Roll-strip Interface Research Articles

A numerical strategy for predicting the interface stresses in metal strip rolling with small reduction

This paper has developed a numerical strategy for predicting the interface stresses in metal strip rolling at a small reduction. Key rolling factors such as the roll elasticity, fluid lubrication, random asperity contacts, asperity-lubricant interaction, rolling speed and reduction were comprehensively integrated into the numerical analysis. The random asperity contact at the roll-strip interface was addressed by a statistical approach with considering the elastoplastic deformations of surface asperities. A transient average Reynolds equation was utilised to deal with the fluid flow subjected to the interruptions caused by the asperity contacts. With the aid of a dynamic explicit finite element analysis, a novel modularised strategy was established to systematically accommodate the mathematical models governing the above key rolling factors, which allows the affordable computation and the effective predictions of contact characteristics in the strip rolling. It was found that both the lubrication and microscale asperity contacts disturb considerably the contact stresses at the rolling interface, and that a proper selection of rolling conditions can reasonably regulate the stresses.

Surface texture transfer in skin-pass rolling with the effect of roll surface wear

This paper presents a novel three-dimensional characterisation of the wear effects of a roll surface on the texture transfer in skin-pass rolling. The method integrates the interactions of the microscale elastic deformation of roll surface asperities with the microscale plastic deformation of strip surface asperities. An elasticity theory and a discrete fast Fourier transform were used to rationally predict the microscale elastic deformation of work roll in the rolling system. The microscale plastic deformation of the asperities on the soft strip surface was calculated by the material redistribution process. The Archard wear law was used to consider the wear effects with the random contact pressure, relative sliding and surface hardness. A multiscale analysis was carried out by the dynamic explicit finite element method (FEM), which fully coupled microscale surface contacts, macroscale elastic deformation of the work roll and the bulk plastic deformation of the metal strip. Due to the complexity associated with numerical simulation, a novel modularised approach was established to complete the calculation in a feasible and efficient manner. The study reveals that the roughness scale of a rolled metal strip decreases gradually due to the roll surface wear and that the rolling force, relative sliding and rolling speed at the roll-strip interface significantly affect the roll surface wear. It is expected that the method established can be used to control the surface texture transfer in skin-pass rolling under the continuous wear of roll surfaces.

Measurement of metal–roll interface during metal rolling using normal and oblique ultrasonic reflection

ABSTRACTIt is important to monitor the roll bite interface during metal rolling to maintain the product size and homogeneity so as to minimize the material wastage. However, the harsh nature of cold rolling makes installation of sensors in metal roll for industrial applications difficult. The present study used a novel ultrasonic measurement technique whereby an ultrasonic signal went through an external sensor layout arrangement to study the metal-roll interface. The reflection coefficient obtained from the roll-strip interface at 0° to the roll surface (normal ultrasonic measurement technique) and 19° (oblique ultrasonic measurement technique) were modelled and experimentally investigated on an instrumented pilot metal rolling mill. Variances of 6.4% and 8.8% were obtained in the reflection coefficient of the techniques from experimental and modelling approaches, respectively. This showed that both techniques could be used to study the effect of the angle of incidence wave on the reflection coefficient.

A multi-field analysis of hydrodynamic lubrication in high speed rolling of metal strips

This paper presents a multi-field analysis of full hydrodynamic lubrication in high speed cold rolling of metal strips. A robust method was developed to solve the fluid field (the lubricant flow at the roll-strip interface) and solid field (the elastoplastic deformation of a metal strip) in a simple step. The elastoplastic deformation of the strip was solved by a dynamic explicit finite element method while the hydrodynamic pressure was calculated by the Reynolds equation. A cavitation boundary was introduced to determine the hydrodynamic pressure when the lubricant film breaks down. In addition, the coupled interaction between the lubricant flow and the elastoplastic deformation of the strip, which was caused by the hydrodynamic pressure and lubricant shear stress at the solid-liquid interface, were successfully treated by an additional penalty term. Both the pressure-driven term and lubricant shearing term were used to calculate the lubricant shear stress. The introduced penalty method enables a stable numerical calculation of Reynolds equation at a high hydrodynamic pressure. The method has overcome the traditional difficulties in solving the interface contact pressure within separated zones and has led to new understanding of the effects of rolling speed and pressure coefficient of the lubricant on the hydrodynamic pressure and lubricant film thickness.

Transient Thermal Analysis of Cold Strip Rolling Considering Rough Surface Contacts

This paper presents a transient thermal analysis of cold strip rolling, integrating both the microscale asperity deformation and macroscale strip deformation. A statistical characterisation was carried out to obtain the contact pressure and thermal contact conductance at the roll-strip interface. To address the effects of rolling speed and temperature rise, Johnson-cook constitutive model of yielding strength was employed to incorporate the strain rate and temperature variation in the analysis. It was found that the developed model can successfully predict both the interface contact stress and temperature rise in the rolling bite.

Interfacial heat transfer under mixed lubrication condition for metal rolling: A theoretical calculation study

Interfacial heat transfer, as an important factor in rolling process, can significantly affect the roll conditions and the product qualities. However, the heat transfer behavior at the roll-strip interface is highly unclear because of various factors including material properties, contact conditions, surface qualities, etc. It is important to gain an insight into the interfacial heat transfer to clarify the heat transfer characteristics in the work zone along the rolling direction. In this paper, a computational analysis of interfacial heat transfer of strip rolling has been carried out based on the heat transfer theory and the mixed lubrication model, which takes the changes of the real contact area and oil film thickness into account. Also, the effects of oxide scale with and without extruded metal on the interfacial heat transfer are discussed. The results of this research suggest that the interfacial heat transfer coefficient (IHTC) at different positions can be determined based on the local contact conditions, and the theoretical calculation is shown to be an effective way to understand the interfacial heat transfer in the work zone under different rolling conditions.

The stribeck curve in cold flat rolling

Cold flat rolling of lubricated steel strips was studied. The three-component system of rolling - the mill, the rolled strip and their interface - was analyzed using a two-step mathematical model, a 1D model and flat rolling experiments. The utilized experimental data have been taken from McConnell and Lenard [1]. The objective of the study was the examination of the interactions of the three components and the development of the Stribeck curve. In the first part of the work, a two-dimensional finite element model was used in which the elastic deformation of the work roll and the elastic–plastic deformation of the strip were considered. Special attention was paid to the events at the lubricated interface where an upgraded version of Levanov’s model was utilized. The model was developed to analyze the local variables at the roll-strip interface. The parameters of the model were determined in an iterative manner, minimizing the differences of the measured and computed roll force and torque. Thus, the relative velocity between the roll and the strip, the roll pressure, the interfacial shear stress and the temperature were obtained. The effect of the temperature on the material parameters of the constitutive equations was also taken into account. Another model was then employed to consider the effects of the local variables on the lubricant’s viscosity. These were then used to obtain the local values of the Sommerfeld number, which in turn led to its average value and to the traditional shape of the Stribeck curve. Further, a simple 1D model of the flat rolling process was also tested for its ability to lead to the Stribeck curve. It was concluded that while both advanced and simple models allowed the development of the curve, removing the simplifying assumptions yielded a more reliable plot.

A novel multi-scale statistical characterization of interface pressure and friction in metal strip rolling

This paper presents a novel multi-scale method to investigate the interface pressure and friction in a 3D model of cold strip rolling. The key innovation is the introduction of an equivalent interfacial layer11Equivalent interfacial layer (EIL). to integrate the effect of lubricant with surface asperity deformation. This multi-scale method also enables to capture the microscopic roll-strip interface deformation associated with the random asperities and to treat all the lubrication regimes from hydrodynamic lubrication to mixed and boundary lubrication in a single program. The proposed method can predict well the experimental measurements, and is applicable to other contact sliding processes involving complex lubrication regimes.

Analysis of Roll Gap Heat Transfers in Hot Steel Strip Rolling through Roll Temperature Sensors and Heat Transfer Models

This paper presents an analysis of roll bite heat transfers during hot steel strip rolling. Two types of temperature sensors (drilled sensor /slot sensor) implemented near roll surface and heat transfer models are used to identify in the roll bite interfacial heat flux, temperature and Heat Transfer Coefficient HTCroll-bite during pilot rolling tests. It is shown that: - the slot type sensor is much more efficient than the drilled type sensor to capture correctly fast roll temperature changes in the bite during hot rolling but life’s duration of the slot sensor is shorter. - average HTCroll-bite, identified with roll sensors temperature signals is within the range 15-26 kW/m2/K: the higher the strip reduction is, the higher the HTCroll-bite is. - scale thickness at strip surface tends to decrease heat transfers from strip to roll in the roll bite. - HTCroll-bite appears not uniform along the roll-strip contact, in contrast to usual assumptions made in existing models - Heat dissipated by friction at roll-strip interface and its partitioning through roll and strip respectively seems over-estimated in the existing thermal roll gap model [1]. Modeling of interfacial friction heat dissipation should be reviewed and verified. The above results show the interest of roll temperature sensors to determine accurately roll bite heat transfers and evaluate more precisely the corresponding roll thermal fatigue degradation.

New concept of friction sensor for strip rolling: Theoretical analysis

A new concept of sensor is proposed to measure friction at roll–strip interface during strip rolling. By measuring elastic strains inside the roll, it is possible to evaluate contact stresses at roll–strip interface using an inverse analysis, with the great advantage not to mark the strip with the sensor. Using simulations, this new sensor is designed and its ability to evaluate contact stresses is discussed as a function of rolling conditions: influences of roll thermal stresses, roll deflection, roll–strip contact length, distance of measurement points to roll surface on contact stress reconstruction by inverse analysis are characterised. Results show that this sensor could be used in various pilot hot and cold rolling conditions for friction evaluation. Next step of this work is a pilot rolling test of the sensor.

The effect of oxide scale of stainless steels on friction and surface roughness in hot rolling

Friction is one of the most significant physical phenomena influencing metal forming. The deformation or fracture of oxide scale significantly affects the roll–strip interface behaviour. Sticking occurs frequently during hot rolling of stainless steels especially ferritic stainless steels, which causes surface defects of both steel products and rolls. It is important to characterize the features of the oxide scale in hot rolling of stainless steel strips, but few studies have been carried out. This paper focuses on the deformation of oxide scale and roll–strip interface characteristics in hot rolling of austenitic stainless steel 304L and ferritic stainless steel 430. Oxidation tests in a short time in humid air with water vapour content of 7.0 vol.% were carried out using Gleeble 3500 thermo-mechanical simulator, and the oxidation characteristics of 304L and 430 steels were obtained. The deformation, surface morphology of oxide scale of the steels, and the friction in hot rolling were studied in hot rolling tests. Thick oxide scale of 304L steel shows high lubricative effect while the breaking of thin oxide scales of 430 steel may make the steel substrate to contact with the roll, which counteracts the lubricative effect of the oxide scale and results in friction coefficient increasing with an increase of reduction. A FEM simulation has been developed to analyse the deformation behaviour of oxide scale and surface roughness transfer during hot rolling of stainless steels and the model describing the rough surface profiles of the scales and steel has been integrated into the simulation.

Adhesive tool wear in the cold roll forming process

Tool wear manifests in change of roughness of roll surfaces and governs the surface quality of both coated and uncoated products. Decrease of friction and wear in the roll–strip interface will stabilize the performance of the cold roll forming (CRF) mill and ensure uniform process output. The scope of this paper includes an experimental study of adhesive wear of hardened tool steel blocks sliding against a mild steel wheel and field observation of tool wear in the CRF of zinc coated mild steel. Wear mechanisms are identified for the materials interacting during the CRF process. The results include an empirical value of tool wear coefficient for D2 steel sliding against unlubricated uncoated hot rolled AISI C1020 mild steel.

Experimental Study on the Deformation of Oxide Scale and Friction during Hot Rolling of Stainless Steel 304L

Contact friction is of crucial importance for the accurate modelling, optimum design and control of industrial rolling processes. Hot rolling tests were carried out to investigate the deformation of oxide scale and friction during hot rolling of stainless steel 304L. The morphology of oxide scale layer and the surface roughness transfer under the conditions of hot rolling were obtained. The friction condition at the roll-strip interface was determined.

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 2.5×10−4m3/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.

Film Formation Analysis with Starvation in the Inlet Zone of Cold Rolling Interface at High Roll Speeds

For successful cold rolling operation at elevated operating conditions, the existence of desirable minimum film thickness at the roll-strip interface and awareness of its controlling by adjusting the process variables such as roll speed, reduction ratio, lubricant types, and roll cooling system etc. are essential. Moreover, the studies of the lubricant's starvation in the inlet zone and its influence on the film thickness at high roll speeds considering viscous shear heating effects are very important issues. Thus, the objectives of this paper are to analyze the inlet zone of cold rolling interface thermohydrodynamically and develop formulas for the prediction of minimum film thickness (for both isothermal and thermal cases) as functions of operating parameters including levels of starvation. Based on the investigations reported herein, it is observed that the existence of starvation up to certain level seems to be beneficial. It causes reduction in the temperature rise in the inlet zone and reduces the quantity of lubricating oil (leading to oil conservation) required during the cold rolling operation provided a thin continuous film exists at the strip-roll interface. Authors believe that the cold rolling industries may be benefited by the formulas reported in this paper.

Analysis of interface temperature, forward slip and lubricant influence on friction and wear in cold rolling

To improve cold rolling processes, it is necessary to understand and to optimise contact at roll–strip interface. Thus a simulation test has been developed in our laboratory. The upsetting rolling test (URT) enables to study friction and iron fines pollution by the reproduction of the main industrial contact conditions such as plastic strain, normal and tangential stresses and forward slip. On the basis of the URT, a new experimental protocol has been developed to reproduce industrial lubrication regime. Moreover, a new heating system has been designed to simulate interface temperature which has a decisive effect on lubricant behaviour. These optimisations permit to analyse contact temperature, forward slip and lubricant influence on friction, iron fine pollution and surface aspects. A great influence of temperature and lubricant on friction and wear has been put forward. Actually an increase of the Coulomb friction coefficient associated with a decrease of the iron fines quantity have been shown with an increase of temperature. These results seem to indicate more adhesive wear when temperature increases.

Thermal and Starvation Effects on the Minimum Film Thickness in Inlet Zone in Cold Rolling

The main objective of this research is to analyze the variation of minimum film thickness in the inlet zone of roll-strip interface by incorporating starvation and viscous shear heating effects at high rolling speeds (5–20m∕s), reduction ratios (0.05–0.20), and slip values (varying up to 20%). An additional objective of this paper is to develop empirical relations for predictions of minimum film thicknesses (both isothermal and thermal) and maximum film temperature rise in the inlet zone of the lubricated roll strip contact as functions of roll-speed, reduction ratio, material parameter, slip, and starvation parameter. An efficient numerical method based on Lobatto quadrature technique is adopted for rigorous analysis of the present problem. The results reveal that the existence of starvation seems to be beneficial in terms of reduction in maximum film temperature rise as well as reduction in quantity of oil required for lubrication provided thin continuous film exists at the contact.

Numerical Simulation of Heat Transfer Problem in Hot and Cold Rolling Process

An efficient numerical model had been developed to model the thermal behaviour of the rolling process. An Eulerian formulation was employed to minimize the number of grid points required. The model is capable to calculate the temperature distribution, the heat penetration depth, the convection heat transfer coefficient of cooling, the flow of metal through the roll gap, and the heat generation by plastic deformation and friction. The roll is assumed to rotate at constant speed, and the temperature variations are assumed to be cyclically steady state and localized with a very thin layer near the surface. The Conventional Finite Difference (CFDM) based on cylindrical coordinates was used to model the roll, and a Generalized Finite Difference Method (GFDM) with non-orthogonal mesh was employed in the deformed strip region and the roll-strip interface area. An upwind differencing scheme was selected to overcome the numerical instability resulting from the high velocity ( high Peclet number ) involved in the rolling process. The equations of the strip and roll are then coupled together and solved simultaneously. Both cold and hot rolling heat transfer behaviours, velocity distribution, and heat generation by deformation and friction under typical rolling conditions were presented to demonstrate the feasibility and capability of the developed numerical model. It has been found that, while the strip is under deformation, the bulk temperature inside the strip increases continuously; this is largely controlled by the deformation energy. On the other hand, the strip surface temperature changes much more drastically and it is mainly controlled by the friction heat and the roll temperature. The roll acts like a heat sink, because the coolant heavily cools it. Thus, as soon as the strip hits the roll its surface temperature drops. Since considerable friction and deformation heat are created along the interface and transferred from the neighboring sub-layer, the surface temperature picks up rapidly. Finally, the results of the temperature distribution for both cold and hot rolling and the heat generation by deformation and friction obtained from the present study were compared with previous published work to verify the validity of the numerical solution. Good acceptable agreements were obtained.

Asperity deformation, lubricant trapping and iron fines formation mechanism in cold rolling processes

Contact conditions in cold rolling are decisive to improve quality and final aspect of rolled strip. The control of contact conditions is obtained by a good combination of process parameters, such as normal and tensile force, forward slip and lubrication regime. To understand and to optimise contact at roll–strip interface, a simulation test was developed in our laboratory. The upsetting rolling test (URT) permits reproduction of the main contact conditions for cold rolling. The URT is able to simulate and to define the influence of rolling parameters on the iron residue quantities, and on the quality of the final surface. In accordance with finite element simulations, we can estimate a mean Coulomb’s friction coefficient. A very good correlation was found between experimental and numerical results. A complementary finite element model has been developed to predict the local behaviour of surface asperities during cold rolling. In this simulation at local scale, the roughness and trapping of lubricant are introduced, in addition to the pressure force and forward slip. It has been found that forward slip and reduction ratio modify the plastic strain behaviour of asperity through pressure and shear force. Thus, we are able to define the quantity of iron residues, as a function of the local plastic strain and the plastic energy solutions.

The cut-groove technique to infer interfacial effects during hot rolling

This article presents a novel experimental technique to infer the coupled effects of friction and heat transfer during the hot rolling of steels. The technique, termed the “cut-groove” method, relates the behavior of the deforming grooves cut on the strip surface to the local effects of friction and heat transfer. Validation of the experimentally observed groove shapes involved developing two-dimensional (2-D) and three-dimensional (3-D) finite-element (FE) models that employed a probabilistic distribution diagram (PDD). The PDD framework modeled the roll-strip interface and accounted for the variations in the oxide scale as distinct states that affect both friction and heat transfer. The numerically predicted groove openings are in good agreement with the experimentally observed groove shapes, particularly for the 2-D case. For the 3-D model, deviations are observed at regions close to the strip edges that are affected by nonplanar strain arising from spread during laboratory rolling.