ZA200604177B

ZA200604177B - Electromagnetic agitation method for continuous casting of metal products having an elongate section

Info

Publication number: ZA200604177B
Application number: ZA200604177A
Authority: ZA
Inventors: Kunstreich Siebo
Original assignee: Rotelec Sa
Priority date: 2003-10-27
Filing date: 2004-10-22
Publication date: 2007-12-27
Also published as: CN100371108C; JP4758903B2; TWI324952B; ATE359886T1; EP1677928B1; US20070074845A1; DE602004006010D1; TW200533437A; CN1863625A; RU2357833C2; KR20060120054A; FR2861324A1; AU2004286877B2; BRPI0415903A; BRPI0415903B1; DE602004006010T2; EP1677928A1; RU2006118350A; JP2007509752A; KR101089261B1

Abstract

The electromagnetic stirring in a continuous casting installation for slabs, in which the mould (1) is equipped with an immersed casting nozzle (4) with lateral outlet holes (5, 5') directed towards the small faces, is carried out with the aid of sliding magnetic fields generated by polyphase induction coils arranged in the proximity of the cast metal. With the primary aim of favoring liquid metal exchanges at the heart of the solidification well (6) between the secondary zone (2) and the mould, one forces the establishment of a longitudinal metal flow in the central region of the cast product along two opposed collinear currents (10a, 10b). An independent claim is also included for a metal product with a elongated straight section from a continuous casting installation using this method of electromagnetic stirring.

Description

- W02005/044487 - 1 - PCT/FR2004/002728

The present invention relates to the continuous casting of metals, especially steel. It relates more particularly to the electromagnetic stirring of flat products (i.e. of elongate cross section) while they 5S are being cast, and even more precisely to the establishment in the metallic liquid pool of a particwlar distribution of the flows by means of applied magnetic fields.

It is reminded that the expression “product of elongate cross section” has to be understood to designate metallurgical products whose width is at least twice the th ickness, especially slabs, narrow slabs, thin slabs, etc.

Coming out the field of continuous steel casting at the start of the seventies, electromagnetic stirr ing has rapidly confirmed its position as an almost indispe nsable tool for controlling the flows in the pool umdergoing solidification. It will be mecalled that tlme principle most commonly employed is tlie well- known prrinciple of MHD (magnetohydrodynamics) wiiich, by means of a moving (rotating or travelling) rmagnetic field «generated by a multiphase inductor, or more general ly by several multiphase inductors, pl aced in the immediate vicinity of the cast product, drives the liquid metal with its displacement. Suitably located on the metallurgical height of the casting machines, these inductors, supplied with electrical current at an adjustaldle frequency, therefore allow various t—ypes of stirring modes that can be matched to the requirements of the mmetallurgist.

Moreover, constant progress in understanding the mechani sms of metal solidification during cormtinuous casting has specifically demonstrated the imnportant role pl ayed by the circulatory movements of thes liquid

REPLACEMENT SHEET (RULE) 26 i - 2 - me®kal on the general quality (i.e. int-ernal soundness, suxface cleanness or lack of inclusions , solidification structure, etc.) of the final solidified product.

In this regard, the movements impartecd to the molten metal during continuous casting may koe schematically classed into two separate categoriess, depending on whether we consider the mould or, keneath it, the secondary cooling stages of the casting machine.

The movements settled on the liquid metal within the mould, at a level where the liquid portcion of the cast metal is greatly predominant, are essemtially designed to control the flows in this critica 1 area. Indeed, : 15 hexe, where the free surface of the cast metal is found, its internal cleanliness dependss greatly on the geometrical shape of this surface. I t is also here where the first solidification skin occurs, the major importance of which being well known as regards both the surface quality of the final cast product and the cormtrol of the casting process itself.

On the other hand, by stirring the metal in the liquid pool beneath the mould, therefore ira the secondary cooling zone (usually called “in the secondary”), the aim is first to improve the internaal metallurgical stxucture of the product via the development of a laxgest equiaxed solidification, this besing known to be fawourable both to the microsegregation of the alloying elements and to the absence of central porosity in the cast product, for example. Thus, electromagnetic stirring is used for the continuous casting of slabs moxe and more frequently whenever produ-cts that require goood internal quality free of porosDdty have to be produced, such as for example thick pl ates for making boilers, or large welded pipes.

It should be only a reminder here, for better unclerstanding of the invention as wiZl be explained below, that it is well known, as shown by the diagram of the appended Figure 3 taken from the document FR 72/20546, to use, in the secondary c ooling zone of a continuous slab casting machine, linear inductors 41, 41’ placed facing each other, on ei-ther side of the large faces of the cast product, and producing transversal magnetic fields that travesl over the width of the product. The aim is thus to set up, within the liquid metal, flows which essentially develop as two adjacent loops rotating in opposite directions. These loops 42, 43 are established parallel to the large faces and extend in stages along the l.ength of the cast product on either side of a common transverse zone of driving action of the magnetic field, the flows of each loop rising along one small face and descending along the opposite small face. Such a movemment configuration is conventionally termed a “Thutterfly wings configuration”.

It is possible, as shown in the appended Figure 4 extracted from document FR 82/1084-4, to multiply, depending on the length of the casting machine, the transverse zones 51, 52 of the driving action of the magnetic fields. In this case, said zones are, pairwise, in opposite directions of rotation, between the closest neighbouring loops, for example so as to generate the largest possible stirred volume for a given available stirring power. Thus , a flow pattern referred to as a “triple-zero c onfiguration” is produced, this being formed from three adjacent loops rotating pairwise in opposite directions, namely a central loop 60 located between the two transverse driving zones 51 and 52, and two oute r loops 61 and 62 on either side of the central loop an.d rotating in the same direction.

Whatever the implementation mode adopted, this can be achieved just as well with inductors placed behind the support rollers of the secondary cooling zone of the casting machine as bet-ween these rollers (FR 72/20547) or inductors housed within the actual rollers (FR 72/20546). The same also applies as regards the means of implementing the immvention, which will be explained below. :

Historically, it seems that the discovery of this type of movement, based orm recirculation of the metal in loops set up in a plamme parallel to the large faces of the slab, stems from the fact that, unlike in long products, in the cont inuous casting of flat products the elongate shape of the cross section of the product does not easily lend =Htself to the establishment of a stable rotational movement about the casting axis. The main reason probably lies in the large velocity gradients that this xequires in the thickness of a product, which barely exceeds some twenty centimeters for the thickest products.

However, a staged-loop configuration of the type shown in Figures 3 and 4, which develops over the metallurgical length parallel to the large faces of the product, does not suffer from such a handicap. It also has the advantage of ensuring better heat exchange between the top and bottom means of the casting machine. The hottest molten metal from the top is driven by forced convection downwards by the descending runnings 42a and 43b, while the rising runnings 42b and 43b seed the top with crystallites of solidified metal that have collected ira the bot tom, thus favouring the early development oXf extensive uniform equiaxed solidification from the periphery right to the centre of the cast product. However, these loops 42, 43 cannot be developed too vigor-cusly near the top as one would wish, owing to the ris k of disturbing the free surface of the metal in the mould. At the present time, it is known in fact how much the preservation of the fragile hydrodynamic equilibr-ium of the in-mould flows prevailing at this level of the mould is necessary for obtaining good quality of the surface, of the sub-skim and of the core of the c¢ ast product.

Precisely, the introduction of the metal to be cast via the top of the mould using a submerged nozzle having lateral discharge outlet-s opening onto the narrow faces sides of the mould "has become virtually genera-l ) practice at the present time, replacing the straight nozzle with a single axial discharge, consequently reserved practically on ly for long products. A majom advantage obtained over in-mould flows lies in the fact that, as shown by the diagram in Figure 1 appended hereto, by means of a rebound effect occurring on the narrow faces of the mould, the jet of hot liquid metal coming out from each l.ateral outlet 27, 27’ in the nozzle 26 is therefore spread out naturally into two fractions. A main fraction 21 is directed downwards, im the direction of extraction of the cast product. The other fraction 22 is reflected upwards so as to provide, near the free surface 23 of the in-mould metal, the enthalpy need ed to prevent the of cast metal solidifying at the meni scus, which is very often the cause of accidental stoppages of the casting process.

The aim is thus to produce, in the mould, a circulation mode called “double roll” as opposed to the “single roll” mode.

The latter mode, shown in Figure 6, is firstly manifested by the phenom enon of metal rising up towards the meniscus upon being discharged from the outlets in the nozzle, very often resulting from an injection off argon to prevent clogging of the nozzle from the casting tundish located. above it. This first upward rise is then continued by a surface current towards each narrow face, and after by a downward-going flow along the latter. In t-his way, the velocity map is quite rapidly established in the mould, in which the velocities are generallly directed downwards in the direction of extractiom of the product, with the absence of the upper roll 22 for supplying “hot” metal to the meniscus.

Howeve vr, the “double roll” mode lasts dur ing casting only if the casting conditions (casting speed, width of the slab, depth of immersion of the cast ing nozzle, flow rate of anti-clogging argon, etc.) lend themselves theret o. Random transitions in “single rol 1* mode may appear during the actual course of castirg if these condit ions fluctuate, which in fact corresponds to a genera l case.

In addition, an essential aspect, in terms of contro 1ling the in-mould “double roll” flows, lies in the preservation within the mould of a “left-right” symmetxy of the recirculating movements at t—he meniscus on either side of the nozzle. This is because it is known that the occurrence of “left-right” asymmetries is the grounds of oscillations in the meta 1 bath that may result in unacceptable rolling of the surface, well known to the operator standing on the castirag platform.

This means that care must be taken to ensure that the partial recirculation currents 22, 22’ near the top are, adove all, steady over time in order t ¢ avoid the occurrence of “left-right” asymmetries. Thesse ascending currents, while still being thermally effec tive enough to deliver the desired heat to the meniscus, must howevex not be too intense from the hydrodynamics standpoint in order to avoid excessive agita.tion of the line of first solidification 25 that forms around the border of the meniscus against the cooled copper wall of thee mould. The regularity of this line of first solidiEication is in fact the warrant of uniformity of formation of the first skin in the top of the mould, without which there is inevitably a risk of break-outs beneath the mould by encrustations of slag «or by local thinnirmg of the thickness of the solidified skin.

Stated more simply, by casting with a subme rged nozzle

4 - 7 - having lat eral discharge outlets, it is possible to achieve, over the course of any one casting run, randomly or, in any case, not necessarily desirable, in-mould £ lows that are either of the “double roll” type, or of the “single roll” type, or unstable flows owing to “l.eft-right” asymmetries.

It is in particular because of these dif ficulties in controlling flows in the top area of contirmuous casting machines t-hat electromagnetic stirring systems have more recentzly appeared that act in the mould, already on the lat-eral discharge jets coming from the nozzle.

As the diagrams of the appended Figures 2a and 2b show, which are extracted from document JP 1 534 702, magnetic £f ields moving horizontally are produced by multiphase linear inductors 30a, 30b and 20a‘, 30b’ placed alormg large faces of the mould 32 facing the discharge ath of the metal jets on either side of the nozzle 31. By adjusting the direction of travel of the fields, it is then possible to slow down th e current of said Jets of metal (countercurrent travel of the fields, goi ng from the small face to the no zzle (Figure 3b) or, om the contrary, to speed it up (cocurrent travel in the direction going from the no=zle towards the small face (Figure 3b). In principle, this allows the amount of enthalpy supplied to the surface of the cast metal to be adjusted, for example accoxxding to the casting coraditions, without excessively dissturbing the in-mould fl ow mode that has to be preserved as a matter of priority .

The above rapid review of the prior art therefore clearly shows the separation, if not the comflict, that exists wherm casting products having an elongate cross section bet ween the stirring of the metal Jn the mould on the ones hand and the stirring in tlme secondary cooling zone on the other.

The object of the present invention is specifically to

A - 8 - overcome such a handicap. Stated another way, applicable to the continuous casting of flat products, particularly slabs, the object of the invention iss, via a studied overall stirring movement of the molten metal over the metallurgical length, to provide good exchange of still-1iquid metal in both directions betwee=n the secondary cooling zone and the mould. This will consequently achieve thermal and chemical uniformity between the top and bottom of the pool of cast liquid metal without disturbing the in-mould flow mode= and, where possdible, without correspondingly being deprived of the cumulative beneficial effects specific to stirring im the mould and to stirring in the secondary cooling zorae respectively.

One complementary object of the invention is to help to improve the metallurgical quality of steel gradess that it is desired to produce with good internal quality, such as grades for thick plate or for large welded pipes, feritic stainless steel, or silicon elexctric steel.

Another complementary object is to be able to vary the flows in t.-he secondary cooling zone in order t o use them level with the casting jets coming out froem the nozzle, ei ther as an accelerating agent or om the contrary ass a braking agent for the metal enterirag the mould, or else as a means for counteracting the “left- right” asymmetry tendencies of the metal movements within the mould.

With these objectives in mind, the subject of the invention DOs a method of electromagnetic stirring in the secondary cooling zone of a plant for- the continuous casting of slabs, or other similar flat products, the mould of which is provided with a submerged casting nozzle having lateral discharge outlets directed towards the narrow faces, which stirring method is implemented by means of travealling magnetic fields generated by multiphase induct ors placed near the cast metal, characterized in that a longitudinal liqui-<d metal flow is forcibly establis.hed in the said secomdary cooling zone, the flow be ing localized in the middle region of the cast product, as two opposing collimear currents.

This one naturally establishes a circulation of the entire liquid metal in the secondary, having the configuration of a “four-leaf clover” with two upper lobes and two lower lobes, the upper lobes of wh ich extend into the mowld right up to the level of the j-ets coming out from the discharge outlets of the cast ing nozzle. . 15

According to one implementation mode, these two longitudinal opposing collinear currents in the middle part of the product, which move away from each other, are created in such a way that the two upper loles which extend into the mould right up to the level of the jets coming owt from the discharge outlets of the casting nozzle merge concurrently with the said jets in order to reinforce them.

According to another implementation mode, these -two longitudinal opposing collinear currents in the middle part of the produc t, which converge on each other, are created in such a way that the two upper lobes that extend into the mould up to the level of the jets coming out from the discharge outlets of the castzing nozzle are superposed countercurrently on the said jets in order to slow tlaem down.

According to one particular embodiment of the method, the location of the central longitudinal flow in the secondary 1s shifted laterally towards one or other of the narrow faces of the cast product so as to counteract the “left-right” asymmetry tendencies of tthe metal movements within the mould.

According to one method of implementation, the longitudinal metal flow in the middle region of the cast product is created as two opposing collinear streams by means of collinear moving magnetic fields that travel longitudinally in the said middle region, either coming closer together, or further apart.

According to the preferred implementation, the longitudinal metal flow in the middle region of the cast product is created as two opposing collinear streams by means of collinear moving magnetic fields that travel transwersely over the width of the cast product, either coming closer together from the edge : 15 towards the centre of the cast product, or moving further apart from the centre towards the edge of the cast product.

According to another preferred implementation, the travelling magnetic fields are generated by means of multiphase linear inductors that are placed facing the large faces of the cast product.

As another implementation mode, the inductors are supplied with electric currents of different intensities, so as to vary, in a different manner, the action on the two opposing collinear metal streams created by the travelling magnetic fields that they generate.

The term “collinear” applied to the travel of the fields or to the metal flows should be understood to mean that the magnetic fields, or alternatively the streams of metal, do not travel parallel to one another but instead travel along the same line, in the manner of two collinear vectors as opposed to two parallel vectors.

As will have been understood, the invention consists,

in its principal basics, in creating, in the secondary cooling zone, &a “stirring cross” having two transverse branches and two longitudinal branches. .The transverse branches (or horizontal branches if it is assumed tchat the casting axzs is vertical) develop across the width of the cast product and the two longitudinal (or vertical) branches develop within the middle region ) (usually the axial region) of the cast product.

Indeed, this st-irring cross in the secondary zone leads to the development of recirculation flows in the licuid pool of quadrillobate configuration, and then creates a global configuration of the movements that also relates to the mould region, such that the aforementioned objectives inteended by the invention are reached.

The invention wrill be more clearly understood and other aspects will become more clearly apparent in the 1i ght of the description that is given with reference to the appended plates of drawings in which: - Figures 1 to 4 are representative of the pr-ior art, already considered above. More precisely: *Figure 1 is a standard diagram showing, in summary form, Gn vertical central section parallel to the large face=s of the mould, the known map of the circulatory movements of the liquid metal entering a mould for thee continuous casting of slabs via a submerged noz=le provided with lateral dischaarge outlets that op en onto the narrow faces; *Figures 2 a, 2b; and 2b; are diagrams, in two vi. ews (on the left im perspective and on the right in cross section), of lxnown in-mould electromagnetic stirr-ing modes for the continuous casting of slabs with a submerged nozzle having lateral discharge outlets (cE.

Fig. 1) by mearis of linear multiphase inductors housed on either side of the nozzle on each large face and producing magnetic fields that travel horizontally in opposed directi ons, pairwise, over the same large face, either in the ssame direction as the discharging jet of s - 12 - metal to which the field is applied (Fig. 2b), or in the opposite direction (Figs 2b; and 2a); *Figure 3 is a simplified diagram showing, in perspective, a slab during continuous casting as can be seen in the secondary cooling zone of the casting machine. This zone is provided with a pair of linemar inductors facing each other on each side of the product over the width of the latter and generating a magnet-ic field gliding horizontally, so as to produce a “butterfly wings” -shap ed electromagnetic stirring mode known is for example from the aforementioned docume=nt

FR 72/20546; *Figure 4 is a diagram similar to the previous ne in figure 3, but. showing a “triple rol 1” electromagnetic stirri ng mode, such as that produced for example by impleme nting the teaching of the afor-e- mentioned document FR 82/10844; - The other fi-gures, numbered 5 to 9, are specific to the invention. More precisely: : 20 *Figure 5 is a general diagram, seen in axi al vertical section paralllel to the large faces of a mould for the continuous ca.sting of slabs, the said mould being provided with a submerged nozzle having later-al discharge outlets that open towards the narrow face s, showing the principle of the global stirring in t he form of a four-leaf clover in the secondary cooli ng zone according to one of the two implementation mod. es of the invention in -which the opposing longitudinal streams move away from each other, and the map of t he circulatory movements of the liquid metal that resul ts therefrom within this zone just below the mould; *Figure 6 is a diagram similar to that of Figu re 5, but in the case in which the in-mould flow mode is no longer of the “double roll” type but is of ¢t he “single roll” type; *Figure 7a 1s a dliagram which, on the basis of a repeat of Figure 5, sh-ows by means of implementing tlhe stirring in the form of a four-leaf clover by means of linear inductors having a horizontally travellimg magneatic field; “wo 000 ANT as *Figure 7b is a diagram similar to Figure 7a, but illustrating another embodiment of this method of implementing the invention, this time using linear inductors having a vertically travellimg magnetic field; *Figure 8 is also a diagram which, on the basis of a repeat of Figure 5, illustrates a preferred embodiment of the invention, setting up a complementary in-mould flow in “double roll” mode by mearas of linear inductors generating a horizontally travel ling field, which act directly on the jets of metal discharging from the outlets in the casting nozzle; and *Figure 9 illustrates another implementation implementation mode of the invention, which consists in creating opposing longitudinal streams in the middle part of the cast product, these no longer being divexgent but convergent.

It will be recalled that Figures 1 to 4 were used to support the explanation of the prior art already made at the beginning of this document. They willl therefore not ke referred to again in the following tesxt.

In ¥igures 5 to 9 representative of t he wmode of stirring in the secondary cooling zone specific to the invemtion in these two implementation modes (divergent or convergent streams at the middle), the travelling magnetic fields, just like the linear inductors that produce them, are represented by thick vertical or hori zontal arrows. The convective movemerats produced are themselves shown by their main paths in the form of line s carrying arrowheads indicating the direction of circulation of the movement over the carryi ng path. The solid lines represent active convection zones, and ther efore circulation zones subjected to the action of the travelling magnetic fields. The broken lines repr esent the passive convection zones, in other words reci rculation zones which are necessarily complementary n - 14 - to the active zones in order to close the 1 oop of the movements.

In these figures, the same elements are «denoted by identical references. Where necessary, in or der not to unnecessar-ily overload certain figures, recurrent references have not been indicated so as te make the essential elements of the invention shown in these figures cl earer.

Each of the figures shows a continuous sl ab casting mould 1 f ollowed beneath it by the secondary cooling zone 2 of the casting machine, here intentiormally shown without tlme support rolls in order not to unmecessarily : 15 reduce the clarity of the drawing. Since the views are in a plane parallel to the large faces of the mould, only the narrow faces are visible at 3 and 3’, these faces determining the narrow faces 18, 18’ of the cast product 6 . Since the large faces are in the plane of the figurees, they are not referenced in the figures.

Moreover, for greater clarity, the referernce 6 will denote either the cast slab itself or its s till-liquid core, more generally called the “liquid pool” .

A submerged nozzle 4 centred on the castdng axis A (which is coincident here, as is conventionally the case, with the longitudinal axis of the casst product) supplies the mould with molten metal from a tundish (not showm) located above it. This nozzle =is provided with latemral discharge outlets 5 and 5’ each facing one or other of the narrow faces 3 and 3' respec tively. The size of tche cast product is determined by the inside dimensionss of the mould that defines the ca sting space into which the molten metal enters in the f orm of jets 7, 7' disscharging from the outlets in thes nozzle 4, conventiorially along a more or less horizontal mean direction, or slightly inclined downwards . The cast product thus advances from the top, level with the meniscus 8, downwards, in the extraction 4d irection of bh - 15 - the casting machine, along the vertical or along a curved path in a plane orthogonal to that in the figures, at an extraction rate (casting rate) usually of the order to one metre per minute. As it advances, the product progressively solidifies from its periphery up to the centre, by extraction of its internal heat, firstly into the mould 1 in contact with the cooled copper walls and then in the secondary cooling zone 2 under the effect of the water spray rails.

It will be xeminded that the metallurgical length (or depth of the liquid pool) is conventionally defined as the differerace in the dimensions along the vertical between the level of the free surface of the cast metal in the mould (or meniscus) and that of the bottom of the liquid pool below the secondary cooling zone, at the point where the finishing solidification fromts, which develop over each of the large faces of the cast product as the solidification progresses, meet.

Located arbi trarily along the longitudinal axis of the product (which is coincident with the casting axis A), about 3 or 4 m below the meniscus 8, and therefore within the s econdary cooling zone 2, is a point P that will be termed the centre of the stirring cross 9 whiich is a specifi«< feature of the invention. This cross 9 is a cross witkh four branches, these being collinear in pairs, namely two longitudinal (and here vertical) branches 10a, 10b, forming a pair aligned with the casting axis A, and two transverse (and here horizontal) branches 1la, 11b forming a pair that develops over the width of the cast product. In each of the two branches of any one pair, the liquid metal circulates therein, pairwise, in opposite directions.

Moreover, the circulation of the metal in one pair is in the opposzite direction to that of the other pair.

Owing to the necessarily “finite” dimensional character of the cast product, these branches, as may be seen,

vo - 16 - are as it wrere connected together by recircullation loops in order to form an overall flow that develops in the plane of the large faces of the cast product in a four-leaf clover configuration, the leaves constit-uting the lobes L1, L2, L3, 14, the upper two of which, Ll and L4, exten-d up to the mould level with the discharge jets 7 and 7°’ .

Thus, in the stirring mode shown in Figures 5 to 8, the pair of vertical branches is of a *divem-gent” convection type - the streams of metal move aways from each other £ rom the centre P. One, 10a, flows away towards the mmould 1 lying above it while the other 10b flows away do wnwards, in the direction of extractdon of the cast product, towards the closure point o f the liquid pool. In the horizontal pair 1lla, 1lb , the convection of the metal is therefore of the “convergent” type - the metal streams flow towardss each other in the direction of the centre of conflueraice P, flowing from the small faces of the product towards the longitudinal -axis A.

As already me ntioned, the metal streams that form these branches are created by travelling magnetic f_delds, which are themselves generated by linear indwuctors placed in the immediate vicinity of the cast product facing these large faces (preferably both sidess). Of course, it is unnecessary for the two pairs of branches to be simultaneously activated by the magnetic f ields.

Only one may" be activated, for example the ve=xxtical branches 10a, 10b, the other branches 1la, 11k» then becoming, of course, the site of recirculati on by reaction, simmce the centre P acts as a current p assage node that mmaintains the mass flow rates ane the movement quantities, and vice versa.

However, in this first stirring mode of the inve ntion, it is important for the vertical branches 10a amd 10b to flow away from each other, as shown in Figures 5 to

8. In the upper lobes L1 and 14 that are close to the mould, the metal rises along the centre and descends along the narrow faces, the opposite being the case in the lower lobes L2 and L3.

Under these conditions, it turns out that the implementation of the invention maximizes the exchange i of metal material between the bottom and top of the liquid pool. Firstly, the circulation of metal in any one lobe takes place in the direction of rotation opposite to that established in the two closest neighbouring lobes. Secondly, since the force of the casting jets 7 and 7' is then systematically reinforced by the cocur rent rising central flux 10a, the recirculation loops L5 and Lé in the mould near the meniscus 8 are in turn reinforced. Consequently, the “double roll” mode L5, L1, L4 and Lé present within the mould are thus additionally stabilized.

It will therefore be readily understood that any liquid metal element (conceptually isolated at an arbitrary point along the metallurgical length) will have a high probability of being present, by randomly following the successive ascending or descending running, at least once in the mould before redescending if it is initially in thre secondary cooling zone, and vice versa if it is initially chosen to be in the mould, it being understood that overall the element will necessarily undergo a mean downward displacement in the direction of extraction with a mean speed equal to the casting speed. In other words, this implementation of the invention maximizes the exchange of molten metal material between the hot zones of the mould and those cooler zones of the secondary and does so by reinforcing, im the mould, the known means suitable for stabilizing the “double roll” mode.

Such an excharmage contributes in particular to better removal of the excess heat and to the initiation of early and ample equiaxed solidification of the metal, without any risk of disturbing the in-mould flow mode, by instead reinforcing the stability of the “left- right” symmetry of the movements on either side of the nozzle, and to do so whatever the local mode present, namely “double roll” (cf. Fig. 5) or “single roll” (cf.

Fig. 6), and therefore counteracting the natural random tendency for transit ion from one mode to the other.

As already mentione d, the branches 10 and 11 of the stirring cross 9 ares generated by the action applied at these points by travelling magnetic fields. The lines of force of these f-elds are orthogonal to the surface of the cast product, or at the least have a . 15 predominantly orthogonal component, in order to maximize the electr-omagnetic coupling with the liquid metal.

It is well known that such fields can be easily produced by conventi onal multiphase linear inductors.

Figure 7a illustrates a first implementation of the invention in which two identical linear inductors 12 and 13 are placed horizontally at the same vertical level on the casting machine (collinear inductors) on either side of the casting axis and mounted in opposition so as to create collinear magnetic fields travelling transversely over the width of the cast product, from the small sides 18, 18‘ towards the centre. These inducttors are advantageously designed so as to each generate a travelling magnetic field, in an active convection branch (lla or 11b), having a length equal to slightly less than one half of the half-width of the cast slab 6.

In this case, the driving force for the stirring is given by the converggent transverse branches lla, 11b of the stirring cross, and the longitudinal diverging flows 10a, 10b ares then obtained after passing the point of confluence P.

Figure 7b illustrates a second mode of implementation, equivalent to the previous ome as regards the effects obtained. According to this s econd implementation mode, the linear inductors 14 and 15, mounted collinearly but in opposition, are placed vertically along the casting axis. In this way, the longitudinal branches 10a and 10b (the presence of which within the secondary is at the very basis of the invention) are this time activated directly, the upper inductor 14 then generating a magnetic field travelling towards the top of the casting machine in the direction of the mould, the lower inductor 15 producing a field that travels downwards towards the bottom of the pool.

Figure 8 illustrates a preferred embodiment of the invention. This consists in converting the upper edge of the upper recirculating lobes L1 and L4, which reinforce the casting jets 7 and 7’, into active convection zones. To do this, added to the pair of inductors already present in the secondary cooling zone, for creating the stirring cross 9, are two additional linear inductozs 16, 17 generating horizontally travelling fieEds, these two inductors being placed collinearly on e ither side of the nozzle 4 level with the jets of metal 7 and 7’ discharging from the outlets 5 and 5’ and travelling cocurrently with the said jets, from the nozzl e towards the narrow faces 3, 3’ of the mould 1. The eff ect of convergence between the jets and the central flow rising up from the bottom is thus further enhanced, amd consequently the local in-mould “double roll” mode likewise.

Figure 9 is similar to Figure 5 but is distinguished therefrom however in an essential manner by the fact that the directions of circul ation of the metal in each of the four branches of the cross 9 are reversed. This

Figure 9 thus illustra tes the second main x - 20 - implementation mode of the irvention, which consists in creating opposing longitudinal collinear currents 20a, 20b in the middle part of the cast product 6, which this time converge on each ot-her towards the point P so as to provide an overall «circulation of the liquid metal that is extended, in the mould 1, by currents rising along the small sides 18, 18’ up to level with the jets of metal 7, 7' coming out from the discharge outlets 5, 5’ in the nozzle, which they oppose as a countercurrent in order to br-ake them.

Overall there is again a stirring configuration in the secondary cooling zone consi sting of four lobes L1 to

L4, the loops of which therefore rotate in opposite directions to those of the first implementation mode.

However, because of the oppeosing effect of the upper lobes Ll1 and L4 on the jets 7 and 7‘, the downward return flows of the metal Dn the middle part of the liquid pool are less channelled and confined, instead much more diffuse and disper sed in that section of the product than in the said first implementation mode.

It will be understood that these two main implementation modes are in fact only two different and complementary facets of the same invention and may be jointly present when impleme=nting the stirring method.

It will in fact be easy to modify, in terms of dynamics, the directions of travel of the acting magnetic fields, for example by reversing the polarities of the inductors that produces them, so as to be able, on demand, to» brake or accelerate the running of the casting jets 7, 7’ by acting on the stirring localized in the secondary, far away from these jets.

It will therefore be seen that a key advantage of the invention is that it ensure s good top/bottom exchange in the liquid pool while still being able to act remotely on the casting jets in the mould, and to do so

. a TE 4 wa? Lo a by a simple and unsophisticated arrangement of the electromagnetic stirring equipment, the components of which are widely available commercially.

As will have been understood, the invention consists, in summary, in judiciously using the electromagnetic stirring means currently available in order to make, in the secondary cooling zone, a cut in the long direction of the product into two juxtaposed strands and, in each strand, to install a butterfly-wings-type stirring configuration. By doing this, an over-all flow system is created in the secondary cooling =z one consisting of four lobes, the core of which is the stirring cross 9 with its centre P.

Preferably, for obvious reasons of symmetry, this division into two strands will take place at mid-width of the cast product, that is two say along the longitudinal axis of the latter, as this axis generally coincides with the casting axis.

This said, it will be sufficient to unbalance the stirring forces between the two transverse branches lia, 11b, for example by a differential adjustment of the intensities of the electrical «currents supplying the inductors 12, 13 in order for the central position of the centre P to be shifted laterally towards one narrow face, 5, or towards the other, 5’, and thus to obtain a more selective effect on the in-mould movements on one side of the nozzle than the other.

Likewise, a similar imbalance in the longitudinal branches 10a, 10b will make it possible, using given stirring equipment, to cause an upward or downward displacement from the centre P of fhe stirring cross without having to modify the positiora of this equipment on the casting machine.

If it is desired to be able to use both these options of

Tt. - 22 - adjusting the position of the centre P of the stirring cross, it will admittedly be necesssary to provide the secondary cooling zone with equipment consisting of four inductors so as to be able to electromagnetically activate each of the four branches 10 a, 10b, lla and 11b.

Whatever its mode of implementat ion, the invention provides overall stirring of the metal over the metallurgical length capable of en suring both thermal and chemical uniformity between the top and bottom of the liquid pool without correspondingly being deprived of the beneficial effects specific to stirring in the mould and stirring in the secordary cooling zone respectively, and without disturbing, indeed by steadying, the local flow mode in tlae mould.

It goes without saying that the invention is not limited to the examples described above, rather it extends to many implementation modes or equivalents provided that its definition, giver in the claims that follow, is respected.

Thus, for example, although the liraear inductors to be used conventionally have a plarae structure, this arrangement is only a preferred one. Also suitable may be inductors of curved shape in order to better match the shape of the surface of the slako at the point where they are placed along the metallurgi.cal length.

Claims

- W02005/044487 - 23 - PCT/FR2004/002728 CLAIMS

1. Method of electromae«gnetic stirring in the secondary cooling zone of a plant for the continuous 5S casting of metal products of elongate cross section, the mould of which is provided with a submerged casting - nozzle having lateral d-ischarge outlets directed towards the narrow faces, which stirring method is implemented by means of -travelling magnetic fields generated by multiphase indwictors placed near the cast metal, characterized in that, for the purpose of promoting liquid metal exchange within the liquid pool (6) between the secondary cooling zone (2) and the mould (1), a longitudinal metal flow is forcibly established in the said secondary cooling zone, the flow being localized in thes middle region of the cast product, as two opposing col linear streams (10a, 10b or 20a, 20b) and providing «circulation of the entire liquid metal as a “four-IJdeaf clover” configuration having two upper lobes and two lower lobes, said upper lobes (Ll, L4) extending into the mould right up to the level of the jets (7, 7’) coming out from the discharge outlets (5, 5’) of the subme rged casting nozzle (4).

2. Stirring method according to Claim 1, characterized in that the said longitudinal opposing collinear streams (10a, 10b) in the middle part of the cast product move away from each other are created in such a way that the said two upper lobes (Ll, L4) which extend into the mould right up to the level of the jets (7, 7') coming out from the discharge outlets (5, 5’) of the casting nozzle merge concurrently with the said jets in order to reinforce t hem.

3. Stirring method according to Claim 1, characterized in that the said longitudinal opposing collinear streams (20a, 20b) in the middle part of the cast product converge on each other are created in such a way that the two upper lobe=s (Ll, L4) that extend into the mould up to the level of the jets (7, 7') coming out from the discharge outlets (5, 5’) of the casting nozzle are superposed countercurrently on the said jets in order to slow them down.

4. Stirring method accore«ding to Claim 1, characterized in that the location of the central longitudinal flow in the secondary cooling zone is shifted laterally towards one or other of the small sides of the cast product.

5. Stirring method according to either of Claims 2 1s and 3, characterized in that the longitudinal metal flow 1s created, in the middle region of the cast product, as two opposing collirmear streams by means of collinear moving magnetic fields that travel longitudinally in the said middIde region, either coming closer together, or further apart.

6. Stirring method according to either of Claims 2 and 3, characterized in that the longitudinal metal flow is created, in the midd le region of the cast product, as two opposing collin ear streams by means of collinear moving magnetic fields that travel transversely over the width of ®he cast product, either coming closer together from the edge towards the centre of the cast product, or moving further apart from the centre towards the edge of the cast product.

7. Stirring method according to any one of the preceding claims, characterized in that the travelling magnetic fields are generated by means of multiphase linear inductors that are placed facing the large faces of the cast product.

8. Method according to Claim "7, characterized in that the inductors are supplied with electric currents of

0 h different intensities.

9. Method according to one of the preceding claims, characterized in that travelling magnetic fields are also used that act directly in the mould (1) on the jets (7, 7’) of metal dischargineg from the outlets (5, 5‘) in the nozzle (4).

10. Flat metal product obtained from a continuous casting plant, the secondary cooling zone of which being the location of an electromagnetic stirring operation according to that defined in Claim 1.