Investigation of the solute transportation coupled with heat transfer and fluid flow during twin-roll strip casting process

Mathematical model of solute [C] distribution in twin-roll strip casting process has been setup successfully with Calcosoft for the first time. Simulation result shows that in the center of the molten steel pool between the two rolls there is a vortex flow, which is a solute enriched area. But the highest solute concentration position is at the solidification front of the columnar grain zone near the cooling roll surface. Another solute enriched position is in the back flow above the nip point. Combined wit...     

Finite-element analysis of a vertical twin-roll casting

Metallic strips can be produced by twin-roll casting, in which the melt is cooled, solidified, and rolled to a specific thickness. In the present study, the rigid-thermoviscoplastic finite-element method was applied to the analysis of complex phenomena including melt flow, heat transfer, solidification, and plastic deformation occurring in a vertical twin-roll casting of magnesium alloy AZ31. The melt was found to be confined in rotational motions of two symmetric vortexes developed at the roll entrance, and thus only the melt near the nozzle wall became solidified and rolled to a sheet. As the nozzle thickness increased, the vortex increased in size resulting in more bifurcation and instability, which are definitely adverse to material properties of the strip. The maximum cooling rate of 600 °C/s was found at the centerline as solidification took place at the roll exit. Other findings are also discussed including roll force, roll torque, and pressure distribution, which were greatly dependent upon the plastic deformation after solidification.     

铸轧镁合金薄带组织演变实验研究与模拟

以镁合金薄带双辊铸轧新技术为背景,开展了铸轧成形过程中组织性能的模拟研究。镁合金薄带双辊铸轧是以液态镁合金为原料,用两个反向转动的水冷铸辊为结晶器,直接制备1．5～3．0mm镁合金薄带的短流程新工艺。对双辊铸轧轻合金薄带的凝固组织演化过程进行数值模拟,用以优化和指导制备技术,具有重要的理论意义和应用前景。项目研究组通过对双辊铸轧过程动态凝固成形的有限元数值模拟研究、镁合金薄带铸轧实验研究以及后续加工过程中的组织演变规律研究,得到了主要工艺参数对铸轧工艺稳定性及组织性能的影响规律。     

双辊薄带连铸过程数值模拟

针对双辊薄带连铸过程,使用了耦合湍流流动和凝固传热的三维数学模型,模拟计算了使用楔型布流系统浇注时,连铸熔池内钢水的流动形态和凝固情况。结果表明,楔型布流系统可使钢水在熔池内合理分布,有利于均匀凝固。     

双辊快速凝固AZ31镁合金薄带试验研究

在等径双辊连铸机上进行了AZ31镁合金薄带连铸的试验研究，掌握了诸如浇铸温度、铸辊转速、辊缝和侧封等合理的工艺参数，成功地铸出了AZ31镁合金薄带并对其显微组织进行了分析。     

Effect of casting parameters and deformation on microstructure evolution of twin-roll casting magnesium alloy AZ31

Twin roll casting method is a promising route to directly produce magnesium alloy strip. It is a rapid solidification process with high temperature gradient combined with thermal flow and roiling deformation in the casting region. As-cast strip with proper mierostructure is requested to serve as next rolling feedstock. However the microstructure of as-cast strip is sensitive for casting conditions during the casting process and the as-cast microstructure greatly affects the mechanical properties. In this work, the effect of casting speed, pouring temperature, deformation as well as anneal process on microstructure and mechanical properties were investigated. The results revels that twin-roll casting process can effectively refine the grain size, improve the morphology and distribution states of Mg1TAl12. The homogenization treatment time can be shorted for the fine microstructure and lower the cost dramatically for the next forming process.     

3D Numerical Simulation on Thermal Flow Coupling Field of Stainless Steel During Twin-Roll Casting

The surface crack and lateral crack of the AISI 304 stainless steel thin strip produced by twin-roll casting were observed. The temperature at the center of outlet during twin-roll-casting process was determined by infrared thermometer. In order to avoid the surface cracks of the casting strip, the thermal flow coupling field of AISI 304 stainless steel during twin-roll-casting process was simulated by a 3D fluid-structure coupling model. According to the simulation result, the effect of the casting speed on thermal flow field was analyzed and the process parameters were optimized. Moreover, by studying heat flux curves, the heat transfer mechanism between molten pool and roll was analyzed. The results show that, with the increase of the casting speed, the temperature of the molten pool increases and the solidification point moves toward the outlet. Meanwhile, the whirlpool above gets larger. Based on the solidification front position, the optimized process parameters are 1500 °C and 0.37 m/s. The heat transfer mechanism between molten pool and roll contains direct contacting heat transfer and air gap heat transfer.     

Development of satellite mill and trial rolling of profiled metal strip

A new type of compact continuous mill called the “Satellite mill” has been developed to produce profiled metal strip from flat strip. This mill consists of one large-diameter driven roll (central roll) and five smaller rolls (satellite rolls) arranged along the periphery of the central roll. A strip is continuously rolled at five gaps between the central roll and the satellite rolls. All rolls are driven at an equal roll peripheral speed to promote transverse metal flow or lateral spread. Guide shoes are provided between the adjacent satellite rolls and are supported with two adjacent satellite rolls. They prevent a strip from bulging or buckling. A test mill was constructed, and rolling experiments have been performed on the production of T-shaped or U-shaped profiled aluminum strip. The deformation- load characteristics were investigated and compared with those of conventional rolling. As a result, in the satellite mill rolling operation, elongation was suppressed. Lateral spread was enhanced, and the profile of the product cross section was significantly improved. Consequently, the new rolling process was found to be forming of profiled metal strip.     

Development and application of coupling model of aluminum thin-gauge high-speed casting

Based on the analyses of aluminum melt flow, solidification, heat transfer during the process of twin-roll casting, a coupling mathematical model of aluminum thin-gauge high-speed casting was developed, which included the casting roller shell. At the same time, Galerkin method was adopted to solve the coupling model. The fluid field and temperature field of aluminum melt in casting zone, the temperature field and thermal stress field of roller shells were simulated by the coupling model. When the casting velocity is 7 m/min, and the thickness of strip is 2 mm, the circumfluent area comes into being in the casting zone, and the mushy zone dominates the casting zone, while the temperature of melt decreases rapidly as it approaches the rollers. The temperature of the roller shell varies periodically with the rotation of roller, and reaches the highest temperature in the casting zone, while the temperature of roller shell decreases gradually as it leaves the casting zone. The difference of thermal stress between the inner surface and outer surface of the roller shell is very large, and the outer surface suffers tensile-compressive stress.     

铝、镁合金进气歧管倾转铸造的特点

采用AnyCasting铸造模拟软件,分别模拟了金属型倾转铸造铝合金和镁合金进气歧管工艺,并对其流动模式和释放热量进行了比较。镁合金熔液在整个充型过程中完全属于层流流动,铝合金熔液也属于层流流动。模拟结果显示铝合金与镁合金进气歧管的冷却曲线变化趋势基本一致,但镁合金熔液比铝合金熔液冷却时间更长。     

锡磷青铜薄板坯水平电磁连铸技术的优化研究

对锡磷青铜薄板坯水平连铸的行波电磁搅拌技术进行了优化研究,提出了在搅拌器的端部放置硅钢片、改变搅拌器的长度以及减小搅拌器的端部电流强度等3种方法,来优化连铸薄板坯端部的流动和凝固过程,并采用数值模拟和工厂实验相结合的方法对其进行了分析。结果表明:相比其他2种方法,通过在行波搅拌器的端部放置硅钢片更有利于改善熔体端部的流动状况,当施加硅钢片的长度为150 mm时,熔体端部的强环流消失,连铸坯端部的凝固坯生长更加均匀,从而有利于防止连铸薄板坯边部的裂纹和偏析缺陷产生。     

3004铝合金铸轧带坯生产实践

生产易拉罐料用的3004铝合金带坯,大多数是用热轧法生产的,用铸轧法生产的难度较大,但是热轧法的加工费是铸轧法的两倍多,因此开发铸轧法生产3004铝合金带坯的技术,具有很大的经济效益和节能效益。采用Φ1003mm×1 900 mm国产双驱动倾斜式铸轧机试生产3004铝合金带坯,通过试验性生产,摸索出了3004铝合金带坯的铸轧生产技术和工艺参数,并稳定批量地给一家生产易拉罐料的铝加工厂供给合格的3004铝合金铸轧带坯。     

镁合金铸轧凝固层焊合点控制及工艺优化

结合镁合金铸轧工艺特点,分析镁合金板坯铸轧过程中凝固层焊合点位置与板坯缺陷的影响规律;针对板坯厚度、铸轧区长度及铸轧速度等关键参数,简化凝壳径向生长及凝固前沿周向转动过程,确定铸轧速度匹配范围,建立凝固层焊合点位置控制模型,并通过工艺试验对控制模型进行验证.结果表明通过理论模型确定工艺匹配范围,可稳定并优化镁合金铸轧工艺,大幅度降低铸轧板坯宏观缺陷,获得表面光洁、质量良好的镁合金铸轧板坯.     

压铸镁合金缺陷带的组织特征及形成机理

研究了压铸镁合金缺陷带的组织形貌及分布特征,建立缺陷带与压室预结晶(ESCs)、气缩孔及压铸工艺参数之间的对应关系,在此基础上探讨了缺陷带的形成及演化机理。结果表明,压铸镁合金截面凝固组织以缺陷带为界可划分为3个有明显组织特征差异的区域。随着压铸低速速度提高,镁合金凝固组织中ESCs含量逐渐降低,而缺陷带组织愈加明显并向压铸件中心靠拢,缺陷带宽度减小,其内部孔洞更加聚集;高速速度越大,压铸镁合金凝固组织中ESCs越分散,缺陷带向压铸件中心靠拢,压铸在无高速速度情况下,镁合金凝固组织横截面出现双缺陷带现象。压铸充型过程中金属液流的形态决定了缺陷带的分布位置和发展趋势,在高速金属液的剧烈冲刷及增压压力的作用下,靠近金属液流外轮廓的晶粒发生破碎或转动,在晶粒间形成大于剩余金属液体积的间隙,随着凝固的进行,形成沿液流轮廓分布孔洞聚集的缺陷带组织。     

Thin strip quality obtained in twin-roll casting

Ferrous alloy strips of width 150 mm and thickness 150–300 μm were obtained by rapidly cooling from the molten state at ～ 105 Ks^?1 with the pressing force of the rolls during casting 1.5 kN mm^?1 and a rotational speed of 8 ms^?1. Typical composition of the strip was Fe—4.5%Si, Fe—23%Cr—5%Al or Fe—12%Al. The roller wear mechanism and strip quality were investigated using two sets of cylindrical, water-cooled, bronze roll moulds. It was found that the width of the strip obtained by rapid quenching in rolls is determined by the consumption of metal and depends on fluctuations in the molten bath. Strip thickness increases with bath depth. The lenticular form of the strip is obtained by bending of the rolls, and the biconcave form, using a thick water-cooled roll sleeve, is a result of the thermal expansion of the roll.

Roller wear and changes to the strip microrelief were investigated using different alloys. The roughness of the surfaces of both strip and roller increased throughout the casting process. The amount of increase depended more on the alloy than on the melt temperature. The average values of the angles of the slopes of micro-peaks were measured and compared and decreased along the strip. Comparison of the coefficient of anisotropy across the width of the strip showed a decrease from the edge to the middle of the strip. The apparent reason for the appearance, on local areas of the strip surface, of oxide pellicles is revealed.     

电磁半连续复合铸造轧辊的思路与实践

为了实现复合轧辊的顺序凝固，研制了电磁半连续复合铸造试验装置，利用该装置进行了一系列电磁半连续复合铸造试验，探索出了一套合理的电磁复合铸造工艺，制造了高铬铸铁／45#钢复合轧辊，并进行了装机试验。结果表明：电磁半连续复合铸造轧辊思路正确，装置简单合理，既实现了轧辊在复合过程中的顺序凝固，减少了铸造缺陷，又使复合界面达到良好结合，提高了轧辊质量和生产效率。     

Twin-roll strip casting of magnesium alloys in China

The development status of twin-roll strip casting for magnesium alloys in China was summarized as well as the new progress when several kinds of twin-roll strip casting technologies were developed and used.Horizontal twin-roll casting (HTRC) of magnesium alloys has attracted much attention and has been industrialized in China.Vertical twin roll casting(VTRC) of the magnesium alloys can reach a speed of higher than 30 m/min and its research and development are just beginning and exhibit exciting potential...     

Microstructure and solidification thermal parameters in thin strip continuous casting of a stainless steel

The present work focuses on the relationships between solidification thermal parameters and the dendritic microstructure of an AISI 304 stainless steel solidified both in a strip casting pilot equipment (twin-roll) and in a directional solidification simulator. Experimental studies were conducted with a stainless steel strip casting obtained in a twin-roll continuous caster pilot equipment and in samples solidified in a directional solidification simulator with two different melt superheats. In both cases, the surface of the substrates was similar, with mean surface roughness of about 0.3 μm. After solidification, the specimens were cut at different positions from the metal/mold interface and etched for metallographic examination. An empirical equation from the literature relating secondary dendrite arm spacing and cooling rates was used to demonstrate the similarity of the cooling efficiency. The results have shown that the simulator can be used in the determination of transient metal/mold interface coefficients (h_i) and in the preprogramming of the strip casting operational conditions as a function of roll materials and surface roughness.     

A coupled model on fluid flow,heat transfer and solidification in continuous casting mold

Fluid flow, heat transfer and solidification of steel in the mold are so complex but crucial, determining the surface quality of the continuous casting slab. In the current study, a 2D numerical model was established by Fluent software to simulate the fluid flow, heat transfer and solidification of the steel in the mold. The VOF model and k-ε model were applied to simulate the flow field of the three phases(steel, slag and air), and solidification model was used to simulate the solidification process. The phenomena at the meniscus were also explored through interfacial tension between the liquid steel and slag as well as the mold oscillation. The model included a 20 mm thick mold to clarify the heat transfer and the temperature distribution of the mold. The simulation results show that the liquid steel flows as upper backflow and lower backflow in the mold, and that a small circulation forms at the meniscus. The liquid slag flows away from the corner at the meniscus or infiltrates into the gap between the mold and the shell with the mold oscillating at the negative strip stage or at the positive strip stage. The simulated pitch and the depth of oscillation marks approximate to the theoretical pitch and measured depth on the slab.     

双辊连续铸轧机铸轧辊铜辊套(2)

本篇是全文的第二部分,对钢-钢、铜-钢(通常上辊辊套为铜合金的、下辊辊套为钢的)、铜-铜三种组合辊套铸轧机铸轧铝合金带坯,其铸轧速度及生产能力进行了对比;对分别采用三种组合辊套生产的1050、8011、3003铝合金带坯的组织进行了对比;介绍了用钢-铜组合辊套铸轧的8006、8011铝合金带坯生产的箔材的组织和性能。显示出铜合金辊套具有很多的优越性。还介绍了铜辊套铸轧机生产铝合金带坯的典型生产工艺。