EP0008376B1 - Method for continuously casting metal in a mould and influence of an electro-magnetic field - Google Patents

Description

The present invention relates to a process for the continuous casting of metal, in which the melt is poured into a mold, the resulting strand having a liquid core is drawn out, guided and further cooled and by means of at least one electro-, magnetic field in the strand-inducing stirrer by means of the Melt acting thrust a turbulent flow is generated in the liquid core.

The structure of a strand produced by the continuous casting process depends, among other things, on the composition of the material and the casting temperature. At casting temperatures of only a few degrees Celsius above the melting temperature, a globular, non-directional structure, and at casting temperatures with more than 15 ° above liquidus, a columnar, directed structure with a strong, central positive segregation of the accompanying elements. In practice, for casting reasons, casting must be carried out at excess temperatures of more than 20 ° C. For this reason, many efforts have already been made to obtain a slab with a predominantly globulitic, undirected structure and little central segregation even with continuous casting at such excess temperatures.

In the continuous casting of steel, it is known to obtain improvements in the quality of the cast material by means of a more or less strong turbulent flow by magnetic stirring of the melt in the liquid core. These improvements have been achieved through various methods of applying the shear forces to the melt.

Alloy and accompanying elements such as C, Si, Mn, P, S etc. are contained in the steel, which can lead to segregation, especially central segregation, when solidified. Such segregations, as well as the crystal structure, are known to include depending on the level of excess temperature. Such segregations are to be prevented by electromagnetic stirring or by the turbulent flow generated. The solidification structure should be influenced in such a way that the largest possible zone of dense, undirected crystal structure is obtained. However, it has been shown that the solidification front is influenced by the strong local movement of the melt in such a way that so-called white bands form. These white bands are negative segregations that can have a negative impact on quality.

When casting billets or blooms, to improve the surface and internal quality, it is known to set the melt in the liquid core with the help of an electromagnetic device in a rotating motion about the longitudinal axis of the beam. The rotation is generated with an imperfect rotating field (three magnetic poles). A fine-grained structure is probably obtained, but the formation of a large white band could not be prevented.

Furthermore, a device is known in which an electromagnetic device with three pole pairs is arranged around the mold tube, which sets the liquid core into a movement rotating about the longitudinal axis of the strand. This rotation, created by a perfect rotating field, has an insufficient turbulence in its flow. The mixing of the liquid steel is imperfect because there is no force acting across the strand due to the uniform magnetic application of the melt. This relatively low turbulence leaves something to be desired in terms of the quality of the cast product in relation to the surface, the distribution of the alloy and accompanying elements, but also to the internal structure.

According to a known method, thrust forces are generated in the direction of the longitudinal axis of the strand with an electromagnetic traveling field, the magnets running around the strand being arranged between the pairs of rollers up to the end of the sump. The flow created along the swamp brings the desired area of non-columnar structure and prevents the occurrence of significant segregations, in particular the central sedimentation and white bands. Such an arrangement requires too much space due to the large number of magnets, hinders sufficient cooling of the strand and is far too complex.

Another known method for slab formats attempts to eliminate these white strips by generating shear forces on the liquid steel with electromagnetic traveling fields, excited by two magnets located on the long sides opposite one another. These shear forces should act transversely to the longitudinal axis of the strand so that the flow is gently knocked against the solidified wall so that the deflected flow is kept within a limited range. This limited range of action results in an insufficient zone of dense, undirected crystal structure: It has further been shown that the white bands can only be poorly eliminated with this process, so that these disadvantages do not allow an optimal product to be obtained, which can e.g. can have a negative impact on the quality of the rolled product.

In experiments with shear forces influenced by symmetry in the phases with straight lines running transversely to the longitudinal direction of the strand Thrust direction, generated by a stirrer arranged on the long side of a slab, the micrographs showed white bands and a wide zone of dendrites, which resulted in an unsatisfactory quality of the cast steel.

It is the object of the present invention to create a method with which an optimal strand quality is obtained. In particular, there should be a sufficient zone of dense, undirected crystal structure. The cast material should not have any white bands and should be low in segregation, particularly with regard to the central segregation.

According to the invention, this object is achieved in that pulsating thrust forces acting differently on the melt are generated by applying a predetermined asymmetry in the phases.

Surprisingly, it was found that with shear forces acting differently on the melt within the fields, such a turbulent flow was generated that despite the high excess temperature there were practically no white bands in the micrograph and that the desired zone of dense, undirected crystal structure was reached without a significant central gradient.

Depending on the given casting parameters, according to an advantageous application of the method, a linear thrust direction can be produced in the melt transversely or longitudinally to the longitudinal axis of the strand from the differently acting shear forces within the fields. In the case of larger strand cross sections with a thrust effect running transversely to the direction of the strand, the space required to produce a stirring effect which is sufficiently long in the direction of the strand is therefore reduced.

For billet and billet formats, according to another advantageous application, the shear forces acting differently within the fields produce a shear direction in the melt that runs in an arc around the longitudinal axis of the strand. When using this process in the mold, the strand surface can be improved in addition to the better internal structure.

According to a further application of the invention, in order to generate the different shear forces, the windings of one phase coil can be acted upon by different currents in comparison to the windings of at least one other phase coil. These different current strengths are advantageously in a range between approximately 10% -25%. The differently acting thrust forces can also be generated by different geometrical designs of the phase coils.

Surprisingly, it has been found that the turbulent flow becomes more efficient if, according to a further feature of the invention, the weaker thrust of one phase takes effect before the stronger thrust of the phase following in the direction of thrust.

At the start of the pouring process, a certain period of time passes before the liquid core can be set into orbital motion. In order to achieve the desired turbulent flow as quickly as possible, the asymmetry in the current application of the phase coils in the start-up period is set from approximately zero to a predetermined maximum value according to an additional criterion. It was thus possible to ensure that the foremost strand section also has the desired metallurgical quality.

In the method according to the invention, due to physical laws, in addition to the force acting in the stirring direction, a transverse force pulsating between zero and a maximum results. This superimposed force has an advantageous effect through an additional amplification of the turbulence if, according to another feature of the invention, by adjusting the direction of thrust of the fields, the force resulting from the asymmetry in the phases and perpendicular to the stirrer surface is directed towards the strand Stirrer surface takes effect. In this way, an additional improvement in quality can be achieved, particularly in the case of a stirrer arrangement on one side.

• Fig. 1 die Anordnung eines Rührers mit geradlinig verlaufender Schubrichtung zur' Durchführung des Verfahrens in einer Bogenanlage,
• Fig. 2 ein Schliffbild eines halben Querschnittes einer Bramme, deren Flüssiger Kern mit einem quer zum Strang wirkenden Wanderfeld gerührt wurde,
• Fig. 3 die Verteilung des Schwefels entlang der Linie 111-111 der Fig. 2,
• Fig. 4 ein Schliffbild einds halben Querschnittes einer Bramme, deren flüssiger Kern gemäss der Erfindung gerührt wurde,
• Fig. 5 die Verteilung des Schwefels entlang der Linie V-V der Fig. 4 und
• Fig. 6 einen Schnitt durch eine Knüppel-oder Vorblockkokille mit einem, die bogenförmig verlaufende Schubrichtung erzeugenden Rührer.

Examples of the invention are described in more detail with reference to figures, these figures illustrating the following:

• 1 shows the arrangement of a stirrer with a linear direction of thrust for 'carrying out the method in a sheet feeder,
• 2 shows a micrograph of a half cross section of a slab, the liquid core of which was stirred with a traveling field acting transversely to the strand,
• 3 shows the distribution of the sulfur along the line 111-111 of FIG. 2,
• 4 shows a micrograph of half a cross section of a slab, the liquid core of which was stirred according to the invention,
• Fig. 5 shows the distribution of the sulfur along the line VV of Fig. 4 and
• 6 shows a section through a billet or bloom block with a stirrer which produces the arcuate direction of thrust.

In Fig. 1, 1 denotes a cooled, curved and oscillating mold for casting a slab, which is supplied with liquid steel from a casting vessel, not shown, via a pouring tube reaching into the mold 1. The strand 2 formed in the mold 1 and having a liquid core 3 is guided and supported in a curved strand path 4 following the mold 1 with a radius of 10 m with the aid of rollers 5. Spray nozzles 6 for further cooling the strand 2 are arranged between the rollers 5. The strand is pulled out and straightened by a driving judge 7.

A stirrer in the form of a traveling field magnet 10 of known construction is arranged on the inside of the strand 4 at a distance of approximately 5 m below the end of the mold. Between the magnet 10 and the inside of the strand 2, rollers 5 'made of an anti-magnetic material, for example stainless steel, are attached. The magnet 10 is constructed in two phases. Three-phase magnets can also be used. The shear forces generated by the stirrer act transversely to the longitudinal axis of the strand.

The slabs cast on the system described had a format of 1550 mm x 270 mm. The pull-out speed was about 0.55 m / min. Both phases were initially at a voltage of 200 V with a frequency of 2 Hz and approx. 1000 amperes, i.e. applied in a known manner, symmetrically. Fig. 2 shows the micrograph of a steel cast at an excess temperature of 29 ° C with 0.15% C, 0.025% S and usual other accompanying elements, wherein, as mentioned, a conventional stirring method was used. The micrograph shows a relatively thin edge zone 20 with a predominantly globulitic structure. This zone 20 is followed by a zone 21 with a columnar structure of dendrites directed towards the center. Zone 21 is followed by zone 22, which has an undirected crystal structure, is lighter and represents a white band. This band can consist of one piece, as the reference number 22 indicates, or can be divided into several bands 23, 24, 25. The zone 22 is followed by a zone 26 with a dense, non-directional crystal structure, which merges into the central reduction 27.

FIG. 3 shows the result of the quantitative analysis of the sulfur fraction length along line 111-111 in FIG. 2. The sulfur content is plotted on the ordinate and the slab thickness on the abscissa. It can be seen from the diagram that the sulfur content in the white band (zones 23, 24, 25) is markedly reduced.

4 illustrates a micrograph of half the cross-section of a slab stirred by the method according to the invention. The format of the slab cross-section, steel quality, pull-out speed, direction of the thrust forces and frequency were the same as described for FIG. 2. The excess temperature was 43 ° C. The strength for the excitation current was 830 A for one phase and 1000 amperes for the other phase. One phase is thus exposed to a current that is approx. 20% higher than the other phase, i.e. the phases of the electromagnetic fields are asymmetrical. A zone 31 with a predominantly globulitic structure can again be seen in the micrograph. This is followed by a zone 32 with dendrites directed towards the center of the slab. A weakly formed zone 33 with a crystal structure that shows no alignment is followed by zone 32. The center of the slab has a zone 34 with an also undirected crystal structure, which is, however, finer and denser than that according to FIG. 2.

FIG. 5 shows the result of the quantitative analysis of the sulfur content along the line V-V of FIG. 4. The analysis reveals that when the liquid core is stirred by the method according to the invention, a relatively uniform distribution of the sulfur is achieved with the turbulent flow generated thereby. Both the positive central increase and the negative segregation in zone 32 largely no longer occur. Only insignificant white bands are present.

The asymmetry described creates shear forces with different shear effects on the melt, i.e. The frequent, frequency-dependent change between stronger and weaker thrust increases the turbulence within the flow in the liquid core significantly. Due to the temporal ranking of the weaker before the stronger thrust, i.e. the phase charged with 830 A takes effect before the phase with 1000 A, increases in the direction of action of the traveling field due to the periodically pulsating thrust to turbulence increases.

On the basis of micrographs, it has been shown that in practical casting operation at the start of the pouring process, a considerably longer period of time elapses before a quality-perfect casting structure is achieved if, instead of an almost symmetrical mode of operation, the stirrer is already operated asymmetrically in the start-up period. In order to achieve the desired turbulence in the melt, closed flow circuits, so-called flow rollers, must first be formed in the strand core, into which additional turbulence is then integrated due to the difference in the thrust forces of the two phases in an asymmetrical mode of operation. In practice, this means that at the start of pouring until sufficient flow has formed in the bottom of the strand, stirring with only a slight difference in the current applied to both phases, e.g. phase 1 is charged with 1000 and phase 2 with approximately 1000 amperes. After forming the necessary flow, i.e. after a turbulent orbiting movement in the liquid core has been reached, the system switches over to the asymmetrical loading already described. In this way, the poorer quality of the first part of the slab could be significantly shortened.

It has also been shown that the running direction of the hiking field also has a decisive influence on the casting quality. For example, stirring transverse to the strand with a linear direction of thrust can run from left to right or vice versa on a broad side of the slab. The stirrer can be arranged on one or on both broad sides. Asymmetry creates a natural shear force that is natural, perpendicular to the main movement component and also perpendicular to the strand pulling direction. In the preferred case, by adjusting the direction of thrust of the fields, the force resulting from the asymmetry in the phases and perpendicular to the stirrer surface should be effective away from the stirrer surface facing the strand. With certain casting parameters, their direction of action can be rotated by 180 °, i.e. away from the middle of the strand towards the strand skin.

When two opposing stirrers interact, e.g. in the case of thick slabs, these can preferably be switched in such a way that their traveling fields run against one another, so that both the transverse force of one and the other traveling field are directed towards the middle of the strand.

The denser, undirected crystal structure produced by the described method and the insignificant white bands result in much better properties of the rolled product when the slabs are rolled out. On top of that, little space is required for the device for generating the optimal turbulent flow.

In the example given, the different shear forces are generated by applying different currents to the windings. However, these different shear forces can also be caused by different geometrical designs of the phase coils, e.g. of the number of turns. The traveling field magnets can also be arranged in such a way that the different shear forces act in the direction of the longitudinal axis of the strand or at an angle to it. Instead of a hiking field acting from one strand side, an additional hiking field can be provided on the other side of the strand. With strands with long liquid cores, more than one moving field can act in the longitudinal direction of the strand. The turbulent flow can also be effective in the mold, the flow can also be effective in the mold, the flow advantageously being kept in such a way that it does not affect the bath level, so as not to have a negative effect on the surface quality of the strand. The asymmetry described can also be achieved by the interaction of several stirring segments in the same stirrer with different loading or geometric design of the phase coils.

The use of the method according to the invention for a billet mold is described with reference to FIG. 6, wherein an arc-shaped thrust direction produces a rotary movement of the melt about the longitudinal axis of the strand. With 51 a mold is referred to on average. It consists of a mold tube 52 made of copper and a mold jacket 53. A cooling jacket 54 is arranged around the tube 52. Cooling water flows through the space between the mold 52 and the cooling jacket 54. A partially solidified strand 60 with a liquid core 61 is shown in the interior of the mold 52. This strand 60 is drawn from the mold by known means and cooled further.

Magnetic poles 70, 71, 72, 73 are provided on each side of the cooling jacket 54, each of which is provided with a turn 74, 75, 76, 77. These magnetic poles are cooled by cooling water in the space between cooling jacket 54 and mold jacket 53. The turns 74, 75, 76, 77 are switched so that a traveling field is created. These magnetic poles form an electromagnetic field in the strand-inducing stirrer. According to the casting parameters, one phase is fed with a 1025% higher current than the other, subsequent phase. For a stick of 100x100 mm, the turns 74 and 76 are at a frequency of 50 Hz and a voltage of 50 V with 400 A and applied windings 75 and 77 with 320 A. The traveling field produces differently acting shear forces in the liquid steel which, due to the arrangement of the magnetic poles described, cause the melt to rotate. If a slower penetration of the stirring action or a lower stirring speed is desired, the frequency is reduced accordingly, especially with large wall thicknesses of the mold tube.

The circuit can, however, also be selected such that the magnetic flux flows between the pole pairs 70, 72 or 71, 73 and the rotary movement is generated in this way with the aid of the magnetic field. The pole pairs 70, 72 are excited, for example, with 400 A and the pole pairs 71, 73 with 320 A.

The number of poles can be increased for larger billet and bloom formats. Instead of the asymmetrical application of current to the turns, the differently acting thrust forces can be generated by different geometrical designs of the phase coils, e.g. by different number of turns or by different formation of the pole irons, such as shape and size of the iron cross sections and / or polar axis directions etc.

The asymmetrical application of electricity or the different geometric Training can also be combined. In the last example cited, stirring was described with an arcuate thrust in the mold. This stirring can also be used in the secondary cooling zone. Instead of the arc-shaped thrust direction, the straight-line thrust direction in the strand longitudinal direction can also be used in the mold.

The method according to the invention can be used for all types of continuous casting plants with continuous molds, also for plants for casting beam pre-profiles and non-ferrous metals. In the case of strands with long, liquid cores, several stirrers can work together.

Claims (9)

1. A method for the continuous casting of metal, wherein molten metal is poured into a mould, the resultant strand, having a molten core, is withdrawn, guided and further cooled, and turbulent flow is set up in the molten core by means of thrust forces acting on the molten metal and produced by at least one stirring device inducing electromagnetic fields in the strand, characterized in that pulsating thrust forces, acting differently on the molten metal, are set up by applying a predetermined asymmetry in the phases.

2. A method according to Claim 1, characterized in that a rectilinear thrust direction, transverse of or along the longitudinal axis of the strand is produced by the differently acting thrust forces within the fields.

3. A method according to Claim 1, characterized in that a thrust direction, extending along an arc about the longitudinal direction of the strand, is produced in the molten metal by the differently acting thrust forces within the fields.

4. A method according to any one of Claims 1-3, characterized in that the windings of one phase coil are acted upon by current strengths which are different from those applied to the windings of at least one other phase coil of the fields.

5. A method according to Claim 4, characterized in that the windings of one phase coil are acted upon by a current which is 10% to 25% higher than that applied to the windings of the other phase coil.

6. A method according to any one of Claims 1-3, characterized in that the differently acting thrust forces are produced by different geometric configurations of the phase coils.

7. A method according to any one of Claims 1-6, characterized in that the lower thrust force of one of the phases becomes effective before the higher thrust force of the next phase occurring in the thrust direction.

8. A method according to any one of Claims 1-7, characterized in that the asymmetry of the current load of the phase coils is adjusted in the start-up period from approximately zero to a predetermined maximum value.

9. A method according to Claim 2 or 6, characterized in that, by adjusting the thrust direction of the fields, the transverse force, occurring at right angles to the surface of the stirring means and resulting from the asymmetry in the phases, becomes effective in the direction away from that face of the stirring means that is presented to the strand.

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