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.     

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.     

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.     

Improved cold-finger measurement of heat flux through solidified mould flux

A new, more accurate approach to estimate thermal properties of mould fluxes is presented, based on the cold-finger technique. This new approach aims to circumvent the dominant effect of convection on steady-state heat flux. Instead, the thickness of the solidified mould flux layer recovered after the tests is correlated with the heat transfer rate during measurements. The cold-finger probe was redesigned to allow reliable measurement of solid film thickness.     

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.     

浇铸主要工艺参数及保护渣性能对模铸钢锭实物质量的影响

对贵钢一炼钢厂2010年1～6月份模铸钢锭裂纹缺陷及冒口部位收缩突出,严重影响钢锭质量的问题,利用钢液结晶凝固的普遍原理优化模铸生产工艺参数,并在改进固体保护渣成分、性能的基础上得以完全解决。     

Effect of solidus temperature and crystalline phase of mould flux on heat transfer in continuous casting mould

Abstract

Pieces of mould flux film obtained from a commercial continuous casting machine and measurements of the mould temperature have been used in an investigation to explain the mechanism of heat transfer in the mould. Comparison with the results of numerical calculations, indicates that the main factor responsible for the reduction of heat transfer with a high basicity flux is the contact resistance at the interface of the mould wall and solid flux film. The results show that a relatively high contact resistance is introduced when using a crystalline, high solidus mould flux because of the stability of the contact resistance.     

Effect of substituting SiO2 with TiO2 on viscosity and crystallisation of mould flux for casting titanium stabilised stainless steel

Abstract

During casting of titanium stabilised stainless steel, a gradual increase in TiO₂ content in the molten mould slag from absorption of TiO₂ inclusions causes changes in properties of the mould flux, such as viscosity, solidification temperature and crystallisation behaviour. To simulate this effect mould fluxes were prepared with SiO₂ being substituted with 5 and 10%TiO₂ and viscosity and crystallisation behaviour studied. Experimental results indicate that the substitution of SiO₂ with TiO₂ leads to suppression of crystallisation of cuspidine and formation of perovskite in glass quenched from mould fluxes. The crystallisation tendency is reduced by the introduction of TiO₂. The high temperature viscosity of mould fluxes shows a decrease with increasing TiO₂. At lower temperature, the viscosity of mould fluxes with 10%TiO₂ is greater than that of TiO₂ free slag, which could be attributed to precipitation of solid particles.     

Analysis of thermomechanical behaviour in billet casting with different mould corner radii

Abstract

A finite element thermal stress model to compute the thermomechanical state of the solidifying shell during continuous casting of steel in a square billet casting mould has been applied to investigate longitudinal cracks. A two-dimensional thermoelastoviscoplastic analysis was carried out within a horizontal slice of the solidifying strand which moves vertically within and just below the mould. The model calculates the temperature distributions, the stresses, the strains in the solidifying shell, and the intermittent air gap between the casting mould and the solidifying strand. Model predictions were verified with both an analytical solution and a plant trial. The model was then applied to study the effect of mould corner radius on longitudinal crack formation for casting in a typical 0·75%/m tapered mould with both oil and mould powder lubrication. With this inadequate linear taper, a gap forms between the shell and the mould in the corner region. As the corner radius of the billet increases from 4 to 15 mm, this gap spreads further around the corner towards the centre of the strand and becomes larger. This leads to more temperature non-uniformity around the billet perimeter as solidification proceeds. Longitudinal corner surface cracks are predicted to form only in the large corner radius billet, owing to tension in the hotter and thinner shell along the corner during solidification in the mould. Off corner internal cracks form more readily in the small corner radius billet. They are caused by bulging below the mould, which bends the thin, weak shell around the corner, creating tensile strain on the solidification front where these longitudinal cracks are ultimately observed.     

Three-dimensional analysis and design of petal-like mould for continuous casting of steels

Abstract

Understanding the shrinkage behaviour of a steel billet is very important for designing the continuous casting mould, and conversely, a well designed mould is beneficial when matching the shrinkage behaviour of the billet. The shrinkage behaviour of a billet is hard to measure in situ during continuous casting but can be calculated by numerical simulation. A three-dimensional finite element model has been built to simulate the thermal and stress fields of the billet in the mould. The dynamic thermal boundary condition, the effect of ferrostatic pressure and the temperature dependent thermophysical parameters have been considered in the model. The shrinkage of billet when considering ferrostatic pressure is on average 0·08 mm smaller than when not considering ferrostatic pressure. The temperature and stress distributions are analysed in the present paper, and based on this analysis, a novel petal-like mould was designed and its taper determined. The designed mould has been tested in industrial practice showing better lifetime and billet quality.     

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。     

Influence of La2O3 on crystallization behavior of free-fluoride mould flux and heat transfer of slag films

In order to research the influence of La2O3 on crystallization behavior of free-fluoride mould flux and the heat transfer of slag film,free-fluoride mould flux with various La2O3 content were investigated by using self-made mould simulator,comprehensive thermal analyzer and SEM-EDS.With the increase of La2O3 content from 0% to 20%,the crystallization ratios of mould flux films were improved from 2% to 90% and the thicknesses of films were also enhanced over two times.Moreover,crystallization temperature was greatly raised and its increasing extent reached 100 oC.It could also decrease the melting temperature of casting powder about 50 oC.However,the undissolved La2O3 particles appeared in slag film if the ratio of La2O3 in free-fluoride mould flux was much too high.     

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.     

Effect of CaO/Al2O3 ratio on viscosity and crystallisation behaviour of mould flux for high Al non-magnetic steel

In order to facilitate the development of CaO–Al₂O₃ based mould flux for casting high aluminium, non-magnetic steel, the effect of CaO/Al₂O₃ ratios from 0.6 to 3.2 on viscosity and crystallisation characteristics were investigated with the aid of a rotational viscometer, Fourier transform infrared spectroscopy, a single hot thermocouple technique and X-ray diffraction analysis. The results showed that, at temperatures above 1543?K (1270°C), the viscosity first decreased and then became stable with increase in the CaO/Al₂O₃ ratio. At temperatures below 1543?K (1270°C), the viscosity again first decreased but then increased, with the CaO/Al₂O₃ ratio. This viscosity behaviour can be attributed to changes in the network structure characteristics and the precipitation of solid particles within the liquid flux. Increase in the CaO/Al₂O₃ ratio also first inhibited and then enhanced crystallisation as demonstrated by the changes in initial crystallisation temperatures and incubation times. The X-ray diffraction results confirmed that, at both low and high CaO/Al₂O₃ ratios, the dominant precipitates were compounds with high melting points. On the other hand, with CaO/Al₂O₃ ratios in the midrange, the dominant precipitates were compounds with relatively low melting temperatures. It is concluded that mould flux with a CaO/Al₂O₃ ratio in the range 1.1–1.6 is the most appropriate for casting high aluminium, non-magnetic steels.     

1.9 t扁锭浇铸D2冷作模具钢凝固过程及内部缺陷预报数学模拟

采用三维铸造软件Procast模拟了钢厂1.9 t扁D2钢锭(%:1.40～1.60C、11.00～13.00Cr、0.70～1.20Mo、≤1.10V)浇铸和凝固过程。模拟结果表明,1.9 t模具扁钢的本体凝固时间为64 min,完全凝固时间为83min,与经验公式的计算结果相吻合;钢锭柱状晶厚度为150 mm左右,等轴晶宽度为260 mm左右;1.9 t扁锭内部无缩孔产生、疏松轻微。     

板坯连铸结晶器钢液流场的数值模拟和优化

用CFD三维计算软件,采用k、ε双方程模型在给定的数值计算条件下,对900 mm高的板坯连铸结晶器内钢液流场进行了数值模拟,研究结晶器工艺参数对结晶器液面湍动能和窄边冲击压力的影响。结果表明:对于断面尺寸为220 mm×900 mm的板坯结晶器,水口插入深度为180 mm,水口倾角为向下15°,拉坯速度为1.6～1.8 m/min时,结晶器内钢液具有较好的流动方式。     

Three-dimensional FEM study of fluid flow in mould for beam blank continuous casting: influence of straight through conduit type SEN

Abstract

In order to increase the beam blank cleanliness, the aim of this work is to analyse the flow field in the mould of beam blank continuous casting, to find the factors influencing the strand cleanliness and then to optimise the process parameters. A three-dimensional steady finite element model was developed to simulate and analyse the turbulent flow field in the mould. The volume of fluid model was used to track the free surface evolution at the meniscus. The influences of processing parameters, such as casting speed and nozzle parameters, on the molten steel flow in the strand (such as vortex location, liquid steel impact depth, velocity and fluctuation of the liquid steel at free surface) were analysed and the optimum processing parameters determined based on mass calculation. The results of this research project have been applied in actual production, and it has been shown that they are very useful and efficient for improving the steel cleanliness and controlling the surface cracks on the beam blank web.     

注流方式对大方坯连铸结晶器内钢水流动与温度状态影响

依据改变水口侧孔射流方向来控制结晶器内钢流流动状态的思想,设计了一种大方坯连铸结晶器新型四分切向水口.对不同类型水口(直通式、四分径向以及四分切向水口)浇注条件下大方坯连铸结晶器内钢水流动形态和温度分布状况进行了流体动力学比较研究.结果表明,新型四分切向水口不仅可以使结晶器内钢水形成上、下两个回流,并能够产生较强的水平旋流.该水平旋流能降低钢水的冲击深度,抑制钢液面波动,促进夹杂物上浮,还具有促进钢水过热耗散的效果.与四分径向水口相比,新型四分切向水口能减弱射流对初生坯壳的冲击,均匀横截面坯壳厚度.此外,该旋流状态使热中心上移,自由液面附近钢水的温度可提高2～6℃,改善化渣条件.     

H型坯连铸结晶器内钢液传递特性的数值模拟

以950 kg/m H型连铸坯结晶器为研究对象,采用FLUENT软件建立三维几何模型,模拟研究了水口浸入深度125 mm和175 mm时拉速（0.6~1.2 m/s）对结晶器内钢液传递特性的影响。结果表明,不同拉速条件下H型坯结晶器内钢液流态相似,但随着拉速的增大,结晶器内钢液流股冲击深度增大和结晶器自由表面流速增大,保护渣熔化状况有改善趋势,同时结晶器液面波动和钢水对凝固坯壳的冲刷有增大趋势。而各粒径夹杂物上浮去除率随拉速的增大而降低,其中大颗粒夹杂物去除率降低显著,当拉速由0.6 m/min增至1.2 m/min时,100μm夹杂物的去除率由16%降至10%。该模拟条件下,20~100μm夹杂物去除率在4%~16%。