301不锈钢板坯结晶器出口热力耦合数学模型

针对某厂301不锈钢连铸坯表面裂纹缺陷发生率高的生产问题,建立了301不锈钢板坯连铸结晶器内凝固坯壳形成及应力和变形的热力耦合模型。该模型考虑了包晶钢高温相变特征及其结晶器内的凝固特点。利用有限元软件ANSYS,采用三维瞬态热传导有限元、生死单元技术及三维热弹塑性接触有限元算法进行求解,对301不锈钢板坯结晶器内凝固过程进行了研究。结果表明,结晶器出口处铸坯宽面中心温度最低,距角部40 mm处温度最高,坯壳最薄,随δ-Fe转变量增加,出口处坯壳温度升高,坯壳厚度变薄。铸坯宽面中心位移变形最小,角部最大,窄面位移量大于宽面。随δ-Fe转变量增加,出口处应力水平下降,热点区附近成裂指数增加,发生纹裂机率增大。     

板坯连铸结晶器内温度场/应力场耦合分析

在考虑结晶器铜板中水槽结构的尺寸和分布的基础上,建立了连铸结晶器内温度场和应力场之间耦合过程的有限元分析模型.通过耦合计算,发现宽窄面方向上的坯壳与结晶器壁间的气隙沿拉坯方向上的变化规律,为分析铸坯在结晶器中产生的质量问题以及设计有关结晶器工艺和结构参数提供了理论研究的手段.     

连铸板坯在结晶器内凝固行为的研究

在考虑结晶器铜板水槽结构尺寸和分布的基础上，建立了连铸板坯在结晶器内温度场和应力场之间耦合过程的有限元分析模型。通过耦合计算，发现板坯在连铸结晶器中宽窄面方向上的坯壳表面温度、坯壳生长及其受力变形等行为沿拉坯方向上的变化规律，为分析和解决铸坯在结晶器中产生的质量问题、设计或优化有关结晶器工艺和结构参数提供了理论依据。     

连铸结晶器内大方坯的热力耦合分析

针对攀钢大方坯连铸机投产初期铸坯表面角部纵裂缺陷,建立了大方坯连铸结晶器内铜板与铸坯问的热力耦合模型,应用模型分析了大方坯连铸结晶器内的传热过程和坯壳的应力分布.在传热模型中,以稳态模型分析结晶器的传热过程,以瞬态模型分析铸坯的传热过程;在力学模型中,考虑铸坯和结晶器的接触边界以处理结晶器角部的气隙,以热弹塑性模型分析铸坯的变形和应力场.2种结构的连铸结晶器中大方坯温度场和应力场计算结果表明,结晶器倒角从25 mm×45°变为12 mm×45°时,可改善铸坯角部的传热条件,降低凝固坯壳角部温度,增加凝固坯壳厚度,有利于减轻和防止铸坯角部裂纹.     

方坯结晶器内气隙分布的数值分析

基于连铸坯壳应力遗传特性建立二维方坯热力耦合模型,利用有限元分析软件ANSYS多载荷步法进行求解。模拟并对比了三种不同结晶器锥度值下的铸坯气隙生成及其分布规律。结果表明：离弯月面60~72mm角部首先出现气隙,随后向铸坯表面中心逐渐扩展,在结晶器出口处仅铸坯表面中心区域坯壳与结晶器存在接触。在结晶器上部300mm内气隙生长速度较快。随着离开弯月面距离增加,气隙生长速度逐渐降低。气隙宽度沿结晶器高度方向分布基本符合抛物线规律。随着锥度增加,气隙出现时机逐渐推迟,气隙宽度和存在范围也相应缩小。     

连铸坯凝固过程热力耦合有限元模拟

 以铸坯和结晶器之间的间隙热阻为纽带,考虑保护渣凝固对接触热阻和渣膜热阻的影响,建立了连铸结晶器与铸坯热力耦合模型并编写了相应的有限元仿真程序。模型预测的坯壳厚度和实验结果吻合良好,两者差值≤2 mm。应用模型分析了大方坯连铸结晶器内的传热过程和坯壳的应力分布。结果表明,随着拉速提高,凝固坯壳厚度减薄,铸坯产生角部裂纹的趋势增加。     

漏斗形薄板坯结晶器内铸坯传热分析

 为了更好地研究FTSC结晶器内铸坯的凝固过程,基于节点温度传递方法建立薄板坯结晶器内铸坯三维瞬态传热数值模型,采用有限元软件ANSYS模拟结晶器内铸坯三维稳态温度场。对比分析平板形及漏斗形结晶器内铸坯传热规律,探讨了漏斗区传热特点。结果表明,漏斗区内钢水存储空间增大,总热容量增加,漏斗区铸坯宽面表面中心温度升高,坯壳厚度减薄。FTSC结晶器宽面出口温度、坯壳厚度连续变化,与漏斗形布置一致。     

结晶器内连铸坯的热和应力状态数值模拟

针对碳钢在连铸结晶器内的凝固过程,考虑铸坯和结晶器内的接触状态,利用ANSYS软件建立了完全热力耦合的三维稳态有限元模型,模拟出结晶器区域内的热和力学状态,包括铸坯应力场、气隙分布规律以及整个结晶器内钢液温度分布等。结果显示,铸坯出结晶器时坯壳外层处于压缩状态、内层处于拉伸状态,内外表面应力分别为279、311 MPa,凝固前沿处应力为3 MPa左右,处于材料的极限强度范围,有产生裂纹的可能。锥度结晶器有利于钢液凝固换热,采用0.7%/m的倒锥度设计后,气隙量较无锥度结晶器最多减少了42%。     

宽厚板坯连铸结晶器流场、温度场及应力场的耦合数值模拟

利用Pro CAST软件对2400 mm×400 mm宽厚板坯结晶器建立三维动态模型,采用移动边界法实现结晶器内流场、温度场及应力场的耦合模拟.结果表明:考虑凝固坯壳的影响,下回流区位置向铸坯中心靠拢,真实反映了钢液在连铸结晶器内的流动情况.自由液面的钢液从窄面流向水口,速度先增大后减小,距水口约0.7 m处,出现最大表面流速,约为0.21 m·s-1.结晶器出口坯壳窄面中心厚度最小且由中心向两侧逐渐增大,最小厚度约为10.4 mm;受流股冲击影响较弱的宽面坯壳与窄面相比生长更均匀,宽面偏角部和中心的坯壳厚度分别为18.9 mm和27.6 mm.铸坯坯壳应力变化趋势与温度基本保持一致,表明初凝坯壳应力主要是热应力.结晶器内铸坯宽窄面上的等效应力均沿着结晶器高度下降方向呈增大趋势,铸坯角部、宽面中心及窄面中心位置的最大应力各约为200、100和25 MPa.      

15CrMoG钢棒材表面缺陷机理分析和工艺改进

分析得出,棒材表面细小纵裂纹和表面裂口缺陷产生于铸坯加热之前,且与结晶器弯月面保护渣有关。利用Thermo-Calc热力学软件计算15CrMoG钢凝固相变过程,结合亚包晶钢连铸凝固特点综合分析15CrMoG钢棒材表面缺陷的产生原因和产生机理。结果表明:15CrMoG钢在固相线温度附近发生包晶反应L+δ→γ和包晶转变δ→γ,不仅导致初生坯壳生长不均匀,而且加剧P、S元素在凝固前沿的偏析。而初生坯壳不均匀是导致棒材表面缺陷根本原因。棒材表面细小纵裂纹产生于结晶器内坯壳薄弱处,经过二冷和轧制工序在夹杂物和硫偏聚处扩展长大。棒材表面裂口缺陷是初生坯壳不均匀导致结晶器内液面波动大,造成铸坯夹渣所致。通过控制[C]0.16%～0.17%、[S]≤0.005%、保护渣碱度1.2、熔点≥1200℃、粘度≥1.0Pa·s,260 mm×30mm铸坯水量150 m³/h,拉速0.5 m/min等措施,裂纹合格探伤合格率由原45%提高至98%。     

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.     

Deformation Prediction of a Heavy Hydro Turbine Blade During the Casting Process with Consideration of Martensitic Transformation

Heavy hydro turbine castings are made of martensitic stainless steel, which undergoes martensitic transformation during the casting process. Therefore, both residual stress and deformation are affected not only by uneven cooling but also by martensitic transformation. In this paper, a coupled thermo-martensitic phase transformation–stress model was established and it was implemented by further development with ABAQUS, which also incorporated the thermal and mechanical boundaries, and the contact pair between the casting and mold. The system was applied to the analysis of a heavy hydro blade casting. Results of stress, displacement, and martensite phase fraction were obtained. It is found that martensitic transformation has a significant effect on the stress and deformation results. The displacement in the normal direction of local areas was calculated to represent deformation in the x, y, and z directions. The deformation of the blade casting occurred mainly at the two thin corners with 18 and 22 mm in opposite tendency. The simulated results were compared with the measured machining allowance, and they are basically in agreement.     

电磁搅拌下连铸圆坯凝固传热及应力的数值模拟

 针对电磁搅拌作用下碳钢在连铸结晶器内的凝固过程,考虑铸坯和结晶器内的接触状态,建立了完全热力耦合的三维稳态有限元模型。结果显示,电磁搅拌加快了钢液热量的释放,使气隙生长延迟,与无外加磁场的情况相比,坯壳内的等效米塞斯应力加大,凝固前沿附近超过了材料能承受的抗拉强度,且处于脆性温度范围区间,可能诱发裂纹形成。     

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。     

不同角部形状特厚矩形坯凝固传热及应变数值模拟

 为了更加有效控制和减少连铸坯的角部横裂纹质量缺陷,根据其形成的机制,针对两种新型铸坯模型,即圆角和倒角模型进行研究。通过建立特厚连铸矩形坯在凝固过程的传热模型并进行数值模拟,得到铸坯在凝固过程沿拉速方向上温度场和坯壳厚度的分布规律,并在此基础上建立热力耦合模型,分析铸坯的应力变化,讨论了产生裂纹的可能性。研究结果表明,通过对比传统直角模型,得出圆角和倒角模型对铸坯角部温度场和应力场两个方面的分布状况都有改善,即新铸坯模型角部温度在连铸矫直段有效避开了钢的高温脆性区,同时降低了铸坯角部的应力值,减小了角部裂纹产生的可能性。     

Multiphase Flow and Thermo-Mechanical Behaviors of Solidifying Shell in Continuous Casting Mold

 The metallurgical phenomena occurring in the continuous casting mold have a significant influence on the performance and the quality of steel product. The multiphase flow phenomena of molten steel, steel/slag interface and gas bubbles in the slab continuous casting mold were described by numerical simulation, and the effect of electromagnetic brake (EMBR) and argon gas blowing on the process were investigated. The relationship between wavy fluctuation height near meniscus and the level fluctuation index F, which reflects the situation of mold flux entrapment, was clarified. Moreover, based on a microsegregation model of solute elements in mushy zone with δ/γ transformation and a thermo-mechanical coupling finite element model of shell solidification, the thermal and mechanical behaviors of solidifying shell including the dynamic distribution laws of air gap and mold flux, temperature and stress of shell in slab continuous casting mold were described.     

Initial development of thermal and stress fields in continuously cast steel billets

A mathematical model has been developed to compute the thermomechanical state of the shell of continuously cast steels in a round billet casting mold. The model determines the temperature distributions, the stresses in and the gap between the casting mold and the solidifying strand. The effect of variations in steel carbon content and mold taper on the thermal, displacement, and stress fields are examined. Comparisons with available experimental observations verify the predictions of the model. The model demonstrates that the thermal shrinkage associated with the phase change from delta-ferrite to austenite in 0.1 Pct C steel accounts for the decreased heat transfer observed in that alloy, as well as its susceptibility to cracking. Formerly Graduate Student, Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign.     

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.