异型坯连铸二冷工艺优化研究

  基于二冷区传热的基本原理和异型坯凝固传热特点,建立了异型坯凝固传热模型;通过现场测试铸坯表面温度和坯壳厚度对数学模型进行修正,得到了异型坯连铸过程中表面温度随时间变化的规律;同时,对原有二冷制度进行了评估,提出了合理的二冷改进措施。通过二冷工艺的优化,提高了二次冷却的均匀性,铸坯表面和内部质量大幅度提高。     

Analysis of flow field and temperature distribution in compact strip production casting process

Abstract

A three-dimensional finite difference model has been formulated, using the commercial code CFX4, to describe the fluid flow and heat transfer in the funnel mould and secondary cooling segment of the compact strip production (CSP) casting process. Through simulation and computation with the established simulation model, influences of the factors such as casting speed, casting superheat, immersion depth of submerged entrance nozzle, amount of cooling water in the secondary cooling segment on flow field and temperature field in the funnel mould and secondary cooling segment of CSP technology were analysed.     

喷嘴喷淋距离对连铸小方坯二冷均匀性的影响

研究了不同喷淋距离下连铸小方坯二冷喷嘴的水量分布,建立了凝固传热模型分析了82B钢连铸坯的热行为。该模型特别考虑了二冷区铸坯表面宽度方向的水流密度分布,并根据铸坯表面测温结果进行了模型校正。采用凝固传热模型研究了喷嘴喷淋距离对连铸二冷均匀性的影响。结果表明:喷嘴喷淋距离的增加有助于提高二冷水横向分布的均匀性,导致铸坯表面温度横向均匀性降低、纵向均匀性提高。这些效果有助于改善铸坯内部裂纹,但是会对角部裂纹产生不利影响。在二冷区前段喷嘴采用低喷淋距离,二冷区末段采用高喷淋距离,既可以提高铸坯角部温度,又能降低表面最大回温速率,有助于同时改善连铸坯角部和内部裂纹。在此基础上,提出了一种连铸小方坯二冷喷嘴布置方式,即二冷区每段喷嘴喷淋距离沿拉坯方向逐渐增加,该方法有助于提高连铸坯“纵?横”冷却均匀性。      

Optimal control of secondary cooling for medium thickness slab continuous casting

Abstract

A software to simulate the solidification, heat transfer and water flowrate distribution in slab continuous casting was developed by establishing a mathematical model for the heat transfer and solidification in medium thickness slab casting. This model was validated by pin shooting and surface temperature measurement experiments. A reasonable target surface control temperature was found by testing the high temperature mechanical properties of Nb bearing ship plate steel, and then the water flowrate of each loop of the secondary cooling zone was determined by the software. The influence of uneven secondary cooling in the slab width direction on the quality of the slab was also investigated, which provided data for the optimisation of the secondary cooling of the slab caster. On the basis of the above research, an optimisation scheme for a secondary cooling system was proposed. Experimental results showed that the quality of the slab was significantly improved after optimisation. The centreline macrosegregation was reduced, and the ratio of equiaxed grains was increased by 3·18%. In addition, the transverse cracking of the slab was almost eliminated.     

基于目标温度的方坯连铸二冷配水方案优化

通过建立铸坯传热数学模型,对连铸凝固过程进行了解析。制定了二冷配水制度优化原则,基于目标温度控制原则开发出二冷配水控制模型。对不同钢种、拉速等工艺条件下的连铸工艺设计出二冷总水量、水量分配和水量与拉速的关系式,并对配水方案进行了凝固与传热的预测和验证。     

Modeling of Dendritic Evolution of Continuously Cast Steel Billet with Cellular Automaton

In order to predict the dendritic evolution during the continuous steel casting process, a simple mechanism to connect the heat transfer at the macroscopic scale and the dendritic growth at the microscopic scale was proposed in the present work. As the core of the across-scale simulation, a two-dimensional cell automaton (CA) model with a decentered square algorithm was developed and parallelized. Apart from nucleation undercooling and probability, a temperature gradient was introduced to deal with the columnar-to-equiaxed transition (CET) by considering its variation during continuous casting. Based on the thermal history, the dendritic evolution in a 4 mm × 40 mm region near the centerline of a SWRH82B steel billet was predicted. The influences of the secondary cooling intensity, superheat, and casting speed on the dendritic structure of the billet were investigated in detail. The results show that the predicted equiaxed dendritic solidification of Fe-5.3Si alloy and columnar dendritic solidification of Fe-0.45C alloy are consistent with in situ experimental results [Yasuda et al. Int J Cast Metals Res 22:15–21 (2009); Yasuda et al. ISIJ Int 51:402–408 (2011)]. Moreover, the predicted dendritic arm spacing and CET location agree well with the actual results in the billet. The primary dendrite arm spacing of columnar dendrites decreases with increasing secondary cooling intensity, or decreasing superheat and casting speed. Meanwhile, the CET is promoted as the secondary cooling intensity and superheat decrease. However, the CET is not influenced by the casting speed, owing to the adjusting of the flow rate of secondary spray water. Compared with the superheat and casting speed, the secondary cooling intensity can influence the cooling rate and temperature gradient in deeper locations, and accordingly exerts a more significant influence on the equiaxed dendritic structure.     

宝钢在线二冷控制模型的研发与应用

针对连铸生产过程中的二冷配水问题,建立了铸坯的凝固传热数学模型。通过实时模拟计算铸坯的温度场,并与PID控制技术相结合,开发了在线二冷控制模型。模型能自动根据钢种、铸坯规格及工艺参数的变化动态调整二冷控制水量,将铸坯的表面温度控制在工艺目标值附近。通过设计合理的控制系统架构,确保了二冷控制系统的稳定性及可靠性。在线测温结果表明,模型具有很高的计算精度。当拉速、浇注钢水过热度变化时,模型能快速将水量调整到目标值,速度快且超调小,从而确保铸坯的表面温度跟踪误差始终限制在较小范围内;当浇注过程处于相对稳态时,铸坯的表面温度保持在目标值。目前,模型已经应用于宝钢内外的多台连铸机,应用效果良好。     

汽车用钢大方坯连铸二冷配水数学模型

应用开发的连铸传热数学模型对石家庄钢铁有限责任公司炼钢厂电炉车间所生产的5个钢种及2种规格铸坯的二次冷却配水进行了模拟计算；根据铸坯凝固冷却过程冶金准则的要求，经反复模拟及优化，得出了二次冷却区各段水量与拉速的二次关系式；综合考虑了过热度对铸坯凝固过程的影响，建立了新的二次冷却水控制模型，并应用于实际生产。     

A measuring method for quick determination of local heat transfer coefficients in spray water cooling within the range of stable film boiling

Within the framework of research investigations described in this paper, a measuring method for the determination of heat transfer coefficients in the range of stable film boiling has been developed for water spray cooling in the secondary cooling zone of continuous casting machines. Based on investigations by ³), a water cooled test sample with a measuring field of 20 × 20 mm² was constructed. For this sample geometry, a special evaluation method has been developed which permits quick evaluation in on-line procedure. Result of measurements are sections for the heat transfer coefficient arising from different spray water distributions. On the one hand, these can be used as calculation figures for the determination of the solidification profile and the solidification rate. On the other hand, the measuring results serve as an assessment basis for nozzle constructions. Practical use of the measuring results confirms the usefulness of the measuring method on a laboratory scale. In addition, the validity of the measuring results has been demonstrated by comparing them with an unsteady state method in accordance with ²).     

Compensation Control Model of Superheat and Cooling Water Temperature for Secondary Cooling of Continuous Casting

In this paper, a compensation control model of secondary cooling process of billet continuous casting for quality steel has been presented. The effects on the spray control of the various parameters such as steel superheat, casting speed, cooling water temperature and chemical component of steel were considered. The parameters of control model were determined to associate with the two‐dimensional heat transfer equation and solved by finite‐difference method. Effects of steel superheat and cooling water temperature on surface temperature, solidification structure and solidifying end point were discussed. Results indicate that steel superheat significantly affects solidification structure and solidifying end point but has a little effect on slab surface temperature. Moreover, secondary cooling water temperature affects surface temperature and solidifying end point but has a little effect on solidification structure. The surface temperature and solidifying end point can be maintain stabilized through applying the compensation control model when steel superheat and cooling water temperature vary. The models have been validated by industrial measurements. The results show that the simulations are in very good agreement with the real casting situation.     

On the development of a three-dimensional transient thermal model to predict ingot cooling behavior during the start-up phase of the direct chill-casting process for an AA5182 aluminum alloy ingot

The control of the heat transfer during the start-up phase of the direct-chill (DC) casting process for aluminum sheet ingots is critical from the standpoint of defect formation. Process control is difficult because of the various inter-related phenomena occurring during the cast start-up. First, the transport of heat to the mold is altered as the ingot base deforms and the sides are pulled inward during the start-up phase. Second, the range of temperatures and water flow conditions occurring on the ingot surface as it emerges from the mold results in the full range of boiling-water heat-transfer conditions—e.g., film boiling, transition boiling, nucleate boiling, and convection—making the rate of transport highly variable. For example, points on the ingot surface below the point of water impingement can experience film boiling, resulting in the water being ejected from the surface, causing a dramatic decrease in heat transfer below the point of ejection. Finally, the water flowing down the ingot sides may enter the gap formed between the ingot base and the bottom block due to butt curl. This process alters the heat transfer from the base of the ingot and, in turn, affects the surface temperature on the ingot faces, due to the transport of heat within the ingot in the vertical direction. A comprehensive mathematical model has been developed to describe heat transfer during the start-up phase of the DC casting process. The model, based on the commercial finite-element package ABAQUS, includes primary cooling via the mold, secondary cooling via the chill water, and ingot-base cooling. The algorithm used to account for secondary cooling to the water includes boiling curves that are a function of ingot-surface temperature, water flow rate, impingement-point temperature, and position relative to the point of water impingement. In addition, a secondary cooling algorithm accounts for water ejection, which can occur at low water flow rates (low heat-extraction rates). The algorithm used to describe ingot-base cooling includes both the drop in contact heat transfer due to gap formation between the ingot base and bottom block (arising from butt curl) as well as the increase in heat transfer due to water incursion within the gap. The model has been validated against temperature measurements obtained from two 711×1680 mm AA5182 ingots, cast under different start-up conditions (nontypical “cold” practice and nontypical “hot” practice). Temperature measurements were taken at various locations on the ingot rolling and narrow faces, ingot base, and top surface of the bottom block. Ingot-based deflection data were also obtained for the two test conditions. Comparison of the model predictions with the data collected from the cast/embedded thermocouples indicates that the model accounts for the processes of water ejection and water incursion and is capable of describing the flow of heat in the early stages of the casting process satisfactorily.     

Film Boiling and Water Film Ejection in the Secondary Cooling Zone of the Direct-Chill Casting Process

Accurate thermal modeling of the direct-chill casting process relies nowadays on increasingly complex boundary conditions for the secondary cooling zone. A two-dimensional axisymmetric finite-element model of the direct-chill casting process was developed to quantify the importance of secondary cooling at the surface compared with internal heat conduction within the billet. Boiling water heat transfer at the surface was found to dominate and be the governing factor only when stable film boiling or water film ejection take place; all other cases were dominated by internal heat conduction. The influence of various parameters (casting speed, cooling water flow rate, and thermophysical properties of the cast material) on the occurrence of water film ejection was analyzed. An exponential relationship was found between the cooling water flow rate and the minimum casting speed at which water film ejection takes place.     

小方坯二冷动态配水模型的研究

建立了小方坯连铸凝固传热过程的二维非稳态数学模型,采取坯龄模型实现了动态配水。对二冷水计算模型计算的动态配水和静态配水进行比较得出拉速变化时动态水量变化较缓和,从而保证铸坯表面温度不发生大的波动。比较目标温度与模型计算的温度,模型计算温度始终在目标温度的周围变动。     

水平连铸结晶器内真实换热边界计算及传递模型

连铸结晶器内冷却水的流动、传热和金属液的传热、凝固全耦合计算时,模型计算效率低、收敛性差,而不考虑真实冷却水流场只进行金属液传热-凝固耦合时,模型计算精度较低,可将冷却水流动-传热和金属液传热-凝固过程分别建模,并基于二次开发的温度场量原位传递程序,将模型串联耦合。结果表明,通过耦合流场计算得到的真实冷却边界替代传统均匀界面换热系数的边界条件施加方式,在保证模型计算效率和计算精度的前提下,可准确模拟铸坯晶粒的尺寸、取向和数量,经试验验证,模拟结果与试验结果高度吻合。     

Spray cooling pattern and microthermomechanical rigidity criterion for improving inner quality in continuously cast steel slabs

Abstract

A new concept has been developed to assess, and a new technique developed to improve, the microthermomechanical rigidity of the solidifying shell in the secondary cooling zones during continuous casting of steel. This is to maximise the coherent solid shell resistance ‘I_c’ against thermometallurgical and mechanical stresses at different levels from meniscus. The idea behind this concept is to optimise the degree of homogeneity of the cooling pattern between a pair of rolls. The effect of the degree of homogeneity of the cooling pattern between a pair of rolls ‘HDCP’, in different spray cooling zones on the different types of centreline segregates has been investigated. A number of plant trials with two different water cooling patterns between a pair of rolls have been performed. The metallographic investigation contains examinations and measurements of the level of centreline segregates of collected samples. A one dimensional (1D) transient finite difference mathematical model of thermal, solidification, solid shell resistance and cooling conditions has been developed to test the different effects of spray patterns on the microthermomechanical rigidity concept and therefore on the mechanism of centreline macrosegregation level. The results indicate that the increase in the degree of homogeneity of the cooling conditions is proportional to the increase in slab internal microquality. The results show that the degree of homogeneity of the cooling pattern affects centreline macrosegregation significantly. This depends significantly on the degree of homogeneity of cooling pattern between a pair of rolls, its location from the meniscus and, generally, on the increase microthermomechanical rigidity concept.     

莱钢4^#矩形坯连铸机二冷水动态控制模型

根据传热学原理,研究从钢水进入结晶器后开始,经过二冷段冷却成铸坯这一阶段的热传导状态,建立传热模型,然后采用差分方法对传热模型进行数字化处理,形成可用于过程控制的连铸机二冷水动态控制模型。该模型在莱钢4^#连铸机上得到具体实现,并取得良好控制效果。     

DYNACS冷却模型在安钢的生产实践

介绍了VAI公司的DYNACS二次冷却模型及其三种铸坯二冷控制方法,并以DYNACS二次冷却模型为开发平台按目标表面温度控制法开发了不同炼钢工艺下不同钢种的二次冷却水表,成功应用于生产实践,提高了铸坯质量、满足了生产要求。     

基于表面温度测量的连铸二冷动态控制模型

二冷动态控制以连铸坯温度的稳定控制为目标,对提高连铸坯质量的稳定性具有重要意义。二冷动态控制的核心内容是实现铸坯表面温度的闭环控制,其关键是实现基于在线凝固传热模型的温度反馈,模型的准确性则是保证二冷动态控制应用效果的基础。为此,提出一种基于表面温度测量,结合射钉修正凝固传热模型的方法,用于辨识凝固传热模型的参数以保证模型的准确性,并应用于二冷动态控制。特别对随机氧化铁皮干扰下的铸坯表面温度的测量方法进行了深入的研究,并开发了能够长期稳定运行于连铸现场的测温装置。     

Determination of CC slab solidification using nail shooting technique

Abstract

The technology of nail shooting was improved and used to study the transverse shape of the solidified shell during steel continuous casting. Three locations across the slab width (1/2, 1/4 and 1/8) were measured by nail shooting and which indicated a larger solidification coefficient and longer liquid core in the slab at higher casting speeds. The solidified shell across the slab width direction was non-uniform due to uneven secondary spray cooling. The point of final solidification at locations 1/8 and 1/4 was much longer than the position between the slab centre and location 1/4, leading to a long solidification end of >2 m, which is poor for the application of dynamic soft reduction. A mathematical model was developed to simulate the growth of the solidified shell and which was in good agreement with the measurements measured by nail shooting. Based on the measurements and simulations, the water spray pattern was improved, making the solidified shell more uniform. Dynamic soft reduction was then optimised resulting in reduced centreline segregation.     

Analysis of Mould,Spray and Radiation Zones of Continuous Billet Caster by Three‐dimensional Mathematical Model based on a Turbulent Fluid Flow

An analysis of mould, spray and radiation zones of a continuous billet caster has been done by a three‐dimensional turbulent fluid flow and heat transfer mathematical model. The aim was to reduce crack susceptibility of the billets and enhance productivity of the billet caster. Enthalpy‐porosity technique is used for the solidification. Turbulence is modelled by a realizable k‐ε model. The three‐dimensional mesh of the billet is generated by Gambit software, and Fluent software is used for the solution of equations. In various zones, different standard boundary conditions are applied. Enhanced wall treatment is used for the turbulence near the wall. In the mould region, Savage and Prichard expression for heat flux is applied. In the spray cooling zone, the heat transfer coefficient for surface cooling of the billet is calculated by knowing the water flow rate and the nozzle configuration of the plant. The model predicts the velocities in the molten pool of a billet, the temperature in the entire volume of billet, the heat transfer coefficient in the mould region, the heat flux in the cooling zone and radiation cooling zone, and the shell thickness at various zones. The model forecasts that the billet surface temperature up to the cutting region is above the austenite‐ferrite transformation temperature (which is accompanied by large volume change). The model predicts a temperature difference of maximum 700 K between the centre and surface of the billet. The entire solidification takes place at 11.0 m length at 3.0 m/min. For the same casting arrangement, increasing the casting speed up to 4.0 m/min has been explored. Based on the simulation results, recommendations to alter the spray water flow rate and spray nozzle diameter are presented to avoid a sudden change of temperature.