High productivity and technological developments at Corus DSP thin slab caster

Abstract

The Corus IJmuiden Direct Sheet Plant focuses on production of thin gauged material through a single casting strand and an (semi-) endless rolling process. The caster has a 90 mm thick funnel shaped mould and is equipped with liquid core reduction, allowing a slab thickness of 70 mm. To reach casting speeds up to 5·4 mm min^?1, steel is fed into the mould using a specially designed submerged entry nozzle and an adjustable multi-pole electromagnetic braking system is installed. Although still increasing, productivity is now at the design level of 1·3 Mt/year. Average casting speed (including start and stop procedures) is >5·0 m min^?1 with sequence lengths up to nine 300 t ladles, representing a maximum planned casting time of 12 h. Significant growth has occurred in the production of (thin gauged) high strength steels. Breakouts have been a constant threat but stabilisation of the casting process has resulted in significant reduction in these events, allowing steady growth to productivity targets. Developments to improve mould fluid flow, mould level control, mould heat transfer and strand lubrication are described.     

方坯高效连铸新型结晶器传热分析与应用

提出了方坯高效连铸结晶器有效结构形式,并通过ansys有限元软件,建立高效连铸结晶器与传统结晶器铜管的传热模型,并对其凝固传热以及温度场进行计算对比,重点讨论不同结构形式的结晶器在传热效率及传热均匀性方面的差异,并讨论其对高拉速下坯壳凝固的影响。结果表明,高效结晶器可以使得结晶器的传热效率提高7.8%,并且使得结晶器铜管热面最高温度降低100℃,热面温差降低到5℃以下。作者根据该理论,通过有限元优化设计,设计制造出方坯高效连铸结晶器,并应用于某钢厂155mm方断面的铸机上,稳定生产拉速达到4m/min,最大拉速达到4.46m/min。     

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。     

Modelling of steel shrinkage and optimisation of mould taper for high speed continuous casting

Abstract

In high speed continuous casting, optimisation of mould taper is key for intensifying heat transfer and for improving the quality of the cast products. Mathematical modelling has been carried out by combining heat transfer, steel shrinkage and parabolic continuous taper model in order to optimise the mould taper profile. These models have been assembled to a set of software, the inputs of which include the steel grade, casting speed, casting temperature, length and the cross-section of mould tube, while the outputs consist of surface temperature of the strand, thickness of solidified shell, thermal linear expansion coefficient, steel shrinkage, distortion of the mould tube wall, the actual air gap, total taper and the continuous taper profile. Optimum mould taper has a parabolic profile which is tapered inwards that changes continuously along the length of the strand in order to achieve reduction in air gap while avoiding distortion of the mould tube.     

High speed continuous casting of steel billets: Part 1: General overview

Abstract

This paper is the first of a series of two, providing an overview of the results of mould heat transfer analyses conducted over years of industrial trials. The effects of casting speed, carbon content, superheat, water flowrate, electromagnetic stirring, and mould lubricant are discussed in terms of the midface mould heat transfer. Mould corner heat transfer is also discussed. Finally, an overview of mould design is given, as well as an analysis that reveals that both excessively steep and shallow meniscus tapers cause high heat transfer. Alternative designs to the conventional straight tapered moulds are discussed briefly. The focus is on the primary cooling zone, and therefore billet quality issues are not extensively covered.     

高拉速连铸用管式结晶器及其在武钢的应用

研究了适用于高效方坯连铸的管式结晶器的材质、长度及倒锥度,并介绍了武钢第一炼钢厂引进的DLAMOLD管式结晶器及其使用情况,指出要发展高效方坯连铸必须研究出在材质、长度、倒锥度等方面都适合于高拉速的管式结晶器,并指出了方坯连铸工作今后的发展方向。     

Analysis of variability and non-uniformity of mould heat extraction for round billet continuous casting in plant trials

Abstract

Maintaining a stable and uniform heat transfer from steel shell to mould is important to produce high quality casting billet. In the present paper, a large amount of measured data of heat flux and temperature for round billet continuous casting mould from a plant trial has been analysed to shed light on the variability and non-uniformity of mould heat transfer around the perimeter. The results show that the variability and non-uniformity of heat extraction from the steel through the mould is affected slightly by operational parameters, such as pouring temperature, casting speed, meniscus, electromagnetic stirring current, but strongly by the steel carbon content and mould powder type. The installation of the mould in caster machine determines the magnitude of non-uniformity of heat transfer to a great extent. The relative root mean square (rRMS) of mould heat flux, presenting the variability and non-uniformity of mould heat transfer around the perimeter in transverse section, has wider range of variation and higher mean value compared with that of temperature. When the abnormality of heat transfer happens, such as deposit, the non-uniformity of mould heat transfer is also studied.     

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.     

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.     

16MnNb钢圆坯连铸二冷制度的研究

 以某钢厂圆坯连铸机为研究对象,建立了连铸坯凝固传热模型。在不同拉速下对280 mm断面圆坯二次冷却过程进行仿真优化,确定了16MnNb钢合适的二冷制度。根据仿真结果,在最小工作拉速(0.9 m/min)下,矫直点处铸坯内弧表面中心温度为947 ℃,有效避开了铸坯的二次低延性区。在最大工作拉速(1.2 m/min)下,铸坯出结晶器时,其凝固坯壳厚度为19 mm,二冷初期产生漏钢等质量问题的可能性较小。不同拉速下,横断面温度场分布均匀。经低倍检测发现,铸坯表面及内部质量良好,无裂纹、疏松、缩孔等质量缺陷。     

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.     

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.     

方坯高速连铸结晶器锥度优化研究

建立了结晶器内腔体积变化的平衡方程和结晶器锥度设计数学模型,分析了钢水凝固收缩和固态相变收缩的规律。将理论分析与工程设计结合起来,开发了一种适用于方坯高速连铸结晶器总锥度和连续锥度设计的新方法。     

合金钢连铸结晶器保护渣的基本功能

分析了合金钢连铸结晶器保护渣对钢水的保温和避免钢液氧化功能、吸收夹杂物的能力、以及润滑铸坯和控制传热的功能。改善保护渣的保温性能，有利于减轻振痕缺陷和铸坯皮下夹渣，提高保护渣吸收夹杂能力，防止高合金钢连铸时的漏钢和减少铸坯夹渣缺陷；而协调保护渣传热与润滑的功能对避免粘结漏钢和减少铸坯表面纵裂纹和微裂纹有着不可低估的作用。     

低温度梯度结晶器的传热分析及工业应用试验

 结晶器内钢液凝固前沿温度梯度的降低可以阻碍柱状晶的生长、促进柱状晶向等轴晶的转变和提高等轴晶率,从而有利于铸坯组织的均质化。基于上述原理和结晶器传热计算,设计和制造了可用于降低钢液凝固前沿温度梯度的带隔热镀层的新型结晶器,并在方坯连铸机上进行了工业试验,分析了其对方坯凝固组织的影响。结果表明,低温度梯度结晶器可满足连铸生产要求;与传统结晶器相比,低温度梯度结晶器出口铸坯表面温度提高了108℃,相近拉速下该结晶器的平均热流密度相对更低,表明其减缓了钢液在凝固初期的传热,从而降低了凝固前沿的温度梯度;通过与低过热度、电磁搅拌技术相结合,低温度梯度结晶器生产的铸坯等轴晶率提高了8%～13%,中心偏析和缩孔严重程度明显降低。     

Abnormal transient phenomena in the continuous casting process: Part 2

Abstract

This paper is the second of a series of two describing abnormal transient phenomena observed during online monitoring of a billet continuous casting machine. Special attention is paid in Part 2 to in mould solidification. A mould heat flux drift phenomenon (HFD) has been detected, but only for mould powder basicities larger than 0·8. The HFD is related to a decrease of the heat flux in the lower part of the mould and an increase in both the billet-mould friction force and mould thermocouple variability. Results of tests changing the mould powder grade during casting have provided help in explaining the HFD. The probable reason for the HFD is crystallisation of the glassy slag layer. The heat flux ratio parameter (HFR), defined as the ratio between the heat flux in the lower part of the mould and the heat flux in the upper part of the mould, has proved to be a good tool for judging the casting performance of a mould powder.     

方坯结晶器多锥度分析

基于坯壳应力遗传特性建立铸坯热力耦合模型,利用ANSYS对结晶器内铸坯进行铸坯传热及应力分析。依据最小气隙原则对结晶器锥度进行优化,分析了单锥度和多锥度对坯壳凝固行为的影响。结果表明:采用两种锥度形式的结晶器,铸坯角部凝固行为存在明显差异,而表面中心区域基本一致。单锥度结晶器内气隙分布较广,角部热流明显降低,在偏离角部12～22 mm处存在"热点"。多锥度结晶器内气隙宽度和存在范围显著减少,"热点"消失,多锥度结晶器更符合坯壳凝固收缩规律。     

钢的连铸过程结晶器与铸坯之间的传热分析

针对连铸过程中的结晶器传热模型，分析总结了结晶器与坯壳之间的热阻构成，并研究了结晶器壁和坯壳界面热流的影响因素，为传热模型的求解和结晶器锥度的优化设计提供了理论依据。     

Analysis of Mould,Spray and Radiation Zones of Continuous Billet Caster by Three‐dimensional Mathematical Model based on a Turbulent Fluid Flow

An analysis of mould, spray and radiation zones of a continuous billet caster has been done by a three‐dimensional turbulent fluid flow and heat transfer mathematical model. The aim was to reduce crack susceptibility of the billets and enhance productivity of the billet caster. Enthalpy‐porosity technique is used for the solidification. Turbulence is modelled by a realizable k‐ε model. The three‐dimensional mesh of the billet is generated by Gambit software, and Fluent software is used for the solution of equations. In various zones, different standard boundary conditions are applied. Enhanced wall treatment is used for the turbulence near the wall. In the mould region, Savage and Prichard expression for heat flux is applied. In the spray cooling zone, the heat transfer coefficient for surface cooling of the billet is calculated by knowing the water flow rate and the nozzle configuration of the plant. The model predicts the velocities in the molten pool of a billet, the temperature in the entire volume of billet, the heat transfer coefficient in the mould region, the heat flux in the cooling zone and radiation cooling zone, and the shell thickness at various zones. The model forecasts that the billet surface temperature up to the cutting region is above the austenite‐ferrite transformation temperature (which is accompanied by large volume change). The model predicts a temperature difference of maximum 700 K between the centre and surface of the billet. The entire solidification takes place at 11.0 m length at 3.0 m/min. For the same casting arrangement, increasing the casting speed up to 4.0 m/min has been explored. Based on the simulation results, recommendations to alter the spray water flow rate and spray nozzle diameter are presented to avoid a sudden change of temperature.     

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.