Effect of alumina pickup on mould flux viscosity in continuous slab casting

Abstract

Mould flux samples were taken from a continuous slab caster mould at various stages during casting sequences. These confirmed that the amount of alumina pickup reaches a steady state level within the first 40 min of casting (in agreement with a simple model of alumina pickup). The steady state level was 3–4 mass-% higher than the original alumina content of the flux. Viscosity measurements carried out on two commercial mould fluxes, with different levels of alumina added, showed that the viscosity increase as a result of a 4 mass-%increase in alumina is not likely to affect mould behaviour. Comparison of the measured viscosities with predictions from literature models showed that no single literature model corresponds consistently to the measured values.     

电磁场对连铸保护渣流变特性的影响

采用自制的电磁场中保护渣黏度测试装置和HF-201型黏度仪,分别对有、无磁场时保护渣的流变特性进行试验研究,并通过Scigress分子动力学模拟研究有、无磁场时连铸保护渣的微观结构,分析保护渣流变特性与微观结构之间的关系。结果表明,在电磁场作用下,连铸保护渣黏度增大,转折温度升高,而其Si-O键长不发生变化,键长为0.162 5 nm,但Si与O的结合能力增强;施加磁场后,熔渣中四配位Si、桥氧以及复杂结构单元Q3的含量增多,熔渣网络结构聚合度增大且复杂、稳定。     

Cold model studies of mixing and mass transfer in steelmaking vessels

Abstract

Room temperature model studies using water to simulate 'metal' and paraffin oil (when required) as 'slag' were conducted to study the extent of mixing and the rate of mass transfer between metal and slag in the 130 t basic oxygen furnaces (BOFs) in operation in Tata Steel. Several systems of gas injection including top blowing, combined blowing and exclusive bottom purging were investigated. Similar work was undertaken in a room temperature model of an 80 t energy optimising furnace (EOF), in operation for a brief period earlier in Tata Steel. Details of the optimum blowing conditions, including the number/distribution of bottom tuyeres for the BOFs, are elaborated in the present paper. How mixing/mass transfer in an EOF compares with the BOF case(s) is also highlighted.     

Fluid flow and heat transfer with nail dipping method in mould during continuous casting process

ABSTRACT

The character and cause of sliver defect on IF steel sheet surface are studied by means of SEM, unstable flow in mould could induce surface velocity and level fluctuations, leading to surface defects during continuous casting of steel. The nozzle clogging is a serious problem during the continuous casting of steel, due to its influence on the casting operations and products quality. In this study, the nail dipping method for measuring surface velocity and flow direction in molten steel were employed. The fluid flow in mould of whole casting sequence was investigated, especially during the nozzle clogging conditions. The results showed that when nozzle clogging occurred in the 7th heat, the flow velocities on R and L side of nozzle were 0.280 and 0.402?m/s, respectively. The surface defect ratio of hot-rolled and cold-rolled plates increases with the increase of heat flux deviation on both sides of the mould copper. The different clogging per cent on both sides of the nozzle will lead to asymmetry flow, the surface velocity is higher with the small clogging per cent side compared to that of relatively large per cent clogging side.     

The effect of in-situ dynamic mold flux melting and crystallization on heat transfer in continuous casting

A mold flux is widely used to modify heat transfer rates in continuous casting,and crystallization of the mold flux has been identified as a primary factor that influences heat flux from the strand to the mold.As the harsh environment and the very high transient nature of the mold caster,the study of dynamic mold flux melting and crystallization as well as their effects on heat transfer has not been conducted widely.By using an infrared radiation emitter,a high level heat flux was applied to a copper mold covered with solid mold flux disk to simulate the heat transfer phenomena in continuous casting.By this technique it is possible to have a liquid layer,a crystalline layer and a glassy layer in contact with one another and,by varying the energy input,it is possible to study the dynamic nature of the film and its effect on the heat transfer rate.A general heat transfer model was also developed to allow the prediction of the effect of varying the thickness of the three potential layers in the flux film.     

神经元网络应用于圆坯连铸结晶器传热计算

以圆坯连铸结晶器温度实测数据为基础,研究应用神经元网络求解结晶器传热反问题的可行性,并尝试将神经元网络与数值计算结合用于结晶器传热计算,探索针对实测数据的结晶器传热在线计算方法.结果表明,数值计算结果与实测温度符合较好,可真实反映实际生产中圆坯结晶器传热的非均匀特性.     

连铸结晶器温度场及热变形的数值模拟

利用MARC有限元软件建立了圆坯结晶器的三维温度场与应力场耦合的热弹性模型，并在此基础上研究了结晶器厚度、冷却水流速以及拉坯速度对结晶器温度场和铜管变形的影响规律．模拟结果表明：拉速提高时，结晶器的温度和变形量都增大，但影响不显著；结晶器厚度增大时，铜管冷面温度降低，热面温度升高，同时变形增大；冷却水流速对铜管温度分布和变形量有较大影响，增大冷却水流速，铜管温度和变形量均下降．     

结晶器与铸坯间保护渣传热性能的研究现状

概述了铸坯与结晶器间的保护渣传热,总结了国内外近年来关于保护渣传热检测方法和传热计算的研究现状,并指出现有保护渣传热性能检测方法为一维传热原理,可以反映保护渣的基本传热性能并作为选择保护渣的依据.认为目前的研究只对保护渣的渣膜结构进行了简单的处理或忽略了结晶器弯月面区域的保护渣传热,需进一步加强铸坯与结晶器间保护渣的传...     

Numerical simulation of fluid flow and solidification in continuous slab casting mould based on inverse heat transfer calculation

Abstract

A mathematical model based on an inverse heat transfer calculation was built to determine the heat flux between the mould and slab based on the measured mould temperatures. With K–? turbulence model, a mathematical model of three-dimensional heat transfer and solidification of molten steel in continuous slab casting mould is developed. Solidification has been taken into consideration, and flow in the mushy zone is modelled according to Darcy’s law as is the case of flow in the porous media. The heat flux prescribed on the boundaries is obtained in the inverse heat conduction calculation; thus, the effect of heat transfer in the mould has been taken into consideration. Results show that the calculated values of mould temperature coincide with the measured ones. Results also reveal that the temperature distribution and shell thickness are affected by the fluid flow and heat transfer of slab which is governed by the heat flux on the mould/slab interface.     

Simulation of heat transfer and strain behaviour in bloom casting mould

Abstract

An optimum casting model was developed to simulate the effect of mould flux on bloom heat transfer and strain behaviour based on a 3D MiLE method, and the influence of casting speed and superheat on bloom heat transfer and lubrication were also investigated. The simulation results showed that solidified shell thickness growth conforms to a Square Root Law, and that the model predicted results are basically in agreement with previous data in the literature, and provide confidence in model. The bloom temperature distribution range in the corner area is smaller than that in the mid-face, and the corner regions form a high cracking risk zone. The hot tearing indicator and effective stress in the corner area are significantly greater than that in the mid-face, so the corner area is the dangerous zone of cracking; The mould flux lubrication in the bloom mid-face is better than for the bloom corner region, due to a higher shell temperature and a fluid slag; The increasing of casting speed can delay air gap formation of the bloom corner area, improving the lubrication conditions, but when the casting speed is changed, it is also necessary for the mould flux viscosity and crystallization temperature be changed also. Increasing the superheat has little influence on the completely solidified distance of liquid flux in the bloom corner area.     

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.     

添加剂对薄板坯连铸保护渣吸附Al_2O_3夹杂的影响

研究了不同添加剂的加入对保护渣吸附Al2O3夹杂后熔化温度的变化,以及添加剂对于保护渣吸附Al2O3夹杂能力的影响。实验结果表明:添加剂w(Li2CO3)由4%增加到10%时,保护渣吸附Al2O3夹杂后熔化温度呈下降趋势;添加NaF和Na2CO3,熔化温度呈上升趋势;添加人造冰晶石和硼砂时,当w(Al2O3)由3%增加到6%时,熔化温度呈上升趋势,当w(Al2O3)6%时,熔化温度呈明显下降趋势。添加剂影响保护渣吸附Al2O3夹杂能力的顺序依次为Li2CO3NaFNa2CO3人造冰晶石硼砂。实验结果对于薄板坯连铸保护渣的设计具有一定的指导意义。     

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.     

Influence of running water on the heat transfer in continuous casting

The influence of running water on spray water cooling is experimentally looked at with the aid of a stationary working experimental plant. The running water was fed separately to the sample from above as a defined water film by using a slot nozzle. In addition to the spray nozzle pre-pressure, the velocity of the running water at the slot nozzle exit and the temperature of the running water, the angle of inclination of the sample plane was also varied. By employing three different full cone spray nozzles and setting different nozzle distances, a range of 450 to 2000 l/(m²·min) of the water impingement density was covered. To estimate the marginal influences, samples of 20 × 20 mm as well as 30 × 30 mm were used. Studies of initially pure spray water cooling without the additional running water film revealed a dependency of the heat transfer on the spray water impingement density, the spray water velocity and also the angle of inclination of the sample plane. For nozzles with a low water impingement density, a superimposing of a running water film on the spray water revealed a sharp increase in the heat transfer coefficients compared to pure spray water cooling. For nozzles with a high water impingement density and, therefore, with a high heat transfer already even for pure spray water cooling, the increase of the heat transfer coefficient is minor.     

Study of heat transfer through layers of casting flux: experiments with a laboratory set-up simulating the conditions in continuous casting

The heat transfer through layers of casting flux has been studied with a laboratory set-up which consists of a water-cooled heat flux probe made of copper and an electrically heated steel plate with a trough to hold the flux sample. The measured heat flux density values were converted to system conductivities k_sys which contain the radiation/conduction conductivity in the flux layer and the contact resistance at the probe side. It was found that k_sys is almost independent from the layer thickness. Ten commercial casting fluxes were investigated and the data for k_sys are given as functions of composition and strand surface temperature.     

基于反算热流的结晶器内流动-传热-凝固耦合模拟

基于Navier-Stokes动量方程和湍流低雷诺数k-ε方程,综合考虑能量守恒和钢液凝固与糊状区对流动过程的影响,建立了描述结晶器内钢液流动、传热及凝固过程的三维耦合数学模型.以实测温度和结晶器反问题模型计算出的热流为边界条件,模拟计算了结晶器内钢水的流动、传热和凝固行为.钢液流动决定结晶器内的温度和热流分布,铸坯凝固受钢液流动和结晶器热流双重因素的影响.建立的模型以及由此得到的铸坯凝固非均匀特征可为进一步考察浇铸过程中纵裂和其他表面缺陷提供借鉴和参考.     

连铸板坯凝固传热过程的计算机模拟

建立了板坯连铸的凝固传热数学模型,充分考虑了弧形铸坯的几何特点,采用有限单元法求解控制方程。通过变参数φ来考虑铸坯内钢水和两相糊状区的导热系数,计算了参数φ、结晶热通量、拉速和二冷强度对铸坯表面温度和坯壳厚度的影响。结果表明,模型的准确性得到提高,拉速对固液界面有明显影响,二冷强度对铸坯表面温度影响非常大。     

Q235B钢浇铸过程保护渣成分及性能研究

针对涟源钢铁集团有限公司Q235B钢板坯连铸过程中发生的粘结漏钢现象,调查了Q235B钢浇铸过程中保护渣的使用情况,并分析和测定了现场保护渣原始渣样、渣条样和漏钢渣样的主要化学成分和物理性能。研究表明,保护渣浇铸前后Al2O3质量分数变化最大为2．7％;浇铸后熔化温度、熔化速度和黏度分别增加15℃、6s和0．04Pa·s,浇铸前后保护渣物理性能变化不大。     

Viscous properties of new mould flux based on aluminate system with CeO_2 for continuous casting of RE alloyed heat resistant steel

The conventional mould fluxes can not be applied to the continuous casting of RE alloyed heat resistant steel, because severe slag-metal interface reactions occur generally in the mold. To restrain the interface reaction and improve conditions for continuous casting, a new mould flux based on aluminate system was devised. The viscous properties were investigated. Scanning electron microscopy and X-ray diffraction were applied to detect and characterize the crystalline phases in the continuous cooling process. The results showed that appropriate addition of CeO_2 could avoid the precipitation of CaO and decrease the viscosity of the mould flux. Increasing the mass ratio of CaO /Al_2O_3, especially to a value exceeding 1, could worsen the stability of the mould flux. With a content of less than 14 wt.%, Li_2O could reduce the viscosity and breaking temperature, but its effect could be weakened for the promoted precipitation of LiAlO_2. To obtain a mould flux with stable viscous properties, such as viscosity and breaking temperature, appropriate contents of CeO_2 and Li_2O should be controlled to around 10 wt.% and 14 wt.%, while the mass ratio of CaO /Al_2O_3 should not be more than 1.     

板坯连铸过程包含相变的三维瞬态热传导数值模拟

利用有限元软件ABAQUS／Standard对连铸结晶器和板坯进行了三维瞬态热传导数值分析，数值模拟计算了温度场．假设相变是在恒温下进行的，三维瞬态热传导方程中引入了潜热，即模拟过程考虑了金属结晶过程中释放的热量．同时也考虑了运动的板坯与结晶器之间的相互作用，从而获得了更准确的连铸过程中结晶器和板坯的温度场分布．