Slag formation during the steel refining process in a ladle furnace combined with the vacuum treatment

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1 Slag formaton durng the steel refnng process n a ladle furnace combned wth the vacuum treatment Jan FALKUS, Katarzyna IŁKOWSKA-PISZCZEK, Lechosław TRĘBACZ, Paweł DROŻDŻ, Tomasz KARGUL Jacob LAUT and Alojz ROZAN ) AGH Unversty of Scence and Technology, Faculty of etals Engneerng and Industral Computer Scence, A. ckewcza 0 Ave, Kraków, Poland; ) Ljubljana Unversty, Ljubljana, Slovena; ) etal Ravne,,Koroška cesta 4, 90 Ravne na Koroškem, Slovena; Abstract: The process of slag formaton and the change of ts chemcal composton durng the steel refnng process s determned by the composton of the slag-formng materals used, the effcency of the furnace slag cutoff and the knetcs of reactons on the metal slag nterface. It s possble to consder all these factors n theoretcal reasonng by usng the so-called hybrd model for calculatons. A concept for modellng the changes n the chemcal composton of metal and slag has been elaborated. The tank theory combned wth the thermodynamc model of reactons on the metal bath-slag and metal bath-gas phase nterfaces were used to develop a hybrd model of the process. The tank theory was used to descrbe the strrng of the metal bath, and ths made t possble to smulate the process n real tme. Some theoretcal bass and the verfcaton of the model were presented, nter ala, durng the last, IX Internatonal Conference on olten Slags n Santago.Ths paper presents a contnuaton of prevous work, addtonally ncludng refnng processes n a vacuum. Specal attenton s pad to the reactons on nterfaces between metal and slag as well as those between metal and the gas phase. In the hybrd model these reactons are smulated usng thermodynamc calculatons, takng nto account the knetcs of the aforementoned reactons. The processes knetcs can be determned by the mxng model based on the tank theory. Another mportant mprovement of the hybrd model s the extenson of the chemcal composton of both metal bath and slag beng consdered n computatons. It enables usng the suggested model to smulate the alloy steel refnng process. The summary presents the verfcaton based on ndustral melts. The verfcaton was conducted for a technologcal lne consstng of a 40-ton ladle furnace workng n parallel wth a vacuum chamber. The typcally stored ndustral database was complemented wth addtonal measurements necessary to conduct the verfcaton. Key words: Process Control, Hybrd odel, Steel Refnng Slag. Introducton Nowadays, steel refnng processes carred out n the ladle consttute a very mportant stage n the technologcal producton lne. These processes are hghly vared and can be dvded nto two major groups: Steel refnng processes runnng under the atmospherc pressure; Steel refnng processes runnng n a vacuum. The subject of ths publcaton s a study of changes takng place n ladle slag durng processes runnng under a reduced pressure. In the majorty of cases, vacuum refnng s ntended to progress wthout the presence of slag, as ths

2 makes steel degassng much easer. However, a certan number of vacuum processes s carred out n two stages, where vacuum refnng represents only a fragment of the technologcal lne for secondary refnng. In ths case, t s nterestng to have nformaton about the mpact of reduced pressure on changes n slag contents and the mpact of these changes on refnng reactons. Ths s partcularly mportant n the producton of alloy steel grades contanng a hgher number of addtves. The research conducted makes t possble to assess changes to the chemcal composton of system phases takng nto account the knetcs of the process carred out. Hence the presented results are not tradtonal calculatons of equlbrum states. The hybrd model used for calculatons had prevously been valdated for a ladle furnace. Key parameters of the model were also establshed n experments conducted on cold models. Ths concerns the percentage share of tanks nto whch the ladle was dvded. The scope of research carred out ncluded two subjects. The frst was to verfy the proposed model n ndustral condtons and assess whether t generated accurate projectons. The second was a seres of vrtual experments to assess the mpact of pressure on changes n the chemcal composton of refnng slag.. Vacuum steel refnng process The proposed soluton of the problem of dynamcally controllng the vacuum steel refnng process s desgned for an electrc furnace shop producng hgh-alloy steel grades. In ths case, the secondary metallurgy stage s completed at two stands, the frst of whch works as a tradtonal ladle furnace, and the second s a vacuum chamber. Both at the frst and the second stage, the metal bath s blown wth argon. The chemcal composton of the bath s manly adjusted n the early perod, when addtves are fed n under atmospherc pressure. At that tme, samples of metal and slag are collected and oxygen actvty s measured. Before vacuum refnng starts, the bath composton should be brought very close to the target composton. Refnng n the ladle furnace lasts between 40 mnutes and hour. Then the bath s refned n a vacuum chamber where t contnues to be mxed by argon blown n at the bottom. Vacuum refnng nfluences not only the chemcal composton of the metal, but also partly changes the composton of slag. go and SO are the components whose contents n slag may be sgnfcantly reduced. Ths automatcally causes the proporton of the remanng consttuents, manly CaO and Al O, to rse. However, t should be noted that the fnal composton of ladle slag after vacuum refnng probably does not reach the equlbrum state and s greatly dependent on the knetcs of processes runnng on the metal/slag nterface.. The hybrd model of vacuum steel refnng The dea of the knetc model of vacuum steel refnng s based on the soluton prevously used for the ladle furnace. [,] The essence of ths soluton s the assumpton that to descrbe process knetcs, one has to use the mxng model, n whch the soluton can be acheved wthn a tme shorter than the actual refnng duraton. The key task s to select the bath mxng model n whch, subsequently, a thermodynamc model wll be embedded, wth the latter enablng the calculaton of the local equlbrum that s reached on the metal/slag nterface.

3 In the soluton adopted, the selected bath mxng model s based on the tank theory. The dea of ths model s based on the assumpton that the ladle can be splt nto so-called tanks, whch are assgned the followng characterstcs: [] The volume of the tank does not change over tme; There s no concentraton gradent wthn the tank; The chemcal composton of the tank changes n steps after the pre-defned Δt tme expres. To valdate the mxng model based on the descrbed theory, the volume of ndvdual tanks and the value of mass fluxes exchanged between them must be defned. The qualty of ths verfcaton s of key mportance for the precson of the projecton generated by the mxng model. Fg. shows the adopted splt of the steel-makng ladle nto tanks. It s noteworthy that the process of blowng argon nto steel s of decsve mportance for ths splt. [4,5] Slag Tank I, Tank II, Tank III, Fg. A dagram of splttng the ladle nto tanks For the splt defned as above, the system of dfferental equatons descrbng the mxng process takes the followng form: m dm = m( + m m + dm = m m( m( m( + () dm = m( m( m m + m j mass of component n tank j, [t] j mj mass of tank j, [t] flow ntensty of metal between the tank and and j [t/mn]

4 The thermodynamc model embedded n the mxng model allows a local equlbrum to be determned on the metal/slag nterface for freely defned chemcal compostons of metal and slag. The tool used to create the thermodynamc model was FactSage, whch allows a subprogram to be generated n the form of a fle that can be embedded n a propretary applcaton. The essence of the proposed soluton s explaned by Fg., n whch the adjacent metal and slag layers achevng equlbrum are marked. The proper defnton of the thckness of both these layers represents the next step determnng the accuracy of model calculatons. { β } α Reaktor I Fg. A dagram of the metal/slag nterface wth the adjacent layers marked [6] In accordance wth the research work completed before, the thckness of the metal and slag layers whch acheve equlbrum should be establshed takng nto account the tme consdered. In the case of numercal calculatons, ths s the computatonal tme step Δt. The value of the α parameter s determned from relatonshp () α = k 00% () I α the coeffcent defnng the percentage of the mass of the frst tank that s transferred to the thermodynamc model, [%] k dmensonless coeffcent, 0. The value of the β parameter was determned based on the precse observaton of changes n slag composton durng expermental research melts. Embeddng the thermodynamc model n the bath mxng model conssts n modfyng the frst equaton of the system of equatons (). Then, the equaton takes the followng fnal form: dm m m = m + + m eq () where:

5 m = m α m ( ) (4) eq eq t m eq ( - the equlbrum content of component calculated by the thermodynamc model, [g] At ths stage, t should be noted that the hybrd model of steel refnng n a ladle, defned above, makes t possble to account for the pressure under whch the analysed process runs. As a result, ths model s sutable for smulatng processes runnng n a vacuum. Ths capablty s due to the need to defne the system pressure n the thermodynamc model. As the thermodynamc model s called up at each tme step of the calculaton, t s also possble to account for the system pressure varablty durng the process. Ths s of major mportance for the realstc representaton of the condtons n whch vacuum steel refnng occurs. 4. Valdatng the steel refnng model based on an ndustral process The model presented above has been valdated for an electrc furnace shop at whch the melt weght amounted to 40 tons. A major challenge was the need to account, n the thermodynamc model, for a wde range of elements, due to the broad range of alloy steel grades produced. In a sense, ths ntroduced tme restrctons n the model, as the procedure of establshng equlbrum states determnes the computaton duraton. In the fnal verson, the actual duraton of computatons represented some 0% of the real process tme. For valdaton purposes, all parameters of the model were determned, and ther values are presented n Table. Table. Key parameters of the model Parameter name Unt Value ass share of tank I % 5 ass share of tank II % 5 ass share of tank III % ass flux between tanks I and II kg/mn 795 ass flux between tanks I and III kg/mn 80 ass flux between tanks II and III kg/mn 894 Coeffcent α value Coeffcent β value Calculaton tme step Δt s 5 The values of man model parameters presented n the table allowed the projecton accuracy to be checked usng the precse recordng of the melt progress. Fg. shows a comparson of smulaton results wth the actual change of alloy element contents n the metal bath. Fg. 4, n turn, presents results of smulatons of changes n the slag chemcal composton for the same melt and the actual changes durng the process. Results presented n Fgures a,b and 4 concern one of the randomly selected valdaton melts. The accuracy acheved s satsfactory and allows the model to be used for vrtual experments.

6 6 5 S, Cr, o [%] 4 S (smulaton) Cr (smulaton) o (smulaton) S (real value) Cr (real value) o (real value) Tme [mn] Fg a. Course of changes S, Cr, o n the chemcal composton of the metal bath 0,5 0,4 C, V [%] 0, 0, C (smulaton) V (smulaton) C (real value) V (real value) 0, Tme [mn] Fg b. Course of changes C and V n the chemcal composton of the metal bath

7 70 60 CaO, AlO, SO, go [%] CaO smulaton AlO smulaton SO smulaton go smulaton CaO (real value) AlO (real value) SO (real value) go (real value) Tme [mn] Fg 4. Course of changes n the chemcal composton of slag 5. Impact of vacuum on changes n the chemcal composton of refnng slag The attempt to assess the mpact of vacuum refnng on changes n the chemcal composton of the slag phase was made usng data from the real process. The mpact of vacuum on go and SO contents, alkalnty and the annesmann s coeffcent was evaluated. In each analysed case, two vacuum degrees,.e and 0.0 bar, were assumed for calculatons. The slag composton change was analysed both for the stage of refnng under atmospherc pressure and of refnng n a vacuum. Fg. 5 shows changes n the go content of slag for varous ntal quanttes of ths oxde and two vacuum degrees. Smlarly, Fg. 6 shows the same relatonshps for SO. Fg. 5 Changes n the go content of slag for varous ntal quanttes of ths oxde and two vacuum degrees

8 Fg 6. Changes n the SO content of slag for varous ntal quanttes of ths oxde and two vacuum degrees The smulaton carred out also allows generatng the relatonshp between another two parameters bascty and so called annesmann s coeffcent - as a functon of tme for vacuum refnng. It s calculated usng the followng formula: % CaO W = % SO % Al O (5) Its value should stay wthn the range of 0, ± 0,4. The mpact of vacuum on the change n the W (annesmann s coeffcen s shown n Fgure 7. 0,4 annesmann's coeffcent 0,5 0, 0,5 0, Tme [mn] Fg 7. annesmann s coeffcent

9 Fulfllng the condton (5) confrms the fact, that the chemcal composton of refnng slag changes n the selected area n the Fgure 8. Fg 8. Phase dagram CaO- Al O - SO 6. Dscusson As expected, vacuum steel refnng n a ladle also mpacts the slag phase, causng ts chemcal composton to change. Although ths mpact s not decsve for the course of the steel degassng process, t causes a new metal/slag equlbrum state to be acheved. A very mportant ssue n calculatons s to account for the fact that solds precptate n slag. For obvous reasons the mxng model accounts only for lqud phases. The thermodynamc model, n turn, allows all phases present n the system to be calculated. From the formal perspectve, the problem descrbed s numercal n nature. Not accountng for precptates would cause errors to be generated n the melt mass balance. We propose solvng ths problem n the model usng a smplfed declaraton of permssble products of chemcal reactons. The error resultng from ths s many tmes smaller than the error caused by gnorng precptates. In ths case, the alternatve soluton consstng n feedng nformaton about the mass of all component comng from the precptates nto the module calculatng the chemcal balance would not be effcent. The number of precptates that can potentally form n slags of alloy steels s very hgh, so an attempt to develop a unversal tool would lengthen the computng tme too much. At present, the model computaton tme averages 5% of the duraton of the real process. The qualty of the projecton from the proposed model depends on the relablty of nput nformaton, such as: The ntal chemcal composton and the mass of slag; The ntal chemcal composton and the mass of the metal bath;

10 The control of the argon flow ntensty; The mass of addtves and the tme of addng them. The model correctly projects the go and SO losses as well as the course of changes n the content of Al O and CaO. Projectons of the chemcal composton of the metallc phase are also accurate. 7. Conclusons The proposed model represents a unversal tool allowng ladle processes, whether carred out n a vacuum or under atmospherc pressure, to be smulated. The structure of the model allows t to be valdated for very dfferent varetes of the process. The man advantages of the proposed soluton nclude:. The model computaton tme s sgnfcantly shorter than the real process tme;. The model allows accountng for the dynamc change of parameters of the process conducted (pressure, temperature, argon flow ntensty);. Vacuum refnng can sgnfcantly mpact the fnal, equlbrum chemcal composton of slag; 4. go SO are the slag consttuents most senstve to loss durng vacuum refnng. The change n the percentage contents of the remanng components s due to changes n proportons caused by the aforementoned loss and the Al O nclusons flowng out. Acknowledgement We would lke to acknowledge the support of ths work by the nstry of Scence and Hgher Educaton, Poland (AGH agreement No ). References []T. Kargul and J. Falkus: A hybrd model of steel refnng n the ladle furnace, Steel Research, 8 (00), [] J. Falkus, T. Kargul and P. Drożdż : A new method for determnng chemcal composton of refnng slag n the ladle furnace, Proceedngs of 7th Internatonal Conference on olten Slags, Fluxes and Salts, Santago, 009, Chle. [] O. Levenspel: Chemcal Reacton Engneerng, New York London Sydney Toronto, Jonh Wley & Son, Inc. 97. [4] J. Falkus: Physcal and athematcal odelng of etal Bath xng Processes, Uczelnane Wydawnctwa Naukowo-Dydaktyczne AGH, Krakow 998, 4. [5] F. Oeters, W. Pluschkell, E. Stenmetz and H. Wlhelm: Flud flow and mxng n secondary metallurgy, Steel Research, 59 (988) vol. 5, 9. [6] T. Kargul: Elaboraton of Hybrd odel of Steel Refnng Process for etallurgcal Technology Assessment, Ph.D thess, AGH Unversty of Scence and Technology, Krakow, 009, 6.