A multi-scale model for friction in cold rolling of aluminium alloy

A simple and robust friction model is proposed for cold metal rolling in the mixed lubrication regime, based on physical phenomena across two length scales. At the primary roughness scale, the evolution of asperity contact area is associated with the asperity flattening process and hydrodynamic entrainment between the roll and strip surfaces. The friction coefficient on the asperity contacts is related to a theoretical oil film thickness and secondary-scale roll surface roughness. The boundary friction coefficient at the “true” asperity contacts is associated with tribo-chemical reactions between fresh metal, metal oxide, boundary additives, the tool and any transfer layer on the tool. The asperity friction model is verified by strip drawing simulations under thin film lubrication conditions with a polished tool, taking the fitting parameter of the boundary lubrication friction factor on the true contact areas equal to 0.1. Predicted values of average friction coefficient, using a boundary friction factor in the range 0.07–0.1, are in good agreement with measurements from laboratory and industrial rolling mill trials.     

Mixture Lubrication with Emulsions in Cold Rolling

A mixture model of lubrication in cold rolling with two adsorbed layers on the solid surfaces and an emulsion layer between them has been developed. The elastic deformation of the strip and the rolls is considered. A series of simulations using this model is carried out, and this model predicted that the value of the inlet film thickness is in good qualitative agreement with the experimental results. There exists a range of roll speeds that provides sufficient friction to perform the rolling process without front tension under the merged conditions and lower roll speeds. At high roll speed, the two adsorbed layers are separated by the emulsion layer, and the pressure hill disappears due to the smaller surface shear stress acting on the strip, so that rolling is no longer possible unless accompanied by a front tension.     

大偏心下滑动轴承热弹流润滑状态及特性*

滑动轴承在大偏心条件下工作时，热效应及弹性变形使得油膜润滑状态发生变化，进而影响摩擦特性。为此建立耦合轴瓦弹性变形、轴颈轴瓦粗糙峰接触、油膜温度分布及黏温-黏压关系的滑动轴承混合润滑模型，采用有限差分法求解得到不同工况下油膜压力场、温度场分布，分析热效应及弹性变形对润滑状态转变及轴承各特性参数的影响；搭建实验台测量试件内表面温度分布，测试结果验证了计算模型的正确性。结果表明：大偏心时热效应和弹性变形使得油膜润滑状态出现转化；粗糙峰的接触使摩擦热增加，且在最小油膜处形成温度峰值；热效应和轴瓦弹性变形使得接触压力峰值集中在轴承两端，承载能力和摩擦力均有所下降。     

Experimental and numerical study of O/W emulsion lubricated strip rolling in mixed film regime

An experimental and numerical study of cold rolling lubricated by O/W emulsion has been carried out. The strip rolling experiment was carried out on a Hille experimental rolling mill with a view to study the performance of emulsion lubrication in terms of practical rolling parameters. Accordingly, rolling parameters such as rolling force and torque were measured. The experimental measurements compare favourably with the computed results from a numerical scheme developed by the authors. The scheme, based on a two-phase lubricant model, is capable of calculating the oil concentration at any point within the inlet zone and work zone, rolling pressure, film thickness, and fractional contact area ratio associated with strip rolling under mixed film lubrication at different rolling speeds. Using this scheme, the intertwined effects of an emulsion’s parameters such as: oil concentration, mean oil droplet size, and rolling speed on strip rolling were investigated. The numerical study encompassed the mixed film regime for speed, S ranges from 10⁻⁴ to 10⁻², supply oil concentration level λ_ds from 1 to 10%, and oil droplet size D _S from 5 to 10. Experimentally, the differences between water, oil and emulsion-lubricated rolling are not discernible except for film thickness. At a low speed of 10 RPM, force and torque of water-lubricated rolling are marginally higher than oil- or emulsion-lubricated ones. However, the difference between emulsion and neat oil is not apparent. The numerical results show the occurrence of a moderate oil concentration increase in the inlet zone followed by a sharp one at the beginning of the work zone. The effect of the concentration process is predominantly seen in the film thickness and the lubricant pressure whilst its effect on the total pressure is less pronounced. The analysis of the results suggests that it is possible to lower the emulsion oil concentration without any adverse effect on the rolling process. This principle can be used to control the outlet lubricant film thickness and hence the surface quality of the rolled strip.     

二次冷轧过程变形区油膜厚度模型

为了定量预报二次冷轧过程轧制变形区油膜厚度,结合二次冷轧机组乳化液直喷系统的设备与工艺特点,分析了带钢表面析出油膜、工作辊表面附着油膜的形成机理,建立了一套二次冷轧过程轧制变形区带钢上下表面油膜厚度模型,定量分析了乳化液流量密度、乳化液浓度、乳化液析出距离、轧机入口轧制速度、轧制咬入角、带钢入口变形抗力、后张力、轧制油初始动力黏度、轧制油压力黏度系数对轧制变形区带钢上下表面油膜厚度的影响,并将该模型应用到某1220二次冷轧机组的生产实践,编制出了相应的模型计算软件,实现了二次冷轧过程变形区油膜厚度的预报,为二次冷轧过程润滑性能的控制奠定了理论基础。     

Calculation of mixed lubrication at piston ring and cylinder liner interface

This paper reports on the theoretical analysis of mixed lubrication for the piston ring. The analytical model is presented by using the average flow and asperity contact model. The cyclic variations of the nominal minimum oil film thickness are obtained by numerical iterative method. The total friction is calculated by using the hydrodynamic and asperity contact theory. The effects of the roughness height, pattern, and engine speed on the nominal minimum film thickness, friction force, and frictional power losses are investigated. As the roughness height increases, the nominal oil film thickness and total friction force increase. Also, the effect of the surface roughness on the boundary friction is dominant at low engine speed and high asperity height. The longitudinal roughness pattern shows lower mean oil film pressure and thinner oil film thickness compared to the case of the isotropic and transverse roughness patterns.     

船用发动机重载工况下凸轮-滚轮副混合热弹流润滑分析

凸轮-滚轮副是大功率船用发动机配气机构的关键摩擦副,除受到弹簧力和自身的惯性力之外,还受到来自喷油器的极高燃油压力,工作条件极为苛刻。为了分析该摩擦副的性能,建立船用发动机重载工况下凸轮-滚轮副的混合热弹流润滑模型,计算燃油压力作用下的摩擦副油膜润滑、摩擦温升和磨损性能。结果表明：喷油器的燃油压力会显著降低凸轮-滚轮摩擦副之间的油膜厚度,同时产生较为严重的微凸体接触;随着环境温度的提高,凸轮-滚轮副的油膜厚度以及油膜温升会有所下降,而微凸体接触压力、摩擦力以及摩擦功率均会显著增加;滚轮打滑会造成凸轮-滚轮摩擦副的油膜厚度下降,同时导致油膜温升以及微凸体接触压力增大和并且致使表面磨损显著加剧。     

点接触混合润滑中两种求解模型的比较

混合润滑是典型零部件主要的润滑状态,根据表面形貌表征方式的不同,混合润滑模型一般分为统计学模型和确定性模型两类.为研究2种模型求解粗糙表面点接触混合润滑性能的差异,通过基于平均流量模型和GW模型的统计模型、基于统一Reynolds方程的确定性模型,分析并比较不同表面粗糙度、卷吸速度、载荷以及润滑油环境黏度时2种模型预测...     

轧制界面非稳态润滑过程系统动力学模型的建立及其数值模拟

运用轧机辊缝动力学的基本理论结合非稳态润滑理论，建立了基于非稳态工作界面的动力学模型。该模型考虑了界面上金属塑性流动过程、界面上部分流体润滑与干摩擦并存的混合摩擦学过程以及工作辊的运动等多重耦合作用。塑性流动过程分析中考虑了界面摩擦状态的动态变化，界面摩擦特性分析中考虑了工作辊运动的动态耦合，因此工作界面上的轧制力模型、界面摩擦模型、工作辊运动模型构成了界面的薄膜约束多重耦合模型。对某大型公司2800轧机垂直系统的自激振动进行了仿真，定量地分析了一些主要参数对轧机垂直自激振动临界速度的影响。     

混合润滑条件下的斯特封高速摩擦与密封特性

为准确研究斯特封高速摩擦与密封特性,基于混合润滑理论,综合流体空化效应、密封接触变形和微观粗糙峰接触等因素影响,建立了斯特封摩擦与密封的数值计算模型。研究了往复运动速度和密封压力对油膜厚度、摩擦力和泄漏量的影响,搭建了往复密封试验台来验证模型的准确性。结果表明：计算摩擦力与实验摩擦力相近。混合润滑模型能更好地模拟高速柱塞副斯特封的摩擦与密封特性,油膜压力与粗糙度接触压力共同影响密封性能,但粗糙度接触摩擦起主导作用。     

Lubrication in steel strip rolling in Japan

Research into, and the state of technology for, lubrication in steel strip rolling in Japan are reviewed. Both cold and hot strip rolling are discussed. Subjects covered include coefficient of friction and oil film thickness, friction pick-up, and roll wear.     

Deterministic solutions and thermal analysis for mixed lubrication in point contacts

A deterministic numerical model has been developed for simulation of mixed lubrication in point contacts. The nominal contact area between rough surfaces can be divided into two parts: the regions for hydrodynamic lubrication and asperity contacts (boundary lubrication). In the area where the film thickness approaches zero the Reynolds equation can be modified into a reduced form and the normal pressure in the region of asperity contacts can be thus determined. As a result, a deterministic numerical solution for the mixed lubrication can be obtained through a unite system of equations and the same numerical scheme. In thermal analysis, the solution for a moving point heat source has been integrated numerically to get surface temperature, provided that shear stresses in both regions of hydrodynamic lubrication and asperity contacts have been predetermined. A rheology model based on the limit shear stress of lubricant is proposed while calculating the shear stress, which gives a smooth transition of friction forces between the hydrodynamic and contact regions. The computations prove the model to be a powerful tool to provide deterministic solutions for mixed lubrication over a wide range of film thickness, from full-film to the lubrication with very low lambda ratio, even down to the region where the asperity contact dominates.     

Multi-factor coupling system characteristic of the dynamic roll gap in the high-speed rolling mill during the unsteady lubrication process

In the present work, a multi-factor coupling dynamic model of a rolling mill system for a dynamic roll gap during an unsteady lubrication process was developed on the basis of the rolling theory, lubrication and the friction theory, and the mechanical vibration theory. The multi-factor coupling model of interfacial film binding was coupled with the rolling force model, dynamic roll gap interface friction model and work roll movement model. The corresponding distributions of friction and pressure at varying surface roughness and times were systematically analyzed during the unsteady mixed lubrication process. The effects of the main processing parameters on the critical speed and amplitude for self-excited vertical vibration were investigated.     

Lubrication of Strip Rolling in the Low-Speed Mixed Regime

An analytical model for strip rolling in the low-speed mixed lubrication regime is developed. An average Reynolds equation for longitudinal saw-tooth surfaces under conditions of high fractional contact area, is combined with an analysis for asperity flattening under conditions of bulk plastic flow, to treat lubrication in the mixed regime. Analyses for the inlet zone and work zone and the influence of pressure on viscosity are included in the model. The model indicates that hydrodynamic lubrication effects are important at much lower speeds than previously considered possible. The film thickness predicted by the model is somewhat smaller than that measured using the oil drop method in rolling aluminum alloy with a mineral oil.     

Mechanics of roll edge contact in cold rolling of thin strip

Simulation of cold rolling of thin strip due to roll edge contact with oil lubrication was performed successfully using a developed influence function method. Roll edge contact and related surface roughness was discussed in this paper. The calculated rolling force, intermediate force and work roll edge contact force increase significantly when the reduction increases. The strip profile becomes poor with a higher reduction, and the calculated rolling forces are consistent with the measured values. A modified edge shape of work roll determined from the roll edge contact length and roll edge flattening value is helpful to reduce the work roll edge wear and to extend the work roll life. Surface roughness and asperity of the rolled strip are characterized by surface profilometer and atomic force microscope. The research shows that the surface roughness reduces with a higher reduction or rolling speed. The effect of the strip width on surface roughness is not significant.     

基于Carreau流变模型的圆柱滚子轴承热混合润滑分析*

为了研究圆柱滚子轴承接触区的混合润滑性能,建立基于Carreau非牛顿流体的热混合润滑模型,求解非牛顿流体线接触热混合润滑数值解。研究滑滚比、卷吸速度及载荷对线接触混合润滑特性的影响,并与相同工况下牛顿流体热混合润滑的结果进行对比。结果表明：随着滑滚比、卷吸速度及载荷的增大,油膜温度都会升高,Carreau非牛顿流体的温度要低于牛顿流体的温度;油膜厚度随着滑滚比、载荷的增大而减小,随着卷吸速度的增大而增大,Carreau非牛顿流体与牛顿流体膜厚相差不大;随着滑滚比的增大,2种流体的平均摩擦因数均增大,随着卷吸速度和载荷的增大,2种流体的载荷比均减小。     

Mixed film lubrication of strip rolling using O/W emulsions

A numerical study on the oil concentration effect of O/W emulsion in cold rolling operating in the mixed film lubrication regime has been carried out. The developed scheme is able to calculate oil concentration at any point within the inlet zone (IZ) and work zone (WZ), rolling pressure, film thickness, and contact ratio for various rolling speeds. Hence the intertwined effects of oil concentration of the supplied emulsion and rolling speed on strip rolling are discussed. The study encompasses mixed film regime with speeds S range from 10⁻⁵ to 10⁻³ and supplied emulsion's oil concentration levels λ_ds range from 5% to 90%. The result shows that a moderate rise in oil concentration occurs in the IZ followed by a rapid one at the beginning of the workzone. In most cases, the oil in the emulsion would have been transformed from disperse phase to continuous phase throughout the WZ. Notwithstanding further concentration, which depends on the oil concentration of the supplied emulsion, could still occur in the WZ. The effect of the concentration process is predominantly seen in the development of the lubricant pressure whilst its effect on the total pressure is less pronounced. The analysis of the results suggests that it is possible to lower the emulsion oil concentration without detrimental effects on the rolling process; and from the analysis of the outlet film thickness, it is shown that the variation of emulsions’ oil concentration could control the exit lubricant film thickness and consequently the strip surface quality.     

Control of surface thermal scratch of strip in tandem cold rolling

The thermal scratch seriously affects the surface quality of the cold rolled stainless steel strip. Some researchers have carried out qualitative and theoretical studies in this field. However, there is currently a lack of research on effective forecast and control of thermal scratch defects in practical production, especially in tandem cold rolling. In order to establish precise mathematical model of oil film thickness in deformation zone, the lubrication in cold rolling process of SUS410L stainless steel strip is studied, and major factors affecting oil film thickness are also analyzed. According to the principle of statistics, mathematical model of critical oil film thickness in deformation zone for thermal scratch is built, with fitting and regression analytical method, and then based on temperature comparison method, the criterion for deciding thermal scratch defects is put forward. Storing and calling data through SQL Server 2010, a software on thermal scratch defects control is developed through Microsoft Visual Studio 2008 by MFC technique for stainless steel in tandem cold rolling, and then it is put into practical production. Statistics indicate that the hit rate of thermal scratch is as high as 92.38%, and the occurrence rate of thermal scratch is decreased by 89.13%. Owing to the application of the software, the rolling speed is increased by approximately 9.3%. The software developed provides an effective solution to the problem of thermal scratch defects in tandem cold rolling, and helps to promote products surface quality of stainless steel strips in practical production.     

Control Scheme Using Forward Slip for a Multi-stand Hot Strip Rolling Mill

Forward slip is an important parameter often used in rolling-speed control models for tandem hot strip rolling mills. In a hot strip mill, on-line measurement of strip speed is inherently very difficult. Therefore, for the set-up of the finishing mill, a forward slip model is used to calculate the strip speed from roll circumferential velocity at each mill stand. Due to its complexity, most previous researches have used semi-empirical methods in determining values for the forward slip. Although these investigations may be useful in process design and control, they do not have a theoretical basis. In the present study, a better forward slip model has been developed, which provides for a better set-up and more precise control of the mill. Factors such as neutral point, friction coefficient, width spread, shape of deformation zone in the roll bite are incorporated into the model. Implementation of the new forward slip model for the control of a 7-stand hot strip tandem rolling mill shows significant improvement in roll speed set-up accuracy.     

冷轧铝工作区的混合润滑特性研究

为研究冷轧铝工作区的混合润滑特性,基于平均流量理论建立考虑表面粗糙度的冷轧铝工作区混合润滑模型,并通过相关文献的数据验证模型的正确性.在不同轧制速度、润滑油黏度以及前后张应力条件下对整个工作区内的润滑特性进行分析,研究轧制工艺参数对油膜厚度、接触面积比以及应力分布的影响.仿真结果表明:随着轧制速度的提高,轧制压力有一定...