THE IMPACT OF CHEMICAL COMPOSITION CHANGE OF MOLD FLUX APPLIED IN LOW CARBON STEEL IN THE WEAR OF SEN

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1 1 THE IMPACT OF CHEMICAL COMPOSITION CHANGE OF MOLD FLUX APPLIED IN LOW CARBON STEEL IN THE WEAR OF SEN A. A. Diego (1), L. M. Demuner (2), G. R. T dos Sntos (2), K. T. de Mtos (2), M. C. C. Bezerr (1), C. A. G Vldres (1), R. de V. Dud (1), F. Vernilli Jr. (3) (1) Croox, Brzil (2) ThyssenKrupp CSA, Brzil (3) University of São Pulo EEL-USP, Brzil Contct dt: Alexndr de Almeid Diego, , lexndr.diego@croox.com Summry The mould flux used in the Continuous Csting of steel is synthetic slg pplied on the surfce of molten steel with the functions of therml insultion, prevention of steel reoxidtion nd inclusions sorption. In the gp etween the mold nd the solidifying steel shell, mould flux lurictes nd controls the het trnsfer. The use of mold flux cuses wer in the region clled slg nd t the SEN s the result of cyclic rections etween the steel, slg nd refrctory. As consequence, it reduces the csting time nd the productivity of the process. In this pper, it ws evluted the impct of the chemicl composition chnge in the mold flux pplied on low cron steel in order to reduce SEN wer. First of ll, oth were chrcterized in order to compre physicochemicl properties. Second, it ws followed the performnce during the industril tril of oth in the sme conditions, which ws collected ech slg film etween the mold nd strnd nd lso the SEN smple. The slg film ws nlyzed y opticl microscopy nd the wer profile of slg nd ws evluted y scnning electron microscopy nd energy dispersive scnning. The result otined presented etter performnce nd decrese of wer refrctory. Key Words Mould flux, sumerged entry nozzle, luriction, erosion nd Continuous Csting of Steel. Introduction In the process of Continuous Csting of Steel, the SEN crries the steel from tundish to the mold, prevents the re-oxidtion of steel nd influences the flow control. The SEN is composed of two prts, the ody of Al 2 O 3 -Grphite nd slg-metl interfce region of ZrO 2 -Grphite, clled slg nd. As demonstrted y [1] the region of slg-metl interfce hs higher wer thn the slg or metl individully. So the slg nd must support this ttck, in order to promote the sfety nd productivity of the process. The function of grphite phse is to increse therml shock resistnce, frcture energy nd resistnce to ttck y slg. The disdvntge is the high susceptiility to oxidtion t high tempertures tht cn decrese the mechnicl strength due to incresed porosity, which promotes reduction in resistnce to ir nd slg penetrtion. In order to keep the cron s long s possile in the structure of the refrctory it mkes use of ntioxidnts s metllic silicon or SiC, mong others, tht rects preferentilly with cron nd reduce the speed of consumption [2]. The ZrO 2 phse hs high resistnce to corrosion nd is stilized y MgO, Y 2 O 3 or CO in order to minimize the therml shock. Studies s [3] nd [4] concluded tht the ntioxidnt my destilize the ZrO 2 nd improving the erosion of the refrctory. The ffinity etween F - nd C 2+ [5] cn contriute to erosion y destilizing Zr 2 O phse. Figure 1. Schemtic representtion in which locl corrosion proceeds [6]. The wer mechnism ws descried y [6] nd discussed in the lter works [7], [8], [9]. Figure 1 descries the erosion process in the interfce slg / metl. Figure 1 shows the oxide phse wetted y the slg, following y dissolution preferentilly in the slg nd fter exposing grphite phse. Figure 1 shows grphite phse wetted y steel tht dissolves it preferentilly in the steel nd exposes the oxide phse nd the cycle egins gin. Muky [6] explined in his work tht in the csting of high cron steel the rection etween refrctory-metl is the rte controlling step, nd for low cron steel the rte controlling step is etween refrctory-slg. Bsed on Mrngoni flow concept [10], the dissolution nd refrctory rsion is locl corrosion cn e induced y vrition of surfce tension nd density etween refrctory nd slg, together with the wettility. The slg in contct with SEN is clled mould flux, which is synthetic slg composed y non-metllic 1

2 2 mterils tht is in contct with liquid steel, melts nd promotes therml insultion, prevention from reoxidtion, sorption of inclusions nd in the interfce metl/strnd lurictes nd controls the het trnsfer [11]. In order to increse the yield of the process of Continuous Csting the first impct is incresing refrctory wer nd s consequence, the cost [12]. In order to improve the csting speed some properties of mould flux re dpted s melt temperture, rek temperture nd reducing the viscosity. The viscosity of mould flux influences directly on erosion of SEN ccording the study done y Nkmur [13]. The reduction of viscosity leds to incresed fluoride nd increse the wer of SEN [1], [14]. But, in other point of view, the study mde y Schevciw [15] shows tht mould fluxes contining ZrO 2 cn e n lterntive for reducing SEN wer. As study done y Cirilli [16], the SEN erosion is relted with the viscosity nd the difference in chemicl composition of slg. As consequence of refrctory wer, it hs een remrked y Bruce [17] tht with increse of ZrO 2 content into slg my led to decrese the surfce qulity due to possile inclusions or even rek outs. This pper evlutes the reduction of SEN wer fter djusting the chemicl composition of the mould flux. Chrcteristics of ech one ws presented, which will e relted to nlysis of OM of slg films smpled etween strnd nd mold nd relted with the nlysis mde in slg nds y SEM nd EDS nlysis s well. Both were pplied in industril scle with stisfctory performnce. Experimentl Procedure 1. Mould flux nlysis The mould fluxes involved in this work ws clled mould flux A nd B. It ws compred oth with chemicl composition nd simultneous therml nlysis, tht ws mde y TG due to mss difference nd DSC y het difference represented y exothermic nd endothermic peks depends on the rection. The simultneous therml nlysis ws conducted in the heting nd cooling, oth considering the rte of 10 o C/min nd mximum temperture of 1200 o C. 2. Industril tril The industril tril ws conducted in the continuous csting mchine numer 2 t ThyssenKrupp Compnhi Siderúrgic do Atlântico Brzil with two strnds. In the strnd numer 3 ws pplied the mould flux B nd strnd numer 4, the mould flux A, t the sme time during sequentil of 5 hets. During the tril it ws checked the consumption rte (kg of mould flux per ton of steel), slg thickness of ech het nd rte of wer of ech SEN. The slg thickness ws mde immersing copper nd steel wires, so the difference ws the mesure of liquid mould flux lyer (mm) in ech het. The consumption ws clculted sed on figure 2. kg/ton Q t = Figure 2. Where: Q t = consumption (kg of mould flux per ton of steel); q = quntity of mould flux (kg) w = Width (mm); t = thickness; 7,6 = density of steel; l = length of csting In the end of 5 th het it ws collected the smples of slg films tken etween the strnd nd mold, when the level of steel decresed in the mold, oth strnds t the sme moment. The wer rte of SEN ws mesured y dividing the dimeter efore nd fter totl hets per totl time of sequentil. Thermogrphies nd grphics were nlyzed vi MoldExpert. 3. Slg film nlysis The slg films ws nlyzed y OM, so it ws necessry to prepre the smples oth y cold mountin crylic resin for emedding of mterilogrfic specimens nd fter polished in sndppers of SiC from 220 to 4000 "mesh". The OM ws the model Axiovert 40 MAT, mrk Zeizz. The mgnifiction ws 5x y red light, 10x y yellow light nd 20x y green light. 4. SEN nlysis The SEN were kept prt in the end of 5 th het, in order to collect the smples from slg nd, oth in the region of higher erosion. Both smples were cut y Isomet The smples ws polished in the sme procedure choose for slg films nd they were nlyzed y SEM, model Hitchi 3000 with EDS linked nd mgnifictions ws 50x nd 1000x. Results nd Discussion q ( wt..7,6). l 1. Mould flux nlysis. Chemicl nlysis Tle 1 compres mould fluxes A nd B nd Tle 2 shows the tempertures clculted [18]. Mould flux B hs N 2 O nd F - contents lower thn mould flux A. The liquidus temperture clculted is lower nd the rek temperture higher thn mould flux A. Tle 1 Comprison of chemicl composition of mould flux A nd B Mould Flux SiO 2 CO N 2O F - CO/SiO 2 A 34,68 30,94 9,20 8,66 0,89 B 33,26 34,19 7,30 5,90 1,03 2

3 3 Tle 2 Liquidus nd Brek tempertures clculted Mould Flux T liq ( o C) T rek ( o C) A B Simultneous therml nlysis Figures 3 nd 4 re the TG-DSC nlysis nd they were summrized in the Tle 3. The exothermic rection is expressed s exo, the opposite is the pek of endothermic rection (endo) nd the curve color red show the temperture. smll decresed nd the rek temperture for version B ws smll incresed ecuse of differences in chemicl composition. The higher sicity cn e relted with higher rek temperture [18]. The differences etween T liq nd T rek etween mould flux A nd B showed the sme tendency of clculted tempertures in Tle Industril tril Tle 4 descries items nlyzed in industril tril. Tle 4. Comprison of items nlyzed during the industril tril Item checked Mould Flux A Mould Flux B Mchine 2 Strnd 4 Strnd 3 Steel grde Low cron Low cron Mould dimentions (mm) Slg thickness (mm) Consumption (kg/ton of steel) Width: 1274 Thickness: ,371 0,370 Width: 1274 Thickness: 255 Figure 3. Simultneous therml nlysis of mould flux A. Wer rte (mm/h) 4,61 3,52 Figure 4. Simultneous therml nlysis of mould flux B. Tle 3. Comprison of results otined from simultneous therml nlysis of mould flux A nd B. Rection / chnges TG Heting: totl loss of weight DSC Heting: decomposition of C free (exo) DSC Heting: loss of CO 2 (endo) DSC Heting: melting rnge (endo) DSC Cooling: rek temperture (exo) Mould Flux A ~12% ( o C) Mould Flux B ~12% ( o C) o C o C ~1000 o C ~1000 o C o C o C 1100 o C 1140 o C The result of TG nlysis during the heting ws similr for oth mould fluxes. DSC nlysis during heting showed decomposition of cron free nd loss of crontes oth in similr rnge of tempertures. The melting rnge of mould flux B ws Figure 5. Thermogrphy nd grphics of Friction Stress x Time, Totl Hetflux x Time nd Csting Signls in mchine #2. )Strnd 04 (Mould Flux A) nd ) Strnd 03 (Mould Flux B). Tle 4 nd Figure 5 show tht the industril performnce of oth mould flux ws the sme relted to slg thickness, consumption nd thermogrphs. It 3

4 4 mens tht oth hd stle performnce during the ppliction. The luriction, showed in consumption result, ws suitle considering the rnge cceptle of 0,35-0,50kg/ton of steel. The melting rte nlyzed y mesurement of slg pool thickness ws good considering the rnge from 10 to 30mm. The thermogrphs in Figure 5 show little difference etween strnds tht could e the differences in life time of mold. The grphs from friction stress, totl het flux nd speed csting in Figure 5 ws quite similr in oth strnds. The wer rte of the mould flux B ws 24% lower which is relted with the djustment mde in chemicl composition s descried in Tle Mould flux slg film nlysis The Figures 6 nd 7 showed the imges of slg films of mould flux A nd B respectively, collected etween mold nd strnd. cn e explined y the difference of cooling. In the mold side the cooling is fster, so there is no time for ordering prticles. In the strnd side, the cooling is less ggressive so there is time to forming homogeneous re. The chemicl composition of mould flux for low cron inhiits the crystl formtion in the slg film in comprison to mould flux pplied in medium cron steel minly ecuse of lower sicity. So the slg film is glssy nd it genertes higher het nd thickness of steel shell, suitle for low cron steel. c Strnd side Figure 7. Anlysis of OM of slg film collected from strnd 3 of mould flux B. ) mgnifiction of 5x red light, ) mgnifiction of 10x yellow light nd c) mgnifiction of 20x green light. c Figure 6. Anlysis of OM of slg film collected from strnd 4 of mould flux A. ) mgnifiction of 5x red light, ) mgnifiction of 10x yellow light nd c) mgnifiction of 20x green light. Figure 6 nd show the slg film of mould flux A spongy nd disrrnge re, in the mold side. Figure 5 show homogeneous structure closer to strnd. It The slg film from mould flux B is similr to A s shown in the Figure 7. There is lower spongy ppernce in mold side nd the trnsition to more homogeneous region is softer. It e concluded tht the ehvior of het flow etween mold nd strnd proly ws quite similr. These conclusions re in ccordnce with the industril tril. 4. SEN nlysis The imges of SEN from strnd 3 nd 4 re presented in Figures 8 nd 9. 4

5 Figure 8. Anlysis of SEM of SEN fter ppliction of mould flux A. ) mgnifiction of 50x nd ) mgnifiction of 1000x showing the plces of punctul nlysis. Element Tle 5. Punctul nlysis of Figure 7. Spectrum C O F * * * * * N * * Mg * * Al Si * * K * * * * C Zr * Fe * * * Element Figure 9. Anlysis of SEM of SEN fter ppliction of mould flux B, strnd 3, ) mgnifiction of 50x nd ) mgnifiction of 1000x showing the plces of punctul nlysis. Spectrum Tle 6. Punctul nlysis of Figure C O F * * * * N Mg * * Al Si K * * * * * C Zr Fe * * * 5

6 6 Figures 8 nd 9 show the shdes of gry tht mens the difference in chemicl composition. Figure 8 nd 9 hd the erosion re highlighted with lck circle. Figure 8 the wer zone is more irregulr, proly the mtrix ws more eroded nd ggregtes were eliminted. Figure 9 shows the wer zone with the re more well-defined, so the erosion ws less ggressive proly ecuse the erosion in mtrix nd ggregtes were similr. Figure 8 hd the thickness verge of wer zone of 1,57mm nd in Figure 9 ws of 2,27mm. Thinner thickness of wer zone in Figure 8 proly cn e explined y the higher erosion nd consequently higher relese of mtrix nd ggregtes from refrctory tht is in ccordnce with the mesure mde during the industril tril. Figures 8 nd 9 showed the elementry punctul nlysis. In oth, the drkest shde of gry is proly the grphite phse ecuse of higher cron content. The lighter shde of gry is proly ggregtes of ZrO 2 stilized with C ecuse of higher percentge of Zr, O nd less of C. The wer zone ws nlyzed in Figure 8 in the point numer 3 tht showed mix of refrctory, slg nd iron ecuse of the presence of Zr, N, K nd other elements found in mould flux nd Fe, respectively. The wer zone ws nlyzed in Figure 9 in points numer 2, 3, 6 nd 7, so ll of then showed the presence of slg ecuse of significnt percentge of N nd O, proly N 2 O nd ggregtes dissolved ecuse of the Zr nd O. Figure 8 the ZrO 2 ggregtes re spongy nd more dissolved thn in Figure 9. It cn e concluded the sme for Figure 9, ut the ZrO 2 ggregtes hd lower porosity nd the percentge of Zr dissolved is lower (points 2 nd 3 Tle 6), proly ecuse wer less ggressive. Conclusion It ws concluded tht: Mould flux A nd B hd similr ehvior in heting nd cooling nlyzed y TG/DSC; Mould flux A nd B hd sme performnce in industril tril s showed in Tle 4 nd Figure 5; The higher F - in Mould flux A proly incresed the SEN wer ecuse the high ffinity etween F- nd C tht destilize the ZrO 2 phse; The lower F - nd N 2 O in mould flux B proly increse the superficil tension tht decresed the wettility [19]; The mechnism of erosion y mould flux A ws the dissolution esily the mtrix, penetrting in the structure round ggregtes nd proly withdrw nd loose ZrO 2 prticles; The mechnism of erosion y mould flux B hd the re well-defined, is more homogeneous nd less ggressive, proly ecuse of the ttck in the mtrix is lower nd consequently withdrw the ggregtes ws decresed. Arevitions DSC EDS OM SEM SEN TG Acknowledgments Differentil Scnning Clorimetry Energy Dispersive Scnning Opticl Microscopy Scnning Electron Microscopy Sumerged Entry Nozzle Thermogrvimetry We would like to thnk the support from University of São Pulo EEL-USP t Loren - Brzil for the SEM nd OM nlysis. References [1] A.-H. Bui, S.-C. Prk, I.-S. Chung e H.-G. Lee, Dissolution Behvior of Zirconi-Refrctories during Continuous Csting of Steel, METALS AND MATERIALS Interntionl, vol. 12, n. 5, pp , [2] A. P. Luz e V. C. Pndolfelli, Artigo Revisão: Atução dos ntioxidntes em refrtários contendo crono, Cerâmic, vol. 53, pp , [3] A. F. Dick, X. Yu, R. J. Pomfret e K. S. Coley, Attck of Sumerged Entry Nozzles y Mould Flux nd Dissolution of Refrctory Oxides in the flux, ISIJ Interntionl, vol. 37, n. 2, pp , [4] A. Kumr, R. Khnn, M. Ikrm ul Hq, J. Spink e V. Shjwll, Effect of Glze on the Corrosion Behvior of ZrO2 Grphite Insert of the Sumerged Entry Nozzle in the Continuous Csting of Steel, steel reserch int., vol. 85, pp. 1-10, [5] M. HAYASHI, T. W. WATANABE, H. NAKADA e K. NAGATA, Effect of N2O on Crystlliztion of Mould Fluxes for Continuous Csting of Steel, ISIJ Interntionl, vol. 46, n. 12, p ,

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