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.     

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 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.     

Investigation of mould thermal behaviour by means of mould instrumentation

Abstract

The thermal behaviour of the continuous casting mould has a critical influence on strand surface quality, casting productivity and operating safety. voestalpine Stahl GmbH has long been interested in the field of research into mould behaviour, and began with the investigation of heat flux in 1995, and then of thermal variability in 1999. Since 2000, an online mould thermal monitoring system for heat flux density, thermal variability and friction has been installed. The heat flow density in the mould is determined by measuring the inflow and outflow temperatures and the throughflow volume of the primary cooling water. Temperature as measured by a thermocouple based breakout detection system in the mould copper plate is used to investigate the thermal variability. These online measured values have been employed to examine the influence of casting parameters, steel analysis and casting powder on the heat flux and thermal variability, and the relationship between these variables and cracking. The knowledge gained through these wide ranging plant based investigations has been used as a major tool in the diagnosis of problems (such as breakout and sticking), for optimisation of the process, particularly in the field of casting powders, and for control of slab quality. In particular, this knowledge has been incorporated into online mould thermal monitoring.     

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.     

Mould heat transfer and continuously cast billet quality with mould flux lubrication Part 2 Quality issues

Abstract

With many billet producers adopting mould powder lubrication, there is a need to clarify the gains in quality that can be achieved with this practice. Over the past three decades considerable research has been conducted to establish the relationship between mould behaviour and defect formation for billets continuously cast with oil lubrication, but little has been done to compare oil cast billets with powder cast billets. In this study, conducted at a Canadian minimill, four faces of a copper mould were instrumented with thermocouples and mould temperatures and billet quality were monitored with mould powder lubrication during casting of 208 × 208 mm billets. In the first part of this two part series (in Ironmaking & Steelmaking No. 1 2000), the results of the mould heat transfer analysis and the influence of variables were presented, together with a comparison between oil and powder lubrication. In the present paper, Part 2, billet quality is examined in detail. The difference in turbulence at the meniscus between oil and powder lubrication is established, and the need to tune mould level sensors when switching to mould powders is demonstrated. Previous work has shown that mould level fluctuations have a strong influence on defects such as offsquareness and transverse depressions, both of which are markedly reduced when casting with mould powders. The inherent stability of the meniscus is improved when employing mould powder lubrication and a submerged entry nozzle. Furthermore, the significant reduction in mould heat transfer at the meniscus, when mould powders are employed, particularly for medium carbon steels has been shown to correlate well with the observed reduction in offsquareness. The paper also elucidates the reasons for the reduction, and in most cases, elimination of transverse depressions in B–Ti grades when casting with mould powders. The mechanism of longitudinal depression formation and subsurface cracking observed in many of the powder cast, medium carbon billets has also been established.     

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.     

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。     

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.     

Abnormal transient phenomena in the continuous casting process: Part 1

Abstract

This paper is the first of a series of two describing online monitoring of the continuous casting machine and the abnormal transient phenomena observed. The present paper, Part 1, pays attention to assessment of the friction between the billet and mould based upon the cam rod force and the electric current intensity of the mould oscillation motor. Information relating to casting parameters before 58 breakouts has been obtained with a monitoring computing program. Almost 40% of the breakouts could have been forecast through assessment of the billet-mould friction force before the breakouts. Transient phenomena in the withdrawal machine have been detected. The jerking of the strand is related to high billet-mould friction; however, other parameters such as the billet cutoff unit and roller bed design also have some effects on this transient phenomenon. Some mechanisms influencing the jerking of the strand are proposed.     

包晶相变对连铸坯初生坯壳凝固收缩的影响

基于Ｆｅ－Ｃ合金的微观组织结构,建立了碳钢线性热膨胀系数计算模型,计算出不同碳含量的钢各在不同温度下的瞬时线性热膨胀系数,并将计算相应用于铸坯热－弹－塑性应力模型,研究了包晶相变对连铸坯初生坯壳凝固收缩的影响,模拟结果表明：浇铸碳含量在０．１％附近的包晶钢时,初生坯壳在靠近弯月面区域和角部区域的收缩很不规划,容易诱发表面缺陷。     

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。     

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．     

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。     

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.     

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.     

BGX—7Ⅱ连铸保护渣的研制

根据包钢圆坯连铸机的浇铸条件,研制出一种圆坯结晶器保护渣。工业试验结果表明;研制的渣能够替代进口产品,满足圆坯Φ２３０－３５０ｍｍ断面连铸生产工艺要求。     

Analytical model for continuous caster profile optimisation

Abstract

As a partial solution of the differential equation of the continuous caster pass line, the analytical relationship between the basic radius and the lengths of the radial and unbending zones was derived. Choosing rationally the length of the unbending zone at a given unbending strain rate, it is possible to reduce the height of a high productivity caster and the ferrostatic pressure within the strand shape to eliminate internal cracking. The software developed allows the temperature, stress, and strain distribution in the strand shell to be determined and, for assumed allowable strains due to bulging and unbending, the caster profile and the positioning of support and unbending rollers to be optimised. For the case of a continuous caster with a vertical mould, the same technique for its profile optimisation was proposed.     

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.