The synthesis and integration of high performance measurement systems, advanced mill actuators and process knowledge into effective control systems is central to the achievement of required results. The high level of capital investment required for hot mill lines means that it is very difficult to justify investment in complete new plant, and achieving improved performance by revamping existing equipment or re-installing re-vamped second-hand equipment is often the only cost effective route.
Continual progress in the development of advanced mill actuators, measurement devices and their integration into effective control schemes demands fresh consideration from producers if they are to maintain or improve their competitive position.
ACTUATORSActuators are the second vital element in that they provide the capability to influence outcomes. Given the different fundamental physical nature of parameters to be controlled, a variety of actuators are needed to achieve product quality objectives.
At a basic level, it is well known that significant benefits can be achieved at relatively low cost by upgrading conventional four-hi hot mills with hydraulic automatic gauge control (HAGC), more powerful roll bend systems (positive and negative) and highly controllable work roll coolant application systems.
However, for profiles and flatness control, these upgrades do have performance limitations, most particularly related to transitional states following delays or product changes. More advanced millstand actuators are required if "first coil-on-target" objectives are to be realized.
The Dynamic Shape Roll (DSR) replaces the back-up roll in a conventional mill and generates rolling load internally via a series of servo-controller hydraulic cylinders arranged across the strip width. Comparison and analysis of the various forms of advanced actuators illustrate that the DSR has significant advantages over all other competitive types. The DSR not only has better performance capability, but also provides fully dynamic integration of gauge, flatness and profile control in one actuator. Unlike competitive actuators, the performance of the DSR is maintained or improved as width increases. This makes the DSR ideally suited to wider mills and higher speed applications, in line with developing product trends.
The first generation of DSR was actually proven in operation on the Neuf Brisach hot line nearly 10 years ago. The DSR has been shown in practice to be a highly reliable and rugged unit with no significant increase in maintenance required when compared to a conventional four-hi mill.
A 1,460 mm diameter DSR has now been in successful operation for a year on the last stand of a 2,840 mm wide tandem hot mill with a load capacity of 2,900 tonnes (Te). This roll, used in conjunction with a Hot Planicim roll, has now fully demonstrated the integrity and versatility of the DSR for aluminum hot rolling applications.
An upgrade of the Huta Konin hot line has been operational since the beginning of 1999. This will incorporate a large, 1,600 mm dia roll with a 2,200 mm face width and a 3,000 Te load capacity. The roll will have a 60 mm working stroke and will be used for gauge, flatness and profile control. Installation of the DSRR on this single stand reversing, slabbing/finishing, twin coiler hot mill will fully prove applicability of the DSRR for the whole range of aluminum hot rolling applications.
Heavy edging mills are often considered an unnecessary luxury in aluminum hot line configurations. However, recent developments of fully hydraulic, heavy duty edging mills (Fig. 2) equipped with hydraulic automatic width control may alter the cost-benefit equation. Benefits include:
- The ability to produce a range of widths from one as-cast width
- Accurate control of width
- Elimination of head and tail flaring
- Improved strip edge consolidation and reduced edge cracking
- Reduced edge trim and improved strip yield.
There are now a significant number of the latest type of hydraulic looper operating successfully in the steel industry. New developments in control technology for hydraulic loopers have enabled the force on the looper to be controlled to precise limits. The looper roll incorporates sensitive load measurements utilizing shear beam load cells.
The multivariable control scheme is illustrated in Fig. 3 and its implementation overcomes one of the remaining elements in the variability of interstand tension. As a consequence, effective control of looper pressure brings significant improvements in dimensional control capability with reduced variability in both longitudinal gauge and width.
There is seen to be an increased interest in the application of strip cooling for both metallurgical and productivity reasons. This encompasses the possible use of laminar jets, water curtains and U-tubes systems. Additionally, a more modulatable cooling actuator is now available for consideration. Adjustable Cooling (ADCO) is a fully automated cooling system which uses a mixture of air and water (Fig. 4) to achieve the wide range of cooling rates necessary to meet the metallurgical requirements of both accelerated cooling and direct quenching.
To date, applications have been directed to steel, but the potential application to aluminum rolling could raise significant benefits.
SYSTEM SYNTHESISMost quality parameters (e.g., profile, temperature, surface quality and microstructure) require that the hot line process be considered as an "integrated whole." The appropriate selection of measurement, actuator and control strategies is reliant on a fundamental appreciation of the complex process interactions and the need to achieve intermediate product targets, e.g. In a roughing mill and tandem mill hot line configuration there is a need for effective control of the transfer bar profile if finished product target profile is to be achieved.
A fully capable mill needs to have an integrated control system implementation philosophy ideally right through Levels 0 to 3. Inevitably, because of the wide range of technologies to be employed, this will involve linking a variety of sub-systems. With modern processing and communications, this is technologically achievable, but does require an appropriate amount of care and attention in system design and implementation.
On-Line Control Models
Model based systems have been in use for some time on aluminum hot mills. Models, typically physically based, provide load, power, temperature, profile and flatness predictions and associated derivatives for setup and control loop gains. As computing capability has increased, model usage has become more extensive and relevant. More sophisticated approaches using adaptation and expert system techniques are gradually finding increasing applicability and yielding improved results.
Historically, resultant microstructure has been an implicit result of control of gross external parameters such as reduction, temperature and speed. Production practices and control systems have been configured to deliver these external parametric targets. However, inevitable variability in the process has meant that underlying microstructure changes are essentially uncontrolled.
With the latest developments in microstructure modeling and the advent of suitably powerful real time computing systems, that direct control of microstructure development is within the bounds of possibility. The use of relatively simple microstructural models, which has proved so successful in steels, is less adequate for aluminum alloys. For aluminum alloys, developments are focused on more physically based models which need to describe a wider range of microstructural variables.
Integrated Profile and Flatness Control
Effective control of strip profile and flatness on aluminum hot lines requires an integrated approach, which considers the development of flatness and profile throughout the rolling schedule. Overall mill performance is improved by the existence of related off-line and on-line systems. The off-line system, incorporating mill models in a dynamic simulator, allows for the development of efficient and practical rolling schedules. In the on-line system, adaptive set-up provides good strip head end performance, and closed loop control results in a tight control band achieving specific targets in the body and tail. Overall, use of the integrated system results in a higher level of performance, both in terms of product quality parameters and, very importantly, system robustness.
This is particularly relevant to single stand twin-coiler applications, but is applicable to all types of hot mills. Earlier versions of automatic steering, using HAGC, relied upon differential load measurement which produced some benefits, but is limited because not all steering errors are load difference related. Introduction of camera based steering systems, integrated with load and position measurements, has meant a much more effective and reliable system.
The potential adverse consequences of poor steering can severely affect production, so the incremental cost of this type of upgrade is usually readily justifiable.
SUMMARYIt is essential in today's market that producers consider the need for improving their plant process capability. Survival depends on it. Clearly the purchase of new plant is desirable, but may not be economically justifiable. The range of technologies covered in this article show that performance of existing hot lines can be successfully upgraded by judicious selection of measurement devices, actuators and integrated control systems to meet the performance requirements that the market demands in an incremental and cost efficient manner.
In making incremental improvements, producers should have a clear future vision and ensure that equipment and control systems will be expandable to meet both current and future needs and hence realize continuing benefit from any investment made. Equally important is to have a good co-operative ongoing relationship between producer and equipment supplier. The hot-line is critical, and it is vital to plan and structure the project and shut down work in such a way that the business is properly protected and the most cost effective implementation is realized.