CSN s blast furnace number 3 (14m diameter, 38

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1 Expnsion phenomen in blst furnce herth fter blow-in Following n emergency stop, CSN s blst furnce number 3 remined seled for five months prior to restrting. Specil instrumenttion nd controls were instlled to try nd ensure tht the furnce herth did not crck s the solid slmnder expnded while the furnce heted up nd iron mking re-commenced. Detils of the instrumenttion, modelling nd operting method re described. The furnce ws successfully restrted. MILLENNIUM STEEL 28 Authors: R vn Oudenllen, K Andreev, M Gerdees, MAG Bentes, AHB de Mello nd F Frnklin Dnieli Corus BV, Corus Group, M Gerdees nd Prtners nd Compnhi Siderúrgic Ntionl (CSN) CSN s blst furnce number 3 (14m dimeter, 38 tuyeres) hd to undergo n emergency, unprepred stop in 26. The slmnder ws tpped fter 1 dys nd only 1 tonnes of hot metl ws removed. It ws decided to sel the furnce completely nd leve the remining burden undisturbed, mintining s much coke inside s possible. After pproximtely five months, the furnce ws redy to be blown-in gin, but the lrge solidified slmnder ws mjor cuse of concern. When the slmnder cooled down, it solidified nd contrcted leving gps in nd round it. During the blowin the molten iron formed could fill these gps, then if the molten iron solidifies in the gps there would be no spce left for therml expnsion of the slmnder which could exert pressure ginst the refrctory (shown schemticlly in Figure 1). The refrctory would then trnsfer this pressure to the shell such tht the resulting circumferentil stress could become so high tht the shell crcked. Furthermore, the pressure exerted by the expnding slmnder might lso dmge the refrctory. Filure of the herth shell of n operting blst furnce will hve serious consequences for cmpign life nd my cuse brekout. During blow-in, the expnsion of the herth ws mesured. Afterwrds, nlyses of the observed phenomen were performed from which nlyticl nd finite element models hve been creted to predict expnsion phenomen. In this rticle, predicted vlues re compred with the herth shell stress nd strin mesurements recorded during blow-in. MEASUREMENTS Mesuring equipment The mesuring equipment plced on the shell to monitor expnsion consisted of: ` Strin guges ` Acoustic emission sensors ` Circumferentil tpe mesure ` Rdil brs Shell Slmnder Refrctories r Fig 1 Expnding slmnder exerts pressure on refrctory nd shell Ten strin guges were mounted onto the herth shell, eqully distributed round the circumference. To protect the guges from the spry cooling wter pieces of sheet metl were ttched to the shell bove the guges (see Figure 2). Acoustic emission sensors cn be used to locte energy concentrtion in the shell. A totl of 24 sensors were mounted onto the herth shell t two different elevtions with spcers used to ensure tht they were positioned out of the spry wter flow. Figure 3 shows tht the mjor prt of the strin energy is locted round tpholes 3 nd 4 nd between tpholes 3 nd 2. During this mesurement, the tpping sequence ws two csts 42

2 RAW MATERIALS AND IRONMAKING through tphole 3 followed by one cst through tphole 2. As well s the loction of the energy concentrtions, the development of the verge energy over time ws monitored. From this informtion, the trnsfer of lod on the shell could be mde visible. The tendencies corresponded with the mesurements over time of the circumferentil strin nd rdil displcement mesurements. Rdil growth of the herth ws monitored s follows: A series of brs were welded perpendiculr to the herth shell, with mtching brs mounted on fixed foundtion r Fig 3 Representtion of energy concentrtions in shell MILLENNIUM STEEL 28 r Fig 2 Strin guge with protective cover (see Figure 4). By mesuring the distnce between the brs on the shell nd the mtching fixed brs t regulr time intervls, herth expnsion could be monitored. At two different elevtions, steel tpes were plced round the herth shell. The tpes were tightened by mens of springs between the ends of the tpe. By mesuring the distnce between the tpe-ends t regulr intervls, the development of the circumferentil strin could be monitored (see Figure 5). Mesurement results Generlly, the coustic mesurements corresponded with the mesurements over time of the circumferentil strin nd rdil strin mesurements. The mesurement with the rdil brs ws not influenced by the herth spry cooling, however, the ccurcy nd reproducibility of the mesurement ws limited. The position nd the ngle of the sliding guge with the verticl strin of the brs on the shell due to therml expnsion mde good nd relible mesurement impossible. The trends of the mesurements of the brs were in greement with the other mesurements. The mesurement of one of the circumferentil tpes ws influenced by the spry cooling, but the mesurement result from the other one ws comprble with the verge of the strin guges. 43

3 r Fig 4 Rdil brs r Fig 5 Circumferentil tpe with spring fstening 12 delt on tpe t EL 837 mm 1 delt on tpe by SG t EL 81 delt on tpe t EL7191 mm mm re 7 f 24 : /j u n : /ju 1 25 n : /ju 1 29 n : /j un 3 2 /j : un 17 1: jul 17 2: jul 17 3: jul 17 4: ju l 14 5: jul 2 6:3 jul 15 7: jul 2 7:3 jul 15 8: jul 3 8: jul 15 9: ju l 1 :3 9-j ul 15 1 : -ju 6 1 l :3 -ju 2 11 l :3 -ju l : -ju l : -ju 13 l -ju l MILLENNIUM STEEL 28 r Fig 6 Mesurement results of circumferentil tpes nd verge of strin guges 44 Figure 6 shows the mesurement results of the circumferentil tpes nd the verge of the strin guges for the first 19 dys fter the blow-in. The tpe t +7,191mm ws influenced by the spry wter-cooling but it cn be seen tht the mesurement results of the tpe t +8,37mm mtches the verge results of the strin guges. Fourteen dys fter the blow-in the circumferentil elongtion reched its mximum vlue of 8mm. At the elevtion of the upper tpe, the shell hs nominl circumference of 5.4m. The corresponding strin is.16. Prt of the mesured circumferentil strin ws cused by therml expnsion of the shell, but becuse the shell temperture ws not monitored, it ws not possible to quntify the tensile stress in the shell from the expnsion of the refrctory nd/or slmnder. ANALYSIS Anlyticl method To quntify the potentil problems of n expnding solidified slmnder, clcultions were mde ssuming: ` Slmnder strt temperture of 1 C ` Shrink gps ssumed to fill with liquids tht then solidify ` Slmnder ssumed to het uniformly ` Slmnder contins 5% coke, 25% free spce A bsic pressure blnce model is creted with the expnding slmnder nd the herth shell (see eqution 1). Therml expnsion of refrctory nd shell re neglected.

4 RAW MATERIALS AND IRONMAKING Stress [N/mm²] circ stress, gp= circ stress, gp=1 circ stress, gp=2 circ stress, gp=3 circ stress, gp= Temperture [ºC] r Fig 7 Shell stress for different gp sizes With the model, the shell stress for different gp sizes is determined. (1) where: σ circ_shell = circumferentil stress in shell [mm 2 ] R S = slmnder rdius t strt temperture [mm] α S = therml expnsion coefficient of slmnder [1/ C] ΔT = increse of temperture [ C] fe = free expnsion of slmnder [mm] E ST = elsticity modulus of slmnder t end temperture [mm 2 ] s = solidity (1 mount of ir in slmnder) [-] E Shell = elsticity modulus of shell [mm 2 ] R Shell = shell rdius [mm] = shell thickness [mm] t Shell It becme obvious tht the mount of free expnsion is criticl for the lod on the shell nd tht if n expnding slmnder is trnsferring the pressure to the shell, the llowble stress level will be esily exceeded. Figure 7 illustrtes shell stress for different gp sizes. Finite element method The expnsion of the slmnder is not the only phenomenon cpble of cusing shell extension or deformtion. Both operting (gs nd hydrosttic) pressures nd expnding refrctory lining cn strin the steel shell. The combined effect of these two prmeters ws ssessed using thermo-mechnicl computer FEM models. The biggest chllenge for the modelling ws n ccurte representtion of therml conditions on the lining hot fce. Two idel trnsient regimes were modelled. According to the first regime the temperture on the hot fce ws tht of the hot metl from the first nlyticl time step. In the second regime the temperture on the hot fce ws grdully rising, the finl temperture being reched fter 2 dys from blow-in. Figure 8 illustrtes modelling nd ctul dt from the 8,37mm tpe, mesured t 24mm from the steel shell (HF is hot fce; CF is cold fce). Model I is with pressure, but without creep, model II is with creep nd without pressure, nd model III is with both pressure nd creep. Over time, lining tempertures were incresing in the monitored herth. According to the first regime the therml stedy stte condition in the lining (temperture stops incresing) is reched fter pproximtely 28 hours. This contrdicts the thermocouple mesurements, which show much slower lining het-up. The second regime produced much better greement with the rel mesurements. This regime ws ccepted for the min study. Clculted therml fields were used by the model to clculte stresses nd displcements in the refrctory nd the shell. The initil model (model 1, see Figure 8) ccounted for the expnsion of refrctory nd gs nd hydrosttic pressures. Temperture dependent mteril MILLENNIUM STEEL 28 45

5 Temperture, C HF - model CF - model CF - mesured Shell rdius extension, m,14,12,1,8,6,4,2 Model I Model II Model III Mesured Time, dys Time, dys r Fig 8 Comprison of the model results with typicl mesured vlues MILLENNIUM STEEL 28 Stress, MP Stress, MP 3 MP 1 sec Strin 8 C 12 C Time, sec r Fig 9 Stress relxtion mesurement of crbonceous mteril properties were used in the model. Mteril stiffness ws determined in series of compressive stress strin mesurements. The predictions mde by the model greed reltively well with the mesurements obtined during the first 1-15 dys since furnce blow-in. Lter, relxtion of the shell displcement occurred in the herth, but the model filed to predict this relxtion. Theoreticlly, shell relxtion cn be cused by severl individul fctors or their combintion. These fctors my include skull building t the hot fce, dissolution of the lining, softening nd creep of the refrctory mterils. Skull cts s n dditionl insulting lyer. When skull lyer is formed, the temperture in the lining decreses, reducing the lining expnsion. In the monitored herth, the expnsion relxtion lsted severl dys, while t the sme time the cold end temperture ws still incresing. Therefore, skull formtion is not probble cuse for the observed relxtion. Dissolution of the lining reduces the thickness, resulting in relxtion of the stresses in the shell. Dissolution strts when refrctory temperture is bove 1,15 C. Interprettion of the temperture mesurements showed tht t the beginning of the mesured shell relxtion the hot fce of the herth hd not yet reched the tempertures needed for dissolution. Therefore, dissolution on its own cnnot cuse the relxtion. Creep of the refrctory hs been mentioned s cuse of blst furnce shell relxtion [1]. No mesurement dt of creep (or stress relxtion) in crbonceous refrctory could be found in the literture. Smples of the herth lining were tested to ssess the mgnitude of this phenomenon. Stress relxtion tests were performed with n initil lod of 3MP (see Figure 9). Within 2 hours, the test smples relxed significntly. The relxtion ws higher t higher test temperture. The existing FEM model ws upgrded with the obtined creep dt. Two clcultions were performed model III ccounted for the furnce pressure, model II did not. The relxtion in the model strted pproximtely the sme time s it ws detected by the monitoring equipment. This llows conclusion tht refrctory creep ws the min cuse of the relxtion. The nlysis lso showed tht the service pressure is responsible for pproximtely 2% of the totl shell expnsion. OPERATIONAL MEASURES For the mesurements described erlier, lrm levels were greed. Green ornge nd red were defined for the vrious instruments (see Figure 1). The red level ws positioned t the theoreticl yield point of the steel. After 1-14 dys 46

6 RAW MATERIALS AND IRONMAKING incresing numbers of the indictors cme into the red zone, indicting tht the slmnder ws heting up nd expnding; indeed there ws moment during which 75% of the indictors were red. The pproch dopted ws tht when red lrm ws reched it ws decided to het up the herth on one side only by csting on one side, or vi two tp holes close to ech other. The im ws to rech the softening point of slmnder loclly (or preferbly even melt wy prt of the slmnder loclly) before the entire slmnder reches criticl temperture. The pressure built-up in the slmnder in the rigid prt due to therml expnsion could thus be relesed in the softened re. After two weeks or so stresses begn to decrese nd therefore it ws thought tht the pplied pproch ws successful. After the blow-in, the models (which were described erlier) were mde using the mesurements tken during blow-in. The nlyses mde cler tht mesured strins could not hve been cused by the expnsion of the slmnder, simply becuse the slmnder expnsion will not be stopped by the reltively thin shell, but will crck it. The observed strin ws cused by the expnsion of the refrctory. The strin in the shell reduces when the temperture of the refrctory reches its creep temperture. The interesting opertionl observtion is tht while the originl thoughts bout the mechnism were not correct, the method pplied seems to be sound: nmely concentrtion of the het input. This hets prt of the refrctory hoop more rpidly (see Figure 11). The loclly heted refrctory reches the creep temperture more quickly while the mjor prt of the refrctory hoop expnds less quickly. When the creep temperture is reched, the therml expnsion of the refrctory hoop is prtly bsorbed by the soft zone nd the pressure exerted on the shell by the refrctory is minimised. Stress level nd mesuring system Shell stress N/mm² < > 22 Rdil Displcement mm < > 9 Strin Guges μm/m < 8 r Fig 1 Alrm levels 8 11 >19 μm/m/hr r Fig 11 Het concentrtion in burden nd refrctory > 11 Circumferentil strp (tpe) mm < > 55 Acoustic Emission energy spectrum Thermocouples C No ction No significnt noise 7% of previous cmpign mximum Little isolted emission energy Equivlent to previous cmpign mximum Wtch very closely, lso wtch rte of increse per hour Reduce blst volume nd blst pressure Lrge locl emission energy Higher CONCLUSIONS From the nlyticl model used for the expnding slmnder clcultions, it cn be concluded tht the liquid iron (generted during blow-in) tht solidifies in the gps of the solid slmnder, is serious thret to the condition of the shell. The expnsion is serious issue, which should be closely monitored with pproprite opertionl ctions nd sfety precutions if lrm levels re reched. In this prticulr cse study, thermo-mechnicl monitoring of the herth hs proven to be very useful exercise. The monitoring dt combined with results of the computer model hve shown tht the cuses of the mesured shell expnsion re the expnsion of the refrctory lining nd the operting pressures. The increse nd decrese of circumferentil strin fter blow-in hs been observed t other blst furnces [2], both new nd existing ones. For exmple, Figure 12 shows the development of the circumferentil strin of CSN s blst furnce number 3 nd nother Brzilin blst furnce. The other blst furnce ws inctive for only two weeks but the observed strin development is comprble to much shorter period compred to CSN s blst furnce. Interpreting the mesurements using thermomechnicl model llows the conclusion to be drwn tht the expnsion of the herth shell is most probbly cused by the expnsion of the refrctory lining nd the operting (gs nd hydrosttic) pressure only. The ltter is responsible for bout 2% of the totl shell expnsion. The monitored expnsion relxtion is most probbly cused by the creep (stress relxtion) of refrctory. Monitoring the herth expnsion nd tempertures, nd verifiction with models results in greter understnding MILLENNIUM STEEL 28 47

7 RAW MATERIALS AND IRONMAKING Strin CSN No. 3 Other Brzilin Furnce of processes tking plce within the furnce. The obtined knowledge cn be used to improve the herth design nd better process control. MS R vn Oudenllen is with Dnieli Corus BV, K Andreev is with Corus RD&T, nd M Gerdees is with M Gerdees nd Prtners, ll in The Netherlnds, nd MAG Bentes, AHB de Mello nd F Frnklin re with Compnhi Siderúrgic Ntionl, Brzil. CONTACT: ron.vn-oudenllen@dnieli-corus.com MILLENNIUM STEEL Blow in period (dys) r Fig 12 Circumferentil strin development fter blow-in This rticle is bsed on pper presented in Pris [3]. REFERENCES [1] H Gudenu, Thermisch-mechnisches Verhlten von H ochofengestellzustellungen, Sthl und Eisen 111(9) (1991) [2] O Milling nd G Everson, Blst furnce herth jcket monitoring nd design, 4th ECIC, Pris 2 [3] R vn Oudenllen, Expnsion phenomen in blst furnce herths fter blow-in, ATS 27, Pris 48