Solid State lonics 40/41 (1990) 737-742 North-Holland

SENSOR AIDED PROCESS CONTROL IN IRON AND STEELMAKING

Alexander McLEAN

Ferrous Metallurgy Research Group. Department of Metallurgy and Materials Science, University of Toronto, Toronto. Ontario M5S IA4. Canada

Control strategies in the Iron and Steel Industry have been based on samples taken from molten metal, molten slag and gas phases. However, with rapid development of dynamic control models, there is a need to place increasing emphasis on the detec- tion of signals transmitted by the various components within the metallurgical system. In this context, during the processing of hot metal and liquid steel, the various reactors can be viewed as transmitting stations within a communications network.

I. Introduction

One hundred years ago in 1890 Henry Mar ion Howe [1] publ ished his classic text ent i t led "The Metal lurgy of Steel". This massive volume was three years in prepara t ion . In spite of the t ime difference, much o f the mater ia l conta ined in this volume is still worthy of s tudy today. Deeply aware o f the oppor- tunities as well as the dangers associated with the new conver ter s teelmaking technology - - the enhanced react ion rates, the process flexibility, the greatly re- duced heat t imes and the difficulties o f metal lurgical control, he went on to write:

The very pl iancy o f the s teelmaking processes de- mands increased watchfulness to prevent and de- tected unsought var ia t ion. Invaluable to the watch- ful and the intelligent, it is a s tumbl ing block to the ignorant and the heedless". These words are even more appl icable today in view of the increased complexi ty o f the s teelmaking pro- cess, involving as it does, complex blast furnace tech- nology, external t rea tment of hot metal , converters with mul t ip le blowing practices, high powered arc furnaces, ladle metallurgy, vacuum processing and cont inuous casting.

It is clearly evident from the writ ings o f Howe that he was keenly aware o f the impor tance o f experi- mental results and the pr inciples o f measurement . He would most surely have endorsed the view of an- o ther eminent scientist of the same period, a Pro- fessor of Natura l Phi losophy at the Univers i ty o f

0167-2738/90/$ 03.50 © Elsevier Science Publishers B.V. ( North-Holland )

Glasgow, Wi l l iam Thomson, bet ter known to us to- day as Lord Kelvin (1824-1907) . F rom his studies in thermodynamics , he establ ished a new scale for the measurement of temperature. In emphasizing the impor tance o f measurement Lord Kelvin stated:

" W h e n you can measure what you are speaking about and express it in numbers , you know some- thing about it. When you cannot measure it, your knowledge is meager and unsat isfactory".

For many decades control strategies in the Iron and Steel Industry have been based on samples taken from the mol ten metal, molten slag and gas phases as well as from solid products . However, with the rapid de- ve lopment o f dynamic control models based on ar- tificial intelligence and expert systems, there is a need to place increasing emphasis on the detect ion o f sig- nals t ransmi t ted by the various components within the metal lurgical system [2] . In this context, during the processing of hot metal and l iquid steel, the var- ious reactors can be viewed as t ransmi t t ing stat ions within a communica t ions network, each generating informat ion in many different forms. For this rea- son, as we move into the nineties, there is a great challenge to develop and implement new diagnost ic sensors which will enable us to moni to r and evaluate signals which pertain not only to the quali ty o f mol- ten mater ia ls during processing but also to the in- tegrity of the processing systems. These sensors should be capable o f detect ing signals which could be chemical , thermal or physical in origin. As with all communica t ion processes, when the signal is re-

738 A. McLean ! Sensor-aided process control in iron

ceived, interpretation is required, followed by eval- uation after which an appropriate response is deliv- ered back to the system. In this way, by sensing the signals transmitted by the system, the process can be more precisely controlled.

2. Control of oxygen and other solutes

The importance of oxygen control in iron and steelmaking cannot be over emphasized. Fig. 1 taken from the work of Eketorp [3] shows schematically how the oxygen potential associated with iron de- creases from iron ore (point A) to blast furnace hot metal (point B) then increases through steelmaking (point C ), decreases again during deoxidation (point D) and may, if precautions are not taken to prevent contact with air, increase again because of reoxida- tion. Throughout the processing steps oxygen control is essential since it has a direct influence on the be-

A ~

deox Idatlon

reoxlda - / / s t e e [ m a k i n g t i on

bla6t fu rnace

I

Fig. 1. Schematic diagram of the change in oxygen potentials dur- ing the conversion of iron ore to steel [ 3 ].

haviour of the various alloying elements as well as C, S,P, N a n d H .

Goto et al. [4] have reviewed the use of oxygen sensors within the Japanese steel industries for de- termination of the oxygen potential associated with metal, slag and gas phases. Measurements have been made in the hot metal and slag flowing from the blast furnace, in the gas, slag and metal phases in con- verters, in gas atmospheres and molten steel within ladle metallurgy stations including vacuum reactors such as RHOB and DH units and within the tundish and molds of continuous casters. Oxygen probes have also been used to continuously monitor the effec- tiveness of inert gas shrouding systems between ladle and tundish, and tundish and mold, during the transfer operations associated with continuous casting.

Some excellent laboratory studies, as well as plant trials, have been conducted by Etsell and co-workers [ 5,6 ] with non-isothermal oxygen probes which per- mit continuous measurements to be made in molten steel as it moves through the tundish. With further development work, it should be possible to use this approach to control the rate of addition of elements such as aluminium or calcium and to monitor the effectiveness of their behaviour based on the activity of the residual oxygen. Heinz and Janke [7] have described the results obtained with an electro-chem- ical sensor for continuous monitoring of oxygen as well as the activity of other elements in molten iron alloys. They also discussed the industrial implica- tions for process control.

An excellent review of solid state sensors used for in situ determinations of elements dissolved in mol- ten iron and steel was published by Iwase [8] in 1989. In this paper, which describes both laboratory experiments as well as plant experiences, results are presented for various electrochemical sensors incor- porating different auxiliary electrodes in order to de- termine the concentrations of solutes such as alu- minium, silicon and phosphorus. Other recent articles pertaining to work of this type include that of Tach- ibana [ 9 ] of NKK who has described a manganese sensor with a double layer solid electrolyte and Oki- mura [10] of Nisshin Steel who has outlined the composition and form of coatings of auxiliary elec- trodes for a chromium sensor.

In a recent monograph, Goto [ 11 ] has discussed

A. McLean/Sensor-aidedprocess control in iron 739

the role of solid state electrochemistry and its ap- plications to sensors and electronic devices. Many of the materials used in the sensors are unstable in mol- ten steel, but they can be used for continuously an- alysing gas compositions, and indirectly provide in- formation on the chemistry of the metal phase. For example, the sulfur and hydrogen contents in steel could be determined in this manner.

3. External treatment of hot metal

During tapping of hot metal from the blast furnace the silicon content can vary by +0.1 to 0.15% or greater. In some cases the hot metal may be in equi- librium with silica and thus the silicon content can be calculated from a knowledge of the oxygen po- tential determined with a conventional oxygen probe and the thermodynamic data for the reaction:

[ % 5 i ] ot [ % C 1 : 1,.5"1.

• I .2 .5 I 2

I I , , 1 . . . . 1 I 550

500

E

450

. . . . I ' ' ' 1 . . . . I " - - - Theoret ico l / line with OMgO= .1~/~°

- / 1723K

4 o o . . . . I I , , I . . . . 1 1 .5 1 2 5 10 20

h s i

30

Fig. 2. Relationship between measured cell potentials and the Henrian activity of silicon obtained with a tri-phasic zirconia electrolyte [ 13 ].

Si (in hot metal ) + 20 (in hot metal ) = SiO2 ( 1 )

In other cases, particularly in Japan, the hot metal may not be equilibrium with silica. For this reason a solid state silicon sensor incorporating MgO-sta- bilized zirconia as an electrolyte, Mo+MoO2 mix- ture as a reference electrode and a mixture of ZrO2 + ZrSiO4 as an auxiliary electrode, has been de- veloped by lwase [12]. Upon immersion in hot metal, local equilibrium between the sensor and hot metal is achieved:

ZrO2 + Si (in hot metal )

+ 20 (in hot metal ) = ZrSiO4 (2)

From eq. (2), it is evident that by measuring the ox- ygen activity, it is possible to determine the silicon content of the hot metal. An alternative approach [13] is to use a tri-phasic solid electrolyte, which consists of three phases, i.e. ZrO2 (monoclinic), ZrOz-MgO (solid solution) and 2MgO-SiO 2. Fig. 2 shows the relationship between measured cell poten- tial and silicon concentration obtained in laboratory experiments.

4. Basic oxygen converter

In the seventies considerable attention was de- voted to the use of oxygen probes as a method of monitoring the carbon content of hot metal during the oxygen blow. However, with the recent devel- opment of a carbon determinator based on liquidus temperature measurements, the importance of oxy- gen probe measurements has shifted from the con- trol of carbon to the control of phosphorus and man- ganese at the end of the oxygen blow.

The application of oxygen probes for phosphorus control in the converter was pioneered by research- ers at NKK [ 14 ], who established a correlation be- tween oxygen activity in the molten steel and the FeO content in the liquid slag. Based on this information, the following equation was derived for the phospho- rus partition ratio:

log (%P2Os)/[%P]2=A{ 11.21og [ (%CaO)

+ 0.03 (%MgO) ] - 0.05 (%FeO)

+29 6 0 0 / T - 36.25+ 5log (%FeO)}, (3)

740 ,4. McLean /Sensor-aided process control in iron

where A is an adjustable parameter and takes a value of 0.95 to 0.98 depending on the extent of bath stir- ring. The equation was derived from plant data. The lime and magnesia concentrations can be obtained from a knowledge of the amounts added during the oxygen blow. Thus, for a particular temperature and FeO content the phosphorus partition ratio can be evaluated. Knowing the amounts of charge material and slag, the phosphorus content of the steel at the end of the oxygen blow can be determined.

5. Secondary processing

In secondary steelmaking, particularly in the case of low carbon aluminum deoxidized steel, control of dissolved aluminum is essential. This can be achieved by the use of conventional oxygen probes, provided the steel bath is in equilibrium with alumina:

2[AI] + 3 [ 0 ] =A1203 . (4)

If the above reaction is not in equilibrium, then an oxygen sensor with an alumina coating on the sur- face of zirconia can be used. The presence of alu- mina will ensure local equilibrium at the molten steel/solid electrolyte interface. Assuming the ab- sence of an a luminum concentration gradient be- tween the electrolyte interface and the bulk of mol- ten steel, then from a measurement of the oxygen activity, the dissolved a luminum concentration can be calculated [ 15,16 ].

Fig. 3 shows the results from some experiments in which molten iron and i ron-chromium alloys con- tained in alumina crucibles at 1723°C were equili- brated for 12 h with water vapour-hydrogen gas mixtures after which samples were quenched and analyzed for a luminum and oxygen [ 17 ]. The bro- ken line in this figure shows the relationship between the activities of oxygen and aluminium for equilib- r ium with alumina, while the solid lines show the re- lationships in terms of the corresponding percent- ages. Deoxidation diagrams of this type are particularly significant for steels containing high concentrations o f alloying elements. In this case, for any particular a luminum level, there is a corre- sponding oxygen activity, the value of which can be determined by an oxygen probe. However, for any

o

.J

- I 0

Wt-°/O ¢hrom,um A 0"0

-I-5 - ~ B 2-0 \ \ c o o \ \ \ ,00

_ 3 . 5 1 I _P 1

-50 -25 -20 -15 -10

LOG [% All

Fig. 3. Effect of chromium on the concentration of aluminum and oxygen in liquid iron in equilibrium with alumina at 1723 ~C; broken line shows activities, solid lines are corresponding per- centages [ 17 ].

particular a luminum level, the actual oxygen con- centration increases as the amount of chromium in- creases due to the reduction in the oxygen activity coefficient. This effect of chromium is well illus- trated in fig. 4 taken from the work of Heinz and Janke [ 7 ] where equilibrium has been established at higher oxygen potentials with chromium oxides. From the viewpoint of the steelmaker, the actual concentration of oxygen in solution is just as critical as the activity, since during cooling and solidifica- tion the oxygen in solution will finally precipitate in the form of inclusions and ultimately determine the cleanliness of the cast product. For this reason, par- ticularly with higher alloy steels, a knowledge of the activity coefficient as well as the oxygen activity is highly desirable. For low alloy steels, coefficients are readily calculated from a knowledge of the interac- tion parameters. However, for higher alloy grades, cross interaction effects can introduce error; and in such cases, the coefficients need to be determined by an additional independent measurement.

A. McLean /Sensor-aidedprocess control in iron 741

c:, 0

0.001

I

0.1

0.01

0001

I

I l l i 1 I I I I I I l 1 I I I

I___,L-_ I I I I I I I I I w t , / , I . - I I I

i i i;i i i i i L._.l I I I I I r ' f ~ l t ~ , , J ~ L I

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1600 C ; i ; ; ; ; a F j jjj I I I I I II

)-- Er203 solurnlion , : : : 1 I I I I I I I | I I l l I I I 1 1

I I I I I I I I l l I I I11 I I I I I 1 I I I I w t , / , J I I I

I I B I : C ~ i : : t I i I l l • _-, , , , . I I I 1 / _ l l r l 1 l l l l

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- ,sso'c I I II1 1 I I I I I 7 1 I I - I

O1 1 I0 100 wl.'l, Cr

Fig. 4. C h r o m i u m - o x y g e n re la t ionships in c h r o m i u m oxide-sat- urated F e - O - C r mel ts [ 7 ].

6. Sensors - - the critical l ink for inte l l igent processing

While this paper has focussed on electrochemical signals, two examples are given here o f physical sen- sors which detect acoustical signals, the first example pertains to an interesting development in sensor technology which has been reported by the Hungar- ian State Research Institute for Iron and Steel [ 18 ], whereby it is possible to continuously moni tor the rate of sulphur removal from molten iron or steel during injection o f appropriate reagents by detecting acoustic vibrations caused by the generation o f bub- bles within the molten metal. It is conceivable that this technology could be used as an on-line detection system to continuously moni tor any change in con-

centration of surface active solutes such as oxygen and sulphur in molten metal.

The second example is concerned with the detec- tion of non-metallic material which can be entrained within molten metal. Molten steel may contain sus- pended reaction products such as oxides, sulphides or nitrides. The particle sizes can vary from less than one micron to greater than 150 microns. When high frequency sound pulses are passed into the liquid, the non-metallic particles interact with the sound pulses [19]. The reflective sound waves are then converted into electrical signals, amplified and dis- played on an oscilloscope. Within the melt, several attenuating mechanisms prevail and they combine to determine the amount of sound which returns to the detection system. Extremely small particles, of the order of 5 microns, interact with the sound waves and although they may not produce enough reflec- tive energy to produce a discrete peak on the oscil- loscope, they invariably decrease the amount o f sound which travels through the melt. They there- fore contribute to the overall attenuation of the re- turn signal.

After three decades of combined efforts by re- searchers, developers, and plant operators on several continents, the electrochemical oxygen sensor is now an established technology. From this particular ex- ample it is very clear that considerable laboratory re- search together with in-plant evaluation is required in order to transform a fundamental scientific con- cept into a reliable sensing device which can be used as an integral component of a process control sys- tem. Thus for truely effectively research in this field it is essential that basic studies be conducted in close co-operation with instrument manufacturers and the steel community. With this tri-partite interaction, (fig. 5 ) there is excellent opportunity for linkages in- volving joint activities in the development, manu- facture and utilization of sensors for enhanced con- trol of iron and steelmaking operations [20]. Successful implementation of a new sensor can have a direct impact on conventional processes and also provide the essential knowledge required for the mo- delling and control of new processes for the 21 st cen- tury. Activities of this type are therefore a critical link in the chain o f quality steelmaking and a mandatory requirement for intelligent processing.

742 A. McLean /Sensor-aidedprocess control in iron

THE ANATOMY OF SENSOR DEVELOPMENT

ELECTRONIC ACADEMIA "s

,NST.OMEN. ' . . . . . . . . / STEEL . . O O U C E . USE"

At

COMMERCIAL ,~1""//£a~" '~ .~( CONTROL _ U M , T ,

OF M~TERI

/ \ OEVICI= DEMANO

ROBUSTNESS VOLUME

Fig. 5. Schematic representation of the co-ordinated activities involving research, manufacturing and utilization sectors for effective development and implementation of a new sensor [ 20 ].

Acknowledgement

T h e a u t h o r is i n d e b t e d to t he N a t u r a l Sc i ences a n d

E n g i n e e r i n g R e s e a r c h C o u n c i l o f C a n a d a for t he

p r o v i s i o n o f f u n d i n g for u n i v e r s i t y r e s e a r c h in t he

f ie ld o f s t e e l m a k i n g sensors . A c k n o w l e d g e m e n t s are

a lso d u e to P r o f e s s o r M. Iwase o f K y o t o U n i v e r s i t y

for he lp fu l d i s c u s s i o n s a n d c o n s i d e r a b l e c o n t r i b u -

t i o n s to t he w o r k o f t he F e r r o u s M e t a l l u r g y R e s e a r c h

G r o u p a t t he U n i v e r s i t y o f T o r o n t o d u r i n g the las t

twe lve years .

References

[ 1 ] H.M. Howe, Metallurgy of steel (The Scientific Publishing Co., New York, 1890 ).

[2 ] A. McLean, Steelmaking Conf. Proc. (Iron and Steel Soc., 1988) p. 3.

[3] S. Eketorp, 210 (1972) 1. [4] K.S. Goto, K. Nagata and M. Susa, in: W.O. Philbrook

Memorial Symp. Proc. (Iron and Steel Soc., 1988)p. 147. [5] T.H. Etsell and C.B. Alcock, Solid State lonics 3/4 ( 1981 )

621.

[6] T.H. Etsell and C.B. Alcock, Can. Met. Quart. 22 (1983) 421.

[ 7 ] M. Heinz and D. Janke, in: W.O. Philbrook Memorial Symp. Proc. (Iron and Steel Soc., 1988) p. 205.

[8] M. Iwase, Tetsu-to-Hagane 75 (1989) 379. [9] K. Tachibana, CAMP-ISIJ 1 (1988) 1135.

[ 10] T. Okimura, CAMP-ISIJ 1 (1988) 1136. [ 11 ] K.S. Goto, Solid state electrochemistry and its applications

to sensors and electronic devices (Elsevier, Amsterdam, 1988).

[ 12] M. lwase, Scand. J. Metal. 18 (1988) 50. [ 13 ] M. lwase, H. Abe and H. lritani, Steel Res. 59 ( 1988 ) 433. [ 14] T. Usui, Y. Kawai, H. lshikawa, Y. Tabata and M. Han-

myou; Tetsu-to-Hagane 73 (1987) 1463. [ 15 ] H. Nakamura, Y. Nakajima and T. Moriya, Trans. ISS/

AIME4 (1984) 63. [ 16] R. Rote, Iron Steel. 10 (1983), January, p. 14. [ 17 ] A McLean and H.B. Bell, J. Iron Steel Inst. 203 ( 1965 ) 123. [ 18 ] G. Pal, G. Endroczi, G. Hollos, G. Nagy, G. Szonyl, B. Laszlo

and Z. Balazs, US Patent, no. 4,617,830, Oct. 21, 1986. [ 19] N.D.G. Mountford, S. Dawson, I.D. Sommerville and A.

McLean, Proc. Vol., Metallurgical processes for the year 2000 and beyond, eds. H.Y. Sohn and E.S. Geskin, (Miner., Met. and Mater. Soc., 1988) p. 745.

120]C.B. Alcock, Forum to identify long-range research opportunities for the North American steel industry (AISI, January 1989).