Incoming Strip Research Articles

Use of Advanced Control Techniques With Virtual Rolling to Improve Threading of the Tandem Cold Mill

The threading of the tandem cold metal rolling mill is a significant function in the control of this process. During the threading process it is important to reduce the excursions in the strip thicknesses and tensions. Additionally, in most cases it is desired by the operational personnel to control the individual mill stand speeds such that, as the head of the strip moves between two adjacent stands, its speed between the two stands is held at the strip speed at the exit of the last stand just before acceleration to run speed. Moreover, it is desired that disturbances in the thickness and the resistance to deformation in the incoming strip are successfully handled by the controller. In this article, we propose a method of improved control for this process that considers all of these requirements. The effectiveness of this method is shown by simulation using data from operating mills.

Modeling and exploiting the strip tension influence on surface imprinting during temper rolling of cold-rolled steel

To produce cold-rolled steel strips with specific mechanical properties and surface roughness typically temper rolling is adopted. In most cases, a uniform roughness pattern on the strip surface is mandatory. Due to the wear of the textured work rolls, their surface roughness (Ra) continuously reduces during the process, which should be accounted for process control. However, conventional temper rolling systems fail to guarantee a uniform surface roughness. In this work, the influence of strip tension on the imprinting of surface roughness during temper rolling is analyzed based on a multi-scale FE modeling concept to explore new ways for surface roughness control. This is done in simulation where, a macroscopic rolling model incorporating strip tension is coupled to a mesoscopic imprinting model and both models are validated using copper rolling trials. The influence of different thickness reductions, strip tensions and incoming strip's surface roughness on imprinting is modeled and compared. The numerical results reveal that a higher strip tension decreases the roughness transfer, which presents potential to control the roughness transfer ratio without changing other process parameters like the prescribed thickness reduction in the future.

Effect of Internal Stress of Incoming Strip on Hot Rolling Deformation Based on Finite Element and Infinite Element Coupling Method

Limited by the deficiency of the finite element method (FEM) to deal with infinite domain problems, the traditional FEM rolling model is not suitable enough to study the effect of the complex internal stress of the incoming strip on hot rolling deformation. To solve this problem, the finite element and infinite element (FE-IE) coupling method is adopted, where the finite element is for the rolling area of the strip and the infinite element is for the elastic constraint of the strip end. Based on the improved rolls-strip coupling model, several internal stresses with typical distribution forms are applied to the strip by a programmed user subroutine, and the effect of the internal stress of the incoming strip on hot rolling deformation is evaluated. The results show that under the same average stress, the various distribution forms of internal stress have little effect on the total roll force, mainly because of the average effect. The uniform internal stress decreases the central thickness and quadratic crown of the strip. Under the symmetric stress and asymmetric stress, the thickness of each fiber along the strip width direction is closely related to the magnitude and distribution of the stress deviation (subtract the average stress from the longitudinal stress). Under the quadratic wave stress, the central thickness and quadratic crown vary almost linearly with the amplitude of the stress deviation. The efficiency coefficients obtained can be treated as a theoretical basis for the further development of an accurate prediction model of hot rolling.

Investigation of correlation between material properties, process parameters and residual stresses in roller levelling

When processing metal strips, roller levelling is the first process the sheet metal undergoes after decoiling. The process of roller levelling mainly aims on the flatness of the material. Additionally, the resulting residual stress distribution after levelling is of great importance since a strip might be flat but have a disadvantageous residual stress distribution for further processing. In this paper the correlations between the incoming strip condition, the roll intermeshes and the residual stresses are evaluated in order to achieve both, a constant flatness as well as a distinct residual stress distribution after levelling. The investigations are focusing on a seven-roll levelling machine with three individual load triangles. The first load triangle ensures a constant plastification of the incoming strip, the load triangle in the middle aims on the residual stress distribution and the last load triangle sets the flatness of the sheet material. Within a numerical model of this setup, a variation of the incoming material properties by means of yield stress and kinematic hardening is done. From the numerical model both global values such as the forces acting on the rolls as well as the local residual stress distribution are obtained. Accompanying experiments are conducted on a laboratory-scale roller levelling machine. Furthermore, the residual stresses across the sheet thickness after levelling are measured using diffraction analysis. The results indicate a distinct relationship between process parameters and the target values of flatness and residual stress distribution. Finally, a strategy for an online process control acting on both flatness and residual stress distribution is introduced based on these results.

Improving the thickness accuracy of cold rolled narrow strip by piezoelectric roll gap control at high rolling speed

Particularly in fast rolling mills, conventional actuators reach their dynamic limits, when longitudinal thickness variations of the incoming strip shall be reduced with high accuracy by model-predictive roll gap control. Accordingly, the applicability of highly dynamic piezoelectric actuators in combination with electromechanical spindles and a high frequency precision measurement of the thickness in front of the roll gap was examined. Rolling tests in a cold rolling mill for narrow slit strips show that this novel concept is suitable to provide the required dynamic actuation especially at high rolling speed.

Analysis of Startup Process and Its Optimization for a Two-Stand Reversible Cold Rolling Mill

Dynamic characteristic analysis of a two-stand reversible cold rolling mill in the startup process was carried out. The delay algorithm of the interstand thickness was proposed. A new method combined with the accelerated secant and the tangent methods was established to solve the simultaneous equations. The thickness and interstand tension transition processes with different static tension establishing processes were analyzed. Both mills were operated under constant rolling force control mode in the above process. The results show that the strip thickness in the rolling gap reduces in the static mill screwdown process. The entry stand runs inversely to establish the static interstand tension. This area becomes an abnormal thickness reduction area of the incoming strip. It results in several abnormal interstand tension increases in the subsequent startup process. The tension increase leads to an impact force on the strip that is the main reason of the strip breakage in the startup process. So the static tension establishing process was optimized, and the interstand tension fluctuation and the strip breakage accidents both reduced significantly. The results are beneficial to the startup process of the two-stand reversible cold rolling mill.

Modelling of strip buckling in the continuous annealing process

This paper considers the equipment and technological characteristics of continuous annealing process (CAP) to solve the issue of strip buckling. It analyses the mechanism of hot buckling and transverse compressive stresses from internal and external forces. A basic description of strip buckling is given together with a new concept of a buckling index. The critical buckling condition is presented, and the model of strip buckling index in the CAP is proposed. The distribution of strip buckling index in different sections of the process is analysed quantitatively along with the influence of incoming strip shape, transverse temperature difference, roller profile, friction coefficient, set tension and strip processing speed on the strip hot buckling index in CAP. Results show that with the influence of thermal stress, the transverse compressive stress that affects the middle part of the strip is largest in the heating section, which is also where hot buckling occurs most easily; centripetal force is dominant in the soaking and slow cooling sections, and in these sections buckling occurs at the roller shoulder. In different sections, the strip buckling index distribution differs, whereas the influence of other parameters on buckling index remains similar.

Development of a Laser Triangulation Gauge for High Precision Strip Thickness Control

Almost all metal strips with thicknesses of < 2 mm are produced by cold rolling. Thickness variations of cold rolled strips are caused by various factors like fluctuation in strength of the material, the eccentricity of the rolls or thickness variation of the incoming strip. As the demands concerning the thickness variation are ever increasing the Institute of Automatic Control and the Institute of Metal Forming aim at reducing the thickness tolerance of thin, cold-rolled steel and copper strips to 1 μm. As high frequency disturbances are expected, it is assumed that this goal can only be achieved by using a predictive controller in combination with a high precision strip thickness gauge and, for roll adjustment, a piezoelectric actuator in addition to the existing electromechanical actuator. The objective of this work is the constructive implementation and the testing of a thickness gauge based on laser triangulation. The gauge includes guide rollers to prevent strip vibration, a C-frame to allow an inline calibration and mechanical adjustment of the measuring range so that even flexible strip thicknesses can be measured. The designed gauge showed a high repeat accuracy of 0.4 μm for two different metal strips. Furthermore the gauge was used to investigate the dynamics of the thickness change of a steel strip at maximum rolling speed of 5 m/s using a Fourier transformation. This frequency analysis supports the need for a piezoelectric actuator that can also subsequently be dimensioned based on the obtained frequency data.

Study of the Tractive Forces Applied to Galvanized Strip on a Straightening Machine in a Continuous Hot-Galvanizing Unit

Relations are presented to determine the tractive force exerted on strip by a straightening machine in a continuous hot-galvanizing unit. The working rollers of the machine have different radii. The relations take this difference into account along with the coefficient of rolling friction in the bearings of the rollers, the number of rollers in the straightening machine, the curvature of the strip, unit back tension, and the strip's mechanical properties and geometric dimensions. Results are presented from a study of how the tractive forces are affected by the cross-sectional area of the strip and the unit back tension. The results are Most of the flat-rolled products made with metallic coatings are strips and sheets with zinc and zinc-bearing coatings. In world practice, rolled strip is galvanized with the use of various types of machines and methods which differ mainly in the processes used for annealing and degreasing the metal (1-4). The accuracy of the geometry of galvanized strip is appreciably influenced by the adjustment of the straightening machine. The goal of present investigation is to examine the tractive forces applied to the strip by a straightening machine in a continuous hot-galvanizing unit (CHGU) and to develop efficient regimes for subjecting the strip to tension. The CHGU at the Mariupol Metallurgical Combine applies a wide range of coatings to strip in a broad range of widths and thicknesses. The unit controls coating thickness through the use of jets, subjects the strip to non-oxidative heat- ing, rapidly cools it in a controlled manner with a protective gas, and is equipped with an automated system that controls the overall production process (4). Among the pieces of equipment in the CHGU: uncoilers; twin shears for cropping the ends of the strip; centering rollers; welding machine; shears for trimming the sides of the strip; a horizontal storage unit that receives incoming strip; preheating furnace, high-speed heating furnace, and soaking unit; sections for rapid, slow, and final cooling of the metal; a sloping trough and a galvanizing bath; a gas cutter to control the thickness of the coating; air cooling section; a vertical unit for storing outgoing strip; temper stand; straightening machine; lubricating unit; coilers; a unit for transverse cutting of the strip; packaging machine. After the strip has been coated, it is straightened to obtain a smooth surface without shape defects. Steel strip with a thick- ness in the range 0.16-0.32 mm is straightened in straightening machines with 40-46-mm-diam. work rollers, while strip thicker than 0.32 mm is sent through straightening machines with work rollers 50-60 mm in diameter. The speed of the rollers is syn- chronized with the overall speed of the hot-galvanizing unit. Tension devices are positioned on both sides of the straightening

Prediction of hot deformation resistance during processing of microalloyed steels in plate rolling process

Mean flow stress of microalloyed high-strength steels during plate rolling has been thoroughly studied. It has been found out by both thermomechanical tests and measurements taken in the industrial plate mill. For this purpose, log data obtained from the plate rolling mills have been converted to mean flow stress using the Sims approach. The agreement between thermomechanical tests and mill data has been tested in order to confirm that thermomechanical testing can provide an easy, convenient and very effective simulation of industrial hot rolling process. The results are analysed and compared to the predictions of some mathematical models developed in literature. Subsequently, the best performing formula, namely the Poliak's equation, has been optimised by means of genetic algorithms and the standard Gauss–Newton method. This latter has allowed a finer tuning of the models' parameters in order to fit at best the available data. The Poliak's formula optimised by genetic algorithms is shown to accurately predict the mean flow stress and therefore it provides a useful tool for the determination of the milling settings before the incoming strip enters the mill.

Influence of Strip Wave-Shape upon Running Deviation inside Continuous Annealing Line

In order to enhance the running deviation during the production of continuous annealing line (CAL) unit, the author conducted a research and revealed that the wave shape of incoming strip had significant effect on the deviation based on the statistic analysis of production data. Via finite element method analysis, a strip-furnace-roller dynamic simulation model was established, in which the perturbation of incoming strip to operation in contact zone was simulated to qualitatively research on effect trend of incoming wave shape on deviation. The result indicates that the strip with single edge wave causes more deviation than strips with other types of wave; in addition, wave height, wave length and wave width have contribution to the influence on the deviation. Accordingly, methods were adopted to optimize the upstream work procedure, and control single edge wave of incoming cold rolls can reduce accidents of slow-down and emergency shutdown caused by deviation.

An efficient model for the crosscut optimisation problem in a wood processing mill

We propose a dynamic programming model for the crosscut optimisation problem. This problem arises in the context of cutting lumber (rip-first) in order to obtain cubic blocks (cut-pieces) with required dimensions and surface characteristics. We propose a novel approach for matching the pattern of defects on all four surfaces of an incoming strip of wood with the surface requirements of the cut-pieces as specified in a given cut-bill, and determine an effective cutting pattern for each incoming strip accordingly. Our proposed dynamic programming model results in fast execution times as shown by the results of a computational experiment. The model is deployed in a software package that has been implemented successfully in several major rough mills.

Research and application of non-symmetrical roll bending control of cold rolling mill

The existing research of the flatness control for strip cold rolling mainly focuses on the calculation of the optimum adjustment of individual flatness actuator in accordance with the flatness deviation, which can be used for general flatness control. However, it does not work for some special rolling processes, such as the elimination of ultra single side edge-waves and the prevention of strip break due to tilting roll control overshooting. For the purpose of solving these problems, the influences of non-symmetrical work roll bending and intermediate roll bending on flatness control were analyzed by studying efficiencies of them. Moreover, impacts of two kinds of non-symmetrical roll bending control on the pressure distribution between rolls were studied theoretically. A non-symmetrical work roll bending model was developed by theoretical analysis in accordance with practical conditions. The model was applied to the revamp of a 1250 6-H reversible universal crown mill (UCM) cold mill. Theoretical study and practical applications show that the coordination utilization of the non-symmetrical work roll bending control and tilting roll control was effective in flatness control when there appeared bad strip single side edge waves, especially when the incoming strip was with a wedge shape. In addition, the risk of strip break due to tilting control overshooting could be reduced. Furthermore, the non-symmetrical roll bending control can reduce the extent of uneven distribution of pressure between rolls caused by intermediate roll shifting in flatness control and slow down roll wear. The non-symmetrical roll bending control technology has important theoretical and practical significance to better flatness control.

Coupled approach for flatness prediction in cold rolling of thin strip

The paper presents a predictive model of the flatness defects, which appear during rolling of thin plates, the origin of which is the roll stack thermo-elastic deformation. The combination of the elastic deflection, the thermal crown and the roll grinding crown results in a non-parallel bite, and if the deformed roll transverse profile is not an affinity of the incoming strip profile, differential elongation results and induces high stresses in the outgoing strip. The latter, combined with the imposed strip tension force, result in a net post-bite stress field which may be sufficiently compressive locally to promote buckling. A variety of non-developable shapes may result, generally occurring as waviness (centre waves, wavy edges, quarter-buckles, etc.). This problem is most of the time addressed in a decoupled way, i.e. as a post-processing of the residual stresses computed by a strip rolling model; the present paper on the contrary describes a fully coupled approach of in-bite plastic deformation and post-bite buckling. For this purpose, a simple buckling criterion has been introduced in a FEM model of strip and roll deformation, Lam3/Tec3; its implementation is documented in details. The capabilities and limits of the present approach are described and discussed. Characterised by its coupled approach, it is primarily devoted to cases where on-line (under tension) manifested defects occur. It is shown that the impact of the post-bite, post-buckled stress field on the in-bite stress and strain fields is quite small in the cases investigated; however, subtle changes appear in the velocity field at bite exit, and this is sufficient to transform completely the post-bite stress field, which is found in much better agreement with measurements if such a coupled treatment is used.

Research and Application of Dynamic Substitution Control of Actuators in Flatness Control of Cold Rolling Mill

AbstractThe work roll bending control has a strong ability to eliminate the symmetrical flatness defects of strip. However, if the incoming strip is coming with severe symmetrical flatness defects, it happens that the position limit of work roll bending is prone to be reached or exceeded during the execution of the displacement, resulting in the residual symmetrical flatness defects without further elimination, as well as a limited flatness control process. Abilities of work roll bending and intermediate roll bending / shifting for flatness control have been analyzed based on the actuator efficiencies for the purpose of solving the problem of work roll bending control reaching threshold. Meanwhile, a self‐learning determination model of actuator efficiency factors, which can be utilized to determine the actuator efficiency factors online accurately, was developed from the measurement data extracted from the rolling mill. Models of intermediate roll bending and intermediate roll shifting substitution control in case of work roll bending control over‐limitation have been developed, in accordance with the practical conditions and the degree of work roll bending over‐limitation. Applications show that the substitution control can play an important role in the residual symmetrical flatness deviation control, which should have been removed completely by the work roll bending control.

A Robust Model for Work Roll Bending Control Reaching Threshold in Flatness Control of Cold Rolling Mill

The work roll bending is one of the most effective flatness actuators in flatness control of cold rolled strips, as a result of its characteristics such as high speed response, powerful effects on the elimination of flatness defects such as edge-waves, central waves and so on. The existing research on flatness control mainly focuses on the calculation of optimal adjustment of individual flatness actuator. However, the final output of a flatness actuator is required to be limited firstly for some special cases. If the incoming strip is coming with big symmetrical flatness defects, it happens that the position limit of work roll bending is prone to be reached or exceeded during the execution of the displacement, resulting in the residual symmetrical flatness defects without further elimination, as well as a limited flatness control process. In order to avoid this situation, substitution control by intermediate roll bending in case of work roll bending control over-limitation have been developed, which is based on actuator efficiencies and in accordance with the practical conditions. Applications show that the substitution control of intermediate roll bending for work roll bending over-limitation can play an important role in the residually symmetrical flatness control.

Performance Analysis of Asymmetrical Roll Bending for Flatness Control of Cold Rolling Mill

When there appeared catastrophic asymmetrical flatness defects in rolling processes, especially when the incoming strip is with a wedge shape, the tilting roll can hardly eliminate these defects completely. Moreover, the overshooting of tilting roll will lead to strip break. In order to improve the ability of cold rolling mill for asymmetrical flatness defects control, performance of the work roll asymmetrical bending as well as the intermediate roll asymmetrical bending has been analyzed, based on the actuator efficiency factors of them. In addition, for the purpose of obtaining accurate efficiency factors matrixes of actuators, a self-learning determination model of actuator efficiency factors was established in accordance with the practical rolling processes. In this paper, a 1250 single stand 6-H reversible UCM cold mill was taken as the object of this study, with efficiency factors of asymmetrical roll bending analyzed, which provides a theoretical basis for better flatness control. Analysis shows that the asymmetrical roll bending is significant for asymmetrical flatness control.

Manifested flatness predictions in thin strip cold rolling

The paper deals with flatness defects prediction in thin plates which appear during rolling. Their origin is the roll stack thermo-elastic deformation. The combination of the elastic deflection, the thermal crown and the roll grinding crown results in a non-parallel bite. If the transverse roll profile is not an affinity of the incoming strip profile, differential elongation results and induces high stresses in the outgoing strip. The latter combine with the imposed strip tension force, resulting in a net post-bite stress field which may be sufficiently compressive locally to promote buckling. A variety of non-developable shapes may result, generally occurring as waviness, and classified as flatness defects (center waves, wavy edges, quarterbuckles…). The purpose of the present paper is to present a coupled approach, following [1]: a simple buckling criterion is introduced in the FEM model of strip and roll deformation, LAM3/TEC3 [2]. The post-bite stress field is in much better agreement with experiments if this treatment is used, as will be demonstrated.

Reaction of 316L stainless steel with a galvanizing bath

Type 316L stainless steel (316L SS), commonly used as pot roll material, was tested in an industrial Zn–Al galvanizing bath, with an effective Al content of 0.2 wt.%. Samples welded to the supporting roll arms and sides of the sink roll experienced heavy dross buildup. Samples placed at locations away from the incoming strip generally experienced much less dross buildup. Dross particles within the buildup on samples attached to supporting roll arms were small in size but numerous while particles on samples welded to the sides of the sink roll were much larger. SEM-EDS analyses indicated that the buildup consisted of two layers. The inner layer was the product of the corrosive reaction of 316L SS with the bath metal, and the outer layer was formed by dross particles built up on the inner reaction layer. The intermetallic phase, which formed at the reaction front of the samples was the inhibition compound Fe2Al5 containing some Mo and Cr. The formation of this intermetallic layer provided a thermodynamically favourable base for the attachment and further buildup of suspended dross particles on the sample surfaces.

Mill-balance Control Technique for Tandem Cold Mill.

This paper describes a mill balance control (MBC) for a tandem cold mill (TCM) with an automatic gagecontrol system (AGC).The stand exit gages are controlled by the AGC, but the motor currents and the rolling forces of stands cannot be controlled.When the setup control is not proper or when the incoming strip has variations in gage and hardness, the distribution pattern of the motor currents and the rolling forces (mill balance) sometimes becomes inappropriate. When a motor current exceeds its rated value, usually all roll speeds are reduced manually, and the productivity decreases. When a rolling force varies to a great extent, strip shape defects may occur.The MBC operates the stand exit gages except for the finish gage and the interstand tensions, and it controls the distribution pattern of the motor currents and the rolling forces while guaranteeing the finish gage accuracy.The MBC controller is designed based on the ILQ design method and is applied to an actual process.Experimental results show that desired mill balance is achieved and the finish gage accuracy is maintained.