Roll Bite Research Articles

Effect of Ageing of Rolling Oil on the Performance of Cold‐Rolled Steel

ABSTRACTInteraction between the cold rolling oil and substrate at the roll bite has always intrigued the tribologists. In plant conditions, it has been observed that occurrences of surface defects on cold‐rolled FHCR coils increase with the ageing of cold rolling emulsion. In this study, the rolling oil was aged at different lab conditions to replicate 6 and 11 months of rolling oil ageing from a particular cold rolling mill. The studies indicated an increase in acid number of the oil. Tribological studies indicated an increase in CoF along with an increase in wear phenomenon on the substrate. The removability of the surface oil becomes difficult with ageing which could lead to potential surface oil residues remaining on the strip after degreasing, resulting in surface defects and dirty coils after annealing. Deterioration in anti‐stain property is also observed as ageing progresses. The deterioration in performance is mainly governed by the depletion in additives and generation of free fatty acids within the rolling oil bath. Proper maintenance of the bath ensures lesser defect formation.

Effect of Roll Diameter on Temperature of Aluminum Alloy Strip during Casting Using a Vertical Type High Speed Twin Roll Caster

The temperature of an aluminum alloy (Al-6%Si) strip cast using a vertical-type high-speed twin-roll caster was measured by inserting a K-type thermocouple into the strip. The effects of the roll diameter on supercooling, cooling rate, and temperature at the roll bite were investigated. It was found that the roll diameter affects the cooling condition of the strip. The thickness of the thermocouple was 0.1 mm and the sampling rate was 100 Hz. Copper rolls with diameters of 300 and 600 mm, respectively, were used. The roll width was 100 mm and the roll speed was 30 m/min. The roll load was 5 or 20 kN.

Reduction of Surface Cracking for Al-4.6%Mg Strips Cast Surface Using an Unequal-Diameter Twin-Roll Caster by Back-Dam Plate with Lip

Cracks occur on the surface of Al-Mg strips cast using a high-speed twin-roll caster. Surface cracking of the as-cast strip decreases with decreasing roll load. However, the difficulty of roll casting increases as the roll load is decreased because the strip becomes brittle and is easily broken because the strip is not completely solidified. In the present study, an unequal-diameter twin-roll caster, which is capable of high-speed casting, was used with a lip attached to a back-dam plate in an attempt to decrease surface cracking without deceasing the roll load. The thickness of the solidified layer was found to decrease as the length of the solidified region decreased. The thickness of the solidified layer at the center of the width direction of a strip on the lower roll decreased by shortening the solidification length by a lip attached to a back dam-plate. The solidified layer at the edges was thicker than that inside the cast strip. The thickness of semisolid metal inside the cast strip increased more than that at the edges of the roll bite. The roll load at the width of the lip decreased. The effect of the lip length on surface cracks was investigated.

Microstructure of Al-2%Si and Al-6%Si Strips Cast Using Vertical-Type High-Speed Twin-Roll Caster

Al-2%Si and Al-6%Si were cast at a roll speed of 30 m/min and a roll load of 5 N/mm using a vertical-type high-speed twin-roll caster. The rolls were suddenly stopped during casting and the molten metal was solidified. The effect of Si content on the microstructure at the cross section near the roll bite was investigated using Weck’s reagent. The cross section of the Al-2%Si strip consisted of five layers (one center layer, two middle layers, and two surface layers) at the roll bite and that of the Al-6%Si strip consisted of three layers (one center layer and two side layers). The a-Al in the center layer was globular for both Al-2%Si and Al-6%Si. The relative thickness of the center layer for Al-6%Si was greater than that for Al-2%Si. In Al-2%Si, the surface layer had a very fine chill-zone microstructure. The Si solid-solution content in the α-Al in the surface layers and the center layer was less than that in the middle layers in the Al-2%Si strip.

Novel Approach to Address Brownish Edge Band in Low Alloy Cold Rolled Steel Used in Tin Plate

Thin film formation of lubricant plays a key role in supporting lubrication and abating wear. During cold rolling of steel, esters, or film formers in rolling oil play an important role in the mixed lubrication regime. The property of the lubricant holds the key to achieving the mean film thickness during the mixed lubrication regimen. Band formation in cold rolling is often related to inadequate acid pickling. During cold rolling operation, the film formation between asperities plays an important role in controlling friction and coil temperatures. A well formulated lubricant ensures smooth transition of lubrication throughout the range of roll bite temperature and also across the lubrication regimes. A sudden spike in coefficient of friction (CoF) can often lead to increased wear and abnormal increase in coil temperatures resulting in band formation. The study highlights degradation of oil sample leading to the problem and the role of key lubricant properties in esters in mitigating the problem.

Effect of Roll Load on Microstructure of Roll Cast Al-10%Mg Strip

The effect of roll load on the microstructure of an Al-10%Mg strip cast using a vertcal-type high-speed twin-roll caster was investigated using roll stop and etching with Weck’s reagent. Roll stop was adopted to investigate the transformation of the microstructute near the roll bite. Weck’s reagent was used to visually determine the of the Mg solid-solution content. The roll load was 2, 88, or 420 N/mm and the roll speed was 30 m/min. A band zone formed at the center of the thickness direction for all roll loads. The Mg solid-solution content in the αAl in the band zone obtained at a roll load of 2 N/mm was greater than that obtained at roll loads of 88 and 420 N/mm. Recrystalization migh occur in the band zone of the strip at a roll load of 2 N/mm. The Mg solid-solution content in αAl near the surface was greater than that inside the strip in the thickness direction. The Mg solid-solution content in αAl near surface obtained at a roll load of 2 N/mm was greater than that obtained at roll loads of 88 and 420 N/mm.

Effect of asymmetric material entrance on lubrication in cold rolling

An Integrated Digital Image Correlation (IDIC) method has been applied to the cold rolling process, the images clearly visualise the geometry of the roll bite and the inlet zone. A key, novel finding is that despite using coils and applying entry tension, the strip can enter asymmetrically in the rolling mill. Moreover, it is shown that asymmetric strip feeding results in an uneven lubricant film thickness between top and bottom strip surface. A method is detailed to feed the strip truly symmetrically in the mill, a procedure that is necessary for a meaningful experimental validation of lubricant film thickness models. For symmetric rolling processes the measured film thickness corresponds significantly better with the theory than for asymmetric processes.

Temperature of Aluminum Alloy Strip During Casting Using a Vertical Type High Speed Twin Roll Caster

The temperature of a strip cast using a vertical type high-speed twin-roll caster was measured by inserting a k-type thermocouple into the strip. The effects of the roll speed and roll load on the cooling rate and temperature at the roll gap were investigated using two aluminum alloys: A383 and Al–4.8%Mg. The thickness of the thermocouple was 0.1 mm, and the sampling time was 0.01 s. The cooling rate increased and the temperature at the roll bite decreased with decreasing roll speed and increasing roll load. The positions of the liquidus and solidus lines were measured, and the reduction of the thickness of the strip was inferred from the roll gap at the solidus line.

Casting Process at Roll Bite in Strip Cast Using Vertical-Type High-Speed Twin-Roll Caster

The solidification process in strip casting using a vertical-type high-speed twin-roll caster was clarified by inserting a 0.1 mm diameter K-type thermocouple into a strip during casting. The roll speeds of the copper rolls were 10 m/min and 30 m/min, and the roll loads were 5 kN and 20 kN. Al-Si alloys with a Si content of 2%, 6%, and 12% were used to investigate the influence of latent heat and viscosity in the semisolid state. The strip temperature increased after exiting the rolls. This indicated that the inside of the strip had not completely solidified at the roll bite, and secondary cooling was necessary to rapidly solidify the inside of the strip. The distance from the roll bite to the solidification starting position ranged from 30 mm to 63 mm. The cooling rate and degree of supercooling increased as the Si content decreased. The cooling rate ranged from 400 °C/s to 4000 °C/s. The cooling rate and degree of supercooling decreased as the viscosity of the semisolid increased.

Experimental Study on the Mechanism of Pinching in Cold‐Rolling Processes

Herein, an experimental investigation into the occurrence of “pinching” defects during cold‐rolling of thin metal sheets is presented. Pinched strips are typically characterized by repetitive ripples and local ruptures, which can cause strip breaks. Even though pinches are a widely experienced phenomenon in both hot‐ and cold‐rolling of steel strips, no previous studies are known that have investigated the underlying mechanism for pinching during continuous rolling processes. Therefore, a set of rolling experiments is performed in a single‐stand pilot mill to create pinches by applying sudden perturbations in the process conditions. For the rolling settings chosen in the performed experiments, it is found that disruptions in the lubrication state are a powerful approach to induce shape defects, which develop as pinches. Herein, a first extensive description of the pinching mechanism is provided, as observed during the trials, by monitoring the strip's behavior and analyzing the damaged sheets. It is shown that ripples in these pinched strips are the result of folds, which form in the roll bite. The folds originate from the waviness of the strip upstream of the bite, being created by nonuniform conditions over the width of the strip.

An advanced model of lubricated cold rolling with its comprehensive pilot mill validation

The increasing demand for thinner and harder steel strips requires the introduction of flexible lubrication systems that continuously adapt the lubrication conditions in existing rolling mills to not saturate the stand capacity by excessive friction. To design these systems, this article introduces one of the most advanced models of lubricated cold rolling since it combines the following features in a single model: elasto-thermo-viscoplasticity of the strip, mixed lubrication by a thermo-piezoviscous lubricant, full-flooded lubrication or starvation, and a complete formulation of non-circular roll flattening. This model is then validated by a new semi-industrial data set, which is one of the most comprehensive ones since it includes: roughness measurements of the rolls and the strips, hardening laws of the strips by plane-strain compression tests, thermo-piezoviscous material laws of the lubricants and a large design space to isolate the influence of individual operating parameters. The results indicate that the changes of the rolling force and the forward slip with the rolling speed and the reduction can be quantitatively predicted except for the decreasing forward slip with the reduction. These predictions require to calibrate the coefficients of boundary friction, thermoplasticity and viscoplasticity for each rolled product as well as the inlet film thickness at each rolling speed, if starvation occurs. Once calibrated, the model therefore allows to predict the influences of various operating conditions, e.g. by how much the rolling force can be reduced if more lubricant becomes available at the entry of the roll bite.

Effect of the elastic deformation of rolls on the surface texture transfer in skin-pass rolling

This paper investigates the surface texture transfer with elastically deformed work rolls in skin-pass rolling. A numerical approach was developed to efficiently overcome the multiscale challenges associated with the numerical characterisations. To couple the microscale plastic deformation of strip surface asperities with the microscale elastic deformation of roll surface asperities, a specific method was first developed to generate the characteristics between normal contact pressure and interface separation. To deal with the three-dimensional deterministic rough surfaces, a material redistribution scheme was used to predict the plastic deformation of the strip surface asperities and a discrete fast Fourier transform algorithm was employed to efficiently calculate the elastic deflection of the roll surface asperities. These enabled the subsequent multiscale analysis in coupling the microscale interface contact with the macroscale deformation of the metal strip and work roll. As such, the stick/slip transition in the rolling bite due to the elastic deformation of the roll as well as the surface topography of the rolled strip have been quantified. The new method has successfully revealed the effects of elastic roll deformation, roll radius, and strip thickness/roughness on the resultant surface texture of strips in skin-pass rolling.

Study of wear particles formation at single asperity contact: An experimental and numerical approach

Wear particles generated due to the relative sliding of the roll and the sheet metal in the roll bite are one of the main factors that contaminate the surface of a cold rolled steel sheet. The details of the wear mechanisms in the contact of individual roll-sheet asperities define the total amount of wear particles generated. In this paper, a micro-mechanical experimental approach is coupled with material point method (MPM) scratch simulations to study the friction and wear behavior of a single roll asperity sliding through a sheet metal. Micro-tribology experiments in the form of single asperity scratch testing showed that ploughing is the dominant wear mechanism in lubricated conditions, while wedge forming was the main wear mechanism in the absence of lubricant. The beneficial influence of chrome plating the rolls on wear particles formation is found to stem from its interaction with the lubricant as the same influence was not observed in non-lubricated test conditions. MPM single asperity simulations revealed that almost all the frictional resistance arises from deforming the substrate in the case of an interfacial shear strength corresponding to the lubricated contact. In contrast, MPM showed that, in unlubricated sliding, the frictional resistance is primarily due to shearing of the adhesion junction and tearing the deforming body. Furthermore, using the degree of wear of the scratch experiments as a benchmark, a critical plastic strain needed to produce wear particles was found to be between 3 to 4 for the investigated interstitial-free steel.

Optimization of Slab Edge in Roughing of Stainless Steel

To reach high demands of a stainless steel surface quality the location of a slab edge is optimized utilizing multiphysical finite element (FE) analysis. The slab edge forms in roughing process when the longitudinal edge of the stainless steel slab moves parallelly towards the center of a transfer strip surface due to several rough and edge rolling passes. Strip spreading and location of the slab edge are managed by edge rolling process which is accomplished concurrently with roughing. Deformation resistance has a significant role characterizing the strip spreading and material flow in the roll bite, thus experimental material compression testing was carried out and the results fitted to the Hensel-Spittel equation. Multiple edger roll profiles were designed, and the most feasible details of the roll profile were iteratively utilized for the new profiles. In this way the location of the slab edge was optimized closer to the edge of the transfer strip by developing a new edger roll profile and resetting edge rolling passes according to results of FE-simulations. To mimic an industrial-scale roughing process an automated pass schedule control was developed in the FE-model. Therefore, multipass simulations require only a pass schedule data to run simulation.

Simulation of plate rolling process using finite element method

Finite Element Method (FEM) is an efficient tool to study high-temperature elastoplastic deformation of material at roll bite in a hot rolling process. The roll bite stress field significantly changes with change in the composition of workpiece material. In this paper, a study of roll bite deformation during a plate rolling process is carried out for microalloyed grade of steel using DEFORM-3D software. Norton-Hoff flow stress constitutive equation, one of the material characteristics equations inbuilt of the software, was used for the simulations. Coefficients and exponents of the constitutive equation were evaluated using multivariable optimization technique from experimental data generated in Gleeble-3500, a dynamic thermo-mechanical simulator. Input parameters like dimensions of roll, slab and roller tables of an industrial plate mill were incorporated in the preprocessor module of DEFORM-3D software. The FEM software calculates stress, strain, roll force and temperature. The stress distribution at roll bite calculated by DEFORM-3D software for microalloyed grade of steel is compared with that of plain carbon grade of steel. Effect of temperature and coefficient of friction on roll bite stress distribution for microalloyed grade of steel is discussed in the paper. Roll force predicted by the FEM software was validated with measured roll force recorded from load cells of the industrial plate mill. The predicted roll force agrees well with the measured values of roll force.

Finite Difference Modeling of the Interstand Evolutions of Profile and Residual Stress during Hot Strip Rolling

Elastic recovery and viscoplastic stress relaxation occur in the interstand of hot rolling, impacting the evolutions of strip profile and residual stress, which are major concerns for obtaining high-quality flat products. A better understanding of the evolutionary mechanisms would help develop shape control strategies. Therefore, a quasi-3D steady-state elasto-viscoplastic rolling model is developed based on the finite difference method. Predictions of spread, profile, and residual stress are validated through comparisons with a two-stand finite element model. The new model is also complemented with a roll stack model and with a viscoplastic constitutive model calibrated by hot compression tests to simulate a seven-stand hot rolling industrial experiment with low carbon steel. Comparisons between the predicted and measured profiles show a satisfactory accuracy. The simulation costs approximately a minute of CPU time, enabling the new model to run massive parametric campaigns for process optimization. It is found that during the interstand elastic recovery, the transverse compressive stress releases and the strip velocity tends to be uniform, revealing residual stress after a significant change of stress pattern. The stress relaxation mainly occurs at the edge near the roll bite and therefore increases the edge drop of the profile; it also decreases the center crown by changing the distribution of the rolling pressure in the roll bite.

Non-intrusive measurement of strip thickness and roll-bite length in metal rolling

It is important to monitor the rolled strip thickness standard to minimize material waste and loss of profit due to strip flatness defects, and also to maintain the product’s size and dimensional homogeneity. Several online measurement techniques are available but none of this can give an in situ measurement within the roll bite. Due to this, a novice experimental method utilizing ultrasonic reflection sensor mounted on one roll was developed for in situ measurement of strip thickness and roll-bite length during cold rolling process. A pitch–catch method was used whereby a piezoelectric element generated an ultrasonic pulse and transmitted to the contact interface evaluated on a pilot mill. The reflected signal was captured by a second piezoelectric element and analysed to determine the condition at the strip–roll interface. This approach was implemented on a pilot mill and reflections from various locations in the roll bite during the rolling were recorded. Recorded signals were used to estimate the rolled strip thickness and roll-bite length after the rolling process.

MODELING THE INFLUENCE OF ROLL WEAR ON THE ROLLING PARAMETERS IN A WIRE ROD BLOCK

A mathematical model of the rolling process in a wire rod block with the determination of the average resultant force of the internal longitudinal forces of the metal in the roll bite deformation is proposed, as a criterion for the rolling process stability, and the analysis of the limits of the process self-regulation from perturbations arising from the wear of the rolls of one of the modules. The influence of the wear of the gauge roll of one of the modules on the rolling parameters in all subsequent modules is revealed, which is manifested in the gradual increase of tension stresses between the modules. Such an increase in tension within the self-regulating equilibrium conditions in the deformation roll bite of the fifth and all subsequent modules leads to a decrease in the rolling moments in these modules and in general for the block. The model allows to determine the maximum wear of the calibers, which maintains the stability of the rolling process, which is estimated by the value of the average resulting longitudinal forces.

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.

Numerical simulation of periodic cracking mechanism in microscopic surface films during roll bonding processes

AbstractThe microscopic surface films existing on the top of metallic layers play an important role in the process of joining by plastic deformation. The bond formation during cold welding processes is basically associated with the fracturing of surface films to produce intimate metallic contacts. The present paper aims at providing a numerical model to describe the cracking pattern of brittle surface films bonded to the ductile substrates. A microscale finite element model is developed which takes into account the fracturing mechanisms of thin surface films in roll bonding processes. The presented model is calibrated by using the existing experimental data for an aluminum alloy covered by a thin layer of oxide film. The model is also validated against a well‐known analytical model for periodic cracking. The distribution of stresses within the fractured surface film demonstrates that the generated cracks in the surface film have essentially a periodic pattern. Moreover, it is shown that the crack spacing is highly dependent on the properties of the surface film. Finally, the obtained results for the roll bonding show that a crack density saturation takes place at the entry of the roll bite where a small surface expansion is applied to the rolled samples.