Modelling and simulation of twin-roll casting of bulk metallic glasses

As an economic and direct route to continuous thin strip production from the melt, twin roll casting (TRC) has been established as an effective process for aluminium alloys. Its adaptation to casting of bulk amorphous alloy strip necessitates matching of the thermal and mechanical behaviour of the cooling multi-component melt to the requirements (especially cooling rate, and strip exit temperature and thermal gradient) of vitrification. Using a dedicated control volume numerical model of TRC, simulation of the casting of 2 mm thick Vit 1 (Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5) alloy strip shows that the acceptable casting speeds are in the range 2.5 to 3.5 cm/s. The effects of varying strip thickness and strip-roll heat transfer coefficient (HTC) on this casting window are assessed. The differences between modelling of conventional alloy solidification and metallic glass formation are presented.     

Steel strips in the context of near net shape casting production

Following the replacement of ingots by continuously cast slabs the next logical step towards near net shape casting has been the development of thin slab casting machines. These are now beginning to match the quality and productivity levels possible with conventional machines. The next step in this evolution towards lower cost operation is high speed thin strip casting machines. The merits of twin roll casters versus belt casting machines for friction free casting of low carbon steels, suggest that a belt caster will be needed for achieving high productivities and associated cooling lengths required for the production of hot strip in the 5–20 mm thickness range.     

板坯连铸二冷动态配水的研究与应用

以津西钢铁集团股份有限公司2~#板坯连铸机为研究对象,建立凝固传热数学模型,分析了连铸坯的温度场及其坯壳厚度以及二冷各段的回温,同时通过凝固坯壳射钉测厚对模型进行校正。采用离线计算在线控制的二冷动态配水模式,在传热模型的基础上通过优化建立拉速、过热度和冷却水量的关系。经过实际应用验证了配水方案的有效性。     

超薄快速铸轧过程轧制压力分布的数学模型与实验研究(Ⅲ)——铸造区与轧制区轧制压力仿真研究

在建立了铸造区，轧制区轧制压力模型的基础上，对常规铸轧条件进行了仿真计算，计算结果与实测数据相吻合，从而验证了模型的正确性，在此基础上，对超薄快速铸轧条件进行了虚拟仿真研究，研究结果表明：不增强铸轧辊内，外冷却能力的条件下，而仅仅减薄铸轧 板坯厚度即可提高铸轧速度，在其它工艺参数保持不变，且保持出口温度与常规铸轧时基本相近的出口温度时，铸轧 厚度 薄时轧制压力比常规铸轧高，随着铸轧速度与铸轧区增大，铸轧坯厚度减薄，铸轧 辊径增大，轧制压力峰值增大。     

Maximum Copper and Tin Contents in LC‐steel Thin Strip ‐ Hot Shortness Model Calculations

For conventional casting processes low copper and tin contents have to be ensured in LC‐steel to avoid hot shortness. It is expected that higher cooling rates, e.g. in thin strip casting, permit higher copper and tin limits. Hot shortness occurs because of selective oxidation of the iron whereby the more noble copper is enriched at the steel‐oxide interface. A liquid metallic copper phase which wets the grain boundaries supports cracking during hot deformation. The enrichment of the liquid copper phase depends on the oxidation temperature: At low temperatures copper is solid, cannot wet the steel surface and is incorporated into the growing oxide layer. At mid temperatures (1083‐1177 °C) the copper phase is liquid, wets the grain boundaries of the steel surface and causes hot shortness. At high temperatures a liquid fayalitic slag is formed in the oxide layer if the steel contains silicon. The fayalitic phase occludes parts of the steel surface and removes copper from the steel surface; then hot shortness is reduced or even avoided. Other mechanisms to remove copper from the steel surface need the presence of Fe₃O₄ and Fe₂O₃ in the oxide layer. These iron oxides are not formed for short oxidation times where linear oxidation takes place. Diffusion of copper into the steel is too slow to reduce hot shortness if copper has an elevated concentration in the steel, e.g. 0.5 wt.‐%. Therefore, only the occlusion mechanism is of importance during linear oxidation. A model is established on the basis of these observations in order to predict an upper copper limit in dependence of the steel strip thickness (cooling behaviour) and the oxygen content in the cooling atmosphere (nitrogen‐oxygen mixture). The model is compared to experimental results from KIMAB which are presented in this issue. It is demonstrated that a copper layer thickness of 0.098 μm at the steel‐oxide interface is sufficient to cause cracks of a depth of more than 0.2 mm. For strip thicknesses below 5 mm a simple approximation can be used to predict the maximum copper content in LC‐steel to avoid hot shortness. For example, thin strip of a thickness of 2 mm will have no cracks (above 0.2 mm) even if 0.7 wt.‐% of copper is contained in the LC‐steel. For atmospheres with a reduced oxygen partial pressure even higher copper contents are possible. Tin is with short oxidation times not a problem concerning hot shortness, as shown by the KIMAB results. This may be explained by the much higher diffusivity of tin in iron compared to copper.     

A transient simulation and dynamic spray cooling control model for continuous steel casting

A two-dimensional heat-transfer model for transient simulation and control of a continuous steel slab caster is presented. Slab temperature and solidification are computed by the model as a function of time-varying casting speed, secondary spray cooling water flow rates and temperature, slab thickness, steel chemistry, and pouring and ambient temperatures. Typically, the solidification path, temperature-solid fraction relationship, is prescribed. However, if these data are not available, a microsegregation solidification model that approximates the effects of steel chemistry and cooling rate is incorporated in the caster model. Measured slab surface temperatures recorded from an operating caster are compared with predictions from the transient model. These demonstrate that the model typically can predict the temperature response at the slab surface within 30 °C. Results of several simulations are given to demonstrate the effects of changing casting conditions on the slab thermal profile, end of liquid pool, and solidification end point. A control methodology and algorithm suitable for online control of a continuous casting machine is described, and the ability to control the surface temperature profile by dynamically adjusting secondary spray cooling flow rates is demonstrated by simulation. Results from a preliminary version of the model that is capable of running in real time are presented and are compared with the slower, but more realistic, version of the model.     

Thin-strip casting of steel with a twin-roll caster – discussion of product defects of ≍ 1 mm-Fe6%Si-strips

After a former detailed study of the thin strip casting process now defects of the strip and their avoidance are discussed. Hence it is deduced that the casting process should be carried out with small rolling forces. Thus the process of strip cooling has to be separated from the process of strip forming, and a cooling device has to be established close to the casting rolls.     

Influence of cooling method on temperatures and solidification during the continuous casting of metal strip

The first part of the present paper explains that the cooling methods of the individual casting processes such as link-type moulds with purely capacitive cooling (e.g. caster-aluminium casting), with spray water cooling involving the Newtonian cooling characteristic (e.g. continuous casting) and also the combination of both types (e.g. Hazelett belt) can be described as borderline cases of a uniform model. In the second part the temperature distributions in the belt mould (for instance, for evaluating the mechanical strength of the belt) and in the solidified material are discussed in greater detail so that belt mould inside and outside surface temperatures can be detected. The third part of this paper shows the influence of various cooling conditions (e.g. different heat-transfer coefficient) and the influence of an insulating layer between belt mould and thin slab on the temperature profiles and on the increase of the solidification thickness as a function of the casting path. In the fourth and last part, the dimensional equations of this paper are made dimensionless with the objective of having the possibility of a comprehensive representation.     

A mathematical model of the splat cooling process using the piston and anvil technique

A mathematical model has been developed to represent the spreading of the slug and its subsequent solidification for the piston and anvil splat cooling technique. In the statement of the problem allowance has been made for fluid flow effects regarding the spreading, convective heat flow and the actual movement of the piston after the impact. In representing heat flow allowance has been made for convection for the two dimensional transient temperature fields and the existence of a mushy zone on solidification. The computed results gave predictions regarding the effect of the piston velocity and of the substrate-splat heat transfer coefficient on both the final splat thickness and on the cooling rates. It was found that the higher the piston velocity and the higher the splat-substrate heat transfer coefficient, the faster are the attainable cooling rates, although there is a tendency to approach limiting values. A critical comparison of the present work with previous modeling efforts shows marked spatial variations in the cooling rates so that the previously used one dimensional models are not likely to be satisfactory. Furthermore, the analysis of the fluid flow phenomena given in the present paper allows a realistic assessment of the final splat thickness, which was not available up to the present.     

带钢生产新工艺

介绍了４种带钢生产的新工艺,即薄板连铸连轧工艺、薄带连铸工艺、连铸冷轧工艺、金属喷镀工艺,对带钢生产工艺的选择及采用有借鉴意义。     

常化炉气雾冷却系统的应用

为了满足高牌号无取向硅钢和高磁感取向硅钢的常化退火工艺要求,设置了常化炉冷却段,通过气雾、空气及喷淋水达到带钢冷却要求。其中气雾冷却段要求最高,故其气雾冷却技术越来越受到关注。目前已投入工业应用的两相流冷却技术主要采用空气雾化水,将雾化后的气体和水滴的混合物直接喷射到带钢表面以较快速率冷却带钢。气雾冷却具有换热系数大、冷却效率高等优点,是一种较为理想的冷却方式。     

带钢超快速冷却条件下的换热过程

 分析了轧后加速冷却过程中带钢表面的局部换热机理，认为冷却系统实现超快速冷却的关键在于扩大带钢表面射流冲击换热区的面积。确定了薄带钢实现超快速冷却所需的对流换热系数，并采用有限元分析工具ANSYS模拟得到了超快速冷却条件下不同厚度带钢的温度场。温度场的分布表明薄带钢在超快速冷却过程中具有较好的温度一致性。同时还表明随着带钢厚度增加，超快速冷却条件下厚度方向的温度梯度显著增大，对于带钢内部组织的均匀性将产生不利的影响。带钢厚度范围应是超快速冷却技术实际应用过程中的重要考虑因素。     

热轧带钢层流冷却控制策略研究

在带钢层流冷却过程中,为实现高品质的冷却效果,针对不同的参数需要采取不同的控制策略。控制的参数主要有:带钢运行速度、冷却区开启阀门数、冷却阀门喷水量、喷水模式等。在实际的生产过程中往往采用设定固定的带钢运行速度值,然后通过控制喷水量的大小来进行冷却控制。针对国内某热轧厂层流冷却实时生产数据进行分析,分析了"速厚积"对冷却效果的影响规律,制定出合适冷却策略,并验证其有效性。     

层流冷却过程中带钢温度场数值模拟

分析了带钢层流冷却过程中的传热，并利用有限元法对层流冷却过程中带钢温度场进行了模拟计算。结果表明：随着轧件厚度的减薄，在带钢厚度方向上的温差逐渐减小；冷却速度不同时，带钢表面温度和中心温度的变化趋势以及波动幅度相应发生变化。在进行模型计算时，应合理考虑带钢厚度及内部热传导的影响。这对提高数学模型的精度，控制卷取温度，提高产品质量以及指导生产具有重要意义。     

Microstructure and precipitation in as cast low carbon Nb–V–Ti microalloyed medium thin slab

Abstract

The solidification structure, austenite and precipitates in a quenched compact strip processing (CSP) medium thin slab (170 mm thick) of Nb–V–Ti microalloyed steel have been studied. It was found that secondary dendrite arm spacing and austenite grain size are slightly larger than that of similar steel produced by CSP thin slab. This is partially attributed to the slower cooling rate caused by the increased slab thickness. On the other hand, the formation of carbonitride during solidification reduces the width of secondary dendrite arm spacing, while TiN particles and alloying elements in solution may inhibit the growth of austenite grain during solidification and subsequent cooling. In addition to the semidendritic, larger cubic and fine cubic precipitates, which can be observed in CSP thin slab, dendritic precipitates were also found in CSP medium thin slab.     

复合铸轧工艺参数对Kiss点的耦合影响

为了获得高质量的层状金属复合材料,依据双辊薄带复合铸轧技术,探究工艺参数间的耦合作用对复合铸轧的影响。采用有限元软件构建了铜和铝的固-液复合铸轧仿真模型,基于该模型,研究了铝板出口厚度、轧制速度、熔池高度、铜带厚度和浇注温度对Kiss点高度的影响,并对正交试验得到的Kiss点高度进行极差分析,得到了铝板出口厚度、轧制速度和铜带厚度3个强耦合参数,给出了这3个参数两两耦合下耦合强度的变化规律。铝板出口厚度和铜带厚度、铜带厚度和轧制速度对Kiss点高度的耦合影响随参数值的增大其效果逐渐减弱,铝板出口厚度和轧制速度对Kiss点高度的耦合影响随参数值的增大其效果逐渐增强。同时,利用复合铸轧熔池物理场的变化,分析了产生耦合的原因。     

Solidification modelling of microstructure in twin-roller thin strip casting of stainless steels

The influence of process parameters on the dendritic microstructure of thin strips cast by the twin-roll method is analyzed in the framework of a one-dimensional solidification model and compared with experimental results. As a relevant characteristic the secondary dendrite arm spacing Λ₂ as a function of the distance x from the roll surface is investigated. The difference between the local dendrite arm spacing near the strip surface and the strip centre, respectively, increases with the strip thickness and only depends on the casting temperature to a small extent. An increase in the strip/roller heat transfer coefficient due to a rising casting velocity or possibly enhanced roll-separating forces leads to a decrease in the dendrite arm spacing. The effect of a sudden decrease in heat transfer during the solidification process, on the Λ₂(x) characteristics, e.g. by a local separation of the solidified shell from the roller surface, is discussed.     

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.     

Investigation of solidification and microsegregation of near-net-shape cast carbon steel

Different near-net-shape casting techniques are investigated in terms of solidification parameters, microstructure, and microsegregation of manganese in carbon steels based on experimental simulation methods: ingot casting of thick and thin slab samples, strip samples, and thin strip. The as-cast thicknesses were between 1.9 and 150 mm. By calculations and measurements data have been determined which lead to the solidification structure like solidification- and cooling-rate, secondary dendrite arm spacings, and the concentrations of microsegregations. Finally, some literature data of material properties of steel strip produced by the investigated methods are given.     

双辊薄带铸轧技术的进展及热点问题评述

美国纽柯公司双辊薄带铸轧生产线2002年投产以来,已经累计生产了近50万t薄带,最薄钢带厚度0.7mm,产品可部分替代冷轧钢板.介绍了纽柯公司双辊薄带铸轧生产线的最新进展,并对一些技术特点和热点问题进行了分析和评述.