薄板坯连铸结晶器锥度设计技术研究及应用

以提高结晶器使用寿命和铸坯表面质量为目标,通过建立漏斗区自有锥度、结晶器总锥度和结晶器局部锥度的计算模型,研究了漏斗区的宽度和深度、窄边铜板偏移量以及磨损对结晶器锥度沿高度方向分布的影响,提出了漏斗区自有锥度和局部锥度沿高度方向分布应与铸坯的凝固收缩相匹配的设计理念,对薄板坯连铸结晶器宽面铜板内腔形状及窄边锥度调控工艺进行了优化设计,为结晶器操作工艺及管理制度的优化提供了理论依据。基于上述研究工作,研发了适合于FTSC工艺的新型结晶器,进行了大量的工业生产试验。结果表明:在浇注宽度为1 520mm的铸坯时,窄边附加偏移量可由12mm降低到7mm,窄边铜板的使用寿命得到了大幅度的提高,过钢量为2.25万t后,窄边铜板最大磨损量由5mm降低到2mm以下;新型结晶器有效控制了铸坯窄面凹陷和表面纵裂纹的发生。     

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.     

薄板坯连铸结晶器铜板热/力行为

基于国内某钢厂CSP漏斗结晶器铜板结构,建立了考虑铜板水槽冷却水流动的薄板坯结晶器铜板三维热/力耦合计算模型,研究分析了典型连铸工艺下结晶器铜板水槽内冷却水的传热特点和铜板温度场与热应力场分布规律,并探讨了冷却水流速及铜板厚度对铜板热/力行为的影响。结果表明,铜板宽面热面与窄面热面最高温度均位于弯月面下约15 mm处,分别达436.5、379.2 ℃。宽面和窄面铜板的最大热应力均位于弯月面下方约25 mm处,分别达876.7、867.8 MPa。宽面铜板的热应力总体比窄面高且分布更为不均匀,螺栓处热面的热应力整体低于其两侧水槽处热面的热应力。增加冷却水流速、减小铜板厚度可减小铜板热面温度与热应力。将螺栓处冷却水缝延长到距结晶器下口30 mm处,可显著改善宽面铜板中下部横向温度分布的均匀性,使其热面横向最大温差减少约19.6 ℃。     

The thermal distortion of continuous-casting billet molds

Preliminary mathematical analyses involving plate bending theory and two-dimensional elastic calculations have revealed that the dominant component contributing to the distortion of continuous-casting billet molds is thermal expansion in the transverse directions. A three-dimensional, elasto-plastic, finite-element analysis of the mold wall has then shown that localized yielding initiates in a region close to the meniscus. The plastic flow is a result of the high thermal stresses induced by the geometric restraint to bending coupled with the locally high temperatures. The resultant distortion profile of the mold down the centerline of a face exhibits a maximum outward bulge of 0.1 to 0.3 mm, which is bounded above by a region of negative taper (1∼2 pct/m) and below by a region of positive taper (∼0.4 pct/m). Measurements of mold wall movement in an operating billet caster using linear displacement transducers compare favorably with model predictions, except in the meniscus region. Case studies of several industrial billet molds have shown that lowering the meniscus level with respect to the location of constraints, or modifying the method of support of the mold tube within its housing so as to reduce the restraint to thermal expansion in the meniscus region, may minimize the extent of permanent distortion. Also, wall thickness can have a significant effect on thermal distortion. Increasing wall thickness results in an increase in both peak wall temperatures and thermal gradients. The former increases the local distortion while the latter causes higher thermal stress levels and possibly permanent distortion. Of the casting variables that can be manipulated to major advantage, cooling water flow rate is the most important. Increasing the water velocity reduces mold wall temperatures, as well as both the total and permanent distortion of the wall.     

Thermal and mechanical behavior of copper molds during thin-slab casting (I): Plant trial and mathematical modeling

Three-dimensional (3-D) finite-element thermal-stress models have been developed to predict temperature, distortion, and residual stress in the mold of continuous casters of thin steel slabs, comparing both funnel-shaped and parallel molds. The mold shape and high casting speed leads to higher mold temperatures and shorter mold life than in conventional slab casters. This study investigates heat flux and the effects of mold shape on distortion and cracking of the thin-slab mold. In Part I of this two-part article, mold wall temperatures measured in the plant were analyzed to determine the corresponding heat-flux profiles in thin-slab molds. This data was then used in an elastic-visco-plastic analysis to investigate the deformation of the molds in service for the two different mold shapes. The model predictions of temperature and distortion during operation match plant observations. During operation, the hot-face temperature reaches 580 °C and heat flux varies from 7 to 4.5 MW/m² when casting at 3.6 m/min. The copper plates bend toward the steel, with a maximum outward distortion of about 0.3 mm. This occurs just above the center of the wide faces and is smaller than the distortion of a conventional slab mold.     

Prediction of Local Heat Flux and Temperature Distribution in a Mould Copper Plate for Flexible Thin Slab Casting Based on In‐plant Temperature Measurements

A three‐dimensional (3D) numerical model has been employed to predict the local temperature and heat flux distribution on the funnel‐shaped Flexible Thin Slab Casting (FTSC) H2 mould plate®. The modelling was combined with in‐plant temperature measurements from thermocouples installed in the mould. For the measurements the thermocouples are arranged to be adapted to the funnel‐curved shape of the mould such that the spatial locations of measured temperature data sets from the thermocouples can be regarded to form a plane. The method divides the geometry of the mould plate into two computational domains along the measured temperature plane in order to utilize the data as the boundary condition by interpolation. The measured data are compared with those obtained by the average heat flux and one‐dimensional (1D) heat conductivity model in the thermal analysis of the mould plate. It is found that the assumption of average heat flux only along the longitudinal direction, used in conventional billet or slab moulds, is not satisfactory to describe the heat transfer of the complex funnel‐shaped mould in the longitudinal and transverse direction. The heat flux values and hot face temperature in funnel‐shaped mould plates obtained by the 3D model are lower in comparison to the 1D model. The structure of cooling water channels has a significant influence on the uniformity of the heat flux. Some fluctuations of heat flux and temperature occur in the junction between the parallel‐curved and funnel‐curved zone along the longitudinal and transverse direction for the present case. It is expected that by utilizing the measured temperature data, the present model can be helpful to understand the thermal behaviour and define the thermal boundary condition of the funnel‐type mould plate.     

Increasing the life of molds for casting copper and its alloys

The work of the molds intended for casting copper and copper alloys in semicontinuous casters for producing flat billets is considered. It is shown that, to increase the resistance of mold plates, the inner space of the mold should have a taper shape toward the casting direction and take into account the shrinkage of the linear dimensions of the ingot during its motion in the mold. The taper shape increases the intensity and uniformity of heat removal due to close contact between the ingot and the mold inner surface. Testing of new design molds under industrial conditions demonstrates that their resistance increases by a factor of 4.0–4.5. The taper effect of the mold plates is much more pronounced in their narrow faces.     

Simulation of slab formation in a continuous-casting machine

A discrete analog of the differential heat-conduction equation permits the use of nonuniform calculation grids in the simulation of continuous casting. That allows the distribution of the temperature gradients in the model to be taken into account, with corresponding increase in the accuracy of the approximation and the results. A mathematical model is developed for the solidification and shrinkage of continuous-cast slab in the mold. The adoption of a nonuniform grid permits the use of elements measuring 1–2 mm in the simulation. This model is used to study the distortion of the slab cross section at the mold walls. Calculation of the geometric profile permits refinement of the thermal and mechanical interaction of the solidifying shell and the mold walls and determination of the optimal mold taper so as to reduce the risk of surface and subsurface cracking in the slabs.     

板坯连铸结晶器热行为与铸坯质量的研究

介绍了板坯连铸结晶器的传热机理,系统分析了结晶器内热流分布特征,探讨了结晶器热行为对铸坯质量的影响.分析结果表明,通过结晶器锥度、窄边形状以及角部结构优化,可以改善结晶器内的传热,有效提高铸坯质量.     

Stress and Friction Distribution around Slab Corner in Continuous Casting Mold with Different Corner Structures

The non-uniform friction and thermal stress in the mold are important as causes of the transverse cracks around strand corner. To analyze the stress distribution features around strand corner, a three-dimensional thermo-elastoplastic finite-element mold model with different corner structures (right-angle, big-chamfer, multi-chamfer, and fillet) was established. The temperature field in the mold was indirectly coupled through a three-dimensional fluid flow and heat transfer model. In addition, the non-uniform mold friction stress loaded on the strand surface was calculated through a friction model. The results show that the stress distribution on the shell is similar to the temperature distribution. The stress concentration appears in the strand corner and the lower part of wide face. The friction stress enhances the corner stress around the edge of the air-gap. For chamfered molds, the stress around the corner between the wide face and chamfer face is larger than that between the narrow face and chamfer face. Around the corner region, both the stress peak and the area of the large stress zone of the right-angle strand are the largest, while those of big-chamfered, multi-chamfered, and fillet strands decrease in that order. The stress peak position of the chamfered strands is closer to the mold exit than that of the right-angle strand. Compared with the use of the right-angle mold, the application of chamfered molds is able to reduce the stress concentration around the strand corner.     

倒角连铸坯角部纵向裂纹形成机制及控制

 倒角结晶器是控制板坯角部横向裂纹的有效方法之一,但角部纵向裂纹是倒角连铸坯易发的缺陷,这成为了倒角结晶器大规模应用的最大障碍。通过研究倒角结晶器生产工艺参数、设备精度以及喷嘴堵塞等因素对倒角连铸坯角部纵向裂纹的影响,确定了角部纵向裂纹发生的机制。研究表明,倒角结晶器窄面锥度不合理、0段和1段连接偏差以及喷嘴堵塞等是角部纵向裂纹产生的主要原因。通过采取有效措施,可以将倒角连铸坯角部纵向裂纹发生率降低到0.6%以下,使得倒角结晶器实现大规模的工业应用。     

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。     

连铸圆坯结晶器锥度优化设计研究

基于圆坯轴对称性和坯壳应力遗传特性,建立二维纵向切片热力耦合模型,利用有限元软件ANSYS对结晶器内铸坯传热及应力进行分析。根据结晶器内坯壳的收缩以及气隙的分布,依据最小气隙原则对结晶器锥度进行优化。经过三次优化计算,得到Q195钢圆坯结晶器锥度曲线。优化后的圆坯结晶器内腔呈抛物线形状,总锥度1.88%/m。在分析工艺条件下,使最大气隙降低为0.021 mm。     

Modeling the thin-slab continuous-casting mold

A three-dimensional mathematical model has been developed to compute the thermomechanical state in the mold of thin-slab continuous casters. The thin-slab mold differs from those used in conventional slab casters in that the upper portion of the broad side walls defines a funnel-shaped chamber which allows the nozzle to be submerged into the liquid metal. The chamber converges with distance down the mold, reducing to the rectangular cross section of the finished casting near the mold exit. The new mold, along with casting speeds up to 6 m/min, allows slabs to be cast 50–60 mm thick, compared with 150 to 350 mm in conventional continuous slab casting. However, the mold shape and high casting speed lead to higher mold temperatures and shorter mold life than are found in conventional slab casters. In this article, we develop mathematical models of the process to determine the role of various process parameters in determining the mold life. Finite-element analysis is used to determine the temperatures in the mold and cast slab, and these data are then used in an elastic-viscoplastic analysis to investigate the deformation of the mold wall in service. Cyclic inelastic strains up to 1.75 Pct are found in a region below the meniscus along the funnel edge. These large strains result from the combination of locally high temperatures coupled with geometric restraint of the mold. The deformation leads to short mold life because of thermal fatigue cracking of the mold. The computed locations and time to failure of the mold in fatigue agree very well with observations of the appearance of mold surface cracks in an operating caster. The models are also used to develop an improved mold design. Formerly Research Assistant, Department of Mechanical and Industrial Engineering, University of Illinois, Urbana, IL     

唐钢新型薄板坯连铸结晶器的设计和使用效果

针对唐钢超薄热带生产线中FTSC工艺的H2长漏斗型结晶器在使用过程出现的问题,开发设计了新的漏斗型结晶器.这种漏斗型结晶器将H2结晶器下部的漏斗区改为平面区,结晶器漏斗区高度由2100 mm缩短为900 mm,并通过模拟计算和优化设计,使结晶器内腔曲面形状各部分平滑过渡,使其具有生产铸坯承受变形应力小的优点.工业应用试验获得了成功,结果表明,使用新型结晶器较好地消除了开浇初期铸坯纵裂现象,提高了铸坯质量,新型结晶器对于钢种和铸坯断面具有良好的适应性,使用寿命达到了进口铜板的水平.     

Mold behavior and its influence on quality in the continuous casting of steel slabs: Part II. Mold heat transfer,mold flux behavior,formation of oscillation marks,longitudinal off-corner depressions,and subsurface cracks

Axial heat-flux profiles have been determined quantitatively from temperature measurements conducted on a slab mold under routine operating conditions. As in earlier studies, the heat flux was observed to have a maximum value at the meniscus and to decline with increasing distance down the mold. The mold heat flux increased with increasing casting speed and was greater with a mold powder having lower viscosity and melting point being applied as lubricant. The heat extraction was largest while casting 0.29 pet carbon steel and least for a 0.09 pet carbon grade; reducing the depth of the submerged entry nozzle increased the heat flux slightly in the upper region of the mold. Most significant was the higher heat flux observed at the meniscus of the outside-radius face, attributable to the locally greater copper plate thickness compared to that of the opposite broad face. All of the measurements can be explained straightforwardly by heat flow in the vicinity of the meniscus and the resulting behavior of the so-called slag rim adjacent to the mold wall. It is postulated that the difference in copper plate thickness between the two broad faces at the meniscus causes the slag rim to be smaller on the outside-radius face which gives rise to shallower oscillation marks, as observed, higher heat transfer, and a slightly thicker solid shell. The dissimilar behavior has implications for quality because the inside-radius shell, experiencing reduced heat extraction, cools and shrinks less than the outside-radius shell. Thus, for a given end-plate taper, the narrow face of the slab adjacent to the inside radius can push against the end plate, accelerating copper wear, and, owing to squeezing of the broad face, cause an off-corner depression and subsurface crack toward the mold exit. If this is correct, maintenance of the same copper plate thickness at the meniscus is fundamental to preventing such an occurrence. Moreover, adjustment of the heat extraction at the meniscus should be achievable by changing copper plate thickness, mold coating thickness/conductivity, cooling water velocity, cooling channel configuration, and mold flux composition for a given steel grade. Formerly Graduate Student, Centre for Metallurgical Process Engineering, The University of British Columbia,     

非均匀冷却型结晶器铜板的传热特性

针对唐钢新型的FTSC非均匀冷却型结晶器,根据连铸结晶器的铜板实测温度、结晶器水量和进出水温差,建立了结晶器热流一维修正计算模型及其分布曲面,分析在生产SS400钢种时不同条件下的结晶器铜板热流分布特点,结果表明,相同条件下热流分布趋势和热面温度分布趋势相似,横向上波动剧烈,在距离铜板中心线400mm处的热流最高,距离弯月面越近波动越剧烈,在结晶器下部,中心部位的热流较低,并且平直段热流比漏斗区域的热流低;沿着结晶器高度方向,热流和热面温度变化梯度较小,在结晶器中部会有一个热流以及热面温度回升趋势;在整个铜板宽面上,热流较高,热面温度较低,这对提高结晶器使用寿命具有积极意义。     

Thermal and mechanical behavior of copper molds during thin-slab casting (II): Mold crack formation

The formation of cracks in a funnel mold of thin-slab casting is investigated using metallographic studies and mathematical models. In Part II of this two-part article on thin-slab casting molds, short longitudinal cracks near the meniscus region of a thin-slab funnel mold are studied metallurgically. X-ray analyses revealed the formation of Cu-Zn brass on the copper matrix at high temperature where the crack initiated. Heat-transfer and thermal-elastic-viscoplastic stress models described in Part I are applied to investigate the temperature and stress fields associated with the cracks. Large cyclic inelastic strains were found in the funnel transition region just below the meniscus due to the slightly higher temperature at that location. The cracks then appear to have propagated by thermal fatigue caused by major level fluctuations at transitions. The stress and strain predictions suggest cycles to failure for molds for various hot-face temperatures.     

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.