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GTN模型的冷轧硅钢边部裂纹扩展研究

针对硅钢板容易在冷轧过程中形成边部裂纹,使用Gurson-Tvergaard-Needleman(GTN)微观损伤模型对带有边部微小缺口的硅钢板在冷轧过程中损伤分布以及裂纹的萌生和扩展进行研究。通过拉伸试验和扫描电镜观察分析得到材料的损伤参数f0、fc和fF,进而通过有限元软件ABAQUS模拟得到各轧制工艺参数对缺口尖端区域损伤分布及微裂纹萌生与扩展的影响,仿真与试验结果表明,在缺口尖端形成两条损伤带,最大损伤值随着压下率的增大而增大,同时裂纹长度随着压下率的增大而迅速增加;当工作辊半径较小时更加容易出现裂纹;沿着轧制方向缺口前侧的比后侧更容易产生裂纹,两侧的裂纹长度都随着摩擦因数的增大而增大;裂纹长度会随着张力的增大而明显增大。研究结果为轧制工艺参数的选择提供理论依据和参考。     

Friction variation in the cold-rolling process

Friction variation along the rolling contact interface was measured by a ‘sensor roll’ embedded with sensors on an experimental rolling mill. The measured results show that the friction coefficient is not constant along the contact length. Empirical formulae describing the friction variation for certain rolling condition were obtained. Experiments were also carried out to study the surface roughness of the strip. It was found that the roll and strip surface roughness can be transferred to each other along the rolling direction. Only slight change of roughness in the transverse direction was observed.     

Analysis of tribological feature of the oxide scale in hot strip rolling

In this paper, the effects of the flow stress of the scale and steel, and the friction coefficients at the scale-steel and the roll-scale interfaces on the final surface roughness have been studied. The surface roughness increases for both oxide scale and steel with an increasing friction coefficient at the roll-scale interface. However, the roughness increment is limited. The calculated roughness is close to the measured value. The temperature affects the scale roughness transfer, and the maximum difference is about 25% when the temperature is 850-1025 °C for rolling speeds 0.12-0.72 m/s. The developed model is applicable in hot strip mills.     

A thermal analysis of strip-rolling in mixed-film lubrication with O/W emulsions

Increase of both roll and strip surface temperatures can significantly affect a rolling process, roll conditions and strip mechanical properties. A comprehensive thermal analysis in cold rolling, especially in a mixed film regime, is needed to understand how thermal fields develop in roll and strip during rolling. It requires a simultaneous solution of the mixed film model for friction in the roll bite and the thermal model for roll and strip thermal fields. This paper presents a numerical procedure to analyse strip rolling process using lubrication with oil-in-water (O/W) emulsions. The thermal model includes the effect of heat generation due to the strip deformation and frictional shear stress at the asperity contacts. The numerical analysis employs a coupled thermal model and a mixed film lubrication model for calculating the friction and the asperity deformation in the bite. The thermal model considers the initial temperatures of the roll and strip, temperature rise due to the strip plastic deformation and friction. While the O/W mixed-film lubrication model takes into account the effect of surface roughness and oil concentration (%vol) of the emulsion. The thermal effect is analysed in terms of strip surface temperature and roll temperature, which are influenced by rolling parameters such as reduction, rolling speed, oil concentration in the emulsion. The results of the parametric study indicate that the effect of oil concentration on the thermal field is relatively small compared to that of reduction ratio and rolling speed. The reduction ratio increases the maximum interface temperature in the roll bite. In the mixed film regime, rolling speed also increases the maximum interface temperature and alters the temperature field of the strip. The numerical procedure was validated against known experimental data and can readily be extended to hot rolling or used to analyse roll strip temperature subjected to different cooling system.     

A non-local fatigue approach to quantify Ti-10V-2Fe-3Al fretting cracking process: Application to grinding and shot peening

This paper presents an experimental study on the tribological behaviour and cracking response of a Ti-10V-2Fe-3Al titanium alloy under fretting loading with a cylinder on plane configuration. Three types of surfaces were investigated: a polished one considered as the reference, a ground one and a shot peened surface. Surfaces were compared with respect to residual stress, hardness and roughness. The first step of this study was to determine sliding conditions and coefficient of friction of the three contact types. Next, fretting tests under stabilized partial slip regime were carried out to investigate crack nucleation and propagation. Results show that whatever surface roughness or residual stress in the material, tribological behaviour is the same. These latter confirm that sliding condition and coefficient of friction in partial slip regime is due to material effect and not to roughness or surface hardness. Then, residual stress induced by grinding or shot peening have no influence on the crack nucleation threshold under fretting solicitation because crack nucleation is only induced by a sufficient tangential loading. The crack nucleation threshold is formalized by applying the Crossland criterion taking into account the stress gradient and the ensuing “size effect”. As expected, cracks propagation is influenced by residual stress under the surface. Compared to the reference case, for a same loading parameters set, residual stress induced by grinding is not sufficient to decrease the crack length reached whereas effects of shot peening decrease highly these latter. So, there is a threshold of residual stress from which residual stresses are useful against cracking.     

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.     

Rolling Friction of Single Balls in a Flat-Ball-Flat-Contact as a Function of Surface Roughness

This paper reports on the dependence of rolling resistance on surface roughness of single balls in a flat-ball-flat configuration at sub-Newton loads using an specially designed rolling resistance tribometer. The tribometer setup features an ultra-stable drive and a two-dimensional force transducer with a high ratio between load and friction force sensitivity. Using this setup, the rolling resistance was examined for different materials as well as a self-mated sapphire contact. Results indicate a very strong dependence of rolling resistance on surface roughness for the tested loads. Rolling resistance was found to decrease monotonically with decreasing roughness. A comparison was also attempted between rolling resistance and sliding friction. Sliding friction measurements show that decreased roughness does not necessarily decrease the coefficient of friction. In fact, a friction minimum is reached at a certain roughness. A further decrease of roughness increases the friction. This indicates that sliding friction measurements do not seem to be transferable to rolling resistance in every case, especially when the average surface roughness is in the range of a few nanometres.     

Service Loading of Rolling Bearings in Wind Turbine Gearboxes: X-ray Diffraction Material Response Analysis of White Etching Crack Premature Failures

Rolling bearings in wind turbine gearboxes occasionally fail prematurely due to white etching cracks. The appearance of preparatively opened fracture faces indicates damage initiation from the surface in most cases due to brittle spontaneous tensile stress–induced cracking. Under the influence of decomposition products of the penetrating lubricant, branching crack growth is subsequently driven by corrosion fatigue. The material response analysis of rolling bearings from all gearbox locations, based on X-ray diffraction residual stress measurements, reveals vibrations in service as the root cause of surface crack initiation. The occurrence of high local friction coefficients in the rolling contact is described by a tribological model. Depth profiles of the equivalent shear and normal stresses are respectively compared with the measured residual stress patterns and a relevant fracture strength. White etching crack failures are experimentally reproduced on a rolling contact fatigue test rig under increased mixed friction. Causative vibration loading is evident from material response analysis. The generation of compressive residual stresses by cold-working the surface is proven to be an effective countermeasure.     

Study on the oxidation of stainless steels 304 and 304L in humid air and the friction during hot rolling

In rolling process, the contact friction is of crucial importance for accurate modeling, optimum design and control of industrial rolling processes. It is important to characterize the features of the oxide scale of stainless steel in hot strip rolling because the scale on the strip surface affects friction coefficient and thermal conductivity coefficient. To some extent, the rolling force and friction condition depend on the thickness and the microstructure of the oxide scale. Oxidation tests of stainless steels 304 and 304L were carried out in a high temperature electric resistance furnace. The humid air in which the water vapour content can be controlled was generated and remained to flow into the chamber of the furnace in 2.5 × 10⁻⁴ m³/s to study the effect of humidity on the oxidation of stainless steels. The microstructure and thickness of oxide scale layer of stainless steels were obtained and two or three oxide layers can be found. The humid air has a significant effect on the growth of oxide scale. Hot rolling tests were carried out on Hille 100 rolling mill. The friction condition at the roll–strip interface during hot rolling of stainless steel was determined and the transfer of surface roughness was discussed.     

Roll-pressure distribution and coefficient of friction in hot rolling by caustics

The optical method of reflected caustics was employed to investigate the variation of the coefficient of friction in the roll gap during strip rolling. Experiments have been carried out on an experimental rolling mill, using lead at room temperature, to obtain quantitative information about pressure distribution and friction, relevant to hot rolling, i.e. when frictional effects are pronounced. The rolls were made of Plexiglas. The normal and tangential pressure distribution and thence the coefficient of friction along the arc of contact at the outer edge of the strip were calculated from the caustic curves formed due to elastic deformation of the rolls, although the rolled strip was deformed plastically.An almost linear decrease of the friction coefficient from a maximum value, at the entry plane to the roll gap. to zero at the neutral plane and then a parabolic, tending to linear, increase of it, towards the exit plane, were predicted, in contradiction to what is up-to-now accepted in the praxis, that the friction coefficient remains constant during hot rolling.     

带钢冷轧机工作辊表面粗糙度实测研究

通过大量跟踪实测工作辊和带钢表面粗糙度,深入分析了带钢表面粗糙度的影响因素和变化规律,运用逐步回归法建立了冷轧带钢表面粗糙度预测数学模型和转印率模型,进而建立了轧辊表面粗糙度衰减规律模型。这些模型可以用于生产中预测轧后带钢表面粗糙度、合理安排带钢轧制顺序、决定工作辊下机时间和修正轧制力计算模型。     

Measurements of friction in strip drawing under thin film lubrication

Strip drawing is used to investigate the friction behaviour under thin film lubrication in metal forming with plastic deformation. Friction coefficients are measured under a wide range of tribological conditions. The surface roughness is measured on an interferometric profilometer. The results show that the friction coefficient decreases with increasing oil film thickness h_w, as estimated using a formula appropriate for smooth tool and workpiece. Measurements of the surface topography show that change in friction is associated with a change in contact ratio between the tool and strip. The effect of strip reduction, strip roughness and die roughness on the friction coefficient is also investigated.     

含横向裂纹简单转子刚度的计算

以两端简支的 Ｊｅｆｆｃｏｔ 转子为对象,运用断裂力学和材料力学理论推导出含横向弓形裂纹的转子刚度计算公式,并在此基础上研究裂纹位置、裂纹深度和轴细长比 Ｒ／ Ｌ等参数对转子刚度的影响。计算结果表明,裂纹位置距转子中心越近、裂纹深度越深,转子的刚度就越小;转轴细长比越大,裂纹对转子刚度的影响就越大;当裂纹较浅时,可忽略平行于裂纹方向的刚度变化,而当裂纹较深时,应同时考虑与裂纹平行和垂直方向上的刚度变化。     

Edge stresses in wide strip hot rolling

In the rolling of wide strip, longitudinal tensile stresses develop along the edges which enhance edge cracking. Theoretical assessment of these stresses and the factors affecting their magnitude and spread are not emphasized in existing solutions, since most of the solutions are based on the assumption of plane strain along the entire width. In the present work a flowline field solution is developed to analyze the flow and stresses in steady state wide strip hot rolling based on the concept of a plane-strain middle zone and two lateral flow edge zones from which edge stresses and edge zones width have been determined. The results obtained show that the maximum value of the longitudinal edge stresses is 1/√3 the flow stress and occurs at the neutral point where the width of edge zone is maximum. The influence of the coefficient of friction at roll-strip interface, roll diameter and thickness reduction ratio on the magnitude and spread of edge stresses is not appreciable.     

TD法VC涂层与陶瓷和钢对摩时摩擦磨损性能对比研究

采用TD法在Cr12MoV冷作模具钢表面制备VC涂层,在摩擦磨损试验机上考察VC涂层与钢球、钢柱和陶瓷球配副时的摩擦磨损性能,利用扫描电镜、粗糙度测量仪和X射线衍射仪(XRD)分析涂层磨损前后表面及界面的形貌、表面粗糙度和物相组成.结果表明,与不同摩擦副配副时,VC涂层摩擦因数随着磨损时间增加先增大后趋于平稳,磨损率随着磨损时间增加而减小,其中与钢柱配副时摩擦因数最小,磨损率最低.与不同摩擦副配副时,VC涂层磨损机制与失效形式不同,与钢球配副时VC涂层磨损机制为磨粒磨损,失效形式为划痕和剥落坑;与钢柱配副时VC涂层磨损机制为黏着磨损和疲劳磨损,失效形式为犁沟和片层状剥落;与陶瓷球配副时VC涂层磨损机制为氧化磨损,失效形式为脆性断裂.     

Occurrence of surface defects on strips during hot rolling process by FEM

During a hot rolling process, surface defects on strips can severely affect the quality of the rolled product, particularly for two conditions: (1) there are initial defects on continuous casting slabs that propagate and/or are inherited from those on the surface of rolled steels from upstream rolling processes; and (2) there are no initial defects on continuous casting slabs, and they consequently appear on the surface of rolled steels due to improper rolling technologies. In this paper, the authors present a new 3D finite element model coupled with constrained node failure to understand better the initiation and growth of surface defects on strips during the hot rolling process for case 2. The strip deformation processes were simulated for various rolling reduction ratios and friction coefficients between the roll and the strip. The occurrence of surface defects on strips was modeled under some rolling conditions. The plastic strain distribution in strips and the rolling forces were obtained. The risk of occurrence of surface defects on strips increases as the friction between the roll and strip increases for the same reduction ratio.     

An innovative development of a four-high foil rolling mill

During foil or ultrathin strip rolling, elastic deflection variation of the work rolls at the contact surface with the strip causes non-uniform thickness distribution of the product in the width direction, which possibly causes a central wrinkling or an edge fracture on the product. This study developed a four-high prototype rolling mill, comprising two work rolls fabricated from an ultrastrength alloy. This prototype rolling mill was composed of a housing frame set, a roll set, a roll gap adjusting mechanism, and a power transmission system. The finite element analysis was used for determining the frame dimensions and work roll diameters. Several experimental rolling passes of stainless steel strips were conducted using this self-developed prototype rolling mill to achieve a 0.05-mm-thick foil product.     

Ring crack propagation in silicon nitride under rolling contact

Silicon nitride has been found to have the optimum combination of properties which are suitable for rolling element bearing applications to withstand high loads, severe environments, and high speeds. Surface ring cracks are difficult to detect but are found on the surface of silicon nitride balls. These ring crack defects decrease the rolling contact fatigue life considerably. This paper presents an experimental study and numerical analysis of ring crack propagation in rolling contact. The contribution of this study is to provide understanding of ring crack propagation behaviour and life prediction in rolling contact. Rolling contact tests are performed on the silicon nitride/steel elements. Silicon nitride ball surfaces are examined before testing using a dye-penetrant technique and optical microscopy. The surfaces are examined using optical microscopy and scanning electron microscopy during testing and after failure. The numerical calculations are based on a 3D model of ring crack growth. The rolling contact loading is simulated by a repeated Hertzian surface load with normal pressure and tangential traction. Fracture mechanics analysis is utilised to determine the stress intensity along the crack front and the stress intensity factors are analysed using a 3D boundary element model. Life predictions from the present calculations are in line with the experimental observations.     

Hydrodynamic analysis of partial film lubrication in the cold rolling process

Lubrication in cold rolling plays an important part for process feasibility and process quality. The hydrodynamic process of lubrication is very complicated and affected by many material and process parameters. This paper examined partial lubrication in the cold rolling process. The average flow Reynolds equation for rolling lubrication was 2set, which considered the pressure?Cviscosity and average flow effects. Lubricating factors such as sidling, surface waviness, lubricant viscosity, surface roughness, and reduction ratio were investigated. The results of the lubrication equation show that sliding, lubricant viscosity, and surfaces roughness affect the values of rolling friction. Surface waviness and reduction ratio also influence both rolling pressure and rolling friction.