板坯连铸铸坯鼓肚变形的仿真研究

合理的控制影响铸坯质量的鼓肚量,有利于铸坯质量的提高。根据板坯实际生产,利用有限元法模拟凝固过程并建立二维热机耦合鼓肚模型,分析了拉速与二冷引起的蠕变时间、坯壳厚度及铸坯温度的变化对鼓肚的影响,为实际生产提供参考。     

板坯连铸三维鼓肚变形仿真研究

通过建立板坯连铸凝固过程的传热模型,获得板坯冷却传热过程的坯壳生长情况,在此基础上利用有限单元法建立了坯壳三维鼓肚变形仿真分析模型。针对实际铸机的设备和工艺状况,计算了板坯凝固过程的鼓肚变形情况,并就三维仿真分析的特点进行了分析。     

1650板坯压下过程热/力学行为及压下工艺优化

为了控制梅钢1 650板坯连铸包晶钢过程铸坯内裂纹发生,基于梅钢1 650板坯连铸机生产实际,建立了1 560mm×230mm断面包晶钢铸坯凝固过程三维热/力耦合有限元模型,揭示了铸坯凝固过程各冷却区内的温度场分布规律和铸坯压下过程应力与变形行为演变规律。结果表明,铸坯在结晶器及零段内冷却强度大,沿拉坯及其垂直方向的温度分布梯度大;在实施铸坯凝固末端压下过程中,铸坯宽面中心与宽向1/4处的表面变形及应力变化较为同步,且靠近铸坯内弧侧凝固前沿的塑性应变最大,铸坯应力最大值集中在角部区域;目前梅钢包晶钢连铸压下区间设置不当,易引发铸坯产生内部裂纹。     

基于有限元法优化低碳钢板坯连铸机结晶器锥度的设计

钢厂1^#连铸机以0.95 m/min拉速生产SS400,D36和X70钢150~180 mm连铸板坯时,易产生角部纵裂纹,发生率最高可达5%。利用商业有限元软件ANSYS,建立了板坯连铸结晶器二维切片式凝固传热数学模型,并采用传热和应力/应变直接耦合的方法对连铸过程结晶器内凝固传热进行计算,分析了各钢种在0.95~1.05m/min拉速下铸坯温度分布以及温度分布不均引起的热应力。工业试验结果表明,根据优化计算结果,将板坯结晶器窄面的锥度系数由原来的1.00%改进为1.10%后,有效地消除了1^#连铸机板坯的角部纵裂纹。     

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.     

板坯鼓肚的控制

连铸生产过程中鼓肚会导致板坯表面变形，应用数学模型能够控制过量鼓肚，消除发生漏钢的可能性，使铸机以最高拉速生产，明显提高其作业率。     

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.     

On the Formation of Centreline Segregation in Continuous Slab Casting of Steel due to Bulging and/or Feeding

Centreline macrosegregation is often observed in continuous slab casting of steel. Two of the main macrosegregation formation mechanisms are bulging and feeding. Both were studied and compared in the current work by using a two‐phase volume averaging model considering only columnar solidification. The casting of the strand itself is modelled by applying a predefined velocity following the casting speed and solid shell deformation (e.g. bulging). Three different cases are simulated and discussed. (i) The first case considers the influence of the feeding flow during solidification without taking bulging into account. Negative macrosegregation is observed in the centre of the casting in this case. (ii) The second case takes the flow caused by series of bulging along the solidifying strand shell into account, and is, therefore, representative for an ideal situation where bulging takes place without solidification shrinkage. In this case positive centreline segregation is found. (iii) The last case shows the results of a simulation which combines both shrinkage‐ and bulging‐induced flows. It is found that under the current casting conditions the bulging effect dominates over the shrinkage effect, and so positive centreline segregation is predicted.     

Improvement in internal quality of boiler plates at Bhilai Steel Plant

Abstract

Bhilai Steel Plant produces plates of boiler, high tensile and other special quality steels via the basic oxygen furnace–vacuum arc degassing–continuous casting route. These plates need to be ultrasonically sound. However, rejection of plates on account of internal defects was high. Metallurgical investigations of defective plates showed the presence of a martensitic zone associated with cracks around the centreline. The martensitic transformation is favoured by segregation of carbon and other elements. The published literature confirms that, even at a normal cooling rate, martensite formation is possible when the concentration of carbon at the central region is high. Bhilai Steel Plant slab casters have certain inherent deficiencies such as a high roll pitch and low machine rigidity, which lead to strand bulging. High strand bulging causes centreline segregation. Segregation during solidification can be controlled by various techniques including low superheat casting, electromagnetic stirring, mechanical soft reduction, and thermal soft reduction. As a major revamping of the caster and the introduction of additional hardware call for a large capital investment, it was planned to improve the cast product quality using a less capital intensive option, thermal soft reduction. In thermal soft reduction, cooling is intensified close to the final stage of solidification. This creates a thermal compaction which allows the segregated liquid to move upwards and dissipate. The intense cooling also strengthens the solid shell, leading to a significant reduction of bulging and thereby reducing macrosegregation. In the present investigation, the secondary water distribution pattern has been redesigned to incorporate intense cooling close to the final stage of solidification, using a mathematical model developed by Steel Authority of India Ltd (SAIL). The modified water distribution pattern is designed to maintain a high rate of cooling at the outer and inner surfaces of the solidified shell in the strand. Plant trials using the modified water distribution pattern have shown a significant improvement in plate quality, confirmed by macro-and microanalysis of the slabs and plates.     

连铸板坯轻压下过程中间裂纹产生机理

针对板坯连铸过程中间裂纹严重的问题,对中间裂纹的形貌、元素偏析等情况进行分析.通过建立有限元模型,对不同压下位置和不同压下量凝固前沿的受力情况进行计算并与临界应力值进行对比.结果表明:C、P、S等元素在晶界处富集只是促使中间裂纹开裂的内因,真正造成铸坯开裂的原因是凝固前沿所承受的拉应力.铸坯通过矫直段时,多处位置的凝固前沿所承受的拉应力超过钢的临界值,导致凝固前沿容易开裂延伸,形成中间裂纹;而弧形段和水平段处凝固前沿所承受的拉应力不超过钢的临界值,无裂纹产生.统计现场大量轻压下的实验结果显示:轻压下避开矫直区进行时,中间裂纹的发生率降低约41.3%.      

Heat Transfer and Deformation Behavior of Shell Solidification in Wide and Thick Slab Continuous Casting Mold

With the considerations of the behaviors of shell deformation, mold flux film and air gap dynamic distribution in shell/mold gap, a two dimensional slice-travel transient thermo-mechanical coupled model of simulation shell solidification in wide and thick slab continuous casting mold was developed by using the commercial program ANSYS. The evolutions of strand-mold system thermal behaviors, including the air gap formation and the mold flux film dynamical distribution in shell/mold gap and shell temperature field, and the evolutions of shell deformation and stress distribution of peritectic steel solidified in a 2120 mm wide and 266 mm thick slab continuous casting mold were analyzed. The results show that the air gap formation and the thick mold flux film distribution mainly concentrate in the regions 0–21 mm and 0–7 mm, 0–120 mm and 0–100 mm off the shell wide and narrow faces corners, and thus the hot spots are given rise to form in the regions 15–55 mm and 15–50 mm off the shell wide and narrow face corners. The shell server deformation occurs in the off-corners in the middle and lower parts of the mold. The stress evolution in shell surface is tensile stress, while that in shell solidification front is compressive stress.     

Effect of Grade on Thermal–Mechanical Behavior of Steel During Initial Solidification

Thermal–mechanical analysis of solidification is important to understand crack formation, shape problems, and other aspects of casting processes. This work investigates the effect of grade on thermal–mechanical behavior during initial solidification of steels during continuous casting of a wide strand. The employed finite element model includes non-linear temperature-, phase-, and carbon content-dependent elastic–viscoplastic constitutive equations. The model is verified using an analytical solution, and a mesh convergence study is performed. Four steel grades are simulated for 30 seconds of casting without friction: ultra-low-carbon, low-carbon, peritectic, and high-carbon steel. All grades show the same general behavior. Initially, rapid cooling causes tensile stress and inelastic strain near the surface of the shell, with slight complementary compression beneath the surface, especially with lower carbon content. As the cooling rate decreases with time, the surface quickly reverses into compression, with a tensile region developing toward the solidification front. Higher stress and inelastic strain are generated in the high-carbon steel, because it contains more high-strength austenite. Stress in the δ-ferrite phase near the solidification front is always very small, owing to the low strength of this phase. This modeling methodology is a step toward designing better mold taper profiles for continuous casting of different steels.     

Using a Two-Phase Columnar Solidification Model to Study the Principle of Mechanical Soft Reduction in Slab Casting

A two-phase columnar solidification model is used to study the principle of mechanical soft reduction (MSR) for the reduction of centerline segregation in slab casting. The two phases treated in the model are the bulk/interdendritic melt and the columnar dendrite trunk. The morphology of the columnar dendrite trunk is simplified as stepwise growing cylinders, with growth kinetics governed by the solute diffusion in the interdendritic melt around the growing cylindrical columnar trunk. The solidifying strand shell moves with a predefined velocity and the shell deforms as a result of bulging and MSR. The motion and deformation of the columnar trunks in response to bulging and MSR is modeled following the work of Miyazawa and Schwerdtfeger from the 1980s. Melt flow, driven by feeding of solidification shrinkage and by deformation of the strand shell and columnar trunks, as well as the induced macrosegregation are solved in the Eulerian frame of reference. A benchmark slab casting (9-m long, 0.215-m thick) of plain carbon steel is simulated. The MSR parameters influencing the centerline segregation are studied to gain a better understanding of the MSR process. Two mechanisms in MSR modify the centerline segregation in a slab casting: one establishes a favorable interdendritic flow field, whereas the other creates a non-divergence-free deformation of the solid dendritic skeleton in the mushy region. The MSR efficiency depends not only on the reduction amount in the slab thickness direction but also strongly on the deformation behavior in the longitudinal (casting) direction. With enhanced computation power the current model can be applied for a parameter study on the MSR efficiency of realistic continuous casting processes.     

Analysis of thin-slab casting by the compact-strip process: Part II. Effect of operating and design parameters on solidification and bulging

The mathematical model to compute the thermal evolution and solidification of thin slabs, previously presented in Part I of this article, was used in combination with a three-dimensional (3-D) finite-element thermomechanical model to analyze how actual operation conditions can lead to excessive deflection and jamming of the slab shell at the pinch rolls. The models suggest that these phenomena arise from a sudden loss of control of the metallurgical length stemming from the coupling of inappropriate steel superheats and casting velocities to deficient heat-extraction conditions at the mold or secondary cooling system. The bulging deformation was calculated with an elastic and creep model that takes into account the temperature distribution across the shell thickness and the different times that shell elements have to creep exposure, i.e., according to the time that rows of elements require to reach their current position in the casting direction at a given casting speed. The last point was simulated by varying the duration of application of the ferrostatic load to the inside surface of each row of elements. The conditions forecast by the models as being responsible for excessive bulging agree very well with those present during the occurrence of these events in the plant. Since bulging after the last containment roll is a major limitation to productivity, this article also presents a parametric evaluation of the casting-speed limits that two compact-strip process (CSP) casters with different supported lengths may have as a function of steel superheat, mold heat-extraction level, water flow rate of the spray and air-mist nozzles, and slab thickness.     

连铸机二冷配水对铸坯鼓肚的改善

部分生产165 mm×280 mm、165 mm×240 mm的R6和R8连铸机,没有二冷夹持段,在提高拉速后铸坯易出现鼓肚现象。通过分析和试验,得出调整和适当增加窄边水量,明显改善了铸坯的鼓肚。通过推导出的坯壳角部最大受力公式,调整凝固系数K值,可防止铸坯各边的变形,以改善铸坯鼓肚现象。     

Thermo-elasto-visco-plastic finite element analysis on formation and propagation of off-corner subsurface cracks in bloom continuous casting

The formation and propagation of the popular off-corner subsurface cracks in bloom continuous casting were investigated through thermo-mechanical analysis using three coupled thermo-mechanical models.A two-dimensional thermo-elasto-visco-plastic finite element model was developed to predict the mould gap evolution,temperature profiles and deformation behavior of the solidified shell in the mould region.Then,a three-dimensional model was adopted to calculate the shell growth,temperature history and the development of stresses and strains of the shell in the following secondary cooling zones.Finally,another three-dimensional model was used to analyze the stress distributions in the straightening region.The results showed that the off-corner cracks in the shell originated from the mould owing to the tensile strain developed in the crack sensitive regions of the solidification front,and they could be driven deeper by the possible severe surface temperature rebound and the extensive tensile stress in the secondary cooling zone,especially upon the straightening operation of the bloom casting.It is revealed that more homogenous shell temperature and thickness can be obtained through optimization of mould corner radius,casting speed and secondary cooling scheme,which help to decrease stress and strain concentration and therefore prevent the initiation of the cracks.     

厚板坯连铸二次冷却传热数学模拟

在考虑二冷边界换热的条件下,建立了与厚板坯连铸机相适应的传热数学模型。用远红外测温仪测试X65管线钢230 mm×1650 mm铸坯表面温度,实验结果同模拟结果吻合较好。应用数学模型,对不同拉速下管线钢的连铸凝固过程进行了仿真计算,分析了拉速对出结晶器坯壳厚度、铸坯表面温度和液芯长度的影响,得出在给定的二冷条件下,为得到合理的铸坯表面温度,管线钢的拉速应为0.9～1.2m/min。     

连铸坯凝固过程中传输现象基础知识系列讲座(待续)──第二讲 连铸坯质量与凝固过程中的传输现象

高效连铸的主要目的是获得高的铸坯无缺陷率，连铸技术工作者一直把连铸坯质量作为连传发展的一个方向。连铸坯存在的缺陷主要表现为裂纹、偏析、非金属夹杂及鼓肚变形等，这些缺陷的形成与连铸坯凝固过程中的传输现象密切相关。     

Analysis of thin-slab casting by the compact-strip process: Part I. Heat extraction and solidification

This article reports on an extensive experimental and modeling study undertaken to elucidate the thermal evolution of thin slabs during their passage through the mold and secondary cooling system of a compact-strip process (CSP) caster. In industrial trials covering a wide range of casting conditions, temperature measurements were carried out at (1) the copper plates of an operating mold and (2) the stainless steel frame of an operating grid. Separately, water-flux and heat-flux distributions generated by the several water and air-mist sprays produced by the different nozzles used in the process were determined in the laboratory. The analysis of these pieces of information, together with a detailed consideration of the geometry of the mold and the arrangement of the rolls and spray nozzles, were used to establish appropriate boundary conditions for a two-dimensional, curvilinear-coordinate, unsteady-state heat-conduction model for predicting the solidification rate of thin slabs. The predicted slab surface temperatures show very good agreement with corresponding measured values taken in plant tests at several locations along and across the secondary cooling system. The validation trials involved a wide range of low- and medium-carbon steel grades, casting speeds, slab widths, and secondary cooling strategies. The second part of this article combines the solidification model with a creep model of the shell to yield useful information about design parameters and casting conditions associated with undesirable bulging behavior of the slab after the last support roll, which causes stoppage of the process by slab clogging at the pinch rolls.     

X80管线钢238 mm x 1 650 mm连铸坯温度场及凝固末端位置的数学模拟

用二维切片跟踪铸坯凝固传热的方法建立了X80管线钢（/%:0.04C,1.85Mn,0.25Si,0.006P,0.003S,0.30Ni,0.21Mo,0.06Nb,0.02V）238 mm×1650 mm板坯连铸过程中垂直拉坯方向传热的数学模型,通过ANSYS对X80管线钢连铸过程中温度场及坯壳厚度的渐变进行计算,得出拉速1.2mm/min时,出结晶器坯壳厚为18.14 mm,铸坯液芯长22.58 m。凝固壳厚度计算值射钉测试结果的相对误差≤2.5%,凝固末端位置的相对误差为0.68%。分析了过热度（25~55℃）,拉速（1.2~1.3m/min）和二冷水量（79.2~96.8 m³/h）对切片各点温度和凝固末端位置的影响。结果表明,增大拉速、减小二冷配水量,连铸坯表面温降变慢,凝固末端位置距离结晶器液面越远,凝固时间变长;该X80管线钢板坯连铸最佳工艺参数为钢水过热度35℃,拉速1.2 m/min和二冷配水量88m³/h。