An Analytical Model for Dependence of Force and Forward Slip on Speed in Cold Rolling

An explicit analytical method for determining approximations for roll force and forward slip for strip rolling is introduced. It reproduces all main predictions of classical strip theory, but is completely free of any numerical algorithms. Nevertheless, for extreme cases of cold rolling, like foil or skin pass rolling, more sophisticated algorithms are still needed. For the friction coefficient in the roll gap and its increase from the entry to the neutral point also explicit relations are proposed. In the framework of this analysis, they are sufficiently precise for cold rolling with one‐phase lubricants and stochastic surfaces. Only a single parameter of adaptation is needed, the microscopic friction coefficient μ_micro. The quality of the total model is illustrated by means of the dependence of roll force and forward slip on railing speed. For this purpose measured process data (four different pass‐schedules, seven to ten coils each) for cold rolling of copper with rolling oil is analyzed.     

Developments of work roll bite lubrication at SSAB Tunnplåt AB hot strip mill

In the strip rolling process friction is needed at first to pull the strip into the roll gap, otherwise slipping occurs. After the neutral plane however, where the velocity of the work roll equals the velocity of the strip, the friction stress alters direction to the opposite. If the friction stress after the neutral plane then is lowered, the total roll force will decrease. This can be done by a properly applicated oil- and water dispersion close to the work roll gap and between the roll cooling wipers. The technique and the key to a successful lubrication is a work roll surface which is as dry as possible. If the roll surface is not dry, neither large changes in the oil concentration, nor changes in the oil composition can make the lubricant stick to the roll surface. The lubricate was partly washed away by the water and only a small roll force decrease could therefore be measured. Only when the roll cooling water on the entry side of the work roll was set off on purpose, the roll force decrease was significant. Due to this result, further test were performed in an upstream stand in the hot strip mill. Here, the work roll wipers were modified and the roll was substituted by one with a larger radius and lubricated. The result was very slight or no water leakage at all. The roll force decrease was then much greater than before and close to those measured for the same reduction in the case of no entry water for the later stand. The electrical current in main drives and the roll wear in the lubricated stand could also be lowered significantly. In addition, the strip surface was greatly improved, under conditions with a high possibility of oxide residual contamination from the roll surface.     

Analysis of secondary oxide-scale failure at entry into the roll gap

Both numerical analysis based on finite-element (FE) modeling and experimental evidence concerning the secondary oxide-scale failure at entry into the roll gap are presented and reviewed for a better understanding of events at the roll-workpiece interface, in turn, leading to better definition of the boundary conditions for process models. Attention is paid to the two limit modes leading to oxide-scale failure, which were observed earlier during tensile testing under rolling conditions. These are considered in relation to the temperature, the oxide-scale thickness, and other hot-rolling parameters. The mathematical model used for the analysis is composed of macro and micro parts, which allow for simulation of metal/scale flow, heat transfer, cracking of the oxide scale, as well as sliding along the oxide/metal interface and spallation of the scale from the metal surface. The different modes of oxide-scale failure were predicted, taking into account stress-directed diffusion, fracture and adhesion of the oxide scale, strain, strain rate, and temperature. Stalled hot-rolling tests under controlled conditions have been used to verify the types of oxide-scale failure and have shown good predictive capabilities of the model. The stock temperature and the oxide-scale thickness are important parameters, which, depending on other rolling conditions, may cause either through-thickness cracking of the scale at the entry or lead to entry of a nonfractured scale when the scale/metal interface is not strong enough to transmit the metal deformation.     

Evaluation on some important force models for cold strip rolling

To improve the accuracy of rolling force prediction, some important force models were evaluated through applied computation for cold rolling of low carbon steel and aluminum alloy according to measured data on lab mill. The effects of model structure and three important variables ‐ flow stress, contact length and friction coefficient ‐ on the precision of computed force were quantitatively studied. Flow stress was measured with plane‐strain compression test, contact length was based on elastic flattening of work‐roll by Hitchcock, and friction‐coefficient was determined by rolling strain and numerical iteration. In steel rolling Bland & Ford integration model and Bryant & Osborn algebraic equation are better in accuracy than Ekelund and Parkins. In aluminum rolling all the models produce large deviations ΔF_R = 10–20% if flow stress, contact length and friction coefficient are determined with the same method as steel rolling. The elastic deformation of aluminum strip is now taken into account for its low elastic modulus. An effective method to determine plastic and elastic contact has been developed in this investigation. The accuracy of force computation is obviously improved for aluminum rolling.     

Effects of interface resistance on heat transfer in steel cold rolling

The thermal contact resistance created in the bite region during rolling depends on surface roughness, contact pressure, and the coolant, lubricant or oxide scale between the roll and strip. Therefore, to estimate temperatures of the roll and the strip accurately, the interface resistance in the contact region should be considered when modelling. The purpose of this study is to more effectively analyze the thermal behaviour of the steel rolling process by considering the interface resistance of the roll and strip in the contact region. Since the interface is very thin in comparison with its length, it is modelled as one-dimensional heat transfer with friction heat generated along the interface. For the estimation of the thermal contact resistance, different surface situations of cold rolling are considered. The finite element method is adopted to evaluate the deformation and friction power dissipated in the rolls and strip during rolling process. Roll and strip thermal properties, such as thermal conductivity and specific heat, are considered to be temperature dependent during the calculation.     

基于辊缝动态摩擦方程的铝板冷轧机垂振机理分析

针对高速铝板轧制过程中频繁出现的冷轧机垂直振动现象,结合轧制工艺润滑原理和机械振动理论,建立基于辊缝动态摩擦方程的轧机垂直振动模型.该模型由辊缝几何形状模型,轧辊-轧件工作界面的动态摩擦模型,变形区内的正向轧制应力、摩擦应力分布模型,以及单机架铝板冷轧机二自由度垂向系统结构模型组成.同时,为研究轧辊-轧件工作界面动态摩擦机制影响下的冷轧机垂振机理及系统稳定性,采用某厂单机架铝轧机设备及工艺参数,搭建Matlab/Simulink平台,分别模拟仿真轧制压力和正向轧制应力曲线,验证该模型的有效性;并讨论分析了变形区混合摩擦状态,轧辊-轧件表面粗糙度、轧件入口厚度与系统稳定性的关系.      

轧辊表面镀铬对冷轧变形区润滑的影响

模拟轧辊表面镀铬工艺,对镀铬轧辊及非镀铬轧辊在同一润滑介质条件下对辊缝变形区润滑的影响进行了研究。结果表明,在同一润滑介质下,钢-钢摩擦副与钢-铬摩擦副的摩擦系数及润滑油成膜性能均有明显不同。钢-铬摩擦副的摩擦系数比钢-钢摩擦副的摩擦系数降低20%以上,在其它实验参数相同的条件下,冷轧润滑油在镀铬表面所形成的油膜厚度比非镀铬表面提高1倍以上。     

热镀锌机组沉没辊系受力模型及影响因素

 热镀锌机组沉没辊系在锌锅内工作状态下受力情况复杂,受外界影响较大。充分考虑热镀锌机组设备与其工艺特点,首先分析沉没辊系在工作状态下受力情况,分别建立矫正辊插入量与包角的关系式、基于辊面锌渣沉积厚度的辊面摩擦因数经验模型。随后采用条元离散法建立沉没辊系辊面接触压力、摩擦驱动力矩模型,基于简支梁模型确定沉没辊系轴端支撑力、轴端摩擦阻力矩模型。最后将沉没辊系受力模型应用于国内某热镀锌机组镀锌工艺段,以典型规格带钢确定沉没辊系受力影响因素,并依据机组实际工况及板形要求来确定生产工艺参数的5种不同水平,来分析其对辊系受力状态的影响规律,为沉没辊系工艺优化提供指导依据。结果表明,4种工艺参数对辊系接触压力及摩擦力矩影响都存在差异性,矫正辊插入量与辊面接触压力呈现非线性关系;带钢张力对各辊接触压力影响程度不同,其中沉没辊受到的影响最大,而其余两辊受到张力几乎不影响;辊面平均摩擦因数、带钢速度对辊的接触压力几乎不影响;辊系摩擦驱动力矩、轴端摩擦阻力矩随着矫正辊插入量及辊面平均摩擦因数增大都呈现出不同程度增大。带钢张力与沉没辊摩擦驱动力矩、轴端摩擦阻力矩呈线性变化关系,而带钢张力对其余两辊影响很小。带钢速度对辊系的摩擦驱动力矩、轴端摩擦阻力矩影响很小。     

Measurements of multiple normal pressure peaks in flat rolling

During normal pressure measurements in flat rolling, conducted in the roll gap of laboratory mills, unexpected multiple pressure peaks appeared. These multiple pressure peaks, such as a double peak, have not only been measured during rolling of billets, but also when rolling slabs or strips. Not explained by the established rolling theory stating just a single maximum point. In order to investigate its physical mechanisms a number of rolling experiments are presented. A model duo mill, load transducers, powder lubrication or by vaseline and wax as model material were used. The normal pressure, the shear stress and its angular movement were measured simultaneously. By first measuring a double normal pressure peak during rolling of a wax specimen and then by consciously making changes in draft, specimen thickness, rolling velocity, specimen width or the friction conditions, respectively, the double peak phenomena were investigated by means of the responding normal pressure distribution. A number of direct measurements in the roll gap by other researchers using different techniques, have also been investigated. The conclusion from the wax rolling experiments and the rewiew of measurements by others is that the peak at entry of contact is caused by the presence of compressive stresses from rigid zones outside the deformation zone, raising the hydrostatic pressure and the normal pressure. The effect is observed by inhomogeneous deformation. The peak value at the entry of contact can then be very high, by analogy to plane indentation with elastic regions present. The resistance of the wax material to the height deformation of the specimen was seen to increase at small reductions or by a specimen with a greater nominal thickness, supporting the “rigid end” theory. The result of this study is also that the exit peak in the double peak is caused by frictional surface stresses at the roll/specimen interface. The effect was seen by changes in frictional influencing factors, such as the reduction or the surface friction condition. Lateral spread of the material rolled was at the same time seen to make the double peak more distinct, decreasing the normal pressure in the “valley” between the two peaks and at exit of contact.     

Effect of steel composition on failure of oxide scales in tension under hot rolling conditions

The differences in failure of oxide scales formed on mild, Si‐Mn, Mn‐Mo and stainless steels were investigated using a high‐temperature tensile test technique over the range of test parameters near to the hot rolling conditions at entry into the roll gap. Temperature, strain and strain rates were 783 – 1200 °C, 2.0 – 5.0 % and 0.2 – 4.0 s^?1 respectively. The scale thickness was maintained within 5 – 250 μm. Mild steel has the highest oxidation rate throughout the temperature range. A slightly thicker scale for the Mn‐Mo steel compared with Si‐Mn steel was observed. The stainless steel has shown the highest resistance to oxidation. Although through‐thickness cracks and sliding were competitive mechanisms for oxide scale failure for the mild steel, the other steel oxides failed only by through‐thickness cracking or were delaminated over the whole temperature range 783 ‐ 1200 °C. Modelling based on the finite‐element method was applied for better understanding of the micro‐events both during uni‐axial tension and just before contact with the rolls. The part of the model related to oxide scale failure has been upgraded taking into account experimental evidence concerning differences in scale failure, due to the steel chemical content, which were observed in the hot tensile tests.     

热连轧粗轧辊磨损的有限元数值模拟和工艺优化

针对轧钢厂GCr15轴承钢240 mm × 240 mm方坯粗轧阶段轧辊磨损较严重的情况,采用Archard磨损数学模型模拟分析了轧件压下量、轧辊硬度、热传导系数及摩擦因子在一道次成形后对轧辊磨损规律的影响。模拟结果表明,轧辊硬度越高,轧辊抗磨损能力越强;热传导系数对轧辊磨损的影响较小;当摩擦因子f＞0.25时,其摩擦因子对轧辊磨损量变化明显;当轧件压下量在△h＜50 mm 时,轧件压下量对轧辊的磨损量影响显著。根据所得结果,结合现场轧制工艺和轧辊材质,将使用的球墨铸铁Ⅰ轧辊[抗拉强度≥400 MPa,硬度HRC值40,热传导系数18 kW/（m²·℃）,摩擦因子0.3]改成球墨铸铁Ⅱ轧辊[抗拉强度≥500 MPa,硬度HRC值45,热传导系数17kW,/（m²·℃）,摩擦因子0.2],并将压下量由70 mm降至50 mm,使轧辊单槽过钢量由优化前10000 t提高至优化后的18000~20000 t。     

冷却水和氧化铁皮对轧辊辊耗的影响

指出当前该厂轧钢生产中,轧辊作为大型工具在降低成本中的重要性。简要分析高温应力掰棍的力学原理,指出高温对轧辊寿命的影响,提出采取优化冷却水控制提高轧辊寿命的措施。并描述轧辊与坯料氧化铁皮之间磨损带来的轧辊缺陷,以及该厂BD2轧辊毛刺与氧化铁皮的关系。对冷却控制和减少氧化铁皮给出一些预防措施。     

冷连轧高速轧制过程中摩擦因数机理模型的研究

在现场试验与理论分析的基础上,针对现有摩擦模型对冷连轧高速轧制过程中摩擦因数的预报精度不高、通用性不强等问题,首先对润滑油膜厚度计算模型与摩擦因数返算方法进行了推导,然后定量研究了辊缝中润滑油膜厚度与摩擦因数之间的关系,最终建立起一套适合冷连轧高速轧制的摩擦因数机理模型,并将其应用于宝钢冷轧薄板厂1220五机架冷连轧机与宝钢冷轧厂2030五机架冷连轧机的生产实践.应用结果表明:提高了轧制的稳定性与带材的控制精度,减小了相关模型的自学习与自适应系数波动,取得了良好的使用效果.     

热轧精轧轧辊摩擦和磨损研究

针对某厂热连轧机采用定值摩擦系数计算轧制力与实际值相差2 000～3 000kN的情况,采用BP神经元网络与NeuroShell2软件对摩擦系数进行预测与检验,建立了符合现场实际的摩擦系数模型,在实际应用中提高了轧制力的预报精度.并根据POMINI公司磨床的辊型曲线,分析了精轧轧辊磨损的基本规律,提出了减小轧辊磨损的具体措施.     

Simulation of rolling friction in the working stands of wide-strip mills

The energy consumed for rolling friction in the interroll contact area in the working stands of cold-rolling and pinch-pass mils intended for the production of wide steel strips has been analyzed. The coefficients and power of rolling friction are obtained for the first time using the databases of the process control systems of operating mills and simulating these quantities. A statistically reliable regression relation is obtained between the coefficient of rolling friction and the significant parameters of rolling and skin rolling (i.e., the interroll force, the roll speed, and the roll body roughness). The power fraction consumed for rolling friction is found to reach 60–80% of the total power of the main drive of working stands for skin rolling and 30–50% for cold rolling. Therefore, it is necessary to take into account these power losses in designing mills and developing technological cold-rolling conditions.     

板宽对4辊CVC热轧机板形控制能力的影响

采用流面条元法分析带材的三维塑性变形，影响系数法分析辊系的弹性变形，并将二者耦合，建立了4辊轧机板形和板凸度的分析计算模型，并针对板宽对4辊CVC热带钢轧机板形控制能力的影响进行了仿真研究。仿真结果表明，随着板宽的增加，出口板凸度先增大后减小，辊缝变得越来越平缓；单位宽度辊间压力增大，且沿横向的变化比较均匀；板带的宽展减小。对热带钢轧制，乃至厚板轧制具有重要的意义。     

Friction Estimation and Roll Force Prediction during Hot Strip Rolling

A mathematical model of friction coefficient was proposed for the roll force calculation of hot-rolled strips. The online numerical solving method of the roll force calculation formula based on the proposed friction model was developed and illustrated by the practical calculation case.Then,the friction coefficient during hot strip rolling was estimated from the measured roll force by force model inversion.And then,the expression of friction model was pro-posed by analyzing the calculation process of stress state coefficient,and the model parameters were determined by the shared parameter multi-model nonlinear optimization method.Finally,the industrial experiments demonstrated the feasibility and effectiveness of the related models.The accuracy of the new roll force model based on the built friction model was much higher than that of the traditional Sims model,and it could be applied in the online hot roll-ing process control.     

Reduction of slab stock by application of natural spread in the hot rolling of plate

In recent years new technologies such as Hi-spread and heavy-edge reductions have been established, in order to achieve a hot link. These technologies are mostly not applicable in existing mills. Hi-spread technology also reduces the number of slab sizes and thus the slab stock. However slab stock can be reduced to some extent by utilizing the natural spread in rolling. The spread mechanism is quite complex and spread is influenced by roll-gap geometry, the friction in roll gap and by the plastic properties of rolled material. The influence of friction and roll-gap geometry are closely connected and have been fairly well investigated earlier, while the influence of plastic properties of rolled material on spread still is unknown. By deduction of a new spread formula and establishment of criteria for edge rolling necessary to obtain plate with straight edges, it was possible to reduce the slab stock for rolling of plate by in order of 10% at Ovako Steel Profiler AB in Luleå.     

大型筒节轧制力预报模型

 根据筒节和轧辊的几何关系,得到了筒节上下表面接触弧长的几何方程;根据现场数据和有限元方法,得到了筒节上下表面接触弧长的变形方程;结合几何方程和变形方程,并基于赫希柯克公式计算了考虑弹性压扁的筒节接触弧长。由于筒节外端对轧制力的影响远远大于接触摩擦的影响,结合接触弧长模型和材料变形抗力模型,基于现场数据和优化算法,优化得到了外端应力状态影响系数,从而建立了大型筒节轧制力预报模型。结果表明：上下辊的接触弧长不等,上辊接触弧长稍大于下辊接触弧长,上辊压下量大于下辊压下量,上辊和下辊的接触弧长之比约为1.3左右;将模型应用到筒节轧制中,计算轧制力与实测轧制力平均误差为9.2%,模型计算精度较高,能够满足工业应用要求。     

Simplified approach for prediction of spread,roll load and torque in grooveless rolling of billets

Abstract

A simplified analytical approach has been developed to predict the spread, roll load and torque in the grooveless rolling of billets under various conditions of billet aspect ratio, height reduction ratio, friction coefficient and roll radius. The results obtained from the simplified analytical model are shown to be in fair agreement with those obtained from three-dimensional finite element simulation under the same conditions. The main merit of the proposed simplified analytical approach, as compared with three-dimensional finite element simulation, lies in the drastic reduction of computational time required in finite element simulation, which favours the effective use of such a simplified approach to design the rolling schedule and apply online control of grooveless rolling.