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ISIJ International, Vol. 60 (2020), No. 1, pp. 106–113

* Corresponding author: E-mail: qliu@ustb.edu.cnDOI: https://doi.org/10.2355/isijinternational.ISIJINT-2019-409

Optimization of Thermal Soft Reduction on Continuous-Casting Billet

Yanshen HAN,1) Wei YAN,1) Jiangshan ZHANG,1) Weiqing CHEN,1) Jun CHEN2) and Qing LIU1)*

1) State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing, 100083 China.2) Xiangtan Iron & Steel Co., Ltd of Hunan Valin., Xiangtan, Hunan, 411101 China.

(Received on July 2, 2019; accepted on July 18, 2019; J-STAGE Advance published date: September 25, 2019)

Thermal soft reduction (TSR) is an effective technique to improve the inner quality of continuous-casting billet, but it may lead to undesired internal and surface cracks. In this work, the technologic parameters of TSR were optimized to ensure its effect and control the cracks of 82A tire cord steel billet. A heat transfer model with comprehensive thermo-physical parameters was established to simulate the thermal behavior of continuous-casting billet. The model was verified by comparing the measured surface tem-peratures and the calculated ones. According to the mechanism of TSR on billet, both the location and water flow rate were comparatively optimized. TSR was determined to locate at 6.96 m–8.46 m from meniscus, where the temperature of billet center dropped rapidly to liquid impenetrable temperature. The water flow rate of TSR was set to 2.2 m3/h, which allowed the reheating rate and surface temperature in a reasonable range and prevented the formation of the cracks. Plant trials were conducted to verify the effect of the optimized TSR. The results showed that the central porosity, V segregation and central seg-regation of the billet were obviously improved by applying TSR. Meanwhile, the internal and surface cracks were well controlled in the billet.

KEY WORDS: thermal soft reduction; technologic parameters; crack; inner quality; central segregation.

1. Introduction

To meet the requirements of high-quality product, contin-uous-casting billets have a growing demand for inner qual-ity, especially for central segregation. Central segregation is an element enriched macroscopic defect formed in billet center, and it cannot be totally eliminated in the subsequent rolling process. The formation of central segregation can be attributed to a flow of enriched liquid in two phase region during final solidification,1–4) and the flow can be caused by a number of factors such as bulging, roll misalignment, solidification and thermal shrinkage during continuous cast-ing. Various methods such as electromagnetic or permanent magnetic stirring,5–7) intensive secondary cooling,8) low superheat,9,10) mechanical soft or heavy reduction11–14) and thermal soft reduction15–20) (TSR) are proposed to improve the central segregation, and remarkable outcomes have been acquired. Among the above methods, TSR is a technique that applies an intensive water spray cooling on billet sur-face at the solidification end. The cooling achieves a rapid temperature drop of the billet surface that corresponds to the billet center, and thus compensates the solidification shrink-age of the central region and inhibits the flow of enriched liquid toward the billet center.1,2,16) As a result, the central segregation can be improved.

In 1990s, Sivesson et al.16–18) conducted a series of inves-tigations about TSR on billet. They applied an intensive water spray cooling until the steel totally solidified, and the central segregation of the billet was decreased evidently. Thereafter, Ray et al.19) adopted TSR to improve the inter-nal quality of boiler plates at Bhilai Steel Plant and found a significant improvement in plate quality. Jung et al.20) inves-tigated TSR on the centerline macro-segregation of stainless steel slab by numerical simulation and industrial test, and both the experimental results and numerical results indicated a decrease of segregation ratio. All the above findings reveal that TSR is an effective method to improve the inner quality of the billets and slabs. However, the investigations mainly focus on the central segregation, and the potential cracks are paid less attention. As TSR is close to the straightening area and achieved by intensive water spray cooling, it is inevita-bly associated with the formation of cracks. Ludlow et al.8) reported that internal reheat-type cracks and surface cracks may form if TSR was applied unsuitably. The cracks also deteriorate the product quality, and therefore they should be taken into consideration on the application of TSR.

Recently, few reports are related to TSR owing to the above-mentioned cracks and a wide application of final electromagnetic stirring. Nevertheless, compared with final electromagnetic stirring, TSR has the advantage of running costs and installation convenience. If the cracks are well controlled, TSR can be a good choice to improve the inner quality of the billet. In the present study, TSR was adopted

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to improve the inner quality of 82A tire cord steel billet, and the control of the surface and internal cracks was also emphasized. A heat transfer model with comprehensive thermo-physical parameters was established to simulate the thermal behavior of continuous-casting billet. The techno-logic parameters of TSR were optimized and regulated to control the cracks. Finally, plant trials were conducted to verify the effect of the optimized TSR.

2. Heat Transfer Model Description

A circular-arc caster with a curved mold was studied. The caster has twelve strands and mainly produces the billet with a section size of 150 mm×150 mm. The schematic diagram of the caster is shown in Fig. 1. The effective lengths of the mold and the secondary cooling zone are 0.84 m and 4.90 m, respectively. At the mold, electromagnetic stirring (MEMS) is applied to improve the quality of the billet and it can generate a rotating magnetic field. In the secondary cooling zone, air-mist nozzles are used to ensure uniform cooling. In the air-cooling zone, TSR is applied to improve the inner quality, while its technical parameters are underdetermined.

2.1. Model EstablishmentA slice moving method was applied to the heat transfer

model. Figure 2 shows the coordinate system and geo-metric model of the slice. To simplify the calculation, the geometric model adopted a quarter of the cross section of the billet with a thickness of 10 mm, namely with a dimen-sion of 75 mm×75 mm×10 mm. The geometric model also considered the round corner according to the final billet size. The radius of the round corner was 6 mm. During the simu-lation, it was assumed the slice moved from the mold to the secondary cooling zone and the air-cooling zone.

2.1.1. AssumptionSome assumptions were made to simplify the heat trans-

fer model.

(1) The heat transfer in the casting direction and meniscus was neglected.

(2) The convective heat transfer was equivalent to con-ductive heat transfer.

(3) The boundary conditions on four sides of the billet at each cooling zone were uniform.

(4) The density, solid fraction and thermal conductivity of the steel were temperature dependent.

2.1.2. Governing EquationBased on the above assumptions, a two-dimensional

unsteady state heat transfer equation was expressed as fol-lows.

��

� �CT

x

T

x y

T

yP eff eff eff,

��

���

��

���

��� �

��

��

�

��

�

�� ......... (1)

where ρ is the density of steel in kg/m3; Cp,eff is the effective specific heat in J/(kg·K); T is the temperature of steel in K; τ is the calculation time in s; λeff is the effective thermal con-ductivity in W/(m2·K); x is the distance from billet center along transverse direction in m, and y is the distance from billet center on inner radius face in thickness direction in m.

The effect of latent heat on the solidification of steel at mushy zone was incorporated into the effective specific heat, and represented by enthalpy, as shown in Eqs. (2) and (3).

C C Lf

Tp eff P

s, � � �

��

h .......................... (2)

H C T L fP

T

s� � �� d h0

1( ) ...................... (3)

where CP is the actual specific heat in J/(kg·K); Lh is the latent heat in J/kg; H is the enthalpy in J/kg and fs is the solid fraction.

2.1.3. Boundary Conditions and Initial ConditionThe billet successively passes the mold, the secondary

cooling zone and the air-cooling zone during continuous casting. The heat extraction from the billet is dominated by different mechanisms in each cooling zone, and conse-quently, the boundary conditions differ greatly.

In the mold, the heat transfer includes the conductive and convective heat transfer in molten steel, the conductive heat transfer from solidified shell to slag film and copper plates,

Fig. 1. Schematic diagram of the billet continuous caster. (Online version in color.)

Fig. 2. The (a) coordinate system and (b) geometric model of the slice. (Online version in color.)

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as well as the radiation heat transfer from solidified shell to the gap and the copper plates. Due to a lack of detailed data, the heat flux was obtained from a heat balance of the cooling water, and equalized to some empirical equations used by other workers, as shown in Eqs. (4) to (6).21) The heat transfer is largely influenced by the gap that formed between billet corner and copper plates.22,23) To consider the effect of the gap on the heat transfer, a decreasing heat flux was used from the billet center to the corner, and the heat flux at the corner decreased with the increase of distance from the meniscus.24)

q b L vm � �2 680 000 / ...................... (4)

b q L va m� � �1 5 2 680 000. ( ) / / .............. (5)

q C W T Sa w m eff� � �� / ....................... (6)

where qm is the heat flux in the mold in W/m2; L is the dis-tance from the meniscus in m; v is the casting speed in m/s; qa is the average heat flux in the mold in W/m2; Lm is the effective length of the mold in m; Cw is the specific heat of the cooling water in J/(kg·K); Wm is the water flow rate in the mold in L/s; ΔT is the temperature difference between the inlet and outlet of the cooling water in K, and Seff is the effective area of the copper plates in m2.

In the secondary cooling zone, the heat transfer includes the impinging water sprays, the radiation, the water evapora-tion and the roller contact cooling. Owing to the complex heat transfer patterns, the heat extraction was represented by an integrated heat transfer coefficient, which was a function of water flux density. The heat flux and the heat transfer coefficient can be expressed by Eqs. (7) and (8).25)

q h T Tws s� �� � ............................. (7)

h Ws s� �220 10 44 0 851. . ........................ (8)

where qs is the heat flux in the secondary cooling zone in W/m2; hs is the integrated heat transfer coefficient in W/(m2·K); T is the temperature of billet surface in K; Tw is the temperature of secondary cooling water in K, and Ws is the water flow rate in L/(m2·min).

In the air-cooling zone, the radiation is the main heat

transfer pattern and the heat extraction can be expressed by Eq. (9).

q T Tr � �� ��� 4 4e ............................ (9)

where qr is the heat flux of radiation in the air-cooling zone in W/m2; σ is Stefan-Boltzmann constant in W/(m2·K4); ε is the radiation coefficient, and Te is the environment tem-perature of the air-cooling zone in K.

The initial condition of the heat transfer model is shown in Eq. (10):

T Tp= ................................... (10)

where Tp is the pouring temperature during casting in K.

2.2. Material Properties82A tire cord steel was studied and its chemical compo-

sition is given in Table 1. The variations of solid fraction, density, enthalpy and thermal conductivity with temperature were calculated using the thermodynamic database from JMatPro software, as shown in Fig. 3. In order to consider the effect of fluid flow on heat flow, the convective heat transfer was equivalent to enhancing the conductive heat transfer by increasing the effective thermal conductivity of the liquid and mushy zone.26,27) According to the type and intensity of the fluid flow, the whole continuous cast-ing process was divided into two zones, namely forced convection zone and natural convection zone. In the forced convection zone, the fluid flow is intense on account of the initial flow from submerged nozzle and external force by MEMS. Therefore, the convective heat transfer accounts for a large proportion. Accordingly, seven times the thermal conductivity of solidus was adopted for the liquid phase, and the thermal conductivity of the mushy zone was assumed to change with temperature linearly. As for the natural convection zone, the convective heat transfer accounts for a small proportion and the thermal conductivity adopted the original values.

Since the fluid flow is quite complex and hard to be measured accurately, it is difficult to distinguish the forced convection zone and the natural convection zone. To quan-tify the length of the forced convection zone, the deflection of columnar crystal was used as the growth direction of the

Table 1. Chemical composition of the 82A steel.

Composition C Si Mn P S Cr Ni Cu Al Ti

Mass fraction (%) 0.83 0.23 0.5 0.01 0.007 0.015 0.008 0.007 0.0017 0.0006

Fig. 3. Thermo-physical properties of 82A steel: (a) solid fraction, (b) density and enthalpy and (c) thermal conductiv-ity. (Online version in color.)

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columnar crystal was largely influenced by the fluid flow. In the forced convection zone, the fluid flow is basically rotary owing to MEMS, and the temperature gradient and concen-tration distribution of solute are changed by the flow.28,29) Accordingly, it exhibits a deflected columnar crystal on the cross section of the billet.

In the current process, a transverse billet was sampled and etched by picric acid to reveal the solidification structure of the billet, as shown in Fig. 4. The columnar crystal deflects until 16.4 mm from billet surface, and then it stops deflect-ing and continues to grow in the direction perpendicular to billet surface. It indicates that when the billet solidifies with a shell of 16.4 mm, the fluid flow is weak enough and the columnar crystal no longer deflects. Accordingly, the forced convection zone can be regarded as an area from meniscus to the place where the billet solidifies with a shell of 16.4 mm, and it is calculated to be 2.22 m from menis-cus by mean of the heat transfer model. Therefore, it can be obtained that the length of the forced convection zone is 2.22 m from meniscus.

2.3. Model ValidationThe main casting parameters of 82A steel are shown in

Table 2. The heat transfer model was initially validated without TSR. When the location and water flow rate of

TSR were determined and the plant trials were conducted, the surface temperatures with TSR were also measured and the measured results also supported the validation of the model. The validation of the model was conducted by com-parisons between the measured temperatures and the calcu-lated ones. The surface temperatures were measured by an infrared radiation pyrometer with an error range of ±1.5%. To reduce the influence of spraying water on the measured results, the measured positions avoided the spraying zone. During the measurement, the pyrometer was perpendicular to the surface center of the side arc and peak values were adopted as the local temperature. The temperature was also calculated by the heat transfer model. Figure 5 shows the temperature profiles of surface center. Without TSR, the surface temperature changes slowly at TSR zone, while the one with TSR drops rapidly. At the position of 11.7 m from meniscus, the temperature of surface center is reduced about 60°C induced by TSR. Under each condition, the measured temperatures agree well with the calculated ones within the error range. It indicates the heat transfer model shows good accuracy, and the model can be used to simulate the thermal behavior of 82A steel billet during continuous casting.

3. Results and Discussion

3.1. Location of TSRIn Sivesson’s investigations,16–18) TSR located at the place

Fig. 4. Deflection of columnar crystal: (a) sampling position, (b) etched solidification structure, and (c) instantaneous solidification state at 2.22 m from meniscus. (Online version in color.)

Table 2. Main casting parameters of 82A steel.

Item Value

Sectional dimension 150 mm ×150 mm

Casting speed 1.8 m/min

Pouring temperature 1 488°C

Water flux of mold cooling 125 m3/h

Temperature difference between7°C

inlet and outlet of mold water

Specific water of secondary cooling 0.5 L/kg

Water flow rate of TSR 0/2.2 m3/h

Current and frequency of MEMS 300 A/4 Hz

Water temperature 35°C

Ambient temperature 25°C Fig. 5. Temperature profiles of surface center. (Online version in color.)

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where the solid fraction of billet center was 0.2 until the billet totally solidified. As the location of TSR was close to the straightening area, the surface temperature of the billet was largely influenced and that it may deteriorate the surface quality. In this work, liquid impenetrable temperature30) (LIT) was introduced to determine the location of TSR. LIT is a characteristic temperature when steel solidifies, and its corresponding solid fraction is 0.85.31) During solidification, when the temperature drops below LIT, the liquid steel can’t penetrate through the solidified dendrites, and thus the flow of enriched liquid can be avoided. In Fig. 3(a), it can be obtained that the LIT of 82A steel is 1 391°C, which is used to determine the end position of TSR. Figure 6 shows the variations of the temperature and solid fraction of bil-let center. The temperature of billet center starts to drop rapidly at about 6.96 m from meniscus, and this site will be the starting position of TSR. Therefore, the location of TSR is determined to 6.96 m–8.46 m from meniscus, where the solid fraction of billet center varies from 0.43 to 0.85. As the suitable position of TSR is closely related to the solidifica-tion state of billet center, the casting parameters should be steadily controlled.

3.2. Water Flow Rate of TSRTemperature variations of the billet during continuous

casting were calculated using the validated heat transfer model. Figure 7 represents the 3D topographies of the sur-face temperature and interior temperature from the meniscus to the air-cooling zone. The surface temperature starts at around 1 500°C and rapidly drops to less than 800°C caused by the intense cooling in the mold. Thereafter the tempera-ture fluctuates in different sections of the secondary cooling zone owing to the change of cooling intensity, and finally decreases slowly in the air-cooling zone. As for the interior temperature, there exists a big difference between billet cen-ter and billet surface. The effect of cooling water becomes weaker with the increase of distance from surface. At the place 20 mm beneath billet surface, the reheating phenom-enon becomes unclear when the surface cooling conditions changes. Near billet center, the temperature keeps nearly

unchanged until the place that 6.96 m from meniscus, and then it begins to drop rapidly. At this position, the surface temperature varies slowly due to only radiation heat transfer in the air-cooling zone.

According to the relationship between the density and temperature of 82A steel shown in Fig. 3(b), the variations of density and volume per unit mass when the billet passes TSR zone were calculated, as shown in Fig. 8. As the sur-face temperature is lower than the interior temperature, the density of the steel increases gradually from billet center to surface center and the volume per unit mass exhibits the opposite results. In the TSR zone, the density and volume per unit mass near surface center keep almost constant, while the ones near billet center change obviously. The steel density of billet center increases from 7 126 kg/m3 to 7 259 kg/m3, accompanied with the corresponding volume change. In order to compensate the solidification shrinkage of billet center, the surface temperature should be suffi-ciently reduced, which is directly determined by the water flow rate of TSR.

Sivesson et al.16–18) reported that the cooling rate of sur-face center need to be twice as large as that of billet center to exert the effect of TSR, and that intensive cooling could

Fig. 6. Variations of the temperature and solid fraction of billet center. (Online version in color.)

Fig. 7. Temperature variations of (a) billet surface and (b) billet interior. (Online version in color.)

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work better. In this work, air-mist nozzles were used to apply TSR and three different water flow rates were initially chosen, including 1.5 m3/h, 2.2 m3/h and 5 m3/h. The heat transfer of the billet was simulated under different water flow rates, and the thermal behaviors in the TSR zone were emphasized. Figure 9 shows the average cooling rates of billet center and surface center when the billet passes TSR zone, and Fig. 10 displays the volume changes per unit mass from billet center to surface center. Without TSR, the cooling rate of billet center and surface center are 1.02°C/s and 0.15°C/s, respectively. That is because there is a fast temperature drop at billet center on account of the release of latent heat at the solidification end. However, the surface temperature varies slowly due to only radiation heat transfer, as shown in Fig. 7(b). Accordingly, there is a large volume shrinkage near billet center and a small one near surface center. With TSR, the cooling rate of billet center changes little, but the cooling rate of surface center increases greatly. When the water flow rate varies from 1.5 m3/h to 5 m3/h, the cooling rate of surface center increases from 3.20°C/s to 6.05°C/s. It indicates each chosen water flow rate can achieve good effect on improving the inner quality of billet. Additionally, the temperature near surface center is reduced and the volume shrinkage is increased by applying TSR. As a result, the solidification shrinkage near billet center can be compensated, and the compensation effect becomes more obvious with the increase of water flow rate. However, it should be noted that excessive cooling of TSR may lead to a low surface temperature and a high reheating rate, and therefore increases the cracking possibility of the billet.

Figure 11 shows the temperature variations of three typi-cal points under different water flow rates. When the water flow rate is 5 m3/h, the reheating rates of surface center and billet corner are 170°C/m and 190°C/m, respectively. There exists high possibility of generating reheat-type internal cracks on account of exceeding 100°C/m.32) Moreover, the corner temperature is 731°C at the straightening point. Under this low temperature, the transformation from austen-ite phase to ferrite phase may occur and carbonitride may precipitate at the grain boundaries,33) and therefore reduces the ductility of the steel. It could lead to high possibility of cracking near billet corner and worsen the surface qual-ity of the billet.34) When the water flow rate is 1.5 m3/h,

Fig. 8. Variations of (a) density and (b) volume per unit mass when the billet passes TSR zone. (Online version in color.)

Fig. 9. Average cooling rates of billet center and surface center when the billet passes TSR zone. (Online version in color.)

Fig. 10. Volume changes per unit mass from billet center to sur-face center when the billet passes TSR zone. (Online ver-sion in color.)

the reheating rate and the surface temperature are in a reasonable range, but it cannot exert the effect of TSR suf-ficiently. Finally, the water flow rate of TSR is determined to 2.2 m3/h, through which the maximum reheating rate of bil-let surface is less than 100°C/m and the surface temperature at the straightening point is above 800°C. Accordingly, the

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internal and surface cracks can be well controlled.

3.3. Plant TrialsPlant trails were conducted at the eleventh strand of the

continuous caster. The casting speed, pouring temperature, and secondary cooling intensity in the trials were 1.8 m/min, 1 488°C and 0.5 L/kg, respectively, and the casting condi-tions were kept at a relatively steady state. In the plant trials, the water flow rates of TSR were adjusted from 0 to 2.2 m3/h and the billets were sampled at each condition. Each sample was etched to reveal macro defects, and also sliced and drilled for chemical analysis.

Aiming at the analysis of the macro defects, the billet slices were sampled in both transverse and longitudinal direction, as shown in Fig. 12(a). Thereafter, the samples were etched 20 minutes using hydrochloric acid solution with a concentration of 50% and a temperature of 70°C, and then photographed, as shown in Fig. 12(b). On the longitudinal samples, there exist inconsecutive central pipes whether TSR is applied or not, but the pipes are not obvi-ous in the transverse samples. Without TSR, V segregation is found on the longitudinal sample. The V segregation exhibits lots of discontinuous black strips and distributes non-uniformly near billet center, which can be regarded

as the flow channels of enriched liquid and has a close relationship with central segregation.35,36) As for transverse sample, severe central porosity is observed around billet center. By applying TSR, the V segregation and the central porosity are alleviated, especially for the central porosity on the transverse sample. This phenomenon indicates the flow of the enriched liquid is inhibited and the compactness of the billet center is increased. Moreover, surface and internal cracks are not observed among all the samples. It indicates that the optimized TSR can effectively avoid the formation of the cracks.

Central segregation degree is represented by a carbon segregation index of billet center. Under each condition, ten successive billet slices with a thickness of 20 mm were sampled, and the center of each sample was drilled to 10 mm depth by alloy drills with a diameter of 5 mm. All the drillings were analyzed by a carbon-sulfur analyzer. The carbon segregation index is defined as C/C0, where C is the carbon content of the drillings and C0 is the carbon content of liquid steel in tundish. Figure 13 shows the distribution of central carbon segregation degree along casting direc-tion. It can be seen that the central segregation degree of each condition distributes non-uniformly. Without TSR, the central carbon segregation degree ranges from 1.03 to 1.30,

Fig. 11. Temperature variations under different water flow rates: (a) 0 and 1.5 m3/h, (b) 2.2 m3/h, and (c) 5 m3/h. (Online version in color.)

Fig. 12. Schematic diagram of (a) sampling positions and (b) macrographs of etched samples. (Online version in color.)

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and the average value is 1.13. When TSR is applied, nega-tive segregation is observed on some samples. Moreover, the maximum value and the average value of the central carbon segregation degree are decreased to 1.18 and 1.06, respectively. It indicates the optimized TSR improved the central segregation of the billet effectively.

4. Conclusions

A heat transfer model with comprehensive thermo-phys-ical parameters was established and validated for 82A tire cord steel billet during continuous casing. The location and water flow rate of TSR were discussed and determined to control the surface cracks and the reheat-type internal cracks of the billet. Thereafter, plant trials were conducted to verify the effect of the optimized TSR on the inner quality of billet. The following conclusions can be drawn.

(1) In the heat transfer model, the deflection of colum-nar crystal was used to quantify the forced convection zone, the length of which was calculated to be 2.22 m from menis-cus. In the forced convection zone, the thermal conductivity of liquid was enlarged by seven times.

(2) TSR was determined to locate at 6.96 m–8.46 m from meniscus, where the temperature of billet center dropped rapidly to liquid impenetrable temperature and the solid fraction of billet center varied from 0.46 to 0.85.

(3) TSR could reduce the temperature near billet surface and compensate the solidification shrinkage near billet cen-ter. With the increase of cooling intensity, the compensation effect becomes more obvious. However, the internal and surface cracks need to be taken into consideration.

(4) The water flow rate of TSR was set to 2.2 m3/h, through which the maximum reheating rate of billet surface could be controlled less than 100°C/m and the surface tem-perature at the straightening point was above 800°C, thus preventing the formation of internal and surface cracks.

(5) Plant trials showed that the inner quality of 82A steel billet was effectively improved by applying the opti-mized TSR, and the average central carbon segregation degree was decreased from 1.13 to 1.06. Moreover, internal and surface cracks were well controlled.

AcknowledgmentsThe authors are grateful to the independent subject of

State Key Laboratory of Advanced Metallurgy, University

Fig. 13. Distribution of central carbon segregation degree along casting direction. (Online version in color.)

of Science and Technology Beijing, China, grant number 41617003, which enabled the research to be carried out successfully.

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