Modeling superheat removal during continuous casting of steel slabs

To investigate superheat dissipation in a continuous slab casting machine, mathematical models have been developed to compute fluid flow velocities, temperature distribution within the liquid pool, heat transfer to the inside of the solidifying shell, and its effect on growth of the shell. Three-dimensional (3-D) velocity and heat-transfer predictions compare reasonably with pre-vious experimental measurements and two-dimensional (2-D) calculations. The results indicate that the maximum heat input to the shell occurs near the impingement point on the narrow face and confirm that most of the superheat is dissipated in or just below the mold. Superheat tem-perature and casting speed have the most important and direct influence on heat flux. The effects of other variables, including mold width, nozzle jet angle, and submergence depth, are also investigated. Calculated heat flux profiles are then input to a one-dimensional (1-D) solidifi-cation model to calculate growth of the shell. Shell thickness profiles down the wide and narrow faces are compared with the predictions of conventional heat conduction models and available measurements.     

Heat flow,gap formation and break-outs in the continuous casting of steel slabs

A theoretical investigation of heat flow and gap formation in the mold of a continuous slab caster has been undertaken using a mathematical model, with the ultimate purpose of predicting the casting conditions which can lead to break-outs. The mathematical model that has been developed for this study is capable of treating the heat flow and air gap as coupled phenomena, and can accept operating variables such as slab size, casting speed, mold taper, and thermal conductivity of the mold powder, as input. Four different cases of slab casting have been investigated; and it has been found that hot spots can form on the surface of the slab within a few centimeters of the corners. Depending on their temperature, it is suggested that these hot spots may give rise to the formation of break-outs off the corners of the slab. From an examination of the behavior of the hot spots, the susceptibility of the cases studied to break-outs has been evaluated. The usefulness of the present analysis in qualitatively ascertaining the location and extent of mold wear under different casting conditions has also been examined. A. Grill was formerly affiliated.     

低碳钢连铸板坯表层凝固钩的特征

对不同浇铸条件下的低碳钢连铸板坯进行了凝固钩（Hook）的特征研究,根据凝固钩的形貌概括了凝固钩的不同类型,统计了凝固钩周围气泡和夹杂物的分布,讨论了不同浇铸参数对铸坯凝固钩深度的影响,并通过Bikerman方程和弯月面凝固理论对凝固钩的不同特征进行了解释.结果表明：按形貌可将凝固钩分为完整叶状、双凝固钩、弯曲截断型和二次凝固型四种类型,其中二次凝固型的凝固钩出现的概率最高为46.8%,而完整叶状、弯曲截断型和二次凝固型的凝固钩出现概率分别为25.3%、7.6%和6.3%;研究发现,凝固钩周围的夹杂物数量明显多于其他区域的夹杂物数量,说明凝固钩能够捕获结晶器内上浮的夹杂物;对比不同浇铸参数发现,采用结晶器电磁制动装置（FC-Mold）、减小结晶器水口浸入深度、增大浇铸拉速均能够减小凝固钩的深度;Bikerman方程的计算结果和弯月面凝固理论能在机理上解释凝固钩的形貌特征.     

管线钢连铸板坯的半宏观偏析和凝固组织

研究了不同凝固组织下管线钢的中心偏析情况.低倍组织腐蚀结果表明,连铸板坯的中心偏析由相对独立的、大小不一的半宏观偏析点组成,且不同凝固组织的半宏观偏析特征有所差别.化学成分分析表明,中心等轴晶时板坯中心为负偏析,且贯穿于整个等轴晶区中,而中心为柱状晶时则表现为正偏析.通过对比化学成分和半宏观偏析面积比发现,板坯的半宏观偏析面积比随C和Mn最大偏析度的增加而增加,由于化学成分分析只能反应局部位置的偏析情况,因此半宏观偏析面积比的方法对生产实践更具有指导意义.对不同过热度下不同凝固组织的半宏观偏析面积比的统计结果显示,在合理使用轻压下技术的前提下,中心为柱状晶更有利于减轻半宏观偏析面积比,进而改善管线钢连铸板坯的中心偏析.     

Metallurgical developments in the continuous casting of austenitic stainless steel slabs

Abstract

The design concept of Atlas Steels’ new integrated sheet/strip mill at Tracy, Quebec involved direct conversion of continuously cast austenitic stainless steel slabs to coiled hot bands. In early production, a high incidence of surface defects, commonly called ‘slivers’, necessitated hotband conditioning greatly in excess of the anticipated level. Electron probe microanalysis of nonmetallics found in the slivers revealed that the majority of such defects resulted from subsurface entrapment of pouring stream reoxidation products in the continuously cast slabs. The problem was brought under operational control by improved steelmaking practice and modifications of the casting operation to permit pouring through refractory stream protection tubes.

Résumé

Aux usines d'Atlas Steel situées it Tracy dans le Québec, le procédé de production prévoit la coulée continue suivie de laminage à chaud de tôles et de bandes. Dès la mise en marche des usines, le trop grand nombre d'inclusions de surface imposa un parachévement imprévu des bandes laminées à chaud. Des analyses faites par micro-sonde ont fait voir que la plupart des inclusions provenaient d'oxydation au jet de la coulée continue. Des modifications apportées à l'élaboration de l'acier et l'emploi de tubes réfract aires au jet de coulée surmonterent les difficultés occasionnées par les inclusions de surface.     

The formation of oscillation marks in the continuous casting of steel slabs

The formation of oscillation marks on the surface of continuously cast slabs has been studied by metallographically examining slab samples and by performing a set of mathematical analyses of heat flow, lubrication, and meniscus shape in the meniscus region of the mold. The metallographic study has revealed that, in agreement with previous work, the oscillation marks can be classified principally according to the presence or absence of a small “hook” in the subsurface structure at the base of individual oscillation marks. The depth of the oscillation marks exhibiting subsurface hooks varies with the carbon content, reaching a maximum at about 0.1 pct carbon, while the oscillation marks without hooks show no carbon dependence. The analysis of heat flow at the meniscus, which is based on a measured mold heat-flux distribution, indicates that depending on the level of superheat, the meniscus may partially freeze within the period of a typical mold oscillation cycle. Lubrication theory has shown that, owing to the geometry of the mold flux channel between the solidifying shell at the meniscus and the straight mold wall, significant pressure gradients capable of deforming the meniscus can be generated in the flux by the reciprocating motion of the mold relative to the shell. A force balance on the interface between the steel and the mold flux has been applied to compute the shape of the meniscus as a function of the pressure developed in the lubricating flux at different stages in the mold oscillation cycle. This has demonstrated that the “contact” point between the meniscus and mold moves out of phase with (by π/2), and has a greater amplitude than, the mold displacement so that just at, or near, the end of the negative strip time molten steel can overflow at the meniscus. From these studies a reasonable mechanism of oscillation-mark formation emerges which involves interaction between the oscillating mold and the meniscusvia pressure gradients in the mold flux, meniscus solidification, and overflow. The mechanism is consistent with industrial observations. E. TAKEUCHI, on study leave from Nippon Steel Corporation     

过热度对高碳钢小方坯宏观偏析影响研究

摘要：基于国内某厂高碳钢小方坯连铸生产过程,首先利用ProCAST软件进行过热度对连铸坯宏观偏析影响的模拟研究,然后对过热度分别为44、39和28℃的连铸坯横断面和纵截面进行宏观偏析和疏松缩孔的研究。模拟结果表明:连铸坯横断面中心碳偏析随着过热度的增加不断增大,过热度超过25℃时,连铸坯横断面中心碳偏析度更加严重;试验结果表明,过热度分别为44、39和28℃ 的连铸坯横断面中心碳最大偏析度分别为1.39、1.32和1.06。结合模拟结果能够有效指导实际生产过程中连铸参数的调整,过热度越高,连铸坯中心碳偏析、条带状疏松缩孔越严重,为了降低连铸坯宏观偏析,建议将过热度控制在25~30℃。     

IF钢开浇阶段铸坯洁净度分析

由于IF钢生产过程中对开浇阶段铸坯质量判定不明确,因此在利用时容易导致产品质量问题而增加生产成本。通过对头坯不同位置进行取样,研究IF钢开浇阶段铸坯沿拉坯方向的洁净度变化。实验结果表明,IF钢开浇阶段铸坯中大型夹杂物主要来源于结晶器卷渣和中间包中来不及上浮的脱氧或二次氧化产物;从距离头坯头部2.5m位置开始,由结晶器卷渣所引入的大型夹杂物含量接近正常坯水平;距离头坯头部7.5m位置处开始N含量与正常坯含量基本持平,簇状Al_2O_3夹杂物数量及尺寸接近正常坯水平;距离头坯头部8.5m位置处开始全氧质量分数保持在20×10~(-6)左右。     

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.     

板坯结晶器内钢液三维流场数学模型的研究

重点研究了浸入式水口浸入深度、拉速等工艺参数对结晶器内钢液流动状态的影响.研究结果表明:改变这些影响因素在一定程度上可以起到改善钢液流动状态的目的,但通过改变这些影响因素并不能完全实现对结晶器内钢液流动状态的有效控制,尤其是对液面波动和对窄面冲击强度的控制.     

Modeling of the solidification process in a continuous casting installation for steel slabs

The development of a computational simulation system for modeling the solidification process in a continuous casting facility for steel slabs is discussed. The system couples a module for solving the direct problem (the calculation of temperatures in the steel strand) with an inverse analysis module that was developed for evaluating the steel/mold heat fluxes from the information provided by thermocouples installed in the continuous casting mold copper plates. In order to cope with the non-uniqueness of the inverse analysis, a priori information on the solution, based on the consideration of the problem physics, is incorporated. The stability of the system predictions are analyzed and the influence of the first trial used to start the evaluation procedure is discussed. An industrial case is analyzed.     

Advantageof anticlogging nozzles over conventional aluminographite nozzles in continuous casting of steel slabs

Abstract

The goal of this work was to establish the advantages of the application of anticlogging nozzles compared with conventional aluminographite nozzles in the continuous casting of steel slabs. Anticlogging nozzles are used in many steelmaking plants to inhibit scab formation at the internal side of the nozzle wall. The formation of aluminate-corundum scab decreases internal nozzle diameter, leading to the reduction of steel flow and eventual blocking of the nozzle. The results of monitoring the behaviour of a large number of conventional aluminographite and anticlogging nozzles in a campaign lasting two months are presented. The data reveal lower average values of internal erosion caused by the steel and reduced thicknesses of scab formed at the internal side of the anticlogging nozzle wall relative to conventional nozzles. At the same time, the average values of external erosion, caused by the action of ungranulated casting powders, were considerably increased relative to external erosion resulting from the action of granulated casting powder.     

Application of swirling flow nozzle and investigation of superheat dissipation casting for bloom continuous casing

A swirling flow nozzle (SFN) has been proposed and designed for bloom continuous casting based on the idea that the melt flow pattern in the mould region can be controlled by changing the jet direction from outlets of the submerged entry nozzle. The simulated and plant trial results show that, as compared to the conventional straight nozzle, the oversized shrinkage porosity along with centre cracking at the strand cross-section is removed, and the maximum segregation degree fluctuation range of solute element C is reduced from 0.17 to 0.05. The positive effect is attributed to the remarkable superheat dissipation effect of horizontal swirling flow generated by SFN, which is reduced by 10.6 K as compared to the normal nozzle. Moreover, the adoption of SFN can further enhance the metallurgical effect of in-mould electromagnetic stirring (M-EMS), where both better chemical homogeneity and soundness of bloom castings can be obtained by the combined adoption of SFN and M-EMS.     

Mathematical modelling of fluid flow,heat transfer and solidification phenomena in continuous casting of steel

A steady state, two-dimensional mathematical model for continuous billet casting operations has been developed. Towards this, governing equations of fluid flow and heat transfer together with their appropriate set of boundary conditions were derived and solved numerically via a control volume based implicit finite difference procedure (e.g., SIMPLE). The effect of various assumptions and procedures applied to modelling of turbulence phenomena, thermal buoyancy, flow through the mushy zone, free surface conditions etc., on the sensitivity of the computed results was investigated computationally. Of all these, modelling of heat and fluid flow phenomena in the mushy region was found to have relatively more effect on the predicted results. In addition to these, a set of three different billet casting operations reported in literature were simulated mathematically and direct comparisons were made between predicted and observed solidified shell profiles. Such comparisons demonstrated reasonable to excellent agreement between theory and experiments.     

A new mechanism of hook formation during continuous casting of ultra-low-carbon steel slabs

The initial stages of solidification near the meniscus during continuous casting of steel slabs involve many complex inter-related transient phenomena, which cause periodic oscillation marks (OMs), subsurface hooks, and related surface defects. This article presents a detailed mechanism for the formation of curved hooks and their associated OMs, based on a careful analysis of numerous specially etched samples from ultra-low-carbon steel slabs combined with previous measurements, observations, and theoretical modeling results. It is demonstrated that hooks form by solidification and dendritic growth at the liquid meniscus during the negative strip time. Oscillation marks form when molten steel overflows over the curved hook and solidifies by nucleation of undercooled liquid. The mechanism has been justified by its explanation of several plant observations, including the variability of hook and OM characteristics under different casting conditions, and the relationships with mold powder consumption and negative/positive strip times.     

Cooling systems in the continuous casting of slabs

The production of rolled bar and complex profiles from continuous-cast slab is considered. The proposed method includes water-based cooling systems and the introduction of consumable macrocooling units in the melt. This approach significantly reduces the axial segregation of chemical elements and the structural inhomogeneity of the blank, improves the plastic properties and improved strength of the rolled bar, and reduces the metal consumption.     

Modeling of transient flow phenomena in continuous casting of steel

This paper describes initial efforts to develop and apply 3D finite-difference models to simulate transient flow in the mold. These transient flow phenomena include flow pattern oscillations caused by sudden changes in nozzle inlet conditions and rapid fluctuations in the molten steel⧹flux interface level at the top surface of the mold. The flow model incorporates interactions with other transport phenomena, including turbulence, superheat removal and argon gas bubble injection. Predictions are shown for the oscillatory evolution of the flow pattern from biased steady flow to symmetrical steady flow after a sudden change in inlet conditions. In addition, the predicted turbulent kinetic energy levels at steady state are shown to correlate with measured surface level fluctuations. The effect of processing conditions are consistent with experimental findings. Without argon, the greatest level fluctuations are found near the narrow face, while increased argon moves the maximum towards the center. Fluctuations decrease with deeper submergence and lower casting speed. These transient phenomena are important because they may lead to defects in the final steel product from entrainment of slag, disruption of solidification at the meniscus and non-uniform heat transfer.     

Research on final- electromagnetic stirring position of 82B steel continuous casting based on superheat variation

A solidification and heat transfer model of continuous casting billet was established and verified by surface temperature measurement for 82B high carbon steel with the section size of 150mm ?? 150mm in a domestic steel plant. Taking solidification ratio fs=0. 7-0. 8 of the billet as the reasonable installation position of final electromagnetic stirring (FEMS), the most suitable position of FEMS was determined at 6. 59-6. 79m distance from meniscus with the variation of superheat from 20?? to 40?? according to numerical results. Combined with the actual situation in the production site, the final- electromagnetic stirrer was installed at the position of 6. 73m distance from meniscus and industrial trials were carried out to verify the effect of FEMS. The results indicate that the quality of billet and wire rod is improved after the application of FEMS. For billet, the average carbon segregation index decreases from 1. 05 to 0. 99, the maximum carbon segregation index decreases from 1. 12 to 1. 05, and the central equiaxed crystal ratio increases from 36% to 39%. For wire rod, the central segregation grade decreases from 2. 5 to 1. 5 and the network cementite grade decreases from 4 to 1.