CONTROL OF COLUMNAR-TO-EQUIAXED TRANSITION IN CONTINUOUS CASTING OF 16% Cr STAINLESS STEEL

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1 Memorie >> Colata continua CONTROL OF COLUMNAR-TO-EQUIAXED TRANSITION IN CONTINUOUS CASTING OF 16% Cr STAINLESS STEEL J.C. Kim, J.J. Kim, J.Y. Choi, J.H.Choi, S.K. Kim POSCO, Korea In continuou cating of 16%Cr ferritic tainle teel, columnar grain undeirably forming among equiaxed grain make non-uniform microtructure and degrade ridging property. Since thi phenomenon reult from the change of olidification condition during continuou cating, we focued on Cellular-to-Equiaxed Tranition (CET) in continuou cating of 16%Cr ferritic tainle teel to control the microtructure of a-cat lab. In order to find the CET condition, we carried out the one dimenional heat tranfer analyi of the melt, and predicted the CET condition by Hunt model. It wa revealed that the econdary columnar grain uually formed at 0.5~0.8 olid fraction, and the formation of them reulted from a teep increae of G/V1/2 value a the melt wa getting out of EMS field; however the increae of G/V1/2 wa readily controlled by changing EMS pattern and the econdary cooling intenity. In reult, econdary columnar grain were eliminated by optimizing the upper and lower EMS intenity, and increaing econdary cooling intenity. We alo invetigated the effect of olidification parameter including uperheat and cating peed a CET condition in continuou cating proce, and the reult were alo dicued by the term of thermal condition of melt. KEYWORDS: Ferritic tainle teel, continuou cating, ridging, columnar-to-equiaxed tranition (cet), econdary columnar tructure, olidification parameter INTRODUCTION Recently, market demand of autenitic tainle teel ha been in tagnation due to higher Ni price in contrat to higher market demand of cheaper ferritic tainle teel with uperior anti-tre-corroion property. However, ferritic tainle teel i inferior to autenitic tainle teel in anti-corroion and forming propertie, o many engineering attempt are focued on developing new ferritic tainle teel and production technique, and improving material quality. In particular, ridging defect ha been typical problem in forming proce of ferritic tainle teel and it i the urface defect reulting from the platic aniotropic difference between recrytallized and non- recrytallized tructure[1,2], which i originated from the non-uniform microtructure made in continuou cating proce[3]. Recent effort have focued to increae equiaxed crytal ratio by retraining columnar grain baed on the reearch, howing that anti-ridging propertie howed ignificant improvement at about 40% of the equiaxed crytal J.C. Kim, J.J. Kim, J.Y. Choi, J.H.Choi, S.K. Kim POSCO, Korea Paper preented at the European Conference Continuou Cating of teel, Riccione, 3-6 June 2008 ratio (ECR) in a-cat lab[4-7]. There were everal way to increae ECR in 400 type STS, for example, making the preferred condition for equiaxed growth by increaing contitutional undercooling or by increaing heterogeneou nucleation ite. Thi tudy focued on the olidification parameter to control the microtructure of a-cat lab for increaing ECR. Typical macro-tructure of a-cat lab of 16%Cr tainle teel exhibited in the order of columnar-equiaxed-econdary columnar-equiaxed after urface chill layer a hown in Fig. 1. In particular, reappearance of columnar tructure (C2) hown among equiaxed region i problem, becaue Fig. 1 Typical macrotructure obtained in lab ection of 16Cr tainle teel. Macrotruttura tipica ottenuta nella ezione di bramma di acciaio inoidabile 16Cr. la metallurgia italiana >> ettembre

2 Colata continua << Memorie Slab Thickne 200~220 mm Superheat 15~50 Cating Speed 0.7~1.0 m/min 2 nd cooling intenity 0.5~1.5 l/kg EMS Intenity Upper Lower 0~1300A 1300 A Tab. 1 Experimental condition for continuou cating of 16Cr tainle teel. Condizioni primentali per la colata continua dell acciaio inoidabile 16Cr. the econdary columnar tructure provoke non-uniformity of macro-tructure, contributing the ridging defect omewhat. Thu, it i neceary to identify the forming condition of econdary columnar tructure and find out the olution to prevent them from generating. In addition, electromagnetic tirring (EMS) technology ha been applied to enure not only homogenou macrotructure, but equiaxed grain tructure[8]. The main role of EMS i to reditribute heat and olute in the molten teel by inducing forced convection and it reult to give ignificant influence on the formation of equiaxed crytal. Moreover, the optimum function of EMS come from harmony with cating condition. Therefore, we invetigated the correlation between EMS and olidification condition, and found out the optimum condition for maximizing ECR conidering CET condition. EXPERIMENTAL METHOD The condition of continuou cating and EMS of 16Cr tainle teel are hown in Tab. 1. In thi tudy, two part of EMS were applied, upper part to control olidification condition at the early tage of olidification and lower part for the control at midterm tage. Each part of EMS wa controlled independently and the effect of the pattern of EMS wa invetigated, which i the combination of EMS power at upper and lower part, on CET condition. After continuou cating, lab ample were cut at the location of 1/4 and 1/2 of the lab width in the thickne direction, and macro tructure are examined. Evaluation of equiaxed crytal ratio i defined in Eq. (1) a the ratio of equiaxed zone to total thickne of lab. The ymbol in equation 1 are noted in Fig. 1. The variation of ECR i defined a ECR index baed on the averaged value of ECR without EMS at the cating peed of 0.9m/min. RESULTS Effect of uperheat Fig. 2 how the variation of ECR a the degree of uperheat. The increae of uperheat reulted to decreae ECR. However, it wa hown that uperheat effect became weak a EMS wa applied. Effect of cating peed The effect of cating peed to ECR wa imilar to the cae with/without the EMS application. A the cating peed increae from 0.8m/min to 0.9m/min in Fig. 3, ECR wa decreaed by 5% in average. Effect of EMS pattern The effect of EMS pattern wa examined by changing the power intenity of upper and lower electromagnetic tirrer independently. Fig. 4 how that higher ECR exited at the condition of low or no power of upper electromagnetic tirrer, in contrat to lower ECR at trong power at (1) Fig. 2 Effect of uperheat in Tundih on ECR. Effetto del urricaldamento nella paniera. Fig. 3 Effect of cating peed on ECR. Effetto della velocità di colata ull ECR. 44 ettembre 2009 << la metallurgia italiana

3 Memorie >> Colata continua Fig. 4 Effect of EMS pattern and intenity on ECR. Effetto di chema e intenità dell EMS ull ECR. Fig. 5 Effect of econdary cooling intenity on ECR. Effetto dell intenità di raffreddamento econdario ull ECR. both electromagnetic tirrer when the lower electromagnetic tirrer wa applied ame power for both cae. In addition, the combination of high power at upper electromagnetic tirrer and low power at lower one gave lower ECR. Effect of econdary cooling intenity ECR increaed by 20% when the econdary cooling condition wa changed from oft to trong cooling condition (Fig. 5). The previou tudy howed that there wa no ignificant change of ECR at trong cooling condition [5], but thi work howed obviou effect of the econdary cooling condition. Thi reult may originate from the trong relation between CET and econdary cooling condition. DISCUSSION Forming mechanim of econdary columnar tructure Many theorie were reported for the tranition from columnar to equiaxed tructure, and alo there have been many reearche for the prediction of microtructure baed on contitutional undercooling theory [9], which wa referred to thi analyi. Growing columnar grain capture or puh equiaxed grain and continuouly grow when the number of generated equiaxed grain wa mall, but they will be uppreed by equiaxed grain occupying remained liquid phae when the volume of equiaxed grain i larger than the pecific value to the liquid phae volume. J.D. Hunt uggeted the tranition condition from columnar to equiaxed tructure a CET model [10], and the condition i defined a below, where, G i the temperature gradient at the dendrite tip, N0 i the nucleation denity of delta ferrite (mm-3), i the contitutional undercooling at the dendrite tip, and (2) Fig. 6 Change of growth condition during cating (a) without EMS (b) with EMS. Cambiamento delle condizioni di crecita durante la colata (a) enza EMS (B) con EMS. i the undercooling for nucleation. With the aumption of i much maller than, the Eq. (2) can be implified a below[6], and it can be expreed by meaurable parameter, i.e. temperature gradient and growth rate. (3) la metallurgia italiana >> ettembre

4 Colata continua << Memorie Fig. 7 Effect of EMS on G/V 1/2 with olid fraction. Effetto dell EMS u G/V 1/2 con frazione olida. gradient and increae of growth rate were oberved in the region of EMS applying, and higher temperature gradient and decreae of growth rate were oberved outide of EMS field. The variation of G/V1/2 with olid fraction wa calculated by applying temperature gradient and growth rate variation to Eq.(3). The area with table columnar tructure exit in the cae of higher G/V1/2 than critical value, and equiaxed growth i more table when the value of G/V1/2 i lower than critical value. The critical value of CET i a criterion to define table equiaxed region, and 0.3 i acquired in thi tudy by comparing calculated G/ V1/2 with real macrotructure of a-cat lab. The obtained value in thi tudy i imilar to the reult of other reearcher [6]. Without applying EMS, grain grew a columnar tructure until the lat tage of olidification (f=0.9), thereafter CET occurred a hown in Fig. 7. On the other hand, CET occurred after f=0.3 with EMS, and tranition to columnar tructure occurred again at f=0.55 with good agreement with oberved value from real macrotructure of a-cat lab at both cae. From the reult, it can be explained why econdary columnar tructure generate, a follow. Strong convection occur in the region of EMS application, reulting to lower temperature gradient at the growing interface. It bring teep decreae of G/V1/2 value, which give the condition for equiaxed grain generation and CET occur. However, temperature gradient and growth rate at the S/L interface will be recovered when the melt leave the EMS field; accordingly econdary columnar tructure develop again due to the change of growth condition. Effect of cating condition on CET In order to increae ECR, two concept were applied in thi tudy. One i to uppre the generation of econdary columnar tructure and the other i to make the condition of the equiaxed crytal generation at early tage of olidification. Baed on thi model, it wa able to predict the Fig. 8 Effect of uperheat on G/V 1/2 with olid fraction. Effetto di uperheat u G/V 1/2 in relazione alla frazione olida. where, V i the growth rate at the dendrite tip, D i the diffuivity of molten teel, C0 i the initial concentration of liquid phae, k i the effective ditribution coefficient, and Γ i the Gibb-Thomon contant. To predict CET condition baed on Eq.(3) in actual continuou cating proce, it i neceary to predict temperature gradient and growth rate. In thi tudy, onedimenional heat tranfer imulation wa carried out in continuou cating proce, and it wa enabled to predict temperature gradient and olidification peed at the pecific location. Fig. 6 how temperature gradient and growth rate at the given point with and without EMS application. Temperature gradient without EMS i about 5 /mm at the early tage of olidification, and become maller a the olidification proceed (Fig. 6a). In the cae of EMS application(fig. 6b), teep drop to maller temperature Fig. 9 Effect of cating peed on G/V 1/2 with olid fraction. Effetto della velocità di colata u G/V 1/2 con frazione olida. 46 ettembre 2009 << la metallurgia italiana

5 Memorie >> Colata continua macrotructure of a-cat lab from calculated reult and thee were compared with experimental one. Effect of uperheating Fig. 8 how the variation of G/V1/2 a uperheat. With higher degree of uperheat, the G/V1/2 value ha larger one in the initial olidification, but the uperheat effect become weak a the olid fraction increae. The area of econdary columnar tructure generation become narrow a uperheat decreaed even though the effect i not ignificant. It correpond with the reult that the effect of uperheat on the variation of CET i not dominant with EMS application. The effect of cating peed A hown in the experimental reult, it wa obviou that the effect of cating peed wa ignificant to CET condition. With increae of cating peed, the growth of columnar dendrite in the early tage of olidification i uppreed due to increae of heat tranfer to the mold, reulting to have horter primary columnar grain with aumption of ame CET condition. However, the increae of cating peed enlarged the area of econdary columnar tructure a hown in Fig. 9, which bring totally maller ECR. It i etimated that the critical cating peed for free-econdary columnar tructure i 0.8m/min at given condition. Fig. 10 how the forming range of econdary columnar tructure with uperheat at tundih. The econdary columnar tructure exited between 0.6~0.9 for olid fraction, which correpond with the value of 65~90mm of oberved ample, improving the reliability of calculated reult. Hence, decreae of ECR with increaing cating peed i originated from the expanion of econdary columnar region, o it i required to decreae temperature gradient for retrain of columnar growth. Effect of econdary cooling condition Fig. 11 how the variation of G/V1/2 with econdary cooling condition. In the condition of 0.9m/min growth rate and oft cooling condition, early generation of econdary columnar grain wa oberved with broader area. Increae of cooling intenity at the ame growth condition delayed generation of econdary columnar grain and it can be explained from Fig. 11. Temperature ditribution of liquid phae decribe a polynomial decay of G/V1/2 a the increae of olid fraction a hown in Fig. 11. With applying EMS, temperature ditribution become uniform by forced convection in EMS field reulting to lower temperature gradient; thereafter temperature decreaed teeply again when the melt got out of EMS field and columnar growth i to be preferred due to the increae of temperature gradient. In thi condition, increae of cooling rate at the ame EMS pattern bring overall decreae of melt temperature; in particular, temperature gradient at the outide of EMS field will be maller by having rather low melt temperature. A a reult, the generation of econdary columnar grain can be delayed; moreover lowering cating peed help to uppre econdary columnar growth. In the experimental reult, the econdary columnar growth wa completely retrained by applying trong cooling intenity at 0.8m/min cating peed and ound lab having higher ECR over 60% were able to obtain. Briefly, to uppre the econdary columnar grain completely, it i propoed to optimize econdary cooling condition in the V, VI region, in which econdary columnar grain ued to generate a hown in Fig. 11. CONCLUSION In thi tudy, parametric tudy wa performed uch a EMS, cating condition, and econdary cooling to increae equiaxed crytal ratio of a-cat lab in ferritic tainle teel, and the following concluion wa derived. The econdary columnar grain, decreaing equiaxed crytal ratio, wa oberved in the area of abence of EMS tirring(6~10m from the menicu), in which the growth Fig. 10 Forming range of econdary columnar tructure. Range di formazione della truttura colonnare econdaria tructure. Fig. 11 FEffect of econdary cooling intenity on G/V 1/2 with olid fraction. Effetto dell intenità di raffreddamento econdario u G/V 1/2 in relazione alla frazione olida. la metallurgia italiana >> ettembre

6 Colata continua << Memorie condition of dendrite tranit from equiaxed to columnar manner a increaing temperature gradient and decreaing growth rate, reulting to the generation of econdary columnar tructure. With EMS application, effect of uperheat on CET condition i no longer ignificant. On the other hand, decreae of cating peed effectively retrained the generation of econdary columnar grain having a tendency increae the equiaxed crytal ratio. In addition, trong econdary cooling wa another way to increae equiaxed crytal ratio, enabling to uppre columnar grain completely at 0.8 m/min cating peed. For the EMS pattern effect, it i oberved that econdary columnar grain were promoted with application of high power of upper electromagnetic tirrer due to increae of melt temperature by flow pattern and the optimum combination of EMS pattern for higher equiaxed crytal ratio i low power of upper electromagnetic tirrer and full power of lower one. REFERENCES 1] H. C. Chao, Tran. ASM, 60 (1967), p 37. 2] R. N. Wright, Met. Tran., 3 (1972), p 83. 3] N. Hirata, H.Ota, Y. Kato, T.Ujoro and O. Furukimi, Tetu-to-Hagane, 89 (2003), p ] Y. Itoh, S. Takao, T. Okajima and K. Tahiro, Tetu-to-Hagane, 6 (1980), p ] M. Okimori, M. Tuchida and K. Inaoka, Tetu-to-Hagane, 80 (1994), T 25. 6] H. Shibata, S. Itoyama, Y. Kihimoto, S. Takeuchi and H. Sekiguchi, ISIJ Int., 46 (2006), p ] J. Hutt, D. StJohn, Int. J. Cat Metal Re., 11 (1998), p 13. 8] M.H. Burden and J.D. Hunt, Metall. Tran. A, 6A (1975), p ] W.C. Winegard, B. Chalmer, Tran. ASM, 46 (1954), p ] J.D. Hunt, Mater. Sci. Eng., 65 (1984), p 75. CONTROLLO DELLA TRANSIZIONE COLONNARE- EQUIASSICO NELLA COLATA CONTINUA DELL ACCIAIO INOSSIDABILE 16% CR Parole chiave: acciaio, colata continua, olidificazione Nella colata continua dell acciaio inoidabile ferritico 16% Cr, gli indeiderabili grani colonnari che i formano tra i grani equiaici rendono la microtruttura non uniforme e compromettono le proprietà di ridging. Poiché queto fenomeno deriva dal cambiamento delle condizioni di olidificazione durante la colata continua, i è concentrata l attenzione ulla Tranizione Cellulare-Equiaico (CET) nella colata continua dell acciaio inoidabile ferritico 16% Cr per controllare la microtruttura della bramma a-cat. Per trovare la condizione CET, nello tudio è tata ABSTRACT effettuata una analii del traferimento di calore monodimenionale del metallo fuo, e una previione della condizione CET econdo il modello di Hunt. Si è oervato che i grani colonnari econdari - olitamente preenti in ragione di una frazione tra 0,5 e 0,8 - e la loro formazione ono il riultato di un bruco aumento del valore G/V1/2 al momento dell ucita del metallo fuo dal campo EMS; tuttavia l aumento del valore G/V1/2 è tato prontamente meo otto controllo cambiando la configurazione dell EMS e aumentando l intenità di raffreddamento econdario. In concluione, i grani colonnari econdari ono tati eliminati ottimizzando l intenità uperiore e inferiore di EMS e aumentando l intenità di raffreddamento econdario. Si è inoltre invetigato l effetto dei parametri di olidificazione inclui il urricaldamento e velocità di colata in termini di tranizione CET nel proceo di colata continua, e i riultati ono tati anche dicui in termini di condizione termica di colata. 48 ettembre 2009 << la metallurgia italiana