Automation and Control in Wire Rod Mill
Automation and Control in Wire Rod Mill
Steel wire rods are the key product of steel industry and have multiple uses. They are used as the material for fasteners, springs, bearings, wire ropes, chains, cables, wire meshes, wire fencings, tyre cord, reinforcement in railway sleepers, and several other uses. They are used for the components needed for the automotive industry, chemical industry, power stations, and machine engineering. Unlike cold-rolled sheets, heavy plates, pipes, sections, and other steel products, wire rods are seldom used as hot rolled for final products, but they are manufactured into machine parts after undergoing one or more stages of so-called post-processing such as heat treatment, wire drawing, and forging at specialist plants. Wire rods are normally drawn down to a specific diameter before being subjected to forging or other forming operations in secondary processing. In several cases, the size of the wire rods before being subjected to these forming operations is to be less than the minimum size of 5.5 mm which can be supplied as rolled from the wire rod mill (WRM).
With the rapid development of modern industrial technology and process automation, the demands on the steel grades, dimensional precision, and quality performance of wire rod products have increased, and the rolling technology is of wire rod rolling is upgraded as a measure of development for meeting the market demand for wire rods.
The economic efficiency of a WRM is strongly correlated to the quality level of the end-rolled products. Rolling of wire rod products is a complex process where the quality of the product is influenced by a range of factors such as incoming material (billet), mechanical and electrical equipment, operating parameters, and automation and control strategies etc. The important quality parameters are rod shape, diameter, rod surface, and the micro-structure. For optimized cost-efficiency and to maximize material usage, tight tolerance for the diameter is necessary, to enable the wire rod to be rolled down as closely as possible to the minimum permissible diameter. Wire rod quality can only be effectively optimized if the mechanical, electrical and instrumentation equipment as well as the control strategy solution combine together well. The combination of automation, process technology, and mechanical equipment is at the heart of the automation and control of the wire rod rolling mill.
The objective of a WRM is to reheat and roll the steel billet (input material) into the product ‘wire rod’. Wire rods are normally rolled in a high-speed rolling mill, where steel temperature is above 1,000 deg C, maximum speed of rolling can go up to 140 metres per second (m/s) and coils of wire rod produced can be up to 2.5 tons (t) of weight and up to 10 kilometers (km) long. During rolling of wire rods, normally 25 passes to 30 passes are taken in a continuous rolling mill. A modern WRM can be either a single strand mill or a two-strand mill.
A WRM needs high levels of productivity for meeting the highly diversified product demand in the present market, while fulfilling stringent quality standards for the wire rod. A WRM basically needs (i) heating facilities for heating the billets to rolling temperatures, (ii) rolling facilities consisting of rolling stands with rolls, chocks, guides and guards, (iii) laying, heat treatment and control cooling, and coiling equipment, and (iv) coil conveying, compacting, and handling equipment. All the equipments are to work in close co-ordination with a control on rolling temperatures, gap time loss, speed loss, cobbles, non-conforming product, and quality deviations which can lead to diversion or even rejection of the wire rod coil. Fig 1 gives schematic diagram of material flow in wire rod mill.
Fig 1 Schematic diagram of material flow in wire rod mill
In order to meet present market demand, a WRM needs high levels of productivity while fulfilling stringent quality standards. In the rolling process of the wire rods in a WRM, the automation and control system play an important role to ensure the correct operation of each link in the equipment chain to achieve the highest sustainable production rate without compromising of the quality. Fig 2 shows benefits of automation and control in wire rod mill.
Fig 2 Benefits of automation and control in wire rod mill
Automation and process control provides a consistent solution for the control high speed WRM. The basic automation systems are integrated with a set of different sensors which are specially designed for quality control and production monitoring. In the design for the automation system, equipments and instrumentation are integrated with each other through a local area network to meet plant production and process requirements. The architecture of the automation system is based on client-server structure, with a single database assuring consistency of the data used to calculate the equipment set-up and to manage the production.
The automation equipment normally includes solenoid valves, control valves, LCD (liquid crystal display) panels, fibre optical cables, remote I/O (input/output) panels, and HART (highway addressable remote transducer) communication bus. Operator work-stations, based on personal computer hardware, support the operator’s decisions displaying the necessary information about process and equipment status. The equipment control automation system is based on distributed architecture of PLC (programmable logic controller) units, where the single unit is dedicated to control single machines or groups of them, simplifying handling and maintenance, and making trouble-shooting easier.
The main functions to be performed by the basic automation and process control system of WRM include (i) material tracking, (ii) automatic sequencing, (iii) speed control, (iv) referencing and set up of drives, (v) tension control, and (vi) temperature control. Some of the additional functions are automatic labelling, consumption accounting, quality monitoring, and equipment life tracking. The large quantity of data acquired from instrumentation and basic automation can be analyzed with a dedicated business intelligence model.
The processing of billet to wire rod in the WRM is achieved through several process steps whose complexity involves mechanical, electrical, and automation and control technologies. Rolling of billet in WRM needs not only mechanical solutions but also appropriate control technologies. The process of rolling in the WRM can be controlled though a standard software and automation architecture which can include four automation levels (Level-1 to Level-4). In all these automation levels which need to cooperate hierarchically in order to achieve the best performances and the highest productivity levels. I WRM, a number of control technologies, mathematical models of physical phenomena and optimization algorithms are implemented. The hierarchical structure of a control automation system normally adopted for WRM is shown in Fig 3.
Fig 3 Typical structure of automation and control system of WRM
The control objectives for the WRM are expressed in terms of throughput and product quality parameters. However, the practical scope of control covers a very wide range of applications ranging from individual local high-speed position control loops with operational speeds at the milli seconds or sub-milli seconds level to the overall work piece scheduling task which operates on an hourly or longer time-scale. All these controls contribute to the overall performance needed from the automation system but the objectives are frequently expressed in terms of sub-goals more appropriate to the time-scale of the particular controller. For example, a position loop’s goal can be expressed in terms of the rate of change and overshoot in response to a demanded position change, while the overall scheduling of products through the WRM can be expressed in terms of speed of satisfying the production plans.
This difference in time-scale and scope of the individual controls is reflected in the multi-level structure of the control systems now widely used in WRM. In Fig 3, the block diagram of such a multi-level system of WRM is shown. The separation of functions between the various levels is not sharp, and whether specific functions are implemented in, say, Level 1 or Level 2 can vary from installation to installation and in response to the development of better control methods and equipment.
The basic level automation system is needed for interlocking, sequencing, switching, mill controls, instrumentation controls for reheating furnace, billet tracking, storage of rolling schedules as look-up tables for manual mill set-up when needed, diagnostics and alarm functions, and data acquisition of important parameters of equipment / process etc. and is normally carried out by distributed control philosophy through PLCs, microprocessor-based systems, and personal computers (PCs).
Special sensors and instrumentation play an important role. Magnetic presence sensors which use the disturbances caused by the metal material being rolled on the magnetic field induced to determine the bar presence, are installed inside the water boxes used for the material thermal treatment. This sensor allows precise material detection and tracking of both hot and quenched bar even in the presence of water and steam.
A control system is used for the on-line measurement of cross section of the wire rods. A magnetic contact-less sensor forms the core of the system. It is sturdy and compact and is designed to be maintenance and wear free. The working principle of the sensor is based on detection of the eddy currents, generated on the surface of the rolled stock by a variable electro-magnetic field. This control system is normally used for several main applications such as (i) weight per metre and section measuring, (ii) section control, and (iii) optimization of the minimum tolerance. The sensors are typically installed downstream each section of the mill (intermediate, and pre-finishing).
A sensor is used for the profile of the wire rod. This sensor is installed at finishing block exit. The sensor is equipped with one or two rotating measurement heads. It provides non-contact on-line profile shape inspection and dimension measurement for the hot wire rods. It is integrated with automation system and it is possible to achieve fast 100 % inspection of shape and dimensions, reading and monitoring of the complete profile of the wire rod. There is additional instrumentation which consists of in-line surface inspection system for the wire rod. It is a no-contact inspection system dedicated to on-line detection of surface defects in rolled wire rods.
The control system allows achievement of several immediate benefits which include (i) reduction of cobbles, (ii) reduction of wear of roller guides, (iii) reduction of breakdowns of roller guides, (iv) precise gap adjustment for wear compensation and easier set-up of a new production, and (v) instant warning in case of out of tolerance events.
The automation system is not the sole determinant of performance of the WRM. However, for any given configuration of mechanical and electrical equipment, the potential performance of the mill is only being achieved with high-performance control and automation. Attention is needed to be focused on throughput and quality, where control is particularly important in achieving good performance. Normally the throughput and quality interact in both positive and negative ways and these interactions are to be considered in designing the control system.
The ultimate throughput which can be achieved in a mill is limited by the capabilities of the mechanical and electrical hardware. To achieve throughputs consistently close to this limit needs high-quality control and automation. At high throughputs, three or more work pieces can be in the rolling mill at different stages of processing at the same time. To avoid catastrophic collisions in the mill, accurate tracking is essential. The tracking system uses signals from mill instrumentation and process information (for example, as a piece is rolled, so its length increases) to maintain a dynamic map of the mill. It is, of course, to be robust against the loss of individual mill instruments.
Throughput control looks ahead at the rolling schedule and determines which part of the mill installation, furnace, roughing mill, intermediate mill, finishing block, or finishing equipment such as laying head, control cooling conveyor, can limit throughput. The limiting process is then controlled to achieve maximum throughput and other parts of the process are controlled to match this throughput. This results in an improvement in energy efficiency and a reduction in wear and tear on the equipment, hence reducing the costs.
Throughput and quality also interact. As throughput increases, control becomes more difficult, and to maintain the needed level of quality and yield needs careful design of the control system. Quality and throughput control also interact in positive ways. For example, for achieving a greater range and accuracy of temperature control out in the finishing block, inter-stand cooling sprays are normally installed. These are to be controlled to maintain the strip temperature at the mill exit but, further, they can be used to increase the speed at which the work-piece is rolled in the finishing block, while maintaining the target exit temperature.
WRM normally consists of roughing mill, intermediate mill, and finishing block for the rolling of the wire rods. Roughing mill and intermediate mills consist of several stands and each stand is driven individually by electrical motors with power supplied from power converters. Typically, one quadrant or four quadrant-controlled DC (direct current) motors are used. For maintaining a stable and high-quality rolling process, it is needed to control the speed of the stands as per the tension conditions between the stands. Speed ratios between these stands are to remain constant to maintain stable material flow. If there is a change in a gap between rolls, it causes a deviation in rolling parameters, what can increase or decrease inter-stand tension. The tension between stands has a large influence on the properties of the work-piece produced in the mill.
There is impact drop compensation for twin module block and multi-drive blocks. A high-speed auto-adaptive speed drop compensation of the drives at biting of the material in each stand (or group of stands) is provided. It allows reducing relative speed variations of between the motors of a multi-drive block down to 0.5 % or even less of the nominal speed. The right tension in the material is maintained along the front-end transient. Hence, the section of the material is kept constant along the whole stock length.
Recent approach is to use ‘inter-stand dimension control (IDC) with the complex system of work-piece cross-sectional area measurements in the inter-stand area. Gauge sensors measure dimensions and IDC control system is used to automatically adjust the gap set–up and speed. WRM normally has a conventional sensor-less control system with the minimum tension control.
Several sources of feed-back signals are normally used in WRM. These include (i) hot metal detector (HMD) which is the most commonly used sensor in the mill lines (it uses infra-red radiation emitted by hot materials which is received by an optical system in the sensor), (ii) loop scanner which is used in the automatic loop control (it optically scans the field to be controlled and does not need any optical adjustments), and (iii) stand threading signal which is generated from the peak torque detector in the drive or PLC unit. Obviously, installation of the first two sensor systems results in additional costs. While the end of the work-piece passes through the roll, the tension of the bar suddenly turns to zero and size of the rod changes.
The changes in cross-sectional area along the work-piece can lead to the cobble in the intermediate mill and finishing block. Massive tension can cause total unloading of the stand which consequently leads to the cobble because of the failure of inter-stand tension control. On the other side, no tension means loopering of the material between the stands, which also leads to cobble in the mill. All these deviations have a big impact on product quality. Even more, these deviations increase the amount of the time spent by the maintenance personnel for the mill maintenance.
The automation and control systems developed for WRM by play an important part in its performance. All the devices controlling the cross-section of the rolled stock, installed at the entry / exit of the pre-finishing block on the two strands, ensure that the cross-section of the rolled wire rod is kept constant. The gap is closed frequently and micro-metrically on the entire cross-section of the rolling mill because of the high level of automation. This allows the mill operators to roll in a consistent, stable, and precise manner without encountering major changes in cross-section, tension, or speed. This type of operation lengthens the life of all the guides, reduces the number of rolling incidents and above all improves the quality of all the rolled wire rods in each of the strand area, hence ensuring a finished product with a high level of quality. The WRM is run in strict compliance with reheating, rolling and heat treatment schedules for each section and type of material because of automation and control equipments.
Dimensional defects are mainly related to the shape and dimensions. During the rolling of wire rods, there can be different defects which can cause the shape to not be as desired or the measurements to not fall within the required tolerances. The majority of the frequent causes of dimensional defects are because of the poor adjustments to the mill control and tension while rolling. For reducing the material tension along the mill, two types of control are applied namely (i) minimum tension control, and (ii) loop control. Each is applied in different parts of the mill. In the first part of mill, roughing and intermediate zone, where the bar section is larger, the minimum tension control is used. In the finishing area, where it is possible to deform the material without damaging it, the loop control is used.
In high-speed WRM with fixed speed ratios between the stands, inter-stand tensions are used to achieve stable rolling conditions. Traditional tension control systems are based on the minimum tension control for roughing mill, the loop control for intermediate mill, and the tension rolling for finishing block. In the case of roughing mill since it is not possible to control the loop between the stands because of the large dimensions of the work-piece in roughing mill, a looper-less control scheme has to be used. So called minimum tension control assumes that the work-piece dimension and material temperature profile remain constant along the whole work-piece. Motor torque feedback during the time interval with and without interaction of the work-piece with the stand is used as a tension indicator.
The tension control measures the material tension using the torque applied by the motors. After calculating the tension of the material, the control system adjusts the reduction ratio to minimize the tension according to the set-point specified by the operator. When the head of the work-piece enters the first stand, before reaching the down-stream stand, the torque is stored. When the work-piece enters the down-stream stand, the current torque is compared with the stored one. Observing whether the value increases or decreases between two the stands determines whether there is a push or pull. The control system acts on the reduction factor correcting the speed difference and hence the tension between the stands.
In the finishing end the material is thinner and it is possible to eliminate the tension with a loop control. Between the stands, a loop table is installed to form the loop when the work-piece passes. When the head of the bar reaches the down-stream stand of the table, the persuader roll rises helping to form the loop. In the table, a loop scanner is placed to measures the height of the loop at all times. The operator specifies the loop height and the control algorithm modifies the mill speed in cascade, acting on the reduction factor. Tension free rolling is achieved by keeping a constant loop height. The good performance of these methods depends primarily on the control system and the process sensors.
Wire rod rolling is a periodical process. The work-pieces pass through roll stands sequentially and one work-piece follows another. For this reason, material tracking system is one of the most important parts of the WRM control structure. It manages the tension control and it is used also for cobble identification. Material tracking function provides accurate information of the work-piece head and tail end positions in the mill. This is a fundamental requirement for automatic control sequence, data collecting systems, main and auxiliary drives and services. Even more, the speed reference distribution, automatic loop control, minimum tension control and automatic cutting of the flying shears are based on precise material tracking.
The correct material tracking is critical for the loop formation and control sequences, especially in a high-speed WRM. The reduction factor control algorithm is also very important to regulate the coordinated mill speed variation. In the last minute, both the tension and loop control operate on the reduction factor (R-Factor) for each of the stands. The control system stores each reduction factor when the process is stable, using it for the next billet. The correct storage of this factor makes it faster for mill adjustments, especially at the start of a new product. Fig 4 gives typical loop control diagram.
Fig 4 Typical loop control diagram
Product change-over stops in WRM are frequent because of the need to accommodate different production orders. The time to stop, change, start and stabilize the production with the new product can be reduced considerably, by automating the processes of change and mill adjustment. The integration of recipe management and the correct stand change automation, consisting of automatic adjustment of the gap and groove positioning, reduces the time needed for the product change-overs. It is also important in order to reduce errors by minimizing human intervention during the adjustment.
There is roller guide calibration and alignment system. It is a calibration system designed to aid the operator during (i) setup of the roller guide in the workshop, (ii) alignment of the roller guide mounting bases with the rolling ring pair groove in the finishing blocks.
During the product change-over, it is necessary to change all or part of the mill configuration. All parameters and settings are stored in the recipe data-base. The operator selects the new product, carrying the new settings to the system. Among the settings are gap and stand groove position. Once the new stands are placed, the system adjusts the gap and groove position automatically according to the values stored in the recipe. When the groove is worn and replacement is needed, the operator chooses to use the new roll groove and the system automatically positions the stand, aligning the new groove. This automatic process reduces 75 % of the time and resources needed for changes and adjustments compared to a manual adjustment.
Once the mill is set-up and adjusted, the next step is to verify that each drive is ready to roll. In this phase, it is very important to have all the information related to the alarms and interlocks. From the HMI (human machine interface), it is possible to check each of the roll conditions necessary to connect and start a particular device or area. The correct description of the faults and interlocks, together with trained work-force, significantly speeds up the start-up phase, reducing trouble-shooting time and break-downs.
Another really useful tool during this startup phase is the ability to simulate the process using a phantom billet. This permit checking the correct function of the sensors and actuators, simulating each of the process sequences. The phantom billet simulation reduces considerably the cobbles during the startup of a new product. Following the adjustment phase, starting and mill simulation comes the stabilization phase which can be defined as the time it takes to reach nominal production output after a mill change. While rolling the first billet, it is necessary to check for any problems in the guiding and if all the mill adjustments are appropriate. The control system, based on the information received from the field sensors, readjustments of the reduction factors are done, as per the mill configuration. The main objective is to minimize tension along the mill and reach nominal production quickly.
Mill performance is affected by a reduced rolling speeds, bottlenecks, and cobbles in the process. The correct mill adjustment and the performance of the control system play an important role in the stability and speed of production, but another very important part of the control system is the material tracking. Tracking is necessary for the proper operation of tension control, loop control, cobble detection, and shear cycles, etc. For the easy tracking of the material, the main sensors used for detection are the HMDs (hot metal detectors) and the torque-motor of the stands. Tracking material is also used for interlock sensors, avoiding false detections. Proper operation and placement of each of the sensors along with the good performance of the control algorithms permits high-speed steady rolling with minimal gap between billets.
The perfect coordination between material tracking and cascade speed control system reduces the number of cobbles and hence reduces losses because of poor performance. For each product, there are factors which prevent increasing production speed. Such characteristics include the necessary cooling time, the performance and power of the main drives or the maximum capacity of the reheating furnace. A WRM is to be treated as a chain, in which the weakest link determines the maximum production. Identification of the equipment which is the bottleneck in the WRM mill and development of a preventive maintenance programme for the same, keep the mill running as long as possible, and hence it increases the efficiency of the mill.
When rolling a billet in WRM, it is necessary that the correct coil dimensions are maintained for complying with the quality standards. For the achievement of correct coil formation, it is very important that the mill exit speed is synchronized with the laying head speed. The laying head has a high inertia which responds slowly to changes in speed and for this reason it is better if a constant finishing block exit speed is maintained. Hence, a two-strand WRM needs special control action.
In a two strand WRM, the roughing mill and intermediate mill is common, where the rolling stands are rolling billets for the two strands simultaneously. After the common roughing mill and intermediate mill, the mill splits into two independent strands. Each of the strands consists of flying shear, crop shear, rolling stands, mini-blocks, and a finishing block. At the output of each block are water boxes and finally a laying head which deposits the wire rod on the cooling conveyor. Differences in the adjustments or wear between the two exit lines necessitates the WRM to have control in both the lines down-stream. This solution causes the laying head speed to change constantly. Another solution is possible, in which the mill up-stream is controlled by making one of the lines as a master. This type of control compensates for the differences between the lines by modifying slightly the exit speed of the follower line. This method maintains, during majority of the time, a constant speed in both laying heads.
The finishing block comprises a number of stands (6 to 10) which are normally driven by a synchronous motor. Due to the high power, high speed (base speed is 850 rpm, maximum speed is 1,600 rpm) and precise static and dynamic characteristics, high demands are made on the design and manufacture of the motor and inverter. The motor drive system is to meet these requirements. Fig 5 shows typical control diagram of two strand finishing mill of WRM.
Fig 5 Typical control diagram of two strand finishing mill of WRM
Present-day WRMs have a sensor which is installed at finishing block exit. The sensor is equipped with one or two rotating measurement heads. It provides non-contact on-line profile shape inspection and dimension measurement for wire rod product. When it is integrated with automation system, it is possible to achieve fast 100 % inspection of shape and dimensions and hence, reading and monitoring of the complete wire rod profile in the coil.
High-speed controls are also provided for high-speed shear rotor-diverter synchronization which allows head and tail cut at the maximum rolling speed of the wire rod mill. Also, the synchronization between the laying head and high-speed shear rotor during at head cut allows a 100 % reliable and precise positioning of the first loop of the wire rod on the control cooling conveyor. This avoids the risk of cobble on the cooling conveyor.
These days in WRMs, in-line surface inspection system is being used. It is a no-contact inspection system for the on-line detection of surface defects in rolled wire rods. Also, used in WRMs is the ‘roller guide calibration and alignment system’. It is a calibration system designed to aid the operator during (i) set-up of the roller guide in the workshop, and (ii) alignment of the roller guide mounting bases with the rolling ring pair groove in the finishing blocks.
Process models
The process models are used in WRM for achieving higher quality and productivity. These models are related to the ‘control cooling technology’ (CCT). One example of an open-loop control system is the controlled cooling technology. The system monitors and controls the whole temperature curve in a wire rod mill from the furnace through to the finished product, exactly and reproducibly. Hence, WRMs are achieving exactly the material properties needed by the customers and increasing the productivity of their production line at the same time.
CCT model consists of off-line CCT system and on-line CCT system. With the off-line CCT system, mill operator draws up cooling programmes and determines the settings and plant parameters for the water cooling sections and the control cooling conveyor (CCC). These values and parameters can be entered directly into the line controller. Further, off line CCT system contains simulation cores for the calculation of the whole temperature curve, including for CCC. With the integrated micro-structure model, mill operator generates, TTT (time-temperature-transformation) diagrams and flow curves for the specific rolling lots.
On-line process CCT system can be installed in addition to off-line CCT system. It monitors and controls the cooling sections in the WRM and ensures that all the target temperatures according to the cooling programmes created in advance using off-line CCT system are maintained. The control system operates in-line, hence, minimizes temperature fluctuations over the product length. Further, irregularities from the heating process can be reduced and the coil quality becomes more homogeneous. On-line CCT system can also be configured to include extensive Level 3 functions such as (i) material tracking, (ii) report generation, (iii) data acquisition and storage, (iv) visualisation, and (v) graphic display of important measurement values.
The benefits of process model include (i) the temperature curve is monitored and controlled (from the furnace through to the finished product), (ii) effective and efficient planning using simulations and models, (iii) selective control routines, (iv) minimization of temperature fluctuations, (v) homogeneous, and reproducible quality, and higher productivity.
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