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 and prevention of microcracking phenomenon occurring during strip casting of an AISI 304 stainless steel

This study was concerned with the effects of microstructural parameters on the microcracking phenomenon occurring during strip casting of an AISI 304 stainless steel. Detailed microstructural analyses of the microcracked regions showed that microcracks were formed mainly along tortoise-shell-shaped depressions and that their number and size were considerably reduced when strip casting was done right after a shot-blasting or pickling treatment of the casting roll surface. This microcracking phenomenon was closely related to the formation of a black oxide layer, which was mainly composed of manganese-rich oxides, on the roll surface. The black oxide layer acted as a barrier of thermal transfer between the rolls and melt, led to an increased gas gap and inhomogeneous solidification of cast strips, and, thus, played a role in forming both tortoise-shell—shaped depressions and microcracks on the strip surface. The installation of brush rolls behind the casting rolls was suggested as a method to prevent microcracks, because the brush rolls could continuously scrape off the black oxide layer affixed on the roll surface during strip casting.     

Formation of Phosphorous Surface Inverse Segregation in Twin-Roll Cast Strips of Low-Carbon Steels

 The formation of phosphorous surface inverse segregation (SIS) in twin-roll cast strips of low-carbon steels was studied. High phosphorous strips were fabricated by using a pilot twin-roll strip caster and a melt/substrate contacting apparatus, respectively. Solidification structures of strips were observed and analyzed, and phosphorus distributions along longitudinal sections of strips were investigated and discussed. The results showed that solidification structures of all strips were columnar grains, either integrated or coarse in the strip made by the melt/substrate contacting apparatus or damaged in some degree in cast strips; and that during cast strip solidification, enrichment of phosphorus occurred between columnar grains, and the average phosphorus concentration near the surface in the strip with 015% of phosphorus was measured to be about 027% which was obviously higher than that in the bulk.     

Analysis and prevention of cracking during strip casting of AISI 304 stainless steel

In this study, a microstructural investigation was conducted on the cracking phenomenon occurring during strip casting of an AISI 304 stainless steel. Detailed microstructural analyses of the cracked regions showed that most of the cracks were deep, sharp, and parallel to the casting direction. They initiated at the tip of dendrites and propagated along the segregated liquid films between primary dendrites, indicating that they were typical solidification cracks. This cracking phenomenon was closely related to the inhomogeneous solidification of cast strips, represented by depressions, i.e., uneven and somewhat concave areas on the strip surface. The depressions, which were unavoidable in flat rolls due to the presence of a gas gap between the roll and the cast strip, were finely and evenly distributed over the cast strip surface by intentionally providing homogeneous roughness on the roll surface; then, the number and size of cracks were considerably reduced. In addition, the nitrogen gas atmosphere, which retained high solubility in the melt during cooling and good wettability with the roll surface, was successfully used to prevent cracking, because the thickness of the gas gap was minimized.     

Numerical Simulation of Filling Process During Twin-Roll Strip Casting

The modeling and controlling of flow and solidification of melt metal in the filling process is important for obtaining the optimal pool level and the formation of the solidified metal layer on the surface of twin-rolls during the twin-roll strip casting. The proper delivery system and processing parameters plays a key role to control flow characteristics in the initial filling stage of the twin-roll strip casting process. In this paper, a commercial CFD software was employed to simulate the transient fluid flow, heat transfer, and solidifications behaviors during the pouring stage of twin-roll strip casting process using different delivery systems. A 3D model was set up to solve the coupled set of governing differential equations for mass, momentum, and energy balance. The transient free-surface problem was treated with the volume of fluid approach, a k–? turbulence model was employed to handle the turbulence effect and an enthalpy method was used to predict phase change during solidification. The predicted results showed that a wedge-shaped delivery system might have a beneficial impact on the distribution of molten steel and solidification. The predicted surface profile agreed well with the measured values in water model.     

M2高速钢铸带组织特征及其后续处理

研究了M2高速钢在不同制备条件下的凝固组织特征以及工业铸带中碳化物在高温热处理、热变形作用下的变化,测量了在不同制备条件下高速钢的凝固速度和共晶碳化物网的厚度,采用透射电镜研究了后续高温热处理、热变形对工业铸带中碳化物相的影响,采用定量金相法分析了制备条件和后续处理工艺对铸带组织的影响.研究结果表明,双辊薄带连铸工艺可以细化高速钢凝固组织的枝晶和共晶碳化物网的厚度,改善碳化物的分布,后续高温热处理和热变形可以进一步优化工业铸带中的碳化物组织.建议在工业铸带的后续处理中同时采用高温热处理和热变形工艺以改善铸带组织.     

The microstructures of strip-cast low-carbon steels and their response to thermal processing

The as-cast microstructure and its modification when subjected to heat treatment is examined for strip-cast low carbon steels. The local solidification rate in the twin-roll strip casting process is estimated to. be 590 to 850 °C/s, and the primary and secondary dendrite arm spacings are approximately 17 to 25 and 10 μm, respectively. The as-cast structure is predominantly Widmanstätten ferrite and, thereby, differs from the conventional hot-rolled sheet. It is suggested that the as-cast morphology is a result of the large initial austenite grain size and the cooling rate and is not a unique characteristic of rapid solidification of strip casting. By restricting the austenite grain size and cooling rate, polygonal ferrite morphology probably can be produced during strip casting. The response to heat treatment depends on the presence of aluminum; with a moderate amount of aluminum, the A1N precipitates in the as-cast structure inhibit the subsequent grain boundary movement and may affect the subsequent recrystallization behavior.     

Metallographic observations of stainless steel sheets produced by twin-roll type strip caster

Microstructures and textures of austenitic stainless steel sheets produced by a twin-roll type strip caster are investigated at various stages of casting, cold-rolling and annealing processes. Strip casting is a rapid solidification process and, therefore, the cast strip sheets have fine columnar dendrites and very sharp (100) textures. After cold-rolling and annealing the strip sheets have equiaxial and somewhat finer grains than those produced by the conventional process, and the texture is composed mainly of the (110)[1?2] orientation.     

双辊薄带连铸0Cr18Ni9不锈钢的直接冷轧研究

为了改善双辊薄带连铸技术得到的0Cr18Ni9不锈钢薄带的表面质量和综合力学性能,对0Cr18Ni9不锈钢薄带进行了直接冷轧的研究.结果表明,不锈钢薄带经直接冷轧后,组织和性能都有明显的改善,各向异性不明显,耐腐蚀性能大大提高,从而论证了0Cr18Ni9不锈钢薄带直接冷轧的可行性.     

Simple model for thermal calculation in twin-roll strip casting process

The numerical model for the solidification process of metal in the twin-roll type strip caster is presented in this paper. In the model the temperature change as well as the eliminating process of latent heat is calculated along the stream lines obtained by a comparatively simple velocity field. The calculations are carried out on the strip casting of type 304 stainless steel under various casting parameters. The results so obtained show a good fit to the experimental ones, and suggest that the control of the height of molten pool is of importance to the establishment of the optimal operation.     

双辊铸轧薄带钢铸轧力计算公式及控制模型

在经典轧制理论和现代连铸理论相结合的基础上,推导出了铸轧力计算公式,并建立了一个非线性状态空间模型,描述双辊结晶器内的金属凝固和塑性变形.此模型可以作为双辊铸轧薄带钢过程铸轧力的实时在线控制模型.     

Modelling of heat flow and interdendritic crack formation in twin-roll strip casting of aluminium alloys

Preliminary mathematical analyses of different interdendritic cracks associated with variation of heat transfer and generation of interdendritic strain in horizontal twin-roll strip casting have been investigated. A 1-D transient finite difference model of heat flow, dendritic solidification and interdendritic thermo-metallurgical strain has been developed. The model contains two cracking criteria to predict qualitatively and quantitatively the tendency of interdendritic crack formation during dendritic solidification of pure aluminium and 6022 aluminium alloy. The model predictions are compared to available analytical methods and previous measurements. This is to verify and calibrate the model where good and reasonable agreements are obtained, respectively. The variations of heat transfer modes during different contact cooling zones and their effects on the generation of interdendritic thermo-metallurgical strain at the surface and central strip locations have been analysed. The model predictions point out that the different contact cooling zones of strip surface and surroundings control the stages of interdendritic crack formation in different mushy regions. The mechanism of interdendritic crack formation in twin-roll strip casting process with previous and present cracking criteria have been explained and discussed. These discussions show the importance of selection of mathematical treatment to predict the stages of interdendritic crack formation.     

The movement of the concave casting surface during mushy-type solidification and its effect on the heat-transfer coefficient

The air gap formation process at the casting/mold interface of a hollow cylinder casting was investigated for alloys solidifying in a mushy type by measuring the displacements of the casting and the mold surfaces during solidification. The formation process of the air gap between the convex casting surface and the outer mold and the heat-transfer coefficient through the gap have been well documented by previous publications. However, the air gap between the concave casting surface and the inner mold, or the core, was found to form differently during mushy solidification, in which the air gap formed during solidification, reached a maximum gap distance, and then decreased due to the contraction of the solidified casting on the expanding inner mold. The gap formation was caused by an inward collapse of the coherent dendrite networks at the concave interface because of low pressure inside of the casting due to solidification shrinkage. The coherent dendrite networks at the convex interface did not collapse inward. The heat-transfer coefficients estimated by measuring the air gap thickness showed a similar tendency to the calculated values obtained by the inverse heat-conduction analysis.     

双辊连铸不锈钢薄带凝固组织特点

 通过金相观察分析了同径双辊薄带连铸机上生产的奥氏体不锈钢薄带的凝固组织,结果表明：铸带凝固组织包括2个柱状晶区和1个等轴晶区,其等轴晶呈近球形或蔷薇形。与传统连铸板坯相比,其柱状晶区一次及二次枝晶的间距较小,等轴晶粒内部为非枝晶结构,其尺寸大约是连铸坯等轴晶的1/10,凝固组织更致密。     

A numerical and experimental study of the solidification rate in a twin-belt caster

A numerical and experimental study was carried out to investigate the solidification process in a twin-belt (Hazelett) caster. The numerical model considers a generalized energy equation that is valid for the solid, liquid, and mushy zones in the cast. Ak-ε turbulence model is used to calculate the turbulent viscosity in the melt pool. The process variables considered are the belt speed, strip thickness, nozzle width, and heat removal rates at the belt-cast interface. From the computed flow and temperature fields, the local cooling rates in the cast and trajectories of inclusions were computed. The cooling rate calculations were used to predict the dendrite arm spacing in the cast. The inclusion trajectories agree with earlier findings on the distribution of inclusion particles for near horizontally cast surfaces. This article also reports the results of an experimental study of the measurement of heat flux values at the belt-cast interface during the solidification of steel and aluminum on a water-cooled surface. High heat fluxes encountered during the solidification process warranted the use of a custom-made heat flux gage. The heat flux data for the belt surface were used as a boundary condition for the numerical model. Objectives of the measurements also included obtaining an estimate of the heat-transfer coefficient distribution at the water-cooled side of the caster belt. Y.G. KIM, formerly Graduate Student, Materials Engineering Department, Drexel University.     

304不锈钢2mm连铸薄带中的裂纹分布和形成分析

用扫描电镜(SEM)观察和分析了双辊连铸法生产的304不锈钢1 200 mm×2 mm铸带表面和内部裂纹的形貌和形成。由于采用导热性好的铜制结晶辊,薄带表面和内部产生了长140 mm、深0.33 mm的裂纹,薄带断面有分层现象。试验结果表明,铜制结晶辊使钢液的冷却速度达到(1~4)×10³K/s,薄带接触辊面温度低,凝固区的温度梯度高达10⁵K/m,温差产生的内应力和应力集中导致裂纹产生;MnS、MnO、Cr₂O₃等夹杂物是裂纹源。     

双辊铸轧凝固组织的数值模拟

对双辊板带铸轧的凝固过程中金属的形核和生长行为进行了数值模拟,建立了一套描述双辊板带凝固组织参数的表征体系,从而为双辊铸轧板带凝固组织特征的定量数学描述奠定了理论基础.同时为该领域的试验研究提供了理论基础.     

Casting of High-Aluminum-Content Mg Alloys Strip by a Horizontal Twin-Roll Caster

This study was aimed to investigate casting of high-aluminum-content Mg alloys strip by a horizontal twin-roll caster. A horizontal-type twin-roll caster was equipped with a nozzle. This nozzle was movable. The roll size was φ300 × W150, and copper rolls were used. The rolling road was very small. It was possible to cast AZ91D and AZ121 magnesium alloys continuously by a horizontal twin-roll caster. There was gloss and no crack. The thickness of as-cast strip of AZ91D was 4.5 mm and that of AZ121 was 4.6 mm, respectively. In the case that roll velocity was 48 m/min, the thickness of as-cast strip of AZ121 was 2.0 mm. A 2.0-mm-thick strip was able to coil, and the diameter was φ500 mm. The microstructures of the as-cast strip of AZ91D and AZ121 magnesium alloys were observed using light optical microscopy. Isometric dendrite crystals were observed at the as-cast strip. The as-cast strip without facing of AZ91D and AZ121 magnesium alloys were able to hot rolling of 75 pct reduction. The surface of the as-rolled sheet was flat and glossy. The tensile strength of the as rolled was 230 MPa and the elongation of as rolled was 4 pct.     

双辊铸轧铝合金中分流块对温度场影响的数值模拟研究

文章采用有限元方法,在大型通用有限元分析软件ANSYS中建立双辊轧制模拟的三维模型。重点研究铸嘴型腔中不同分流块的数目及分布对温度场、出口温度和出口速度的影响,以及对不同工艺条件下凝固行为的分析。研究结果表明,铝熔体在单个分流块的作用下分布更加均衡,避免了宽度方向上温差过大以及流动过程中出现较大的波动,其中板带表面和心部的出口温度、出口速度均匀性对于提高铸轧铝合金板带的质量有很大影响。实验结果为初步的,为提高铸轧板带质量提供理论和技术支撑。     

双辊薄带连铸生产取向硅钢的技术分析

 双辊薄带连铸工艺具有流程短、单位投资低、能耗低、劳动生产率高等特点,而取向硅钢被认为是双辊法连铸工艺中最具有发展前途的钢种之一。针对此介绍了双辊薄带连铸技术的工作原理及其研究现状,总结了目前国外双辊薄带连铸生产取向硅钢相关方面的专利,并分析了双辊薄带连铸生产取向硅钢与传统工艺在组织、织构以及抑制剂控制方面的差异性,指出该流程在生产取向硅钢方面具有独特的优势,其工业化应用将会对取向硅钢的生产和发展起到革命性的推动作用。