Decarburization mechanism of RH-MFB refining process

Journal of University of Science and Technology BejingVolume I3, Number 3, June 2006, Page 218MetallurgyDecarburization mechanism of RH-MFB refining processChuanji Han), Liqun AiP), Bosong Liu2), Jun Zhang"), Yanping Bao", and Kaike Cail)1) Metallurgical and Ecological Engineering School, University of Science and Technology Beijing, Bejjing 100083, China2) Mtallurgy and Energy Sources School, Hebei Polytechnic University, Tangshan 063009, China(Received 2005-06-15)Abstract: The overall decarburization mechanism can be divided into the decarburization in bulk molten steel and float-of each conception the RH-MFB decarburization mathematical model has been built according to the thermodynamic andmass conservation principle, and contributions of every decarburization mechanism were discussed and analyzed.Key words: RH-MFB; refining; decarburization mechanismNomenclatureakc- _Volumetric coefficient of decarburization, m'.min- ;Q- -Circulation of molten steel, t:min-'ako -Volumetric coefficient of deoxidation, m'.min^ |;Qfo - Decarburization amount of bulk molten steel, 10 4%;C-[C%] in molten steel, 10 4%;Qpur- Decarburization amount caused by gas blowing, 10 4%;Dc- -Mass transfer cofficient ofC, cm-st;QS - _Decarburization amount by splashing particles, 10 49%;f- Cofficient of decarburization;R- - -Radius of the particles, mm;FO2- Rate of MFB lance, m' min-!K - -Equilibrium constant of C-O reaction;T- Molten steel temperature, K;W- -Mass of molten steel, t;M- -Molar mass of C, gmol ;M.- Molar mass of O, g:mol );Wan- - Mass of molten steel in the degassing vessel, t;War- -Mass of splashing particles in the degassing vessel, t;N- -Number of particles;a- Coefficient of C-O reaction;0-[0%] in molten seel, 10 4%;Pr- Pressure in degassing vessel, Pa;βOxygen yield;Po - CO partial pressure, Pa;ρ Density of molten steel, tm 3Poo- Pressure of CO in bbles, Pa;1. IntroductionFormer studies were mostly concentrated in the re-fining course on original RH equipment, including re-As a secondary refining technique, RH can exactlystricting step and velocity as well as the correspondingcontrol and quickly achieve the anticipated metallurgi-volume (surface) mass transfer coefficient and so on,cal targets, with a lesser temperature loss, so it is anbut not much in the refining reactions inside the multi-absolutely necessary process of production of low car-functional RH equipment [8-9]. Determination of thebon and ultra low carbon steel. Because the RH-MFBreaction mechanism is particularly important for atechnique is a combination of RH with a multifunction-mathematical model. In most of the former studies, de-al oxygen lance, it can quickly reduce [C] to a very lowcarburization was supposed to happen only in the vac-extent by blowing oxygen into the melt, with high car-uum tank and the positions of escape were the surfacebon and low oxygen under vacuum. In this way, theof argon bubbles, the surface of CO bubbles, and therequested tapping condition can be reached more easilyfree surface of molten steel in the degassing vessel. Inand converter steelmaking is lightened. A lot of studiesfact, during the H refinigreat amount ofhave been carried out on RH, especially on mathemati-particles were s中国煤化工ng vessel. Socal models of the decarburization process, to simulatein this paper, thHC N M H Gmechanism isthe decarburization process [1-7].divided into the decarburization in bulk molten steelCorresponding author: Chuanji Han, E-mail: hanchuanji@metall.ustb.edu.cn.C.J. Han et al, Decarburization mechanism of RH-MFB refining process219and floating of CO against static pressure, the decar-3.3. Decarburization of splashing particlesburization on the surface of argon bubbles and splash-Suppose that all particles are round and [C%] at theing particles. In this way the RH-MFB decarburizationsurface is Cs after the decarburization, equilibriummathematical model has been built to simulate the de-[C%] is C, so the decarburization of splashing parti-carburization process.cles can be expressed as follows:2. Assumption of the model8C_(02C . 20C(0≤t≤0,0≤r≤R) (6;To simplify the model, the following assumptionsOtAt2" r Otwere made:Dc = 5.2x10-3 exp(-11700x4.184.RT 5) [10](7)(1) Molten steel both in the degassing vessel and la-dle were perfectly mixed;Dl600°C = 2.24x10-4.(2) The degassing vessel was the unique decarburi-Corresponding to the initial conditions andzation reaction site;boundary conditions, C(r , 0)=Cv, C(0, t)=Cv, C(R,t)=(3) The contents of C and 0 at the gas-liquid inter-Cs, the resolution of this problem is:face were in equilibrium with CO partial pressure in thegas phase;C(r,t)=CvR(Cs-Cv)x(4) The decarburization rate was controlled by the| (2n+1)R-r「 (2n+1)R+rmass transfer of C and O in molten steel.2erfc- erfc(8)2JDct2√Dct3. Decarburization mechanismSo C。can be obtained:3.1. Decarburization in bulk molten steel and1R 4rur2C(r,t)drfloating of CO against static pressureCe(t)=[。4rπr2drThe CO decarburization rate inside bulk molten steelcan be expressed as follows:n2π2Dct)C,(1)+6(Cv =C()5- lexp((9)_d[%C])π=n^R2dt )= a(CvOvK- Pco)(1)Ignore the variation of Cs with time:Pco=Pv + pgh +(2σ1r)2)Ce(0)=Cs -6(Cv-Cs)5_ 1_n2πDcθThe decarburization quantity in the degassing vesselπ2 22 exp|R2}(1)isThe decarburization amount of every particle in thed[%C])Qoi = Wchi(3)time of 0:3.2. Decarburization on the surface of argon bub-q=Cv-Ce=bles号之-exp_n2π2Dcθ(Cv-Cs) (11)π2台n2RThis mechanism is a kind of compound controllingmodel, which includes three stages: difusion of C andThe total decarburization amount:O onto the surface of argon bubbles, chemical reactionon the surface of argon bubbles, and mass transfer inQ& =Na(Cv-C,)(12)gas. Generally speaking, the equation of decarburiza-tion velocity on the surface of argon bubbles can be4. Establishment and verification of theexpressed as follows:mathematical model of RH-MFB decarburi-Gs; CvOvKfzation processd[%C]\0.0224 .(4)4.1. Establishment of the overall mathematicaldt )2100McWchi_(CvOvKf - Pco)modelDecarburization amount on the surface of argonThe mass equation of C and O both in ladle and RHbubbles in a unit is expressed as follows: .processes can I中国煤化工d[%CQur = Wchi(5)wdCL=Q(C\MHCNMH G(13)lt )2d.220J. Univ. Sci. Technol. Bejing, Vol.13, No.3, Jun 2006350w dOL=Q(Ov-On)(14)■Measureddi300●Calculated(Wai+ wa)9Y=Q(CL-Cv)-(Q% +Qfur +Q& )200(15)50 t100(Wain+Wa)=QY=Q(Or-Ov)-5(M(+Q +5)+1429βxFO2 (16)0)20 25McTimc / minakc(Cv -Cs )/Mc = ako(Ov -03)/Mo(17)Fig. 1. Comparison between calculated and measured re-sults.The C- 0 connection at the interface of gas-liquid:5. Contributions of every decarburizationlg(CsOs / Po)=-(1160/T - 5.997)(18)mechanism to decarburization effectSubscript L, S, V represent ladle, interface, andFig. 2 shows the capability comparison of three de-degassing vessel respectively.carburization mechanisms during the RH process: bulkIf the post combustion is not considered, themolten steel, splashing particles, and the surface of ar-multi-function burner (MFB) oxygen blowing op-gon bubbles. Fig. 3 shows the capability of threeeration does not influence the essence of the reaction amechanisms at the end of the decarburization process.lot, the direct effect is observed only in the increase ofAs Fig. 2 shows, at the beginning of the decarburiza-[O%] in molten steel at the beginning of the treatment.tion, the rates are low because of a slow drop of pres-So during MFB oxygen blowing, the oxygen yield insure; after about 4-5 min, the decarburization rate formolten steel is defined as follows:every mechanism reaches the top then slows downTotal weight of blown oxygen=1 .429x10 3βxFO2xgradually.Table 1 shows the decarburization amount of theblowing time.three decarburization mechanisms. The decarburiza-According to the experience at Baosteel, the freetion process is divided into three stages: 0-5 min is de-oxygen is increased by (1.8-2.2)x10 4% for every 1 m3fined as the early stage, 6-18 min is the medium stageblown oxygen.and 19-20 min the last stage. From the comparison, it is1.429x 103βxFO2xblowing time=300x( 1.8-2.2)xclear the decarburization in bulk molten steel is the10 4%xFO2xblowing time, so β<45%, thus β>45% ismain one in early and medium stages, whereas, the de-accepted in calculation.carburization of splashing particles becomes the lead-ing one in the last stage, the rate of the decarburizationThe Kuwabara expression is adopted to calculate theon the surface of argon-bubbles is small during wholecirculation rate because it takes into account the effectprocess and is reduced with time. The effect of the de-of pressure on circulation,carburization in bulk molten steel is greater than that onQ=114GI/3D4/3 {In(P/P)}I319)the surface of argon bubbles and that of splashing par-ticles in vacuum, and nearly accounts for half of the4.2. Establishment and verification of the mathe-total decarburization amount. But the effect of the de-matical modelcarburization on the surface of argon bubbles and theThe RH-MFB decarburization model is described bydecarburization of splashing particles are also great;formulae (13) to (18). Put formulae (3), (5), (12), (17),especially as one third of the carbon is removed byand (18) into (13)-(16), a quaternion equation is ob-splashing particles with relatively small mass, this alsotained. In this group of equations, Q is calculated withindicates that defining the decarburization on the sur-formula (19). There are four unknown variables CL, OL,face of splashing particles as one of decarburizationCv, and Ov, this constant differential equation groupmechanisms is very reasonable. It can also be seenfrom Fig. 3 that at the end of decarburization, the de-can be resolved by Runge-K utta.carburization capability of splashing particles gradu-Calculated values agree with measured ones perally catches up中国煤化工1 bulk moltenfectly, as shown in Fig. 1, which means that the modelsteel and becolYHC N M H Gdecarburiza-is feasible.tion..C.J. Han et al, Decarburization mechanism of RH-MFB refining process221-2(. Total decarburization rateburization, and decarburization of splashing particles- Decarburization rate of bulk moltenare three decarburization mechanisms, and the decar-- Decarburization rate of splashingburization by splashing particles contributes a lot and4 Decarburization rale of Ar gas inoccupies a chief position at the last stage of the decar-bubbles' surfaceburization process.3|tI0.8-1- Total dearburication ratc1015202- Decarburization rate of bulk molten stccl0.6一3- Decarburication ratc of splashing dropsTime / min4 Dcarburization matce of Ar gas in bubbls'。 surfaceFig.2. Comparison of decarburization amount by three de-0.4-carburization mechanisms.2 0.26. Conclusions(1) The present RH-MFB decarburization model can名0.0I8nicely simulate the actual process of decarburizationand forecast the end carbon of the process.Fig. 3. Comparison of decarburization amount by three de-(2) The decarburization in bulk molten steel, the de-carburization mechanisms at the end of decarburizationcarburization on the surface of argon-bubble decar-process.Table. 1. Comparison of decarburization amount by three decarburization mechanismsRatio of decarburization amount toDecarburization amount / (10 'wt%)the total/ wt%_Total quantity /Stagein bulk(10- *wt%)on the surface of of splashingmoltersurface of of splashingmolten steel argon bubblesparticlesargonsteelbubbles0-5 min169.788.025.356.451.8614.9133.236-18 min129.461.61 3.953.947.6010.7441.65.19-20 min .4.1.82.43.907.3248.78_Whole process303.2151.439.5112.349.9313.0337.04References .[6] M. Kamo, K. Adachi, and M. Nambu, Longer life in RHwith oxygen top blowing system, Steelmaking Conf. Proc,[1] Y. Kita, Refining technology for inerstitial free steel in Ka-80(997), p.483.kogawa works, Steelmaking Conf. 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