Electromagnetic Stirring in Continuous Casting Process
Electromagnetic Stirring in Continuous Casting Process
In the continuous casting process of liquid steel, methods of improving the quality of the cast steel product are always important. This has also remained important for the development of the process. In addition to modifying the jet flow angle and remaking the submerged entry nozzle (SEN) shape, an electromagnetic technique, which is able to control the fluid flow without contact between the liquid steel and a stirrer, has been used as a flow control technique. One type of electromagnetic technique is the electromagnetic stirring (EMS), which generates a fluid flow by the Lorenz force provided by a linear induction motor. EMS technology has been used in the continuous casting of steel for several years but the effect of the application and subsequent benefits of stirring the liquid core depends very much on section size, steel grade, and product application.
Since the first application of the principle of continuous casting to steel in the test continuous casting machine of Junghans of former West Germany, the quality of the continuous cast product has been paid more and more attention. In recent years with the stress on the production of clean steels, there are higher requirements for the microstructure and the composition homogenization of the cast product. The chemical composition, solidification conditions, and the nature of the liquid steel flow in the mould affect the surface quality and the inner structure of the cast product. The application of EMS technique promotes the formation of an equiaxed crystallic zone in the strand. It causes the refinement of the solidification structure, the reduction in the content of inclusions, and improvement in the quality of the surface, sub surface, and the inner structure of the cast product.
In the continuous casting process, liquid steel is injected into the mould. The final steel shell is obtained after the solidification which starts in the mould and continues in the strands. Electromagnetic devices such as stirrers and brakers are well known technologies used to improve both the quality of the final cast product and the casting speed. The main defects of the final shell in terms of microstructure and surface cracking can be directly related to in-mould phenomena such as temperature variation, velocity and pressure of liquid steel, free-surface behaviour, and slag entrainment which are some of the main causes of defects in the final product. It is worth mentioning that there is one other type of electromagnetic device which is also used in continuous casting machine and it is called electromagnetic braking (EMBR). This device is mostly installed in the mould and resembles a linear EMS but has a direct current instead of an alternating current.
EMS is a direct and powerful technique for controlling the solidification process in the continuous casting of liquid steel. A significant, but not the only advantage of EMS, is improved quality and uniformity of structure and chemistry at the centre line of the cast product. Productivity advantages accompany the quality improvements. Experimental results have shown a beneficial effect from the EMS on the microstructure of the steel, for example by increasing the equiaxed zone width. Several types of defects in the strand can be effectively decreased in magnitude with the application of EMS. Bubbles and porosities are also expected to be considerably affected by the EMS. It is further reported that the EMS increases the yield and the productivity of the continuous casting process.
The application of EMS to the continuous casting process has comparatively long history and first trial of EMS went back to 1960s. EMS was introduced to the continuous casting machine roughly a decade after continuous casting process started industrially producing steels. It has been shown that EMS also affects the inclusions and bubbles. Strand EMS opened the door of real application of EMS to continuous casting machine at the late 1970s. The objective of strand EMS is to acquire the high equiaxed zone ratio for the purpose of suppression of the centre segregation. After that, in-mould EMS (Fig 1) had been developed to improve the surface quality of the cast steel product by suppression of entrapment of non-metallic inclusions and argon bubbles to the solidified shell.
Fig 1 Electromagnetic stirrer
In order to attain the high productivity of the continuous casting process, EMBR had been developed in 1980s to stabilize the liquid steel flow in the continuous casting mould. The first type of EMBR is to create local magnetic field for which a pair of DC (direct current) magnets have been installed at the neighborhood of the SEN port. The imposed field ‘brakes’ the discharged flow from SEN directly. The second type of EMBR is the level magnetic field which uniform magnetic field in the width direction of mould develops plug- like flow below magnetic field area. One kind of second type of EMBR is the flow control (FC) mould (which the imposition of a pair of level magnetic field at the upper part of the continuous casting mould, simultaneously stabilize the meniscus flow and descending flow along the narrow face of mould). At present, the combination of EMS and EMBR has also been developed.
With the purpose of controlling the process and preventing the final product from defects, the process has been improved with electromagnetic devices such as EMS and EMBR. The main difference is that the stirrers work under the supply of AC (alternating current) current and produce dynamics magnetic fields. Brakers are permanent magnets or circuits fed by DC (direct current) current. Hence they produce constant magnetic fields. Despite the differences they are based on the same idea that the superposition of a magnetic field to the metal flow generates Lorentz forces which can drive the flow in accordance with the process design. The physical phenomena occurring in the mould is a multi-physics issue which includes liquid flow, multiphase analyses, electromagnetic computation, heat transfer and solidification processes where each of these physics depends on the others.
It is well known that an alternating magnetic field (either single phase or multiphase) applied to a conductor, whether solid or fluid, induces electric currents in the conductor, and hence a Lorentz force distribution. This Lorentz force is in general rotational, and if the conductor is fluid, it is set in motion. Thus the magnetic field acts as a non-intrusive stirring device and it can, in principle, be engineered to provide any desired pattern of stirring. Stirring can also be affected through the interaction of a steady current distribution driven through a fluid and the associated magnetic field. When the field frequency is high, the Lorentz force is confined to a thin electromagnetic boundary layer, and the net effect of the magnetic field is to induce either a tangential velocity or a tangential stress just inside the boundary layer. The distribution of velocity or stress is related to the structure of the applied field. Symmetric configurations can lead to patterns of stirring in which the streamlines lie on toroidal surfaces, however more normally, the streamline pattern is chaotic.
Flow in the mould region is controlled by the nozzle and mould geometry, casting speed, nozzle submergence depth, argon gas injection, and the application of electromagnetic forces. Electromagnetic forces are optionally applied as either static or moving magnetic fields through the thickness of the strand. Static (DC) electromagnetic fields induce current in the conducting liquid steel, which in turn, generates forces which directly oppose the flow, so are they referred to as ‘brakes’, or ‘EMBR’. EMBR fields include local cylindrical-shaped fields, wide ‘ruler-shaped’ magnetic fields across the entire mould width, and double-ruler fields, sometimes referred to as ‘flow-control’, or ‘FC-mould’ fields.
Electromagnetic forces are an important tool to control fluid flow in the mould, combined with other casting conditions, nozzle, and mould geometry. Methods include static magnetic fields (local and ruler EMBR), and time-varying magnetic fields, such as EMS, multi-mode EMS, electromagnetic level stabilizers (EMLS), and electromagnetic level accelerators (EMLA). Optimal use can stabilize flow, leading to fewer surface defects, fewer inclusions, and improved microstructure.
Moving (AC) fields originated with electromagnetic stirring (EMS), where phase-shifting the fields from several series of magnetics to make the net field move in opposite directions on opposite sides of the strand induces rotating flow, normally in the transverse plane in the mould (M-EMS) or electromagnetic rotary stirring (EMRS). Making the fields move in the same direction, sometimes called ‘multi-mode EMS’, can induce accelerating flow (EMLA), or decelerating flow (EMLS). Electromagnetic forces offer an advantage over other flow-control parameters since the induced force varies with the strength of the liquid steel flow, giving the system the theoretical ability to be self-stabilizing for turbulent flow variations. In practice, this is difficult to achieve.
Principle of EMS
EMS utilizes the principle of a linear motor. It differs from the conventional mechanical and decompression types as it is a non-contact stirrer in which no part touches the liquid steel. As shown in the Fig 2a, a coil is installed at the bottom of the furnace generates a moving magnetic field (H), if a 3-phase AC voltage is applied to this coil (inductor). Electric power force is generated in the liquid steel due to the action of the magnetic field and causes induction current (I) to the flow (Fleming’s right hand rule). This current then acts with the magnetic field of the inductor to induce electromagnetic force (F) in the liquid steel as per the Fleming’s left-hand rule. This force is known as Lorentz force.
Fig 2 Principle of EMS
The rotational electromagnetic stirrer is equivalent to an asynchronous motor stator. It is normally supplied by a three-phase or sometimes a two phase frequency converter. A rotating magnetic field is generated whose variation inside the liquid steel produces eddy currents, which, interacting with the magnetic field, generate a force (Lorentz force). The final result is the occurrence of a torque which induces the steel rotation. The generated torque depends in several factors namely (i) intensity of the supplied current, (ii) number of windings forming a coil, (iii) frequency, and (iv) system geometry. These parameters change depending on the stirrer type M-EMS (mould electromagnetic stirrer), S-EMS (strand electromagnetic stirrer), and (iii) F-EMS (final electromagnetic stirrer).
Hence, the magnetic field acts as a non-intrusive stirring device and it can, in principle, be engineered to provide any desired pattern of stirring. The stirrer design, size, and position etc. depend on the continuous casting machine data, the steel grades to be produced, and the casting parameters.
EMS systems create a rotating magnetic induction field with an induction of B, which induces eddy current j in a direction perpendicular to B, whose velocity is v. Induction B and current j create the electromagnetic force, which works on every unit of volume of steel and bring about a stirring motion in the liquid steel. The vector product (v x B) demonstrate a connection between the electromagnetic field and the flow of the liquid steel. The speeds of the liquid steel caused by the EMS is somewhere in the range of 0.1 meter per second (m/s) to 1.0 m/s.
When an electric current j(x,t) flows through a conducting body, whether solid or fluid, in the presence of a magnetic field B,(x,t), there is a force F per unit volume (the Lorentz force) given by equation F=j x B which acts upon the conductor. In general, this force is rotational, i.e., curl F is not equal to zero, and, if the conductor is fluid, it cannot be compensated by a pressure gradient. In these circumstances, the fluid must move in response to the force. This, in its simplest terms, is the principle, of electromagnetic stirring.
By considering an incompressible liquid confined to a bounded volume V with surface S, and let V+ be the exterior region. Within the conductor, B and j are related by Ampere’s law (Mo)j = curl B, V x B = 0, where Mo = 4 (pi) x (10)-7 (in SI units). The magnetic field can also have external sources, e.g., currents (AC or DC) in coils in the exterior domain V+. The normal situation which can be considered is sketched in Fig 2b. Currents in the external coils C, C’, through Faraday’s law, induce a current distribution in the conductor. This current can be augmented by the direct application of potential differences between electrodes E, E’ imbedded in the boundary S. Hence the current can be induced through application of a time dependent magnetic field, or electrically, or both. A very wide range of physical conditions and an equally wide range of applications particularly in the field of metallurgical processing can be considered.
Despite the great practical importance of some of these applications, and the fact that the fundamental principles of EMS are well understood, understanding of the flows generated by EMS in all but the most idealized circumstances is still at a fairly primitive level.
Categories of EMS
EMS can be categorized based on where it is installed in the casting machine. According to the setup position and metallurgical aspects, all electromagnetic stirrers can be classified into three types. These three possible stirrer applications according to the position and the needed effects on the cast steel product are (i) M-EMS, (ii) S-EMS), and (iii) F-EMS. M-EMS is located in the mould, as the name suggest. It is the in- mould stirring (sometimes termed as primary EMS). S-EMS is located below the mould in the secondary cooling region. It is the stirring below the mould where there remains a large percentage of liquid steel (sometimes termed as secondary EMS or below mould stirring). FEMS is located at the end of the metallurgical length (just before solidification is complete) It is the stirring just prior to the final solidification point (termed as final EMS). Fig 3 shows the three principal types of stirring.
Fig 3 Types of EMS
M-EMS – A rotary type M-EMS is normally the first choice when selecting billet / bloom stirring equipment. The rotating magnetic field produced gives a circular motion in the liquid steel (Fig 3). The central equiaxed zone is enlarged since the rotational flow promotes the fracturing of the tips of the columnar dendrites, which then serve as nuclei for equiaxed crystal formations in the central zone. Further, the rotational flow flushes the solidification front, hence preventing inclusions and gas bubbles from being entrapped. Still further, the centrifugal force developed results in the lighter phases (i.e. inclusions and gas bubbles moving towards the centre of the strand away from the solidification front.
Linear M-EMS is used for larger rectangular strand sections. Two stirrers are then placed horizontally along the cast product wide sides, and the benefits are similar to those obtained with rotary stirring. M-EMS was traditionally built into the mould in an internal design, where the coil was removed from the caster with the mould. For each mould exchange, electric cables and possibly water hoses were to be connected / disconnected to the coil. New casting machines have external design in which the coil is built around the mould and remains in the caster during mould exchange.
M-EMS is normally installed in the lower part of the mould for stirring of the liquid steel in the mould. It improves surface, sub surface, and inner strand quality. The application of M-EMS results into reduction of pinholes, centre porosity, and segregation in the cast product. It improves the solidification structure, reduces the surface roughness, and increases the heat delivery rate. M-EMS is either of round or square design and it can be installed internally or externally. For providing flexible control of stirring speed in the mould meniscus, dual coil M-EMS (Fig 6) has been developed. The dual coil M- EMS consists of two independent EMS. The upper EMS is meant for flow control in the meniscus and the lower EMS performs the stirring of main metal in the mould. The reduction in the liquid steel speed in the meniscus is achieved by rotating the upper EMS magnetic field in the opposite direction to that of the lower EMS. Such a design of dual M-EMS widens the opportunities for using the EMS technique under various conditions of continuous casting of steel.
S-EMS – In a linear S-EMS, the electromagnetic coil is installed along one side of the strand and produces a vertical circulation liquid metal flow pattern in the strand (Fig 3). As the stirrer is placed along one side of the strand, it can be used for very different strand sizes. The increase in the central equiaxed crystal zone is obtained by the same mechanism as that obtained by the rotary stirrer. Inclusions, which normally are concentrated in a band close to the upper surface in curved mould continuous casting machines, are also more uniformly distributed. The rotary S-EMS which is placed in the optimum position high below the mould is sensitive to breakouts.
S-EMS produces a stirring force which pushes the liquid steel horizontally along the cast product width and generates a butterfly type flow pattern in the liquid steel. When S-EMS can be placed behind the support rollers (Fig 4) then it is not dependent on a minimum support roller diameter and hence in this case can be optimally placed along the strand from the metallurgical point of view. S-EMS when built into the support rollers needs a minimum roller diameter to include the iron core and windings. In this case the stirrer is placed at a distance from the meniscus and hence is less effective. S-EMS operates at low frequency to ensure good penetration of the stirrer force through the strand. As a result, the liquid steel has transverse stirring as shown in Fig 3. S-EMS is normally used in combination with M-EMS. S-EMS can be of either linear or rotary type stirrer. Most common is the linear stirrer, which is easy to install and protect against heat radiation and possible breakouts. S-EMS promotes the formation of equiaxed structure. It promotes grain refinement in the cast product and reduces the shrinkage cavity, centre segregation, and internal cracks. It also removes superheat effectively.
Fig 4 Location of S-EMS
F- EMS – There is equally strong interest in using EMS to stir far below the mould in the final solidification zone of a continuous casting strand. However, conventional EMS systems have proven to be somewhat ineffective when applied in this region. As a potential solution, there recently has been considerable interest in applying modulated Lorentz forces to develop a broadly distributed vigorous stirring in the final zone. F-EMS is normally installed in combination with M-EMS or S-EMS to reduce and cut peaks in centre segregation. F-EMS is particularly efficient when casting high carbon or high alloy steel grades. Also with the use of F-EMS, it is found that the solidification structure of the cast product is improved and there is increase in the ratio of the equiaxed structure and the inner porosity. The shrinkage is reduced, and the ratio of central carbon segregation is decreased. Further the secondary dendrite arm spacing (SDAS) is improved, and the ratio of central equiaxed grain is considerably increased, which results in finer grains. Hence, the quality of the cast product is improved with the F-EMS.
Basically, there are two types of stirring as applied to continuous casting ‘rotary’ stirring and ‘up and down’ (or axial) stirring (Fig 5). In the recent past, several versions of these types of stirring have been proposed in many patents, some more sophisticated than others, but all or nearly all of them can be classified in either of the categories mentioned above.
Fig 5 Rotary and axial stirring
The electromagnetic field in the EMS is created in three different ways which include linear stirrers, rotary stirrers, and conductive stirrers. The magnetic poles of the linear stirrer are situated on a straight line and the magnetic poles of the rotary stirrer are situated on a circle. Both linear and rotary electromagnetic stirrers employ AC to produce the magnetic fields and the desired effects. Linear and rotary electromagnetic stirrers induce a current in the steels. Conductive stirring, on the other, hand utilizes a conduction current plus the induction current to produce the electromagnetic field and the desired effects. Rotary EMS is installed both in the mould and in the secondary cooling region, while linear EMS is mostly used as S-EMS devices. Conductive stirring is a niche compared to the other two modes.
Rotary stirring – The original work on rotary stirring was done by a group of investigators in Austria. Billets cast in a round mould were stirred at the mould level or just below the mould. Indeed, the mould is the only area that rotary stirring might make sense. Stirring substantially below the mould in a rotary fashion can create more problems than it can solve. As stated, there is some merit in rotary stirring of rounds in the mould. Solid inclusions are removed from the surface of the casting and heat transfer is enhanced by forcing the solid skin of the strand to be in better contact with the mould. There is no danger of rupturing the skin by rotary stirring in the mould. But the major advantage of rotary stirring is in the ease of equipment design. Electrical engineers are very familiar with this type of electromagnetically induced motion since it is the same as that of nearly all electric motors in use today.
While rotary motion, presents no engineering design problem, it may not be the best type of motion from the metallurgical point of view. One of the basic problems with rotary stirring is that the liquid is subjected to centrifugal forces which tend to segregate its light constituents (inclusions for example) toward the centre (Fig 5). This imposes an upper limit on the velocity of the liquid, which is not necessarily the same limit set by the appearance of the ‘picture frame’ effect. Sometimes these considerations can be ignored, as when casting rounds for seamless pipes, possibly. However, another more inflexible limitation cannot be ignored. Raising the circular velocity of the liquid disproportionately increases the pressure on the solid shell, which can then rupture. This danger is particularly acute when casting a steel grade which contains highly segregating elements, such as phosphorus, selenium, and lead. The low melting liquid that these constituents form occupies space between the dendrites reducing any strength the shell can have.
Another undesirable situation which arises when stirring below the mould in a rotary fashion, particularly when it is done in one level only, is the effective separation of the liquid pool into two parts, an upper (hot) part above the stirring level and a lower (cold) part below the stirring level. Besides disrupting the natural flow in the pool, this partitioning can cause bridging problems. Bridging, in turn, intensifies macro segregation as can be demonstrated by some problems reported recently when rotary stirring has been applied below the mould only. Such a problem is particularly likely to develop when stirring with low velocities. Large fragments of dendrites separated from the mushy zone in the stirred region cannot be reduced in size because of the low intensity of stirring. These large fragments sink to the lower (cold) part of the pool where they have a chance to grow, form clusters, and they cause bridging.
There can be another disadvantage to rotary stirring. Early data have indicated that to reduce substantially the inclusion size and content of steel by EMS, the velocity of the liquid is to exceed a certain lower limit. For example, for AISI 4335 steel grade, this limit has been shown to be over 0.5 m/s, which is comparable to the velocities occurring during the rimming action in large ingots where a clean skin is also produced. There is a near certainty that the previously mentioned upper limits, for safe rotary liquid motion, conflict with the high speed requirements for inclusion reduction. The same is true to produce the new solidification structures, namely the fibrous structure and the flow modified or thamnitic structures, which also need high velocities. Segments of the steel industry which have been aggressively pursuing new developments for quality, in general, and induction stirring can soon be pursuing these structures through high velocity stirring.
Axial stirring – The axial or ‘up and down’ version of stirring provides for moving the liquid portion of a solidifying strand in a direction parallel to the axis of the strand, this type of induced motion can be used to intensify the naturally occurring, thermally induced, convective flow patterns. In the mould area, there are reasons to reverse the natural flow. In the continuous casting of steel, the mould area constitutes a small part of liquid pool which can be as deep as 15 m or more (depending on the speed and size of the machine). Below the mould, the flow is ‘down’ adjacent to the solid skin and ‘up’ at the centre of the strand.
The ‘up and down’ version of EMS is the most appropriate from the metallurgical point of view. The velocity of the liquid is practically unlimited in this technique, which provides ample freedom for the application of desired controls. The danger of break-outs is minimized, because the electromagnetically induced forces tend to contain the liquid rather than force it against the solid shell. There are other major benefits. The hot liquid from the top is brought quickly to the bottom of the pool, which tends to reduce somewhat the thickness of the shell and to keep the temperature gradient high across the mushy zone. Both these effects improve the heat flow, which in turn can be useful toward increasing the productivity of the continuous casting machine. There is another way the productivity can be improved with this version of EMS. The contour of the solid shell can be modified to form a round bottom and the depth of the pool reduced. This allows higher casting speeds. The extent of centre-line shrinkage and segregation can be reduced as well, since the isotherms are changed and growth at the centre of the strand has an increased upward component.
Finally, even the inclusions which form during solidification, such as the notorious alumina clusters, are not allowed to be entrapped in the solid, they are swept quickly to the top of the pool where they have a chance to join the slag (i.e. floating over the meniscus) and, hence, be eliminated. This type of flow is rather difficult to implement, particularly if stirring is to be applied over a considerable part along the metallurgical length, i.e., the continuous version of ‘up and down’ stirring. It is to be emphasized, however, that the difficulties in this case are in the electrical engineering side of the problem. It is relatively difficult to implement one-directional flow without major perturbations which show up either between coils or at the end of a series of coils which form a linear motor. These anomalies are reflected in cast structures as bands of either positive or negative segregation. Further, ‘up and down’ stirring normally needs a rather large area of the strand free of support rolls, or at least modification of the rolls, so that they do not interfere with the fields of the linear motors used. Finally, the linear motors used for this type of stirring have very low efficiency (of the order of 1 % or even less), primarily because of the high resistance of their electromagnetic loops (large air gaps and solid metal-skin gaps).
Intermittently reversing stirring – A variation of the rotary stirring mode has been suggested, originally by some Japanese investigators. The technique provides for intermittently reversing flow direction which, it is claimed, improves the size of the equiaxed zone. The above discussion for rotary stirring applies here, with a few more qualifications. Intermittent motion does waste energy, but it has merit in meeting one objective of EMS, frustrating columnar growth. Reversing flow in stirring not only can break dendrites into smaller fragments by shear in local turbulence cells, it can also frustrate the unidirectional growth of columnar dendrites, as these dendrites attempt to grow into the flow (upstream) all the time. It is doubtful, however, that other possible benefits of EMS can be derived with this technique.
Effect of electromagnetic stirring on the quality of the cast steel products
The chemical composition, solidification conditions and nature of the liquid steel flow in the mould essentially affect the surface quality and the inner structure of the strand. The process of strand formation includes solidification of the liquid steel in the mould and in the secondary cooling zone (SCZ). Rotating or travelling magnetic fields affect the nature of flows in the liquid and intensify the heat-mass transfer processes. The degree of influence of electromagnetic stirring on strand quality depends on the technical characteristics of the EMS and on its arrangement along the continuous casting bending axis. EMS can successfully be installed in the mould, in the SCZ and in the final solidification zone (FCZ).
For improving the surface, subsurface, and inner strand quality, the liquid steel stirring has to take place in the mould. M-EMS is either of round or square design and it can be installed internally or externally. The result of applying M-EMS is a reduction in centre porosity and segregation in the cast product. To provide flexible control of stirring speed in the mould meniscus, the dual-coil M-EMS (Fig 6) has been developed. It consists of two independent EMS. The upper EMS is intended for flow control in the meniscus. The lower EMS performs the main metal stirring in the mould. The reduction in metal speed in the meniscus is achieved by rotating the upper EMS magnetic field in the opposite direction to that of the lower EMS. Such an M-EMS design widens the opportunities for using the technique under various conditions of continuous casting of liquid steel.
Fig 6 Quality of strand without stirring, with SMS, and dual coil M-EMS
The application of electromagnetic stirring of steels promotes the formation of an equiaxed crystallic zone in the strand. The stirring improves strand quality, even in steel casting with overheating. To further reduce and cut peaks in centre segregation, F-EMS, in combination with M-EMS or S-EMS, has to be used. F-EMS is particularly efficient when casting high carbon or high alloy steel grades. F-EMS and M-EMS combinations reduce the areas with the highest carbon content, where cementite and martensite otherwise can form. It has been found that stainless steels, solidifying with primary ferrite, have a sound centre at a reduction ratio of 3.6 when using S-EMS and F-EMS. The application of S-EMS increases the equiaxed crystallic zone instead of columnar structure and reduces cracks in the steel strand. The benefits available by using one or more EMS in combination are listed in Tab 1.
|Tab 1 Benefits available with using one or more EMS|
|M-EMS||M-EMS + F-EMS||M-EMS + S-EMS + F-EMS||M-EMS + S-EMS||S-EMS||S-EMS + F-EMS|
|Pinhole and blowhole||+++||+++||+++||+++||–||–|
|Surface and subsurface cracks||+++||+++||+++||+++||–||–|
|Surface cracks (round)||++||++||++||++||–||–|
|Solidification structure and internal cracks||++||++||+++||++||+**||+**|
|Centre line segregation, and centre porosity||++||+++||+++||++||++||+++|
|* S-EMS in high position|
|** Better structure only in centre part of the product, after position of S-EMS, worse structure in external part compared to application of M-EMS. Risks of negative segregation when excessive stirring applied.|
|*** with S-EMS in low position|
For more demanding qualities the use of EMS can be justified when the costs of the quality defects, conditioning or rejections, or the costs of casting larger sections are too large. Rotary stirring is used for carbon steel with carbon less than 0.2 %. In some cases, in-mould stirring is preferred than the secondary stirring since in the secondary stirring the negative segregation is found. In-fact negative segregation does not have any effect on the mechanical properties but one minor exception is that it can cause local variation in the hardenability which is not appreciated. Carbon content between 0.2 % and 0.5 %, two-stage stirring is used. It is better to complement the in-mould stirring with the secondary stirring or final stirring. For carbon content higher than 0.5 % and alloy steels with a large solidification range, three-stage stirring is used.
Any benefits from EMS for slabs can be negated from the poor geometry. So, care is to be taken for the machining. Method of reducing submerged nozzle convection currents with the EMBR for improving cleanness. This consists of two sets of coils placed along the outer walls of the mould faces. The magnetic field reduces the liquid steel velocity and impurities float to the surface where they are trapped by the mould powder. The roll gap geometry of bloom casters and more considerably slab casters can have a major influence on the internal quality of continuous cast semis and on various types of segregation and consequently the increased levels of some elements in these segregated areas. The main types of segregation caused by deviations from the true roll gaps are (i) inter columnar macro segregation, (ii) centre line macro segregation, and (iii) off centre line semi macro segregation (also termed V segregation or spot segregation).
In the temperature range 1,300 deg C up to the solidus the ductility of steel is very low. This is due to the liquid phases of FeS and MnS which have segregated to the boundaries between dendrites. FeS and MnS both have melting points much lower than steel and hence these weak boundaries open at quite low tensile strains.
One of the metallurgical problems found in continuously cast products is the development of large columnar dendritic zones. The effect of columnar growth on the mechanical properties such as loss of ductility in steel has been investigated by Weiser. Alberney, have shown that centre line defects in the continuous casting can be considerably reduced by controlling the columnar growth regions. The control of columnar growth is crucial in producing good quality strand cast products.
Essentially, induction stirring causes a sweeping flow along the solid-liquid interface which affects the final solidification structure since it influences the local growth conditions such as the temperature gradient, the boundary layer thickness, and the structure and size of the ‘mushy zone’. Since macro-segregation is known to result from inter-dendritic fluid flow, reduction in the length of the ‘mushy zone is to effectively reduce the extent of macro-segregation, particularly along the centre line. Several studies have shown that EMS is an effective means of improving continuously cast steel solidification structures by preventing columnar growth.
The size of columnar zones and associated inter-dendritic segregation and shrinkage porosity are greatly reduced by the use of in-strand or in-mould electromagnetic stirring. The latter technique effectively increases the size of the equi-axed solidification zone and greatly reduces the amount of centre line shrinkage (Fig 6). The relative size of columnar and equiaxed zones in a cast cross section are also affected by superheating of liquid steel. High superheating in unstirred billets increases the size of the columnar zone because the nucleation of equiaxed dendrites is retarded. EMS reduces the effects of high superheats but does not completely compensate for the increased size of columnar zones developed by high superheat temperatures.
Superheat was one of the most fundamental factors recognized from the early years of continuous casting especially for medium and high carbon steels. In an early report, pilot plant tests were performed casting 150 mm x 150 mm billets of high carbon steels. It was proven that at low superheats or even sub-liquidus temperatures of casting, the centre line segregation was minimized. The electromagnetic stirring at the mould (M-EMS) exhibited some benefits, and the application of EMS at the strand (S) and final (F) stages of solidification started being installed in some casters. It was found that the combination of EMS, that is, (S+F)-EMS for blooms and (M+S+F)-EMS for billets, is the most effective method for reducing macro-segregation among various EMS conditions, causing them to solidify more rapidly during the final stages of solidification, providing more finely distributed porosities and segregation spots along the central region. The optimum liquid pool thickness was found to decrease as the carbon content increased, which can be attributed to longer solidification times in the solid fraction (fs) range from fs= 0.3 to 0.7. The effect of superheat on the solidification structure has been analyzed, verifying the empirical fact that increasing superheat the columnar dendritic growth increases against the equiaxed one. They concluded that convection effects influenced micro-segregation behaviour of the studied high carbon (C less than or equal to 0.7 %), and high manganese steels.
The effect of F-EMS parameters with current intensity increasing from 300 A (ampere) to 400 A and frequency increasing from 4 Hz (hertz) to 12 Hz, on the electromagnetic forces and carbon concentration distribution in the central cross section of 70 steel square billet has been studied. The optimal F-EMS parameter to make uniform the central cross-sectional carbon concentration and minimize the centre carbon segregation of 70 steel billets has been obtained with a current intensity of 280 A and frequency of 12 Hz. Under this stirring parameter, the carbon segregation indexes for all sampling points are in the range of 0.92–1.05, which is attributed to the fact that its stirring intensity is more suitable for decreasing the strand centre temperature and increasing the solidification rate of the billet. Hence, the rejected solute element has limited time to transport after electromagnetic stirring which promotes the reduction of centre segregation.
It is well known that porosities and shrinkage cavity occur in the central part of continuous cast blooms and billets. Although there are good results in carbon segregation levels at a stirring current and frequency of 280 A and 12 Hz, respectively, further investigations have shown that the F-EMS has a considerable impact on the other internal qualities of a square billet.
The effect of F-EMS parameters on centre segregation was studied in 140 mm × 140 mm billet continuous casting process. In the model, the initial growth of equiaxed grains which can move freely with liquid was treated as slurry, while the coherent equiaxed zone was regarded as porous media. The results show that the stirring velocity is not the main factor influencing centre segregation improvement, which is more affected by current intensity and stirring pool width. Because solute transport is controlled by solidification rate as stirring pool width, centre segregation declines continuously with current intensity increasing. As liquid pool width decreases and less latent heat needs to dissipate in the later solidification, the centre segregation can be improved more obviously by F-EMS. Due to centre liquid solute enrichment and liquid phase accumulation in the stirring zone, centre segregation turns to rise reversely with higher current intensity and becomes more serious with stirring pool width further decreasing, it forms positive segregation and solute can be concentrate with weak stirring, leading to centre segregation deterioration. With the optimized current intensity, centre segregation improvement is better with respect to F-EMS.
Some F-EMS stirring techniques are more effective than others in terms of structure morphological transformation from original dendritic to globulitic and in its refining. Macrostructure of casts without the use of stirring is different from the one with the use of stirring. The structure can be obtained with conventional stirring is largely globule-shaped with some presence of dendrites and dendrite fragments. The structure obtained with modulated stirring consists of entirely globule-shaped crystals and structure appears to be more refined.
Grain size can be varied by applying different stirring setting. With F-EMS conventional stirring, the grain diameter is reduced in both cast mid radius and in central area with comparison with the unstirred structure. A further grain diameter reduction has been achieved with counter-rotating modulated and unmodulated stirring. However, the smallest grain diameter in the casts has been obtained with unidirectional modulated stirring, in comparison with the grain diameter in the cast without stirring.
In general, the microstructure of samples using F-EMS consists of globules and elongated grains in the structure obtained with stirring, and fine inter-granular eutectic network containing different compounds. The coarse dendritic structure of the cast products cast without stirring can be 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 grains. This structure also appears to be more refined in comparison with that obtained with the conventional stirring.
The globule mean area and length in the microstructure of the combined mid-radius and centre area of the cast obtained with conventional stirring is when compared with the structure of the other casts. The globule mean area in the structure can be reduced, but not in case of structure obtained without stirring. The structure obtained with unidirectional modulated stirring in the casts, the globule mean area in these casts is reduced in comparison with conventional stirring. A similar trend is determined in reduction of the globule length. Concurrent with globule size reduction, their density has increased. The effect of the M-EMS on the solidification structures has been obtained under fixed superheat, casting speed, secondary cooling intensity, and M-EMS frequency. The ratio of the central equiaxed grain zone was found to increase with decreasing superheat, increasing casting speed, decreasing secondary cooling intensity, and increasing M-EMS current. But the equiaxed zone is limited for M-EMS, since it has more responsibility towards columnar zone. The grain size obviously decreased with decreasing superheat and increasing M-EMS current but was less sensitive to the casting speed and secondary cooling intensity.
White band segregation – The increasing use of electromagnetic stirring (EMS) over recent years has brought with it increased interest in the problem known as white bands. The white band is a zone of negative segregation (appearing white on sulphur prints) frequently found in S-EMS stirred products and corresponding to the position of the solidification front during stirring. The visual appearance of segregation has not only given rise to the name but is probably also the white band’s most undesirable feature. The extent of negative segregation at the white band is less than the positive segregation at the centre line, but it is continued presence after hot working can result in a deterrent to customer acceptance, mostly on cosmetic grounds. Kor has suggested an explanation, in which the white band is the result of changes in growth rate at the start and end of strand stirring. White band is due to the solute washing mechanism which was firstly found by Bridge and Rogers. This proposes that the turbulent flows caused by EMS penetrate the dendrite mesh and sweep out enriched inter-dendritic liquid (Fig 7). However, in order to maintain this action it is necessary to assume that the removed solute is very rapidly dispersed throughout the remaining liquid. This being so, it is difficult then to explain the observed solute enrichment at the end of stirring.
Fig 7 Schematic of the solute washing mechanism for white band formation
In tandem, mathematical modelling has played an important role in the implementation of EMS, as regards to providing a deeper understanding of the effects of stirring on, for example, the heat and fluid flow. A series of studies by Schwerdtfeger and co-workers have formed the cornerstone of the modelling in this area. Specifically, they have explored, both experimentally and theoretically, the effect of stirring in the round billet, rectangular bloom and slab geometries which are characteristic for the continuous casting of steel. These models consist of the Navier Stokes equations for the velocity field of the liquid metal and Maxwell’s equations for the induced magnetic flux density. In principle, these are two-way coupled, since the alternating magnetic field gives rise to a Lorentz force which drives the velocity field. This, in turn, can affect the magnetic field. Moreover, the frequency of the magnetic field is typically large enough to allow the use of the time-averaged value of the Lorentz force as input to the Navier Stokes equations.
Recent study by Vynnycky revisited the problem of a rotary EMS applied to round-billet continuous casting and found that the method used originally to determine the components of the Lorentz force led to a non-unique solution. This has been a consequence of the fact that the normal component of the induced magnetic flux density, rather than the tangential ones, has been prescribed as the boundary condition. Moreover, since the normal component has been prescribed in models for the case of longitudinal stirring for rectangular blooms also, it is natural to expect non-uniqueness in those models too. Furthermore, since the expressions for the components of the Lorentz force are still frequently used, it is clear that a resolution of the issue is still timely, especially in view of modern-day interest in modulated EMS. In this case, magnetic fields of different frequencies are applied and it is the intention that the resulting Lorentz force is to have a constant time-averaged part and a time varying one. It goes without saying that posing the correct boundary conditions for the magnetic field is important for achieving meaningful results from modelling.
Since the early industrial implementation of EMS, it has been recognized that demanding steel grades, especially those with a wide solidification range, benefit from stirring both within the casting mould and also at a later solidification stage. This type of stirring, in continuous casting of liquid steel, became known as final solidification zone stirring or F-EMS. Despite early reports on F-EMS effectiveness with respect to improving the cast strand internal quality, especially the structural soundness and segregation, in the long run it has been realized that the metallurgical performance of F-EMS lacked in both the effectiveness and consistency, which can be attributed to a number of defining factors. First, it is important to position the F-EMS with respect to the solidification stage which 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 steel products 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. 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 equi-axial solidification structure, but has very little impact on mixing in the melt volume, especially near its central region. 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 improvement of the RMF based stirring at a later solidification stage through enhancement of the secondary fluid flow in the radial-axial plane. Hence, intermittent and alternating stirring schemes, both of which use sequential forced and dormant periods, have been introduced in the 1980s. Kojima and co-workers, demonstrated experimentally, while Davidson and Boysan confirmed theoretically that strong recirculatory flow occurs 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 considerable improvement of F-EMS performance. The reasons for that can be found in the recent study by S Eckert and co-workers who have shown that the occurrence of strong recirculatory flows is contingent on a provision of a narrow range of stirring and casting parameters. Non-compliance 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 component. These recent developments have been theoretical and laboratory-scale in nature and none has been implemented into production practice. Counter-rotating magnetic fields have also been tested for stirring a solidifying aluminum alloy in laboratory experiments conducted by Vives. Considerable improvements in solidification structure have been achieved by using this stirring method.
Advantages of EMS
Advantages of EMS in the final product depend on the application and some examples are (i) better hot workability, during extrusion forging of the bars the frequency of internal failures is lower, (ii) improved shearing ability by avoiding the structure which causes cracks, (iii) improved hardenability because of improved homogeneity, (iv) improved wire rod drawing performances with a low frequency of cup and cone breakages, and (v) higher and more consistent fatigue properties of bars.
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