Decarburization mechanism of RH-MFB refining process

Journal of UniverSitg of Science and Technologl B e g Volume 13, Number 3, June 2006, Page 218

Metallurgy

Decarburization mechanism of RH-MFB r e f h g process Chuanji Han”, Liqun Ai2), Bosong Liu2’,Jun Zhang”, Yanping Baol’, and Kaike Cai” I) Metallurgical and Ecological Engineering School, University of Science and Technology Beijing, Beijing 100083, China 2) Metallurgy and Energy Sources School, Hebei Polytechnic University, Tmgshan 063009, China (Received 2005-06-15)

Abstract: The overall decarburization mechanism can be divided into the decarburization in bulk molten steel and floating of CO against static pressure, the decarburizationon the surface of argon bubbles and splashing particles. On the basis of each conception the RH-MFB decarburizationmathematical model has been built according to the thermodynamicand mass conservation principle, and contributions of every decarburization mechanism were discussed and analyzed.

Key words: RH-MFB; refining; decarburization mechanism

Nomenclature akc-Volumetric coefficient of decarburization,m3.min-’; ako-Volumetric coefficient of deoxidation, m3.mid; C+C%] in molten steel, 10-4%; D d a s s transfer coefficient of C, cms-’; f-coefficient of decarburization; FO,-Rate of MFB lance, m’min-’; K -EQuilibrium constant of C - 0 reaction; Mc-Molar mass of C, gmol-’; kfo-Molar mass of 0, g.mo1-l; N-umber of particles; O+O%] in molten steel, lo4%; P-essure in degassing vessel, Pa; P& 20 partial pressure, Pa; P,,-Pressure of CO in bubbles, Pa;

1. Introduction As a secondary refining technique, RH can exactly control and quickly achieve the anticipated metallurgical targets, with a lesser temperature loss, so it is an absolutely necessary process of production of low carbon and ultra low carbon steel. Because the RH-MFB technique is a combination of RH with a multifunctional oxygen lance, it can quickly reduce [C] to a very low extent by blowing oxygen into the melt, with high carbon and low oxygen under vacuum. In this way, the requested tapping condition can be reached more easily and converter steelmaking is lightened. A lot of studies have been carried out on RH, especially on mathematical models of the decarburization process, to simulate the decarburization process [ 1-71. Corresponding author: Chuanji Han, E-mail: [email protected]

Q-Circulation of molten steel, tmin-’; QLi -Decarburization amount of bulk molten steel, 1 P % ; Q&-Decarburization amount caused by gas blowing, 1W%; Qki 4ecarburization amount by splashing particles, 1 P % ; R-Radius of the particles, nun; T-olten steel temperature, K , W-Mass of molten steel, t; W,,--hlass of molten steel in the degassing vessel, t; Wb+4ass of splashing particles in the degassing vessel, t; M o e f f i c i e n t of C-0 reaction; &Oxygen yield; w e n s i t y of molten steel, t.m-3.

Former studies were mostly concentrated in the refining course on original RH equipment, including restricting step and velocity as well as the corresponding volume (surface) mass transfer coefficient and so on, but not much in the refining reactions inside the multifunctional RH equipment [8-91. Determination of the reaction mechanism is particularly important for a mathematical model. In most of the former studies, decarburization was supposed to happen only in the vacuum tank and the positions of escape were the surface of argon bubbles, the surface of CO bubbles, and the free surface of molten steel in the degassing vessel. In fact, during the RH refining process, a great amount of particles were splashed inside the degassing vessel. So in this paper, the overall decarburization mechanism is divided into the decarburization in bulk molten steel

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C.J. Hun et ul., Decarburization mechanism of RH-MFB refining process

and floating of CO against static pressure, the decarburization on the surface of argon bubbles and splashing particles. In this way the RH-MFB decarburization mathematical model has been built to simulate the decarburization process.

2. Assumption of the model

3.3. Decarburization of splashing particles Suppose that all particles are round and [C%] at the surface is C, after the decarburization, equilibrium [C%] is Ce, so the decarburization of splashing particles can be expressed as follows:

ac

To simplify the model, the following assumptions were made: (1) Molten steel both in the degassing vessel and ladle were perfectly mixed;

(2)The degassing vessel was the unique decarburization reaction site; ( 3 ) The contents of C and 0 at the gas-liquid interface were in equilibrium with CO partial pressure in the gas phase;

,(O
at

11700~4.184 Dc =5.2~1O-~exp(- RT [lo1

(6)

(7)

DAW'" = 2 . 2 4 ~ 1 0 ~ .

Corresponding to the initial conditions and boundary conditions, C(r, O)=Cv,C(0, t)=Cv, C(R, r)= C,, the resolution of this problem is: r

(4)The decarburization rate was controlled by the mass transfer of C and 0 in molten steel.

3. Decarburization mechanism

So C, can be obtained:

3.1. Decarburization in bulk molten steel and floating of CO against static pressure

Ce (t) = f4nrzC(r,t)dr -

The CO decarburization rate inside bulk molten steel can be expressed as follows:

(-7)

4x1-2dr

(9)

= a ( C v O v K - Pco)

Ignore the variation of C, with time:

I

Pco = Pv + p g h + ( 2 o / r )

The decarburization quantity in the degassing vessel is

( d'yl)

Q& = W . -chi

(3)

I

3.2. Decarburization on the surface of argon bubbles This mechanism is a kind of compound controlling model, which includes three stages: diffusion of C and 0 onto the surface of argon bubbles, chemical reaction on the surface of argon bubbles, and mass transfer in gas. Generally speaking, the equation of decarburization velocity on the surface of argon bubbles can be expressed as follows: Gs CvOvKf 0.0224 wchi (CvOvKf - Pco) 1OOM c

The decarburization amount of every particle in the time of 8:

q=cv-c, =

The total decarburization amount:

4. Establishment and verification of the mathematical model of RH-MFB decarburization process

(4)

4.1. Establishment of the overall mathematical model

Decarburization amount on the surface of argon bubbles in a unit is expressed as follows:

The mass equation of C and 0 both in ladle and RH processes can be expressed as follows:

2

-

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J. Univ. Sci. Technol. BeQing, Vo1.13, No.3, Jun 2006 350 I

I

I

rn Measured

3001

-

0

Calculated

0

I001

50 -

I I

akc(Cv - C s ) I M c =ako(Ov - 0 s ) l M o

,

I

(17)

The C-0 connection at the interface of gas-liquid: lg ( CSOS/ P& ) = - (1 160/ T - 5.997)

(18)

Subscript L, S, V represent ladle, interface, and degassing vessel respectively.

5. Contributions of every decarburization mechanism to decarburization effect

Fig. 2 shows the capability comparison of three decarburization mechanisms during the RH process: bulk molten steel, splashing particles, and the surface of arIf the post combustion is not considered, the gon bubbles. Fig. 3 shows the capability of three multi-function burner (MFB) oxygen blowing opmechanisms at the end of the decarburization process. eration does not influence the essence of the reaction a As Fig. 2 shows, at the beginning of the decarburizalot, the direct effect is observed only in the increase of tion, the rates are low because of a slow drop of pres[O%] in molten steel at the beginning of the treatment. sure; after about 4-5 min, the decarburization rate for So during MFB oxygen blowing, the oxygen yield in every mechanism reaches the top then slows down molten steel is defined as follows: gradually. Total weight of blown oxygen=l.429~lO-~/?xFO,x Table 1 shows the decarburization amount of the blowing time. three decarburization mechanisms. The decarburizaAccording to the experience at Baosteel, the free tion process is divided into three stages: 0-5 min is deoxygen is increased by (1 3-2.2) x104% for every 1 m3 fined as the early stage, 6- 18 min is the medium stage blown oxygen. and 19-20 min the last stage. From the comparison, it is 1.429~1 O~3~xF0,xblowing time=300x( 1.8-2.2)~ clear the decarburization in bulk molten steel is the main one in early and medium stages, whereas, the de104%xF0,xblowing time, so p=45%, thus /?=45% is carburization of splashing particles becomes the leadaccepted in calculation. ing one in the last stage, the rate of the decarburization The Kuwabara expression is adopted to calculate the on the surface of argon-bubbles is small during whole circulation rate because it takes into account the effect process and is reduced with time. The effect of the deof pressure on circulation, carburization in bulk molten steel is greater than that on the surface of argon bubbles and that of splashing par/P2)}1'3 (19) Q=114G'~3D4'3{ln(Pl ticles in vacuum, and nearly accounts for half of the 4.2. Establishment and verification of the mathetotal decarburization amount. But the effect of the dematical model carburization on the surface of argon bubbles and the decarburization of splashing particles are also great; The RH-MFB decarburization model is described by especially as one third of the carbon is removed by formulae (13) to (18). Put formulae (3), (5), (12), (17), splashing particles with relatively small mass, this also and ( I 8) into (13)-( 16), a quaternion equation is obindicates that defining the decarburization on the surtained. In this group of equations, Q is calculated with face of splashing particles as one of decarburization formula (1 9). There are four unknown variables C,, OL, mechanisms is very reasonable. It can also be seen Cv, and Ov, this constant differential equation group from Fig. 3 that at the end of decarburization, the decan be resolved by Runge-Kutta. carburization capability of splashing particles graduCalculated values agree with measured ones perally catches up with or even exceeds that in bulk molten fectly, as shown in Fig. 1, which means that the model steel and becomes a dominant factor of decarburizais feasible. tion.

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C.J. Hun et ul., Decarburization mechanism of RH-MFB refining process

2o

burization, and decarburization of splashing particles are three decarburization mechanisms, and the decarburization by splashing particles contributes a lot and occupies a chief position at the last stage of the decarburization process.

1- Total decarburization rate

I

3- Decarburization rate of splashing

0

5

10 Time / min

15

E

1- Total decarbunzation rate

2 0.6

3- Decarburization rate of splashing drops 4- Decarburization rate of Ar gas in bubbles'

,

.i

20

0

Fig. 2. Comparison of decarburizationamount by three decarburization mechanisms.

6. Conclusions

5

(2) The decarburization in bulk molten steel, the decarburization on the surface of argon-bubble decar-

surface 4

i

.-L

2

( I ) The present RH-MFB decarburization model can nicely simulate the actual process of decarburization and forecast the end carbon of the process.

.

0.2

0" 0.0'

18

I

I

19 Time / min

20

Fig. 3. Comparison of decarburization amount by three decarburization mechanisms at the end of decarburization process.

Table. 1. Comparison of decarburization amount by three decarburization mechanisms Decarburization amount / ( 104wt%) Stage

Total quantity / (lOAwt%)

0-5 min 6-18 niin 19-20 min Whole process

169.7 129.4 4.1 303.2

in bulk molten steel

on the surface of argon bubbles

of splashing particles

88.0 61.6 1.8 151.4

25.3 13.9 0.3 39.5

56.4 53.9

References Y. Kita, Refining technology for inerstitial free steel in Kakogawa works, Steelmaking Con5 Proc., 73( 1990), p.79. Koji Yamaguchi, Effect of refining conditions for ultra low carbon steel on decarburization reaction in RH degasser, ISIJ Int., 32( 1992), No. 1, p. 126. M.S. Chiang, C.T. Lin, C.L. Chou, et al., Application of off-gas analysis for RH decarburizationprocess, SEAISI Q., 26(1997), No.2, p.27. M.Yano and S. Kitamura, Improvement of RH refining technology for the production of ultra low carbon and low nitrogen steel, Steelmaking Con$ Proc., 77( 1994), p.117. y. Kate, Development of rapid decarburization techno@Y by combined process of converter and RH degasser for ultra low carbon steel, Kawasaki Steel Tech. Rep., 32( 1995). No.5, p.25.

2.0 112.3

Ratio of decarburization amount to the total / wt% on the in bulk surface of of splashing molten argon particles steel bubbles

5 1.86 47.60 43.90 49.93

14.91 10.74 7.32 13.03

33.23 41.65 48.78 37.04

[61 M. Kamo, K. Adachi, and M. Nambu, Longer life in RH with oxygen top blowing system, Steelmaking Con$ Proc., 80(1997), p.483. [71 Didier Huh, et a / . , Kinetics of vacuum decarburization of ultra low carbon steels, Steelmaking Conference Proceedings, 2001, p.601. [81 J.H. Wei and N.W. Yu, Study on mathematical modelling for RH and RH-KTB refining process of molten steel: mathematical model of the processes, J. Baotuo Univ. Iron Steel Technol. (in Chinese), 21(2002), No.3, p.242. 191 Tatsuro Kuwabara, Investigation of decarburization behavior in RH-reactor and its operation improvement, Trans. ISM, 28(1988), p.305. [ 101 J.X. Chen, Common Diagram and Data Manual of Steelmaking (in Chinese), Metallurgy Industry Press, Beijing, 1984, p.654.