Influence of electromagnetic stirring on transport phenomena in round billet continuous casting mould and macrostructure of high carbon steel billet

Abstract

The characteristics of magnetic field, flow field, temperature profile and inclusion trajectories in a round billet continuous casting mould with electromagnetic stirring (M-EMS) were numerically simulated. An industrial plant trial was conducted to verify the magnetic field characteristics and investigate the influence of M-EMS on the solidification macrostructure of high carbon steel. The results indicate that the predicted magnetic field was in good agreement with the measured data, and the velocity patterns, temperature, inclusion trajectory distributions and macrostructure of steel are significantly modified using M-EMS during casting.     

圆坯结晶器应用电磁搅拌的物理模拟

 以某钢厂 500mm圆坯连铸结晶器为原型,建立了1∶1的物理模型,通过机械搅拌模拟结晶器电磁搅拌,分析研究了电磁搅拌对结晶器内钢液的流动、夹杂物去除率的影响。试验研究表明：电磁搅拌使结晶器内的钢液有一定的水平切向速度,使搅拌区钢水温度的分布更加均匀,有利于结晶器内保护渣的均匀熔化。电磁搅拌能够把水口冲击的轴向速度变为径向速度,从而大大降低流股的冲击深度,能有效地促进夹杂物的上浮和热区中心的上移。但并不是搅拌强度越大,夹杂物的去除率就越高,试验表明,在相同时间内,当搅拌强度为48r/min左右时,夹杂物的上浮率最高。     

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.     

Electromagnetic torque detecting for optimization of in-mould electromagnetic stirring in the billet and bloom continuous casting

ABSTRACT

In this paper, an electromagnetic torque device based on the theoretical model was established to determine the optimum frequency accurately and conveniently in billet and bloom continuous casting with in-mould electromagnetic stirring (M-EMS). Magnetic characteristics with M-EMS was investigated by numerical simulation and physical experiment with the application of electromagnetic torque device and gauss meter. In addition, the effects of stirring frequency with M-EMS on macro segregation and equiaxed crystal ratio were compared and analysed for 55SiCr with 150?mm?×?150?mm billet caster and BU with 310?×?360?mm bloom caster by a series of plant trials, respectively. The results showed that maximum magnetic flux density and maximum electromagnetic torque occurred with different frequency and same current in M-EMS, and central equiaxed crystal ratio and macro segregation has been significantly improved by optimum frequency with M-EMS.     

结晶器电磁搅拌对60钢铸坯质量的影响

对60钢(0.57%～0.62%C)采用结晶器电磁搅拌工艺前后的150 mm×150 mm铸坯质量进行了系统研究,并优化了电磁搅拌工艺。结果表明,采用较强结晶器电磁搅拌强度(250 A)、低过热度(≤250℃)浇铸工艺能显著改善铸坯质量,60钢铸坯中心等轴晶面积比率由平均27.65%增加到52.58%,中心碳偏析指数由1.07降至1.01,铸坯夹杂物含量和尺寸明显减少,钢水中的氧化物夹杂得到有效去除。     

连铸结晶器电磁搅拌对夹杂物的影响

在连铸过程中 ,夹杂物对钢的质量有着重要影响。本文分析了结晶器电磁搅拌对夹杂物的作用机理 ,简要叙述了结晶器电磁搅拌对铸坯内夹杂物含量、大小、分布及其形态的影响     

结晶器电磁搅拌强度对含硫易切不锈钢连铸坯质量的影响

结合东北特殊钢集团大连特殊钢有限责任公司连铸机的实际生产状况,在不同结晶器电磁搅拌强度下,对SUS416易切不锈钢铸坯质量的影响进行了试验分析。试验结果表明：结晶器电磁搅拌强度为200A／5Hz时,能够降低钢中氧含量,皮下气孔以及卷渣数量得以大幅度减少,同时明显改善铸坯中心缩孔程度。     

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.     

浸入式水口结构对连铸大圆坯质量的影响

为研究不同结构的浸入式水口对大规格连铸圆坯质量的影响,以某钢厂生产断面直径为?500 mm的42CrMo连铸圆坯为背景,对使用侧孔浸入式水口和传统直通浸入式水口的使用效果开展研究。结果表明,采用侧孔浸入式水口浇铸时,结晶器进出水温差由传统直通水口的3.30℃提高至3.54℃;连铸圆坯中心疏松由1.5级改善至1.0级,中心偏析指数由0.93～1.21降低到0.98～1.02,近表层至近中心碳极差由0.050%～0.075%降至0.035%～0.053%,使用侧孔浸入式水口的连铸圆坯碳偏析得到改善;铸坯内弧侧表层至3/4R处氧化物夹杂物总量减少0.5个/mm²;铸坯从1/4R处向内长度不小于13μm的硫化物数量减少0.35个/mm²;轧材全氧质量分数平均降低0.000 12%,夹杂物中B类氧化物夹杂均在1.5级以内,钢中大尺寸夹杂物明显减少,钢的洁净度得到改善。     

Novel concept of fine inclusion removal using carbonate powder injection through the ladle shroud

ABSTRACT

Tundish is the last refractory vessel in the steelmaking process. The fluid phenomena in tundish have a strong effect on the separation of non-metallic inclusions. In this paper, a new inclusion removal approach using powder injection through the shroud has been proposed and the devices have been set up. The plant trials have been carried out and the ruling factors have been analysed. The results indicate that this novel technique is beneficial to separate the small non-metallic inclusion removal from the molten steel. Compared with conventional inclusion removal technology, the number of the oxide inclusions can be decreased to a lower level and the inclusion becomes finer.     

高端抗硫管线钢非金属夹杂物研究

采用扫描电镜和大样电解等检验方法对抗硫管线钢的冶炼过程试样和连铸坯中夹杂物的数量、尺寸、成分、形貌进行系统分析。结果表明:钢液经过LF精炼后,显微夹杂物的面积比降低了34.7%;中间包钢液的夹杂物面积比较VD出站增加了6.1%。LF进站钢液中的夹杂物主要为Al_2O_3夹杂物,在LF精炼和VD真空处理过程中由于钢渣间的相互作用,形成以CaO、MgO、Al_2O_3为主要组成的复合型夹杂物。钙处理后夹杂物中的CaO和Al_2O_3的物质的量比接近12∶7,并与钢液发生了脱硫反应,形成了含CaS的复合夹杂物。中间包开浇阶段铸坯中的显微夹杂物和大型夹杂物都明显高于稳定浇铸状态;在稳定浇铸状态下,铸坯中的w(T[O])小于15×10~(-6),大型夹杂物的含量小于0.2 mg/kg;大型夹杂物的主要来源是钢包引流砂、结晶器保护渣。     

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.     

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.     

浸入式水口结构对连铸大圆坯质量的影响

为研究不同结构的浸入式水口对大规格连铸圆坯质量的影响,以某钢厂生产断面直径为500 mm的 42CrMo连铸圆坯为背景,对使用侧孔浸入式水口和传统直通浸入式水口的使用效果开展研究。结果表明,采用侧孔浸入式水口浇铸时,结晶器进出水温差由传统直通水口的3.30 ℃提高至3.54 ℃;连铸圆坯中心疏松由1.5级改善至1.0级,中心偏析指数由0.93~1.21降低到0.98~1.02,近表层至近中心碳极差由0.050%~0.075%降至0.035%~0.053%,使用侧孔浸入式水口的连铸圆坯碳偏析得到改善;铸坯内弧侧表层至3/4R处氧化物夹杂物总量减少0.5个/mm2;铸坯从1/4R处向内长度不小于13 μm的硫化物数量减少0.35个/mm2;轧材全氧质量分数平均降低0.000 12%,夹杂物中B类氧化物夹杂均在1.5级以内,钢中大尺寸夹杂物明显减少,钢的洁净度得到改善。     

方坯连铸曲面结晶器横截面形状的研究

分析了方坯连铸直面结晶器的工艺缺陷及曲面结晶器的工艺效果,探讨了曲面结晶器横截面的设计与选择,确定了我故土丕和对结晶器横截面形状设计的影响,提出了相应的方坯连铸的经济断面。     

Investigations of Inclusion Modification in High Grade Pipeline Steel

SiCa line was injected into liquid pipeline steel at the end of LF refining as calcium treatment, and samples were taken from liquid steel in ladles before and after calcium treatment, liquid steel in mould, and slabs. Results show that Ca content in steel decreased obviously after calcium treatment with production, and the compositions, morphologies and sizes of inclusions in steel also varied with time; Al2O3 inclusions turned to be fine irregular CaS-CaO-Al2O3 compound inclusions after calcium treatment, and then to be fine globular CaO-Al2O3 inclusions in mould, and finally changed to be a few larger irregular CaS-CaO-Al2O3 complex inclusions in slabs; reoxidation of molten steel treated by Ca deteriorates effect of calcium treatment. Thermodynamic study revealed that inclusion variations were related with preferential reactions among Ca, Al2O3 and S and precipitation of S in high sulfur capacity CaO-Al2O3 inclusions. New calcium treatment controlling rules were put forward according to the inclusion variations and requirements of pipeline on inclusion controlling.     

Fluid Flow,Heat Transfer and Inclusion Motion in a Four‐Strand Billet Continuous Casting Tundish

Inclusions in the steel in a four‐strand continuous casting tundish, billet and wire products are firstly investigated with industrial trials, and the fraction of inclusions removed in terms of total oxygen in the tundish is measured. Then the 3‐dimenional fluid flow, heat transfer and inclusion motion in the tundish are numerically simulated. The κ‐? two‐equation model is used to model turbulence. Inclusion motion and trajectories are calculated by considering drag force and buoyancy force, coupling the effect of turbulent fluctuation (Random Walk Model). The effect of strands‐blocking on the fluid flow, heat transfer and inclusion removal is studied. A new design of tundish is proposed focusing on removing more inclusions from the molten steel.     

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.     

连铸坯夹杂原因分析

造成连铸坯宏观夹杂的原因是钢液被二次氧化形成夹杂以及中间包中的夹杂物卷入钢液进入结晶器而形成铸坯夹杂，采取保护浇铸坯夹杂保证中间包液面达到一定高度，及时捞渣等措施，可减少铸坯夹杂，改善铸坯质量。     

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。