Uneven Solidification during Wide-thick Slab Continuous Casting Process and its Influence on Soft Reduction Zone

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1 ISIJ Internatonal, Vol. 54 (2014), No. 1, pp Uneven Soldfcaton durng Wde-thck Slab Contnuous Castng Process and ts Influence on Soft Reducton Zone Cheng JI, 1,2) * Sen LUO, 1) Maoyong ZHU 1) and Yogeshwar SAHAI 2) 1) School of Materals and Metallurgy, Northeastern Unversty, 3-11, Wenhua Road, Shenyang, Laonng, Chna. 2) Department of Materals Scence and Engneerng, The Oho State Unversty, 177 Watts Hall, 2041 College Rd, Columbus, OH, USA. (Receved on June 25, 2013; accepted on August 19, 2013) A two-dmensonal heat transfer model was developed to analyze the uneven soldfcaton, partcularly the shape of soldfcaton end n wde-thck slab contnuous castng process. In order to ensure the accuracy of smulaton, materal propertes of pertectc steel were calculated by weghted averagng of phase fractons, and the boundary condtons were obtaned by realstc water flux dstrbuton on the slab transverse surface. The model was verfed by nal shootng results at dfferent locatons of strand, and the absolute value of relatve error was found to be no more than 2.12%. Accordng to the smulaton results, the lqud core around 1/8 locaton of slab wdth ncreased after the water flux was decreased around the slab corner to reduce transverse corner cracks. The soft reducton (SR) zone was optmzed to reduce macro-segregaton both at the slab center and 1/8th locaton. The plant results showed that the nner qualty of wde-thck slab was sgnfcantly mproved n the whole transverse secton wth the optmzed SR zone parameters. KEY WORDS: wde-thck slab; contnuous castng; nozzle arrangement; soft reducton zone; soldfcaton end; pertectc steel; nal shootng. 1. Introducton The basc prncple of soft reducton (SR) s to apply a certan amount of reducton at the soldfcaton end of the strand for compensatng lqud core shrnkage and preventng the flow of resdual molten steel toward the center of the strand. As a result, the central segregaton and porosty of strand would be reduced. 1,2) SR zone s one of the most mportant SR parameters, whch decdes where the SR should be mplemented, and t s usually determned and presented by sold fracton of the strand centerlne. 3) However, n some cases, ths determnaton method would lmt the mplementaton effects of SR because the uneven soldfcaton end of slab could not be predcted accurately only by the sold fracton of strand centerlne, and the center macro-segregaton would not be reduced effectvely except around the slab center as shown n Fg. 1. The uneven shell thckness at the slab soldfcaton end Fg. 1. The macrograph of centerlne macro-segregaton n mm 180 mm slab transverse secton. * Correspondng author: E-mal: jc@smm.neu.edu.cn DOI: s usually caused by the non-unform dstrbuton of secondary coolng water flux along the slab transverse surface. Due to the larger wdth, the non-unformty of shell thckness s more serous concern n wde-thck slab contnuous castng process. Shen et al. 4,5) bult a 3-D heat transfer model whch consdered the poston and dstrbuton of spray nozzles, and the predcted results were n better agreement wth the measured results than those by usng average boundary condtons. F. Ramstorfer et al. 6) carred out a seres of testng experments of water flux and ts correlaton wth heat transfer coeffcents at dfferent ar/water pressures and dfferent dstances from the nozzle tp to the surface, and the correlatons between water flux and heat transfer coeffcent were presented. Q. Wang et al. 7) presented ther experence on slab corner temperature control and reducton n transverse cracks by use of asymmetrc nozzles. M. J. Long et al. 8) studed the non-unform behavor of shell growth by nal shootng method, and optmzed the nozzle arrangement n order to obtan unform shell thckness at the soldfcaton end and good soft reducton effect. 9) However, n order to avod the slab corner fallng nto the brttle temperature range durng the straghtenng process, especally for some mcro-alloyed steel and pertectc steels, the water flux has to be decreased around the slab corner, 10) whch resulted n the reduced growth of slab shell. In ths case, the soft reducton (SR) zone has to be adjusted to ft the rregular morphology of slab soldfcaton end. In the present work, wth the practcal parameters of wde-thck contnuous castng machne, a two-dmensonal ISIJ

2 ISIJ Internatonal, Vol. 54 (2014), No. 1 heat transfer model was developed to predct the morphologes of slab soldfcaton end. In ths model, the materal propertes of pertectc steel between soldus and lqudus temperature were calculated from the relatonshp of phase fractons and temperature, whch n turn were derved by mcro-segregaton model. In order to descrbe the uneven soldfcaton shell accurately, the boundary condtons of secondary coolng zones were derved by real water flux dstrbuton of secondary coolng nozzles, and fnally, the shell thckness was verfed by means of the nal shootng method. The non-unformty of soldfcaton end under dfferent castng condtons for dfferent castng speeds and nozzles arrangement, were smulated and analyzed. Thus the poston and length of SR zone were optmzed. Fnally, the plant results before and after the applcaton of optmzed SR zone confguraton were compared. 2. Heat Transfer Model As shown n Fg. 2, the wde-thck slab contnuous castng machne s composed of 1 bender, 6 bow segments, 2 straghtener segments, and 6 horzontal segments from mold end to caster end, and t s dvded nto 10 coolng zones. All segments are composed of 7 roller pars and could execute soft reducton functon by adjustng the taper of segment nner and outer frames. 7 to10 nozzles are arranged between every two neghborng rollers n the 1 st to 4 th secondary coolng zones, whle there are only flat armst coolng nozzles n one row from the 5 th to 8 th coolng zones. Based on some smplfed assumptons, 11) the mathematcal heat transfer model of quarter slab transverse secton was developed to predct the temperature feld of wde-thck slab. A two-dmensonal transent heat conducton equaton was employed to descrbe the heat transfer behavor as follow: T T T ρ( T) c( T) λ T λ T... (1) t = ( ) x x + ( ) y y Where, T and t are temperature, C and calculaton tme, s, respectvely. ρ (T), c(t) and λ(t) are the densty, kg/m 3, specfc heat, J/(kg C), and heat conductvty, w/(m C), respectvely. The explct fnte dfference method was adopted to calculate Eq. (1) wth non-unform grd method. In the present work, pertectc steel wth castng temperature C was chosen as specfc research steel grade, and ts man compostons was 0.17 Wt Pct C, 0.15 Wt Pct S, 0.60 Wt Pct Mn, Wt Pct P, and Wt Pct S Materal Propertes In order to obtan more accurate materal propertes of pertectc steel between the soldus and lqudus temperatures range, a one-dmensonal drect fnte-dfference model was developed to track the δ /γ transformaton and to predct the solute redstrbuton. Based on the assumpton of Ueshma et al., 12) the transverse secton of the growng dendrte was a regular hexagon as shown n Fg. 3(a), and one sxth of a dendrte was taken as the modelng doman wth a length of λ /2, whch was dvded nto 100 nodes as shown n Fgs. 3(b) and 3(c). Accordng to the Fck s second law, the solute dffuson control equaton was expressed as follows s = ( ) s C... (2) t x D s T C x s Where D ( T ) s the dffuson coeffcent of solute element n sold phase, m 2 /s. C s s the solute concentraton of element n the sold phase. The lqud moton of nter-dendrte was neglected, and the total mass of solute element was assumed to be constant durng the dendrte growth process. The prmary dendrte armng space, λ, was calculated by: 13) n λ = CR wc... (3) Where, C R s the effectve coolng rate, and t s set as 1.78 C/s accordng to the prevous research of present authors; 14) w c s nomnal steel carbon content; n s ndex obtaned from the followng equaton: CR CR n = (4) CR < CR 1. 0 The lqudus temperature, T l and the δ /γ phase transformaton temperature, T Ar4 were determned by the solute concentratons n the nterface nodes as gven below: 12,15) T m wt% C N = ( ) l T n wt% C Ar 4 N = ( )... (5) Fg. 2. The dvson of coolng zones for the contnuous caster. Fg. 3. Schematc dagram of the dendrtes morphology and calculaton doman: (a) an overvew of the dendrtes array, (b) transverse sectons of dendrtes, and (c) the structure of the modelng doman ISIJ 104

3 ISIJ Internatonal, Vol. 54 (2014), No. 1 Where m and n are the slope of the lqudus and the nterface between δ and γ phase n the Fe- bnary phase dagram; N s the number of solute elements consdered n the model. The rates of dffuson nto sold and lqud phases were determned by dffuson coeffcents and equlbrum dstrbuton coeffcents of the elements. 12,15) The fundamental equatons were descrbed n detal by Ueshma et al. 12) Fgure 4(a) shows the phase fracton evoluton of pertectc steel durng ts soldfcaton, and (b), (c), and (d) show ts conductvty, densty and enthalpy, respectvely, whch were calculated by weghted phase fracton equatons that were descrbed n detal by L and Thomas. 16) It should be noted that the thermal conductvty of lqud steel s usually ncreased compared to ts sold state conductvty due to ts flow and convecton 11,17) n the calculatons Boundary Condtons The boundary condtons for heat transfer are dvded as mold, secondary coolng, and ar coolng from mold level to the end of strand. (1) Mold coolng The heat extracton n mold was calculated as follows: λ( T ) T... (6) = q mold n Where, n represents length dmentons ( x for the slab narrow face and y for the slab wde face); q mold s mold heat flux, Mw/m 2. The heat flux was decreased from slab surface center to the corner because the shrnkage of slab corner would ncrease heat resstance between the slab and the mold. Therefore, q mold was calculated as follows: qmold = qcenter ( 1 exp( a1x a2))... (7) Where, a 1 and a 2 are parameters accordng to dfferent heght n mold; x s poston from surface center to corner, m; the q center s the heat flux of the surface center along the castng drecton, Mw/m 2. It was calculated from the Eq. (8) proposed by Savage and Prtchard 18) q = A center B t... (8) Where, q center s the heat flux on the broad face center, Mw/m 2 ; A and B are coeffcents dependng on the mold coolng condtons; t s tme on the mold, s. The fnal calculaton results were calbrated and verfed by the authors prevous work, whch gave the heat flux dstrbuton between the mold and slab surface and slab temperature dstrbuton based on the dstrbuton of mold flux, the ar gap dstrbuton and slab soldfcaton shrnkage. 19) (2) Secondary coolng In the 1 st to 8 th coolng zone, the water was sprayed drectly towards the slab. In ths area, the heat of the strand s manly taken away by the coolng water n the form of convecton, and an equvalent convecton coeffcent, q sec, was taken to calculate heat extracton. λ( ) T T... (9) = q = ( ) sec hsec Tsurf Tamb n Where, T surf and T amb are the surface temperature of the strand and the ambent temperature, respectvely, n C n h w and h ec calculatons, and the unt s K n h rad calculatons; h sec s the effectve heat transfer coeffcent, w/(m 2 C), and t could be expressed by: h = h + h + sec rad ec hw... (10) Where, h rad, h ec and h w s the heat transfer coeffcent of radaton, roller contact and water spary n th coolng zone, w/(m 2 C). The h rad s calculated as follows: hrad = σ ε (( Tsurf + 273) + ( Tamb + 273))...(11) 2 (( T + 273) + ( T + 273) 2 ) surf amb Fg. 4. Phase fracton and materal propertes of pertectc steel: (a) phase fracton, (b) conductvty, (c) densty, and (d) enthalpy ISIJ

4 ISIJ Internatonal, Vol. 54 (2014), No. 1 Fg. 5. Water flux dstrbuton characterstcs of two typcal nozzles: (a) orgnal nozzle (110 ), (b) new nozzle (90 ). Fg. 6. The nozzles layout of bow and straghten segments on transverse drecton. Fg. 7. Transverse water flux dstrbutons n the 8 th coolng zone under dfferent nozzle arrangements. Where, σ s Stefan-Boltzmann constant, w/ (m 2 K4 ); ε s steel emssvty. The h ec s calculated as follows: hc NR RL h... (12) ec = ZL Where, N R s the roller number of th coolng zone; R L s the contact length between roller and slab n th coolng zone, m; Z L s the length of the th coolng zone, m. Accordng to the prevous research, 17,20,21) h c s set as Kw/(m 2 C), and R L s equal to 0.02 m. Accordng to the expermental results of F. Ramstorfer et al. 6) and Nozak et al., 11) the h w s the functon of the water flux dstrbuton, and t could be expressed as: hw = α W( x) 055. ( Tw)... (13) Where, α s a modfed parameter of th coolng zone, whch depends on the caster parameters; T w s the coolng water temperature, C; W (x) s the water dstrbuton flux n th coolng zone (l/(m 2 mn)), where x s the dstance from the slab surface center to corner. In order to mprove the accuracy of boundary condtons n the secondary coolng zones, the water-ar flow rate and water flux dstrbuton of all nozzles were measured, and the Fg. 5 gves the water flux dstrbuton of two typcal nozzles. The 110 nozzle was orgnally used n the 7 th and 8 th coolng zones, and the 90 nozzle was used to compare ts coolng effects wth that of orgnally nozzle. The measured results show that the water flow dstrbuton of nozzles are both subject to the Gaussan dstrbuton, and the hghest pont of water flow s n or around the nozzle centerlne poston. Based on the smooth Gaussan fttng data and the nozzle arrangement, the W (x) was derved. Fgure 6 gves the nozzles arrangements of 5 th to 8 th coolng zones, and Fg. 7 presents the example of water flux dstrbuton of dfferent nozzle arrangements n 8 th coolng zone whle the ar and water pressure are both equal to 0.1 MPa. (3) Ar coolng In 9 th and 10 th coolng zones, the water spray was towards the rollers nstead of the slab, so only radaton and roller contact were consdered. The heat extracton q ar s calculated as follows: λ( T ) T = = ( + q h h n ) ( T T ) ar rad ec surf amb... (14) 2.3. Model Valdaton A typcal example of the steady-state castng process s presented to verfy the model. The pertectc steel was cast at 0.9 m/mn, whle the castng temperature was C and the slab secton was mm 250 mm at the room temperature. Fgure 8 shows the profle of the mushy zone n the cross secton of the mddle of strand. In the castng drecton, the solne of f s=0 s almost unform from the slab center to about x=0.85 m, but the solnes of f s around 1/8 slab wdth locaton are prolonged wth ncrease of f s. In the transverse drecton, the shell (f s=0) develops almost equally from the slab center to x=0.55 m due to the stable coolng ntensty as shown n Fg. 7, but the lqud core s prolonged gradually from locaton x=0.55 to 0.82 m due to the contnuous decrease of coolng ntensty. Fgure 9 shows the temperature profle of the slab transverse secton at the end of 8 th and 9 th segments, whch are located at m and m from the menscus, respec ISIJ 106

5 ISIJ Internatonal, Vol. 54 (2014), No. 1 tvely, and t clearly ndcates the dumbbell-shaped soldfcaton end. In order to verfy the uneven shape of soldfed shell, the nal shootng was carred out at dfferent locatons n the Fg. 8. The solnes under dfferent f s on the mddle secton of the strand. transverse and castng drectons. The measured results are shown n Fg. 10. Nal shootng s a knd of smple and effectve method to measure shell thckness, and t had been successfully used by many researchers. 8,22 24) The prncple of the nal shootng method s that sulphur was embedded n the slot of the nal surface, and after the nal was shot nto the slab, the nal could not melt n the soldfed shell whle sulphur hardly redstrbuted. However, the nal would gradually melt from soldus to the lqud core, and the dffuson of sulphur would be ncreased correspondngly. Therefore, the shell thckness s the length wth the clear mark of nal boundary, and the reference lnes, whch are perpendcular to the slab thckness drecton, as shown n Fg. 10 to ndcate shell thckness. The shootng locatons, predcted results, measurements, and the errors are lsted n Table 1 and are compared n Fg. 11. The predcted results of the shell thckness agreed well wth the nal shootng measurement results, and the absolute value of relatve errors were found to be less than 2.12% Fg. 9. The temperature profle of transverse secton at m and m from the menscus. Fg. 11. Comparson between predcted results and measurements of shell thckness. Fg. 10. Nal shootng results of dfferent locaton: (a) 1/2 wde of slab (slab center) at the 7th segment end; (b), (c), and (d) were 1/2, 1/4 and 1/8 wdth of slab at the 8th segment end respectvely. Table 1. Comparson between the predcted results and measurements. Strand poston Predcted Shootng results (mm) Error From menscus (m) Of slab wdth results (mm) Measured Calbrated Absolute (mm) Relatve (%) / / / / ISIJ

6 ISIJ Internatonal, Vol. 54 (2014), No. 1 whch may be caused by the delay of nal meltng and measurement errors. 8) 3. Optmzng the Soft Reducton Zone Accordng to the research by Takahash, 25) the flow ablty of metal would decrease wth the ncrease of the sold fracton, but lqud steel could flow freely n the low sold fracton regons, whle n the hgh sold fracton the solute segregaton would ncrease due to no supply of the lqud steel. Therefore the soft reducton should be appled to hgh sold fracton regon, so the solute-enrched lqud could be squeezed out of the slab center. Based on the prevous research by the present authors, 2,3) the soft reducton parameters of pertectc steel were chosen from f s=0.6 to 0.9 n slab contnuous castng process n present work. Furthermore, the locaton of 1/8 slab wdth, where ts length of lqud core was almost equal to the max length on the whole slab wdth as shown n Fg. 8, was chosen to present the maxmum length of uneven soldfcaton end compared to the other locatons Effect of Dfferent Castng Speeds The pertectc steel slab of mm 250 mm secton was smulated wth dfferent castng speed, whle the castng temperature was C. Fgure 12(a) presents the solnes of SR zone n the mddle of the thckness cross secton of the strand. Wth the ncrease of the castng speed, the secondary coolng ntensty enhances, however the tme for heat release decreases, and therefore the strand poston of the SR zone moves towards cast end ether on the 1/2 or 1/8 locaton of slab wdth. For nstance, when the castng speed ncreases from 0.9 to 1.0 m/mn, the SR start and end pont of 1/2 locaton (slab center) move towards cast end wth 2.08 and 2.28 m, respectvely. Fgure 12(b) ndcates that the solnes of SR zone are more and more non-unform n the transverse drecton wth ncrease of the castng speed owng to the hysteress of heat transfer. The strand poston of SR start pont of 1/8 locaton are prolonged to that of 1/2 locaton (slab center) wth 1.57, 1.81 and 2.09 m, and extended to the SR end pont at 2.00, 2.28 and 2.52 m, when the castng speed s 0.9, 1.0 and 1.1 m/mn, respectvely. The length of SR zone of 1/8 locaton s longer than that of 1/2 locaton due to the prolonged lqud core as shown n Fg. 12(b). It s obvous that the orgnal SR zone, whch was desgned based on the sold fracton at the slab centerlne, could not reduce the segregaton around 1/8 locaton of slab wdth. In the present work, orgnal SR start poston s consdered as that of whole slab wdth, but the SR end pont of whole slab wdth was mproved accordng to that of 1/8 locaton. As shown n Fg. 13 the length of SR zone ncreased almost lnearly about 0.24 m, 0.23 m, and 0.51 m for 1/2 locaton, 1/8 locaton, and for whole slab wdth, respectvely, when the castng speed ncreases by 0.1 m/mn Effect of Dfferent Nozzle Arrangements The overcoolng on slab corners reduces the ductlty of the slab, whch would cause the corner transverse cracks durng the straghtenng stage. The nozzles arrangement could effect the water flux dstrbuton sgnfcantly, and n order to reduce the transverse corner cracks of mm wdth slab, 90 ar-mst nozzles were chosen nstead of the orgnal 110 one n the 7 th and 8 th coolng zones. 26) The water flux dstrbuton characterstcs for these nozzles are shown n Fg. 5(b), Fgures 14(a) and 14(b) compare the solnes of SR zone of mm 250 mm slab before and after changng the nozzle arrangement n the mddle of wdth and thckness Fg. 13. Length of SR zone for dfferent slab locatons. Fg. 12. The solnes of SR zone of mm 250 mm slab n the mddle of thckness (a) and wdth (b) cross secton of strand for dfferent castng speed ISIJ 108

7 ISIJ Internatonal, Vol. 54 (2014), No. 1 Fg. 14. The solnes of SR zone of mm 250 mm slab n the mddle of wdth (a) and thckness (b) cross secton of strand wth dfferent nozzle arrangements. Fg. 15. The solnes of SR zone of mm 250 mm slab n the mddle of wdth (a) and thckness (b) cross secton of strand wth dfferent nozzle arrangements. cross secton of strand, when the castng speed s 0.9 m/mn. Fgure 14(a) ndcates the poston of SR zone of 1/2 locaton s not qute dfferent under two knds of nozzles arrangements. At the slab center, the water flux under 90 nozzoles arrangement s lower than that of 110 ones as shown n Fg. 7, whch caused the SR start and end ponts move towards cast end wth m and m after the 90 nozzles used. Because the water flux of 90 nozzles s hgher than that of 110 ones at the locaton of 1/8 shown n Fg. 7, the SR start and end ponts under 90 nozzles move forward to menscus by 0.22 m and 0.12 m than that of 110 nozzles, respectvely. As shown n Fg. 14(b) the solnes of SR zone from x=0 (slab center) to x=600 mm are almost same under two dfferent nozzles arrangements, because the water flux wth the two nozzle arrangements s nearly same as shown n Fg. 7. From x=600 mm to x=850 mm, the water flux of 90 nozzles s hgher than that of 110 ones, whch reduced the length of lqud core. In the last 200 mm near the slab corner, the solnes of SR zone are almost unchanged before and after the 90 nozzles were appled. Ths s because the coolng effect of the narrow surface s more sgnfcant than that of the wde surface and the water flux does not change much after nozzle arrangement was adjusted. Therefore, the SR zone s almost nvarable after the 90 nozzles were replaced wth the 110 ones n mm wdth slab contnuous castng process. In order to meet dfferent customer s demands, the wdethck slab contnuous castng machne was also used to produce narrower secton slabs, such as mm wdth, and the overcoolng of slab corner s more serous as shown n Fg. 7. Therefore, the nozzles arrangement of the 7 th and 8 th coolng zones was modfed n the mm wdth slab castng process n that only the center nozzle was left wth reduced water flow rate. 26) Fgure 15 compares the solnes of SR zone of mm 250 mm slab before and after adjustment of nozzles arrangement, whle the castng speed was 1.0 m/mn. Fgure 15(a) ndcates that the SR zone of 1/2 and 1/8 locatons are almost same under three 110 nozzle arrangements due to the nearly same coolng ntensty at these two locatons as shown n Fg. 7. After the sde nozzles were removed n 7 th and 8 th coolng zones, the coolng ntensty of 1/8 locaton was reduced, whch prolonged the SR start and end poston wth 1.8 m and 2.2 m towards the cast end, respectvely. It s can be seen from Fg. 15(b) clearly that the SR zone s almost constant from x=0 (slab center) to x=0.6 m under three nozzle arrangements. But when the sde nozzles were removed, the SR prolonged gradually from the slab center to x=0.55 m due to the contnuously decrease of water flux. Therefore, after the sde nozzles were removed n the 7 th and 8 th coolng zone, the length of SR zone gradually prolonged from the slab center to around locaton of 1/8 wdth, and SR should be fnshed by 9 th and 10 th segments together nstead of only the 9 th segment. Comparng Fgs. 14(b) and 15(b), t can be seen that the ISIJ

8 ISIJ Internatonal, Vol. 54 (2014), No. 1 Table 2. The defect nspecton statstc results of slabs before and after optmzaton. Before optmzaton After optmzaton Month total total Samplng number Central segregaton % 56.35% 62.42% 62.00% 93.91% % 93.13% Central porosty % 90.61% 89.93% 91.14% 100% 100% 100% 100% effect of nozzle arrangements on the SR zone of mm wdth slab s more sgnfcant than that of mm wdth slab. On one hand, the slab corner water flux before and after nozzle arrangement optmzed for mm wdth slab was changed more than that of mm wdth slab, and therefore the effect of constant narrow sde coolng plays a more sgnfcant role on the heat extracton around slab corner n castng of mm wde slab. In fact, at the end of 8 th coolng zone the slab corner temperature of the mm wdth slab s mproved more than 160 C, whle that of the mm wdth slab s only about 60 C. 26) On the other hand, as shown n Fg. 7, the water flux changed on the whole transverse surface of mm wdth slab after adjustment of the nozzles arrangement, but n mm wdth t only changed around the slab corner. As a result, the wder range of uneven water flux caused more sgnfcant non-unformty of the SR zone. Accordng to the smulaton results, t can be concluded that the lqud core around 1/8 locaton of slab wdth s sgnfcantly longer than that of slab center both for mm and mm wde slabs due to the reduced water flux around slab corner. 4. Results n Plant Applcaton Accordng to the smulaton results, n mm 250 mm secton slab contnuous castng process, the SR was usually executed by two segments, the frst segment was used to elmnate the macro-segregaton from 1/2 to 1/4 locaton of slab wdth, and the macro-segregaton around 1/8 locaton was reduced by the next segment. To be specfc, SR were executed by 8 th and 9 th segment for castng speed 0.9 m/mn, 9 th and 10 th segment for castng speed 1.0 m/mn, and 10 th and 11 th segment for castng speed 1.1 m/mn. The orgnally desgned SR rate was adopted for the frst segment, and t was usually set as mm/m accordng to the slab thckness and specfc strand poston. However, there s a brttle temperature range between the zero strength temperature (ZST) and zero ductlty temperature (ZDT) at the soldfcaton end due to the presence of nterdendrtc lqud flms caused by the mcro segregaton of solute elements. 3) If the SR s appled on ths area wth large SR rate, the center cracks would be nduced. In order to solve ths contradcton, the SR rate of the second segment was dropped to mm/m. For the mm 250 mm secton slab, because the SR zone was prolonged by change of nozzles arrangement, SR should also be executed by two segments. The SR rate lmtaton s smlar to that of mm wdth slab. The optmzed SR zone wth adjusted SR rate was appled n practce, and the transverse macrographs of the mm 250 mm slab before and after the optmzaton are Fg. 16. The transverse macrographs of the mm 250 mm slab before (a) and after (b) the optmzaton of SR zone. shown n Fg. 16. It can be seen that the centerlne macrosegregaton s serous before the optmzaton especally on the range of 1/4 to 1/8 locaton of the slab wdth due to the prolonged soldfcaton end, and t s amelorated sgnfcantly after the optmzaton. The defect nspecton statstcal results of slabs durng three months each before and after optmzaton are lsted n Table 2. It can be seen that the proporton of centeral segregaton wth defects level no hgher than 1.0 level ncreases from 62.00% to 93.13%, and the defect level of nearly all of the centeral porosty does not exceed Conclusons A two-dmensonal heat transfer model was developed to descrbe the uneven dstrbuton of SR zone on the transverse drecton n wde-thck slab contnuous castng process. In order to obtan accurate materal propertes, a mcrosegregaton model was used to calculate the relatonshp between phase fracton and temperature, and the boundary condtons were derved by the realstc water flux. The model was verfed by the nal shootng results of dfferent strand locatons, and the absolute values of relatve errors were found to be no more than 2.12%. The lqud core around locaton of the 1/8 slab wdth prolonged when the water flux around the slab corner was decreased, whch was necessary to avod the slab transverse cracks. For the mm wdth slab, the strand poston of SR zone moved towards the cast end wth ncrease of castng speed and length of the SR zone ncreased almost lnearly about 0.24 m, 0.23 m, and 0.51 m for 1/2 locaton, 1/8 locaton and whole slab wdth, respectvely when the castng speed was ncreased by 0.1 m/mn. For mm wdth slab, the SR zone gradually prolonged from the slab center to around 1/8 locaton of slab wdth because the water flux reduced after the sde nozzles were removed n 7 th and 8 th coolng zones. At last, the SR zone was optmzed based on the predcted 2014 ISIJ 110

9 ISIJ Internatonal, Vol. 54 (2014), No. 1 results, and the SR rate was adjusted accordngly. The plant results showed that the macro-segregaton was sgnfcantly mproved after optmzaton, and the proportons of centeral segregaton and centeral porosty whch the defects grade s not hgher than 1.0 reach 93.13% and 100%, respectvely. Acknowledgments The authors sncerely acknowledge the fnancal support from Natonal Natural Scence Foundaton of Chna No and No REFERENCES 1) Y. Tsuchda, M. Nakada, I. Sugawara, S. Myahara, K. Murakam and S. Tokushge: Trans. Iron Steel Inst. Jpn., 24 (1984), ) C. J and M. Y. Zhu: 139th TMS Annual Meetng & Exhbton, Mnerals, Metals and Materals Socety/AIME, Warrendale, PA, (2010), ) S. Luo, M. Y. Zhu, C. J and Y. Chen: Ironmakng Steelmakng, 37 (2010), ) H. F. Shen, R. A. Hardn, R. MacKenze and C. Beckermann: J. Mater. Sc. Technol., 18 (2002), ) L. L. Guo, Y. Tan, M. Yao and H. F. Shen: Int. J. Mn. Met. Mater., 16 (2009), ) F. Ramstorfer, J. Roland, C. Chman and K. Morwald: Int. J. Cast. Metal. Res., 22 (2009), 39. 7) Y. J. Lu, Q. Wang, Y. G. L, S. P. He, Y. M. He, S. S. Pan, J. G. Zhang and B. Hu: Ironmakng Steelmakng, 38 (2011), ) M. J. Long, D. F. Chen, Q. X. Wang, D. H. Luo, Z. W. Han, Q. Lu and W. X. Gao: Ironmakng Steelmakng, 39 (2012), ) M. J. Long, D. F. Chen, L. F. Zhang, Y. Zhao and Q. Lu: Metall Int., 16 (2011), ) B. Mntz and D. N. Crowther: Int. Mater. Rev., 36 (1991), ) T. Nozak, J. Matsuno, K. Murata, H. Oo and M. Kodama: Trans. Iron Steel Inst. Jpn., 18 (1978), ) Y. Ueshma, S. Mzoguch, T. Matsumya and H. Kajoka: Metall. Trans. B, 17 (1986), ) M. El-Bealy and B. G. Thomas: Metall. Mater. Trans. B, 27 (1996), ) W. L. Wang, M. Y. Zhu, Z. Z. Ca, S. Luo and C. J: Steel Res. Int., 83 (2012), ) Y. M. Won and B. G. Thomas: Metall. Mater. Trans. A, 32 (2001), ) C. S. L and B. G. Thomas: Metall. Mater. Trans. B, 35 (2004), ) S. Louhenklp, J. Mettnen and L. Holappa: ISIJ Int., 46 (2006), ) J. Savage and W. H. Prtchard: J. Iron Steel Inst., 178 (1954), ) Z. Z. Ca and M. Y. Zhu: Acta Metall. Sn, 47 (2011), ) B. Petrus, K. Zheng, X. Zhou, B. G. Thomas and J. Bentsman: Metall. Mater. Trans. B, 42 (2011), ) G. Xa and A. Schefermuller: Steel Res. Int., 81 (2010), ) L. H. Feng, M. Y. Zhu and K. Lu: Fundamental of Chemcal. Engneerng, Vol Trans Tech Publcaton, Swtzerland, (2011), ) J. C. Ma, Z. Xe and G. L. Ja: ISIJ Int., 48 (2008), ) W. H. Lu, Z. Xe, Z. P. J, B. Wang, Z. Y. La and G. L. Ja: J. Iron Steel Res. Int., 15 (2008), ) T. Takahash, M. Kudoh and K. Ichkawa: Trans. Jpn. Inst. Met., 21 (1980), ) C. J, M. Zhu, Z. Ca and Y. Saha: 8th Pacfc Rm Int. Cong. on Advanced Materals and Processng, Mnerals, Metals and Materals Socety/AIME, Warrendale, PA, (2013), ISIJ