UDK 669.1:666.187:669.787 Izvirni znanstveni èlanek
ISSN 1580-2949 MATER. TEHNOL. 35(3-4)105(2001)
V. GROZDANI]: REOXIDATION OF STEEL BY ADDING AN OVERCRITICAL CONCENTRATION OF DEOXIDIZER
REOXIDATION OF STEEL BY ADDING AN OVERCRITICAL CONCENTRATION OF DEOXIDIZER
PONOVNA OKSIDACIJA JEKLA Z DODATKOM NADKRITIÈNE VSEBNOSTI DEOKSIDANTA
Vladimir Grozdani}
University of Zagreb, Metallurgical Faculty, Aleja narodnih heroja 3, 44103 Sisak, Croatia
Prejem rokopisa - received: 2000-09-20; sprejem za objavo - accepted for publication: 2001-06-16
At deoxidation of molten steel using silicon or aluminium appears an paradox. Namely, increasing concentration of deoxidizer in the melt over some critical value results in increase of dissolved oxigen. Thermodynamical conditions for appearance of this phenomenon has been discussed in detail.
Key words: deoxidation, reoxidation, deoxidiser content
Dezoksidacija staljenega jekla s silicijem ali aluminijem lahko privede do paradoksa. S poveèanjem vsebnosti dezoksidanta nad neko kritièno mejo se opaža poveèanje vsebnosti raztopljenega kisika. Analizirani so termodinamièni pogoji, ki privedejo do tega pojava.
Kljuène besede: dezoksidacija, reoksidacija, vsebnost dezoksidanta
1 INTRODUCTION
Deoxidation is the removal of excess oxygen from molten metal. The procedure involves adding materials with a high affinity for oxygen, the oxides of which are either gaseous or readily form slags 1. Thedeoxidation of steel is usually performed by adding Mn, Si and Al, or rarely by adding Cr, V, Ti, Zr and B. The deoxidation of molten steel exhibits a paradox. By increasing the concentration of deoxidizer in the melt over some critical value reoxidation of steel takes place. A few examples of the reoxidation of steel by adding the usual deoxidizers (Si and Al) are examined in this paper.
2 REOXIDATION OF STEELBY ADDING SILICON
Silicon is a strong deoxidizer. If Š%Si\ > 0.2, Š%C] < 0.15, and thetemperatureof themelt is 1873 K the allowing reaction is possible:
ŠSi] + 2Š0] = (Si02)                       (1)
Theequilibrium constant of thereaction (1) is
If thestandard stateis solid silica, then in thethermodynamic calculation for the deoxidation of steel by means of silicon aSi02 is taken as being equal to one. According to Kramarov 2 if theslag is saturated with silica then:
logtfa-0=log------—1-------- = 31000 -1215    (3)
Š%Si]Š%0]  fJl        T
MATERIALI IN TEHNOLOGIJE 35 (2001) 3-4
So the deoxidation ability of silicon depends on its concentration and the temperature of the metal, but also on theactivity aSi02. At high temperature and the some valueof Š%Si], Š%Mn] and Š%C], its deoxidation ability is something smaller than that of carbon, but considerably higher than manganese.
At somepoint during theincreasein theconcentration of silicon in the Fe-Si melt the minimum concentration of oxygen Š%0]min is obtained, any further increase in Š%Si] results in an increase of Š%0]. At first sight this suggests that the product of the activity coefficients (foSi)2fsiSi decrease faster than the increase of Š%Si]. The result of that is that, in agreement with the expression for theequilibrium constant (2), theextremevaluefor the concentration of silicon Š%Si]extr. corresponds to the minimum concentration of Š%0]min..
When calculating thecritical valueof Š%Omin. and Š%Si]extr. it is necessary to express equation (2) in the following way
ln ŠeO ] = - ln a ,i0  - - ln K „ „ - - ln Š % Si ] -
L            2       a°2     2                  2
--lnf?-lnfnSi                          (4)
2
The logarithms of the coefficients of activity are:
ln f0Si =2.3 log fo  = 23 e o Š% Si]              (5)
ln /I =2.3 log f* = 2.3 e f Š% Si]              (6)
and the interaction parameters are e0Si = -0.133 and eSiSi = 0.098. The interaction parameters e0° and eSi° are neglected because the concentration of oxygen is relatively small (log fo° « 0, log fSi° = eSi°Š%O] « 0,
105
V. GROZDANI]: REOXIDATION OF STEEL BY ADDING AN OVERCRITICAL CONCENTRATION OF DEOXIDIZER
and follows  f0° ˜ 1 and fSi° ˜ 1), and theactivity asi02=1.
To obtain Š%Si]extr. it is necessary to take the first derivative of the concentration of oxygen divided by the concentration of silicon and set the equation equal to zero under specific conditions:
?lnŠ%0]            1
Si
2,3 — 2
2,3e; =0
?Š%Si]        2Š%Si]
The second derivative is greater than zero: ?2 lnŠ%0]             1
?Š%Si]2          2Š%Si]2
>0
(7)
(8)
Consequently, the function has a minimum. The concentration of silicon at which Š%O] is a minimum:
Š%Si]
 e
1
-2,3e*'-4,6È'
(9)
On thebasis of thequoted valueof theinteraction parameters for T=1873 K and aSiO2=1 Š%Si]extr. a valueof 2.59% is obtained. In that case fSiSi = 1.794 and fOSi = 0.452. For steel melts there is also the influence of the other elements on fSi and fO. At 1873 K theequilibrium constant KSi-O = 25099. Theminimum concentration of oxygen in the melt is:
Š%Si]
 ri
1
JKSiŠ%si]f*(f0si)2
(10)
By inserting reasonable values into equation (10) a minimum concentration of oxygen Š%0]min = 6.5×103 % is obtained.
3 REOXIDATION OF STEELBY ADDING
ALUMINIUM
The deoxidation of steel by aluminum proceeds according to thereaction:
2ŠAl] + 3Š0] = (Al203)                   (11)
The equlibrium constant may be determined from:
logKA
log
Š%A/]2Š%0]3/i/
3
23
O
(12)
At 1873 K theequilibrium constant KA1.0 = 4.9×1013. In thecalculation it is assumed that aAi2œ = 1, because at a small valueof Š%Al] (0.001-0.01) theequlibrium concentration (%FeO) in the slag is neglected, so the products of deoxidation are solid Al2O3 or an oxide phase saturated with aluminia. The coefficient of activity of aluminum fM = f£ f° ˜1, and the coefficient of oxygen f0 = f° f„ ˜ f£, which is an approximation, because the influence of the other elements present on/0
is very strong, and fO is <1. The equilibrium concentration of oxygen in the melt may be expressed with:
Š%0]
1
3Š%A]2(C)3
(13)
K
An analysis of theequation (13) shows that at some increment of the concentration of aluminium the activity factor fOAl decreases faster than Š%Al]2/3 increases. The result is that at somevalueof Š%Al] the Š%O]min. is obtained. A further increase in Š%Al] results in an increase of Š%O].
To obtain Š%Al]extr., thefirst derivativeof the logarithm of the concentration of oxygen divided by the concentration of aluminum must be set equal to zero:
1
?2 lnŠ%0] = ?Š%Al]          3Š%Al]
23 e o =0
Š%AÏ]
 e
0.29
(14)
(15)
According to Kulikov 3, interaction parameter e0Al = -1.17, which corresponds to Š%Al]extr. = 0.248. According to formula (13) the minimum concentration of oxygen Š%O]min = 1.35×10-4 % is obtained.
4 CONCLUSION
During the deoxidation of steel with common deoxidizers, Si and Al, a paradox occurs. For a particular concentration of deoxidizer there exists a minimum concentration of oxygen dissolved in the steel which increases with an increase in the concentration of the deoxidizers. This is explained by the fact that in the equation for the equilibrium constant the factor of activity decreases faster than the increment of the value of the concentration of dissolved deoxidizer. For the deoxidation of steel with silicon the critical concentration of Si is 2.59%, which corresponds to a minimum concentration of oxygen of 6.5×10-3 %. In thecaseof aluminum, the critical concentration of Al dissolved in the steel is 0.248%, which corresponds to a minimum concentration of oxygen of 1.35×10-4 %, under the defined conditions (T=1873 K, aSiO2=1, aAl2O3=1, neglected influence of other elements present in the melt).
5 REFERENCES
1 D. C. Gould, AFS Metalcasting Dictionary, American Foundrymen’s Society, Des Plaines, 1968.
2 A. D. Kramarov, A. N. Sokolov, Elektrometallurgija stalji i ferro-splavov, Metallurgija, Moskva, 1976.
3 I. S. Kulikov, Raskislenije metallov, Metalurgija, Moskva, 1975.

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