A novel FEMS system for continuous casting of steel billets and blooms

Value Paper Authors: Len S. Beitelman, Douglas Lavers, Göte Tallbäck, Christopher Curran

2 Novel FEMS system | ABB Value Paper

A Novel FEMS system for continuous casting of steel billets and blooms

IntroductionSince the early industrial implementation of electromagnetic stirring (EMS) it was recognized that demanding steel grades, especially those with a wide solidification range, would benefit from stirring both within the casting mold and also at a later solidification stage. This type of stirring, in continuous casting of steel billets and blooms, became known as final solidification zone stirring or FEMS. Notwithstanding early reports on FEMS effectiveness with respect to improving the cast strand internal quality, especially the structural soundness and segregation(1,2), in the long run it was realized that the metallurgical performance of FEMS lacked in both the effectiveness and consistency, which can be attributed to a number of defining factors. First, it is important to position the FEMS with respect to the solidification stage that corresponds to a certain solid fraction level in the melt volume. Second, the stirring at this solidification stage is being performed under conditions of progressively diminishing stirring torque and increasing melt viscosity. The former occurs due to a reduction of the stirring pool radius, while the latter is due to an increase in the solid fraction of the melt. There is also an additional important factor impacting on the stirring effectiveness, arising due to the nature of the magnetic field used for stirring. The stirring systems currently employed in the production of continuously cast billets and blooms are based on application of a rotating magnetic field (RMF). Such fields have limitations in their application at a later, or advanced, solidification stage, arising from the fact that the resulting angular velocity is very nearly constant with respect to radial position(3) This flow pattern is characterized by intensive shear force and turbulence at the solid-liquid interface which is highly effective in terms of dendrite fragmentation and the subsequent development of an equiaxal solidification structure(4,5), but has very little impact on mixing in the melt volume, especially near its central region(6). In contrast, intensive turbulence and mixing throughout the melt volume is required at a late solidification stage in order to

disrupt formation of the crystalline network and, associated with it, the development onset of structural defects such as porosity, fissures, and solutal segregation.

There have been numerous developments aimed at improve-ment of the RMF based stirring at a later solidification stage through enhancement of the secondary fluid flow in the radial-axial plane. Thus, intermittent and alternating stirring schemes, both of which use sequential forced and dormant periods, were introduced in the 1980s(7,8). Kojima et al, demonstrated experimentally(7), while Davidson and Boysan confirmed theo-retically(9), that strong recirculatory flow would occur in the radial-axial directions during the dormant periods (i.e. without active stirring) due to the initial axial gradient of the swirl flow. However these stirring methods have not resulted in a signifi-cant improvement of FEMS performance. The reasons for that can be found in the recent work by S. Eckert and his co-work-ers(10) who showed that the occurrence of strong recirculatory flows is contingent on a provision of a narrow range of stirring and casting parameters. Noncompliance with those provisions can negatively impact on stirring performance and even render it useless or harmful. There have been several recent attempts to intensify turbulence and mixing in the bulk of the solidifying melt by using modulated electric currents to energize the stirring coils. The objective is to produce a modulated electromagnetic field which consists of both a time-averaged and a time-varying components. These recent developments have been theoretical and laboratory-scale in nature (11,12,13). and none has been implemented into production practice. Counter-rotating magnetic fields were also tested for stirring a solidifying aluminum alloy in laboratory experiments conducted by Vives(14). Significant im-provements in solidification structure were achieved by using this stirring method. This paper describes the design and operating principles of a novel stirring system developed by ABB Inc.(15), together with test results obtained with an industrial scale version of this system used for stirring of aluminum-silicon alloy A357.

ABB Value Paper | Novel FEMS system 3

Stirring system design and operation principlesThe newly developed stirring system consists of either two or three inductors arranged axially at a certain distance from each other, as schematically shown in Fig.1. A two-coil version of such an arrangement is similar (in principal) to the dual-coil EMS used for stirring in the casting mold(16). This stirring coil arrangement provides for the superposition of substantial

portions of the adjacent magnetic fields, as shown in Fig. 2 (for the two-coil arrangement). The superposed resultant magnetic field becomes modulated when the adjacent original magnetic fields are of different frequency. An example of such a modulated magnetic field is presented in Fig.3.

As explained elsewhere(17) and summarized below, the resultant magnetic field has multiple time-varying and time-independent components. At any point within the range of the superposed magnetic fields, the total net magnetic flux density and the total current density induced in the melt will be the vector sum of each adjacent stirrer contribution. Thus, for

825750700650600550500450400350300250200150100500

600 650 700 750 8000 50 100 150 200 250 300 350 400 450 500 550

Gau

ss (r

ms)

Distance from the stirrer top plate

Figure 1. A schematic of a three-coil arrangement in the new stirring system. The numbers on the right identify the stirring coils. The numbers on the central axis denote locations used in numerical simulation of axial velocity and turbulent viscosity (Figures 8 and 9).

Figure 3. Examples of modulated magnetic flux density at different points on the stirrer central axis. a and c – at a distance of 50 and 100 mm. respectively, from the mid-distance between the adjacent coils mid-planes. b – at the mid-distance.

Figure 2. An example of magnetic fields superposition produced by two adjacent stirring coils.

0.25

0.20

0.15

0.10

0.05

0

-0.05

-0.10

-0.15

-0.20

Volta

ge, V

0.25

0.20

0.15

0.10

0.05

0

-0.05

-0.10

-0.15

Volta

ge, V

a 0.25

0.20

0.15

0.10

0.05

0

-0.05

-0.10

-0.15

-0.20

Volta

ge, V

a

b

c

4 Novel FEMS system | ABB Value Paper

two magnetic field having the same rotational direction, these parameters will be as follows:

→B =

→B1 +

→B2 (1)

→J =

→J1 +

→J2 (2)

where:

→B1 and

→B2 are the magnetic flux density of individual stirrers

within the range of their magnetic field superposition.

→J1 and

→J2 are the density of current induced in the melt by the

magnetic fields of respective stirrers. The magnetic force (i.e. the Lorentz force) will also be defined as the vector product of the vector sums of the magnetic flux and current densities:

→f1 =

→J ×

→B (3)

As a result of contributions from the two adjacent magnetic fields, the Lorentz force will have multiple terms. These terms are defined as follows:

− Two time–independent or DC terms, one for each stirrer. − Two double frequency terms, one for each stirrer. − Two time-varying terms, at angular frequencies of (ω1

+ ω2) and (ω2 + ω1), due to cross-coupling between the two adjacent stirrers. Two time–varying terms, at angular frequencies of (ω1−ω2) and (ω2−ω1), also due to cross- coupling.

We should note that the high frequency components of magnetic force i.e. the double frequency terms and those at (ω1+ω2) and (ω2+ω1) will essentially be filtered out by the melt inertial effects. Hence, of the above multiple components, only the DC and low frequency ±(ω1−ω2) terms effectively act on molten metal. A key feature of low frequency components (i.e. the forces at ± (ω1−ω2)), is that both are proportional to the induced current density, and thus to either ω1 or ω2. Both frequencies can be chosen to be arbitrarily large since a highly conductive copper mold is not present Therefore, the frequency differential force components can also be large. Fig. 4 shows an example of the time-average and the time-varying X-components of magnetic force produced by a two-stator stirring system. This system is comprised of the two identical individual stirrers, each producing magnetic fields having a common rotational direction. As shown in the figure, the time-varying force component, i.e. the modulating force, has a reasonable magnitude and spatial span. The modulated stirring produced by this force is characterized by large oscillations of angular velocity, as shown in Fig. 5. The large primary flow oscillations, in turn, produce strong oscillations of the

-0.600 -0.400 -0.200 0.000 0.200 0.400 0.600

X-C

ompo

nent

of F

orte

Den

sity

[N/m

^3]

Position w.r.t. Mid-Plane [m]

Time Average Force0.5 Hz Force

50.0

45.0

40.0

35.0

30.0

25.0

20.0

15.0

10.0

5.0

0.0

4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0

Om

ega

[rad/

sec]

Time [sec]

Figure 4. Axial distribution of the time averaged | Figure. 5. Oscillating angular velocity produced in the melt and low frequency time-varying by modulated stirring. (Numerical simulations) (modulated) component of the Lorentz force.(Numerical simulations) | Figure 6. Time-varying components of the Lorentz force produced by counter-rotating magnetic fields. | Figure 7. Axial profile of angular velocity with counter-rotating stirring. (measured in a column of mercury). A-magnetic fields of common rotational direction, f1=f2=18.0 Hz. B-magnetic fields of opposing rotational direction, f1=18.0 Hz, f2=17.5 Hz.

X-C

ompo

nent

of F

orte

Den

sity

[N/m

^3]

Position w.r.t. Mid-Plane [m]

Upper & Lower OnlySuperimpose AllDifference

18

16

14

12

10

8

6

4

2

0

0 200 400 600 800 1000

Ang

ular

vel

ocity

(rad

/s)

Depth below meniscus (mm)

5

7

4

6

ABB Value Paper | Novel FEMS system 5

recirculating secondary flow in the radial-axial plane, with the resultant effect that high intensity turbulence is produced across the stirring pool volume. With counter-rotating superposed magnetic fields, the time-varying components of the force produced by the respective original magnetic fields will cancel each other, as illustrated by Fig. 6(17). In this instance, intensive oscillations of the secondary flow, and the resultant turbulence, are produced due to a steep angular velocity gradient caused by the reversal of the stirring flow direction, as show in Fig. 7.

Fig. 8 shows temporal profiles of axial velocity in liquid steel produced by a counter-rotating modulated magnetic field. Each profile is identified with the locations on the stirring pool central axis, as shown in Fig. 1. The strongest velocity oscillations occur at the mid-distance between the two stirrers, denoted by 3 in Fig. 1. This location corresponds to the interaction between the two opposing swirl flows and changing angular velocity direction, as shown in Fig. 7. The angular velocity gradient was identified as the major driving force in generating recirculatory secondary flow(9). As a result of the highly oscillating secondary

flow, high intensity turbulence is being developed in the melt bulk(18).

Fig. 9 shows profiles of turbulent viscosity corresponding the same stirring conditions as those pertinent to Fig. 8. In this case the strongest turbulence has also been developed at the mid-distance between the two adjacent stirrers (profile 3). For comparison, the turbulent viscosity produced by a conventional unidirectional, unmodulated, magnetic field of the similar strength and frequency is also shown. Turbulent viscosity in the region between the stirrers, in this instance, is less by a factor of 5 than that produced by the counter-rotating stirring. Thus, stirring obtained with modulated magnetic fields of common rotational direction and either modulated or unmodulated counter-rotating magnetic fields is characterized by strong oscillating secondary flow and turbulence within the melt volume. This condition of fluid flow should result in improvement in the solidification structure. To verify the effectiveness of the new stirring concept, a series of trials with 357 aluminum alloy were carried out at a different stirring modes and operating parameter settings. The results of these trials will be discussed in the next sections of the paper.

Experimental Experimental set- up A general view of the experimental set-up is shown on the photograph in Fig. 10. It is comprised of an induction crucible furnace, electromagnetic stirrer, power source and the cooling water system (the latter is not shown in the photograph). A 357 aluminum alloy (Al-7%Si-0.5% Mg) was melted in the furnace and poured via a fused silica launder into an air-cooled stainless steel mold arranged inside the stirrer. The pouring temperature was in the range of 690 to 730°C. The material used for all casts was from the same batch in order to avoid an effect of chemistry variations on solidification. Each cast produced a 22 kg cylindrical shape ingot (0.115m diameter and 0.86m height). To facilitate ingot extraction from the mold, it was constructed from two longitudinal halves bolted together. The inner walls of the mold were coated with boron nitrate to prevent ingot sticking during solidification. To simulate the stirring conditions

0.72

3 4 1 and 5

0.5

0.3

0.1

-0.1

-0.3

-0.50 50 100 150 200

Ang

ular

vel

ocity

(rad

/s)

Time, (s)

Axial Velocity, (m/s)

Figure 8. Temporal profiles of axial velocity in liquid steel produced by modulated counter-rotating stirring (results of numerical simulation). The numbers indicate locations on the central axis of the stirring pool, as shown in Fig. 1.

Figure 9. Temporal profiles of turbulent viscosity in liquid steel stirred by modulated counter-rotating magnetic field and conventional rotating magnetic field (results of numerical simulation). The numbers identify locations on the stirring pool central axis, as shown in Fig. 1.

3

2

2.5

3

1.5

1

0.5

0

0 10050 150 200

Time, (s)

Turbulent viscosity, (Ns/m^2)

Counter-Rotating Magnetic Field

3 1 and 5 2 4

2

2.5

1.5

1

0.5

0

0 20 40 60 80 100 120 140 160 180 200

Time, (s)

Turbulent viscosity, (Ns/m^2)at the stirring pool center

Conventional Rotating Magnetic Field

6 Novel FEMS system | ABB Value Paper

occurring at the final solidification stage of production for continuously cast billets and blooms, the effect of meniscus and its deformation due to stirring motion had to be eliminated. This was achieved by placing a graphite cover on the melt surface at the end of pouring. The two K-type thermocouples were mounted on the graphite cover and arranged at the mold mid-radius and center, respectively, at a distance of 0.2m below the meniscus to monitor the solidification progress.

Temperature monitoring As the melt temperature measurements were carried out in the presence of strong A.C. magnetic fields, the thermocouple signal had to be conditioned to prevent magnetic field interference. A National Instruments data acquisition system SCXI-1000 was used for this purpose. The system provides signal preamplification, low-pass filtering and cold-junction compensation. The Lab VIEW Express processor incorporated into the system allowed for the real-time temperature reading and recording. One temperature reading was based on averaging of 100 samples/second. The purpose of temperature monitoring was two-fold. First, it provided reasonably accurate information on evolution of the solid in the melt with solidification progress, and it also made available the melt cooling rate. Both parameters are important for control of

stirring and solidification conditions and also for the assessment of the effects these conditions produce on the solidification structure. Thus, the effects of stirring intensity and cooling rate on critical level of solid fraction and grain morphology and size are well established(19,20,21) To isolate the effect of cooling rate on solidification, it was maintained within a reasonably narrow range within a group of castings, whereas the stirring parameters were set up within a wide range. In the previously shown examples of fluid flow numerical simulation, the presence of the solid fraction in the melt was disregarded. In reality, however, the solid fraction, upon reaching a certain critical level, affects fluid flow, including turbulence, as the melt viscosity increases(19,21) For a solid fraction within the range of 0.1 to 0.2, A crystalline network starts to form(19) in many alloys. There is a broad agreement that under most industrial conditions of solidification and stirring, and also in some laboratory instances, at a certain critical level of solid fraction which is typically in the range of 0.30 to 0.35,(22,23,24) stirring motion in the melt cannot be sustained. Under the prevailing experimental conditions of these trials, this situation occurred within the temperature range of 585 to 590°C which, in accordance with the solid fraction versus temperature diagram, corresponds to a solid fraction of 0.40.

Cast No. Electric Current and Frequency Applied Stirring Method

Upper Stirrer Lower Stirrer

Group A

1 No stirring

2 0 140A,11Hz Unidirect. conventional

3 160A, 8Hz 140A, 11Hz C-R modulated

4 150A, 10.5 Hz 140A, 11Hz C-R modulated

5 160A, 10 Hz 140A, 10Hz C-R unmodulated

6 160A, 8 Hz 140A, 11Hz Unidirect. modulated

7 150A, 10.5 Hz 140A, 11Hz Unidirect modulated

Group B

8 145A, 10.5Hz 140A, 11 Hz C-R modulated

9 145A, 11 Hz 140A, 11 Hz Unidirect. unmodulated

10 145A, 10.5 Hz 140A, 11 Hz Unidirect. modulated

11 145A, 10.25 Hz 140A, 11 Hz Unidirect. modulated

12 140A, 11 Hz 140A, 11 Hz C-R unmodulated

Footnote: C-R is the counter-rotating stirring, Unidirect. is unidirectional stirring.

Table I. Stirring Settings Used in Casting Trials.

Figure 10. Experimental set-up used for stirring A357 aluminum alloy.

610

600

590

580

570

0 10 20 30 40 50 60 70 80 90 100 110 120

Tem

pera

ture

, C

Time, s

EMS On

b

a

Figure 11. An example of the cooling curve obtained during solidification of A357 alloy ingots. a- the temperature plateau, b- the temperature spike.

ABB Value Paper | Novel FEMS system 7

As seen from the cooling curve shown in Fig. 11, after a steep temperature decline followed the commencement of stirring, there is a short plateau within that temperature range followed by a slower temperature decline. In some instances, a temperature spike precedes the start of that decline. These characteristics of the cooling curves reflect dynamics of both stirring intensity and solidification progress. Thus, the rapid temperature decline is a result of intensive mixing of the melt and a rapid heat transfer from the solidifying shell into the melt bulk. With solidification progress, the rate of latent heat release accelerates, whereas stirring intensity declines due to the progressive increasing of melt viscosity and the diminishing radius of the stirring pool. This, in turn, results in a reduction of the heat transfer within the melt, which is evident in a plateau on the temperature curves. These conditions typically have taken place within an interval of 25 to 40 seconds into the solidification. The stirring was discontinued as the temperature reached 570°C.

Electromagnetic Stirring The stirring system used in the trials with A357 alloy is similar to that shown in Fig. 1, except that only two stirring coils were used in the arrangement, i.e. the upper coil number 1 and the lower coil number 2. Each stirring coil was energized from a separate, single-frequency power source. The A357 alloy casts were characterized in terms of two groups, i.e. Group A and Group B, which were carried out with different stirring modes and parameter settings. The melt pouring temperature was

also different for each group of casts. For the casts in group A, this temperature was in the range of 620 to 650°C, and for the casts of Group B, it was 710-720°C. The stirring settings used in the casting trials are summarized in Table I.

As seen, from Table I, the stirring methods used on 11 casts varied from a conventional single frequency provided by a single stirring coil (cast No.2), to modulated and unmodulated with either unidirectional or counter-rotating magnetic fields. In addition, for the purpose of comparison, the cast No.1 was processed without any stirring.

Figure 12. Macrostructure of A357 ingots cast at different stirring conditions. Magnification 20 X a) mid-radius location, b) center location.

1.5

1.0

0.5

0

Dia

met

er, m

m

No

EM

S

Con

vent

iona

l EM

S

Cou

nter

Rot

atin

g M

odul

ated

Cou

nter

Rot

atin

g M

odul

ated

Cou

nter

Rot

atin

g U

nmod

ulat

ed

Uni

dire

ctio

nal M

odul

ated

Uni

dire

ctio

nal M

odul

ated

Casts

Figure 13. Average grain diameter in A357 macrostructure obtained at different stirring conditions.

No EMS Conventional EMS Unidirectional Modulated EMS

a

b

8 Novel FEMS system | ABB Value Paper

Evaluation of the Solidification Structure The solidification structure of A357, both macro and micro, was evaluated in the mid-radius and the center regions of the cast ingot. For this purpose, a transverse sample was cut out from the middle portion of the ingot. For macrostructure evaluation, the specimens were ground to an 800 grit finish and etched with Poulton’s reagent (60% HCl, 30% HNO3 and 5% HF) to reveal the macrostructure. Directional lighting was also applied to enhance the structure details. The macrostructure was evaluated with an optical microscope at magnification of 20.

The effect of different stirring settings on the macrostructure was assessed via grain diameter size using a linear intercept method. The microstructure was evaluated in the specimens which were either polished but not etched or etched with 0.5% HF. There was no noticeable difference found in the structure detail obtained by either of these two methods. The quantitative and morphological characteristics of the microstructure were assessed with an image analysis performed with Clemex Vision Professional Edition, Version 5 system attached to an Olympus PMG3 optical metallographic microscope. The microstructure

Figure 14. Microstructure of A357 ingots cast at different stirring conditions .Magnification 50 X a) mid-radius location, b) center location.

Cast No. Location

on the

Sample

Mean Area, μm2 Mean Length, μm Mean Circular

Diameter, μm

Mean Spherical

Diameter, μm

Mean Width, μm Mean Density,

Globule/mm2

Group A

1 M-R C 17,577 18,734 175 179 135 138 165 169 117 121 44.62 42.45

2 M-R C 13,100 12,788 153 151 120 118 147 146 105 104 60.48 60.96

3 M-R C 12,841 13,036 150 153 118 119 145 146 104 115 59.6 59.55

4 M-R C 13,101 13,078 153 152 119 119 146 146 104 105 60.82 59.77

5 M-R C 12,735 12,449 149 148 116 116 143 142 102 102 59.66 59.24

6 M-R C 11,671 11,435 144 143 114 113 139 138 100 99 65.94 68.33

7 M-R C 11,409 11,471 142 143 112 113 137 139 99 99 66.59 66.27

Group B

8 M-R C 16,982 16,040 170 167 134 131 164 161 118 116 46.71 48.15

9 M-R C 15,563 15,549 164 164 129 129 158 158 114 114 54.61 51.60

10 M-R C 14,833 14,383 161 159 127 126 156 154 112 111 53.89 53.63

11 M-R C 14,469 14,739 159 161 126 127 154 155 111 112 54.55 54.41

12 M-R C 12,601 12,100 149 146 117 115 143 141 102 101 64.25 61.99

*M-R is the mid-radius area of the specimen. **C is the center area of the specimen.

Table II. Alpha Aluminum Globule Dimensions and Density Evaluation with Clemex Image Analysis System.

No EMS Conventional EMS Unidirectional Modulated EMS

a

b

ABB Value Paper | Novel FEMS system 9

was evaluated at optical magnification of 50. Only the specimens etched with 0.5 % HF were used for the evaluation.

Results and discussion Both metallographic and quantitative evaluations of the solidification structure of A 357 casts indicate that some applied stirring techniques are more effective than others in terms of structure morphological transformation from the original dendritic to globulitic and also in its refining.

Figure 12 shows the examples of macrostructure of the ingots cast without any stirring and with different stirring methods, i.e. conventional and unidirectional modulated. As seen, the structure in mid-radius of the ingot cast without stirring is solely comprised of large, fully developed dendrites (Fig. 12a). The structure in the center of this ingot is a mixture of the dendrites, the whole and fragmented, and some globule and elongated-shaped crystals (Fig.12b). The structure obtained with conventional stirring (Table I, cast. No. 2) is largely globule-shaped with some presence of the dendrites and dendrite fragments. The structure of the ingot cast with modulated stirring (Table I, cast No. 7) consists of entirely globule-shaped crystals. This structure also appears to be more refined. The large porosity seeing on the photographs as black spots results from the air entrainment during pouring of the melt into the mold and they are not taken into consideration for the structure evaluation. Quantitatively, the macrostructure can be characterized by the grain diameter. The effect of different stirring settings on the average grain diameter is shown in Fig. 13.

As seen, with conventional stirring, the average grain diameter was reduced by approximately 23% in the ingot mid-radius and 16% in the central area, respectively, in comparison with that of the unstirred structure. A further grain diameter reduction was achieved with counter-rotating modulated and unmodulated stirring. However, the smallest grain diameter in this group of the casts was obtained with unidirectional modulated stirring (Table I, cast No.7 and Figure12a and b). In comparison with the grain diameter in the cast without stirring, it was reduced by 28% in the mid-radius and 24% in the center, respectively. The same samples, used for macrostructure evaluation, were also evaluated for the microstructure.

In general, the microstructure of all samples consists of prime alpha aluminum crystals: (dendrites in the structure obtained without stirring) and globules and elongated graines in the structure obtained with stirring, and fine intergranular eutectic network containing silicon and manganese compounds. The coarse dendritic structure of the ingots cast without stirring, shown in Fig. 14a and b, has been transformed into mainly globular one with some rosette-shaped as a result of the conventional stirring application. The structure obtained with unidirectional modulated stirring consists of a mixture of fine round-shape globules and large elongated graines. This structure also appears to be more refined in comparison with that obtained with the conventional stirring presented in Figure 14 a,b. The microstructure was further evaluated with the

Clemet Image Analysis System. The results of that evaluation are summarized in Table II.

Figures 15 and 16 show the globule mean area and length in the microstructure of the combined mid-radius and center area of the casts Group A (ingots 1 to 7). The structure obtained with conventional stirring was selected as the benchmark for comparison with the structure of the other casts. As seen, the globule mean area in that structure was reduced by 34% over that value in the structure obtained without stirring. The other stirring settings produced no noticeable difference (it was in the range of 2-3%), except that obtained with unidirectional modulated stirring in the casts 6 and 7. The globule mean area in these casts was reduced by 11 to 13% in comparison with conventional stirring. A similar trend was determined in reduction of the globule length. Concurrent with globule size reduction, their density has increased. Thus it was increased

Figure 15. Aluminum globule mean area obtained with different Figure16. Aluminum globule mean length obtained stirring methods. Group A casts. with different stirring methods. Group A casts. | Figure 17. Aluminum globule density (globule/mm2) obtained with different stirring methods. Group A casts.

140134

100 98 10097

89 87

%

130

120

110

100

90

80

70

60

0

No

EM

S

Con

vent

iona

l EM

S

Cou

nter

Rot

. Mod

ulat

ed

Cou

nter

Rot

. Mod

ulat

ed

Cou

nter

Rot

. Unm

od.

Uni

dire

ctio

nal M

od.

Uni

dire

ctio

nal M

od.

Casts

120

72

100 98 99 98

%

100

80

60

40

20

0

No

EM

S

Con

vent

iona

l EM

S

Cou

nter

Rot

. Mod

ulat

ed

Cou

nter

Rot

. Mod

ulat

ed

Cou

nter

Rot

. Unm

od.

Uni

dire

ctio

nal M

od.

Uni

dire

ctio

nal M

od.

Casts

110 109

120115

100 98 10095

%

110

100

90

80

70

60

50

No

EM

S

Con

vent

iona

l EM

S

Cou

nter

Rot

. Mod

ulat

ed

Cou

nter

Rot

. Mod

ulat

ed

Cou

nter

Rot

. Unm

od.

Uni

dire

ctio

nal M

od.

Uni

dire

ctio

nal M

od.

Casts

9287

15

16

17

10 Novel FEMS system | ABB Value Paper

by 28% in the structure obtained with conventional stirring, as shown in Fig. 17, and a further increase by approximately 10% was achieved with unidirectional modulated stirring.

In Group B, the benchmark of microstructure was assigned to that obtained with unidirectional unmodulated stirring (Table I, cast No. 9). From conventional stirring used in the Group A, this stirring differs in that it was produced with two stirring coils, whereas conventional stirring is based on operating only one stirring coil. In terms of microstructure refinement, as seen from Figures 18 and 19, this stirring setting was found to be less effective than unidirectional modulated method used in the casts 10 and 11, and especially in comparison with counter-rotating unmodulated stirring applied to the cast No. 12. The latter stirring method resulted in globule area and length reduction of 20% and 10% respectively, in comparison with the same parameters of the benchmark structure. The globule mean density was also markedly increased by 22% in the cast No. 12 (Fig. 20).

It is interesting to note that the effectiveness of counter-rotating unmodulated stirring on microstructure of the casts in Group A was not so prominent. It resulted in a modest 3 to 5% reduction of the globule size and practically no change in globule density in comparison with the results produced by conventional stirring method. On the basis of macro and microstructure evaluation of the two groups of casts solidified under different thermal and stirring conditions, we can conclude that unidirectional modulated and counter-rotating unmodulated stirring methods are more effective in comparison with conventional and counter-rotating modulated stirring. The results of this structural evaluation may be considered as an experimental validation of the physical and numerical models of magnetics and fluid flow which showed quantitatively a marked enhancement of the secondary radial-axial flow and concomitant turbulence produced by these stirring methods. (25) The results obtained with A357 alloy provide the reason to expect similar effects of the unidirectional modulated and counter-rotating stirring on the solidification structure of steel. Steel and aluminum alloys,

as it was shown experimentally (20), have many common solidification characteristics. For example, they have almost the same critical solid fraction at the same shear rate. A globule and rosette-shaped microstructure can also be obtained in rheocast steel, (26) as shown in the example of AISI4340 in Fig. 18.

The electromagnetic stirring system used in those experiments was based on the two conventional stirrers which makes us think that the new stirring system should perform even better.

Conclusions Metallurgical evaluation of different stirring methods with respect to their effect on A357 alloy solidification structure have convincingly demonstrated marked improvements achieved with unidirectional modulated and counter-rotating unmodulated stirring in comparison with conventional, RMF-based stirring. Both stirring methods drastically enhance the secondary flow resulting in intensive turbulence and mixing. The new stirring system provides advantages of combining both of the above stirring methods which broadens its operating flexibility. Additional benefits of the new system arise from the fact that it is comprised of the components of conventional, currently used equipment such as single-frequency power sources and inductors. It does not require special controllers of the current frequency.

Future work The next stage of the new stirring system development should be carried out in the production setting of continuous casting of quality demanding steel billets or blooms.

Acknowledgement We wish to acknowledge the contribution to this study made my Mr. Mel Van Harten of Bodycote Testing Group, Cambridge, Ontario in his evaluation of the A357 alloy solidification structure.

Figure 18. Microstructure of AISI 4340 steel (Ref. K.E. Blazek et al).

Dendritic structure Dendritic structure

ABB Value Paper | Novel FEMS system 11

References 1. K. Ayata, T. Fujimoto, T. Mori, I. Wakasugi, S. Kojima, T.

Ueoka, S. Kato and S. Dotani, “Improvement of Internal Quality of Continuously Cast Billets by Combination Electromagnetic Stirring.” Trans. ISIJ, Vol. 23, No.12, 1983 (B-410-411).

2. H. Mizukami, M. Kamatsu, T. Kitagawa and K. Kawakami, “Effect of Electromagnetic Stirring at the Final Stage of Solidification of Continuously Cast Strand,” Tetsu-to-Hagane, 70 (1984), No.2, p.194.

3. P.A. Davidson, “Estimation of turbulence velocities induced by magnetic stirring during continuous casting,” Material Science & Technology, vol. 1, 1985, November, pp. 994-999.

4. S. Kobayashi, S. Ishimura, M. Yoshihara and Y. Sugitani, “Factors Affecting Equiaxed Zone Generation in Electromagnetic Stirring,” Trans. ISIJ, vol.28, (1998), No. 11 pp 939-944.

5. H. Esaka, T. Wakabayashi, K. Shinozuka, and M. Tamura, “Origin of Equiaxed Grains and their Motion in the Liquid Phase,” ISIJ Int., vol. 43 (2003), No. 9. pp 1415-1420.

6. K.-H. Spitzer, M. Dubke, K. Schwerdtfeger, “Rotational Electromagnetic Stirring in Continuous Casting of Round Strands,” Met. Trans.B, vol. 17B (1986) pp. 119-131.

7. S. Kojima, T. Ohnishi, T. Mori, K. Shiwaku, I. Wakasugi and M. Ohgami, “Application of Advanced Mold Stirring to a New Bloom Caster,” Steelmaking Proceedings, vol. 66. Atlanta, 1983, pp 127-131.

8. A. Ishizaka, A. Yamagami, A. Kuribayashi, K. Taguchi, I. Sugawara, E. Sunami, “Effects of Electromagnetic Stirring at the Final Stage of Solidification on the Quality of Large Section Blooms,” Trans. ISIJ, vol. 24, No. 12, 1984, [B-387].

9. P.A. Davidson and F. Boysan. Ironmaking and Steelmaking, vol. 18 (1991), p 245.

10. S. Eckert, P.A. Nikrityuk, D. Räbiger, K. Eckert and G. Gerbeth, “Efficient Melt Stirring Using Pulse Sequences of Rotating Magnetic Field: Part I: Flow Field in a Liquid Metal Column.” Met. Mater.Trans.B, vol. 39B. (2008) pp 375-386.

11. H. Branover, E. Golbraikh, A. Kapusta, B. Mikhailovich, I. Dardik, R. Thomson, S. Lesin, M. Khavkin, “On the potentialities of intensification of electromagnetic stirring of melts,” Magnetohydrodynamics, vol. 42, No. 23 (2006), P. 3-9.

12. S. Eckert, G. Gerbeth, D. Räbiger, B. Willers, C. Zhang, “Experimental Modeling Using Low Melting Point Metallic Melts: Relevance to Metallurgical Engineering,” Steel Res. Int., 78 (2007) No. 5, pp. 419-425.

13. J. Pal, S. Eckert, C. Zhang, G. Gerbeth, “Velocity Measurements in Metallic Melts Driven by AC Magnetic Fields in a Square Vessel, ”The 5th Symposium on Electromagnetic Processing of Materials, 2006, Sendai, Japan, ISIJ, pp 725-730.

14. C. Vives, “Free Surface Shape, Fluid Flow and Crystallization of Thixotropic Aluminum Alloy Slurries and Composites in the Presence of Contra-Rotating Magnetic Fields,” The 3rd International Symposium on Electromagnetic Processing of Materials,” Nagoya, Japan, 2000, ISIJ, pp 445-450.

15. L.S. Beitelman, J.D. Lavers, G.R. Tallbäck and C.P. Curran. “Modulated Electromagnetic Stirring of Metals at Advanced Stage of Solidification,” U.S. Patent Application No. 12/076,954, March 25, 2008.

16. L. Beitelman, J.A. Mulcahy, “Method and Apparatus for Control of Stirring in Continuous Casting of Metals,” U.S. Patent No. 5.699.850, Dec. 23,1997.

17. J.D. Lavers, L.S. Beitelman, G.R. Tallbäck, C.P. Curran, “A Dual EMS System for Stirring Liquid Metals at an Advanced Solidification Stage,” International Scientific Colloquium on Modelling for Electromagnetic Processing, Hannover, Germany, October 27-29, 2008.

18. G.R. Tallbäck, Internal ABB Report, “Counter-Rotating EMS and Standard EMS. A Study of Differences,” Feb. 2003.

19. M.C. Flemings, “Behavior of Metal Alloys in The Semisolid State,” Met. Trans B, vol. 22, June 1991, pp 269-291.

20. Z. Fan, “Semisolid metal processing,” Int. Mater. Rev., 2008, vol. 47. No. 2, pp 49-85.

21. M. Hirai, K. Takebayashi, Y. Yoshikawa and R. Yamaguchi, “Apparent Viscosity of Al-10 mass % Cu Semi-Solid Alloys,” ISIJ Int., vol. 33 (1993), No. 3, pp 405-412.

22. H. Mizukami, M. Komatsu, T. Kitagawa, K. Kawakami, “Effect of Electromagnetic Stirring at the Final Stage of Solidification of Continuously Cast Strand,” Trans. ISIJ, vol. 24, No. 11 (1984), pp 923-930.

23. J.K. Roplekar and J.A. Dantzig, “A Study of Solidification with a Rotating Magnetic Field,” Int. J. Cast. Met.Res.14 (2001), pp.79-95

24. A.A. Tzavaras, H.D. Brody, “Electromagnetic Stirring and Continuous Casting-Achievements, Problems and Goals,” J. Metals, March 1984, pp 31-37.

25. L. Beitelman, D. Lavers, G. Tallbäck, C. Curran, “A Novel Method of Electromagnetic Stirring of Liquid Metals at An Advanced Solidification Stage,” The 6th European Conference on Continuous Casting 2008, Riccione, Italy, 3-6 June 2008.

26. K.E. Blazek, J.E. Kelly, and N.S. Pettore, “The Development of a Continuous Rheocaster for Ferrous and High Melting Point Alloys,”ISIJInt.,vol.35 (1995), No.6,pp.813-8

Contact us

3BU

S09

5011

en

A

BB

US

CS

170

4ABB LtdAffolternstrasse 44 CH-8050 Zurich, Switzerlandwww.abb.com