高速方坯连铸结晶器冶金理论问题探讨

提出并论述了高速方坯连铸结晶器冶金中结晶器热流、钢的凝固收缩、结晶器变形、锥度设计和圆角半径等问题,重点讨论了拉速、碳含量及润滑剂对热流的影响,不同碳含量的钢凝固收缩和相变特征以及结晶器热变形规律,提出了不同钢种的凝固收缩和相变收缩的计算方法和结晶器锥度、圆角半径的设计思路.     

Study on the non-uniform slab shrinkage of special steel during slab continuous casting

A full-scale finite-element stress model of a slab and its mould was developed to analyse the non-uniform slab shrinkage of special steel in a vertical caster during slab continuous casting. The stress model was based on the heat transfer model which was validated by comparing the calculated temperature with the measured ones. According to the shrinkage calculation, the single linear taper is not suitable for the continuous casting of this special steel, and the narrow face taper need to be calculated and optimised. It will provide a helpful tool for further improving the casting parameters and operations for special steel.     

Thermomechanical finite-element model of shell behavior in continuous casting of steel

A coupled finite-element model, CON2D, has been developed to simulate temperature, stress, and shape development during the continuous casting of steel, both in and below the mold. The model simulates a transverse section of the strand in generalized plane strain as it moves down at the casting speed. It includes the effects of heat conduction, solidification, nonuniform superheat dissipation due to turbulent fluid flow, mutual dependence of the heat transfer and shrinkage on the size of the interfacial gap, the taper of the mold wall, and the thermal distortion of the mold. The stress model features an elastic-viscoplastic creep constitutive equation that accounts for the different responses of the liquid, semisolid, delta-ferrite, and austenite phases. Functions depending on temperature and composition are employed for properties such as thermal linear expansion. A contact algorithm is used to prevent penetration of the shell into the mold wall due to the internal liquid pressure. An efficient two-step algorithm is used to integrate these highly nonlinear equations. The model is validated with an analytical solution for both temperature and stress in a solidifying slab. It is applied to simulate continuous casting of a 120 mm billet and compares favorably with plant measurements of mold wall temperature, total heat removal, and shell thickness, including thinning of the corner. The model is ready to investigate issues in continuous casting such as mold taper optimization, minimum shell thickness to avoid breakouts, and maximum casting speed to avoid hot-tear crack formation due to submold bulging.     

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。     

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.     

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。     

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.     

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.     

奥氏体不锈钢板坯连铸结晶器锥度的优化

针对奥氏体不锈钢连铸中的质量问题 ,对太钢三炼钢 1260mm× 160mm板坯结晶器的锥度进行了分析。通过不锈钢铸坯在结晶器内收缩的计算和拉坯速度、过热度等工艺参数对铸坯收缩影响的分析 ,得出采用双锥度结晶器比单锥度结晶器更符合铸坯在结晶器内的收缩规律 :液面附近 80～200mm区域采用较大锥度 ,液面下 200～800mm的结晶器下部 ,采用较小锥度 ,并在此基础上设计了曲线锥度结晶器。生产试验表明 ,双锥度设计显著改善了铸坯质量 ,消除了窄面鼓肚和中间凹陷等缺陷     

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。     

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.     

板坯连铸结晶器倒角结构和锥度研究

针对板坯连铸铸坯常出现角部横裂纹缺陷,建立了板坯连铸结晶器与铸坯二维瞬态热力耦合有限元模型,研究了板坯结晶器窄面铜板在不同倒角结构和不同锥度情况下铸坯的凝固收缩行为,计算了铸坯在结晶器内的温度和应力分布情况。模型较全面地考虑了保护渣和气隙对传热的影响。数值模拟结果表明：结晶器窄面铜板倒角过大或过小,都不利于铸坯温度的均匀分布;对断面厚度为230mm的铸坯,窄面铜板采用20mm～25mm ×45°;倒角及抛物线锥度时,铸坯表面温度分布最均匀,最大平面主应力分布较合理,角部出现横裂纹的可能性会大大降低。     

连铸圆坯结晶器锥度优化设计研究

基于圆坯轴对称性和坯壳应力遗传特性,建立二维纵向切片热力耦合模型,利用有限元软件ANSYS对结晶器内铸坯传热及应力进行分析。根据结晶器内坯壳的收缩以及气隙的分布,依据最小气隙原则对结晶器锥度进行优化。经过三次优化计算,得到Q195钢圆坯结晶器锥度曲线。优化后的圆坯结晶器内腔呈抛物线形状,总锥度1.88%/m。在分析工艺条件下,使最大气隙降低为0.021 mm。     

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.     

Optimisation of cutting length of continuously cast strands

Abstract

Weight per metre and strand circumference were measured, using sensitive methods, on 177 mm round strands, with particular attention being paid to phase transformation and thermal shrinkage in the vicinity of the peritectic. The following results were achieved for a broad range of steel grades. With constant mould dimensions, the weight per metre of round continuously cast strand increases as casting speed rises. Soft steels, such as grade S35 containing 0·10%C, have a low weight per metre, whereas harder grades, such as C60 with 0·60%C, have the highest weight per metre. Low alloyed steels and oilfield tubular grades occupy rankings between these extremes. Martensitic and more highly resulphurised round billets have a conspicuously low weight per metre. The following definitive influencing factors on weight per metre became apparent: expansion of the mould tube under exposure to heat; shrinkage as a result of δ - γ transformation; creep processes under exposure to ferrostatic pressure; density of the compact steel; and porosity in the strand centre. These influences lessen in the order in which they are listed; they are, in some cases, contradictory, and balance one another out.     

A Study about the Influence of Carbon Content in the Steel on the Casting Behavior

In the casting process of steels with a C‐content ranging from 0.09 to 0.53 mass%, austenite is formed as secondary crystal phase by peritectic reaction between crystal of δ ferrite and residual melt. For unalloyed or micro‐alloyed steels the C‐content or C‐equivalent influences the casting behavior of steel in the mould, such as strand shell growth, crack formation, heat transfer, temperature fluctuation in the copper plate, mould level fluctuation and oscillation marks formation. The negative casting behavior like the uneven strand shell growth, the deep oscillation mark formation, the high mould level fluctuation, the crack formation on the strand surface were found mostly for steel with C‐content or Cp between 0.10–0.13 mass%. The strand shell structure (strand shell growth, mushy zone, δ + γ phase transformation) and shrinkage of the strand shell were simulated depending on the C‐content by means of mathematical simulation. On the basis of the simulation results and of the measured high temperature strength of steel the dependence of stiffness and the irregularity of the shrinkage of strand shell on the C‐content was investigated. It was found that the stiffness and irregularity of the shrinkage of the strand shell reach the maximum value at a C‐content of about 0.12 mass%.     

Design of corners of mould in square billet casting

Abstract

In continuous casting, heat flow optimisation in the mould is key to improving the quality of the product and production savings. The heat flow influences, and is influenced by, several phenomena of a mechanical or metallurgical nature, so its optimisation should include these. In particular, shrinkage of the strand and solid phase formation are among the most influencing factors affecting the cooling of the solidifying product. The present paper describes a model implemented by a software tool that can carry out simulation of shell formation of carbon steel within the mould, for rectangular shapes. The first aim of the software is to simulate formation of the solid shell in the strand and the deformations to which this solid is subjected. Deformations are a result of both thermal shrinkage, related to phase changes, and stresses caused by metallostatic pressure or the mould-shell interaction. The output of the model consists of temperature maps of the strand, maps of formation of the shell and the ideal mould contour.     

小方坯连铸结晶器内热力行为研究

通过THERCAST软件建立了考虑结晶器保护渣和气隙热阻的三维有限元模型,分析了150 mm×150 mm小方坯在连铸结晶器中的热力学状况,发现拉速对热流密度的影响很大,热流密度会随着拉速的增加而增加,不过拉速越大,热流密度增加越缓慢,然而,过热度对于热流密度的影响却很小。由于钢水静压力的作用,气隙沿横向分布非常不均匀,表面中心气隙很小,仅为0.2 mm,角部气隙却很大,最大值达到了1.15 mm。根据模拟结果和参考前人设计结晶器的经验,建议设计同时带有横向和纵向锥度的结晶器。     

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.