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A contemplative stance on the automation of continuous casting in steel processing
Copyright Cl IFAC Future Trends in Automation in Mineral and Metal Processing, Finland, 2000
A CONTEMPLATIVE STANCE ON THE AUTOMATION OF CONTINUOUS CASTING IN STEEL PROCESSING I. K. Craig", F. R. Camisani-Calzolari', P. C. Pistorius" •Department of Electrical and Electronic Engineering, University of Pretoria, Pretoria, 0002, South Africa. Tel. +27 124202712; Fax. +27 123625000; email:[email protected]; [email protected] •• Department ofMaterials Science and Metallurgical Engineering, University of Pretoria, Pretoria, 0002, South Africa. Tel. +27 124203922; Fax. +27 123625304; email:[email protected]
Abstract: This paper gives a contemplative stance on the automation of continuous casting in steel processing. It gives a brief overview of the control techniques used in this industry, typical applications of such techniques, and discusses supportive technologies that form a critical part of the implementation of such techniques. The casting process and operational issues are described. Particular attention is paid to production planning and scheduling, computer-aided quality control, and the crucial role that instrumentation plays in the automation of a caster. The importance of integrating casting automation systems into plant wide production systems, and the development of new instrumentation, is highlighted. Copyright © 2000 IFAC Keywords: Steel industry; continuous casting; industrial control; automation; quality control; production planning; scheduling; computer-aided quality control, instrumentation.
excellent references on continuous casting and the application of process control and automation in the operation of such a machine (see e.g. Hiebler, 1992; Lankford, Samways, Craven, and McGannon, 1985).
I. INTRODUCTION This document forms part of a larger study by the IFAC Technical Committee on Mining, Metals, and Minerals (MMM) processing in which a contemplative stance on the MMM industry is being prepared. The purpose of such a contemplative stance is to give a brief overview of the control techniques or methods used in this industry, to give typical applications of such techniques, and to discuss supportive technologies that form a critical part of the implementation of such techniques. It also aims to identify techniques, applications and supportive technologies that are actively used or that are emerging as potential winners in the future. A contemplative stance is given in this document on automation of continuous casting in metal processing. It is by no means a comprehensive literature survey on the subject. There are many
An overview of the continuous casting process is given in Section 2. Section 3 covers important operational issues of casters, and continuous casting automation is discussed in Section 4. Production planning and scheduling is discussed in Section 5 and computer-aided quality control in Section 6. Typical supervisory control loops are discussed in Section 7 and Section 8 discusses caster instrumentation. Conclusions are drawn in Section 9.
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2. PROCESS OVERVIEW
the mould. Thirdly, the tundish can be designed to provide liquid steel to several moulds as is the case with multistrand casters.
The description of the process is given for a bow type continuous caster but is in principle applicable to most types of casters. A caster is depicted in Fig. 1.
At the bottom of the tundish there are mechanisms to allow the flow of steel into the mould. One of these mechanisms is a metering nozzle. A metering nozzle delivers an almost steady flow rate of steel into the mould. Control of the level in the mould must be exercised through casting speed manipulation when a metering nozzle is used. The second method is a slide-gate with submerged entry nozzle (SEN). The slide-gate opens and closes to regulate the flow of steel into the mould. The third method, and by far the most popular, is the stopper rod. The hole at the bottom of the tundish through which molten steel flows, is "plugged up" by a stopper rod according to the mould requirements. The shape of the " plug" is designed to allow a near-linear characteristic between the position of the plug and the flow rate. The level of steel in the mould must not vary, typically, by more than 4 mm. Larger variations tend to cause surface imperfections such as depressed regions filled with mould flux.
Ladle
I
'--_ _...-~- TlulcIi ..
~.lk;-- SI!N
~
i
S1rDnd
RacIi • .,. CooU ..
Fig. I. The bow continuous casting process.
2.3 Mould
2.1 Ladle
The mould is usually a water cooled copper sheath. The level of molten steel in the mould is known as the meniscus and can be measured by either a radiometric device or an eddy current device. As the liquid steel moves down the mould, it forms a shell that is strong enough to withstand the ferrostatic pressure of the liquid steel within the strand. The mould is typically about one metre long. To ensure lubrication of the solidified shell within the mould mould powders or oil are added at the top of th~ mould. These additives form a liquid layer between the steel and the copper to reduce friction. They also provide insulation from the atmosphere at the top of the mould to prevent oxidation. The mould oscillates to aid in the extraction of the solidified strand. S~m~times the mould flux is insufficiently dIstrIbuted along the mould, or the mould is too hot so that the strand surface fuses with the mould surface (this is known as a sticker). It results in the tearing of the strand shell and can cause a breakout so that liquid steel pours out from beneath the mould. Breakouts are extremely costly because of losses due t? s~rapping of steel and destruction of equipment by lIqUId metal that results in down-time. Special thermocouples are sometimes inserted in the mould to measure temperature gradients from the top to the bottom of the mould. Stickers cause characteristic changes in temperature with time. The thermocouples hence act as a breakout-detector, warning the operator of possible breakouts.
Molten steel arrives at the continuous casting machine in a refractory-lined container known as a ladle. In steel casters, the ladle contains approximately 70-300 tons of molten steel at between 1500 and 1600°C (typically 30-40 °C above its melting point). The ladle is then placed on one end of a rotating platform known as the turntable. The turntable can accommodate at least two ladles simultaneously. Thus, when all the steel from one ladle is cast, the turntable swings around and casting proceeds from the second ladle. This method of ladle switching ensures that steel is usually available for casting heats in sequence. The turntable method is the most widely used mechanism to switch ladles. Some casters use a slide mechanism by which the ladle is slid into position before casting commences. However, this method causes a long delay between switching because one ladle has to be extracted before another ladle can be slid into position. At the bottom of the ladle there is usually a slide-gate mechanism that controls the rate of flow of molten steel into another container known as the tundish. 2.2 Tundish
The tundish acts as a reservoir of molten steel. The reason that liquid metal is not poured directly into the mould from the ladle is three-fold. Firstly, if a ladle is not available for casting, continuity is not assured. Secondly, the tundish is designed to accommodate complex mechanisms to control the flow of steel into
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2.4 Secondary cooling zone
Lubrication may be lost for a number of reasons, including inadequate feeding of the mould powder, large fluctuations in metal level in the mould, and changes in the mould flux composition (for example, by alumina pick-up). Severe loss of lubrication may lead to a breakout, and well before this occurs, adverse effects on surface quality are likely.
On exit from the mould, the strand enters the secondary cooling zone that ranges in length from 6 to 20 metres. Directly below the mould there is usually a device known as the spray ring which provides a great amount of water flow to the strand to aid in cooling, and to ensure a smooth heat-transfer gradient between the mould and secondary cooling zone. In the secondary cooling zone, rollers support the strand and aid in bending and straightening in the case of bow type casters. Water sprays extract the heat from the strand and are grouped in three to six spray zones. Valves independently control the water flow in each spray zone.
Mould level control is similarly essential for stable operation, and has been well studied as reported in Section 7. 4. CONTINOUS CASTING AUTOMATION There are many drivers of process and automation technology including increasing customer demands, increased competition amongst producers, more stringent environmental regulations, and increased safety requirements. These drivers force producers to make optimum use of their steel plants, and have led to the development of overall production systems that ensure process consistency between steelmaking, casting and rolling (Holleis, 1992). A continuous casting automation system should therefore form part of an overall steel plant production system and perform essential tasks including production planning and scheduling, quality assurance, and the more conventional supervisory control functions. These issues are discussed in the sections that follow .
2.5 Radiation zone
On exit from the secondary cooling zone (SCZ), the strand moves into the radiation zone where the strand cools off naturally. Once the entire cross section (transversal slice) is below the solidus temperature, the strand is cut and transported to a fmishing process, typically rolling. 3. OPERATIONAL ISSUES The main operational issues in continuous casting relate to achieving a stable operation following startup, and then maintaining stability. Achieving stable operation involves appropriate use of the dummy bar, the correct starting lubricant, and the applicable sequence of ramping up to the casting speed. Standard procedures are available for this.
According to the traditional plant automation hierarchy, plant production system tasks are performed at a relatively high level (level 2 and higher). Lower level functions such as measurement and actuation, and slave loop and supervisory control are generally well understood and technologically mature. There is however significant room for improvement in the operation of casting machines even at these lower levels, as discussed in Sections 7 and 8.
Maintaining stability depends firstly on machine maintenance -including, for example, the cooling water of the mould, the mould shape, the mould oscillation mechanism, alignment of rollers and sections in the secondary cooling zone, and ensuring that all the sprays in the secondary cooling zone are functioning. Changes in these tend to affect operational stability in the longer term. Shorter-term upsets can arise from changes in metal feeding into the mould, transfer of oxide inclusions to the mould, loss of lubrication, and poor mould level control. Metal feeding is adversely affected by build-up of alumina inclusions in the submerged-entry nozzle (for aluminium-killed steels). These inclusions change the flow behaviour into the mould, so affecting mould level control. If such a build-up breaks away and arrives in the mould, it can give a sudden increase in the alumina content of the mould flux (with an effect on its viscosity and heat transfer properties), or it may be entrapped in the solidifying steel, forming a defect. The rate at which alumina inclusions arrive in the submerged-entry nozzle is in turn strongly affected by reoxidation of the steel, which occurs readily at any point of air entrainment. For this reason, shrouding the steel during its passage from ladle to tundish and mould is essential.
5. PRODUCTION PLANNING AND SCHEDULING Production planning and scheduling is a mature field as is evident from the many papers on production systems that were presented at the 14th IFAC World Congress in Beijing (see e.g. Mo, & Xiao, 1999; Ve1.a, Belak, & Babic, 1999). These papers are fairly general in nature and do not deal with steel plants per se. In contrast, papers on production systems that are applied to steel plants were presented at the 9th IF AC Symposium on Automation in Mining, Minerals and Metal processing held in Cologne in 1998 (see e.g. Reisch, Weller, & Pirron, 1998; Lipp, Bachmann, Bematzki, & Limbeck, 1998). Production scheduling papers are strongly represented at IFAC events with for example three sessions dedicated to this topic at the 14th IFAC 69
World Congress in Beijing. These sessions dealt with fuzzy and genetic approaches to scheduling, scheduling and queuing theory, and scheduling and control of manufacturing systems. Papers presented in these sessions are however quite general in nature and do not deal specifically with continuous casting or even steel production in general. An exception is the paper by Peng, Yang & XU (1999) which deals with continuous casting planning. Various rolling mill scheduling papers were also presented (see e.g. Tang, Rong & Yang, 1999). Nothing was however presented at this World Congress on plant wide scheduling of a steel manufacturing operation.
(Ho lie is, Bumberger, Fastner, Hirschmanner & Schwaha, 1985). If hot charging is practised, only limited visual inspection of slab surfaces for defects is possible, and slab grinding cannot be applied. As a result, computer-aided quality control (CAQC) is increasingly being used. CAQC systems utilise knowledge of the interactions between casting conditions and strand defect formation to predict product quality (Wolf, 1992). The aim of quality control is to assure the required product characteristics, typically expressed in terms of steel analysis, strand dimensions, surface quality, and internal soundness. From a control perspective, continuous on-line measurement of these product characteristics is required for corrective action to be timeous and effective. Unfortunately this is not possible for most of these product characteristics as sensors that can provide real-time measurements with high reliability have yet to be developed.
Traditionally, scheduling has been the domain of the industrial engineer who was tasked with ensuring that optimal use was made of process equipment suchas a caster by minimising waiting time resulting from e.g. machine set-up time. Increasing pressures from process and automation drivers mentioned earlier, are resulting in a blurring of the traditional boundaries between the automation and industrial engineering disciplines. This fact, together with the many production scheduling papers that are now presented at IFAC events, should further result in an increased awareness of the importance of plant wide production systems, also for steel production, in which techniques or methods from different disciplines are brought together.
Critical deviations in casting conditions that can be measured, such as excessive mould level variations, and varying or too low casting speed are therefore used as indications that surface defects will result. Models that use such casting conditions to predict product quality are therefore often used in CA QC systems (see also discussion in Section 7.). The lack of sensors mentioned above is perhaps the biggest hindrance to the development of more effective CAQC systems. This does not however prevent researchers form improving the quality of strand defect prediction models in the interim. For example, new prediction models can be developed from experimental data, and additional secondary sensors can be integrated into such models. An example of such secondary sensors could be additional mould thermocouples to measure the meniscus temperature.
The field of production planning and scheduling appears to be mature from a theoretical point of view. There does however seem to be significant scope for research in integrating casting automation systems into plant wide production systems. One of the reasons being that optimal planning and scheduling of a process unit such as a casting machine cannot be done in isolation as it is possible that an optimal unit schedule could conflict with an optimal plant wide schedule. For example the practice of sequence casting (casting a series of heats in succession without interrupting the casting process), is used to improve caster productivity as it avoids frequent casting machine set-up. Sequence casting can however be in conflict with the scheduling of different slab sizes required by the fmishing mills (Lankford et aI., 1985). A plant wide strategy is therefore required to achieve optimum results.
7. SUPERVISORY CONTROL Supervisory control usually refers to controllers or optimisation routines that provide setpoints for lower level control loops. Lower level control loops include mould and spray water flow control. Supervisory controllers include tundish and mould level control and secondary cooling control (Baumann, 1992).
6. COMPUTER AIDED QUALITY CONTROL
Possibly the most famous control problem in continuous casting from the perspective of the control community is that of mould level control. Many papers have been published on this topic, and although casting levels are difficult to control, this problem has largely been solved (Graebe, Goodwin, & Elsley, 1995). New casting technologies such as strip casting might however provide new level control challenges.
Making optimum use of a steel plant often results in process dynamics and process systems that are more difficult to control. This is particularly relevant to continuous casting. In order to improve productivity, buffering in the form of a storage yard between the continuous casting machines and the hot rolling mills has been eliminated in modern steel mills. This lack of buffering has resulted in the development of automation strategies to facilitate direct rolling and hot charging of continuously cast semiproducts
70
Another interesting supervisory control problem is that of secondary cooling control. This becomes particularly important when varying casting speeds, ladle changeovers, and spray nozzle blockages require an adjustment in steady-state spray water patterns. Given the absence of reliable and permanent strand surface temperature measurements in the secondary cooling zone, heat transfer models and optimisation routines can be used to determine optimum spray patterns (Camisani-Calzolari, Craig, & Pistorius, 1998). The increased importance of direct charging might justify a new look at secondary cooling control.
into CAQC systems that facilitates direct rolling and hot charging strategies. In general, the lack of direct measurements is often partly compensated for by the use of soft sensors which infer the desired quantity from measuring a related variable (see e.g. Casali, Vallebuona, Bustos, Gonzalez & P. Gimenez, 1998). In addition, process models are used to estimate variables that cannot be measured directly, or for which instruments are prohibitively expensive. For example, a model relating mould parameters to hot slab quality can be used in a CAQC system to assign a quality rating for each slab, as discussed in Section 6.
8. CONTINUOUS CASTER INSTRUMENTATION 9. CONCLUSIONS Instruments are used extensively in continuous casting operations for monitoring variables in the ladle, tundish, mould, secondary cooling zone, In fact, the radiation zone and run-out table. increased use of instruments to measure and control casting parameters have been credited as one of the major contributors to the large gains achieved in caster productivity and quality over the last 20 years. Ozgu (1996) gives a detailed review of continuous caster instrumentation. Of the 112 references quoted by Ozgu, only I refers to an IF AC meeting. This seems to indicate that caster instrumentation is not a popular topic amongst those who attend IFAC events, or that this topic is covered adequately by other organisations.
This paper gave a contemplative stance on the automation of continuous casting in steel processing. A brief overview of the control techniques used in this industry was given together with typical Supportive applications of such techniques. technologies, such as instrumentation, that form a critical part of the implementation of such techniques, were also discussed. The casting process and operational issues were highlighted. Particular attention was paid to production planning and scheduling, computer-aided quality control, and the crucial role that instrumentation plays in the automation of a caster. One of the major and largely unclaimed benefits in casting automation is the integration of casting automation systems into plant wide production systems. Significant advances in casting automation at a unit process level are hampered by the lack of instrumentation, and can be expected to occur once such instrumentation has been developed.
As discussed above, instruments are crucial to any control and automation system and their contribution to achieving modem productivity and quality standards cannot be overestimated. Instruments are the "eyes" of the control and automation system, and with current technology permanent instrumentation is available for the process and quality control system to "see" the most important ladle, tun dish and mould variables.
ACKNOWLEDGEMENTS The authors would also like to thank the South African National Research Foundation for its continued financial support.
Given the hazardous environment found within the spray chamber of the secondary cooling zone of a caster, it is not surprising that caster control systems are often "blind" to changes in critical process variables, such as the strand surface temperature, in this zone. Instrumentation used here is usually of a temporary nature and is therefore employed on an experimental basis. Other such experimental instrumentation include those used to measure interroll bulging, thickness of the solidified shell, and mould/strand friction (Baumann, 1992).
REFERENCES Baumann, H.G. (1992). Automation of casting plants, in Practice of Steelmaking, Part 5: Continuous casting, in the 4th edition of the Metallurgy of Iron vol. 11, A supplement to the Gmelin Handbook of Inorganic Chemistry, Springer-Verlag, Berlin, pp. 335a-353a. Camisani-Calzolari, F.R., Craig, I.K., & Pistorius, P.C. (1998). Specification framework for control of the secondary cooling zone in continuous casting, ISIJ International, 38(5),447-453. Casali, A., Gonzalez, G., Torres, F., Vallebuona, G., Castelli, L., & Gimenez, P. (1998). Particle size
With the increased emphasis on automation strategies to facilitate direct rolling and hot charging of continuously cast semiproducts, more effort should be put into developing permanent secondary cooling zone instrumentation. In addition, more emphasis should also be placed on developing affordable hot slab inspection technologies which can be integrated
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World Congress, Vol. M Se-04-S, Elsevier, Oxford, U.K., pp. Ill-liS. Wolf, M. (1992). Metallurgy of continuous casting: Quality control, in Practice of Steelmaking, Part S: Continuous casting, in the 4th edition of the Metallurgy of Iron vol. 11, A supplement to the Gmelin Handbook of Inorganic Chemistry, Springer-Verlag, Berlin, pp. 314a-319a.
distribution soft-sensor for a grinding circuit, Powder Technology, 99(1), IS-21. Graebe, S.F., Goodwin, G.C., & Elsley, G. (199S). Control design and implementation in continuous steel casting, IEEE Control Systems Magazine, August, 1995, 64-71. Hiebler, H. (1992). Practice of Steelmaking, Part 5: Continuous casting, in the 4th edition of the Metallurgy of Iron vol. 11, A supplement to the Gmelin Handbook of Inorganic Chemistry, Springer-Verlag, Berlin. Engineering of casters: Holleis, G. (1992). Integrated process automation, in Practice of Steelmaking, Part S: Continuous casting, in the 4th edition of the Metallurgy of Iron vol. 11, A supplement to the Gmelin Handbook of Inorganic Chemistry, Springer-Verlag, Berlin, pp. I 06a-1 09a. Holleis, G., Bumberger, H., Fastner, T., Hirschmanner F., & Schwaha, K. (198S). Prerequisites for production of continuously cast semis for direct rolling and hot charging, in proceedings of Continuous Casting '85, pp. 46.146.7. Lankford, W.T., Samways, N.L., Craven, R.F., & McGannon, H.E. (198S). The making, shaping and treating of steel, lOth edition, AISE Steel Foundation, Pittsburgh. Lipp, H.-P., Bachmann, S., Bematzki, K.-P., & Limbeck, W. (1998). A process controller with fuzzy petri nets for operative production management in a steel making process, In preprints of 9th IFAC Symposium on automation in mining, mineral and metal processing, Sep., pp. 87-93. Mo, Y. & Xiao, D. (1999). A model driven approach to information integration in continuous process industries, in Proceedindgs of the 14'h IFAC World Congress, Vol. A la-04-S, Elsevier, Oxford, U.K., pp. 11S-120. Ozgu, M.R. (1996). Continuous caster instrumentation: State-of-the-art review, Canadian Metallurgical Quarterly, 3S(3), 199223 . Peng, Z., Yang, Y. & Xu, X. (1999). A two stage heuristic approach for continuous casting planning, in Proceedindgs of the 14'h IFAC World Congress, Vol. 0 7b 03-4, Elsevier, Oxford, U.K., pp. 19-24. Reisch, 0., Weller, R., & Pirron, 1. (1998). TopazDecentralised cooperative production management, In preprints of 9th IFAC
Symposium on automation in mining, mineral and metal processing, Sep., pp. 81-8S. Tang, L., Rong, A. & Yang, Z. (1999). A MTSP model for hot rolling scheduling in Baosteel complex, in Proceedindgs of the 14'h IFAC World Congress, Vol. N 7b-OS-I, Elsevier, Oxford, U.K., pp. 4S1-4S6. Vefa, I., Belak, V. & Babic, Z. (1999). Development of inventory production system in turbulent environment, in Proceedindgs of the 14'h IFAC
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A CONTEMPLATIVE STANCE ON THE AUTOMATION OF CONTINUOUS CASTING IN STEEL PROCESSING I. K. Craig", F. R. Camisani-Calzolari', P. C. Pistorius" •Department of Electrical and Electronic Engineering, University of Pretoria, Pretoria, 0002, South Africa. Tel. +27 124202712; Fax. +27 123625000; email:[email protected]; [email protected] •• Department ofMaterials Science and Metallurgical Engineering, University of Pretoria, Pretoria, 0002, South Africa. Tel. +27 124203922; Fax. +27 123625304; email:[email protected]
Abstract: This paper gives a contemplative stance on the automation of continuous casting in steel processing. It gives a brief overview of the control techniques used in this industry, typical applications of such techniques, and discusses supportive technologies that form a critical part of the implementation of such techniques. The casting process and operational issues are described. Particular attention is paid to production planning and scheduling, computer-aided quality control, and the crucial role that instrumentation plays in the automation of a caster. The importance of integrating casting automation systems into plant wide production systems, and the development of new instrumentation, is highlighted. Copyright © 2000 IFAC Keywords: Steel industry; continuous casting; industrial control; automation; quality control; production planning; scheduling; computer-aided quality control, instrumentation.
excellent references on continuous casting and the application of process control and automation in the operation of such a machine (see e.g. Hiebler, 1992; Lankford, Samways, Craven, and McGannon, 1985).
I. INTRODUCTION This document forms part of a larger study by the IFAC Technical Committee on Mining, Metals, and Minerals (MMM) processing in which a contemplative stance on the MMM industry is being prepared. The purpose of such a contemplative stance is to give a brief overview of the control techniques or methods used in this industry, to give typical applications of such techniques, and to discuss supportive technologies that form a critical part of the implementation of such techniques. It also aims to identify techniques, applications and supportive technologies that are actively used or that are emerging as potential winners in the future. A contemplative stance is given in this document on automation of continuous casting in metal processing. It is by no means a comprehensive literature survey on the subject. There are many
An overview of the continuous casting process is given in Section 2. Section 3 covers important operational issues of casters, and continuous casting automation is discussed in Section 4. Production planning and scheduling is discussed in Section 5 and computer-aided quality control in Section 6. Typical supervisory control loops are discussed in Section 7 and Section 8 discusses caster instrumentation. Conclusions are drawn in Section 9.
67
2. PROCESS OVERVIEW
the mould. Thirdly, the tundish can be designed to provide liquid steel to several moulds as is the case with multistrand casters.
The description of the process is given for a bow type continuous caster but is in principle applicable to most types of casters. A caster is depicted in Fig. 1.
At the bottom of the tundish there are mechanisms to allow the flow of steel into the mould. One of these mechanisms is a metering nozzle. A metering nozzle delivers an almost steady flow rate of steel into the mould. Control of the level in the mould must be exercised through casting speed manipulation when a metering nozzle is used. The second method is a slide-gate with submerged entry nozzle (SEN). The slide-gate opens and closes to regulate the flow of steel into the mould. The third method, and by far the most popular, is the stopper rod. The hole at the bottom of the tundish through which molten steel flows, is "plugged up" by a stopper rod according to the mould requirements. The shape of the " plug" is designed to allow a near-linear characteristic between the position of the plug and the flow rate. The level of steel in the mould must not vary, typically, by more than 4 mm. Larger variations tend to cause surface imperfections such as depressed regions filled with mould flux.
Ladle
I
'--_ _...-~- TlulcIi ..
~.lk;-- SI!N
~
i
S1rDnd
RacIi • .,. CooU ..
Fig. I. The bow continuous casting process.
2.3 Mould
2.1 Ladle
The mould is usually a water cooled copper sheath. The level of molten steel in the mould is known as the meniscus and can be measured by either a radiometric device or an eddy current device. As the liquid steel moves down the mould, it forms a shell that is strong enough to withstand the ferrostatic pressure of the liquid steel within the strand. The mould is typically about one metre long. To ensure lubrication of the solidified shell within the mould mould powders or oil are added at the top of th~ mould. These additives form a liquid layer between the steel and the copper to reduce friction. They also provide insulation from the atmosphere at the top of the mould to prevent oxidation. The mould oscillates to aid in the extraction of the solidified strand. S~m~times the mould flux is insufficiently dIstrIbuted along the mould, or the mould is too hot so that the strand surface fuses with the mould surface (this is known as a sticker). It results in the tearing of the strand shell and can cause a breakout so that liquid steel pours out from beneath the mould. Breakouts are extremely costly because of losses due t? s~rapping of steel and destruction of equipment by lIqUId metal that results in down-time. Special thermocouples are sometimes inserted in the mould to measure temperature gradients from the top to the bottom of the mould. Stickers cause characteristic changes in temperature with time. The thermocouples hence act as a breakout-detector, warning the operator of possible breakouts.
Molten steel arrives at the continuous casting machine in a refractory-lined container known as a ladle. In steel casters, the ladle contains approximately 70-300 tons of molten steel at between 1500 and 1600°C (typically 30-40 °C above its melting point). The ladle is then placed on one end of a rotating platform known as the turntable. The turntable can accommodate at least two ladles simultaneously. Thus, when all the steel from one ladle is cast, the turntable swings around and casting proceeds from the second ladle. This method of ladle switching ensures that steel is usually available for casting heats in sequence. The turntable method is the most widely used mechanism to switch ladles. Some casters use a slide mechanism by which the ladle is slid into position before casting commences. However, this method causes a long delay between switching because one ladle has to be extracted before another ladle can be slid into position. At the bottom of the ladle there is usually a slide-gate mechanism that controls the rate of flow of molten steel into another container known as the tundish. 2.2 Tundish
The tundish acts as a reservoir of molten steel. The reason that liquid metal is not poured directly into the mould from the ladle is three-fold. Firstly, if a ladle is not available for casting, continuity is not assured. Secondly, the tundish is designed to accommodate complex mechanisms to control the flow of steel into
68
2.4 Secondary cooling zone
Lubrication may be lost for a number of reasons, including inadequate feeding of the mould powder, large fluctuations in metal level in the mould, and changes in the mould flux composition (for example, by alumina pick-up). Severe loss of lubrication may lead to a breakout, and well before this occurs, adverse effects on surface quality are likely.
On exit from the mould, the strand enters the secondary cooling zone that ranges in length from 6 to 20 metres. Directly below the mould there is usually a device known as the spray ring which provides a great amount of water flow to the strand to aid in cooling, and to ensure a smooth heat-transfer gradient between the mould and secondary cooling zone. In the secondary cooling zone, rollers support the strand and aid in bending and straightening in the case of bow type casters. Water sprays extract the heat from the strand and are grouped in three to six spray zones. Valves independently control the water flow in each spray zone.
Mould level control is similarly essential for stable operation, and has been well studied as reported in Section 7. 4. CONTINOUS CASTING AUTOMATION There are many drivers of process and automation technology including increasing customer demands, increased competition amongst producers, more stringent environmental regulations, and increased safety requirements. These drivers force producers to make optimum use of their steel plants, and have led to the development of overall production systems that ensure process consistency between steelmaking, casting and rolling (Holleis, 1992). A continuous casting automation system should therefore form part of an overall steel plant production system and perform essential tasks including production planning and scheduling, quality assurance, and the more conventional supervisory control functions. These issues are discussed in the sections that follow .
2.5 Radiation zone
On exit from the secondary cooling zone (SCZ), the strand moves into the radiation zone where the strand cools off naturally. Once the entire cross section (transversal slice) is below the solidus temperature, the strand is cut and transported to a fmishing process, typically rolling. 3. OPERATIONAL ISSUES The main operational issues in continuous casting relate to achieving a stable operation following startup, and then maintaining stability. Achieving stable operation involves appropriate use of the dummy bar, the correct starting lubricant, and the applicable sequence of ramping up to the casting speed. Standard procedures are available for this.
According to the traditional plant automation hierarchy, plant production system tasks are performed at a relatively high level (level 2 and higher). Lower level functions such as measurement and actuation, and slave loop and supervisory control are generally well understood and technologically mature. There is however significant room for improvement in the operation of casting machines even at these lower levels, as discussed in Sections 7 and 8.
Maintaining stability depends firstly on machine maintenance -including, for example, the cooling water of the mould, the mould shape, the mould oscillation mechanism, alignment of rollers and sections in the secondary cooling zone, and ensuring that all the sprays in the secondary cooling zone are functioning. Changes in these tend to affect operational stability in the longer term. Shorter-term upsets can arise from changes in metal feeding into the mould, transfer of oxide inclusions to the mould, loss of lubrication, and poor mould level control. Metal feeding is adversely affected by build-up of alumina inclusions in the submerged-entry nozzle (for aluminium-killed steels). These inclusions change the flow behaviour into the mould, so affecting mould level control. If such a build-up breaks away and arrives in the mould, it can give a sudden increase in the alumina content of the mould flux (with an effect on its viscosity and heat transfer properties), or it may be entrapped in the solidifying steel, forming a defect. The rate at which alumina inclusions arrive in the submerged-entry nozzle is in turn strongly affected by reoxidation of the steel, which occurs readily at any point of air entrainment. For this reason, shrouding the steel during its passage from ladle to tundish and mould is essential.
5. PRODUCTION PLANNING AND SCHEDULING Production planning and scheduling is a mature field as is evident from the many papers on production systems that were presented at the 14th IFAC World Congress in Beijing (see e.g. Mo, & Xiao, 1999; Ve1.a, Belak, & Babic, 1999). These papers are fairly general in nature and do not deal with steel plants per se. In contrast, papers on production systems that are applied to steel plants were presented at the 9th IF AC Symposium on Automation in Mining, Minerals and Metal processing held in Cologne in 1998 (see e.g. Reisch, Weller, & Pirron, 1998; Lipp, Bachmann, Bematzki, & Limbeck, 1998). Production scheduling papers are strongly represented at IFAC events with for example three sessions dedicated to this topic at the 14th IFAC 69
World Congress in Beijing. These sessions dealt with fuzzy and genetic approaches to scheduling, scheduling and queuing theory, and scheduling and control of manufacturing systems. Papers presented in these sessions are however quite general in nature and do not deal specifically with continuous casting or even steel production in general. An exception is the paper by Peng, Yang & XU (1999) which deals with continuous casting planning. Various rolling mill scheduling papers were also presented (see e.g. Tang, Rong & Yang, 1999). Nothing was however presented at this World Congress on plant wide scheduling of a steel manufacturing operation.
(Ho lie is, Bumberger, Fastner, Hirschmanner & Schwaha, 1985). If hot charging is practised, only limited visual inspection of slab surfaces for defects is possible, and slab grinding cannot be applied. As a result, computer-aided quality control (CAQC) is increasingly being used. CAQC systems utilise knowledge of the interactions between casting conditions and strand defect formation to predict product quality (Wolf, 1992). The aim of quality control is to assure the required product characteristics, typically expressed in terms of steel analysis, strand dimensions, surface quality, and internal soundness. From a control perspective, continuous on-line measurement of these product characteristics is required for corrective action to be timeous and effective. Unfortunately this is not possible for most of these product characteristics as sensors that can provide real-time measurements with high reliability have yet to be developed.
Traditionally, scheduling has been the domain of the industrial engineer who was tasked with ensuring that optimal use was made of process equipment suchas a caster by minimising waiting time resulting from e.g. machine set-up time. Increasing pressures from process and automation drivers mentioned earlier, are resulting in a blurring of the traditional boundaries between the automation and industrial engineering disciplines. This fact, together with the many production scheduling papers that are now presented at IFAC events, should further result in an increased awareness of the importance of plant wide production systems, also for steel production, in which techniques or methods from different disciplines are brought together.
Critical deviations in casting conditions that can be measured, such as excessive mould level variations, and varying or too low casting speed are therefore used as indications that surface defects will result. Models that use such casting conditions to predict product quality are therefore often used in CA QC systems (see also discussion in Section 7.). The lack of sensors mentioned above is perhaps the biggest hindrance to the development of more effective CAQC systems. This does not however prevent researchers form improving the quality of strand defect prediction models in the interim. For example, new prediction models can be developed from experimental data, and additional secondary sensors can be integrated into such models. An example of such secondary sensors could be additional mould thermocouples to measure the meniscus temperature.
The field of production planning and scheduling appears to be mature from a theoretical point of view. There does however seem to be significant scope for research in integrating casting automation systems into plant wide production systems. One of the reasons being that optimal planning and scheduling of a process unit such as a casting machine cannot be done in isolation as it is possible that an optimal unit schedule could conflict with an optimal plant wide schedule. For example the practice of sequence casting (casting a series of heats in succession without interrupting the casting process), is used to improve caster productivity as it avoids frequent casting machine set-up. Sequence casting can however be in conflict with the scheduling of different slab sizes required by the fmishing mills (Lankford et aI., 1985). A plant wide strategy is therefore required to achieve optimum results.
7. SUPERVISORY CONTROL Supervisory control usually refers to controllers or optimisation routines that provide setpoints for lower level control loops. Lower level control loops include mould and spray water flow control. Supervisory controllers include tundish and mould level control and secondary cooling control (Baumann, 1992).
6. COMPUTER AIDED QUALITY CONTROL
Possibly the most famous control problem in continuous casting from the perspective of the control community is that of mould level control. Many papers have been published on this topic, and although casting levels are difficult to control, this problem has largely been solved (Graebe, Goodwin, & Elsley, 1995). New casting technologies such as strip casting might however provide new level control challenges.
Making optimum use of a steel plant often results in process dynamics and process systems that are more difficult to control. This is particularly relevant to continuous casting. In order to improve productivity, buffering in the form of a storage yard between the continuous casting machines and the hot rolling mills has been eliminated in modern steel mills. This lack of buffering has resulted in the development of automation strategies to facilitate direct rolling and hot charging of continuously cast semiproducts
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Another interesting supervisory control problem is that of secondary cooling control. This becomes particularly important when varying casting speeds, ladle changeovers, and spray nozzle blockages require an adjustment in steady-state spray water patterns. Given the absence of reliable and permanent strand surface temperature measurements in the secondary cooling zone, heat transfer models and optimisation routines can be used to determine optimum spray patterns (Camisani-Calzolari, Craig, & Pistorius, 1998). The increased importance of direct charging might justify a new look at secondary cooling control.
into CAQC systems that facilitates direct rolling and hot charging strategies. In general, the lack of direct measurements is often partly compensated for by the use of soft sensors which infer the desired quantity from measuring a related variable (see e.g. Casali, Vallebuona, Bustos, Gonzalez & P. Gimenez, 1998). In addition, process models are used to estimate variables that cannot be measured directly, or for which instruments are prohibitively expensive. For example, a model relating mould parameters to hot slab quality can be used in a CAQC system to assign a quality rating for each slab, as discussed in Section 6.
8. CONTINUOUS CASTER INSTRUMENTATION 9. CONCLUSIONS Instruments are used extensively in continuous casting operations for monitoring variables in the ladle, tundish, mould, secondary cooling zone, In fact, the radiation zone and run-out table. increased use of instruments to measure and control casting parameters have been credited as one of the major contributors to the large gains achieved in caster productivity and quality over the last 20 years. Ozgu (1996) gives a detailed review of continuous caster instrumentation. Of the 112 references quoted by Ozgu, only I refers to an IF AC meeting. This seems to indicate that caster instrumentation is not a popular topic amongst those who attend IFAC events, or that this topic is covered adequately by other organisations.
This paper gave a contemplative stance on the automation of continuous casting in steel processing. A brief overview of the control techniques used in this industry was given together with typical Supportive applications of such techniques. technologies, such as instrumentation, that form a critical part of the implementation of such techniques, were also discussed. The casting process and operational issues were highlighted. Particular attention was paid to production planning and scheduling, computer-aided quality control, and the crucial role that instrumentation plays in the automation of a caster. One of the major and largely unclaimed benefits in casting automation is the integration of casting automation systems into plant wide production systems. Significant advances in casting automation at a unit process level are hampered by the lack of instrumentation, and can be expected to occur once such instrumentation has been developed.
As discussed above, instruments are crucial to any control and automation system and their contribution to achieving modem productivity and quality standards cannot be overestimated. Instruments are the "eyes" of the control and automation system, and with current technology permanent instrumentation is available for the process and quality control system to "see" the most important ladle, tun dish and mould variables.
ACKNOWLEDGEMENTS The authors would also like to thank the South African National Research Foundation for its continued financial support.
Given the hazardous environment found within the spray chamber of the secondary cooling zone of a caster, it is not surprising that caster control systems are often "blind" to changes in critical process variables, such as the strand surface temperature, in this zone. Instrumentation used here is usually of a temporary nature and is therefore employed on an experimental basis. Other such experimental instrumentation include those used to measure interroll bulging, thickness of the solidified shell, and mould/strand friction (Baumann, 1992).
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With the increased emphasis on automation strategies to facilitate direct rolling and hot charging of continuously cast semiproducts, more effort should be put into developing permanent secondary cooling zone instrumentation. In addition, more emphasis should also be placed on developing affordable hot slab inspection technologies which can be integrated
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