扁坯窄面鼓肚变形的分析和避免措施

扁坯由于厚度较薄,理论上一般不会出现窄面鼓肚的问题,但是实际生产中这种情况时有发生。本文通过研究分析,发现当铸坯宽面夹持辊间距过大引起铸坯鼓肚后,鼓肚的铸坯通过下一对宽面夹持辊时,由于坯壳角部的扭转变形会造成铸坯窄面出现明显的鼓肚。高碳钢连铸过程中,当二次冷却比水量不合适时,也有可能导致铸坯的窄面鼓肚,同时产生三角区、中间裂纹。通过分析两个钢厂出现的问题,给出了相应的解决方法,可以为同类连铸机的设计、维护起到一定的参考作用。     

中薄板铸机生产高碳刃具钢铸坯缺陷控制

针对鞍钢股份有限公司炼钢总厂中薄板铸机生产高碳刃具钢时出现铸坯鼓肚和纵裂等缺陷的问题,采用了双联工艺控制钢水温度和成分,调整了保护渣碱度和铸机拉速,采取二冷水强冷却的措施。采取措施后,铸坯未出现鼓肚和纵裂,内部质量也得到改善。     

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。     

铸坯鼓肚对辊列的影响

在实际生产过程中，连铸机扇形段下线维修很大程度上是因为辊列支撑轴承的失效两造成辊子塌或辊子不转，我厂1600mm直弧型连锊机扇形段采用的是二分节辊的辊列排布方式，在使用过程中，长辊支撑轴承失效率是短辊的三倍，为此，就浇注过程中的铸坯鼓肚的存在及其存在形态进行了论证，并通过受力分析对理想状态下铸坯对辊列的载荷和发生锊坯鼓肚时铸坯对辊列的载荷做了分析和对比，论证了二分节辊辊列排布受铸坯鼓肚的影响较大，而新型辊采用的三分节辊辊列排布形式可以减小铸坯鼓肚对辊列的影响，同时就二分节辊辊列的日常维护和线下维任提出相关建议。     

辊子错位对铸坯鼓肚变形的影响

 由于加工、安装、变形与磨损等原因,连铸机辊列中的辊子会偏离设计位置而产生错位,这对铸坯鼓肚变形产生较大影响。基于高温铸坯黏弹塑性本构方程,建立两辊间距内的铸坯坯壳动态鼓肚数学模型,并利用模型计算试验铸机的铸坯坯壳鼓肚曲线,依照实测数据验证了模型的有效性。根据奥钢联工业连铸机的设备及工艺参数,计算铸机不同扇形段内铸坯坯壳在不同辊子错位量情况下的鼓肚变形,并讨论在辊子发生错位的情况下辊间距对铸坯坯壳固液交界面处最大应变的影响,并给出铸机辊间距的确定方法。     

板坯连铸机"滞坯"原因分析与控制

随着汽车消费产业的高速发展,汽车用钢的需求量逐年增加,对钢材质量和性能的要求也越来越高。2017年以来,河钢邯宝炼钢厂随着含Nb、Ti、B等低合金高强钢的大量生产,连铸生产过程中"滞坯"现象频繁发生,轻者影响生产组织,重者会导致漏钢,从而影响铸坯质量。为控制连铸频繁"滞坯"现象,保证生产顺行,减少因"滞坯"影响铸坯质量的发生,结合现场生产实际,通过对扇形段设备进行改进,在末端扇形段增设喷淋,使扇形段铸坯上下表面温度均匀,减少因铸坯扣头或上翘引起的"滞坯"现象。另外,对二冷配水进行优化,减少在连铸生产过程中因铸坯"鼓肚"造成的"滞坯"现象。通过采取上述措施,在连铸生产中"滞坯"现象大大减少,为连铸稳态浇注提供了保障。     

连铸板坯的“鼓肚”、应变与设备工艺参数的关系

为防止板坯连铸中的缺陷,铸机的设计及生产必须考虑到控制铸坯上的应力和应变。本文给出板坯宽面“鼓肚”的蠕变解,蠕变“鼓肚”的近似计算式以及坯壳内外表面的应变计算式。为铸机设计和生产提供重要理论依据。     

420CL钢板坯连铸结晶器液面波动分析及改进

结合邯钢420CL钢的生产实际,比较系统地分析了420CL钢板坯结晶器液面波动的原因及影响因素,认为凝固过程中的包晶相变反应使铸坯初生坯壳不均匀是导致结晶器液面波动的根本原因。采用碳钢包晶钢专用保护渣,适当降低结晶器水量、提高二冷区上部冷却强度、抑制铸坯鼓肚等措施,可以减少结晶器的液面波动,为现场420CL钢的稳定生产提供依据。     

连铸板坯的“鼓肚”、应变与设备工艺参数的关系

为防止板坯连铸中的缺陷,铸机的设计及生产必须考虑到控制铸坯上的应力和应变。本文给出板坯宽面“鼓肚”的蠕变解,蠕变“鼓肚”的近似计算式以及坯壳内外表面的应变计算式。为铸机设计和生产提供重要理论依据。     

宽厚板连铸坯鼓肚变形数值计算研究

连铸过程中铸坯已凝固,坯壳在钢水静压力作用下发生鼓肚变形,影响浇铸过程顺行与铸坯质量。以宽厚板连铸坯为对象,采用数值计算方法定量研究了其在连铸过程三个典型铸流位置L1 (弯曲位置)、L2 (弧形段中间位置)、L3 (矫直位置)的鼓肚变形规律。由L1至L3,坯壳鼓肚变形及其导致的凝固前沿拉伸应变均不断增加。凝固前沿厚度方向拉伸应变εxx、拉坯方向拉伸应变εyy与宽度方向拉伸应变εzz呈集中分布趋势,三者分别加剧三角区裂纹、中间裂纹及角部裂纹风险。随着拉速由0.7 m/min增大至0.9 m/min,宽面坯壳鼓肚变形与εxx、εzz先增加后减小,而窄面坯壳鼓肚变形与εyy持续增大。     

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.     

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.     

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。     

连铸坯实测反问题热弹黏塑性建模与分析

为了研究连铸板坯的传热和力学特征,通过建立考虑蠕变的铸坯热弹黏塑性全尺寸有限元数值模型,以实测温度和传热反问题获得的热流为依据,计算和探讨了结晶器内铸坯的传热、应力和应变行为。结果表明,弯月面下100 ~ 200 mm铸坯表面承受拉应力,宽面距角部40 ~ 90 mm的偏角部区域温度较高,坯壳厚度也较薄,距角部约400 mm处温度相对较低,收缩量及应力应变较大。窄面近角部区域的应力和应变总体上低于宽面,距角部越近,窄面铸坯表面的应变越高。偏角部区域坯壳厚度、应力、应变的非均匀性及存在的过大差异,是探讨近角部裂纹成因需要考虑的重要因素。     

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.     

Simulation of thermal behavior during peritectic steel solidification in slab continuous casting mold

Thermal behavior of the solidifying shell in continuous casting mold is very important to final steel products.In the present work,one two-dimension transient thermal-mechanical finite element model was developed to simulate the thermal behavior of peritectic steel solidifying in slab continuous casting mold by using the sequential coupling method.In this model,the steel physical properties at high temperature was gotten from the micro-segregation model withδ/γtransformation in mushy zone,and the heat flux was obtained according to the displacement between the surface of solidifying shell and the hot face of mold as solidification contraction,the liquid-solid structure and distribution of mold flux,and the temperature distribution of slab surface and mold hot face,in addition,the rate-dependent elastic-viscoplastic constitutive equation was applied to account for the evolution of shell stress in the mold.With this model,the variation characteristics of surface temperature,heat flux, and growth of the solidifying shell corner,as well as the thickness distribution of the liquid flux,solidified flux,air gap and the corresponding thermal resistance were described.     

模拟连铸过程的数学模型

开发了一种描述连铸坯凝固过程的热传输数学模型。该模型考虑了物料参数随温度的变化以及凝固时的相变潜热,采用了VisualC 语言编写程序模块。根据此模型计算了铸坯断面的温度、坯壳厚度等参数,并与生产试验数据作了对比。结果表明二者相当吻合,此模型可用于优化连铸工艺参数并进行在线控制模型的开发。     

宁钢板坯窄面鼓肚原因分析

以宁钢板坯窄面鼓肚为分析对象,用经验方法对结晶器的冷却能力进行了验算,验算结果与漏钢坯壳实测结果基本吻合。结合相近条件下板坯结晶器内流场与温度场的计算机仿真结果,认为宁钢板坯窄面鼓肚的主要原因是结晶器冷却能力偏弱,结晶器的锥度漂移、浸入式水口结瘤、拉速偏低且波动大是宁钢板坯窄面鼓肚的操作因素。提出了防止板坯窄面产生鼓肚的措施,指导连铸机的改造与连铸生产。     

Analysis of bulging for high speed continuous casting

In order to research the temperature distribution and mechanical deformation of slab bulging during high speed continuous casting,mathematical models have been developed to analyze the thermal and mechanical behavior of the slab.The thermal history of the slab has been predicted by a two-dimensional transient finite element heat transfer model,whose results serve as the input to the stress model.The stress model has been formulated for a two-dimensional longitudinal plane.In this case,the maximum tensile strain during the bulging process is located at the solidification front just past the top of the upstream roll,which may contribute to crack formation.The maximum tensile stresses are located at the cold surface in the middle of the two back-up rolls,just at the point of the maximum bulging.Stresses near the solidification front are small because of the high temperatures which produce lower elastic modulus values.Finally,the effect of the casting speed on the bulging deformation is discussed.