numerical simulation of delayed pouring technique for a 360t heavy steel ingot

Numerical simulation of delayed pouring technique for a 360t heavy steel ingot

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2012 IOP Conf. Ser.: Mater. Sci. Eng. 33 012092

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Numerical simulation of delayed pouring technique for a 360t heavy steel ingot

J Li1, D R Liu2, X H Kang1 and D Z Li1, 3 1 Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, No.72 Wen Hua Road, Shenyang 110016, China 2 School of Materials Science and Engineering, Harbin University of Science and Technology, No.4 Lin Yuan Road, Xiang Fang District, Harbin 150040, China

E-mail: [email protected]

Abstract. A continuum mathematical model for the transport phenomena in the solidification systems has been established to study the central axial macrosegregation in a 360t multi concentration pouring (MCP) steel ingot. A time explicit finite difference scheme is adopted to calculate the coupling of the temperature, concentration and velocity flow fields. The flow equations are solved by the solution algorithm for transient fluid flow (SOLA) technique. Simulations of Fe-C-Si-Mn quaternary alloy are performed. The established model has been validated by comparing with the experimental result of a 360t MCP steel ingot. The simulated carbon concentration profile along the centreline of the ingot shows a fair agreement with the measurement. The influence of the delay time for the last ladle on the macrosegregation along the centreline has been investigated. Simulation results show that the delay time for the last ladle has a significant effect on the macrosegregation, especially for the positive segregation below the hot top. A novel criterion of selecting the delay time for the last ladle has been proposed to reduce the macrosegregation. By selecting the proper delay time for the last ladle, the carbon concentration along the centreline in the ingot body can be controlled within the range of the industrial limit.

1. Introduction Macrosegregation is a phenomenon of the deviation from the average composition over a distance larger than the dendrite arm spacing, which occurs during the solidification process of the melt [1]. It is an unavoidable phenomenon due to the different solubilities of elements in the liquid and solid phases [2]. For large ingots and castings, macrosegregation is more likely to happen due to long solidification time. Additionally, it is far more difficult to eliminate subsequently by thermo-mechanical post-treatments, and results in fateful and permanent defects in forgings [3]. Therefore, suppressing macrosegregation during the casting process has always been the focus of casting research [4-7]. In the past decades, scientists and engineers have tried many different techniques such as continuous casting [8], squeeze casting [9], electromagnetic stirring [10] and multi concentration pouring (MCP) technique to restrain the formation of the macrosegregation during the casting process. Among these techniques, the MCP technique developed in Japan in 1982 and characterized as the sequential pouring of the melt with different carbon concentrations [11], is considered to be a very promising technique due to its good maneuverability and notable effect, especially for the gigantic 3 Author to whom any correspondence should be addressed.

MCWASP XIII IOP PublishingIOP Conf. Series: Materials Science and Engineering 33 (2012) 012092 doi:10.1088/1757-899X/33/1/012092

Published under licence by IOP Publishing Ltd 1

mould casting ingots (300-600 t). The MCP technique has been adopted by many heavy machinery plants around the world recently

to reduce the macrosegregation in huge steel ingot. However, since the influence of the parameters on the macrosegregation is still not clear, the MCP casting parameters are generally selected according to experience, which directly results in poor application. With respect to the high experimental cost of the large steel ingot, it is more economical to study the formation of the macrosegregation during the MCP process and optimize the pouring technique using numerical simulation method. However, little information about the mathematical modeling of the MCP process has been found in the literature due to its complexity. Only Liu et al. [12] have performed the correlative simulations, with Fe-C binary alloy calculated in the model.

In this paper, a continuum mathematical model for the transport phenomena in the solidification systems has been established to investigate the formation of the macrosegregation in a 360 t MCP steel ingot. A time explicit scheme is used to solve the coupling of the temperature, concentration and velocity flow fields. Based on the explicit finite difference scheme, flow equations are solved by the SOLA technique, rather than the well-known time implicit SIMPLER algorithm. Fe-C-Si-Mn quaternary alloy is calculated in the simulation. The influence of the delay time for the last ladle on the macrosegregation in the large steel ingot has been reasonably studied and a novel criterion for selecting the delay time has been proposed.

2. Mathematical and numerical modelling The mathematical model is referred to that proposed by Pardeshi et al. [13] and Voller et al. [14]. Here only a brief description of the model is described.

2.1. Governing equations Mixture energy conservation equation

p

Hc UT U H T

t

(1)

1s s s lH f h f h (2)

where l s , sh is the solid phase enthalpy, lh is the liquid phase enthalpy, H is the mixture

enthalpy and is the heat conductivity. Mixture solute conservation equation

( ) 0k

klt

CUC

(3)

(1 )s s s s l lC f C f C (4)

where C is the mixture concentration, sC and lC are the average solid and liquid concentration

respectively and the superscript k is a marker for the kth component of an m component system. Continuity

0U

(5)

Momentum equations are based on the mixture theory, V-momentum

l

v PUv Kv v

t x

(6)

W-momentum


2

1

mj j j

l T ref c l refj

w PUw Kw w g T T C C

t z

(7)

1

1

ni i

l ladlei

ref ni

ladlei

T VT

V

(8) 0

1

1

ni i

ladlei

ref ni

ladlei

C VC

V

(9)

where m=3, is the number of elements (C, Si and Mn), n is the number of ladles, refT is the average

liquidus temperature of ladles, refC is the average initial concentration of ladles.

The mushy region is modelled by Carman-Kozeny relationship

20

3

KK=

1s

s

f

f (10) 0 2

180 l

s

Kd

(11)

2.2. Numerical implementation Conservation equations are numerically solved on a staggered Cartesian mesh using a finite volume approach and uniform spaced grids. Velocity components are defined on the interfaces of the control volume and other dependent variables are defined at the center of the control volume. The upwind scheme is adopted to evaluate the contribution of the convection. Diffusion terms are discretized with standard second-order finite difference procedure. In the present study, during each macro-scale time step, an explicit-time-stepping scheme is used to solve the coupling of the temperature, concentration and momentum equations, and the continuity equations are solved by the SOLA technique. The time-step is determined by the maximum of the velocity at the previous time step. The grid size is selected to be 30 mm×30 mm.

It is worth mentioning that in this work the filling process is not taken into consideration, since the simulated ingot has a very large volume. The filling time is generally 40~50 minutes, which is negligible compared to the 70-hour solidifying time. Thus the pouring of next ladle is simply treated as putting melt above the melt of former ladles in a simultaneous and quiescent way. After pouring (actually the pouring time is zero), the calculation domain is expanded. Velocity distribution in the melt of the former ladles is inherited and set as the initial condition for the following calculation. Then, the melt from different ladles is considered as a whole and participates in the calculation of all conservation equations.

3. Validation of the established model Figure 1 shows the schematic of a 360 t large steel ingot produced by the MCP technique. In the hot top mould, a 160 mm thick layer of fireclay and a 20 mm thick layer of insulation board were used at the circumference of the hot top. The top of the hot top was covered by a 300 mm thick layer of covering flux. The thermophysical data of the insulation materials in the hot top mold are listed in table 1. The chemical composition of the experimental steel are listed in table 2. The nominal carbon concentration of the experimental steel is 0.18 wt%. To counteract the traditional negative segregation at the lower part and positive segregation at the upper part of the ingot, the carbon concentration in ladle I and III was 0.21 wt% and 0.14 wt%, respectively. The carbon concentration in ladle II was equal to the nominal value. Additionally, the contents of Si and Mn in each ladle were uniform. The mass of the melt in each lade was 160 t (I), 160 t (II) and 40 t (III), respectively. Three ladles with different carbon concentrations were poured in succession, with uniform pouring velocity of about 8 t/min. In the experiment research, some specimens were taken from the centerline between point A and B after solidification and the carbon concentration was measured, as illustrated in figure 1. The


3

measured carbon concentration profile along line AB is shown in figure 2. The measured result indicates a negative segregation at the lower part and a positive segregation at the upper part of the ingot. The positive segregation at the upper part of the ingot body is severe and the maximum of the carbon concentration is 0.26 wt%, which is 0.04 wt% higher than the maximum of the industrial limit (0.16-0.22 wt%).

Figure 1. Schematic of vertical central plane of the 360 t steel ingot in two-dimensional.

Figure 2. Comparison between experimental and simulated carbon content profile along line AB.

Table 1. Thermophysical data of the insulation materials in the hot top mold.

ρ (Kgm-3) λ (Wm-1K-1) cp (KJKg-1K-1)Fireclay 2570 70 0.92 Insulation board 500 2.3 1.1 Covering flux 500 60 0.6

Table 2. Chemical composition of the experimental steel (wt%).

C Si Mn P S

0.16-0.22 0.15-0.30 1.2-1.5 ≤0.01 ≤0.015

In order to validate the established simulation model, solidification process of the 360 t MCP steel ingot with the experimental processing parameters has been simulated. Because of the axial symmetry of the steel ingot, the simulation model was restricted to the vertical central plane of the ingot and a two dimension simulation was performed. After the final ladle poured, a layer of covering flux was overlaid at the top of the ingot and then heat transfer was calculated at the covering flux/hot top interface. Before this process was finished, the top surface of the melt was treated as insulated. The experimental steel was simplified to Fe-0.18 wt%C-0.2 wt%Si-1.4 wt%Mn quaternary alloy in the simulations and the carbon concentration distribution was chosen to evaluate the macrosegregation. Thermophysical data of the Fe-C-Si-Mn alloy and the thermal boundary conditions used in simulation are referred to reference [7]. Comparison of the carbon concentration profile along the centerline between the simulation and the measured result is shown in figure 2. The simulation result presents a similar profile compared to the measurement, with only minute difference in details. It means that the established continuum mathematical model and the developed numerical scheme are reasonable and can be adopted to simulate the formation of the macrosegregation in the heavy steel ingot during the MCP processing.

4. Results and discussion

4.1. Optimization of the insulation condition in the hot top mold Besides the conventional negative and positive segregation, the measured carbon concentration profile along the centerline of the experimental steel ingot shows another interesting phenomenon, i.e., the largest carbon content is in the ingot body rather than in the hot top as usual. This phenomenon indicates that the final solidification region is in the ingot body, which is directly caused by the poor


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insulation condition of the hot top. This solidification sequence would definitely induce serious positive segregation and shrinkage porosity in the ingot body. Therefore, the insulation condition of the hot top should be optimized before improving the pouring technique to reduce the macrosegregation. Better insulation materials are chosen to ensure the better insulation condition of the hot top. The thermophysical data of optimized insulation materials are listed in table 3.

Table 3. Thermophysical data of optimized insulation materials in the hot top mold.

ρ (Kgm-3) λ (Wm-1K-1) cp (KJKg-1K-1)Fireclay 1000 0.6 1.13 Insulation board 200 0.3 1.13 Covering flux 200 0.4 1.13

Figure 3 shows the carbon concentration profile along the centerline of the 360 t MCP ingot after optimizing the insulation condition of the hot top. It clearly indicates that the point of the maximal positive segregation is shifted into the hot top, which means that a perfect sequential solidification is achieved. At the same time, the macrosegregation in the ingot body is reduced, the maximal carbon content decreases from 0.26 wt% to 0.24 wt%. Unfortunately, the 0.24 wt% carbon concentration in the ingot body is still beyond the range of the industrial limit (0.16-0.22 wt%). Namely, the carbon concentration cannot be controlled within the industrial limit for such gigantic ingot only by optimizing the insulation condition of the hot top. Therefore, pouring technique should be further optimized to reduce the macrosegregation after the insulation optimization. The following simulations are based on the insulation optimization model.

Figure 3. Carbon concentration profile along the centerline for MCP after insulation optimization.

Figure 4. Comparison of carbon concentration profile along the centerline between MCP and SCP.

4.2. Comparison of MCP and single concentration pouring (SCP) In order to clarify the effect of the pouring technique on the macrosegregation, first the solidification process of the 360 t steel ingot with the SCP technique is simulated. The initial carbon concentration in the ladle is 0.18 wt%, equaling to the nominal concentration of the experimental steel. The result is compared with that of the traditional MCP technique, in which three ladles with different carbon contents are poured in succession, as shown in figure 4. Surprisingly, there is almost no significant difference between these two curves, which means that the macrosegregation cannot be reduced efficiently by the traditional MCP technique. Figure 5 shows the solid fraction and the carbon concentration distribution in the ingot three hours after the last ladle poured in the MCP process. It can be seen that, at this moment, the solidification ratio of the whole ingot is only about 10%, while the carbon distribution is almost homogeneous in the remaining melt. The melt of different carbon concentrations from different ladles mixes quickly by natural convection before solidification. The homogenous mixing of the melt eliminates the dilute effect of the last ladle with lower carbon concentration. Thus in order to achieve the reduction of the macrosegregation, a proper interval between pouring ladles should be set to efficiently use the concentration gradient of different ladles. Generally, in order to avoid crack, there should be no interval between ladles when pouring the ingot


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body. So the interval between the second and the last ladle, viz., the delay time for the last ladle becomes the key point to be investigated and adjusted to control the macrosegregation. In the following discussion the multi concentration delayed pouring (MCDP) is used to describe a MCP case with a delay time for the last ladle.

Figure 5. (a) Solid fraction and (b) carbon concentration distribution in the ingot 3 hours after the last ladle poured in the MCP process.

4.3. The criterion of selecting the delay time for the last ladle In order to investigate the influence of the delay time for the last ladle on the macrosegregation in ingot, simulations are carried out for solidification in two MCDP cases with different delay time: (1) 6 hours and (2) 30 hours. The results are compared with the result of the MCP case (delay 0 hour), as shown in figure 6.

Figure 6. Comparison of carbon concentration profile along the centerline with different delay time.

Figure 7. Carbon concentration profile along the centerline 30 hours after the first two ladles poured.

It can be seen that, when the delay time is 30 hours, viz. the last ladle is poured 30 hours after the first two ladles poured, a negative-positive-negative-positive (NPNP) style segregation formed from the bottom to the hot top along the centerline. Although the positive segregation below the hot top is reduced remarkably, this result is still not ideal due to the large concentration fluctuation and uneven concentration distribution in the ingot body. Figure 7 shows the carbon concentration profile along the centerline at the moment just before the last ladle poured, which can explain the above result clearly. The carbon content changes sharply from point A to B in figure 7, indicating the existence of the solid-liquid interface in this region at the moment. When the melt with lower carbon content in the last ladle is poured, the negative-positive segregation below point A cannot be influenced any more, as the metal here has solidified. While the upper residual melt would definitely mixes with the new poured melt to even mixed melt quickly. Then a new solidification mode is established in the upper part of the ingot, where the new negative-positive segregation will appear again after the solidification. Finally the NPNP style segregation forms in the whole ingot body. It can be concluded that a too long delay time for the last ladle will induce the NPNP style segregation. However, when the delay time is 6 hours, it can be seen that no noticeable improvement of the macrosegregation is achieved and the largest carbon concentration is still beyond the industrial limit. This is because the delay time is so


6

short that the melt with lower carbon content mixes homogeneously with the residual carbon enriched melt before solidification, similar to the condition of the MCP described in section 4.2. It seems that the delay time of 6 hours is still not long enough to reduce the macrosegregation and a longer delay time should be considered.

From the above discussion, it can be concluded that in order to control the carbon macrosegregation in the ingot body efficiently, the delay time for the last ladle should be neither too long nor too short. To ensure the quality of the ingot, not only the carbon concentration should be controlled within the industrial limit but also the large concentration fluctuation should be avoided. So, these two facts taken together, a criterion of selecting the delay time for the last ladle is proposed. The delay time for the last ladle should be as long as possible with the precondition of preventing the appearance of the NPNP style segregation, viz., the melt with lower carbon concentration in the last ladle should be poured at the moment that the segregation along the centerline turns from negative to positive. At this moment, the melt with lower carbon content is poured to dilute the remaining higher carbon concentration melt, to efficiently control the macrosegregation in the ingot.

4.4. The selection of the delay time for the 360 t steel ingot Firstly, a case with only the first two ladles poured is simulated, in order to eliminate the influence of the last ladle. The carbon concentration profile along the centerline after solidification is showed in figure 7. The profile presents the same pattern with the case of pouring three ladles, the macrosegregation along the centerline turns from negative to positive at the height of 1590 mm in the ingot body. In order to determine this transition time, solid fraction along the centerline is traced. It is found that after 17-hour solidification, the solidification height along the centerline just reaches 1590 mm. Consequently, the 17-hour delay time is selected for the last ladle for the 360 t steel ingot.

Figure 8. Carbon concentration profile along the centerline for only the first two ladles poured.

Figure 9. Comparison of carbon content profile along the centerline for MCP and MCDP (delay 17 hours).

Finally, the solidification process of the MCDP technique with 17-hour delay time is simulated. Figure 9 shows the carbon profiles along the centerline of this MCDP and the traditional MCP, which are different only on the delay time for the last ladle. It can be seen that the carbon segregation along the centerline is reduced significantly with the MCDP technique, especially for the positive segregation below the hot top. The carbon concentration of the ingot body is controlled within the range of the industrial limit (0.16-0.22 wt%). The maximal carbon content in the ingot body decreases from 0.24 wt% to 0.22 wt% and the integral area of the positive segregation along the centerline decreases significantly. Meanwhile, the NPNP style segregation fluctuation is avoided and the carbon concentration in the ingot body is more uniform. Consequently the macrosegregation in the 360 t steel ingot can be controlled efficiently by using the MCDP technique with 17-hour delay time for the last ladle.

It should be mentioned that the formation and movement of the equiaxed grains, which may influence the formation of the segregation, are not considered in the simulations. If they were considered, the area of the negative segregation at the lower part of the ingot may be enlarged in


7

theory, due to the cloud of equiaxed grains formed at the beginning stage during the solidification. The height，where the segregation along the centerline turns from negative to positive, may be increased, and hence the optimized delay time in the MCDP technique should be prolonged accordingly. However, it cannot be quantificationally predicted that how the equiaxed grains influence the optimization of the delay time arbitrarily. In fact the simulation would become very complicated when the equiaxed grains are considered, which will be studied in the future work.

5. Conclusions A mathematical model for the calculation of the macrosegregation in large steel ingot during the MCP process has been established and validated by comparing with the experimental result. The formation of the macrosegregation in the 360 t steel ingot has been investigated using the established model. The conclusions are summarized as follows: 1. The traditional MCP technique with no interval between ladles has no significant effect on reducing the macrosegregation in the large steel ingot. The delay pouring time for the last ladle has remarkable influence on the macrosegregation distribution, especially for the positive segregation below the hot top. In order to effectively reduce the macrosegregation, a proper delay time for the last ladle should be selected. 2. If the delay time is too long, the NPNP style segregation appears. If the delay time is too short, the positive segregation cannot be reduced noticeably. To control the macrosegregation in the large steel ingot efficiently, a criterion of selecting the delay time for the last ladle has been proposed. The last ladle should be poured at the moment when the segregation along the centerline turns from negative to positive. 3. Based on the selecting principle, the 17-hour delay time is decided for the 360 t steel ingot. With the 17-hour delayed pouring technique, the carbon concentration along the centerline in the ingot body is controlled within the industrial limit and the macrosegregation distribution is improved remarkably in the 360 t steel ingot.

Acknowledge The authors gratefully acknowledge the financial supports from the Main Direction Program of Knowledge Innovation of Chinese Academy of Sciences (Project No.KGCX2-YW-221).

References [1] Olsson A, West R and Fredriksson H 1986 Scandinavian Journal of Metallurgy 15 104-12 [2] Maidorn C and Blind D 1985 Nuclear Engineering and Design 84 285-96 [3] Lesoult G 2005 Materials Science and Engineering A 413-414 19-29 [4] Gu J P and Beckermann C 1999 Metallurgical and Materials Transactions A 30 1357-66 [5] Flemings M C 2000 ISIJ International 40 833-41 [6] Hebditch D J and Hunt J D 1974 Metallurgical Transactions 5 1557-64 [7] Combeau H, Zaloznik M, Hans S and Richy P E 2009 Metallurgical and Materials

Transactions B 40 289-304 [8] Choudhary S K and Ganguly S 2007 ISIJ International 47 1759-66 [9] Zhong Y, Su G Y and Yang K 2003 Journal of Materials Science & Technology 19 413-6 [10] Griffiths W D and McCartney D G 1996 T Materials Science and Engineering A 216 47-60 [11] Tanaka Y and Sato I 2011 Journal of Nuclear Materials 417 845-59 [12] Liu D R, Sang B G, Kang X H and Li D Z 2010 International Journal of Cast Metals Research

23 354-63 [13] Pardeshi R, Voller V R, Singh A K and Dutta P 2008 International Journal of Heat and Mass

Transfer 51 3399-409 [14] Voller V R, Mouchmov A and Cross M 2004 Applied Mathematical Modelling 28 79-94


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numerical simulation of delayed pouring technique for a 360t heavy steel ingot

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