The Technological Study of Dynamically Detecting Molten Steel Weight

Copyright © IFAC Automation in Mining. Mineral and Metal Processing. Beijing. PRC. 1992

THE TECHNOLOGICAL STUDY OF DYNAMICALLY DETECTING MOLTEN STEEL WEIGHT D.A. Yu, R.G. Liu, X.G. Li and c.Y. Zhang Department of Automatic Control, Northeast University of Technology. p,a, Box 321, Shenyang 110006, PRC

Abstract. There are some dozens of continuous casting machines of smaller square billet in China. But the devices detecting dynamical ly molten stee l weight in steel ladle can not run normally because of technology condition and bad environment . so the success rate of continuously casting steel is decreased. In this paper. the device detecting weight of molten steel in steel ladle was designed which was suitable for bad environment. Some methods was given to overcome the high temperature of environment and the heat exchange of sensor were calculated. The detecting device has been running very well in an iron - steel plant since October. I ~8~. !<_eL~r~.

Molten steel weight. Dynamically detect ing . Continuou s ly casting. Strong wind

cooling. Calculation of heat exchange,

INTRODUCTION

furnace provides molten steel meeting the need of given

machine. To solve the problem, we designed and made the device detect ing dynamically molten steel weight in steel ladle. which was suitable for bad environment. The device was installed in the Anshan Iron - Steel Company, China. It has been running very well since October.

chemical components and temperature for the molten

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.'\s we know. the production process of continuous cast ing stee l is as follow: the converter or open -

hearth

steel ladle. The molten steel in steel ladle flows in to crystallizer through middle pot. and is cast into smaller square steel billet. Therefore. the ability of continuous

DETECTING SYSTEM

casting machine is directly relative to the providing abi lity of molten steel and dispatch of molten steel. If the

be shortened and the production quantity will be in -

The s ketch of detecting device is showed in Fig. 1. The detecting device is composed of three parts: pressure sensor (with wind cooling system) . detecting and indicating meters and large displator. They are introduced in

creased. Meantime. the metal recovery ratio and the

detail as follows.

molten steel is provided on time. the effective operating time w ill be enhanced. the auxiliary operating time will

consumptions of heat - resisting material and auxiliary material must be reached better level. The dispatch of molten steel is decided by the left capacity ( weight) of

Anti - Shock \lethods

molten stee l in steel ladle and the casting speed . If the

Key

left capacity of molten steel in steel ladle is not gotten preCisely on time, th e production dispatch and the oper-

.'\nti - Biassed Load :\lethods

ation of continuous casting steel are difficult. sometimes

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re jects are made and even accidents will happen. Strong Wind Cooling There are some dozens of continuous casting machines

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of smaller square billet in China. But many continuous

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,-\ \'eraging Heat :\1ethods

casting machines are not installed with the de\'ices detecting dynamically molten steel weight in steel ladle or

Heat Isulation :\lethods

the devices can not run normally because of the limits of technology condition and bad environment (for exampie: strong flux of heat radiation. strong shock . much powder and st rong electomagnetical disturbance. ). So the detecting of left capacity of molten steel became a difficult problem w hi ch affected the abili ty of casting

Fig. 1. The Sketch of Detecting System

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Pressure Sensor

Detecting Instrument

Four pressure sensors of shear - bridge of QS - 2 type are selected which measurement range is 60 (tons) and the accuracy is O. 05 %. First of all, they must meet the need of whole measurement range of 200(tons) • bigger security coefficient and high ability of anti -shock. Sec· ondly, the whole installation height of sensor (including the protection box ) mu st be lower than 260 (mm) because of the limit of technology condition, but the other types of sensor can not meet the size requirement.

WSC - 4 n type of intelligence weighting instrument reformed is selected as detecting instrument. The instru ment adoptes the stable-zero by itself applifier ICL 7650 , double integral A I D changer ICL -7135 and single chip computer 8031. The ability of anti - disturbing and calculating accuracy are enhanced. and the accuracy need of whole detecting system is met. The instru ment has the functions of removing weight of empty lad le , clearing zero drift, indicating gross weight, net weight, setting peel weight of ladle and etc.

Four sensors are electrified by one high precision power. The method of parallel output is used. and the whole output signal after paralleling is: e = 1/ 4 (e, ez e3 e,) Where the e, ,ez. e~ and e, are the output signals of four se nsors se perativel y. Due to parallel output, the output resistance is reduced and anti - common - mode di st urbing ability of the syste m is improved. Only one common - power is needed to electrity sensor. The equipments are sa ved. The reliability of the system is improved. In the case . the auxiliary error reSUlting from eccentric loading can be avoided through making output resistance of sensors equal by adding standard electric resistance. The method of parallel output improves greatly the ability of anti - eccentric load.

DYNAMICALL Y MEASURING RESULTS

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After this system had run normally for one year. re lative measuring supervisory department tested and evalu ated it on spot in November. 1990. The results are fol lows: 1. Measuring range 0- 150 (tons) 0.1 (tons) 2. Resolution : 3. Accuracy : O. 26 % (biggest relative error) 4. Ability of anti-shock: 260 % 5. Biggest temperature drift : 0.3 (tons)

The installati on sketch of continuous casting machine is showed in Fig. 2. in which, No. 1, No. 2, No. 3 and No. 4 indicate se nsors se peratively, No. 5 indicates the frame of car, No . 6 indi cates side shield of limiting position when molten steel ladle is put down, No. 7 indicates molten steel ladle. No. 8 indicates middle pot. Four sensors are installed on one horizontal plane as requested. The error of hori zontal height is lower than 2(mm / m ). So the weight of molden steel ladle can be loaded on sensor equally.

HOW TO OVERCOME THE HIGH TEMPERATURE When steel is cast continuously . the flowing molten steel and the molten steel in middl e pot radiate the heat to the se nsor, car - frame and s urrounding a ir. The tempera ture of sensor will ri se due to heat radiation, heat con du ct ion and heat convection. Before any methods lowerin g temperature of se nsor were not taken, the temperature on outside faces of sensor protectors on car - frame

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Fig. 2. The Sketch of Continuous Casting Machine

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Table 1

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2 3 4 1 2 3 4

1 2 3 4

sensor on the market has the function 'of temperature compensating . if there is not temperature gradient in elastic body . If there is temperature gradient, most sensors can not compensate automatica ll y the influence of temperature gradient. It can be calculated th at if there was temperature differential of 1 'C between R , ,R 3 ,and R 2 , R, in bridge circuit,the false value of 100(uV) would appear. Therefore . the some methods must be taken to overcome the influence of temperature.

The Temperature of Measuring Points

A

B

C

D

E

F

G

76 70 71 94

76 80 66 96 57 66

77 77

66 79 68 89 56

55

53

94

51

54

97 89

Remarks

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69

36

54 101 44 71 38 75

50 98 99

67 64

71 8] 64 106 68 ]0 5

92 ]09 ]02 116 ]16

76 79

73 78

58 63 52 ;);) 54 61 100 98 93 106

103 57 71 56 62 98 98

96 100 92

69 54

56 45

105 130

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The Methods of Insu lating Heat

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The asbestos board is put on the bottom of protection box of se nsor to retard the speed of heat conduction from car - frame. Because the heat conducti on coefficient of asbestos board is O. 12 (J / m • s ' C) . the heat condu ction coefficient of steel is 45. 3(J / m' s · 'C). the difference of two coeffi cients is more than 300 dou ble. So long as the thickness of asbestos board is enough. the speed of heat conduct ion can be reduced greatly.

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The Methods of A "eraging Heat

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The Side Radia ted

In order to make temperature of side radiated and side not radiated of senso r equa l and reduce temperature gradient . a piece thin coppe r board is wrapped round the bottom and two sides of sensor. Because copper conducts heat well. it can qu ick ly conduct heat from higher temperature area to lower temperature a rea to reduce temperature gradient and make sensor be heated equally.

The Side Not Radiated

Fig. 3. The Measuring Points of T emperature was measured. The resu lts a re showed in Table I. In the table. the A, B, C and D indicate measuring temperature points of sides radiated of sensor protectors, which are s howed in Fig. 3. The E and F indicate measuring temperature points of side not radiated, which are showed in Fi g. 3. The G indicates measu r ing temperatu re point of sensor top of bearing bod y touching with bearing bod y of molten steel ladle. Th e numbers of sensors in Table I is the same with the number s of se nsors in Fig. 2 .

The Methods of An ti- Heat Radiation The a luminium board polished is installed on the outside of protection box as reflecting board of heat radiation because heat absorbing coefficient of aluminium board is lower than that of other materials.

W e take No. I sensor in T able 1 as example to analyze. Af ter casti ng 90 minut es continuously , the average temperature of side radiated is 73.8 C , and the average temperature of side not radiated is 54 'C ,the tempera ture differentia l is ] g. 8 ('. The average tempera ture of ba,e body of sensor is 63. 9 C. The temperature of the point G on the top is hi gher more than 30 'C than that of ba,e body . :\fte r casting 310 minutes cont inu ously. the ;!\'Crage te mperature or ,ide radiated rise to 98. ;) C . the a\'erage temperature of ,ide not radiated ri se to G5. ;) C . Th e temperature differential reach to 33 C. Th e average temperature of base body i, 82 (' . The temperat urt' of point G is 106 (' . It can be ,een from T able I that 1\ 0 . ,I ,enso r is most influenced by temperature. Thi s is be\'ause the 'iensor is closest to the molten steel mouth of molten steel ladle.

The Methods of Wind Cooling Se\'eral methods above mentioned ca n only retard the speed of heating sensor . but yet. the sensors are heated constantly. So strong wind cooling methods mu st be taken. :\ cuboid blower is designed and installed front side and back side of every sensor. On the blower. some dozen' of holes which diameter i, Cl) 10 (mm) ;Jre opened to \'001 ,erlso r ;Jnd protection bo.'\ equally,

H EA T CO ~V ECTIO~ CALClJ LA TIO~ OF SENSOR Bec;Ju se actu31 process of getting heat :lnd heat (On\'el'tion is \'ery complicated on spot . it only can be calculated appro.'\imateJy the heat quantity of getting and heat quantity brought away by wind cooli ng blower during the process of casting steel continuously.

Two results can be gotten from above analyzing : First ly. when steel cast continuously . the tempera ture of sensor would rise and exceed its normal using temperature range - 30 'C 70 'C. Secondly. there is bigge r temperature gradient in the body of sensor. The general

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This is the heat radiation quantity got ten by one sensor. Because molten steel ladle can keep out about 95 % heat radiation in actual production, the heat quantity radiating to sensor is about 5 % of theoretical calculating value,that is 711. 6(J / s).

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Actual Calculation of Getting Heat Quantity of Sensor '~

The getting heat quantity of one sensor is calculated by using actual measuring data in Table 1. The data of the No. 4 sensor is used which is measured immediately after casting steel 90 minutes. According to the formula of getting heat. the heat quantity Q' " gotten by sensor is : Q' m = m X (' ><. ':-'T = 3339. 4 (kJ) (6) where: m = 95 (kg) : The mass of sensor and protection box. c=O. 47 (kJ / kg' C):Specific heat of steel, ':-'T = 72 . 5 ( C ): Rising temperature of sensor. The formula (6) is divided by the casting time t (t = 90 /. 60 (s» getting: q' '" = Q ' ,,, I t = 618.4 (J / s) (7) The value of formula ( () indicates approximately the average speed of getting heat of sensor. It is also approx imately equal to the result of theoretical calculation.

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Fig. 4. The Space Position of Heat Radiation Plane

The Theoretical Calculation of Getting Heat Quantity of Sensor In fact, the sensor is heated by four ways. They are heat radiation coming from molten steel in middle pot, heat conduction from car - frame. heat conduction from ladle bearing body and heat convection of high environment temperature. Because there are thick asbestos board under the sensor and other reasons, the heating speed of last three ways are much slower than that of first way, So we o:lly calculate approximately the getting heat quantity from heat radiation,

Theoretical Calculation of Heat Convection of Ventilating Cooling

Fig. 4 is a schematic diagram of space position of heat radiation plane F" of molten steel in middle pot and absorbing heat plane F, of sensor. When F, is at right an gles to F " the an gle radiation coefficient f" of F, to F , is obtained from ha ndbook(Yang. 1982). f, , = l/:1 / A, (f, lb-(J.,d ' + ',") - f, (b - u ,d ' --i-o' ) + f , (b - 'j .c' +0' ) - f , (b - u ,c' ..- "") f , (b -(3 .c' + -r') - f, (b- 13 .c' + ,',' ) +f 2(b -li ,d " +h' ) - f, (b - (3,d ' + '," ) + f, (a -[3,d ' - f, (a - (3,d ' +6' ) f, (a - 13 .c'+0' ) - f, (a - [i .d '+'," ) + f 2 (a - u .c '~/ ) - f, (a - u,c' + o' ) f, (a - u .d ' + 0' ) - f:,(a - u ,d'+ '," ) = 0. 3264 (2) where a = 0.b = l.c = 0,d = 3. u= O. 138, 13= O. 724, .( = 2. ,~= 2.25. (Unit is meters separately) A , = 0,0986 (m ' ) • A,= 3 (m ' ) (To be the areas of Ft and F,)

The q ' vor is set as the heat quantit y brought away by wind cooling through blower. Because the size of blower is special and the section size of air flowing is complicated and variable. there is not theoretical or experimental formula of calculating heat convection at present. Therefore the heat convection formula of turnulent flow in pipe is used approximately. According to flowing speed of air in blower. moving viscosity coefficient of air and the equivalent diameter of effective flowing section area of blower. The Reynolas number Re is gotten. Re = 21000. The Nusselt number Nu is calculated from formula(8) (Gao.1986). Nu = 0.023· Re o., . Pr"" , [, ' ER ' C, = 197.86 (8) where :Pr = O. 704 : Prandtl number. , , = J. 5 : Pipe length correction coefficient. = 2. 3 : Pipe curvature correction coefficient. [, = 1 : Temperature differential correction coefficient. Nu =u 'd /I (9) while where : I) Exothermic coefficient. d Equi valent diameter.d = 0 . 06 2 (m ) • Heat conduction coefficient of air. •. =0 .0257 (J / m • s · C) . therefore : U = Nu ;"'" i ./ d = 82, 0 1 (\V I m ' · C ) (10) From Newton heat convection formul a gen ing: Q 'v,' = u (t, - t,) = 16 4 0,2 (\V / m ' ) (ll ) where : L: Average temperature of high temperature \\'all.t.= 50 ( C ): t,: Input temperature of air. t, = 30 ( C ), When the area of heat convection is about O. 448 (m').

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f, (x.y) =O . 5x· / ytg " (X ! / y ) + J / 4' x' Inlx'+ y ' ) + 1I8' (x "+ yJln(x '--l- y) The angle coefficient f" of F , to F t is : (3) f" = f l2 • A ,/ A, = 0.0107 The heat quantity qo", radiated in unit time by molten steel in middle pot is (Gao. 1986) : qo', ' = A , ' E ' 0 ,, ' T ' = J. 33 /. 10 6 U / s) (4) where : ,= O. 7 : Balack body coefficient of steel.
(5)

266

whole heat convection quantity q' OUI of one sensor is : q' OUI = 734.81 (J/s) (12) Comparing formula (7) with formula ( 12 ) , it can be seen that the heat convection quantity of wind cooling in unit time is approximately equal to that gotten by sensor. After the structure is given, the major factor influencing heat convection quantity is air Reynolas number which is relative to air flowing speed. When Re = 21000,the air flow speed is about 5. 25(m/s) in pipe and whole air flow vol ume is about O. 47 (m 3 / s) . The quantity of air flow volume in ventilating pipe can be adjusted by valves. After continuously casting about 930 minutes, the surface temperature of sensor is smaller than 45 'C due to ventilating cooling. Examined at spot for two years, the detecting system come up to designing request. The practice proves that the design of wind cooling system is reasonable.

CONCLUSION

1. Theoretical calculation is in accord with actual measu ring results, which is valuable of reference for designing similar device. 2. Examined at spot at different seasons of spring, summe r, autumn and winter, for two years, the system can run steadly and reliably. It can improve success rate of continuous casting steel, and has notably economic and social benefit.

REFERENCE

Qian, R. D. (1984). Engineering Hyd rom echanics. National Defence Industry Press ,China . Chap. 6, pp. 15 5-207 Gao,K. M. (1986). Heat Transfer. Northeast University of Technology Press, China. Chap. 2,pp. 38-43,Chap. 3,pp. 58-66. Yang, X. R. (1982). Hand Book of Angle Coefficient on Heat Radiation. Mechanica l Industr y Press, China. Chap. 3, pp. 77 - 81.

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