Thermo-Mechanical Behavior of the Continuous CastingBloom in the Heavy Reduction Process

CHENG JI,1 CHEN-HUI WU,1 and MIAO-YONG ZHU1,2

1.—School of Metallurgy, Northeastern University of China, No. 3-11, Wenhua Road, HepingDistrict, Shenyang 110819, Liaoning, China. 2.—e-mail: myzhu@mail.neu.edu.cn

A two-stage sequential heavy reduction (HR) method, in which the reductionamount was increased both before and after the solidification end, is presentedto simultaneously improve the homogeneity and compactness of the continu-ous casting bloom. With bearing steel GCr15 chosen as the specific researchsteel, a three-dimensional thermal–mechanical finite element model wasdeveloped to simulate and analyze the thermal and mechanical behaviors ofthe continuous casting bloom during the HR process. In order to ensure theaccuracy of the simulation, the constitutive model parameters were derivedfrom the experimental results. The predicted temperature distribution andshell thickness were verified using a thermal infrared camera and nailshooting results, respectively. The real measured relationship between the HRpressure and amount were applied to verify the mechanical model. Theexplorative application results showed that the quality of the bloom center andcompactness of rolled bars have both been significantly improved after the HRwas applied.

INTRODUCTION

Soft reduction (SR) technology is an effectivemethod to reduce the slab and bloom centerlinesegregation as well as porosity in the continuouscasting process.1,2 SR is usually implemented in themushy zone to compensate for liquid core shrinkageand to prevent the solute-enriched liquid flowingtoward the center of the strand. However, it isdifficult to conquer the solidification shrinkagecompletely after the SR zone. Furthermore, in thecontinuous casting process of blooms or slabs withlarger section sizes, the regular SR amount is notenough to conquer the dissipation of the reductionenergy caused by the thicker shell deformation, andthe center quality could not be efficiently improved.On the other hand, the temperature differencebetween the strand surface and center is approxi-mately 400�C at the solidification end, indicatingthat the compactness of the strand center could beeffectively improved by means of increasing thereduction amount at and after the solidification enddue to the temperature gradient. According to theabove-mentioned reasons, heavy reduction (HR)technology, which imposes a larger reduction rate/amount at and after the solidification end of the

strand, was developed for healing the solidificationshrinkage cavity and for improving the centerdensity.

In the early 1990s, Kawasaki Steel Corp. pro-posed and applied continuous forging technology inthe bloom continuous casting process to control theporosity and segregation of the center.3,4 In thismethod, the solidifying shells were pressed intocontact with each other by forging them down witha pair of anvils installed at the solidification end,and the center segregation and porosity were suc-cessfully improved. Increasing numbers of research-ers have realized that the quality of the center of theslab and bloom could be significantly improved bymeans of increasing the reduction amount/rate atthe solidification end. Nippon Steel presented NSBloom Large Reduction Technology,5 which applieda large reduction amount after the complete solid-ification end of the continuous casting bloom by apair of convex-shaped rolls. With these, the reduc-tion energy could be concentrated onto the bloomcenter, and the center porosity could be efficientlyreduced. Sumitomo Metal Industries also developeda HR method called porosity control of the castingslab (PCCS).6,7 With the PCCS technology, thecenter porosity of the very thick continuous casting

JOM, Vol. 68, No. 12, 2016

DOI: 10.1007/s11837-016-2041-8� 2016 The Author(s). This article is published with open access at Springerlink.com

(Published online August 2, 2016) 3107

slab could be significantly decreased by the largeroll reduction just before the end of solidification.Yu et al.8 verified the effect of the temperaturegradient of the rolling process on improving thecenter porosity of the ultra-heavy plate by meansof a physical rolling experiment and numericalsimulation, and this process could be regarded as atype of HR method if the reduction positions werelocated before the torch cutoff of the caster. In theabove-mentioned methods, the HR position wasusually fixed on the strand, and the internalquality of the slab or bloom was hardly improvedunder any circumstances due to the unfixed solid-ification end, especially when the casting speed orthe casting steel grade was changed. Therefore,POSCO provided a type of flexible HR method,which was executed by several segments at thestrand solidification end, and the maximum reduc-tion rate was 30 mm/min.9 Zhao et al.10,11 proposeda new HR approach, called the heavy reductionprocess to improve segregation and porosity(HRPISP), and they simulated the deformationand dynamic recrystallization (DRX) behavior ofthe HR process of the extra-thick slab by using theTHERCAST.

It can be inferred that powerful mechanical,hydraulic, and motor-driven equipment is usuallyrequired to implement HR, and the correspondinginvestment cost of upgrading the caster is muchhigher. In the present work, a two-stage sequen-tial HR method was designed while still makingfull use of the original withdrawal and straight-ening units’ capacities. In the first stage, thereduction amount was greatly enhanced before thesolidification end to improve the homogeneity ofthe bloom, and in the second stage, the bloom wasgreatly compressed at and after solidification bythe following units with their maximum reductioncapability to improve the compactness of thebloom. In this method, the upgrade of equipmentis not necessary, and the reduction parameterscould be adjusted flexibly according to the specificcasting conditions. With the specific parameters ofa bloom continuous casting machine, a three-dimensional thermo-mechanical coupled finite ele-ment model was built to describe the effect of theHR on the thermal and mechanical behaviors ofthe bearing steel GCr15 bloom. In order toimprove the accuracy of the simulation results,the constitutive model for the GCr15 bloom wasderived from the experimental results. Themechanical model was validated by the realmeasured relationship between the reduction pres-sure and the amount of withdrawal and straight-ening units, and the predicted temperaturedistribution as well as shell thickness were veri-fied by the results of a thermal infrared cameraand nail shooting, respectively. A typical HR caseof the steady-state casting process was simulated,and the temperature distribution, shell thickness,reduction amount and equivalent strain at the

bloom surface and inside were analyzed andcompared. Finally, the results of explorativeindustrial application were presented.

MATHEMATICAL MODELING

The deformation behavior of the strand shell hasa significant effect on the fluid flow and macro-segregation in the mushy zone, which had beenproven by the pioneering work of Miyazawa andSchwerdtfeger12 and the work of Kajitani et al.13

Wu et al.14 developed a two-phase columnar solid-ification model to describe the effect of shell defor-mation on the macro-segregation behavior, and themechanism of SR was derived according to thesimulation results. However, the shell deformationbehavior of the HR process is different from that ofthe SR process due to its dramatically increasedreduction amount, so in this work, a 3-D thermal–mechanical coupled FE model was developed todescribe the shell deformation behavior during thesequential reduction process.

In the present work, a 4-strand arc bloom contin-uous casting machine was chosen as the specificresearch objective as shown in Fig. 1. The bloomsection is 380 mm 9 490 mm at room temperature,and the reduction is executed by withdrawal andstraightening units located in the air cooling zone.The caster was originally designed by CONCAST,and the maximum cylinder pressure of the with-drawal and straightening units is 1900 kN. A type ofbearing steel, GCr15, was chosen as the specific steelgrade used in this research, and the main composi-tion was 1.00 wt.% C, 0.25 wt.% Si, 0.30 wt.% Mn,0.01 wt.% P, 0.01 wt.% S, and 1.45 wt.% Cr.

Due to the symmetry of the bloom in the widthdirection, one-half of the bloom, with dimensions of1200 mm 9 245 mm 9 380 mm, was chosen as theresearch object as shown in Fig. 2. Based on somesimplified assumptions,15,16 a 3-D thermal–

1 2 3 4 5 6 7 8 9 10 11 12

800m

m

16524.5mmMold

`

R=1

6500

mm

R=2

3000

mm

R=4

6000

mm

17500mm

31670mm

Unit

1-12 Units number

Secondary cooling zone

Air cooling zone

1683

7.2m

m

84.8°

3.5°

1.74°

Distance from meniscus 1# 17500mm 7# 24640mm 2# 18740mm 8# 26050mm 3# 19920mm 9# 27410mm 4# 21000mm 10# 28830mm 5# 22280mm 11# 30250mm 6# 23640mm 12# 31670mm

Length of secondary cooling zones

Zone 1 380mm Zone 2 1420mm Zone 3 1780mm Zone 4 2330mm Zone 5 2340mm

Fig. 1. The schematic of the bloom continuous casting machine.

Ji, Wu, and Zhu3108

mechanical coupled FE model was developed usingthe commercial software Msc.Marc. Hexahedralfinite elements were used to mesh the computationdomain, and the thermal and mechanical analyseswere carried out by using the sequential couplingmethod when the bloom goes through the strandfrom the meniscus to the caster end with astable casting speed of 0.45 m/min. The heat fluxand displacement at the symmetrical face of themodel are assumed to be zero. The HR was executedby withdrawal and straightening units in the aircooling zone. In order to improve the calculationefficiency of the coupled model, only heat transferwas calculated before the bloom arrived at the firstunit. The thermal and mechanical behavior of thetypical points at the middle slice of the model wereinvestigated and compared during the HR process.The locations of the typical points are shown anddescribed in Fig. 2.

Heat Transfer Model

A 3-D transient heat conduction equation wasemployed to describe the heat transfer behavior asfollows:

q Tð Þc Tð Þ @T@t

¼ @

@xk Tð Þ @T

@x

� �þ @

@yk Tð Þ @T

@y

� �

þ @

@zk Tð Þ @T

@z

� �: ½1�

where T and t are the temperature in �C andcalculation time in s, respectively. q(T), c(T), andk(T) are the density in kg/m3, specific heat in J/(kg �C), and heat conductivity in W/(m �C), respec-tively x, y and z are the width, thickness, and lengthof the bloom, respectively.

Based on the present authors’ previous work,1 amicrosegregation model was established for acquir-ing the material properties of GCr15 in the temper-ature range between the solidus and liquidustemperatures. The thermal parameters, such as

the conductivity, enthalpy, density, etc., were cal-culated by weighted averaging of the phase fractionwith the specific calculation equations, which havebeen described in detail by Li and Thomas.17

The boundary condition of heat transfer has beendescribed in the present authors’ previous work.1 Inthe mold, the heat flux decreases from the surfacecenter to the corner due to the shrinkage of theshell. In the secondary cooling zone, the equivalentconvection coefficients of each of the cooling zoneswere calculated according to the measured waterflux distribution of the nozzles. The heat transfercaused by the roll–bloom contact was also consid-ered. Before the first withdrawal and straighteningunit, the heat extraction of the roll–bloom contactwas considered by the average equivalent heattransfer coefficient method due to the smallercontact length. As for the withdrawal and straight-ening units, due to the larger radius of the rolls andlonger roll–bloom contact length, the simulationdomain between two neighboring rolls was dividedaccurately by different heat extraction methods.The heat extraction between the rolls and bloom,qec, could be calculated as follows:

qec ¼ hiec � ðTsurf � Ti

rollÞ: ð2Þ

where hiec is the equivalent convection coefficient

between the ith roll and bloom in w/(m2 �C) and Tiroll

is the temperature of the ith roll. The hiec is

approximately 1.0 kW/(m2 �C)–1.25 kW/(m2 �C)based on the previous work of Xia and Schiefermul-ler,18 and the Tiroll is approximately 150–300�Caccording to the measured results of a thermalinfrared camera.

Constitutive Model

The bloom deformation was mainly caused by HR,and the thermal strain is not considered in thiswork. An Anand constitutive model was used todescribe the relationship between the inelasticstrain rate, temperature and flow stress of GCr15steel at high temperature.19,20 In this model, thereis no explicit yield surface, and the loading andunloading criteria are not needed. A single scalarvariable (deformation resistance, s) is used torepresent the isotropic resistance to inelastic strain.The constitutive equation is given as follows:

_ep ¼ A exp � Q

RT

� �sinh n

rs

� �h i 1m

: ð3Þ

where the evolution equation for s is expressed as:

ds

dt¼ h0 1 � s

s�

��� ���asign 1 � s

s�

� �h i_ep: ð4Þ

with

s� ¼ ~s_ep

Aexp

Q

RT

� �� �n

: ð5Þ

Fig. 2. 3-D thermal–mechanical coupled FE model and the locationsof typical points in the model.

Thermo-Mechanical Behavior of the Continuous Casting Bloom in the Heavy Reduction Process 3109

where _ep is the inelastic strain rate in s�1; s* is thedeformation resistance saturation value in MPa; s isthe deformation resistance in MPa; Q is the activa-tion energy of hot deformation of 350.10 kJ/mol; A isthe pre-exponential factor of 5.55 9 1013 s�1; n isthe multiplier of stress of 2.32; m is the strain ratesensitivity of stress of 0.331; h0 is the harden-ing/softening constant of 6025 MPa; ~s is the defor-mation resistance saturation coefficient of238.6 MPa; n is the strain rate sensitivity of satu-ration of 0.0046; a is the strain rate sensitivity ofhardening/softening of 1.1; and s0 is the initial valuefor deformation resistance of 64 MPa.

The parameters of the constitutive model werederived from the experimental results, which weremainly quoted from the previous work of Huaet al.21 and complemented by the present authors’experiments with temperatures of 750�C, 850�C,1200�C and 1250�C. The mean relative errorbetween the calculated and experimental flow stresscurves is lower than 2.06%, which indicates that theconstitutive equation could accurately describe themechanical behavior of GCr15 steel. The details ofthe constitutive model will be described and dis-cussed in detail in future publications.

In the present work, the temperature-dependentelastic modulus was calculated as the followingrelationship based on the experimental data fromMizukami:22

where E(T) is the elastic modulus in GPa. Ts andTZST are the solidus and zero strength temperaturein �C, respectively. ES is the elastic module whenthe temperature is equal to TS in GPa. EZST is theelastic module when the temperature is equal toTZST, and it is set as 0.01 GPa.

The temperature-dependent Poisson’s ratio, t(T),is calculated as follows:23

tðTÞ ¼8:23 � 10�5T þ 0:278 600�C � T � TS

ðT�TsÞtZSTþðTZST�TÞtS

TZST�TSTS <T � TZST

tZST T >TZST

8<:

ð7Þ

where tS is Poisson’s ratio when the temperature isequal to TS. tZST is Poisson’s ratio when thetemperature is equal to TZST. The tZST could notbe equal to 0.5 due to the numerical difficulty for thesolver, and it is set as 0.499.

In the continuous casting process, the mushy zonedeformed together with the solidified shell, and thereare two typical approaches to take into account the

influence of the mushy zone on the strand deforma-tion. Bellet et al.24,25 considered that the slab could bedivided as liquid part and a solid part by a so-calledcoherency temperature. The liquid part is modeledusing a viscoplastic law, and the solid part isdescribed by an elastic-viscoplastic law. However,Bellet et al.24 also noted that the constitutive equa-tion of the coherent solid phase in the mushy zone iscontested by the lack of experimental data. On theother hand, many researchers have applied theferrostatic pressure as the mechanical boundarycondition on the inside surface of the shell insteadof the liquid core.13,16,17,26,27 Due to lack of accuratemechanical properties in the mushy zone of theGCr15 bloom, the latter method was adopted in thepresent work, and the ferrostatic pressure was takenas a mechanical boundary condition at the isothermof the liquid impenetrable temperature (LIT).27 Theferrostatic pressure is calculated by

P ¼ qlghi: ð8Þ

whereP is the ferrostatic pressure in Pa;ql is the steeldensity at the liquidus temperature in kg/m3; g is thegravity acceleration in m/s2; and hi is the verticalheight from the meniscus to the ith unit in m.

The roll rotates around its axis at the castingspeed, and the bloom was driven by the frictionalforce between the rolls and the bloom while thecoefficient of friction is constant at 0.3.16 The unit

rolls were considered as rigid bodies, and the nodeto segment contact algorithm was used to detect thecontact between the rolls and bloom.17 The toler-ance for the roll penetration was chosen to be0.25 mm, and a penalty constraint was applied toavoid excessive roll penetration.

Parameters

A typical case of the steady-state casting processwas simulated, in which the GCr15 steel was cast at0.45 m/min with a casting temperature of 1497�C.In order to investigate the thermal and mechanicalbehaviors of the GCr15 bloom during the sequentialreduction process, the reduction amounts of thesecond to eighth units were all assumed to be 4 mmin this case, and the total reduction amount was28 mm.

Model Validation

The heat transfer behavior was verified by themeasured surface temperature and shell thickness.The surface temperature was measured by a

EðTÞ ¼ 968 � 2:33T þ 1:9 � 10�3 � T2 � 5:18 � 10�7T3 600�C � T � TSðT�TsÞEZSTþðTZST�TÞES

TZST�TSTS <T � TZST

EZST T >TZST

8>><>>:

ð6Þ

Ji, Wu, and Zhu3110

thermal infrared camera (A40, FLIR), and the shellthickness was measured by the nail shootingmethod.28 Figure 3 shows the comparison betweenthe predicted and measured results. As shown inFig. 3, the relative error between the predicted andmeasured temperatures on the bloom narrow sur-face center was <0.91%, while the relative errorbetween the predicated shell thickness and nailshooting results was<2.67%.

Figure 4 shows the actual measured and predictedrelationship between the reduction amount andpressure of each unit when the casting speed is0.45 m/min. With increasing pressure, the reductionamount increased approximately as a quadraticfunction. The calculated pressure with a reductionamount of 4 mm was compared with the measuredresult to verify the accuracy of the mechanical model.As shown in Fig. 4, the relative errors between thecalculated and measured pressures of each unit wereall <3.00% except those of the third and seventhunits, which was possibly due to the original error ofthe magnetostrictive position sensors. The meanrelative error of all of the units is 2.95%.

RESULTS AND DISCUSSION

Distribution of the Surface Temperature andSolidified Shell

Figure 5 shows the temperature distribution oftypical points on the inner surface during the HRprocess. Due to the contact between the bloom androlls, the temperature dips under the roll–bloomcontact areas were obvious. As for points B and C,the temperature dips under each unit are bothnearly equal to 80�C. Because the deformation-resistant ability of the bloom corner is far higherthan that of other positions due to it having thelowest temperature on the bloom inner surface, thecontact length between the bloom corner and rolls isthe smallest on the inner surface. Therefore, thetemperature dips of point A were far smaller than

those of points B and C, and they decreasedgradually along the casting direction due to thedecreasing contact length.

The solidus isolines with and without HR in themiddle of the longitudinal section of the strand arecompared in Fig. 6. The solidus isoline was com-pressed toward the outer surface due to the HR, andthe amount of compression on the upper half of thebloom was larger than that on the lower half of thebloom because the HR was implemented by theupper roll of the units. The solidification end afterHR deviated 12.15 mm from the original bloomcenter toward the bottom surface of the bloom.

In the bloom HR process, the contact area betweenthe bloom and rolls was evidently prolonged, and theheat transfer distance from the liquid core to thebloom surface was continuously reduced. Therefore,

0 5 10 15 20 25700

800

900

1000

1100

1200

1300

1400

1500 1# 2# 3# 4# 5# 6# 7# 8#

Strand position [m]

Tem

pera

ture

[o C]

Predicted temperature [ oC]Wide surface centerNarrow surface centerCorner

Measured temperature [ oC]Narrow surface center

-20

020406080100120140160180200220240

Shell thickness [mm] Predicted Nail shooting

Thic

knes

s [m

m]

Fig. 3. Comparison between the predicted and measured surfacetemperature and shell thickness.

450 600 750 900 1050 1200 1350 1500 16500123456789

101112

Predicted with error bar

MeasuredUnits Data Quadratic fitting8#7#6#5#4#3#2#

Red

uctio

n am

ount

[m

m]

Pressure [kN]Fig. 4. Measured and predicted relationship between the reductionamount and pressure of each unit.

17 18 19 20 21 22 23 24 25 26 27

700

750

800

850

900

950

1000 1# 2# 3# 4# 5# 6# 7# 8#Te

mpe

raut

re [o C

]

Distance from meniscus [m]

Point A, CornerPoint B, 1/4 location of inner surfacePoint C, Inner surface center

Fig. 5. The temperature distribution of typical points during the HRprocess.

Thermo-Mechanical Behavior of the Continuous Casting Bloom in the Heavy Reduction Process 3111

the heat extraction during the HR process wasenhanced, and the solidification end moved towardthe meniscus by 0.295 m as shown in Fig. 6.

Distribution of the Reduction Amount

Figure 7 shows the distribution of the HR amountof the typical points. As shown in Fig. 7a, there areobvious surface rebounds between each of the twoneighboring rolls, which are mainly caused by thealternate HR process of ‘‘compression–relaxation–compression’’, and the corner rebound is lower thanother points due to its higher deformation-resistantability. As for points B and C, the bulging occurredjust before the roll–bloom contact area. The broad-ening deformation of point B is blocked by thesolidified bloom narrow side, and the ‘‘accumulationeffect’’ obviously enhanced the bulging. The broad-ening deformation in the bloom corner could bereleased freely in the width direction, and nobulging occurs at point A.

In the direction of the bloom thickness, as shownin Fig. 7b, the HR amount transferred from thebloom surface to the center gradually, and in thiscase, the transfer efficiencies of the HR amount inthe bloom quarter and center are approximately85% and 50%, respectively. Between two neighbor-ing rolls, the rebound in the bloom inside is lowerthan that at the surface due to its longer HRtransfer distance, and the bulging just before theroll–bloom contact areas is almost invisible at pointsD and E due to the same reason.

Distribution of the Equivalent Strain

The equivalent strains of typical points are shownin Fig. 8, and they all increased stepwise becausethe reduction in the deformation of the bloomoccurred only under the units.

Figure 8a compares the equivalent strains of thepoints on the bloom inner surface. Because thecorner temperature is lowest on the bloom cross-

section, the deformation-resistant ability of point Ais the highest, and, correspondingly, the equivalentstrain of point A is the lowest. Because the temper-ature decreases along the casting direction, theincremental equivalent strain of point A under thesequential units decreases gradually. The tempera-tures of the width 1/4 location and center location onthe inner surface are almost the same, but theequivalent strain of point B is much higher thanthat of point C due to its larger broadening defor-mation in addition to the reduction deformation.The equivalent strains of points B and C decreasedbefore the sharp increases due to the obviousbulging just before the roll-bloom contact areas asshown in Fig. 7a.

As shown in Fig. 8b, the equivalent strain of theinner surface is the largest because the HR acts onthe inner surface directly. Because the deformation-resistant ability at the bloom center is the lowest,the equivalent strain of point E is higher than thatof point D (the bloom quarter). It could be inferredthat the lowest equivalent strain in the HR processis not located at the bloom center, and the HR couldbe transmitted to the bloom center successfully.

Results in Plant Application

Based on the simulation results of the FE modeland the reduction amount calculation method,1 thepreliminary reduction parameters of the two-stagesequential HR method were designed with a castingspeed of 0.45 m/min. Before the solidification end,the total reduction amount was approximately13.5 mm, which was executed by the third to thesixth units. At and after the solidification end, theHR was executed by the seventh to the ninth units,and the total reduction amount was 15.5 mmaccording to the maximum reduction ability of thecaster. The total reduction amount of the wholestrand is 29 mm, and the reduction ratio is 7.63%for a 380-mm thickness GCr15 bloom.

The designed HR parameters have been appliedto the above-mentioned bloom continuous castingmachine. The blooms with and without HR weresampled at the same time but from different castingstrands, and the macrographs of the blooms androlled bars (/ = 110 mm) transverse section withand without HR are compared in Fig. 9. It can beclearly seen that the serious porosity and shrinkagecavities in the bloom center were evidently elimi-nated after the HR was applied, and, correspond-ingly, the homogeneity and compactness of therolled bar were improved.

CONCLUSION

A two-stage sequential HR method has beenproposed to simultaneously improve the homogene-ity and compactness of the continuous castingbloom, and a 3-D thermal–mechanical coupled FEmodel was built to investigate the effect of the two-stage sequential HR on the thermal and mechanical

17 18 19 20 21 22 23 24 25 26 27 28

-80

-60

-40

-20

0

20

40

60

808#7#6#5#4#3#

Blo

om th

ickn

ess

[mm

]

Strand position [m]

Without reduction With reduction

Strand centerline without reduction

Solidificationend

2#

Fig. 6. Solidus isolines in the bloom thickness direction with andwithout HR.

Ji, Wu, and Zhu3112

behaviors of the GCr15 bloom. An Anand constitu-tive model was chosen to describe the relationshipbetween the inelastic strain rate, temperature andflow stress of the GCr15 bloom, and the modelparameters were derived from the experimentalresults. In order to verify the accuracy of the modelwith respect to the thermal behavior, the predictedsurface temperature and shell thickness were ver-ified by the thermal infrared camera and nailshooting results with relative errors <0.91% and2.67%, respectively. The real measured relationshipbetween the pressure and reduction amount of eachunit were applied to verify the accuracy of themechanical model, and the mean relative error is2.95%.

The typical casting case was simulated andanalyzed, in which the reduction amounts of thesecond to eighth units were all assumed to be 4 mm

with a casting speed of 0.45 m/min. In the GCr15bloom HR process, the heat extraction of the roll–bloom contact was enhanced, and the solidificationend of the strand moves toward the meniscus by0.295 m. The surface rebound between two neigh-boring rolls was caused by the alternate reductionprocess of ‘‘compression–relaxation–compression’’.The ‘‘accumulation effect’’ caused by the blockedbroadening deformation enhanced the bugling of thebloom inner surface just before the roll–bloomcontact area, and the bloom corner has no bulgingdue to the freely released broadening deformation.In this case, the transfer efficiencies of the reductionamounts in the bloom quarter and center areapproximately 85% and 50%, respectively, and theequivalent strain of the bloom center is higher thanthat of the quarter due to its lower deformation-resistant ability.

18 19 20 21 22 23 24 25 26-30

-25

-20

-15

-10

-5

0

Point A, Corner Point B, 1/4 location of inner surface Piont C, Inner surface center

Red

uctio

n am

ount

[mm

]

Distance from meniscus [m]

8#7#6#5#4#3#2#

23.40 23.55 23.70 23.85

-18

-20

-16

18 19 20 21 22 23 24 25 26-30

-25

-20

-15

-10

-5

0

Distance from meniscus [m]

Red

uctio

n am

ount

[mm

]

Point C, Inner surface center Point D, 1/4 from inner surface Point E, Center

8#7#6#5#4#3#2#(a) (b)

Fig. 7. Evolution of the HR amount at typical points: (a) points A, B and C; (b) points C, D and E.

18 19 20 21 22 23 24 25 260.000.050.100.150.200.250.300.350.400.450.50

24.5 24.6 24.7 24.80.050.100.150.200.250.300.350.40

(a)

Distance from meniscus [m]

Point A, corner Point B, 1/4 location on the inner surface Point C, inner surface center

Equi

vale

nt s

trai

n

8#7#6#5#4#3#2#

18 19 20 21 22 23 24 25 26-0.020.000.020.040.060.080.100.120.140.160.180.20(b)

Point C, inner surface center Point E, bloom center

Point D, 1/4 from inner surface

Equi

vale

nt s

trai

n

Distance from meniscus, m

8#7#6#5#4#3#2#

23.55 23.60 23.65 23.700.04

0.06

0.08

0.10

0.12

Fig. 8. Evolution of the equivalent strain at typical points: (a) points A, B and C; (b) points C, D and E.

Thermo-Mechanical Behavior of the Continuous Casting Bloom in the Heavy Reduction Process 3113

The explorative industrial application showedthat the bloom center quality and compactness ofthe rolled bars were improved significantly with thenew HR method. Furthermore, through combiningit with more industrial experiments, the modelcould be applied to investigate the two-stagesequential HR process systematically, such as opti-mization of the reduction amount, calculation of thereduction efficiency, calculation of the maximumreduction amount to avoid internal cracks, etc.

ACKNOWLEDGEMENTS

The present work is financially supported by theNational Natural Science Foundation of China No.51,474,058 and U1560208, the Program for Liaon-ing Excellent Talents in University (LJQ2015036)and Fundamental Research Funds for the CentralUniversities of China (N150205002). Special thanksare due to our cooperating company for industrialtrials and applilcattion.

OPEN ACCESS

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