CA1045823A - Static method of controlling the refining reactions of pig iron for steel making purposes in an oxygen top blowing converter - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A static method of controlling the refining reactions of pig iron used for steel making purposes in an oxygen top blowing converter and in which a slag composition is achieved which, at the end of the blowing procedure, is substantially in chemical equilibrium with the molten metal (hereinafter referred to as the bath ), the composition of the bath approach-ing as closely as possible the desired composition of the steel, especially a steel having maximum limiting amounts of manganese and phosphorus. The method of the invention is particularly suitable for the production of a steel of a quality such that a substantial elimination of the elements accompanying the pig iron is achieved. Such steel is, for example, deep-drawing steel having a carbon content between 0.03 and 0.06% and a manganese content between 0.20 and 0.30%.

Description

The present invention relates to the refining of pig iron to produce steel.
More particularly, the invention relates to a static method of controlling the refining reactions of pig iron used for stèel making purposes in an oxygen top blowing converter and in which a slag composition is achieved which, at the end of the blowing procedure, is substantially in chemical equilibrium with the molten metal (hereinafter referred to as the bath), the composition of the bath approaching as closely as possible the desired composition of the steel, especially a steel having maximum limiting amounts of manganese and phosphorus. The method of the invention is parti-cularly suitable for the production of a steel of a quality such that a substantial elimination of the elements accompanying the pig iron is achieved.
Such steel is, for example, deep-drawing steel having a carbon content between 0.03 and 0.06% and a manganese content between 0.20 and 0.30%.
The prior art describes in detail how slag composition is related to bath composition, and theoretical models are known which attempt to bring about the desired bath composition by adjusting the corresponding slag composition.
A considerable problem in calculating the amounts of the various components which make up a charge is the determination of the quantity of lime which will dissolve. Onlythlt portion of lime which has dissolved by the end of the blowing procedure is of importance in determining the position of the slag in the CaO-FeO-SiO2- system and thus also in determining the equilibrium between bath and slag. The hitherto known models do not use calculations which are related to each specific charge for determining the -lime dissolution and cannot therefore predict exactly the chemical composition and quantit~ of the final slag, because the quantity of lime dissolved depends on the special conditions of charging and blowing. German Offen-le~ungsschrift 2 111503 describes an attempt to use a dynamic method ~ -~

. . - - ~ ., " , ., , ,, :
.

10~5823 incorporating continuous measurement to influence the slag path in the CaO-FeO-SiO2- system without predieting from the beginning of the proeess the position of the final slag.
In contrast thereto the present invention aims at determining as exaetly as possible before the ~efining process starts the position of the slag composition on the lime saturation line in the CaO-FeO-SiO2- system.
Aeeording to the invention this is made possible by calculating, for each particular charge, the quantity of lime (CaO) that might be expected to dissolve. The reason why the exact pre-determination is so important is that the CaO-content of the slag and the FeO-content of the slag are functionally related and the FeO-content of the slag det~rmines (for a given temperature) the distribution of the iron-acc~mpanying elements, in parti-cular manganese and phosphorus, between bath and slag.
In eontrast to the method described in the German Offenlegungs-schrift 2 104 067, the objeet of which is a static method of controlling the carbon content of a steel, the manganese and the phosphorus content of the bath are the controlling factors of the steel composition of the present process. In steels having a low carbon content, for which the method of the invention is particularly suited, the carbon content is of lesser importance and only serves as an indicator for the oxygen dissolved in the bath.
At this juncture it should be understood that the most cost-saving ~
refining conditions (i.e. charge and blowing eonditions) for a desired ~ -steel quality have to be set before the beginning of ~he blowing proeedure and eannot be adjusted subsequently. An exaet stipulation of the refining eonditions prior to the beginning of the eharge is therefore of paramount importanee. Dynamie models do not ealeulate the initial eharge whieh is needed to produee a steel of a desired eomposition; they merely control the eourse of the proeess and attempt eorreetion measures if the desired steel

-2-' ' 1045823 ;
would not otherwise be produced. A regulating influence generally causes a less favorable charge development than would normally be possible; moreover, on account of the rapid course of the refining process, the margin for correction during the blowing procedure is very narrow. Furthermore the costs and the utility of continuous-measuring devices detracts from the cost-effectiveness of such methods.
` These shortcomings, which are also described e.g. in German - Offenlegungsschrift 2 104 067, also apply to the invention disclosed in German Offenlegungsschrift 2 lll 503. In contrast to the present invention, the invention according to German Offenlegungsschrift 2 lll 503 permits only an approximate indication of the final value of the FeO-content of the slag in the CaO-FeO-SiO2- system. Therefore the value of this known method is limited.
The difficulties that occur with dynamic models are avoided by the present invention. Accordingly, the present invention provides a static method of controlling the refining of pig iron in an oxygen top-blowing steel making process in which a bath comprising molten pig iron having a given manganese, phosphorus and silicon content ( ~ ,PR,SiR), a fluxing agent and slag containing lime of a given granulation and composition and having a given capacity to oxidise constituents of the bath as manifest by the effective FeO-content of the slag is top blown with a blast of oxygen from a blowing lance whereby desired values for the Mn and P contents in the finished steel (Mn ,P ) and a desired final temperature (t) are achieved, -the refining process being one in which the refining proceeds to completion, the method comprising (a) introducing an estimated value for the quantity of dissolved lime (CaOfl) and the desired final temperature into pre-established empirical relationships and values to give initial calculated values for the Mn and P content of the finished steel; (b) if the initial calculated values for the Mn and P content of the finished steel are not . . ,

3-104~8Z3 substantially equal to the desired values, changing the said capacity of oxidation of the slag, and, if desired, the solubility of the lime until final calculated values substantially equal to the desired values for the desired Mn and P content are achieved; and (c) using pre-established empirical relationships and values and the final calculated ~alues for the Mn and P contents, the ultimately chosen capacity of oxidation of the slag and the ultimately chosen solubility of the lime to calculate the quantity of lime to be added for a given set of refining conditions.
Thus, with the method of the present invention a steel of a desired quality having a preset concentration of iron - accompanying elements, in particular manganese and phosphorus can be prepared by first calculating the charge conditions (quantity of pig iron, quantity of scrap for cooling ; and quantity of lime, ore and fluxing agent) with sufficient accuracy that when the refining process has proceeded to completion the composition of the slag has to lie on the lime saturation line in the CaO-FeO-SiO2- system.
Then the optimum blowing conditions are found by reference to known data for the particular charge, and then by using a comparison between nominal values and actual values, correction values are determined at the end of each charge and are used to adjust the model to changing operating conditions.
It is a prerequisite of the method of the invention that the refining process proceeds to completion. In practice the criterion for the completion of the refining process is the fact that the flame at the mouth ;~
of the converter goes out.
When the refining reaction has proceeded to completion, the factors characterising the equilibrium between the slag and bath will be suitable as control parameters. If the charges and the blowing conditions are chosen in such a way that at the end of the refining process these control parameters have reached precalculated values, the desired distribu-tion of the iron-accompanying elements between bath and slag will be achieved.

' ~ , :, ,, The desired final composition of the crude steel at a certain, also desired, temperature corresponds to a certain precalculable slag composition.
One important problem in calculating the charge to be used to produce a slag of a given composition (and therefore a steel of a given composition) is the quantity of lime which will dissolve for any particular set of reaction conditions. A significant proportion of the lime has not dissolved by the end of the refining process and only that portion of lime which has dissolved by the end of the process is important as far as the equilibrium between bath and slag is concerned. For a more precise deter-mination of the final state it is not sufficient to assume that always thesame amount of lime will dissolve, e.g. about 80%; on the contrary, the amount of lime which will dissolve has to be determined for each different charge if major deviations from the desired steel composition are to be avoided. In contrast to known methods in the method of the present invention ~-the position of the final slag on the lime saturation line in the (CaO)-(FeO)-(SiO2)- system is determined as precisely as possible. According to the invention this is made possible by precalculating the amount of lime that is to be expected to dissolve under the conditions of the reaction.
This precalculation is necessary because the FeO-content of the slag and thus the distribution of the iron-accompanying elements, in particular Mn and P, at a particular temperature is fixed by the amount of dissolved lime.
It is also to be noted that for a desired steel composition the most cost-saving ~onditions are set before the refining process starts and that these conditions cannot be adjusted subsequently. Therefore the precise determina- -tion of the refining conditions before the beginning of the charge is a decisive advantage.
To obtain the quantity of lime required the expected dissolution ~-of the lime charged and the capacity of the slag to oxidise constituents of the bath (hereinafter referred to as the "capacity of oxidation") must be , ~

- , : , - :
- .. . ~ .: . .. . .

calculated first. The calculation is based on known empirical values preferably stored in a computer and takes into account the chemical com-position and the granulation of the lime used, the kind and quantity of the fluxing agents selected and blowing parameters such as the height of the lance relative to the bath, the blowing time and the oxygen flow.
The relation between the quantity of dissolved lime (CaOfl) and the quantity of lime added (CaO ); also referred to as the lime charge) gives the efficiency of the lime:
. C O
~ CaO = - (1) CaO
ges ; Factors which affect the dissolution of the lime are the amount of lime added, the quantity and kind of fluxing agent and the height of the lance above the bath at the beginning of blowing. The lowering scheme of the lance in relation to the original hei~ht is usually fixed.
The slag components coming from the pig iron, mainly SiO2 and MnO, -have also to be taken into account. The lime efficiency ~ CaO can therefore be expressed as an empirical function of the process factors: CaO es (the quantity of added lime), SiO2 (SiO2 - quantity from slagging of the Si-content of the pig iron, from the SiO2-content of the lime of the fluxing agent and of the mixer slag in kg/metric ton of pig iron), MnO (MnO-quantity in kg/metric ton of pig iron deriving from the slagging of the Mn-content in the pig iron), DStB (original lance position in m above the bath), F ~, (quantity of fluxing agent in kg/metric ton of pig iron):
CaO = f (CaOges, SiO2, MnO, DStB, Fm, ~ ~ CaO) (2) ~ ~ CaO being a correction value for ~ CaO. The qualitative influences of these process parameters on the lime efficiency is known, but the quantitative calculation of the lime dissolution has hi-therto not been carried out in a static control process.
From the knowledge about the qualitative influences of the relevant k, - 6-.

process paramaters on the rate of dissolution of the lime and by using calculations as to the velocities of formation of the reaction products derived from the law of mass action, a theoretical model for the rate of dissolution of the lime which is sufficiently accurate for practical application has been developed.
Figure 1 illustrates schematically the quantity of lime dissolved in the slag in kilograms per ton of pig iron ([kg/tR~ ) as a function of the blowing time tB in minutes for two different states of operation, the upper curve corresponding to the state of operation at reference conditions (when ~ ~ CaO = O).
Figure 2 also illustrates purely schematically, i.e. not true to scale, the rate of dissolution of the lime, i.e. the amount of lime in kg/metric ton of pig iron dissolved in the slag per unit time (min) for both states of operation, the upper curve again corresponding to the state of operation at reference conditlons.
In Figure 3a the logarithm of the manganese distribution (MnO) (log ~
[Mn]
as ordinate is plotted against the logarithm of the total iron oxide in the slag (log(FeO)) as abscissa for three states of operation, wherein (MnO) and [Mn] are as hereinafter defined. The line hl corresponds to the state of operation at reference conditions, whereas the lines hl' and hl" illustrate deviations therefrom, which are caused for example by an alteration of the position of the nczzle of the lance, whereby, as is known, the rate of oxidation of the bath is changedO
Figure 3b, similarly to Figure 3a illustrates the logarithm of the phosphorus distribution (P205 ) log [p]2 .... .
. ~ ~', .
- .

as ordinate against the logarithm of the total iron oxide in the slag (log (FeO)) as abscissa. According to theory, the lines h2, h2' and h2"
of the phosphorus distribution are five times as steep as the corresponding lines for the manganese distribution. Line h2 in the middle refers to reference conditions.
The illustration in Figures 3a and 3b corresponds to the law of mass action applied to the respective slagging of manganese represented by the equation [Mn~ + (FeO)~ (MnO) + Fe or of phosphorus represented by the equation [ 2] + 5(FeO)~ (P205) + 5 ~F~ ~
Application of the law of mass action to these chemical equations : ~-leads to the following respective relationships:

(MnO) K
M rMn~ (FeO) wherein KM is the tempera~ure-dependent equilibrium constant of the mangan-ese slagging, (MnO) is the MnO-content of the slag, rMn] is the Mn-content of the bath in % at the end of the refining process and (FeO) is the FeO- ;~

content of the slag, and ~ (p O ) rP~ . (FeO) wherein Kp is the temperature-depcndent equilibrium constant of the phosphor-us slagging, (P205) is the phosphate content of the slag, rP~ is the phos-phorus content of the bath at the end of the refining process and (FeO) is the FeO-content of the slag.
As can be seen from Figure 1, the curve of the rate of dissolution -of lime at reference conditions (upper curve in Figure 1) in the beginning ; runs steeply and then becomes gradually flatter until a point of inflection is reached, whereupon the curve is flat again, and finally becomes progress-:.

ively steeper.
The lower curve shows the rate of dissolution of the lime in a deviation from reference conditions.
From published data on the lime dissolution rate it is known that after an initial stage which has not been considered here and in which the major part of silicon and manganese burns off, the lime dissolution first happens rapidly; the dissolution then slows down (approximately in the second third of the blowing time) because of the formation of dicalcium silicate she~ls around the lime particles; and finally it accelerates again towards the end of the process, because at increasing temperature the sili-cate shells melt. This gives the characteristic curves shown in Figure 1.
When these curves are differentiated with respect to time a relation between ~ ~

the dissolution velocity of the lime ~ -d CaO
( dt and the blowing time is obtained (see Figure 2). When the blowing conditions (and thus the conditions for the lime dissolution) change, the curves according to Figure 1 and Figure 2 are shifted to form the lower curves in Figures 1 and 2.
From Figure 1 and 2 it can also be observed that the velocity of lime dissolution (CaOfl) (kg/t. min) at at the beginning of the blowing procedure is proportionate to the quantity of lime dissolved by the end of the refining process (at approx. constant blowing time), because the two ordinates of the rectang~es in Figure 2 are proportionate to the gradient of the tangents in Figure 1.
By fixing the specific rates of lime dissolution at the beginning of the refining process by means of corresponding precalculable charge _g_ :

conditions and by means of directed deviations from the reference conditions while using a constant lance lowering scheme and a constant oxygen consump-tion per metric ton of pig iron and per min., then the behaviour of the charge through the course of the process and the final state reached at the end of the refining process (final temperature and final composition) are set from the very beginning. ~y a comparison with corresponding measured values of previous melts, an adjustment of the static control method may be carried out, which allows for the effect of parameters not actually measured and thus adjusts the process control to the operational conditions that change in the course of a process. Thus the high costs of a dynamic process control may be avoided by using the method of the invention.
The invention is thus based on the knowledge that the variation in the quantity of the lime dissolved at the end of a refining process, and thus the v~riation in the lime efficiency, can be determined from the initial rate of the dissolution of the lime at the beginning of the process, i.e.
from the charging conditions, from the conditions in the converter and from the blowing sche~e (lance position and quantity of blast oxygen). The rate of lime dissolution itself is related to the rates of formation of the reaction products, which rates can be derived from the law of mass action.
One of the partial reactions is the reaction of silicon dioxide with the lime. The reaction rates for this reaction are:

d CaO
dt Kl . (CaO) . (SiO2) For wollastonite (C20W) ' ,, K = K2 . (CaO) . (SiO2) for calcium ortho silicate dt (CaOK).

The latter however, does not exist as a liquid phase at the temperatures prevailing in the first and second thirds of the refining process, but it forms a reaction-preventing silicate shell around the lime ' " '`~ : ' ,' ;; ' '' ', :: ,. ' ~ ` . . , , -particles. The velocity of the lime dissolution therefore results as difference of the two reactions running parallel:

d Cafl = Kl (CaO) . (SiO2) - K2 . (CaO) . (SiO2) (3)-dt In homogeneous solutions, the initial velocity of a reaction is determined by the concentration of reactants (mole fractions or mole per cent, per cent by weight in solvent, etc.) at a given temperature. In the present case where a heterogeneous system is present it is better to describe the reacting molecules of the slag components (e.g. solid CaO, liquid SiO2) by their quantity in kg per metric ton of pig iron. Later on when referring to the effect of fluxing agents t~is will be described in more detail.
From practical experience, the determination of the functional dependence of the lime efficiency ~ CaO on the various parameters is best carried out with the aid of the manganese slagging (as~uming constant blowing conditions). The molten or dissolved CaO in the slag determined via the manganese slagging proved to be more representative than the analytical value of CaO in the slag.
As is known the following equilibrium constant KM applies for the manganese slagging:

K ( ) - thus ( ) = KMn (FeO) (4).
Mn [Mn] . (FeO) ~Mn]

For the range that is of interest, the lime saturation line in the (CaO)-(FeO)-(SiO2)- system can be described by the following formula:

CaOfl (CaO) (FeO) = A. _ + B = A. + B (5), sio2 (sio2) ~.. :.. :-wherein A = 18.24 and B = -51.04 at 1600 C and wherein the sum of (CaO), (SiO2) and (FeO) in the slag is assumed constant at approx. 80%. (This formula only applies for FeO ~ 16~; i.e. for a slag ratio i.e. (Cao):(SiO2)~

-11- ~ '' 1045~323 2.8; when the slag ratio is lower than 2.8, FeO is set equal to 16%).
The combination of the formulae 4 and 5 gives the quantity of lime CaOfl dissolved at a state of equilibrium:
(MnO) (FeO) - B ~ . K -B
fl A . SiO2 = A Mn . SiO2 (6) In equations 4 to 6:
KM is the equilibrium constant for the manganese slagging (temperature-dependent);
(MnO) is the MnO-content of the slag (in %);
[Mn] is the Mn-content of the bath (in %) at the end of the refining reaction 10 = [Mn~:
(FeO), (CaO) and (SiO2) are the respective contents of FeO, CaO, and SiO2 of the slag in the (CaO)-(FeO)-(SiO2)~ system (in %), and A,B are the constants of the lime saturation line.
The quantity of SiO2 results from an SiO2-balance. Taking into consideration the SiO2-quantity coming from the mixer slag the following empirical equation can be determined from operational values:

SiO2 = 2.45 (21.4 SiR + CaOfl . f) (kg/tRE) ---- (6a), wherein SiR indicates the Si-content of the pig iron and f denotes the proportion factor % SiO2 in relation to % CaO in the lime.
The equilibrium constants for the manganese and the phosphorus slagging were also calculated from operational values and the following dependence upon the temperature has been found:

lo K 6257 - 3.10085 (7a), and log Kp = t + 273 15 - 9.69437 (7b) '~ -12-. . . :.: :
: ~ :, . . . . .
. .

All the other values can be observed from the slag~and pig iron analysis. The oxide iron of the slag determined analytically is converted entirely to FeO
(FeO) = 1.29 (Fe).
The quantities of dissolved lime determined in this way and the lime efficiencies of a number of charges are subjected~to a suitable evalu-ation with the factors which are expected to have an influence on the lime efficiency in accordance with equations (1) and (2), so as to obtain an equation for the lime dissolution. A statistical determination of the coefficiencies of the set-up renders a high degree of certainty and confirms that the set-up is justified.
By processing data which is specific to the plant in question the following form for equation (2) was determined:

Cafl = Cages ~ = Cages (0-125 SiO2 - 0.00124 CaO SiO2 +

0.187 ~ - 0.02 + ~ ~ ) (2') ;
(for reference conditions ~ ~ = O, otherwise alteration of the amount of fluxing agents added and other corrections are taken into consi~eration).
The coefficients of equation 2' depend on each particular refining process and plant and must be determined individually for each set-up.
` In the previous deliberations it has always been assumed that the refining conditions are kept constant and that only the constants enter into the model equations. It may, however, be necessary or suitable to change the refining conditions by prescribing a different addition of fluxing agent, by changing the sort of lime or by modifying the lance lowering s~heme.
When the influence of an alteration of the refining conditions upon the lime dissolution, and thus the blowing result, is known, there is still the opportunity of obtaining, according to the invention, a particular state of equilibrium. For a sufficient determination of the state of equilibrium one -".
~ ;~ ' ' ~'~ ' : . .; -has, first of all, to control the lime dissolution, but one must not neglect the capacity of oxidation of the slag either.
As can be seen from Figure 2 a change in the initial velocity of the lime dissolution (if not too big a variation) is proportionate to the alteration of the areas of the rectangles below the two curves and is thus proportional to the change in the quantity of dissolved lime because the areas below the curves of Figure 2 indicate the quantity of dissolved lime.
The influence of the sizes with not too large a range of variation can - therefore be taken into account as an additive correction for the quantity of dissolved lime or for the lime efficiency.
This mode of operation will now be substantiated theoretically - with reference to the addition of fluxing agent: `
The process begins with the fact that the lime molecules first have to be separated from the solid lime grain under the formation of a low-melting phase.
; The quantity of CaO available for this purpose depends on the surface of the lime particles, i.e. for a given lime quantity it depends on the granulation. For a given grain size, the number of CaO-molecules ; available is proportional to the quantity of lime supplied in the beginning.
The lowering of the melting point and the acceleration of the lime dis-solution achieved by the addition of fluxing agents can therefore, according to considerations similar to those which lead equations (1) and (2), be expressed by a term containing the partial reactions of the process and also -other values (e.g. the granulation) and can thus enter into the calculation of the lime efficiency.
Practical tests have shown for example that an increase in the addition of fluorspar (~ Fm) of 1 kg per metric ton of pig iron results in an increase in the lime dissolution (~ ~ ) of 2.5%:
., ~ ~ = 0.025 Fm (8).

14- ~-. .
: . .

The capacity of the slag to oxidise constituents of the bath is directly related to the amount of oxidic iron (FeO)w which becomes effective in the slagging of manganese and phosphorus. As long as the capacity of oxidation remains constant ( = reference condition ), it need not be taken into consideration explicity in the equations. The relationship between the slagging ratio (MnO/Mn) and (FeO) is a straight line in a log-log-~pl~t at a particular temperature and at a particular degree of oxidation ¦see Figure 3a for the slagging ratio of manganese). Properly speaking (FeO) stands for (FeO)w at reference conditions:
the line hl in Figure 3a is considered as the reference line at reference conditions (d FeO - 1). An alteration of the capacity of oxidation can be allowed for by multiplying (FeO) by a factor~ F o- This factor CF O is a family parameter whnch takes into account the varying capacity of oxidation of the slag. The multiplication corresponds to a translatory shift ~f the reference line hl in the graph of Figure 3a. The capacity of ;
oxidation is determined decisively by different original positions of the lance at a constant lance lowering scheme, as well as by different oxygen flows (Nm3/min). Statistical evaluations of operational results have determined the following dependence of c<F O upon the original lance position DStB (in meters above the bath): ~-~F O = 1.824 (1.39 - DStB) + 1 (9) (for an oxygen flow and lance lowering scheme at reference conditions).
At a constant capacity of oxidation, the adjustment of the desired manganese content in the crude steel also fixes the phosphorus content and vice versa. Therefore, the manganese and phosphorus contents cannot be prescribed independently of one another.- However, by changing the capacity of oxidation it is possible to achieve a certain balancing: in the illu- --stration according to Figure 3b, the reference line h2 for the phosphorus slagging is five times as steep, and the translatory shift assigned to a -. ,; --15--~.i, , - : - ,: ,. - .: ., . .-change in the capacity of oxidation is five times as big as the correspond-ing values for the manganese slagging, because the influence of the iron oxide concentration [Fe~ on the slagging of manganese varies directly with LFe~ but its influence on the slagging of phosphorus varies as ([Fe~ )5, which means that a change in the capacity of oxidation affects the slagging of the manganese and of the phosphorus to a different degree Within a certain, technically suitable, range it is therefore possible to control the variables of the process in such a way that the desired values for man-ganese and phosphorus in the crude steel can simultaneously be maintained.
In the illustrations according to Figures 3a and 3b the temperature is assumed to be constant; it is taken into account in the equations for the slagging of manganese and phosphorus; its influence upon the position of the lime saturation line in the (CaO)-(FeO)-(SiO2)- system may,~however, be neglected.
The capacity of oxidation of the slag need not be present as a mathematical function; it may be in the form of a table or of a family of functions (for instance one function for each lance lowering scheme). The same also applies for the lime efficiency, for which the parameters - e.g.
; different sorts of lime and/or different granulations - may be tabulated.
The slagging values ~ and (P205) contain the bath concentration in the denominator and the concentration in the slag in the numeratorO In evaluations of melting report data the slag analysis and, possibly even the slag weight are given. These data permit an examination of the required constancy of the conditions in order to evaluate the influence of changes.

When putting the model into practical use, naturally no slag data are available prior to the beginning of the refining process. Therefore - ~

104~8~23 one has to calculate these data. Since the composition of the pig iron and the values of the first sampling analysis (Mnv, Pv) are known, the quantities of Mn and P that have to enter into the slag can be determined which gives the quantities of MnO and P205 that have to form in the slag. For deter-mining the concentration of MnO and P205 in the slag one needs, however, the slag weight. From the equations for the manganese-and phosphorus slagging [M~l Mn . (FeO)~ ~nd [ ]2 = Kp . (FeO)S

the slag weight may be calculated from the following equations:
(MnR - Mnv) S (MnO) [ ~ Mn ( )W 100 [Mn~ (lOa) lo22.9 [ ]2 = Kp . (FeO)w 100 = 2 (10 This is made easier by the knowledge that the sum of the analytical-ly determined concentrations of (CaO), (SiO2) and (FeO) in the slag for a given raw material is roughly constant and amounts to 80~. Thus the missing slag weight may be substituted in the above equations lOa and lOb from the following equation (11) (CaO) + (FeO) + (SiO2) = constant (approx. 80~), from which follows:
CaO 1 + SiO2 S = f . 100 (lla) 80 - (FeO) In these equations: -MnR, PR are the respective contents of manganese and phosphorus in the pig iron (%);
Mnv, PV are the respective contents of manganese and phosphorus in a first sampling analysis (desired values, operands) (%);
S is the slag weight (kg/metric ton of pig iron);

-` ,: . ' . , ., - . ~, , :

KM ~ Kp are the respective equilibrium constants for Mn- and P-slagging;
and (FeO)w is the effective FeO-content of the slag (%), wherein the follow-ing relation applies:
(FeO)w = ~ FeO (FeO). ~-~
As the quantity of SiO2 is known from the SiO2-balance equation (6a), equation (lla), together with the equation (4), allows the determina-tion of the quantity of dissolved lime (CaOfl) required for the slagging at reference conditions : (FeO)w = (FeO), to which quantity a lime charge according to equation (2) or equation (2'), respectively can be assigned.
Changes in the capacity of the slag to oxidise the bath and in the lime efficiency (e.g. on account of the addition of fluxing agents) enter into the calculation procedure at the appropriate place. Also demands as to the ~-quality and the question of costs play an important role, so that from the number of technically possible solutions, the optimum one may be chosen.
The sulfur-and the carbon contents of the crude steel are not adjusted but are only calculated in a known manner from the charge chosen and are taken into consideration in the balances.
On account of these calculations a very exact quantitative balance of the slag can be drawn up which brings about greater exactness and better reproducibility for the subsequent oxygen, heat-and iron balance. This means that the desired final temperature and the quantity of the liquid steel can be maintained better than it would be possible with known static control systems. Experience has shown that the requir~d quantity of blowing oxygen can be predicted so exactly that the moment at which the calculated quantity is used up practically coincides with the moment at which the flame dies down. After having blown the calculated oxygen quantity one can there-fore finish off the charge without having to worry.
In the theoretical model it is assumed that all values that do not explicitly occur in the equations can be kept constant or have no ,; :

r~ 18--~ .

influence. This assumption is, however, only valid as a timed average, the values being subject to statistical fluctuations. For this reason deviations occur between the desired values and the values achieved in reality. From these deviations correction values are calculated and are stored, together with the values of some first melts. From the correction values of the first melts the deviations for the melt to be calculated can be predicted and can be taken into account. Thus the theoretical model follows not only the operational fluctuations, but it also detects and considers a trend.
Correction values are established for the lime dissolution, for the cap- ;
ability of oxidation of the bath and for the oxygen, heat-and iron balance.
This adjustment built up according to known mathematical processes further improves the accuracy of the model.
The amount of pig iron required for a certain charge results from the desired output weight taking into consideration the portion of scrap iron; this amount of pig iron then determines the total demand of lime and other additions.
All of the calculations can be carried out in a process computer and the correction values can be stored in ~he computer. The only purpose of the computer is to speed up the determination of the proper lime charge and not to dynamically control the process. Hence, the calculations can also be carried out by hand.
The following Examples illustrate the invention: -In the Examples it is assumed that a standard operation is given in which the reference relations are fulfilled. Also, the slag ratio (CaO) : (SiO2) is always assumed to be above the technologically meaningful lower l;m;t of 2.8 ( = 16~ FeO in the ternary s~stem CaO-FeO-SiO2) and the lime solubility is assumed to be sufficient to obtain the required amount of CaOfl in the slag. Standard operation is present when the initial noz~le position (DStB) and the fluxing agent charge (in kg/metric ton of -19~
- - . , , ., ~ . :

pig iron) have the values determined on the basis of operational experience.
I~ it is not possible to obtain the desired analysis values for phosphorus and/or manganese under the standard conditions, the desired analysis values are obtained by altering the initial nozzle position (DStB) and the fluxing agent charge. Depending on the individual case, various strategies as set forth in the Examples can be applied in arriving at the desired values.
All three Examples deal with the production of low-carbon steels (approx. 0005% C) in a converter having a nominal capacity of 50 metric tons.
Since the carbon content is only calculated and taken into consideration, ~ ~ -but is not adjusted in achieving the results, it is not mentioned in the Examples. The same applies for the sulfur content, which amounted to 0.035%
in the pig iron and was therefore very low. The original lance position DStB was 1.39 m in all cases, the oxygen flow was 160 Nm3/minO Also 1.7 kg of fluorspar and 7 kg of granulated slag per metric t~n of pig iron are given ~ -~
as standard conditions.
Exam~le 1:
In this Example, a specified manganese content in the crude steel, i.e. prior to alloying, and the maximum permissible phosphorus content are desiredO If with the desired manganese content the permissible phosphorus content were exceeded, the charge depends on the P-content of the crude steel; a manganese content that is lower than the desired one is taken in such a caseO
Charge: pig iron 0.37% Si, 1.10% Mn, 0.096% P:
lime 92% CaO, 0.75% SiO2 (f = 0.75/92);
desired values: 0.32% ~ , 0.015% Pv~ t = 1610C. ~ -This leads to the following equations:

log KMn = 6257/(t + 273.15) - 3.10085 (7a) -log Kp = 12857/(t + 273.15) - 9.69437 (7b) SiO2 = 2.45 (21.4 SiR + CaOfl . f)0-7 (6a) - . :. . . . -:

12-9 ( ~ - Mnv) / Mnv = KMn . (FeO)w / (lOa) ( R PV) / Pv = Kp (FeO)w . S/loo (lOb) (FeO) = (A. CaO/SiO2 + B) . 0.8 (5) S = 100 (CaOfl + SiO2) / (80 - (FeO)) (lla) From analysis of the slag, mean values of A and B in equation 5 are determined to be 22.8 and -6308.
By inserting the desired temperature of the crude steel into equation (7a), and (7b), respectively one obtains KM = 1.66638, and Kp =
0000135838 respectively. The calculation is now carried out as follows:
First a median empirical value within a specified range is assumed for the value C~Ofl (e.g. 45 kg out of the range 30 to 60 kg) and is inserted into equation (6a), whereby a calculated value for SiO2 results. CaOfl is selected in a ~anner such that CaO/SiO2 is ~ 2.8. In Equation 6(a) not only the SiO2-content derived from the pig iron is taken into account, but also the amount stemming from the dissolved lime and that introduced with the pig iron slag. With this value, via equation (5), a value for (FeO) is obtained from Equation (lla) and can be used to obtain a value for S. If the first estimated value for CaOfl is correct, Equation (lOa) has no remainder. The first calculation rendered the following data:
CaO(=CaOfl) = 42.4120 kg per metric ton of pig iron, SiO2 = 10,~446 kg per metric ton o~ pig iron, FeO = 20.9585% in the slag, S = 90.0325 kg of slag per metric ton of pig iron.
From Equation (lOa) or (lOb), respectively, one obtains the values for Mnv = 0.32% and for PV = 0.0188949 (0.018%) i.e. too high a value for the P-content.
Now the calculation process is repeated with PV as the target.
This leads to a higher value for CaOfl than in the first calculation process, and the following is obtained~

_21-,~ ... .- . : :

::

iO45823 CaO(=CaOfl)= 43.6218 kg per metric ton of pig iron, SiO2 = 10.7536 kg per metric ton of pig iron, FeO = 22.9506% in the slag, S = 95.3127 kg of slag per metric ton of pig iron.
By inserting these values into the Equations (lOa) and (lOb), respectively, one obtains the values for ~ = 0.287527% and for PV = 0.015%.
The occurring slag ratio is calculated to be -fl SiO = 4.0565 and is thus bigger than the minimum slag ratio (2.8) and therefore falls into the range of validity of the lime saturation line. Since the CaO is not entirely dissolved the CaO s to be added has to be calculated.
The calculation of the amount of CaO s necessary to obtain the required amount of CaOfl is carried out according to Equation 2'.
From Equation (2') the value for CaO s is found to be 52.9477 kg/
metric ton of pig iron. Then ~ CaO is found from Equation (1) to equal 0.82386. The lime charge is then calculated from the CaO-content of the lime to be 57.6 kg/metric ton of pig iron, i.e. CaO es/92% CaO.
The lime charge necessary to introduce the required amount of CaO s results from the amount of CaO s while taking into consideration the CaO content of the lime. In calculating the amount of CaO , the ges limitations for the lime solubility have to be considered and ~ must not exceed the value 1. Therefore, the relation CaOfl ~ CaO s ~ CaO holds .
true and CaO is the maximum value which CaO s may take in Equation (2) for a real solution.
For this charge the following results were obtained: -Desired Obtained t 1610C 1608C
0.2g% 0.28%

PV 0.015% 0.014%

A ~ 22 .,............................... .. ~ ~ . .

This good consistency proves the good practicability of the method of the invention.
Example 2:
In this melt a manganese content as high as possible is to be achieved in the finished steel and the phosphorus content is not to exceed a certain limit. For this purpose the lowest possible lime charge is chosen;
however, the minimum slag ratio of 2.8 and a minimum slag quantity have to be maintained. An attempt is made to get the phosphorus content below the desired limit via a change in the capacity of oxidation of the slag.
The original conditions are the same as in Example 1.
Charge: pig iron 0.38% Si, 1.05% Mn, o.ogl%'`P, lime 92% CaO, 0.75% SiO2, f = 0.75/92;
desired values: t = 1600C, Mn : as high as possible, P = max. 0.025% - ;
Since the MnO : Mn ratio is to be kept as small as possible, a slag lying in the lower limit range of the slag ratio (2.8) of the lime saturation line is desired, and therefore the iron oxide content of the slag is fixed at 16%.
By inserting the desired temperature of the crude steel there results from (7a) and (7b), respectively: KM = 1.73585, respectively Kp = 0.0014773.
From the Equations (6) and (6a) having two unknown quantities each one obtains the values for CaOfl and SiO2 and thus the value for FeO
is obtained from Equation 5, and with this value the value for S is obtained from Equation (lla).
The first calculation rendered the following values: -S = 64.4421 kg per metric ton of pig iron, CaOfl = 30.3796 kg per metric ton of pig iron, SiO2 = 10.8499 kg per metric ton of pig iron, FeO = 16% (prescribed), and by insertion into the Equations (lOa), -and (lOb), respectively, ~ = 0.439803% and PV = 0.035637%. This means that too much phosphorus is contained in the steel. In order to achieve the permissible maximum value of 0.025% P, the capacity of oxidation of the slag has to be raised from 1 to 1.19358, according to Equation (lOb). As a measure for influencing the capacity of oxidation of the slag the nozzle position can be changed, which change can be calculated from Equation (9);
therefore the original lance position is changed from 1.39 m above the bath to 1.28 m, which distance lies within the technically permissible range.
The calculation of the lime efficiency and of the lime change is carried out analogously to Example l; the necessary lime charge is calculated to be 30.3796/0.92 = 33.02 kg per metric ton of pig iron.
For this charge the following results were obtained:
desired obtained t 1600C 1595C
Mnv 0.40% 0-39%
Pv 0.025% 0.023%
~` Example 3:
In this melt it was desired to achieve a low phosphorus content in the steel, the manganese eontent not falling below a minimum value.
Sinee the maximum quantity of lime whieh is soluble under reference conditions is not suffieient for aehieving the desired dephosphorisation, the solubili-ty has to be inereased by the addition of fluxing agents (fluorspar). Only an inferior sort of lime eontaining 86% CaO, 1.05% SiO2 (f = 1.05/86) was available. The original eonditions are the same as in Example 1.
Charge: pig iron 0.40% Si, 1.08% Mn, 0.~31% P;
lime 86% CaO, 1.05% SiO2, f = 1.05/86.
desired values: t = 1610C, Mn : min. 0.25%, P : max. 0.015%.
The first ealeulation, similar to the one earried out in Example 1, rendered the following values: ; -.. . . . . . . . . .

KMn = 1.66638, Kp = 0.0013538, CaOfl = 47.5321 kg per metric ton of pig iron, - SiO2 = 11.5303 kg per metric ton of pig iron, FeO = 24.1522% in the slag, S = 105.756 kg per metric ton of pig iron, = 00251192%, PV = 0.015%.
The results correspond to the specification and a variation of the lance height, as in Example 2, is not necessar~. It is, however, not possible 10 to reach the required quantity of CaOfl, with the mentioned quantity of SiO2. Assuming its complete effectiveness, a maximum of 46.0729 kg of free (dissolved) CaO per metric ton of pig iron could be obtained, which is less than the calculated value for CaOfl. In order to achieve the desired quantity of dissolved CaO one has to raise the solubility of the lime ( ~ ~ ) by 2.57052% according to Equation (2') by adding fluorspar. For this empirical Equation (8) applies. By means of Equation (8) it is determined that the fluxing agent added has to be increased by 1.032 kg per metric ton ~
of pig iron. Thus a total fluxing agent charge of 2.732 kg per metric ton ~ ~ -of pig iron and a ~ CaO of 0.840101 results, which gives a CaO s-demand of 56.5791 kg per metric ton of pig iron and a lime charge (taking into con-sideration the CaO-content of the lime) of 56.5791. 100 65.8 kg per metric ton of pig iron. The slag ratio turns out to be 4.12238.
For this charge the following results were obtained:
desired obtained t 1610C 1615C
0.25% 0.27%
PV 0.015% 0,014% -~
It is to be noted that in each case, for achieving the desired steel temperature, the quantity of scrap (per metric ton of pig iron) is -, - , , . -- -- ., ~ , : .

determined from the heat balance and the demand of oxygen is determined via the melting loss of the iron-accompanying elements and of the iron. The duration of the refining process results from the oxygen demand and from the oxygen flow.

.-,~ , . .

Claims (12)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A static method of controlling the refining of pig iron in an oxygen top-blowing steel making process in which a bath comprising molten pig iron having a given manganese, phosphorus and silicon content (MnR,PR,SiR), a fluxing agent and slag containing lime of a given granulation and composition and having a given capacity to oxidise constituents of the bath as manifest by the effective FeO-content of the slag is top-blown with a blast of oxygen from a blowing lance whereby desired values for the Mn and P contents in the finished steel (Mnv,Pv) and a desired final temperature (t) are achieved, the refining process being one which proceeds to completion, the method comprising (a) introducing an estimated value for the quantity of dissolved lime (CaOfl) and the desired final temperature into pre-established empirical relationships and values to give initial calculated values for the Mn and P content of the finished steel; (b) if the initial calculated values for the Mn and P content of the finished steel are not substantially equal to the desired values, changing the said capacity of oxi-dation of the slag, and, if desired, the solubility of the lime until cal-culated values substantially equal to the desired values for the desired Mn and P content are achieved; and (c) using pre-established empirical relationships and values and the final calculated values for the Mn and P
contents, the ultimately chosen capacity of oxidation of the slag and the ultimately chosen solubility of the lime to calculate the quantity of lime to be added for a given set of refining conditions.

2. A method according to claim 1, wherein the calculations are per-formed on a computer.

3. A method according to claim 1, wherein the change in the solubil-ity of the lime is achieved by changing the granulation of the lime.

4. A method according to claim 1 or claim 2, wherein the change in the solubility of the lime is achieved by changing the quantity of fluxing agent in the bath.

5. A method according to claim 1, wherein the empirical relationships are established by collecting the usual characteristic charge data of the melting operation and the quantity of lime dissolved (CaOfl) at the end of the refining process and the lime efficiency are calculated from this data using empirical relationships based on the variables,CaOges which represents the quantity of lime added in kg per metric ton of pig iron, SiO2 which represents the quantity of SiO2 derived from the slagging of the silicon content of the pig iron, of the lime, of the fluxing agent and of the mixer slag, MnO which represents the quantity of MnO in kg per metric ton of pig iron derived from the slagging of the Mn-content of the pig iron, DstB which represents the original lance position in meters above the bath and Fm which represents the quantity of fluxing agent in kg per metric ton of pig iron.

6. A method according to claim 5, wherein the characteristic charge data of the melting operation are the amount, chemical composition and temperature of the pig iron and of the crude steel, the amount and chemical composition of lime and other additions, the quantity of scrap, the chemical composition and quantity of slag, and blowing parameters, chosen from at least one of the height of the lance relative to the bath, oxygen consumption and blowing time.

7. A method according to claim 5, wherein when lime saturation occurs, the empirical relationships include (5) when the slag ratio [CaOfl/SiO2] ? 2.8 and in which relationship A = 18.24 and B = -51.4 assuming that the sum of (CaO), (FeO) and SiO2 is approximately constant at 80%, (FeO) being set at 16% when the slag ratio is lower than 2.8.

8. A method according to claim 5, wherein the empirical relationships include the functional dependence of the lime efficiency ? CaO which is determined with the aid of the following formula derived from the law of mass action applied to the manganese slagging:

wherein KMn is the temperature-dependent equilibrium constant of the man-ganese slagging, (MnO) is the MnO-content of the slag, [Mn] is the Mn-content of the bath in % at the end of the refining process and (FeO) is the FeO-content of the slag.

9. A method according to claim 5 wherein the empirical relationships include the functional dependence of the lime efficiency ? CaO which is determined with the aid of the formula derived from the law of mass action applied to the phosphorus slagging:

wherein Kp is the temperature-dependent equilibrium constant of the phos-phorus slagging, (P205) is the phosphate content of the slag, [P] is the phosphorus content of the bath at the end of the refining process and (FeO) is the FeO-content of the slag.

10. A method according to claim 1, wherein the capacity of the slag to oxidise constituents of the bath is changed by changing the conditions of the blowing process and is taken into account on the basis of the follow-ing empirically determined, mathematical relationship .alpha.FeO = 1.824 (1.39 - DStB) + 1 (9), wherein .alpha.FeO denotes the factor of (FeO) as against reference conditions and DStB denotes the original lance position in meters above the bath.

11. A method according to claims 1 or 2, wherein correction values for the lime dissolution, for the said capacity of oxida-tion of the slag, and for the oxygen-, iron- and heat-balance are calculated from the deviations observed between the corresponding calculated and observed values, whereby more accurate empirical relationships can be set up.

12. A method according to any one of claims 1 or 2, wherein the empirical relationships are as follows:

log KMn = 6257/ (t + 273.15) - 3.10085 log Kp = 12857/ (t + 273.15) - 9.6437 SiO2 = 2.45 (21.4 SiR + CaOfl . f) 0.7 12-9 (MnR - Mnv) / Mnv = KMn . (FeO)W . S/100 22.9 (PR - PV) / Pv2 = Kp . (FeO)? . S/100 (FeO) = (22.8. CaO/SiO2 - 63.8) . 0.8 and S = 100 (CaOfl + SiO2) / (80 - (FeO) ), wherein KM is the temperature-dependent equilibrium constant of the man-ganese slagging, t is the desired temperature in degrees centigrated, Kp is the temperature-dependent equilibrium constant of the phosphorus slagging, SiR is the Si-content of the pig iron, CaOfl is the quantity of dissolved lime, f is the proportion factor % SiO2 in relation to % CaO in the lime, MnR and PR are the Mn and P content of the pig iron, Mnv and PV are the desired values for manganese and phosphorus in the finished steel, (FeO)w is the effective FeO content in percent of the slag and S is the slag weight in kg/metric ton of pig iron.