I E E E T R A N S A C T I O N S O N I N D U S T R I A L E L E C T R O N I C S A N D C O N T R O L I N S T R U M E N T A T I O N V O L . IECT-1 3, N O . 1 A P R I L , 1966

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Fig. 11. Control of moisture.

Grade Change

Some paper mills run one grade of paper continuously, e.g., newsprint, while others may change the product they produce as often as five or ten times a day. On mills

where changes are frequent or of long durat ion, a con­venient computer application is the supervision and im­plementation of the grade change. It is possible to com­pute the various time lags through the mill and the amount of change to be made in each of the manipula ted variables.

The changes may then be scheduled using a P E R T type program or some simplification thereof, such tha t the change is made in the minimum amount of t ime. Of course, any automat ic change of this type must not re­sult in breaking the sheet of paper on the machine. I t is possible to design the program so that this will not happen and the progress can be monitored continually throughout the change to assure that the transient period does not produce any disruption.

The Spread of Computer Control in Steel Mills ALLEN S. Β ROWER

Abstract—The application of digital computers in the metals industry has experienced an extremely rapid growth over the past five years, both in this country and in Europe. Indicative of this trend is the fact that the six new continuous hot strip mills most recently built, or being built, in the United States were all designed -for and purchased with process computer control. The typical functions in such systems include reheat furnace control, slab tracking, roughing and finishing mill setup, finishing and coiling temperature control, and production logging. Other more recent applications have been made in the steel making and primary mills area, both of which have found ready acceptance.

Results from early applications now available indicate that pro­jected economic justification in terms of quality improvement i s valid. Several in-service installations are described and the results obtained are discussed.

Examination of the steel industry processes leads to a belief that, over the next decade, almost all parts of the steel plant will be com­puter-controlled. This continuing growth will be limited only by our ability to understand and rationalize the process to be controlled.

Presently on the horizon is overall control of the steel plant from customer order to product shipment. Multiple computers, both busi­ness and process control, will be linked together to achieve inte­grated production control throughout the production cycle. The re­sults will be improved efficiency of operation and shortened delivery cycle time.

Τ Ν T H E P E R I O D when we have come to accept J[ manned satellites soaring overhead as common­

place, a quiet and virtually unheralded explosion has burst upon the industrial scene. While digital computa-

Manuscr ip t received September 15, 1965. ^ τ T h e au thor is with the General Electric Company , Schenectady,

tion devices and techniques have extended man ' s reach into outer space, the same digital technologies have sharpened his senses and improved his abilities to con­trol his environment in the inner space wherein he has lived and worked for many centuries.

Digital computers for the control of industrial pro­cesses have, in a relatively short period of time, achieved a position of importance and acceptance unparalleled by any other industrial change in the last four decades. Moreover, the steel industry has, by conscious decision, placed itself in the vanguard of such change.

I t is difficult to fix the b i r thda te of this latest phase of technological revolution. However, 1958 is probably close enough to illustrate the point being made here. The growth rate of digital computer applications to control of industrial processes can only be described as phe­nomenal. In 1963, Control Engineering published a tabu­lation of 340 known installations, of which 55 were in the metals industry. Two years later, an updated listing showed a total of 765 applications, with 144 in the metals industry. Of this latter, 110 are identifiable in the steel industry.

Great difficulty exists in t rying to maintain up-to-date and accurate figures on computer installations due to the proprietary na ture of the information in some in­stances, and the rapidi ty of change through addit ions. Though no claim can be made for exactness of numbers , Fig. 1 illustrates graphically the trend in the rapid pop­ulation growth of process computers in the steel indus­t ry .

16

BROWER: COMPUTER CONTROL IN STEEL MILLS 17

I20i

1958 1959 I960 1961 1962 1963 1964 1965 YEAR

ACCUMULATED TOTAL PROCESS COMPUTERS INSTALLED OR ON ORDER BY STEEL INDUSTRY SINCE 1958

Fig. 1. Growth curve for process control computers in the steel industry .

While two years ago the number of foreign installa­tions outnumbered the domestic ones, this si tuation no longer exists as the modernization of American steel p lants has gained in momentum.

T h e question arises as to the definition of a process control computer and, therefore, which instal lat ions to include in such a tabulat ion. There is continued uncer­t a in ty and debate on this point because many indus­trial control-type computers are not being used to con­trol ci process directly. Some are used only to collect and refine data for off-line analysis; some are being used completely off-line as laboratory research calcula­tors ; and some are used on-line to prepare and display instructions to human operators.

For our purposes here, it has been decided to recog­nize as on-line control computer systems those installa­tions t ha t satisfy at least these three condit ions:

1) They are built a round a digital computer with mathematical capabil i ty, designated by the manu­facturer as an on-line industrial control model.

2) They are directl\ r connected to a process or opera­tion through sensing devices, ac tuators , or opera­tor signals.

3) They can control, or are capable of future con­trolling, a process or some portion of a process.

These criteria eliminate those computers used for off­line research or analysis and all special-purpose da ta accumulators such as those widely used for tinning-line production logging.

FACTORS I N F L U E N C I N G APPLICATION

Examinat ion of computer application in the steel indus t ry should s tar t with the motives behind such ap­plication. Rationally, any process is sui table for com­puter control if any of the following condit ions apply :

1) The product throughput value is high enough to justify the added cost of the computer system.

2) The quality of product produced has a decisive effect on following process efficiency.

3) The process decisively determines the end product quali ty regardless of succeeding processes.

4) The cost of the basic process equipment is so large tha t a moderate increase in production efficiency can eliminate or postpone the need for construct­ing an additional facility.

5) The process is complex, having many variables re­quiring frequent, rapid, and accurate adjus tment for most economic operat ion.

6) The process is inherently unstable, subject to rapid drifts requiring continual manual a t ten t ion .

7) The process contains many unknown operat ing factors and relationships t h a t can be improved or solved by rational s tudy and analysis.

Most on-line computers now installed or on order have been structured as a par t of a new facility or as a par t of a major modernization of an existing facility. However, in recent months, on-line computer systems have begun to be applied to existing plants as self-justified projects. The majority of future projects will probably be additions to existing mills and processes.

Figure 2 illustrates the processes in a modern in­tegrated steel plant producing str ip and t inplate. Based on the foregoing criteria, we would expect to find com­puter applications to all bu t the pickle line and the temper mill. The former is excluded due to the relat ive simplicity of the process and its low contr ibuted process value, while the temper mill in i ts present mechanical form is not readily amenable to practical computer control.

The tabulat ion of Fig. 3 suppor ts this conclusion. Though a greater number of installations are shown for

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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS AND CONTROL INSTRUMENTATION

Shipping Tinning line

Fig. 2. Steel plant processes for sheet and str ip production.

POWER AND UTILITIES 5

RAW MATERIAL PREPARATION 5

BLAST FURNACES 10

STEEL MAKING PROCESSES 32

REVERSING HOT MILLS 14

HOT STRIP MILLS 20

COLD STRIP MILLS 7

PROCESSING L INES 14

PRODUCTION SCHEDULING AND CONTROL 3

TOTAL 110

Fig. 3. Process applicat ions of digital computers in the steel indus t ry (September 1, 1965).

18 APRIL

1966 BROWF.R: COMPUTER CONTROL IN STEEL MILLS 19

steel making processes (the bulk of these on basic oxygen furnaces), in fact few of these actually function to control the process directly. The greatest number of actual control computers is found for the hot strip mill, followed by the reversing hot mill and the process­ing lines. The high production rate and high contr ibuted product value, especially of the hot strip mill, provides the economic return required, while the extensive prior work on au tomat ion and control has provided the process model and know-how required to develop the computer instruction program.

T h e amount of effort being expended on data logging and investigation of operation is rapidly adding to the knowledge and unders tanding of the Basic Oxygen steel making process. I t is very probable that many, if not all, of the installations presently fitted with digital computers for da ta logging, or for predictive calcula­tions for the operator ' s guidance, will soon become di­rect closed-loop controllers with adapt ive feedback.

There are many types of reversing hot mills, possess­ing varying apt i tudes for computer control. Most uni­versal slabbing mills, s t ructural mills, and blooming mills can be operated very satisfactorily by card pro­gram systems and, until recently, there has been little pressure to go beyond this level of sophistication.

Plate mills are, however, finishing mills. They have operat ing characteristics tha t tax the ability of human operators . Program control systems are of little merit since the incoming ingot characteristics are variable, and it is impossible to predict consistently rolling schedules t ha t will make the width required in the finishing plate. Then, too, the finished plate is a final shipped product , having qual i ty requirements for flat­ness, cross-sectional shape, and accuracy of gauge and width. Thus , there is considerable merit in on-line com­pute r systems for plate mills and other reversing finishing mills.

On-line computer control systems for large continuous processing lines are a natural outgrowth of the recent rise of coil-form orders for strip mill products. Such product ion created a need for continuous digital-type product ion da ta analyzers and accumulators. Upgrad­ing these machines to computer systems has not been an unnecessarily expensive step, and has usually re­sulted in a system possessing much greater capabilities with economic advantage . Applications are found pri­marily on tinning, continuous annealing, and shear and cuti iρ lines.

T H E H O T S T R I P M I L L

T h e world is now in the midst of a hot strip mill construction boom, and so it is no surprise to see twenty hot str ip mill computer control systems installed or on order. More are being planned.

A modern continuous, or semicontinuous, hot strip

mill is an extremely expensive facility. The most recent installations are capable of producing over 3.5-million tons of finished coils in a full production year. This combination of high capital cost and high productive capacity is such that few steel producing corporations possess more than one such unit at any one plant site.

Each hot strip mill suppor ts a much greater number of succeeding finishing mills and processes, which, how­ever, operate at much slower production rates and exist in multiple units such tha t disturbances in any one unit are less critical. The hot strip mill, however, occupies a production defile. Disturbances in its production rate and quality are quickly and seriously felt in succeeding processes and in plant profit. By its nature , the hot strip mill is its owner's most impor tant possession. Hence, it is entirely logical tha t efforts towards im­provement should have been exerted here first.

ITot str ip mill computer systems are probably the most elaborate systems of this type now being designed for the steel industry. In a typical system (see Fig. 4), the computer performs a number of interrelated but separate functions: p r imary da ta input, slab tracking, mill pacing, mill setup, temperature control, and pro­duction logging.

Other nonprocess-oriented functions such as off-normal monitoring, engineering logging, etc., are also included, but these are not basically operating functions.

Primary Data Input {Fig. 5) The computer requires rolling schedules similar to

those provided to the human operators, but they must be prepared in machine language and form. Punched tape or cards can be used equally well.

Cards or tape are made up and da ta entered into the computer before the arrival of the first slab on that schedule a t the furnace entry. Operator display and control stations are provided at both furnace entry and discharge to allow for break-ins and al terat ions to the schedule.

Slab Tracking (Fig. 6) After discharge from the furnace, each slab is tracked

through the mill by means of metal sensors located so as to establish zones which can contain no more than one entire slab at any t ime. The slab-tracking program is thus the "executive" function which ties the computer to the process in real-time. It establishes when necessary calculations must be made ; references to subsystems are t ransmi t ted ; it maintains correct slab identities in the various operating pulpit displays; and, finally, it corre­lates all production and process da ta for logging.

Mill Setup (Fig. 7) The mill setup function will determine a compatible

set of speed and roll openings for the entire mill and t ransmi t them to the proper regulating subsystems at

20 IEEE T R A N S A C T I O N S O N I N D U S T R I A L E L E C T R O N I C S A N D C O N T R O L I N S T R U M E N T A T I O N APRIL

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Fig. 8. Tempera tu re control function.

KEY POINT FURNACES

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QUALITY LOG

Fig. 9. Mill pacing function. Fig. 10. Quality logging function.

1966 B R O W E R : C O M P U T E R C O N T R O L IN STEEL MILLS 21

the appropriate moment in t ime for each change in roll­ing schedule. Within the l imitat ions of the mill, main drive power, and specified rolling practice, the com­puter distributes drafting through the mill to establish required finish gauge at the head end with desired stand load distribution.

This function also includes width control. For given incoming slab dimensions, the computer determines the roll settings for the edgers, d is t r ibut ing reductions in a predetermined fashion. Feedback from width gauges a t strategic locations will correct sett ings and compen­sate for roll \vear and other changing mill conditions.

Temperature Control (Fig. S)

Certain physical properties of the hot rolled steel are set by the temperatures a t which finish rolling and coil­ing occur. The computer, through both the mill pacing and mill setup functions, acts to mainta in the tempera­tures of these points within t h e specified limits through the length of each coil. With the assistance of temper­a ture measuring sensors at several points in the mill, the finish mill delivery speed is modified as required for this purpose. Once the head end of the coil is in the coiler, the computer will control the mill speed to over­come temperature rundown. Runou t table sprays are computer controlled with spray pa t t e rns varied accord­ing to rolling conditions and mill speed.

Mill Facing (Fig. 9)

Another computer function sets the rate at which slabs are processed in order to a t t a in a maximum mill utilization. Taking account of physical limitations of design, conditions of the mill components , and the other functions of computer control , the program will main­tain a minimum separation between slabs which will jus t avoid collision at any point . This control is achieved primarily by signaling the furnace push operation. The computer will also have the abil i ty to s top the run-in and interstand tables and to control the holding table speed to further regulate separat ion. The objective of the mill pacing function is maximum tonnage without damage to mill or product .

Production Logging (Fig. 10) Finally, the ability of the digital computer to com­

municate, as well as control, is utilized in a production logging function. In-process da t a is collected, and strip quali ty is logged and tallied. These are analyzed to yield product quali ty and production reports . This function is synchronized with the s lab-tracking function so t h a t coil ident i ty da ta and process readings are properly correlated with coil quali ty and product ion data .

O P E R A T I N G R E S U L T S

The value in return from such a system is shown by the bar graph of Fig. 11 for d a t a obtained from one such system in operation. T h e computer has provided a

8EF0RE MODERNIZATION

GAUGE CONTROL ONLY

85

COMPUTER AND GAUGE CONTROL

94

37

19 19

-4-2 1*2+4 -4 -2 2*4 -2 1+2

PERCENT OF PRODUCTION VS GAUGE DEVIATION BAND

Fig. 11. Quality improvement of p roduc t through au tomat ion .

GAUGE IN .001 NO. OF 8ARS % OF TOTAL > + 4

+ 2 TO + 4 -M TO+2 ii - I TO-2 - 2 TO-4

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DATA C0LLECTED-I2 χ t SEC SCANS TWO SECS AFTER HEAD END HAS PASSED X-RAY HMD

HISTQRAMS -HEAD END GAUGE

Fig. 12. Improvement in mill setup wi th process computer .

9 percent increase in total production within ± 2 mil gauge tolerances, over and above the Automatic Gauge Control action. Why this should be so is illustrated by-da ta from another installation (Fig. 12). The ability of the computer to achieve consistently proper mill setup far excels tha t of an operator. This ability is part icu­larly significant in tha t it removes restrictions on schedules limiting gauge changes between successive orders. A change of gauge from 0.144 inch to 0.312 inch has been accomplished b}' the compute r with a head-end gauge within 0.002 inch of target .

Results of the control of finishing tempera ture over several bars are shown by Fig. 13. By determining when and a t what ra te to accelerate the mill (i.e., raise the strip delivery speed), an average t empera tu re rundown, head to tail, of only 22°F has been achieved. Thus , a consistently improved product, coupled with increased productivity and reduced mater ia l loss due to mill wrecks, has provided the economic advantage which was projected to justify the added inves tment in com­puter control.

22 IEEE T R A N S A C T I O N S ON I N D U S T R I A L E L E C T R O N I C S A N D C O N T R O L I N S T R U M E N T A T I O N A P R I L

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INTEGRATKD PLANT CONTROL

The clerical and administrat ive procedures required to translate management directives into practice, and customers' orders into coordinated process instructions, occupy a work area between business operations and the production processes. Today, these functions are usu­ally performed using tabulat ing machines and /or small off-line computers. Advances in this work area do not appear to have kept pace with improvements in the business sector or the production processes. The auto­mation of this sector consti tutes a fresh market for com­puter systems.

Accurate inventory control and scheduling are vital to efficient plant operation. The orderly flow of ma­terial from the raw7 materials stock yard to the shipping platform should be a planned and scheduled operation. Although material in process may look absolutely uni­form, the dimensions, analyses, and properties will vary considerably in various lots or orders. Each customer's order must be scheduled and poured at the steel plant ; i ts identi ty must be retained as it receives the pre­scribed processing a t each step on its way to the shipping platform. The steel in each order increases in value at each step in the process, bu t it also suffers losses due to cropping, shearing, t r imming, and accidents.

In recent years the manufacturing processes them­selves have been drastically improved and the time

required to perform each operation has been reduced; modern material handling facilities have reduced losses and t ranspor t time. Automat ion of individual processes has t remendously improved the quali ty of the product , reduced waste, and further reduced processing t ime.

The manual data collecting methods of the past for order t racking and accounting are gradually giving way to da t a input stat ions on the factory floor which t rans­mit operator reports on material flow by a wired tie to the business computer . Digital process control com­puters are capable of collecting all inventory and ac­counting da ta per t inent to the process, reducing and convert ing the data to the desired form, and transmit­ting it to the plant accounting system computer quickly and accurately.

We are now aware t h a t the performance of highly au tomated computer controlled mills and processes is limited by the quality and timeliness of the production instructions supplied by production control operations. We have stated earlier t ha t those instructions must be prepared and t ransmit ted in machine-sensible form, i.e., punched cards, tape , or electrical coded signals. Though the production control function has become an essential par t of a control system, it is presently the weakest par t because it is still largely a manual opera­tion. M a n y plants use tabulat ing machines for this work. Some use small off-line computers, bu t in all cases d a t a collection, schedule preparat ion, etc., is done by

1966 BROWER: COMPUTER CONTROL IN STEEL MILLS 23

ADVANCE PLANNING AND MANAGEMENT LEVEL

W O R K S CO-ORDINATION 8 MANAGEMENT

R E P O R T S ^

ADVANCED SCHEDULING AND PLANT OPTIMISATION L E V E L

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COLD 1

REDUCTION MILL

PRODUCTION

TINPLATE PRODUCTION

Fig. 14. Hierarchy control for integrated steel plant .

batch methods at times set by the facility and human convenience. Considered as a closed-loop dynamic sys­tem, the longest time constants and most inaccurate ele­ments are now in the production control operation.

As a result, we are beginning to see the emergence of integrated p lant control systems, structured as shown in Fig. 14. The process control computers are linked directly to the business computers through data links. Order service and inventory control in the business com­puters result in process scheduling and control instruc­tions t ransmit ted directly to the process controller for execution. With ident i ty maintained within the com­puter system, the material is tracked through each pro­

cess by the control computers. Data collected in process is t ransmit ted back to the business computer for colla­tion with other applicable data, verification of order completion, and generation of billing and shipping state­ments, production reports, and other necessary manage­ment documents. Several such, systems are in various stages of implementation at the present time.

Thus , the need to integrate production control func­tions and the processes into one dynamic on-line system has found recognition. These systems represent a signif­icant advance in economic gain and in productivi ty, and a further step in the spread of computer control in the steel industry.