Shape and Gauge Control of Strip in a Cold Rolling Mill

Shape and Gauge Control of Strip in a Cold Rolling Mill

The economic efficiency of metal rolling processes is strongly correlated to the quality level of the end-rolled products. The latest efforts to increase the quality of the end rolled products in rolling processes have been mainly focused on the large-scale application and use of automation control advanced methods. The high proportion of thin strip production has become very important for a cold rolling mill. In this case, the thickness control and an advanced strip shape control is necessary for strip quality which is determined by the variation of the strip thickness and the strip shape.

Rolling of flat steel products is a complex process where the quality of the product is influenced by a range of factors such as incoming material, mechanical and electrical equipment, lubrication, and control strategies etc. The significant quality parameters are material thickness, material shape and surface, and the homogeneity of stress distribution. For optimized cost-efficiency and to maximize material usage, tight tolerances for the thickness are essential, to enable the strip to be rolled down as closely as possible to the minimum permissible thickness. Product quality can only be effectively optimized if the mechanical, electrical and instrumentation equipment as well as the control strategy solution combine together well.

The cold rolling of metal strip is one process in which a sequence of processes is performed to convert the raw materials into a finished product. It is a deformation process in which the thickness of the strip is reduced by compressive forces exerted by two opposing rolls (normally in a four-high arrangement). The roll rotates to pull and simultaneously squeeze the strip between them. The strip is rolled in several passes either in a reversing mill or in a tandem mill. Each work roll is supported by a backup roll of larger diameter. As the strip passes through the pairs of work rolls in each pass, the thickness is successively reduced. The reduction in thickness is caused by very high compression stress in a small region (denoted as the roll gap, or the roll bite) between the work rolls. In this region the metal is plastically deformed, and there is slipping between the strip and the work roll surface. The necessary compression force is applied by hydraulic rams or in many older mills by a screw arrangement driven by an electric motor.

Cold rolling is done to further reduce thickness of the hot rolled strip and achieve material properties which are suitable for the obtaining of rolled products where higher thickness precision, suitable flatness profiles and higher surface quality of the strip are required. Strip thickness reduction by means of cold rolling can be achieved mainly by means of three types of processes which needs different automation solutions in terms of sensors and control technologies.

The three types of the process used in cold rolling mill include (i) single stand cold reversing mill where the flat metal strip is processed in several passes (from 3 to 7) and the coil is uncoiled–recoiled by two reels installed in proximity of the stand, (ii) two stand cold reversing mill where the reduction of the thickness is achieved with a reversing process but the number of passes (from 1 to 3) is reduced because of increasing of the number of stands, and (iii) tandem cold rolling mill or simply tandem mill where the thickness reduction is achieved with a number of non-reversing stands (typically ranging from 3 to 7 non reversible stands). In some cases the tandem mill is coupled with the pickling process in order to increase the productivity. In this case the process is known as continuous tandem cold rolling mill since coils are welded together and the process is expected to stop only for maintenance reasons. In this case even the weld between a coil and the following one is subject to rolling.

The thickness deviations at the output of the stand are derived from two sources. The first source is due to the material properties, which can be produced (i) by the thickness deviations of the feeding strip, and (ii) by the deformation resistance. These are determined mainly by the material hardness during various passes, but also by sheet chemical composition on strip length. The second source is generated by the rolling mill. These deviations appear, mainly due to the stand yielding, depending on the stand elasticity module. The friction coefficient variation of the working rolls with the rolling strip can also influence the thickness deviations.

In cold rolling mills (and in particular in tandem rolling mill) the thickness control i.e. the automatic gauge control (AGC) regulation is achieved with sophisticated controllers which need to take into account that loopers are not present (as in the case of hot strip mill) and hence the regulation activity of all the stands is to be coordinated in order to guarantee stability of the rolling process. Further, the basic controls are to be distinguished from the external controls. The basic controls are hydraulic gap control (HGC), speed controller (SC), and torque controller (TC) do not depend on the type of rolling process whereas the external controllers can change significantly according to the structure of the process and the availability of sensors.

Single stand cold reversing mill

A typical thickness controller for a single stand cold reversing mill and the most common configuration of sensors is shown in Fig 1. Here, the thickness gauge sensors are based on x-ray technology and are aimed at measuring the thickness in the centerline (and seldom the thickness profile). Speed gauge sensors are based on laser technologies or are simply encoders. In general, the use of laser technology (much more expensive) is preferred when the needed measuring precision is to be ensured also in presence of fast acceleration / declaration periods, that is, when an encoder can lose contact with the material. Load cells are normally installed in each inter-stand in order to get a direct measurement of the inter-stand tension. As shown in Fig 1, it is quite normal to see single stand cold reversing mill is provided with thickness and speed sensors (possibly encoders) on both sides of the mill.

Fig 1 Thickness control in a single stand cold reversing mill

For the single stand cold reversing mill, the external controllers are (i) the tension control by torque (TCT) in which the entry / exit tensions are kept constant through the torque regulated by the TC that, in turn, exploits the motors applied to the coiler / uncoiler reels, (ii) the gauge control, feed-back via gap (GBG) in which the controller generates a trim for the HGC reference on the basis of the thickness measurement ‘H x-ray  out’ and is available downstream the stand , (iii) the gauge control, feed-forward via gap (GFG) in which the controller generates a trim for the HGC reference in order to anticipate the thickness deviations of the incoming strip to be rolled though the x-ray installed on the entry side and produces the measurement ‘H x-ray in’, and (iv) the gauge control, mass flow via gap (GMC) in which the controller aims at compensating the deviations of thickness ‘H x-ray out’ by exploiting the mass flow principle and so the speed measurements of the strip at the entry side and exit side (‘V in’ and ‘V out’ ).

More precisely, since strip width variations are negligible the mass flow balance equation is expected to be satisfied (‘H x-ray in’ x ‘V in’ = ‘H x-ray out ’x ‘V out’). On the basis of this equation, it is possible to track the measurement of ‘H x-ray in’ at the entry side of the stand and then get another measurement of the thickness at the exit of the considered stand ‘H MF out’ = (‘H x-ray in’ x ‘V in’/ V out’).  The GMC, by controlling the signal ‘H MF out’ instead of the signal ‘H x-ray out’ , ensures a wider stability margin and better performances than the GBC, since there is no transport delay affecting the measure represented by ‘H MF out’.

Two stand cold reversing mill

In the two stand cold reversing mill, HGC applied on stand number 1 (Fig 2) does not aim at regulating the stand number 1 exit thickness directly. Indeed, some regulators are introduced in the two stand cold reversing mill case in order to keep, as much as possible, constant the inter-stand tension between stand number 0 and stand number 1 in order to avoid the generation of disturbances for the GMC / GBC acting on stand number 0.

Fig 2 Thickness control in two stand cold reversing mill

Moreover, the thickness at the exit of stand number 1 is regulated by the GBS (gauge control, feed-back via speed). This regulator acts on the speed reference used by the SC applied on stand number 1 and, possibly, on the speed reference used by the SC applied on stand number 0. The inter-stand tension is indeed controlled by two mutually exclusive controllers namely (i) TCS (the tension control via speed) controller which regulates the inter-stand tension by varying the speed reference for the SC applied on the stand number 0, and TCG (the tension control via gap) controller which acts on the gap reference for the HGC applied on stand number 1.

The selection between keeping active the TCG or the TCS depends on the mill speed. Indeed, at low speed the TCS results are in a more prompt controller but, of course, it can interfere with the GBS which is in charge of ensuring the final thickness. Hence, a suitable logic is implemented in order to switch, as soon as possible, from TCS to TCG when the speed reaches a threshold. Of course, in the two-stand cold reversing mill, when the rolling direction is reversed the roles of then stands number 0 and number 1 are reversed and the external controllers are applied with a symmetrical logic.

Tandem cold rolling mill

In the tandem cold rolling mill, the control logic which is applied to the two stand cold reversing mill is further extended in order to take into account the contribution of more stands (Fig 3) and the corresponding availability of sensors. A typical tandem cold rolling mill installation is provided with the sensors such as (i) thickness x-ray at the entry side of stand number 0 and at the exit of stand number 0, (ii) thickness x-ray at the exit of last stand, (iii) laser speed-meters are in general installed only on entry / exit of stand number 0, (iv) all the inter-stand speeds and the coiling speeds are measured through encoders, and (v) all the inter-stand tensions are measured by load cells.

As in the case of two stand cold reversing mill, GMC / GBC / GFC is applied to the first stand of the tandem stand number 0 (Fig 3) whereas the GBS, in charge of regulating the final thickness, can act on the speed references for all the stands. Moreover, as in the case of the two stand cold reversing mill, all the inter-stand tensions are regulated by TCG or TCS.

The speeds of the stands and of the coiler and uncoiler are to be coordinated in order to ensure the stability of the mill. This feed forward controller is known as ‘speed master’. The ‘speed master controller’ is to be implemented in order to coordinate the speeds of the various entities in the mill. This is particularly important in two stand cold reversing mill / tandem cold rolling mill where the inter-stand tension regulation achieved by TCG / TCS is not as fast as that achieved by the TCT.

Fig 3 Thickness control in tandem cold rolling mill

Flatness control in cold rolling mill

The flatness control in cold rolling mill is carried out through automatic flatness control (AFC). The control tasks which are to be achieved in the Level 1 closed-loop control for cold rolling mill concerns not only the thickness (AGC) but also the flatness (AFC).

For a strip subject to cold rolling, the flatness is defined as the amount of internal stress difference along the width of the material. The measurement of the strip internal stresses (the so-called shape) during coiling can be taken through suitable flatness sensors named shape-meters or stressometers which until now represent a significant investment. Due to the cost of these sensors seldom a plant is equipped with more than one flatness sensor, that is, the shape-meter installed at the exit of the mill.

The flatness sensor is the most important part of the AFC system. The contact roll type sensor (stressometers) is applied in this system in consideration of the stability and the response of the output. Earlier, the contact roller type flatness sensor was not used for the high speed and ultra thin gauge cold rolling mill such as six stand tandem cold rolling mill, because of the anxiety of the scratches between the sensor and the strip. In this system, a more sophisticated helper driving system of sensor roller which permits a synchronization of the sensor speed and the strip speed prevents the scratches.

The AFC task is usually performed by exploiting in closed-loop the flatness actuators of the last stand only, since it is the nearest to the shape-meter and it has the most immediate and predictable effect on the coil final flatness. The rolling stands used for performing cold rolling have normally advanced flatness actuators. In general in tandem cold rolling mill / two-stand cold reversing mill the stands can be of 4- high type or 6-high type (i.e., stands with 6 rolls). The single stand cold reversing mill process can be achieved (in particular for stainless steel) with stands of 20-high type also known as cluster mill or Sendzimir mill.

Strip shape measurement system

Strip shape, also referred to as strip flatness, is becoming more of a concern for all involved in the rolling mill industry. Poor strip shape can increase scrap since products made from strip with poor shape can be defective. With the increasing speed and sophistication of process lines, poor shape feed stock can damage machinery or slow down production.

Strip shape becomes increasingly difficult to control as the width to thickness ratio increases and also as the material becomes harder. It is normally accepted that strip shape defects are caused by a differential percentage reduction across the strip width. This causes a differential elongation of adjacent portion of strip, which sets up internal stresses, leading to buckling. There are four major strip shape defects produced be differential reductions. These are termed as (i) loose (wavy) edges, (ii) quarter buckle, (iii) centre fullness, and (iv) herringbone (ripple).

The introduction of new rolling mills, such as the continuous variable crown (CVC) and pair cross (PC) and the work rolls crossing and shifting (RCS) have been developed to improve the strip shape and profile, as the mills have an ability to work as a shifting roll, crossing roll and bending roll.

The shape control capability of a particular type of rolling mill exerts a decisive effect on strip quality. According to product positioning, analyzing and comparing the control of shape control for cold rolling strip performances of various rolling mill types, and selecting the appropriate rolling mill type is essential.

Reasonable design of rolling process parameters is the basis to ensure shape quality. Improving strip shape through optimizing the rolling process is a traditional technological method. However, numerous new applications of this method have been established. Shape quality is improved by optimizing the emulsion flux of the process cooling and multi-zone cooling. The steel-sticking phenomenon caused by the bad shape of the strip head is avoided by optimizing the roll-bending force.

The shape quality, mechanical properties, and surface roughness of a strip rolled by a cold rolling mill are achieved by optimizing the rolling force and tension. Comprehensive improvement of strip shape and surface quality can also be achieved by optimizing the rolling force, strip elongation rate, and tension. Under the logic of ensuring good strip shape, the flux, concentration, and temperature of the emulsion are optimized to prevent slipping and thermal scratching between the strip and the rolls. As a result, the cleaning degree of the strip surface is improved, and the emulsion consumption is reduced.

The local shape control is difficult since its scope is small. ‘Convex rib’ is a typical local shape defect. It has become the focus of production units of cold rolled strips in recent years. Convex rib is due to the existence of a local high point along the strip width, forming an apparent bulge corresponding to the position of the local high point. This defect leads to longitudinal convex rib at the local position after decoiling the coil and thus seriously affects product quality. A large amount of industrial data shows that the convex rib of cold rolled strip is mainly caused by the inheritance of hot rolled strip local high points, which points out the origin of preventing the convex rib.

The edge drop control technology can reduce the cutting loss and increase the yield. The application of edge drop control technology to silicon steel has received increasing attention in recent years. The achievements of edge drop control technology are mainly embodied in developing automatic control systems and designing of the roll profiles.

Optimizing the rolling process parameters is a common and effective technological measure to improve surface quality. Surface quality control is frequently combined with shape control. Comprehensive control of the surface roughness and shape of a two stand cold reversing mill is achieved by the optimization of the process parameters, such as rolling force and tension. The rolling force, strip elongation rate, and tension of rolling mill are optimized to achieve comprehensive control of the surface quality and shape of the strip. In addition, the surface cleanliness and shape of the strip are improved by optimizing the flux, concentration, and temperature of the emulsion. At present, the application of the comprehensive control technology is quite rare.

Shape detection is the basis or achieving shape closed-loop control and is the key to improve shape quality. The strip shape-meter is the ‘eye’ of online detection. It is a high end measuring instrument of the rolling process. The strip shape-meter consists of two main parts, namely, shape detection roller and shape signal transmission processor. The development of a shape-meter is so difficult that it has been monopolized for a long time by a few companies. In the past 10 years, a major breakthrough on the shape-meter has achieved. The seamless shape detection roller and wireless shape signal transmission processor have been independently developed and successfully applied.

A conventional shape-meter used in cold rolling consists of an array of load cells distributed along the width of the strip. Each load cell produces a signal representing the pressure exercised by the slice of strip in contact with it. As a result, the shape-meter produces an array of tension signals whose dimension is the amount of load cells placed on the sensor [Shape = (T1 . . . Tn)]. Recently, contactless sensors based on ultrasound are available and provide a quite similar array of signals. It is worth pointing out that the presence of a gradient in the specific tension associated to two different strip slices implies that the two slices present different elongation values. In turn, an excessive difference in the elongation between the strip slices can imply a manifested flatness defect which is required to be corrected.

The internationally popular sectional shape detection roller is sectional shape detection roller (Fig 4). This roller consists of a core shaft, outer rings and piezoelectric sensors. The strip surrounds the shape detection roller to form a certain angle. Strip tension T acts on the shape detection roller and results in pressure N. A series of outer rings and sensors are arranged along the axial direction of the shape detection roller, and pressure N is transmitted and detected. The axial pressure distribution is converted into tension distribution by the signal-processing computer so that the strip shape can be calculated. This kind of shape detection roller can crush and scratch the strip surface for two reasons. First, a gap exists among the outer rings. Second, the axial temperature of the shape detection roller is different. The temperature difference between the meter roller’s middle and edge is around dozens of degrees Celsius, resulting in radial thermal expansion differences among different outer rings.

An inlayed block shape detection roller (Fig 4) has been developed to overcome the short comings of sectional shape detection roller. Two rectangular grooves are machined on the body of the detection roller, and a set of elastic blocks with sensors is installed in each rectangular groove. The structure effectively avoids scratching the strip surface caused by the uneven thermal expansion of the sectional detection roller. However, the ‘skin effect’ of the current easily leads to uneven hardness of the rectangular groove edge and other part of the roller body when heat treatment is applied. The broken strip can easily damage the soft part of the roller body, and the damaged roller surface can scratch the strip surface in further production.

A new type of seamless shape detection roller (Fig 4) has been developed to completely solve the above mentioned problems. Two to four precise through-holes are machined along the circumferential direction near the roller surface in the roller body. The wall thickness between the hole and the outer surface of the roller is between 6 mm and 8 mm. The wall thickness is not only to be conducive to the pressure transfer, but also retain enough thickness for the roller grinding. A series of sensors are arranged inside each through-hole. The roller material is high carbon chromium manganese steel and the roller surface hardness exceeds 60 HRC after quenching. The hardened layer thickness is 4 mm or more. This shape detection roller presents the advantages of seamless surface, high hardness, and deep hardened layer, which completely solve the technical problems of crushing and scratching strip surface.

A certain pre-pressure is applied on the sensor by an interference fit manner to keep the sensor working in the linearity range. In fact, a certain temperature difference and thermal deformation difference exist between the outer surface of the detection roller and the internal sensor in the rolling process, and these differences can weaken or reduce the magnitude of interference and pre-pressure between sensors and the inner wall of through-holes, resulting in detection signal distortion. Hence, the assembly of sensors is to have an adequate magnitude of interference and pre-pressure. To solve the problem, the temperature field, thermal deformation, and pressure transfer of the detection roller and sensors in the rolling process are simulated with the finite element software. The structure size of the roller and the magnitude of interference are optimized.

The carbon brush and slip ring are the popular structures for signal transmission and power supply. A series of copper slip rings are fixed on the rotating end of the detection roller’s neck, and a series of carbon brushes are fixed on the cover which is connected with the bearing chock. The signals of the detection roller are transmitted to the slip rings via wires. The rotating slip rings then transmit the signals to the fixed carbon brushes. Also, the signals are transmitted to the remote terminals via long-distance wires. The principle of power supply is similar with that of signal transmission. Owing to the friction between the carbon brush and slip ring, wear, vibration, and serious signal distortion can occur. The carbon brush and slip ring needed to be maintained frequently and equipped with cooling and cleaning devices. The remote transmission of analog signal can be easily interfered by electromagnetic, vibration, temperature, and other factors, which can increase the error.

Fig 4 Types of detection rollers and wireless signal processing system

A wireless and integrated signal transmission processor (Fig 4) has been developed to solve the above problems. The signal processor consists of a rotating head and a fixed cover. The rotating head is connected to the detection roller and rotates synchronously. An excitation power board, an analog acquisition board, a digital processing board, a wireless transmission board, and an inner magnetic ring are arranged on the rotating head. The cover is fixed on the bearing chock, and a wireless receiving board, an outer magnetic ring, and a voltage-stabilizing circuit board are installed on the cover.

The working principle is that power is supplied to the wireless receiving board and the outer magnetic ring on the cover by the cable and voltage-stabilizing circuit board. The rotating inner magnetic ring receives the voltage-stabilizing signal by wireless induction transmission, and all sensors in the detection roller and all the circuit boards on the rotating head are powered by the excitation power board. The output signals of sensor are gathered by the analog acquisition board, and the digital processing board implements analog-to-digital conversion. Digital processing and coding are performed under the conditions of magnetic coupling isolation and photoelectric isolation. The signals are then delivered to the wireless transmission board. The wireless transmission board converts the signals into high-frequency wireless signals and transmits the wireless signals to the wireless receiving board. The wireless receiving board converts the wireless signals to wired signals. Finally, the signals are transmitted to the remote signal-processing computer via the shielded cable.

Shape detection is affected by vibration, electromagnetic, temperature difference, detection roller installation error, deformation, coil shape changes, and other factors, so the shape detection signals need noise cancellation and error compensation to ensure the precision. A discrete-time tracking differentiator is applied to eliminate the noise of the shape detection signal.

The shape control system can be divided into pre-setting control system and closed-loop control system according to the control sequence, and the modelling of shape control system includes mechanism and intelligent models. Presetting control is a predictive control, and closed-loop control is a monitoring control. The two control systems complement each other and ensure the strip shape quality. Shape pre-setting control is the foundation of closed-loop control. Closed-loop control precision, speed, and stability are directly influenced by the precision of pre-setting control.

The core and key in shape closed-loop control are the control models, which include the control strategy and model algorithm. When the strip shape signal is detected, the control model rapidly calculates the adjustment amount of control means (e.g., roll tilting amount, roll bending force, multi-zone cooling spray) to achieve timely and accurate adjustment of the strip shape based on quadratic optimization and hysteresis compensation.

Roll tilting and bending are the most flexible and fast shape control means. The fuzzy-neural PID (proportional integral derivative) model of roll tilting and bending adjustment has been established based on the coordination of the two intelligent methods of fuzzy theory and neural network. The conventional PID control algorithm cannot be directly adapted to the complex and changeable rolling process, so the fuzzy theory, which is good at dealing with unknown models, is combined with the neural network with a strong self-learning capability to improve the shape control effect.

Comments on Post (1)

  • Milind Joshi

    Good information very useful.

    • Posted: 10 October, 2020 at 17:45 pm
    • Reply

Leave a Comment