Computer Control of LD Converter Process With Pretreated Hot Metal

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COMPUTER CONTROL OF LD CONVERTER PROCESS WITH PRETREATED HOT METAL H. Miyahara*, Y. Yamada*, M. Yoshino*, H. Ishikawa and T. Hasegawa** " I'roU'\.\ CUlllrul /)1'1)(/1'11111' 111. ~(' ihill I r()rk,1 , ,\'~~ C()rpurali()II , ,\lill{//I/;;l'IIlarir/a-l'iw, I-I, ~a,, 'a.\{/ki-kl/, ~"'I'II,lIIki, 21(), japall ""' S/r' .. llIIakillg /)"p{/rllll('l(I, ~"ihill 1I '()rk,I, ,\,~~ C()/jJ()raliulI, ,\lillalll;;l'IIlarir/a-rhu, I - I, ~(/,, '{/ .\aki-kl/, ~m"{/,lIIki, 21{), jalmll

Abstract. A computer control technique for LD converter process with pretreated hot metal has been developed and practicized at Keihin Works. A new ladle dephosphorization plant has started its operation in 1986 and consequently, less slag blowing by LD converter has been put into operation. The main feature of less slag blowing is the direct reduction of manganese ore, which realize high recovery of manganese and reduction of ferromanganese alloy consumption. Both static and dynamic control model have been reconstructed to attain high manganese content and stable yield by metallurgical reaction background altogether with operational data analysis. The end point estimation model by sublance measurement has been also developed. The computer control technique has greatly contributed to realize high hitting ratio, high [Mn] at end point, stable manganese yield, less consumption of ferro-alloy and total cost reduction. Keywords. Steel industry; computer control; dynamic con trol; models.

control;

INTRODUCTION

converters;

process

control;

static

into operation at Keihin Works in November, 1986. Figure 1 shows the hot metal pretreatment flow from blast furnace to LD converter after the opening of the dephosphorization plant. The hot metal from No. 2 Blast Furnace is totally desiliconized and dephosphorized to the gross product phosphorus level. The pretreated hot metal is then slagged off by the slag dragger and charged to LD converter. The LD converters in Keihin Works are combined blowing process with bottom stirring gas (Ar, N2 and C,02) for bet ter performance of the vessel reactl.on. The converter operation with pretreated hot metal is called "less slag blowing", whereas slag volume is less than 10 kg/t. The basic benefit of less slag blowing is the direct reduction of low cost manganese ore, which result in less consumption or even no addition of high cost ferromanganese alloy to adjust gross product manganese content. In addition to this benefit, total flux consumption is reduced. From these aspects, less slag blowing enables reduction of refining cost to a great extent.

The steelmaking plant occupies an extremely important position in the integrated steel works since the qualities of steel are determined and entire production of steel starts hence. Particularly, operational result of LD converter has great influence on both material production flow and steel quality. At NKK, fundamental research and development of blowing control of the LD converter with an objective of raising the end point hitting ratio was started in the early 1960's, and an online computer control system using the direct measurement by sublance was successfully established by the 1970's. On the other hand, users' demands for higher quality and higher grade steel have become rigorously intensified in recent years, together with the necessity of total cost reduction by rationalization of operations . In response to these demands, NKK has consolidated the secondary refining facilities and reconstructed all LD converters to combined blowing processes. In 1986, a new hot metal pretreatment facility started its operation at Keihin Works. Accordingly, the blowing operation of LD converter with pretreated hot metal has been put into practice. Since then, great attention has been paid to realize the maximum merit of less slag blowing, such as high and stable manganese yield and less consumption of ferro-alloy . It is comprehensible that computer control plays an important role in realization of these effects. This paper deals with a computer control system developed for LD converter blowing with pretreated hot metal, mainly its strageties for development and modification of control models. Moreover" the benefits of its applicable results are discussed.

STRAGETIES FOR COMPUTER CONTROL MODEL DEVELOPMENT As mentioned above, the goal of less slag blowing is to attain manganese content at end point as high as possible to realize less addition of ferromanganese alloy at tapping_ This operation can be achieved only by high and precise manganese end point control, and the development of a new computer control technique was inevitable. After all, the steelmaking shop in Keihin Works deals with wide gross products for hot strip, plate mills and seamless pipe mills. Therefore, local conditions have been considered in the development of the new control model.

BASIC CONCEPTS OF HOT METAL PRETREATMENT AND LESS SLAG BLOWING

(1)

A new ladle dephosphorization plant has been put

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Both [Cl and [Mn] at end point distributes widely at great extent_

(2)

namely Gross product manganese content, aimed [MnJ at end point is high in average (Appro. 0.85%) level.

From these aspects, computer control strageties have been focused to two viewpoints as mentioned below. (1)

Both wide ranged [Cl and [Mn] at end point must be controlled with high precision by unique computer control model.

(2)

High and stable manganese yield must be obtained in order to attain high [Mn] at end po in t.

The conventional computer control model may not be applied directly to less slag blowing control in account of its different blowing conditions. Accordingly, the computer control model has been reconstructed based upon fundamental metallurgical reaction and operational data analysis with the above consideration.

a

: aimed temperature at end point; WST ' Ws, Ws', WMO welght of plg iron, steel, slag, residue slag and manganese ore, respectively; manganese proport ion of manganese ore.

In this case, (MnO) , Ws and W 0 are the unknown values. These nonlinear equat~ons may be solved instantly by process computer, accompanied by a mathematical method such as Newton-Raphson method. The main characteristic of this calculation is the consideration of manganese in residue slag from the previous heat, because (MnO) is very high and its influence may not be negligible in less slag blowing. For precision, the slag calculation involves estimated residue slag volume and slag composition of the previous heat, calculated from the end point estimation model described later in this paper.

Heat Balance Calculation CONSTRUCTION OF COMPUTER CONTROL SYSTEM A summary of the computer control system is shown in Fig. 2. Naturally, the main purpose of this computer control system is to realize aimed chemical components and aimed temperature at end point without additional actions such as reblows or coolant addition. The computer control model consists of four subsystems, namely, 1) static control model, 2) dynamic control model, 3) end point estimation model and 4) feedback control model. The functions of each control model are reviewed in Table 1. The detail description of the computer control models is shown hereafter.

The aim carbon content in less slag blowing distributes widely from 0.04% to 0.5%. In general, less slag blowing requires heat compensation with regard to its lower carbon and lower silicon content in hot metal compared to conventional blowing. Therefore, the heat compensation calculation is carried out utilizing a heat balance model. For operation conditions, coke addition is practicized for heat compensation. Furthermore, remaining slag volume and slag composition of the previous heat are reflected in the heat balance model to improve accuracy.

DYNAMIC CONTROL MODEL Decarburization Model

STATIC CONTROL MODEL Manganese Ore Calculation The essential aspect of manganese ore addition is to get aimed manganese content at end point by the optimal combination of manganese source. Therefore, fundamental research of manganese reaction has been carried out initially. The slag-metal distribution of manganese in a steelbath depends upon molten steel temperature and oxygen potential. Furthermore, oxygen potential has a strong relationship with carbon content. From this viewpoint, a manganese equilibrium model in connection with end point temperature and carbon has been derived. In practice, this manganese equilibrium model is solved by adopt ing the manganese balance model and slag calculation, as shown below: (1)

(2) (3)

manganese equilibrium model 10g(MnO)/[Mn]EP=f([C]EP,T

EP

)

(1)

manganese balance model Wp[Mn] p+aWMO=WST[Mn] EP+Ws (MnO)

(2)

slag calculation Ws=g (Ws' , (MnO) , , (M) , ,[Mn] EP)

(3)

Generally, the dynamic control model is represented by two models; decarburization model and temperature rise model. It is natural that in order to get high and stable manganese yield, high accuracy control of carbon is required. In less slag blowing, the decarburization characteristic near the end of blow varies from ordinary blowing on account of the different blowing conditions such as less slag volume and high manganese content. From this viewpoint, a new de carburization model has been developed to match the decarburization curve in less slag blowing: -dC/dQ={a/(8-y) HC+8-2y-,!(C 8)2+4(8 y) (c-y)} (4) where, C: carbon content; Q: oxygen value; a, B , y , 5: model parameters.

In this model, a new model parameter y plays an important role in determination of decarburization curve. If y =O, then Eq. (4) resembles the conventional decarburization model. The parameter y is set from 0 to 15 based on slag volume in the vessel (Fig. 3). Manganese Behavior Model

where, (MnO) , (MnO)': manganese oxide in slag, residue slag respectively; [Mn]EP' [C]EP: aimed manganese/carbon content at end point; [Mn]p: manganese content in pig iron; (M)' : M component in residue slag;

influence of manganese content on The decarburization rate must be considered in less slag blowing. The oxygen consumption for manganese oxidation loss is calculated and reflected in total oxygen consumption as below: (5)

COlllpllter Comro\ of LD COIl\TrttT Process oxygen consumption for manganese oxidation loss; IHn]SL: estimated [Mn] at sublance measurement; [Mn ]Ep: aimed [Mn] at end point; b : constant value.

where, 60

Mn

for estimation is obtained by sub lance measurement with an oxygen sensor probe, which measures temperature, carbon (by carbon determinator) and oxygen activity in molten steel. These measurement values are used to estimate [Mn] at end point by the following equations:

( 6) (1) where, 60 : total oxygen consumpti o n; TOTAL 60 : oxygen consumption for C decarburizati o n.

(2)

(3)

Sublance CD[C] Modification In general, carbon content in molten steel is measuring solidification determined by sublance sample using the temperature of a following equation: (7)

where,

[C]CD: carbon content by carbon determinator; TCD : solidification temperature; P ,ql: constant values. l

Basically, [Cl D is influenced by other components in s£eel such as manganese, sulfur, phosphorus, etc. but usually neglected on account of their small content. This assumption will not be true under less slag blowing condition because of high manganese content. Thus, manganese content is reflected in Eq. (7) as: (8)

where,

[Mn]SL: estimated [Mn] at sub lance measurement; P ' q2' r : constant values. 2 2

It is noted that [Mn] SL is calculated from Eq. (1), (2), (3), the same set of equa tions for calculating manganese ore. In this case, carbon content and temperature measured by sublance are substituted in the manganese equilbrum model to solve [Mn]SL'

Feedback Control Model The decarburization model is modified to resemble the actual decarburization curve by the online feedback control model. In this case, carbon transient point (B) is selected as a feedback parameter. After the blow end of every heat, actual B is calculated backward by the operational data. Then the new feedback parameter is modified for the next heat: S (i+l ) = uS (i) + vB (i)

( 9)

where, 8( i+l): 8 for (i+l)th heat; fl U) 8 for i the heat; 8 (i) actual S calculated backwards for i the heat; u,v constant values.

manganese equilibrium log (MnO)![Mn]CAL = f( [O)vp, T)

(10)

manganese balance Wp[Mn]p + aW = WST[Mn]CAL + Ws(MnO) MO

(11)

slag calculation Ws = g(Ws', (M)', W ) f

(12)

where,

oxygen activity; estimated [Mnl at end point; flux weight.

Note that these equations shown above are very similar to the manganese ore calculation. where actual temperature and oxygen activity are adopted instead of aimed temperature and carbon. The results of the estimation are shown in Fig. 4. I t is clear that end point [Mn lCAL is estimated within the precision of +0 . 10%, which is acceptable for practical use. -

APPLICABLE RESULTS The operational results of applying the computer control to less slag blowing are shown in Fig. 5. The computer control system has raised up both carbon and temperature simultaneous hitting ratio to 90%, drawing out high and stable manganese yield. Accordingly, [Mnl at end point has reached up to 0.70% in average, making it possible to adjust high manganese steel by LD converter process without ferromanganese alloy addition . Figure 6 presents the transition of ferromanganese alloy consumption in Keihin Works. It should be noted that total consumption of alloy has been reduced from 8.0 kg/t to 2.6 kg/t in proportion with the improvement of blowing control. Thus, the computer control system has withdrawn remarkably high advantages in LD converter process, both in total cost reduction and operational stabilization.

CONCLUSION The new computer control technique for blowing operation with pretreated hot metal has been developed and practicized since the opening of the new ladle dephosphorization plant. The operational results prove high accuracy of computer control, such as high efficiency, high [Mn] at end point, high and stable manganese yield and less consumption of ferromanganese alloy. Consequently, total refining cost has been reduced to great extent.

REFERENCES END POINT ESTIHATION MODEL Fujii,

The end point estimation of chemical components of molten steel is the essential factor for the realization of tapping without end point sampling. Under this condition, the end point estimation model by sub lance measurement has been developed. The components to be estimated are focused to [Hn] at end point, the most important component to judge the possibility for direct tapping in less slag blowing. The information

s.

(1977). Converter Automation and Instrumentation. Nippon Kokan Technical Report-Overseas, April. Yamase, O. (1987). Hot Metal Pretreatment and Less Slag Refining Technology at Fukuyama Works. Nippon Kokan Technical Report, No. 118. (Japanese). Hasegawa. T. (1988). Modification of Computer Control Hodel for Less Slag Blowing. Zairyo-to-Process, Vol. 1, (Japanese).

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Function

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Sublance meas urement (in blow)

End point estimation

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Estimation of end point

Feed back con tro l

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Tapping

Modification of contro l parameters for each model

measurement

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Final determination of total amount of oxygen

Calculation of additional co olant Control of end point ca rbon and temperature [Mnl.

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