of mould fluxes for continuous casting of special steels

The melting behaviour of mould powder during continuous casting is an important consideration with respect to caster performance, production rate and steel quality. Two important factors, powder composition and carbon addition, are critical to control the properties and melting behaviour of the mould flux. In this study, the effect of different carbonaceous materials on the melting characteristics of mould powders was evaluated. Correlations were established between the structural factors and chemical reactivity of carbon and melting behaviour of mould flux. In addition, two examples are given of the effect of flux composition on casting performance for specific steels. A flux with reasonable basicity and additives was designed for the casting of heat-resistant steel (Incoloy 800) to reduce surface cracks. Another flux was designed for the casting of non-magnetic steel containing high aluminium by partially replacing SiO₂ with Al₂O₃ to limit aluminium oxidation by SiO₂.     

Fluorine‐Free Mould Powders for Billet Casting – Technological Parameters and Industrial Tests

There is a clear trend in industry to eliminate fluorine from metallurgical slags. Regarding the continuous casting process, fluorine in mould powders is undesirable due to the following reasons: (i) evolves easily from slags producing health‐injurious gaseous substances like hydrofluoric acid, (ii) creates problems for storage and utilization of solid waste, and (iii) causes machinery corrosion. A collaboration among universities, mould powder supplier, and steel producer was established, aiming for the development of a new F‐free mould powder for billet casting. Firstly, technological parameters for a particular composition were determined: melting characteristics, viscosity, melting behavior, and crystallization tendency. Afterwards, industrial trials were performed in a Brazilian steelworks, doing some evaluations during the continuous casting process, such as ΔT between water cooling inlet and outlet, slag pool thickness, melting performance, powder consumption, Al₂O₃ pick‐up, and effect on SEN erosion. Comparable surface quality for the billets was obtained when using the F‐free mould powder, since the amount of scrap produced during the rolling process was not critical.     

CSP薄板坯连铸低碳钢结晶器保护渣的研究

高拉速薄板坯连铸保护渣与常规板坯连铸保护渣在物理性能上有较大差异，通过对保护渣理化性能、熔化特性的研究，确定了适合高拉速薄板坯低碳钢连铸用保护渣的理化指标。     

Three-dimensional analysis of molten steel flow in slab continuous casting mould with rotated ports in submerged entry nozzle

Abstract

The purpose of this study is to develop a three-dimensional (3D) analysis system capable of analysing the flow field of molten steel in the slab continuous casting mould with rotated ports in the submerged entry nozzle. The ultimate goal is to obtain the optimal design for the entry ports of the submerged nozzle, which can introduce favourable flow patterns to remove non-metallic inclusions and avoid entrapment of molten slag and casting powder to produce steel slab of high cleanliness. In this study, a computational fluid dynamics technique, Sola-Surf, is employed to conduct the 3D fluid flow analysis. The technique has the capability of treating fluid flow problems with a free surface that slightly vibrates. The slightly vibrating free surface presents fairly accurately the behaviour of the molten slag–casting powder layer in the continuous casting mould. The developed simulation system is then tested on a slab continuous casting mould to analyse the fluid flow behaviour of molten steel under various nozzle designs. The design conditions include submerged depth of the nozzle, tilted angle of the nozzle port, and rotated angle of the nozzle port. The results of the simulations show that of the various design factors rotation of the nozzle entry ports has the greatest effect on the flow pattern. It can prolong the residence time of the molten steel and stabilise the molten slag–casting powder layer, which is very favourable for obtaining continuous casting slag of high cleanliness.     

FTSC薄板坯连铸中碳钢保护渣研究及开发

针对唐钢FTSC薄板坯连铸所产生的板坯裂纹、表面夹渣、卷渣及漏钢现象进行保护渣生产试验研究与理论分析。研究结果表明,组分变化对保护渣熔化温度和粘度等指标有着重要的影响。通过实验室对保护渣组份及其变化对其性能影响的研究,结合唐钢FTSC薄板坯连铸自身特点,设计出保护渣基本配方,且在不断试验改进中,最终开发出适合唐钢薄板坯连铸用中碳钢保护渣C2.     

Effects of crystallisation behaviour of mould fluxes on properties of liquid slag film

Abstract

During the continuous casting of medium carbon steel slabs, stable liquid slag film performance is necessary to ensure that the process runs smoothly. In this paper, the effects of mould flux crystallisation behaviour on the properties of leftover liquid slag films were examined. First, the compositions of the leftover liquid slags of high fluorine, CaO–SiO₂–TiO₂ and CaO–SiO₂–Na₂O based mould fluxes after a single crystalline phase was separated were calculated. Then, the viscosity, melting temperature, break temperature and degree of crystallisation of each leftover liquid flux were measured. The results revealed that the properties of leftover liquid slag films of CaO–SiO₂–TiO₂ and CaO–SiO₂–Na₂O based mould fluxes are not as stable as those of high fluorine films after crystallisation. This paper provides a new method for developing fluorine free mould fluxes and for determining the most suitable type of crystalline phase that should be utilised in the continuous casting of medium carbon steel slabs.     

Continuous casting mould powder evaluation

Abstract

A continuous casting mould powder must satisfy various requirements including thermal insulation, chemical insulation, inclusion absorption, lubrication, and promotion of uniform heat transfer from the solidifying steel strand to the copper mould. The relative importance of these properties varies according to the type of steel cast, the prevailing casting conditions, and the end steel requirements. Development of mould powders at Corus UK Ltd involves plant based trials of different powders whose properties influence the above characteristics of which heat transfer is a major consideration. Particular use is made of mould thermal monitoring which, although primarily designed for sticker breakout detection, is now finding increasing use as an investigative tool in mould slag assessment. Both static and dynamic plant data are now available relating mould slag heat transfer performance to casting conditions and this paper describes some recent investigative work carried out by the Aluminium & Steel Casting Department of Teesside Technology Centre, Corus UK Ltd. The study shows that using a combination of plant based observation and laboratory analysis, a greater depth in understanding of mould powder performance can be realised, yielding important information for future powder development.     

The Influence of Carbonaceous Material on the Melting Behaviour of Mould Powder

The melting behaviour of mould powder during continuous casting is an important consideration with respect to caster performance, production rate and steel quality. In this experimental study the effect of different carbonaceous materials on the melting characteristics of mould powders was evaluated. Using X‐ray diffraction, different types of carbon were quantitatively characterized in terms of their internal structure and reactivity experiments were conducted to investigate potential relationships between the structural morphology of carbons and their reactivity. High temperature microscopy and drip test experiments were then used to investigate the melting behaviour of mould powders containing different carbonaceous materials. From the results obtained, correlations were established between the structural factors, chemical reactivity and melting behaviour.     

宽厚板坯连铸自动加渣机的应用实践

介绍了鞍钢股份有限公司鲅鱼圈钢铁分公司宽厚板坯连铸机结晶器自动加渣系统的工作原理、设备组成、系统技术特点及控制功能。实践表明,自动加渣方式下的保护渣铺展及熔化状态、液渣层厚度及结晶器传热速度控制的稳定性均优于人工推渣方式,浇注各钢种的平均罐数漏钢报警率和铸坯表面纵裂及凹陷发生率均明显降低。     

A short history of mould powders

The success of the continuous casting process owes much to the performance of the mould powder. Mould powders have been used for more than 50 years and our knowledge of how they work and perform has increased steadily over this time. The performance of the mould powder should always be judged by how it affects the steel shell. Since this is frequently due to the nature of the slag film formed between the shell and the mould, the performance of the mould powder can also be judged by the nature of the slag film formed. However, other factors (such as the depth of the molten pool) must also be taken into account since they affect the formation of defects. Our current state of knowledge on how mould powders work is summarised.     

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.     

CAS OB精炼和连铸过程钢中夹杂物来源示踪研究

通过添加示踪剂研究CAS OB的精炼过程和连铸过程以及板坯钢样夹杂物来源。结果表明,精炼过程和钙处理后的钢中显微夹杂物均含有钢包渣示踪元素镧;中间包钢样中的显微夹杂物同时含有镧和中间包示踪元素铈;板坯中的显微夹杂物含有镧、铈、钾、钠中的几种,说明钢液脱氧产物与钢包渣、中间包覆盖剂、结晶器保护渣均产生了反应。板坯中大型夹杂物主要源于结晶器保护渣,其次源于钢包渣,少部分源于中间包覆盖剂;大型夹杂物同时含有钾、钠、镧、铈中的几种元素,说明大型夹杂物是脱氧产物与卷入钢液中的钢包渣、中间包覆盖剂、保护渣或中间包内衬蚀损产物相互反应的复杂夹杂物。     

Application of high-basicity mould fluxes for continuous casting of large steel slabs

Controlling the formation of longitudinal cracks on hypo-peritectic steel slab surfaces is one of the key challenges in continuous casting worldwide. Based on the production in Chongqing Iron & Steel Co., mould fluxes with high basicity ranging from 1.7 to 1.8 were put forward and the effects of compositions on the basic properties of mould flux were studied in the present paper. After laboratory experiments, a high-basicity mould flux with an increased crystallisation speed to abate the heat-transfer capability in the meniscus area was fully applied in the first steelmaking plant of Chongqing Iron & Steel Co. Using the high-basicity mould flux, for more than 3 million tonnes of slab production, the large longitudinal crack was eliminated and the rate of formation of small longitudinal cracks decreased greatly. During the production process, sticking that impedes the smooth running of continuous casting was infrequent, and the spot check for longitudinal cracks on micro alloy steel surface was simplified in the slab finishing process. Furthermore, since the application effect of mass production is remarkable, the hot charging process is better implemented, and the contract can be accomplished in time. In summary, the problem of longitudinal cracks on hypo-peritectic steel slab surfaces is finally resolved and the high-basicity mould fluxes have become indispensable auxiliary materials during continuous casting of hypo-peritectic steel.     

CaO-Al2O3基低反应性保护渣熔化及流动特性的成分控制区域研究

摘要：传统CaO-SiO2系保护渣在浇铸高锰高铝钢时,渣中SiO2易被钢中Al还原,造成保护渣成分改变和性能恶化,危害铸坯表面质量和连铸过程顺行。为了抑制钢 渣反应,旨在减少渣中氧化性组分的低反应性,CaO-Al2O3基渣系是重要选择方向。在评估高锰高铝钢凝固特性和传统反应性保护渣基础上,提出了低反应性保护渣基本性能要求,并采用单纯形法设计了CaO-Al2O3基保护渣系的试样组成。通过测试实验渣样的熔化特性和流动特性,获得了5组低反应性连铸保护渣熔化流动特性的成分控制区域。典型区域基本性能为：熔化温度（半球点温度）900～1100℃,1300℃的黏度0.1～0.2Pa·s,转折温度900～1150℃。     

萤石含量对结晶器保护渣物理性质和渣膜矿相结构的影响

针对亚包晶钢连铸板坯易发生表面纵裂现象,研究和分析了萤石配加量对保护渣（/%:2937水泥熟料,8硼砂,20石英砂,15硅灰石,12纯碱,816萤石）熔化温度、黏度和平均热流密度以及渣膜矿相结构的影响。结果表明,随着萤石含量的增加,渣膜结晶率和枪晶石含量升高,硅灰石含量降低,同时保护渣熔化温度、黏度、平均热流密度降低;提出萤石配加量为12%时,能满足亚包晶连铸板坯对渣膜矿相结构的要求,并改善了铸坯质量。