Stirrer Selection Guide

CVS MAK NAAAT SAN. ve T C.LTD.

theMill for Minimills

Dilovas Organize Sanayi Bölgesi 3. K m Muallimköy Cad. No:21Gebze 41400 Kocaeli / TÜRK YE

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Tel. +90 262 759 1505 Fax. +90 262 759 1880 [email protected] www.cvs.com.tr

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CVS MAKINASTIRRERS

SELECTIONGUIDE

mailto:[email protected]

http://www.cvs.com.tr




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1. Introduction ...............................................................................................................................3

2. Working principle ......................................................................................................................6

3. Mould electromagnetic stirrer (M-EMS) ..................................................................................7

3.1 Metallurgical effects ..........................................................................................................7

3.2 Electromagnetic characteristics ........................................................................................7

4. Strand electromagnetic stirrer (S-EMS) ..................................................................................9

4.1 Metallurgical effects ..........................................................................................................9

4.2 Electromagnetic characteristics ........................................................................................9

5. Final electromagnetic stirrers (F-EMS) .................................................................................. 10

5.1 Metallurgical effects ........................................................................................................ 10

5.2 Electromagnetic characteristics ...................................................................................... 11

6. Stirrers selection ..................................................................................................................... 12

7. Stirrer electrical characteristics. ............................................................................................. 14

8. Stirrer reliability ....................................................................................................................... 15

8.1 Stirrer power supply ........................................................................................................ 15

8.2 Stirrer cooling .................................................................................................................. 16

9. Custom-made stirrers design ................................................................................................. 17






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1. Introduction

The continuous casting is a process by which, without temporal solution of continuity, the cast

steel is solidified into a semifinished product that has different names, depending on the section

type (bloom, billet, slab, ring, etc.). The steel coming from the steel shop is carried through a

ladle. From the ladle the liquid steel is poured in the tundish, that feeds one or more castingstrands at the same time. From the tundish the steel flows into a water cooled copper moulds,

where the steel starts its solidification process, taking the section shape imposed by the mould.

From the lower part of the mould, the bar is pulled out. Pulling it gradually away from the

mould, the liquid section is reduced progressively due to the cooling and at a certain distance,

where the product is completely solidified, the automatic cutting operations take place, by

thermal or mechanic cutting in the relative stations. During the cooling process there are three

stages at which the steel begins and finishes its solidification process, briefly described below.

STAGE 1 – MOULD COOLING.

It is known as primary cooling and takes place quickly through a direct contact between the

steel, the copper wall and the water. At this stage a first solid steel layer is formed (generally

known as “skin”) with fine and equiaxial crystals. The mould is designed in such a way that the

skin at the outlet of the mould has a homogeneous thickness, able to bear the liquid steel

pressure.

Sect. A

Sect. A-A

Liquid

steel

Liquid

steel

Crystallizer (Copper

mould)

Cooling water

First solidification skin

Steel level (meniscus)

Detachment from the wall

A






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STAGE 2 – SECONDARY COOLING

In this area the cooling is obtained by water spray (sometimes air + water) directly on the billet

or bloom surfaces. However this type of cooling process is milder than mould cooling, so the

solidification is slower, favouring the creation of dendritical crystals oriented from the wall

towards the centre. This dendritical columnar area is critical for the creation of intercrystalline

cracks.

STAGE 3 – SECONDARY COOLING – CONE CLOSING AREA.

The solidification of the last portion of liquid steel is completed in this area. The final

solidifications at the central part of the billet or bloom occur in this area, the type of

solidification structure depends also on the steel grade. This central area, can be analyzed by

the Baumann or macro-etch tests, both in the transversal and longitudinal sections.

Billet core

closing without

equiaxial area.

Reduced inner

high quality area.

End of solidification End of solidification

Billet core

closing with

equiaxial area.

Good inner

quality.

Sect. A

Sect. B

Liquid

steel

Sect.

Sect. B-Liquid steel

Foot

rollers

spray

Dendrits

A

B






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CONTINUOUS CASTING STIRRERS

The electromagnetic stirrers are mainly used in the continuous casting to improve the product

quality and meet the requirements of increasingly demanding applications.

There are three possible stirrer application ways:

- M-EMS (Mould Electromagnetic Stirrers)

- S-EMS (Strand Electromagnetic Stirrers)

- F-EMS (Final Electromagnetic Stirrers).






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2. Working principle

The rotational electromagnetic stirrer is equivalent to an asyncrhonus motor stator. It isgenerally supplied by a three-phase or sometimes two-phase driver at a given frequency. A

rotating magnetic field is generated, whose variation inside the steel produces eddy currents

that, interacting with the magnetic field, generate a force (Lorentz force). The final result is the

occurrence of a torque that induce the steel rotation.

The generated torque depends on the following factors:

Intensity of current circulating in the stirrer.

Number of windings forming a coil.

Rotating frequency.

System geometry.

These parameters change depending on the stirrer type M-EMS, S-EMS or F-EMS. Within asingle class of stirrers, however, there may be differences depending on the steel grade, the

casting mode and the production sizes.

B- rotating

magnetic field.

J- eddy

current

B -Induced

steel stirring

Billet/bloom






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3. Mould electromagnetic stirrer (M-EMS)

3.1 METALLURGICAL EFFECTS

Mould electromagnetic stirrers are used for the following reasons:

Surface slags inclusions reduction.

Subsurface slags inclusions reduction.

Pinholes reduction.

Blowholes reduction.

Breakout risk reduction.

Primary solidification structure improvement (Equiaxed structure).

Internal cracks reduction (Internal and star cracks).

Center line segregation reduction.

Center porosities reduction.

Generally speaking, the casting parameters mutually connected are the following:

Tundish superheat.

Casting speed.

Magnetic field intensity.

The M-EMS also allows to cast with higher superheats with the same speed or increase thespeed with the same superheat maintaining the same steel quality.

The stirrer position must be evaluated depending on casting practice: protected cast or open

stream casting. Protected casting generally requires to lower the stirrer position in order to

reduce the possibility of powder entrapment and the nozzle wear. On the contrary, with open

stream casting the stirrer position can be maintained higher and near the meniscus.

3.2 ELECTROMAGNETIC CHARACTERISTICS

The magnetic induction intensity generated by the mould electromagnetic stirrer inside the

liquid steel depends on the frequency. The presence of a copper crystalizzator generates

reaction currents that tend to reduce the inner magnetic field intensity. It is possible to observe

directly from the Maxwell equations that the electromagnetic torque that generates the stirring,

as a first approximation, has the following functional dependence:

2fBfC [N m]






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where B represents the magnetic induction field and f its rotating frequency.

Therefore the presence of the mould will give to the generated torque the dependence on the

frequency which is shown in the following picture.

Therefore, the choice of the frequency parameter depends on the size that is intended to cast.

The best or optimal frequency is around 7 Hz for the size 130 mm and decreases rapidly for

greater sizes.

The generated torque is an integral effect and it is therefore conceptually wrong to link the

stirrer performances to the magnetic field measure in some specific point inside the crystallizer.

Therefore the couple depends on the following parameters:

Current intensity supplied to the stirrer.

Rotating magnetic field frequency.

Cast size.

Stirrer coils geometry.

Number of stirrer poles/phases.Referring to the last item it is possible to state that the three-phase systems generally generate

a more homogeneous rotating field distribution and therefore a more uniform torque.

C [N m]

Couple curves at a specific stirrer supply current and for differentcrystallizer copper thickness.

f [Hz]

Thinner copper moulds

Thicker copper moulds

f1 f2






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4. Strand electromagnetic stirrer (S-EMS)


Strand electromagnetic stirrers are used for the following reasons:

Primary solidification structures improvement.

Internal cracks reduction.

Center line segregation reduction.

Center porosities and cavity reduction.

The position of the strand stirrer is chosen according to the metallurgical structure required for

the final solidification. The position is different also in case of high-carbon steels or where a

final electromagnetic stirrer is present. In general the casting variables mutually connected are

identical to the ones described in item 3.1


Strand electromagnetic stirrers present characteristics similar to those mentioned in item 3.2,

referring to mould electromagnetic stirrers. However the absence of copper moves completely

the frequency to higher values, since its electrical conductivity is greatly higher than the steel

conductivity.

Generally for the billets the stirrer supply frequency is set out at 50 Hz, while for the blooms thefrequency decreases to about 20 Hz or less for very big casting sections.






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5. Final electromagnetic stirrers (F-EMS)


Basically, the use of F-EMS is strictly necessary when there are highly qualitative requirements

for the centre of billets or blooms.

The aim is to:

Improve the central solidification area.

Reduce the “V” segregation.

Reduce the center line segregation, in particular for high-carbon steels and high alloysteels;

Reduce the central porosity and cavities.The stirrer position must be chosen in an appropriate way, in order to act in the area where the

last solidification phase begins, as described in the following picture.

Too high positionfor the stirrer

Optimal positionfor the stirrer

Too low position forthe stirrer

Liquid steel

Steel undersolidification

Solidified steel






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Final electromagnetic stirrers present similar characteristics to those mentioned in the item 4.2,referring to strand electromagnetic stirrers.






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6. Stirrers selection

MEMS is generally used as most effective for improving the solidification structure, as it canhelp to break the growing dendritic structure and create more nuclei which are helpful for an

equiaxed structure to be formed. MEMS is anyhow also beneficial for reduction of segregation,

reduction in number of pin holes and blow holes, and improvement on surface and subsurface

quality. MEMS is also effective on centre segregation.

SEMS is active also on centre segregation, reducing the average value and cutting segregation

peaks, although more effective on solidification structure (increase of equiaxed zone) and in

improvement of internal quality (reduction of cracks and center porosity).

FEMS is mainly used against centre/V segregation, is further reducing segregation values and

bringing it to more stable values, with lower differences between minimum and maximum

values. FEMS is also active against centre porosity.

MEMS, SEMS and FEMS can be either used alone, or in combination of two or three of them

(M+S; S+F; M+F; M+S+F), according final quality requirements and possibilities of installation.

Due to different position along the strand, and different amount of liquid still present, also

different effects are deriving from application of one or the other stirring equipment, or their

combination.

In following figures examples of such features are reported.






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7. Stirrer electrical characteristics.

The electrical parameters that characterize the stirrer are the following:

Current I.Voltage V.

Power A.

Frequency f.Power factor or cos .

The current is a set value that may vary with continuity till a maximum value of amperes.

Similarly the frequency may be set out in a specific range between fmin and fmax.

On the contrary the voltage is not a value that may be set out, but it is the result of the two

previous parameters. In fact, as a first approximation, the stirrer impedance is given by a

resistive part, that substantially corresponds to the copper winding electrical resistance, while

on the other hand by 2 f L, where f is the frequency and L the system inductance. The power

supplied to the stirrer must be divided into apparent (A in VoltAmpere), reactive (Q in Var) and

active power (P in Watt); they are connected by the following formula A2 = P2+Q2. Finally, from

the relationship between active power and the apparent power, the power factor can beobtained.

Due to a great air gap extension, stirrers are inevitably little efficient machines, with cosvalues lower than 0.5.






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8. Stirrer reliability

8.1 STIRRER POWER SUPPLY

The supply unit called inverter generates a stirrer supply voltage through PWM techniques, that

consist in voltage steps of variable duration, at a fixed height. This technique, even though by

now essential for the satisfaction of drivers requested flexibility, introduces great stresses into

the copper winding insulations. In fact the voltage steps lead to inevitable over-voltages, as

shown by the following picture.

The over-voltages that are generated may be great and even twice the applied voltage. They

depend on:

Stirrer electrical characteristics.

Voltage rise time till the maximum value. Mainly connected with the drivercharacteristics.

Cable length.

In particular the over-voltage is well known to assume a typical S curve trend, if drawn as a

function of the supply cables length.

Appliedvoltage

time

Ideal step

Real step

Over-voltage

Rise time

Cables lenght

Ove

r-vo

ltag

e






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Typically 100 meters represent the maximum cable length. Anyway when building the plant it is

good to reduce as much as possible this parameter.

The over-voltages play a key role for the duration of the winding, not only because they subject

the insulation to electric stresses at a voltage higher than the nominal machine working voltage,

but also because they promote the partial discharge, by now very known phenomenon for the

PWM drivers (voltage square waves in a variable period). Furthermore, the sudden supply

voltage variation introduces not only the fundamental frequency stress, but also a widerfrequency spectrum stresses. Typically, the insulations can be tested by standard electrical tests

at 50 Hz in sinusoidal regime. On the contrary, due to the different frequencies and high

temperatures generated by intense current densities, their dielectric characteristics may

dramatically change, reducing to values clearly lower than those measured in standard

conditions.

8.2 STIRRER COOLING

The stirrer cooling may be either internal or external to the copper conducting wires. Generally

the second solution is preferred, where the copper windings are directly submerged in a water

circuit under pressure. In order to minimize the ground losses and to increase the stirrer

duration, demineralized water is preferably used.

The insulation water uptake depends on the type of material used and on its working

temperature. By optimizing the insulation material and the insulation manufacturing process, it

is possible to use also industrial water of the same quality as that used in the copper crystallizer

cooling circuit, without compromising the stirrer duration.

For all the reasons described in item 8.1 and 8.2, it has paid particular attention to the copper

winding resin impregnation process, developing a proprietary operational procedure and

optimizing the resin formulation. The performances achievable with this new formulation are

considerably higher than those of the resins available on the market. The final results are then

a resin and an impregnation process able to guarantee:

A longer duration of the windings.

A higher resistance to the voltage transient overshoots.

A high wear resistance due to the micro-particles present in the cooling water.






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9. Custom-made stirrers design

It is able to design, build, install and start complete stirrer systems for continuous castingmachines for blooms and billets, in accordance with the product qualitative needs.

The design is made through the use of FEM techniques that allow to optimize the product in

accordance with the client’s specific requests.

FEM building of a system consisting of stirrer+ mould + steel.

Visualization of a detail of the magnetic inductionflux lines, for a single electric phase.



Documents

Stirrer Selection Guide