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.     

一种有效的骨架图毛刺去除算法及其在带钢裂纹检测中的应用

如何有效去除毛刺是图像处理中的一个经典问题,也是精确计算带钢裂纹长度的基础。传统毛刺去除方法需要人工定义毛刺的长度阈值,这对于同一幅样板图像中细小裂纹和粗大裂纹并存的带钢裂纹检测并不适用。本文利用纹理二值模型,将骨架图中的像素点分为端点、节点和一般骨架点。首先根据毛刺与端点和节点的关系,去除骨架图中的大部分毛刺,然后去除剩余节点的最短分支,最终得到无毛刺的骨架图。实验表明,该方法避免了单一阈值的选取,可有效去除毛刺并得到精确反映目标几何结构的骨架图。     

热轧带钢Q235B边裂缺陷成因及控制

通过对热轧带钢Q235B边裂缺陷进行检验和分析,认为造成带钢边裂的主要原因是快换中包期间连铸拉速陡然变化时,各工艺参数不匹配,铸坯角部过冷,在振痕波谷处产生角部横裂纹,这些横裂纹在后续工序中进一步恶化,最终演变为边裂缺陷.采取优化后的连铸工艺可有效改善板坯角部横裂纹,消除热轧带钢边裂缺陷;同时板坯角部缺陷经修磨后再轧制可避免边裂缺陷产生.     

Meandering Instability of a Vertical Plume

The meandering instability of a two-dimensional vertical plume is considered in this paper. The base flow profile is derived using boundary layer approximation and similarity hypothesis, in the spirit of Blasius boundary layer solution. Using linear stability analysis, we derive a modified version of the Orr-Sommerfeld equation governing the disturbances. The resulting eigenvalue problem is solved by the Galerkin method. It is found that a vertical plume is unstable to meandering, and in particular, three different wavelengths of meanders exist. A meandering instability of long wavelength ensues at the lowest portion of a vertically rising plume, where R<94. Beyond R = 94 and R = 192, the most unstable wavelength shifts to an intermediate and to a short wavelength meandering during the course of rising motion.     

森吉米尔轧机轧件板形与边部减薄预报研究

利用接触元双坐标法（CEM）建立了双AS-U-Roll系统森吉米尔轧机轧制过程中轧件及辊系变形行为的CEM函数矩阵,开发了SM4SM软件。应用SM4SM分析了第一中间辊锥角、锥度长度和窜辊长度对轧件截面形状和轧件边部减薄控制的影响。研究结果对冷轧过程控制轧件板形和边部减薄具有重要的参考价值。     

Enterotoxin-binding glycoproteins in a proteose-peptone fraction of heated bovine milk

The binding of Escherichia coli heat-labile enterotoxin to caseins, whey proteins, milk fat globule membrane, and proteose-peptone fraction from bovine milk was studied by using the Western blot technique. Two toxin-binding glycoproteins, pp16k and pp20k, with molecular weights of 15,500 and 20,000, respectively, were detected only in a proteose-peptone fraction. These glycoproteins were partially purified by ammonium sulfate precipitation and Toyopearl HW 55 gel filtration chromatography. The binding ability to the toxin was destroyed by periodate treatment or beta-galactosidase treatment, indicating that a carbohydrate moiety, particularly a terminal galactosyl residue, was essential for the binding of the toxin. In contrast, the binding ability was not changed by mild acid treatment, and these glycoproteins did not bind cholera toxin, which can bind to ganglioside GM1, suggesting that the carbohydrate structure of the glycoproteins is different from that of GM1. The N-terminal amino acid sequence and immunoblot analysis indicated that the protein moieties of pp16k and pp20k are identical to alpha-lactalbumin and beta-lactoglobulin, respectively. These toxin-binding glycoproteins were not detected in whey proteins isolated from unheated skim milk, suggesting that they are newly generated during heat treatment of skim milk before the preparation of a proteose-peptone fraction.     

Fluid flow in a propagating crack

The fluid flow in a sharp, propagating crack is shown to have a large pressure drop near the tip for finite crack velocities. An analysis of the effect of liquid surface tension is made to determine the possibility of cavitation of the liquid. The analysis indicates that cavitation is possible for certain liquid metal embrittlement systems. Other systems apparently cannot support cavitation, but the fluid flow will still be important since it influences mass transport within the propagating crack.     

Design of a high-precision twin-roll strip caster

During strip casting,the thickness deviation of casting strip directly affects the quality of the hot-rolled sheet because the casting strip needs to be rolled only once. To accurately control the gap between casting rolls and reduce the thickness deviation of the casting strip,a conceptual design of a prestressed twin-roll caster is proposed herein; this caster involves the setting of a constant pressure balancing cylinder between the chocks of two casting rolls. The balancing cylinder hydraulic system adopts a three-way servo valve that controls the hydraulic cylinder.The parameters of the hydraulic power components are determined using the principle of optimal power matching,and the hydraulic system model of the balancing cylinder is thus established. The dynamic analysis and simulation of the hydraulic system are performed using finite element model to obtain relevant evaluation indexes. Finally,the hydraulic system is corrected using proportional-integral-derivative( PID) correction to achieve a well-controlled effect. Thus,the engineering requirements of the strip casting process can be met.     

Wear analysis of a side dam in twin-roll strip casting

Reducing wear on a side dam can prolong the casting operation life of a twin-roll strip casting process,thus reducing production cost and improving casting stability. To lengthen the service life of the side dam,it is necessary to understand the wear performance of the side dam material. To investigate the wear behavior mechanism of the side dam,in this study,the UMT-2 friction and wear tester was used to determine the relationship between the wear rate of the side-dam material and various parameters. Based on the roughness of the contact area between the side dam and the end of the casting rolls as well as on the amount of deformation of the side dam,which was derived using a thermal-deformation simulation model,the reasons for the uneven wear of the side dam were obtained.     

Precision online model for prediction of strip temperature in hot strip rolling

Abstract

Investigated via a series of finite element (FE) process simulation is the effect of diverse process variables on some selected non-dimensional parameters characterising the thermomechanical behaviour of the roll and strip in hot strip rolling. Then, on the basis of these parameters, online models are derived for precise prediction of the temperature changes occurring in the bite zone as well as in the interstand zone in a finishing mill. The prediction accuracy of the proposed models is examined through comparison with predictions from a FE process model.